STRONG METAL-SUPPORT INTERACTIONS high-temperature reduction

The structure of supported rhodium catalysts has been the subject of intensive research during the last decade. Rhodium is the component of the automotive exhaust catalyst (the three-way catalyst) responsible for the reduction of NO by CO [1], In addition, it exhibits a number of fundamentally interesting phenomena, such as strong metal-support interaction after high temperature treatment in hydrogen [21, and particle disintegration under carbon monoxide [3]. In this section we illustrate how techniques such as XPS, STMS, EXAFS, TEM and infrared spectroscopy have led to a fairly detailed understanding of supported rhodium catalysts. 

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

Ceria/noble metal (such as Ru, Rh, and Pd) catalysts are composed of noble metal species such as nanoparticles and clusters dispersed on the ceria supports. The catalysts show typical strong metal-support interactions (SMSI) (Bernal et al., 1999), that is, the catalysts exhibit a number of features for SMSI effects including (1) reducible supports (2) "high temperature" reduction treatments (3) heavily disturbed chemical properties and significant changes in catalytic behavior of the dispersed metal phase (4) reversible for recovering the conventional behavior of the supported metal phase. In these cases, the reducibility of ceria NPs is greatly enhanced by the noble metal species and the catalytic activities of the noble metals are enhanced by ceria NPs. 

Beard BC, Ross PN (1986) Pt-Ti alloy formation from high-temperature reduction of a titania-impregnated Pt catalyst - implications for strong metal support interaction. J Phys Chem 90 6811... 

Ti02 catalysts reduced at 700°K and is presently accepted as the key step in the induction of strong metal-support interaction (SMSI). The present observation appears to be the first case in which SMSI can be induced at normal (500 K) catalyst reduction temperatures. High resolution electron energy loss spectroscopy (EELS) experiments of CO on Ni/Mn0x reduced at 500°K showed dramatic reduction of the C-0 stretching frequency of adsorbed CO. These EELS results are discussed in light of... 

The work of Tauster and coworkers (1,2) showed that hydrogen chemisorption is suppressed on group VIII metals supported on a series of oxides after these samples have been reduced at high temperatures. The term strong metal-support interactions (SMSI) was introduced to describe this behavior. A similar suppression in hydrogen chemisorption has since been reported for many other supported metal systems 0-5). However, the use of other chemical probes (4, 5) demonstrated that different mechanisms of metal-support interactions could exist for different types of oxides. Furthermore, even for a so-called SMSI oxide, the degree of interaction could be influenced by many parameters such as crystallite size and reduction temperature. It would thus be desirable to find an approach to systematically compare catalytic behavior of different systems. 

The selective hydrogenation of a,P-unsaturated aldehydes is used as a probe reaction in studying the strong metal/support interaction (SMSI). Ceria is able to form oxygen vacancies and interme-tallic compounds after reduction treatment at relatively high temperatures. 

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