Role of CeO2–ZrO2 in diesel soot oxidation and thermal stability of potassium catalyst

Potassium nitrate catalysts supported on different oxides (CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂) were prepared for diesel soot combustion. The ageing treatment was performed at 800 °C for 24 h and the catalytic activity was evaluated by a temperature-programmed oxidation technique. The results demonstrated that, compared with CeO₂ and ZrO₂, Ce_0.5Zr_0.5O₂ presented good redox properties, a high surface area and available potassium-holding capacity at an elevated temperature. For aged K/Ce_0.5Zr_0.5O₂, the combustion temperature of soot particle was 359 °C under tight contact conditions and 455 °C under loose contact conditions. Thus, ceria–zirconia mixed oxides were considered as good candidate supports for diesel soot oxidation catalysis.     

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.     

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     

Innovative means for the catalytic regeneration of particulate traps for diesel exhaust cleaning

Catalytic traps for diesel particulate removal are multifunctional reactors coupling filtration and catalytic combustion of soot. This paper reviews the most recent developments carried out at Politecnico di Torino concerning two different trap types: zirconia-toughened-alumina foams catalysed with Cs–V catalysts, operating according to a deep filtration mechanism, and cordierite or SiC wall-flow filters catalysed with perovskite catalysts (e.g. LaCr_0.9O₃), enabling shallow-bed filtration. The preparation and characterisation of these two trap types are described and the performance of the traps (filtration efficiency, pressure drops, etc.) evaluated on a diesel engine bench under various operating conditions. A final critical assessment points out that most chances of practical application in mobile sources lie in wall-flow type traps for their superior filtration efficiency (>95%) and their compatibility with active trap regeneration means (e.g. fuel post-injection) that can occasionally rise on purpose the exhaust gas temperature to accelerate the catalytic combustion of trapped soot. Conversely, completely passive solutions based on deep filtration catalytic traps show only promise for stationary applications at temperatures higher than 350°C, due to insufficient catalyst activity at lower temperatures.     

Bench Scale Experiments of Diesel Soot Oxidation Using Pr0.7Sr0.2K0.1MnO3 Perovskite Type Catalyst Coated on Ceramic Foam Filters

Potassium and strontium substituted praseodymium manganate type perovskite catalyst coated on ceramic foam filters have been studied for diesel particulate removal. The synthesized catalyst coated filter pieces have been characterized by using XRD, SEM and TG analysis, whereas their catalytic activity towards soot oxidation was tested using a bench scale facility with real diesel engine exhaust. The catalyst coated filters decrease the soot oxidation T_initial value by 150 °C and T_final by 100 °C as compared to bare soot oxidation reaction, which can be considered as high activity under the actual conditions of diesel engine. The catalytic materials show good thermal stability, while their low cost will also add to their potential for practical applications. Although perovskites have been studied for laboratory evaluations of catalytic soot oxidation, present results further substantiate the possibility of using low-cost, supported, non-noble metal based catalysts for diesel exhaust emission control applications, especially for the cost-effective retrofitment of in-use vehicles with old generation engines.     

Catalytic oxidation of carbon black under the conditions of a weak contact in the presence of M/Ce0.72Zr0.18Pr0.1O2, where m is platinum,palladium, and ruthenium

Nanodisperse M/Ce_0.72Zr_0.18Pr_0.1O₂ catalysts, where M is Pt, Pd, and Ru in amounts of 0.5, 1.0, and 2.0 wt %, for the afterburning of carbon black in the exhaust fumes of diesel engines under the conditions of a weak contact are synthesized. The structural, textural, and catalytic properties of the samples are studied by means of energy dispersive X-ray spectroscopy (EDX), synchrotron X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), transmission electronic microscopy (TEM), low-temperature nitrogen adsorption, and thermogravimetric analysis-differential scanning calorimetry (TG-DSC). It is shown that metal-support interaction, growing in the order Pt → Pd → Ru, occurs as platinum metals are impregnated into the surface of a Ce_0.72Zr_0.18Pr_0.1O₂ support. Ruthenium-containing catalysts are the ones most active for the afterburning of carbon black, due not only by the nature of the impregnated component, but also to the relatively ill-defined metal-support interaction, as compared to platinum and palladium samples. An ～190°C drop in the temperature of oxidation onset and an ～120°C drop in the temperature of complete oxidation, relative to these same parameters for a support free of platinum group elements, is characteristic of these. The high activity of 0.5%Ru/Ce_0.72Zr_0.18Pr_0.1O₂ enables us to purify diesel exhausts of carbon black effectively while using relatively small amounts of the noble metal, making it possible to lower the price of catalyst converters of diesel exhausts.     

NOx Adsorption/Desorption Processes Over Ce0.76Zr0.24O2 and Their Influence on DeSoot Activity: Effect of the Catalyst Calcination Temperature

The catalytic activity of a Ce_0.76Zr_0.24O₂ mixed oxide, calcined at different temperatures, for soot oxidation under NO _x /O₂ was correlated with the catalytic activity for NO₂ production. The Ce_0.76Zr_0.24O₂ mixed oxide samples were prepared by co-precipitation and calcination at different temperatures: 500–1000 °C. A satisfactory correlation between the total amount of NO₂ desorbed after NO + O₂ adsorption at 60 °C and the T50% for soot combustion was found. The NO _x adsorption process was also explored by in situ DRIFTS.     

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.     

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.     

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.     

