TiO2 Catalyst with Preferentially Exposed Anatase

ACS Catal. , 2016, 6 (9), pp 5807–5815. DOI: 10.1021/acscatal.6b01121. Publication Date (Web): July 25, 2016. Copyright © 2016 American Chemical So...
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Advanced MnOx/TiO2 Catalyst with Preferentially Exposed Anatase {001} Facet for Low-Temperature SCR of NO Shengcai Deng,† Tingting Meng,† Bolian Xu,† Fei Gao,† Yuanhua Ding,‡ Lei Yu,*,†,‡ and Yining Fan*,† †

Key Laboratory of Mesoscopic Chemistry of MOE, Jiangsu Provincial Key Laboratory of Vehicle Emissions Control, Jiangsu Provincial Key Laboratory of Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Jiangsu Provincial Key Laboratory of Environmental Material and Environmental Engineering, School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, People’s Republic of China S Supporting Information *

ABSTRACT: MnOx/TiO2 (anatase) nanosheets (NS) with a preferentially exposed {001} facet was found to be a better catalyst for selective catalytic reduction (SCR) of NO than conventionally employed MnOx/TiO2 nanoparticles (NP) with the {101} facet preferentially exposed, affording both high NO conversion and high N2 selectivity at 80−280 °C. Further investigations indicated that Mn3+ as the major species on TiO2 (NS) was incorporated into octahedral vacancies with a lower polymerization degree, resulting in high catalytic activity for SCR and low activity for NH3 oxidation, thus restraining the undesirable N2O generation. In comparison, on the surface of TiO2 (NP), Mn4+ as the major species was incorporated into tetrahedral vacancies in a highly polymerized state, leading to lower NO conversion and lower N2 selectivity. The results indicate that it is possible to enhance the low-temperature SCR activity of the catalysts by tailoring the preferentially exposed facet of TiO2. KEYWORDS: selective catalytic reduction, nitric oxide, MnOx/TiO2 catalyst, TiO2 preferentially exposed facets, Mn dispersion, isotopic labeling experiment

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INTRODUCTION Nitric oxide (NO) is one of the major air pollutants in the atmosphere that cause acidic rain, photochemical smog, ozone depletion, greenhouse effects, etc. Elimination of this pollutant is considered to be a practical research topic with imperious demands.1−5 During the past decade, manganese oxide (MnOx) based catalysts have attracted much attention in this field because of their unique properties for the selective catalytic reduction (SCR) of NO with NH3 to harmless N2 at low temperatures, which allows SCR units to be placed at backward positions of dust removal devices or even of desulfurization units without reheating the flue gas.6−10 Among the reported works, MnOx/TiO2 has been shown to be one of the most promising MnOx-based catalysts due to their high catalytic activities in low-temperature SCR,11−13 owing to the unique acidity and redox properties of MnOx species over an anatase support.14 However, one major defect of MnOx/TiO2-catalyzed SCR is the generation of the byproduct N2O.15,16 A series of metal elements, such as Ce, Fe, Ni, Co, Cu, Ca, W, etc. were introduced as promoters to improve MnOx/TiO2 catalysts.17−19 Continuous investigations have shown that lowtemperature SCR performance was closely related to the MnOx dispersion state in the catalyst, which could be adjusted by introducing metal promoters.20,21 However, there have been few works on the relationship of the exposed facet of TiO2 © XXXX American Chemical Society

supports to MnOx/TiO2 activity. Generally, TiO2 (anatase) prepared through a sol−gel method has turned out to have primarily {101} facets exposed (>90%) according to the Wulff construction principle.22 Recently, methods to fabricate anatase with preferentially exposed {001} facets have been developed by introducing fluoride ion.23−27 Xie et al. prepared novel TiO2 nanosheets (NS) with 89% {001} facets exposed through a conventional hydrothermal method by adding hydrofluoric acid (HF).28 The mature fabrication method of TiO2 (NS) with preferentially exposed {001} facets facilitates further applications of this material in catalysis. Our group aims to develop green technologies with industrial application potential.29−33 Recently, we prepared a highly dispersed MnO x /TiO 2 (anatase) catalyst with preferentially exposed {001} facets and employed it in the SCR of NO. Herein, we wish to report our findings.

