Strong metal support interaction SMSI effect

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. 

It is now well established that spillover-backspillover phenomena play an important role in numerous catalytic systems. It is worth reminding that the effect of strong-metal-support interactions (SMSI), which was discovered by Tauster74 and attracted the intense interest of the catalytic community for the least a decade75 was eventually shown to be due to backspillover of ionic species from the Ti02 support onto the supported metal surfaces. 

The use of supports such as Ti02, where the effect of a strong metal-support interaction (SMSI) was observed at high reduchon temperature, is one of the recommended routes. It is proposed that TiO having coordinatively unsaturated Ti cations that could interact with the electron pair donor site of the C=0 bond, facilitates adsorption of the unsaturated aldehyde in a favorable way to produce UOL [74, 75]. As for the metaUic phase, both theoretical and experimental studies indicate that larger particles improve the selectivity to UOL. In effect, it has been... 

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. 

Fig. 4.38. The effects of various pretreatments (oxidative and reductive) on CO oxidation on a 40-nm Pt/ceria model catalyst prepared by colloidal lithography as measured by the temperature of 50% of CO conversion and the apparent activation energy from the Arrhenius plot. CO reduction was made in 0.5% CO for Ih at 573K, H2 oxidation (a-treatment) was done at a = Ph2/(.Ph.2 + P02) = 0.33 at 573 K for 1 h, and finally /3 = CO oxidation (/3-treatment) was done in the O-rich regime (oxidative conditions), /3 = Pco/ Pco + P02) = 0.2 with 0.3% CO and 1.2% O2 at temperatures between 300 and 673 K. It is seen that reduction leads to a lower Tbo and activation energy, while sustained CO oxidation leads to an increase of the activation energy, which is not recovered by reductive treatments. The latter is explained in terms of strong-metal-support interactions (SMSI) and particle reshaping (see text)...

The reducibility of platinum should be affected both by SOOI effect (see above) and strong metal-support interaction (SMSI) between platinum and transition metal oxide monolayer. Both mentioned effects were observed by means of temperature-programmed techniques. TPR investigations put in evidence that the reducibility of platinum strongly depended on the type of primary carrier as well as on the type of transition metal monolayer. The results of TPR investigations are presented in Fig. 1 and Table 2. 

Bonding modifiers are employed to weaken or strengthen the chemisorption bonds of reactants and products. Strong electron donors (such as potassium) or electron acceptors (such as chlorine) that are coadsorbed on the catalyst surface are often used for this purpose. Alloying may create new active sites (mixed metal sites) that can greatly modify activity and selectivity. New catalytically active sites can also be created at the interface between the metal and the high-surface-area oxide support. In this circumstance the catalyst exhibits the so-called strong metal-support interaction (SMSI). Titanium oxide frequently shows this effect when used as a support for catalysis by transition metals. Often the sites created at the oxide-metal interface are much more active than the sites on the transition metal. 

Thus, in the particular case of metal catalysts supported on some reducible oxides, the occurrence of so-caUed metal-support interaction (SMSI) effects has been reported [75]. In order to minimize the metal-support interaction, stable oxides (not reducible) such as alumina, sihca or zirconia are most frequently used but as mentioned above, it is quite possible that the metal has one or several chemical bonds with the support (Scheme 18.4). For particles having a diameter of more than 2 nm the support is not beUeved to have a strong electronic effect on the particle because the number of metaUic atoms becomes much more important than the number of chemical bonds between the particle and the support... 

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