Catalytic honeycomb monoliths

Fig. 17.18 Illustration of a catalytic honeycomb monolith and an enlarged view of the singlechannel model problem. In the model problem, the channel diameter (d = 2 mm) is much smaller than the channel length (L = 100 mm). For clarity, the illustration shows a large channel diameter.

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

One of the potentially wide-spread applications under development is catalytic filters for air pollution control. This combines separation and catalytic oxidation into one unit operation. One possibility is the oxidation of volatile organic carbon (VOC) by employing a porous honeycomb monolithic ceramic membrane filter. Inside the pores are deposited an oxidation catalyst such as precious metals. The resulting VOC removal efficiency can exceed 99% [Bishop et al., 1994]. 

Membranes that arc catalytically active or impregnated with catalyst do not suffer from any potential catalyst loss or attrition as much as other membrane reactor configurations. This and the above advantage have the implication that the former requires a lower catalyst concentration per unit volume than the latter. It should be mentioned that the catalyst concentration per unit volume can be further increased by selecting a high "packing density" (surface area per unit volume) membrane element such as a honeycomb monolith or hollow fiber shape. 

The substrate is an integral part of the catalytic converter system. Its primary function is to bring the active catalyst into maximum effective exposure with the exhaust gases. In addition, it must withstand a variety of severe operating conditions, namely, rapid changes in temperature, gas pulsations from the engine, chassis vibrations, and road shocks. As noted earlier, pellets of cylindrical and spherical geometry and honeycomb monoliths both became available for catalyst supports. 

Selective catalytic reduction honeycomb monoliths (typically characterized by a square channel section) are obtained by extruding a mass of pastelike catalytic material. The ele-... 

Catalytic Fuel Combustion in Honeycomb Monolith Reactors... 

IV. PRACTICAL APPLICATIONS OF HONEYCOMB MONOLITHS IN CATALYTIC COMBUSTORS... 

In quite a different application, a novel approach for producing olefins via a hydrocarbon-steam cracking process, without the use of a catalyst, was demonstrated to benefit from the use of a honeycomb monolithic catalytic reactor [28]. A typical problem associated with cracking processes of this type is maintaining the appropriate combination of heat transfer and residence time, which, if not balanced, will lead to either poor conversion... 

A new avenue under study is the use of honeycomb monoliths and granular catalytic media both as reformers and particulate filters. Experimental work is underway in Sweden and Canada respectively. 

Greene [90] investigated the oxidative destruction of methylene chloride by zeolite Y in the H form and ion exchanged with cerium and chromium. The zeolite was supported on a low surface area cordierite honeycomb monolith by application as a wash coat. Below 425°C the catalytic activity decreased in the order ... 

SYNTHESIS AND STUDY OF HONEYCOMB MONOLITHIC CATALYSTS FOR CATALYTIC COMBUSTION... 

Catalytic afterburning (off-gas purification) Pt/Pd LaCeCoO (perovsldte) oxides of y W, Cu, Mn, Ife supported catalyst (honeycomb monolith or catalyst bed) or bulk catalyst 150-400 °C 200-700 "C... 

Honeycomb monoliths (Fig. 6.1) are structured catalyst supports consisting of parallel straight capillary channels. Nowadays, they are widely used for the catalytic exhaust converter in the automobile industry and end-of-pipe gas cleaning. The gas-only application of monoliths stems from the fact that the pressure drop is low using the surface area of the catalyst as a criterion, the pressure drop in a monolith is an order of magnitude lower than in randomly packed beds. The channels are about a millimeter in diameter, and on the wall (-100 pm) a wash-coat of catalytic material (-50 pm) is applied. 

This work focuses on the kinetic study of the methane catalytic combustion in a honeycomb monolith wash-coated with Pd/y-AhOa (homemade). The experimental conditions were chosen to adequately represent the operation of a domestic-scale catalytic heater, i.e. relatively high volumetric flowrates and high methane molar fractions (Lopez et al., 2000). From experimental data (obtained at conditions of negligible mass transfer resistances) and using a mathematical model for the laboratory reactor, the intrinsic kinetic parameters are calculated for a power law type rate expression. 

Honeycomb monoliths are obtained by extruding a paste made by catalytic material, whereas plate catalysts are made by depositing the catalytic material onto a stainless steel net or a perforated metal plate. Composite ceramic monolith catalysts, consisting of a monolith matrix made of cordierite coated with metal oxide SCR material, are also offered. However, they may suffer adhesion problems in the presence of dust and their use may be preferably limited to clean environment. Coated metal monoliths are constructed of thin metal foil and are characterized by large cell densities. In view of this, they are used exclusively in dust-free applications. 

Some of the most active catalysts used in the Selective Catalytic Reduction (SCR) processes to remove nitrogen oxides (NOx) from exhaust gas streams are those based on vanadia supported on titania [1]. In order to avoid the problems associated with pressure drop and diffusional limitations, encountered with conventional peletted catalysts in forms of cylinders and spheres etc., the supports should ideally be configured as honeycomb monoliths for these reactions which normally take place with high space velocities due to the large volumes of gas to be treated [2]. However, the difficulties encountered in the preparation of monoliths based solely on titania makes the inclusion of binders to both improve the rheological properties of the paste prior to extruding and the soundness of the monolith with subsequent thermal treatment a necessity [3]. 

Diehm C Catalytic reforming offuels over noble metal-coated honeycomb monoliths capillary-based in-situ sampling technique, Dissertation (Ph.D. Thesis), 2013, Faculty of Chemistry and Biosciences, Karlsruhe Institute for Technology. 

Monolith reactor model—For validation purposes, the kinetic models of the SCR catalyst [6] and of the PGM catalyst [18] were used to simulate catalytic activity runs over honeycomb monoliths coated with the SCR and the PGM component, respectively, of the studied ASC system. In the case of the SCR catalyst, the kinetics were implemented in a heterogeneous dynamic ID -I- ID model of a single monolith channel, accounting both for external (gas-solid) and internal (intra-porous) mass transfer resistances [12, 25, 26]. Model simulations... 

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