Oxidic support materials

The raw materials needed to supply about ten million new automobiles a year do not impose a difficult problem except in the case of the noble metals. Present technology indicates that each car may need up to ten pounds of pellets, two pounds of monoliths, or two pounds of metal alloys. The refractory oxide support materials are usually a mixture of silica, alumina, magnesia, lithium oxide, and zirconium oxide. Fifty thousand tons of such materials a year do not raise serious problems (47). The base metal oxides requirement per car may be 0.1 to 1 lb per car, or up to five thousand tons a year. The current U.S. annual consumption of copper, manganese, and chromium is above a million tons per year, and the consumption of nickel and tungsten above a hundred thousand tons per year. The only important metals used at the low rate of five thousand tons per year are cobalt, vanadium, and the rare earths. 

Besides supported (transition) metal catalysts, structure sensitivity can also be observed with bare (oxidic) support materials, too. In 2003, Hinrichsen et al. [39] investigated methanol synthesis at 30 bar and 300 °C over differently prepared zinc oxides, namely by precipitation, coprecipitation with alumina, and thermolysis of zinc siloxide precursor. Particle sizes, as determined by N2 physisorpt-ion and XRD, varied from 261 nm for a commercial material to 7.0 nm for the thermolytically obtained material. Plotting the areal rates against BET surface areas (Figure 3) reveals enhanced activity for the low surface area zinc... 

It was not until recently that Chen and Goodman probed the influence of the oxide support material on the intrinsic properties at the metal surface. By covering a titania support with one or two flat atomic layers of gold they eliminated, direct adsorbate-support interactions as well as particle size and shape effects. Their results definitively showed that the electronic properties at the metal surface changed due to charge transfer between the support and the metal. Furthermore, their comparison of one- and two-layer films highlighted the dependence of these effects on the thickness of the metal slab. 

Section I reviews the new concepts and applications of nanotechnology for catalysis. Chapter 1 provides an overview on how nanotechnology impacts catalyst preparation with more control of active sites, phases, and environment of actives sites. The values of catalysis in advancing development of nanotechnology where catalysts are used to facilitate the production of carbon nanotubes, and catalytic reactions to provide the driving force for motions in nano-machines are also reviewed. Chapter 2 investigates the role of oxide support materials in modifying the electronic stmcture at the surface of a metal, and discusses how metal surface structure and properties influence the reactivity at molecular level. Chapter 3 describes a nanomotor driven by catalysis of chemical reactions. 

Si02 is the best catalyst support for this reaction. As AI2O3 is incorporated into the Si02 the catalyst activity and selectivity decreases significantly. In these mixed oxide support materials Bronsted acid sites favour acylation, whilst the presence of Lewis acid sites results in hydrolysis of the anhydride. 

Reproducibility of film preparation and stability of the resulting films are important issues for practical applications. Cleanliness of the IRE before metal deposition can play a decisive role in determining reproducibility. Depending on the conditions, metal films may not be stable and may peel off (36,37). The stability and reproducibility of metal films can be enhanced by evaporating a metal oxide support material (such as AI2O3) prior to evaporation of the desired metal. Contaminants on the IRE are covered or displaced by evaporation of the metal oxide. It was reported that a 50-100-nm-thick AI2O3 layer deposited on a Ge IRE by electron beam physical vapor deposition hardly affected the reflectivity in an ATR experiment. Thin platinum films directly deposited onto it were found to be rather stable under catalytic reaction conditions (26,38). 

A simple oxide catalyst can be used in either the bulk state or supported on an inert oxide support material. The bulk oxides are usually prepared using a precipitation-calcination sequence similar to those described in Chapter 9 for the preparation of support oxides. " In general, the simple semiconductor oxides are not very good catalysts for synthetic reactions. The insulator oxides, however, can be used as solid acids and bases for a number of reactions. Alumina has been used as an acid catalyst for the vapor phase rearrangement of cyclohexanone oxime to caprolactam (Eqn. 10.9). Modification of the y-alumina surface by the addition of 10-20% of B2O3 increased its activity for this reaction, giving caprolactam in 80% selectivity even after several hours of continuous operation. "... 

