Evidences of the Cerium Oxide-Catalysed DPF Regeneration in a Real Diesel Engine Exhaust

The active phase Ce_0.5Pr_0.5O₂ has been loaded on commercial substrates (SiC DPF and cordierite honeycomb monolith) to perform DPF regeneration experiments in the exhaust of a diesel engine. Also, a powder sample has been prepared to carry out soot combustion experiments at laboratory. Experiments performed in the real diesel exhaust demonstrated the catalytic activity of the Ce–Pr mixed oxide for the combustion of soot, lowering the DPF regeneration temperature with regard to a counterpart catalyst-free DPF. The temperature for active regeneration of the Ce_0.5Pr_0.5O₂-containing DPF when the soot content is low is in the range of 500–550 °C. When the Ce_0.5Pr_0.5O₂-containing DPF is saturated with a high amount of soot, pressure drop and soot load at the filter reach equilibrium at around 360 °C under steady state engine operation due to passive regeneration. The uncoated DPF reached this equilibrium at around 440 °C. Comparing results at real exhaust with those at laboratory allow concluding that the Ce_0.5Pr_0.5O₂-catalysed soot combustion in the real exhaust is not based on the NO₂-assisted mechanism but is most likely occurring by the active oxygen-based mechanism.     

Measurements of diesel soot oxidation kinetics in an isothermal flow reactor–catalytic effects using Pt based coatings

Despite the significant progress in soot oxidation studies, there is still high uncertainty regarding the rate expressions to model the reactions in diesel particulate filters (DPF). This uncertainty arises from inherent difficulties in sampling and measuring the reaction rate in a realistic way, as well as different properties of the examined soot. In this context, the scope of this study is the development of a novel experimental set-up capable of overcoming existing experimental difficulties. The developed set-up allows for real diesel soot oxidation studies in an isothermal flow reactor. The reaction of soot with oxygen and NO₂ is studied with synthetic gas and with real diesel exhaust and the reaction kinetics are derived for both bare and Pt-based catalyzed substrates by combining experimental and model results.     

Effect of Biofuels on Catalyzed Diesel Particulate Filter Regeneration

Under the terms of the Renewable Energy Directive, EU member states are required to use 10 % of transport energy sourced from renewable sources, mainly biofuels, by 2020. The purpose is to reduce greenhouse gas (GHG) emissions from the transport sector. However, biodiesel used as fuel has a significant impact on emissions, as related by most of the literature on the subject. In particular, nitric oxides (NOx) and particulate matter (PM) emissions from current diesel technologies are critical factors because they are already close to the limits permitted by regulations and both limits will be even more stringent in the near future. Soot particles are trapped on a diesel particulate filter (DPF). If the DPF is catalyzed like in this study, the soot is then burned by reaction with NO₂ (CDPF continuous regeneration) which occurs at lower temperatures than reaction with O₂ (active regeneration). Tests of ultra-low sulfur diesel blended with rapeseed-biodiesel at 30 % (B30) and Fischer–Tropsch diesel (FT30) were conducted. The Fischer–Tropsch diesel was chosen to represent a biomass-to-liquid fuel. This work investigated the impact of these two biofuels on engine polluting emissions and the resulting CDPF ability to regenerate. When compared with similar inlet conditions on a synthetic gas bench, an impact of fuel was observed on soot reactivity: the CDPF loaded with FT30 soot regenerated slightly faster. Engine bench tests were also performed to combine the effects of fuel on engine emissions and soot reactivity and to evaluate the CDPF. The increase in NOx and decrease in PM emissions observed for B30 appeared to significantly improve CDPF continuous regeneration by NO₂.     

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

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

Detailed investigation of non‐catalytic DPF regeneration

The present investigation concerns the phenomena that occur during the non‐catalytic regeneration of Diesel Particulate Filters (DPFs). The temperature evolution in the filter has been correlated to the emissions of CO, HC, NO, and NO₂ during the loading and regeneration process. The emissions were assessed over both the diesel oxidation catalyst (DOC) and the DPF, in order to characterise the chemical species evolution inside the after‐treatment line. Different regeneration temperatures, which have been found to have a strong impact on the evolution of the soot oxidation rate, have been assessed. Finally, the particulate emissions during regeneration have been measured on a number and size basis.     

