Intermediates in the Selective Reduction of NO by Propene over Cu

J. Phys. Chem. B 2000, 104, 2885-2893

2885

Intermediates in the Selective Reduction of NO by Propene over Cu-Al2O3 Catalysts: Transient in-Situ FTIR Study Ken-ichi Shimizu,* Hisaya Kawabata, Hajime Maeshima, Atsushi Satsuma,* and Tadashi Hattori Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan ReceiVed: August 30, 1999; In Final Form: NoVember 23, 1999

The mechanism of the selective catalytic reduction (SCR) of NO by C3H6 on Cu-Al2O3 catalysts, which consist of highly dispersed Cu2+ ions in the surface aluminate phase, are investigated by in-situ FTIR spectroscopy. During NO + C3H6 + O2 reaction, the acetate is produced via the partial oxidation of C3H6 and becomes the predominant adspecies in the steady-state condition at 473-623 K. The acetate, which is stable in NO, is quite reactive with NO + O2, leading to the formation of isocyanate species (Cu-NCO) on the surface and N2 and CO2 in the gas phase. The rate of acetate reaction in NO + O2 is close to the steadystate rate of NO reduction over wide range of temperature, indicating that the acetate is an intermediate in the SCR and takes part in the rate-determining stage. A mechanism is proposed; the acetate and nitrates, formed by NO + O2, react to generate the Cu-NCO species, then Cu-NCO reacts with nitrates or NO to produce N2 and CO2. This mechanism explains the role of oxygen in facilitating SCR. Cu2+ ion is the principal active component in Cu-Al2O3 catalysts; it plays crucial roles in all the important steps, including the reaction of the acetate with nitrates.

Introduction Selective catalytic reduction of nitrogen oxides by hydrocarbons (HC-SCR) has attracted much attention, because it has a potential ability to remove NOx from diesel and other oxygen rich exhausts.1,2 Although the majority of the HC-SCR catalysts reported are metal-containing zeolites, such as Cu-ZSM-5,1,2 we have recently reported that Cu-aluminate catalysts, containing highly dispersed Cu2+ ions in the aluminate phase, showed high de-NOx activity comparable to Cu-ZSM-5 and higher hydrothermal stability than Cu-ZSM-5.3 Extensive studies have been devoted for understanding the mechanism of HC-SCR. In earlier studies, the reaction mechanism was proposed on the basis of some empirical observations. For example, the observation that reaction does not take place without oxygen has been discussed in terms of the role of oxygen and attributed to different causes. One is the possible formation of NO2 as a key intermediate.3,4 It has been also suggested that the partial oxidation of hydrocarbon molecules to generate HxCyOz species could be the active reductant for NOx.3,5,6 Afterward, various surface species, such as NOx adspecies,7,8 NCO,7,9 and CN7,9,10 species, formed during adsorption of reagents were identified by using IR, and they were suggested to be possible intermediates. As Matyshak et al. noted,11 most of the early mechanistic studies were performed under conditions that were not close to those of catalytic reaction. Recently some studies have succeeded to observe surface complexes in situ (under reaction conditions). However, observed species are not guaranteed to be directly involved in the reaction pathway, unless dynamic response of them is studied. Further, the kinetics of the reaction of the adsorbed species should be tested and compared with the rate of the steady-state reaction to obtain quantitative evidence on the * Address to whom correspondence should be addressed.

intermediacy of the adsorbed species.11-13 As for the mechanistic study of HC-SCR, very few attempts were devoted to this aspect, and hence the reaction mechanism of HC-SCR has not yet been fully clarified. In addition, most of the detailed mechanistic studies have been focused on the metal-exchanged zeolites,14-20 while rather few have focused on the mechanism over the metal oxide based catalysts.21-23 The role of transition metal and the support in the reaction is also a matter of controversy. For example, Hamada et al.24 studied SCR by C3H6 on Co/Al2O3, and suggested that the transition metal and the support play a different role; NO is oxidized to NO2 on impregnated cobalt species, and NO2 reacts with C3H6 to form N2 on Al2O3. On the other hand, we have recently reported a reaction study of HC-SCR on the transition metal aluminate catalysts, and suggested that a primary role of Al2O3 support is to atomically disperse the transition metal cations, which are responsible for the activity of HC-SCR.3 In this study, we have carried out systematic in-situ FTIR studies of the formation and the reaction of adsorbed species in C3H6-SCR on Cu-Al2O3 catalyst. It was of particular interest to clarify the role of adsorbed acetate; the transient reactions of acetate with NO + O2 were followed by IR and GC analysis, and the reaction rates of surface and gas-phase molecules were compared. Further, the transient reaction rate is compared with the steady-state rate of NO reduction to prove that the acetate is an intermediate of this reaction. By comparison with the results on Al2O3, the role of dispersed Cu ions and the Al2O3 support in each reaction steps is discussed. Experimental Section Catalysts. Al2O3 (gamma type, a reference catalyst JRCALO-1A25) was supplied from the Catalysis Society of Japan. Supported catalysts, Cu-Al2O3, were prepared by impregnating

10.1021/jp9930705 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/09/2000

2886 J. Phys. Chem. B, Vol. 104, No. 13, 2000

Shimizu et al.

TABLE 1: Physical Characteristics of the Catalysts catalystsa

bulk Cu mol %

surface Cu mol %b

surface area m2 g-1

Al2O3 Cu(2)-Al2O3 Cu(4)-Al2O3 Cu(8)-Al2O3

0 1.6 3.2 6.5

0 4.2 6.5 10.0

160 98 82 76

a Cu content (wt %). b Determined from Al 2s and Cu 2p3/2 peaks in XPS.

