Catalysts For Diesel Soot Oxidation Research Articles

Catalytic activity of Zr/CeO2-Al2O3 catalyst for diesel soot oxidation: synthesis, characterization, and performance evaluation.

Diesel soot is a significant contributor to air pollution. Soot particles present in diesel engine exhaust have a negative impact on the environment and human health. Diesel oxidation catalysts (DOCs) and diesel particulate filters (DPFs) currently use noble metal-based catalysts for soot oxidation. Due to the use of noble metals in the catalyst, the cost of diesel after-treatment systems is steadily rising. As a result, diesel vehicles have become commercially less viable than gasoline vehicles and electronic vehicles. The study focuses on an alternative diesel oxidation catalyst with efficiency similar to that of a noble metal catalyst but with a much lower cost. CeO2-Al2O3 catalysts are known for their oxygen storage capacity and high redox activity, making them suitable for soot oxidation. Adding Zr to these catalysts has been shown to influence their structural and chemical properties, significantly affecting their catalytic behavior. Therefore, the current study is focused on using Zr/CeO2-Al2O3 as a substitute for noble metal-based catalysts to enhance its performance for diesel soot oxidation in automotive exhaust. Evaporation-induced self-assembly (EISA) was used to prepare 1, 3, and 5 weight (wt) % Zr supported mesoporous CeO2-Al2O3 catalysts. Morphological, structural, and physicochemical properties of the synthesized catalysts were examined using Brunauer-Emmett-Teller (BET) absolute isotherm, Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Temperature programmed reduction (TPR), and Temperature-programmed desorption of ammonia (NH3-TPD). XRD, BET, and SEM data confirmed that the catalysts were mesoporous and low-crystalline with a high surface area. The soot oxidation activity of the catalysts was evaluated using a thermogravimetric analysis (TGA) technique. The loose contacts soot oxidation activity test suggested that 50% oxidation of soot occurred at 390°C in the absence of a catalyst. T50 of CeO2-Al2O3 catalyzed soot oxidation was 296°C. Adding Zr to the catalyst significantly improved catalytic activity for diesel soot oxidation. We observed a further drastic change in T50 of soot over 1, 3, and 5% Zr/CeO2-Al2O3, which were 220°C, 210°C, and 193°C, respectively. According to these results, incorporating Zr into the CeO2-Al2O3 catalyst significantly improved the oxidation process of soot.

Bimetallic catalysts Ag-K(Cs)/ZSM-5 for efficient soot oxidation via synergistically regulating active species and support of catalyst

Diesel soot oxidation is a typical gas (O2)-solid (soot)-solid (catalyst) three-phase oxidation, which requires both high contact efficiency between the three-phase substances and excellent intrinsic catalytic activity of catalysts. We proposed a new strategy for improving the catalytic performances of Ag/HZSM-5 catalyst for diesel soot oxidation via synergistically regulating both active species and support of catalyst. One is constructing hierarchical pore structure on HZSM-5 (denoted as Z) support by lye treatment with various concentration (0.1 M, 0.2 M and 0.5 M); the other is incorporating alkali-metal (K or Cs) into the Ag/Z catalyst to form the bimetallic catalysts. Internally, the bimetallic catalysts Ag-K/Z-0.1 M and Ag-Cs/Z-0.1 M exhibited the superior catalytic activities. The CO2 selectivity reached 98.6% and 95.6% for Ag-K/Z-0.1 M and Ag-Cs/Z-0.1 M, respectively. In addition, Ag-Cs/Z-0.1 M obtained excellent stability, maintaining almost 100% activity without any recession after 10 cycles. Moreover, both bimetallic catalysts exhibited excellent sulfur and water resistance. The physicochemical properties of the bimetallic catalyst systems were investigated by SEM, N2 adsorption–desorption, XRD, XPS, O2-TPD, etc. The linear relationship between metallic silver and active oxygen content, and their correlation with catalysts activities have been discussed. Finally, possible dual synergistic effects in the bimetallic systems have been proposed.

