Reaction Mechanism of H2-Promoted Selective Catalytic Reduction

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J. Phys. Chem. C 2007, 111, 2259-2264

2259

Reaction Mechanism of H2-Promoted Selective Catalytic Reduction of NO with NH3 over Ag/Al2O3 Ken-ichi Shimizu* and Atsushi Satsuma Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan ReceiVed: October 17, 2006; In Final Form: NoVember 22, 2006

The rate of the selective catalytic reduction of NO with ammonia (NH3-SCR) on Ag/Al2O3 at 473 K increased by a factor of 630, when H2 (1%) was added to the reaction gas mixture. The mechanistic cause of this dramatic activity improvement was investigated by kinetic and spectroscopic studies. Kinetic studies indicate that H2 addition increases the rate of NH3 reaction in NH3 + O2 below 673 K and decreases the activation energy for NH3 + O2 reaction, indicating that H2 addition is effective for the oxidative activation of NH3. In situ UV-vis results under reaction conditions show that the Ag+ ion and Agnδ+ cluster (n e 8) coexist during H2-assisted NH3-SCR reaction (H2-NH3-SCR). ESR spectra after exposing H2-NH3-SCR reaction gas mixture at 423 K show that the superoxide ion is formed on the catalyst, and its amount increased with H2 concentration. The steady-state NO reduction rate and relative amount of the cluster during the reaction increased with H2 concentration, and the rate correlates fairly well with the relative amount of the cluster, indicating that the cluster acts as active species in H2-NH3-SCR. It is concluded that Agnδ+ cluster and protons formed by H2 reduction of Ag+ ions are involved in the reductive activation of molecular oxygen into superoxide ion, which should act as effective oxidant for N-H activation of NH3 to NHx (x e 2) intermediate.

Introduction Silver-loaded alumina, Ag/Al2O3, is among the most active and selective catalyst for selective catalytic reduction of NO by hydrocarbons (HC-SCR)1-4 including SCR by alcohol4 or by higher hydrocarbons.2,3 Very recently, much attention is focused on Ag/Al2O3 catalyst5-16 after the finding of a dramatic improvement of HC-SCR activity by H2 addition reported by Satokawa5 and our group.3,6-10 On the other hand, the selective catalytic reduction of NO by urea (urea-SCR), using urea as the ammonia source, is one of the most promising methods for NOx removal from mobile diesel engines.2 Although great efforts have been made to develop a new catalyst for urea-SCR and NH3-SCR, new catalysts with a wide range of operating temperature is needed for reducing the catalyst volume. More recently, Richter et al.15,16 found that hydrogen addition leads to drastic increase in the activity of Ag/Al2O3 for NH3-SCR. Much attention has been focused on mechanistic causes of H2 effect on the activity of Ag/Al2O3 for HC-SCR6-14 and NH3SCR16 though the comprehensive reason for the hydrogen effect is still unclear. As Ag/Al2O3 does not catalyze the NO selective reduction by H2 in excess O2,6 hydrogen does not act as a reducing agent of NO but as a promoter of the HC-SCR and NH3-SCR. For the H2-assisted NH3-SCR (H2-NH3-SCR) reaction on Ag/Al2O3, Kondratenko et al.16 reported transient kinetic evidence indicating that the role of H2 is to transform oxidized Ag species to reduced species that can be active sites for O2 and NO. In the research area of the selective alkane oxidation catalysis, several studies demonstrated that H2 as coreductant plays an important role in the reductive activation of O2.17-20 From these facts, it is expected that the reductive * Corresponding author. Fax +81-52-789-3193, e-mail: kshimizu@ apchem.nagoya-u.ac.jp.

