The reaction of hydrogen with TiO 2(110) supported rhodium gem-dicarbonyl

Abstract

The reaction of hydrogen with the rhodium gem-dicarbonyl species Rh(CO) 2 supported on TiO 2(110) has been investigated using FT-RAIRS and XPS. The Rh(CO) 2 species is prepared by the dissociative adsorption of {Rh(CO) 2Cl} 2 which also produces adsorbed Cl. The reaction is observed to go through two sequential stages. Initial reaction with hydrogen at 300 K produces a mono-dispersed mono-carbonyl intermediate which is characterised by an IR transmission band ν(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 downward shift in the C(1s) binding energy for the remaining carbonyl of 0.9 eV. A concomitant shift in the Rh(3 d 5 2 ) binding energy from 309.1 eV in the gem-dicarbonyl to 308.2 eV in the intermediate is also observed. Predissociation of hydrogen on neighbouring clusters of metallic rhodium, or by excitation in the gas phase, significantly increases the rate of reaction. We suggest that the intermediate is formed through the reaction of a hydrogen atom migrating on the TiO 2(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 accompanied by a reduction of the carbon in the intermediate and a further reduction of the rhodium, is significantly increased at 425 K. The identity of the reaction product formed in the second stage is discussed in the light of the suggested mechanisms for CO hydrogenation over supported rhodium catalysts.