ADSORPTION AND REACTION OF DIMETHYL DISULFIDE ON THE NI(111) SURFACE

The adsorption and reactions of dimethyl disulfide, CH3S-SCH3, have be en analyzed on the Ni(lll) surface using high-resolution electron ener gy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), Auger electron spectroscopy ( AES), temperature programmed desorption (TPD), and deuterium labeling. All of the S-S bonds in dimethyl disulfide (DMDS) are broken below 15 0 K, forming methyl thiolate (CH3S) as the primary surface intermediat e. Condensed DMDS desorbs at 166 K. Methane, ethane, and hydrogen are the main desorption products from the reaction of the adsorbed disulfi de with Ni(lll). The methane desorption following adsorption of DMDS a nd CH3SH is remarkably similar. Like methanethiol, total decomposition is favored for DMDS at low coverages, while hydrocarbon formation is the main reaction pathway for higher coverages. The methane desorption profiles for DMDS coadsorbed with hydrogen are similar to those obser ved from methanethiol coadsorbed with hydrogen. Coadsorption of deuter ium with high coverages of DMDS results in an increased temperature (22 K) for the methane formation reaction, indicating that C-H(D) bond formation is the rate-limiting step in CH4 formation at high DMDS cove rages. Disproportionation and coupling reactions between the adsorbed thiolates are the main reaction mechanisms for methane and ethane form ation, respectively. Analysis of the C Is XPS peak areas and TPD inten sities suggests that by 550 K approximately 85% of the saturated surfa ce thiolate desorbs as gaseous methane and ethane. Annealing a saturat ion exposure of DMDS (>0.33 ML) on the Ni(lll) surface results in a co mplex LEED pattern as a result of the reconstruction of the top Ni lay er. Surface reconstruction starts below room temperature and for S cov erages as low as 0.10 ML.