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Formation of Ammonia during the NO-H2 Reaction over Pt/ZrO2 T. Nanba,*,† F. C. Meunier,§,‡ C. Hardacre,‡ J. P. Breen,‡ R. Burch,‡ S. Masukawa,† J. Uchisawa,† and A. Obuchi† Research Center for New Fuels and Vehicle Technology, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan, and School of Chemistry, Centre for Theory and Application of Catalysis (CenTACat), Queens UniVersity Belfast, Belfast BT9 5AG, U.K. ReceiVed: July 9, 2008; ReVised Manuscript ReceiVed: September 16, 2008
The mechanism for the formation of NH3 during the NO-H2 reaction over Pt/ZrO2 was studied. Steady-state isotopic transient kinetic analysis was carried out with isotopic switching from 15NO-D2 to 14NO-D2, and the results revealed that formation of N2 and N2O was associated with linearly adsorbed NO on the Pt surface, whereas ammonia formation was associated with NDx species adsorbed on ZrO2. The adsorbed NHx species were not observed on the surface of ZrO2 during NH3 adsorption. From transient kinetic experiments, the formation rates of NHx species and of gaseous NH3 agreed with each other, suggesting that the NHx species on ZrO2 was an ammonia intermediate. The NDx species did not react with D2 directly, but H-D exchange occurred easily. The addition of H2O to the NO-H2 feed gas enhanced the formation of NH3. In situ diffuse reflectance spectra and transient kinetic analysis revealed that H2O enhanced the conversion of NHx species to NH3. Introduction Methods for removing NOx (NO + NO2) from diesel and lean-burn engine exhausts at low temperature are desirable for the emission control of these exhausts. One promising NOx removal method is selective catalytic reduction by H2. Although H2 has often been regarded as a nonselective reductant,1 Yokota et al. have found that Pt catalysts are highly active in the removal of NOx at around 100 °C in the presence of excess O2.2 Ueda et al. reported that Pd catalysts are also active at around 100 °C for the same reaction.3 NO reduction by H2 on Pt and Pd catalysts have been studied in detail.4-12 In contrast, Rh- and Ir-based catalysts exhibit activity for NOx removal at 200-300 °C,13 though the activity of the latter metals is promoted by small amount of SO2.14 In summary, platinum-group metals are effective for the removal of NOx with H2 in the presence of excess O2, and Pt-based catalysts, in particular, are the most active at around 100 °C. One problem with Pt catalysts is that a high selectivity toward N2O is typically observed at low temperatures.5 To decrease N2O formation at low temperature, several researchers have added promoter materials to Pt catalyst systems. For example, the addition of Mo and Na to Pt/Al2O3 effectively decreases N2O formation.15 N2O formation also can be suppressed by the use of perovskite-type materials as supports.8,16,17 A physical mixture of Pt catalyst and zeolite has been proposed as an effective system for low-temperature NOx removal with low N2O selectivity.18,19 This system is a simple combination of catalysts possessing different functions: Pt and zeolite act as catalysts for the formation of NH3 and for selective catalytic reduction with NH3 (NH3-SCR), respectively. Honda Ltd. recently presented a double-layered catalytic converter system for NOx * Corresponding author. Tel: +81 29 861 8288, fax: +81 29 861 8259, e-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Queens University Belfast. § Present address: University of Caen, LCS-ENSICAEN-CNRS, 14050 Caen, France.
