Preparation and characterization of diesel particle filter (DPF) with hierarchical microstructure for catalytic combustion of soot

Mullite whiskers were prepared on the wall of cordierite honeycomb ceramics using the gas-phase growth method. Subsequently, Co/Ce_0.75Zr_0.25O₂ catalyst was loaded on the mullite whiskers to form hierarchical microstructure by the sol-gel technology for catalytic combustion of soot. Scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform-infrared spectroscopy, X-ray diffraction and Transmission electron microscopy were used to characterize the structural morphology and phase compositions. The filtration capability can be improved by mullite whiskers growth on the wall of cordierite honeycomb ceramics, and the microstructural feature is similar to the pseudostratified ciliated epithelium on the tracheal cavity surface (one section of human respiratory system). The Co/Ce_0.75Zr_0.25O₂ catalyst decreased the ignition temperature of soot particles oxidation, and the cycle stability experiment confirmed that Co/CZ@M/C sample has good structural stability and stable catalytic performance. The high-efficiency filtration and the low-temperature catalytic oxidation of soot particles are combined with the hierarchical microstructure of Co/CZ@M/C, which has potential application in the diesel particulate filter (DPF) field.     

Catalytic soot oxidation in microscale experiments

The oxidation of soot agglomerates over catalytically active surfaces is of interest for the development of catalytic reactors for the control of soot emissions. The process involves the transport and deposition of nanoparticle aggregates to a surface on which catalyst particles are deposited. To simulate this process, graphitized carbon nanoparticles and platinum nanoparticles were separately deposited on an oxidized silicon wafer by laser ablation and electro hydro dynamic atomization. Changes in particle morphology produced by the reaction were visualized ex situ by scanning electron microscopy. In this way chemical reaction data could be correlated with the local surface coverage and particle size of the catalytically active material and the morphology of the reacting particles, resulting in detailed local information on their interaction, which is not available in studies on bulk samples. The contact between catalyst and soot particles was loose, simulating the behavior of catalyst systems used in practice. The activation energy of the oxidation in air was found to be 40 kJ/mol whereas the activation energy in air/NO was found to be 160 kJ/mol, both in presence of Pt deposited on a SiO₂ support. Notwithstanding the higher activation energy, the reaction rate of soot oxidation in air/NO is about two to three orders of magnitude higher than in air. A linear relationship between the relative Pt surface and reaction rate was found for the oxidation in an air/NO atmosphere. In air, the relationship has a minimum which indicates that there are different simultaneous mechanisms of reaction. Although activation energies are different from other studies, the oxidation temperatures are comparable. The EHDA and laser ablation produced platinum catalysts behave similarly and show potential to be used as model catalyst.     

Co3O4–CeO2 mixed oxide-based catalytic materials for diesel soot oxidation

Co₃O₄–CeO₂ type mixed oxide catalyst compositions have been prepared by using co-precipitation method and, their catalytic activity towards diesel particulate matter (PM)/carbon oxidation has been evaluated under both loose and tight contact conditions. These catalysts show excellent catalytic activity for PM/carbon oxidation, despite their low surface area. The activation energy observed for non-catalyzed and catalyzed reactions are 163 kJ/mol and 140 kJ/mol, respectively, which also confirm the catalytic activity of catalyst for carbon/soot oxidation. The promotional effects of an optimum amount of cobalt oxide incorporation in ceria and presence of a small amount of potassium appears to be responsible for the excellent soot oxidation activity of this mixed oxide type material. The catalytic materials show good thermal stability, while their low cost will also add to their potential for practical applications.     

Effect of experimental conditions on the parameters used for evaluating the performance of the catalyst Mo/Al2O3 in diesel soot combustion

The literature reported different studies of soot combustion reaction under very distinct experimental conditions, which can include different values of catalyst:soot weight ratios, gas flow and heating rates. Therefore, avoiding screening of innumerable catalysts or empirical experiments, this work aims to present a general methodology based on a statistical experimental design of experiments with soot combustion, evaluating different reaction conditions and parameters that can be used for any other similar study. In this way, the effect of experimental conditions on the parameters used for evaluating the performance of Mo/Al₂O₃, a promising system previously studied, and Pt/Al₂O₃, a notorious catalytic system, were studied by a complete factorial experimental design. The results have shown that the experimental conditions strongly interfere with the parameters used for evaluating the catalytic performance and then it may generate incorrect conclusions.The effects of interaction between different conditions on the activity and mainly on the selectivity of CO₂ permitted to explain the performance of catalysts on soot combustion and to distinguish different pathways of catalytic and non-catalytic reactions under specific reaction conditions.The most appropriate conditions for studying soot combustion seem to be high cat:soot ratios, low heating rates and high gas flow rates, which, according to this work, must be equal to: 95:1, 2 K min⁻¹ and 115 mL min⁻¹, respectively.     

