BY-NC-ND 3.0 license Open Access Published by De Gruyter January 21, 2016

Emerging nanostructured materials for the catalytic removal of volatile organic compounds

Jiaqi Li, Hui Liu, Yuzhou Deng, Gang Liu, Yunfa Chen and Jun Yang
From the journal Nanotechnology Reviews

Abstract

The strong growing interest in using catalytic oxidation to remove volatile organic compounds (VOCs), which seriously threaten the health of human being, is rooted in its desirable features such as relative energy savings, low cost, operation safety and environmental friendliness. Within the last decades, the development of manufacturing processes, characterization techniques and testing methods has led to the blossom of research in synthesis and application of various nanostructured materials, which creates great opportunities and also a tremendous challenge to apply these materials for highly efficient catalytic removal of VOCs. We herein will systematically introduce the latest research developments of nanostructured materials for the catalytic degradation of VOCs so as to provide the readers a coherent picture of the field, mainly focusing on noble metals and metal oxides, which are currently two primary types of VOC catalysts. This review will focus on synthesis, fabrication and processing of nanostructured noble metals and metal oxides as well as the fundamentals and technical approaches in catalytic removal of VOCs, providing technical strategies for effectively developing novel nanostructured catalysts with low cost, enhanced activity and high stability for pollutant removal from surrounding environments.

1 Introduction

It has been well recognized that the exposure to volatile organic compounds (VOCs), for example, benzene, phenol, toluene and formaldehyde, may trigger many serious health problems, such as irritation of eyes, skin and respiratory tract, headaches, pneumonia and cancer, even at very low concentrations [1–4]. Therefore, there is an increasing interest in investigating the origins, reducing the emissions and developing the purification technologies for VOCs. VOCs are mainly emitted from factories or industrial processes including chemical, power and pharmaceutical plants, gas stations, petroleum refining, printing, shoe making, food processing, automobile, and furniture and textile manufacturing [5]. In addition, VOCs are also the most abundant and harmful chemical pollutants in indoor air [2]. Among the indoor sources, solvents, glue, insulating materials, and cooking and tobacco smoke are considered as the major indoor contributors to VOCs [6]. Currently, the existing purification technologies are divided into two major categories: adsorption and degradation. However, in the adsorption process, secondary pollution is easily caused when the used adsorbent is disposed. On the other hand, degradation technologies include photodegradation [7–9], plasma degradation [10–12], biodegradation [13–15] and catalytic degradation [16–18]. Taking into account rapidness, efficiency and energy saving, catalytic degradation is the optimal strategy for VOC abatement because VOCs can be completely and quickly oxidized into CO2 and H2O over certain catalysts at a considerably lower temperature. Therefore, the fabrication of highly efficient and stable catalysts is crucial for the development of catalytic degradation.

In recent years, great efforts have been devoted to investigating the application of nanostructured materials for the catalytic removal of VOCs from our surrounding environments, focusing on the relationship between the performance and physical or chemical features of nanomaterials in order to develop efficient and stable catalysts for reducing the temperature for the complete oxidation of VOCs. Generally, there are two major types of efficient nanomaterials developed for the catalytic oxidation of VOCs: supported noble metals and transition-metal oxides. Usually, nanostructured noble metals have a better catalytic activity for VOC oxidation, but their wide application is still limited by the high cost. Thus, there is increasing interest in enhancing the catalytic performance of per unit noble metal to reduce the total metal loading on the supporting substrates. On the other hand, more and more highly efficient single or multiple transition-metal oxides at nanometer scale are developed in order to replace noble metals as catalysts for VOC oxidation. Owing to the unremitting efforts of scientific communities, significant advances have been successfully made for catalytic oxidation of VOCs at low temperatures, even at room temperature in some cases [6]. Therefore, in the following, after a brief update of the literature, we prefer to devote this review to summarizing supported noble metals and transition-metal oxides with various nanostructures, including their synthesis, characterization and potential applications as catalysts for the complete oxidation of VOCs so as to provide the readers a systematic and coherent picture of the field. Most of these studies have only been carried out in the last several years, particularly by the authors in different laboratories. Regarding the creation of great opportunities and tremendous challenges due to the accumulation in nanostructured noble metals and transition-metal oxides, we will make some perspectives in the last section of this review for the future developments in using these nanostructured materials for the VOC removal. We hope the understanding of the fundamentals of VOC catalytic oxidation would be helpful for the establishment of more effective technical approaches for the pollutant removal from surrounding environments.

2 General methods for catalyst synthesis

2.1 Methods for the synthesis of nanostructured noble metals

2.1.1 Reduction of metal salts

Metallic nanoparticles are usually synthesized by the reduction of metal salts. During the reaction, synthesis media such as organic solvents [19–22], microemulsions [23–28] or aqueous solutions [29–32], proper reducing agents and surfactants all have a significant effect on particle formation. The synthesis media have a significant influence on the final particle size, morphology, uniformity and distribution. As a typical example, Yang et al. synthesized stellated Pt nanoparticles with Ag cores using oleylamine as a reducing agent and capping surfactants, as shown in Figure 1, which have uniform size and enhanced activity for catalyzing methanol oxidation reaction [33]. The choice of reducing agents is of importance for controlling the reaction rate as well as the particle size. Chen and co-workers prepared a series of highly active TiO2-supported Pt nanoparticles (Pt/TiO2) with various sizes using different reducing agents, for example, hydrogen (H2), sodium borohydride (NaBH4) and sodium citrate, which exhibit different catalytic activity for the decomposition of benzene [34]. Nearly 100% benzene oxidation was achieved over the Pt/TiO2 catalysts obtained by the sodium citrate reduction at approximately 160°C due to the negative charges and rich chemisorbed oxygen on their surface. The commonly used reducing agents include NaBH4 [28, 29, 31, 32, 35, 36], hydrazine [37–40], hydrogen [34, 41–44], sodium citrate [34, 45], some organic ammonium salts [19, 21, 22] and polyol [30, 46, 47]. Among them, NaBH4 has high reducing capability with a reducing potential -1.24 V versus standard hydrogen electrode at pH of 14 and 0.48 V at pH of 0. It can reduce metal salts in either aqueous or non-aqueous media at any pH values. In addition, one NaBH4 molecule can supply eight electrons for the reduction reaction if water (H2O) is the product, and hence NaBH4 is a highly efficient reducing agent [48]. Hydrazine is another commonly used reducing agent, which has a standard reduction potential of -0.23 V in an acidic and -1.23 V in a basic medium. Because the standard reduction potential of metals normally varies from -1 to any positive value, hydrazine is mainly used in an alkaline medium and the pH value for the maximum reduction efficiency of hydrazine is 11 or higher [49]. As illustrated in numerous publications, surfactants are mainly used for controlling the size and morphology of particles as well as modifying or passivating their surface properties to prevent them from agglomeration, such as polyvinylpyrrolidone [30, 50, 51].

Figure 1: Representative TEM images (A, B, G, J, M), HRTEM images (B, E, H, K, N) and corresponding EDX spectra (C, F, I, L, O) of the stellated Ag-Pt nanoparticles made after the reaction for 30 min (A, B, C), 60 min (D, E, F), 90 min (G, H, I), 120 min (J, K, L) and 180 min (M, N, O), respectively. Reproduced from [33] with permission from Nature Publishing Group.

Figure 1:

Representative TEM images (A, B, G, J, M), HRTEM images (B, E, H, K, N) and corresponding EDX spectra (C, F, I, L, O) of the stellated Ag-Pt nanoparticles made after the reaction for 30 min (A, B, C), 60 min (D, E, F), 90 min (G, H, I), 120 min (J, K, L) and 180 min (M, N, O), respectively. Reproduced from [33] with permission from Nature Publishing Group.

2.1.2 Thermal decomposition of metal salts

Impregnation [34, 52–57] or deposition-precipitation (DP) [58–61] followed by thermal decomposition of metal salts at elevated temperature is another widely used method to prepare supported noble metal nanoparticles for catalytic removal of VOCs. For an impregnation process, metal nitrates, which can be easily decomposed, are often used instead of metal chlorides, other usual metal salts. During the DP process, the choice of precipitant and its concentration would have an obvious influence on the speed of the reaction, thus affecting the final size and/or morphology of noble metal particles. For example, Shi and co-workers prepared two kinds of CeO2-supported Au nanoparticles (Au/CeO2), as shown in Figure 2 for the transmission electron microscopy (TEM) images, by DP using urea or NaOH as a precipitant for the formaldehyde (HCHO) oxidation [62]. Due to the generation of increased amounts of active surface oxygen species resulting from the strong Au-CeO2 interaction, the Au/CeO2 catalysts using urea as a precipitant show higher activity than that of the Au/CeO2 catalysts using NaOH as a precipitant, leading to 100% conversion of HCHO into CO2 and H2O at room temperature, even in the presence of water and at high gaseous hourly space velocity (GHSV, 143,000 h-1). Other commonly used precipitants include strong alkali [53, 59, 63], ammonium hydroxide [64] and carbonate [58, 65].

Figure 2: The HRTEM images of Au/CeO2 catalysts. (A) Au/CeO2 (DPU); (B) Au/CeO2 (DPN). Reproduced from [62] with permission from Elsevier BV.

Figure 2:

The HRTEM images of Au/CeO2 catalysts. (A) Au/CeO2 (DPU); (B) Au/CeO2 (DPN). Reproduced from [62] with permission from Elsevier BV.

2.2 Methods for the synthesis of nanostructured metal oxides

2.2.1 Sol-gel method

The sol-gel method refers to producing solid materials from small molecules. The method is applicable for the fabrication of metal oxides, especially the oxides of silicon and titanium. The process is conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. This process is a combination of two steps, hydrolysis and polycondensation of suitable molecular precursors, which leads to the gradual formation of a solid-phase network. Typical precursors are metal alkoxides. The reactions are affected by some physical parameters such as pH, substituent and solvent [48]. Then to obtain the final sample, the gel is dried and calcinated to remove the organic components. Li and co-workers first prepared titania (TiO2) and alumina (Al2O3) sols separately, and then they mixed them together, dried and calcined to obtain the final Al2O3-TiO2 substrates, which were used to deposit nanostructrued Pd metal for the total oxidation of ethanol [55]. They found that the addition of 5 wt.% Al2O3 to TiO2 plays an important role in high stability of the anatase structure of TiO2, which was ascribed to the formation of Al-O-Ti chemical bonds in Al2O3-TiO2 crystals. Meanwhile, the Al-O-Ti chemical bonds could effectively promote the dispersion of Pd on the surface of the Al2O3-TiO2 substrates, and hence the activity of Pd/Al2O3-TiO2 for the total oxidation of ethanol is distinctly higher than that of Pd/TiO2.

2.2.2 Thermal decomposition technique

Analogous to the preparation of nanostructured noble metals, the thermal decomposition technique is also often used to produce metal oxides. In brief, metal salts were first deposited on appropriate substrates by impregnation [66] or DP [67], and then they were calcinated at high temperature to obtain the final metal oxide products. Typically, Babu et al. synthesized CeO2 nanoparticles by chemical combustion using glycine as a fuel [68]. Cerium nitrate and glycine were dissolved in double-distilled water. The mixture was kept in a hot plate and evaporated to form dry powder. Subsequently, the mixture was heated at 300°C to initiate the ignition of fuel for the occurrence of the decomposition process. The resulting powder was centrifuged, washed and dried to obtain CeO2 nanoparticles. Recently, Chen et al. reported the preparation of Pt/SiO2 catalysts by flame spray pyrolysis (FSP) using tetraethoxysilane (TEOS) as a precursor of SiO2 [41]. Platinum acetylacetonate and TEOS were dissolved in toluene with a certain ratio to prepare the precursor solution, which was then dispersed to a flame to initiate combustion. Finally, the as-formed Pt/SiO2 particles were collected by a glass fiber filter with the aid of a vacuum pump. The advantage of FSP is easy to scale up, and the production process does not require washing, filtration, drying and calcination.

2.2.3 Hydrothermal method

Hydrothermal synthesis is a technique to crystallize substances from high-temperature aqueous solutions under high vapor pressure in which generally insoluble materials can be dissolved. In the autoclave, a temperature gradient is maintained between the opposite ends of the growth chamber. The nutrient solute dissolves at the hotter end, while deposits on a seed crystal at the cooler end, resulting in the formation of desired products. This method is particularly suitable for the growth of large good-quality crystals accompanied by maintaining good control over their composition. Zhang et al. employed MnSO4·H2O, KMnO4 and (NH4)2S2O8 as precursors, respectively, to prepare α-, β-, γ- and δ-MnO2 (Figure 3) using the hydrothermal method for the catalytic oxidation of HCHO at low temperature [69]. They found that there are dramatic differences in activities among these MnO2 catalysts with different crystal structures. δ-MnO2 exhibits the best activity among the four catalysts and achieves nearly complete HCHO conversion at 80°C due to the tunnel structure and active lattice oxygen species, while α-, β- and γ-MnO2 obtain 100% HCHO conversion at 125°C, 200°C and 150°C, respectively.

Figure 3: SEM images of the α-, β-, γ- and δ-MnO2 samples; 1 and 2 refer to different magnifications of one sample. Reproduced from [69] with permission from the Royal Society of Chemistry.

Figure 3:

SEM images of the α-, β-, γ- and δ-MnO2 samples; 1 and 2 refer to different magnifications of one sample. Reproduced from [69] with permission from the Royal Society of Chemistry.

3 Air purification by supported noble metal nanostructures

Noble metal-based nanostructures are the preferred ones for VOC catalytic oxidation because of their superior activities, as summarized in Table 1 for the publications in recent five years. Improving the dispersion of noble metals on the surface of substrates is an effective strategy to enhance their utilization. For noble metal catalysts supported on conventional substrates such as Al2O3, SiO2 or TiO2, changing the electronic structure or chemical state of noble metals by altering the particle size, morphology and structure or adding suitable assistants is the way to activate oxygen for the VOC oxidation, while for noble metal catalysts supported on transition-metal oxides, especially those with variable valences, the activation of oxygen can be achieved by increasing the amount of the adsorbed oxygen and improving the mobility of lattice oxygen through doping other transition metals to the substrates. It is noteworthy that noble metal catalysts are easily deactivated at high temperature due to sintering. Hence, many leading research groups devote themselves to improving the stability of these catalysts.

Table 1

Summary of publications in recent five years on noble metal-based nanostructures for catalytic oxidation of VOCs.

YearCatalystsLoading (wt.%)Loading methodaCatalyst mass (mg)VOC typeVOC conc. (ppm)WHSVb (mL gcat-1 h-1)T90c (°C)Refs.
2012Na-Pt/TiO21IMHCHO600120,000RT[70]
2015Pt/TiO20.01–1IM250HCHO22±260,000RT[71]
2015Pt/MnO2/TiNTd0.2IM200HCHO~5030,000<30[72]
2012Pt/MnO20–2Reduction100HCHO46020,000<80[50]
2015Pt/ZSM-50.5Reduction100Toluene100060,000174[30]
Pt/ZSM-51Reduction100Toluene100060,000149[30]
Pt/ZSM-52Reduction100Toluene100060,000135[30]
2014Pt/HZSM-5-600.5IM100Toluene100060,000185[73]
Pt/NaZSM-5-600.5IM100Toluene100060,000180[73]
Pt/KZSM-5-600.5IM100Toluene100060,000169[73]
Pt/CsZSM-5-600.5IM100Toluene100060,000169[73]
2013Pt-R/Beta-He1IM100Toluene100060,000190[74]
Pt-O/Beta-H1IM100Toluene100060,000200[74]
Pt-R/Beta1IM100Toluene100060,000211[74]
Pt-O/Beta1IM100Toluene100060,000214[74]
2014Pt/SiO20.21Flame spray pyrolysis100Benzene10060,000149[41]
2014Pt/CeO2, commercial ceria0.35IM140n-Butanol100060,000135[75]
Pt/Al2O30.25IM140n-Butanol100060,000165[75]
Pt/23% CeO2-Al2O30.25IM140n-Butanol100060,000167[75]
Pt/CeO20.35IM140CH3COOH100060,000204[75]
Pt/Al2O30.25IM140CH3COOH100060,000286[75]
Pt/23% CeO2-Al2O30.25IM140CH3COOH100060,000221[75]
2011Pd/TiO21IM+

Reduction
500HCHO10120,000RT[76]
Pd/TiO21DP+

Reduction
500HCHO10120,000RT[76]
2014Na-Pd/TiO21IM60HCHO14095,000RT[77]
2015Pd/CeO2-Cube1Reduction100HCHO60010,00014[78]
Pd/CeO2-Oct1Reduction100HCHO60010,00048[78]
Pd/CeO2-Rod1Reduction100HCHO60010,00080[78]
2015Pd-Au/FeCeIM1DP0.5 mLBenzene12,0624000<150[58]
Pd-Au/FeCeMM1DP0.5 mLBenzene12,0624000150[58]
Pd/FeCeIM1DP0.5 mLBenzene12,0624000<150[58]
Pd/FeCeMM1DP0.5 mLBenzene12,0624000170[58]
2014Pd-Pt/Ce/KL-NYf0.17+0.03IM0.375 mLBenzene100020,000205[79]
2011Au/CeO23DP50HCHO500120,000RT[80]
2011Au/CeO20.56DP200HCHO60066,000<60[81]
2011Au/CoOx4DP250C3H8800012,000206[82]
2011Au/CuMnOx-3001CPC3H8500045,000280[83]
2012Au/Co-UVM-71.21DP70C3H8100040,000360[84]
Au/Co-UVM-71.21DP70Toluene100040,000280[84]
2011Au/CeO21.5DP200Toluene100036,000265[85]
Au/CeO21.5DP200C3H6100036,000175[85]
2011Au/TiO21DP200C3H6100035,000261[86]
2011Au/CeO21.7DP200Benzene500015,000178[87]
2015Ag/TiO28IM60HCHO110100,00091[52]
2014Ag/CeO2-N2.6Hydrothermal50HCHO81084,000107[88]
Ag/CeO2-P2.3IM50HCHO81084,000175[88]
2013Ag/CeO2/SiO25IM145HCHO18,000–22,00069,000147[89]

aIM, impregnation; DP, deposition-precipitation; CP, co-precipitation.

bWHSV, weight hourly space velocity.

cT90, the temperature for the 90% conversion of VOC; RT, room temperature.

dTiNT, TiO2 nanotube arrays.

eBeta-H, mesoporous Beta zeolite; Beta, conventional Beta zeolite.

fKL-NY, kaolin/NaY composite.

