C–H Bond Activation Mechanism by a Pd(II)–(μ-O)–Au(0) Structure Unique to Heterogeneous Catalysts

Abstract

We focused on identifying a catalytic active site structure at the atomic level and elucidating the mechanism at the elementary reaction level of liquid-phase organic reactions with a heterogeneous catalyst. In this study, we experimentally and computationally investigated efficient C–H bond activation for the selective aerobic α,β-dehydrogenation of saturated ketones by using a Pd–Au bimetallic nanoparticle catalyst supported on CeO2 (Pd/Au/CeO2) as a case study. Detailed characterization of the catalyst with various observation methods revealed that bimetallic nanoparticles formed on the CeO2 support with an average size of about 2.5 nm and comprised a Au nanoparticle core and PdO nanospecies dispersed on the core. The formation mechanism of the nanoparticles was clarified through using several CeO2-supported controlled catalysts. Activity tests and detailed characterizations demonstrated that the dehydrogenation activity increased with the coordination numbers of Pd–O species in the presence of Au(0) species. Such experimental evidence suggests that a Pd(II)–(μ-O)–Au(0) structure is the true active site for this reaction. Based on density functional theory calculations using a suitable Pd1O2Au12 cluster model with the Pd(II)–(μ-O)–Au(0) structure, we propose a C–H bond activation mechanism via concerted catalysis in which the Pd atom acts as a Lewis acid and the adjacent μ-oxo species acts as a Brønsted base simultaneously. The calculated results reproduced the experimental results for the selective formation of 2-cyclohexen-1-one from cyclohexanone without forming phenol, the regioselectivity of the reaction, the turnover-limiting step, and the activation energy.