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Acetylene Adsorption on Pd–Ag Alloys: Evidence for Limited Island Formation and Strong Reverse Segregation from Monte Carlo Simulations

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journal contribution
posted on 2018-06-07, 00:00 authored by Emanuele Vignola, Stephan N. Steinmann, Katell Le Mapihan, Bart D. Vandegehuchte, Daniel Curulla, Philippe Sautet
Restructuring of alloy surfaces induced by strongly bound adsorbates is a well-established phenomenon occurring in catalysis and membrane science. In catalytic processes, this restructuring can have profound effects because it alters the ensemble distribution between the as-prepared state of the catalyst and the catalytic surface under operando conditions. This work assesses the restructuring of Pd–Ag alloys induced by adsorption of acetylene in the framework of the ensemble formalism. A detailed Ising-type model Hamiltonian of the (111) surface plane is fitted to extensive density functional theory computations. The equilibrium distributions under a realistic environment are then evaluated by a Monte Carlo approach as a function of temperature and alloy composition. Acetylene induces a strong reverse segregation within the relevant range of temperature. Therefore, the surface of Pd–Ag catalysts is almost entirely covered by Pd for bulk ratios <0.8 Ag–Pd, which is, in general, detrimental to the selectivity of Pd–Ag catalysts. Despite the very strong vertical segregation, acetylene only induces marginal in-plane ordering, that is, the surface triangular ensembles follow random distributions as a function of the surface layer Ag fraction quite closely. This can be explained by two factors: first, triangular sites are not sufficient to fully capture the diversity of acetylene binding energies on Pd–Ag alloy surfaces. Rather, an extended environment including the first coordination sphere is necessary and leads to an overlap in terms of binding energy between weakly binding Pd3 ensembles and strongly binding Pd2Ag ensembles. The second critical aspect is related to lateral interactions, which preclude adsorption of acetylene molecules on nearest neighbor triangular sites. Therefore, in a Pd3 island, roughly two thirds of Pd3 sites would be lost. Our study suggests that the equilibrium structure of these alloy catalysts under operando conditions is far from the state targeted by the catalyst design, revealing a nearly unavoidable reason for loss of selectivity of the catalyst with time of operation.

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