A modified confined catalyst with CeO2 on the interior and CuO on the exterior surface of TiO2 nanotubes (Ce-in-TNT-Cu-out) was prepared and investigated for the combustion of butane catalytically. Compared with the Ce-in-TNT and TNT-Cu-out, the Ce-in-TNT-Cu-out presents a higher activity for butane oxidation, with a conversion of 10% at 200°C and a conversion of 90%) at 300°C. XPS analysis indicates that more Ce(IV) and Cu(I) components exist in the Ce-in-TNT-Cu-out catalyst. It is proposed that electron transfer ability between encapsulated CeO2 and loaded CuO is significantly enhanced by the confinement effect of the TiO2 nanotubes, facilitating the formation and migration of active oxygen species in the catalyst. This result shows that modulating the electronic property of the active component can further improve the catalytic combustion performance of the confined catalysts.
In recent decades, the excessive emission of volatile organic compounds (VOCs) has caused a serious environmental pollution problem. Many techniques have already have been adopted to remove VOCs, including adsorption, condensation, photocatalysis, biotreatment, catalytic combustion, etc. [1-2]. Among these technologies, catalytic combustion is one of the most promising technologies for the removal of VOCs due to its efficient cleaning and no secondary pollution .
As is well known, catalysts play a vital role in the further application of catalytic combustion technology. In particular, noble metal catalysts, such as Pt and Pd, show high activities for the oxidation of VOCs . However, an extremely high cost and susceptibility to deactivation limit their application in industry. In recent years, many efforts have been devoted to the development of more efficient, stable and cheaper non-noble metal oxides [5-6]. Metal oxides have attracted increasing attention as potential oxidation catalysts owing to their unique redox properties and high oxygen storage capacity . It is well known that CeO2 can form oxygen vacancies in the surface or bulk, thus providing lattice oxygen for catalytic combustion reactions. Moreover, CeO2 can store or release oxygen through the cycle of oxidation reduction between Ce4+ and Ce3+ [8,9,10]. Based on these unique properties, CeO2 is widely applied as a promoter in three-way catalysts (TWCs) for the elimination of toxic auto-exhaust gases (NOx, CO and hydrocarbon) . Recently, researchers also have found that bimetallic cerium-copper oxide exhibits a better performance on the catalytic combustion of VOCs [12-13]. Zhou et al. prepared different mole ratios of Ce/Cu by a hard-template method and (being designated as CeCu- HTx, where x is the Ce/Cu molar ratio) investigated their catalytic properties towards the catalytic combustion of phenyl VOCs (benzene, toluene, xylene, and ethylbenzene) in air. It was observed that the CeCu-HT3 catalyst showed a high stability and efficiency for the catalytic combustion of toluene in air. At the reaction temperature of 225°C, the conversion of toluene on CeCu-HT3 exceeded 90% . He et al. synthesized mesostructured Cu-Mn-Ce-O mixed oxide catalysts using a homogeneous coprecipitation (hcp) method, and reported that the Cu-Mn-Ce-O mixed oxide (hcpcmc30) had the best reducibility with most of the reducible species at about 153 °C. The superior catalytic performance can be ascribed to the higher surface oxygen concentration, better oxygen mobility and the adsorption ability of chlorobenzene (CB) molecules. In addition, the incorporation of CuO and MnOx can effectively inhibit the formation of organic byproducts, such as phenolates, maleates, and ortho-benzoquinone type species .
In addition to bimetallic oxide catalysts, confining oxide nanoparticles into porous materials or tubular materials could be another effective approach to improve the catalytic combustion performance. Bao’s group reported the tuning of the redox properties of metal and oxide nanoparticles by encapsulation within carbon nanotubes (CNTs) . They discovered that the confinement effect of CNTs can hinder sintering process of the entrapped active component, facilitated the electron transfer and significantly enhanced the activity and stability of the catalysts . Compared with CNTs, TiO2 nanotubes (TNTs) were easy to synthesize and stable in thermal reactions in the presence of oxygen, which significantly extended the area of possible application fields . Wang et al. prepared titanium nanotube-confined ceria and investigated the catalytic performance for selective catalytic reduction (SCR) of NO with ammonia. In comparison with the catalysts supported by TiO2 nanoparticles, the confined ceria showed superiority in the SCR of NO due to the improved redox potential . Our previous work has investigated the nanometer confinement effect in catalytic combustion using Pd-in-TiO2 and MnO2-in-TiO2 nanotube catalysts [20-21]. It was proved that TiO2 nanotube confinement can reduce the catalytic combustion temperature of butane, which can be related to the tuning of the electronic character resulting from the confinement effect. According to the above results, a consensus was reached that modulating the electronic property of the active component can improve catalytic performance . The modulation of the electronic property of TiO2 nanotube confined catalysts in catalytic combustion reactions and the relationship between metal oxides and tubular supports have not been investigated. Copper oxide and CeO2 are considered highly active and promising catalysts for the combustion of VOCs .
