Energetics of Reaction Pathways Enabled by N and H Radicals during Catalytic, Plasma-Assisted NH3 Synthesis

• Published in: ACS sustainable chemistry & engineering Vol. 10; no. 6; pp. 2034 - 2051
• Main Authors: Liu, Tsung-Wei, Gorky, Fnu, Carreon, Maria L, Gómez-Gualdrón, Diego A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14.02.2022
• Subjects: DFT; Eley−Rideal reactions; catalyst descriptor; plasma composition; DBD; scaling relationships; OES
• ISSN: 2168-0485; 2168-0485
• DOI: 10.1021/acssuschemeng.1c05660

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, optical emission spectroscopy is consistent with only a weak effect of the metal on plasma composition and with the presence of small concentrations of plasma radicals in N2/H2 mixtures in dielectric barrier discharge (DBD) reactors, which are bound to enable new catalyst-involved pathways not considered in previous kinetic models for NH3 synthesis. Thus, we comprehensively examined, via density functional theory 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 the following: (i) plausible NNH formation and consequent prominent role of the associative pathway to form NH3 (consistent with some experimental reports detecting surface-bound N X H Y 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 ones. The energetics herein presented will allow thoroughly studying the pathway competition in future kinetic models, but numbers calculated here already suggest that the dominant pathway may change with the metal identity. For instance, N2H Y dissociation favorability is more likely to become competitive with ER hydrogenation earlier in the hydrogenation sequence 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 DBD reactor linearly correlate with ΔE rxn for the ER hydrogenation reaction H• + HNNH2 * → HNNH3 *. This descriptor may be robust to exact synthesis conditions, as its correlation with TOFs was maintained for earlier TOF data in a sub-atmospheric radio frequency reactor.