CO oxidation on nanosized Au/Al2O3 by surface hydroxyl groups and in the absence of O2, studied by inverse gas chromatography

In catalysis by gold, the term "decomposition" is used in order to describe CO oxidation by surface hydroxyl groups in the absence of O-2, at which carbon dioxide is formed without the deposition of carbon. This process is not only important due to its contribution in the activity of CO oxidation but also because it can offer significant information concerning the mechanism of CO oxidation on gold catalysts. In the present work reversed-flow inverse gas chromatography (RF-IGC) is utilized in order to compare the kinetics of CO oxidation by surface hydroxyl groups, the adsorptive capacity and the nature of the active sites related to CO sorption over bare gamma-Al2O3 and Au/gamma-Al2O3 catalyst, in a wide temperature range (50-300 degrees C). The rates related to CO surface binding exhibit exactly the same behavior, as the activity of CO oxidation by surface hydroxyl groups does over bare gamma-alumina and Au/Al2O3 respectively, indicating that at higher temperatures a much stronger binding of CO molecules over gamma-Al2O3 can occur. While, the strong bonding of CO over Au/Al2O3 catalyst, is attributed to CO on active sites of cationic Aux+ near the alumina support, in the case of bare gamma-Al2O3 is related to CO insertion in surface Al-hydroxyls, which are inactive at lower temperatures. Rise of temperature affects the topography in a different way resulting in homogeneity in the case of gamma-Al2O3 and heterogeneity in that of Au/Al2O3. Gold phase enhances the sorption of bigger amounts of CO over Au/Al2O3 catalyst in comparison to bare gamma-Al2O3 support, in agreement with the fact that CO is activated preferably at small particles of metallic Au. Although, a certain amount of CO is stored on the studied solids as inactive spectators of CO oxidation e. g. as carbonate like species during CO adsorption particularly over gamma-Al2O3, Au containing catalyst exhibits higher adsorptive capacity towards CO. Slight decrease of the amount of surface CO with rising temperature is observed due to the enhancement of CO oxidation by surface hydroxyl groups over both bare alumina and Au/Al2O3. CO oxidation by surface hydroxyl groups indicates a "periphery" mechanism in which CO molecule activated on metallic gold (Au-0...CO) is attacked by an hydroxyl group either on a support cation or on a peripheral Au-III ion, forming an intermediate carboxylate group attached to the latter. (C) 2014 Elsevier B.V. All rights reserved.