Doping an Oxophilic Metal into a Metal Carbide: Unravelling the Synergy between the Microstructure of the Catalyst and Its Activity and Selectivity for Hydrodeoxygenation
Sagar Bathla, Chi‐Cong Tran, Serge Kaliaguine, Samir H. Mushrif
Abstract
Transition metal carbide catalysts (Mo2C, W2C) have emerged as promising alternatives to precious-metal-based catalysts like Pt, Pd, and Ru for the hydrodeoxygenation (HDO) of biomass-derived species, including bio-oil. However, low selectivity toward deoxygenation and excessive hydrogenation are major limitations of monometallic carbide catalysts. Hence, using the popular molybdenum carbide as an example, this work demonstrates how the doping of an oxophilic metal like tungsten (W) into it can alter its structure, activity, and selectivity. By combining density functional theory calculations of mixing energies and relative stabilities with experimental characterization data (X-ray diffraction, X-ray photoelectron spectroscopy), we confirmed that for a tungsten-to-molybdenum ratio of 5:3, a mixed metal carbide bulk with overlayers of metallic tungsten (MoWC) is the most stable microstructure. We also computed core-level shifts in the binding energy of Mo and W, validating the experimentally observed X-ray photoelectron spectroscopy shifts of +0.3 and −0.1 eV in the W 4f and Mo 3d spectra, respectively, which correspond to a decrease in the electron density of W atom. In addition, a linear correlation was confirmed between the shifts in the d-band center and core-level shifts in binding energies to further validate the suggested MoWC structure. Furthermore, stronger CO adsorption on MoWC agrees well with the experimental findings. Investigations into the reaction mechanisms and pathways for the HDO of guaiacol revealed that the presence of tungsten overlayers on bulk MoWC catalyst creates a step-like structure and significantly brings down the direct deoxygenation barrier from 60 kJ/mol on Mo2C to only 18 kJ/mol on MoWC. The subsequent dehydroxylation of the cresol intermediate is also kinetically and thermodynamically more facile on MoWC in comparison to Mo2C. Unlike Mo2C, ring hydrogenation of benzene and other intermediates to ring saturated products is not favored on MoWC, explaining the experimentally observed higher selectivity toward benzene on MoWC, as compared to that on the monometallic Mo2C.