Enhanced reducibility of Ce1-xTixO2 compared to that of CeO2 and higher redox catalytic activity of Ce1-x-yTixPtyO2-δ compared to that of Ce1-xPtxO2-δ

Article abstract of DOI:10.1021/jp0565103

Nanocrystalline Ce_1-xTi_xO₂ (0 ≤ x ≤ 0.4) and Ce_1-x-yTi_xPt_yO_2-δ (x = 0.15, y = 0.01, 0.02) solid solutions crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature programmed reduction and XPS study of Ce_1-xTi_xO ₂ (x = 0.0-04) show complete reduction of Ti⁴⁺ to Ti ³⁺ and reduction of ～20% Ce⁴⁺ to Ce³⁺ state compared to 8% Ce⁴⁺ to Ce³⁺ in the case of pure CeO ₂ below 675°C. The substitution of Ti ions in CeO₂ enhances the reducibility of CeO₂. Ce_0.84Ti _0.15Pt_0.01O_2-δ crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at 30°C over Ce_0.84Ti_0.15Pt _0.01O_2-δ is 5 and that over Ce_0.99Pt _0.01O_2-δ is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce_1-x-yTi_xPt_yO₂ (x = 0.15, y = 0.01, 0.02) compared to Ce_1-xPt_xO₂ (x = 0.01, 0.02). Synergistic involvement of Pt²⁺/Pt^o and Ti ⁴⁺/Ti³⁺ redox couples in addition to Ce ⁴⁺/Ce³⁺ due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near E_F is shown to be responsible for improved redox property and higher catalytic activity.

Full text of DOI:10.1021/jp0565103

5262
J. Phys. Chem. B 2006, 110, 5262-5272
Enhanced Reducibility of Ce1-xTixO2 Compared to That of CeO2 and Higher Redox
Catalytic Activity of Ce1-x-yTixPtyO2-δ Compared to That of Ce1-xPtxO2-δ
Tinku Baidya,† Arup Gayen,† M. S. Hegde,*,† N. Ravishankar,‡ and Loic Dupont§
Solid State and Structural Chemistry Unit and Material Research Centre, Indian Institute of Science,
Bangalore, India, and Laboratoire de ReactiVite et de Chimie des Solids, UniVersity of Picardie, 33,
rue Saint Leu, 80039 Amiens Cedex, France
ReceiVed: NoVember 10, 2005; In Final Form: January 16, 2006
Nanocrystalline Ce1-xTixO2 (0 e x e 0.4) and Ce1-x-yTixPtyO2-δ (x ) 0.15, y ) 0.01, 0.02) solid solutions
crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature
programmed reduction and XPS study of Ce1-xTixO2 (x ) 0.0-04) show complete reduction of Ti4+ to Ti3+
and reduction of ∼20% Ce4+ to Ce3+ state compared to 8% Ce4+ to Ce3+ in the case of pure CeO2 below 675
°C. The substitution of Ti ions in CeO2 enhances the reducibility of CeO2. Ce0.84Ti0.15Pt0.01O2-δ crystallizes
in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at
30 °C over Ce0.84Ti0.15Pt0.01O2-δ is 5 and that over Ce0.99Pt0.01O2-δ is 4 against just 0.078 for 8 nm Pt metal
particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce1-x-yTixPtyO2 (x )
0.15, y ) 0.01, 0.02) compared to Ce1-xPtxO2 (x ) 0.01, 0.02). Synergistic involvement of Pt2+/Pt° and
Ti4+/Ti3+ redox couples in addition to Ce4+/Ce3+ due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near
EF is shown to be responsible for improved redox property and higher catalytic activity.
Introduction
are created. Enhanced catalytic activity toward CO oxidation,
hydrocarbon oxidation, and hydrogen adsorption were observed
compared to that of Pt/Al2O3 or Pt° (metal particles) over CeO2
itself. Synergistic involvement of Pt2+/Pt° and Ce4+/Ce3+ redox
couples mediated by oxide ion vacancy is shown to be
responsible for higher catalytic activity.23 Therefore, if Pt2+ ions
can be substituted for Ce4+ or Ti4+ in Ce1-xTixO2, Pt2+/Pt° along
with Ce4+/Ce3+ and Ti4+/Ti3+ redox couples from the support
can enhance the redox-catalytic activity further compared to
Precious metals Pt, Pd, Rh, and Ru are dispersed over an
oxide support generally by impregnation, drying, and reduction
in hydrogen at ∼400-500 °C. The commonly used oxide
supports are R-Al2O3, γ-Al2O3,1,2 and SiO2,3 which are not
reducible by hydrogen, and the precious metals disperse as fine
metal particles on these supports. On the other hand, CeO2, TiO2,
and SnO2 are reducible oxide supports,4-13 and therefore, metals
supported on these oxides can become oxidized. Nano-CeO2
can be partially reduced by hydrogen at ∼400-500 °C to
CeO2-δ (δ ∼ 0.1) and to Ce2O3 above 900 °C. Oxygen storage
capacity (OSC) was introduced on a quantitative basis, which
is the capacity of ceria based support to release oxygen under
reducing condition and to uptake oxygen under oxidizing
condition.14 While ZrO2 cannot be reduced by hydrogen,
reducibility of CeO2 is enhanced in the Ce1-xZrxO2 solid
solution, thereby increasing OSC.15-17 Only the Ce4+ ion
is known to be reduced to a greater extent in the presence of
Zr ions. If Ce1-xTixO2 solid solution can be prepared, both
Ce and Ti can be reduced and therefore large OSC is expected.
Indeed, the formation of Ce1-xTixO2 (x ) 0.4) oxides has
been reported to be promising toward oxygen storage
capacity.18
Ce1-xPtxO2-δ
.
Here we report the redox property of Ce1-xTixO2 (x ) 0.1-
0.4) in H2 and a systematic study of the effect of Pt2+
substitution in Ce1-xTixO2 forming Ce1-x-yTixPtyO2-δ prepared
by a single step solution combustion method. Just as the
reducibility of Ce1-xZrxO2 is higher than that of CeO2, Ce1-xTixO2
shows higher reducibility than CeO2 or TiO2. Further, Ce1-x-yTix-
PtyO2-δ showed higher rates of CO, C2H2, C2H4, and C3H8
oxidation activity compared to Ce1-xPtxO2-δ
.
Experimental Section
TiCl4 (10 cm3) was added to ice cold (4 °C) acidified water
(1 mL of concentrated HCl in 200 mL of H2O) slowly with
continuous stirring forming TiOCl2. Dilute ammonium hydrox-
ide was added to this solution giving a white precipitate of TiO-
(OH)2. The precipitate was filtered and washed thoroughly to
remove chloride ion. Then, TiO(OH)2 was dissolved in a
minimum amount of concentrated nitric acid to obtain the nitrate
precursor TiO(NO3)2. The Ti ion concentration in the solution
was estimated by the colorimetric method.24
In an attempt to understand noble metal-CeO2 interaction,
Pd2+, Cu2+, Pt2+, and Rh3+ ions were substituted for Ce4+ in
CeO2 forming a Ce1-xMxO2-δ solid solution for small values
of x ) 0.01-0.02.19-22 For charge compensation, on lower
valent ionic substitution for Ce4+ in CeO2, oxide ion vacancies
* Address correspondence to this author. E-mail: mshegde@
sscu.iisc.ernet.in. Phone: +91-80-2293-2614. Fax: +91-80-2360-1310.
