Coverage Effect on the Activity of the Acetylene Semihydrogenation over Pd-Sn Catalysts: A Density Functional Theory Study

The existence of acetylene impurities in ethylene feedstock is harmful to downstream polymerization reactions. The removal of acetylene can be achieved via semihydrogenation reaction, which is normally catalyzed by Pd-based catalysts. This paper describes the coverage effect on the activity of acetylene hydrogenation reactions over Pd and PdSn alloy surfaces. High-coverage models are presented to construct coverage-dependent adsorption energies of C2H2, C2H4, and H-2 on Pd(111) and Pd3Sn(111) surfaces. It has been validated that the downshift of d-band center caused by preadsorbed molecules makes the adsorption weaker along with the increase of coverage, and the geometric effect can be neglected. An iterative method has been applied to predict surface coverages of reaction intermediates. Previous calculations with low-coverage models indicate that alloying Pd with late or post-transition metals, in general, enhances ethylene selectivity, accompanied with lower hydrogenation activity. However, by applying a high-coverage model, we show that the predicted hydrogenation barriers are comparable over Pd(111) and Pd3Sn(111) surfaces.