Thermodynamics and Kinetics of Molecular Hydrogen Adsorption and Dissociation on MXenes: Relevance to Heterogeneously Catalyzed Hydrogenation Reactions

Abstract

The interaction of molecular hydrogen with a series of 28 two-dimensional (2D) carbides and nitrides, known as MXenes, has been studied by means of periodic density functional calculations. This study shows that trends in atomic and molecular adsorption energies can be rationalized in terms of the electrostatic potential above the surface site and the Bader charge on the surface metal atoms. For all systems, molecular hydrogen is found to dissociate with almost negligible barriers, meaning that at low temperature the MXene surface will be passivated by adsorbed atomic hydrogen. The conditions at which the MXene surface is partly covered and, hence, able to participate in hydrogenation reactions are investigated by means of ab initio thermodynamics and phase diagrams derived from microkinetic simulations. The first provide the equilibrium conditions for a given H coverage on the MXene of interest, whereas the second provides the conditions at which a given configuration is reachable at the working conditions. For fast enough processes, both approaches necessarily lead to the same result, but this may differ when high energy barriers are involved, as it the case here for the H adatoms recombination step. With this suite, we show that Fe2C, W2N, and Mo2C are promising hydrogenation catalysts. This work serves as a first step toward the rational design and implementation of MXene-based hydrogenation catalysts.