CO oxidation and CO2 reduction on carbon supported PtWO3 catalyst

N.P. Lebedeva

V. Rosca G.J.M. Janssen

Presented at the 7th Spring Meeting of ISE, 22-25 March 2009, Szczyrk, Poland

ECN-L--09-179

December 2009

CO oxidation and CO 2 reduction on carbon supported PtWO 3 catalyst

N.P. Lebedeva , V. Rosca, G.J.M. Janssen Energy research Centre of the Netherlands (ECN), Petten, The Netherlands

7th Spring Meeting of ISE, Szczyrk 2009, Poland

Motivation

• PEM fuel cells are attractive power source• H2 - O2 as fuel, Pt as a catalyst

however

• H2 often contains traces of CO• reformate contains CO and up to 25% of CO 2

• CO tolerance can be improved by alloying Pt-M• CO2 is often not inert then!

Case study: PtWO3/C

Why PtWO 3/C ?

• PtWO3/C is a CO tolerant catalyst• CO tolerance is through bifunctional mechanism

H-C≡O

PtWO3PtWO3PtWO3Pt WO3

OH OHOH OHH

O

HCO2

-C≡O

H-H

-C≡O

• Some report good performance in reformate-fed PEMFC• Stability – WO 3 is insoluble in acids (PEMFC)

• Not as widely implemented as PtRu/C

Reductive co-precipitation

Pt precursorW precursor

Vulcan XC-72suspension

Reducing agent

• filtering• drying under

N2 flow

• Formic acid• H2PtCl6·xH2O

• Na2WO4 ·2H2O

US 006007934 A; PI 9702816-9 A

Catalyst preparation

PtWO3/C (Pt:W=6:4)

Catalyst characterisation

20 30 40 50 60 70

2 Theta, degree

C002

Pt111

Pt200 Pt

220

XRDd = 4.4 nm

TEM

3200

4200

5200

6200

7200

8200

9200

10200

11200

253035404550

Binding energy / eV

W 4f7/2

W 4f5/2

W 5p3/2

O 2s

5600

6600

7600

8600

9600

10600

11600

12600

13600

14600

15600

60708090

Binding energy / eV

Pt 4f7/2Pt 4f5/2

XPS Pt is present as Pt metalW as amorphous WO 3no alloying Pt-W found

surface slightly enriched with WO 3Pt0.70W0.30 (XPS) vs. Pt 0.77W0.23 (EDX)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

I /

mA

E / V vs RHE

Catalyst characterisationBronze re-oxidation charge increases with scan rate decreasing ⇒⇒⇒⇒ slow process

Pt + H+ + e PtHads

xPtHads + WO3·yH2O xPt + HxWO3·yH2O

PtHads Pt + H+ + e

HxWO3·yH2O xH+ + WO3·yH2O + xe

References:Tseung, Kulesza, Tsirlina, Kondo

PtWO3/C, 0.5 M H2SO4;10 mV/s

Catalyst characterisation

On-line electrochemical mass spectrometry setup

A.H. Wonders, et al. J. Appl. Electrochem. 36 (2006) 1215

CO adlayer oxidation

Oxidation of a saturated adlayer of CO starts at low overpotentials, ~ 0.25 V

Small fraction of the adlayer, ca. 2 %, is being removed

The major part oxidizes at potentials typical for Pt, ca. 0.7 V

0.0 0.2 0.4 0.6 0.8 1.0

-60

-40

-20

0

20

40

60

80

100

I /

µµ µµA

0.1 0.2 0.3 0.4 0.5 0.6

-40

-20

0

20

40

0.0 0.2 0.4 0.6 0.8 1.00.0

5.0x10-8

1.0x10-7

1.5x10-7m/z 44 (CO

2)

Ion

curr

ent /

a.u

.

E / V vs. RHE

0.1 0.2 0.3 0.4 0.5 0.62.1x10-8

2.2x10-8

2.3x10-8

2.4x10-8

2.5x10-8

m/z 44 (CO2)

E / V vs. RHE

T. Nagel et al, J. Solid State Electrochem., 7 (2003) 614.

On-line MS0.5 M H2SO4; 2mV/s

saturated CO adlayer

Continuous CO oxidation

Oxidation of the dissolved CO on PtWO3/C starts already at ~ 0.12 V vs. RHE

0.0 0.2 0.4 0.6 0.8 1.0

-60

-40

-20

0

20

40

60

80

100

I /

µµ µµA

0.1 0.2 0.3 0.4 0.5 0.6

-40

-20

0

20

40

0.0 0.2 0.4 0.6 0.8 1.00.0

5.0x10-8

1.0x10-7

1.5x10-7

2.0x10-7

2.5x10-7

m/z 44 (CO2)

Ioni

c cu

rren

t / a

.u.

