Molybdenum oxides, surface oxygen vacancies

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. 

The first research group to propose a description of the structure of CoMo catalysts was led by Schuit and Gates (13). This group introduced the so-called monolayer model directly derived from the physical studies of CoMo oxide precursors supported on y-alumina carried out by J. T. Richardson (14) (Richardson first proposed the existence of a special Co/Mo entity.) In this model the upper or first layer contained only sulfur atoms, each bonded to a molybdenum atom of the second layer (below the first one), these molybdenum atoms being bonded to two oxygen atoms also located in this second layer. When a sulfur atom was removed by reduction (H2 flow) of Mo5+ to Mo3+, a vacancy was formed at the surface and became the preferential adsorption site of a sulfur atom in the organic gas phase. The presence of cobalt incorporated into underlying layers of the alumina... 

The role of the surface oxide was mainly in influencing the accessibility of the metal to the molybdenum disulphide. With copper, for example, the oxygen is not strongly bonded to the metal and can be displaced. On the other hand, with titanium the oxide is strongly held, but lattice vacancies are present which expose titanium metal to the molybdenum disulphide. 

Some of the oxides of vanadium and molybdenum catalyze the selective oxidation of hydrocarbons to produce valuable chemical intermediates. In a reaction path proposed by Mars and van Krevelen (see Section 10.5), the hydrocarbon first reduces the surface of the metal oxide catalyst by reaction with lattice oxygen atoms. The resulting surface vacancies are subsequently re-oxidized by gaseous O2. The elementary steps of this process are shown below. Electrons are added to the sequence to illustrate the redox nature of this reaction. 

N2O in the feed, about 2% of the molybdenum on the surface is present as Mo + after the catalytic reaction. On the contrary, in the absence of N2O, no reduced molybdenum was observed. When N2O is added, hydroxyls decrease and the bonds corresponding to M=0 increase further. This effect is more marked when the amount of N2O introduced is higher. In the presence of O2 only, the number of M=0 bonds formed increases further. Molybdenum is in a higher oxidation state in the presence of O2. On the contrary, when N2O is added. Mo is less oxidized. N2O inhibits the oxidation of molybdenum, promoting its reduction. N2O could be adsorbed in the same vacancies where O2 adsorption usually occurs. The inhibition in the adsorption of O2 can have as a consequence, to limit (or inhibit) the formation of more electrophilic oxygen species (as 0 , O2), formed from the dissociation of molecular oxygen. These species could promote the non-selective attack of the hydrocarbons. A higher amount of N2O deeply reduces the catalysts and promotes the formation of (reduced) sites where carbonaceous products could be formed. ... 

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