PNNL

Abstract

Aqueous-phase electrocatalytic hydrogenation of benzaldehyde on Cu leads not only to benzyl alcohol (the carbonyl hydrogenation product), but Cu also catalyzes carbon–carbon coupling to hydrobenzoin. In the absence of an organic substrate, H2 evolution proceeds via the Volmer–Tafel mechanism on Cu/C, with the Tafel step being rate-determining. In the presence of benzaldehyde, the catalyst surface is primarily covered with the organic substrate, while H* coverage is low. Mechanistically, the first H addition to the carbonyl O of an adsorbed benzaldehyde molecule leads to a surface-bound hydroxy intermediate. The hydroxy intermediate then undergoes a second and rate-determining H addition to its a-C to form benzyl alcohol. The H additions occur predominantly via the proton-coupled electron transfer mechanism. In a parallel reaction, the radical a-C of the hydroxy intermediate attacks the electrophilic carbonyl C of a physisorbed benzaldehyde molecule to form the C–C bond, which is rate-determining. The C–C coupling is accompanied by the protonation of the formed alkoxy radical intermediate, coupled with electron transfer from the surface of Cu, to form hydrobenzoin. H.C. is grateful to the Chinese Scholarship Council for the financial support. J.A.L. was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences (Transdisciplinary Approaches to Realize Novel Catalytic Pathways to Energy Carriers, FWP 47319). M.A.H. would like to acknowledge the Humboldt Research Fellowship for experienced researchers for the financial support. This work was supported by resources provided by the Pawsey Supercomputing Research Centre with funding from the Australian Government and the Government of Western Australia and High-Performance Computational Facility at Indian Institute of Technology Delhi (IIT Delhi). The authors would like to acknowledge support from the Scheme for Promotion of Academic and Research Collaboration (SPARC) Phase III, Government of India (Project No. 3829).