Different Roles of Surface Nitrates in the Selective Catalytic Reduction

Dec 13, 2010 - Different Roles of Surface Nitrates in the Selective Catalytic Reduction of NO by Propane over CoOx/Al2O3 and Ga2O3/Al2O3: Intermediate...
0 downloads 0 Views 641KB Size
Download PDF
1248

J. Phys. Chem. C 2011, 115, 1248–1254

Different Roles of Surface Nitrates in the Selective Catalytic Reduction of NO by Propane over CoOx/Al2O3 and Ga2O3/Al2O3: Intermediates or Spectators† Chuanhua He‡ and Klaus Ko¨hler* Department of Chemistry, Technische UniVersita¨t Mu¨nchen, Lichtenbergstrasse 4, D-85747 Garching, Germany ReceiVed: July 15, 2010; ReVised Manuscript ReceiVed: NoVember 12, 2010

The different role of nitrate species formed on CoOx/Al2O3 and Ga2O3/Al2O3 in the selective catalytic reduction of nitric oxide by propane is demonstrated in a comparative study using in situ Fourier transform infrared spectroscopy. The investigations are accompanied and supported by detailed measurements of conversion and selectivity (exact number of electrons transferred on the basis of a complete nitrogen balance). Surface nitrates formed under NO + O2 and C3H8 + NO + O2 streams over both CoOx/Al2O3 and Ga2O3/Al2O3. The reactivity of surface nitrates under C3H8 and C3H8 + O2 streams was found to be substantially different in both cases. Surface nitrates on CoOx/Al2O3 are involved in the partial oxidation of C3H8 to surface oxygenates and the formation of surface nitrogen containing organic species (through reaction with surface oxygenates). Surface nitrates on Ga2O3/Al2O3 are chemically inert to C3H8 but show low reactivity to surface oxygenates at 598 K. The results of time dependent IR experiments and stoichiometric calculations of the SCR reaction suggest that surface nitrates are important intermediates in the SCR of NO by C3H8 over CoOx/Al2O3 but are only spectators over Ga2O3/Al2O3. The presence of redox active metal oxide surface species (CoOx) seems to be a prerequisite for the catalytic reduction of surface nitrates by (gaseous) hydrocarbons. On Ga2O3/Al2O3, the missing conversion of surface nitrates and the direct conversion of nitrogen oxide species of lower nitrogen oxidation states explain the extraordinarily high efficiency of the reducing agent (propane) in the SCR of NO in this case (one molecule of propane converts more than four NO molecules to N2). Introduction Alumina and alumina supported metals or metal oxides, in particular Ag/Al2O3, CoOx/Al2O3, and Ga2O3/Al2O3, have been studied as promising catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) by hydrocarbons in the presence of O2, H2O, and low concentration of SO2.1-20 The investigation and elucidation of the rate determining step and the overall reaction mechanism of the SCR of NOx by hydrocarbons is motivated by both industry and academia. The SCR of NOx by hydrocarbons is a potential technology to abate toxic nitrogen oxides in the emissions of highly fuel-efficient diesel engines and power plants.21-27 A better understanding of the reaction mechanism would help in the design of more active and more stable catalysts for this process. The formation of diatomic nitrogen (N2) needs the combination of two nitrogen atoms (Nox ) 0)28,29 or a pair of nitrogen containing ions (Nox ) +3 and Nox ) -3).30-32 How nitrogen atoms in nitrogen oxide (Nox ) +2) are reduced to Nox ) 0 or Nox ) -3 and how diatomic nitrogen is formed are significant scientific issues. Hamada et al.,6 Lick et al.,7 and Shimizu et al.13 reported that the conversion of NOx is extremely low in a reaction stream of NOx and hydrocarbons (without oxygen) over CoOx/Al2O3 and Ga2O3/Al2O3. The authors proposed that oxygen plays an important role in the formation of strongly bound reaction intermediates by the activation of NO to NO2 or the partial oxidation of hydrocarbons to surface carbonaceous species †

Part of the “Alfons Baiker Festschrift”. * Corresponding author. Fax: + 49 89 289 13473. E-mail: klaus.koehler@ ch.tum.de. ‡ Present address: Evonik Degussa (China) Co., Ltd., 12/F Taikang Financial Tower, 38# Dongsanhuanbei Road, Chaoyang District, Beijing 100026, P. R. China.

(CxHyOz). More recently, various surface species, such as nitrates, hydrocarbon oxygenates (acetates, formates), organonitro and nitrite compounds, cyanides, and isocyanides were detected by in situ IR spectroscopy during the SCR of NO using hydrocarbons or hydrocarbon oxygen derivates.15-18,33-42 However, there are some disagreements on the formation and the role of these detected species. Liu et al.15 and Haneda et al.43 investigated the mechanism of the SCR of NO with C3H6 over CoOx/Al2O3 and Ga2O3/Al2O3 and suggested that NO was first oxidized and adsorbed as a NOx- species (NO2-, NO3-). Subsequently, the adsorbed NOx- species were reduced to N2 by reductants or derivatives generated from the reaction between oxygen and the reductants. Shimizu et al.44 further showed that nitrate species were converted to N2 during exposure to the reductant at rates that were similar to those of the steady-state reduction of NO. On the other hand, Zuzaniuk et al.45 suggested in their studies on the surface reactions of the SCR of NO by C3H6 over Al2O3, Ag/Al2O3 and Co/Al2O3 that the formation of NO2 was not achieved through direct oxidation of NO by O2. Obuchi et al.23 proposed in their studies on the reduction of NO using MTBE over γ-alumina that the first step in the SCR reaction is the formation of surface carbonaceous species through partial oxidation of hydrocarbons. This mechanism is further supported by Meunier et al.,46 Shibata et al.,47 and He et al.48 by studies of the SCR of NO using C3H6 or C3H8 over Ag/γ-Al2O3 and CoOx/Al2O3. To elucidate the reaction mechanism, it is important to investigate the formation and reactivity of the observed surface species over the given catalyst systems. In the present work, the formation of surface nitrates under NO + O2 and C3H8 + NO + O2 streams over CoOx/Al2O3 and Ga2O3/Al2O3 at the reaction temperature is evidenced by in situ IR spectroscopy. The reactivity of surface nitrates to C3H8 and

