J. Phys. Chem. 1988, 92, 4120-4127
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Surface Chemistry of Ketene on Ru(001). 2. Surface Processes M. A. Henderson, P. L. Radloff, C. M. Greenlief, J. M. White,* Department of Chemistry, University of Texas, Austin, Texas 78712
and C . A. Mims Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: August 18, 1987; In Final Form: February I , 1988)
The chemistry of ketene (CH2CO)on Ru(001) was studied by static secondary ion mass spectrometry (SSIMS) and temperature programmed desorption (TPD). Ketene adsorption at 105 K is both molecular and dissociative and leads to H2, CO, and C02, b l t no hydrocarbons, in TPD. Between 200 and 250 K, molecular ketene is hydrogenated to adsorbed oxygenates, q2(C,0) acetaldehyde (CH3CHO) and q2(C,0) acetyl (CH3CO). The hydrogen is supplied by the decomposition of methylene, the latter produced by ketene decomposition. Hydrogenation to CH,CHO is favored at low coverages of ketene, while at high coverages, hydrogenation to CH3C0is favored. Pre- or postadsorbed H2enhances the hydrogenation of ketene to CH,CHO but does not lead to any hydrocarbon desorption. Between 200 and 250 K, ethylidyne (CCH,) forms, particularly at high initial ketene coverages. During dosing at 350 K, ketene decomposes but only H2 desorbs. The accumulated adsorbed species include CO and CCH,. While dosing at 400 K, both CO and H2 desorb, significantly more ketene decomposes than at lower temperatures, and CCH and CH accumulate.
1. Introduction This paper deals with the processes, as detected by TPD and SSIMS, by which surface structure derived from ketene evolve on Ru(OO1). The companion paper (paper 1) deals with the identification of the structures. Surface methylene groups have been proposed as key C, fragments in Fischer-Tropsch synthesis.’-3 The work reported here forms part of a series of surface science studies we have undertaken using C H 2 C 0 as a small C-, H-, and 0-containing molecule whose reaction on single-crystal metals may have relevance for the understanding of catalytic C, chemistry.k6 Several catalytic studies have utilized ketene and diazomethane (CH2N2) as methylene sources to probe hydrocarbon chain growth.’,’ In addition to our own work, a few studies of ketene and diazomethane have been done on well-defined metal surfaces under UHV conditions. McBreen et aL8 identified methylene fragments on Fe( 110) by HREELS from both ketene and diazomethane decomposition. Some methylene was stable on Fe( l 10) till 520 K, and vinyl (CCH,) formation was observed from its reaction with C above 400 K. Berlowitz et aL9 observed that CHzNzdecomposition on P t ( l l 1 ) yielded methane (by CH, hydrogenation) and ethylene (presumably by CH2 coupling) in TPD. Ruthenium was chosen for the present study because it is more active than Pt for the synthesis of hydrocarbons from CO and H2.lo Identification of the relevant surface structures relies heavily on high-resolution electron energy loss spectroscopy (HREELS) and additional SSIMS results that are reported in a companion paper” (hereafter referred to as paper 1). In brief, paper 1 shows that at 105 K ketene adsorbs both molecularly, as q3(C,C,0) and q2(C,C) species, and dissociatively (to CO and CH,) at low (1) Brady, R. C., 11; Pettit, R. J . Am. Chem. Soc. 1980, 102, 6181; J . Am. Chem. SOC.1981, 103, 1287. (2) Fischer, F.; Tropsch, H. Erennsf. Chem. 1926, 7 , 97. (3) Craxford, S.R.; Rideal, E. K. J . Chem. Soc. 1939, 1604 (4) Radloff, P. L.; Mitchell, G. E.; Greenlief, C. M.; White, J M.; Mims, C. A. Surf. Sci. 1987. 