Low Cost, Ceria Promoted Perovskite Type Catalysts for Diesel Soot Oxidation

Perovskite type catalysts with SrCoO₃ and Sr_0.8Ce_0.2CoO₃ compositions have been prepared by co-precipitation and other methods and, their catalytic activity towards diesel particulate matter (PM)/carbon oxidation has been evaluated under the loose contact condition. These catalysts show excellent catalytic activity for PM/carbon oxidation, despite their low surface area and under the loose contact condition. The synergistic effects of Ce incorporation in perovskite and presence of a small amount of potassium appears to be responsible for the high soot oxidation activity of these perovskite type materials. The Ce incorporation seems to be contributing by enhancing the redox property of the catalyst, while it appears unlikely that potassium is contributing by improving the catalyst–soot contact through its volatization. The catalysts show excellent thermal stability and stable activity under repeated cycles of use.     

The effect of the contact between platinum and soot particles on the catalytic oxidation of soot deposits on a diesel particle filter

Experiments were performed with two model soot aerosols brought into different forms of contact with Pt aerosol particles, to investigate the effectiveness of this contact in lowering the catalytic soot oxidation temperature. The contact was either generated between individual particles in the aerosol state (Pt-doped soot to simulate a fuel borne catalyst), or by sequential or simultaneous deposition of separately generated soot and Pt aerosols onto a sintered metal filter. (Formation of a soot cake on previously deposited Pt aerosol would simulate a catalyst coated diesel particle filter.) The catalytic activity was determined in all cases from temperature ramped oxidation in air of the filtered particles, and defined as the 50% conversion temperature.

It was found that Pt-doped soot and simultaneously filtered aerosols were both equally effective in reducing the oxidation temperature by up to 140–250 °C for the spark discharge soot (with 3–47 wt% Pt concentration in the soot cake), and by up to 140 °C for the pyrolysis soot (3 wt% Pt). Conversely, the deposition of a thin soot layer of 5–10 μm thickness onto Pt, or vice versa, produced only a slight temperature reduction on the order of about 13–42 °C. These results suggest that the distance between soot and Pt particles plays a key role in promoting an effective oxidation on the filter, which is consistent with the role of Pt particles as local generators of activated oxygen.     

Preparation and Application of Perovskite Catalysts for Diesel Soot Emissions Control: An Overview

The diesel engines are energy efficient (1), but their particulate matter (soot) emissions are still a matter of concern even though major advances in their control are being made. For soot abatement, catalytic diesel particulate filter (DPF) technique is widely employed to trap and burn the soot. Many types of catalysts have been investigated for the soot combustion i.e. platinum group metal (PGM) based, perovskite-type oxides, spinel-type oxides, rare earth metal oxides, and mixed transient metal oxides etc. The cost of PGM catalysts is high and their availability is questionable. Further they are susceptible to poisoning and have low thermal stability. On the other hand perovskite catalysts show potential as effective soot oxidation catalyst for the DPF because of their low cost, high thermal stability and tailoring flexibility. Many papers related to soot oxidation over perovskite catalysts have been published but no review paper appears in the literature that is dedicated to soot oxidation. Thus, this article provides a summary of published information regarding pure and substituted perovskite catalyst, preparation methods, properties, and their application for diesel soot emission control.     

Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

This study addresses the catalytic reaction of NO_x and soot into N₂ and CO₂ under O₂-rich conditions. To elucidate the mechanism of the soot/NO_x/O₂ reaction and particularly the role of the catalyst -Fe₂O₃ is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O₂ and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe₂O₃ catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N₂. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe₂O₃ catalyst. This model includes the dissociative adsorption of O₂ on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe₂O₃ catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.     

Influence of Potassium Doping on the Activity and the Sulfur Poisoning Resistance of Soot Oxidation Catalysts

Soot oxidation on potassium nitrate impregnated cerium oxide and manganese cerium mixed oxide was investigated using TG-FTIR. It was found, that potassium nitrate promotes the soot oxidation by enhancing the contact between the soot and the catalyst, as soot ignition temperature corresponds to the melting point of potassium nitrate for both the individual and the mixed oxide. Since potassium nitrate is easier sulfatized than cerium or manganese, the susceptibility of the catalyst to sulfur poisoning is considerably decreased.     

Performances of a catalytic foam trap for soot abatement

A catalytic trap for soot particles was prepared by deposition of Cu–V–K–Cl catalyst on a ceramic foam. Catalytic trap performances were evaluated by treating the exhaust of a gas oil burner under different operating conditions. The results obtained showed that ceramic foam is a particularly suitable support for this application since it yields low gas pressure drop, good soot collection efficiency (“deep bed” filtration mechanism), high thermal shock resistance and good contact throughout the filter between soot particles and catalyst surface. In addition, the catalytic foam trap is able to spontaneously regenerate at operating conditions comparable to those typical of diesel engine exhaust and after more than 70 test hours it retains its activity towards soot oxidation.     

