Ionic dispersion of Pt and Pd on CeO2 by combustion method: Effect of metal-ceria interaction on catalytic activities for NO reduction and CO and hydrocarbon oxidation

Article abstract of DOI:10.1006/jcat.2000.3048

Ceria-supported Pt and Pd catalysts were synthesized by the combustion method and were characterized by XRD, transmission electron microscopy, and XPS. Pt and Pd metals were ionically dispersed on the CeO₂ surface of crystallite sizes in the ra

Full text of DOI:10.1006/jcat.2000.3048

Journal of Catalysis 196, 293–301 (2000)
doi:10.1006/jcat.2000.3048, available online at http://www.idealibrary.com on
Ionic Dispersion of Pt and Pd on CeO2 by Combustion Method: Effect
of Metal–Ceria Interaction on Catalytic Activities for NO Reduction
and CO and Hydrocarbon Oxidation
,1
Parthasarathi Bera, K. C. Patil,† V. Jayaram, G. N. Subbanna,‡ and M. S. Hegde
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India; †Department of Inorganic and Physical Chemistry,
Indian Institute of Science, Bangalore 560012, India; and ‡Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
Received April 13, 2000; revised August 21, 2000; accepted August 28, 2000
Therefore, CeO2 seems to have a promoting effect on the
metal in addition to acting as a support. Besides these in-
vestigations, several authors have reported different kinds
of interaction between noble metals and ceria and their
effects on catalytic activities (10–18). These studies have
shown that the nature and extent of interaction depend on
particular noble metal, catalyst pretreatment, preparation
technique, size of CeO2 crystallite, lattice oxygen, and so
on. Despite these studies, the electronic structure of noble
metals on the CeO2 surface and the role played by CeO2 in
promoting the catalytic reactions are not well understood.
Recently, we have reported the preparation by the com-
bustion technique of Cu/CeO2 (19) and nanosize noble
metal particles dispersed on -Al2O3 (20, 21) which are
active catalysts for NO reduction and CO and hydrocar-
bon oxidation. Here we report the synthesis of M/CeO2
(M = Pt, Pd) catalysts by the combustion method for the
first time. Interestingly, unlike -Al2O3 both Pt and Pd have
been found to be ionically dispersed over CeO2 support.
The objectives of the present studies are to investigate the
interaction between noble metals and ceria in M/CeO2 and
its comparative effects on reactivity toward NO reduction
and CO, CH4, and C3H8 oxidation with M/Al2O3.
Ceria-supported Pt and Pd catalysts have been synthesized by
the combustion method. The catalysts are characterized by X-ray
diffraction (XRD), transmission electron microscopy (TEM), and
X-ray photoelectron spectroscopy (XPS). Pt and Pd metals are ion-
ically dispersed on the CeO2 surface of crystallite sizes in the range
of 15–20 nm. In 1% Pt/CeO2 Pt is found to be in the + 2 and + 4
oxidation states whereas Pd is in the + 2 state in 1% Pd/CeO2. Cata-
lytic activities for NO reduction by CO, NH3, CH4, and C3H8 and
CO, CH4, and C3H8 oxidation by O2 have been investigated over all
these catalysts using the temperature-programmed reaction tech-
nique. The results are compared with Pt and Pd metals dispersed on
-Al2O3 support prepared by the combustion technique. Both oxi-
dation and reduction reactions occur at much lower temperatures
over M/CeO2 compared to those over M/Al2O3 (M = Pt, Pd). The
rate and turnover frequency of NO + CO and CO + O2 reactions
over M/CeO2 are higher than those over M/Al2O3. The observed en-
hanced catalytic activity of M/CeO2 has been attributed to the ionic
dispersion of Pt and Pd on CeO2 leading to a strong metal–ceria in-
teraction in the form of solid solution, Ce1 xMxO2
n)x/2, having
(4
linkages of the type –O2 –Ce4+–O2 –Mn+–O2 (n = 2, 4) on the
c
CeO2 surface.
2000 Academic Press
Key Words: combustion synthesis; metal–ceria interaction; Pt/
CeO2; Pd/CeO2; NO; CO; hydrocarbon.
INTRODUCTION
EXPERIMENTAL
In recent years, much attention has been focused on
CeO2-supported noble metal catalysts due to their appli-
cations in automotive exhausts. Noble metal catalysts sup-
ported on Al2O3 (1) are well-known three-way catalysts
(TWC). CeO2 is added as a promoter to TWC for its oxy-
gen storage capacity (OSC) and stabilization of the metal
dispersion (2–7). CeO2 along with BaO is added to Al2O3-
supported noble metal catalysts to improve its performance
(8). Recently, Ferna´ndez-Garc´ıa et al. (9) have shown the
influence of ceria on Pd activity for the CO + O2 reaction.
Preparation of Catalysts
The combustion mixture for the preparation of
1% Pt/CeO2 contained (NH4)2Ce(NO3)6, H2PtCl6, and
C2H6N4O2 (oxalyldihydrazide) in the mole ratio 0.99 :
0.01 : 2.33. Oxalyldihydrazide (ODH) prepared from di-
ethyl oxalate and hydrazine hydrate was used as the fuel
(22). In a typical preparation, 10 g of (NH4)2Ce(NO3)6 (E.
