1

G. Magesh

CY04D012

Photocatalytic and catalytic routes for removal of pollutants present in water and air

2

Chapter 1: Introduction

Chapter 2: Materials and methods

Chapter 3: Characterization and photocatalytic activity of Ce modified TiO2

Chapter 4: Characterization and photocatalytic studies of carbon-TiO2 composites

Chapter 5: Characterization, photocatalytic and electrochemical studies of CdSnO3 and Cd2SnO4

Chapter 6: Characterization and CO oxidation activity of Au/TiO2

Contents of the thesis

33

Photocatalysis - reaction assisted by photons in the presence of a catalyst

In photo catalysis - simultaneous oxidation and reduction

Light excites electrons from valence to conduction band - electrons and holes

Light induced excitation processes in a photo catalyst

Photocatalysis

Environmental pollution

Environmental pollution is having a deadly effect on humans and ecosystems

Water pollution is mostly due to pesticides, oil, sewage, dyes, and heavy metals

Air pollution is mostly due to automobile and industrial exhaust

4

Objectives

Factors to be considered in a photocatalyst Recombination of electrons and holes

Amount of visible light utilized (Bandgap)

Stability against photo-corrosion

Position of VB and CB

To use heterogeneous photocatalysts for degrading/oxidizing organic pollutants in water effectively.

To expand the range of radiation required in TiO2 for the photocatalytic redox process to visible region.

To increase adsorption capacity of photocatalyst towards organic pollutants.

To investigate new materials with suitable properties for their photocatalytic activity in visible light.

To study new support materials for Au as catalyts for oxidation of CO.

5

Preparation, characterization and photocatalytic activity of Ce modified TiO2

Chapter - 3

6

TiO2 is a widely studied and applied photocatalyst because of its favorable

properties

Solar radiation contains only 7 % UV light & pure TiO2 inactive in sunlight

Various methods have been attempted to improve the visible light absorption

- dye sensitization, doping of metal/non-metallic ions

- coupling of two semiconductors

CeO2 having a bandgap of 2.8 eV will increase visible light activity by coupled

semiconductor mechanism

The Ce3+\Ce4+ redox couple is expected to increase charge transfer. This will lead

to reduction in recombination

Cerium modified TiO2

7

Ammonium ceric nitrate in water at 0.5 ml/min

Aq. NH3 (pH 12.7)

Washed, centrifuged

Calcined 600 oC, air 6 h

0.25 %, 0.5 %, 1 %, 2 %, 3 %, 5 % and 9 % CeO2 modified TiO2, pure TiO2 and pure CeO2 were prepared

Titanium(IV) isopropoxide inCH2Cl2 at 1.5 ml/min

Sol

Stirred 12 h

Dried

Preparation of cerium modified TiO2

8

X-ray diffraction patterns of CeO2-TiO2 samples (a) CeO2 (b) 9% CeO2-TiO2 (c) 5% CeO2-TiO2 (d) 3% CeO2-TiO2

(e) 2% CeO2-TiO2 (f) 1% CeO2-TiO2 (g) 0.5% CeO2-TiO2 (h) 0.25% CeO2-TiO2 (i) TiO2

XRD patterns of the samples

Peaks corresponding to CeO2 start to appear at 2.0 % CeO2 loading

XRD patterns

9

Particle size ranges from 10 – 50 nm

Maximum no. of particles are around 25 nm in size

TEM images of 3 % CeO2-TiO2

10

Agglomerates of particles were observed in SEM

EDAX confirms presence of Cerium

SEM image of 3 % CeO2-TiO2

11

Red shift observed with CeO2-modified samples

Increase in red shift with increase in % of CeO2

Diffuse reflectance UV-Visible spectra

12

Photocatalytic reaction conditions

Amount of catalyst: 100 mg

Duration : 90 minutes

Methylene blue : 80 ml of 20 ppm solution

Visible light source : 400 W high pressure Hg lamp ( > 420 nm using filter)

UV light used : Eight 8 W Hg lamps ( = 365 nm)

Analysis : Measuring max of methylene blue at 662 nm by UV-visible spectrophotometry

