Molybdenum complexes Raman spectroscopy

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

Spatially resolved Raman spectroscopy has provided insights into the physicochemical processes that determine the distribution of the H2PMoOi iCoOV active phase in alumina pellets (Bergwerff et al., 2005). Molybdenum and cobalt complexes were found to diffuse through the pore structure of the alumina pellets at different rates the transport of cobalt complexes was fast, whereas molybdenum complexes required several hours to reach an equilibrated distribution. Spatially resolved Raman monitoring provides information about how preparation conditions affect the distribution of molybdenum ions (Bergwerff et al., 2005). 

Bulk Mixed Oxide Catalysts. - Raman spectroscopy of bulk transition metal oxides encompasses a vast and well-established area of knowledge. Hie fundamental vibrational modes for many of the transitional metal oxide complexes have already been assigned and tabulated for systems in the solid and solution phases. Perhaps the most well-known and established of the metal oxides are the tungsten and molybdenum oxides because of their excellent Raman signals and applications in hydrotreating and oxidation catalysis. Examples of these two very important metal-oxide systems are presented below for bulk bismuth molybdate catalysts, in this section, and surface (two-dimensional) tungstate species in a later section. 

Molybdenum(Vl) complexes, 1256,1375-1414 alkyl, 1407 alkylidyne, 1407 alkylimido, 1396 arylimido, 1396 bipyridyldibromodioxo, 1388 bis(acetylacetonate) dioxo, 1388 catecholate dioxo, 1389 dimethyl, 1406 dinuclcar, 1408 IR spectroscopy, 1412 O or S bridge, 1411 preparation, 1411 Raman spectroscopy, 1412 single 0X0 bridge, 1408 structure, 1408 triple bridge, 1410 dioxo... 

A study carried out by ab initio calculations showed that the structure is certainly more complex than that described by Remskar [79]. Various stoichiometries of Mo-S-I systems were simulated, but the most probable structure today is not known and all the simulated structures can be synthesized. Vrbanic et al. [80] presume that this structure is not that described by Remskar but rather similar to a Cheviel structure [81], according to results obtained by Raman spectroscopy analyses. The structure proposed by Vrbanic is a wire composed of a core made of sulfur atoms, surrounded by molybdenum atoms and finally iodine atoms (see Figure 2.54) [82]. 

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