Insights into the CO2 deoxygenation to CO over oxygen vacancies of CeO2

Abstract

Cerium oxide (CeO₂) is known to exhibit oxygen vacancy enhanced catalytic activity by itself or as a support. The particle shape effect on CO or VOC oxidation is also documented. This study reports the CO₂ deoxygenation to CO over oxygen vacancies of CeO₂ of different shapes, which can be a viable approach for CO₂ utilization. A sequential experimental approach using a combination of TPR (temperature-programmed reduction under H₂ up to 900 °C), TPO (temperature-programmed oxidation under 20% O₂ up to 900 °C), and CO₂ pulse reaction is performed. Tube and polyhedra CeO₂ exhibit shape stability after TPR while cube and rod turn into polyhedral grains. Within a TPR-TPO-TPR or a TPR-CO₂-TPR-CO₂ test sequence, the tube sample shows the best reversibility in oxygen vacancy formation (based on the H₂ consumption ratio in the two TPRs) and the highest oxygen vacancy capacity (based on the second TPR) among the four samples. The CO₂ deoxygenation to CO occurs over the four TPR-treated samples and the initial per pulse CO₂ conversion is steady, implying that the surface reaction governs the rate and that the diffusion of stripped oxygen into the bulk phase is relatively fast. The deoxygenation reactivity is reproducible in the two CO₂ pulsing tests in TPR-CO₂-TPR-CO₂ experiments, indicating the importance of reversibly generated oxygen vacancies. The results also indicate that the deoxygenation activity of surface oxygen vacancies can be influenced by subsurface vacancies.