Investigation of the role of surface lattice oxygen and bulk lattice oxygen migration of cerium-based oxygen carriers: XPS and designed H2-TPR characterization

The relationship between the oxygen species of cerium-based oxygen carriers and catalytic behavior, namely the correlation between catalytic activity and surface lattice oxygen (OS-L) and that between catalytic stability and bulk lattice oxygen (OB-L), was investigated by using CH3SH and Ce1-xYxO2-δ (x = 0, 0.25, 0.50, 0.75, and 1.0) solid solutions as examples. Activity and stability experimental studies with corresponding XPS were performed to assess the role of definite surface oxygen in cerium-based oxygen carriers. The surface lattice oxygen (OS-L), rather than the surface adsorbed oxygen (OS-A), was observed to be responsible for the catalytic decomposition of CH3SH. Further, the difference in catalytic activity between CeO2 and Y-doped samples is closely associated with the insertion of Y3+ ion into the lattice of CeO2 leading to the loss of surface lattice oxygen (OS-L). H2-temperature programmed reduction (TPR), a specially designed H2-TPR, X-ray photoelectron spectroscopy, reaction product (CO and CO2) analysis, and oxygen storage capacity tests were performed to demonstrate the migration of bulk lattice oxygen, which was directly related to the catalytic stability of CeO2 and Y-doped catalysts. Direct evidences of the migration of bulk lattice oxygen over cerium-based oxygen carriers were obtained. Additionally, the migration rate of bulk lattice oxygen (OB-L) within Ce0.75Y0.25O2-δ was faster compared to the migration rate of bulk lattice oxygen (OB-L) of CeO2. Finally, improvements in catalytic stability are closely associated with the fact that bulk lattice oxygen (OB-L) participates in the decomposition of CH3SH through its faster migration to replenish surface lattice oxygen (OS-L). The factors that influenced the migration rate of bulk lattice oxygen (OB-L) were thus also subsequently investigated and discussed.