Engineering Spin State of Atomic Iron Centers for High-Performance Photocatalytic Nitrogen Fixation.

Electronic structures fundamentally influence material properties, with electron spin playing a pivotal role in defining catalytic activity and reaction pathways. However, the precise spin-mediated mechanisms of adsorption energies and nitrogen-nitrogen transition states on the catalyst surface, remain unclear due to the complexity of spin-mediated promotion factors. Herein, we demonstrate that tuning the spin state of single iron (Fe) sites on TiO2 can significantly enhance photocatalytic nitrogen reduction reaction (NRR). Our theoretical predictions reveal that low spin states of single Fe sites facilitate N2 adsorption and intermediate formation, thereby activating more catalytic sites on TiO2 for efficient nitrogen fixation. By manipulating the crystal phase and incorporating fluorine dopants, we systematically modulate the spin states of Fe sites, achieving optimized N2 adsorption and desorption kinetics and suppressing charge recombination. Experimental results combined with density functional theory (DFT) calculations confirm that these modifications reduce the magnetic moment of Fe sites, lower free energy barriers, and strengthen electronic interactions with key intermediates, particularly during N─NH formation. Intriguingly, we find that weakening N2 adsorption via reduced Fe magnetization enhances catalytic performance, challenging conventional assumptions that stronger N─N bond activation necessarily improves NRR efficiency. Our experimental results corroborate these findings, showing a remarkable 72-fold increase in ammonia production rate compared to pristine TiO2. This work highlights the crucial role of electron spin engineering in designing highly efficient NRR catalysts and provides a new paradigm for rational catalyst design.