Order In NO Research Articles

Upon further analysis, neither cytochrome c554 from Nitrosomonas europaea nor its F156A variant display NO reductase activity, though both proteins bind nitric oxide reversibly.

A re-investigation of the interaction with NO of the small tetraheme protein cytochrome c554 (C554) from Nitrosomonas europaea has shown that the 5-coordinate heme II of the two- or four-electron-reduced protein will nitrosylate reversibly. The process is first order in C554, first order in NO, and second-order overall. The rate constant for NO binding to the heme is 3000 ± 140M-1s-1, while that for dissociation is 0.034 ± 0.009s-1; the degree of protein reduction does not appear to significantly influence the nitrosylation rate. In contrast to a previous report (Upadhyay AK, et al. J Am Chem Soc 128:4330, 2006), this study found no evidence of C554-catalyzed NO reduction, either with [Formula: see text] or with [Formula: see text] Some sub-stoichiometric oxidation of the lowest potential heme IV was detected when [Formula: see text] was exposed to an excess of NO, but this is believed to arise from partial intramolecular electron transfer that generates {Fe(NO)}8 at heme II. The vacant heme II coordination site of C554 is crowded by three non-bonding hydrophobic amino acids. After replacing one of these (Phe156) with the smaller alanine, the nitrosylation rate for F156A2- and F156A4- was about 400× faster than for the wild type, though the rate of the reverse denitrosylation process was almost unchanged. Unlike in the wild-type C554, the 6-coordinate low-spin hemes of F156A4- oxidized over the course of several minutes after exposure to NO. Concomitant formation of N2O could explain this heme oxidation, though alternative explanations are equally plausible given the available data.

Oxidation of zeolite acid sites in NO/O2 mixtures and the catalytic properties of the new site in NO oxidation

The NO oxidation reaction kinetics over a number of zeolite structures were investigated at temperatures above 423K using plug-flow microreactors and in situ FTIR spectroscopy. Fast catalytic NO oxidation rates (up to 1000×) are observed over H- and Na-exchanged zeolite frameworks (BEA, MFI, and CHA) at temperatures above 423K with respect to the gas-phase reaction. NO oxidation rates increase with temperature over H- and Na-zeolites, while siliceous zeolites display little activity. Activation energies scale in the order of CHA<MFI<BEA (40.8–55.5kJmol−1), and application of transition state theory shows that the formation of the activated complex is an enthalpically controlled process that benefits from smaller zeolite pore size. In all samples, rates are proportional to NO and O2 concentrations, in contrast to low-temperature catalytic rates Loiland et al. [33], which are second order in NO and first order in O2. This difference indicates that a new reaction mechanism is operative above 423K. In-situ FTIR studies reveal that NO+ coordinated at framework sites in the zeolite pores (SiO−(NO+)Al) plays a direct role in the catalysis, and they also reveal that the NO oxidation rate is proportional to the amount of NO+ present in the sample. Furthermore, it is found that NO+ is in equilibrium with gas-phase NO and that desorption of NO+ (as NO) yields an oxidized acid site (SiOAl). No evidence of NO+ formation is observed over the siliceous zeolite samples. A working model of the reaction mechanism for the high-temperature NO oxidation reaction is proposed for acidic zeolites that is consistent with the observed form of the rate equation and the observed NO+ reaction intermediate.