Herein, we prepared a modified confined catalyst with CeO2 inside and CuO on the outside surface of TiO2 nanotubes (Ce-in-TNT-Cu-out) for butane combustion. The performance of catalytic combustion for butane based on the modified confined catalyst was studied in comparison with the non-modified catalyst. This research focuses on a hypothesis that the catalytic combustion performance of CeO2 can be further improved through modulating the electronic structure of the active component by loading CuO on the exterior surface of TiO2 nanotubes.
2.1 Catalyst preparation
TiO2 nanotubes (TNTs) were synthesized using a hydrothermal method according to previous literature . The CeO2 catalyst was confined inside TiO2 nanotubes (named as Ce-in-TNT) by a vacuum-assisted impregnation method. 5 mL of Ce(NO3)3⋅6H2O solution (10 g/L) was dropped into an Erlenmeyer beaker containing 1 g TNTs under stirring. The resulting mixture was stirred for 2 h under vacuum-assisted and ultrasonic conditions to entrap Ce ions into the inner cavity of TNTs. Based on the Ce-in-TNT catalyst, the CuO modified confined catalyst (named as Ce-in-TNT-Cu-out) was prepared as follows: 5 mL of Cu(NO3)2⋅3H2O solution (10 g/L) was dropped into an Erlenmeyer beaker containing the Ce-in-TNT catalyst under stirring. The resulting mixture was stirred for 2 h under atmospheric pressure to deposit Cu ions on the outer walls of the TNT. Finally, the products were dried overnight at 60°C, followed by calcination in static air at 450°C for 2 h. For comparison, the catalysts of TNT supported CeO2 (Ce-out-TNT) and CuO (Cu-out-TNT) were also prepared by the same method.
2.2 Catalyst characterization
X-ray diffraction (XRD) patterns were recorded using an X’ Pert-PRO MPD diffractometer equipped with Cu Kα radiation. High revolution transmission electron microscopy (HRTEM) was carried out on a JEM-2010HR microscope (JEOL, Japan) with an acceleration voltage of 200 kV. The specific surface area, pore diameter and pore volume of the catalysts were measured by SI-MP-10/Pore Master-33 BET. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250XI to examine the surface element composition and electronic state of the catalysts. The C 1s line was taken as an internal standard at 284.6 eV. The hydrogen temperature-programmed reduction (H2-TPR) was analyzed using ASIQACIV 200-2 AUTOSORB-iQ-C chemisorption analyzer. First, 100 mg of catalyst was pretreated under an Ar atmosphere at 300°C for 1 h, and was then cooled to room temperature. Finally the sample was heated from room temperature to 1000 °C with a heating rate of 10°C/min under a gas mixture of 5% H2/95% Ar (v/v).
2.3 Catalytic activity test
The performance of prepared catalysts was investigated in a fixed-bed quartz reactor (ID 5mm) at atmospheric pressure. A temperature programmer (YUDIAN, model 708P, China) was used to control the heating rate at 1°C/ min. The feed gas (5 vol% C4H10, 50 vol% O2 and 45 vol% N2) passed through the catalyst (0.1 g) in the reactor with a flow rate of 50 mL/min. The gaseous products were analysed by gas chromatography (GC, Agilent, 7890 A) to assay the content of C4H10, O2, N2 and CO2 according to the calibrated curves of standard gases.
The conducted research is not related to either human or animals use.
3 Results and discussion
The XRD patterns of the Ce-in-TNT, Cu-out-TNT and Ce-in-TNT-Cu-out are presented on Figure 1. All of the samples show characteristic diffraction peaks of anatase TiO2 at 2θ=25.34°, 38.09°, 48.11°, 53.69°, 55.07° and 62.78° (JCPDS 21-1272). For the Cu-out-TNT catalyst, the diffraction peaks become sharp and intense, which may be due to the fact that the addition of Cu metal oxide results in the growth of TiO2 crystallite. For the three catalysts, no distinct diffraction peaks of Ce and/or Cu metal oxides are observed, indicating that the metal oxides are highly dispersed and their sizes are too small to be detected [25-26].
