photocatalysis: fundamentals and applications - unito.it · ed elettrochimica . tio 2 electronic...

Elena Selli

[email protected]

Effects of surface modification and doping on

the photocatalytic activity of TiO2

DIPARTIMENTO DI CHIMICA FISICA

ED ELETTROCHIMICA

TiO2 electronic excitation

A + e-CB → A-•

O2 ads + eCB O2

ads

Reduction

D + h+VB → D+•

OHads + h+

VB OHads

Oxidation

A-•

D

Conduction band

Valence band

-

+

Eg

A

D+•

En

erg

y

hn

Recombination e-

CB + h+VB → heat

Main Applications

• Down-hill reactions, e.g. oxidative

degradation

• Up-hill reactions, e.g. water splitting

TiO2 modification

The photocatalytic efficiency of semiconductors is greatly affected by surface modifications, by altering

• The adsorption properties of inorganic and organic species

• Electron transfer paths occurring at the semiconductor surface under irradiation

Effects of

• Noble metal particles deposition

• Surface fluorination and doping with fluorine

Investigated reactions

• Photocatalytic degradation of organic molecules

• Hydrogen production from water vapour

TiO2 preparation and characterization

Synthetic methods • Sol-gel synthesis

• Flame spray pyrolysis

• Deposition-precipitation (DP)

• Deposition of stabilised noble

metal (NM) nanoparticles

• Photodeposition

Characterization

• XRD

• HRTEM

• DR UV-vis spectroscopy

• XPS

• BET

… photocatalytic activity tests …

Surface modification by NM deposition

Acid Red 1 (AR1) Formic Acid (FA)

HCOOH OH NHCOCH3

SO3NaSO3Na

N=N

Evolution of H2O2,

generated from O2 reduction by eCB

Spectrophotometric analysis

lmax = 531 nm

Ion

Chromatography

Indirect spectrofluorimetric

analysis 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

200 300 400 500 600 700 800

l (nm)

Ab

so

rban

ce

Down-hill reactions: Photocatalytic oxidative degradation on TiO2

Photocatalysis on Au/TiO2

Gold metal nanoparticles capture photopromoted electrons because their Fermi level is lower than CB of TiO2

interface electron transfer

efficiency of electron-hole pairs separation

hn e-

h+ VB

CB

2H+

1/2 O2 + 2H+

H2

H2O

Au

TiO2 Bandgap

Au/TiO2 characterisation

Uv-vis diffuse reflectance spectra

PLASMONIC BAND

(550 nm)

Au weight PERCENT

Au nanoparticles DIMENSIONS

0.0

0.5

1.0

1.5

2.0

2.5

200 300 400 500 600 700 800l (nm)

ADP 2.5%

DP 0.5%

DP 0.38%

DP 0.13%

DP 0.06%

DP 0%

P25

HRTEM

H2 1.3 %

NaBH4 1%

DP UREA 0.5%

DP 0.5%

AR1 photodegradation

• No Au/TiO2 sample more efficient than P25

• Negligible Au deposition effects

1.00 0.98

0.61

0.90 0.85

0.79 0.76

0.91

0.68 0.64

0.76

0.69 0.66

0.74

0.79

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

P25 DP DP

0.06%

DP

0.13%

DP

0.38%

DP

0.5%

DP

2.5%

UREA

0%

UREA

0.5%

UREA

2.5%

NaBH4

0%

NaBH4

1%

H2 H2

1.3% 150°C

H2

1.3% 200°C

k0 /k0P25

DP SERIES

NaBH4 SERIES

DP urea

SERIES

H2 SERIES

0% 0%

M.V. Dozzi, L. Prati, P. Canton, E. Selli, PCCP, 2009, 11, 7171

H2O2 evolution during AR1 photodegradation

Increase in H2O2 production rate in the presence of gold

nanoparticles

Controversial effect of Au on TiO2: favored eCB- transfer to O2

leading to H2O2, but not parallel increase of AR1 photodegradation

0

2

4

6

8

10

0 2000 4000 6000 8000 10000 12000 14000 16000

time (s)

