Inhibition Of CO Oxidation Research Articles

The Effect of Adsorbed Water in CO Oxidation on Au/TiO2(110)

The effect of moisture on CO oxidation on Au/TiO 2 (110) model catalysts is investigated using temperature-programmed desorption and molecular beam reactive scattering under ultrahigh vacuum (UHV) conditions. Oxygen exchange is observed between adsorbed atomic oxygen and isotopically labeled water. Coadsorbed water (H 2 18 O) takes part in CO oxidation on O a precovered Au/TiO 2 (110) model catalysts, leading to the formation of C 16 O 18 O and C 16 O 16 O. The amount of C 16 O 18 O produced increases with increasing water coverages; however, the total amount of CO 2 produced decreases. Although coadsorbed O a and H 2 O have a minimal influence on the initial adsorption probability of CO, the total uptake of CO decreases as H 2 O coverages increase. Interestingly, the adsorption of water induces desorption of predeposited molecularly chemisorbed O 2 . Thus, adsorbed water slightly inhibits CO Oxidation on atomic oxygen precovered Au/TiO 2 (110) model catalysts under UHV conditions.

SO 2 effects on CO oxidation in a CO 2 atmosphere, characteristic of oxy-fuel conditions

SO 2 is an important pollutant formed during many combustion processes which may affect the oxidation of the fuel. The current work focuses on the evaluation of the effect of the presence of SO 2 on CO oxidation in a CO 2 atmosphere, found in oxy-fuel combustion, in comparison to a N 2 dilution characteristic of air combustion. A flow reactor experimental and kinetic modeling study of the gas-phase interaction of SO 2 with the radical pool during CO oxidation under CO 2 and N 2 diluted conditions has been carried out for different stoichiometries in the 800–1800 K temperature range. The presence of SO 2 is seen to inhibit the oxidation of CO in both atmospheres for all the stoichiometries studied, although the inhibition is significantly higher as the stoichiometry becomes more reducing. The inhibiting effect of SO 2 in the CO 2 atmosphere is caused by similar radical recombination mechanisms than those occurring with N 2. Under fuel-rich conditions, the inhibition is governed by the SO 2/HOSO inter-conversion and is less pronounced as the CO 2 concentration increases. However, as the conditions are more fuel-lean, the SO 2 inhibiting effect is comparable in both atmospheres due to the higher importance of the SO 2 + O recombination. As the SO 2 concentration increases, its interaction with the radical pool is enhanced and the inhibition of CO oxidation is higher. An updated kinetic mechanism is used for calculations and provides a very good description of the experimental results for all the operating conditions analyzed.

The Effects of Water on CO Oxidation over TiO2 Supported Au Catalysts

Polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) was used to investi- gate the effects of water on CO oxidation catalyzed by TiO2 supported Au. The introduction of water into the reactant mixture caused two effects: (1) a decrease in the CO coverage; and (2) the appearance of a carbonate spe- cies. Since both effects inhibit CO oxidation, the promo- tional effects of water found by previous researchers can only be rationalized by its effects on molecular oxygen adsorption and activation. Although this argument is not confirmed by the PM-IRAS data alone, this conclusion is consistent with previous studies. It is further inferred from this study that carbonate species first form on TiO2 sites and then migrate to Au sites. Surface hydroxyl species may play the same promotional role as adsorbed water.

Influence of HCl on CO and NO emissions in combustion

The influence of HCl on CO and NO emissions was experimentally investigated in an entrained flow reactor (EFR) and an internally circulating fluidized bed (ICFB). The results in EFR show the addition of HCl inhibits CO oxidation and NO formation at 1073 K and 1123 K. At the lower temperature (1073 K) the inhibition of HCl becomes more obvious. In ICFB, chlorine-containing plastic (PVC) was added to increase the concentration of HCl during the combustion of coal or coke. Results show that HCl is likely to enhance the reduction of NO and N 2O. HCl greatly increases CO and CH 4 emission in the flue gas. A detailed mechanism of CO/NO/HCl/SO 2 system was used to model the effect of HCl in combustion. The results indicate that HCl not only promotes the recombination of radicals O, H, and OH, but also accelerates the chemical equilibration of radicals. The influence of HCl on the radicals mainly occurs at 800–1200 K.

