Nanocasting oxides with

Figure 17.9 TEM images of C03O4 nanocasted oxides. (Reprinted with permission from Ref. [31]. Copyright 2010, Elsevier.)...

The performance of various nanocasted oxides v/as studied by Vasconcelos et al. in the gas-phase dehydration of glycerol to produce hydroxyacetone (Table 13.7). The CeZr catalyst exhibited the best catalytic performance, CeOs being the worst. The binary oxide CeOj-ZrOg, with moderate basicity, proved to have a cooperative acid-base character in the Ceo.8Zro202 phase, resulting in a higher production of hydroxyacetone. 

It is well known that the nanocasting strategy with mesoporous silica or carbon as hard template is an effective technique to fabricate mesoporous metal and metal oxides [236-238]. 

Ordered mesoporous silica seems to be an ideal hard template, which can be used as a mold for other mesostructures with various compositions, such as ordered mesoporous carbon and metal oxides. Mesoporous silicas with various different structures are available, and silica is relatively easily dissolved in HF or NaOH. Alternatively, mesoporous carbons with a solid skeleton structure are also suitable choices as hard templates due to their excellent structural stability on thermal or hydrothermal and chemical treatment. A pronounced advantage of carbon is the fact that it is much easier to remove than silica by simple combustion. The nanocasting synthesis of mesoporous carbon by using mesoporous silica as template will be discussed in detail in the section on mesoporous carbon. In many cases, silica is unsuitable for synthesizing framework compositions other than carbon, since the leaching of the silica typically affects the material which is filled into the silica pore system. 

Besides cooperative pathways, also tme liquid crystal templating (TLCT) and the hard template route (Section 9.3.7) have been developed for the synthesis of ordered mesoporous materials. In the case of the TLCT, a preformed surfactant liquid crystalline mesophase is loaded with the precursor for the inorganic materials (140). The nanocasting route, on the other hand, is a clearly distinct method (141). Here, no soft surfactant template is used but, instead, the pore system of an ordered mesoporous solid is used as the hard template serving as a mold for preparing varieties of new mesostructured materials, for example, metals, carbons, or transition metal oxides. 

Many experiments were designed to obtain ordered mesoporous materials with completely different compositions of the network no longer correlated to silica. Also here, nanocasting is beneficial. Due to their high relevance in many areas of catalysis and their variable redox- and magnetic properties, much work was devoted to the creation of stable ordered mesoporous transition-metal oxides. In the meantime, many compositions with Ti, Zr, V, Ta, Mo, W, Mn, and Y, as the central element were introduced." ... 

Nanocasting can be considered as the best method to obtain a narrow pore size distribution. This is a very important feature because carbon supports with tunable properties allow the modeling of the system, in order to optimize the processes of methanol oxidation (adsorption and re-dissolution) and mass transport in fuel cells. 

The use of oxide-type perovskites as dry reforming catalysts will be dealt with extensively in Volume II of this book. In this chapter, we report one example of such catalyst in order to highlight the potentiahty of the nanocasting preparation method as a way to control bulk and smface properties of perovskite-derived catalysts. 

For many oxides, including mixed metal oxides, of catalytic interest, preparing a high-surface-area solid with thermally stable porous structure was extremely difficult until recentiy. The hard templating procedure provides a systematic solution to this problem. Nanocasting that makes use of a mesostructured solid template is a special case. Both mesostructured silica and mesostructured carbon have been demonstrated so fer as hard templates. Rather precise replicas of these nanomolds have been obtained for a variety of oxide-type perovskites. Unprecedented specific surface areas in the 150-200 m /g were reached. 

New Au C materials have been obtained using a nanocasting method, starting form preliminary functionalized silicas. Excellent dispersions were obtained, with Au particles partially embedded in carbon walls. This route is very promising for the preparation of catalysts for the aerobic oxidation of olefins in the liquid phase. 

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