Catalyst-support interactions alumina supported metals

Alumina supported Sn-Ru and Re-Pt catalysts were found to be highly active and selective in liquid phase hydrogenolysis of ethyl dodecanoate and butyl acetate to the corresponding alcohol. The content of ionic forms of the second metals strongly affected the performance of these catalysts. The interaction between two metals in these catalysts has been shown by different experimental methods. 

Partial oxidation reactions are usually carried out over transition metal oxides capable of changing their valent state during their interaction with reacting molecules. Naturally, zeolites with their alumina-silicate composition did not prove themselves as good oxidation catalysts. They failed also to serve as efScient catalyst supporters, since transition metals being introduced into the zeolite matrix lose their ability to activate dioxygen [3,4],... 

Supported metal catalysts are used in a large number of commercially important processes for chemical and pharmaceutical production, pollution control and abatement, and energy production. In order to maximize catalytic activity it is necessary in most cases to synthesize small metal crystallites, typically less than about 1 to 10 nm, anchored to a thermally stable, high-surface-area support such as alumina, silica, or carbon. The efficiency of metal utilization is commonly defined as dispersion, which is the fraction of metal atoms at the surface of a metal particle (and thus available to interact with adsorbing reaction intermediates), divided by the total number of metal atoms. Metal dispersion and crystallite size are inversely proportional nanoparticles about 1 nm in diameter or smaller have dispersions of 100%, that is, every metal atom on the support is available for catalytic reaction, whereas particles of diameter 10 nm have dispersions of about 10%, with 90% of the metal unavailable for the reaction. 

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. 

In situ ETEM permits direct probing of particle sintering mechanisms and the effect of gas environments on supported metal-particle catalysts under reaction conditions. Here we present some examples of metals supported on non-wetting or irreducible ceramic supports, such as alumina and silica. The experiments are important in understanding metal-support interactions on irreducibe ceramics. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

The surface saturation by sulfur has to be compared to the irreversible adsorbed sulfur introduced by Menon and Prasad (22) and Apesteg-uia et al. (23). The study of H2S adsorption on supported catalysts was carried out by Menon and Prasad (22) and Apesteguia et al., Parera et al., and Barbier et al. and Marecot (23-25). For alumina supports, it was shown (23-25) that chlorine inhibits the adsorption of H2S on the support. Yet this adsorption on pure alumina is wholly reversible at 500°C, as is shown in Fig. 2. On Pt/Al203 at 500°C, only a fraction of the adsorbed sulfur is quickly desorbed in a hydrogen atmosphere. This result enabled the preceding authors (22-25) to develop the notion of reversible and irreversible adsorbed sulfur. The irreversible form, which does not exist on pure alumina, would interact with the metal. The quantity of irreversible sulfur, determined after 30 h of desorption under hydrogen flow at 500°C, does not depend on the sulfiding conditions (Table I). 

It is also possible to simulate supported metal catalysts by the vapor deposition of metal on a flat surface of silica, alumina, etc. The particle size distribution can be closely controlled and the results verified by various electron spectroscopies, for example (SI). For the reverse situation of a flat metal surface decorated by oxide particles, one can simulate catalysts in the strong metal-support interaction state (32). 

Another ion exchange procedure involves the interaction of a metal acetylacetonate (acac) with an oxide support. Virtually all acetylacetonate complexes, except those of rhodium and ruthenium, react with the coordinatively unsaturated surface sites of 7 alumina to produce stable catalyst precursors. On thermal treatment and reduction these give alumina supported metal catalysts having relatively high dispersions. 38 Acetylacetonate complexes which are stable in the presence of acid or base such as Pd(acac)2, Pt(acac)2 and Co(acac)3, react only with the Lewis acid, Al" 3 sites, on the alumina. Complexes which decompose in base but not in acid react not only with the Al 3 sites but also with the surface hydroxy groups. Complexes that are sensitive to acid but not to base react only slightly, if at all, with the hydroxy groups on the surface. It appears that this is the reason the rhodium and ruthenium complexes fail to adsorb on an alumina surface. 38... 

Model 23 R-16 (NiMo) is a new HDS promoted catalyst developed by ICERP to be used in desulphurization units in order to reach the new diesel fuel specifications. The catalyst is based on a new type of alumina obtained by an original preparing method which offers a correct interaction degree between metal and its support. The acidic property and the pore size were improved by the addition of promoteurs such as silica and phosphorus (P2O5). Some of the properties of new 23 R-16 (NiMo) promoted catalyst in comparison with the standard hydrofining catalyst are listed in Table 2. 

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,... 

The theoretical results have also indicated that when metal atoms are bound to specific defects their chemical activity may change, in particular can increase. This is likely to be true also for small metal clusters. This has not been fully appreciated so far. In fact, even inert supports, like silica, alumina, or magnesia, can interact strongly with the supported metal if this is bound at a defect site and can have a direct role in the chemistry of the supported species. Some preliminary calculations on supported clusters, however, suggest that the effect of the defect on the cluster electronic structure is restricted to very small, really nanometric clusters of about ten atoms size [224]. Should the size of active catalysts in real applications go down to this size, the specific interaction with the substrate could no longer be ignored in the interpretation of the catalytic activity. 

The carbon nanostructured support provides both a high activity and a high selectivity when compared to what is usually observed on traditional supports such as alumina or activated charcoal. Such catalytic behavior is attributed to the presence of a peculiar electronic interaction between the carbon nanofilaments and the metal which constitutes the active phase. This leads to a metallic site with unexpected catalytic performances [6,7]. In addition, due to their small dimensions, typically of about hundred of nanometers or less, the carbon nanofilaments display an extremely high external surface area which makes them a catalyst support of choice for liquid phase reactions. Due to the low difiusion coefficients of gaseous reactants in liquids, mass transfer phenomena become predominant in the liquid phase. The l%h external surfece area considerably decreases the... 

Supported metals are used extensively in heterogeneous catalysis. In the present investigation platinum is loaded onto titania and titania-alumina supports to study the SMSI effects in detail. The catalysts were characterized by X-ray Diffraction(XRD), Stepwise Temperature Programmed Reduction (STPR) and chemisorption measurements. All the samples exhibit eharacteristic behaviour showing SMSI effect after HTR, though there is only moderate interaction in the mixed oxide sample. From STPR studies, the reducibility of platinum and the support in supported platinum systems is shown to depend on the extent of the interaction at the interface. 

With supported metal catalysts that have to be treated in a reducing gas flow at elevated temperatures to convert the catalytic precursor into the desired metal, it is important to assess the extent of reduction. Often the oxidic phase of the cata-lytically active precursor is stabilized by interaction with the support. It is even possible for a finely divided precursor to react with the support to a compound much more stable than the corresponding metal oxide. An example is cobalt oxide, which can react with alumina to form cobalt aluminate, which is very difficult to reduce to metallic cobalt and alumina. Another example is silica-supported iron oxide. Usually the reduction of iron(III) to iron(II) proceeds readily, because the reduction to iron(II) is hardly thermodynamically limited by the presence of water vapor. Iron(ll), however, reacts rapidly with silica to iron(II) silicate, which is almost impossible to reduce. 

We consider now highly divided (FE > 0.5) supported metals, which may be model or actual working catalysts. Clusters supported by frozen inert gas matrices are not discussed here. Possible interactions with the support now complicate the interpretation of experiments, but we shall see that for silica and alumina the behavior of supported and unsupported clusters seem to be similar. 

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). 

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