1200%
J. Phys. Chem. 1993,97, 12008-12013
Adsorption and Reaction of Methanethiol on Ru( 0001) D. R. Mullins' and P. F. Lyman Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6201 Received: June 10, 1993; In Final Form: August 24, 1993"
Methanethiol (CH3SH) adsorbs dissociatively on Ru(0001) at 100 K forming two molecular intermediates. These species have been identified by S 2p soft X-ray photoelectron spectroscopy (SXPS) as methylthiolate moieties adsorbed in different bonding sites. At low coverages only a high coordination adsorption site is occupied. The high coordination site is saturated at about 0.20 monolayer of CH3SH, and then a low coordination site is populated. Upon heating, the thiolates totally decompose into atomic C, S,and gaseous Hz with a competing pathway producing atomic S and gaseous CH4 at high coverage. The methylthiolate in the low coordination site decomposes more readily than the methylthiolate in the high coordination site. Some surface hydrocarbon fragments are formed following C S bond scission which totally decompose at higher temperatures. Isotopic labeling indicates that the desorption of the sulfhydryl hydrogen precedes the decomposition of the thiolate at high coverage and that there is exchange of hydrogen between the surface and the methyl group.
Introduction The adsorptionand reaction of methanethiol,CH3SH, on clean metal surfaces has become a very well studied system in hopes of understanding the interactions between sulfur-containing adsorbates and metallic substrates. Methanethiol can serve as a probe molecule for studying thiol-surface interactions because its adsorption and reaction path frequentlycontainsseveral similar steps on most metal surfaces. In general, methanethiol adsorbs as methylthiolate, CH&, even at low temperatures. As the substrate temperature is raised, the carbon-sulfur bond cleaves resulting in either total decomposition to adsorbed atomic C, atomic S, and desorbed Hz, or partial decomposition to atomic S and desorbed CH4. Although the reactionson most of the metals studied are similar in the most general sense, the details of the reaction mechanisms can be quite different. The desorption temperature of CH4, an indirect measure of the C S bond stability, ranges from ca. 270 and Fe( K on the Ni ~urfacesl-~ to ca. 300 K on Mo( 1 >350 K on W(001)6and Cu(100),7 and to >400 K for one of the states on Pt( 11 l).899 The branching between the total and partial decomposition pathways also varies. The total decomposition pathway remains fairly significant on W(O01),6 W(21 1),Io and Mo( 1lO)4at high coverages. The amounts of hydrogen produced in the total decomposition pathway decreases significantly on Fe( 100)s and Ni( at high coverage. On most surfaces only a single type of intermediate has been proposed, CH,S-; however on Pt( 1 11) the partially dehydrogenated species CH2S- has been suggested.8 Minor amounts of longer chain hydrocarbons have also been observed in the desorption products from some surfaces. 1,29799 The aim of this paper is to compare the adsorption and reaction of CH3SH on two dissimilar metal substrates. Ru(0001) serves as an ideal compliment to our previous study of CH3SH on W(O01).6 The close-packed Ru(0001) surface has a much higher density of first layer metal atoms compared to the more open W(OO1) surface. In addition, measurements of hydrodesulfurization (HDS) activity on metal sulfides have shown a characteristic "volcano" behavior if HDS activity is plotted vs position in the periodic table across a transition-metal series." RuSz is found at the peak of this volcano indicating high HDS activity, whereas WS2 is found at the base (low activity). Similar relative activities between Ru and W have been observed for other types *Abstract published in Aduunce ACS Absrrucrs, October 1, 1993.
0022-3654/93/2097- 12008$04.00/0
of catalytic reactions on the clean metal surfaces.12 W and Ru therefore serve as prototypesfor a low-activity and a high-activity substrate, respectively. Using a combination of temperatureprogrammed desorption and photoelectron spectroscopy,we can study the reaction branching as a function of coverage and the stability of the C S bond and identify the reaction intermediates. In general, the adsorption and decomposition of methanethiol behaves similarly on the two surfaces. The decomposition of methanethiol is restricted to the total decomposition and methane production pathways discussed above. Two reaction intermediates are observed on both surfacesand are identified as methylthiolate, CH3S-, adsorbed in different bonding sites. The C S bond is significantly less stable on Ru(0001) compared to W(OO1). The relative stability of the thiolates adsorbed in the different sites is reversed on the two surfaces.
