Surface Chemistry of Ketene on Ru(001). 1 ... - ACS Publications

J. Phys. Chem. 1988, 92, 41 11-41 19

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Surface Chemistry of Ketene on Ru(001). 1. Surface Structures M. A. Henderson, P. L. Radloff, 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 1 , 1988)

Ketene (CH2CO) adsorption and reaction on Ru(O0 1) were studied by high-resolution electron energy loss spectroscopy (HREELS), static secondary ion mass spectrometry (SSIMS), and temperature programmed desorption (TPD). Only H2, CO, and C 0 2 are observed in TPD. At 105 K, low ketene exposures dissociate to CO and methylene and adsorb molecularly as an $(C,C,O) species. At higher exposures ketene adsorbs molecularly as $(C,C). Above 200 K, adsorbed ketene hydrogenates to two distinct acyl species: q2(C,0) acetaldehyde (CH3CHO), and $(C,O) acetyl (CH,CO). Adsorbed acetate is also considered. The hydrogen required for hydrogenation is supplied from decomposition of CH2 and ketene. Above 200 K, ethylidyne (CCH,) is formed. Methylidyne (CH) and acetylide (CCH) are formed by decomposition of all the above oxygenate and hydrocarbon species. Dosing ketene on Ru(001) at 350 K, where CO accumulates, favors ethylidyne formation. The adsorbed CO stabilizes ethylidyne, which otherwise decomposes at 320 K. Dosing at 400 K, which is above the threshold of CO desorption, results in CH and CCH.

1. Introduction This paper deals with the surface structures derived from ketene (CH2CO) on single-crystal ruthenium, Ru(001). Ketene as an adsorbate is interesting, among other reasons, for its relevance to catalytic Cl chemistry. Ruthenium is an active catalyst for higher hydrocarbon formation in Fischer-Tropsch (FT) synthesis. The latter is thought take place by the polymerization of surface CHI groups-structures that might be supplied by the decomposition of ketene. In this study we show structural data obtained by HREELS for the adsorption and reaction of ketene on Ru(OO1). These data are correlated with SSIMS and TPD results and with literature references for analogous organometallic and surface species. A companion paper,' hereafter referred to as paper 2, focuses on the processes associated with the formation and decomposition of these species. When dealing with complex molecular adsorbate systems, such as ketene, it is difficult to determine structures on the basis of surface science information alone. In these cases comparisons with organometallic c o m p l e x e ~ often ~ . ~ provide the key to the identification. In a study of the adsorption and reaction of ketene on Pt( 11 1),4 organometallic data were valuable in interpreting that complex system, and in this paper we use a similar approach in the study of ketene on Ru(001). On Pt( 11 l ) , we determined that the structure of ketene adsorbed at 105 K was v,(C,C)~based on v(CH,) and v(C0) frequencies similar to those measured by Miyashita et aLS for the complex (PPh,),Pt-$(C,C) CH,CO. Upon annealing above 250 K, q2(C,C) ketene on Pt(l11) both hydrogenates to $(C) C H 3 C 0 and decomposes to C O and CH2. Assignment of vl(C) C H 3 C 0 was based partially on $(C) CH,CO complexes of Pt characterized by Adams and Booth.6 The CH2, however, is transient and unobservable by HREELS. It decomposes to C H and H, hydrogenates to CH4, or reacts to C2H4.' McBreen et a1.* have (1) Henderson, M. A.; Radloff, P. L.; Greenlief, C. M.; White, J. M.; Mims, C. A. J . Phys. Chem., following paper in this issue. (2) Herrmann, W. A. Adu. Organomet. Chem. 1982, 20, 159. (3) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1982, 21, 117. (4) Mitchell, G . E.; Radloff, P. L.; Greenlief, C. M.; Henderson, M. A,; White, J. M. Surj. Sci. 1987, 183, 403. (5) Miyashita, A.; Shitara, H.; Nohira, H. Organometallics 1985, 4, 1463. (6) Adams, D. M.; Booth, G. J . Chem. SOC.1962, 1112. (7) Radloff, P. L.; Mitchell, G. E.; Greenlief, C. M.; White, J. M.; Mims, C. A. Surf. Sci. 1987, 183, 377. (8) McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1984, 148, 292.

0022-3654/88/2092-4111$01.50/0

observed v2(C,C) ketene on Fe(ll0) at 120 K. Its decomposition yields C O and CH2, the latter of which is stable on Fe( 110) to 520 K. Fortunately, many organometallic complexes of ketene have been studied and complexes with q2(C,C),s,"s 772(C,0),'G'9 and 73(C,C,0)20structures have been characterized. In some cases these ketene complexes are synthesized by C-C bond formation between bridging carbenes and carbonyl^.^^-^^ Facile C-C bond cleavage and hydrogenation to acyl ligands have also been ob~erved.'~-~~,~' On Ru(001), we find that ketene adsorption at 105 K is complex, showing a variety of surface structures depending on the exposure. These range from dissociative (yielding C O and CH2) to molecular adsorption in at least two coordinations ($(C,C,O) and $(C,C)). Annealing the molecular species above 200 K results in further decomposition to C O and CH2 as well as hydrogenation to $(C,O) C H 3 C H 0 and $(C,O) CH3C0. We also consider adsorbed acetate $(O,O)CH3CO0. Ethylidyne is formed above 200 K.

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 .The ~ system is ion-pumped (700 L/s) with a working base pressure of 1X Torr. A 170 L/s turbomolecular pump is used on the gas handling system. (9) Schorpp, K.; Beck, W. 2.Naturforsch., B 1973, 28, 738. (10) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1974, 13, 335. (11) Miyashita, A,; Shitara, H.; Nohira, H. J . Chem. SOC.,Chem. Commun. 1985, 850.

(12) Herrmann, W. A,; Plank, J.; Ziegler, M. L.; Weidenhammer, K. J . Am. Chem. SOC.1979, 101, 3133.

(13) Morrison, E. D.; Steinmetz, G. R.; Geoffroy, G.L.; Fultz, W. C.; Rheingold, A. L. J . Am. Chem. SOC.1984, 106, 4783. (14) Lin, Y. C.; Calabrese, J. C.; Wreford, S. S. J. Am. Chem. SOC.1983, 105. 1679. Redhouse, A. D.; Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 5. 615. -. ., ~.

