Promotional Mechanism of Sulfation on Selective Catalytic Reduction

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J. Phys. Chem. C 2008, 112, 16052–16059

Promotional Mechanism of Sulfation on Selective Catalytic Reduction of NO by Methane in Excess Oxygen: A Comparative Study of Rh/Al2O3 and Rh/Al2O3/SO42– Shicheng Xu,† Junhua Li,*,†,‡ Dong Yang,† and Jiming Hao† Department of EnVironmental Science and Engineering, Tsinghua UniVersity, Beijing, China, 100084, and Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136 ReceiVed: July 16, 2008; ReVised Manuscript ReceiVed: August 9, 2008

The catalytic performance and reaction mechanism for selective catalytic reduction of NOx by methane in excess oxygen over Rh-loaded sulfated alumina were investigated. Sulfation was observed to influence the properties of catalysts in two aspects: (i) modify the dispersion and electronic state of surface rhodium species and (ii) determine the surface NOy groups. Differences in surface nitrates and their behaviors lead to different mechanisms for CH4-SCR of NO. On Rh/Al2O3, the adsorbed monodentate nitrates (1558 cm-1) could react with methane in the absence of oxygen, while remaining almost inert to the mixture of methane and an excess amount of oxygen. However, nitrates on Rh/Al2O3/SO42-, most probably bidentate nitrates (1600-1598 cm-1), were able to react with methane in the presence of oxygen. Moreover, a proportional relationship between NO conversion rates and intensities of nitrosyl (1936 cm-1) bands was found on Rh/Al2O3/SO42-, and the rate-determining step of NO conversion was probably the reduction of nitrosyl. 1. Introduction As one major air pollutant from fossil fuel combustion, nitric oxide (NO) is the precursor of acid rain and photochemical smog. Selective catalytic reduction (SCR) of NO with ammonia is an efficient way to remove NO from flue gas of stationary sources and diesel engines; however, alternative reductants are continually being studied due to corrosion and leakage of ammonia. Since methane is the main component of natural gas, it gradually becomes appealing1 for cost and safety concerns,2,3 and many reports have been focused on selective catalytic reduction of NO with methane (CH4-SCR) during the past decade. Among various catalysts for CH4-SCR reactions, noble metalbased catalysts have been proven to be effective to remove NO in the low-temperature region. Burch et al.4 reported a comparative study on various noble metal loaded alumina catalysts. Rhloaded alumina showed excellent selectivity toward N2 formation over Pd and Pt, on which the formation of N2O was observed obviously.5,6 In Ohtsuka’s work on noble-metal loaded sulfated zirconia,7 Rh/SZ has been proven to have an overall higher activity in a larger temperature range. Also, the author categorized Pd and Rh into the same group, which could be labeled by high activity for NO2 reduction to N2. Bahamonde and Mendioroz8,9 reported the effective performance of Rh supported on aluminum pillared clays over CH4-SCR reaction in excess oxygen, and they identified the active site as exchanged Rh3+ and oxidized Rh2Ox assisted by the acid sites. Loughran and Resasco10 testified to the importance of acidity in SCR reactions by adding sulfated zirconia in a mechanical mixture with Pd/SiO2. The effect of sulfate zirconia in promoting conversion of NO to N2 was explained by a two-step bifunctional mechanism, the first step of which involves the oxidation of NO to NO2 on the acid sites. Since then, solid superacids * Corresponding author. Tel:+1-734-6473884, +86-10-62782030. Fax: +86-10-62785687. E-mail: [email protected], [email protected]. † Tsinghua University. ‡ University of Michigan.

such as sulfated zirconia (SZ)11-14 and tungstate zirconia (WZ)2,15-19 have been proven to be effective supports for CH4SCR catalysts. The importance of the acid site20-23 has been widely accepted for its bifunctional effect: chemisorptions of intermediate species and stabilization of active sites.16 Li et al.24,25 have found Pd/Al2O3/SO42- exhibits good activity toward CH4-SCR reaction. Since γ-Al2O3 is widely used as a support for TWC catalysts, sulfated alumina will be a promising commercial support for CH4-SCR as long as excellent catalysts based on it could be discovered. However, few studies on CH4SCR have focused on sulfated alumina except for Pd/Al2O3/ SO42-. The involvement of oxygen in the CH4-SCR reaction results in the formation of adsorbed nitrates and nitro species,26-28 and their reacting with activated methane would be influential to the overall catalytic activity. Exchange of NO2 moiety between adsorbed NO2 and monodentate nitrates has been detected on Cu-ZSM-5.29 In CH4-SCR reactions over Co, Mn, and Cu loaded ZSM-5,30 NO abatement rates have been found to be proportional to the concentration of monodentate nitrates. All these could explain the importance31-34 of NO2 in CH4-SCR reactions. On the other hand, Djega-Mariadassou and Berger35 applied three catalytic cycles into CH4-SCR reactions, in which NO, instead of NO2, react with oxygenated hydrocarbons and lead to N2 formation. Although methanol and formaldehyde have been detected by GC-MS during TPSR, the role of surface NOx complexes in selective catalytic reduction of NO with CH4 has not been elucidated. This paper reported the activities of Rh-loaded sulfated alumina on CH4-SCR reactions in excess oxygen. Textural properties of the samples were studied by XPS and H2-TPR. The reaction intermediates were analyzed by a comparative DRIFTS study between Rh/Al2O3/SO42- and Rh/γ-Al2O3, and the differences in reactivity of surface nitrates and the relationship between nitrosyl and conversion rate were extensively discussed, which could be helpful in understanding CH4-SCR reactions in excess oxygen.