3.1 Platinum (Pt)-based nanostructures

Pt-based nanomaterials have always been used for catalytic oxidation of VOCs such as benzene, toluene, xylene, formaldehyde and methane, due to their outstanding catalytic performance.

Pt nanoparticles can be easily well dispersed on porous substrates. For instance, Jaroniec et al. prepared mesoporous AlOOH nanoflakes for the deposition of Pt (Pt/AlOOH) [32]. Compared with Pt supported on commercial Al2O3 and TiO2 (P25), the Pt/AlOOH catalysts with Pt deposition of 0.8 wt.% exhibit the highest activity for the decomposition of HCHO at room temperature, as shown in Figure 4. The excellent performance of Pt/AlOOH could be attributed to the abundance of surface hydroxyls, high dispersion of Pt nanoparticles, excellent adsorption behavior of porous AlOOH substrates, and their high specific surface area (SSA) and large pore volume. The results suggest that highly efficient catalysts for indoor air purification at room temperature, including HCHO decomposition, could be designed by depositing nanoparticles of noble metals on mesoporous supports with abundance of surface hydroxyls and large surface area.

Figure 4: (A) The concentration change of formaldehyde as a function of test time on AlOOH, AlOOH-c (calcined AlOOH), c-AlOOH (commercial AlOOH) and c-Al2O3 absorbents and (B) the concentration change of HCHO and CO2 as a function of reaction time for Pt/AlOOH, Pt/AlOOH-c, Pt/c-Al2O3 and Pt/TiO2 catalysts. Reproduced from [32] with permission from Elsevier BV.

Figure 4:

(A) The concentration change of formaldehyde as a function of test time on AlOOH, AlOOH-c (calcined AlOOH), c-AlOOH (commercial AlOOH) and c-Al2O3 absorbents and (B) the concentration change of HCHO and CO2 as a function of reaction time for Pt/AlOOH, Pt/AlOOH-c, Pt/c-Al2O3 and Pt/TiO2 catalysts. Reproduced from [32] with permission from Elsevier BV.

Most recently, Yu et al. synthesized hierarchical macro-mesoporous Pt/NiO hollow microspheres for catalytic decomposition of formaldehyde in air at room temperature [31], as displayed in Figure 5 for scanning electron microscopy (SEM) images. They first prepared hierarchically structured Ni(OH)2 hollow spheres from Ni(NO3)2 by hydrothermal treatment, and then calcined the as-obtained Ni(OH)2 hollow spheres at 300°C, 400°C, 500°C or 600°C for 2 h in air to obtain macro-mesoporous NiO hollow spheres. Pt deposition on NiO hollow spheres was achieved using a combined NaOH-assisted impregnation of supports with Pt precursor and NaBH4 reduction. The obtained Pt/NiO macro-mesoporous samples were labeled as Ni300P, Ni400P, Ni500P and Ni600P, respectively, for the NiO substrates prepared at different temperatures. The order of catalytic activity is Ni400P > Ni300P >Ni500P >Ni600P. The highest activity for Ni400P is mainly due to its bimodal macro-mesoporous structure with open and accessible pores, which provides a large specific surface for HCHO adsorption and Pt dispersion, enables the fast diffusion and transport of gas molecules, and ensures the accessibility of active sites on Pt nanoparticles for gas reactants.

Figure 5: SEM images and the corresponding high-magnification SEM images (insets) of the samples: Ni80 (A), Ni400 (B) and Ni600 (C); TEM image of the Ni400P sample (D), HR-TEM image of an individual nanosheet in the Ni400P sample (E) and HAADF-STEM image of Ni400P (F). Inset scale bars=500 nm. Reproduced from [31] with permission from Wiley-VCH.

Figure 5:

SEM images and the corresponding high-magnification SEM images (insets) of the samples: Ni80 (A), Ni400 (B) and Ni600 (C); TEM image of the Ni400P sample (D), HR-TEM image of an individual nanosheet in the Ni400P sample (E) and HAADF-STEM image of Ni400P (F). Inset scale bars=500 nm. Reproduced from [31] with permission from Wiley-VCH.

Analogously, Chen and co-workers adopted porous SBA-15 silica with high surface area as a substrate for Pt loading through a deposition-reduction or impregnation method for deep oxidation of benzene [45]. The porous SBA-15 substrate provides sufficient room for the dispersion of Pt nanoparticles. The authors found that the synthesis routes have a great influence on the physicochemical properties and catalytic performances of Pt/SBA-15 catalysts. The catalysts reduced by NaBH4 and H2 exhibit higher activities (Figure 6) than those of the catalysts obtained by sodium citrate reduction because of their smaller crystallite size, higher dispersion and negatively charged surface induced by strong metal-support interaction. Complete benzene conversion to CO2 has been achieved at ca. 145°C over the best NaBH4-derived Pt/SBA-15 catalyst, which is 82°C lower than the temperature for total benzene oxidation over the catalyst prepared by sodium citrate reduction.

Figure 6: Benzene conversion (%) as a function of temperature (°C) over the Pt/SBA-15 catalysts prepared by different methods. (A) Reduced by NaBH4 without calcination, (B) impregnated and treated at 450°C in air, (C) impregnated and treated at 450°C in H2 and (D) reduced by sodium citrate without calcination. Benzene concentration=1000 ppm in air, catalyst weight=100 mg (mesh=40–60) and SV=60,000 mL g-1 h-1. Reproduced from [45] with permission from Springer.

Figure 6:

Benzene conversion (%) as a function of temperature (°C) over the Pt/SBA-15 catalysts prepared by different methods. (A) Reduced by NaBH4 without calcination, (B) impregnated and treated at 450°C in air, (C) impregnated and treated at 450°C in H2 and (D) reduced by sodium citrate without calcination. Benzene concentration=1000 ppm in air, catalyst weight=100 mg (mesh=40–60) and SV=60,000 mL g-1 h-1. Reproduced from [45] with permission from Springer.

The size of Pt particle is often regarded as a sensitive parameter for the catalytic oxidation of VOCs. Xiao et al. successfully loaded 1 wt.% Pt nanoparticles ranging from 1.3 to 2.3 nm onto the surface of ZSM-5 (Pt-x/ZSM-5, where x is the mean diameter of the Pt nanoparticles) for catalytic oxidation of toluene [30], and the results are shown in Figure 7. They found that Pt-1.9/ZSM-5 has the highest activity due to a balance of Pt dispersion and Pt0 proportion, which leads to the oxidation of 98% toluene at 155°C.

Figure 7: (A) Dependence of the catalytic activity on reaction temperature and (B) the dependence of T5, T50 and T98 of toluene on Pt particle size in the complete oxidation of toluene over the Pt-x/ZSM-5 catalysts. Reproduced from [30] with permission from the Royal Society of Chemistry.

Figure 7:

(A) Dependence of the catalytic activity on reaction temperature and (B) the dependence of T5, T50 and T98 of toluene on Pt particle size in the complete oxidation of toluene over the Pt-x/ZSM-5 catalysts. Reproduced from [30] with permission from the Royal Society of Chemistry.

Further, Huang et al. synthesized a series of 1 wt.% Pt/TiO2 catalysts with various Pt particle sizes ranging from 1.54 to 22.3 nm by NaBH4 reduction and thermal treatment at different temperatures for the catalytic oxidation of gaseous HCHO at ambient temperature [35]. As illustrated in Figure 8, the turnover frequencies (TOFs) of nanoparticles are apparently affected by the Pt particle size. The TOFs are nearly linearly increased with the increase of the Pt particle size, reaching the maximum value of 4.39 s-1 on the Pt/TiO2-R-650 (Pt/TiO2-R-T, where R and T represent the reduction treatment and heating temperature, respectively) catalyst with the Pt particle size of 10.1 nm. A further increase in the Pt particle size would lead to the decrease of the TOFs. Pt particles with small size can offer high surface area and maximize Pt utilization, both of which are essential for effective catalysis. However, the particle size-dependent catalytic activity of Pt is not quite straightforward in the range of a few nanometers. Specifically, as the size decreases, Pt nanoparticles on the supports probably have been aggregated or dissolved from the substrate, resulting in the decrease of activity. For Pt particles with large sizes, the number of Pt atoms on (100) and (111) crystal facets increases with respect to that at edges and corners, and the (100) and (111) crystal facets of Pt particles provide appropriate sites for the catalytic reaction. The interface determined mainly by the size of Pt nanoparticles plays an essential role in achieving high reactivity. The weak strength of the Pt-O bond for larger particles might be helpful for the formation of more active adsorbed oxygen species. However, the decrease in activity for large Pt particles could also be expected because of their too small surface area [6].

Figure 8: TOFs as a function of Pt particle size (calculated from CO chemisorption). Reproduced from [35] with permission from Elsevier BV.

Figure 8:

TOFs as a function of Pt particle size (calculated from CO chemisorption). Reproduced from [35] with permission from Elsevier BV.

Forming noble metal alloys is an effective strategy to improve the activity of the catalysts for VOC removal. Huang and Qian prepared a series of Au-Pd/SiO2 catalysts by the DP method followed by calcination at 200°C in air for 4 h [90]. The authors found that the preparation procedure leads to the formation of highly dispersed PdO supported on SiO2 in Pd/SiO2, but to the formation of both highly dispersed PdO and AuPd alloy nanoparticles in Au-Pd/SiO2 catalysts. The fraction of metallic Pd in Pd species increases with the increase of the Au/Pd molar ratio in Au-Pd/SiO2 catalyst, indicating that the alloying of Au and Pd could enhance the formation of metallic Pd by promoting the thermal decomposition of PdO supported on SiO2. Because of the presence of metallic Pd, Au-Pd/SiO2 catalysts are more active in CO oxidation than Pd/SiO2 for CO oxidation.

In 2014, Fan et al. prepared sub-10 nm AuPtPd alloy trimetallic nanoparticles (TMNPs) with a high oxidation-resistant property by photodeposition followed by a high temperature (700°C–900°C) air annealing process for the catalytic oxidation of VOCs [91], as shown in Figure 9 for detailed structural characterizations. In order to evaluate their durability in long-term operation, all samples were pre-treated by aging at 800°C in air for 5 h prior to the catalytic testing. The fresh Pt and Pd samples (4 wt.% loading) show good catalytic activity for n-hexane oxidation, with T50 (the temperature for 50% hexane conversion) <240°C. However, after thermal aging at 800°C in air, a sharp decrease in their oxidation activity is observed, and their T50 increases to above 400°C (as shown in Figure 10). The activation temperatures (T15) of the Pt and Pd catalysts also increase to 305°C and 260°C, respectively, suggesting a very poor activity for n-hexane oxidation. In contrast, all AuPtPd alloy TMNPs exhibit good activity even though their metal loading amount is only one-fourth of the Pt or Pd catalyst. Among them, the Au50Pt25Pd25 sample, which has the highest Pd0 content, shows the best catalytic activity with the lowest T50 (286°C) and T15 (203°C). The authors believe that the sub-10 nm size and the stable metallic Pd state of the supported Au50Pt25Pd25 alloy TMNPs are responsible for their excellent catalytic performance.

Figure 9: HAADF-STEM images and particle size distribution, typical HAADF-STEM images, corresponding EDS mappings and cross-sectional line-scanning profiles of Au25Pt25Pd50/EP-TiO2 after the photodeposition (A–D) and after annealing at 800°C (E–H). The inset in (E) shows the typical HR-TEM image of Au25Pt25Pd50/EP-TiO2 after annealing at 800°C. Reproduced from [91] with permission from the Royal Society of Chemistry.

Figure 9:

HAADF-STEM images and particle size distribution, typical HAADF-STEM images, corresponding EDS mappings and cross-sectional line-scanning profiles of Au25Pt25Pd50/EP-TiO2 after the photodeposition (A–D) and after annealing at 800°C (E–H). The inset in (E) shows the typical HR-TEM image of Au25Pt25Pd50/EP-TiO2 after annealing at 800°C. Reproduced from [91] with permission from the Royal Society of Chemistry.

Figure 10: (A) Immiscible area isotherms of the bulk Au-Pt-Pd phase diagram. (B) XRD patterns of samples 1–5 annealed at 800°C. (C) The reaction temperatures for 50% (T50) and 15% (T15) conversion of n-hexane for samples 1–6 (1 wt.%) and 7–10 (4 wt.%). Reproduced from [91] with permission from the Royal Society of Chemistry.

Figure 10:

(A) Immiscible area isotherms of the bulk Au-Pt-Pd phase diagram. (B) XRD patterns of samples 1–5 annealed at 800°C. (C) The reaction temperatures for 50% (T50) and 15% (T15) conversion of n-hexane for samples 1–6 (1 wt.%) and 7–10 (4 wt.%). Reproduced from [91] with permission from the Royal Society of Chemistry.

Suitable additives usually increase the dispersion of the noble metal and induce additional synergetic effects, which can enhance the activity of catalysts by helping them to absorb active species, even by changing the reaction route. He et al. reported a novel alkali-metal-promoted Pt/TiO2 catalyst for the ambient oxidation of HCHO [70]. Their studies suggested that the addition of alkali-metal ions (such as Li+, Na+ or K+) to the Pt/TiO2 catalyst could stabilize an atomically dispersed Pt-O(OH)x-alkali-metal species on the catalyst surface and open a new low-temperature reaction pathway, significantly promoting the activity for the HCHO oxidation (Figure 11) by activating H2O and catalyzing the facile reaction between surface OH and formate species to total oxidation products.

Figure 11: (A) HCHO conversion η over x wt.% Na-1 wt.% Pt/TiO2 (x=0, 1 and 2) catalysts as a function of temperature T. Reaction conditions: HCHO δ=600 ppm, O2 20 vol.%, relative humidity: about 50%, He balance, total flow rate of 50 cm3 min-1 and GHSV 120,000 h-1 (inset: stability test of 2% Na-1% Pt/TiO2 at 25°C, GHSV 300,000 h-1 with same other reaction conditions). (B) H2-TPR profiles of x wt.% Na-1% Pt/TiO2 (x=0, 1 and 2). Reproduced from [70] with permission from Wiley-VCH.

Figure 11:

(A) HCHO conversion η over x wt.% Na-1 wt.% Pt/TiO2 (x=0, 1 and 2) catalysts as a function of temperature T. Reaction conditions: HCHO δ=600 ppm, O2 20 vol.%, relative humidity: about 50%, He balance, total flow rate of 50 cm3 min-1 and GHSV 120,000 h-1 (inset: stability test of 2% Na-1% Pt/TiO2 at 25°C, GHSV 300,000 h-1 with same other reaction conditions). (B) H2-TPR profiles of x wt.% Na-1% Pt/TiO2 (x=0, 1 and 2). Reproduced from [70] with permission from Wiley-VCH.

