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PROPERTIES 79 Conventionally brittle materials can be made ductile by exploiting enhanced diffusional creep at low temperatures if the grain size is very small. At low temperatures, where atomic diffusion along grain boundaries dominates, the deformation rate, , is given by where is the tensile stress, is the atomic volume, d is the average crystal size, B is a numerical constant, Db is the boundary diffusivity, kT has the usual meaning, and is the thickness of the boundaries. The diffusional creep rate of a polycrystal, therefore, may be enhanced by reducing the crystal size, d, and by increasing the boundary diffusivity, Db. Recent studies on the boundary diffusivity in nanocrystalline metals with grain sizes around 5 nm (Horváth et al., 1987; Birringer et al., 1988) indicate a thousand-fold enhancement of grain-boundary diffusion over conventional polycrystals. This enhancement (103), coupled with a reduction in crystal grain size (from 10 µm to 10 nm), could increase creep rates by approximately 1012. The enhanced plasticity at low temperature may be utilized for processing by conventional extrusion or rolling methods. If desired, the material may be subsequently converted back to conventional-scale structures. For example, the surface may be laser-annealed to develop a much coarser surface grain structure with properties different from the unrecrystallized interior. Finally, it is also useful to think of some of these artificial multilayers as custom-made composites that allow extension of the well-known advantages of composites to a much smaller length scale. The combination of a ductile but weak metallic phase with a strong but brittle compound is a classic example (e.g., pearlite). It is now possible to create metal-oxide or metal-nitride films artificially in which the spacing of the phases is much smaller than was possible by conventional methods. CATALYTIC PROPERTIES The nature of the surface sites that are responsible for the catalytic reaction (active sites) has been the subject of catalysis research studies for many years, and several models have been proposed for describing the catalytically active sites. These models have been based on different characteristics of the sites, such as (1) the geometry of the sites (i.e., the distance between atoms around the site), (2) the coordination number of the catalytic sites that are associated with individual atoms, (3) sites associated with a critical ensemble of atoms, and (4) electronic structure
PROPERTIES 80 models that assume that a critical electronic configuration imparts activity. Research aimed at characterizing catalytically active sites has included studies of the nature of the adsorption and coordination of reactants with surface atoms, the arrangement of the reactants and intermediates on the surface, the size and configuration of the surface site, and the acid or base nature of the site. Detailed understanding of the relationships between catalyst structure and properties has in the past been hampered by limited structural information on catalysts. New and improved characterization techniques combined with the potential for synthesizing catalysts with controlled microstructures hold potential for making connections between structure and properties in model catalyst systems that closely mimic real catalysts. Such studies also hold potential for synthesizing materials with improved catalytic properties. A great deal of attention has been given to catalyst particle size, and many catalytic reactions have been classified as structure-sensitive or structure-insensitive, depending on the active site requirement of various catalytic reactions. Variation of the particle size of supported metal catalysts in the range of 1 to 10 nm can have dramatic effects on the activity and selectivity. Reactions that have been reported to exhibit structure sensitivity include these broad classes: oxidation (generally rates decrease with decreasing particle size), alkane transformation such as hydrogenolysis and skeletal isomerization (usually, but not always, rates increase with decreasing particle size), and some isotopic exchange and hydrogenation reactions (rates usually decrease with decreasing particle size) (Bond, 1985). An example from the recent literature is the effect of iron particle size for iron supported on MgO as the determining factor in the activity and selectivity of the iron for hydrocarbon synthesis from CO-H2 (McDonald et al., 1986). A topic of major interest is the determination of how the catalyst surface is affected by pretreatments. Some supported metal catalysts have been found to exhibit widely different activity following specific pretreatments. For example, Ru supported on alumina readily catalyzes the water gas shift reaction and ammonia decomposition following a brief oxidation of the catalyst, but this activity is lost following reduction (Taylor et al., 1974). Progress has been made in understanding pretreatment effects through the combined use of activity and microscopy measurements. The surfaces of Ni/SiO2 (Lee et al., 1986) and Rh/SiO2 (Lee and Schmidt, 1986) have been examined in detail. Oxidation and reduction pretreatments were used to change the catalyst microstructure, and activity changes in the catalyst were shown to be associated with morphological changes in the active site. The effect of oxidation-reduction pretreatments on the activity of a series of Pt/SiO2 and Pt/Al2O3 catalysts has been examined in detail for
PROPERTIES 81 several reactions, including isotopic exchange between cyclopentane and deuterium and between methylcyclopropane hydrogenation and hydrogenolysis (Inoue et al., 1978; Otero-Schipper et al., 1978; Wong et al., 1980; Pitchai et al., 1985; Burch and Garla, 1982). Factors discussed as potentially responsible for the various pretreatment effects include oxide formation and incomplete reduction, morphological changes depending on the temperature of exposure to hydrogen, the degree of dehydroxylation of the support, and the presence of some inhibitive form of adsorbed hydrogen. The effect of the treatment of supported metal catalysts with hydrogen on surface morphology, catalyst activity, and adsorption properties is in a state of confusion at the present time (Paal and Menon, 1983). Bimetallic catalysts that exhibit activity or selectivity for a particular reaction or sequence of reactions that cannot be explained by the activity of the individual metals are said to exhibit synergistic effects. An example is the reduction of nitric oxide by carbon monoxide over Pt-Ni/Al2O3 and Pd-Ni/Al2O3; the bimetallic catalysts are significantly more effective for reducing nitric oxide than Pt, Pd, or Ni alone (Klimisch and Taylor, 1973). Likewise, the activity of a bimetallic Pt-Rh/Al2O3 catalyst was shown to be substantially higher than that of a physical mixture of the two noble-metal catalysts for promoting the oxidation of carbon monoxide in a CO-NO-O2 feed at low temperature (Oh and Carpenter, 1986). The term âbifunctional catalystsâ refers to catalysts that perform more than one type of reaction. Generally, separate components of the catalyst are responsible for the separate functions, and product formation is the result of sequential reactions on the catalyst. Catalytic reactions carried out on single-crystal surfaces using various surface science techniques have advanced understanding of the mechanisms of elementary surface reactions and have provided understanding of the relationship of the structure and composition of the surface to catalytic properties. Comparisons of the properties of supported catalysts with single-crystal catalysts with known structure can provide understanding of structural features that are important for the catalysis. A significant body of such research is only just emerging; future progress will make use of the ability to prepare and characterize model catalysts. The importance of this research for understanding the catalytic properties of supported metal catalysts is demonstrated by comparisons of the adsorption properties and catalytic activity of single-crystal and supported noble-metal catalysts. In the past, most research with single-crystals was carried out under ultrahigh-vacuum conditions. Recent studies comparing nitric oxide reduction over single-crystal Rh(111) catalysts and over alumina-supported Rh catalysts at realistic pressures have revealed that the two types of Rh catalyst exhibit substantially different kinetics, and analysis of the rate data suggests that
