Production of Hydrogen Using Titania Based Photocatalysts Tit i B - - PowerPoint PPT Presentation

Production of Hydrogen Using Tit i B d Ph t t l t Titania Based Photocatalysts

Wonyong Choi

School of Environmental Science and Engineering D f Ch i l E i i

• Dept. of Chemical Engineering

Pohang University of Science and Technology (POSTECH) Pohang, KOREA

Solar Energy Based Hydrogen Economy Solar Energy Based Hydrogen Economy

H2 production CO2-free CO2-neutral CO2-producing

Electrolysis

(alkaline, PEM

Photoelectrolysis Photocatalysis

(PEC semiconductor

Thermo- chemical

Natural Oil Coal

Thermo- chemical Biological process

H2 from fossil fuels Water splitting H2 from biomass

Wind

( , electrolyzer) (PEC, semiconductor powder/colloid)

cycles

(S-I cycle, etc)

PV+

Storage of l i i High-T electrolysis

Solar th l Nuclear th l

Natural gas Oil Coal Gasification Steam

process (gasification, pyrolysis, reforming) process (anaerobic digestion, fermentation)

+

Wind +EL

Proven technology but costly

PV+ EL

electricity as H2 electrolysis (solar, nuclear)

thermal thermal

reforming/ water gas shift CO2 H2

but costly process

H2 CO2

+

Current Ideal solar-to- hydrogen conversion process but far from i l Current industrial process C & S commercial realization Capture & Storage (CCS)

Solar Energy

Solar Energy

5

1.2 x 105 TW

(10,000 x Current world demands)

• Abundant
• Environment-friendly energy source
• Safe and Clean

Earth ~ 0.1% of the Earth’s surface f (5 times as big as South Korea) + 10% i ffi i Global need

13 TW

~ 10% conversion efficiency

13 TW

Photocatalysis as a mean of solar energy conversion

A/A- H O/H

e- e-

H2O/H2

ΔG0(H O H + 1/2O )

hν

ΔG0(H2O → H2 + 1/2O2) =56.7 kcal/mol

H2O/O2 D/D+

e- e-

Photocatalyst (usually semiconductors)

Water Splitting

• n a Photocatalyst Particle
• 1. Photon absorption

Generation of e- and h+ with sufficient potentials for

• 3. Reduction

for H2 evolution Photon with sufficient potentials for water splitting (Band Engineering) H O H2

2

e- h+

+ H2O

e- h+

+

• 2. Charge Separation

and migration to surface reaction sites x reaction sites x

• 4. Oxidation

H2O O2 for O2 evolution

2

Band Gap Positions in Various Semiconductors

Vacuum level E vs. NHE

eV

• 3.0
• 1.0
• 4.0
• 5.0
• 4.5

2.3 1.1 3.0 2.5 H+/H2

• 6.0

3.0 3.2 3.2 3.4 2.8 2.2 3.2 3.8 GaP Si 2

+2 0 +1.0

• 7.0
• 8 0

iO TiO Z O Fe2O3 GaP SiC CdS

+2.0 +3.0

• 8.0

TiO2

Rutile

TiO2

Anatase

SrTiO3 Nb2O5 WO3 ZnO2 SnO SnO2

@ pH 0

Common Strategies for D l i Vi ibl Li ht Ph t t l t Developing Visible Light Photocatalysts

1. Impurity Doping in Wide Band-gap Oxide Semiconductors

• transition metal ions (cations)
• nitrogen, carbon (anions)

2. Sensitization of Wide Band-gap Oxide Semiconductors

• organometallic complexes (e.g., ruthenium bipyridyl derivatives)
• organic dyes
• rganic dyes
• inorganic quantum dots (e.g., CdS)

3 Nanohybrid Systems 3. Nanohybrid Systems (metal oxides & chalcogenides, metal nanoparticles, organic & inorganic sensitizers, polymers, etc.)