removal that contains both platinum and zeolite-based catalysts; this system meets stringent U.S. Environmental Protection Agency (EPA) Tier II Bin 5 emission requirements.20 The NOx removal mechanism of this system involves in situ NH3 generation over Pt catalysts under fuel-rich conditions and subsequent NH3-SCR over zeolite-based catalysts under lean conditions. Because NH3 reacts efficiently with NO and O2 to form N2 selectively on zeolite,21 NH3 is a key intermediate species for decreasing N2O selectivity and improving NOx abatement (deNOx) activity in catalytic converters. Satsuma et al. also suggested that with Pt/zeolite, NH3 is formed from the NO-H2 reaction over Pt and then NH3-SCR proceeds on the acidic site of the zeolite support.9,22 In situ NH3 generation is important for high performance of deNOx converters, and clarification of NH3 formation pathways is important to improve the catalytic activity of these converters. NO reduction by H2 is generally believed to proceed as follows: NO dissociatively adsorbs on noble metal surfaces to form Nad and Oad, and the recombination of Nad, the reaction of Nad with nondissociatively adsorbed NO, and the reaction of Nad with Had form N2, N2O, and NH3, respectively.23,24 In the presence of O2, however, other reaction pathways have been proposed; one such pathway is the reaction between nitrate/ nitrite and H2 in the presence of excess oxygen.25 It has also been reported that the NO-H2 reaction may proceed through the decomposition of ammonium nitrate in the presence of O2.26 Involvement of species adsorbed and accumulated on the surface of support materials appears to further complicate the mechanism for the NO-H2 reaction in the presence of O2. Clarification of the reaction of adsorbed species on the support surface leads to better understanding of the NO-H2 reaction mechanism. In this study, we aimed to clarify whether NH3 formation proceeded on support surfaces in a simple NO-H2 reaction. The catalyst used here was Pt/ZrO2, which has a higher capacity for NH3 formation than Pt/Al2O3 and Pt/TiO2 even in the presence of O2.13 We performed steady-state isotopic
10.1021/jp8060812 CCC: $40.75 2008 American Chemical Society Published on Web 10/29/2008
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transient kinetic analysis, and we report here that adsorbed species on the ZrO2 surface were involved in the formation of NH3. Experimental Section The method by which the Pt/ZrO2 catalysts were prepared is described elsewhere.18 The catalysts used had a Pt loading of 0.4 wt%, a Pt dispersion of 61%, and a Brunauer-Emmett-Teller specific surface area of 36 m2/g. The catalytic activity was measured by means of a conventional plug flow reactor. The weight of the catalysts was 0.1 g. The feed gas was composed of 0.01-0.6% 14NO, 0.27 or 0.5% H2, 0 or 0.5% D2, and 0 or 1% H2O with balance He, and the flow rate was 200 mL/min. Before the activity measurements, the catalysts were pretreated with 10% O2/He at 300 °C for 1 h and then with 10% H2/He at 300 °C for 1 h. The product gas was analyzed by a Fourier-transform infrared spectrometer (FTIR; Magna 560, Nicolet) equipped with a multireflectance gas cell for NO, NO2, N2O, and NH3 analysis and a microgas chromatograph (micro-GC; M200, Agilent) equipped with an MS-5A column and a thermal conductivity detector for N2 analysis. The reaction temperature was maintained at 150 °C. Steady-state isotopic transient kinetic analysis (SSITKA) was carried out in a diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy cell in an FT-IR (VERTEX70, Bruker) equipped with a mercury-cadmium-telluride detector. For SSITKA, the weight of the catalysts was 18 mg, and the feed gas was composed of 0.5% 14NO or 15NO and 2.5% D2 or H2 with balance Ar. If necessary, each gas component could be diminished, or 0.12% H2O was added. H2O was supplied by bubbling Ar through H2O maintained at 0 °C prior to introducing Ar to the feed gas mixture. The total flow rate was 35 mL/min, which corresponded to the same space velocity as that of the plug flow reactor. For the measurements, the switching of feed gases in the catalyst bed was completed within 20 s. Prior to measurements, the samples were pretreated with 10% H2 or D2 at 300 °C for 30 min. A background spectrum was obtained at a resolution of 4 cm-1 by integrating over 50 scans after pretreatment. For the measurements, integration was carried out over 5 scans. The effluent gas from the DRIFT cell was analyzed by a mass spectrometer (HPR20, Hiden). The signal intensities of the mass spectra and the areas under the peaks of the surface species were normalized on the basis of values under steadystate conditions. Results and Discussion NO-H2 Activity. Figure 1 shows the effect of NO concentration on NO-H2 activity over Pt/ZrO2. The H2 concentration was kept constant at 2700 ppm. The stoichiometries of the NO-H2 reactions are:
