CHEMICAL AND BIO-CERAMICS JOURNAL OF MATERIALS SCIENCE 38 (2 003)4661 – 4675 Ceramics for catalysis M. A. KEANE Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506-0046, USA E-mail: [email protected] The use of ceramics as heterogeneous catalysts represents an extension of “non-traditional” ceramics applications and is now a burgeoning topic of research. In this Review, the principles of heterogeneous catalysis are presented and discussed in terms of surface reactivity and catalyst structure in general. Catalytic selectivity, rate enhancement and catalyst deactivation are addressed. The critical (bulk and surface) structural features that impact on catalyst performance are identified along with a survey of catalyst characterization techniques. Ceramics applications in catalysis are divided into (i) direct use as catalysts and (ii) use as support materials (substrates) to anchor and disperse a variety of active metals. Practical ceramic catalysts are typically complex metal oxides containing at least two different cations which offer enormous compositional flexibility, as is discussed in the case of perovskite oxides. Taking a broad definition of ceramics as “any inorganic nonmetallic material,” there is a wide array of catalysts that can be termed ceramics with disparate end uses. For the purposes of illustration, three established systems are discussed that each illustrates the role of ceramic materials in practical heterogeneous catalysis: (i) catalysis using zeolites; (ii) catalytic converters; (iii) solid oxide fuel cells. C 2003 Kluwer Academic Publishers 1. Introduction rial. The classification of ceramics into “traditional” Ceramics encompass such a vast array of materials that (clay products, glass, cement, etc.) and “advanced” a concise definition is difficult to formulate. As a work- (carbides, oxides, nitrides, etc.) is now common par- able explanation, ceramics can be considered as inor- lance in the materials research community. The appli- ganic, nonmetallic materials and can be crystalline or cation of ceramics in catalysis certainly falls within the amorphous (glassy). Ceramic compounds are formed non-traditional category. Heterogeneous catalysis and between metallic and nonmetallic elements such as ceramics overlap in terms of solid state inorganic chem- aluminum and oxygen (alumina-Al2O3), calcium and istry but the development of ceramic materials as effec- oxygen (calcia-CaO), and silicon and nitrogen (silicon tive catalyst supports and catalytic agents has meant a nitride-Si3N4). Ceramic materials display a wide range closer synergy in terms of research and development. of properties which facilitate their use in many differ- The latter has focused on a tuning of the chemical re- ent product areas. Modern ceramics include structural activity of the oxide surfaces to modify the interaction clay products such as bricks, sewer pipes, tiles, white- with adsorbed reacting species. It is estimated that 90% wares (dinnerware, sanitary fixtures, porcelain, deco- of the new processes commercialized in the chemical rative ceramics, etc.), refractories, glasses (including industry in the last 50 years have been based on catal- fibers for communication), abrasives and cements. The ysis. Catalytic chemistry finds applications in the pro- so-called advanced ceramics comprise structural ma- duction of commodity chemicals, fuels, polymers, and terials used for engine components, coatings, cutting pharmaceuticals, as well as in environmental applica- tools and the bioceramics used as bone and tooth re- tions for pollution abatement [3]. Given the number placements. The latter applications require a high re- of ceramic materials that exhibit catalytic properties, a sistance to wear/corrosion, features that set ceramics full enumeration of the associated catalytic applications apart from most metals/alloys [1]. While metals weaken is beyond the scope of this paper. Three established rapidly at temperatures above ca. 800◦C, ceramic mate- systems present themselves as suitable case studies rials retain their mechanical properties at much higher to illustrate the role of ceramic materials in practi- temperatures and such thermal resistance has been put cal heterogeneous catalysis: (i) catalysis using zeolites; to good effect in a number of applications [2]. Ad- (ii) catalytic converters; (iii) solid oxide fuel cells. vanced ceramics are used in capacitors, resistors, in- sulators, piezoelectrics, magnets, superconductors, and electrolytes. Such materials require a level of process- 2. Fundamentals of heterogeneous catalysis ing science and engineering far beyond that used in the The development of catalysis, from the Greek (kata, production of conventional ceramics. wholly; lyein,toloosen), as a