Role of cerium-zirconium mixed oxides as catalysts for car pollution: A short review

Article abstract of DOI:10.1016/S0925-8388(98)00477-0

This article focuses on the role of mixed oxides in the Ce-Zr-O system, which is important as an excellent promoter for oxygen storage capacity in automotive three-way catalysts. In a model of Pt-alumina catalysts, there is a clear effect of CeO₂-ZrO₂ solid solution on NO and CO removal activities under dynamic air/fuel conditions. It is suggested that the design and development of automotive catalyst can be realized through research on complex oxides in Ce-Zr-O and related systems.

Full text of DOI:10.1016/S0925-8388(98)00477-0

Journal of Alloys and Compounds 275–277 (1998) 886–890
L
Role of cerium–zirconium mixed oxides as catalysts for car pollution:
A short review1
*
Masakuni Ozawa
Nagoya Institute of Technology, CRL, Tajimi, Gifu 507-0071, Japan
Abstract
This article focuses on the role of mixed oxides in the Ce–Zr–O system, which is important as an excellent promoter for oxygen
storage capacity in automotive three-way catalysts. In a model of Pt–alumina catalysts, there is a clear effect of CeO2 –ZrO2 solid solution
on NO and CO removal activities under dynamic air/fuel conditions. It is suggested that the design and development of automotive
catalyst can be realized through research on complex oxides in Ce–Zr–O and related systems.
1998 Elsevier Science S.A.
Keywords: Automobile exhaust; Catalysis; Ce–Zr mixed oxide
1. Background
models. Emission regulations have been applied in Euro-
pean countries since 1988, and more rigorous regulations
are planned for the USA. This paper is a short review of
the role of mixed oxides and possible material aspects of
Ce–Zr–O and related systems in industrial rare-earth
products for car pollution.
The automotive three-way catalyst system (TWC) gen-
erally operates under a certain air–fuel ratio (A/F) range.
The A/F is controlled by an oxygen sensor device so that
the catalyst maintains high purifying activity. Common
three-way catalysts consist of precious metals such as Pt,
Rh and Pd, promoters and a transition–alumina support.
Gandhi and coworkers [1] reported the role of ceria
(CeO2); the addition of CeO2 promotes dynamic per-
formance in purifying carbon monoxide (CO), nitrogen
oxides (NOx) and hydrocarbons (HC) under conditions of
rich–lean A/F in automotive exhaust, which is called the
oxygen storage capacity (OSC). CeO2 provides oxygen for
oxidizing CO and HC under rich A/F conditions and
removes it from the exhaust gas phase for reducing NOx
under lean A/F. Ce41 in the CeO2 lattice is readily
converted to Ce31 due to its nonstoichiometric behavior
[2–4]. The ceria in TWC has several roles including
promotion of the water gas shift reaction, the dispersion of
precious metals as well as inhibition of the sintering of the
alumina support [5–10]. Practical use under thermally
harsh conditions resulted in significant degradation of the
catalyst, including loss of surface area of the support,
sintering of precious metals and deactivation of the ceria.
Durability of catalysts is required for the advanced engine
systems now being introduced in various new automobile
2. Effect of cerium–zirconium mixed oxides on three-
way catalysts
Several articles have examined the effect of a zirconium
promoter in three-way catalysts, however only a few
critical papers deal directly with the OSC of the Ce–Zr–O
system in the initial stage of development. Stepien and
coworkers studied the effect of a ZrO2 support for Rh,
which had better durability than the Rh–Al2O3 system
[11]. Funabiki and Yamada described the co-addition of
Ce, Zr and Ba to precious metals catalyst [12]. They
indicated the formation of several complex oxides in this
system which should inhibit sintering of the catalysts.
Matsumoto and coworkers argued the role of solid solu-
tions of Ce–Zr–O systems as well as Ce–La–O as OSC
promoters in TWC, specifically leading to the improve-
ment of heat durability [13,14]. Masuda and coworkers
reported that a ZrO2 catalyst containing Ce and Pt from the
sol–gel method showed lower thermal stability and CO
conversion than a Ce–Pt catalyst [15]. Huuska and
Maunula examined the effect of Zr, Si, La and Ba on the
thermal aging of precious metal catalysts using the TPD
technique [16]. Ozawa and coworkers reported the applica-
*
Tel.: 181 572 27 6811; fax: 181 572 27 6812; e-mail:
ozawa@crl.nitech.ac.jp
1An invited paper.
0925-8388/98/$19.00
1998 Elsevier Science S.A. All rights reserved.
PII: S0925-8388(98)00477-0

