Competitive no,co and hydrocarbon oxidation reactions over a diesel oxidation catalyst

The oxidation of NO, CO and hydrocarbons (HC) individually, in mixtures, and with NO₂ were investigated over a monolith‐supported Pt/Al₂O₃ catalyst under oxidising conditions. Although competitive adsorption and inhibition by other species on oxidation reactions is a relatively well‐known phenomenon, this study represents a more comprehensive examination of such effects between key components in vehicle exhaust gases. NO₂ was completely reduced by CO and C₃H₆, under NO₂ limited conditions, at temperatures as low as 110°C and at temperatures above 140°C with dodecane and m‐xylene. NO₂ was then again observed once the extent of oxidation of the other species by oxygen was significant. Under the conditions tested, NO, CO and HC oxidation was inhibited by NO₂ in the feed gas mixture. HC were also found to inhibit the oxidation of NO and other HC species due to site adsorption competition. For CO, HC did not change the onset of oxidation, but did inhibit the extent after their light off began. At low temperatures, CO was initially found to inhibit NO oxidation, but at higher temperatures, once CO oxidation was significant, CO promoted NO conversion to NO₂. The observed inhibition effects of the different gases on HC oxidation were not additive, indicating one species would cause inhibition, but once its inhibition ended, another species could still then cause inhibition. The combined effect of C₃H₆, NO and NO₂ on CO conversion was found to be additive. This is because CO oxidation started prior C₃H₆. © 2011 Canadian Society for Chemical Engineering     

Detailed kinetic modeling of NOx adsorption and NO oxidation over Cu-ZSM-5

Detailed kinetic modeling was used in combination with flow reactor experiments to investigate the NO_x adsorption/desorption and NO oxidation over Cu-ZSM-5. NO oxidation is likely an important step for selective catalytic reduction (SCR) using urea and hydrocarbons, and thus was investigated separately. First the NO₂ adsorption on Brönstedt acid sites in H-ZSM-5 was modeled using an NO₂ temperature programmed desorption (TPD) experiment. These results, together with the results of the NO₂ TPD and NO oxidation experiments, were used in developing the model for Cu-ZSM-5. A substantial amount of NO₂ was adsorbed on the catalyst. However, the results from a corresponding NO TPD experiment showed that only very small amounts of NO were adsorbed on the catalyst and therefore this step was not included in the model. The model consists of reversible steps for NO₂ and O₂ adsorption, O₂ dissociation, NO oxidation and two steps for nitrate formation. The first nitrate formation step was disproportionation of NO₂ to form NO and nitrates. This step enabled us to describe the NO production during NO₂ adsorption. Further, in the reverse step the NO reacts with the nitrates and decreased their stability. Without this step the nitrates blocked the surface resulting in to low NO oxidation activity. However, we observe that nitrates can be decomposed also without the presence of NO and in the second reversible step were the nitrates decomposed to form NO₂ and oxygen on the copper. These steps enabled us to describe both the TPD and activity measurement results. NO oxidation was observed even at room temperature. Interestingly, the NO₂ decreased when increasing the temperature up to 100 °C and then increased as the temperature increased further. We suggest that this low-temperature NO oxidation occurs with species loosely bound on the surface and that is included in the detailed mechanism. An additional NO₂ TPD at 30 °C was also modeled to describe the loosely bound NO₂ on the surface. The detailed model correctly describes NO₂ storage, NO oxidation and low-temperature NO oxidation.     

Catalytic oxidation of nitric oxide over modified carbide slag: Experimental and theoretical studies

In this work, experimental and theoretical methods were used to investigate the catalytic effect of K₂O on NO oxidation. Experimental results indicated that K₂O was the main active component. It enhanced the removal of NO and decreased the reaction temperature. Theoretical results indicated that the K₂O (001)-O surface was more suitable for the adsorption of NO and O₂. Also, NO and O₂ on the Ca(OH)₂ surface may migrate to the K₂O surface for reaction. NO₂ was formed via the interaction of (NO)₂ and O atoms. The formation of ON NO was the key step for NO oxidation, which was more conducive to the subsequent oxidation of NO.     

