The influence of the active component and support nature, gas mixture composition on physicochemical and catalytic properties of catalysts for soot oxidation

Physicochemical and catalytic properties of compositions Fe(Ce)–Mn–O/support (gamma-, theta-, alpha-Al₂O₃, SiO₂ as the support) and Pt/CeO₂/theta-Al₂O₃ for oxidation of soot were characterized. It was established that the phase composition of the initial catalysts depended mainly on the nature of the active component and preparation conditions. Non-isothermal treatment of the soot–catalyst compositions at the temperature up to 1000 °C resulted in a change in the phase composition depending mainly on the final treatment temperature. The catalyst surface area was determined by the support nature. It was established that catalyst activities for oxidation of soot are determined by both catalyst nature and composition of gas mixture. The process of the soot oxidation is thought to involve oxygen from the catalyst surface. The higher proportion of weakly bound surface oxygen, the higher was the catalyst activity. An increase in the oxygen concentration from 5% O₂/N₂ to 15% O₂/N₂ is shown to lead to a decrease of the temperature of the soot oxidation. The influence of the oxygen concentration on the process of soot oxidation becomes weaker in the presence of water vapor. Results showed that the presence of NO in the gas mixture favors a decrease in the oxidation temperature of the soot, the higher being the nitrogen oxide concentration, the more pronounced effect. Introduction of SO₂ in amount of 50 ppm in the gas mixture has no noticeable effect on the process of the soot oxidation. Among the catalysts under study, Fe–Mn–K–O/gamma-Al₂O₃ is most effective to oxidation of the soot at otherwise identical conditions.     

Diesel soot oxidation by nitrogen dioxide,oxygen and water under engine exhaust conditions: Kinetics data related to the reaction mechanism

Experimental studies on diesel soot oxidation under a wide range of conditions relevant for modern diesel engine exhaust and continuously regenerating particle trap were performed. Hence, reactivity tests were carried out in a fixed bed reactor for various temperatures and different concentrations of oxygen, NO₂ and water (300–600 °C, 0–10% O₂, 0–600 ppm NO₂, 0–10% H₂O). The soot oxidation rate was determined by measuring the concentration of CO and CO₂ product gases. The parametric study shows that the overall oxidation process can be described by three parallel reactions: a direct C–NO₂ reaction, a direct C–O₂ reaction and a cooperative C–NO₂–O₂ reaction. C–NO₂ and C–NO₂–O₂ are the main reactions for soot oxidation between 300 and 450 °C. Water vapour acts as a catalyst on the direct C–NO₂ reaction. This catalytic effect decreases with the increase of temperature until 450 °C. Above 450 °C, the direct C–O₂ reaction contributes to the global soot oxidation rate. Water vapour has also a catalytic effect on the direct C–O₂ reaction between 450 °C and 600 °C. Above 600 °C, the direct C–O₂ reaction is the only main reaction for soot oxidation. Taking into account the established reaction mechanism, a one-dimensional model of soot oxidation was proposed. The roles of NO₂, O₂ and H₂O were considered and the kinetic constants were obtained. The suggested kinetic model may be useful for simulating the behaviour of a diesel particulate filter system during the regeneration process.     

Diesel Soot Combustion over Mn2O3 Catalysts with Different Morphologies: Elucidating the Role of Active Oxygen Species in Soot Combustion

Catalytic diesel soot combustion was examined using a series of Mn₂O₃ catalysts with different morphologies, including plate, prism, hollow spheres and powders. The plate‐shaped Mn₂O₃ (Mn₂O₃‐plate) exhibited superior carbon soot combustion activity compared to the prism‐shaped, hollow‐structured and powdery Mn₂O₃ under both tight and loose contact modes at soot combustion temperatures (T₅₀) of 327 °C and 457 °C, respectively. Comprehensive characterization studies using scanning electron microscopy, scanning transmission electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, temperature‐programmed reduction and oxygen release measurements, revealed that the improved activity of Mn₂O₃‐plate was mainly attributed to the high oxygen release rate of surface‐adsorbed active oxygen species, which originated from oxygen vacancy sites introduced during the catalyst preparation, rather than specific surface‐exposed planes. The study provides new insights for the design and synthesis of efficient oxidation catalysts for carbon soot combustion as well as for other oxidation reactions of harmful hydrocarbon compounds.     

