Diesel Soot Combustion Research Articles

Precise casting of biomorphic La0.9K0.1CoO3 catalysts derived from pinewood for diesel soot combustion

A biomorphic perovskite-type oxide, possessing a large surface area, was successfully fabricated using pinewood as a template. The porous structure prompted catalytic activity towards soot combustion under low temperature.

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.

Synthesis of K-doped three-dimensionally ordered macroporous Mn0.5Ce0.5Oδcatalysts and their catalytic performance for soot oxidation

A series of K-doped Mn0.5Ce0.5Oδ (K-MCO) catalysts with three-dimensionally ordered macroporous (3DOM) structure and different K loadings were successfully synthesized using simple methods. These catalysts exhibited well-defined 3DOM nanostructure, which consisted of extensive interconnecting networks of spherical voids. The effects of the calcination temperature and calcination time on the morphological characteristics and crystalline forms of the catalysts were systematically studied. The catalysts showed high catalytic activity for the combustion of soot. 3DOM 20% K-MCO-4h catalyst, in particular, showed the highest catalytic activity of all of the catalysts studied (e.g., T50 = 331 °C and SmCO2 = 95.3%). The occurrence of structural and synergistic effects among the K, Mn, and Ce atoms in the catalysts was favorable for enhancing their catalytic activity towards the combustion of diesel soot. Furthermore, the temperatures required for the complete combustion of the soot (<400 °C) were well within the exhaust temperature range (175–400 °C), which means that the accumulated soot can be removed under the conditions of the diesel exhaust gas. These catalysts could therefore be used in numerous practical applications because they are easy to synthesize, exhibit high catalytic activity, and can be made from low cost materials.

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.

Shape-selective oriented cerium oxide nanocrystals permit assessment of the effect of the exposed facets on catalytic activity and oxygen storage capacity.

The catalytic performance of a range of nanocrystalline CeO2 samples, prepared to have different morphologies, was measured using two accepted indicators; oxygen storage and diesel soot combustion. The same powders were characterized in detail by HR-TEM, XRD, XPS, and Raman methods. The study demonstrates that activity is determined by the relative fraction of the active crystallographic planes, not by the specific surface area of the powders. The physical study is a step toward quantitative evaluation of the relative contribution to activity of the different facets. The synthetic protocol permits fabrication of CeO2 nanostructures with preferentially grown active planes, and therefore has potential in developing catalytic applications and in nanocompositing.

Structured catalysts based on sepiolite with tailored porosity to remove diesel soot

Wall-flow monolithic filters are commonly made of silicon carbide or cordierite and present high porosity, but low surface area. Alternatively, wall-flow monoliths with high surface area can be employed as support for active phases that catalyze the elimination of the particles deposited in the pores. In this study, mesoporous monoliths with tailored macroporosity are obtained by extrusion using sepiolite clay and different pore generating agents. Cobalt, barium and potassium were incorporated as active phases during the kneading of the dough or by impregnation of the structured material to give the monoliths the capacity for catalytically burning soot under the conditions of diesel engine exhausts. The incorporated and impregnated catalytic monoliths were morphologically, mechanically and physical-chemically characterized at different preparation stages to study the influence of support porosity and the method of addition of active phases in the catalytic activity for diesel soot combustion. The most active catalyst was obtained impregnating the active components to a previously extruded monolith calcined at 850°C, and then calcining the assembly at 600°C, for which the temperature of maximum soot combustion rate was 385°C.

Active oxygen by Ce–Pr mixed oxide nanoparticles outperform diesel soot combustion Pt catalysts

A Ce0.5Pr0.5O2 mixed oxide has been prepared with the highest surface area and smallest particle size ever reported (125m2/g and 7nm, respectively), also being the most active diesel soot combustion catalyst ever tested under realistic conditions if catalysts forming highly volatile species are ruled out. This Ce–Pr mixed oxide is even more active than a reference platinum-based commercial catalyst. This study provides an example of the efficient participation of oxygen species released by a ceria catalyst in a heterogeneous catalysis reaction where both the catalyst and one of the reactants (soot) are solids. It has been concluded that both the ceria-based catalyst composition (nature and amount of dopant) and the particle size play key roles in the combustion of soot through the active oxygen-based mechanism. The composition determines the production of active oxygen and the particle size the transfer of such active oxygen species from catalyst to soot.