activation of molecular oxygen is the essential role of H2 in the promotion of NH3-SCR on Ag/Al2O3. As for H2-assisted C3H8-SCR, we reported a series of spectroscopic and kinetic studies on the hydrogen effect6-10 and proposed that Agnδ+ cluster (n e 8) and protons formed by H2 reduction of Ag+ ions are involved in the reductive activation of molecular oxygen into superoxide ion, which should act as an effective oxidant for C3H8 activation to the partially oxidized intermediate.10 Materials with clusters21-30 have interesting chemical properties unusual for bulk solids. Size-specific catalysis of supported gold clusters is a well-known example.21-23 Among numerous reports on the synthesis and properties of metal clusters, silver clusters appear to be very popular in this research field, and several review articles were published.24,25 For example, silver clusters can be easily synthesized in the cages and channels of zeolites by vacuum dehydration, by reduction with reducing reagents, and by γ- or X-ray irradiation.8,24-29 Although the preparation of Ag clusters has been demonstrated extensively, very little is known about a unique catalysis of Ag clusters. In this paper, we show catalytic and spectroscopic studies of H2-NH3-SCR with Ag/Al2O3 to give experimental evidence on the mechanistic causes of the hydrogen effect. The relationship between the NO reduction rate and the relative amount of the Agnδ+ cluster, estimated by in situ UV-vis spectroscopy, is examined. Combined with ESR and kinetic results, comprehensive reasons for the hydrogen effect in NH3-SCR is discussed, focusing on the role of the Agnδ+ cluster and protons on the reductive O2 activation to O2- and subsequent N-H activation of NH3. Experimental Section Ag/Al2O3 catalysts were prepared by impregnating γ-AlOOH with an aqueous solution of silver nitrate followed by evapora-

10.1021/jp0668100 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007

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Figure 1. Arrhenius plot for the reactions of NH3-SCR, H2-NH3SCR, NH3 oxidation, and H2-assisted NH3 oxidation on Ag/Al2O3-2. Typical gas composition is NO/NH3/O2/H2 ) 0.1%/0.1%/10%/1%.

tion to dryness at 393 K and by calcination in air at 873 K for 4 h. The Ag/Al2O3 catalysts are designated as Ag/Al2O3-x, where x is the silver loading (wt. %). The H2-NH3-SCR (NO + NH3 + O2 + H2) and NH3 + O2 + H2 reactions were performed in a fixed-bed flow reactor with 0.01-1.0 g catalyst at a flow rate of 100 cm3 min-1. Typical composition of the feed gas was NO/NH3/O2/H2/He ) 0.1%/0.1%/10%/1%/balance, respectively. Kinetic studies were made under the condition where NO and H2 conversions were below 50% by changing catalyst amount. The effluent gas was analyzed by GC and NOx analyzer (Best BCL-100uH). Reaction rates of NO and H2 in H2-NH3-SCR were calculated using the amount of N2 and H2 determined by GC. N2O yield was below 1% in H2-NH3-SCR reaction with Ag/Al2O3 under the conditions of this study. Products of NH3 + O2 + H2 reaction was N2 and N2O detected by GC and NO2 and NO detected by NOx analyzer. The NO conversion in transient catalytic experiment (Figure 4) was estimated with NOx analyzer. Diffuse reflectance UV-vis measurements were made at 373-773 K with UV-vis spectrometer (JASCO V-550) equipped with an in situ flow cell with quartz window used in our previous study.9 A diffuse reflectance sample cell is connected with a gas flow system. The light source is led to the center of an integrating sphere by an optical fiber. Reflectance was converted to pseudo-absorbance using the Kubelka-Munk function. BaSO4 was used to collect a background spectrum. Various gas mixtures were fed at a flow rate of 100 cm3 min-1 to the Ag/ Al2O3 sample (50 mg), which was pretreated in a flow of O2(10%)/He at 823 K. In situ UV-vis spectra were recorded under the same reactant composition as that of the catalytic tests. ESR spectra were measured by a X-band JEOL JES-TE200 spectrometer at a microwave power level (1.0 mW) at which microwave power-saturation of the signals did not occur. The magnetic field was calibrated with a JEOL NMR Field Meter ES-FC5. Prior to the ESR measurements, the sample was exposed to a flow of NO/NH3/O2/H2/He (0.1%/0.1%/10%/1%/ balance) or O2/H2/He (10%/1%/balance) gas mixture at 473 K for 0.5 h and was cooled to room temperature under a flow of the same gas mixture. Then, ESR spectra of the quenched sample were measured after moving the catalyst powder into the suprasil quartz tube without exposure to air. Results NH3-SCR Activity Tests. Figure 1 shows the Arrenius plot for four different reactions over Ag/Al2O3-2: NH3-SCR, H2assisted NH3-SCR (H2-NH3-SCR), NH3 oxidation by O2, H2assisted NH3 oxidation by O2. For each reaction, the reaction rate gave fairly good straight lines, and thus apparent activation

