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Some Fundamental Challenges in Electrocatalysis

Some Fundamental Challenges in Electrocatalysis David J. Schiffrin Chemistry Department University of Liverpool UK. Summary, or what is going on? And why?. Instrumentation and techniques Surface spectroscopies In situ analysis e.g., mass spectroscopy, XAFS

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Some Fundamental Challenges in Electrocatalysis

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  1. Some Fundamental Challenges in Electrocatalysis David J. SchiffrinChemistry DepartmentUniversity of LiverpoolUK

  2. Summary, or what is going on? And why? Instrumentation and techniques Surface spectroscopies In situ analysis e.g., mass spectroscopy, XAFS Availability of large scale facilities Surface science techniques, high resolution XPS Imaging, surface probe microscopies, STEM, ultrahigh resolution TEM Theory Theoretical advances in electron transfer and reactivity Computational methods e.g., quantum chemical calculations Materials Alloys Nanoparticles Non Pt based electrocatalysts Applications Electroanalysis Fuel cells Electrosynthesis Economic and social drivers Environmental issues (“green” chemistry) Hydrogen transport Water treatment

  3. Some scientific issues Nanoparticles Alloy properties: how to predict structures and properties Importance of size effects Prediction of reactivity of different surfaces Synthesis: control of size and geometry Reactivity Use biologically inspired synthetic strategies Transposition of single crystal studies to nanostructured surfaces Quantum chemical calculations for the prediction of reactivity Extension of theory of electron transfer to reactions on nanostructured surfaces Metal-reactant interactions Reactions of interest Oxygen reduction Oxygen evolution Nitrate reduction Carbon dioxide reduction

  4. Fuel cell buses in Europe and North America A fuel cell engine

  5. Cathode (+) Anode (-) Pt Nanoparticles O2 H2O + 4 e- Carbon Carbon 2 H2 4H+ + 4 e- Running a car on nanoparticles What about the Pt-carbon contact? How do we study the properties of well-defined nanoparticle-electrode systems?

  6. Preparation of nanoparticulate electrode surfaces

  7. Nanoparticles at surfaces • Problem: in order to study the electrochemical properties of nanoparticles (e.g., size effects), we need to attach them to an electrode surface. • Two approaches: • Synthesise and then fix them • In-situ growth

  8. Attachment by ligands of nanoparticles Dithiols: Stability problems with Pt Diamines: Poor long term stability Alternative strategy: In situ growth

  9. Electrochemical growth of nanoparticles on carbon surfaces Separate control of nucleation and growth to achieve uniform size distribution R Penner, J. Phys. Chem. B 2002, 106, 3339-3353

  10. In-situ nucleation and growth on HOPG Guojin Lu and Giovanni Zangari, J. Phys. Chem. B 2005, 109, 7998 AFM image of Pt nanoparticles prepared using the potential pulse sequence shown. Average particle height = 26 nm. Average particle height = 16 nm. Bayati, Abad, Nichols, Schiffrin, in preparation

  11. Creation of growth sites by oxidation Oxidised HOPG at 2.0 V for 1 s AFM image of electrodeposited nanoparticles on electrochemically oxidized HOPG. Size between 2 and 5 nm depending on growth time. Oxidation in H2SO4 leads to a large disruption of the surface. Bad news Bayati, Abad, Nichols, Schiffrin, in preparation

  12. Chemical growth of nanoparticles on electrode surfaces

  13. Chemical functionalisation strategy of carbon surfaces Dao-jun Guo, Hu-lin Li, Electrochemistry Communications 6 (2004) 999–1003 Waje etg al., Nanotechnology 16 (2005) S395–S400

  14. Construction of 2-D arrays of Pt particles. Strategy followed NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 Pt nuclei Redn Electrochemical or chemical growth Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

  15. Construction of 2-D arrays of Pt particles. (I) Amino termination NO NO NO NO 2 2 2 2 Diazonium chemistry N2 + + + N N e - NO NH NH NO NH NO 2 2 2 2 2 2 Sn(II), HCl Chemical reduction XPS spectra of the N1s region for Pt/-NH2 modified HOPG after background subtraction. Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

  16. Conductance measurements for BDMT (To find out if a phenyl group is a high conductance linker) Zero bias conductance = 7 nS (140 MΩ per molecule) or approx. 5x10-6Ω cm-2 Very small resistance!! OK Van Zalinge, et al., Nanotechnology 17 (2006) 3333–3339

  17. Mechanisms of dediazotation reactions heterolytic Rate of the heterolytic reaction is very slow, 1/2 of 6-9 hs! homolytic The radical mechanism is the preferred route for the electrochemical functionalisation. Spontaneous attachment? Source of electrons?

