18 Catalytic Technology for Soot and Gaseous Pollution Control

Transcription

1 Druckfreigabe/approval for printing 0 0 Without corrections/ ohne Korrekturen After corrections/ nach Ausfçhrung der Korrekturen Date/Datum:... Signature/Zeichen:... Catalytic Technology for Soot and Gaseous Pollution Control Olaf Deutschmann and Athanasios G. Konstandopoulos. Introduction Pollutant emissions from stationary sources have long been a public topic, with the British Alkali Act requiring soda ash plants to cut acid gas emissions by % as far back as. Since then, the anthropogenic emission of pollutants has drastically increased, leading to severe health risks and climate changes. With the general public having been aware of such risks for a long time, technologies have developed continuously to reduce the amounts of gases and particles emitted into the atmosphere, though these are often still enforced by legislation. Today, pollutants are emitted continuously from both stationary and mobile sources worldwide. Catalysis has developed as a major technology also for the reduction of pollutant emissions. This chapter will present a survey of todays technology and challenges for the catalytic control of soot and gaseous pollutants, detailing mobile and, to a certain extent, also stationary sources. Topics related to the reduction of CO emissions and catalytic combustion technologies as primary measures for reducing the formation of pollutants will not be included at this point... Pollutant Emissions from Stationary Sources ` ` Stationary sources for soot and gaseous emissions range from private homes, small industrial sites and waste-incineration facilities to large, coal-fired power plants. These may have no, little, or extensive flue-gas cleaning technologies, many of which will include catalytic processes. The main pollutants are nitrogen oxides (NO x ), sulfur oxides (SO x ), particles/soot, carbon monoxide (CO), hydrocarbons (HCs), volatile organic compounds (VOC), ammonia (NH ), heavy metals, and others. A survey of the typical ranges of gaseous emissions from stationary sources without flue gas cleaning is provided in Table.. Emissions of NO x,so x, and NH cause acidification of the environment and the formation of smog, while the greenhouse gases CO,N O, and CH are made j Handbook of Combustion Vol.: Combustion Diagnostics and Pollutants Edited by Maximilian Lackner, Franz Winter, and Avinash K. Agarwal Copyright Ó 0 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: ----

2 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Q Table. Typical ranges of gaseous emissions from stationary sources without flue gas cleaning []. Industry Source Fuel type Gas emissions Heat and power generation Levels/pp mv a) Boiler Coal NO x 0 0 Oil/Pet coke SO x 0 00 Particles b) mg Nm NO x 0 00 SO x Particles b) mg Nm Biofuel NO x 0 0 SO x 0 0 Gas turbine Gas NO x 0 CO 00 Diesel engine Oil NO x SO CO 0 00 Hydrocarbons 0 00 ppm C Incineration Municipal waste, NO x 0 0 sewage sludge SO 0 CO 0 Dioxins ng Nm Particles b) Process industry Nitric acid plants NO x N O 0 00 HNO Trace NH Trace Cement Gas þ solid NO x 0 00 calcination Ethylene burners Gas NO x 0 Smelters Gas SO x NO x 0 Heavy metals < (after scrubber) Coal gasification Coal NO x 0 Industry heaters Gas SO x 0 Glass furnaces Gas NO x FCC catalyst Gas NO x regeneration Printing industry, etc. Solvents VOC g Nm a) b)

3 . Introduction j 0 0 responsible for global warming. Several other gaseous emissions such as dioxins, VOCs, H S, and heavy metals present direct health risks. The topic of catalytic technologies for exhaust-gas after-treatment of stationary sources has also been described in many books, book chapters, and review articles. For further survey information, the reader is referred to reviews by Gabrielsson and Pedersen [], Spivey [], and others... Pollutant Emissions from Mobile Sources Internal combustion engines in automobiles represent a major source for the emission of NO x, CO, and unburnt HCs, whereas diesel engines contribute also to the emission of soot. The most appropriate way to minimize these air pollutants is to modify the combustion process. However, aside from those primary measures, current legislative emission standards can only be met by additional, secondary measures for exhaust purification by application of catalysts. The importance of environmental catalysis will further increase in future due to the tightening of emission limits, and an increasing number of automobiles. Already today, environmental applications exhibit a worldwide market share of % among all catalytic processes, whereas over 0 million automotive catalysts devices are produced per year. The catalytic system used for after-treatment of the exhaust gas depends primarily on the fuel used (gasoline, diesel, biofuels) and the operating conditions. Typical engine raw emissions are listed in Table.. In principle, there is a need to distinguish between stoichiometric-operated gasoline engines, lean-operated gasoline engines, and diesel engines which exhibiting different major pollutants, namely CO/NO x /HCs, NO x, and NO x /soot, respectively. The topic of catalytic technologies for exhaust-gas after-treatment of mobile sources, automotive exhaust catalysis, has also been detailed in many books and reviews. Further survey information is available in the reviews of Lox et al. [], Heck and Farrauto [], Votsmeier et al. [], Eigenberger et al. [], and many others. Table. Typical raw emissions of a spark-ignition (SI) (Otto) and a diesel internal combustion engine []. Diesel engine N (vol.%).. O (vol.%) 0. CO (vol.%). H O (vol.%). NO x (vol.%) CO (ppm) HC (ppm) 00 0 Otto engine

4 j Catalytic Technology for Soot and Gaseous Pollution Control Spark-Ignition Internal Combustion Engines The concept and design of catalysts for gasoline engines have always been coupled with changes in engine design and operation, and with legislation. The raw emission of gasoline-fueled engines depends on the air fuel ratio. In general, leaner conditions lead to higher NO x emissions, while richer conditions favor CO and HC emissions. The chemical composition of the HCs emitted is rather complex, and depends on the fuel composition, the air fuel ratio, and the conditions during combustion. It is, therefore, not straightforward to find a model exhaust gas in laboratory studies that can match the properties of the HC raw emissions. Today, most studies use a mixture of propylene and methane, where the first component represents the fast-reacting HC species, and the latter component the rather more stable species. Today, indirect-injection spark-ignition (SI) engines are operated at stoichiometric air fuel ratios (i.e., l ), because the application of a three-way catalyst (TWC) requires such an operation. In fact, it was the development of highly efficient TWC exhaust purification systems that led to the coupling between exhaust-gas aftertreatment and motor management. The operation of a SI engine under lean conditions that is, with excess air leads to a reduction in fuel consumption and, as a consequence, reduced CO emissions. The major pollutants here are NO x, primarily NO. During the past few years, the NO x storage reduction catalyst (NSC) which is also referred to as the NO x -Adsorber or lean NO x trap has been the favored concept for the reduction of NO x emissions from lean-operated gasoline-fueled engines. Yet, during the same time, the selective catalytic reduction (SCR) concept has also been under consideration for leanoperated SI engines.... Diesel-Operated Internal Combustion Engines The internal combustion engine requires ignition of the mixture of fuel and oxygen, either by SI (i.e., gasoline engines) or compression-ignition (CI); these include traditional diesel and various premixed diesel combustion variants known as HCCI (homogeneous charge compression ignition) engines, PCCI (premixed charge compression ignition) and LTC (low-temperature combustion). The diesel engine takes in air, and utilizes the heat of compression to ignite a small quantity of fuel, which is injected into the cylinder. In the case of the diesel premixed mode engines, an air fuel mixture is compressed until it reaches the point of autoignition. This promises not only to result in lower emissions, but also to exhibit an improved fuel economy. Unfortunately, these new diesel engine technologies are often very difficult to control at higher engine loads, and it is for this reason that practical development is focusing on the production of mixed-mode diesel combustion engines that operate with a premixed fuel air charge at low engine loads, but under conventional (nonpremixed) diesel combustion at higher loads. Diesel engines are endowed with a high energy efficiency, fuel economy, durability and, provided that an emission control technology is applied to reduce emissions of NO x, HCs, CO, and particulate matter (PM or soot), the diesel engine can become the dominant automotive powertrain, not only for heavy-duty and commercial vehicles, but also for passenger cars, as recent trends in Europe have already demonstrated.