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RESULTS AND DISCUSSION The catalyst supports TiO2 (NS) and TiO2 (NP) were initially prepared through a HF hydrothermal method and a conventional sol−gel method, respectively. Transmission electron Received: April 19, 2016 Revised: July 23, 2016

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of (200) diffraction for TiO2 (NS) are attributed to the anisotropic growth of crystals along the [100] direction, indicating a predominant exposure of the {001} facets. The catalyst supports TiO2 (NS) and TiO2 (NP) were then impregnated with aqueous Mn(NO3)2 solution, dried, and then calcined at 400 °C to prepare the corresponding MnOx/TiO2 catalysts.36 Independent SCR reactions using MnOx/TiO2 (NS) and MnOx/TiO2 (NP) were initially performed in a low-temperature window (80−280 °C) with gas hourly space velocity (GHSV) of 50000 h−1. As shown in Figure 3, NO

microscope (TEM) images and high-resolution transmission electron microscope (HR-TEM) images are shown in Figure 1

Figure 1. TEM and HR-TEM images of TiO2 (NS) and TiO2 (NP): (A) TEM for morphology and proposed morphology model of TiO2 (NS); (B, C) HR-TEM of TiO2 (NS); (D) TEM morphology and proposed morphology model for TiO2 (NP); (E, F) HR-TEM of TiO2 (NP).

and Figure S1 in the Supporting Information.34 TiO2 (NS) clearly exhibited a uniform sheetlike morphology with an average length of ca. 50 nm and a mean thickness of ca. 7 nm (Figure 1A). Figure 1B,C confirmed the primary {001} facet exposure. It can be seen from Figure 1B,C that the d(101) spacing is 0.35 nm and the d(001) spacing is 0.235 nm, which are consistent with the values reported in the literature.35 For TiO2 (NP), Figure 1D shows an irregularly truncated octahedral morphology with average sizes of ca. 20 nm, while Figure 1E,F shows a d(101) spacing of 0.35 nm, indicating the preferential exposure of {101} facets. X-ray diffraction (XRD) was used to further verify the preferentially exposed facet of TiO2 (NS). The XRD patterns in Figure 2 showed the characteristic TiO2 (anatase) peaks of TiO2 (NS) and TiO2 (NP) samples. The observed broad peak of (004) diffraction and the narrower peak

Figure 3. SCR performances of (a) MnOx/TiO2 (NS) and (b) MnOx/ TiO2 (NP) at different temperatures with Mn loading at 1.6 mmolMnOx/100 m2TiO2.

conversions on MnOx/TiO2 (NS) catalyst at different temperatures were higher than those on MnOx/TiO2 (NP) (Figure 3A, curve a vs b). In comparison with conventional V2O5/TiO2 catalyst,5,37,38 MnOx/TiO2 catalyst could be employed at lower temperatures (100−300 °C vs 300−400 °C) and the NO conversion reached its highest value of 80% at ca. 200 °C (Figure 3A, curve a). The NH3 consumptions over both catalysts have also been investigated, and the results are shown in Figure 3A. For the MnOx/TiO2 (NS) catalyst, NH3 conversion was slightly lower than NO conversion, due to a slight excess of introduced ammonia in the feedstock (1100 ppm of NH3/1000 ppm of NO). However, for MnOx/TiO2 (NP), the NH3 conversion was obviously higher than NO conversion, which could be ascribed to the oxidation of ammonia by O2. N2 selectivityies of MnOx/TiO2 (NS)-

Figure 2. XRD patterns of TiO2 and MnOx/TiO2 catalysts with Mn loading at 1.6 mmol of MnOx/100 m2 of TiO2. 5808

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remarkable drop in SCR activity.40 Thus, the influence of SO2 and H2O on the catalytic performance was investigated and is shown in Figure 5. As shown in Figure 5A, the NO conversions

catalyzed reactions were also very remarkable, remaining higher than 90% even at 200 °C (Figure 3B, curve a). However, for MnOx/TiO2 (NP)-catalyzed reactions, the N2 selectivity decreased obviously with increasing reaction temperature and decreased to below 85% at 200 °C (Figure 3B, curve b). To further elucidate the relationships of MnOx loading amounts with catalyst activities, a series of catalysts with different Mn contents were tested in SCR at 160 °C (Figure 4).

