STRONG METAL-SUPPORT INTERACTIONS reduced catalysts

To Illustrate the utility of the technique, we have addressed the question of the anomalous chemlsorptlve behavior of tltanla-supported group VIII metals reduced at high temperatures. The suppression of strong H2 chemisorption on these catalysts has been ascribed to a strong-metal-support Interaction (SMSI) ( ). It has also been found that the reaction activity and selectivity patterns of the catalysts are different In normal and SMSI states... 

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. 

While some stability issues have been identified on the supported Au catalysts, a major progress was made by the discovery that Au/Ce02 showed remarkable performance stability.50 Among the catalysts with several Au loadings (Fig. 6.4), it was found that 3% Au showed the most stable operation over a 3-week period of tests due to an optimal ratio of surface Au and ceria sites. Evidence of strong metal-support interaction was correlated with the enhanced reducibility of ceria in the presence of Au nanoparticles. Further research into different catalyst preparation methods for Au/Ce02 showed that Au dispersion and the WGS activity are extremely sensitive to minor variations in the preparation procedure.51... 

One of the main aspeets that determine the properties of a given catalyst is the nature of the interaction between the oxide support and the dispersed active metal. The influence of this interaction on the activity, selectivity and stability of the catalyst is determined by factors such as the preparation method, the atmosphere and temperature of the calcination and reduction stages (1,2), and the specific metal-support system studied. The latter is especially important for catalysts based on transition metals supported on partially reducible oxides (3). Such catalysts displaying strong metal-support interaction (SMSI) exhibit suppression of chemisorption of H2 and CO (4-6) when reduced at temperatures up to 773 K. 

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... 

Recently titania appeared as a non-conventional support for noble metal catalysts, since it was found to induce perturbations in their H2 or CO adsorption capacities as well as in their catalytic activities, This phenomenon, discovered by the EXXON group, was denoted "Strong Metal-Support Interactions" (SMSI effect) (1) and later extended to other reducible oxide supports (2). Two symposia were devoted to SMSI, one in Lyon-Ecully (1982) (3) and the present one in Miami (1985) (4) and presently, two main explanations are generally proposed to account for SMSI (i) either the occurence of an electronic effect (2,5-13) or (ii) the migration of suboxide species on the metal particles (14-20). The second hypothesis was essentially illustrated on model catalysts with spectroscopic techniques.lt can be noted that both possibilities do not necessarily exclude each other and can be considered simultaneously (21). 

A study of the metal-support interaction effect has been carried out for a Pt-Ti02 catalyst prepared by microemulsion. Isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane were chosen as model reactions for studying the influence of the catalyst nature on the strong metal-support interaction (SMSI) effect. A comparison between the cata-lyticbehaviour of similar catalysts prepared by microemulsion and incipient wetness method,was reported. The catalysts were reduced at 200 and 390 °C since it is well known that the reduction temperature plays a predominant role in the SMSI effect l The behaviour of thePt-TiO microemulsion-prepared catalyst was found to be similar to that of a Pt/AbOs catalyst. Also, it was found that the microemulsion-prepared catalysts require a higher temperature of reduction to induce metal-support interactions. 

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