Shimizu and Satsuma energies can be estimated. For the NH3-SCR reaction, H2 addition in the reaction gas mixture dramatically increased the rate of NO reduction but did not decrease the apparent activation energy. For the NH3 + O2 reaction, H2 addition increased the rate of NH3 reaction to N2, N2O, NO2, and NO below 673 K and decreased the apparent activation energy from 165 kJ mol-1 to 22 kJ mol-1. This indicates that H2 addition enhances the oxidative activation of NH3 by reducing the activation energy of the rate-determining step. On Ag/Al2O3-2, the rates of the H2+NH3+O2 reaction were lower than those of the H2-NH3SCR reaction above 473 K. Hence, it is reasonable to assume that the activation energy of 22 kJ/mol for the H2+NH3+O2 reaction is estimated not under mass transfer-limited regimes but under kinetic control regimes. Figure 2 shows the effect of reactant concentrations on the rates of NO reduction to N2 and H2 consumption in H2-NH3SCR with Ag/Al2O3-2. The H2 consumption rate linearly increased with H2 concentration to 5%, while the NO reduction rate linearly increased with H2 concentration to 1% and gradually increased with a further increase in H2 concentration (Figure 2A). This indicates that H2 oxidation to H2O becomes dominant at a higher H2 concentration region. Empirical reaction orders for NO reduction with respect to H2 was estimated from the data; below 1% H2 the reaction order in H2 was ca. 1.1. Note that the result in Figure 2A rules out a possible explanation of the hydrogen effect by the formation of hot-spots by H2 oxidation to H2O. As shown in Figure 2B, the NO reduction rate steeply increased with O2 concentration in a range of 1.4% to 2%, and it was almost unchanged above 4%. The empirical reaction order in O2 below 2% O2 is ca. 2.1, and the order is nearly zero at higher concentration. As shown in Figure 2C, the NO reduction rate linearly increased with an increase in NO concentration to 1500 ppm. The reaction order in NO was ca. 1.1. The effect of NH3 concentration on the reaction rates are shown in Figure 2D. Ag/Al2O3 did not catalyze the NO selective reduction by H2 in excess O2 (NH3 free condition), confirming that hydrogen does not act as a reducing agent of NO but as a promoter of the NH3-SCR. In the NH3 concentration range of 500-3000 ppm, the NO reduction rate decreased with the concentration, and the reaction order in NH3 was ca. -0.43. This suggests that NH3-derived surface species are strongly adsorbed by the active site and inhibit the H2-NH3SCR reaction. For a series of Ag/Al2O3 catalysts with different Ag loadings, NO reduction rates in the H2-assisted NH3-SCR reaction (H2NH3-SCR) at 473 K were measured under the condition where NO and H2 conversions were below 50%. As shown in Figure 3, the rates gradually increase for the lower loadings and increase sharply to 3 wt %. A possible interpretation of this finding is that the important step in the selective NO reduction proceeds only under cooperation of several adjacent Ag atoms, as discussed below. Figure 4 shows the time dependence of NO conversion in NH3-SCR at 473 K over Ag/Al2O3-3. The NO conversion was low (3%) under the NH3-SCR condition (t < 0 s). Upon the addition of H2 at t ) 0 s, the NO conversion immediately increased and finally reached to 96%. The removal of H2 from the reaction mixture at t ) 600 s resulted in a decrease in NO conversion. In Situ UV-vis. Diffuse reflectance UV-vis spectroscopy has been conventionally used for characterizing silver clusters formed by reducing Ag+ ions in oxides, inert gas-solid matrices, and solutions. It is accepted that the bands in a range 40 000-52 000 cm-1 corresponds to 4d10 to the 4d95s1 transition of Ag+ ions, and the bands between 25 000 and 40 000 cm-1