Figure 5. SO2 resistance and 2.5 vol % H2O test over (a) MnOx/TiO2 (NS) and (b) MnOx/TiO2 (NP) catalyst with Mn loading at 1.6 mmol of MnOx/100 m2 of TiO2.

of MnOx/TiO2 (NS) and MnOx/TiO2 (NP) catalysts at 300 °C were initially 90% and 75%, respectively, and fell to 40% and 15% after treatment with 100 ppm of SO2 for 12 h and could not be recovered after cutting off SO2. The NO conversion showed a remarkable drop and decreased to 36% and 18% in 2 h in the presence of 250 ppm of SO2. To get a deeper insight into the intrinsic reasons that lead to the different SO2 resistance abilities, tests of temperature-programmed oxidation of SO2 by O2 were performed on both catalysts to investigate the oxidation ability of SO2 over MnOx/TiO2, and the results are shown in Figure 6; the results indicated that MnOx/TiO2 (NS) exhibited higher SO2 oxidation ability than MnOx/TiO2 (NP), implying that the SO2 oxidation ability might not be crucial for the distinct difference of SO2 resistance. However, the TG-DTA-MS results shown in Figure 7 and Figure S2 in the Supporting Information revealed that the amount of Mn sulfate over MnOx/TiO2 (NS) was lower and the as-formed Mn sulfate was easier to decompose in comparison to that over MnOx/TiO2 (NP), which leads to higher SO2 resistance of the MnOx/TiO2(NS) catalyst. As shown in Figure 5B, the activity of MnOx/TiO2 catalysts remained almost unchanged after the

Figure 4. NO conversion and N2 selectivity of (a) MnOx/TiO2 (NS) and (b) MnOx/TiO2 (NP) at 160 °C with different Mn loadings.

The results indicated that the MnOx/TiO2 (NS) catalysts were favorable than the MnOx/TiO2 (NP) catalysts (Figure 4A, curves a and b). The MnOx/TiO2 (NS) catalyst with Mn loading at 1.6 mmol of MnOx/100 m2 of TiO2 was shown to be the best catalyst, giving the highest NO conversion of ca. 60% (Figure 4A, curve a) and an excellent N2 selectivity of ca. 95% (Figure 4B, curve a). Under practical application conditions, the exhaust fumes contain trace amounts of SO2 and H2O that can induce catalyst poisoning and deactivation even after passing through the desulfurization devices.39 With the existence of SO2 in the NH3-SCR process, SO2 can be catalytically oxidized to SO3 by the catalyst, then SO3 can react with catalysts to irreversibly form metal sulfates or ammonium sulfates, i.e. NH4HSO4 and (NH4)2SO4, and these ammonium sulfates can physically block the catalyst bed and most of the active sites, resulting in a 5809

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MnOx/TiO2 (NS) was still observed after MnOx uploading, indicating that the support TiO2 (NS) was stable at 400 °C in air. The supported MnOx was suggested to be in its amorphous state because no characteristic diffraction peaks were observed. The BET surface areas of MnOx/TiO2 (NS) and MnOx/TiO2 (NP) are also given in Table 1. The surface areas of MnOx/ Table 1. Surface States and Properties of the Catalysts Mn loading catalyst

Mn/areaa

wt %

surface areab

NO TOFc

MnOx/TiO2 (NS)

0.4 0.8 1.6 2.0 0.4 0.8 1.6 2.0

1.2 2.4 4.8 6.0 1.2 2.4 4.8 6.0

53 53 53 48 48 46 38 30

1.1 0.8 0.7 0.4 0.9 0.5 0.4 0.3

MnOx/TiO2 (NP)

Figure 6. Temperature-programmed oxidation of SO2 over the MnOx/TiO2 catalysts with Mn loading at 1.6 mmol MnOx/100 m2 TiO2. a

Mn/area = mmol of MnOx/100 m2 of TiO2. bBET surface area (m2/ g): STiO2(NP) = 56 m2/g; STiO2(NS) = 53 m2/g. cTurnover frequency of SCR at 160 °C: (mol of NO converted)/((mol of Mn amount) s) × 10−3.

