<i>In Situ</i> Investigations on Structural Evolutions during the Facile Synthesis of Cubic α-MoC_1–<i>x</i> Catalysts

Abstract

Cubic α-phase molybdenum carbides (α-MoC_1-<i>x</i>) exhibit great potential in hydrogen production at low temperatures due to their excellent activity in water dissociation. However, the design strategies of α-MoC_1-<i>x</i> are severely restricted by the harsh synthesis conditions, which involve multistep ammonification and carburization or the utilization of a significant amount of noble metals. Herein, high-purity α-MoC_1-<i>x</i> synthesis in a one-step carburization process was achieved with the assistance of a trace amount of Rh (0.02%). The structural evolution of Mo species during phase transition was monitored via qualitative and quantitative analysis by <i>in situ</i> X-ray diffraction (XRD) and <i>in situ</i> X-ray absorption spectroscopy (XAS), respectively. Environmental transmission electron microscopy (ETEM) was used to follow the visual changes. We reveal that the reduction of monoclinic MoO₃ to cubic oxygen-deficient Mo oxide (MoO_<i>x</i>) at low temperatures owing to the promoted H₂ activation on Rh sites is vital to the following carbon atom insertion and transformation to α-MoC_1-<i>x</i>, making the carburization follow the topological route. The systematic analysis of the relationship between the reduction behavior and the structural evolution supplies a feasible strategy for the α-MoC_1-<i>x</i> synthesis, and <i>in situ</i> characterizations shed light on controlling the phase transformation during carburization.