Effect of Ceria Precursor on the Physicochemical and Catalytic

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Effect of Ceria Precursor on the Physicochemical and Catalytic Properties of Mn−W/CeO2 Nanocatalysts for NH3 SCR at Low Temperature Xiuyun Wang,† Kai Zhang,† Weitao Zhao,† Yangyu Zhang,† Zhixin Lan,† Tianhua Zhang,† Yihong Xiao,† Yongfan Zhang,† Huazhen Chang,‡ and Lilong Jiang*,† †

National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, China School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China

‡

ABSTRACT: A series of Mn−W/CeO2 catalysts were synthesized by employing Ce−NO-R nanorods [Ce(NO3)3· 6H2O as the precursor] and Ce−Cl-C nanocubes and Ce−ClR nanorods (CeCl3·7H2O as the precursor) as the supports, and their structures and catalytic performances in NOx removal were investigated. TEM results demonstrate that the morphology of the CeO2 species is associated with the Ce precursor. The activity results show that the NOx conversion in Mn−W/Ce−NO-R is greater than 96% at 150 °C, which is much higher than those in Mn−W/Ce−Cl-R and Mn−W/ Ce−Cl-C. At the same time, Mn−W/Ce−NO-R exhibits strong SO2 tolerance and excellent H2O resistance. DFT modeling shows that NO adsorbs over Mn−W/Ce−NO-R{110} by forming ONO*, whereas NO prefers to adsorb on the O vacancies in Mn−W/Ce−Cl-C{100}. Moreover, the superior catalytic performance of Mn−W/Ce−NO-R is associated with its greater amounts of surface Ce3+ and Mn4+ species, surface acid sites, and active oxygen species, resulting in the promotion of the reactions according to the Langmuir−Hinshelwood and Eley−Rideal mechanisms.

1. INTRODUCTION Nitrogen oxides (NOx) are considered to be primary atmospheric pollutants that can cause serious harm to the environment and human health.1−3 The selective catalytic reduction (SCR) of NOx with NH3 is a well-established and efficient process for NOx abatement in coke-oven flue gas (COFG) and stationary applications.4−7 V2O5−WO3(MO3)/ TiO2 is extensively used as a commercial SCR catalyst,8−10 but its narrow manipulation temperature window (300−400 °C) and overoxidation of SO2 limit its further application. Therefore, it is necessary to search for alternative catalysts to solve these problems, particularly the catalyst manipulation temperature being lower than 200 °C, which is beneficial for resolving the problems of SO2 oxidation and poisoning. CeO2 nanomaterials have attracted much interest because of their excellent oxygen storage capabilities and a special Ce4+/ Ce3+ redox cycle that promotes the activation of reactants on the ceria surface.11,12 Recently, many studies have shown that the morphology of CeO2 can significantly influence its catalytic activity in many reactions, such as CO oxidation,13 propane oxidation,14 and the water−gas shift reaction .15 Generally, CeO2 materials with different morphologies have been synthesized by a variety of methods including thermal evaporation, coprecipitation, and sol−gel techniques.16−18 To the best of our knowledge, the effect on NOx removal of the morphology of CeO2 tuned by changing the cerium precursor © XXXX American Chemical Society

is still unexplored. Notably, pure CeO2 nanomaterials cannot satisfy the requirements for practical use, owing to the facts that they have poor thermal stability and are easily poisoned by SO2, thereby limiting their practical application. The combination of CeO2 and oxides of transition metals including Mn,19 Fe,20 Nb,21 Co,22 Ni,23 and Ti24 can usually improve the catalytic performance. Among such oxides, manganese oxides have attracted substantial attention because of their low costs, readily available raw materials, and good redox properties. It was reported that the doping of Mn cations in CeO2 can increase the surface Ce3+ species and oxygen vacancies by aggravating the lattice distortion, which is beneficial for facilitating the adsorption and oxidation of NO, resulting in an improvement in the low-temperature activity. Additionally, the addition of W cations to CeO2 can induce more Lewis acid sites, leading to an enhancement of the amount of the adsorbed NH3 species, favoring the improvement of N2 selectivity and SO2 tolerance.6 Therefore, the combined use of low-cost manganese and/or tungsten oxides and CeO2 would be a promising pathway for the development of inexpensive NOx removal catalysts that can Received: Revised: Accepted: Published: A