TiO2 (NS) with different Mn loadings were very similar, but for MnOx/TiO2 (NP), the surface area decreased obviously with increased Mn loadings, indicating that TiO2 (NP) particles were much easier to aggregate with increasing MnOx loading on its surface during calcination. X-ray photoelectron spectroscopy (XPS) experiments were undertaken to investigate the valence state of Mn species. As shown in Figure 8 and Table S1 in the Supporting Information,

Figure 7. TG-DTA-MS curves of (A) MnOx/TiO2 (NS) and (B) MnOx/TiO2 (NP) catalysts treated by SO2 during SCR reactions with Mn loading at 1.6 mmol of MnOx/100 m2 of TiO2.

Figure 8. X-ray photoelectron spectra of Mn 2p for (top) MnOx/TiO2 (NS) and (bottom) MnOx/TiO2 (NP) catalysts with Mn loading at 1.0 mmol of MnOx/100 m2 of TiO2.

water vapor was introduced, implying the superior waterresistance ability of both catalysts. XRD analysis was then utilized to investigate the crystalline structure of MnOx/TiO2 (NP) and MnOx/TiO2 (NS) (Figure S3 in the Supporting Information).34 Only TiO2 (anatase) characteristic diffraction peaks (2θ = 25.4, 37.9, 48.1°, JCPDS #21-1272) were found on MnOx/TiO2 (NS) and MnOx/TiO2 (NP) (Mn loading: 1.6 mmol of MnOx/100 m2 of TiO2). The sharp peak at 2θ = 48.1° indicated that the (002) diffraction of

Mn 2p3/2 peaks were deconvoluted to Mn4+ (642.0 eV), Mn3+ (641.0 eV), and Mn2+ (644.6 eV) in the surface layer of MnOx/ TiO2 catalysts.34,41−43 Deconvolution results in Figure 8 showed that Mn3+ oxide existed on MnOx/TiO2 (NS) as the major species. However, for MnOx/TiO2 (NP), Mn4+ was the major species with a small amount of of Mn3+. For MnOx/TiO2 (NS) catalyst, a very weak shoulder peak around 647 eV could 5810

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220 and 360 °C originating from desorption of NH3 adsorbed on weakly acidic sites and strongly acidic sites, respectively. It has been confirmed that NH3 adsorbed on weakly acidic sites of MnOx could react with NO to produce N2 through the Eley− Rideal mechanism at low temperatures, while NH3 adsorbed on the strongly acidic sites was considered to not be involved in low-temperature SCR.48 The in situ NH3-adsorption IR and NH3-TPD results showed that the amount of weak Lewis acid sites on MnOx/TiO2 (NS) was higher than that on MnOx/ TiO2 (NP), leading to the higher low-temperature SCR activity of MnOx/TiO2 (NS) catalyst. Figure 10 shows the Mn/Ti molar ratios on surface layers of the catalysts determined by XPS. It is noteworthy that Mn/Ti

be ascribed to the shakeup satellite of Mn3+ species.44,45 However, no shakeup satellite showed up over MnOx/TiO2 (NP), as the surface MnOx was constituted mainly of Mn4+ species. It should be noted that the dispersed MnOx species on the anatase titania prepared by a conventional sol−gel method existed mainly as Mn4+.18,41,42 In comparison, the dispersed MnOx species on the anatase titania with preferentially exposed {001} facets existed mainly as Mn3+, which is possibly one of the key factors that is responsible for the high activity and selectivity of MnOx/TiO2 (NS) catalysts. In the NH3-SCR process, the adsorption and activation of NH3 on surface acid sites of the catalysts were generally considered to be the primary processes of the reaction and were closely related to the MnOx state.46 Thus, in situ NH3 adsorption IR and NH3-temperature-programmed desorption (NH3-TPD) experiments were then carried out to further investigate the acidic properties of the materials. In situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) of NH3 adsorption were measured at different temperatures, and the corresponding results are shown in Figure 9. Both catalysts mainly consist of Lewis acidic sites, as the absorption bands appeared at 1600−1610 cm−1 and 1170−1190 cm−1.47 Furthermore, the NH3-TPD curves in Figure S4 in the Supporting Information reveal that the number of Lewis acidic sites on MnOx/TiO2 (NS) is higher than that on MnOx/TiO2 (NP).34 The NH3-TPD curves were fitted into two peaks at