2NO + H2 f H2O + H2O
(1)
2NO + 2H2 f H2 + 2H2O
(2)
2NO + 5H2 f 2NH3 + 2H2O
(3)
Thus, the stoichiometric concentrations of NO for the formation of N2O, N2, and NH3 were 5400, 2700, and 1080 ppm, respectively, in the present case. As shown in the figure, NOx conversion was nearly 100% at NO concentrations below 1900 ppm; the conversion decreased with increasing NO concentration. NH3 and N2 formation increased with increasing NO feed concentration, and N2O formation was essentially negligible for concentrations less than 1900 ppm. Above 1900 ppm NO, the formation of N2O increased markedly, whereas
Figure 1. Dependence of 14NO-H2 activity on NO concentration over Pt/ZrO2. Catalyst weight: 0.1 g; feed gas composition: 0.01-0.6% 14NO and 0.27% H2 with balance He; total flow rate: 200 mL/min. Symbols indicate NOx conversion (b) and conversion from NOx to N2 (O), N2O (0), and NH3 (4).
NH3 formation decreased and N2 formation leveled off. Clearly, NH3 was the major product formed over Pt/ZrO2 at NO concentrations below 3000 ppm, which is a higher NO concentration than that needed for stoichiometric N2 formation. The observed high selectivity toward NH3 agreed with the characteristics of Pt catalysts reported previously.27,28 DRIFT Spectra for the Steady-State Reaction of Isotopic NO-H2. Prior to SSITKA experiments, we confirmed the surface species present in the steady-state NO-H2 reaction by replacing each component of the feed gas with its respective isotope. In situ DRIFT spectra for the steady-state reactions of 14NO-D and 15NO-D are shown in Figure 2a. For bands 2 2 characteristic of nitrogen-containing adsorbed species, a redshift was observed from 2502 to 2490 cm-1 and from 1751 to 1718 cm-1 during the isotopic switch from 14NO to 15NO. The former bands were assigned to NDx species,29,30 whereas the latter bands were characteristic of NO linearly adsorbed on Pt.31,32 The band observed at 2400-2300 cm-1, which did not exhibit a shift concurrent with isotopic switching, was attributed to gaseous CO2, C-D, or a combination of both;33 and the negative band at 2010 cm-1 was attributed to the consumption of carbonyl in ZrO2. These C-D and carbonyl species were most likely formed from the reaction between D2 and surface carbonates. Figure 2b shows the spectra for the steady-state reactions of 15NO-H and 14NO-H over Pt/ZrO . A red-shift from 3336 2 2 2 to 3327 cm-1 was observed after the isotope was switched. The band at 3250 cm-1 showed no isotope shift, indicating that this band was characteristic of -OH.34 The ratio of peak frequencies of 3336 cm-1 to 14NDx band in Figure 2a was 1.333. For stretching within an amine group, an isotopic shift of 1.330 has been reported.35 Because our coefficient was near this reported value, we concluded that the band at 3336 cm-1 was due to 14NH . x Steady-State Isotopic Transient Kinetic Analysis of the NO-H2 Reaction with NO Isotope. Figure 3 shows the evolution of N2 and N2O, as well as the peak position of the linear Pt-NO band, as a function of time during the isotopic switch from 15NO-D2 to 14NO-D2. The time at which the switching was carried out was designated as t ) 0 s; after this time, the amount of 15N2 (m/z 30) quickly decreased. 14N15N (m/z 29) formation reached a maximum at 7 s and then decreased. 14N2 (m/z 28) formation began at 5 s and then became constant at 10 s. For N2O formation, 15N2O (m/z 46) quickly
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Figure 2. In situ DRIFT spectra for the steady-state reactions of 15NO-D2 and 14NO-D2 (a) and for composition: 0.5% 14NO or 15NO and 2.5% D2 or H2 with balance Ar; reaction temperature: 150 °C.
