Wu Nsf(c) Presentation

NSFC-NSF Workshop’ 2009

Synergetic effect between copper oxide and ceria for soot catalytic oxidation Xiaodong Wu Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Changzhou, China 2009-10-17

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Beijing

EcoMaterials Group

Essential research field: automobile exhaust catalysts   

Three-way catalysts Soot oxidation catalysts Oxidation catalysts



deNOx catalysts (NH3-SCR, LNT)



Supports and coating technologies

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Outline Background Studies on Cu-Ce mixed oxide catalysts  Choice of support material;  Optimization of the Cu/Ce ratio;  Modification of Cu-Ce catalysts. Challenges

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Schematic of Diesel Particulate Matters (PM) PM ： important pollutant to human health and environment Vapor Phase Hydrocarbons

Solids (Soot)

Solid Carbon Spheres (0.01 -0.08 µ m diameter) from to make Solid Particle Agglomerates (0.05-1.0 µ m diameter) With Adsorbed Hydrocarbons Adsorbed Hydrocarbons

Soluble Organic Fraction (SOF) /Particle Phase Hydrocarbons

Liquid Condensed Hydrocarbon Particles Sulfate with Hydration

Adsorbed Hydrocarbons

Vuk and Johnson, SAE 760131 (1976)

Sulfate (SO4)

Background Most promising method ： DPF or POC Key techniques: (catalyst, fuel additive, external heater…) regeneration

ation ener Reg

∆p

l Fi

n io t tra

time Diesel particulate filter

Exhaust back pressure vs. time 5

Reported work NOx storage

NOx-assisted soot oxidation Modern diesel engines

• Low NOx emission • Low exhaust temperature

Comparison of the soot-TPO profiles on Mn25Ce75 (■) and a commercial platinum catalyst (▲). Reactant gas: 10% O2 + 1000 ppm NO + 3% H2O in N2.

K. Tikhomirov, et al, Appl. Catal. B 64 (2006) 72.

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Objective CuOx

• Cheap • Redox activity To form Cu-Ce mixed oxides CeO2

• Cheap • Oxygen storage capacity Experimental conditions: F. E. López-Suárez, et al, Appl. Catal. B 84 (2008) 651. A. Setiabudi, et al, Appl. Catal. B 51 (2004) 9.

• • • •

Activity and selectivity Hydrothermal resistance Resistance to SO2 …

• In the presence of NO+O2 • Fast heating rate: 10-20℃/min; • High flow rate: 500ml/min; • Loose catalyst-soot contact 7 conditions.

Choice of support: CeO2 or Ce-Zr? Demonstrating the importance of the Cu-Ce interactions Cu-Ce-Zr (a )

Ce-Zr Cu-Ce

Cu-Ce

Cu-Ce-Zr CeO2

CeO2 Ce-Zr

TPO curves of (a) fresh and (b) aged catalysts . Reaction atmosphere: 10% O2+1000ppm NO + N2. Aging treatment: in static air at 800oC for 20h

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Optimization of the Cu/Ce ratio (a) Without catalyst

(b) Fresh Aged

Cu ： 5-10% Fresh Aged

Without catalyst

Samples

Samples

(a) Activity and (b) selectivity of Cu-Ce catalysts. Aging condition: 7%steam/air, 800oC for 20h Ti ： at which the conversion of soot reaches 5% CO2 × 100 Selectivity to CO2: SCO 2 (%) = CO + CO2

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Structure of Cu-Ce mixed oxides (a)

Fresh

Segregation and growth of CuOx at high temperatures

(b)

Aged

CuOx clusters are finely dispersed on ceria particles instead of forming Cu-Ce solid solutions.

XRD patterns of (a) fresh and (b) aged catalysts

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Synergistic effect between Cu and Ce Surface composition and valence state of the samples Samples

Cu/Ce (Nominal)

Cu/Ce (measured)

CeO2-F

-

Cu5-F

Ce4+ Ce3+ + Ce4+

Cu 0 + Cu + Cu 0 + Cu + + Cu 2+

-

0.83

-

0.053

0.194

0.85

1

Cu20-F

0.25

0.492

0.87

0.44

CeO2-A

-

-

0.84

-

Cu5-A

0.053

0.42

0.85

0.89

Cu20-A

0.25

0.552

0.87

0.3

Cu+

Ce4+ O2-

Ce4+ CeO2

O2Ce4+

O2Ce

3+

CuO1-x

Cu+ Ce4+ O2-

Ce

4+

Ce3+ O2-

Ce

4+

Ce 4+ − O − Cu 2+ Ce 4+ − VO.. − Cu + Ce 3+ − VO.. − Cu +

active sites Scheme of the Cu-Ce interface

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Strong interaction between Cu and Ce A