(16) Fachinetti, G.; Biran, C.; Floriani, C.; Chiesi-Villa, A,; Guastini, C. J . Am. Chem. SOC.1978, 100, 1921. (17) Gambarotta, S.; Pasquali, M.; Floriani, C.; Chiesi-Villa, A,; Guastini, C. Inorg. Chem. 1981, 20, i173. (18) Straw, D. A.; Grubbs, R. H. J . Am. Chem. SOC.1982, 104, 5499. (19) Casey, C. P.; OConner, J. M. J . Am. Chem. SOC.1983, 105, 2919. (20) Holmgren, J. S.; Shapley, J. R.; Wilson, S. R.; Pennington, W. T. J . Am. Chem. SOC.1986, 108, 508. (21) Lindner, E.; Berke, H. Z . Naturforsch., B 1974, 29, 275

0 1988 American Chemical Society

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The Journal of Physical Chemistry, Vol. 92, No. 14, 1988

The HREELS spectrometer is a 127’ cylindrical sector type with stationary monochromator and analyzer defining a total scattering angle of 120O. All spectra were taken with a primary beam energy of 6.8 f 0.3 eV and resolution of 10 mV full width at half-maximum. Because of the fixed angle of the spectrometer, off-specular measurements were performed by rotating the sample about an axis in the plane of the surface until the elastic peak decreased to 1 / 100 of its specular intensity. The off-specular scattering angles were not determined, since the only purposes of such experiments were (1) to identify losses that were obstructed by intense dipole modes and (2) to determine which modes were predominantly dipole scattered. TPD and SSIMS data were taken with an Extrel quadrupole mass spectrometer identical with that used in the ketene/Pt( 111) study.7 Positive SSIM spectra were obtained with a rastered 1-keV Ar ion beam with currents between 2.5 and 8.0 nA/cm2. The TPD ramp rate was 6.2 K/s. The surface of the Ru crystal (1.3 mm thick by 10.0 mm diameter) was oriented to within f0.5’ of the (001) face. The crystal was spotwelded to two 0.5-mm-diameter tantalum wires for resistive heating and was cooled to 105 K by conduction to a liquid nitrogen reservoir. The temperature was monitored by a Chromel-Alumel thermocouple spot-welded to the side of the crystal. Rigorous sample cleaning was done by Ar+ bombardment to remove irreducible oxides and common surface impurities (K, Na, Si, and AI). Routine cleaning of surface carbon was accomplished by cycling the crystal temperature between 900 and 1450 K in front of a doser (see below) with an O2 flux which resulted in a 1 X IO-* Torr pressure rise as measured by the system’s ion gauge. This was followed by annealing at 1550 K to remove adsorbed 0. Surface cleanliness was monitored by HREELS and AES (single pass C M A with coaxial electron gun). Ketene preparation and purification have been described prev i o ~ s l y .Ketene ~ was dosed through a directional doser consisting of a 0.2-cm-i.d. stainless steel tube connected by a leak valve to the gas handling ~ y s t e m .(A ~ second doser of this type delivered O2 for surface cleaning.) During dosing the sample face was perpendicular to the cylinder axis of the doser tube and was about 1 cm from its end. Exposures were based on the time spent in front of the doser at a ketene flux which resulted in a 1 X 1O-Io Torr pressure rise detected at the system’s ion gauge (prior to rotating the sample in front of the doser). The doses were calibrated by backfilling experiments. The dosing temperature was 105 K unless otherwise noted. For dosing at higher temperatures, the sample was cleaned and its temperature adjusted to the desired value. Being certain the sample was rotated away from the doser, the ketene leak valve was then opened until a steady system pressure of 1 X l 0-9 Torr was reached. To initiate the dose, the sample was rotated in front of the doser while the transient gas-phase signals were monitored.

3. Results 3.1. Overview of KetenelRu(001). Figure 1 shows an overview of ketene chemistry derived from this study. Accompanying the scheme are the TPD spectra of 6 langmuirs (1 langmuir = 10” Torrs) of ketene on Ru(OO1). There are two dominant channels. The first involves hydrogenation of molecularly adsorbed ketene to $(C,O) C H 3 C H 0 and s2(C,0) C H 3 C 0 (Figure 1 at 250 K). Acetate, a 2 ( 0 , 0 )CH3CO0, may also be formed, at least transiently, in this channel. The second channel involves reactions of the dissociated CH2 fragments to form CH, species and CCH3 (250 K). Only H2, CO, and C 0 2 are observed in TPD, and the peaks correlate with the decomposition of the surface species. The purpose of this paper is to verify the presence of the species in Figure 1. To do so we rely mainly on the HREELS and SSIMS results although supporting kinetic evidence from paper 2 will be mentioned. 3.2. Ketene Adsorption at 105 K. Figure 2 shows the HREEL spectra of various exposures of ketene on Ru(001) at 105 K. For 0.25 langmuirs of ketene (Figure 2b) the major losses are from atop CO at 1995 ( v ( C 0 ) )and 440 cm-’ ( ~ ( R u C ) ) .The clean

Henderson et al.

330

K

I

380 440 K

1-

m I

I

T

Surface Species

Temperature

Figure 1. Reaction scheme of 6 langmuirs of ketene dosed on Ru(001) at 105 K. There are two channels-adsorbed oxygenates (1) on the left and hydrocarbon fragments (2) on the right. At the extreme left, the temperatures indicate where the species listed form and decompose. On the extreme right are qualitative TPD spectra for H,, CO,,and CO.

I

0

1000

2000

3000

Electron Energy Loss Icn-ll

Figure 2. HREEL spectra of various ketene exposures on Ru(001) at 105 K: (a) clean surface; (b) 0.25 langmuir; (c) 0.75 langmuir; (d) 1.5 langmuirs; (e) 3.0 langmuirs; (f) 6.0 langmuirs.

surface spectrum (Figure 2a) obtained over the same time period as Figure 2b shows that about one-third as much accumulates by adsorption from the background. Additional losses in Figure 2b are at 2945-2870, 1295, 1065, 890, and 590 cm-’. The HREEL spectra change in a complex manner with increasing ketene exposure. At intermediate coverages (Figure 2d) features appear at 3145, 1655, 1105, 900, and 585 cm-I. A broad

Surface Chemistry of Ketene on Ru(001). 1

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I

R" +

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3 c

. 108

I

0

25

50

75

100

113

125

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Figure 3. (a) +SSIM spectrum (1 keV, 4.5 nA/cm2) of 3 langmuirs of ketene on Ru(001) at 105 K, (b) same as (a) except with a different rf generator; (c) +SSIM spectrum (1 keV, 4.5 nA/cm2) after heating (a) to 200 K; (d) same as (c) but with the rf generator of (b).

