Takei D, Yatabe T, Yabe T, Miyazaki R, Hasegawa JY, Yamaguchi K. C-H Bond Activation Mechanism by a Pd(II)-(μ-O)-Au(0) Structure Unique to Heterogeneous Catalysts.
JACS AU 2022;
2:394-406. [PMID:
35252989 PMCID:
PMC8889553 DOI:
10.1021/jacsau.1c00433]
[Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 10/01/2021] [Indexed: 06/14/2023]
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.
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