FTIR, XANES and DFT Calculations

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FTIR, XANES and DFT Calculations On Pt Based
Fuel Cell Catalysts
Eugene S. Smotkin Department of Chemical
Engineering Chemistry ARO Workshop on
Application of First-Principles-Based
Computational Methods to the Design of
Electrochemical Power Systems Berkeley, CA
August 31, 2001
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Overview

• Lattice Parameter Study of Fuel Cell Catalysts
  and Arc-melted Alloys
• Potential Dependent FTIR of CO adsorbed on
  Arc-melted Alloys
• Potential Dependent FTIR of CO adsorbed on
  membrane electrode assemblies
• XANES of supported PtRu Catalysts
• Density Functional Theory Calculations Pt and
  PtRu

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Reformate Fuel Cells

• Hydrogen is produced by reforming of hydrocarbon
  fuels.
• Fuel Processor yields 10 50 ppm CO
• Water activation is needed for CO Oxidation.

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Effect of CO concentration
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The Distribution and Composition of the Phases
are Dependent on the Synthetic Method
Watanabe or Borohydride reduction
Arc melted alloys
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FCC Lattice Spacings of Catalysts and Arc-Melted
Alloys
Gurao et. al, J. Phys. Chem. B 1998, 102, 9997
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Local Maxima

• Borohydride PtRuOs gt PtRu JES, 144, 1543,
  1997
• Borohydride PtRuOsIr gtPtRu Science, 280, 1735,
  1998
• Watanabe PtRu gt PtRuOsIr Fuel cell testing IIT
• Arc-melted alloys PtRuOs gt PtRu JES, 144, 1543,
  1997
• High Surface Area PtRu(5050) gt PtRu 7030
  Commercial catalysts
• Well Defined Alloys PtRu 7030 gt PtRu(5050)
  JES, 141, 1795, 1994

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FCC Lattice Spacings of Catalysts and Arc-Melted
Alloys
Ref. J. Phys. Chem. B 1998, 102, 9997
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Adsorbed CO as a Probe of Electronic Structure

• Blyholder Mechanism, J. Phys. Chem 1964, 68, 2772
• Donation of the 5? MO of CO into the metal
• Back donation of the metal d-band into the 2?
  MO of CO
• Coverage effect
• Dipole-dipole coupling

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FTIR of Adsorbed CO on Arc-melted Pt and Pt based
alloy electrodes
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Pure Pt, 50 CO
CO on arc-melted Pt
.05 V
0.7 V
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Ru, 50 CO
0.05 V
0.7 V
CO on arc-melted Ru
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In-situ Specular Reflectance Spectroscopy of Fuel
Cell Anodes
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CO on PtRu in a DMFC
Ru phase
Alloy phase
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Possibilities

• Alloying causes increased back donation into the
  2? MO of CO, reducing the bond order and
  lowering the stretching frequency.
• Alloying reduces coverage and dipole-dipole
  coupling. Stretching frequencies increase with
  crowding.
• Its still an open question

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Study the bulk electronic structure with XAS
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Schematic of XAFS experiment
I0 Incident Flux I Transmitted Flux x
Sample Thickness µ(E) Absorption Coefficient
at photon energy E
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In-situ XAFS Fuel Cell Schematic
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Selected Previous Fuel Cell XAFS Work

• J. McBreen and S. Mukerjee ( J.E.S 1995 Vol 142,
  No.10 p 3399) studied in-situ X-Ray absorption on
  a Pt-Ru electrocatalyst in 1 M HClO4.
• Andrea Russel et.al (J Phys.Chem B
  2000,104,1998-2004) studied EXAFS of CO oxidation
  on a Pt/C catalyst in 1M H2SO4.
• W.E. OGrady et.al. (Langmuir 2001,17,3047-3050)
  performed ex-situ analysis of Pt-Ru
  electrocatalyst of an as prepared catalyst and an
  MEA that was run in a DMFC.

