Loose Contact Conditions Research Articles

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.

Effect of the morphological and surface properties of CeO2-based catalysts on the soot oxidation activity

In diesel particulate filters, a solid (soot)–solid (catalyst) reaction occurs and, in these cases, the catalyst morphology plays a relevant role, being the number of contact points a critical feature for the resulting reaction rate.For this reason, two different ceria-based catalyst morphologies have been investigated in this work, for the soot oxidation reaction: first, a ceria catalyst was prepared by solution combustion synthesis (SCS), which lead to a specific surface area of 29m2/g. On the other hand, three-dimensional self-assembled (SA) stars ceria, so-called due to their branched morphology starting from a central nucleus, exhibited a higher specific surface area (124m2/g), a high availability of contact points between soot particles and the catalyst itself, due to the concavities of the obtained morphology, and interesting surface features at the nanoscale. The synthesis method was tuned in order to resort to a desired and well-defined morphology in terms of shape and size.Aged stars were also taken into account, in order to fully evaluate the advantages and drawbacks of this new material against classic SCS catalysts.Compared to the SCS ceria catalyst, the SA stars have a much higher microporous volume, being of about 0.04cm3/g, and a finer crystallite size. SA stars showed a better activity towards soot oxidation: the peak oxidation temperature dropped from 614°C (in the non-catalytic case) to 403°C in tight contact conditions, and to 552°C in loose ones. XPS analysis elucidated that this can be due to a higher surface oxygen availability as compared to SCS ceria, while HRTEM revealed that surface crystallites show oxygen defective structures, almost absent at the SCS catalyst surface, which also might involve a higher redox capability.In loose contact conditions, being the most realistic testing condition because soot deposits on the catalytic layer of the diesel particulate filter without any force exertion, the SA stars exhibited the lowest onset temperature, even after aging, thus proving their higher intrinsic activity. Moreover, aged stars, which are less active than fresh SCS powders in tight contact conditions, regain good results in the loose ones, thanks to intrinsic capability to catch soot particles, peculiar of the SA star morphology.In the perspective of applying such engineered morphologies in diesel particulate filters, the three-dimensional shape of SA stars may involve an enhancement of the number of contact points between the cake layer and the catalyst one, thus fostering the final activity.

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

As morphology plays a relevant role in solid/solid catalysis, where the number of contact points is a critical feature in this kind of reaction, three different ceria morphologies have been investigated in this work as soot oxidation catalysts: ceria nanofibers, which can become organized as a catalytic network inside diesel particulate filter channels and thus trap soot particles at several contact points but have a very low specific surface area (4 m2/g); solution combustion synthesis ceria, which has an uncontrolled morphology but a specific surface area of 31 m2/g; and three-dimensional self-assembled (SA) ceria stars, which have both high specific surface area (105 m2/g) and a high availability of contact points. A high microporous volume of 0.03 cm3/g and a finer crystallite size compared to the other morphologies suggested that self-assembled stars could improve their redox cycling capability and their soot oxidation properties. In this comparison, self-assembled stars have shown the best tendency towards soot oxidation, and the temperature of non-catalytic soot oxidation has dropped from 614°C to 403°C in tight and to 552°C in loose contact conditions, respectively. As far as the loose contact results are concerned, this condition being the most realistic and hence the most significant, self-assembled stars have exhibited the lowest T10% onset temperature of this trio (even after ageing), thus proving their higher intrinsic activity. Furthermore, the three-dimensional shape of self-assembled stars may involve more of the soot cake layer than the solution combustion synthesis or nanofibers of ceria and thus enhance the total number of contact points. The results obtained through this work have encouraged our efforts to understand soot oxidation and to transpose these results to real diesel particulate filters.

