Catalysis of silica sol–gel reactions using a PdCl2 precursor

Abstract

This work shows for the first time that palladium chloride, PdCl2, can influence the sequencing of sol–gel reactions involving tetraethyl orthosilicate (TEOS). A three-step procedure was utilised to create porous silica materials: liquid-phase sol reaction, drying and calcination. Evidence from 1H Nuclear Magnetic Resonance (NMR) spectroscopy revealed that PdCl2 had negligible influence on liquid-phase sol–gel reactions. During drying, 29Si NMR data showed that the silica sols doped with PdCl2 underwent more condensation reactions than those without. Variations in parameters known to effect sol–gel reactions could not account for the magnitude of the observed changes. Evidence from differential scanning calorimetry indicates that palladium catalyses silica hydrolysis during the drying stage, which promotes condensation reactions. Despite being more condensed after drying, 29Si NMR analysis revealed that the palladium silica structure became less condensed (compared with non-doped silica) after calcination. It is hypothesised that the interaction between palladium oxide and silanol groups inhibits condensation during the calcination process. The differences in sol–gel bonding seems to have minimal influence on the porosity of the calcined materials, though the presence of palladium nanoparticles reduced the total pore volume. This work has important implications for the design and optimisation of porous palladium silica materials. It also challenges the common assumption that metal dopants do not interact with silica sol–gel reactions.