Carbon Monoxide Oxidation as a Probe for PtRu Particle Surface Structure

The carbon monoxide molecule was used to probe the surface properties of PtRu(1:1)/C catalysts with 5, 30, and 60 wt % metal. The spectroscopic IR study performed in the external reflection configuration clearly showed that linearly bonded CO dominates the spectra for all catalysts, although the presence of bridged adsorbates was also revealed. These nanostructures, consisting of metal grains interconnected via grain boundaries, and formed at high metal loadings on carbon support, strongly influence the catalytic oxidation of the adsorbed carbon monoxide. It is proposed that in the potential interval 0.3 < E < 0.45 V, O(H)ads formation occurs preferentially at the active sites which are presumably located at the emergence of the grain boundaries at the particle surfaces. Dissolved CO strongly inhibits CO oxidation on multigrained materials while for highly dispersed isolated PtRu nanoparticles it shifts the oxidation onset negative. The spatial distribution of the adsorbed CO molecules on the catalyst sur...

Temperature-Programmed Reaction of CO Adsorbed on Oxygen-Covered Pt(100): Reactivity of High-Coverage Oxygen Phases

We utilized temperature-programmed reaction spectroscopy (TPRS) to investigate the reactivity of oxygen-covered Pt(100) toward the oxidation of CO. The reaction is facile on oxygen phases that form below coverages of about 1 ML (monolayers), producing complicated CO2 desorption traces. However, the reaction is inefficient when three-dimensional (3D) oxide particles cover the surface. We observe CO2 production in roughly three temperature regimes during TPRS. Between 120 and 220 K, O atoms react with CO molecules adsorbed within oxygen phase domains. In this regime, the reactivity toward CO is highest for the two-dimensional (2D) surface oxide and decreases in the order of metastable O phase, O chemisorbed in a disordered (3 × 1) phase, and 3D oxide. Decreases in the CO and O binding strengths generally correlate with increasing reactivity of the oxygen phases below 220 K. At temperatures from about 225 to 335 K, reaction involves species at the boundaries of separate O and CO domains. As the temperature surpasses 335 K, CO2 production occurs between CO and disordered or isolated O adatoms. Within the second temperature regime, our data suggests that both Pt and O atoms migrate away from 2D and 3D oxide domains and into CO domains where reaction occurs. Kinetic barriers associated with the creation and transport of Pt adatoms likely inhibit CO oxidation by the 3D oxide. We also find that the generation of Pt adatoms facilitates significant surface defect formation and oxygen loss to the bulk during reaction. The results of this investigation clearly demonstrate that the surface oxygen phase distribution strongly influences the temperature-programmed reaction of coadsorbed CO and O on Pt(100).

Abatement of CO from relatively simple and complex mixtures: III. Oxidation on Pd–CeO2/C and CeO2/C catalysts

Oxidation of carbon monoxide on CeO2/C and Pd/CeO2/C catalysts was investigated at temperatures ranging from 50 to 700°C until the catalysts disintegrated. The CeO2/C catalyst, which was stored in air for an extended period of time, was found to be more active than the same catalyst which was annealed in inert atmosphere, which suggested that the catalyst could accumulate active surface oxygen over time. The effects of surface oxidation were further evident after preparing more active Pd catalysts on both oxidized and annealed forms of CeO2/C, with the latter catalyst showing a higher activity. Nitric oxide was found to inhibit CO oxidation on ceria and promote it on supported Pd catalysts. ESR spectra showed the presence of active oxygen, possibly in the forms of superoxide and peroxide on the surface of carbon and ceria. The reduced Pd clusters could generate superoxide radicals whereas they were stabilized by partially reduced Ce3+ and carbon. Adsorbed water most likely protected ceria sites from a complete oxidation into Ce4+. Long-term storage of a sealed container of Pd/CeO2/C at room temperature aged the catalyst, which possibly involved a Pd-assisted slow partial reduction of Ce4+ by carbon and an accumulation of Ce3+⋯O2− complexes.