SCR catalyst coated on low-cost monolith support for flue gas denitration of industrial furnaces

NH3–SCR honeycomb catalyst was prepared by coating powdery active metal components on the surface of blank monolith supports and tested at the temperatures of 250–350°C, and superficial gas velocities of 5.3–6.3m/s. The tests were performed with a simulated gas mixture containing NO, SO2, O2, H2O and N2 or a real flue gas produced on line. The results showed that the coated honeycomb catalyst has an extraordinary high activity for low-temperature deNOx in the presence of 5vol.% steam and 1000ppm SO2. Its activity increased with increasing the coating thickness and reached at 110μm thickness as comparable as that of the 100% active components-molded catalyst, suggesting the deNOx reaction proceeded mainly in the surface layers of the honeycomb cell wall and hence coated honeycomb catalyst should be more cost-efficient than 100% active components-molded one. In a real coal-burned flue gas stream the catalyst maintained essentially its low temperature activity and stability, further verifying its high industrial applicability. Then, the effects of NO concentration and NH3/NO ratio on NO conversion were investigated to seek a suitable kinetic model for prediction of the catalyst’s deNOx rate and reactor design for pilot plant research. The data obtained showed that a simple power-law kinetic equation with first order in NO and zero order in ammonia can be applied to the coated honeycomb catalyst, and was used for determination of the kinetic parameters. Finally, simulation calculation was performed under arbitrary conditions to provide valuable information for design of pilot-plant scale reactors.

Efficient nitrosation of glutathione by nitric oxide

Nitrosothiols are increasingly regarded as important participants in a range of physiological processes, yet little is known about their biological generation. Nitrosothiols can be formed from the corresponding thiols by nitric oxide in a reaction that requires the presence of oxygen and is mediated by reactive intermediates (NO2 or N2O3) formed in the course of NO autoxidation. Because the autoxidation of NO is second order in NO, it is extremely slow at submicromolar NO concentrations, casting doubt on its physiological relevance. In this paper we present evidence that at submicromolar NO concentrations the aerobic nitrosation of glutathione does not involve NO autoxidation but a reaction that is first order in NO. We show that this reaction produces nitrosoglutathione efficiently in a reaction that is strongly stimulated by physiological concentrations of Mg2+. These observations suggest that direct aerobic nitrosation may represent a physiologically relevant pathway of nitrosothiol formation.

Catalytic NO Oxidation Pathways and Redox Cycles on Dispersed Oxides of Rhodium and Cobalt

AbstractThe elementary steps and site requirements for the oxidation of NO on Rh and Co and the oxidation state of the catalysts were probed by isotopic tracers, chemisorption methods, and kinetic measurements of the effects of the pressures of NO, O2, and NO2 on turnover rates. On both catalysts, NO oxidation rates were first order in NO and O2 and were inversely proportional to NO2 pressure, as observed on Pt and PdO. These data implied that O2 activation on an isolated vacancy (*) on the catalyst surfaces that were saturated with oxygen (O*) was the kinetically relevant step. Quasi‐equilibrated NO–NO2 interconversion steps established the coverage of * and O* and the chemical potential of oxygen during the catalysis. These chemical potentials set the oxidation state of Rh and Co clusters and were described by an O2 virtual pressure, which was determined from the formalism of non‐equilibrium thermodynamics. RhO2 and Co3O4 were the phases that were present during NO oxidation, which had several consequences for catalysis. Turnover rates increased with increasing cluster size because the vacancies that were needed for O2 activation were more abundant on large oxide clusters, which delocalized electrons better than small clusters. NO oxidation turnover rates on RhO2 and Co3O4 were higher than expected from the oxygen‐binding energy on Rh and Co metal surfaces and from the reduction potentials of Rh3+ and Co2+. These NO oxidation rates were consistent with the rates on Pt and PdO when one‐electron‐reduction processes, which were accessible for Rh4+ and Co3+ but not for Pt2+ and Pd2+, were used to describe the reactivity of RhO2 and Co3O4. One‐electron redox cycles caused the 16O2–18O2 exchange rates to be higher than the NO oxidation rates, in contrast with their analogous values on Pt and PdO, although O2 activation on the vacancies limited NO oxidation and O2 exchange on all of the catalysts. One‐electron redox cycles allowed electron sharing between metal cations and a facile route to form vacancies on RhO2 and Co3O4. This interpretation of the data highlighted the role of vacancies in kinetically relevant O2‐activation steps to explain the higher reactivity of larger metal and oxide clusters and to provide a common framework to describe NO oxidation and the active species on catalysts of practical interest.