August 21, 2017 November 7, 2017 December 11, 2017 December 11, 2017 DOI: 10.1021/acs.iecr.7b03466 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

response method (TRM) at 200 °C was applied to measure the catalytic activities of Mn−W/Ce−Cl-C and Mn−W/Ce−NOR. Before TRM testing, a 0.1-g sample was treated in flowing air (50 mL/min) at 350 °C for 30 min. After the system had cooled to 200 °C, 167 ppm NO was provided, and the reactor was maintained at a steady state for 30 min; then a feed gas containing 167 ppm NO, 167 ppm NH3, 3 vol % O2, and a balance of Ar was employed in the next step. The outlet gases of NO, N2, NH3, NO2, and N2O were monitored with a mass spectrometer (HIDEN HPR20). 2.3. Characterization. Powder X-ray diffraction (XRD) was carried out on a Panalytical X’Pert Pro diffractometer using Co Kα radiation. N2 physisorption measurements were performed using an ASAP 2020 apparatus. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis was carried out using an Ultima2 spectrometer. H2 TPR experiments were performed on an AutoChem II 2920 apparatus, with the sample treated under an air flow (30 mL/min) at 400 °C for 30 min and then purged with Ar (30 mL/min) at the same temperature for 0.5 h. After cooling to room temperature, the temperature was increased to 900 °C at 5 °C/min in a gas flow of 10 vol % H2/Ar (30 mL/min). NH3 TPD-MS was also tested on an AutoChem 2920 instrument. A 50 mg sample was first pretreated in argon at 500 °C for 60 min. After the sample had been cooled to 50 °C, it was heated in a 1.01% NH3/Ar stream for 30 min, then flushed with argon at 200 °C to remove physically adsorbed NH3, and finally cooled to 50 °C. NH3 TPD was then performed from 50 to 800 °C in N2 stream. NH3 TPD quadruple mass spectroscopy (Q-MS) was used to evaluate the evolving gases. The signals for NH3 (m/z = 17), NO (m/z = 30), NO2 (m/z = 46), N2O (m/z = 44), and N2 (m/z = 28) were monitored by using a QIC20 benchtop gas analysis system. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Physical Electronics Quantum 2000 scanning X-ray photoelectron spectrometer, equipped with a monochromatic Al Kα source (1486.6 eV) and a charge neutralizer. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) analyses were performed on a JEM-2010 microscope. Scanning TEM (STEM)−energydispersive X-ray spectroscopy (EDS) was carried out on a JEM2010 microscope operating at 200 kV in bright-field mode. O2 TPD experiment was carried out from room temperature to 800 °C at 5 °C/min. O2 TPD quadruple mass spectroscopy (Q-MS) was used to investigate evolving gases. The signals for O2 (m/z = 32), H2O (m/z = 18), and CO2 (m/z = 44) were monitored using a QIC20 benchtop gas analysis system connected to an AutoChem II 2920 outlet. 2.4. In Situ DRIFTS. In situ DRIFTS was carried out on a Nicolet Nexus FTIR spectrometer in the range of 700−4000 cm−1 with a resolution of 4 cm−1. Before each experiment, the sample was treated at 350 °C for 30 min in a N2 stream to eliminate physically adsorbed impurities and then cooled to 50 °C. Background spectra were collected in flowing N2. Then, target preadsorbed gas was provided to the cell at a flow rate of 30 mL/min at 100 °C for 30 min to ensure that complete absorption saturation occurred. After the physically adsorbed target gas had been eliminated under an argon flow at 200 °C for 3 h, the DRIFTS spectrum was collected at 50−350 °C. Typically, after the adsorption of 500 ppm NH3 or 500 ppm NO + 3 vol % O2, the catalyst was pretreated at 200 °C under an Ar flow at 20 mL/min for 1 h; subsequently, 500 ppm NO + 3 vol % O2/Ar or 500 ppm NH3 (20 mL/min) was introduced

