Magnesia active sites

Magnesia may also be activated by molecular H2 at 430°C, without the need of a Pt/AI203 activator. However, the sites active for hydrogenation of ethylene, unlike those on alumina or silica, were destroyed by oxidation at 430°C. Spiltover hydrogen (but not molecular H2) activates MgO already at 200°C and the active sites are insensitive to 02 treatment but are blocked by NH3. The active sites are therefore not the same as those created on Si02 and... 

Here we intended to study the DC conductivity, oxygen isotope exchange, andTPD techniques which give the active site information at the high temperature [10]. Two kinds of magnesia catalysts, i.e., MgO and Li/MgO were studi. The defect notations follow after Kroger [11]. 

Higher in activity, but also more costly, are catalysts that contain precious metals such as rhodium, ruthenium, platinum, palladium and rhenium or mixtures thereof [107], while alumina or magnesia [214] and rare earth oxides such as ceria and zirconia or mixtures thereof serve as the carrier material. Rare earth metals have an oxygen storage capability, they interact with the precious metal and generate active sites for hydrocarbon activation [164]. 

Leveies, L., Seshan, K., Lercher, J.A., and Leffert, L. Oxidative conversion of propane over hthium-promoted magnesia catalyst n. Active site characterization and hydrocarbon activation. J. Catal. 2003, 218, 307. 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

The theoretical results have also indicated that when metal atoms are bound to specific defects their chemical activity may change, in particular can increase. This is likely to be true also for small metal clusters. This has not been fully appreciated so far. In fact, even inert supports, like silica, alumina, or magnesia, can interact strongly with the supported metal if this is bound at a defect site and can have a direct role in the chemistry of the supported species. Some preliminary calculations on supported clusters, however, suggest that the effect of the defect on the cluster electronic structure is restricted to very small, really nanometric clusters of about ten atoms size [224]. Should the size of active catalysts in real applications go down to this size, the specific interaction with the substrate could no longer be ignored in the interpretation of the catalytic activity. 

A CO oxidation mechanism involving gold particles only is proposed especially when the support is non-reducible. For instance, Schubert et al. [54] deduced from experiments of CO-O2 time resolved titration and of isotopic exchange, that when gold is supported on magnesia, silica or alumina, the O2 adsorption takes place on low coordinated sites of the gold surface, but leads to a lower activity than when oxygen can be activated on a reducible support. 

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