Regulation of Rh Single-Atom Coordination for Enhanced Reverse Hydrogen Spillover and Efficient Electrochemical Dechlorination

Reverse hydrogen spillover (RHS) from the surface oxygen of titanium oxide to single-atom catalytic centers enables efficient electrochemical hydrogenation via atomic hydrogen (H*) transfer, a process critically dependent on the coordination environment and electronic structure of the active site. In this study, we reveal that a four-oxygen-coordinated Rh single-atom electrode (Rh₁O₄) exhibits superior RHS capability during water electrolysis of titanium foam compared to its five- or three-coordinated counterparts (Rh₁O₅ or Rh₁O₃). The Rh-O coordination number directly modulates the relative position of the Rh d-band center to the Fermi level, thereby regulating H* adsorption on Rh and the RHS efficiency between titanium oxide's surface oxygen and the Rh single atom on the titanium foam electrode. Remarkably, the four-coordinated Rh₁O₄ configuration achieves an optimized hydrogen adsorption Gibbs free energy (ΔG_H*) of +0.08 eV, approaching that of the coordinating oxygen atoms (-0.47 eV), which drastically reduces the RHS energy barrier to +0.55 eV, significantly lower than those of Rh₁O₅ and Rh₁O₃. This structural optimization translates to exceptional electrochemical hydrogenation performance, exemplified by a 4-chlorophenol degradation rate constant of 4.65 h^-1, surpassing Rh₁O₅ (1.18 h^-1) and Rh₁O₃ (0.16 h^-1) by 4- and 29-folds, respectively. Our findings highlight the pivotal role of single-atom coordination engineering in tailoring atomic-level hydrogen transfer dynamics and provide a strategic framework for designing high-performance single-atom electrocatalysts for sustainable electrochemical hydrogenation applications.