Catalytic Reduction of Nitrogen Oxides by Methane over Pd(110)

Transcription

1 15242 J. Phys. Chem. 1996, 100, Catalytic Reduction of Nitrogen Oxides by Methane over Pd(110) S. M. Vesecky, J. Paul, and D. W. Goodman* Department of Chemistry, Texas A&M UniVersity, College Station, Texas ReceiVed: June 5, 1996 X The catalytic reduction of NO with CH 4 has been studied over Pd(110) in the temperature range K at a total pressure of 10 Torr. The rate of NO reduction is at a maximum at a high CH 4 /NO stoichiometry (8/1) and at a minimum at a low (1/4) CH 4 /NO ratio. Correspondingly, the activation energy for NO reduction decreases with increasing CH 4 /NO ratio, and the reaction orders are -1.0 in NO and +1.5 in CH 4. Unlike NO, the rate of N 2 O reduction is at a maximum at low CH 4 pressures, and the reaction orders are positive in N 2 O and negative in CH 4. In the presence of oxygen, NO is oxidized to NO 2, which is in turn reduced back to NO by CH 4. The NO/NO 2 oxidation/reduction cycle has the effect of delaying NO reduction to N 2 (or N 2 O) until all of the O 2 is consumed. 1. Introduction The impact of environmental catalysis is becoming increasingly more important due to stricter regulations concerning automobile and flue gas emissions. 1 The subfield of environmental catalysis concerned with air quality control involves the reduction of NO x species and the oxidation of CO and volatile organic compounds (VOC s) produced in mobile and stationary sources 2 There are many stationary sources of environmental gas phase pollutants. Methane is perhaps the largest pollutant by volume, emitted from sources such as livestock, gas wells, and landfills. Another serious source of pollution is coal-fired power plants, which emit large quantities of NO x species that subsequently react with HO 2 and OH - to form HNO 3 in acid rain. 3 Current technology for limiting NO x emissions involves the selective catalytic reduction (SCR) or NO x with NH 4 3 NO x + NH 3 + O 2 f N 2 + H 2 O (1) Although this process is efficient, it involves the long-range transport of NH 3 to the stationary NO x sources. Transportation of ammonia through pipelines poses serious safety concerns, the most important being that corrosion of the pipes can lead to catastrophic releases of ammonia in populated areas en route to the NO x sources (namely power plants). The use of methane (and higher hydrocarbons) to reduce NO x species is a promising alternative to current SCR technology. 5 The reaction of CH 4 and NO x to form CO 2, N 2, and H 2 O removes two serious greenhouse pollutants at once. Additionally, CH 4 is cheaper and much safer to transport than NH 3 and is often found in close proximity to the stationary sources. Furthermore, excess methane can be used directly to operate many conventional power plants. In order to be useful in practical applications, however, methane must be proven to have reduction efficiencies similar to those of ammonia. Typically, Pt/Rh and Pd catalysts dispersed on high surface area supports have been used for the reduction of NO with CO. 6-8 In the case of the CH 4 + NO reaction, however, oxidation of methane either by the oxide support or by ambient Present address: Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO KTH/the Royal Institute of Technology, Physics III, Stockholm, Sweden * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, August 15, S (96) CCC: $12.00 oxygen is a serious concern. If too much methane is oxidized to CO 2, the efficiency of the NO x reduction process will suffer. Several catalysts appear to have promising activities for CH 4 + NO conversion; among them are titania-supported Pd and Pd/Cu particles. 9 The goal of these studies is to deconvolute the catalytic properties of Pd, Cu, and the oxide support. Currently, there is very little spectroscopic data for NO x and CH 4 chemisorption on these catalysts. By characterizing the catalyst structure, the chemisorption properties, and the catalytic activities and selectivities, some insight may be gained on the basic mechanism of the CH 4 + NO reaction on Pd. As a first step in understanding the CH 4 + NO reaction over palladium catalysts, this study addresses only the effects of the Pd(110) surface in mediating the reaction kinetics. The single crystal surface has the advantage over more complex catalysts of allowing a fundamental understanding of the interactions of Pd with CH 4 and NO over a range of temperature