Suppression of Nitrogen Oxide Dissociation by Gold on Pt(335)

Delphi Research Labs, Warren, Michigan 48090, and General Motors Research and ... Prototype NOx sensors for use in automotive exhaust can contain Au/P...
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J. Phys. Chem. B 2001, 105, 204-209

Suppression of Nitrogen Oxide Dissociation by Gold on Pt(335) D. C. Skelton,†,⊥,# Hong Wang,‡ R. G. Tobin,§,† David K. Lambert,*,|,† Craig L. DiMaggio,| and Galen B. Fisher|,† Center for Sensor Materials, Michigan State UniVersity, East Lansing, Michigan 48824-1116, Seagate Technology, 7801 Computer AVenue South, Bloomington, Minnesota 55345, Department of Physics and Astronomy, Tufts UniVersity, Medford, Massachusetts 02155, Delphi Research Labs, Warren, Michigan 48090, and General Motors Research and DeVelopment Laboratory, Warren, Michigan 48090 ReceiVed: July 30, 2000; In Final Form: October 31, 2000

Prototype NOx sensors for use in automotive exhaust can contain Au/Pt electrodes that dissociate O2 for removal by electrochemical pumping, but these electrodes should not dissociate NO. In these sensors, the NO diffuses to a second chamber where it is then detected. To understand the process that occurs at the Au/Pt electrode we have used temperature-programmed desorption to study the dissociation of NO and O2 on Pt(335) and on partially Au covered Pt(335). With a gold coverage of 0.15 ML, the dissociation probability of NO is decreased by a factor of 5 relative to bare Pt. Increasing Au coverage to 0.3 ML decreases NO dissociation to an undetectable level. At 0.3 ML Au, the saturation coverage of atomic O is reduced by only 20%. The variation of NO dissociation with Au coverage occurs in the range where all of the Au atoms are at steps, but Au is more effective at decreasing NO dissociation than can be explained by a simple siteblocking model. The effect of adsorbed O on NO dissociation was also investigated. Both Au and O decrease NO dissociation, but Au is much more effective. Blocking 40% of the step sites with O reduces NO dissociation by 70%smore than site-blocking, but much less than a comparable coverage of Au. Our observations show how the Au/Pt electrode in a NOx sensor is able to dissociate oxygen ∼103 times more efficiently than NO.

1. Introduction Tightened automotive emission regulations and increased demand for fuel efficiency are motivating the development of new emission control systems.1-4 Internal combustion engines with improved fuel efficiency, such as the direct-injection diesel and lean-burn gasoline engines, are being developed, but because they operate lean it is difficult to control NOx.2,4 One promising approach to NOx control for such engines involves the use of a NOx adsorber-catalyst system that traps the NOx while the exhaust is lean. The adsorber-catalyst is periodically regenerated by enriching the exhaust with hydrocarbons and CO that react with the absorbed form of NOx to form N2, CO2, and water. In many systems, a NOx sensor is used to detect the breakthrough of NOx and signal the need for catalyst regeneration. An exhaust NOx sensor can be constructed using technology developed for flat plate exhaust oxygen sensors.3 The NOx sensor detects the O atoms released when NO dissociates on a catalytically active electrode. To operate successfully, the NOx, which is typically present at concentrations from 10 to 1000 ppm in the untreated exhaust, must be separated from oxygen at concentrations of 1-15% (104-105 ppm).4 A zirconia-based NOx sensor has been developed that is capable of detecting NOx under these conditions.5-8 As shown in Figure 1, the sensor * Corresponding author. Fax +1 810 986 0886; E-mail: david.k.lambert@ delphiauto.com † Michigan State University. ‡ Seagate Technology. § Tufts University. | Delphi Research Labs. ⊥ General Motors Research and Development Laboratory. # Present address: Kimball Physics, 311 Kimball Hill Rd., Wilton, NH 03086.