simultaneously exhibit superior low-temperature activities and excellent SO2 tolerance performances. Herein, we report a series of Mn−W/CeO2 catalysts that were synthesized by employing Ce−NO-R nanorods [Ce(NO3)3·6H2O as the precursor] and Ce−Cl-C nanocubes and Ce−Cl-R nanorods (CeCl3·7H2O as the precursor) as the supports and investigated in NOx removal. The obtained Mn− W/Ce−NO-R catalyst exhibited 96% NOx conversion at 150 °C, strong SO2 tolerance, and excellent H2O resistance. The structure, distribution, reducibility, and surface properties of the catalysts were studied systematically by transmission electron microscopy (TEM)/high-resolution TEM (HR-TEM), highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM), Raman spectroscopy, N 2 physisorption, X-ray diffraction (XRD), H2 temperatureprogrammed reduction (H2 TPR), NH3 temperature-programmed desorption-mass spectrometry (TPD-MS), O 2 TPD-MS, and X-ray photoelectron spectroscopy (XPS). The effects of Mn and W addition to CeO2 on the NH3/NO adsorption abilities and the formation energies of surface oxygen vacancies were clarified by DFT calculations. Moreover, in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study the intermediate species and the transformation of these surface-adsorbed species, and reaction mechanisms for the catalyst are proposed.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The procedures for the synnthses of Ce−Cl-R, Ce−Cl-C, and Ce−NO-R involve several steps. Briefly, CeO2 nanocubes (Ce−Cl-C) and nanorods (Ce−NO-R) were prepared by a hydrothermal process. Two millimoles of CeCl3·7H2O or Ce(NO3)3·6H2O was added to 40 mL of NaOH (6 M) aqueous solution, and the mixture was then transferred to 100 mL Teflon-lined stainless steel autoclaves and heated at 180 °C for 24 h. Thereafter, the obtained precipitates were filtere and then subjected to vacuum freeze-drying at −46 °C for 10 h and subsequent drying at 80 °C for 12 h. Finally, the powders were calcined at 400 °C for 3 h, yielding Ce−Cl-C or Ce−NO-R. The synthetic procedure for Ce−Cl-R was similar to that used for Ce−Cl-C, except that the hydrothermal temperature process was carried out at 110 °C for 24 h. Mn−W/Ce-C-R, Mn−W/Ce−Cl-R, and Mn−W/Ce−NO-R catalysts were prepared by the successive wet impregnation of Mn (5 wt % Mn) and W (5 wt % W) on as-prepared Ce−C-R, Ce−Cl-R, and Ce−NO-R supports, respectively. 2.2. Catalytic Activity Tests. Catalytic activity tests were carried out in the fixed bed of a stainless steel reactor with an inner diameter of 8 mm. Before each SCR test, 0.2 g of sample was first pretreated in Ar at 400 °C for 2 h. After the sample had cooled to the test temperature, inlet gas containing 498 ppm NO, 500 ppm NH3, and 3 vol % O2/Ar was applied at a gas hourly space velocity (GHSV) is 90000 h−1. SO2 tolerance was examined by exposing catalysts to 498 ppm NO, 500 ppm NH3, 100 ppm SO2, and 3 vol % O2/Ar at 200 °C. Resistance against H2O and SO2 was tested by introducing 5.5 vol % H2O and 100 ppm SO2 into the reaction gas at 200 °C. The concentrations of NOx were detected online with a NO−NO2− NOx analyzer (model 42i-HL). The outlet NO, NO2, N2O, and NH3 concentrations were also measured with an FTIR spectrometer (Nicolet Nexus 6700) using a heated, multiplepath gas cell. NOx conversion was obtained 1 h after the catalytic system had reached a steady state. The transient B

DOI: 10.1021/acs.iecr.7b03466 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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models of CeO2 materials with nanorod and nanocubic structures are shown in Figure 1M. The results show CeO2 nanorods with prior growth directions along the {110} and {111} facets, and CeO2 nanocubes (Figure 1D) with {100} facets. Interestingly, no obvious changes take place in the morphologies of the M- and W-doped CeO2. However, the HR-TEM images of Mn/W-doped CeO2 shows that the spacing distance between two fringes becomes larger owing to the insertion of Mn and W into the CeO2 lattice. 3.1.2. XRD and Raman Spectroscopy. The crystallinities and phase compositions of the as-prepared samples were investigated by XRD. As shown in Figure 2, the XRD patterns

to investigate the reactivity of preadsorbed NH3 with NO + O2 species or preadsorbed NO + O2 with NH3 species. 2.5. DFT Calculations. DFT calculations were performed using the Vienna ab Initio Simulation Program (VASP) with the gradient-corrected PW91 exchange-correction function. For valence electrons, a tight convergence of the plane-wave expansion was obtained with a kinetic energy cutoff of 500 eV, and the ionic cores were described by the projector-augmented wave (PAW) method. The Brillouin zone of the Monkhorst− Pack grid was set at 2 × 2 × 1. The calculated electronic energy should be converged to 10−5 eV, and the position of each atom was permitted to relax until all forces were less than 0.02 eV/Å.

