NO−C2H4 Reactions on the Surface of Stepped Pt(332) - The Journal

J. Phys. Chem. C 2007, 111, 9919-9926

9919

NO-C2H4 Reactions on the Surface of Stepped Pt(332) Yuhai Hu and Keith Griffiths* Department of Chemistry, The UniVersity of Western Ontario, London N6A 5B7, Canada ReceiVed: February 16, 2007; In Final Form: May 3, 2007

NO-C2H4 interactions on the surface of stepped Pt(332) have been studied using Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). IR data show that pre-dosed C2H4 molecules suppress the adsorption of NO on the surface of Pt(332) to an extent depending on both C2H4 coverage and the temperatures to which C2H4 pre-adlayers are annealed. At 90 K, the adsorption of NO on step sites is significantly suppressed by C2H4 following exposures greater than 0.32 L. This siteblocking effect persists and is even enhanced when annealing C2H4 pre-adlayers to 200 K, a temperature at which the adsorbed C2H4 molecules are not dissociated. As annealing temperatures are increased beyond 260 K, an ethylidyne species forms and is located on terraces. Consequently, the adsorption of NO on step sites is restored but to an extent smaller than that on a clean Pt(332) surface. The IR spectra also indicate that there are no detectable intermediates resulting from direct chemical reactions between NO and C2H4/C2H4derived hydrocarbons, which can promote N2 production. The co-adsorption of C2H4- and C2H4-derived hydrocarbons does significantly promote N2 desorption, being dependent on the temperatures to which predosed C2H4 adlayers are annealed. Annealing C2H4 adlayers to temperatures e300 K significantly enhances N2 desorption at temperatures below 400 K, giving rise to a peak at about 340-380 K. This low-temperature N2 desorption disappears completely after annealing the C2H4 adlayers to >350 K. N2 desorption at ∼460 K appears to be slightly enhanced. NO dissociation is the rate-limiting step in the reduction of NO by C2H4and C2H4-derived hydrocarbons. The contribution of C2H4- and C2H4-derived hydrocarbons to N2 desorption is mainly attributed to 1) weakening of N-O bonds through an electron-donation effect; and 2) providing a source of reductants, i.e., H, CHx, C2Hx, and even C, which react with the atomic O from NO dissociation, leaving the surface with more vacant sites for further NO dissociation. The generation of CHx and C2Hx therefore plays a central role in the NO reduction mechanism.

Introduction The selective catalytic reduction (SCR) of NO in the presence of hydrocarbons is one of the important ways to suppress NO emission. A large number of studies have been done with the intent of screening optimum catalysts and exploring the reaction mechanism. It is generally acknowledged that metal ionexchanged zeolites, base metal oxides, and supported metals are active for this reaction.1,2 The reactions proceed mainly through three mechanisms, which are intimately related to the catalysts and hydrocarbon species:1,2 cyanide or isocyanate intermediate surface species; organo-nitro and related species as intermediates; NO decomposition and subsequent oxygen removal by reaction with hydrocarbon reductants. Nonetheless, many important questions still remain unanswered, in particular, the elucidation of the detailed reaction mechanisms. The SCR of NO in the presence of CO, NH3, and H2 under ultrahigh vacuum (UHV) conditions has been extensively investigated previously,2,3-8 offering significant insights into the reaction mechanisms. In contrast, fewer studies have addressed the SCR of NO in the presence of hydrocarbons under the same conditions. The adsorption and reaction of individual hydrocarbon species such as ethylene, benzene, acetylene, etc. has been more extensively studied.9-15 Van Hardeveld et al. studied the reaction of NO and C2H4 on Rh(111) using thermal desorption spectroscopy (TDS) and * To whom correspondence should be addressed. E-mail: [email protected].

secondary ion mass spectrometry (SIMS).16 H2, H2O, NO, CO, N2, and HCN were detected. The production of these species is highly dependent on the relative coverage of NO and C2H4. Mullins and Zhang studied the interaction between NO and C2H4 on Rh/CeOx(111) catalysts using TDS.17 They suggested that the products are in intimate association with the oxidizing states of the catalysts, and adsorbed NO and C2H4 do not strongly interact to form new intermediates on Rh/CeOx(111). Zebisch et al. has carried out a very detailed study on the co-adsorption of benzene and NO on the surface of Ni(111) using TDS and LEED.18 They concluded that various ordered structures could be formed in the co-adlayers, depending on the relative coverage and surface temperature. Increasing NO exposure suppressed the dissociation of benzene upon heating, and under certain conditions, benzene dissociation was completely suppressed. These studies are very helpful for a better understanding of the reaction of hydrocarbons with NO. However, much more information is still needed. The adsorbed C2H4 molecules undergo a series of changes upon thermal activation, producing various hydrocarbon species and H atoms, depending on the surface temperature. Both C2H4- and C2H4-derived species are capable of promoting NO reduction under certain conditions. So far, it is still not clear how these reductants react with NO, and their contributions to N2 production. These questions are hopefully answered using thermal desorption spectroscopy combined with a surface-sensitive spectroscopy, such as FTIRRAS. Under certain circumstances, identification of adsorbed species using vibrational spectroscopy can be misleading.19