Adsorption studies were carried out for the same duration without irradiation

Reaction conditions for irradiation and dark studies

13

Amount of MB adsorbed in dark after 90 minutes of stirring

Catalyst Amt adsorbed (× 10-7 mol /

0.1 g catalyst)

TiO2 9.10

0.25 % CeO2-TiO2 8.34

0.50 % CeO2-TiO2 7.27

1.00 % CeO2-TiO2 6.53

2.00 % CeO2-TiO2 5.78

3.00 % CeO2-TiO2 5.46

5.00 % CeO2-TiO2 4.60

9.00 % CeO2-TiO2 4.18

CeO2 3.42

Adsorption of MB decreases with increase in CeO2 loading

Pure CeO2 shows about 1/3 adsorption of TiO2

Amount of MB adsorbed in dark

14

Catalyst Amount degraded ( x 10-7 mol / 0.1 g catalyst)

Visible UV

Overall Photocatalytic(Overall-Dark)

Overall Photocatalytic(Overall-Dark)

TiO2 9.63 0.53 32.40 23.30

0.25 % CeO2-TiO2

14.65 6.31 34.56 26.22

0.50 % CeO2-TiO2

17.01 9.74 37.28 30.01

1.00 % CeO2-TiO2

16.80 10.27 40.45 33.92

2.00 % CeO2-TiO2

14.65 8.87 39.22 33.44

3.00 % CeO2-TiO2

14.12 8.66 31.61 26.18

5.00 % CeO2-TiO2

11.34 6.74 28.17 23.57

9.00 % CeO2-TiO2

9.73 5.55 26.83 22.65

CeO2 8.66 5.24 5.18 1.76

Overall and photocatalytic decrease in MB under UV and visible irradiation

UV light

15

Electronegativity of TiO2, (TiO2) = [(Ti) 2(O)]1/3

VB energy = Ionisation energy, IE(TiO2) = EVB(TiO2) = (TiO2) + ½ Eg

CB energy = Electron affinity, EA(TiO2) = ECB(TiO2) = (TiO2) – ½ Eg

ECB(TiO2) (in NHE) = ECB(TiO2) – 4.5 eV (in Absolute vacuum scale)

where (TiO2), (Ti), and (O) are the electronegativities of TiO2, titanium, and oxygen respectively

Calculation of band position

Band positions of TiO2, CeO2 and Ce2O3 were calculated

Y. Xu, M.A.A. Schoonen, Am. Mineral., 85 (2000) 543

No considerable change in d-value for CeO2-TiO2 compared to pure TiO2

16

Mechanism in visible light

G. Magesh, B. Viswanathan, R.P. Viswanath, T.K. Varadarajan, Ind. J. Chem. A, 48A (2009) 480

Bandgap, conduction and valence band energy positions of the various oxides

SemiconductorBandgap

(in eV)ECB in NHE

(in eV)

EVB in NHE

(in eV)

TiO2 3.20 -0.29 2.91

CeO2 2.76 -0.32 2.44

Ce2O3 2.40 -0.47 1.93

17

CeO2-TiO2 prepared by co-precipitation method

No new phase observed due to CeO2 loading

On loading CeO2 red shift of upto 75 nm was observed in UV-visible spectrum compared to TiO2

CeO2-TiO2 composite shows higher activity in visible light and UV light

CeO2 has conduction band position more negative than that of TiO2

CeO2-TiO2- works in visible and UV light by coupled semiconductor mechanism

Summary

18

Preparation, characterization and photocatalytic studies of carbon-TiO2 composites

Chapter - 4

19

Adsorption:

Adsorption - important step in photocatalysis

TiO2 has less adsorption capacity

Improving adsorption leads to

Electron and hole transferred quickly to adsorbed compounds

Leads to reduction in recombination

Improving adsorption:

