Comparison of the Structures and Mechanism of Arsenic Deactivation

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Comparison of the Structures and Mechanism of Arsenic Deactivation of CeO2–MoO3 and CeO2–WO3 SCR Catalysts Xiang Li, Junhua Li, Yue Peng, Xiansheng Li, Kezhi Li, and Jiming Hao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03687 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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The Journal of Physical Chemistry

Comparison of the Structures and Mechanism of Arsenic Deactivation of CeO2–MoO3 and CeO2–WO3 SCR Catalysts Xiang Lia, Junhua Lia,*,Yue Penga, Xiansheng Li, Kezhi Li and Jiming Haoa a

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China

* Corresponding author: E-mail address: [email protected] Tel.: +86 10 62771093, fax: +86 10 62771093

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Abstract The mechanism of arsenic poisoning of CeO2–WO3 (CW) and CeO2–MoO3 (CM) catalysts during the selective catalytic reduction (SCR) of NOx with NH3 was investigated. It was found that the ratio of activity loss of the CW catalyst decreases as the temperature increases, while the opposite tendency was observed for the CM catalyst. The fresh and poisoned catalysts were characterized using X-ray diffraction (XRD) temperature-programmed reduction with H2 (H2-TPR), X-ray photoelectron spectra (XPS), NH3-temperature-programmed desorption (NH3-TPD), in situ DRIFTS and in situ Raman spectroscopy. The results indicate that arsenic oxide primarily destroys the structure of the surface CeOx species in the CM catalyst but prefers to interact with WO3 in the CW catalyst. Additionally, the BET surface area, the number and stability of Lewis acid sites and the NOx adsorption for these two types of catalysts clearly decrease after deactivation. According to the DRIFTS and Raman investigations, at low temperatures, the greater number of sites with adsorbed NH3 in the poisoned CM catalyst leads to less loss of activity than the poisoned CW catalyst. However, at high temperatures, the greater number of Lewis acid sites remaining in the poisoned CW catalyst may play an important role in maintaining the activity of this catalyst.

1. Introduction NOx are a significant type of air pollutants that are associated with many environmental problems, such as dust haze, climatic change, photochemical smog and so on.1-3 Among the technologies for NOx abatement, selective catalytic reduction (SCR) with NH3 is regarded as 2 ACS Paragon Plus Environment

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the most effective method and has been widely used in coal-fired power plants and industrial boilers since the 1970s.4-8 However, the traditional V2O5–WO3(MoO3)/TiO2 catalysts have some inevitable problems, namely, the biological toxicity of vanadium species, an easy deactivation under specific flue gas conditions and excessive working temperatures. After considerable comparison and testing, cerium-based catalysts have been widely accepted as promising candidate substitutes for conventional catalysts because these materials are nontoxic and have a large BET surface area and remarkable oxygen storage capacity in the NH3-SCR reaction.9-17 Compared with other varieties of Ce-based composite oxide catalysts, the CeO2– WO3 (CW) and CeO2–MoO3 (CM) catalysts present better NH3-SCR activity, with nearly 100% N2 selectivity, a wider temperature range (250-400 °C) and stronger resistance to H2O and SO2.12, 14 Arsenicals that exist as arsenic trioxide in flue gas are dangerous toxicants for commercial SCR catalysts.18 Recently, several reports have focused on the role of the arsenic poisoning mechanism in V-based catalysts. Hums et al. have argued that gaseous arsenic oxide, existing as As2O3 or its dimer As4O6, could diffuse into the pore channels of catalysts and block their micropores.19 Based on an X-ray absorption and diffuse reflectance infrared spectroscopy study, Hilbrig et al. have found that arsenic in a high oxidation state (V) can interact with active sites in vanadium, thus depleting the surface acid sites.20 Additionally, our recent work on As poisoning and regeneration indicated that the deactivation is related to the concentration of As2O3, the number of Lewis acid sites and the reducibility of the catalyst.21 However, few studies have systematically elucidated the factors influencing active sites and the deactivation 3 ACS Paragon Plus Environment


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mechanism in As-poisoned Ce-based catalysts. It is well known that MoO3 has greater resistance than WO3 to the As poisoning of V2O5/TiO2, but there is still no agreement on the specific mechanism of Mo improvement effect. Recently, on the basis of experiments and DFT calculations, Peng et al. proposed that the greater dispersion of MoO3 on the TiO2 support and reactivity of the surface Mo–O–Mo and Mo=O groups than those in WO3, were the reasons for the resistance of MoO3.22 To better improve the applicability of CW and CM catalysts, it is necessary to investigate and compare the effect of As on these two types of catalysts. Although MoO3 has been proven to be more effective for resisting deactivation by As in V-based catalysts, it is important to study whether this effect is also present in various Ce-based catalysts. Moreover, the study of the deactivation of CW and CM catalysts is very different from traditional V-based catalysts. Firstly, WO3 or MoO3 should be considered the primary active center rather than the catalytic promoter for these two catalysts. Secondly, Ce-based catalysts have wider temperature windows; therefore, the deactivation effect and mechanism at both low and high temperatures are worthy of intense investigation. In this study, fresh and poisoned CW and CM catalysts were prepared to investigate the poisoning mechanism by comparing changes in the structural features, redox properties, acidity, surface-adsorbed species and active sites during the SCR process. We also attempted to clarify the relationship between the surface species and deactivation mechanism at both low and high temperatures.