M. Ozawa / Journal of Alloys and Compounds 275–277 (1998) 886–890
887
tion of Ce–Zr oxide solid solution to three-way catalysts.
An excellent improvement in dynamic performance was
achieved by the OSC effect of CeO2 –ZrO2 solid solution
formed in Pt/Al2O3 and practical catalysts [17].
Here, I summarize several factors for the OSC promoter
in the Ce–Zr–O system.
and a high reaction rate over the modified CeO2 particles.
Fig. 1 shows experimental results for the CO oxidation
reaction (in He) with a pulse technique over CeO2 and
modified CeO2 powders heated at 10008C. CO is oxidized
to CO2 with active oxygen from the oxide particles. The
0.9 g samples were pre-heated at 8008C for 1 h in flowing
oxygen, cooled and then reacted by pulsed CO of 8 mmol
at intervals of 3 min in He at a constant heating rate. Solid
solutions of Ce0.8Zr0.2O2 are formed in these powders
after heat treatment at 10008C. The results of the pulse
reaction indicate excellent improvement of the activity of
CeO2 with the addition of ZrO2 in both the mixed oxides
themselves and alumina-supported oxides.
In situ X-ray diffraction experiments indicate that the
steady-state reduction of both CeO2 and Ce12xZrxO2
powders is described by the simple kinetic equation kt 5
h1 2 (1 2 a)1 / 3j2, where k is the kinetic constant, a a
nonstoichiometric parameter for CeO22a, and t the time of
reaction in CO/N2. The reaction was controlled by the
diffusion of lattice oxygen to the surface, the same as for
Ce–La mixed oxides [23,24]. The formation of Ce–Zr
mixed oxides enhances the diffusion rate of lattice oxygen
for CO oxidation as well for inhibition of sintering of
CeO2. Such observations of diffusion via aspects of solid
state chemistry suggests, in principle, the possibility of
more excellent compositions or states existing in the Ce–
Zr–O and related systems.
2.1. Inhibition of sintering
The thermal deactivation of TWC is caused by several
factors. The main phenomenon is the sintering of the fine
particles of precious metals and alumina supports as well
as ceria when the catalyst is subjected to exhaust at
elevated temperatures for prolonged periods. One recent
requirement for materials is thermal stability and durability
for hot exhaust at around or above 10008C. CeO2 powder
readily sinters at elevated temperatures, although it is a
good refractory oxide with a high melting point. The
addition of zirconium, especially the formation of Ce–Zr
mixed oxides, is effective in the inhibition of the sintering
of ceria. Simple experiments indicate that Zr modification
of CeO2 powder, followed by solid state reactions, has the
effect of improving the thermal stability of CeO2 promoter
[13]. Chemical synthesis processes as well as impregnation
are expected to lead to excellent inhibition of the thermal
deactivation (sintering) of CeO2.
On the other hand, the catalytic reaction for the OSC is
the reaction between a reactant such as CO and oxygen at
a surface active site of CeO2. Enhanced oxygen diffusion
is an important factor for improving and developing OSC
promoters for TWC, however investigation of the pure
catalysis of OSC is required for future development.
Recently, several researchers have reported a better com-
position, Ce12xZrxO2, through systematic OSC measure-
ments [25–28].
2.2. Compounds (phases)
Many researchers have reported the phase diagrams of
the CeO2 –ZrO2 system [18–22]. In the Ce-rich region of
the diagrams, a cubic solid solution of Ce12xZrxO2 ap-
pears, while tetragonal and monoclinic solid solutions form
in the tight regions of the Zr-rich region. In the central
region, there are several structures such as tetragonal
Ce2Zr3O10 and other metastable mixed oxides. However,
the compounds or mixed oxides of the Ce–Zr–O system
are often obtained in their metastable states, which are also
useful for catalytic application. It is interesting that the
mixed fractions (compositions) of metals in the Ce–Zr–O
system form several different kinds of metastable and/or
stable solid solutions. In addition, the resulting phases
depend on the synthesis route of the oxides. One of the key
points for industrial development is the application of these
metastable compounds or phases which can be derived by
various advanced powder processing technologies.
2.3. Oxygen evolution and nonstochiometry
Oxygen evolution and/or uptake originates from the
nonstoichiometry and oxygen diffusion in the surface and
lattice of Ce12xZrxO2. The OSC promoter should satisfy
two factors: a wide operation range for redox between
Ce31 and Ce41 in reducing and oxidizing atmospheres,
Fig. 1. Conversion (%) to CO2 for the CO pulse reaction over powders of
(s) CeO2, (d) Ce0.8Zr0.2O2, (h) Al2O3-supported CeO2 (33 wt%), and
(j) Al2O3-supported Ce0.8Zr0.2O2 (33 wt%). All samples were heated at
10008C for 3 h in air.

