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Morphology and Crystal-Plane Effects of CeO2 on TiO2/CeO2 Catalysts during NH3-SCR Reaction Xiaojiang Yao, Li Chen, Jun Cao, Fumo Yang, Wei Tan, and Lin Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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Industrial & Engineering Chemistry Research

Morphology and Crystal-Plane Effects of CeO2 on TiO2/CeO2 Catalysts during NH3-SCR Reaction

Xiaojiang Yao,† Li Chen,† Jun Cao,† Fumo Yang,*,‡,§ Wei Tan,┴ and Lin Dong┴

†

Research Center for Atmospheric Environment, Chongqing Institute of Green and Intelligent

Technology, Chinese Academy of Sciences, Chongqing 400714, PR China ‡

School of Architecture and Environment, Sichuan University, Chengdu 610065, PR China

§

Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment,

Chinese Academy of Sciences, Xiamen 361021, PR China ┴

Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing

University, Nanjing 210093, PR China

1

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ABSTRACT A series of supported TiO2/CeO2 catalysts were synthesized to investigate the morphology and crystal-plane effects of CeO2 on these TiO2/CeO2 catalysts for NH3-SCR. The experiment results show that TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts present nano-cubes (mainly exposed {100} facet), nano-polyhedrons (mainly exposed {111} and {100} facets), and nano-rods (mainly exposed {110} and {100} facets), respectively. Furthermore, TiO2/CeO2-NR catalyst exhibits the best dispersion of TiO2 species, the most excellent reduction behavior and surface acidity, and the largest amount of Ce3+ ions and chemisorbed oxygen, which is due to the largest specific surface area and pore volume of CeO2-NR suppot, as well as the strongest interaction between the surface dispersed TiO2 species and {110} facet of CeO2-NR support. All of them result in the best denitration performance of TiO2/CeO2-NR catalyst during NH3-SCR reaction.

KEYWORDS: TiO2/CeO2 catalysts; Morphology and crystal-plane effects; Reduction behavior; Surface acidity; NH3-SCR reaction

2


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1. INTRODUCTION Nitrogen oxides (i.e., NOx) originated from the use of fossil fuel caused serious atmospheric pollution and further endangered to human health.1,2 Selective catalytic reduction of NOx by ammonia (i.e., NH3-SCR) is recognized as the most cost-effective

treatment

technology

for

stationary

sources.3

Commercial

V2O5-WO3/TiO2 catalysts have been widely applied for NH3-SCR process in the thermal power plants and coal-fired boilers due to excellent denitration and sulfur-resistance performance during 300-400 °C.4-6 But, there are several typical drawbacks for vanadium-based catalysts due to the toxicity of V2O5, oxidation of SO2, and generation of N2O, etc..7-9 Therefore, the development of non-vanadium-based catalysts becomes a hot research topic in recent years. CeO2-based catalysts were systematically studied during NH3-SCR reaction because of their excellent denitration performance, strong oxygen storage ability, and good redox behavior, etc..10-12 For instance, Zhang et al. synthesized several CeO2/zirconium phosphate catalysts for NH3-SCR, and found that the interaction of active species and support improved the corresponding redox property and surface acidity, and further led to excellent catalytic performance.11 Li et al. identified the active sites of CeO2-WO3 catalysts during NH3-SCR reaction, and reported that the reaction mechanism involved a redox cycle based on strong oxygen storage ability and excellent redox behavior of CeO2, and an acid cycle originated from W-O-W species of Ce2(WO4)3.13 He et al. synthesized an environmentally-benign CeO2-TiO2 catalyst for NH3-SCR reaction, and pointed out that the good dispersion of CeO2 and the synergetic interaction of CeO2 and TiO2 resulted in the enhanced denitration performance.14 On the other hand, it is well known that the morphologies and crystal-planes of catalysts can significantly influence their catalytic performance in some redox reactions due to different surface energies, atom arrangements, and synergistic interactions.15-18 Similarly, morphology and crystal-plane effects can be also found in NH3-SCR reaction. For example, Zhang et al. prepared two kinds of Fe2O3/TiO2-NS (nanosheets) and Fe2O3/TiO2-NSP (nanospindles) catalysts, which mainly exposed 3



{001} and {101} facets, respectively.19 They found that Fe2O3/TiO2-NS catalyst with {001} facet exhibited better catalytic performance than Fe2O3/TiO2-NSP catalyst with {101} facet for NH3-SCR reaction due to more oxygen defects, active oxygen species, acid sites, and adsorbed nitrate species, as well as lower NH3/NO adsorption energy. Shen et al. explored the reason for the different catalytic performances of γ-Fe2O3 nanorods and α-Fe2O3 nanorods during NH3-SCR reaction, and reported that γ-Fe2O3 nanorods with {110} and {001} facets displayed better catalytic performance than α-Fe2O3 nanorods with {210} and {001} facets, which is due to that {110} and {001} facets of γ-Fe2O3 nanorods contain iron and oxygen ions simultaneously, while {210} and {001} facets of α-Fe2O3 nanorods only provide iron ions for NH3/NO adsorption but lack neighboring oxygen ions for NH3/NO activation.20 In recent years, CeO2 with special morphology and crystal-plane has been successfully synthesized and used for CO oxidation, photocatalytic hydrogen evolution, NO reduction by CO, methane dry reforming, and water-gas shift reactions, etc..21-25 On the other hand, TiO2 was widely used as one of the important components of the denitration catalysts during NH3-SCR reaction due to its good sulfur-resistance. Especially, CeO2-TiO2 catalysts were systematically studied during NH3-SCR reaction because of the combination of the advantages of CeO2 and TiO2.14,26-29 However, the influence of morphology and crystal-plane of CeO2 on the denitration performance of CeO2-TiO2 catalysts is still unclear and lacks deep investigation. So, we attempt to synthesize CeO2 supports with different morphologies and crystal-planes for preparing supported TiO2/CeO2 catalysts, and clarify the morphology and crystal-plane effects of CeO2 on TiO2/CeO2 catalysts during NH3-SCR reaction.

