water-gas shift on gold- and copper-oxide catalysts...

Chemistry Department

Water-Gas Shift on Gold- and Copper-Oxide Catalysts:Active Sites and Reaction Mechanism

José A. Rodriguez

* Collaborators:BNL: P. Liu, J. Hanson, J. Hrbek, X. Wang, J. GracianiTufts University: M. StephanopoulosUniversidad Central de Venezuela: M. Perez, J. EvansUniversity of Seville: J. Fernandez-Sanz, J. Graciani

$ US-DOE


Outline

• Motivation/objective

• Water-gas shift reaction on Au-CeO2 powders

• Water-gas shift on Au/CeO2(110) surfaces

• Water-gas shift on CeOx/Au(111) surfaces

Ce O

Crude Oil

Coal

Naturalgas

Wood

Organic waste

Biomass

Crude Oil

Coal

Natural gas

Wood

Organic waste

Biomass

Others

Reformer WGS COoxidation

H2

nuclear

Geothermal

Fuel Cell

Otherapplications

SolarHydroWindWave

Generator

Power plant

CO2 sequestration

~95%

Most hydrogen is currently derived from the steam reforming ofhydrocarbons: CnHm + nH2O nCO + (n-m/2)H2

The reformed fuel contains 1-10% CO, which degrades the performance of the Pt electrode used in fuel cell systems.

The water-gas shift (CO + H2O H2 + CO2) is used to remove the CO.

Motivation:

Recent studies indicate that ceria impregnated with gold nanoparticles is very active as a catalyst for the water-gas shift reaction being betterthan commercial Cu/ZnO catalysts - Fu et al, Science 301 (2003) 935

To improve the performance of the Au-ceria catalysts, we must: - Identify what are the active phases. What active sites are making the catalysis? - Determine the nature of the Au-CeO2 interactions?

Image of transmissionelectron microscopy

CeO2 nanoparticlesAu

Our studies of XRD, photoemission and XANES/EXAFS showtwo types of nanoparticles in the gold-ceria catalysts:

-nanoparticles of metallic gold-nanoparticles of gold oxide

What is the active phase of Au for the water-gas shift reaction? -Stephanopoulus et al propose Au+1 and Au+3 as the active centers, Science 301 (2003) 935.

-In the early 1990s, Campbell, Norskov and coworkers found that Cu is the active phase in Cu/ZnO catalysts, - J. Chem. Soc. Faraday Trans. 86 (1990) 2725 - J. Catal. 134 (1992) 445 In-situ XRD and XAFS by Clausen et al corroborated this - J. Catal. 198 (2001) 56

To solve this controversy the water-gas shift was investigatedover the following systems

• Water-gas shift reaction on Au-CeO2 powders

• Water-gas shift on Au/CeO2(110) surfaces

• Water-gas shift on CeOx/Au(111) surfaces

Ce O

Au

Integral approach to catalysis:

Quantum-chemical modeling(DFT, MD, kinetic MC)

Chemistry associated with catalysis(surface science studies)

In-situ characterization ofcatalysts

(xanes/exafs, tr-xrd)

Fundamental understanding of the behavior of active sites:Rational design of better catalysts

National Synchrotron Light Source

Photoemission XAFS

XRD


* Work with single crystals - Photoemission - X-ray absorption near-edge spectroscopy

* Work with high-surface area catalysts - In-situ Time-resolved x-ray diffraction

- In-situ X-ray absorption near-edge spectroscopy


Water-gas shift reaction on:

• High surface area Au-CeO2 and Cu-CeO2 catalysts

11900 11920 11940 11960 119800.0

0.2

0.4

0.6

0.8

1.0

1.2

Inte

nsity /

a.u

.

Energy / eV

0.5% Au 1.8% Au 2.4% Au 5.0% Au Au foil

Au L3 edge powder Au/CeO2 catalysts

Before reaction, as prepared:

The relative amount of AuOxincreases when the totalamount of gold decreases.

AuCeO2 nanoparticles

11920 11940 11960 119800.0

0.2

0.4

0.6

0.8

1.0

1.2

25oC

30oC

68oC

100oC

160oC

300oC

Au foil

Norm

aliz

ed !µ(

E)

Energy / eV

Au L3 edge

In-situ measurements under atmospheric pressure for WGS reaction. TheAuOx becomes Au. The active phase consist of metal nanoparticles dispersedon CeO2-x.

0 5 10 15 20 25

! 200oC

300oC

25oC

500oC

400oC

Pro

du

ct

rela

tive

co

nce

ntr

atio

n

Time / hour

H2

CO2

2.4%Au

Photon Energy (eV)

5660 5680 5700 5720 5740 5760 5780 5800 5820 5840 5860 5880

Flu

ore

sce

nce

Yie

ld (a

rb u

nits

)

Ce L3 edgeXANES

CeO2

Ce3+

Au-CeO2, WGS 300 C

Au-CeO2, WGS 150 C

In-situ XANES at Ce L3-edge

Ceria is reduced under reactionconditions.