Catalytic combustion of toluene over Fe–Mn mixed oxides supported on cordierite

The catalytic combustion of toluene over Fe–Mn mixed oxides supported on cordierite was investigated. The catalysts were synthesized by the impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and BET specific surface area measurement. The effects of the mole ratio of Fe to Mn, the loading of Fe–Mn mixed oxides on the catalyst support and the calcination temperature were all investigated. The results indicate that Fe–Mn/cordierite catalysts with a 4 mol ratio of Fe to Mn, used with 10 wt% loading, and calcined at 500 ^°C showed the highest catalytic activities as measured by the oxidation of toluene. Compared to unsupported powder catalysts of Fe–Mn mixed oxides, the Fe–Mn/cordierite catalyst showed higher activity for the catalytic combustion of toluene with less active component.     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.     

A screening study on the activation energy of vanadate‐based catalysts for diesel soot combustion

The activation energy of carbon combustion catalysed by alkali vanadates or alkali vanadates/chlorides mixtures is assessed by the Ozawa method. The most active catalyst, Cs₄V₂O₇, entails more than 50% decrease of the activation energy compared to non‐catalytic combustion (from 157 down to 75 kJ/mol). The catalyst performance is enhanced when the catalyst is dissolved in a eutectic liquid (e.g., AgCl + CsCl), which likely improves the catalyst/carbon contact conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

The influence of NOx on the oxidation of metal activated diesel soot

The influence of NO on the oxidation of metal (cerium, copper, and iron)-activated soot was studied. Without NO in the gas phase, the activation energy of soot is ≈170 kJ/mol, independent of the type of metal applied in the soot. The rate-limiting step in the oxidation with oxygen is probably the decomposition of surface oxygen complexes. In presence of NO, the oxidation rate of soot mixed with a supported platinum catalyst is increased significantly, especially for cerium-activated soot. The activation energy of the oxidation reaction is decreased by the presence of NO in the gas phase. The increase in reaction rate as a result of NO and a platinum catalyst is explained by a cycle of two catalytic reactions, where platinum oxidises NO to NO₂, which subsequently oxidises soot using cerium as a catalyst, forming NO which can participate in the reaction more than once. This oxidation mechanism can be put into practice by combining a platinum-activated particulate trap with a combination of platinum and cerium fuel additives. This combination might be a breakthrough in the search for an applicable catalytic soot removal system.     

Kinetics of the dehydrogenation of ethylbenzene to styrene over unpromoted and K-promoted model iron oxide catalysts

The dehydrogenation of ethylbenzene to styrene over unpromoted and potassium-promoted model iron oxide catalysts has been studied using ultrahigh vacuum techniques in conjunction with elevated pressure reaction kinetics. Model iron oxide catalysts were prepared by oxidizing a polycrystalline Fe sample that was subsequently dosed with metallic potassium. At 875 K the unpromoted catalyst exhibited a turnover frequency of 5×10^–4 molecules/ site s and an activation energy of 39 kcal/mol, both in excellent agreement with the results found for an analogous iron oxide powder catalyst. Potassium promotion increased the turnover frequency to 1.0×10^–3 molecules/site s and lowered the activation energy to 36 kcal/mol for the dehydrogenation reaction. Similarities between the activation energies on the unpromoted and promoted catalysts indicate that the active site is the same on both catalysts. Creation of the active site was dependent upon the formation of an Fe³⁺ metastable species, consistent with the formation of a KFeO₂ phase, upon the addition of potassium.     

