Structural and Electronic properties of platinum nanoparticles … · 2005. 1. 18. · Pt-OH at...
Transcript of Structural and Electronic properties of platinum nanoparticles … · 2005. 1. 18. · Pt-OH at...
The 4th SUNBEAM Workshop
Structural and Electronic properties of Structural and Electronic properties of platinum platinum nanoparticlesnanoparticles studied bystudied by
in situ xin situ x--ray diffraction and ray diffraction and in situ xin situ x--ray absorption spectroscopyray absorption spectroscopy
Hideto ImaiHideto ImaiFundamental and Environmental Fundamental and Environmental
Research LaboratoriesResearch Laboratories
NEC CorporationNEC Corporation
Micro Fuel CellsMicro Fuel Cells Portable batteries in the Ubiquitous Networking SocietyPortable batteries in the Ubiquitous Networking Society
Fuel (Methanol or hydrogen)
Potential successor to Potential successor to Li Li -- ion batteriesion batteries
Higher energy capacityHigher energy capacityLonger usage periodLonger usage periodSmaller sizeSmaller size
Instantly rechargeable
CO2
Cathode catalysts OO22+ 4H+ 4H++ + 4e+ 4e-- →→ 2H2H22OO
CHCH33OH + HOH + H22O O →→COCO22 + 6H+ 6H++ + 6e+ 6e--
e-
H+
Anode catalysts
Instantly rechargeableProton conducting
membrane
Air (O2) H2OFuel cell-powered laptop computer
(A prototype)
Technological hurdlesTechnological hurdles
Energy Loss at the cathode electrodeEnergy Loss at the cathode electrode
Cathode ReactionCathode ReactionOOxygen xygen RReduction eduction RReactioneaction
OO22+ 4H+ 4H++ + 4e+ 4e-- = 2H= 2H22OOEE00 = 1.23 V= 1.23 V
Catalytic activity of the current Catalytic activity of the current catalyst (Pt) is not enoughcatalyst (Pt) is not enough
Elucidation of ORR mechanismElucidation of ORR mechanismSurface morphologySurface morphologyIntermediate speciesIntermediate speciesElectronic state of platinumElectronic state of platinum
Anode
Cathode
1.2
1.0
0.8
0.6
0.4
0.2
0.0Po
tent
ial (
V)
1.00.50.0Current density (A/cm2)
Cell Voltage(PEFCs) Cell Voltage
(DMFCs)
CHCH33OH + HOH + H22O O →→COCO22 + 6H+ 6H++ + 6e+ 6e--
Voltage lossat cathode
HH2 2 →→ 2H2H+ + + 2e+ 2e--
Current vs. Voltage (Polarization) curve for PEFCs and DMFCs
Stability of platinum in aqueous systems Stability of platinum in aqueous systems
-1.0x10-2
-0.5
0.0
0.5
1.0
Cur
rent
(A)
1.51.00.50.0Potential (V vs. RHE)
-1.0x10-2
-0.5
0.0
0.5
1.0
Cur
rent
(A)
1.51.00.50.0Potential (V vs. RHE)
22HH22OO →→ OO22+ 4H+ 4H++ + 4e+ 4e--
Oxide formation
Hydrogen adsorption
Double layer
22HH++ + 2e+ 2e-- →→ HH2 2
Pt-H Pt Pt-OH?Pt-O?
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
Pote
ntia
l (V
)
14121086420pH
PtPtO or Pt(OH)2
PtO2
PtO3
O2+4H++4e-=2H2O
2H++2e-=H2
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
Pote
ntia
l (V
)
14121086420pH
PtPtO or Pt(OH)2
PtO2
PtO3
O2+4H++4e-=2H2O
2H++2e-=H2
Fuel cell operating region
The potential - pH equilibrium diagram for the system platinum - water
Cyclic voltammogram for carbon supported platinum nanoparticles in 0.5 M H2SO4
In situ XRD and XAFS measurementsIn situ XRD and XAFS measurements
Platinum Platinum nanoparticlesnanoparticles (supported on a carbon)(supported on a carbon)ca. 3 nm, 50 wt% ca. 3 nm, 50 wt% -- Pt loading Pt loading
Potential controlPotential controlHome made three electrode electrochemical cellHome made three electrode electrochemical cell0.5M H0.5M H22SOSO44 electrolyteelectrolytePotentiostaticPotentiostatic modemode
In situ XRD BL16XUIn situ XRD BL16XUPhoton energy 30 Photon energy 30 keV keV ( ( λ λ = 0.041nm= 0.041nm ))Transmission mode, Imaging plateTransmission mode, Imaging plate
In situ XAFS BL16B2In situ XAFS BL16B2LL33 and and LL22 absorption edgesabsorption edgesTransmission modeTransmission mode
In situ In situ xx--ray diffractionray diffraction
10x106
8
6
4
2
0
Inte
nsity
(a.u
.)
222018161412108 2θ (degree)
(111
)
(200
)
(220
) (311
)
(222
)PtO
PtO
0.0 V vs. RHE 0.4 V 0.7 V 0.9 V 1.0 V 1.2 V 1.4 V Pt (bulk) PtO (bulk)
10x106
8
6
4
2
0In
tens
ity (a
.u.)
