Mechanism and kinetics for both thermal and electrochemical reduction of N2 catalysed by Ru(0001) based on quantum mechanics

Abstract

The conversion of N_2(g) to NH_3(g) is an important industrial process that plays a vital role in sustaining the current human population. This chemical transformation relies heavily on the Haber–Bosch process (N₂ thermal reduction, N₂TR), which requires enormous quantities of energy (2% of the world supply) and extreme conditions (200 atm and 500 °C). Alternatively, N_2(g) can be reduced to NH_3(g) through electrochemical means (N₂ER), which may be a less energy intensive and lower-capital approach since the H atoms come from H₂O not H₂. However, N₂ER efficiency is far from satisfactory. In order to provide the basis for developing a new generation of energy efficient processes, we report the detailed atomistic mechanism and kinetics for N₂ER on Ru(0001) along with a comparison to N₂TR. We obtained these results using a new electrochemical model for quantum mechanics (QM) calculations to obtain free energy surfaces for all plausible reaction pathways for N₂ER under a constant electrode potential of 0.0 V_SHE. For both processes, the elementary steps involve several steps of breaking of the NN bonds, hydrogenation of surface N₂H_X or NH_X, and NH₃ release. We find similar energetics for the NN cleavage steps for both systems. However, the hydrogenation steps are very different, leading to much lower free energy barriers for N₂ER compared to N₂TR. Thus, N₂ER favors an associative route where successive hydrogen atoms are added to N₂ prior to breaking the NN bonds rather than the dissociative route preferred by N₂TR, where the NN bonds are broken first followed by the addition of Hs. Our QM results provide the detailed free energy surfaces for N₂ER and N₂TR, suggesting a strategy for improving the efficiency of N₂ER.