Using composite or doped metal oxide substrates as supports for nanostructured noble metals could increase the amount of the adsorbed oxygen, improve the mobility of lattice oxygen and further decrease the catalytic temperature. Typically, Ji et al. used highly ordered pore-through TiO2 nanotube (TiNT) arrays modified with 0.5 wt.% MnO2 as the support to prepare the Pt/MnO2/TiNT catalyst [72]. The introduction of a MnO2 modification layer promotes the HCHO adsorption over the support surface, the dispersion of Pt particles and the surface enrichment of Pt and chemisorbed oxygen. Finally, the monolith-like Pt/MnO2/TiNT holds an enhanced performance for HCHO oxidation in comparison with that of Pt/TiNT. A HCHO conversion of 95% with more than 100 h stable performance was achieved over the Pt/MnO2/TiNT catalysts at 30°C with a low 0.2 wt.% Pt loading amount, as shown in Figure 12.

Figure 12: (A) Dependence of HCHO conversion on reaction temperature for TiNT, MnO2/TiNT, 0.16% Pt/TiNT, 0.16% Pt/MnO2/TiNT, 0.20% Pt/TiNT and 0.20% Pt/MnO2/TiNT; (B) long-term test for trace HCHO oxidation over 0.20% Pt/MnO2/TiNT at 30°C. Reproduced from [72] with permission from the American Chemical Society.

Figure 12:

(A) Dependence of HCHO conversion on reaction temperature for TiNT, MnO2/TiNT, 0.16% Pt/TiNT, 0.16% Pt/MnO2/TiNT, 0.20% Pt/TiNT and 0.20% Pt/MnO2/TiNT; (B) long-term test for trace HCHO oxidation over 0.20% Pt/MnO2/TiNT at 30°C. Reproduced from [72] with permission from the American Chemical Society.

In 2012, Imanaka et al. prepared novel Pt/CeO2-ZrO2-SnO2/γ-Al2O3 catalysts by a co-precipitation method for the catalytic combustion of VOCs such as ethylene, toluene and acetaldehyde [92]. The introduction of a small amount of SnO2 within the CeO2-ZrO2 lattice as a promoter is considerably effective to enhance the capabilities for oxygen release and storage of the catalysts, and therefore the complete oxidation of VOCs is markedly activated. The improvement of the reducibility of the catalyst could be ascribed to the simultaneous reduction of Ce4+ and Sn4+ in CeO2-ZrO2-SnO2 solid solutions. By optimizing the composition and the Pt amount, complete oxidation of ethylene, toluene and acetaldehyde was realized at temperatures as low as 55°C, 110°C and 140°C over a 10 wt.% Pt/16 wt.% Ce0.68Zr0.17Sn0.15O2.0/γ-Al2O3 catalyst, respectively, as displayed in Figure 13.

Figure 13: Temperature dependences of (A) toluene, (B) acetaldehyde and (C) ethylene oxidation over the Pt/CZ/Al2O3, Pt/CZB/Al2O3 and Pt/CZS/Al2O3 catalysts (CZ, Ce0.79Zr0.21O2.0; CZB, Ce0.64Zr0.16Bi0.20O1.9; CZS, Ce0.68Zr0.17Sn0.15O2.0). Reproduced from [92] with permission from the Chemical Society of Japan.

Figure 13:

Temperature dependences of (A) toluene, (B) acetaldehyde and (C) ethylene oxidation over the Pt/CZ/Al2O3, Pt/CZB/Al2O3 and Pt/CZS/Al2O3 catalysts (CZ, Ce0.79Zr0.21O2.0; CZB, Ce0.64Zr0.16Bi0.20O1.9; CZS, Ce0.68Zr0.17Sn0.15O2.0). Reproduced from [92] with permission from the Chemical Society of Japan.

In all, the appropriate particle size, the surface enrichment and well-dispersity of Pt nanoparticles, the strong metal-support-interaction and the negatively charged metallic Pt, which can facilitate the electron transfer and the formation of active oxygen to provide the active sites for VOCs oxidation, collectively lead to the good performance of the Pt-based catalyst in VOC oxidation.

3.2 Palladium (Pd)-based nanostructures

Although Pd has relatively lower activity in comparison with that of Pt for the catalytic oxidation of most pollutants in VOCs, it is also extensively used as an active component in a number of industrial catalytic formulations for the removal of air pollutants because of its excellent performance in catalyzing the oxidation of methane [93–95], toluene [96] and halocarbons [97], good resistance to sintering and low sensitivity to inhibition by choride and water [98]. Developing Pd-based catalysts for VOC removal is receiving considerable attention.

Zhou et al. loaded 1 wt.% Pd on the ceria (CeO2) supports with different morphologies to eliminate the indoor HCHO pollution [78]. Specifically, the Pd nanoparticles loaded on {100}-faceted CeO2 nanocubes exhibit much higher activity than that of the Pd particles loaded on {111}-faceted CeO2 nanooctahedrons and nanorods (enclosed by {100} and {111} facets, as shown in Figure 14). HCHO could be fully converted into CO2 over the Pd/CeO2 nanocubes at a GHSV of 10,000 h-1 and an HCHO inlet concentration of 600 ppm at ambient temperature. According to the Pd 3d XPS (X-ray photoelectron spectroscopy) spectra and H2-TPR (temperature programmed reduction) results, the status of the Pd species is dependent on the morphologies of the supports. The {100} facets of CeO2 could maintain the metallic Pd species rather than the {111} facets, which promotes the catalytic combustion of HCHO. The Raman and O 1s XPS results reveal that nanorods with more defect sites and oxygen vacancies are responsible for the easy oxidation of the Pd species, which results in low catalytic activity.

Figure 14: HRTEM images of (A, B) Pd/Cube, (C, D) Pd/Oct, (E, F) Pd/Rod with low and high resolution, respectively. Reproduced from [78] with permission from the American Chemical Society.

Figure 14:

HRTEM images of (A, B) Pd/Cube, (C, D) Pd/Oct, (E, F) Pd/Rod with low and high resolution, respectively. Reproduced from [78] with permission from the American Chemical Society.

Tabakova et al. prepared Fe-doped CeO2 materials as supports for mono- (Au, Pd) and bimetallic Au-Pd catalysts with the aim to increase the mobility of the surface lattice oxygen, as well as to facilitate the nucleation of noble metal particles on the surface [58]. The analyses of the structural characteristics and the catalytic performance in complete benzene oxidation show that the method of CeO2 modification (impregnation or mechanochemical mixing) influences the catalytic properties of mono- (Au, Pd) and bimetallic catalysts. The Pd-based catalysts exhibit similar but significantly higher activities as compared with the Au catalysts. The best catalytic behavior and positive effect of Pd deposition on the already deposited gold were observed over the bimetallic sample supported on the Fe-Ce oxide prepared by impregnation: 100% complete benzene oxidation is achieved at 200°C, as shown in Figure 15.

Figure 15: The conversion of benzene over Au, Pd and Pd-Au catalysts on Fe-modified ceria. Reproduced from [58] with permission from Elsevier BV. over Au, Pd and Pd-Au catalysts on Fe-modified ceria. Reproduced from [58] with permission from Elsevier BV.

Figure 15:

The conversion of benzene over Au, Pd and Pd-Au catalysts on Fe-modified ceria. Reproduced from [58] with permission from Elsevier BV. over Au, Pd and Pd-Au catalysts on Fe-modified ceria. Reproduced from [58] with permission from Elsevier BV.

Qi et al. used shell powder (SP, pearl shell) as a carrier for CeO2 and Pd catalysts for the complete oxidation of benzene [37]. The effects of adding CeO2 on the catalytic activity of Pd/SP for benzene deep oxidation are shown in Figure 16. From the results, they deduced that the addition of CeO2 significantly affects the catalytic activity of Pd/SP. The SP support exhibits very low catalytic activity, and the conversion was <20% at a reaction temperature of 500°C. For 6% Ce/SP, the conversion is over 90% at approximately 500°C, and the catalyst activity is also low. Because Pd impregnates directly onto SP, the temperature of complete benzene oxidation is approximately 420°C. After the addition of CeO2 and Pd, the catalytic activity is considerably greater than that of Ce/SP and Pd/SP. Figure 16 shows that the catalysts with 6–8 wt.% Ce loading have the highest activity. In particular, Pd/6%Ce/SP shows a conversion of over 80% at a reaction temperature of 280°C and complete oxidation at approximately 300°C. In summary, CeO2 acting as a promoter in benzene oxidation could greatly improve the catalytic activity of Pd/SP catalysts.

Figure 16: Temperature dependence of benzene conversion over the catalysts: effect of adding ceria on Pd/SP (500). Reproduced from [37] with permission from Elsevier BV.

Figure 16:

Temperature dependence of benzene conversion over the catalysts: effect of adding ceria on Pd/SP (500). Reproduced from [37] with permission from Elsevier BV.

Chen et al. synthesized Pd-based catalyst by sodium citrate reduction and hydrothermal process on the CexMn1-x composite oxide supports for benzene catalytic oxidation [99]. The CexMn1-x composite oxides promote the migration of oxygen vacancy through their interaction with deposited Pd, which contributes to the improvement of catalytic performance. In detail, as shown in Figure 17, benzene was oxidized on CexMn1-x oxides along three paths. In path I, benzene was oxidized by oxygen released from the CeO2 phase; in path II, benzene was oxidized by oxygen released from CexMn1-x oxides, due to the formation of Ce-O-Mn solid solution; in path III, benzene was oxidized by the active oxygen species from gas oxygen molecules. Part of the activated oxygen directly oxidizes benzene at the surface of CeO2 and Ce-O-Mn, and then other activated oxygen species are transported to the surface of Mn2O3 through oxygen vacancies and oxidizes the benzene adsorbed on Mn2O3. The synergistic effect between CeO2 and Mn2O3 could also enhance the migration of oxygen vacancy to improve the catalytic activity. For PdO/CexMn1-x catalysts, the interaction between active phase and supports plays an important and even determined role in the catalytic process, which could influence the surface energy of supports. Gas oxygen molecules are easily activated at the surface of supports to enhance catalytic capability. Meanwhile, lattice oxygen of supports is also easily released to produce more oxygen vacancies due to the decrease of the surface energy barrier. Therefore, the intrinsic properties of supports and interaction between Pd species and the supports are important to enhance the catalytic behaviors.

Figure 17: The mechanism of benzene catalytic oxidation for PdO/CexMn1-x catalysts. Reproduced from [99] with permission from Springer.

Figure 17:

The mechanism of benzene catalytic oxidation for PdO/CexMn1-x catalysts. Reproduced from [99] with permission from Springer.

Wei et al. prepared Al2O3-Ce0.3Zr0.7O2 (ACZ) and 3 wt.% BaO-doped Al2O3-Ce0.3Zr0.7O2 (ACZBa) by co-precipitation to load 0.25 wt.% Pd as the catalysts for the complete oxidation of methanol [100]. The work dealt with a combined investigation of the effect of adding barium on the characteristics of the catalyst and on its catalytic performance for methanol oxidation. Figure 18 presents the activities of catalysts for the complete oxidation of methanol. As indicated, the barium-doped catalyst is more reactive in the selective conversion of methanol to CO2. The light-off temperatures (T50) of Pd/ACZ and Pd/ACZBa are 56°C and 48°C, respectively. Furthermore, at reaction temperatures of 50°C and 60°C, the barium doping has a pronounced effect on the complete oxidation of methanol with an increased conversion by ca. 20%. Apart from CO2, major partial oxidation products (HCHO) are also monitored on both catalysts at low temperature. Although both catalysts show an inhibiting effect on the elimination of the partial oxidation product HCHO with increasing temperature, the level of emitted HCHO is lower on Pd/ACZBa at each tested temperature. Combining these results, it is reasonable to believe that the doping barium would enhance the complete oxidation of methanol due to formation of more active sites and more basic sites, which are needed for deep oxidation of methanol and could facilitate the formation of methoxy groups, a decisive rate step for catalytic oxidation of methanol.

Figure 18: (A) Conversion of methanol oxidation to CO2 and (b) the trend of formaldehyde in the exit gas at different temperatures over Pd/ACZ and Pd/ACZBa. Reproduced from [100] with permission from Elsevier BV.

Figure 18:

(A) Conversion of methanol oxidation to CO2 and (b) the trend of formaldehyde in the exit gas at different temperatures over Pd/ACZ and Pd/ACZBa. Reproduced from [100] with permission from Elsevier BV.

Analogous to Pt-based catalysts, well-dispersed Pd nanoparticles on the supports, the appropriate Pd state, the strong metal-support interaction and the speeded-up mobility of oxygen vacancy (or lattice oxygen) can facilitate the VOC oxidation over Pd-based catalysts.

3.3 Gold (Au)-based nanostructures

In the field of catalysis, gold (Au) was originally considered to be a poor catalyst due to its chemical inertness toward reactive molecules such as oxygen and hydrogen. However, this conception has been changed since Haruta and co-workers prepared novel gold catalysts, which are smaller than 10 nm and uniformly dispersed on transition-metal oxides, by co-precipitation for the oxidation of CO even at a temperature as low as -70°C [101, 102]. Recently, Au nanoparticles exhibiting unpredictable and unique catalytic properties have been widely investigated for dealing with VOC purification. In 2014, Yu and co-workers prepared Au nanoparticles supported on well-defined CeO2 nanorods with exposed {110} and {100} facets by a DP method [63]. Both nanometer and subnanometer Au particles are found to coexist on CeO2 supports with various Au contents (0.01–5.4 wt.%, as shown in Figure 19). The catalytic performance of Au/CeO2 catalysts was examined for converting HCHO into CO2 and H2O at room temperature and is shown to be Au content dependent. The catalysts with 1.8 wt.% Au show the best performance. The high reactivity and stability of Au/CeO2 catalysts are mainly attributed to well-defined CeO2 nanorods with {110} and {100} facets, which have relatively low energy for the formation of oxygen vacancy. Furthermore, Au nanoparticles could induce the weakened Ce-O bond, which in turn promotes HCHO oxidation.

Figure 19: TEM image (A) and HRTEM image (B) of CeO2 nanorods; HRTEM image of an Au NP for 1.8 wt.% Au/CeO2 catalysts (C); and (D–F) HAADF-STEM images of Au/CeO2 composites with different gold content. Inset: the distribution of Au NPs: (D) 1.8 wt.%, (E) 3.0 wt.% and (F) 5.4 wt.%. Reproduced from [63] with permission from the American Chemical Society.

Figure 19:

TEM image (A) and HRTEM image (B) of CeO2 nanorods; HRTEM image of an Au NP for 1.8 wt.% Au/CeO2 catalysts (C); and (D–F) HAADF-STEM images of Au/CeO2 composites with different gold content. Inset: the distribution of Au NPs: (D) 1.8 wt.%, (E) 3.0 wt.% and (F) 5.4 wt.%. Reproduced from [63] with permission from the American Chemical Society.

Fei et al. deposited gold nanoparticles (3–4 nm) on Mn3O4 nanocrystallites with three distinct morphologies (cubic, hexagonal and octahedral) [60], as shown in Figure 20. The resulting structures were characterized, and their activities for benzene combustion were evaluated. The dominant exposed facets for the three kinds of Mn3O4 polyhedrons follow the activity order: (103)≈(200)>(101). A similar activity order is derived for the interfaces between the Au and the Mn3O4 facet: Au/(200)≈Au/(103)>Au/(101). The metal-support interactions between Au nanoclusters and specific facets of Mn3O4 polyhedrons lead to a unique interfacial synergism, in which the electronic modification of Au nanoparticles and the morphology of the Mn3O4 substrate have a joint effect that is responsible for a significant enhancement in the catalytic activity of Au/Mn3O4 systems.

Figure 20: HRTEM images of (A) Mn3O4 CPs (cubic), (B) Mn3O4 Ops (octahedral), (C) Mn3O4 HPs (hexagonal), (D) Au/Mn3O4 CPs, (E) Au/Mn3O4 OPs and (F) Au/Mn3O4 HPs. The insets of (A)–(C) are the geometric models of the Mn3O4 nanocrystallites. Reproduced from [60] with permission from Wiley-VCH.

Figure 20:

HRTEM images of (A) Mn3O4 CPs (cubic), (B) Mn3O4 Ops (octahedral), (C) Mn3O4 HPs (hexagonal), (D) Au/Mn3O4 CPs, (E) Au/Mn3O4 OPs and (F) Au/Mn3O4 HPs. The insets of (A)–(C) are the geometric models of the Mn3O4 nanocrystallites. Reproduced from [60] with permission from Wiley-VCH.

Idakiev et al. synthesized mesoporous oxides TiO2 and ZrO2, labeled as MTi and MZr, respectively, by surfactant templating via a neutral C13(EO)6-Zr(OC3H7)4 assembly pathway, and CeO2-modified TiO2 and ZrO2 (CeMTi and CeMZr, respectively) by a DP method as supports for Au catalysts, featuring their high surface areas and uniform pore size distributions [103]. The supported gold catalysts were assessed for the catalytic abatement of air pollutants, that is, CO, methanol (CH3OH) and acetone ((CH3)2O). The gold catalysts supported on CeO2-modified mesoporous ZrO2 display superior catalytic activity, achieving 100% conversion of CO at 10°C and 100% conversion of methanol at 60°C. The catalytic activity toward the oxidation of CO decreases in the order of Au/CeMTi>Au/MTi>CeMTi>MTi and Au/CeMZr>Au/MZr>CeMZr>MZr, respectively. The high catalytic activity could be attributed to interaction between the CeO2 additive and the mesoporous oxides resulting in improvement of the reducibility of the supports. Moreover, the synergy between gold and CeO2 additive significantly enhanced the capability for oxygen activation and reducibility of the catalysts that result in improvement of the catalytic activities.