[H2O

2]. 1

05(M

)

P25

DP 0%

DPU 0%

DPN 0%

DPH 0%

DP 0.5%

DP 2.5%

DPU 2.5%

1.00

0.53

1.44

1.08

0.93

1.05 1.04

0.81

1.01

0.78 0.85

1.36

1.13

0.88

0.61

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

P25 DP

0%

DP 0.06%

DP 0.13%

DP 0.38%

DP 0.5%

DP 2.5%

UREA 0%

UREA 0.5%

UREA 2.5%

NaBH4

0% NaBH4

1% H2

0%

H2

1.3% 150°C

H2

1.3% 200°C

k 0

/k 0

P25

DP SERIES

NaBH4

SERIES

DP UREA

SERIES

H2 SERIES

HCOOH photocatalytic mineralization

• Best performance for NaBH4 and H2 – reduced Au/TiO2 samples

• Calcination at 450°C reduces the photocatalytic activity

• Optimal Au loading

• Beneficial role of Au in HCOOH degradation

H2O2 evolution during FA photodegradation

0

5

10

15

20

25

30

0 2000 4000 6000 8000 10000 12000 14000time (s)

[H2O

2]·

10

5 (M

)

P25

DP 0%

DP 0.06%

DP 0.13%

DP 0.38%

DP 0.5%

DP 2.5%

No H2O2 could be detected during FA mineralization on bare TiO2

Relatively high H2O2 amounts, increasing with gold content up to

0.5%, could be detected on Au/TiO2

H2O2 concentrations much higher than those detected during

AR1 degradation were attained

M.V. Dozzi, L. Prati, P. Canton, E. Selli, PCCP, 2009, 11, 7171

0.612.4DPH 1.%

25.43.3DPN 1%

26.93.8DPU 2.5%

24.47.4DP 2.5%

27.97.9DP 0.5%

21.74.1DP 0.38%

1.13.6DP 0.13%

0.523.2DP 0.06%

0.072.9P25

FAFAAR1AR1

0.612.4DPH 1.%

25.43.3DPN 1%

26.93.8DPU 2.5%

24.47.4DP 2.5%

27.97.9DP 0.5%

21.74.1DP 0.38%

1.13.6DP 0.13%

0.523.2DP 0.06%

0.072.9P25

FAFAAR1AR1

105 [H2O2]max/M

HCOOH photomineralization mechanism

Reduction path:

Prevailing photodegradation mechanism:

direct oxidation through valence band

holes at the TiO2 surface

HCOO-ads + h+

VB CO2- + H+

CO2- + O2 CO2 + O2

-

O2 + e-CB O2

-

O2- + e-

CB + 2 H+ H2O2

AR1

PHOTODEGRADATION

INHIBITED

AR1 photodegradation mechanism

PROCESS MEDIATED BY ∙OH RADICALS

RED-OX at Au/TiO2 interface

E (V) vs NHE pH 7

+2.64

+1.99

+1.68

-0.50

/AuAu0

OHOH /

VBh

CBe

-0.33

22/OO

•Au0 AuI oxidation

•AuI Au0 reduction

h+VB + H2O OH + H+

h+VB + Au Au+

OH + H+ + Au Au+ + H2O

e-CB + Au+ Au

Limiting factors of TiO2 photocatalytic efficiency

1. The band gap of TiO2 is 3.2 eV, i.e. it absorbs light in the UV

region, so that only a small portion (5%) of the sunlight can

be used for photocatalytic processes.