Gas-phase reactions of nitric oxide with atomic lanthanide cations: Room-temperature kinetics and periodicity in reactivity

An inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer has been employed in a systematic survey of the room-temperature kinetics of reactions of NO with 13 atomic lanthanide cations from Ce + to Lu + (excluding Pm +). The atomic ions are produced at ca. 5500 K in an ICP source and are allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction in helium buffer gas at 0.35 ± 0.01 Torr and 295 ± 2 K. All lanthanide cations were observed to exhibit some reactivity towards NO, almost exclusively resulting in LnO + formation. Reactions were observed that are both first and second order in NO. Periodic trends in the measured reaction efficiencies for direct exothermic O atom transfer correlate with trends in the energy required to promote an electron in Ln + to achieve either a d 1s 1 or a d 2 excited electronic configuration in which two non-f-electrons are available for bonding. No such correlation is apparent for the remaining reactions.

Nitric Oxide as an Electron Donor, an Atom Donor, an Atom Acceptor, and a Ligand in Reactions with Atomic Transition-Metal and Main-Group Cations in the Gas Phase

The room-temperature reactions of nitric oxide with 46 atomic cations have been surveyed systematically across and down the periodic table using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and product distributions were measured for the reactions of first-row cations from K+ to Se+, of second-row cations from Rb+ to Te+ (excluding Tc+), and of third-row cations from Cs+ to Bi+. Reactions both first and second order in NO were identified. The observed bimolecular reactions were thermodynamically controlled. Efficient exothermic electron transfer was observed with Zn+, As+, Se+, Au+, and Hg+. Bimolecular O-atom transfer was observed with Sc+, Ti+, Y+, Zr+, Nb+, La+, Hf+, Ta+, and W+. Of the remaining 32 atomic ions, all but 8 react in novel termolecular reactions second order in NO to produce NO+ and the metal-nitrosyl molecule, the metal-monoxide cation and nitrous oxide, and/or the metal-nitrosyl cation. K+, Rb+, Cs+, Ga+, In+, Tl+, Pb+, and Bi+ are totally unreactive. Further reactions with NO produce the dioxide cations CaO2+, TiO2+, VO2+, CrO2+, SrO2+, ZrO2+, NbO2+, RuO2+, BaO2+, HfO2+, TaO2+, WO2+, ReO2+, and OsO2+ and the still higher order oxides WO3+, ReO3+, and ReO4+. NO ligation was observed in the formation of CaO+(NO), ScO+(NO), TiO+(NO), VO+(NO)(1-3), VO2+(NO)(1-3), SrO+(NO), SrO2+(NO)1,2, RuO+(NO)(1-3), RuO2+(NO)1,2, OsO+(NO)(1-3), and IrO+(NO). The reported reactivities for bare atomic ions provide a benchmark for reactivities of ligated atomic ions and point to possible second-order NO chemistry in biometallic and metal-surface environments leading to the conversion of NO to N2O and the production of metal-nitrosyl molecules.

Effects of precursor and support variation in the performance of uranium oxide catalysts for CO oxidation and selective reduction of NO

Uranium oxide catalysts supported upon γ-Al 2O 3 and amorphous and mesoporous silica have been tested for CO/O 2 and NO/CO conversions. Pre-calcination is deleterious to catalysts on a mesoporous support due to extrusion of the active uranium phase into poorly dispersed uranium oxide particles. Developing the catalysts under CO/O 2/He or CO/NO/He avoids extrusion and results in superior mesoporous catalysts (from uranyl nitrate precursor) which are comparable to Pt/Al 2O 3 catalysts. Chloride is found generally to be deleterious to catalyst performance. The exception to this in UCl 4/γ-Al 2O 3 calcined at 873 K for CO/O 2. In this system, the total activity is correlated to the level of chlorine retention and average uranium phase crystallite size within the catalyst. The net rate expression for selective reduction of NO by CO over the best mesoporous catalyst is zero order in NO and proportional to [CO] 1.4 in contrast to bulk U 3O 8 where the net rate depends upon [NO]. The activation energy ( E (a)) and pre-exponential factors ( ν) are highly correlated both to each other and the net dispersion of the UO 2.2 phase recovered after catalysis; the most active systems are highly dispersed where the E (a) values are more than compensated for by higher values of ν.

