Ni based mixed oxide materials for CH4 oxidation under redox cycle conditions

The preparation, characterization and redox properties of Ni-Al-O and Ni-Mg-Al-O mixed oxides for CH4 chemical looping combustion (CLC) is addressed in this study. Ni-Al-O samples having different Ni/Al ratios (0.5-2.25), prepared by coprecipitation, consist after calcination at 1000degreesC of cubic NiO and NiAl2O4 spinel. A similar phase composition is obtained for Ni-Mg-Al-O, with Mg partitioned in the two phases. The presence of NiAl2O4 prevents the crystal size growth of NiO with respect to pure NiO; further limit of the sintering of the cubic oxide was observed in presence of Mg. Reduction of the samples by Hi occurs in two steps, associated with reduction of Ni2+ in NiO and NiAl2O4, Mg stabilizes Ni2+ in both the cubic oxide and the spinel phase and improves regenerability upon repeated redox cycles. Temperature programmed reduction with CH4 (CH4-TPR) experiments showed poor selectivity to CO2 and H2O, being CO and H-2 the most abundant products. Also, formation of coke is observed over the samples. The same behavior is observed in CH4/O-2 pulse experiments; however, in the case of the Mg-containing system, coke formation can be avoided by co-feeding H2O along with CH4. (C) 2003 Elsevier Science B.V. All rights reserved.