Molybdena oxidized state

The individual techniques used to characterize molybdena catalysts are now considered. Table II presents a listing of articles concerning the characterization of molybdena catalysts. Unless otherwise specified, we implicitly refer to Mo and/or Co supported on an activated alumina, commonly y-AlaOs. Most work has been done on the calcined (oxidized) state of the catalyst because of ease of sample handling. Reduced and sulfided catalysts are more difficult to work with since for meaningful results, exposure of these samples to air or moisture should be rigorously avoided. Therefore, sample transfer or special in situ treatment facilities must be provided. 

Another degree of modification of the catalysts can be achieved by introduction of components which on one hand affect the dispersion of the noble metal similarly to the ceria discussed earlier, but also possess catalytic activities of their own. One example of such an additive explored in depth at Ford Research is molybdenum oxide. Molybdena, similar to ceria, forms a two-dimensional phase on 7-AI2O3 and thereby also affects the Pt dispersion and its catalytic properties. Platinum, in turn, affects strongly the reducibility of molybdena, as shown in Fig. 4, using ESCA to characterize the oxidation state after reduction in the absence and presence of Pt [7]. 

A direct confirmation of this behavior is obtained by TPR [8], One can deduce that in the presence of Pt the average oxidation state of surface molybdenum ions will be lower in an operating catalyst. It is possible to postulate the existence of surface complexes of the type PtMoOx, where below 600°C x may range from 0 to 2 depending on the reaction temperature. Recent preliminary EXAFS results seems to corroborate such a picture [9J. Conversely, one can consider the surface Pt in such complexes as being more oxidized (electron deficient) than when dispersed in the absence of modifiers such as molybdena or ceria. This is found to affect the catalytic properties of Pt. A similar behavior prevails in other systems as well. For instance, it was recently reported that addition of ceria to a Pd/AlgC catalyst results in a Pd surface state which is more difficult to reduce [10]. 

Molybdena catalysts generally need to be activated by reduction or sulfidation in order to obtain an active catalyst for most reactions in which they are employed (except for oxidation-type reactions). Therefore, it is important to determine what changes occur in the state of the oxidized catalyst when it is subjected to these activation pretreatments. 

Indeed, Lund and Dumesic (5-8) reported that the water-gas shift activity of an iron oxide catalyst is reduced by several orders of magnitude when supported on silica. Strong interactions between molybdena and alumina have been documented for the calcined states of hydrotreating catalysts (e.g., 9-11). Also, interaction is manifested in many mixed oxides by enhanced acidity, compared to the acidities of the pure component oxides (12-14). 

Rather than survey all of the possible modifications that can be made to an alumina surface, we will focus on a subset involved in two different types of surface-catalyzed chemical reactions, namely, the partial oxidation of ethylene to ethylene oxide (EO) and hydrodesulfurization (HDS) processes. Both of these catalytic systems have functional points in common, in that alumina serves as a support (a-alumina for the EO process and 7-alumina for the HDS process) and alkali-metal salts serve as promoters for both reactions. To illustrate this commonality, this section will be divided into three parts (1) the adsorption of alkali-metal salts to 7-alumina, as reflected in the Rb and Cs solid-state NMR spectroscopy of these systems (2) the absorption of ethylene to silver supported on aluminas in the presence and absence of cesium salts, as followed by C NMR spectroscopy, and (3) the solid-state Mo NMR of fresh and reduced/ sulfided molybdena-alumina catalysts. 

In all studies, increase of Scat was observed with increasing temperature up to 673 K. It was stated by the Tokyo Group (T Kabe and colleagues) that the maximal Scat was approached at 473 K on the non-promoted molybdena, whereas the maximal uptake by Co- and Ni-promoted MoOx/Al20 was reached only at 573 K and 673K.[ d7] j. concluded that molybdenum oxide was preferably sulfided in the lower temperature interval, whereas higher temperatures were necessary for sulfiding nickel and cobalt species. 

In the past few years, in situ Raman spectroscopy studies of supported metal oxide catalysts have focused on the state of the surface metal oxide species during catalytic oxidation reactions (see Table 2). As mentioned earlier, there has been a growing application of supported metal oxide catalysts for oxidation reactions. The influence of different reaction environments upon the surface molybdena species on Si02 was nicely demonstrated in two comparative oxidation reaction studies (see Fig. 4). The dehydrated surface molybdena on silica is composed of isolated species (no Raman bands due to bridging Mo—O—Mo bonds at —250 cm ) with one terminal Mo=0 bond that vibrates at —980 cm" The additional Raman bands present at —800, —600, and 500-300 cm in the dehydrated sample are due to the silica support. During methane oxidation, the surface... 

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