10.1021/jp071340a CCC: $37.00 © 2007 American Chemical Society Published on Web 06/09/2007

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Hu and Griffiths

However, IR spectroscopy, in particular FTIR-RAS, is still widely used in interrogating the surface chemistry of NOx and hydrocarbons, offering some well-defined information that is essential for exploring catalytic reactions involving these two kinds of species.20 To our knowledge, no vibrational spectroscopic study has been done in the interrogation of C2H4-NO interactions on single crystals. In this paper, the reduction of NO by C2H4 on the surface of Pt(332) is investigated using FTIR-RAS and TDS, with special attention paid to the influence of the pretreatment of C2H4 adlayers on the NO reduction yield. Pt(332) is a stepped surface with a 6(111)x(111) structure.21 The (111) plane is predominant in practical metal-containing catalysts for deNOx because of its stability. Previous studies have established that the dissociation of NO mainly occurs at defect sites (e.g., steps) on Pt surfaces at temperatures higher than 300 K.22-24 This surface can therefore serve as an ideal model for simulating the surfaces of true catalysts in the reaction atmosphere. Experiment All the experiments were carried out in a standard UHV system described elsewhere.25,26 Briefly, a stainless steel UHV chamber is pumped by a turbo-molecular pump, which is in turn backed by an oil diffusion pump. A base pressure of 1 × 10-10 Torr is routinely attained. The chamber is fitted with potassium chloride windows for grazing incidence surface IR spectroscopy. The Pt(332) crystal was cleaned by repeated cycles of Ar+ ion bombardment and oxidation. The final cleanliness was judged by the CO infra red spectrum, which is sensitive to the cleanliness and the state of perfection of the surface.21,27 IR spectra were obtained in a single reflection geometry at 7° grazing incidence using a Digilab FTS 7000 spectrometer. One thousand scans, which takes less than 2 min, were co-added at a resolution of 8 cm-1. Spectra are ratioed to the reflectance from the clean surface under identical temperature and geometry conditions to produce absorbance data. A narrow band mercury cadmium telluride (MCT) detector was used throughout, with a cutoff around 1000 cm-1. Although some IR data is shown extending down to 800 cm-1, the steep spectral function (cutoff) in the frequency region 800-950 cm-1 makes the data unreliable below about 950 cm-1. Both ethylene and NO were introduced into the chamber by backfilling. The highly efficient pumping system can restore the background to workable levels quickly. Thermal desorption spectra were recorded by resistively heating the crystal at a rate of 4 K/s. Results 1. IR Spectra of NO-C2H4 Co-Adlayers. Figure 1 shows IR spectra recorded following an exposure of a clean Pt(332) surface at 90 K to various amounts of C2H4 (first) and then 0.8 L NO. The adsorption of 0.8 L NO on a clean Pt(332) surface gives rise to three peaks at ∼1488, ∼1628, and ∼1690 cm-1, which have previously been assigned to the characteristic vibrations of NO molecules adsorbed in bridge sites on terraces (1488 cm-1) and in atop sites on terraces (1690 cm-1) and in bridge sites on steps (1628 cm-1) respectively.19,28,29 The small peak at ∼1536 cm-1 is probably caused by the adsorption of NO on other defect sites on the surface. In the presence of predosed C2H4, the peak at ∼1628 cm-1 undergoes a disproportionately bigger change than the other two peaks. The peak intensity is attenuated quickly with increasing C2H4 exposure. The vibrational frequency is also evidently red-shifted. No other

Figure 1. IR spectra recorded following exposure of a clean Pt(332) surface to various amounts of C2H4 (pre-dosed) and then 0.8 L NO at 90 K.

Figure 2. IR spectra of co-adlayers with exposures of 0.16 L C2H4 and 0.8 L NO, in which pre-dosed C2H4 was annealed to various temperatures as indicated before being exposed to NO at 90 K. Spectra were taken at 90 K.

significant changes are observed for the peaks associated with NO molecules adsorbed on terraces, with the exception that the vibrational frequencies are slightly red-shifted. In order to obtain insight into the interactions between C2H4derived species and NO, the Pt(332) surface was exposed at 90 K to 0.16 L C2H4 (or 0.8 L C2H4) and then annealed to various target temperatures before being exposed to 0.8 L NO at 90 K. IR spectra of the resultant co-adlayers were recorded at 90 K. The spectra for the co-adlayers starting with 0.16 L C2H4 are shown in Figure 2. Compared to the 90 K spectrum (no anneal), annealing the pre-dosed C2H4 adlayer to 200 K causes the peak at 1623 cm-1 to decrease fairly significantly. Thereafter, this peak appears largely unchanged at higher annealing temperatures. In this temperature program, the peaks associated with NO molecules on terraces (∼1485 and 1690 cm-1) show no significant changes, except that the peak at ∼1483 cm-1 is enhanced appreciably following annealing temperatures of 350 and 400 K, eventually achieving an intensity comparable to the

