Using synchrotron radiation to study catalysis Carlo Segre Physics Department & Center for Synchrotron Radiation Research and Instrumentation Illinois Institute of Technology March 18, 2014 Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 1 / 35
Outline Synchrotron radiation X-ray absorption spectroscopy Operando synchrotron fuel cells Oxygen reduction at the NiPt cathode Methanol oxidation on a PtRu catalyst Ru@Pt core-shell methanol catalysts Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 2 / 35
History of x-ray sources Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 3 / 35
History of x-ray sources 1895 x-rays discovered by William Röntgen Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 3 / 35
History of x-ray sources 1895 x-rays discovered by William Röntgen 1 st generation synchrotrons initially used in parasitic mode (SSRL, CHESS) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 3 / 35
History of x-ray sources 1895 x-rays discovered by William Röntgen 1 st generation synchrotrons initially used in parasitic mode (SSRL, CHESS) 2 nd generation were dedicated sources (NSLS, SRC, CAMD) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 3 / 35
History of x-ray sources 1895 x-rays discovered by William Röntgen 1 st generation synchrotrons initially used in parasitic mode (SSRL, CHESS) 2 nd generation were dedicated sources (NSLS, SRC, CAMD) 3 rd generation featured insertion devices (APS, ESRF, ALS) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 3 / 35
History of x-ray sources 1895 x-rays discovered by William Röntgen 1 st generation synchrotrons initially used in parasitic mode (SSRL, CHESS) 2 nd generation were dedicated sources (NSLS, SRC, CAMD) 3 rd generation featured insertion devices (APS, ESRF, ALS) 4 th generation are free electron lasers (LCLS) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 3 / 35
What is a synchrotron? Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 4 / 35
Headlight effect In electron rest frame: emission is symmetric about the axis of the acceleration vector Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 5 / 35
Headlight effect In electron rest frame: In lab frame: emission is symmetric about the axis of the acceleration vector emission is pushed into the direction of motion of the electron Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 5 / 35
Synchrotron source characteristics Bending magnet Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
Synchrotron source characteristics Bending magnet Wide horizontal beam Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
Synchrotron source characteristics Bending magnet Wide horizontal beam Broad spectrum to high energies Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
Synchrotron source characteristics Bending magnet Wide horizontal beam Broad spectrum to high energies Undulator Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
Synchrotron source characteristics Bending magnet Wide horizontal beam Broad spectrum to high energies Undulator Highly collimated beam Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
Synchrotron source characteristics Bending magnet Wide horizontal beam Broad spectrum to high energies Undulator Highly collimated beam Peaked spectrum with harmonics Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
Synchrotron source characteristics Bending magnet Wide horizontal beam Broad spectrum to high energies Undulator Highly collimated beam Peaked spectrum with harmonics Tunable energies Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
Synchrotron source characteristics Bending magnet Undulator Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 6 / 35
