In Situ/Operando Methods for Understanding Electrocatalytic Nitrate Reduction Reaction

Ir NCs Embedded Co-MOF Nanosheets for Boosting Electrochemical Nitrate Reduction to Ammonia Performance

In this study, Ir nanoclusters adorned with abundant p-mercaptobenzoic acid (p-MBA) ligands were employed to fabricate an electrocatalytic material consisting of Ir nanoclusters embedded within two-dimensional Co-MOF nanosheets (Ir NCs@Co-MOF) for the electrocatalytic NO3- reduction reaction (NO3-RR). TEM analysis confirmed that Ir nanoclusters are uniformly distributed in 2D Co-MOF nanosheets, with an average diameter of about 1.8 nm. At a potential of -0.8 V vs RHE, the Ir NCs@Co-MOF catalyst achieved a nitrate conversion rate, ammonia selectivity, and yield of 92.5, 81.4%, and 230.1 μg·h-1·cm-2, respectively, over a reaction duration of 120 min. The strong interaction between Ir nanoclusters and Co-MOF serves to enhance electrocatalytic activity and accelerate the rate of nitrate reduction. Stability tests indicated that after 20 cycles, both the nitrate conversion and ammonia selectivity of the Ir NCs@Co-MOF catalyst demonstrated relative stability, thereby indicating a robust performance for this catalytic system. The results of EPR and TBA quenching experiments indicate that *H plays a key role in the NO3-RR process. In situ DEMS investigations revealed that during the NO3-RR process, the reaction pathway was as follows: *NO3 → *NO2 → *NO → *NOH → *NH2OH → *NH2 → *NH3 → NH3.

Anion-Exchange Strategy for Ru/RuO2-Embedded N/S-Co-Doped Porous Carbon Composites for Electrochemical Nitrogen Fixation

Ionic porous polymers have been widely utilized efficiently to anchor various metal atoms for the preparation of metal-embedded heteroatom-doped porous carbon composites as the active materials for electrocatalytic applications. However, the rational design of the heteroatom and metal elements in HPC-based composites remains a significant challenge, due to the tendency of the aggregation of metal nanoparticles during pyrolysis. In this study, a nitrogen (N)- and sulfur (S)-enriched ionic covalent organic framework (iCOF) incorporating viologen and thieno[3,4-b] thiophene (TbT) was constructed via Zincke-type polycondensation. The synthesized iCOF possesses a crystalline porous structure with a pore size of 3.05 nm, a low optical band gap of 1.88 eV, and superior ionic conductivity of 10-2.672 S cm-1 at 333 K, confirming the ionic and conjugated nature of our novel iCOF. By applying the iCOF as the precursor, a ruthenium and ruthenium(IV) oxide (Ru/RuO2) nanoparticle-embedded N/S-co-doped porous carbon composite (NSPC-Ru) was prepared by using a two-step sequence of anion-exchange and pyrolysis processes. In the electrochemical nitrogen reduction reaction (eNRR) application, the NSPC-Ru achieves an impressive NH3 yield rate of 32.0 μg h-1 mg-1 and a Faradaic efficiency of 13.2% at -0.34 V vs. RHE. Thus, this innovative approach proposes a new route for the design of iCOF-derived metal-embedded porous carbon composites for enhanced NRR performance.

Hydride Formation and Decomposition on Cu(111) in HClO4

Cu electrodeposition and the electrocatalysis of hydrogenation reactions thereupon involve significant interactions with adsorbed hydrogen. Electrochemical mass spectrometry (EC-MS) is used to explore the formation and decomposition of surface hydride on Cu(111) in 0.1 mol L-1 HClO4. Hydride formation is associated with two reduction waves that reflect the potential-dependent Hads coverage and its reconstruction. Voltammetric cycling reveals an additional oxidative and reductive feature at ≈ -0.05 V versus the reversible hydrogen electrode (RHE) that reflects the state of the 2D surface hydride. Extending the voltammetric window to more negative potentials results in an increase in Hads coverage and surface reconstruction that subsequently leads to accelerated hydride decomposition at positive potentials. Voltammetric and chronoamperometric analysis of hydride formation indicates a Hads coverage of ≈0.75 monolayers (ML) between -0.225 V vs RHE and -0.275 V vs RHE with further increases in Hads observed with the onset and acceleration of the HER at more negative potentials. Returning to more positive potentials, hydride decomposition begins above -0.05 V vs RHE. Recombination of Hads to form H2 accounts for desorption of ≈0.5 ML of Hads while its oxidation to H3O+ consumes between ≈0.15 and ≈0.4 ML of Hads, depending on the specific electrochemical conditions. The potential-dependent Hads coverage and surface reconstruction are congruent with trends identified in recent computational and electrochemical scanning tunneling microscopy studies. In contrast to perchloric acid, the presence of strongly adsorbing anions, such as sulfate or halides, favors hydride decomposition via the recombination pathway.

