J . Phys. Chem. 1984,88, 1996-2003
1996
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U S . Department of Energy under Contract No. DE-AC03-76SF00098. Helpful conversations with Lin Rongfu, R. J. Koestner, and M. A. Van Hove are gratefully acknowledged. B.E.K. acknowledges the support of the Miller Institute for Basic Research in Science in the form of a Miller Fellowship. Appendix The surface geometry and site symmetry of an adsorbed molecule can be determined through the use of the principles of group theory in conjunction with the metalsurface dipole selection rule operable in HREELS. The symmetry assignment is made by comparing the number, frequency, and intensity of the dipole-active modes observed with the correlation table of the point group for the gas-phase molecule. We describe here the symmetry determination of benzene adsorbed on Rh( 111) in the ~ ( 2 4 X3 4)rect structure. First one must determine which modes of adsorbed benzene are dipole active, Le., are due to dipole scattering. Two inelastic scattering mechanisms can be responsible for the observed electron energy loss peaks in HREELS: dipole scattering and impact scattering. Losses that are the result of dipole scattering satisfy a surface dipole selection rule and appear in a scattering lobe which is sharply peaked in the specular direction. Impact scattering produces a broad angular scattering distribution and the losses do not satisfy the above selection rule. The contributions of the two mechanisms can be distinguished by studies of the angular dependence of the losses in HREELS. The results in Figure 6 show that all of the observed loss peaks in Figure 2 are dipole active. The assignment in Table I of these peaks to vibrational modes derived from gas-phase benzene modes is made by using the isotope shift and the results of force field calculations for benzene comp1exes.l’ We now consider the correlation table for the symmetry point group of gas-phase benzene, D6h. By using the relationships between the representation of D6h and its subgroups, we can predict
which modes would be observed as dipole h i v e in HREELS upon reduction of the gas-phase benzene symmetry. Only those modes that belong to totally symmetric representations (Al, A’, or A) satisfy the surface dipole selection rule and are observed in dipole scattering. Gas-phase benzene vibrational frequencies are given in Table IV for those modes that have AI, A’, or A representation under the given symmetry at the surface. Six additional vibrational modes should also appear in HREELS, since the benzene molecules become fixed in space upon adsorption, removing the translational and rotational degrees of freedom. These new modes involve dipole moment changes perpendicular (Azusymmetry mode T,) and parallel (Elusymmetry degenerate modes T, and Ty) to the ring plane and may be dipole active. Only a few D6hsubgroup symmetries are considered in Table IV. The strong intensity of the dipole-active -yCH(A2Jmode and the weak intensity of the other dipole-active modes that involve dipole moment changes in the ring plane indicate that the molecular ring plane is parallel to the metal surface and the in-plane modes are effectively screened by the metal surface. This general orientation makes only a few symmetries possible, as indicated in Table IV. Symmetry groups lower than C3, can be ruled out, since only seven vibrational modes are observed to be dipole active. Clearly, the number and frequency of the observed dipole-active modes favor the C3”point groups. The 1130- and 1320-cm-I dipole-active modes lead to the conclusion that the adsorption site symmetry is C3,(ad). If one includes neighboring molecules in the ~ ( 2 4 3 X 4)rect structure, the site symmetry is C,. However, the observation of only seven dipole modes indicates that the effect of neighboring molecules is negligible. The appearance of the 1420-cm-I mode ( q 3El,,) , to be dipole active in Figure 6 is not explained by this symmetry assignment. Possibly, several unresolved impact scattering losses which have small differences in their angular dependences comprise this peak. The observation of two dipole-active uRhX modes instead of only one dipole active VRh-C mode is also not explained by this symmetry assignment. Registry No. C6H6,71-43-2; Rh, 7440-16-6; hydrogen, 1333-74-0; carbon, 7440-44-0.
Surface Coordination Chemistry of Ruthenlum. A Survey of Ruthenium(001) Surface Chemlstry K. L. Shanahan* and E. L. Muettertiest Department of Chemistry, University of California, Berkeley, California 94720 (Received: December 15, 1983)
The chemisorption behavior of a range of organic and inorganic molecules on the basal plane of ruthenium, Ru(001), was examined. The organic molecules included arenes, heteroaromatics, nitriles, and simple oxygen derivatives of hydrocarbons, such as acids, aldehydes, and ketones. This close-packed ruthenium surface proved to be exceedingly reactive: most of the molecules studied irreversibly chemisorbed at 25 OC. Only CO, PF3, CF3CN, HCN, and (CN), showed some degree of molecular thermal desorption after adsorption at 25 “C.
Introduction In an attempt to understand the coordination chemistry of metal surfaces, we have been studying the reactions of hydrocarbons and simple inorganic molecules with clean metal surfaces. This surface chemistry is known to be a sensitive function of surface topography and surface composition.’ In earlier studies, we have further documented this sensitivity to surface structure and composition for nickel group metal reactions with certain classes of organic molecules.* These studies are now being extended to the earlier transition metals to gain some measure of effects due to d level filling. We describe here our initial studies of iron group +Deceased January 12, 1984.
0022-3654/84/2088-1996$01.50/0
metals and specifically those of ruthenium. The early transition metals are more reactive than the late members of the series. Generally, the tendency toward dissociative (1) (a) Somorjai, G. A. “Chemistry in Two Dimensions: Surfaces”;Cornell University Press: Ithaca, 1981. (b) “The Nature of the Surface Chemical Bond”; Rhodin, T. N.; Ertl, G. Ed.; North-Holland: Amsterdam, 1978. (c) “Treatise on Solid State Chemistry”;Hannay, N. B., Ed.; Plenum Press: New York, 1976; Vol. 6 A and 6B. (2) (a) Friend, C. M.; Muetterties, E. L ; Gland, J. L. J . Phys. Chem. 1981, 85, 3256. (b) Friend, C. M.; Stein, J.; Muetterties, E. L. J . Am. Chem. SOC.1981, 103, 767. (c) Friend, C. M.; Muetterties, E. L. Ibid.1981, 103, 773. (d) Wexler, R. M.; Tsai, M.-C.; Friend, C. M.; Muetterties, E. L. Ibid. 1982, 104,2034. (e) Tsai, M.-C.; Muetterties, E. L. Ibid.1982, 104, 2534. (f) Gentle, T. M.; Muetterties, E. L J . Phys. Chem. 1983, 87, 2469.
