VARIATION OF THE NUMBER OF METAL ATOMS INVOLVED IN ACTIVE-SITES AND OF THE TRUE ACTIVATION-ENERGY OF HYDROCARBON CONVERSION AND CO HYDROGENATION OVER METALS

According to the model of ensemble, most of the kinetic features of a catalytic reaction can be described by a simple equation involving onl y two adjustable parameters, the true activation energy, E(o), and the nuclearity, X, of the active site which consists of an ensemble compo sed of X adjacent free metal atoms. This review paper shows that for o ne given reaction, X does not change from one metal to another (CH4 - D-2 exchange over Ni and Rh). In contrast, it varies dramatically from one reaction to another, addition reaction being associated with low X values, and reactions involving C - H and C - C bond cleavage requir ing progressively larger nuclearities. In contrast, E(o) varies little with the type of reaction which is considered. In the same way, q, th e energy characteristic of active sites, deduced from the slope of the curve obtained by plotting the apparent activation energy, E(a), vers us y, the order with respect to the reactant which is the most abundan t on the surface of the catalyst (Temkin's equation) does not depend m uch on the type of reaction. This yields a unique curve when consideri ng a series of reactions over several metals in a E(a) vs y representa tion. Finally, it is observed that E(o) = q for the case of five react ions, for still unclear reasons.