  18. Spontaneous Grafting of Glassy Carbon with Fast Red Fast Red ACETONITRILE 0.05 M H2SO4 H2O unbuffered phosphate buffer (pH 7) Seinberg, Kullapere, Mäeorg, Maschion, Maia, Schiffrin, Tammeveskia, J. Electroanal Chem, in press

  19. Construction of 2-D arrays of Pt particles. Strategy followed NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 Pt nuclei Redn Electrochemical or chemical growth Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

  20. XRD data analysis- Pt nanoparticles on HOPG (b) Size calculations using the Scherrer equation sp3 carbon? Fitting to data Residuals XRD (a) XRD patterns for Pt nanoparticles on Ar-NH2 modified HOPG of 4.0 (1), 2.7 (2) and 2.0 nm (3) average size (measured by TEM). (b) Analysis for the 2.0 nm Pt nanoparticles using the Rietveld refinement. βi is the width in radians of the diffraction peaks measured at half their maximum intensity (FWHM) and corrected for instrumental broadening Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

  21. sp2- sp3 evidence from single CNT conductance CNTs were functionalised using diazonium chemistry. “Upon annealing, the functionalization is removed, restoring the electronic properties of the nanotubes.”

  22. sp2 sp3 surface changes on grafting sp3 sites Graphene sheet - sp2 sites X-Ray diffraction of a functionalised HOPG surface Surface functionalisation by bonding changes the sp2 sites on the graphene sheet into sp3 sites. CNTs become non-conducting on side functionalisation Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

  23. Pt nanoparticles prepared by chemical reduction TEM image of Pt nanoparticles on Ar-NH2 modified HOPG after one nucleation-growth cycle. The insets show the size distribution. Average sizes: a) (2.7 ± 0.4); b) (4.0 ± 0.5) nm. Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

  24. XPS of Pt nanoparticles attached to HOPG Fitted data • CPS a a’ b’ b Original data Pt 2 nm Two contributions observed: core and surface The Pt core has a lower value of the FWHM: Uniform environment Surface BE corresponds to Pt(II) present in a range of environments Deconvoluted XPS spectra of the 4f core level for Pt/Ar-NH2 modified HOPG electrodes. Diameter = 2 nm. Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem 623 (2008) 19

  25. XRD analysis for 4 nm particles Fitting to data Residuals Rietveld refinement for the analysis of the XRD data for 4.0 nm Pt nanoparticles on -NH2 modified HOPG. Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

  26. Size effects: Yes! CO Methanol Potential sweep voltammograms at 20 mV/s in 0.5 M H2SO4 + 1 M methanol at a Pt/Ar-NH2 modified HOPG electrode containing nanoparticles with sizes of 4.0 ± 0.5 (1), 2.7 ± 0.4 (2) and 2.0 ± 0.5 nm (3). CO stripping voltammogram in 0.5 M H2SO4 (sweep rate = 20 mV/s) at a Pt/Ar-NH2 modified HOPG containing nanoparticles with sizes of 4.0 ± 0.5 (1), 2.7 ± 0.4 (2) and 2.0 ± 0.5 nm (3).

  27. Control of shape, size and composition of nanoparticles

  28. Nanoparticle Alloys Synthesis “Polyol” method: reduction of a Pt complex by a di-alcohol in the presence of metal carbonyls with oleic acid or oleylamine as stabilisers. T = 200 oC Pt Mainly (111) planes! Pt-Co Giersig et al. J. Phys. Chem. B 2003, 107, 7351-7354

  29. Shape control Ag nanocubes formed by reduction in ethylene glycol used both as a reducing agent and solvent. Stabilising agent = poly(vinylpyrrolidone) SunY, XiaY, Science 2002, 298, 2176–79