0 1984 American Chemical Society
Surface Coordination Chemistry of Ruthenium chemisorption appears to increase as the d level filling is decreased.' This is notable even when comparing iron with nickel group metals. For example, C O chemisorption is molecular and fully reversible on all nickel group metals whereas this process is dissociative on iron.3 Ruthenium is less reactive than iron at least as evidenced by the fully molecular character of C O chemisorption; nevertheless, studiesk7 of the chemisorption of other classes of molecules on ruthenium do suggest a higher reactivity for this metal with respect to nickel group metals. We explore here this reactivity aspect using the close-packed Ru(001) surface with small inorganic molecules and with several classes of organic molecules, classes that had been examined in some detail for nickel group metals. Basically, our exploratory studies of ruthenium can be divided into five general areas: (a) a comparison of C O and PF3 chemisorption, (b) benzene and benzene derivatives, (3) heteroaromatic molecules, thiophene, pyridine, and furan, (d) organic nitriles (cyanides) and inorganic cyanides, and (e) a family of hydrocarbon derivatives that contain oxygen (phenomenologically related in that all decompose on Ru(001) to form carbon monoxide and hydrogen). Fairly complete data are available for these areas in nickel group metal chemistry thus allowing intercomparisons of metal chemistry.
(3) (a) Textor, M.; Gay, I. D.; Mason, R.; F. R. S. Proc. R. SOC.London, Ser. A 1977,356, 37. (b) Rhodin, T. N.; Brucker, C. F. Solid State Commun. 1977, 23, 275. (c) Brundle, C. R. I B M J . Res. Deuel. 1978, 22, 235. (d) Brodtn, G.; Gafner, G.; Bonzel, H. P. Appl. Phys. 1977, 13, 333. (4) Diatomic and triatomic molecules studied on Ru(001) have included hydrogen,8oxygen:, nitrogen,8b-'0carbon monoxide," nitric oxide,12nitrous oxide,I3 water," and hydrogen sulfide.15 (5) Miscellaneouspolyatomic inorganic molecules studied on Ru(001) are sulfur hexafluoride,16phosphorus trifluoride," and ammonia.18 (6) Saturated hydrocarbons that have been investigated on Ru(001) are ethane,I9 c y c l o p r ~ p a n e , 'cy~lohexane,l~~*~ ~~~ and ~yclooctane.'~ (7) Unsaturated hydrocarbons and hydrocaron derivatives have also been examined on Ru(001). These include benzene?3 diazomethane,22 and formic a ~ i d . 2 ~ (8) (a) Schwarz, J. A. Surf.Sei. 1979, 87, 525. (b) Danielson, L. R.; Dresser, M. J.; Donaldson, E. E.; Dickinson, J. T. Surf.Sci. 1978, 71, 599. (c) References contained in ref Sa and 8b. (9) (a) Praline, G.; Koel, B. E.; Lee, H.-I.; White, J. M. Appl. Surf Sci. 1980,5,296. (b) pahman, T. S.; Anton, A. B.; Avery, N. R.; Weinberg, W. H. to be published. (c) References contained in ref 9a and 9b. (10) Anton, A. B.; Avery, N. R.; Toby, B. H.; Weinberg, W. H. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 181, and references therein. (11) (a) Thomas, G. E.; Weinberg, W. H. J. Chem. Phys. 1979, 70, 954. (b) Brown, A.; Vickerman, J. C. Surf.Sei. 1982, 117, 154. (c) Williams, E. D.; Weinberg, W. H.; Sobrero, A. C. J. Chem. Phys. 1982, 76, 1150. (d) References contained in ref 1la-c. (12) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M. Surf.Sci. 1983, 125, 366, and references contained therein. (13) Shi, S.-K.; White, J. M. J. Chem. Phys. 1980, 73, 5889. (14) (a) Thiel, P. A.; Hoffmann, F. M.; Weinberg, W. H. Phys. Reu. Lett. 1982,49, 501. (b) Doering, D. L.; Madey, T. E. Surf.Sci. 1982, 123, 305. (c) References contained in ref 14a and 14b. (15) Kelemen, S. R.; Fischer, T. E. Surf.Sei. 1979, 87, 53. (16) Fisher, G. B.; Erikson, N. E.; Madey, T. E.; Yates, J. T., Jr. Surf. Sei. 1977, 65, 210. (17) Nitschkt, F.; Ertl, G.; Ktippers, J. J . Chem. Phys. 1981, 74, 5911. (18) (a) Danielson, L.R.; Dresser, M. J.; Donaldson, E. E.; Sandstrom, D. R. Surf.Sci. 1978, 71, 615. (b) Madey, T. E.; Yates, J. T., Jr. in "Proceedings of the Seventh International Vacuum Congress and Third International Conference on Solid Surfaces"; Vienna, 1977; p 1183. (c) References contained in ref 18a and 18b. (19) Madey, T. E.; Yates, J. T.; Jr. Surf.Sei. 1978, 76, 397. (20) Felter, T. E.; Hoffman, F. M.; Thiel, P. A.; Weinberg,, W. H. Surf. Sei. 1983, 130, 163, and references therein. (21) Hoffmann, F. M.; Felter, T. E.; Thiel, P. A.; Weinberg, W. H. Surf. Sci. 1983, 130, 173, and references therein. (22) George, P. M.; Avery, N. R.; Weinberg, W. H.; Tebbe, F. N. J . Am. Chem. SOC.1983, 105, 1393. (23) Kelemen, S.R.; Fischer, T. E. Surf. Sei. 1981, 102, 45. (24) Avery, N. R.; Weinberg, W. H.; Toby, B. H.; Anton, A. B. J . Electron Spectrosc. Relat. Phenom. 1983, 29, 233. (25) Avery, N. R.; Toby, B. H.; Anton, A. B.; Weinberg, W. H. Surf.Sci. 1982, 122, L574.
