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Catalytic reduction of NO in the presence of benzene on a Pt(3 3 2) surface
Applied Surface Science 254 (2008) 1666–1675 www.elsevier.com/locate/apsusc
Catalytic reduction of NO in the presence of benzene on a Pt(3 3 2) surface Yuhai Hu, Keith Griffiths * Department of Chemistry, The University of Western Ontario, London N6A 5B7, Canada Received 28 February 2007; accepted 12 July 2007 Available online 20 July 2007
Abstract The catalytic reduction of NO in the presence of benzene on the surface of Pt(3 3 2) has been studied using Fourier transform infra red reflection-absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). IR spectra show that while the presence of benzene molecules at low coverage (e.g., following an exposure of just 0.25 L) promotes NO–Pt interaction, the adsorption of NO on Pt(3 3 2) at higher benzene coverages is suppressed. It is also shown that there are no strong interactions between the adsorbed NO molecules and the benzene itself or benzene-derived hydrocarbons, which can lead to the formation of intermediate species that are essential for N2 production. TDS results show that the adsorbed benzene molecules undergo dehydrogenation accompanied by hydrogen desorption starting at 300 K and achieving a maximum at 394 K. Subsequent dehydrogenation of the benzene-derived hydrocarbons then begins with hydrogen desorption starting at 500 K. N2 desorption from NO adlayers on clean Pt(3 3 2) surface becomes significant at temperatures higher than 400 K, giving rise to a peak at 465 K. This peak corresponds to N2 desorption from NO dissociation on step sites. The presence of benzene promotes N2 desorption, depending on the benzene coverage. When the benzene exposure is 0.25 L, the N2 desorption peak at 459 K is dramatically increased. Increasing benzene coverage also results in the intensification of N2 desorption at 410 K. At benzene exposures of 2.4 L, N2 desorption develops as a broad peak with a maximum at 439 K. It is concluded that the catalytic reduction of NO by platinum in the presence of benzene proceeds by NO decomposition and subsequent oxygen removal at temperatures lower than 500 K, and NO dissociation is a rate-limiting step. The contribution of benzene to N2 desorption is mainly attributed to providing a source of H, which quickly reacts with NO-derived atomic O, leaving the surface with more vacant sites for further NO dissociation. # 2007 Elsevier B.V. All rights reserved. Keywords: NO; Platinum; Benzene; deNOx; Hydrocarbon; Selective catalytic reduction
1. Introduction With a reaction atmosphere similar to that in automobile exhausts, the selective catalytic reduction (SCR) of NO in the presence of hydrocarbons is a desirable option to eliminate nitrogen oxides. In the past two decades, a large number of studies have been done toward screening optimum catalysts and exploring the reaction mechanisms for the SCR of NO by hydrocarbons [1,2]. Metal ion-exchanged zeolites, base metal oxides and supported metals are found to be active for this reaction. Their activities are dependent on the properties of hydrocarbons, such as degree of saturation and chain length
* Corresponding author. Tel.: +1 519 661 4141. E-mail address: [email protected] (K. Griffiths). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.113
[1–5]. These, in turn, lead to the difficulty in approaching a consensus on the reaction mechanism for this reaction. Adsorption and reaction of small molecules on the surface of a single crystal under UHV condition has proven to be a powerful method in catalytic research, providing very detailed information on the reaction mechanisms [6,7]. The SCR of NO in the presence of CO, NH3 and H2 on the surfaces of various single crystals has been thoroughly investigated previously [8–13]. In contrast, many fewer investigations have been carried out toward the SCR of NO by hydrocarbons under the same conditions. The adsorption and reaction of individual hydrocarbon species has been better studied, such as benzene, ethylene, acetylene, etc. [14–19]. To this end, we are attempting to approach the mechanism of SCR of NO in the presence of hydrocarbons under UHV conditions. In this paper, the reduction of NO with benzene on
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the surface of Pt(3 3 2) is investigated. Pt(3 3 2) is a stepped surface with a 6(1 1 1) (1 1 1) structure [20]. The (1 1 1) plane is predominant in the practical metal-containing catalysts for deNOx because of its stability. This surface can therefore serve as a model for simulating the surfaces of true catalysts. On Pt surfaces, the adsorption and dissociation of NO molecules intimately depends on the surface structure [21–23]. The adsorption initially proceeds on step sites, and extends to terrace sites at higher coverages. The dissociation of NO is believed to occur primarily at defect sites (e.g., steps) at temperatures higher than 300 K [21–23]. The adsorption of benzene on platinum, in particular on flat surfaces, has also been thoroughly investigated using vibrational spectroscopy and temperature programmed desorption [14,15,24–29]. It is generally acknowledged that benzene molecules are adsorbed with the molecular plane parallel to the metal surface at lower coverages, and tilted to the metal surface at higher coverages, e.g., in condensed layers. Besides molecular desorption, the adsorbed benzene molecules undergo step-wise dehydrogenation, intimately associated with the surface temperature and even the surface structure. This well-defined information is of great help for better understanding the coadsorption and reaction of benzene with NO at various temperatures and, in particular, the reaction mechanism.
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grazing incidence using a Digilab FTS 7000 spectrometer. 1000 scans, which takes less than 2 min, were coadded at a resolution of 8 cm 1. A narrow band mercury cadmium telluride (MCT) detector was used throughout, with a cut-off around 1000 cm 1. Although data can be obtained below this region, the steep combined spectral function of the detector and associated optics make the data unreliable and often unreproducible. Therefore, in this paper, IR spectra are shown only in the region beyond 1000 cm 1. Both benzene 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 achieved by resistively heating the crystal at a linear rate of 4 K/s. 3. Results 3.1. Adsorption of benzene
All the experiments were carried out in a standard UHV system described elsewhere [30,31]. 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(3 3 2) 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 [20,32]. IR spectra were obtained in a single reflection geometry at 78
3.1.1. IR spectra Fig. 1 shows IR spectra recorded following exposure of the clean Pt(3 3 2) surface to various amounts of benzene at 90 K and then scanning. No peaks associated with adsorbed benzene molecules are observed when the benzene exposure was 3.8 L. According to previous studies [29,33], it can be suggested that the adsorption of benzene at these exposures is lower than the saturation (monolayer) amount, and benzene molecules are adsorbed with their molecular planes parallel to the surface. A peak at 1480 cm 1 appears at a benzene exposure of 5.8 L, and increases with increasing benzene exposure. A new peak at 1036 cm 1 appears as the benzene exposure is increased to 9.8 L. Increasing the benzene exposure to 30 L gives rise to another two peaks at 3037 and 3089 cm 1, respectively. All these peaks have previously been assigned to the characteristic vibrations of second and multilayer benzene on platinum surface [29,33]. Fig. 2 shows IR spectra of benzene on Pt(3 3 2) after annealing temperatures ranging from 90 to 600 K. In this experiment, the clean Pt(3 3 2) surface is exposed to 3.8 L
Fig. 1. IR spectra recorded following exposure of a clean Pt(3 3 2) to various amounts of benzene at 90 K. (A) 1000–2000 cm 1; (B) 2000–3500 cm 1.
Fig. 2. IR spectra of benzene after being annealed to various temperatures as indicated (spectra were recoded at 90 K). (A) 1000–2500 cm 1; (B) 2500– 3500 cm 1.
2. Experiment
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benzene at 90 K and then annealed to the target temperatures as indicated. IR spectra were recorded after cooling the surface down to 90 K from the target temperature. It can be seen that in this temperature range, no peaks in connection with molecular benzene vibration are detected. When the annealing temperature is below 400 K, a very weak peak at 1950 cm 1 appears, and grows steadily. It is most likely that this peak is due to the vibration of CO molecules, which adsorb on the surface during the experimental process and which are affected by the presence of benzene and benzene-derived hydrocarbons [34]. These results suggest that the hydrocarbon species derived from the dehydrogenation of benzene on the surface of Pt(3 3 2) are probably IR-inactive, at least in the spectral range >1000 cm 1 [19]. It is worth noting that a broad peak ranging from 3100 to 3300 cm 1 appears, and appears to increase with the annealing temperature. In reality, this peak increases also as a function of time, independent of any other experimental factors and is caused by the condensation of water in the IR detector as a function of elapsed time and can be ignored.
from the lowest benzene exposure when the surface temperature is increased to 300 K. At a benzene exposure of 0.25 L, there are two desorption peaks at 394 and 557 K, corresponding to dehydrogenation of adsorbed benzene molecules and the subsequent dehydrogenation of benzene-derived hydrocarbons, respectively [27]. Increasing benzene exposure to higher values leads to evident changes in the desorption spectra. The peak at 394 K is dramatically enhanced, but the peak maximum position does not changed. In contrast, hydrogen desorption grows very rapidly on the higher-temperature side of the peak at 557 K, merging with it into a broad peak at a benzene exposure of 9.6 L. 3.2. Adsorption of NO
3.1.2. TDS results Thermal desorption spectra of benzene and hydrogen were recorded following exposure of the clean Pt(3 3 2) surface to various amounts of benzene at 90 K, as shown in Fig. 3. No molecular benzene desorption is detected when the benzene exposure is 1.2 L, indicating that all the adsorbed benzene molecules are completely decomposed. As the benzene exposure is increased to 2.4 L, molecular benzene desorption appears with a maximum at 610 K. Continuing to increase the benzene exposure induces significant enhancement in benzene desorption. Due to strong lateral repulsive interactions between adsorbed benzene molecules, benzene desorption is shifted to lower temperatures with increasing benzene exposure. The two peaks at 150 and 390 K, at a benzene exposure of 30 L are corresponding to the desorption of benzene from multilayers [27]. As compared to benzene desorption, hydrogen desorption is much more complicated. The desorption becomes discernable
3.2.1. IR spectra Fig. 4 shows IR spectra recorded following exposure of the clean Pt(3 3 2) surface to various amounts of NO at 90 K. When the NO exposure is 0.8 L, there are three main peaks at 1483– 1488, 1623, and 1687–1695 cm 1, respectively. The peak at 1623 cm 1 is initially predominant, but is decreased slightly with increasing NO exposure. The other two peaks increase as a function of exposure. The peaks at 1483–1488 and 1687– 1695 cm 1 are assigned to vibrations of NO molecules adsorbed in bridge and in atop sites on terraces, respectively [21,35,36], and the peak at 1623 cm 1 to vibrations of NO molecules in bridge sites on steps [21,35,36]. These results corroborate previous conclusions that NO molecules adsorbed on step sites are the most stable species on the stepped surface [21,35,36]. The decrease in intensity with increasing NO exposure (1623 cm 1) is due to site exchange of the adsorbed NO molecules because of repulsive lateral interactions [21,35,36]. Increasing the NO exposure to 1.6 L greatly attenuates the peaks at 1485 and 1627 cm 1, but enhances the peak now above 1700 cm 1, eventually giving rise to two peaks at 1701 and 1730 cm 1, respectively. When the NO exposure is increased to 2.4 L, the two peaks above 1700 cm 1 are significantly enhanced with the peak at 1727 cm 1 being predominant. The peak at 1630 cm 1 becomes much smaller.
