A study of nitrous oxide decomposition over calcium oxide

A study of N₂O decomposition reaction in a fixed bed a reactor over bed of CaO particles has been conducted. Effects of parameters such as concentration of inlet N₂O gas, reacting temperature and content of CO₂/ CO gas present in the reacting materials on the decomposition reaction have been investigated. The results showed that the conversion of N₂O decomposition was accelerated by the increase of reaction temperature, and the existence of CO, while the rate was hindered by the existence of CO₂. Heterogeneous gas solid reaction kinetics was proposed for N₂O decomposition and compared with homogeneous reaction kinetics. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Apparatus for steady-state kinetic measurements of the catalytic reduction of nitric oxide by ammonia on iron oxide

The reduction of nitric oxide with ammonia on an unsupported iron oxide catalyst has been studied in a continuous-flow recycle reactor using simulated flue gas. The responses of the employed reactor system to step and pulse inputs of tracer indicate that the system could be regarded as a continuous stirred tank reactor (CSTR). Preliminary tests were carried out to determine the effect of temperature and particle size on the measured reaction rates. Additional experiments were performed in order to study the influence of oxygen and water concentration on these rates. A gas chromatographic system has been developed to analyze the gas components NO, N₂O, NO₂, NH₃, H₂O, O₂, CO₂ and N₂. In addition, the concentrations of NO and NO₂ were measured with a nondisperse infrared (NDUV/NDIR) analyzer.     

Adsorbed species and reaction rates for NO-CO-O2 over Rh(111)

We have studied the NO-CO-O₂ reaction over a Rh(111) catalyst by monitoring the reaction products (CO₂, N₂O, and N₂) and the infrared (IR) intensity of surface CO and NO at various partial pressures of NO, CO and O₂, and sample temperatures. The selectivity for N₂O formation, apparent activation energy for product formation, and NO consumption rate during NO-CO-O₂ are identical to those measured during the NO-CO reaction. The IR measurements show that during NO-CO-O₂ the same two adsorbed species, NO at 1640 cm^-1 and linear CO at ~2040 cm^-1, are present in the same surface concentrations as during NO-CO. For this reason the NO-CO-O₂ kinetics are dominated by the NO-CO kinetics, the NO consumption is rate limited by dissociation of adsorbed NO, and the N₂O selectivity is dominated by surface NO coverage. In contrast, O₂ consumption is adsorption rate limited with the NO-CO adsorption-desorption equilibrium controlling the vacant sites required to dissociatively adsorb O₂. These kinetic and IR data of the CO-NO-O₂ reaction and our interpretation of them agree with previous studies over supported Rh catalysts and thus confirm the previously proposed explanation. From RAIRS and kinetic data we estimate the rate constant for the CO+O→CO₂ elementary step. The pre-exponential factor for this rate is 2×10¹⁰ s^-1, a factor of 50 smaller than previous estimates. This rate constant is important to the NO-CO-O₂ kinetics because it affects O coverage, which, under certain conditions, inhibits NO consumption. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhancement of nitric oxide removal by ammonia on a low-rank coal based carbon by sulphuric acid treatment

This work presents a study of the effect of wet sulphuric acid treatment and gas-phase treatment with SO₂ + O₂ + H₂O on the catalytic activity of a low-rank coal-based carbon for the nitric oxide reduction with ammonia. Carbons were characterized by N₂ adsorption, TPD, and FTIR in order to assess how the surface chemistry and the texture of carbons change after the treatments. A great amount of oxygenated functional groups both CO₂ and CO evolving ones are produced by liquid-phase sulphuric acid treatment. However, the amount of those groups after gas-phase treatment with SO₂ + O₂ + H₂O is lower, in particular the CO₂ evolving groups. The catalytic activity of carbons was examined in a fixed bed reactor at 150 °C in a gas flow containing NO, O₂, N₂ and NH₃, the effluent concentration being monitored continuously during the reaction. The obtained results indicate that an appropriate balance between the type of oxygen functional groups and surface area available to the reactant gas are required to reach high levels of NO conversion.     