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     

Kinetics of catalyzed and non-catalyzed oxidation of soot from a diesel engine

To comply with the new regulations on particulate matter, car manufacturers more and more commonly use diesel particulate filters (DPF). The working of these systems needs to periodically burn soot that has been accumulated during the loading of the DPF. This paper describes the kinetics of the non-catalytic and catalytic oxidation of real diesel soot with oxygen. From these experiments, mechanisms for catalyzed and non-catalyzed soot oxidation have been proposed.     

Nanosized Pt-Perovskite Catalyst for the Regeneration of a Wall-Flow Filter for Soot Removal from Diesel Exhaust Gases

Perovskite-type catalysts have been investigated for diesel soot combustion: (i) the LaCr_0.9O_3– substoichiometric perovskite, (ii) K–La partially substituted chromites; (iii) Pt added ii-type perovskites. The catalysts prepared showed a progressively higher activity and potential for practical application in diesel particulate traps. Engine bench tests performed on a SiC wall-flow trap (Ibiden) lined with the La_0.9K_0.1Cr_0.9O_3– + 1 wt%Pt catalyst showed that the catalyst not only speeds up soot combustion on occasional trap heating (regeneration phase) but also prolongs the trap loading phase (soot accumulation during normal operation) as Pt active sites promote NO–NO₂ oxidation, followed by the non-catalytic reaction of NO₂ with the trapped soot.     

Development of a Surface Area Dependent Rate Expression for Soot Oxidation in Diesel Particulate Filters

We collected soot from diesel engine exhaust on miniature particulate filter samples and evaluated soot oxidation rates on an automated flow reactor system. A series of isothermal pulsed oxidation experiments quantified reaction rates as a function of gas composition, temperature, flow rate, and soot consumption. An O₂ chemisorption method measured the soot active surface area as filter regeneration progressed. We developed a rate law with an explicit dependence on carbon surface area and estimated the associated kinetic parameters from the pulsed oxidation data. The resulting rate expression successfully captures the soot oxidation behavior over a wide range of operating conditions.     

Chemical composition of diesel particulate matter and its control

In this review, we have systematically discussed diesel particulate composition and its formation, understanding of which is essential to design the effective catalyst compositions. The most commonly used after treatment strategies such as diesel oxidation catalysts, diesel particulate filters, and partial flow filters are described followed by chronological and category-wise discussions on various groups of reported soot oxidation catalysts. A detailed review is also presented on mechanistic and kinetics aspects of non-catalytic direct particulate matter (PM) or soot oxidation in air/O₂ and NO₂. Recent progress in catalyst development with a focus on the low-cost catalyst for diesel PM oxidation has been given more emphasis considering their renewed importance.     

Sequence of monolithic converters DOC–CDPF–NSRC for lean exhaust gas detoxification: A simulation study

With increasingly strict automotive emission regulations the exhaust gas aftertreatment becomes more complex and expensive. Mathematical modelling and simulations play an important role in design of the aftertreatment systems consisting of multiple catalytic devices, reducing the time and cost demands of the system design. In this paper a combined exhaust gas aftertreatment system for diesel engines is studied. It consists of a diesel oxidation catalyst (DOC) for CO and hydrocarbons oxidation, a catalyzed diesel particulate filter (CDPF) for soot filtration, and an NO_x storage and reduction catalyst (NSRC, also called lean NO_x trap, LNT) for NO_x abatement. Effective mathematical models of the individual converters are presented and used first to demonstrate the functionalities of the system, and then to conduct a parametric simulation study. The aim of this study is to map the influence of the individual components on the performance of the entire system in standard test driving cycle. The sizes of the DOC, CDPF, and NSRC converters are varied while the overall volume of the combined system is kept constant. The resulting maps of pressure drop, CO, HC, particulate matter, and NO_x conversions show non-linear dependences on the sizes of individual converters. Co-operative and competitive effects occurring in the combined system are discussed. Suitable reactors sizes are found that enable high conversions of all controlled exhaust gas components.     