NO-C2H4 Reactions on Stepped Pt(332)

Figure 3. IR spectra of co-adlayers with exposures of 0.8 L C2H4 and 0.8 L NO, in which pre-dosed C2H4 was annealed to various temperatures as indicated before being exposed to NO at 90 K. Spectra were taken at 90 K. For comparison, the spectrum of just 0.8 L C2H4 annealed to 300 K is shown at top.

‘steps’ peak (at 1612-1620 cm-1). In noting this, when comparing different peaks within a spectrum, we make no particular connection between the intensity of a peak and the number of molecules that gives rise to it. The influence of C2H4-derived hydrocarbons on the adsorption of NO on Pt(332) becomes much more significant and distinct when C2H4 pre-exposure is increased to 0.8 L, as shown in Figure 3. At an annealing temperature of 200 K, the peak at ∼1601 cm-1, which can still be observed at 90 K (no anneal), disappears completely, but a weak peak appears at ∼1561 cm-1. The peak at 1601 cm-1 most likely corresponds to “bent” NO molecules on step sites according to a previous assignment.28,29 The peak at 1561 cm-1 can be tentatively assigned to the vibration of NO molecules on step sites that directly interact with C2H4 molecules, as have been observed in the spectra of C6H6-NO and CH3OH-NO co-adlayers.30,31 The other two peaks associated with NO molecules on terraces remain almost unchanged except that the vibrational frequencies are evidently red-shifted. For the annealing temperatures beyond 200 K, while the peak at 1561-1574 cm-1 does not change significantly, the peak at 1448-1461 cm-1 disappears completely. In this annealing temperature region, the peak at 1624-1639 cm-1, which is close to that of NO molecules on steps on the clean surface, is the dominant peak. These results suggest that the presence of C2H4 derivatives arising from an annealing of 0.8 L C2H4 to temperatures beyond 250 K primarily suppresses the adsorption of NO on terraces. Moreover, the peaks associated with C2H4-related species also show appreciable changes in the presence of NO. Annealing the adlayer following an exposure of 0.8 L C2H4 to temperatures above 250 K results in the formation of ethylidyne with two characteristic peaks at ∼1126 and ∼1339 cm-1, respectively (in the top spectrum in Figure 3). In the presence of post-dosed NO of 0.8 L at 90 K, the peak associated with C-C stretch (∼1126 cm-1)32-34 disappears completely. In contrast, the peak associated the symmetric deformation of CH3 (∼1339 cm-1)32-34 remains unchanged, indicating that ethylidyne is still presented in the co-adlayers. The loss of the C-C vibration may result from a change in peak width (or cross section) and/ or a reorientation of the C-C axis induced by NO.

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Figure 4. IR spectra recorded following exposure of a clean Pt(332) surface to 0.16 L C2H4 (pre-dosed) and then 0.8 L NO at various temperatures as indicated.

Investigation was extended further to interrogate the interactions between NO and C2H4 (and derived hydrocarbons) at various adsorption temperatures. IR spectra were recorded after exposing the Pt(332) surface to 0.16 L C2H4 (first) and then 0.8 L NO at the temperatures as indicated. The data are shown in Figure 4. Comparing to 90 K, the spectra of co-adlayers formed at adsorption temperatures g200 K show several features: (1) the peak at 1474-1480 cm-1 decreases steadily and disappears at 350 K; (2) the peak at 1687 cm-1 disappears completely at only 200 K; (3) a new peak at 1808-1817 cm-1 appears, and its intensity changes as the adsorption temperature varies. This peak has previously been assigned to vibrations of an NO-O-like complex.29 We carefully interrogated NO and 18O interactions on Pt(332),35 and found that oxidation of NO 2 by 18O2 (or 18O) does not take place, though oxygen-exchange reaction is indeed observed, giving rise to a weak peak with vibration frequency of about 50 cm-1 lower than that of N16O. Combining this result with the fact that this peak appears at 200 K or lower (as shown in this figure), at which NO dissociation is not appreciable, we tentatively assign it to the vibration of adsorbed NO molecules on the steps in an atop mode; 4) a peak at 2050-2063 cm-1 appears. Blank experiment without exposure of NO and C2H4, which takes the same time as that required for recording an individual spectrum in Figure 4, proves that no CO peak is observed. Accordingly, it can be suggested that this amount of CO cannot merely be attributed to the contribution of CO molecules adsorbing on the surface during the process of spectrum acquisition. On the other hand, by comparing this peak with those from CO exposure to various values, it should be emphasized that this amount of CO is very small, and no evident desorption peak is observed in a thermal desorption spectrum. IR measurements were also taken of co-adlayers prepared by annealing the Pt(332) surface, exposed at 90 K to 0.32 L C2H4 (first) and then 1.6 L NO to various target temperatures. IR spectra were recorded after cooling the annealed co-adlayers down to 90 K, and are shown in Figure 5. At these exposures, there is one dominant peak at ∼1690 cm-1, which remains essentially unchanged for annealing temperatures e220 K. The appearance and predominance of this peak, and the disappearance of the other peaks connected with NO molecules in any bridge sites indicates that the adsorption of NO in bridge sites, both on steps and on terraces is strongly suppressed by