and partial pressure conditions. The ideal stoichiometry for this reaction should be CH 4 + 4NO f CO 2 + 2N 2 + 2H 2 O (2) Under oxidizing conditions, the partial reduction of NO should proceed as CH 4 + 6NO f CO + 3N 2 O + 2H 2 O (3) The secondary reduction of partially reduced NO x species should then continue as CH 4 + 4N 2 O f CO 2 + 4N 2 + 2H 2 O (4) The key feature to all of these reactions is that they are highly energetically favorable, with H s of reaction of -200 to -300 kcal/mol. 10 Therefore, if a suitable catalyst can be found, the reduction of NO x species by CH 4 should proceed completely to N 2. In this study, infrared reflection-absorption spectroscopy (IRAS) is used to determine the surface coverages of CH 4 and NO at reaction conditions. IRAS is also used in conjunction with transmission IR for acquiring kinetics data. The reaction kinetics data presented show the utility of using gas phase IR for studying relatively uncomplicated reactions in situ and in real time. 2. Experimental Section 2.1. Apparatus. The experiments were performed in a combined ultrahigh-vacuum (UHV) surface analysis chamber 1996 American Chemical Society

2 Catalytic Reduction of NO with CH 4 J. Phys. Chem., Vol. 100, No. 37, and high-pressure reactor (e1 atm). 11 The UHV surface analysis chamber is equipped with an array of analytical techniques for determining surface composition and cleanliness. For this study, Auger electron spectroscopy (AES) was used to monitor carbon, oxygen, and sulfur contamination. Since the energy for the Auger transition of carbon overlaps one of the main Auger signals from palladium, AES cannot be used to verify the absence of carbon on Pd. Instead, temperatureprogrammed desorption (TPD) of O 2 was used to monitor residual carbon contamination. If CO and CO 2 desorption features are seen in the O 2 /Pd TPD spectrum, then the presence of surface carbon is confirmed. The absence of any gas phase CO or CO 2 in the O 2 TPD spectrum is the most reliable means of confirming a carbon-free palladium surface. 12 The high-pressure reactor cell is separated from the UHV analysis chamber by a series of differentially pumped sliding seals. The sealing surface is formed by the 1 in. diameter sample probe compressing against a set of three 1 in. diameter spring-loaded Teflon seals. This allows the analysis chamber to be maintained at UHV while running reactions at pressures up to 1 atm in the high-pressure cell. The reactor cell has a volume of about 350 cm 3 and is quickly pumped by a turbomolecular pump from atmospheric pressures to pressures of less than 10-8 Torr. The sample can be transferred from the high-pressure cell to the UHV surface analysis chamber within 2 min after evacuation of the reactor, thus allowing postreaction analysis of the surface at pressures of 10-9 Torr. The high-pressure cell is coupled to a Fourier transform infrared spectrometer, operating in the reflection-absorption mode. This allows adsorbed molecules with dipoles perpendicular to the surface, such as NO and N 2 O, to be monitored with infrared reflection-absorption spectroscopy (IRAS). Gas phase infrared spectra can also be obtained in the reflectionabsorption geometry, thus allowing the kinetics of gas phase product formation and reactant depletion to be followed with IR Sample Preparation. The Pd(110) sample was polished with 1 µm diamond paste and 0.05 µm alumina following standard polishing procedures. The sample was then cleaned in the UHV chamber by repeated cycles of oxidation at 900 K (1 min at 10-6 Torr of O 2 ) followed by flash annealing to 1200 K. 13 After this procedure, the sample was found to be free of bulk carbon contamination. O 2 TPD, however, showed the presence of a small amount of residual surface carbon, as seen by the presence of CO and CO 2 in the O 2 TPD spectrum. This residual carbon was easily removed by running the CO oxidation reaction on the crystal at elevated pressures (g1 Torr) at a temperature of about 600 K. The ratio of CO/O 2 must be kept at stoichiometric (2/1) or slightly oxidizing conditions to prevent the deposition of additional carbon. TPD of O 2 /Pd following this procedure indicates the removal of residual surface carbon on all low index facets of Pd CH 4 + NO Reaction. Reactions were performed in the batch mode. The same results were obtained whether the reactants were introduced into the high-pressure cell individually or mixed in the manifold prior to