Metal-support interaction electronic effects

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). 

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... 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Improvements in the resolution and versatility of microscopic techniques have come about rapidly. TEM, STEM, and high-resolution electron microscopy have helped the catalytic chemist to analyze the effects of metal-support interactions and particle-size effects—developments that will probably lead to improvements in commercial technologies. Several novel analytical methods, arising from very clever experimentation, were discussed at the... 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

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... 

Small metal particles are frequently expected (however, the evidence is sometimes questionable) to experience an electron transfer with the carrier, which modifies the adsorption and catalytic properties of the metal particles [sometimes called the Schwab effect (108-116)]. In other cases, by special conditions under preparations of the catalysts, a so-called strong metal support interaction effect (SMSI) (117-121) was evoked. In particular, with zeolites as carriers, there are pieces of experimental evidence reported (115, 116) in support of the existence of such transfer (for remarks on those conclusions, see 122, 123). 

While effects of film thickness have also been reported [275], metal-support interactions seem to be important in governing the properties of the overlayer. For instance, Ni chemically deposited on MgO has shown in particular higher resistance to oxidation as a consequence of electronic interactions with MgO [276]. 

The electrocatalytic effect of small crystallites is attributed to the metal-support interaction [326, 330], a well known and widely discussed topic in catalysis [334]. In particular, since precious metals have high electron work functions, electrons are injected from the support into the crystallites thus modifying considerably the electron density which becomes a function of the crystal size. The fact that the effect of different supports is not visible [335] can be explained in terms of a large Abetween different supports are very small [326]. However, in the case of carbonaceous supports, different preparations of the support may result in sizable effects because the morphology of the overlayer can be influenced [336]. 

Although the effect of the support on the catalytic properties of the supported metal particles has been well established, the nature of this metal-support interaction has been the subject of much debate. Explanations have involved the formation of metal-proton adducts on Bronsted acidic supports5,12, electron transfer between support and particle8,13,14, the polarization of the metal particle by nearby cations15 and a rehybridization and polarization within the particle by the Madelung potential of the support16,17. 

Metal-support interaction can be revealed by differences in XAS chemical shifts for metal particles of the same size but on different supports. As the particle size decreases the perturbation of the electronic structure by the particle environment (support effect) becomes larger. Such problems have recently also been investigated by means of XPS (129) for a series of Pt and Rh catalysts supported on y-Al203 and TiOz. 

The rdle ofd ects and dopants on the metal-support interactions. It is well known that defects and dopants change both the electronic and the geometric structure of the interface hence an understanding of these effects is of particular interest. 

When considering metal-support interaction effects, the whole set of Electron Microscopy data presented in the previous section point out some important differences between the behaviour of noble metal catalysts supported on ceria and that of titania-supported catalysts. Much higher reduction temperatures are required in the case of ceria-type supports to observe nanostructural features similar to those described for the so called SMS I efTect. 

To summarise the results concerning the study of reversibility of metal-support interaction states, we could first state that the classic reoxidation treatment at 773 K does not allow the recovery of the NM/Ce02 catalysts from the decorated or alloyed states. The noble metal/ceria phase separation may only be achieved upon reoxidation at temperatures well above 773 K. This observation represents an additional major difference between titania and ceria supported noble metal catalysts. Moreover, the likely regeneration of NM/CcOi catalysts reduced at 773 K by reoxidation at 773 K would actually prove, in good agreement with earlier HREM studies on the reduced catalysts (117,194), that the observed deactivation effects are not due to decoration or alloying phenomena, rather consisting of purely electronic effects (105). 

It is quite challenging to rmderstand in what way the zeolite influences the metal compared to other supports. The electronic changes that could be induced by the pore system are quite subtle and metal particle size effects may overrule these changes [200]. hi comparison to metal-support interactions on macroporous oxides, the interaction between metal particles and the supporting zeolite matrix seems more pronounced. This may be because the metal particles interact with the zeolite lattice over a substantial fraction of their surfece. It has also been suggested that in addition to the intrinsic electronic effects due to the small size of the metal particles in the zeolite cage, a modification of the electronic structure of the metal by the acidic zeolite framework has to be considered [201,202]. 

---