3. RESULTS AND DISCUSSION 3.1. Structural and Textural Properties. 3.1.1. TEM and HR-TEM. TEM and high-resolution TEM (HR-TEM) images of Ce−Cl-R, Ce−Cl-C, Ce−NO-R, Mn−W/Ce−Cl-R, Mn−W/ Ce−Cl-C, and Mn−W/Ce−NO-R are presented in Figure 1.

Figure 2. XRD patterns of the prepared samples: Ce−NO-R, Ce−ClR, Ce−Cl-C; Mn−W/Ce−NO-R, Mn−W/Ce−Cl-R, and Mn−W/ Ce−Cl-C.

of Ce−Cl-R, Ce−Cl-C, and Ce−NO-R exhibit diffraction peaks at 33.28°, 38.61°, 55.76°, 66.47°, and 69.87°, which can be indexed as the (111), (200), (220), (311), and (222) planes, respectively, of cubic CeO2. Notably, each Ce atom is arranged in a face-centered-cubic pattern surrounded by eight oxygen atoms in the cubic fluorite structure of CeO2 (Figure 1N). At the same time, oxygen atoms occupy all of the tetrahedral positions, and each oxygen atom is coordinated with four Ce atoms. The average crystalline sizes for Ce−Cl-R, Ce−Cl-C, and Ce−NO-R are 12.9, 13.0, and 13.2 nm, respectively (Table 1). No obvious diffraction peaks assignable to the Mn and/or W phases are observed in any of the catalysts, indicating that these phases are highly dispersed on CeO2 or incorporated into the CeO2 structure to produce Ce−M−O (M = Mn and/or W) solid solutions.23,25 The existence of Ce−M−O solid solutions can restrain the growth of these oxide crystals and facilitate the activation of oxygen species,26,27 which is beneficial to the promotion of NO oxidation to NO2 (vide infra). To further clarify the structural characterization of the asprepared samples, Raman spectra of the prepared samples were acquired and are displayed in Figure 3A. The band at 461 cm−1 corresponds to the F2g mode of CeO2, whereas the band at 602 cm−1 can be assigned to the oxygen-defect-induced mode of CeO2.24,26 No obvious manganese and tungsten oxide peaks are observed after the addition of Mn and W, and one reason is that the Raman bands of manganese and tungsten oxides are much weaker than those of CeO2, owing to the strong absorbance of CeO2 in the wavenumber range of Raman spectra. Another reason is the generation of Ce−Mn (W)−O solid solutions.28 Additionally, the relative intensity ratio of the defect band (Ia) and the F2g band (Ib) in the CeO2 fluorite

Figure 1. (A,C,E,G,I,K) TEM and (B,D,F,H,J,L) HR-TEM images of (A,B) Ce−Cl-R, (C,D) Ce−Cl-C, (E,F) Ce−NO-R, (G,H) Mn−W/ Ce−Cl-R, (I,J) Mn−W/Ce−NO-R, and (K,L) Mn−W/Ce−Cl-C. (M) Geometric models of a CeO2 nanorod and a CeO2 nanocube. (N) Simulated structure of unit cells of CeO2.

Ce−Cl-R (Figure 1A) and Ce−NO-R (Figure 1E) nanorods exhibit an average width of 10−16 nm and an average length of 70−180 nm, whereas Ce−Cl-C features a cubic morphology (Figure 1C). As can be seen in the HR-TEM images of Ce−ClR (Figure 1B) and Ce−NO-R (Figure 1F), lattice distances of 0.191−0.195 and 0.316−0.319 nm were observed, corresponding to the {110} and {111} planes, respectively, of CeO2. The distance between the adjacent fringes in Ce−Cl-C is in the range of 0.266−0.271 nm, which is in conformity with {100} planes. Thus, the morphologies of the CeO2 nanomaterials are associated with the cerium precursors. Furthermore, geometric C


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Industrial & Engineering Chemistry Research Table 1. Textural Properties, Crystalline Sizes, and Amounts of NH3 Desorbed from As-Prepared Samples sample

BET surface area (m2/g)

pore volume (cm3/g)

pore diameter (nm)

crystalline size (nm)

amount of NH3 desorbed (mmol/g)

Ce−NO-R Ce−Cl-R Ce−Cl-C Mn−W/Ce−NO-R Mn−W/Ce−Cl-R Mn−W/Ce−Cl-C

69 66 64 56 53 57

0.19 0.21 0.18 0.23 0.22 0.24

13.00 14.39 15.27 16.28 16.53 17.18

13.2 12.9 13.4 13.5 14.0 14.2

0.36 0.22 0.24 1.40 0.91 0.94

Figure 3. (A) Raman spectra and (B) Ia/Ib ratios of Ce−Cl-R, Ce−Cl-C, Ce−NO-R, Mn−W/Ce−Cl-R, Mn−W/Ce−Cl-C, and Mn−W/Ce−NOR.

