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, CH 3 SH) at different O 2 /CH 3 SH 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 CH 3 SH/O 2 /Ar RF plasma were SO 2, CS 2, OCS, CO, CO 2, CH 4, C 2 H 4 C 2 H 2, H 2, H 2 O, HCOH, and CH 3 OH. However, CS 2, CH 4, C 2 H 4, C 2 H 2, H 2, H 2 S, CH 3 SCH 3 (DMS), and CH 3 S 2 CH 3 (DMDS) were detected in the CH 3 SH/Ar RF plasma. In the CH 3 SH/Ar plasma, over 83.7% of the total sulfur input was converted into CS 2 at 60 W; this is due to the lack of competition between O and S and the thermodynamic stability of CS 2 . In the oxygen-rich conditions of the CH 3 SH/O 2 /Ar plasma, the most predominant sulfur-containing compound was SO 2 . As the feed O 2 /CH 3 SH ratio was increased, M SO 2 was increased, while M CS 2 was decreased simultaneously. M OCS was reduced by increasing either the O 2 /CH 3 SH ratio or the applied power. From the decay of CS 2 and the generation of CO at a lower O 2 /CH 3 SH ratio of 0.6, CS, CS 2, and CO were suggested as the primary species to react with O, OH, O 2, S, or S 2 and then to form OCS. This study provides useful insight into the reaction mechanisms involved in the decomposition of CH 3 SH and, mainly, the formation of CS 2, CH 4, C 2 H 2, C 2 H 4, SO 2, CO, CO 2, and OCS in CH 3 SH/Ar and CH 3 SH/O 2 /Ar plasmas.