Dai et al. prepared Ce0.6Zr0.3Y0.1O2 (CZY) nanorods to support gold and palladium alloy (zAuxPdy/CZY, z=0.80–0.93 wt.%, x or y=0, 1, 2) nanoparticles using cetyltrimethyl ammonium bromide (CTAB)-assisted hydrothermal and polyvinyl alcohol (PVA)-protected reduction methods, respectively [104]. The CZY in zAuxPdy/CZY has a cubic crystal structure shown in Figure 21. Au-Pd nanoparticles with 4.6–5.6 nm average size are uniformly dispersed on the nanorod-like CZY surface. The combination of gold and palladium provides an interesting possibility of generating a catalyst with high activity for the oxidation of toluene. The 0.90 wt.% Au1Pd2/CZY sample with the highest concentration of adsorbed oxygen species and the best low-temperature reducibility exhibits the best catalytic performance for toluene oxidation, giving rise to T50 and T90 of 190°C and 218°C at a space velocity (SV) of 20,000 mL g-1 h-1, respectively. The active sites might be the surface oxygen vacancies on CZY, oxidized noble metal nanoparticles and/or noble metal nanoparticle-CZY interfaces. Thus, the high concentration of adsorbed oxygen species, good low-temperature reducibility and strong interaction between Au-Pd alloy nanoparticles and CZY nanorods, as well as good dispersion of Au-Pd alloy nanoparticles, are responsible for the excellent catalytic performance of the 0.90 wt.% Au1Pd2/CZY sample.

Figure 21: (A, B) SEM and (C–L) TEM images as well as SAED patterns (insets) of (A, B) CZY, (C, D) 0.90 wt.% Au/CZY, (E, F) 0.80 wt.% Pd/CZY, (G, H) 0.93 wt.% Au1Pd1/CZY, (I, J) 0.90 wt.% Au1Pd2/CZY and (K, L) 0.91 wt.% Au2Pd1/CZY. Reproduced from [104] with permission from the Royal Society of Chemistry.

Figure 21:

(A, B) SEM and (C–L) TEM images as well as SAED patterns (insets) of (A, B) CZY, (C, D) 0.90 wt.% Au/CZY, (E, F) 0.80 wt.% Pd/CZY, (G, H) 0.93 wt.% Au1Pd1/CZY, (I, J) 0.90 wt.% Au1Pd2/CZY and (K, L) 0.91 wt.% Au2Pd1/CZY. Reproduced from [104] with permission from the Royal Society of Chemistry.

The same as that of the Pt-O bond, the strength of the surface Au-O bond increases with the decrease in the Au particle size. However, as summarized by Huang et al., it is noteworthy that Au and Pt on the nanoscale show different behavior during the catalytic oxidation of VOCs, although both metals bind reactants more strongly as the particle size becomes smaller [6]. The binding of reactants with Pt becomes so strong that the reaction never proceeds at low temperatures. As for gold, however, the weaker binding and flexibility of the nanoparticles promotes catalytic reaction.

3.4 Silver(Ag)-based nanostructures

Ag also attracts much attention, especially in the catalytic oxidation of HCHO, although its catalytic activity is lower than that of the other noble metals such as Pt, Pd and Au. Typically, Dai and co-workers fabricated 0.13 wt.% Ag/Mn2O3 nanowires (0.13Ag/Mn2O3-ms) using a mixture of NaNO3 and NaF as molten salt and MnSO4 and AgNO3 as metal precursors [36]. The counterparts derived via the impregnation method (0.12Ag/Mn2O3-imp) and PVA-protected reduction route (0.12Ag/Mn2O3-redn) as well as the bulk Mn2O3-supported (0.23Ag/Mn2O3-bulk) Ag catalyst were also prepared. At a toluene/oxygen molar ratio of 1/400 and an SV of 40,000 mL g-1 h-1, toluene could be completely oxidized into CO2 and H2O at 220°C over the 0.13Ag/Mn2O3-ms catalyst. And the catalytic activity decreases in the sequence of 0.13Ag/Mn2O3-ms>0.12Ag/Mn2O3-imp>0.12Ag/Mn2O3-redn>Mn2O3-ms>0.23Ag/Mn2O3-bulk. It was concluded that the excellent catalytic activity of 0.13Ag/Mn2O3-ms is associated with its high dispersion of Ag nanoparticles on the surface of Mn2O3 nanowires and good low-temperature reducibility.

Zhang et al. prepared Ag-based catalysts loaded on different supports (TiO2, Al2O3 and CeO2) by the impregnation method for the catalytic oxidation of HCHO at low temperature [52]. The Ag/TiO2 catalyst shows the distinctive catalytic performance, achieving the complete HCHO conversion at around 95°C. In contrast, Ag/Al2O3 and Ag/CeO2 catalysts display much lower activity and the 100% conversion was reached at 110°C and higher than 125°C, respectively. The characterization results reveal that the supports have a dramatic influence on the Ag particle size and dispersion. Kinetic tests show that Ag-based catalysts on TiO2, Al2O3 or CeO2 supports have a similar apparent activation energy of 65 kJ mol-1, indicating that the catalytic mechanism keeps immutability over these three catalysts. Therefore, the Ag particle size and dispersion are confirmed to be the main factors affecting the catalytic performance for HCHO oxidation. The Ag/TiO2 catalyst has the highest Ag dispersion and the smallest Ag particle size, and accordingly exhibits the best catalytic performance for HCHO oxidation.

Li et al. prepared three-dimensional (3D) ordered mesoporous Ag/Co3O4 and K-Ag/Co3O4 catalysts on the basis of 3D-Co3O4 [105]. Ag nanoparticles are uniformly distributed on polycrystalline pore walls. The addition of Ag nanoparticles and K+ ions do not affect the mesoporous structure and only decrease the surface areas, pore diameters and pore volumes. Compared with Ag/Co3O4, the TOF of K-Ag/Co3O4 is much higher, which is attributed to the surface OH- species provided by K+ ions, abundant Ag(111) active faces, Co3+ cations and surface lattice oxygen (O2-) species generated by the strong interaction of Ag and Co and anion lattice defects. Ag(111) facets are active planes; surface OH-and O2- are active species. At low temperature (<80°C), to a large extent the HCHO catalytic activity on the K-Ag/Co3O4 catalyst depends on the surface OH- species around the Ag(111) facets. At relatively high temperature (>80°C), the surface OH- species are consumed and replaced quickly, and their supplement relies on the migration of O2- species from the 3D-Co3O4 support. The pathway of the reaction for HCHO oxidation on K-Ag/Co3O4 follows the HCHO→CHOO-+OH-→CO2+H2O route, as schematically illustrated in Figure 22.

Figure 22: Reaction pathway of the K-Ag/Co3O4 catalyst. Reproduced from [105] with permission from the American Chemical Society.

Figure 22:

Reaction pathway of the K-Ag/Co3O4 catalyst. Reproduced from [105] with permission from the American Chemical Society.

In the current literature works, the catalytic activity of Ag-based materials is not very perfect compared with other noble metal-based catalysts. The studies in this field are mainly focused on the reaction mechanism of the HCHO catalytic oxidation besides the investigation of the catalytic performance as mentioned above.

Photocatalysis based on TiO2 or TiO2-supported noble metal catalysts is also widely used to decompose VOCs as a green environmental remediation technology under ambient conditions, that is, room temperature and atmospheric pressure. However, the fundamentals in photocatalysis for VOC removal are quite different. In the strategies to increase the activity of catalysts, photocatalysis focuses on narrowing the band gap and hindering the high recombination of photogenerated electron-hole pairs, that is, prolonging the lifetime of photoexcited charge carriers [81]. However, thermal catalysis reviewed in this work devotes to adjusting the electronic interaction between the catalyst and oxygen to improve the ability of the catalyst for activating oxygen. Besides, the catalytic reaction mechanism, products, efficiency and circumstances (liquid phase for photocatalysis and gas phase for thermal catalysis) are also different between photocatalysis and thermal catalysis. Therefore, photocatalysis is not included in this review and will have to be summarized separately.

4 Air purification by metal oxides

Although great successes have been made in using noble metal-based nanostructures to catalyze the decomposition of VOCs, the high cost significantly limits their practical application. Therefore, as summarized in Table 2, in recent years, efforts have been devoted to developing efficient non-noble metal oxides, especially nanostructured transition-metal oxides such as V2O5, Cr2O3, MnOx, Fe2O3, Co3O4, NiO, CuO, ZrO2, La2O3 and CeO2, which have comparably high activity and much lower cost, for the deep catalytic oxidation of VOCs for replacing noble metal catalysts as well as decreasing the resource consumption.

Table 2

Summary of publications in recent five years on metal oxide-based nanostructures for catalytic oxidation of VOCs.

YearCatalystsPreparation methodaCatalyst mass (mg)VOC typeVOC conc. (ppm)WHSVb (mL gcat-1 h-1)T90c (°C)Refs.
2012CoMnCP150HCHO8060,00073[106]
20145% CuOx/Mn0.5Ce0.5O2IM300HCHO3310,000210[107]
2011Zr0.4Ce0.48Mn0.12O2Sol-gel200Butanol80012,000175[108]
2015Mn-doped-CeO2Hydrothermal50CO10,00072,000162.5[109]
Ni-doped-CeO2Hydrothermal50CO10,00072,000190[109]
Co-doped-CeO2Hydrothermal50CO10,00072,000197[109]
Cu-doped-CeO2Hydrothermal50CO10,00072,000214[109]
2011CeCuTiHydrothermal500Toluene39015,000202[110]
CeCrTiHydrothermal500Toluene39015,000217[110]
CeMnTiHydrothermal500Toluene39015,000250[110]
2013Co3O4-HT-CTABHydrothermal100Toluene100020,000215[111]
Co3O4-HT-PEGHydrothermal100Toluene100020,000226[111]
Co3O4-HTHydrothermal100Toluene100020,000260[111]
2013Mn0.85Ce0.15Hydrothermal300Toluene100032,000216[112]
2015Cu1Mn2Ce4Sol-gel500Toluene121724,000219[113]
2014Mn-Co (1:2)Two-step hydrothermal100Toluene100030,000240[114]
MnOxHydrothermal100Toluene100030,000263[114]
Co3O4Hydrothermal100Toluene100030,000271[114]
201210% Ni-Mn/cordieriteIM2700Toluene100010,000283[115]
201310% Fe-Mn/cordieriteIM5000Toluene100010,000300[116]
2014Mn5Co5Sol-gel50n-Hexane1000120,000209[117]
Co3O4Sol-gel50n-Hexane1000120,000236[117]
MnOxSol-gel50n-Hexane1000120,000252[117]
Mn5Co5Sol-gel50Ethyl acetate1000120,000190[117]
MnOxSol-gel50Ethyl acetate1000120,000202[117]
Co3O4Sol-gel50Ethyl acetate1000120,000220[117]
2013Cu/CeO2IM50Ethyl acetate46760,000255[118]
Cu/Ce0.75Sm0.25O1.875IM50Ethyl acetate46760,000286[118]
Cu/Sm2O3IM50Ethyl acetate46760,000340[118]
20117% CuO/Ce0.7Mn0.3O2IM+CP500Benzene100012,000231[119]
2015Cu0.6MnNanocasting100Benzene100060,000234[120]
2014Mn5Co5Sol-gel50Benzene1000120,000237[121]
MnOxSol-gel50Benzene1000120,000278[121]
Co3O4Sol-gel50Benzene1000120,000282[121]
2013Ce12.5Mn87.5Flame spray pyrolysis100Benzene100060,000250[122]
20113D Co16Ce1Nanocasting50Benzene1000100,000263[123]
2D Co16Ce1Nanocasting50Benzene1000100,000270[123]
2013Ce0.3Mn0.7Hydrothermal100Benzene100060,000325[124]
2015La0.9Ce0.1MnO3Flame spray pyrolysis100Benzene100060,000426[125]
2012Cu-Mn/TiO2IM100Benzene90030,000307[126]
Cu-Mn/ZrO2IM100Benzene90030,000315[126]
Cu-Mn/Al2O3IM100Benzene90030,000357[126]
Cu-Mn/Al2O3IM100Acetaldehyde50030,000190[126]
Cu-Mn/TiO2IM100Acetaldehyde50030,000171[126]
Cu-Mn/ZrO2IM100Acetaldehyde50030,000172[126]
2015ZnFe2O4Hydrothermal100C3H8200013,000402[127]
Zn(FeAl)O4Hydrothermal100C3H8200013,000500[127]
CoAl2O4Hydrothermal100C3H8200013,000515[127]
Zn0.5Co0.5Al2O4Hydrothermal100C3H8200013,000540[127]
ZnAl2O4Hydrothermal100C3H8200013,000555[127]

aIM, impregnation; CP, co-precipitation.

bWHSV, weight hourly space velocity.

cT90, the temperature for the 90% conversion of VOC.

According to the investigations summarized in many publications, the catalytic activity of transition-metal oxides for VOC oxidation is dependent on the morphology [128–130] and crystal phase [60, 69, 131]. Meanwhile, the design and synthesis of porous structures have also been proven to be an effective way to maximize the number of active sites for enhancing their catalytic behavior [117, 121, 131–134]. In addition, composite metal oxides and doped metal oxides provide another way to improve the catalytic performance by enhancing the mobility of lattice oxygen [109, 117, 120–122, 124, 125, 135].

4.1 Single metal oxides

In 2014, three different types of MnO2 microspheres, such as hierarchical hollow β-MnO2 microspheres consisting of uniform nanorods, hierarchical double-walled hollow β/α-MnO2 microspheres assembled by two-categorical nanorods, and hierarchical hollow α-MnO2 microspheres constructed by nanorods and nanowires, were synthesized by a facile hydrothermal method without employing any templates, catalysts, surfactants or calcinations for the catalytic oxidation of benzene by Chen and co-workers [131]. Their catalytic abilities for benzene oxidation decreased with the order of hierarchical hollow α-MnO2 microspheres >hierarchical two-wall β/α-MnO2 microspheres >hierarchical hollow β-MnO2microspheres, suggesting that the catalytic activity is quite relied on the structure of metal oxide catalysts.

In an early study, Dai et al. fabricated 3D ordered mesoporous cubic Co3O4 (denoted as Co-KIT6 and Co-SBA16) by adopting the KIT-6- and SBA-16-templating strategies, respectively, as shown in Figure 23 [136]. Co-KIT6 and Co-SBA16 possess large surface areas (118–121 m2 g-1), high oxygen species concentrations and well low-temperature reducibility compared with the nonporous Co3O4 catalyst. Under the conditions of VOC (toluene or methanol) concentration of 1000 ppm, VOC/O2 molar ratio of 1/20, and SV of 20,000 mL g-1 h-1, 90% toluene and methanol conversions could be, respectively achieved at 180°C and 139°C over Co-KIT6, much outperforming the bulk Co.

Figure 23: HRTEM images of (A) Co-KIT6 and (B) Co-SBA16. Reproduced from [136] with permission from Elsevier BV.

Figure 23:

HRTEM images of (A) Co-KIT6 and (B) Co-SBA16. Reproduced from [136] with permission from Elsevier BV.

Most recently, Chen and co-workers developed a template-free sol-gel chelating method to synthesize a porous hierarchical layer-stacking CeO2 nanostructure [137]. The morphologies of the as-prepared CeO2 are spindle-like and flower-like, as shown in Figure 24, which are both assembled by many porous nanoflakes. The basic unit, nanoflake, is organized by numerous nanoparticles with an average size of 10 nm. The as-prepared CeO2 possesses a high surface area of 171.6 m2 g-1, a high pore volume of 0.31 cm3 g-1 and a narrow pore size of 4.0 nm. Compared with the commercial CeO2 (T50=332°C), this novel structure with the T50 of 239°C exhibits much higher activity for CO oxidation, which could be ascribed to its higher surface area, small crystal size and novel hierarchical porous structure. Moreover, the same authors also applied a novel template-free oxalate route to synthesize manganese oxides with high surface areas (355 m2 g-1) and well-defined mesopores for the catalytic oxidation of benzene [128]. The effects of calcination temperature on the features of catalyst structure and catalytic activity were investigated. Manganese oxides prepared by oxalate route demonstrate better catalytic activities for complete oxidation of benzene, toluene and o-xylene as compared with related manganese oxides prepared by other methods (NaOH route, NH4HCO3 route and nanocasting strategy). Especially, the temperature for 90% benzene conversion on the oxalate-derived manganese oxide catalysts is 209°C, which is 132°C lower than the temperature required for the catalyst prepared by the NaOH route. The catalytic performance of manganese oxide is correlated with the surface area, pore size, low-temperature reducibility and distribution of surface species. The mole ratio of Mn4+/Mn2+ in the samples, which have better catalytic activity, is close to 1.0. This is good for the redox process of Mn4+ ↔ Mn3+ ↔ Mn2+, which is the key factor in determining the activity of MnOx, further indicating that the oxalate route is good for keeping the distribution of manganese oxidation states at an appropriate degree. A possible process for the complete oxidation of VOCs on manganese oxide catalysts is schematically shown in Figure 25. In addition, the best catalyst is highly stable for prolonged time in stream and is resistant to water vapor.

