Characteristics on HDS over amorphous silica–alumina in single and dual catalytic bed system for gas oil

Combustion temperatures of particulate matter of Diesel automobile engines under lean conditions in laboratory experiments depend on a number of parameters: e.g. model gas composition and flow rate, catalyst composition and micro structure, soot/catalyst ratio as well as model soot type (composition, particle size and size distribution) and contact type. Especially the last two factors are often underestimated. In the literature most commonly Printex U and loose or tight contact are used. Here we report on the effect of these two parameters by varying the soot used and contact type and with nano-scaled ceria as catalyst due to its known Oxygen Storage Capacity (OSC). Apart from investigating the influence of these factors our second main objective was to find a preparation technique for the soot-catalyst contact, which fulfills the criteria of high reproducibility, high comparability to real conditions and facile automation. The last criterion is the basis for its use in high-throughput techniques (HTT) for parallelized discovery and optimization studies of new combustion catalysts.     

Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

This study addresses the catalytic reaction of NO_x and soot into N₂ and CO₂ under O₂-rich conditions. To elucidate the mechanism of the soot/NO_x/O₂ reaction and particularly the role of the catalyst -Fe₂O₃ is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O₂ and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe₂O₃ catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N₂. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe₂O₃ catalyst. This model includes the dissociative adsorption of O₂ on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe₂O₃ catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.     

High-throughput approach to the catalytic combustion of diesel soot

A methodology for the evaluation of diesel soot oxidation catalysts by high-throughput (HT) screening was developed. The optimal experimental conditions (soot amount, catalyst/soot ratio, type of contact, composition and flow rate of gas reactants) ensuring a reliable and reproducible detection of light-off temperatures in a 16 parallel channels reactor were set up. The temperature profile measured in the catalyst/soot bed under TPO conditions when the exothermic combustion of soot takes place was shown to provide an accurate measurement of the ignition. Its reproducibility and relevance were checked. The results obtained with a reference noble metal free catalyst (La_0.8Cr_0.8Li_0.2O₃ perovskite) agree very well with literature data. Qualitative mechanistic features could be derived from these experiments, stressing the likely limiting step of oxygen transfer from catalyst surface to soot particulates to ignite the soot combustion. Ceria material was shown to be more appropriate than perovskite one. From an HT screening of a large diverse library (over 100 mixed oxides catalysts) under optimized conditions, about 10 new formulations were found to perform better than selected noble metal free reference materials.     

Realistic contact for soot with an oxidation catalyst for laboratory studies

Laboratory temperature programmed oxidation (TPO) is popular for diesel soot oxidation catalyst screening. The method of depositing soot on the catalyst can be critical for the measured catalyst performance. Different methods for bringing the soot in physical contact with the catalyst were evaluated in order to determine which methods give a realistic contact. Suitable methods are filtration from a (artificial) soot aerosol, shaking in a sample bottle, mixing with a spatula, and dipping in a soot dispersion. For these preparation methods Printex U synthetic soot is a suitable model compound.     

Catalytic combustion of soot with a O2/NO mixture. KNO3/ZrO2 catalysts

Catalysts containing 0.25–20% KNO₃ supported on ZrO₂ have been studied for diesel soot combustion. The addition of KNO₃ to ZrO₂ support enhances its activity due to the increased contact between soot and catalyst and also because the KNO₃ acts as catalyst. The combustion temperature has been measured for “loose” and “tight” contact, between soot and catalyst, and the difference was 10 K for KNO₃(20)/ZrO₂ catalyst. This finding is very important because under practical conditions the contact between soot and catalyst is poor and this contact resembles the contact denoted as “loose contact”.     

Performances of a catalytic foam trap for soot abatement

A catalytic trap for soot particles was prepared by deposition of Cu–V–K–Cl catalyst on a ceramic foam. Catalytic trap performances were evaluated by treating the exhaust of a gas oil burner under different operating conditions. The results obtained showed that ceramic foam is a particularly suitable support for this application since it yields low gas pressure drop, good soot collection efficiency (“deep bed” filtration mechanism), high thermal shock resistance and good contact throughout the filter between soot particles and catalyst surface. In addition, the catalytic foam trap is able to spontaneously regenerate at operating conditions comparable to those typical of diesel engine exhaust and after more than 70 test hours it retains its activity towards soot oxidation.     

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.     

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).     

Candle Soot as Efficient Support for Proton Exchange Membrane Fuel Cell Catalyst

Candle soot deposited from the candle flame was used as a catalyst support for an anode catalyst in a proton exchange membrane fuel cell. The results showed that Pt/soot hybrids prepared by magnetron sputtering of 5 nm platinum films on candle soot exhibit very high mass activity in the fuel cell, which is more than one order of magnitude higher than that for commercial catalyst. The elementary preparation, high surface‐to‐volume ratio, good conductivity and hydrophobicity make candle soot a promising type of the support for PEMFCs catalyst.     

