Kinetics of catalyzed and non‐catalyzed soot oxidation with nitrogen dioxide under regeneration particle trap conditions

BACKGROUND: For compliance with the regulations on diesel particulate matter, car manufacturers have developed diesel particulate filters (DPF). These technologies require a regeneration method which oxidizes soot deposits in the filter. In diesel exhaust emissions there are two suitable oxidizing gases: oxygen and nitrogen dioxide. Nitrogen dioxide is much more active than O₂ and can directly attack the carbon surface. This work describes the kinetics of the oxidation of soot by NO₂ over a wide range of conditions relevant for DPF. RESULTS: The catalyzed and the non‐catalyzed oxidation of soot have been performed in a fixed‐bed reactor. The experimental results show that the overall oxidation process can be described by two additive parallel reactions: a direct C ? NO₂ reaction catalyzed by H₂O and a cooperative C ? NO₂ ? O₂ reaction catalyzed by the Pt/Al₂O₃ catalyst. The results obtained allow to propose the following kinetic law for the specific rates of the catalyzed and the non‐catalyzed oxidation of soot in the regeneration filter conditions: CONCLUSION: The kinetic parameters describing the oxidation rate of soot by NO₂ over a range of temperature and gas composition have been obtained. The extracted kinetics data are relevant for modeling the removal of trapping soot in automotive gas exhaust technology. Copyright © 2009 Society of Chemical Industry     

Ceria‐coated diesel particulate filters for continuous regeneration

The potential of diesel particulate filters wash‐coated with highly dispersed nano‐metric ceria particles for continuous regeneration has been investigated. To this end, catalytic filters were prepared, soot‐loaded (avoiding the formation of the cake layer), and regenerated—under isothermal conditions—at temperature ranging from 200–600°C. Results have shown that catalytic oxidation of soot starts from 300°C and, at all temperatures, the selectivity to CO₂ is higher than 99%. 475°C is the minimum temperature at which the filter is regenerated via catalytic path. At this temperature, the catalytic filter maintains substantially the same performance over repeated cycles of soot loading and regeneration, indicating that the thermal stability of ceria is preserved. This has been further confirmed by comparison between the outcomes obtained from characterization (X‐ray powder diffraction, N₂ adsorption at 77 K, Hg intrusion porosimetry, and scanning electron microscope/energy dispersive X‐ray analysis) of fresh filter and filter subjected to repeated regeneration tests. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3442–3449, 2017     

Simulation of NOx and soot abatement with Cu‐Cha and Fe‐ZSM5 catalysts

The embodiment of the NO_x selective catalytic reduction (SCR) functionality in a diesel particulate filter (DPF), so‐called SCR‐on‐Filter (SCRoF), is investigated through numerical modeling with SCR kinetics corresponding to Cu‐Chabazite and Fe‐ZSM5 catalysts. The results of the simulations of the SCR activity, performed in the absence and presence of soot, indicate that the presence of soot negligibly affects the NO_x conversion efficiency, given the slow dynamics of passive regeneration. Conversely, the reduction in cake thickness by soot passive oxidation is significantly different in the absence of SCR activity (uncatalyzed DPF) compared to that in its presence (SCRoF). In fact, in the SCRoF only 60–80% of the original soot consumption obtained in the absence of SCR reaction over 1 h can be achieved. Individual Cu‐Chabazite and Fe‐ZSM5 catalysts, as well as in‐series layers of the two catalysts, are investigated to devise the widest temperature window for SCRoF. © 2016 American Institute of Chemical Engineers AIChE J, 63: 238–248, 2017     

Biological oxidation of ferrous iron: study of bioreactor efficiency

The bio‐oxidation of ferrous iron is a potential industrial process for the regeneration of ferric iron in the removal of H₂S. In the first stage, H₂S is selectively oxidized to elemental sulfur using ferric sulfate. The ferrous sulfate produced is oxidized to ferric sulfate using Thiobacillus ferrooxidans for recycle and reuse in the process. The aim of the work described here was to investigate continuous oxidation of ferrous iron by immobilized T ferrooxidans and the factors which can directly affect the oxidation rate in order to assess the feasibility of this technique on an industrial scale. An analysis of the evolution of bioreactor performance with time (125 days) was performed in order to assess the feasibility of this technique on an industrial scale. A good oxidation rate was obtained despite the transport problems encountered due to occlusion of the porous support. On the other hand, the toxic effects due to absorption in the ferric solution of one or more compounds from the gas digester were studied using a ferric iron solution from the absorption process. The results indicate the feasibility of the biological system for the regeneration of the ferric‐absorbing solution. Finally, a previous study for the design of an industrial bioreactor to regenerate ferric sulfate solutions, used to remove H₂S from biogas in a wastewater‐treatment plant (Jerez de la Frontera, Spain), is introduced. Good biological oxidation performances have been obtained using a pilot plant bioreactor of 500 dm³. Copyright © 2003 Society of Chemical Industry     