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Figure 2. Effect of (A) H2, (B) O2, (C) NO, or (D) NH3 concentration on the rates of (O) NO reduction to N2 and (b) H2 consumption for H2-C3H8-SCR over Ag/Al2O3-2 at 423 K. Typical gas composition is NO/NH3/O2/H2 ) 0.1%/0.1%/10%/1%.

Figure 3. Effect of Ag loading of Ag/Al2O3 on (O) NO reduction rate in H2-NH3-SCR at 473 K and (b) area of UV-vis band at 28600 cm-1 during the reaction at 473 K (from Figure 7). Gas composition is NO/NH3/O2/H2 ) 0.1%/0.1%/10%/1%.

Figure 5. (A) In situ UV-vis spectra of Ag/Al2O3-3 at 473 K in (a) O2(10%) after oxidation at 823 K and during (b) NH3-SCR and (c) H2-NH3-SCR reactions, and (d) difference spectrum of c subtracted by a. (B) Curve-fitting analysis of the difference spectrum d. Figure 4. Effect of hydrogen switching on/off on NO conversion and the UV-vis band height at 28600 cm-1 during NH3-SCR over Ag/ Al2O3-3 at 473 K. Gas composition is NO/NH3/O2/H2 ) 0.1%/0.1%/ 10%/1%, and catalyst weight is 50 mg.

are due to Agn clusters with different size and oxidation states.8,9,12,30 To investigate the structure of the working catalyst, we adopted in situ diffuse reflectance UV-vis spectroscopy, which was used in our preliminary study.9 The UV-vis spectrum of preoxidized Ag/Al2O3-3 in O2 at 473 K (Figure 5) showed a tail of a band centered above 40 000 cm-1 due to Ag+ ions. During the NH3-SCR reaction at 473 K the spectral feature was very close to that of the preoxidized sample, indicating that Ag+ ions are the predominant Ag species during

the NH3-SCR reaction. During the steady-state H2-NH3-SCR reaction, the band due to Ag+ ion and bands in a range of 25 000-40 000 cm-1 appeared. As shown in Figure 5B, unresolved bands in a difference spectrum during the H2-NH3SCR reaction were deconvoluted into three Gaussian curves. A broad at 20 000 cm-1 is assigned to Ag metal particle and bands at 28 600 and 35 000 cm-1 are assigned to Agnδ+ (n e 8) clusters.8,9,12,30 Thus, it is found that the Agnδ+ cluster and Ag+ coexist on the catalyst under the H2-NH3-SCR condition. Note that large parts of Ag+ ions can be “invisible” for in situ UVvis spectroscopy used in this study because of the following reasons: the spectral data above 40 000 cm-1 is not available due to large noise of our in situ UV-vis apparatus, and some

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Shimizu and Satsuma

Figure 8. ESR spectra recorded at 77 K for Ag/Al2O3-2 (a-d) after H2-NH3-SCR reaction at 423 K with different H2 concentrations: (a) 0%, (b) 0.5%, (c) 1%, (d) 2%. Spectrum e is recorded after H2 + O2 reaction at 423 K. Figure 6. (A) In situ UV-vis spectra of Ag/Al2O3-2 at 423 K during H2-NH3-SCR reaction with different H2 concentrations (vol %). Each spectrum was subtracted by the spectrum in O2(10%) at 423 K. (B) Effect of H2 concentration on the area of UV-vis bands at 20000, 28600, and 35000 cm-1 during the reaction at 423 K. Gas composition is NO/NH3/O2/H2 ) 0.1%/0.1%/10%/0-5%.