SO2对La0.8K0.2Cu0.05Mn0.95O3钙钛矿催化剂氧化碳烟的影响

采用柠檬酸配位法制备K、Cu掺杂的Lu0.8K0.2Cu0.05Mn0.95O3钙钛矿催化剂,运用程序升温氧化(TPO)考察在不同浓度SO2气氛下La0.8K0.2Cu0.05Mn0.95O3催化剂催化氧化模拟碳黑的性能,并用XRD、FFIR和XPS等进行表征.结果表明,催化剂在0～0.1%的SO2气氛中呈现出不同活性,φSO2≤0.05%的气氛可促进催化剂催化氧化碳黑的活性,当φSO2=0303%,催化剂活性最高;引入φSO2≥0.06%时催化剂活性明显下降.XPS说明表面活性氧的增加是低浓度的SO2促进催化活性的原因,同时XRD、FTIR结果表明高浓度的SO2所产生的大量SO42-是抑制催化剂活性的原因.     

Pt/Ce0.6Zr0.4O2催化剂催化氧化碳烟过程中活性氧对NO-NO2循环及表面含氧物种的分解作用简

通过在Ce_0.6Zr_0.4O₂载体上浸渍Pt（NO₃）₂制得Pt/Ce_0.6Zr_0.4O₂催化剂,该催化剂在松散接触条件下,于NO+O₂或O₂气氛中均表现出比Pt/Al₂O₃更好的碳烟氧化性能. 进一步研究表明,Pt/Ce_0.6Zr_0.4O₂催化剂中的Pt 与Ce_0.6Zr_0.4O₂存在相互作用,使得催化剂在一定温度范围内对活性氧的利用率大为提高,从而促进了气氛中NO↔NO₂的循环,乃至碳烟与NO₂的反应和碳烟表面含氧中间物种的生成;更重要的是,这部分活性氧本身可加速含氧中间物种的分解. 因此,在NO + O₂的气氛中,Pt/Ce_0.6Zr_0.4O₂催化剂的碳烟起燃温度比Pt/Al₂O₃降低了34 ℃.     

Stationary Kinetics of Light Olefin Oxidation by Hydrogen Peroxide in Aqueous Alkali Solutions in the Presence of Fe(III) Oxide

The steady-state kinetics of ethylene and propylene oxidation by hydrogen peroxide in the presence of Fe(III) oxide in aqueous solutions with the permanent adding of H₂O₂ to the reaction medium was studied. The use of an original method for the study of the steady-state reaction kinetics with gas chromatographic detection of substrate consumption from the gas phase made it possible to estimate the apparent rate constants of ethylene oxidation, the ratio of the rate constants of propylene and ethylene oxidation, the reaction orders with respect to the substrate and oxidant concentration, the dependence of the apparent rate constant of ethylene oxidation on the catalyst weight and on the pH of solution, and the apparent activation energy of the process under condition of substrate distribution between the gas and liquid phases. It was found that the kinetic isotope effect in ethylene oxidation is almost absent when completely deuterated ethylene is used.     

Catalytic combustion of diesel soot over perovskite-type catalyst: Potassium titanates

Potassium titanates with a high crystallinity were successfully prepared by the sol-gel method and characterized by XRD, SEM, and BET surface area measurements. K₆Ti₄O₁₁, K₂Ti₂O₅, K₂Ti₄O₉ were found to have better soot oxidation performance compared with Pt/TiO₂ and CeO₂ based catalysts. K₂Ti₂O₅ may be an excellent candidate for soot oxidation due to its high oxidation activity, water-stability, resistance to sulfur poisoning and economical advantages. Certain amount of NO _x can contribute to the catalytic combustion of diesel over potassium titanates, implying that K₂TiO₅ may be a kind of catalyst for simultaneous removal of NO _x and soot.     