Thermally stable metal ruthenate based soot oxidation catalyst for diesel exhaust emission control

Lanthanum ruthenate materials with perovskite type structure can be easily synthesized with ruthenium in 4+ oxidation state. La_3.5Ru_4.0O₁₃ type perovskite has been synthesized in unsupported and supported forms by using various methods. This perovskite type La_3.5Ru_4.0O₁₃ phase shows high thermal stability and can therefore be used as a catalyst for high temperature applications, including those for auto-exhaust emission control. The material shows good catalytic activity for the carbon/soot oxidation in view of its possible application in diesel soot oxidation for regeneration of Diesel Particulate Filter.     

Nitric oxide emissions and temperature evolution during the char grate‐fired process

Understanding the char grate‐fired process is key to developing a low‐nitric oxide (NO) technology for industrial boilers. In this work, char combustion and NO emissions during a grate‐fired process were studied in a small‐scale one‐dimensional fixed‐bed system by adjusting the char/oxygen (O₂) ratio. Evolution of the surface temperature of the char bed was measured using an infrared temperature measurement system. As the char/O₂ ratio increased, a reaction layering of the char bed occurred. The char bed can be divided into oxygen‐absent and ‐present parts in time, and into reduction and oxidation layers in space. This kind of division was determined by the complete oxidation layer that could deplete all O₂. The reduction layer could reduce NO emissions well. With the increase of the char/O₂ ratio, the char mass proportion of the oxygen‐absent part increased, while that of the oxygen‐present decreased; and the NO emissions and conversion rate of char nitrogen decreased. When combustion began, char started to burn and released a large amount of heat, and the surface temperatures of both the oxidation and reduction layers increased, with a larger rise of the former of about 260 °C. As the reaction proceeded, the surface temperature of the oxidation layer gradually decreased, while that of the reduction layer increased until the char bed was burnt through.     

Ceria‐based nanomaterials as catalysts for CO oxidation and soot combustion: Effect of Zr‐Pr doping and structural properties on the catalytic activity

In this work, we investigated a set of ceria‐based catalysts prepared by the hydrothermal and solution combustion synthesis. For the first time to our knowledge, we synthesized nanocubes of ceria doped with zirconium and praseodymium. The catalysts were tested for the CO and soot oxidation reactions. These materials exhibited different surface reducibility, as measured by H₂‐TPR, CO‐TPR and Soot‐TPR, despite their comparable chemical compositions. As a whole, Soot‐TPR appears a suitable characterization technique for the soot oxidation catalysts, whereas CO‐TPR technique allows to better discriminate among the CO oxidation activities. Praseodymium contributes positively toward the soot oxidation. On the other hand, it has an adverse effect on the CO oxidation over the same catalysts, as compared to pure ceria. The incorporation of zirconium into the ceria lattice does not have a direct beneficial effect on the soot oxidation activity, although it increases the catalyst performances for CO oxidation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 216–225, 2017     

Studies on the Effect of Physico‐Chemical Soot Properties and Feed Gas Composition on the Kinetics of Soot Oxidation on Fe2O3 Catalyst

Three commercial carbon black samples as well as self‐made C₃H₆ soot were investigated for their reactivity in the oxidation on an α‐Fe₂O₃ catalyst. These studies were performed by temperature programmed oxidation (TPO) using a packed bed. For reference purposes, TPO studies in the absence of the catalyst were made as well. The carbon black samples were characterized towards the content of C, H, N and O as well as higher heating value, specific surface area, moisture and volatile matter and were deemed to be suitable model substances for diesel soot of different maturity. The correlation of these physico‐chemical properties with the kinetics in catalytic TPO indicated that the soot oxidation on Fe₂O₃ is significantly affected by the initial number of surface oxygen compounds of the soot. The decomposition of these surface species causes the formation of active carbon sites, which are supposed to accelerate the soot oxidation.     

Emissions from a diesel car during regeneration of an active diesel particulate filter

The California Air Resources Board (CARB) and the Joint Research Center of the European Commission (JRC) have collaborated on emissions testing of a light duty diesel vehicle, which is Euro 4 compliant and comes equipped with a diesel particulate filter (DPF). The California testing included an investigation of the regeneration of the DPF over cruise conditions and NEDC test cycles. DPF regeneration is caused by the buildup of soot in the filter, and for the present test vehicle the regeneration process is assisted by a fuel borne catalyst. Regulated exhaust emissions increased substantially during the regeneration events; however, PM emissions levels were below California LEVII emissions standards. There was a very large increase of volatile particles between 5 and 10 nm, and these volatile particles were generated during all of the observed regeneration events. It appears that the particle number instruments that use the PMP methodology do not capture the PM mass increase during DPF regeneration; however, for one regeneration event there was an apparent large increase in solid particles below the PMP size limit. The PM mass increase associated with regeneration appears to be due to semi-volatile particles collected on filters. During the testing, the regeneration events exhibited considerable variations in the time for regeneration as well as the amount of PM emissions. From this investigation, several questions have been posed concerning the emission of very small (<20 nm) volatile and solid particles during DPF regeneration that need further investigation.     