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Reformate/ H2-Air fuel cell performance in the
in-situ XAFS cell
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Pt LIII-edge 2p3/2 to 5d orbital ANL, transmission
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Ru K-edge of References s to p orbital
transition, BNL
Ru transmission Pt80Ru20 alloy electron
yield Ruthenium Oxides transmission
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Ru K-edge s to p orbital transition
ANL in-situ transmission
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Electrode Conditioning

• Conditioning has no effect on Pt edge.
• Conditioning has no effect on Ru edge.
• Fresh electrodes yield very poor performance.
• Conditioning is an issue of wetting.

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Ex-situ and in-situ Ru K-edge
Ex-situ
In-situ
References
Ex-situ data BNL, fluorescence Ru and RuO2
BNL, transmission In-situ data ANL, transmission
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Conclusions

• In-situ XANES data suggest that both Pt and Ru
  are metallic in the electrocatalyst and present
  as a mixed alloy under real fuel cell operating
  conditions.
• Conditioning the fuel cell does not change the
  gross chemistry of the dispersed Pt-Ru catalyst
  particles.
• Alloying increases the Pt-d band vacancies in the
  catalyst.
• The ex-situ MEA XANES show reduction of some RuOX
  to metallic Ru with conditioning.
• Increased vacancies as a result of alloying
  supports a coverage effect explanation for
  reduced stretching frequencies versus a bulk
  electronic explanation.

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DFT Calculations

• DFT calculations were performed under B3LYP in
  various Pt and Pt-Ru absorbed CO clusters
• Size cluster effect studied first 3 layers
  needed and at least 13 surface atoms for size
  independent CO normal modes.
• When Pt is alloyed with Ru CO stretching
  frequency drops.
• Jaguar 4.1 by Schrödinger Inc.

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No of atoms 5 Pt-C1.805 Å , C-O1.150 Å CO
Stretching Frequency 2147.6 cm-1
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No of atoms9 Pt-C1.80 Å , C-O1.145 Å CO
Stretching Frequency 2166.05 cm-1
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No of atoms13 CO Stretching Frequency 2173.0
cm-1
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No of atoms21 Pt-C1.808 Å , C-O1.148 Å CO
Stretching Frequency 2166.16 cm-1
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No of atoms25 CO Stretching Frequency 2172.6
cm-1 ONE LAYER HAS CONVERGED TO 2168 cm-1
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No of atoms 13 (1s layer 9, 2nd layer
4) Pt-C1.847 Å , C-O1.149 Å CO Stretching
Frequency 2131.07 cm-1
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No of atoms 17(1s layer 13,2nd layer
4) Pt-C1.846 Å , C-O1.150 Å CO Stretching
Frequency 2128.96 cm-1
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No of atoms 25(1s layer13, 2nd layer 12) Pt-C
1.836, C-O1.151 CO Stretching Frequency 2121.16
cm-1
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No of atoms 37(1s layer25,2nd layer 12) Pt-C
1.848, C-O1.151 CO Stretching Frequency 2124.33
cm-1
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No of atoms 14(1s layer9,2nd layer 4,3rd layer
1) Pt-C 1.827, C-O1.148 CO Stretching
Frequency 2148.94 cm-1
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No of atoms 18(1s layer9,2nd layer 4,3rd layer
5) Pt-C 1.823, C-O1.148 CO Stretching
Frequency 2151.4 cm-1
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No of atoms 13 (9 Pt atoms, 4 Ru atoms) Pt-C
1.881, C-O1.151 CO Stretching Frequency 2107.69
cm-1
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DFT Calculations of CO on Pt and Pt-Ru clusters
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Conclusions

• DFT results predict reduction of CO stretching
  frequencies upon alloying.
• XANES predicts increase in d-band vacancies upon
  alloying and all is metal.
• Conditioning doesnt change bulk.
• FTIR results are consistent with DFT results but
  the question is still open.

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Acknowledgments

• Rameshkrishnan Viswananthan, IIT
• Guoyan Hou, IIT
• Aili Bo, IIT
• Renxuan Liu, NuVant Systems/IIT
• Simon Bare, UOP
• Carlo Segre, IIT Physics
• Bogdan Gurau
• Nick Dimakis, IIT Physics
• Funding Provided by UOP, NuVant Systems ARO