† Solid State and Structural Chemistry Unit, Indian Institute of Science.
‡ Material Research Centre, Indian Institute of Science.
§ University of Picardie.
Ce1-xTixO2 (x ) 0.1-0.4) were prepared by taking stoichio-
metric amounts of (NH4)2Ce(NO3)6‚6H2O (E. Mark India Ltd.),
TiO(NO3)2, and glycene and heating at 400 °C to give a flaming
combustion.25,26
10.1021/jp0565103 CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/24/2006

Reducibility of Ce1-xTixO2 Compared to That of CeO2
J. Phys. Chem. B, Vol. 110, No. 11, 2006 5263
Chemical reactions occurring in the combustion process can
be written as follows:
3(NH4)2Ce(NO3)6 + 8C2H5NO2 f
3CeO2 + 10N2 + 16CO2 + 32H2O (1)
9TiO(NO3)2 + 10C2H5NO2 f
9TiO2 + 14N2 + 20CO2 + 25H2O (2)
9(1 - x)(NH4)2Ce(NO3)6 + 9xTiO(NO3)2 + 10C2H5NO2 +
14(1 - x) C2H5NO2 f 9Ce1-xTixO2 + 20CO2 +
28(1 - x)CO2 + 14N2 + 34(1 - x)N2 + 25H2O +
71(1 - x)H2O (3)
Figure 1. (a) Percent CO conversion as a function of temperature over
Ce0.85Ti0.15O2, and Pt, Ti substituted oxides under the following reaction
conditions: CO ) 2 vol %, O2 ) 2 vol %, Ft ) 100 sccm, GHSV )
43 000 h-1, W ) 25 mg.
Pt/Ce0.85Ti0.15O2 (1 atom %) was prepared by the solution
combustion method taking (NH4)2Ce(NO3)6‚6H2O, Pt(NH3)4-
(NO3)2 (ABCR, France), TiO(NO3)2, and glycene in the mole
ratio of 0.8415:0.01:0.1485:2.42. In a typical preparation, 3 g
of (NH4)2Ce(NO3)6‚6H2O, 0.0252 g of Pt(NH3)4(NO3)2, 0.1823
g of TiO(NO3)2 (in solution), and 1.1814 g of glycine were taken
in a 300 mL Pyrex dish. Similarly, 2 atom % Pt/Ce0.85Ti0.15O2
was also prepared. To compare the catalytic activity of 1 atom
% Pt impregnated over combustion, synthesized Ce0.85Ti0.15O2
was prepared by reducing H2PtCl6 dispersed over the oxide
support by hydrazine.
X-ray diffraction (XRD) data of all the mixed oxides and
the Pt-substituted oxides were recorded on a SIEMENS D5005
diffractometer at a scan rate of 0.5° min-1 in the 2θ range
between 20° and 100°. For the structural refinement by the
Rietveld method, the XRD data were recorded in a Philips
X’Pert diffractometer at a scan rate of 0.5° min-1 with a 0.02°
step size in the 2θ range between 20° and 100°. The refinement
was done with the FullProf-fp2k program27 varying 17 param-
eters simultaneously.
X-ray photoelectron spectra (XPS) of the prepared samples
and a few hydrogen reduced materials were recorded in an
ESCA-3 Mark II spectrometer (VG Scientific Ltd., England),
using Al KR radiation (1486.6 eV). Binding energies were
calibrated with respect to C(1s) at 285 eV with a precision of
(0.2 eV. For XPS analysis the powder samples were made into
0.5 mm thick, 8 mm diameter pallets and placed into an ultrahigh
vacuum (UHV) chamber at 10-9 Torr housing the analyzer. The
obtained spectra were curve fitted with Gaussian peaks after
subtracting a linear background.
cm × 0.4 cm) at atmospheric pressure. Typically, 50 mg of the
catalyst (40/80 mesh size) diluted with SiO2 (30/60 mesh size)
was loaded in the reactor to obtain a column length of 1.1 cm
and the ends were plugged with ceramic wool. For all the
reactions, the total flow was kept fixed at 100 sccm to achieve
a gas hourly space velocity (GHSV) of 43 000 h-1. Actual rates
of the reactions were obtained following the percent conversion
as a function of W/F, where W is the weight of the catalyst and
F is the gas flow rate in moles/second. Before the catalytic test,
the as-prepared catalyst was heated in O2 flow at 200 °C for 1
h followed by degassing in He flow to the experimental
temperature. The reactions were carried out as a function of
temperature with a linear heating rate of 10 deg min-1
.
Results
Catalytic Test. CO oxidation over the catalysts are carried
out with 25 mg of each catalyst at space velocity of 43 000 h-1
with 2:2 vol % of CO and O2. The heating rate was also kept
constant at 10 deg/min. In Figure 1, CO conversion vs
temperature plots are shown. As can be seen from the figure,
Ce0.84Ti0.15Pt0.01O2-δ catalyst has a much lower light-off tem-
perature with T50 ) 170 °C compared to Ce0.99Pt0.01O2-δ with
T50 ) 260 °C. Further, T50 of 2 atom % Pt in CeO2 or Ce0.85
Ti0.15O2 are lower compared to the corresponding 1 atom % Pt
oxides. A 1 atom % Pt metal impregnated catalyst over Ce0.85
-
-
Ti0.15O2 shows T50 of 260 °C compared to 170 °C in the
combustion synthesized catalyst. Having observed higher cata-
lytic activity from the light-off curves for Pt in Ce1-xTixO2
compared to Pt in CeO2, actual rates were estimated by running
the reactor under differential condition: rate ) FCOx/W, where
FCO in the molar flow rate (moles/second), W is the weight of
the catalyst (grams), and x is the fractional conversion. A series
of reactions were carried out by varying W keeping the CO:O2
ratio of 2:6 vol % constant. Total gas flow was kept constant at
100 sccm made up with He. Space velocity was kept constant
at 43 000 h-1. In Figure 2a, fractional CO conversion vs W/F
plots is shown at various temparatures. From the figure it can
be noted that even up to 40% conversion, x vs W/F is linear.
Accordingly, from the slopes, rates were estimated. CO oxida-
tion experiments were carried out over Ce0.99Pt0.01O2-δ under
the same experimental conditions, and here also up to 40%
conversion, conversion was linear with W/F. In Figure 2b rates
of CO conversion (µmol‚gm-1‚s-1) vs temperature are plotted.
Clearly, there is a huge difference in the CO oxidation rates
between Pt in Ce1-xTixO2 and Pt in CeO2. At 155 °C, the rate
Hydrogen Uptake Measurement and Catalytic Reactions
Hydrogen uptake measurements were performed in a continu-
ous flow microreactor of length 30 and 0.4 cm internal diameter
with 5.49% H2/Ar (certified calibration gas mixture from
Bhuruka Gases Ltd., Bangalore) flowing at 30 sccm at a linear
heating rate of 10 deg min-1 up to 700 °C. For Pt-substituted
samples, the low temperature was achieved through liquid
nitrogen cooling. Hydrogen concentration was monitored by a
thermal conductivity detector (TCD). The volume of hydrogen
consumed was calibrated against the reduction of a known
amount of CuO. The gas flow was monitored with a mass flow
sensor (AWM3100V) calibrated with a standard bubble flow
meter. Hydrogen uptake over Pt-substituted samples also has
been carried out under isothermal condition.
