Promotional Effects of Ti on a CeO2âMoO3 Catalyst for the Selective

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Promotional Effects of Ti on a CeO2-MoO3 Catalyst for the Selective Catalytic Reduction of NOx with NH3 Yang Geng, Xiaoling Chen, Shijian Yang, Fudong Liu, and Wenpo Shan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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ACS Applied Materials & Interfaces

Promotional Effects of Ti on a CeO2-MoO3 Catalyst for the Selective Catalytic Reduction of NOx with NH3 Yang Geng a, Xiaoling Chen a, Shijian Yang a, Fudong Liu b,§, Wenpo Shan a,* a

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China b

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley 94720, California, United States

§

Current address: BASF Corporation, 25 Middlesex Essex Turnpike, Iselin, New Jersey 08830, United States

*Corresponding author. Fax: +86 25 84315173; Tel: +86 18012920637; E-mail: [email protected]

1

ACS Paragon Plus Environment


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Abstract In this study, Ti was doped to CeO2-MoO3 to promote the catalytic performance for the selective catalytic reduction of NOx with NH3 (NH3-SCR). The preparation method for CeMo0.5TiaOx (a = 0, 1, 2, 5, 10) catalysts was a stepwise precipitation method. When Ti was doped, all of the Ce-Mo-Ti catalysts exhibited remarkably improved NOx conversion and N2 selectivity than the CeMo0.5Ox without Ti. The CeMo0.5Ti5Ox with excellent activity in a broad temperature range was selected as an optimal catalyst to investigate the effects of Ti addition. The formation process analysis of the CeMo0.5Ti5Ox showed that, the Mo and Ti species firstly precipitated together from the mixed solution with the increase of pH, and then Ce species precipitated onto the Mo-Ti precipitates. The obtained catalyst exhibited remarkably facilitated NOx and NH3 adsorption, enhanced charge imbalance, promoted redox property, and improved surface acidity, which are all important reasons for the excellent catalytic performance of an NH3-SCR catalyst. Keywords: Ce-based catalyst; selective catalytic reduction; surface acidity; stepwise precipitation; nitrogen oxides abatement

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1. Introduction Nitrogen oxides (NOx) have significant impact on the environment and the health of human. NOx emission control from mobile diesel engines is increasingly required globally, particularly for the developing countries like China with severe air pollution problems.1-3 Selective catalytic reduction of NOx with NH3 (NH3-SCR) has become the dominant technology for diesel vehicles to meet the ever tightened emission standards.4-5 Catalyst plays an important role in the NH3-SCR technology.6-7 V2O5-WO3/TiO2 is an extensively used NH3-SCR catalyst for stationary sources, and it is also applied for the NOx control of diesel vehicle.1, 8 However, there are still some problems with this catalyst, for example the toxicity of active V species and the narrow active window. Although China and some other developing countries currently still permit the use of vanadium-based catalyst on diesel vehicles, this catalyst will be phased out gradually due to its inevitable problems. Therefore, it is very important to develop novel catalyst systems without vanadium.6, 9 Many transition metal based catalysts, including oxides and zeolites, were developed for NH3-SCR reaction.6, 10 These catalysts are mainly Fe-based oxides11-12, Cu-based oxides13, Mn-based oxides14-15, Fe-based zeolites16-17, and Cu-based zeolites9, 18-20. In addition, Ce as a rare earth metal attracted much attention for its application in NH3-SCR catalysts, due to its unique redox, oxygen storage, and acid-base properties.21 Ce-based composite oxide catalysts, such as Ce-Ti22-23, Ce-W24-25, Ce-Mo26-27, Ce-W-Ti28-29, and Ce-Cu-Ti30 oxides, have been shown to be 3



