HYDROGEN A PROBE MOLECULE AND A REACTANT ON PLATINUM CATALYSTS

The main results in the literature concerning the thermodynamics, the kinetics and the stoichiometry of adsorption of hydrogen in gas phase are compared to those obtained in liquid phase by transient electroche mical methods on platinum catalysts. Two states of adsorbed hydrogen a re identified : one strongly bound and not very mobile, the other weak ly bound and mobile on the surface of the catalyst. The difference in the binding energy of that two states is close to 50kJ./mol. The catal ytic hydrogenation of unsaturated compounds proves that only weakly bo und hydrogen (or the most mobile) is active. The nature of the hydroge n (or the binding energy Pt-H) involved in the reaction varies with th e nature of the unsaturated compounds under consideration. By controll ing the nature of the adsorbed hydrogen (by means of the potential con trol of the catalyst) it is possible to orient the selectivity of the hydrogenation reactions.