The catalytic test was done in a temperature programmed
reaction (TPR) system equipped with a quadrupole mass
spectrometer SX200 (VG Scientific Ltd., England) for product
analysis in a packed bed tubular quartz reactor (dimensions 25
of CO conversion over Ce0.84Ti0.15Pt0.01O2-δ is 45 times higher

5264 J. Phys. Chem. B, Vol. 110, No. 11, 2006
Baidya et al.
Figure 2. (a) x vs W/FCO plot of Ce0.84Ti0.15Pt0.01
temperature from a W/FCO plot and the light-off curve for Ce0.99Pt0.01O2-δ
and Ce0.84Ti0.15Pt0.01O2-δ
O2-δ. (b) Rate vs
Figure 3. (a) Arhenious plot [ln(TOF) vs 1/T] for CO oxidation over
.
Ce0.99Pt0.01O2-δ and Ce0.84Ti0.15Pt0.01O2-δ. (b) Rate vs O2 partial pressure
dependence over Ce0.84Ti0.15Pt0.01O2-δ
.
than that over Ce0.99Pt0.01O2-δ. In the same plot, rates derived
mg of catalysts and in Figure 5, the percent NO conversion vs
temperature is given. NO reduction activity over Ce0.84Ti0.15
from light-off curves taking conversion up to 25% are plotted
for the two catalysts. The rates obtained from the light-off curve
below 15% conversion are in good agreement with the rates
obtained under differential condition. Further, the conversion
up to 15% is linear with W up to 150 mg of catalyst. Therefore,
-
Pt0.01O2-δ and Ce0.99Pt0.01O2-δ is similar. The rate of conversion
at 135 °C (at 15% conversion) is 2 × 10-7 mol gm-1 s-1
.
Activation energies are 23 and 24 kcal mol-1 for Ce0.99Pt0.01O2-δ
rates of CO oxidation as a function of PO , rates of hydrocarbon
and Ce0.84Ti0.15Pt0.01O2-δ, respectively.
2
oxidation, and NO reduction reported here are taken for
Hydrogen Uptake Studies. Figure 6a shows the temperature
programmed reduction profiles of Ce1-xTixO2 (x ) 0.0-0.4)
along with pure TiO2. Notice the Y-scales are different in the
plots for different compositions. Pure CeO2 shows H2 uptake
from 350 °C with a low-temperature peak at ∼500 °C, which
has been attributed to surface cerium reduction14 followed by
bulk cerium reduction beyond 550 °C. The area under the peak
up to 675 °C corresponds to 5.2 cm3 of H2 per g of CeO2. Upon
Ti substitution, the reduction starts at ∼300 °C. The total
reduction up to 675 °C has been evaluated for all the composi-
tions and the data are given in Table 1. Clearly, the extent of
reduction increases with increase in Ti content. The samples
were reoxidized by passing oxygen at 300 °C for 1 h. The
hydrogen uptake of the reoxidized sample in the subsequent
cycle was about the same as that in the first cycle. From the
conversions less than 15% by running the reaction with 100-
150 mg of the catalyst with GHSV of 43 000h-1
.
In Figure 3a, activation energy of CO oxidation is given
taking rates from W/F plots and activation energy is 13.2 kcal/
mol with Ce0.84Ti0.15Pt0.01O2-δ, and it is much lower than that
over Ce0.99Pt0.01O2-δ with a value of 19.6 kcal/mol. Thus,
Ce1-xTixO2 is a better oxide support for Pt than pure CeO2.
Partial pressure dependence of oxygen over CO oxidation is
shown in Figure 3b where 2 vol % of CO is kept constant and
the volume percent of oxygen was varied from 0.2 to 10.
Reaction was carried out with 50 mg of Ce0.84Ti0.15Pt0.01O2-δ
catalyst. From the log(rate) vs log(PO ), we find that when the
2
partial pressure of oxygen is lower than the of CO, the order is
0.78 and in excess oxygen condition, the order is 0.28.
volume of hydrogen taken up by Ce0.6Ti0.4O2 up to 675 °C, the
In Figure 4a light-off curves of C2H2, C2H4, and C3H8
oxidation over 100 mg of the two catalysts are given. In each
of the reactions, 1 vol % of “HC” was taken and the “HC” to
oxygen ratio was 1:5 with a total flow of 100 sccm. In Figure
4b, Arrhenius plots are given for all the reactions with the two
catalysts taking the rates estimated below 15% conversion.
Clearly, Ce0.84Ti0.15Pt0.01O2-δ shows higher catalytic activity
compared to Ce0.99Pt0.01O2-δ. The activation energies are lower
for the Ce0.84Ti0.15Pt0.01O2-δ compared to Ce0.99Pt0.01O2-δ for
oxidation of all the hydrocarbons studied here.
+3
stoichiometry of the reduced oxide is Ce0.46+4Ce0.14+3Ti0.4
O
,
1.73
on the basis of 100% reduction of Ti4+ to the Ti3+ state. That
the Ti4+ ion is fully reduced to Ti3+ in the TPR is substantiated
by XPS studies presented later. While only 16% Ti is reduced
from Ti4+ to Ti3+ up to 675 °C in TiO2, 100% Ti from Ti4+ is
reduced to 3+ in the solid solution. Only about 8% Ce is
reduced in pure CeO2 up to 675 °C but over 20% Ce4+ is
reduced in Ce1-xTixO2. Thus, reducibility of both Ce4+ and Ti4+
is enhanced in Ce1-xTixO2.
NO reduction by CO was carried out taking 0.25 vol % of
NO and 0.25 vol % of CO with a 40 sccm total flow over 150
Figure 6b shows the TPR profiles of 1-2 atom % Pt/Ce0.85
-
Ti0.15O2 from -100 to 650 °C. There are three regions of

Reducibility of Ce1-xTixO2 Compared to That of CeO2
J. Phys. Chem. B, Vol. 110, No. 11, 2006 5265
Figure 6. (a) TPR profiles of pure CeO2 (curve A), Ce1-xTixO2 (x )
01-0.4) (curves B, C, D, E) solid solutions, and pure TiO2 (curve F);
(b) TPR profile of Ce0.99Pt0.01O2-δ (curve A), Ce0.84Ti0.15Pt0.01O2-δ (curve
B), Ce0.83Ti0.15Pt0.02 2-δ (curve C), and Ce0.83Ti0.15Pt0.02
O
O2-δ (curve C′,
cycle-2). Notice the difference in the Y-scale for different samples.