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very effective for NH3-SCR reaction. Among these oxides, the CeO2-MoO3 was firstly reported by Peng et al.24 The structure-activity relationship of the catalyst was investigated, and the effects of calcination temperature and phosphorus compounds on catalyst activity were analyzed.26, 31-32 This catalyst was also used for simultaneous elimination of NOx and Hg0.33 Recently, Wang et al. found that the N2O formation is notable at high temperatures during NH3-SCR reaction over CeO2-MoO3 catalyst.34 Many studies have proved that surface acidity has a significant impact on the catalytic performance of an NH3-SCR catalyst, through the adsorption and activation of ammonia.8, 29, 35 Therefore, Ti was doped to CeO2-MoO3 to promote the surface acid sites in this study. Interestingly, we found that the introduction of Ti remarkably affected the structure of the catalyst. The obtained catalyst, showed facilitated NOx and NH3 adsorption, enhanced charge imbalance, promoted redox property, and improved surface acidity. Thus, the introduction of Ti remarkably improved the catalytic performance. 2. Experimental 2.1 Catalyst synthesis The CeMo0.5TiaOx (Mo/Ce molar ratio = 0.5; “a” represents the Ti/Ce molar ratio, and a = 1, 2, 5, 10) catalyst was prepared by a stepwise precipitation method.29 Ti(SO4)2, (NH4)6Mo7O24, and Ce(NO3)3·6H2O, with specific Ce/Mo/Ti molar ratio, were dissolved into DI water together, and then excessive urea was added. After 12 h vigorous stirring at 90 ºC, precipitant was obtained by filtration of the mixed solution and washing with DI water. At last, the collected samples were calcined at 500 ºC in 4


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air for 5 h after drying at 100 ºC for 12 h. The CeMo0.5Ti5Ox catalyst with the highest NH3-SCR activity was also denoted as CeOx/MoO3-TiO2. A CeMo0.5Ox catalyst without Ti was prepared with the same approach as CeMo0.5TiaOx samples. The stoichiometry of the catalysts were confirmed by XRF analysis (see Table S1 in the Supporting Information). In addition, a CeO2-MoO3/TiO2 (10% CeO2 and 5% MoO3) was also prepared as a reference catalyst by the standard impregnation method. 2.2 Catalytic performance test The weight of the catalyst (40-60 mesh) used for the test was 0.11 g. The typical reactant gas composition was controlled as follows: 500 ppm NO, 500 ppm NH3, 5 vol.% O2, 5 vol.% H2O (when used), 100 ppm SO2 (when used), N2 balance, and 200 mL/min total flow rate. The GHSV during the test of 100,000 h-1. The concentrations of NO, NO2, NH3 and N2O were continuously measured by a Nicolet Antaris IGS analyzer. 2.3 Characterizations The surface area, X-ray photoelectron spectroscopy (XPS), and H2-temperature programmed reduction (H2-TPR) tests of the samples were performed on Micromeritics

ASAP

2020,

Thermo-VG

Scientific

ESCALAB

250,

and

Micromeritics AutoChem_II_2920 instruments, respectively. The experiment details have been introduced in our previous study.36 The X-ray diffraction (XRD) was performed on a Bruker-AXS D8 diffractometer with Cu Kα radiation, a step size of 0.03 o/s. Transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HR-TEM) was performed at 200 kV on a JEOL JEM-2100 5



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microscope, with a point resolution of 0.23 nm. The samples were ultrasonically suspended in alcohol, then deposited on the copper grid. Raman spectra was collected on a HORIBA Jobin Yvon LabRAM ARAMIS Raman Spectrometer, with a 532 nm DPSS diode-pump solid semiconductor laser and 1800 Grating lines/mm Resolution spatial at room temperature. Temperature programmed desorption of ammonia (NH3-TPD) and temperature programmed desorption of NOx (NOx-TPD) were both carried out on the reaction system of catalytic performance test. 100 mg sample and a gas flow rate of 200 mL/min were used in a typical experiment of NH3-TPD. Before TPD test, the catalyst was pretreated under 5 vol.% O2/N2 at 350 oC for 1h. Then, the catalyst was exposed to 500 ppm NH3/N2 for 1 h at 50 °C and purged with N2 for another 1 h. The NH3-TPD result was recorded with the raise of temperature to 500 oC at the rate of 10 o