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2. Experimental 2.1. Catalyst preparation and poisoning CW and CM catalysts with Ce/W and Ce/Mo molar ratios of 3:2 were prepared by a conventional co-precipitation method that has been previously reported.12,

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In a typical

synthesis, 2.605 g of Ce(NO3)3·6H2O and 1.078 g of (NH4)6W7O24·6H2O (CW) or 0.706 g (NH4)6Mo7O24·4H2O (CM) were dissolved in water, and excess (NH4)2CO3 solution was slowly added dropwise to the solution mixture with vigorous stirring until the pH was 9. After stirring the solution for 6 h, the precipitate was filtered and then washed with DI water several times, followed by drying overnight at 120 °C and calcination at 500 °C for 4 h. The poisoned samples were obtained by immersing the samples in As2O3 solutions. In a typical synthesis, 30 mL of the As2O3 aqueous solution (2.1 g/L) was added to a 50 mL beaker containing 2 g of fresh catalyst. The mixture was stirred for 2 h, dried at 100 °C for 10 h, and then calcined at 450 °C for 3 h. The poisoned samples of the CW and CM catalysts were denoted by “CWP” and “CMP”, respectively. 2.2. Catalyst activity Activity measurements were performed in a fixed-bed quartz reactor (I. D. = 6 mm) using 100 mg of catalyst at atmospheric pressure. The feed gas mixture was controlled as follows: 500 ppm NO, 500 ppm NH3, 3% O2, 200 ppm SO2 (when used), 5% H2O (when used) and N2 as the balance gas. The outlet gas concentrations of NO, NO2, NH3, and N2O were continually monitored using a Fourier transform infrared (FTIR) spectrometer (MultiGas TM 2030 FTIR Continuous Gas Analyzer). The data for the steady-state activity of the catalysts were recorded 5 ACS Paragon Plus Environment


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after 40 min at each temperature. 2.3. Catalyst characterization The N2 sorption isotherm and BET surface area of the samples were acquired at 77 K with a Micromeritics ASAP 2020 apparatus. The crystal structure was determined with an X-ray diffractometer (Rigaku, D/max-2200/PC). X-ray photoelectron spectra (XPS) were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific. H2-TPR and NH3-TPD experiments were performed using a chemisorption analyzer (Micromeritics, ChemiSorb 2920 TPx) and an FTIR spectrometer (MKS, MultiGas 2030HS), respectively. In situ IR spectra were recorded on a FTIR spectrometer (Nicolet NEXUS 6700). The spectra were recorded as averages of 32 scans at a resolution of 4 cm-1. Additionally, in situ Raman spectra were recorded with a Raman microscope (InVia Reflex, Renishaw) equipped with a deep-depleted, thermoelectrically cooled, charge-coupled device (CCD) array detector, and the 532-nm line of the laser was used for recording the Raman spectra. 3. Results 3.1. Catalytic activity The ratios of NO conversion normalized to the BET surface area for two types of catalysts under a high GHSV of 240,000 h-1 were calculated and are presented in Fig. 1; their balanced NO conversions are shown in Fig. S1. It is noteworthy that the loss ratio for the CW catalyst decreases with an increase in temperature (from 59.0% at 200 °C to 0.9% at 450 °C), although the difference between the fresh and poisoned catalysts narrows. In contrast, the reverse trend is observed for the CM catalyst (from 15.5% at 250 °C to 34.6% at 450 °C). Despite a reduction of over 30% in the ratio of NO conversion at 200 °C, the CM catalyst exhibits better low-temperature anti-poisoning performance than CW because 6 ACS Paragon Plus Environment