Effect of Ru-Based Catalytic Additives on NO and CO Formed during Regeneration of Spent FCC Catalyst

A series of Ru-based additives were developed to simultaneously reduce the NO and CO emissions during the regeneration of spent fluid catalytic cracking (FCC) catalyst. The evaluation of the additives was performed in a bench-scale fluid-bed reactor, while the parameters under study were the catalytic support and the Ru loading. The additives' performance was compared to that of a commercial, Pt-based, CO promoter. It was found that different types of alumina supports affected the catalytic performance. The promotion of the alumina support with cerium (Ce) enhanced the CO oxidation but had no significant influence on the deNOx performance. Decreasing Ru loading improved NO reduction but inhibited CO oxidation. A combined performance of CO oxidation and NO reduction was achieved using a Ru(0.1%)/Ce(10%)alumina additive. Moreover, in the present work, the stability of the Ru-based additive was examined by submitting the best additive to a redox pretreatment. The presence of O2 inhibited, while the presence of CO improved, its NO reduction activity. This improvement was cumulative until 100% NO reduction was achieved and suggests that reduced Ru is the required active phase. Selected Ru-based additives were tested in a pilot plant scale FCCU. The pilot plant data showed that Ru/alumina exhibited an unstable performance and high levels of CO emissions. However, low Ru loadings on Ce-promoted alumina were more stable and efficient for reducing both NO and CO emissions. Pilot plant results also validated that submitting the additive in a reducing pretreatment further enhanced its performance.

Inhibition of CO oxidation on RuO2(110) by adsorbed H2O molecules

Catalytic CO oxidation on the RuO(2)(110) surface was studied at 300 K by scanning tunneling microscopy (STM), high-resolution electron-energy-loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). Upon repeatedly exposing the surface to several 10 L of CO and O(2) at 300 K, STM shows that unreactive features accumulate with each CO and O(2) titration run. HREELS and TDS show formation of increasing amounts of H(2)O, retarded formation of O-cus atoms and incomplete removal of CO-bridge molecules during O(2) dosing, and a changing ratio of single- and double-bonded CO-bridge molecules. It is concluded that H(2)O (presumably from the residual gas) is accumulating at the Ru-cus sites thus blocking them, so that the dissociative adsorption of oxygen is prevented and the CO oxidation reaction is suppressed. Some 10% CO- bridge remains on the surface even during oxygen exposure. Consistent with this interpretation, deactivation of the surface is suppressed at 350 K, at the onset of H(2)O desorption.

Surfactant-controlled synthesis of Pd/Ce 0.6Zr 0.4O 2 catalyst for NO reduction by CO with excess oxygen

For the first time, this work reports a surfactant-controlled synthetic method to obtain a nanophase of mesoporous ceria–zirconia solid solution containing cationic defects in the crystalline structure. The incorporation of a cationic surfactant (myristyltrimethylammonium bromide) into the ceria–zirconia solid network not only controlled the pore diameter distribution but also induced creation of the lattice defect. Ceria–zirconia solid solution showed crystal microstrain and structural distortion that varied with the calcination temperature. Compared to pure ceria, the addition of zirconium to the ceria promoted the bulk oxygen reducibility and enhanced the thermal stability of the solid. Hydrogen could be stored into or released from the PdO/Ce 0.6Zr 0.4O 2 catalyst during the TPR procedure, which is associated to the formation/decomposition of a PdH x phase, due to the hydrogen dissociation catalyzed by metallic Pd. At cool start of reaction, NO reduction by CO with excess oxygen over the Pd/Ce 0.6Zr 0.4O 2 catalyst showed selectivity around 100% to N 2. A competition between NO reduction by CO and CO oxidation by O 2 was observed: at reaction temperatures below 200 °C, NO inhibited CO oxidation activity; however, at reaction temperatures above 200 °C, high activity of CO oxidation resulted in an inhibition effect on NO reduction.

Interactions of CO, HCl, and SO x in pulverised coal flames

The effect of HCl and SO 2 on CO oxidation in pulverised coal flames was investigated experimentally and kinetically in an entrained flow combustion reactor. Two bituminous coals (German ‘Goettelborn’ and a Polish coal) were used as fuels with a feeding rate of 1 or 1.5 kg/h. HCl or SO 2 is introduced into the reactor premixed with the primary air. Experimental results indicate that HCl addition may inhibit CO oxidation in coal flames and increases CO emission. Reducing temperature in the reactor will enhance the inhibitory effect of HCl on CO oxidation. The measured CO profiles along the reactor height clearly show that the addition of HCl may inhibit CO oxidation. In the experimental range of SO 2 addition, the inhibiting effect of SO 2 on CO oxidation is less significant than HCl. A detailed kinetic mechanism is used to model the reactions, and the controlling reactions are analysed.

