Catalysts For Diesel Soot Combustion Research Articles

Recent advances in performances and mechanisms of cerium-based oxide catalysts for catalytic combustion of soot particles released from diesel engines

<p indent=0mm>With the increasingly strict environmental protection regulations and the improvement of people’s awareness of environmental protection, the serious environmental pollution caused by soot particles from diesel vehicle exhaust has attracted people’s attention. Catalytic purification technology is one of the most effective ways to eliminate soot particles and the research and development of highly effective catalysts is one of the key factors for determining the application of this technology. In recent years, cerium-based oxide catalysts have been widely used for catalytic combustion of diesel soot particles due to their excellent oxidation-reduction performance, suitable surface acidity, and high oxygen storage/release capacity. The researches focusing on highly active essence of the cerium-based oxide catalysts in soot combustion are helpful to understand the fundamental theories of catalytic processes of soot combustion and provide scientific guidances for the optimization of existing catalysts and design of new-type catalysts. In this review, the recent research progress of cerium-based oxide catalysts for diesel soot combustion is summarized, including single cerium oxide catalyst and cerium-based oxide catalysts modified by rare earth metals, transition metals, noble metals and alkali (alkaline earth) metals. Meanwhile, the reaction mechanisms of above-mentioned catalysts are also summarized. Finally, the problems and development directions of cerium-based oxide catalysts for soot combustion are proposed and prospected.

Pt-free CoAl2O4 catalyst for soot combustion with NOx/O2

Cobalt aluminate spinel compounds prepared by different methods were characterized in terms of crystalline structure, morphology, textural properties and acidity, and evaluated as catalysts for diesel soot combustion in NOx/O2 mixtures. The activation of commercial CoAl2O4 microparticles by ball milling reduced the onset temperature of this material considerably and yielded an efficient catalyst for soot combustion that exhibited a temperature of half conversion, 411 °C, similar to that obtained with the reference Pt/Al2O3 catalyst. Moreover, like Pt, the milled CoAl2O4 suppressed the CO formation observed in non-catalytic soot combustion. The significant modification of the particle size, the emergence of new exposed crystal facets and the creation of defects are responsible for the appearance of new active sites and high acidity that improved catalyst-to-soot contact and reactivity. The findings open a new gate toward the use of efficient Pt-free catalyst for soot combustion.

Nanostructured ceria-praseodymia catalysts for diesel soot combustion

Nanostructured ceria-praseodymia catalysts with different praseodymium contents have been prepared through hydrothermal synthesis to study the effect of Pr as a dopant and the effect of morphology towards soot combustion under “loose” and “tight” soot-catalyst conditions. Samples synthesized through solution combustion synthesis (SCS) have also been prepared as comparative materials. Studies in physicochemical properties of the catalysts have been carried out using complementary techniques. The present work also resorts to soot-TPR as an unconventional method of investigating the ability of solid catalysts to initiate soot oxidation in the absence of bulk oxygen. Ce50Pr50 catalyst (where 50 indicates the atomic percentage of cerium as well as of praseodymium) with mixed structures of nanorods and nanocubes has attained the best catalytic performances, thanks to the high lattice oxygen mobility and the easy reducibility. The insertion of Pr cations to the ceria framework enhances the number of redox sites on the surface, thus generating more oxygen vacancies. As a whole, activity tests in general have proven that despite having relatively low surface areas, ceria-praseodymia nanocubes and nanorods facilitated soot combustion reaction more actively than SCS-based ceria-praseodymia catalysts with larger surface areas. This evidences the beneficial effect of well-defined nanostructures in soot combustion, due to their possession of highly reactive low-index facets (100) and (110). Within SCS-based samples, however, the specific surface area overshadows the importance of praseodymium. This eventually marks the synergistic combination of well-defined nanostructures and praseodymium as a dopant.

