Mesoporous transition metal oxide materials

An important feature of both manganese oxides and the Mo-V oxide is that they are redox active. Therefore, applications as catalysts, conductive materials, and electrode materials have been investigated. For example, both the manganese oxides and the Mo-V oxide have excellent catalytic activity for selective oxidation of organic molecules. However, catalytic activity characteristic of ordered porosity has not yet been reported, because pores are so small that only very small organic molecules can enter the pores. 

Both microporous manganese oxides and the Mo-V were prepared by heating aqueous solutions of the metal precursors. In order to produce pure samples, conditions (pH, temperature, reaction time, and concentration of metal precursor) must be carefully controlled. Further investigation is needed to demonstrate the importance of these bifunctional (redox and microporosity) materials for understanding the formation mechanism of these microporous materials and for the development of a new strategy to form other microporous transition metal oxides. 

Ordered mesoporous crystalline metal oxides have been synthesised using template methods, which are generally divided into the soft template method and the hard template method , depending on the nature of the templates. 

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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... 

The recent discovery of mesoporous silica or in general mesoporous molecular sieves (MMS) has attracted a great deal of attention [57, 58]. The adjustable porosity of silica-based MMS allows large reactant molecules to penetrate into the internal void space to be processed at the active-acid sites and then diffuse out freely as products. Because of the low acidity of the silanol groups of such materials nonsilica mesoporous transition-metal oxide materials have been recently prepared for catalytic purposes. 

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... 

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. 

Many experiments were designed to obtain ordered mesoporous materials with completely different compositions of the network no longer correlated to silica. Also here, nanocasting is beneficial. Due to their high relevance in many areas of catalysis and their variable redox- and magnetic properties, much work was devoted to the creation of stable ordered mesoporous transition-metal oxides. In the meantime, many compositions with Ti, Zr, V, Ta, Mo, W, Mn, and Y, as the central element were introduced." ... 

The difficulty in direct synthesis of mesoporous transition metal oxides by soft templating (surfactant micelles) arises from their air- and moisture-sensitive sol-gel chemistry [4,10,11]. On the other hand, mesoporous silica materials can be synthesized in nimierous different solvent systems (i.e., water or water-alcohol mixtures), various synthetic conditions (Le., acidic or basic, various concentration and temperature ranges), and in the presence of organic (Le., TMB) and inorganic additives (e.g., CT, SO, and NOs ) [12-15]. The flexibility in synthesis conditions allows one to synthesize mesoporous silica materials with tunable pore sizes (2-50 nm), mesostructures (Le., 2D Hexagonal, FCC, and BCC), bimodal porosity, and morphologies (Le., spheres, rods, ropes, and cubes) [12,14,16-19]. Such a control on the physicochemical parameters of mesoporous TM oxides is desired for enhanced catalytic, electronic, magnetic, and optical properties. Therefore, use... 

Recently, we have introduced a new approach for the synthesis of mono-modal mesoporous transition metal oxides so called University of Connecticut (UCT) mesoporous materials [4]. The developed generic method uses inverse surfactant micelles as nanoreactors where the entire sol-gel chemistry is carried out to form oxide materials. The advantage of using inverse surfactant micelles is to overcome the problems associated by relatively weak S-l hydrogen-bonding interactions. Since the TM sols are confined in the inverse micelles, there is a physical barrier between the neighboring TM... 

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... 

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. 

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. 

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. 

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