Fig. 3. Thermal desorption spectra of benzene (A) and H2 (B). Exposures as indicated in the figure.
Fig. 4. IR spectra recorded following exposure of the clean Pt(3 3 2) to various amounts of NO at 90 K.
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Fig. 5. IR spectra recorded following an exposure of Pt(3 3 2) to 0.8 L NO at the temperatures as indicated.
The peak at 1483–1488 cm 1 observed at lower NO coverage disappears completely. Since much stronger lateral repulsive interactions exists among adsorbed NO molecules at higher coverage, the two peaks above 1700 cm 1 can be assigned to atop NO on step sites (1727 cm 1) and on terrace sites (1701 cm 1), respectively [21,35,36]. Fig. 5 shows IR spectra recorded following exposure of the clean Pt(3 3 2) surface to 0.8 L NO at the indicated temperature and then cooling to 90 K. The spectra change dramatically with changing the adsorption temperature. Comparing to the spectrum at 90 K, the spectra for adsorption temperatures between 200 and 350 K give rise to a new peak at 1821– 1840 cm 1. The assignment of this peak is not determined. A previous study tentatively assigned it to an O–NO species [21,36]. Considering that this peak gains significant intensity at 200 K, at which NO dissociation does not occur, we assign this peak to vibration of NO molecules adsorbed on step edge, probably in atop mode. The vibrational frequency of NO molecules adsorbed in bridge sites on terraces is shifted slightly to 1478 cm 1. The peak intensity is also diminished. The atop NO on terraces with a vibrational frequency of 1695 cm 1 disappears completely. The peak at 1629 cm 1 is split into two peaks at 1591 and 1625 cm 1, respectively. The new peak at 1591 cm 1 is assigned to bridge or bent NO on step edges [36]. At 350 K, only two peaks are evident, at 1606 and 1824 cm 1, respectively, suggesting that the adsorption of NO at this high temperature is only localized on the step sites. When the exposure is performed with a surface temperature of 420 K, only a very small peak at 1606 cm 1 is observed. The red-shift of the vibration of bridge NO from 1625 to 1606 cm 1 with increasing temperature from 90 K to temperatures higher than 350 K, can be attributed to the reduced concentration of NO molecules on the step sites, leading to weaker lateral repulsive interaction among adsorbed NO molecules. No peak is found in the spectrum when the surface temperature is increased to 550 K. 3.2.2. TDS results Fig. 6 shows thermal desorption spectra of NO and N2 recorded following exposure of the clean Pt(3 3 2) to various
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amounts of NO at 90 K. At low exposure, i.e., 0.1 L, NO desorption mainly occurs at 451 K. This peak grows quickly with increasing NO exposure. Simultaneously, a second desorption peak appears at 352 K, also gaining intensity quickly. Combining these TDS results with the above IR results, it is reasonable to attribute the peak at 451 K to NO desorption from step sites, and the peak at 352 K to the desorption from terrace sites. A small peak at 149 K is mainly caused by desorption of NO molecules from platinum wires on which the crystal is mounted. N2 desorption becomes discernable when the surface temperature is increased to 300 K, giving rise to a desorption maximum at 465 K. Moreover, it is worth noting that the peak intensity is not increased dramatically with increasing NO coverage. This suggests that only a fixed amount of NO is dissociated. Since it has been shown that NO decomposes to only a very minor extent on the planar Pt(1 1 1) surface, this nitrogen mainly corresponds to recombination of N atoms resulting from NO dissociation on step sites. 3.3. Coadsorption of benzene and NO 3.3.1. IR spectra Fig. 7 shows IR spectra recorded following exposure of Pt(3 3 2) to various amounts of benzene (first) and then 1.6 L NO at 90 K and then scanning. IR spectrum of the coadlayer with a benzene exposure of 0.25 L does not vary greatly from that of NO adlayer on the clean surface, except that the peak intensities are slightly decreased, and that the vibration frequencies are red-shifted. The peak at 1730 cm 1 seen for the same exposure of NO on the clean surface in Fig. 4 disappears almost completely. Increasing the benzene preexposure to 1.2 L significantly attenuates the peaks at 1466 and 1611 cm 1, which become almost invisible at a benzene exposure of 2.4 L. The peak at 1679 cm 1 still retains significant intensity. These results, together with that of the NO adlayers on clean surface, show that the presence of benzene molecules suppresses the adsorption of NO on the surface of Pt(3 3 2). This suppression effect is however,
Fig. 6. Thermal desorption spectra of NO (A) and N2 (B) following exposure of the Pt(3 3 2) surface as indicated at 90 K.
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Fig. 7. IR spectra recorded following exposure of the clean Pt(3 3 2) to various amounts of benzene (first) and then 1.6 L NO at 90 K and then scanning.
different on the different ‘types’ of adsorbed NO. Exposing Pt to 0.25 L benzene completely suppresses the formation of atop NO on the step edge (1726 cm 1). As the benzene exposure is increased to higher values, the suppression is much stronger on the bridged NO on both the steps and on the terraces than on the atop NO on the terraces. The different suppressing effect of benzene on the adsorption of NO on Pt(3 3 2) surface can be explained by the adsorption geometry of benzene on the (1 1 1) planes. It is known that the adsorbed benzene molecules on planar (1 1 1) surfaces are prone to bond to the surface with carbon atoms located in the hollow sites at lower coverage, but with carbon atoms located in the bridge sites at higher coverage [37–39]. This leads to changes both in orientation of the benzene molecules and in the structure of the adlayers. As a consequence, it can be expected that the adsorption of NO molecules in bridge sites is more significantly affected by a low/ moderate coverage of pre-adsorbed benzene than those in atop sites. This results in more evident changes in the vibrations of NO molecules in bridge sites. However, it should be noted that it is hard to know exactly where benzene molecules are located on the surface of Pt(3 3 2) at the initial stage of adsorption. The rapid loss of atop NO on the step sites (1726 cm 1) at lower benzene coverage, e.g., 0.25 L, suggests that the adsorption probably proceeds initially at step edges. Fig. 8 shows IR spectra recorded following exposure of Pt(3 3 2) to 1.2 L benzene (first) and then either 0.8 or 1.6 L NO. All of the exposures and scanning take place at the temperature indicated in the figure. The spectra change dramatically when the surface temperature is varied. For the coadlayer with 1.2 L benzene and 0.8 L NO at 90 K, four peaks are observed, at 1465, 1597, 1623, and 1682 cm 1, respectively. According to the above assignments, it can be suggested that bridge and atop NO on the terraces, and bridge NO on the steps and step edges (bent NO) are formed in this coadlayer at 90 K. When the adsorption/exposure temperature is increased to 300 K, the spectra undergo almost the same changes as that of NO adlayers on clean surface, except that all the peaks are
Fig. 8. IR spectra recorded following exposure of the clean Pt(3 3 2) to benzene (first) and then NO with the sample held at the indicated temperature. Scanning also take place at the indicated temperature. Exposures as indicated in the figure.
red-shifted. Only one peak at 1568 cm 1 is observed at 350 and 400 K. Compared to the benzene-free adlayer, this vibration is red-shifted by 34 cm 1, suggesting that pre-dosed benzene or benzene-derived hydrocarbons strengthens the interaction between NO and Pt (to be discussed later). Increasing the NO exposure to 1.6 L does not change the spectra greatly, but enhances the intensities of N–O vibrations. Besides the significant red-shift of the N–O vibration at temperatures higher than 350 K, another two interesting results can also be seen: (1) doubling the NO exposure does not induce a significant red-shift of N–O vibrations; (2) the peak at 1800 cm 1 observed on the benzene-free surface at 350 K disappears completely in the coadlayers at the same temperature. Since our TDS results have shown that benzene dissociation on Pt(3 3 2) surface starts at a temperature higher than 300 K, becoming significant at 350 K and completing at 400 K, it can be suggested that the adsorbed benzene layer has converted into a mixture of benzene and benzene-derived hydrocarbons/carbon at temperature of 350 K and higher. The hydrocarbon species probably accumulate on the step edge due to their preference to occupy the coordination-unsaturated sites [40], preventing the adsorption of NO on such kind of sites. 3.3.2. TDS results Thermal desorption spectra of benzene and H2 from benzene-NO coadlayers are shown in Fig. 9. The coadlayers were prepared by exposing Pt(3 3 2) to various amounts of
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Fig. 9. Thermal desorption spectra of benzene (A) and H2 (B) from various coadlayers produced at 90K. The order of, and magnitude of exposure are as given in the figure.
benzene (first) and then 1.6 L NO at 90 K. For comparison, benzene and H2 desorption from benzene-O2 coadlayers prepared at 90 K is also shown. Benzene desorption from both kinds of coadlayers does not vary greatly, except that a slightly larger amount of desorption from the benzene-NO coadlayers is observed. Another remarkable feature is that benzene desorption is observed from the coadlayers even with very low benzene exposure, i.e., 0.25 L, indicating that benzene dissociation is suppressed by NO and O2. This corroborates the previous studies that the presence of a second molecule, such as CO and NO, suppresses the dissociation of the adsorbed benzene molecules under certain conditions [38]. Thermal desorption spectra of hydrogen from both kinds of coadlayers show a complete loss of the peak at 394 K. Hydrogen desorption appears only at temperatures higher than 450 K. The suppression of benzene dissociation by the coadsorbed NO molecules can partially contribute to the loss of hydrogen desorption at temperature below 400 K. However, this contribution is not significant, because a large amount of H2 desorption from the dehydrogenation of benzene-derived hydrocarbons is observed at temperatures higher than 450 K, proving that benzene dissociation occurs for the C6H6-NO coadlayers. This result in turn, indicates that hydrogen derived from the dehydrogenation of benzene has fully reacted with NO or O. Another interesting observation conveyed by the spectra is that hydrogen desorption from benzene-NO coadlayers is larger than that from the benzene-O2 coadlayers. This mainly results from oxygen deficiency in the benzene-NO coadlayers at high temperatures at which NO has desorbed completely. Fig. 10 shows thermal desorption of N2 recorded following exposure of Pt(3 3 2) to various amounts of benzene (first) and then 1.6 L NO at 90 K. For comparison, N2 desorption from H2NO coadlayer is also shown (see figure caption for details). Obviously, N2 desorption is enhanced for both kinds of coadlayers, compared to that from NO layers on clean surface, and is intimately dependent on the reducing species and the coverage. For the H2-NO coadlayer, the enhancement mainly occurs at temperatures lower than 400 K, covering an extensive temperature range. The intensity of the peak at 466 K is not
Fig. 10. Thermal desorption spectra of N2 from various coadlayers: (a) 1.6 L NO; (b) 0.8 L H2 + 1.6 L NO; for (c–e), 1.6 L NO with benzene exposures of 0.25 (c), 1.2 (d) and 2.4 L (e); (f) 1.2 L benzene + 1.6 L NO with benzene adlayer pre-annealed to 400 K.