Sn催化剂对柴油车排气颗粒去除效果

制备了以Sn为活性组分,以Cu、K、V为助催化剂,以TiO₂/γ-Al₂O₃/堇青石为载体的催化剂,催化剂在700℃下于空气中在马弗炉活化3h.采用DSC/TG测试方法确定催化剂的活性.研究发现,以Sn为活性组分的催化剂能显著降低颗粒的起燃温度和扩大燃烧温度范围,Cu、 K、V的添加能进一步降低起燃温度,而燃烧温度范围却稍有所变窄.活化会降低催化活性,活化导致活性下降的原因不是由催化剂的烧结引起的,而是由活性组分的挥发流失造成的.     

不同水热条件下MnO_2的制备及其催化炭烟颗粒燃烧性能

采用简单的水热法,通过控制不同锰源、酸碱性及水热温度等条件制备一系列MnO_2催化剂,利用SEM、XRD、N_2吸附-脱附和H_2-TPR等对MnO_2催化剂的物化性能进行表征,考察MnO_2催化剂催化炭烟颗粒燃烧的性能。结果表明,制备的纳米棒状MnO_2催化剂均具有良好的催化燃烧炭烟活性,在水热温度120℃、锰源为硝酸锰和酸性条件下,制备的MnO_2催化剂具有最佳催化燃烧炭烟颗粒的性能。     

CATALYTIC PRODUCTION OF ELEMENTAL SULFUR FROM THE THERMAL DECOMPOSITION OF H2S IN THE PRESENCE OF CO2

The feasibility of using a cobalt-molybdenum (Co-Mo) sulfide catalyst that was prepared from a commercial Co-Mo oxide catalyst for the production of elemental sulfur from hydrogen sulfide (H₂S) and carbon dioxide (CO₂) in a packed bed catalytic reactor was studied. It was demonstrated that the desired sulfide catalyst could be prepared by first reducing, then sulphiding the corresponding oxide. The results showed that the prepared catalyst was capable of producing elemental sulfur from the thermal decomposition of H₂S in the presence of CO₂ over a temperature range of 465-700°C and at atmospheric pressure. A specific rate coefficient was calculated as well as the Arrhenius parameters for the non-equilibrated reaction. The H₂S decomposition reaction was found to be a second order reaction and have an activation energy of 114.4kJ/mol(27.3kcal/mol).     

自蔓延燃烧法制备锰氧化物催化剂及其催化燃烧炭烟颗粒

柴油机尾气中的炭烟颗粒是城市雾霾的主要来源之一,严重污染环境和危害人体的健康。因此,降低和消除柴油车尾气中的炭烟颗粒具有重要的意义。本文以高锰酸钾和一水柠檬酸为原料,通过自蔓延燃烧法成功制备了一系列锰氧化物催化剂。采用X射线衍射（XRD）、扫描电子显微镜（SEM）、N₂吸附-脱附、H₂程序升温还原（H₂-TPR）、O₂程序升温氧化（O₂-TPD）和X射线光电子能谱（XPS）等手段对催化剂进行了表征,并考察了该系列催化剂催化炭烟颗粒燃烧的活性。结果表明,制备的锰氧化物催化剂均具有良好的催化燃烧炭烟活性。当高锰酸钾与一水柠檬酸的摩尔比为12∶1、煅烧温度为450℃时,制备的催化剂具有较低的还原峰温度,较大的比表面积和孔径以及化学吸附氧和Mn⁴⁺含量,从而表现出最佳催化燃烧炭烟颗粒的性能,其催化燃烧炭烟温度T₁₀、T₅₀和T₉₀分别为284℃、327℃和360℃。     

Abatement of diesel-exhaust pollutants: NOx storage and soot combustion on K/La2O3 catalysts

Potassium-loaded lanthana is a promising catalyst to be used for the simultaneous abatement of soot and NO_x, which are the main diesel-exhaust pollutants. With potassium loadings between 4.5 and 10 wt.% and calcination temperatures between 400 and 700 °C, this catalyst mixed with soot gave maximum combustion rates between 350 and 400 °C in TPO experiments, showing a good hydrothermal stability. There was no difference in activity when it was either mixed by grinding in an agate mortar or mixed by shaking in a sample bottle (tight and loose conditions, respectively). Moreover, when the K-loaded La₂O₃ is used as washcoat for a cordierite monolith, there were found no significant differences in the catalytic behaviour of the system, which implies its potentiality for practical purposes.