B

C

Mixture

noi t cO uC dor p2

Cu-Ce synergetic effect CuOx nanoparticles CuOx clusters Temperature

CO-TPR profiles

 A: Cu-Ce interaction

CuO particles

 B: Bulk CuO  C: CeO2

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Oxidative adsorption of NO in sootTPO Cux+: oxidizing NO to NO2 Without catalyst

Cex+: coordinated to adjacent sites to store NO2derived species

Evolution of NO concentration Reaction atmosphere:1000ppmNO+10%O2+N2

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Reaction mechanism in NO+O2 +

(1)

NO

O

O

C-O* O O

N

N N O O O O O M M M M

O2

(2) + O2

(3) NO2* spillover

(4) COx (g)

Soot

Catalyst

Scheme of Cu-Ce+soot reaction in NO+O2 （1 ） （2 ）

NO + O2 + Cat . → Cat . − NO3−

Cat . − NO3− + O2 → Cat . − O + NO2

（3 ）

NO2 + Cf → C (O) + NO

（4 ）

C (O) + O 2 → CO 2 + Cf

? Cat. − NO

− 3

+ C f → Cat . + C (O) + NO

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Enhanced thermal stability of Cu-CeAl CuCeAl2 CuCeAl2 CuCe

√

Fig. TPO curves of the aged catalysts under loose contact conditions.

Fig. TPO curves of the fresh, hydrothermal aged and sulfated Cu-Ce-Al under loose contact.

Reaction conditions: 1000ppm NO+10%O2+N2 Ageing conditions: in dry air at 800°C for 10h

×

Reaction conditions: 1000ppm NO + 10% O2 + N2.

√

Hydrothermal ageing: in 10H2O/air at 800°C for 10h

×

SO2 poisoning: in 1000ppmSO2/10%H2O/air at 800°C, 10h 15

Active surface nitrates Symmetric nitrates

Inert

Asymmetric nitrates

Active

Time evolution of FT-IR spectra of the hydrothermal aged (a) Cu–Ce and (b) Cu–Ce–Al exposed to 1000 ppm NO + 10% O2 at 50℃.

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Improved resistance to SO2 by adding K  Preparation: impregnation, 500oC for 3h  SO2 treatment: in 400ppmSO2/air at 400oC for 2 h

Tm and CO2 selectivity of the fresh and poisoned catalysts Sample (wt.%) Cu5K2/CeO2 Cu2K5/CeO2

Tm (ºC)

S (%)

Fresh

378

81

Poisoned

410

85

Fresh

359

58

Poisoned

380

86

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Preferential sulfurization of potassium Nitrates Carbonates Sulfates SO2

IR spectra of the fresh and SO2-poisoned catalysts.

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Combination of POC and LNT K salts

• Solubility; • Volatility; • Corrosiveness.

Replaced by Ba

Tm of CuCe and Ba1CuCe in the presence of different NO concentrations

Catalysts preparation: Ce(NO3)3, Cu(NO3)2

Citric acid sol-gel 500 ℃, 3h

CuCe

Loading Ba(Ac)2

BaCuCe

550 ℃, 1h 19

Conclusions Cu-Ce is a good candidate as NO2-assisted soot oxidation catalyst with the Ce4+ –O2- –Cu2+ pairs at the interface as active sites, which presents high activity comparable to commercial PM catalysts. By modification of alumina, potassium or barium, the resistances to hydrothermal ageing and sulfur dioxide of Cu-Ce catalysts can be enhanced.

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Challenges in mixed oxide catalysts Performance:  The performance of catalysts are still not satisfying  low-temperature activity and lifetime (like thermal stability and resistance to SO2)

 How to improve the activity of catalysts  in trace NO or without NO? Mechanism:  Active NOx species  surface nitrates, the decomposed NO2, the oxidized NO2 or just NO itself?

 Possible and important intermediates during soot oxidation  surface oxygen complexes (SOCs) and surface nitrogen complexes (SNCs) if possible  Corresponding in situ characterizations

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Acknowledgements Financed by the Ministry of Science and Technology of China. Thanks to: Team leader: Prof. Duan Weng Ph.D candidate: Jia Li Master: Qing Liang and Haibo Xu Master candidate: Fan Lin

Thank you for your attention! 22