XPS studies on the oxygen species of LaMn 1− xCu xO 3+ λ

Abstract

The electronic states of LaMn 1− x Cu x O 3+ λ ( x=0–0.4) have been studied with X-ray photoelectron spectroscopy (XPS). The valence states of substituted copper ions were Cu 2+ and the manganese ions were a highly mixed state of Mn 3+ and Mn 4+. The nonstoichiometry and electronic state of lattice oxygen have been studied. The samples at x=0 and 0.1 had an excess of lattice oxygen but those at x=0.2–0.4 had lattice oxygen deficiency. A modified Auger parameter (Δ α′) was used to evaluate the electronic states of oxygen ions. The Δ α′ of lattice oxygen increased with increasing substitute quantity. This increase of Δ α′ reflected the decrease of ionic bond character of lattice oxygen. The adsorbed oxygen species on LaMn 1− x Cu x O 3+ λ was assigned mainly as O − from the peak positions of spectra for the O 1s and O KLL levels, and the Δ α′ of this O − decreased with x. This decrease, i.e., the increase of ionic bond character of adsorbed oxygen was correlated well with the value of nonstoichiometry of lattice oxygen. The rate of CO oxidation at 448 K was increased by the substitution till x=0.4. We consider that this enhancement of reactivity comes from the change of electronic state of adsorbed oxygen, O − itself, i.e., a weak interaction between O − and low coordinated metal site brings about a high reactivity.