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the CW catalyst has approximately twice the loss ratio of CM at 200 °C. This indicates that the arsenic oxide deactivation effect is different for the two types of catalysts and seems to be related to the reaction temperature. 3.2. Structural characteristics (N2-physisorption, XRD and Raman spectra) As listed in Table 1, the BET surface areas and pore volumes for poisoned catalysts are obviously reduced compared with fresh catalysts, whereas their pore diameters increase slightly for both types of catalysts. According to the N2 adsorption curves shown in Fig. S2, the poisoned samples have lower N2 adsorption capacity with H3-type hysteresis loops.24 It can be deduced that the deposition of As2O3 blocks the channels to some degree, which hinders the progress of the reaction. The XRD patterns of the catalysts are presented in Fig. 2. All the curves display typical diffraction patterns for cerianite (PDF-ICDD 43-1002). Several peaks, attributed to Ce2Mo3O13 (PDF-ICDD 31-0331), hexagonal-phase Ce2O3 (PDF-ICDD 44-1086) and cubic-phase Ce2O3 (PDF-ICDD 49-1458), were observed for the CM catalysts. No visible phase of the WO3 species was observed in either the fresh or poisoned CW catalysts, signifying that WO3 can be better incorporated into the CeO2 lattice to form WxCe1−xO2−δ solid solutions with a short-range order.25 The grain sizes of CeO2 on the CM and CW catalysts were also calculated using the Scherrer equation based on the (111) plane (Table 1). A larger CeO2 crystallite size was acquired in the CMP catalyst. However, for the CWP catalyst, the crystallite size was nearly unchanged after the deposition of arsenic oxide. It was postulated that the As-poisoning effect on Ce is likely more serious for CMP than CWP. To confirm this conjecture, Raman spectroscopy was used to explore the changes in chemical bonds and functional groups 7 ACS Paragon Plus Environment


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before and after deactivation. As shown in Fig. S3, for the CM system, the main peak of the cubic-phase CeO2 at 460 cm-1, due to the F2g mode, is reduced sharply after deactivation, but this peak remains nearly the same after As poisoning of the CW catalyst.26 Moreover, compared with the fresh catalyst, CWP exhibits an obvious blue shift from 958 cm−1 to 968 cm−1,corresponding to the W=O stretching modes of the isopolytungstate species. Additionally, some new peaks at 804 [νas(W–O)] and 636 cm−1 [νs(W–O–W)] attributed to the octahedral unit can also be shown.27-28 These results indicate that the deactivation of CeO2 can be mitigated by the introduction of WO3 due to the stronger interaction between As2O3 and WO3. 3.3. Redox properties (H2-TPR and XPS) H2-TPR is a good method to explore both the reducibility of metal oxide catalysts and the potential to take up oxygen, based on the changes in peak position and hydrogen consumption. Fig. 3 presents the H2-TPR profiles of fresh and poisoned catalysts. The pristine CM catalyst has two apparent reduction peaks centered at 574 and 787 °C, assigned to the reduction of surface Ce4+ (to Ce3+) and bulk Ce4+ (to Ce3+), respectively.29-30 In comparison, the CW catalyst exhibits a weaker second reduction peak with the same onset consumption temperature (365 °C). However, after the introduction of As, the Ce4+ reduction peaks of CW are replaced by a new, large band centered at 636 °C. Under the same conditions, the first peak of CM moves to a higher temperature (610 °C), and the second peak shifts to a lower temperature (707 °C). Moreover, the amounts of H2 consumption, normalized according to the surface area, for both series of samples increased significantly after doping with reducible arsenic (Table 1). Therefore, this indicates that the reducibility of the catalyst increases due to the simultaneous 8 ACS Paragon Plus Environment

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reduction of As2O3, which may promote the production of N2O as a side reaction, similar to previous reports of As-poisoned V-based catalysts.21 Surface information such as element valences and atomic concentrations can be obtained through XPS. The deconvoluted Ce3d XPS results of two series of catalysts are shown in Fig. 4(a). As illustrated in the figure, the spin-splitting peaks correspond to the Ce3d5/2 spin-orbit components (denoted by ‘‘v’’) and the Ce3d3/2 spin-orbit components (denoted by ‘‘u’’). The bands marked by v, v’’, v’’’, u, u’’, and u’’’ (blue colored curves) can be ascribed to surface Ce4+ species with the 3d104f0 electronic state, whereas the others, labeled as v’ and u’ (red colored curves), can be attributed to surface Ce3+ atoms corresponding to the 3d104f1 initial electronic configuration.31 It can be seen that the peak positions of Ce3d are minimally changed in the CWP catalyst with only reductive surface Ce4+. In contrast, these positions for the CMP catalyst are shifted toward a lower BE by 0.7 eV, with a decreased surface Ce4+/Ce3+ ratio. Considering that the surface atomic concentrations of Ce4+ and the ratio of Ce4+/Ce3+ do not change significantly (Table 2), either the surface coverage resulting from As atoms or the toxic effect on WOx may play the key role in CW catalyst deactivation. However, because the surface Ce4+ concentration decreases more sharply than the surface Ce3+ concentration for the CM catalyst (Table 2), the greater loss of surface Ce4+ atoms may be the main reason for the deactivation of the CM catalyst. The binding energies of W 4f7/2 and W 4f5/2, centered at 35.6 and 37.8 eV (Fig. 4(b)) for the CW catalyst, were identified as the energies of W6+, the values of which are close to those reported by other researchers.12, 32 Based on previous reports, a shift to a lower BE in XPS is 9 ACS Paragon Plus Environment