The EXAFS experiment I o = incident intensity I t = transmitted intensity I f = fluorescence intensity x = sample thickness µ(e) = absorption coefficient I t = I o e µ(e)x µ(e)x = ln ( Io I t ) µ(e) I f I o Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 7 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). k hν hν t Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. hν k t hν Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. Any excess energy from the x-ray is given to an ejected photoelectron hν k t hν Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. Any excess energy from the x-ray is given to an ejected photoelectron, which expands as a spherical wave Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. Any excess energy from the x-ray is given to an ejected photoelectron, which expands as a spherical wave Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. Any excess energy from the x-ray is given to an ejected photoelectron, which expands as a spherical wave, reaches the neighboring electron clouds Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. Any excess energy from the x-ray is given to an ejected photoelectron, which expands as a spherical wave, reaches the neighboring electron clouds, and scatters back to the core hole Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. Any excess energy from the x-ray is given to an ejected photoelectron, which expands as a spherical wave, reaches the neighboring electron clouds, and scatters back to the core hole Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
The x-ray absorption process An x-ray is absorbed by an atom when the energy of the x-ray is transferred to a core-level electron (K, L, or M shell). The atom is in an excited state with an empty electronic level: a core hole. Any excess energy from the x-ray is given to an ejected photoelectron, which expands as a spherical wave, reaches the neighboring electron clouds, and scatters back to the core hole, creating interference patterns called XANES and EX- AFS. ln(i o /I) 0.5 0-0.5-1 -1.5 XANES / NEXAFS -2 11500 12000 12500 E(eV) EXAFS Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 8 / 35
XANES edge shifts and pre-edge peaks 1.2 V 2 O 3 1 ln(i o /I) 0.8 0.6 V 2 O 5 0.4 V metal 0.2 LiVOPO 4 0 5460 5470 5480 5490 5500 E(eV) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 9 / 35
XANES edge shifts and pre-edge peaks 1.2 1 V 2 O 3 Edge shifts give valence state 0.8 ln(i o /I) 0.6 V 2 O 5 0.4 V metal 0.2 LiVOPO 4 0 5460 5470 5480 5490 5500 E(eV) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 9 / 35
XANES edge shifts and pre-edge peaks 1.2 1 V 2 O 3 Edge shifts give valence state 0.8 Pre-edge peaks indicate geometry ln(i o /I) 0.6 V 2 O 5 0.4 V metal 0.2 LiVOPO 4 0 5460 5470 5480 5490 5500 E(eV) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 9 / 35
XANES edge shifts and pre-edge peaks 1.2 1 V 2 O 3 Edge shifts give valence state 0.8 Pre-edge peaks indicate geometry ln(i o /I) 0.6 0.4 V metal V 2 O 5 Linear combination fitting for phase composition 0.2 LiVOPO 4 0 5460 5470 5480 5490 5500 E(eV) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 9 / 35
XANES edge shifts and pre-edge peaks 1.2 1 V 2 O 3 Edge shifts give valence state 0.8 Pre-edge peaks indicate geometry ln(i o /I) 0.6 0.4 V metal V 2 O 5 Linear combination fitting for phase composition 0.2 LiVOPO 4 0 5460 5470 5480 5490 5500 E(eV) Complete description requires full quantum codes Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 9 / 35
The EXAFS equation The EXAFS oscillations can be modelled and interpreted using a conceptually simple equation (the details are more subtle!) χ(k) = j N j S0 2f j(k) krj 2 e 2k2 σ 2 j e 2R j /λ(k) sin [2R j + δ j (k)] The sum could be over shells of atoms (Pt-Pt, Pt-Ni) or over scattering paths for the photo-electron. f j (k): λ(k): δ j (k): N j : R j : σ j : scattering factor for the path photoelectron mean free path phase shift for the j th path number of paths of type j half path length path disorder Ni Ni Pt Pt Pt Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 10 / 35