Figure 9. Effect of H2 concentration on (O) NO reduction rate on Ag/Al2O3-2 in H2-NH3-SCR at 423 K (data from Figure 2A) and (b) area of ESR signal due to O2- after the reaction (from Figure 8).

Figure 7. In situ UV-vis spectra of Ag/Al2O3 with different Ag loading (wt %) at 473 K during H2-NH3-SCR reaction. Each spectrum was subtracted by the spectrum recorded in O2(10%) at 473 K. Gas composition is NO/NH3/O2/H2 ) 0.1%/0.1%/10%/1%.

types of Ag+ ions are known to have a main absorption band above 50 000 cm-1.12 This point as well as the very high differences in extinction coefficients for Ag+ ions and Agnδ+ clusters12 accounts for the absence of a decrease in the spectrum intensity of Ag+ ions at 40 000 cm-1 when Agnδ+ clusters are formed. Another explanation is that a minor part of the Ag+ ion species is reduced to Agnδ+ clusters under the reaction conditions. Difference UV-vis spectra of Ag/Al2O3-2 during the H2NH3-SCR reaction at 423 K were recorded with different H2 concentrations (Figure 6A). Curve-fitting analysis of each spectrum was performed using Gaussian curves centered at 20 000, 28 600, and 35 000 cm-1, and the peak area of each band is plotted as a function of H2 concentration in Figure 6B. Difference UV-vis spectra during the H2-NH3-SCR reaction at 473 K on Ag/Al2O3 with different Ag loadings are shown in Figure 7. Intensity of the band at 28 600 cm-1 assignable to the Agnδ+ cluster increased with loading. Curve-fitting analysis of each spectrum was performed using Gaussian curves centered

at 20 000, 28 600, and 35 000 cm-1, and the peak area of the band due to the Agnδ+ cluster (28 600 cm-1) is plotted as a function of Ag loading in Figure 3. Figure 4 shows the time dependence of the band height for Agnδ+ cluster (28 600 cm-1) during the NH3-SCR reaction at 473 K over Ag/Al2O3-3. The measurement was performed under the same GHSV condition and reactant compositions as those of the catalytic test in Figure 4. Upon H2 addition, the band height increased and finally reached a constant. The removal of H2 from the reaction mixture at t ) 600 s resulted in a decrease in the band height. ESR Evidence for O2- Formation. Figure 8 shows ESR spectra recorded at 77 K for Ag/Al2O3-2 after various pretreatments. After the NO + NH3 + O2 (NH3-SCR) reaction (spectrum a) at 423 K as well as after calcination in O2 (result not shown), the sample showed no ESR signals. After the H2NH3-SCR reaction (spectrum b-d), the Ag/Al2O3-2 sample showed ESR spectrum with anisotropic g values (gxx ) 2.025, gyy ) 2.009, and gzz ) 2.001). This signal has been previously identified as O2- (super oxide) ion on silver catalysts.35-37 To investigate the effect of H2 concentration in H2-NH3-SCR reaction on the relative amount of O2- radicals, the O2- ESR signal intensity was quantified from the double integration of the signal. As plotted in Figure 9, the relative amount of O2increased with H2 concentration. As reported in our previous study,10 the ESR signal due to O2- ion was also observed after treating Ag/Al2O3-2 catalyst in H2 (0.5%) + O2 (10%) at 423 K (spectrum e).

Catalytic Reduction of NO with NH3

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Figure 10. NO reduction rate for H2-NH3-SCR at 473 K on Ag/ Al2O3 of various Ag loadings as a function of area of UV-vis band at 28600 cm-1. Numbers in the figure denote Ag loading (wt%).

Figure 11. Rates of NO reduction in H2-NH3-SCR over Ag/Al2O3-2 at 423 K with different H2 concentration (data from Figure 2A) vs area of UV-vis band at 28600 cm-1 during the reaction at 423 K (data from Figure 6). Numbers in the figure denote H2 concentration.