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Figure 5. IR spectra of the co-adlayer prepared at 90 K with 0.32 L C2H4 (first) and 1.6 L NO, then followed by annealing to various temperatures as indicated. Spectra were taken after cooling down to 90 K.

the adsorption of 0.32 L C2H4.Site-switching of NO molecules, due to the increase in NO coverage (density) and the resultant strong repulsive interaction among the adsorbed NO molecules,30 also likely plays an role. Dramatic changes are observed when the co-adlayer is annealed to 270 K. Six weak peaks appear, at about 1450, 1579, 1624, 1687, 1717, and 1801 cm-1, respectively. All the peaks correspond to the vibrations of adsorbed NO molecules at different temperatures, as has been identified above. At an annealing temperature of 300 K, the spectrum is dominated by two peaks at ∼1630 and ∼1797 cm-1, indicating that the adsorption of NO on step sites is restored at this temperature. Three other peaks are also observed, i.e., 1465, 1579, and 2050 cm-1, but are much weaker. The surface still likely contains NO molecules adsorbed in all possible sites. As the annealing temperature is increased, the peaks associated with NO molecules on steps remain, but the frequencies are shifted, probably due to changes in surface coverage (NO desorption and decomposition has started at these high temperatures). Only one weak peak at 1626 cm-1 appears when annealing the co-adlayers to 500 K. It is worth noting that in this procedure, neither the peak associated with the symmetric deformation of CH3 (1339 cm-1) nor the peak associated with C-C bond (1126 cm-1) of ethylidyne appears. Moreover, as in the data in Figure 4, a CO peak (2050 cm-1) is also observed, but the peak becomes discernible only when the annealing temperature is increased to 300 K. These results suggest that the presence of NO greatly affects the process of C2H4 dehydrogenation, but the oxidation of C2H4 does not proceed as efficiently as the oxidation of C2H4drivatives (to be discussed in detail in next part). 2. TDS Results. 2.1 TDS Results of C2H4. Annealing Pt(332) exposed to C2H4 at 90 K gives rise to several primary products, such as C2H4, C2H2, and H2, being consistent with the result from other studies.36 Considering that some fragments arise from the facile C2H4 dissociation in the ion source of the mass spectrometer and not from the surface, in this paper, we will mainly focus on C2H4 and H2 desorption. Molecular C2H4 desorption is detected following an exposure of 0.32 L, becoming significant when the exposure is beyond 0.8 L. The desorption maximum appears at 210 K, as shown in Figure 6A.

Hu and Griffiths

Figure 6. Thermal desorption spectra of C2H4 (A) and H2 (B) from the C2H4 adlayers on the clean (NO-free) surface.

H2 desorption is also strongly related to C2H4 coverage. When the C2H4 exposure is e0.16 L, H2 desorption begins at 280 K and reaches a maximum at 350 K, finishing at ∼500 K, as shown in Figure 6B. H2 desorption becomes complex as the C2H4 exposure is increased to 0.32 L and higher. Three main peaks can be observed, at about 284, 350 and 451 K respectively. The peaks do not increase significantly with increasing C2H4 exposure from 0.8 to 2 L. These characteristics are different from H2 desorption arising from C2H4 on non-stepped Pt surfaces, but is quite consistent with that observed on the stepped Pt(210) surface by Masel.37 The peak at 284 K can be mainly attributed to H2 desorption from the dehydrogenation of C2H4, and the other two peaks to H2 desorption from the subsequent dehydrogenation of the C2H4-derived hydrocarbons. 2.2 N2 and H2 Desorption from C2H4-NO Co-Adlayers. Before interrogating N2 desorption (mass 28) from the NOC2H4 co-adlayers, several experiments were performed to help distinguish the contribution of CO to mass 28. (1) CO desorption following a CO exposure of 0.4 L starts when surface temperature is higher than 400 K, giving rise to maximum at ∼510 K. (2) When annealing Pt(332) exposed to saturation with C2H4 at 90 K in the presence of 3 × 10-7 Torr 18O2, CO desorption becomes discernible when the surface temperature is increased to >400 K, reaching a maximum at about 600 K. (3) When annealing Pt(332) exposed to 0.08 L C2H4 (first) and 3 L O2 at 90 K, there is no significant CO desorption in the temperatures ranging from 200 to 500 K. As a consequence, it is reasonable to conclude that mass 28 desorption from the NO-C2H4 coadlayers at surface temperatures lower than 470 K (the highesttemperature N2 desorption from NO/Pt(332) system) is primarily corresponding to N2. Figure 7 shows N2 and H2 desorption from co-adlayers prepared at 90 K with 0.08 L C2H4 (pre-dosed) and various amounts of NO. Two peaks are observed in the thermal desorption spectra of N2, at ∼355 and ∼460 K, respectively. Both peaks increase with increasing NO exposure, but the higher-temperature peak grows much more rapidly. It should be noted that only a fixed amount of NO is decomposed on a clean Pt(332) surface, giving rise to one peak at ∼460 K, which achieves a maximum at a NO exposure of 0.8 L (see the top desorption spectrum in Figure 7A). The appearance of the lowertemperature N2 desorption suggests that the presence of C2H4 molecules facilitates NO reduction.