Figure 2 shows the HRTEM images of the Ce-in-TNT and Ce-in-TNT-Cu-out catalysts. It is observed from the TEM images that the TNT retains its one-dimensional nanotubular morphology after the addition of the Ce and Cu components. For both catalysts, CeO2 particles with a diameter of about 2-3 nm are mostly entrapped into the inner cavity of the TNT support by vacuum-assisted impregnation. The measured lattice spacings of 0.31 nm and 0.19 nm are consistent with the (111) and (220) planes of CeO2 (see the insets of Figure 2) . For the Ce-in-TNT-Cu-out, CuO with a particle size of about 3-4 nm are mostly deposited on the exterior surface of the TNT support. The recorded HRTEM image from the marked red circle region (Figure 2b) shows the lattice spacing of 0.27 nm corresponding to the CuO (110) plane .
The nitrogen adsorption/desorption isotherms and pore-size distribution curves for the Ce-in-TNT, Cu-out- TNT and Ce-in-TNT-Cu-out catalysts is presented on Figure 3. All samples present type (IV) isotherm curves, which is characteristic of mesopore structure . The surface area, average pore size and pore volume of the catalysts were estimated using the desorption branch of the isotherm, as listed in Table 1. Among the three catalysts, the Cu-out-TNT exhibits the biggest SBET area with a value of 143.2 m2/g. The Ce-in-TNT and the Ce-in-TNT-Cu-out catalysts have similar SBET areas of about 100 m2/g. At the same time, pore size and pore volume of the catalysts will also change after the addition Ce and Cu metal oxides. For the Ce-in-TNT and the Ce-in-TNT-Cu-out, SBET area and pore volume are obviously lower than the Cu-out-TNT. This may be due to the fact that CeO2 particles are entrapped into the inner cavity of TNT, taking up some of the pore volume.
The surface elemental composition and electronic state of the prepared catalysts were determined by XPS. The XPS spectra of Ce 3d, Cu 2p, O 1s and Ti 2p are shown in Figure 4. For the Ce-in-TNT and Ce-in-TNT-Cu-out catalysts, the Ce 3d XPS spectra can be adequately deconvolved into eight peaks (Figure 4a), in which the marked u′ and v′ components are attributed to 3d3/2 and 3d5/2 features of Ce3+ 3d, assigned to Ce(III) species. The v/v″/v′″ and u/u′/u′″ components correspond to the Ce4+ 3d3/2 and Ce4+ 3d5/2 respectively can be assigned to Ce(IV) species, which is in good agreement with other reported works [30, 31, 32]. It can be seen on Figure 4b the Cu 2p spectra for the Cu-out-TNT and Ce-in-TNT-Cu-out catalysts. The peaks corresponding to Cu2+ and Cu+ at 931.4 and 934.1 eV can be identified in the Cu 2p spectra [33-34]. The surface atom ratios of Ce4+/ Ce3+ and Cu2+/Cu+ are obtained according to XPS spectral analysis, as clearly listed in Table 1. Compared with the Ce-in-TNT and Cu-out-TNT, the Ce-in-TNT-Cu-out exhibits a much higher surface atom ratio for Ce(IV) and Cu(I). That is, more Ce(IV) and Cu(I) components exist in the CuO modified confined catalyst. As for the O1s spectra, it can be resolved into a lattice oxygen peak at 530 eV and a weakly adsorbed oxygen peak at 532 eV . In the Ti 2p spectra, a shift (0.1 eV) towards lower binding energy is observed for the CuO modified confined catalyst. Based on the XPS analysis, it is proposed that the surface atom ratio of Ce4+/Ce3+ was changed after loading CuO on the outer surface of TiO2 nanotube. There exists more Ce(IV) and Cu(I) components in the Ce-in-TNT-Cu-out catalyst.
Figure 5 gives the H2-TPR curves of the Ce-in-TNT, Ce-in-TNT-Cu-out and Cu-out-TNT catalysts. The Ce-in-TNT shows an overlapping H2 reduction peak at 300-800°C. According to the literatures, pure CeO2 has two reduction peaks at about 500°C and 800°C, corresponding to the reduction of surface and bulk oxygen of CeO2, respectively [36, 37, 38, 39]. For the Ce-in-TNT-Cu-out, the TPR curve is strongly modified owing to the addition of Cu metal oxide, showing the reduction peak belonging to surface oxide of CeO2 at temperature much lower than that of Ce-in-TNT. The TPR curves of the Ce-in-TNT-Cu-out and Cu-out-TNT show two reduction peaks in the range of 150-280°C. The low temperature peak is attributed to the reduction of highly dispersed CuO clusters on the surface of TiO2, and one at higher temperature is ascribed to the reduction of larger CuO particles . For the Ce-in-TNT-Cu-out, the H2-TPR peak temperature resulting from CuO is much lower than that for the Cu-out-TNT. The above results indicate that the CuO modified confined catalyst is easier to reduce, which may be attributed to the enhancement of electron transfer ability between encapsulated CeO2 and loaded CuO caused by the confinement effect of the TiO2 nanotubes.