C/min. The NOx-TPD was performed with a similar procedure of NH3-TPD, except

that 300 mg sample was used for the test and 500 ppm NO/N2 + 5 vol.% O2 was applied for the adsorption. The in situ DRIFTS experiment was performed on a Nicolet IS 50 FTIR spectrometer, equipped with an MCT/A detector. The results were recorded after the sample was treated with 500 ppm NH3/N2 (200 mL/min) for 0.5 h and purge with N2 for another 0.5 h. 3. Results and discussion 3.1 NH3-SCR performance Figure 1(A) shows the NOx conversion as a function of temperature in the 6


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NH3-SCR reaction over CeMo0.5TiaOx catalysts under a GHSV of 100,000 h-1. The NOx conversion over CeMo0.5Ox was relatively low. However, the addition of Ti remarkably expanded the operation temperature window for NH3-SCR reaction and also improved the NOx conversion. Particularly, the CeMo0.5Ti5Ox showed the best NH3-SCR activity at low temperatures with the NOx conversion above 80% from 200 to 425 °C. Meanwhile, the NOx conversion of CeMo0.5Ti5Ox was obviously higher than that of CeO2-MoO3/TiO2 (prepared by impregnation method). As show in Figure 1(B), the NH3 conversion is consistent with the NOx conversion below 300 °C, implying that the consumption of NOx and NH3 was almost equal with each other. Above 300 °C, the NH3 over CeMo0.5TiaOx catalysts was almost completely consumed due to the catalytic oxidation reaction. Figure 1(C) showed the N2 selectivity of the catalysts. High level of N2O was formed over CeMo0.5Ox at high temperature, inducing a decrease of N2 selectivity. With the addition of Ti, the N2 selectivity was enhanced gradually. In addition, the N2 selectivity of CeMo0.5Ti5Ox was clearly higher than that of CeO2-MoO3/TiO2. The CeMo0.5Ti5Ox was selected as the optimal catalyst, and the effects of H2O were tested (Figure 2). When 5 vol.% H2O was added into the reaction gas, the activity of CeMo0.5Ti5Ox catalyst decreased obviously from 150 to 300 °C, while the NOx conversion above 350 °C was clearly enhanced. The NOx conversion over 90% was observed from 225 to 450 °C. The addition of H2O could decrease the oxidation ability, which led to the inhibition of the low-temperature SCR activity. However, the addition of H2O also could inhibit the NH3 oxidation reaction, which led to the 7



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promotion of the high-temperature NOx conversion.37 The influence of SO2 on CeMo0.5Ox and CeMo0.5Ti5Ox was tested and compared (Figure 3). After exposure to SO2 at 300 oC for 12 h, the NO conversion over CeMo0.5Ox decreased gradually from 82.8 to 52.5%, and the loss of activity was not recovered even after the removal of SO2. However, there was almost no significant activity loss detected on CeMo0.5Ti5Ox under the same test condition, indicating that the addition of Ti could remarkably promote the SO2 resistance of the catalyst. 3.2 TEM / HR-TEM Figure 4 shows the TEM images of CeMo0.5Ox and CeMo0.5Ti5Ox catalysts. The selected area electron diffraction patterns (SAED) of both CeMo0.5Ox and CeMo0.5Ti5Ox catalysts showed obvious polycrystalline diffraction rings. The HR-TEM images of CeMo0.5Ox and CeMo0.5Ti5Ox catalysts are shown in Figure 5. The fringes of appearing in the micrographs could be used to determine the crystallographic spacing of the crystals. Some straight fringes were observed in the CeMo0.5Ox with a lattice distance of 0.312 nm, corresponding to the (111) plane of CeO2 crystals. With the addition of Ti, the straight fringes corresponding to the CeO2 crystals became obscure, while some straight fringes with a lattice distance of 0.351 nm, corresponding to the (101) plane of anatase TiO2 crystals, were observed. The HR-TEM images indicated that the addition Ti could limit the growth of CeO2 crystallites. 3.3 XRD and Raman results XRD patterns of the CeMo0.5TiaOx catalysts with various Ti/Ce molar ratios are 8


