J. Phys. Chem. 1994, 98, 11786- 11790
11786
Conversion of Methyl Radicals to Methanol and Formaldehyde over Molybdenum Oxide Catalysts Sergei Pak, Michael P. Rosynek, and Jack H. Lunsford' Department of Chemistry, Texas A%M University, College Station, Texas 77843 Received: August 3, 1994; In Final Form: August 31, 1994@
Methyl radicals, formed by the thermal decomposition of azomethane, were allowed to react over both Moo3 and MoOs/SiO2 catalysts. In the continuous flow mode, the dominant reaction was the homogeneous coupling of CHy radicals to form C2H.5; however, significant amounts of HCHO and CH30H were also observed. Over Moo3 at 300 OC, CH30H was the principal oxygenate, but at 500 "C the CH30H was converted mainly to HCHO. At both temperatures, HCHO was the favored product over MoOdSiO2. The addition of H2O to the reactants enhanced the formation of CH30H at 300 "C. During temperature-programmed reaction studies, CH3*radicals were first allowed to react with MoOdSi01 at 150 "C. Methanol began to appear in the gas phase at about 170 "C and was the main product up to 400 "C. As a result of adding H2O continuously during the TPR experiment, CH30H was produced at 80 "C, and the integral amount of CH30H formed over the temperature range from 80 to 500 "C increased. These results are consistent with the view that CH3radicals react with Moo3 to form surface methoxide ions. The methoxide ions may either decompose to form HCHO or they may react with surface protons or water to form CH30H.
+
Introduction
CH3*(g) MoV'02-
The direct oxidation of methane to methanol and formaldehyde has been achieved over silica-supported molybdena and vanadia catalysts, although high selectivities to the desired oxygenates occur only at small (< 2%) methane conversion levels.'-1° Unless excess water is present, the production of formaldehyde greatly exceeds that of methanol. Liu et ai.' proposed that the mechanism involves the formation of CH3. radicals, which react with the surface to yield methoxide (CH30-) ions. These ions may either decompose to HCHO and surface OH- ions, or they may react with H20 to produce CH30H. It was demonstrated that MoO(OCH3)4 reacts with water to yield CH30H. Evidence for the role of methoxide ions in the formation of formaldehyde comes mainly from studies on the partial oxidation of methanol over supported and unsupported molybdenum oxide.11,12Groff13 demonstrated by infrared spectroscopy that CH30- ions were formed when CH30H reacted with the Moo3 catalyst surface. The observed kinetic isotope effects indicate that the rate-limiting step in the catalytic process is the loss of hydrogen from the surface methoxide ions. By contrast, during the partial oxidation of CHq, the rate-limiting step becomes the abstraction of hydrogen from the alkane. This is a much more demanding reaction that requires higher temperatures. Thus, the steady-state concentration of methoxide ions formed from CHq probably would be less than the limits of detection by infrared spectroscopy. To circumvent the difficulties that result from the activation of CH4, alternative methods have been used to generate methyl radicals.'J4 In one case, surface 0- ions formed on an Moos/ Si02 catalyst were allowed to react with CHq, and the resulting infrared spectrum provided evidence for the formation of CH3Oions at 25 "C.' In another case, CH3*radicals were formed over a Sm2O3 catalyst at elevated temperatures, and the radicals subsequently reacted at 25 "C with the MoOslSi02 catalyst. The reduction of MoV*to MoVwas followed by ESR spectro~copy.'~ Together, these results suggest the reaction EJ Abstract
published in Advance ACS Abstracts, October 15, 1994.