An Experimental and Modeling Study of the Inhibition of Benzene Combustion by HCI, in a Fluidized Bed

Fluidized beds often burn fuels containing aromatic hydrocarbons and chlorine. This work uses a bench-scale fluidized bed and some detailed calculations to investigate the inhibition of benzene oxidation by hydrogen chloride. Carbon monoxide and carbon dioxide were measured in the freeboard of the fluidized bed reactor over a range of HCI and oxygen concentrations and bed temperatures. For fuel-lean conditions, the presence of HCI causes the CO emissions to increase significantly. HCI addition also increases the unburned hydrocarbon levels. For fuel-rich conditions, the inhibitory effect of HCI is reduced. The experimental measurements indicate, in spite of the large surface areas available to quench radicals, most of the oxidation occurs within the fluidized bed and not in the freeboard. Model predictions agree qualitatively with experimental measurements made in the bench-scale fluidized bed; the trends with HCI concentration, equivalence ratio and temperature are correct. However, the model tends to under-predict the HCI inhibition of hydrocarbon and carbon monoxide oxidation. The model suggests that the inhibition of CO oxidation by HCI is the result of reduced radical levels due to reduced chain branching. The model shows less HCI inhibition of hydrocarbon oxidation because of CI radical reactions with benzene and intermediate hydrocarbons.

The effect of carbon monoxide on the oxidation of four C6 to C8 hydrocarbons over platinum, palladium and rhodium

The oxidations of benzene, toluene and 1-hexene, alone and in mixtures with isooctane and carbon monoxide, have been investigated over alumina-supported platinum, palladium and rhodium under three different conditions. In the absence of CO, oxidation of all hydrocarbons is complete at the lowest temperature with platinum. However, inclusion of CO greatly inhibits hydrocarbon oxidation on platinum. There is also moderate inhibition of palladium, but with rhodium oxidation of the aromatics is somewhat enhanced by the presence of carbon monoxide. The activity order then becomes Rh>Pd>Pt. Conversely, unsaturates inhibit CO oxidation to a lesser extent, this effect being greatest for hexene and toluene with palladium and least with benzene on rhodium. In mixtures of the four hydrocarbons, with or without CO present, the order of removal is always hexene followed by toluene, benzene and isooctane with rhodium exhibiting the largest separation between toluene and benzene. This order appears to be largely governed by competitive adsorption between hydrocarbons of different adsorption strength since the pattern of reactivity is very different when the unsaturates are oxidised alone. Under very lean conditions platinum is the most active for the oxidation of benzene alone, palladium for toluene and rhodium for hexene alone. However, relative activities are strongly dependent on conditions since the reactions exhibit very different kinetic orders in oxygen with each metal. The oxidations of hexene and toluene are positive order in oxygen over Pt and Pd but negative order over Rh. On the other hand the oxidation of benzene seems to be positive order in oxygen for Pt, negative order with Pd and near zero order with Rh. Some of these trends can also be rationalised in terms of relative strengths of adsorption.

Co-combustion of coal and municipal solid waste in a circulating fluidized bed

The emissions of NO, N 2O, HCl, SO 2 and CO were investigated during the combustion of coal and municipal refuse mixtures in a circulating fluidized bed boiler. The results show that the addition of municipal solid waste (MSW) leads to lower N 2O and SO 2 emissions. On the other hand, hydrogen chloride and nitric oxide concentrations increased with the amount of waste added. The variation of NO emissions with the mixture composition has been simply described by a model considering that NO is especially reduced by CO on the char surface. CO emissions slightly decrease when the MSW proportion is low, but when the chlorine supply is significant, inhibition of CO oxidation by HCl provokes the opposite trend: a growth in CO concentration.

The effect of halides on emissions from circulating fluidized bed combustion of fossil fuels

