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

Abstract

This work focuses on characterizing the structural and surface properties of Ti X Ce 1- X O 2 (TiO 2 −CeO 2 ) mixed oxides using XRD, XPS, BET, H 2 -TPR, and NH 3 -TPD techniques. The Ti X Ce 1- X O 2 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 nm when X = 0.1− 0.3, which are significantly smaller than TiO 2 and CeO 2 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 Ce into TiO 2, and Ti into CeO 2, respectively. This change indicates distortion of structure and was attributed to reduction of Ti 4+ to Ti 3+, and Ce 4+ to Ce 3+ . 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 Ce into TiO 2, and Ti into CeO 2 . The structural and surface modification by introducing Ce into TiO 2 increased the reducibility of mixed oxides in H 2 -TPR. NH 3 -TPD showed that increasing Ti content in Ti X Ce 1- X O 2 enhanced surface acidity. Furthermore, H 2 O and N 2 formation from NH 3 was detected by mass spectrometry, which was attributed to the oxidation activity of the Ti X Ce 1- X O 2 mixed oxides. The highest NH 3 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 Ti X Ce 1- X O 2 mixed oxides can be tailored by controlling their composition by the urea coprecipitation procedure.