10.1021/jp806301z CCC: $40.75  2008 American Chemical Society Published on Web 09/23/2008

A Comparative Study of Rh/Al2O3 and Rh/Al2O3/SO42– 2. Experimental Methods 2.1. Catalysts Preparation. γ-Al2O3 was prepared by calcining alumina phosphide (Aluminum Corporation of China Limited) in air at 650 °C for 5 h. Sulfated alumina (SA) was obtained by impregnating the prepared γ-Al2O3 with an aqueous solution of 0.5 M H2SO4, with the ratio of 15 mL/g. The slurry was sequently stirred at room temperature for half an hour and the residue was dried at 110 °C overnight and calcined in air at 600 °C for 5 h. Rh loading was performed by incipient wetness impregnation of the obtained SA or γ-Al2O3 with an aqueous solution of RhCl3 · 3H2O. All the samples were dried at 110 °C overnight, and then calcined in air at 600 °C for 5 h. The catalysts loaded with Rh are identified as xRhSA or xRhA, where x indicates the weight content of rhodium, while SA suggests sulfated alumina where A serves as the abbreviation of γ-Al2O3. For instance, 1RhSA represents sulfated alumina loaded with 1 wt % rhodium. 2.2. Catalytic Activity Measurements. The activity measurements were carried out with a fixed-bed quartz reactor (i.d. 8 mm) in a temperature range of 300 to 600 °C. The feed gas was a mixture of 1000 ppm NO, 3000 ppm CH4, 10% O2, and N2 as the balance gas, and the space velocity was approximately 12 000 h-1. NO and NO2 concentration were analyzed with a chemiluminescence NO/NO2 analyzer (Thermal Environmental Instruments, model 42C), and CH4 concentration was measured by gas chromatography (Shimadzu GC 17A). 2.3. Characterization Methods. BET surface areas were measured by the N2 adsorption-desorption method, using a NOVA 3200e analyzer. X-ray diffraction (XRD) patterns were determined by using a Rigaku D/max-RB diffractometer. The analysis was performed with Cu target (40 kV and 100 mA); a typical scan speed was 6°/min with a step of 0.002° in the range from 20° to 70°. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a PHI15300/ESCA system with Al Ka radiation (1484.6 eV), which was calibrated internally by carbon deposit C1s binding energy (BE) at 284.6 eV. H2-TPR measurements were carried out with a Micromeritics Chemisorb 2720 pulse chemisorptions system. Each sample (∼0.1 g) was first calcined at 500 °C for 1 h and then cooled to room temperature. With the reducing mixture (5% H2/Ar), the reaction was carried out by heating the sample from 50 to 600 °C at 10 deg/min. The consumption of H2 was quantified with a TCD detector. 2.4. DRIFTS Studies. The DRIFTS spectra were recorded on a Nicolet Fourier Infrared Spectrum with a diffuse reflection situ pool and highly sensitive MCT detector. The diffuse reflectance FT-IR measurements were carried out in situ in a high-temperature cell fitted with ZnSe windows. The samples were finely ground, directly placed in a ceramic crucible, and manually pressed. The feed gas streamed into the cell at a total flow rate of 100 mL/min. The temperature in the cell can be programmed from 20 to 800 °C. Prior to analysis, all samples were first calcined at 550 °C in a 100 mL/min flow of 10% O2/N2 for 1 h. The background spectra were collected after dwelling for 30 min at a given temperature. If not specified, the sample spectra reported here were collected after dwelling for 30 min. The spectra were recorded at a resolution of 4 cm-1. 3. Results 3.1. Catalytic Activity. Panels a and b of Figure 1 present NO conversion and CH4 conversion, respectively, on RhSA catalysts with different rhodium content as a function of temperature. On 1RhSA, with increasing temperature, the

