LIQUID-PHASE CATALYTIC-HYDROGENATION OF UNSATURATED-COMPOUNDS - A NEW VIEW ON THE CAUSES OF THE ZERO ORDER OF THE REACTION

The hydrogenation of maleic acid has been studied as a model hydrogenation of unsaturated double-bond compounds in aqueous and alcoholic 1 M H2SO4 solutions. The curves of hydrogenation uptake time and catalyst potential time show that as the reaction proceeds the degree of surface coverage by the organic substance decreases with the logarithm of its bulk concentration. The kinetic equation derived for the reaction indicates that the decrease in the surface coverage is offset by a change in the catalyst potential or in the chemical potential of adsorbed hydrogen, resulting in a constant hydrogenation rate and zero-order reaction with respect to the unsaturated compound. © 1990.