Potassium Stability in Soot Combustion Perovskite Catalysts

The activity for soot combustion in NO _x /O₂ and stability, under reaction conditions, of two potassium-perovskite catalysts (K/SrTiO₃ and Sr_0.8K_0.2TiO₃) and a potassium-copper perovskite catalyst (K–Cu/SrTiO₃) has been studied. In fresh catalysts, potassium is more active than copper. However copper is stable under reaction conditions while potassium-catalysts are progressively deactivated due to the loss of this metal during consecutive TPR cycles.     

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.     

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.     

含氯挥发性有机化合物催化燃烧催化剂的研究进展

含氯挥发性有机化合物(CVOCs)是一类重要的大气污染物,催化燃烧是实现CVOCs高效减排的一种主流处理技术,但工艺过程中存在氯元素易吸附在催化剂表面致使催化剂失活的问题。本文从催化燃烧CVOCs的反应机理、催化剂活性组分、催化剂载体等几个方面,对近年来催化燃烧处理CVOCs的研究进行了综述,其中催化活性组分可分为以钌、钯为主的贵金属催化剂和集中在高活性的过渡金属复合氧化物、钙钛矿型非贵金属催化剂,并重点阐述了水蒸气对催化燃烧CVOCs反应活性的影响及机制。根据国内外研究状况和技术水平,提出了催化燃烧技术的研究及发展方向,充分利用一定浓度水蒸气的优点抑制催化剂氯中毒及产生较低含量的副产物,为CVOCs高效工业化处理提供了重要的参考。     

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.     

天然气催化燃烧催化剂的研究(Ⅱ）

介绍了天然气催化燃烧钙钛矿型氧化物催化剂、六铝酸盐型催化剂以及负载型非贵金属催化剂的研究现状。对于钙钛矿型氧化物催化剂,利用A位取代或调整B位元素的种类及配比、新的技术和方法制备高比表面积或具有纳米结构的钙钛矿型氧化物,是提高其甲烷催化燃烧活性的重要手段。六铝酸盐型催化剂具有很高的热稳定性和甲烷燃烧活性,但起燃温度较高,通过采用将金属Pd负载到六铝酸盐上或改变制备方法,提高其比表面积,以提高其低温反应活性。负载型非贵金属催化剂研究最多的是过渡族金属,其氧化活性、抗毒性能和耐久性都存在问题,需进一步研究。     

Nanosized Pt-Perovskite Catalyst for the Regeneration of a Wall-Flow Filter for Soot Removal from Diesel Exhaust Gases

Perovskite-type catalysts have been investigated for diesel soot combustion: (i) the LaCr_0.9O_3– substoichiometric perovskite, (ii) K–La partially substituted chromites; (iii) Pt added ii-type perovskites. The catalysts prepared showed a progressively higher activity and potential for practical application in diesel particulate traps. Engine bench tests performed on a SiC wall-flow trap (Ibiden) lined with the La_0.9K_0.1Cr_0.9O_3– + 1 wt%Pt catalyst showed that the catalyst not only speeds up soot combustion on occasional trap heating (regeneration phase) but also prolongs the trap loading phase (soot accumulation during normal operation) as Pt active sites promote NO–NO₂ oxidation, followed by the non-catalytic reaction of NO₂ with the trapped soot.     

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.     

HT-search for alkaline- and noble-metal-free mixed oxide catalysts for soot oxidation

High-throughput techniques have been applied to search for soot oxidation catalysts free of noble metals and alkaline metals. Libraries consisting of up to 206 catalysts were screened for relative heats of reaction by emissivity-corrected infrared thermography. Using this approach, new catalyst formulations capable of combusting soot at relatively low temperatures were discovered. The hits identified via the high-throughput experimentation were successfully synthesized and tested conventionally using thermogravimetric analysis. The highest activity of the Pb₁₀La₅Co₈₅O_x catalyst was found to be maintained after calcination at 700 °C. A good correlation between XRD and transmission electron microscope (TEM) measurements was achieved. Electron dispersive X-ray (EDX) analysis at specific points revealed a homogeneous distribution of the various constituents of the catalysts as expected based on the sol–gel method used.     