5 . Catalytic Technology for Soot Pollution Control j 0 0 To meet the ever-increasing, stringent diesel emission regulations, emission control technologies have been developed to address each pollutant. In the case of NO x emission reduction there are several routes:. Selective catalytic reduction (with ammonia resulting from urea injection and decomposition in the exhaust).. NO x -absorbing materials that store NO x and release them under appropriate (for the reduction of NO x to nitrogen) conditions in the exhaust.. Selective NO x reduction with the aid of HCs. In the case of CO and HCs, diesel oxidation catalysts (DOCs) are used for the oxidation of these gases to CO, while soot emissions are reduced by employing diesel particulate filters (DPFs). In this chapter, attention will be focused on catalytic technologies for emission control for mobile applications. First, technologies for controlling soot emissions will be discussed, focusing on DPFs and DOCs, after which gaseous pollutants and the concepts of TWCs, SCR, and NSCs will be discussed. A final section will detail the control technologies used to reduce gaseous pollutants from stationary applications.. Catalytic Technology for Soot Pollution Control.. Introduction... Diesel Soot The development of efficient particulate reduction technologies requires the prior characterization of soot particles emitted from diesel engines. Representative recent studies conducted in this area [, ] have indicated that the characterization of the soot particulates depends on several factors. From a control technology point of view, the state of the soot particles in the raw exhaust upstream of the DPF is of great interest. The diesel particulates are a complex entity (see Figure.) that contain carbonaceous solid aggregates and organic fractions that may form nuclei particles or aggregates. Their state depends on the exhaust temperature (which may induce condensation of the soluble organic fraction; SOF), on the sampling conditions (dilution process), and on various other parameters []. Most modern DPF systems are preceded by a DOC, the role of which is to oxidize exhaust HCs (either injected into the engine cylinder, or in the exhaust), and thus raise the exhaust temperature to the levels needed for DPF regeneration (i.e., the process of removing the accumulated soot by oxidation; see below). The DOC eliminates almost completely any SOF that might be present on the soot particles. The composition and morphology of diesel soot particles in the exhaust is very important. Such morphology will affect the structure of any deposits [] formed in the DPF (hence the engine backpressure), while their composition (predominantly carbonaceous) affects their oxidation potential and the ease of DPF

6 0j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Figure. Schematic of diesel particulate matter. Note: This pictorial representation appeared originally for the first time in Ref. [], and has been adopted since then in several subsequent publications, but without reference to the original source. regeneration [ ]. The substructure of soot primary particles obtained by different combustion sources and fuels has been studied using high-resolution transmission electron microscopy (TEM) [,, ], and is known to be connected to reactivity of the soot. Soot aggregates are assemblies of primary particles formed from carbon platelets arranged in so-called turbostratic structures (Figure.). More specifically, soot particles are composed of a distribution of graphitic (G) and diamond (D) -like carbon domains (Figure.) that correspond, respectively, to ordered and amorphous carbon structures that, in turn, are largely responsible for the variation in the reactivity of soot particles. Figure. Primary carbon particle structure [, 0].

7 . Catalytic Technology for Soot Pollution Control j 0 0 Figure. Typical Raman spectrum of soot and its analysis in component peaks [, ]. With regards to the size of the soot particles, a variety of measurements have indicated that the majority of emitted solid diesel aggregate particles have electrical mobility and aerodynamic diameters in the range of 00 nm. The primary particle diameters are found to lie in the range of 0 nm [,, ]. Other investigations into the effect of the different types of diesel engine, of fuels, and of engine-operating conditions on the size distribution of the soot particles have been conducted []. Notably, the soot aggregate size distributions were monitored in the exhaust of five turbo-charged, direct-injection diesel engines (model years 00), with displacements in the range of. to. liters and with advanced fuel injection systems [, ] The results showed that, for a constant shape, the size distribution was increased in line with the steady-state solution of the soot aggregate population, achieving a balance that accounted for the coagulation and oxidative fragmentation processes []. Naturally, the simultaneous occurrence of these two processes would lead to a distribution of soot aggregate morphologies. It was also shown how the fractal dimension of diesel soot aggregates (Figure.) was distributed according to their electrical mobility diameter []. When these measurements were conducted on so-called Euro II and Euro III diesel engines operating under different conditions, there appeared to be a very robust pattern over the different engines and operation conditions tested (for a review, see Ref. []). Here, Euro X refers to the series of automotive emission regulations specified by the European Union (see Ref. []).... Diesel Particulate Filters Several materials and configurations have been employed as DPFs [,, ], with the combination of materials and geometries determining the performance of the filter with regards to pressure drop evolution during soot loading, filtration efficiency, regeneration, and so on. The most popular configuration for DPF applications is that

8 Color Fig.:. j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 fractal dimension of the ceramic honeycomb wall-flow monolith, with alternately plugged channels. This functions primarily as a separator, collecting the soot particles inside and on the walls of the channel, while allowing the gases to exit freely through the walls of the filter. Its popularity is based mainly on its advantageous performance with respect to pressure drop evolution during soot loading and particulate filtration efficiency, as it exhibits a compact arrangement that can achieve very high filtration efficiencies (>0%) with a small penalty in pressure drop... Soot Loading and Oxidation mobility diameter (nm) EURO III () 00 rpm bar EURO III() 00 rpm. bar EURO III() 00 rpm bar EURO III() 00 rpm bar EURO III() 00 rpm bar EURO III () 00 rpm bar EURO III () 000 rpm bar EURO II 00 rpm bar EURO II 00 rpm bar EURO II 00 rpm bar EURO II 000 rpm bar Van Gulijk et al (00)-. kw Figure. Fractal dimension of soot aggregates with different mobility diameters obtained with different engines and operation conditions [, ]. One factor that determines the operating efficiency of a DPF is its permeability (i.e., resistance to flow), which is in turn closely related to the geometric and microstructural characteristics of the filters []. This resistance of the DPF to flow is expressed by its pressure drop, which can be very accurately described by summing the individual pressure drop contributions (see Figure.). Simulations of the evolution of the pressure drop of the filter can provide a significant means of developing improved configurations [,, ]. Konstandopoulos and Johnson [], based on the fundamental principles of fluid mechanics and flow through porous media, published the first analytical solutions for flow fields and pressure drop of wall-flow monoliths in terms of filter micro

9 . Catalytic Technology for Soot Pollution Control j 0 0 Figure. Schematic of filter inlet and outlet adjacent channels, depicting local pressure values for the derivation of the effects of compressibility on pressure drop. structure and geometric configurations; these were subsequently validated experimentally for a particular extruded monolith design. The analytical model (extended for non-darcian flow effects) was later shown to be in excellent agreement with threedimensional (-D) computational fluid dynamics (CFD) simulations, and was further validated against a wider variety of filter media [,, ]. When the Konstandopoulos and Johnson [] model was tested extensively against a range of filter samples, it was reported to give excellent a priori predictions of pressure drop, thereby opening new development possibilities [ ]. This approach was later extended to include the influence of the accumulated soot, which accounted explicitly for the soot layer microstructure and its dependence on the operating conditions of the DPF [, ].... Soot Accumulation on the Filters The development of the pressure drop during soot accumulation on the filter is characterized by two filtration modes deep bed and cake filtration that depend on the microstructure of the filter []. The deep bed filtration mode is characteristic of all filter structures, and is expressed by a nonlinear increase of the pressure drop (Figure.). This is due to the initial deposition of soot particles inside the porous structure of the filter wall, which block locally the flow paths. Depending on the microstructure of the porous filter, even a small amount of deposited soot may have a huge effect on the pressure drop, as it may block a disproportionately large part of the pore structure hence the nonlinear character of the pressure drop evolution. As the porous wall becomes blocked by the deposited soot there will be a smooth transition to the cake filtration mode, where a macroscopic soot layer grows on top of the filter wall, characterized by a linear dependence of the pressure drop on the accumulated particulate mass in the filter. Konstandopoulos et al. [] demonstrated for the first time that, during filter loading, the microstructure of the soot cake was determined by the relative strength