introduction into the reactor cell. For power law studies, the pressure of either CH 4 or NO was kept constant at 0.9 Torr while the pressure of the other reactant was varied between and 7.2 Torr. The reactant stoichiometries (CH 4 /NO) ranged from 1/4 to 8/1, with the total pressure kept constant at 10 Torr by filling with argon. The reaction was studied over a temperature range of K, at 25 K increments. Extreme care was taken to eliminate background reactions from the heater legs at these high temperatures. Figure 1. Reaction profile for 0.9 Torr of NO Torr of CH 4 (10 Torr total pressure, fill with Ar) at 725 K (batch mode). Figure 2. Reaction profile for 0.9 Torr of NO Torr of CH 4 (10 Torr total pressure, fill with Ar) at 725 K (batch mode). The reaction kinetics were followed with infrared spectroscopy by monitoring the reduction in gas phase NO and CH 4, while simultaneously monitoring the evolution of gas phase N 2 O and CO 2. After all of the NO was consumed, the reaction of N 2 O + CH 4 could be observed similarly by monitoring the reduction in gas phase N 2 O and CH 4 and the evolution of gas phase CO 2. The IR peak areas of each measured species were converted into the corresponding partial pressures using a set of calibration curves. The calibration curves were obtained by plotting the gas phase infrared intensity of a given molecule versus the partial pressure of that species. 3. Results and Discussion 3.1. CH 4 + NO on Pd(110). Figures 1 and 2 show the profiles for the reaction of CH 4 and NO at 725 K on Pd(110) as a function of time. All reactions performed on model catalysts in the high-pressure cell (350 cm 3 ) were run in the batch mode. As a result, the reactions are at equilibrium only at low conversions (10-20%) where the effect of the change in concentration of each reactant is negligible. The gas phase concentrations of CH 4, NO, and N 2 O were monitored simultaneously with infrared spectroscopy, and the IR intensities were converted to the number of moles produced (or consumed) per second by the formula n/s ) ( P/s)(V/RT) (5) P/s was determined by multiplying the slopes of the lines shown in Figures 1 and 2 by the conversion factor (1 min/60 s). By sampling the gas phase infrared spectra at fixed time

3 15244 J. Phys. Chem., Vol. 100, No. 37, 1996 Vesecky et al. intervals (every 30 s), we obtained values of P/s for each reactant and product species. V is the volume of the cell, 350 ml, R is the universal gas constant, (Torr ml)/ (mol K), and T is the sample temperature, in kelvin. The rate of the reaction is given as the turnover frequency (TOF), which corresponds to the number of molecules produced (or consumed) per surface site per second. In this case, the overall reaction rate was determined from the TOF for NO reduction. This value was calculated by normalizing the moles of NO molecules consumed per second ( n/s) to the total number of surface sites, given by the equation TOF ) ( n/s)( )/( ) (6) where is the total number of surface atoms on the front face of the Pd(110) single crystal used in these experiments. For these studies, the Pd atoms on the back face of the crystal were rendered inactive by depositing a multilayer of an inert oxide such as silica or alumina onto the back face. Under all conditions studied (including Figures 1 and 2), a significant amount of N 2 O was formed, most likely because reaction 3 was more favorable than reaction 2. IRAS data of adsorbed NO at reaction conditions show that the coverage of NO was relatively unaffected by the presence of CH 4, even at highly reducing conditions (CH 4 :NO > 8:1). The heat of adsorption of NO/Pd(110) was determined isosterically to be 30 ( 1 kcal/mol. The heat of adsorption of CH 4 /Pd(110), on the other hand, is only on the order of about 10 kcal/mol. These values for the heats of adsorption, combined with the qualitative IRAS data which show the surface coverage of NO to be only slightly lowered at high CH 4 :NO ratios (>4:1), point to the fact that reaction 3 should be favored over reaction 2 under all conditions due to the much higher coverage of NO versus CH 4. Figures 1 and 2 support this idea, showing a selectivity for reaction 3 over reaction 2 of about 90% and 70%, respectively. Any CO produced in reaction 3 is quickly converted to CO 2 by CO + NO f CO 2 + N 2 + N 2 O (or) (7) CO + N 2 O f CO 2 + N 2 (8) Each of these reactions is significantly faster than the CH 4 + NO reaction, thus accounting for the fact that no gas phase CO is observed at these reaction conditions. 