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.     

Kinetics of the oxidation of carbon black by NO2: Influence of the presence of water and oxygen

In a fixed bed reactor, the rate of carbon black oxidation by NO₂ is significant for temperatures above 300°C, leading to NO, CO and CO₂ formation. The presence of O₂ in the feed gas increases the rate of oxidation, as well as the presence of water. A cumulative effect is observed when both water and oxygen are present. An oxygen balance shows that oxygen atoms of water molecules are not consumed. Water acts as a catalyst for the C-NO₂ reaction. A kinetic mechanism in which intermediate nitro-oxygenated species are formed in the presence of NO₂ during an initial step is in agreement with all these observations. Oxygen and NO₂ are able to react with these species at 300°C. A parametric study of the effects of the temperature, NO₂, O₂ and H₂O concentrations was performed. With a one-dimensional model of NO₂ consumption along the thickness of the carbon black bed, kinetic constants were derived and a phenomenological law was proposed, accounting for the effect of the presence of oxygen and water.     

Oxidation mechanism of carbon black by NO2: Effect of water vapour

The oxidation mechanism of carbon by automotive exhaust gasses (O₂, H₂O and NO₂) was investigated in the range of temperature (300–400 °C) corresponding to the operation conditions of the continuous regeneration regime of the soot trap. No carbon gasification occurs when O₂ is injected in absence of NO₂ below 450 °C. However, when O₂ (10 vol.%) and NO₂ (100–600 ppmv) are both present in the gas mixture, two distinct carbon oxidation reactions take place. A direct reaction occurs between NO₂ and the carbon surface along with a co-operative one involving simultaneously O₂ and NO₂. In the latter case, NO₂ promotes the decomposition of the intermediate C(O) complex.Injection of water vapour increases the overall oxidation rate but does not modify the global mechanism. The promoting effect of water is attributed to the intermediate formation of traces of nitric and nitrous acids which enhance the direct reaction C–NO₂ rate. In contrast, water has no measurable effect on the co-operative C–NO₂–O₂ reaction. A relationship between the water promoting effect on the direct oxidation rate and the amount of theoretical formation of HNO₂ and HNO₃ was established.     

Kinetics of catalyzed and non‐catalyzed soot oxidation with nitrogen dioxide under regeneration particle trap conditions

BACKGROUND: For compliance with the regulations on diesel particulate matter, car manufacturers have developed diesel particulate filters (DPF). These technologies require a regeneration method which oxidizes soot deposits in the filter. In diesel exhaust emissions there are two suitable oxidizing gases: oxygen and nitrogen dioxide. Nitrogen dioxide is much more active than O₂ and can directly attack the carbon surface. This work describes the kinetics of the oxidation of soot by NO₂ over a wide range of conditions relevant for DPF. RESULTS: The catalyzed and the non‐catalyzed oxidation of soot have been performed in a fixed‐bed reactor. The experimental results show that the overall oxidation process can be described by two additive parallel reactions: a direct C ? NO₂ reaction catalyzed by H₂O and a cooperative C ? NO₂ ? O₂ reaction catalyzed by the Pt/Al₂O₃ catalyst. The results obtained allow to propose the following kinetic law for the specific rates of the catalyzed and the non‐catalyzed oxidation of soot in the regeneration filter conditions: CONCLUSION: The kinetic parameters describing the oxidation rate of soot by NO₂ over a range of temperature and gas composition have been obtained. The extracted kinetics data are relevant for modeling the removal of trapping soot in automotive gas exhaust technology. Copyright © 2009 Society of Chemical Industry     

Effect of pretreatment and regeneration conditions of Ru/γ-Al2O3 catalysts for N2O decomposition and/or reduction in O2-rich atmospheres and in the presence of NOX, SO2 and H2O

