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.     

Effect of Ag, Cu, and Au Incorporation on the Diesel Soot Oxidation Behavior of SiO2: Role of Metallic Ag

The catalytic behaviors of Ag, Cu, and Au loaded fumed SiO₂ have been investigated for diesel soot oxidation. The diesel soot generated by burning pure Mexican diesel in laboratory was oxidized under air flow in presence of catalyst inside a tubular quartz reactor in between 25 and 600 °C. UV–Vis optical spectroscopy was utilized to study the electronic states of Ag, Cu, and Au(M) in M/SiO₂ catalysts. The soot oxidation was seen to be strongly enhanced by the presence of metallic silver on 3 % Ag/SiO₂ surface, probably due to the formation of atomic oxygen species during the soot oxidation process. The catalyst is very stable due to the stability of Ag⁰ species on the catalyst surface and high thermal stability of SiO₂. Obtained results reveal that though the freshly prepared 3 % Cu/SiO₂ is active for soot oxidation, it gets deactivated at high temperatures in oxidizing conditions. On the other hand, 3 % Au/SiO₂ catalyst does not present activity for diesel soot oxidation in the conventional soot oxidation temperature range. The catalytic behaviors of the supported catalyst samples have been explained considering the electron donating ability of the metals to generate atomic oxygen species at their surface.     

Advances in the development of filterless soot deposition systems for the continuous removal of diesel particulate matter

A model catalytic converter system has been developed to investigate and characterize novel catalyst structures for filterless diesel particulate matter deposition and oxidation in modern heavy duty vehicle diesel engine exhaust systems. The particulate traps are designed for low exhaust gas back pressures and to avoid the clogging effects observed in ceramic filters. In experiments under realistic flow conditions deposition efficiencies of up to 70% have been achieved for submicrometer particles in stacks of corrugated stainless steel foil with microsphere surface coating. The model catalytic converter system is also used to study the reaction kinetics of soot oxidation and volatilization by oxygen and nitrogen oxides under a wide range of reaction conditions, for real diesel engine soot, different model soot substances, and different types of converter surfaces.

     

Advances in the Development of Filterless Soot Deposition Systems for the Continuous Removal of Diesel Particulate Matter

A model catalytic converter system has been developed to investigate and characterize novel catalyst structures for filterless diesel particulate matter deposition and oxidation in modern heavy duty vehicle diesel engine exhaust systems. The particulate traps are designed for low exhaust gas back pressures and to avoid the clogging effects observed in ceramic filters. In experiments under realistic flow conditions deposition efficiencies of up to 70% have been achieved for submicrometer particles in stacks of corrugated stainless steel foil with microsphere surface coating. The model catalytic converter system is also used to study the reaction kinetics of soot oxidation and volatilization by oxygen and nitrogen oxides under a wide range of reaction conditions, for real diesel engine soot, different model soot substances, and different types of converter surfaces.     

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.     

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.     

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.     

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     

Observations on the Aging Environment Dependent NO Oxidation Activity of Model Pt/Al2O3 Diesel Oxidation Catalyst

The influence of aging environment of model diesel oxidation catalyst Pt/Al₂O₃ on the NO oxidation activity is studied. The fresh catalyst Pt/Al/F (calcined in air at 500 °C) is aged with or without phosphorus (P) poisoning (7.5 wt%) at 800 °C either in air (P/Pt/Al/O or Pt/Al/O) or in simulated diesel exhaust (P/Pt/Al/R or Pt/Al/R). Catalyst aged under diesel exhaust environment (Pt/Al/R) surprisingly presents the best NO oxidation activity under excess of O₂ followed by the fresh (Pt/Al/F) and thermally aged (Pt/Al/O) catalysts. The activity difference between the catalysts is quite large, especially between Pt/Al/R and Pt/Al/O that are aged at the same temperatures but under different environments suggesting the importance of the aging environment for the catalytic activity. The NO oxidation activity of P poisoned catalysts P/Pt/Al/R and P/Pt/Al/O is minute as compared to their P free counter parts indicating that chemical aging is more detrimental for catalytic efficiency than thermal aging.     

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.     

Influence of the Alkali Promoter on the Activity and Stability of Transition Metal (Cu, Co, Fe) Based Structured Catalysts for the Simultaneous Removal of Soot and NOx

Structured catalysts prepared by means of coating cordierite monoliths with alumina-based suspensions containing transition metals such as Cu, Co and Fe and alkali/alkali-earth promoters such as K and Ba. Textural and structural features of these catalysts were analyzed by means of N₂ adsorption and SEM. Their activity in the simultaneous removal of soot and NO_x was assayed in a lab-scale installation, using a carbon black as diesel surrogate. Catalysts exhibited significant activity in deNO_x and soot oxidation. K and Ba enhanced both NO_x adsorption and soot–catalyst contact. However Ba contributed to a greater extent to the adsorption of N-species, which moreover presented higher thermal stability than on K-catalysts, and K showed higher mobility than Ba. Thus, Ba-containing catalysts showed increased activity towards NO_x reduction but shifted to higher temperatures in comparison to K-catalysts, which on the other hand resulted more active towards soot oxidation than Ba-ones. Fe-based catalyst turned out to be less active both in soot oxidation and NO_x reduction than Co and Cu-based ones. Intensive calcination of the catalysts at 800 °C for 5 h resulted in substantial loss of K and Ba. Loss of promoter depends, however, on the metal contained in the catalyst. In this sense Fe-containing catalysts showed higher stability. Calcination has a substantial effect on catalytic activity. Catalyst significantly lost their NO_x adsorption capacity and showed similar activity than a catalyst prepared in absence of promoter, pointing to a substantial change in reaction mechanism and reaction predominantly occurring on metallic sites upon the loss of alkali/alkali-earth compound.     

