Ammonia Nitrite Research Articles

Reduction over zeolite-based catalysts of nitrogen oxides in emissions containing excess oxygen: Unraveling the reaction mechanism

Reduction of NO x to N 2 over zeolite-based catalysts is a multi-step reaction for which transition metal (TM) ions are helpful but not indispensable. The present state of understanding the mechanism is reviewed for the catalyzed NO x reduction with ammonia or alkanes; reduction with acetaldehyde is also sketched, but will be described in detail in a separate paper. A decisive step is the interaction of two reaction intermediates containing N atoms in different oxidation states. A favorable reaction path uses an organic molecule to reduce part of the NO x to ammonia, this product then reacts with NO+NO 2 to give N 2. Isotopic labeling shows that each N 2 molecule has one N atom from the NH 3 intermediate, the other from NO x . N 2 is formed at room temperature, if an 1:1 mixture of NO and NO 2 is in contact with an Fe/MFI catalyst covered with NH 3. During NO x reduction with CH 4 over Pd/zeolite catalysts, H/D exchange of methane has been observed, indicating that methane is dissociatively adsorbed on Pd 0 clusters that are present in the steady-state of surface oxidation and reduction steps. BaNaY catalysts, containing negligible impurities of TM elements, catalyze NO x reduction with acetaldehyde at 200 °C. In the prevailing mechanism acetaldehyde is transformed via acetate ions and nitromethane to isocyanic acid, which is hydrolyzed to NH 3. Again, N 2 is ultimately produced from NH 3 and N 2O 3 via ammonium nitrite.

The unprecedented nos gene cluster of Wolinella succinogenes encodes a novel respiratory electron transfer pathway to cytochrome c nitrous oxide reductase

The ε-proteobacterium Wolinella succinogenes grows anaerobically by respiratory nitrite ammonification but not by denitrification. Nevertheless, it is capable of N2O reduction to N2. Recently, the genome sequence of W. succinogenes revealed a nos gene cluster with intriguing features encoding a new type of N2O reductase. The predicted enzyme is similar to other N2O reductases exhibiting conservation of all residues ligating the two multinuclear copper centers but carries an unprecedented C-terminal monoheme cytochrome c domain. Notably, the N2O reductase pre-protein is synthesized with a Sec-dependent signal peptide, rather than the usually observed twin-arginine signal sequence, implying that the copper and heme cofactors are both incorporated in the periplasm. The nos gene cluster further consists of four adjacent open reading frames which are predicted to encode two monoheme c-type cytochromes as well as homologs of NapG and NapH. The latter proteins are thought to function in quinol oxidation coupled to cytochrome c reduction in electron transport to periplasmic nitrate reductase. While the accessory genes nosD, -F, -Y and -L are present in W. succinogenes, homologs of nosR and nosX are absent from the genome. We hypothesize that the nos gene cluster of W. succinogenes encodes a complete electron transport chain catalyzing N2O reduction by menaquinol, a pathway which might also be relevant to other bacteria.

Nanomaterials for Heterogeneous Combustion

AbstractNanophase materials and nanocomposites, characterized by an ultra fine grain size (less than 100 nm) have attracted wide spread interest in recent years by virtue of their unusual mechanical, electrical, optical, magnetic, and energetic properties. Studies have shown that the thermal behavior of nano‐scaled materials is quite different from micron‐sized powders. Nanosized metallic and explosive powders have been used as solid propellant and explosive mixtures to increase efficiency. At the same time recent studies reveal that the presence of nanosized metals in propellants does not necessary translate into an increased burning rate and burning temperature. The reasons of this effect are far from being clear. This paper presents a new approach to the production of nanocomposites of some energetic materials – ammonium nitrite, cyclotrimethylene trinitramine (RDX), and aluminum – by the vacuum co‐deposition technique. The thermal behavior of the synthesized nanopowder and nanocomposites is investigated. A substantial difference in burning rate of RDX nanopowder has been found in comparison to micron‐sized material. Experimental results allow investigating the effects of nanosized materials on the combustion characteristics.

