ABSTRACT:Restructuring of alloy surfaces induced by strongly bound adsorbates is a well-establish phenomenon occurring in catalysis and membrane science. In catalytic processes this restructuring can have profound effects since 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, i.e., 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 weakling binding Pd 3 ensembles and strongly binding Pd 2 Ag ensembles. The second critical aspect is related to lateral interactions, which preclude adsorption of acetylene molecules on nearest neighbor triangular sites. Therefore, in a Pd 3 island, roughly two thirds of Pd 3 sites would be lost. Our study suggests that the equilibrium structure of these alloy catalysts under operando conditions are far from the state targeted by catalyst design, revealing a nearly unavoidable reason for loss of selectivity of the catalyst with time of operation.

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