Merck India Ltd., 99.9% ), 0.095 g of H2PtCl6 (Ranbaxy
Laboratories Ltd., 99% ), and 5.175 g of ODH were dis-
solved in the minimum volume of water in a borosilicate
dish of 130 cm3 capacity. The dish containing the redox
mixture was introduced into a muffle furnace maintained
1 To whom correspondence should be addressed. Fax: +91-80-3601310.
E-mail: mshegde@sscu.iisc.ernet.in.
293
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

294
BERA ET AL.
at 350 C. Initially the solution boiled with frothing and mass spectrometer QXK300 (VG Scientific Ltd., England)
foaming and underwent dehydration. At the point of com- for product analysis in a packed-bed tubular reactor. The
plete dehydration, the surface ignited, burning with a flame details of this home-made TPR set-up have been described
( 1000 C) and yielding a voluminous solid product within elsewhere (23). Typically 0.1–0.2 g of the catalyst was
5min. Similarly, 1% Pd/CeO2 hasbeen prepared. In the case loaded in a quartz tube reactor of 20 cm length and 6 mm
of Pd the precursor used was PdCl2 (Glaxo India Ltd. 99% ). diameter. The reactor can be heated from 30 to 750 C at a
The color of 1% Pt/CeO2 is light gray and 1% Pd/CeO2 rate of 15 C min 1 and the sample temperature was mea-
is dull white. Similarly, 2–5% Pt/CeO2 and 5% and 10% sured by a fine chromel–alumel thermocouple immersed
Pd/CeO2 were also prepared by this method. Details of in the catalyst. The quartz tube was evacuated to 10 6 Torr.
the preparation of -Al2O3-supported metal particles have The gaseous products were sampled through a fine control
been described earlier (20). These compounds were pre- leak valve to an ultrahigh-vacuum (UHV) system housing
pared in an open muffle furnace kept in a fume cupboard the quadrupole mass spectrometer at 10 9 Torr. The gases
with exhaust. The reaction can be controlled by carrying were passed over the catalyst at a flow rate of 25 mol
1
1
out the combustion in an open vessel. As-prepared com-
s
and the flow rate can be varied from 10–40 mol s
.
5
bustion products without further reduction by H2 are used Accordingly, space velocity was in the range of 5.0 10
4
1
for all the studies including the catalytic reactions.
to 2.0 10 mol g
s
1. The dynamic pressure of gases
was in the range of 1 to 20 Torr in the reaction system in
all of the experiments. All the masses were scanned every
10 s. At the end of the reaction, the intensity of each mass
as a function of temperature (thermogram) was generated.
NO, NH3, CO, CH4, C3H8, and O2 gases were obtained
from Bhoruka Gases Ltd., Bangalore. Their purities were
better than 99% as analyzed by the quadrupole mass
spectrometer.
Characterization of Catalysts
XRD patterns of the catalysts were recorded on a
Siemens D5005 diffractometer using CuK radiation with
a scan rate of 2 min 1. A JEOL JEM-200CX transmission
electron microscope operated at 200 kV was used to carry
out TEM studies. Samples for TEM studies were prepared
by taking a small quantity of powder in ethanol/acetone
medium and was ultrasonicated for a few minutes. The re-
sultant slurry was then deposited on a holey carbon-coated
200 mesh copper grid. Suitable regions in the grid were se-
lected for electron diffraction and microscopy.
RESULTS
Structural Studies
XPS of these materials were recorded in an ESCA-3
Mark II spectrometer (VG Scientific Ltd., England) using
AlK radiation (1486.6 eV). Binding energies were calcu-
lated with respect to C(1s) at 285 eV. Binding energies were
measured with a precision of 0.2 eV. For XPS analysis the
powder samples were made into pellets of 8 mm diameter
and placed into an ultrahigh-vacuum (UHV) chamber at
10 9 Torr housing the analyzer. Prior to mounting the sam-
ple into the analyzing chamber it was kept in the prepara-
XRD patterns of 1–3% Pt/CeO2 are shown in Fig. 1. The
patterns can be indexed for CeO2 with fluorite structure.
The diffraction lines are broad and the crystallite sizes cal-
culated by using the Debye–Scherrer method are in the
range of 15–25 nm. Diffraction lines due to Pt metal or any
platinum oxides have not been seen. Even the XRD pattern
ofas-prepared 5% Pt/CeO2 doesnot contain Pt metalpeaks.
The 1–5% Pt/CeO2 samples when heated in air at 500 C for
40 h do not show any Pt metal or oxide peaks. However, on
heating at 800 C for 40 h, 2–5% Pt/CeO2 samples show Pt
metalpeaksin the XRD patternswhere Pt peak intensityin-
creases as the concentration of Pt increases. XRD patterns
of 1% , 5% , and 10% Pd/CeO2 are given in Fig. 2. Here also
Pd metal or PdO peaks are not observed in the as-prepared
samples. Crystallites of CeO2 calculated from the XRD pat-
tern are in the range of 20 nm. Pd metal or PdO could not
be detected on heating of 1% and 5% Pd/CeO2 catalysts at
800 C for 24 h. However, 10% Pd/CeO2 sample shows weak
diffraction lines due to PdO on heating at 800 C for 24 h.