0022-3654/94/2098- 11786$04.50/0
-
MoVOCH3-
(1)
which is believed to be the origin of methoxide ions during the partial oxidation of methane. Otsuka et a1.15 studied the conversion of CHq to HCHO over an Fe2(Mo04)3 catalyst and concluded that CH30H is the precursor for HCHO. That is, they do not believed that HCHO may be formed directly from a surface intermediate, although they suggest that adsorbed CH3O may be an intermediate in the formation of CH30H. Their conclusion is based mainly on the high activity and selectivity that Fe2(Mo04)3 has for the oxidation of CH30H to HCHO, which is an argument that also could be made for the silica-supported molybdena catalyst. In order to more fully understand the role of surface methoxide ions in the formation of methanol and formaldehyde, methyl radicals, derived from the decomposition of azomethane (CH3N=NCH3), were allowed to react with pure Moo3 and with an Mo03/SiO2 catalyst. The products that were formed during one of three reaction modes were then followed. These modes include (i) the continuous reduction of the catalyst by the CH3radicals, (ii) a catalytic reaction with 0 2 as the oxidant, and (iii) temperature-programmed reaction (TPR) of the surface species. As a potentially practical application of this methodology, one can visualize a reactor in which CH3- radicals are formed at elevated temperatures over one of the many catalysts that are effective in the oxidative coupling of C&.16 The radicals then would react at a lower temperature with a second catalyst that would be suitable for the formation of CH30H or HCHO. Sun et ai.'' have employed a similar reactor system using a double-layered catalyst bed that consisted of SrLa203 followed by MoO3/SiO2. They did not, however, keep the two catalysts at different temperatures. When the radical-generating catalyst, SrLa203, was added to the reactor that contained MoO3/SiO2, the space-time yield for HCHO increased but the selectivity decreased. Since they operated at atmospheric pressure, it is not surprising that much of the C% was converted to the coupling products, Cz& and C2H6. Because the coupling reaction requires a third body, it could be minimized by 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 45, 1994 11787
Conversion of Methyl Radicals decreasing the total pressure, but in doing so one would limit the practical value of the process. Experimental Section Molybdenum oxide (99.99%,Johnson Matthey) had a surface area of 0.3 m2/g. The 6.6 wt % Mo03/Si02 catalyst was prepared by the addition of Davison Grade 57 silica gel (330 m2/g) to an aqueous solution of ammonium heptamolybdate. The slurry was evaporated to dryness, and the catalyst was heated in air for 16 h at 500 "C to decompose the ammonium heptamolybdate. The resulting material had a surface area of 136 mz/g. Assuming to a coverage of one atomic layer, ca. 28% of the Si02 was covered with Moos, which corresponds to 2 Mo/nm.2 Before exposure to the reactants, both catalysts were treated in situ in 1.2 Torr of flowing 0 2 for 1 h at 500 "C. Methyl radicals were generated by thermally decomposing azomethane at 950 "C in a 2-mm-i.d. fused-quartz tube. In agreement with the results of Stair and co-workers,18 the decomposition of CH3NxNCH3 was essentially complete under these conditions, and the initial products were mainly CH3. and N2. We have demonstrated, using a matrix isolation electron spin resonance system (MIESR), that the flux of CHy radicals exiting the quartz tube was in good agreement with the amount of azomethane that was decomposed.lg The azomethane, which was prepared by a variation of the method described by Renaud and was diluted in argon to a ratio of argonhomethane = 10. The experiments were carried out in a fused-quartz reactor which consisted of two sections. The lower section was the region in which the azomethane decomposition took place. The upper section, which was 8-mm id., contained the catalyst and was heated separately. The catalyst (100-150 mg) was placed on a thin layer of quartz wool, located about 15 mm above the exit of the 2-mm quartz tube. The radicals entered the upper section at a rate of 1 x 10l6 s-l and the residence time in the catalyst bed was ca. 1 ms. When desirable, 0 2 could be added before the other gases passed through the catalyst. During the flow reactions, the pressure below the catalyst bed was typically 35 mTorr, and the pressure drop across the bed was about 5 mTorr. When 0 2 was added, the total pressure was kept constant at 35 mTorr. The