888
M. Ozawa / Journal of Alloys and Compounds 275–277 (1998) 886–890
2.4. Dynamic performance of Pt catalysts given by high
oxygen storage capacity
Figs. 2 and 3 and Table 1 show examples of the
three-way activities of model catalysts: Pt/g-Al2O3, Pt/
CeO2 /g-Al2O3 and Pt/Ce0.8Zr0.2O2 /g-Al2O3. They were
prepared by the physical mixing of g-Al2O3-supported Pt
and CeO2 or Pt and Ce0.8Zr0.2O2. Starting powders of
Pt/g-Al2O3, Pt/CeO2 /g-Al2O3 and Ce0.8Zr0.2O2 /g-
Al2O3 were prepared by the impregnation of aqueous
solutions of Pt(NO2)2(NH3)2, Ce(NO3)3 and a mixture of
Ce(NO3) and ZrO(NO3)2 with powdered g-Al2O3. Pt/g-
Al2O3 was heated at 7008C for 3 h in air, and CeO2 /g-
Al2O3 and Ce0.8Zr0.2O2 /g-Al2O3 powders were heated at
10008C for 5 h in air before mixing. The heated samples
simulated the degree of sintering in the practical use of
Pt-based TWC at high temperatures. XRD measurements
of these powders confirmed the formation of crystalline
CeO2 and Ce0.8Zr0.2O2 (solid solution). The three-way
activity of the catalysts was tested using a simulated
mixture of gases (CO, NO, C3H6, CO2, H2, H2O, O2, N2
Fig. 3. Three-way removal efficiency vs. temperature for a model exhaust
(seven compositions) with A/F modulation over Al2O3-supported Pt with
CeO2 –ZrO2; mixture of powdered (5 wt% Pt/Al2O3)0.1(33 wt%
Ce0.8Zr0.2O2 /Al2O3)0.9. Conversion of (———) CO, (? ? ?) NO, (– ? –)
HC.
balance) with and without variation of the O2 /CO ratio.
The results indicate that the activities of Pt catalysts under
conditions of A/F modulation were enhanced by the
addition of CeO2 and much more so by the addition of
Ce0.8Zr0.2O2. The activities without modulation were the
same regardless of the addition of CeO2 or Ce0.8Zr0.2O2,
because, in this case, the preparation should not influence
Pt which is separated physically from CeO2 and
Ce0.8Zr0.2O2. The experiment of the model catalysts shows
directly the effect of the mixed oxides themselves on the
activities under A/F modulation.
As matter of fact, the state of the precious metal is very
important for designing three-way removal performance.
The interaction of precious metals, CeO2 –ZrO2 and the
Al2O3 support should be examined with respect to SMSI
in oxidizing and reducing atmospheres. There are several
studies of ceria and related oxide-supported noble metals
promoting the activity of surface oxygen [27–30].
2.5. Thermal durability of the catalyst
Fig. 4 shows the average conversion of CO and NO for
two Pt/Rh/Al2O3 catalysts using CeO2 and (Ce,Zr)O2
[14,31]. They were subjected to high-temperature exhaust
at 800–10008C before catalytic evaluation. Removal ef-
ficiencies were measured for a simulated mixture of gases
(CO, NO, C3H6, CO2, H2, H2O, O2, N2 balance). The data
indicate that the use of (Ce,Zr)O2 improves the durability,
Table 1
Removal conversion efficiency (%) of CO, NO and hydrocarbon (HC) for
Pt catalysts at 5508C (see text)
Catalyst
CO
NO
HC
Fig. 2. Three-way removal efficiency vs. temperature for a model exhaust
(seven compositions) (a) with and (b) without A/F modulation over
Pt/Al2O3
Pt/Al2O3 1Ce/Al2O3
Pt/Al2O3 1Ce–Zr/Al2O3
95
97
98
84
91
99
100
100
100
Al2O3-supported
Pt;
mixture
of
powdered
(5 wt%
Pt/
Al2O3)0.1(Al2O3)0.9. Conversion of (———) CO, (? ? ?) NO, (– ? –) HC.

