16 Interactions and Surface Phenomena in Supported Metal Catalysts
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E. Ruckenstein Department of Chemical Engineering, State University of New York at Buffalo, Amherst, NY 14260 Transmission Electron Microscopy was used to investigate the behaviour of Fe/alumina catalysts in various environments (such as H2 and O2). The results indicate that depending on the environment, the crystallites can extend, spread or contract upon the substrate and also acquire toroidal shapes or fracture into smaller units. At high temperatures and in an O2 atmosphere, films spread out from the crystallites and a (multilayer) contiguous film coexists with the crystallites. These wetting and spreading phenomena are a result of the interactions between the metal, substrate and atmosphere. The strong chemical interactions between the substrate and the compounds formed between the active metal and the chemical atmosphere can enormously decrease the interfacial free energy between the crystallites and substrate. This then leads to a rapid spreading of the crystallites, to torus formation and also fragmentation of the crystallites. New mechanisms of sintering based on wetting and spreading have been suggested to explain some experimental observations. Considerations based on spreading are used to explain the room temperature suppression of CO and H2 chemisorption upon any of the Group VIII metals supported on TiO2 when the supported metal is prereduced in H2 at 500°C (The Tauster effect). From the condition that a layer of oxide should spread over the metal, one concludes that only those combinations of oxides and metals for which the interaction energy per unit interfacial area is greater than twice the surface free energy of the oxide can manifest the Tauster effect. A thin film thermodynamic treatment is employed in which the free energy of formation of the oxide film is expressed in terms of the film thickness. The minimization of this free energy with respect to the film thickness provides an ex0097^6156/86/0298-0152$06.00/0 © 1986 American Chemical Society
Baker et al.; Strong Metal-Support Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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Surface Phenomena in Catalysts
pression for the thickness of the spreading film. One concludes that a monomolecular film of oxide is likely to spread upon the surface of the crystallites driven by the strong interactions between the oxide species (TiO ) and the metal. Various implications of a physical nature, such as the occurrence of raft-like crystallites (pillbox morphology), and of a chemical nature, such as the possibility of an optimum coverage of the metal surface by ΤiΟ to obtain maximum activity, are pointed out. x
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Supported metal catalysts contain small metal crystallites dispersed over the internal surface area of refractory metal oxides. The traditional view was that the support constitutes an inert carrier whose role is merely to ensure a high dispersion of the metal. However, while the commonly used refractory oxide supports, silica and alumina, increase the metal dispersion, they are not inert, especially toward the non-noble metals and less conspicuously also toward the noble metals. The physical and chemical interactions between the active metal, the oxide support and the environment affect the surface properties of the catalyst and consequently influence the shape of crystallites and the particle size distribution. Two sets of experimental observations involving surface phenomena are of interest in the present context: (1) The average size of the crystallites increases in time, thereby decreasing the surface area of metal exposed to the chemical atmosphere. This sintering process, reviewed recently in Reference 1, occurs in order to decrease the free energy of the system and is therefore affected by the interfacial free energies involved and hence by the interactions (both physical and chemical) between the crystallites, substrate and atmosphere. (2) Tauster and Fung C2»3) observed that when Group VIII metals are supported on Ti02 and the catalysts reduced at 5Q0°C, their normal ability to chemisorb H2 and CO at room temperature is almost completely suppressed. This suppression is reported to be absent when the catalysts are prereduced at the lower temperature of 200°C. This Tauster effect was explained by the formation of a metal-metal bonding between the reduced cations of titanium and any of the Group VIII metals. An alternate explanation was associated with the blockage of the metallic surface by TiO species resulting from the reduction of the Ti02 substrate (4-6). The scope of the present paper is to emphasize that the inter actions between support, metal and atmosphere are responsible for both the physical (size distribution, shape of the crystallites, wettability of the substrate by the crystallites and vice versa), the chemical and the catalytic (suppression of chemisorption, increased activity for methanation, etc.) manifestations of the supported metal catalysts. In the next section of the paper, a few experimental results concerning the behaviour of iron crystallites on alumina are presented to illustrate the role of the strong chemical interactions between the substrate and the compounds of the metal formed in the chemical atmosphere. Surface energetic considerations, similar to those already employed by the author (7,8), are then used to explain some of the observed phenomena. Subsequently, the Tauster effect is explained as a result of the migration, driven by strong interactions, x
Baker et al.; Strong Metal-Support Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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of T i O species over the surface of the ioetal and thermodynamic arguments are adduced to support the premise that the layer of ΤΙΟχ on the metal i s monomolecular i n thickness. F i n a l l y , the insight gained from the above considerations i s used to explain the occur rence of extended planar shapes of the c r y s t a l l i t e s on the substrate ( p i l l b o x morphology) and to point out some c a t a l y t i c implications of the spreading of T i O over the surface of the metal. x
x
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Experimental Numerous experiments have been carried out i n t h i s laboratory using p a r t i c u l a r l y alumina as substrate (but also T1O2) and Pt, Pd, N i , Co, Fe and some of t h e i r a l l o y s as metals (8-12). Only a few r e s u l t s obtained with Fe/Al203 are reported here; more d e t a i l s are included in Reference 8. The experiments are based on transmission electron microscopy, which provides information on the physical behaviour of the c r y s t a l l i t e s , and on electron d i f f r a c t i o n , which provides i n f o r mation on the chemical compounds formed as a result of the i n t e r actions between atmosphere, substrate and metal. Changes occurring i n model Fe/Al203 catalysts were recorded upon t h e i r heating i n hydrogen or oxygen atmospheres at temperatures ranging from 300 to 900°C. Two kinds of hydrogen have been employed for reduction. In some experiments, ultrahigh pure hydrogen, which contains a. . and φ. . = » for r