Catalytic reduction of NO by hydrocarbons over a mechanical mixture of spinel Ni–Ga oxide and manganese oxide

NO reduction with propylene over Mn₂O₃, spinel Ni–Ga oxide and their mechanical mixtures has been investigated. Mn₂O₃ has no activity to NO reduction, but has a high activity for NO oxidation to NO₂. Spinel Ni–Ga oxide showed an apparent activity to NO reduction only at temperatures above 400°C. Mixing of Mn₂O₃ to the Ni–Ga oxide resulted in a significant enhancement of NO reduction in the temperature range of 250–450°C. The optimal Mn₂O₃ content in the mixture catalyst was about 10–20 wt%. It is suggested that the synergetic effect of Mn₂O₃ and Ni–Ga oxide plays an important role in the catalysis of NO reduction. The Ni–Ga oxide and Mn₂O₃ mixture catalyst is superior to Pt/Al₂O₃ and Cu-ZSM-5 by showing a higher NO reduction conversion, resistance to water and negligible harmful by-product formation. Other lower hydrocarbons C₂H₄, C₂H₆ and C₃H₈ also give a maximum NO reduction conversion as high as 50%. The difference from using C₃H₆ is that the temperature at the maximum NO reduction is higher than it is with C₃H₆. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pt–Ce-soot generated from fuel-borne catalysts: soot oxidation mechanism

Soot containing Ce-, Pt-, Pt–Ce-, Fe-, and Cu-fuel-borne catalysts is generated in a diesel engine, is characterised by XRD, and studied in oxidation with O₂ and NO + O₂ under various reaction conditions. Fe-, Pt–Ce- and Ce-soot are oxidised at lower temperature with O₂, compared with Pt-soot, and the opposite trend is observed with NO + O₂. NO is oxidised to NO₂ more efficiently over Pt-soot and decreased the soot oxidation temperature by about 150 °C, compared with Ce-, Fe- or Pt–Ce-soot. On the other hand, NO₂ is most efficiently utilised over the Pt–Ce- and Ce-soot. The soot oxidation under the different feed gas conditions demonstrates that nitrate species are involved in the oxidation of Ce- and Pt–Ce-soot. The oxidation species with the decreasing order of activity are: (1) nitrates, (2) NO₂, (3) lattice oxygen, and (4) gas-phase oxygen. All the above species are involved in the oxidation of Pt–Ce-soot.     

The Effect of a Changing Lean Gas Composition on the Ability of NO2 Formation and NOx Reduction over Supported Pt Catalysts

Three model catalysts (Pt/Al₂O₃, Pt/TiO₂, Pt/V₂O₅/TiO₂) were examined in regard to their NO₂ formation ability under a changing lean gas composition. The results show that the NO to NO₂ oxidation function as well as the NO _x reduction under lean gas conditions is affected by a change in the lean gas atmosphere. The NO oxidation activity also decreased with time, for Pt/Al₂O₃ and Pt/TiO₂, and a possible explanation may be platinum oxide formation. This deactivation was not observed for Pt/V₂O₅/TiO₂.     

Basic rules of NO oxidation by Fe2+/H2O2/AA directional decomposition system

Basic rules of NO oxidation by a Fe²⁺/H₂O₂/AA directional decomposition system were researched based on the technical background of flue gas NO_x removal. Effects of gas‐liquid interfacial area, main gas, and solution parameters on NO oxidation efficiency (η) were analyzed. The results showed that adequate contact area was the precondition for high η by a Fe²⁺/H₂O₂/AA system. η decreased with the increase in NO concentration, which illustrated that this method would be efficient in oxidizing NO at a low concentration. η tended to decrease linearly with the growth in gas flow, however, the NO oxidation rate (v) rose with the increase in NO concentration and gas flow. η increased with the initial concentrations of H₂O₂ and Fe²⁺, but the amplitude decreased. Controlling the initial concentrations of H₂O₂ and Fe²⁺ to achieve reasonable synergies between generation rate and consumption rate of ·OH could weaken the invalid consumption of reactants. η increased with the increase in temperature in the range 30–60 °C, but it nearly did not change with temperature after 60 °C. This oxidation technology and the traditional wet flue gas desulphurization technology exhibited temperature synergy. Under typical pH of wet desulphurization, η and H₂O₂ consumption rate did not change obviously.     

