Single vs double atom catalyst for N-2 activation in nitrogen reduction reaction: A DFT perspective

Ammonia synthesis through electrochemical reduction of nitrogen molecules is a promising strategy to significantly reduce the energy consumption in traditional industrial process. Detailed mechanism study of multistep complex nitrogen reduction reaction is prerequisite for the design of highly efficient catalyst. Stable atomically dispersed catalyst with unique geometric and electronic structure is suitable for the mechanism clarification of such a complex reaction. In this study, d-block transition-metal (TM) anchored C2N single layer catalyst is investigated by the density functional theory (DFT) calculation. Both single TM-anchored single atom catalyst (SAC) and double TM-anchored double atom catalyst (DAC) exhibit good thermodynamic stability in atomically dispersed catalyst. In the case of SACs, IVB metals (Ti, Zr, Hf) exhibit the highest reactivity and lowest overpotential. While in the case of DACs, CrCr system leads to the NH3 formation, but VV system leads to the N2H4 formation. The SACs show much lower overpotential and stronger activation of N-2 molecule than the DACs due to the different activation mechanisms: traditional sigma-donation/-backdonation N-2 activation mechanism is found in SACs, while a new pi-donation/pi-backdonation N-2 activation mechanism is found in the DACs. The present work demonstrates that the different catalytic effect for NRR between SAC and DAC and their corresponding electronic structure origin, which gives more insight into the single atom catalyst.