Preparation, characterization and photocatalytic properties of cerium doped TiO2: On the effect of Ce loading on the photocatalytic reduction of carbon dioxide

Abstract

The parent TiO2 and cerium doped TiO2 photocatalysts with Ce loadings 0.28–10 mol.% were prepared by the sol-gel method controlled within reverse micelles of nonionic surfactant Triton X-114. Photocatalysts were comprehensively characterized using nitrogen physisorption, XRD, XPS, contact potential difference measurements, Raman spectroscopy, and DR UV–vis spectroscopy and their performance was explored in the CO2 photocatalytic reduction for the first time. Concerning photocatalysts properties, it was revealed that the inhibiting effect of cerium on the TiO2 crystallites growth occurred only up to 3 mol.% of Ce when the incorporation of Ce4+ into the anatase lattice took place. This phenomenon was correlated with the expansion of anatase cell volume. At higher Ce loadings (≥5 mol.%) the anatase lattice was saturated and the formation/separation of amorphous ceria and/or ceria (∼1 nm) nucleation occurred, accompanied by the increase of TiO2 anatase crystallite-size and the limitation of value of anatase cell volume. Further, it was found out that the mesoporosity of photocatalysts may be preferentially attributed to voids existing between the individual crystallites and thus can be influenced by changes in the crystallite size. The modification of TiO2 with cerium affected also the spectral response of photocatalysts, shifting it to the visible light region. However, this property itself was not crucial in the CO2 photocatalytic reduction. The key role in the CO2 photocatalytic reduction played the energies of electrons and holes within the electronic structure of photocatalysts, which were markedly affected by the Ce atoms addition. For 0.28 mol.%Ce/TiO2, both electrons and holes have required potentials for the photocatalytic reduction of CO2, while for 3 mol.% and higher Ce loadings the energy of electrons was already below H+ reduction potential and thus the photocatalytic performance of these catalysts was decreasing.