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 1997 Experimental Section Reagents. All chemicals were reagent grade. Commercial sources were Aldrich Chemical Co. (benzene, benzene-d,, toluene, toluene-d8, hexafluorobenzene, pivalonitrile, acrylonitrile, and acetaldehyde); Mallinckrodt (pyridine, acetonitrile, methanol, acetone, and acetic acid); Merck and Co. (CD,CN, pyridine-d,, I2C'*O, CD3C&, and CH3C6D5);Pfaltz & Bauer (CF3CN); Ozark-Mahoning (PF3); Matheson Gas Products (cyanogen and carbon monoxide); Fumico (HCN); Biorad Laboratories (13CO); Eastman Chemical Products (methyl acetate). Trimethylphosphine was prepared and purified as described previously.2c Apparatus and Procedure. The ultrahigh vacuum system consisted of a standard Varian chamber equipped with a Varian single-pass cylindrical mirror analyzer and glancing incidence electron gun for Auger electron spectroscopy, Varian four-grid low-energy electron diffraction optics and integral electron gun, and a UTI lOOC quadrupole mass spectrometer interfaced with a UTI programmable peak selector for thermal desorption spectroscopy. Evacuation was achieved with a 200 L s-l ion pump, a titanium sublimation pump, and a liquid-nitrogen-cooled cryoplate. During operation, the chamber pressure was in the low torr range with carbon monoxide, hydrogen, and argon the major background molecules. Two Ru(001) crystals were used interchangeably in these studies. These were obtained through Professor T. N . Rhodin from the Cornel1 Materials Science Center. The crystals were cleaned and polished by standard procedures. Weinberg et aL2, have shown that oxygen can be a substantial surface contaminant if the crystal is not annealed at high temperatures for sufficient time periods. In all our studies, the crystal was annealed at 1100 OC for a minimum of 3 min to remove oxygen. This treatment was judged to be sufficient to remove surface oxygen in that recombination of surface carbon, supplied by decomposed hydrocarbon molecules, and oxygen to form C O (desorption in the 200-350 "C range) was not observed. The crystal was heated by electron bombardment, and in thermal desorption studies, heating rates of 20 OC s-l were employed typically. Linear heating rates were achieved over the 25-400 O C range. With more extended heating ranges there was nonlinearity at the high-temperature end of the range. Temperatures were measured with a chromel-alumel thermocouple spot welded to the crystal and exposures were effected with small needle dosers. Adsorption was done either with the crystal at 25 or at -160 "C. Thermal desorption studies initiated at -160 "C were complicated by background problems (desorption from crystal supports) for some molecules. In these cases, an accurate assignment of low-temperature molecular desorption maxima from the crystal, Le., multilayer desorption, was not feasible. Measurement of carbon and sulfur contaminant levels by Auger electron spectroscopy was complicated by ruthenium transitions near the characteristic carbon and sulfur transitions. The ratio of the 152-eV peak-to-peak height to the ruthenium 231-eV peak-to-peak height was used as measure of sulfur. For estimates of carbon levels, two parameters were used: the peak height ratio of the 272-eV transition to the 231-eV transition and the ratio of the negative portion to the positive portion of the 272-eV transition. Low sensitivity to carbon in the Auger experiment further lowered the accuracy of the carbon determination at low levels of carbon coverage. Another problem was encountered with fluorine, namely, electron stimulated desorption: with Ru(001)-NCCF3, all fluorine was removed from the surface in 20 min with a 2000-eV electron gun and all molecular desorption from Ru(OO1)-NCCF, was suppressed by one full Auger scan. For these reasons, Auger spectra were recorded at the lowest practical beam current (estimated at 0.5 PA) using glancing incidence to maximize surface sensitivity. Decomposition of organic molecules on Ru(OO1) usually led to carbon formation on the surface. Diffusion of oxygen atoms ~~
~
(26) (a) Thomas, G. E.; Weinberg, W. H. J . Chem. Phys. 1978,69, 3611. (b) Thiel, P. A.; Weinberg, W. H.; Yates, J. T., Jr. Chem. Phys. Lett. 1979, 67, 403.
1998 The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
to the surface from the bulk is an established thermal p r o c e ~ s . ~ When such surfaces were heated rapidly to high temperatures, carbon monoxide formed and desorbed; the desorption peak was broad and was centered at 550-575 "C. Clean surfaces when flashed showed no CO or O2desorption peaks. Because of the possible high-temperature desorption of CO, the analysis of the thermal behavior of nitrogen-containing compounds was complicated because N 2 (same mass as CO) often desorbed in the 500-600 OC range. However, the combination of mass spectrometric analysis and Auger analysis (to monitor loss of nitrogen from the surface) removed the ambiguity as to whether N2or CO was desorbing in most of the systems studied. General procedures for thermal desorption and chemical displacement experiments have been described in earlier articlesS2 Figures provided in the text are on the same sensitivity scale, with the following exceptions: Figures 1, 13, and 14 are presented on a five times lower sensitivity scale and the C O desorption profiles in Figure 11 are on a 2.5 times lower sensitivity scale. All exposures are expressed in langmuirs (1 langmuir = 10" torr s) and are presented for relative comparison only. Multilayer desorptions are not presented due to the aforementioned potential desorption from the crystal supports. Results and Discussion Carbon Monoxide and Phosphorus Trifluoride. As a calibration of our ruthenium studies with respect to earlier Ru(001) investigations and to other transition metals, the chemisorption of CO and of PF3on Ru(001) was reexamined. In addition, the effect of surface carbon contaminants on the Ru(OO1)XO thermal desorption process was established to enable an interpreation of the data relating to the decomposition process (which led to C O formation) of oxygen derivatives of hydrocarbons on Ru(O0 1). Our results with CO were in agreement with those of earlier workers." There were two thermal desorption maxima, one at 135-140 OC and the second at 185-195 "C. Carbon and oxygen atoms were not detectably present on the surface after the desorption experiment. These results are consistent with a fully reversible chemisorption process, a pervasive quality of CO chemisorption on nickel group metals. To further distinguish between associative and reversible dissociative chemisorption processes, the thermal desorption of C O from Ru(OO~)-'~C'~O12C'80was examined. There was no detectable increase in 13C1s0 or l2CI6Oamong the carbon monoxide molecules desorbed from this surface. Hence, within experimental detection limits, the carbon monoxide chemisorption process on Ru(001) is fully associative (molecular) in character. The two thermal desorption maxima for Ru(OOl)-CO may reflect the presence of two stereochemically differentiable chemisorption states.27 To assess this possibility, the high-temperature "state" was populated with l2CI6Oand the low-temperature "state" with I3CL6Oby sequentially exposing the Ru(001) surface to l2Cl6Oand then I3Cl6O. In the subsequent thermal desorption experiment, the ratios of 12C'60to 13C160in the two thermal desorption maxima were found to be identical within experimental error. The permissable conclusion from this experiment is either that there is only one chemisorption state or that there are two or more states and the chemisorbed CO molecules are mobile and rapidly undergo surface diffusion between states. Phosphorus trifluoride is formally analogous to carbon monoxide in its bonding capability: both molecules are poor o donors and strong ?r acceptors. Ertl and co-workers17 concluded from a general study of PF3 chemisorption that M-CO and M-PF3 bond energies are comparable with the latter typicaly being somewhat larger for a given metal. Specifically, they studied Ru(001)-PF3 and suggested that PF3 is more strongly bonded to ruthenium than is CO because the PF3 thermal desorption maximum was 245 "C compared to a high-temperature maximum of 190 "C for CO at saturation coverages.28 We confirmed the PF, desorption maxima ~~
~
(27) For a more complete discussion of the use of isotopically labeled
compounds to detect stereochemically differentiable chemisorption states, see ref 2.
