Radiation Induced Chemical Activity at Iron and Copper Oxide Surfaces

The radiolysis of three iron oxides, two copper oxides, and aluminum oxide with varying amounts of water were performed using γ-rays and 5 MeV ⁴He ions. The adsorbed water on the surfaces was characterized using temperature programmed desorption and diffuse reflectance infrared spectroscopy, which indicated that all of the oxides had chemisorbed water on the surface. Physisorbed water was observed on the Fe₂O₃ and Al₂O₃ surfaces as well. Molecular hydrogen was produced from adsorbed water only on Fe₂O₃ and Al₂O₃, while the other compounds did not show any hydrogen production due to the low amounts of water on the surfaces. Slurries of varying amounts of water were also examined for hydrogen production, and they showed yields that were greater than the yield for bulk water. However, the yields of hydrogen from the copper compounds were much lower than those of the iron suggesting that the copper oxides are relatively inert to radiation induced damage to nearby water.

X-ray diffraction measurements did not show any indication of changes to the bulk crystal structure due to radiolysis for any of the oxides. The surfaces of the oxides were analyzed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). For the iron samples, FeO and Fe₃O₄, Raman spectroscopy revealed areas of Fe₂O₃ had formed following irradiation with He ions. XPS indicated the formation of a new oxygen species on the iron oxide surfaces. Raman spectroscopy of the copper oxides did not reveal any changes in the surface composition, however, XPS measurements showed a decrease in the amount of OH groups on the surface of Cu₂O, while for the CuO samples the amount of OH groups were found to increase following radiolysis. Pristine Al₂O₃ showed the presence of a surface oxyhydroxide layer which was observed to decrease following radiolysis, consistent with the formation of molecular hydrogen.