Al2O3 with an aqueous solution of copper acetate followed by evaporation to dryness at 395 K and by calcination in air at 1073 K for 12 h. The calcination condition of Cu-Al2O3, which is similar to that employed by Hierl et al.26,27 would result in the formation of the copper aluminate phase on Al2O3 surface as will be confirmed later. The specific surface area of the sample was determined using nitrogen adsorption (BET method). X-ray diffraction patterns of the powdered catalysts were taken in a Rigaku RINT 1100. UV-vis spectra were measured using UV-vis spectrometer (JASCO V-750) in a diffuse reflectance mode. XPS measurements were performed with AXIS-HSi photoelectron spectrometer using Al KR radiation (1485.9 eV). In-Situ FTIR. The samples were pressed into 0.04-0.07 g of self-supporting wafers and mounted into a quartz IR cell with CaF2 windows. In situ IR spectra were recorded on a JASCO FT/IR-620 equipped with the IR cell connected to a conventional flow reaction system, which was used in our previous studies.18,22,23 All the spectra were measured at the reaction temperatures accumulating 100 scans at a resolution of 2 cm-1. A reference spectrum of the catalyst in He was subtracted from each spectrum. Prior to each experiment the catalyst was (1) heated in 6.7% O2/He at 773 K for 1 h, (2) cooled to the desired temperature and purged for 30 min with He, (3) and then various gas mixtures were fed at a flow rate of 42 cm3 min-1. The concentration of NO, C3H6, and O2 in the gas mixtures were 1000 ppm, 1000 ppm, and 6.7% with He balance, respectively. The extinction coefficient of the acetate and nitrates on the catalyst were determined in the same fashion as described in our previous study.23 The extinction coefficient of the acetate and nitrates were 1.7 × 10-17 and 1.3 × 10-17 cm-1‚cm2/ molecule, respectively. They were independent of the temperature of IR measurement and the Cu loading on the catalyst within an experimental error ((7%). Catalytic Test. To determine the steady-state and transient rate of the gaseous product formation, separate experiments were performed in a conventional flow reactor with 0.05-2.0 g of a catalyst at a flow rate of 100 cm3 min-1 under the condition where both NO and C3H6 conversions were below 30%. Product analysis was performed by a NOx analyzer (Best BCL-100 uH) and a gas chromatograph with 13X molecular sieve and Porapak Q columns. The pretreatment of the sample and the concentrations of reaction mixtures were the same as in the corresponding in-situ IR experiments. It should be noted that no formation of CO or N2O were observed in all the reaction experiments. Results Structure of the Catalysts. Table 1 lists the bulk and surface concentration of Cu and the BET surface area of the catalysts employed. For all the Cu-Al2O3 catalysts, the surface concentration of Cu atom as determined by XPS was higher than the bulk Cu concentration, indicating a surface enhancement of Cu. The XRD analysis of Cu-Al2O3 samples showed diffraction lines due to a spinel phase, but no lines due to CuO.3 Figure 1 shows UV-vis diffuse reflectance spectrum of samples. There

Figure 1. UV-vis diffuse reflectance spectra of Cu-Al2O3 catalysts of various Cu loading.

were two absorption bands at 1450 nm and 750 nm, which have been assigned to the electric transitions in Cu2+ in tetrahedral and octahedral sites of spinel lattice, respectively.27 These results are similar to those observed by Hierl et al.26,27 for Al2O3supported Cu catalyst, whose calculation condition is similar to that employed in this study. According to their proposal, the most probable picture of the Cu species in Cu-Al2O3 catalysts is the surface spinel, in which Cu2+ ions are incorporated in the alumina surface.26,27 Therefore, it was shown that Cu-Al2O3 catalysts employed in this study consist of the highly dispersed Cu2+ species surrounded by Al2O3. Formation of Adsorbed Species during SCR Reaction. Figure 2 shows the IR spectra of adsorbed species during NO + C3H6 + O2 reaction at various temperatures (473-623 K) in the steady-states. Figure 2a includes the spectra recorded at 523 K during C3H6 + O2 reaction and after an introduction of the acetic acid (120 µmol/g-catalyst) for comparison. In flowing NO + C3H6 + O2, two strong bands at around 1580-1590 and 1452 cm-1 were observed. These bands were in good agreement with the bands observed in flowing C3H6 + O2, and those of acetic acid adsorbed on the same catalyst. These bands are very close to those assigned to νas(COO) and νs(COO) of adsorbed acetate, which were observed during C3H6-SCR on Al2O3 in our previous studies.22,23 Thus, the bands at around 1580-1590 and 1452 cm-1 were assigned to νas(COO) and νs(COO) of the adsorbed acetate, respectively. Weak bands at 1720, 1642, 1392, 1376, 1311, and 1278 cm-1 observed in NO + C3H6 + O2 were in good agreement with the bands observed in C3H6 + O2. These bands are therefore the nitrogen-free species and can be assigned to carbonyl (1720 cm-1),18 formate (1392 and 1376 cm-1),22,23 and carbonate species (1642, 1311, and 1278 cm-1).28-30 A small band at 1300 cm-1 observed in NO + C3H6 + O2 may be assigned to the unidentate nitrate.23 In the region of 23002100 cm-1 (Figure 1b), small bands at 2236, 2198, and 2150 cm-1 were observed. The band at 2198 cm-1, which is similar to that assigned by Kung et al.10 to NCO species on Cu-ZrO2 (2190 cm-1) and that assigned by Solymosi et al.31 to NCO species bound to the Cu ions (2180-2201 cm-1), is assigned

Intermediates in Selective Reduction of NO by Propene

J. Phys. Chem. B, Vol. 104, No. 13, 2000 2887

Figure 4. Time dependence of the acetate concentration in (O) NO + C3H6 +O2 and in (b) C3H6 + O2 on Cu(8)-Al2O3 and in (4) NO + C3H6 + O2 on Al2O3 at 523 K. Figure 2. IR spectra of adsorbed species in a flow of NO + C3H6 + O2 for 200 min over Cu(8)-Al2O3 at various temperatures. Figure 2b includes the spectrum of adsorbed acetic acid and that in flowing C3H6 + O2 for 200 min at 523 K.