To determine whether cobalt oxide support materials may be able to facilitate oxidation reactions we conducted reactivity experiments on cobalt oxide clusters with CO. Oxygen atom transfer to CO was observed for all anionic cobalt oxide clusters containing one to three cobalt atoms. 

Series of supported Ru, Pt, and Au catalysts have been tested for the selective oxidation of CO. Table 3 shows a comparison of three typical catalysts reported recently. Since Pt group metals are essentially more active for H2 oxidation than for CO oxidation, support materials [77,78] and bimetallic systems [79] were exploited to suppress H2 oxidation. Better selectivities to CO oxidation (around 50%) are... 

Fig. 1.56. In the example depicted in this figure, a bifunctional catalyst like platinum on silicon oxide enables the isomerization of methyl-cyclopropane to buten-2 as well as its hydrogenation to butane [349]. The isomerization proceeds on the oxide support. Hydrogen dissociates on the metal particle and the H-atoms spillover onto the oxide support material where they are mobile to react with butene...

The most relevant results on catalytic oxide support materials have been summarized in Table 6. 

The manufacture of heterogeneous catalysts from pre-prepared nanometal colloids as precursors via the so-called precursor concept ll has attracted industrial inter-est.l l An obvious advantage of the new mode of preparation compared with the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors can be tailored for special applications independently of the support. In addition, the metal particle surface can be modified by lipophilic or hydrophilic protective shells, and covered with intermediate layers, e.g. of oxide. The addition of dopants to the precursor is also possible. The second step of the manufacture of the catalyst consists in the simple adsorption of the pre-prepared particles by dipping the supports into organic or aqueous precursor solutions at ambient temperature. This has been demonstrated, e.g., for charcoal, various oxidic support materials, even low-surface materials such as quartz, sapphire, and highly oriented pyrolytic graphite. A subsequent calcination step is not required (see Fig. 1). 

Ways to circumvent these issues will be (i) the application of alternative polymeric or oxidic support materials, (ii) rigorous control of the operation conditions, and (iri) surface modification and/or graphitization ofthe routinely applied carbon supports. Since most metal oxides offer only comparatively low electronic conductivities, are difficult to disperse into an ink for electrode preparation and also form... 

In nanoparticle electrocatalysis, the area that Michael entered just some time ago in Munich, he and his coworkers rationalized the sensitivity of electrocatalytic processes to the stmcture of nanoparticles and interfaces. Studies of catalytic effects of metal oxide support materials revealed intriguing electronic structure effects on thin films of Pt, metal oxides, and graphene. In the realm of nanoparticle dissolution and degradation modeling, Michael s group has developed a comprehensive theory of Pt mass balance in catalyst layers. This theory relates surface tension, surface oxidation state, and dissolution kinetics of Pt. 

In the following, the different methods to create libraries of oxide materials will be discussed. We place only little emphasis on the synthesis of supported catalysts or modified zeolites by ion-exchange, wet impregnation, or other methods. Such procedures usually rely on custom manufactured prefabricated, typically oxidic support materials and oxides are not necessarily formed. To a much larger extent than in the carrier beads, which are used in the split-and-pool approach (see Section 12.3.6), the support material plays an essential role in the final materials, be it for high dispersion of a noble metal compound or by providing catalytically active sites as in a zeolite. 

However, while the above was a rather crude approach to fabricate more porous electrode layers with oxidic support materials, a more elegant way was chosen for home-made ATO by 3D morphology engineering. ATO powder with unique hollow-sphere morphology was synthesized by ultrasonic spray p)irolysis (USP). Depending on precursor concentration and temperature, this process yields a powder composed of individual nano-crystallites forming the shells of hollow spheres with a controlled nano- and microporosity [98]. This offers efficient mass transport and is assumed to prevent the collapse of the electrode structure with time during operation. 

Considerable progress has been made in model catalysts of planar well-ordered extended surfaces, but under UHV conditions. Atomic structure and electronic properties of many metal surfaces or metal oxide-supported materials are now accessible, related to the physical and chemical properties of adsorbate species on surfaces, in particular binding sites. However, there are several problems related to the application of models with real catalysts, such as metal-support interactions and... 

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