Shanahan and Muetterties of -245 OC but also detected a competing thermal decomposition process. Auger experiments (phosphorus) after the desorption experiments indicated that extent of decomposition was about 5%. A ( 4 3 X 43)R3Oo low-energy electron diffraction pattern was observed for Ru(001)-PF3 at 25 "C; which is the same pattern observed for Ru(OOl)-CO at moderate coverages. The closepacked (1 11) planes of nickel and palladium both form ordered (2 X 2) states with PF3.17 There are several general and comparative comments that can be made concerning CO and PF, metal chemistry: (a) the metal surface bond to CO energetically is similar to (or slightly less than that of) the surface bond to PF3for a given metal, (b) there is not a consistent and large energy difference in M-CO (or M-PF,) bonds between iron group metals and nickel group metals (for comparisons within a transition series), and (c) the reactivity of the iron group metals is higher than that of the nickel group for both C O and PF,. Chemical displacement of CO from Ru(OO1)-CO did not occur with displacing agents like PF3 and P(CH3)3although CO was from Ni( 111)-CO by exposure to P(CH3)3. Basically, the PF3 results should have been anticipated for the following reasons: (i) PF3 and CO are similar in an electronic context, (ii) both C O and PF3 form an ordered ( 4 3 X 43)R3Oo unit cell on Ru(001), and (iii) the chemisorption bond energies for CO and PF3 are similar on Ru(001). Although P(CH3)3did not displace C O from Ru(OOl)-CO, it did affect the shape of CO thermal desorption peak (the two characteristic peaks merged into a single sharp peak). The effect, however, was probably not from molecular P(CH3),; this molecule decomposes on Ru(001) and its decomposition products of C and P were the species that affected the shape of the CO thermal desorption spectrum. Because our experiments with oxygen-containing molecules indicated substantial decomposition to form carbon monoxide and because the carbon monoxide thermal desorption maxima showed a great deal of variability, we examined the effect of surface carbon on the carbon monoxide thermal desorption. We found that surface carbon substantially affected the chemisorption behavior of carbon monoxide on Ru(001). Explicably, the carbon surface impurities decreased the CO saturation coverage. However, the most interesting and significant effect was on the relative intensities of the two C O desorption peaks from a saturated layer of CO. With increasing carbon coverage, the intensity of the high-temperature CO desorption peak was decreased relative to that of the low-temperature peak and eventually, at high carbon covera g e ~the , ~desorption ~ spectrum consisted of one broad assymetric desorption with a maximum at 105-110 "C, a shift of -25 O C from the maximum of the low-temperature peak for the clean surface (Figure 1). Analogous effects on the CO desorption behavior were observed with carbon derived from either benzene or acetaldehyde decomposition at 500 "C. In addition, the desorption maxima from CO coverages of approximately one-half monolayer were shifted down by 25-50 OC, depending on the coverage of the surface carbon. Thus, the CO signature desorption peaks are much affected3I by carbon, a result that is relevant to the interpretation of the CO formation and subsequent desorption from oxygen-containing hydrocarbons adsorbed on Ru(001). Benzene and Benzene Deriatives. Chemical and physical studies have shown that benzene is bound molecularly to surface metal (28) Thermal desorption experiments provide kinetic parameters (activation energies). These cannot be presumed to correlate uniformly with chemisorption bond energies. (29) Friend, C. M., Ph.D. Thesis, University of California, Berkeley, CA,
_1981. _
(30) Because of the overlapping Ru and C Auger transitions, the absolute coverage cannot be accurately estimated. The carbon from acetaldehyde did not appear to be graphitic in character (Auger and LEED criteria). On the other hand, the carbon from benzene decomposition may have been graphitic as indicated by Auger analysis but showed no LEED pattern indicative of graphitic surface carbon. Coverage was probably higher with the carbon from benzene and saturation coverages here were in the range of 1 to 1.5 carbon atoms per ruthenium atom. (31) A molecular interpretation of these carbon effects cannot be presented because a structural characterization of these mixed chemisorption states has not been made.
Surface Coordination Chemistry of Ruthenium I
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200 300 T ("C)
Figure 1. Carbon monoxide thermal desorption spectra were sensitive to surface carbon contamination. By adsorbing either benzene or acetaldehyde on the surface and heating to 400-500 'C, carbon was deposited on the surface. Illustrated here are the CO desorption spectra for Ru(OOl)-C-CO prepared by sequential doses of 0.5, 2.5, and 2.5 langmuirs of benzene (spectra b-d,respectively), then thermolysis at 500 'C, and finally CO (3 langmuirs) adsorption at 25 "C. Spectrum a is for the clean surface. The sensitivity scale on this figure is one-fifth that of all other figures.
atoms at 25 "C on Ag, Ni, Pd, Pt, Rh, Ir, Fe, and Ru surfaces.32 Extensive spectroscopic studies of many of these systems have established that the benzene molecule is oriented in a plane parallel to the surface.23~33-37~391 Accordingly, no significant differences were expected for Ru(001)-C6H6. Consistent with this projection, we found from low-energy electron diffraction studies that benzene chemisorbed on Ru(001) at 25 "C formed an ordered p(3 X 3) overlayer at all coverages. This unit cell just envelops the van der Waals extension for benzene, and would allow close packing of benzene molecules in a plane parallel to this surface basal plane of ruthenium. Kelemen and F i ~ c h e had r ~ ~presented a convincing case for molecular chemisorption of benzene on Ru(001) at 25 "C, based on an ultraviolet photoelectron spectroscopic study. However, the thermal reactivity of benzene on Ru(001) is very high and only hydrogen (or deuterium) molecules were detected in thermal desorption experiments with Ru(O0 1)-C6H6 or (C&) formed (322 Namely, A ~ ( l l l ) Ni(111),2c,34*35 ,~~ Ni(100),2c335Ni(llO),zc Pdl l ) , fs37 Pd(100),2*36~37 Pt(111),2e~34 Pt(100),38,39 Rh(111),40Ir(100),41 Fe11),42 and R U ( O O ~ ) . ~ ~ (33) Avouris, P.; Demuth, J. E. J . Chem. Phys. 1981, 75, 4783. (34) Lehwald, S.;Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577. (35) Bertolini, J. C.; Rousseau, J. Surf.Sci. 1979, 89, 467. (36) Hofmann, P.; Horn, K.; Bradshaw, A. M. Surf.Sci. 1981,105, L260. (37) Llovd. D. R.:. Ouinn. _ . C. M.: Richardson. N. V. Solid Srate Commun. 1977, 23, 151: (38) Tsai, M.-C.; Muetterties, E. L. J . Phys. Chem. 1982, 86, 5067. (39) Fischer, T. E.; Kelemen, S. R.; Bonzel, H. P. Surf.Sci. 1977, 64, 157. (40) Koel, B. E.; Somorjai, G. A. J . Electron Spectrosc. Relat. Phenom. 1983, 29, 287. (41) BrcdCn, G.; Rhodin, T.; Capehart, W . Surf. Sci. 1976, 61, 143. (42) Mason, R.; Textor, M. Proc. R . SOC.London, Ser. A 1977, 356, 47.