Figure 3. IR spectra of adsorbed species on Al2O3 and Cu-Al2O3 with various Cu loading in a flow of NO + C3H6 + O2 for 200 min at 523 K. (The spectra were normalized per gram and the surface area of the catalysts.)

to NCO species bound to the Cu ions (Cu-NCO). The band at 2236 cm-1 can be assigned to NCO species bound to Al (AlNCO), since the band has been observed on Al2O3 by Ukisu et al.32 The band at 2150 cm-1 can be assigned to CN species.10,32,33 Figure 3 shows the IR spectra recorded during NO + C3H6 + O2 reaction in a steady-state at 523 K on Cu-Al2O3 with various Cu loading and on Al2O3. In the case of Cu-Al2O3, two main bands due to the acetate (1580-1590 cm-1 and 1452 cm-1) were observed, together with small bands due to carbonyl (1720 cm-1), binyl (1642 cm-1), formate (1392 and 1376 cm-1), carbonate (1311 and 1278 cm-1), Al-NCO (2236 cm-1), CuNCO (2198 cm-1), and CN species (2150 cm-1). A shoulder at around 2260 cm-1 can be assigned to Al-NCO.31,32,34 These results indicate that the acetate should be the main adspecies during the reaction on a series of the catalysts tested. In the case of Al2O3, νs(COO) of the acetate band is centered at 1465 cm-1, which is higher than those on Cu-Al2O3 catalysts (1452 cm-1). In the O-H stretching region of the spectra (Figure 3c), bands with negative absorbance appeared above 3700 cm-1. In

our previous study of C3H6-SCR on Al2O3,23 this phenomena has been explained that the specific OH groups are reactive and removed or exchanged upon formation of acetate, which results in a coordination of acetate on the coordinatively unsaturated M-O site (MCUS-O site). For Cu-Al2O3, the negative bands appeared at around 3740 ( 4 cm-1, while two bands appeared at 3764 and 3720 cm-1 for Al2O3. This suggests that the adsorption sites of acetate on Cu-Al2O3 and Al2O3 are different in nature from each other. Together with the result that position of the acetate band for Al2O3 was higher than that for CuAl2O3, it is suggested that the acetate on Cu-Al2O3 catalysts is not associated with the AlCUS-O site but the CuCUS-O site. Figure 4 shows the time course of the acetate concentration in a flow of NO + C3H6 + O2 on Cu(8)-Al2O3 at 523 K. The acetate concentration was estimated by using integrated intensities of the acetate band (1452 cm-1) and the extinction coefficient of acetate. During C3H6-SCR reaction, the acetate concentration increased with time-on-stream and became constant. It should be noted that intensity of the negative band at 3740 cm-1 increased with time-on-stream in a flow of NO + C3H6 + O2 (results not shown). This trend is likely to coincide with an increase of the acetate concentration during NO + C3H6 + O2 reaction, suggesting again the coordination of the acetate on CuCUS-O sites. As shown in Figure 2, the acetate was also formed in C3H6 + O2. Figure 4 includes the change in the acetate band during the reaction of C3H6 + O2 at 523 K. The steadystate concentration of acetate in NO + C3H6 + O2 was almost the same as in C3H6 + O2. The initial rate of acetate formation in NO + C3H6 + O2, determined from the slope of the curve was 2.0 nmol/m2 s, and was 2 times higher than that in C3H6 + O2 (1.0 nmol/m2 s). Change in the acetate concentration during NO + C3H6 + O2 reaction on Al2O3 is also shown in Figure 4. The initial rate of acetate formation on Al2O3 (0.15 nmol/m2 s) was about 13 times lower than that on Cu(8)-Al2O3. Formation and Reaction of Nitrates. In a flow of NO + O2 over Cu(8)-Al2O3 catalyst (Figure 5A, spectrum c), couples of bands in the region of 1650-1500 cm-1 and 1170-1300 cm-1 were observed. These bands are generally assigned to the ν3 split stretching vibration of the nitrates on the metal oxides.22,23,35-37 As shown in the figure, these bands are close to those assignable to nitrates on Al2O3 (spectrum b) but slightly differ in their peak positions. According to the literature,37 the


Figure 5. (A) IR spectra of adsorbed species on Cu(8)-Al2O3 (a, c-f) and on Al2O3 (b) at 523 K. The spectra were measured (a) in a flow of NO for 60 min, (b, c) in a flow of NO + O2 for 180 min, (d) He purge for 10 min after (c), (e) in a flow of C3H6 for 10 min after (c), and (d) in a flow of C3H6 + O2 for 10 min after (c). (B) Time dependence of nitrate concentration on (O) Cu(8)-Al2O3 and (4) Al2O3.