Kinetics of NO reduction over Ag/alumina by higher hydrocarbon in excess of oxygen

Continuous NO x reduction by octane under lean conditions over a highly active Ag/alumina catalyst was studied. Experimental data, observed at steady-state conditions, was used to produce a phenomenological kinetic model to determine the rate orders in NO, hydrocarbons and O 2 . The results showed that the reaction order in NO was equal to zero in the temperature range of 300–550 °C. However, at the same time reaction orders in octane exceeded unity, and were dependent on the NO concentration. Mechanistic model was developed, which takes into account kinetic regularities.

Gas-Phase Kinetic Measurements of the Ligation of Ni+, Cu+, Ni(Pyrrole)1,2+ and Cu(Pyrrole)1,2+ with CO2, D2O, NH3 and NO

The rate and equilibrium kinetics of the reactions of the biologically important metal species M(+), M(+)(pyrrole) and M(+)(pyrrole)(2) (M = Ni, Cu) have been investigated with the biological gases CO(2), D(2)O, NH(3) and NO in the gas phase at 295 +/- 2 K in helium buffer-gas at a pressure of 0.35 +/- 0.01 Torr. The measurements were taken with an Inductively Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. Only ligation was observed for the reactions of bare Ni(+) and Cu(+) with CO(2), D(2)O and NH(3) with rates consistent with the known strengths of the resulting ligand-metal bonds. Both metal cations appeared to be oxidized and produce N(2)O in interesting reactions that are second order in NO. One pyrrole ligand was observed to increase the rate of ligation by as much as a factor of 100 and to switch off the oxidation with NO. Equilibrium was achieved for the ligation of CO(2), D(2)O and NO to both Ni(+)(pyrrole) and Cu(+)(pyrrole), and so it was possible to determine absolute values for the standard free energies of ligation. No ligand substitution was observed with M(+)(pyrrole). M(+)(pyrrole)(2) was observed to be generally unreactive towards the small molecules investigated: a notable exception is ammonia. Very fast ligand substitution reactions were observed for reactions of M(+)(pyrrole)(2) with NH(2).

FTIR Studies of the Reaction of Gaseous NO with HNO3 on Porous Glass: Implications for Conversion of HNO3 to Photochemically Active NOx in the Atmosphere

The heterogeneous reaction of HNO3 adsorbed on porous glass surfaces with gaseous NO was investigated using transmission Fourier transform infrared (FTIR) spectroscopy at room temperature. The amount of adsorbed HNO3 varied from (4.2−110) × 1017 molecules of HNO3 on a 6 cm2 porous glass plate whose BET surface area was measured to be 28.5 ± 0.6 m2 (2σ). The initial concentration of gaseous NO varied from (0.2−6) × 1017 molecules cm-3. A rapid release of NO2 into the gas phase was observed to occur simultaneously with a decrease in adsorbed HNO3. A trace amount of gaseous HONO was also formed. The measured yields of NO2 and loss of HNO3 and NO are consistent with the net reaction 2HNO3 + NO → 3NO2 + H2O which is due to HNO3 + NO → HONO + NO2, followed by HONO + HNO3 → 2NO2 + H2O and/or 2 × (HNO3 + NO → HONO + NO2) followed by 2HONO → NO + NO2 + H2O. Both 15NO2 and 14NO2 were observed as reaction products when 15NO and 14HNO3 were used as the reagent species, indicating that some of the NO2 produced originates in HNO3. The measured decay rates for adsorbed HNO3 were first order in NO. The rates initially increase with increasing HNO3 but tend to plateau, consistent with complete surface coverage and the formation of multilayers of HNO3, perhaps in part in the pores. Extrapolation of these results to atmospheric NO levels suggests that this heterogeneous reaction may serve as a mechanism to regenerate photochemically active forms of NOx and nitrous acid from HNO3 in the atmosphere.

