Hydrocarbon cracking reactions on nickel

Abstract

The cracking reactions of several low molecular weight hydrocarbons on a nickel carbide catalyst have been studied in the absence of added hydrogen. Kinetic measurements have shown that the Arrhenius parameters for product methane formation, from different reactants, were closely similar, and this is considered to be evidence that the reaction which immediately precedes the product methane desorption step is rate-controlling. During such cracking reactions, in which product carbon was deposited on the solid phase, a small concentration of metallic nickel was maintained in the catalyst. It seems probable, therefore, that crystalline nickel was the active solid in cracking and that crystalline nickel carbide was not a necessary intermediate in nickel-catalyzed cracking and hydrocracking reactions. The results obtained in the present study are compared with data for other reactions occurring at nickel surfaces and the significance of the obedience of the Arrhenius parameters, measured for these reactions, to the compensation law is considered. It is concluded that the compensation effect results from the establishment of surface equilibria, the position of which is determined largely by hydrogen availability, on the active surface of the solid catalyst. Such surface equilibria involve species formed by dissociation of gaseous products and reactants and (possibly) also surface metal atoms.