Discussion In our previous report on the mechanistic cause of activity improvement of Ag/Al2O3 by H2 addition for the selective catalytic reduction of NO with propane (C3H8-SCR), structural changes of Ag species upon H2 reduction and subsequent O2 reoxidation were investigated by various spectroscopic characterizations.10 Summarizing the results of in situ UV-vis, in situ Ag K-edge EXAFS, and D2 adsorption IR, it was shown that reduction of Ag+ ions on Ag/Al2O3-2 by H2 at 573 K yields protons on alumina and a partially reduced Agnδ+ cluster (n e 8). The H2 adsorption microcalorimetric experiment showed the following stoichiometry for the H2 reduction of Ag+ ions:

4xAg+ + xH2 f Ag4x2x+ + 2xH+ From in situ EXAFS and UV-vis results, it was also shown that part of the Ag+ ions on Ag/Al2O3-2 are reduced to Agnδ+ clusters (n e 8) during H2 + O2 reaction.10 As for the structure of Ag species during H2-NH3-SCR reaction, the in situ UVvis result provides structural information of the working catalyst. The position of the UV-vis band at 28600 cm-1 observed under the H2-NH3-SCR condition is same as that observed during H2 + O2 reaction,10 indicating that the Agnδ+ cluster is present on the catalyst during the H2-NH3-SCR reaction. From the data in Figure 3, the rate of NO reduction in the H2-NH3SCR reaction over Ag/Al2O3 with different Ag loadings is plotted as a function of intensity of the UV-vis band due to Agnδ+ cluster during the H2-NH3-SCR reaction (Figure 10). The rate increased with the relative amount of Agnδ+ cluster under the reaction condition. From the data in Figure 7, the rate of NO reduction in H2-NH3-SCR over Ag/Al2O3-2 under different H2 concentrations is plotted in Figure 11 as a function of intensity of UV-vis band due to the Agnδ+ cluster. The rate

increased linearly with the band area, that is, the relative amount of Agnδ+ cluster. These results indicate that the Agnδ+ cluster is the catalytically important species for the H2-NH3-SCR reaction under a steady state. In our previous UV-vis and EXAFS studies, it was established that the exposure of the Agnδ+ cluster to O2 results in the reoxidation and redispersion of the cluster to Ag+ ion.10 IR results showed that D2 treatment of Ag/Al2O3-2 resulted in the formation of acidic OD groups, and the generated acidic OD groups were consumed by the reaction with O2 to yield D2O.10 These results indicate that the Agnδ+ cluster and protons, formed by the H2 reaction of Ag+ ions on Ag/Al2O3, react with O2 to produce H2O and Ag+ ion. The kinetic results indicate that the H2 addition results in (1) a significant increase in the rate of NH3-SCR, (2) an increase in the rate of NH3 reaction in NH3 + O2 below 673 K, and (3) a decrease in the activation energy (from 165 to 22 kJ mol-1) for the NH3 + O2 reaction. Gang et al.31 studied the mechanism of ammonia oxidation on silver catalyst, and their NH3-TPD and TPR results showed that on oxidized silver surface NH3 dissociatively adsorbs as an NHx species with one or two hydrogen abstracted by surface oxygen. Assuming that this reaction is the initial step for ammonia oxidation on Ag/Al2O3, it is deduced that H2 addition promotes the oxidative hydrogen abstraction of NH3 by surface oxygen. Previously, we reported that H2 addition promotes the oxidation of NO to adsorbed nitrates or NO2.6,7 These results suggest that H2 plays an important role in the oxidative activation of NH3 and NO, which should result in the promotion of NH3-SCR. Recently, Kondratenko et al.16 reported a transient isotope study on mechanism of H2-NH3-SCR on Ag/Al2O3. On the basis of the result that H2-prereduced catalyst shows considerably higher transient activity in NH3-SCR, they proposed that O2 is activated by reduced Ag species to generate reactive oxygen species, which then dehydrogenates NH3 to NHx (x e 2) intermediates. Numerous mechanistic studies on the NH3-SCR reaction over various catalysts proposed that the oxidative N-H activation of NH3 to produce NHx (x e 2) is an important step.31-34 Our ESR results showed direct evidence on the reductive activation of molecular oxygen into O2- during H2NH3-SCR and H2 + O2 reactions on Ag/Al2O3-2. As shown in Figure 9, the NO reduction rate correlates fairly well with the relative amount of superoxide ions produced in the reaction condition, indicating that O2- plays an important role in H2NH3-SCR. In summary, the following role of hydrogen on the promoting of NH3-SCR is presented. In the absence of hydrogen, the activation of molecular oxygen on the surface into reactive oxygen species is slow. The hydrogen addition promotes the reaction of molecular oxygen into reactive oxygen species, O2-, that is involved in the oxidative activation of NO and NH3 to NO2 and NHx (x e 2), which is involved in the N2 formation. The mechanism of H2-assisted NH3-SCR is presented in Scheme 1. The reaction consists of the following steps: (1) the H2 dissociation on the Ag site, (2) spillover of the H atom to form proton on alumina, (3) aggregation of isolated Ag species to form reduced the Agnδ+ cluster (n e 8), (4) O2 reduction under the cooperation of the cluster and H+ to

