The role of rare earth metals in lanthanide-incorporated mesoporous titania

Cerium and lanthanum-incorporated mesoporous TiO2 with up to 10 mol% metal were synthesized by a sol–gel method using a neutral structure-directing agent. The textural properties and surface structure of samples were investigated by means of N2 sorption measurements, SEM images and XRD patterns. The doping of lanthanide elements resulted in not only an increase in the surface area, pore volume and average pore size of mesoporous TiO2, but also in inhibition of mesostructure collapse and the growth of TiO2 nanocrystallites. In both cerium and lanthanum-incorporated mesoporous TiO2, the fact that only the anatase TiO2 phase was present indicated substantial incorporation of metal species into the framework or significant dispersion inside the channel. However, Ce and La-incorporated mesoporous TiO2 differed with regard to the surface chemical states of the metal elements. According to XPS of xCe-TiO2 powders, Ti mainly existed in a coordinatively unsaturated titanium (IV) state for low doping amounts, whereas the Ti3+ state was observed when the Ce/Ti ratio exceeded one. In contrast, no apparent difference in peak intensity and position was seen for lanthanum samples. While only the La3+ state was found in xLa–TiO2, Ce 3d spectra showed a mixture of Ce3+/4+ oxidation states on the surface which led to changes in the chemical environment of oxygen.