2. EXPERIMENTAL PROCESS 2.1. Preparation of catalysts CeO2 supports were prepared by a hydrothermal method based on adjusting the hydrothermal reaction temperature and alkaline solution concentration. In detail, Ce(NO3)3·6H2O and NaOH were mixed together in deionized water under magnetic stirring. After that, the obtained suspension was put in the Teflon-lined stainless steel 4


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autoclave to react at different temperatures for 24 h. The hydrothermal reaction temperature and NaOH concentration were fixed at 180 °C and 6 M, 180 °C and 0.1 M, 100 °C and 6 M for CeO2 nano-cubes (CeO2-NC), CeO2 nano-polyhedrons (CeO2-NP), CeO2 nano-rods (CeO2-NR), respectively. Subsequently, the precipitate was centrifuged and washed until pH = 7, and dried at 60 °C overnight during a vacuum oven. Finally, all of these supports were put in a muffle furnace and calcined at 400 °C for 3 h. TiO2/CeO2 catalysts with different morphologies were obtained by a hydrolysis impregnation method with butyl titanate (C16H36O4Ti). Mole ratio of Ti:Ce was set as 1:9 for TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts. In brief, CeO2-NC, CeO2-NP, and CeO2-NR supports were respectively dispersed into the ethanol solution containing appropriate amount of C16H36O4Ti with magnetic stirring for 1 h during an ice-water bath. After that, the desired amount of deionized water was dropped in the above-mentioned suspension with magnetic stirring for 3 h to promote the hydrolysis of C16H36O4Ti. Further, these samples were heated at 100 °C to vaporize the solvent. And then, they were oven dried at 100 °C overnight, and finally calcined at 400 °C for 3 h to obtain TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts. 2.2. Characterization of catalysts Transmission electron microscopy (TEM) images of these CeO2 supports and TiO2/CeO2 catalysts were performed by a JEM-2100 instrument working at 200 kV. Each sample was dispersed in absolute ethanol with ultrasonic oscillation for 30 min, and deposited on the carbon-covered copper grid for test. X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert3 Pro diffractometer with Cu-Kα source (wavelength = 0.15418 nm). In which, the working voltage and current were fixed at 40 kV and 40 mA. Raman spectra were collected by a Renishaw inVia Reflex Laser Raman spectrometer. In which, the wavelength and power were 532 nm and 5 mW, while the resolution of spectra is ±1 cm–1. Textural data were got via N2-physisorption by the Belsorp-max analyzer 5



according to BET and BJH methods. Prior to analysis, the sample was vacuum pretreated at 300 °C for 4 h to achieve 10 µm Hg. H2-temperature programmed reduction (H2-TPR) profiles were performed by the TP-5076 dynamic sorption analyzer equipped a thermal conductivity detector (TCD). 7% H2-Ar mixture was chosen as the reductant. Firstly, 50 mg catalyst was pretreated by N2 at 300 °C for 1 h. After cooling to room temperature, the reduction process was started. NH3-temperature programmed desorption (NH3-TPD) profiles were also recorded on the TP-5076 dynamic sorption analyzer with a thermal conductivity detector (TCD). Firstly, 0.2 g catalyst was pretreated by nitrogen at 300 °C for 1 h. Secondly, the catalyst was saturated by 1% NH3-N2 mixture at 100 °C, and then flushed with N2 for 1 h. Finally, the TPD process conducted from ambient temperature. X-ray photoelectron spectroscopy (XPS) tests were performed on the PHI 5000 VersaProbe system equipped monochromatic Al-Kα source (1486.6 eV). Before the test, each sample was pretreated in the UHV chamber (below 5×10–7 Pa) at room temperature. Furthermore, charging effect was compensated by calibrating the binding energy with C 1s at 284.6 eV. In situ diffuse reflectance infrared Fourier transform spectra (In situ DRIFTS) were performed on a Nicolet 5700 FT-IR spectrometer. Each sample was loaded in the cell and pretreated with nitrogen at 450 ºC for 1 h. Sample background was recorded in the cooling step. The catalyst was saturated by NH3-N2 (500 ppm NH3) or NO-O2-N2 (500 ppm NO and 5% O2) or NO-NH3-O2-N2 (500 ppm NO, 500 ppm NH3, and 5% O2) mixture at ambient temperature. And then, the spectra were collected at each target temperature via subtracting sample background. 2.3. Test of catalytic performance Evaluation of denitration performance for NH3-SCR was carried out at a steady state. The composition of reaction gas is NO (500 ppm), NH3 (500 ppm), O2 (5%), SO2 (100 ppm, when needed), and H2O (5%, when needed), while nitrogen is the rest. Firstly, 0.2 g sample was loaded in the reaction tube and pretreated by nitrogen at 6


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300 °C for 1 h. Subsequently, reactions were conducted at each temperature under the space velocity of 60000 ml·g–1·h–1. Moreover, NO and NO2 were measured via the flue gas analyzer, while N2O was monitored via the N2O analyzer. Finally, NOx conversion was obtained by the following formula: NOx conversion (%) =

[ NO]in + [ NO2 ]in − [ NO ]out − [ NO2 ]out × 100% [ NO ]in + [ NO2 ]in

3. RESULTS AND DISCUSSION 3.1. Morphological and crystal-plane characteristics (TEM and HRTEM) In order to determine the morphologies and crystal-planes of these CeO2 supports, TEM and high resolution TEM (HRTEM) characterizations were performed, and the images are presented in Figure 1. We can see from Figure 1a that CeO2-NC exhibits a perfect cubic shape with smooth surface and uniform size distribution during 10-28 nm (Figure S1 in the Supporting Information). Moreover, static analysis of TEM indicates that the average size of CeO2-NC is ca. 20.5 nm, which is listed in Table 1. HRTEM image of CeO2-NC in Figure 1b displays that all of the lattice fringes correspond to (200) crystal-plane with an inter-plane spacing of 0.267 nm, which belongs to {100} facet. Therefore, the morphology of CeO2-NC particles is compatible with the cube enclosed with six {100} facets.23,24,30 Figure 1c presents the TEM image of CeO2-NP as a uniform polyhedral shape with uniform size distribution during 8-17 nm (Figure S1 in the Supporting Information), and the average size is about 11.9 nm (Table 1). HRTEM image of CeO2-NP (Figure 1d) shows the clear lattice fringes of (111) and (200) crystal-planes with inter-plane spacing of 0.310 and 0.267 nm, which indicate that CeO2-NP particles are similar to the truncated octahedron enclosed with {111} and {100} facets.23,25 For CeO2-NR, TEM image (Figure 1e) gives a rod shape with the length of 30-150 nm and the diameter of 5-10 nm (as shown in Figure S1 in the Supporting Information). Furthermore, the average length and diameter is approximately 68.1 nm and 6.9 nm, respectively (Table 1). Two kinds of lattice fringes with inter-plane spacing of 0.190 and 0.267 nm ascribed to (220) and (200) crystal-planes can be observed in Figure 1f, which indicate that 7



CeO2-NR particles preferentially grow along [110] crystalline direction and mainly exposed {110} and {100} facets.21,24 In summary, these CeO2 supports exhibit different morphologies and crystal-planes, which suggest that they have different atomic arrangements and coordination states. Furthermore, TEM and HRTEM images of TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts are displayed in Figure 2, which still maintain the original morphologies and crystal-planes of these CeO2 supports after the loading of TiO2. The size distribution and average size of these catalysts have no obvious changes compared with their corresponding supports (Figure S2 in the Supporting Information and Table 1). In addition, the lattice fringes and crystal boundaries of these TiO2/CeO2 catalysts are more blurry than the corresponding CeO2 supports due to the introduction of TiO2 on the surface of CeO2. However, there are no lattice fringes of crystalline TiO2 can be observed in this figure, which suggests that TiO2 species may be highly dispersed on these CeO2 supports.