Figure 1. Schematic diagram of a two-stage NOx sensor. The two pumping cells reduce the oxygen concentration by ∼105 without removing the NOx. The Rh measuring electrode dissociates NOx, allowing the released oxygen to be detected. After refs 5 and 7.

consists of two cavities connected by a diffusion barrier. Exhaust gas diffuses into the first cavity, where oxygen is dissociated on a Au/Pt alloy electrode5 and pumped away, decreasing the oxygen concentration by a factor of about 103. The oxygendepleted gas then diffuses into the second cavity, which contains another stage of oxygen pumping that reduces the oxygen concentration by a further factor of 102. It is crucial to the operation of the sensor that these drastic reductions in oxygen concentration take place without substantially lowering the NOx concentration. The second cavity also contains a NOx detection cell with a Rh electrode. The sensor produces a linear response to NOx concentration at levels below 1000 ppm, with little crosssensitivity to oxygen.8 The key to the operation of this sensor is the selective dissociation of NO and O2 at the various electrodes. Rhodium is a natural choice for the NOx-detection electrode, because it

10.1021/jp002716m CCC: $20.00 © 2001 American Chemical Society Published on Web 12/07/2000

Suppression of Nitrogen Oxide Dissociation is an efficient catalyst for NO dissociation.1,9-11 If there is to be any NO left to measure at that point, however, the Au/Pt pumping electrodes must effectively dissociate and remove O2 from the feed stream while letting NO pass undisturbed. We have studied the dissociation of both NO and oxygen on a well characterized single crystal of Pt with a controlled Au coverage to mimic the effects that occur on the Au/Pt alloy electrodes in the actual NOx sensor. The crystal surface we use is Pt(335), which consists ideally of (111)-oriented terraces four atomic rows wide separated by monatomic-height (100)-oriented steps. Deposited Pt films are likely to be largely (111)-oriented, since the close-packed (111) face has the lowest surface energy, but the films will contain a high density of atomic-scale defects. The (335) surface is therefore a reasonable model for a real electrode. We have shown previously that Au that has been deposited on Pt(335) blocks the step sites,12 which we show in the present study to be the only active sites for NO dissociation on Pt(111), in agreement with previously reported results.13,14 As a result, 0.30 ML Au, which is enough to fill the step sites, virtually eliminates NO dissociation. Oxygen also dissociates at step sites on bare Pt.15 However, 0.30 ML of Au lowers the saturation oxygen coverage by only 20% compared to bare Pt(335). Because a partial Au overlayer has a much weaker effect on O2 dissociation than on NO dissociation, a Pt(335) surface with 0.45 ML Au is at least 100 times more active for O2 dissociation than for NO dissociation. This is just the selectivity needed for the oxygen-pumping electrodes in the NOx sensor shown in Figure 1. 2. Experimental Section The experiments were carried out in an ultrahigh vacuum chamber housing a double-pass Auger electron spectrometer, a quadrupole mass spectrometer, a four-screen front-view apparatus for low-energy electron diffraction (LEED), a highresolution electron energy loss spectrometer (HREELS), and four parallel-array dosers. Details of the preparation, cleaning, and characterization of Pt(335) and partially Au-covered Pt(335) can be found in refs 12 and 16, and references therein. Data for this work were collected with temperature programmed desorption (TPD) using both normal and isotopically substituted NO. In the studies of clean Pt(335), 15N16O was dosed at ∼100 K on a clean Pt(335) crystal to coverages in the range 0.003 to 0.25 ML. Desorption of 15N16O and 15N2, as well as several other gases, was monitored as the sample temperature was increased at 10 K/s. The integrated areas of these desorption traces were used to determine the NO coverage and the percentage of NO dissociation. The saturation coverage of NO on Pt(335) was assumed to be 0.46 ML.13 Normal 14N16O was used in the studies of Au/Pt(335). Both NO and O2 were dosed using parallel-array dosers that enhanced the flux by a factor of 100 over background dosing. The saturation coverage of atomic O was obtained by exposing the sample to O2 as it cooled from 1000 to 100 K. Lower O-coverages were obtained by saturating the surface with O2 at 100 K and then heating to a selected temperature. Heating to 750 K, for example, removes all of the O2 and all the terrace atomic O, leaving only atomic O at step sites.15 The Au was deposited by thermal evaporation from a resistively heated filament. Gold coverage (θAu) was determined with Auger spectroscopy. Details of the Au deposition procedure and the coverage determination have been given elsewhere.12 This paper is concerned primarily with the variations in NO dissociation probability caused by surface modifications, so our