K-supported catalysts for diesel soot combustion: Making a balance between activity and stability

Two typical K-supported oxides (K/Al2O3 and K/TiO2) were selected as catalysts for diesel soot combustion in order to evaluate the relationship between activity and stability. The surface species were first characterized by X-ray powder diffraction (XRD), X-ray absorption fine-structure (XAFS), infrared analysis (IR), X-ray photoelectron spectroscopy (XPS), in situ IR of CO2 adsorption and temperature-programmed desorption of CO2 (CO2-TPD). Then the activity was studied by temperature-programmed oxidation (TPO), temperature-programmed reduction with soot (Soot-TPR) and in situ IR of soot combustion reactions. Lastly, the stability of the catalyst was checked by washing with water and analysis for K. This showed that K/TiO2 displayed negligible activity due to the immobilization of K+ ions in TiO2 to form K2Ti6O13. However, K/Al2O3 exhibited superior activity owing to the presence of active free K+ ions on the surface. The downside is the leaching of K+ ions into the effluent containing some water vapor, resulting in its poor stability – the other side of the coin for the highly active free K species. One of the suggested strategies for making a balance between activity and stability is encapsulating active K species in the channel of the catalyst as demonstrated by the tunneled cryptomelane.

Structure and catalytic property of CeO2-ZrO2-Fe2O3 mixed oxide catalysts for diesel soot combustion: Effect of preparation method

A series of Ce0.5Fe0.30Zr0.20O2 catalysts were prepared by different methods (co-precipitations method, citric acid sol-gel method, impregnation method, physical mixed method, and hydrothermal method) and characterized by X-ray diffraction (XRD), Raman spectroscopy, Brunauer-Emmett-Teller (BET) and H2-TPR measurements. Potential of the catalysts in the soot oxidation was evaluated in a temperature-programmed oxidation (TPO) apparatus. The results showed that all the Fe3+ and Zr4+ were incorporated into ceria lattice to form a pure Ce-Fe-Zr-O solid solution for the co-precipitation sample, but two kinds of Fe phases existed in the Ce-Fe-Zr-O catalysts prepared by other methods: Fe3+ incorporated into CeO2 lattice and dispersed Fe2O3 clusters. The free Fe2O3 clusters could improve the activity of catalysts for soot oxidation comparing with the pure Ce-Fe-Zr-O solid solution owing to the synergetic effect between free Fe2O3 and surface oxygen vacancies. In addition, the activity of catalysts strongly relied on the surface reducibility of free Fe2O3 particles. Holding both abundant free Fe2O3 particles and high oxygen vacancy concentration, the hydrothermal Ce0.5Fe0.3Zr0.2O2 catalyst presented the lowest Ti (251 °C, ignition temperature of soot oxidation) and Tm (310 °C, maximum oxidation rate temperature) for soot combustion (with tight-contact between soot and catalysts) among the five samples. Even after aging at 800 °C for 10 h, the Ti and Tm were still relatively low, at 273 and 361 °C, respectively, indicating high catalytic stability.

Zirconia-Supported Copper and KNO3 Catalysts for Diesel Soot Combustion. Deactivation by Hydrotreatment and SO2

In this study, the sulfur tolerance and hydrothermal stability of Cu-KNO3/ZrO2 catalysts in catalytic diesel soot oxidation were investigated. Supported alkaline nitrate catalysts have shown excellent activity for soot oxidation in the presence of O2 or NO/O2. High-temperature water vapor treatments do not affect the catalytic activity, suggesting a good hydrothermal stability of the catalysts. Treatments performed at high SO2 concentration lead to a partial deactivation of the catalysts for the combustion reaction in the presence of O2. The presence of copper improves the sulfur tolerance of the KNO3/ZrO2 catalyst for the combustion of soot in the presence of NOx. NO/O2 present in the feed flow of the reactor can regenerate in situ nitrate anions that provide the redox cycles required for the reaction. Keywords: Diesel soot, Combustion, Hydrothermal stability, SO2.

Potassium-promoted Ce0.65Zr0.35O2 monolithic catalysts for diesel soot combustion

This work presents a comparative study of Ce0.65Zr0.35O2 and CeO2 catalysts supported on cordierite for soot combustion, and the effect of potassium on the catalytic performance. The catalysts were prepared by sequential impregnation over the monolith. The activity in the soot combustion by O2 was studied by successive temperature programmed oxidation cycles, using a loose-contact mode between the soot and the catalyst. A deactivation was observed after ten cycles of combustion for potassium catalysts. This deactivation was attributed to potassium loss at high temperatures. On the other hand, the catalysts without K showed activation after the ten soot combustions. When catalysts with the same cerium molar loading were compared, an improvement in the activity of the mixed oxide compared with the pure ceria was observed. The soot combustion was improved for K/Ce0.65Zr0.35O2/cordierite and K/CeO2/cordierite with NO/O2 presence. However, the catalyst K/CeO2/cordierite was still more active. By X-ray diffraction analyses, it was found that insertion of zirconium into the CeO2 structure took place during preparation. However, a Raman and SEM/EPMA studies indicated heterogeneity in Ce0.65Zr0.35O2. Domains of cerium–zirconium inside the cordierite channel with a ratio different from the nominal value were observed by scanning electron microscopy analyses. This heterogeneity could decrease the catalytic activity of the cerium–zirconium oxide mixture. The catalytic activity in supported catalysts strongly depends on the effect of Zr, the number of Ce4+/Ce3+ redox surface sites, the BET area and the structural changes induced by high temperatures.