Engineering the Local Electronic Configuration of Diatomic Iron-Nickel Site for Enhanced Nitrate and Ammonia Co-Electrolysis Activity

Anthropogenic activities have caused a significant rise in nitrate and ammonia nitrogen levels in natural water bodies, disrupting the balance of the nitrogen cycle. The electrocatalytic reduction of nitrate and the oxidation of ammonia are promising strategies for converting polyvalent nitrogen into nontoxic and harmless N2. Herein, a bifunctional electrode loaded with diatomic iron-nickel site on porous N-doped carbon (FeNi-NC) is designed and successfully applied for the co-electrolysis of nitrate and ammonia. The incorporation of the Fe atom shifts the partial density of states of Ni 3d away from the Fermi level and suppresses the 3d-2π* coupling between Ni sites and superficial N2, leading to the easy desorption of N2 intermediates. Consequently, the Faradaic efficiencies of FeNi-NC for N2 production at the cathode and anode are 90.3% and 99.4% at 1.8 V, respectively, and an electricity consumption saving of 19.4% is achieved. This work provides a feasible strategy to regulate the electronic configuration of atomically dispersed catalysts for sewage treatment.

Ammonia Synthesis with Visible Light and Quantum Dots

Light-assisted synthesis of ammonia from nitrate and nitrite sources is a sustainable approach to reduce the burden of the energy-intensive Haber-Bosch process. However, poor selectivity and the need for UV-active photocatalysts are the current bottlenecks in the synthesis of ammonia from nitrate and nitrite sources. Herein, we introduce selective visible-light-driven ammonia production from nitrate and nitrite ions with indium phosphide quantum dots (InP QDs) as the photocatalyst. The presence of catalytic indium sites and microenvironment modulation through an interplay of catalyst-reactant interactions resulted in efficient and selective ammonia formation under visible light. Ammonia was produced in an attractive yield of ∼94% in both aqueous and gaseous phases within 2 h of visible-light irradiation at room temperature. A decent formation of ammonia was observed under sunlight as well, strengthening the translational prospects of InP QD photocatalysts. Mechanistic investigations ascertained a negligible role of competing hydrogen evolution in direct nitrate reduction, confirming the active participation of photoexcited charge carriers from InP QDs in the ammonia synthesis. Kinetic studies revealed the energetically challenging nitrate-to-nitrite conversion as the rate-determining step, with subsequent reactions proceeding with ∼100% conversion to yield ammonia. A series of experiments concluded that water is the proton source in the InP QD-photocatalyzed synthesis of ammonia. Our study shows the impact of the rationally designed core and surface of InP QD-based photocatalysts in developing sustainable routes to produce ammonia beyond the Haber-Bosch process.

Oxygen vacancies in Bi2WO6 enable robust nitrate reduction reaction catalysis

The faradaic efficiency of Bi2WO6 is 62.0% at -0.3 V vs. RHE, an improvement of 1.4- and 1.8-times relative to Bi2O3 and WO3 for the nitrate reduction reaction. In addition, the catalysis occurring on Bi2WO6 follows the pathway: *NO3-*NO2-*NO-*NOH-*HNOH-*H2NOH-*NH3, as verified by operando Raman spectroscopy and theoretical calculations.

Recent progress in the advanced strategies, rational design, and engineering of electrocatalysts for nitrate reduction toward ammonia

Ammonia is a valuable feedstock for most chemicals, pharmaceuticals, and fertilizer products. It is a promising carbon-free energy source. Under severe experimental circumstances (high temperature and high pressure), ammonia is manufactured industrially using the standard Haber-Bosch process. This process uses a lot of energy and emits a huge amount of CO2 into the environment. One method that is seen to be promising and could eventually replace the Haber-Bosch process is the electrocatalytic production of ammonia. However, in ambient conditions, the cleavage of the nitrogen molecule is exceedingly difficult. As a result, the yield of ammonia remains modest and the study's scope is still restricted to the lab. When the catalytic performance is significantly increased, nitrate and nitrite contaminations in water systems can be effectively removed and simultaneously transformed into energy sources if nitrites or nitrates are employed as nitrogen sources instead of nitrogen gas. This may become a new substitute for the synthesis of ammonia, but nitrate and nitrite reduction are not getting enough attention. In this review, we discuss the performance of the electrocatalytic nitrate reduction reaction, which includes cycling stability, reactivity, selectivity, and faradaic efficiency. Following this summary, we look into the crucial elements, the rate-determining step, and the reaction mechanisms that govern the performance of the nitrate reduction reaction. In order to support the practical use of the electrocatalytic nitrate reduction reaction, we finally provided a summary of the challenges and future directions guiding the design of efficient catalyst and reaction systems.