Figure 24: (A, B, C) SEM images and (D, E, F,) TEM images of the as-prepared sample CeO2-HL. Reproduced from [137] with permission from Elsevier BV.

Figure 24:

(A, B, C) SEM images and (D, E, F,) TEM images of the as-prepared sample CeO2-HL. Reproduced from [137] with permission from Elsevier BV.

Figure 25: The possible pathways for benzene oxidation on the MnOx catalyst. Reproduced from [128] with permission from the Royal Society of Chemistry.

Figure 25:

The possible pathways for benzene oxidation on the MnOx catalyst. Reproduced from [128] with permission from the Royal Society of Chemistry.

4.2 Multiple complex oxides

4.2.1 Composite oxide

The synergistic effects occurred in composite oxides composed of two or more active components may have significant contribution to the catalytic oxidation of VOCs. Typically, Lamonier et al. conducted acid treatment on MnOx-CeO2 mixed oxides synthesized by surfactant-assisted wet chemistry [138]. The substitution of Ce4+ by Mn species in the fluorite structure to form a solid solution is demonstrated in calcined and acid-treated samples with a Mn solubility limit of 50% in the fluorite structure. Textural and redox properties are found to be strongly altered by acid treatment (10 m), especially when the Mn solubility limit is exceeded, as shown in Figure 26. Using H2SO4 as a reactive agent for the acid treatment, the SSA and pore volume are greatly increased owing to the dissolution of Mn2+ species presenting among the particles, unveiling the primary porosity and the oxidation of manganese species in the solid into a higher oxidation state. Among the calcined samples, mixed Mn/Ce oxides facilitate the complete oxidation of HCHO at lower temperature in comparison with pure CeO2 and MnOx oxides, owing to an increase in SSA and enhancement in the redox properties of the mixed metal oxide solid solutions. Moreover, a mechanistic study of HCHO reactivity on the best catalyst (chemically activated pure manganese oxide) for HCHO oxidation confirms that the oxygen from manganese oxide through the formation of both monodentate and bidentate-bridging formate species has different reactivity.

Figure 26: (A) TEM images of calcined n=1 sample, (B) TEM images of acid-treated n=1 sample, (C) HRTEM of the acid-treated sample and (D) particle size distribution of the calcined and acid-treated n=1 sample from TEM images. Reproduced from [138] with permission from the American Chemical Society.

Figure 26:

(A) TEM images of calcined n=1 sample, (B) TEM images of acid-treated n=1 sample, (C) HRTEM of the acid-treated sample and (D) particle size distribution of the calcined and acid-treated n=1 sample from TEM images. Reproduced from [138] with permission from the American Chemical Society.

Ce-Mn oxides with different cerium-to-manganese ratios were also synthesized in one step through the FSP method using cerium acetate and manganese acetate as precursors for catalytic oxidation of benzene [122]. Mn ions in the Ce-Mn oxides are evidenced in multiple chemical states. Crystalline Ce-Mn oxides consisted of particles with size smaller than 40 nm have SSAs of 20–50 m2 g-1. As shown in Figure 27, compared with other Ce-Mn oxides, flame-made 12.5% Ce-Mn oxide exhibits excellent catalytic activity at relatively low temperatures (260°C for T95), ascribed to the small size of the catalyst particles and the synergetic effect of Ce and Mn due to the well mixing of Ce and Mn during the high-temperature flame process.

Figure 27: Benzene conversion as a function of temperature over flame-made CeO2-MnOx oxides with different Ce-to-Mn ratios: benzene=1000 ppm, synthetic air (20 vol.% O2 and 80 vol.% N2) balance, GHSV=60,000 mLgcat-1·h-1. Reproduced from [122] with permission from Elsevier BV.

Figure 27:

Benzene conversion as a function of temperature over flame-made CeO2-MnOx oxides with different Ce-to-Mn ratios: benzene=1000 ppm, synthetic air (20 vol.% O2 and 80 vol.% N2) balance, GHSV=60,000 mLgcat-1·h-1. Reproduced from [122] with permission from Elsevier BV.

In 2014, as shown in Figure 28, Tang and co-workers fabricated a series of Mn-Co mixed oxide nanorod with porous structure and high surface area by an oxalate route for deep oxidation of VOCs [117, 121]. Compared with the single MnOx or Co3O4, the Mn-Co mixed oxides exhibit an enhanced activity for the catalytic oxidation of ethyl acetate and n-hexane, with T90 is as low as 194°C and 210°C at a high SV, respectively. The formation of solid solution with spinel structure inhibits the growth of nanoparticles, and leads to their higher surface area, smaller particle size and more rich porous structure. Meanwhile, the strong synergistic effect between Mn and Co species in the oxides has great contribution to its low-temperature reducibility, which promotes the oxidation of VOCs. Besides, they also successfully synthesized mesoporous Cu-Mn oxides with high surface area (121–266 m2 g-1) for deep oxidation of benzene through a simple co-nanocasting approach using siliceous SBA-15 mesoporous material as a hard template [120]. As shown in Figure 29, compared with the referenced Cu-Mn oxides prepared by traditional methods, which have a T90 at 277°C–365°C for deep oxidation of benzene and a high apparent activation energy (Ea) values (89.1–113.1 kJ mol-1), the nanocasted catalyst exhibits the best activity with T90 at 234°C and the lowest Ea values (45.0 kJ mol-1), which could be attributed to its high surface area, rich porous structure, abundant oxygen adsorbed species, low-temperature reducibility and strong interaction between Cu and Mn species promoted by the nanocasting method. In particular, nanocasted catalysts show better activity when water vapor is introduced in the feed gas.

Figure 28: TEM images of the prepared oxides (a and a1: MnOx, b and b1: Mn5Co5Ox, c and c1: Co3O4). Reproduced from [117] with permission from Elsevier BV.

Figure 28:

TEM images of the prepared oxides (a and a1: MnOx, b and b1: Mn5Co5Ox, c and c1: Co3O4). Reproduced from [117] with permission from Elsevier BV.

Figure 29: (A, B) Benzene conversion as a function of reaction temperature over Cu-Mn composite oxides prepared by nanocasting strategy and other methods, (C) Arrhenius plots for the oxidation of benzene over the Cu-Mn composite oxides synthesized by different methods. Reproduced from [120] with permission from Elsevier BV.

Figure 29:

(A, B) Benzene conversion as a function of reaction temperature over Cu-Mn composite oxides prepared by nanocasting strategy and other methods, (C) Arrhenius plots for the oxidation of benzene over the Cu-Mn composite oxides synthesized by different methods. Reproduced from [120] with permission from Elsevier BV.

4.2.2 Doped oxides

By doping different metal atoms into the crystal lattice of the original metal oxides, the mobility of lattice oxygen, the adsorption ability of surface oxygen and the reducibility of the catalysts could be promoted, and consequently their catalytic performance is improved.

In 2015, Yang et al. developed a facile strategy to synthesize a series of transition-metal-doped (Cu, Co, Ni, Mn) CeO2 nanoparticles assembled from hollow nanocones for the catalytic CO oxidation, as shown in Figure 30 [109]. The appropriate order (after the formation of the Ce(HCOO)3 precursor) for the addition of the Mnδ+ solution to the reaction system is the key to obtain new hierarchical nanostructures. Transition-metal cations could be incorporated into the CeO2 lattice during phase transformation from Ce(HCOO)3 to CeO2. The obtained series of Mn-doped CeO2 samples demonstrate enhanced catalytic properties toward CO oxidation, which could be attributed to the increased concentration of oxygen vacancies.

Figure 30: Conversion of CO over as-prepared samples (M0, CeO2; M1, Cu-doped CeO2; M2, Co-doped CeO2; M3, Ni-doped CeO2; M4, Mn-doped CeO2) and commercial CeO2. Reproduced from [109] with permission from Wiley-VCH.

Figure 30:

Conversion of CO over as-prepared samples (M0, CeO2; M1, Cu-doped CeO2; M2, Co-doped CeO2; M3, Ni-doped CeO2; M4, Mn-doped CeO2) and commercial CeO2. Reproduced from [109] with permission from Wiley-VCH.

Also in 2015, Balzer et al. reported the syntheses of nanorods of a Ce1-x CoxO2 system, with x=0, 0.05, 0.10, 0.15 and 0.20, employing a microwave-assisted hydrothermal method and investigations into their catalytic activity in the total oxidation of three VOCs (benzene, toluene and o-xylene) [139]. The physicochemical characterizations show that the inclusion of cobalt leads to an increase in the oxygen vacancies, which could result in an enhancement of the bulk and surface oxygen mobility in the system. Also, an increase in the catalytic activity is observed with the progressive incorporation of cobalt into the ceria matrix. The results reveal that the Ce0.80Co0.20O2 catalyst is able to remove 100% of the benzene, nearly 100% of the toluene and around 70% of the o-xylene at about 750°C. The higher oxidation activity observed for the catalyst with the highest cobalt load could be attributed to a combination of several factors, including a large number of active sites of the cobalt being exposed and improved mobility of the active oxygen species.

Recently, Chen et al. prepared perovskite-type La1–xCexMnO3 (x=0%–10%) catalysts by FSP and their activities during the catalytic oxidation of benzene were examined over the temperature range of 100°C–450°C [125]. The incorporation of Ce was found to improve the benzene oxidation activity, and the perovskite, in which x is 10% (Figure 31), exhibits the highest activity. Phase composition and surface elemental analysis indicate that non-stoichiometric compounds are present. The substitution of La3+ by Ce4+ results in an increase in the surface Mn4+/Mn3+ ratio and a decrease in the surface Oads/Olatt ratio due to the charge neutralization. These trends in the Mn4+/Mn3+ and Oads/Olatt ratios have good correlation with the catalytic activity during benzene oxidation, indicating that the Ce4+-induced modification of the Mn4+/Mn3+ratio and the oxygen species is accompanied by enhanced catalytic activity.

Figure 31: TEM images of (A) pure LaMnO3 and (B) La0.900Ce0.100MnO3 perovskites; HRTEM images of (C) pure LaMnO3 and (D) La0.900Ce0.100MnO3 perovskites; and (E) a corresponding EDX image of La0.900Ce0.100MnO3 perovskites. Reproduced from [125] with permission from Elsevier BV.

Figure 31:

TEM images of (A) pure LaMnO3 and (B) La0.900Ce0.100MnO3 perovskites; HRTEM images of (C) pure LaMnO3 and (D) La0.900Ce0.100MnO3 perovskites; and (E) a corresponding EDX image of La0.900Ce0.100MnO3 perovskites. Reproduced from [125] with permission from Elsevier BV.

In an early study, Chen et al. reported that CeO2-based materials, as shown in Figure 32, including CeO2-NFs (nanofibers), CeO2-NCs (nanocubes), Sm-CeO2-NCs and Gd-CeO2-NCs, were synthesized for CO catalytic oxidation by a simple hydrothermal process [135]. The catalytic activity of the CeO2-NFs is higher than that of the CeO2-NCs because of the combined effect of particle size and oxygen vacancies for pure CeO2. For doped CeO2, the Sm-CeO2-NCs show a higher activity compared with the CeO2-NCs as a result of an increase in the number of oxygen vacancies due to the substitution of Ce4+ species with Sm3+ ions. In contrast, Gd doping for the Gd-CeO2-NCs has a negative effect on the CO catalytic oxidation due to the special electron configuration of Gd3+ (4f7), although the number of oxygen vacancies is enhanced after doping. Hence, the oxygen vacancies in pure CeO2 and the electron configuration of the dopants in doped CeO2 play important roles in CO oxidation.

Figure 32: TEM images of CeO2-NFs (A, B), CeO2-NCs (C, D) and Sm-CeO2-NCs (E, F). Reproduced from [135] with permission from Wiley-VCH.

Figure 32:

TEM images of CeO2-NFs (A, B), CeO2-NCs (C, D) and Sm-CeO2-NCs (E, F). Reproduced from [135] with permission from Wiley-VCH.

5 Monolithic structure

For practical applications, structured catalysts are needed for a catalytic reactor with good structural and thermal stability, minimum pressure drop and without reactor blocking. Thus, the monolithic catalysts including metal wire mesh/foil [140, 141] and ceramic [116, 142–150], a feasible replacement of conventional catalysts in heterogeneous catalysis, have increasingly attracted the attention due to their strong mechanical strength and high heat transfer capacity. Moreover, the ceramic monolithic catalysts consisting of parallel, non-intersecting channels of an inert oxide, for example, cordierite 2MgO-2Al2O3-5SiO2 with an abundant resource and low cost, are promising for reactions with a high SV and a large amount of heat exchange. Usually, for monolithic catalysts it is necessary to use a pre-coat of an inorganic oxide called the washcoat to create a higher surface area and to act as a second support for the active phase (noble metals and/or transitions metals). The most common washcoat material is γ-Al2O3, but La2O3, ZrO2, TiO2 and CeO2 have also been exploited [115, 151–154].

As a typical example, in 2012, Li et al. used the electroless plating method to prepare Pd-based FeCrAl wire mesh monolithic catalysts without alumina interlayer film (Figure 33) [141]. The as-prepared 0.3–0.4 wt.% Pd/FeCrAl catalysts calcined at a suitable temperature of 800°C show excellent low-temperature catalytic oxidation activity and good catalytic stability in the 70 h durability test, which might be contributed to the formation of a molten appearance for PdO active particles in 0.1–1 μm size scale on the surface of the catalyst. Such a molten appearance could induce the increase of catalytic activity as well as the prevention of the catalyst from deactivation in toluene oxidation reaction.

Figure 33: SEM photographs of 0.4 wt.% Pd/FeCrAl catalysts prepared at different calcination temperatures: (A) 400°C, (B) 600°C, (C) 800°C and (D) 1,000°C. Reproduced from [141] with permission from Elsevier BV.

Figure 33:

SEM photographs of 0.4 wt.% Pd/FeCrAl catalysts prepared at different calcination temperatures: (A) 400°C, (B) 600°C, (C) 800°C and (D) 1,000°C. Reproduced from [141] with permission from Elsevier BV.

Actually, early in 2010, Ji et al. prepared a series of CuxCo1–x/Al2O3/FeCrAl (x=0–1) catalysts, as shown in Figure 34, for the catalytic oxidation of toluene in a conventional fixed-bed quartz reactor [140]. Using FeCrAl alloy foils as supports benefits the dispersion of Cu. A Cu-Co-O solid solution phase is present when the content of Cu in the catalysts is low and a CuO phase is present when the content of Cu is high. The surface morphology of the monolithic catalysts changes significantly with the ratios of Cu and Co. Moreover, when x<0.5, the introduction of copper oxide makes the reduction temperature of the catalysts shift to the lower temperature, suggesting that the introduction of a proper amount of copper oxide could improve the reducibility of the cobalt oxide and thus enhanced the catalytic activity of the catalysts. Therefore, the Cu0.5Co0.5/Al2O3/FeCrAl catalyst exhibiting the best catalytic activity could make toluene totally oxidized at 374°C with an SV of 5.6×104 mL g-1 h-1.

Figure 34: SEM images of the samples: (A) FeCrAl; (B) FeCrAl foil pre-oxidized at 950°C for 15 h; (1–7) CuxCo1-x /Al2O3/FeCrAl catalysts, (1) x=1, (2) x=0.9, (3) x=0.75, (4) x=0.5, (5), x=0.25, (6) x=0.1, (7) x=0. Reproduced from [140] with permission from the Editorial office of Acta Physico-Chimica Sinica.

Figure 34:

SEM images of the samples: (A) FeCrAl; (B) FeCrAl foil pre-oxidized at 950°C for 15 h; (1–7) CuxCo1-x /Al2O3/FeCrAl catalysts, (1) x=1, (2) x=0.9, (3) x=0.75, (4) x=0.5, (5), x=0.25, (6) x=0.1, (7) x=0. Reproduced from [140] with permission from the Editorial office of Acta Physico-Chimica Sinica.

