An EPR study on the formation of the superoxide radical ion on monoclinic zirconia

Zirconium oxide is capable of forming and stabilizing the superoxide radical ion O,- at its surface according to various routes. These include both an electron transfer from the solid to molecular oxygen and the capture of superoxide ions formed along a different surface process like the catalytic disproportionation of hydrogen peroxide. The EPR spectrum is characterized by an anisotropic g tensor with g,,=2.0336, g,.,,= 2.0096 and gXX= 2.0034. A less abundant species is sometimes observed with g,,=2.0255. The superoxide formation from molecular oxygen is favored by partial rehydration of the surface.