A temperature-adjustable in situ infrared diffuse reflectance spectroscopy system for catalysts

We introduce an innovative in situ infrared diffuse reflection rapid detection system, endowed with a temperature regulation function. This system is adept at conducting rapid infrared spectra scanning as well as simulating the catalytic environment of diverse reaction systems. The infrared absorption spectra of four kinds of Pt-based catalysts under vacuum conditions across a wide temperature spectrum ranging from -180 to 300 °C are obtained and analysed through IR correlation spectroscopy. A key finding is the notable variance in peak intensity within Pt/CeO2/CNT catalysts, highlighting a robust adsorption capacity for oxygen-containing groups at lower temperatures and a marked desorption at higher temperatures. By enabling rapid and accurate assessments of catalyst behavior under varying temperatures, it not only accelerates the evaluation process but also provides valuable insights that can guide the synthesis of more efficient catalysts.

Nitrogenous Intermediates in NOx-involved Electrocatalytic Reactions

Chemical manufacturing utilizing renewable sources and energy emerges as a promising path towards sustainability and carbon neutrality. The electrocatalytic reactions involving nitrogen oxides (NOx) offered a potential strategy for synthesizing various nitrogenous chemicals. However, it is currently hindered by low selectivity/efficiency and limited reaction pathways, mainly due to the difficulties in controllable generation and utilization of nitrogenous intermediates. In this minireview, focusing on nitrogenous intermediates in NOx-involved electrocatalytic reactions, we discuss newly developed methodologies for studying and controlling the generation, conversion, and utilizing of nitrogenous intermediates, which enable recent developments in NOx-involved electrocatalytic reactions that yield various products, including ammonia (NH3), organonitrogen molecules, and nitrogenous compounds exhibiting unconventional oxidation states. Furthermore, we also make an outlook to highlight future directions in the emerging field of NOx-involved electrocatalytic reactions.

Ambient Electrochemical Ammonia Synthesis: From Theoretical Guidance to Catalyst Design

Ammonia, a vital component in the synthesis of fertilizers, plastics, and explosives, is traditionally produced via the energy-intensive and environmentally detrimental Haber-Bosch process. Given its considerable energy consumption and significant greenhouse gas emissions, there is a growing shift toward electrocatalytic ammonia synthesis as an eco-friendly alternative. However, developing efficient electrocatalysts capable of achieving high selectivity, Faraday efficiency, and yield under ambient conditions remains a significant challenge. This review delves into the decades-long research into electrocatalytic ammonia synthesis, highlighting the evolution of fundamental principles, theoretical descriptors, and reaction mechanisms. An in-depth analysis of the nitrogen reduction reaction (NRR) and nitrate reduction reaction (NitRR) is provided, with a focus on their electrocatalysts. Additionally, the theories behind electrocatalyst design for ammonia synthesis are examined, including the Gibbs free energy approach, Sabatier principle, d-band center theory, and orbital spin states. The review culminates in a comprehensive overview of the current challenges and prospective future directions in electrocatalyst development for NRR and NitRR, paving the way for more sustainable methods of ammonia production.

Tandem Nitrate-to-Ammonia Conversion on Atomically Precise Silver Nanocluster/MXene Electrocatalyst

Electrocatalytic reduction of nitrate (NO3 RR) to synthesize ammonia (NH3 ) provides a competitive manner for carbon neutrality and decentralized NH3 synthesis. Atomically precise nanoclusters, as an advantageous platform for investigating the NO3 RR mechanisms and actual active sites, remain largely underexplored due to the poor stability. Herein, we report a (NH4 )9 [Ag9 (mba)9 ] nanoclusters (Ag9 NCs) loaded on Ti3 C2 MXene (Ag9 /MXene) for highly efficient NO3 RR performance towards ambient NH3 synthesis with improved stability in neutral medium. The composite structure of MXene and Ag9 NCs enables a tandem catalysis process for nitrate reduction, significantly increasing the selectivity and FE of NH3 . Besides, compared with individual Ag9 NCs, Ag9 /MXene has better stability with the current density performed no decay after 108 hours of reaction. This work provides a strategy for improving the catalytic activity and stability of atomically precise metal NCs, expanding the mechanism research and application of metal NCs.

Advances in Cu nanocluster catalyst design: recent progress and promising applications

The quest for cleaner pathways to the production of fuels and chemicals from non-fossil feedstock, efficient transformation of raw materials to value-added chemicals under mild conditions, and control over the activity and selectivity of chemical processes are driving the state-of-the-art approaches to the construction and precise chemical modification of sustainable nanocatalysts. As a burgeoning category of atomically precise noble metal nanoclusters, copper nanoclusters (Cu NCs) benefitting from their exclusive structural architecture, ingenious designability of active sites and high surface-to-volume ratio qualify as potential rationally-designed catalysts. In this Minireview, we present a detailed coverage of the optimal design strategies and controlled synthesis of Cu NC catalysts with a focus on tuning of active sites at the atomic level, the implications of cluster size, shape and structure, the ligands and heteroatom doping on catalytic activity, and reaction scope ranging from chemical catalysis to emerging photocatalysis and electrocatalysis.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.