10 Color Fig.:. j Catalytic Technology for Soot and Gaseous Pollution Control 0 Figure. Filtration modes, from deep-bed to cake (surface) filtration. of convective versus diffusive transport of the soot aggregates towards the deposit, as quantified by the dimensionless mass transfer Peclet number (the ratio of convective to diffusive mass transfer) (see Figure.). This showed that the properties of the soot layer (i.e., packing density, permeability of the soot layer) were dynamic, that they depended on the deposit growth mechanism and its history, and that the soot deposit microstructure may be deformed under sufficiently high pressure drop values [, ]. 0 Figure. Soot deposit growth mechanism [].

11 . Catalytic Technology for Soot Pollution Control j Soot Oxidation: DPF Regeneration As soot accumulates on the wall of the filter, it alters the permeability of the filter and increases the pressure drop, to a point where the backpressure would induce a very high fuel consumption and hinder the engines performance. Therefore, the soot loaded filter must be regenerated by oxidizing the accumulated soot a process termed DPF regeneration. Unfortunately, the oxidation of soot must be initiated and maintained at very high temperatures (0 0 C) that do not normally occur in the exhaust of a diesel engine. However, several methods are available by which the oxidation of soot particles can be achieved at lower temperatures (00 0 C). These methods are classified as either active or passive [, ]. The passive approach involves the use of a catalyst, either in fuel-borne form or as a catalytic coating. During regeneration tests with an increasing temperature, the pressure drop of the sootloaded filters was shown to decrease as the soot collected inside the filter was oxidized, emitting CO and CO gases. Based on such gas evolution, the soot oxidation rate (Figure.) can be calculated as a function of the temperature, thus providing a means of evaluating the regeneration behavior of different DPF technologies. The normalized soot oxidation rate (s )isdefined as: _r soot ¼ m 0 dm dt ð:þ where m 0 is the initial amount of soot mass collected. The soot consumption rate dm/ dt is typically computed by summing the CO and CO produced during the oxidation in a synthetic exhaust gas stream which does not contain CO nor CO in order to detect the soot-derived CO/CO. The evolution of the normalized soot oxidation rate as a function of temperature provides a means to compare and evaluate different DPF Figure. Soot oxidation rate as a function of temperature.

12 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 technologies with respect to their soot oxidation activity. The CO selectivity (Equation.) can be also calculated, which is an important parameter because it affects the total heat release during regeneration. CO f CO ¼ CO þ CO.. Catalytic Diesel Particulate Filters (CDPFs) ð:þ Catalytic soot oxidation has a long history [], the details of which have been recently reviewed [, ]. The CDPFs currently available commercially are coated with Ptgroup metal (PGM) catalysts that aim at the indirect oxidation of soot by NO generated from the oxidation of NO (known as an NO -assisted system []). The operation of the PGM-coated CDPFs depends on the balance between NO and soot in the exhaust. In contrast, soot oxidation catalysts, which are based on metal oxides, are thought to act through redox and/or spill-over mechanisms []. These oxidize the soot directly and, when combined with a PGM catalyst, can cover a wide range of operating conditions in the exhaust, thus broadening the soot oxidation temperature window and increasing the catalytic reactivity of the CDPF. However, one challenge that the direct soot oxidation catalysts face is the poor sootto-catalyst contact. The importance of the contact of soot with the catalyst, in diesel emission control, was recognized almost three decades ago [] as a barrier for active catalytic filter development, and it has become popularized in more recent laboratory studies of powdered carbon black catalyst mixtures (the introduction of loose and tight contact studies [, ]). Details of soot catalyst contact effects on diesel soot oxidation in filters have been reviewed [].... Direct Soot Oxidation Catalysts The firststepinthedevelopmentofasootoxidationcatalystisitssynthesis,which can be conducted via different techniques employing either solid or liquid precursors (e.g., solid-state reactions, sol gel precipitation, combustion methods, aerosol methods). The evaluation of the synthesized catalysts is conducted initially at the laboratory scale, as mentioned before, by producing powder mixtures of soot with the catalyst and employing, for example, a thermogravimetric analysis (TGA) technique. In this method, the loss in the weight of the mixture, caused by the oxidation of soot, is measured as a function of temperature; this change is interpreted, further, as the soot oxidation rate. The difference between the plain soot powder and the soot catalyst mixtures determines the reactivity of the catalysts (Figure.). However, as noted above, the performance of the catalysts measured in the soot catalyst mixtures on a laboratory scale, provides only a rough idea of the differences between the catalysts, which mostly are affected by the specific catalyst chemistry.

13 Color Fig.:.. Catalytic Technology for Soot Pollution Control j 0 0 /m o (dm/dt) (s - ).E-0.0E-0.E-0.0E-0.0E-0 0.0E Catalyst D Catalyst C Under realistic conditions on the filters, the catalysts are affected also by the filters geometry, as this will determine the degree of contact between the soot and the catalyst. A mathematical description of the incomplete soot catalyst contact the so-called Two-Layer model [] was introduced more than a decade ago, and has since been incorporated into state-of-the-art DPF simulators [0 ]. This model considers that the contact of soot with the catalyst is determined by the details of catalyst distribution in the filter (a type of frozen randomness), and the details of soot particle deposition and resultant deposit microstructure, as well as soot deposit restructuring (a type of evolving randomness). This forms the basic tool for the analysis of soot oxidation rates in CDPFs (Figure.), and can be used to describe the soot oxidation on catalytic filters to within % [, ]. 0 Catalyst B Temperature ( C) Catalyst A Figure. Effect of different catalyst formulations on the oxidation of soot []. Figure. Application of the two-layer model to experimental data for a catalyzed filter sample with respect to (a) soot oxidation rate and (b) conversion []. 0 soot 0