background pressure in the system was '1 mTorr. The stable products exiting the catalyst bed were sampled by allowing a fraction of the stream to leak into a UHV system containing an INFICON Quadrex 200 mass spectrometer. A matrix effect was introduced by the presence of oxygen and the large excess of argon in the system; therefore, it was necessary to calibrate the mass spectrometer using known mixtures of organic molecules, oxygen, and argon. In preparation for the TPR experiments, 150 mg of the oxidized catalyst were exposed to the reagent stream containing CH3*radicals for 30 min at a specified temperature. After this period, an inlet valve was closed, and the sample was heated in vacuo for 10 min at the temperature of adsorption. The temperature was then increased at a rate of 23 "C/min. Results and Discussion
Flow Experiments. At the residence times and CHy radical concentrations used in these experiments, extensive coupling of CH3Js would be expected to occur before the gases reached the ionization region of the mass spectrometer. Part of this coupling occurred in the reactor, and if the CH3- radicals did not react with the catalyst, additional coupling occurred downstream from the reactor. At a total pressure of 35 mTorr, a radical concentration of 1.3 x IOl4 radicals/cm3, and a
TABLE 1: Major Products Formed in an Empty Reactor at Room Temperature selectivities, % ~~
02/CHs"
Cz&
HCHO
Cfi
COz
AMB,b%
0 0.17 1.13 3.66
97.2 95.0 93.5 93.8
0 0.3 0.7 0.7
2.8 4.2 4.5 4.7
'0.1
4 0.8 10 12
0.5 1.4 0.8
a The total pressure was ca. 35 mTorr. AMB is the difference in the carbon balance between the amount expected from azomethane decomposition and that detected by mass spectrometry.
TABLE 2: Product Distribution in the Reaction of Methyl Radicals over Silica selectivities, % T,"C
02/CH3
CzH6
HCHO
Cfi
COz
AMB,%
300
0 0.33 2.5 0 0.19 1.03 3.2
96.2 95.3 92.8 93.4 92.6 90.8 86.4
0 1.7 2.8 0 0.5 2.0 4.2
3.2 2.6 3.4 6.1 6.3 5.5 6.2
0.5 0.3
6 11 17 0.7 4 8 14
500
1.0 0.5 0.6 1.7 3.2
residence time of 0.02 s, it can be shown that 66% of the radicals should couple before they reach the catalyst bed. This value was determined by using an equation which allows one to calculate the rate constant for radical coupling as a function of temperature and pressure.21 The extent of coupling, even at these low pressures and CH3. radical concentrations, illustrates the problem that would be encountered in designing a system requiring transport of the radicals from one catalyst bed to another. The selectivities for the various carbon-containing products obtained by passing mixtures of decomposed azomethane, argon, and oxygen through the empty reactor at 25 "C are shown in Table 1. Carbon monoxide is not included in the table because of the interference from the NZpeak at 28 amu. The column labeled AMB refers to the deficit in the carbon mass balance, which is defined as the difference between the expected amount of carbon, based on the amount of azomethane decomposed, and the amount of carbon detected by the product analysis. A positive AMB indicates that less carbon was detected than expected, and part of this carbon may have been present as CO. The values of AMB were generally less than 20%, which indicates that if CO were present, it was not a major product. The coupling of CHr radicals to form C2H6 was the dominant reaction in the empty reactor at 25 "C, even when substantial amounts of 0 2 were present. The only other significant product detected was C&. The origin of the C& is uncertain, although it could have resulted from the reaction of CH3- with hydrogen on the metal walls of the UHV system. Such reactions are known to occur, for example, on Ni s ~ r f a c e s . ~ ~ ~ ~ ~ As indicated in Table 2, when 150 mg of silica gel was placed in the reactor at 300 or 500 "C, the results were similar to those obtained in the empty reactor, except that somewhat more CHq was formed at 500 "C. In the absence of 02, there was no evidence for the formation of methanol, and only a small amount of formaldehyde was detected, which is of interest because silica itself is a moderately good catalyst for the oxidation of methane to formaldehyde.8 Even in the presence of 0 2 , only a small amount of HCHO was formed. The addition of ca. 200 mg of pure Moo3 to the reactor resulted in the conversion of the CH3. radicals to CH30H and HCHO, as shown in Table 3. The decrease in the C2I& selectivity, relative to that found in the open reactor, indicates
11788 J. Phys. Chem., Vol. 98, No. 45, 1994
Pak et al.
