Rational design and reduction kinetics of efficient Ce-Co oxygen carriers for chemical looping reforming of methane

Abstract

One of the most significant topics in chemical looping reforming technology is to design and prepare appropriate oxygen carriers with high reactivity and excellent stability. Starting from preparation strategy and doping engineering, this work focuses on the study for chemical looping reforming of methane over the cobalt-doped Ce-based oxygen carriers synthesized via solution combustion method with the assistance of coconut shell. The reactivity of as-prepared oxygen carriers was evaluated by isothermal redox experiments, and the apparent reduction kinetics of CH4 over representative oxygen carriers was investigated on the fixed bed. A series of characterization analyses (including XRD, XPS, H2-TPR, BET, SEM, TEM and Raman) indicated that the introduction of cobalt decreases the crystallite size of composite oxygen carrier, as well increases their oxygen vacancy concentration and lattice oxygen mobility compared with the pure CeO2. Moreover, benefitting from the interaction effect between carbon material and microwave field, a moderate amount of coconut shell additive could not only further facilitate above positive changes, but enhance the interaction between Ce and Co. As consequence, for the Ce9Co1Oδ-10CS oxygen carrier, syngas with a H2/CO ratio of about 2.2 can be stably obtained, and both CH4 conversion and CO selectivity exceed 90%, and it maintains high stability in consecutive redox cycles. From the kinetic study, it has been demonstrated that the reduction of CeO2, Ce9Co1Oδ and Ce9Co1Oδ-10CS oxygen carriers are well represented by the phase-boundary controlled (contracting cylinder) mechanism, and the activation energies are calculated to be 128.73, 91.18 and 46.82 kJ/mol in turn, which also corresponds well to the reaction performance of oxygen carriers.