THE REACTION OF HYDROGEN WITH TIO2(110) SUPPORTED RHODIUM GEM-DICARBONYL

The reaction of hydrogen with the rhodium gem-dicarbonyl species Rh(CO )(2) supported on TiO2(110) has been investigating using FT-RAIRS and XPS. The Rh(CO)(2) species is prepared by the dissociative adsorption of {Rh(CO)(2)Cl}(2) which also produces adsorbed Cl. The reaction is o bserved to go through two sequential stages. Initial reaction with hyd rogen at 300 K produces a mono-dispersed mono-carbonyl intermediate wh ich is characterised by an IR transmission band nu(C O) at 2065 cm(-1) . XPS confirms that formation of this intermediate is accompanied by a loss of one of the two CO molecules from the gem-dicarbonyl, and a do wnward shift in the C(ls) binding energy for the remaining carbonyl of 0.9 eV. A concomitant shift in the Rh(3d(5,2)) binding energy from 30 9.1 eV in the gem-dicarbonyl to 308.2 eV in the intermediate is also o bserved. Predissociation of hydrogen on neighbouring clusters of metal lic rhodium, or by excitation in the gas phase, significantly increase s the rate of reaction. We suggest that the intermediate is formed thr ough the reaction of a hydrogen aom migrating on the TiO2(110) surface with Rh(CO)(2) to form Rh(H)CO. This intermediate is shown to undergo a slow reconversion to the gem-dicarbonyl on exposure to CO. The rate of the second stage of the reaction with hydrogen, which is accompani ed by a reduction of the carbon in the intermediate and a further redu ction of the carbon in the intermediate and a further reduction of the rhodium, is significantly increased at 425 K. The identity of the rea ction product formed in the second stage is discussed in the light of the suggested mechanisms for CO hydrogenation over supported rhodium c atalysts. (C) 1998 Elsevier Science B.V.