3.2. Structural and textural properties (XRD, Raman, and N2-physisorption) Crystal structures of CeO2 supports and TiO2/CeO2 catalysts with different morphologies were characterized via XRD, as exhibited in Figure 3. Figure 3a shows that all the samples of CeO2-NC, CeO2-NP, and CeO2-NR exhibit several diffraction peaks, which are ascribed to CeO2 with cubic fluorite structure (PDF-ICDD 34-0394).31,32 According to Debye-Scherrer equation (Dβ = Kλ/βcosθ), the average crystallite size of CeO2-NC and CeO2-NP supports was calculated from the strongest diffraction peak, and given in Table 1, which is consistent with TEM results. However, the average crystallite size of CeO2-NR support is not given in Table 1 due to the large size-difference between each dimension, which is not suitable to be calculated by Debye-Scherrer equation. Furthermore, the intensity of diffraction peaks for these CeO2 supports is different each other, and in the order of CeO2-NC > CeO2-NP > CeO2-NR, which may be related to the crystallite size of CeO2 (supported by the results of TEM). The lattice constant of these CeO2 supports was also calculated by Bragg equation (2dsinθ = nλ), and summarized in Table 1, which is very close to each other. In addition, Figure 3b shows that only the characteristic peaks of CeO2 are 8


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observed on TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts, while the diffraction peaks of crystalline TiO2 are absent, which suggest that TiO2 may be highly dispersed on these CeO2 supports and/or existed in the form of cluster beyond the detection limit of XRD.33,34 This phenomenon is consistent with the results of HRTEM. Interestingly, the lattice constant of these TiO2/CeO2 catalysts is slightly smaller than that of the corresponding CeO2 supports due to the smaller ionic radius of Ti4+ (0.68 Å) than Ce4+ (0.92 Å), which indicates that a part of Ti4+ maybe doped into the surface lattice of these CeO2 supports. Figure 4 presents the Raman spectra of these CeO2 supports and TiO2/CeO2 catalysts with different morphologies. Figure 4a shows that the Raman spectra of CeO2-NC, CeO2-NP, and CeO2-NR are very similar, which exhibit a strong band at 464 cm–1 and a weak shoulder around 600 cm–1. According to the literatures,35-37 the former is assigned to F2g vibration mode of CeO2 with cubic fluorite structure, while the latter is related to the defect-induced mode (denoted as D) of CeO2 due to the existence of Ce3+. Furthermore, oxygen vacancy is conducive to the decomposition of NO molecules, which can improve the catalytic performance for NO removal.38,39 We all know that the ratio between the integrated peak area at 600 and 464 cm–1 (i.e., A600/A464) is usually applied for quantifying the relative content of oxygen vacancy in CeO2.37 However, the D band in the present work is too weak to calculate the corresponding peak area, because it will lead to larger errors. As a result, the relative content of oxygen vacancy in CeO2-NC, CeO2-NP, and CeO2-NR is not calculated. Although crystalline TiO2 is not detected by XRD over TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts, Figure 4b exhibits that besides F2g and D vibration bands, a new band is observed at 143 cm–1 on TiO2/CeO2-NC and TiO2/CeO2-NP catalysts, which can be assigned to Eg vibration band of anatase TiO2.40,41 Therefore, this new band may be resulted from larger clustered anatase TiO2, and further, its intensity in TiO2/CeO2-NC catalyst is remarkably stronger than that of TiO2/CeO2-NP catalyst, while it is absent over TiO2/CeO2-NR catalyst, which suggest that the dispersion of TiO2 species on CeO2-NR support is the best among these TiO2/CeO2 catalysts, while on CeO2-NC support is the worst. 9



Textural characteristics of these CeO2 supports and TiO2/CeO2 catalysts with different morphologies were tested via N2-physisorption experiment, and the results are given in Figure 5. With regard to Figure 5a, CeO2-NC and CeO2-NP exhibit the type IV isotherm with the H2 hysteresis loop resulted from the wormhole-like mesostructure and interstice mesoporous structure generated via nanoparticle accumulation, while CeO2-NR shows the type IV isotherm with the H3 hysteresis loop owing to the slit-like mesopores generated via the aggregation of nano-rod particles.42-44 Moreover, the illustration in Figure 5a exhibits that the pore size of CeO2-NC, CeO2-NP, and CeO2-NR locates at the mesoporous range of 2-50 nm, which further confirms that all of these CeO2 supports contain mesopores. Table 1 displays that BET specific surface area of these CeO2 supports is different each other, and ranked by CeO2-NR > CeO2-NP > CeO2-NC. Especially that CeO2-NR not only possesses the largest BET specific surface area, but also exhibits the largest total pore volume due to its rod-like structure, which can promote the dispersion of TiO2 (supported by Raman results). Figure 5b shows that isotherms and pore size distribution results of these TiO2/CeO2 catalysts have no obvious difference with those of CeO2 supports, which suggest that the loading of TiO2 almost doesn’t change the textural characteristics of CeO2 supports. However, we can see from Table 1 that the total pore volume of these CeO2 supports decreases to some extent after the loading of TiO2, which might be because that the dispersion of TiO2 on CeO2 supports and/or in the mesopores occupies some pore volume.

3.3. Redox performance and surface acidity (H2-TPR and NH3-TPD) We all know that the redox performance and surface acidity of catalysts are key factors for their denitration performance in NH3-SCR reaction, which are usually characterized by H2-TPR and NH3-TPD, respectively. Figure 6a displays that H2-TPR curves of these TiO2/CeO2 catalysts with different morphologies are similar to each other, which exhibit two broad reduction peaks during 300-630 °C (i.e., peak α in Region I) and 630-900 °C (i.e., peak β in Region II), respectively. Furthermore, the low-temperature reduction peak α can be attributed to the reduction of surface CeO2 and dispersed TiO2, while the peak β is related to the reduction of bulk CeO2.33,45-47 10


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Interestingly, Table 2 shows that the peak temperature (i.e., Tα and Tβ) of these TiO2/CeO2 catalysts is ranked by TiO2/CeO2-NC > TiO2/CeO2-NP > TiO2/CeO2-NR, while the experimental H2 consumption (i.e., Sα+Sβ) is in the opposite sequence, which indicate that TiO2/CeO2-NR exhibits the best reduction behavior. Moreover, the experimental H2 consumption of these TiO2/CeO2 catalysts is remarkably smaller than theoretical H2 consumption of Ti4+→Ti3+ and Ce4+→Ce3+ (given in the parentheses in Table 2), which suggests that only a part of TiO2 and CeO2 can be reduced in the current H2-TPR process. Especially, the ratio of the low-temperature reduction peak (i.e., Sα/(Sα+Sβ)) is also in the sequence of TiO2/CeO2-NR > TiO2/CeO2-NP > TiO2/CeO2-NC, which further confirms the best reduction behavior of TiO2/CeO2-NR. In other words, the formation of oxygen vacancy on TiO2/CeO2-NR is the easiest during reduction process, which can accelerate the decomposition of NO molecules.38,39 These phenomena may be related to the exposed crystal-planes of CeO2 supports, because that different exposed crystal-planes exhibit different atomic arrangements, and further lead to different interaction with the surface dispersed TiO2. Moreover, previous density functional theory studies show that the formation energy of oxygen vacancy on the surface of CeO2 follows the sequence of {111} > {100} > {110} facet, which suggests that the formation of oxygen vacancy over {110} facet of CeO2 is the easiest.1,23,48,49 Just right, HRTEM image presents that TiO2/CeO2-NR catalyst mainly exposes {110} facet of CeO2, which means that our H2-TPR results are consistent with the above-mentioned density functional theory studies. The adsorption and activation of NH3 species over the catalysts play an important role in NH3-SCR reaction, which are highly dependent on the acid strength and acid amount of catalysts. Furthermore, the temperature and integral area of desorption peaks in NH3-TPD profiles can represent the acid strength and acid amount, respectively. As shown in Figure 6b, all of these TiO2/CeO2 catalysts with different morphologies exhibit three desorption peaks during 50-750 °C, which are denoted to I, II, and III with the increase of desorption temperature. The low-temperature desorption peak I is attributed to NH3 species desorbed from weak acid sites; peak II is assigned to the desorption of NH3 species from medium-strong acid sites; while 11



