Influence of synthesis method on molybdenum carbide crystal structure and catalytic performance in stearic acid hydrodeoxygenation

The role of the synthesis method of molybdenum carbide nanoparticle catalysts supported on carbon nanofibers on crystal structure and on catalytic performance in hydrodeoxygenation of stearic acid was investigated. We obtained the cubic phase of molybdenum carbide (α-MoC_1-x) by impregnating carbon nanofibers with a solution of (NH₄)₂MoO₄, then exposing them to 20% CH₄/H₂ at 650 °C for 2 h. When increasing the Mo loading from 7.5 wt% to 20 wt% or using the carbothermal reduction method, i.e. using carbon from the support to reduce the (NH₄)₂MoO₄ precursor at 800 °C for 6 h, the hexagonal phase (β-Mo₂C) resulted. Experiments with stearic acid hydrodeoxygenation showed that both phases (7.5 wt% Mo) displayed similar intrinsic activities. However, α-MoC_1-x/CNF reached 80% stearic acid conversion after 240 min while the β-Mo₂C/CNF catalyst attained the same conversion after 360 min. CO chemisorption results showed that α-MoC_1-x/CNF and β-Mo₂C/CNF have a similar number of potential active sites (66 and 56 μmol g⁻¹, respectively). We attribute the difference in catalytic performance between α-MoC_1-x/CNF and β-Mo₂C/CNF to differences in the catalyst's crystal structure, more specifically, the associated site density. The face-centered cubic α-MoC_1-x/CNF has a lower site density (0.1096 Mo atoms Å⁻²) than the hexagonal close-packed β-Mo₂C/CNF (0.1402 Mo atoms Å⁻²), making the Mo atoms at the surface of the α-MoC_1-x phase more accessible for large reactant molecules such as stearic acid thus allowing its convertion in shorter times.