Insight into the synergy effect on the improved reactivity of spinel NiMn2O4 oxygen carrier in chemical looping combustion

Abstract

Spinel NiMn2O4 is a promising oxygen carrier due to the synergistically enhanced performance, but it remains elusive how the synergy effect acts on the reactivity. Herein, the synergistically improved reactivity of spinel NiMn2O4 was systematically studied by thermogravimetric analysis (TGA) tests and theoretical calculations. Experimental results suggested that the reduction of NiMn2O4 by CO is divided into two stages, which corresponds to the successive reduction steps of NiO and Mn2O3. The reduction products of NiMn2O4 are Ni and MnO, and the former will synergistically promote the reduction of Mn2O3 into MnO. Owing to the presence of Ni atoms, the reactivity of NiMn2O4 is superior to Mn2O3. Moreover, the reaction mechanism of NiMn2O4 with CO was unraveled by the density functional theory (DFT) approach. The results indicate that CO prefers the chemisorption on the Ni–O bridge site. Two-step reactions are involved in the CO oxidation reaction: CO adsorption and CO2 desorption, and the latter is the rate-determining step. Three types of O atoms exhibit different reactivity because they bond with different metal atoms. The activation energy of CO oxidation via the three types of O atoms are 18.20, 36.53 and 67.53 kJ·mol−1. The O atom bonding with two Ni and one Mn atoms has the highest reactivity, implying that the Ni atom can improve the reactivity of Mn-based oxygen carrier. This work reveals a new insight on the synergistically improved reactivity of spinel NiMn2O4.