The effect of the pretreatment (inert, oxidative, and reducing) of Ru/γ-Al₂O₃ catalyst on its activity and stability in the decomposition of N₂O in the absence or presence of O₂, SO₂, H₂O and NO_X was studied in the present work. Decomposition of pure N₂O was slightly enhanced by the H₂-pretreated catalyst (metallic Ru) compared to the O₂- or He-pretreated ones, owing to a cyclic oxidation–reduction pathway of metallic Ru. The observed decrease of activity by O₂ or H₂O addition was reversible compared to SO₂ which caused a strong, irreversible deactivation of the catalyst, irrespective of the type of pretreatment. This was attributed to the formation of stable sulphates, mainly those on RuO₂ surface, which could only be removed by regeneration under reducing (H₂ in He) atmosphere at temperatures of ca. 500 °C. Oxidative or inert regeneration required very high temperatures (i.e. >700 °C) in order to decompose these sulphates. A method of retaining N₂O conversion activity very high (≥98%) for long reaction times is suggested and is based on frequent and short-time (ca. 10 min) regenerations of the catalyst under reducing atmosphere (ca. 5% H₂ in He). The effect of co-feeding various reducing agents, such as CO or C₃H₆, on the N₂O conversion activity in the presence of O₂, SO₂, H₂O and NO_X is negligible, mainly because they are oxidized at relatively low temperatures in the O₂-rich feeds used in this study.     

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.     

Preferential CO Oxidation in the Presence of H2, H2O and CO2 at Short Contact-times

High selectivities and conversions in the preferential oxidation of CO in the presence of large quantities of H₂, H₂O and CO₂ are demonstrated on noble metal catalysts at millisecond contact times (~10–15 ms) for temperatures between 150 and 500 °C. With a simulated water-gas shift product stream containing 0.5% CO and varying amounts of H₂, H₂O and CO₂, we are able to achieve ~90% CO conversions on a Ru catalyst at temperatures of ~300 °C using a stoichiometric amount of O₂ (0.25%). Experiments with and without O₂ and with varying H₂O reveal that significant water-gas shift occurs on Pt and Pt-ceria catalysts at temperatures between 250 and 400 °C, while significant CH₄ is formed on Ru and Rh catalysts at temperatures greater than 250 and 350 °C, respectively. The presence of H₂O blocks H₂ adsorption and allows preferential CO oxidation at higher temperatures where rates are high. We propose that a multistage preferential oxidation reactor using these catalysts can be used to bring down CO content from 5000 ppm at the reactor entrance to less than 100 ppm at very short contact-times.     

Partial Oxidation of Methane to Synthesis Gas Over Ru-Loaded Y2O3 Catalyst

Ru-loaded Y₂O₃ catalyst was investigated for the partial oxidation of methane to synthesis gas. Ru(0.5 wt%)/Y₂O₃ catalyst afforded a high CH₄ conversion of 27% at a CH₄:O₂ ratio of 5 to give nearly a 1:2 ratio of CO and H₂ with a selectivity of 75% at 873 K. Ru(0.5 wt%)/Y₂O₃ catalyst maintained high catalytic activity over 10 h in the partial oxidation of methane. Carbon deposition of the catalyst surface in the reaction of CH₄ was examined by thermogravimetric analyses, and it was found that no carbon deposition occurred on the Ru(0.5 wt%)/Y₂O₃ catalyst. The synthesis-gas production proceeded basically via a two-step reaction consisting of methane combustion to give H₂O and CO₂, followed by the reforming of methane from CO₂ and steam.     

Studies on the Effect of Physico‐Chemical Soot Properties and Feed Gas Composition on the Kinetics of Soot Oxidation on Fe2O3 Catalyst

Three commercial carbon black samples as well as self‐made C₃H₆ soot were investigated for their reactivity in the oxidation on an α‐Fe₂O₃ catalyst. These studies were performed by temperature programmed oxidation (TPO) using a packed bed. For reference purposes, TPO studies in the absence of the catalyst were made as well. The carbon black samples were characterized towards the content of C, H, N and O as well as higher heating value, specific surface area, moisture and volatile matter and were deemed to be suitable model substances for diesel soot of different maturity. The correlation of these physico‐chemical properties with the kinetics in catalytic TPO indicated that the soot oxidation on Fe₂O₃ is significantly affected by the initial number of surface oxygen compounds of the soot. The decomposition of these surface species causes the formation of active carbon sites, which are supposed to accelerate the soot oxidation.     