Dye Dye-

• Sensitized TiO

Sensitized TiO2 Solar Cell Solar Cell

R

Schematics of DSSC Performance of DSSC

Jsc : short circuit current

Dye* (-) ECB

e e-

• Voc: open circuit voltage

ff : fill factor

ff = J V

Pmax

Dye+ / 0

y (eV)

Pt ( )

A- A-

Jsc×Voc

Current Jsc

Dye+ / 0

Energy

(+ )

A

EVB Eg(TiO2) ≈ 3.2 eV

e e-

• I 3
• + 2e- ↔ 3I -

I I 3

3

• + 2e

+ 2e-

• ↔

↔ 3I

3I -

• Jsc = 18mA/cm2

Voc = 0.74V

C Voc

Semiconductor Solution

(+ )

VB

3 3 3

ff = 73%

Voltage

H2 Production on Dye Production on Dye-

• Sensitized TiO

Sensitized TiO2 e- e-

Pt Pt

CB CB

S+

+ / S

/ S*

H20 1/ 2H 1/ 2H + OH + OH-

S+

+ / S

/ S

D/ D D/ D+ 1/ 2H 1/ 2H2 + OH + OH

VB VB

/

2H+ + 2e- H2 (E0 = -0.41 V) (1) 2H2O O2 + 4H+ + 4e- (E0 = + 0.82 V) (2) H O H + 1/2O ( G 1 23 V) (3) Δ H2O H2 + 1/2O2 ( G = 1.23 V) (3) Δ

Hydrogen Production with Dye-Sensitized TiO2 Controlling/Modifying Interfacial Properties :

• Sensitizer anchoring mode
• Ion-exchange resin coating
• Barrier layer coating
• Hybridization with carbon nanotubes
• Non-Ruthenium Dye sensitized systems

Anchoring Group Anchoring Group

Different ways of anchoring molecules on surfaces Different ways of anchoring molecules on surfaces

HOOC COOH

c-RuIIL3

N

Ru

N HOOC N N COOH COOH

3

N N HOOC

mol g-1)

15 20 c-RuL3 p-RuL3

COOH

Tris(4,4’-dicarboxy-2,2’-bipyridyl) Ruthenium(II)

uL3]ad (μm

5 10

H

2 4 6 8 10 12

[Ru

5

N N

p-RuIIL3

pH-dependent adsorption of the Ru-sensitizer on TiO2

pH

N N N N

Ru

([TiO2] = 0.5 g/L, [RuL3] = 10 μM)

P P O O OH HO OH HO

(B ) (4 4’ bi ( h h t ) 2 2’ bi id l) R th i (II) (Bpy)2(4,4’-bis(phosphonato)-2,2’-bipyridyl) Ruthenium(II)

Bae et al., J. Phys. Chem. B 2004, 108, 14903

Anchoring Groups in Ru-Sensitizers

O OH O OH O OH O OH O OH OH

Carboxyl

N N Ru N N N N N N Ru N N N N O HO O OH N N Ru N N N N O HO O OH O O O O O

C2 C4 C6

P P O OH O OH OH P P O OH OH O OH OH P O OH OH

Phosphonic

N N Ru N N N N P P P OH O HO O OH OH N N Ru N N N N P P P OH O HO O OH OH N N Ru N N N N P O HO

p

P P OH O OH OH O OH OH OH OH OH

P6 P2 P4

A h i G Eff t H d d t H d P d ti Anchoring Group Effect: pH-dependent Hydrogen Production

• n RuII/Pt-TiO2 under Visible-light Illumination

30 160

• l)

20 25

C2

l)

100 120 140

P2 P4 P6

H2 (μmo

10 15

C2 C4 C6

H2 (μmo

60 80 2 4 6 8 10 5 2 4 6 8 10 20 40

pH

2 4 6 8 10

pH

2 4 6 8 10

[Pt/TiO2] = 0.5 g/L; [RuLx]i = 10 μM; [EDTA] = 10 mM; λ > 420 nm [Pt/TiO2] 0.5 g/L; [RuLx]i 10 μM; [EDTA] 10 mM; λ 420 nm (Bae and Choi, J. Phys. Chem. B 2006, 110, 14792)

TiO TiO2 Surface Modification w ith Nafion Surface Modification w ith Nafion

(CF2CF2)x (CFCF2) (

2 2)x

(

2)

(CF2CFO)CF2CF2 O O O SO-

–

CF3 O

Monomer unit of nafion

– – – –

Cation-exchanger

Stable against photocatalytic oxidation

TiO2

– – – –

5.0 nm hydrophobic zone

– – – –

4.0 nm 1.0 nm aqueous zone Interface

Nafion-Coated TiO2 Particle ( H P k d W Ch i J Ph Ch B

sulfonic anion fluorocarbon chain zone

( H. Park and W. Choi, J. Phys. Chem. B 2005, 109, 11667 )

Ru(dcbpy) Ru(dcbpy)3-TiO TiO2 vs.