band develops in the region 1200-1400 cm-' with peaks assigned at 1260 and 1365 cm-'. The growth of the 585-cm-' loss is particularly noteworthy (Figure 2c,d). Upon deuteriation (not shown), the losses at 1260, 1105, and 585 cm-' (Figure 2c,d) do not shift significantly. The first layer becomes saturated and multilayers form for ketene exposures between 3 and 6 langmuirs (paper 2). These exposures produce identical HREEL spectra. Although multilayers of ketene form at 105 K, they desorb during tuning of the spectrometer and are not observed by HREELS. Surface disordering at a 6-langmuir exposure decreases the elastic peak count rate by an order of magnitude from the 1.5-langmuir ketene spectrum (Figure 2d). The dominant losses (besides C O losses at 1970 and 410 cm-l) are now at 2930,1380,1080,930, and 640 cm-I with smaller bands at 3120 and 1755 cm-' (Figure 2f). Figure 3 shows the SSIMS (1 keV, 4.5 nA/cm2) of 3 langmuirs of ketene on Ru(001) before and after heating from 105 to 200 K. The major ions in the low mass region are m / e 1 (H'), 14 (CH,+), 15 (CH,+) and 43 ((CH,CO)H+) (Figure 3a). The m / e 1 signal while demonstrating the presence of hydrogen at the surface does not necessarily indicate H bonded to Ru(001). The ions in the range m / e 24-30 include C2 species mixed with Al+ ( m l e 27). No ions were detected between m / e 45 and 96. The ions in the 96-104 range are from the Ru substrate. The group of ions between m / e 110 and 118 are assignable to RuCH2+. To examine ions with m / e >120 (Figure 3b), a different rf generator (with poorer resolution) was used. Sets of ions above m / e 120 are attributable to RuCO' ( m / e 124-132) and Ru(CH2CO)+ ( m l e 138-146). The assignments of the ions containing H were verified by SSIMS of deuteriated ketene (CD,CO). The SSIM spectrum changes significantly after heating a 3langmuir dose to 200 K (Figure 3c,d). The CH3+/CH2+intensity ratio increases, the distribution of ions within the m / e 110-120 (RuCH,+) region changes and the overall intensity of this region drops, the intensity in the 124-132 range (RuCO') increases, and the intensity in the 138-146 range (RuCH,CO+) drops. Some of these intensity changes, especially the drop in CH2CO+intensity, are the result of desorption of ketene multilayers at 125 K (paper 2). Unlike the HREELS measurements, multilayer ketene can be retained long enough to get a SSIM spectrum. The SSIMS ion yields at 105 K for m l e 14, 15,42, and 43 are plotted in Figure 4 as a function of ketene exposure. The dominant ion at 0.25 langmuir is m / e 14 (CH,'). The ions associated with molecular ketene ( m / e 42 and 43) are the smallest. Above 0.50 langmuir the ion yields of m / e 15 and 43 increase abruptly while those of m l e 14 and 42 increase slowly. 3.3. HREELS Annealing Set. The HREELS annealing set for 6 langmuirs of ketene on Ru(001) between 105 and 300 K is shown in Figure 5. Spectra were obtained after heating to the

Figure 4. +SSIMS ion yields for m / e 14, 15, 42, and 43 from various exposures of ketene on Ru(001) at 105 K.

& 1975

CH2CO/RU (001)

1275,

x 1000250 K

1135

930

x 400

2930

IC)

x 400

105 K

x 100

I

(a1

6 0

1000 2000 3000 E l e c t r o n Energy Loss W11

Figure 5. HREEL spectra of 6 langmuirs of ketene on Ru(001) at 105 K (a) and after heating to (b) 200 and (c) 250 K. Spectra b and c were obtained after recooling to 105 K.

desired temperature and cooling to 105 K. At 150 K (not shown) the spectrum of Figure 5a is retained. However, after annealing to 200 K (Figure 5b), there are significant changes of relative intensity and an %fold increase in the elastic peak count rate. After heating to 250 K (Figure 5c), the spectrum simplifies somewhat as features at 3120, 1755, 1080, 930, and 640 cm-' disappear (Figure Sa). The major features occur at 2930, 2755, 1365, 1275, 1135, 975, and 655 cm-' (aside from those of CO). With deuteriated ketene (CD2CO) (Figure 6b), the 1200-1400cm-I region is resolved into two modes at 1420 and 1260 cm-l. Off-specular measurements indicate that these two losses are predominantly dipole scattered. When C H 2 C 0 is used, a loss at 1370 cm-' is resolved in off-specular measurements. The deuteriated analogue is a t 1050 cm-' (Figure 6b). The HREELS annealing set of Figure 5 is continued in Figure 7. The 300 K spectrum (Figure 7a) is very much like the 250 K spectrum (Figure 5c). When the sample is annealed to 350 K (Figure 7b), there are significant changes accompanied by the first product desorption state in TPD (Figure 1) and by changes in SSIMS (paper 2). The losses at 1355 and 1275 cm-l disappear, leaving a clearly resolved feature at 1425 cm-I. The 960- and 655-cm-I features attenuate while the intensity of atop C O in-

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0

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1000

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E l e c t r o n Energy Loss ( c n - l l

Figure 6. HREEL spectra of 6 langmuirs of ketene (a) and 6 langmuirs of deuteriated ketene (CD,CO) (b) on Ru(001) after heating from 105 to 250 K and cooling to 105 K.

Figure 8. HREEL spectra of 6 langmuirs of ketene (a) and 6 langmuirs of deuteriated ketene (CD2CO) (b) on Ru(001) after heating from 105 to 400 K and recooling to 105 K.

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Figure 7. HREEL spectra of 6 langmuirs of ketene on Ru(001) at 105 K heated to: (a) 300, (b) 350, (c) 400, and (e) 465 K. Spectrum d is the subtraction of (c) from (b). Spectra were obtained after recooling to 105 K.

E l e c t r o n Energy Loss I c i l I Figure 9. HREEL spectra of 3 langmuirs of ketene on 5 langmuirs of preadsorbed H2 on Ru(001) at 105 K (a) and after heating to (b) 250 K and (c) 320 K. Spectra b and c were obtained after cooling to 105 K.

creases and a bridged CO loss appears at 1800 cm-I. Only subtle changes take place between the 350 and 400 K HREEL spectra (Figure 7b,c) even though the largest H2 desorption state occurs by heating the sample above 380 K (Figure 1). Two notable changes are the disappearance of the 1800-cm-I loss (Figure 7b) and the appearance of the 780-cm-' loss (Figure 7c). Subtraction of Figure 7c (400 K) from Figure 7b (350 K), by normalization of the 1425-cm-I loss intensities, reveals two weak losses at 1345 and 1110 em-' (Figure 7d). This difference spectrum shows that heating to 400 K removes intensity from these regions. The v(CH) feature at 2915 cm-I shifts to 2995 em-'.

The losses remaining after heating ketene to 400 K (Figure 7c) are compared with those of deuteriated ketene heated to 400 K in Figure 8. Losses at 2995, 2925, 1340 (off-specular), 985, and 795 cm-I shift with deuteriation while those at 1425 and 675 cm-' do not. Upon annealing to 465 K (Figure 7e), the features at 1425 and 675 cm-l disappear completely. These changes accompany the desorption of H2 and C 0 2 . The CO features are severely attenuated. The residual intensity at 2000 and 455 cm-' is mainly from background adsorption. The remaining losses (Figure 7e) are at 3000, 1175, and 785 cm-'.