Preparation of Cerium-Bismuth Oxide Catalysts for Diesel Soot Oxidation Including Evaluation of an Automated Soot-Catalyst Contact Mode.

Cerium‐bismuth oxides have emerged as promising candidates for Diesel soot oxidation. The catalysts are synthesized via automated co‐precipitation methods. T50 values, where 50 % of soot is oxidized, and the dynamic oxygen storage capacity (OSCdyn) are used to compare the catalytic activity. The activity is measured by thermogravimetric methods. The synthesized catalysts are characterized through powder X‐ray diffraction (PXRD), Raman spectroscopy, and specific surface area (SBET) measurements. This work investigates the influence of the contact mode between soot and catalyst. The literature‐known manual contact modes “loose”, “tight”, and “wet” are compared with our developed automated contact mode, using a dual asymmetric centrifuge. The rotation speed rs and mixing time tM have been varied independently. Both factors influence the T50 value. A continuous transition from loose to tight contact mode with increasing rotation speed rs can be shown. Furthermore, the automated contact mode shows better reproducibility behavior compared to manual contact modes.

Improved Hydrothermal Stability in Glass Diesel Soot Oxidation Catalysts

The hydrothermal stability of K-Ca-Si-O glass soot oxidation catalysts has been improved by substitution of Ce and Zr for Ca. This work demonstrates that glasses can be tailored to withstand the challenging diesel exhaust hydrothermal environment by considering the field strengths and partial molar free energies of the hydration reactions (ΔGi) of the cation species in the glass. The result is a glass that shows less formation of precipitates after 2 h hydrothermal exposure in air with 7% H2O at temperatures ranging from 300–700 °C. A K-Ca-Si-O glass with a soot T50 (the temperature when 50% of the soot is oxidized) of 394 °C was found to degrade to 468 °C after a 2 h, 700 °C hydrothermal exposure, whereas the improved K-Ce-Zr-Si-O glass only changed from 407 °C to 427 °C after the same treatment.

Study on oxygen species characteristics of CexMn1-xO2 catalyst for diesel soot oxidation

ABSTRACT CexMn1-xO2 series catalysts were prepared with the sol–gel method. The catalytic activity for soot oxidation was investigated by thermogravimetric analysis(TGA). All catalysts were elevated by X-ray diffraction (XRD), nitrogen adsorption techniques, temperature programmed desorption (TPD), Raman spectroscopy, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The influence of the catalysts on soot oxidation characteristics and the relationship between the catalytic oxygen species and the combustion of soot were mainly discussed. The results revealed that the crystal structure of the catalyst did not change after Mn doped CeO2. Mn inhibited the growth of CeO2 cell and promoted the formation of oxygen vacancies on the catalyst surface, which was conducive to the activation and migration of oxygen species. Ce0.5Mn0.5O2 had the most absorptive atomic oxygen O−, and the ignition temperature and peak temperature of Ce0.5Mn0.5O2 were decreased by 37 and 50°C compared to CeO2, respectively.

Effect of the Electronic State of Cu, Ag, and Au on Diesel Soot Abatement: Performance of Cu/ZnO, Ag/ZnO, and Au/ZnO Catalysts.

