Towards a structure-activity relationship for oxide supported metals

The chemistry of [RhCl(CO)(2)](2) on rutile single crystal surfaces and high area oxides is described. Dissociative chemisorption leads to RhCl(CO)(2)/oxide in most cases, but when the favoured site is removed. as in rutile(l I 0)(I x 2), reductive decomposition to the metal is rapid. In that favoured site, the surface provides a Lewis base site (0) for coordination to Rh and a Lewis acid site (Ti) for Cl. Metal particles formed by the thermolysis can attain SMSI effects. Using energy dispersive EXAFS (EDE) in conjunction with mass spectrometry, the reactions of Rh(CO)(2)Cl/alumina and Rh/alumina with NO have been studied in detail using the Rh K-edge. Both react rapidly with NO under mild conditions. For the dicarbonyl, the kinetics of the changes in the Rh structure and the gas phase compositions reveal two different stages in the reaction towards a bent nitrosyl complex, formulated as RhCl(NO)/alumina. For Rh/alumina, there is rapid disruption of the metal particles; 0, similarly rapidly disrupts supported rhodium. This fluidity of the metal structures strengthens the importance of synchronous structure/reactivity observations. It has long been the aim to derive a molecular level understanding of the heterogeneous catalysts. Such descriptions are often thwarted by the structural complexity and also by the absence of techniques that can be suitably applied to such materials under reaction conditions. We have sought to tackle this problem by a multi-facetted approach stretching from detailed studies of the chemisorption of organo-rhodium compounds onto the surfaces of single-crystal surfaces through the interaction of organometallics onto the surfaces of high area supports to the improved characterisation of oxide-supported metal catalysts. In this report, we review progress towards the elusive structure-reactivity relationship. (C) 2002 Elsevier Science B.V All rights reserved.