TABLE 1: Rietveld Refined Parameters for CeO2,
Ce1-xTixO2 (x ) 0.0-0.4) Compounds
compd
RBragg
RF
RP
ø2
a (Å)
CeO2
1.17
1.58
1.01
2.45
1.77
1.98
0.90
0.98
1.01
1.81
1.25
1.32
6.00
5.16
6.06
7.06
5.25
5.65
1.02
1.10
1.16
1.04
1.09
1.11
5.4131(6)
5.4061(6)
5.4032(2)
5.3989(2)
5.3892(5)
5.3720(3)
Ce0.9Ti0.1O2
Ce0.85Ti0.15O2
Ce0.8Ti0.2O2
Ce0.7Ti0.3O2
Ce0.6Ti0.4O2
Figure 4. (a) Percent CO2 formation as a function of temperature for
different hydrocarbon oxidation over Ce0.99Pt0.01O2-δ and Ce0.84Ti0.15
-
Pt0.01O2-δ. Reaction conditions: “HC” ) 1 vol %, O2 ) 5 vol %, Ft )
100 sccm, GHSV ) 43 000 h-1, W ) 100 mg. (b) Arrhenius plot [ln-
(rate) vs 1/T] for corresponding hydrocarbon oxidation as in part a.
material is oxidized when exposed to air even at room
temperature, as observed from the amount of hydrogen taken
up by the reduced sample exposed to air. After the first cycle,
O2 was passed at 300 °C for 1 h followed by purging with pure
Ar and a second TPR run was carried out. The H2 uptake curves
in the second cycle were similar to those in the first cycle
[compare curves C and C′ of Figure 6b].
The H2 uptake experiment was also carried out under
isothermal condition at 30 °C as a function of time. The
experiment was repeated for reduction-oxidation cycles at 30
°C to check for the reversibility of H2 uptake. First, H2 uptake
was measured at 30 °C over 1 atom % Pt/Ce0.85Ti0.15O2. When
O2 was passed at 30 °C over the H2 adsorbed catalysts,
temperature increased to 38 °C indicating H2 + O2 to H2O
reaction. The reactor was purged with pure Ar and again H2/Ar
was passed. Adsorption of H2 followed by the H2 + O2 reaction
cycle was repeated over 50 times and each time the volume of
H2 adsorbed was the same (Figure S1, Supporting Information).
However, the H2 adsorbed sample was purged with Ar and a
repeat H2/Ar run did not show any H2 uptake. The H2/Pt molar
ratios were 2.5 and 1.5 for 1 and 2 atom % Pt in Ce0.85Ti0.15O2.
Figure 5. Percent NO conversion vs temperature for the reaction NO
+ CO under the following conditions: NO ) 0.25 vol %, CO ) 0.25
vol %, Ft ) 40 sccm, GHSV) 20 000 h-1, W ) 150 mg.
hydrogen uptake: first one extends from -50 to 120 °C; the
second region extends from ∼120 to 300 °C; and the third region
is above 300 °C. The low-temperature region corresponds to
hydrogen uptake related to Pt species in the compound.23 The
second and the third regions of the H2 uptake can be attributed
to the reduction of the Ti4+ and the Ce4+ state, respectively.
Reduction of both Ti4+ and Ce4+ ions occurs at a lower
temperature upon Pt substitution and the extent of reduction of
Ce4+ is increased. The H2 uptake curve for Ce0.99Pt0.01O2-δ is
also given for comparison in the figure. The H2/Pt molar ratio
in the first region of 1 atom % Pt/Ce0.85Ti0.15O2 is 12.0, which
is 3 times higher than that observed over 1 atom % Pt/CeO2.23
For 2 atom % Pt/Ce0.85Ti0.15O2, the ratio is 7.5 under region 1.
Total H2 uptake up to 650 °C is also much higher than that of
the corresponding Ce1-xTixO2 oxides without Pt. The reduced
The H2/Pt molar ratio at 30 °C for Ce0.99Pt0.01O2-δ was 2. This
means for 1 atom % Pt/Ce0.85Ti0.15O2 the number of H atoms
per Pt atom adsorbed at 30 °C is 5. The H2 adsorption was
carried out at 30 °C over ∼80 Å Pt metal particles (prepared
by the polyol reduction method) in a separate experiment. The
H2/Pt molar ratio was 0.078. Thus, at 30 °C, there is 25 times
enhancement of H2/Pt over Ce0.99Pt0.01O2-δ and 32 times over
Ce0.84Ti0.15Pt0.01O2-δ compared to Pt metal particles. The number
of H atoms per surface Pt atom (estimated from ∼80 Å Pt metal
particles) turnes out to be 0.97, which is close to 1. This is
what is expected for Pt metal particles. To confirm whether the
adsorbed H2 reacts with O2 at 30 °C, O2 was passed over the
H2 adsorbed catalyst in the catalytic reactor equipped with QMS.
First H2/He was passed over 1 atom % Pt/Ce0.85Ti0.15O2, the

5266 J. Phys. Chem. B, Vol. 110, No. 11, 2006
Baidya et al.
Figure 7. (a) XRD patterns of (A) CeO2, (B) Ce0.9Ti0.1O2, (C) Ce0.8Ti0.2O2, (D) Ce0.7Ti0.3O2, (E) Ce0.6Ti0.4O2, and (F) Ce0.5Ti0.5O2. The asterisk
indicates the anatase TiO2 100% peak. (b) Rietveld refined observed (O), calculated (+), and difference XRD patterns of Ce0.6Ti0.4O2.
sample was purged with He, and then O2/He was passed over
the catalyst. Formation of H2O is detected by the mass
spectrometer. When H2/He and O2/He are passed simulta-
neously, H2O is formed at 30 °C, but the temperature increased
rapidly due to the exothermic nature of the reaction H2 + O2
f H2O. This clearly suggests that the H2 adsorbed is either
dissociated or a precursor for dissociation. Thus, the isothermal
H2 uptake experiment also shows higher H2 uptake for Pt in
Ce0.85Ti0.15O2 than Pt in CeO2. Enhanced reducibility of CeO2
due to Ti ion substitution by hydrogen in both the presence
and the absence of Pt and enhanced catalytic activity of
Ce1-x-yTixPtyO2-δ compared to Ce1-xPtxO2-δ is to be attributed
to the structure of Ce1-xTixO2 and Pt/Ce1-xTixO2. Therefore, a
detailed study of the structure of the oxides was undertaken.
XRD Studies. Figure 7a shows the XRD pattern of Ce1-xTixO2
for x ) 0.0-0.5. All the XRD patterns can be indexed to the
fluorite structure. Peaks due to TiO2 in anatase or rutile phases
are not detectable up to x ) 0.4 even when the pattern is
expanded in the Y-scale. For x ) 0.5, the TiO2 (110) peak is
visible. There is a systematic increase in the 2θ values of the
diffraction lines with the increase in Ti indicative of solid
solution formation. The background is flat for 2θ ) 20-90°,
indicating the material is crystalline. The line width increased
with increasing Tix for x ) 0.2, which further supports the idea
of solid solution. To confirm further, Rietveld refinements were
carried out while simultaneously varying 17 parameters such
as overall scale factor and background parameter. Observed,
calculated, and difference plots are given for the x ) 0.4 sample
in Figure 7b (see Figure S2, Supporting Information, for x )
0.1 composition). The structural parameters are given in Table
2. Lattice parameter “a” decreased from 5.4131(6) Å for CeO2

Reducibility of Ce1-xTixO2 Compared to That of CeO2
J. Phys. Chem. B, Vol. 110, No. 11, 2006 5267
TABLE 2: Composition of CeO2, Ce1-xTixO2, and
Ce1-x-yTixPtyO2-δ Compounds Reduced by Hydrogen up to
675 °C
H2 uptake
% Ce4+
reduced
to Ce3+
up to 675 °C,
formula of the
reduced species
compd
mol/g (×10-4
)
CeO2
TiO2
Ce0.9Ti0.1O2
Ce0.85Ti0.15O2
Ce0.8Ti0.2O2
Ce0.7Ti0.4O2
2.3
10.0
7.4
CeO1.96
TiO1.92
8
Ce0.9Ti0.1O1.88
Ce0.85Ti0.15O1.86
Ce0.8Ti0.2O1.83
Ce0.7Ti0.3O1.77
Ce0.6Ti0.4O1.73
Ce0.99Pt0.01O1.95
Ce0.84Ti0.15Pt0.01O1.75
Ce0.83Ti0.15Pt0.02O1.63
15.5
15
17.5
22
23.5
10
36
8.8
11.1
15.9
20.0
2.9
15.7
23.2
Ce0.6Ti0.4O2
Ce0.99Pt0.01O2-δ
Ce0.84Ti0.15Pt0.01O2-δ
Ce0.83Ti0.15Pt0.02O2-δ
60
to 5.3720(3) for x ) 0.4. A systematic decrease in the lattice
parameter confirms Ce1-xTixO2 solid solution formation.
