On the Limited Role of Electronic Support Effects in Selective Alkyne Hydrogenation: A Kinetic Study of Au/MO\u3csub\u3ex\u3c/sub\u3e Catalysts Prepared from Oleylamine-Capped Colloidal Nanoparticles

We report a quantitative kinetic evaluation and study of support effects for partial alkyne hydrogenation using oleylaminecapped Au colloids as catalyst precursors. The amine capping agents can be removed under reducing conditions, generating supported Au nanoparticles of ~2.5 nm in diameter. The catalysts showed high alkene selectivity (\u3e90%) at all conversions during alkyne partial hydrogenation. Catalytic activity, observed rate constants, and apparent activation energies (25– 40 kJ/mol) were similar for all Au catalysts, indicating support effects are relatively small. Alkyne adsorption, probed with FTIR and DFT, showed adsorption on the support was associated with hydrogen-bonding interactions. DFT calculations indicate strong alkyne adsorption on Au sites, with the strongest adsorption sites at the metal-support interface (MSI). The catalysts had similar hydrogen reaction orders (0.7–0.9), and 1- octyne reaction orders (~ 0.2), suggesting a common mechanism. The reaction kinetics are most consistent with a mechanism involving the non-competitive activated adsorption of H2 on an alkyne-covered Au surface

Supported Ni-Au Colloid Precursors for Active, Selective, and Stable Alkyne Partial Hydrogenation Catalysts

Bimetallic NiAu catalysts have garnered broad interest for a variety of reactions including automotive emissions, selective hydrogenation, selective oxidation, hydrodechlorination, and biomass conversion. However, the bulk immiscibility of the two metals, complicating catalyst synthesis, has limited studies of this bimetallic system. We report a solution-phase synthesis for Ni and bimetallic NiAu heterogeneous catalysts. Using oleylamine as a capping agent, an optimized synthesis for Ni catalysts led to supported particles with a narrow size distribution (4.7 ± 0.4 nm). Gold was added to the Ni nanoparticles via galvanic displacement of Ni in organic solution, the particles were deposited onto commercial alumina, and oleylamine capping agent was removed. The catalytic activity of the bimetallic materials in 1-octyne partial hydrogenation was in between the activity of monometallic Ni and Au catalysts. At high space velocity, the bimetallic catalysts largely maintained the high alkene selectivity associated with Au catalysts (\u3e90% alkene selectivity at a 95% conversion). At lower space velocities, the NiAu catalysts also had a reduced propensity to overhydrogenate the alkene (relative to Ni). A simple catalyst performance parameter, which combined activity, selectivity, and space velocity, was developed and used to describe the overall performance of each catalyst under varying reaction conditions. By this metric, the bimetallic catalysts had considerably better performance than monometallic Ni. The most active bimetallic catalyst was examined with a week-long stability test; it showed no activity loss with a 100% carbon balance. Catalysts were characterized by transmission electron microscopy, X-ray diffraction, H2 and N2 adsorption, and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The reactivity and characterization studies suggest the active catalysts are likely composed of bimetallic NiAu surfaces. The incorporation of Au into the catalysts suppresses H2 adsorption on Ni, leading to lower hydrogen coverage during catalysis; this contributes to slowing undesirable alkene hydrogenation and improving catalyst selectivity