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shown in Figure 6. The typical peaks of CeMo0.5Ox corresponded very well to the cubic fluorite phase of CeO2 (PDF# 34-0394). With the increase of Ti, the peaks of CeO2 became weaker, while the peaks associated with anatase TiO2 (PDF# 21-1272) were observed, implying that Ti addition could inhibit the recrystallization of CeO2 by dilution effect. No obvious diffraction peaks attributed to Mo and Ce species were detected in the CeMo0.5Ti5Ox, indicating that Mo and Ce species were probably dispersed on/in the catalysts and existed as crystallite phase with very small particle size (as shown by the HR-TEM).24 This might be the main reason for CeMo0.5Ti5Ox showing the best NH3-SCR activity at low temperatures. The Raman spectra for the two catalysts are shown in Figure 7. Only cubic CeO2 was observed on CeMo0.5Ox catalyst, while the typical peaks of CeMo0.5Ti5Ox corresponded to TiO2.38-39 The XRD and Raman results are in good agreement with the TEM / HR-TEM analysis. 3.4 Formation process of the catalyst To further analyze the structure of the catalyst, the formation processes of the CeMo0.5Ox and CeMo0.5Ti5Ox were investigated in detail. The variation of pH during the precipitation process of CeMo0.5Ox is shown in Figure 8(A). The initial pH of the mixed solution was about 3.1. It rapidly increased to 4.4 in the first hour, and some suspended yellow-green (grossly) particles formed with the hydrolysis of urea. After 4 h, the pH increased to about 6.5, and the color of the suspended particles changed to be white. The pH reached 7.1 after 6 h and kept stable. The NOx conversion over the CeMo0.5Ox samples with different precipitation 9



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time is also shown in Figure 8(A). We can see that, the NOx conversion over all of the samples was relatively low. The variation of pH during the precipitation process of CeMo0.5Ti5Ox catalyst is shown in Figure 8(B). Due to the acidity of Ti(SO4)2, the initial pH of the mixed solution was only 1.2. The pH value increased with time due to the hydrolysis of urea, and the color of the suspended particles changed from white to light yellow gradually. Table 1 shows the surface atomic concentrations and surface Ce/Ti and Mo/Ti atomic ratio of the CeMo0.5Ti5Ox with different precipitation time. No Ce was detected on the surface of the 1 h sample. With the increase of time, the concentration of the surface Ce species and Ce/Ti atomic ratio increased gradually. However, the Mo/Ti atomic ratio of the CeMo0.5Ti5Ox samples with different precipitation time kept stable. Based on the above-mentioned analysis, the precipitation process during the preparation of the CeMo0.5Ti5Ox catalyst can be proposed as: the Mo and Ti species firstly precipitated out from the mixed solution together, and then Ce species precipitated onto the Mo-Ti precipitates with the increase of pH. Therefore, the CeMo0.5Ti5Ox catalyst would be better denoted as CeOx/MoO3-TiO2, i.e., CeOx highly dispersed on MoO3-TiO2. Figure 8(B) also shows the NOx conversion over the CeOx/MoO3-TiO2 samples with different precipitation time under a GHSV of 100,000 h-1. The low-temperature SCR activities of the 1 h and 2 h CeOx/MoO3-TiO2 samples were lower than that of the 1 h CeMo0.5Ox sample, which was associated with the lack of active Ce species. With the increase of surface Ce species, the 4 h and 12 h CeOx/MoO3-TiO2 samples 10


