Enhancing nitrobenzene reduction to azoxybenzene by regulating the O-vacancy defects over rationally tailored CeO2 nanocrystals

The precise control of the selectivity of intermediates in nitrobenzene reduction is a time-honored challenge. Herein, we report a regulatory method for improving the selectivity of azoxybenzene using four rationally tailored CeO2 catalysts, whose exposed planes, acid–base chemistries, and defect properties in terms of Ce3+ cations and O-vacancies are optimized. Among the four catalysts, Spindle-structured CeO2, which has the largest concentration of Ce3+ cations and O-vacancy defects exhibits the highest nitrobenzene conversion rate. However, it exhibits a lower azoxybenzene selectivity (70.5%) than Rod-CeO2 (90.4%). Based on the semi-quantitative characterization results and the initial rates of the rate-determining step, the defect densities of the CeO2 catalysts are demonstrated to correspond to their catalytic selectivity. Density functional theory (DFT) calculations and in situ capping tests further reveal that the parent Ce3+ cations of O-vacancy defects serve as adsorption-activation sites for nitro groups of nitrobenzene. Moreover, the O-vacancy defects serve as the main active sites and conduce the over-reduction of azoxybenzene, which should be inhibited to improve the azoxybenzene selectivity. Therefore, regulating the O-vacancy defects at the moderate concentration is responsible for the satisfactory azoxybenzene yield of 90.4% exhibited by Rod-CeO2 nanocrystals.