10.1021/jp1065852  2011 American Chemical Society Published on Web 12/13/2010

Catalytic Reduction of NO by Propane

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1249

C3H8 + O2 over CoOx/Al2O3 and Ga2O3/Al2O3 are compared by time dependent IR experiments under the same conditions. On the basis of the investigation on the reactivity of surface nitrates in combination with stoichiometric calculations (based on catalytic measurements including a complete nitrogen balance), the differences in the roles of surface nitrates during the SCR of NO by C3H8 over CoOx/Al2O3 and Ga2O3/Al2O3 could be clarified. Experimental Section Catalyst Preparation. The catalysts applied in this work (10 mol % Ga2O3/Al2O3 and 1 mol % CoOx/Al2O3) were prepared by the impregnation method. A certain amount of γ-Al2O3 support (deliverer: Sasol, specific surface area (BrunauerEmmett-Teller): 198 m2/g) was soaked in a solution of cobalt nitrate hexahydrate (Merck, >99%) or gallium nitrate hydrate (Merck, >99%) in a predetermined amount of H2O and was allowed to stand at room temperature for 2 h with frequent stirring. Then, the water was removed by evaporation under reduced pressure. The catalyst precursor thus obtained was dried at 393 K overnight, followed by calcination at 723 K in static air for 4 h. Catalytic Activity Measurements. The catalytic activity was measured using a fixed bed flow reactor. The reaction mixture consisted of 1000 ppm of NO, 1000 ppm of C3H8, and 10% O2 (helium as balance gas). The gas flow rate GHSV was 10 000 h-1. The analysis of feed gas and products was carried out with a mass spectrometer (Balzers MSC 200, capillary length 1 m, software: Quadstarplus) for C3H8, O2, NO, and NO2, a CLD (chemoluminescence detector) NOx analyzer (EC 700, Ecophysics) for NOx, a NDIR CO analyzer (nondispersive infrared detector, Maihak, UNOR 610) for CO, and a gas micro chromatograph (MIT P200, two channels, Software: EZChrom 200) for N2O, CO2, and CxHy, with a thermal conductivity detector and PoraPLOT Q and molecular sieve A (5 Å) as separation column allowing the very exact and complete calculation of the (nitrogen and carbon) balance. The catalytic activity was evaluated in terms of NO conversion to N2, C3H8 conversion to COx (CO + CO2), and the ratio between NO conversion and C3H8 conversion. The formation of N2O was found to be negligible in the present work. In Situ FTIR Experiments. Catalyst powder (5 mg) was pressed into self-supporting wafers with a diameter of 8 mm (10 mg/cm2) and placed in a high-temperature flow cell with two CaF2 windows. The “homemade” IR flow cell used in this study was made of stainless steel. The design and schematic drawing of the IR cell were published in a previous work.48 IR spectra were recorded on an FTIR spectrometer (FTS-575C, BIO-RAD) with 16 scans at a resolution of 2 cm-1. Prior to each experiment, the catalyst wafer was first activated in situ by heating in a 10% O2 stream (total flow: 50 mL/min) at 673 K for 2 h, followed by cooling to the desired temperature and purging in He for 30 min. Then, the spectrum of the clean surface was recorded which was used as the background for the in situ experiment. The IR spectra of surface species under a flow of various gas mixture (total flow: 50 mL/min) were recorded as a function of time. All gases applied in this study were purchased from Messer Griesheim GmbH. The purities of the gases are 99.996% (He), 99.998% (O2), 99.5% (NO) and 99.5% (C3H8). In addition to the steady-state reaction, time dependent experiments were employed to clarify the role and reactivity of surface nitrates. The catalyst was first exposed to NO + O2 at 598 K for 120 min to accumulate surface nitrates. After purging

Figure 1. Catalytic performance of CoOx/Al2O3 (filled symbols: 9, b, 2) and Ga2O3/Al2O3 (empty symbols: 0, O, ∆) in the selective catalytic reduction of NO using C3H8. (9, 0): conversion of NOx to N2; (b, O): conversion of C3H8; (2, ∆): reductant efficiency (XNO: XC3H8). Reaction conditions: NO ) 1000 ppm, C3H8 ) 1000 ppm, O2 ) 10%, GHSV ) 10 000 h-1.