EXAFS analysis 0.5 0 ln(i o /I) -0.5-1 -1.5-2 11500 12000 12500 E(eV) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
EXAFS analysis 1 ln(i o /I) 0.5 0 11500 12000 12500 E(eV) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
EXAFS analysis 1 ln(i o /I) 0.5 0 11500 12000 12500 E(eV) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
EXAFS analysis 0.2 1 remove background 0.1 ln(i o /I) 0.5 χ 0-0.1 0 11500 12000 12500 E(eV) -0.2 0 5 10 15 k(å -1 ) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
EXAFS analysis 0.2 ln(i o /I) 1 0.5 remove background and apply k-weighting χ 0.1 0-0.1 0 11500 12000 12500 E(eV) -0.2 0 5 10 15 k(å -1 ) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
EXAFS analysis 0.2 ln(i o /I) 1 0.5 remove background and apply k-weighting χ 0.1 0-0.1 0 11500 12000 12500 E(eV) 30-0.2 0 5 10 15 k(å -1 ) 25 20 χ(r) 15 10 take Fourier Transform 5 0 0 2 4 6 R(Å) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
EXAFS analysis 0.2 ln(i o /I) 1 0.5 remove background and apply k-weighting χ 0.1 0-0.1 0 11500 12000 12500 E(eV) 30-0.2 0 5 10 15 k(å -1 ) 25 extract structural parameters for first shell χ(r) 20 15 10 5 take Fourier Transform and fit with a structural model 0 0 2 4 6 R(Å) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
EXAFS analysis 0.2 ln(i o /I) 1 0.5 remove background and apply k-weighting χ 0.1 0-0.1 0 11500 12000 12500 E(eV) 30-0.2 0 5 10 15 k(å -1 ) 25 extract structural parameters for first shell or more distant atoms as appropriate χ(r) 20 15 10 5 take Fourier Transform and fit with a structural model 0 0 2 4 6 R(Å) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 11 / 35
Mark I operando fuel cell Transmission mode <1 mm of graphite Pt/Ru on anode Pd on cathode 35 C operating temp 1-2 min scan time R. Viswanathan et al., In-situ XANES study of carbon supported Pt-Ru anode electrocatalysts for reformate-air polymer electrolyte fuel cells, J. Phys. Chem. B 106, 3458 (2002). Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 12 / 35
Mark II operando fuel cell FTIR & x-rays Air-breathing cathode Pd on anode 1.2 mg/cm2 loading 50 C operating temp Pt L3 and Ni K edges Continuous scan mode @ E.A. Lewis et al., Operando x-ray absorption and infrared fuel cell spectroscopy, Electrochim. Acta. 56, 8827 (2011). Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 13 / 35
Oxygen reduction at a PtNi cathode Anode: 0 V vs. SHE 2 H 2 4 H + + 4 e Cathode: 1.23 V s. SHE O 2 + 4 H + + 4 e 2 H 2 O breaking O O bond is rate limiting U.S. Department of Defense (DoD) Fuel Cell Test and Evaluation Center (FCTec) O Pt O O Pt O + H e H +. H H O Pt + H e + 2H + 2e (a) (b) (c) (d) O O Pt O 2H O 2 Pt Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 14 / 35
Fuel cell performance and open questions 1200 1229 mv Potential [mv] 1000 800 600 400 860 mv 780 mv 200 0 0 20 40 60 80 100 Current Density [ma/cm 2 ] PtNi/Pd has higher open circuit voltage, similar performance to Pt/Pd. Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 15 / 35
Fuel cell performance and open questions Potential [mv] 1200 1000 800 600 400 200 860 mv 1229 mv 780 mv 0 0 20 40 60 80 100 Current Density [ma/cm 2 ] PtNi/Pd has higher open circuit voltage, similar performance to Pt/Pd. Pt: How do reactants adsorb on platinum surface? Do all faces of Pt adsorb equally well? Is there a change in location with coverage? PtNi: Why is ORR improved with bimetallic catalyst? Pt electronic structure modified Pt catalyst geometric structure modified Static oxygen adsorbates inhibited Overpotential reduced How do real catalysts differ from model systems? Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 15 / 35
