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 N-2 adsorption-desorption at -196 degrees C, CHS elemental analysis, solid state Si-29 CPMAS NMR spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy of CO at -196 degrees 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 degrees C and subsequently treated in H-2/He at 600 degrees C or directly treated in H-2/He at 600 degrees 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. (C) 2015 Elsevier B.V. All rights reserved.