Active oxygen-assisted NO-NO2 recycling and decomposition of surface oxygenated species on diesel soot with Pt/Ce0.6Zr0.4O2 catalyst

A Pt/Ce0.6Zr0.4O2 catalyst was synthesized by impregnating a Ce0.6Zr0.4O2 support with Pt(NO3)2 solution. This material exhibits higher catalytic activity than Pt/Al2O3 for soot oxidation under loose contact conditions in both NO + O2 and O2 atmospheres. The results show that the pronounced activity of this catalyst is attributed to a synergistic effect arising from the combination of Pt and Ce0.6Zr0.4O2 solid solution, leading to a remarkable increase in the availability of active oxygen at temperatures as low as 450 °C. The NONO2 recycling efficiency is improved over Pt/Ce0.6Zr0.4O2 catalyst, enhancing the ability of the NO2-soot reaction to generate surface oxygenated complexes (SOCs) on the soot. More importantly, the catalytic oxidation of these SOCs can be accelerated with the participation of active oxygen from the CeO2-ZrO2 solid solution. As a result, during soot oxidation under NO + O2, the light-off temperature of the Pt/Ce0.6Zr0.4O2 catalyst is lowered by 34 °C than that of Pt/Al2O3 catalyst.

Catalytic evaluation of promoted CeO2-ZrO2 by transition, alkali, and alkaline-earth metal oxides for diesel soot oxidation

Series of mixed metal oxides were synthesized by gel-combustion method and their catalytic activities for soot oxidation were investigated. The catalysts were M-Ce-Zr (M = Mn, Cu, Fe, K, Ba, Sr), and xK-20Mn-Ce-Zr (x = 0, 5, 10, 20), they were characterized by XRD, SEM, TPR and BET surface area techniques. The results of soot temperature programmed oxidation (TPO) in an O2 oxidizing atmosphere indicate that K-Ce-Zr has the highest catalytic activity for soot oxidation under loose contact condition, due to enhancement of the soot and catalyst contacts. On the other hand, under a tight contact condition, Mn-Ce-Zr and Cu-Ce-Zr nano-composites have high activities for soot oxidation and lower the soot TPO peak temperatures by about 280 and 270°C, respectively, as compared to non-catalytic soot oxidation. Furthermore, the addition of up to 10 wt.% potassium oxides into Mn-Ce-Zr increases its catalytic activity and further reduces the soot TPO peak temperature by about 40°C under loose contact condition.

Application of glass soot catalysts on metal supports to achieve low soot oxidation temperature

K–Ca–Si–O glass was applied to metal supports for use as a catalyst for diesel soot combustion. Glasses were processed from the melt and by a sol–gel route. Catalyst activity for the oxidation of diesel exhaust soot and flame soot from an oil lamp was compared by thermogravimetric analysis (TGA). The results show that a K-based catalytic glass coating on metal substrates can reduce the temperature where half of the engine soot is oxidized (T50) to as low as 360°C under loose contact conditions, and offers catalytic stability for long term combustion cycling. Scanning electron microscopy observations show that sol–gel glass processing is effective for coating complex wire mesh shapes without pore clogging.

Sulfation of Pt/Al2O3 catalyst for soot oxidation: High utilization of NO2 and oxidation of surface oxygenated complexes

The sulfated Pt/Al2O3 catalyst was synthesized by impregnating H2SO4 on the Pt/Al2O3 catalyst. It exhibits higher catalytic activity than Pt/Al2O3 for soot oxidation under loose contact conditions in a feed flow containing NO and O2. Further studies ascribe its good activity to a synergistic effect between sulfate species and platinum. Firstly, the electronegative sulfate species may improve the oxidation-resistance of metallic platinum in the oxidizing atmosphere, which is important for achieving high NO↔NO2↔soot recycling efficiency; Secondly, the sulfate species inhibit the NOx adsorption on the alumina support, leading to a preferential adsorption of NOx on the surface of soot rather than on the catalyst, and thus provide more chances for NO2-soot reaction to generate the surface oxygenated complexes (SOCs); Finally, the sulfate species can promote the further decomposition of these SOCs species, resulting in a high catalytic efficiency for soot oxidation.

Soot combustion improvement in diesel particulate filters catalyzed with ceria nanofibers