Thermal stability of Cu-K-V catalyst for diesel soot combustion

This paper reports a study on the thermal stability of a Cu-K-V catalyst, which showed particular promise for low temperature combustion of diesel particulate. Prolonged treatments were performed at high temperatures (400–1000°C) for periods up to 15 days under different gaseous atmospheres. The effect of such treatments on the catalyst composition was investigated by means of weight-decrease measurements and composition analysis (atomic absorption, X-ray diffraction, etc.), whereas the catalyst activity towards soot combustion was determined via thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The apparent activation energy of the soot combustion process was calculated for a collection of catalyst samples, thermally treated according to several different representative conditions, by the Ozawa method on the basis of the DTA results. Some of the thermal treatments (especially those performed at high temperatures: 900–1000°C) resulted in a reduction of the catalyst activity as shown by the increase of both the activation energy and the soot ignition temperature, as a consequence of the volatilisation of at least some of the active compounds of the catalyst itself (KCl, CuCl₂, etc.). Any periodic thermal regeneration of a catalytically-activated trap for diesel emissions (leading to such high temperatures) performed to eliminate any accumulated soot, has thus to be avoided by designing a trap capable of burning out all the soot produced at the diesel exhaust temperatures (< 400°C).     

Soot differentiation by laser derivatization

Combustion produced soot is highly variable in its composition and nanostructure, both of which are dependent upon combustion conditions. Quantification of high-resolution transmission electron microscopy (HRTEM) images for nanostructure parameters shows that soot nanostructure is dependent upon its source. In principle, this permits identification of the soot and its contribution to any pollution monitoring receptor site. Many structural and chemical aspects are subtle, unaccounted for in direct nanostructure quantification. The process of pulsed laser annealing is demonstrated to enhance slight differences in nanostructure and chemical composition. Chemistry-based limitations imposed due to nanosecond heating and microsecond cooling timescales highlight these initial compositional and structural differences—as dependent upon source-specific formation conditions. This study demonstrates laser-based heating as an analytical tool for soot differentiation by formation conditions/source by identifying operational parameters for optimal derivatization. Nanostructure changes are qualitatively shown using HRTEM and quantified using image-based fringe analysis for real and model soots.

Copyright © 2019 American Association for Aerosol Research     

The effect of the contact between platinum and soot particles on the catalytic oxidation of soot deposits on a diesel particle filter

Experiments were performed with two model soot aerosols brought into different forms of contact with Pt aerosol particles, to investigate the effectiveness of this contact in lowering the catalytic soot oxidation temperature. The contact was either generated between individual particles in the aerosol state (Pt-doped soot to simulate a fuel borne catalyst), or by sequential or simultaneous deposition of separately generated soot and Pt aerosols onto a sintered metal filter. (Formation of a soot cake on previously deposited Pt aerosol would simulate a catalyst coated diesel particle filter.) The catalytic activity was determined in all cases from temperature ramped oxidation in air of the filtered particles, and defined as the 50% conversion temperature.

It was found that Pt-doped soot and simultaneously filtered aerosols were both equally effective in reducing the oxidation temperature by up to 140–250 °C for the spark discharge soot (with 3–47 wt% Pt concentration in the soot cake), and by up to 140 °C for the pyrolysis soot (3 wt% Pt). Conversely, the deposition of a thin soot layer of 5–10 μm thickness onto Pt, or vice versa, produced only a slight temperature reduction on the order of about 13–42 °C. These results suggest that the distance between soot and Pt particles plays a key role in promoting an effective oxidation on the filter, which is consistent with the role of Pt particles as local generators of activated oxygen.     

钙钛矿La1-xKxCoO3纳米晶的制备与结构及其对炭烟的催化燃烧性能

采用溶胶-凝胶法制备了K+掺杂的La1-xKxCoO3系列钙钛矿结构柴油车尾气炭烟氧化催化剂,用XRD, TG-DTA及程序升温反应等技术详细研究了K+掺杂量及焙烧温度对催化剂结构和炭烟燃烧性能的影响,初步探讨了催化剂结构与性能之间的相关性. 实验结果表明,以蔗糖为络合剂在600℃下可以得到纯钙钛矿结构的La1-xKxCoO3纳米晶,其中菱方相为LaCoO3系钙钛矿的高温稳定相,升高焙烧温度及增加K+掺杂量都会促进钙钛矿结构由立方相转变为菱方相. K+的掺杂可以降低炭烟的燃烧温度,一定量的K可以提高炭烟的燃烧速率. 700℃焙烧的具有菱方相钙钛矿结构的La0.9K0.1CoO3具有最好的催化性能,对炭烟的起燃点和燃尽温度分别为240及387℃,可以通过柴油车自身的排气热量来实现炭烟的催化燃烧过程.     

