This article elucidates the differences in activity and selectivity of Mo/W-(oxy)carbides. We find that the O* domains on the Mo/W-oxycarbide surface promote C-O scission in the HDO reaction. Also, the Mo/W-oxycarbides are selective toward n-alkane products, whereas the Mo/W-carbides are selective toward n-alkene products.

Abstract

Hydrodeoxygenation (HDO) reactions are among the most important reactions for the valorization of biomass to value-added chemicals. Transition metal carbides (TMCs) are promising alternative HDO catalysts to platinum group metals. However, it is known that these TMCs have the tendency to partially oxidize themselves in the presence of oxygen or oxygen-containing compounds, which makes it a challenge to identify the nature of the true active site under relevant reaction conditions. In this paper, first, we analyze the surface structure of transition metal oxycarbides (TMOCs), that is, Mo₂CO_x and W₂CO_x. Further, taking the HDO of n-butyric acid to n-butane with hydrogen as an example, we rationalize the differences in HDO performances of TMOCs as compared to TMCs (Mo₂C and W₂C) using density functional theory, ab initio molecular dynamics, and microkinetic modelling. It is found that the O* domains on the surface of TMOCs enhance the HDO activity by easing the dissociation of the C-O bond and promoting the hydrogenation reactions, as compared to pure TMCs. Furthermore, microkinetic modelling analysis shows that Mo₂CO_x is a more active and selective catalyst for alkane production compared to Mo₂C, W₂C, and W₂CO_x. These insights could guide the manipulation of efficient carbide-based HDO catalysts for biomass conversion.