Dendrimer-mediated synthesis of subnanometer-sized Rh particles supported on ZrO_2

摘要

We have achieved the synthesis of ZrO_2-supported Rh particles with sub-nanometer dimensions and very narrow particle size distributions using G4OH-PAMAM dendrimers as templates. The different steps of this process were characterized by EXAFS and HRTEM measurements. The results indicate that partially hydrolyzed Rh~(3+) species in an aqueous solution can interact with amide/amine groups in the interior of G4OH dendrimers to form (Rh~(3+))_x-G4OH complexes. Rh_3 clusters were subsequently formed in the same solution following room temperature reduction of the (Rh~(3+))_x-G4OH complexes with NaBH_4. These Rh_3 clusters can be delivered intact on the surface of the ZrO_2 support. The formation of highly dispersed and nearly uniform Rh particles with sizes below 1 nm was observed following thermal removal of the dendrimer component. The structure-sensitive reaction of ethane hydrogenolysis was used for the evaluation of Rh/ZrO_2 samples with different metal particle sizes. A substantial decrease in turnover frequency was observed when the particle diameter decreased below 1.5 nm, consistent with mechanistic explanations advanced previously in the literature for this reaction.