The synthesis and catalytic performances of three-dimensionally ordered macroporous perovskite-type LaMn1−xFexO3 complex oxide catalysts with different pore diameters for diesel soot combustion

Polymethyl methacrylate (PMMA) microspheres with diameters of 300–800nm were synthesized by a modified emulsifier-free emulsion polymerization technique with water–oil biphase double initiators. The obtained PMMA microspheres did not aggregate and their size distribution was narrow. Three-dimensionally ordered macroporous (3DOM) LaMn1−xFexO3 (x=0, 0.05, 0.1, 0.15) with different pore diameters were prepared by the means of colloidal crystal template method using PMMA microspheres with different diameters as templates. The microspheres and 3DOM catalysts were characterized by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformation infrared spectroscopy (FT-IR), nitrogen adsorption–desorption and laser particle size analysis. All the catalysts possessed 3DOM structure with hexagonal well-ordered array. Their macroporous diameters were in the range of 140–600nm. The catalytic performances of the catalysts for diesel soot combustion were investigated by temperature-programmed oxidation (TPO) reaction. The 3DOM LaMn1−xFexO3 catalysts had good catalytic performances for diesel soot combustion, and their catalytic activities increased with the increasing of pore diameters.

Comparison of catalytic performance of Ag- and K-based catalysts for diesel soot combustion

Soot combustion performance of supported Ag (Ag/TiO2 and Ag/CeO2) and K (K/TiO2 and K/CeO2) catalysts under tight and loose contact conditions was evaluated by thermogravimetric analysis (TG) and the determination of the apparent activation energy as a function of soot conversion. The soot combustion performance strongly depended on the active species and contact conditions. The soot combustion proceeded by catalytic and non-catalytic combustion processes, depending on the contact conditions and the active species. Almost all the soot was catalytically combusted over Ag and K supported catalysts when they were tightly contacted with the soot, and Ag was more active catalytic species for soot oxidation than K. Under the loose contact condition, the supported K catalyst was superior to the supported Ag catalysts in the fraction of catalytically combusted soot and propagation of the combustion to the non-contact soot. Therefore, the supported K catalyst showed higher performance than the supported Ag catalyst for the removal of the majority of soot under the loose contact condition.

Alkali Carbonate Stabilized on Aluminosilicate via Solid Ion Exchange as a Catalyst for Diesel Soot Combustion

We report the successful preparation of alkali-carbonate-supported aluminosilicate catalysts for diesel soot combustion. From the views of practical feasibility as well as economic and environmental concerns, utilizing alkali metals is preferable for diesel soot combustion. We overcome the drawback that the alkali compounds used as catalysts are easily soluble in water. Na2CO3 was stabilized on sodium- and potassium-containing nepheline ((Na, K)-nepheline) with water tolerance. This result suggests that carbonates are stabilized on the supports with higher energy than hydration energy and van der Waals force by thermal treatment of K2CO3/Na-nepheline via solid ion exchange. Na2CO3 on the surface of thermally treated K2CO3/Na-nepheline is likely to be a catalytically active species for carbon black combustion as (Na, K) mixed carbonate. Na2CO3 stabilized on (Na, K)-nepheline is recovered by CO2, which is likely to be derived from catalytic action on combustion of carbon black.

On the role of lattice/surface oxygen in ceria–zirconia catalysts for diesel soot combustion

A series of ceria and ceria–zirconia catalysts with varying composition and surface area have been systematically investigated in the oxidation of soot in the range of temperature 600 K < T < 800 K. The samples have been characterized by their textural and structural properties and formation of homogeneous solid solutions has been confirmed. The activity in soot combustion has been measured both in the presence and absence of gas-phase O 2 in order to characterize the contribution of surface and bulk properties of the materials. It was shown that the number of active surface oxygen linked to ceria plays a major role, while oxygen storage capacity, governed by introduction of zirconia, can be important when and where oxidation of soot particles is operated in the absence or defect of gaseous oxygen. The role of Zr in stabilizing surface area of ceria (thus increasing the number of surface active sites) is also important when evaluating the effect of composition on soot oxidation activity.

