Catalyst-support interactions silica supported metals

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. 

S. H. Overbury, L. Ortiz-Soto, H. G. Zhu, B. Lee, M. D. Amiridis, and S. Dai, Comparison of Au catalysts supported on mesoporous titania and silica Investigation of Au particle size effects and metal-support interactions, Catal. Lett. 95(3-4), 99-106 (2004). 

The focus of these studies has been on identifying mild activation conditions to prevent nanoparticle agglomeration. Infrared spectroscopy indicated that titania plays an active role in dendrimer adsorption and decomposition in contrast, adsorption of DENs on silica is dominated by metal-support interactions. Relatively mild (150° C) activation conditions were identified and optimized for Pt and Au catalysts. Comparable conditions yield clean nanoparticles that are active CO oxidation catalysts. Supported Pt catalysts are also active in toluene hydrogenation test reactions. 

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. 

Among the transition metals, Pd, Pt, Ir, and Rh in various forms have been reported active in methanol synthesis (32-34), as noted in Section II. Palladium, platinum, and iridium metals were supported on silica, and it has recently been suggested that palladium is present in its valence state Pd(II) which is the active form of the catalyst (70). Under the synthesis conditions the Pd(II) ions could not survive the highly reducing atmosphere of the CO/H2 synthesis gas, and so this valence state would have to be induced by the presence of silicon dioxide. Should this be a general case, silica would not act merely as an inert support, and the silica-supported transition metals would have to be considered binary catalysts whose active state is formed by a support-metal interaction. ... 

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). 

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. 

It has been reported that methanol is formed from C0-H2 reaction over silica-supported Pd, Pt, Ir catalysts (5). The behaviour of the metal was found to be influenced by the carrier (6,7,8, 9). The selectivity in methanol was discussed in terms of acid-base properties of the support which influenced the non dissociative adsorption of CO on the metal required for oxygenated hydrocarbon formation, in terms of electronic interaction between the metal and the support or in terms of stabilization by the carrier of oxidized metal cations which would adsorb CO non dissociatively. We have studied the C0-H2 reaction at 553 K, 30 atmospheres over Pt supported on a variety of oxides. The characteristics of the catalysts are given in table 2... 

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. 

Various Pd based catalysts supported on silica and containing manganese have been prepared and characterized. Two Mn species have been detected, i) reduced Mn in direct interaction with Pd on the metallic particles ii) oxidised Mn layed on Si02 and showing reactive oxygens An increase in activity for reduction of NO is observed on the catalysts containing Mn with an optimum for a Pd/Mn atomic ratio of about one. This enhancement of activity is due to either the presence of Pd-Mn dual sites or to a bifiinctional mechanism between reduced Pd and oxidised Mn at the vicinity of Pd. 

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. 

Metals or metal oxides are usually supported on the supports such as silica and alumina when they are used as catalysts. The deposition of metal species on the supports often results in the improvement of catalytic activity and selectivity, and/or in the inhibition of their sintering at high temperatures due to the chemical interaction between the metal species and the supports. Because the supported metal or metal oxides are prepared conventionally with an impregnation method, the metal species are mainly supported on the surface of supports. On the other hand, we have prepared the supported metal catalysts by using microemulsion systems [1,2]. The preparation methods can produce the metal or metal oxide particles uniformly covered with silica layers. Thus, the metal species interact strongly with silica. In addition, because metals or metal oxides which work as active sites for the catalytic reactions are covered with silica layers with porous structures, new functions such as shape selectivity could be expected in the catalytic reaction over them. 

A great variety of oxides can be used as supports. These materials are chemically stable, but in some cases, interactions between the metallic particles and the support can occur. Thus, in the particular case of metal catalysts supported on some reducible oxides, the occurrence of so-called metal-support interaction effects has been reported. " " In order to minimize the metal-support interaction, stable oxides (not reducible), such as alumina or silica, are used. Moreover, the size-dependent electronic, structural, and chemical properties of metal nanoparticles on oxide supports are an important aspect of heterogeneous catalysis. " " ... 

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