Electrochemical Oxidation Of Ammonia Research Articles

In-situ phase transformation of silver oxide nanoparticles encapsulated in reduced graphene oxide (Ag/rGO) for direct electrochemical ammonia oxidation

Silver nanoparticle intercalated in graphene oxide substrate (Ag/GO400 and Ag/rGO400) was synthesized by electroplating and thermal methods for electrochemical oxidation of ammonia in dilute aqueous solutions. Electrochemical characterization, including voltammetry and Raman spectra, identified the solid-state phase transformation of silver, i.e., Ag(0)⇌ Ag(I)⇌ Ag(II), which were electron mediators for the conversion of ammonia to other nitrogen compounds, specifically dinitrogen. The electrochemical-chemical processes (EC) controlled the catalytic yields of N2 and NO3− in the applied potential regions of Ag2O and AgO formation, respectively. The intercalation of highly dispersed Ag nanoparticles/nanoclusters in lamellar sheets of reduced graphene oxide (rGO) enhanced the electrochemical reactivity, leading to the improved faradaic efficiency and N2 selectivity (SN2). GO reduction to rGO decreased surface oxygen content, thereby lowering the yield of nitrite and nitrate oxyanions on Ag/rGO400 than Ag and Ag/GO400 electrodes. Ag/rGO400 exhibited an ammonia removal efficiency of 97% and SN2 of 80% at + 1.0 V (vs. Hg/HgO, pH 11). The Ag/rGO400 anode, equipped with Cu(111)/Ni cathode in an asymmetric and undivided electrolyzer, was applied to treat real wastewater containing around 150 mg-N/L NH3. At an optimal anode to cathode number ratio of 2 to 2, SN2 was>90% in continuous flow mode, which confirmed the stability of the synthesized electrodes.

Self-supporting CuM LDH (M = Ni, Fe, Co) carbon cloth electrodes for selective electrochemical ammonia oxidation to nitrogen

Electrochemical ammonia oxidation reaction (eAOR) has important applications in ammonia fuel cells and wastewater treatment due to its environmental friendliness and convenient operation. In this study, hydrothermal synthesis was used to create self-supported CuM (M = Ni, Fe, Co) carbon cloth electrodes. After three hours of electrolysis at 1.65 V vs. RHE, a CuCo/CC electrode with the ideal Cu/Co ratio (1:2) achieved the maximum dinitrogen selectivity of 90.0% and Faraday efficiency of 77.4%. The reaction mechanism was investigated using Raman and in-situ IR spectroscopy. Cu serves as NH3 adsorption sites and Co catalyzes the oxidation, and together achieve high efficiency.

Electrochemical Ammonia Oxidation with Molecular Catalysts

Electrocatalytic ammonia oxidation (AO) provides an attractive route to a carbon-neutral fuel economy and the energy-efficient production of value-added feedstocks. The design and study of molecular catalysts can give a deeper mechanistic understanding of AO, which will aid in optimizing catalysts for overpotential, efficiency, and chemoselectivity. This Perspective introduces the topic with biological AO chemistry and then surveys the recent developments of molecular AO electrocatalysts, followed by a discussion of design strategies for new catalysts.

Efficient and highly selective direct electrochemical oxidation of ammonia to dinitrogen facilitated by NiCu diatomic site catalysts

Direct electrochemical oxidation has recently received increasing attention in ammonia-nitrogen wastewater treatment. However, how to catalyze the oxidation of ammonia to N2 with high selectivity retains a tremendous challenge. In this work, NiCu3-N-C diatomic site catalyst (DAC) anchored in nitrogen-doped carbon was synthesized via solid-phase pyrolysis. HAADF-STEM and XAFS characterizations demonstrated the high dispersion of adjacent Ni/Cu diatomic sites and the micro-environment of Ni-N4/Cu-N4 groups. The experiment results exhibit that NiCu3-N-C DAC has an eminent intrinsic activity towards AOR with a TOF of 3.64 s−1 at 1.50 V vs. RHE. Consequently, in a real wastewater with 616 mg/L NH4+-N, 99.52% of removal efficiency and 86.60% of faradaic efficiency (FE) after a 6-hour degradation experiment were acquired with a high N2 selectivity of 97.87%, confirming its excellent catalytic performance. This work furnishes a good example of rationally designing functionally coordinated diatomic site catalysts (DACs) for the efficient treatment of ammonia-nitrogen wastewater.