Ag/Ce0.75Zr0.25O2催化剂中Ag的负载量对碳烟燃烧活性的影响

开发低温下高催化活性的柴油机碳烟颗粒燃烧催化剂是当前环境催化领域的热点问题。利用共沉淀的方法制备了用于碳烟催化燃烧反应的Ag/Ce_0.75Zr_0.25O₂催化剂。活性评价结果表明,相对于Ce_0.75Zr_0.25O₂催化剂,Ag的引入可显著降低碳烟催化燃烧温度。而且,Ag的负载量存在一个最佳值。以XRD、in-situ XRD、BET、TPR等表征手段探究了该系列催化剂结构性质及其变化产生的影响。结果表明,Ag与Ce物种间的相互作用可显著降低催化剂（特别是CeO₂表面氧）的还原温度。该相互作用使Ag/Ce_0.75Zr_0.25O₂催化剂在一定温度下（>200℃）就表现出Ag⁺的性质。这些性质与该催化剂具有较高的碳烟氧化活性相关。而且,该催化剂也表现出良好的稳定性。     

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.     

High-throughput approach to the catalytic combustion of diesel soot

A methodology for the evaluation of diesel soot oxidation catalysts by high-throughput (HT) screening was developed. The optimal experimental conditions (soot amount, catalyst/soot ratio, type of contact, composition and flow rate of gas reactants) ensuring a reliable and reproducible detection of light-off temperatures in a 16 parallel channels reactor were set up. The temperature profile measured in the catalyst/soot bed under TPO conditions when the exothermic combustion of soot takes place was shown to provide an accurate measurement of the ignition. Its reproducibility and relevance were checked. The results obtained with a reference noble metal free catalyst (La_0.8Cr_0.8Li_0.2O₃ perovskite) agree very well with literature data. Qualitative mechanistic features could be derived from these experiments, stressing the likely limiting step of oxygen transfer from catalyst surface to soot particulates to ignite the soot combustion. Ceria material was shown to be more appropriate than perovskite one. From an HT screening of a large diverse library (over 100 mixed oxides catalysts) under optimized conditions, about 10 new formulations were found to perform better than selected noble metal free reference materials.     

Zirconia supported Ru–Co bimetallic catalysts for diesel soot oxidation

Various amounts of ruthenia–cobaltate bimetallic catalyst supported on zirconia have been prepared by co-impregnation method and their catalytic activity towards soot/carbon oxidation has been evaluated using TG technique under the loose contact condition. These catalysts show good activity for carbon/soot oxidation, which is observed to be a factor of ruthenia content. The thermal stability experiments confirmed the stability of catalytic materials in air at least up to 900 °C. In this way, ruthenia can be easily dispersed on zirconia possibly through the solid solution formation, while its thermal stability can be significantly improved by introducing a transition metal namely cobalt. Formation of Ru–Co bimetallic clusters over zirconia is probably responsible for its thermal stability, while dissociative adsorption of oxygen on catalyst surface appears to be responsible for their catalytic activity.     

Potential rare earth modified CeO2 catalysts for soot oxidation: I. Characterisation and catalytic activity with O2

Ceria (CeO₂) and rare-earth modified ceria (CeReO_x with Re = La, Pr, Sm, Y) catalysts are prepared by nitrate precursor calcination and are characterised by BET surface area, XRD, H₂-TPR, and Raman spectroscopy. Potential of the catalysts in the soot oxidation is evaluated in TGA with a feed gas containing O₂. Seven hundred degree Celsius calcination leads to a decrease in the surface area of the rare-earth modified CeO₂ compared with CeO₂. However, an increase in the meso/macro pore volume, an important parameter for the soot oxidation with O₂, is observed. Rare-earth ion doping led to the stabilisation of the CeO₂ surface area when calcined at 1000 °C. XRD, H₂-TPR, and Raman characterisation show a solid solution formation in most of the mixed oxide catalysts. Surface segregation of dopant and even separate phases, in CeSmO_x and CeYO_x catalysts, are, however, observed. CePrO_x and CeLaO_x catalysts show superior soot oxidation activity (100% soot oxidation below 550 °C) compared with CeSmO_x, CeYO_x, and CeO₂. The improved soot oxidation activity of rare-earth doped CeO₂ catalysts with O₂ can be correlated with the increased meso/micro pore volume and stabilisation of external surface area. The segregation of the phases and the enrichment of the catalyst surface with unreducible dopant decrease the intrinsic soot oxidation activity of the potential CeO₂ catalytic sites. Doping CeO₂ with a reducible ion such as Pr^4+/3+ shows an increase in the soot oxidation. However, the ease of catalyst reduction and the bulk oxygen-storage capacity is not a critical parameter in the determination of the soot oxidation activity. During the soot oxidation with O₂, the function of the catalyst is to increase the ‘active oxygen’ transfer to the soot surface, but it does not change the rate-determining step, as evident from the unchanged apparent activation energy (around 150 kJ mol⁻¹), for the catalysed and un-catalysed soot oxidation. Spill over of oxygen on the soot surface and its subsequent adsorption at the active carbon sites is an important intermediate step in the soot oxidation mechanism.     

