A Spectroscopic Study on Nitrogen Electrooxidation to Nitrate

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Sulphur-Boosted Active Hydrogen on Copper for Enhanced Electrocatalytic Nitrate-to-Ammonia Selectivity

Electrocatalytic nitrate reduction to ammonia is a promising approach in term of pollutant appreciation. Cu-based catalysts performs a leading-edge advantage for nitrate reduction due to its favorable adsorption with *NO3. However, the formation of active hydrogen (*H) on Cu surface is difficult and insufficient, leading to the significant generation of by-product NO2 -. Herein, sulphur doped Cu (Cu-S) is prepared via an electrochemical conversion strategy and used for nitrate electroreduction. The high Faradaic efficiency (FE) of ammonia (~98.3 %) and an extremely low FE of nitrite (~1.4 %) are achieved on Cu-S, obviously superior to its counterpart of Cu (FENH3: 70.4 %, FENO2 -: 18.8 %). Electrochemical in situ characterizations and theoretical calculations indicate that a small amount of S doping on Cu surface can promote the kinetics of H2O dissociation to active hydrogen. The optimized hydrogen affinity validly decreases the hydrogenation kinetic energy barrier of *NO2, leading to an enhanced NH3 selectivity.

Synergy of Pd2+/S2--Doped TiO2 Supported on 2-Methylimidazolium-Functionalized Polypyrrole/Graphene Oxide for Enhanced Nitrogen Electrooxidation

The electrosynthesis of nitrate catalyzed by electrochemical nitrogen oxidation reaction (NOR) is considered as an alternative and sustainable approach to the conventional industrial manufacture, but optimizing the electrocatalytic NOR performance and fabricating the efficient NOR electrocatalysts at the design level still encounter great challenges. Herein, unique Pd2+- and S2--doped TiO2 (Pb/S-TiO2) nanoparticles are successfully in situ grown on the surface of 2-methylimidazolium-functionalized polypyrrole/graphene oxide (2-MeIm/PPy/GO), which present the typical hierarchical micro-nanostructures, resulting in the excellent electrocatalytic NOR performance with the highest NO3 - yield of 72.69 µg h-1 mg-1 act. and the maximum Faraday efficiency of 8.92% at 2.04 V (vs reversible hydrogen electrode) due to the synergistic effect of each component. Due to the doping effect, the appropriate oxygen evolution reaction (OER) activity is achieved by Ti-site, where OER principally occurs, providing *O during the non-electrochemical step of NOR, while the electrocatalytic NOR process as the electrochemical conversion of inert N2 to active *NO intermediates mainly occurs at the Pd-site. Especially, the sulfate radicals in situ formed on Pb/S-TiO2@2-MeIm/PPy/GO further promote nitrogen adsorption and decrease the reaction energy barrier, resulting in the acceleration of NOR. It provides theoretical and practical experience for the design and preparation of NOR electrocatalysts.

Controllable Assembly of Highly Oxidized Cobalt on Graphdiyne Surface for Efficient Conversion of Nitrogen into Nitric Acid

The manufacture of nitric acid (HNO3 ) consumes large amounts of energy and causes serious environmental pollution. Electrochemical synthesis is regarded as a key way to eliminate carbon emissions from the chemicals industry. The selective electrosynthesis of HNO3 from nitrogen was achieved by controllable assembly of cobalt metal on graphdiyne surface using a powerful tool of electrochemistry at ambient conditions. As an advanced material, graphdiyne (GDY) has a large conjugated structure on its surface and is rich in sp-C triple bond skeleton, which can achieve strong interaction with metal atoms, resulting in incomplete charge transfer between graphdiyne and cobalt atoms. The experimental and theoretical calculation results show that the highly oxidized cobalt on graphdiyne (HOCo/GDY) can selectively and efficiently activate and convert the nitrogen into the key intermediate *NO, which promotes the efficient overall conversion performance of nitrogen to nitric acid. Thus, the highest nitric acid yield (192.0 μg h-1  mg-1 ) and Faradaic efficiency (21.5 %) were achieved at low potentials.

Monitoring SEIRAS on a Graphitic Electrode for Surface-Sensitive Electrochemistry: Real-Time Electrografting