Kinetics of the Selective Catalytic Reduction of NO over HZSM-5

We have studied the kinetics of the selective catalytic reduction (SCR) of NO by NH3 over HZSM-5. We find that between 350 and 450°C the reaction is first order in NO and oxygen concentration but is negative order in ammonia. The rate data can be modeled by assuming that ammonia inhibits the reaction by blocking sites necessary for NO oxidation, which appears to be the rate-determining step. Ammonia oxidation is measurable at all conditions studied and becomes increasingly important as temperature and oxygen concentration increase or as NO concentration decreases; the oxidation of ammonia is more likely to produce N2 than NO. We have developed a kinetic model that accurately describes the rate of N2 formation over a wide range of conditions. We suggest that the active site for NO oxidation may be highly acidic extra-framework aluminum rather than the framework Brønsted aluminum sites.

Kinetics of the conversion of NO to N 2 during the oxidation of iron particles by NO in a hot fluidised bed

The kinetics of the conversion of poisonous NO gas to N 2 in Fe+q NO→ FeO q+ q 2 N 2 have been measured in hot fluidised beds of sand from 973 to 1173 K at 1 bar. The reaction involves NO (in N 2) reacting with tiny (diam. 139–400 μm) iron particles and, because NO is thereby converted to harmless N 2, the reaction is of interest for removing NO from, e.g. fluidised bed or other combustors. Whether the oxide produced is FeO, Fe 3O 4 or Fe 2O 3 is one subject of discussion and it is concluded that the product is mainly FeO. Two stages were found for the reaction. Initially, the rate of oxidation of iron is first order in NO and, in fact, is kinetically controlled. The outer layer of oxide produced on each iron particle is found to be very porous at the highest temperature (1173 K), but much less porous at the lowest temperature (973 K). The associated rate constant has a low activation energy (41±4 kJ/mol) and a pre-exponential factor close to the theoretical upper limit of the collision frequency. During the second stage of reaction, the observed rate is independent of the concentration of NO. This is because the rate is controlled by diffusion of Fe 2; ions and electrons from a particle’s metallic core, through vacancies in the layer of FeO (covering the unreacted Fe), to the outer surface of the particle. Interestingly, the second stage has a larger activation energy (155±20 kJ/mol) than the first stage. At the highest temperature (900°C) these iron particles can be oxidised completely, but at 700°C the reaction stops after only a conversion of ∼20% of Fe to FeO.

Rapid NO 2 formation in diluted petrol-fuelled engine exhaust—A source of NO 2 in winter smog episodes

Substantially enhanced rates of oxidation of nitric oxide have been found in petrol engine exhaust diluted with air in the dark. The main product of oxidation is nitrogen dioxide, but up to 20% of the oxidised NO can be in the form of other NO y species. Similarly enhanced NO oxidation rates are also found in the presence of petrol vapour. The kinetics of NO oxidation are close to zero order in NO, the rate being appreciably faster than the known thermal reaction 2NO + O 2 → 2NO 2. Attempts to isolate the chemical species responsible for this rate enhancement from petrol vapour and engine exhaust have led to identification of 1-methyl-1,3-cyclopentadiene and 1,3-cyclohexadiene. A mechanism has been developed in which addition of NO 2 to a conjugated diene causes initiation of a free-radical chain oxidation. A simple numerical model incorporating this process is capable of explaining the observed rate enhancements.