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 1999 at 25 or -160 O C , except for benzene desorption from multilayers at -1 10 "C after adsorption at -160 "C. At low coverages, the H2 desorption comprised a relatively sharp peak at 180 "C that "tailed" and dropped to baseline levels at -450 "C. With increased benzene coverage, the H2 maximum broadened and shifted to 150 "C (Figure 2, see paragraph at end of text regarding supplementary material). In contrast, benzene exhibits partial thermal desorption from clean nickel,2cpalladium,2f and platinum surfaces;2e decomposition is a competing thermal process and occurs at a measurable rate in the 110-240 "C range depending upon the metal and the surface topography. In our earlier studies of benzene chemisorption on nickel group metals, we showed that molecules like P(CH3)3,2C,f,e PF3,43and CH3NC2Ccould displace molecularly bound benzene from these metal surfaces. In fact, benzene displacement from Ni( 11 by P(CH3)32Cwas quantitative from 20-90 OC (above -90 "C, benzene thermal desorption and decomposition proceed at high rates). However, no benzene was detectably displaced from Ru(001)-C6H6 at 25 OC or below by either P(CH& or PF3. On heating RU(001)-C6H6-P(CH3)3 formed at 25 OC, there was no evidence of benzene molecular desorption. As in the case of Ru(OOl)-CO, the activation energy for the displacement reaction appears to be relatively high. That at least some benzene must be present as the molecule on Ru(001) at low or moderate temperatures was indicated by a thermal desorption experiment for RU(OO1)-C6H6-P(CH3)3 formed at -160 "c by benzene adsorption followed by phosphine adsorption. In this experiment, a small benzene desorption maximum was observed at -20 OC. Hexafluorobenzene displayed nearly complete thermal reversibility in chemisorption on Pt( 11l).44 However, this molecule irreversibly adsorbed on Ru(001) at 25 O C . Although nothing detectably desorbed in the thermal desorption experiment, fluorine but not carbon disappeared from the surface at -700 "C as judged by Auger electron spectroscopy. Probably, the fluorine was removed from the surface as a volatile ruthenium fluoride which could not be detected by mass spectrometry due to sensitivity limitations rather than by diffusion of fluorine into the bulk. Ru(001)-C6F6 formed at -160 "cdid show hexafluorobenzene desorption peak from multilayers at -1 10 "C and from another very weak molecular desorption peak which appeared at -60 O C at low coverage (this desorption peak was obscured by the multilayer peak at higher coverages). Ru(001)-C6F6 formed at 25 'C or lower temperatures gave no evidence of ordering by low-energy electron diffraction; but an ordered state could have been missed because of the sensitivity of fluorinated molecules to electron beam damage. Toluene chemisorption on Ru(001) was irreversible and resembled that of benzene. A poorly ordered p(3 X 3) low-energy electron diffraction pattern was observed for Ru(001)-toluene surfaces formed at 25 "C and also for surfaces formed by annealing at 0 "C after initial adsorption at -160 "C. A remarkable feature of some nickel surfaces? specifically Ni( 111 ) and Ni( loo), was the regiospecific character of C-H bond breaking in the surface-mediated thermal decomposition of the toluene; all aliphatic C-H bonds were cleaved before aromatic C-H bonds were broken. Some degree of regiospecificity in toluene C-H bond breaking was detected for Ru(001)-toluene but the overall chemistry was not nearly so straightforward. As shown in Figure 3, the hydrogen desorption from toluene decomposition on Ru(001) was complex and very sensitive to initial toluene coverage. Most notably, the desorption spectra did not simply consist of two peaks of relative intensities 3 and 5 as found2" for Ni( 111) and Ni( 100). Nevertheless, to probe the issue of regioselectivity more sensitively, the thermal decomposition of CD3C6H5and CH3C6D5was studied on Ru(001). There was in these experiments some indication of selectivity in that the lowand high-temperature desorption regions for Ru(001)-CD3C6H5 contained proportionately more deuterium and hydrogen, respectively (Figure 4, supplementary material). The CH3C6D5 (43) Unpublished results, this laboratory. (44) Johnson, A. J.; Muetterties, E. L., to be published.
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Shanahan and Muetterties containing oxygen and produced H2, CO, and Ru(OOl)-C. Since a large group of oxygen-containing hydrocarbons are reviewed in a later section, the full discussion of Ru(OOl)-furan is deferred to that section. Thiophene irreversibly chemisorbed on Ru(001). On the basis of thermal desorption experiments with Ru(O01)-thiophene-2,5-d2, the a-C-H bonds are broken first in the overall thermal decomposition process. Thus, there is full regiospecificity in this thiophene decomposition on Ru(001). A complete characterization of this chemistry will be reported in a separate article dedicated to thiophene surface chemistry on iron, cobalt, and nickel group metals. Inorganic and Organic Cyanides. In molecular transition-metal coordination chemistry, inorganic and organic cyanides exhibit a number of reaction courses. Cyanide ion itself is one of the strongest field ligands representing the negative member of the isoelectronic triad CN-, NO+, and CO. In some cases, H C N and even cyanogen, (CN),, will oxidatively add to a zerovalent metal atom in a coordinately unsaturated complex to form a hydridocyano or dicyano complex:45 ML, (CN)2 L,M(CN)2
+
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100 200 300 400 500 600 T ("C) Figure Like benzene, toluene decomposes quantitativel, I hydrogen and su..,ce carbon when adsorbed on the clean Ru(001) surface and then heated. Illustrated here is the coverage dependence of the deuterium desorption (amu 4) from perdeuteriotoluenedecomposition. Spectra a-e, respectively, represent desorption from exposures of 0.1,0.5,0.8, 1.5, and 2.5 langmuir on the clean Ru(001) surface at 25 O C .
