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

Abstract

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 Mo 2 CT x 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-Mo 2 C that is complete at ca. 730 °C. We demonstrate that Mo 2 CT x is a highly stable and active catalyst for the water–gas shift reaction with a selectivity >99% toward CO 2 and H 2 at 500 °C. The conversion of carbon monoxide on Mo 2 CT x 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 Mo 2 CT x and serve as a guideline for its future catalytic applications.