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

The conversion of N to NH 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 (N thermal reduction, NTR), which requires enormous quantities of energy (2% of the world supply) and extreme conditions (200 atm and 500 °C). Alternatively, N can be reduced to NH through electrochemical means (NER), which may be a less energy intensive and lower-capital approach since the H atoms come from HO not H. However, NER 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 NER on Ru(0001) along with a comparison to NTR. 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 NER under a constant electrode potential of 0.0 V. For both processes, the elementary steps involve several steps of breaking of the NN bonds, hydrogenation of surface NH or NH, and NH 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 NER compared to NTR. Thus, NER favors an associative route where successive hydrogen atoms are added to N prior to breaking the NN bonds rather than the dissociative route preferred by NTR, where the NN bonds are broken first followed by the addition of Hs. Our QM results provide the detailed free energy surfaces for NER and NTR, suggesting a strategy for improving the efficiency of NER.