peak III is related to strong acid sites.45,50 Interestingly, the desorption peak temperature of these TiO2/CeO2 catalysts is sequenced by TiO2/CeO2-NR > TiO2/CeO2-NP > TiO2/CeO2-NC, which suggests that the acid strength of TiO2/CeO2-NR is stronger than the other two catalysts. Moreover, the quantitative data of NH3-TPD in Table 2 show that the acid amount of these TiO2/CeO2 catalysts is also in the same sequence, which indicates that TiO2/CeO2-NR has the most acid sites among these catalysts. These experiment results suggest that the adsorption and activation of NH3 species over TiO2/CeO2-NR may be easier than the other two catalysts. The possible reasons are as follows: (1) specific surface area and pore volume of CeO2-NR are significantly larger than those of CeO2-NP and CeO2-NC supports, which can promote the dispersion of TiO2 to provide more sites for the adsorption and activation of NH3 species; (2) the generation of oxygen vacancy on {110} facet of TiO2/CeO2-NR is easier than {111} and {100} facets of TiO2/CeO2-NP and TiO2/CeO2-NC catalysts, which means that the migration of oxygen on {110} facet of TiO2/CeO2-NR is the easiest, and further benefits to enhance the interaction between CeO2-NR support and surface dispersed TiO2 species to accelerate the adsorption and activation of NH3.

3.4. Surface chemical state analysis (XPS) As we all know, surface property of the catalysts can significantly influence the catalytic performance for gas-solid phase reactions, which is because that these reactions mainly proceed on the surface of catalysts. Therefore, in the present work, surface property of these TiO2/CeO2 catalysts with different morphologies was analyzed by XPS, as shown in Figure 7. The Ce 3d spectra in Figure 7a are deconvoluted into eight well-resolved bands, which can be classified into two groups of spin-orbital multiplets, denoted as u and v, on behalf of Ce 3d3/2 and Ce 3d5/2, respectively.51,52 u′ and v′ bands are resulted from Ce3+ ions, while the other bands are related to Ce4+ ions, which indicate that Ce3+ and Ce4+ ions exist over these TiO2/CeO2 catalysts simultaneously. Moreover, the relative content of Ce3+ over these TiO2/CeO2 catalysts can be calculated from the integrated peak area ratio of Ce3+ to the total Ce as follows:51 Ce3+ (%) = (Su′+Sv′)/∑(Su+Sv) × 100%, which is listed in 12


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Table 3. It shows that Ce3+ content of TiO2/CeO2-NR is obviously larger than that of other two catalysts, which might be due to that the generation of oxygen vacancy over TiO2/CeO2-NR catalyst is the easiest. For Ti 2p spectra in Figure 7b, all of these TiO2/CeO2 catalysts exhibit a strong binding energy band at 458.1-458.4 eV and a weak one at 463.8-464.1 eV, which are ascribed to Ti 2p3/2 and Ti 2p1/2, respectively. The binding energy of Ti 2p3/2 and Ti 2p1/2 over TiO2/CeO2-NC is consistent with that of Ti4+ ions (458.4 and 464.1 eV),53 while the binding energy of TiO2/CeO2-NP and TiO2/CeO2-NR catalysts is slightly lower, which indicates that a part of Ti3+ species might be existed over TiO2/CeO2-NP and TiO2/CeO2-NR catalysts because of the electron interaction of TiO2 and CeO2-NP (CeO2-NR). Especially, TiO2/CeO2-NR catalyst exhibits the lowest binding energy of Ti 2p3/2 and Ti 2p1/2 among these TiO2/CeO2 catalysts, which indicates that the electron interaction between TiO2 and CeO2-NR is the strongest. Furthermore, Table 3 shows that the ratio of Ti/(Ti+Ce) on these TiO2/CeO2 catalysts can be sequenced by TiO2/CeO2-NR > TiO2/CeO2-NP > TiO2/CeO2-NC, indicating the best dispersion of TiO2 on CeO2-NR support, which is consistent with N2-physisorption and Raman results. The best dispersion of TiO2 also benefits to strengthen the electron interaction between TiO2 and CeO2-NR support. Figure 7c shows that two binding energy bands can be observed in O 1s spectra of these TiO2/CeO2 catalysts, which labeled as O′ and O″, respectively. In detail, the former is resulted from lattice oxygen species, while the latter is assigned to chemisorbed oxygen.54 It is well known that the chemisorbed oxygen usually adsorbs on the defective sites of catalysts, its migration ability is very excellent, which benefits to oxidize NO to NO2, and subsequently enhances the catalytic performance for NH3-SCR by the “fast SCR” approach.39 So, the relative content of O″ (i.e., atomic ratio of O″/(O″+O′)) is calculated, and given in Table 3. It shows that O″ content of these TiO2/CeO2 catalysts is sorted by TiO2/CeO2-NR > TiO2/CeO2-NP > TiO2/CeO2-NC, which suggests that TiO2/CeO2-NR catalyst has the most chemisorbed oxygen species.

