First-Principles Insights into Tungsten Semicarbide-Based Single-Atom Catalysts: Single-Atom Migration and Mechanisms in Oxygen Reduction.

Understanding the structural evolution of single-atom catalysts (SACs) in catalytic reactions is crucial for unraveling their catalytic mechanisms. In this study, we utilize density functional theory calculations to delve into the active phase evolution and the oxygen reduction reaction (ORR) mechanism of tungsten semicarbide-based transition metal SACs (TM 1 /W 2 C). The stable crystal phases and optimal surface exposures of W 2 C are identified by using ab initio atomistic thermodynamics simulations. Focusing on the W-terminated (001) surface, we screen 13 stable TM 1 /W 2 C variants, ultimately selecting Pt 1 /W 2 C(001) as our primary model. The surface Pourbaix diagram, mapped for this model under ORR conditions, reveals dynamic Pt 1 migration on the surface, triggered by surface oxidation. This discovery suggests a novel single-atom evolution pathway. Remarkably, this single-atom migration behavior is also discerned in seven other group VIII SACs, enhancing both their catalytic activity and their stability. Our findings offer insights into the evolution of active phases in SACs, considering substrate structural arrangement, single-atom incorporation, and self-optimization of catalysts under various conditions.