However, generally the washcoat and active phase on the metal wire mesh/foil are easy to fall off due to the different thermal expansion coefficients among them. Therefore, the matching among thermal expansion coefficients becomes important, and cordierite, which has low thermal expansion coefficient, is favorable. Luo et al. prepared a series of CexY1-x Oδ (CY) washcoat on cordierite honeycomb by an impregnation method, which was used as a support to prepare Pd catalysts [151]. A model reaction of the complete combustion of toluene was conducted to evaluate the performance of the developed Pd/CY catalyst. The results show that compared with conventional washcoat, the CY washcoat has better adhesion and higher vibration and heat resistance. The CY washcoat could anchor well Pd onto the cordierite honeycomb substrate. A CeO2-Y2O3 solid solution forms and the steady presence of PdO is noticed at high calcination temperatures, resulting in a better thermal stability. On a Pd/Ce0.8Y0.2Oδ catalyst, toluene could be completely oxidized at 210°C, and the catalyst shows good stability up to 30 h reaction.

In 2013, Lamonier et al. synthesized cordierite monoliths coated with Mn, Ce and Zr elements for the n-butanol catalytic oxidation [154]. The one-pot synthesis (Ce0.12Zr0.40Mn0.48O2/cordierite monoliths or CeZrMn(0.48)/M) leads to a homogeneous distribution of Mn species, whereas the impregnation of Mn over the Ce-Zr washcoated phase (Mn(y)/CeZr/M, CeZr for Ce0.6Zr0.4O2 and y for the precursor of Mn: y=Nit, Mn(NO3)2; y=Na, NaMnO4) cannot. Ultrasound experiments used to evaluate the coating adherence show that the washcoated mixed oxide is well anchored on the cordierite. The introduction mode of Mn species is crucial for catalytic activity. The specific reaction rates of n-butanol oxidation slightly increased over Mn(y)/CeZr/M. However, the huge catalytic activity improvement is observed over CeZrMn(0.48)/M, as shown in Figure 35, due to the high SSA, good dispersion and reducibility of the mixed oxides coated as a thin layer over the cordierite.

Figure 35: Specific reaction rates of CO2 production as a function of the reaction temperature and the monolithic catalyst. Reproduced from [154] with permission from Elsevier BV.

Figure 35:

Specific reaction rates of CO2 production as a function of the reaction temperature and the monolithic catalyst. Reproduced from [154] with permission from Elsevier BV.

In a study reported little earlier, Shen et al. investigated the catalytic combustion of toluene over the Ni-Mn mixed complex supported on industrial cordierite [115]. The catalysts were prepared by the wet impregnation method. The catalytic activity toward the complete oxidation of toluene to CO2 and H2O strongly depends on the molar ratio of Ni/Mn, loading amount of Ni-Mn oxides and calcination temperature. When the molar ratio of Ni/Mn is 0.5, the catalyst has a better performance because of the formation of NiMnO3 nanocrystallite. The catalyst with 10 wt.% loadings of Ni-Mn mixed oxides shows a higher activity due to the highly dispersed Ni-Mn oxide sites on the surface of cordierite. The catalytic activity of the catalysts descends with increasing calcination temperature due to the sintering of the active phase. Thus, calcination at 400°C is suitable, and the best catalyst could make the conversion of toluene reach 92.1% at 300°C.

6 Conclusions and perspectives

Catalytic oxidation of VOCs is highly desirable for the low-temperature purification technology in terms of energy savings, low cost, operation safety and environmental friendliness. The highly efficient catalysts mainly include nanostructured noble metals and metal oxides, especially transition-metal oxides. The supported noble metal exhibits superior activity toward the catalytic oxidation of VOCs at low temperature, even at room temperature. The catalytic performance of supported noble metals is generally governed by many factors including the properties of supports and noble metals, dispersion, particle size, morphology and valence of metallic particles. Therefore, there are numerous studies focusing on how the properties of support and metal, metal precursor, preparation and pretreatment method, reaction conditions, etc., affect the performance of catalysts for the oxidation of VOCs. Transition-metal oxides also have outstanding performances as the catalysts for the removal of VOCs. Their catalytic activities quite rely on the structure and morphology. Researchers are therefore devoting themselves to selectively exposing a larger fraction of the active facets where the active sites can be enriched and tuned.

The efforts of many leading research groups have led to a rich variety of catalytic materials, and their accumulation creates great opportunities and also a tremendous challenge to apply these materials in the complete oxidation of VOCs. In addition to structural advantages, the welcome features for a multiphase structure including the synergistic effect among different species of the catalysts have been successfully acquired. Future research challenges may include the following:

  1. The most common strategies for fabricating noble metal catalysts are traditional impregnation methods or FSP, but the powerful and versatile solution-based approaches which can well, fine control the uniform and ideal particle size, crystal plane and morphology, for example, core-shell, hollow interiors, cage-bell construction, stellated/dendritic structures and heterostructure, by adjusting the reaction parameters including the concentration of reactants, the mole ratio between precursors and surfactants, and the reaction temperature and time are yet to be investigated.

  2. One of the greatest challenges is the deactivation of catalysts under realistic process conditions. In some realistic industrial reactions such as the catalytic combustion widely used in the tail gas processing and the oil refining process, the catalysts can maintain their activities in the reaction at only 300°C–400°C for most of the time, but sometimes the reaction temperature can reach as high as nearly 1000°C at one moment owing to the inevitably excessive combustion or other reasons. Although this situation may only last for several seconds, the momentary process brings serious irreversible degradation of the catalyst. So in the preparation process of the practical catalysts, the catalysts must be aged under harsh pretreatment conditions before being used in order to make sure that they are durable and stable enough.

  3. The mechanistic understanding of the underlying chemistry for the catalytic systems, for example, the synergetic effect and interface boundary sites, may be valuable for the development of the highly efficient and stable catalysts with interesting architectures and tailored functionalities. Currently, the mechanisms for catalytic oxidation of small VOC molecules, such as formaldehyde, have been well documented, and most of the intermediate products can be determined [18, 32, 62, 70]. However, the mechanisms for catalytic oxidation of large VOC molecules are yet to be fully investigated because the reaction pathways are too complicated. The reaction order quite depending on the nature of the catalyst is often used as an entry point to understand the reaction mechanism. The reaction order with respect to benzene, butanol, ethyl acetate, n-hexane and toluene over Pt/Al2O3-supported catalysts is positive with values in the 0–1 range, while the reaction order with respect to oxygen is observed to be close to zero or negative [155, 156]. Depending on the partial reaction order, different reaction mechanisms have been proposed for VOC oxidation including the Langmuir-Hinshelwood mechanism, in which the controlling step is the surface reaction between two adsorbed molecules on analogous active sites, the Eley-Rideal mechanism, in which the controlling step is the reaction between an adsorbed molecule and a molecule from the gas phase, and the Mars-van Krevelen mechanism, which is an oxidative-reductive mechanism involving the reaction of VOC molecule and oxygen on different redox sites [5]. The investigation of reaction mechanisms will accelerate the fast developing characterization techniques, especially in situ analysis instruments.

  4. In addition, few studies have been carried out in terms of catalytic oxidation of mixed VOCs from industrial processes and highly diluted VOCs from indoor environments, the performance and mechanism of which are probably quite different from those of catalytic oxidation of single VOC with high concentration. Therefore, more academic and industrial efforts must be devoted before the extensive application of nanostructures as catalysts for VOC removal.

  5. Explore other scientific-related issues. Many interesting scientific findings might be derived from the synthesis and characterization of nanomaterials, which would not only satisfy everlasting human curiosity, but also promise new advances in nanoscience and nanotechnology.


Corresponding authors: Yunfa Chen and Jun Yang, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China, Phone: +86-10-8254-4896, Fax: +86-10-8254-4919, e-mail: (Y. Chen), Phone: +86-10-8254-4915, Fax: +86-10-8254-4915, e-mail: (J. Yang)

Acknowledgments

The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (NSFC) (Nos. 51272253, 21173226, 21376247 and 21573240), the DBN Young Scientists Foundation (1014115004), the Comprehensive Reforming Project to Promote Talents Training of BUA (BNRC and YX201413), the National High Technology Research and Development Program of China (Grant No. 2012AA062702), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB05050300), the Knowledge Innovation Project of CAS (No. KZCX2-EW-403) and the Hundred Talents Program of the Chinese Academy of Sciences (No. MPCS-2014-C-01).

References

[1] He Z, Li G, Chen J, Huang Y, An T, Zhang C. Pollution characteristics and health risk assessment of volatile organic compounds emitted from different plastic solid waste recycling workshops. Environ. Int. 2015, 77, 85–94. Search in Google Scholar

[2] Villanueva F, Tapia A, Amo-Salas M, Notario A, Cabanas B, Martinez E. Levels and sources of volatile organic compounds including carbonyls in indoor air of homes of Puertollano, the most industrialized city in central Iberian Peninsula. Estimation of health risk. Int. J. Hyg. Environ. Health 2015, 218, 522–534. Search in Google Scholar

[3] Zhang Z, Wang X, Zhang Y, Lu S, Huang Z, Huang X, Wang Y. Ambient air benzene at background sites in China’s most developed coastal regions: exposure levels, source implications and health risks. Sci. Total Environ. 2015, 511, 792–800. Search in Google Scholar

[4] Tang X, Bai Y, Duong A, Smith MT, Li L, Zhang L. Formaldehyde in China: production, consumption, exposure levels, and health effects. Environ. Int. 2009, 35, 1210–1224. Search in Google Scholar

[5] Liotta LF. Catalytic oxidation of volatile organic compounds on supported noble metals. Appl. Catal. B Environ. 2010, 100, 403–412. Search in Google Scholar

[6] Huang H, Xu Y, Feng Q, Leung DYC. Low temperature catalytic oxidation of volatile organic compounds: a review. Catal. Sci. Technol. 2015, 5, 2649–2669. Search in Google Scholar

[7] Le Bechec M, Kinadjian N, Ollis D, Backov R, Lacombe S. Comparison of kinetics of acetone, heptane and toluene photocatalytic mineralization over TiO2 microfibers and Quartzel® mats. Appl. Catal. B Environ. 2015, 179, 78–87. Search in Google Scholar

[8] Zhang F, Li X, Zhao Q, Zhang Q, Tade M, Liu S. Fabrication of α-Fe2O3/In2O3 composite hollow microspheres: a novel hybrid photocatalyst for toluene degradation under visible light. J. Colloid Interface Sci. 2015, 457, 18–26. Search in Google Scholar

[9] Zhu XB, Chang DL, Li XS, Sun ZG, Deng XQ, Zhu AM. Inherent rate constants and humidity impact factors of anatase TiO2 film in photocatalytic removal of formaldehyde from air. Chem. Eng. J. 2015, 279, 897–903. Search in Google Scholar

[10] Zhu T, Chen R, Xia N, Li X, He X, Zhao W, Carr T. Volatile organic compounds emission control in industrial pollution source using plasma technology coupled with F-TiO2/γ-Al2O3. Environ. Technol. 2015, 36, 1405–1413. Search in Google Scholar

[11] Huang R, Lu M, Wang P, Chen Y, Wu J, Fu M, Chen L, Ye D. Enhancement of the non-thermal plasma-catalytic system with different zeolites for toluene removal. RSC Adv. 2015, 5, 72113–72120. Search in Google Scholar

[12] Abedi K, Ghorbani-Shahna F, Jaleh B, Bahrami A, Yarahmadi R, Haddadi R, Gandomi M. Decomposition of chlorinated volatile organic compounds (CVOCs) using NTP coupled with TiO2/GAC, ZnO/GAC, and TiO2-ZnO/GAC in a plasma-assisted catalysis system. J. Electrostat. 2015, 73, 80–88. Search in Google Scholar

[13] Xiao B, Chen M, Tan Z, Liu Q, Zhou H, Li X. Isolation and characterization of bacillus sp. BZ-001H capable of biodegrading formaldehyde at high concentration. Environ. Eng. Sci. 2015, 32, 824–830. Search in Google Scholar

[14] Tassi F, Venturi S, Cabassi J, Vaselli O, Gelli I, Cinti D, Capecchiacci F. Biodegradation of CO2, CH4 and volatile organic compounds (VOCs) in soil gas from the Vicano-Cimino hydrothermal system (central Italy). Org. Geochem. 2015, 86, 81–93. Search in Google Scholar

[15] Jiang B, Zhou Z, Dong Y, Tao W, Wang B, Jiang J, Guan X. Biodegradation of benzene, toluene, ethylbenzene, and o-, m-, and p-xylenes by the newly isolated bacterium comamonas sp. JB. Appl. Biochem. Biotech. 2015, 176, 1700–1708. Search in Google Scholar

[16] Liotta LF, Wu H, Pantaleo G, Venezia AM. Co3O4 nanocrystals and Co3O4-MOx binary oxides for CO, CH4 and VOC oxidation at low temperatures: a review. Catal. Sci. Technol. 2013, 3, 3085–3102. Search in Google Scholar

[17] Li WB, Wang JX, Gong H. Catalytic combustion of VOCs on non-noble metal catalysts. Catal. Today 2009, 148, 81–87. Search in Google Scholar

[18] Scire S, Liotta LF.Supported gold catalysts for the total oxidation of volatile organic compounds. Appl. Catal. B Environ. 2012, 125, 222–246. Search in Google Scholar

[19] Liu H, Feng Y, Chen D, Li C, Cui P, Yang J. Noble metal-based composite nanomaterials fabricated via solution-based approaches. J. Mater. Chem. A 2015, 3, 3182–3223. Search in Google Scholar

[20] Ye F, Liu H, Yang J, Cao H, Yang J. Morphology and structure controlled synthesis of ruthenium nanoparticles in oleylamine. Dalton Trans. 2013, 42, 12309–12316. Search in Google Scholar

[21] Feng Y, Ma X, Han L, Peng Z, Yang J. A universal approach to the synthesis of nanodendrites of noble metals. Nanoscale 2014, 6, 6173–6179. Search in Google Scholar

[22] Chen D, Li C, Liu H, Ye F, Yang J. Core-shell Au@Pd nanoparticles with enhanced catalytic activity for oxygen reduction reaction via core-shell Au@Ag/Pd constructions. Sci. Rep. 2015, 5, 11949. Search in Google Scholar

[23] Barroso F, Tojo C. Modelling of nano-alloying and structural evolution of bimetallic core-shell nanoparticles obtained via the microemulsion route. J. Colloid Interface Sci. 2011, 363, 73–83. Search in Google Scholar

[24] He Q, Mukerjee S. Electrocatalysis of oxygen reduction on carbon-supported PtCo catalysts prepared by water-in-oil micro-emulsion. Electrochim. Acta 2010, 55, 1709–1719. Search in Google Scholar

[25] Chen D, Liu S, Li J, Zhao N, Shi C, Du X, Sheng J. Nanometre Ni and core/shell Ni/Au nanoparticles with controllable dimensions synthesized in reverse microemulsion. J. Alloy. Compd. 2009, 475, 494–500. Search in Google Scholar

[26] Ahmed J, Sharma S, Ramanujachary KV, Lofland SE, Ganguli AK. Microemulsion-mediated synthesis of cobalt (pure fcc and hexagonal phases) and cobalt-nickel alloy nanoparticles. J. Colloid Interface Sci. 2009, 336, 814–819. Search in Google Scholar

[27] Yadav OP, Palmqvist A, Cruise N, Holmberg K. Synthesis of platinum nanoparticles in microemulsions and their catalytic activity for the oxidation of carbon monoxide. Colloid Surface A 2003, 221, 131–134. Search in Google Scholar

[28] Yan Z, Xu Z, Yu J, Jaroniec M. Highly active mesoporous ferrihydrite supported Pt catalyst for formaldehyde removal at room temperature. Environ. Sci. Technol. 2015, 49, 6637–6644. Search in Google Scholar

[29] Ye F, Liu H, Hu W, Zhong J, Chen Y, Cao H, Yang J. Heterogeneous Au-Pt nanostructures with enhanced catalytic activity toward oxygen reduction. Dalton Trans. 2012, 41, 2898–2903. Search in Google Scholar

[30] Chen C, Chen F, Zhang L, Pan S, Bian C, Zheng X, Meng X, Xiao F. Importance of platinum particle size for complete oxidation of toluene over Pt/ZSM-5 catalysts. Chem. Commun. 2015, 51, 5936–5938. Search in Google Scholar

[31] Qi L, Cheng B, Ho W, Liu G, Yu J. Hierarchical Pt/NiO hollow microspheres with enhanced catalytic performance. ChemNanoMat 2015, 1, 58–67. Search in Google Scholar

[32] Xu Z, Yu J, Jaroniec M. Efficient catalytic removal of formaldehyde at room temperature using AlOOH nanoflakes with deposited Pt. Appl. Catal. B Environ. 2015, 163, 306–312. Search in Google Scholar