3.5. Catalytic performance and H2O+SO2 tolerance (NH3-SCR reaction) 13



Morphology and crystal-plane effects of CeO2 on the denitration performance of these TiO2/CeO2 catalysts were tested via NH3-SCR reaction, as displayed in Figure 8. Figure 8a shows that the difference among NOx conversion over these CeO2 supports is not obvious, in which, CeO2-NC and CeO2-NP almost exhibit the same catalytic activity. Moreover, NOx conversion of these CeO2 supports is very poor, which is below 40%. However, a remarkable increase of NOx conversion can be observed after the introduction of TiO2 on these CeO2 supports. Furthermore, Figure 8a shows that NOx conversion of these TiO2/CeO2 catalysts is sorted by TiO2/CeO2-NR > TiO2/CeO2-NP > TiO2/CeO2-NC (below 400 °C). Especially, TiO2/CeO2-NR catalyst possesses the optimal catalytic activity, which exhibits above 80% NOx conversion during 250-350 °C. According to the above-mentioned characterization results, we think that the possible reasons are as follows: (1) CeO2-NR support with rod-like structure exhibits the largest specific surface area and pore volume, which can promote the dispersion of TiO2 on CeO2-NR to increase the active sites and strengthen the electron interaction of TiO2 and CeO2-NR; (2) TiO2/CeO2-NR catalyst mainly exposes {110} facet, which is beneficial to generate oxygen vacancy to strengthen the electron interaction between TiO2 and CeO2-NR through oxygen migration, and further improves the redox performance and surface acidity. In detail, the improvement of redox performance is conducive to generate more Ce3+ ions and oxygen vacancies in the reaction process, which can weaken N-O bond to accelerate the decomposition of NO molecules. Moreover, oxygen molecules can be adsorbed and activated on oxygen vacancy to form the chemisorbed oxygen species, which combined with the improved redox performance, benefit to oxidize NO to NO2, and subsequently enhance the catalytic performance for NH3-SCR by the “fast SCR” approach. Furthermore, the improvement of surface acidity benefits for the adsorption and activation of NH3 molecules. All the factors result in good denitration activity of TiO2/CeO2-NR catalyst during NH3-SCR reaction. On the other hand, N2O is the dominant by-product in NH3-SCR, which can decrease N2 selectivity of denitration catalysts. Therefore, N2O concentration is also a detection indicator in the whole process of catalytic performance test, as shown in 14


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Figure 8b. We can find that these CeO2 supports exhibit an appreciable amount of N2O when the reaction temperature is up to 400 °C, while no N2O is found on these TiO2/CeO2 catalysts even at 450 °C, which suggests that the loading of TiO2 also increases the N2 selectivity of these CeO2 supports. It is well known that water vapor and sulfur dioxide inevitably exist in the practical coal-fired flue gas, which can lead to the deactivation of denitration catalysts in different degrees. Therefore, we measured H2O+SO2 tolerance of these TiO2/CeO2 catalysts with different morphologies at 300 °C, and the obtained results are given in Figure 9. We can find that the variation trend of NOx conversion over these TiO2/CeO2 catalysts is very similar. In detail, in the first 120 min, all of these TiO2/CeO2 catalysts exhibit stable NOx conversion in the absence of H2O and SO2; once H2O and SO2 are introduced, NOx conversion declines gradually and then trends to be stable from 120 to 600 min, which indicates that H2O and SO2 result in the partial deactivation of these TiO2/CeO2 catalysts because of competitive adsorption of H2O+SO2 and reactant molecules, deposition of sulfates, and sulfation of active species;12,55 finally, NOx conversion recovers partially when H2O and SO2 are cut off in the last 120 min, but is lower than the initial level, which suggests that the decrease of NOx conversion caused by H2O and SO2 contains two parts of reversible and irreversible deactivation. Interestingly, TiO2/CeO2-NR catalyst still exhibits the highest NOx conversion (ca. 70%) among these catalysts even existing H2O and SO2, which suggests that it is expected to be applied for actual denitration.

3.6. Interaction with reactant molecules (in situ DRIFTS) For the purpose of exploring NH3-SCR reaction mechanism on these TiO2/CeO2 catalysts with different morphologies, in situ DRIFTS measurements were conducted to understand the interaction between these TiO2/CeO2 catalysts and reactant molecules. NH3 adsorption in situ DRIFTS are presented in Figure 10. For TiO2/CeO2-NC catalyst in Figure 10a, several vibration bands can be found during 1000-2000 cm–1. The bands at 1111, 1151, and 1604 cm–1 are assigned to the bending vibration of N-H bond in NH3 species coordinated to Lewis (L) acid, while the bands at 1381, 1445, 1477, and 1678 cm–1 are resulted from NH4+ ions on Brønsted (B) 15



acid.39,40,54 Some interesting phenomena are found in the heating step: (1) all of the bands related to B acid gradually weaken with temperature increasing, and completely disappear at 300 °C; (2) the bands for L acid also decrease with temperature elevating, but still exist even at 450 °C. These results suggest that the thermal stability of L acid is significantly stronger than that of B acid. With regard to TiO2/CeO2-NP (Figure 10b) and TiO2/CeO2-NR catalysts (Figure 10c), the vibration bands of L acid and B acid can be also detected. The variation trend of these bands over TiO2/CeO2-NP and TiO2/CeO2-NR catalysts with the increase of temperature is very similar to the observations on TiO2/CeO2-NC catalyst. Interestingly, the intensity of the bands for L acid over these TiO2/CeO2 catalysts is sorted by TiO2/CeO2-NR > TiO2/CeO2-NP > TiO2/CeO2-NC, while the intensity of the bands for B acid exhibits the opposite sequence.

Combining with the obtained results of catalytic performance

(TiO2/CeO2-NR > TiO2/CeO2-NP > TiO2/CeO2-NC), it suggests that L acid of these TiO2/CeO2 catalysts plays a more significant role than B acid in NH3-SCR reaction. The adsorption and activation of NO+O2 on these TiO2/CeO2 catalysts were characterized via NO+O2 adsorption in situ DRIFTS, as presented in Figure 11. Several vibration bands are detected over TiO2/CeO2-NC catalyst (Figure 11a) when it is saturated by NO+O2 molecules. According to the literatures,9,31,33,56 the bands at 1016, 1235, and 1582 cm–1 are resulted from bidentate nitrates; the band at 1209 cm–1 is assigned to monodentate nitrites; the band at 1279 cm–1 is ascribed to monodentate nitrates; while the strong band at 1622 cm–1 is related to bridging bidentate nitrates. We can find that all of these bands weaken with the increase of temperature due to the desorption/dissociation/transformation of these nitrites and nitrates. Similarly, the bands of bidentate nitrates, monodentate nitrites, monodentate nitrates, and bridging bidentate nitrates can be also observed over TiO2/CeO2-NP (Figure 11b) and TiO2/CeO2-NR catalysts (Figure 11c). Especially, TiO2/CeO2-NR catalyst exhibits a new band of monodentate nitrates at 1451 cm–1,31 which might be related to the good dispersion of TiO2 over CeO2-NR support. Furthermore, it can be seen from Figure 11c that some interesting phenomena are observed in the heating step: (1) the bands of monodentate nitrates (1282 and 1451 cm–1) not only weaken with temperature 16