  30. Nanorods -Seeded growth C J Murphy et al J. Phys. Chem. B,. 2005109, 13857

  31. Nanorods-Seeded growth C J Murphy et al J. Phys. Chem. B,. 2005109, 13857

  32. Nanosphere lithography Insulating spheres Conducting substrate Controlled metal deposition Template dissolution P D Van Duyne, J. Phys. Chem. B 2001, 105, 5599-5611

  33. Synthesis of nanotriangles For use in enhanced Raman spectroscopy: high field localisation to produce “hot spots”, regions of high electromagnetic field

  34. Polymer stabilised Au nanoparticles (2-5 nm) Alkanethiol Transfer to organic solvents Langmuir 2007, 23, 885-895

  35. Polymers used Langmuir 2007, 23, 885-895

  36. Size-controlled Synthesis (Irshad Hussain, M Brust, A Cooper, Liverpool, Langmuir 2007, 23, 885-895 ) HAuCl4 in Water PMAA-DDT Co-polymer NaBH4 Gold nanoparticles of different sizes

  37. Polymer stabilised Au nanoparticles (2-5 nm) Alkanethiol Transfer to organic solvents Langmuir 2007, 23, 885-895

  38. Reduction/fixation of carbon dioxide

  39. Electrochemical reduction of CO2 Direct reduction Transition metal ions catalysed reduction Biologically inspired-Calvin cycle

  40. Direct reduction Main products (older work): CO, formic acid, oxalic acid Difficulties: direct electron transfer leads to reactive radicals CO2 + e- ⇌ CO2- CO2- + H2O ⇌ COOH COOH is readily reduced: COOH + e-  HCOO- (Formate!)

  41. Reduction from the adsorbed state From Pt, the product is adsorbed CO, which is strongly attached to the Pt surface. From Cu, a very large number of products are formed in low yields and very irreproducibly. Alcohols, hydrocarbons, acids, etc have been reported. The mechanism is not understood.

  42. Reduction on complexes Transition metals macrocylic complexes produce a large zoological garden of compounds. The mechanisms are not clear. The yields are in general low.

  43. Calvin Cycle approach Try to reproduce sections of the cycle by which plants reduce and fix carbon dioxide

  44. Looking at biology-CO2 reduction

  45. The reduction step in the Calvin Cycle Reduction Step The real reduction step is the reduction of a carboxylate group to yield an aldehyde

  46. Accessing the electrocatalytic metal centre of redox proteins with nanoparticles

  47. Does the incorporation of a nanoparticle within a molecular wire change the rate of ET? e- e- Compare with: Jensen, Chi, F. Grumsen, Abad, Horsewell, Schiffrin, Ulstrup J. Phys. Chem. C, 2007, 111, 6124-6132

  48. Au Electronic coupling of redox proteins by AuNPs Direct Electron Transfer mediated by gold nanoparticles Galactose oxidase Nanoparticle–biomolecule conjugates J. Abad, M.Gass, A. Bleloch and D. J. Schiffrin, in preparation

  49. R-CHO R-CH2-OH +e- +e- Cu2+- Tyr· Cu2+- Tyr Cu+- Tyr -e- -e- (GalODox) (GalODsemi) (GalODred) CuII O2 H2O2 Tyr-495 His-496 His-581 H20 Tyr-272 Redox protein studied Galactose oxidase Cu(II) centre Firbank, S.J., Rogers, M., Hurtado-Guerrero, R., Dooley, D.M., Halcrow,M.A., Phillips, S.E.V., Knowles, P.F., McPherson, M.J., Biochem. Soc. T2003,31, 506–509. Labile ligand. Can be replaced by COO- J. Abad, M.Gass, A. Bleloch and D. J. Schiffrin, in preparation

  50. High Angle Annular Dark Field (HAADF) STEM Tomography The strong Coulomb interaction of the electrons with the potential of an atom core, which leads to high angle scattering (designated as Rutherford scattering) and even to back-scattering, is employed in STEM (Z-contrast imaging) and in SEM. By the HAADF-STEM method, small clusters (or even single atoms) of heavy atoms can be imaged in a matrix of light atoms since the contrast is approximately proportional to Z2 (Z: atomic number). HAADF detectorThe high-angle annular dark field detector is also a disk with a hole, to detect electrons that are scattered to higher angles and almost only incoherent Rutherford scattering contributes to the image. Thereby, Z contrast is achieved

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