Surface phase structures responsible for the activity and deactivation of the fcc MoC (111)-Mo surface in steam reforming: a systematic kinetic and thermodynamic investigation

Abstract

The surface phase structure evolution on a fcc MoC (111)-Mo terminated surface under a H₂O/H₂-rich environment typical in steam reforming (SR) reactions was systematically investigated by periodic density functional theory (DFT) calculations and an ab initio thermodynamic method. The configurations of surface adsorbates (H₂O*, OH*, O*, and H*) at different coverages (1/9 ML ≤ θ ≤ 1 ML) were explored. For θ_H2O ≤2/3 ML, the adsorption of H₂O is mainly through a Mo–O coordinating interaction, while further captured H₂O interacts only through hydrogen bonds with the surface coordinating H₂O* at θ_H2O >2/3 ML. For θ_OH ≤4/9 ML, the surface OH* all possess cavity site configurations (coordinating with three surface Mo atoms) with relatively strong surface binding strength and significant steric hindrance, while the top site OH* with much weaker surface binding strength and small steric hindrance emerge at θ_OH ≥5/9 ML. For all θ_O, the surface O* always have cavity site configurations with obviously stronger surface binding strength than the top site OH*. The order of surface binding strength of stable adsorbates is O* (cavity site) > OH* (cavity site) > OH* (bridge site) > OH* (top site) > H₂O* > H* > , indicating the top site OH* with moderate surface binding strength as the probable reactive surface species in SR reactions. The energy barriers and elementary reaction rates at different coverages and temperatures show that the H₂O dissociation resulting in OH* is really facile, and the reaction rates are several orders of magnitude larger than those of surface O* formation by OH* dissociation, signifying that the (111)-Mo surface is more prone to be covered with OH* instead of O* at least in the initial SR reaction stage. The ab initio thermodynamic calculations under constrained-equilibrium conditions further confirm the existence of top site OH* under typical SR reaction conditions, which can deliver a reactive surface phase structure. Though the O* formation is kinetically unfavourable, its gradual accumulation by thermodynamic driving force will lead to surface Mo–O layers, the formation of which is detrimental to activity in long-term running.