Catalytic decomposition of mercaptans on metal films of iron, nickel, palladium, aluminium and copper

Abstract

Adsorption and decomposition of methanethiol, ethanethiol and n-propanethiol on clean and oxidized films of Fe, Pd, Ni, Al and Cu have been studied in the temperature range 193–500 K. Mainly molecular chemisorption of the mercaptans occurred on the films at 193 K. All the mercaptans were chemisorbed dissociatively on all surfaces above 300 K by the rupture of S—H, C—S, C—H and C—C bonds. The proposed dominant pathway involved decomposition of the mercaptan to form a thiolate (—SR) on a surface site and a hydrogen adatom on the adjacent site. In another less important pathway, sulphhydryl (—SH) and alkyl groups are formed. Surface recombination reactions of the bound species took place at temperatures > 300 K yielding hydrogen, hydrocarbons together with some alkyl sulphide. The gaseous products subsequent to the decomposition of the mercaptans on the oxidized metal films also involved other gases, including CO, H₂O and olefins. The pattern of the products formed and their relative concentrations at temperatures 300–500 K indicated that the extent of mercaptan dissociation decreased in the order S—H, C—S > C—H > C—C. The rate of mercaptan adsorption directly depended on its pressure and the kinetic data revealed the operation of a compensation effect throughout the interaction of the mercaptan with various films. The transition-metal films showed greater activity than Al and Cu, and among the former metals Fe exhibited the greatest activity for mercaptan adsorption.