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.     

Molten Salts Are Promising Catalysts. How to Apply in Practice?

A diesel soot filter with a Cs₂SO₄V₂O₅ molten salt diesel soot oxidation catalyst has been developed. An engine test-bench was used to test it in diesel exhaust gas with ELPI analysis and to deposit diesel soot on filters for temperature programmed oxidation experiments. Molten salt (Cs₂SO₄V₂O₅) based catalytic foam has an onset temperature for catalytic oxidation of 320°C. This is a promising temperature for continuous filter-regeneration applications. Unfortunately the liquid state of the catalyst makes it unfit for the very effective wall-flow monolith filter, and necessitates the use of a foam filter as support. The onset temperature of the catalytic foam of 320°C is still too high to justify a change from wall-flow monolith to foam, as ceramic foam is a less effective filter than the wall-flow monolith. Foams are no absolute filters, and should be optimized for each application.     

Characteristics of Oxidation of Diesel Paticulate Matter over a Spinel Type Cu0.95K0.05Fe2O4 Catalyst

This paper presents an experimental study on oxidation of diesel paticulate matter (PM) and aims at investigating the characteristics of PM oxidation. The experiments were performed over a Cu_0.95K_0.05Fe₂O₄ catalyst which is attributed to a spinel type metal oxide. The effects of O₂ on PM oxidation as well as on NO_x reduction were studied and the roles of O₂ in PM oxidation and in NO_x reduction, respectively, are discussed. During the temperature‐programmed oxidation of PM, SOF oxidation and soot oxidation lead to two CO₂ peaks at different temperatures. It was found that the presence of O₂ benefits PM oxidation but suppresses the reduction of NO_x into N₂ by consuming the soot. This study revealed that the appearance of PM oxidation is different from that of soot oxidation. The mechanisms on PM oxidation and NO_x reduction are discussed.     

Potassium‐Activated Wire Mesh: A Stable Monolithic Catalyst for Diesel Soot Combustion

Potassium‐modified FeCrAl alloy wire mesh was developed as a catalytic diesel particulate filter to suppress the emission of soot from a diesel engine. Potassium species were deposited on wire mesh by a chemical vapor deposition method, in which a model soot was used to convert KOH into metallic K at high temperatures to subsequently activate the wire mesh. Tests showed that metallic K reacted with the enriched Al₂O₃ component on the surface derived from segregation and successive oxidation during precalcination. The resulting layer of K‐O‐Al species offers remarkable activity and stability for the catalytic oxidation of diesel soot. The K‐activated wire mesh could lower the initial temperature of soot combustion and maintain the activity for several cycles.     

Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration

The so-called 4-way catalytic converter (4WCC) has the ability to simultaneously convert CO, HC, NOx and particulate matter on a single support. It allows diesel vehicles to obey to increasingly stringent emission regulations while at the same time decreasing the space needed by the exhaust aftertreatment system. It is combined with fine engine control strategies so as to ensure conversion of all pollutants. It is hence associated with a large number of catalytic reactions which interact with each other and compete for active sites. The behavior of a commercial 4WCC was characterized on a synthetic gas bench. Gas composition, temperatures and gas hourly space velocity were chosen close to real engine operating conditions. Samples were loaded with soot on an engine bench test. Oxidation reactions were dominant in a lean environment: CO oxidation by NO₂ at low temperature followed by H₂, CO, NO and HC oxidation by O₂. NOx were stored on barium storage sites. In rich conditions H₂, CO and HC were used to reduce NOx. NH₃ production from H₂ was also observed. It could be used to further reduce NOx in lean conditions if stored on a downstream SCR system like in the Honda system [1]. A further conversion of HC was obtained at high temperature due to steam reforming. Interactions and inhibitions were also found. NOx storage appeared to be inhibited by CO oxidation with NO₂ at low temperatures and also by HC, maybe through competition for storage sites with CO₂ produced during HC oxidation. Catalytic reactions were affected by the soot deposit. Continuous oxidation of soot by NO₂ also induced a slower NOx storage rate.     