greatly enhanced because much less hydrogen is available at temperatures beyond 400 K. For the benzene-NO coadlayers, N2 desorption changes significantly with changing benzene exposure. At a benzene exposure of 0.25 L, the peak at 459 K is greatly enhanced. A shoulder at 410 K is also observed. Increasing the benzene exposure to 1.2 L enhances the peak at 410 K greatly, resulting in a broad N2 desorption, as shown in Fig. 10d. At a benzene exposure of 2.4 L, N2 desorption develops into a broader peak with the peak maximum at 439 K. Fig. 10f shows N2 desorption from the coadlayer prepared by annealing Pt(3 3 2) exposed to 1.2 L benzene at 90 K to 400 K and then exposing the annealed adlayer to 1.6 L NO after cooling the surface down to 90 K. N2 desorption strongly resembles that in Fig. 10d. Consequently, it can be suggested that the dehydrogenation of benzene and the derived hydrocarbons are important for promoting N2 desorption. Fig. 11 shows thermal desorption of N2 from benzene-NO coadlayers, which are prepared by exposing Pt(3 3 2) to 1.2 L benzene and then either 0.8 or 1.6 L NO at the various temperature as indicated in the figure. For the coadlayers with 0.8 L NO prepared at 90 and 300 K (lower panel), N2 desorption is identical. When the adsorption temperature is increased to 350 K, the peak at 410 K is decreased, but the peak at 460 K remains almost unchanged. Only the peak at 460 K is observed from the coadlayer prepared at 400 K. The peak intensity is also obviously decreased. N2 desorption from the coadlayers with 1.2 L benzene and 1.6 L NO at various temperatures (upper panel) is in general agreement with that from the coadlayers with 0.8 L NO. The essence of this figure lies in that it presents N2 desorption from coadlayers other than NO + undissociated benzene, i.e., the coadlayer with mixtures of benzene, benzene-derived hydrocarbons and NO (350 K) or coadlayers with just benzene-derived hydrocarbons and NO
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Fig. 11. Thermal desorption spectra of N2 from benzene-NO coadlayers prepared at various temperatures. Exposures as given in the upper and lower panels.
(400 K). Assuming that benzene-derived hydrocarbons promote NO reduction by directly reacting with the NO molecules, it might be expected that N2 desorption from the coadlayers with adsorption temperatures higher than 350 K should be significantly enhanced, or at least, not decreased. This is, however, not observed in the thermal desorption spectra. A previous study concluded that the aromatic ring is retained in the process of the dehydrogenation of adsorbed benzene molecules at temperatures lower than 500 K [27]. In other word, when the surface temperature is lower than 500 K, there are not many lower-carbon fragments on the surface, such as C1 or C2, which are active to react with O or N atoms. We also observed that annealing benzene adlayers under 5 10 7 torr O2 results in significant CO and CO2 desorption at temperature up to about 600 K (not shown here). There results indicate that the benzene-derived hydrocarbons do not contribute much to NO reduction by direct reaction with NO. Fig. 12A shows thermal desorption spectra at mass 44 (corresponding to either molecular desorption of N2O or CO2) for various coadlayers produced at 90 K. Benzene or hydrogen is dosed first according to the exposures given in the figure. For the adlayers with just 1.6 L NO, the desorption starts at 200 K and lasts to 500 K. Introduction of 0.8 L H2 mainly enhances the desorption at 430 K. Since there is no carbon source on these surfaces, this desorption is attributed to N2O. For the benzene-O2 coadlayers, the desorption appears at temperature higher than 350 K, with maximum above 400 K. The desorption is suppressed with increasing benzene coverage. This desorption is attributed to CO2 from the oxidation of benzene or benzene-derived hydrocarbons. It is worth noting that compared to CO2 desorption from annealing benzene
Fig. 12. Thermal desorption spectra at mass 44 from various coadlayers. (A) coadlayers produced at 90 K, (B) coadlayers produced at various temperatures as indicated.
adlayers under 5 10 7 torr O2, CO2 desorption from these coadlayers is much smaller. For the benzene-NO coadlayer with 0.25 L benzene, the desorption resembles that from H2NO coadlayer. Increasing benzene coverage leads to enhancement in the desorption at both 300 and 460 K. Compared with the desorption from the benzene-O2 coadlayers, the desorption at mass 44 from the benzene-NO coadlayers can be mainly attributed to N2O. The desorption of mass 44 from the benzene-NO coadlayers prepared at various temperatures is also shown in Fig. 12B. The desorption is strongly dependent on the adsorption temperature and NO coverage. Increasing the NO exposure from 0.8 to 1.6 L significantly enhances the desorption both at lower (295 K) and at higher (450 K) temperatures. Increasing the adsorption temperature to 300 K or higher removes the lowertemperature peak completely for both kinds of coadlayers, but the higher-temperature peak can still be observed for the coadlayer with 1.6 L NO. The peak intensity is slightly decreased. Both of the two peaks are attributed N2O desorption. Finally, it should be mentioned that almost no water was detected from the benzene-NO and benzene-O2 coadlayers. Thermal desorption of water from H2-O2 coadlayers 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.
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4. Discussion Generally, three distinct mechanisms have been proposed for the SCR of NO in the presence of hydrocarbons, i.e., the intermediate of a cyanide or isocyanate surface species, the intermediate of an organo-nitro and related species or NO decomposition and subsequent oxygen removal by reaction with hydrocarbon reductants [1,2]. One of the main difficulties in approaching the reaction mechanism for this type of reaction is that it is hard to know exactly the identity of hydrogen-carbon species on the surface of a catalyst. Unlike small molecules such as CO and H2 (or H), hydrocarbon species cannot retain their structures in the free molecular state, decomposing into various species upon thermal activation. Employment of a hydrocarbon, which is stable at temperatures close to NO dissociation, can be very helpful for disentangling the reaction mechanisms. A previous study has shown that the adsorbed benzene molecules on Pt(3 3 2) undergo dehydrogenation of benzene and then subsequent dehydrogenation of benzenederived hydrocarbon [27], but the two steps are well separated with a temperature difference of about 100 K. Moreover, the dehydrogenation of benzene-derived hydrocarbons is also about 40 K higher than N2 desorption. This information on the identities of benzene related species enables benzene to be an ideal molecule for this kind of study. Zebisch et al. has carried out a very detailed study on the coadsorption of benzene and NO on Ni(111) surface [38]. They concluded that various ordered structures could be formed in the coadlayers, 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. However, the dissociation of NO and the desorption of N2, as well as the connection with benzene coverage and surface temperature was not addressed. These questions are intimately related to the SCR of NO by hydrocarbons. Fig. 3 shows that upon annealing, the adsorbed benzene molecules undergo dehydrogenation with hydrogen desorption starting at 300 K and achieving a maximum at 394 K, and subsequent dehydrogenation of the benzene-derived hydrocarbons with hydrogen desorption starting at 500 K and completing at 700 K. The two ‘kinds’ of H2 are well separated even when the benzene coverage is above the saturation amount. On the other hand, Fig. 6 has indicated that on Pt(3 3 2) surface, N2 desorption is discernable when the surface temperature is increased to 300 K, and becomes significant at temperatures higher than 400 K, with a maximum at 465 K. N2 desorption finishes at 500 K. Figs. 10 and 11 clearly show that N2 desorption from various benzene-NO coadlayers is much more than that from the benzene-free adlayers, but the desorption becomes significant exclusively at temperature close to 400 K, with a maximum at 410 K or higher, at which benzene-derived H2 has completely desorbed. These results indicate that the desorption of N2 from the benzene-NO coadlayers is more related to NO dissociation than to benzene dissociation. In order to make the reaction mechanism more clear, a detailed comparison for the desorption of NO, N2, N2O and H2
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is shown in Fig. 13. The spectra are selected from the above thermal desorption spectra as indicated in the figure. The annotation within the figure indicates the desorbing species followed by the layer from which it is produced. NO desorption at temperatures higher than 400 K from the benzene-NO coadlayer is decreased significantly compared to NO desorption from a layer of NO alone, indicating that the adsorbed NO molecules on the step sites are playing an essential role in the production of N2 from benzene-NO reaction on the surface of Pt(3 3 2). This is corroborated by the desorption of N2 from the same coadlayers, which becomes significant at temperature higher than 400 K. The desorption of N2O gives rise to two peaks. Obviously, the lower-temperature desorption (305 K) does not contribute to the yield of, and desorption of N2 because of the big temperature difference in between. At higher temperature, e.g., 400 K, the generation of N2O from NO-containing adlayers is mainly through the reaction between N (resulting from NO dissociation) and adsorbed NO [2]. Due to its very weak interaction with Pt surface, N2O desorbs immediately after formation. Previous studies on NO-CO reactions on Pt(1 1 2) surface have shown that CO can also promote N2 yield, resulting in new N2 desorption at 420 K [41], which is lower than that without CO by about 80 K. That study also confirmed that NO-derived N atoms desorb immediately at temperature at which they are generated. Another study showed that N2 desorption from the recombination of atomic N was detected even at temperatures at about 180 K [42]. Therefore, it is
Fig. 13. Thermal desorption spectra of NO, N2, N2O and H2 under various conditions as indicated in the figure.
