Hydrodeoxygenation of acetic acid over Ni-promoted Cu-based catalysts: a theoretical mechanism and kinetic study

Abstract

Developing low-cost catalysts for hydrodeoxygenation (HDO) of carboxylic acids is meaningful in scientific research and applied fields. NiCu catalysts are promising replacements for the common noble-metal-based catalysts based on experimental tests, but micro-level understanding of the mechanism and role of the Ni promoter is lacking. In this paper, a NiCu bimetallic surface was built to investigate the HDO process through DFT combined with kinetic methods, with acetic acid as the model reactant. The reaction followed the direct deoxygenation mechanism to produce the acetaldehyde intermediate on the Ni site, with C–O bond cleavage as the rate-limiting step, which was distinctly improved on the Ni site due to the intrinsic strong oxophilicity. The subsequent hydrogenation of CH₃CHO and reduction of O(H) species happened on both the Ni and Cu sites simultaneously. The production rate of ethanol increased by more than an order of magnitude when a small amount of metallic Ni promoter was introduced on the copper surface. Further studies on the C–O bond scission revealed that the oxophilicity of the metal dopant (represented by oxygen binding energy) could serve as a descriptor to evaluate the HDO activity of many bimetallic catalysts. The promising catalyst should meet the balance conditions between the barriers of oxygen formation and reduction. This work provided micro-scale understanding and insight into the active site and mechanism, which could guide the subsequent design of similar nonprecious-metal-based catalysts.