Pt/C and PtNi/C comparison µ(e) 1.0 0.5 550 mv 750 mv 900 mv 0.02 0.00 0.02 0.00 µ(e) 0.0 11550 11575 11550 11575 Energy (ev) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 16 / 35
PtNi Structural model Attempt to get global information about the oxygen Fit all potentials with same metal core parameters for each catalyst Simultaneous fit of Pt and Ni edges in PtNi/C with constraint on Pt-Ni distance Fit in k, k 2, and k 3 weighting simultaneously M-O path constraints length common across potentials σ 2 fixed to 0.01 Pt-O in PtNi/C are refined with a single occupation # Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 17 / 35
Example fits 1.0 Pt in Pt/C 1 0.5 0 χ(r) [Å -3 ] 0.0 1.0 0.5 0.0 1.0 Pt in PtNi/C Ni in PtNi/C Pt-O Pt-O 1 0 1 χ(q) [Å -2 ] 0.5 0 0.0 0 2 4 6 R [Å] Ni-O 0 4 8 12 q [Å -1 ] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 18 / 35
Fit results Pt/C PtNi/C Pt Ni N Pt 8.7 ± 0.2 N Pt 6.1 ± 0.3 N Ni 3.7 ± 0.2 R Pt-Pt 2.749 ± 0.001 R Pt-Pt 2.692 ± 0.003 R Ni-Ni 2.572 ± 0.006 N Ni 3.4 ± 0.1 N Pt 8.9 ± 0.5 R Pt-Ni 2.635 ± 0.004 N Total 9.5 ± 0.4 N Total 12.6 ± 0.7 R Pt-O 2.02 ± 0.01 R Pt-O 2.09 ± 0.03 R Ni-O 1.90 ± 0.01 Note the Pt-Pt and Pt-O bond lengths as well as total metal near neighbors Q. Jia et al, In Situ XAFS studies of the oxygen reduction reaction on carbon supported Pt and PtNi(1:1) catalysts, J. Phys. Conf. Series 190, 012157 (2009). Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 19 / 35
What does Ni really do? Resides predominantly in metal core of nanoparticle Eliminates static Pt-O bonds at all potentials Number of O near neighbors increases with potential Lengthens Pt-O and shortens Pt-Pt bond Reduces Pt white line in most reduced state (0 mv) Open circuit voltage is increased (reduction in overpotential) Can we use modeling to establish specific mechanism? Pt-Pt bond reduction (weakening of Pt-O bond)? Electron donation to Pt d-band (weakening of Pt-O bond)? Stronger affinity for oxygen? Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 20 / 35
Pt cluster modeling By using FEFF8.4, which performs full multiple-scattering selfconsistent calculations, we can explore the implications of the ligand effect and the strain effect on the electronic state of Pt. Separate the effects of (a) Shorter Pt-Pt distance (b) charge transfer from subsurface Ni Use experimentally determined distances Calculate local density of states Calculate XANES spectrum Q. Jia, et al., Structure-property-activity correlations of Pt-bimetallic nanoparticles: a theoretical study Electrochimica Acta bf 88, 604 (2013). Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 21 / 35
Cluster calculation results Strain effect shorter Pt-Pt bond broader and lower Pt d-band serves to weaken the Pt-O bond White line at absorption edge is reduced In agreement with DFT calculations (Nørskov et al.) Ligand effect subsurface Ni sharpens and raises Pt d-band Raises chemisorption energy Increases white line Net effect dominated by strain effect Predictive ability Moving down periodic table (Ru, Ag) Moving left across periodic table (towards Mn) Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 22 / 35
Methanol oxidation by a PtRu anode Anode: 0.02 V vs. SHE CH 3 OH + H 2 O 6 H + CO 2 + 6 e Cathode: 1.23 V s. SHE 3 2 O 2 + 6 H + + 6 e 3 H 2 O U.S. Department of Defense (DoD) Fuel Cell Test and Evaluation Center (FCTec) Pt surface poisoned by CO The presence of Ru promotes CO oxidation through a bifunctional mechanism Pt (CO) ads + Ru OH Pt + RuCO 2 + H + + 2 e Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 23 / 35
