Sustainable Non-Thermal Plasma-Assisted Nitrogen Fixation-Synergistic Catalysis

The Support can Disguise the Catalytic Effect: The Case of Silver on Alumina in Plasma Ammonia Synthesis

Plasma catalysis combines the high-energy chemistry of plasma with the speed and selectivity of chemical reactions in catalysis. However, unlike well-established thermal catalysis, a better understanding of fundamental mechanisms is needed, as evidenced by the contrasting results reported in the literature. One main challenge is that not only the genuine catalytic effect may play a role, but both the support and the catalyst also impact the plasma, complicating the understanding. In this study, exploring the impact of support by comparing a single metal on various substrates made of the same material is focused on. Herein, silver on γ-alumina is used to investigate ammonia synthesis in N2/H2 plasma discharges. Beyond confirming the beneficial role of silver in ammonia formation, it is also found that the influence of support is crucial and it is affected by the preparation method. These findings contribute to clarifying the discrepancies in the literature results despite using the same materials.

Understanding the Nitrogen Reduction Reaction Mechanism on CuFeO2 Photocathodes

This study investigates the reaction pathways for the conversion ofN 3 ${{\rm{N}}_3 }$ toNH 2 ${{\rm{NH}}_2 }$ onCuFeO 2 ${{\rm{CuFeO}}_2 }$ (CFO) by employing density functional theory (DFT) calculations. Concentrating on the most stable (012) surface orientation, two systems were examined: the pristine (012) surface and the corresponding oxygen defective surface. To find the thermodynamic stable pathway, the associative Heyrovský mechanism was considered, containing four different reaction pathways. The reaction intermediates predominantly interact with the iron sites on the surface, following the distal alternating reaction pathway via the formation of hydrazine. Introducing oxygen defects changes the reaction mechanism to a Mars-van-Krevelen-type mechanism, avoiding the formation of hydrazine, while the Gibbs free energy of the first hydrogenation step is lowered by 1.17 eV (from 2.17 to 1.00 eV). Analyzing the charge density distribution reveals that an oxygen defective surface enables CFO to facilitate a π ${\pi }$ -backdonation between iron sites and the reaction intermediates, increasing the intermediate-surface interaction. This indicates an enhanced catalytic activity for the nitrogen reduction reaction (NRR) by generating oxygen lattice defects in CFO.

Understanding the Enhanced Catalytic Desulfurization Mechanism: Gas-Phase and Surface Reactions with a CuCeOx Catalyst under Nonthermal Plasma Conditions

To understand the mechanisms of enhanced catalytic technologies under nonthermal plasma (NTP) conditions, complex surface processes must be assessed. However, the predictive capability of the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) processes is limited by various factors. The present study aimed to clarify the interaction mechanisms between NTP and catalysts in the enhancement process, explore the specific pathways of the enhancement process based on E-R and L-H model validations, and obtain data to support the rational design of NTP-enhanced catalytic processes. We investigated CuCeOx catalysts and SO2 removal reaction as a probing reaction using two enhancement scheme configurations, combined with gas-phase reaction process simulations. During the gas-phase reaction stage of the enhancement process, no significant differences were observed among the different configurations caused by the generation of radicals that were induced by N2 (A3Σu+)-excited species. However, introducing CuCeOx catalysts altered the enhancement process, and the placement of the catalyst influenced the corresponding desulfurization mechanism.

Observation and Characterization of Vibrationally Active Surface Species Accessed with Nonthermal Nitrogen Plasmas

Polycrystalline Ni, Pd, Cu, Ag, and Au foils exposed to nonthermal plasma (NTP)-activated N2 are found to exhibit a vibrational feature near 2200 cm-1 in polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS) observations that are not present in the same materials exposed to N2 under nonplasma conditions. The feature is similar to that reported elsewhere and is typically assigned to chemisorbed N2. We employ a combination of temperature-dependent experiments, sequential dosing, X-ray photoelectron spectroscopy, isotopic labeling, and density functional theory calculations to characterize the feature. Results are most consistent with a triatomic species, likely NCO, with the C and O likely originating from ppm-level impurities in the ultrahigh-purity (UHP) Ar and/or N2 gas cylinders. The work highlights the potential for nonthermal plasmas to access adsorbates inaccessible thermally as well as the potential contributions of ppm-level impurities to corrupt the interpretation of plasma catalytic chemistry.

A Bifunctional Catalyst for Green Ammonia Synthesis from Ubiquitous Air and Water

Ammonia (NH3 ) is essential for modern agriculture and industry, and, due to its high hydrogen density and no carbon emission, it is also expected to be the next-generation of "clean" energy carrier. Herein, directly from air and water, a plasma-electrocatalytic reaction system for NH3 production, which combines two steps of plasma-air-to-NOx - and electrochemical NOx - reduction reaction (eNOx RR) with a bifunctional catalyst, is successfully established. Especially, the bifunctional catalyst of CuCo2 O4 /Ni can simultaneously promote plasma-air-to-NOx - and eNOx RR processes. The easy adsorption and activation of O2 by CuCo2 O4 /Ni greatly improve the NOx - production rate at the first step. Further, CuCo2 O4 /Ni can also resolve the overbonding of the key intermediate of * NO, and thus reduce the energy barrier of the second step of eNOx RR. Finally, the "green" NH3 production achieves excellent FENH3 (96.8%) and record-high NH3 yield rate of 145.8 mg h-1  cm-2 with large partial current density (1384.7 mA cm-2 ). Moreover, an enlarged self-made H-type electrolyzer improves the NH3 yield to 3.6 g h-1 , and the obtained NH3 is then rapidly converted to a solid of magnesium ammonium phosphate hexahydrate, which favors the easy storage and transportation of NH3 .