Reduction of lean NO2 with diesel soot over metal-exchanged ZSM5, perovskite and γ-alumina catalysts

The catalytic performances of metal-exchanged ZSM5, perovskite and γ-alumina catalysts for the reduction of nitrogen dioxide (NO₂) by diesel soot were investigated. The reaction tests were performed through temperature-programmed reaction (TPR), in which NO₂ and O₂ were passed through a fixed bed of catalyst-soot mixture. On the three types of catalyst, NO₂ was reduced to N₂ by model soot (Printex-U) and most of the soot was converted into CO₂. Pt-, Cu- and Co-exchanged ZSM5 catalysts exhibited reduction activities with conversions of NO₂ into N₂ of about 20%. Among the perovskite catalysts tested, La_0.9K_0.1FeO₃ showed a 32% conversion of NO₂ into N₂. The catalytic activities of the perovskite catalysts were largely influenced by the number and stability of oxygen vacancies. For the γ-alumina catalyst, the peak reduction activity appeared at a relatively high temperature of around 500 °C, but the NO₂ reduction was more effective than the NO reduction, in contrast to the results of the ZSM-5 and perovskite catalysts.     

Hydrodeoxygenation of guaiacol on noble metal catalysts

Hydrodeoxygenation (HDO) performed at high temperatures and pressures is one alternative for upgrading of pyrolysis oils from biomass. Studies on zirconia-supported mono- and bimetallic noble metal (Rh, Pd, Pt) catalysts showed these catalysts to be active and selective in the hydrogenation of guaiacol (GUA) at 100 °C and in the HDO of GUA at 300 °C. GUA was used as model compound for wood-based pyrolysis oil. At the temperatures tested, the performance of the noble metal catalysts, especially the Rh-containing catalysts was similar or better than that of the conventional sulfided CoMo/Al₂O₃ catalyst. The carbon deposition on the noble metal catalysts was lower than that on the sulfided CoMo/Al₂O₃ catalyst. The performance of the Rh-containing catalysts in the reactions of GUA at the tested conditions demonstrates their potential in the upgrading of wood-based pyrolysis oils.     

LaMnO3 Perovskite Supported Noble Metal Catalysts for the Total Oxidation of Methane

Two series of LaMnO₃ supported noble metal (Pt, Pd, Rh) catalysts prepared by the citrate method and calcined in air at 600 and 800 °C, respectively, were investigated. The catalysts resulting from method A were prepared by simultaneous incorporation of the noble metals during perovskite preparation and those following method B were generated by impregnation of the calcined perovskites with the noble metal compounds. The noble metals form solid solutions with the perovskite lattice. Reduction of the catalysts with hydrogen prior to the catalytic reaction led to a significant enhancement of the catalytic activity. During the catalytic reaction, the noble metal clusters are partially transformed to highly dispersed noble metal oxides or nonstoichiometric noble metal oxide phases, which are the catalytically active phases for the total oxidation of methane. The best results were obtained with the Pd containing catalysts prepared by method B.     

Screening and further investigations on promising bi-functional catalysts for metal–air batteries with an aqueous alkaline electrolyte

A wide range catalyst screening with noble metal and oxide catalysts for a metal–air battery with an aqueous alkaline electrolyte was carried out. Suitable catalysts reduce overpotentials during the charge and discharge process, and therefore improve the round-trip efficiency of the battery. In this case, the electrodes will be used as optimized cathodes for a future lithium–air battery with an aqueous alkaline electrolyte. Oxide catalysts were synthesized via atmospheric plasma spraying. The screening showed that IrO₂, RuO₂, La_0.6Ca_0.4Co₃, Mn₃O₄, and Co₃O₄ are promising bi-functional catalysts. Considering the high price for the noble metal catalysts further investigations of the oxide catalysts were carried out to analyze their electrochemical behavior at varied temperatures, molarities, and in case of La_1?x Ca _x CoO₃ a varying calcium content. Additionally all catalysts were tested in a longterm test to proof cyclability at varied molarities. Further investigations showed that Co₃O₄ seems to be the most promising bi-functional catalyst of the tested oxide catalysts. Furthermore, it was shown that a calcium content of x = 0.4 in LCCO has the best performance.     

Microwave-assisted in-situ regeneration of a perovskite coated diesel soot filter

Dielectric heating may be used as an in situ technique for the periodic regeneration of soot filters, as those used in Diesel engines. As generally the Diesel exhaust temperatures are below the soot light-off temperature, passive regeneration is not possible. Presently, we have investigated the dielectric heating of a monolithic soot filter, coated with a La_0.8Ce_0.2MnO₃ perovskite. This type of perovskite has suitable dielectric properties, i.e., a high dielectric loss factor, and simultaneously acts as an oxidation catalyst. It is shown that a perovskite coated cordierite filter, covered with synthetic carbon, can be fully regenerated with selectivity towards CO₂ close to 100%. In contrast, the same filter without such a coating can only be partially regenerated. Moreover, considerable amounts of CO are formed. The La_0.8Ce_0.2MnO₃ coating is shown to have excellent temperature shock resistance and thermo-chemical stability. It is shown that the microwave cavity design is crucial to obtain a uniform filter heating. Following the development of such a design, microwave assisted soot filter regeneration becomes feasible.     