ANION DOPING with p-BLOCK elements (B, C, N …)

inserted in the TiO2 structure

POSSIBLE CREATION OF DIFFERENT TYPES OF INTRA BAND GAP STATES

Sol gel synthesis

S-doped TiO2 F-doped TiO2

Dopant: Thiourea Dopant: NH4F

Reference

Undoped TiO2

E_0 series

Calcination

• 500 °C

• 600 °C

• 700 °C

Dopant/Ti molar ratio

• 20

• 8

• 2

Preparation of S/F doped TiO2

M.V. Dozzi, S. Livraghi, E. Giamello, E. Selli, Photochem. Photobiol. Sci. 2011, 10, 343

Characterisation of S/F doped TiO2

XRD analysis

20 30 40 50 60 70 802q

E_0_500

E_0_600

E_0_700

20 30 40 50 60 70 802q

F_20_700

F_20_600

F_20_500

• Both types of doping

inhibit the phase transition

from anatase into rutile


F_20_700 E_0_500

HRTEM analysis


BET analysis

• Surface area decreases with increasing calcination temperature

• Doping seems to limit particles sintering effects, especially in case

of samples calcined at 500°C

B.E.T Specific Surface Area OF F - DOPED TiO2 SAMPLES

7

15

43

21

63

9

31

23

1216

6

12

0.0

20.0

40.0

60.0

80.0

E_0 F_20 F_8 F_2

SSA (m2∙g

-1)

500°C

600°C

700°C

Photoactivity of S/F doped TiO2 in FA degradation

• S-doped TiO2 show a photocatalytic activity quite similar to that of undoped materials. Insertion of S appears rather difficult

• Beneficial role of F-doping: bulk modification of TiO2

• Reaction rate increase with increasing calcination temperature: high cristallinity of F-TiO2 samples composed of pure anatase thermal energy can trigger the introduction of F- for O2-

low surface area and possible formation of Ti3+ centers

• High doping levels limit photoactivity

0.40.4

0.4

0.3

0.6

0.4

0.7

0.4 0.30.4

0.1

0.9

0.1

0.30.4

1.0

0.5

0.9

0.5

2.0 2.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

E_0 S_20 S_8 S_2 F_20 F_8 F_2

k0·10

7 (M·s

-1)

500°C

600°C

700°C

UNDOPED

TiO2

S - DOPED

TiO2

F - DOPED

TiO2

EPR characterisation

Detected species:

• nitric oxide (NO) radical in micro-voids

• nitrogen atoms (N) trapped in the bulk

•Ti3+ species

3250 3300 3350 3400 3450 3500 3550 3600

10mw 77K

S20-700

S20-600

S20-500

Ti3+

(I)

?

*10

B/Gauss

EPR spectra of S-doped samples at 77 K E

F_2

0 F_8 F_2

S_2

0 S_8 S_2

500°C -

NO

N

Ti3+

NO

Ti3+

NO

N

Ti3+

NO

N NO NO

600°C - N

Ti3+

NO

Ti3+

NO

Ti3+ NO NO

NO

Ti3+

700°C - NO

Ti3+ NO NO Ti3+

NO

Ti3+

NO

Ti3+ M.V. Dozzi, S. Livraghi, E. Giamello, E. Selli, Photochem. Photobiol. Sci. 2011, 10, 343

Photoactivity of NH4F-doped TiO2

ACETIC ACID PHOTOOXIDATION ACETALDEHYDE PHOTOOXIDATION

0.7

0.6

0.4 0.4

0.3

0.4

0.3 0.3

0.9

0.5

0.6

0.50.4

0.3

0.2

1.00.9

0.6

0.0

0.3

0.6

0.9

1.2

1.5

D_0 D_3 D_5 D_12 D_25

rate

(m

mo

l CO

2 ∙

min

-1)

500°C

600°C

700°C

P25

A45

JRC-8

Commercial

3.64.0

3.0

1.51.1

4.7

1.2 1.1

3.4

1.6

3.5

1.31.7

0.3

5.5

3.1

4.1

0

2

4

6

8

D_0 D_3 D_5 D_12 D_25

rate

∙10

2 (

min

-1)