Synthesis of three-dimensionally ordered macroporous Al–Ce mixed oxide catalysts with high catalytic activity and stability for diesel soot combustion

A series of three-dimensionally ordered macroporous (3DOM) Al–Ce mixed oxide catalysts were prepared by the colloidal crystal templating method. The obtained catalysts were characterized by means of XRD, SEM, BET, H2-TPR, and Raman spectroscopy. The effects of an Al/Ce ratio on the phase structure and catalytic performances were investigated. Al–Ce mixed oxide catalysts could form solid solutions with moderate aluminum content. 3DOM samples showed higher catalytic activity for soot combustion than corresponding disordered macroporous (DM) ones due to enhanced contact efficiency between the soot and the catalyst. 3DOM mixed oxide catalyst with aluminum to cerium ratio of 3:7 shows the highest catalytic activity for soot combustion, in which the values of T10, T50, and T90 are 297, 384, and 427°C, respectively. The doping of Al into CeO2 catalysts can enhance the catalytic stability of the catalyst. After 6-cycle runs, the T50 value of 3DOM samples with alumina:cerium (3:7) and alumina:cerium (6:4) have no significant change. Furthermore, the effect of thermal treatments on the catalytic properties for soot combustion was also studied. Compared with 3DOM bare CeO2, 3DOM Al–Ce mixed oxide catalysts exhibited higher thermal stability because aluminum can prevent cerium particles from agglomeration.

Catalytic performance of CuO/Ce0.8Zr0.2O2 loaded onto SiC-DPF in NOx-assisted combustion of diesel soot

A copper/ceria-zirconia catalyst was incorporated onto a DPF following an environmentally-friendly impregnation procedure. Its catalytic activity was studied for diesel exhaust NO oxidation and soot combustion reactions.

Three-dimensionally ordered macroporous SiO2-supported transition metal oxide catalysts: facile synthesis and high catalytic activity for diesel soot combustion

Three-dimensionally ordered macroporous (3DOM) SiO2-supported transition metal oxide catalysts were successfully synthesized, and they exhibit high catalytic activities for soot combustion.

Oxidation of diesel soot on binary oxide CuCr(Co)-based monoliths

Binary oxide systems (CuCr2O4, CuCo2O4), deposited onto cordierite monoliths of honeycomb structure with a second support (finely dispersed Al2O3), were prepared as filters for catalytic combustion of diesel soot using internal combustion engine's gas exhausts (O2, NOx, H2O, CO2) and O3 as oxidizing agents. It is shown that the second support increases soot capacity of aforementioned filters, and causes dispersion of the particles of spinel phases as active components enhancing thereby catalyst activity and selectivity of soot combustion to CO2. Oxidants used can be arranged with reference to decreasing their activity in a following series: O3≫NO2>H2O>NO>O2>CO2. Ozone proved to be the most efficient oxidizing agent: the diesel soot combustion by O3 occurs intensively (in the presence of copper chromite based catalyst) even at closing to ambient temperatures. Results obtained give a basis for the conclusion that using a catalytic coating on soot filters in the form of aforementioned binary oxide systems and ozone as the initiator of the oxidation processes is a promising approach in solving the problem of comprehensive purification of automotive exhaust gases at relatively low temperatures, known as the “cold start” problem.

The structural catalyst CuMoO4/TiO2/TiO2 + SiO2/Ti for diesel soot combustion

A Ti surface treated by plasma electrolytic oxidation (PEO) in silicate electrolyte has been coated with the nanostructural TiO2 layer and then impregnated using the sol–gel method with Cu and Mo, which are active ingredients for soot combustion. The layer TiO2 consists of anatase crystals of up to 50nm, and has a mosaic-like structure with interconnected surface cracks. The quantity and volume of microcracks depend on both a concentration of the TiO2 colloidal suspension and a regime of its heat treatment. After impregnation with Cu and Mo and calcination at 550°С, the surface of the TiO2 layer is evenly covered by the CuMoO4 particles of 150–350nm. The catalytic performance for diesel soot combustion was analyzed under loose contact conditions of soot and catalyst. The structured catalysts CuMoO4/TiO2/TiO2+SiO2/Ti provide diesel soot combustion above 280°С with two maximum rates of 340–360°С (soot combustion in volume of microcracks) and 397–405°С (soot combustion on flat sides of a surface). The PEO coating on the titanium substrate showed a good adherence when subjected to an ultrasonic test, and improved after using weakly concentrated colloidal suspension for the formation of a thin TiO2 layer.

Characterization of the NO-soot combustion process over La0.8Ce0.2Mn0.7Bi0.3O3 catalyst

The combustion of diesel soot in NO+O2/He atmospheres was investigated by temperature-programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS), and in-situ diffuse reflectance Fourier transform (DRIFT) technology, with the aim to demonstrate the participation of nitric oxide species and surface carbon-oxygenated complexes in the process. According to the TPO results, two main adsorption periods for nitric oxide species were observed: one was from 50 to about 180°C and the other occurred in the temperature range of 180–400°C. The DRIFT results indicated that the first adsorption period was mainly associated with weakly adsorbed NO, bidentate nitrite and unidentate nitrate, monodentate nitrate, chelating nitrate and bridging nitrate, while the second uptake was related to nitrite and free nitrate species. Based on the XPS and DRIFT results, possible reaction processes for the NOx-soot combustion are proposed based on the Langmuir–Hinshelwood mechanism. First, the carbon-oxygenated groups, including carboxyls, lactones, and anhydrides were formed at free carbon sites by the oxidation of dissociative nitrates. Such moieties were so stable that they could not be further oxidized, even at 400°C, without the presence of a catalyst. Second, the adsorption of NO+O2 resulted in the formation of nitrates on the MnOx surface of the catalyst. Then, the surface-activated nitrates, with activity decreasing in the order: chelating nitrates>bridging nitrates>monodentate nitrates>free ionic nitrates, further oxidized these carbon-oxygenated complexes with subsequent release of CO2, NO and N2.