Kinetic and Mechanistic Study of NOxReduction by NH3over H-Form Zeolites

A study of the kinetics and mechanism of the selective catalytic reduction (SCR) of NO by NH3over H-ZSM-5 was undertaken. Steady-state kinetic experiments performed at temperatures above 330°C with and without the presence of H2O indicate that H2O does not affect SCR by NH3at these temperatures. Analysis of the kinetic data indicates that SCR is positive order in NO and O2and inhibited by NH3. A series of transient tests were also performed to determine the roles of NO, O2, and NH3in the mechanism of NO SCR. Transient test results indicate that O2reacts with NO to form an active intermediate species (possibly NO2or NO+) which reacts with adsorbed NH3. Transient test results also suggest that although adsorbed NH3is necessary for NO reduction to proceed, excess gaseous NH3inhibits SCR. Comparisons of SCR activity exhibited by samples of H-ZSM-5 and H-mordenite of different Si/Al ratios suggest that a limiting Al content is necessary for H-form zeolites to be active. This result suggests that pairs of neighboring Brønsted acid sites are necessary for adsorption of NH3, such that the adsorbed NH3molecules are close enough together that they can both bond with other reactant molecules.

NO absorption in a CrO3/H2SO4/H2O solution: experimental and theoretical investigations in a laboratory and a pilot plant

Abstract The absorption rates of NO in a CrO3/H2SO4/H2O solution have been investigated. A gas/liquid stirring cell and a pilot packing column were used. The experiments, which covered a wide range of gas and liquid concentration, are discussed using the film theory of gas absorption accompanied by a fast chemical reaction. The reaction was found to be second order in NO and first order in CrO, The rate of NO absorption increases if either the H2SO4 or CrO3 concentration or the NO partial pressure is increased. Good agreement between experimental data and theory is only obtained when the concentration terms in the model are replaced by activities. In a further step, a scale-up from stirring-cell experiments to a pilot packed column was performed. A high surplus of reactants (500 g l−1 H2SO4 and 50–150 g l−1 CrO3), which is common in technical applications, led to a simple expression for the mass transfer term.

Selective Reduction of NO by NH3over Chromia on Titania Catalyst: Investigation and Modeling of the Kinetic Behavior

The kinetics and the parametric sensitivity of the selective catalytic reduction (SCR) of NO by NH3were investigated over a chromia on titania catalyst. The chromium oxide phase was made up predominantly of X-ray amorphous Cr2O3. High SCR activity and selectivity to N2was attained at low temperatures. The high selectivity is attributed to the absence of significant amounts of CrO2and crystalline α-Cr2O3which favor N2O formation. The selectivity to N2O increased with higher temperature. Addition of up to 6% H2O to the dry feed reduced the rate of NO conversion and decreased the undesired formation of N2O. The effect of water on the catalytic behavior was reversible. In the absence of oxygen, the reaction between NO and NH3became marginal, independently whether H2O was present or not. Small amounts of oxygen were sufficient to restore SCR activity. Admission of SO2to the SCR feed resulted in a severe loss of activity. The poisoning of the catalyst by SO2was already notable for low SO2concentrations (30 ppm) and for temperatures up to 573 K. X-ray photoelectron and FTIR spectroscopy revealed the presence of sulfate species on the catalyst surface. Analysis of the kinetic data indicated that the SCR reaction is first order in NO and zeroth order in NH3for temperatures in the range 400–520 K. The estimated activation energies for dry and wet feed amounted to 60.0±1.6 kJ/mol (95% confidence limits). For temperatures in the range 400–520 K, and for a SO2free feed, the steady-state kinetic data could be well described with a model based on an Eley–Rideal type reaction between activated ammonia surface species and gaseous or weakly adsorbed NO.