The influence of poisons as water and SO₂ was investigated. While water does not affect the soot combustion activity, SO₂ slightly shift the TPO peak to higher temperature. Surface basicity, which is a key factor, was analysed by measuring the interactions of the catalytic surface with CO₂ using the high frequency CO₂ pulses technique, which proved to be very sensitive, detecting minor changes by modifications in the dynamics of the CO₂ adsorption–desorption process. Water diminishes the interaction with CO₂, probably as a consequence of an adsorption competition. The SO₂ treated catalyst is equilibrated with the CO₂ atmosphere more rapidly if compared with the untreated one, also showing a lower interaction. The lower the interaction with the CO₂, the lower the activity.

Differential scanning calorimetric (DSC) results indicate that the soot combustion reaction coexists with the thermal decomposition of hydroxide and carbonate species, occurring in the same temperature range (350–460 °C). The presence of potassium increases surface basicity shifting the endothermic decomposition signal to higher temperatures.

We also found that NO₂ strongly interacts with both La₂O₃ and K/La₂O₃ solids, probably through the formation of monodentate nitrate species which are stable under He atmosphere until 490 °C. These nitrate species further react with the solid to form bulk nitrate compounds. The addition of Cobalt decreases the nitrates stability and catalyses the NO_x to N₂ reduction under a reducing atmosphere, which is a necessary step for a working NO_x catalytic trap. Preliminary studies performed in this work demonstrated the feasibility of using these catalysts to simultaneously remove NO_x and soot particles from diesel exhausts. The nitrate formation is still observed during the catalytic combustion of soot in the presence of NO_x, making our K/La₂O₃ a very interesting system for practical applications in simultaneous soot combustion and NO_x storage in diesel exhausts.     

Determination of kinetic parameters of the oxidehydrogenation of ethane with CO2 on nanosized calcium-doped ceria under fast deactivation processes

Kinetic parameters of the oxidehydrogenation of ethane with CO₂ on nanosized Ca-doped CeO₂ have been investigated. During reaction, interaction of the reactants with the oxide catalyst causes a fast deactivation process. Overlapping of this fast deactivation with catalytic reaction makes quite difficult the reliable determination of kinetic parameters. This handicap can be overcome by getting sufficient data in a short testing time, thus reducing the degree of deactivation. The kinetics of catalytic reaction and of catalyst deactivation have been studied by conducting a series of consecutive tests at increasing temperatures in steps of 20 °C and monitoring the evolution of the reaction for periods of 30 min on stream at each temperature, with full product analysis every 3 min. At temperatures above 680 °C, catalytic rates decreased linearly with run time at isothermal operation, while deactivation rates increased with increasing temperature. Analysis of the results allows to uncouple catalyst deactivation and catalytic reaction and to obtain the kinetic parameters of both processes (i.e., steady-state and deactivation rates, and their apparent activation energies). Deactivation rate of CO formation is one order of magnitude faster than of ethene formation but both processes show the same apparent activation energy, ca. 47 kcal/mol. The apparent activation energy values for the catalytic reaction are 32 ± 4 and 26 ± 2 kcal/mol for the rates of formation of ethene and CO, respectively.     

Single step direct coating of 3-way catalysts on cordierite monolith by solution combustion method: High catalytic activity of Ce 0.98Pd0.02O2-δ

A new process of coating of active exhaust catalyst over γ-Al₂O₃ coated cordierite honeycomb is reported here. The process consists of (a) growing γ-Al₂O₃ on cordierite by solution combustion of Al(NO₃)3 and oxylyldihydrazide (ODH) at 600 °C and active catalyst phase Ce_0.98Pd_0.02O_2-δ on γ-Al₂O₃ coated cordierite again by combustion of ceric ammonium nitrate and ODH with 1.2 × 10⁻³ M PdCl₂ solution at 500 °C. Weight of active catalyst can be varied from 0.02 to 2 wt% which is sufficient but can be loaded even up to 12 wt% by repeating dip dry combustion. Adhesion of catalyst to cordierite surface is via oxide growth which is very strong. About 100% conversion of CO is achieved below 80 °C at a space velocity of 880 h⁻¹. At much higher space velocity of 21,000 h⁻¹, 100% conversion is obtained below 245 °C. Activation energy for CO oxidation is 8.4 kcal/mol. At a space velocity of 880 h⁻¹ 100% NO conversion is attained below 185 °C and 100% conversion of ‘HC’ C₂H₂ below 220 °C. At same space velocity 3-way catalytic performance over Ce_0.98Pd_0.02O_2-δ coated monolith shows 100% conversion of all the pollutants below 220 °C with 15% of excess oxygen. In this method, handling of nano material – powder is avoided.     

