Morphology Effects of CeO2 Nanomaterials on the Catalytic Combustion of Toluene: A Combined Kinetics and Diffuse Reflectance Infrared Fourier Transform Spectroscopy Study

CeO2 with varied morphologies (nanopolyhedra, nanorods, and nanocubes) were synthesized via a hydrothermal method and applied to the catalytic combustion of toluene. The physicochemical properties of all catalysts were characterized by transmission electron microscopy, N2-physisorption, X-ray diffraction, H2-temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The CeO2 nanopolyhedra showed superior catalytic activity compared to CeO2 nanorods and nanocubes. Kinetic studies showed that the catalytic combustion of toluene processed through the Mars–van-Krevelen (MvK) mechanism and the oxidation of the reduced CeO2 surface by molecular oxygen are the rate-determining steps in the low-temperature region. The XPS and H2-TPR results showed that CeO2 with varied morphologies had distinct oxygen distribution, especially surface lattice oxygen. A linear relationship between surface lattice oxygen and catalytic activity was observed, indicating that surface lattice oxygen played an important role in the catalytic activity of toluene combustion. Furthermore, the in situ DRIFTS results provided a full roadmap of catalytic combustion of toluene: toluene first rapidly adsorbed onto the surface of CeO2 to form molecularly adsorbed toluene, which then reacted with the surface hydroxyl groups to generate benzyl species without O2. The benzyl species could be further oxidized to benzyloxy, benzaldehyde, and benzoate species, and finally fully oxidized to CO2 and H2O. All these results indicated that the reactivity of surface lattice oxygen was crucial on the catalytic combustion of toluene, providing a basic understanding for the catalytic combustion of toluene on CeO2 with varied morphologies.