Catalytic oxidation. I. The oxidation of ethylene over Pd and PdAu alloys

The gross kinetics for the oxidation of ethylene were determined over Pd and PdAu alloys in a steady-state flow reactor and in a recirculating reactor in which the products were stripped with each pass. The partial oxidation products over Pd included acetic acid, acetic anhydride, acetaldehyde, and (a trace) ethylene oxide. Initial selectivity for partial oxidation varied between 25 and 45%, depending upon reaction conditions; steady-state values fell in the range, 6-25%. Both the total rate and selectivity increased with O₂ pressure. The rates of both total and partial oxidation passed through maxima near 5 Torr with varying C₂H₄ pressure, changing from first-order at lower pressures to negative-order at higher. The apparent activation energy, based on initial rate data was 20 ± 2 kcal/mole for both complete and partial oxidation, but was 30 ± 2 kcal/mole for steady-state measurements. The difference may reflect effects of poisoning by products. When small amounts of acetaldehyde-¹⁴C were added to the reaction, 80% of the radioactivity was recovered in the acetic acid and 10% as CO₂. Added ethylene oxide-¹⁴C was recovered unreacted. The C₂H₄ reacted about 1.4 times faster than C₂D₄. Scrambling in the unreacted ethylenes during oxidation of equimolar mixtures was minimal at low ethylene pressures. No ¹⁶O¹⁸O was found when mixtures of ¹⁶O₂ and ¹⁸O₂ were used to oxidize ethylene, but the isotopic distribution in the product CO₂ was near equilibrium. The PdAu alloys were much less active than pure Pd and much more active than Au. The selectivity, however, went through a maximum (~50%) near 20% Pd. A similar maximum was found for fast O₂ chemisorption.