Figure 4. STEM mapping of (a−f) Mn−W/Ce−NO-R drift-corrected spectrum images scanning the (b) Mn L-edge, (c) W M-edge, (d) Ce Medge, and (e) O K-edge. (f) HAADF-STEM image.

Table 2. Surface Mn4+/Mn3+, Ce4+/Ce, Ce3+/Ce, and Oads/(Olatt + Oads) Ratios element contenta (%)

a

catalyst

Mn4+/Mn3+

Ce4+/Ce (%)

Ce3+/Ce (%)

Oads/(Olatt + Oads) (%)

Mn

W

Mn−W/Ce−NO-R Mn−W/Ce−Cl-R Mn−W/Ce−Cl-C

1.78 1.01 1.78

85.90 87.24 88.32

14.10 12.76 11.68

69 57 64

4.57 4.62 4.58

4.29 4.40 4.28

ICP-AES results.

3.1.3. STEM Mapping and N2 Physisorption. Generally, the dispersion of metal oxides is closely associated with their catalytic activity.29 Higher dispersion generally results in better catalytic performance. STEM-EDS analysis (Figure 4) was carried out to obtain direct insight into the elemental distribution in Mn−W/Ce−NO-R. The elemental mappings confirm that Mn and W are both well dispersed in CeO2, which is consistent with the XRD results. Moreover, the bulk composition of Mn−W/Ce−NO-R was further analyzed

phase can be used as an indicator of the oxygen defect density. The Ia/Ib ratios of the as-prepared samples are presented in Figure 3B and follow the order Mn−W/Ce−NO-R > Mn−W/ Ce−Cl-R > Mn−W/Ce−Cl-C > Ce−NO-R > Ce−Cl-R > Ce− Cl-C, suggesting that Mn−W/Ce−NO-R contains the highest amounts of defects. These results also imply that the interaction of manganese/tungsten oxides with Ce−NO-R is stronger than those of manganese/tungsten oxides with Ce−Cl-R and Ce− Cl-C. D


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Figure 5. (A) NO-to-NO2 conversion (reaction conditions: 498 ppm NO and 3 vol % O2 balanced with Ar, GHSV = 90000 h−1). (B) NOx conversions of Ce−Cl-R, Ce−Cl-C, Ce−NO-R, Mn−W/Ce−Ce-R, Mn−W/Ce−Cl-C, and Mn−W/Ce−NO-R. (C) N2 selectivities of Mn−W/ Ce−NO-R at different temperatures. (D) Outlet concentrations of NO, NH3, and N2O in Mn−W/Ce−NO-R at 200 °C (reaction conditions: 498 ppm NO, 500 ppm NH3, 5.5 vol % H2O, and 3 vol % O2 balanced with Ar, GHSV = 90000 h−1). (E) NOx conversion of Mn−W/Ce−NO-R in the presence of 5.5 vol % H2O and 100 ppm SO2 at 200 °C. (F) NOx conversion of Mn−W/Ce−Cl-C in the presence of 5% H2O and 100 ppm SO2 at 200 °C.

catalysts are all obviously increased, which can promote the adsorption and activation of the reaction molecules (i.e., NO, NH3, and O2) on the internal surfaces of these catalysts. 3.2. Catalytic Performance. 3.2.1. Catalytic Activity. The NO-to-NO2 conversions of the catalysts were investigated, and the results are shown in Figure 5A. The NO conversions at low temperatures are low because of kinetic limitations, and the conversion attains a maximum at 250−300 °C, after which a decrease in conversion ocurs because of thermodynamic limitations. Notably, the NO-to-NO2 conversion in Mn−W/ Ce−NO-R is higher than those in Mn−W/Ce−Cl-C and Mn− W/Ce−Cl-R at 50−300 °C. The catalytic activities of the as-

using ICP-AES, as reported in Table 2. The results indicate that the mass fractions of Mn and W metals in the mixed oxide catalysts are slightly lower than the nominal values. Additionally, textural properties such as surface area are also important parameters for catalysis. It was found that the surface areas of Ce−Cl-R, Ce−Cl-C, and Ce−NO-R are close to each other and in the range of 64−69 m2/g (Table 1). These results suggest that only minor textural property changes occur for CeO2 materials with different shapes. After the addition of Mn and W, the Brunauer−Emmett−Teller (BET) surface area decreases to the range of 53−57 m2/g. Compared with that of pure CeO2, the pore volumes of the Mn- and W-doped E


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Figure 6. TRM profiles of (A) Mn−W/Ce−Cl-C and (B) Mn−W/Ce−NO-R at 200 °C.