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reasonable to suggest that N2 desorption from the combination of two N atoms is a very rapid process, and the formation of N2 does not require N2O as an intermediate, at least, at higher temperatures like 400 K. Hydrogen desorption occurs at temperatures quite close to that at which N2 desorption is enhanced. Since benzene-derived hydrocarbons do not contribute a lot to the enhancement (Fig. 11), it can be concluded that the release of H from benzene dehydrogenation plays a key role in the promotion of N2 desorption. In general, no matter what kind of mechanism the NO reduction reaction follows, the yield and desorption of N2 from a Pt surface mainly proceeds through three routes: (I) recombination of atomic N from direct NO dissociation; (II) decomposition of N2O forming from the reaction of atomic N with NO; (III) direct decomposition of intermediate species between NO and reductants. Route (III) is sometimes entangled with the route (I). For the reaction between benzene and NO, the above analysis has deduced that the formation of N2 does not require N2O as the intermediate, i.e., route (II). Combined with the TDS data, IR results are expected to disentangle the contribution of route (I) and (III) in the yield of N2 from the catalytic reduction of NO by benzene, and provide insight into the role of benzene. The adsorption of NO on Pt(3 3 2) surface preferentially occupies the step sites, with a typical vibration band at about 1600 cm 1. The adsorbed NO molecules on step sites can undergo some changes with changing NO coverage and surface temperature, as has shown in Figs. 4 and 5. IR spectra of benzene on Pt(3 3 2) surface, which are shown in Fig. 1 are in agreement with that on non-stepped Pt surface, studied previously [29,33]. However, unlike the dissociation of smaller hydrocarbons such as ethane, ethylene, propene, etc., the hydrocarbons derived from the dehydrogenation of benzene on Pt(3 3 2) do not give rise to any peaks in the region with frequencies above 1000 cm 1. IR spectra of the benzene-NO coadlayers undergo some changes as compared to that of individual NO and benzene on Pt(3 3 2). The most obvious change is that the adsorption of benzene decreases the vibrational frequency of the N–O bond. This can be explained by the difference between the interactions of benzene and NO with Pt surfaces. The adsorbed benzene donates electron density to Pt atoms, decreasing the work function of the Pt surface [43]. The adsorbed NO is an electron accepter that increases the work function of the surface [44]. Therefore, on the coadlayers, more d-electron density of the Pt atoms is overlapping with anti-bond orbital of the adsorbed NO molecule, leading to the weakening of N–O bond. This effect, in turn, may facilitate the dissociation of the adsorbed NO molecules on the step sites, partially contributing to the enhancement in N2 desorption. Nonetheless, the adsorbed benzene does not chemically react with the adsorbed NO to form intermediates that are essential for N2 production, because there are no new vibrational bands (besides that of N–O bonds) appearing in the IR spectra. The IR spectra also verify that the benzene-derived hydrocarbons do not react with NO directly either, as has been shown by the data in Fig. 8. Moreover, a
previous study concluded that the decomposition of CH3ONO on Pt(1 1 1) surface gives rise to NO but not N2, indicating the scission of C–ON bond is easier than N–O in this kind of molecule [45]. As a consequence, it can be concluded that the yield and desorption of N2 from the benzene-NO coadlayers is not through route (III), but through route (I), i.e., the recombination of atomic N. The reaction mechanism for the catalytic reduction of NO in the presence of benzene can, therefore, be described as: the adsorbed NO molecules decompose when the surface temperature is increased higher than 400 K, giving rise to atomic N and O. The N atoms recombine and desorb quickly. Part of these N atoms can also react with the adsorbed NO to form N2O. When the surface is lacking in reducing species, the O atoms will stay on the surface till 800 K, preventing further dissociation of NO. On the other hand, the presence of benzene provides H atoms through dehydrogenation, which react with O atoms quickly, leaving the surface with more sites for NO dissociation. On this aspect, it can be assumed that benzene mainly serves as a source of H for the removal of O. The removal of atomic O by directly oxidizing benzene or benzenederived hydrocarbons can play a role, but not essentially. 5. Conclusion Coadsorption and reaction of benzene and NO on the surface of Pt(3 3 2) was studied using FTIR-RAS and TDS. NO dissociation mainly occurs on the step sites, giving rise to a desorption peak at 465 K. Adsorbed benzene molecules undergo dehydrogenation at temperature higher than 300 K, the resultant hydrocarbons further dehydrogenate at temperatures higher than 500 K. The adsorption of benzene enhances the interaction between NO and Pt, decreasing the strength of N–O bond. N2 desorption is greatly promoted in the presence of benzene, occurring mainly at temperatures higher than 400 K. It is concluded that NO dissociation is the rate-limiting step in NO reduction. The reaction mainly proceeds through NO dissociation on step sites, followed by the quick removal of the atomic O by hydrogen from benzene dehydrogenation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
P. Burch, J.P. Breen, F.C. Meunier, Appl. Catal. B 39 (2002) 283–303. V.I. Parvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233–316. F. Witzel, G.A. Sill, W.K. Hall, J. Catal. 149 (1994) 229–237. S. Matsumoto, K. Yokota, H. Doi, M. Kimura, K. Sekizawa, S. Kasazawa, Catal. Today 22 (1994) 127–146. E. Joubert, X. Courtois, P. Marecot, D. Duprez, Appl. Catal. B 64 (2006) 103–110. G.A. Somorjai, J. Phys. Chem. B 104 (2000) 2969–2979. J.H. Sinfelt, Surf. Sci. 500 (2002) 923–946. V.P. Zhdanov, B. Kasemo, Surf. Sci. Rep. 29 (1997) 35–90. E. Ozensoy, C. Hess, D.W. Goodman, Top. Catal. 28 (2004) 13–23. A.J. Hallock, C.M. Matthews, F. Balzer, R.N. Zare, J. Phys. Chem. B 105 (2001) 8725–8728. G. Veser, F. Esch, R. Imbihl, Catal. Lett. 13 (1992) 371–382. S.J. Lombardo, T. Fink, R. Imbihl, J. Chem. Phys. 98 (1993) 5526–5539. V.P. Zhdanov, B. Kasemo, Appl. Catal. A 187 (1999) 61–71. N.C. Chen, L.P. Ford, R.I. Masel, Catal. Lett. 56 (1998) 105–109. C.M. Friend, E.L. Muetterties, J. Am. Chem. Soc. 103 (1981) 773–779.
Y. Hu, K. Griffiths / Applied Surface Science 254 (2008) 1666–1675 [16] R.M. van Hardeveld, A.J.G.W. Schmidt, R.A. van Santen, J.W. Niemantsverdriet, J. Vac. Sci. Technol. A 15 (1997) 1642–1646. [17] J. Kim, H.B. Zhao, C. Panja, A. Olivas, B.E. Koel, Surf. Sci. 564 (2004) 53–61. [18] D.J. Burnett, A.M. Gabelnick, D.A. Fischer, A.L. Marsh, J.L. Gland, J. Catal. 230 (2005) 282–290. [19] Z. Ma, F. Zaera, Surf. Sci. Rep. 61 (2006) 229–281. [20] H. Hopster, H. Ibach, Surf. Sci. 77 (1978) 109–117. [21] W.A. Brown, D.A. King, J. Phys. Chem. B 104 (2000) 2578–2595. [22] Z. Jiang, W. Huang, D. Tan, R. Zhai, Z. Bao, Surf. Sci. 600 (2006) 4860– 4869. [23] S. Sugai, K. Shimozu, H. Watanabe, H. Miki, K. Kawasaki, Surf. Sci. 282 (1993) 67–75. [24] S. Haq, D.A. King, J. Phys. Chem. 100 (1996) 16957–16965. [25] F.S. Thomas, N.S. Chen, L.P. Ford, R.I. Masel, Surf. Sci. 486 (2001) 1–8. [26] J.M. Campbell, S. Seimanides, C.T. Campbell, J. Phys. Chem. 93 (1989) 815–826. [27] M.C. Tsai, E.L. Muetterties, J. Am. Chem. Soc. 124 (1982) 2534–2539. [28] S. Lehwald, H. Ibach, J.E. Demuth, Surf. Sci. 78 (1978) 577–590. [29] C. Lutterloh, J. Biener, K. Po¨hlmann, A. Schenk, J. Ku¨ppers, Surf. Sci. 352–354 (1996) 133–137, and reference therein. [30] B.W. Callen, K. Griffiths, P.R. Norton, Phys. Rev. Lett. 66 (1991) 1634– 1637. [31] B.W. Callen, K. Griffiths, P.R. Norton, Surf. Sci. 261 (1992) L44–L48.
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[32] V.K. Agrawal, M. Trenary, Surf. Sci. 259 (1991) 116–128. [33] J. Breitbach, D. Franke, G. Hamm, C. Bekker, K. Wandelt, Surf. Sci. 507– 510 (2002) 18–22. [34] E. Ozensoy, C. Hess, D.W. Goodman, J. Am. Chem. Soc. 124 (2002) 8524–8525. [35] N. Tsukahara, K. Mukai, Y. Yamashita, J. Yoshinobu, H. Aizawa, Surf. Sci. 600 (2006) 3477–3483. [36] R.J. Mukerji, A.S. Bolina, W.A. Brown, Z.P. Liu, P.J. Hu, J. Phys. Chem. B 108 (2004) 289–296. [37] G. Hamm, T. Schmidt, J. Breitbach, D. Franke, C. Becker, K. Wandelt, Surf. Sci. 562 (2004) 170–182. [38] P. Zebisch, W. Huber, H.P. Steinruck, Surf. Sci. 258 (1991) 1–15. [39] A. Wander, G. Held, R.Q. Hwang, G.S. Blackman, M.L. Xu, P. de Andres, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 249 (1991) 21–34. [40] J.C. Charlier, A. De Vita, X. Blase, R. Car, Science 275 (1997) 646–649, and reference therein. [41] Y.H. Hu, S. Han, H. Horino, B.E. Nieuwenhuys, A. Hiratsuka, Y. Ohno, K. Ivan, T. Matsushima, Surf. Sci. 526 (2003) 159–165. [42] H. Wang, R.G. Tobin, C.L. Dimaggio, G.B. Fisher, D.K. Lambert, J. Chem. Phys. 107 (1997) 9569–9576. [43] N. Kizhakevariam, I. Villegas, M.J. Weaver, Surf. Sci. 336 (1995) 37–54. [44] T. Fukushima, M.B. Song, M. Ito, Surf. Sci. 464 (2000) 193–199. [45] J.W. Peck, D.I. Mahon, D.E. Beck, B. Bansenaur, B.E. Koel, Surf. Sci. 410 (1998) 214–227.
