The effects of branching on the reactive decomposition of sulfur-containing organic species on clean and modified iron(100) surface

The thermal decomposition of neopentyl thiol, n-alkyl sulfides, and 2,2-dimethyldodecane thiol (2-DMDT) on clean and oxygen modified Fe(100) surfaces has been investigated. These compounds were chosen as model systems to probe the effect of molecular structure on corrosion inhibitor thermal stability. These systems were characterized using AES (Auger electron spectroscopy), HREELS (high resolution electron energy loss spectroscopy), and TPRS (temperature programmed reaction spectroscopy). On the clean Fe(100) surface upon adsorption at 100 K, the S-H and C-S bonds of the neopentyl thiol cleave resulting in adsorbed S, H and neopentyl alkyl fragments. Upon heating, some neopentyl alkyl fragments react with adsorbed surface hydrogen to evolve neopentane at 260 K. On the oxygen modified surface only S-H bond cleavage is observed. and a neopentyl thiolate surface species is formed. Neopentane evolution is not observed from the oxygen modified surface upon initial heating. On both surfaces isobutene and trace amounts of neopentane are seen to desorb at 380 K. Small hydrocarbon fragments remain on the surface until they finally decompose to leave atomic carbon and sulfur on the iron surface at 500 K. The adsorption, desorption, and decomposition of ethyl and butyl sulfides has also been investigated on the Fe(100) surface. On the clean Fe(100) surface alkyl sulfides adsorb to form a chemisorbed layer, or additionally at higher exposures a physisorbed multilayer. The desorption energy of this physisorbed layer is chain length dependent. The chemisorbed layer of ethyl sulfide also partially desorbs molecularly, however butyl sulfide only decomposes reactively. These species form a mixture of alkane thiolates and alkyl species on the surface. The chemisorbed layer for both sulfides decomposes by reductive elimination and hydrogen recombination to evolve the alkene and corresponding alkane at 300K with no dependence on chain length. Atomic sulfur and carbon are left on the surface. The adsorption and subsequent decomposition of 2,2-dimethyidodecane thiol (2-DMDT) was investigated on the clean Fe(100) surface. This molecule was designed to retain the beta-branching of neopentyl thiol and to add a C10 alkyl chain to potentially minimize alkyl-hydrogen recombination. Upon adsorption, this species desorbs molecularly at 200 K and 240 K. The 200 K desorption is due to the molecule physisorbed to the bare surface, and the 240 K desorption is due to a multilayer state. Further heating results in reactive decomposition, with the major decomposition occurring at 590 K. This is a much higher temperature than has been observed in the neopentyl thiol or alkyl sulfide cases. The mechanistic origins of this enhancement in thermal stability is discussed.