ranrui, china-us workshop 2009

8/14/2019 Ranrui, China-US Workshop 2009

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Modification and oxygen storage propertiesof CeO2-ZrO2 mixed oxides used for

automotive emission purification

Rui RANDept. Mater. Sci. & Eng., Tsinghua University, Beijing 100084

[email protected]

Introduction;Oxygen storage material design;

Modification mechanism;

Application in catalysts;

Summary.

China-US Workshop 2009Session 5 Environment B


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Automotive emission pollution becomes one of thebiggest environmental problems

Emission:

CO~ 33562 104t/y

HC~ 331 104

t/y NOx~4188 104t/y

PM10~895 104t/y

Auto productions of China in the recent

decade


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Catalytic convertor and the key materials Oxygen storage materials: CeO

2-ZrO

2mixed oxides

Zr increase the thermal stability andoxygen mobility.

Three-way catalytic convertor

CO,HC

NOx,O

2

CO2, H2ON2


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Static OSC: CZP2~CZN2~CZ;

Dynamic OSC: CZP5>CZP2>CZP8>CZN2>CZ;

Oxygen storage material design: Doping by RE

OSC results of the aged Pr, Nd doped samples (500)

XD Wu, D Weng, R Ran. Solid States Science 2007, 9 (7): 636-643


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OSC results of the aged (a) Pr-doped; (b) Pr-loaded samples (500).CZ: CeO2-ZrO2; CZPx: Pr-doped CZ: PCZx: Pr-loaded CZ

3wt.% Pr6O11 loaded CZ

Promotion of Pr

Promotion of Pr

(b)(a)

Oxygen storage material design: impregnation


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Samples OSCsurf*

(mol CO2 gcat1 )

OSCC(mol CO2 gcat

1 )

CZa 39.9 466.6

CZSa 52.8 536.6

SCZa 60.0 601.9

Sr can effectively promote the dynamic OSC of CeO2-ZrO2.

The results suggested that the impregnated Sr should affect thesurface/ interface structure.

* OSCsurf (mol CO2 gcat 1) = SBET 3.8 mol CO2 m

2

Dynamic OSC of aged Sr-modified samplesAgeing condition 1050 6h

Oxygen storage material design: impregnation

J Fan, XD Wu, R RAN, D Weng. J Catal, 2008, 258: 177186


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The Pr ions affect the properties only by influencing the structure and the

status ofCe, Pd species which are directly related to the OSC of catalysts.

(a) (b)

CO-TPR curve for aged samples: (a) supports; (b) Pd catalysts

Pr can be considered as an inert species under 500C in reduction process.

Ce reduction peak

Ce reduction peak

PdOreduction peak

Adsorbedoxygen

Modification mechanism: Oxygen mobility


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XPS patterns for aged CZPw and w derive from Pr3+ ;w and w derive from Pr4+

XANES spectra at Ce L3-edge for Pr6O11 and aged CZP7a

XD Wu, D Weng, R Ran. Solid States Science 2007, 9 (7): 636-643

Pr3+

Pr4+

Pr mainly exists in Pr3+ over aged CZP samples.

Modification mechanism: ionic valence


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Samples c/a Samples c/a

CZa 1.013

CZP2a 1.011 CZN2a 1.013

CZP5a 1.008 CZN5a 1.012

CZP8a 1 CZN8a 1

Tetragonality (c/a) of aged samples (XRD results)

b

a

c

ab

c

t: pseudo-cubic structure

tetragonal structure

Pr3+ cation has larger ion radius than Nd3+ .

Its insertion should lead more lattice distortionin the bulk.

43 3 '

Pr / Nd Pr 2Ce

Ce oV

++ +

+

43 3 '

Pr / Nd Pr 2Zr

Zr oV

++ +

+

Vacancy formation:

Modification mechanism: bulk structure


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Lattice constants and crystal sizes of the Pr-contained samples

Samples Lattice constants () SBET(m2/g)

Crystallitesize ()

Fresh Aged Aged Aged

a a c c/a

CZ 5.215(2) 5.172(2) 5.254(3) 1.016 22 196

CZP2 5.227(1) 5.189(1) 5.248(1) 1.011 16 196

CZP5 5.235(1) 5.210(1) 5.250(2) 1.008 13 197

CZP8 5.241(1) 5.238(1) / 1 9 182

PCZ1 5.212(3) 5.180(1) 5.259(2) 1.015 22 153

PCZ3 5.216(1) 5.185(1) 5.246(1) 1.012 30 129PCZ5 5.212(1) 5.200(1) 5.250(1) 1.010 24 127

Pr is mainly dispersedon the surface.

Pr is partially insertedinto the ceria lattice

Modification mechanism: crystallite size


11/14J Fan, XD Wu, R RAN, D Weng. J Catal, 2008, 258: 177186

Doping

Loading

Take Sr for anexample:

Modification mechanism: model


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Summary

Appropriate Pr/Nd/Sr doping or loading can promote the OSC of

CeO2-ZrO2 mixed oxides. It is found that 3wt% of Pr-loaded CZ byimpregnation is an optimized route to improve the oxygen storagecapacity (OSC) and thermal stability of CZ.

The promotion of Pr/Nd/Sr is related to their more oxygen vacancies

and larger surface Ce3+content, which are benefit to the oxygenmigration and surface reaction. Larger Pr3+/Nd3+/Sr2+ cations dopingleads more lattice distortion to accelerate oxygen migration.

Surfacial modification can also promote the oxygen storage velocity

by creating structure defects at grain boundaries and suppress thecrystallite growing during the ageing process.


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Acknowledgement

This work was supported by the National Natural ScienceFoundation of China, Project No. 50972069.

I would like to acknowledge my team colleagues: Prof. Duan

Weng, Dr. Xiaodong Wu, Dr. Jun Fan, Miss Xiaodi Wu, Miss LeiWang, and those who made great efforts on this work.


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Thank You for Your Attention!

ranrui, china-us workshop 2009

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