Catalytic soot oxidation

CATALYTIC SOOT OXIDATION 2.1. The contact between soot and catalyst... 

Since it is difiScuh to obtain diesd soot with constant prop es (the composition dq> ids on the engine load) a model soot was applied (Printex-U, a flame soot kindly provided by Degussa). This soot has a N2-BET surface area of 96 mV and contains approximately 5 wt% of adsorbed hydrocarbons and 0 2-0.4 wt% sulfur. Catalytic soot oxidation temperatures were d ermined in a thermobalance (STA 1500H). About 4 mg catalyst, 2 mg soot and 54 mg SiC were applied as a sample. A heating rate of 10 K/min and a flow rate of 50 ml/min 21 vol% O2 in N2 wa"e used. The maximum of the DSC curve was defined as the oxidation temperature. Samples refored to as tight contact were intensively milled in a ball mill for one hour, before dilution with SiC and thermal analysis, whereas loose contact was established by simply mixing of the catalyst and soot with a spatula. 

Soot oxidation profiles as obtained fi r K2M0O4 in tight contact and loose contact are shown in figure 1. The maximum of the exothomic heat effect is located at 685 K for the ball milled sample ( tight contact ) and 790 K for the spatula mixture ( loose contact ). Apparently the milling procure lowers the catalytic soot oxidation temperature by approximately 100 K, and is essential for a high soot oxidation activity. Non-catalytic soot oxidation occurs at 875 K. Neeft et aL [14]... 

Figure 2. Correlation between the mdting point of metal ddorides and the corresponding soot oxidation temperature. Solid dot well defined melting point. Open dot deccanpoation or oxidation takes place before melting. The horizontal solid line indicates the non-catalytic soot oxidation temperature.

As FeOCl was not carbothermally reduced, it is suggested that FeOCl activates and transfers ojqrgen, just like CuCl. Interestingly, the application of FeOCl leads to the firrmation of surfiice oxygen complexes, whereas catalytic soot oxidation by Fe203 does not [19]. This is another indication that chlorine diemically affects the catalytic soot oxidation activity of metal oxides and that the previous scheme also holds for FeOCl. 

DPFs to increase the oxidation rate, prevent uncontrollable soot combustion, and decrease instrumental and energetic loads for the regeneration process. There have been excellent reviews concerning research on catalytic soot oxidation since 1980, when R D of DPFs started. Therefore, after a brief explanation of the commonly used methods for soot oxidation catalyst evaluation, we will classify the recently investigated catalysts into several types, mostly those introduced since 2000, and give detailed explanations of their reaction mechanisms, materials, and situations for practical application. 

Method for Evaluation of Catalytic Soot Oxidation Activity... 

For a= 1, soot in the catalytic layer is oxidized fast leaving the soot in the thermal layer unreacted. This has been observed with some early catalytic filters. As a decreases the soot from the top layer replaces more rapidly the soot oxidized in the catalytic layer increasing the global oxidation rate. The corresponding soot layer thickness evolution is shown in Fig. 22. For values of a close to 1 (e.g. 0.9) the catalytic layer is totally depleted from soot at some instances, followed by sudden penetration events from the soot of the thermal layer. These events are clearly shown in the thickness evolution for oc = 0.9 in... 

Fig. 22. For smaller values of a, equilibrium between the soot entering the catalytic layer and the soot oxidized in it is established leading to a constant global oxidation rate.

The two-layer model for soot oxidation with N02 in a catalytic filter can be written as follows ... 

Based on the N02-soot oxidation data promoted by the catalytic coating tested in Konstandopoulos et al. (2000) an R value of about 3.4 has been computed. This represents a significant enhancement. 

Konstandopoulos, A. G., and Kostoglou, M. A mathematical model of soot oxidation on catalytically coated ceramic filters . Advances in Vehicle Control and Safety (AVCS 98), Amiens, France, July 1-3, 1998. 

In the presence of a catalyst and with a finite selectivity for CO production, soot oxidation is described with the following global reactions, one for the thermal and one for the catalytic path (i.e. oxidation of soot by oxygen transferred from the catalyst by a redox and/or spill-over mechanism). Konstandopoulos and Kostoglou (1999b, 2000) ... 

It is fair to state that by and large the most important application of structured reactors is in environmental catalysis. The major applications are in automotive emission reduction. For diesel exhaust gases a complication is that it is overall oxidizing and contains soot. The three-way catalyst does not work under the conditions of the diesel exhaust gas. The cleaning of exhaust gas from stationary sources is also done in structured catalytic reactors. Important areas are reduction of NOv from power plants and the oxidation of volatile organic compounds (VOCs). Structured reactors also suggest themselves in synthesis gas production, for instance, in catalytic partial oxidation (CPO) of methane. 

More recent investigations have been conducted into NOx-assisted soot oxidation [55, 56]. NO2 was suggested as an oxidation agent to assist the combustion of particulate matter in the presence of oxygen in the exhaust gas [57]. NO2 can be produced by catalytic oxidation of NO prior to the catalytic filter. 

The use of CeOs-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO. O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology. 