[33] Liu H, Ye F, Yao Q, Cao H, Xie J, Lee JY, Yang J. Stellated Ag-Pt bimetallic nanoparticles: an effective platform for catalytic activity tuning. Sci. Rep. 2014, 4, 3969. Search in Google Scholar

[34] Li Z, Yang K, Liu G, Deng G, Li J, Li G, Yue R, Yang J, Chen Y. Effect of reduction treatment on structural properties of TiO2 supported Pt nanoparticles and their catalytic activity for benzene oxidation. Catal. Lett. 2014, 144, 1080–1087. Search in Google Scholar

[35] Huang H, Hu P, Huang H, Chen J, Ye X, Leung DYC. Highly dispersed and active supported Pt nanoparticles for gaseous formaldehyde oxidation: influence of particle size. Chem. Eng. J. 2014, 252, 320–326. Search in Google Scholar

[36] Deng J, He S, Xie S, Yang H, Liu Y, Guo G, Dai H. Ultralow loading of silver nanoparticles on Mn2O3 nanowires derived with molten salts: a high-efficiency catalyst for the oxidative removal of toluene. Environ. Sci. Technol. 2015, 49, 11089–11095. Search in Google Scholar

[37] Zuo S, Du Y, Liu F, Han D, Qi C. Influence of ceria promoter on shell-powder-supported Pd catalyst for the complete oxidation of benzene. Appl. Catal. A Gen. 2013, 451, 65–70. Search in Google Scholar

[38] Kahani SA, Molaei H. Synthesis of nickel metal nanoparticles via a chemical reduction of nickel ammine and alkylamine complexes by hydrazine. J. Iran. Chem. Soc. 2013, 10, 1263–1270. Search in Google Scholar

[39] Wang Y, Shi YF, Chen YB, Wu LM. Hydrazine reduction of metal ions to porous submicro-structures of Ag, Pd, Cu, Ni, and Bi. J. Solid State Chem. 2012, 191, 19–26. Search in Google Scholar

[40] Gui Z, Fan R, Mo WQ, Chen XH, Yang L, Hu Y. Synthesis and characterization of reduced transition metal oxides and nanophase metals with hydrazine in aqueous solution. Mater. Res. Bull. 2003, 38, 169–176. Search in Google Scholar

[41] Liu G, Yang K, Li J, Tang W, Xu J, Liu H, Yue R, Chen Y. Surface diffusion of pt clusters in/on SiO2 matrix at elevated temperatures and their improved catalytic activities in benzene oxidation. J. Phys. Chem. C 2014, 118, 22719–22729. Search in Google Scholar

[42] He C, Yue L, Zhang X, Li P, Dou B, Ma C, Hao Z. Deep catalytic oxidation of benzene, toluene, ethyl acetate over Pd/SBA-15 catalyst: reaction behaviors and kinetics. Asia-Pac. J. Chem. Eng. 2012, 7, 705–715. Search in Google Scholar

[43] Pint CL, Kim SM, Stach EA, Hauge RH. Rapid and scalable reduction of dense surface-supported metal-oxide catalyst with hydrazine vapor. ACS Nano 2009, 3, 1897–1905. Search in Google Scholar

[44] Kim HS, Kim TW, Koh HL, Lee SH, Min BR. Complete benzene oxidation over Pt-Pd bimetal catalyst supported on γ-alumina: influence of Pt-Pd ratio on the catalytic activity. Appl. Catal. A Gen. 2005, 280, 125–131. Search in Google Scholar

[45] Tang W, Wu X, Chen Y. Catalytic removal of gaseous benzene over Pt/SBA-15 catalyst: the effect of the preparation method. React. Kinet. Mech. Catal. 2015, 114, 711–723. Search in Google Scholar

[46] An N, Li S, Duchesne PN, Wu P, Zhang W, Lee JF, Cheng S, Zhang P, Jia M, Zhang W. Size effects of platinum colloid particles on the structure and CO oxidation properties of supported Pt/Fe2O3 catalysts. J. Phys. Chem. C 2013, 117, 21254–21262. Search in Google Scholar

[47] Wang Y, Ren J, Deng K, Gui L, Tang Y. Preparation of tractable platinum, rhodium, and ruthenium nanoclusters with small particle size in organic media. Chem. Mater. 2000, 12, 1622–1627. Search in Google Scholar

[48] Chaudhuri RG, Paria S. Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications. Chem. Rev. 2012, 112, 2373–2433. Search in Google Scholar

[49] Chen JP, Lim LL. Key factors in chemical reduction by hydrazine for recovery of precious metals. Chemosphere 2002, 49, 363–370. Search in Google Scholar

[50] Yu X, He J, Wang D, Hu Y, Tian H, He Z. Facile controlled synthesis of Pt/MnO2 nanostructured catalysts and their catalytic performance for oxidative decomposition of formaldehyde. J. Phys. Chem. C 2012, 116, 851–860. Search in Google Scholar

[51] Chen C, Wu Q, Chen F, Zhang L, Pan S, Bian C, Zheng X, Meng X, Xiao F-S. Aluminum-rich beta zeolite-supported platinum nanoparticles for the low-temperature catalytic removal of toluene. J. Mater. Chem. A 2015, 3, 5556–5562. Search in Google Scholar

[52] Zhang J, Li Y, Zhang Y, Chen M, Wang L, Zhang C, He H. Effect of support on the activity of Ag-based catalysts for formaldehyde oxidation. Sci. Rep. 2015, 5, 12950. Search in Google Scholar

[53] Na H, Zhu T, Liu Z. Effect of preparation method on the performance of Pt-Au/TiO2 catalysts for the catalytic co-oxidation of HCHO and CO. Catal. Sci. Technol. 2014, 4, 2051–2057. Search in Google Scholar

[54] Chen H, Rui Z, Ji H. Monolith-like TiO2 nanotube array supported Pt catalyst for HCHO removal under mild conditions. Ind. Eng. Chem. Res. 2014, 53, 7629–7636. Search in Google Scholar

[55] Zhao J, Wang Y, Lou X, Li K, Li Z, Huang W. Effects of adding Al2O3 on the crystal structure of TiO2 and the performance of Pd-based catalysts supported on the composite for the total oxidation of ethanol. Inorg. Chim. Acta 2013, 405, 395–399. Search in Google Scholar

[56] Tang X, Chen J, Huang X, Xu Y, Shen W. Pt/MnOx-CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature. Appl. Catal. B Environ. 2008, 81, 115–121. Search in Google Scholar

[57] Garcia T, Solsona B, Cazorla-Amorós D, Linares-Solano Á, Taylor SH. Total oxidation of volatile organic compounds by vanadium promoted palladium-titania catalysts: comparison of aromatic and polyaromatic compounds. Appl. Catal. B Environ. 2006, 62, 66–76. Search in Google Scholar

[58] Tabakova T, Ilieva L, Petrova P, Venezia AM, Avdeev G, Zanella R, Karakirova Y. Complete benzene oxidation over mono and bimetallic Au-Pd catalysts supported on Fe-modified ceria. Chem. Eng. J. 2015, 260, 133–141. Search in Google Scholar

[59] Rui Z, Chen L, Chen H, Ji H. Strong metal-support interaction in Pt/TiO2 induced by mild HCHO and NaBH4 solution reduction and its effect on catalytic toluene combustion. Ind. Eng. Chem. Res. 2014, 53, 15879–15888. Search in Google Scholar

[60] Fei ZY, Sun B, Zhao L, Ji WJ, Au CT. Strong morphological effect of Mn3O4 nanocrystallites on the catalytic activity of Mn3O4 and Au/Mn3O4 in benzene combustion. Chem. Eur. J. 2013, 19, 6480–6487. Search in Google Scholar

[61] Huang H, Leung DYC, Ye D. Effect of reduction treatment on structural properties of TiO2 supported Pt nanoparticles and their catalytic activity for formaldehyde oxidation. J. Mater. Chem. 2011, 21, 9647–9652. Search in Google Scholar

[62] Chen BB, Shi C, Crocker M, Wang Y, Zhu AM. Catalytic removal of formaldehyde at room temperature over supported gold catalysts. Appl. Catal. B Environ. 2013, 132, 245–255. Search in Google Scholar

[63] Xu Q, Lei W, Li X, Qi X, Yu J, Liu G, Wang J, Zhang P. Efficient removal of formaldehyde by nanosized gold on well-defined CeO2 nanorods at room temperature. Environ. Sci. Technol. 2014, 48, 9702–9708. Search in Google Scholar

[64] Chen S, Luo L, Jiang Z, Huang W. Size-dependent reaction pathways of low-temperature CO oxidation on Au/CeO2 catalysts. ACS Catal. 2015, 5, 1653–1662. Search in Google Scholar

[65] Liu B, Liu Y, Li C, Hu W, Jing P, Wang Q, Zhang J. Three-dimensionally ordered macroporous Au/CeO2-Co3O4 catalysts with nanoporous walls for enhanced catalytic oxidation of formaldehyde. Appl. Catal. B Environ. 2012, 127, 47–58. Search in Google Scholar

[66] Yang SM, Liu DM, Liu SY. Catalytic combustion of benzene over Au supported on ceria and vanadia promoted ceria. Top. Catal. 2008, 47, 101–108. Search in Google Scholar

[67] Li S, Jia M, Gao J, Wu P, Yang M, Huang S, Dou X, Yang Y, Zhang W. Infrared studies of the promoting role of water on the reactivity of Pt/FeOx catalyst in low-temperature oxidation of carbon monoxide. J. Phys. Chem. C 2015, 119, 2483–2490. Search in Google Scholar

[68] Anandkumar M, Ramamurthy CH, Thirunavukkarasu C, Babu KS. Influence of age on the free-radical scavenging ability of CeO2 and Au/CeO2 nanoparticles. J. Mater. Sci. 2015, 50, 2522–2531. Search in Google Scholar

[69] Zhang J, Li Y, Wang L, Zhang C, He H. Catalytic oxidation of formaldehyde over manganese oxides with different crystal structures. Catal. Sci. Technol. 2015, 5, 2305–2313. Search in Google Scholar

[70] Zhang C, Liu F, Zhai Y, Ariga H, Yi N, Liu Y, Asakura K, Flytzani-Stephanopoulos M, He H. Alkali-metal-promoted Pt/TiO2 opens a more efficient pathway to formaldehyde oxidation at ambient temperatures. Angew. Chem. Int. Ed. 2012, 51, 9628–9632. Search in Google Scholar

[71] Kwon DW, Seo PW, Kim GJ, Hong SC. Characteristics of the HCHO oxidation reaction over Pt/TiO2 catalysts at room temperature: the effect of relative humidity on catalytic activity. Appl. Catal. B Environ. 2015, 163, 436–443. Search in Google Scholar

[72] Chen H, Tang M, Rui Z, Ji H. MnO2 promoted TiO2 nanotube array supported Pt catalyst for formaldehyde oxidation with enhanced efficiency. Ind. Eng. Chem. Res. 2015, 54, 8900–8907. Search in Google Scholar

[73] Chen C, Wang X, Zhang J, Pan S, Bian C, Wang L, Chen F, Meng X, Zheng X, Gao X, Xiao F. Superior performance in catalytic combustion of toluene over KZSM-5 zeolite supported platinum catalyst. Catal. Lett. 2014, 144, 1851–1859. Search in Google Scholar

[74] Chen C, Zhu J, Chen F, Meng X, Zheng X, Gao X, Xiao F. Enhanced performance in catalytic combustion of toluene over mesoporous Beta zeolite-supported platinum catalyst. Appl. Catal. B Environ. 2013, 140, 199–205. Search in Google Scholar

[75] Sedjame HJ, Fontaine C, Lafaye G, Barbier JJ. On the promoting effect of the addition of ceria to platinum based alumina catalysts for VOCs oxidation. Appl. Catal. B Environ. 2014, 144, 233–242. Search in Google Scholar

[76] Huang H, Leung DYC. Complete oxidation of formaldehyde at room temperature using TiO2 supported metallic Pd nanoparticles. ACS Catal. 2011, 1, 348–354. Search in Google Scholar

[77] Zhang C, Li Y, Wang Y, He H. Sodium-promoted Pd/TiO2 for catalytic oxidation of formaldehyde at ambient temperature. Environ. Sci. Technol. 2014, 48, 5816–5822. Search in Google Scholar

[78] Tan H, Wang J, Yu S, Zhou K. Support morphology-dependent catalytic activity of Pd/CeO2 for formaldehyde oxidation. Environ. Sci. Technol. 2015, 49, 8675–8682. Search in Google Scholar

[79] Zuo S, Sun X, Lv N, Qi C. Rare earth-modified kaolin/NaY-supported Pd-Pt bimetallic catalyst for the catalytic combustion of benzene. ACS Appl. Mater. Interfaces 2014, 6, 11988–11996. Search in Google Scholar

[80] Li H, Zhang N, Chen P, Luo M, Lu J. High surface area Au/CeO2 catalysts for low temperature formaldehyde oxidation. Appl. Catal. B Environ. 2011, 110, 279–285. Search in Google Scholar

[81] Liu B, Li C, Zhang Y, Liu Y, Hu W, Wang Q, Han L, Zhang J. Investigation of catalytic mechanism of formaldehyde oxidation over three-dimensionally ordered macroporous Au/CeO2 catalyst. Appl. Catal. B Environ. 2012, 111–112, 467–475. Search in Google Scholar

[82] Solsona B, Aylón E, Murillo R, Mastral AM, Monzonís A, Agouram S, Davies TE, Taylor SH, Garcia T. Deep oxidation of pollutants using gold deposited on a high surface area cobalt oxide prepared by a nanocasting route. J. Hazard. Mater. 2011, 187, 544–552. Search in Google Scholar

[83] Solsona B, Garcia T, Agouram S, Hutchings GJ, Taylor SH. The effect of gold addition on the catalytic performance of copper manganese oxide catalysts for the total oxidation of propane. Appl. Catal. B Environ. 2011, 101, 388–396. Search in Google Scholar

[84] Solsona B, Pérez-Cabero M, Vázquez I, Dejoz A, García T, Álvarez-Rodríguez J, El-Haskouri J, Beltrán D, Amorós P. Total oxidation of VOCs on Au nanoparticles anchored on Co doped mesoporous UVM-7 silica. Chem. Eng. J. 2012, 187, 391–400. Search in Google Scholar

[85] Ousmane M, Liotta LF, Carlo GD, Pantaleo G, Venezia AM, Deganello G, Retailleau L, Boreave A, Giroir-Fendler A. Supported Au catalysts for low-temperature abatement of propene and toluene, as model VOCs: support effect. Appl. Catal. B Environ. 2011, 101, 629–637. Search in Google Scholar

[86] Ousmane M, Liotta LF, Pantaleo G, Venezia AM, Di Carlo G, Aouine M, Retailleau L, Giroir-Fendler A. Supported Au catalysts for propene total oxidation: study of support morphology and gold particle size effects. Catal. Today 2011, 176, 7–13. Search in Google Scholar

[87] Ying F, Wang S, Au CT, Lai SY. Highly active and stable mesoporous Au/CeO2 catalysts prepared from MCM-48 hard-template. Microporous Mesoporous Mater. 2011, 142, 308–315. Search in Google Scholar

[88] Ma L, Wang D, Li J, Bai B, Fu L, Li Y. Ag/CeO2 nanospheres: efficient catalysts for formaldehyde oxidation. Appl. Catal. B Environ. 2014, 148, 36–43. Search in Google Scholar

[89] Kharlamova T, Mamontov G, Salaev M, Zaikovskii V, Popova G, Sobolev V, Knyazev A, Vodyankina O. Silica-supported silver catalysts modified by cerium/manganese oxides for total oxidation of formaldehyde. Appl. Catal. A-Gen. 2013, 467, 519–529. Search in Google Scholar

[90] Qian K, Huang W. Au-Pd alloying-promoted thermal decomposition of PdO supported on SiO2 and its effect on the catalytic performance in CO oxidation. Catal. Today 2011, 164, 320–324. Search in Google Scholar

[91] Qiao P, Xu S, Zhang D, Li R, Zou S, Liu J, Yi W, Li J, Fan J. Sub-10 nm Au-Pt-Pd alloy trimetallic nanoparticles with a high oxidation-resistant property as efficient and durable VOC oxidation catalysts. Chem. Commun. 2014, 50, 11713–11716. Search in Google Scholar

[92] Yasuda K, Yoshimura A, Katsuma A, Masui T, Imanaka N. Low-temperature complete combustion of volatile organic compounds over novel Pt/CeO2-ZrO2-SnO2/γ-Al2O3 catalysts. Bull. Chem. Soc. Jpn. 2012, 85, 522–526. Search in Google Scholar

[93] Abbasi R, Huang G, Istratescu GM, Wu L, Hayes RE. Methane oxidation over Pt, Pt:Pd, and Pd based catalysts: effects of pre-treatment. Can. J. Chem. Eng. 2015, 93, 1474–1482. Search in Google Scholar