Figure 10. Mn/Ti atomic ratio in the surface layer of the catalyst determined by XPS analysis: (A) MnOx/TiO2 (NS) and (B) MnOx/ TiO2 (NP) catalysts.

molar ratios increase linearly along with Mn loadings but at different rates, and the two straight lines come across at different Mn loadings, from which the dispersion capacity of MnOx on TiO2 was determined to be 1.6 mmol of MnOx/100 m2 of TiO2 for MnOx/TiO2 (NP) and 1.2 mmol of MnOx/100 m2 of TiO2 for MnOx/TiO2 (NS), respectively. To discuss the dispersion state of oxide species, the surface structure of supports as one of the key factors should be taken into consideration. As suggested by the incorporation model, dispersed metal cations might incorporate into the vacant sites on the surface of the support with the accompanying

Figure 9. In situ NH3 adsorption DRIFT spectra over (A) MnOx/ TiO2 (NS) and (B) MnOx/TiO2 (NP) catalysts with Mn loading at 2.4 mmol of MnOx/100 m2 of TiO2. 5811

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ACS Catalysis anions as capping anions for charge compensation.49 It has been well established that anatase has a distorted NaCl structure. As shown in Figure 11A,B, the surface vacancy on

similar six-line hyperfine structures, while the Mn3+ species was ESR silent. The hyperfine coupling constant was about 76 G for Mn4+ and 80−100 G for Mn2+.53 As indicated in Figure S6 in the Supporting Information, the characteristic Mn4+ ESR hyperfine structure (g = 2.0, A = 75 G) over MnOx/TiO2 (NP) disappeared due to the strong electron dipole−dipole interaction among the dense MnOx species on the TiO2{101} surface. Thus, the ESR signal of MnOx/TiO2 (NP) only exhibited a single line with Lorentzian shape.34,54,55 The MnOx species over MnOx/TiO2 (NS) catalyst were mainly incorporated into sparse octahedral vacancies, leading to a lower polymerized degree of MnOx species and more acidic sites with weaker acidity and, thus, leading to the higher SCR activity. As given in Table 1, the obtained NO-TOF value for MnOx/TiO2 (NS) estimated on the basis of the total amount of MnOx was higher than that of MnOx/TiO2 (NP). H2-temperature-programmed reduction (H2-TPR) was taken to investigate the redox properties of MnOx species over MnOx/TiO2 catalysts, and the results of a series of samples are shown in Figure 12. Figure 12C shows that pure MnO2 displays two peaks at ca. 350 and 520 °C, which can be assigned to be the successive reduction steps of MnO2 to Mn2O3 and Mn2O3 to MnO, respectively.41,56,57 As shown in Figure 12A,B, there were two reduction peaks for MnOx/TiO2 catalyst: the broad peak around 200−500 °C indicated the step reduction from MnO2 to MnO, while the H2 consumption peaks around 590 °C in Figure 12B and around 650 °C in Figure 12C were the characteristic signals of the reduction of Ti4+ to Ti3+. The H2 consumption peaks apprearing at 325 and 406 °C for MnOx/ TiO2 (NP) catalysts could be ascribed to the reduction of MnO2 to Mn2O3 and Mn2O3 to MnO.43,44 Obviously, in comparison with MnOx/TiO2 (NP) catalyst, the characteristic H2 consumption peak corresponding to the reduction of Mn4+ to Mn3+ on MnOx/TiO2 (NS) catalyst shifted to higher temperature region (e.g, 350 vs 325 °C), indicating a decrease in its reducibility. In order to provide more information on the reduction steps of MnOx in the TPR process, ex situ XPS tests were performed after the MnOx/TiO2 catalyst was treated at 300 °C with 5 vol % H2/Ar for 10 min, and the results wereare shown in Figure S7 in the Supporting Information. For the fresh MnOx/TiO2 (NP) sample, the MnOx was composed of mainly Mn4+ with a small amount of Mn3+. Most of the Mn4+ species over MnOx/TiO2 (NP) was reduced to Mn3+ after H2/ Ar treatment, and the characteristic shakeup satellite of Mn3+ at 647 eV appeared. In comparison, the valence state of MnOx species over MnOx/TiO2 (NS) showed no obvious variation. It has been revealed that a MnOx species with low polymerization degree has lower reducibility in comparison to a highly polymerized MnOx species.58,59 In addition, it should be noted that more Mn 3+ species lead to lower total H 2 consumption peak area for MnOx/TiO2 (NS) catalyst, as revealed in Figure 12A. The TPR result strongly suggests that lattice oxygen ions in the MnOx/TiO2 (NP) catalyst have mobility higher than those of MnOx/TiO2 (NS), which results in different reactivities for oxidation of NH3 and thus different selectivities for the SCR reaction. As illustrated by eqs 1−7 in Figure 13, in the SCR process, NH3 was first oxidized to activated intermediate species NH2 (eq 1), which led to N2 through a reaction with NO (eq 2) or through a dimerization process (eq 3). However, the deep oxidation of NH3 might generate NH (eq 4), which leads to the undesired byproduct N2O through the reaction with NO (eq 5) or O2− (eqs 6 and 7).46,60,61 The higher mobility of lattice