Thus, 1% Pt/CeO2 and 1% , 5% , and 10% Pd/CeO2 samples
are devoid of Pt or Pd metal particles or their oxides in the
as-prepared as well as heated samples. Catalytic studies are
therefore carried out using 1% Pt/CeO2 and 1% Pd/CeO2
catalysts.
9
tion chamber at 10 Torr for 5 h in order to desorb any
volatile species present on the sample.
Adsorption Studies
Adsorption of CO on 1% Pt/CeO2 and 1% Pt/Al2O3 was
carried out in a constant volume glass system. Typically 0.5 g
of the catalyst was degassed at 250 C in a vacuum system
(10 6 Torr) for 3 h, and then cooled to 0 C. A known vol-
ume ofCO waspassed into the sample tube and the pressure
was measured by a digital pirani gauge calibrated against
N2. The volume of CO adsorbed expressed in cubic cen-
timeters (STP) was accurate within 10% .
Temperature-Programmed Reaction (TPR)
The catalytic reactions were carried out in a temperature-
TEM of the 1% M/CeO2 shows that the crystallite sizes
programmed reaction system equipped with a quadrupole are in the range of 15–20 nm which agrees well with the

IONIC DISPERSION OF Pt AND Pd ON CeO2
295
sets of spin orbital doublets. Accordingly, Pt(4f) peaks at
71.7 and 74.9 eV and 74.0 and 77.2 eV in 1% , 3% , and 4%
Pt/CeO2 can be assigned to Pt2+ and Pt4+ oxidation states
(12, 24). Peaks due to Pt(4f) metal at 71.0 and 74.2 eV are
not observed in as-prepared 1–5% Pt/CeO2 catalysts. Thus,
Pt metal in 1–5% Pt/CeO2 is in the +2 and +4 oxidation
states. However, 2–5% samples when heated to 800 C for
40 h do show Pt(4f) peaks due to Pt0, Pt2+, and Pt4+. The
Pt(4f) region could be resolved into three sets of spin orbit
peaks and indeed Pt0 (4f7/2,5/2) peaks at 71.0 and 74.2 eV
were seen. The intensity of the Pt0 peak is about 25% of the
total Pt(4f) envelope in the 2% Pt/CeO2 heated sample.
Pd(3d) spectra are given in Fig. 5 for 1% Pd/CeO2, 5%
Pd/CeO2, 10% Pd/CeO2, and 1% Pd/Al2O3. Pd(3d5/2,3/2
)
peaks observed at 338.0 and 342.8 eV in Pd/CeO2 cata-
lysts are shifted by 2 eV with respect to Pd metal in 1%
Pd/Al2O3. Binding energies of Pd(3d) peaks in Pd/CeO2
catalysts agree well with those of Pd2+ in PdCl2 indicating
that Pd is in the +2 state (25). Thus, the shifted Pd2+ peaks
show that Pd on CeO2 is in the highly ionic state. Even in
10% Pd/CeO2, Pd metal peaks are not seen. This observa-
tion agrees well with the XRD and TEM results. In Fig. 6
Ce(3d) peaks obtained from 1% Pt/CeO2 and 1% Pd/CeO2
are shown. The spectra with satellite features (marked in
the figure) correspond to CeO2 with Ce in the +4 oxidation
state (12, 26).
FIG. 1. XRD patterns of as-prepared (a) 1% Pt/CeO2, (b) 2% Pt/
CeO2, and (c) 3% Pt/CeO2 catalysts and (d) 3% Pt/CeO2 catalyst after
heating at 800 C for 40 h.
XRD measurements. Typical TEM images of 1% M/CeO2
are given in Fig. 3. The polycrystalline diffraction pattern
observed could be indexed only to CeO2 with fluorite struc-
ture. The morphology of CeO2 crystallites is cubic. Pt or Pd
metal diffraction rings are completely absent in the pattern.
Even high-resolution images of as-prepared 4% Pt/CeO2
give lattice fringes corresponding to (111) and (100) planes
of CeO2. However, 2–5% Pt/CeO2 samples heated at 800 C
for 40 h reveal the presence of Pt particles of 2–4 nm in the
image and a ring is observed due to Pt metal in the elec-
tron diffraction. In contrast to Pt and Pd on CeO2, nanosize
metal particles dispersed on -Al2O3 can be seen. TEM im-
ages of 1% Pt/Al2O3 and 1% Pd/Al2O3 are given in Figs. 3c
and 3d for comparison and the average sizes of Pt and Pd
particles are 7 and 10 nm, respectively. TEM results confirm
that the metal particles are not present in the as-prepared
1% M/CeO2 catalysts.
XPS Studies
Photoelectron spectra of the Pt(4f) core level region of
Pt metal and 1% , 3% , and 4% Pt/CeO2 are given in Fig. 4.
In the Pt metal, 4f7/2,5/2 peaks are observed at 71.0 and
74.3 eV respectively. The Pt(4f) region shows peaks due
to multiple oxidation states in the Pt/CeO2 catalysts. The
Pt(4f7/2,5/2) peaks in Pt/CeO2 catalysts were resolved into
FIG. 2. XRD patterns of as-prepared (a) 1% Pd/CeO2, (b) 5% Pd/
CeO2, and (c) 10% Pd/CeO2 catalysts.

