Achieving High Selectivity for Alkyne Hydrogenation at High Conversions with Compositionally Optimized PdAu Nanoparticle Catalysts in Raspberry Colloid-Templated SiO2

Improving the selectivity for catalytic hydrogenation of alkynes is a key step in upgrading feedstocks for olefin polymerization. Herein, dilute PdxAu1-x alloy nanoparticles embedded in raspberry colloid-templated silica (x = 0.02, 0.04, and 0.09) are demonstrated to be highly active and selective for the gas-phase hydrogenation of 1-hexyne, exhibiting higher selectivity than pure Pd at high conversion. The conversion of 1-hexyne remains high even for the very low amounts of Pd in Pd0.02Au0.98. These catalysts are highly resistant to sintering-addressing a long-standing challenge in the use of Au-based catalysts. Clear evidence is presented that the addition of the second hydrogen to the half-hydrogenated intermediate is the rate-limiting step and that the stability of the half-hydrogenated intermediate of the alkyne is higher than the half-hydrogenated alkene, which explains the high selectivity even at high conversions. Moreover, of the three compositions investigated, optimum selectivity and activity are observed for the nanoparticles containing 4% Pd. The apparent activation energy for production of 1-hexene from 1-hexyne is measured to be 38 kJ mol(-1) for the Pd0.04Au0.96 catalysts, which is similar to 14 kJ mol(-1) lower than for pure Pd. The hydrogenation is completely, but reversibly, suppressed by adding CO to the reactant mixture, indicating that the Pd centers are the active sites for reaction. The method of templating used in preparation of the catalysts is highly customizable and versatile. This study demonstrates that the composition of the nanoparticles as defined by the dilution ratio of Pd in Au and by the method used to make the supported catalyst is an important tunable parameter that can be used to optimize activity and selectivity of bimetallic systems.