Tailored Bimetallic Ni-Sn Catalyst for Electrochemical Ammonia Oxidation to Dinitrogen with High Selectivity.

Direct electrochemical ammonia oxidation reaction (eAOR) is an efficient and sustainable strategy to process wastewater containing ammonia, and it endures overoxidation and severely competitive oxygen evolution reaction (OER). Herein, we synthesized a Ni(OH)2/SnO2 composite catalyst by a multistep strategy and applied it to the eAOR process. Ni(OH)2/SnO2 exhibited a N2-N Faradaic efficiency (FEN2-N) of 84.2%, with a N2 partial current density (jN2-N) of 2.7 mA cm-2 at 1.55 V vs reversible hydrogen electrode (RHE) in 0.5 M K2SO4 with 10 mM NH3-N (pH 11). The oxophilic Sn promoted NH3 absorption on Ni sites while suppressing the OER. As the active species, NiOOH accelerated the dimerization of intermediates (*NH2 or *NH) to form N2.

Interpretable design of Ir-free trimetallic electrocatalysts for ammonia oxidation with graph neural networks

The electrochemical ammonia oxidation to dinitrogen as a means for energy and environmental applications is a key technology toward the realization of a sustainable nitrogen cycle. The state-of-the-art metal catalysts including Pt and its bimetallics with Ir show promising activity, albeit suffering from high overpotentials for appreciable current densities and the soaring price of precious metals. Herein, the immense design space of ternary Pt alloy nanostructures is explored by graph neural networks trained on ab initio data for concurrently predicting site reactivity, surface stability, and catalyst synthesizability descriptors. Among a few Ir-free candidates that emerge from the active learning workflow, Pt3Ru-M (M: Fe, Co, or Ni) alloys were successfully synthesized and experimentally verified to be more active toward ammonia oxidation than Pt, Pt3Ir, and Pt3Ru. More importantly, feature attribution analyses using the machine-learned representation of site motifs provide fundamental insights into chemical bonding at metal surfaces and shed light on design strategies for high-performance catalytic systems beyond the d-band center metric of binding sites.

Active chlorine mediated ammonia oxidation in an electrified SnO2–Sb filter: Reactivity, mechanisms and response to matrix effects

Ammonia pollution control has gained increasing attention as a result of the existential threat to the environment. The electrochemical advanced oxidation process has demonstrated to be an attractive and environmentally benign platform for aqueous ammonia removal because it only requires electricity and water as a clean “reagent”. Herein, we developed a porous SnO2–Sb filter toward high-efficient ammonia conversion. Despite the introduction of enhanced convection, chlorine-mediated ammonia oxidation dominated the reaction mechanism of the electrified SnO2–Sb filter at circumneutral pH. At an extremely short hydraulic contact time (1.0–5.1 min), ammonia was primarily oxidized to N2 with no chloramines detected. Matrix effect assays showed that the chloride–to–ammonia ratio was vital to determine the removal rate and product selectivity. A pH buffer, HCO3−, might prompt ammonia removal whilst inhibiting chlorate formation. In contrast, the ammonia removal performance of the SnO2–Sb filter deteriorated in the presence of dissolved organic matter (e.g., humic acid). The matrix effects significantly influenced the process efficiency to treat municipal sewage and lake water. The results of this study can advance our understanding of electrochemical ammonia oxidation and provide insight into the effective implementation and optimization of the flow-through process.

Green Chemistry: Advanced Electrocatalysts and System Design for Ammonia Oxidation

The hazards associated with handling hydrogen fuels have driven people to consider alternative clean and sustainable fuel types. Ammonia shows significant potential for this task as both a direct fuel and hydrogen carrier due to its unique features of facile transportation and low cost. Regarding this, electrochemical ammonia oxidation reaction (AOR) is the essential process for utilizing ammonia for energy applications, either for hydrogen production via ammonia splitting or energy generation via direct ammonia fuel cells, which is highly commercially promising. On this basis, the development of high‐performance and economic electrocatalysts for AOR is critical. In this review, the kinetics and mechanism of ammonia electrooxidation are first discussed to provide a foundation to understand the current issues associated with this technology, and then a comprehensive presentation on the different types of electrocatalysts for AOR is illustrated. Afterward, an outlook is presented and the possible research directions for AOR electrocatalysis are proposed, which is expected to shed light on the future development of this promising technology.

