Experimental and theoretical insights into the reaction mechanism of spinel CuMn2O4 with CO in chemical-looping combustion

Chemical-looping combustion has been regarded as a promising technology to realize low-cost CO2 separation. CuMn2O4 homogeneously combining Cu and Mn into spinel matrix is a potential oxygen carrier owing to the synergistically improved performance. The involved reaction mechanism of spinel CuMn2O4 with CO was systematically studied by experiments and theoretical calculations. Thermo-gravimetric analysis results suggested that CuMn2O4 can be reduced into Cu and MnO. The reactivity of CuMn2O4 is superior than that of Mn2O3 due to the existence of Cu. To further understand the improved reactivity of CuMn2O4, the interaction between CO and spinel CuMn2O4 was studied by density functional theory calculations. The results indicated that CO prefers to chemisorb at Mn site. The oxidation of CO on CuMn2O4 surface is found to be a three-step reaction including CO adsorption, CO diffusion and CO2 desorption. CO oxidation on CuMn2O4 surface is an exothermic reaction mainly limited by the CO diffusion step. The activation energy for oxidizing CO upon Mn-terminated surface is 64.59 kJ/mol. However, the rate-limiting-step possesses a lower activation energy (58.83 kJ/mol) over Cu-terminated surface due to the high reactivity of Cu. The calculated results can well explain the improved reactivity of spinel CuMn2O4 observed in the experiments.