Simulation of the Radiation-Chemical Conversion of Mercury Vapor in Power-Plant Flue Gases in the Presence of Hydrogen Chloride

The effect of HCl on the conversion of mercury vapor in the electron-beam treatment of power-plant flue gases for removing nitrogen and sulfur oxides was investigated. A kinetic scheme for the process consists of the liquid-phase oxidation of Hg by O₃molecules and OH radicals followed by the adsorption of liquid-phase oxidation products on soot particles, which are removed from the gas flow using filters. It was found that almost complete removal of mercury vapor is attained at typical radiation doses and process temperatures at soot concentrations higher than 100 g/m³(STP). At a soot concentration lower than 100 g/m³(STP) and an HCl concentration higher than 50 mg/m³(STP) in the gas phase, biologically active HgCl₂is formed in considerable amounts.     

In Situ Oxidation of Nb22O54 in a Gas Reaction Cell High-Resolution Transmission Electron Microscope

A nonstoichiometric block structure oxide, Nb₂₂O₅₄ (NbO_2.455), with monoclinic symmetry was characterized following in situ oxidation in a controlled environment, high-resolution electron microscope. This instrument is based on a JEOL 4000EX electron microscope equipped with a gas reaction cell. The oxidation reaction was carried out by introducing ca. 20 mb of oxygen gas to the specimen region and heating the specimen by means of a focussed electron beam. The resulting structures obtained after the oxidation reaction were not totally homogeneous. (1) Mainly microdomains of Nb₁₀O₂₅ were found, which consists of [3×3] blocks of octahedra linked through tetrahedrally coordinated sites—such a structure is isostructural with PNb₉O₂₅. Nb₁₀O₂₅ seems to be a metastable phase in the Nb-O system, and was found here for the first time as extended domains. (2) A highly disordered structure was observed in some areas of the crystal after oxidation, with a corresponding electron diffraction pattern similar to an “X” phase interpreted as two-dimensionally disordered H-Nb₂O₅. Very clear lamellar defects were also found after oxidation.     

Soot formation from C2H2 and C2H4 pyrolysis at different temperatures

A study of the pyrolysis of two hydrocarbons, C₂H₂ and C₂H₄, at different temperatures has been carried out in order to compare their behaviour in terms of soot and gas yields and gas composition. Pyrolysis experiments have been performed in the same conditions for both hydrocarbons: an inlet hydrocarbon concentration of 15,000 ppmv and a temperature range of 1000–1200 °C. For C₂H₂ and C₂H₄ pyrolysis tests, the results present the same trend when increasing the temperature: an increase in soot yield, a decrease in gas yield and a similar evolution of the outlet gases. Comparatively, it can be observed that acetylene is a more sooting hydrocarbon than ethylene for a given temperature. Additionally, the study of soot reactivity with O₂ and NO shows that the soot samples obtained from ethylene show a slightly higher reactivity towards O₂ and NO than the soot samples formed from acetylene.     

Highly Productive Oxidative Biocatalysis in Continuous Flow by Enhancing the Aqueous Equilibrium Solubility of Oxygen

We report a simple, mild, and synthetically clean approach to accelerate the rate of enzymatic oxidation reactions by a factor of up to 100 when compared to conventional batch gas/liquid systems. Biocatalytic decomposition of H₂O₂ is used to produce a soluble source of O₂ directly in reaction media, thereby enabling the concentration of aqueous O₂ to be increased beyond equilibrium solubility under safe and practical conditions. To best exploit this method, a novel flow reactor was developed to maximize productivity (g product L^?1 h^?1). This scalable benchtop method provides a distinct advantage over conventional bio‐oxidation in that no pressurized gas or specialist equipment is employed. The method is general across different oxidase enzymes and compatible with a variety of functional groups. These results culminate in record space‐time yields for bio‐oxidation.     