J. Phys. Chem. B, Vol. 105, No. 1, 2001 205 conclusions do not depend on quantitative knowledge of the fraction of NO that dissociates. A quantitative determination of dissociation probability requires that the relative sensitivity of the mass spectrometer to NO and N2 be known. As described in greater detail in ref 13, to determine the relative sensitivity, a surface was prepared with the step sites saturated with NO. During a TPD experiment, step site NO either desorbs as NO or dissociates to later recombine and desorb as N2. To convert more of the adsorbed NO to N, electron beam stimulated NO dissociation (ESD) was used, following Belton et al.17 Because the electron beam could not have increased the total amount of nitrogen, the data set a lower limit on the sensitivity ratio (N2/ NO) of 72 ( 8. Since some NO could have desorbed during the bombardment, we could not set an upper limit. For the present paper we use a value of 72. Several unrelated experiments were conducted in the vacuum chamber between the experiments on bare Pt(335) and those on partially Au-covered Pt(335). The chamber was vented and baked as needed for maintenance, and the mass spectrometer was disassembled and the internal parts were bead blasted. The sample underwent many cycles of gold deposition and cleaning. When TPD experiments with NO were repeated on the same bare Pt(335) crystal, the N2 desorption peak was much smaller than had been observed previously. This change could result either from changes in the sample, such as a reduced defect density, or from a change in the sensitivity ratio of the mass spectrometer. The dissociation probabilities given here for Au/ Pt are based on the assumption that the dissociation probability on the clean sample without electron bombardment was the same as in the earlier experiments, which gives a new N2/NO sensitivity ratio of 4.0 ( 0.4. This ratio agrees well with the ratio 4.0 ( 0.2 measured for NO dissociation on Rh in much earlier experiments using the same chamber.18 If it is instead assumed that the sensitivity ratio was unchanged, the dissociation probabilities for the Au/Pt experiments are a factor of 18 lower, but our conclusions regarding the effects of Au remain unchanged. 3. Results 3.1. NO on Pt(335). A representative set of 15N16O and 15N2 TPD spectra from Pt(335), taken simultaneously, are shown in Figure 2. Each trace represents a successively higher dose of NO at ∼100 K in the NO coverage range θNO ) 0.003 ML to 0.25 ML. The spectra are not scaled to account for mass spectrometer sensitivity. At θNO near zero, NO adsorbs exclusively at step sites (peak s in Figure 2a) with a desorption temperature of 510 K.13 With increased NO exposure, the step desorption peak continues to grow until ∼0.03 ML. At ∼0.05 ML, adsorption of NO begins at terrace sites (peak γ), and the terrace peak is clearly visible at 0.07 ML. As θNO increases, additional terrace peaks, (R and β), appear in the TPD spectrum until at saturation coverage there are four distinct peaks. At saturation NO coverage, about 20% of the NO desorbs from the step sites (s), in rough proportion to their density on the surface. The N2 desorption curves are dominated by a single peak with a desorption temperature that ranges from 525 to 510 K, assigned to the recombination of atomic N derived from thermally dissociated step NO.13 This peak increases in intensity up to θNO ) 0.03 ML, at which the desorption of step NO saturates. At higher θNO, no increase in dissociation is observed. The trends in the dissociation of NO and in the desorption of N2 are shown graphically in Figure 3. The fraction of NO

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Figure 2. Desorption spectra of 15N16O and 15N2 taken simultaneously from bare Pt(335). At low coverages NO shows a single desorption peak (s) at ∼ 500 K due to NO adsorbed at step sites. The s peak saturates at θNO ) 0.03 ML. With increasing coverage, NO fills three desorption peaks in order of decreasing temperature, γ, β and R. NO dissociation, complete at coverages near zero, produces a single N2 peak in the temperature range 510 K to 525 K that saturates at θNO ) 0.03 ML once the step sites are filled with NO.

Figure 3. Fraction of NO dissociated shown on the left axis (b), and total amount of N2 in arbitrary units shown on the right axis (0). The dissociation probability is 1.0 for θNO ∼ 0, but drops rapidly to a final value of 0.025 ( 0.003.