[94] Ohtsuka H. The oxidation of methane at low temperatures over zirconia-supported Pd, Ir and Pt catalysts and deactivation by sulfur poisoning. Catal. Lett. 2011, 141, 413–419. Search in Google Scholar

[95] Machocki A, Rotko M, Gac W. Steady state isotopic transient kinetic analysis of flameless methane combustion over Pd/Al2O3 and Pt/Al2O3 catalysts. Top. Catal. 2009, 52, 1085–1097. Search in Google Scholar

[96] Bendahou K, Cherif L, Siffert S, Tidahy HL, Benaïssa H, Aboukaïs A. The effect of the use of lanthanum-doped mesoporous SBA-15 on the performance of Pt/SBA-15 and Pd/SBA-15 catalysts for total oxidation of toluene. Appl. Catal. A Gen. 2008, 351, 82–87. Search in Google Scholar

[97] Jin LY, Ma RH, Lin JJ, Meng L, Wang YJ, Luo MF. Bifunctional Pd/Cr2O3-ZrO2 catalyst for the oxidation of volatile organic compounds. Ind. Eng. Chem. Res. 2011, 50, 10878–10882. Search in Google Scholar

[98] Centi G. Supported palladium catalysts in environmental catalytic technologies for gaseous emissions. J. Mol. Catal. A Chem. 2001, 173, 287–312. Search in Google Scholar

[99] Wang Z, Yang M, Shen G, Liu H, Chen Y, Wang Q. Catalytic removal of benzene over CeO2-MnOx composite oxides with rod-like morphology supporting PdO. J. Nanopart. Res. 2014, 16, 2367. Search in Google Scholar

[100] Luo Y, Xiao Y, Cai G, Zheng Y, Wei K. A study of barium doped Pd/Al2O3-Ce0.3Zr0.7O2 catalyst for complete methanol oxidation. Catal. Commun. 2012, 27, 134–137. Search in Google Scholar

[101] Haruta M, Yamada N, Kobayashi T, Iijima S. Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide. J. Catal. 1989, 115, 301–309. Search in Google Scholar

[102] Haruta M, Kobayashi T, Sano H, Yamada N. Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0°C. Chem. Lett. 1987, 16, 405–408. Search in Google Scholar

[103] Idakiev V, Dimitrov D, Tabakova T, Ivanov K, Yuan ZY, Su BL. Catalytic abatement of CO and volatile organic compounds in waste gases by gold catalysts supported on ceria-modified mesoporous titania and zirconia. Chin. J. Catal. 2015, 36, 579–587. Search in Google Scholar

[104] Tan W, Deng J, Xie S, Yang H, Jiang Y, Guo G, Dai H. Ce0.6Zr0.3Y0.1O2 nanorod supported gold and palladium alloy nanoparticles: high-performance catalysts for toluene oxidation. Nanoscale 2015, 7, 8510–8523. Search in Google Scholar

[105] Bai B, Li J. Positive effects of K+ ions on three-dimensional mesoporous Ag/Co3O4 catalyst for HCHO oxidation. ACS Catal. 2014, 4, 2753–2762. Search in Google Scholar

[106] Shi C, Wang Y, Zhu A, Chen B, Au C. MnxCo3-xO4 solid solution as high-efficient catalysts for low-temperature oxidation of formaldehyde. Catal. Commun. 2012, 28, 18–22. Search in Google Scholar

[107] Li J, Pan K, Yu S, Yan S, Chang MB. Removal of formaldehyde over MnxCe1-x O2 catalysts: thermal catalytic oxidation versus ozone catalytic oxidation. J. Environ. Sci. China 2014, 26, 2546–2553. Search in Google Scholar

[108] Azalim S, Franco M, Brahmi R, Giraudon JM, Lamonier JF. Removal of oxygenated volatile organic compounds by catalytic oxidation over Zr-Ce-Mn catalysts. J. Hazard. Mater. 2011, 188, 422–427. Search in Google Scholar

[109] Zhang J, Guo J, Liu W, Wang S, Xie A, Liu X, Wang J, Yang Y. Facile preparation of Mn+-doped (M = Cu, Co, Ni, Mn) hierarchically mesoporous CeO2 nanoparticles with enhanced catalytic activity for CO oxidation. Eur. J. Inorg. Chem. 2015, 6, 969–976. Search in Google Scholar

[110] Weng X, Zhang J, Wu Z, Liu Y. Continuous hydrothermal flow syntheses of transition metal oxide doped CexTiO2 nanopowders for catalytic oxidation of toluene. Catal. Today 2011, 175, 386–392. Search in Google Scholar

[111] Bai G, Dai H, Deng J, Liu Y, Wang F, Zhao Z, Qiu W, Au CT. Porous Co3O4 nanowires and nanorods: highly active catalysts for the combustion of toluene. Appl. Catal. A Gen. 2013, 450, 42–49. Search in Google Scholar

[112] Liao Y, Fu M, Chen L, Wu J, Huang B, Ye D. Catalytic oxidation of toluene over nanorod-structured Mn-Ce mixed oxides. Catal. Today 2013, 216, 220–228. Search in Google Scholar

[113] Lu H, Kong X, Huang H, Zhou Y, Chen Y. Cu-Mn-Ce ternary mixed-oxide catalysts for catalytic combustion of toluene. J. Environ. Sci. China 2015, 32, 102–107. Search in Google Scholar

[114] Qu Z, Gao K, Fu Q, Qin Y. Low-temperature catalytic oxidation of toluene over nanocrystal-like Mn-Co oxides prepared by two-step hydrothermal method. Catal. Commun. 2014, 52, 31–35. Search in Google Scholar

[115] Huang Q, Zhang Z, Ma W, Chen Y, Zhu S, Shen S. A novel catalyst of Ni-Mn complex oxides supported on cordierite for catalytic oxidation of toluene at low temperature. J. Ind. Eng. Chem. 2012, 18, 757–762. Search in Google Scholar

[116] Ma WJ, Huang Q, Xu Y, Chen YW, Zhu SM, Shen SB. Catalytic combustion of toluene over Fe-Mn mixed oxides supported on cordierite. Ceram. Int. 2013, 39, 277–281. Search in Google Scholar

[117] Tang W, Wu X, Li S, Li W, Chen Y. Porous Mn-Co mixed oxide nanorod as a novel catalyst with enhanced catalytic activity for removal of VOCs. Catal. Commun. 2014, 56, 134–138. Search in Google Scholar

[118] Konsolakis M, Carabineiro SAC, Tavares PB, Figueiredo JL. Redox properties and VOC oxidation activity of Cu catalysts supported on Ce1-x SmxO delta mixed oxides. J. Hazard. Mater. 2013, 261, 512–521. Search in Google Scholar

[119] Li T, Chiang S, Liaw B, Chen Y. Catalytic oxidation of benzene over CuO/Ce1-x MnxO2 catalysts. Appl. Catal. B Environ. 2011, 103, 143–148. Search in Google Scholar

[120] Tang W, Wu X, Li S, Shan X, Liu G, Chen Y. Co-nanocasting synthesis of mesoporous Cu-Mn composite oxides and their promoted catalytic activities for gaseous benzene removal. Appl. Catal. B Environ. 2015, 162, 110–121. Search in Google Scholar

[121] Tang W, Li W, Li D, Liu G, Wu X, Chen Y. Synergistic effects in porous Mn-Co mixed oxide nanorods enhance catalytic deep oxidation of benzene. Catal. Lett. 2014, 144, 1900–1910. Search in Google Scholar

[122] Liu G, Yue R, Jia Y, Ni Y, Yang J, Liu H, Wang Z, Wu X, Chen Y. Catalytic oxidation of benzene over Ce-Mn oxides synthesized by flame spray pyrolysis. Particuology 2013, 11, 454–459. Search in Google Scholar

[123] Ma C, Mu Z, He C, Li P, Li J, Hao Z. Catalytic oxidation of benzene over nanostructured porous Co3O4-CeO2 composite catalysts. J. Environ. Sci. China 2011, 23, 2078–2086. Search in Google Scholar

[124] Wang Z, Shen G, Li J, Liu H, Wang Q, Chen Y. Catalytic removal of benzene over CeO2-MnOx composite oxides prepared by hydrothermal method. Appl. Catal. B Environ. 2013, 138, 253–259. Search in Google Scholar

[125] Liu G, Li J, Yang K, Tang W, Liu H, Yang J, Yue R, Chen Y. Effects of cerium incorporation on the catalytic oxidation of benzene over flame-made perovskite La1-x CexMnO3 catalysts. Particuology 2015, 19, 60–68. Search in Google Scholar

[126] Doggali P, Teraoka Y, Mungse P, Shah IK, Rayalu S, Labhsetwar N. Combustion of volatile organic compounds over Cu-Mn based mixed oxide type catalysts supported on mesoporous Al2O3, TiO2 and ZrO2. J. Mol. Catal. A Chem. 2012, 358, 23–30. Search in Google Scholar

[127] Zawadzki M, Okal J. Effect of Co and Fe substitution on catalytic VOCs removal on zinc aluminate. Catal. Today 2015, 257, 136–143. Search in Google Scholar

[128] Tang W, Wu X, Li D, Wang Z, Liu G, Liu H, Chen Y. Oxalate route for promoting activity of manganese oxide catalysts in total VOCs’ oxidation: effect of calcination temperature and preparation method. J. Mater. Chem. A 2014, 2, 2544–2554. Search in Google Scholar

[129] Li D, Shen G, Tang W, Liu H and Chen Y. Large-scale synthesis of hierarchical MnO2 for benzene catalytic oxidation. Particuology 2014, 14, 71–75. Search in Google Scholar

[130] Li D, Wu X, Chen Y. Synthesis of hierarchical hollow MnO2 microspheres and potential application in abatement of VOCs. J. Phys. Chem. C 2013, 117, 11040–11046. Search in Google Scholar

[131] Li D, Yang J, Tang W, Wu X, Wei L, Chen Y. Controlled synthesis of hierarchical MnO2 microspheres with hollow interiors for the removal of benzene. RSC Adv. 2014, 4, 26796–26803. Search in Google Scholar

[132] Tang W, Liu G, Li D, Liu H, Wu X, Han N, Chen Y. Design and synthesis of porous non-noble metal oxides for catalytic removal of VOCs. Sci. China Chem. 2015, 58, 1359–1366. Search in Google Scholar

[133] Liu J, Xia Y, Bao D, Zeng L. Mesoporous Cr2O3 with high surface area: preparation and catalytic activity performance for toluene combustion. Chin. J. Inorg. Chem. 2014, 30, 353–358. Search in Google Scholar

[134] Wang Y, Zhu X, Crocker M, Chen B, Shi C. A comparative study of the catalytic oxidation of HCHO and CO over Mn0.75Co2.25O4 catalyst: the effect of moisture. Appl. Catal. B-Environ. 2014, 160, 542–551. Search in Google Scholar

[135] Wang Z, Wang Q, Liao YC, Shen GL, Gong XZ, Han N, Liu HD, Chen YF. Comparative study of CeO2 and doped CeO2 with tailored oxygen vacancies for CO oxidation. ChemPhysChem 2011, 12, 2763–2770. Search in Google Scholar

[136] Xia Y, Dai H, Jiang H, Zhang L. Three-dimensional ordered mesoporous cobalt oxides: highly active catalysts for the oxidation of toluene and methanol. Catal. Commun. 2010, 11, 1171–1175. Search in Google Scholar

[137] Tang W, Li W, Shan X, Wu X, Chen Y. Template-free synthesis of hierarchical layer-stacking CeO2 nanostructure with enhanced catalytic oxidation activity. Mater. Lett. 2015, 140, 95–98. Search in Google Scholar

[138] Quiroz J, Giraudon JM, Gervasini A, Dujardin C, Lancelot C, Trentesaux M, Lamonier JF. Total oxidation of formaldehyde over MnOx-CeO2 catalysts: the effect of acid treatment. ACS Catal. 2015, 5, 2260–2269. Search in Google Scholar

[139] Balzer R, Probst LFD, Cantarero A, de Lima MM, Araujo VD, Bernardi MIB, Avansi W, Arenal R, Fajardo HV. Ce1-x CoxO2 nanorods prepared by microwave-assisted hydrothermal method: novel catalysts for removal of volatile organic compounds. Sci. Adv. Mater. 2015, 7, 1406–1414. Search in Google Scholar

[140] Zhao FZ, Zeng PH, Ji SF, Yang X, Li CY.Catalytic combustion of toluene over CuxCo1-x /Al2O3/FeCrAl monolithic catalysts. Acta Phys. Chim. Sin. 2010, 26, 3285–3290. Search in Google Scholar

[141] Li Y, Li Y, Yu Q, Yu L. The catalytic oxidation of toluene over Pd-based FeCrAl wire mesh monolithic catalysts prepared by electroless plating method. Catal. Commun. 2012, 29, 127–131. Search in Google Scholar

[142] Jiang L, Yang N, Zhu J, Song C. Preparation of monolithic Pt-Pd bimetallic catalyst and its performance in catalytic combustion of benzene series. Catal. Today 2013, 216, 71–75. Search in Google Scholar

[143] Wu D, Jiang W, Zhou J. Effect of drying and calcination on the toluene combustion activity of a monolithic CuMnAg/γ-Al2O3/cordierite catalyst. J. Chem. Technol. Biot. 2010, 85, 569–576. Search in Google Scholar

[144] Ma R, Su X, Jin L, Lu J, Luo M. Preparation and characterizations of Ce-Cu-O monolithic catalysts for ethyl acetate catalytic combustion. J. Rare Earth. 2010, 28, 383–386. Search in Google Scholar

[145] Benard S, Giroir-Fendler A, Vernoux P, Guilhaume N, Fiaty K. Comparing monolithic and membrane reactors in catalytic oxidation of propene and toluene in excess of oxygen. Catal. Today 2010, 156, 301–305. Search in Google Scholar

[146] Yue L, Zhao L, Zhang Q, Zhang T, Luo M. Catalytic combustion of toluene over Pd-based monolithic catalysts with a novel washcoat Ce0.8Zr0.15La0.05Oδ. J. Rare Earth. 2009, 27, 733–738. Search in Google Scholar

[147] Zhang Q, Zhao L, Yue L. Toluene combustion over Pd-based monolithic catalysts with a novel CexZr1-x O2 washcoat. React. Kinet. Catal. Lett. 2008, 93, 27–33. Search in Google Scholar

[148] Zhao L, Zhang Q, Luo M, Teng B. Toluene combustion over Pd-Ce0.4Zr0.6O2 directly washcoated monolithic catalysts. J. Rare Earth 2007, 25, 715–720. Search in Google Scholar

[149] Barresi AA, Cittadini M, Zucca A. Investigation of deep catalytic oxidation of toluene over a Pt-based monolithic catalyst by dynamic experiments. Appl. Catal. B Environ. 2003, 43, 27–42. Search in Google Scholar

[150] Musialik-Piotrowska A, Syczewska K. Combustion of volatile organic compounds in two-component mixtures over monolithic perovskite catalysts. Catal. Today 2000, 59, 269–278. Search in Google Scholar

[151] Luo MF, He M, Xie YL, Fang P, Jin LY. Toluene oxidation on Pd catalysts supported by CeO2-Y2O3 washcoated cordierite honeycomb. Appl. Catal. B Environ. 2007, 69, 213–218. Search in Google Scholar

[152] Lu H, Zhou Y, Huang H, Zhang B, Chen Y. In-situ synthesis of monolithic Cu-Mn-Ce/cordierite catalysts towards VOCs combustion. J. Rare Earth 2011, 29, 855–860. Search in Google Scholar

[153] Zhao F, Zhang G, Zeng P, Yang X, Ji S. Preparation of CuxCo1-x /Al2O3/cordierite monolithic catalysts and the catalytic combustion of toluene. Chin. J. Catal. 2011, 32, 821–826. Search in Google Scholar

[154] Azalim S, Brahmi R, Agunaou M, Beaurain A, Giraudon JM, Lamonier JF. Washcoating of cordierite honeycomb with Ce-Zr-Mn mixed oxides for VOC catalytic oxidation. Chem. Eng. J. 2013, 223, 536–546. Search in Google Scholar

[155] Papaefthimiou P, Ioannides T, Verykios XE. Combustion of non-halogenated volatile organic compounds over group VIII metal catalysts. Appl. Catal. B Environ. 1997, 13, 175–184. Search in Google Scholar

[156] Radic N, Grbic B, Terlecki-Baricevic A. Kinetics of deep oxidation of n-hexane and toluene over Pt/Al2O3 catalysts: platinum crystallite size effect. Appl. Catal. B Environ. 2004, 50, 153–159. Search in Google Scholar

Received: 2015-9-28
Accepted: 2015-11-7
Published Online: 2016-1-21
Published in Print: 2016-2-1

©2016 by De Gruyter

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