183. 403. ( 5 ) Mhchell, G. E.; Radloff, P. L.; Greenlief, C. M.; Henderson, M. A,; White. J. M. Surf. Sci. 1987., 183. ~ - -377. - . . (6)’Radloff, P: L.; Greenlief, C. M.; Henderson, M. A,; Mitchell, G. E.; White, J. M.; Mims, C. A. Chem. Phys. Lert. 1986, 132, 8 8 . (7) Blyholder, G.; Emmett, P. H. J . Phys. Chem. 1959, 63, 962; J . Phys. Chem. 1960,64, 470. (8) McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1984, 148, 292. (9) Berlowitz, P.; Yang, B. L.; Butt, J. B.; Kung, H. H. Surf. Sci. 1985, 159, 540. (10) Vannice, M . A. J . Cutal. 1975, 37, 449. ( I 1) Henderson, M. A,; Radloff, P. L.; White, J. M.; Mims, C. A. J . Phys. Chem., preceding paper in this issue. ~~
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coverages, but only molecularly at higher coverages. Two C, oxygenated species resulting from the hydrogenation of ketene were identified after annealing to various temperatures: q2(C,0) acetaldehyde (CH3CHO), and q2(C,0) acetyl (CH,CO). Another C2 oxygenate, bidentate acetate, was considered in paper 1 and may make some contribution. Two additional C2 hydrocarbon fragments were identified: ethylidyne (CCH3) and acetylide (CCH). These involve C-C bond formation but no C-0 bond cleavage. 2. Experimental Section The ultra-high-vacuum chamber used in this study and the methods of data collection have been detailed e l ~ e w h e r e .TPD ~ and SSIMS data were taken as described in paper 1. Typical ramp rates were 2.1 K/s for TPSSIMS and 6.2 K/s for TPD. For direct comparison with TPSSIMS, a TPD ramp rate of 2.1 K/s was occasionally used. Adsorption was usually done a t 105 K; however, some hightemperature dosing experiments were performed. Transient gas-phase signals were monitored during dosing at higher temperatures. Relevant additional experimental details are given in the companion paper.”
3. Results 3.1. TPD Results. In the TPD experiments, we searched carefully for C1 through C4 hydrocarbons but found none, regardless of the ketene exposure. This important observation holds for ketene dosed at 105, 350, and 400 K as well as ketene dosed with H2. Additionally, no oxygenates other than C O and CO, desorbed. This includes H 2 0 , H,CO, and CH30H. After dosing at 105 K, the TPD products were limited to relatively large amounts of CO and Hz, small amounts of C02, and, at high doses, molecular CH2C0. After dosing at 350 K, TPD gave only C O and H,. After a 400 K dose, the TPD was dominated by H2 but also contained small amounts of CO and C 0 2 . With this overview in mind, we turn to the details of TPD following 105 K ketene exposures (Figure 1). For a 0.25-langmuir ketene exposure, mainly desorption-limited H2 (380 K) and C O (488 K) are observed, with a very small amount of reaction-limited H2 desorption at 488 K. At higher ketene exposures, other reaction-limited H2desorption states develop at 325, 381, 443, 495, and 595 K. The peak temperatures of these states do not change significantly with increasing ketene exposure, implying that first-order steps control the dehydrogenations of various species derived from ketene. The 325 K state saturates at a ketene exposure of about 1.0 langmuir. The 381 and 443 K states begin 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4121
Surface Chemistry of Ketene on Ru(OO1). 2 126
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Figure 1. TPD spectra of various exposures of ketene on Ru(001) at 105 K. The exposures are listed at the right-hand end of each curve. The four sets of spectra are for H2, CO, C 0 2 ,and CH2C0.