TABLE 3: Product Distribution in the Reaction of Methyl Radicals over Moo3 selectivities, % T,"C 300
500
0z/CH3 CzHs HCHO
0 0.2 1.2 2.6 0 0.1 1.0 2.6
73.3 83.2 80.0 77.6 68.1 81.6 80.4 78.1
0.9 2.7 5.7 6.1 12.8 8.9 10.4 13.1
CH30H
C&
COz
22.8 11.6 11.1 13.2 14.9 6.8 6.3 6.9
2.5 2.0 2.3 2.0 2.7 2.0 2.1 1.0
0.5 0.4 0.7 1.1 1.5 0.6 0.8 1.0
AMB,% -13 9 9 11 -8 5 5 12
TABLE 4: Product Distribution in the Reaction of Methyl Radicals over MoOJ/SiOz selectivities, % T,"C
0dCHs
(22%
300
0 0, Hz0" 0.28 1.4 3.1 0 0, HzO" 0.36 2.4 3.95 4.2 18.6
91.7 69.7 83.0 86.3 78.3 69.6 76.5 76.4 71.9 72.3 63.0 53.8
500
HCHO 5.0 3.6 6.5 7.9 13.2 27.4 12.5 18.9 24.0 25.4 29.0 39.5
CH3OH
CH4
COz
0
3.0 8.5 6.1 3.3 2.6 1.9 5.8 2.8 2.5 0.4 4.6 4.3
0.2 4.1 0.8 1.2 1.9 1.0 4.2 1.8 1.6 1.9 3.4 2.3
14.0 3.6 1.4 3.9 0 1.0
0 0 0 0 0
AMB,% 9
50
'
40
.
30
r-
10 21 17
2o T
8
0 0
10 14 19 13 25
The CHfl20 ratio was about 0.5-0.7. Trace amounts of 0 2 were
added via the HzO. that reactions other than coupling were occurring. In the absence of 0 2 , the oxygenated product at 300 "C was mainly CH30H, but at 500 "C, both CH30H and HCHO were formed in nearly equal amounts. Based on these results, one cannot exclude the possibility that HCHO is derived from CH30H. The partial oxidation of CH30H to HCHO over Moo3 in the presence of 0 2 becomes significant even at 200 OC.llJz When 0 2 was added to the system in progressively larger amounts, the selectivity to CH30H decreased at 300 "C, and the amount of HCHO increased. At 500 "C, the presence of 0 2 significantly increased the formaldehyde-to-methanol ratio, which is not surprising in view of the fact that catalytic conversion of CH30H to HCHO is half-order in 0 2 . l 2 The amount of CO2 remained small, which indicates that the complete oxidation of HCHO and CH30H was limited. Analogous experiments were carried out over the Mo03/SiOz catalyst and the results are given in Table 4. At 300 "C, in the absence of 0 2 , a small amount of HCHO and no CH30H was detected. At 500 "C, more HCHO was formed, but again no CH30H was detected. In this case, the amount of HCHO was nearly equivalent to the sum of the HCHO and CH30H formed over MoO3. In the absence of 0 2 , the production of HCHO decreased with time on stream, as shown in Figure la, because oxygen was depleted from the catalyst. Initially, the selectivity to HCHO was 28%, but after 44 min the amount of HCHO decreased to only 4%. In Tables 3 and 4, the data reported without 0 2 were obtained after ca. 3 min on stream. In a separate experiment, CH30H, at a partial pressure approximately equivalent to 30% of the CH3*radical pressure, was passed together with Ar over an oxidized MoO3/SiO2 catalyst. At 300 "C, 67% of the CH30H reacted, and at 500 "C, 93% reacted. The principal product was HCHO. This result demonstrates that if CH30H were formed from CH3- radicals, it would be largely converted to HCHO. At 500 "C, the selectivity for C2H6 in the absence of 0 2 is consistent with the calculated extent of CH3- radical coupling
5i 10
20
30
40
50
Time, min
Figure 1. Variation in product selectivity with respect to time on stream during the reaction of CH3. radicals over MoOr/SiOz (a) without 0 2 and (b) with 0dCHy = 3.9: W, CZH6; A, HCHO; A, cH4; 0 , COz.