Catalytic generation of nitric oxide from nitrite at the interface of polymeric films doped with lipophilic Cu(II)-complex: a potential route to the preparation of thromboresistant coatings

A novel approach potentially useful for the development of more thromboresistant polymeric materials is examined. The method is based on the catalytic generation of nitric oxide (NO) via Cu(I) mediated reduction of nitrite ions. Preliminary solution phase studies demonstrate that ascorbate or thiolate anions can generate Cu(I) from Cu(II) with subsequent catalytic conversion of any nitrite ions present to NO by the unstable Cu(I) species. Incorporation of this same chemistry within a hydrophobic polymeric material requires immobilizing Cu(II) ions into a polymeric phase via use of a lipophilic Cu(II) chelating ligand (dibenzo [e,k]-2,3,8,9-tetraphenyl-1,4,7,10-tetraaza-cyclododeca-1,3,7,9-tetraene (DTTCT)). It is shown that this complex can be reduced to its Cu(I) form by appropriate reducing equivalents present in the bathing solution. The resulting Cu(I) complex can then reduce nitrite to NO with the NO generation occurring at the polymer/solution interface at physiological pH. Data from chemiluminescence experiments indicate that the flux of NO at the polymer surface is comparable to that of endothelial cells (⩾1×10 −10 mol/cm 2 min) when 0.5 m m nitrite/1 m m ascorbate are present in the bathing solution. Potentially more useful NO generation can be achieved by doping the polymer film with the Cu(II) complex along with a lipophilic quaternary ammonium nitrite salt. In this case reducing equivalents within the aqueous phase enable the nitrite derived from the polymer to be converted into NO by the Cu(II/I) ligand complex. Films of this type are shown to generate NO for at least 6 h in PBS buffer with fluxes on the order of 1.5×10 −10 mol/cm 2 min. Physiologically relevant levels of NO release are also shown to exist at the polymer interface when films are soaked in fresh plasma as well as undiluted whole blood, indicating that endogenous reducing equivalents present in blood can efficiently reduce the Cu(II)-ligand within the polymer film. The prospects of using these new NO releasing films to devise more biocompatible polymeric coatings for biomedical applications are discussed.

Reclamation of Treated Domestic Wastewater Using Biological Membrane Assisted Carbon Filtration (Biomac)

Membrane processes are increasingly used as an advanced treatment technique for the reclamation of treated domestic wastewater. Despite their inherent advantages, fouling remains an operational problem, while the removal of dissolved organic components such as volatile organic compounds is negligible. In the present work, the addition of a partially non-submerged biological granular activated carbon filtration to a microfiltration lab-scale reactor was investigated. It was observed that the reactor could be operated under stable flux conditions although regular hydraulic backwashing was necessary. Preferential attached growth of nitrifiers on the activated carbon particles allowed for a complete and very stable nitrification, with permeate total ammonium nitrogen and nitrite levels below 0.2 mg l−1 regardless of influent concentrations. Chemical oxygen demand of the permeate averaged 5.26 mg O2 l−1 which is below the Environmental Protection Agency guideline for wastewater reuse. Using an electronic nose, elimination of volatile compounds was assessed. The combined process resulted in complete odour removal, with the permeate odour levels equalling the reference samples (tap water), even during periods of increased reactor load (shock load experiment). A 4.2 log10CFU and 3.7 log10CFU removal were observed for total coliforms and E. coli, respectively.

Enzymology and bioenergetics of respiratory nitrite ammonification.

Nitrite is widely used by bacteria as an electron acceptor under anaerobic conditions. In respiratory nitrite ammonification an electrochemical proton potential across the membrane is generated by electron transport from a non-fermentable substrate like formate or H(2) to nitrite. The corresponding electron transport chain minimally comprises formate dehydrogenase or hydrogenase, a respiratory quinone and cytochrome c nitrite reductase. The catalytic subunit of the latter enzyme (NrfA) catalyzes nitrite reduction to ammonia without liberating intermediate products. This review focuses on recent progress that has been made in understanding the enzymology and bioenergetics of respiratory nitrite ammonification. High-resolution structures of NrfA proteins from different bacteria have been determined, and many nrf operons sequenced, leading to the prediction of electron transfer pathways from the quinone pool to NrfA. Furthermore, the coupled electron transport chain from formate to nitrite of Wolinella succinogenes has been reconstituted by incorporating the purified enzymes into liposomes. The NrfH protein of W. succinogenes, a tetraheme c-type cytochrome of the NapC/NirT family, forms a stable complex with NrfA in the membrane and serves in passing electrons from menaquinol to NrfA. Proteins similar to NrfH are predicted by open reading frames of several bacterial nrf gene clusters. In gamma-proteobacteria, however, NrfH is thought to be replaced by the nrfBCD gene products. The active site heme c group of NrfA proteins from different bacteria is covalently bound via the cysteine residues of a unique CXXCK motif. The lysine residue of this motif serves as an axial ligand to the heme iron thus replacing the conventional histidine residue. The attachment of the lysine-ligated heme group requires specialized proteins in W. succinogenes and Escherichia coli that are encoded by accessory nrf genes. The proteins predicted by these genes are unrelated in the two bacteria but similar to proteins of the respective conventional cytochrome c biogenesis systems.