Catalytic reduction of NO in the presence of benzene on a Pt(3 3 2) surface Yuhai Hu, Keith Griffiths * Department of Chemistry, The University of Western Ontario, London N6A 5B7, Canada Received 28 February 2007; accepted 12 July 2007 Available online 20 July 2007
Abstract The catalytic reduction of NO in the presence of benzene on the surface of Pt(3 3 2) has been studied using Fourier transform infra red reflection-absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). IR spectra show that while the presence of benzene molecules at low coverage (e.g., following an exposure of just 0.25 L) promotes NO–Pt interaction, the adsorption of NO on Pt(3 3 2) at higher benzene coverages is suppressed. It is also shown that there are no strong interactions between the adsorbed NO molecules and the benzene itself or benzene-derived hydrocarbons, which can lead to the formation of intermediate species that are essential for N2 production. TDS results show that the adsorbed benzene molecules undergo dehydrogenation accompanied by hydrogen desorption starting at 300 K and achieving a maximum at 394 K. Subsequent dehydrogenation of the benzene-derived hydrocarbons then begins with hydrogen desorption starting at 500 K. N2 desorption from NO adlayers on clean Pt(3 3 2) surface becomes significant at temperatures higher than 400 K, giving rise to a peak at 465 K. This peak corresponds to N2 desorption from NO dissociation on step sites. The presence of benzene promotes N2 desorption, depending on the benzene coverage. When the benzene exposure is 0.25 L, the N2 desorption peak at 459 K is dramatically increased. Increasing benzene coverage also results in the intensification of N2 desorption at 410 K. At benzene exposures of 2.4 L, N2 desorption develops as a broad peak with a maximum at 439 K. It is concluded that the catalytic reduction of NO by platinum in the presence of benzene proceeds by NO decomposition and subsequent oxygen removal at temperatures lower than 500 K, and NO dissociation is a rate-limiting step. The contribution of benzene to N2 desorption is mainly attributed to providing a source of H, which quickly reacts with NO-derived atomic O, leaving the surface with more vacant sites for further NO dissociation. # 2007 Elsevier B.V. All rights reserved. Keywords: NO; Platinum; Benzene; deNOx; Hydrocarbon; Selective catalytic reduction
1. Introduction With a reaction atmosphere similar to that in automobile exhausts, the selective catalytic reduction (SCR) of NO in the presence of hydrocarbons is a desirable option to eliminate nitrogen oxides. In the past two decades, a large number of studies have been done toward screening optimum catalysts and exploring the reaction mechanisms for the SCR of NO by hydrocarbons [1,2]. Metal ion-exchanged zeolites, base metal oxides and supported metals are found to be active for this reaction. Their activities are dependent on the properties of hydrocarbons, such as degree of saturation and chain length
* Corresponding author. Tel.: +1 519 661 4141. E-mail address: [email protected] (K. Griffiths). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.113
[1–5]. These, in turn, lead to the difficulty in approaching a consensus on the reaction mechanism for this reaction. Adsorption and reaction of small molecules on the surface of a single crystal under UHV condition has proven to be a powerful method in catalytic research, providing very detailed information on the reaction mechanisms [6,7]. The SCR of NO in the presence of CO, NH3 and H2 on the surfaces of various single crystals has been thoroughly investigated previously [8–13]. In contrast, many fewer investigations have been carried out toward the SCR of NO by hydrocarbons under the same conditions. The adsorption and reaction of individual hydrocarbon species has been better studied, such as benzene, ethylene, acetylene, etc. [14–19]. To this end, we are attempting to approach the mechanism of SCR of NO in the presence of hydrocarbons under UHV conditions. In this paper, the reduction of NO with benzene on
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the surface of Pt(3 3 2) is investigated. Pt(3 3 2) is a stepped surface with a 6(1 1 1) (1 1 1) structure [20]. The (1 1 1) plane is predominant in the practical metal-containing catalysts for deNOx because of its stability. This surface can therefore serve as a model for simulating the surfaces of true catalysts. On Pt surfaces, the adsorption and dissociation of NO molecules intimately depends on the surface structure [21–23]. The adsorption initially proceeds on step sites, and extends to terrace sites at higher coverages. The dissociation of NO is believed to occur primarily at defect sites (e.g., steps) at temperatures higher than 300 K [21–23]. The adsorption of benzene on platinum, in particular on flat surfaces, has also been thoroughly investigated using vibrational spectroscopy and temperature programmed desorption [14,15,24–29]. It is generally acknowledged that benzene molecules are adsorbed with the molecular plane parallel to the metal surface at lower coverages, and tilted to the metal surface at higher coverages, e.g., in condensed layers. Besides molecular desorption, the adsorbed benzene molecules undergo step-wise dehydrogenation, intimately associated with the surface temperature and even the surface structure. This well-defined information is of great help for better understanding the coadsorption and reaction of benzene with NO at various temperatures and, in particular, the reaction mechanism.
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grazing incidence using a Digilab FTS 7000 spectrometer. 1000 scans, which takes less than 2 min, were coadded at a resolution of 8 cm 1. A narrow band mercury cadmium telluride (MCT) detector was used throughout, with a cut-off around 1000 cm 1. Although data can be obtained below this region, the steep combined spectral function of the detector and associated optics make the data unreliable and often unreproducible. Therefore, in this paper, IR spectra are shown only in the region beyond 1000 cm 1. Both benzene 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 achieved by resistively heating the crystal at a linear rate of 4 K/s. 3. Results 3.1. Adsorption of benzene
All the experiments were carried out in a standard UHV system described elsewhere [30,31]. 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(3 3 2) 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 [20,32]. IR spectra were obtained in a single reflection geometry at 78
3.1.1. IR spectra Fig. 1 shows IR spectra recorded following exposure of the clean Pt(3 3 2) surface to various amounts of benzene at 90 K and then scanning. No peaks associated with adsorbed benzene molecules are observed when the benzene exposure was 3.8 L. According to previous studies [29,33], it can be suggested that the adsorption of benzene at these exposures is lower than the saturation (monolayer) amount, and benzene molecules are adsorbed with their molecular planes parallel to the surface. A peak at 1480 cm 1 appears at a benzene exposure of 5.8 L, and increases with increasing benzene exposure. A new peak at 1036 cm 1 appears as the benzene exposure is increased to 9.8 L. Increasing the benzene exposure to 30 L gives rise to another two peaks at 3037 and 3089 cm 1, respectively. All these peaks have previously been assigned to the characteristic vibrations of second and multilayer benzene on platinum surface [29,33]. Fig. 2 shows IR spectra of benzene on Pt(3 3 2) after annealing temperatures ranging from 90 to 600 K. In this experiment, the clean Pt(3 3 2) surface is exposed to 3.8 L
Fig. 1. IR spectra recorded following exposure of a clean Pt(3 3 2) to various amounts of benzene at 90 K. (A) 1000–2000 cm 1; (B) 2000–3500 cm 1.
Fig. 2. IR spectra of benzene after being annealed to various temperatures as indicated (spectra were recoded at 90 K). (A) 1000–2500 cm 1; (B) 2500– 3500 cm 1.
2. Experiment
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benzene at 90 K and then annealed to the target temperatures as indicated. IR spectra were recorded after cooling the surface down to 90 K from the target temperature. It can be seen that in this temperature range, no peaks in connection with molecular benzene vibration are detected. When the annealing temperature is below 400 K, a very weak peak at 1950 cm 1 appears, and grows steadily. It is most likely that this peak is due to the vibration of CO molecules, which adsorb on the surface during the experimental process and which are affected by the presence of benzene and benzene-derived hydrocarbons [34]. These results suggest that the hydrocarbon species derived from the dehydrogenation of benzene on the surface of Pt(3 3 2) are probably IR-inactive, at least in the spectral range >1000 cm 1 [19]. It is worth noting that a broad peak ranging from 3100 to 3300 cm 1 appears, and appears to increase with the annealing temperature. In reality, this peak increases also as a function of time, independent of any other experimental factors and is caused by the condensation of water in the IR detector as a function of elapsed time and can be ignored.
from the lowest benzene exposure when the surface temperature is increased to 300 K. At a benzene exposure of 0.25 L, there are two desorption peaks at 394 and 557 K, corresponding to dehydrogenation of adsorbed benzene molecules and the subsequent dehydrogenation of benzene-derived hydrocarbons, respectively [27]. Increasing benzene exposure to higher values leads to evident changes in the desorption spectra. The peak at 394 K is dramatically enhanced, but the peak maximum position does not changed. In contrast, hydrogen desorption grows very rapidly on the higher-temperature side of the peak at 557 K, merging with it into a broad peak at a benzene exposure of 9.6 L. 3.2. Adsorption of NO
3.1.2. TDS results Thermal desorption spectra of benzene and hydrogen were recorded following exposure of the clean Pt(3 3 2) surface to various amounts of benzene at 90 K, as shown in Fig. 3. No molecular benzene desorption is detected when the benzene exposure is 1.2 L, indicating that all the adsorbed benzene molecules are completely decomposed. As the benzene exposure is increased to 2.4 L, molecular benzene desorption appears with a maximum at 610 K. Continuing to increase the benzene exposure induces significant enhancement in benzene desorption. Due to strong lateral repulsive interactions between adsorbed benzene molecules, benzene desorption is shifted to lower temperatures with increasing benzene exposure. The two peaks at 150 and 390 K, at a benzene exposure of 30 L are corresponding to the desorption of benzene from multilayers [27]. As compared to benzene desorption, hydrogen desorption is much more complicated. The desorption becomes discernable
3.2.1. IR spectra Fig. 4 shows IR spectra recorded following exposure of the clean Pt(3 3 2) surface to various amounts of NO at 90 K. When the NO exposure is 0.8 L, there are three main peaks at 1483– 1488, 1623, and 1687–1695 cm 1, respectively. The peak at 1623 cm 1 is initially predominant, but is decreased slightly with increasing NO exposure. The other two peaks increase as a function of exposure. The peaks at 1483–1488 and 1687– 1695 cm 1 are assigned to vibrations of NO molecules adsorbed in bridge and in atop sites on terraces, respectively [21,35,36], and the peak at 1623 cm 1 to vibrations of NO molecules in bridge sites on steps [21,35,36]. These results corroborate previous conclusions that NO molecules adsorbed on step sites are the most stable species on the stepped surface [21,35,36]. The decrease in intensity with increasing NO exposure (1623 cm 1) is due to site exchange of the adsorbed NO molecules because of repulsive lateral interactions [21,35,36]. Increasing the NO exposure to 1.6 L greatly attenuates the peaks at 1485 and 1627 cm 1, but enhances the peak now above 1700 cm 1, eventually giving rise to two peaks at 1701 and 1730 cm 1, respectively. When the NO exposure is increased to 2.4 L, the two peaks above 1700 cm 1 are significantly enhanced with the peak at 1727 cm 1 being predominant. The peak at 1630 cm 1 becomes much smaller.
