some fundamental challenges in electrocatalysis david j. schiffrin chemistry department university...
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Some Fundamental Challenges in Electrocatalysis
David J. SchiffrinChemistry DepartmentUniversity of LiverpoolUK
Summary, or what is going on? And why?Instrumentation and techniquesSurface spectroscopiesIn situ analysis e.g., mass spectroscopy, XAFSAvailability of large scale facilitiesSurface science techniques, high resolution XPSImaging, surface probe microscopies, STEM, ultrahigh resolution TEMTheoryTheoretical advances in electron transfer and reactivity Computational methods e.g., quantum chemical calculationsMaterialsAlloysNanoparticlesNon Pt based electrocatalystsApplicationsElectroanalysisFuel cellsElectrosynthesisEconomic and social driversEnvironmental issues (“green” chemistry)Hydrogen transportWater treatment
Some scientific issues
Nanoparticles Alloy properties: how to predict structures and properties Importance of size effectsPrediction of reactivity of different surfacesSynthesis: control of size and geometry
ReactivityUse biologically inspired synthetic strategiesTransposition of single crystal studies to nanostructured surfacesQuantum chemical calculations for the prediction of reactivityExtension of theory of electron transfer to reactions on nanostructured surfacesMetal-reactant interactions
Reactions of interest Oxygen reductionOxygen evolution Nitrate reductionCarbon dioxide reduction
Running a car on nanoparticles
CarbonCarbon
O2
H2O + 4 e-
2 H2
4H+ + 4 e-
Cathode (+) Anode (-)Pt Nanoparticles
What about the Pt-carbon contact? How do we study the properties of well-defined nanoparticle-electrode systems?
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:
1. Synthesise and then fix them
2. In-situ growth
Attachment by ligands of nanoparticles
Dithiols: Stability problems with Pt
Diamines: Poor long term stability
Alternative strategy: In situ growth
Electrochemical growth of nanoparticles on carbon surfaces
R Penner, J. Phys. Chem. B 2002, 106, 3339-3353
Separate control of nucleation and growth to achieve uniform size distribution
In-situ nucleation and growth on HOPG
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
Guojin Lu and Giovanni Zangari, J. Phys. Chem. B 2005, 109, 7998
Creation of growth sites by oxidation
Bayati, Abad, Nichols, Schiffrin, in preparation
Oxidation in H2SO4 leads to a large disruption of the surface. Bad news
AFM image of electrodeposited nanoparticles on electrochemically oxidized HOPG. Size between 2 and 5 nm depending on growth time.
Oxidised HOPG at 2.0 V for 1 s
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
NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2
NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2
Electrochemical or chemical growth
Pt nuclei
24PtCl
Redn
Construction of 2-D arrays of Pt particles. Strategy followed
Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press
Construction of 2-D arrays of Pt particles. (I) Amino termination
+
e-
+ N2
N+
N
NO2
NO2 NO 2 NO 2
Diazonium chemistry
NO2 NO 2 NO 2 NH2 NH 2 NH 2
Sn(II), HCl Chemical reduction
396 398 400 402 404 406
0
50
100
150
200
103 x
Co
un
ts
Binding Energy / eV
XPS spectra of the N1s region for Pt/-NH2 modified HOPG after background subtraction.
Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press
Conductance measurements for BDMT (To find out if a phenyl group is a high conductance linker)
Van Zalinge, et al., Nanotechnology 17 (2006) 3333–3339
Zero bias conductance = 7 nS (140 MΩ
per molecule) or approx. 5x10-6 Ω cm-2
Very small resistance!! OK
Mechanisms of dediazotation reactions
N2+ + N2
N2+ + Red . + N2 + Ox (concerted)
N2 + Ox.
(stepwise)
heterolytic
homolytic
Rate of the heterolytic reaction is very slow, 1/2 of 6-9 hs!
The radical mechanism is the preferred route for the electrochemical functionalisation. Spontaneous attachment? Source of electrons?