under a He stream for 30 min, the feed gas was switched to C3H8 or C3H8 + O2. The changes in the intensity of the IR bands were measured with time on the stream. Results and Discussion 1. Catalytic Performance of the SCR of NO by C3H8 over CoOx/Al2O3 and Ga2O3/Al2O3. The catalytic activity was measured by using a fixed bed flow reactor. The analysis of feed gas and products was carried out with a mass spectrometer (Balzers MSC 200, capillary length 1 m, software: Quadstarplus) for C3H8, O2, NO, and NO2, a CLD (chemoluminescence detector) NOx analyzer (EC 700, Ecophysics) for NOx, a NDIR CO analyzer (nondispersive infrared detector, Maihak, UNOR 610) for CO, and a gas microchromatograph (MIT P200, two channels, Software: EZChrom 200) for N2O, CO2 and CxHy, with a thermal conductivity detector and PoraPLOT Q and molecular sieve A (5 Å) as separation column allowing the very exact and complete calculation of the (nitrogen and carbon) balance. The catalytic activity was evaluated in terms of NO conversion to N2, C3H8 conversion to COx (CO + CO2), and the ratio between NO conversion and C3H8 conversion. The formation of N2O was found to be negligible in the present work. Figure 1 shows the catalytic performance of CoOx/Al2O3 and Ga2O3/Al2O3 catalysts for the selective catalytic reduction of NO using C3H8 in a fixed bed continuous flow reactor at temperatures between 573 and 723 K. Both CoOx/Al2O3 and Ga2O3/Al2O3 showed promising activities in the selective catalytic reduction of NO using propane as the reductant. The catalysts achieved a maximum NO conversion of 100% at 723 K. Interestingly, CoOx/Al2O3 exhibits much higher C3H8 conversion than Ga2O3/Al2O3 at the temperature with the maximum NO conversion. The conversion of C3H8 was 60.9% and 24.1% at 723 K over CoOx/Al2O3 and Ga2O3/Al2O3, respectively. To compare the reductant efficiency of the SCR of NO by C3H8 over CoOx/Al2O3 and Ga2O3/Al2O3, the conversion ratio of NO to C3H8 (XNO:XC3H8) was calculated and is shown in Figure 1 (dotted line). CoOx/Al2O3 shows lower reductant efficiency (XNO: XC3H8) than Ga2O3/Al2O3 at the same temperatures (between 573 and 723 K). The difference in the reductant efficiency (XNO: XC3H8) indicates that the reaction paths of the SCR of NO by C3H8 over these two catalysts may be different. 2. Formation of Surface Species under NO + O2 and NO + C3H8 + O2 Streams. The IR spectra of adsorbed species over CoOx/Al2O3 and Ga2O3/Al2O3 under a NO + O2 stream at

1250

J. Phys. Chem. C, Vol. 115, No. 4, 2011

Figure 2. IR spectra of adsorbed species under a NO + O2 stream over (a) CoOx/Al2O3 and (b) Ga2O3/Al2O3 at 598 K after 90 min.

Figure 3. Results of the curve fitting procedure (Origin 8.5) applied to the FTIR spectra (in the region 1700-1100 cm-1) of adsorbed species under a NO + O2 stream over (a) CoOx/Al2O3 and (b) Ga2O3/Al2O3 at 598 K after 90 min.

598 K are shown in Figure 2. CoOx/Al2O3 and Ga2O3/Al2O3 give similar IR spectra under a NO + O2 stream at 598 K. Specifically, three strong bands at 1552, 1296, and 1247 cm-1 and two weak bands at 1713 and 1036 cm-1 were observed under a NO + O2 stream over CoOx/Al2O3 (Figure 2, spectrum a). These bands slightly shift to 1552, 1292, and 1239 cm-1 under a NO + O2 stream over Ga2O3/Al2O3 (Figure 2, spectrum b). After the curve-fitting procedure (Origin 8.5) applied to the IR spectra in the region of 1700-1100 cm-1, three shoulders at 1610, 1582, and 1518 cm-1 were identified over CoOx/Al2O3 (Figure 3a). These bands slightly shift to 1608, 1579, and 1522 cm-1 over Ga2O3/Al2O3 (Figure 3b). Shimizu et al.44 and Haneda et al.43 investigated the spectra of adsorbed species under a

