Theoretical study on the reaction mechanism and selectivity of acetylene semi-hydrogenation on Ni–Sn intermetallic catalysts

Abstract

Recently, Ni–Sn intermetallic compounds (IMCs) with unique geometric structures have been proved to be selective catalysts for acetylene hydrogenation to ethylene, but the origin of the selectivity remains unclear. In this work, a density functional theory (DFT) study has been carried out to investigate the mechanism of acetylene hydrogenation on six surfaces of Ni–Sn IMCs, and the geometric effects towards ethylene selectivity were revealed. Two key parameters (adsorption energy and the hydrogenation barrier of ethylene), which determine the ethylene selectivity, were studied quantitatively. The adsorption sites for C₂H_y (y = 2, 3, 4) can be classified into three types: Type 1 (Ni₃Sn(111) and Ni₃Sn₂(101)-2) with Ni trimers, Type 2 (Ni₃Sn(001) and Ni₃Sn₂(001)) with Ni monomers, and Type 3 (Ni₃Sn₂(101) and Ni₃Sn₂(001)-2) with reconstructed metal trimers. The adsorption energy (E_ad) decreases following the order: Type 1 > Type 3 > Type 2, which indicates that the adsorption strength depends significantly on site ensemble: a more isolated Ni site would facilitate the desorption of ethylene. However, the surface roughness mainly dominates the hydrogenation barrier of ethylene. Either low or high roughness decreases the interactions between H and C₂H₄ (E_int), resulting in an enhanced energy barrier for over-hydrogenation of C₂H₄ (E_a,hydr); while moderate roughness benefits E_int and lowers E_a,hydr. The selectivity to ethylene is denoted as ΔE_a = E_a,hydr − |E_ad|, thus depending on the interplay of site ensemble effects and surface roughness. From this point of view, Ni₃Sn(001) and Ni₃Sn₂(101) surfaces with well-isolated Ni ensembles and low (or high) surface roughness exhibit decreased |E_ad| and increased E_a,hydr, giving rise to excellent selectivity to ethylene. This work provides significant understanding of the origin of ethylene selectivity in terms of geometric effects, which gives helpful instruction for the design and preparation of intermetallic catalysts for acetylene semi-hydrogenation.