in the volume before the catalyst bed. Moreover, the initial selectivity for HCHO suggests that a large fraction of the remaining CH3- radicals was converted to the aldehyde. Based on the reactive sticking coefficients obtained from the MIESR results,14it can be shown that essentially all of the CHy radicals that reached the catalyst bed reacted with the fully oxidized MoO3. Following the addition of 0 2 , a small amount of CH30H was detected at 300 "C, but at 500 "C, no CH30H was observed, even at an OdCH3. ratio of 19. As the pressure of 0 2 was increased, while holding the pressure of CH3- radicals constant (results not shown in Table 4),the amount of HCHO increased and the amount of C2H6 decreased. Except at the largest 0 2 / CH3. ratio, there was no significant increase in the amount of C02. With 0 2 present in the gas phase, the rate of HCHO formation remained constant over a period of 28 min, as shown in Figure lb. Because of the possible role of water in converting methoxide ions to methanol,' water was added to the reagents. As indicated in Table 4, the presence of water resulted in the formation of CH30H at 300 "C and, to a lesser extent, at 500 "C. At 300 "C, the amount was almost 4 times as great as that attained by the addition of 0 2 at an O2/CH3 ratio of 0.28. The amount of HCHO significantly decreased upon the addition of H20, which suggests that H20 might poison the conversion of CH30H to HCHO at this temperature or (more likely) that H20 reacts with a precursor of HCHO (Le., methoxide ions), converting it to CH30H. The presence of H20 also appeared to increase the formation of COz. TPR Experiments. In order to differentiate between the effects of homogeneous reactions and reactions that occur on the surface, TPR results were obtained subsequent to the reaction of CH3. radicals with the molybdenum oxide catalysts. The initial experiments were carried out following the adsorption of azomethane on Si02 and MoOs/SiOz. These experiments provided information on the influence of unreacted azomethane ( e1%) on the TPR results. The decomposition products were affected by the partial pressure of azomethane, as well as by
J. Phys. Chem., Vol. 98, No. 45, 1994 11789
Conversion of Methyl Radicals
1.4
a
rz a Y)
n
-l
0.6
f i
0.4
0.2 0.0 0
100
200
300
400
500
600
Temperature, OC
Figure 2. TPR results after reaction of 3 mTorr of CH3. with Mood Si02 for 30 min at 150 "C: 0, CH30H; A, HCHO; 0, CO; 0 , COz.
