Reduction of N2 to NH3 by TiO2-supported Ni cluster catalysts: a DFT study

Abstract

Electrochemical techniques for ammonia synthesis are considered as an encouraging energy conversion technology to efficiently meet the challenge of nitrogen cycle balance. Herein, we find that TiO₂(101)_-supported Ni₄ and Ni₁₃ clusters can serve as efficient catalysts for electrocatalytic N₂ reduction based on theoretical calculations. Electronic property calculations reveal the formation of electron-deficient Ni clusters on the TiO₂ surface, which provides multiple active sites for N₂ adsorption and activation. Theoretical calculation identifies the strongest activated configuration of N₂* on the catalysts and confirms the potential-limiting step in the nitrogen reduction reaction (NRR). On Ni₄–TiO₂(101), N₂* → NNH* is the potential-limiting step with a very small free energy increase (ΔG) of 0.24 eV (the corresponding overpotential is 0.33 V), while on Ni1₃–TiO₂(101) the potential-limiting step occurs at NH* → NH₂* with ΔG of 0.49 eV (the corresponding overpotential is 0.58 V). Moreover, the Ni_n–TiO₂(101) catalyst, especially Ni₁₃–TiO₂(101), involves in a highly selective NRR even at the corresponding NRR overpotential. This work will enlighten material design to construct metal oxide supported transition metal clusters for the highly efficient NRR and NH₃ synthesis.