Figure 11. Incorporation model for exposed facets along with the local configuration of vacancy sites on the TiO2 surface.

{101} facets and {001} facets consists of a tetrahedral vacancy and an octahedral vacancy, respectively.50,51 On the basis of the lattice constants of anatase TiO2, i.e., a = b = 0.37852 nm and c = 0.95139 nm,52 it is estimated that the density of the tetrahedral vacancy site on the {101} facet is 1.70 mmol of sites/100 m2 of TiO2 and the density of the octahedral vacancy site on the {001} facet is 1.16 mmol of sites/100 m2 of TiO2, as shown in Figure S5 in the Supporting Information, respectively.34 These values are almost identical with the experimentally measured dispersion capacities of MnOx on TiO2 (NP) and TiO2 (NS), as mentioned above. The incorporation model suggested that the dispersed manganese cations might locate on the vacant site on anatase with the accompanied oxygen anions positioning on the top, as shown schematically in Figure 11C,D. Furthermore, it should be noted that the surface tetrahedral vacancy site density on the TiO2 {101} surface was about 1.5 times as much as that of the octahedral vacancy sites on the TiO2 {001} surface, leading to a higher density of dispersed MnOx species on TiO2 {101} surface in comparison to that on TiO2 {001}. The electron spin resonance (ESR) experiments were taken to monitor the paramagnetic MnOx species. It was clear that isolated Mn2+ and Mn4+ species with high dispersion exhibited 5812

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oxygen ions in the MnOx/TiO2 (NP) catalyst facilitated the cleavage of N−H bonds of NH3 molecules to give N2O formation through the reaction of −NH species with gaseous NO. In contrast, the lower mobility of lattice oxygen ions in the MnOx/TiO2 (NS) catalyst resulted in more −NH2 species to react with gaseous NO to give more N2 formation, which was further verified by NH3 oxidation TPSR experiments. As shown in Figure 14A, NH3 consumptions on MnOx/TiO2 (NS) were slightly lower than those on MnOx/TiO2 (NP),

Figure 14. TPSR profiles of NH3 oxidation over MnOx/TiO2 catalysts with Mn loading at 2.4 mmol of MnOx/100 m2 of TiO2.