Monitoreo de parámetros fisico-químicos, en la cuenca alta del río Bogotá

La Universidad Militar "Nueva Granada" en su constante interés por el desarrollo de nuevas técnicas que permitan la preservación del medio ambiente, inició en el año 2000 la investigación "IDENTIFICACIÓN Y PRUEBADEBIOINDICADORESy RECUPERADORES PARA LA DESCONTAMINACIÓN DEL RÍO BOGOTÁ". En la primera fase de este estudio se hace un diagnóstico de la situación actual de la cuenca alta del Río Bogotá, a través de los proyectos de grado presentados por los estudiantes de la Universidad y dirigidos por un grupo de profesores. Se realizó una investigación como Tesis de Grado para optar al título de Ingeniero Civil,donde se analiza el comportamiento de los pará-. metros fisico-químicos en la cuenca alta del Río Bogotá; estos parámetros son de gran importancia ya que establecen las modificaciones que sufre una corriente superficial, como consecuencia de los diferentes usos del agua (causados por actividades industriales, domésticas, recreativas y de transporte). Eltítulo del trabajo de grado es "MONITOREODE PARÁMETROSFISICO-QUÍMICOS,EN LA CUENCAALTA DEL RÍO BOGOTÁ". Los aspectos fisicos que se tienen en cuenta en este trabajo son: turbiedad, temperatura y conductividad, y como aspectos químicos oxígeno disuelto, demanda bioquímica de oxígeno (DBO), demanda química de oxígeno (DQO), pH, alcalinidad total, acidez, dureza, cIoruros, amonio, nitritos, nitratos, fosfatos altos y bajos, sulfatos, cromo, cobre y cloro total.

Modification of heme c binding motifs in the small subunit (NrfH) of the Wolinella succinogenes cytochrome c nitrite reductase complex

The two multiheme c-type cytochromes NrfH and NrfA form a membrane-bound complex that catalyzes menaquinol oxidation by nitrite during respiratory nitrite ammonification of Wolinella succinogenes. Each cysteine residue of the four NrfH heme c binding motifs was individually replaced by serine. Of the resulting eight W. succinogenes mutants, only one is able to grow by nitrite respiration although its electron transport activity from formate to nitrite is decreased. NrfH from this mutant was shown by matrix-assisted laser desorption/ionization mass spectrometry to carry four covalently bound heme groups like wild-type NrfH indicating that the cytochrome c biogenesis system II organism W. succinogenes is able to attach heme to an SXXCH motif.

Effect of anaerobic pretreatment on environmental and physicochemical characteristics of duckweed based stabilization ponds

Duckweed based stabilization ponds, an alternative for wastewater treatment, are attracting a growing interest from researchers because they are basically a low cost technology, easy to built and operate, and produce tertiary quality effluents. Besides, this technology offers the possibility of resource recovery by producing high quality duckweed protein, which can be of further use. Since the technology is rather new, there are many aspects to be studied before its full-scale implementation. It is necessary to gain sound knowledge of the basic principles of the complex processes occurring in the system, as well as of the practical aspects of design and operation. The presence of a layer of duckweed on the surface is expected to produce different environmental and physicochemical conditions in the water from those found in conventional stabilization ponds. These environmental and physicochemical conditions affect both plant growth and biological treatment processes in the system, therefore it is important to determine their behavior in a duckweed system and how they can be affected by an anaerobic pretreatment. Continuous flow pilot plants composed of seven ponds in series were operated with artificial substrate under two different conditions: with anaerobic pretreatment and without anaerobic pretreatment. The flow was kept constant during the operation. Conditions such as pH, temperature, dissolved oxygen, alkalinity, conductivity, chemical oxygen demand, biochemical oxygen demand, total and ammonium nitrogen, nitrites and nitrates, and phosphorus were evaluated in the system under steady state conditions. The main conclusions from the study include the following: pH, temperature and oxygen profiles are more stable in duckweed ponds than in conventional stabilization ponds; anaerobic pretreatment has a significant effect on the oxygen concentration in the system and on the organic matter removal but not on the nutrient removal.