• vs. Ru(bpy)

Ru(bpy)3

2+ 2+/Nafion

Nafion/TiO /TiO2

H+ Pt

O OH O OH

L2'RuII(bpy) C O Ti

e-

H+ H2

hv

Pt

(a)

N N Ru N N N N O HO O OH

L2 Ru (bpy) C Ti

TiO2

O H2

O O O O

e-

• 2+

H+ - H+

hv

[H+]Nf > [H+]aq

(b)

RuII(dcbpy)3

• RuL3

2+

RuL 2+ H+ H+ H+

• -

H+ H2

Ru N N N N

TiO2

• RuL3

2

H+ H+

• H+

Nafion layer Nafion layer TiO2 Solution

N N Ru N N

RuII(bpy)3

2+

Photo-sensitized H 2 Production in tw o anchoring systems tw o anchoring systems

25 30

Ru(bpy)3

2+ + TiO2

Ru(dcbpy) + TiO2

)

20

mol h-1)

15 20 25

Ru(bpy)3

2+ + Nf-TiO2

ex]ads (μM)

10 15

[H2 ] (μm

5 10 15

Ru-comple

5 10 2 3 4 5 6 7 8 5 2 4 6 8 10

[R pH pH

( H. Park and W. Choi, Langmuir 2006, 22, 2906 )

Photoelectrochemical Hydrogen Production

Carbon Nanotube Assisted Generation of Hydrogen in Dye-Sensitized Photoelectrochemical Cell under Visible Light

e- e-

(-0.62 VNHE)

e- e-

(-0.62 VNHE)

½ H2 H+ dye*/dye+ ½ H2 H+ dye*/dye+

fi

hv

CNT TiO2 CB TiO2 CB dye e- e- hv

(~ 0 VNHE) (-0.5 VNHE)

fi

hv

CNT TiO2 CB TiO2 CB dye e- e- hv

(~ 0 VNHE) (-0.5 VNHE)

D D+ dye/dye+ Pt FTO D D+ dye/dye+ Pt FTO

nafion matrix dye e- e- e- e- FTO nafion matrix dye e- e- e- e- FTO

TiO2/Nafion/CNT/Dye electrode

( J P k d W Ch i J Ph Ch C

1 µm

( J. Park and W. Choi, J. Phys. Chem. C 2009, 113, 20974)

Photoelectrochemical Hydrogen Production

1.2 10 12 14

{CNT+TiO2}/Nf {CNT+TiO2}/Nf/Ru(bpy)3

2+

{TiO2}/Nf

(mA)

0.30

l)

0 8 1.0 1.2 E1 (current) E2 (current) E1 (H2) E2 (H ) (1-R)2/2R 6 8 10

tocurrent

0.20

ΔH2 (μmol

0 4 0.6 0.8 E2 (H2)

CNT CNT

2 4

Phot

0 00 0.10

Δ

0 0 0.2 0.4

CNT CNT

Wavelength (nm) 300 400 500 600 700

Irradiation time (min)

10 20 30 40 50 60 0.00 0.0

E1: TiO2/Nf/RuL3 with CNT E2: without CNT

Dye-Sensitized TiO2 with Thin Overcoat of Al2O3 Dye-Sensitized TiO2 with Thin Overcoat of Al2O3

) 80 100 [H2] (μmol 40 60 20

Al2O3/TiO2/Pt TiO2/Pt

50 100 150 200

Al 2p

Al O /TiO /Pt

0.25 nm

Al 2p Intensity (a

Al2O3/TiO2/Pt TiO2/Pt

Al2O3

• arb. unit)

68 70 72 74 76 78 80

XPS Binding Energy (eV)

(W. Kim et al., J. Phys. Chem. C 2009, 113, 10603)

Organic Dye Organic Dye

Donor Acceptor

Strong Intra-molecular Charge Transfer

Organic Dye

O O

(LUMO)

g y

N S S O O COOH NC O

e− CB hν

O

VB

TiO2

hν

22

VB (HOMO)

(Y. Park et al., Chem. Commun. 2010, 46, 2477)