Noble metals such as Au, Ag, and Cu supported over semiconducting ZnO are well-known heterogeneous oxidation catalysts. All of them have been utilized for the oxidation of diesel soot with varied success. However, Au-supported ZnO is seen to be superior among them. Here, we present a comparative study of all these three catalysts for diesel soot oxidation to explain why Au/ZnO is the best among them, demonstrating the contribution of electronic states of metals in composite catalysts. The electronic states of Cu, Ag, and Au determined by X-ray photoelectron spectroscopy on 1 wt % Cu/ZnO, 1 wt % Ag/ZnO, and 1 wt % Au/ZnO catalysts were correlated with their diesel soot oxidation activities. Although all three catalysts present reasonable diesel soot oxidation activities at relatively low temperature, 1% Cu/ZnO and 1% Ag/ZnO oxidize only about 60% of the deposited diesel soot around 250 °C and 1% Au/ZnO oxidizes 100% of the deposited diesel soot, at a temperature as low as 230 °C. The activity of the catalysts is attributed to the formation of stable M0–Mδ+ bifunctional catalytic sites at the metal–ZnO interface, which enhances the contact efficiency of solid diesel soot on Mδ+ and generates the superoxide species on M0 moieties. The stability of the bifunctional M0–Mδ+ sites is controlled by the electronic interactions between the metal (M) and n-type semiconductor ZnO at their interface. Very high activity of 1% Au/ZnO is attributed to the presence of Au3+ at the catalyst surface, which generates a stronger Coulombic force with diesel soot electrons. We demonstrate a direct relation between the diesel soot oxidation activity of these three metals and their electronic states at the catalyst surface.

Synthesis and Performance of Transition Metal Based Perovskite Catalysts for Diesel Soot Oxidation

In present investigation, the effect of the intrinsic factors including the structure, nature of B-site ions in the four systems LaCoO3, LaNiO3, LaFeO3 and LaZnOy perovskite-type oxide catalysts, and the external factors of catalyst-soot contacting model, and the operating parameters such as air flow rate and temperature on the catalytic performances for the combustion of diesel soot were reported. The catalysts were characterized by XRD, FTIR, SEM, and N2-sorption. Activity of the catalyst for soot oxidation was evaluated on the basis of light off temperature characteristics Ti, T50 and T100. LaCoO3, LaFeO3 and LaNiO3 samples possessed the perovskite structure, and gave high activities for the total oxidation of soot below 445 oC. Whereas, LaZnOy catalyst was not indicating the ABO3 perovskite structure and existed as a mixture of metal oxides. The activity order in decreasing sequence of the catalyst was as follows: LaCoO3>LaFeO3>LaNiO3>LaZnOy. SEM pictures of the perovskite samples showed that the particles sizes were close to 100 nm.

Enhanced Catalytic Performance of Manganese and Cobalt Co–doped CeO2 Catalysts for Diesel Soot Oxidation

AbstractNanosized manganese and cobalt codoped ceria (Ce0.8Mn0.1Co0.1O2−δ) solid solutions have been prepared through simple coprecipitation method to study the effect of Mn and Co bi‐dopants towards diesel soot oxidation. Pure ceria (CeO2), manganese doped ceria (Ce0.8Mn0.2O2−δ), and cobalt doped ceria (Ce0.8Co0.2O2−δ) solid solutions have also been prepared for comparison. XRD results revealed the formation of solid solutions in doped samples. XPS results confirmed the presence of more Ce3+ ions and surface adsorbed oxygen in codoped ceria. H2‐TPR study disclosed that the codoping of ceria exceptionally improves the redox behaviour of CeO2. The catalytic activity results demonstrate that Ce0.8Mn0.1Co0.1O2−δ shows much better activity (T50 ∼ 636 K) while compared to pure CeO2 (T50 ∼ 874 K) in tight contact conditions. A larger surface area, facile redox behaviour, and high concentration of surface adsorbed oxygen species are proposed to be the crucial factors for the enhanced catalytic activity.

Effect of Y on improving the thermal stability of MnOx-CeO2 catalysts for diesel soot oxidation

A series of MnOx-CeO2-Y2O3 catalysts with different Y loadings (0, 1, 3, 6, and 10 wt%) were prepared by a co-precipitation method and investigated for NOx-assisted soot oxidation. The thermal stabilities of these catalysts were evaluated by treating them at 800 °C for 12 h under dry air flow. The catalysts were characterized by X-ray diffraction, N2 adsorption-desorption, Raman spectroscopy, H2 temperature-programmed reduction, oxygen storage capacity, NO temperature-programmed oxidation, X-ray photoelectron spectroscopy, and soot temperature-programmed oxidation. The addition of Y led to decreased BET surface areas and poor low-temperature reduction abilities and oxygen storage capacities, which affected NO and soot oxidation activities. However, after aging, the doping of Y had effectively enhanced the stability of the catalytic activities for NO and soot oxidations, where the addition of 6%–10% Y achieved the optimum result because the maximal soot oxidation rate temperature was increased by only 34–35 °C. Additionally, the catalytic activity and deactivation of MnOx-CeO2-containing catalysts were closely related to the presence of Mn4+ and oxygen species on the surface.

