Molybdenum oxides, surface oxygen

In the following, we discuss recent concepts and theoretical results concerning microscopic properties of vanadium and molybdenum oxide surfaces. While the two elements form different classes of oxides their surfaces exhibit numerous stmctural and electronic similarities, such as microscopic surface binding, adsorption, or oxygen vacancies, which we will point out accordingly. 

The existence of additional occupied states of Mo character, located above the O 2sp derived valence region, is relevant for the interpretation of experimental photoemission spectra of molybdenum oxide surfaces. According to the results of the cluster studies additional photoemission intensity above the valence band region may be indicative of chemical reduction of the metal centers, leading to lower oxidation states, where the effect can be introduced by oxygen vacancies or by different chemical composition of the oxide. This has been verified in UPS experiments on differently prepared MoOsCOlO) surfaces in comparison with measurements of other single and mixed valency molybdenum oxide samples [212]. 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

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. 

Tribochemical reaction The formation of an iron sulfide tribofilm may be displaced by other surface active elements such as oxygen, where the oxide of the iron heat of formation AHf = -2.82 eV is thermodynamically more stable than the sulfide AHf = -1.04 eV. Using the data from Table 5.11, compare the heat of formation of molybdenum oxide and molybdenum sulfide. 

The mechanism by which the sublimed oxygen-containing Mo collector leads to better performances is not well understood. It is commonly assumed that Cs reduces the molybdenum oxide formed during the sublimation, the resulting volatile CS2O then dissociating on the emitter surface. To bypass the molybdenum oxide reduction step, the first approach tried was to load commercial "cesium oxide" (composition close to that of CSO2) in hollow collectors. 

Ally alcohol oxidation into acrolein on the rhombic phase of molybdenum oxide modified with vanadium oxide has been studied by the kinetic molhod ami Ijy ESR of ions in situ. II was shown, that active sites for this reaction are V ions situated in the bulk of the catalyst, or near its surface, but not at the surface. Fast diffusion of e.lectrons and a more slower diffusion of oxygen ions between the surface and the bulk occur during the reaction. 

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