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increasing, but also completely disappear at 200 °C; (2) two new bands for monodentate nitrites (1349 cm–1) and bidentate nitrates (1538 cm–1) appear at 200 and 400 °C, respectively,9,31 and further enhance with the elevation of temperature because of the transformation of the adsorbed NOx species. These experiment results indicate that TiO2/CeO2-NR catalyst can adsorb and activate NO and O2 molecules more efficiently than TiO2/CeO2-NC and TiO2/CeO2-NP catalysts. In situ DRIFTS experiments of NO+NH3+O2 adsorption were also performed on these TiO2/CeO2 catalysts, and the corresponding results are displayed in Figure 12. Combining with NH3 adsorption (Figure 10) and NO+O2 adsorption (Figure 11) in situ DRIFTS results, Figure 12 shows that all the TiO2/CeO2 catalysts present the vibration bands of L acid (1090-1093, 1596-1609 cm–1), B acid (1437-1445, 1678-1683 cm–1), and monodentate nitrites (1188-1208 cm–1) when they are exposed to the simulated reaction atmosphere of NO+NH3+O2. This phenomenon suggests that both of NH3 and NO+O2 molecules are adsorbed over these TiO2/CeO2 catalysts simultaneously,

which

confirms

that

NH3-SCR

reaction

follows

Langmuir-Hinshelwood (L-H) mechanism over these TiO2/CeO2 catalysts. That is to say, the possible reaction pathway of NH3-SCR on these TiO2/CeO2 catalysts is as follows: NH3(g) + * → *NH3(a)

[1]

2NO(g) + O2(g) + 2* → 2*NO2(a)

[2]

8*NH3(a) + 6*NO2(a) → 7N2(g) + 12H2O(g) + 14*

[3] (* is the adsorption sites)

Interestingly, Figure 12 displays that all of these vibration bands on the catalysts weaken with temperature increasing due to the reaction among NO, NH3, and O2. Moreover, we can find that the remarkable decrease of the vibration band for monodentate nitrites (1188-1208 cm–1) occurs at 200, 200, and 150 °C over TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts, respectively. These observations suggest that NH3-SCR reaction is easier to proceed on TiO2/CeO2-NR catalyst than that on TiO2/CeO2-NC and TiO2/CeO2-NP catalysts, which coincides with catalytic performance results.

17



4. CONCLUSIONS Three kinds of CeO2 were successfully synthesized via a hydrothermal method, and used as supports to prepare TiO2/CeO2 catalysts for NH3-SCR. Some interesting conclusions are obtained from the characterization of physicochemical property and evaluation of catalytic performance, as follows: (1) TEM and HRTEM images display that CeO2-NC, CeO2-NP, and CeO2-NR supports exhibit the morphologies of nano-cubes (mainly exposed {100} facet), nano-polyhedrons (mainly exposed {111} and {100} facets), and nano-rods (mainly exposed {110} and {100} facets), respectively. Furthermore, TiO2/CeO2-NC, TiO2/CeO2-NP and TiO2/CeO2-NR catalysts still maintain the original morphologies and exposed facets of the corresponding CeO2 supports. (2) Based on HRTEM, XRD, Raman, and N2-physisorption results, TiO2 is highly dispersed over these CeO2 supports with different morphologies. Especially, TiO2/CeO2-NR catalyst exhibits the best dispersion of TiO2 species due to the largest specific surface area and pore volume of CeO2-NR support. (3) H2-TPR and NH3-TPD profiles show that the reduction behavior and surface acidity of TiO2/CeO2-NR catalyst are better than the other two catalysts due to the best dispersion of TiO2 over CeO2-NR support and the strongest interaction between the surface dispersed TiO2 species and {110} facet of CeO2-NR support. (4) XPS results present that TiO2/CeO2-NR catalyst exhibits the most Ce3+ ions and chemisorbed oxygen among these TiO2/CeO2 catalysts due to the strongest interaction between TiO2 species and CeO2-NR support as well as the easiest formation of oxygen vacancy on {110} facet of TiO2/CeO2-NR catalyst. (5) The catalytic performance of TiO2/CeO2-NR catalyst is remarkably better than the other two catalysts, which benefits from the best dispersion of TiO2 species, the most excellent reduction behavior and surface acidity, and the largest amount of Ce3+ ions and chemisorbed oxygen. (6) In situ DRIFTS results suggest that NH3-SCR reaction follows L-H mechanism over these TiO2/CeO2 catalysts.

18


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ASSOCIATED CONTENT Supporting Information Additional information about the size distribution of these CeO2 supports and TiO2/CeO2 catalysts (Figure S1 and Figure S2).

AUTHOR INFORMATION Corresponding Author * Sichuan University, South Section 24# of the First Ring Road, Chengdu 610065, PR China. Tel.: +86 28 85 467800. E-mail address: [email protected] (F.M. Yang).

ORCID Fumo Yang: 0000-0001-6895-0086

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21507130) and Chongqing Science & Technology Commission (cstc2016jcyjA0070, cstckjcxljrc13).

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2014, 150-151, 315-329. (46) Ma, Z.W.; Zhao, S.L.; Pei, X.P.; Xiong, X.M.; Hu, B., New insights into the support morphology-dependent ammonia synthesis activity of Ru/CeO2 catalysts. Catal. Sci. Technol. 2017, 7, 191-199. 24


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(47) You, R.; Zhang, X.Y.; Luo, L.F.; Pan, Y.; Pan, H.B.; Yang, J.Z.; Wu, L.H.; Zheng, X.S.; Jin, Y.K.; Huang, W.X., NbOx/CeO2-rods catalysts for oxidative dehydrogenation of propane: Nb-CeO2 interaction and reaction mechanism. J. Catal. 2017, 348, 189-199. (48)Wang, Z.L.; Feng, X.D., Polyhedral shapes of CeO2 nanoparticles. J. Phys. Chem. B 2003, 107, 13563-13566. (49) Sayle, T.X.T.; Parker, S.C.; Sayle, D.C., Oxidising CO to CO2 using ceria nanoparticles. Phys. Chem. Chem. Phys. 2005, 7, 2936-2941. (50) Fang, C.; Zhang, D.S.; Cai, S.X.; Zhang, L.; Huang, L.; Li, H.R.; Maitarad, P.; Shi, L.Y.; Gao, R.H.; Zhang, J.P., Low-temperature selective catalytic reduction of NO with NH3 over nanoflaky MnOx on carbon nanotubes in situ prepared via a chemical bath deposition route. Nanoscale 2013, 5, 9199-9207. (51) Sudarsanam, P.; Hillary, B.; Amin, M.H.; Hamid, S.B.A.; Bhargava, S.K., Structure-activity relationships of nanoscale MnOx/CeO2 heterostructured catalysts for selective oxidation of amines under eco-friendly conditions. Appl. Catal. B: Environ. 2016, 185, 213-224. (52) Yao, X.J.; Tang, C.J.; Ji, Z.Y.; Dai, Y.; Cao, Y.; Gao, F.; Dong, L.; Chen, Y., Investigation of the physicochemical properties and catalytic activities of Ce0.67M0.33O2 (M = Zr4+, Ti4+, Sn4+) solid solutions for NO removal by CO. Catal. Sci. Technol. 2013, 3, 688-698. (53) Siavash Moakhar, R.; Goh, G.K.L.; Dolati, A.; Ghorbani, M., Sunlight-driven photoelectrochemical sensor for direct determination of hexavalent chromium based on Au decorated rutile TiO2 nanorods. Appl. Catal. B: Environ. 2017, 201, 411-418. (54) Ma, L.; Seo, C.Y.; Nahata, M.; Chen, X.Y.; Li, J.H.; Schwank, J.W., Shape dependence and sulfate promotion of CeO2 for selective catalytic reduction of NOx with NH3. Appl. Catal. B: Environ. 2018, 232, 246-259. (55) Li, J.H.; Chang, H.Z.; Ma, L.; Hao, J.M.; Yang, R.T., Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts—A review. Catal. Today 2011, 175, 147-156. 25



(56) Hadjiivanov, K.I., Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal. Rev. 2000, 42, 71-144.