14 j Catalytic Technology for Soot and Gaseous Pollution Control Deposition of Catalysts on Filters In order for the catalysts to be evaluated under realistic conditions in the exhaust of a diesel engine, they must be deposited on DPFs. Initially, segments of catalyst-coated DPFs are evaluated on an engine test cell bench, employing side-stream reactor technology [,, ]. The CDPF segment with the best catalyst formulation is further upgraded into a full-size CDPF, which is placed directly in the exhaust of a diesel engine and evaluated according to a certain methodology [, ]. Deposition of the catalyst on the DPF segments can be conducted via impregnation in a slurry of the catalyst powder, in a catalyst precursor solution and subsequent firing, or via in situ synthesis and deposition of the catalyst on the filter with an aerosol route [, ]. In the latter case the catalyst is deposited on the monolith channels with the same mechanism as the soot particles, a feature which is expected to maximize the contact of the soot particles with the predeposited catalyst sites.... Fuel-Borne Catalysts A different approach towards the geometry and soot catalyst contact problem is the concept of fuel-borne catalysts, also known as fuel additives. These are based on the formation of metal oxides, during combustion of the fuel in the engine, that can either remove the precursors that may cause the formation of the particulate matter or, alternatively, inhibit the nucleation of such particle precursors [, ]. Some fuel additives can also form metal oxides along with or inside the carbonaceous particulate matter, and thereby increase the soot reactivity for oxidation. In this way, they are in close proximity to the soot particles, so that they achieve an increased contact and therefore a more effective catalytic performance. One side effect of such a technology, however, is the formation of incombustible metal oxide deposits on the DPFs (ash accumulation), which causes an irreversible increase of the pressure drop of the filter and affects its durability and lifetime. A second disadvantage is the possible emission of metal oxide nanoparticles in the atmosphere, that could pose a significant threat for health and the environment. However, no such phenomena have been reported in the open literature to date... Assessment of DPF Technologies... Filtration Efficiency The filtration efficiency of a filter is the characteristic that determines its ability to remove soot particles from the exhaust stream. It is defined by measuring the concentration and size distribution of the soot particles before (upstream) and after (downstream) the filter [, ]. The filtration efficiency of a DPF may change with the addition of the catalyst, as the latter alters the microstructure of the DPF (e.g., blocking of the pores of the channel walls). It has been shown that increasing the amount of catalyst on the filter increases, as anticipated, the initial filtration efficiency of the filter [] (Figure.). In all cases, the filtration efficiency of the uncoated and coated filters appears to increase as soon as a small amount of soot load accumulates on them (Figure.).

15 Color Fig.:. and.. Catalytic Technology for Soot Pollution Control j 0 0 dn/dlogdp (#/cm ).00E+0.0E+0.00E+0.0E+0.00E+0.0E+0.00E+0.00E E+00 Figure. uncoated x g/m xg/m x g/m x g/m x g/m x g/m 0 dp (nm) uncoated Particle size distribution downstream of the filters for the same soot mass load.... Soot Loading The overall pressure drop across a filter, during the accumulation of soot, can be formally decomposed into the sum of the pressure losses that occur due to the gas flow through the clean porous medium, and the additional pressure losses that are Filtration Efficiency xg/m xg/m xg/m uncoated 0 dp (nm) x g/m x g/m x g/m uncoated Figure. Change in the filtration efficiency of filters with different catalyst amounts for a small amount of soot mass load

16 Color Fig.:. 0j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Pressure drop (mbar) induced due to the catalyst and soot deposition. The latter contains contributions from the filter pore blockage during the deep-bed filtration mode, as well as contributions from the layer of particles on its surface generated during the cake filtration mode. Both. the microstructure of the monolith (e.g., porosity, pore size), and the way in which catalyst is deposited on the monolith walls, affect the development of the pressure drop during soot accumulation. The conventional way to deposit a catalyst on a filter is by using wet chemistry (sol gel and slurry) techniques; in this way, the catalyst is deposited mainly inside the walls of the filter, blocking the pores, and subsequently causing a decrease in the permeability of the filter. This, in turn, will result in an increase of the pressure drop of the filter, which is enhanced during soot accumulation. An example of the effect of different filter microstructure and different direct soot oxidation catalyst deposition techniques (e.g., wet chemistry or aerosol) on the development of the pressure drop of the filter during soot accumulation is given in Ref. [0]. Here, it was shown that the way in which the catalyst is deposited on the filter may even cause a lower pressure drop compared to an uncoated filter (as in the case of the aerosol-coated filter) (Figure.). The layer of the catalyst operates as a filter, onto which the majority of the soot is deposited. Consequently, the pressure drop of the aerosol-coated filter will arise only from the resistance of the soot cake, the catalyst layer, the soot inside the catalyst cake, and from a small contribution of the filter wall. When the filter is not coated, or when the catalyst is not uniformly Soot mass load (gm - ). uncoated aerosol slurry Figure. Effect of the coating technique on the development of pressure drop during soot loading on standard filters..0.

17 Color Fig.:.. Catalytic Technology for Soot Pollution Control j 0 0 Pressure drop (mbar) 0 0 deposited and the catalyst layer is not sufficiently porous that soot can enter inside the layer, then the total pressure drop will be higher. In this case, the contribution of the pressure drop that has developed due to soot being deposited inside the filter wall, will causes an increased resistance to flow. This can be more easily understood by considering the characteristics of cake versus deep-bed filtration, as modeled in Refs [, ]. The aforementioned advantageous effect of the aerosol-coated filter was further investigated [0] for the case of an increase in the amount of a direct soot oxidation catalyst on the soot loading behavior. It was observed that, even with a quadruple amount of catalyst on the filter, the pressure drop remained at a lower level compared to the uncoated filter (Figure.).... Regeneration 0.. Soot mass load (gm - ) uncoated x g/m x g/m x g/m x g/m Figure. Pressure drop development during soot loading of standard filters coated via an aerosol technique at different amounts of catalyst.... Direct Soot Oxidation As noted above, the soot catalyst contact has a significant effect on the regeneration of the filter. It has been shown that the increased difference between several catalytic formulations in the powder scale is eliminated at the filter scale [,, 0] (Figure.). Whilst this is slightly affected by the chemistry, the factor that most determines the catalytic efficiency of the filter is its geometry. It has also been observed [0] that the coating technique, except from the effect on the pressure drop during soot loading, can determine the degree of contact between soot and the catalytic layer, thus affecting the soots oxidation activity (Figure.). Another issue observed [0] was the presence of complex multisite kinetics deriving from more than one different mode of contact of soot with the catalyst. *.

18 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 /m o (dm/dt) (s - ) /m o (dm/dt) (s - ).00E-0.00E-0.00E-0.00E-0.00E-0.00E-0.00E-0.00E-0.00E E+00 0 Figure..E-0.0E-0.0E-0.0E-0.0E-0.0E-0 0.0E+00 0 Figure. Catalyst C Catalyst B Catalyst D uncoated Temperature (ºC) Soot oxidation rate on monoliths coated with different catalysts. uncoated aerosol slurry Temperature (ºC) Effect of the coating technique on the soot oxidation rate. 00

19 . Catalytic Technology for Soot Pollution Control j 0 0 lnk Temperature (ºC) A detail of the soot oxidation rate plotted as an Arrhenius curve shows regions where changes in the slope of the curve are obvious (Figure.), and could be associated with different modes of contact.... Combination of Direct Soot Oxidation Catalyst with Pt The combination of direct soot oxidation catalysts with Pt would broaden the operating window of the CDPF. By utilizing Pt for the conversion of NO to NO, the indirect oxidation of soot could be achieved at temperatures lower than those of direct soot oxidation. Under conditions where the NO concentration is poor in the exhaust, the direct soot oxidation catalyst could, in turn, be utilized to make up for the absence of NOx. Pt is also considered to enhance the oxygen transfer and, therefore, to enhance the performance of the metal oxide used as a direct soot oxidation catalyst. An investigation of the Pt deposition technique on CDPFs, combined with a direct soot oxidation, has been conducted [] (Figure.).... Conversion-Dependent Phenomena Microstructural phenomena that occur during catalytic soot oxidation were investigated [], employing fuel-borne catalysts (FBCs) deriving from fuels with different concentrations of additives. These studies involved the oxidation of soot under isothermal conditions, with the oxidation rates being obtained as a function of soot conversion. These data allow the study of conversion-dependent phenomena, which are of some importance when it is 00. /T*00 (K - ). 00 x g m - x g m - Figure. Effect of the presence of different kinds of soot-to-catalyst contact on the behavior of soot oxidation on soot loaded monoliths coated with one-fold g m and four-fold g m.. 0.