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presented remarkably enhanced low-temperature activities. 3.5 XPS Figure 9(A) shows the XPS spectra of Ce 3d of the CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts. Eight peaks can be obtained by fitting the curves: v (882.4-882.7 eV), v' (885.2-886.0 eV), v'' (888.3-889.4 eV), v''' (898.2-898.3 eV), u (900.5-901.5 eV), u' (903.8-904.0 eV), u '' (907.3-907.9 eV), and u''' (916.7-916.8 eV). The peaks labeled with v' and u' represent the 3d104f1 initial electronic state of Ce3+, while the peaks labeled with v, v'', v''', u, u '' and u''' represent the 3d104f0 state, corresponding to Ce4+.40 For both of the catalysts, the co-existence of Ce3+ and Ce4+ species could generate Ce4+/Ce3+ redox electron pair on the surface, which is beneficial for the storage and release of reactive oxygen species and the oxidation of NO to NO2. For both of the catalysts, we can see that Ce4+ is predominant and the peaks for Ce3+ are relatively weak. Higher Ce3+ ratio implies that the catalyst surface have more oxygen vacancies, which would facilitate the adsorption of reactive species.41-43 According to calculation, the Ce3+ ratio of CeOx/MoO3-TiO2 (32.0%) was significantly higher than that of CeMo0.5Ox (15.9%). The high Ce3+ ratio of CeOx/MoO3-TiO2 could promote the electron transfer and provide more oxygen vacancies, thus promote the adsorption and activation of reactants. The XPS spectra of O 1s are shown in Figure 9(B). Three peaks can be obtained by fitting the curves: the peaks at 529.5-530.0 eV are attributed to the lattice oxygen O2− (denoted as Oβ); the peaks at 530.2-530.6 eV are assigned to the surface adsorbed oxygen (denoted as Oα); and the peaks at 532.3-532.6 eV are belonging to 11



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chemisorbed water (denoted as Oα´), respectively.44-45 Oα has high oxidation ability due to its higher mobility.46 Many researchers believe that high ratio of Oα would beneficial for the oxidation of NO to NO2, which could facilitate the NOx conversion through "fast SCR" reaction.46-47 With the addition of Ti, the Oα ratio of CeOx/MoO3-TiO2 increased to 41.2% from that of 33.6% on CeMo0.5Ox. This result confirmed that the high Ce3+ ratio of CeOx/MoO3-TiO2 induced more oxygen vacancies and thus adsorbed more active oxygen species, which would be beneficial for the conversion of NOx. According to the XPS results of Mo 3d and Ti 2p, the Mo of CeMo0.5Ox and CeOx/MoO3-TiO2 were both mainly in the formal (VI) oxidation state, and the Ti of CeOx/MoO3-TiO2 was mainly in the formal (IV) oxidation state (see Figure S2 in the Supporting Information). 3.6 H2-TPR The H2-TPR profiles of CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts were shown in Figure 10. The two sharp peaks at 636 oC for CeMo0.5Ox and 526 oC for CeOx/MoO3-TiO2 were both attributed to the reduction of surface Ce4+ to Ce3+.27, 48 The small peak for CeMo0.5Ox at 442 oC was assigned to the reduction of well dispersed surface Mo species, while the broad peak at 818 oC for CeOx/MoO3-TiO2 was due to the bulk reduction of CeO2 (such as the CeO2 crystals detected by HR-TEM).49-50

Compared

with

CeMo0.5Ox,

the

reduction

temperature

of

CeOx/MoO3-TiO2 was shifted to lower temperature. The H2 consumption on CeOx/MoO3-TiO2 started at 255 oC, which is lower than that of CeMo0.5Ox (at 292 oC). 12


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These results strongly indicated the better redox property of CeOx/MoO3-TiO2 than that of CeMo0.5Ox. Previous studies have shown that the low-temperature SCR activity is mainly controlled by the redox ability of the catalyst.51-52 In this study, the NO oxidation to NO2 (associated with the redox properties) over CeOx/MoO3-TiO2 is higher than that over CeMo0.5Ox (see Figure S3 in the Supporting Information), which indicates that CeOx/MoO3-TiO2 is more efficient for NOx conversion through facilitating the “fast SCR” process (2 NH3 + NO + NO2 → 2 N2 + 3 H2O).53-55 Therefore, the enhanced redox property of CeOx/MoO3-TiO2 induced by the addition of Ti would be beneficial for the low temperature catalytic activity. 3.7 NOx/NH3-TPD Figure 11(A) illustrates the TPD profiles of NOx on the CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts. A desorption peak of NOx at 260 oC was observed on CeOx/MoO3-TiO2 catalyst, which was associated with the decomposition of chemsorbed NOx species.49, 56 However, no obvious desorption peak was observed on the CeMo0.5Ox. Previous studies have proved that the reaction of adsorbed NOx with adsorbed NH3 species (Langmuir-Hinshelwood mechanism) is an important reaction route for NH3-SCR over Ce-based catalysts, especially at low temperature.37, 57 The NOx adsorption capacity of CeOx/MoO3-TiO2 was much higher than that of CeMo0.5Ox (for about 8 μmol/g). Therefore, the addition of Ti and the resulted structure of CeOx/MoO3-TiO2 remarkably enhanced its NOx adsorption ability. Figure 11(B) illustrates the TPD profiles of NH3 on the CeMo0.5Ox and 13