Figure 3. Formation of various isotopes of N2 (a) and N2O (b), along with the position of the peak absorbance of linear Pt-NO (c), as a function of time following isotopic switching of 15NO to 14NO. Feed gas composition: 0.5% 14NO or 15NO and 2.5% D2 with balance Ar; reaction temperature: 150 °C.
decreased. 14N15NO (m/z 45) formation reached a maximum at 6 s and then decreased. 14N2O (m/z 44) formation began at 5 s and then became constant at 10 s. The position of the Pt-NO band shifted from 1718 to 1751 cm-1 within 15 s following the isotopic switch. This time interval agreed with that observed for the formation of the various isotopes of N2 and N2O, suggesting that N2 and N2O formation was concurrent with the formation and reaction of linearly adsorbed Pt-NO. Figure 4 shows the formation of ND3 and the change in the position of the NDx band as a function of time during the isotopic switch from 15NO-D2 to 14NO-D2. Kr was used as an indicator to monitor the purging of 15NO following the isotopic switch. The formation of 15ND3 (m/z 21) increased somewhat and then decreased gradually, even though 15NO was purged immediately after isotopic switching. The intensity of 14ND (m/z 20) was calculated by subtraction of the value before 3 isotopic switching. Since the reaction reached steady state before isotopic switching, the change in the intensity of m/z 20 after isotopic switching was attributed to 14ND3 formation. 14ND3 formation increased 1 min after the switch. The switch from 15ND to 14ND was completed in about 7 min, as evidenced 3 3 by the position of the maximum of the NDx band, which shifted gradually and eventually reached 2502 cm-1 6 min after the switch. The switching of intensities for the formation of the ND3 isotopes occurred on the same time scale as the NDx band
15
NO-H2 and
14
NO-H2 (b). Feed gas
Figure 4. Formation of deuterated ammonia species (a) and the position of the peak absorbance of the NDx band as a function of time following isotopic switching of 15NO to 14NO. Feed gas composition: 0.5% 14NO or 15NO and 2.5% D2 with balance Ar; reaction temperature: 150 °C.
Figure 5. In situ DRIFT spectra for 14NH3 adsorption and for the steady-state reaction of 14NO-H2. Feed gas composition was 1000 ppm 14 NH3 for 14NH3 adsorption and 0.5% 14NO and 2.5% H2 for 14NO-H2 reaction; reaction temperature: 150 °C. The background spectra for both samples were obtained at 150 °C after H2 pretreatment at 300 °C.
shift, suggesting that ND3 formation was related to the formation and reaction of the NDx species. The time interval for switching the isotopes of the NDx species was much longer than that observed for the linear Pt-NO species. This comparatively longer time interval suggests that the NDx species was mainly absorbed on the ZrO2 support.
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Figure 6. Variation with time of the normalized area under the 15NDx band and the normalized signal intensity of m/z 21 (a) and the change in spectra between 0 and 8 min as measured by means of a conventional flow-reactor system (b). The sample was pretreated with 10% H2 or D2 at 300 °C for 30 min. The feed gas composition was 0.5% 15NO and 2.5% D2 with balance Ar for Figure 6a, and 0.1% 14NO and 0.5% D2 with balance He for Figure 6b; reaction temperature: 150 °C.
