Macroporous transition metal oxide preparation

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. 

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]. 

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