Gold nanoparticles supported on conventional silica as catalysts for the low-temperature CO oxidation

tA catalytic system where gold nanoparticles are dispersed on the surface of a conventional mesoporoussilica was investigated. The sample (2.25 wt% Au) was prepared by functionalizing the silica surfacewith 3-mercaptopropyltrimethoxysilane, anchoring gold from an HAuCl4solution and then reducingit with sodium citrate. Prior to the catalytic runs, the Au/SiO2–SH system was submitted to differentthermal treatments. The catalyst, either as-made or after each thermal treatment, was characterizedby N2adsorption–desorption at −196◦C, CHS elemental analysis, solid state29Si CPMAS NMR spec-troscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, andFourier transform infrared spectroscopy of CO at −196◦C. The bare and functionalized supports werealso characterized. Large Au nanoparticles (ca. 30 nm) form readily by adding the HAuCl4solution tothe functionalized support and, besides such particles, a second family of small ones originates uponthermal treatment (2–5 nm, depending on the temperature and the atmosphere), leading to a bimodalgold particle size distribution. Remarkable CO conversion is obtained when the catalyst is calcined inair at 560◦C and subsequently treated in H2/He at 600◦C or directly treated in H2/He at 600◦C. Basedon the treatment-induced modifications in the physico-chemical properties of both the carrier and thesupported phase, a reaction mechanism is outlined.