Role of Electronic Factor in Soot Oxidation Process Over Tunnelled and Layered Potassium Iron Oxide Catalysts

This paper describes the investigations of the catalytic activity in soot oxidation over well-defined iron oxide based materials. The nanostructuration of iron oxide by potassium into tunnelled (KFeO₂) and layered (K₂Fe₂₂O₃₄) ferrites and the surface promotion with CeO₂ results in the marked increase in the catalytic activity (decrease of the ignition temperature down to 210 °C and T _10 % to 310 °C). The measurements of the catalysts work function showed that both nanostructuration and surface promotion with ceria of the best KFeO₂ phase led to increase of the electron availability (decrease of the work function). Strong correlation of the catalytic activity in soot combustion of the Ce–K–Fe–O systems with the work function value was revealed for the first time in the model studies, and can be used as a guideline for optimisation of the real catalytic filters.     

A highly efficient and porous catalyst for simultaneous removal of NOx and diesel soot

A highly efficient and porous catalyst (La_0.8K_0.2Cu_0.05Mn_0.95O₃) for simultaneous removal of nitrogen oxides (NOx) and diesel soot was synthesized and the sample was characterized by XRD, BET, SEM–EDS, XRF, and XPS. The results indicate that this catalyst is perovskite and it possesses the very good characteristics for multiphase catalytic reactions. The catalytic properties were appraised under simulated diesel engine exhaust by temperature programmed reaction (TPR). TPR results indicate that this catalyst is a promising candidate for simultaneous removal of NOx and diesel soot. The maximum NO conversion into N₂ and the ignition temperature of soot are 54.8% and 260 °C, respectively. Comparing with those in previous reports, the comprehensive performance of this catalyst is greatly improved by partial substitution of La with K and Mn with Cu at the same time.     

Diesel particulate emission control

This paper reviews the emission control of particulates from diesel exhaust gases. The efficiency and exhaust emissions of diesel engines will be compared with those of otto engines (petrol engines). The formation of particulates (or “soot”), one of the main nuisances of diesel exhaust gases, will be briefly outlined. The effects of various emission components on human health and the environment will be described, and subsequently the emission standards for particulates and for NO_x, which have been introduced worldwide, will be summarized. Possible measures for reducing exhaust emissions of particulates and NO_x will be discussed, such as the use of alternative fuels, modifications to the engine and the use of aftertreatment devices. It will be made clear that aftertreatment devices may become necessary as diesel emission standards become more stringent, in spite of important progress in the other fields of reducing exhaust emissions. Selective catalytic reduction via hydrocarbons, ammonia or urea, a possible aftertreatment method for NO_x emission control, will be discussed briefly. Filters for collecting particulates from diesel exhaust gases will be examined in more detail and aftertreatment control systems for particulate removal will be reviewed. These can be divided into (i) non-catalytic filter based systems which use burners and electric heaters to burn the soot once it has been collected on the filter; (ii) catalytic filter-based systems which consist of filters with a catalyst coating, or filters used in combination with catalytically active precursor compounds added to the diesel fuel; and (iii) catalytic non-filter-based systems in which gaseous hydrocarbons, carbon monoxide and part of the hydrocarbon fraction of the particulates are oxidized in the exhaust gases. Finally, recent trends in diesel particulate emission control will be discussed, indicating the growing importance of the catalytic solutions: the fast introduction of non-filter-based catalysts for diesel engines and the possible application of filters in combination with catalytically active precursor compounds added to diesel fuel.     

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.     

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     

PdO x /Pd at Work in a Model Three-Way Catalyst for Methane Abatement Monitored by Operando XANES

The oxidation state of palladium in a model Pd/ACZ three-way catalyst was monitored by synchronous XANES and mass spectrometry during two consecutive heating (to 850 °C) and cooling (to 100 °C) cycles under stoichiometric conditions simulating exhaust after treatment of a natural gas engine. During heating in the first cycle, PdO reduction occurred around 500 °C and the initial fully oxidized state of Pd was never recovered upon heating and cooling cycles. A mixed Pd²⁺/Pd oxidation state was at work in the second cycle. Hence, the operando XANES study reveals that the PdO _x /Pd pair exists in a working catalyst but is less active than the catalyst in its initial state of fully oxidized palladium. It is also evident from XANES spectra that ceria–zirconia promotes re-oxidation of metallic Pd, thus reasonably sustaining catalytic activity after exposure to high temperatures.     

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.     

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.     

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.