Dendrimer-mediated synthesis of subnanometer-sized Rh particles supported on ZrO2

We have achieved the synthesis of ZrO2-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 Rh3+ species in an aqueous solution can interact with amide/amine groups in the interior of G4OH dendrimers to form (Rh3+)x-G4OH complexes. Rh3 clusters were subsequently formed in the same solution following room temperature reduction of the (Rh3+)x-G4OH complexes with NaBH4. These Rh3 clusters can be delivered intact on the surface of the ZrO2 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/ZrO2 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.