Evolution of the surface sulfur composition of a Ru sulfide particle during CH3SH condensation reaction: sulfur migration from the bulk to the surface

Abstract

The condensation of methanethiol (CH₃SH) into dimethylsulfide (CH₃SCH₃), which is a useful test reaction for probing the acid–base character of transition metal sulfide catalysts for carbon–heteroatom hydrogenolysis reactions, shows a systematic deactivation process at the beginning of the reaction before reaching steady-state activity. The solid surface of RuS₂ catalyst in equilibrium with the gas phase was examined by various techniques with the aim of understanding the mechanism responsible for this loss of activity, e.g. coke formation, poisoning by the reactant or the products, change in particle size, etc. X-Ray diffraction and elemental analysis showed neither the formation of carbonaceous deposits nor a sintering effect during the catalytic run. In contrast, a modification of the surface composition of the catalyst induced by the reaction was observed using temperature programmed reduction (TPR). This technique showed that large amounts of H₂S are detectable after performing the catalytic test. Sulfur mass balance analyses demonstrate that these sulfur species are an integral part of the total sulfur content of the ruthenium sulfide particles. These surface modifications arise from sulfur migration from the bulk to the surface of the particles during reation. Based on these experimental data and on a crystallographic model developed for RuS₂, it is proposed that the methanethiol condensation reaction proceeds on monovacant Ru sites.