NO-C2H4 Reactions on Stepped Pt(332)

Figure 7. Thermal desorption spectra of N2 (A) and H2 (B) from the co-adlayers prepared at 90 K with 0.08 L C2H4 (pre-dosed) and various amounts of NO.

Figure 8. Thermal desorption spectra of N2 (A) and H2 (B) from coadlayers with 0.16 L C2H4 and 0.8 L NO, in which pre-dosed C2H4 is annealed to the temperatures as indicated before being exposed to 0.8 L NO at 90 K.

H2 desorption simultaneously decreases with increasing NO exposure, and disappears completely when the NO exposure is increased to 1.6 L. Nonetheless, appearance of H2 desorption at a NO exposure of 0.8 L indicates that the presence of NO does not completely suppress C2H4 dissociation. N2 and H2 desorption from co-adlayers with exposures of 0.16 L C2H4 and 0.8 L NO are shown in Figure 8. In this experiment, Pt(332) surface is exposed to C2H4 (first) at 90 K, annealed to various target temperatures, and then NO was dosed after cooling the annealed adlayer down to 90 K. It is quite remarkable that this preannealing procedure leads to a significant enhancement in N2 desorption at 350-380 K, providing that the annealing temperature is not beyond 300 K. This suggests that the presence of C2H4 derivatives facilitates NO reduction. This is consistent with the IR results above in Figure 5, i.e., the oxidation of C2H4 derivatives produced at moderate temperatures is more efficient than the oxidation of C2H4 itself. According to previous studies,38 at these moderate temperatures, i.e., e 300 K, most of the C-C bonds are retained in these derivatives. At annealing temperatures of 350 and 400 K, this lower-temperature N2 desorption is greatly decreased and

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Figure 9. Thermal desorption spectra of N2 (A) and H2 (B) from coadlayers with 0.4 L C2H4 and 0.8 L NO, in which C2H4 is pre-dosed at the surface temperatures as indicated before being exposed to 0.8 L NO at 90 K.

eventually disappears. In contrast, the higher-temperature desorption appears almost unchanged. H2 desorption from these co-adlayers is also intimately related to the annealing temperature. When the annealing temperature is e300 K, three peaks appear, with maxima at about 320330, 396-430, and 520-546 K, respectively. The peak intensities increase only slightly with increasing the annealing temperature. The low-temperature peaks disappear when the annealing temperature is increased to 350 K and higher. H2 desorption from these co-adlayers corroborates the suggestion that the presence of NO does not significantly suppress C2H4 dissociation. Because of a considerable amount of C2H4 desorption (also mass 28), N2 desorption from co-adlayers with C2H4 exposures of 0.8 L and higher, and when the annealing temperature is lower than 250 K, cannot be reliably resolved. Figure 9 shows N2 desorption from co-adlayers with exposures of 0.4 L C2H4 (pre-dosed) and 0.8 L NO. (In this experiment, C2H4 exposures were performed at surface temperatures of 250 K and higher. Only after the base pressure had decreased to less than 3 × 10-9 Torr was the surface cooled down to 90 K. Thermal desorption spectra indicated that there was almost no mass 28 desorption following this procedure. N2 desorption was recorded after exposing the resultant surface to 0.8 L NO at 90 K.) Like that in Figure 8, the desorption falls into two parts with maxima at very roughly 380 K and 450-456 K, respectively. For the 250 K ethylene exposure, the N2 peak at ∼380 K is dominant in the TDS. The desorption at ∼450 K only appears to be a shoulder or very minor contribution to the total signal. The lower-temperature desorption is decreased significantly with increasing the surface temperature at which C2H4 is pre-dosed, and the 350 K ethylene-exposure spectrum is dominated by desorption at ∼456 K. Compared with the data in Figure 8A, the significant enhancement in low-temperature N2 desorption under these conditions indicates that the higher coverage of C2H4 promotes NO reduction to much larger extent. H2 desorption from the same co-adlayers is shown in Figure 9B. With the C2H4-exposure-temperature e300 K, H2 desorption does not change significantly. H2 desorption below 400 K disappears completely when the C2H4-exposure temperature is increase to 350 K, at which two peaks at ∼433 and ∼536 K are observed. Combining this result with the IR results in Figure 3 and TDS results in Figure 9A, it is reasonable to suggest that