296
BERA ET AL.
FIG. 3. TEM of (a) 1% Pt/CeO2, (b) 1% Pd/CeO2, (c) 1% Pt/Al2O3, and (d) 1% Pd/Al2O3.
Surface concentrations of metal ions in M/CeO2 have isotherm on 1% Pt/Al2O3 isalso shown for comparison. This
been estimated by the relation
experiment was carried out to examine the dispersion of Pt
on CeO2 and Al2O3. The isotherms follow Langmuir type of
adsorption. Saturation CO adsorption is0.15cm3 g 1 for 1%
Pt/CeO2 and it is 3 times more than that on 1% Pt/Al2O3.
Translating these data into the number of CO molecules
adsorbed per Pt atom gives 0.11 and 0.012 CO molecules in
1% Pt/CeO2 and 1% Pt/Al2O3, respectively. Nearlyan order
of magnitude increase in CO adsorption over Pt/CeO2 with
respect to Pt/Al2O3 can be rationalized from fine Pt metal
particles (7 nm) dispersed on Al2O3 compared to Pt ions on
CeO2. The total number of surface Pt atoms was calculated
by taking (111) and (100) faces of Pt in 7-nm Pt clusters.
The ratio of the number of CO molecules to the number of
Pt surface atoms is 0.13. Therefore, the sticking coefficient
of CO on Pt/Al2O3 observed here is 0.13 which agrees well
with the value reported by Gruber (29).
XM
IM
Ce DE (Ce)
M DE (M)
Ce
M
=
,
[1]
XCe
ICe
where X, I, , , and DE are the surface concentration, in-
tensity, photoionization cross section, mean escape depth,
and geometric factors, respectively. Integrated intensities of
Pt(4f), Pd(3d), and Ce(3d) peaks have been taken into ac-
count to estimate the concentration. Photoionization cross
section and mean escape depths have been obtained from
Scofield (27) and Penn (28), respectively. The surface con-
centration of Pt is 15% in 1% Pt/CeO2 whereas it is 5% in
1% Pd/CeO2.
Adsorption Studies
The adsorption isotherm of CO over 1% Pt/CeO2 at 0 C
is given in Fig. 7. In the same figure, the CO adsorption
Assuming Pt taken in 1% Pt/CeO2 is dispersed as ions
on 15-nm surfaces of CeO2 crystallites and Ce4+ ions are

IONIC DISPERSION OF Pt AND Pd ON CeO2
297
FIG. 6. XPS of Ce(3d) core level region of (a) 1% Pt/CeO2 and
(b) 1% Pd/CeO2.
FIG. 4. XPS of Pt(4f) core level region of (a) Pt metal foil, (b) 1%
Pt/CeO2, (c) 3% Pt/CeO2, and (d) 4% Pt/CeO2.
Temperature-Programmed Reaction (TPR)
NO reduction by CO, NH3, CH4, and C3H8 and CO, CH4,
and C3H8 oxidation by O2 have been carried out over 1%
Pt/CeO2 and 1% Pd/CeO2 catalysts.
replaced by Pt2+ or Pt4+ ions, 20% of Ce4+ ions can be re-
placed by Pt2+ or Pt4+ ions. Further, the ratio of the number
of CO molecules to the number of Pt ions on 1% Pt/CeO2
is 0.12. Thus, the sticking coefficients of CO on 1% Pt/CeO2
and 1% Pt/Al2O3 are in the same range. Nearly 10 times in-
crease in CO adsorption on 1% Pt/CeO2 compared to 1%
Pt/Al2O3 is therefore primarily due to a higher dispersion of
Pt ions on CeO2. On the other hand, pure CeO2 and Al2O3
do not show CO adsorption at 0 C.
NO reduction by CO. An equimolar mixture of NO and
CO passed over these catalysts shows N2 and CO2 forma-
tion. A sharp decrease in NO concentration occurs at 140 C
and below 175 C complete conversion of NO has been ob-
served. An increase in CO2 (m/z = 44) concentration with
a simultaneous decrease in NO (m/z = 30) concentration is
taken as the measure of NO conversion. Since N2 and CO
FIG. 5. XPS of Pd(3d) core level region of (a) 1% Pd/Al2O3, (b) 1%
FIG. 7. Adsorption isothermsofCO on 1% Pt/CeO2 and 1% Pt/Al2O3
Pd/CeO2, (c) 5% Pd/CeO2, and (d) 10% Pd/CeO2.
at 0 C.

298
BERA ET AL.
have the same mass, above 175 C m/z = 28 is due to N2.
Similarly, total conversion of NO is observed below 270 C
on 1% Pt/CeO2. The reaction can be written as
2NO + 2CO → N2 + 2CO2.
[2]
NO reduction by NH3. NO reduction by NH3 over these
catalysts was carried out with NO + NH3 in 6 : 4 molar ra-
tios. A sharp decrease in NO concentration occurs at 210 C
in Pt/CeO2 and 100% NO conversion occurs below 225 C.
Here N2 and H2O are the only products. N2O is not ob-
served. The reaction on M/CeO2 can be given by
6NO + 4NH3 → 5N2 + 6H2O.
