Hierarchically metal oxides

An excellent example that shows the potential of combining various bottom-up techniques is the joint work by Whitesides and Stucky [4]. Hierarchical metallic oxides were produced by combining (i) sol-gel self-assembly of neutral surfactants, (ii) spherical polystyrene templates, and (iii) molds with micrometric cavities (micromolding). Figure 3.12 shows how the described materials are hierarchically organized at several scales ranging from a few nanometers to hundreds of micrometers. 

Mesoporous, Macroporous, and Hierarchical Metal Oxide Structures... 

Figure 33.10 Hierarchical metal oxide structure composed of a Si02-CTAB/Ti02-F127 ID photonic structure with Ag inclusions in TI02 slab, (a) STEM image, (b) XRR and SAXS signals. (c) UV-vis response, (d) Raman spectroscopy signal for the periodic and nonperiodic...

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

The area of catalyst immobilization has received considerable attention as can be judged from the available literature reviews.[1 30] Immobilization of oxidation catalysts shows intrinsic advantages over other catalysts as the tendency for selfoxidation will decrease. Moreover, complexes with generally low solubility, such as heme-type transition metal complexes, can be dispersed molecularly on supports. It is the aim of the present work to overview the state of knowledge on the immobilization of transition metal complexes using microporous supports, such as zeolites and laminar supports like clays. The wealth of information available for complexes immobilized on LDHs or tethered to the mesopore walls in hierarchically organized oxides will not be dealt with. 

Soler-Illia, G.J.A.A. Sanchez, C. Lebeau, B. Patarin, J. Chemical strategies to design textured silica and metal oxide-based organised networks from nanostructured networks to hierarchical structures. Chem. Rev. 2002, 102, 4093. 

Conducting polymers can be mixed with or electrochemically deposited onto transition metal oxides to achieve better performances as electrode materials. However, the wrapping of conducting polymers on metal oxides may reduce the effective contact area between metal oxides and electrolytes. This will further impede insertion of alkali ions into metal oxides and result in poor rate performance with low specific capacitance. A rational strategy is to use conducting polymers as active electrode materials directly and optimize their structures. To this end, a hierarchically nanostructured conductive polymer hydrogel was synthesized by a facile interfacial polymerization (Shi et al., 2014). 

Cell assemblies. As macroporous templates, these provide a facile bioinspired method for the synthesis of hierarchical macro-mesoporous titania with tunable macroporous morphology and enhanced photocatalytic activity [134]. This is also a simple and facile technique that can be used to prepare many types of metal oxide porous materials with good control over the pore size and morphology. 

Metal oxides are found in rocks and minerals, where atoms or ions are packed together in a regular fashion. Nature combines hard and soft materials in hierarchical... 

E.S. Toberer, J.D. Epping, B.F. Chmelka, R. Seshadri, Hierarchically porous rutile titania harnessing spontaneous compositional change in mixed-metal oxides, Chem. Mater. 18 (2006) 6345-6351. 

More complex in preparation and expensive, hierarchically nanostructured porous group V metal oxides synthesized via a spontaneous autoformation process from alkoxide precursors were used as supports for palladium in total oxidation of toluene (Figure 18.8) [40]. In addition to the effect of the support cation, multimodal porosities of these supports facilitate both the dispersion of the noble metal nanoparticles and the internal transport of the substrates. 

Rooke, J.C., Barakat, T., Brunet, Li, Y Finol, M.F., Lamonier, J.-F., Giraudon, J.-M., Cousin, R., Siffert, S., and Su, B.L. (2015) Hierarchically nanostructured porous group Vb metal oxides from alkoxide precursors and their role in the catalytic remediation of VOCs. Appl Catal B Environ., 162, 300-309. 

Smatt, J.-H., Weidenthaler, C., Rosenholm, J.B., and Linden, M. (2006) Hierarchically porous metal oxide monoliths prepared by the nanocasting route. Chem. Mater.,... 

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