Hierarchically porous metal oxides

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

Hierarchically porous metal oxide networks can be formed via a spontaneous self-formation phenomenon from metal alkoxides in aqueous solution [113]. Two chemical processes, hydrolysis and condensation, are involved in this spontaneous self-formation procedure to target hierarchically porous structures [114,115]. In fact, the hydrolysis and condensation rates are generally comparable for metal alkoxides [116]. The condensation rate is directly proportional to the rapid hydrolysis rate of reactive metal alkoxides [117,118]. It is well known that the rapid reaction rate of metal alkoxides plays the key role in the formation of hierarchically porous metal oxides [119,120]. The self-formation procedure to form hierarchically porous materials can be achieved by dropping liquid metal alkoxide precursors into an aqueous solution. In this section, the features of self-formation procediu-e and the resulting hierarchically porous materials are summarized. 

Recently, the self-formation phenomenon used for the formation of hierarchically porous metal oxides was ako applied to the formation of hierarchically micro-macroporous niobium oxides [12,141,144]. These synthesized niobium oxide particles with amorphous nature are mainly 100 in size with a regular array of parallel macropores (Figure 32.13a). The macropores with the pore size in the range of 0.3-10 pm extend through almost the whole particle. The macropore walls are constructed from accessible micropores. The hierarchically porous niobium oxides can be obtained via a self-formation phenomenon under different pH values (2, 7, and 12) [141,144]. 

From further studies, it is proved that the surfactants or polymers are not directiy involved in the formation of hierarchically porous structures. Hierarchically porous metal oxides can be obtained via a spontaneous self-formation procedure in the absence of any external template. 

Besides the biomaterials mentioned above, the cuttlebone [49] and chito-san [50] with unique structure have also been used as templates for the formation of hierarchically porous materials, which maintained the biological structure. Starch gel and dextran were also used to produce hierarchically sponge-like micro-, meso/macroporous monoliths of silicalite and meso/macro-porous metal oxides [51,52]. 

The search for better catalysts has been facilitated in recent years by molecular modeling. We are seeing here a step change. This is the subject of Chapter 1 (Molecular Catalytic Kinetics Concepts). New types of catalysts appeared to be more selective and active than conventional ones. Tuned mesoporous catalysts, gold catalysts, and metal organic frameworks (MOFs) that are discussed in Chapter 2 (Hierarchical Porous Zeolites by Demetallation, 3 (Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature), and 4 (The Fascinating Structure... 

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. 

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. 

Rooke, J.C., Barakat, T., Finol, M.F., Billemont, P., De Weireld, G., Li, Y., Cousin, R., Giraudon, J.M., Siffert, S., Lamonier, J.F., and Su, B.L. (2013) Influence of hierarchically porous niobium doped Ti02 supports in the total catalytic oxidation of model VOCs over noble metal nanoparticles. Appl. Catal. B Environ., 142-143, 149-160. 

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. 

Route C mobilizes routes A and B to combine complex precursor systems (aerosol cocktails) in different ratios and yield multifunctional materials with hierarchically structured porosity [18-21]. For instance, latex beads can be combined with a diluted sol-gel surfactant dispersion that contains small inorganic nanoparticles of metallic oxides, metals, and so on. This approach then leack by EISA to micrometer-sized porous spheres with an independent control of dimensions at four levels of size. 

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. 

Tunable chemical composition Hierarchically porous materials with single or multiple chemical compositions can be prepared via this spontaneous selfformation procedure by using single or multiple alkoxide precursors. Pure oxide materials can be obtained by starting with just the alkoxide or alkylme-tal precursors in aqueous solutions, as opposed to other synthesis strategies where the resulting products are often contaminated by residual species. Furthermore, metal phosphates with hierarchically porous structures can also be prepared by this self-formation process. 

It is found that hierarchically meso/macroporous metal oxides can be synthesized even without the use of any external macrotemplate. In fact, great efforts have been made by scientists to promote development of hierarchically porous materials via the spontaneous self-formation phenomenon from metal alkoxides during the past decade. In this section, we will review the history of self-formation phenomenon to target hierarchically porous materials based on metal alkoxides. 

Meso-Macroporous Mixed Oxides Multicomponent oxides play a central role in chemical and petrochemical processing as catalysts and as supports for catalytically active species [145]. It is known that the catalytic efficiency of metal oxides can be improved by doping them with a metal or combining them with another metal oxide [145]. The strategy based on the self-formation phenomenon to fabricate the porous hierarchy demonstrated its simplicity and superiority in the synthesis of hierarchically meso-macroporous metal oxides with multiple compositions. 

For all processes that involve the adsorption step, such as physical processes of separation or catalytic transformations, the usage of solid materials with optimised activity as adsorbents and catalysts is necessary. Various solids, such as porous materials (zeolites—molecular sieves with hierarchical porosities and natural clays), activated carbons, mesoporous silica-based materials, pillared clays and metal oxides, have shown the ability to act as adsorbents or as catalysts for the conversions of previously mentioned atmospheric pollutants. Solid materials are also used for the removal of pollutants that can be found in wastewaters. The possibilities to remove polyaromatic hydrocarbons (PAHs) and heavy metal particles using the adsorptive characteristics of activated carbon and porous materials from wastewaters have been proven [15-17]. The same classes of solids are used for the elimination of organic pollutants form wastewaters by heterogeneous catalytic oxidation processes one of the most important tasks is to eliminate phenolic compounds [13]. 

Because of the attractive physicochemical properties and potential applications in catalysis, biotechnology, adsorption, and separation, fabrication of hierarchically porous (macro/mesoporous) materials, especially for the three-dimensional ordered macro/ mesoporous (3DOM) materials, has been a focus in the research on materials science and engineering in recent years [99,199,200], By using close-packed arrays of monodisperse spheres, such as polystyrene (PS), poly(methyl methacrylate) (PMMA), and silica as template, metals [201,202], metal oxides [203-208], metal chalcogenides [209], silica [204,210,211], carbon [212,213], polymers [214,215], and hydroxyapatite [216] with 3DOM structures have been generated. 

The above approach of integrating analytically (under certain assumptions) across the porous wall the species balances to obtain local soot consumption rates can be extended for the case of more reactions occurring in the porous wall. In the presence of a precious metal catalyst, the hydrocarbons and the carbon monoxide of the exhaust gases are also oxidized. It can be assumed that all the reactions in the porous wall occur hierarchically (according to their... 

Nb-based catalysts are among the investigated systems in total removal of -butanol due to the capacity of niobium to adopt variable oxidation states. Thus, Nb-doped hierarchically micro(meso) macroporous Ti02 (anatase phase) (Figure 17.10) synthesized via a self-formation procedure showed close performances with catalysts in which the same materials served as supports for noble metals [46]. The efficiency of these catalytic systems in the total oxidation of butan-l-ol was enhanced by the improved diffusion through the intrinsic macro-porous network. An effect of the niobium content was evidenced as welL... 

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