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

Transcription

1 CO oxidation and CO 2 reduction on carbon supported PtWO 3 catalyst N.P. Lebedeva V. Rosca G.J.M. Janssen Presented at the 7 th Spring Meeting of ISE, March 2009, Szczyrk, Poland ECN-L December 2009

2 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

3 Motivation PEM fuel cells are attractive power source H 2 - O 2 as fuel, Pt as a catalyst however H 2 often contains traces of CO reformate contains CO and up to 25% of CO 2 CO tolerance can be improved by alloying Pt-M CO 2 is often not inert then! Case study: PtWO 3 /C

4 Why PtWO 3 /C? PtWO 3 /C is a CO tolerant catalyst CO tolerance is through bifunctional mechanism -C O CO 2 OH -C O H-H OH Pt WO 3 Pt WO 3 Pt WO 3 Pt WO 3 H O H H OH -C O OH Some report good performance in reformate-fed PEMFC Stability WO 3 is insoluble in acids (PEMFC) Not as widely implemented as PtRu/C

5 Catalyst preparation Reductive co-precipitation W precursor Pt precursor Reducing agent PtWO 3 /C (Pt:W=6:4) Vulcan XC-72 suspension filtering drying under N 2 flow H 2 PtCl 6 xh 2 O Formic acid Na 2 WO 4 2H 2 O US A; PI A

6 Catalyst characterisation TEM XRD Pt 111 C d = 4.4 nm Pt Pt Theta, degree Pt 4f5/2 Pt 4f7/2 O 2s XPS W 4f7/2 W 4f5/ Pt is present as Pt metal W as amorphous WO3 no alloying Pt-W found 8600 W 5p3/ Binding energy / ev Binding energy / ev 60 surface slightly enriched with WO3 Pt0.70W0.30 (XPS) vs. Pt0.77W0.23 (EDX)

7 Catalyst characterisation PtH ads Pt + H + + e H x WO 3 yh 2 O xh + + WO 3 yh 2 O + xe Bronze re-oxidation charge increases with scan rate decreasing slow process I / ma References: Tseung, Kulesza, Tsirlina, Kondo Pt + H + + e PtH ads xpth ads + WO 3 yh 2 O E / V vs RHE xpt + H x WO 3 yh 2 O PtWO 3 /C, 0.5 M H 2 SO 4 ; 10 mv/s

8 Catalyst characterisation On-line electrochemical mass spectrometry setup A.H. Wonders, et al. J. Appl. Electrochem. 36 (2006) 1215

9 CO adlayer oxidation Ion current / a.u. I / µa x10-7 m/z 44 (CO 2 ) 1.0x x E / V vs. RHE x10-8 m/z 44 (CO 2 ) 2.4x x x x E / V vs. RHE 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 On-line MS 0.5 M H 2 SO 4 ; 2mV/s saturated CO adlayer T. Nagel et al, J. Solid State Electrochem., 7 (2003) 614.

10 Continuous CO oxidation I / µa Oxidation of the dissolved CO on PtWO 3 /C starts already at ~ 0.12 V vs. RHE x10-7 m/z 44 (CO 2 ) m/z 44 (CO 2 ) Ionic current / a.u. 2.0x x x x E E-008 On-line MS 0.5 M H 2 SO 4 ; 2mV/s E / V vs. RHE 2.10E E / V vs. RHE saturated CO solution T. Nagel et al, J. Solid State Electrochem., 7 (2003) 614.

11 Continuous CO oxidation I / ma exp 1 exp 2 exp 3 Gradual deactivation of the catalyst is observed Steady-state resembles Pt What happens? 0 PtWO 3 /C deactivation during continuous CO oxidation 0.5 M H 2 SO 4 ; 2 mv/s E / V vs RHE

12 Continuous CO oxidation Bronze peak disappears Dissolution WO 3? Deactivation? I / ma blank CV (before) blank CV (after) E / V vs RHE PtWO 3 /C: blank CVs before and after continuous CO oxidation 0.5 M H 2 SO 4 ; 10 mv/s

13 Continuous CO oxidation I / ma After exposure to hydrogen evolution conditions Bronze peak re-appears No dissolution of WO RT (Reversible) deactivation! -2 inactive catalyst reactivated catalyst PtWO 3 /C catalyst reactivation ( V) M H 2 SO 4, 20 mv/s E / V vs RHE

14 Mechanism of the CO oxidation Pt + H + + e PtH ads xpth ads + WO 3 yh 2 O Pt + H x WO 3 yh 2 O CO Pt + OH HxWO3 CO 2 + WO 3 + Pt CO sol CO ads H x WO 3 H + sol Pt CO ads CO 2 WO 3 H ads Where does O come from lattice or H 2 O and what is the role of H x WO 3 yh 2 O?

15 CO 2 reduction After exposure to CO 2 saturated electrolyte an adsorbate forms I / ma Suppresses Hupd on Pt Oxidizes similarly to CO CO E / V vs. RHE PtWO 3 /C catalyst, CO 2 - saturated 0.5 M H 2 SO 4, E ads = 0.1 V vs RHE CO 2 + 2H ads CO + H 2 O

16 CO 2 reduction max ΞCO / ΞCO Rate of the adsorbate formation is much lower on PtWO 3 /C than on Pt/C: - lower Hupd concentration however 0.2 PtW/C Pt/C Eventually full coverage will be RT t / s CO 2 + 2H ads CO + H 2 O

17 Conclusions PtWO 3 /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 H x WO 3 is the active component & oxygen donor; formation of the bronze is slow in the PtWO 3 /C catalyst In the presence of CO, the bronze formation (via H spill-over from Pt to WO 3 ) is inhibited; (reversible) catalyst deactivation. PtWO 3 /C catalyst reduces CO 2 to CO at much lower rate than Pt/C. Still full coverage is expected at prolonged exposure to CO 2 at RT. PtWO 3 /C has a limited operating window (ca V to ca 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: WO 3 phase as crystalline as possible for quick recharging; well dispersed with Pt Drying induces ageing slowing down of recharging lower catalyst activity

18 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