Fig. 3. Thermal desorption spectra of benzene (A) and H2 (B). Exposures as indicated in the figure.
Fig. 4. IR spectra recorded following exposure of the clean Pt(3 3 2) to various amounts of NO at 90 K.
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Fig. 5. IR spectra recorded following an exposure of Pt(3 3 2) to 0.8 L NO at the temperatures as indicated.
The peak at 1483–1488 cm 1 observed at lower NO coverage disappears completely. Since much stronger lateral repulsive interactions exists among adsorbed NO molecules at higher coverage, the two peaks above 1700 cm 1 can be assigned to atop NO on step sites (1727 cm 1) and on terrace sites (1701 cm 1), respectively [21,35,36]. Fig. 5 shows IR spectra recorded following exposure of the clean Pt(3 3 2) surface to 0.8 L NO at the indicated temperature and then cooling to 90 K. The spectra change dramatically with changing the adsorption temperature. Comparing to the spectrum at 90 K, the spectra for adsorption temperatures between 200 and 350 K give rise to a new peak at 1821– 1840 cm 1. The assignment of this peak is not determined. A previous study tentatively assigned it to an O–NO species [21,36]. Considering that this peak gains significant intensity at 200 K, at which NO dissociation does not occur, we assign this peak to vibration of NO molecules adsorbed on step edge, probably in atop mode. The vibrational frequency of NO molecules adsorbed in bridge sites on terraces is shifted slightly to 1478 cm 1. The peak intensity is also diminished. The atop NO on terraces with a vibrational frequency of 1695 cm 1 disappears completely. The peak at 1629 cm 1 is split into two peaks at 1591 and 1625 cm 1, respectively. The new peak at 1591 cm 1 is assigned to bridge or bent NO on step edges [36]. At 350 K, only two peaks are evident, at 1606 and 1824 cm 1, respectively, suggesting that the adsorption of NO at this high temperature is only localized on the step sites. When the exposure is performed with a surface temperature of 420 K, only a very small peak at 1606 cm 1 is observed. The red-shift of the vibration of bridge NO from 1625 to 1606 cm 1 with increasing temperature from 90 K to temperatures higher than 350 K, can be attributed to the reduced concentration of NO molecules on the step sites, leading to weaker lateral repulsive interaction among adsorbed NO molecules. No peak is found in the spectrum when the surface temperature is increased to 550 K. 3.2.2. TDS results Fig. 6 shows thermal desorption spectra of NO and N2 recorded following exposure of the clean Pt(3 3 2) to various
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amounts of NO at 90 K. At low exposure, i.e., 0.1 L, NO desorption mainly occurs at 451 K. This peak grows quickly with increasing NO exposure. Simultaneously, a second desorption peak appears at 352 K, also gaining intensity quickly. Combining these TDS results with the above IR results, it is reasonable to attribute the peak at 451 K to NO desorption from step sites, and the peak at 352 K to the desorption from terrace sites. A small peak at 149 K is mainly caused by desorption of NO molecules from platinum wires on which the crystal is mounted. N2 desorption becomes discernable when the surface temperature is increased to 300 K, giving rise to a desorption maximum at 465 K. Moreover, it is worth noting that the peak intensity is not increased dramatically with increasing NO coverage. This suggests that only a fixed amount of NO is dissociated. Since it has been shown that NO decomposes to only a very minor extent on the planar Pt(1 1 1) surface, this nitrogen mainly corresponds to recombination of N atoms resulting from NO dissociation on step sites. 3.3. Coadsorption of benzene and NO 3.3.1. IR spectra Fig. 7 shows IR spectra recorded following exposure of Pt(3 3 2) to various amounts of benzene (first) and then 1.6 L NO at 90 K and then scanning. IR spectrum of the coadlayer with a benzene exposure of 0.25 L does not vary greatly from that of NO adlayer on the clean surface, except that the peak intensities are slightly decreased, and that the vibration frequencies are red-shifted. The peak at 1730 cm 1 seen for the same exposure of NO on the clean surface in Fig. 4 disappears almost completely. Increasing the benzene preexposure to 1.2 L significantly attenuates the peaks at 1466 and 1611 cm 1, which become almost invisible at a benzene exposure of 2.4 L. The peak at 1679 cm 1 still retains significant intensity. These results, together with that of the NO adlayers on clean surface, show that the presence of benzene molecules suppresses the adsorption of NO on the surface of Pt(3 3 2). This suppression effect is however,
Fig. 6. Thermal desorption spectra of NO (A) and N2 (B) following exposure of the Pt(3 3 2) surface as indicated at 90 K.
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Fig. 7. IR spectra recorded following exposure of the clean Pt(3 3 2) to various amounts of benzene (first) and then 1.6 L NO at 90 K and then scanning.
different on the different ‘types’ of adsorbed NO. Exposing Pt to 0.25 L benzene completely suppresses the formation of atop NO on the step edge (1726 cm 1). As the benzene exposure is increased to higher values, the suppression is much stronger on the bridged NO on both the steps and on the terraces than on the atop NO on the terraces. The different suppressing effect of benzene on the adsorption of NO on Pt(3 3 2) surface can be explained by the adsorption geometry of benzene on the (1 1 1) planes. It is known that the adsorbed benzene molecules on planar (1 1 1) surfaces are prone to bond to the surface with carbon atoms located in the hollow sites at lower coverage, but with carbon atoms located in the bridge sites at higher coverage [37–39]. This leads to changes both in orientation of the benzene molecules and in the structure of the adlayers. As a consequence, it can be expected that the adsorption of NO molecules in bridge sites is more significantly affected by a low/ moderate coverage of pre-adsorbed benzene than those in atop sites. This results in more evident changes in the vibrations of NO molecules in bridge sites. However, it should be noted that it is hard to know exactly where benzene molecules are located on the surface of Pt(3 3 2) at the initial stage of adsorption. The rapid loss of atop NO on the step sites (1726 cm 1) at lower benzene coverage, e.g., 0.25 L, suggests that the adsorption probably proceeds initially at step edges. Fig. 8 shows IR spectra recorded following exposure of Pt(3 3 2) to 1.2 L benzene (first) and then either 0.8 or 1.6 L NO. All of the exposures and scanning take place at the temperature indicated in the figure. The spectra change dramatically when the surface temperature is varied. For the coadlayer with 1.2 L benzene and 0.8 L NO at 90 K, four peaks are observed, at 1465, 1597, 1623, and 1682 cm 1, respectively. According to the above assignments, it can be suggested that bridge and atop NO on the terraces, and bridge NO on the steps and step edges (bent NO) are formed in this coadlayer at 90 K. When the adsorption/exposure temperature is increased to 300 K, the spectra undergo almost the same changes as that of NO adlayers on clean surface, except that all the peaks are
Fig. 8. IR spectra recorded following exposure of the clean Pt(3 3 2) to benzene (first) and then NO with the sample held at the indicated temperature. Scanning also take place at the indicated temperature. Exposures as indicated in the figure.
red-shifted. Only one peak at 1568 cm 1 is observed at 350 and 400 K. Compared to the benzene-free adlayer, this vibration is red-shifted by 34 cm 1, suggesting that pre-dosed benzene or benzene-derived hydrocarbons strengthens the interaction between NO and Pt (to be discussed later). Increasing the NO exposure to 1.6 L does not change the spectra greatly, but enhances the intensities of N–O vibrations. Besides the significant red-shift of the N–O vibration at temperatures higher than 350 K, another two interesting results can also be seen: (1) doubling the NO exposure does not induce a significant red-shift of N–O vibrations; (2) the peak at 1800 cm 1 observed on the benzene-free surface at 350 K disappears completely in the coadlayers at the same temperature. Since our TDS results have shown that benzene dissociation on Pt(3 3 2) surface starts at a temperature higher than 300 K, becoming significant at 350 K and completing at 400 K, it can be suggested that the adsorbed benzene layer has converted into a mixture of benzene and benzene-derived hydrocarbons/carbon at temperature of 350 K and higher. The hydrocarbon species probably accumulate on the step edge due to their preference to occupy the coordination-unsaturated sites [40], preventing the adsorption of NO on such kind of sites. 3.3.2. TDS results Thermal desorption spectra of benzene and H2 from benzene-NO coadlayers are shown in Fig. 9. The coadlayers were prepared by exposing Pt(3 3 2) to various amounts of
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Fig. 9. Thermal desorption spectra of benzene (A) and H2 (B) from various coadlayers produced at 90K. The order of, and magnitude of exposure are as given in the figure.
benzene (first) and then 1.6 L NO at 90 K. For comparison, benzene and H2 desorption from benzene-O2 coadlayers prepared at 90 K is also shown. Benzene desorption from both kinds of coadlayers does not vary greatly, except that a slightly larger amount of desorption from the benzene-NO coadlayers is observed. Another remarkable feature is that benzene desorption is observed from the coadlayers even with very low benzene exposure, i.e., 0.25 L, indicating that benzene dissociation is suppressed by NO and O2. This corroborates the previous studies that the presence of a second molecule, such as CO and NO, suppresses the dissociation of the adsorbed benzene molecules under certain conditions [38]. Thermal desorption spectra of hydrogen from both kinds of coadlayers show a complete loss of the peak at 394 K. Hydrogen desorption appears only at temperatures higher than 450 K. The suppression of benzene dissociation by the coadsorbed NO molecules can partially contribute to the loss of hydrogen desorption at temperature below 400 K. However, this contribution is not significant, because a large amount of H2 desorption from the dehydrogenation of benzene-derived hydrocarbons is observed at temperatures higher than 450 K, proving that benzene dissociation occurs for the C6H6-NO coadlayers. This result in turn, indicates that hydrogen derived from the dehydrogenation of benzene has fully reacted with NO or O. Another interesting observation conveyed by the spectra is that hydrogen desorption from benzene-NO coadlayers is larger than that from the benzene-O2 coadlayers. This mainly results from oxygen deficiency in the benzene-NO coadlayers at high temperatures at which NO has desorbed completely. Fig. 10 shows thermal desorption of N2 recorded following exposure of Pt(3 3 2) to various amounts of benzene (first) and then 1.6 L NO at 90 K. For comparison, N2 desorption from H2NO coadlayer is also shown (see figure caption for details). Obviously, N2 desorption is enhanced for both kinds of coadlayers, compared to that from NO layers on clean surface, and is intimately dependent on the reducing species and the coverage. For the H2-NO coadlayer, the enhancement mainly occurs at temperatures lower than 400 K, covering an extensive temperature range. The intensity of the peak at 466 K is not
Fig. 10. Thermal desorption spectra of N2 from various coadlayers: (a) 1.6 L NO; (b) 0.8 L H2 + 1.6 L NO; for (c–e), 1.6 L NO with benzene exposures of 0.25 (c), 1.2 (d) and 2.4 L (e); (f) 1.2 L benzene + 1.6 L NO with benzene adlayer pre-annealed to 400 K.
