Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis

Abstract

Correlations between the energies of elementary steps limit the rates of thermally catalysed reactions at surfaces. Here, we show how these limitations can be circumvented in ammonia synthesis by coupling catalysts to a non-thermal plasma. We postulate that plasma-induced vibrational excitations in N2 decrease dissociation barriers without influencing subsequent reaction steps. We develop a density-functional-theory-based microkinetic model to incorporate this effect, and parameterize the model using N2 vibrational excitations observed in a dielectric-barrier-discharge plasma. We predict plasma enhancement to be particularly great on metals that bind nitrogen too weakly to be active thermally. Ammonia synthesis rates observed in a dielectric-barrier-discharge plasma reactor are consistent with predicted enhancements and predicted changes in the optimal metal catalyst. The results provide guidance for optimizing catalysts for application with plasmas.

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Fig. 1: Modelled ammonia synthesis rates during thermal catalysis.
Fig. 2: Relationships between the N2 vibrational state and effective dissociation barrier.
Fig. 3: Plasma-induced vibrational excitation of N2.
Fig. 4: Rate enhancements with plasma-induced N2 vibrational excitation.
Fig. 5: Experimental plasma-catalytic kinetics.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Sustainable Ammonia Synthesis Program, under award number DE-SC-0016543. W.F.S. acknowledges additional support under award number DE-FG02-06ER15839. Computational resources were provided by the Notre Dame Center for Research Computing. We thank the Notre Dame Energy Materials Characterization Facility and Notre Dame Integrated Imaging Facility for use of the X-ray diffractometer and transmission electron microscope, respectively. P.M. acknowledges support through the Eilers Graduate Fellowship for Energy Related Research.

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P.M. developed the microkinetic model. P.B. and J.K. performed the ammonia synthesis rate experiments. F.A.H. and P.R. performed the plasma characterization. P.M., P.B., F.A.H., D.B.G, J.C.H. and W.F.S. co-wrote the manuscript.

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Correspondence to David B. Go or Jason C. Hicks or William F. Schneider.

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Supplementary Methods, Supplementary Figures 1–11, Supplementary Table 1 and Supplementary References.

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Mehta, P., Barboun, P., Herrera, F.A. et al. Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis. Nat Catal 1, 269–275 (2018). https://doi.org/10.1038/s41929-018-0045-1

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