Oxygen storage–release behavior of delafossite-type CuCr1−x M x O2 (M = Fe, Ga)

Abstract

The oxygen storage capacity (OSC) and oxidation–reduction properties of delafossite-type CuCr1−xMxO2 (M = Fe and Ga) were investigated. The solid solutions having 3R-type delafossite-type structure were synthesized by a solid-state reaction method at 860–1100 °C in N2 atmosphere. Substitution of Fe3+ and Ga3+ into Cr3+ site of CuCrO2 improved their OSC and stability under cyclic oxidative/reductive atmosphere. CuCr0.5Fe0.5O2 and CuCr0.3Ga0.7O2 exhibited the largest OSC for M = Fe and Ga, respectively. The thermogravimetry in an oxidative atmosphere and XRD analyses revealed that CuCr1−xMxO2 oxidized to Cu(Cr1−xMx)2O4 spinel and CuO. Under 5 % H2/He atmosphere, CuCr1−xFexO2 was reduced to Cu and (Cr, Fe)3O4 spinel phase, which was eventually reduced to Fe and Cr2O3. In the M = Ga system, the delafossite phase was reduced to Cu + Cr2O3 solid solution (x = 0–0.1) or Cu + Ga2O3 solid solution (x = 0.9–1.0). In the region x = 0.3–0.7, metallic Cu was formed, coexisting with the delafossite phase, without the formation of Cr and Ga oxides. These results suggest that oxygen release behaviors were attributed to reductive decomposition for CuCr1−xFexO2 and deposition of Cu from the delafossite phase maintaining the structure for CuCr1−xGaxO2 (x = 0.3–0.7).