Preparation, characterization and reactivity of Me-hexaaluminate (Me=Mn, Co, Fe, Ni, Cr) catalysts in the catalytic combustion of NH3-containing gasified biomasses

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 1000 ◦C 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 H2 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 H2 the most abundant products. Also, formation of coke is observed over the samples. The same behavior is observed in CH4/O2 pulse experiments; however, in the case of the Mg-containing system, coke formation can be avoided by co-feeding H2O along with CH4