SIZABLE DEACTIVATION EFFECT FOR THE 1,3-BUTADIENE HYDROGENATION ON VAPOR-DEPOSITED PD AGGREGATES ON GRAPHITE

The catalytic activity of Pd aggregates vapor-deposited on graphite ha s been measured in the 1,3-butadiene hydrogenation. It is generally de creasing with particle size. But, such a decrease is largely depending upon the hydrogen and the hydrocarbon pressure. Indeed, with low hydr ocarbon pressure (about 1 Torr) and in presence of large excess of hyd rogen (pH(2)/PC4H6 = 100) the activity of the smaller aggregates is co mparable to that of larger particles. The different behaviour is conse quently attributed to a size dependent deactivation. Such a deactivati on could be due to carbon deposits generated by dehydrogenation of the diene admolecules. This behaviour is tentatively related with marked differences observed by XPS on the smaller aggregates, both with respe ct to the core level energies and to the density of states into the va lence band.