AMn 2 O 4 (A = Ni, Co, Cu) oxygen carrier chemical looping reforming of benzene: Migration pathways of reactive oxygen species by experimental and DFT investigations

Chemical looping reforming (CLR) provides a novel solution for clean and efficient utilization of biomass tar. The versatility of oxygen carrier (OC) is essential for improving reforming efficiency. The properties of Mn-based spinel OCs (AMn 2 O 4, A = Ni, Co, Cu) were investigated in the CLR process using benzene as a tar model compound. Detailed characterization and experimental results demonstrate the excellent structural stability of Mn-based spinel. The NiMn 2 O 4 showed the most prominent reforming effect on benzene with the highest conversion of 95.77 % at 850 °C, S/C = 1.0, and WHSV = 3.0 h −1. After 40 cycles, NiMn 2 O 4 and CoMn 2 O 4 maintained significant catalytic activity for benzene reforming, achieving conversions of 92.96 % and 90.07 %, respectively, in the final cycle. Density functional theory (DFT) calculations demonstrate that the addition of H 2 O increases the activity of NiMn 2 O 4. Compared to benzene adsorption alone, the adsorption energy decreased from −2.20 eV to −2.54 eV after the addition of H 2 O. The migration path of NiMn 2 O 4 (100) reactive oxygen species in the presence or absence of H 2 O is directly demonstrated. In the absence of H 2 O, the activation energy barrier for direct oxidation of C 6 H 5 * by NiMn 2 O 4 lattice oxygen is dominant (0.98 eV), but OH* produced by dissociation of H 2 O exhibits high activity, and oxidation of C 6 H 5 * to produce the key intermediate product C 6 H 5 O* has an activation energy barrier of only 0.35 eV. In addition, H 2 O has a predominant role in the replenishment of oxygen vacancies. The elucidation of the oxygen migration mechanism provides new guidance for the design of efficient OCs for catalytic oxidation.