Synergistic and size effects in selective hydrogenation of alkynes on gold nanocomposites

Au/Al2O3 nanocomposites possess high activity and stability in the selective hydrogenation of phenylacetylene at 423 K. Strong dependences of the activity and selectivity on the particle size were revealed: as the diameter of gold nanoparticles was decreased from 30 to 2.5 nm, the TOF of phenylacetylene hydrogenation increased from 0.028 to 0.142 s−1 and the selectivity of styrene formation increased by an order of magnitude. The same trends were observed in the hydrogenation of acetylene into ethylene on supported gold. The nature of the obtained dependences is discussed in terms of “Geometric” and “Electronic” size effects. A synergistic catalytic effect was revealed for Au-Ni/Al2O3: the conversion of acetylene on the Au-Ni catalysts was higher than the sum of conversions on Au and Ni catalysts by an order of magnitude. The appearance of synergistic activity in Au-Ni/Al2O3 is discussed in terms of formation of new Aσ+ catalytic sites with improved adsorption and catalytic properties due to electron transfer from gold to oxidized nickel.