The hydrogenation of o-chloronitrobenzene over Au/TiO2 was investigated in supercritical carbon dioxide (scCO(2)), ethanol, H2O, and H2O/CO2 at 140 degrees C. The reaction rate followed the order of H2O > H2O/CO2 > ethanol > scCO(2). Au/TiO2 was deactivated in the systems containing CO2. Changes of Au/TiO2 catalysts were investigated by means of transmission electron microscopy (TEM), in situ infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance ultraviolet-visible spectra (DR-UV/Vis). Aggregation of Au particles was excluded from origins of deactivation by TEM. Carbonate-like species formed on gold in the presence of CO2 and H-2 were detected by in situ IR spectroscopy. The results of XPS and DR-UV/Vis reveal that the active species of Au-0 was oxidized in the presence of CO2. The possible formation of CO and its influence on the activity of Au/TiO2 were also discussed. Accordingly, the formation of carbonates, the oxidation of surface Au-0 by CO2, and the CO formation are proposed to be the possible factors for the deactivation of Au/TiO2. (C) 2010 Elsevier B.V. All rights reserved.