Effects of a Pt/Ce0.68Zr0.32O2 catalyst and NO2 on the kinetics of diesel soot oxidation from thermogravimetric analyses

In this study, the reactivity of well-characterized diesel soot samples is investigated by thermogravimetry under different kinds of oxidizing atmospheres (20% O₂ or 10% O₂ + 700 ppm NO₂) either under catalyzed or non-catalyzed conditions. Whatever the atmosphere used, the catalyst Pt/ceria-zirconia was able to lower significantly the ignition temperature of soot, but the catalytic effect was found to be more pronounced when the oxidation process was assisted by NOx. This is due mainly to the efficiency of both catalyst components (the noble metal and the OSC material) in recycling the NO released after attack of the soot by NO₂. By contrast, the NO₂ is of very limited use in the absence of catalyst under our experimental conditions. The global kinetic parameters representative of the carbonaceous matrix oxidation are determined using a methodological approach combining thermogravimetric experiments and non-linear multivariate regression. The kinetic parameters obtained are consistent both with the literature results and the postulated mechanistic pathways for soot oxidation assisted or not by NOx.     

Characterization of Particulate Matter from Direct Injected Gasoline Engines

The reactivity and reaction kinetics of particulate matter (PM) from direct injected gasoline (GDI) engines has been studied by O₂ and NO₂ based temperature programmed and isothermal step-response experiments, and the PM nano-structure has been characterized using HRTEM. The reactivity of the PM samples collected in filters during on-road driving was found to increase in the following order: Printex U < diesel < gasoline PI ≈ gasoline DI < ethanol for O₂ based combustion. The activation energies for O₂ and NO₂ based oxidation of PM collected from a GDI engine in an engine bench set-up was estimated to 146 and 71 kJ/mol respectively, which is comparable to corresponding values reported for diesel and model soot. Similar nano-structure features (crystallites plane dimensions, curvature and relative orientation) as observed for diesel soot were observed for gasoline PM.     

Mechanistic Study of Carbon Oxidation with NO2 and O2 in the Presence of a Ru/Na‐Y Catalyst

The oxidation of model soot by NO₂ and O₂ in the presence of a Ru/Na‐Y catalyst under conditions close to automotive exhaust gas after‐treatment systems is investigated. Isothermal oxidation experiments of a physical mixture of carbon black and catalyst were performed in a temperature range of 300–400 °C. A remarkable increase of the oxidation rate by NO₂ and O₂ in the presence of the Ru/Na‐Y catalyst was observed. An overall mechanism involving oxygen transfer from the Ru catalyst to the carbon surface leading to an increase of C(O) complexes is proposed. These C(O) complexes are destabilized in the presence of NO₂ increasing the carbon oxidation rate.     

Impact of ferrocene on the structure of diesel exhaust soot as probed with wide-angle X-ray scattering and C(1s) NEXAFS spectroscopy

We report on the structure of a set of diesel exhaust samples that were obtained from reference diesel fuel and diesel fuel mixed with ferrocene. Characterization was carried out with X-ray absorption spectroscopy (C(1s) NEXAFS) and wide-angle X-ray scattering (WAXS). The reference diesel soot shows a pronounced graphite-like microstructure and molecular structure, with a strong (0 0 2) graphite Bragg reflex and a strong aromatic CC resonance at 285 eV. The mineral matter in the reference soot could be identified as Fe₂O₃ hematite. The soot specimen from the diesel mixed with ferrocene has an entirely different structure and lacks significantly in graphite-like characteristics. NEXAFS spectra of such soot barely show aromatics but pronounced contributions from aliphatic structures. WAXS patterns show almost no intensity at the Bragg (0 0 2) reflection of graphite, but a strong aliphatic γ-side band. The iron from the ferrocene transforms to Fe₂O₃ maghemite.