The influence of aluminum phosphates on graphite oxidation

Aluminum phosphates are of interest for use as inhibitors for the oxidation of carbonaceous materials. This investigation analyzes three aluminum phosphates of nominal compositions, Al₂O₃ · P₂O₅, Al₂O₃ · 3P₂O₅, and Al₂O₃ · 9P₂O₅, in terms of thermal stability and efficacy to inhibit the thermal oxidation of graphite flake in pure oxygen. Temperature programmed oxidation reveals that the onset temperature for oxidation is increased by 75-100 °C as a result of the aluminum phosphate treatments. Isothermal oxidation rate measurements show that while the overall oxidation rate constants are lowered by the aluminum phosphate treatments, the apparent activation energy remains constant (54-59 kcal/mol), which indicates that the reduction in rate constant with temperature is due to a lowering of the pre-exponential term.     

The role of acid sites in cobalt zeolite catalysts for selective catalytic reduction of NOx

The role of the acidic support in ion-exchanged cobalt-zeolite, lean NO_x catalysts has been determined by studying the individual steps in the selective reduction pathway. At a GHSV of 10,000 and reaction temperatures below 400°C, NO oxidation is not sufficiently rapid to obtain equilibrium over, for example, 1–4 wt% Co-mordenite catalysts. The NO oxidation rate increases in the order H⁺Co²⁺ Co oxide, and neither the number, nor the strength of the acid sites affects the specific rate of the Co²⁺ ions. For reduction of NO₂ by propylene at 300°C and methane at 400°C, the formation of N₂ is suggested to occur at support protons sites. In addition, the rate of N₂ formation increases linearly with an increase in the number of acid sites, and the specific activity increases with an increase in acid strength. Cobalt (2+) ions do not contribute significantly to the formation of N₂, but do non-selectively reduce NO₂ to NO. It is proposed that the formation of N₂ occurs by protonation of the reducing agent followed by attack of the carbocation by gas phase NO₂. Thus, the selective reduction of NO requires two catalytic functions, metal and acid sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Removal of Nitric Oxide in a Pulsed Corona Discharge Reactor

Conversion of nitric oxide (NO) in a pulsed corona discharge reactor was investigated. Relative importance of each of the active species such as O, OH, HO₂ and O₃ produced by corona discharge was evaluated with respect to the oxidation of NO to NO₂. Of those species, O₃ was found to be the most important one in the oxidation of NO. In order to reduce the energy required to convert NO, olefins (ethylene, propylene) were used as additive, and the scheme for the oxidation of NO promoted by the hydrocarbon was discussed. In the presence of hydrocarbon, ozone also played very important role in the reaction mechanism. The concentration of ozone was measured at the reactor outlet to verify the importance of ozone in the oxidation chemistry. Compared to ethylene, propylene gave much better performance in the conversion of NO at the same specific energy. When propylene was added to the gas stream at identical amount to initial NO_x (300 ppm), 60 % of NO could be converted with a specific energy of only 2.6 Wh m^–3.     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.     

UV/H2O2氧化联合Ca(OH)2吸收同时脱硫脱硝

在小型紫外光-鼓泡床反应器中,对UV/H₂O₂氧化联合Ca(OH)₂吸收同时脱除燃煤烟气中NO与SO₂的主要影响因素[H₂O₂浓度、紫外光辐射强度、Ca(OH)₂浓度、NO浓度、溶液温度、烟气流量以及SO₂浓度]进行了考察。采用烟气分析仪和离子色谱仪分别对尾气中的NO₂和液相阴离子作了检测分析。结果显示:在本文所有实验条件下,SO₂均能实现完全脱除。随着H₂O₂浓度、紫外光辐射强度和Ca(OH)₂浓度的增加,NO的脱除效率均呈现先大幅度增加后轻微变化的趋势。NO脱除效率随烟气流量和NO浓度的增加均有大幅度下降。随着溶液温度和SO₂浓度的增加,NO脱除效率仅有微小的下降。离子色谱分析表明,反应产物主要是SO₄^2-和NO₃^-,同时有少量的NO₂^-产生。尾气中未能检测到有害气体NO₂。     

Effects of Addition of Na and S to Alumina Catalyst for the Selective Reduction of Nitrogen Monoxide with Ethene in Excess Oxygen

