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.

Redox-Induced Self-Modification of Cermet Anodes of Ni–CeO2-Based Oxide for Solid Oxide Fuel Cells

“…This means that the chemical state of cerium changed significantly upon the redox treatment. Then, the spectra of Ce 3d were deconvoluted [25][26][27][28][29][30] and the relative concentration of Ce 3+ at the surface was semi-quantitatively calculated according to the procedure reported by Korsvik et al 31 The obtained results are summarized in Table I. For comparison, the results of GDC powders subjected to the same treatment are also listed.…”

The interaction of oxygen with the surface of CeO2–TiO2mixed systems: an example of fully reversible surface-to-molecule electron transfer

“…The intensities were obtained in the 2θ ranges between 20° and 70°. The X'Pert High-Score 40 software was used for data handling. The MAUD 2.26 41 software was used for Rietveld refinement.…”

“…µ-Raman analysis has, furthermore, a good spatial resolution. Therefore, to check the homogeneity of the materials, five acquisitions in different 40 regions were performed for each sample.…”

“…6a) shows the typical features 40 already reported for this species 57 with the low field component of the g tensor at g zz = 2.039 (the z direction corresponds to the oxygen internuclear axis) and with an anomalous value of the high field component that, instead of laying around the free spin value (g e =2.0023) as expected by the ionic model of adsorbed 45 superoxide 58 , is at g xx =2.010 and very close to g yy so producing a sort of unresolved perpendicular line. The anomalous value of g xx shown by superoxide ions on cerium dioxide was already discussed 57,59 in terms of a model involving some degree of covalence in the interaction of adsorbed oxygen with the 4f 50 cerium orbitals and causing deviations from the purely ionic model.…”

“…Oxygen storage capacity makes ceria suitable for several applications such as those in automotive three way catalysts [5][6][7] and, in general, in oxidation catalysis [8][9][10][11][12][13] . Cerium dioxide is also used as sensor 14,15 , in fuel 40 cell technology as a solid state electrolyte 16,17 , and even in cosmetics 18 . The oxygen storage (and release) in ceria seems to be favoured by its fluorite structure.…”