Theoretical and Computational Chemistry

Energetics of pathways enabled by experimentally detected radicals during catalytic, plasma-assisted NH3 synthesis



Plasma-assisted catalysis is emerging as an alternative to several thermocatalytic processes. For ammonia synthesis, it could make the process milder, which would help production decentralization and compatibility with renewable energy. However, one major obstacle preventing optimization of the plasma-assisted process is the incipient mechanistic understanding of ammonia formation on plasma-exposed catalysts. Here, emission spectroscopy detects N• and H• radicals in plasma phas-es generated from N2/H2 mixtures even at atmospheric pressure, which are bound to enable new catalyst involved pathways not considered in previously reported kinetic models for ammonia synthesis. Thus, we comprehensively examined, via den-sity functional theory (DFT) calculations, the energetics (favorability) of 51 reactions on Fe, Ni, Co, Pd, Ga, Sn, Cu, Au, and Ag. Enthalpic barriers for Eley-Rideal (ER) reactions involving N• and H• radicals were found to be negligible and hence supportive of: i) plausible NNH formation and consequent prominent role of the associative pathway to form NH3 (con-sistent with some experimental reports detecting surface-bound NXHY species), ii) likelihood of N• adsorption taking over N2* dissociation as the primary source of surface bound N*, and iii) probable dominance of ER hydrogenation reactions over Langmuir-Hinshelwood (LH) ones. The energetics herein presented will allow thoroughly studying pathway competition in future kinetic models, but numbers calculated here already suggest that the dominant pathway may change with metal identity. For instance, N2HY dissociation favorability becomes competitive with ER hydrogenation earlier in the hydrogena-tion sequence in the more nitrophilic the metals. Yet, the calculated favorability of ER reactions is also already consistent with the weaker dependence of initial NH3 turnover frequencies (TOFs) on metal identity compared to the thermocatalytic scenario. With practical implications for computational catalyst screening, TOFs experimentally measured herein for an atmospheric dielectric barrier discharge (DBD) reactor linearly correlate with ΔErxn for the ER hydrogenation reaction H• + HNNH2* → HNNH3*. This descriptor may be robust to exact synthesis conditions, as its correlation with TOFs was main-tained for earlier TOF data in a sub-atmospheric radio frequency (RF) reactor


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Supplementary material

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Supplementary information: Energetics of pathways enabled by experimentally detected radicals during catalytic, plasma-assisted NH3 synthesis
Complementary optical emission spectra. Complementary snapshots of optimized reactant, product and transition state geometries. Scaling relationships. Tabulated reaction energetics data. R2 for correlation of TOFs with different reaction energies.