[3]
On the other hand, M/Al2O3 catalysts give N2O as a product
along with N2 and H2O. At higher temperatures N2O de-
composes. The reaction on M/Al2O3 catalysts can be written
as
4NO + 2NH3 → 2N2 + N2O + 3H2O.
[4]
NO reduction by CH4. A remarkable low-temperature
(350 C) NO reduction by CH4 occurs over 1% Pt/CeO2
catalyst giving N2, CO2, and H2O as the products. The cor-
responding 1% Pd/CeO2 catalyst shows complete NO con-
version at 450 C. The reaction can be written as
FIG. 8. Mass spectra of NO + C3H8 reaction over 1% Pt/CeO2 at
(a) 25 C and (b) 325 C.
4NO + CH4 → 2N2 + CO2 + 2H2O.
[5]
over Pt/CeO2 and Pd/CeO2 occurs at 110 C and 230 C, re-
spectively. The products are H2O and CO2. The reactions
can be written as
NO reduction by C3H8. NO reduction by C3H8 over 1%
Pt/CeO2 occurs above 325 C. Mass spectra of NO and C3H8
at 25 C and products N2, H2O, and CO2 at 325 C are shown
in Figs. 8a and 8b. At 325 C NO is fully utilized by C3H8.
The reaction can be written as
CH4 + 2O2 → CO2 + 2H2O
C3H8 + 5O2 → 3CO2 + 4H2O.
[8]
[9]
10NO + C3H8 → 5N2 + 3CO2 + 4H2O.
[6]
Temperatures of nearly 100% conversion of NO, CO,
CH4, and C3H8 for all the reactions over 1% Pt/CeO2 and
1% Pd/CeO2 catalysts are summarized in Table 1. Results
on the same reaction over 1% Pt/Al2O3 and 1% Pd/Al2O3
catalysts are also given in Table 1 for comparison. It may be
Formation of N2O (m/z = 44) in small quantity cannot
be ruled out in the NO reduction by CO, CH4, and C3H8.
However, dissociation of N2O over Pt in the NO + NH3
reaction has been reported by Takoudls and Schmidt (30).
Here also even if N2O is formed it may be dissociated at
higher temperature.
TABLE 1
CO oxidation by O2. CO oxidation was carried out over
1% Pt/CeO2 and 1% Pd/CeO2 catalysts in the presence of
O2. A sharp decrease in CO concentration is observed at
140 C in the case of 1% Pd/CeO2 and 100% conversion of
CO occurs below 200 C. Similarly, 100% CO conversion
occurs at 180 C over 1% Pt/CeO2. The reaction is written
as follows:
Nearly 100% Conversion Temperatures ( C) of NO, CO, CH4,
and C3H8 for the Following Reactions on CeO2- and Al2O3-
Supported Pt and Pd Catalysts
Reactions 1% Pt/CeO2 1% Pt/Al2O3 1% Pd/CeO2 1% Pd/Al2O3
NO + CO
NO + NH3
NO + CH4
NO + C3H8
CO + O2
270
225
350
325
180
400
110
400
280
425
475
230
425
175
175
275
450
330
175
330
230
350
330
475
525
230
350
300
2CO + O2 → 2CO2.
[7]
Hydrocarbon oxidation. Complete CH4 oxidation oc-
curs over 1% Pt/CeO2 and 1% Pd/CeO2 at 390 C and
330 C, respectively, whereas complete oxidation of C3H8
CH4 + O2
C3H8 + O2

IONIC DISPERSION OF Pt AND Pd ON CeO2
299
noted from the table that nearly 100% conversion occurs
at a much lower temperature with M/CeO2 catalysts com-
pared to that with M/Al2O3 catalysts for all the reactions.
All the reactions over pure CeO2 occur at much higher tem-
peratures (19).
Temperature-programmed desorption of 1% Pt/CeO2
and 1% Pd/CeO2 catalysts was carried out. Oxygen des-
orption has not been observed up to 550 C. Further, reac-
tion with H2 over the desorbed catalysts did not show H2O
formation even up to 600 C. This result suggests that O2
associated with Pt or Pd is strongly bound to the CeO2 lat-
tice and in NO reduction as well as CO and hydrocarbon
oxidation studies it is the feed O2 which is utilized during
the reaction.
Kinetics
The rate of the reactions is calculated by using the equa-
tion (31)
FIG. 10. Rate of CO conversion over CeO2- and Al2O3-based cata-
lysts for CO + O2 reaction.
F X
rate =
,
[10]
vW
catalyst. For example, 5.74 10 5 mol of Pt is present in 1 g
of1% Pt/CeO2 catalyst. The rate and TOF data ofNO + CO
and CO + O2 reactions over Pt/CeO2 and Pd/CeO2 cata-
lysts are summarized in Table 2. TOF values are higher for
M/CeO2 catalysts compared to those for M/Al2O3 catalysts.
TOF of CO oxidation on combustion-synthesized Pd/Al2O3
is higher than that of the same reaction on same catalyst
prepared by the incipient wetness method (32, 33).