the adsorption time and temperature. In the desorption temperature range of 100-230 "C, molecular CH3N-NCH3 and a small amount of NH3 appeared. At higher temperatures, there was evidence in the mass spectrum for N2, HCN, CH3CN, and CH~NTH,but no C2H6 or C2H4 was detected. Methanol and formaldehyde, if present at all, were in very low concentrations. Following the thermal decomposition of azomethane at 950 "C and the reaction of CH3. radicals with Si02 or pure Moo3 at 150 "C, no TPR products were detected, except for trace amounts that might be attributed to nitrogen-containing compounds. The failure of Moo3 to yield detectable amounts of CH30H and HCHO probably results from its small surface area, which was about a factor of 10 less than that of molybdenum oxide on MoOs/SiOz. This difference in surface area was not a factor in the catalytic experiments because the temperature for the reaction of the CH3. radicals with the catalyst was much greater. Following the reaction of CH3- radicals with MoOdSiO2 at 150 "C, the TPR results of Figure 2 demonstrate the formation of CH30H, HCHO, CO, and C02. In addition, trace amounts of acetaldehyde, acetone, and other products were also formed. There was no C2H6, which confirms that this hydrocarbon was formed by the coupling of CH3- radicals in the gas phase and that the reaction of CH3- radicals with molybdena was not reversible. Since NZ was not present, it was possible to determine the amount of CO, but with limited accuracy because HCHO and CH30H also contribute to the peak at 28 amu. At temperatures less than 500 "C, more CO was formed than C02. The amount of CH30H reached a maximum at 300 "C, and at temperatures greater than 400 "C, HCHO became the principal product. This observation is consistent with the conversion of CH30H to HCHO, even in the absence of gas-phase 0 2 . The effect of H20 is evident in the results of Figure 3a. After CH3- radicals had reacted with the MoOs/Si02 catalyst at 150 "C, H20 was adsorbed at 50 "C. As the temperature was increased, water was released for interaction with surface species. By comparing the results of Figure 3a and Figure 2, it is evident that the appearance of CH30H is shifted from ca. 170 "C to 100 "C, and the maximum is shifted by about the same amount. The CH30H/HCHO ratio is significantly greater at the CH30H maximum when water is present. The role of water is further demonstrated in Figure 3b, which illustrates the results obtained when the TPR experiment was carried out with the catalyst being continuously exposed to 28 mTorr of H20. In this case, an appreciable amount of CH30H was produced, even at 80 "C, and the total amount of CH30H formed over the temperature range from 80 to 500 "C was much greater than in the previous experiment. It is important to note
0.4
0.0
Temperature, OC
Figure 3. TPR results after reaction of 3 mTorr of CH3. with Mood Si02 for 30 min at 150 "C, followed by (a) adsorption of 30 mTorr of H20 for 20 min at 50 "C and (b) heating the sample in a flow of 28 mTon of HzO: 0, CH3OH; HCHO; A, HCHO; 0, CO; 0, C02.
that the CH30H selectivity was quite good in the 100-200 "C temperature range. These results are consistent with the proposal that water, or perhaps protons derived from the water, react with surface methoxide ions to form methanol. As a result of this reaction pathway, less of the methoxide is converted directly to formaldehyde. At 80 "C, the temperature is too low to effect the conversion of CH30H to HCHO; therefore, the greater selectivity for CH30H cannot be attributed to the poisoning of this reaction by H20. But at higher temperatures, of course, the secondary reaction of CH30H to HCHO occurs, again via CH3O- ions.