Catalytic Oxidation of Methanol by Aqueous POM on Al2O3 Supported Catalysts and Electrochemical Performance of POM

Phosphomolybdic acid (H₃PMo₁₂O₄₀, POM) was attempted to be used as the energy‐storage agent in this paper to avoid some problems of the direct methanol fuel cell (DMFC), such as catalyst poisoning and methanol permeation. Catalytic oxidation of methanol by aqueous POM on Al₂O₃ supported catalysts with Pt and Ru active metal was evaluated in the presence of liquid water. The process takes advantage of the high catalytic activities of platinum for methanol oxidation. The effects of temperature, reaction time, and methanol concentration on activity were observed. The catalytic activity of Pt/Al₂O₃ is better than that of Ru/Al₂O₃ for the oxidation of methanol by POM. The methanol conversion rate reached 93.55% on the Pt/Al₂O₃ at 80 °C after reaction for 1 h. The electrochemical experiments indicate that POM shows a larger current density in redox processes on an Au electrode than methanol. The redox process of reduced POM is a reversible multi‐electron transfer process.     

Highly stable Fe/γ‐Al2O3 catalyst for catalytic wet peroxide oxidation

BACKGROUND: A highly stable Fe/γ‐Al₂O₃ catalyst for catalytic wet peroxide oxidation has been studied using phenol as target pollutant. The catalyst was prepared by incipient wetness impregnation of γ‐Al₂O₃ with an aqueous solution of Fe(NO₃)₃· 9H₂O. The influence of pH, temperature, catalyst and H₂O₂ doses, as well as the initial phenol concentration has been analyzed. RESULTS: The reaction temperature and initial pH significantly affect both phenol conversion and total organic carbon removal. Working at 50 °C, an initial pH of 3, 100 mg L^?1 of phenol, a dose of H₂O₂ corresponding to the stoichiometric amount and 1250 mg L^?1 of catalyst, complete phenol conversion and a total organic carbon removal efficiency close to 80% were achieved. When the initial phenol concentration was increased to 1500 mg L^?1, a decreased efficiency in total organic carbon removal was observed with increased leaching of iron that can be related to a higher concentration of oxalic acid, as by‐product from catalytic wet peroxide oxidation of phenol. CONCLUSION: A laboratory synthesized γ‐Al₂O₃ supported Fe has shown potential application in catalytic wet peroxide oxidation of phenolic wastewaters. The catalyst showed remarkable stability in long‐term continuous experiments with limited Fe leaching, < 3% of the initial loading. Copyright © 2010 Society of Chemical Industry     

Improving the Activity of Rh/Al2O3 Catalyst for NO Reduction by Na Addition in the Presences of H2O and O2

The effect of Na addition on the performance of Rh/Al₂O₃ catalyst for NO reduction with CO in the presence of H₂O and O₂ was investigated. The reacted catalysts were analyzed by the FTIR technique to identify the products for further investigation on the possible catalytic reaction mechanisms and the reasons behind the H₂O poisoning. Experimental results show that the removal efficiency of NO by Rh/Al₂O₃ catalyst was 63% at 250 °C but that decreased as the H₂O content increased. Adding Na to modify the Rh/Al₂O₃ catalyst significantly enhanced the conversion of NO to 99% at 250–300 °C even as the H₂O content was 1.6 vol%. The FTIR analyses results reveal that the abundant H₂O in the flue gas can compete with NO to adsorb on the surfaces of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts and further enhance the formation of NO₃ that reacts with H. The effects of H₂O on Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts can be eliminated by increasing the reaction temperature to higher than 300 °C. Rh-Na/Al₂O₃ is a feasible catalyst for NO reduction at such condition with relative high H₂O and O₂ contents.     

Comparison of the activity of Ru and Pt catalysts for the oxidation of carbon by NO2

The role of two catalysts Pt/Al₂O₃ and Ru/NaY on the oxidation of carbon by NO₂ was investigated in the temperature range 300–400 °C. In the case of Pt/Al₂O₃ no significant catalytic effect on the carbon oxidation rate is observed although decomposition of NO₂ takes place on the noble metal and leads to the formation of NO. This result suggests that the amount of the oxygen atoms transferred from the metallic surface sites to the carbon surface to form C(O) complex is negligible. In contrast, in presence of Ru/NaY the oxidation rate of carbon by NO₂ is markedly increased. Hence, a significant part of the formed O through catalytic decomposition of NO₂ on Ru surface sites is transferred to the carbon surface leading to a larger amount of C(O) complexes on the carbon surface. Thus, the ruthenium surface is a generator of active oxygen species that are spilled over on the carbon surface at 350 °C.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

An exploratory study of diesel soot oxidation with NO2 and O2 on supported metal oxide catalysts

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.     