The activation energy (Ea) has been calculated from
Arrhenius plots of ln(rate) vs 1/T for NO + CO and
CO + O2 reactions over these catalysts. Here, CO2 forma-
tion rates were taken into account to calculate the acti-
vation energy. There is a remarkable difference in activa-
tion energy between CeO2- and Al2O3-supported Pt and Pd
catalysts. For CO + O2 reaction over Pt/CeO2 Ea is 31.2 kJ
mol 1 whereas that for Pt/Al2O3 is 53.9 kJ mol 1. Similarly,
for NO + CO reaction over Pd/CeO2 and Pd/Al2O3 Ea are
where F is the inlet molar flow rate of the particular gas, X is
the fractional conversion of the gas at a particular temper-
ature, v is the stoichiometric coefficient of the gas, and W is
the weight of the catalyst. Rate is expressed in micromoles
per gram per second. The rates of NO + CO and CO + O2
are given in Figs. 9 and 10. The turnover frequencies (TOF)
of NO and CO conversion at different temperatures have
been calculated after dividing the rate ( mol g 1 s 1) by the
active site concentration ( molg 1). TOF isexpressed in in-
verse seconds. The active metal site concentration has been
obtained from the amount of the metal present in 1 g of the
TABLE 2
Rate ( mol g 1 s 1) and TOF (s 1) Data of NO + CO and CO + O2
Reactions over CeO2- and Al2O3-Supported Pt and Pd Catalysts
NO + CO
Rate
CO + O2
Catalysts
TOF
Rate
TOF
1% Pt/CeO2 27.10 (225 C) 0.47 (225 C) 25.00 (150 C) 0.44 (150 C)
47.50 (250 C) 0.83 (250 C) 57.08 (175 C) 0.99 (175 C)
1% Pd/CeO2 11.88 (125 C) 0.21 (125 C) 43.75 (150 C) 0.76 (150 C)
47.50 (150 C) 0.82 (150 C) 56.67 (175 C) 0.98 (175 C)
1% Pt/Al2O3 40.42 (375 C) 0.42 (375 C) 12.92 (200 C) 0.13 (200 C)
54.58 (400 C) 0.57 (400 C) 48.33 (225 C) 0.50 (225 C)
1% Pd/Al2O3 23.31 (325 C) 0.24 (325 C) 17.10 (225 C) 0.18 (225 C)
56.25 (350 C) 0.58 (350 C) 50.83 (250 C) 0.52 (250 C)
FIG. 9. Rate of NO conversion over CeO2- and Al2O3-based catalysts
for NO + CO reaction.

300
BERA ET AL.
52.6 and 60.6 kJ mol 1, respectively. Activation energies ically dispersed on the CeO2 crystallite surface. Unlike the
of CO oxidation on Pd/Al2O3, Pt/Al2O3 and Pt/CeO2 are case of Pt/CeO2, the surface concentration of Pd is only 5%
lower than the values reported in the literature (33–36).
for 1% Pd/CeO2. Pd2+ in Pd/CeO2 is highly ionic in nature.
Further, Pd2+ is a more stable ion. Therefore, it is possible
that Pd2+ ion can form solid solution Ce1 xPdxO2 x indicat-
ing that all the Pd metal taken in the preparation may not
be dispersed on the CeO2 crystallite surface. This is further
supported by the fact that even up to 10% Pd/CeO2 could
be prepared without any PdO or Pd metal formation by the
combustion method. From these studies we propose the
presence of –O2 –Ce4+–O2 –Mn+–O2 – type of linkages
on the CeO2 surface. Oxygen ion vacancies are created due
to lower valent ionic substitution for Ce4+ ions. Oxygen ion
vacancy should have beneficial effects on catalytic activities
due to the increase in oxygen ion mobility.
All the reactions over CeO2-supported Pt and Pd cata-
lysts occur at much lower temperature compared to those
over Al2O3-supported Pt and Pd catalysts (Table 1). But
CeO2-promoted Pd/Al2O3 is shown to suppress the alkane
oxidation (38–40). In these studies, addition of CeO2 to
Pd/Al2O3 enhances PdO formation in the presence of O2
along with CeAlO3 phase formation. In Pd/CeO2 catalysts
prepared here, formation of a PdO phase has not been ob-
served. The higher activity of the Pd/CeO2 catalyst noticed
here may be due to the availability of dissociated oxygen
via oxide ion defects formed in Ce1 xPdxO2 x. In general,
the reasons for the drastic decrease in reaction tempera-
ture over M/CeO2 compared to M/Al2O3 can be attributed
to a strong metal–ceria interaction. Ionic dispersion of Pt
and Pd metals in the form of a solid solution brings about
metal ion incorporation in the CeO2 matrix. Since only 1%
of Ce4+ ions are substituted by Pt2+ or Pd2+ ions, O2 ions
associated with the metal ions are bonded to Ce4+ ions.
This seems to be the reason for the stability of Pt2+ and
Pd2+ ions toward reduction. As pure CeO2 does not show
CO adsorption or any of the catalytic reactions at such low
temperatures, therefore, the active adsorption sites for NO,
CO, NH3, and other gases have to be Pt2+ and Pd2+ ions on
CeO2 surface. A nearly 10 times increase in CO adsorption
per mole of Pt in 1% Pt/CeO2 compared to 1% Pt/Al2O3 is
due to the higher dispersion of Pt on the CeO2 surface. Ac-
cordingly, CO oxidation and NO reduction rates and TOFs
are higher on Pt/CeO2 compared to those on Pt/Al2O3.