indicating a lower catalytic activity for NH3 oxidation, which resulted in higher N2 selectivity (Figure 14B) and restrained the generation of overoxidation products N2O and NO (Figure 14C,D). During the NH3 oxidation process, the surface oxygen consumed by NH3 over the surface Mn−O bond can be compensated by gaseous oxygen. On the basis of the Eley− Rideal mechanism, N2O and N2 were mainly produced through the coupling reaction of gaseous NO with oxidation NH3 intermediate −NH or −NH2 species; their selectivity was consistent with the NH3 oxidation ability.62,63 The temperature-programmed surface reaction (TPSR) of isotopically labeled 15NH3 with NO and O2 was then employed to provide essential hints. As depicted in Figure 15B, for the reaction catalyzed by MnOx/TiO2 (NP), 15N14NO was detected at 150 °C, indicating that the generation of 15 14 N NO from the reaction of 15NH3 with NO started at this temperature. The oxidation of 15NH3 by O2 began at 225 °C, where the characteristic 15N15NO signal was observed. However, for reactions using MnOx/TiO2 (NS) catalyst (Figure 15A), the intensity of 15N14NO was lower, while the generation of 15N15NO and 15NO vanished, suggesting a decrease in NH3 overoxidation. Thus, it could be concluded that the lower polymerization degree Mn species on MnOx/ TiO2 (NS) possessed weaker NH3 oxidation ability, leading to

Figure 12. H2-TPR profiles of (A) MnOx/TiO2 (NS) and (B) MnOx/ TiO2 (NP) catalyst with different MnOx loadings (mmol of MnOx/ 100 m2 of TiO2, marked above the curves) and (C) pure TiO2 and MnO2 as reference.

Figure 13. Proposed reaction scheme for the SCR reaction.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01121. Detailed experimental procedures and additional spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for L.Y.: [email protected]. *E-mail for Y.F.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP-2012009111001), the NNSFC (21202141, 21173182), the Opening Foundation of the Key Laboratory of Environmental Materials and Engineering of Jiangsu Province (K14010), the Key Science & Technology Specific Projects of Yangzhou (YZ20122029), and Yangzhou Nature Science Foundation (YZ2014040) for financial support.

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ABBREVIATIONS NO, nitric oxide; SCR, selective catalytic reduction; GHSV, gas hourly space velocity; HR-TEM, high-resolution transmission electron microscopy; ESR, electron spin resonance; XRD, X-ray powder diffraction; TOF, turnover frequency; TPD, temperature-programmed desorption; XPS, X-ray photoelectron spectroscopy; TPR, temperature-programmed reduction; TPSR, temperature-programmed surface reaction; in situ DRIFTS, in situ diffuse reflectance infrared Fourier transform spectra; TG-DTA, thermogravimetry-differential thermal analysis

Figure 15. Isotopically labeled TPSR results for the reaction of 14NO with 15NH3 + O2 for (A) MnOx/TiO2 (NS) and (B) MnOx/TiO2 (NP) catalysts with Mn loading at 1.2 mmol of MnOx/100 m2 of TiO2.

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REFERENCES

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lower activity for the oxidation of NH3 to N2O and thus leading to higher N2 selectivity in the SCR reaction. In conclusion, MnOx/TiO2 (NS) with preferentially exposed {001} facets was found to be a better catalyst for the SCR reaction of NO than the conventionally employed MnOx/TiO2 (NP) with {101} facets preferentially exposed. The mechanisms have been well investigated in this article through a series of experiments. It was found that, in MnOx/TiO2 (NP), MnOx was mainly incorporated into tetrahedral vacancy with Mn4+ as highly polymerized species with monolayer dispersion capacity of 1.6 mmol of MnOx/100 m2 of TiO2. However, for MnOx/ TiO2 (NS) catalyst, MnOx was mainly incorporated into octahedral vacancy sites with major Mn3+ species with a monolayer dispersion capacity of 1.2 mmol of MnOx/100 m2 of TiO2. The MnOx/TiO2 (NS) catalyst possessed lower polymerization degree MnOx species and more weak Lewis acid sites than MnOx/TiO2 (NP), leading to higher SCR activity at low temperatures. In addition, the lower polymerization degree Mn species on MnOx/TiO2 (NS) catalyst possessed weaker NH3 oxidation ability to cleave H atoms from NH3 to form N2O with gaseous NO and thus resulted in higher N2 selectivity. 5814

DOI: 10.1021/acscatal.6b01121 ACS Catal. 2016, 6, 5807−5815

Research Article

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DOI: 10.1021/acscatal.6b01121 ACS Catal. 2016, 6, 5807−5815