Spontaneous Grafting of Glassy Carbon with Fast Red
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015(b) 3
2
1
I / m
A
E / V-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015(c) 2
1
I / m
AE / V
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
(d)
21
I / m
A
E / V
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015 3
21(e)
I / m
A
E / V
0.05 M H2SO4 ACETONITRILE
phosphate buffer (pH 7) H2O unbuffered
Seinberg, Kullapere, Mäeorg, Maschion, Maia, Schiffrin, Tammeveskia, J. Electroanal Chem, in press
Fast Red
NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2
NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2
Electrochemical or chemical growth
Pt nuclei
24PtCl
Redn
Construction of 2-D arrays of Pt particles. Strategy followed
Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press
XRD data analysis- Pt nanoparticles on HOPG
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.
(b)
sp3 carbon?
Size calculations using the Scherrer equation ii
i θcosβ
λD
βi is the width in radians of the diffraction peaks measured at half their maximum intensity (FWHM) and corrected for instrumental broadening
Fitting to data
Residuals
Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press
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.”
sp2 sp3 surface changes on grafting
Graphene sheet - sp2 sites
Surface functionalisation by bonding changes the sp2 sites on the graphene sheet into sp3 sites. CNTs become non-conducting on side functionalisation
40 45 50 55 600
20000
40000
60000
80000
100000
Inte
nsi
ty/ A
rb u
nit
s
2 / deg
X-Ray diffraction of a functionalised HOPG surface
sp3 sites
Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press
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
XPS of Pt nanoparticles attached to HOPG
CPS
Original data
b’ b
a’a
Fitted data
Deconvoluted XPS spectra of the 4f core level for Pt/Ar-NH2 modified HOPG electrodes. Diameter = 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
Pt 2 nm
Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem 623 (2008) 19
XRD analysis for 4 nm particles
2θ/deg
Inte
nsit
y/A
.U.
C
Rietveld refinement for the analysis of the XRD data for 4.0 nm Pt nanoparticles on -NH2 modified HOPG.
Fitting to data
Residuals
Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press
Size effects: Yes!
-0.6 -0.4 -0.2 0.0 0.2 0.4
Cu
rren
t D
ensi
ty /
mc
m-2
E / V vs. MSE
0.4mAcm-2
4 nm
2.7 nm
2 nm
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).
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 Methanol
Nanoparticle Alloys Synthesis
Giersig et al. J. Phys. Chem. B 2003, 107, 7351-7354
“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
Pt-Co
Mainly (111) planes!
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
Nanosphere lithography
P D Van Duyne, J. Phys. Chem. B 2001, 105, 5599-5611
Insulating spheres
Conducting substrate Controlled metal deposition
Template dissolution
Synthesis of nanotriangles
For use in enhanced Raman spectroscopy: high field localisation to produce “hot
spots”, regions of high electromagnetic field
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
2.5
3.0
Abs
orba
nce
Wavelength (nm)
Au : Polymer = 1 : 0.006 Au : Polymer = 1 : 0.03 Au : Polymer = 1 : 0.18 Au : Polymer = 1 : 0.6 Au : Polymer = 1 : 1.8 Au : Polymer = 1 : 3.6
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
2.5
3.0
Abs
orba
nce
Wavelength (nm)
Au : Polymer = 1 : 0.006 Au : Polymer = 1 : 0.03 Au : Polymer = 1 : 0.18 Au : Polymer = 1 : 0.6 Au : Polymer = 1 : 1.8 Au : Polymer = 1 : 3.6
Alkanethiol
Transfer to organic solvents
Langmuir 2007, 23, 885-895
Polymer stabilised Au nanoparticles (2-5 nm)
CO2H
S x9
HAuCl4 in Water
PMAA-DDT Co-polymerNaBH4
Gold nanoparticles of different sizes
Size-controlled Synthesis (Irshad Hussain, M Brust, A Cooper, Liverpool, Langmuir 2007, 23, 885-895 )
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
2.5
3.0
Abs
orba
nce
Wavelength (nm)
Au : Polymer = 1 : 0.006 Au : Polymer = 1 : 0.03 Au : Polymer = 1 : 0.18 Au : Polymer = 1 : 0.6 Au : Polymer = 1 : 1.8 Au : Polymer = 1 : 3.6
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
2.5
3.0
Abs
orba
nce
Wavelength (nm)
Au : Polymer = 1 : 0.006 Au : Polymer = 1 : 0.03 Au : Polymer = 1 : 0.18 Au : Polymer = 1 : 0.6 Au : Polymer = 1 : 1.8 Au : Polymer = 1 : 3.6
Alkanethiol
Transfer to organic solvents
Langmuir 2007, 23, 885-895
Polymer stabilised Au nanoparticles (2-5 nm)
Electrochemical reduction of CO2
1.Direct reduction2.Transition metal ions catalysed reduction3.Biologically inspired-Calvin cycle
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!)
