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.     

Alumina Supported Co–K–Mo Based Catalytic Material for Diesel Soot Oxidation

Alumina supported Co–K–Mo based mixed metal oxide type catalytic materials have been prepared by co-impregnation. These catalysts show excellent activity for carbon as well as diesel soot oxidation, which could be due to the redox properties of Mo and Co as well as to a synergistic effect of molybdenum, cobalt, and K contents. The catalyst containing 5 wt% molybdenum shows a lowering of carbon oxidation by about 190 °C under loose contact conditions as compared to the non-catalyzed reaction, as well as to bare alumina. Characterization studies suggest a composite nature of these materials, while thermal stability investigations confirm the stable nature. The selected catalyst has been studied by XPS, however, it is difficult to conclude which are the important factors contributing to the catalytic activity. It appears to be a synergistic effect of Co, K, and Mo components as these catalysts show much improved activity as compared to the individual components in supported and unsupported forms.     

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.     

Thermally stable ceria–zirconia catalysts for soot oxidation by O2

Ce–Zr mixed oxides calcined at 1000 °C are more active catalysts for soot oxidation than pure CeO₂ calcined at the same temperature, both in loose and tight contact between soot and catalyst. 1000 °C sinterised-CeO₂ presents a very low surface area (2 m²/g), a large crystal size (110 nm) and a lack of surface redox properties. Ce–Zr mixed oxides present higher BET surface areas (typically 17–19 m²/g), smaller crystal sizes and enhanced redox properties. The Zr molar fraction does not affect appreciably the catalytic activity of Ce–Zr mixed oxides in the range studied (Zr molar fraction from 0.11 to 0.51).     

Catalytic Oxidation of CO over Nanocrystalline La1–xCexNiO3 Perovskite‐Type Oxides

Nanocrystalline La_1–xCe_xNiO₃ (x = 0.1, 0.3, 0.5, 0.7, 0.9) perovskite‐type oxide catalysts prepared by the Pechini method were employed in catalytic CO oxidation and the effect of substitution of La by Ce on CO conversion was evaluated. The results indicated the remarkable effect of La substitution with Ce on the catalytic performance at low temperatures. The reaction temperature had a significant influence on the stability of the catalysts. The La_0.1Ce_0.9NiO₃ sample exhibited the highest activity among the prepared catalysts in CO oxidation reaction. In addition, the influence of different parameters including pretreatment condition, feed ratio, and gas hourly space velocity (GHSV) on the catalytic performance was examined. The optimum catalyst proved high stability under severe reaction conditions in the presence of water vapor and CO₂ in the feed stream.     

Nanometric La1−x K x MnO3 Perovskite-type oxides – highly active catalysts for the combustion of diesel soot particle under loose contact conditions

The La_1−xK_xMnO₃ perovskite-type oxides whose sizes were in nanometric range were prepared by the citric acid-ligated method. The structures of these perovskite-type oxides were examined by XRD and FT-IR. The catalytic activity for the combustion of soot particulate was evaluated by a technique of the temperature-programmed reaction. In the LaMnO₃ catalyst, the partial substitution of K for La at A-site enhanced the catalytic activity for the combustion of soot particle. In the La_1−xK_xMnO₃ catalysts, the combustion temperature of soot particle decreases with increasing x values. The La_1−xK_xMnO₃ oxides with the substitution quantity between x=0.20 and x=0.25 are good candidate catalysts for the soot particle removal reaction, and the combustion temperature of soot particle is between 285 and 430 °C when the contact of catalysts and soot is loose, and their catalytic activities for the combustion of soot particle are as good as supported Pt catalysts, which is the best catalyst system so far reported for soot combustion under loose contact conditions.     

Catalytic behavior of potassium containing compounds for diesel soot combustion

Alkali doped oxides were synthesized and tested as catalysts for diesel soot combustion using a combinatorial method. It has been found that potassium shows better promotion of the catalytic activity than other alkali elements, and most of the potassium-rich oxides showed similar catalytic behaviors when catalysts and soot were mixed in a slurry. The influence of different mixing methods, including loose contact, tight contact and slurry (wet) mixing with different soot suspensions, on the catalytic behavior of some transition metal oxides, alkali metal carbonates and potassium-containing oxides were studied through thermogravimetry and XRD. The high activity of potassium-containing catalysts is found to be due to the intimate contact between soot and potassium cations caused by polar solvents. Potassium containing catalysts degraded after repeated thermal cycles due to the loss of potassium. It was also found that the addition of transition elements can inhibit the loss of potassium.     

