Metal-support interactions alumina supported metals

Figure 34. The role of Ge and chloride may be interpreted on the basis of the crystallite migration mechanism. IR studies of CO chemisorbed on Pt/Al203 show that chloride accentuates the metal-support interaction. Metals or metal oxides can have the same effect by modifying the acidity of alumina, or may only play the...

Interest, because of their nonuniformity and will not be discussed further. Schmidt ( ) and Baker (11) have elaborated on the platlnum-slllca mobility question and the strength of metal-support Interactions. Sample C, Pt on alumina, was used as a reference for the H/Pt measurements, and beyond that Is of little relevance In these discussions. 

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

Alumina is one the best known catalyst support materials and is frequently used in both research and industrial applications not only for its relatively high surface area on which active metal atoms/crystallites can spread out as reaction sites, but also for its enhancement of productivity and/or selectivity through metal-support interaction and spillover phenomena. 

Cheekatamarla and Lane [62, 63] studied the effect of the presence of Ni or Pd in addition to Pt in the formulation of catalysts for the ATR of synthetic diesel. For both metals, a promotional effect with respect to catalytic activity and sulfur poisoning resistance was found when either alumina or ceria was used as the support. Surface analysis of these formulations suggests that the enhanced stability is due to strong metal-metal and metal-support interactions in the catalyst. 

SMSI is also thought to affect methanation catalysts (normally transition metal or noble metals supported on alumina), which are used in the producton of substitute natural gas (SNG). In general, heating in H2 causes sintering on alumina and silica supports and heating in O2 or steam can cause dispersion and particle coalescence at 200 °C (Rukenstein and Lee 1984,1987, Nakayama et al 1984). The data have been based on ex situ EM studies. Here EM methods, especially under dynamic reaction conditions, can provide a wealth of new insights into metal-support interactions under reaction conditions. 

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. 

The initial reaction rate values show the effect of metal support interaction, the extent of reduction and method of preparation. For alumina supported sample the increase in reduction temperature, and an increase in the degree of reduction, results in a fourfold increase in initial... 

In the Au/Al203/NiAl(100) system, hemispherical particles occur even at low coverage,7 unlike the situation with titania size distribution was narrow, and particles were stable to 600 K, implying low mobility of adsorbed atoms. Paradoxically, on alumina large particles migrate and coalesce faster than small ones, presumably because the metal-support interaction is weaker but with Au/FeO the diffusivity of atoms is higher due to a lower concentration of surface defects. 

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... 

In addition, the present results clearly show that y-alumina improves the catalytic activity and selectivity to long chain hydrocarbons. Particularly, when the Fe-K is supported on y-alumina, the selectivity to olefins is improved as well as the activity and chain growth ability. It is likely that the marked improvements achieved by using both K and alumina is due to well dispersed active phase Fe-K. The better dispersion of Fe and K should provide the efficient interaction of Fe and K as well more active sites for the CO2 hydrogenation. The improved dispersion seems to be the result of strong metal-support interaction. 

The scope of the present paper is to emphasize that the interactions between support, metal and atmosphere are responsible for both the physical (size distribution, shape of the crystallites, wettability of the substrate by the crystallites and vice versa), the chemical and the catalytic (suppression of chemisorption, increased activity for methanation, etc.) manifestations of the supported metal catalysts. In the next section of the paper, a few experimental results concerning the behaviour of iron crystallites on alumina are presented to illustrate the role of the strong chemical interactions between the substrate and the compounds of the metal formed in the chemical atmosphere. Surface energetic considerations, similar to those already employed by the author (7,8), are then used to explain some of the observed phenomena. Subsequently, the Tauster effect is explained as a result of the migration, driven by strong interactions,... 

R535 B. N. Shelimov, J.-F. Lambert, M. Che and B. Didillon, Molecular-Level Studies of Transition Metal - Support Interactions During the First Steps of Catalysts Preparation Platinum Speciation in the Hexa-chloroplatinate/Alumina System , /. Mol. Catal. A Chem., 2000,158,91... 

Zirconia is considered a neutral support due to its dual superficial characteristics, oxidizing or reducing and basic or acid behaviour. Moreover, zirconia is a more reactive support than silica and alumina and hence with a stronger metal-support interaction. 

The role of the support material in determining the activity and selectivity of precious metal catalysts is critical and there is now a significant literature on metal support interactions. The effect may be illustrated for rhodium by considering alumina and ceria as support phases. In the case of alumina the metal support interaction was investigated by firing lXRh/Al O samples in air over a range of temperatures (table 7). 

The rhodium dispersion becomes progressively worse on the higher temperature and, therefore, lower surface area alumina phases, NO uptake also falls sharply as the ageing temperature of each Rh/A1 0 phase is increased. The lower NO uptake can be explained partially by rhodium sintering (as the oxide) and also by a metal support interaction (Ref. 36). The interaction is less for the high temperature, less reactive alumina phases but even here NO absorption is not measurable after ageing at 850 C. The rhodium/alumina interaction is also observed when temperature programmed reduction (TPR) is performed (Fig. 14(A) and (B). 

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