Redox behaviour of copper( II ) ion pairs as surface species on CuTh oxide catalysts. Electron paramagnetic resonance and X-ray photoelectron spectroscopy study

EPR and XPS have been used to study the redox reaction on CuTh oxide catalysts with different Cu/Th atomic ratios. Apart from the substitutional (S) sites occupied by Cu2+ ions both in the bulk and on the surface of thoria, other Cu2+ ions, occupying different sites on the surface were susceptible to H2 and/or O2 treatment at different temperatures. In particular, copper(II) ion pairs which can be obtained at high copper contents (Cu/Th > 0.01) are formed from one copper(II) ion, reducible by hydrogen, and another, corresponding to substitutional surface sites (Ss), which remains stable towards hydrogen. The migration of some Cu2+ ions, from sites very susceptible to the cyclic redox treatment, is responsible for the growth of CuO crystallites on the surface of CuTh oxide catalysts.