Structured glass catalysts for diesel particulate filters

Efforts to create a structured glass catalyst coating for use in diesel particulate filters (DPF) are described. Several methods for producing porous ceramics were investigated as possible routes to increase the active surface area of a K–Ca–Si–O (KCS) glass catalyst coating applied via a sol-gel route. A carbon pitch template yielded a desirable fibrous glass catalyst structure, but the fibers lay predominantly parallel to the substrate surface, minimizing their effectiveness in capturing soot particles and blocking the cordierite pore structure. A mesophase carbon microbead template yielded a structure that was too fragile for practical use. A chemical blowing agent (CBA) based on ethylene cellulose encapsulated KHCO₃ was developed that offered a means to orient glass catalyst fibers away from the surface. However the viscosity of the CBA/fiber loaded sol makes it unsuited for applying into DPF channels. Finally, phase separation by addition of polypropylene glycol (PPG) was successful for creating a microporous K–Ca–Si–O glass catalyst coating on a cordierite filter. A PPG/KCS coated cordierite filter provides 2.8 times higher active surface area than a non-porous KCS coated cordierite filter, and a T₅₀ (the temperature when half of the carbon is oxidized) that is 30°C lower than a non-porous KCS-coated cordierite filter. The excellent performance of the PPG/KCS coated cordierite filter is attributed to its higher catalyzed surface area and more intensive soot-catalyst interaction.     

Metal-Free Catalytic Wet Oxidation: From Powder to Structured Catalyst Using N-Doped Carbon Nanotubes

This work shows a promising N-doped carbon catalyst for the oxidation of oxalic acid by catalytic wet oxidation, which is able to compete with the traditional noble metal and metal oxide catalysts used in the process. After preliminary studies conducted in batch mode, the catalytic performance of the metal-free carbon nanotubes, both in powder form and supported on a macrostructured carrier (a cordierite monolith), was evaluated under continuous operating conditions. The ability of the N-functionalities to promote activation and chemisorption of oxygen led not only to fast oxalic acid mineralization under batch mode (5 min of reaction to reach full mineralization), but also to good performance under continuous operation (more than 90% conversion of oxalic acid in the steady state, using the powder and around 55% using the catalyst immobilized on a honeycomb cordierite monolith).     

V-W-Ti-Si/堇青石催化剂柴油机脱硝反应动力学

采用固定床反应器,以自制的V-W-Ti-Si/堇青石为催化剂,在消除内外扩散影响的基础上,考察了氨选择性催化还原反应(NH_3-SCR)过程,建立了幂指数型的脱硝反应动力学模型,并通过线性拟合确定了模型参数。结果表明,在自制的V-W-Ti-Si/堇青石催化剂上,NO,NH_3和O_2的反应级数分别为0.93,0,0.24,活化能为25.99 kJ/mol,模型拟合的相对误差在10%左右,可用于指导柴油机尾气脱硝催化剂反应过程研究与开发。     

Evaluation of hectorites,synthesized in different conditions,as soot combustion catalysts after impregnation with copper

Two microporous hectorites were prepared by conventional and microwave heating, and a delaminated mesoporous hectorite by an ultrasound-assisted synthesis. These three hectorites were impregnated with copper. The characterization techniques used were XRD, N₂ adsorption, TEM and H₂ reduction after selective surface copper oxidation by N₂O (to determine copper dispersion). The catalytic activity for soot combustion of the copper-free and the copper-containing hectorites was tested under a gas mixture of 500 ppm NOx/5% O₂/N₂ (and 5% O₂/N₂ in some cases), evaluating their stability through three consecutive soot combustion experiments.The delaminated hectorite showed the highest surface area (353 m²/g) allowing the highest dispersion of copper. This copper-containing catalyst was the most active for soot combustion among those prepared and tested in this study. We have also concluded that the Cu/hectorite-catalyzed soot combustion mechanism is based on the activation of the O₂ molecule and not on the NO₂-assisted soot combustion.