dissociated to form N2, filled circles, and the total amount of N2 produced, open squares, were obtained from the integrated areas of the TPD data in Figure 2. Near zero coverage, with NO only at step sites, all of the NO dissociates, as shown in the lowest coverage trace in Figure 2. At θNO ) 0.003 ML the dissociation probability is 1.0 and the amount of N2 produced is already at one-half its maximum value. At θNO ) 0.05 ML, the coverage at which the step sites are saturated, both values have gone through a knee, with the NO fraction that dissociates decreasing to approximately 0.1, and the amount of N2 reaching its maximum. At saturation θNO only 2% of the total NO on the surface dissociates, which is 9% of the NO at step sites. It is clear from these data that NO dissociation occurs only at step sites, but is additionally constrained. Fifty percent of the maximum N2 is produced with only 10% of the step sites filled. Filling the remaining 90% of the steps only doubles the yield of N2. The rapid decrease in dissociation probability as the steps fill indicates a self-poisoning effect for NO. Neighbor-

ing NO molecules could prevent dissociation or the dissociation products could block special sites, such as kinks, required for NO dissociation.19,20 To further investigate the relationship between the step sites and NO dissociation, we performed TPD experiments using the two highest temperature NO desorption peaks (γ and s peaks in Figure 2a) and varied the amount of atomic O preadsorbed on the surface. As described earlier, the oxygen was dosed to saturation at 100 K and the sample was heated to selectively desorb oxygen and reach the desired oxygen coverage. The NO was dosed at 300 K after the oxygen coverage had been prepared. This temperature is above the R and β desorption peaks of NO. We have shown that preadsorbed atomic O blocks CO from step sites on Pt(335),21 and Agrawal and Trenary have shown that atomic O blocks NO from defect sites on Pt(111).22 The TPD data for 15N16O and 15N2 after oxygen predosing are shown in Figure 4. We used three O coverages: total saturation, saturation of the step sites, and 40% saturation of the step sites. Bare Pt was also used for comparison. On a surface with 40% of the step sites blocked by O, dissociation of NO is reduced by 70% when compared to dissociation on bare Pt. At the two highest O coverages, where the step sites are saturated by O, N2 desorption is almost completely suppressed and shows no detectable peak in the temperature range near 500 K where N2 desorption is clearly visible at lower O coverages. This confirms that the dissociation of NO takes place at the step sites. It also further demonstrates that blocking step sites has a disproportionate effect on the NO dissociation rate: an O coverage far less than saturation of the step sites greatly decreases NO dissociation. The strong sensitivity of NO dissociation to poisoning of the step sites helps explain how the oxygen-pumping stages of the NOx sensor achieve the required selectivity. 3.2. NO on Au/Pt(335). The oxygen preadsorption experiments in the previous section demonstrate that atomic O effectively blocks NO adsorption in the step sites, which in turn blocks the dissociation of NO. Previous studies have shown that Au deposited on Pt(335) also blocks the adsorption of O, H, and CO at step sites.12,16 In this section we present a series of experiments designed to test the effect of Au deposition on the desorption and dissociation of NO. Desorption traces for NO

Suppression of Nitrogen Oxide Dissociation

Figure 4. Desorption of 15N16O and 15N2 from Pt(335) as a function of oxygen precoverage. The figure shows three precoverages of oxygen plus bare Pt. Saturating 40% of the step sites with oxygen reduces NO dissociation by 70%. An oxygen coverage sufficient to fill the step sites blocks dissociation of NO.

Figure 5. Desorption of NO from partially Au-covered Pt(335) as a function of θAu. The NO dose was sufficient to fill the step sites with just a small amount on the terrace on bare Pt(335). As θAu is increased, the proportion of NO in the step and terrace sites changes, and at 0.45 ML Au, desorption of NO is entirely from the terraces.

as a function of Au coverage are shown in Figure 5. On bare Pt(335), the initial NO dose was sufficient to fill the step sites, with only a small amount of coverage spillover onto the terraces. The same NO dose was used for all Au coverages. For each of the five NO desorption curves the integrated area is equal to within 10%, indicating that at least at low θNO, Au does not inhibit NO adsorption. On bare Pt(335), for the dose used, 90% of the adsorbed NO is at step sites. After deposition of 0.15 ML Au, the distribution has changed to 60% step NO and 40% terrace NO. At 0.30 ML Au, desorption from the steps is

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Figure 6. Desorption of q/m ) 28 and 14 taken simultaneously with the data of Figure 5. The 500 K peak for mass 28 has constant integrated area above 0.30 ML Au, and this peak is assumed to be only CO. This area was subtracted from the other traces to determine the amount of desorbed N2.

