Transition metal oxides oxide materials Mesoporous

Both aluminum oxide and zirconium oxide are catalytically interesting materials. Pure zirconium oxide is a weak acid catalyst and to increase its acid strength and thermal stability it is usually modified with anions such as phosphates. In the context of mesoporous zirconia prepared from zirconium sulfate using the S+X I+ synthesis route it was found that by ion exchanging sulfate counter-anions in the product with phosphates, thermally stable microporous zirconium oxo-phosphates could be obtained [30-32]. Thermally stable mesoporous zirconium phosphate, zirconium oxo-phosphate and sulfate were synthesized in a similar way [33, 34], The often-encountered thermal instability of transition metal oxide mesoporous materials was circumvented in these studies by delayed crystallization caused by the presence of phosphate or sulfate anions. 

A different approach was reported by the Wang group. They observed that, when comparing the catalytic behavior of transition metal oxides deposited over mesoporous materials, the Cu-containing catalyst was the most effective. After this preliminary result, they studied the influence of the catalyst preparation on catalytic behavior, reporting a formaldehyde selectivity of ccl 60-70% at methane conversion of 2% when working at 500-650°C. They proposed a relatively different mechanism with the stabilization of Cu"+ species during the catalytic tests. ... 

In the following section, we restrict our discussion to templated mesoporous solids that are of potential interest as battery electrodes, including many transition-metal oxides and carbon. This slice of the literature still points the interested reader to many articles on the synthesis and physical characterization of relevant mesoporous materials. A much smaller number of electrochemical studies with templated mesoporous electrodes have been published, and these studies in particular will be noted. 

Since pure mesoporous silica phases does not show any catalytic activity many successful attempts have been made to vary the inorganic composition towards transition metal oxides or metal chalcogenides [5-12], In particular the semiconducting properties of the latter offer a great range of possible applications in materials chemistry. 

Mesoporous materials with a transition metal oxide framework have immense potential for applications in catalysis, photocatalysis, sensors, and electrode materials because of their characteristic catalytic, optical, and electronic properties. However, for some applications, this potential can only be maximized in the highly crystalline... 

In order to elucidate the importance of the role of in situ formed carbon in the formation of well-organized, highly crystalline mesoporous transition metal oxides, as-synthesized Ti02 was directly calcined under air to 700°C while keeping all other conditions the same as for the CASH method. As expected, the BET surface area of the resulting material was only 0.2 m2 g-1 and no porous structure could be detected by TEM imaging. This implies that the mesostructure completely collapsed. The crystallite size of this sample, heat treated to 700°C in air is 31.5 nm (calculated... 

Since mesoporous materials contain pores from 2 nm upwards, these materials are not restricted to the catalysis of small molecules only, as is the case for zeolites. Therefore, mesoporous materials have great potential in catalytic/separation technology applications in the fine chemical and pharmaceutical industries. The first mesoporous materials were pure silicates and aluminosilicates. More recently, the addition of key metallic or molecular species into or onto the siliceous mesoporous framework, and the synthesis of various other mesoporous transition metal oxide materials, has extended their applications to very diverse areas of technology. Potential uses for mesoporous smart materials in sensors, solar cells, nanoelectrodes, optical devices, batteries, fuel cells and electrochromic devices, amongst other applications, have been suggested in the literature.11 51... 

Non-aqueous synthetic methods have recently been used to assemble mesoporous transition metal oxides and sulfides. This approach may afford greater control over the condensation-polymerization chemistry of precursor species and lead to enhanced surface area materials and well ordered structures [38, 39], For the first time, a rational synthesis of mesostructured metal germanium sulfides from the co-assembly of adamantanoid [Ge4S ()]4 cluster precursors was reported [38], Formamide was used as a solvent to co-assemble surfactant and adamantanoid clusters, while M2+/1+ transition metal ions were used to link the clusters (see Fig. 2.2). This produced exceptionally well-ordered mesostructured metal germanium sulfide materials, which could find application in detoxification of heavy metals, sensing of sulfurous vapors and the formation of semiconductor quantum anti-dot devices. 