He and Ko¨hler stream of NO + O2 over Al2O3, Cu-Al2O3 and Ga2O3/Al2O3. The authors assigned the bands at 1544-1560 and 1290-1296 cm-1 and the bands at 1574-1586 and 1238-1256 cm-1 to unidentated nitrates and bidentated nitrates, respectively. Thus, we attribute the band at 1579 cm-1 over Ga2O3/Al2O3 and the band at 1582 cm-1 over CoOx/Al2O3 to ν(NdO) of bidentate nitrates and the band at 1239 cm-1 over Ga2O3/Al2O3 and the band at 1247 cm-1 over CoOx/Al2O3 to νas(ONO) of bidentate nitrates. The band at 1552 cm-1 was assigned to ν(NdO) of unidentate nitrates. The band at 1292 cm-1 over Ga2O3/Al2O3 and the band at 1296 cm-1 over CoOx/Al2O3 were assigned to νas(ONO) of unidentate nitrates. The band at 1036 cm-1 was assigned to νs(NO2) of adsorbed nitrate species according to reports by Kantcheva et al.49 and Sadykov et al.,50 Shimizu et al.,44 and Iglesias-Juez et al.51 attributed the bands at 1609-1616 cm-1 to surface bridging nitrates or adsorbed NO2 in their studies on the adsorbed species under a NO + O2 + C3H6 stream over Al2O3 and 6 wt % Ag/Al2O3. Accordingly, we attribute the bands centered at 1610 and 1608 cm-1 to surface bridging nitrates over CoOx/Al2O3 and Ga2O3/Al2O3, respectively. Figure 4 shows the IR spectra of adsorbed species on CoOx/ Al2O3 and Ga2O3/Al2O3 under a NO + C3H8 + O2 stream at 598 K. Other than the bands observed in a NO + O2 stream, a broad band at 3550 cm-1, a shoulder at 1620 cm-1, three bands of intermediate intensity at 1460, 1392, and 1378 cm-1, and a weak band at 2238 cm-1 were observed under flowing NO + C3H8 + O2 over both catalysts. In the C-H stretching vibration region, three bands at 2988, 2968, and 2908 cm-1 were observed. The broad band at 3550 cm-1 was assigned to the bridging hydroxyl group perturbed by the interaction with the organic species generated from the C3H8 + O2 reaction as specified in our previous work.16,48 The band at 2908 cm-1 is in good agreement with that of formate species adsorbed on Ga2O3/Al2O3 and Ag/Al2O3 as reported by Haneda et al.43 Meunier et al.46 and Shimize et al.44 investigated the surface oxygenates in their study of C3H6-SCR of NO over Ga2O3/ Al2O3. Amenomiya et al.52 studied the adsorbed species in the water gas shift reaction over Al2O3. The authors assigned the band at 1392 cm-1 to δ(C-H) and the band at 1378 cm-1 to νs(COO). Accordingly, we attribute the bands at 2908, 1392, and 1378 cm-1 to ν(C-H), δ(C-H), and νs(COO) of surface formates over alumina, respectively. The band at 1460 cm-1 is in good agreement with those of adsorbed acetate on Ag/Al2O3 and Al2O3 reported by Meunier et al.46 and Shimizu et al.44 Thus, we attribute the band at 1460 cm-1 to νas(COO) of the surface acetate over Al2O3. According to the literature;43,44,46-48 the band at 2238 cm-1 is assigned to surface isocyanate species. Two negative bands at 3760 and 3670 cm-1 were observed in a NO + O2 + C3H8 stream over CoOx/Al2O3. Similar bands at 3750-3770 and 3760-3780 cm-1 were observed in a C3H6 + O2 + NO stream over Al2O3 and Ga2O3-Al2O3 catalysts reported by Shimizu et al.44 and Haneda et al.,43 respectively. These bands are assigned to catalyst surface OH groups consumed by the formation of surface nitrates and carboxylates or by the interaction with the surface nitrates and carboxylates formed.16,48,53 These negative bands shift to 3760 and 3680 cm-1, respectively, in a NO + O2 + C3H8 stream over Ga2O3/Al2O3. 3. Reactivity of Surface Nitrates in a C3H8 Stream. The reactivity of surface nitrates toward propane over CoOx/Al2O3 and Ga2O3/Al2O3 was compared by transient response of the IR spectra at 598 K. First, the catalysts were exposed to a flow of NO + O2 for 90 min and purged in a He stream for 30 min; then, the gas flow was switched to C3H8 for 60 min. Figure 5 shows the time dependent changes in the IR spectra of the

Catalytic Reduction of NO by Propane

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1251

Figure 4. FTIR spectra of adsorbed species on (a) CoOx/Al2O3 and (b) Ga2O3/Al2O3 after 90 min of treatment in a stream of NO (1000 ppm) + C3H8 (3000 ppm) + O2 (10%) at 598 K.

Figure 5. FTIR spectra of adsorbed species on CoOx/Al2O3 at 598 K taken after (a) soaking the catalysts in a flow of NO (1000 ppm) + O2 (10%) for 90 min followed by purging in He for 30 min and then changing the gas to C3H8 (3000 ppm) for (b) 2, (c) 5, (d) 15, (e) 30, and (f) 60 min.

adsorbed species on CoOx/Al2O3 after the gas flow was switched to C3H8. In the IR spectrum of the adsorbed species in a NO + O2 stream (Figure 5, spectrum a), four strong bands centered at 1582 and 1246 cm-1 as well as 1552 and 1296 cm-1 characteristic of surface bidentate and monodentate nitrate species, respectively, were observed. After changing the gas flow to C3H8, the bands characteristic of surface nitrates decreased with time on stream and disappeared after 60 min (Figure 5, spectrum f). At the same time, the band at 1460 cm-1, characteristic of surface acetate, and the bands at 2906, 1590, 1392, and 1376 cm-1, characteristic of surface formate, were observed and gradually increased with time on stream. The decrease of surface nitrates and the formation of surface carboxylates indicates that surface nitrates are chemically reactive toward C3H8 and take part in the oxidation of propane to carboxylates over the CoOx/ Al2O3 catalyst. More interestingly, the band at 2242 cm-1, characteristic of surface nitrogen containing organic species (e.g., -NCO), was also observed. Surface nitrogen containing organic species are reported to be an important intermediate in the formation of NH3 by hydrolysis in the low temperature SCR using oxygenates. NH3 is one of the most important final intermediates in the formation of N2 though the reaction 2NH3 + NO + NO2 f 2N2 + 3H2O.56-59 Figure 6 shows the time dependent changes in the IR spectra of adsorbed species on Ga2O3/Al2O3 after the gas flow was

Figure 6. FTIR spectra of adsorbed species on Ga2O3/Al2O3 at 598 K taken after (a) soaking the catalysts in a flow of NO (1000 ppm) + O2 (10%) for 90 min followed by purging in He for 30 min and then changing the gas to C3H8 (3000 ppm) for (b) 5, (c) 15, (d) 30, and (e) 60 min.

