Activation by O2 of Ag X Pd1-x Alloy Catalysts for Ethylene Hydrogenation

A composition spread alloy film (CSAF) spanning all of AgxPd1-x composition space, x(Pd) = 0 -> 1, was used to study catalytic ethylene hydrogenation with and without the presence of O-2 in the feed gas. High-throughput measurements of the ethylene hydrogenation activity of AgxPd1-x alloys were performed at 100 Pd compositions spanning x(Pd) = 0 -> 1. The extent of ethylene hydrogenation was measured versus x(Pd) at reaction temperatures spanning T = 300 -> 405 K and inlet hydrogen partial pressures spanning P-H2(in) = 70 -> 690 Torr. The inlet ethylene partial pressure was constant at P-C2H4(in) = 25 Torr, and the O-2 inlet partial pressure was either P-O2(in) = 0 or 15 Torr. When P-O2(in) = 0 Torr, only those alloys with x(Pd) >= 0.90 displayed observable ethylene hydrogenation activity. As expected, the most active catalyst was pure Pd, which yielded a maximum conversion of similar to 0.4 at T = 405 K and P-H2(in) = 690 Torr. Adding a constant O-2 partial pressure of P-O2(in) = 15 Torr to the feed stream dramatically increased the catalytic activity across the CSAF at all experimental conditions and catalyst compositions without inducing catalytic ethylene combustion and without measurable O-2 consumption. The presence of P-O2(in) = 15 Torr more than doubled the maximum achievable conversion on Pd to similar to 0.9 and activated alloys with as little as x(Pd) = 0.6 for ethylene hydrogenation. Measurement of the reaction order with respect to hydrogen, n(H2), showed that n(H2) approximate to 0 when P-O2(in) = 15 Torr on high x(Pd) alloys but that n(H2) increases to values between 0.5 and 1 as x(Pd) decreases or when P-O2(in) = 0 Torr. We attribute this P-O2(in)-induced change in n(H2) to a change in the reaction mechanism resulting from different functional catalyst surfaces: one that is O-2-activated and Pd-rich and one that is Ag-capped with low activity. Both are extremely sensitive to the bulk alloy composition, x(Pd), and the reaction temperature, T. These results show that the activity of AgPd catalysts for ethylene hydrogenation depends strongly on the operational conditions. Furthermore, we demonstrate that the exposure of AgPd catalysts to 15 Torr of O-2 at moderate temperatures leads to enhanced catalyst performance, presumably by stimulating both Pd segregation to the topmost surface and Pd activation for ethylene hydrogenation.