completely blocked. As θAu is increased further, NO is forced into terrace desorption peaks with a lower binding energy. These peaks, however, occur at the same temperatures as for high coverage desorption of NO from bare Pt(335), as seen in Figure 2a; the Au does not appear to change the kinetics of NO desorption from the terraces. Desorption of N2 during an NO TPD experiment indicates NO dissociation, but isotopic labeling was not used for the data in this section and 14N2 desorption peaks can be confused with CO peaks at the same mass. Two independent methods were used to determine the dissociation probability: comparison of the q/m ) 14 and 28 desorption peaks, and scaling the q/m ) 14 peak to the NO peak at q/m ) 30 by assuming a constant cracking fraction. The desorption traces for mass 14 and mass 28 as a function of θAu, taken simultaneously with the data of Figure 5, are shown in Figure 6. Several steps are required in order to analyze the data. Mass 28 comes from two sources: CO and 14N2. The signal at q/m ) 14 includes contributions from both 14NO and 14N2. The presence of a peak at the same temperature in both mass channels is therefore a signature of N2 desorption. Examination of Figure 6 shows that only on bare Pt(335) do the desorption spectra exhibit definite peaks for both mass 14 and mass 28 at the same temperature. The desorption peak for mass 28 at 0.6 ML Au is attributed entirely to CO adsorbed from the background (because there is no coincident mass 14 peak), and we assume that an equal amount of CO desorption is present in all the spectra. This area is subtracted from the integrated area of the mass 28 spectra at the four lower Au coverages to find the amount of desorbed 14N2. To compare the mass 14 and mass 30 desorption curves, their ratio with pure NO was separately measured to be 0.075 ( 0.003. In the absence of NO dissociation the TPD signal at mass 14 should have the same shape as the mass 30 signal, with an intensity ratio of 0.075; any excess mass 14 signal is from N2. Figure 7 compares the mass 14 desorption curve with the mass 30 desorption scaled by 0.075 for clean Pt(335) and θAu ) 0.15 ML. The traces for the two masses are nearly indistinguishable

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Figure 7. Comparison of NO (q/m ) 30) and the mixed NO and N2 (q/m ) 14) desorption data taken simultaneously with the data of Figure 5, for bare Pt and 0.15 ML Au/Pt. The NO data are scaled by a factor of 0.075 to determine the q/m ) 14 signal due to cracking of NO; the residual q/m ) 14 signal is assumed to be from N2 (see text). At all θAu g 0.15 ML, the two curves are nearly indistinguishable, indicating minimal N2 desorption.

Figure 8. NO dissociation probability (b) and saturation oxygen coverage, normalized to the saturation coverage on bare Pt(335) (0) as a function of θAu. NO dissociation is much more strongly affected by Au than is oxygen adsorption.

for θAu g 0.15 ML, indicating little or no dissociation. At θAu ) 0.0 ML the mass 14 curve noticeably exceeds the scaled NO curve near 500 K, where N2 desorbs from Pt(335) during dissociation. (See Figure 2b.) This method, with no adjustable parameters, gives dissociation probabilities consistent with those obtained from the mass 28 curves, but with greater uncertainty because of the small mass 14 signal and the uncertainty in the cracking fraction. The filled circles in Figure 8 show the NO dissociation probability, calculated from the mass 28 desorption as described above, as a function of θAu. The error bars are dominated by systematic errors related to the choice of temperature range over which the desorption traces are integrated. As a further consistency check, the dissociation probability for bare Pt(335)

Skelton et al.

Figure 9. Desorption of atomic O from bare Pt(335) and at four coverages of Au. The two peaks above 600 K correspond to recombinative desorption of atomic O, with the peak at 850 K coming from oxygen on step sites. Step desorption appears to be eliminated by 0.30 ML Au, but the total amount of adsorbed oxygen decreases strongly only at 0.6 ML Au and above.