Surface Chemistry of Ketene on Ru(001). 1 CH$O/RU (001)

TABLE I: HREELS Frequencies (in cm-') of Adsorbed Methylenes' CH2/CH2C0/

425

Ru(O0 1) b

780 dosed a t 400 K

lbl

va(CH2) u,(CHJ 6(CHz) w(CH2) r(CHz) p(CH2) U,(MC) u,(MC)

2945 2870 1295 1065

(2205) (2150) (1120) (915)

890

(710)

590e

(540)'

CH2/CH2N2/ Ru(001)' 2965 1165 785

CH2/CH$O/ Fe(1 lo)* 2970 1430 1020 930 790 650

stretch; 6, deformation; w , wag; T , twist; p , rock. bThis work. Frequencies in parentheses are for CD2. cReference23. "Reference 8. 'This loss may result from the r ( C C 0 ) mode of $(C,C,O) ketene although none of its other losses are observed. 'u,

0

lo00

2000

3000

Electron Energy Loss Ica-ll Figure 10. HREEL spectra of 30 langmuirs of ketene dosed on Ru(001). Dosing temperatures were (a) 350 and (b) 400 K. Spectra were obtained after recooling to 105 K. See text for dosing procedure.

3.4. Coadsorption with H2. The adsorption of 3 langmuirs of ketene with 5 langmuirs of preadsorbed H2at 105 K on Ru(001) yields the HREEL spectrum of Figure 9a. C O losses from ketene decomposition are at 1960 and 435 cm-'. Additional losses appear at 2940, 1795, 1375, 1105, 920, 720, and 615 cm-l. The features of Figure 9a resemble those of 1.5 langmuirs of ketene on clean Ru(001) (Figure 2d) with the exception of the shape of the band around 1300 cm-l and the intensity at 1755 cm-'. The 1795-cm-I loss is 40 cm-I higher in frequency than the 1755-cm-l loss of Figure 2f and is also more intense. After heating to 250 K (Figure 9b), the surface becomes ordered and there are changes in the HREELS peaks, the most notable of which is the attenuation of the feature at 1795 cm-'. Some intensity is also lost in the region 700-1 100 cm-I. The major features are at 2925, 1370, 1275 (shoulder), 1125,975, 795, and 655 cm-'. A weak 1780-cm-I band remains. With the exception of the small 1125- and 795-cm-I losses, the spectrum of Figure 9b is very similar to that of 6 langmuirs of ketene heated to 250 K (Figure 6a). After heating to 320 K (Figure 9c), CO losses at 2000 and 450 c m - I double in intensity. Losses at 2910, 1420, and 655 cm-I from another species resemble those of 6 langmuirs of ketene heated to 400 K (Figure 8). 3.5. Elevated Dosing Temperatures. Dosing at elevated surface temperatures produces a dynamic balance among adsorption, decomposition, reaction, and desorption. We expect, therefore, a different surface composition than that produced by vacuum annealing after a low-temperature dose. The results of dosing ketene with the substrate at 350 and 400 K are shown in Figure 10. The temperature choices were based on CO desorption given the importance of CO-guided reactions.2z The lower temperature allows CO to accumulate while at 400 K C O desorbs during the dose. We anticipate that different hydrocarbon fragments may accumulate at these two temperatures. While dosing at 350 K, the only gaseous reaction product was H2, while at 400 K both H2 and C O were detected. In neither case were hydrocarbons detected. The surface became saturated at both 350 and 400 K with a 30-langmuir dose. The ketene valve was then closed and the sample was cooled to 105 K for the HREELS measurement. (22) Akhter, S.;White, J. M. Surf. Sci. 1986, 180, 19.

The major loss features after dosing about 30 langmuirs of ketene at 350 K on Ru(OO1) are at 2010 and 430 cm-I (atop CO) and at 2955, 1355, 1135,970, and 780 cm-' (Figure loa). Bridged C O is also observed at 1795 cm-'. Dosing 30 langmuirs of ketene at 400 K on Ru(001) results in the HREEL spectrum of Figure lob. The dominant losses are at 2995, 1140, and 780 cm-'. A small amount of C O (1945 and 425 cm-') and weaker losses at 2870, 1380, and 1310 cm-' are also observed. 4. Discussion

The adsorption and subsequent decomposition of ketene on Ru(001) produces a complex variety of adsorbate species. This is shown by the sequential appearance with increasing coverage and temperature of different loss features in HREELS and is supported by SSIMS. We identify these species below. 4.1. Ketene Adsorption at 105 K . Ketene chemisorption on Ru(001) at 105 K follows two routes, dissociative and molecular. Dissociation is signaled by the v(C0) and v(RuC) modes of atop C O at 1995 and 440 cm-I (Figure 2b). Some, but certainly not all, of this intensity may arise from background adsorption. During the time period of Figure 2b, no more than a third of the observed intensity accumulates from background and ketene dosing does not significantly increase the background C O pressure. We suggest that CH, is the other C H 2 C 0 decomposition product and is responsible for the losses at 2945-2870, 1295, 1065, and 890 cm-I in Figure 2b. These losses are assigned in Table I and are compared with those of CH2/Ru(O01) obtained by Hill et al.23from CH2Nzdecomposition and of CH2/Fe(l 10) obtained by McBreen et aL8 from ketene and CH2NZdecomposition. Hill et aLZ3did not observe a CHz scissors mode, but we assign the 1295-cm-' loss to it even though it is over 100 cm-' red-shifted from typical values. The 1430-cm-' assignment by McBreen et aL8 is more typical. A H-C-H- -Ru interaction (1) can explain

liiy.

H - CH2

1

the low 6(CH2) scissors frequency and the broad v(CH,) stretch (2870-2950 cm-I). Consistent with this, we note that Demuth and I b a ~ have h ~ ~previously assigned a 1300-cm-' loss to the 6(CH2) of a H-C-H--Ni species obtained from the hydrogenation of CH. At least two types of molecularly adsorbed ketene are observed after dosing at 105 K. Their loss features grow in more or less sequentially with increasing ketene exposure. The first type is formed at low exposures and is characterized by losses at 1260, (23) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W. H. J . Am. Chem. SOC.1986, 108, 3554. (24) Demuth, J. E.; Ibach, H. Surf. Sci. 1978, 78, L238.

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TABLE II: Comparison of the HREELS Frequencies (in cm-') of $((C,C) Ketene on Ru(001), with q2(C,C) Ketene on P t ( l l l ) , Ketene on Fe(llO), and di-a C2H4 on Ru(001)" V2(C,C) CHzCO/Ru(OOI)* 105 K %(CHZ) VACHZ)

3120 2930 1755

V2(CC) CH,CO/Pt( 111)' 100 K

(2330) (2190) (1775)

CH2CO/Fe(l I O ) d 120 K

2954 2875 1717 1347 1150 1040 937

(2229) (2135) (1717) (1040 os) (977) (835 os) (827)

654 48 1 457 307

(512 os) (433) (299)

3090 2990 1980. 1690? 1420 970 880 1140 720 600 370 510

di-a C2H,/Ru(001)' 120 K 2940 1415 1110 940 870 440

Frequencies in parentheses are for CD2C0. *This work. CReference 4. dReference 8. eReference 25.