2θ

Tim

e / h

CuCeO2

CuO

200oC300oC

400oC500oC

2θ

Tim

e / h

CuCeO2

CuO

2θ

Tim

e / h

CuCeO2

CuO

200oC300oC

400oC500oC

0 2 4 6 8 10 12 14

200oC 500

oC

300oC

400oC

300oC

400oC

300oC

500oC

400oC

CuO

Cu0.2Ce0.8O2

H2

rela

tive c

oncentr

aio

n

Time / hour

5%Cu/CeO2

Water-gas shift reaction (CO + H2O H2 + CO2) on CuO/CeO2

Figure1.TR-XRDpatternsfor5%CuO/CeO2Figure2.RelativeH2concentrationsintheduring the WGS reaction.(λ=0.921Å) products. .

metallic Cu is the active phaseIn agreement with the results of Campbell, Norskov, Clausen et all for Cu/ZnO: -J. Chem. Soc. Faraday Trans. 86 (1990) 2725. - J. Catal. 134 (1992) 445; 198 (2001) 56.

Summary

In-situ XANES and XRD studies show that the activephase of Au-CeO2 and Cu-CeO2 catalysts for the WGSreaction consist of metal nanoparticles dispersed onpartially reduced CeO2-x

Integral approach to catalysis:

Quantum-chemical modeling(DFT, MD, kinetic MC)

Chemistry associated with catalysis(surface science studies)

In-situ characterization ofcatalysts

(xanes/exafs, tr-xrd)

Fundamental understanding of the behavior of active sites:Rational design of better catalysts


Water-gas shift reaction on:

• Au and Cu nanoparticles supported on CeO2(111), ZnO(0001) and TiO2(110)

CeO

CeO2(111) structure

Sideview

Topview

For Au on CeO2(111) , the results of STMshow 3D islands of Au with sizes of 2-4nm (0.3-0.8 ML) - Freund et al, Catal. Lett. 114 (2007) 8.

Model systems: Au and Cu on CeO2(111)

150x150 nm2

Au coverage / ML

0.0 0.4 0.8 1.2 1.6 2.0 2.4

mo

lecu

les p

rod

uce

d / 1

017

mo

lec c

m-2

0

2

4

6

8

10

1220 Torr CO, 10 Torr H2O

625 K

H2

CO2

H2

CO2

on CeO2(111)

on ZnO(0001)

Cu coverage / ML

0.0 0.4 0.8 1.2 1.6 2.0 2.4

mo

lecu

les p

rod

uce

d / 1

017

mo

lec c

m-2

0

2

4

6

8

10


625 K

H2CO2

on CeO2(111)

on ZnO(0001)

After exposing Au/CeO2(111) surfaces tomixtures of {CO + H2O} under WGSreaction conditions, post-reaction XPScharacterization shows Au and partiallyreduced CeO2-x(111)

Rodriguez, Liu, Hrbek et al, Angew. Chemie, 46 (2007) 1329

Particle size matters: At the maximumactivity metal particles with 2-4 nm insize.

Question: What is the role of the oxide?

H2

pro

du

ce

d / 1

017

mo

lecu

le c

m-2

0

2

4

6

8

10

12

Au data Cu data

Au/CeO2 Cu/CeO2Au/ZnO Au Cu/ZnO Cu

* Amounts of H2 produced during the WGS reaction on 0.5 ML of gold or

copper deposited on CeO2(111) and ZnO(000ī). * 20 Torr of CO and 10 Torr of H2O at 625 K for 5 minutes in a batch reactor.

H2

mo

lecu

les

pro

du

ce

d / 1

01

7 m

ole

cu

le c

m-2

0

2

4

6

8

10

Cu/TiO2 Cu/ZnO CuCu/MoO2 Cu/CeO2 Cu/MgO

Oxide support matters.Question: What is the role of the oxide?

Rodriguez et al, J. Phys. Chem. C, in press (2009)


What is the role of the oxide in the water-gas shift reaction?

Density functional calculations were performed to answer thisquestion. Model catalysts: - Cu(100) and Au(100) surfaces - Free Cu and Au nanoparticles - Cu/TiO2-x(110) and Au/TiO2-x(110) surfaces

Liu and Rodriguez, J. Chem. Phys. 126 (2007) 164705Rodriguez et al, J. Phys. Chem. C, in press (2009).


Density functional calculations were performed with

-DMol3 code -numerical basis set -relativistic effective-core potentials -PW91 functional

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

*CO+O

*+H2O*+

2H*

TS3

TS2

TS1

Cu29

En

erg

y (

eV

)

Cu(100)

CO2

+H2+H

2O

CO2

+2H*+

H2O*

*CO+2

H*+

2HO*

*CO+2

H2O*

*CO+2

H2O

CO+2

H2O

TS3

TS2

TS1

HOCO*+

H*

CO2

+H2

CO2

+2H*

CO*+

H*+

OH*

CO*+

H2O*

CO*+

H2O

CO+H2

O

WGS on Cu(100) and on a Cu29 nanoparticle

Liu and Rodriguez, J. Chem. Phys. 126 (2007) 164705

Cu surfaces and nanoparticle are able to catalyze the WGS reaction. Inagreement with other works: - Campbell et al, J. Chem. Soc. Faraday Trans. 86 (1990) 2725 - Norskov et al, J. Catal. 134 (1992) 445 - Clausen et al. J. Catal. 198 (2001) 56 - Mavrikakis et al, J. Am. Chem. Soc. 130 (2008) 1402


