Effects of carbon and sulfur on the decomposition of hydrocarbons on nickel

Abstract

The kinetics and mechanisms of the decomposition of C2H2, C2H4, and n-C4H10 on polycrystalline Ni resulting in the carburization of the metal have been studied at 1300 to 1500 K and 10−4–10−1 Pa. Special attention has been given to the reaction-retarding effect of surface-segregated carbon. Simultaneous investigations of the reaction rates (by measuring partial pressure changes) and of the nonmetal surface coverages (by applying Auger electron spectroscopy) were carried out in an UHV flow reactor. It was found that hydrocarbon decomposition and carburization are blocked at a certain temperature-dependent concentration of dissolved carbon by the formation of a surface graphitic carbon monolayer in which two C atoms are adsorbed per Ni atom. This monolayer formation is accompanied by surface faceting. The influence of adsorbed sulfur on CH4 decomposition was studied in a flow apparatus by carburizing thin Ni foils in CH4–H2 and CH4–H2–H2S mixtures (1060 to 1280 K, 1-atm total pressure) under conditions where surface reactions are rate controlling. The reaction kinetics were monitored by applying the resistance–relaxation method. For pH2S/pH2 ratios between 8×10−9 and 9×10−5 the initial rate of carburization was found to decrease by a factor of up to 100. The kinetic data are interpreted in terms of the poisoning of active surface sites by adsorbed sulfur. This analysis yields information on the adsorption isotherm for sulfur.