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.     

Potential rare-earth modified CeO2 catalysts for soot oxidation

The physico-chemical properties of ceria (CeO₂) and rare earth modified ceria (with La, Pr, Sm, Y) catalysts are studied and correlated with the soot oxidation activity with using O₂ and O₂ + NO. CeO₂ modified with La and Pr shows superior soot oxidation activity with O₂ compared with the unmodified catalyst. The improved soot oxidation activity of rare earth doped CeO₂ catalysts can be correlated to the increased meso/micro pore volume and the stabilisation of the external surface area. On the other hand, unreducible ions decrease the intrinsic soot oxidation activity of rare earth modified ceria with both O₂ and NO + O₂ due to the decreased amount of redox surface sites. The catalyst bulk oxygen storage capacity is not a critical parameter in determining the soot oxidation activity. The modification with Pr shows the best soot oxidation with both O₂ and O₂ + NO compared with all other catalysts.     

Characteristics on HDS over amorphous silica–alumina in single and dual catalytic bed system for gas oil

Combustion temperatures of particulate matter of Diesel automobile engines under lean conditions in laboratory experiments depend on a number of parameters: e.g. model gas composition and flow rate, catalyst composition and micro structure, soot/catalyst ratio as well as model soot type (composition, particle size and size distribution) and contact type. Especially the last two factors are often underestimated. In the literature most commonly Printex U and loose or tight contact are used. Here we report on the effect of these two parameters by varying the soot used and contact type and with nano-scaled ceria as catalyst due to its known Oxygen Storage Capacity (OSC). Apart from investigating the influence of these factors our second main objective was to find a preparation technique for the soot-catalyst contact, which fulfills the criteria of high reproducibility, high comparability to real conditions and facile automation. The last criterion is the basis for its use in high-throughput techniques (HTT) for parallelized discovery and optimization studies of new combustion catalysts.     

同时消除柴油机尾气排放炭颗粒和NOx催化剂的研究进展

介绍了简单氧化物、复合氧化物(尖晶石和钙钛矿)催化剂都具有同时消除柴油机尾气中的炭颗粒和NO的活性．但是炭颗粒的起燃温度较高,生成N2的选择性差。在消除炭颗粒和氮氧化物时,有N2O生成,造成二次污染,炭颗粒与催化剂的接触形式直接影响炭颗粒的燃烧温度。     

A novel catalyst for diesel soot oxidation

In this study, cobalt and lead based mixed oxide catalysts were tested for their soot oxidation ability. In addition to a mixed oxide formerly marketed as ceramic paint, a home made set was also prepared by incipient wetness impregnation of a cobalt oxide powder with a lead acetate solution and subsequent calcination. The materials investigated in this study were shown to decrease the peak combustion temperature of home made soot from 500 to 385 °C in air. Soot oxidation tests under inert (N₂) atmospheres revealed that the oxidation took place by using the lattice oxygen of the catalyst. Reaction temperature could be further decreased when these mixed oxide catalysts were impregnated with platinum. An optimum platinum loading was determined as 0.5 wt% based on the peak combustion temperature of the soot. The role of Pt was to assist the oxygen transfer from the gas phase to the lattice. It was observed that NO₂ is a better oxidizing agent as compared to air whereas NO had hardly any activity against soot oxidation reaction. When the mixed oxide catalyst was impregnated with platinum, the peak combustion temperature was measured as 310 °C in the presence of NO_x and air. The catalyst's unique performance was in terms of the rate of soot oxidation. Under the experimental conditions studied here, the soot oxidation was so facile that the oxygen in the gas phase was completely depleted. This stream of oxygen depleted and CO enriched gas phase can be used to reduce NO_x in the presence of a downstream or a co-catalyst.     

