Mixed metals unsupported

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

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. 

A systematic study of the Ti and Zr complexes with tripodal amido ligands showed that they are good building blocks for stable bimetallic complexes that contain polar metal-metal bonds. A representative example of this class of compound is the Fe-Ti mixed-metal complex The Fe-Ti distance for this unsupported metal-metal bond is 2.433 A, which is comparable to that in the supported system 4. 

Since the strategy was initially based on catalytic purposes, the surfaces considered initially were mostly (i) highly divided oxides (here are included simple oxides, mixed oxides, zeoUtic materials, mesoporous systems, hybrid organic inorganic materials, metal organic frameworks, etc.) and (ii) highly divided metals (supported or unsupported small metal particles). 

Before bismuth-promotion the Pt-on-alumina catalyst was pre-reduced in water with hydrogen. The pH was decreased to 3 with acetic acid and the appropriate amount of bismuth nitrate dissolved in water (10 - lO " M) was added into the mixed slurry in 15-20 min, in a hydrogen atmosphere. Promotion of unsupported Pt was carried out similarly. The metal composition of the bimetallic catalysts was determined by atomic absorption spectroscopy. 

Teraoka et al. [41, 42] have applied the amorphous citrate process to prepare unsupported (or neat) and supported perovskites of the type LaMn, LaCo, LaMnCu, LaCoFe, LaCaCo, LaCaMn, LaSrMn, LaSrCo, LaSrCoCu, and LaSrCoFe. The use of the citrate method for preparing atomically homogeneous oxides on supports has been a very important extension of the citrate method over the years. Nitrates of constituent metals of the required perovskite were dissolved in water and mixed with an aqueous solution of citric acid (molar ratio of citric acid to total metals 1 1). Water was evaporated from the mixed solution... 

A problem, different in nature, that needs additional attention refers to the characterization of the active centers involved in adsorption and catalytic processes and particularly to the estimation of the number of metallic centers and the exposed surface in supported and unsupported perovskites. A number of chemical and physical methods have been used for metals and oxides, and those based on selective chemisorption of probe molecules seem to be the most promising for this purpose (307). However, while considerable progress has been made for supported metals, no method has been accepted for oxides. This has been caused by the comparatively complex nature of these latter compounds where oxide ions and metal ions of different oxidation states may be present. As probe molecules, O2, CO, and NO were the most frequently used (307) the 02 chemisorption presents the problems inherent to any method based on gas adsorption at low temperatures (a large fraction of physisorbed gas accompanying the chemisorption). On the other hand, its symmetric character renders this molecule unamenable to study by IR spectroscopy. Nonetheless, this method has been used with some success by Weller et al. (308-310) on simple oxides, and its possible application to perovskites and other mixed oxides should be explored. Previous chemisorption work... 

Unsupported carbon fibers are formed in a continuous process [47-48] where the metal catalyst particles are continuously mixed into the flow of the feed gases, and where fibers carrying a metal catalyst particle in their tips are continuously removed. A specific process [51] that is currently under commercial development uses iron pentacarbonyl as a catalyst and hydrocarbons such as methane, natural gas, or others which can be derived from coal, recyclate and discarded rubber tires. These fibers are only several pim long and have much lower diameters (0.1 to 0.2 jm). Apparently, less time is available for side growth (or thickening) in a continuous process. 

The mixed-valent, square-planar cation [Rh (CO)2(CH3CN)2] was reported to form one-dimensional stack in the solid state. The chain is composed of the repeat unit [ Rh2 (02CCF3)2(C0)4 Rh2 °(02CCF3)4 Rh (02CCF3)2(C0)4 ] , in which an Rh",Rh" unit is connected with two dimeric Rh, Rh units. A linear chain composed of dirhodium units is not limited to acetonitriles and carbonyls. Metal chains of the type [ Rh2(OAc)2(LL)2 (BF4)] (LL = 1,10-phenanthroline 2,2 -bipyridyl) have been prepared by one-electron reduction of the corresponding dirhodium(II,II) precursors [Rh2(OAc)2 (LL)2] in aqueous ethanol. For the phen compound, three different Rh-Rh separations are noted in the Rh chain. The carboxylate bridged Rh-Rh distance is 2.652 A the unsupported interdimer Rh-Rh distance is 2.739 A and the resultant tetranuclear fragments are linked into an infinite chain by Rh-Rh... 

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