Formation Rate of NHx Species and NH3. ZrO2 is known to have an NH3 adsorption site.13,36-38 A likely reason for the long tailing of deuterated ammonia observed in Figure 4 is repetitive adsorption of ammonia. We first compared an in situ DRIFT spectrum acquired in a flow of 14NH3 at 150 °C with a spectrum for the 14NO-H2 reaction (Figure 5). The bands of adsorbed species in the 14NH3 spectrum appear at 3250 cm-1 with shoulders at 3300 and 3164 cm-1. Since the band at 3250 cm-1 in the spectrum for the 14NO-H2 reaction was attributed to -OH band (Figure 2b), the main band of NH3 adsorption overlapped the -OH band. The main 14NHx band for the 14NO-H reaction, that is, the band at 3336 cm-1, was not well2 defined in the spectrum for 14NH3 adsorption. Therefore, the NHx species was not formed by simple adsorption of molecular NH3. In other words, the correlation between NHx species and ammonia formation in SSITKA experiments cannot be represented by a simple readsorption of NH3 formed as a product of the NO-H2 reaction. To obtain information about NHx species on ZrO2, the formation rates of adsorbed 15NDx species in the 15NO-D2 reaction over D2- and H2-pretreated Pt/ZrO2 were compared, and the results are shown in Figure 6a. The experiment was performed by using SSITKA system. For the D2-pretreated sample, the 15NDx band increased immediately after addition of the feed gas. For the H2-pretreated sample, the 15NDx band also increased initially, but reached a plateau at 20 s before increasing again after 1 min. The difference in these bands resulted from the H-D exchange between the NDx species and H adsorbed on ZrO2. The portion of the 15NDx band between 0 and 20 s was identical for both H2- and D2-pretreated samples. The initial increase in intensity of the 15NDx band in the first 20 s suggests that NDx species were formed on the Pt surface. The formation of NH3 through adsorbed NHx species on Pt surfaces has been extensively studied.39 However, the reaction of NHx species to form NH3 on noble metal surfaces is reported to be slow.40 H-D exchange in NDx species on Pt caused by migration of H from ZrO2 was excluded from consideration, because the amount of D2 used here should have fully covered the Pt surface with D within 1 s. Therefore, the stepwise increase in NDx species indicated that these species migrated to ZrO2. The formation of gaseous 15ND3 (m/z 21) was observed over D2- and H2-pretreated Pt/ZrO2 simultaneously, and these results are also shown in Figure 6a. For the D2-pretreated sample, notable 15ND3 formation began after 1.5 min. For the H2pretreated sample, 15ND3 formed gradually after 3 min. The
Figure 7. Variation in the formation of 15ND3 and in the area under the 15NDx band after components were eliminated from the 15NO-D2 feed gas. Feed gas composition prior to elimination of components: 0.5% 15NO and 2.5% D2 with balance Ar; reaction temperature: 150 °C.
slopes observed as a function of time in the profiles of increase in 15NDx band and 15ND3 formation were in agreement for both D2- and H2-pretreated samples (Figure 6a). Using a conventional flow-reactor system, we confirmed that ammonia isotopes were formed in the 14NO-D2 reaction over the H2-pretreated catalyst (Figure 6b). The spectra clearly show the presence of specific bands at 967 and 932 cm-1 for 14NH3, 896 and 874 cm-1 for 14NDH , 818 and 809 cm-1 for 14ND H, and 749 and 745 cm-1 2 2 for 14ND3. Therefore, the delay of ND3 formation observed for the H2-pretreated sample resulted from the formation of ammonia isotopes. Moreover, bands corresponding to the formation of NHx species were not observed during the delay period for NDx species. These results suggest that the surface species formed by means of H-D exchange in NDx species was not stably adsorbed on ZrO2 and rapidly reacted to form ammonia isotopes. Reactivity of NHx Species on ZrO2. The NHx species formed first on Pt, so NH3 release from the Pt surface should have occurred. Although the NH3 formation rate on Pt was not estimated, the results above suggest that NHx species on ZrO2 had a large contribution to NH3 formation. So, we focused on the mechanism of the subsequent reaction of the NHx species on ZrO2 to form NH3, and the 15NO feed was turned off during the steady-state 15NO-D2 reaction. Figure 7 shows the changes in the formation of 15ND3 and the area under the band of the 15ND band after the 15NO was turned off. The amount of 15ND x 3
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Figure 8. Variation of the areas under the NDx and NHx bands as a function of time after the switch from 15NO-D2 to 15NO-H2 (a) and absorbance of resulting ammonia isotopes (b). Feed gas composition: 0.5% 15NO and 2.5% D2 or H2 with balance Ar for Figure 7a, and 0.1% 14NO and 0.5% H2 or D2 with balance He for Figure 7b; reaction temperature: 150 °C.
