C2H2-Induced Surface Restructuring of Pd–Ag Catalysts: Insights from Theoretical Modeling
In this paper, the influence of acetylene chemisorption on the equilibrium distribution of atoms in the (111) surface layer of Pd–Ag alloys is studied. All possible surface layer compositions are considered from a statistical mechanics based model with parameters determined from first-principles computations. Surface restructuring is investigated at three different temperatures. In conditions relevant for the catalytic acetylene selective hydrogenation, the equilibrium surface distribution of alloy atom type strongly deviates from the smooth binomial random case and specific ensembles are favored, with sharp transition zones between them as a function of Ag content. Indeed, the chemisorption energy of acetylene on Pd rich ensembles provides a contribution to the free energy that counterbalances the entropic gain from the random alloy distribution. The molecule coverage and the associated energetic term are weakened by thermal effects and lateral interactions between adsorbates.