Figure 7. H2 TPR profiles of (A) Ce−NO-R, Ce−Cl-C, and Ce−Cl-R and (B) Mn−W/Ce−NO-R, Mn−W/Ce−Cl-C, and Mn−W/Ce−Cl-C.

NOx conversion returns to 94%. Additionally, the presence of both 5.5 vol % H2O and 100 ppm SO2 results in a decrement of the NOx conversion from 94 % to 61% in Mn−W/Ce−NO-R (Figure 5E). Notably, the NOx conversion returns to 94% after the supply of SO2 is stopped. However, Mn−W/Ce−Cl-C exhibits poor sulfur and H2O resistance performances (Figure 5F). These reaction patterns indicate that Mn−W/Ce−NO-R exhibits high sulfur resistance and excellent H2O resistance abilities. 3.2.3. Transient Reactions. In general, NH4NO3 is generated at low temperatures in the presence of NH3 and NO2.31−33 Therefore, a transient response method (TRM) study of Mn−W/Ce−Cl-C and Mn−W/Ce−NO-R at 200 °C was employed to indirectly determine the amount of NH4NO3 based on the principle of N balance, and the results are presented in Figure 6. When 167 ppm NO, 167 ppm NH3, and 3 vol % O2 were provided into the systems, the amounts of N2 and N2O formed were 91 and 10 ppm, respectively (Figure 6A). After the system achieved a steady state, the outlet NO, NO2, and NH3 concentrations were 8, 1, and 16 ppm, respectively. The lack of 107 ppm in the total amount of N confirms the production of 53.5 ppm NH4NO3 in Mn−W/ClCl-C. Similarly, the concentration of NH4NO3 formed in Mn− W/Ce−NO-R (Figure 6B) is near the same level, indicating that the cerium precursor has little influence over the distribution of reaction products.

prepared samples are shown in Figure 5B. Pure Ce−Cl-R, Ce− Cl-C, and Ce−NO-R show low catalytic activities, and their NOx conversions are less than 20% at the test temperatures. The NOx conversions of Mn−W/Ce−Cl-R, Mn−W/Ce−Cl-C, and Mn−W/Ce−NO-R increased when the temperature was raised from 50 °C, and they reached 90% at temperatures of 250, 200, and 150 °C, respectively. The NOx conversions at 150 °C in Mn−W/Ce−Cl-R, Mn−W/Ce−Cl-C, and Mn−W/ Ce−NO-R were 19%, 82%, and 96%, respectively. Apparently, Mn−W/Ce−NO-R shows much higher NOx conversion than Mn−W/Ce−Cl-R and Mn−W/Ce−Cl-C, indicating that the cerium precursors have a significant impact on the catalytic performances of the resulting catalysts. Moreover, Mn−W/ Ce−Cl-R also shows higher NOx conversion than Mn−W/Ce− Cl-C at 50−250 °C, demonstrating a clear structural dependence. Additionally, the N2 selectivity in Mn−W/Ce−NO-R was maintained at more than 91% for temperatures of 50−350 °C (Figure 5C). 3.2.2. Effects of H2O and SO2. Under real exhaust conditions, SO2 and water vapor would be present and would have a strong effect on the NOx removal efficiency.1,29,30 Thus, it is necessary to investigate the impact of H2O/SO2 on the NOx conversion over catalysts. For Mn−W/Ce−NO-R, the outlet N2O concentration is less than 2 ppm in the presence of 5.5 vol % H2O (Figure 5D). At the same time, the presence of 5.5 vol % H2O lowers the NOx conversion to 71% for Mn−W/ Ce−NO-R (Figure 5E). After the H2O supply is cut off, the F


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Figure 8. Mn 2p, W 4f, Ce 3d, and O 1s XPS spectra of Mn−W/Ce−Cl-C, Mn−W/Ce−NO-R, and Mn−W/Ce−Cl-R .

