Activation by O₂ of AgPd_1- Alloy Catalysts for Ethylene Hydrogenation

A composition spread alloy film (CSAF) spanning all of AgPd_1- composition space, x_Pd = 0 → 1, was used to study catalytic ethylene hydrogenation with and without the presence of O₂ in the feed gas. High-throughput measurements of the ethylene hydrogenation activity of AgPd_1- 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_H2ⁱⁿ = 70 → 690 Torr. The inlet ethylene partial pressure was constant at P_C2H4ⁱⁿ = 25 Torr, and the O₂ inlet partial pressure was either P_O2ⁱⁿ = 0 or 15 Torr. When P_O2ⁱⁿ = 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 ∼0.4 at T = 405 K and P_H2ⁱⁿ = 690 Torr. Adding a constant O₂ partial pressure of P_O2ⁱⁿ = 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 O₂ consumption. The presence of P_O2ⁱⁿ = 15 Torr more than doubled the maximum achievable conversion on Pd 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 ≈ 0 when P_O2ⁱⁿ = 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_O2ⁱⁿ = 0 Torr. We attribute this P_O2ⁱⁿ-induced change in n_H2 to a change in the reaction mechanism resulting from different functional catalyst surfaces: one that is O₂-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 O₂ at moderate temperatures leads to enhanced catalyst performance, presumably by stimulating both Pd segregation to the topmost surface and Pd activation for ethylene hydrogenation.