Preparation of nickel-copper layered double hydroxide on nickel foam as a non-noble catalyst for efficient electrooxidation of ammonia

Electrochemical oxidation of ammonia is an energy-effective and environment-friendly technology for ammonia-containing wastewater treatment. The key challenge is to explore the high activity and low-price catalyst for ammonia electrooxidation. NiCu layered double hydroxide (NiCu-LDH) on nickel foam (NF) was synthesized for ammonia oxidation via a facile hydrothermal reaction. The Ni0.9Cu0.1-LDH/NF achieved a maximum current density of 78 mA·cm−2 at 1.76 V vs. RHE and a lowest Tafel slope of 195.64 MV·dec−1 in comparison with other catalysts, which was prepared by using different molar ratios of Ni2+ and Cu2+. The Ni0.9Cu0.1-LDH/NF showed smaller and denser nanosheets compared with that of Ni(OH)2/NF, indicating it had more electrochemical active areas and active sites. Chronoamperometry tests showed that the Ni0.9Cu0.1-LDH /NF reached a current density of 23 mA·cm−2 after 120 mins, which was four times as high as Ni(OH)2/NF (6 mA·cm−2). The above results demonstrated that incorporating Cu into the Ni-based layered hydroxides could improve the electrochemical activity of the catalyst, showing Ni0.9Cu0.1-LDH /NF to be a prospective inexpensive, and steady catalyst for high-performance of ammonia electrooxidation.

Heterogeneous Electrochemical Ammonia Oxidation with a Ru-bda Oligomer Anchored on Graphitic Electrodes via CH-π Interactions.

Molecular catalysts can promote ammonia oxidation, providing mechanistic insights into the electrochemical N2 cycle for a carbon-free fuel economy. We report the ammonia oxidation activity of carbon anodes functionalized with the oligomer {[RuII(bda-κ-N 2 O 2)(4,4'-bpy)]10(4,4'-bpy)}, Rubda-10, where bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine. Electrocatalytic studies in propylene carbonate demonstrate that the Ru-based hybrid anode used in a 3-electrode configuration transforms NH3 to N2 and H2 in a 1:3 ratio with near-unity faradaic efficiency at an applied potential of 0.1 V vs Fc+/0, reaching turnover numbers of 7500. X-ray absorption spectroscopic analysis after bulk electrolysis confirms the molecular integrity of the catalyst. Based on computational studies together with electrochemical evidence, ammonia nucleophilic attack is proposed as the primary pathway that leads to critical N-N bond formation.

Electrochemical characteristics of silver/nickel oxide (Ag/Ni) for direct ammonia oxidation and nitrogen selectivity in paired electrode system

Electrochemical engineering processes are emerging alternatives for ammonia removal owing to high efficiency and low waste product. Ag electrodes were synthesized by electroplating metallic silver nanoparticles (Ag NPs) on different substrates (Ni, Ti, and graphite) for the electrochemical oxidation of ammonia in aqueous solutions. Among tested electrodes, Ag/Ni400 was the best in NH3 oxidation and N2 yield. Behaviors of the two reversible couples, AgO/Ag2O and NiOOH/Ni(OH)2, catalytically mediated the faradaic process were analyzed by voltammetry. The performance of direct electro-oxidation in terms of current efficiency and energy consumption was investigated in an asymmetric cell using Ag/Ni400 anode and copper cathode (Cu/Ni). Parameters of batch constant current electrolysis included initial concentration and current density. The ratio of paired electrodes significantly affected the N2 selectivity. The Ag/Ni400-Cu/Ni electrode system was further applied for the treatment of a real opto-electronic wastewater containing NH3 of around 250 mg-N/L and low concentration of Cl- ion (<50 mg L-1); under an electrode ratio of 2:1, N2 yield reached to>95 % at 10 mA cm−2.