bands at 1544 and 1296 cm-1, the bands at 1574 and 1256 cm-1 and the bands at 1600 and 1208 cm-1 observed in this study are assigned to unidentate, bidentate, and bridging nitrates, respectively. In a flow of NO (spectrum a), bands assignable to nitrates were observed at around 1600-1574 cm-1, though their intensity was much less than in NO + O2. This indicates that the formation of nitrates is greatly promoted by the presence of O2. The reactivity of nitrates toward hydrocarbon reductant was evaluated by the transient response of the IR spectra at 523 K. The nitrate bands (in the region 1170-1300 cm-1) formed in a flow of NO + O2, which was relatively stable in He (spectrum d), disappeared by exposing to a flow of C3H6 (spectrum e) or C3H6 + O2 (spectrum f) for 10 min, and the bands due to acetate (1452 cm-1), formate (1392, 1376 cm-1), and carbonates (1311, 1276 cm-1) simultaneously appeared. This indicates that nitrates can chemically interact with hydrocarbon species and take part in hydrocarbon oxidation to the carboxylates and carbonates. In addition, the bands due to Al-NCO (2236 cm-1), Cu-NCO (2198 cm-1), and CN (2150 cm-1) also appeared by the reaction of nitrates with C3H6 or C3H6 + O2, which indicates the N-insertion from the nitrates to hydrocarbon or partially oxidized hydrocarbon species. Figure 5B shows the time course of the nitrate concentration during the nitrate formation in NO + O2 and the nitrate reaction with C3H6 mentioned above. The reaction rate of nitrate in a flow of C3H6 estimated from the slope of the curve (2.0 nmol/ m2 s) was higher than that of nitrate formation in NO + O2 (0.87 nmol/m2 s). Change in the nitrate concentration during the same experiment on Al2O3 is also shown in Figure 5B. The rate of nitrate formation on Al2O3 (0.44 nmol/m2 s) was about 2 times lower than that on Cu(8)-Al2O3. The rate of nitrate reaction with C3H6 over Al2O3 (0.13 nmol/m2 s) was about 15 times lower than that over Cu(8)-Al2O3. By using the flow reactor, the rate of NO2 formation in NO + O2 reaction over Cu(8)-Al2O3 catalyst was measured at 523 K. The rate is listed in Table 2, together with the rates of other reactions on Cu(8)-Al2O3 catalyst at 523 K determined by insitu IR and flow reaction experiments. The rate of NO2 formation (0.35 nmol/m2 s) was lower that the rate of NO3formation (0.87 nmol/m2 s). Reaction of Adsorbed Acetate with NO + O2. The reactivity of the acetate toward NOx species was examined by the in-situ IR experiment at 523 K. Acetic acid (120 µmol/g) was first introduced to the IR disc of Cu(8)-Al2O3 as a pulse in the He carrier at 523 K, which resulted in the formation of the adsorbed acetate (the spectrum shown in Figure 2). Then,

Shimizu et al. the catalyst was exposed to a flow of NO + O2, while recording IR spectra as a function of time. Figure 6A shows the changes in the integrated intensities of the bands due to acetate, CuNCO, and nitrates with time on stream. The intensity of acetate band steeply dropped by exposure to a flow of NO + O2. Simultaneously, the band due to Cu-NCO appeared, and its intensity increased with time up to 9 min and then decreased. The nitrate band appeared after an induction period of 15 min, and its intensity increased gradually with time. Figure 6B shows the results of another set of experiments to determine the rate of gaseous product formation in the reaction of adsorbed acetate with NO + O2. When the catalyst, pretreated by the introduction of acetic acid (120 µmol/g) at 523 K, was exposed to a flow of NO + O2, the formation of N2 and CO2 was observed. The formation rates of N2 and CO2 first increased with time and passed through a maximum at 17 min, and then decreased. From a comparison with the results in Figure 6A, it is likely that N2 and CO2 are produced through a consecutive reaction of the acetate with NO + O2 (or possibly nitrates) via Cu-NCO species. On the other hand, when the adsorbed acetate on the catalyst was exposed to a flow of NO, the acetate band hardly decreased and N2 and CO2 were hardly produced (results not shown). From these results, it was shown that the acetate is active as a reductant and reacts with NO + O2 (possibly with nitrates) to produce N2 and CO2 via the formation and reaction of Cu-NCO species. It should be noted that the reaction of the formate, formed by introducing the formic acid (120 µmol/g) on the same catalyst, with NO + O2 at 523 K did not produce N2, but resulted in a rapid desorption and/or oxidation of the formate to CO2. Transient Reaction. The reactivity of adsorbed species formed during C3H6-SCR reaction toward NOx species can be evaluated by the transient response of IR spectra. The wafer of Cu(8)-Al2O3 catalyst was first exposed to a flow of NO + C3H6 + O2 for 200 min at 523 K, and then the flowing gas was switched to He, NO, O2, or NO + O2. Figure 7 shows the change in the IR spectra as a function of time after the feed was switched to NO + O2. As NO + O2 was passed over the catalyst, there was a gradual decrease in acetate band (1452 cm-1), rather steep decrease in Cu-NCO (2198 cm-1) and CN bands (2150 cm-1), and in contrast, a slow decrease in Al-NCO band (2235 cm-1). Meanwhile, nitrate band was progressively formed after 12 min. Figure 8 shows the change in the integrated intensity of acetate bands. The intensity hardly decreased in He or in NO, indicating that the acetate is stable in an inert gas and in NO. The acetate band fairly decreased in O2, indicating that the acetate fairly reacts with O2. The band significantly decreased in NO + O2, indicating that the acetate has high reactivity with NO + O2. It should be noted that Cu-NCO band (2198 cm-1) also decreases in a flow of NO, O2, or He with rather lower rates than in NO + O2. Figure 9 shows the results of another set of experiments to determine the rate of gaseous products formed during the same series of transient reactions. The Cu(8)-Al2O3 catalyst was first pretreated at 523 K for 200 min in a flow of NO + C3H6 + O2 to obtain the steady-state activity. The catalyst was then exposed to a stream of He, O2, NO, or NO + O2 at 523 K. When the pretreated catalyst was exposed to a flow of NO + O2, the formation of N2 and CO2 was observed with a gradual decrease in their formation rates. N2 was hardly produced in a flow of O2, but CO2 was produced with a slightly lower formation rate than in NO + O2. In a flow of He, the formation of N2 and CO2 was hardly observed. In a flow of NO, the formation of N2 and CO2 was observed, though their formation rates were



