3:15 PM - EN06.10.05
Towards the Efficient Electrosynthesis of Hydrogen Peroxide and Ammonia on Solid Surfaces
Imperial College London1
Electrochemical synthesis methods can provide sustainable routes to our most coveted chemicals. In this contribution, I will discuss our recent progress in the heterogeneous electrocatalysis of H2O2 and NH3 production. Both chemicals are currently synthesised in enormous quantities, in large centralised reactors, far from the point-of-consumption. The logistical and safety challenges intrinsic to the transportation of these chemicals set them beyond the reach of many end users. In contrast, their electrochemical synthesis is far more attractive.
Efficient electrochemical H2O2 production requires a catalyst that can steer the reaction away from the formation of the most thermodynamically stable product, H2O. To this end, porphyrin like structures offer advantages over pure metal structures by lieu of their isolated reactive sites. We emulated the reactivity of porphyrins by producing mercury-based alloys, which demonstrated unprecedented selectivity, activity and stability.1,2 I will provide an overview of the factors controlling H2O2 production, focussing on the role of (i) catalyst structure at the atomic scale (ii) electrode mesotructure and (iii) electrolyte pH. I will also compare results from model rotating ring disk electrode to tests in real electrolytic devices.3
Current ammonia production via the Haber Bosch process requires high pressures, high temperatures and consumes >1% of our current fossil fuel production. To the contrary, nitrogenase in nature catalyses N2 reduction at room temperature and atmospheric pressures. At present, sold electrodes are rather inactive and unselective. The translation of the activity and selectivity of nitrogenase to a solid inorganic surface would enable the efficient on-site on demand synthesis of ammonia, powered by renewable electricity.
While we have made huge progress in the electrocatalysis of H2O2 production, there remains much to be learnt about N2 electroreduction. We recently used quantitative isotopic labelling experiments to provide unequivocal proof that Li-based electrodes are able to reduce N2 to NH3 in non-aqueous electrolytes.4Our work provides a solid foundation for further progress, and enables scientists to distinguish false positives from true breakthroughs. Nonetheless, the efficiency we reported is rather low, i.e. there is still ample room for improvement. Drawing insight from the nitrogenase enzyme, I will discuss different avenues towards more efficient N2 reduction.
1 Siahrostami, S., Verdaguer-Casadevall, A., Karamad, M. R., Deiana, D., Malacrida, P., Wickman, B., Escudero-Escribano, M., Paoli, E. A., Frydendal, R., Hansen, T. W., Chorkendorff, I., Stephens, I. E. L. & Rossmeisl, J. Nature Materials12, 1137, (2013).
2 Verdaguer-Casadevall, A., Deiana, D., Karamad, M., Siahrostami, S., Malacrida, P., Hansen, T. W., Rossmeisl, J., Chorkendorff, I. & Stephens, I. E. L. Nano Letters 14, 1603, (2014).
3 Yang, S., Verdaguer-Casadevall, A., Arnarson, L., Silvioli, L., Čolić, V., Frydendal, R., Rossmeisl, J., Chorkendorff, I. & Stephens, I. E. L. ACS Catalysis, 4064, (2018).
4 Andersen, S. Z., Čolić, V., Yang, S., Schwalbe, J. A., Nielander, A. C., McEnaney, J. M., Enemark-Rasmussen, K., Baker, J. G., Singh, A. R., Rohr, B. A., Statt, M. J., Blair, S. J., Mezzavilla, S., Kibsgaard, J., Vesborg, P. C. K., Cargnello, M., Bent, S. F., Jaramillo, T. F., Stephens, I. E. L., Nørskov, J. K. & Chorkendorff, I. Nature (2019) DOI: 10.1038/s41586-019-1260-x.