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.     

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     

New Tool for Experimental Analysis of Diesel Particulate Filter Loading

The loading of a diesel particulate filters (DPFs) entails the need of trap regeneration by particulate combustion, whose efficiency and frequency are somehow affected by the way soot is deposited along the channels. Great efforts are thus spent to improve the understanding of the filtration process of DPFs, aimed at obtaining a deeper insight into the relationship between engine performance and filter loading so as to take advantage of this insight for DPF design and optimization purposes. Small lab-scale 300 cpsi DPF samples were loaded downstream the Diesel oxidation catalyst (DOC) in an ad hoc designed reactor capable of hosting five samples with part of the entire flow produced by an automotive diesel engine at the 2500 × 8 BMEP operating condition, selected to be representative as one of the critical engine points of the New European Driving Cycle (NEDC). Soot layer thickness was estimated by means of Field emission scanning electron microscope (FESEM) observations after sample sectioning at progressive locations, obtained through a procedure defined not to affect the distribution of the soot inside the filter and to enable estimation of the actual soot thickness along the channel length. This is a pre-requisite to get suitable data for the validation of the DPF models required for trap design and optimisation.     

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.     

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.     

Numerical simulation of soot filtration and combustion within diesel particulate filters

The numerous benefits offered by diesel engines, compared to gasoline ones, are balanced by a drawback of increasing concern, namely soot emissions. Nowadays, soot emissions can be reduced by physically trapping the particles within on-board diesel particulate filters (DPF). The filter gets progressively loaded by filtering the soot laden flue gases, thus causing an increasing pressure drop, until regeneration takes place. The aim of this work is to develop a fully predictive three-dimensional mathematical model able to accurately describe the soot deposition process into the filter, the consequent gradual modification of the properties of the filter itself (i.e. permeability and porosity), the formation of a soot filtration cake, and the final regeneration step. The commercial computational fluid dynamics (CFD) code Fluent 6.2.16, based on a finite-volume numerical scheme, is used to simulate the gas and particulate flow fields in the DPF, whereas particle filtration sub-models and regeneration kinetics are implemented through user-defined-subroutines (UDS).Model predictions highlight uneven soot deposition profiles in the first steps of the filtration process; however, the very high resistance to the gas flow of the readily formed cake layer determines the evolution into an almost constant layer of soot particles. The ignition of the loaded soot was simulated under different operating conditions, and two regeneration strategies were investigated: a “mild regeneration” at low temperature and oxygen concentration, that operated a spatially homogeneous ignition of the deposited soot, and a “fast regeneration”, with an uneven soot combustion along the axial coordinate of the filter, due to strong temperature gradients inside the filter itself. These findings are supported by comparison and validation with experimental data.     

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     

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.     

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.     

In-Operation Monitoring of the Soot Load of Diesel Particulate Filters: Initial Tests

Diesel particulate filters (DPF) are indispensable parts of modern automotive exhaust gas aftertreatment systems due to the stringent emissions legislation. For a fuel-efficient control strategy, it would be beneficial to determine directly and in-operation their actual trapped soot mass. Two novel approaches—based on the electrical conductivity of trapped soot particles—emerged recently. By measuring the electrical resistance between different single walls inside the filter, the soot load is determined with local resolution. The microwave-based technique is a contactless approach that gives an integral value depending on the soot mass in the DPF. We present investigations on loading and regeneration of DPFs in a dynamometer test bench applying both methods. The results are compared with each other and correlated with the differential pressure and the soot mass. Especially the microwave-based technique has a potential for serial application.     

Evaluation of Performance of Diesel Particulate Filters Through Integrated Multi-Scale Computer Calculations

Wall-flow channel models and soot deposition models based on micro scale considerations are integrated into global 3D diesel particulate filter simulations. In addition, transient and steady-state simulations are combined to understand at the same time short- and long-time behaviour of the diesel particulate filter (DPF). The functionality of the simulation tool is achieved and correlations with measured data encourage the use of the model as a tool to predict DPF behaviour.     