In Figure 8a-c, XRD patterns of 1 atom % Pt in Ce0.85
-
Figure 8. XRD patterns of (a) Ce0.85Ti0.15O2, (b) Ce0.84Ti0.15Pt0.01O2-δ
,
Ti0.15O2 made by the combustion method and 1 atom % Pt
impregnated over combustion synthesized Ce0.85Ti0.15O2 are
shown by expanding the Y-scale 25 times after normalizing the
intensities with respect to the (111) peak. The position of the
100% peak expected due to anatase TiO2 at 2θ ) 25.3° [marked
arrow in Figure 8a] is absent in the pattern even at an expansion
of 25 times in the Y-scale. The Pt(111) peak is to be observed
at 2θ ) 39.8° and expansion of the curve in Figure 8b shows
that the intensity in the Pt(111) peak region is negligibly small;
it is in the noise level. But a good Pt(111) peak is observed for
the Pt/Ce0.85Ti0.15O2 impregnated sample (Figure 8c). Pt in H2-
PtCl6 taken in the combustion synthesis is converted into Pt°,
PtO, or PtO2; at least 100% intensity peak due to these oxides
should be of detectable intensity as in the Pt/Ce0.85Ti0.15O2
(impregnated) sample, but no such peak could be detected.
Therefore, Pt is finely dispersed as either atoms or ions. As is
shown later, Pt is dispersed as ions from XPS studies. Therefore,
the natural position for the Pt ions in CeO2 is the Ce4+ site.
and (c) 1 atom % Pt impregnated Ce0.85Ti0.15O2 (all the curves were
magnified by 25 times in the Y-scale).
high-resolution image confirm Pt ion substitution in the
combustion synthesized Pt/Ce0.85Ti0.15O2.
XPS Studies. To gain confidence in the XPS of Pt metal
peaks vs oxidized Pt in CeO2, Pt(4f) of Pt metal foil in metallic
contact with the XPS probe, a Pt metal foil insulated from the
XPS probe by the ZrO2 pallet were recorded. In addition, Pt
metal particles made into a pallet in contact with the probe, Pt
metal impregnated over Ce0.85Ti0.15O2, (NH3)4PtII(NO3)2, and
PtO2 (in the form of compact 1 mm thick pallets), were also
recorded. Pt metal foil and Pt metal particles did not show any
charging. Pt(4f) peaks from Pt metal foil that was insulated over
the ZrO2 pallet showed a charging shift of 2.6 eV. Pt metal
impregnated over Ce0.85Ti0.15O2 showed a charging shift of 6.4
eV. The Ce0.99Pt0.01O2-δ pallet showed charging of 6.3 eV.
Correction for the shift due to charging was done taking C(1s)
at 285 eV; since compact pallets are used for powder samples,
there is no differential charging. In Figure 10a, Pt(4f) of the
above samples are shown. Pt(4f7/2,5/2) peaks due to Pt metal foil
with no charging shift and with a charging shift after correction
overlap fully with binding energies at 71.1 and 74.2 eV. There
is no measurable increase in the width of the Pt(4f) peaks due
to charging. Pt(4f) peaks from the Pt metal impregnated sample
show an indication of surface oxidation from the decrease in
4f7/2,5/2 peak resolution, even though the main peaks are due to
Pt in the metallic state. The binding energy of Pt2+(4f7/2,5/2) from
(NH3)4PtII(NO3)2 is at 72.4 and 75.6 eV (curve D, Figure 10).
Thus, filling up the valley between (4f7/2,5/2) peaks in Pt metal
impregnated over Ce0.85Ti0.15O2 (curve C, Figure 10) is due to
surface oxidation of Pt° to the Pt2+ state. In curve E in Figure
10, the Pt(4f) peak from R-PtO2 is plotted where Pt is in the
4+ state. In this compound, there was no charging shift because
it is a conducting oxide. Binding energies of Pt(4f7/2,5/2) are at
75.0 and 78.2 eV, respectively. The Pt(4f) spectrum from
The structure of Ce0.99Pt0.01O
2-δ by EXAFS indeed showed Pt2+
ion substitution for Ce4+ in CeO2 where a unique Pt-Ce
distance is obtained at 3.28 Å.21 Rietveld refinement of XRD
profiles of Ce0.85Ti0.15O2 and Ce0.84Ti0.15Pt0.01O2-δ was done (see
Figure S3a,b, Supporting Information, for 1% and 2% Pt in
Ce0.85Ti0.15O2) and lattice parameter “a” is 5.4032(2) and 5.4052-
(3) Å, respectively. The increase in the lattice parameter is
significant, which indicates Pt ionic substitution in Ce1-xTixO2.
High-Resolution TEM Study. The high-resolution TEM
image and the electron diffraction pattern of combustion
synthesized Ce0.84Ti0.15Pt0.01O2-δ are shown in Figure 9, parts
a and b, respectively. The fringes spaced at ∼3.20 Å correspond
to CeO2(111) layers. Fringes due to Pt metal particles separated
at 2.30 Å are absent. The electron diffraction pattern clearly
shows the fluorite structure of the compound. No spot or ring
due to Pt metal is detectable. The average particle size is ∼150
Å, which agrees well with that calculated from X-ray diffraction
with use of the Scherrer formula. EDAX analysis on these
images showed a Pt:Ti:Ce ratio close to 76:23:1 as shown in
Figure 9c, which is close to the bulk composition within the
experimental error. In Figure 9d the high-resolution TEM image
of Pt metal impregnated over Ce0.85Ti0.15O2 is shown. Clearly,
Pt metal particles with Pt(111) fringes at 2.30 Å are observed
and are marked in the figure. The lattice fringes with 3.20 Å
are due to Ce0.85Ti0.15O2. Thus, the HRTEM study confirmed
the presence of Pt metal particles in the Pt impregnated over
Ce0.85Ti0.15O2. Therefore, the absence of Pt metal particles in
the TEM image and the presence of Pt X-ray emission on the
Ce0.99Pt0.01O2-δ is distinctly different and indicative of oxidized
Pt with more than one oxidation state as seen from Figure 10b.
Under such condition, it is common to resolve the spin-orbit
doublets into components due to different oxidation states.