14 In addition to the CO oxidation reactions, (7) and (8), the water gas shift reaction may remove some CO by CO + H 2 O a CO 2 + H 2 (9) As seen in Figures 1 and 2, the concentration of gas phase N 2 O steadily increases while the concentration of gas phase NO is decreasing. N 2 O is not reduced at a significant rate by CH 4 (reaction 4) until after all the NO is reduced because N 2 O adsorption on palladium is inhibited by NO(a), as seen by IRAS. The surface coverage of N 2 O/Pd(110) is only measurable at low temperatures (<200 K) and relatively high pressures (>10-3 Torr). Similar to the case of CH 4, this low θ N2O relative to θ NO along with a heat of adsorption for N 2 O of only about kcal/mol favors reaction 3 over reaction 4. The rate of N 2 O reduction is at a maximum at a 1:1 CH 4 :NO stoichiometry (Figure 1). As the CH 4 :NO ratio increases, the rate of NO reduction also increases, but the rate of N 2 O reduction decreases (Figure 2). The increase in the rate of NO reduction with increasing P CH4 can be rationalized by the more competitive adsorption of CH 4 with NO at reducing conditions (seen qualitatively with IRAS). The decrease in the rate of N 2 O Figure 3. Arrhenius plots for the rates of NO reduction as a function of CH 4:NO ratio. Figure 4. Apparent activation energy for NO reduction as a function of CH 4:NO ratio. reduction at reducing conditions, however, is most likely caused by a higher θ CH4 versus θ N2O. Since both the heat of adsorption and the surface coverage of N 2 O are lower than for NO, it should be expected that the secondary reduction reaction 4 can more easily be poisoned at reducing conditions by CH 4 (Figure 2). Figure 3 shows a series of Arrhenius plots for the rates of NO reduction as a function of P CH4. It can readily be seen from this figure that the activity for NO reduction increases with increasing P CH4, whereas the apparent activation energy for NO reduction decreases with increasing CH 4 :NO ratio. Figure 4 shows the apparent activation energy for the reaction over a much broader range of CH 4 :NO pressure ratios. Again, it can clearly be seen that the apparent activation energy for the reaction is strongly dependent on the CH 4 :NO pressure ratio. At higher CH 4 :NO ratios, the desorption of NO is facilitated because CH 4 is able to compete more effectively with NO for surface sites. This, in turn, decreases the apparent activation energy for the reaction and increases the rate of reaction because the activation energy for NO desorption becomes less of a limiting factor. Figure 5 shows the power laws of the reactants for NO reduction at 725 K. The negative order in NO strongly supports the idea that θ NO and the desorption of NO from the Pd(110) surface limit the rate and in turn increase the apparent activation energy for the reaction (Figures 3 and 4). The positive order in CH 4 corresponds to the IRAS data which show that θ NO decreases slightly with increasing P CH4. The fact that the power law in CH 4 is greater than 1 may imply that each CH 4 is displacing more than one NO and/or that the equilibrium shifts from reaction 3 toward reaction 2 with increasing P CH4.

4 Catalytic Reduction of NO with CH 4 J. Phys. Chem., Vol. 100, No. 37, Figure 7. Reaction profile for 1 Torr of NO + 1 Torr of CH / 2 Torr of O 2 (10 Torr total pressure, fill with Ar) at 800 K (batch mode). Figure 5. Reaction power laws in NO and CH 4 at 725 K. The pressure of one component was held constant at 0.9 Torr while the pressure of the other component was varied from to 3.6 Torr. The total pressure was 10 Torr. enough concentrations, thus promoting the activity and lowering the apparent activation energy for the reaction CH 4 + NO + O 2 on Pd(110). For practical applications, the reaction of CH 4 + NO must be considered in the presence of O 2. The first reaction which occurs in the presence of O 2 is the direct gas phase oxidation of NO to NO Since both NO and NO 2 can act as oxidants for CH 4, the reaction profile for CH 4 + NO + O 2 becomes somewhat more complicated (Figure 7). In the absence of O 2, the reaction profile shows only two regimes: one for NO reduction and one for N 2 O reduction (Figures 1 and 2). In the presence of O 2, an additional reaction regime is formed by the reaction of NO 2 + CH 4 (first region of Figure 7). Since NO 2 is a better oxidant than NO (and N 2 