Operando NAP-XPS unveils differences in MoO3 and Mo2C during hydrodeoxygenation

Abstract

MoO3 and Mo2C have emerged as remarkable catalysts for the selective hydrodeoxygenation (HDO) of a wide range of oxygenates at low temperatures (that is, ≤673 K) and H2 pressures (that is, ≤1 bar). Although both catalysts can selectively cleave C–O bonds, the nature of their active sites remains unclear. Here we used operando near-ambient pressure X-ray photoelectron spectroscopy to reveal important differences in the Mo 3d oxidation states between the two catalysts during the hydrodeoxygenation of anisole. This technique revealed that, although both catalysts featured a surface oxycarbidic phase, the oxygen content and the underlying phase of the material impacted the reactivity and product selectivity during the hydrodeoxygenation. MoO3 transitioned between 5+ and 6+ oxidation states during the operation, consistent with an oxygen-vacancy driven mechanism wherein the oxygenate is activated at undercoordinated Mo sites. In contrast, Mo2C showed negligible oxidation state changes during hydrodeoxygenation and maintained mostly 2+ states throughout the reaction. The nature of the active sites of molybdenum trioxide and molybdenum carbide, two related catalysts with great potential for hydrodeoxygenation reactions, is still under debate. Now, a comparative operando near-ambient-pressure XPS study during hydrodeoxygenation of anisole reveals important differences between these two materials.