Rational design of highly efficient MXene-based catalysts for the water-gas-shift reaction.

Water molecules linked by hydrogen bonds are responsible for the high efficiency of bi-functional catalysts for the water-gas-shift (WGS) reaction because water can act as a proton transfer medium. Herein, we propose an associative pathway for the WGS reaction assisted by water to realize hydrogen production. Based on this pathway, we show by first-principles calculations that a large family of oxygen-terminated two-dimensional transition metal carbides and nitrides (MXenes) deposited on Au clusters are promising catalysts for the WGS reaction. Remarkably, the rate-determining barriers for *CO → *COOH on Au/MXO are in the range from 0.15 eV to 0.39 eV, indicating that WGS can occur at much lower temperatures. Furthermore, a comprehensive microkinetic model is constructed to describe the turnover frequencies (TOF) for the product under the steady-state conditions. More importantly, there is a perfect linear scaling relationship between the rate-determining barriers of the WGS and the free energy of the adsorbed hydrogen. Besides, the potential energy diagrams for CO reforming reveal that the F terminations introduced in experiments have only a slight influence on the catalytic performance of the oxygen-terminated MXenes. Our work not only opens a new avenue towards the WGS reaction but also provides many ideal catalysts for hydrogen production.