Surface modification of CeO2 nanoflakes by low temperature plasma treatment to enhance imine yield: Influences of different plasma atmospheres

Solvothermal synthesized CeO2 nanoflakes were plasma treated under different atmospheres (combination of Ar, H2, O2 and NH3), and further used for imine synthesis. The plasma processing can improve the amount of surface oxygen vacancies, Ce3+ ratio, and the peroxy anions O22− ratio. All the three parameters can influence the CeO2 catalytic ability of imine conversion to different extent. The plasma with working gas of H2 and/or O2 can increase the peroxy anions O22− ratio greatly (from 3.14% to about 40%), while the reducing condition (Ar + H2 and Ar + NH3 as working gas) can significantly improve the Ce3+ ratio (∼28.54%) and the surface oxygen vacancies. The reaction rate of imine synthesis can be improved up to 75% of the pristine CeO2 when Ar + H2 mixture gas was used as the plasma atmosphere. The amount of CeO2 surface oxygen vacancies is the most important parameter for imine conversion. The enhancement of catalytic ability can be mainly attributed to the improved surface oxygen vacancies and the Ce3+ ratio, and slightly to the rising of peroxy anions O22− ratio, although the value of the last parameter increased most.