Selective CO oxidation on a Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in the surface ignition regime: 1. Fine purification of hydrogen-containing gases

Selective CO oxidation in a mixture simulating the methanol steam reforming product with an air admixture was studied over Ru/Al₂O₃ catalysts in a quasi-adiabatic reactor. On-line monitoring of the gas temperature in the catalyst bed and of the residual CO concentration at different reaction conditions made it possible to observe the ignition and quenching of the catalyst surface, including transitional regimes. A sharp decrease in the residual CO concentration takes place when the reaction passes to the ignition regime. The evolution of the temperature distribution in the catalyst bed in the ignition regime and the specific features of the steady-state and transitional regimes are considered, including the effect of the sample history. In selective CO oxidation and in H₂ oxidation in the absence of CO, the catalyst is deactivated slowly because of ruthenium oxidation. In both reactions, the deactivated catalyst can be reactivated by short-term treatment with hydrogen. A 0.1% Ru/Al₂O₃ catalyst is suggested. In the surface ignition regime, this catalyst can reduce the residual CO concentration from 0.8 vol % to 10–15 ppm at O₂/CO = 1 even in the presence of H₂O and CO₂ (up to ～20 vol %) at a volumetric flow rate of ～100 1 (g Cat)^?1 h^?1, which is one magnitude higher than the flow rates reported for this process in the literature.     

Study on photosensitized oxidative desulfurization of thiophene by riboflavin

Photosensitized oxidative desulfurization of thiophene in n-octane/water extraction system with riboflavin as photosensitizer and O₂ in air as oxidant was studied. A 500 W high pressure Hg lamp was used as the light source for irradiation and air was introduced by a gas pump for supplying O₂. The desulfurization yield of thiophene was 85.4% for a 3 h photoirradiation under the conditions with an air flow of 150 mL/min, water/oil ratio of 1:1 and riboflavin concentration of 30 μmol/L. The mechanism of photosensitized oxidation of thiophene is¹O₂ exited from ³O₂ by the addition of riboflavin. Under the above conditions, the photooxidation kinetics of thiophene with O₂/riboflavin is first-order with a rate constant of 0.3277 h⁻¹ and half-time of 1.37 h.     

Catalytic removal of soot particles over MnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> catalysts prepared by the auto-combustion method

Soot removal for exhaust gas from diesel engine has been addressed due to the more stringent legislation and environmental concerns. MnCo₂O₄ catalysts were systematically prepared using glucose as a fuel via the auto-combustion method and applied for soot removal. The as-prepared samples were characterized by X-ray diffraction (XRD), O₂-temperature-programmed oxidation (TPO) reaction and H₂-temperature-programmed reduction reaction (H₂-TPR). The catalytic activities for soot combustion were evaluated by micro activity test (MAT) with a tight contact mode between soot and catalysts. Compared with catalysts prepared by the solid state method without glucose, auto-combustion method in the presence of glucose can decrease the synthetic temperature, avoiding high temperature treatment and sintering. The catalysts prepared with glucose could catalyze soot oxidation effectively and the derived values of T₁₀, T₅₀, and T₉₀ were 326, 408, and 468 °C in a tight contact mode, respectively, showing a significant drop of T₁₀, T₅₀, and T₉₀ by 156, 177, and 178 °C for non-catalytic reaction.     

Catalytic activity of Cu-containing oxide systems with K2CO3 additives in the oxidation of diesel soot by oxygen

