Regulating photocatalytic nitrogen reduction via d-electron-mediated synergistic adsorption in dual-atom catalysts.

Understanding the N2 adsorption mechanism is pivotal for the rational design of high-performance photocatalysts for the nitrogen reduction reaction (NRR). Using density functional theory (DFT), we present a systematic investigation of 21 homo- and heteronuclear transition-metal dual-atom catalysts anchored onto graphitic carbon nitride (TM1TM2@g-CN; TM = Zr, Nb, Mo, Ru, Rh, and Pd), revealing a predictive electronic descriptor governing their structural and adsorption behavior. We demonstrate that the TM pairs with a total d-electron count (Nd) less than 12 preferentially form 6-coordination configurations, while the others form 4-coordination configurations. Notably, catalysts with Nd < 12 enable a unique synergistic bridge-on adsorption, where one TM atom simultaneously interacts with both N atoms through the dxz orbital, and the other TM atom binds with one N atom, facilitating enhanced π-backdonation into N2 antibonding orbitals, which was confirmed through crystal orbital Hamilton population (COHP) and Bader charge analyses. This mechanism underpins the superior NRR performance of NbRu@g-CN, which exhibits the lowest limiting potential (-0.43 V), while MoMo@g-CN and RuPd@g-CN display superior photocatalytic driving force and NRR selectivity over the HER. These findings establish a predictive d-electron rule for rational dual-atom catalyst (DAC) design.