Structures metal oxide catalysts

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

The hydrogen-bonding interactions within the complexes W2CI4-(y-OR)2(OR)2(ROH)2 and V Cli y-OR)2(ORT)2(Rf0H)2 may provide the molecular analogues with which to model the structure and reactivities of transition metal oxide catalysts that possess surface hydroxyl groups. The thermal treatment which is often carried out in the pretreatment of metal oxides (leading to the loss of -OH... 

The decomposition of nitrous oxide over various metal oxides has been widely investigated by many investigators (1-3). Dell, Stone and Tiley (4) have compared the reactivity of metal oxides and shown that in general p-type oxides were the best catalysts and n-type the worst, with insulators occupying an intermediate position. It has been generally accepted (5) that this correlation indicates that the electronic structure of the catalyst is an important factor in the mechanism of the decomposition of nitrous oxide over metal oxides catalysts. The reaction is usually written (4) as... 

An important attribute of these materials comes from the two distinct cationic sites (Oh and Td), and the feasibility of cations migration among them due to the redox nature of metalions, while keeping the spinel structure intact. This particular aspect helps to avoid possible segregation/sintering of metal-ions and remain stable for longer period, compared to a mixed metal oxide catalyst. This unique property of spinels makes them an attractive candidate for number of catalytic reactions. 

These motivations strengthen the interest for eatalysis towards the development of ordered assemblies of ID nanostruetures for oxide materials, e.g. metal-oxide catalysts in which the 3D macro-structure is constituted by an ordered assembling of regular ID structures with nanometric size. Note that this type of structure is significantly different from that of metal-oxide supported over other metal-oxides, such as monolayer-type V202/Ti02 materials. See also later, when the concept of nanostructured metal-oxide films is defined. 

To investigate the effect of the synthesis method on the structure-reactivity relationship of the supported metal oxide catalysts, a series of V205/Ti02 catalysts were synthesized by equilibrium adsorption, vanadium oxalate, vanadium alkoxides and vanadium oxychloride grafting [14]. The dehydrated Raman spectra of all these catalysts exhibit a sharp band at 1030 cm characteristic of the isolated surface vanadium oxide species described previously. Reactivity studies with... 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

Lanthanide-containing porous materials have found many applications in various fields [20-22], They are known as active and selective catalysts for synthesis of higher hydrocarbons (mostly ethane and ethylene) from methane [23], which is of considerable importance for utilizing the reserves of natural gas around the World. Cerium oxide has been employed as a catalyst or as a structural promoter for supported metal oxide catalysts... 

Banares, M.A. and Wachs, I.E. (2002) Molecular structures of supported metal oxide catalysts under different environments. /. Raman Spectrosc., 33, 359. 

There is evidence for isomerization of chemisorbed propylene oxide to acrolein on silver and for surface polymer formation on metal oxide catalysts (11,12). Formation of a surface polymeric structure has also been observed during propylene oxidation on silver (13). It appears likely that the rate oscillations are related to the ability of chemisorbed propylene oxide to form relatively stable polymeric structures. Thus chemisorbed monomer could account for the steady state kinetics discussed above whereas the superimposed fluctuations on the rate could originate from periodic formation and combustion of surface polymeric residues. 

Multiwall carbon nanotubes (MWNT) were obtained according to the method described in [4, 5]. The structure of MWNT and PTFE-MWNT composites was studied with use of transmssion electron microscope JEM-100CXII. Average diameter of nanotubes was 10-20 nm, surface area (determined by argon desorption method) - 250-400 m2/g, bulk density of MWNT powder 20-40 g/dm3. As-obtained MWNT were used which contained 6-20% of minerals (rests of metal oxide catalyst). 

Lin MM. Complex metal oxide catalysts for selective oxidation of propane and derivatives. II. The relationship among catalyst preparation, structure and catalytic properties. Applied Catalysis, A General. 2003 250(2) 287-303. 

Behrens M, et al. The potential of micro structural optimization in metal/oxide catalysts higher intrinsic activity of copper by partial embedding of copper nanoparticles. Chem-CatChem. 2010 2(7) 816-18. 

As more Raman spectra of supported metal oxide catalysts appeared in the literature, many contradictory models for the dispersed metal oxide structure were proposed. It was observed in 1983-1984 by Wang and Hall (1983), Chan et al. (1984), and Stencel et al. (1984) that supported Re207, M0O3, and WO3-V2O5 were in hydrated states during ambient Raman measurements. However, the molecular structures of the various hydrated dispersed metal oxide species on oxide supports were not fully understood at that time. 

It is emphasized that the PZC theory for the prediction of the molecular structure of hydrated polyoxo anions holds true only under ambient conditions when the oxide surfaces are extensively hydrated. This condition is not satisfied when the supported metal oxide catalysts are heated... 

The first Raman spectra of bulk metal oxide catalysts were reported in 1971 by Leroy et al. (1971), who characterized the mixed metal oxide Fe2(MoC>4)3. In subsequent years, the Raman spectra of numerous pure and mixed bulk metal oxides were reported a summary in chronological order can be found in the 2002 review by Wachs (Wachs, 2002). Bulk metal oxide phases are readily observed by Raman spectroscopy, in both the unsupported and supported forms. Investigations of the effects of moisture on the molecular structures of supported transition metal oxides have provided insights into the structural dynamics of these catalysts. It is important to know the molecular states of a catalyst as they depend on the conditions, such as the reactive environment. 

VO-CH3) and 665 cm"1 (V-O-CH3 vibrations). (B) The intensity of the Raman bands assigned to V-OCH3 methyl vibrations at 2930 and 2830 cm"1 increase with respect to those of the Si-OCH3 vibrations at 2960 and 2860 cm"1 with surface vanadium coverage. (Adapted from M.A. Banares, I.E. WachsJ. Raman Spectrosc. 33, 359 (2002) Molecular Structures of Supported Metal Oxide Catalysts Under Different Environments ). 

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