In recent years ordered mesoporous materials have attracted great attention [1] for their interesting property [2] and these materials have numerous potential applications in many fields such as separation, catalysis, and biomaterial engineering [3,4]. Much interest is being focused on the preparation of transition-metal oxides using several templating pathways besides silica-based materials. 

For redox catalysis, efforts have been spent on preparing transition metal modified mesoporous materials. These materials are capable of extending the catalytic oxidation chemistry to large molecules. The selective catalytic activity has also been demonstrated, for example, in the oxidation of aromatic compounds by using titanium-containing mesoporous silica (Ti-MCM-41 and Ti-HMS). ... 

It seems to be possible that most transition metal oxides can be made in porous crystals with different morphologies using various mesoporous silicas as templates. It is expected that these materials have potential in applications such as catalysis, fuel cell, gas sensors and Li-batteries. Their physical properties would fall in between nanoparticles and bulk specimens, although our knowledge about these properties is still very limited. 

The history of mesoporous material synthesis is unintentionally or intentionally duplicating the development of zeolites and microporous molecular sieve. It starts from silicate and aluminosilicate, through heteroatom substitution, to other oxide compounds and sulfides. It is worth mentioning that many unavailable compositions for zeolite (e.g., certain transition metal oxides, even pure metals and carbon) can be made in mesoporous material form. 

This paper describes a new synthesis strategy of preparing thermally stable mesostructured transition metal oxides, namely, two-step synthesis (TSS). Basically, the synthesis course involves two steps (1) formation of a mesostructured transition metal oxide solid mediated by surfactant in a basic aqueous solution and (2) treatment of the solid product in an acidic organic solvent containing the respective precursor from which the solid product was produced. The final material synthesized according to such a method is thermally stable and structurally mesoporous with high surface area and uniform pores arranged disorderedly. 

Besides cooperative pathways, also tme liquid crystal templating (TLCT) and the hard template route (Section 9.3.7) have been developed for the synthesis of ordered mesoporous materials. In the case of the TLCT, a preformed surfactant liquid crystalline mesophase is loaded with the precursor for the inorganic materials (140). The nanocasting route, on the other hand, is a clearly distinct method (141). Here, no soft surfactant template is used but, instead, the pore system of an ordered mesoporous solid is used as the hard template serving as a mold for preparing varieties of new mesostructured materials, for example, metals, carbons, or transition metal oxides. 

Compared to sihca-based networks, nonsiliceous ordered mesoporous materials have attracted less attention, due to the relative difficulty to apply the principles employed to create mesoporous silica to nonsilica compositions. Other framework compositions are much more sensitive than silica to redox reactions, hydrolysis, or phase transformations. The reactivity of the inorganic precursors is much more difficult to control in the case of transition metal oxides, the reaction kinetics being much faster. Also, crystalline nonsiliceous frameworks are less prone to adapt the curvature of micellar aggregates, whereas the amorphous nature of silica allows for certain flexibility. 

In this chapter, the progress being made in the development of highly dispersed transition metal oxides (Ti, V, Cr, Mo) or ions (Ag ) as single-site heterogeneous photocatalysts within the frameworks or cavities of zeolites and mesoporous materials will be reviewed. 

It has thus been elucidated that well-ordered micro- or mesopores of zeolites or mesoporous materials can accommodate transition metal oxides or ions in an isolated state as single-site photocatalysts to realize unique and selective photocatalytic reactions essentially different from those on semiconducting photocatalysts such as Ti02. It was observed that zeolite or mesoporous frameworks offer one of the most promising molecular reaction fields and approaches in the development of effective new photocatalytic systems that can contribute to the reduction of global air pollution and utilize solar energy as a clean, safe and abundant resource. 

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