500°C

600°C

700°C

P25

A45

Commercial

LIQUID PHASE GAS PHASE

• Beneficial effect of NH4F doping

• Reaction rate increase with increasing calcination temperature:

NH4F - doping inhibits the anatase into rutile phase transition

high crystallinity of NH4F-TiO2 samples composed of pure anatase

• Home made NH4F-doped samples more active than commercial ones

• Too high doping levels limit the TiO2 photoactivity

M.V. Dozzi, B. Ohtani, E. Selli, PCCP, 2011, 13, 18217

Action spectra analysis: AcOH photooxidation

OH 2 CO 2 O 2 COOHCH2223

Substrate: transparent acetic acid

Wavelength range: 370-460 nm

Samples: D_5, D_12 and D_25

Wavelength step: 10 nm

0

5

10

15

20

360 370 380 390 400 410 420 430 440 450 460 470

l (nm)

Fa

pp (

%)

D_5_500

D_5_600

D_5_700

0

5

10

15

20

360 370 380 390 400 410 420 430 440 450 460 470

l (nm)

Fa

pp (

%)

D_12_500

D_12_600

D_12_700

0

5

10

15

20

360 370 380 390 400 410 420 430 440 450 460 470

l (nm)

Fa

pp (

%)

D_25_500

D_25_600

D_25_700

0.0

0.2

0.4

0.6

0.8

1.0

250 300 350 400 450 500 550

l (nm)

Ab

so

rpti

on

D_5_500

D_5_600

D_5_700

0.0

0.2

0.4

0.6

0.8

1.0

250 300 350 400 450 500 550

l (nm)

Ab

so

rpti

on

D_12_500

D_12_600

D_12_700

0.0

0.2

0.4

0.6

0.8

1.0

250 300 350 400 450 500 550

l (nm)

Ab

so

rpti

on

D_25_500

D_25_600

D_25_700

DRS spectra: comparison model vs. calculation

BAND A BAND B

500°C 0 Maximum

600°C Small Small

700°C Maximum 0

Active BAND A (365 nm)

Inactive BAND B (420 nm)

0.0

0.1

0.2

0.3

300 350 400 450 500 550

l (nm)

Ab

so

rpti

on

D_5_500

D_5_600

D_5_700

0.0

0.1

0.2

0.3

300 350 400 450 500 550

l (nm)

Ab

so

rpti

on

D_12_500

D_12_600

D_12_700

0.0

0.1

0.2

0.3

300 350 400 450 500 550

l (nm)

Ab

so

rpti

on

D_25_500

D_25_600

D_25_700

BAND A

BAND B BAND A

BAND B

BAND A

BAND B

Experimental action spectra

results well fit with the

qualitative model

Correlation between DRS and action spectra

Subtraction of the action spectra obtained with samples calcined at 500°C

from those obtained with samples calcined at 600 and 700°C

-2.0

0.0

2.0

4.0

6.0

360 380 400 420 440 460

l (nm)

D F

ap

p (

%)

D_5_700 - D_5_500

D_5_600 - D_5_500

-2.0

0.0

2.0

4.0

6.0

360 380 400 420 440 460

l (nm)

D F

ap

p (

%)

D_12_700 - D_12_500

D_12_600 - D_12_500

-2.0

0.0

2.0

4.0

6.0

360 380 400 420 440 460

l (nm)

D F

ap

p (

%)