The gas-solid reaction involved in non-catalytic soot combustion is a relatively slow process.76,84 Catalytic assistance (gas-solid-solid process) provides an increase in the rate of soot oxidation, although the process efficiency is mainly determined by the type of contact (tight or loose) established between the soot and the catalyst.76,78,84,85 As mentioned... 

Nevertheless, the use of a supported noble metal catalyst (usually Pt) for soot oxidation under loose contact conditions (proposed to be closer to the practical condition) results in a significant decrease in the soot oxidation temperature.76,94,97,98 Thus, incorporation of the soot in a Pt/SiC foam catalyst allows the soot oxidation rate to be doubled (and also to decrease the maximum rate temperature) with respect to a non-catalysed situation in which the soot is incorporated into the Pt-free SiC foam (with Pt/SiC foam located upstream to promote NO oxidation). In turn, a considerable decrease in the maximum rate temperature is observed when employing NO + O2 instead of O2 as oxidant in the Pt/SiC-soot configuration.98 On the basis mainly of these results, a catalytic role for NO is proposed in a recycle reaction as follows ... 

The removal of soot from diesel exhaust gas is preferably done catalytically. Fuel additives and supported molten salts are promising catalyst for this application. NO in the exhaust gas can be used to increase the soot oxidation rate. 

The NOx that is present in diesel exhaust gas, can be an advantage in the oxidation of soot. Several studies report an important increase of catalytic soot combustion by addition of NO to the gas phase [2, 7], Probably an oxidation cycle with NO as an intermediate for oxygen transfer plays an important role. This cycle can be catalysed at one or more stages. The oxidation of NO to NO2 can be catalysed, but also the subsequent oxidation of soot by NO2 can be catalysed. This observation is confirmed by very recent experiments with bi-metallic fuel additives [8]. 

The NO is probably enhancing the oxidation of soot by an oxidation cycle. This cycle is discussed by Mul [2] and Hawker [6]. Mul proposed a cycle as shown in figure 4. The NO is oxidised catalytically into NO2, whereas the soot is oxidised by NO2 uncatalysed. The oxidation by NO2 can also be catalysed. This could be a possible application of two different catalytic systems. One metal then catalyses the oxidation of NO to NO2 and the other metal catalyses the soot oxidation. Recent work with fuel additives supports the existence of such a mechanism [7]. 

No catalyst was involved in this study. Hillenbrand and Trayser [Ref. 8] took soot collected from an engine, mixed it with metal salts (Cu, Na, Co, and Mn), and burned it in a laboratory reactor. A substantial lowering of the combustion temperature was observed with the use of such salts. McCabe and Sinkevitch [Ref. 9] also looked at mixing base metal additives either with the soot or the fuel and then determined the effect on soot combustion temperature. Finally, Goldenberg, et al. [Ref. 10] looked at soot oxidation either alone or on a catalytic material. 

Of the different catalytic coatings tested sample No. 3 did not show any evidence of soot oxidation. Incipient soot oxidation was evident on samples No. 4, 5 (a less steep pressure drop trace increase than sample No. 3). Samples No. 6, 7 (foam) exhibited clearly soot oxidation as their pressure drop trace was decreasing with temperature. All pressure drop decreases with the catalytic filters have been observed above 650°C under oxygen deficient conditions and therefore it can be stated that the soot is oxidized catalytically by other species than oxygen in these experiments. The best catalyst formulation appears to be the combination of reducible oxide/alkali metal and precious metal. 

A number of metal oxides was screened upon catalytic activity for soot oxidation by means of TGA/DSC. Several metal oxides appeared to be active soot oxidation catalysts. Contact between catalyst and soot was found to play a major role in this solid-solid-gas reaction varying this contact, activities for several catalysts ranged from active to hardly any activity. It is fUrther tentatively suggested that contact of soot, deposited on catalytic coated particulate filters, is poor, which has major implications for the development of soot oxidation catalysts under diesel operation conditions. 

In literature, not always sufficient attention is paid to these parameters. In catalytic carbon and grapliite studies, as reviewed by McKee [3], experimental conditions are usually tliorouglily controlled and described. In soot oxidation shidies for diesel aftertreatment purposes (e.g. [1,4-7]), experimental conditions are often less carefiilly reported. As also tlie intrinsic parameters in tliese two types of studies differ (low oxygen and catalyst concentrations in tlie former type of experiments versus liigh... 

Tlie objective of tliis study is to investigate tlie influence of above mentioned intrinsic parameters of catalyst and soot on tlie catalytic combustion of soot. In order to be able to study these parameters, first the activity of different soot oxidation catalysts has to be defined properly. The screening of catalyst materials, as presented in this paper, aims to fulfil tliis need. 

A number of ways was employed to make contact between soot and catalyst. It was shown tliat the contact between soot and catalyst is of prime importance for tlie reactivity of the catalyzed soot oxidation. Several catalysts were found to be unable enhancing the soot oxidation rate when the contact between soot and catalyst is poor. This explains why catalyst coated particulate filters have been found quite inactive under practical conditions. For catalytic systems wliich seem to be able to decrease soot combustion temperatures a possible explanation can be probably found in catalyst mobility. 

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