TABLE 2: Rates in Steady-State and Transient Reactions at 523 K over Cu(8)-Al2O3 rate nmol/m2 s reaction conditions

species

formation

C3H6 + NO + O2 C3H6 + O2 NO + O2 flow after steady-state in C3H6 + NO + O2 NO + O2 NO + O2 C3H6 flow after NO + O2 Steady-state in C3H6 + NO + O2

acetate acetate acetate nitrate NO2 nitrate N2 CO2 N2 CO2

2.0a 1.0a

NO + O2 flow after acetic acid adsorption a

consumption

0.26a 0.87a 0.35b 2.0a 0.14b 1.9b 0.19c 1.9c

comments Figure 4 Figure 4 Figure 10 Figure 5B Figure 5B Figure 9A Figure 9B Figure 6B

Initial rates in transient conditions. b Rates in steady-state conditions. c Rates at t ) 17 min in Figure 6B.

Figure 7. Dynamic changes in the IR spectra as a function of time in flowing NO + O2 on Cu(8)-Al2O3 at 523 K. Before the measurement, the catalyst was pre-exposed to a flow of NO + C3H6 + O2 for 200 min at 523 K.

Figure 6. Dynamic changes in the (A) surface and (B) gas-phase molecular as a function of time in a flow of NO + O2 at 523 K over Cu(8)-Al2O3, on which the acetic acid (120 µmol/g) was preadsorbed: intensity of the IR bands due to (b) acetate, (1) Cu-NCO, and (]) NO3-, rates of (O) N2 and (4) CO2 formation.

much less than in NO + O2. As mentioned above, the acetate band hardly decreased, but Cu-NCO band decreased in a flow of NO. Thus, the results should indicate that the reaction of Cu-NCO species with NO resulted in N2 formation. This interpretation is supported by the results of Iwamoto et al.,38 in which the NCO species on Cu-ZSM-5 catalyst gave N2 by the reaction with NO. Figure 10 shows the first-order plots of NO reduction to N2 and CO2 formation during the transient reaction of surface adspecies with NO + O2. The result gave fairly good straight lines. It should be noted that the extrapolation of the first-order plot for NO reduction rate to the intercept was consistent with

the steady-state rate constant of NO reduction, which indicates that the steady-state N2 formation occurs by the reaction of the adsorbed intermediates with NO + O2. On the other hand, the rate of acetate consumption, d[CH3COO-]/dt, during the transient reaction of CH3COO- with NO + O2, which was determined by using the extinction coefficient of the acetate band and by numerically differentiating the IR data in Figure 8, was plotted in Figure 10. Clearly, the rate of acetate consumption was very close to the transient rate of NO reduction to N2 over a wide range of reaction time. In addition, the rate of acetate consumption was the same order of magnitude as the rates of CO2 formation during the same transient reaction. The first-order rate constant of acetate consumption was estimated to be 4.6 × 10-4 s-1, which was close to those of N2 (2.5 × 10-4 s-1) and CO2 (2.8 × 10-4 s-1) formation. These results confirm that the NO reduction to N2 occurs by the reaction of the adsorbed acetate with NO + O2. Comparison of Steady-State and Transient Reaction Rates. As shown in Figure 2, the acetate was present as a predominant adsorbed species in the steady-state condition over a wide temperature range (473-623 K). For each temperature, transient rates of acetate consumption in NO + O2 were determined from the transient IR experiments in the same manner as mentioned above. Figure 11 shows the initial rates of acetate consumption for each temperature as an Arrenius plot. Figure 11 includes the steady-state-rates of NO reduction to N2 and C3H6 oxidation to CO2 in NO + C3H6 + O2 reaction over Cu(8)-Al2O3. It is shown that the initial rates of acetate reaction were close to the steady-state rates of N2 and CO2 formation over a wide range of temperatures. Furthermore, Arrenius plots for these rates gave good straight lines, and the apparent


Figure 8. Time dependence of the relative intensities of the acetate band normalized by the initial intensity in (O) NO + O2, (2) O2, (4) NO, and (b) He. Before the measurements, the Cu(8)-Al2O3 catalyst was pretreated in NO + C3H6 + O2 for 200 min at 523 K.

Shimizu et al.

Figure 10. First-order plots of acetate consumption (dotted line), (O) NO reduction to N2 and (4) CO2 formation in flowing NO + O2 at 523 K over Cu(8)-Al2O3, which was pre-exposed to a flow of NO + C3H6 + O2 for 200 min at 523 K.