Effect of Exhaust Gas Temperature on Oxidation of Marine Diesel Emission Particulates with Nonthermal-Plasma-Induced Ozone

Because the regulations governing diesel engine emissions are becoming more stringent, effective aftertreatment is needed for particulate matter. Although diesel particulate filters (DPFs) are a leading technology used in automobiles, there remains a problem with DPF regeneration for marine diesel engines that use heavy oil fuel. In the present study, pilot-scale experiments were conducted to develop a particulate oxidation technology for marine diesel engine emissions using DPF regeneration by nonthermal-plasma-induced ozone injection. It has been shown that particulate oxidation depends on the exhaust gas temperature, and regeneration can be performed most effectively at a temperature of approximately 300 °C.     

Characterization of polyaniline via pyrolysis mass spectrometry

In this work, direct insertion probe pyrolysis mass spectrometry technique was applied to investigate the thermal and the structural characteristics of electrochemically prepared HCl and HNO₃‐doped polyaniline (PANI) films. It has been determined that the thermal degradation of both samples showed three main thermal degradation stages. The first stage around 50–60°C was associated with evolution of solvent and low‐molecular‐weight species adsorbed on the polymer, the second stage just above 150°C was attributed to evolution of dopant‐based products, and the final degradation stage at moderate and elevated temperatures was associated with evolution of degradation products of the polymer. Chlorination and nitrolysis of aniline during the electrochemical polymerization were detected. Extent of substitution increased as the electrolysis period was increased. Furthermore, for the HNO₃‐doped PANI, the evolution of CO₂ at elevated temperatures confirmed oxidation of the polymer film during electrolysis. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Oxidation of carbon by CeO2: Effect of the contact between carbon and catalyst particles

The oxidation of carbon black, CB, in presence of CeO₂ is investigated to gain a better understanding of the effect of the contact between the two solids during this reaction. Different CB/CeO₂ mixtures are tested in a fixed bed reactor. The experimental data are used to propose a model for CB oxidation in presence of CeO₂. It accounts for the size distribution of CeO₂ particles, the contact area between CB and CeO₂, the mass of CB in the sample and the initial ratio CB/CeO₂. Corresponding kinetic parameters are determined.     

Combined effects of soot load and catalyst activity on the regeneration dynamics of catalytic diesel particulate filters

The combined effects of soot load and catalyst activity on the regeneration dynamics of a catalytic diesel particulate filter have been investigated through transient CFD‐based simulations of soot combustion in a single‐channel configuration. The soot load was changed by varying the amount of soot accumulated as cake layer, while keeping the amount of soot trapped inside the catalytic wall constant. Substantially uniform soot combustion that allows reasonably fast regeneration of the filter under controlled temperature conditions has been simulated only in the absence of cake and at relatively low catalyst activity. Conversely, in the presence of cake, numerical predictions have shown that, regardless of both soot load and catalyst activity, fast regeneration always occurs by propagation of sharp reaction fronts that result in high temperature rises. These findings highlight the importance of avoiding the cake formation, while properly optimizing the catalyst activity, to conduct a safe and effective regeneration of catalytic filters. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1714–1722, 2018     

Microstructure evolution of a W‐doped ZrB2 ceramic upon high‐temperature oxidation

This basic research deals with the microstructure evolution of a W‐doped ZrB₂ ceramic, as‐sintered and upon oxidation at 1650°C. Transmission electron microscopy enabled to disclose microstructural features occurred during oxidation never observed before. In the pristine material, (Zr,W)B₂ solid solutions surround the original ZrB₂ nuclei, whereas refractory W‐compounds at triple junctions and clean grain boundaries are distinctive of this ceramic. After oxidation, the microstructure is typified by intragranular nanostructures, in which nanosized W inclusions remained trapped within ZrO₂ grains, or decorate their surfaces. The understanding of the oxidation reactions occurring in the system as a function of the oxygen partial pressure was fundamental to conclude that W‐based compounds do not notably suppress or retard the oxidation of ZrB₂ ceramics.     

Electrochemical oxidation of ammonia (NH4/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH₄⁺/NH₃) in sodium perchlorate was investigated on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples.     