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usually associated with an increased electron density or reduction to a lower oxidation state.16 Compared with the fresh catalyst, the W 4f peak position of the CWP catalyst shifts by 0.23 eV toward a lower BE, suggesting that some W6+ could be reduced by low-valent As species. However, no significant changes in the Mo3d peak positions were observed in the CMP sample compared with the CM sample. These findings suggest that the chemical environment of W is influenced more easily than that of Mo after the deposition of As. The XPS of As3d for the deactivated catalysts are shown in Fig. 4(c). Two characteristic peaks can be observed by fitting the curves: the first peak, centered at 44.6-44.9 eV, can be assigned to As3+, and the second peak, centered at approximately 45.6 eV, corresponds to As5+.33 According to the estimation of the peak areas, 6.49% of the As5+ species exist on the surface of the CMP catalyst, while the surface As atoms on the CWP catalyst are mainly composed of the As3+ species. Additionally, the O1s XPS for the investigated catalysts are presented in Fig. S4. It is clear that the surface concentration of oxygen ‘‘Oβ’’ increases in the deactivated catalysts, which may be primarily due to As–OH deposited on the catalyst surface. Therefore, the suppressive effect of chemisorbed oxygen species provided by CeO2 active sites could not be obviously observed in the O1s XPS due to As–OH compensation. 3.4 Surface acidity Surface acidity is another critical aspect for evaluating the SCR performance. The influence of the added As species on the amount and strength of the acid, based on NH3-TPD experiments, is illustrated in Fig. 5. Both the fresh and poisoned samples exhibit one NH3 desorption peak centered below 280 °C, which corresponds to medium-strong acid site for the catalysts. It can 10 ACS Paragon Plus Environment

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be seen that the desorption peak areas of the CW catalysts clearly decrease over the entire temperature range after the introduction of As. Moreover, there is a greater loss of the CMP catalyst at low temperatures than at high temperatures, resulting in a peak shift toward higher temperatures, in contrast to the CWP catalyst. The above results imply that the addition of As2O3 decreases both the strong and weak acid sites of the CW catalyst, while in the CM catalyst, there is a greater loss of weak acid sites than of strong acid sites. In situ DRIFTS experiments were also employed to further explore the NH3 desorption behavior of the catalysts, as shown in Fig. S5. The bands at 1192 cm−1 and 1584 cm−1 can be assigned to the Lewis acid sites in the CM catalyst, while those at 1204 cm−1 and 1595 cm−1 are assigned to the Lewis acid sites in the CW catalyst. Additionally, the peaks centered at 1420 and 1662 cm−1 can be attributed to the Brønsted acid sites in the CM catalyst, and those at 1426 and 1677 cm−1 can be attributed to the Brønsted acid sites in the CW catalyst34. In the region of stretching vibrations, three small peaks in the range of 3150-3580 cm−1 belong to the νas (N–H), νs (N–H), 2δas (H–N–H) and 2δs (H–N–H) modes of ammonia adsorbed onto the Lewis acid sites.35-36 Moreover, the band at 1540 cm-1 is ascribed to the amide species, which is an important intermediate species in the SCR process and can be observed for both types of catalysts.37 Two negative bands are also found at approximately 3620 and 3665 cm−1, and these may be ascribed to doubly coordinated and bridging O–H stretching modes, respectively.38-39 On the one hand, after poisoning by As2O3, the peaks corresponding to Lewis acid sites and amide species diminished remarkably and almost vanished as the temperature continually increased. Considering the NH3-TPD results, the loss of weakly adsorbed ammonia species 11 ACS Paragon Plus Environment


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(weak acid sites) may be related to the Lewis acid sites. Additionally, a new characteristic peak at 1534 cm−1 was observed above 250 °C, which may be associated with the oxidation intermediates of NH3, originating from arsenic oxide species.40 These results indicate that poisoning by As2O3 destroys the number and stability of Lewis acid sites as well as the reaction mechanism. On the other hand, the bands of the poisoned catalysts attributed to Brønsted acid sites seemed to increase slightly at low temperatures (below 100 °C), although these diminished more easily at elevated temperatures. At the same time, the negative band, attributed to the bridging –OH stretching frequency, was preserved up to 250 °C for the CWP catalyst. In contrast, the doubly coordinated –OH of the CMP catalyst was susceptible to temperature. Consequently, newly formed As–OH sites from As oxide deposition can supplement a portion of the Brønsted acid sites, but these As–OH sites break the original surface –OH and Brønsted acid sites. Furthermore, the stability of the Brønsted acid sites is decreased after deactivation. 3.5 NO + O2 adsorption Because adsorbed NOx can participate in the low-temperature SCR process through the Langmuir-Hinshelwood mechanism, the NOx adsorption behavior over the two types of catalysts was investigated using in situ DRIFTS.41-43 As shown in Fig. 6, after the adsorption of NO + O2 onto CW, seven bands appeared in the range of 1100-1700 cm-1, at 1597, 1579, 1546, 1527, 1453, 1301 and 1218 cm−1. The bands centered at 1301 and 1527 cm−1 can be ascribed to monodentate nitrate, while those at 1546 and 1579 cm-1 may imply that two types of bidentate nitrate exist. Furthermore, the bands at 1218, 1453 and 1597 cm−1 are assigned to chelating nitro compounds, monodentate nitrite and bridging nitrate, respectively.11, 12 ACS Paragon Plus Environment

