Abstract
The co-adsorption of hydrogen and carbon monoxide on Pd3Ag(111) alloy surfaces has been studied as a model system for Pd-Ag alloys in membrane and catalysis applications using periodic density functional theory calculations (PW91-GGA). We explored the effects of Pd–Ag surface composition, since segregation of silver towards and away from the surface has been suggested to explain the experimentally observed changes in H2 activation, CO inhibition and reactivity. We found that CO pre-adsorbed on the surface weakens the adsorption of H on Pd3Ag(111) alloy surfaces irrespective of whether the surface termination corresponds to the bulk Pd3Ag composition, or is purely Pd-terminated. A higher coverage of H with CO present is obtained for the Pd-terminated surface; this surface also exhibits a larger range of chemical potentials for co-adsorbed hydrogen and CO. The barrier for H2 activation increases with increasing CO coverage, but the surface composition has the largest impact on H2 activation at intermediate CO coverage. The results imply that Pd-based membranes with typically ~ 23 wt% Ag are less prone to CO poisoning if the surface becomes Pd-terminated.
Highlights
Palladium and its alloys are important because of their catalytic properties in hydrogenation, oxidation and other reactions
We have long been involved in experimental work on thin, polycrystalline (1–10 μm) Pd alloy membranes, that may enable highly efficient separation of hydrogen from mixtures containing CO, CO2, H2O and CH4; essentially allowing a CO2 rich retentate on the high-pressure side that is suitable for C O2 capture and sequestration (CCS) [11]
Through periodic density functional theory (DFT) calculations, we have addressed the effect of surface composition and segregation on co-adsorption of hydrogen and CO to understand the reactivity
Summary
Palladium and its alloys are important because of their catalytic properties in hydrogenation, oxidation and other reactions. Understanding the energetics for adsorption of relevant atomic and molecular species as well as the segregation behavior of Pd–Ag alloys, and how these mutually affect each other, is important. The interaction of CO + H2 with P d1−xAgx surfaces is of particular relevance to membrane separation of hydrogen produced from carbon containing feedstock (natural gas, biomass or coal), as well as CO hydrogenation catalysis and the electro-oxidation of alcohols [9, 10]. Due to the low thickness and fabrication by sputtering that allows elimination of bulk and gas phase transport limitations in the membrane separation configuration, the effect of surface phenomena becomes more apparent. That the CO inhibition effect on ~ 3 μm thin
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