20 Color Figs.:. and. j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Figure. Comparison of direct soot oxidation-pt filters coated via different methods with respect to: (a) NO to NO conversion; and (b) indirect soot oxidation. considered that soot is not a uniform entity, but rather exhibits a sub-primary particle nanostructure [,,,, ]. The conversion of soot could be visualized as a successive motion along a radius of a primary particle, which uncovers its internal parts (possibly with different reactivities). In these experiments it was noted that, depending on the concentration of the FBC and the temperature, different intraparticle phenomena set in (e.g., the uncovering of catalyst sites, catalyst particle mobility inside the soot particle creating channels or pits, similar to the situation for oxide catalyst particles on carbon [ ]) that determine the profile of the soot oxidation rate (Figures. and.0). The soot oxidation behavior of catalytic filters coated using different techniques was also investigated under isothermal conditions [], visualizing the geometric relationship between soot and the catalytic layer through the difference in the profiles of the soot oxidation rate as a function of conversion (Figure.). /m(dm/dt) (s - ).E-0.0E-0.E-0.0E-0.0E-0 0.0E Figure. 0% 0.% 0.% 0.0%.0%.0% Conversion 0.0 Soot oxidation rate at C for all fuel-borne catalyst concentrations..00

21 Color Figs.:.0 and.. Catalytic Technology for Soot Pollution Control j 0 0 /m(dm/dt) (s - ).E-0.E-0.E-0.E-0.0E-0.0E-0.0E-0.0E-0.0E-0 0.0E Figure.0 0% 0.% 0.% 0.0%.0%.0%.. Simulation Approaches Conversion Diesel particulate filter design, system integration and control, based on a traditional design of experiments approach, may become very time-consuming and costly, due to the high number of tests required. This provides a privileged window of opportunity for the application of simulation. Recent reviews of advances in DPF simulation technology are available in Refs [, ]. While the interested reader should consult these references (and their cited literature) for more detailed information on the underlying assumptions regarding the treatment of the various physico-chemical phenomena (including soot particle transport, deposition, and oxidation), an overview of the mode of use of different simulation models in practice is available in Ref. []. /m(dm/dt) (s - ).E-0.E-0.0E-0 0.0E Soot oxidation rate at 0 C for all fuel-borne catalyst concentrations. Advanced Coating (Material A) Advanced Coating (Material B) Conventional Coating (Material A) Conventional Coating (Material B) Conversion Figure. Comparison of soot oxidation rate at 0 C, on material A and B, coated with the two methods (conventional and advanced)

22 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Figure. Computer reconstruction of various porous filters. Examples of the Microflow Simulation approach include a computer-reconstructed porous DPF (Figure.) and the flow field and, subsequently, soot deposition inside the filter porous wall (Figure.) via the reconstructed porous material and employing Lattice Boltzmann-based methods [,, ]. DPF models coupled with the simulation of other emission control devices are, therefore, the appropriate tools for system optimization and control. An example of a coupled simulation of a DOC and a DPF in series is provided in Ref. []. Important issues for the DPF performance at this level are the development of transversal soot loading non-uniformities, induced by inlet cone flow and temperature maldistributions, and/or by heat losses to the environment from the DPF external surface. In addition, incomplete regenerations of the DPF must be described. The continuum multichannel approach [0 ] represents a computationally tractable and accurate tool to address the previously mentioned issues. The development of highly integrated simulators of multifunctional exhaust emission control systems requires the interfacing of multichannel models of DPFs, as well as of other honeycomb-type converters, to standard CFD solvers. Recent advances in this area have included the rigorous integration into the continuum multichannel framework of segmented filter designs, computationally efficient discretizations of

23 Color Figs.:. and.. Catalytic Technology for Soot Pollution Control Figure. Visualization of: (a) soot deposition; and (b) velocity through the filter wall, at different surface mass loads in an extruded ceramic (granular) filter wall (width of frame is 0 mm []). 0 non-axisymmetric filter geometries (e.g., oval and trapezoidal shapes) and intelligent coupling to general-purpose CFD solvers to account for spatial distributions of inlet conditions brought about by specific upstream exhaust piping layouts, as well as to temporal variations due to engine operation. An example of a -D simulation of a DPF is shown in Figure.. 0 Figure. Example of temperature field evolution in a mm mm in DPF during regeneration. j

24 Color Fig.:. j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Figure. NO concentration through the reconstructed CDPF wall with catalyst and at 0 C. A previously developed -D simulation framework for porous materials [ 0] was applied to the case of NO NO turnover in a granular silicon carbide CDPF. The detailed geometry of the CDPF wall was digitally reconstructed, and microsimulation methods were used to obtain detailed descriptions of the concentration and transport of the NO and NO species in both the reacting environment of the soot cake, and in the catalyst-coated pores of the CDPF wall. From these results, NO NO turnover was quantified and a comparison made with the turnover predicted by an approximate zero-dimensional analytical model. A good agreement was found between the two models, thereby justifying use of the approximate but fast analytical model in large-scale simulations of CDPFs. Details of the simulation framework, the approximate analytical solution, and the simulation of NO NO turnover in a simplified -D wall pore and in a reconstructed filter wall are available in Ref. [] (Figure.). The use of approximate but fast analytical approaches to describe the coupled transport and reaction phenomena in filter walls [, ] opens new possibilities for embedding multifunctionally catalyzed DPFs in large-scale, multidimensional simulators at a fraction of the time required for conventional, brute-force computational approaches. In addition, it becomes possible to consider the deployment of efficient on-board monitoring and control algorithms, implementable in computationally limited engine control units (ECUs)... Effect of Ash Accumulation As noted in Section..., the use of additives in both the fuel and lubricants, as well as deterioration of the engine, causes the formation of ash particles that are collected on the DPF. These particles are incombustible, and as they are accumulated on the DPF they cause an irreversible increase of the pressure drop.

25 Rapid Ash Aging Method The evaluation of the effects of ash aging on filter performance is a time- and costconsuming task that has slowed down the process of creating innovative filter structures and designs. Consequently, a development of a methodology to produce filter samples that have been aged by accumulating ash, produced by the controlled pyrolysis of oil fuel mixtures, would accelerate the evaluation of filters with respect to ash aging []. The artificial ash particles obtained in this way can be compared to those from real engine operation [], and they bear both morphological (size) and compositional similarities to ash particles collected from engine-aged DPFs. The ashloaded samples are then evaluated with regards to their soot-loading behavior, filtration efficiency, and regeneration performance. It was observed that, for the uncoated filters, an ash mass load up to a certain level led to a decreased overall pressure drop during soot loading, whereas in the case of coated filters the ash deposit led to either an increased or decreased overall pressure drop, depending on the type of catalytic coating used (Figure.). Ash particle deposition may also influence the filter regeneration performance (Figure.). Finally, the ash particle accumulation caused substantial improvement in the filtration efficiency of the tested samples.... Ash Aging Simulation To predict the behavior of the ash-aged filter with respect to pressure drop, the DPF pressure drop model has been extended to account for the presence of ashes in the channels of the DPF [,, ]. Ash deposit growth dynamics was described with a mechanistic model that exhibits different ash-deposition profiles, namely deposition along the filter channel walls, and deposition at the end of the filter channel. The Pressure drop (mbar) Catalytic Technology for Soot Pollution Control j Catalyst P Ash-free Challenge mass load (gm - ) Ash-loaded, g/m oil-derived ash Figure. Effect of the ash loading on a coated (designated as Catalyst P based on Pt-group metals) filter sample [].