Sr 1− xK xTiO 3 catalysts for diesel soot combustion

Strontium titanates partially substituted with potassium (Sr 1− x K x TiO 3, x = 0.1–0.5) were prepared by sol–gel citric method. The obtained materials were characterized by XRD, S BET and cyclohexanol (CHOL) decomposition measurements, their catalytic activity in soot combustion was determined. It has been shown that substitution of strontium by potassium lowers temperature of soot ignition temperature by 270 °C in a 10% O 2 in Ar atmosphere and ca 400 °C in a 1500 ppm of NO 2, 10% O 2 in Ar atmosphere for Sr 0.2K 0.2TiO 3. The potassium promoting effect is discussed in terms of surface basicity and creation of oxygen vacancies where molecular oxygen or (NO, NO 2) is adsorbed forming basic surface oxygen species active for soot oxidation.

Cobalt and KNO 3 supported on alumina catalysts for diesel soot combustion

The catalytic combustion of diesel soot was studied in the presence of fresh and aged catalysts: Co/Al 2O 3, KNO 3/Al 2O 3 and Co/KNO 3/Al 2O 3. The catalysts were prepared by impregnation using nitrate solutions. The catalysts were characterized by X-ray diffraction, thermal programmed reduction, vibrational spectroscopy and X-ray photoelectron spectroscopy. Fresh and aged catalysts present high activity in presence of O 2 and O 2/NO. The values of the combustion temperature decrease more than 200 °C with respect to that observed in the process without catalysis. The activity is associated with the presence of KNO 3 and the role of this salt can be attributed to the contribution of NO 3 −/NO 2 − redox cycle.

Three-dimensionally ordered macroporous LaCo xFe 1− xO 3 perovskite-type complex oxide catalysts for diesel soot combustion

Polymethyl methacrylate (PMMA) microspheres with diameters of 300–500 nm were prepared by using a modified emulsifier-free emulsion polymerization technique with water–oil biphase double initiators. A series of three-dimensionally ordered macroporous (3DOM) LaCo x Fe 1− x O 3 ( x = 0–0.5) perovskite-type catalysts were synthesized by using colloid crystal template (CCT) method with the PMMA microspheres as template. The microspheres and 3DOM catalysts were characterized by means of XRD, FT-IR, SEM and Laser particle size analyzer, and the catalytic performances of the catalysts for diesel soot combustion were investigated by temperature-programmed oxidation (TPO) reaction. The results indicated that the obtained PMMA microspheres did not aggregate and their solid content (i.e., the mass percentage of polymer microspheres in the whole emulsion polymer) reached 9%. Their size distribution was narrow and the microspheres with different sizes can be designed under different conditions. Except LaCo 0.7Fe 0.3O 3, all the catalysts formed 3DOM structure and possessed a hexagonally well-ordered array. Their macroporous diameters were in the range of 250–300 nm. The 3DOM catalysts had better catalytic performances than the nonmacroporous particle catalysts for diesel soot combustion. Among the studied catalysts, the 3DOM LaCo 0.5Fe 0.5O 3 catalyst had the highest catalytic activity, and its T 50 and S C O 2 m were 397 °C and 99.7%, respectively.

Studies of Zn-Al-Ce mixed oxides as catalysts for diesel soot combustion

A series of Zn-Al-Ce mixed oxides was synthesized by a co-precipitation method. The obtained samples were characterized with respect to composition (XRF), structure (XRD, FT-IR) and texture (BET). Zn-Al-Ce mixed oxides were tested as catalysts of diesel soot combustion. The best catalytic activity was found for Zn2Ce oxide system, which operated in the temperature range of 350-500°C.