One way of improving adsorption is carbon- TiO2 catalysts

Carbon is a good adsorbent

Carbon - conducting and improves charge transfer

Preparing carbon-TiO2

Literature shows carbon prepared over TiO2 and TiO2 prepared over carbon

Preparing TiO2 and carbon together is expected to have better activity

Carbon-TiO2

20

Sucrose + Titanium trichloride solution

Dissolved in water

Kept in oven at 150 °C for 15 h

Calcined at 300 °C for 4 h in air

Calcined at 300, 400, 500, and 600 °C in N2 for 6 h to vary the amount of carbon

Preparation of carbon-TiO2

XRD patterns

XRD pattern of C-TiO2 calcined at 300 oC in air; at various temperatures in N2

21C-TiO2 calcined at 300 oC air ; 600 oC N2

SEM images

TEM images

22

C-TiO2 calcined at 300 oC in air- at 600 oC in N2

Prepared carbon graphitic in nature

Raman spectra

Diffuse reflectance UV visible spectra

C-TiO2 calcined in air 300 oC ; in N2 different temperatures

Carbon-TiO2 shows no absorbance in visible region

No doping of carbon is taking place

23

Source : 400 W Hg lamp Pollutant : 80ml 50ppm methylene blue

Irradiation : 90 min Catalyst : 0.1 g

Catalyst % C % MB conc. decrease

under irradiation

% MB conc.

decrease in dark

% Photocatalytic (Irradiation –

Dark)

TiO2

600 oC

NA 34.0 13.0 21.0

C-TiO2 300 oC

5.4 88.0 41.3 46.7

C-TiO2 400 oC

3.0 87.7 31.8 55.9

C-TiO2 500 oC

2.1 74.5 26.8 47.7

C-TiO2 600 oC

1.4 72.7 23.8 48.9

Absorbance at 662 nm was monitored by UV-visible spectroscopy

All C-TiO2 samples showed at least 25 % increase in activity than TiO2

Photocatalytic activity of C-TiO2 from TiCl3 and sucrose

24

Sucrose + P25 TiO2

Dispersed in water

Kept in oven at 150 °C for 15h

Calcined at 360, 365, 370, 375 and 400 °C for 4 h in air

Preparation of carbon-P25 TiO2

XRD pattern

25

Carbon – P25 TiO2 calcined at 370 oC

TEM images

Diffuse reflectance UV-visible spectra

C-P25 TiO2 from sucrose calcined at different temps in air with varying amounts of carbon

No shift in UV-visible absorption was observed

This shows absence of C doping

26

Source : 400W Hg lamp Pollutant : 80ml 50ppm Methylene blue

Irradiation : 90 min Catalyst : 0.1 g

Catalyst % C % MB conc. decrease under

irradiation

% MB conc. decrease in dark

% Photocatalytic (Irradiation –

Dark)

P25 400 oC air NA 55.2 9.4 45.8

C-P25 365 oC 2.3 95.4 38.8 56.6

C-P25 370 oC 0.5 90.5 25.5 65.0

C-P25 375 oC 0.2 59.5 12.2 47.3

Carbon-P25 TiO2 showed higher activity than P25 treated under similar conditions

Up to 20 % improvement in activity observed

Photocatalytic activity of C-TiO2 from sucrose and P25 TiO2

27

Carbon and TiO2 were prepared together using sucrose and TiCl3

Carbon was prepared over commercial P25 TiO2

SEM and TEM images confirmed the existence of carbon and TiO2 together

Amount of carbon was varied by changing the calcination temperatures

Photocatalytic studies for the degradation of methylene blue showed that carbon and TiO2 prepared together showed better activity than carbon prepared over commercial TiO2

Summary

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Preparation, characterization, photocatalytic and electrochemical studies of CdSnO3 and Cd2SnO4

Chapter - 5

Choice of materials for new visible light photocatalysts

Semiconductor valence band are composed of d-orbitals and p-orbitals

Conduction band is composed of s-orbitals and p-orbitals

Materials containing elements with completely filled d-orbitals (d10) have VB edge at higher energy and hence small bandgap

13

Al

14

Si

15

P

28

Ni

29

Cu

30

Zn

31

Ga

32

Ge

33

As

46

Pd

47

Ag

48

Cd

49

In

50

Sn

51

Sb

78

Pt

79

Au

80

Hg

81

Tl

82

Pb

83

Bi

Elements whose compounds show small bandgap

30

Aqueous SnCl4.5H2O

solution

Aq. 3CdSO4. 8H2O solution

Added simultaneously

Aq. NaOH solution

Stirred overnight

Washed, dried, calcined 850 oC air 6 h

CdSnO3

Preparation of CdSnO3

31

Rhombohedral (JCPDS no.