Innovative means for the catalytic regeneration of particulate traps for diesel exhaust cleaning

Catalytic traps for diesel particulate removal are multifunctional reactors coupling filtration and catalytic combustion of soot. This paper reviews the most recent developments carried out at Politecnico di Torino concerning two different trap types: zirconia-toughened-alumina foams catalysed with Cs–V catalysts, operating according to a deep filtration mechanism, and cordierite or SiC wall-flow filters catalysed with perovskite catalysts (e.g. LaCr_0.9O₃), enabling shallow-bed filtration. The preparation and characterisation of these two trap types are described and the performance of the traps (filtration efficiency, pressure drops, etc.) evaluated on a diesel engine bench under various operating conditions. A final critical assessment points out that most chances of practical application in mobile sources lie in wall-flow type traps for their superior filtration efficiency (>95%) and their compatibility with active trap regeneration means (e.g. fuel post-injection) that can occasionally rise on purpose the exhaust gas temperature to accelerate the catalytic combustion of trapped soot. Conversely, completely passive solutions based on deep filtration catalytic traps show only promise for stationary applications at temperatures higher than 350°C, due to insufficient catalyst activity at lower temperatures.     

Catalytic removal of methane over thermal-proof nanostructured catalysts for CNG engines

Five spinel-type catalysts AB₂O₄ (Co_0.8Cr₂O₄, CoCr₂O₄, MnCr₂O₄, MgFe₂O₄ and CoFe₂O₄) were prepared and characterized by XRD, BET and FESEM techniques. The activity of these catalysts towards the combustion of methane was evaluated in a Temperature Programmed Combustion (TPC) apparatus. The half conversion temperature of methane over the Co_0.8Cr₂O₄ catalyst was 369 °C with a W/F = 0.12 g s/cm³. On the basis of Temperature Programmed Desorption (TPD) of oxygen as well as of catalytic combustion runs, the prevalent activity of the Co_0.8Cr₂O₄ catalyst could be explained by its higher capability to deliver suprafacial chemisorbed oxygen species. This catalyst, promoted by the presence of 1 wt% of Pd, deposited by wet impregnation, was lined on cordierite monoliths and then tested in a lab-scale test rig. The combination of Pd and Co_0.8Cr₂O₄ catalysts enables half methane conversion at 340 °C (GHSV = 10,000 h⁻¹), a performance similar to that of conventional 4 wt% Pd-γ Al₂O₃ catalysts but guaranteed with just a four-fold lower amount of noble metal. Both the catalysts in powder and the monolith hosting the Co_0.8Cr₂O₄ + 1 wt% Pd catalyst, submitted to a thermal ageing treatment in air at 700 °C for 12 h, displayed a negligible deactivation.     

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.     

Catalytic combustion of soot on metal oxides and their supported metal chlorides

Gravimetric temperature programmed oxidation was used to study the combustion of a soot mixed with various metal oxides and their supported metal chloride catalysts. It is found that the catalytic effect of metal oxide on soot combustion varies depending on property of oxides. CuO and Cr₂O₃ are better catalysts. Addition of some chloride salts (FeCl₃, NaCl and KCl) increases the catalytic activity and KCl exhibits the highest promoting effect by reducing the T_max for about 200 °C. Metal chlorides can also show a synergistic effect on soot combustion. FeCl₃–KCl/CuO can reduce the T_max of carbon oxidation from 780 to 500 °C. Investigation also demonstrates that FeCl₃–KCl/CuO is effective for NO reduction at low temperatures.     

Synergistic catalysis effect of Mn-promoted BaAl2O4 catalysts on catalytic performance for soot combustion

A series of Mn-promoted BaAl₂O₄ catalysts were developed for catalytic soot combustion. These catalysts were characterized using XRD, BET, H₂-TPR, O₂-TPD, and in-situ DRIFTS. The results showed that an optimized combustion catalyst should accommodate barium and manganese with proper balance to promote soot combustion. A synergistic mechanism was suggested to explain the synergistic effect of manganese and barium: NO was trapped mainly as nitrites by barium, while the generated nitrites further transformed to nitrates facilitated by manganese, which finally oxidized soot into CO₂.     

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.