Figure 9. Effect of H2O addition on 14NO-H2 activity. Catalyst weight: 0.1 g; feed gas composition: 0.11% 14NO, 0.5% H2, and 0 or 1% H2O with balance He; total flow rate: 200 mL/min. Symbols indicate NOx conversion (b) and concentrations of N2 (O), N2O (0), and NH3 (4).
formation decreased rapidly after the 15NO feed was eliminated, and the normalized signal intensity reached 0.1 at 2 min. The area under the 15NDx band decreased gradually and reached ca. 0.9 at 20 min. The trend of 15ND3 formation after both 15NO and D2 were eliminated from the feed gas was the same as that observed after elimination of only 15NO. This similarity in 15ND3 behavior suggests that direct reaction of the NDx species with ZrO2 and D2 was negligible. Figure 8a shows changes in the band intensities of 15NDx and 15NHx as a function of time after the switch from 15NO-D2 to 15NO-H2. The results suggest that H-D exchange in NHx species was very fast. Figure 8b shows ammonia isotopes formation after the switch from 14NO-H2 to 14NO-D2 in the conventional flow reactor. The ammonia species observed exhibited maximum intensity at different times in the order of NH3 f NDH2 f ND2H f ND3, and the switching rate was much slower than the rate of NDx species formation. This slow shift in species composition was attributed to H-D exchange between ammonia and the hydroxyl group of ZrO2. Considering the results shown in Figures 7 and 8, we concluded that the reaction between NHx species on ZrO2 and H2 consisted solely of a simple H-D exchange. Effect of H2O Addition. The other possible source for the supply of hydrogen to the NHx species was H2O, which was
Figure 10. Variation in the normalized areas under the NDx and NHx bands as a function of time following the introduction of H2O to the feed gas. Feed gas composition: 0.5% 15NO, 2.5% D2, 0.12% H2O with balance Ar; reaction temperature: 150 °C.
formed during the NO-H2 reaction. Therefore, we examined the effect of adding H2O to the 14NO-H2 reaction in the conventional flow-reactor system, and the results are shown in Figure 9. When H2O was added, NH3 formation increased sharply and then reached a plateau after approximately 30 min. N2O increased and N2 decreased, and both reached steady states after approximately 30 min. When H2O was eliminated from the feed gas, the concentrations of products gradually returned to the amounts that had been observed before H2O addition. This result clearly suggests that H2O reversibly promoted the formation of NH3. Figure 10 shows the normalized areas under the 15NDx and 15NH bands as a function of time after H O addition. The 15NH x 2 x band was integrated by baseline subtraction in the region from 3310 to 3400 cm-1. The area under the 15NDx band decreased just after the introduction of H2O and reached a plateau at 0.1 after 20 min. The area under the 15NHx band remained nearly constant for ∼1 min following the introduction of H2O and then increased to a plateau with a normalized value of 0.15. The trend observed for the formation of the NHx species suggests that the hydrogen in H2O was exchanged with deuterium in the NDx species. Notably, the sum of the normalized areas of 15NDx and 15NHx was much less than unity, suggesting that the addition of H2O enhanced the conversion of NHx species to ammonia. We observed the formation behavior of ammonia isotopes and surface NDx and NHx species after D2 was eliminated from
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Figure 11. Variation in formation profiles of ammonia isotopes (a) and in the areas under the 15NDx and 15NHx bands (b) as a function of time after D2 was cut off from the feed gas for the 15NO-D2-H2O reaction. For parts a and b, the reaction conditions were the same as those used for Figures 9 and 10, respectively. Symbols in part a indicate 14ND3 (O), 14ND2H (0), 14NDH2 (9), and 14NH3 (b).