1252

J. Phys. Chem. C, Vol. 115, No. 4, 2011

He and Ko¨hler

Figure 8. Time dependent IR spectra of adsorbed species on CoOx/ Al2O3 at 598 K. Before the experiment, the catalyst was soaked in a NO + O2 stream for 90 min to accumulate enough surface nitrate/ nitrite species. After purging in a He stream for 30 min, the gas flow was changed to a C3H8 + O2 stream. The IR spectra were recorded every 2 min after the gas flow was changed to C3H8 + O2.

Figure 7. Time dependence of the relative intensities of surface nitrates in the range 1330-1180 cm-1 normalized by initial flow intensity He (O, b) and C3H8 (0, 9) over CoOx/Al2O3 (empty symbols) and Ga2O3/ Al2O3 (filled symbols). Before the measurements, the catalysts were pretreated in NO (1000 ppm) + O2 (10%) for 90 min and purged with He for 30 min at 598 K.

switched to C3H8. Four bands centered at 1580 and 1240 cm-1 as well as 1554 and 1292 cm-1 characteristic of surface bidentate and monodentate nitrate species, respectively, were observed under a NO + O2 stream. A broad band at 1388 cm-1 was also observed after soaking Ga2O3/Al2O3 in NO + O2 for 90 min (Figure 6, spectrum a). Prinetto et al.54 and Sedlmair et al.55 investigated the adsorbed species over BaO/Al2O3 and BaOBaCO3/Al2O3 and assigned the band at 1380 cm-1 to hyponitrite (N2O2)2- formed on basic sites. Accordingly, we assign the band at 1388 cm-1 to surface hyponitrite (N2O2)2-. After soaking in a C3H8 stream for 60 min, only small quantitative changes in the IR bands were observed. The bands characteristic of surface carboxylates were not observed over Ga2O3/Al2O3. This result implies that the adsorbed nitrates did not react with C3H8 over Ga2O3/Al2O3 at 598 K. Figure 7 shows the changes in the integrated intensity of the nitrate bands in the range of 1330-1180 cm-1 (normalized by initial intensity) as a function of time after the feed gas was switched from NO + O2 to He or C3H8 at 598 K. The intensity of the IR bands characteristic of surface nitrates decreased only slightly under a He stream over CoOx/Al2O3. The small decrease in the concentration of surface nitrates can be interpreted as the desorption of adsorbed species. However, the intensity of the surface nitrate species significantly decreased when the feed gas was switched to C3H8 over CoOx/Al2O3. We investigated the IR spectra of adsorbed species under a C3H8 stream over CoOx/Al2O3 and reported that propane itself cannot be adsorbed onto the catalyst surface at the reaction temperature.48 Thus, the decrease of surface nitrates must be explained by the reaction of surface nitrates with propane rather than the site competition of surface nitrates with C3H8. The reactivity of surface nitrates toward propane indicates that nitrates could be relevant reaction intermediates in the SCR reaction over CoOx/Al2O3. The intensity of the IR bands characteristic of surface nitrates decreased only slightly under He and/or C3H8 streams over Ga2O3/Al2O3. The small decrease in the concentration of surface nitrates is again explained by desorption of the adsorbed species. A significant difference in the consumption rate of surface nitrates between He and C3H8 streams was not observed. This

Figure 9. Time dependent IR spectra of adsorbed species on Ga2O3/ Al2O3 at 598 K. Before the experiment, the catalyst was soaked in a NO + O2 stream for 90 min to accumulate enough surface nitrate/ nitrite species. After purging under a He stream for 30 min, the gas flow was changed to a C3H8 + O2 stream. The IR spectra were recorded every 2.5 min after the gas flow was changed to C3H8 + O2.

indicates that the adsorbed nitrate species are not reactive toward C3H8 over Ga2O3/Al2O3 at the experimental temperature. 4. Reactivity of Surface Nitrates under a C3H8 + O2 Stream. The reactivity of surface nitrates toward C3H8 + O2 over CoOx/Al2O3 and Ga2O3/Al2O3 was also investigated by transient response of the IR spectra at 598 K. First, the catalysts were exposed to a flow of NO + O2 for 90 min and purged in a He stream for 30 min. Then, the gas flow was switched to C3H8 + O2 for 60 min. Figure 8 shows the time dependent IR spectra of adsorbed species on CoOx/Al2O3 at 598 K. After the gas flow was switched to C3H8 + O2, the bands characteristic of surface nitrates gradually decreased and disappeared with time on stream. Simultaneously, new bands characteristic of surface acetate and formate were observed and continuously increased. These results suggest that surface nitrates are chemically reactive toward propane or other organic species generated from the C3H8 + O2 reaction. The time dependent IR spectra of adsorbed species on Ga2O3/ Al2O3 at 598 K are illustrated in Figure 9. The intensity of the IR bands characteristic of surface nitrates decreased only slightly with time on stream, until 60 min. The bands characteristic of surface acetate and formate were observed and increased with time on stream. Note that the bands characteristic of surface acetate were not observed with the “nitrate covered Ga2O3/Al2O3 surface in a flow of C3H8” (see above and Figure 6). We suggest that these adsorbed oxygenates are generated from C3H8 + O2