The addition of Na and S into alumina catalysts brought about a decrease in the catalytic activity for the reduction of NO with ethane in excess oxygen. Aluminas containing Na or S in different amounts were subjected to activity tests for the related reactions to elucidate the causes of the suppressive effects of the addition of Na and S on the reduction of NO. The reactions taken as test reactions were the oxidation of NO with oxygen, the reaction of NO₂ with ethene in the absence of oxygen, and the reaction of ethene with oxygen. The addition of Na suppressed the oxidation of NO, and the reaction of NO₂ with ethene to form N₂, but promoted the reaction of ethene with oxygen to a great extent. The addition of Na also caused the formation of NO in the reaction of NO₂ with ethene. The changes which the addition of Na brought about are all unfavorable directions for the reduction of NO. The most important effect of the addition of Na on the decrease in the reduction of NO is suggested to be due to the enhancement of the reaction of ethene with oxygen. The addition of S suppressed the oxidation of NO to a great extent, but did not affect much the reaction of ethene with oxygen. Like the case of the Na addition, the addition of S caused the formation of NO in the reaction of NO₂ with ethene.     

Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration

The so-called 4-way catalytic converter (4WCC) has the ability to simultaneously convert CO, HC, NOx and particulate matter on a single support. It allows diesel vehicles to obey to increasingly stringent emission regulations while at the same time decreasing the space needed by the exhaust aftertreatment system. It is combined with fine engine control strategies so as to ensure conversion of all pollutants. It is hence associated with a large number of catalytic reactions which interact with each other and compete for active sites. The behavior of a commercial 4WCC was characterized on a synthetic gas bench. Gas composition, temperatures and gas hourly space velocity were chosen close to real engine operating conditions. Samples were loaded with soot on an engine bench test. Oxidation reactions were dominant in a lean environment: CO oxidation by NO₂ at low temperature followed by H₂, CO, NO and HC oxidation by O₂. NOx were stored on barium storage sites. In rich conditions H₂, CO and HC were used to reduce NOx. NH₃ production from H₂ was also observed. It could be used to further reduce NOx in lean conditions if stored on a downstream SCR system like in the Honda system [1]. A further conversion of HC was obtained at high temperature due to steam reforming. Interactions and inhibitions were also found. NOx storage appeared to be inhibited by CO oxidation with NO₂ at low temperatures and also by HC, maybe through competition for storage sites with CO₂ produced during HC oxidation. Catalytic reactions were affected by the soot deposit. Continuous oxidation of soot by NO₂ also induced a slower NOx storage rate.     

NH3 oxidation catalysed by calcined limestone—a kinetic study

The oxidation of NH₃ catalysed by calcined limestone was studied in a small fixed bed reactor at fluidized bed combustion temperatures (750-950 °C) using three types of limestones: Faxe Bryozo, Stevns Chalk and Ignaberga. It was shown that the limestones are very active catalysts for the oxidation of NH₃. Experiments were carried out with O₂ concentrations between 0 and 4.5 vol%. At low O₂ concentrations (<1 vol%), a strong increase in the NH₃ oxidation rate was observed with increasing O₂ concentration. At O₂ concentrations above 1 vol% no further effect of increasing the O₂ concentration was observed.The selectivity for NO formation found in most experiments is about 0.65-0.70 for all limestones used when the O₂ concentration is above 0.5 vol%. This means that the oxidation of NH₃ to NO is about two times faster than the oxidation to N₂. Below 0.5 vol% O₂, the selectivity for NO decreases with decreasing O₂ concentration. The selectivity for NO is not influenced by inlet concentrations of NH₃ or NO.The experimental observations are explained by a reaction scheme initially proposed by de Soete [Proc. of the 5th International Workshop on Nitrous Oxide Emissions, 1992] and modified in this study.The experiments showed that the NH₃ oxidation is first order in NH₃ and that the reaction rate is not influenced by NO. The influence of O₂ is modelled by a Langmuir model for O₂ adsorption on the CaO surface. Based on the kinetic model, reaction rate expressions for NH₃ oxidation to NO and N₂ were obtained from the experimental data. To our knowledge, this is the first time that validated reaction rate expressions, which can describe the NH₃ oxidation over calcined limestone over a broad O₂ range and a broad temperature range, are reported in the literature. The kinetics obtained in this work may be used to improve the modelling of NO emissions from fluidized bed combustors.     