1105, and 585 cm-' (Figure 2c,d). These do not shift with deuteriation and are assigned to the CO stretch, the CC stretch, and the out-of-plane CCO bend of $(C,C,O) ketene (2). The other

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@+#) TOP

W 2

$(C,C,O) C H 2 C 0 losses at 2935, 1365, and 900 cm-I are assigned to the v(CH2), 6(CH2), and p(CH2) modes, all of which shift with deuteriation. The wagging mode w(CD2) is resolved at 960 cm-l with CD2C0. The dominant C H losses at 2935 and 1365 cm-' are sp3-like, consistent with the $(C,C,O) form. The second type of chemisorbed ketene is observed at exposures above 1.5 langmuirs (Figure 2e,f). The losses of Figure 2f are assigned to v2(C,C) ketene (3). They are compared in Table I1

T2(C,C)C H 2 C 0 3

and with those of v2(C,C) ketene/Pt( 11 1),4 ketene/Fe( 1 di-a-bonded C2H4/R~(001).25The v(C0) loss at 1755 cm-l is key to the assignment and is in agreement with the Pt( 111) work.4 Several v2(C,C) ketene complexes are known which also exhibit C O stretching frequencies between 1700 and 1800 cm-1.49%12We include C2H4 on Ru(001) in Table I1 because of the similar bonding of the methylene portion of the molecule. v3(C,C,0) ketene, CH2, and CO are still present on the surface in Figure 2e,f although the losses of the first two are obscured by s2(C,C) ketene. The 1260-cm-*stretch of $(C,C,O) ketene is resolved for 6 langmuirs of C D 2 C 0 (not shown) indicating that v3 is not converted into sz a t higher coverages. The lack of significant changes in the HREELS upon annealing to 150 K indicates that multilayer ketene does not contribute (25) Henderson, M. A.; Mitchell, G. E.; White, J. M., Surf. Sci., in press.

significantlyto the spectrum in Figure 5a. As noted earlier, most of its desorbs during the tuning of the HREELS spectrometer. SSIMS ion yield changes as a function of coverage (Figure 4) support the ketene structures suggested by HREELS. The dominant ion below 0.5 langmuir of ketene is CH2+,consistent with the HREELS assignment of C H 2 and CO (Figure 2b). Above 0.5 langmuir of ketene, ions associated with molecular ketene dominate. Figure 4, however, makes clear the difficulty of relating SSIMS ions to surface structures. The high (CH2CO)H+and CH3+intensities do not necessarily reflect a high concentration of CH3-containing surface species, because protonation and other ion-molecule chemistry can occur during the SSIMS event. Nonetheless, the character of Figure 4, particularly at low ketene exposures, is helpful in identifying regimes where different species appear and/or different SSIMS processes occur. Coupled with TPD (paper 2) and HREELS, these data are quite useful. The SSIMS data of Figure 3 also support dissociative ketene adsorption on Ru(001) at 105 K. The distribution of ions in the m / e 110-1 18 region is almost entirely given by adding CHI (14) to the distribution of Ru isotopes between m / e 96 and 104. These ions may not be due entirely to CH2 from ketene decomposition since molecular ketene could contribute. It is interesting, however, that the dominant C1 ion is CH3+ while the dominant RuC, ion is RuCH2+. The SSIMS signals as a function of coverage (Figure 4) suggest that the CH3+is from SSIMS protonation of adsorbed ketene and the CH2+is from adsorbed CHI. Although a m / e 15 ion signal is usually suggestive of CH3 groups,26 there is not evidence from Figure 2 that hydrogenation of ketene has occurred at 105 K. The two other sets of ions above m / e 120 (Figure 3) are attributable to RuCO' ( m / e 124-132) and Ru(CH2CO)+ ( m / e 138-146). There is very little contribution from Ru(CH3CO)+in this latter set, a fact which parallels the RuC, ion behavior. Similar behavior for the CH, and CH,CO ions was noted for ketene on Pt(l1 l).' The relative yields observed here differ significantly from those for ketene adsorbed at 100 K on multilayer Cu/Ru(001) where the major ion is C U ( C H ~ C O ) + . ~ ' There are differences in the structural chemistry of ketene on Ru(001) and that of other systems. On Ru(001), our results show that the adsorption state varies significantly with exposure from dissociative (CH2 + CO) to molecular with $(C,C,O) and T~(C,C) bonding. At about 2 langmuirs the Ru(001) surface is crowded enough that sufficient sites are no longer available to form additional v3(C,C,0) ketene; instead, q2(C,C) ketene forms, which requires fewer sites. This finding suggests a reinterpretation of the results of McBreen et aL8 from ketene on Fe( 110). The 1980-cm-' loss is more consistent with the v(C0) of atop C 0 2 * resulting from dissociative ketene adsorption rather than from (26) Ogle, K. M.;White, J. M. Swf~Sci. 1986, 165, 234. (27) Henderson, M. A,; Zhou, Y.;White, J. M.; Mims, C. A,, work in

progress. (28) Erly, W. J . Vac. Sci. Technol. 1981, 18, 472.

Surface Chemistry of Ketene on Ru(OO1). 1


TABLE 111: Comparison of the HREELS Frequencies (in cm-') for n3(C,0) C H E H O on Ru(001) and s2(C,0) (CH1)FO on Ru(001)' a2(C,0) CH3CHO/ Ru(OO1)b up(CH3) v,(CH~) V(CH) Sa(CH3) 6,(CH3) V(C0) V(CC) P(CH3) NCH) y(CC0) w(CC0)

3035 2930 2755

OS

1365 1275 1135 975 795 os 655 575 os

(2230 OS) (2200) (nr) (1050) (1 260) (1095) (850) (nr) (620) (nr)

TABLE I V HREELS Frequencies (in cm-') of ?*(C,O) CHBCOon Ru(001) and of q2(0,O)CHICOO on C U ( ~ O O ) ~

.12(c,o) (CH3)2CO/

.1 (0IO ) CH3COOb

?2(C,0) CHpCO

Ru(OO1)C 2970

(2220)

1370 1280

(1070) (1280)

990

(820)

670

(620)

T*

va(CH3) v,(CH3) va(Co0) VACOO) V(C0) MCH,) 6,(CH3) V(W p(CH3) r(cco) S(OC0) ?r(CCO)

2925, nrc

(nf)

1425

(1420)

1340 OS 1160 985

(1070) (nr) (820)

675

(645)

3025 1435

(2218) (1 602) (1413)

1041

(1061)

677

(648)

Tz

"nr, not resolved; os, off specular. Frequencies in parentheses are for deuteriated species. bThis work. Reference 29.