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Figure 1. (a) NO conversion and (b) CH4 conversion in CH4-SCR over RhSA catalysts with different Rh content in the temperature range of 300-600 °C. (c) Relationship between rhodium content and NO/ CH4 conversion at 450 °C. Gas mixture: 1000 ppm NO, 3000 ppm CH4, 10% O2, and N2 as balance, GHSV ) 12 000 h-1.

maximum NO conversion reached 36% around 450 °C, and then decreased at higher temperature, and this was due to combustion of CH4 by O2. The Tmax (temperature for maximum NO conversion) was shifted to higher temperatures upon decreasing the Rh content in the catalyst. Conversion rates of catalysts by function of rhodium contents at 450 °C are compared in Figure 1c. Methane conversion increased almost linearly with the increment of Rh content. A similar increase of NO conversion rate could be found when Rh content was below 1 wt %.

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Xu et al. TABLE 1: BET Surface Area of Various Samples sample

BET surface area (m2/g)

γ-Al2O3 SA 1RhA 1RhSA

191.5 201.3 199.6 189.2

TABLE 2: XPS Results of Rh Catalysts sample

Rh 3d5/2 (eV)

Al 2p (eV)

S 2p (eV)

S/Al ratio (×10-2)

Rh/Al ratio (×10-3)

1RhA 1RhSA 2RhSA

309.9 310.4 310.4

74.6 74.8 74.8

169.6 169.8

8.13 7.77

6.85 11.3 15.9

TABLE 3: XPS Results of Various Rh Species in Reference formula Figure 2. Comparison of NO and CH4 conversion over (a) 1RhA and (b) 1RhSA in the temperature range of 300-600 °C. Gas mixture: 1000 ppm NO, 3000 ppm CH4, 10% O2, and N2 as balance, GHSV ) 12 000 h-1.

Figure 3. Effect of O2 concentration on NO conversion over 1RhSA and 1RhA at 450 °C. Gas mixture: 1000 ppm NO, 3000 ppm CH4, 0-20% O2, and N2 as balance, GHSV ) 12 000 h-1.

However, 2RhSA exhibited approximately the same NO conversion as 1RhSA at 450 °C. The effects of sulfation on the catalytic activity over 1RhA in excess oxygen are illustrated in Figure 2. It is obvious that sulfation promoted the catalytic ability of 1RhSA. On the other hand, methane conversion on 1RhA was apparently higher than that on 1RhSA. The role of oxygen was further investigated by changing the O2 concentration up to 20% in the inlet gas at the temperature of 450 °C, and the results are presented in Figure 3. In agreement with literature,4 NO conversion reached almost 100% on 1RhA in the absence of oxygen. However, the poisoning effect of oxygen was quite remarkable, and its catalytic effect dropped drastically with the increase of O2 concentration, especially when it surpassed 1%. On the contrary, oxygen promoted NO conversion on 1RhSA. 3.2. Characterization. 3.2.1. BET and XRD. Table 1 summarizes the BET surface areas of the samples derived from γ-Al2O3. The results indicate that sulfation and impregnation affected surface areas of the investigated catalysts slightly. The XRD patterns (not shown) of 1RhA, 1RhSA, or SA show similar structures of γ-Al2O3, and no peaks of rhodium species were detected. The main reason

Rh Rh2O3 RhO2 RhCl3 · 3H2O Rh+ Rh3+ Rh3+

Rh 3d5/2(eV)