D_25_700 - D_25_500

D_25_600 - D_25_500

Band B absorption in the Vis

region is not photoactive in AcOH

decomposition

Band A is responsible for the

enhanced photoefficiency obtained

with samples calcined at 700°C

Conclusions

• Photoactivity improvement of F-doped materials must be related to

bulk modifications that ensure a better charged separation, most

probably related to high crystallinity; in fact only surface

fluorination was shown to produce a marked decrease of FA

oxidation rate

• Inactive band B, being most evident in the absorption spectra of

doped samples calcined at 500°C and decreasing in intensity with

increasing the calcination temperature, can safely be attributed to

nitrogen doping

M.V. Dozzi, B. Ohtani, E. Selli, PCCP, 2011, 13, 18217

• Band A, and the activity in acetic acid decomposition, increased

with increasing the calcination temperature. It might be attributed

to extrinsic absorption originating from surface oxygen vacancies or

surface defects

VB

CB

TiO2

Bandgap

-1

-0,5

0

0,5

1

1,5

2

2,5

Po

ten

tia

l / V

V

2H2O → O2 + 4H+ + 4e-

e-

4H+ + 4 e- → 2H2

ECB

EVB

TiO2

anatase

H2O (pH 7)

hn

-

+

e-

h+

e- 2H+

H2

1/2 O2 + 2H+

H2O

hn

ΔG = +237 kJ mol-1 222 O2

1Hh 2OH 2

TiOν

Up-hill reactions: photocatalytic water splitting

mol kJ 3.9ΔG COH 3OH OHCH

organics of Reforming

122

hν23

HRTEM analysis

5 nm

5 nm

A

B

5 nm

5 nm

5 nm

5 nm

A

B

FP-0.5%Pt/TiO2

1%Ag/TiO2

Flame-made TiO2 photocatalyst containing 0.5 wt.% Pt

1.0 wt.% Ag deposited on P25 TiO2

h

hν

e e e e e

NM

TiO2

EF

TEM investigation of Au/TiO2

FP- 1 wt% Au/TiO2 THPC 1 wt% Au/TiO2

Photocatalytic H2 production from vapor

340 360 380 400 420 440 460

Co

un

ts /

a.u

.

l nm

emission spectrum

of the light source

H2 out H2 in

H2O in

H2O out

D

GF

E2

E1

N2 out

N2 in

hn

PI

AB

CH

L

30°C

MN

TI

Reaction conditions:

14 mg of catalyst on sieved quartz

T = 60°C

P = 1.2 bar

Feeding mixture: 40 ml min-1 of 1.8% CH3OH/3.1% H2O/N2

A = Photoreactor

B = Photocatalyst bed

C = Pyrex- glass window

D = GC or QMS detector

F = pump

G = thermostated bubbler

G. L. Chiarello, L. Forni, E. Selli, Catal. Today, 2009, 144, 69

Effects of methanol addition and of Au on TiO2

in H2O + 6 vol% CH3OH

7900

290

160

H2 production rate

(μmol H2 g-1 h-1)

46.8

1.7

1.0

Relative rate

151

59

56

Relative rate with respect to H2O splitting

FP-TiO2

TiO2 (P25)

Photocatalyst

FP-1%Au/TiO2

0

4

8

12

16

20

0 1 2 3 4 5 6 7 8

irradiation time / h

mm

ol H

2 /

g c

ata

lyst

purging

in the dark FP-Au/TiO2

FP-TiO2

ca 200 Nml H2 gcat-1 h-1

In the presence of methanol, the H2 production rate increased up to 60

times for FP-TiO2 and up to 150 times for FP-1% Au/TiO2 photocatalysts

22

hν

2x COHOH CH

organics of Reforming

G.L. Chiarello, L. Forni, E. Selli, Catal. Today, 2009, 144, 69

Reaction Sequence

CH3OH 2TiO ,hn H2CO + H2 (1)

H2CO + H2O 2TiO ,hn HCO2H + H2 (2)

HCO2H 2TiO ,hn CO2 + H2 (3)

H2CO 2TiO ,hn CO + H2 (4)