HRTEM Study of diesel soot collected from diesel particulate filters

HRTEM study of several soot samples collected on Diesel Particulate Filters (DPF) under conditions relevant to practical applications of DPF technology, revealed nano-structure, to our knowledge, not reported previously for diesel soot. In particular, some of the primary particles were found to have hollow interior, and the outer shell exhibiting evidence of graphitization, with a higher crystallinity compared to the non-hollowed particles. The percentage of such particles varied between different soot samples and tentatively appeared to be related to the oxidation history of the sample. Remarkably, similar effect was not reproduced for a carbon black sample, Printex-U, suggesting that propensity to such oxidation-induced graphitization is related to the original nano-structure of the particle. These initial observations were independently confirmed for the same set of soot samples by two different HRTEM facilities, at NASA-Glenn and PNNL.     

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.     

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.     

Study on regeneration of diesel particulate filter using a laboratory reactor

Regeneration of diesel particulate filters can be accomplished by complete combustion of a collected particulate. A reactor has been developed for study of the regeneration in the presence of catalysts, additives and ignition aids. This reactor allows an accurate measurement of soot ignition temperatures and a kinetic study of soot oxidation with an undisturbed soot layer and under a defined gas composition. Results of various investigations carried out with this reactor are presented.     

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 T_10% 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.     

Preparation and characterization of diesel particle filter (DPF) with hierarchical microstructure for catalytic combustion of soot

Mullite whiskers were prepared on the wall of cordierite honeycomb ceramics using the gas-phase growth method. Subsequently, Co/Ce_0.75Zr_0.25O₂ catalyst was loaded on the mullite whiskers to form hierarchical microstructure by the sol-gel technology for catalytic combustion of soot. Scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform-infrared spectroscopy, X-ray diffraction and Transmission electron microscopy were used to characterize the structural morphology and phase compositions. The filtration capability can be improved by mullite whiskers growth on the wall of cordierite honeycomb ceramics, and the microstructural feature is similar to the pseudostratified ciliated epithelium on the tracheal cavity surface (one section of human respiratory system). The Co/Ce_0.75Zr_0.25O₂ catalyst decreased the ignition temperature of soot particles oxidation, and the cycle stability experiment confirmed that Co/CZ@M/C sample has good structural stability and stable catalytic performance. The high-efficiency filtration and the low-temperature catalytic oxidation of soot particles are combined with the hierarchical microstructure of Co/CZ@M/C, which has potential application in the diesel particulate filter (DPF) field.     

Characteristics of the simultaneous removal of PM and NOx using CuNb-ZSM-5 coated on diesel particulate filter

The purpose of this study is to investigate the characteristics of the simultaneous removal of PM and NOx on the CuNb-ZSM-5 SCR/DPF catalysts coated onto DPF substrate. NOx conversion by the CuNb-ZSM-5 catalyst was higher than those by Cu- or Fe-ZSM-5 catalysts. NOx conversion of the SCR/DPF catalyst with a wall-flow (plugged) was considerably lower under 450 °C than that of the SCR/DPF catalyst with a channel-flow (unplugged). The de-NOx performance of the SCR/DPF catalyst coated with CuNb-ZSM-5 was highest among the catalysts examined. SCR/DPF catalyst coated with CuNb-ZSM-5 had superior PM oxidation performance compared to the other SCR/DPF catalysts.     

Counter‐intuitive temperature excursions during regeneration of a diesel particulate filter

A major technological challenge in the regeneration of diesel particulate filters (DPFs) is that sometimes local high temperature excursions melt the cordierite ceramic filter. The cause of this melting is still an open question as the highest temperature attained under stationary (constant feed) combustion of the accumulated particulate matter is too low to cause this melting (melting temperature ～1250°C). We recently conjectured that the high temperature excursions are a counterintuitive response to a rapid deceleration, which decreases the exhaust gas temperature and flow rate and increases the oxygen concentration. Infrared measurements of the spatiotemporal temperature during soot combustion on a single‐layer DPF showed that a simultaneous step change of the feed temperature, flow rate, and oxygen concentration can lead to a transient temperature that exceeds the highest attained for stationary operation under either the initial or the final operation conditions. The experiments revealed that the magnitude of the temperature rise depends in a complex way on several factors, such as the direction of movement of the propagating temperature front. The amplitude of the temperature rise is a monotonic decreasing function of the distance that the temperature front moved before the step change. The rapid response to the feed oxygen concentration increases initially the moving front temperature. The slow response of the ceramic DPF to a decrease in the feed temperature may eventually decrease the moving front temperature and even lead to premature extinction and partial regeneration. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

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.