Desulfurization of ethylene sulfide on Mo(100): The roles of ring size and strain in adsorbate reaction selectivity

Abstract

The reactions, under ultrahigh vacuum, of ethylene sulfide ( c-C 2H 4S) on Mo(110) have been examined, by isothermal reaction spectroscopy, temperature programmed reaction spectroscopy, and X-ray photoelectron spectroscopy, Auger electron spectroscopy, and low energy electron diffraction. The dominant reaction channel (~ 85%) is intramolecular elimination of ethylene sulfide to gaseous ethylene. Approximately 70% of the ethylene formation occurs upon ethylene sulfide adsorption for all crystal temperatures examined (100, 120, and 140 K). At high coverages, small amounts of ethylene are formed at ~ 200 K, produced during the decomposition of chemisorbed ethylene sulfide. A minor reaction channel (~15%) for ethylene sulfide on Mo(110) is complete decomposition to surface carbon, surface sulfur, and gaseous dihydrogen. For high ethylene sulfide coverages, the dihydrogen produced during decomposition evolves at 375, 460, and 575 K. Ethylene sulfide desulfurization results in the deposition of 0.35 monolayer of atomic sulfur at reaction saturation for adsorption temperatures of <140 K, as measured by Auger electron and X-ray photoelectron spectroscopies. The high ring strain in ethylene sulfide is proposed to account for the low reaction barrier leading to ethylene. The ethylene product does not trap on the surface even for temperatures of 100 K, attributed to formation of ethylene via a concerted transition state with a large component of momentum perpendicular to the surface. A scheme for the reaction is proposed, and the reaction is compared to those of other cyclic Sulfides on Mo(110).