One of the major obstacles of chemical looping combustion (CLC) with solid fuels is the low reaction rate of solid fuels with an oxygen carrier. Alkali metal catalysts can be used to enhance the reaction rate. In this work, the catalytic reduction mechanism of a K2CO3 modified iron-based oxygen carrier by activated carbon (AC) and the multicycle CLC tests were investigated. The results confirm that the deep reduction of K2CO3 added iron-based oxygen carrier occurred at a high rate. The energy dispersive spectrometer (EDS) analysis shows that the catalytically active species of K2CO3 migrated from the oxygen carrier to the surface of AC during reduction, which could then catalyze the rate-limiting step (C-CO2 reaction) of the reduction process. Besides the catalytic effect of K2CO3, a remarkable autocatalytic effect of iron oxide itself on the reduction was also observed. The active species could also travel back to the oxygen carrier after reduction, which guaranteed the persisting catalytic effect in the multicycle tests. The loss of potassium primarily contributed to the deactivation of the oxygen carrier, but this deactivation can be partly retrieved and improved by the supplement of K2CO3. (C) 2014 Elsevier B.V. All rights reserved.