Selective hydrogenation of acetylene in excess ethylene using Ag- and Au–Pd/SiO2 bimetallic catalysts prepared by electroless deposition

Selective hydrogenation of acetylene in excess ethylene has been investigated for a series of Ag- and Au–Pd/SiO2 bimetallic catalysts having incremental surface coverages of Ag and Au on the Pd surface. The catalysts were prepared by electroless deposition and characterized by atomic absorption spectroscopy and selective chemisorption. Both Ag- and Au–Pd/SiO2 catalysts showed increased selectivity of acetylene conversion to form ethylene, rather than ethane, at high coverages. The catalyst performance results suggest that at high coverages of Ag or Au on Pd, that result in small ensembles of Pd sites, acetylene is adsorbed as a π-bonded species that favors hydrogenation to ethylene. At low coverages, where ensemble sizes of contiguous Pd surface sites are much larger, acetylene is strongly adsorbed as a multi-σ-bonded species which preferentially forms ethane, lowering the selectivity to ethylene. Both kinetic analyses and the calculated turnover frequencies of acetylene conversion and ethane formation were consistent with the above explanation. The similar performance trends for Ag- and Au–Pd/SiO2 suggest that the bimetallic effect for these catalysts is geometric and not electronic in nature.