ref

307.2 308.2-309.4 309.8-309.9 309.4 310.4 307.6-309.6 308.8-311.3 310.4

36 36 9 37 36 38 38 this work

is that contents of rhodium in the observed samples were relatively low (e2 wt %). 3.2.2. XPS. The binding energy of Rh, Al, S, and their relative content in the surface were quantified by XPS and presented in Table 2. Highly dispersed rhodium species have been found on sulfated samples. The great binding energies detected on RhSA samples provided evidence for interaction between rhodium species and sulfate groups. However, the Rh 3d5/2 binding energy of Rh compounds varies vastly among different experimental conditions, which makes the Rh identification rather difficult. Table 3 summarizes the Rh 3d5/2 binding energy of different Rh species reported in the literature.9,36-38 Despite the complexity of possibly existing rhodium compounds on 1RhA, the 0.5 eV shift of the Rh 3d5/2 binding energy from 1RhA (309.9 eV) to 1RhSA and 2RhSA (310.4 eV) was quite noticeable. This exceptionally high binding energy could be explained by the absorbed sulfate species with strong electronegativity that withdraw the electron atmosphere39 of Rh-O. Hence, it is most likely that sulfate groups stabilized the surface Rh3+ ions. Moreover, by comparing the Rh/Al ratio of the observed samples, it could also be found that sulfation of support improved the dispersion of Rh species on the surface. Conglomeration possibly occurred on 2RhSA, which showed a relatively low dispersion (15.9/11.3 ≈ 1.4 < 2) in comparison to 1RhSA. 3.2.3. H2-TPR. H2-TPR curves of 1RhA, 1RhSA, SA, and γ-Al2O3 are presented in Figure 4. Hydrogen consumption was not observed on γ-Al2O3 below 600 °C, while the reduction peak on sulfated SA occurred above 500 °C, which was probably due to the sulfuric species on the surface. For Rh-loaded samples, hydrogen consumption peaks at lower temperatures appeared. On 1RhA, the flat reduction peak of rhodium species was probably due to rhodium aluminates [Rh(AlO2)y] which formed after Rh-loaded alumina had been calcined at high temperature.40,41 On the contrary, on 1RhSA, the noticeable hydrogen consumption peak at around 295 °C showed that rhodium species on sulfated alumina were more reducible. Moreover, reduction peaks of sulfate groups shifted to around 480 °C with a broader shoulder, and this could be due to the possibly existing rhodium sulfate. In sum, highly charged

A Comparative Study of Rh/Al2O3 and Rh/Al2O3/SO42–

Figure 4. H2-TPR results of various catalysts: (a) SA, (b) 1RhSA, (c) 1RhA, and (d) γ-Al2O3.

Figure 5. DRIFTS spectra of CH4 + O2 reaction on 1RhA, 1RhSA, and SA after 15 min. Reaction condition: 3000 ppm CH4, 10% O2, N2 as balance gas. Temperature: 450 °C.

rhodium ions were stabilized by the surface sulfate species and the interaction between them made rhodium species more reducible. 3.3. DRIFTS Studies. 3.3.1. Interaction between CH4 and O2 on the Surface. This approach was carried out by flowing a 100 mL/min mixture of 3000 ppm CH4 and 10% O2 (N2 as the balance gas). The spectra of 1RhA, 1RhSA, and SA are presented in Figure 5. Rhodiumloaded alumina has been extensively applied as an effective catalyst for partial oxidation of methane,42,43 and from its spectra it could be seen that methane was easily oxidized by oxygen. Bands raised at 1885, 1594, and 1460 cm-1 are signals of absorbed CO,44 formate, and carbonates,45 respectively. In contrast, adsorption and oxidation of methane on SA were more difficult. The weak and broad bands around 1560 cm-1 could be indicators of activated hydrocarbon species,45-47 and the negative band at 1427 cm-1 corresponded to sulfated species12 which were partially covered by adsorbents. According to most literature, formate45,48-50 raises its band at 1650-1590 cm-1, thus bands at 1640 cm-1 on 1RhSA could be due to υ(CdO) vibration mode of formate.2 It is also possible that the δ(H2O) vibration mode of adsorption of water contributed to this huge band around 1600 cm-1. The band at 1480 cm-1 could be

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Figure 6. DRIFTS spectra of NO + O2 coadsorption on 1RhA, 1RhSA, and SA for 15 min. Reaction condition: 1000 ppm NO, 10% O2, N2 as balance gas. Temperature: 450 °C.