Impacts of acid gases on mercury oxidation across SCR catalyst

A series of bench-scale experiments were completed to evaluate acid gases of HCl, SO₂, and SO₃ on mercury oxidation across a commercial selective catalytic reduction (SCR) catalyst. The SCR catalyst was placed in a simulated flue gas stream containing O₂, CO₂, H₂O, NO, NO₂, and NH₃, and N₂. HCl, SO₂, and SO₃ were added to the gas stream either separately or in combination to investigate their interactions with mercury over the SCR catalyst. The compositions of the simulated flue gas represent a medium-sulfur and low- to medium-chlorine coal that could represent either bituminous or subbituminous. The experimental data indicated that 5–50 ppm HCl in flue gas enhanced mercury oxidation within the SCR catalyst, possibly because of the reactive chlorine species formed through catalytic reactions. An addition of 5 ppm HCl in the simulated flue gas resulted in mercury oxidation of 45% across the SCR compared to only 4% mercury oxidation when 1 ppm HCl is in the flue gas. As HCl concentration increased to 50 ppm, 63% of Hg oxidation was reached. SO₂ and SO₃ showed a mitigating effect on mercury chlorination to some degree, depending on the concentrations of SO₂ and SO₃, by competing against HCl for SCR adsorption sites. High levels of acid gases of HCl (50 ppm), SO₂ (2000 ppm), and SO₃ (50 ppm) in the flue gas deteriorate mercury adsorption on the SCR catalyst.     

Lean selective catalytic reduction of NO2 by methane over Co‐zeolites

The reaction of NO₂, CH₄ and O₂ was studied using low levels of methane compared to NO₂ and O₂ over protonic and cobalt‐exchanged ferrierite, ZSM‐5 and mordenite zeolites. Results suggest that two reaction pathways at low and high temperatures may be involved in the lean selective catalytic reduction (SCR) of NO₂ by methane. At low temperatures, the reduction of NO₂ to NO and N₂ might be the initial reaction step. It is likely that NO₂ or its adsorbed precursors initiate the reaction of methane at low temperatures. At high temperatures, the oxidation of NO and combustion of methane with oxygen might be involved. No appreciable differences were observed in the reduction of NO₂ over Co‐zeolites as compared to known results of NO reduction over these materials. However, enhanced N₂ formation rate was observed on H‐zeolites starting from NO₂ instead of data reported for NO. Furthermore, it appears that the active sites for SCR are both acid and metal sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Co-adsorption of nitrogen monoxide and nitrogen dioxide in zeolitic de-NOx catalysts

Room temperature adsorption and temperature programmed desorption (TPD) of NO, NO with O₂, and NO after NO₂ saturation were investigated for Na/Y, Na/ZSM5, Cu/ZSM5 and SiO₂ gel. The adsorbates were characterized by TPD, using mass spectrometry to identify the desorbed molecules. In the presence of O₂, the adsorption of NO and NO₂ is non-additive; co-desorption of nearly equal amounts of NO and NO₂ takes place suggesting the formation of a complex with an overall composition N₂O₃ within the zeolite. Since SiO₂ gel does not adsorb NO nor NO₂, the adsorption capacity of the zeolites is a function of their specific structure. The oxidation of NO to NO₂ is catalyzed by Na/Y and Na/ZSM5.     

Catalytic effect of platinum on the kinetics of carbon oxidation by NO2 and O2

The effect of a commercial Pt/Al₂O₃ catalyst on the oxidation by NO₂ and O₂ of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O₂ and NO₂ is observed. However, in presence of NO₂–O₂ mixture, it is found that besides the well established catalytic reoxidation of NO into NO₂, Pt also exerts a catalytic effect on the cooperative carbon–NO₂–O₂ oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO₂ leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications.     

Reduction of nitrogen oxides by ozonization-catalysis hybrid process

Treatment of nitrogen oxides (NO_x) by using a hybrid process consisting of ozonization and catalysis was investigated. The ozonization method may be an alternative for the oxidation of NO to NO₂. It was found that nitric oxide (NO) was easily oxidized to nitrogen dioxide (NO₂) in the ozonization chamber without using any hydrocarbon additive. In a temperature range of 443 to 503 K, the decomposition of ozone into molecular oxygen was not significant, and one mole of ozone approximately reacted with one mole of NO. A kinetic study revealed that the oxidation of NO to NO₂ by ozone was very fast, almost completed in a few tens of milliseconds. When the amount of ozone added was less than stoichiometric ratio with respect to the initial concentration of NO, negligible NO₃ and N₂O₅ were formed. The oxidation of a part of NO to NO₂ in the ozonization chamber enhanced the selective reduction of NO_x to N₂ by a catalyst (V₂O₅/TiO₂), indicating that the mixture of NO and NO₂ reacts faster than NO.