Ceria nanofibers were prepared and deposited on SiC diesel particulate filters (DPFs), aiming at improving the soot-catalyst contact conditions and promote soot combustion at lower temperatures than in the noncatalytic case. In particular, the nanofibers have been found to be very active with respect to other ceria catalyst morphologies, due to their arrangement in a network which enhances the number of soot-fiber contact points.This effect was initially elucidated in a series of tests of soot temperature programmed combustion, which were carried out on the catalysts powders mixed with soot in loose contact conditions: a specific sub-set of nanofibers exhibited a 112°C anticipation of the onset oxidation temperature (10% of total soot combustion) with respect to the non-catalytic test, and 38°C with respect to ceria nanopowders obtained with the so-called Solution Combustion Synthesis (SCS).The nanofibers were then supported on Alumina washcoated DPFs, which were loaded with soot for 1h, and subsequently subjected to a progressive temperature increase to induce soot ignition. Both CO2 concentration in the outlet gas, and the pressure drop, were recorded during these tests. The main advantage given by the nanofiber catalyzed DPF, with respect to the other investigated morphologies, was not related to the maximum rate of soot oxidation, which was similar for all ceria catalysts, but again to the onset temperature. In fact, the pressure drop curve started to decrease more than 50°C before the DPF catalyzed with ceria through in situ SCS.This behavior could greatly improve the soot oxidation activity especially for DPF passive regeneration purposes.

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.

Easy synthesis of three-dimensionally ordered macroporous La 1−xK xCoO 3 catalysts and their high activities for the catalytic combustion of soot

Three-dimensionally ordered macroporous (3DOM) La 1− x K x CoO 3 ( x = 0–0.3) perovskite-type oxide catalysts with large pore sizes and interconnected macroporous frameworks were successfully synthesized by a novel method of carboxy-modified colloidal crystal templates (CMCCT). The obtained catalysts were characterized by means of XRD, FT-IR, SEM, TEM, BET, XPS, and H 2 TPR. The detailed preparation process was investigated using in situ TG–DSC, Raman, and EDS analyses. The sp 2-hybridized carbon atoms of the modified polymer templates can be converted to sturdy amorphous carbon materials by calcination under an inert atmosphere to prevent the macrostructure from collapsing. The active site densities of catalysts were determined by isothermal anaerobic titration, and their turnover frequency (TOF) values were then calculated. The catalytic performances for soot combustion were evaluated using temperature-programmed oxidation (TPO) reaction technology. 3DOM La 1− x K x CoO 3 catalysts exhibited catalytic activities as high as that of noble metal Pt catalysts under loose contact conditions.

Catalytic combustion of soot particulates over La 2- xK xNiMnO 6 catalysts

Nanosized La 2- x K x NiMnO 6 catalysts with ABO 3 type perovskite-like structure were prepared by auto-combustion method using citric acid as a ligand to control particle size and morphology. The structures and properties of these perovskite-like oxides were investigated by X-ray powder diffraction (XRD) and temperature-programmed reduction (TPR). The catalytic activities for soot combustion were evaluated by temperature-programmed oxidation (TPO) with pure O 2 and O 2/NO x as oxidant, respectively. In the La 2- x K x NiMnO 6 catalysts, the partial substitution of K at A-site leads to an increase of the concentrations of high valence cation and oxygen vacancy, which enhance the catalytic activity for soot combustion. The optimal substitution amount of K was equal to x = 0.4 among these samples. T p (peak temperature) in O 2-TPO profile was 420 °C and T p in O 2/NO x -TPO profile was 370 °C over La 1.6K 0.4NiMnO 6 catalyst for soot particulates combustion under loose contact conditions between catalyst and soot.

Catalytic performance of supported precious metal catalysts for the combustion of diesel particulate matter

CeO 2 and TiO 2 supported precious metal (Ag, Pt and Pd) catalysts were prepared by an impregnation method, and their catalytic performance for oxidation of simulated diesel particulate matter (PM) was investigated by thermogravimetry (TG), differential thermal analysis (DTA) and mass spectrometry (MS) under both tight-contact (TC) and loose-contact (LC) conditions. The simulated PM was prepared by liquid-phase adsorption of SOF (eicosane, C 20H 42) on soot (carbon black). Catalytic performance for the oxidation of simulated PM depended on both metal species and supports. CeO 2 was inherently active for both SOF and soot oxidation, and TiO 2 was nearly inactive. Pt was the most active species for SOF oxidation, followed by Pd and Ag. The SOF oxidation was independent of contact condition between the simulated PM and catalysts. Soot oxidation performance, on the other hand, drastically affected by the contact conditions, and Ag was far more active than Pt and Pd under both TC and LC conditions. Ag/CeO 2 was the most active soot oxidation catalyst. The oxidation of soot was influenced by the coexisting SOF depending on catalytic materials and contact modes.

