Boron-based material, featuring B-dependent reactivity and diverse phases, is emerging as a promising catalyst system. However, catalytic mechanism on many borides remains poorly understood due to complex surface reconstructions under reaction conditions. Here, we investigate the MoB2 surface in conditions of hydrogen evolution reaction in acidic media, using grand canonical global optimization, grand canonical density functional theory, ab initio molecular dynamics, free energy surface sampling, and an analytical model for electrochemical barrier evaluation. We propose a boron enrichment strategy to tune the surface reactivity of the hexagonal face of MoB2. We reveal the dynamic nature of the B-enriched surface under H coverage and kinetic trapping of the system in the metastable regime, with an extensive examination on the deactivation pathways. The metastable center B site on B-enriched surfaces, featuring buckled-up configuration and a usual relaxation effect, is found to be highly active towards HER via Volmer-Heyrovsky mechanism. This work demonstrates how off-equilibrium behaviors can arise from the interplay between adsorbate coverage and surface reconstruction on a seemingly simple surface, and we present a theoretical framework and computational workflows to address them, along with other realistic complexities, in kinetics simulations.