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-copper perovskite catalysts for mild temperature diesel soot combustion

The activity and stability of potassium-copper perovskite catalysts for soot combustion were analysed at 450°C in a NOx/O2 gas mixture. Both impregnated (K-Cu/SrTiO3) and substituted (SrKTiCuO3) copper-potassium catalysts were prepared. For comparative purpose, also a potassium substituted catalyst (SrKTiO3) was studied. The high activity of the fresh potassium catalyst (SrKTiO3) is based on the volatile potassium species reaching the surface of soot. However, this catalyst is not useful because potassium is progressively lost during the reaction. Potassium was stabilized by copper and, as a consequence, the two potassium-copper perovskite catalysts present a constant activity for the soot combustion. The activity of the potassium-copper perovskite catalysts, which is maintained during consecutive reactions, is related to their NO2 production capacity. The metal addition method seems to influence the catalysts performance, the substituted catalyst (SrKTiCuO3) being the most active and stable.

Effects of Synthesis Routes on the States and Catalytic Performance of Manganese Oxides Used for Diesel Soot Combustion

The catalyst obtained through the co-precipitation of Mn(Ac)(2) and (NH4)(2)CO3 (denoted as Mn-Ac-NH4) displays the highest performance for soot oxidation. Compared with other MnOx catalysts, the catalyst Mn-Ac-NH4 exhibits much better reducibility and more uniform sponge-like nanostructure. The characterization results indicate that the oxygen species on the surface of Mn-Ac-NH4 are more active for soot combustion. The NO species can also be captured and oxidized by these oxygen species, generating more reactive NO2 which can enhance soot oxidation.

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.

Ceramic papers as flexible structures for the development of novel diesel soot combustion catalysts

Currently, the most adequate technology for the abatement of particles coming from diesel engines is the one that employs catalytic filters. This work proposes the preparation of catalytic ceramic papers to be employed in the development of new catalytic filters. To this end, the papermaking technique was employed. The resulting papers were flexible, easy to handle and presented suitable mechanical properties to resist the tests from a test bench in which they were placed inside a metal housing, at the exhaust pipe outlet of a Corsa 1.7 vehicle. These optimal mechanical properties were obtained through the incorporation of a suspension of CeO2 nanoparticles during the papermaking process. It was found that the nanoparticles covered the ceramic fibers completely and that their excess accumulated under the form of patches. In order to promote the filter continuous regeneration, once the ceramic papers were formed they were impregnated with a cobalt salt which, after a calcination stage, produced oxidic clusters in tight contact with the CeO2 nanoparticles. The papers thus obtained exhibited a maximum soot combustion temperature close to 400 °C, being T50 = 376 °C for the more active catalytic ceramic paper, in the presence of NO (1000 ppm) in the feed. Even though more tests are necessary to determine filtration efficiency, preliminary experiments carried out in the test bench proved that the ceramic papers properly resisted the high gas flow rates of fumes emitted by a real diesel engine (approximately 2 m3 min−1). From the TPO experiments performed on samples of catalytic ceramic papers taken from the test bench and in tight contact with the soot retained, it was possible to obtain a maximum soot combustion temperature equal to 428 °C. It was also found that the presence of ceria aggregates caused the papers to deactivate to a lesser degree after being treated at 900 °C.

Development of a stacked wire-mesh structure for diesel soot combustion

Abstract This work describes the development of stacked wire-mesh structures and their washcoating method to prepare a structured system for the catalytic combustion of soot. For this purpose, a metallic structured substrate was designed and built using wire-mesh discs of AISI 304. The coating of this substrate was optimized by investigating various strategies and parameters of the washcoating such as the suspension formulation (sol–gel coating with posterior impregnation of the active phase, the all-in-one strategy using catalyst precursors and the preparation of the catalyst slurry with a ready-made catalyst), the amount of Co, Ba, K/CeO 2 catalyst and the packing material (wire diameter and number of packed discs). The coating proposed strategies were analyzed measuring textural properties, catalyst coating adherence and morphology, and pressure drop of the catalytic substrate. Finally, the coated wire-mesh substrates were tested in the combustion of soot showing promising catalytic activity.