Organic Dye vs. Organic Dye vs. Ru Ru-

• com plex Dye

com plex Dye

Ru-dye (RuL3) Organic-dye (OD)

λ Dye λmax (nm) εmax (M-1 cm-1) ΔE (V) E0(dye/dye·+) (VNHE) E0(dye*/dye·+) (VNHE) OD 445 24500 2.45 1.35

• 1.0

Metal Metal-free organic dye sensitizers free organic dye sensitizers

RuL3

c

465 19500 2.20 1.39

• 0.81

Low-cost production High visible light absorption

Metal Metal free organic dye sensitizers free organic dye sensitizers

23

High visible light absorption Facile molecular design

H2 Production using a Dye Sensitized TiO2 System

Hydrophilicity of Organic Dyes

NC

Lee at al., Org. Lett. 2010, 12, 460

reference

O O O O O O O O

N S S COOH

HOD 1 (5a) HOD 2 (5b) HOD 3 (5c) HOD 4 (5d)

N S S O O O O O COOH NC N S S O O O COOH NC N S S O O O COOH NC N S S O O COOH NC O

HOD-1 (5a) HOD-2 (5b) HOD-3 (5c) HOD-4 (5d)

100 120

H2

60 80 100

mol) @ 5h

TiO2

Pt

2 H+

O O O 20 40

[H2] (μm

N O O HOD-4 HOD-3 HOD-2 HOD-1 Reference

Hydrophilic group length

Enhanced proximity of proton source to the reduction center (Pt)

O [Dye/Pt/TiO2]= 10 μmol/g, [EDTA] = 10 mM, [Cat] =1g/L , pH0=3, λ>420nm

Fullerol/TiO2 Charge Transfer Mediated Visible Light Photocatalysis

Fullerol (C60(OH)x) C60(OH)x / TiO2

g t

• tocata ys s
• O

O-

(

60(

)x)

60(

)x /

2

O- O O

• O
• O

O H H H

TiO

O

O

• O

O O O O- H H H H

TiO2 C60

O- O-

• O

Surface-Complex Formation

Water Soluble !

• Polyhydroxylate water-soluble form of the fullerene

C60

p Ligand(C60) to Metal (Ti) charge transfer (LMCT)

60

• C60(OH)x(ONa)y (x+y=24) y generally around 10-15

Visible light activity

Theoretical Calculation of Fullerol/TiO2 Complex

<Fullerol/TiO > <Fullerol + TiO > <Fullerol/TiO2> <Fullerol + TiO2>

Charge Transfer Transition

LUMO

Transition

hν HOMO

• These absorption spectra are calculated using intermediate neglect of differential overlap (INDO) model

parameterized for spectroscopy at the configuration interaction (CI) level of theory (ZINDO/S-CIS)

Photocatalytic Activity of Fullerol/TiO Fullerol/TiO2

100

Photocurrent

20

H2 Production

m-2

60 80 C60(OH)x/ TiO2

• l)

15

C60(OH)x /Pt /TiO2 C60 /Pt /TiO2 Pt /TiO2

I ph / μA cm

40 60

[H2] (μmo

5 10 1000 2000 3000 4000 20 C60/ TiO2 TiO2 1 2 3 4 5 6 5

t / s

Collector Electrode

Visible Light Illumination Time (h)

[EDTA] = 10 mM, [Cat] =1g/L, pH0=3, λ>420nm

e-

Fe3+ Fe2+ photo- catalyst

e-

Light [Fe3+] = 1 mM, [LiClO4] = 0.1 M, pH 1.8, [Cat]=1g/L, λ>420nm,E= 0.7 V, RE: Ag/AgCl, CE: Graphite rod, WE: Pt plate

• Y. Park et al., Chem. Eur. J. 2009, 15,

10843

Conclusions

Dye-sensitized TiO2 nanoparticles can be modified in various ways for

Dye sensitized TiO2 nanoparticles can be modified in various ways for H2 production.

• The hydrogen production on dye-sensitized TiO2 is critically

y g p y O2 y influenced by the kind of surface anchoring groups of the dye.

• Nafion-coated TiO2 can anchor non-derivatized ruthenium bipyridyl

2

py y complexes via ion exchange for efficient hydrogen production.

• The presence of alumina overcoat on TiO2 enhanced the efficiency of

p

2

y dye-sensitization for hydrogen production.