Direct and mediated electrochemical oxidation of ammonia on boron-doped diamond electrode

Direct (non-mediated) electrochemical oxidation of ammonia on boron-doped diamond (BDD) electrode proceeds mainly at high pH (> 8) via free ammonia (NH₃) oxidation. To enhance ammonia oxidation on BDD at low pH (< 8), where mainly ammonium (NH₄⁺) is present, oxidation of ammonia was mediated by active free chlorine. In this process, electro-generated in situ active chlorine rapidly reacts with ammonia instead of being further electro-oxidized to chlorate at the electrode surface. Thus, active chlorine effectively removes ammonia from an acidic solution, while the formation of by-products such as chlorate and possibly perchlorate is minimized.     

Enhanced ammonia oxidation on BDD induced by inhibition of oxygen evolution reaction

Electrochemical oxidation of ammonia (NH₃ and NH₄ ⁺ ) on boron-doped diamond (BDD) electrode was studied using differential electrochemical mass-spectrometry (DEMS) and chronoamperometry. Electro-oxidation of ammonia induces inhibition of the oxygen evolution reaction (OER) due to adsorption of the ammonia oxidation products on the BDD surface. The inhibition of the OER enhances ammonia electro-oxidation, which becomes the main reaction. The amino radicals, formed during ammonia oxidation, trigger a reaction chain in which molecular oxygen dissolved in solution is involved in the ammonia electro-oxidation. Nitrogen, nitrous oxide, and nitrogen dioxide were detected as the ammonia oxidation products, with nitrogen being the main gaseous product of the oxidation.     

Surface and Bulk Reactions of Solid Nickel Dimethylglyoximate with Electron-Active Gases

The action of ammonia on the surface of solid nickel dimethylglyoximate Ni(HDm)_d2 is reversible and is accompanied by a change in the electrophysical properties of the surface layer of the complex in proportion to the ammonia content in the air. The action of chlorine on the complex gives rise to bulk chemical reactions yielding the Ni(III) complex {[Ni^III(HDm)₂(H₂O)]₂Cl⁰₂}Cl^I₂ with a bridging μ-dichloride group, which is further oxidized to a substance presumably containing such ligands as one dimethylglyoxime group and products of its oxidation by chlorine.     

Electrochemical behavior of ammonia at Ni/Ni(OH)2 electrode

The electrochemical oxidation of ammonia was investigated on a Ni/Ni(OH)₂ electrode prepared by potential cycling of a Ni electrode in 1 M NaClO₄. It was found that oxidation of ammonia is strongly pH dependent and proceeds mainly at pH values above 7. This indicates that NH₃ rather than ${\text{NH}}_4^{+}$ is oxidized on nickel electrodes. Oxidation of ammonia occurs in the potential region of Ni(II)/Ni(III) redox activity resulting in formation of a clear peak. Ni/Ni(OH)₂ is not deactivated during ammonia oxidation even at high ammonia concentrations. A considerable fraction of the ammonia was oxidized to nitrate (11%), while the rest were gaseous nitrogen compounds. It is postulated that nitrogen was formed via a mechanism involving direct electron transfer from ammonia to the anode whereas the formation of nitrate involved oxygen transfer from water to an ammonia molecule.     

Electrochemical Ammonia Gas Sensing in Nonaqueous Systems: A Comparison of Propylene Carbonate with Room Temperature Ionic Liquids

First, the direct and indirect electrochemical oxidation of ammonia has been studied by cyclic voltammetry at glassy carbon electrodes in propylene carbonate. In the case of the indirect oxidation of ammonia, its analytical utility of indirect for ammonia sensing was examined in the range from 10 and 100 ppm by measuring the peak current of new wave resulting from reaction between ammonia and hydroquinone, as function of ammonia concentration, giving a sensitivity 1.29×10^?7 A ppm^?1 (r²=0.999) and limit‐of‐detection 5 ppm ammonia. Further, the direct oxidation of ammonia has been investigated in several room temperature ionic liquids (RTILs), namely 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([C₄mim] [BF₄]), 1‐butyl‐3‐methylimidazolium trifluoromethylsulfonate ([C₄mim] [OTf]), 1‐Ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim] [NTf₂]), 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim] [NTf₂]) and 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([C₄mim] [PF₆]) on a 10 μm diameter Pt microdisk electrode. In four of the RTILs studied, the cyclic voltammetric analysis suggests that ammonia is initially oxidized to nitrogen, N₂, and protons, which are transferred to an ammonia molecule, forming NH via the protonation of the anion(s) (A^?). However, in [C₄mim] [PF₆], the protonated anion was formed first, followed by NH . In all five RTILs, both HA and NH are reduced at the electrode surface, forming hydrogen gas, which is then oxidized. The analytical ability of this work has also been explored further, giving a limit‐of‐detection close to 50 ppm in [C₂mim] [NTf₂], [C₄mim] [OTf], [C₄mim] [BF₄], with a sensitivity of ca. 6×10^?7 A ppm^?1 (r²=0.999) for all three ionic liquids, showing that the limit of detection was ca. ten times larger than that in propylene carbonate since ammonia in propylene carbonate might be more soluble in comparison with RTILs when considering the higher viscosity of RTILs.     

