Tunneling effects in confined gold nanoparticle hydrogenation catalysts

Abstract

Clean surface gold nanoparticles (AuNPs) of ∼6.6 nm that were confined in ionic liquid (IL) cages of hybrid γ-alumina (γ-Al₂O₃) displayed hydrogenation pathways in the reduction of trans-cinnamaldehyde distinct from those imprinted directly onto γ-Al₂O₃. Hydrogen activation proceeded via homolytic activation in IL-encapsulated AuNPs and via heterolytic cleavage for IL-free supported AuNPs. Higher negative apparent entropy (ΔS_app) values were obtained for the IL-confined AuNPs compared to the non-hybrid catalyst (Au/γ-Al₂O₃), 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.