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.
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.
Calvin Cycle approach
Try to reproduce sections of the cycle by which plants reduce and fix carbon dioxide
The reduction step in the Calvin Cycle
H2C-OPO32-
HC-OH
CO2 --
-
3-Phosphoglyceric acid
ATP ADP
PGK = Phosphoglycerate kinase
H2C-OPO32-
HC-OH
O=C-OPO32-
1,3-Bisphosphoglycerate
NADH NAD++ Pi
GAPDH = glyceraldehyde 3-phosphatedehydrogenase
H2C-OPO32-
HC-OH
HC=O
Glyceraldehyde 3-phosphate
HC-OH
H2C-OPO32-
C=O
HC-OH
H2C-OPO32-
Ribulose-1,5-bisphosphate
CO2
Rubisco = Ribulose-1,5- bisphosphate carboxylase/oxygenase
Rubisco
GAPDH
CarboxylationReduction of Phosphoglycerate
Regenaration of CO2 Acceptor
To glucose phosphate and other products
PGK
The real reduction step is the reduction of a carboxylate group to yield an aldehyde
The real reduction step is the reduction of a carboxylate group to yield an aldehyde
ReductionStep
Does the incorporation of a nanoparticle within a molecular wire change the rate
of ET?
Jensen, Chi, F. Grumsen, Abad, Horsewell, Schiffrin, Ulstrup J. Phys. Chem. C, 2007, 111, 6124-6132
e- e-
Compare with:
Au
SH SH
C
S
OS
O
SH SH
C
S
OS
O
SH SH
C
S
OS
O
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
Redox protein studied
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. T 2003, 31, 506–509.
(GalODox) (GalODsemi) (GalODred)
Cu2+- Tyr· Cu2+- Tyr Cu+- Tyr
O2 H2O2
R-CH2-OH R-CHO
+e-
-e-
+e-
-e-
CuII
Tyr-495
His-581
Tyr-272
His-496
H20
Galactose oxidase
Cu(II) centre
Labile ligand. Can be replaced by COO-
J. Abad, M.Gass, A. Bleloch and D. J. Schiffrin, in preparation
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
HAADF-STEM redox protein-nanoparticle hybrid images
Copper
5 nm
(a) (b)
HAADF-STEM image for galactose oxidase (a) and 3D molecular display of the protein (b) obtained from the Jmol http://www.jmol.org/.
Copper
(a)
5 nm
HAADF-STEM images for TA-AuNPs~Galactose oxidase hybrid systems and 3D molecular display of the protein in different orientations
J. Abad, M.Gass, A. Bleloch and D. J. Schiffrin, in preparation
Wiring Redox Proteins to AuNPs for Direct Electron Transfer
Au
SH SH
SH SH
SH SH
Cu(II) centre
-0.4 -0.2 0.0 0.2 0.4 0.6-2
-1
0
1
Cu
rre
nt
(A
)
Potential / V vs. SCE
No electron transfer!
J. Abad, M.Gass, A. Bleloch and D. J. Schiffrin, in preparation
Electronic coupling of redox proteins to AuNPs
Cyclic voltammogram shows that
electron transfer to the copper centre is
very fast when the molecular wire
connecting it to the electrode surface
contains a nanoparticle as shown
above.
J. Abad, M.Gass, A. Bleloch and D. J. Schiffrin, in preparation
Conclusions
It might be possible to either drive biological processes
electrochemically or at least to use the information form these in the
design of electrocatalytic materials
The inclusion of nanoparticles within molecular wires change the
transfer function, increasing conductance
Modern instrumental techniques and computational methods are having
a profound effect on our understanding of fundamental aspects of
electrocatalytic reactions.