ADSORBED STATES OF ACETONE AND THEIR REACTIONS ON RH(111) AND RH(111)-(2X2)O SURFACES

The binding and decomposition of acetone on clean and modified Rh(111) surfaces were studied by using TPD (temperature-programmed desorption) and HREELS (high-resolution electron energy loss spectroscopy). On the clean surface the eta-2(C,O) configuration was the dominant form of acetone. This species was characterized by a nu-(CO) frequency of 1380 cm-1. This value was 430 cm-1 below the nu-(CO) frequency of liquid acetone, indicating a substantial reduction in bond order arising from the binding of acetone to the metal surface via the pi and pi-* orbitals of the carbonyl. The strength of this interaction was also indicated by the reactivity of the eta-2(C,O)-acetone intermediate. All acetone molecules adsorbed in the first monolayer on the clean surface decomposed to CO, H2, and surface carbon. This decomposition exhibited a primary kinetic isotope effect upon deuterium substitution in both TPD and HREELS experiments. Thus C-H rather than C-C scission was the rate-determining step in acetone decomposition on Rh(111). Three different methods of estimating the acetone decomposition kinetics from the hydrogen TPD were compared. Accurate modeling of the kinetics required inclusion of the effect of hydrogen atom recombination kinetics on the rate of hydrogen evolution from the surface. Modification of the surface by addition of a (2 x 2) overlayer of oxygen resulted in a change in the acetone binding configuration from eta-2(C,O) to eta-1(O). This shift was the result of electronic modification of the surface by the electronegative oxygen atoms. The nu-(CO) frequency of acetone observed on the oxygen-predosed surface, 1660 cm-1, was much higher than that observed on the clean surface. The reduced interaction of acetone with the modified surface was also reflected in the decreased reactivity of this absorbate. eta-1(O)-Acetone tended to desorb rather than decompose as observed for eta-2(C,O)-acetone.