AMn2O4 (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 (AMn2O4, 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 NiMn2O4 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, NiMn2O4 and CoMn2O4 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 H2O increases the activity of NiMn2O4. Compared to benzene adsorption alone, the adsorption energy decreased from −2.20 eV to −2.54 eV after the addition of H2O. The migration path of NiMn2O4 (100) reactive oxygen species in the presence or absence of H2O is directly demonstrated. In the absence of H2O, the activation energy barrier for direct oxidation of C6H5* by NiMn2O4 lattice oxygen is dominant (0.98 eV), but OH* produced by dissociation of H2O exhibits high activity, and oxidation of C6H5* to produce the key intermediate product C6H5O* has an activation energy barrier of only 0.35 eV. In addition, H2O 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. © 2024 Elsevier B.V.