Effect of barium loading on CuO x–CeO 2 catalysts: NO x storage capacity, NO oxidation ability and soot oxidation activity

A series of Ba–Cu–Ce catalysts were prepared by loading different amounts of Ba(Ac) 2 on the sol–gel-synthesized CuO x –CeO 2 mixed oxides. The activities of the catalysts for soot oxidation were evaluated in 1000 ppm NO/10% O 2/N 2 under loose contact conditions. The catalysts were characterized by XRD, BET, H2-TPR, in situ-DRIFTS and NO-TPO measurements. When Ba loading is between 6 and 10 mol.% of Cu + Ce, the catalysts exhibit an onset temperature ( T i) as low as 376 °C. NO is oxidized at Cu x+ and Ce x+ sites and then stored on the adjacent Ba species in form of barium nitrate. Ba(NO 3) 3 begins to decompose, releasing abundant NO 2 at around 370 °C under the driving of soot as the reductant. The nitrate-derived NO 2 and the NO-derived NO 2 initiate the soot combustion over the Ba-modified catalysts with a significantly lowered T i. The ternary catalysts lose their NO x storage capacity after hydrothermally aged at 800 °C for 10 h due to the formation of BaCeO 3. However, Ba restrains the sintering of (Cu,Ce)O x , resulting in relatively more favorable redox properties and thus higher activity for NO oxidation. The barium-modified catalysts show lower onset temperatures (470–480 °C) of soot oxidation than pure CuCe (509 °C) after aging.

Introduction to the Catalytic Combustion of Diesel Soot

The heterogeneous catalytic reaction containing solid particle as reactant is a class of important and complex chemical reaction. The solid particles such as soot emitted from diesel engines, are becoming increasingly important in the context of cleaning air and lowering automotive emissions. Diesel soot particulates are defined by the US Environmental Protection Agency (EPA) as “all compound collected on a pre-condition filter in diluted diesel exhaust gases at a maximum temperature of 50°C”. The catalytic combustion is considered as one of effective method for purifying these diesel soot pollutants. Supported oxide catalysts were originally developed for the catalytic combustion, but they are not especially active at low temperatures (<400°C) under the loose contact condition between catalysts and soot [1,2]. Noble metal catalysts such as platinum and gold, on the other hand, are efficient and exhibits high activity [3,4]. However, Pt is very expensive and causes the great emissions of sulfates. Thus, the development of active and stable catalysts without noble metals for the catalytic combustion of soot particles at low-temperature remains a significant challenge.

The highly active catalysts of nanocomposite K–Co–CeO2 for soot combustion

Nanocomposite K-Co-CeO(2) catalysts were synthesized by the controllable micropore-diffused co-reaction method. Due to the size-match nanometre effect between CeO(2) support and active components and formation of NO(2), these cheap nanocomposite catalysts exhibit super catalytic performances which are as good as a supported Pt catalyst under loose contact conditions for soot combustion.

Low temperature DPF regeneration by delafossite catalysts

Several Li–Cr delafossite catalysts (LiCrO 2, LiCr 0.9O 2, LiCr 0.8O 2, LiCr 0.7O 2, Li 0.9CrO 2, Li 0.8CrO 2 and Li 0.7CrO 2) were prepared via a highly exothermic and self-sustaining reaction, the so-called “solution combustion synthesis (SCS)” method, and characterized by means of XRD, BET, FESEM-EDS, H 2-temperature programmed reduction (TPR) and XPS analyses, as catalysts for the combustion of soot, a major pollutant emitted by diesel engines. These catalysts already showed appreciable activity at 350 °C towards the catalytic combustion of soot even under loose contact conditions. The best prepared catalyst (LiCr 0.9O 2) could ignite soot combustion well below 350 °C, which is inside the range of temperatures reached at the exhaust line of a diesel engine. The correlation between the activity order and the capability to provide surface adsorbed oxygen (O −) by the prepared delafossite catalysts, enabled by a shift of the chromium from a high valence to a low valence state, is pointed out as a peculiar feature of these catalysts. An in situ SCS method was tailored to the preparation of a LiCr 0.9O 2-catalyzed trap based on a SiC wall-flow monolith. Engine bench tests on these catalytic traps (trap loading and regeneration by induced temperature increase) showed that the presence of the catalyst enabled both a more complete regeneration and a one-third fold reduction of the regeneration time compared to the case of a bare, non-catalytic trap. The catalyzed trap was finally characterized in terms of soot emissions during both the loading and the regeneration phase.

