Abstract
Theoretical investigations of electrochemical production of ammonia at ambient temperature and pressure on nitrogen covered molybdenum nanoparticles are presented. Density functional theory calculations are used in combination with the computational hydrogen electrode approach to calculate the free energy profile for electrochemical protonation of N2 and N adatoms on cuboctahedral Mo13 nanoparticles. Pathways for electrochemical ammonia production via direct protonation of N adatoms and N2 admolecules with an onset potential as low as -0.5 V and generally lower than -0.8 V on both a nitrogen covered or a clean Mo nanoparticle. Calculations presented here show that nitrogen dissociation at either nitrogen vacancies on a nitrogen covered molybdenum particle or at a clean molybdenum particle is unlikely to occur under ambient conditions due to very high activation barriers of 1.8 eV. The calculations suggest that the nitrogen will be favored at the surface compared to hydrogen even at potentials of -0.8 V and the Faradaic losses due to HER should be low.
Highlights
Numerous experimental[7,8,9,10,11,12,13,14,15,16,17,18,19,20,21] and theoretical[22,23,24,25,26,27,28,29,30,31,32,33,34] studies have examined ammonia synthesis and they offer excellent insight into the challenges faced when developing new catalytic materials for ammonia synthesis. It has been shown in previous studies that ammonia synthesis is very structure sensitive on metal surfaces and primarily occurs on the surface steps of Fe and Ru.[22,35,36]
The adsorption free energies of the first eight nitrogen atoms are shown in Fig. 2 where 0 is the free energy of the clean Mo13 nanocluster and the respective number of 1/2N2 in gas phase
The graph shows that the adsorption free energies are strongest for the first additions and are reduced when more nitrogen are added to the surface
Summary
Numerous experimental[7,8,9,10,11,12,13,14,15,16,17,18,19,20,21] and theoretical[22,23,24,25,26,27,28,29,30,31,32,33,34] studies have examined ammonia synthesis and they offer excellent insight into the challenges faced when developing new catalytic materials for ammonia synthesis It has been shown in previous studies that ammonia synthesis is very structure sensitive on metal surfaces and primarily occurs on the surface steps of Fe and Ru.[22,35,36] Whereas, the competing hydrogen evolution reaction (HER) is structure insensitive.[37] Nanoclusters offer a way to increase the selectivity for NH3 production. The dissociation barrier for N2 molecules at various nitrogen coverages will be presented
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