Real Metal-Support Interactions

Schwab also noticed that activation energies for formic acid decomposition were significantly lower for Cu/MgO, Ag/MgO, and Ni/Al203 com- 

Adamiec, S. E. Wanke, B. Tesche, and U. Klengler, in Metal-Support and Metal-Additive Effects in Catalysis , ed. B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G. -A. Martin, and J. Vedrine, (Studies in Surface Science and Catalysis 11), Elsevier, Amsterdam, 1982, p. 77. 

In summary, apparent metal-support interactions may arise through the operation of a specific particle size effect, or of bifunctionality or spillover, or through the support acting as a source or sink of a catalytic poison. Deliberately added promoters constitute an additional complication. Real interactions not due to these or similar causes may be attributed either to electronic or geometric effects, the latter embracing possible differences in crystal habit, or to the creation of phases which contain the active component in some form but which are hard to reduce. 

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A prime example of the sensitivity of a metal to the nature of its support is provided by the synthesis of methanol and other oxygenated products from CO + H2 on Pd and Rh catalysts. " It is clearly necessary to use a support having basic character in order to achieve high selectivity, but whether this is because of a spillover or bifunctional mechanism, or because of a real metal-support interaction, is unclear at the present time, but in view of the proven involvement of the support in CO2 hydrogenation the former possibility certainly cannot be discounted. This subject is being covered by Ponec and Poels in a companion article in this volume and will therefore not be taken further here. 

Hayek K, Fuchs M, Klotzer B, Reichl W, Rupprechter G (2000) Studies of metal-support interactions with real and inverted model systems Reactions of CO and small hydrocarbons with hydrogen on noble metals in contact with oxides. Topics Catal 13 55... 

Freshly prepared Pd-Ru and Pd-Ni on a monolithic support converted all the NO (lOOOp.p.m.) with less than 5% ammonia formation in 0.4% O2 (1% CO, 250p.p.m. CaHg) at 753 and 873 K, respectively. As a further example of the metal-support interactions discussed in Section 6, the well known formation of nickel aluminate at high temperatures (in part) caused substantial deactivation of Ni-Pd in real exhaust, whereas Ni-Al204 was a more stable non-interacting support allowing a better performance to be maintained. After lOOh under net reducing conditions there was still a 50% loss of Ru from Pd-Ru catalysts. 

More recent work has concentrated on the study of model systems in which the characteristic metal-support interaction effects were reproduced by depositing controlled amounts of titania species on metal surfaces (116-124). To ascertain the chemisorption and catalytic consequences of the TiOx overlayer, the evaporation has usually been carried out under 10-7-10-6 torr of oxygen. In this section we analyze the more relevant metaI/Ti02 and TiOx/metal model studies and their impact on our understanding of the metal-support interactions taking place on real supported catalysts. 

Considerable progress has been made in model catalysts of planar well-ordered extended surfaces, but under UHV conditions. Atomic structure and electronic properties of many metal surfaces or metal oxide-supported materials are now accessible, related to the physical and chemical properties of adsorbate species on surfaces, in particular binding sites. However, there are several problems related to the application of models with real catalysts, such as metal-support interactions and... 

Besides electronic effects, structure sensitivity phenomena can be understood on the basis of geometric effects. The shape of (metal) nanoparticles is determined by the minimization of the particles free surface energy. According to Wulffs law, this requirement is met if (on condition of thermodynamic equilibrium) for all surfaces that delimit the (crystalline) particle, the ratio between their corresponding energies cr, and their distance to the particle center hi is constant [153]. In (non-model) catalysts, the particles real structure however is furthermore determined by the interaction with the support [154] and by the formation of defects for which Figure 14 shows an example. 

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