-2.0

-1.5

-1.0

-0.5

0.0

0.5

TS3

TS2

TS1

Cu29

HOCO*+

H*

CO2

+H2

CO2

+2H*

CO*+

H*+

OH*

CO*+

H2O*

CO*+

H2O

Energ

y (

eV

)

CO+H2

O

Au29

A Au29 nanoparticle cannot catalyze the WGS reaction


Au systems exhibit a very low reactivity for thebonding or dissociation of H2O

• For the Au-TiO2 catalysts the oxide must play a very important role inthe WGS process.

Au-TiO2 surfaces exhibit high catalytic activity for the water-gas shift

Model for Au/TiO2-x(110)

Model used by Norskov et al tostudy the oxidation of CO

Angew. Chem. Int. Ed. 44 (2005)1824.

We used this model to study the WGS reaction on Au/TiO2-x(110).

Rodriguez et al, J. Phys. Chem. C, in press (2009).

•The Au-TiO2 interface helps in the dissociation of water and in theformation of a key HOCO intermediate.

DFT results for WGS on Cu(100) and Au(100)

Au(100) and Au(111) do not dissociate water well, butsubsequent steps of the WGS are fine.

Performance of CeOx/Au(111) as a WGS catalyst?

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

TS3TS2

TS1

*CO+O*+H2O*+2H*

HOOC*+*OH+2H*

Cu(100)

Energ

y (e

V)

Au(100)

CO2+H2

+H2O

CO2+2H*+

H2O

*

*CO

+2H*+2HO

*

*CO

+2H2O

*

*CO

+2H2O

CO+2H2

O

Images of scanning tunneling microscopy for nanoparticlesof CeO2 dispersed on a Au(111) surface

A B

200 nm x 200 nm30 nm x 25 nm

Model catalyst for the water-gas shift

amorphous ceria

Fraction of Au(111) covered by CeO2

0.0 0.2 0.4 0.6 0.8 1.0

H2

pro

du

ced

/ 10

17

mo

lecu

le c

m-2

0

1

2

3

4

5

Cu(100)


Production of hydrogen through the water-gas shift (CO + H2O H2 + CO2) on CeO2/Au(111) catalysts

Rodriguez et al, Science, 318 (2007) 1757

u'

v'

v0after WGS reaction

!Ce ~ 30% of surface

Binding Energy (eV)

880890900910920

Inte

nsity

(arb

. un

its)

Ce 3d XPS u'''

u''

uv'''

v''

v

v'

v0

u0

u'

CeO2/Au(111)

CeO2/Au(111) + WGS

Ce2O3/Au(111)

Fraction of Au(111) covered

0.0 0.2 0.4 0.6 0.8 1.0

Perc

en

tag

e o

f O v

acan

cie

s in

CeO

2

2

4

6

8

10

12

14

16

18

After WGS reaction

Fig 3

(a)

(b)

v'u'

v0

Photoemission indicatesthat there is a correlationbetween the concentrationof O vacancies in theceria nanoparticles and thecatalytic activity

Fraction of Au(111) covered by CeO2

0.0 0.2 0.4 0.6 0.8 1.0

H2

pro

du

ce

d / 1

017

mo

lec

ule

cm

-2

0

1

2

3

4

5

Cu(100)


Rodriguez et al, Science, 318 (2007) 1757

CeO2-x with clusters of O vacancies. It dissociateswater.

Au(111)

Image of scanning tunneling microscopy for nanoparticle ofCeO2-x dispersed on a Au(111) surface

30 nm x 30 nm

Proposed mechanism for the water-gas shift reaction

active site

Water dissociates on O vacancies of the oxide, COadsorbs on Au sites located nearby, and subsequentreaction takes place at the metal-oxide interface


Summary

In-situ XRD and XANES studies show that the active phase of Au-CeO2 andCu-CeO2 catalysts for the WGS reaction consist of metal nanoparticlesdispersed on partially reduced CeO2-x

The catalytic activity of WGS catalysts containing Au or Cu supported onCeO2(111) and TiO2(110) depends strongly on the size of the metalnanoparticles.

Au nanoparticles alone are inactive for the water-gas shift because they cannotdissociate O-H bonds. The metal-TiO2 and metal-CeO2 interfaces play a directrole in the dissociation of water accelerating the WGS process.

- Collaborators:

BNL: P.Liu, J. Hanson, J. Hrbek, X. Wang, J. GracianiTufts Univ: M. StephanopoulosUniv Central Venezuela: M. Perez, J. EvansUniversity of Seville: J. Fernandez-Sanz

- Financial support at BNL: $ US-DOE

water-gas shift on gold- and copper-oxide catalysts...

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