The effect of particle size on nitric oxide decomposition and reaction with carbon monoxide on palladium catalysts

The effect of palladium particle size on its catalytic activity was investigated by the decomposition of chemisorbed nitric oxide and the reaction of nitric oxide with carbon monoxide in flow conditions. Palladium particles (30–500 Å) were prepared on silica thin films (100 Å) which were supported on a Mo(110) surface. The reactivity of the supported palladium varied with the metal particle size. On large palladium particles, nitric oxide (NO) reacts to form nitrous oxide (N₂O), dinitrogen (N₂) and atomic oxygen during temperature-programmed reaction, whereas on small particles (< 50 Å), nitrous oxide is not formed. Similarly, reactions of NO with CO on large particles, in flow conditions produce N₂O, N₂ and CO₂, whereas N₂O is not produced on small particles. In addition, more extensive NO decomposition is observed on the smaller particles.     

The reactions of nitromethane in the gas phase and on a Co-ZSM5 catalyst under the conditions of the methane/NOx SCR reaction

The rate of reaction of nitromethane in the gas phase is little changed by the presence of O₂ or NO. The major products are CO, NO and H₂O, plus NO₂ if O₂ is included. The corresponding catalytic reaction over fresh Co-ZSM5 commences at a much lower temperature and gives primarily CO₂ and NH₃, the latter being oxidised to N₂ above 340°C. As the catalyst ages below 320°C HNCO eventually becomes the major nitrogen-containing product although it can be hydrolysed by added water. The results demonstrate one possible route for the methane-SCR reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO Oxidation and the CO/NO Reaction on Pd(110) Studied Using “Fast” XPS and a Molecular Beam Reactor

We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to correlate changes in the rate of CO oxidation and the CO–NO reaction with the coverages of the adsorbates and intermediates on the surface. In the reactor, both reactions exhibit an isothermal “light-off” phenomenon in which the rate autocatalytically increases with time. In the case of the CO oxidation reaction this is due to the desorption of CO which releases extra sites for O₂ dissociation which, in turn, removes more CO, and hence the acceleration. In effect the reaction can be written as 2CO_a + O_2g + 2S → 2CO_2g + 4S, the acceleration coming from the release of extra adsorption sites S, which are involved in the reaction itself. “Fast XPS”, carried out in situ during the course of the reaction, shows domination of the surface by CO_a below 390 K and domination by O_a above that temperature, with a rapid change in surface coverage over a very narrow temperature window. On high surface area samples this acceleration is further reinforced due to a rapid temperature increase because of the highly exothermic nature of the overall reaction. The situation for the CO–NO reaction is broadly similar, except that the surface is dominated by NO at low temperature, not CO which tends to be displaced from the surface by NO. “Light-off” is dictated by the onset of the dissociation of NO_a, which occurs at ～400 K. Once N_a and O_a are formed, N₂O production is immediate and accelerates due to the creation of vacant sites for both NO and CO adsorption, the latter removing O_a as CO_2g. Again, the reaction self-accelerates and there is a rapid change of surface coverage from NO_a to O_a at ～450 K. The overall self acceleration is due to the following overall reaction, 2NO_a + CO_g + S → N₂O_g + CO_2g + 3S, again producing more adsorption sites (S) in carrying out the reaction step. The rate is reduced at high temperature due to domination of the surface by O_a and to the reduced coverages of the molecular species.     

Kinetic Investigation of the Photocatalytic Reduction of Nitric Oxide by Carbon Monoxide at Low Pressure on Silica-Supported Molybdenum Oxide

The kinetics of photoinduced reactions that occur upon UV irradiation (<360 nm) of a MoO₃/SiO₂ catalyst (2.5 wt% Mo) in CO-NO mixtures and CO alone are studied at gas pressures from 0.05 to 2 torr and for CO/NO ratios from 0.3 to 3.0 in the temperature interval 20-150C. The data obtained are consistent with a previously proposed two-stage redox mechanism. In the first stage NO is reduced to N₂O through the reaction CO+2NO CO₂+N₂O, while in the second stage the N₂O formed is further reduced to N₂ via the reaction CO+N₂O CO₂+N₂. The ratio of rate constants for quenching of a transient excited state (Mo⁵⁺-O^-)* by NO and CO molecules is found to be 2.8. The reaction rates decrease with increasing temperature, apparently because of a lower concentration of adsorbed species and/or a reduction of the steady-state concentration of (Mo⁵⁺-O^-)*.     