44-45

After the

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introduction of As2O3, only the bridging nitrate species and nitro compounds were observed below 300 °C; the adsorption of NOx species was thus obviously suppressed. Above 300 °C, the band corresponding to bridging nitrates shifts to a higher wavenumber (1618 cm-1), with a weaker signal intensity. Unlike the CW catalyst, the CM catalyst has a single peak at 1564 cm-1 for bidentate nitrate and a new band at 1209 cm-1, which belongs to bidentate nitrite.34 In contrast, the inhibitory effect of As oxide on NOx adsorption appears to be more evident in the CMP than in the CWP catalyst because all the ad-NOx nearly vanished above 200 °C. However, previous studies have reported that stably adsorbed nitrate species can suppress the NH3 activation and weaken the low-temperature SCR process with the CM catalyst. Therefore, this may be a reasonable explanation for the higher low-temperature SCR activity of the CMP catalyst. 3.6 Investigation of reaction mechanism and deactivation details Further studies on changes in the reaction mechanism were carried out using in situ DRIFTS and in situ Raman spectroscopy. Fig. 7 and Fig. 8 display the DRIFTS spectra of two series of catalysts under a reactive atmosphere at 200 °C. As illustrated in Fig. 7(a), when the CW catalyst is treated with NH3, coordinated NH3 species (1172 and 1593 cm-1) and NH4+ species (1455 and 1677 cm-1) form on the CW surface.38 After switching the gas to NO + O2, both Lewis and Brønsted acid sites rapidly vanish in 3 min and are then replaced by new bands, attributed to NOx species (1242, 1488, 1577 and 1605 cm-1). When the reactants are introduced in a reversed order (Fig. 7(b)), the chelating nitro compounds (1236 cm-1), monodentate nitrate (1523 cm-1) and bidentate nitrate (1542 and 1579 cm-1) disappear within 1 min after the NH3 is 13 ACS Paragon Plus Environment


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purged. These results indicate that both Lewis and Brønsted acid sites can participate in the SCR reaction with ad-NOx species through the L-H process at low temperatures. However, although Brønsted acid sites (1438 and 1671 cm-1) still exist in poisoned catalysts (Fig. 7(c)), Lewis acid sites (1172 and 1593 cm-1) decrease significantly. After the catalyst is exposed to NH3, a small new band of adsorbed NH3 appears at 1302 cm-1, which may be related to the arsenic species. When the catalyst is purged with NO + O2, this band remains almost constant as the Brønsted acid sites are slowly consumed. Therefore, arsenic oxide not only suppresses the adsorption of NH3 on both Lewis and Brønsted acid sites but also provides nonreactive ad-NH3 sites. Furthermore, when the gaseous order is reversed, only a few weak bands due to monodentate (1296 cm-1), bidentate (1583 cm-1) and bridging nitrate (1605 cm-1) can be observed after the adsorption of NO + O2.34 These bands are soon overlapped by the adsorbed NH3. It can thus be concluded that the adsorption of nitrate species is also vastly inhibited due to the As-poisoning effect, and these species thus have minimal participation in the low-temperature SCR process. The DRIFT spectra of the CM and CMP catalysts were also recorded under the same reaction conditions (Fig. 8). For the fresh sample, NH3 adsorption onto Brønsted and Lewis acid sites and a few chelating species of nitro compounds were involved in the low-temperature SCR, which may primarily underwent the Eley-Rideal process between the adsorbed NH3 and gaseous NOx. However, for the CMP catalyst, the reaction rate of ad-NH3 on the Brønsted acid sites was slowed. Additionally, Lewis acid sites and nitro compound species disappeared, and only weak monodentate and bridging nitrates could be seen. Considering that the major active 14 ACS Paragon Plus Environment