3.3. Reducibility. H2 TPR was used to investigate the redox properties of the as-prepared samples, and the results are presented in Figure 7. For Ce−Cl-R, Ce−Cl-C, and Ce−NO-R, the reduction peaks at 405−518 °C are related to the reduction of surface CeO2, whereas the peaks located at 762−782 °C are associated with the reduction of bulk CeO2 oxides.1,24 Notably, the temperature of surface CeO2 reduction in Ce−NO-R is lower than those in Ce−Cl-R and Ce−Cl-C, suggesting better low-temperature reducibility and stronger lattice oxygen mobility in Ce−NO-R. After the addition of manganese and tungsten oxides, an additional peak at 210−234 °C is observed in Mn−W/CeO2, corresponding to the successive reduction processes of MnO2 → Mn2O3 → MnO.34 At the same time, the reductive temperatures of surface and bulk CeO2 in Mn−W/ Ce−NO-R are shifted down to 420 and 735 °C. These results indicate that the introduction of Mn and W promotes metal−

support interactions and further facilitates the reduction of the catalysts. According to the H2 TPR results, the incorporation of manganese and tungsten oxides into CeO2 can induce lattice distortion and the formation of oxygen vacancies, as confirmed by the XRD and Raman results, leading to an increase in the oxygen mobility and, thereby, a higher redox efficiency. Furthermore, the lower reductive temperatures and higher peak areas in the case of Mn−W/Ce−NO-R (210 and 420 °C) compared to the cases of Mn−W/Ce−Cl-R (234 and 464 °C) and Mn−W/Ce−Cl-C (234 and 423 °C) suggest the generation of more reducible Mn and Ce species in the former catalyst, which contributes to the improvement of the lowtemperature catalytic activity. 3.4. Surface Properties. 3.4.1. XPS Studies. XPS measurements were performed to obtain information about the surface elemental compositions and valence states of the catalysts.35 G


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Figure 9. (A,C,E) Top and (B,D,F) side views of the optimized geometries of (A,B) Ce{111}, (C,D) Ce{110}, and (E,F) Ce{100} in oxygen vacancies (gray, Ce; red, O; blue, N; white, O atoms).

Ce{100}, are presented in Figure 9. The energy for the formation of an oxygen vacancy was calculated according to the equation Ev = Eoxygen vacancy system + 1/2EO2 − Efull system, where Eoxygen vacancy system, EO2, and Efull system represent the energy of the system containing an oxygen vacancy, the energy of an O2 molecule, and the energy of the full system,39,40 respectively.42,43 The calculated Ev values in CeO2 follow the trend Ce{111} (2.65 eV) > Ce{100} (2.04 eV) ≈ Ce{110} (2.02 eV). These results demonstrate that the generation of oxygen vacancies occurs more easily on the Ce{110} and Ce{100} surfaces, resulting in a rapid reaction between NO and O2. Notably, the Ev value of Mn−W/Ce{110} (1.40 eV) is more negative than that of pure CeO2{110}, demonstrating that oxygen vacancies are more easily generated in CeO2 after the addition of manganese and tungsten oxides, which is consistent with the Raman results. 3.5. O2 TPD-MS. Furthermore, the oxygen species in the redox reaction are usually transformed by the following pathways: O2(ads) → O2−(ads) → O−(ads) → O2−(latt).41,42 O2 TPD-MS experiments were performed on the as-prepared samples to demonstrate the presence of adsorbed oxygen species and the mobility of corresponding surface oxygen species (see Figure 10). Oxygen species desorbed at 99−161, 265−515, and 750−787 °C are ascribed to superoxide (O2−), O −(ads), and O2−(latt), respectively.42 Several oxygen