the feed gas in the NO-D2-H2O reaction. During the steadystate reaction of NO-D2-H2O, ammonia isotopes were observed byproduct gas analysis in the order ND3 > ND2H > NDH2 > NH3 (Figure 11a). When D2 was shut off, tailing was observed in the ammonia isotopes profiles; this tailing became longer with decreasing molecular weight of ammonia isotope, and 14NH3 exhibited a peak at 4 min. Simple H-D exchange would exhibit no peak in the formation of all ammonia species. Therefore, the peculiar peak of NH3 at 4 min indicates that NH3 was formed from other reaction. Figure 11b shows variations in the areas under the NDx and NHx bands as a function of time after D2 was switched off. The NDx band decreased quickly, concurrent with the H-D exchange shown in Figure 8. These results suggest that the NDx species were quickly exchanged with the NHx species in the presence of H2O. After D2 was shut off, the NHx band increased slightly and then decreased after 2 min. The increase in NHx species up to 2 min reflected the formation of these species through H-D exchange of NDx species. The decrease in NHx species after 2 min was concurrent with the peak of NH3 formation at 4 min shown in Figure 8a. Notably, gaseous D2 was completely purged from the system within 2 min. Therefore, the results suggest that H2O reacted with NHx to form NH3 after 2 min, even in the absence of gaseous H2. Speculated Mechanism of Ammonia Formation over ZrO2. These results suggest that the NHx species adsorbed on ZrO2 were intermediate species in the formation of NH3 by the NO-H2 reaction. A possible adsorption site for the NHx species was a multicoordinated Zr-OD site. Figure 12 shows spectra in the range of -OD absorption. The spectrum obtained after D2 pretreatment exhibited a broad absorption at 2800-2100 cm-1 and an additional absorption peak at 2704 cm-1 (spectrum a). These bands were ascribed to -OD and multicoordinated Zr-OD, respectively.41-43 Under steady-state reaction conditions in the 15NO-D2 reaction, the absorption at 2704 cm-1 decreased (spectrum b). Spectrum c was obtained for the 15NO-D reaction over a sample pretreated with D . A sharp 2 2 decrease in absorbance was observed at 2704 cm-1, suggesting that the multicoordinated Zr-OD site had interacted with some adsorbed species. Multicoordinated Zr-OD sites are associated with the formation of an oxygen defect.41 Additionally, hydrogen spillover proceeds easily on Pt/ZrO2,44,45 and the adsorption site for this spillover is an oxygen defect site on ZrO2.46 In the present
Figure 12. In situ DRIFT spectra of -OH region after D2 pretreatment (spectrum a), for the 15NO-D2 reaction after H2 pretreatment (spectrum b), and for the 15NO-D2 reaction after D2 pretreatment (spectrum c). Feed gas composition: 0.5% 15NO and 2.5% D2 with balance Ar; reaction temperature: 150 °C.
studies, one of the Zr-O bonds of a multicoordinated Zr-OH site might have dissociated, allowing NHx species to be adsorbed on Zr. The adsorbed NHx species was likely Me-NH2 or Me2-NH, and Fripiat et al. have assigned the bands above 3300 cm-1 to these amide and imide species.47 The enhancement of NH3 formation and the decrease in band intensity of the NHx species upon the addition of H2O are suspected to be due to the following reaction with an amide species: M-NH2 + H2O f NH3 + M-OH.47 This reaction is also known to occur for alkaline metals, such as Li, Na, Ca, and Mg.48,49 The displacement of NH3 adsorbed on ZrO2 by H2O occurs readily.50 Furthermore, the Me-NH2 species is more stable than molecularly adsorbed NH3 and NH4+ species on ZrO2.47 H2O is formed in the NO-H2 reaction and would play an important role in the formation of ammonia from NHx on ZrO2. We concluded that one NH3 formation pathway in the NO-H2 reaction was the spillover of NHx from Pt to ZrO2, followed by the formation of NH3 through reaction of NHx with H2O formed during the NO-H2 reaction.