molecular ketene. The loss they observed at 1690 cm-l is probably the v ( C 0 ) of q2(C,C) ketene. Thus, unless adsorption of background C O is responsible for all of the 1980 cm-' feature, both dissociative and molecular adsorption take place on Fe( 110). Consistent with dissociative adsorption, several known ketene complexes undergo facile C-C bond cleavage to C O and carbe n e ~ . * ~No - ' ~variation in ketene chemistry or bonding structure is observed on Pt( 11 1) as a function of exposure at 100 K where only q2(C,C) ketene is formed. Changes in the distribution of species formed during adsorption on Ru(001) at 105 K are important because they control the distribution of surface products realized upon annealing (Figure 1). This topic is discussed in more detail in paper 2. 4.2. Annealing to T I 250 K . As noted above, most of any multilayer concentration desorbs before the HREELS data is taken. Changes in relative intensity of SSIMS signals do accompany multilayer desorption at 125 K. Significant surface chemistry does occur during heating from 105 to 200 K. At 200 K the m / e 108-1 19 ions cannot be assigned to a single species. While a detailed assignment cannot be made, the intensity at m / e 108 indicates RuC+ and that at m / e 119 suggests RuCH3+. The increased CH3+/CH2+ratio is consistent with hydrogenation of ketene to CH3-containing species such as methyl, acetyl, ethylidyne, and acetaldehyde, which are discussed further below. Additional ketene decomposition in this temperature regime is indicated by the increased intensity in the RuCO+ region in SIMS and by additional intensity for atop C O in HREELS. The drop in the Ru(CH2CO)+ region in SSIMS, a significant part of which occurs after multilayer desorption (paper 2), is also consistent with ketene reactions. Hydrogenation of ketene to acetyl and/or acetaldehyde should also lead to ions in the m / e 138-150 range, but the yields are very small. After annealing 6 langmuirs of ketene to 250 K, there are multiple species on the surface. These are indicated in the HREEL spectra and separated by the sequential loss of their features in the HREELS as they decompose to produce hydrogen. Kinetic information (paper 2) also supports these assignments. We discuss their identification below. 4.3. q2(C,0)CH3CH0. Table I11 lists the HREELS features of ketene heated to 250 K (Figure 6) which are assigned to q2(C,0) acetaldehyde (CH3CHO) (4). These features are lost during the first H2desorption state at 325 K. Table I11 also shows

Tt2 ( C , O ) C H 3 C H 0

'nr, not resolved; os, off specular. Frequencies in parentheses are for deuteriated species. bReference 46. cObstructed by the v(CH)/v(CD) modes of CH/CD.

that these q2(C,0) CH3CH0assignments compare well with those obtained by Avery and co-workers for q2(C,0) acetone ((CH3)2CO) on R U ( O O ~ ) .Both ~ ~ q2(C,0)C H 3 C H 0 and q2(C,0) (CH3)2C0exhibit decreases in the u(C0) frequency of about 500 cm-I from their solid-phase values. The CO stretches of the adsorbed species are indicative of single bonds. The CH3stretching and bending frequencies are also similar for q2(C,0) C H 3 C H 0 and (CH3)2C0. The u(C0) assignment for q2(C,0) C H 3 C H 0 in Table I11 is also in agreement with a weak mode at 1240 cm-' observed by McCabe et al. and ascribed to q2(C,0) CD3CD0 on Pt(S)[6( 11l)X(100)].30 The kinetics of acetalydehyde decomposition described in paper 2 also support the assignment of these HREEL spectra to adsorbed acetaldehyde. HREELS of low C H 3 C H 0 exposures on Ru(001) at 105 K yield spectra very similar to that assigned to q2(C,O)CH,CHO in Figure 6.31 4.4. q 2 ( C , 0 )C H 3 C 0and q2(0,0) CH3CO0. In this section, we discuss the losses measured from ketene heated to 400 K (Figure 8). HREELS (Table IV), TPD, and SSIMS support two CH,COO ( 6 ) . possibilities-q2(C,0) C H 3 C 0 (5) and q2(0,0)

FH3

0-6

'Q2 ( C , O ) C H 3 C 0

OR

m O=$

T12 ( 0 , O )C H 3 C O 0 6

5

We favor the first (acetyl) of these for reasons outlined below but cannot rule out the latter (acetate). The losses in Figure 8 assigned to qz(C,O) C H 3 C 0 disappear after heating above 440 K, indicating that decomposition of this species is responsible for the desorption of H2 and C 0 2 (Figure 1) and the sharp attenuation of the TPSSIMS m / e 15 and 43 ions at 440 K (paper 2). Bonding of the 0 to the surface lowers the C O stretch by over 200 cm-l from that of q'(C) C H 3 C 0 on (29) (a) Avery, N. R.; Anton, A. B.; Toby, B. H.; Weinberg, W. H. J . Electron Spectrosc. Relaf. Phenom. 1983, 29, 233. (b) Avery, N. R.; Weinberg, W. H.; Anton, A. B.; Toby, B. H. Phys. Rev. Left. 1983, 51, 682. (c) Avery, N. R. J . Vac. Sci. Technol., A 1985, 3, 1459. (d) Avery, N. R. Surf. Sei. 1983, 125, 771. (30) McCabe, R. W.; DiMaggio, C. L.; Madix, R. J. J . Phys. Chem. 1985, 89, 854.

4

(31) Henderson, M. A,; Zhou, Y.; White, J. M., manuscript in preparation.

4118


Henderson et al. TABLE V Comparison of the CO Stretching Frequencies (in em-') of CHKHO and H,CO SDecies on Ru(001)"

3' ( C )

CH3C0

7

X = CH3 1720 (1712)b 1275 (1260)' 1425 (CH3CHO) X = H (H2CO) 1720 (1690)d 980 (1020)d 1180 central carbon hybridization

P t ( l l 1 ) (7).4 We previously observed a loss at 1395 cm-I from deuteriated ketene on Pt(111) heated to 300 K which was assigned to the v(C0) of v2(C,0) CD3C0.4 Interference from v'(C) CD3C0, the dominant surface species, prevented assignment of other modes. There are abundant literature references of bridging v2(C,0) acety132-39and v2(C,0) acylwZ organometallic complexes possessing u(C0) modes between 1300 and 1550 cm-I. Additionally, Sexton and Avery4, and Friend et have observed vz(C,N) structures of acetonitrile (CH,CN) on P t ( l l 1 ) and Ni(l1 l ) , respectively. Particularly because COz is a reaction product associated with the decomposition at 440 K in Figure 1, we also considered the bidentate acetate species, v 2 ( 0 , 0 )CH3CO0. This species has been studied on Al( 111),45Cu( and Ni(100).47 There is a very good match of the vibrational modes, assigned by these authors to OCO bends and stretches, with the modes we observe in Figure 8 (Table IV). The following observations partially based on recent results of CH3COOH and CH,C(O)Cl on R u ( O O ~ ) ~argue * against a significant nontransient accumulation of adsorbed acetate. (1) CH3COOH decomposition on Ru(001) produces C 0 2 and H2 TPD peaks at 440 K and HREELS losses at 400 K very similar to those of Figure 8. However, significant quantities of reaction-limited C O desorption (from C 0) above 550 K in TPD of CH3COOH and the evolution of a strong u(Ru-0) loss at 500 cm-l between 300 and 400 K suggest that C-0 bond cleavage of C H 3 C O 0 results in C H 3 C 0 on Ru(001). (2) The CH, stretching frequencies measured for a ~ e t a t e ~are ' , ~all ~ at 3000 cm-l or above. While we find a mode in this region, there is also a mode at a lower frequency (2925 cm-') in our work (Figures 7c, 8a, and 9c) which we prefer to assign to the sp3-hybridized carbon in v2(C,0) CH,CO, leaving the 3000-cm-' band for carbons with lower hybridization, for example, CH and CCH. (3) If C H 3 C O 0 were present in greater than C, symmetry, the asymmetric OCO stretch would be absent in the specular direction