O), NO is reduced at a measurable rate only after all of the NO 2 and O 2 have been consumed (Figure 7). The stoichiometry of this additional reaction pathway should ideally follow as CH 4 + 2NO 2 f N 2 + CO 2 + 2H 2 O (11) Figure 6. NO:CH 4 reaction rate versus initial CH 4:NO concentration. Although the microscopic effect on CH 4 (a) on NO (a) is difficult to determine, a slight shift in the selectivity toward reaction 2 with increasing P CH4 can be seen by comparing the maximum concentration of N 2 O in Figure 2 (0.36 Torr) versus Figure 1 (0.41 Torr). Repeated experiments confirm that the selectivity for N 2 O decreases with increasing P CH4. The shift in the selectivity between the partial reduction (3) and the optimum stoichiometry (2) can also be seen by plotting the TOF for NO reduction versus the TOF for CH 4 oxidation as a function of the initial partial pressure conditions (Figure 6). At oxidizing conditions (low CH 4 :NO ratios), over eight NO molecules are reduced (or at least partially reduced to N 2 O) for every CH 4 molecule oxidized. This, in fact, is a higher stoichiometry than that predicted by eq 3. The additional NO reduction may occur first by NO dissociation into N(a) and O(a) followed by NO(a) + N(a) f N 2 O(a) f N 2 O(g) (10) The O(a) may then react with either CH 4 or CO to form CO 2. As the CH 4 concentration increases, the consumption of NO relative to CH 4 decreases. At an 8:1 excess of CH 4 :NO, the number of NO molecules reduced per CH 4 oxidized approaches the ideal stoichiometry (4:1 NO:CH 4 ) of reaction 2. These results again support the idea that CH 4 can displace NO at high Alternatively, NO 2 may only be partially reduced, giving a stoichiometry of CH 4 + 4NO 2 f 4NO + CO 2 + 2H 2 O (12) Since these reactions were run in the batch mode, NO produced by reaction 12 was reoxidized to NO 2, thus repeating a catalytic cycle, with NO as the catalyst: NO + 1 / 2 O 2 a NO 2 (13) The effect of this catalytic cycle in the presence of oxygen is to prevent the reduction of NO by CH 4. NO reduction readily occurs at these conditions in the absence of oxygen (Figures 1 and 2), implying that either O(a) or NO 2 (a) inhibits the NO + CH 4 reaction. Once all of O 2 was consumed, the reduction of NO by CH 4 proceeded as in the absence of O 2 (region 2 of Figure 7). The fall in the concentration of NO 2 corresponds to the consumption of O 2 either by reaction 13 or by direct oxidation of CH 4 and CO. Region 1 of Figure 7 shows that N 2 O production was suppressed in the presence of oxygen. This selectivity for N 2 versus N 2 O in the presence of O 2 is encouraging, implying that eq 11 is indeed viable. Unfortunately, at the conditions studied, only a small fraction of the NO was converted to NO 2. As seen in region 2 of Figure 7, about 0.9 Torr of the original 1 Torr of NO remained after all of the NO 2 had been consumed. Still, the 0.1 Torr of NO reduced (indirectly) by eq 11 showed complete selectivity for N 2 versus N 2 O.

5 15246 J. Phys. Chem., Vol. 100, No. 37, 1996 Vesecky et al. Figure 8. NO reduction rates as a function of O 2 partial pressure. An attempt was made to optimize the NO 2 concentration by varying the partial pressure of oxygen. Figure 8 shows the results of the measured NO reduction rates as a function of oxygen partial pressure at the same temperature and pressure conditions shown in Figure 7. For oxygen partial pressures between 0 and 0.5 Torr, the primary effect seen for a batch mode reaction is to delay the onset of the direct reduction of NO by CH 4. Even at partial pressures of O 2 of more than 100 Torr, the equilibrium concentration of NO 2 never exceeded more than 10% of the concentration of NO. The most likely cause of this low concentration of NO 2 is the reverse reaction 13 caused by the thermal dissociation of NO 2 back to NO. At high enough oxygen pressures, all NO should eventually be reduced to N 2, indirectly, through reaction 11. As the partial pressure of oxygen increases, however, the rate of NO 2 reduction decreases, perhaps due to increased surface poisoning by O(a). At higher temperatures (>800 K), the reaction should run faster. The equilibrium between NO 2 and NO, however, increasingly favors NO due to thermal dissociation. The power laws of NO 2, CH 4, NO, and O 2 for this system have yet to be determined. For practical applications in the presence of oxygen, however, the complete reduction of NO to N 2 over Pd only catalysts does not appear feasible. 