Ammonia Formation in NO-H2 Reaction Conclusion The reaction mechanism for the formation of ammonia in the NO-H2 reaction over Pt/ZrO2 was studied. By evaluating catalytic activity in a conventional flow-reactor system, we showed that Pt/ZrO2 had a high capacity to form NH3. Steadystate isotopic transient kinetic analysis with isotopic switching between 15NO and 14NO in the NO-D2 reaction revealed that N2 and N2O formation were caused by linearly adsorbed NO on the Pt surface and that ammonia formation was related to the NDx species. The NDx species exhibited a longer tailing after the switch from 15NDx to 14NDx than did linearly adsorbed NO, suggesting that the NDx species adsorbed on the ZrO2. The NHx species in the NO-H2 reaction was not a major species in the adsorption of molecular NH3, so the long tailing of NDx species was not due to readsorption of NH3 as a product. Samples pretreated with H2 or D2 exhibited different trends for the formation of ND3 and NDx species. NDx species formed rapidly in both samples just after the feed gas was introduced, but the sample pretreated with H2 showed a reduction in the rate of formation after 20 s. This observation suggests that NHx species were formed on Pt first and then migrated to the ZrO2 surface. ND3 formation for the H2-preteated sample exhibited delay. The slopes observed as a function of time in the formation profiles of ND3 and NDx species agreed with each other for both samples. These results suggest that ammonia was formed from NHx species on ZrO2. The formation of 15ND3 after the 15NO supply was eliminated from the 15NO-D feed was 2 identical to that observed after the feed gas was shut off completely. The absorption of NDx species decreased slightly when the 15NO supply was eliminated. These results suggest that the NDx species did not react with D2. From steady-state isotopic transient kinetic analysis involving the switching of D2 and H2, we found that H-D exchange in the NDx species occurred quickly. The addition of H2O to the NO-H2 feed gas enhanced NH3 formation. In situ DRIFT spectra revealed the formation of NHx species and a decrease in NDx absorption after H2O was added to the NO-D2 feed gas. The sum of the intensities of the NDx and NHx absorption bands was greatly reduced after H2O addition. These results suggest that H2O improved the conversion of surface NHx species to NH3. When D2 was removed from the 14NO-D2-H2O feed gas, ammonia isotopes of lower molecular weight exhibited longer tailing in their formation profiles, and NH3 exhibited a peculiar peak in its formation profile. The formation of NHx species increased before decreasing after D2 was shut off. The peak of NH3 formation and the onset of the decrease in NHx absorbance were concurrent each other. We concluded that the ammonia formation pathway in the NO-H2 reaction included the following reaction on ZrO2: M-NH2 + H2O f NH3 + M-OH. Acknowledgment. This research was financially supported by the Scientist Exchange Program between the Japan Society for the Promotion of Science (JSPS) and the Royal Society. References and Notes (1) Unland, M. L. J. Catal. 1973, 31, 459. (2) Yokota, K.; Fukui, M.; Tanaka, T. Appl. Surf. Sci. 1997, 121122, 273. (3) Ueda, A.; Nakano, T.; Azuma, M.; Kobayashi, T. Catal. Today 1998, 45, 135. (4) Frank, B.; Emig, G.; Renken, A. Appl. Catal., B 1998, 19, 45. (5) Burch, R.; Coleman, M. D. Appl. Catal., B 1999, 23, 115.
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