Summary and Conclusions Methyl radials are converted to methanol and formaldehyde over Moo3 or MoOs/SiOs catalysts at temperatures as low as 300 "C. Higher temperatures and larger O2/CH3*ratios generally favor the production of HCHO. The HCHO is formed, in part, by the secondary oxidation of CH30H, although methoxide ions may provide a direct pathway for HCHO formation. The methoxide ions are formed by the reaction of CH3- radicals with MoO3. From the data presented in this study, it is not possible to conclude that HCHO is formed directly from methoxide ions; however, if one considers the results of Sleight and co-workers" on the conversion of CH30H to HCHO, it is evident that methoxide ions are the intermediate. The addition of H20 to the system enhances the selectivity for CH30H, and in a TPR experiment CH30H is formed at a much lower temperature. These results are consistent with the fact that water reacts with a pure molybdenum methoxide compound to yield methanol.' On the surface of Moos, in the absence of H20, the methoxide ions are stable at temperatures up to 275 "C, but they react with H20 to form CH30H at temperatures less than 80 "C. This result suggests that if C& could be converted to CH3. radicals at moderate temperatures, perhaps by electrochemical or photolytic methods, it would be
11790 J. Phys. Chem., Vol. 98, No. 45, 1994 possible to achieve high yields of methanol. Ward et followed the formation of CH30H from C& and 02 during irradiation of a Cun-modified Moo3 catalyst at 100 "C. Although the reaction rate was small, the selectivity to CH30H was 100%. Acknowledgment. This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. References and Notes (1) Liu, H.-F.; Liu, R.4.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J. Am. Chem. SOC. 1984, 106, 4117. (2) Khan, M. M.; Somorjai, G. A. J . Catal. 1985, 91, 263. (3) Zhan, K. J.; Khan, M. M.; Mak, C. H.; Lewis, K. B.; Somorjai, G. A. J. Catal. 1985, 94, 501. (4) Spencer, N. D. J. Catal. 1988, 109, 187. (5) Barbaux, Y.; Elamrani, A. R.; Payen, E.; Gengembre, L.; Bonnelle, J. P.; Grzybowska, B. Appl. Catal. 1988, 44, 117. (6) Spencer, N. D.; Pereira, C. J . Catal. 1989, 116, 399. (7) Banares, M. A.; Fierro, J. L. G.; Moffat, J. B. J . Catal. 1993, 143, 262. (8) Parmaliana, A.; Frusteri, F.; Mezzapica, A,; Miceli, D.; Scurrell, M. S.; Giordano, N. J . Catal. 1993, 143, 262. (9) Smith, M. R.; Ozkan, U. S. J. Catal. 1993, 141, 124.
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(11) Cheng, W.-H.; Chowdhry, U.; Ferretti, A.; Firment, L. E.; Groff, R. P.; Machiels, C. J.; McCarron, E. M.; Ohuchi, F., Staley, R. H.; Sleight, A. W. In Heterogeneous Catalysis; Shapiro, B. L., Ed.; Texas A&M Univer. Press: College Station, TX, 1984; pp 165-181. (12) Yang, T.-J.; Lunsford, J. H. J . Catal. 1987, 103, 5 5 . (13) Groff, R. P. J . Catal. 1984, 86, 215. (14) Tong, Y.; Lunsford, J. H. J . Am. Chem. SOC.1991, 113, 4741. (15) Otsuka, K.; Wang, Y.; Yamanaka, I.; Morikawa, A. J . Chem. Soc. Faraday Trans. 1993, 89, 4225. (16) Maitra, A. M. Appl. Catal. A: Gen. 1993, 104, 11. (17) Sun, Q.;DiCosimo, J. I.; Herman, R. G.; Klier, K.; Bhasin, M. Catal. Lett. 1992, IS, 371. (18) Peng, X.-D.; Viswanathan, R.; Smudde, G. H.; Stair, P. C. Rev. Sci. Instrum. 1992, 63, 3930: Smudde, G. H.; Peng, X. D.; Viswanathan, R.; Stair, P. C. J . Vac. Sci. Technol. 1991, A9, 1885. (19) Xu, M.; Ballinger, T. H.; Rosynek, M. P.; Lunsford, J. H., to be published. (20) Renaud, R.; hitch, L. G. Can. J . Chem. 1954, 32, 545. (21) Slagle, I. R.; Gutman, D.; Davies, J. W.; Pilling J . Phys. Chem. 1988, 92, 2455. (22) Hall, R. B.; Castro, M.; Kim, C.-M.; Chen, J.; Mims, C. A. Petrol. Div.Prepr. 1994, 39, 282. (23) Tjandra, S.; Zoera, F. J . Catal. 1994, 147, 598. (24) Ward, M. D.; Brazdil, J. F.; Mehandru, S. P.; Anderson, A. B. J . Phys. Chem. 1987, 91, 6515.