Combustion tests involved addition of chlorides and bromides while burning Highvale coal were carried out in a pilot-scale (152 mm square × 7.3 m tall) circulating fluidized bed combustor (CFBC). The halogens were added in the form of hydrochloric acid (HCl) and calcium bromide ( CaBr 2. 1 2 H 2O ) solutions (10 wt%). The thermodynamic equilibrium composition of CFBC products was calculated using the ASPEN database package. The calculations predict 12% conversion of HCl to CaCl 2 for an HCl feed rate of 9.78 kg h −1 and complete conversion of CaBr 2 to HBr in the vapour phase throughout the range of CaBr 2 solution feed rate investigated. The experimental results indicate that chloride and bromide additions increase the CO and SO 2 concentrations in the flue gases, with corresponding decreases in the NO x level. The halides have no significant effect on N 2O emission. The CO level increased from 27 to 230 ppmv when the chloride concentration in the reactor was ∼4200 ppmv (Cl/fuel = 4.58 wt%). The effect of bromide on CO emission was more dramatic, the CO emission jumping from 56 to 480 ppmv for lower concentrations of bromide (20–400 ppmv). The experiments confirm previous work showing that halide-containing species inhibit CO oxidation through interaction with the hydrogen-oxygen radical pool. The decrease in NO concentration with chloride addition can be explained by surface modification of CaO particles due to formation of a liquid calcium chloride phase favoured by high HCl concentrations near the feed point. The formation of the liquid calcium chloride phase has the potential to make the CaO surface unavailable, thereby reducing catalytic oxidation of volatile nitrogen to NO.

Spectroscopic states of the CO oxidation/CO2 reduction active site of carbon monoxide dehydrogenase and mechanistic implications.

CO dehydrogenases catalyze the reversible oxidation of CO to CO2, at an active site (called the C-cluster) composed of an Fe4S4 cube with what appears to be a 5-coordinate Fe (called FCII), linked to a Ni (Hu, Z., Spangler, N. J., Anderson, M. E., Xia, J., Ludden, P. W., Lindahl, P. A., & Münck, E. (1996) J. Am. Chem. Soc. 118, 830-845). During catalysis, electrons are transferred from the C-cluster to an [Fe4S4]2+/1+ electron-transfer cluster called the B-cluster. An S = 1/2 form of the C-cluster (called Cred1) converts to another S = 1/2 form (called Cred2) upon reduction with CO, at a rate well within the turnover frequency of the enzyme (Kumar, M., Lu, W.-P., Liu, L., & Ragsdale, S. W. (1993) J. Am. Chem. Soc. 115, 11646-11647). This suggests that the conversion is part of the catalytic mechanism. Dithionite is reported in this paper to effect this conversion as well, but at a much slower rate (kso = 5.3 x 10(-2) M-1 s-1 for dithionite vs 4.4 x 10(6) M-1 s-1 for CO). By contrast, dithionite reduces the oxidized B-cluster much faster, possibly within the turnover frequency of the enzyme. Dithionite apparently effects the Cred1/Cred2 conversion directly, rather than through an intermediate. The conversion rate varies with dithionite concentration. The Cred1/Cred2 conversion occurs at least 10(2) times faster in the presence of CO2 than in its absence. CO2 alters the g values of the gav = 1.82 signal, indicating that CO2 binds to a C-cluster-sensitive site at mild potentials. CN- inhibits CO oxidation by binding to FCII (Hu et al., 1996), and CO, CO2 in the presence of dithionite, or CS2 in dithionite accelerate CN- dissociation from this site (Anderson, M. E., & Lindahl, P. A. (1994) Biochemistry 33, 8702-8711). The effect of CO, CO2, and CS2 on CN- dissociation suggested that these molecules bind at a site (called the modulator) other than that to which CN- binds. The effects of CO2, CS2, CO, and dithionite on the Cred1/Cred2 conversion rate followed a similar pattern, suggesting that this rate is also influenced by modulator binding. Some batches of enzyme cannot convert to the Cred2 form using dithionite, but pretreatment with CO or CO2/dithionite effectively "cures" such batches of this disability. The results presented suggest that the Ni of the C-cluster is the modulator and the substrate binding site for CO/CO2. The inhibitor CS2 in the presence of dithionite also accelerates the decline of Cred1, leading first to an EPR-silent state of the C-cluster, and eventually to a state yielding an EPR signal with gav = 1.66. CS2 binding thus shares some resemblance to CO2 binding. Approximately 90% of the absorbance changes at 420 nm that occur when oxidized CODHCt is reduced by dithionite occur within 2 min at 10 degrees C. This absorbance change occurs in concert with the gav = 1.94 signal development. The remaining 10% of the A420 changes occur over the course of approximately 50 min, apparently coincident with the Cred1/Cred2 conversion. One possibility is that the conversion involves reduction of an (unidentified) Fe-S cluster. A three-state model of catalysis is proposed in which Cred1 binds and oxidizes CO, Cred2 is two electrons more reduced than Cred1 and is the state that binds and reduces CO2, and Cint is a one-electron-reduced state that is proposed to exist because of constraints imposed by the nature of the CO/CO2 reaction and the properties of the clusters involved in catalysis.

