Carbon Monoxide-Induced Mobility of Isolated Palladium Species on Silicon Dioxide at Room Temperature

Abstract

Carbon monoxide is a well-established probe molecule and common reactant. However, it can also influence sintering, especially for highly dispersed systems. In this study, we investigated the reactivity of carbon monoxide with two well-defined, silica-supported palladium surface complexes, differing in the donor–acceptor properties of their ligands. In fact, the presence of a phosphine or amine ligand clearly determined the reactivity and final dispersion of palladium in the supported catalyst model systems. For the amine surface complex ≡SiO-PdMe(tmeda) (tmeda: N, N, N’, N’- tetramethylenediamine), reductive decomposition and agglomeration to palladium particles were observed at room temperature by in situ IR spectroscopy and X-ray absorption spectroscopy. In contrast, the fast aggregation was prevented for ≡SiO-PdMe(dppe) (dppe: 1,2-bis(diphenylphosphino)ethane). The insertion of CO into the Pd-methyl bond was identified by in situ IR spectroscopy in both cases. Density functional theory calculations addressed the differences in reactivity, discovering a lower activation barrier of the first CO insertion for the tmeda complex. The study draws a detailed mechanism of structural changes at silica-supported palladium species under carbon monoxide, which must be considered when highly dispersed palladium metal species are handled under CO, e.g., for catalyst characterization by chemisorption and preparation procedures, even at very mild conditions.