ascribed to carbonates.47 Nevertheless, compared to spectra of 1RhA, those of 1RhSA exhibited weak signals of CO and carbonates, and this implied that the oxidative ability of rhodium alumina was weakened by sulfation. Also, adsorption of oxygenated species, especially formate species, became easier on 1RhSA. 3.3.2. NO and O2 Coadsorption. NO and O2 coadsorption was performed in a 100 mL/min flow mixture of 1000 ppm NO and 10% O2, with N2 as the balance gas. In Figure 6, the huge band at 1560 cm-1 and the abutting shoulder at 1581 cm-1 on 1RhA indicated the formation of nitrates.51 The small band at 1901 cm-1 could be ascribed to nitrosyl44,52-54 adsorbed on Rh+. The band appearing on SA after coadsorption at 1581 cm-1 could be ascribed to the chelating bidentate nitrate, and another band peak at 1471 cm-1 could be assigned to nitrito groups.54 On 1RhSA, bands of nitrosyl shifted to 1930 cm-1, which was in agreement with the high binding energy of Rh 3d5/2 found in the XPS tests, and it could also be inferred that the nitrosyl bands raised on 1RhSA resulted from the adsorption of NO on highly charged rhodium ions, most probably Rh3+ ions. Two additional bands developed at 1630 and 1598 cm-1, corresponding to NOy groups. To further demonstrate the higher Rh dispersion on the 1RhSA sample, we compared the IR bands of nitrosyl species on 1RhA and 1RhSA, which resulted from adsorption of NO on Rh sites. Since the molecular adsorption corresponds to a 1:1 stoichiometry, the IR absorbance of nitrosyls after NO adsorption could be used to estimate Rh dispersion.55 The integrated area of the nitrosyl band of 1RhSA was 0.93, which was approximately 1.6 times as great as that of 1RhA (0.56). This result was in agreement with the comparison of Rh/Al ratios of these two samples (11.3/6.85 ≈ 1.65). With respect to nitrates which could bond to metal ions on the surface in different ways,54 their bands varied in respectful IR range depending on the metal ions they attached to. Despite the difficulties in the assignment, we compared the data on thermal stability and reactivity of formed NOy and attempted to assign the bands as follows. Adsorbed NO2 and bridging bidentate nitrates usually raise their bands in the IR range of 1650-1600 cm-1, while bridging monodentate nitrates and chelating bidentate nitrates develop their bands at 1580-1500 and 1560-1480 cm-1. Splitting of the υ3 mode is most significant for bridging bidentate nitrate, less stronger for

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Xu et al.

TABLE 4: DRIFTS Results and Assignment of NOy Groups

chelating bidentate nitrate, and least for bridging monodentate nitrate,54 and this makes the frequency of their υas(NO2) mode decrease in the same sequence. Hence, bands at 1558 cm-1 could be due to bridging monodentate nitrate56 and that at 1581 cm-1 could be ascribed to the chelating bidentate nitrate.56,57 On the other hand, according to the literature,58 the adjacent bands found at 1628 and 1594 cm-1 could be assigned respectively to NO2 and nitrate. Thus, we tentatively assigned NOy groups as shown in Table 4. Although no evidence could exclude the possibility that the nitrates on sulfated/unsulfated samples were of the same type while bonded to different cations, their different reactivity is manifest as will be demonstrated in the following parts. Comparison of 1RhSA and SA spectra showed that the NOy groups formed mainly on the supports. Also, by comparing intensities and positions of nitrate signals on 1RhA and 1RhSA, it makes clear that sulfation of the support switched the preference of adsorption NOy groups on the surface, and more importantly, reduced NO adsorption. Probably, the formation of nitrate was determined by the number and strength of acid sites on the support. Moreover, the results of NO and O2 coadsorption showed that rhodium species promoted the adsorption of ad-NO2 and nitrosyl. In addition, since the nitrito band on 1RhSA are less significant than that on SA, it could be implied that rhodium species could stabilize formed nitrates. 3.3.3. ReactiWity of NOy Groups on Rh/Al2O3. The sample (1RhA) was first treated in a 100 mL/min flow of N2 containing 1000 ppm NO for 15 min, and then heated to the desired temperatures in a 100 mL/min flow of pure N2. Since the NOy groups were unstable at 450 °C, their reactivity properties were investigated under the temperature of 400 °C. The spectra were recorded after 30 min, until the signals of formed NOy groups remained unchanged. Figure 7 presents spectra for the reaction between methane and NOy groups with and without oxygen. To find out the reason why NO conversion on RhA was rather low in the presence of oxygen, a flow of N2 containing 10% O2 and 3000 ppm CH4 was injected in the cell after the formation of stable NOy groups, and the spectra are presented in Figure 7a. Bands representing methane and carbon dioxide increased significantly after the appearance of methane, and bands at 1590 cm-1 also increased, indicating the formation of formate species. However, no obvious change of nitrosyl bands was observed, and bands of the NOy groups also eroded sluggishly.

On the contrary, when the above approach was used with the removal of oxygen, a different picture emerges (see Figure 7b). A dramatic change of bands in the range of 2100-1800 cm-1 could be noticed after methane was injected, and at the same time, bands of monodentate nitrate (1585 cm-1) vanished

Figure 7. DRIFTS spectra of transient response of formed stable NOy groups on 1RhA upon a flowing mixture of 3000 ppm CH4 and (a) 10% or (b) 0% O2 in N2 at 400 °C.