greatly enhanced because much less hydrogen is available at temperatures beyond 400 K. For the benzene-NO coadlayers, N2 desorption changes significantly with changing benzene exposure. At a benzene exposure of 0.25 L, the peak at 459 K is greatly enhanced. A shoulder at 410 K is also observed. Increasing the benzene exposure to 1.2 L enhances the peak at 410 K greatly, resulting in a broad N2 desorption, as shown in Fig. 10d. At a benzene exposure of 2.4 L, N2 desorption develops into a broader peak with the peak maximum at 439 K. Fig. 10f shows N2 desorption from the coadlayer prepared by annealing Pt(3 3 2) exposed to 1.2 L benzene at 90 K to 400 K and then exposing the annealed adlayer to 1.6 L NO after cooling the surface down to 90 K. N2 desorption strongly resembles that in Fig. 10d. Consequently, it can be suggested that the dehydrogenation of benzene and the derived hydrocarbons are important for promoting N2 desorption. Fig. 11 shows thermal desorption of N2 from benzene-NO coadlayers, which are prepared by exposing Pt(3 3 2) to 1.2 L benzene and then either 0.8 or 1.6 L NO at the various temperature as indicated in the figure. For the coadlayers with 0.8 L NO prepared at 90 and 300 K (lower panel), N2 desorption is identical. When the adsorption temperature is increased to 350 K, the peak at 410 K is decreased, but the peak at 460 K remains almost unchanged. Only the peak at 460 K is observed from the coadlayer prepared at 400 K. The peak intensity is also obviously decreased. N2 desorption from the coadlayers with 1.2 L benzene and 1.6 L NO at various temperatures (upper panel) is in general agreement with that from the coadlayers with 0.8 L NO. The essence of this figure lies in that it presents N2 desorption from coadlayers other than NO + undissociated benzene, i.e., the coadlayer with mixtures of benzene, benzene-derived hydrocarbons and NO (350 K) or coadlayers with just benzene-derived hydrocarbons and NO
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Fig. 11. Thermal desorption spectra of N2 from benzene-NO coadlayers prepared at various temperatures. Exposures as given in the upper and lower panels.
(400 K). Assuming that benzene-derived hydrocarbons promote NO reduction by directly reacting with the NO molecules, it might be expected that N2 desorption from the coadlayers with adsorption temperatures higher than 350 K should be significantly enhanced, or at least, not decreased. This is, however, not observed in the thermal desorption spectra. A previous study concluded that the aromatic ring is retained in the process of the dehydrogenation of adsorbed benzene molecules at temperatures lower than 500 K [27]. In other word, when the surface temperature is lower than 500 K, there are not many lower-carbon fragments on the surface, such as C1 or C2, which are active to react with O or N atoms. We also observed that annealing benzene adlayers under 5 10 7 torr O2 results in significant CO and CO2 desorption at temperature up to about 600 K (not shown here). There results indicate that the benzene-derived hydrocarbons do not contribute much to NO reduction by direct reaction with NO. Fig. 12A shows thermal desorption spectra at mass 44 (corresponding to either molecular desorption of N2O or CO2) for various coadlayers produced at 90 K. Benzene or hydrogen is dosed first according to the exposures given in the figure. For the adlayers with just 1.6 L NO, the desorption starts at 200 K and lasts to 500 K. Introduction of 0.8 L H2 mainly enhances the desorption at 430 K. Since there is no carbon source on these surfaces, this desorption is attributed to N2O. For the benzene-O2 coadlayers, the desorption appears at temperature higher than 350 K, with maximum above 400 K. The desorption is suppressed with increasing benzene coverage. This desorption is attributed to CO2 from the oxidation of benzene or benzene-derived hydrocarbons. It is worth noting that compared to CO2 desorption from annealing benzene
Fig. 12. Thermal desorption spectra at mass 44 from various coadlayers. (A) coadlayers produced at 90 K, (B) coadlayers produced at various temperatures as indicated.
adlayers under 5 10 7 torr O2, CO2 desorption from these coadlayers is much smaller. For the benzene-NO coadlayer with 0.25 L benzene, the desorption resembles that from H2NO coadlayer. Increasing benzene coverage leads to enhancement in the desorption at both 300 and 460 K. Compared with the desorption from the benzene-O2 coadlayers, the desorption at mass 44 from the benzene-NO coadlayers can be mainly attributed to N2O. The desorption of mass 44 from the benzene-NO coadlayers prepared at various temperatures is also shown in Fig. 12B. The desorption is strongly dependent on the adsorption temperature and NO coverage. Increasing the NO exposure from 0.8 to 1.6 L significantly enhances the desorption both at lower (295 K) and at higher (450 K) temperatures. Increasing the adsorption temperature to 300 K or higher removes the lowertemperature peak completely for both kinds of coadlayers, but the higher-temperature peak can still be observed for the coadlayer with 1.6 L NO. The peak intensity is slightly decreased. Both of the two peaks are attributed N2O desorption. Finally, it should be mentioned that almost no water was detected from the benzene-NO and benzene-O2 coadlayers. Thermal desorption of water from H2-O2 coadlayers 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.
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4. Discussion Generally, three distinct mechanisms have been proposed for the SCR of NO in the presence of hydrocarbons, i.e., the intermediate of a cyanide or isocyanate surface species, the intermediate of an organo-nitro and related species or NO decomposition and subsequent oxygen removal by reaction with hydrocarbon reductants [1,2]. One of the main difficulties in approaching the reaction mechanism for this type of reaction is that it is hard to know exactly the identity of hydrogen-carbon species on the surface of a catalyst. Unlike small molecules such as CO and H2 (or H), hydrocarbon species cannot retain their structures in the free molecular state, decomposing into various species upon thermal activation. Employment of a hydrocarbon, which is stable at temperatures close to NO dissociation, can be very helpful for disentangling the reaction mechanisms. A previous study has shown that the adsorbed benzene molecules on Pt(3 3 2) undergo dehydrogenation of benzene and then subsequent dehydrogenation of benzenederived hydrocarbon [27], but the two steps are well separated with a temperature difference of about 100 K. Moreover, the dehydrogenation of benzene-derived hydrocarbons is also about 40 K higher than N2 desorption. This information on the identities of benzene related species enables benzene to be an ideal molecule for this kind of study. Zebisch et al. has carried out a very detailed study on the coadsorption of benzene and NO on Ni(111) surface [38]. They concluded that various ordered structures could be formed in the coadlayers, 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. However, the dissociation of NO and the desorption of N2, as well as the connection with benzene coverage and surface temperature was not addressed. These questions are intimately related to the SCR of NO by hydrocarbons. Fig. 3 shows that upon annealing, the adsorbed benzene molecules undergo dehydrogenation with hydrogen desorption starting at 300 K and achieving a maximum at 394 K, and subsequent dehydrogenation of the benzene-derived hydrocarbons with hydrogen desorption starting at 500 K and completing at 700 K. The two ‘kinds’ of H2 are well separated even when the benzene coverage is above the saturation amount. On the other hand, Fig. 6 has indicated that on Pt(3 3 2) surface, N2 desorption is discernable when the surface temperature is increased to 300 K, and becomes significant at temperatures higher than 400 K, with a maximum at 465 K. N2 desorption finishes at 500 K. Figs. 10 and 11 clearly show that N2 desorption from various benzene-NO coadlayers is much more than that from the benzene-free adlayers, but the desorption becomes significant exclusively at temperature close to 400 K, with a maximum at 410 K or higher, at which benzene-derived H2 has completely desorbed. These results indicate that the desorption of N2 from the benzene-NO coadlayers is more related to NO dissociation than to benzene dissociation. In order to make the reaction mechanism more clear, a detailed comparison for the desorption of NO, N2, N2O and H2
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is shown in Fig. 13. The spectra are selected from the above thermal desorption spectra as indicated in the figure. The annotation within the figure indicates the desorbing species followed by the layer from which it is produced. NO desorption at temperatures higher than 400 K from the benzene-NO coadlayer is decreased significantly compared to NO desorption from a layer of NO alone, indicating that the adsorbed NO molecules on the step sites are playing an essential role in the production of N2 from benzene-NO reaction on the surface of Pt(3 3 2). This is corroborated by the desorption of N2 from the same coadlayers, which becomes significant at temperature higher than 400 K. The desorption of N2O gives rise to two peaks. Obviously, the lower-temperature desorption (305 K) does not contribute to the yield of, and desorption of N2 because of the big temperature difference in between. At higher temperature, e.g., 400 K, the generation of N2O from NO-containing adlayers is mainly through the reaction between N (resulting from NO dissociation) and adsorbed NO [2]. Due to its very weak interaction with Pt surface, N2O desorbs immediately after formation. Previous studies on NO-CO reactions on Pt(1 1 2) surface have shown that CO can also promote N2 yield, resulting in new N2 desorption at 420 K [41], which is lower than that without CO by about 80 K. That study also confirmed that NO-derived N atoms desorb immediately at temperature at which they are generated. Another study showed that N2 desorption from the recombination of atomic N was detected even at temperatures at about 180 K [42]. Therefore, it is
Fig. 13. Thermal desorption spectra of NO, N2, N2O and H2 under various conditions as indicated in the figure.