Some new laboratory approaches to studying tropospheric heterogeneous reactions

Assessing the potential for global climate change requires a detailed understanding of the fundamental chemical and physical processes controlling the concentrations of key gases as well as particles in the atmosphere. Laboratory studies are used to obtain the basic kinetic and mechanistic data needed for inputs to models as well as for interpreting field observations. While gas-phase reactions are reasonably well-understood, “heterogeneous” processes involving gases and solids are not. We briefly describe applications of three approaches to laboratory studies of heterogeneous atmospheric reactions which have not been widely used for this purpose in the past: diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDS), and X-ray photoelectron spectroscopy (XPS). The application of these techniques to studying the reactions of the oxides of nitrogen with NaCl and NaBr found in sea-salt particles is described and used to illustrate their utility in obtaining both kinetic and mechanistic data. The reaction of NO 2 with NaBr is found to be approximately second order in NO 2, suggesting that the dimer N 2O 4 may be the reacting species. If this is the case, a preliminary value for the reaction probability for the N 2O 4NaBr reaction at 298 K is 2 × 10 −4 with an uncertainty of a factor of three. That for the HNO 3NaCl reaction was found using XPS to be (4 ± 2) × 10 −4. The kinetic data from these studies indicate that the NO 2 reaction is too slow to be competitive with the N 2O 5 and HNO 3 reactions. Mechanistically, both the DRIFTS and TEM-EDS studies show that water vapor even at relative humidities well below the deliquescence point causes a selective recrystallization of surface nitrate into microcrystallites of NaNO 3, regenerating a fresh salt surface. This may explain field observations of some marine particles which are essentially totally devoid of chloride ions.

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH4 over 4%Sr/La2O3 in the absence and presence of O2 were examined between 773 and 973 K, and N2O decomposition was also studied. The presence of CH4 greatly increased the conversion of NO to N2 and this activity was further enhanced by co-fed O2. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH4 in the absence of O2, and reduction with 1% O2 in the feed were 8.3·10−4, 4.6·10−3, and 1.3·10−2 μmol N2/s m2, respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N2 formation by decomposition was inhibited by O2 in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH4 continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O2 with equal activation energies of 26 kcal/mol. The addition of O2 increased the reaction order in CH4 at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O2 was 0.26 up to 0.5% O2 during which time the CH4 concentration was not decreased significantly. N2O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10−4 μmol N2/s m2 at 623 K and 1220 ppm N2O and an activation energy of 24 kcal/mol. The addition of CH4 inhibits this decomposition reaction. Finally, the use of either CO or H2 as the reductant (no O2) produced specific activities at 773 K that were almost 5 times greater than that with CH4 and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H2 to reduce NO to N2 over these REO catalysts.

Selective Catalytic Reduction of NO with Methane on Gallium Catalysts

Gallium species supported on H-ZSM-5 and on H-mordenite are active catalysts for NO reduction with methane in the presence of excess O 2. Over Ga-H-ZSM-5, the NO reduction rate is first order in NO with a variable, fractional order in CH 4. The distinct feature of these gallium-based catalysts is their very high CH 4 selectivities for NO x reduction. Ga-H-Y, on the other hand, is a poor catalyst for this reaction. The preparation method, ion exchange or wet impregnation, does not appear to be critical for the performance of a Ga/H-ZSM-5 catalyst. Gallium-exchanged and impregnated H-ZSM-5 have similar activities for the NO reduction. Comparable Brønsted acid sites were found on the H-ZSM5 and the H-ZSM-5 supported gallium catalysts via temperature-programmed desorption measurements with ammonia and isopropylamine and by infrared measurements. On the other hand, the presence of H + in zeolite is important; Ga/Na-ZSM-5 was inactive for the NO reduction. It is apparent that a synergism exists between gallium species and H +, and this serves to activate CH 4 as a reductant for NO in the presence of O 2. While Ga-H-ZSM-5 is more selective than Co-ZSM-5 for NO reduction, it is more sensitive to water, which makes it less attractive for commercial applications.