N2O‐Vermeidung bei der Behandlung hoch stickstoffreicher Abwässer

In comparison to a single‐stage deammonification system, large differences of N₂O emissions in double stage treatment with nitrification have been documented. Experiences are presented from pilot‐scale nitrification plants employing continuous feeding and clarification as well as sequencing‐batch reactor systems. During treatment of digestion centrate with high NH₄‐N concentrations, nitrous oxide gas was identified in reactors and exhaust gases. With similar NH₄ reduction, the results revealed an order of a magnitude lower N₂O emissions during wastewater treatment in a single‐stage deammonification system.     

Photoluminescence Properties of Er/Pr‐Doped K0.5Na0.5NbO3 Ferroelectric Ceramics

Er/Pr‐doped K_0.5Na_0.5NbO₃ ceramics have been fabricated and the effects of Pr³⁺ on their photoluminescence properties have been investigated systematically. The visible upconversion emissions, near‐infrared and mid‐infrared downconversion emissions of Er³⁺ ions under the excitation of 980 nm have been studied in detail. The effects of Pr³⁺ on PL properties and energy‐transfer processes have also been elucidated. By selecting an appropriate excitation source, simultaneous visible downconversion emissions of Er³⁺ and Pr³⁺ ions can be realized, and the emission colors of the ceramics can be tuned via the concentration of Pr³⁺ ions in a wide range from yellowish green to yellow. Our results also reveal that the photoluminescence emissions of the ceramics can be enhanced by the alignment of polarization of the ferroelectric host.     

Preparation and Application of Perovskite Catalysts for Diesel Soot Emissions Control: An Overview

The diesel engines are energy efficient (1), but their particulate matter (soot) emissions are still a matter of concern even though major advances in their control are being made. For soot abatement, catalytic diesel particulate filter (DPF) technique is widely employed to trap and burn the soot. Many types of catalysts have been investigated for the soot combustion i.e. platinum group metal (PGM) based, perovskite-type oxides, spinel-type oxides, rare earth metal oxides, and mixed transient metal oxides etc. The cost of PGM catalysts is high and their availability is questionable. Further they are susceptible to poisoning and have low thermal stability. On the other hand perovskite catalysts show potential as effective soot oxidation catalyst for the DPF because of their low cost, high thermal stability and tailoring flexibility. Many papers related to soot oxidation over perovskite catalysts have been published but no review paper appears in the literature that is dedicated to soot oxidation. Thus, this article provides a summary of published information regarding pure and substituted perovskite catalyst, preparation methods, properties, and their application for diesel soot emission control.     

Adsorbent Regeneration by Non‐thermal Plasma for Elimination of Odorous Compounds from Indoor Air

The non‐thermal plasma (NTP) technique was shown to be a method to improve indoor air quality. In particular in kitchens, odorous emissions can be removed by NTP. A combined concept of adsorption of volatile organic compounds (VOCs) and plasma regeneration of the adsorber was tested in adsorption‐regeneration‐adsorption cycles. As reference VOCs, 2‐methylthiophene, 2‐methylpyrazine, 2‐acetylthiazole, nonanal, and trans‐2‐nonenal were selected in humid air streams. These odorous compounds are emitted during cooking and frying processes. The adsorption‐regeneration concept was also tested during a simulated frying process with garlic in rape oil. A hydrophobic zeolite was chosen as adsorber material and placed directly into the discharge zone of a plasma reactor.     

Recent Advances on TiO2‐ZrO2 Mixed Oxides as Catalysts and Catalyst Supports

This is the first review of titanium dioxide‐zirconium dioxide (TiO₂‐ZrO₂) mixed oxides, which are frequently employed as catalysts and catalyst supports. In this review many details pertaining to the synthesis of these mixed oxides by various conventional and nonconventional methods and their characterization by several techniques, as reported in the literature, are assessed. These mixed oxides have been synthesized by different preparative analogies and were extensively characterized by employing various spectroscopic and nonspectroscopic techniques. The TiO₂‐ZrO₂ mixed oxides are also extensively used as supports with metals, nonmetals, and metal oxides for various catalytic applications. These supported catalysts have also been thoroughly investigated by different techniques. The influence of TiO₂‐ZrO₂ on the dispersion and surface structure of the supported active components as examined by various techniques in the literature has been contemplated. A variety of reactions catalyzed by TiO₂‐ZrO₂ and supported titania‐zirconia mixed oxides, namely; dehydrogenation, decomposition of chlorofluoro carbons (CFCs), alcohols from epoxides, synthesis of ?‐caprolactam, partial oxidation, deep oxidation, hydrogenation, hydroprocessing, organic transformations, NO_x abatement, and photo catalytic VOC oxidations that have been pursued in the literature are presented with relevant references.