In Situ XANES/XRD Study of the Structural Stability of Two-Dimensional Molybdenum Carbide Mo2CTx: Implications for the Catalytic Activity in the Water–Gas Shift Reaction

MXenes, a recently discovered family of two-dimensional (2D) materials, are promising catalysts and supports for applications in heterogeneous catalysis; however, the thermal stability of MXenes and their surface chemistry are not fully explored. Here, we report that 2D molybdenum carbide Mo2CTx remains stable and shows no appreciable sintering up to ca. 550–600 °C in a reducing environment, as assessed by a combined in situ X-ray absorption near-edge spectroscopy (XANES) and powder X-ray diffraction (XRD) study during a temperature-programmed reduction (TPR) experiment. At higher temperatures, the passivating oxo, hydroxy, and fluoro groups defunctionalize the molybdenum-terminated surface, inducing a transformation to bulk β-Mo2C that is complete at ca. 730 °C. We demonstrate that Mo2CTx is a highly stable and active catalyst for the water–gas shift reaction with a selectivity >99% toward CO2 and H2 at 500 °C. The conversion of carbon monoxide on Mo2CTx starts to decline at temperatures that are associated with the decrease of the interlayer distance between the carbide sheets, as determined by the XRD-probed TPR, indicative of increasing mass transfer limitations at these conditions. Our results provide an insight into the thermal stability and reducibility of Mo2CTx and serve as a guideline for its future catalytic applications.