Midtemperature CO2 Deoxygenation to CO over Oxygen Vacancies of Doped CeO2

Abstract

CO₂ capture and utilization are a must for easing the global warming caused by the use of fossil fuels. Previous studies demonstrate the possibility of thermal deoxygenation of CO₂ to CO over the vacancies of CeO₂. This study examines the influence of the dopant to CeO₂ on the deoxygenation of CO₂ to CO, wherein the examined dopants include Zr, Gd, Sm, and In. Only In-doped CeO₂ exhibits significant reactivity for CO₂ deoxygenation in our sequential temperature-programmed reduction (TPR) and CO₂-TPRx (temperature-programmed reaction) tests up to 700 °C. In_0.5Ce_0.5O_y after H₂-TPR demonstrates a deoxygenation onset temperature as low as 400 °C and it can maintain a stable performance in cycle tests. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses indicate that In exsolutes from the fluorite framework during TPR and the exsoluted In⁰ become oxidized in the subsequent CO₂ deoxygenation reaction. XPS indicates that the redox of Ce also occurs during TPR-CO₂-TPRx with In_0.5Ce_0.5O_y. In₂O₃ by itself demonstrates a higher deoxygenation onset temperature, a lower per gram deoxygenation capacity, and a poorer stability than In_0.5Ce_0.5O_y under the same test conditions, while CeO₂ is inactive. The results suggest a synergy between exsoluted In and the fluorite substrate, leading to the observed deoxygenation activity of In-doped CeO₂.

Keywords: CO2; CeO2; deoxygenation; dopant; oxygen vacancy.

Figure 1

X-ray diffraction patterns of selected as-prepared M_xCe_1–xO_y samples in the (a) 20–80° 2θ range and (b) expanded 2θ range revealing the shift in the fluorite(111) peak.

Figure 2

CO₂ deoxygenation results of selected samples in 2-cycle tests using (a) an isothermal pulse input of 5% CO₂ at 700 °C after the sample was activated by TPR up to 900 °C and (b) TPRx from 50 to 700 °C under a 5% CO₂ flow after the sample was activated by TPR up to 700 °C.

Figure 3

Ex situ XPS spectra of selected as-prepared samples: (a) Ce 3d, showing the fitting results of the combination of Ce³⁺ and Ce⁴⁺ (fitting results are shown in Table S2), and (b) In 3d.

Figure 4

Ex situ XPS analysis of In_0.5Ce_0.5O_y and CeO₂ samples at different stages during 2 cycles of the TPR deoxygenation sequential test. (a) Ce 3d, (b) In 3d, (c) O 1s of In_0.5Ce_0.5O_y, and (d) Ce 3d of CeO₂. Fresh: as-prepared, reduced: after 1st TPR, 1 cycle: after 1st TPR and 1st TPRx deoxygenation, and 2 cycle: after 2nd TPR and 2nd TPRx deoxygenation. All XPS data were analyzed after sample cooling to room temperature and then exposure to air.

Figure 5

XRD analysis of CeO₂, In_0.2Ce_0.8O_y, and In_0.5Ce_0.5O_y samples at different stages during 2 cycles of the TPR deoxygenation sequential test. Fresh: as-prepared, reduced: after 1st TPR, 1 cycle: after 1st TPR (to 700 °C) and 1st TPRx deoxygenation, and 2 cycle: after 2nd TPR and 2nd TPRx deoxygenation. All XRD data were analyzed after sample cooling to room temperature and then exposure to air.

Figure 6

In situ DRIFTS analysis during CO₂-TPRx over (a) reduced In_0.5Ce_0.5O_y and (b) reduced CeO₂. Both samples were in line with reduced samples by TPR up to 700 °C. The background spectra were recorded over reduced samples under Ar at the specified temperature of the spectrum.

Figure 7

Mass spectrometry signals during 5 cycles of isothermal CO₂ deoxygenation in 5 consecutive cycles of the sequential H₂-reduction-CO₂-deoxygenation test over In_0.5Ce_0.5O_x at 700 °C. The “blank” indicates the results in the absence of In_0.5Ce_0.5O_x (i.e., empty tube).