26 0j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Conversion (-) Uncoated, ash-free Uncoated, ash-loaded Catalyst P, ash-free Catalyst P, ash-loaded Temperature ( C) Figure. Effect of ash particles on the soot conversion rate for uncoated and coated filters []. results showed good quantitative agreement with experimental data, and the model could be used to describe the dynamic ash transport and deposition phenomena inside the DPF []. By considering two idealized modes of ash accumulation, namely: (i) ash only on the wall (forming a layer in series with the porous wall and the soot cake); and (ii) ash only at the end of the DPF channel (forming a plug that reduces the DPF length), it was possible to derive [] analytic approximations for the optimum cell density of wall-flow filters that minimize the DPF pressure drop, under the combined constraints of a prescribed filter volume, exhaust flow, temperature, as well as different soot and ash loadings inside the filter, thus facilitating the task of selection and the reliable deployment of DPFs over the vehicle life-cycle.. Catalytic Technology for Gaseous Pollution Control.. Reduction of Gaseous Emissions from Mobile Sources... The Three-Way Catalyst (TWC) The most frequent type of catalytic converter in automobiles is the so-called TWC for stoichiometric-operated gasoline engines, with a yearly worldwide production of over 0 million units. TWC systems have been applied to gasoline engines since the 0s, and contain either Pt/Rh or Pd/Rh in the mass ratio of approximately :, with a total loading of precious metals of about. g l. The TWCs simultaneously convert NO x, CO, and HC into N,CO, and H O[, ]. The catalytic components are supported

27 Color Fig.:.. Catalytic Technology for Gaseous Pollution Control j 0 0 Figure. Scheme of a monolith-based automotive catalytic converter. by a cordierite honeycomb monolith coated with high-surface-area alumina (see Figure.). This washcoat layer additionally contains thermal stabilizers (e.g., La O ), as well as the oxygen-storage component, ceria (CeO ). The ceria is able to release oxygen under rich conditions, thus maintaining HC and CO abatement and avoiding the emission of H S. The TWC process occurs exclusively within a narrow range of O content that is close to stoichiometric combustion conditions; that is, when the air coefficient l ranges from 0. to.0 (Figures. and.). To realize these conditions, an oxygen sensor is used which measures the air coefficient of the exhaust stream, forcing the engine management system to regulate the air fuel ratio. The TWC process involves a complex network of numerous elementary reactions [, ], where the effectiveness of the catalyst is closely related to the specific activity of the precious metals, as well as to their surface coverage. The transfer of TWC technology to lean-burn engines that is, lean-operated gasoline and diesel motors is problematic because of the insufficient NO x abatement. This is associated with the lower raw emissions of reducing agents, as well as the high content of O, which enhances the oxidation of HCs and CO thus suppresses NO x reduction. Therefore, alternative concepts are required for the reduction of NO x under leanburn conditions. For this purpose, SCR by NH and NSCs are considered in the automotive industry, as discussed below. The catalytic CO oxidation is an essential reaction of TWCs and NSCs, and has also been applied in diesel engines since the 0s, using so-called DOC. Furthermore, the catalytic abatement of CO is also a state-of-the-art technology for gas turbine engines fed by natural gas. DOCs usually contain Pt as the active component and reveal outstanding performance, although the highly expensive

28 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Figure. Typical engine raw emission of a gasoline operated engine. platinum can be substituted by the less active, but more inexpensive, palladium. The precious metal load of a DOC is approximately g l. DOCs are also capable of oxidizing gaseous HCs as well as HCs adsorbed onto soot particles (as discussed above). Figure. Dependence of the post-twc catalyst emissions on the air value in gasoline-operated automobiles.

29 Color Fig.:.. Catalytic Technology for Gaseous Pollution Control j 0 0 Over the past decade, a variety of computational tools has been developed to simulate the transient behavior of the TWC under operating conditions [ ], using sophisticated models for chemistry and mass and heat transport. These models today allow for the simulation of driving cycles [, ] taking spatially nonuniform inlet conditions into account [].... Selective Catalytic Reduction of NO x in Mobile Applications... Introduction Although, the SCR of NO x by NH has already been used in nonmobile applications for over twenty years, it has also been introduced as the technology of choice for NO x removal in lean-burn engines, and in particular diesel engines. Indeed, the SCR process covers the relevant temperature range of diesel engines to provide effective NO x abatement and has, in the past few years, also advanced to a state-of-the-art technology for heavy-duty vehicles. Unfortunately, in mobile applications the storage of NH is problematic, and consequently an aqueous solution of urea (. wt%) referred to as AdBlue is currently used. In this case, the urea solution is sprayed into the exhaust tailpipe, where ammonia is produced after thermolysis and hydrolysis of the vaporizing urea water droplets (Figure.). Much of the current research is focused on optimizing the dosing system, and the development of vanadia-free catalysts [0], although the use of alternative reducing agents, such as HCs and hydrogen, has also been discussed.... Pre-Catalyst Processes When the urea water solution (UWS) is sprayed into the hot exhaust stream [], the subsequent generation of NH proceeds in three steps [, ]: ) Evaporation of water from a fine spray of UWS droplets: ðnh Þ COðaqÞ!ðNH Þ COðsorÞþ:H OðgÞ; ) Thermolysis of urea into ammonia and isocyanic acid: ðnh Þ COðsorlÞ!ðNH ÞðgÞþHNCOðgÞ Figure. SCR-Denoxtronic-systems at Robert Bosch GmbH, using an urea solution as ammonia source. Illustration courtesy of Robert Bosch GmbH.

30 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 Table. Exhaust gas properties and spray parameters for urea dosing system using UWS in automotive applications []. Location ) Hydrolysis of isocyanic acid: HNCOðgÞþH OðgÞ!NH ðgþþco ðgþ: Value Exhaust Exhaust velocity (m s ) 0 Exhaust temperature (K) Wall temperature (K) 0 00 Spray Sauter mean diameter (mm) 0 0 Injection velocity (m s ) Injection temperature (K) 0 0 As the evaporation and spatial distribution of the reducing agent upstream of the catalyst are crucial factors for the conversion of NO x, the dosing system must ensure the correct preparation of the reducing agent, under all operating conditions. An overview of the different exhaust gas and spray characteristics that occur in passenger cars and trucks is provided in Table.. Appropriate spray properties of the urea solution will also avoid the deposition of urea on walls, which could lead to the formation of melamine complexes []. The chain of effects, from the injection point to catalyst entrance, which includes the injection of the UWS, the evaporation and thermolysis and hydrolysis of the UWS droplets, the impingements of droplets on the tailpipe wall, and the potential formation of films, is shown schematically in Figure.. The films may evaporate, Figure. Chain of effects from the injection point to catalyst entrance.

31 Color Fig.:.. Catalytic Technology for Gaseous Pollution Control j 0 0 but may also lead to the solid depositions mentioned above. Meanwhile, CFD simulations can be used to comprehend the interaction of spray injection and the turbulent flow of the exhaust gas in the tailpipe. The numerically predicted thickness of the wall film, and its composition, is shown in Figure.. As urea and water have different boiling points, the urea concentration in the droplets will vary with time. As the evaporation time and, consequently, the droplet composition also depends on the initial droplet size and the interaction with the hot exhaust gas stream, the impinging droplets will vary not only in size but also in urea concentration. Meanwhile, CFD Figure. CFD simulation of interactions of flow, spray, and wall describes wall film formation; wall film thickness (a) and urea fraction in film (b) [].

