AMn2O4 (A = Ni, Co, Cu) oxygen carrier chemical looping reforming of benzene: Migration pathways of reactive oxygen species by experimental and DFT investigations

Abstract

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.