Ag supported on electrospun macro-structure CeO2 fibrous mats for diesel soot oxidation

Ceria oxide (CeO2) nanofibers with diameters of 241–253nm were produced via the electrospinning method. They were then applied as new support materials for silver nanoparticles and their effectiveness as diesel soot oxidation catalysts was examined. The crystal structure of the Ag was confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), and energy dispersive spectroscopy (EDS). The diesel soot oxidation experiment was performed using a mixture of carbon black and the fabricated catalyst. In the tight contact and loose contact modes, the highest temperatures of both the CeO2-fiber-supported Ag catalysts (Ag/CeO2) calcined at 500°C and the CeO2 fibers calcined at 500°C were shifted to a lower temperature range, compared with JRC-CEO-3 (a commercial catalyst). In addition, the activation energy also showed similar behavior. Thus, the fibrous CeO2 and Ag/CeO2 have been shown to be effective and promising catalysts for soot oxidation.

High NO oxidation catalytic activity on non-noble metal based cobalt-ceria catalyst for diesel soot oxidation

In this study, we have explored commercial ceria, commercial cobalt oxide (Co3O4) and a laboratory synthesized nano-ceria supported cobalt oxide (Co3O4) for catalytic NO oxidation, which is an important reaction for soot oxidation in diesel particulate filters (DPF), used for vehicle exhaust emissions control. Ceria supported cobalt oxide has been synthesized by incipient wetness impregnation method, and shows high catalytic activity for NO oxidation at moderate temperature. The catalytic activity is even comparable to reported noble metal based NO oxidation catalysts, and could be mainly due to the high dispersion of Co3O4 nanoparticles on ceria support with enhanced catalyst support interaction. This has also resulted in improved redox properties of Co3O4-ceria catalyst.

Promising ceria–samaria-based nano-oxides for low temperature soot oxidation: a combined study of structure–activity properties

Nanostructured Ce–Sm mixed oxides are promising catalysts for diesel soot oxidation through facile active oxygen participation.

The effect of Ca2+ and Al3+ additions on the stability of potassium disilicate glass as a soot oxidation catalyst

Potassium disilicate (K2Si2O5) glasses substituted with additions of Ca2+ and Al3+ have been prepared, characterized and tested as diesel soot oxidation catalysts. Ca and Al modify the silicate glass network, resulting in improved stability for catalyzing oxidation of diesel soot. TGA and water immersion studies showed that substituting K2O with CaO can improve the chemical stability with a small loss of catalytic activity. (K2O)0.5(CaO)0.5(SiO2)2 had a soot ignition temperature (Tig) around 410°C and also excellent catalytic stability with repeated soot oxidation cycling. Similarly, substituting with Al3+ gave improved stability, with (K2O)0.7(Al2O3)0.3(SiO2)2 exhibiting a Tig around 370°C and better stability than K2O·2Si2O2. The activity results are interpreted with insights gained from FTIR characterization.

A Novel α-Al2O3 Diesel Particulate Filter with Alkali Metal-Based Catalyst for Diesel Soot Oxidation

A diesel particulate filter (DPF) substrate was fabricated from alkali-resistant α-Al2O3 to make a practical use of alkali metal catalysts for diesel soot oxidation. The fundamental properties of the α-Al2O3-DPF, including its particle filtration efficiency, pressure drop, and thermal durability were comparable to those of the conventional DPFs. The diesel soot oxidation activity of the catalyzed α-Al2O3-DPF with a washcoat of alkali metal-based catalyst was, even after thermal aging, much higher than that of the conventional catalyzed-DPF with platinum group metal catalysts.