Reduction mechanism of cobalt(II)-ammonia complexes on a dropping mercury electrode

The mechanism of the polarographic reduction of cobalt(II) complexes with ammonia at a dropping mercury electrode over a wide ligand concentration range was investigated. It was shown that the Co(II) aquo ion and the Co(NH₃)²⁺ and Co(NH₃²⁺₂ complexes participate in the electrode process. Transfer coefficients, α, for these species and the electrode reaction rates were evaluated. Stability constants of Co(II) complexes with ammonia in 0.5 M ammonium perchlorate were determined on the basis of the polarographic wave equation of totally irreversible reduction of complex specie.     

Fabrication of TiO2/Ti nanotube electrode and the photoelectrochemical behaviors in NaCl solutions

Titanium oxide nanotube electrodes were successfully prepared by anodic oxidation on pure Ti sheets in 0.5 wt.% NH₄F + 1 wt.% (NH₄)₂SO₄ + 90 wt.% glycerol mixed solutions. Nanotubes with diameter 40–60 nm and length 7.4 μm were observed by field emission scanning electron microscope. The electrochemical and photoelectrochemical characteristics of TiO₂ nanotube electrode were investigated using linear polarization and electrochemical impedance spectroscopy techniques. The open-circuit potential dropped markedly under irradiation and with the increase of Cl⁻ concentrations. A saturated photocurrent of approximately 1.3 mA cm⁻² was observed under 10-W low-mercury lamp irradiation in 0.1 M NaCl solution, which was much higher than film electrode. Meanwhile, the highest photocurrent in NaCl solution implied that the photogenerated holes preferred to combine with Cl⁻. Thus, a significant synergetic effect on active chlorine production was observed in photoelectrocatalytic processes. Furthermore, the generation efficiency for active chlorine was about two times that using TiO₂/Ti film electrode by sol–gel method. Finally, the effects of initial pH and Cl⁻ concentration were also discussed.     

Characterization of gold nanoparticles electrochemically deposited on amine-functioned mesoporous silica films and electrocatalytic oxidation of glucose

This study reports the preparation and characterization of gold nanoparticles deposited on amine-functioned hexagonal mesoporous silica (NH₂–HSM) films and the electrocatalytic oxidation of glucose. Gold nanoparticles are fabricated by electrochemically reducing chloroauric acid on the surface of NH₂–HSM film, using potential step technology. The gold nanoparticles deposited have an average diameter of 80 nm and show high electroactivity. Prussian blue film can form easily on them while cycling the potential between −0.2 and 0.6 V (vs saturated calomel electrode) in single ferricyanide solution. The gold nanoparticles loading NH₂–HSM-film-coated glassy carbon electrode (Au–NH₂–HSM/GCE) shows strong catalysis to the oxidation of glucose, and according to the cathodic oxidation peak at about 0.16 V, the catalytic current is about 2.5 μA mM⁻¹. Under optimized conditions, the peak current of the cathodic oxidation peak is linear to the concentration of glucose in the range of 0.2 to 70 mM. The detection limit is estimated to be 0.1 mM. In addition, some electrochemical parameters about glucose oxidation are estimated.     