Reaction mechanism of NO decomposition over alkali metal-doped cobalt oxide catalysts

The kinetics of NO decomposition were investigated over alkali metal-doped Co₃O₄ catalysts. For all the alkali metal-doped Co₃O₄ catalysts tested, the presence of O₂ caused a decrease in the N₂ formation rate with reaction orders between −0.26 and −0.40. The reaction orders with respect to NO were between 1.21 and 1.47, which are higher than unity, suggesting that NO decomposition proceeds via a bimolecular reaction. The observation by in situ Fourier transform infrared (FT-IR) spectroscopy confirmed the presence of nitrite (NO₂⁻) species on the surface under NO decomposition conditions. Isotopic transient kinetic analysis performed using ¹⁴NO and ¹⁵NO revealed that a surface-adsorbed species, probably NO₂⁻, serves as an intermediate during NO decomposition. We proposed a reaction mechanism in which the reaction is initiated by NO adsorption onto alkali metals to form NO₂⁻ species, which migrates to the interface between the alkali metals and Co₃O₄, the active sites, and then react with the adsorbed NO species to form N₂.     

Catalytic effect of biomass ash on CO, CH4 and HCN oxidation under fluidised bed combustor conditions

In fluidised bed combustion heterogeneous reactions catalysed by the bed material, CaO, and char are significant for the emission levels for instance of NO, N₂O, and CO. The catalysts present in the bed affect significantly the selectivity of HCN and NH₃ oxidation, which are known as precursors of NO_x (i.e. NO and NO₂) and N₂O emissions from solid fuel combustion. Thus the catalytic activity of biomass ashes may also be responsible for the negligible N₂O emissions from biomass combustion due to the presence of a large amount of solids in fluidised bed combustion, homogeneous oxidation may be suppressed within the bed by the quenching of the radicals. For this reason the catalytic oxidation of hydrocarbons and CO on the bed material may be of significance for the total burnout within the fluidised bed combustor.Within this study the effect of different ashes from spruce wood, peat, and for comparison bituminous coal on the oxidation of CH₄, CO, and HCN was studied. The different ashes were shown to have a strong catalytic activity for the oxidation of CH₄, CO, and HCN. In HCN oxidation the selectivity towards NO is high, whereas very little N₂O is formed. The activity of the ashes is strongly dependent on the fuel, which may be explained by their composition.The kinetics of the oxidation of CO and HCN in the temperature range relevant for fluidised bed combustion, i.e. 800-900 °C, has been evaluated for spruce wood ash.     

Highly active and stable bimetallic Ir/Fe-USY catalysts for direct and NO-assisted N2O decomposition

The current research investigated N₂O decompositions over the catalysts Ir/Fe-USY, Fe-USY and Ir-USY under various conditions, and found that a trace amount of iridium (0.1 wt%) incorporated into Fe-USY significantly enhanced N₂O decomposition activity. The decomposition of N₂O over this catalyst (Ir/Fe-USY-0.1%) was also partly assisted by NO present in the gas mixture, in contrast to the negative effect of NO over noble metal catalysts. Moreover, Ir/Fe-USY-0.1% can decompose more than 90% at 400 °C (i.e. the normal exhaust temperature) under simulated conditions of a typical nitric acid plant, e.g. 5000 ppm N₂O, 5% O₂, 700 ppm NO and 2% H₂O in balance He, and such an activity can be kept for over 110 h under these strict conditions. The excellent properties of bimetallic Ir/Fe-USY-0.1% catalyst are presumably related to the good dispersion of Fe and Ir on the zeolite framework, the formation of framework Al–O–Fe species and the electronic synergy between the Ir and Fe sites. The reaction mechanism for N₂O decomposition has been further discussed on the temperature-programmed desorption profiles of O₂, N₂ and NO₂.     

Reduction of nitric oxide by carbon monoxide on copper catalysts

An experimental study has been carried out of the kinetics and mechanism of the reduction of NO by reaction with CO on unsupported and supported copper catalysts. At low CO concentrations ($\text{3 <}\text{CO}\text{NO}\text{< 10}$), the kinetics of this reaction exhibit an effective reaction order of unity with respect to NO; hence, the fractional conversion is independent of the reactant level. However, relatively high CO concentrations ($\text{CO}\text{NO}\text{> 10}$) in the gas stream cause reaction inhibition. On the basis of the experimental results a reaction mechanism is proposed involving dissociative chemisorption of nitric oxide. The formation of an isocyanate (NCO) species on the copper surface is responsible for reaction inhibition. This species acts as the precursor for ammonia formation in the presence of water vapor. The dissociative chemisorption step of NO on a copper surface has an activation energy of 9.3 kcal/mole, a value unaffected by such supports as SiO₂ or CuAl₂O₄.     