M. Ozawa / Journal of Alloys and Compounds 275–277 (1998) 886–890
889
addition, selection of the Ce/Zr ratio is important for
powders and composite materials, as described above [25–
28]. Furthermore, third compositions will be expected to
improve the OSC of oxides [35–38]. More heat-stable and
more OSC-effective compositions should be developed
using coprecipitation and modified processes. The phase
diagram of CeO2 –ZrO2 –Ce2O3 is also useful for describ-
ing solid-state redox behavior [39]. Another approach is
the examination of structures and the local correlation
between atoms in the oxides through advanced characteri-
zation methods such as neutron scattering [40]. Of course,
detailed studies of catalytic precious metals/oxides system
is required [30].
The use of a OSC promoter in TWC is a well-known
technology since proposed by Gandhi et al. Recent re-
search reveals that the Ce–Zr–O system and related mixed
oxides are more effective than pure CeO2. In practice, new
CeO2 and/or Ce–Zr mixed oxide powders or other mixed-
state materials are expected to be developed as a major
group of rare-earth related products. They are essential due
to the more stringent exhaust regulations in developed
countries, future air pollution standards as well for the
economic benefit of automobile-related industries.
Fig. 4. Average removal efficiency of CO and NO over Pt/Rh/Al2O3
catalysts with CeO2 (h) and (Ce,Zr)O2 (j) after durability tests using
engine exhaust gases at 800, 900 and 10008C.
specifically the heat-stability, of the catalyst, as well as the
initial activity. For practical three-way catalysts with
various promoters, it is expected that the durability and
performance will be improved by CeO2 –ZrO2 as OSC
promoter without complex rearrangement of other com-
positions or the catalyst fabrication process.
4. Summary
2.6. Other effects
This article describes the role of mixed oxides of the
Ce–Zr–O system which are important OSC promoters in
automotive three-way catalysts. In a model Pt–alumina
catalyst, the effect of CeO2 –ZrO2 solid solution on activity
was clearly observed under dynamic A/F conditions. It is
suggested that materials research to develop novel OSC
promoters of Ce-based complex oxides is required for
rare-earth related products for advanced automotive
catalysts.
Other possible roles of Ce–Zr mixed oxides should be
investigated, including (a) OSC in low-temperature opera-
tion, (b) the water gas shift reaction and other reactions
over precious metals supported on CeO2 –ZrO2, and (c)
high dispersion, SMSI and structural features of precious
metals on CeO2 –ZrO2.
3. Mixed oxide products for car pollution
Material–chemical aspects for automotive parts have
recently been reviewed by Narula and coworkers [32].
Catalytic materials are important inorganic or ceramic
products which are very much in demand, because they are
widely used as components in automotive catalytic conver-
ters. For example, automotive catalytic heat-stable
aluminas must be manufactured using a specialized syn-
thesis method, and are additionally modified in the catalyst
fabrication process. Since Ce–Zr mixed oxides and modi-
fied CeO2 promoters have been applied in practice, there is
extensive research and development of the oxides them-
selves as catalytic materials. An inexpensive coprecipita-
tion technique is useful for preparing mixed oxide powders
with moderate surface areas in these oxide systems
[33,34]. Research using more advanced fabrication tech-
niques should be carried out with the aim of producing fine
particles which are highly dispersed and with a surface
area as large as the original single and/or mixed oxides. In
Acknowledgements
The author would like to thank the many engineers and
scientists of Toyota Motor Co. and Toyota C.R.D Labs.
(Note: in this article, no patent applications are described,
although they are informative and very useful for car
pollution technology.)
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