Selective oxidation of soot over Cu doped ceria/ceria–zirconia catalysts

Copper doped ceria and ceria–zirconia mixed oxides were prepared using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification helped to improve the activity and selectivity of ceria and ceria–zirconia for soot catalytic oxidation. The CO-TPR results showed that Cu–Ce had a better reducibility than pure ceria at low temperatures. After ageing at 800 °C for 20 h in flow air, CuO–CeO₂ showed the maximum soot oxidation rate at 378 and 519 °C under tight and loose contact conditions, respectively, achieving a nearly 100% selectivity to CO₂ production. This effect may be attributed to the existence of well dispersed copper oxide species strongly interacting with the ceria surface, which may decrease the activation energy of soot oxidation. A conceivable mechanism of this synergetic effect was proposed.     

Thermally stable metal ruthenate based soot oxidation catalyst for diesel exhaust emission control

Lanthanum ruthenate materials with perovskite type structure can be easily synthesized with ruthenium in 4+ oxidation state. La_3.5Ru_4.0O₁₃ type perovskite has been synthesized in unsupported and supported forms by using various methods. This perovskite type La_3.5Ru_4.0O₁₃ phase shows high thermal stability and can therefore be used as a catalyst for high temperature applications, including those for auto-exhaust emission control. The material shows good catalytic activity for the carbon/soot oxidation in view of its possible application in diesel soot oxidation for regeneration of Diesel Particulate Filter.     

Bench Scale Experiments of Diesel Soot Oxidation Using Pr0.7Sr0.2K0.1MnO3 Perovskite Type Catalyst Coated on Ceramic Foam Filters

Potassium and strontium substituted praseodymium manganate type perovskite catalyst coated on ceramic foam filters have been studied for diesel particulate removal. The synthesized catalyst coated filter pieces have been characterized by using XRD, SEM and TG analysis, whereas their catalytic activity towards soot oxidation was tested using a bench scale facility with real diesel engine exhaust. The catalyst coated filters decrease the soot oxidation T_initial value by 150 °C and T_final by 100 °C as compared to bare soot oxidation reaction, which can be considered as high activity under the actual conditions of diesel engine. The catalytic materials show good thermal stability, while their low cost will also add to their potential for practical applications. Although perovskites have been studied for laboratory evaluations of catalytic soot oxidation, present results further substantiate the possibility of using low-cost, supported, non-noble metal based catalysts for diesel exhaust emission control applications, especially for the cost-effective retrofitment of in-use vehicles with old generation engines.     

Highly Active La1−x K x CoO3 Perovskite-type Complex Oxide Catalysts for the Simultaneous Removal of Diesel Soot and Nitrogen Oxides Under Loose Contact Conditions

The nanometric La_1?x K _x CoO₃ (x = 0–0.30) perovskite-type oxides were prepared by a citric acid-ligated method. The catalysts were characterized by means of XRD, IR, BET, XPS and SEM. The catalytic activity for the simultaneous removal of soot and nitrogen oxides was evaluated by a technique of the temperature-programmed oxidation reaction. In the LaCoO₃ catalyst, the partial substitution of La³⁺ at A-site by alkali metal K⁺ enhanced the catalytic activity for the oxidation of soot particle and reduction of NO _x . The La_0.70K_0.30CoO₃ oxides are good candidate catalysts for the simultaneous removal of soot particle and NO _x . The combustion temperatures for soot particles over the La_0.70K_0.30CoO₃ catalyst are in the range from 289 to 461 °C, the selectivity of CO₂ is 98.4% and the conversion of NO to N₂ is 34.6% under loose contact conditions. The possible reasons that can lead to the activity enhancement for the K-substitution samples compared to the unsubstituted sample (LaCoO₃) were given. The particle size has a large effect on its catalytic performance for the simultaneous removal of diesel soot and nitrogen oxides.     

Study of perovskite‐type oxides and their supported Ag derivatives for catalytic oxidation of diesel soot

A series of perovskite‐type oxides and derived Ag catalysts were prepared, and characterized by N₂‐adsorption, X‐ray diffraction and X‐ray photoelectron spectroscopy. The influences of pretreatment and Ag loading on catalytic activity for diesel soot oxidation were also investigated. Prereduction resulted in a decrease in catalytic activity. An increase in activity with Ag addition was observed, especially with more than 5% Ag loading. This catalyst could be a promising candidate for the catalytic elimination of diesel soot. © 2002 Society of Chemical Industry     

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.     