Experimental Section The experiments were performed in a stainless steel ultrahigh vacuum chamber equipped with cryogenic and turbomolecular Torr. The pumps. The base pressure was less than 1 X Ru(0001) sample was cut from a single-crystal rod, aligned to within O S o of the (0001) direction by Laue back reflection and polished with alumina powder. The sample was mounted on W wires and could be cooled by liquid nitrogen to 100 K and heated resistively to 1500 K. A chromel-alumel thermocouple was attached to the sample to monitor the temperature. Sample cleaning was performed before each experiment by argon ion sputtering followed by heating to 1500 K. Cleanliness was monitored by Auger electron spectroscopy. Carbon contamination was monitored by measuring the ratio of the Auger peak intensity at 270 eV ( R u ~+~ C273) o to the Auger peak intensity at 230 eV (R~z30).The ratio of Ru270to Ruz3o on a carbon-free surface was determined by measuring the Auger spectrum after exposing the Ru sample to oxygen at 1000 K to remove the carbon. In a similar manner, sulfur contamination was monitored by the ratio (Rulso + S I S I ) / R U ~A~ O sulfur-free . surface, which was used for establishing a baseline for this ratio, was verified by soft X-ray photoemission which had a sensitivity to less than .02MLof sulfur. TheCH3SH(Matheson, 99.5%) andCH3SD (MSD, 98.3 atom %) were purified by several freeze/pump/thaw cycles and the purity checked by the mass spectrometer. Due to the rapid exchange of the S-H hydrogen with the gas manifold,l.S the isotopicpurity of the CH3SD was estimated to be only 70% based on the mass spectrometer cracking pattern. The surface was 0 1993 American Chemical Society
Methanethiol on Ru(0001)
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
exposed to CH3SH by admitting the gas through a directed doser while the sample temperature was maintained at < 100 K. The dosing method ensures that the back of the sample is not exposed to CH3SH. The dosing system has been described p r e v i o ~ s l y . ~ J ~ Thermal desorption spectra were recorded by a UTI lOOc mass spectrometer which could be multiplexed to collect up to eight separate masses in a given desorption spectrum. The sample was heated resistively at a rate of 6 K/s. Photoemission spectra were obtained using a VSW CLASS 100hemispherical analyzer. The binding energies were measured with respect to the Fermi level. The accuracy of our reported binding energies is f0.025%.6J3 208-eV excitation radiation for the S 2p core level spectroscopy was obtained from the Grasshopper Mark I1 monochromator at the Synchrotron Radiation Center in Stoughton, WI. He1 radiation at 21.22 eV was used for valence level photoemission. The instrumental resolution was estimated to be 0.50 f 0.05 eV for S 2p and 0.10 f 0.02 eV for the valence level. Auger electron spectra were collected with a Physical Electronics single-pass cylindrical mirror analyzer operating with a modulation voltage of 4 eV.
Results
Coverage. The amount of CH3SH that adsorbed and reacted on the Ru(0001) surface was determined by measuring the sulfur AES peak intensity after the sample was heated to >600 K. The SXPS intensities were constant as the sample was annealed and the thiol decomposed, indicating that no sulfur containing products desorbed, nor did S dissolve into the bulk. The sulfur AES response was calibrated by measuring the Auger spectrum from a sample exposed to H2S at room temperature and then annealed to >500 K. This produces a sulfur coverage of 0.5 monolayer (ML; 1 ML = 1.57 X 1015~ m - 9 . I ~ To determine the Auger intensity due to sulfur at 151 eV, the Ru peak at 150 eV must be compensated for. This was done by assuming that Ru150 is proportional to Ru230 and that the ratio does not change with adsorbate coverage.15 This assumption appears to be valid since the Ru peak ratio did not change when various coverages of oxygen were adsorbed. The sulfur signal is normalized to the R~230intensity from the clean substrate, measured just before adsorbate exposure, to account for long term drifts in the analyzer sensitivity. The sulfur AES intensity is thus determined by
1230(clcan)
The various sources of experimental error result in an accuracy of f0.03 ML in the determination of the sulfur coverage. The maximum amount of CH3SH that could be chemisorbed on the surface was 0.40 f 0.05 ML. In principle, the carbon coverage and therefore the amount of total decomposition could be determined in the same manner by correcting for R~270. In practice this is difficult because the intense R~270peak and the weak C272 peak result in a greater experimental error. Thermal Desorption. The only desorption products from Ru(0001) following exposure to CH3SH at