Arc discharge synthesis of carbon nanotubes: Comprehensive review

In quest to synthesize high quality carbon nanotubes in bulk, different routes have been proposed and established over the last two decades. Arc discharge is the oldest and among the best techniques to produce high quality carbon nanotubes. Though this synthesis technique has been explored for a long time, the nanotube growth mechanism is still unclear and the growth conditions lack strong correlation with the synthesized product. In this review, we attempt to present the mechanism of nanotube growth in arc discharge and the factors affecting its formation. In order to understand the physics of this mechanism, the effect of experimental parameters such as setup modification, power supply, arc current, catalyst, pressure, grain size, electrode geometry and temperature on size and yield of the nanotubes has been detailed. The variation in synthesis parameters employed in literature has been presented along with challenges and gaps that persist in the technique.     

Alumina Supported Co–K–Mo Based Catalytic Material for Diesel Soot Oxidation

Alumina supported Co–K–Mo based mixed metal oxide type catalytic materials have been prepared by co-impregnation. These catalysts show excellent activity for carbon as well as diesel soot oxidation, which could be due to the redox properties of Mo and Co as well as to a synergistic effect of molybdenum, cobalt, and K contents. The catalyst containing 5 wt% molybdenum shows a lowering of carbon oxidation by about 190 °C under loose contact conditions as compared to the non-catalyzed reaction, as well as to bare alumina. Characterization studies suggest a composite nature of these materials, while thermal stability investigations confirm the stable nature. The selected catalyst has been studied by XPS, however, it is difficult to conclude which are the important factors contributing to the catalytic activity. It appears to be a synergistic effect of Co, K, and Mo components as these catalysts show much improved activity as compared to the individual components in supported and unsupported forms.     

Catalytic soot oxidation in microscale experiments

The oxidation of soot agglomerates over catalytically active surfaces is of interest for the development of catalytic reactors for the control of soot emissions. The process involves the transport and deposition of nanoparticle aggregates to a surface on which catalyst particles are deposited. To simulate this process, graphitized carbon nanoparticles and platinum nanoparticles were separately deposited on an oxidized silicon wafer by laser ablation and electro hydro dynamic atomization. Changes in particle morphology produced by the reaction were visualized ex situ by scanning electron microscopy. In this way chemical reaction data could be correlated with the local surface coverage and particle size of the catalytically active material and the morphology of the reacting particles, resulting in detailed local information on their interaction, which is not available in studies on bulk samples. The contact between catalyst and soot particles was loose, simulating the behavior of catalyst systems used in practice. The activation energy of the oxidation in air was found to be 40 kJ/mol whereas the activation energy in air/NO was found to be 160 kJ/mol, both in presence of Pt deposited on a SiO₂ support. Notwithstanding the higher activation energy, the reaction rate of soot oxidation in air/NO is about two to three orders of magnitude higher than in air. A linear relationship between the relative Pt surface and reaction rate was found for the oxidation in an air/NO atmosphere. In air, the relationship has a minimum which indicates that there are different simultaneous mechanisms of reaction. Although activation energies are different from other studies, the oxidation temperatures are comparable. The EHDA and laser ablation produced platinum catalysts behave similarly and show potential to be used as model catalyst.     

EPR characterisation of carbon black in loose and tight contact with Al2O3 and CeO2 catalysts

The intensity of contact between soot and catalyst is one of the major parameters that determine the soot oxidation reactivity. In the present work the EPR study is focused on the mixtures of carbon black with Al₂O₃ and CeO₂ in loose and tight contacts in order to provide the physicochemical characterisation of the contact. For tight contact CB-catalyst mixtures a new paramagnetic species is observed and can be considered as a fingerprint of the contact between the two solids. These new paramagnetic species increase the catalytic reactivity of CB combustion in the presence of cerium oxide.     

NO2-aided Soot Oxidation on LaMn0.7Ni0.3O3 Perovkite-type Catalyst

The LaMnO₃-based perovskite-type mixed oxides were studied for both trapping of NO _x and combustion of diesel soot. The LaMn_0.7Ni_0.3O₃ (LMN3) perovskite shows high NO _x adsorption capacity, quick adsorption rate and efficient adsorbed species. After the catalyst interacts with NO at low temperature around 325 °C, decomposition of the nitrates leads to the decrease of the maximum soot oxidation temperature to 430 °C. The fine crystallite size, increased surface area and readily reducibility at low temperature also favor the oxidation of soot over LMN3 under loose contact conditions.     

Kinetic model of the catalytic reforming of gasolines in moving-bed reactors

A mathematical model of the catalytic reforming of gasolines in a reactor with continuous catalyst regeneration is proposed. The model takes into account the motion of the catalyst, changes in its activity along the bed height, and the dependence of its activity on the circulation ratio. The kinetic parameters of the Pt-Sn catalyst are determined under operational conditions by solving the inverse kinetic problem. The composition of the reformate component as calculated by the model coincides with the experimental data within the accuracy of chromatographic analysis. The proposed model is invariant to the composition of raw materials and can be used for predictive calculations.     

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.