Ru EXAFS fitting Addition of Ru-O/C neighbors improves the EXAFS fit The peak at about 1.3 Å is ascribed to oxygen bound to Ru The asymmetric distribution of the Ru-O/C peak is consistent with disorder Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 24 / 35
Metal nanoparticle structure First shell analysis Fit Pt and Ru EXAFS simultaneously at each potential. No potential dependence observed Simultaneously fit Pt and Ru data at all potentials. Identical overall average coordination was observed Use fractional coordination numbers, x (Pt around Ru) and y (Ru around Pt) and total coordination number about each atom, N Bond lengths and Debye-Waller factors are consistent with literature values for C supported Pt-Ru catalyst (Russel 2001, Camara 2002) N 8.2 ± 0.2 x 0.54 ± 0.02 y 0.27 ± 0.02 [Ru] [Pt] = y x = 0.50 Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 25 / 35
Metal core restucturing As-received catalyst Ru oxidation 58 % N = 5.6 [Ru] [Pt] = y x = 0.44 Pt-O bonds present # Ru-O bonds 2.8 In-situ catalyst Ru oxidation 15 % N = 8.2 [Ru] [Pt] = y x = 0.50 No Pt-O bonds # Ru-O/C bonds 0.24 Inner core has more Pt than Ru Ru on surface and outside of metallic nanoparticle S. Stoupin, et al., Pt and Ru X-ray absorption spectroscopy of PtRu anode catalysts in operating direct methanol fuel cells J. Phys. Chem. 110, 9932 (2006). S. Stoupin, et al., Structural analysis of sonochemically prepared PtRu versus Johnson Matthey PtRu in operating direct methanol fuel cells Phys. Chem. Chem. Phys. 10, 6430 (2008). Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 26 / 35
Role of Ru in CO oxidation? PtRu bifunctional catalyst improves performance In commercial PtRu catalysts there is always a lot of inactive Ru-oxide (?) Ru signal dominated by metallic Ru environment How does Ru behave in the presence of reactants adsorbed on platinum surface? Core-shell nanoparticles can resolve these questions Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 27 / 35
Ru-decorated Pt nanoparticles Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 28 / 35
Electrochemical performance 0.002 PtRu Without Methanol Low V peaks are H + stripping Dip at 0.5 V is oxygen stripping Current [A] 0 Pt Ru shifts potential on all peaks -0.002-0.004-0.2 0 0.2 0.4 0.6 0.8 Potential [V] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 29 / 35
Electrochemical performance Current [A] 0.08 0.06 0.04 0.02 0 PtRu -0.2 0 0.2 0.4 0.6 0.8 Potential [V] Pt Without Methanol Low V peaks are H + stripping Dip at 0.5 V is oxygen stripping Ru shifts potential on all peaks With Methanol Continual current growth is due to methanol oxidation Ru improves current by removing the CO which blocks active sites Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 29 / 35
Ru EXAFS 1 No methanol +675 mv 0.8 +575 mv χ(r) [Å -3 ] 0.6 0.4 +375 mv +175 mv -225 mv 0.2 0 0 1 2 3 4 5 R [Å] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 30 / 35
Ru EXAFS No methanol With methanol 1 +675 mv 1 0.8 +575 mv 0.8 χ(r) [Å -3 ] 0.6 0.4 +375 mv +175 mv -225 mv χ(r) [Å -3 ] 0.6 0.4-225 mv +375 mv +675 mv 0.2 0.2 0 0 1 2 3 4 5 R [Å] 0 0 1 2 3 4 5 R [Å] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 30 / 35
Ru-M paths 3.1 3 (a) Ru-Pt (b) Ru-Ru Without methanol 2.9 R [Å] 2.8 2.7 With methanol 2.6 2.5-0.2 0 0.2 0.4 0.6-0.2 0 0.2 0.4 0.6 Applied Potential [V vs Ag/AgCl] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 31 / 35
Ru-M paths 3.1 3 2.9 (a) Ru-Pt (b) Ru-Ru Without methanol Ru-M distances are longer and RuO 2 is formed at high potentials R [Å] 2.8 2.7 With methanol 2.6 2.5-0.2 0 0.2 0.4 0.6-0.2 0 0.2 0.4 0.6 Applied Potential [V vs Ag/AgCl] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 31 / 35