was compared with the point at θNO ) 0.05 ML in Figure 3 (using 15NO and 15N2), because the NO coverages were similar. The results are consistent, confirming the validity of the analysis. The NO dissociation probability at this coverage on bare Pt(335) is 0.09 ( 0.01, but it is lowered by a factor of 3 at 0.15 ML Au and by at least a factor of 10 at 0.45 ML Au. 3.3. Oxygen on Au/Pt(335). The open squares in Figure 8 show the total amount of oxygen that desorbs as a function of θAu, normalized to the saturation coverage on Pt(335). These data were arrived at as follows. The details of oxygen adsorption on bare Pt(335) and on 0.7 ML Au/Pt(335) have been discussed in detail elsewhere.12,15 Briefly, on bare Pt(335) oxygen adsorbs dissociatively above 200 K with recombinative desorption peaks at 700 and 850 K from the terrace and step sites, respectively.15 Oxygen dissociation takes place predominantly at the step sites, with only a 5% probability of dissociation on the terrace sites. On the 0.7 ML Au/Pt(335) surface, the step sites are blocked by Au and the adsorption of atomic O is reduced to 10% of its value on bare Pt. Desorption of oxygen is also exclusively from the terrace areas of the surface that remain unblocked by the Au. Although the sites at which oxygen would initially dissociate on bare Pt are filled with Au, the oxygen molecules still find dissociation sites, although far less efficiently. Our NO dissociation experiments show that a partial monolayer of Au is highly effective at blocking the dissociation of NO. For exhaust NOx sensors to operate successfully, however, the Au/Pt pumping electrodes must retain the ability to dissociate oxygen. We also investigated the desorption and dissociation of oxygen from Pt(335) as a function of θAu. Following each Au dose, the sample was heated to 1000 K and oxygen was adsorbed while the sample cooled to 100 K; TPD experiments were done upon reaching 100 K. Desorption of oxygen at five Au coverages is shown in Figure 9. The peak near 850 K, characteristic of desorption from steps,15 is no longer visible for Au coverages of 0.20 ML and above, though it may be present as a shoulder. As θAu increases, the recombinative

Suppression of Nitrogen Oxide Dissociation desorption signal approaches the shape of a single second-order desorption peak, suggesting that step desorption is eliminated. 4. Conclusion The Pt(335) surface is active for the dissociation at step sites of both O2 and NO.13-15 Blocking the step sites with Au, however, has a greater effect on the dissociation of NO than it does on O2, as seen in Figure 8. Oxygen adsorption is significantly affected only at very high θAu. Even at coverages where not only the step sites but also a large percentage of the terraces are covered with Au, the partially Au-covered surface still dissociates oxygen, as evidenced by the fact that oxygen still adsorbs on 0.6 ML Au/Pt(335), where no Pt steps are available. We concluded in an earlier paper that O2 dissociates at the boundary between the Au and Pt areas on 0.7 ML Au/Pt(335).12 The boundary provides sites that are active for oxygen dissociation, but not for NO dissociation. While it may seem obvious that Au should passivate the Pt surface, Au/Pt alloys have been shown to catalyze the production of light hydrocarbons from alkanes.23 Gold in NaY zeolite has been shown24,25 to catalyze the reaction of NO and CO to form isocyanate in the presence of H2. Gold particles supported on metal oxides catalyze26 the reduction of NO by H2. Moreover, Au supported on Al2O3 has been shown27 to catalyze the reduction of NOx to N2 using hydrocarbon in the presence of O2. The dissociation of NO on metals is a notoriously complex process exhibiting a strong dependence on metal, crystal face, coverage, and temperature.14,28 Over Pt the dissociation rate can vary by 1021 depending on the crystal face.14 Masel has emphasized that NO dissociation is a “special case”, with energetics and orbital symmetry that make it unusually sensitive to local electronic structure.14 Our results extend that sensitivity to the effects of “poisons” at the steps. With θAu ) 0.15 ML the NO dissociation probability is reduced by a factor of 4; with 40% of the step sites blocked with oxygen the NO dissociation is reduced by 70%. A NO dissociation rate that is not simply proportional to the number of empty step sites indicates that NO dissociation is dependent on the surface bonding configuration. A likely explanation is that NO dissociation requires either multiple adjacent step sites, or special step sites, such as kinks or missing atoms, where previously adsorbed atoms are also most strongly bound. The quantities plotted in Figure 8 are not identicals dissociation probability for NO, and saturation coverage for oxygensso quantitative comparisons must be made carefully. Nevertheless, it is suggestive to estimate the dissociation probability for oxygen, using a value of 0.45 for oxygen at step sites on bare Pt(335),15 and assuming that the dissociation probability scales with the ability of the surface to adsorb oxygen. We then find dissociation probabilities for oxygen at θAu ) 0.30 and 0.45 ML to be 0.4 and 0.3, respectively. When compared to the upper limit dissociation probabilities for NO