32 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 simulations have also been used for the rapid prototyping of SCR-systems. In particular, the position and angle of the injection nozzle, and the position of the catalyst in relation to the tailpipe, must be selected so as to produce an optimum distribution of the reducing agent at the front face of the catalyst.... Catalytic Conversion of NO x by NH [] As the ammonia is formed, it becomes available as a reducing agent for NO x, mainly NO and NO, which is carried out in a structured catalyst integrated into the tailpipe; this is very similar to the TWC approach discussed above. At higher temperatures (>0 C), however, the ammonia reacts with oxygen in an undesirable parallel reaction to produce N,N O, or NO. In contrast, at temperatures below 00 C, ammonia and NO x may form solid deposits of ammonium nitrate and nitrite. NO x can be reduced continuously by NH on a SCR catalyst, resulting in the selective formation of nitrogen and water. At this point, it should be mentioned that the SCR procedure is the only technique converting NO x selectively into N, even under strongly oxidizing conditions. Hence, SCR was considered the technology of choice when deno x became an issue for lean-burn engines. In fact, the SCR process covers the relevant temperature range of diesel engines, providing effective NO x abatement, to a point where during the past few years it has advanced to become state-of-the-art deno x technology for heavy-duty vehicles. Whilst the operational range of the SCR procedure is limited at low temperatures (<00 C) by the kinetics of the catalyst, above approximately 0 C it is slightly restricted by the oxidation of NH. The stoichiometry of the main desired reactions can be described as follows [ ]: NOþ NH þ O! N þ H O NO þ NH! N þ H O NO þ NO þ NH! N þ H O ðstandard SCRÞ ðno SCRÞ ðfast SCRÞ The standard SCR reaction is most important if NO x originates from hightemperature combustion processes, where very little NO is present. However, in exhaust streams containing larger amounts of NO x, the fast SCR reaction proceeding with at least a -fold higher rate than the standard SCR reaction may become the predominant reaction []. DOC systems integrated into the exhaust-gas aftertreatment systems (see Figure.) convert NO to NO, thus changing the NO /NO x ratio at the entrance of the SCR catalyst drastically and leading to major increase in SCR, particularly below 0 C. However, when designing this precatalyst it must be remembered that molar ratios of NO /NO x > 0. can lead to NH NO deposits, thus deactivating the SCR catalyst. As the hydrolysis of urea may not be fully completed at the entrance of the SCR catalyst, the front section of the SCR structure is composed of a urea hydrolysis catalyst, such as alumina or titania. The subsequent part, which forms the real SCR catalyst, is commonly extended by an NH oxidation catalyst to avoid any slip of NH.

33 . Catalytic Technology for Gaseous Pollution Control j 0 0 Figure. Mechanism of the SCR reaction on V O /TiO catalysts []. The most common SCR catalyst used today is a TiO -supported WO /V O ; this is normally used in the form of an homogeneous monolith, although in a few applications charcoal catalysts may also be used. The mechanism of the SCR reaction on a V O catalyst was elucidated both experimentally [ ] and by using quantum-mechanical calculations []. The reaction follows an Eley Ridealtype mechanism, where two different active sites are involved (Figure.) that are in such close proximity that they represent a Brønsted acid site (V þ OH) and a(v þ ¼O) redox site. In the first step, NH is adsorbed onto the Brønsted site to produce NH þ ; this subsequently interacts with the neighboring redox site, leading to a reduction of the latter. The gaseous or weakly adsorbed NO then reacts with the activated N species to form N and H O. In the final stage, the (V þ OH) group is reoxidized into (V þ ¼O), again resulting in the production of H O.... Alternate Catalysts [] Currently, a major trend in automotive SCR is the substitution of V O catalysts by harmless materials. In the meantime, catalytic converters based on V O have been prohibited in Japan and California, on the basis of the toxicity of the active component; similar discussions regarding this point are currently ongoing in the European Union. An additional problem is that V O demonstrates a limited high-temperature stability, which may cause difficulties when coupling SCR with particulate filter systems in passenger cars. At present, Fe-ZSM zeolite is considered to be the most-favored type of catalyst as an efficient substitute for the classical V O patterns [0, 0 ].... Alternative Reducing Agents Alternative reducing agents have been investigated extensively over the past two decades, mainly hydrocarbons (HC-SCR) and hydrogen (H -SCR). In the case of HCs, additional fuel may be injected in the raw exhaust or the exhaust line upstream of the SCR catalyst. Unfortunately, difficulties in achieving a high conversion and selectivity over the catalysts [], the catalyst stability at high temperature, and problems in meeting HC limits have so far prevented HC-SCR from becoming widespread in terms of its applications.

34 j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 H -SCR, which is currently still undergoing fundamental development, is of particular interest for low-temperature exhaust gases []. The conversion of NO x by H already operates at stoichiometric (TWC) and rich (NSC in regeneration phase) conditions. However, low-temperature H -SCR has also been reported for strongly oxidizing conditions using Pt catalysts. Although a very good performance is obtained below 0 C, the narrow range in activity and the high selectivity towards N O remain challenging issues. The mechanism of the NO reaction by H on Pt/ Al O, under lean conditions, implies the dissociative adsorption of NO on reduced Pt sites []. The recombination of two N atoms leads to the evolution of N, while the oxygen is retained on the Pt surface. The production of N O is explained by the combination of a surface N atom with NO adsorbed onto neighboring Pt sites. Finally, the effect of the hydrogen is to regenerate the active Pt sites. A potential source for the onboard production of H is the processing of fuel by catalytic partial oxidation or steam reforming [ ]. Additionally, the temporary generation of H might also occur by engine management; that is, after the injection of fuel.... NO x Storage Catalysts NO x storage reduction catalysts were originally developed for lean SI engines, and are currently being transferred for use in diesel-powered passenger cars. The NSC procedure is based upon the periodic adsorption and reduction of NO x [, ], the principle of which is illustrated in Figure.. The catalysts consist of Pt, Pd, and Rh in the mass ratio of approximately : :, with a total precious metal load of approximately g l. The NSC contains basic adsorbents such as Al O (0 g l ), CeO ( g l ) and BaCO ( g l, denoted as Figure. Reduction of NO x emissions of lean operated engines. Principles of a NO x storage/ reduction catalyst (NSC).

35 . Catalytic Technology for Gaseous Pollution Control j 0 0 BaO equivalent) []. In the lean phase of the engine (general operation mode), the NO x of the exhaust is adsorbed onto the basic components of the NSC, mainly on the barium carbonate, to form a nitrate. When the storage capacity is reached, the engine is operated at rich conditions (l 0.) for a few seconds, and this leads to an exhaust containing CO, HCs, and H as reducing agents for catalyst regeneration (backtransformation of the nitrate to the carbonate): Storage (lean) phase:. NO oxidation over noble metal: NO þ O! NO j. NO x storage on Ba sites: BaCO þ NO þ O >BaðNO Þ þ CO. Regeneration (rich) phase: BaðNO Þ þ CO=H =HC>BaCO þ NO þ CO =H O Obviously, this global scheme is an oversimplification, and many research investigations have been devoted to elucidating the intrinsic kinetics [ ]. The effect of the Ba component is to adsorb NO x at temperatures above 0 C, whereas substantial storage is also provided by Al O and CeO at lower temperatures []. However, below 0 C the effectiveness of NSC catalysts is limited by the kinetics of the NO production on the Pt component, while above 00 C the thermal stability of the NO x surface species represents the limiting factor. For the NO adsorption on the Ba sites, two parallel pathways of nitrate formation are suggested []. The first route involves the adsorption of NO on Ba to form nitrites, which are subsequently oxidized by gas phase O into nitrates. The second route includes the catalytic NO oxidation on Pt into NO, followed by its immediate adsorption in the form of nitrates. It has also been suggested that barium peroxide species might serve as the crucial sites for nitrate formation []. As compared to SCR, the most important advantage of the NSC technique is the fact that no additional liquid tank (Urea-SCR), additional injection management/ system (HC-SCR), or chemical reactor (H -SCR) is needed. However, a substantial constraint is the susceptibility to sulfur poisoning. In parallel to NO, SO is also oxidized on Pt, followed by the adsorption of SO onto the basic substrate. The sulfate species produced lead to a drastic deactivation of the NO x storage sites. Although, the poisoned sites of ceria and alumina can be regenerated thermally above 0 C, the released SO x is readsorbed selectively onto the Ba species. The regeneration of poisoned Ba sites is inadequate, even under rich conditions and high temperatures, as the sulfate groups are partially converted into highly stable BaS []. In recent years, several groups have coupled detailed kinetic models and CFD to provide a numerical simulation of NO x storage catalysts under varying conditions and using different modeling approaches [, 0 ]. These approaches have