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Figure and Table captions Figure 1. TEM and HRTEM images of (a, b) CeO2-NC, (c, d) CeO2-NP, and (e, f) CeO2-NR supports.

Figure 2. TEM and HRTEM images of (a, b) TiO2/CeO2-NC, (c, d) TiO2/CeO2-NP, and (e, f) TiO2/CeO2-NR catalysts.

Figure 3. XRD patterns of these (a) CeO2 supports and (b) TiO2/CeO2 catalysts with different morphologies.

Figure 4. Raman spectra of these (a) CeO2 supports and (b) TiO2/CeO2 catalysts with different morphologies.

Figure 5. N2-physisorption results of these (a) CeO2 supports and (b) TiO2/CeO2 catalysts with different morphologies.

Figure 6. (a) H2-TPR and (b) NH3-TPD profiles of these TiO2/CeO2 catalysts with different morphologies.

Figure 7. XPS spectra of these TiO2/CeO2 catalysts with different morphologies: (a) Ce 3d, (b) Ti 2p, (c) and O 1s.

Figure 8. (a) NOx conversion and (b) N2O concentration of these CeO2 supports and TiO2/CeO2 catalysts with different morphologies.

Figure 9. H2O+SO2 tolerance of these TiO2/CeO2 catalysts with different morphologies at 300 °C.

Figure 10. In situ DRIFTS of NH3 adsorption over (a) TiO2/CeO2-NC, (b) TiO2/CeO2-NP, and (c) TiO2/CeO2-NR catalysts.

Figure 11. In situ DRIFTS of NO+O2 adsorption over (a) TiO2/CeO2-NC, (b) TiO2/CeO2-NP, and (c) TiO2/CeO2-NR catalysts.

Figure 12. In situ DRIFTS of NO+NH3+O2 adsorption over (a) TiO2/CeO2-NC, (b) TiO2/CeO2-NP, and (c) TiO2/CeO2-NR catalysts.

Table 1 Average size, lattice constant, and textural data of these CeO2 supports and TiO2/CeO2 catalysts with different morphologies.

Table 2 Quantitative data of these TiO2/CeO2 catalysts with different morphologies obtained from H2-TPR and NH3-TPD experiments.

Table 3 The surface atomic concentration and atomic ratio of these TiO2/CeO2 27



catalysts with different morphologies calculated from XPS.

28


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Figure 1. TEM and HRTEM images of (a, b) CeO2-NC, (c, d) CeO2-NP, and (e, f) CeO2-NR supports.

29



Figure 2. TEM and HRTEM images of (a, b) TiO2/CeO2-NC, (c, d) TiO2/CeO2-NP, and (e, f) TiO2/CeO2-NR catalysts.

30


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Page 31 of 44

Figure 3. XRD patterns of these (a) CeO2 supports and (b) TiO2/CeO2 catalysts with different morphologies.

(a)

CeO -N 2 R

(311)

(220)

(200)

Intensity (a.u.)

(111)

Supports

CeO -N 2 P CeO -N 2 C

10

20

30

40

50

60

70

80

60

70

80

2Theta (degree)

(b)

TiO /C 2 eO -NR 2

(311)

(220)

(111)

Catalysts

(200)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO /C 2 eO -NP 2 TiO /C 2 eO -NC 2

10

20

30

40

50

2Theta (degree)

31



Figure 4. Raman spectra of these (a) CeO2 supports and (b) TiO2/CeO2 catalysts with different morphologies.

(a)

Supports

4000

Intensity (a.u.)

F2g

D CeO2-NR CeO2-NP CeO2-NC

200

400

600

800

1000

−1

Raman shift (cm )

(b)

Catalysts

4000

F2g

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 44

D TiO2/CeO2-NR

Eg

TiO2/CeO2-NP TiO2/CeO2-NC

200

400

600

800 −1

Raman shift (cm )

32


1000

Page 33 of 44

Figure 5. N2-physisorption results of these (a) CeO2 supports and (b) TiO2/CeO2 catalysts with different morphologies.

(a)

Supports dV/dD (a.u.)

Volume adsorbed (a.u.)

CeO2-NR

CeO2-NP CeO2-NC

10

20

30

40

50

Pore diameter (nm)

CeO2-NR CeO2-NP CeO2-NC

0.0

0.2

0.4

0.6

0.8

1.0

0.8

1.0

Relative pressure (P/P0)

(b)

Catalysts dV/dD (a.u.)

TiO2/CeO2-NR

Volume adsorbed (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2/CeO2-NP TiO2/CeO2-NC

10

20

30

40

50

Pore diameter (nm)

TiO2/CeO2-NR TiO2/CeO2-NP TiO2/CeO2-NC

0.0

0.2

0.4

0.6

Relative pressure (P/P0)

33



Figure 6. (a) H2-TPR and (b) NH3-TPD profiles of these TiO2/CeO2 catalysts with different morphologies.

(a)

H2-TPR

3

β

H2 consumption (a.u.)

α

TiO2/CeO2-NR TiO2/CeO2-NP TiO2/CeO2-NC Region II

Region I 100

200

300

400

500

600

700

800

900

o

Temperature ( C)

(b)

NH3-TPD

5 I

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 44

II

III

TiO2/CeO2-NR

I II

III

TiO2/CeO2-NP

I II

0

100

200

300

III

400

500

TiO2/CeO2-NC 600

700

o

Temperature ( C)

34


800

900

Page 35 of 44

Figure 7. XPS spectra of these TiO2/CeO2 catalysts with different morphologies: (a) Ce 3d, (b) Ti 2p, (c) and O 1s.

(a)

(b)

Ce 3d u''' v''' 0 TiO /CeO u ,u 2 2 -NR u'' u'

Ti 2p 458.1

0

Intensity (a.u.)

v ,v v'' v'

TiO /CeO 2 2 -NP

TiO /C 2 eO -NC 2

TiO2/CeO -NR 2 463.8 458.3

TiO2/CeO -NP 2 464.0 458.4

TiO2/CeO2-NC 464.1

Ce 3d3/2 920

910

Ce 3d5/2 900

890

468

880

466

464

462

(c)

O 1s O'

O'' TiO2/CeO2-NR

TiO2/CeO2-NP

TiO2/CeO2-NC

536

534

460

458

Binding Energy (eV)

Binding Energy (eV)

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


532

530

528

526

Binding Energy (eV)

35


524

456

454


Figure 8. (a) NOx conversion and (b) N2O concentration of these CeO2 supports and TiO2/CeO2 catalysts with different morphologies.