Higher activity of Diesel soot oxidation over polycrystalline ceria and ceria–zirconia solid solutions from more reactive surface planes

High temperature treatment under air of samples of polycrystalline ceria and ceria–zirconia is shown to induce a rearrangement of crystals, with a surface structural evolution of the oxides toward the formation of more reactive exposed planes belonging to the {100} and {110} families, associated with a decrease of the exposure of the less reactive {111} facets. This is dependent on the amount of ceria, being more evident with pure CeO2, and less important with increasing ZrO2 content. A correlation between the exposure of more reactive planes and the specific rate of carbon soot oxidation is found, in agreement with previous results obtained in monodimensional nanoshaped and polycrystalline ceria samples for CO oxidation. This suggests that Diesel soot oxidation catalysts might be designed by keeping into consideration also these effects.

Catalytic Activity and Stability of K/CeO2 Catalysts for Diesel Soot Oxidation

A series of CeO 2 supported potassium nitrate and carbonate catalysts were prepared by an impregnation method. The catalytic activity and stability for soot oxidation were investigated by thermal gravimetric/differential thermal analysis and temperature-programmed oxidation cycle techniques. K/CeO 2 substantially promotes the catalytic activity, which can be gauged by the lowering of the soot total removal temperature by more than 200 °C compared with the uncatalyzed reaction. The influence of K precursor on the catalytic activity and stability was addressed. The effect is discussed in terms of the combined functions of K and CeO 2 , potassium mobility and basicity. For potassium-promoted ceria catalysts the potassium nitrate can preserve the high catalytic activity and good stability through several cycles. When K was present as K 2 CO 3 , the activity and stability of catalyst were lower than those of KNO 3 /CeO 2 . This behavior is attributed to the higher melting point and stronger basicity of K 2 CO 3 which lead to irreversible adsorption of CO 2 generated during soot combustion on the catalyst. 采用不同的钾盐前体制备了一系列 K/CeO 2 催化剂, 利用热重和程序升温氧化 (TPO) 等技术考察了其催化性能及稳定性. 结果表明, K/CeO 2 催化剂可使碳黑完全燃烧温度降低近 200 ºC. 钾盐前体对催化活性和稳定性具有较大影响, 由于硝酸钾熔点低, 金属在载体上的流动性强, 有利于催化剂与碳黑的有效接触, 因而表现了较高的活性, 三次 TPO 循环试验中催化活性稳定. 碳酸钾的熔点高且碱性较强, 使碳黑燃烧生成的 CO 2 不可逆吸附在其表面, 导致反应活性低, TPO 循环实验表明其反应速率降低, 失活明显. The activity and stability of KNO 3 /CeO 2 catalysts were higher than those of K 2 CO 3 /CeO 2 for diesel soot oxidation. This behavior is attributed to the lower melting point and high surface mobility of KNO 3 .

Deposition of a Cu/Mo/Ce catalyst for diesel soot oxidation on a sintered metal fiber filter with a CeO2 anti corrosion coating

Abstract A three-layer metallic soot filter was coated with a CeO 2 bufferlayer and a soot oxidation catalyst based on copper, molybdenum and cerium oxides. Deposition on the diesel particulate filter was performed by dipcoating from aqueous sol–gel media. The effect of dipcoating speed and use of several additives on the shielding effectiveness of the CeO 2 thin film was studied. Thereby corrosion by the catalyst could be avoided. The catalytic activity and thermal stability were studied by thermogravimetric analysis on powder mixtures while catalyst coated filters were analyzed using a flow through reactor. The combustion temperature was lowered with at least 75 °C.