A Comparative Study of Rh/Al2O3 and Rh/Al2O3/SO42–

Figure 8. DRIFTS spectra of transient response of formed stable NOy groups on 1RhSA upon a flowing mixture of 3000 ppm CH4 and 10% O2 in N2 at 450 °C.

quickly. Until they were flattened, bands at 2091, 2025, and 1854 cm-1, which represent different types of adsorbed CO59,60 bonding to different cations, developed drastically. These phenomena showed that monodentate nitrate could become involved in the reaction with methane in the absence of oxygen. Since partial oxidation of methane into carbon monoxide had been found in CH4-SCR over rhodium alumina,4 the generation of CO might be due to the partial oxidation of methane, caused by an insufficient number of NOy groups. Nevertheless, since CO was not detected as soon as methane was fed in, and at the beginning, the generation of CO2 without CO was also detected, monodentate nitrate could react with methane and oxidize it to CO2. In conclusion, on 1RhA, monodentate nitrate could only react with methane in the absence of oxygen. 3.3.4. ReactiWity of NOy Groups on Rh/Al2O3/SO42-. Similar to the previous approaches, NO adsorption was performed by flowing 100 mL/min flow of N2 containing 1000 ppm NO for 15 min under 250 °C, and then, with the removal of NO, the reaction cell was heated to the desired temperatures to form the stable NOy groups. It is interesting that both bands of adNO2 and bridging bidentate nitrates vanished when the temperature was above 400 °C. At 450 °C, besides nitrosyl bands (1936 cm-1), there only remained bands around 1576 and 1544 cm-1, which could probably be signals of nitro species51 or those of monondentate nitrates and chelating bidentate nitrates on 1RhSA. After NO adsorption for 15 min at 450 °C and purging N2 to remove unstable NOy groups for another 30 min, 10% O2 and 3000 ppm of CH4 was added to the cell. The reactivity of these stable NOy groups was investigated, and the corresponding spectra are presented in Figure 8. With the presence of methane and oxygen, bands at 1637 cm-1 developed, indicating the formation of formate, while that of nitrosyl and NOy bands remained unchanged. These phenomena implied that nitrosyls and stable NOy groups are inert to the mixture of methane and oxygen. Remaining unchanged in the presence of oxygen, the monondentate nitrates and chelating bidentate nitrates on 1RhSA performed similarly to nitrates discovered on 1RhA. To obtain information about reactivity of ad-NO2 and bridging bidentate nitrates, transient reactions were performed in the following approach. NO and O2 coadsorption was first performed on 1RhSA at 450 °C for 15 min, then 3000 ppm CH4 was added to the mixture, without removing NO or O2. The recorded spectra are presented in Figure 9. As soon as methane

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Figure 9. DRIFTS spectra of reactions on NOy groups on 1RhSA. NO + O2 coadsorption was first performed under a flowing mixture of 1000 ppm NO and 10% O2 in N2 at 450 °C for 15 min. The spectra were recorded after the addition of CH4 into the gas mixture, without removal of NO or O2.

was fed in, bands of nitrosyl (1936 cm-1) increased, while that of bridging bidentate nitrates (1600 cm-1) decreased. This phenomenon led to the fact that bridging bidentate nitrates could respond to methane in the presence of oxygen. The increase of the nitrosyl band could indicate the formation of NO,57 or more precisely, the generation of NO through interaction between bridging bidentate nitrates and oxygen with methane. Since bands of formate and ad-NO2 were adjacent to each other around 1630 cm-1, an unambiguous interpretation of the change of the 1633 cm-1 band is difficult. Although the slight shift of the band peak around 1633 cm-1 was observed, the tiny change could not manifest nor deny the reaction occurred on ad-NO2. To sum up, on 1RhSA, bridging bidentate nitrates could react with methane in the presence of oxygen. 3.3.5. Role of Nitrosyl on Rh/Al2O3/SO4.2 The role of nitrosyl was studied by observing the intensity of its band at different temperatures, under the 100 mL/min gas mixture of 1000 ppm NO, 3000 ppm CH4, and 10% O2, which was the same as that used in catalytic tests. The results were summarized in Figure 10. As is shown in Figure 10a, intensities of the nitrosyl band (1936 cm-1) peaked at 450 °C. However, bands around 1633 cm-1 decreased as temperature increased. The proportional relationship between nitrosyl bands and NO conversion as a function of temperature is demonstrated in Figure 10b. The result implied that nitrosyl was involved in the rate-determining step of the reaction. Nitrosyl is formed via the coordinative bond between NO, which is an electron donor, and the metal ion that plays the role of an electron acceptor.54 Thus it is reasonable that the formation of N2 was finalized through the reduction of NO.6 4. Discussion 4.1. Active Rhodium Species. The XPS and H2-TPR results showed that the active rhodium species on sulfated samples were more reducible and probably highly charged. Figure 1c also implied that surface rhodium species were responsible for NO conversion. According to the literature,12 the aggregation of rhodium oxides may prevail on high-loading samples due to the limited number of acidic sites on the support. After comparing rhodium dispersion on 1RhSA and 2RhSA with their catalytic performances, it could be inferred that only highly

