Single-Atom Centers of MXenes for Electrochemical Ammonia Oxidation: Moving Beyond Thermodynamic Descriptors

The ammonia oxidation reaction (AOR) presents a promising route for clean energy conversion and wastewater remediation, yet it currently relies on scarce and expensive platinum-based catalysts. In this study, we explore electrochemically formed single-atom centers on MXenes (MXene-SACs) under anodic polarization as a material class of Earth-abundant elements for electrochemical ammonia oxidation. These systems offer well-defined active sites at the atomic scale, providing benefits in controlling the catalytic interface and guiding selective N–N coupling. To investigate the kinetics of N–N bond formation as a function of the coupling position in the reaction mechanism, a comprehensive series of transition state calculations was performed. Electrocatalytic activity is assessed by employing two key descriptors, namely G_max(U) ─ a thermodynamic representation of the free-energy span model ─ and G^‡(U), which considers the N–N coupling transition state relative to the most stable intermediate in the definition of the energetic span. This dual-descriptor approach reveals that different MXene-SACs engage in N–N coupling through distinct mechanistic pathways and at different stages of hydrogenation. In particular, W- and Mo-based MXene-SACs, particularly in their nitride forms, exhibit low N–N coupling barriers and favorable mechanistic profiles, making them promising candidates for AOR. Distinct Brønsted–Evans–Polanyi (BEP) relationships are observed for the different reaction intermediates in the AOR. While a strong correlation between thermodynamics and kinetics is witnessed for hydrogen-rich intermediates such as *NH₂–*NH₂, these correlations deteriorate as the degree of hydrogenation decreases, emphasizing the inadequacy of thermodynamic analysis alone. In this context, the G^‡(U) descriptor serves as a mechanistically relevant metric that bridges the gap between thermodynamic favorability and kinetic feasibility and provides guidance for the rational design of advanced AOR catalysts.