Activity and stability of BaKCo/CeO2 catalysts for diesel soot oxidation

The activity and stability of Ba,K,Co/CeO2 catalyst in the diesel soot oxidation reaction are studied. Different modes of preparation are analyzed, varying both the cobalt precursor and its order of impregnation. In order to study the stability in a diesel exhaust atmosphere, the catalysts were pretreated in streams of CO2, H2O, NO and SO2. The fresh and the treated catalysts were characterized by FTIR and XRD techniques. The catalytic activity was measured by TPO of soot–catalyst mixtures. The tight contact was used to analyze the intrinsic activity. It was found that the activity was higher for the catalysts prepared using Co(NO3)2 compared with the catalysts prepared using Co(AcO)2. This is because in the former, KNO3 is present on the catalyst, being this compound very active for this reaction. The thermal stability is lower for the catalyst prepared with Co(NO3)2. This catalyst displays a higher K lost when treated at high temperature. When K is present as K2CO3, as is the case of the catalysts prepared with Co(AcO)2, the thermal stability is higher since K2CO3 is less volatile than KNO3. All the catalysts are stable in the presence of mixtures of (CO2+H2O+NO+O2) having a composition similar to a real diesel exhaust. Under these conditions, the K mantains its original chemical state, either carbonate or nitrate, depending on the precursor used in the catalyst preparation. In presence of SO2, all the catalysts deactivate due to K2SO4 formation, which is not active for soot combustion. However, the sensitivity to SO2 depends on the precursors used to prepare the catalyst. The Ba,K,Co/CeO2 catalyst prepared impregnating Co(NO3)2 on the Ba,K/CeO2 catalyst has a higher resistance to the deactivation by SO2, since the following reaction occurs: Ba(NO3)2+K2SO4→BaSO4+KNO3, which implies that active KNO3 will dissapear slower from the catalytic surface, thus maintaining the activity. TPO experiments of the catalyst in loose contact with soot were also carried out. The Ba,K,Co/CeO2 catalyst prepared impregnating Co(NO3)2 on the Ba,K/CeO2 catalyst showed the higher activity in loose contact mode.

Comparison of Preparation Methods of Copper Based PGMFree Diesel-Soot Oxidation Catalysts

CuO-CeO2 systems have been proposed as a promising catalyst for low temperature diesel-soot oxidation. CuO-CeO2 catalysts prepared by various methods were examined for air oxidation of the soot in a semi batch tubular flow reactor. The air oxidation of soot was carried out under tight contact with soot/catalyst ratio of 1/10. Air flow rate was 150 ml/min, soot-catalyst mixture was 110 mg, heating rate was 5 0C/min. Prepared catalysts were calcined at 500 0C and their stability was examined by further heating to 800 0C for 4 hours. It was found that the selectivity of all the catalysts was nearly 100% to CO2 production. It was observed that the activity and stability of the catalysts greatly influenced by the preparation methods. The strong interaction between CuO and CeO2 is closely related to the preparation route that plays a crucial role in the soot oxidation over the CuO-CeO2 catalysts. The ranking order of the preparation methods of the catalysts in the soot oxidation performance is as follows: sol-gel > urea nitrate combustion > Urea gelation method > thermal decomposition > co-precipitation. © 2011 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

High-throughput approach to the catalytic combustion of diesel soot II: Screening of oxide-based catalysts

Following the development of a high-throughput (HT) methodology for the evaluation of diesel soot oxidation catalysts in a 16 parallel channels reactor, a library of over 60 catalysts was tested under optimized conditions. The catalyst compositions were chosen to include solids which specific properties like oxygen storage capacity, oxygen mobility and ionic conductivity. The key parameters for high activity appear related to the presence of active and mobile surface oxygen species, and to an appropriate catalyst particle size in order to favour the number of contacts with the soot. In contrast, high oxygen storage capacity and bulk oxygen ion mobility do not appear as relevant properties for high catalytic activity. Nine new formulations were found to perform better than the reference catalyst “high surface area (HSA) ceria” (Rhodia).