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

The mechanism of selective catalytic reduction (SCR) of NOx with NH3 over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH3 then to flowing NO + O2 or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH4NO2. In situ FTIR results in flowing NO + NH3 + O2 at different temperatures show that NH3 is strongly adsorbed and reacts with impinging NOx. The intensity of the NH4NO2 bands initially increases with temperature, but passes through a maximum at 120 °C because the nitrite decomposes to N2 + H2O. The mechanistic model rationalizes that the consumption ratio of NO and NH3 is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H_2O enhances the rate because it is needed to form NH4NO2. At low temperature, when adsorbed H2O is abundant it lowers the rate because it competes with NOx for adsorption sites.

Nitrite in rain and dew in Santiago city, Chile. Its possible impact on the early morning start of the photochemical smog

Cations (pH, potassium, sodium, calcium, magnesium, and ammonium) and anions (sulfate, nitrate, nitrite, and chloride) concentrations were measured in Santiago city rain and dew waters collected during the 1995 to 1999. Concentrations measured in dews are considerably higher than those measured in rains. The high ionic concentration present in dew waters could contribute to their corrosion potential. Natural dust makes an important contribution to the ions present in dews, but the presence of rather high sulfate concentrations (up to 900 μeq/l) indicate a significant contribution of anthropogenic sources. A peculiar characteristic of dew waters is the relatively high nitrite concentrations (up to 180 μeq/l). This nitrite can be resuspended into the boundary layer after dew water evaporation, possibly due to the relatively high volatily of ammonium nitrite. This upward flux could constitute an important source of hydroxyl radicals in the early morning, contributing so to the initial steps of the observed photochemical smog.

Reduction of NOx with Ammonia over Fe/MFI: Reaction Mechanism Based on Isotopic Labeling

The selective catalytic reduction of nitrogen oxides has been studied in the presence of O2 over Fe/MFI catalysts with Fe/Al∼1, prepared by sublimation of FeCl3 vapor onto HMFI. Reduction of NOx is much faster with NH3 than with isobutane. A 100% NOx reduction to N2 is observed for an NH3/NO ratio above unity and 90% for a 1/1 dosage. The catalyst is very active and stable in the presence of water vapor. Isotopic labeling with 15NO shows that the preferred path for the reduction of NOx with NH3 is via NH4NO2. This nitrite decomposes to N2+2H2O. A 100% pure 15N14N product is obtained by circulating a 1/1 mixture of 15NO and 15NO2 over a catalyst covered with adsorbed 14NH3. In contrast, 14N2 is formed by oxidation of 14NH3 with O2 or N2Ox with x>3. The high efficiency of this catalyst is due to the fact that ammonia swiftly intercepts the oxidation of NO when the value of x=3 is reached. At high temperature when little adsorbed water is present, water vapor promotes activity because it is needed for the formation of ammonium nitrite. At low temperature it acts kinetically as a mere competitor for the active sites.

High-resolution structures of the oxidized and reduced states of cytochrome c554 from Nitrosomonas europaea.

Cytochrome c554 (cyt c554) is a tetra-heme cytochrome involved in the oxidation of NH3 by Nitrosomonas europaea. The X-ray crystal structures of both the oxidized and dithionite-reduced states of cyt c554 in a new, rhombohedral crystal form have been solved by molecular replacement, at 1.6 A and 1.8 A resolution, respectively. Upon reduction, the conformation of the polypeptide chain changes between residues 175 and 179, which are adjacent to hemes III and IV. Cyt c554 displays conserved heme-packing motifs that are present in other heme-containing proteins. Comparisons to hydroxylamine oxidoreductase, the electron donor to cyt c554, and cytochrome c nitrite reductase, an enzyme involved in nitrite ammonification, reveal substantial structural similarity in the polypeptide chain surrounding the heme core environment. The structural determinants of these heme-packing motifs extend to the buried water molecules that hydrogen bond to the histidine ligands to the heme iron. In the original structure determination of a tetragonal crystal form, a cis peptide bond between His129 and Phe130 was identified that appeared to be stabilized by crystal contacts. In the rhombohedral crystal form used in the present high-resolution structure determination, this peptide bond adopts the trans conformation, but with disallowed angles of phi and psi.

