Molybdenum oxide, single

Cover Illustration Atomic force microscopy image of molybdenum oxide particles on flat, silicon dioxide substrate, which serves as a model system for a supported catalyst. The area shown corresponds to one square micrometer the maximum difference in height is approximately 10 nanometer. The superimposed curve is the secondary ion mass spectrum of the model catalyst, showing the caracteristic isotopic patterns of single molybdenum ions and of molybdenum oxide cluster ions. 

Acrolein and Acrylic Acid. Acrolein and acrylic acid are manufactured by the direct catalytic air oxidation of propylene. In a related process called ammoxida-tion, heterogeneous oxidation of propylene by oxygen in the presence of ammonia yields acrylonitrile (see Section 9.5.3). Similar catalysts based mainly on metal oxides of Mo and Sb are used in all three transformations. A wide array of single-phase systems such as bismuth molybdate or uranyl antimonate and multicomponent catalysts, such as iron oxide-antimony oxide or bismuth oxide-molybdenum oxide with other metal ions (Ce, Co, Ni), may be employed.939 The first commercial process to produce acrolein through the oxidation of propylene, however, was developed by Shell applying cuprous oxide on Si-C catalyst in the presence of I2 promoter. 

Molybdenum(IV) Oxide and Tungsten(IV) Oxide Single Crystals 149... 

MOLYBDENUM(IV) OXIDE AND TUNGSTEN(IV) OXIDE SINGLE CRYSTALS... 

The photoluminescence spectra of anchored molybdenum oxide/Si02 and molybdenum oxide/PVG catalyst recorded at 77 K decay as a single exponential, as evidenced by the straight line obtained in the logarithmic plots. The lifetime (i.e., t at 77 K) of the charge-transfer excited triplet state was determined to be 1.9 x 10 - s (33, 63, 126). 

On the other hand, the decay curves of the photoluminescence spectra of the molybdenum oxide/Si02 and molybdenum oxide/PVG catalyst prepared by the impregnation method with an aqueous solution of (NH4)6M<)7024 show some deviation from a single exponential decay, indicated by two components with different lifetimes (i.e., = 1.6 x 10 s and t2 = 0.8 x 10 s, respectively), in the logarithmic plots. These results clearly show that there are at least two different kinds of emitting dioxo-molybdenum species (33, 126). 

Theoretical studies on molybdenum oxide surfaces have focused exclusively on single crystal surfaces of M0O3 and M0O2 that are described by low Miller indices. This is due to their relatively simple geometry considering the... 

Up to now, theoretical work on molybdenum oxide surfaces has been restricted to only few periodic slab as well as local cluster studies on the single valence oxide surfaces Mo03(010), (100) and MoO2(011). As for vanadium oxide, there is very little experimental information on quantitative microscopic details of these complex surfaces despite their importance concerning catalytic applications. The following discussion of theoretical results will be based exclusively on cluster studies that have yielded rather detailed microscopic insight. 

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]. 

Two octamolybdenum compounds, containing both bridging pyrazolate and terminal pyrazole ligands, [Mo8(pz)6(0),8(Hpz)6], 177, and [Mo8(pz)6-(0)2,(Hpz)6], 178, were prepared by the reaction of molten pyrazole with molybdenum oxides and their structures were determined by single-crystal X-ray diffraction (133). The overall geometries of 177 and 178 are similar. Both complexes consist of an octametallic framework that may be described as two bas-ally connected flattered trigonal pyramids. 

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