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CeOx/MoO3-TiO2 catalysts. Both of the NH3 desorption from the two catalysts showed a broad peak from 75 to 400 oC. The NH3 adsorption capacity of CeOx/MoO3-TiO2 (319 μmol/g) was much higher than that of CeMo0.5Ox (75 μmol/g), implying that the addition of Ti and the resulted structure of CeOx/MoO3-TiO2 remarkably enhanced its NH3 adsorption ability. The NOx/NH3 adsorption capacity of CeOx/MoO3-TiO2 is much higher than that of CeMo0.5Ox, while the BET surface area of CeOx/MoO3-TiO2 (97.6 m2/g) is only 0.6 times higher than that of CeMo0.5Ox (62.5 m2/g), indicating that some other reasons (such as the enhanced Ce dispersion and introduced Ti species) have contributed to the improvement of reactants (NOx and NH3) adsorption abilities. 3.8 In situ DRIFTS To examine the effects of Ti addition on the surface acid sites, in situ DRIFTS was applied. Figure 12 shows the DRIFTS of NH3 adsorption on the CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts at 250 °C. The bands centered at 1602/1584, 1243/1234, 1212, and 1188 cm-1 were associated with NH3 species coordinated to Lewis acid sites.58-60 The bands at 1676 and 1416/1434 cm-1 attributed to NH4+ bound to Brønsted acid sites.35, 56, 61 The bands at 3383/3375, 3345, and 3259/3258 cm-1 are assigned to the stretching vibration modes of N-H associated with NH4+.56, 60 In addition, the negative bands at 3658/3667 and 3624/3645 cm-1 can be ascribed to the consumption of surface hydroxyl species due to the interaction between the hydroxyl groups and NH3 to form NH4+.56, 59 With the adsorption of NH3, the bands associated with NH3 and NH4+ on 14


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CeOx/MoO3-TiO2 were obviously higher than the bands on CeMo0.5Ox. This result indicated that Ti addition could provide more Brønsted and Lewis acid sites on the surface of the catalyst. Many studies have proved that surface acidity are very important for the NH3-SCR reaction.29, 51-52, 62 Particularly, the high temperature SCR reaction activity is almost determined by the acidity of catalyst.29 Thus, the enhanced acidity by Ti introduction has significantly contributed to the improvement of high temperature activity. It is well recognized that the coordinated NH3 at Lewis sites on the commercial V2O5-WO3/TiO2 and V2O5-MoO3/TiO2 catalysts are associated with the surface Ti species, and the Ti species could strongly adsorb NH3.8, 63 In this study, the added surface Ti species on CeOx/MoO3-TiO2 could act as Lewis acid sites to adsorb NH3 species, thus facilitating the SCR reaction.8 Besides, the enhancement of acidity possibly inhibited the over-oxidization of adsorbed NH3 on CeOx/MoO3-TiO2, thus suppressing the formation of N2O and increasing the N2 selectivity.57, 64 4. Conclusions A stepwise precipitation approach was used to prepare CeMo0.5TiaOx (a = 0, 1, 2, 5, 10) catalysts for NH3-SCR, and the promotional functions of Ti for this reaction was investigated in detail. The CeMo0.5Ox without Ti only exhibited relatively low catalytic activity. When Ti was added, the Ce-Mo-Ti catalysts (especially the CeMo0.5Ti5Ox catalyst) showed much better NOx conversion, N2 selectivity and SO2 resistance. 15