concept has been ac- This paper deals with an application of ceramics corded to Berzelius (1779–1848) who rationalized the that is growing in importance, i.e., as catalytic mate- “catalytic power” of certain substances as an ability 0022–2461 C 2003 Kluwer Academic Publishers 4661 CHEMICAL AND BIO-CERAMICS solution. Examples of homogeneous catalysts include transition metal ions, transition metal complexes, in- organic acids/bases and enzymes. In a heterogeneous reaction, the catalyst is in a different phase from the reactants, where the reaction occurs at the surface of a solid (catalyst) particle in contact with the gaseous or liquid solution. The main disadvantage associated with heterogeneous when compared with homogeneous cat- alyst operation is the lower effective concentration of catalyst as the reaction occurs only on the exposed ac- tive surface. Catalyst recovery and reuse is, however, far more facile in the case of heterogeneous operation. The prototypical example of heterogeneous catalysis is the formation of hydrocarbons from the reaction of Figure 1 Energy diagram for a catalyzed and uncatalyzed chemical hydrogen and carbon monoxide over a suitable solid reaction. catalyst. The simple methanation reaction to “awaken affinities, which are asleep at a particular 3H + CO ↔ CH + H O (1) temperature, by their mere presence and not by their 2 4 2 ownaffinity” [4, 5]. Various definitions of catalysis has an associated G0 =−96 kJ mol−1 at 500 K, have been proposed but an early definition offered by which represents a favorable equilibrium ratio. How- Wilhelm Ostwald in 1895 is still widely in use: “Cat- ever, the homogenous reaction rate between CO and alysts are substances which change the velocity of a H in the gas phase is very low and only proceeds to reaction without modification of the energy factors of 2 any appreciable degree in the presence of metal cata- the reaction.” The latter serves to exclude substances lysts at temperatures in the range 523–723 K [8]. that accelerate the rate of reaction by entering into re- Catalysis involving ceramics falls within the remit action with a resultant disruption of the reaction equi- of heterogeneous systems used to enhance reactions librium. A catalyst works by forming chemical bonds between reactants in the gas and (to a lesser extent) to one or more reactants which facilitates their con- liquid phase by use of a solid catalyst. Any discussion version but the catalyst does not significantly affect of the role of ceramics in heterogeneous catalysis must the reaction mechanism [6]. A more rigorous defini- be preceded by a consideration of surface reactivity and tion of a catalyst is then “a substance that increases the catalyst structure in general. rate of reaction without modifying the overall standard Gibbs energy change in the reaction.” This is illustrated schematically in Fig. 1. In an uncatalyzed transforma- tion collisions between participating molecules must 2.1. Catalysis on surfaces possess sufficient (activation) energy to pass over the Any reaction that is promoted by heterogeneous catal- energy barrier that is characteristic for that reaction. In ysis involves the following steps: terms of the more realistic “transition state theory” orig- Step 1: reactant + catalyst inated by Eyring and co-workers, the reactants form a Step 2: reactant/catalyst complex short-lived transition state or activated complex which Step 3: product/catalyst complex reacts to give product(s) [7]. Catalyst/reactant interac- Step 4: product + catalyst tions serve to lower the energy barrier (transition state is at a lower energy) and facilitate reaction under more The surface reaction is facilitated by reactant/catalyst moderate reaction conditions. The catalyst reduces the interaction(s) that generates a reactive reactant/catalyst enthalpy of activation for the forward reaction by ex- complex. These interactions may be weak, of the ‘non- actly the same amount as it reduces the enthalpy for the bonding’ type with the reactant staying intact while reverse reaction, there is no entropy change and the sticking to (or absorbing on) the surface. Alterna- free energy change/position of equilibrium remains tively, the interactions may involve the formation of the same for the catalyzed and uncatalyzed processes. new chemical bonds between the surface atoms and The incorporation of catalysts into any reaction system the adsorbed molecule, which necessarily involves ex- will impact on reaction rate with the result that reaction tensive reorganization of the bonding within the re- kinetics is central to catalysis, providing the quantita- actant. This reactive type of interaction is generally tive framework for an assessment of catalyst activity. known as chemisorption in