Lanthanum cobaltite catalysts for diesel soot combustion

This paper deals with the preparation (by solution combustion synthesis), the characterization (by XRD, AAS, BET, FESEM, XPS and TPD/R analyses), the catalytic activity evaluation (in a temperature-programmed combustion microreactor), and the conception of a possible reaction mechanism of a series of nano-structured soot combustion catalysts based on La–Co substoichiometric or alkali metal-substituted perovskites (La0.9CoO3, La0.9Na0.1CoO3, La0.9K0.1CoO3, La0.9Rb0.1CoO3), whose performance has been compared with that of the standard LaCoO3. Significant catalytic activities have been measured in the 350–450°C range, even after a specific thermal ageing protocol (at 850°C for 16h in the presence of water) or repeated combustion cycles (up to 9). The La0.9Rb0.1CoO3 catalyst has been found to allow the best compromise between satisfactory catalytic activity and stability. This catalyst has been then deposited on an SiC wall-flow trap, submitted to the same ageing treatment and then tested on a diesel engine bench (trap loading and regeneration inducing a temperature increase due to the catalytic combustion of the on purpose post-injected fuel). The presence of the catalyst in the wall-flow trap has enabled both a more complete regeneration and a significant reduction of the regeneration time compared to that of a non-catalytic trap, even after ageing, with a consequent potential saving of post-injected fuel.

On the effect of La–Cr–O– phase composition on diesel soot catalytic combustion

A series of samples of La–Cr–O– perovskites were designed as catalysts for diesel soot combustion. They were prepared by using co-precipitation method, at ambient temperature using ammonia followed by a hydrothermal treatment (T = 200 °C, P = 20 atm, t = 24 h). All the chromium-containing precursors were then calcined at high temperature to develop the oxide catalyst. Two phase composition 86%LaCrO3–(14%) La2CrO6 or 94%LaCrO3–6%La2O3 were formed depending on the atmosphere of calcination (oxygen or hydrogen respectively) used. Their respective BET surface areas were 1.1 and 6.5 m2 g−1. Highly crystalline, pure phase of LaCrO3 and La2CrO6 powders were also prepared, with BET area of 4 and 3 m2 g−1, respectively. The crystalline structure and properties of all samples were characterised by X-ray diffraction (XRD), using Rietveld refinement, and temperature-programmed reduction (TPR) techniques. O2-TPD measurements performed on all samples showed the presence of suprafacial, weakly chemisorbed oxygen only for LaCrO3, which contributes actively to soot combustion. TPR study performed on all catalysts showed that while pure LaCrO3 and La2O3 samples did not reduce, the biphasic catalysts showed the presence of oxygen depletion peaks characteristic of lattice oxygen mobility in the samples at ca. 665 °C. Catalytic combustion of diesel soot was studied over all catalysts. The results showed that pure LaCrO3 exhibited significant catalytic activity which was sensitively affected by the modifier La2CrO6 or La2O3.

Alkaline‐Earth Metal Doped K‐V Catalysts for Diesel Soot Combustion–Preparation, Characterization and Catalytic Properties

The K‐V‐alkaline‐earth metal catalysts supported on α‐alumina ceramic substrate have been prepared. The morphology of the soot particulates deposited on prepared catalysts supported on α‐alumina ceramic substrate was described by scanning electron microscopy (SEM). The XRD was used to characterize the catalysts and their catalytic activities were evaluated by soot oxidation reaction using the TPR system. The SEM photographs presented that soot particles have a particle size of about 100 nm in diameter forming a loose contact with catalyst, which is resemble to the real situation for the catalyst application. The XRD and TPR study showed that the catalytic activities of the catalysts were improved through the cooperation of KNO3 and some alkaline earth metal compounds. The soot onset ignition temperature at 310°C is the lowest by the cooperation of the crystalline phases KNO3, KCaVO4, and Ca3(VO4)2 for the K‐V‐Ca catalyst with a molar ratio of 6:1:1. In addition, the catalyst containing higher KNO3 content has an adsorption for CO2. The all prepared K‐V‐Ba catalysts can adsorb more CO2 at room temperature.

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.

Characterization and catalytic activity of unsupported FeV and K-FeV catalysts for diesel soot combustion

A microstructural characterization by XRD, FT-IR and temperature programmed reduction (TPR) of unsupported FeV and K-FeV catalysts, prepared by solid-state synthesis, is reported. The formation of FexVyOz mixed phases of different compositions has been evidenced depending on the V/Fe ratio and calcination temperature. The catalytic activity of the samples in the combustion of diesel soot has been investigated with the aim to understand the role of FexVyOz mixed phases in the title reaction.