Molybdena active site

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

Numerous adsorption measurements have been made on molybdena catalysts to ascertain the nature of the active sites that may be present. These generally involved volumetric or gravimetric determinations. 

No one particular site or specie has been identified as the active site for the various reactions which occur on reduced molybdena catalysts. It may be that different sites are used for different types of reactions, e.g., hydrogenation and isomerization. Phase studies and measurement of Mo... 

The high acidity of molybdena-alumina composites appear to be derived mainly not from the intrinsic acidity of the alumina but from active sites on a surface phase containing molybdenum atoms. The amount of LB and DTA formation that is observed initially over molybdenum-alumina is at least 10 times greater than observed over the bare support when the latter reacts with 8% 1-dodecene. In this case, the bare support was alumina B listed in Table II. 

There have been several theoretical studies on the olefin metathesis reaction and the structures of homogeneous catalysts [1-5]. However, we have not found examples of theoretical modelling of olefin metathesis active sites on heterogeneous catalysts. In the case of the heterogeneous molybdena catalysts, the active centres contain probably Mo [6,7], but other Mo valences are also possible (e.g., Mo [8]). 

Handzlik, J. and Ogonowski, J. (2000) Theoretical study on active sites of molybdena-alumina catalyst for olefin metathesis, in Corma, A., Melo, F.V., Mendioroz, S. and Fierro, J.L.G. (eds.). Studies in Surface Science and Catalysis 130, pp. 1181-1186. 

It has also been proposed that methanol adsorption and its oxidation to formaldehyde occurs at coordinatively unsaturated sites, possessing four-fold coordination, rather than coordinatively saturated sites, possessing six-fold coordination [19]. Unfortunately, the surface vanadia species predominantly possess four-fold coordination which prevents this issue from being address with the current data. However, supported molybdena catalysts possesses both four-fold and six-fold coordination and their TOFs for methanol oxidation have been measured [27]. It was found that, contrary to above hypothesis, the coordinatively saturated surface molybdena species is approximately four times more active than the coordinatively unsaturated molybdena species for titania supported molybdena catalysts. Thus, methanol oxidation proceeds on both coordinatively saturated and coordinatively unsaturated sites at relatively comparable reaction rates (TOFs). 

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