Tunneling effects in confined gold nanoparticle hydrogenation catalysts

Clean surface gold nanoparticles (AuNPs) of ∼6.6 nm that were confined in ionic liquid (IL) cages of hybrid γ-alumina (γ-Al 2 O 3 ) displayed hydrogenation pathways in the reduction of trans -cinnamaldehyde distinct from those imprinted directly onto γ-Al 2 O 3 . Hydrogen activation proceeded via homolytic activation in IL-encapsulated AuNPs and via heterolytic cleavage for IL-free supported AuNPs. Higher negative apparent entropy (Δ S app ) values were obtained for the IL-confined AuNPs compared to the non-hybrid catalyst (Au/γ-Al 2 O 3 ), suggesting a decrease in the number of microstates induced by the nano-confined environment. High kinetic isotope effect (KIE) values ( k H / k D = 2.5-2.9 at 273 K) and Arrhenius convex curves were observed. Furthermore, differences of 5.6 and 6.2 kJ mol −1 between the apparent activation energies of the deuteration and hydrogenation reactions ( E D a-app − E H a-app ) associated with pre-exponential factor ratios ( A D / A H ) of 4.6 and 5.1 provided strong evidence of the possible involvement of a tunneling pathway in the case of the confined AuNPs. H 2 activation proceeds via homolytic activation over AuNPs confined in ionic liquid hybrid supports and display high kinetic isotope effects and convex curves in the Arrhenius plots, which suggests the possible involvement of a tunneling pathway.