Theoretical Insights into Electrocatalytic CO2 Reduction to C2H4 via Bridging-Mode Asymmetric Coupling over Bimetallic Triatomic Clusters

Bridging-mode asymmetric coupling of *CHO and *CO governs C2H4 selectivity over Cu2TM/β-TBN bimetallic triatomic clusters for electrocatalytic CO2 reduction. In these catalysts, two Cu atoms and a transition metal (TM = Ti ~ Zn, Zr ~ Cd) dopant are anchored on a β-phase tetragonal borophene nanosheet. After screening fourth- and fifth-period TM dopants for thermodynamic stability and CO2 activation, twelve systems were examined. CO2 adsorbs preferentially in a bridging geometry at the Cu(1)–TM site, its adsorption energy scaling linearly with the TM d-band center. Of the three possible C–C coupling routes, the most favorable is the asymmetric coupling of bridge-adsorbed *CHO with adjacent *CO, yielding *CHOCO[1,2,3], in which the three metal sites jointly coordinate the oxygen and carbon atoms. This route, enabled by the simultaneous engagement of all three metal sites, is inaccessible on single-site or dual-site catalysts. Thermodynamic stability alone proves an insufficient guide. The formation free energy of the *CHO+*CO precursor, ΔG(*CHO+*CO), and the C–C coupling transition-state energy barrier, ETS, are inversely correlated. Excess stabilization of the intermediate raises the kinetic barrier, overstabilizing the precursor in a manner that hinders the coupling step rather than promoting it. This thermodynamic-kinetic trade-off demands a balanced intermediate affinity. Resolving this trade-off reveals a period-dependent divergence in regulatory mechanism. For fourth-period TMs, electron transfer between adsorbate and surface predominates, with geometric displacement playing a secondary role. For fifth-period TMs, vertical displacement of the TM atom dominates, as confirmed by cross-correlation analysis. Electronic and geometric controls, distinct in origin yet convergent in function, define a unified design space for C–C coupling catalysts.