+

sp2

SP3

(1420)' (1160)d

sp2 to sp'

O s , solid. Frequencies in parentheses are for deuteriated species. bReference 30. 'This work. dReference 49.

but present in the off-specular direction. In preliminary work involving acetic acid adsorption on Ru(001), we do find a mode at 1610 cm-I under conditions where acetate is expected. For ketene, we made a very careful search in the off-specular direction for a mode around 1600 cm-I and found none. (4) Acetyl chloride (CH3C(0)C1) on Ru(001) yields Hz and COz TPD peaks and HREELS losses similar to those we have assigned to CH,CO from ketene. This evidence is, admittedly, circumstantial and points to the desirability of a thorough study of acetic acid and acetyl chlorine adsorption on Ru(001). At present, however, we proceed on the basis that the weight of evidence favors v2(C,0) CH,CO. Transient populations of acetate may form and lead to CO,. This is discussed further in paper 2. An interesting comparison exists between the C O stretching frequencies of the structures resulting from ketene hydrogenation on Ru(001) and those of v2(C,0) H 2 C 0 and v2(C,0) HCO observed by Anton and c o - w ~ r k e r for s ~ ~formaldehyde (H2CO) on Ru(001). Table V compares the u(C0) frequencies for the series: C H , C H 0 ( ~ o l i d ) ~ ~ - v ~ ( CCH3CHO-v2(C,0) ,0) C H 3 C 0 with those of H2CO(solid)-v2(C,0) H2CO-v2(C,0) HCO. The u(C0) frequencies for the adsorbed HzCO species are considerably less than those for C H 3 C H 0 . The CH,CHO species are also more stable, decomposing above 300 K while the HzCO species decompose below 300 K. Although the u(C0) frequencies for the two series differ significantly, they follow the same trend from XHCO(s) to v2(C,0) XHCO to vz(C,O) XCO (X = CH3 or H). The large change in the v(C0) from XHCO(s) to vz(C,O) XHCO reflects the lower C O bond order in the latter. 4.5. Ethylidyne. The losses in Figure 7d (obtained by subtraction of Figure 7c from Figure 7b) at 1345 and 11 10 cm-' are assigned to the b(CHJ and u(CC) modes of ethylidyne (8), the

y 3 (32) Morrison, E. D.; Bassner, S. L.; Geoffroy, G. L. Organometallics 1986, 5, 408. (33) Longato, B.; Norton, J. R.; Huffman, J. C.; Marsella, J. A,; Caulton, K. G. J . Am. Chem. SOC.1981, 103, 209. (34) Yu, Y.-F.; Gallucci, J.; Wojcicki, A. J . Am. Chem. SOC.1983, 105, 4826. (35),Butts, S . B.; Holt, E. M.; Strauss, S. H.; Alcock, N. W.; Stimson, R. E.; Shriver, D. F. J . Am. Chem. SOC.1979, 101, 5864. (36) Ferguson, G. S.; Wolczanski, P. T. J . Am. Chem. SOC.1986, 108, 8293. (37) Siinkel, K.; Schloter, K.; Beck, W.; Ackermann, K.; Schubert, U. J . Organomet. Chem. 1983, 241, 333. (38) Marsella, J. A.; Huffman, J. C.; Caulton, K. G.; Longato, B.; Norton, J. R. J . Am. Chem. SOC.1982, 104, 6360. (39) Wong, W. K.; Chiu, K. W.; Wilkinson, G.; Galas, A. M. R.; Thorton-Pett, M.; Hursthouse, M. B. J . Chem. SOC.,Dalton Trans. 1983, 1557. (40) Kampe, C. E.; Boag, N. M.; Kaesz, H. D. J . Am. Chem. SOC.1983, 105, 2896. Kiener, V.; Burnbury, D. St. P.; Frank, E.; Lindley, (41) Fischer, E. 0.; P. F.; Mills, 0. S. Chem. Commun. 1968, 1378. (42) Merlino, S.; Montagnoli, G.; Braca, G.: Sbrana, G. Znorg. Chim. Acta 1978, 27, 233. (43) Sexton, B. A.; Avery, N. R. Surf. Sci. 1983, 129, 21. (44) Friend, C. M.; Muetterties, E. L.; Gland, J. L. J . Phys. Chem. 1981, 85, 3256. (45) Chen, J. G . ;Crowell, J. E.; Yates, J. T., Jr. Surf. Sci. 1986, 172, 733. (46) Sexton, B. A. Chem. Phys. Lett. 1979, 65, 469. (47) Scharpes, E. W.; Benziger, J. B., submitted for publication in J . Phys. Chem.; private communication. (48) Henderson, M. A.; Zhou, Y.; White, J. M., work in progress

bt3-CCH3

8

decomposition of which is responsible for the 380 K H2 TPD state. These assignments are in good agreement with those of CCH3 from CzH, decomposition on Ru(001),23,50 from ketene dosed at 350 K (discussed below), and from the ethylidyne complex p3CH3CCo3(C0)9.51The appearance of a C,H species (2995 and 780 cm-l) from CCH, decompositionz5and the disappearance of bridged CO (1800 cm-I) (Figure 7b), which is associated with the stabilization of CCH3 above its normal decomposition temalso suggest that the perature on clean Ru(001) (350 K),25,52,53 (49) Anton, A. B.; Parmeter, J. E.; Weinberg, W. H. J . Am. Chem. SOC. 1985, 108, 1823; J . Am. Chem. SOC.1986, 108, 1823. (50) Barteau, M. A,; Broughton, J. Q,; Menzel, D. Appl. Surf. Sci. 1984, 19, 92. (51) Skinner, P.; Howard, M. W.; Oxton, I . A,; Kettle, S. F. A,; Powell, D. B.; Sheppard, N. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1203.


Surface Chemistry of Ketene on Ru(001). 1 losses of Figure 7d are due to CCH,. Bridged C O is not observed from HREELS of C O on clean R u ( O O ~ ) . ~ ~ The HREEL spectrum (Figure loa) after 30 langmuirs of ketene on Ru(001) at 350 K clearly shows the typical losses of ethylidyne as well as those of atop C O (2010 and 430 cm-I). As is the case with ketene adsorbed at 105 K and heated to 350 K, there is a loss at 1795 cm-I due to bridged CO. This loss disappears along with the ethylidyne features after decomposition of ethylidyne at 380 K (not shown). A loss at 780 cm-' in Figure 10a is the 6(CH) mode of either C H and/or CCH, which are additional products of CH,CO adsorption at 350 K and which persist beyond 380 K (see below). 4.6. C,H Species. We assign the losses at 3000 and 785 cm-I from ketene adsorbed at 105 K and annealed to 465 K (Figure 7e) to the v(CH) and 6(CH) modes of methylidyne (CH, s a ) .