Catalysts for electrochemical ammonia oxidation: Trend, challenge, and promise

Developing sustainable and clean energy technology is critial to address the current energy and environmental crisis. As an energy carrier, hydrogen has been getting attention due to its abundance, high energy density, and zero carbon emission. Due to the difficulty of hydrogen storage and transport, the electro-oxidation of ammonia shows great potential for renewable energy since it satisfies both energy supply and environmental protection as a carbon-free fuel with high hydrogen content. However, ammonia electro-oxidation is a sluggish reaction and shows low durability. Understanding the complicated mechanism and intermediates is conducive to developing efficient electrocatalysts with high catalytic activity and durability. This review begins with the ammonia oxidation technologies and their principles. The two typical mechanisms for ammonia electro-oxidation, Oswin-Salomon mechanism and Gerischer-Mauerer mechanism, are discussed, and attention to the poisoning species is included. Then, the electrocatalysts for ammonia reaction are classified into Pt-based catalysts and non-noble metal-based catalysts, and their performances are reviewed with various strategies. Based on the progress on the electrochemical ammonia oxidation catalysts, the challenges and future insight on ammonia oxidation reaction are discussed and proposed.

Electrooxidation of ammonia on A-site deficient perovskite oxide La0.9Ni0.6Cu0.35Fe0.05O3-δ for wastewater treatment

Wastewater can contain high amounts of ammonia which can pose as a great safety threat if released into natural waters. The electrochemical oxidation of ammonia offers a viable strategy to remove high concentrations and provides an attractive method for wastewater treatment. However, finding a highly efficient and low-cost catalyst is imperative for overcoming the sluggish nature of ammonia oxidation reaction. Herein, a modified A- and B-site perovskite is proposed as a catalyst for the oxidation of ammonia, making it suitable as an anode in an ammonia electrolyser. A series of La1-yNi0.6Cu0.4-xFexO3-δ (x = 0, 0.05 and 0.10; y = 0, 0.05 and 0.10) perovskite materials were synthesised by a conventional sol–gel method. Amongst those tested oxides, La0.9Ni0.6Cu0.35Fe0.05O3-δ was found to have superior activity towards the electrooxidation of ammonia due to an optimised amount of Fe doping and the presence of oxygen vacancies introduced by an A-site deficiency. Subsequently, La0.9Ni0.6Cu0.35Fe0.05O3-δ was employed as an anode in an ammonia electrolyser where the ammonia removal efficiency reached 95.4 % in simulated wastewater after 80 hr and a substantial reduction in real wastewater was also observed. These results demonstrate that the A-site deficient perovskite materials are a viable electrode for the removal of ammonia in a practical energy setting and paves way for future applications.

CuSn Double‐Metal Hydroxides for Direct Electrochemical Ammonia Oxidation to Dinitrogen

AbstractDirect electrochemical ammonia oxidation reaction (dAOR) to form N2 is of great significance to process ammonia‐containing wastewater or to make efficient ammonia fuel cells. Herein, CuSn double‐metal hydroxides are synthesized via a co‐precipitation method and used in dAOR with 10 mM −N Among the synthesized hydroxides, CuSn(OH)6 shows a decent −N partial current density (1.2 mA cm−2) and a substantially higher −N Faradaic efficiency (84.5 %). In situ infrared spectroscopy confirms that the intermediacy of dimeric species facilitates the formation of desired N2 product during the dAOR process. The synergy between Cu and Sn is critical for the enhanced dAOR activity, as Cu serves as the catalytic center while Sn adsorbs −N intensively.

Pathways of ammonia electrooxidation on nickel hydroxide anodes and an alternative route towards recycled fertilizers

Ni(OH)2-catalyzed electrochemical oxidation of ammonia can be used for the synthesis S- or P-containing NH4NO3-based fertilizers with up to 72% faradaic efficiency and up to 98% ammonia removal efficiency.

Electrocatalytic oxidation of ammonia on Pt: Mechanistic insights into the formation of N2 in alkaline media

Electrochemical ammonia oxidation reaction (AOR) is one of the most effective methods to utilize ammonia. Currently, the favorable coupling process during AOR and most important intermediates are still under debate. Herein we used ab initio molecular dynamics (AIMD) method combined with deliberately designed free-energy calculations to thoroughly study the energetics of elementary steps during AOR. The main pathway of N2 formation during AOR is initiated by the dehydrogenation of NH3 to NH2, and subsequent NH2 + NH2 coupling to form N2H4, followed by stepwise dehydrogenation of N2H4 to N2. The Gerischer-Mauerer mechanism was found to be favored in the process. The dehydrogenation of NH3 to NH2 was determined to be the rate-limiting step, and NH2 is the most important intermediate. Through this work, the overall reaction network and corresponding energetics of AOR is revealed, which may enlighten the understanding of AOR and further accelerate corresponding catalyst innovation.