2 CH3OH 2TiO ,hn HCO2CH3 + 2 H2 (5)

2 CH3OH 2TiO ,hn CH3CHO + 2 H2 (6)

2 CH3OH 2TiO ,hn CH3OCH3 + H2O (7)

Time Profile of Products Evolution

0

5

10

15

20

25

0,0 0,5 1,0 1,5 2,0 2,5

H2

CO2

CO

mm

ol

gca

t-1

Irradiation time / h

0

5

10

15

20

25

0,0 0,5 1,0 1,5 2,0 2,5

H2

CO2

CO

mm

ol

gca

t-1


0

50

100

150

200

250

300

350

0,0 0,5 1,0 1,5 2,0 2,5

H2CO

HCO2HGC

pea

k a

rea


0

50

100

150

200

250

300

350

0,0 0,5 1,0 1,5 2,0 2,5

H2CO

HCO2HGC

pea

k a

rea


H2, CO2 and CO accumulate at constant rate in the re-circulating gas phase

H2CO and HCOOH were identified in the gas phase, but they did not accumulate there, but in the liquid phase

G. L. Chiarello, M. H. Aguirre, E. Selli, J. Catal., 2010, 273, 182

Production rates and selectivity

Production rate / mmol h-1 gcat-1

Selectivity in relation to H2 production

/ %

Photocatalyst H2 CO2 CO H2CO HCO2H CH4 CO2 CO H2CO HCO2H Sum

TiO2 0.36 0.013 0.032 0.25 tr. 9.1 10-4 10.7 17.7 69.4 - 98

FP-TiO2 0.72 0.022 0.044 0.48 tr. 7.6 10-4 9.1 12.2 67.0 - 88

1%Ag/TiO2 1.17 0.032 0.056 0.81 0.06 1.1 10-3 8.2 9.6 68.8 11.0 98

1%Au/TiO2 13.30 1.61 0.479 5.17 0.84 1.2 10-3 36.3 7.2 38.9 12.7 95

1%Au-

1%Ag/TiO2 12.82 1.24 0.603 5.23 1.11 1.6 10-3 29.0 9.4 40.8 17.3 96

1%Pt/TiO2 18.60 2.88 0.468 7.16 1.23 8.7 10-3 46.4 5.0 38.5 13.2 103

0.5%Pt/TiO2 7.75 0.45 0.322 4.40 0.58 4.4 10-3 17.5 8.3 56.7 14.9 98

0.5%Pt/FP-TiO2 8.36 0.70 0.190 4.65 0.49 2.8 10-3 25.1 4.6 55.6 11.8 97

FP-0.5%Pt/TiO2 14.23 1.02 0.078 7.85 1.44 2.3 10-3 21.6 1.1 55.1 20.3 98


1003

2

22

H

COCO

r

rS Good mass balance agreement !

H2 production rate and selectivities

rH2 greatly increased upon NM deposition

Ag (F = 4.74 eV) is the poorest co-catalyst, followed by Au (5.31 eV) and Pt (5.93 eV)

rH2 doubled upon doubling the Pt amount on TiO2

FP-made photocatalysts perform better than those prepared by deposition of NM nanoparticles

0

20

40

60

80

100

FP-T

iO2

TiO2

1%Ag/

TiO2

1%Au/

TiO2

Au-

Ag/

TiO2

1%Pt/T

iO2

0.5%

Pt/T

iO2

0.5%

Pt/F

P-T

iO2

FP-0

.5%

Pt/T

iO2

0

4

8

12

16

20

r H2

/ m

mo

l H2

h-1

gcat-1

Sele

cti

vit

y / %

rH2

SCO2

SCO

0

20

40

60

80

100

FP-TiO

2TiO

2

1%Ag/

TiO2

1%Au/

TiO2

Au-Ag/

TiO2

1%Pt/T

iO2

0.5%

Pt/TiO

2

0.5%

Pt/FP-T

iO2

FP-0.5

%Pt/T

iO2

0

4

8

12

16

20

r H2

/ m

mo

l H2

h-1

gcat-1

Sele

cti

vit

y / %

rH2rH2

SCO2

SCO

0

4

8

12

16

20

0.5%

Au/D

00.

5%A

u/D

50.

5%A

u/D

70.

5%A

u/D

120.

5%P

t/D

00.

5%P

t/D

50.

5%P

t/D

70.

5%P

t/D

120.