Review of low-temperature plasma nitrogen fixation technology

Nitrogen fixation is essential for all forms of life, as nitrogen is required to biosynthesize fundamental building blocks of creatures, plants, and other life forms. As the main method of artificial nitrogen fixation, Haber–Bosch process (ammonia synthesis) has been supporting the agriculture and chemical industries since the 1910s. However, the disadvantages inherent to the Haber–Bosch process, such as high energy consumption and high emissions, cannot be ignored. Therefore, developing a green nitrogen fixation process has always been a research hotspot. Among the various technologies, plasma-assisted nitrogen fixation technology is very promising due to its small scale, mild reaction conditions, and flexible parameters. In the present work, the basic principles of plasma nitrogen fixation technology and its associated research progress are reviewed. The production efficiency of various plasmas is summarized and compared. Eventually, the prospect of nitrogen fixation using low-temperature plasma in the future was proposed.

Direct Oxidative Nitrogen Fixation from Air and H2 O by a Water Falling Film Dielectric Barrier Discharge Reactor at Ambient Pressure and Temperature

Current industrial production of HNO₃ relies on the Ostwald process via catalytic oxidation of NH₃ , which is responsible for the vast bulk of CO₂ emission. An attractive alternative route to HNO₃ is direct N₂ oxidation to aqueous HNO₃ , which avoids the NH₃ intermediate. Herein, we for the first time report a non-thermal plasma-assisted nitrogen fixation process characteristic of a large gas-liquid contact based on the water falling film dielectric barrier discharge, wherein HNO₃ is produced directly from ambient air and H₂ O at atmospheric pressure and room temperature without the presence of any catalytic material. By optimizing the plasma reaction conditions, a relatively high synthesis rate and low energy consumption was achieved at the same time with good product selectivity.

Plasma-catalytic ammonia synthesis beyond thermal equilibrium on Ru-based catalysts in non-thermal plasma

The barrier for N₂ dissociation on Ru can be decreased by plasma-activation, or the barrier can be removed completely by the formation of N radicals, resulting in NH₃ formation beyond the thermal equilibrium on Ru-catalysts.

Plasma-Assisted Chain Reactions of Rh3+ Clusters with Dinitrogen: N≡N Bond Dissociation

Dinitrogen activation is known as one of the most challenging subjects in chemistry. A number of well-defined metal complexes, nitrides, and clusters have been studied that show catalysis for dinitrogen activation. However, direct evidence of a complete cleavage of the N≡N triple bond at mild conditions is rather limited to date. Herein, we report a study on the dissociation of N₂ on small rhodium clusters assisted by surface plasma radiation. From mass spectrometry observation, a few rhodium nitride clusters with an odd number of nitrogen atoms are produced, such as the Rh₃N_2m-1⁺ (m = 1-5) series, indicative of N≡N bond dissociation in the mild plasma atmosphere. Interestingly, Rh₃N₇⁺ is identified with outstanding mass abundance among the Rh_nN_2m-1⁺ products, and its ground-state structure is in the form of Rh₃N(N₂)₃⁺ by capping a nitrogen atom on the top of Rh₃⁺ plane and hanging three N₂ molecules beneath the three Rh atoms respectively, giving rise to a C_3v symmetry and excellent stability. We demonstrate the catalysis of such a three-atom rhodium cluster and reveal a dinitrogen activation strategy by thermodynamics- and dynamics- favorable chain reactions of multiple N₂ molecules with two rhodium clusters under plasma atmosphere.

Feasibility Study of Plasma-Catalytic Ammonia Synthesis for Energy Storage Applications

Plasma catalysis has recently gained traction as an alternative to ammonia synthesis. The current research is mostly fundamental and little attention has been given to the technical and economic feasibility of plasma-catalytic ammonia synthesis. In this study, the feasibility of plasma-catalytic ammonia is assessed for small-scale ammonia synthesis. A brief summary of the state of the art of plasma catalysis is provided as well as a targets and potential avenues for improvement in the conversion to ammonia, ammonia separation and a higher energy efficiency. A best-case scenario is provided for plasma-catalytic ammonia synthesis and this is compared to the Haber-Bosch ammonia process operated with a synthesis loop. An ammonia outlet concentration of at least 1.0 mol. % is required to limit the recycle size and to allow for efficient product separation. From the analysis, it follows that plasma-catalytic ammonia synthesis cannot compete with the conventional process even in the best-case scenario. Plasma catalysis potentially has a fast response to intermittent renewable electricity, although low pressure absorbent-enhanced Haber-Bosch processes are also expected to have fast responses to load variations. Low-temperature thermochemical ammonia synthesis is expected to be a more feasible alternative to intermittent decentralized ammonia synthesis than plasma-catalytic ammonia synthesis due to its superior energy efficiency.