Characterization of Structural and Surface Properties of Nanocrystalline TiO2-CeO2 Mixed Oxides by XRD, XPS, TPR, and TPD

This work focuses on characterizing the structural and surface properties of TixCe1-xO2 (TiO2-CeO2) mixed oxides using XRD, XPS, BET, H-2-TPR, and NH3-TPD techniques. The TixCe1-xO2 mixed oxides synthesized by the urea coprecipitation method showed unimodal nanoporous structure with pore sizes increasing from 3.7 nm for X = 0.9 to 5.3 nm for X = 0.1. Concomitant with their higher surface area and pore volume, the mixed oxides were nanocrystalline, about 4.0 nm in crystallite size when X = 0.9, and 4.8-5.4 rim when X = 0.1-0.3, which are significantly smaller than TiO2 and CeO2 single oxides prepared by the same method (8.1 to 8.4 nm). A dominant anatase phase was detected by XRD when X was 0.9 or higher while a cubic fluorite phase was dominant when X was 0.3 or lower. Lattice parameters were changed by incorporating Cc into TiO2, and Ti into CeO2, respectively. This change indicates distortion of structure and was attributed to reduction of Ti4+ to Ti3+, and Ce4+ to Ce3+. XPS (Ce 3d, Ti 2p, O 1s) and H-2-TPR revealed that the oxidation state of surface cations decreased, and oxygen deficiency of the surface was significantly enhanced by introducing Cc into TiO2, and Ti into CeO2. The structural and surface modification by introducing Cc into TiO2 increased the reducibility of mixed oxides in H-2-TPR. NH3-TPD showed that increasing Ti content in TixCe1-xO2 enhanced surface acidity. Furthermore, H2O and N-2 formation from NH3 was detected by mass spectrometry, which was attributed to the oxidation activity of the TixCe1-xO2 mixed oxides. The highest NH3 oxidation activity was observed when X = 0.9. The present study clearly established that the structural (crystal phase, crystal size, nanoporosity, pore size) and surface properties (reducibility, oxygen deficiency, acidity, oxidation activity of the TixCe1-xO2 mixed oxides can be tailored by controlling their composition by the urea coprecipitation procedure.