E / V vs. RHE

0.1 0.2 0.3 0.4 0.5 0.6

2.10E-008

2.80E-008

3.50E-008

m/z 44 (CO2)

E / V vs. RHE

On-line MS0.5 M H2SO4; 2mV/s

saturated CO solution

T. Nagel et al, J. Solid State Electrochem., 7 (2003) 614.

Continuous CO oxidation

Gradual deactivation of the catalyst is observed

Steady-state resembles Pt

What happens?

0.0 0.2 0.4 0.6 0.8 1.0

0

1

2

3

4

exp 1 exp 2 exp 3I

/ m

A

E / V vs RHE

PtWO3/C deactivation during continuous CO oxidation0.5 M H2SO4; 2 mV/s

Continuous CO oxidation

Bronze peak disappears

Dissolution WO3 ?

Deactivation ?

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

blank CV (before) blank CV (after)

I /

mA

E / V vs RHE

PtWO3/C: blank CVs before and after continuous CO oxidation

0.5 M H2SO4; 10 mV/s

Continuous CO oxidation

After exposure to hydrogen evolution conditionsBronze peak re-appears

No dissolution of WO3 @ RT

(Reversible) deactivation !

0.0 0.2 0.4 0.6 0.8-4

-2

0

2

4

I / m

A

E / V vs RHE

inactive catalyst reactivated catalyst

PtWO3/C catalyst reactivation(20 min. @ -0.05 V)

0.5 M H2SO4, 20 mV/s

Mechanism of the CO oxidation

COsol

Hads

H+sol Pt□COads

CO2WO3

HxWO3

COads

Pt + H+ + e PtHads

xPtHads + WO3·yH2O Pt + HxWO3·yH2O

COPt + OHHxWO3 CO2 + WO3 + Pt

Where does “O” come from – “lattice” or H2O – and what is the role of HxWO3·yH2O ?

CO2 reduction

After exposure to CO2 –saturated electrolyte an adsorbate forms

Suppresses Hupd on PtOxidizes similarly to CO

CO

-20.00

-10.00

0.00

10.00

20.00

0.00 0.20 0.40 0.60 0.80 1.00

E / V vs. RHE

I / m

A

PtWO3/C catalyst, CO2 -saturated 0.5 M H2SO4, Eads = 0.1 V vs RHE

CO2 + 2Hads CO + H2O

CO2 reduction

Rate of the adsorbate formation is much lower on PtWO3/C than on Pt/C:

- lower Hupd concentration

however

Eventually full coverage will be reached @ RT

0

0.2

0.4

0.6

0.8

1

0 500 1000 1500 2000 2500 3000 3500 4000

t / s

ΞΞ ΞΞ CO

/ ΞΞ ΞΞ C

Om

ax

PtW/C

Pt/C

CO2 + 2Hads CO + H2O

Conclusions

• PtWO3/C is highly active in the oxidation of CO: adlayer begins to oxidize at 0.25 V and dissolved CO at 0.12 V vs. RHE

• HxWO3 is the active component & oxygen donor; formation of the bronze is slow in the PtWO3/C catalyst

• In the presence of CO, the bronze formation (via H spill-over from Pt to WO3) is inhibited;

⇒⇒⇒⇒ (reversible) catalyst deactivation.

• PtWO3/C catalyst reduces CO2 to CO at much lower rate than Pt/C. Still full coverage is expected at prolonged exposure to CO2 at RT.• PtWO3/C has a limited operating window (ca. 0.10 V to ca. 0.45 V). In case of high overpotential on the anode (start-up, fuel starvation etc.) and/or peak of high CO concentration – possible death of the catalyst.

General recommendations:• WO3 phase as crystalline as possible for quick recharging; well dispersed with Pt

• Drying induces ageing ⇒⇒⇒⇒ slowing down of recharging ⇒⇒⇒⇒ lower catalyst activity

Acknowledgement

Marijke Roos, Energy Research Centre of the Netherlands (ECN), for EDX measurements

Vera Smit-Groen, Nuclear Research and Consultancy Group (NRG), for XRD measurements

Onne Wouters, Nuclear Research and Consultancy Group (NRG), for TEM imaging

Ad Wonders, Eindhoven University of Technology (TUE), for assistance with on-line MS

Tiny Verhoeven, Eindhoven University of Technology (TUE), for XPS measurements