2264 J. Phys. Chem. C, Vol. 111, No. 5, 2007 yield O2-, H2O, and Ag species with higher oxidation state than Agnδ+ cluster (Agn(δ+x)+ or Ag+), (5) N-H activation of NH3 by O2- to yield NHx (x e 2), (6) NO oxidation by O2- to NO2, (7) reaction of NHx with NO2 to yield N2 and H2O. It is interesting to note that the promotion effect of hydrogen for the redox catalysis was also reported in the studies on the partial oxidation of hydrocarbons.17-20 Wang et al.17 proposed that hydrogen, as the electron donor as well as the proton donor, reductively activates molecular oxygen. For H2-assisted allyl alcohol epoxidation over a titanosilicate catalyst, the formation of O2- intermediates via H2 + O2 reaction is proposed as a key step.18 In general, the term “reductive oxidation” stands for the process shown:

O2 + 2H+ + 2e- f [O] + H2O where [O] represents reactive oxygen species, and a good example employing this process in hydrocarbon oxidation is given in a group of enzymes called monooxygenases.20 The formation of water is a key to yield unstable reactive oxygen atoms in a thermodynamically favorable manner. As shown in Figure 3, the rate of NO reduction also depends strongly on Ag loading. The lowest loading catalyst is nearly inactive. The rate increased sharply above 1 wt % loading. In our previous paper, the rate of Ag+ reduction in H2 to silver clusters was evaluated by the time-resolved UV-vis method, and it was found that the reduction rate increased with Ag loading. Richter et al. reported H2-TPR data of preoxidized Ag/Al2O3 with different Ag loadings and showed that the catalyst with higher Ag loading has higher reducibility of Ag+ species.13 From these findings, the following conclusions on the structure-activity relationship can be drawn. The mononuclear Ag+ species on the lowest loading catalyst (0.5 wt %) have low reducibility during the H2-NH3-SCR reaction (Figure 6), and consequently the reductive O2 activation does not occur. Over the higher loading catalyst (1-3 wt %), having Ag+ ions with higher reducibility, the cluster and cooperating protons formed by H2 reduction react with O2 to yield O2-, which is an important intermediate in the NO reduction. Conclusions During the H2-assisted NH3-SCR reaction over Ag/Al2O3, the Agnδ+ cluster (n e 8) and the superoxide ion are produced on the catalyst. The steady-state NO reduction rate correlates fairly well with the relative amount of the cluster during the reaction. The rate also correlates fairly well with the relative amount of superoxide ions produced in the reaction. It is concluded that the Agnδ+ cluster and superoxide ion act as active species in the H2-assisted NH3-SCR reaction. The Agnδ+ cluster and protons formed by H2 reduction of Ag+ ions are involved

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