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sites, Brønsted acid sites, remain, the influence of As on low-temperature activity is less serious than that in the CWP catalyst, which is in accordance with the activity data shown in Fig. 1. Fig. S6 presents the DRIFTS spectra of two series of catalysts in a flow of NO + O2 after the catalyst was pre-exposed to a NH3 atmosphere for 30 min at 350 °C; the Eley-Rideal process is widely regarded as the high-temperature SCR mechanism for CW and CM catalysts14, 25. When exposed to NH3, the CW, CWP and CM catalysts exhibited both Brønsted and Lewis acid sites, whereas CMP had only Brønsted acid sites. However, when NO and O2 were introduced, chelating nitro compounds, monodentate nitrate, bidentate nitrates and bridging nitrate (1236, 1481, 1544/1576 and 1598 cm-1) gradually emerged. Additionally, NH4NO3 (1368 cm-1) and H2O (1619 cm-1) also appeared as intermediates during the SCR reaction, with adsorbed NH3 species decreasing on the CW and CWP catalyst surfaces.41, 44 It must be noted that Lewis acid sites rarely participate in the reaction, and the reaction rate on Brønsted acid sites slows down with the emergence of the characteristic peak of NH3 oxidation (1534 cm-1) in the CWP catalyst. Despite the few remaining Brønsted acid sites on the CMP catalyst, the acid sites are greatly reduced because of the deposition of arsenic oxide. Therefore, the smaller amount of acid may play the key role in the greater loss of activity at high temperatures with the CMP catalyst. To further investigate the changes in active sites on the surface during the reaction mechanism, in situ Raman spectra were are also collected following the same procedure. For the CW catalyst (Fig. 9(a)), when the catalyst was purged with NO + O2 after pretreatment with NH3, the ʋs mode of the W=O vibrations due to WOx (998 cm-1) reemerged, while the characteristic crystalline CeO2 vibrations (457 and 248 cm-1) were obviously enhanced, 15 ACS Paragon Plus Environment


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indicating that NH3 can be chemisorbed onto the surface of both WOx and CeOx at low temperatures. Moreover, when the gaseous order was reversed, the band at 930 cm-1, attributed to the W=O vibrations of Ce2(WO4)3 compounds, and the band at 817 cm-1 due to the oxotungstate species were strengthened to some extent after the NH3 was introduced (Fig. 9(b)).25-26 Therefore, Ce2(WO4)3 compounds and W–O bonds from [WO4] or [WO6] units may also be active sites during the SCR reaction. Nevertheless, the changes described above become inconspicuous after deactivation by As oxide because the peak for the W=O vibration of WOx weakens. Compared to the results of the IR spectra, we tentatively assigned the Lewis acid sites to CeO2 and W=O from amorphous WOx. Correspondingly, we assigned the Brønsted acid sites to the hydroxyl formed on the W–O or W=O modes of both WOx and Ce2(WO4)3. Based on the acidic nature of tungsten along with the basic nature of ceria, these materials have different adsorption functions during the SCR reaction. Therefore, after the introduction of basic As2O3, the acid-base balance of fresh catalysts can be destroyed, thus affecting the NH3 adsorption onto WO3. The in situ sequential Raman spectra of the CM catalyst were also acquired under the same conditions (see Fig. S7), and similar results were observed, apart from the substitution of Ce2Mo3O13 (935 cm-1) for Ce2(WO4)3.46 Additionally, it should be noted that the band for the Mo=O stretching vibration at 962 cm-1 decreases with As deactivation. This decreasing trend is in accordance with the loss of Lewis acid sites shown before (Fig. 10).47 Therefore, the Lewis acid sites of the catalysts may be mainly provided by Mo=O bonds, although these sites seem to be less important than Brønsted acid sites for the low-temperature SCR process. Fig. 16 ACS Paragon Plus Environment

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S7 shows the in situ sequential Raman spectra of the two types of catalysts in a flow of NO + O2 after the catalyst is pre-exposed to a NH3 atmosphere at 350 °C. As shown in the figure, all the CeO2, WOx or MoOx and Ce2(WO4)3 compounds and Ce2Mo3O13 still participate in the high-temperature SCR reaction as NH3 adsorptive sites. After deactivation, the intensity of the Raman spectra of the CWP catalyst decreased under the same reaction atmosphere. Additionally, both the reaction rate and peak intensity of the CMP catalyst weakened despite a sustained reaction, suggesting a loss of Lewis acid sites originating from Mo=O that are essential for the high-temperature activity of the CM catalyst. These results also convincingly reflect the difference between the two catalysts in the high-temperature activity by deactivation. In particular, the deactivation effect of the CMP catalyst is more severe than that of the CWP catalyst. 4. Discussion 4.1 Influence of As on structure and properties Unlike our previous research on the As poisoning of the CeO2–WO3/TiO2 catalyst, no characteristic XRD peaks belonging to the arsenic oxide species occurred with the CMP and CWP catalysts (Fig. 2), suggesting that the arsenic oxide species can be highly dispersed on the surface of catalysts.48 Considering that the radius of As3+ (0.58 Å) or As5+ (0.46 Å) is less than that of Ce4+ (0.87 Å) combined with W6+ (0.62 Å) or Mo6+ (0.64 Å), the replacement of Ce4+ combined with W6+ or Mo6+ by As3+ or As5+ to form a solid solution is unexpected. Moreover, based on the Raman spectra (Fig. S3), the Ce–O bonds of the CM catalyst are remarkably influenced by As species; therefore, the deactivation effect on CeOx should be the major reason 17 ACS Paragon Plus Environment