Y W

M 3-

CH

9a SIDE

TOP

n

U

p4,'-Q2(C,C)

CCH

9b

This assignment is in agreement with results from C2H4 decomposition on Ru(001) by Hills et al.23and by Barteau et aLsoHills et al. suggest that C C H (9b) decomposes on Ru(001) at 380 K to C H and C. Obstruction by the losses of $(C,O) CH,CO does not permit the identification of the v(CC) mode of C C H below 440 K. The loss at 1175 cm-l (v(CC)) in Figure 7e (465 K) suggests that some C C H may also be present above 380 K. This loss is, however, about 100 cm-I red-shifted from that observed by Hills et aL2, at 1290 cm-l. There is evidence of methylidyne (785 cm-l) in addition to ethylidyne formation when ketene is adsorbed at 350 K on Ru(001). Ketene adsorbed with the surface at 400 K produces C H (2995 and 780 cm-I, Figure lob) but not CCH,. Additional losses at 2870, 1380, 1310, and 1140 cm-I are due to C H stretch, C C stretch, C O stretch, and/or CH, bending modes of one or more unidentified C,H,,O, species. TPD after ketene adsorption at 400 K (paper 2) yields a small CO, desorption state at 461 K indicating that an oxygenate is present in the HREEL spectra of Figure lob. The losses of Figure 10b do not match those of v2(C,0) C H 3 C 0 listed in Table IV. 4.7. Other (Unobserved) Species. No evidence of ketene dimers was found by HREELS or SSIMS in agreement with results from Pt(l1 l).' This species is expected to produce SSIMS ions in the m / e 70-85 range. If dimers form, they do so transiently, without accumulation of products in amounts sufficient for detection by either HREELS or SSIMS. In addition, no ions showing evidence of CHI coupling to form C, species larger than ethylidyne were seen in SSIMS. Although occasionally a weak loss at 1640 cm-' (Figure 6) appeared after heating ketene to 250 K, assignment (52) Greenlief, C . M.; Radloff, P. L.; Zhou, X.; White, J. M. Surf. Sci. 1987. 191. 93. (53) Hills, M. M.; Parmeter, J. E.; Weinberg, W. H. J . Am. Chem. Soc. 1986, 108, 7215. (54) Thomas, G . E.; Weinberg, W. H. J . Chem. Phys. 1979, 70, 1437.

of this loss to the u(C0) of vl(C) C H 3 C 0 is dubious because of the signal-to-noise level. 4.8. Coadsorption of Ketene with H2. The preadsorption of 5 langmuirs H, does not have a major effect on the bonding of ketene at 105 K. A significant amount of ketene is dissociatively adsorbed, as evidence by atop CO modes at 1960 and 435 cm-I. An intense loss at 1795 cm-I which is 40 cm-I higher than the v(C0) of v2(C,C) ketene is assigned to the v(C0) of bridged CO. The absence of any resolvable losses between 1600 and 1780 cm-l suggests that undissociated ketene is adsorbed predominantly as $(C,C,O). We assign the major losses in Figure 9a to v(CH,) (2940), 6(CH2) (1375), v(CC) (1105), p(CH,) (920), and S(CC0) (615). With the exception of the u(C0) mode, which is probably hidden by the broad 6(CH2) mode, there is agreement between these assignments and those of Figure 2d. The 850-cm-I loss (not labeled in Figure 9a) is unassigned. The 720-cm-I loss is probably from the u(Ru-H) of adsorbed H.s5 Although the ketene exposure in Figure 9 was 3 langmuirs, the amount of adsorbed ketene determined from CO TPD (paper 2) is equivalent to about a 0.5-langmuir exposure on the clean surface. This exposure on the clean surface produces the same adsorbate structures (dissociative as C O and CH2 and molecular as v3(C,C,O)) as with preadsorbed H2 although the amount of dissociation may be different. The combined effect of H and v3(C,C,O) ketene does result in a large amount of bridged CO, which is not observed in the absence of preadsorbed H (Figure 2) or for C O on the clean surface.s4 Although preadsorption of hydrogen does not significantly alter the initial adsorption, it does influence the relative amounts of the various surface intermediate produced upon heating. The dominant surface species after heating to 250 K is q2(C,0) C H 3 C H 0 (2925, 1370, 1275 (shoulder), 975, and 655 cm-') in agreement with TPD results (paper 2). Additionally, atop C O (1985 and 435 cm-I), bridged C O (1780 cm-I) and adsorbed H (1125 and 795 cm-1)55 are observed. After decomposition of 02(C,0) CH3CH0 (Figure 9c, 320 K) only atop CO and q2(C,0) C H 3 C 0 are present on the surface. The intensity of the 20OO-cm-' CO loss is doubled due to q2(C,0) CH,CHO decomposition. No ethylidyne is observed in agreement with TPD results (paper 2).

5. Summary The results of this work can be summarized as follows: (1) Ketene adsorption on Ru(001) at 105 K is both dissociative and molecular, with the former favored at low exposures. (2) Dissociation at 105 K forms CH2 and CO. (3) Molecular ketene adsorption below 1.5 langmuirs is predominantly as v3(C,C,0). Above 1.5 langmuirs, ketene adsorbs in the v2(C,C) form. Preadsorbed H2does not affect the bonding of ketene significantly. (4) Hydrogenation of molecular ketene, by heating above 200 K, results in v2(C,0) C H 3 C H 0 and q2(C,0) C H 3 C 0 . Preadsorption of H, favors the former species. (5) Above 200 K, CCH, forms, presumably by CH, coupling. (6) CCH, is formed when ketene is dosed at 350 K. At this temperature C O accumulates on the surface, blocking CCH, decomposition which occurs at 320 K on clean Ru(001). Dosing ketene at 400 K, above the threshold of C O desorption, results in CH, CCH, and an unidentified C,H,O, species. (7) There is no evidence for the formation of C, species larger than n = 2. Acknowledgment. J.M.W. gratefully acknowledges the support of the National Science Foundation under Grant CHE-8505413. We thank Ying Zhou for assistance in collecting some of the data presented here. Registry No. Ru, 7440-18-8; D2,7782-39-0; H2, 1333-74-0; CO, 630-08-0; CH2, 2465-56-7; CH3CH0, 75-07-0; CH3C0, 3170-69-2; ketene, 463-51-4; ethylidyne, 67624-57-1; methylidyne, 3315-37-5.

~

(55) Barteau, M. A.; Broughton, J. Q.;Menzel, D.Surf. Sci. 1983, 133, 443.

Surface Chemistry of Ketene on Ru(001). 1 ... - ACS Publications

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