Figure 9. Formation rates of N2 (A) and CO2 (B) as a function of time in flowing (O) NO + O2, (2) O2, (4) NO, and (b) He over Cu(8)-Al2O3. Before the measurements, the catalyst was pre-exposed to a flow of NO + C3H6 + O2 for 200 min at 523 K.

activation energy for the acetate reaction rate (76 kJ/mol) were close to that of the steady-state rates of N2 formation (81 kJ/ mol). For Cu-Al2O3 catalysts with various Cu loading, the initial rates of acetate consumption in NO + O2 at 523 K were determined from the transient IR experiments. These rates were plotted in Figure 12 together with the steady-state NO reduction and C3H6 conversion rates. Again, the rate of acetate reaction with NO + O2 and NO reduction rate were very close to each other for each Cu-Al2O3 samples. In addition, these two rates increased in a similar manner with an increase in Cu loading. Taking into account the result in Table 1 that the surface concentration of Cu2+ ion increases with an increase in Cu loading, the above results indicate that Cu2+ ion is the principal active component, which plays an important role in the reaction of the adsorbed acetate with NO + O2.

Figure 11. Arrhenius plots of (b) the initial rate of acetate consumption in NO + O2, and the steady-state rates of (O) NO reduction to N2 and (4) C3H6 conversion to CO2 for NO + C3H6 + O2 reaction over Cu(8)-Al2O3.

Discussion Role of Acetate and Nitrates. As shown above, the formation of nitrates is greatly promoted by the presence of O2 (Figure 5A). In our previous study,23 the same result was obtained in the case of the Al2O3 catalyst, and it was suggested that the formation of nitrates proceeds by NO oxidation and the subsequent adsorption of NO2 on basic oxygen sites. The nitrates were quite reactive toward C3H6 at 523 K, leading to the



Figure 13. Proposed mechanism of C3H6-SCR over Cu-Al2O3 catalysts.

Figure 12. Effect of Cu loading on (b) the initial rate of acetate consumption in NO + O2, the steady-state rates of (O) NO reduction to N2, and (4) C3H6 conversion to CO2 for NO + C3H6 + O2 reaction at 523 K.

formation of the acetate (Figure 5A). As shown in Figure 2, the acetate was formed in NO + C3H6 + O2 and in C3H6 + O2, and the rate of acetate formation was 2 times higher in the former reaction. This result should indicate that during NO + C3H6 + O2 reaction both oxygen and nitrates take part in the C3H6 oxidation to the acetate. The acetate was the dominant adspecies in NO + C3H6 + O2 reaction on Cu-Al2O3 at 473-623 K (Figure 2). As shown in Figure 6, the acetate formed by introducing the acetic acid is an active reductant, which reacts preferentially with NO + O2 to produce N2 and CO2 via the formation and reaction of Cu-NCO species. On the other hand, the surface species formed during NO + C3H6 + O2 reaction in a steady-state reacted preferentially with NO + O2 to produce N2 and CO2 (Figure 9). It should be noted that the N2 formation rate in this transient reaction (Figure 9) is very close to that in the reaction of adsorbed acetic acid with NO + O2 (Figure 6). Therefore, it is the acetate formed during SCR reaction that acts as the dominant reductant, which reacts with NO + O2 to produce N2 and CO2. This conclusion is quantitatively confirmed by the result in Figure 10, in which the transient rate of acetate consumption in NO + O2 was the same order of magnitude as the transient rates of NO reduction and CO2 formation over a wide range of reaction time. As shown in Figure 10, the extrapolation of the first-order plot for NO reduction rate to the intercept was consistent with the steady-state rate constant of NO reduction, which indicates that the steady-state N2 formation occurs by the reaction of the adsorbed intermediates with NO + O2. As shown in Figure 11, the initial rates of acetate reaction with NO + O2 were close to the steady-state rates of NO reduction over a wide range of temperature, and the apparent activation energy for the former reaction was also close to that of the latter reaction. Furthermore, for Cu-Al2O3 samples with various Cu loading (0 - 8 wt %), the rates of acetate reaction with NO + O2 and the steady-state NO reduction were very close to each other. Taking into account

the result that the rate of acetate formation in NO + C3H6 + O2 (2.0 nmol/m2 s) was about 8 times higher than that of the acetate reaction in NO + O2 (0.26 nmol/m2 s), the above results lead to the conclusion that the acetate is an intermediate of this reaction on a series of Cu-Al2O3 catalysts and possibly takes part in the rate-determining stage over a wide temperature range. As shown in Figure 5A, the reaction of nitrates with C3H6 or C3H6 + O2 results in the formation of N-containing organic species, Al-NCO, Cu-NCO, and CN species, on the surface. This indicates the N insertion from the nitrates to hydrocarbon or carboxylate species. Combined with the above-mentioned discussions and the result that nitrates were immediately formed in NO + O2 (Figure 5B), it can be concluded that the acetate react with nitrates to produce N2 and CO2 via Cu-NCO species, though the presence of nitrates was not clearly confirmed during SCR reaction (Figure 2). The absence of nitrate bands during SCR reaction can be explained by the result shown in Figure 5B that the reaction rate of nitrate with hydrocarbon species is higher than that of nitrates formation in NO + O2. The intermediacy of nitrates is in agreement with the findings in our previous studies on C3H6-SCR mechanism over Na-Hmordenite18 and Al2O3,23 where it was also shown that the nitrates are involved in the reaction with organic species on the surface, which finally results in N2 formation. There have been suggestions in the literature that gaseous NO2 formed from the oxidation of NO is a possible intermediate of SCR reaction, which subsequently reacts with hydrocarbons or surface organic species.3,4 However, considering the results that the NO2 formation rate in NO + O2 reaction (0.35 nmol/m2 s) was lower that the rate of nitrates formation (0.87 nmol/m2 s) and nitrates reaction in C3H6 (2.0 nmol/m2 s), and that the nitrates were fairly stable in He purge, the reactive NOx species in the present system should not be the gaseous NO2 but the nitrates (adsorbed NO2) on the catalyst surface. Reaction Mechanism of C3H6-SCR over Cu-Al2O3. On the basis of the above results, a reaction mechanism for the C3H6-SCR on Cu-Al2O3 catalysts is presented as shown in Figure 13. The reaction begins with the formation of adsorbed nitrates via the NO oxidation by O2 and the formation of acetate via the partial oxidation of propene with O2 and possibly with nitrates. The acetate, which acts as a surface reductant, reduces nitrates to N2. In the course of this step, N-containing hydrocarbon species, Al-NCO, Cu-NCO, and CN adspecies, are formed. Cu-NCO and possibly CN species rapidly react with nitrates and NO to produce N2 and CO2, and thus may be regarded as the final intermediates. This is in agreement with the results by Kung et al.10 in which NCO and CN species were observed as possible intermediates for C3H6-SCR over CuZrO2 catalyst. In contrast, Al-NCO species is rather stable in NO + O2, and thus can be a relatively inert spectator on the surface. This conclusion is contrary to the earlier finding9,32 that Al-NCO species (2270-2230 cm-1) was proposed as the intermediate for HC-SCR over Cu-based catalysts. The reaction pathway in Figure 13 is similar to that reported in our previous