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for the loss of activity. In contrast, for the CWP catalyst, the W=O bond is shortened, with the appearance of a more distorted octahedral geometry and augmented isopolytungstate groups, indicating that WO3 is preferentially affected by As2O3. In other words, WO3 can be sacrificed to partly prevent the deactivation of CeO2. Therefore, a reasonable explanation for the structural changes is that a portion of the arsenic oxide can be primarily bonded as amorphous structures to the surface CeO2 (CM) and WO3 (CW) via Ce–O–As and W–O–As linkages, respectively. Additionally, because the electronegativity of As (2.18) is stronger than that of Ce (1.12), a portion of the surface Ce4+ species of CMP can be reduced to the Ce3+ species after the introduction of As due to the electron-withdrawing nature of As. These results, combined with the XPS results (Fig. 4 and Table 2), which indicates the formation of less excess surface Ce3+ on CMP, suggest that a redox process between Ce4+ and As3+ may occur and inhibit the self-redox process of the Ce species, hindering the SCR reaction. In contrast, W(IV) hinders the reduction of Ce4+ by As3+, and the strong interaction between W and As may be the dominant reason for the deactivation of CWP. Previous research on CW and CM catalysts had indicated that surface Ce atoms and unsaturated Wn+ cations are the main contributors of Lewis acid sites, while WOx (with an appropriate doping amount) or polymeric MoOx units promoted the generation of Brønsted acid sites.14, 25 Combined with the structure effects discussed above, the loss of Lewis acid sites derived from a decreased number of surface Ce atoms and distorted WOx species may be the key reason for the deactivation of the CMP and CWP catalysts, respectively. Thus, it is concluded that the surface structures of CWP and CMP are influenced by As species in two different ways. 18 ACS Paragon Plus Environment

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4.2 Influence of As on the reaction mechanism For the CM catalyst, when the reaction temperature is below 250 °C, previous research and our DRIFTS results (Fig. 8) reveal that NH3 adsorption is vital at low temperatures (via the E-R process).14 Therefore, excessive NOx adsorption has an adverse effect on low-temperature activity because of competitive adsorption. However, when As is introduced to the CM catalyst, the NOx adsorption of the poisoned sample becomes weaker (Fig. 6). Some of the ad-NH3 may be released to retard the loss of activity due to deactivation. In contrast, for the CW catalyst, NOx adsorption can also participate in the low-temperature SCR process (via the L-H process).49 The loss of ad-NOx sites has a great impact on the deactivation of the CW catalyst. In addition, based on the structural analysis of the CMP catalyst, the deposition of arsenic atoms leads to the loss of surface Ce4+ and thus suppresses the conversion cycle of Ce4+ to Ce3+. Nevertheless, the existing W(IV) atoms on the CW surface hinder the reduction of Ce4+ by As3+ because of the formation of stronger interactions between W and As. However, the NOx adsorption sites for both types of catalysts are mainly provided by the CeO2 species due to its basic surface. Therefore, the lower NOx adsorption of CMP than of CWP at low temperatures due to the structural change of CeOx mitigates the deactivation of CMP. When the reaction temperature is above 300 °C, the NOx adsorptive sites become unimportant, and the reaction between ad-NH3 and gaseous NO and O2 (the E-R process) is the crucial SCR mechanism for both the CW and CM catalysts (Fig. S6 and Fig. S7). Although some Brønsted acid sites provided by As species can be found, these sites are inactive during the SCR reaction. Therefore, the loss of acid sites may play a key role in the high-temperature deactivation. 19 ACS Paragon Plus Environment


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Furthermore, for the CMP catalyst, the reduced quantity of acid and Lewis acid sites, originating from the damage of the surface Ce species and Mo=O bonds, may play the major role in the greater loss of activity of this catalyst than CWP.

