The application of monoliths for gas phase catalytic reactions
Introduction
Monolithic supports are uni-body structures composed of interconnected repeating cells or channels. They are most commonly composed of ceramic or metal materials but some can also be made of plastic. The most important physical characteristics when used as a catalyst support is the size of the channel through which the gaseous reactants and products traverse. The catalyst is composed of a high surface area inorganic oxide carrier, i.e., γ-Al2O3, upon which catalytic metals or metal oxides are dispersed. Every catalyst company has their own proprietary technology for catalyzing the walls of the monolith but a typical method is to impregnate the Al2O3 (or other suitable carrier) with salts of the catalytic components. The catalyzed carrier is then milled to a particle size less than about 10 microns in an aqueous media sometimes with a small amount of acid. The slurry is usually about 30–40 wt.% solids but is adjusted to obtain the proper loading. The ceramic monolith is then dipped into the slurry and the wet gain recorded. After drying at 110°C a dry weight gain is obtained. If acceptable the catalyzed monolith is calcined to about 400–500°C in air in order to decompose the salts and to insure good bonding between the carrier and the monolith. For metal monoliths it may be necessary to use a pre-coat of an inorganic oxide to insure good bonding between the carrier and the metal monolith. When deposited on the walls it is referred to as the catalyst washcoat. The exact procedures vary with the type of monolith and the specific application.
Reactants enter each of the channels, interact with the catalyst on the walls and the resulting products continue down the channel and exit. A cartoon of the monolith channel coated with a catalyst is shown in Fig. 1.
The number of channels, their diameters and wall thickness determine the cell density, expressed as cells per square inch (cpsi), which in turn allows the calculation of the geometric surface area; the sum of the areas of all the channel walls upon which the catalyst is deposited. This leads to one of the most important advantages of the monolith in that it has a large open frontal area resulting in very little resistance to flow and hence low pressure drop. The lower the pressure drop the lower the resistance to flow or back pressure on the system and hence lower the energy loss.
Metal monoliths can be made with even thinner walls, with open frontal areas approaching 90% resulting in larger channel diameters and offering even lower pressure drop than ceramics at comparable or greater geometric areas. Factors such as cost, weight, maximum temperature capability, heat management, etc. dictate which material is used in a specific application. Fig. 2 shows some typical ceramic and metal monoliths.
The first large-scale use of the ceramic monolith came in mid-1970s when the catalytic converter was installed on new vehicles in US (1). The material of choice was an extruded multi-cell ceramic called cordierite (2MgO 2Al2O3 5SiO2) with low thermal expansion with the resulting property of high resistance to fracture due to thermal shock. To implement this structure there was a need to develop an entirely new technology for depositing the catalytic component but the advantages greatly outweighed the cost associated with developing the new way of manufacturing catalysts. The presence of the catalyst in the exhaust of the automobile in the form of an open structured monolith (or honeycomb) offers less resistance to flow decreasing power loss compared to a typical bed of particulate bead catalysts. The open structure allows their use in high dust environments, such as coal-fired power plants and diesel exhausts, without concern for plugging. For extreme operating conditions where there is a heavy accumulation of dust the monolith allows ease of cleaning by air lancing or chemical washing.
They offered high geometric surface areas with a lighter and more compact reactor than beads. Lighter weight allows more rapid warm up of the catalyst favoring conversion of pollutants in a shorter period of time once the engine is started. High geometric surface area favors high conversion of pollutants when the rate is controlled by bulk mass transfer a condition which exists for most operating conditions of the warmed up automobile. Because of their uni-body structure they are more resistant to mechanical vibrations and attrition experienced in normal driving than a packed bed of particulate beads. Their structure allows greater freedom of orientation in the exhaust.
By roughly around 1980 and still today essentially all automobile manufacturers design their catalytic converters using ceramic monoliths. There are, however, niche vehicular markets where metal monoliths are preferred over ceramics such as in heavy-duty trucks and in some high performance vehicles where monoliths with open frontal areas of 90% reduce the pressure drop close to zero. Today, the monolith is the support of choice for almost all environmental applications where high flow rates and low pressure are required with all the other benefits as indicated above. Monoliths are now taken on different forms tailored for specific applications. Metal heat exchangers or radiators are coated with a catalyst and function to convert dilute pollutant containing gas streams at high flow rates with minimal pressure drop. Ceramic and metal monoliths can be constructed with protrusions in the channel to enhance turbulence and hence increase mass transfer to the walls of the channel upon which the catalyst is deposited.
Certainly the successful application of the monolith as a support for the catalytic converter in the automobile gave great confidence to other industries to design their pollution abatement systems with them. Section 5 gives a brief summary of these with appropriate references for additional information. Section 6 highlights hydrocarbon fuel processing for the fuel cell as the next major application of monolithic structures.
Although, the advantages of the monolith are many there are some disadvantages that prevent extensive use outside the environmental applications. The parallel channel monolith is essentially an adiabatic reactor limiting the control of temperature. For many exothermic or endothermic chemical and petroleum reactions selectivity is governed by temperature and therefore, these types of monoliths are not well suited. One can employ a metal heat exchanger or metallic foam to control temperature but the amount of catalyst on the walls in a given volume of monolith is much less than a comparable volume of small diameter beads or extrudates. Therefore, for chemical controlled reactions the monolith may not contain sufficient catalyst to yield the desired conversion efficiencies.
Section snippets
Chemical and physical properties of monoliths
Monolith materials as supports for catalysts in the automotive exhaust catalytic converter were required to meet very severe operating conditions of temperatures over 1000°C with resistance to thermal shock. Ceramic materials such as cordierite, 2MgO–Al2O3–5SiO2 (14% MgO, 35% Al2O3 and 51% SiO2) have high melting temperatures 1465°C, resistance to oxidation, and can be made to have excellent thermal shock resistance (low expansion coefficients). This requirement stems from the rapidly changing
Packaging
The monolith must be packaged in the exhaust of an automobile, power plant, restaurant, etc. The typical converter package consists of a resilient mat to hold the substrate, end seals to prevent gas leakage, a stainless steel can to house the system and sometimes a heat shield for highly exothermic applications to protect adjacent components [3].
A robust converter package provides positive holding pressure on the ceramic substrate, promotes symmetric entry of inlet gases and provides adequate
Kinetics
Kinetic analysis of the reaction rate on catalyzed monoliths follows the same chemical engineering principles as packed bed reactors. Because the reaction rate is many times very fast, the reaction itself is controlled by the transfer of the reactant species to the surface. Mass transfer controlled reactions can be obtained through the use of more active catalysts, higher catalyst loading or higher operating temperatures (Fig. 3). The basic test is to obtain the conversion versus operating
Three way catalysts
The three way gasoline catalyst converter (TWC) simultaneously converts CO, HC and NOx to CO2, H2O and N2 when operated in the stoichiometric air to fuel ratio in the exhaust of the internal combustion engine [1]. It is the most dominant application of the washcoated cordierite monolith due to the high geometric surface area, low pressure drop, mechanical integrity and thermal shock resistance. It is primarily used in the under-floor position in the exhaust (under the driver), however, it is
Hydrogen generation for the fuel cell
The proton exchange membrane (PEM) fuel cell is being intensely investigated for homes and vehicles promising high efficiency and clean power generation. The PEM fuel cell requires H2 for the anode. Fuel processing of hydrocarbons to make H2 will likely involve the use of ceramic and/or metal monoliths and/or heat exchangers catalyzed with the appropriate catalyst [15]. Ceramic and/or metal monoliths offer a wide variety of advantages over packed beds for almost all unit operations mainly
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