experiment yielded complementary results (proportionately more H in the low-temperature peak and more D in the high-temperature peak). Adsorption at -160 O C did not alter these results except for the appearance of a multilayer desorption a t -65 O C . Heteroaromatic Molecules. Arenes and heteroaromatics because of their rigid ring conformations exhibit a high potential for regiospecific reactions with the crystal planes of metals. As discussed in the previous section, benzene binds with metal atoms of flat metal crystal planes so as to place the ring plane parallel to the crystal plane. Pyridine in contrast binds initially through the ?r system centered on nitrogen so as to place the C5N ring nearly normal to the surface plane; and in its stepwise thermal decomposition, it first goes through an a-pyridyl species with the C5N ring plane normal to the metal surface plane. This appears to be most typical of the nickel group metals, particularly Ni( 100).2d On Ru(001), three heteroaromatic molecules were investigated: thiophene, pyridine, and furan. Pyridine chemistry on Ru(001) was complex. The pyridine chemisorption was irreversible and the decomposition products were hydrogen, hydrogen cyanide, and nitrogen. For example, Ru(001)-NC5Ds, formed at 25 "C with a 0.3-langmuir exposure, yielded a broad D2 desorption with maxima at 110 and 400 'C, DCN desorption at 400 OC (confirmed as DCN by experiments with CsHsN which gave HCN), and N2 desorption above 500 "C (also CO from bulk oxygen). The character of the desorption spectra (maxima and also the peak widths) were dependent upon the adsorption temperature but the products did not change except for the appearance of molecular pyridine desorption maxima (multilayer pyridine) near -1 00 O C for Ru(OO1)-NCSHS formed at -160 OC. On some surfaces, particularly Ni( pyridine readily forms a 2-pyridyl complex by scission of an a-C-H bond. This feature was readily discernable from specifically labeled pyridine molecules. A thermal desorption study of Ru(001)-NCSH3D2-2,6-d, indicated some degree of regiospecificity, namely, scission of an a-C-H bond, in the early stages of pyridine decomposition on this surface. N o ordered low-energy electron diffraction patterns were detected for Ru(001)-NCsH5 either for adsorption at 25 or at -160 "C. Furan irreversibly chemisorbed on Ru(001) and gave no evidence of regiospecificity in the C-H bond-breaking processes. Furan behaved thermally like all other hydrocarbon molecules
A similar oxidative addition reaction has been observed for nitriles (organic cy ani de^):^^ Pt[P(C2H5)3]3 C6H5CN C6H5Pt(CN: [P(C2Hs)3]2 4- P(C2H5)3
+
However, the typical outcome of the reaction on an organic cyanide with a coordinately unsaturated mononuclear metal complex would be the formation of an adduct4' typically with the nitrogen atom bonded to the metal atom, e.g., Os3(CO)11NCCH3!8 A structural alternative with both the C and the N atoms of the cyanide bonded to metal atoms has been postulated and crystallographically defined for a metal cluster49 and postulated but not crystallographically defined for mononuclear metal complexes.50 Nitrile metal surface chemistry has only recently been studied by modern surface techniques. It was recently proposed5' that for Pt( 11 1)-NCCH3, the cyanide bond vector is parallel to the surface plane. This proposal was based on interpretation of HREELS and XPS data. An analogous structure has been proposed for Ni( 1 11)51but this does not seem totally consistent with the observed chemistry.2b The chemical point with respect to a normal and a parallel C N on an atomically flat surface is that, in the normal form A, the hydrogen atoms are relatively far
B
A
removed from the surface whereas in the parallel form B, they would be very close. The reactivity of the B form should be much higher than for A because the activation energy for C-H bond scission on a clean transition-metal surface is intrinsically low. To gain some quantitative measure of ruthenium surface chemistry for cyanides we used two inorganic sources of CN, hydrogen cyanide and cyanogen, and four organic cyanides that provided varying measures of electronic and steric effects and ~~
~
~~
~
(45) Argento, B. J.: Fitton, P.; McKeon, J. E.; Ruk, E. A. Chem. Commun. 1969, 1427. (46) Kane, A. R.; Muetterties, E. L. J . Am. Chem. SOC.1971, 93, 1041. (47) Kouba, J. K.; Muetterties, E. L.; Thompson, M. R.; Day, V. W. Organometallics 1983, 2, 1065-73. (48) Dawson, P. A.; Johnson, B. F.G.; Lewis, J.; Puga, J.; Raithby, P. R.; Rosales, M. J. J . Chem. SOC.,Dalton Trans, 1982, 233. (49) Andrews, M. A.; Kaesz, H. D. J . Am. Chem. SOC.1979,101,7255. (50) Storhoff, B. N.; Lewis, H. C., Jr. Coord. Chem. Rev. 1977, 23, 1. (51) Sexton, B. A,; Avery, N. R. Surf. Sci. 1983, 129, 21.
Surface Coordination Chemistry of Ruthenium
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2001
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Figure 5. Cyanogen was one of the few molecules studied that exhibited molecular desorption after adsorption at room temperature. The cyanogen desorption was coverage dependent as shown above in the molecular thermal desorption spectra for cyanogen (amu 52) adsorbed on Ru(001) at 25 OC for exposures of 2.5, 5, 20, and 40 langmuir (spectra a-d, respectively).
additional functional sites, namely, acetonitrile, pivalonitrile (tert-butyl cyanide), acrylonitrile (vinyl cyanide), and trifluoroacetonitrile. Cyanogen chemisorption on Ru(001) was partially reversible and only (CN), and N 2 (T,,, at 470 "C) were detected in the thermal desorption experiment. The desorption spectra were dependent on coverage and on the adsorption temperature as shown in Figure 5 (and Figure 6, supplementary material). If hydrogen was present on the surface, then H C N was produced in the thermal desorption experiment-the HCN desorption maxima were at 120 and 320 OC. Gudde and Lamberts2 reported similar results for Ru( 100)-C2N2 and characterized the chemisorption state as associative (molecular). Our data do not allow distinctions to be made between associative and dissociative chemisorption for cyanogen but such issues will be explored later through spectroscopic studies (HREELS and NEXAFS). N o ordered low-energy electron diffraction patterns could be detected for Ru(001)-C2N2, either at 25 or -160 Hydrogen cyanide adsorbed on Ru(001) at 25 "C produced a sharp ( 4 3 X 43)R3Oo low-energy electron diffraction pattern. The thermal desorption experiments for Ru(OOl)-HCN showed complex molecular HCN desorption behavior as well as a coverage dependence. As illustrated in Figure 7, the molecular desorption spectra showed maxima at 110 and 155 "C with significant intensity out to -400 OC. In addition, a 28-amu peak was observed at 450-500 OC that was due to N2 (decomposition) as judged by Auger measurements of the surface, and an H2 peak at 115 "C. Adsorption a t -160 "C did not alter these results. Acetonitrile formed on Ru(001) at 25 OC an ordered chemisorption state of p(2 X 2) form but no molecular desorption was detected. Decomposition products were H2, N2, and HCN (T,,, = 110, 530, and 350 "C, respectively). After adsorption of CH3CN on Ru(001) at -160 OC, the thermal desorption yielded molecular desorption a t 50 OC in addition to the decomposition products. Heating Ru(001)-NCCH3-NCCD3 produced only CH,CN and CD3CN in the 50 OC molecular desorption region; thus there was no significant reversible carbon-hydrogen bond breaking. The character of the hydrogen desorption from Ru(001)-NCCH, formed at -160 OC was different from that for Ru(OO1)-NCCH, formed at 25 "C in that hydrogen began to appear with the molecular acetonitrile desorption at 50 OC and ( 5 2 ) Gudde, N. J.; Lambert, R. M. Surf. Sci. 1983, 124, 372.
(53) This is in contrast to the results of Gudde and Lambert,s2 who observed multiple low-energy electron diffraction patterns for Ru( 100)-(CN)*.
T ("C)
Figure 7. Hydrogen cyanide also underwent reversible desorption after adsorption at 25 or -160 OC. Unlike the case of cyanogen, however, no significant changes were noted upon changing the initial adsorption temperatures. Hydrogen cyanide thermal desorption profiles from Ru(001)-HCN (amu 27) formed at exposures of 0.5, 1, and 3 langmuirs (spectra a-c, respectively) at 25 "C on clean Ru(001) are presented above to illustrate the coverage dependence of the molecular desorption. I
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Ru(O0 1 ) - CD,CN
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200 300 T VC) Figure 8. Acetonitrile adsorbed on clean Ru(001) at -160 OC underwent both molecular desorption and decomposition in the thermal desorption experiment. This is illustrated above for CD3CN on Ru(001) for states formed at -160 OC with initial exposures of 0.05 and 0.1 langmuir (spectra a and b, respectively). Atomic mass units 44 and 4 represent CD3CN and D2, respectively. Desorption from multilayers is not shown due to interfering desorption from the crystal supports.