Rotating-disc electrode studies of the anodic oxidation of iodide in concentrated iodine + iodide solutions

The anodic oxidation of iodide on platinum in concentrated iodine + iodide solutions has been investigated using a rotating disc electrode. The conventional limiting diffusion current, which is produced by the diffusion of iodide ions towards the electrode, was not observed due to the formation of an iodine film on the electrode. On the other hand, the steady-state anodic current after a current/time transient is the genuine limiting diffusion current in the anodic oxidation due to diffusion of iodine species from the electrode surface towards the bulk solution. Thus, the dissolution-diffusion control mechanism of the iodine film is confirmed. This is interesting as a typical example of an anodic process in a redox system governed by diffusion of the anodic product species from the electrode surface towards the bulk solution. When an iodine film is formed on the electrode, the maximum driving force of the iodine species is Δm_I2,max, which is defined as the extent of unsaturation of the iodine, and the limiting current of the anodic oxidation of iodide is always directly proportional to Δm_I2,max, regardless of the forms of iodine species in the solution, which may be I₂, I⁻₃, i₅⁻, etc. δm_I2,max is clearly determined by the solution composition and temperature, and it is different in definition and value from the usual degree of unsaturation of iodine.     

Preparation of an Electrode Modified with an Electropolymerized Film as a Mediator of NADH Oxidation and Its Application in an Ethanol/O2 Biofuel Cell

This paper reports a novel mediator for the oxidation of β‐nicotinamide adenine dinucleotide (NAD⁺/NADH), an electropolymeric film (pAPRu) of [Ru(NH₂‐phen)₃]²⁺. A pAPRu‐modified electrode was prepared via electropolymerization and exhibited catalytic activity toward the electrochemical oxidation of NADH due to the imine moieties of pAPRu. The electrochemical oxidation of ethanol was observed using an alcohol dehydrogenase (ADH)‐immobilized electrode. A compartmentless ethanol/O₂ biofuel cell composed of an ADH anode and a bilirubin oxidase cathode was constructed. The maximum current density and the maximum power density of the biofuel cell were 190 µA cm^?2 and 31 µW cm^?2 (at 0.29 V), respectively.     

Highly Efficient Electrochemical Nitrate and Nitrogen Reduction to Ammonia under Ambient Conditions on Electrodeposited Cu-Nanosphere Electrode

The electrochemical reduction reaction of nitrogenous species such as NO₃⁻ (NO₃RR) and N₂ (NRR) is a promising strategy for producing ammonia under ambient conditions. However, low activity and poor selectivity of both NO₃RR and NRR remain the biggest problem of all current electrocatalysts. In this work, we fabricated Cu-nanosphere film with a high surface area and dominant with a Cu(200) facet by simple electrodeposition method. The Cu-nanosphere film exhibits high electrocatalytic activity for NO₃RR and NRR to ammonia under ambient conditions. In the nitrate environment, the Cu-nanosphere electrode reduced NO₃⁻ to yield NH₃ at a rate of 5.2 mg/h cm², with a Faradaic efficiency of 85 % at −1.3 V. In the N₂-saturated environment, the Cu-nanosphere electrode reduced N₂ to yield NH₃ with the highest yield rate of 16.2 μg/h cm² at −0.5 V, and the highest NH₃ Faradaic efficiency of 41.6 % at −0.4 V. Furthermore, the Cu-nanosphere exhibits excellent stability with the NH₃ yield rate, and the Faradaic efficiency remains stable after 10 consecutive cycles. Such high levels of NH₃ yield, selectivity, and stability at low applied potential are among the best values currently reported in the literature.     

NH_4~+对甲酸在炭载Pd催化剂上氧化性能的影响

为了解HClO4、NH4ClO4和NaClO4电解液对炭载Pd(Pd/C)催化剂电极对甲酸氧化的电催化性能的影响,在用X射线衍射(XRD)谱、能量色散谱(EDS)和透射电子显微镜(TEM)对Pd/C催化剂进行表征的基础上,采用电化学方法测量了Pd/C催化剂在不同电解液中对甲酸氧化的电催化性能.发现在不同电解液中,Pd/C催化剂对甲酸氧化的电催化活性和稳定性按NH4ClO4NaClO4HClO4的次序降低.由于甲酸的存在,不同电解液的pH相差较小,因此,电解液的pH影响较小,而阳离子的影响较大.在NaClO4电解液中的性能优于在HClO4电解液中的性能是pH的影响.在NH4ClO4电解液中的性能优于在NaClO4电解液中是由于NH4+能降低CO在Pd/C催化剂电极上的吸附强度和吸附量,这一发现对提高直接甲酸燃料电池(DFAFC)的性能很有意义.     

