SMSI effect

All of these results are consistent with the notion that surface migration of titanium oxide species Is an Important factor that contributes to the suppression of carbon monoxide chemisorption. The H2 chemisorption experiments on 1-2 ML of Ft, where no migration Is observed, strongly Indicate that electronic (bonding) Interactions are also occurring. Thus, for the tltanla system, both electronic Interactions and surface site blocking due to titanium oxide species must be considered In Interpreting SMSI effects. 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

Figure 4 The different modes of action of electronic promotors in Co-based Fischer-Tropsch catalysis (A) promoter metal oxide decoration of the cobalt surface (B) the SMSI effect and (C) cobalt-promoter alloy formation...

The SMSI effect in Mn-promoted Ru/Ti02 catalysts was studied in more detail making use of the SSIMS technique, as well as with TEM, and selective chemisorption experiments. The SSIMS technique revealed the presence of TiO c forming two new surface sites, TiO -Ru and TiO-Mn. These species were found to be located at the immediate vicinity of the Ru nanoparticles. These new surface sites were considered to alter the electronic properties of the Ru metal surface and, as a consequence, the product selectivity. 

It is perhaps worthwhile noting that the promoter patches on the metal can be either created during the wet steps of the catalyst preparation, or when a transition metal oxide is used as a support, they can be created by migration of the support material on the metal upon high temperature reduction. The metal surface can be kept almost completely covered in vacuo (SMSI effect) [52], but in the presence of CO or of the reaction mixture, the layer of oxide recrystallizes and the metal surface becomes accessible again from the gas phase [33]. 

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. 

For catalysts containing reducible oxide supports, as is the case of systems, the chemisorption studies may also be used for detecting the metal deactivation effects due to the occurrence of a SMSI effect (300,301). On MT1O2 catalysts, the classic SMSI systems, it is now well established that reduction at about 773 K strongly inhibits the metal chemisorptive capability (171,302,318-320). The chemisorption data reported for M/Cc(M)02. catalysts have also suggested the occurrence of such an effect. It is certainly an interesting question which deserves some further discussion in this chapter. 

The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/TiOj, reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titania-supported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce02 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states. 

Coke deactivation on Pt/Al203 catalysts have been studied intensively in the literature. Previous works have focused on the kinetics of catalyst deactivation [7] the influence of additives on coke formation [8] the coke deposition on different morphologic surfaces [9] the structure [10] and chemical composition of coke [11]. Deactivation by coke deposition on niobia supported catalysts, or even on other reducible supports which promote SMSI effect has not been studied. 

The surface properties of dilute solid solutions of titanium in platinum should therefore closely resemble pure platinum, so alloy formation as such, if it occurs, can provide no ready explanation of SMSI effects. 

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