5%A

u/P

25

0.5%

Pt/P

25

H2 CO2 CO

NAKED

Au/TiO2

Pt/TiO2D0 D5 D7 D12 P25

0.0

0.2

0.4

0.6Au or Pt/P25

rate

/ m

mol h

-1g

cat-1

Effects of doping and of NM loading

The rate of H2 and CO2 production greatly increased upon 0.5 wt.% Au or Pt nanoparticles deposition

Pt is a better co-catalyst compared to Au, in agreement with the work function values Φ(Pt) = 5.93 eV and Ф(Au) = 5.31 eV

Doping of TiO2 enhanced the H2 production rate, with an identical bell-shaped trend with increasing the dopant content for the naked, Au- and Pt-modified titania series

5% NH4F-doped TiO2 (D5) was the best photocatalyst within each series

NH4F-doped TiO2

Effect of methanol molar fraction x

rH2 SCO2 SHCO2H

0

20

40

60

80

100

0.00

5

0.00

9

0.02

3

0.04

7

0.10

0

0.22

9

0.30

8

0.64

0

0.80

0

0.89

4

0.95

6

1.00

0

0

4

8

12

16

20

r H2

/ m

mo

l H2

h-1

gcat-1

Se

lec

tivit

y /

%

1%Au / TiO2

CH3OH molar fraction (x)

0

20

40

60

80

100

0.00

5

0.00

9

0.02

3

0.04

7

0.10

0

0.22

9

0.30

8

0.64

0

0.80

0

0.89

4

0.95

6

1.00

0

0

4

8

12

16

20

r H2

/ m

mo

l H2

h-1

gcat-1

Se

lec

tivit

y /

%

1%Au / TiO2


0

20

40

60

80

100

0.00

5

0.00

9

0.02

3

0.04

7

0.10

0

0.22

9

0.40

0

0.64

0

0.80

0

0.89

4

0.95

6

1.00

0

0

4

8

12

16

20

r H2

/ m

mo

l H2

h-1

gcat-1

Sele

cti

vit

y / %

FP-0.5%Pt / TiO2


0

20

40

60

80

100

0.00

5

0.00

9

0.02

3

0.04

7

0.10

0

0.22

9

0.40

0

0.64

0

0.80

0

0.89

4

0.95

6

1.00

0

0

4

8

12

16

20

r H2

/ m

mo

l H2

h-1

gcat-1

Sele

cti

vit

y / %

FP-0.5%Pt / TiO2



SSA = 70 m2 g-1

53% anatase, 47% rutile

SSA = 48 m2 g-1

80% anatase, 20% rutile


0,0 0,2 0,4 0,6 0,8 1,0

0

2

4

6

8

10

rate

/ m

mo

l h

-1 g

-1 ca

t

CH3OH molar fraction

H2CO

0,0 0,2 0,4 0,6 0,8 1,0

0,0

0,5

1,0

1,5

2,0

2,5

rate

/ m

mo

l h

-1 g

-1 ca

t


CO2

0,0 0,2 0,4 0,6 0,8 1,0

0,0

0,5

1,0

1,5

2,0

2,5

rate

/ m

mo

l h

-1 g

-1 ca

t


HCO2H

0,0 0,2 0,4 0,6 0,8 1,0

0

4

8

12

16

20

rate

/ m

mo

l h

-1 g

-1 ca

t


H2


FP-0.5%Pt/TiO2

Reaction Mechanism

G. L. Chiarello, D. Ferri, E. Selli, J. Catal., 2011, 280, 168

H2Oads + h+VB → OHads + H+

ads

2 H+ads + 2 e-

CB → H2

CO 2 (g) CH 3 OH ad H 2 CO ad HCO 2 H ad CO 2 ad

H 2 CO (g)

HCO 2 H (g)

CH 3 OH (g)

TiO 2 surface h VB

+ (or • OH)

H + (or H 2 O)

h VB + + H 2 O ad

(or • OH )

