Summary of Mechanism Studies

Summary of Mechanism Studies.—The recent research does not solve old problems. There are areas of disagreement in the findings, no doubt due to different catalysts and experimental conditions. No single primary product can be identified. Carbon oxides are often the major products leaving the catalyst surface, in relative concentrations dependent upon the efficiency of the catalyst for the CO shift reaction (4). On Rh the CO content tends to be high and the CO 2 low, probably indicative of lower activity for the shift reactions compared with Ni, where CO tends to be lower. Methane however can be formed directly from the higher hydrocarbons at 600 °C and above, as well as by methanation of carbon oxides. 

Most workers favour a 2-site mechanism, involving adsorption and disruption of the hydrocarbon molecule on the metal to produce single-carbon species such 

Kasaoka, M. Omoto, T. Watamabe, and K. Takamatsu, Nippon Kagaku Kaishi, 1975, 8, 1418. 

Formation of lead soaps appears to be the mechanism by which lead-based paints inhibit corrosion of clean steel. When formulated with linseed oil, lead reacts with components of the oil to form soaps in the cured film in the presence of water and oxygen, these soaps degrade to, among other things, salts of a variety of mono- and di-basic aliphatic acids. The lead salts of azelaic, suberic, and pelargonic acid act as corrosion inhibitors lead azelate is of particular importance in LBP. These acids may inhibit corrosion by bringing about the formation of insoluble ferric salts, which reinforce the metal s oxide film until it becomes impermeable to ferrous ions, thus suppressing the corrosion mechanism. 

The formation of lead soaps is believed to be the critical corrosion-protection step for both new (clean) steel and rusted steel. 

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