Adsorption and reaction of dimethyl disulfide on the Ni(111) surface

The adsorption and reactions of dimethyl disulfide, CH3S-SCH3, have been analyzed on the Ni(lll) surface using high-resolution electron energy-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 150 K, forming methyl thiolate (CH3S) as the primary surface intermediate. Condensed DMDS desorbs at 166 K. Methane, ethane, and hydrogen are the main desorption products from the reaction of the adsorbed disulfide with Ni(lll). The methane desorption following adsorption of DMDS and 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 observed from methanethiol coadsorbed with hydrogen. Coadsorption of deuterium 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 coverages. Disproportionation and coupling reactions between the adsorbed thiolates are the main reaction mechanisms for methane and ethane formation, respectively. Analysis of the C Is XPS peak areas and TPD intensities suggests that by 550 K approximately 85% of the saturated surface thiolate desorbs as gaseous methane and ethane. Annealing a saturation exposure of DMDS (>0.33 ML) on the Ni(lll) surface results in a complex LEED pattern as a result of the reconstruction of the top Ni layer. Surface reconstruction starts below room temperature and for S coverages as low as 0.10 ML.