2892 J. Phys. Chem. B, Vol. 104, No. 13, 2000 study18 where the consecutive reaction of surface organic species, including N-containing hydrocarbon species, with the nitrate was proposed. This scheme also includes the route of the competitive oxidation of the C3H6 with O2 as follows. The formation of formate and carbonates adspecies also occurs via the partial oxidation of propene (Figure 2 and 5A). These species do not act as the surface reductant, but disrobe or react with O2 to produce CO2. Acetate can also react with O2 to produce CO2 (Figures 5B and 6B). These pathways contribute the unselective oxidation of the C3H6 with O2, and compete with the SCR reaction for the consumption of C3H6 reductant. As shown in Figure 8, the rate of acetate oxidation by NO + O2 (nitrates) was larger than that by O2. This result, along with the result that nitrates were immediately formed in NO + O2, explains why, in the presence of great excess of oxygen, hydrocarbons interact selectively with NOx, instead of oxygen. Dissociative adsorption of oxygen on isolated Cu2+ ions may be hindered as postulated by Kung et al.,39 whereas NO forms nitrates rather easily, which could lead to the selective oxidation of the acetate intermediate by nitrates. It is generally accepted that the role of oxygen is primary of importance to discuss the reaction mechanism of SCR. The proposed mechanism explains the role of oxygen in facilitating the reduction of NO by C3H6; oxygen activates both NO and C3H6 into the reactive species in the initial steps of the reaction. In addition, as Marquez-Alvarez et al. clarified, another role of oxygen should be to maintain the copper atoms in an appropriate oxidation state (Cu2+).40 Role of Copper Ions in C3H6-SCR. The role of Cu ions in the SCR reaction has been of another interest, and a number of possibilities have been proposed. Among them are activation of hydrocarbon, oxidation of NO to NO2, and the reaction of activated hydrocarbon with NOx species and subsequent formation of N-containing reaction intermediates that results in N2 production. The mechanism shown in Figure 13 is basically consistent with that proposed for Al2O3,23 which implies that the presence of Cu ions does not markedly change the reaction mechanism. However, the rates of acetate formation (Figure 4), nitrate reaction (Figure 5B), and acetate reaction with NO + O2 (Figure 12) were significantly promoted (by a factor of 101102) by the presence of Cu ions. The rate of nitrate formation on Cu(8)-Al2O3 was also higher (by a factor of 2) than Al2O3 (Figure 5B). The results in Figure 3 suggested that the adsorption site of the acetate intermediate for Cu-Al2O3 catalysts is not associated with AlCUS-O site but CuCUS-O site. Further, Cu ions were found to be the adsorption site of the most probable N-containing reaction intermediates (Cu-NCO) that can results in N2 production. From these results, the surface Cu ions should take part in all the important steps; (1) the activation of hydrocarbon to the acetate; (2) oxidation of NO to nitrates; (3) the reaction of the acetate with nitrates; (4) subsequent formation of the final intermediates (Cu-NCO) that results in N2 production. This could be due to the superior redox property of Cu ions. Thus, the C3H6-SCR reaction proceeds primarily on the surface Cu ions. As pointed out in our previous study,3 the primary role of the Al2O3 support should be to disperse Cu2+ ions in an atomic level. Conclusions The reaction mechanism of C3H6-SCR on Cu-Al2O3 catalysts, which consists of highly dispersed Cu2+ ions in the surface aluminate phase, was investigated by in-situ FTIR spectroscopy, and the following conclusions can be drawn. The reaction begins with the formation of adsorbed nitrates via the NO oxidation