to develop at ketene exposures near 0.75 langmuir, and they saturate at 2 3 langmuirs. The higher temperature states are present throughout most, if not all, of the exposure range. They also saturate at 1 3 langmuirs. On the basis of first-order kinetics and an assumed preexponential factor of lOI3 sd1,I2the activation energies (E,) for three of these reaction-limited hydrogen desorption processes are 19.5 kcal/mol (325 K), 23.0 kcal/mol (381 K), and 26.9 kcal/mol (443 K). Activation energies determined from SSIMS data for two of these processes will be presented below. The C O TPD (Figure 1) is desorption-limited regardless of ketene exposure. Thus, there is no oxygen-containing intermediate which decomposes to CO(a) above the normal CO/Ru(001) desorption temperature (450-480 K). C02appears only for ketene exposures exceeding 0.75 langmuir and is always concomitant with the 443 K H2 peak, suggesting that they are related to a common surface species (see below). Molecular ketene ( m / e 42) desorbs in a single TPD state at 126 K and appears only at high exposures ( 2 3 langmuirs) where the first layer is saturated (see below). This TPD state is due to ketene physisorbed either in the first layer or in a m ~ l t i l a y e r . ~ The TPD yields of H2, CO, and C 0 2 are plotted in Figure 2 versus ketene exposure. Absolute coverages in monolayers (ML), (defined as the number of species per surface Ru atom) were calibrated by H2 and CO TPD and were calculated assuming saturation 0 (H) = 1.0 ML13J4 and saturation 0 (CO) = 0.68 ML.IS Coverages of C 0 2 were estimated in two ways. In the first, we used the stoichiometric relations: B(CH2CO) = )/20(H) =qco)
+ 20(C02)
O(COz) = l/qO(H) - l/,0(CO> where O(CH2CO) is the coverage of decomposed ketene. The second equality arises from the observation that no oxygen remained after TPD. This estimate of CO, (0.05 ML) is subject (12) Redhead, P. A. Vacuum 1962, 12, 203. (13) Peebles, D. E.; Schreifels, J. A.; White, J. M. Surf. Sci. 1982, 116, 117. (14) Lindroos, M.; Pfniir, H.: Feulner, P.; Menzel, D. Su$. Sci. 1987, 180, 237. (15) Pfniir, H.; Menzel, D. J . Chem. Phys. 1983, 79, 2400.
0
, 1
2
3
4
5
6
Ketene Exposure ILI
Figure 2. TPD yields of H2, CO, and C 0 2 as a function of ketene exposure on Ru(001) at 105 K. See text for calibration procedure. 1 ML = 1 adsorbed species per surface Ru atom.
to considerable error because it is calculated as the difference of two relatively large numbers and because there are small amounts of background adsorption, particularly H2. This would overestimate the amount of C 0 2 . A second assessment of the relative amounts of C O and C 0 2 desorbed, based on TPD peak areas and mass analyzer sensitivity ratios, leads to a C 0 2 coverage of 0.01 ML. As noted above, the saturation coverage of hydrogen derived from ketene decomposition may be lower than shown in Figure 2 because of small amounts of background hydrogen adsorption. We estimate this to be no more than 0.05 ML which would lower the calculated amount of CO,. On the basis of our experience, we are confident that the saturation CO, coverage lies between 0.01 and 0.05 ML and is probably closer to 0.05 ML. With these calibrations, the yields of H,, CO, and CO, saturate for ketene exposures above 3 langmuirs at 0.46, 0.34, and 0.05 ML, respectively. Regardless of the uncertainty in the absolute coverages of COz and H,, the c o 2 ( 4 4 3 K):H2(443 K) TPD peak area ratio is independent of ketene dose between 0.75 langmuir and saturation (3 langmuirs). Preadsorption of 0.5 langmuir of C i 8 0 with 6 langmuirs of ketene at 105 K resulted in a C'602:C160180 TPD peak area ratio of greater than 30:l although the ratio of desorbed CI6O to C l 8 0 was about 1.7:l. This shows that the C 0 2 derives mainly from some species other than CO. Furthermore, the postexposure of 2 langmuirs of 0, on 6 langmuirs of ketene (previously heated to 400 K and cooled to 105 K) did not alter the amount of H2 and C 0 2 in the 443 K TPD peaks. Instead, the added 0 reacted with C to give a reaction-limited CO desorption state at 635 K, a state never seen when only ketene is dosed (Figure 1). These two experiments suggest that c o 2 ( 4 4 0 K) and H,(440 K) arise from a common intermediate which is derived solely from ketene. When Hz is coadsorbed with ketene, there are changes in the H2 TPD but still no hydrocarbons desorb. Figure 3 shows (from bottom to top) the H2 ( m / e 2) TPD spectra of 5 langmuirs of H,, of three exposures of ketene each followed by 5 langmuirs of H2, of 5 langmuirs of H2 followed 6 langmuirs of ketene, and of 6 langmuirs of ketene. Just as in the absence of H2, the TPD of C O (not shown) is always desorption limited. Generally speaking, the H2 features are superpositions of spectra from H2 and C H 2 C 0 . The most important distinction is the relative intensity of the 314 K peak, which is higher whenever the surface
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The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 r
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5 L H2 700
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Figure 3. H2TPD spectra of ketene and hydrogen coadsorbed on Ru(001) at 105 K: (a) 5 langmuirs of H2 alone; (b) 5 langmuirs of H2 on 0.25 langmuir of ketene; (c) 5 langmuirs of H2on 0.75 langmuir of ketene; (d) 5 langmuirs of H2 on 1.5 langmuirs of ketene; (e) 6 langmuirs ketene on 5 langmuirs of H,;and (f) 6 langmuirs of ketene alone.