Kinetics and Mechanism of Electroreduction of Ammonia and Hydroxyammonia Complexes of Zinc(II) on a Dropping Mercury Electrode

It is found that the equilibrium potential of the Zn(Hg)/Zn(II) system depends on the concentration of ammonia molecules and solution pH. The dependence conforms to the literature data on the stability constants for ammonia and hydroxyammonia complexes of zinc. Their reduction on a dropping mercury electrode in solutions of pH 9.2–12 and [NH₃] = 0.05–2 M yields one irreversible cathodic wave with a diffusion limiting current. In dilute supporting electrolytes, the plateau of the latter is preceded by a maximum due to accumulation of insoluble reduction products on the surface of the mercury drop. The pH and [NH₃] dependences of the half-wave potential of waves that are undistorted by a maximum are analyzed with allowance made for a change in the composition of zinc(II) complexes in the bulk solution. According to the analysis, the slow two-electron electrochemical stage involves complexes Zn(NH₃)₂ ²⁺ that form from complexes present in solution in preceding reversible chemical reactions. The effect the supporting-electrolyte concentration has on the electroreduction rate of zinc(II) complexes and the mechanism of the electrochemical stage is discussed.     

Determination of hemoglobin at a novel NH2/ITO ion implantation modified electrode

A novel electrode was prepared by implanting NH₂ ⁺ into an ITO film (NH₂/ITO). Gold nanoparticles were deposited on the surface of NH₂/ITO electrode. The NH₂/ITO and Au/NH₂/ITO electrodes were used to determine hemoglobin (Hb) immobilized on the electrodes surfaces. The relationship of the reductive peak current value of Hb among different electrodes was: Hb/ITO:Hb/Au/ITO:Hb/NH₂/ITO:Hb/Au/NH₂/ITO=1:1.5:2:4. The linkage between the –NH₂ implanted into ITO film and the –COOH of Hb was recognized to be the reason for the increase of active Hb coverage on NH₂/ITO electrode compared with the ITO electrode. Increase of active Hb coverage on Au/NH₂/ITO compared with Au/ITO was attributed to the different amount of gold nanoparticles deposited. The determination of Hb at an Au/NH₂/ITO electrode was optimized. Calibration curve was obtained over the range of 1.0 × 10⁻⁸ – 1.0 × 10⁻⁶ mol · L⁻¹ with a detection limit of 1.0 × 10⁻⁸ mol · L⁻¹. Results showed that the novel NH₂/ITO and Au/NH₂/ITO electrodes exhibited good stability, reproducibility besides better electrochemical performance. Correspondence: Jing Bo Hu, Department of Chemistry, Beijing Normal University, Beijing 100875, China     

Catalytic Effect on Silver Electrodeposition of Gold Deposited on Carbon Electrodes

A new methodology, based on silver electrocatalytic deposition and designed to quantify gold deposited onto carbon paste electrode (CPE) and glassy carbon electrode (GCE), has been developed in this work. Silver (prepared in 1.0 M NH₃) electrodeposition at ?0.13 V occurs only when gold is previously deposited at an adequate potential on the electrode surface for a fixed period of time. When a CPE is used as working electrode, an adequate oxidation of gold is necessary. This oxidation is carried out in both 0.1 M NaOH and 0.1 M H₂SO₄ at oxidation potentials. When a GCE is used as working electrode, the oxidation steps are not necessary. Moreover, a cleaning step in KCN, which removes gold from electrode surface, is included. To obtain reproducibility in the analytical signal, the surface of the electrodes must be suitably pretreated; this electrodic pretreatment depends on the kind of electrode used as working electrode. Low detection limits (5.0×10^?10 M) for short gold deposition times (10 min for CPE and 5 min for GCE) were achieved with this novel methodology. Finally, sodium aurothiomalate can be quantified using silver electrocatalytic deposition and GCE as working electrode. Good linear relationship between silver anodic stripping peak and aurothiomalate concentration was found from 5.0×10^?10 M to 1.0×10^?8 M.     