J. Phys. Chem. B, Vol. 105, No. 1, 2001 209 at the same Au coverages, 0.02 and 0.01, respectively, the surface dissociates oxygen at least 20 times more efficiently than NO at θAu ) 0.30 ML and at least 30 times more efficiently at 0.45 ML. We emphasize that for θAu above 0.4 ML these ratios are lower limits, since NO dissociation at high θAu was below the detection threshold. For the exhaust NOx sensor shown in Figure 1 to operate successfully, an initial O2/NO ratio of ∼103 in the exhaust gas must be reduced to ∼10-3 at the measuring electrode. To achieve this reduction, each pumping electrode must be ∼103 times more active for oxygen dissociation than for NO dissociation. Our study indicates that a Au/Pt alloy provides this level of discrimination because of the unusual sensitivity of NO dissociation to the local environment, and suggests that the optimal θAu ∼ 0.5 ML. Acknowledgment. This work was supported by the NSF under grants DMR-9606233 and DMR-9400417 (MRSEC). References and Notes (1) Taylor, K. C. Catal. ReV. Sci. Eng. 1993, 35, 457. (2) Fritz, A.; Pitchon, V. Appl. Catal. B 1997, 13, 1. (3) Woestman, J. T.; Logothetis, E. M. Ind. Phys. 1995, 1, 20. (4) Tamaru, K.; Mills, G. A. Catal. Today 1994, 22, 349. (5) Kato, N.; Hamada, Y.; Kurachi, H. SAE Paper 970858. (6) Kato, N.; Kurachi, H.; Hamada, Y. SAE Paper 980170. (7) Inagaki, H.; Oshima, T.; Miyata, S.; Kondo, N. SAE Paper 980266. (8) Kato, N.; Kokune, N.; Lemire, B.; Walde, T. SAE Paper 199901-0202. (9) Paˆrvulescu, V. I.; Grange, P., Delmon, B. Catal. Today 1998, 46, 233. (10) Root, T. W.; Schmidt, L. D.; Fisher, G. B. Surf. Sci. 1983, 134, 30. (11) Root, T. W.; Schmidt, L. D.; Fisher, G. B. Surf. Sci. 1985, 150, 173. (12) Skelton, D. C.; Tobin, R. G.; Lambert, D. K.; DiMaggio, C. L.; Fisher, G. B. J. Phys. Chem. B 1999, 103, 964. (13) Wang, H.; Tobin, R. G.; DiMaggio, C. L.; Fisher, G. B.; Lambert, D. K. J. Chem. Phys. 1997, 107, 9569. (14) Masel, R. I. Catal. ReV. Sci. Eng. 1986, 28, 335, and references therein. (15) Wang, H.; Tobin, R. G.; Lambert, D. K.; DiMaggio, C. L.; Fisher, G. B. Surf. Sci. 1997, 372, 267. (16) Skelton, D. C.; Tobin, R. G.; Lambert, D. K.; DiMaggio, C. L.; Fisher, G. B. J. Phys. Chem. B 2000, 104, 548. (17) Belton, D. N.; DiMaggio, C. L.; Schmieg, S. J.; Ng, K. Y. S. J. Catal. 1995, 157, 559. (18) DiMaggio, C. L., unpublished. (19) Gohndrone, J. M.; Masel, R. I. Surf. Sci. 1989, 209, 44. (20) Gorte, R. J.; Schmidt, L. D.; Gland, J. L. Surf. Sci. 1981, 102, 348. (21) Wang, H.; Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. J. Chem. Phys. 1995, 103, 2711. (22) Agrawal, V. K.; Trenary, M. Surf. Sci. 1991, 259, 116. (23) Chandler, B. D.; Schabel, A. B.; Blanford, C. F.; Pignolet, L. H. J. Catal. 1999, 187, 367. (24) Salama, T. M.; Ohnishi, R.; Shido, T.; Ichikawa, M. J. Catal. 1996, 162, 169. (25) Salama, T. M.; Ohnishi, R.; Ichikawa, M. J. Chem. Soc., Faraday Trans. 1996, 92, 301. (26) Galvagno, S.; Parravano, G. J. Catal. 1978, 55, 178. (27) Kung, M. C.; Bethke, K. A.; Yan, J.; Lee, J.-H.; Kung, H. H. Appl. Surf. Sci. 1997, 121/122, 261. (28) Brown, W. A.; King, D. A. J. Phys. Chem. B 2000, 104, 2578.