Catalytic Reduction of NO by Propane

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1253

instead of C3H8 + *-NOx- over Ga2O3/Al2O3 (see below). The slight decrease in intensity of the IR bands characteristic of surface nitrates may result from the desorption of the adsorbed species and the chemical reaction between surface nitrates and newly formed surface oxygenates. 5. Discussion: Role of Surface Nitrates in the SCR of NO by C3H8 over CoOx/Al2O3 and Ga2O3/Al2O3. There are two oxidizing agents (NO and O2) and one reducing agent (C3H8) in the SCR feed, and three reactions (eqs I-III) may occur simultaneously and competitively.

C3H8 + 10NO f 5N2 + 3CO2 + 4H2O

(I)

C3H8 + 5O2 f 3CO2 + 4H2O

(II)

C3H8 + xNO + (10 - x)/2O2 f 3CO2 + x/2N2 + 4H2O (III) Equation I represents the reduction of NO by propane directly, which gives the highest reductant efficiency. However, this reaction can be excluded from the main reaction routes according to the catalysis evaluation results published by Hamada et al.,6 Lick et al.7 and Shimizu et al.13 The authors reported that the reduction of NO to N2 cannot be achieved over CoOx/Al2O3 and Ga2O3/Al2O3 without the presence of O2 in the SCR feed. Equation II represents the combustion of propane, which gives the lowest reductant efficiency because NO is not reduced at all. This reaction is the main competitor of the reduction of NO by C3H8 and may lead to the decrease of reduction efficiency (XNO:XC3H8) at higher temperatures. However, the catalysts investigated in this work (1 mol % of CoOx/ Al2O3 and 10 mol % of Ga2O3/Al2O3) are not known catalysts for the total oxidation of propane at temperatures of 723 K and below.16,48 Equation III represents the selective catalytic reduction of NO by propane in the presence of excessive oxygen. This equation suggests that C3H8 or NO or both C3H8 and NO are first oxidized to active intermediates which subsequently react with another species to form N2, H2O, and CO2. Figure 4 illustrated that surface nitrates are detected over both CoOx/ Al2O3 and Ga2O3/Al2O3 during the SCR of NO by C3H8 in the presence of O2. Although the structures of surface nitrates over both catalysts are similar, the reactivity of surface nitrates toward C3H8 and the role of surface nitrates in the SCR reaction are quite different. Over CoOx/Al2O3, the intensity of the IR bands characteristic of surface nitrates decreased significantly and disappeared completely after 90 min under a C3H8 stream (as shown in Figure 5). Meanwhile, the bands characteristic of surface acetate and formate were observed and increased with time on stream. These results suggest that surface nitrates are chemically reactive to C3H8 and play an important role in the oxidation of C3H8 to form surface oxygenates. Surface oxygenates have been reported as the most important intermediates to form reduced nitrogen containing species such as -NCO and -CN.15,16,44-48 Over Ga2O3/Al2O3, a significant difference in the consumption rate of surface nitrates between He and C3H8 streams was not observed. This indicates that the adsorbed nitrate species are not chemically reactive toward C3H8 at 598 K (Figures 6 and 7). Figure 9 illustrates that surface acetate and formate are formed under a C3H8 + O2 stream over the “nitrate covered Ga2O3/Al2O3”. The formation of surface oxygenates may result from (i) oxidation of C3H8 by O2 or (ii) oxidation of C3H8 by

