Engineering of Ag@Pd/Al2O3 with varied Pd-shell thickness: Dynamic evolution of ligand and strain effects on acetylene selective hydrogenation

Abstract

Bimetallic nanoparticles exhibit a synergistic effect that critically depends on their surface composition, but such promotion mechanisms become vague with varying surface compositions. Here, alumina supported Ag@Pd core–shell and PdAg alloy structure with controlled size and surface compositions were prepared to demonstrate synergetic mechanisms, particularly, ligand and strain effects on activity and ethylene selectivity for acetylene hydrogenation. The performance evaluation indicates that Ag@Pd catalysts with well-controlled Pd-shell thickness can effectively lower apparent activation energy and improve ethylene selectivity. Hydrogenation activity increases from 0.019 to 0.062 s−1 with decreasing Pd-shell thickness under mild conditions, which is 3–6 times higher than their alloyed and monometallic counterparts. Combined characterizations and density functional theory are conducted to reveal such shell-thickness-dependent performance. The ligand effect arising from Ag alloying in the interface of Ag@Pd2ML observes the strongest binding of acetylene, but it diminished sharply and the strain effect gets more prevailing with increasing shell thickness. The competition of ethylene desorption and deep-hydrogenation were also investigated to understand the selectivity governing factors, and the selectivity descriptor (0.5BE(C2H4) – BE(H)) was built to match the contribution of ligand and strain effect on the different surfaces of Pd-Ag bimetallic NPs. The exploration of synergetic mechanisms among bimetallic NPs with varied structure and surface compositions in this work can help us to deepen the understanding catalyst structure–activity relationship and provide a feasible way to optimize the overall catalytic performance.