Emerging nanostructured materials for the catalytic removal of volatile organic compounds

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.

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.

2.2 Methods for the synthesis of nanostructured metal oxides

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.

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.

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.

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.

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.

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.

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 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.

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.

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.% Ce_0.68Zr_0.17Sn_0.15O_2.0/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, Ce_0.79Zr_0.21O_2.0; CZB, Ce_0.64Zr_0.16Bi_0.20O_1.9; CZS, Ce_0.68Zr_0.17Sn_0.15O_2.0). 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.

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.

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.

Chen et al. synthesized Pd-based catalyst by sodium citrate reduction and hydrothermal process on the Ce_xMn_1-x composite oxide supports for benzene catalytic oxidation [99]. The Ce_xMn_1-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 Ce_xMn_1-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 Ce_xMn_1-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/Ce_xMn_1-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/Ce_xMn_1-x catalysts. Reproduced from [99] with permission from Springer.

Wei et al. prepared Al₂O₃-Ce_0.3Zr_0.7O₂ (ACZ) and 3 wt.% BaO-doped Al₂O₃-Ce_0.3Zr_0.7O₂ (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.

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.

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.

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 Ce_0.6Zr_0.3Y_0.1O₂ (CZY) nanorods to support gold and palladium alloy (zAu_xPd_y/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 zAu_xPd_y/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.

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 (O^2-) species generated by the strong interaction of Ag and Co and anion lattice defects. Ag(111) facets are active planes; surface OH^-and O^2- 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 O^2- 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.

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.

4.1 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.

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 MnO_x, 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 25:

The possible pathways for benzene oxidation on the MnO_x 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 MnO_x-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 MnO_x 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.

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-MnO_x oxides with different Ce-to-Mn ratios: benzene=1000 ppm, synthetic air (20 vol.% O₂ and 80 vol.% N₂) balance, GHSV=60,000 mLg_cat^-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 MnO_x 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 (E_a) values (89.1–113.1 kJ mol^-1), the nanocasted catalyst exhibits the best activity with T₉₀ at 234°C and the lowest E_a 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: MnO_x, b and b1: Mn₅Co₅O_x, 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.

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 Mn^δ+ 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.

Also in 2015, Balzer et al. reported the syntheses of nanorods of a Ce_1-x Co_xO₂ 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 Ce_0.80Co_0.20O₂ 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 La_1–xCe_xMnO₃ (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 O_ads/O_latt ratio due to the charge neutralization. These trends in the Mn⁴⁺/Mn³⁺ and O_ads/O_latt 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) La_0.900Ce_0.100MnO₃ perovskites; HRTEM images of (C) pure LaMnO₃ and (D) La_0.900Ce_0.100MnO₃ perovskites; and (E) a corresponding EDX image of La_0.900Ce_0.100MnO₃ 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.