Role of Electronic Factor in Soot Oxidation Process Over Tunnelled and Layered Potassium Iron Oxide Catalysts

This paper describes the investigations of the catalytic activity in soot oxidation over well-defined iron oxide based materials. The nanostructuration of iron oxide by potassium into tunnelled (KFeO₂) and layered (K₂Fe₂₂O₃₄) ferrites and the surface promotion with CeO₂ results in the marked increase in the catalytic activity (decrease of the ignition temperature down to 210 °C and T _10 % to 310 °C). The measurements of the catalysts work function showed that both nanostructuration and surface promotion with ceria of the best KFeO₂ phase led to increase of the electron availability (decrease of the work function). Strong correlation of the catalytic activity in soot combustion of the Ce–K–Fe–O systems with the work function value was revealed for the first time in the model studies, and can be used as a guideline for optimisation of the real catalytic filters.     

Role of CeO2–ZrO2 in diesel soot oxidation and thermal stability of potassium catalyst

Potassium nitrate catalysts supported on different oxides (CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂) were prepared for diesel soot combustion. The ageing treatment was performed at 800 °C for 24 h and the catalytic activity was evaluated by a temperature-programmed oxidation technique. The results demonstrated that, compared with CeO₂ and ZrO₂, Ce_0.5Zr_0.5O₂ presented good redox properties, a high surface area and available potassium-holding capacity at an elevated temperature. For aged K/Ce_0.5Zr_0.5O₂, the combustion temperature of soot particle was 359 °C under tight contact conditions and 455 °C under loose contact conditions. Thus, ceria–zirconia mixed oxides were considered as good candidate supports for diesel soot oxidation catalysis.     

CeO2-based catalysts with engineered morphologies for soot oxidation to enhance soot-catalyst contact

As morphology plays a relevant role in solid/solid catalysis, where the number of contact points is a critical feature in this kind of reaction, three different ceria morphologies have been investigated in this work as soot oxidation catalysts: ceria nanofibers, which can become organized as a catalytic network inside diesel particulate filter channels and thus trap soot particles at several contact points but have a very low specific surface area (4 m²/g); solution combustion synthesis ceria, which has an uncontrolled morphology but a specific surface area of 31 m²/g; and three-dimensional self-assembled (SA) ceria stars, which have both high specific surface area (105 m²/g) and a high availability of contact points. A high microporous volume of 0.03 cm³/g and a finer crystallite size compared to the other morphologies suggested that self-assembled stars could improve their redox cycling capability and their soot oxidation properties. In this comparison, self-assembled stars have shown the best tendency towards soot oxidation, and the temperature of non-catalytic soot oxidation has dropped from 614°C to 403°C in tight and to 552°C in loose contact conditions, respectively. As far as the loose contact results are concerned, this condition being the most realistic and hence the most significant, self-assembled stars have exhibited the lowest T_10% onset temperature of this trio (even after ageing), thus proving their higher intrinsic activity. Furthermore, the three-dimensional shape of self-assembled stars may involve more of the soot cake layer than the solution combustion synthesis or nanofibers of ceria and thus enhance the total number of contact points. The results obtained through this work have encouraged our efforts to understand soot oxidation and to transpose these results to real diesel particulate filters.     

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.     

NO2-aided Soot Oxidation on LaMn0.7Ni0.3O3 Perovkite-type Catalyst

The LaMnO₃-based perovskite-type mixed oxides were studied for both trapping of NO _x and combustion of diesel soot. The LaMn_0.7Ni_0.3O₃ (LMN3) perovskite shows high NO _x adsorption capacity, quick adsorption rate and efficient adsorbed species. After the catalyst interacts with NO at low temperature around 325 °C, decomposition of the nitrates leads to the decrease of the maximum soot oxidation temperature to 430 °C. The fine crystallite size, increased surface area and readily reducibility at low temperature also favor the oxidation of soot over LMN3 under loose contact conditions.     

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.     

Influence of Potassium and NO Addition on Catalytic Activity in Soot Combustion and Surface Properties of Iron and Manganese Spinels

Two series of (0–4 wt%) potassium doped oxide catalysts based on iron and manganese spinel were prepared. The synthesized materials were characterized in terms of their structure (XRD, Raman spectroscopy) and surface electronic properties (work function measurements). The catalytic activity towards soot combustion was determined by temperature programmed oxidation of a physical mixture of soot and catalyst in tight contact in gas oxygen mixtures with and without NO addition. For iron spinel based materials, where potassium is localized at the surface, the catalytic activity correlates with the work function lowering upon K doping, while for manganese spinel based materials, where potassium is incorporated into the bulk (formation of KMn₄O₈ or KMn₈O₁₆), the correlation was not found. The presence of NO in the gas mixture leads to a systematic decrease of soot ignition temperature for all samples.