Shimizu et al. by O2 and the formation of the acetate via the partial oxidation of C3H6 with O2 and nitrates. The acetate, which is the predominant adsorbed species in the steady-state condition, is the important intermediate of C3H6-SCR on a series of CuAl2O3; it acts as an active reductant on the surface and reduces nitrates to N2, which could be the rate-determining stage over a wide temperature range. In the course of this step, Al-NCO, Cu-NCO, and CN species are produced. Cu-NCO and possibly CN species are reactive toward nitrates and NO to produce N2 and CO2 and thus can be regarded as the final intermediates, while Al-NCO species can be a relatively inert spectator on the surface. The mechanism presented in Figure 13 has been proposed, which explains the role of oxygen in facilitating SCR; oxygen can activate both NO and C3H6 into the reactive species in the initial steps of the reaction. Cu2+ ion is the principal active component in Cu-Al2O3 catalysts. It plays an important role in all the important steps: (1) the activation of hydrocarbon to the acetate; (2) oxidation of NO to nitrates; (3) the reaction of the acetate with nitrates; (4) subsequent formation of the final intermediate (Cu-NCO) that results in N2 production. Acknowledgment. K.S. acknowledges support by the Fellowship of JSPD for Japanese Junior Scientists. References and Notes (1) Iwamoto, M.; Yahiro, H. Catal. Today 1994, 22, 5. (2) Shelef, M. Chem. ReV. 1995, 95, 209. (3) Shimizu, K.; Maeshima, H.; Satsuma, A.; Hattori, T. Appl. Catal. B 1998, 18, 163. (4) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1991, 70, L15. (5) Bennett, C. J.; Bennett, P. S.; Golunski, S. E.; Hayes, J. W.; Walker, A. P. Appl. Catal. A 1992, 86, L1. (6) Jewell, L.; Sokolovskii, V. D.; Coville, N. J.; Glasser, D.; Hilderadt, D. Chem. Commun. 1996, 2081. (7) Bamwenda, G. R.; Ogata, A.; Obuchi, A.; Oi, J.; Mizuno, K.; Skrzypek, J. Appl. Catal. B 1995, 6, 311. (8) Hadjiivanov, K.; Klissurski, D.; Ramis, G.; Busca, G. Appl. Catal. B 1996, 7, 251. (9) Ukisu, Y.; Sato, S.; Muramatsu, G.; Yoshida, K. Catal. Lett. 1991, 11, 177. (10) Li, C.; Bethke, K. A.; Kung, H. H.; Kung, M. C. J. Chem. Soc., Chem. Commun. 1995, 813. (11) Matyshak, V. A.; Il’ichev, A. N.; Ukharsky, A. A.; Korchak, V. N. J. Catal. 1997, 171, 245. (12) Tamaru, K. Dynamic Heterogeneous Catalysis; Academic Press: London, 1978. (13) Matyshak, V. A.; Krylov, O. V. Catal. Today 1995, 25, 1. (14) Adelman, B. J.; Beutel, T.; Lei, G.-D.; Sachtler, W. M. H. J. Catal. 1996, 158, 327. (15) Aylor, A. W.; Lobree, L. J.; Reimer, J. A.; Bell, A. T. Stud. Surf. Sci. Catal. 1996, 101, 661. (16) Lobree, L. J.; Aylor, A. W.; Reimer, J. A.; Bell, A. T. J. Catal. 1997, 169, 188. (17) Xin, M.; Hwang, I. C.; Woo, S. I. J. Phys. Chem. B 1997, 101, 9005. (18) Satsuma, A.; Enjoji, T.; Shimizu, K.; Sato, K.; Yoshida, H.; Hattori, T. J. Chem. Soc., Farady Trans. 1998, 94, 301. (19) Chen, H.-Y.; Voskoboinikov, T.; Sachtler, W. M. H. J. Catal. 1998, 180, 171. (20) Lobree, L. J.; Aylor, A. W.; Reimer, J. A.; Bell, A. T. J. Catal. 1999, 181, 189. (21) Long, R. Q.; Yang, R. T. J. Phys. Chem. B 1999, 103, 2232. (22) Shimizu, K.; Kawabata, H.; Satsuma, A.; Hattori, T. Appl. Catal. B 1998, 19, L87. (23) Shimizu, K.; Kawabata, H.; Satsuma, A.; Hattori, T. J. Phys. Chem. B 1999, 103, 1542. (24) Hamada, H. Catal. Today 1994, 22, 21. (25) Murakami, Y. Stud. Surf. Sci. Catal. 1983, 16, 775. (26) Ertl, G.; Hierl, R.; Knozinger, H.; Thiele, N.; Urbach, H.-P. Appl. Surf. Sci. 1980, 5, 49. (27) Hierl, R.; Knozinger, H.; Urbach, H.-P. J. Catal. 1981, 69, 475. (28) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497.

Intermediates in Selective Reduction of NO by Propene (29) van Neer, F. J. R.; van der Linden, B.; Bliek, A. Catal. Today 1997, 38, 115. (30) Bando, K. K.; Sayama, K.; Kusama, H.; Okabe, K., Arakawa, H. Appl. Catal. A 1997, 165, 391. (31) Solymosi, F.; Bansagi, T. J. Catal. 1995, 156, 75. (32) Ukisu, Y.; Sato, S.; Muramatsu, G.; Yoshida, K. Catal. Lett. 1992, 16, 11. (33) Captain, D. K.; Amiridis, M. D. J. Catal. 1999, 184, 377. (34) Sumiya, S.; He, H.; Abe, A.; Takezawa, N.; Yoshida, K. J. Chem. Soc., Faraday Trans. 1998, 94, 2217.

J. Phys. Chem. B, Vol. 104, No. 13, 2000 2893 (35) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides; Wiley: New York, 1990; p 53. (36) Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Appl. Catal. 1990, 64, 243. (37) Parkyns, N. D. Proc. 5th Int. Congr. Catal. 1972, 1, 255. (38) Takeda, H.; Iwamoto, M. Catal. Lett. 1996, 38, 21. (39) Kung, H. H.; Kung, M. C. Catal. Today 1996, 30, 5. (40) Marquez-Alvarez, C.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A.; Haller, G. L.; Fernandez-Garcia, M. J. Am. Chem. 1997, 119, 2905.

Intermediates in the Selective Reduction of NO by Propene over Cu

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