1210 2θ (degree)
(111
)
(200
)
PtO
0.0V0.7V, 0.9V
1.0V
1.2V
1.4V
0.0 V vs. RHE 0.4 V 0.7 V 0.9 V 1.0 V 1.2 V 1.4 V Pt (bulk) PtO (bulk)
The potential dependence of x-ray diffraction patterns for carbon supported platinum nanoparticles
In situ In situ xx--ray absorption spectroscopyray absorption spectroscopy
L3 absorption edge L2 absorption edge
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0A
bsor
ptio
n (a
.u.)
13.4013.3513.3013.2513.20Photon Energy (keV)
0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V
1.5
1.0
0.5
0.0
Abs
orpt
ion
(a.u
.)
11.711.611.5Photon Energy (keV)
0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V
The potential dependence of XAS spectra at the L3 absorption edge
The potential dependence of XAS spectra at the L2 absorption edge
Fourier transform of Pt LFourier transform of Pt L33 EXAFSEXAFS
0.2
0.1
0.0
Four
ier T
rans
form
543210 R (Å)
Pt-Pt
Pt-O
0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V
30
20
10
0
Four
ier T
rans
form
4321 R (Å)
Pt-Pt
Pt-O
0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V
0.2
0.1
0.0Fo
urie
r Tra
nsfo
rm543210
R (Å)
Pt-Pt
Pt-O
0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V
The k3-weighted Fourier transform of Pt L3EXAFS
The k-weighted Fourier transform of Pt L3EXAFS
Structural models for the platinum Structural models for the platinum nanoparticlenanoparticle surfacesurface
cf. Pt-O distance 2.58Å (bulk PtO2)0.5 ~ 0.7 V Clean Pt surface (Double layer region) Surface reconstruction
hollow site Pt-O~2.06Å
0.8 ~1.0 V Irreversible OH (or atomic O) adsorption
1.1 ~ 1.4 V Amorphous like PtOx(several layers)
bridge site Pt-OH~ 2.26Å
on-top site Pt-OH~ 2.02Å
XANES spectraXANES spectra
1.5
1.0
0.5
0.0
Abs
orpt
ion
(a.u
.)
11.5911.5811.5711.5611.55Photon Energy (keV)
0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V
1.2
1.0
0.8
0.6
0.4
0.2
0.0A
bsor
ptio
n (a
.u.)
13.3013.2913.2813.2713.2613.25Photon Energy (keV)
0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V
The potential dependence of XANES spectra at the L3 absorption edge
The potential dependence of XANES spectra at the L2 absorption edge
LL33 and and LL22 absorptions in platinumabsorptions in platinum
Conduction band
5d 5/2
5d 3/2
Spin-orbit coupling1.6 eV
L3
EF
EThe fractional change in the number of unoccupied d states from that of bulk Pt
fd =∆ hT
hTPt=
(∆ A3 + 1.11∆Α2)
(A3 + 1.11Α2)Pt
hT = ( 1 + fd ) hTPt
Ai : edge area for the i th edge∆ Ai = Aisample - AiPthT : total number of unoccupied d states∆hT = hTsample – hTPthTPt = 0.30 (tight-binding +
spin-orbit coupling)
2p 3/2L2
2p 1/2
Electronic states Electronic states Unoccupied states in platinum 5Unoccupied states in platinum 5dd orbitalsorbitals
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0Q
(C)
1.00.50.0Potential (V vs. RHE)
Hydrogen adsorption
1.06V
1.52V
Surface oxidation
0.02V
Pt-H PtOx
Pt-OH
0.45
0.40
0.35
0.30Pt 5
d u
nocc
upie
d st
ates
1.51.00.50.0Potential (vs. RHE)
8x10-3
6
4
2
0
-2
-4C
urrent (A)
bulk platinum
The total charge due to the hydrogen adsorption and the surface oxide formation estimated from the cyclic voltammogram
The potential dependence of unoccupied states in platinum 5d orbitals
SummarySummaryIn situ XRD and XAFS studies clearly showed the followings :
In the oxygen reduction reaction potential regionIn the oxygen reduction reaction potential region……..
Surface reconstruction, adsorption of oxygen species, Surface reconstruction, adsorption of oxygen species, and formation of platinum oxidesand formation of platinum oxides
Irreversible OH Irreversible OH adsorptionadsorption (0.8 ~ 1.0V)(0.8 ~ 1.0V)PtPt--OH at onOH at on--top sitetop sitePtPt--O at 3 fold or 4 fold hollow sitesO at 3 fold or 4 fold hollow sites
Surface oxide formation (1.1 ~ 1.4 V)Surface oxide formation (1.1 ~ 1.4 V)amorphous like amorphous like PtOPtOx x (several layers)(several layers)
Charge transfer from platinumCharge transfer from platinumThe number of unoccupied states in Pt 5The number of unoccupied states in Pt 5dd orbitalsorbitals linearly linearly depends on the potential and the coverage of oxygen speciesdepends on the potential and the coverage of oxygen species
The formation of surface oxygen species may significantly reducethe catalytic activity of platinum on oxygen reduction reaction