NOx conversion (%)

(a) 100

CeO2-NC

80

CeO2-NP

60

TiO2/CeO2-NC

40

TiO2/CeO2-NR

CeO2-NR TiO2/CeO2-NP

20 0 -20 100

200

300

400

500

400

500

o

Temperature ( C)

(b)

30 CeO2-NC 25

N2O concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 44

CeO2-NP CeO2-NR

20

TiO2/CeO2-NC TiO2/CeO2-NP

15

TiO2/CeO2-NR

10 5 0 100

200

300 o

Temperature ( C)

36


Page 37 of 44

Figure 9. H2O+SO2 tolerance of these TiO2/CeO2 catalysts with different morphologies at 300 °C.

100 5%H2O + 100ppm SO2 on 80

NOx conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 5%H2O + 100ppm SO2 off

40

o

Reaction temperature: 300 C TiO2/CeO2-NC

20

TiO2/CeO2-NP TiO2/CeO2-NR

0 0

100

200

300

400

500

Reaction time (min)

37


600

700


Figure 10. In situ DRIFTS of NH3 adsorption over (a) TiO2/CeO2-NC, (b) TiO2/CeO2-NP, and (c) TiO2/CeO2-NR catalysts.

(a)

(b)

TiO2/CeO2-NC

150 C o 200 C o 250 C o 300 C o 350 C o 400 C o 450 C

B

L

BB B

1800

1600

1400

1200

2000

1000

L B B

L

L

1800

1600

1400

1200 −1

Wavenumber (cm ) TiO2/CeO2-NR

0.05

1103 1159

1662 1604

o

25 o C 50 C o 100 C o 150 o C 200 oC 250 oC 300 o C 350 oC 400 oC 450 C

B

1440

B

L

L

L 2000

1461 1395 1289

L

−1

(c)

1567

B

Wavenumber (cm )

Kubelka-Munk (a.u.)

2000

1667

o

25 C o 50 C o 100 C o 150 C o 200 C o 250 oC 300 o C 350 oC 400 oC 450 C

L

L

1103 1148

1151 1111

1445 1678 1477 1604 1381

o 25 C o 50 C o 100 C o

TiO2/CeO2-NP

0.05

Kubelka-Munk (a.u.)

0.05

Kubelka-Munk (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 44

1800

1600

1400

1200 −1

Wavenumber (cm )

38


1000

1000

Page 39 of 44

Figure 11. In situ DRIFTS of NO+O2 adsorption over (a) TiO2/CeO2-NC, (b) TiO2/CeO2-NP, and (c) TiO2/CeO2-NR catalysts.

(a)

(b)

TiO2/CeO2-NC

1

TiO2/CeO2-NP

1

1622

Kubelka-Munk (a.u.)

1582

1016

o

25 C o 50 C o 100 C o 150 C o 200 C o 250 C o 300 C o 350 C o 400 C o 450 C

2000

1235 1209

1628 1235 1279 1209

1585 1016 o 25 C o 50 C o 100 C o 150 C o 200 oC 250 o C 300 C o 350 oC 400 o C

450 C

1800

1600

1400

1200

1000

2000

1800

−1

(c)

1600

1200


1

1579 1617 1451

1282 1241 1212

1016

o

25 C o 50 C o 100 C o 150 C o 200 C o 250 C o 300 C o 350 C o 400 C o 450 C

2000

1400

−1

Wavenumber (cm )

Kubelka-Munk (a.u.)

Kubelka-Munk (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1538

1800

1600

1349 1400

1200 −1

Wavenumber (cm )

39


1000

1000


Figure 12. In situ DRIFTS of NO+NH3+O2 adsorption over (a) TiO2/CeO2-NC, (b) TiO2/CeO2-NP, and (c) TiO2/CeO2-NR catalysts.

(a)

(b)

TiO2/CeO2-NC

0.4

TiO2/CeO2-NP

0.4

1188

Kubelka-Munk (a.u.)

1191

1093 1683 1609

o

25 C o 50 C o 100 oC 150 o C 200 oC 250 oC 300 o C 350 oC 400 oC 450 C 2000

1437

B

1093 o

L

1800

1600

1400

1200

2000

1000

B

L

B

1600

1400

1200 −1


0.4

1208 1445 1090

1683 1604

o

25 C o 50 C o 100 C o 150 C o 200 C o 250 C o 300 o C 350 oC 400 oC 450 C

2000

1442

L

1800

−1

Wavenumber (cm )

(c)

1678 1596

25 o C 50 C o 100 C o 150 o C 200 oC 250 oC 300 o C 350 oC 400 oC 450 C

L

B

Kubelka-Munk (a.u.)

Kubelka-Munk (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 44

B L

B L

1800

1600

1400

1200 −1

Wavenumber (cm )

40


1000

1000

Page 41 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 1 Average size, lattice constant, and textural data of these CeO2 supports and TiO2/CeO2 catalysts with different morphologies.

Average size by

Average size by

Lattice

BET specific surface

Total pore volume

Mean pore

TEM (nm)

XRD (nm)

constant (Å)

area (m2·g–1)

(cm3·g–1)

diameter (nm)

CeO2-NC

20.5

24.4

5.410

40

0.171

17.2

CeO2-NP

11.9

13.8

5.414

72

0.148

8.2

/

5.414

104

0.331

12.7

Sample

CeO2-NR

Length: 68.1; Diameter: 6.9

TiO2/CeO2-NC

21.1

24.7

5.405

41

0.169

15.9

TiO2/CeO2-NP

12.3

14.0

5.406

67

0.133

7.9

/

5.403

71

0.293

16.5

TiO2/CeO2-NR

Length: 57.4; Diameter: 7.0

41


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 42 of 44

Table 2 Quantitative data of these TiO2/CeO2 catalysts with different morphologies obtained from H2-TPR and NH3-TPD experiments.

H2-TPR Sample

Peak temperature (°C)

NH3-TPD

H2 consumption (µmol·g–1)

Sα/(Sα+Sβ)

Acid amount (a.u.)

Tα

Tβ

Sα

Sβ

Sα+Sβ

TiO2/CeO2-NC

576

808

488

904

1392 (3072)

0.35

265 551 248

1064

TiO2/CeO2-NP

545

802

739

889

1628 (3072)

0.45

445 583 357

1385

TiO2/CeO2-NR

540

775

943

694

1637 (3072)

0.58

631 823 451

1905

42


SI

SII

SIII

SI+SII+SIII

Page 43 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 3 The surface atomic concentration and atomic ratio of these TiO2/CeO2 catalysts with different morphologies calculated from XPS.

Sample

Atomic concentration (at.%)

Atomic ratio (%) Ti/(Ti+Ce) Ce3+/(Ce3++Ce4+) O″/(O″+O′)

Ce

Ti

O

TiO2/CeO2-NC

19.2

6.7

74.1

25.9

14.4

21.1

TiO2/CeO2-NP

17.3

9.9

72.8

36.4

15.8

22.2

TiO2/CeO2-NR

12.2

14.5

73.3

54.3

17.3

25.3

43



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