Effect of different reagents to adjust the pH on the synthesis, structure, and properties of Au/SiO2 catalysts obtained from aqueous HAuCl4

We compared the structures and properties of Au/SiO2 catalysts obtained from aqueous HAuCl4 using different reagents to adjust the pH value. The speciation of aqueous HAuCl4, Au colloids, and Au/SiO2 catalysts were characterized in detail by ultraviolet–visible (UV–Vis) absorption spectroscopy, ion chromatography, dynamic light scattering (DLS) measurements, X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS). The catalytic activity and stability of the Au/SiO2 catalysts in CO oxidation were also evaluated. The results show that the Au species in aqueous HAuCl4 and the morphology of Au colloids were apparently affected by the solution pH, but were independent of the reagent used to adjust the pH value. The catalytic activity of Au/SiO2 catalysts in low-temperature CO oxidation inevitably exhibited a volcano shape, depending on not only the pH value of the aqueous HAuCl4, but also the specific reagent used to adjust the pH value, while the stability of the Au/SiO2 catalysts depended only on the coexisting elements.