Enhanced oxygen storage capacity of cation-ordered cerium-zirconium oxide induced by titanium substitution

1. Experimental section 1.1 Material synthesis Polycrystalline samples of Ce0.5Zr0.5−xTixO2 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized following the solution combustion method, using (NH4)2Ce(NO3)6 (99.5%, Alfa Aesar, US), ZrO(NO3)2·xH2O (99.99%, Sigma-Aldrich, US), TiO(NO3)2 aqueous solution, and glycine (99.0%, Wako Chemicals, Japan) in a 0.5:(0.5−x):x:2 molar ratio. TiO(NO3)2 aqueous solution was prepared by filtering the precipitate TiO(OH)2, which was obtained by hydrolyzing Ti(OC3H7)4 (95%, Wako Chemicals, Japan) in deionized water, and subsequently dissolving it in a minimum amount of HNO3 aqueous solution (65%, Wako Chemicals, Japan). All reagents were dissolved in minimum amounts of deionized water in crucibles and heated to 400 oC at a rate of 2 oC/min. In order to compare it with Ce0.5Zr0.4Ti0.1O2, a polycrystalline sample of κ-Ce2Zr2O8 was synthesized by reduction of Ce0.5Zr0.5O2 at 1200 oC for 4 h under CO flow and a subsequent oxidation at 500 oC for 3 h under air. Pt/Ce0.5Zr0.5−xTixO2 and Pt/κ-Ce2Zr2O8 were prepared by impregnation with Pt(NO3)2(NH3)2 aqueous solution (Tanaka Kikinzoku Kogyo, Japan) followed by calcination at 500 oC for 3 h under air. The Pt loading amount on the metal basis was 1 wt%.

Electron diffraction (ED) patterns were collected from the samples supported on copper grids using field emission transmission electron microscope (TEM; HF-2100, Hitachi, Japan) operating at 200 kV.The specific surface areas (SSAs) were estimated by the single point Brunauer-Emmett-Teller (BET) analysis S1 at −196 ºC using an automatic surface analyzer (Micro Sorp 4232III, Micro Data, Japan).Pretreatment of the samples was carried out at 250 ºC for 20 min under N 2 flow in order to remove any residual water and gases.

High-resolution synchrotron XRD measurements
Synchrotron XRD measurements were performed using a Debye-Scherrer camera with a twodimensional detector (PILATUS 100K) in the BL5S2 beamline at the Aichi Synchrotron Radiation Center.The incident beams from the bending magnet were monochromatized to λ = 0.689591 Å.
The capillaries were rotated during the measurements to reduce any preferential orientations.The collected XRD patterns were analyzed with the Rietveld method using the program JANA2006 (Institute of Physics, Praha, Czech Republic).S2 1.4 X-ray absorption near edge structure (XANES) measurements XANES spectra were recorded in transmission mode at the BL33XU in Spring-8, JASRI.The incident beams from the bending magnet were monochromatized using Si(111) for the Ce L 3 -edge.
Samples mixed with boron nitride were pressed into disks (10 mm in diameter, 0.1 mm in thickness) and introduced to an in situ cell with a gas switching system.Spectra in were collected 5 min after the atmosphere inside the in situ cell was changed from 3% O 2 /He to 3% H 2 /He at 600 ºC.The timeresolved changes in the edge positions were recorded every 0.2 s after the atmosphere was changed.

Oxygen storage/release capacity (OSC) measurements
OSCs were estimated using a thermogravimeter (TGA-50, Shimadzu, Japan) connected to a dual gas supplying system.Samples (10 mg) were put into platinum pans and heated to the target temperatures at a rate of 50 ºC/min under 5% O 2 /N 2 flow.Then, the weight changes of the samples were monitored upon switching between 5% O 2 /N 2 and 5% H 2 /N 2 flow every 5 min.The OSCs were estimated from the average of 5 measurements of the weight losses of the samples.

Fig. S1 .
Fig. S1.X-ray diffraction patterns of the obtained samples for Ce 0.5 Zr 0.5−x Ti x O 2 (0  x  0.5) as target compositions at room temperature.Asterisks (*) indicate peaks arising from the superstructure.CeO 2 and an unknown phase are contained in the samples of x = 0.3, 0.4, and 0.5.

Fig. S3 .
Fig. S3.Time-resolved absorption edge intensities in Ce L 3 -edge XANES spectra after switching the atmosphere from 3% O 2 /He to 3% H 2 /He at 600 °C.Data were collected every 0.2 s.

a
Calculated from the crystallographic data obtained by the Rietveld refinement.b Calculated from the crystallographic data reported in reference S3.

Table S2 .
Bond valence sums calculated for various anion sites.