XPS spectra of thhe Mn 2p, W 4f, Ce 3d, and O 1s regions for Mn−W/Ce−Cl-R, Mn−W/Ce−Cl-C, and Mn−W/Ce−NO-R are shown in Figure 8. The Mn 2p3/2 XPS spectra suggest that Mn4+ (644.9 eV) and Mn3+ (641.9 eV) are the primary species, along with a third phase (647.1 eV), which is ascribed to the incomplete decomposition of manganese nitrate owing to the relatively low calcination temperature.35 The Mn4+/Mn3+ ratios in the as-prepared catalysts are listed in Table 2. The values for Mn−W/Ce−NO-R (1.78) and Mn−W/Ce−Cl-C (1.78) are higher than that for Mn−W/Ce−Cl-R (1.01). Moreover, it was reported that the binding energies of W6+, W5+, and W4+ are 37.3, 36.2, and 35.2 eV, respectively, for W 4f5/2 and 35.1, 33.6, and 32.9 eV, respectively, for W 4f7/2.36 The W 4f XPS spectra of Mn−W/Ce−Cl-C, Mn−W/Ce−NO-R, and Mn−W/Ce− Cl-R indicate that W is present in the form of W6+. For the Ce 3d XPS spectra, the peaks assigned as U′ and V′ are indicative of the 3d104f1 initial electronic configuration, matching well with surface Ce3+ species, whereas the as peaks assigned U, U″, U‴, V, V″, and V‴ are the representative of the 3d104f0 electronic configuration corresponding to surface Ce4+ species. The Ce3+/Ce ratio was calculated, and the results are included in Table 2. The value was found to be 14.10% for Mn−W/Ce−NO-R, but a bit lower, at 12.76% and 11.68% for Mn−W/Ce−Cl-R and Mn−W/Ce−Cl-C, respectively. The presence of the Ce3+ species could cause a charge imbalance and unsaturated chemical bonds on the catalyst surface, thereby giving rise to the promotion of surface oxygen vacancies.1,23 Surface oxygen vacancies are known to play a significant role in the adsorption and dissociation of O2, leading to the generation of highly active electrophilic O2.37 In the XPS spectra of the O 1s region, the peak at 530.0−530.2 eV can be assigned to lattice oxygen (Olatt),38 whereas that at 532.5−532.7 eV can be assigned to surface oxygen species (O2−, O22−, or O−) resulting from the adsorption of gaseous O2 into oxygen vacancies. The Oads/(Olatt + Oads) ratios (Table 2) of the catalysts follow the trend Mn−W/Ce−NO-R (69%) > Mn− W/Ce−Cl-C (64%) > Mn−W/Ce−Cl-R (57%), which is in good agreement with the sequence of their catalytic activities. Notably, the greater amount of oxygen vacancies in Mn−W/ Ce−NO-R is beneficial to the adsorption and activation of NO species, and NO reacts with active O species to produce adsorbed NO2, which is beneficial to the improvement of the low-temperature catalytic activity. 3.4.2. DFT Calculations of Surface Oxygen Vacancies. The optimized oxygen vacancy models of Ce{111}, Ce{110}, and

Figure 10. O2 TPD-MS of as-prepared samples: (a) Ce−NO-R, (b) Ce−Cl-R, (c) Ce−Cl-C, (d) Mn−W/Ce−NO-R, (e) Mn−W/Ce−ClR, and (f) Mn−W/Ce−Cl-C. H


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Figure 11. NH3 TPD-MS of (A) Ce−NO-R, Ce−Cl-R, and Ce−Cl-C and (B) Mn−W/Ce−NO-R, Mn−W/Ce−Cl-C, and Mn−W/Ce−Cl-R.

Figure 12. In situ DRIFTS of NH3 adsorption at different temperatures in (A) Mn−W/Ce−NO-R and (B) Mn−W/Ce−Cl-C (test conditions: 500 ppm NH3 and Ar).

desorption peaks are observed in Ce−Cl-R (142 and 265 °C), Ce−Cl-C (99 and 462 °C), and Ce−NO-R (161, 515, and 787 °C). All of the desorption peaks of Mn−W/Ce shift to lower temperatures compared to their counterparts for CeO2, owing to the substitution of Mn and W cations into the CeO2 lattice, which leads to a disordered surface structure and, in turn, enhances the mobility of the oxygen atoms and generates surface-active oxygen species. Additionally, the peak area at 200−500 °C in Mn−W/Ce−NO-R is higher than those in Mn−W/Ce−Cl-R and Mn−W/Ce−Cl-C, suggesting that the former catalyst contains more O−(ads) species. Additionally, these surface oxygen vacancies play important roles in the activation of O2 to form O2−, O22−, and O2− species. These partially reduced oxygen species are known to be electrophilic and catalytically active species in NOx removal for strengthening of the N−O bonds and weakening of the N−N bonds. 3.6. NH3 and NO adsorption. 3.6.1. NH3 TPD-MS. Surface acidity is crucial to the catalytic activity in low-temperature NOx removal;43 therefore, NH3 TPD-MS was employed to study the amounts of surface acid sites in the as-prepared samples. As shown in Figure 11, an NH3 desorption peak at low temperature (

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