36 00j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 differed especially in the applied reaction mechanisms, and the treatment of mass transfer in the single channels and the washcoat. Furthermore, most studies have considered NO oxidation only in the lean phase, and its reduction in the rich phase. The applied gas matrix is sometimes far from a realistic exhaust gas, because it contains neither water nor CO, both of which have a major influence on the catalytic activity of the noble metal and the morphology of barium. As the storage of nitrogen oxides is a slow process compared to the subsequent reduction, the observed behavior was explained by the different molar volumes of BaCO and Ba(NO ), the so-called shrinking core model [, 0]. State-of-the art simulations have included detailed reaction mechanisms, radial mass transfer limitations in the catalytic channels and the washcoat, and have been capable of describing the transient behavior of the NSC at varying inlet conditions. As an example, Figure. shows the numerical predicted and experimentally measured NO and NO profiles in a laboratory flat-bed reactor, using a commercially manufactured Pt/Ba/Al O model NSC as function of axial position and storage time. The effect of NO formation in the first section of the catalyst, and its subsequent storage, is clearly captured. The model applied a detailed reaction scheme for the processes on the noble metal Pt [], and a lumped scheme coupled with the shrinking core model for the storage component Ba... Reduction of Gaseous Emissions From Stationary Sources... Catalytic Technologies for NOx Removal [] Nitrogen oxides arise from the oxidation of nitrogen-containing compounds of the fuel (fuel NO x ), the oxidation of atmospheric nitrogen from combustion with air (thermal NO x ), and by the oxidation of intermediate combustion species (prompt NO x ). Often, a combination of combustion modifications and catalytic gas cleaning is used, for example, low-no x burners, and SCR. In addition, a selective noncatalytic reduction (SNCR) step can be applied by injecting ammonia into the furnace. Primary NO x formation can substantially improved by oxy-fuel combustion that is, combustion with pure oxygen or enriched air. Interestingly, the potent greenhouse gas nitrous oxide (N O) cannot be removed by the normal SCR process. This emerges as a particular in nitric acid production plants, where the threat of environmental harm is inevitable, unless innovative end-of-pipe N O removal technologies can be developed to effect the reduction of the N O produced as waste. The current approach to this problem has focused on the use of transition-metal, ion-exchanged zeolites for the decomposition of N O. SCR with ammonia is by far the most relevant technology for the catalytic removal of NO x from stationary sources, and has been implemented since the 0, notably to deal with NO x produced not only by power plants but also by industrial boilers and gas turbines. It should be noted that SCR with ammonia would utilize a similar approach for both mobile and stationary applications, except that the size of the structured catalyst would be much larger in the latter case.

37 Color Fig.:.. Catalytic Technology for Gaseous Pollution Control j0 0 0 Figure. Axial profile of NO (a) and NO (b) concentration at varying storage time in the lean phase of Pt/Ba/Al O catalyst at 0 C [, ]. The selective catalytic reduction of NO x was first carried out using Pt catalysts although, due to the high N O selectivity of this catalyst, base metal catalysts have subsequently been developed for NO x reduction. Vanadia supported on titania (in the anatase form) and promoted with tungsten or molybdenum oxide exhibits the best catalytic properties. Although BASF were the first to describe vanadia as an active component for SCR, a TiO -supported vanadia for the treatment of exhaust gases was

38 0j Catalytic Technology for Soot and Gaseous Pollution Control 0 0 also developed in Japan. Anatase is the preferred support for SCR catalysts for two main reasons. First, it is only moderately sulfated under real exhaust gas conditions; in fact, its catalytic activity even increases after sulfation []. Second, vanadia can be spread in thin layers on the anatase support, which in turn leads to highly active structures with large surface areas. Unfortunately, the amount of vanadia in a technical catalyst is limited to only a few weight%, because it is also catalytically active for SO oxidation. The mechanism of the standard SCR reaction over vanadia-based catalysts is generally assumed to proceed via an Eley Rideal mechanism that involves adsorbed ammonia and gas-phase NO (as described above). As the rate of the SCR reaction under industrially relevant conditions is quite high, external and intraparticle diffusion resistances play an important role, especially for the frequently used honeycomb monolith or plate-type catalyst geometry operating in a laminar flow regime. These geometries must be used to minimize the pressure drop over the catalyst bed. Monolithic elements usually have channel sized of mm, a crosssection of cm, and lengths of 0 0 cm. Monoliths or packages of plate catalysts are assembled into standard modules which are then placed in the SCR reactors as layers. Notably, these modules can be easily replaced to introduce fresh or regenerated catalysts. SCR reactors can be used in different configurations, depending on the fuel type, the flue gas composition, the NO x threshold, and other factors. The first possibility is a location directly after the boiler (a high-dust arrangement), where the flue gas usually has the optimal temperature for the catalytic reaction. However, dust deposition and erosion, as well as catalyst deactivation, will be more pronounced than in other configurations. A second option, which is common in Japan, is to place the SCR reactor after a high-temperature electrostatic precipitator for dust removal (low-dust arrangement). In this case, although damage of the catalyst by dust can be prevented, the deposition of ammonium sulfate (which in the high-dust configuration mainly occurs on the PM in the gas stream) may become more critical. It is for this reason that especially low limits for ammonia slip must to be met. Finally, the SCR reactor may be located in the cold part after the flue gas desulfurization unit, in the so-called tail-end arrangement. In this case, in order to achieve the required reaction temperature the exhaust gases must be reheated by means of a regenerative heat exchanger and an additional burner. A major benefit here, however, is that catalysts with very high activities can be used, as no poisons will be present and SO oxidation need not be considered. New promising catalysts for the removal of NO x include iron-exchanged zeolites, such as MFI and BEA. Although field tests in the flue gases of power plants have shown a quite strong deactivation, notably by mercury [], these catalysts appear to be especially suited for clean exhaust gases, such as in nitric acid plants. The main advantages of iron zeolite catalysts include a broader temperature window for operation, and the ability also to reduce N O emissions. Uhde GmbH has recently developed the EnviNOx Ò process for the simultaneous reduction of NO x and N O, which uses iron zeolite catalysts provided by S ud- Chemie AG.

39 . Outlook j Technologies for Removal of Other Emissions Catalytic technologies may play a minor role in the removal of other gaseous emissions from stationary sources; however, for the sake of brevity, the reader should consult comprehensive reviews produced by Gabrielsson and Pedersen [] and by Spivey []. Catalytic combustion may be applied to remove VOCs [], and also to reduce the formation of gaseous pollutants, in particular NO x [, ]. Further details on these topics are available in the reviews of Forzatti et al. [] and Hayes and Kolaczkowski [].. Outlook The currents trends in the implementation of exhaust-gas after-treatment systems for cars and trucks are determined by an increasing complexity. The system often includes several of the components described above. As an example, Figure. illustrates the recently developed system that includes DOC, NSC, DPF, and SCR technology. Whilst the management of this chemical plant underneath a car represents a major challenge, the even lower legislative emission limits planned for the future may require even greater complexity of designs and operation strategies. Yet, on the other hand, integrated systems are becoming increasingly attractive, with Kolios et al. having recently proposed a heat-integrated reactor concept for catalytic reforming and automotive exhaust purification []. Figure. Exhaust-gas treatment of the E BLUETEC. Illustration courtesy of Daimler AG.