Catalytic oxidation II. Silica supported noble metals for the oxidation of ethylene and propylene

The catalytic oxidation of C2H4 and C3H6 was studied over silica-supported Pt, Pd, Ir, Ru, and Rh. The specific activities fell in the order listed and correlated with the percentage d-character of the metal and fairly well with its atomic radius. The partial oxidation products were characterized. Acetaldehyde and acetic acid were produced from C2H4 in all cases, but only with Pd and Ir were the quantities significant. Products from C3H6 were more varied. C3 products in substantial amounts were formed over Rh, Ru, and Pd, the selectivity decreasing in the order given. Somewhat smaller amounts of C2 products were also formed over these catalysts. The principal product over Ir was acetic acid; the most active catalyst, Pt, was relatively unselective. Three different types of pressure dependencies were found: the rate of total oxidation decreased with olefin pressure and increased with oxygen pressure over Pt and Pd; the rate increased with olefin pressure and was inhibited by oxygen over Rh and Ir; and the rate increased with oxygen pressure, but was nearly independent of olefin pressure over Ru. Activation energies varied from 16 to 29 kcal/mole and tended to correlate inversely with the catalytic activity. The order of activity depended upon the ability of the metal to activate O2.