Figure 6 presents the butane conversion as a function of reaction temperature over the Ce-in-TNT-Cu-out, Ce-in-TNT, Cu-out-TNT and Ce-out-TNT catalysts. Among the prepared catalysts, the Ce-in-TNT-Cu-out presents the highest activity for butane oxidation, with a conversion of 10% at about 200°C. A steep increase in conversion at higher temperatures is observed and a conversion of 90% is reached at 300°C for the Ce-in-TNT-Cu-out. The other three catalysts exhibit moderate oxidation activity for butane below 300°C. A conversion of 90% for butane is achieved at 340°C for the Ce-in-TNT. The Cu-out-TNT and Ce-out-TNT catalysts exhibit relatively poor activity for butane oxidation, with a conversion below 90% at 360°C. The above results demonstrate that the performance of catalytic combustion butane on TiO2 nanotube confined catalysts can be further improved by CuO modification.
It is reported that the TiO2 nanotubes (TNTs) have a similar character with carbon nanotubes (CNTs) . Previous studies show that π-electron density derived from the deviation of graphene layers planarity shifted from the concave inner surface to the convex outer surface due to the unique tubular structure, and finally led to an electron-deficient interior CNTs surface and an electron-enriched exterior surface [40, 41, 42]. The same electronic character is observed in the TNTs, which have an electron-deficient interior surface and an electron-enriched exterior surface . When nanoparticles are encapsulated within TiO2 nanotubes, the confined nanoparticles possess more electron deficiency, resulting in an enhanced catalytic activity [20-21]. Our previous research has found that the confined MnO2 catalyst presents a better combustion activity for butane than the non-confined one . In this study, the same result has also been proved for the confined CeO2 catalyst (see Figure 6). Based on the confined CeO2 catalyst, CuO was used to further regulate the electron density distribution of TNTs. Due to transformation of Cu2+-Cu+, electron density is shifted from the interior to the exterior surface of TNTs. Using a matrix layer covered by nanoparticles instead of a mixture of nanoparticles is beneficial for efficient electron transformation . As a result, the entrapped CeO2 will be in an electron-deficient state, which can provide a great quantity of lattice oxygen for catalytic combustion. A higher percentage for Ce(IV) and Cu(I) in the modified confined CeO2 catalyst has been deduced from XPS analysis (see Figure 4 and Table 1). TPR results further confirm that the Ce-in-TNT-Cu-out catalyst is easier to reduce (see Figure 5), which is consistent with literature [44-45]. The CeO2 materials with more Ce(IV) are believed to be promising for exhibiting higher oxidation activity . For the butane combustion over the CeO2– based catalysts, firstly, a butane molecule is adsorbed on the catalyst surface and oxidized by the surface lattice oxygen. This will lead to the transformation of Ce4+-Ce3+ and form oxygen vacancies. Finally, the formation of oxygen vacancies is provided by the oxygen molecules from the feed gas in the redox cycle. The confinement effect changes the balance of Ce4+/Ce3+ ion pairs and accelerates the activation of the oxygen reactants. Furthermore, owing to unique structure of the catalyst with CeO2 on the inside and CuO on the outside surface of TNTs, the transformation of Cu2+-Cu+ will accelerate the conversion of Ce3+-Ce4+, facilitating the formation and migration of active oxygen species in the catalyst. Thus, the catalytic activity for butane oxidation over the Ce-in-TNT-Cu-out is significantly improved by CuO modification.
The catalytic performance towards butane oxidation of the Ce-in-TNT-Cu-out, with CeO2 inside and CuO on the outside surface of TNTs, was investigated for the first time. The Ce-in-TNT-Cu-out exhibits higher oxidation activity and a lower combustion temperature than the Ce-in-TNT. The enhanced catalytic combustion activity can be ascribed to the tuning of the electronic property of the active component CeO2 by loading CuO on the exterior surface of the TiO2 nanotubes. This study provides a prospect to use modified TiO2 nanotube confined non-noble metal oxide catalysts for dealing with VOCs in industrial applications.
The authors thank the National Natural Science Foundation of China (No. 51661145022), Science and Technology Plan Project of Guangdong Province (NO. 2017A050501044), the Natural Science Foundation of Guangdong (No. 2015A030313715) for financial support.
Conflict of interest: Authors state no conflict of interest.
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