Structure-selectivity relationships surface

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

B. Notari, in Structure-Activity and Selectivity Relationship in Heterogeneous Catalysis, Studies in Surface Science and Catalysis, Vol. 67, R. K. Grasselli and A. W. Sleight, eds., Elsevier, Amsterdam, 1991, p. 243. 

The observed effects of structure on rate and on orientation, confirmed by the Brown selectivity relationship, show that there is no basic difference between heterogeneous catalytic alkylation of aromatic compounds and homogeneous electrophilic aromatic substitution, cf. nitration, sul-phonation etc. This agreement allows the formulation of the alkylation mechanism as an electrophilic attack by carbonium ion-like species formed on the surface from the alkene on Br0nsted acidic sites. The state of the aromatic compound attacked is not clear it may react directly from the gas phase (Rideal mechanism ) [348] or be adsorbed weakly on the surface [359]. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

Figure A3.10.22 Relationship between selectivity and surface structure for w-butane hydrogenolysis on iridium, (a) Illustrations of the Ir(l 10)-(1 x 2) and Ir(l 11) surfaces. The z-axis is perpendicular to the plane of the surface, (b) Selectivity for 2 production (mol% total products) for w-butane hydrogenolysis on both Ni single crystals and supported catalysts at 475 K. The effective particle size for the single crystal surfaces is based on the specified geometric shapes [43]. A Ir/Al203 Ir/Si02-...

Venuto, P.B., 1997, Structure-reactivity-selectivity relationships in reaction of organics over zeolite catalysts, in Progress in Zeolite and Microporous Materials, eds H. Chon, S.-K. Ihm and Y.S. Uh, Vol. 105 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 811-852. 

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