880287)

XRD pattern of CdSnO3

SEM images

32

Catalyst : 50 mg Light source : 480 W Hg lamp

Irradiation time : 90 min Model pollutant : 50 ml 25 ppm p-chlorophenol

Visible light : > 420 nm (HOYA L-42 filter)

Catalyst % Degradation

UV-Visible Visible

CdSnO3 94.47 0.00

Absorbance starts 415 nm

Bandgap 3.0 eV

Diffuse reflectance UV-visible spectrum

Photocatalytic decontamination of water

33

Aq. SnCl4.5H2O solution Aq. NaOH solution

Mixed together

Sn(OH)4 precipitate

Dissolved in con. H2SO4

Washed till absence of Cl-

Mixed with aq. 3CdSO4 . 8H2O solution

Dried calcined air 900 oC

Cd2SnO4

Preparation of Cd2SnO4

Precipitated with NaOH

Precipitate washed till absence of SO42-

34

Orthorhombic (JCPDS no. 801467)

XRD pattern

SEM images of Cd2SnO4

35

Absorbance starts at 532 nm

Bandgap : 2.3 eV

Catalyst : 50 mg Light source : 480 W Hg lamp

Irradiation time : 90 min Model pollutant : 50 ml 25 ppm p-chlorophenol

Visible light : > 420 nm (HOYA L-42 filter)

Catalyst % Degradation

UV-Visible Visible

Cd2SnO4 75.81 24.94

Diffuse reflectance UV-visible spectrum

Photocatalytic decontamination of p-chlorophenol

36

For H2 evolution

Conduction band potential - more negative than 0.00 V vs NHE

For O2 evolution

Valence band potential - more positive than +1.23 V vs NHE

Energy

-ve

+ve

Band positions of various types of semiconductors

Potential

Reduction (H+ /H2) 0.00 V

Oxidation (HO-/O2) +1.23 V

Types of semiconductors suitable for water splitting

37

Impedance measurementsCoated on Ti plates using PVDF as binder

Frequency : 0.01 – 10000 Hz Reference electrode : Ag/AgCl

Counter electrode : Pt Amplitude : 0.005 V

Electrolyte : 0.5 M Na2SO4 Potential range : 0 V to 0.9 V

Flat band potential : 0.15 V vs Ag/AgCl 0.35 V vs NHE

Cannot evolve H2 and only O2 evolution possible

Determination of band potential by Mott-Schottky plot

MS plot of CdSnO3 MS plot of Cd2SnO4

Flat band potential : 0.23 V vs Ag/AgCl 0.43 V

vs NHE

Cannot evolve H2 and only O2 evolution possible

38

Hydrogen evolution reaction using CdSnO3 and Cd2SnO4

Medium : 35 ml Water-methanol (5:1 ratio)

Catalyst : 50 mg

Light source : 480 W Hg lamp

No hydrogen evolution occurred in UV-visible and visible irradiation

Photocatalytic water splitting studies

39

Rhombhohedral CdSnO3 and orthorhombic Cd2SnO4 were prepared by co-precipitation method

Diffuse reflectance measurements showed bandgaps of 3.0 and 2.3 eV for CdSnO3 and Cd2SnO4 respectively

Photocatalytic p-chlorophenol degradation measurements showed both catalyst were effective in UV-visible radiation

Only Cd2SnO4 was found to be photoactive in visible radiation ( > 420 nm)

Mott-schottky plots showed flat band potentials of 0.35 and 0.43 V (vs NHE) for CdSnO3 and Cd2SnO4 respectively

Water splitting studies showed no H2 evolution in accordance with measured flat band potentials

Summary

40

Preparation, characterization and CO oxidation activity of Au/TiO2

Chapter - 6

41

CO is a toxic gas from the partial combustion of fuel from Internal Combustion Engines