1
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reached a maximum at 6C-70 "C (Figure 8). This system exhibits the relatively common phenomenon for chemisorbed hydrocarbon derivatives of competitive thermal desorption and decomposition. Chemically, it more closely resembles Pt( 11 l)-NCCH,54 than
2002
The Journal of Physical Chemistry, Vol. 88,No. 10, 1984 I
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RU (0011 -CF,CN
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T VC) Figure 9. Trifluoroacetonitrile reversibly chemisorbed (partially) on Ru(001). Here is the coverage dependence of the molecular desorption (amu 69)from Ru(001)-CF3CNformed at 25 OC (the molecular peak, amu 95,showed the same profile as amu 69 but at reduced sensitivity). Spectra a-e are for initial exposures of 1.5, 2, 2.5,5, and 11 langmuirs, respectively. Ni( 111)-NCCH3;2a,bs55the latter exhibits nearly complete reversible chemisorption and the former largely irreversible chemisorption. Pivalonitrile behaved analogously to acetonitrile although it did not yield low-energy electron diffraction evidence of an ordered chemisorption state on Ru(001). After pivalonitrile adsorption at -160 "C, the thermal desorption experiment yielded molecular pivalonitrile desorption at 35 "C as well as molecular desorption from multilayers at -100 "C. Hydrogen, hydrogen cyanide, and nitrogen also formed ( TmX= 110, 345, and 550 "C, respectively). Acrylonitrile adsorbed on Ru(001) at 25 "C gave no low-energy electron diffraction evidence of ordering and the adsorption process was irreversible, even after adsorption at -160 "C (except for desorption from multilayer configurations in the region of -1 00 "C). Hydrogen, hydrogen cyanide (Tmx= 270 "C), and nitrogen (T,,, = 570 "C) were, as with the other nitriles, thermal decomposition products. With adsorption at -160 "C, the thermal decomposition began at -25 "C for low coverages and shifted to 0-10 "C for high coverages, as signaled by the hydrogen desorption. With adsorption at 25 "C, hydrogen desorption was noted immediately upon initiation of the thermal desorption experiment. These data provide no information as to what functional group atoms are bonded to the surface metal atoms. Trifluoroacetonitrile chemisorption behavior on Ru(001) was complex and was substantially different from that of the alkyl cyanides described above. The thermal desorption spectra for Ru(001)-NCCF3 formed at 25 "C are illustrated in Figure 9 (see Figure 10, supplementary material, for spectrum of Ru(OOl)-NCCF3 formed at -160 "C). Adsorption was partially reversible but thermal decomposition was the major process(es) on this surface. Decomposition products included FCN and N2 with respective T,,, of 400 and 600 "C. Fluorine was present on the surface to -600 OC and then was fully lost by 900 OC presumably as a ruthenium fluoride, or by diffusion into the bulk. The curious features of the CF3CN molecular desorption was its multiple peak character with a temperature spread in the peak maxima ranging from 85 to 340 "C and the dependence of the number of peaks on the adsorption temperature (Figures 9 and (54) Garwood, G. A., Jr.; Hubbard, A. T. Surf. Sci. 1982, 118, 223.
(55) Wexler, R.M.; Ph.D. thesis, University of California, Berkeley, CA 1983.
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Figure 11. Acetaldehyde and acetone adsorbed on Ru(001) at 25 "C decomposed quantitatively to yield H2, CO,and surface carbon in the thermal desorption experiment. Illustrated above are the Hz (amu 2) and CO (amu 28)thermal desorption spectra for 1.2-langmuirexposures of acetaldehyde (a) and acetone (b) on clean Ru(001)and 25 OC. The CO desorption spectra in this figure are on a 2.5 times lower sensitivity scale than all other figures. 10, supplementary material). In this system, there may be a recombination of radical species at high temperatures whereby molecular CF3CN is regenerated (and desorbed). N o evidence of ordered arrays of trifluoroacetonitrile was obtained from lowenergy electron diffraction studies; however, this could have been missed due to the extreme sensitivity of this compound to electron beam damage. All the Ru(OOl)-NCR ( R = H , CN, alkyl, etc.) results can be directly compared with those for Ni(l1 1)-NCRSS5 Nitrile chemisorption on Ru(001) is much less reversible than on Ni( 111). Nevertheless, there are some similarities. Reversibilities of fluoronitriles are higher than their hydrocarbon analogues. The temperature of fluoronitrile desorption (maximum rate) is high. CF3CN has a T,, of 200 "C on Ni( 111) and a T ,, ranging from 85 to 340 "C for Ru(001). Acrylonitrile showed irreversible chemisorption on both N i ( l l 1 ) and Ru(001). Oxygen-ContainingHydrocarbons. All oxygen-based hydrocarbon derivatives investigated underwent largely thermal decomposition after chemisorption on Ru(O0 l); and explicably, hydrogen and carbon monoxide were the common decomposition products in agreement with studies of Ru(OOl)-formic acidz5and ku(110)-methanol.s6 As carbon monoxide forms an ordered ( d 3 X d 3 ) R 3 0 ° on clean Ru(001), low-energy electron diffraction was employed to sense temperatures where decomposition was extensive and formation of the ordered CO state was achieved. Very faint ( d 3 X d3)R3Oo patterns were observed for methanol at 25 "C and for all other chemisorbed molecules except acetic acid after warming to 100-150 OC. These data suggest that thermal decomposition of these chemisorbed species begin at relatively low temperatures. The molecules studied were methanol, acetaldehyde, acetone, acetic acid, methyl acetate, furan, and tetrahydrofuran. With the exception of acetic acid, the thermal ( 5 6 ) (a) Goodman, D. W.; Yates, J. T., Jr.; Madey, T. E. Chem. Phys. Lett. 1978,53,479. (b) Fisher, G.B.; Madey, T. E.; Waclawsk, B. J.; Yates, J. T., Jr. In "Proceedings of the Seventh International Vacuum Congress and Third International Conference on Solid Surfaces"; Vienna, 1977;p 1071. (c) Yates, J. T., Jr.; Goodman, D. W.; Madey, T. E. Ibid. p 1133.