Accordingly, the Ce0.99Pt0.01O2-δ is deconvoluted into sets of
Pt(4f7/2,5/2) spin-orbit doublets taking into account fwhm from
Pt metal 4f7/2,5/2 states, spin-orbit splitting of 3.2 eV, and
7
5
relative intensity of /2 and /2 states. The Pt(4f) spectrum of
Ce0.99Pt0.01 2-δ deconvoluted into three sets of spin-orbit states
is shown in Figure 10b. The Pt(4f7/2) peaks at 71.1, 72.4, and
O

5268 J. Phys. Chem. B, Vol. 110, No. 11, 2006
Baidya et al.
Figure 9. (a) HRTEM image, (b) the corresponding ED pattern, (c) EDAX of Ce0.84Ti0.15Pt0.01O2, and (d) HRTEM image of 1 atom % Pt impregnated
over Ce0.85Ti0.15O2.
75 eV coincide with model compounds with Pt in 0, 2+, and
4+ oxidation states. If the peak due to Pt° is not taken into
account, fwhm becomes too high for Pt2+ and Pt4+, which is
not realistic.
In the XPS study of the Pt(4f) core level spectrum in
Ce0.84Ti0.15Pt0.01O2-δ, the Pt(4f) was deconvoluted into two sets
presented. In Ce0.84Ti0.15Pt0.01O2-δ, 63% Pt is in the 2+ state
and 37% in the 4+ state. For 2 atom % Pt-substituted sample,
about 7% Pt is found in the Pt° state and 65% Pt in the Pt2+
state. In the H2-reduced sample, the peak intensity at 71.1 eV
due to Pt° is 49% and that of Pt2+ is 45%. There is a decrease
in the Pt4+ peak intensity and an increase in the Pt° state upon
H2 reduction in the reduced sample. Peaks due to the Pt2+ state
persist. Ce0.83Ti0.15 Pt0.02O2-δ upon hydrogen reduction turns
black. The sample upon exposure to air even at room temper-
ature changes to gray, which is the same color as the “as-
prepared” sample. Thus, in combustion synthesized 1 and 2 atom
% Pt/Ce0.85Ti0.15O2, Pt is present mainly in the 2+ state.
The surface concentration of Pt in all the as-prepared samples
was calculated employing the following equation:28
of spin-orbit doublets, since intensity due to Pt° is not observed.
Accordingly, the Pt(4f7/2,5/2) peaks at 72.6, 75.8 and 75.0, 78.2
eV are due to Pt in 2+ and 4+ oxidation states, respectively
(Figure 11a). In Ce0.83Ti0.15Pt0.02O2-δ (Figure 11b), the Pt(4f7/2,5/2)
region was deconvoluted into three sets of spin-orbit doublets
since the intensity due to Pt° was significant. Accordingly,
Pt(4f7/2) peaks at 71.1, 72.7, and 74.9 eV are assigned to Pt°,
Pt2+, and Pt4+ oxidation states, respectively. The Ce0.83Ti0.15
-
Pt0.02O2-δ was reduced in a tubular furnace in H2 at 675 °C for
6 h and the XPS of the reduced sample showed a much higher
intensity of the Pt° state (Figure 11c). The same sample upon
air oxidation at 500 °C shows peak positions and intensities
due to Pt°, Pt2+, and Pt4+ as in the “as-prepared” sample (Figure
11d). In Table 3 the binding energies of Pt(4f7/2) in the model
compounds and the catalysts, full width at half-maximum
XPt
IPt(σCeλCe + σTiλTi)
(ICe + ITi)σPtλPt
)
(4)
X(Ce+Ti)
where X, I, σ, λ, and DE are the surface concentration, intensity,
photoionization cross-section, mean escape depth, and geometric
factors, respectively. The integrated area under the respective
core level peak after subtracting a linear background was taken
as the measure of observed intensity. The photoionization cross-
(fwhm) of the 4f7/2 peak, and relative intensities of Pt°, Pt2+
and Pt4+ states calculated from the respective areas are
,

Reducibility of Ce1-xTixO2 Compared to That of CeO2
J. Phys. Chem. B, Vol. 110, No. 11, 2006 5269
TABLE 3: Binding Energies of Pt(4f7/2), Relative Intensity,
and fwhms of Different Pt-Substituted Samples
binding energy rel intensity fwhm
catalyst
species of Pt4f7/2 (eV)
(%)
(eV)
Pt metal
Pt°
71.1
72.4
75
71.0
72.3
74.5
2.2
2.4
2.4
2.4
2.8
2.9
(NH3)4PtII(NO3)2
R-PtO2
Pt2+
Pt4+
Pt°
Ce0.99Pt0.01O2-δ
7
71
22
Pt2+
Pt4+
Pt°
Ce0.84Ti0.15Pt0.01O2-δ
Ce0.83Ti0.15Pt0.02O2-δ
Pt2+
Pt4+
Pt°
72.6
75
62.7
37.3
7.6
67.1
25.3
49.0
45.0
7.0
2.7
3.0
2.2
2.7
2.9
2.2
2.7
2.9
2.2
2.7
2.9
71.1
72.7
74.9
71.1
72.6
74.8
71.0
72.5
74.9
Pt2+
Pt4+
Pt°
Ce0.83Ti0.15Pt0.02O2-δ
(reduced at 675 °C
for 6 h)
Pt2+
Pt4
Ce0.83Ti0.15Pt0.02O2-δ
Pt°
12.4
62.1
25.5
(reoxidized in air at Pt2+
Figure 10. (a) XPS of the Pt(4f) core level in (A) Pt metal foil in
contact with the XPS probe, (B) Pt metal foil over the insulating ZrO2
pallet, (C) 1 atom % Pt impregnated over Ce0.85Ti0.15O2, (D) (NH3)4-
PtII(NO3)2, and (E) PtO2. (b) XPS of the Pt(4f) spectrum of Ce0.99Pt0.01O2-δ
deconvoluted into Pt°, Pt2+, and Pt4+ states.
500 °C for 1 h)
Pt4+
to 675 °C) are shown in Figure 12a-f. The binding energy of
Ti(2p3/2,1/2) at 459.0 and 464.8 eV in TiO2 corresponds to Ti in
the 4+ state (Figure 12a). Ti(2p3/2,1/2) peaks are observed at
459.0 and 464.8 eV showing Ti in the 4+ state in Ce0.6Ti0.4O2
(Figure 12b). The Ti4+ ion in Ce1-xTixO2, upon hydrogen
reduction in the TPR up to 675 °C, is fully reduced to the 3+
state as can be seen from the peaks at 458.2 and 464.0 eV
sections and mean escape depth were taken from the litera-
ture.29,30 The surface concentrations of Pt in the as-prepared 1
and 2 atom % Pt/Ce0.85Ti0.15O2 are 3.5% and 4.0%, respectively.
Thus, the surface concentration of Pt is higher than that in the
bulk, indicating significant surface segregation of platinum in
the as-prepared samples. Surface concentration of Ti in Ce1-xTixO2
was close to the bulk composition within experimental error of
(3% and no surface segregation of Ti is observed.
(Figure 12c).31 The Ti(2p3/2 1/2
, ) peaks in as-prepared 1 and 2
atom % Pt/Ce0.85Ti0.15O2 are observed at 458.8 and 464.7 eV
(Figure 12d,e) and Ti is essentially in the 4+ state. In the TPR
reduced 2 atom % Pt/Ce0.85Ti0.15O2 sample, Ti(2p3/2,1/2) peaks
are observed at 458.2 and 464.0 eV (Figure 12f) corresponding
to Ti in the 3+ state. This shows that all the Ti4+ are reduced
to the 3+ state.
