Decomposition of CH₃SH in a RF plasma reactor: Reaction products and mechanisms

Application of the RF (radio frequency) cold plasma method to the decomposition of methanethiol (methyl mercaptan, CH₃SH) at different O₂/CH₃SH ratios (0-4.5), with various input powers (20-90 W), and at constant operating pressure (30 Torr) was investigated. The species detected in the CH₃SH/O₂/Ar RF plasma were SO₂, CS₂, OCS, CO, CO₂, CH₄, C₂H₄ C₂H₂, H₂, H₂O, HCOH, and CH₃OH. However, CS₂, CH₄, C₂H₄, C₂H₂, H₂, H₂S, CH₃SCH₃ (DMS), and CH₃S₂CH₃ (DMDS) were detected in the CH₃SH/Ar RF plasma. In the CH₃SH/Ar plasma, over 83.7% of the total sulfur input was converted into CS₂ at 60 W; this is due to the lack of competition between O and S and the thermodynamic stability of CS₂. In the oxygen-rich conditions of the CH₃SH/O₂/Ar plasma, the most predominant sulfur-containing compound was SO₂. As the feed O₂/CH₃SH ratio was increased, M_SO2 was increased, while M_CS2 was decreased simultaneously. M_OCS was reduced by increasing either the O₂/CH₃SH ratio or the applied power. From the decay of CS₂ and the generation of CO at a lower O₂/CH₃SH ratio of 0.6, CS, CS₂, and CO were suggested as the primary species to react with O, OH, O₂, S, or S₂ and then to form OCS. This study provides useful insight into the reaction mechanisms involved in the decomposition of CH₃SH and, mainly, the formation of CS₂, CH₄, C₂H₂, C₂H₄, SO₂, MCO₂, and OCS in CH₃SH/Ar and CH₃SH/O₂/Ar plasmas.