NO x Reduction by Ethanol on Pd/Sulphated Zirconia

The NO reduction by ethanol was studied on palladium catalyst supported on sulphated zirconia. Temperature programmed desorption of NO and ethanol (TPD) and temperature programmed surface reaction (TPSR) analyses as well as catalytic tests in reducing and oxidizing conditions (O₂ presence), besides diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed the formation of intermediate species during the reaction, such as ethoxy species that reacted forming ethylene. Besides dehydrogenate formed adsorbed acetate species, which than decompose and/or react with hydroxyls of the support. The sulphated zirconia support increased the acid sites with the formation of strong Brönsted sites, favoring the formation of ethoxy species. Acetate species also react with NO adsorbed on Pd forming N₂, N₂O, CO and CO₂. The excess of O₂ favored ethanol oxidation to CO₂, consequently less ethanol was available to react with NO _x .     

The formation of N2O during the reduction of NO by NH3

The process of selective non-catalytic reduction of NO, SNCR, is important for limiting emissions of nitrogen oxides from coal-fired power plants. Such a process has been studied for many years, both in the laboratory and under practical conditions. This work was an attempt at elucidating some of the problems associated with the method when used under circulating fluidized bed (CFB) conditions and in particular, the formation of the N₂O by-product. The NO + NH₃ reaction has been studied in the laboratory, over quartz sand in a heated fixed bed flow reactor. In comparison with a combustion environment, the composition of the gas phase was drastically simplified and limited to NO and NH₃, in nitrogen as the carrier gas, with O2 added in some experiments. The product gases were analyzed for NO, N₂O and NH₃. The effects the following parameters were studied: temperature inside the reactor between 850 and 1250 K, height of the sand bed, NH₃/NO molar ratio over the range 0.54–2.0 and the addition of 1 or 2% of O₂ in volume. Baseline tests with an empty reactor were also made. With no sand in the reactor, the results were both qualitatively and quantitatively different. The sand helped to increase the efficiency of NO reduction, particularly at lower temperatures, but N₂O formation also appeared to be strongly enhanced, except at the highest temperatures. Higher molar NH₃/NO ratios favored NO reduction and N₂O production, both with and without sand. The reduction of NO did not appear to require the presence of O₂, but the introduction of 1% or 2% of O₂ gave some benefit. The results confirmed that under practical conditions more attention should be paid to the role of the bed solids in the SNCR process.     

The selective non-catalytic reduction of nitric oxide using ammonia at up to 15% oxygen

Selective non-catalytic reduction of nitric oxide (NO) using ammonia was studied with up to 15% (by volume) oxygen at 102 kPa. The experiments were conducted in an electrically heated laminar-flow, quartz reactor using mixtures of N₂, O₂, NO, and CO to simulate exhaust gas. The base case condition included 330 ppmv of NO, 495 ppmv of NH₃, and 15% O₂. At a reactor temperature of 1050 K, 77% of the NO was removed. For a lower oxygen concentration of 1%, the NO removal was as high as 98% at 1100 K. The degraded performance at high oxygen concentrations is attributed to increases in the oxidation reactions. A major result of this work was the quantification of the amount of N₂O in the treated gases. For the base case conditions, 21 ppmv of N₂O was measured for a reactor temperature of 1075 K. Increasing the ratio of NH₃ to NO (by increasing the NH₃ concentration) increased the maximum NO removal and decreased the temperature at which this level of NO removal was achieved. For the higher NH₃ concentrations, however, the N₂O concentration increased to as high as 54 ppmv. The oxidation products of ammonia (in the absence of NO) for these conditions were found to include first N₂O beginning at 900 K and then NO beginning at 1050 K. Comparisons between these experimental results and predictions from the Miller and Bowman (1989) model indicate that further enhancements of the model may be necessary to incorporate the features of high oxygen conditions.     

Kinetic modeling of non-hydrocarbon/nitric oxide interactions in a flow reactor above 1,400K

The reduction of nitric oxide by reaction with non-hydrocarbon fuels under reducing conditions at comparatively higher temperature has been studied with a detailed chemical kinetic model. The reaction mechanism consists of 337 elementary reactions between 65 chemical species based on the newest rate coefficients. The experimental data were adopted from previous work. Analyses by comparing existing experimental data with the modeling predictions of this kinetic mechanism indicate that, at comparatively high temperature, apart from the reaction path NO→HNO→NH→N₂, NO+N→N₂ is also prominent. In the presence of CO, NO is partly converted to N by reaction with CO. Based on present model, the reduction of NO at high temperature, which was usually underestimated by previous work, can be improved to some extent. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Kinetic relationship between NO/N2O reduction and O2 consumption during flue-gas recycling coal combustion in a bubbling fluidized-bed

