Effect of CeO2 Nanostructures Morphology on Facet(s) Reactivity for Direct Oxidation of Kenaf Stalks to Vanillin

Abstract

Ceria’s exceptional reversible redox properties between Ce4+ and Ce3+ contributes to its high oxygen storage capacity and release capacity has received attention as an oxidation catalyst for a variety of process. However, ceria’s ability in catalytic performance can differ, corresponding to the selective exposition of different Ce facets in different nanostructures by controlling the shape of ceria crystal. The different CeO2 structures with the reactive Ce(100), Ce(110) and Ce(111) facets were synthesized using hydrothermal method and were characterized using XRD, BET, RAMAN, TPR, TPO and XPS. All synthesized CeO2 nanostructure shows diffraction peaks corresponding to the formation of cubic fluorite. N2 adsorption-desorption isotherms showed that all catalysts possess Type IV isotherm, indicating a mesoporous structure. The Raman spectra indicate a peak that dominated the characteristic of the F2g mode of cubic fluorite structure. The TPR and TPO analysis display formation peak corresponding to the surface-to-bulk ratio of reducibility and oxidized oxygen, which is responsible for the redox properties of ceria nanostructures. The XPS analysis of CeO2 nanoparticles proved that Ce exists in Ce3+ and Ce4+ oxidation states. All catalysts were tested for direct oxidation of kenaf stalks under 300 W of microwave irradiation using H2O2 as the oxidizing agent at pH 11.5 and temperatures of 170 °C for 20 min with 15% catalyst loading. The highest vanillin yield obtained by CeO2-Nps-400 heterogeneous catalyst was 3.93%, whereas 4.82% vanillin was produced using 2N NaOH as a homogeneous catalyst. The results clearly revealed the crucial role of catalyst morphology on the physicochemical properties and the catalytic behavior of the CeO2 nanostructures, with the particle-shaped catalyst exhibiting the highest catalytic performance.