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Figure 10. Ball-and-stick model of the adsorption geometry of NO on Pt(332).

the presence of 0.8 L NO leads to significant changes in the dehydrogenation processes of C2H4-related hydrocarbons. Finally, it should be mentioned that almost no water was detected from the NO-C2H4 and O2-C2H4 co-adlayers. Thermal desorption of water from H2-O2 co-adlayers was recorded to monitor the behavior of water in the chamber. However, water desorption for this reaction only gave rise to a very weak peak, suggesting that there is a very poor sensitivity for the detection of water. In these experiments, the mass spectrometer is not mounted line-of-sight to the crystal face. The vacuum system contains many surfaces, which are held at liquid nitrogen temperature, providing an almost infinite pumping speed for water. Unfortunately, the geometry of the chamber cannot be reconfigured at this time for several reasons. It is worth noting that Friend et al. also reported difficulty in detecting water from methanol-NO2 reaction.39 Discussion 1. Co-Adsorption of C2H4 and NO on Pt(332). Compared to NO, the surface chemistry of C2H4 on stepped Pt surfaces has not been so thoroughly studied. We do not focus on trying to distinguish the location of C2H4 and C2H4-derived hydrocarbons on Pt(332), although this information can be, to some extent, tracked via the IR spectra of C2H4-NO co-adlayers on the Pt(332) surface. The adsorption of NO on the surface of Pt(332) proceeds with NO molecules preferentially located on the steps at lower coverage. The adsorption on terraces begins at higher coverage. As a guide, a simple model for the various adsorption sites for NO on Pt(332) is presented in Figure 10. The adsorbed NO molecules also undergo site switching with increasing coverage because of the increased lateral repulsive interactions, leading to frequency shifts in the IR spectra. The adsorption of NO is also intimately dependent on adsorption temperature. NO molecules are mainly located in sites on the steps at temperatures higher than 350 K. The IR spectra of C2H4 on Pt(332) show peaks identical to that on Pt(111) surfaces at 90 K (not shown). For the C2H4NO co-adlayers, the spectral changes are associated with the relative coverages of the two adsorbed species, the temperature to which pre-dosed C2H4 was annealed, and are more obvious for NO vibrations. The data in Figure 1 shows that at 90 K, the presence of pre-dosed C2H4 decreases the NO peak associated with step sites much more than the other two peaks of NO associated with terraces. The adsorption of NO on step sites is blocked by pre-adsorbed C2H4 molecules. This site-blocking effect not only holds, but proceeds to a larger extent when the C2H4 covered surface is annealed to 200 K. At this temperature, the dissociation of adsorbed C2H4 molecules does not occur, as has been shown in Figures 2 and 3. We can conclude that adsorbed C2H4 on Pt(332) prefers step sites. The enhancement in the site-blocking effect for annealing temperature at 200 K can be explained by the particular

