Beyond the thermodynamic volcano picture in the nitrogen reduction reaction over transition-metal oxides: Implications for materials screening

Abstract

Electrocatalytic production of ammonia from dinitrogen is considered as a sustainable alternative to the energy-demanding and pollutive Haber-Bosch process. A promising class of materials for selective nitrogen reduction (NRR) corresponds to transition-metal oxides given that these electrodes do not show a high activity toward the competing hydrogen evolution reaction. So far, density functional theory calculations have been used to comprehend trends in a class of materials by using the concept of scaling relations and volcano plots. This thermodynamic theory pinpoints that either the formation of the *NNH adsorbate or the formation of ammonia are reconciled with the potential-determining reaction steps. Thus, the development of NRR catalyst has largely focused on the optimization of these two elementary processes. In the present contribution, overpotential and kinetic effects are factored into the volcano plot for the NRR over transition-metal oxides by making use of the recently introduced activity descriptor Gmax(η). It is illustrated that the thermodynamic volcano picture is too simplistic as the limiting reaction step may alter close to the volcano apex: there, particularly surface reactions may govern the reaction rate. In addition, it is demonstrated how to include the formation of hydrazine as a competing side reaction into the volcano plot, which is of importance for weak binding *NNH catalysts where the formation of hydrazine may compete with the formation of ammonia. Given that the outlined methodology in this manuscript is universal and not restricted to the class of transition-metal oxides, the presented kinetic volcano picture may contribute to the development of NRR catalysts for nitrogen fixation.