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

Abstract

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 phases 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 density 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 (consistent 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 hydrogenation 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 maintained for earlier TOF data in a subatmospheric radio frequency (RF) reactor.

Mechanochemical Synthesis of RuCo/MgTiO3 Catalysts for Nonthermal Plasma-Assisted Ammonia Synthesis

“…45,46 To further understand the difference between the NTPcatalytic and plasma-alone systems under investigation, OES spectra at different wavelength ranges were analyzed in detail (as shown in Figure 8b− plasma-alone system. 14,47,48 Although MgTiO 3 (which has high electronegativity and dielectric constant) could positively promote the homogeneity of the plasma discharge and the concentration of electrons, 12 the MgTiO 3 packing occupied part of the reactor volume, thus reducing the plasma discharge volume. However, as shown in Figure 8b, the plasma-catalytic systems (employing Ru/MgTiO 3 -6-500, Co/MgTiO 3 -6-500, and RuCo/MgTiO 3 -6-500) showed comparatively higher intensities of relevant species in comparison with the plasmaalone and MgTiO 3 -packed systems.…”

“…14,49 Figure 8b also shows the relatively higher peak signal of NH species (336 nm) in the plasmacatalytic systems, which is linked with the concentrated N 2 + , N atomic , and H α in the presence of the catalysts. 14,48 Previous studies have proposed that the formation of NH species is the key step for NH 3 generation under NTP conditions. 50 In this work, however, the peak intensity of NH, N 2 *, N 2 + , and H α species is not positively related to the ammonia synthesis rate of the NTP systems, e.g., the RuCo/MgTiO 3 -6-500 catalyst has the highest activity while the peak intensities of relevant species are rather comparable with that of the Ru/MgTiO 3 -6-500 (Figure 8b).…”

“…To obtain plausible surface reaction mechanisms, advanced in situ characterization methods such as electron impact molecular-beam mass spectrometer (EI-MBMS) and Fourier transform infrared (FTIR) spectroscopy should be further employed. 48,51,52 ■ CONCLUSIONS In this work, a simple mechanochemical ball milling method was used to prepare the supported Ru and Co on MgTiO 3 catalysts for NTP-assisted ammonia synthesis in a DBD reactor at ambient conditions. Systematic investigation showed that (i) the mechanochemical method needs optimization regarding the synthesis duration and rotational speed to improve the metal loading and reduce the loss of crystallinity of the MgTiO 3 support and (ii) the bimetallic RuCo catalyst (i.e., RuCo/MgTiO 3 -6-500) was superior to the monometallic catalysts in the NTP-catalytic NH 3 , showing the performance of ∼50 μmol min −1 g cat −1 at 12 kV.…”

Sustainable ammonia synthesis from nitrogen wet with sea water vapor by single-step plasma catalysis

“…And even a minute presence of N radicals can offer a pivotal effect in the synthesis of ammonia from N2 and H2. [65] However, most of the reported mechanisms for ammonia synthesis from N2 and H2 include the formation of plasma-induced vibrationally excited N2() species and the excitation of adsorbed N(ads) on the catalyst surface to react with H2 to form NH3 via either Eley-Rideal (E-R) or Langmuir-Hinshelwood (L-H) [23,61,62,[65][66][67]. With both the E-R and L-H mechanisms involved in many reactions to form ammonia.…”