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reasonable to suggest that N2 desorption from the combination of two N atoms is a very rapid process, and the formation of N2 does not require N2O as an intermediate, at least, at higher temperatures like 400 K. Hydrogen desorption occurs at temperatures quite close to that at which N2 desorption is enhanced. Since benzene-derived hydrocarbons do not contribute a lot to the enhancement (Fig. 11), it can be concluded that the release of H from benzene dehydrogenation plays a key role in the promotion of N2 desorption. In general, no matter what kind of mechanism the NO reduction reaction follows, the yield and desorption of N2 from a Pt surface mainly proceeds through three routes: (I) recombination of atomic N from direct NO dissociation; (II) decomposition of N2O forming from the reaction of atomic N with NO; (III) direct decomposition of intermediate species between NO and reductants. Route (III) is sometimes entangled with the route (I). For the reaction between benzene and NO, the above analysis has deduced that the formation of N2 does not require N2O as the intermediate, i.e., route (II). Combined with the TDS data, IR results are expected to disentangle the contribution of route (I) and (III) in the yield of N2 from the catalytic reduction of NO by benzene, and provide insight into the role of benzene. The adsorption of NO on Pt(3 3 2) surface preferentially occupies the step sites, with a typical vibration band at about 1600 cm 1. The adsorbed NO molecules on step sites can undergo some changes with changing NO coverage and surface temperature, as has shown in Figs. 4 and 5. IR spectra of benzene on Pt(3 3 2) surface, which are shown in Fig. 1 are in agreement with that on non-stepped Pt surface, studied previously [29,33]. However, unlike the dissociation of smaller hydrocarbons such as ethane, ethylene, propene, etc., the hydrocarbons derived from the dehydrogenation of benzene on Pt(3 3 2) do not give rise to any peaks in the region with frequencies above 1000 cm 1. IR spectra of the benzene-NO coadlayers undergo some changes as compared to that of individual NO and benzene on Pt(3 3 2). The most obvious change is that the adsorption of benzene decreases the vibrational frequency of the N–O bond. This can be explained by the difference between the interactions of benzene and NO with Pt surfaces. The adsorbed benzene donates electron density to Pt atoms, decreasing the work function of the Pt surface [43]. The adsorbed NO is an electron accepter that increases the work function of the surface [44]. Therefore, on the coadlayers, more d-electron density of the Pt atoms is overlapping with anti-bond orbital of the adsorbed NO molecule, leading to the weakening of N–O bond. This effect, in turn, may facilitate the dissociation of the adsorbed NO molecules on the step sites, partially contributing to the enhancement in N2 desorption. Nonetheless, the adsorbed benzene does not chemically react with the adsorbed NO to form intermediates that are essential for N2 production, because there are no new vibrational bands (besides that of N–O bonds) appearing in the IR spectra. The IR spectra also verify that the benzene-derived hydrocarbons do not react with NO directly either, as has been shown by the data in Fig. 8. Moreover, a
previous study concluded that the decomposition of CH3ONO on Pt(1 1 1) surface gives rise to NO but not N2, indicating the scission of C–ON bond is easier than N–O in this kind of molecule [45]. As a consequence, it can be concluded that the yield and desorption of N2 from the benzene-NO coadlayers is not through route (III), but through route (I), i.e., the recombination of atomic N. The reaction mechanism for the catalytic reduction of NO in the presence of benzene can, therefore, be described as: the adsorbed NO molecules decompose when the surface temperature is increased higher than 400 K, giving rise to atomic N and O. The N atoms recombine and desorb quickly. Part of these N atoms can also react with the adsorbed NO to form N2O. When the surface is lacking in reducing species, the O atoms will stay on the surface till 800 K, preventing further dissociation of NO. On the other hand, the presence of benzene provides H atoms through dehydrogenation, which react with O atoms quickly, leaving the surface with more sites for NO dissociation. On this aspect, it can be assumed that benzene mainly serves as a source of H for the removal of O. The removal of atomic O by directly oxidizing benzene or benzenederived hydrocarbons can play a role, but not essentially. 5. Conclusion Coadsorption and reaction of benzene and NO on the surface of Pt(3 3 2) was studied using FTIR-RAS and TDS. NO dissociation mainly occurs on the step sites, giving rise to a desorption peak at 465 K. Adsorbed benzene molecules undergo dehydrogenation at temperature higher than 300 K, the resultant hydrocarbons further dehydrogenate at temperatures higher than 500 K. The adsorption of benzene enhances the interaction between NO and Pt, decreasing the strength of N–O bond. N2 desorption is greatly promoted in the presence of benzene, occurring mainly at temperatures higher than 400 K. It is concluded that NO dissociation is the rate-limiting step in NO reduction. The reaction mainly proceeds through NO dissociation on step sites, followed by the quick removal of the atomic O by hydrogen from benzene dehydrogenation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
P. Burch, J.P. Breen, F.C. Meunier, Appl. Catal. B 39 (2002) 283–303. V.I. Parvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233–316. F. Witzel, G.A. Sill, W.K. Hall, J. Catal. 149 (1994) 229–237. S. Matsumoto, K. Yokota, H. Doi, M. Kimura, K. Sekizawa, S. Kasazawa, Catal. Today 22 (1994) 127–146. E. Joubert, X. Courtois, P. Marecot, D. Duprez, Appl. Catal. B 64 (2006) 103–110. G.A. Somorjai, J. Phys. Chem. B 104 (2000) 2969–2979. J.H. Sinfelt, Surf. Sci. 500 (2002) 923–946. V.P. Zhdanov, B. Kasemo, Surf. Sci. Rep. 29 (1997) 35–90. E. Ozensoy, C. Hess, D.W. Goodman, Top. Catal. 28 (2004) 13–23. A.J. Hallock, C.M. Matthews, F. Balzer, R.N. Zare, J. Phys. Chem. B 105 (2001) 8725–8728. G. Veser, F. Esch, R. Imbihl, Catal. Lett. 13 (1992) 371–382. S.J. Lombardo, T. Fink, R. Imbihl, J. Chem. Phys. 98 (1993) 5526–5539. V.P. Zhdanov, B. Kasemo, Appl. Catal. A 187 (1999) 61–71. N.C. Chen, L.P. Ford, R.I. Masel, Catal. Lett. 56 (1998) 105–109. C.M. Friend, E.L. Muetterties, J. Am. Chem. Soc. 103 (1981) 773–779.
Y. Hu, K. Griffiths / Applied Surface Science 254 (2008) 1666–1675 [16] R.M. van Hardeveld, A.J.G.W. Schmidt, R.A. van Santen, J.W. Niemantsverdriet, J. Vac. Sci. Technol. A 15 (1997) 1642–1646. [17] J. Kim, H.B. Zhao, C. Panja, A. Olivas, B.E. Koel, Surf. Sci. 564 (2004) 53–61. [18] D.J. Burnett, A.M. Gabelnick, D.A. Fischer, A.L. Marsh, J.L. Gland, J. Catal. 230 (2005) 282–290. [19] Z. Ma, F. Zaera, Surf. Sci. Rep. 61 (2006) 229–281. [20] H. Hopster, H. Ibach, Surf. Sci. 77 (1978) 109–117. [21] W.A. Brown, D.A. King, J. Phys. Chem. B 104 (2000) 2578–2595. [22] Z. Jiang, W. Huang, D. Tan, R. Zhai, Z. Bao, Surf. Sci. 600 (2006) 4860– 4869. [23] S. Sugai, K. Shimozu, H. Watanabe, H. Miki, K. Kawasaki, Surf. Sci. 282 (1993) 67–75. [24] S. Haq, D.A. King, J. Phys. Chem. 100 (1996) 16957–16965. [25] F.S. Thomas, N.S. Chen, L.P. Ford, R.I. Masel, Surf. Sci. 486 (2001) 1–8. [26] J.M. Campbell, S. Seimanides, C.T. Campbell, J. Phys. Chem. 93 (1989) 815–826. [27] M.C. Tsai, E.L. Muetterties, J. Am. Chem. Soc. 124 (1982) 2534–2539. [28] S. Lehwald, H. Ibach, J.E. Demuth, Surf. Sci. 78 (1978) 577–590. [29] C. Lutterloh, J. Biener, K. Po¨hlmann, A. Schenk, J. Ku¨ppers, Surf. Sci. 352–354 (1996) 133–137, and reference therein. [30] B.W. Callen, K. Griffiths, P.R. Norton, Phys. Rev. Lett. 66 (1991) 1634– 1637. [31] B.W. Callen, K. Griffiths, P.R. Norton, Surf. Sci. 261 (1992) L44–L48.
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[32] V.K. Agrawal, M. Trenary, Surf. Sci. 259 (1991) 116–128. [33] J. Breitbach, D. Franke, G. Hamm, C. Bekker, K. Wandelt, Surf. Sci. 507– 510 (2002) 18–22. [34] E. Ozensoy, C. Hess, D.W. Goodman, J. Am. Chem. Soc. 124 (2002) 8524–8525. [35] N. Tsukahara, K. Mukai, Y. Yamashita, J. Yoshinobu, H. Aizawa, Surf. Sci. 600 (2006) 3477–3483. [36] R.J. Mukerji, A.S. Bolina, W.A. Brown, Z.P. Liu, P.J. Hu, J. Phys. Chem. B 108 (2004) 289–296. [37] G. Hamm, T. Schmidt, J. Breitbach, D. Franke, C. Becker, K. Wandelt, Surf. Sci. 562 (2004) 170–182. [38] P. Zebisch, W. Huber, H.P. Steinruck, Surf. Sci. 258 (1991) 1–15. [39] A. Wander, G. Held, R.Q. Hwang, G.S. Blackman, M.L. Xu, P. de Andres, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 249 (1991) 21–34. [40] J.C. Charlier, A. De Vita, X. Blase, R. Car, Science 275 (1997) 646–649, and reference therein. [41] Y.H. Hu, S. Han, H. Horino, B.E. Nieuwenhuys, A. Hiratsuka, Y. Ohno, K. Ivan, T. Matsushima, Surf. Sci. 526 (2003) 159–165. [42] H. Wang, R.G. Tobin, C.L. Dimaggio, G.B. Fisher, D.K. Lambert, J. Chem. Phys. 107 (1997) 9569–9576. [43] N. Kizhakevariam, I. Villegas, M.J. Weaver, Surf. Sci. 336 (1995) 37–54. [44] T. Fukushima, M.B. Song, M. Ito, Surf. Sci. 464 (2000) 193–199. [45] J.W. Peck, D.I. Mahon, D.E. Beck, B. Bansenaur, B.E. Koel, Surf. Sci. 410 (1998) 214–227.