Electronic interaction involving Ce4+(4f), O2 (2p), and
Pd2+(4d) or Pt2+(5d) orbitals may facilitate the redox reac-
tions. For example, in the NO + NH3 redox reaction, elec-
trons released from NH3 may be transferred to NO via
–M2+–O2 –Ce4+–M2+–linkagesleadingto redoxexchange:
DISCUSSIONS
The combustion method involves rapid heating of an
aqueous solution containing stoichiometric amounts of cor-
responding metal nitrates and hydrazine-based fuels. Dur-
ing the combustion the temperature reached for a short pe-
riod (1–2 min) is about 1000 C. Evolution of a large amount
of gases during the process is responsible for fine crystal-
lite formation. So in combustion-derived M/CeO2, Pt or Pd
ions should either be separated into Pt or Pd metal clusters
(as in Pt/Al2O3) or Pt, Pd oxide clusters separated from
oxide support or ions should be incorporated. In the case
of Pt/CeO2 and Pd/CeO2, Pt and Pd ions seem to be in-
corporated due to low metal loading (1% by mole). Solid
solution of the type Ce1 xMxO2 is one of the routes of
x
incorporation of ions in CeO2. In contrast to this, in the
impregnation method for example, Pt2+, Pd2+ ions in solu-
tion are dispersed on already prepared CeO2 substrate and
dried at relatively low temperature (<500 C). Thus, some
of the important merits of the combustion method are fine
crystallites formation, easy incorporation of dopant ions in
the support/host lattice, no need of a hydrogen reduction
step, and purity of the catalysts.
In the present study all the catalytic reactions were car-
ried out over as-prepared 1% Pt/CeO2 and 1% Pd/CeO2
samples and therefore, investigation of chemical states of
Pt or Pd in these samples is important. It is clear from the
XRD as well as TEM studies that neither the metal parti-
cles nor any of the oxide phases of Pt or Pd on CeO2 are
observed in 1% Pt/CeO2 and 1% Pd/CeO2. XPS studies
show that Pt is in the +2 and +4 and Pd is in the +2 oxida-
tion states in 1% Pt/CeO2 and 1% Pd/CeO2, respectively.
Even in 10% Pd/CeO2, Pd(3d) peaks observed from XPS
correspond to the Pd2+ state. These observations suggest
that Pt and Pd ions are incorporated in CeO2 in the form of
a solid solution, Ce1 xMxO2
n)x/2. The ionic radii (37) of
(4
Pt2+ (80 pm) and Pd2+ (86 pm) are closer to that of Ce4+ (97
pm). Therefore, if these metal ions are formed in the prepa-
ration condition, Pt2+, Pd2+ ions can form solid solution of
the type Ce1 xMxO2 x (M = Pt2+, Pd2+).
By the combustion method Pt and Pd ions are homoge-
neously dispersed on nanosize CeO2 crystallites in a single
step. Considering a 15-nm mean size of CeO2 crystallites,
the Ce4+ ion concentration on the crystallite surfaces can
be calculated from its lattice parameter a. Assuming that Pt
ions are ionically dispersed on the CeO2 surface we find that
20% Ce4+ ions can be replaced by Pt ions in 1% Pt/CeO2.
Observations of Pt in the +2 and +4 states and 15% surface
–Ce4+–O2 –M2+– + e ⇔ –Ce4+–O2 –M+–
⇔ –Ce3+–O2 –M2+– ⇔ –Ce4+–O2 –M2+– + e.
concentration of Pt from XPS studies (against 1% taken in In anycase, such a possibilitydoesnot exist in M/Al2O3 cata-
the preparation) suggest that almost all the Pt ions are ion- lysts. Therefore, the metal–ceria interaction appears to play

IONIC DISPERSION OF Pt AND Pd ON CeO2
301
an important role in lowering the reaction temperature.
Detailed mechanism of these reactions involving Pt and
Pd ions over CeO2 needs further investigations.
9. Ferna´ndez-Garc´ıa, M., Martı´nez-Arias, A., Salamanca, L. N.,
Coronado, J. M., Anderson, J. A., Conesa, J. C., and Soria, J., J. Catal.
187, 474 (1999).
10. Summers, J. C., and Ausen, S. A., J. Catal. 58, 131 (1979).
11. Jin, T., Okuhara, T., Mains, G. J., and White, J. M., J. Phys. Chem. 91,
3310 (1987).
CONCLUSIONS
12. Jin, T., Zhou, Y., Mains, G. J., and White, J. M., J. Phys. Chem. 91, 5931
(1987).
13. Harrison, B., Diwell, A. F., and Hallett, C., Platinum Metals Rev. 32,
73 (1988).
14. Lo¨o¨f, P., Kasemo, B., Andersson, S., and Frestad, A., J. Catal. 130, 181
(1991).
15. Nunan, J. G., Robota, H. J., Cohn, M. J., and Bradley, S. A., J. Catal.
133, 309 (1992).