Co,Ba,K/ZrO 2 coated onto metallic foam (AISI 314) as a structured catalyst for soot combustion: Catalytic activity and stability

This work presents results on the catalytic activity and stability in the diesel soot combustion of a structured catalyst composed of a Co,Ba,K/ZrO 2 layer coated onto a metallic foam (AISI 314). The washcoated ZrO 2 layer, mainly of tetragonal structure, adopted the form of mosaics (flakes) with interconnected surface cracks. After the impregnation of the active components, the fresh catalyst was constituted by a complex mixture of phases originated in the chemical compositions of both the metallic foam and the active ingredients, the main crystalline phases detected being Cr 2O 3, FeCr 2O 4, Mn 1.5Cr 1.5O 4, ZrO 2, BaCO 3, K 2CrO 4 and BaCrO 4. The performance of the structured catalyst was maintained within a wide range of soot:catalyst ratios, which is an important factor to be considered for practical applications due to the variable soot amounts that are released by the engine during its operation under different conditions. The stability of the catalyst was evaluated under loose contact conditions during 120 h of time-on-stream, showing that the catalytic activity remained practically constant. The characterization of the fresh and used catalysts indicated the migration of both Co and K towards more exposed positions after TPO runs, which could be a positive factor given the oxidizing properties of Co oxides and the improvement of the soot-to-catalyst contact given by K species. However, a longer duration or severe ageing temperatures might lead to sintering and loss of soot oxidation ability.

Textural–structural properties and soot oxidation activity of MnO -CeO2 mixed oxides

Abstract A series of MnOx-CeO2 mixed oxides were prepared using a sol-gel method followed by calcinations at various temperatures. The soot oxidation activities were measured in different NO concentrations under loose contact conditions. A quasi-parabolic curve is observed for the apparent dependency of activity on the catalyst surface area. As the surface area is below 8 m2/g, the soot oxidation is limited by the availability of NO2 due to the separation and sintering of the mixed oxides. The rate-determining step turns to be NO2 reaction with soot for the high-surface-area catalysts.

Role of stable nitrates stored on BaCoCe in soot catalytic oxidation

Abstract BaCoCe catalysts were prepared by impregnating different amounts of barium acetate on Co–Ce mixed oxides. The optimal Ba loading amount is 10 mol.% of Ce, and the obtained catalyst presents a maximal soot oxidation rate temperature at 378 °C in NO + O 2 under loose contact conditions. The NO x storage capacity of BaCoCe is twice of that of CoCe, and the decomposition of the stored nitrates is pronouncedly delayed. The soot is ignited at 358 °C by NO 2 produced from decomposition of the relatively stable nitrates with heat transfer limitations.

Effects of adsorbed and gaseous NOx species on catalytic oxidation of diesel soot with MnOx–CeO2 mixed oxides

MnOx–CeO2, MnOx and CeO2 catalysts were synthesized by the sol–gel method. The structural, redox and adsorption properties of the individual and mixed oxides were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), CO temperature-programmed reduction (TPR), NO temperature-programmed oxidation (TPO), NOx temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The TPO tests were performed with the soot–catalyst mixture under loose contact conditions to evaluate the catalytic activity of the oxide catalysts for soot oxidation. The MnOx–CeO2 mixed oxide catalyst presents the lowest soot oxidation temperature among the catalysts investigated in the presence of NO and O2. The synergetic effect between manganese oxide and ceria restrains the growth of oxide crystallites, increases the specific surface area and improves the low-temperature redox property. Especially, the activity of MnOx–CeO2 mixed oxides for NO oxidation and its capacity for NO2 storage in the form of surface nitrates are greatly enhanced. Not only the NO2 released from decomposition of surface nitrates but also that formed by catalytic oxidation of gaseous or adsorbed NO on the catalyst is confirmed important for soot oxidation. It is found by the in situ DRIFTS tests with the soot–catalyst mixture that the generation of surface oxygen complexes (SOCs), such as carboxylic anhydrides, lactones, quinine, ceto-enol groups, ethers and phenols, occurs at about 100°C lower temperature with exposure to NO than in the absence of NO, which is an important step for soot oxidation.