Hu and Griffiths adsorption geometry of adsorbed C2H4 molecules on the stepped surface at this temperature. A previous study has suggested that the adsorption of C2H4 at this temperature on stepped surfaces can proceed with CdC π-bound to the step corner and with the molecular axis perpendicular to the step edge.37 As the annealing temperature moves beyond 250 K, the dehydrogenation of C2H4 has started. The primary surface species is initially ethylidyne, which is preferentially located in terrace sites with its molecular axis parallel to the terrace normal. This is the reason that adsorption of NO in step sites is restored and the adsorption of NO on terraces is suppressed (in Figure 3, 4, and 5) at the higher temperatures to which the C2H4 adlayers are annealed. Besides the site-blocking effect, other information on the interaction between C2H4 and NO can be elucidated from the IR spectra: First, NO does have some interaction with predosed C2H4-related species. The presence of NO, under certain conditions, results in complete loss of the peaks associated with ethylidyne arising from C2H4 dissociation (Figure 4 and 5). Second, CO is produced in the co-adlayers under certain conditions, though the amount is very little (both in Figure 4 and in Figure 5). (The generation of this CO is to be discussed in the next part.) Third, even though significant changes have been observed in the IR spectra of co-adlayers prepared following different procedures, no characteristic peaks associated with surface species that are proposed to be intermediates for N2 production are detected, such as 1246-1292 cm-1 for NO3-, 1300-1336 cm-1 and 1410 cm-1 for carbonate, 2232 cm-1 for -NCO, and 2162-2130 cm-1 for -CN, etc.20 Consequently, it can also be concluded that the reduction of NO in presence of C2H4 and C2H4-derived hydrocarbons on the surface of Pt(332) does not proceed through cyanide or isocyanate related surface species or intermediates of organo-nitro and related species as suggested before.1 2. Reaction between NO and C2H4/C2H4-Derived Hydrocarbons. Since the IR spectra do not support a mechanism involving intermediates (stable or otherwise), the reaction more likely proceeds through a mechanism of NO decomposition and subsequent oxygen removal by reaction with hydrocarbon reductants. This mechanism is also corroborated by the thermal desorption spectra of N2, which are direct clues for tracking the transfer of N in NO molecules in the process of surface reaction. Thermal desorption spectroscopy shows that N2 desorption from NO adlayers on the clean Pt(332) surface starts at about 300 K, becoming significant at ∼400 K and reaching maximum at ∼460 K. The contribution of C2H4 and its derivatives to N2 production is manifested by a significant enhancement in lowertemperature N2 desorption (below 400 K). It has also been confirmed that the derivatives generated from C2H4 dissociation at moderate temperatures (below 300 K) are more reactive toward NO than C2H4 itself, as has been shown in Figure 8 and 9. This C2H4-derivative-induced lower-temperature N2 desorption is in good agreement with previous studies under ambient conditions.1 It has been confirmed that, no matter what parameter is considered, such as the structure of zeolite, the nature of metal, the nature of hydrocarbon, the method of catalyst preparation, etc., the temperature at which NO conversion reaches higher values is lower than that needed for NO decomposition. However, little explanation was offered previously. Compared to the reaction systems studied under ambient conditions, C2H4-NO reaction on the surface of Pt(332) is easier to deal with (though still complex and open to interpretation).

NO-C2H4 Reactions on Stepped Pt(332) This effect of promoting N2 desorption at low temperature can be addressed by taking the chemisorption properties of C2H4 and NO into consideration. It has been established that adsorbed C2H4 molecules (like most hydrocarbons) donate electron density to metal atoms, decreasing the work function of the metal surface.40,41 Under certain circumstances, NO molecules behave as an electron acceptor. Using a Kelvin probe, we measured the change of work function of Pt(332) following NO exposure. At 90 K, NO increases the work function of Pt(332) by ∼100 mV. Accordingly, in the co-adlayers, it can be speculated that more π electron density from C2H4 is donated into the Pt surface, while more d-electron density of the Pt atoms is back-donated into the antibonding orbital of the coadsorbed NO molecules. This effect will lead the bonds of both coadsorbed C2H4 and NO molecules to be weakened to a larger extent than either of the individual adsorbed molecules on the surface, facilitating the dissociation of both molecules in the co-adlayers. This effect has previously been observed in C2H4NO co-adlayers on Rh(111).16 In the present work, besides lowtemperature N2 desorption, the red shift of all N-O vibrations in the presence of C2H4, also corroborates this effect. Following the above analysis, we can elucidate the reaction of hydrocarbon species with NO in the presence of Pt at temperatures lower than 500 K, in some detail. Upon heating, the adsorbed C2H4 and NO molecules both decompose, leaving the surface with a primary mixture of H, CHx, C2Hx, C, O, N, and NO. Besides direct desorption of things such as H2 and N2, some reactions happen. The resultant H atoms quickly react with atomic O (under these conditions we would not expect to observe an -OH vibration). C-containing species can also react with atomic O or even NO to form CO, which does not leave the surface immediately because of the strong interaction between CO and Pt.21 Obuchi et al. have given serious consideration to the mechanism of NO reduction with hydrocarbons, and suggested that carbonaceous radicals can react directly with NO to produce N2.42 Illan-Gomez and co-workers also carefully interrogated the reduction of NO by C and CHx, and concluded that this reaction can take place efficiently on various metal surfaces.43 This is one of the main reasons that a small CO peak at 2063 cm-1 (or 2050 cm-1) is observed in the IR spectra of the co-adlayer at the adsorption temperature of 350 K. Given the very small amount of CO present in the coadlayers, it should be emphasized that we cannot exclude the possibility of direct oxidation of ethylene-related species into CO2, which cannot be detected during TDS due to the high pumping speed of the system. The contribution of C2H4 and its derivatives to the catalytic reduction of NO on the surface of Pt(332) can be therefore described as (1) promoting NO dissociation through an electron back-donating effect and (2) providing a source of reductants, i.e., H, CHx, C2Hx, and even C, which remove the atomic O from NO dissociation or react directly with NO and leave the surface with more vacant sites for further NO dissociation. It should be emphasized that this analysis is very simplistic and somewhat idealized. Both O and NO can directly react with CHx or C2Hx, but it is difficult to distinguish these reactions currently. Nonetheless, considering the fact that C2H4 derivatives are more reactive toward NO (or O) than C2H4, and that lowertemperature N2 desorption (below 400 K) is significantly increased, we can conclude that the reaction between NO (O) and C2Hx (or CHx) plays a central role in promoting NO reduction.