DIESEL SOOT COMBUSTION WITH PEROVSKITE CATALYSTS

Diesel soot emissions from stationary or mobile sources can be reduced through physical trapping in particulate filters until periodical in situ combustion takes place. This study focuses on the development of several perovskites for the catalytic combustion of diesel particulates in multifunctional catalytic reactors. Several perovskites, with BET surface areas of 20–30 m²/g, were prepared by the solution combustion synthesis method and were characterized by XRD, SEM, TEM, and TPD techniques. Catalytic activity tests have shown that the most promising catalysts, namely, perovskite catalysts with Cr in the B site and Tb or Pr in the A site, can ignite soot combustion well below 400°C, i.e., at a temperature 200°–250°C lower than that of noncatalytic diesel soot combustion. The best catalytic formulation was deposited on a full-scale wall-flow filter and tested against the soot emissions of a diesel engine, resulting in reduced regeneration time and substantial fuel consumption saving compared to the corresponding bare filter performance.     

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.     

The Influence of O2, Hydrocarbons, CO, H2, NO x , SO2, and Water Vapor Molecules on Soot Combustion over LaCoO3 Perovskite

The effect of various gases (O₂, hydrocarbons, CO, H₂, NO _x , SO₂, and H₂O vapor) presenting in the diesel exhaust on soot combustion using LaCoO₃ as a catalytic material was investigated in this paper. A significant promotion of the combustion rate was found following a trend of 10% H₂O addition > 3,000 ppm NO _x  > 1% H₂ or 3,000 ppm C₃H₆ addition, while the improvement in soot oxidation due to the introduction of 3,000 ppm CO or 3,000 ppm CH₄ into the reactant gas is relatively less. The wet pretreatment of LaCoO₃ with 10% steam before soot oxidation hardly affects the combustion behavior. Interestingly, 10% water vapor in the reaction feed produced a significant promoting effect on combustion. In contrast, 30 ppm SO₂ treating led to an obvious deactivation likely owing to the coverage of active sites by sulfate compounds.     

Secondary nanoparticle emissions during diesel particulate trap regeneration

Two nanostructured mixed oxide catalysts (the CoCr₂O₄ spinel and the LiCrO₂ delafossite) have been recently developed for diesel soot combustion. The catalysts have been deposited via in situ combustion synthesis over SiC wallflow trap by CTI (Salindres, F). Bench tests proved that, after soot loading, both the developed traps enable a faster and more complete regeneration at 550 °C than the non-catalysed trap. However, a specific study on the particles distribution after the SiC trap, carried out via SMPS analysis, showed that secondary nanoparticles (<20 nm) are emitted during the regeneration promoted by the highly-active CoCr₂O₄ catalytic trap, as opposed to the LiCrO₂-catalysed and the virgin counter parts. This phenomenon has been investigated vs. the regeneration temperature and some sampling conditions so as to draw preliminary indications on the nature of these undesired particles.     

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.     

Further Insights into the Key Features of Ceria–Zirconia Mixed Oxides Governing the Catalysed Soot Combustion Under NO x /O2

Eighteen Ce_0.76Zr_0.24O₂ samples have been prepared by different procedures and characterised by N₂ adsorption at –196 °C, XRD, Raman spectroscopy, H₂-TPR and XPS. The catalytic activity for soot combustion depends both on the BET surface area and on the Ce/Zr atomic surface ratio, which strongly depends on the preparation procedure.     

Role of Surface Area in Oxygen Storage Capacity of Ceria–Zirconia as Soot Combustion Catalyst

Two thermal stable phases, Ce_0.75Zr_0.25O₂ and Ce_0.16Zr_0.84O₂, with different surface area were prepared by coprecipitation. The oxygen storage capacity (OSC) measurements were carried out at 500 °C under both transient (CO–O₂ cycle at 0.05, 0.1 and 0.25 Hz) and stationary reaction conditions (CO pulse). In the oxygen storage/release process, the rate-determine step is surface reactions when the specific surface area is lower than 60 m²/g. When the surface area increases further, the influence of surface area is less important. The increased surface area favors diesel soot catalytic combustion via providing more redox sites to activate adsorbed oxygen. Nevertheless, this effect is less important when the specific surface area is larger than 40 m²/g, especially under loose contact condition.     

Impact of Biodiesel-Based Na on the Selective Catalytic Reduction of NOx by NH3 Over Cu–Zeolite Catalysts

A single-cylinder diesel engine was used to investigate the impact of Na on Cu–zeolite SCR catalysts using 20 % bio- and petrol-diesel fuel blend (B20) with elevated levels of Na. The Na exposure was performed on light-duty (DOC–SCR–DPF) and heavy-duty (DOC–DPF–SCR) configurations of the diesel emissions control devices. The accelerated Na aging is achieved by exposing the system to elevated levels of Na that represent full useful life exposure (700,000 km) and periodically increasing the exhaust temperature to replicate DPF regeneration. After aging, the NO_x performance and relevant chemistry of the SCR catalysts were evaluated in a bench flow reactor. The SCR in the DOC–SCR–DPF configuration was found to be severely affected by Na contamination, especially when NO was the only NO_x species in the simulated exhaust gases. In the DOC–DPF–SCR configuration, no impact is observed in the SCR NO_x reduction activity. Electron microprobe analysis (EPMA) reveals that Na contamination on the SCR samples in the DOC–SCR–DPF configuration is present throughout the length of the catalysts.     

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.     

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.