Active metal oxides

Design, Preparation and Characterization of Highly Active Metal Oxide Photocatalysts... 

Fig. 3.17 Screening protocol showing library designs (top) and post reaction images of TLC detection wafers (bottom). Note that the white TLC plates appear black and the red spots appear white in the photo, (a) Binaries of redox active metal oxides (b) extension of binaries into ternaries by adding dopants (c) focus ternaries of best hits (d) noble metal doping of MoVNb ternary. Compositional details are given in the text. Reaction temperatures for (a-c) =375 °C, (d) =325 °C.

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

The redistribution reaction in lead compounds is straightforward and there are no appreciable side reactions. It is normally carried out commercially in the liquid phase at substantially room temperature. However, a catalyst is required to effect the reaction with lead compounds. A number of catalysts have been patented, but the exact procedure as practiced commercially has never been revealed. Among the effective catalysts are activated alumina and other activated metal oxides, triethyllead chloride, triethyllead iodide, phosphorus trichloride, arsenic trichloride, bismuth trichloride, iron(III)chloride, zirconium(IV)-chloride, tin(IV)chloride, zinc chloride, zinc fluoride, mercury(II)chloride, boron trifluoride, aluminum chloride, aluminum bromide, dimethyl-aluminum chloride, and platinum(IV)chloride 43,70-72,79,80,97,117, 131,31s) A separate catalyst compound is not required for the exchange between R.jPb and R3PbX compounds however, this type of uncatalyzed exchange is rather slow. Again, the products are practically a random mixture. 

A further development in the 1980s/1990s was the introduction of some newer catalysts. Narrow range or peaked ethoxylates can be made using acid activated metal alkoxides, metal phosphates or activated metal oxides as catalyst. These catalysts are insoluble and therefore heterogeneous in nature and the major process difference is that catalyst slurry is added to the reactor after which the conditions are exactly as with normal alkaline catalysts. The reactions are slightly quicker and need less catalyst but it must be filtered out. Most producers [ 12-20 ] have patents on these systems, the advantages of which are seen in the finished products as ... 

The oxidation of carbon monoxide on the activated metal oxide near room temperature is primarily a reaction between reversibly absorbed carbon monoxide and adsorbed oxygen atoms or molecules. The two gases are probably adsorbed on adjacent sites on the active areas. An unstable carbonate ion is first formed, which finally breaks down to give C02. Since CO is much more strongly held than oxygen on the active areas, the reaction is independent of the CO pressure and is proportional to the oxygen pressure. 

Figure 3. Scanning electron micrographs of catalyst microspheres before and after leaching with acids, (a) Commercial MCM powder made with ca. 50% colloidal silica 22 nm particle size, (b) Same powder after acid leaching the active metal oxide components. Continued on next page.

Olefin metathesis began as an industrial process involving ill-defined heterogeneous catalysts comprising high oxidation state metal salts and various activating metal oxides [3]. Due to low concentrations of the active species, no spectroscopic evidence could be obtained and little mechanistic data was avail-... 

Figure 5 Is a histogram showing the distribution of pore volume vs. pore diameter for alumina carrier, fresh cobalt molybdenum catalyst and used cobalt molybdenum catalyst. There was a slight change In mode diameter when the carrier was loaded with about 20% active metal oxides. The pore volume was reduced from 0,60 to 0.53 ml/g. However, accumulation of about 17% coke during the processing of West Coast resld greatly shifted the mode downward and reduced the total pore volume from 0.53 to 0.30 ml/g. (All of these pore volumes have been normalized to 1.0 gram of alumina).

However, quartz reactor was Inert to methane combustion (1). Hence, for this reason, no quantitative comparlson/concluslon can be drawn from the data presented here regarding the selectivity to C2s. However, seml-quantltatlvely the more active metal oxides were of Mn, Cd, Sn, Tl, Pt, Ce,... 

The less active metal oxides were those of Mo, Cu, Sb, L1, and Mg (Table 4) of these Sb and Mg oxides exhibited higher C2 activities than the blank reactor. 

Studies are being undertaken to measure the catalytic activity of other metals supported on AIPO4-5. Of particular interest will be a study of the use of AlPO s to support the active metal oxide catalysts reported elsewhere ( 1 5 ). 

Plachenov and co-workers (Technological Institute, Leningrad) (94, 95) developed methods of synthesizing active metal oxides, including silicas. A systematic study of the secondary structure of porous substances was carried out (96, 97). 

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