Surface Coordination Chemistry of Ruthenium decomposition spectra were remarkably similar as illustrated in Figure 11 for acetaldehyde and acetone, with variations in CO desorption maxima being attributable to differences in relative sticking coefficients of the adsorbing oxygenated hydrocarbons, and differences in the resulting amount of surface carbon produced by the d e c o m p o s i t i ~ n . ~ ~ Methanol adsorbed on Ru(001) a t -160 O C produced a molecular methanol desorption peak at -60 OC. Carbon monoxide and hydrogen were the only decomposition products detected with maxima at 215 and 105 OC, respectively. Decomposition probably occurs a t temperatures at least as low as 25 0C;58an analogous conclusion had been presented for Ru( 110)-CH30H.56No carbon or oxygen were detected on the crystal surface after the thermal desorption experiment. Acetaldehyde decomposed quantitatively on Ru(001) to form CO a t 205 OC, H 2 at 110 OC, and Ru(001)-C (Figure 11). Acetone also decomposed quantitatively again to form CO, H2, and Ru(001)-C with T,,, = 205 and 90 OC for CO and H2, respectively (Figure 11). Adsorption at -160 OC produced multilayers of acetaldehyde and acetone, which molecularly desorbed at -120 OC in both cases. Methyl acetate and acetic acid both decomposed on Ru(001) to produce H2 and C O although methyl acetate after adsorption at -160 O C produced a molecular desorption peak that shifted in T,,, from -0 to -90 OC with increased coverage (Figure 12, supplementary material). Mechanistically, the thermal decomposition processes of the acid and the ester are not related. Methyl acetate decomposition was analogous to other oxygen derivatives of hydrocarbons in that hydrogen appeared at low temperatures (TmaX 100°C) and CO at -205 'CS9 On the other hand, acetic acid decomposition (Figures 13 and 14, supplementary material) at low coverages on a clean Ru(001) surface yielded H2 and C O desorption maxima a t -220 OC and a small COz desorption maximum at -180 "C. Higher coverages shifted the CO and the H2maxima down to 180 O C and the C 0 2 maximum up to -220 OC. Also, additional C O desorptions were observed in the 250-400 O C region which is most likely due to recombination of surface C and 0 atoms to form C0.60,61 After the
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(57) See the section on CO and PF, for details regarding the effect of surface carbon on CO desor tions. (58) (a) Dr. Hrbek etb!la have conducted a detailed study of methanol chemisorption with thermal desorption and electron energy loss spectroscopy. They concluded that methanol adsorbs as a methoxide species and further decomposes to CO and H2 near 25 "C. (b) Hrbek, J.; dePaola, R.; Hoffmann, F., to be submitted for publication. (59) There was one perplexing feature to methyl acetate decomposition on Ru(001). After the crystal was heated to -400 "C, no carbon would be detected on the surface. Furthermore, an extensive search for other carboncontaining molecules (CH,, C2Hq,C2H4, C2H2,CH,OH, and C02) desorbing from the crystal was negative. Either carbon uniquely diffused into the bulk or methyl acetate decomposed on adsorption at -160 OC to produce a molecule that would not remain adsorbed at -160 "C (methane would not). An argument against the latter rationale is that an ordered CO LEED pattern did not appear until -+lo0 "C. (60) Adsorption of hydrocarbon molecules on a Ru(001) surface contaminated with trace amounts of oxygen (sometimes not detectable by Auger) also would produce CO desorptions in the 200-400 "C region in the thermal desorption experiments and, as mentioned previously, would always give CO desorptions at approximately 550 "C.
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2003 thermal desorption experiments, the surface contained both carbon and oxygen as judged by Auger electron spectroscopy. Readsorption of acetic acid on these Ru(OOl)-C-O surfaces yielded thermal desorption spectra in which CO, COz, and H2 desorptions all peaked a t -205-210 O C with analogous peak shapes. Multilayer desorption at -80 OC was noted for adsorption of acetic acid at -160 OC. N o ordered LEED patterns were observed upon adsorption of acetic acid at 25 or -160 OC; however, faint (2 X 2) patterns could occasionally be observed after warming Ru(OO1)-CH,COzH to 150 OC. Furan adsorbed on Ru(001) at 25 OC produced carbon monoxide in thermal desorption experiments with CO maxima at 170-220 "C, depending on the exposure (and the resulting carbon coverage), and hydrogen desorption maxima at 70-90 "C. Adsorption at -160 OC produced a molecular peak at -50 "C for low coverages which was obscured at higher coverages by the multilayer desorption peak at --120 OC. Tetrahydrofuran adsorbed on Ru(001) at 25 OC also decomposed quantitatively to CO, H2 (T,,, = 170-220 and 70-90 OC, respectively), and Ru(OOl)-C. Adsorption at -160 OC produced a surface that had a coverage-dependent molecular desorption peak that ranged from 5 OC for low coverages to -80 OC for high coverages (merged with the multilayer peak). No ordered LEED patterns were observed upon adsorption at 25 or -160 OC. Generally, the Ru(001) surface was more reactive toward hydrocarbon derivatives containing oxygen than those of the nickel group62which tend to undergo molecular desorption.
Acknowledgment. This research was supported by the National Science Foundation. We also acknowledge Professor Thor Rhodin for the loan of the ruthenium crystals and for helpful discussions, Professor Neil Bartlett for a sample of cyanogen, and Professor W. H. Weinberg and Dr. Jan Hrbek for advance information regarding their Ru(001) studies. Registry No. CO, 630-08-0; CF,CN, 353-85-5; PF,, 7783-55-3; HCN, 74-90-8; ruthenium, 7440-1 8-8; benzene, 7 1-43-2; toluene, 10888-3; hexafluorobenzene, 392-56-3; pivalonitrile, 630- 18-2; acrylonitrile, 107-13-1; acetaldehyde, 75-07-0; pyridine, 110-86-1; acetonitrile, 75-05-8; methanol, 67-56-1; acetone, 67-64-1; acetic acid, 64-19-7; cyanogen, 460-19-5; methyl acetate, 79-20-9; trimethylphosphine, 594-09-2.
Supplementary Material Available: Figures 2 , 4, 6, 10, 12, 13, and 14 show additional desorption spectra (1 1 pages). Ordering information is given on any current masthead page. (61) It should be noted that this behavior is quite different from that of formic acid (ref 25). With formic acid, the hydrogen produced desorbed with a maximum at 182 "C for low coverages and at 102 "C for high coverages. No such drift was observed for the H,desorption arising from acetic acid decomposition. (62) See, for example: (a) Madix, R. J.; Falconer, J. L.; Suszko, A. M. Surf. Sci. 1976, 54, 6. (b) Liith, H.; Rubloff, G. W.; Grobman, W. D. Ibid. 1977,63, 325. (c) Goodman, D. W.; Yates, J. T., Jr.; Madey, T. E. Ibid. 1980, 93, L135. (d) Baudais, F. L.; Borschke, A. J.; Fedyk, J. D.; Dignam, M. J. Ibid. 1980,100,210. (e) Christmann, K.; Demuth, J. E. J. Chern. Phys. 1982, 76, 6308,6318. (f) Rendulic, K. D.; Sexton, B. A. J. Catal. 1982, 78, 126. (9) Avery, N. R. Surf.Sci.1983, 125, 771. (h) See ref 56c.
