Macroporous transition metal oxide

Figure 2. Synthetic scheme for macroporous transition metal oxides. Double templating is achieved by the co-existence of rod-like micelles and vesicles or microbubbles.

SEM and TEM investigations of macroporous and toroidal mesostructured transition metal oxides... 

Ordered macroporous materials with pore sizes of more than 50 nm appeared in the late 1990s with the development of a method using colloidal crystals of monodisperse spheres as a new template.The walls of macroporous materials are larger than those of mesoporous materials, and a number of well-ordered macroporous crystalline transition metal oxides have been prepared.The preparation method... 

In the case of other transition metals, their alkoxides are not suitable as metal sources. Furthermore, a two-step method in which deposited metal salts are solidified by a reaction with oxalate or a base is not suitable for the preparation of ordered macroporous mixed metal oxides. Each metal has a different reactivity with oxalic acid or a base, and the produced oxalate or metal hydoxide has different solubilities in the reacting media, which causes a mixed metal oxide with an undesired metal ratio.On the other hand, in situ methods, in which an additive such as EG, citric acid or EDTA is present with mixed metals, ensure the chemical homogeneity of the products and are suitable methods for producing ordered macroporous mixed metal oxides with a desired ratio. Synthesis methods, structural characterisation and applications of macroporous mixed metal oxides are summarised in Table 3.4. 

The synthesis of ordered macroporous crystalline materials has been attracting much attention. Walls of macroporous materials are larger than those of mesoporous materials, and this macroporosity can be introduced into a wide variety of transition metal oxides. Potential applications of these materials include photonic materials, catalysts and electrode materials. The ordering scale is close to the wavelength of light, and interest has therefore been shown in photonic materials. In some cases, introduction of macroporosity increases the surface area, and these materials show better catalytic performance than that of nonporous materials. Similar to mesoporous materials, macropores are favoured for diffusion of reactants compared with nonporous materials and many applications, such as in a Li battery electrode, have been reported. 

Sadakane, M., Sasaki, K., Nakamura, H., Yamamoto, T., Ninomiya, W., and Ueda, W. (2012) Important property of polymer spheres for the preparation of three-dimensionally ordered macroporous (3DOM) metal oxides by the ethylene glycol method the glass-transition temperature. Langmuir, 51, 17766-17770. 

Activity and selectivity of monometallic Ag catalysts can be controlled by the preparation conditions leading to micro- and meso- to macroporous catalysts which are active and selective in the hydrogenation of crotonaldehyde. In Ag catalysts modified by a second metal, bimetallic sites exhibiting surface polarity and Ag particles in close contact with a partially reduced early transition metal or a rare earth element, or Ag species stabilized and incorporated in these oxides were concluded to be the active species in the working state of these catalysts. Simultaneous introduction of both metals during the sol-gel process under optimized hydrolyzing conditions could further increase the metal-promoter interaction and lead to well-tailored new hydrogenation catalysts. 

Holland et al. extended the possible oxide structures to include not only silica, mesoporous silica, titania, zirconia, a yttria stabilized zirconia, and alumina but also oxides of W, Fe, V, and Sb [21]. These latter transition metals formed less ordered structures, containing areas of non-porous material. Different dilutions of alkoxide in alcohol resulted in various inorganic loadings, and moderate control in the wall thickness and window sizes between spherical voids [21 ]. SEM images of a series of macroporous titania structures obtained with different alkoxide dilutions in ethanol are shown in Fig. 3. Gundiah and Rao have also prepared macroporous materials of ternary mixed oxides, PdTiOj and Pb(ZrTi)03 [22]. 

Like alkoxides of late transition metals, lanthanide alkoxides are not suitable for production of ordered macroporous lanthanide oxides. Common lanthanide nitrates are also not suitable. Addition of citric acid or EDTA is necessary to produce ordered macroporous LaiOlCOsli, La203, Ce02, Nd202C03, Nd203, Sm203, and EU2O3 materials. ... 

To overcome the stability problem, it is obvious that an alternative approach to material synthesis would be needed. In this respect a recent report suggested that iron oxide encapsulated in meso- and macroporous carbon can be used as anode in Li batteries and reaches a greatly improved reversibility and rate performance. At the same time, a similar structure for a cathode material based on iron and lithium fluoride was synthesized and investigated. It was demonstrated that encapsulation of transition metal-metal fluoride in nanocarbon might be an effective strategy to improve the cycling performance of such a cathode material. ... 

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