Expansion and shrinkage in the redox process of (Mg, Ni) O solid solutions for preparation of steam reforming catalysts in SOFCs

Abstract

(Mg, Ni) O solid solutions were synthesized by calcination of MgO and NiO mixtures (MgO: NiO = 65:35 in mol) at 1500°C for 2 h in air. When the samples were firstly reduced at 1000°C in a gas mixture of 4 vol% hydrogen and nitrogen, a volume expansion was observed. The results of powder X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDX) analyses suggested that the expansion after reduction was possibly caused by the creation of space between (Mg, Ni) O grains by segregation of nickel particles on (Mg, Ni) O grains. When the reduced samples were reoxidized at 1000°C in air and then reduced again at 1000°C in a gas mixture of 4 vol% hydrogen and nitrogen, the samples revealed an accelerated expansion, which was much larger than that of the samples reduced once. The morphological characteristics of (Mg, Ni) O grains and segregated nickel particles of the samples those were reduced twice differed to those of the samples reduced once: however, the phases of the all samples were similar, as verified by XRD and SEM analyses. The accelerated expansion after re-oxidation and reduction might be related to changes in the microstructure of the (Mg, Ni) O grains with the nickel particles segregated. © 2009 The Ceramic Society of Japan. All rights reserved.