Temperature-Programmed Desorption/Surface Reaction (TPD/TPSR) Study of Fe-Exchanged ZSM-5 for Selective Catalytic Reduction of Nitric Oxide by Ammonia

Temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR) were employed to study Fe-exchanged ZSM-5 for selective catalytic reduction (SCR) of NO with ammonia. TPD profiles of ammonia and NOx showed that both NOx and NH3 adsorbed on Fe–ZSM-5. Physisorbed NOx and NH3 were not affected significantly by iron content. With increasing iron content, chemisorbed NOx (mainly NO2 bonded to iron sites) increased while chemisorbed NH3 (mainly NH+4 on Brønsted acid sites) decreased due to substitution of protons by iron ions. The TPSR results indicated that ammonia adsorbed species were quite active in reacting with NO, O2, NO+O2, and NO2 (producing H2O, N2 and/or N2O), following the reactivity rank order NO2∼NO+O2>NO>O2. NOx adsorbed species were also reactive to NH3 at high temperatures. With NH3 and NOx coadsorbed on Fe–ZSM-5, TPSR with gaseous He, NO, and NO2 showed two kinds of reactions for N2 formation. One reaction near 55°C originated from decomposition of ammonium nitrite, which was not affected by Fe3+ content. The other reaction at higher temperatures (170–245°C) was due to an adsorbed complex, probably [NH+4]2NO2, reacting with NO or NO2. A possible reaction path was proposed for NO reduction involving NO2 and [NH+4]2NO2 as intermediates. Since the reactivity of [NH+4]2NO2 to NO (producing only N2 at 170°C) was higher than that to NO2 (producing both N2 and N2O at 200°C), it is reasonable to deduce that [NH+4]2NO2 prefers to react with NO and not NO2, both of which are present in the SCR reaction. This may be the reason for N2 being the only product for SCR on Fe–ZSM-5.

Studies on the thermal behavior of ammonium permanganate

The thermal decomposition of ammonium permanganate was studied by thermogravimetry-mass spectrometry (TG-MS). A Perkin Elmer TGS-2 thermobalance and a Hiden’s HAL 3F/PIC mass spectrometer equipped with a fast ion counter was used. Products of thermal decomposition of ammonium permanganate are oxygen, manganese oxides, water and ammonia. There is no gas phase oxidation of ammonia by oxygen. The possible intermediates of low-temperature decomposition reaction are ammonium nitrite or nitrate which transform into N 2, N 2O, or H 2O. The oxidation of adsorbed ammonia by manganese oxides via the superoxide active centers takes place on the surface of manganese oxide.

Kinetic and mechanistic analysis of the reactions in the aqueous system pentacyanoferrate(II)–ammonia–nitrite

The pentacyanoferrate(II)–ammonia–nitrite system was subjected to a kinetic and mechanistic study as a function of NH3 and NO2– concentration, pH, temperature and pressure. A detailed study was performed on the formation of [FeII(CN)5(NO)]2– in the system consisting of NO2– and [FeII(CN)5L]3– (L = H2O or NH3). Two equilibria were taken into consideration, viz. the pH dependent aquation of the ammine complex, and the transformation of the nitro into the nitrosyl complex. Based on the reported activation parameters, the substitution of water or ammonia in [FeII(CN)5L]3– (L = H2O or NH3) by nitrite follows a limiting dissociative mechanism. The reaction of [FeII(CN)5(NO)]2– with ammonia leading to the formation of dinitrogen was investigated in a more qualitative way, and found to depend strongly on pH with a maximum nitrogen yield at a pH of ca. 10.5. The trend was accounted for on the basis that the process involves nucleophilic attack of NH3 on the N (NO) atom in [FeII(CN)5(NO)]2–. This reaction was found to be the rate-limiting step in the catalytic cycle occurring in this system and leading to the overall reduction of NOx to dinitrogen.