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Xu et al. SCHEME 1: Proposed Reaction Stages in CH4-SCR over Rh/Al2O3/SO42- in Excess Oxygen

Figure 10. (a) DRIFTS spectra of NO-CH4-O2 reactions on 1RhSA under different temperatures. (b) Integrated area of nitrosyl band and NO conversion in the temperature range of 300-550 °C. Gas mixture: 1000 ppm NO, 3000 ppm CH4, 10% O2, and N2 as balance.

dispersed rhodium species were responsible for NO conversion. On the other hand, since within all rhodium-content samples tested in the experiment rhodium content was almost linearly proportional to CH4 conversion, rhodium species, whether highly dispersed or not, dominated the oxidation of methane. Since the oxygenated species after methane activation was probably the reductant for the reduction of NO, this could also explain the Tmax shift that was observed in Figure 1a. The catalysts with lower Rh content oxidized methane into the oxygenate species at higher temperatures, thus their Tmax shifted to higher temperatures. 4.2. Oxygen Poisoning Mechanisms. As has been demonstrated in the previous sections, the monodentate nitrates formed on 1RhA could react with methane in the absence of oxygen. The mechanism of selective NO reduction with methane61 could be simplified as the following steps:

2NO + O2 f 2NO2 2NO2+CH4 f CO2+2H2O + N2 According to literature, monodentate nitrates have proven the ability to exchang the NO2 moiety with adsorbed NO2,29 and its concentration has been found to be proportional to NO conversion rates on some zeolite catalysts.30 Hence, it is probable that monodentate nitrates are involved in the surface reaction. On the basis of the catalytic performance of 1RhA presented in

Figure 3, it could be inferred that, when the oxygen concentration is low, the mechanism of reaction on rhodium alumina could follow the above-mentioned route. In addition, in this mechanism, the oxidation of NO into NO232-34 has been considered as the rate-determining step. However, the presence of excess oxygen significantly inhibited the oxidation of methane by monodentate nitrates. On the basis of the fact that methane was activated into formate by excess oxygen, it is likely that monodentate nitrate is inactive toward formate. 4.3. Reaction Route on Rh/Al2O3/SO42-. The engagement of oxygen changed the oxygenated hydrocarbon and also directed another reaction route, which was similar to the three catalytic cycles raised by Djega-Mariadassou et al.35 On the basis of the result of FTIR studies, the reaction route could be proposed as the following: NO was first oxidized into bridging bidentate nitrates, and then mild oxidation of methane created the oxygenated hydrocarbon and NO. The abatement of NO was completed by reaction between oxidized methane derivatives and nitric oxide. Probably, the nitric oxide that was involved in the reaction should be one that attached on the surface, namely nitrosyl. This inference is also in agreement with the relationship between rhodium loading and NO conversion presented in Figure 1c, because the dispersed rhodium ions were the only metal cations that could lead to the formation of nitrosyls on 1RhSA. In Kantcheva’s study on Pd/WO3-ZrO2,2,17 nitrosyl is also found to be the reactant toward formate, and that approach leads to the formation of N2. However, in our experiments, the oxygenated species was difficult to clarify. No convincing evidence of its formation via transformation of nitro methane was found. It had also been shown in Figure 8 that nitrosyl could not react with the generated formate. Perhaps the difficulties of finding the active oxygenated species correspond to the comparatively mean performances of RhSA catalysts. Or perhaps formate is not the right intermediate that reacts with nitrosyl. Whatsoever, the last step of the above-mentioned route, which involves the reaction between nitrosyls and oxygenated species, was probably the rate-determining one, and this was supported by the correlation between DRIFTS and catalytic test results. The reaction pathways could be proposed as shown in Scheme 1. Data on thermal stabilities of formed NOy groups and NO + O2 coadsorption on 1RhA and 1RhSA reflected that the formed nitrates, whether the monodentate form or the bidentate one, were unstable at the temperatures when the catalysts exhibited their best performance. It is also under these temperatures that they appeared in respectfully large amount during the NO + O2 coadsorption. Moreover, the transient reactions manifested the fast reaction between them and methane. Hence, these steps are probably not the rate-determining steps. Nonetheless, the change of reaction mechanism originated from the property of the nitrates. It is the distinctive reactivity of different nitrates that affected the intermediates during the reaction, and resulted in the change of mechanism. 4.4. Effect of Sulfation. Sulfation affects the properties of support in several aspects. First, sulfation of γ-Al2O3 increases the acidity of the support,24,25 which is responsible for the activity of oxygenated hydrocarbon.9 On 1RhSA, sulfation weakened the oxidative ability and deterred the further oxidation of methane. Second, SO42- on the surface stabilized the active rhodium species, namely Rh3+ ions. Chin et al.11 suggested the