Flue-gas recycling combustion of a sub-bituminous coal and its rapid pyrolysis char at 1120 K has been simulated experimentally in a bubbling fluidized-bed. O₂, CO₂ and H₂O, and NO or N₂O were pre-mixed and fed into the bed together with coal/char particles with the O₂ concentration in the exit gas maintained at 3.5 vol%. Increasing the inlet O₂ concentration, thus increasing the O₂ consumption rate and decreasing the flue-gas recycling ratio, caused the once-through conversion of fuel-bound nitrogen into N₂O to decrease while the conversion to NO to remain unchanged. The in-bed reductions of NO and N₂O were both first order with respect to the respective nitrogen oxide, with the rate constants to increase linearly with the rate of O₂ consumption in the bed and thus also with that of char/volatiles consumption. This finding, which indicated linear increase in the concentrations of reactive species involved in NO/N₂O reduction with the rate of O₂ consumption, enabled consideration that the homogeneous and heterogeneous reduction rates of NO and N₂O were proportional to the consumption rates of O₂ by the volatiles and char, respectively. The rate analysis of the kinetic data revealed the relative importance of burning volatiles and char as the agents for the reduction of NO and N₂O. While the reduction in the gas phase was fully responsible for the NO-to-N₂O conversion, the reactions over the char surface governed the NO-to-N₂ reduction. The volatiles and char had comparable contributions to the reduction of N₂O to N₂. The NO-to-N₂ and N₂O-to-N₂ reductions over the char surface were, respectively, accelerated and decelerated by increasing the H₂O concentration.     

Catalytic reduction of nitrous oxide with carbon monoxide over calcined Co–Mn–Al hydrotalcite

The N₂O catalytic reduction by carbon monoxide over Co–Mn–Al calcined hydrotalcite was studied. The effect of oxygen and that of CO/N₂O molar ratio on the rate of N₂O decomposition was examined. CO strongly enhanced N₂O conversion when O₂ was absent in the feed gas. In the presence of oxygen, carbon monoxide acts as a non-selective reductant thereby inhibiting N₂O destruction. Continuing excess of CO over N₂O without presence of O₂ led to a very slow reduction of the catalyst, which caused noticeable N₂O conversion decrease with progressing catalyst reduction. The simultaneous reduction of N₂O by CO and direct N₂O decomposition took place when CO was limiting reactant (CO/N₂O < 1) but only at temperatures, at which the direct decomposition is possible without presence of reductant. Simple reduction was observed when reactants ratio was CO/N₂O ≥ 1.     

NO decomposition and reduction on Pt/Al2O3 powder and monolith catalysts using the TAP reactor

A systematic study over Pt/Al₂O₃ powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H₂. NO pulsing and NO–H₂ pump-probe experiments demonstrate the effect of catalyst temperature, NO–H₂ pulse delay time and H₂/NO ratio on N₂, N₂O and NH₃ selectivity. At lower temperature (150 °C) decomposition of NO is negligible in the absence of H₂, indicating that N–O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H₂ at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N₂O or recombine to form N₂. In excess H₂, higher temperatures and longer delay times favor the production of N₂. The longer delay enables NO decomposition on reduced Pt with the role of H₂ being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N₂ formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination.     

Kinetics of the reaction of oxygen with carbon monoxide adsorbed on palladium

A study of the kinetics of the reaction of adsorbed carbon monoxide with oxygen on polycrystalline palladium is reported in which a pressure jump method was used to induce transients in the carbon dioxide production. Through an analysis of these transients under a variety of conditions of temperature and oxygen pressure, some details of the kinetics have been delineated. At relatively low temperatures and under a significant O₂ pressure, CO(a) is desorbed more readily as CO₂, via the reaction CO(a) + O(a) → CO₂, than as CO. The reaction is first order in oxygen and the rate is limited by the rate of adsorption of oxygen onto sites which are in close proximity to CO(a). Oxygen adsorption at sites which are further than a critical distance from CO(a) are unreactive. The critical distance increases with temperature reflecting increased mobility. Under conditions where both CO(a) and O(a) are significant and both CO(g) and O₂(g) are small the rate is limited by the mobility of CO(a) and/or O(a). The amount of CO(a) during the course of the steady-state oxidation reaction can be determined by analyzing the transient CO₂ production which occurs following a pressure jump in carbon monoxide.