Research method

X-ray absorption
fine structure (XAFS)

Realization of Environmentally-Friendly
High-Performance Three-Way Catalyst
Clarification of two major mechanisms of catalysts for automotive exhaust purificationRole of SPring-8

Achievements

Clarification of the mechanism behind the storage and release of oxygen to
optimize the activities of three-way catalysts* for the purification of automotive
exhaust gas
Elucidation of the aggregation suppression mechanism of catalytic
noble-metal particles to provide long-lasting catalytic activities
Contribution to environmental cleanup by the installation of high-performance
three-way catalysts developed using these analyses in gas-powered cars
R&D facility: Toyota Motor Corporation and Toyota Central R&D Labs., Inc.
*Three-way catalyst: Harmful components in vehicular exhaust gas are oxidized or reduced by the action of catalysts and thus the exhaust gas ispurified. Three-way catalysts promote the simultaneous detoxification of three harmful components, that is, carbon monoxide (CO), hydrocarbons(HCs), and nitrogen oxide (NOx), and are used most frequently in gasoline-powered vehicles. Three-way catalysts comprise noble metals causing
catalytic reactions, supports for dispersing these noble metal particles in the nanometer scale, and catalytic promoters assisting thenoble-metal-induced catalytic reactions.

BackgroundResults

The components of exhaust gas fromautomobiles are continuously changing; hence, toappropriately control the concentration of oxygenin accordance with the changing components,materials with a high capability to promptly store
and release oxygen are required. CZ supports weredeveloped as a material with such a function.
However, the factors behind the improvement in
the function were unknown.
Moreover, in the process of development, it
was revealed that the aggregation of platinum (Pt)particles was suppressed by combining CZ supports
with Pt, and thus the catalytic activity wasmaintained. We needed atomic-level analysis toclarify the mechanism behind this result.

A high-brilliance, high-energy X-ray must be
used to examine the structure of CZ supports indetail because their constituents are heavyelements. The results of the XAFS analysis at
SPring-8 revealed that the ability of CZ supports tostore and release oxygen varies depending on thearrangement of heavy metal atoms.
Moreover, the catalysts containing both CZ
supports and Pt form Pt-O-Ce bonds when
exposed to high-temperature exhaust gas,
preventing the aggregation of Pt particles. The
mechanism underlying this suppression ofaggregation was clarified by XAFS analysis.
From these analyses, we succeeded inimproving the catalytic materials containing Pt/CZ
and in realizing a high-performance three-way
catalyst.

Publication: Y. Nagai et al.; Catalysis Today 74, 225-234 (2002) Y. Nagai et al.; Journal of Catalysis 242 (1), 103-109 (2006)
T. Tanabe et al.; Journal of Catalysis 257 (1), 117-124 (2008)

Action of Pt/CZ-containing three-way catalyst

Relationship between structure of CZ support
and oxygen storage/release performance

Emission purification devices (catalytic convertors) are installed in exhaust pipes. The
catalysts used in such devices contain platinum (Pt) as a noble metal and CZ (a solid
solution of ceria CeO2 and zirconia ZrO2) supports as a catalytic promoter. These powdery
catalytic components are trapped in a ceramic support with a honeycomb structure usedto enlarge the area in contact with exhaust gas. CZ appropriately controls theconcentration of oxygen and promotes the catalytic activity, while the aggregation of Pt
particles is suppressed by the interaction between CZ and Pt.
Carbon dioxide

Water

Carbonmonoxide
Nitrogenoxide

The significant effect of the atomic-level uniformity andconfiguration of Ce and Zr on the performance of storing andreleasing oxygen was clarified. Subsequently, the mechanismof the deterioration of CZ supports was also clarified, and we
elucidated a CZ support structure that sustains the highperformance of storing and releasing oxygen over a longtime (joint research with Kyoto University).
Zirconium
CeriumOxygenNitrogen

When conventional catalysts are exposed to high-temperature gas, theirPt particles aggregate and grow. As a result, the surface area of theparticles decreases, and hence the area in contact with exhaust gas
decreases, causing a deterioration of the purification performance.
20-30 nm2

Orderedsolid solution

Low

Oxygen storage/release performanceHigh

Suppression mechanism of catalytic noble-metal particle aggregation

Hydrocarbons

Emissionflow path
CZ support with highoxygen storage capacity

Disorderedsolid solution

Emissionfrom exhaust pipe

Emission from engine

Control by storing andOxygen
releasing oxygen

MixtureCeramic support

Platinum (Pt)
CZ support with high oxygenstorage capacity
Other catalytic components

When Pt/CZ catalysts are exposed to high-temperature gas, Pt-O-Cebonds are formed by the interaction between Pt and CZ, preventing theaggregation of Pt particles. Under normal conditions, catalytic reactions
are induced when the bonds break, resulting in the lasting activity of thecatalysts.
Pt-O-Ce bondsare formed

particles

Conventional support

Conventional support

CZ support

Initial state

Under high-temperature
exhaust gas

Initial state

CZsupport
Under high-temperature
exhaust gas

3