The ubiquity of graphitic materials in electrochemistry makes it highly desirable to probe their interfacial behavior under electrochemical control. Probing the dynamics of molecules at the electrode/electrolyte interface is possible through spectroelectrochemical approaches involving surface-enhanced infrared absorption spectroscopy (SEIRAS). Usually, this technique can only be done on plasmonic metals such as gold or carbon nanoribbons, but a more convenient substrate for carbon electrochemical studies is needed. Here, we expanded the scope of SEIRAS by introducing a robust hybrid graphene-on-gold substrate, where we monitored electrografting processes occurring at the graphene/electrolyte interface. These electrodes consist of graphene deposited onto a roughened gold-sputtered internal reflection element (IRE) for attenuated total reflectance (ATR) SEIRAS. The capabilities of the graphene-gold IRE were demonstrated by successfully monitoring the electrografting of 4-amino-2,2,6,6-tetramethyl-1-piperidine N-oxyl (4-amino-TEMPO) and 4-nitrobenzene diazonium (4-NBD) in real time. These grafts were characterized using cyclic voltammetry and ATR-SEIRAS, clearly showing the 1520 and 1350 cm-1 NO2 stretches for 4-NBD and the 1240 cm-1 C-C, C-C-H, and N-Ȯ stretch for 4-amino-TEMPO. Successful grafts on graphene did not show the SEIRAS effect, while grafting on gold was not stable for TEMPO and had poorer resolution than on graphene-gold for 4-NBD, highlighting the uniqueness of our approach. The graphene-gold IRE is proficient at resolving the spectral responses of redox transformations, unambiguously demonstrating the real-time detection of surface processes on a graphitic electrode. This work provides ample future directions for real-time spectroelectrochemical investigations of carbon electrodes used for sensing, energy storage, electrocatalysis, and environmental applications.

Energizing Co Active Sites via d-Band Center Engineering in CeO2 -Co3 O4 Heterostructures: Interfacial Charge Transfer Enabling Efficient Nitrate Electrosynthesis

The electrochemical nitrogen oxidation reaction (NOR) holds significant potential to revolutionize the traditional nitrate synthesis processes. However, the progression in NOR has been notably stymied due to the sluggish kinetics of initial N2 adsorption and activation processes. Herein, the research embarks on the development of a CeO2 -Co3 O4 heterostructure, strategically engineered to facilitate the electron transfer from CeO2 to Co3 O4 . This orchestrated transfer operates to amplify the d-band center of the Co active sites, thereby enhancing N2 adsorption and activation dynamics by strengthening the Co─N bond and diminishing the resilience of the N≡N bond. The synthesized CeO2 -Co3 O4 manifests promising prospects, showcasing a significant HNO3 yield of 37.96 µg h-1 mgcat -1 and an elevated Faradaic efficiency (FE) of 29.30% in a 0.1 m Na2 SO4 solution at 1.81 V versus RHE. Further substantiating these findings, an array of in situ methodologies coupled with DFT calculations vividly illustrate the augmented adsorption and activation of N2 on the surface of CeO2 -Co3 O4 heterostructure, resulting in a substantial reduction in the energy barrier pertinent to the rate-determining step within the NOR pathway. This research carves a promising pathway to amplify N2 adsorption throughout the electrochemical NOR operations and delineates a blueprint for crafting highly efficient NOR electrocatalysts.

Activity Trend and Selectivity of Electrochemical Ammonia Synthesis in Reverse Artificial Nitrogen Cycle

Ammonia is important for modern agriculture and food production as it is a major source of fertilizer. Electrochemical ammonia synthesis (EAS) with sustainable energy generated electricity and decentralized reactors has been considered as environmentally friendly process. Several nitrogen sources have been considered and intensively studied in experiments and computations. Recently, it has been proposed and demonstrated that nitrogen oxides (NOx ) electroreduction for selective ammonia production is feasible. Fundamental insights on experimental observation are necessary for more rational design of catalysts and reactors in the future. In this concept, we review the theoretical and computational insights of electrochemical nitrogen oxides reduction, particularly, the activity trend over diverse transition metal catalysts and products selectivity at varying potentials. Finally, we address the opportunities and challenges in the reverse artificial nitrogen cycle, as well as fundamental issues in electrochemical reaction modelling.

Selective Electrocatalytic Oxidation of Nitrogen to Nitric Acid Using Manganese Phthalocyanine

Ammonia is produced through the energy-intensive Haber-Bosch process, which undergoes catalytic oxidation for the production of commercial nitric acid by the senescent Ostwald process. The two energy-intensive industrial processes demand for process sustainability. Hence, single-step electrocatalysis offers a promising approach toward a more environmentally friendly solution. Herein, we report a 10-electron pathway associated one-step electrochemical dinitrogen oxidation reaction (N₂OR) to nitric acid by manganese phthalocyanine (MnPc) hollow nano-structures under ambient conditions. The catalyst delivers a nitric acid yield of 513.2 μmol h^-1 g_cat^-1 with 33.9% Faradaic efficiency @ 2.1 V versus reversible hydrogen electrode. The excellent N₂OR performances are achieved due to the specific-selectivity, presence of greater number of exposed active sites, recyclability, and long period stability. The extended X-ray absorption fine structure confirms that Mn atoms are coordinated to the pyrrolic and pyridinic nitrogen via Mn-N₄ coordination. Density functional theory-based theoretical calculations confirm that the Mn-N₄ site of MnPc is the main active center for N₂OR, which suppresses the oxygen evolution reaction. This work provides a new arena about the successful example of one step nitric acid production utilizing a Mn-N₄ active site-based metal phthalocyanine electrocatalyst by dinitrogen oxidation for the development of a carbon-neutral sustainable society.