The synthesis of highly dispersed noble and base metals on silica via strong electrostatic adsorption: II. Mesoporous silica SBA-15

Abstract

The catalyst synthesis method of strong electrostatic adsorption (SEA) was applied to mesoporous silica SBA-15. pH shift control experiments were performed to determine the point of zero charge (PZC) of SBA-15. Adsorption–pH surveys of various noble and base metal ammines (Pt, Pd, Cu, Co, Ru ammines) on SBA-15 were carried out to determine the pH of maximum uptake. Adsorption kinetics also were examined. SBA-15-supported Pt, Pd, and Co materials were prepared via SEA and dry impregnation (DI) methods for further characterization via TPR and STEM. The results suggest that the adsorption mechanism of all metal ammines is largely electrostatic, and that the adsorption can approach equilibrium in a very short time (within 5 minutes). Metal complexes deposit homogeneously on the internal surface area of SBA-15. Low-temperature-reducing metals, such as Pt and Pd, form very well-dispersed (1.3 to 2.0 nm) nanoparticles after reduction, with small standard deviation, at loadings on the order of 10 wt%. Cobalt particles were larger (7.3–9.2 nm), because the much higher temperature required for reduction of the adsorbed cobalt ammine complex led to metal sintering. The SEA-prepared Co particles were still much smaller than the DI-prepared particles, which could be reduced at much lower temperature.