co oxidation and co reduction on carbon supported ptwo3 … · v. rosca g.j.m. janssen presented at...
Post on 10-Nov-2018
213 Views
Preview:
TRANSCRIPT
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
top related