Ru-M paths 3.1 3 2.9 (a) Ru-Pt (b) Ru-Ru Without methanol Ru-M distances are longer and RuO 2 is formed at high potentials R [Å] 2.8 2.7 2.6 With methanol Ru-M distances are shorter and remain the same at all potentials 2.5-0.2 0 0.2 0.4 0.6-0.2 0 0.2 0.4 0.6 Applied Potential [V vs Ag/AgCl] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 31 / 35
Ru-O/C paths R [Å] 2.1 2.0 (a) Without methanol 1.9 # of Neighbors 5 4 3 2 1 (b) With methanol -0.2 0 0.2 0.4 0.6 Applied Potential [V vs. Ag/AgCl] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 32 / 35
Ru-O/C paths R [Å] 2.1 2.0 1.9 (a) Without methanol Above 375 mv Ru-O paths appear and total number of Ru-O neighbors increases to that of RuO 2 # of Neighbors 5 4 3 2 1 (b) With methanol -0.2 0 0.2 0.4 0.6 Applied Potential [V vs. Ag/AgCl] Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 32 / 35
Ru-O/C paths R [Å] 2.1 2.0 1.9 (a) Without methanol Above 375 mv Ru-O paths appear and total number of Ru-O neighbors increases to that of RuO 2 # of Neighbors 5 4 3 2 1 (b) -0.2 0 0.2 0.4 0.6 Applied Potential [V vs. Ag/AgCl] With methanol Ru has one low Z neighbor at all potentials (carbon); a second above 175 mv (oxygen) with constant bond lengths and slightly increasing numbers Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 32 / 35
Bi-functional mechanism H H O C H H +H+ H + H + H O 3 H H + H + C O O 1 2 4e - C 4 O C O 2e - Ru Pt Pt Pt 1 Pt 1 x-rays EXAFS Pt Pt Pt 2 Pt C. Pelliccione et al., In situ Ru K-Edge x-ray absorption spectroscopy study of methanol oxidation mechanisms on model submonolayer Ru on Pt nanoparticle electrocatalyst J. Phys. Chem. C 117, 18904 (2013). Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 33 / 35
Principal collaborators - Thank you! Pt/Ru core-shell Christopher Pelliccione IIT (Ph.D. student) John Katsoudas IIT (MRCAT staff) Elena Timofeeva Argonne, Energy Systems Pt/Ru anode catalyst Stanislav Stoupin IIT (Ph.D. Student) Soma Chattopadhyay IIT (MRCAT staff) Harry Rodriguez UPR (Ph.D. student) PtNi cathode catalyst Qingying Jia IIT (Ph.D. Student) Eugene Smotkin Northeastern Univ. Emily Lewis Northeastern Univ. (M.S. Student) Supported by the Department of Energy Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 34 / 35
Abstract X-ray absorption spectroscopy using synchrotron radiation has become an essential tool for operando studies of catalysts. I will discuss the fundamentals of synchrotron radiation and x-ray absorption spectroscopy along with several examples of how it is applied to the study of polymer electrolyte fuel cell catalysts. Over the past decade, we have studied the anode and cathode of hydrogen/air and direct methanol fuel cells in operando. Most recently, we have conducted in situ experiments of Ru-decorated Pt catalysts (Ru@Pt) to elucidate the details of the bifunctional mechanism of methanol oxidation. The presence of Ru atoms only at the surface of nanoparticles permits correlation of the x-ray absorption fine structure (XAFS) at the Ru K-edge to the role of Ru atoms in the methanol oxidation process. In-situ XAS spectra of the Ru@Pt nanoparticles were collected at various potentials in background electrolyte, and then in 1 M solution of methanol in the same electrolyte. Significant differences in the catalyst state have been revealed between these two environments. In the background electrolyte, Ru gradually oxidizes from mostly metallic to a Ru(III)/Ru(IV) mixture at the highest potentials. In the presence of methanol, the Ru oxidation state remains a mixture of metallic and Ru(III) even at the highest potentials. CO-type species were found adsorbed on Ru atoms at all potentials, and co-adsorbed with OH species at potentials 0.175 V vs Ag/AgCl and higher. These results provide a better understanding of the role of surface Ru atoms in methanol oxidation by bi-metallic catalysts. Illinois Institute of Technology Villanova Univ. - Chemistry Colloquium March 18, 2014 35 / 35