Characteristics of Oxidation of Diesel Paticulate Matter over a Spinel Type Cu0.95K0.05Fe2O4 Catalyst

This paper presents an experimental study on oxidation of diesel paticulate matter (PM) and aims at investigating the characteristics of PM oxidation. The experiments were performed over a Cu_0.95K_0.05Fe₂O₄ catalyst which is attributed to a spinel type metal oxide. The effects of O₂ on PM oxidation as well as on NO_x reduction were studied and the roles of O₂ in PM oxidation and in NO_x reduction, respectively, are discussed. During the temperature‐programmed oxidation of PM, SOF oxidation and soot oxidation lead to two CO₂ peaks at different temperatures. It was found that the presence of O₂ benefits PM oxidation but suppresses the reduction of NO_x into N₂ by consuming the soot. This study revealed that the appearance of PM oxidation is different from that of soot oxidation. The mechanisms on PM oxidation and NO_x reduction are discussed.     

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.     

Potassium‐catalyzed steam gasification of ash‐free lignite coal with CO2 capture

The purpose of this research was to study steam gasification of ash‐free coal integrated with CO₂ capture in the presence of a K₂O catalyst for enhancement of the key water‐gas shift reaction and promotion of hydrogen production. To achieve this goal, gasification experiments on ash‐free coal (AFC) were carried out at varying temperatures (600, 650, 675, 700, and 750 °C) with a sorbent‐to‐carbon (CaO/C) ratio of 2 and a catalyst (K₂O) loading of 0.2 g/g (20 weight percent (wt%)) in a fixed‐bed reactor equipped with a gas chromatography analyzer. The sorbent‐to‐carbon (CaO/C) ratio of 2 is based on dry and ash‐free basis. The CaO/C ratio and K₂O wt% were chosen to maximize hydrogen production based on our previously determined optimal values. The AFC was originally extracted from raw lignite coal using organic solvents, which allowed the sorption‐enhanced gasification to be conducted with minimal ash‐catalyst interactions. The effect of temperature on the yield and the initial reaction rate were investigated. The optimal reaction temperature of 675 °C was determined. Carbon balance and final carbon conversions were calculated based on the residue analysis. Activation energy was also calculated using intrinsic kinetics of the reaction. In this study, using AFC offered the potential advantage of operating the gasification process with catalyst recycle.     

Application of NOX storage/release materials based on alkali-earth oxides supported on Al2O3 for high-temperature diesel soot oxidation

Alkali-earth oxides and nitrates supported on alumina were studied as model systems for NO_X storage/release. Their impact on the high-temperature soot oxidation has been investigated. The stability of surface nitrates and temperature of NO_X release increase parallel to the basicity of the cation. The presence of soot decreases the temperature of NO_X release. The storage capacity depends on the several factors, such as basicity, dispersion of the cation, and pre-treating conditions. Adsorption of NO with O₂ at 200 °C leads to the formation of surface nitrates that mainly exist as ionic nitrates. Stored nitrates contribute to the soot oxidation and assist to lower the temperature of soot oxidation up to almost 100 °C. In the presence of only NO_X storage material the efficiency of NO_X utilization is, however, quite low, around 30%. Therefore, the presence of an oxidation catalyst is essential to increase the efficiency of NO_X utilization for soot oxidation up to 140% and selectivity towards CO₂. A combination of oxidation catalyst with NO_X storage materials enables to lower the temperature of soot oxidation more than 100 °C for the Sr- and Ca-based systems.     

Temperature‐Programmed Oxidation of Soot in a Hybrid Catalysis‐Plasma System

Non‐thermal plasma (NTP) technology was applied to promote the temperature‐programmed oxidation (TPO) of soot over a perovskites type of La_0.8K_0.2MnO₃ catalyst. The O radicals originating from the decomposition of O₂, as well as NO dissociation if nitrogen oxide were involved, reduce the ignition temperatures of soot. In NO‐O₂‐He, for example, the ignition temperature decreased to 240 °C from 290 °C as the voltage increased from 0 kV to 15 kV. The higher voltage also benefited the adsorption of NO molecules onto the catalyst surface (NO_ad). As a result, the maximum N₂/NO ratio (conversion ratio of NO into N₂) rose from 23 % to 53 %. Some of the NO molecules were dissociated into N and O radicals in plasma, and hence, the N₂/NO ratio was further enhanced due to the combination of N atoms. In any case, the redox process between NO_x and soot proved to be important in soot oxidation.