Weak metal-support interaction

Metal-support interactions can be defined as being weak, medium or strong.28 Non-reducible metal oxides such as silica, alumina, and magnesia as well as carbon or graphite are considered to exert only a weak influence on the metal and, thus, exhibit only a weak metal-support interaction (WMSI). Zeolites (Chapters 10 and 13) exert a medium metal-support interaction (MMS1)28 while metals supported on reducible oxides when reduced at high temperatures exhibit a strong metal-support interaction (SMSI).27-32... 

The application of selective chemisorption to supported Pt catalysts is well established but there have been valuable additional studies of the use of hydrogen in the pulse-flow technique and of CO adsorption using TPD and carbon monoxide. Recently the usual assumption about the stoicheiometry for hydrogen adsorption, Ptg/H = 1 has been questioned. For the Council of Europe Pt-Si02 catalyst, where a weak metal-support interaction was postulated, 1.75 hydrogen atoms per surface metal atom were found at 300 K in two adsorbed forms (the formation of jSa was activated). Recent work on selective chemisorption applied to metals of catalytic interest other than platinum will now be examined. 

Furthermore, the (Ni-Al-Ti)sg and (Ni-Al-Ti)imp samples have equal percentages of reduction and their selectivity patterns are quite different. Therefore, the size of the metallic particles and the metal-support interaction could be the cause of the individual behaviour of each catalyst regarding selectivity. This hypothesis is confirmed by the fact that after reduction at 773 K, the evolution of the selectivity of each catalyst (except Ni-Ti) is similar to that shown by (Ni-Al-Ti)imp after reduction at 573 K. Thus, it seems clear that large metallic particles and therefore weak metal-support interactions are responsible for the evolution in the selectivity to ethylene. 

It is even harder to prepare catalysts with a narrow Ag particle size distribution supported on alumina with a small surface. In all cases, the preparation of Ag catalysts on a-Al203 via conventional deposition and drying produces a polydisperse Ag particle size distribution. Probably, this results from a weak metal-support interaction. Indeed, Ag particles are known to migrate over dumina surface even under rather mild conditions [8]. 

When supported metal oxide phases are concerned, the kind of surface species that can be present at the surface depends on the support nature, being active a strong or weak metal-support interaction. CuO is an important catalytic phase, easy to disperse on acid supports that can interact with it with strong metal-support bond this interaction can influence the redox properties of CuO. Modified silicas with amount of alumina (SA), titania (ST), and zirconia (SZ) in 12-14 wt.% concentration were used to support CuO (8-9 wt.%) [20] a commercial silica-alumina support was comparatively studied (SAG). On the different supports, the CuO redox properties were controlled by combining TPR and successive TPO experiments. 

In general, encapsulated metal particles were observed on all graphite-supported catalysts. According to Ref. [4] it can be the result of a rather weak metal-graphite interaction. We mention the existence of two types of encapsulated metal particles those enclosed in filaments (Fig. 1) and those encapsulated by graphite. It is interesting to note that graphite layers were parallel to the surface of the encapsulated particles. 

The crystalline structure of the metal is also affected by the metal-support interaction. Metal particles supported on CNFs have a highly crystalline structure due to strong metal-support interaction [155], whereas Pt particles supported on Vulcan and OMCs have a more dense globular morphology due to weak metal-support... 

The EXAFS results have implications for the metal support interaction. The data in Table 9.1 indicate that the main interaction between rhodium and alumina occurs between reduced metal atoms and two to three oxygen ions in the surface of the support at a distance of 0.27 nm. It appears logical, therefore, to attribute the metal support interaction to bonding between oxygen ions of the support and induced dipoles inside the rhodium particle [19]. Although such bonding is weak on a per atom basis, the cumulative bond for the whole particle may be significant. 

Because the size regime of n=l-6 atoms is of great practical significance to the spectroscopic, chemical and catalytic properties of supported metal clusters in both weakly and strongly interacting environments (28), it is important to study very small metal clusters in various types of substrate as well as in the gas phase. In this way, one can hope to develop a scale of metal cluster-support effects (guest-host interactions) and evaluate the role that they play in diverse technological phenomena. 

---