Potential rare-earth modified CeO2 catalysts for soot oxidation part II: Characterisation and catalytic activity with NO + O2

Ceria (CeO₂) and rare-earth modified ceria (CeReO_x with Re = La³⁺, Pr^3+/4+, Sm³⁺, Y³⁺) supports and Pt impregnated supports are studied for the soot oxidation under a loose contact with the catalyst with the feed gas, containing NO + O₂. The catalysts are characterised by XRD, H₂-TPR, DRIFT and Raman spectroscopy. Among the single component oxides, CeO₂ is significantly more active compared with the other lanthanide oxides used in this study. Doping CeO₂ with Pr^3+/4+ and La³⁺ improved, however, the soot oxidation activity of the resulting solid solutions. This improvement is correlated with the surface area in the case of CeLaO_x and to the surface area and redox properties of CePrO_x catalyst. The NO conversion to NO₂ over these catalysts is responsible for the soot oxidation activity. If the activity per unit surface area is compared CePrO_x is the most active one. This indicates that though La³⁺ can stabilise the surface area of the catalyst in fact it decreases the soot oxidation activity of Ce⁴⁺. The lattice oxygen participates in NO conversion to NO₂ and the rate of this lattice oxygen transfer is much faster on CePrO_x. In general, the improvement of the soot oxidation is observed over the Pt impregnated CeO₂ and CeReO_x catalysts, and can be correlated to the presence of Pt°. The surface reduction of the supports in the presence of Pt occurred below 100 °C. The surface redox properties of the support in the Pt catalysts do not have a significant role in the NO to NO₂ conversion. In spite of the lower surface area, the Pt/CeYO_x and Pt/CeO₂ catalysts are found to be more active due to larger Pt crystal sizes. The presence of Pt also improved the CO conversion to CO₂ over these catalysts. The activation energy for the soot oxidation with NO + O₂ is found to be around 50 kJ/mol.     

Effect of Ag, Cu, and Au Incorporation on the Diesel Soot Oxidation Behavior of SiO2: Role of Metallic Ag

The catalytic behaviors of Ag, Cu, and Au loaded fumed SiO₂ have been investigated for diesel soot oxidation. The diesel soot generated by burning pure Mexican diesel in laboratory was oxidized under air flow in presence of catalyst inside a tubular quartz reactor in between 25 and 600 °C. UV–Vis optical spectroscopy was utilized to study the electronic states of Ag, Cu, and Au(M) in M/SiO₂ catalysts. The soot oxidation was seen to be strongly enhanced by the presence of metallic silver on 3 % Ag/SiO₂ surface, probably due to the formation of atomic oxygen species during the soot oxidation process. The catalyst is very stable due to the stability of Ag⁰ species on the catalyst surface and high thermal stability of SiO₂. Obtained results reveal that though the freshly prepared 3 % Cu/SiO₂ is active for soot oxidation, it gets deactivated at high temperatures in oxidizing conditions. On the other hand, 3 % Au/SiO₂ catalyst does not present activity for diesel soot oxidation in the conventional soot oxidation temperature range. The catalytic behaviors of the supported catalyst samples have been explained considering the electron donating ability of the metals to generate atomic oxygen species at their surface.     

EPR characterisation of carbon black in loose and tight contact with Al2O3 and CeO2 catalysts

The intensity of contact between soot and catalyst is one of the major parameters that determine the soot oxidation reactivity. In the present work the EPR study is focused on the mixtures of carbon black with Al₂O₃ and CeO₂ in loose and tight contacts in order to provide the physicochemical characterisation of the contact. For tight contact CB-catalyst mixtures a new paramagnetic species is observed and can be considered as a fingerprint of the contact between the two solids. These new paramagnetic species increase the catalytic reactivity of CB combustion in the presence of cerium oxide.     

Influence of the Alkali Promoter on the Activity and Stability of Transition Metal (Cu, Co, Fe) Based Structured Catalysts for the Simultaneous Removal of Soot and NOx

Structured catalysts prepared by means of coating cordierite monoliths with alumina-based suspensions containing transition metals such as Cu, Co and Fe and alkali/alkali-earth promoters such as K and Ba. Textural and structural features of these catalysts were analyzed by means of N₂ adsorption and SEM. Their activity in the simultaneous removal of soot and NO_x was assayed in a lab-scale installation, using a carbon black as diesel surrogate. Catalysts exhibited significant activity in deNO_x and soot oxidation. K and Ba enhanced both NO_x adsorption and soot–catalyst contact. However Ba contributed to a greater extent to the adsorption of N-species, which moreover presented higher thermal stability than on K-catalysts, and K showed higher mobility than Ba. Thus, Ba-containing catalysts showed increased activity towards NO_x reduction but shifted to higher temperatures in comparison to K-catalysts, which on the other hand resulted more active towards soot oxidation than Ba-ones. Fe-based catalyst turned out to be less active both in soot oxidation and NO_x reduction than Co and Cu-based ones. Intensive calcination of the catalysts at 800 °C for 5 h resulted in substantial loss of K and Ba. Loss of promoter depends, however, on the metal contained in the catalyst. In this sense Fe-containing catalysts showed higher stability. Calcination has a substantial effect on catalytic activity. Catalyst significantly lost their NO_x adsorption capacity and showed similar activity than a catalyst prepared in absence of promoter, pointing to a substantial change in reaction mechanism and reaction predominantly occurring on metallic sites upon the loss of alkali/alkali-earth compound.