Oxidation to CO2 is one of the ways of removing CO

Gold nanoparticles supported on TiO2 is a suitable catalyst

TiO2 exists in different crystalline forms

Mostly anatase and rutile were studied as supports

Report shows brookite phase of TiO2 gives a higher activity than anatase and rutile

W. Yan, B. Chen, S.M. Mahurin, S. Dai and S.H. Overbury, Chem. Commun., (2004) 1918.

Carbon monoxide oxidation

42

40 ml HAuCl4 (5 millimoles) in 600 ml water

32 ml 1 % sodium citrate + 8 ml 1 % tannin + 120 ml water. pH adjusted to 8 using 4 %

Na2CO3

Heat 60 oC Heat 60 oC

Both solutions mixed, stirred maintained at 60 oC for 30 mins

Pink colored gold sol

TiO2 was prepared from TiCl4 and TiCl3 and were labeled as BRT4 and BRT3 respectively

Preparation of TiO2

Preparation of Au/TiO2 – sol deposition

Gold sol was deposited with the help of poly(diallyldimethylammonium chloride) (PDDA)

Calculated amount of gold loading – 2 wt %

Gold loaded on BRT4, BRT3 and Degussa P25 TiO2

43

Catalyst% Au loading based on ICP

Au/BRT4-Sol 2.22

Au/P25-Sol 2.15

Au/BRT3-Sol 2.22

Catalyst Average size in nm

(No. of particles)

Au/BRT4-asprep 15.2 (56)

Au/BRT4-used 17.0 (167)

Au/P25-asprep 15.1 (59)

Au/P25-used 15.0 (149)

Catalyst XRD Surface area

(m2/g) %

Anatase%

Rutile%

Brookite

BRT4 0 0 100 114

BRT3 55 0 45 197

P25 75 25 0 50

XRD analysis and surface area

Gold estimation by ICP

Particle size from TEM

44

TEM images of Au/TiO2 samples prepared by sol method (A) Au/BRT4-sol-asprepared (B) Au/BRT4-sol-after reaction (C) Au/P25-sol-asprepared and (D) Au/P25-sol-after reaction

Au particles on Au/BRT4 were agglomerated after reaction

No change in size observed in Au particles on Au/P25 after reaction

TEM images of Au/TiO2 prepared by sol method

45

100 % conversion is achieved at 100 oC, 200 oC and 220 oC for Au/P25, Au/BRT3 (anatase + brookite) and Au/BRT4 (brookite) respectively

Activity of Au/P25 is retained after calcination whereas considerable decrease observed in Au/BRT4 and a slight decrease in Au/BRT3

CO oxidation activity of catalysts

Reaction mixture 35 ml/min of gas flow (0.5 vol. % CO, 9.4 % O2, 51.9 % He and 38.2 % Ar) and at a ramp rate of 4 oC/min

Reaction performed before and after calcination in O2

60 mg of catalyst calcined at 400 oC in 20 % O2 in Ar for 1 h (10 oC /min heating rate, 30 ml/min gas flow)

Products monitored online by mass spectrometer

CO oxidation activity results

46

W. Yan, B. Chen, S.M. Mahurin, S. Dai and S.H. Overbury, Chem. Commun., (2004) 1918.

15 ml of 0.0254 M HAuCl4.3 H2O soln. + 10 ml water in a beaker

pH adjusted to 8 using 1 M KOH

Heated up to 60 oC with stirring

500 mg TiO2 added

Stirred at 60 oC for 2 h

Centrifuged 5000 RPM 10 mins

Washed & centrifuged 3 times in water and once in ethanol

Dried 60 oC for 12 h

Au/TiO2

Calculated gold loading – 2.2 wt %

Gold loaded on P25 and BRT4

Preparation of Au/TiO2 by deposition-precipitation method

47

Temperature programmed reaction was performed with 27.5 ml/min of gas flow (0.5 vol. % CO, 9.4 % O2, 51.9 % He and 38.2 % Ar) and at a ramp rate of 5 oC/min

30 mg of catalyst calcined at 400 oC in 20 % O2 in Ar for 1 h (10 oC/min heating rate, 12.5 ml/min gas flow)