The Ti(2p) spectra of pure TiO2, Ce0.6Ti0.4O2, Ce0.6Ti0.4O1.73
(reduced up to 675 °C in the TPR), as-prepared 1 to 2 atom %
Pt/Ce0.85Ti0.15O2, and 2 atom % Pt/Ce0.85Ti0.15O2 (reduced up
Figure 11. XPS of the Pt(4f) core level in (a) Ce0.84Ti0.15Pt0.01O2-δ, (b) Ce0.83Ti0.15Pt0.02O2-δ, (c) Ce0.83Ti0.15Pt0.02O2-δ (reduced up to 675 °C in H2),
and (d) Ce0.83Ti0.15Pt0.02O2-δ (reoxidized in air at 500 °C).

5270 J. Phys. Chem. B, Vol. 110, No. 11, 2006
Baidya et al.
Figure 12. XPS of the Ti(2p) core level from (a) TiO2, (b) Ce0.6Ti0.4O2, (c) Ce0.6Ti0.4O2 (reduced up to 675 °C), (d) Ce0.84Ti0.15Pt0.01
O2-δ, (e)
Ce0.83Ti0.15Pt0.02 2-δ, and (f) Ce0.83Ti0.15Pt0.02O2-δ (reduced up to 675 °C).
O
Figure 14. Valence band spectra of (a) pure CeO2, (b) Ce0.6Ti0.4O2
(reduced up to 675 °C), (c) Ce0.83Ti0.15Pt0.02 2-δ, (d) Ce0.83Ti0.15Pt0.02O2-δ
(reduced up to 675 °C), (e) PtO2, and (f) Pt metal.
Figure 13. XPS of the Ce(3d) core level of (a) Ce0.6Ti0.4O2, (b) Ce0.6
-
O
Ti0.4O2 (reduced in H2 up to 675 °C), (c) Ce0.83Ti0.15Pt0.02 2-δ (reduced
O
up to 675 °C), and (d) Ce0.83Ti0.15Pt0.02O2-δ (reoxidized in air at 500
°C).
a plus sign). However, TPR reduced Ce0.83Ti0.15Pt0.02O2-δ show
The Ce(3d) core level regions of Ce0.6Ti0.4O2, Ce0.6Ti0.4O1.73
(H2 reduced up to 675 °C), 2 atom % Pt/Ce0.85Ti0.15O2 (reduced
in H2 up to 675 °C), and the same sample air oxidized at 500
°C are given in Figure 13, curves a-d, respectively. Ce is
present in the 4+ state in Ce1-xTixO2. The Ce(3d) spectrum is
similar to that of CeO2 with 3d5/2 peaks at 882.9 and 901.3 eV
and the corresponding satellites are at 889 and 899 eV for 3d5/2.32
After TPR up to 675 °C, Ce in the 3+ as well as the 4+ states
are observed as can be seen from the satellites due to the 3+
state (marked with an asterisk) and the 4+ state (marked with
Ce mostly in the 4+ state (compare curves a and c in Figure
13c). The same sample upon air oxidation shows Ce in the 4+
state. Significant reduction of Ce4+ to Ce3+ in the TPR reduced
Ce0.83Ti0.15Pt0.02O2-δ is not observed. The TPR reduced sample
was black, but upon exposure to air while transferring to the
XPS chamber the sample turned gray and, therefore, the Ce
ion seems to become oxidized even at room temperature to the
4+ state.
In Figure 14, valence band spectra of CeO2, Ce0.6Ti0.4O2
reduced up to 675 °C, Ce0.83Ti0.15Pt0.02O2-δ, and the H2-reduced

Reducibility of Ce1-xTixO2 Compared to That of CeO2
J. Phys. Chem. B, Vol. 110, No. 11, 2006 5271
Ce0.83Ti0.15Pt0.02O2-δ are given. VB of Pt metal and PtO2 are
also given in the figure. In CeO2, the valence band consists of
O(2p) peaked at 6.5 eV and no electron density up to ∼3 eV
from EF. Pure TiO2 also shows a similar valence band due to
O(2p).33 On hydrogen reduction, the Ti3+ state with the Ti-
(3d1) band with measurable electron density below EF and above
the O(2p) band edge is observed as can be seen by curve B in
Figure 14. In the Pt ion substituted sample and the reduced
sample (curves C and D in Figure 14) also, an increase in the
electron density below EF is visible. Very high electron density
at EF is seen in Pt metal and the valence band extends up to 6
eV. PtO2 also shows high electron density just below EF due to
the Pt(5d) band. Since the extent of Ce4+ reduction is small,
the Ce(4f1) peak at ∼2 eV is not discernible in the VB region.
Therefore, we can write the formula of the compound as
Ce0.85-xPtxTi0.15O2-δ. The exact composition of oxide ions is
difficult to ascertain. However, this compound can be reduced
to a greater extent than Ce0.85Ti0.15O2. While Ce0.85Ti0.15O2 can
3+
be reduced to Ce0.704+Ce0.153+Ti0.15
O
1.86
, Ce0.84Ti0.15Pt0.01O2-δ
can be reduced to Ce0.52+4Ce0.32+3Ti0.15+3Pt0.01
O
1.75
up to 675
0/+2
°C. This means, more Ce4+ can be reduced with Pt2+ ionic
substitution in Ce1-xTixO2.
The Ce(4f) band lies about 2 eV below the EF as observed
from the valence band spectrum of Ce2O3.32 In CeO2 it is empty.
Pt metal has a high electron density at EF (Figure 14). Even in
PtO2 where Pt is in the 4+ state, Pt(5d) electrons have
sufficiently high electron density at EF. Certainly, Pt in the 2+
state should have electron density sufficiently high near EF. In
TiO2, the Ti(3d) band lying below the EF is empty. The distinct
Ti3+(3d) band peak at ∼1.5 eV below EF is well separated from
the O(2p) band in Ti2O3.33 Thus, Ti(3d), Pt(5d), and Ce(4f)
bands overlap below EF and, therefore, any change in the
electron density due to reduction of Pt2+ ion to Pt° should
populate Ti(3d) and Ce(4f) levels. Since the Pt° (6s2+5d8) band
has high electron density at EF and the Ti(3d) band lying below
is empty in the Ti4+ state, oxidation of Pt° to Pt2+ should occur,
simultaneously reducing Ti4+ to the Ti3+ state. Increased
reduction of Ti4+ and Ce4+ ion in the Pt-substituted sample is
to be attributed to such an electronic interaction. Increase of
CO oxidation rate over Ce0.84Ti0.15Pt0.01O2-δ compared to
Ce0.99Pt0.01O2-δ should then be due to facile electron/hole
transfer from Pt to Ti and Ce sites. Accordingly, the CO
oxidation reaction can be written as follows:
Discussion
Our interest in this study was to see how Ti4+ ionic
substitution for Ce4+ in CeO2 influences redox properties of
CeO2 and Ce1-xPtxO2-δ. In our earlier studies, we had shown
that -Ce4+-O-Pt2+-0-Ce4+- interaction in Ce1-xPtxO2-δ
is responsible for dissociated hydrogen adsorption at low
temperature. Further, CO adsorption over Pt2+ sites and higher
dissociative chemisorption of oxygen over oxide ion vacancy
sites around Pt2+ leading to higher CO oxidation activity were
shown. Here, formation of Ce1-xTixO2 (x ) 0.1-0.4) solid
solution by the combustion method is confirmed from XRD
studies. Pure CeO2 is reduced to CeO1.96 in H2 up to 675 °C
and TiO2 to TiO1.92. However, Ce1-xTixO2 can be reduced to a
greater extent. That Ti4+ in Ce1-xTixO2 is reduced to the Ti3+
state fully is shown by XPS. With 15% Ti4+ ion substitution,
Ce0.85Ti0.15O2 could be reduced to Ce0.85Ti0.15O1.86. This means
15% of Ti4+ is reduced to the Ti3+ state and 15% of Ce4+ can
be reduced to the Ce3+ state compared with just 8% in pure
O2 + 2V•O• f 2Oox + 4h•
-
2
2[Ce4+-O-Pt2+-[Oox]-Ti4+] + 2COads f
CeO2 and 16% of Ti4+ in TiO2. Ce0.6Ti0.4O2 could be reduced
2CO2 + 2[Ce4+-O-Pt°-0-Ti4+] + 2V•O•
2-
3+
to Ce0.714+Ce0.143+Ti0.4
O
1.73
retaining the gross fluorite struc-
ture that can be oxidized back to Ce0.6Ti0.4O2. Thus, reducibility
of Ce1-xTixO2 is higher than that of CeO2 or TiO2.