surface nitrates. All of the experimental observations in this paper and the results from the literature support that surface oxygenates are generated from the oxidation of C3H8 by O2 (point i). The bands characteristic of surface oxygenates were not observed under a C3H8 stream over the “nitrate covered Ga2O3/Al2O3” (Figure 6). The formation of surface acetate and formate starts at 523 K over Ga2O3/Al2O3 under a C3H8 + O2 stream.16 Shimizu et al.44 investigated the role of surface acetates and nitrates in the SCR of NO by C3H6 over Al2O3 and proposed that surface nitrates were reduced by propene and/or propene derived species. The reaction between surface nitrates and surface carboxylates over Ga2O3/Al2O3 could be excluded from the main reaction path by electron balance calculations. In this case, one molecule of C3H8 can reduce only two molecules of NO which contrasts our experimental finding of XC3H8:XNO ≈ 1:4 (Figure 1). Considering the experimental results and discussion above, we suggest that surface nitrates are not involved in the main reaction path of the SCR of NO by C3H8 over Ga2O3/Al2O3 at 598 K. This reaction path (mechanism) is applicable in the temperature range 573-723 K because one mole of C3H8 can convert about 4 mols of NO as shown in Figure 1. The difference in the reactivity of surface nitrates over CoOx/ Al2O3 and Ga2O3/Al2O3 may result from the presence of redox active metal in the catalyst. Over CoOx/Al2O3, surface nitrates are involved in the oxidation of hydrocarbons and reduced to nitrogen containing organics (e.g., -NCO), which are reported to be an important intermediate in the formation of NH3 by hydrolysis in the low temperature SCR using oxygenates. If a suitable redox active metal oxide component is missing in the catalyst, as is the case for Ga2O3/Al2O3, the reaction may start from the oxidation of hydrocarbons. It is the oxygenates obtained from the partial oxidation of hydrocarbons that react with gaseous NO or NO2 and reduce nitrogen to a reducing species like -NCO. In the later reaction path, surface nitrates are spectators or even potential catalyst poisons. Conclusions The formation and reactivity of surface nitrates in the SCR of NO by C3H8 over CoOx/Al2O3 and Ga2O3/Al2O3 were investigated by in situ IR spectroscopy. The IR spectra of surface nitrates over CoOx/Al2O3 and Ga2O3/Al2O3 are similar. However, the reactivity of surface nitrates toward to C3H8 and/or surface oxygenates is much different. Surface nitrates over CoOx/Al2O3 are involved in the partial oxidation of C3H8 to surface oxygenates and/or the formation of surface nitrogen containing organic species (through reaction with surface oxygenates). Surface nitrates over Ga2O3/Al2O3 are chemically inert to C3H8 but show low reactivity to surface oxygenates. The reaction of surface nitrates and oxygenates is excluded from the main route of C3H8-SCR of NO over Ga2O3/Al2O3 by electron balance calculations. The results of time dependent IR experiments and stoichiometric calculations of the SCR reaction allowed for interesting conclusions on the role of surface nitrates. Surface nitrates are important intermediates in the SCR of NO by C3H8 over CoOx/Al2O3, but surface nitrates are only spectators in the SCR of NO by C3H8 over Ga2O3/Al2O3. The presence of redox active metal oxide surface species seems to be a prerequisite for the catalytic reduction of surface nitrates by (gaseous) hydrocarbons. The missing conversion of surface nitrates and the direct conversion of nitrogen oxide species of lower nitrogen oxidation states explain the extraordinarily high efficiency of the reducing agent (propane) in the SCR of NO over Ga2O3/ Al2O3.

1254

J. Phys. Chem. C, Vol. 115, No. 4, 2011

Acknowledgment. We thank the Bayerische Forschungsstiftung for a financial support. C.H.H. acknowledges the Bayerische Forschungsstiftung for a grant. Dr. Martin Paulus, Dr. Josef Find, Dr. Julius A. Nickl, Dr. Hans-Ju¨rgen Eberle, and Dr. Jo¨rg Spengler are acknowledged for their fruitful cooperation and useful discussion. References and Notes (1) Fokema, M. D.; Ying, J. Y. Catal. ReV. 2001, 43, 1. (2) Zhang, R.; Kaliaguine, S. Appl. Catal., B 2008, 78, 275. (3) Carucci, J. R. H.; Arve, K.; Eraenen, K.; Murzin, D. Y.; Salmi, T. Catal. Today 2008, 133-135, 448. (4) Shimizu, K. I.; Satsuma, A.; Hattoi, T. Appl. Catal., B 2000, 25, 239. (5) Kameoka, S.; Ukisu, Y.; Miyadera, T. Phys. Chem. Chem. Phys. 2000, 2, 367. (6) Hamada, H. Catal. Today 1994, 22, 21. (7) Lick, I. D.; Ponzi, E. N. React. Kinet. Catal. Lett. 2003, 79, 197. (8) Yan, J. Y.; Kung, M. C.; Sachtler, W. M. H.; Kung, H. H. J. Catal. 1997, 172, 178. (9) Horiuchi, T.; Fujiwara, T.; Chen, L.; Suzuki, K.; Mori, T. Catal. Lett. 2002, 78, 379. (10) Liotta, L. F.; Pantaleo, G.; Macaluso, A.; Di Carlo, G.; Deganello, G. Appl. Catal., A 2003, 245, 167. (11) Haneda, M.; Kintaichi, Y.; Mizushima, T.; Kakuta, N.; Hamada, H. Appl. Catal., B 2001, 31, 81. (12) He, C.; Paulus, M.; Chu, W.; Find, J.; Nickl, J. A.; Koehler, K. Catal. Today 2008, 131, 305. (13) Shimizu, K. I.; Satsuma, A.; Hattori, T. Appl. Catal., B 1998, 16, 319. (14) Haneda, M.; Kintaichi, Y.; Shimada, H.; Hamada, H. J. Catal. 2000, 192, 137. (15) Liu, Z.; Hao, J.; Fu, L.; Zhu, T.; Li, J.; Cui, X. Appl. Catal., B 2004, 48, 37. (16) He, C.; Paulus, M.; Find, J.; Nickl, J. A.; Eberle, H. J.; Spengler, J.; Chu, W.; Koehler, K. J. Phys. Chem. B 2005, 109, 15906. (17) Takahashi, M.; Nakatani, T.; Iwamoto, S.; Watanabe, T.; Inoue, M. Appl. Catal., B 2007, 70, 73. (18) Miyahara, Y.; Takahashi, M.; Masuda, T.; Imamura, S.; Kanai, H.; Iwamoto, S.; Watanabe, T.; Inoue, M. Appl. Catal., B 2008, 84, 289. (19) Shimizu, K.; Higashimata, T.; Tsuzuki, M.; Satsuma, A. J. Catal. 2006, 239, 117. (20) Burch, R.; Halpin, E.; Sullivan, J. A. Appl. Catal., B 1998, 17, 115. (21) Kintaichi, Y.; Hamada, H.; Tabata, M.; Sasaki, M.; Ito, T. Catal. Lett. 1990, 6, 239. (22) Kikuchi, E.; Yogo, K.; Tamaka, S.; Abe, M. Chem. Lett. 1991, 6, 1063. (23) Obuchi, A.; Ogata, A.; Ohi, K.; Nakamura, M.; Ohuchi, H. J. Chem. Soc., Chem. Commun. 1992, 247. (24) Li, Y.; Armor, J. N. Appl. Catal., B 1992, 1, L31. (25) Iwamoto, M.; Hamada, H. Catal. Today 1991, 10, 57. (26) Armor, J. N. Catal. Today 1995, 26, 99. (27) Burch, R.; Breen, J. P.; Meunier, F. C. Appl. Catal., B 2002, 39, 283. (28) Yahiro, H.; Iwamoto, M. Appl. Catal., A 2001, 222, 163.