Effect of inorganic anions on the titanium dioxide-based photocatalytic oxidation of aqueous ammonia and nitrite

In this study, we investigated the effects of four inorganic anions (Cl⁻, SO₄²⁻, H₂PO₄⁻/HPO₄²⁻, and HCO₃⁻/CO₃²⁻) on titanium dioxide (TiO₂)-based photocatalytic oxidation of aqueous ammonia (NH₄⁺/NH₃) at pH ∼ 9 and ∼10 and nitrite (NO₂⁻) over the pH range of 4–11. The initial rates of NH₄⁺/NH₃ and NO₂⁻ photocatalytic oxidation are dependent on both the pH and the anion species. Our results indicate that, except for CO₃²⁻, which decreased the homogeneous oxidation rate of NH₄⁺/NH₃ by UV-illuminated hydrogen peroxide, OH scavenging by anions and/or direct oxidation of NH₄⁺/NH₃ and NO₂⁻ by anion radicals did not affect rates of TiO₂ photocatalytic oxidation. While HPO₄²⁻ enhanced NH₄⁺/NH₃ photocatalytic oxidation at pH ∼ 9 and ∼10, H₂PO₄⁻/HPO₄²⁻ inhibited NO₂⁻ oxidation at low to neutral pH values. The presence of Cl⁻, SO₄²⁻, and HCO₃⁻ had no effect on NH₄⁺/NH₃ and NO₂⁻ photocatalytic oxidation at pH ∼ 9 and ∼10, whereas CO₃²⁻ slowed NH₄⁺/NH₃ but not NO₂⁻ photocatalytic oxidation at pH ∼ 11. Photocatalytic oxidation of NH₄⁺/NH₃ to NO₂⁻ is the rate-limiting step in the complete oxidation of NH₄⁺/NH₃ to NO₃⁻ in the presence of common wastewater anions. Therefore, in photocatalytic oxidation treatment, we should choose conditions such as alkaline pH that will maximize the NH₄⁺/NH₃ oxidation rate.     

Electrocatalysis of N2 to NH3 by HKUST‐1 with High NH3 Yield

Electrolytic ammonia synthesis from nitrogen at ambient conditions is appearing as a promising alternative to the Haber‐Bosch process which is consuming high energy and emitting CO₂. Here, a typical MOF material, HKUST‐1 (Cu?BTC, BTC=benzene‐1,3,5‐tricarboxylate), was selected as an electrocatalyst for the reaction of converting N₂ to NH₃ under ambient conditions. At ?0.75 V vs. reversible hydrogen electrode, it achieves excellent catalytic performance in the electrochemical synthesis of ammonia with high NH₃ yield (46.63 μg h^?1 mg^?1 _cat. or 4.66 μg h^?1 cm^?2) and good Faraday efficiency (2.45%). It is indicated that the good performance of the HKUST‐1 catalyst may originate from the formation of Cu(I). In addition, the catalyst also has good selectivity for N₂ to NH₃.     

Nanostructured Platinum‐iridium Alloy Microelectrode for Ammonia Determination

Nanostructured platinum‐iridium alloy microelectrode with high surface area was successfully prepared by applying successive potential cycles to a conventional PtIr microdisc in ionic liquid electrolyte containing ZnCl₂ at elevated temperature. Scanning‐electron microscope studies show that a very thin nanostructured film was created on the electrode upon 20 potential cycles between −2.0 and 0.75 V versus a Ag pseudo‐reference electrode. The film nanostructures are characteristic of regular hill‐like nano‐spacings separated by valley‐like nano‐cracks, and a roughness factor of approximately 40. The nanostructured electrode is highly active towards electrochemical oxidation of ammonia, and generates a linear relation between voltammetric peak currents (or chronoamperometric currents), and logarithm of ammonia concentration in a range of approximately 1 ppm to 10000 ppm. It has been proposed that the Temkin adsorption of ammonia from the bulk solution onto the electrode surfaces was involved in its electrochemical oxidation and could be responsible for the linear current‐logarithmic concentration relation.     

Bioelectrochemical Haber–Bosch Process: An Ammonia‐Producing H2/N2 Fuel Cell

Nitrogenases are the only enzymes known to reduce molecular nitrogen (N₂) to ammonia (NH₃). By using methyl viologen (N ,N ′‐dimethyl‐4,4′‐bipyridinium) to shuttle electrons to nitrogenase, N₂ reduction to NH₃ can be mediated at an electrode surface. The coupling of this nitrogenase cathode with a bioanode that utilizes the enzyme hydrogenase to oxidize molecular hydrogen (H₂) results in an enzymatic fuel cell (EFC) that is able to produce NH₃ from H₂ and N₂ while simultaneously producing an electrical current. To demonstrate this, a charge of 60 mC was passed across H₂ /N₂ EFCs, which resulted in the formation of 286 nmol NH₃ mg⁻¹ MoFe protein, corresponding to a Faradaic efficiency of 26.4 %.