Nanostructured supports

The utilization of large surface areas and, to a certain extent, controllable surface properties make carbon materials an ideal support for finely dispersed catalyst nanoparticles, as discussed in Section 15.2. The special features of nanocarbons for this purpose will be highlighted in the following section. Starting with the controlled synthesis of a variety of nanocarbon-inorganic hybrids, some examples will be discussed, where the superior catalytic performance arises from the unique properties of the nanostructured support. 

The following sections are organized into two parts. The first presents a brief overview of several approaches to preparing nanostructured supports and catalysts that are under current development. Approaches that incorporate building blocks in either part of the system (catalyst or support) will be highlighted. The second part describes a new approach to preparing nanostructured catalysts that we are currently developing in our laboratories. 

The features of the more important characterisation techniques described here are summarised in Table 4.2. As is evident from this review, there is no technique which is universally applicable for the characterisation of the porous properties of all materials. The choice is made on the basis of many criteria, such as the range of pore size, the nature of the material and its form, together with the application envisaged. Frequently, more than one technique is required in a detailed examination. In the case of membranes, particular problems are encountered because of their form and the small quantity of active material involved. Furthermore, other complexities arise in the case of microporous thin films, although, as we have noted here, currently this is an area of active progress. This involves the development of new techniques and advances in phenomenological theories to describe the properties of such nanostructured supported materials. 

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... 

For the anode and cathode catalyst development, the unique nanostructured support and catalyst coating methods offer many combinations of materials and process conditions to generate new... 

M.S. Wong, Nanostructured supported metal oxides, in J.L.G. Fierro (Ed.) Metal oxides Chemistry and applications, Taylor Francis Group, LLC Boca Raton, FL, pp. 31-54, 2006. 

Several other examples have been reported in the literature in which one single BCP has been used. On the contrary, studies using blends are rather scarce. Krausch and coworkers [57] adsorbed a linear ABC triblock copolymer of poly(styrene-b-2-vinylpyridine-b-tert-butylmethacrylate) onto the SiO surface of a silicon wafer to fabricate a nanostructured support and analyze the microphase separation of a polymer blend on this substrate. The BCP used exhibits a lateral microphase separation as a consequence of the strong affinity of the middle block [ie, poly(vinylpyrrolidone) (PVP)] to the boundary surface. Interestingly, the phase separation of the homopolymers... 

When precursors are sonicated in high-boiling alkanes, e.g., decane or hexa-decane, nanostructured powders are formed. Using a polymeric ligand [e.g., polyvinylpyrrolidone (PVP)] or inorganic supports (silica, alumina, etc.), nanophase metal colloids, and nanostructured supported metal catalysts are generated, with very interesting catalytic activity. 

Figure 3. Labeling of the different inequivalent sites of Fe nanostructures supported on Fe(lOO) as used in Table V.

The high cost of platinum means that the car industry strives to reduce the amount of catalyst required in a PEM fuel cell. Current strategies focus on the use of PtM nanoparticles (M = another ri-block metal), nanoparticles with a PtM core encased in Pt atoms, and thin Pt films dispersed on nanostructured supports. Iron-based catalysts would be much cheaper, but their performance is usually poor. A promising advance (still at the research stage) is in the application of microporous carbon-sup-ported iron-based catalysts in which the iron cations are thought to be coordinated in Fe(phen)2p sites, the phenanthroline-units being incorporated into graphitic sheets. ... 

Although it can be concluded from the above evidence that greater edge plane exposure and number of defected sites usually have a positive impact on catalytic activity of the carbon nanostructured supports, it is also known that the corrosion of carbon materials is initiated at these edge planes. This relationship between catalyst support enhancement and carbon corrosion needs to be kept in mind when developing novel nanostructured carbon catalysts. Unfortunately, the influence of defects in CNTs and CNFs on the durability of these structures is stiU not quite known and little research is published in this area. 

Examples of dense silica, hybrid silica, metal oxides, solid-state metal oxide solutions, or colloidal self-assembly are unlimited. However, the recent developments to accurately control processing conditions (e.g., atmosphere, temperature, and motion) led to films with unique properties (see Figure 9.6) [52,53]. These progresses concern mesoporous coatings with controlled pore size and structure [26], hard template infiltration and/or replication [54-58], nanostructured epitaxial low-quartz thin films [59], ultrathin nanostructured supported networks [60,61], ultrathick porous Ti02 layer prepared from aqueous solutions [51], coatings with hierarchical porosity [62], multilayer porous stacks [63], colloidal MOF layers [64,65], pillar planar nanochannels (PPNs) for nanofluidics [66], and so on. 

Formo E, Peng ZM, Lee E, Lu XM, Yang H, Xia YN (2008) Direct oxidation of metheinol on pt nanostructures supported on electrospun nanofibers of anatase. J Phys Chem C 112(27) 9970-9975... 

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