Solid-State Kinetics of Reduction of NiO/Ce – γAl ₂O ₃ Oxygen Carriers for Chemical-Looping Combustion

The modification effects of Ce on the reactivity/stability of NiO/Ce –γAl ₂O ₃ oxygen carriers are investigated. Before nickel loading, γAl ₂O ₃ is stabilized by introducing small amount of ceria (1–5%). The main active component, nickel is loaded on Ce –γAl ₂O ₃ following an incipient impregnation method. The morphology of the prepared oxygen carries is examined by XRD analysis. The reduction and re-oxidation characteristics of the NiO/Ce –γAl ₂O ₃ samples are evaluated in TPR/TPO cycles (TPR: temperature-programmed reduction; TPO: temperature-programmed oxidation). XRD analysis confirms the formation of easily reducible NiO species over the Ce –γAl ₂O ₃ support. The TPR/TPO experiments indicates that the Ce modification helps to decrease the metal-support interaction, resulting minimum nickel aluminate formation. Over, 95% of the nickel oxide on Ce –γAl ₂O ₃ are reducible, while only 70% of the loaded nickel oxide on γAl ₂O ₃ are available for reduction. Incorporation of Ce also provides consistent reduction and re-oxidation enactment during the cyclic oxidation-reduction experiments, such characteristics are very important for a successful oxygen carrier for chemical-looping combustion process. It is believed that the stable redox behavior of the NiO/Ce –γAl ₂O ₃ sample is due to nonexistence of agglomeration of nickel crystals as observed in pulse chemisorptions. The solid-state kinetics of the reduction of the NiO/Ce –γAl ₂O ₃ oxygen carriers is expressed by a nucleation and crystal growth model. The developed model fits the experimental data adequately. The estimated energy of activation for the reduction of NiO/Ce –γAl ₂O ₃ is found to be in the range of 52–55 kJ/mol, which is consistent to what is reported in the open literature.