is H-rich. For example, the intensity of this peak is strong when 5 langmuirs of H2 is dosed after 0.25 langmuirs of ketene (Figure 3b) but there is m intensity in this region for TPD of 0.25 langmuir ketene alone (Figure 1). The 3 14 K peak is also particularly strong when 6 langmuirs of ketene is dosed after 5 langmuirs of H2 (Figure 3e). When ketene is dosed first and in increasing amounts (Figure 3b-d), the H2 TPD peak positions and yields become like those of 6 langmuirs of ketene alone (compare parts d and f of Figure 3). Coadsorption of H 2 and CH&O also tends to decrease the amount of C 0 2 desorbed (not shown) but it increases in the series b, c, d, and f of Figure 3 and correlates with an increasing relative amount of C H 2 C 0 adsorption. For the conditions of Figure 3b,e, no CO, desorbs. Thus, for Figure 3e, the amount of CO desorbed (0.12 ML) gives the coverage of ketene decomposed. In the absence of preadsorbed hydrogen, the decomposition of this coverage of ketene yields predominantly desorption-limited H 2 (as in Figure 1 at 0.25-langmuir dose) whereas here it is mostly reaction limited because the deposition of H on the H-covered surface results in immediate H2 desorption. The amount of H2 desorbing in the 314 K peak of Figure 3e is approximately 0.28 ML of H(a) = 0.14 ML of H2 which exceeds the amount of ketene decomposed. Thus, under these conditions, some of the preadsorbed H(a) which would have appeared as desorption-limited H2 in the absence of ketene (Figure 3a) recombines rapidly with hydrogen atoms made available by the decomposition of a hydrogenated ketene species to give the reaction-limited H2 peak at 314 K. The appearance of this H2 is the result of surface overcrowding and is consistent with the small H2 state at 234 K in Figure 3e. The latter is due to desorption-limited H 2 from a compressed coadsorbed overlayer as in the coadsorption of H, and C O on R u ( O O ~ ) . ' ~ 3.2. Acetaldehyde, Acetyl Chloride, and Acetic Acid. Preliminary results from a study of adsorbed acetaldehyde, acetyl chloride, and acetic acid on Ru(001)I6 are important to the discussion that follows. The relevant observations are (1) Decomposition of acetaldehyde dosed on Ru(001) occurred with the most intense H2TPD feature at 300-310 K in agreement with the 320 (16) Henderson, M. A,; Zhou, Y.; White, J. M., manuscript in preparation.
100
300 500 700 Tenperature IKI
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Figure 4. Lower four curves: +TPSSIM spectra (1 keV, 4.5 nA/cm2, 2.1 K/s) of 6 langmuirs of ketene on Ru(001) at 105 K. The upper four curves are the comparison TPD (2.1 K/s) spectra.
K feature here. (2) Very minor (