In conclusion, we have shown a new process of dispers-
ing Pt and Pd in the ionic form on a CeO2 surface by
the combustion method. Characterization techniques em-
ployed here are XRD, TEM, and XPS. Catalytic activities
for NO reduction and CO and hydrocarbon oxidation over
these catalysts have been examined by TPR. The salient
features of our studies are the following.
16. Serre, C., Garin, F., Belot, G., and Maire, G., J. Catal. 141, 1 (1993).
17. Serre, C., Garin, F., Belot, G., and Maire, G., J. Catal. 141, 9 (1993).
(1) Pt and Pd ions dispersed over CeO2 crystallites of 15– 18. Martı´nez-Arias, A., Coronado, J. M., Catalun˜a, R., Conesa, J. C., and
Soria, J., J. Phys. Chem. B 102, 4357 (1998).
19. Bera, P., Aruna, S. T., Patil, K. C., and Hegde, M. S., J. Catal. 186, 36
20 nm have been synthesized by the combustion technique.
(2) Pt is in the +2 and +4 states in 1% Pt/CeO2 and Pd
(1999).
is found to be in the +2 state in 1% Pd/CeO2.
20. Bera, P., Patil, K. C., Jayaram, V., Hegde, M. S., and Subbanna, G. N.,
(3) There is a metal–ceria interaction which leads to the
J. Mater. Chem. 9, 1801 (1999).
formation of a solid solution, Ce1 xMxO2
(4) Active adsorption sites for the catalytic reactions are
noble metal ions in M/CeO2 catalysts.
(5) NO reduction and CO and hydrocarbon oxidation
temperatures are much lower over M/CeO2 catalysts com-
pared to those over M/Al2O3 catalysts.
(6) M/CeO2 catalysts show higher rates and TOFs for
NO + CO and CO + O2 reactions.
.
21. Bera, P., Patil, K. C., and Hegde, M. S., Phys. Chem. Chem. Phys. 2,
373 (2000).
22. Graw, G., Anal. Chim. Acta 14, 150 (1956).
23. Hegde, M. S., Ramesh, S., and Ramesh, G., Proc. Indian Acad. Sci.
Chem. Sci. 104, 591 (1992).
24. Shyu, J. Z., and Otto, K., J. Catal. 115, 16 (1989).
25. Briggs, D., and Seah, M. P., “Practical Surface Analysis by Auger and
X-ray Photoelectron Spectroscopy,” Appendix 4. Wiley, New York,
1984.
(4 n)x/2
26. Sarma, D. D., Hegde, M. S., and Rao, C. N. R., J. Chem. Soc., Faraday
Trans. 2 77, 1509 (1981).
ACKNOWLEDGMENTS
27. Scofield, J. H., J. Electron Spectrosc. Relat. Phenom. 8, 129 (1976).
28. Penn, D. R., J. Electron Spectrosc. Relat. Phenom. 29, 9 (1976).
29. Gruber, H. L., J. Phys. Chem. 66, 48 (1962).
30. Takoudls, C. G., and Schmidt, L. D., J. Phys. Chem. 87, 958 (1983).
31. Burwell, R. L., Jr., Pure Appl. Chem. 46, 71 (1976).
32. Choi, K. I., and Vannice, M. A., J. Catal. 131, 1 (1991).
33. Rainer, D. R., Koranne, M., Vesecky, S. M., and Goodman, D. W.,
J. Phys. Chem. B 101, 10769 (1997).
P.B. and K.C.P. thank the Council of Scientific and Industrial Research
(CSIR), Govt. of India, for financial support. The Department of Atomic
Energy (DAE), Govt. of India, is gratefully acknowledged for funding this
research.
REFERENCES
34. Yao, Y.-F. Y., J. Catal. 87, 152 (1984).
1. Howitt, C., Pitchon, V., and Maire, G., J. Catal. 154, 47 (1995).
2. Taylor, K. C., Catal. Rev.-Sci. Eng. 35, 457 (1993).
3. Yao, H. C., and Yao, Y.-F. Y., J. Catal. 86, 254 (1984).
4. Kummer, J. T., J. Phys. Chem. 90, 4747 (1986).
35. Cant, N. W., Hicks, P. C., and Lennon, B. S., J. Catal. 54, 372 (1978).
36. Hardacre, C., Ormerod, R. M., and Lambert, R. M., J. Phys. Chem.
98, 10901 (1994).
37. Shannon, R. D., Acta Crystallogr. A 32, 751 (1976).
38. Yao, Y.-F. Y., Ind. Eng. Chem. Prod. Res. Dev. 19, 293 (1980).
39. Shyu, J. Z., Otto, K., Watkins, W. L. H., Graham, G. W., Belitz, R. K.,
and Gandhi, H. S., J. Catal. 114, 23 (1988).
5. Oh, S. H., J. Catal. 124, 477 (1990).
6. Oh, S. H., and Eickel, C. C., J. Catal. 112, 543 (1988).
7. Lo¨o¨f, B., Kasemo, B., and Keck, K.-E., J. Catal. 118, 339 (1989).
8. Thomas, J. M., and Thomas, W. J., “Principles and Practice of Hetero-
geneous Catalysis,” p. 577. VCH, Weinheim, 1997.
40. Oh, S. H., Mitchell, P. J., and Siewert, R. M., J. Catal. 132, 287
(1991).