Chromium oxidation states and XPS analysis of the chromia/zirconia system

X‐ray photoelectron spectroscopy (XPS) has been applied to the study of the oxidation state variations of Cr ions in the chromia/zirconia system (CrOx/ZrO2) submitted to redox cycles: O2 at 773 K (standard oxidation), CO at 673 K (standard reduction) and H2O after standard reduction at 743 K. The XPS results are compared with those coming from the average oxidation number determination (n) by redox cycling and with electron spin resonance (ESR) analysis, particularly with regard to the Cr(V) identification. The results show that equilibrium adsorption of chromic acid on zirconia gives Cr(VI) species (binding energy ∼ 580 eV). The XP Cr(VI) feature shows a continuous evolution towards reduced chromium states as the samples age. After oxygen treatment, in addition to Cr(VI), the presence of a component at 1 eV lower binding energy is required to reproduce the spectral feature. This component was assigned to Cr(V) on the basis of ESR and average oxidation state results. Thermal treatment in CO (standard reduction) results in a broad (4.5−5.5 eV) and unresolved Cr 2p feature. The presence of Cr(II) at 576.0 ± 0.2 eV was established by curve fitting procedures. Cr(II) is destroyed by H2O treatment at 743 K, with ensuing formation of Cr(III) species.