The effect of the extent of oxidation of copper on the activity for co-oxidation

Abstract

In flow-experiments CO-oxidation has been investigated on bulk and silica-supported copper (oxide). On copper (II) oxide the reaction proceeds with an activation energy of 15 kcal Mol— 1 at a limited number of sites. A thin layer of copper (II) oxide on metallic copper exhibits a larger number of unstable active sites of the same activation energy. A less severely oxidized copper surface shows an activation energy of 11 kcal Mol— 1 and an almost bare copper surface an energy of 1 to 2 kcal Mol— 1 only. To study the activity of adsorbed oxygen pulse-flow experiments with supported catalysts and static experiments at low pressures with single crystals have been combined. At coverages from 0.5 to 0.1 oxygen adsorbed on Cu (111), (100), and (110) surfaces reacts with an activation energy of (7 ± 1) kcal Mol— 1 to carbon dioxide. This reaction also occurs on a limited number of sites.The heat of formation of the oxides or of adsorption of oxygen does not correlate with the experimental activation energies. The state of adsorbed oxygen (e.g. [math], O— , O2— ) and the adsorption energy of carbon monoxide are determining the activity. The reactivity of adsorbed oxygen is smaller than that of oxygen admitted simultaneously with carbon monoxide to metallic copper. It is argued that oxygen passes through one or more active states before being adsorbed as a relatively stable oxygen ion. When carbon monoxide is present during the adsorption of oxygen, it can rapidly react with the transient active oxygen species.