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9925 Conclusion Co-adsorption and reaction of C2H4 and NO on the surface of Pt(332) was studied using FTIR-RAS and TDS. There is no direct chemical reaction between NO and C2H4- and C2H4derived hydrocarbons, which can lead to the formation of intermediate surface species that are essential for N2 production. However, coadsorbed C2H4 molecules greatly promotes the dissociation of NO and the desorption of N2, depending on the temperature to which C2H4 adlayers are annealed. Compared to just NO chemisorbed on Pt(332), N2 desorption is more dramatically enhanced at temperatures below 400 K after annealing C2H4 adlayers to temperature e300 K. This lowtemperature desorption disappears completely when the annealing temperature is greater than 350 K, while the desorption at ∼460 K is enhanced slightly. The reduction of NO in the presence of C2H4 and the derived hydrocarbons proceeds through a mechanism of NO decomposition and subsequent removal of atomic O by reductants. The contribution of C2H4 and derived hydrocarbons to N2 desorption is mainly attributed to weakening N-O bonds through electron backdonation and providing a source of reductants, i.e., H, CHx, C2Hx, and C, which react with the atomic O from NO dissociation, leaving the surface with more vacant sites for further NO dissociation. References and Notes (1) Burch, R.; Breen, J. P.; Meunier, F. C. Appl. Catal., B 2002, 39, 283-303. (2) Parvulescu, V. I.; Grange, P.; Delmon, B. Catal. Today 1998, 46, 233-316. (3) Zhdanov, V. P.; Kasemo, B. Surf. Sci. Rep. 1997, 29, 35-90. (4) Ozensoy, E.; Hess, C.; Goodman, D. W. J. Am. Chem. Soc. 2002, 124, 8524-8525. (5) Jug, K.; Homann, T.; Bredow, T. J. Phys. Chem. A 2004, 108, 2966-2971. (6) Irurzun, I. M.; Imbihl, R.; Vicente, J. L.; Mola, E. E. Chem. Phys. Lett. 2004, 389, 212-217. (7) Lombardo, S. J.; Fink, T.; Imbihl, R. J. Chem. Phys. 1993, 98, 5526-5539. (8) Siera, J.; Cobden, P.; Tanaka, K.; Nieuwenhuys, B. E. Catal. Lett. 1991, 10, 335-342. (9) Marsh, A. L.; Gland, J. L. Catal. Lett. 2004, 93, 165-170. (10) Velic, D.; Hotzel, A.; Wolf, M.; Ertl, G. J. Chem. Phys. 1998, 109, 9155-9165. (11) Ma, Z.; Zaera, F. Surf. Sci. Rep. 2006, 61, 229-281. (12) Deng, R. P.; Herceg, E.; Trenary, M. J. Am. Chem. Soc. 2005, 127, 17628-17633. (13) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. Chem. Phys. Lett. 1978, 56, 267-271. (14) Demuth, J. E. Chem. Phys. Lett. 1977, 45, 12-17. (15) Arvanitis, D.; Dobler, U.; Wenzel, L.; Baberschke, K.; Stohr, J. Surf. Sci. 1986, 178, 686-692. (16) Van Hardeveld, R. M.; Schmidt, A. J. G. W.; van Santen, R. A.; Niemantsverdriet, J. W. J. Vac. Sci. Technol., A 1997, 15, 16421646. (17) Mullins, D. R.; Zhang, K. J. Phys. Chem. B 2001, 105, 13741380. (18) Zebisch, P.; Huber, W.; Steinruck, H. P. Surf. Sci. 1991, 258, 1-15. (19) Brown, W. A.; King, D. A. J. Phys. Chem. B 2000, 104, 25782595. (20) Shimizu, K.; Satsuma, A. Phys. Chem. Chem. Phys. 2006, 8, 26772695. (21) Hopster, H.; Ibach, H. Surf. Sci. 1978, 77, 109-117. (22) Mukerji, R. J.; Bolina, A. S.; Brown, W. A. J. Chem. Phys. 2003, 119, 10844-10852. (23) Gohndrone, J. M.; Masel, R. I. Surf. Sci. 1989, 209, 44-56. (24) Wang, H.; Tobin, R. G.; DiMaggio, C. L.; Fisher, G. B.; Lambert, D. K. J. Chem. Phys. 1997, 107, 9569-9576. (25) Callen, B. W.; Griffiths, K.; Norton, P. R. Phys. ReV. Lett. 1991, 66, 1634-1637. (26) Callen, B. W.; Griffiths, K.; Norton, P. R.; Harrington, D. A. J. Phys. Chem. 1992, 96, 10905-10913. (27) Agrawal, V. K.; Trenary, M. Surf. Sci. 1991, 259, 116-128.

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