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

A catalytic system where gold nanoparticles are dispersed on the surface of a conventional mesoporous silica was investigated. The sample (2.25 wt% Au) was prepared by functionalizing the silica surface with 3-mercaptopropyltrimethoxysilane, anchoring gold from an HAuCl4 solution and then reducing it with sodium citrate. Prior to the catalytic runs, the Au/SiO2-SH system was submitted to different thermal treatments. The catalyst, either as-made or after each thermal treatment, was characterized by N2 adsorption-desorption at -196 °C, CHS elemental analysis, solid state 29Si CPMAS NMR spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy of CO at -196°C. The bare and functionalized supports were also characterized. Large Au nanoparticles (ca. 30 nm) form readily by adding the HAuCl4 solution to the functionalized support and, besides such particles, a second family of small ones originates upon thermal treatment (2-5 nm, depending on the temperature and the atmosphere), leading to a bimodal gold particle size distribution. Remarkable CO conversion is obtained when the catalyst is calcined in air at 560°C and subsequently treated in H2/He at 600°C or directly treated in H2/He at 600°C. Based on the treatment-induced modifications in the physico-chemical properties of both the carrier and the supported phase, a reaction mechanism is outlined