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The analyses of TEM, XRD and Raman proved that the addition of Ti could limit the growth of CeO2 crystallites and only very small CeO2 crystal domains were formed in CeMoTi5Ox, below the detection limit of XRD. Formation process analysis of the CeMo0.5Ti5Ox showed that, the Mo and Ti species firstly precipitated out together from the mixed solution with the increase of pH, and then Ce species precipitated afterwards onto the Mo-Ti precipitates. This stepwise precipitation process is very important for the generation of small CeO2 domains on MoO3-TiO2. Comparative characterizations of the CeMo0.5Ox and CeMo0.5Ti5Ox catalysts were carried out to show the effects of Ti addition. The CeMo0.5Ti5Ox catalyst showed remarkably facilitated NOx and NH3 adsorptions, enhanced charge imbalance, promoted redox property, and improved surface acidity, which are associated with the excellent catalytic performance of the catalyst. Particularly, the promoting effects of Ti addition on the redox and surface acid functions of the catalyst played key roles in the improvement of low and high temperature activities (together with N2 selectivity), respectively.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51308296), the Fundamental Research Funds for the Central Universities (30920140111012) and the Qing Lan Project.

Supporting Information Supporting Information Available: Elemental molar ratios of the catalysts 16


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obtained by XRF, SO2 resistance of CeMo0.5Ti5Ox at 250 oC, XPS results of Mo 3d and Ti 2p, separated NO oxidation, and in situ DRIFTS investigation of NO oxidation.

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Figure captions: Figure 1. (A) NOx conversions, (B) NH3 conversions, and (C) N2 selectivity over the CeMo0.5TiaOx (a = 0, 1, 2, 5, 10) and CeO2-MoO3/TiO2 catalysts. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol.%, N2 balance, and GHSV = 100,000 h-1. Figure 2. Effect of H2O on NOx conversion and N2O production over the CeMo0.5Ti5Ox catalyst. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol.%, [H2O] = 5 vol.% (when used), N2 balance. Figure 3. NOx conversion over CeMo0.5Ox and CeMo0.5Ti5Ox catalysts in the presence of SO2 at 300 oC. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol.%, [SO2] = 100 ppm, N2 balance, and GHSV=100,000 h-1. Figure 4. TEM images and SAED pattern (inset) of the CeMo0.5Ox and CeMo0.5Ti5Ox catalysts. Figure 5. HR-TEM images of the CeMo0.5Ox and CeMo0.5Ti5Ox catalysts. Figure 6. XRD patterns of the CeMo0.5TiaOx (a = 0, 1, 2, 5, 10) catalysts. Figure 7. Raman spectra (λex=532nm) of CeMo0.5Ox and CeMo0.5Ti5Ox catalysts. Figure 8. Variations of the pH during the preparations of (A) CeMo0.5Ox and (B) CeMo0.5Ti5Ox catalysts. NOx conversions over the samples with different precipitation time were inserted. Figure 9. XPS results of (A) Ce 3d and (B) O 1s of the CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts. Figure 10. H2-TPR profiles of the CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts. Figure 11. (A) NOx-TPD and (B) NH3-TPD profiles of the CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts. 22


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Figure 12. In situ DRIFTS of NH3 adsorption at 250 oC on the CeMo0.5Ox and CeOx/MoO3-TiO2 catalysts.

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Table 1 Surface atomic concentrations and Ce/Ti and Mo/Ti atomic ratio of the CeMo0.5Ti5Ox with different precipitation time Surface atomic concentrations a (At %) Sample

Surface Ce/Ti

Surface Mo/Ti

atomic ratio

atomic ratio

Ce

Mo

Ti

O

CeMo0.5Ti5Ox-1 h

0

3.6

20.8

75.6

0.00:1

0.17:1

CeMo0.5Ti5Ox-2 h

0.7

3.6

21.4

74.3

0.03:1

0.17:1

CeMo0.5Ti5Ox-4 h

3.2

2.9

18.4

75.5

0.17:1

0.16:1

CeMo0.5Ti5Ox-12 h

4.6

3.1

19.0

73.3

0.24:1

0.16:1

a

according to XPS result

24


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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 8

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Figure 10

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Figure 11

1

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Figure 12

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Promotional Effects of Ti on a CeO2âMoO3 Catalyst for the Selective

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