4. Conclusions The mechanism of poisoning of CM and CW catalysts by arsenic oxide was investigated, compared and analyzed in this work. It was found that for the CMP catalysts, the degree of activity loss increases with temperature, while the tendency is reversed for the CWP catalysts. By exploring the changes in the physical structure, redox properties, acidity, surface-adsorbed species and reaction mechanism before and after deactivation, we determined that the reasons for these results are as follows: (1) As2O3 has a major impact on CeOx by oxidizing a portion of the Ce3+ into Ce4+ species for the CMP catalyst. However, As2O3 prefers to interact with WO3 through W–O–As linkages in the CWP catalyst. (2) As2O3 decreases the strength, number and stability of acid sites for the two types of catalysts. Moreover, CMP exhibits a more serious loss of Lewis acid sites with an increase in temperature compared with CWP. (3) According to the results of in situ DRIFTS and Raman spectra, more Brønsted acid sites and reduced NOx adsorption mitigate the deactivation levels in CMP catalysts compared with CWP at low temperatures. At high temperatures, the greater quantity of remnant Lewis acid sites in CWP originates from the protection of the surface Ce species by WO3 and may play the key role in the reduced loss of activity of this catalyst compared with CMP. 20 ACS Paragon Plus Environment

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Acknowledgements This work was financially supported by National Natural Science Foundation of China (21325731, 21407088 and 51478241), National High-Tech Research and Development (863) Program of China (2013AA065401 and 2013AA065304), and China Postdoctoral Science Foundation (2013M530643). Supporting information Figures of activities, N2 adsorption curves, Raman spectra, XPS spectra and additional DRIFTS spectra.

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Figure captions Fig. 1. Rates of NO conversion normalized to the BET. Fig. 2. XRD of the fresh and poisoned catalysts calcined at 500 °C. Fig. 3. H2-TPR profiles of the fresh and poisoned catalysts. Fig. 4. XPS spectra of the fresh and poisoned catalysts over the spectral regions of Ce 3d (a), Mo 3d and W4f (b) and As 3d (c). Fig.5. NH3-TPD curves of the fresh and poisoned catalysts. Fig. 6. DRIFTS spectra of the NOx desorption of CW (a), CWP (b), CM (c) and CMP (d) catalysts at 100-350 °C. Fig. 7. Sequential DRIFTS spectra of CW (a, b) and CWP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C. Fig. 8. Sequential DRIFTS spectra of CM (a, b) and CMP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C. Fig. 9. Sequential Raman spectra of CW (a, b) and CWP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C. Fig. 10. Sequential Raman spectra of CM (a, b) and CMP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C.



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Fig. 1. Rates of NO conversion normalized to the BET. Reaction condition: [NO]=[NH3]=500ppm, [O2] =3%, N2 balance, total flow rate=400 mL/min, GHSV=240000 h-1.


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Fig. 2. XRD curves of the fresh and poisoned catalysts calcined at 500 °C.



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Fig. 3. H2-TPR profiles of the fresh and poisoned catalysts.


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Fig. 4. XPS spectra of the fresh and poisoned catalysts over the spectral regions of Ce 3d(a), Mo 3d and W4f (b) and As 3d(c).



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Fig.5. NH3-TPD curves of the fresh and poisoned catalysts.


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. Fig. 6. DRIFTS spectra of the NOx desorption of CW(a), CWP(b), CM(c) and CMP(d) catalysts at 100-350 °C.



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Fig. 7. Sequential DRIFTS spectra of CW (a, b) and CWP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C.


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Fig. 8. Sequential DRIFTS spectra of CM (a, b) and CMP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C.



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Fig. 9. Sequential Raman spectra of CW (a, b) and CWP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C.


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Fig. 10. Sequential Raman spectra of CM (a, b) and CMP (c, d) recorded under various atmospheres: the dehydrated catalyst is first treated by NH3, then NO + O2 is added and the reversed order at 200 °C.



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Captions for Tables

Table 1 Structural and textural parameters measured by N2 adsorption, XRD, Raman and H2-TPR. Table 2 XPS results of fresh and poisoned catalysts.


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Table 1 Structural and textural parameters measured by N2 adsorption, XRD, Raman and H2-TPR. Sample

BET (m2/g)

Pore size (nm)

Pore volume (cm3/g)

Crystallite size a (nm)

Position of F2g (cm-1)

H2 consumption (µmol/m2)

CW CWP CM CMP

85.2 70.9 32.6 24.4

11 11.8 23.8 24.5

0.23 0.21 0.19 0.15

10.9 10.5 12.2 14.4

459 459 461 461

9.6 32.7 42.6 66.1



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Table 2 XPS results of fresh and poisoned catalysts. Surface atomic concentration (%) Catalyst

Ce4+

Ce4+/

Cetotal

Oα

Oβ

Ototal

As5+

As3+

Astotal

W

Mo

Ce3+ CW

14.35

2.46

20.18

31.73

42.77

74.5

-

-

-

5.31

-

CWP

12.04

3.08

15.95

33.44

41.23

74.67

2.38

2.64

5.02

4.36

-

CM

15.26

5.02

18.3

32.43

41.61

74.04

-

-

-

-

7.66

CMP

8.48

2.42

11.98

37.43

31.38

68.81

6.49

4.19

10.68

-

8.35


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Graphic Abstract


Comparison of the Structures and Mechanism of Arsenic Deactivation

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