Inhibition of CO oxidation on hydroxyapatite by tetrachloromethane

The introduction of tetrachloromethane (TCM) into the feedstream of CO and O2 inhibits the formation of CO2 on hydroxyapatite. TCM interacts with the surface of hydroxyapatite to form its chlorinated analogue, chlorapatite, which suppresses the oxidation of CO to CO2. Thus, in the oxidative coupling of methane the beneficial effects of the addition of TCM to the feedstream result, at least in part, from the suppression of the further oxidation of CO.

Impact of SO2 and NO on CO oxidation under post-flame conditions

An experimental and theoretical study on the effect of SO2 on moist CO oxidation with and without NO present has been carried out. The experiments were performed in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 800–1300 K. Inlet concentrations of SO2 ranged from 0 to 1800 ppmv, while the NO ranged between 0, 100, or 1500 ppm. SO2 inhibits CO oxidation under the conditions investigated, shifting the fast oxidation regime 20–40 K towards higher temperatures at 1500 ppm SO2. The inhibition is most pronounced at high O atom levels. The experimental data supported by model analysis suggest that SO2 primarily reacts with O atoms forming SO3, which is subsequently consumed mainly by reaction with O and HO2. Addition of NO significantly diminishes the effect of SO2. Since NO is usually present in combustion flue gases, the impact of SO2 on CO burnout in most practical systems is projected to be small. The H/S/O thermochemistry and reaction subset has been revised based on recent experimental and theoretical results, and a chemical kinetic model has been established. The model provides a reasonable overall description of the effect of SO2 and NO on moist CO oxidation, while the SO3/SO2 ratio is well predicted over the range of conditions investigated. In order to enhance model performance further, rate constants for a number of SO2 and SO3 reactions need to be determined with higher accuracy. © 1996 John Wiley & Sons, Inc.

Interactions of CO, NOxand H2O Under Post-Flame Conditions

ABSTRACT An experimental and theoretical study of the impact of NO, NO2 and H2O on CO oxidation has been carried out. The experiments were performed in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 800-1400 K. Inlet concentrations of NO and NO2 were 0-1 % (vol) and 0-622 ppmv, respectively, while the water vapor level was varied in the ranges 1-32% (NO absent) and 1-10% (NO present). The results show that the concentration of water vapor has a strong effect on the CO oxidation process, partly because it controls the O/OH ratio of the radical pool, and partly due to the high efficiency of H2O in promoting H + O2 recombination, which causes a strong inhibition of CO oxidation at high levels. Presence of NO and NO2 has a significant impact on moist CO oxidation, In low concentrations NO enhances CO consumption in the 900-1100 K range by converting HO2 to OH. In higher concentrations NO may catalyze recombination of radicals, thereby inhibiting the CO oxidation. The overal...

A flow reactor study of HNCO oxidation chemistry

An experimental and theoretical study of HNCO oxidation has been carried out. The experiments were performed in an isothermal quartz flow reactor, and the effects of temperature, CO concentration, and NO concentration were investigated at atmospheric pressure in the temperature range 1025–1425 K. The reaction mechanism for RAPRENOx proposed by Miller and Bowman (1991) has been updated based on the present result as well as recent advances in the understanding of important elementary steps. Model predictions with the revised mechanism are in good agreement with our experimental data as well as data from the literature. Oxidation of HNCO proceeds mainly through NCO, which subsequently is oxidized to NO or reacts with NO to form N2 and N2O. This sequence of reactions is chain terminating, and for reaction to occur, radicals must be generated either by alternative oxidation pathways or by the presence of other combustibles. A chain-branching oxidation route initiated by reaction of HNCO with O2 is proposed in order to explain the observed HNCO decay in the absence of inlet CO. Addition of CO enhances HNCO oxidation and the RAPRENOx chemistry, since CO oxidation acts to replenish the radical pool. The experimental results show that the mutual presence of HNCO and NO strongly inhibits CO oxidation at lower temperatures. In addition to the chain terminating HNCO/NCO reactions, a second inhibition mechanism involving NO is necessary to explain this behavior. This mechanism is presently believed to be NONO2 interconversion, but additional work is needed to confirm this. Further progress in the understanding of the HNCO chemistry is dependent on an accurate determination of the rate and/or mechanism of a number of key reactions, including HNCO + OH, HNCO + O2, NCO + NO and NO + O + M.