Electrochemical ammonia oxidation reaction on defect-rich TiO nanofibers: Experimental and theoretical studies

Ammonia is a promising platform molecule for the future renewable energy infrastructure owing to its high hydrogen content and carbon-free nature. The development of inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) is essential to ammonia-based economy. In this paper, a self-supporting AOR catalyst of ultra-elongated defective TiO nanofibers grown on Ti foil (TiO/Ti foil) is designed. By the comparisons of electrochemical measurement results, the efficient AOR activity of TiO/Ti foil has been presented in alkaline ammonia solution when the voltage is higher than 0.4 V. Density functional theory (DFT) calculations illustrate that the surface oxygen vacancies in TiO play a vital role in facilitating AOR activity by reducing the energy barrier in the rate-determining ∗HNNH2 formation step and also promoting N2 desorption. These results demonstrate that defective TiO is a promising low-cost and stable catalyst for AOR, which is a potential electrode for efficient H2 production by ammonia electrolysis and direct ammonia fuel cells.

Ammonia oxidation on iridium electrode in alkaline media: An in situ ATR-SEIRAS study

• ATR-SEIRAS is extended to study the AOR on Ir electrode in basic media. • NH x ,ad species is attributed to the pivot intermediate. • N 2 H x+y ,ad species is identified as the key active intermediate to generate N 2. • NO 2 − and NO 3 − byproducts arise from NO ad oxidation. Mechanistic understanding of electrochemical ammonia oxidation reaction (AOR) is important for designing efficient AOR catalysts. In this work, in situ attenuated total reflection surface-enhanced infrared spectroscopy (ATR-SEIRAS) is employed to study the AOR at varied potentials on an Ir electrode in alkaline media. The AOR on Ir contains two sequential oxidation peaks in the anodic voltammograms. The in situ ATR-SEIRAS results indicate that the small Peak I at a lower potential is mainly due to dehydrogenation of interfacial ammonia and dimerization of as-generated NH x, ad species to N 2 H x+y, ad species. And the broad and intense Peak II at a higher potential mainly involves the N 2 production from N 2 H x+y, ad species, NO ad species generation from NH x ,ad species and further successive oxidation into the NO 2 − and NO 3 − by-products at higher potentials. It is thus inferred that NH x species are the pivotal intermediates for AOR on Ir electrode, and the N 2 H x+y ,ad and NO ad species the key intermediates for the production of N 2 , NO 2 − and NO 3 − , respectively.

Coupling photocatalytic and electrocatalytic oxidation towards simultaneous removal of humic acid and ammonia−nitrogen in landscape water

Aiming to bring photocatalytic and electrocatalytic oxidation processes into solving practical issue of organics and ammonia−nitrogen pollution in landscape water that resulting in algae bloom and eutrophication, this work firstly investigates photocatalytic oxidation of humic acid and electrochemical oxidation of ammonia upon optimization of each process parameters, respectively. The platinum modified titania (Pt/TiO2) exhibits improved activity than pure titania and CuOx, MnOx and NiOx modified titania for decomposition of humic acid. As an application−oriented study, this work has developed a simple and effective brushing and annealing method for immobilization of TiO2 and Pt/TiO2 onto ceramic foam for further application. In addition, the RuO2–IrO2/Ti electrode presents the best electrocatalytic activity compared with RuO2/Ti and IrO2/Ti electrodes in terms of ammonia oxidation, and the ammonia conversion pathways have been studied. Lastly, an integrated and enlarged reactor system employing optimized photocatalytic ceramic foam and stable electrodes has been developed for simultaneous oxidation of humic acid and ammonia−nitrogen in water circulated flow condition, based on cooperative production of reactive oxidant species between photocatalysis and electrocatalysis. The results show that coupled photocatalytic and electrocatalytic oxidation is a promising approach for treatment of organic matter and inorganic ammonia nitrogen in landscape water.

High entropy spinel oxide for efficient electrochemical oxidation of ammonia

High entropy spinel oxide for efficient electrochemical oxidation of ammonia