He and Ko¨hler (29) Shelef, M. Chem. ReV. 1995, 95, 209. (30) Yeom, Y. H.; Wen, B.; Sachtler, W. M. H.; Weitz, E. J. Phys. Chem. B 2004, 108, 5386. (31) Sun, Q.; Gao, Z. X.; Chen, H. Y.; Sachtler, W. M. H. J. Catal. 2001, 201, 89. (32) Satsuma, A.; Cowan, A. D.; Cant, B. W.; Trimm, D. L. J. Catal. 1999, 181, 165. (33) Yu, Y.; Zhang, X.; He, H. Appl. Catal., B 2007, 75, 298. (34) Gorce, O.; Baudin, F.; Thomas, C.; Da Costa, P.; Djega-Mariadassou, G. Appl. Catal., B 2004, 54, 69. (35) Yu, Y.; Song, X.; He, H. J. Catal. 2010, 271, 343. (36) Takahashi, A.; Haneda, M.; Fujitani, T.; Hamada, H. J. Mol. Catal. A 2007, 261, 6. (37) Burdeinaya, T. N.; Matyshak, V. A.; Tretyakov, V. F.; Glebov, L. S.; Zakirova, A. G.; Carvajal Garcia, M. A.; Arias Villanueva, M. E. Appl. Catal., B 2007, 70, 128. (38) Shimizu, K.; Shibata, J.; Satsuma, A. J. Catal. 2006, 239, 402. (39) Xu, S.; Li, J.; Yang, D.; Hao, J. J. Phys. Chem. C 2008, 112, 16052. (40) Kantcheva, M.; Cayirtepe, I. Catal. Lett. 2007, 115, 148. (41) Radtke, F.; Koeppel, R. A.; Minard, E. G.; Baiker, A. J. Catal. 1997, 167, 127. (42) Wang, X.; Yu, S.; Yang, H.; Zhang, S. Appl. Catal., B 2007, 71, 246. (43) Haneda, M.; Bion, N.; Daturi, M.; Saussey, J.; Lavalley, J.; Duprez, D.; Hamada, H. J. Catal. 2002, 206, 114. (44) Shimizu, K.; Kawabata, H.; Satsuma, A.; Hattori, T. J. Phys. Chem. B 1999, 103, 5240. (45) Zuzaniuk, V.; Meunier, F. C.; Ross, J. R. H. J. Catal. 2001, 202, 340. (46) Meunier, F. C.; Breen, J. P.; Zuzaniuk, V.; Olsson, M.; Ross, J. R. H. J. Catal. 1999, 187, 493. (47) Shibata, J.; Shimizu, K.; Satokawa, S.; Satsuma, A.; Hattori, T. Phys. Chem. Chem. Phys. 2003, 5, 2154. (48) He, C.; Koehler, K. Phys. Chem. Chem. Phys. 2006, 8, 898. (49) Kantcheva, M.; Vakkasoglu, A. S. J. Catal. 2004, 223, 352. (50) Sadykov, V. A.; Lumin, V. V.; Matyshak, V. A.; Paukshtis, E. A.; Rozovskii, A. Y.; Bulgakov, N. N.; Ross, J. R. H. Kinet. Catal. 2003, 44, 412. (51) Iglesias-Juez, A.; Hungria, A. B.; Martinez-Arias, A.; Fuerte, A.; Fernandez-Garcia, M.; Anderson, J. A.; Conesa, J. C.; Soria, J. J. Catal. 2003, 217, 310. (52) Amenomiya, Y. J. J. Catal. 1979, 57, 64. (53) Pushkar, Y. N.; Sinitsky, A.; Parenago, O. O.; Kharlanov, A. N.; Lunina, E. V. Appl. Surf. Sci. 2000, 167, 69. (54) Prinetto, F.; Ghiotti, G.; Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. J. Phys. Chem. B 2001, 105, 12732. (55) Sedlmair, Ch.; Seshan, K.; Jentys, A.; Lercher, J. A. J. Catal. 2003, 214, 308. (56) Yeom, Y.; Li, M.; Savara, A.; Sachtler, W.; Weitz, E. Catal. Today 2008, 136, 55. (57) Yeom, Y. H.; Li, M.; Sachtler, W. M. H.; Weitz, E. J. Catal. 2006, 238, 100. (58) Savara, A.; Li, M.; Sachtler, W. M. H.; Weitz, E. Appl. Catal. B: EnViron. 2008, 81, 251. (59) Forzatti, P.; Lietti, L.; Nova, I.; Tronconi, E. Catal. Today 2010, 151, 202.

JP1065852