Activation by O₂ of Ag_xPd_1–x Alloy Catalysts for Ethylene Hydrogenation

Abstract

A composition spread alloy film (CSAF) spanning all of Ag x Pd 1−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 Ag x Pd 1−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 ∼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 ∼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 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.

H2–D2 Exchange Activity and Electronic Structure of AgxPd1–x Alloy Catalysts Spanning Composition Space

“…While surface enrichment of Ag is reported for Ag x Pd 1– x alloys under vacuum and inert atmospheres due to its lower surface free energy, , inverse surface segregation phenomena have been reported both experimentally and computationally , for Ag x Pd 1– x alloys in H 2 -rich atmospheres. In fact, we have recently shown that a Ag x Pd 1– x CSAF nearly identical to the one studied here could be activated for ethylene hydrogenation by inducing inverse surface segregation phenomena using trace amounts of a spectator species in the reaction mixture . The key point to make is that the surface structure and electronic environment of Ag x Pd 1– x alloys can be highly variable, resulting in significant changes to catalytic properties under reaction conditions.…”

“…Alloys and multicomponent materials are often used as catalysts because they possess catalytic properties superior to those of their pure components . The beneficial properties of alloys arise from changes in ligand, strain, and ensemble effects, which influence the physical and electronic characteristics of the material to enhance its catalytic activity, selectivity, and resistance to poisoning. , As a result, the catalytic activity of an alloy catalyst can be tuned using its composition. , To properly understand how alloying affects the kinetics of chemical reactions, it is necessary to measure reaction rates across a wide range of catalyst compositions.…”