4. Conclusions Overall, these results show that the ideal stoichiometry of 4 NO + 1 CH 4 predicted by reaction 2 is followed only at highly reducing conditions due to the much higher heat of adsorption of NO versus CH 4. At an 8:1 excess of CH 4 :NO, the apparent activation energy of the reaction is about 27 kcal/mol lower than at the ideal stoichiometry of 1:4 CH 4 :NO (Figure 4). This difference in the apparent activation energy for the reaction at reducing versus oxidizing conditions corresponds very closely to the measured heat of adsorption of NO/Pd(110) 30 kcal/ mol. Although an 8:1 excess of CH 4 :NO is optimum for NO reduction (2), the secondary reduction of N 2 O (4) is inhibited at highly reducing conditions due to displacement of N 2 O by CH 4. In the presence of oxygen, the rate of NO (and N 2 O) reduction by CH 4 is negligible due to displacement of NO/Pd by either NO 2 (a) or O(a). Oxygen catalyzes the formation of NO 2, which then either goes on to react (slowly) with CH 4 or thermally dissociates back into NO. When O 2 and NO 2 are present in the CH 4 + NO system, the selectivity for NO reduction goes completely to N 2. Once all of the O 2 has been depleted, however, the NO + CH 4 reaction follows the same activity and selectivity pattern as if O 2 had never been present. To optimize this reaction, ideally all NO should be converted to NO 2. Unfortunately, the rate of the NO 2 + CH 4 reaction is so slow that extremely high temperatures must be employed. Not only do these high temperatures make this reaction impractical and inefficient, they also lead to the thermal dissociation of most NO 2 back to NO. Although the NO + CH 4 reaction is more efficient, the selectivity for N 2 versus N 2 O was extremely poor under the conditions studied (10-30%). Although Pd only catalysts do not appear practical for the reduction of NO by CH 4, the role of the Pd surface in a bimetallic Pd/Cu or an oxide supported system may be much different. The fundamental kinetics data obtained on the Pd(110) surface provides a starting point for interpreting kinetics results in more complicated systems. Acknowledgment. The authors acknowledge, with pleasure, the support of this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. S.M.V. gratefully acknowledges the support of an Eastman Chemical graduate fellowship. References and Notes (1) Armor, J. N. In EnVironmental Catalysis; Armor, J. N., Ed.; ACS Symposium Series 552; American Chemical Society: Washington, DC, 1993; p 1. (2) Summers, J. C.; Sawyer, J. E.; Frost, A. C. In Catalytic Control of Air Pollution; Silver, R. G., Sawyer, J. E., Summers, J. C., Ed.; ACS Symposium Series 495; American Chemical Society: Washington, DC, 1992; p 98. (3) Edney, E. D.; Stiles, D. C.; Spence, J. W.; Hayne, F. H.; Wilson, W. E. In Materials Degradation Caused by Acid Rain; Baboian, R., Ed.; ACS Symposium Series 318; American Chemical Society: Washington, DC, 1986; p 172. (4) Spitznagel, G. W.; Huttenhofer, K.; Beer, J. K. In EnVironmental Catalysis; Armor, J. N., Ed.; ACS Symposium Series 552; American Chemical Society: Washington, DC, 1993; p 172. (5) Vesecky, S. M.; Nordlander, P.; Ohman, L.-O.; Persson, P.; Bjornbom, E.; Zadeh, B. G.; Lunsford, J. H.; Goodman, D. W.; Keiski, R. G.; Paul, J. Proceedings of the 11th World Clean Air Conference, Helsinki, (6) Shelef, M.; Graham, G. W. Catal. ReV.sSci. Eng. 1994, 36, 433. (7) Taylor, K. C. Catal. ReV.sSci. Eng. 1993, 35, 457. (8) Fisher, G. B.; Oh, S. H.; Carpenter, J. E.; Goodman, D. W. J. Catal. 1986, 100. (9) Paul, J.; Ohman, L. O. Proc. JECAT 95; Lyon, (10) Lide, D. R.; Kehiaian, H. V. CRC Handbook of Thermophysical and Thermochemical Data; CRC Press: Boca Raton, FL, 1994; p 197. (11) Campbell, R. A.; Goodman, D. W. ReV. Sci. Instrum. 1992, 63, 172. (12) Vesecky, S. M.; Chen, P. J.; Xu, X.; Goodman, D. W. J. Vac. Sci. Technol. A 1995, 13, (13) Grunze, M.; Ruppender, H.; Elshazly, O. J. Vac. Sci. Technol. A. 1988, 6, (14) The rate of the CO + NO reaction to form CO 2, N 2, and N 2O is almost 2 orders of magnitude faster than the CH 4 + NO reaction at the most favorable (highly reducing) conditions studied. Similarly, the rate of the CO + N 2O reaction is about 1 order of magnitude faster than the CH 4 + NO reaction. (15) Calvert, J. G.; Stockwell, D. R. In SO 2, NO and NO 2 Oxidation Mechanisms: Atmospheric Considerations; Calvert, J. G., Ed.; Butterworth: Boston, 1984; p 1. JP961644P