2[Ce4+-O-Pt°-0-Ti4+] + 2h• f 2[Ce3+-O-Pt2+-0-Ti3+]
The Ti4+ ion is too small to occupy the 8-coordinated Ce4+
site in the CeO2 structure. The Ce-O bond length is 2.34 Å.
With the lattice parameter of 5.372 Å for Ce0.6Ti0.4O2, Ce-O
and Ti-O bond lengths should be 2.32 Å. The Ti4+ ion with
size 0.74 Å, being much smaller than Ce4+ (0.98 Å), prefers 6
coordination as in anatase or rutile phases. Ti-O bond distances
are 1.96 and 1.93 Å in the TiO2 anatase phase. The Ce1-xTixO2
phase shows large line broadening in the XRD (see Figure 7)
suggesting that oxide ion sublattice may be distorted leading to
a change in the local coordination around the Ti4+ ion. It is
possible that local coordination of Ti can be 6+2. This means
6 O2- of the 8 oxide ions come closer from the mean of 2.32
Å to ∼2 Å due to the smaller Ti4+ ion and the remaining two
oxide ions may move to longer distances. The oxide ions moving
away from the shorter Ti-O distances must be easier to remove
by H2 thus explaining the higher reduciablity of Ce1-xTixO2.
Since oxide ion is bonded to both Ti4+ and Ce4+ ions, both of
them are reduced. In Ce1-xCuxO2-δ, for example, the extent of
Ce4+ ion reduction was higher due to Cu2+ ion substitution.20
In Ce1-xZrxO2, 4+4 coordination for the Zr4+ ion is suggested
to explain the high reducibility of the Ce4+ ion due to the Zr4+
ionic substitution.34 A detailed local structure of the Ti4+ ion
in CeO2 by EXAFS is essential to understand higher chemical
reactivity. A more interesting point is its reducibility in the Pt
ion substituted Ce0.85Ti0.15O2. XRD, TEM, and XPS studies
indicated clearly that Pt is in the ionic state. Since Pt is present
mainly in the 2+ state, oxide ion vacancies need to be created.
2[Ce3+-O-Pt2+-0-Ti3+] + 2h• f [Ce4+-O-Pt2+-0-Ti4+]
2CO + O2 ) 2CO2
From our XPS and hydrogen reduction studies, the reaction
O-Pt°-0-Ti4+-O + h• f O-Pt2+-0-Ti3+-O
is more facile than Ce4+-O-Pt°-0- + h• f Ce4+-O-Pt2+-0-. This
should be expected because the Ti(3d) band is closer to the
Pt(5d) band and electron or hole transfer from Pt(5d) to Ti(3d)
is more facile. Thus, the reason for higher catalytic activity for
CO oxidation is due to energetically more favorable electron
transfer from Pt(5d) to Ti(3d) than Pt(5d) to Ce(4f). If such a
proposition is correct, Ti1-xPtxO2-δ should be much more active
for CO oxidation than the Pt-substituted Ce1-xTixO2. Indeed,
Ti0.99Pt0.01O2-δ was prepared by the solution combustion method
and the formation of Ti0.99Pt0.01O2-δ in the anatase phase with
Pt in the 2+ and the 4+ states has been well characterized.35
The CO oxidation over Ti0.99Pt0.01O2-δ under the same experi-
mental condition as in Figure 1 indeed shows T50 at 140 °C
(see Figure 1). This confirms that the Pt(5d) f Ti(3d) interaction
is responsible for the enhanced catalytic behavior.
While TiO2 (anatase) itself is a good reducible support, the
oxide transforms to TiO2 (rutile) above 650 °C36 whereas
Ce1-xTixO2 is stable up to 750 °C without any TiO2 separation.
Pt ion substitution in Ce1-xTixO2 has been possible and the

5272 J. Phys. Chem. B, Vol. 110, No. 11, 2006
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in the oxidized state, viz., Pt2+, Ti4+, and Ce4+, as in the above
reaction scheme is essential for high TOF. The Ce3+ ion is more
easily oxidizable than Ti3+ to Ti4+. Therefore, from the catalysis
consideration, Ce1-xTixO2 as a support is more important than
pure TiO2.
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Conclusions
(a) Ce1-xTixO2 (x ) 0.1-0.4) solid solutions were synthesized
by the solution combustion method.
(b) Ce1-x-yTixPtyO2 (x ) 0.15; y ) 0.01-0.02) crystallites
synthesized by a single step synthesis show that Pt is mainly in
the Pt2+ state.
(c) H/Pt molar ratios are 5 and 4 over Ce0.84Ti0.15Pt0.01O2 and
Ce0.99Pt0.01O2-δ, respectively, and the ratio is just 0.078 over
about 8 nm Pt metal particles at 30 °C.
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(d) Reducibility of CeO2 is enhanced by Ti4+ ion substitution
in Ce1-xTixO2 (x ) 0.0-0.4), and Pt ion substitution in
Ce1-xTixO2 enhances its reducibility further compared to that
of the Pt ion in CeO2.
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(e) The rate of CO conversion over Ce0.84Ti0.15Pt0.01O2-δ is
45 times higher than that over Ce0.99Pt0.01O2-δ at 155 °C.
(f) Electronic interaction between Ce4+-Ce3+, Ti4+-Ti3+
,
and Pt2+-Pt° via overlap of Ce(4f), Pt(5d), and Ti(3d) states
in the valence band of Ce1-x-yTixPtyO2-δ is suggested to be
responsible for faster electron transfer for higher CO + O2
catalytic activity.
Acknowledgment. The authors gratefully acknowledge
financial support from the Department of Science and Technol-
ogy, Government of India.
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Supporting Information Available: Hydrogen uptake as a
function of time at 30 °C over Ce0.83Ti0.15Pt0.02O2 is given in
Figure S1, the Rietveld refined XRD profile of Ce0.9Ti0.1O2 in
Figure S2, and the Rierveld refined XRD profile for 1% and
2% Pt in Ce0.85Ti0.15O2 in Figure S3a and S3b, respectively.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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