It is shown by XRD that mixed oxide phases Cu_0.92Co_2.08O₄ and Cu₄MgO₅ are formed along with the oxides CuO, Co₃O₄, MgO, and CaO under certain conditions. The positive catalytic effect of individual oxides (CuO and Co₃O₄) and mixed oxide systems (CuO-Cu_0.92Co_2.08O₄, CuO-CaO, and Cu-MgO-Cu₄MgO₅) on the oxidation of diesel soot at 280–580°C is established, and a series of catalytic activities CuO-Cu_0.92Co_2.08O₄ > CuO-MgO-Cu₄MgO₅ > CuO-CaO = CuO > Co₃O₄ is revealed. Using TEM, the surface micromorphology of crystallites that form oxide systems is characterized. It is found that a catalytic system’s activity increases as the size and surface smoothness of crystallites diminishes. According to data from X-ray photoelectron spectroscopy, a considerable increase in the concentration of O₂ in soot with CuO and Co₃O₄ additives after its oxidation by oxygen is observed without changing the oxidation state of Cu and Co oxidation. The promoting effect of potassium additives in the form of K₂CO₃ on the investigated catalytic systems during soot oxidation is revealed.     

Development of a TG-FTIR system for investigations with condensable and corrosive gases

A thermogravimetric analyzer and a Fourier-transform infrared (FTIR) spectrometer were combined and redesigned for investigations with corrosive and condensable reactive gases. The standard gas inlet and outlet of the thermogravimetric analyzer were changed in order to heat the gas tubes, which are lead through the flanges, and avoid condensation in these parts of the system. Furthermore, all tubes upstream and downstream of the thermogravimetric analyzer were trace heated up to 180 °C. The gas measuring cell of the FTIR spectrometer was designed such that an optimum compromise between the small flow rates through the thermogravimetric analyzer and a short residence time of the gases in the gas measuring cell could be achieved. The gas supply allows the dosage of different gas compositions containing nitrogen, oxygen, water, NH₃, and NO₂, for example. The system was validated by analyzing the composition of a diesel particulate matter (PM) sample with a temperature-programmed desorption followed by oxidation (TPD/O) experiment, which showed good agreement with the established analysis methods. The reactivity of the PM sample was investigated by temperature-programmed oxidation (TPO) experiments with different reactive gas mixtures of oxygen, water, and NO₂ in nitrogen. By adding NO₂, the soot oxidation started at lower temperatures and the addition of water lead to a shift of the maxima of the carbon oxidation rates to lower temperatures. The ratio of formed CO₂ and CO was shifted to higher values by the addition of NO₂ and water whereby the influence of water was much more pronounced.     

Radiation-chemical treatment of flue gases from thermal power stations for the removal of mercury vapor

The efficiency of radiation-chemical treatment of flue gases from thermal power stations for removing nitrogen and sulfur oxides was examined as applied to the removal of mercury vapor from the gases. A kinetic scheme of the process was developed. It involves the liquid-phase oxidation of Hg by O₃ molecules formed under the action of ionizing radiation on the gas macrocomponents followed by adsorption of the oxidation products at soot particles. It was found that almost complete removal of mercury vapor is attained at typical radiation doses and soot concentrations in the flue gases.     

Oxidation of Finely Dispersed Carbon on Coated Oxide Catalysts

The catalytic activity of soot filters for internal combustion gas exhausts made from synthetic cordierite with a catalytic covering of transition metal oxides (Co₃O₄, MnO₂, CuO, Cr₂O₃) or binary oxides (CuCr₂O₄, CuCo₃O₅, Co₃MnO₆, CuMnO₃) has been studied. The most active catalyst for the oxidation of CO and hexane and for the combustion of soot forming compounds in the exhaust gases is copper chromite. A secondary carrier based on -Al₂O₃ increased the soot capacity of the filters and increased the selectivity the process of combustion of soot to CO₂.     

DTA,TG studies of catalytic oxidation of carbon particles over M2 III O3 (MIII=Al,Cr, Fe,Ni)

Carbonizate as a model soot has been submitted to oxidation using Al₂O₃, Cr₂O₃, Ni₂O₃ and Fe₂O₃ as catalysts in the temperature range from RT up to 1000°C. The results obtained indicate that Fe₂O₃ is the most active catalyst in soot oxidation. However, all the catalysts examined are active in transformation of carbonizate components. It has been shown that DTA and TG methods can be used as fast methods testing the carbonizate oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.