Reduction kinetics and carbon deposit for Cu-doped Fe-based oxygen carriers: Role of Cu

By combining macroscopic kinetic modeling and density functional theory (DFT) calculations, reduction kinetics and carbon formation of Cu-doped Fe-based oxygen carriers are investigated to reveal the role played by Cu. Compared to undoped Fe-Zr oxygen carrier, 1 mol% Cu dopant enhances reduction rate in the first two reactions (Fe2O3 → Fe3O4 (R1) and Fe3O4 → Fe1-xO (R2)) and hinders carbon formation. This is a result of Cu doping reducing the activation energies of R1 and R2 (i.e. R1: 90.6 → 56.5 kJ∙mol−1, R2: 225.9 → 192.2 kJ∙mol−1), and increasing the formation energy of carbon–carbon chain on the surface of reduced material. Grain model indicates that Cu dopant promotes oxygen ion migration in bulk material and improves reduction reaction rate constant in R1 and R2. However, higher activation energy is found for R3 (Fe1-xO → Fe), because the formation of an oxygen vacancy becomes harder as the reduction reaction processes.