CO oxidation activity of samples from DP method

Au/Brookite shows higher activity in DP method

Au/P25 shows higher activity in sol method

Brookite shows considerable decrease in activity after calcination in both cases

Important observations

48

Peaks corresponding to Au were observed

Au (200) peak showed an increase in intensity after reaction

Other phases of TiO2 not observed after reaction

No peaks corresponding to Au were observed

XRD pattern of Au/BRT4 and Au/P25 prepared by sol method

49

Au supported on brookite and P25 TiO2 were prepared by deposition-precipitation and sol deposition methods

CO oxidation studies were carried out with the catalysts

Au/P25 more active in sol deposition method

Au/Brookite showed better activity in deposition-precipitation method

Au/Brookite prepared by both methods showed decrease in activity after calcination

Summary

Conclusions

CeO2-TiO2 showed redshift up to 75 nm and higher activity than TiO2 in visible light and UV light. CeO2 has a conduction band position more negative than that of TiO2 and CeO2-TiO2 works in visible and UV light by coupled semiconductor mechanism.

Carbon-TiO2 composites were prepared by two different methods namely preparation of carbon and TiO2 together and preparation of carbon over commercial P25 TiO2. Photocatalytic degradation of methylene blue experiments showed that carbon and TiO2 prepared together showed better activity than carbon prepared over commercial TiO2.

Photocatalytic p-chlorophenol degradation studies showed that both Cd2SnO4 and CdSnO3 were active in UV-visible radiation whereas, Cd2SnO4 alone was active in visible radiation. Mott-Schottky plots showed that both CdSnO3 and Cd2SnO4 have flat band potentials lower in energy than the H2 evolution potential. Photocatalytic water splitting experiments showed no H2 evolution.

Au supported on brookite and P25 TiO2 (Anatase+Rutile) were prepared by deposition precipitation and sol deposition methods. Au/P25 was found to be more active in sol deposition method whereas Au/brookite showed better activity in deposition-precipitation method. Au/brookite prepared by both the methods showed decrease in activity after calcination at higher temperature.

51

Grateful thanks are due to

(Late) Prof. R.P. Viswanath

Prof. T.K. Varadarajan

Prof. B. Viswanathan

The current and past Heads of Department of Chemistry

The Doctoral committee members and faculty of the Department of Chemistry

The supporting staff

Colleagues and friends

DST and CSIR for fellowships

Acknowledgements

52

LIST OF PUBLICATIONS

REFEREED JOURNALS

Magesh, G., B. Viswanathan, R.P. Viswanath and T.K. Varadarajan (2009) Photocatalytic behavior of CeO2-TiO2 system for the degradation of methylene blue. Indian J. Chem., Sec A, 48A, 480-488.

OTHER PUBLICATIONS

Magesh, G., B. Viswanathan, R.P. Viswanath and T.K. Varadarajan (2007) Photocatalytic routes for chemicals. Photo/Electrochemistry & Photobiology in the Environment, Energy and Fuel, 321-357.

PRESENTATIONS IN SYMPOSIUM/CONFERENCE

Magesh, G., B. Viswanathan, R. P. Viswanath and T. K. Varadarajan, ‘Visible light photocatalytic activity of Ce modified TiO2 nanoparticles for methylene blue decomposition’, International Conference on Nanomaterials and its Applications (Poster presentation), February 4-6th 2007, NIT, Trichy, India.

Magesh, G., B. Viswanathan, T.K. Varadarajan and R.P. Viswanath, ‘CeO2-TiO2 system as visible light photocatalyst for the degradation 4-chlorophenol’, Catworkshop-2008 (Poster Presentation), February 18-20, 2008, IMMT, Bhubaneswar, India.

Magesh, G., T.K. Varadarajan and R.P. Viswanath, ‘Enhanced photocatalytic activity of carbon-TiO2 composites towards pollutant removal’, CATSYMP-19, (Poster presentation) January 18-21, 2009, National Chemical Laboratory, Pune, India.

Magesh, G., B. Viswanathan, T.K. Varadarajan and R.P. Viswanath, ‘Cadmium stannates as photocatalysts for decontamination of water’, Indo-Hungarian workshop on future frontiers in catalysis (poster presentation) February 16-18, 2010, IIT Madras, Chennai, India.