Carbon monoxide oxidation over three different oxidation states of copper: Development of a model metal oxide catalyst

Carbon monoxide oxidation was performed over the three different oxidation states of copper: metallic (Cu), copper (I) oxide (Cu{dollar}sb2{dollar}O), and copper (II) oxide (CuO) as a test case for developing a model metal oxide catalyst amenable to study by the methods of modern surface science and catalysis. Copper was deposited and oxidized on oxidized supports of aluminum, silicon, molybdenum, tantalum, stainless steel, and iron as well as on graphite. The oxides of copper showed initial activity on the oxidized supports but became deactivated due to compound formation. Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) were used to characterize the oxidation states of the supported copper. Graphite-supported copper was used to determine the kinetics of CO oxidation over Cu, Cu{dollar}sb2{dollar}O, and CuO. Cu forms three dimensional islands on graphite which, upon oxidation, were found to wet the graphite surface. Copper was observed to oxidize slowly to CuO in a 66/33 Torr CO/O{dollar}sb2{dollar} gas mixture at 275{dollar}spcirc{dollar}C during a catalytic reaction. Hence, kinetic measurements were made over Cu, Cu{dollar}sb2{dollar}O, and CuO under 97/3, 90/10, and 66/33 Torr CO/O{dollar}sb2{dollar} gas mixtures, respectively, in order to maintain the catalyst oxidation state during the reaction. The catalytic activity was found to decrease with increasing oxidation state (Cu {dollar}>{dollar} Cu{dollar}sb2{dollar}O {dollar}>{dollar} CuO) and the activation energy increased with increasing oxidation state (Cu 9 kcal/mol {dollar}<{dollar} Cu{dollar}sb2{dollar}O 14 kcal/mol {dollar}<{dollar} CuO 17 kcal/mol). Reaction mechanisms were determined for the different oxidation states. CO{dollar}sb2{dollar} was found to inhibit CO oxidation over Cu by dissociating on the catalyst surface, and over Cu{dollar}sb2{dollar}O and CuO by blocking CO adsorption. The reduction of CuO in ultra high vacuum (UHV = absence of O{dollar}sb2{dollar}) and in CO, and the oxidation of Cu in CO{dollar}sb2{dollar} and O{dollar}sb2{dollar} were studied by XPS to determine the interaction of each component in the carbon monoxide oxidation reaction with the catalyst surface. The activation energy for reduction of CuO to Cu{dollar}sb2{dollar}O in UHV and in CO was found to be 26 and 9 kcal/mol respectively. A UHV environment was unable to further reduce Cu{dollar}sb2{dollar}O to Cu. CO was able to completely reduce CuO to Cu with the rate of reduction from CuO to Cu{dollar}sb2{dollar}O being faster than the rate from Cu{dollar}sb2{dollar}O to Cu. The oxidation of Cu by O{dollar}sb2{dollar} and CO{dollar}sb2{dollar} was found to be an activated process. Exposure of Cu to O{dollar}sb2{dollar} at 25{dollar}spcirc{dollar}C resulted in adsorbed and subsurface oxygen, while at 100{dollar}spcirc{dollar}C Cu{dollar}sb2{dollar}O was formed. CO{dollar}sb2{dollar} at 200{dollar}spcirc{dollar}C was able to oxidize Cu to Cu{dollar}sb2{dollar}O but was unable to further oxidize Cu{dollar}sb2{dollar}O to CuO. Lastly, NO reduction by CO was studied. A Cu and CuO catalyst were exposed to an equal mixture of CO and NO at 300-350{dollar}spcirc{dollar}C to observe the production of N{dollar}sb2{dollar} and CO{dollar}sb2{dollar}. At the end of each reaction, the catalyst was found to be Cu{dollar}sb2{dollar}O. There is a need to study the kinetics of this reaction over the different oxidation states of copper.