A Comparative Study of Rh/Al2O3 and Rh/Al2O3/SO42– stabilizing of Pd2+ ions on sulfated zirconia by sulfated groups associated with protons. In this work, XPS and H2-TPR results also showed that the surface rhodium species on 1RhSA was more reducible (compared to 1RhA), highly dispersed, and of a greater binding energy. The shift of nitrosyl adsorbed on 1RhSA also provided evidence for the existence of highly charged cations. Third, mutual effects of sulfation and the loading of active species changed the property of the catalysts. Strong acid sites were increased by loading Co in the study of Co/sulfated zirconia by Li.25 In this work, it is evident that sulfation changed the bonding patterns of formed nitrates. Monodentate nitrates were favored by rhodium alumina, while on Rh/Al2O3/SO42- sulfation resulted in the preference to bridging bidentate nitrates, which could oxidize methane in the assistance of oxygen. In this way, sulfation changed the mechanism of the reactions. 5. Conclusion The activity of Rh/Al2O3/SO42- for CH4-SCR in excess oxygen was promoted by sulfation. The interaction between the surface sulfated groups and rhodium species resulted in the stabilization of well-dispersed Rh3+ ions which could be the active sites in the reaction. Oxygen concentration, temperature, and acidity of the catalysts influenced the way that methane was involved in the reaction, and mild oxidation of methane was facilitated by sulfation. Different from the monodentate nitrates on Rh/Al2O3, nitrates on Rh/Al2O3/SO42- appeared to be in a bridging bidentate form and could react with methane in the presence of oxygen, which caused the generation of nitrosyl. The reaction between nitrosyl and oxygenated hydrocarbon could be the rate-determining step for NO conversion. Acknowledgment. The work was financially supported by the National Natural Science Fund of China (Grant NO. 20677034), the National High-Tech Research and Development (863) Program of China (Grant No. 2006AA060301), and the New Century Excellent Talents in University of China (NCET-2005). References and Notes (1) Li, Y.; Armor, J. N. Appl. Catal. B 1992, 1, 31. (2) Kantcheva, M.; Cayirtepe, I. Catal. Lett. 2007, 115, 148. (3) Fokema, M. D.; Ying, J. Y. Catal. ReV. Sci. Eng. 2001, 43, 1. (4) Burch, R.; Ramli, A. Appl. Catal. B 1998, 15, 49. (5) Burch, R.; Scire, S. Appl. Catal. B 1994, 3, 295. (6) Pietraszek, A.; Da Costa, P.; Marques, R.; Kornelak, P.; Hansen, T. W.; Camra, J.; Najbar, M. Catal. Today 2007, 119, 187. (7) Ohtsuka, H. Appl. Catal. B 2001, 33, 325. (8) Bahamonde, A.; Mohino, F.; Rebollar, M.; Yates, M.; Avila, P.; Mendioroz, S. Catal. Today 2001, 69, 233. (9) Mendioroz, S.; Martin-Rojo, A. B.; Rivera, F.; Martin, J. C.; Bahamonde, A.; Yates, M. Appl. Catal. B 2006, 64, 161. (10) Loughran, C. J.; Resasco, D. E. Appl. Catal. B 1995, 7, 113. (11) Chin, Y. H.; Pisanu, A.; Serventi, L.; Alvarez, W. E.; Resasco, D. E. Catal. Today 1999, 54, 419. (12) Cordoba, L. F.; Sachtler, W. M. H.; de Correa, C. M. Appl. Catal. B 2005, 56, 269. (13) Quincoces, C. E.; Guerrero, S.; Araya, P.; Gonzalez, M. G. Catal. Commun. 2005, 6, 75. (14) Bautista, P.; Faraldos, M.; Yates, M.; Bahamonde, A. Appl. Catal. B 2007, 71, 254. (15) Chin, Y. H.; Alvarez, W. E.; Resasco, D. E. Catal. Today 2000, 62, 291. (16) Okumura, K.; Kusakabe, T.; Niwa, M. Appl. Catal. B 2003, 41, 137. (17) Kantcheva, M.; Cayirtepe, I. J. Mol. Catal. A: Chem. 2006, 247, 88.

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