H + (or H 2 O)

h VB + (or • OH)

H + (or H 2 O)

G.L. Chiarello, D. Ferri, E. Selli, J. Catal., 2011, 280, 168

By assuming that

• the rate of each oxidation step occurring on the photocatalyst surface is the sum of the rates of the direct and of the hydroxyl-radical mediated paths

• the steady state concentration of photopromoted electrons equals that of photoproduced holes

• the intermediate surface coverage can be expressed according to the Langmuir adsorption isotherm

• the activity coefficients can be calculated by the van Laar equation

an expression is obtained for the rate of the oxidation products’ formation via the OH radical mediated path with a maximum located at very low x values, followed by a decay trend with increasing x.

The hole-mediated oxidation path exhibits a typical saturation behaviour with increasing x.

A water-mediated path is at work!

Reaction Mechanism

rate

/ m

mol H

2C

O h

-1 g

cat-1

0

2

4

6

8

10H2CO _ FP-0.5%Pt / TiO2


0.0 0.2 0.4 0.6 0.8 1.0

rate

/ m

mol C

O2

h-1

gcat-1

0.0

0.5

1.0

1.5

2.0

2.5CO2 _ FP-0.5%Pt / TiO2

rate

/ m

mol H

CO

2H h

-1 g

ca

t-1

0.0

0.5

1.0

1.5

2.0 HCO2H _ FP-0.5%Pt / TiO2

rate

/ m

mol H

2C

O h

-1 g

ca

t-1

0

2

4

6

8

10H2CO _ 1%Au / P25

rate

/ m

mol H

CO

2H h

-1 g

cat-1

0.0

0.5

1.0

1.5

2.0 HCO2H _ 1%Au / P25


0.0 0.2 0.4 0.6 0.8 1.0

rate

/ m

mol C

O2

h-1

gca

t-1

0.0

0.5

1.0

1.5

2.0

2.5

3.0 CO2 _ 1%Au / P25

rate

/ m

mol H

2C

O h

-1 g

cat-1

0

2

4

6

8



0.0 0.2 0.4 0.6 0.8 1.0

rate

/ m

mol C

O2

h-1

gcat-1

0.0

0.5

1.0

1.5

2.0

2.5CO2 _ FP-0.5%Pt / TiO2

rate

/ m

mol H

CO

2H h

-1 g

ca

t-1

0.0

0.5

1.0

1.5

2.0 HCO2H _ FP-0.5%Pt / TiO2

rate

/ m

mol H

2C

O h

-1 g

cat-1

0

2

4

6

8



0.0 0.2 0.4 0.6 0.8 1.0

rate

/ m

mol C

O2

h-1

gcat-1

0.0

0.5

1.0

1.5

2.0

2.5CO2 _ FP-0.5%Pt / TiO2

rate

/ m

mol H

CO

2H h

-1 g

ca

t-1

0.0

0.5

1.0

1.5

2.0 HCO2H _ FP-0.5%Pt / TiO2

rate

/ m

mol H

2C

O h

-1 g

ca

t-1

0

2

4

6

8

10H2CO _ 1%Au / P25

rate

/ m

mol H

CO

2H h

-1 g

cat-1

0.0

0.5

1.0

1.5

2.0 HCO2H _ 1%Au / P25


0.0 0.2 0.4 0.6 0.8 1.0

rate

/ m

mol C

O2

h-1

gca

t-1

0.0

0.5

1.0

1.5

2.0

2.5

3.0 CO2 _ 1%Au / P25

G.L. Chiarello, D. Ferri, E. Selli, J. Catal., 2011, 280, 168

Each elementary step occurs through either

• an indirect OH radical mediated path (……….)

• a hole mediated direct path (-----)

• a water assisted path (_ _ __ )


Acknowledgements

Thank you!

Cariplo Foundation for financial support

photocatalysis: fundamentals and applications - unito.it · ed elettrochimica . tio 2 electronic...

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