Reactions of benzene with rhodium cluster cations: Competition between chemisorption and physisorption

The reactions of Rh_n⁺ cations, n = 1-40, with C₆H₆ and C₆D₆ were investigated under single collision conditions in a Fourier-transform-ion-cyclotron-resonance mass spectrometer. For clusters with up to 18 rhodium atoms, dissociative chemisorption, accompanied by total or partial dehydrogenation in competition with nondissociative adsorption of an intact benzene molecule (physisorption) was observed. For clusters with n>18 only the nondissociative adsorption was observed in the primary reaction step. Besides some enhancement of the nondissociative adsorption channel for cluster sizes n=9, 11, 12, and 13, deuteration has little effect upon the observed reactions. The atomic rhodium cation reacts after a few seconds induction time to an arene complex, which then forms a bis-arene complex in a secondary reaction step. Rhodium dimer alone exhibits some cleavage of the Rh-Rh bond, resulting also in the Rh-C₆H₆⁺ complex.