Synthesis, Properties and Crystal Structure of Bis(tetra(n-butyl)ammonium) Bisnitrophthalocyaninatoruthenate(II)

Tetra(n-butyl)ammonium nitrite reacts with ‘dichlorophthalocyaninatoruthenium(III) acid’ to yield bis(tetra(n-butyl)ammonium) bisnitrophthalocyaninatoruthenate(II). It crystallizes monoclinic with crystal data: a = 15.114(4) Å, b = 22.34(3) Å, c = 18.206 (11) Å, β = 90.88(5)°; space group P1 21/n 1, Z = 4, and its X-ray crystal structure has been determined. Ru is located in the centre of the slightly distorted phthalocyaninate dianion, and is coordinated to two nitrite ions through their nitrogen (N) atoms in a staggered trans-configuration. The average Ru - N p ( Np pyrrolic N atoms) and Ru -N distance is 1.978(6) and 2.068(5) Å, respectively. The electrochemical and spectroscopic properties (IR, resonance Raman, UV/vis) of the compound are in full agreement with its molecular structure.

Solvent and deuterium isotope effects on the decomposition of ammonium nitrite solutions

The rate of decomposition of NH4NO2 solutions, at pH 5–7, equals k[NH3] [HNO2]2 or k[NH 4 + ] [NO 2 − ][HNO2]. A plausible mechanism involves a ratedetermining attack of N2O3, derived from HNO2, on NH3. ΔH++ and ΔS++ are 82 kJ-mol−1 and −27 J-mol−1-K−1, respectively. On partially replacing the solvent water by methanol or ethanol, the change δΔG++, coupled with the calculated standard Gibbs energy of transfer of the reactants from water to the mixed solvent indicated that, in the latter, there is a greater destabilization of the transition state compared to that of the reactants. This can be explained by assuming two hydrogen bonds from the same water molecule to the transition state and hence a loss of hydrogen bond energy in the mixed solvent compared to the aqueous solution. The rate constant for the reaction of ND4NO2 in D2O compared to the reaction of NH4NO2 in water, gave a composite isotope effect involving two acid-base equilibria, suggested in the proposed mechanism; in addition to primary isotope effects in the equilibrium: 2 HNO2ΦN2O3+H2O.

Effects of cosolvents cetyltrimethylammonium bromide on the thresholds for nucleation of nitrogen in aqueous solutions

The decomposition of ammonium nitrite in water creates a supersaturated solution of nitrogen. The same process occurs in water-organic solvent mixtures. Acetone, dioxane, dimethylsulfoxide (DMSO) and dimethylformamide (DMF) are the cosolvents used in this study. The limits of supersaturation of nitrogen (C SL /mol L−1) were determined in all of these solvent mixtures by releasing the dissolved gas sonicationally and measuring the volume of released gas. C SL was generally increased in the presence of cosolvents. The effectiveness sequence of organic solvents was found to be as DMF<DMSO<Dioxanet≅Acetone. Transportation period of small bubbles formed during sonication is changed by compositions of solvent mixtures. These periods may depend on the viscosity of the solution. Effects of the presence of cetyltrimethylammonium bromide were also studied. It was concluded that there may be a relation between the strength of the watercosolvent H-bonds and C SL and all of the measured quantities of this study were generally affected by micelle formation.

The UV-enhanced decomposition of aqueous ammonium nitrite

As the decomposition of NH 4NO 2 to N 2 and H 2O is a potential pathway for the loss of N 2 from the aquatic environment, the above reaction has been studied by volumetric gas analysis. Experiments performed with UV irradiation were approximately four times faster than the rate of the dark reaction. However, the increase in the rate of decomposition with light intensity was not strictly linear. The rate of gas evolution increased with [NO 2 −] and, to a lesser extent, with [NH 4 +]. An optimum rate was observed at pH 8–9. A proposed mechanism involves the formation of NO and NO 2 from dark reactions of the excited NO 2 − ions. N 2O 3, formed from NO and NO 2, is thought to react with NH 3, derived from NH 4 +, to produce an intermediate which dissociates to the products in fast steps. The decline in the rate of gas evolution at pH values greater than 9 is ascribed to the hydrolysis of N 2O 3, giving NO 2 − ions. An estimated activation energy was 26 kJ mol −1. In the presence of anatase or colloidal Fe(OH) 3, the rate of gas evolution dropped as the concentration of the semiconductor increased. Quantum yields, estimated using uranyl oxalate actinometry, were φ 240–248=0.058, φ 254=0.041, φ 257–300=0.047 and φ 300–400=0.016.