PHOTOCATALYTIC AND ELECTROCHEMICAL PROCESSES FOR GENERATION OF HYDROGEN AND DECONTAMINATION OF WATER M. Sathish CYD

 Photocatalytic generation of hydrogen – CdS nanoparticles  Photocatalytic decontamination of water – anion doped visible light active TiO 2 photocatalyst  Electrolytic generation of hydrogen – compartmentalized electrolytic cell  Electrolytic decontamination of water – compartmentalized electrolytic cell CONTENTS 2

HYDROGEN PRODUCTION BY WATER SPLITTING ProcessesDrawback  Photocatalytic decomposition– Suitable catalyst  Electrolytic decomposition– Over potential  Thermal decomposition– High temperature  Biological decomposition– Infancy 3

MECHANISM OF PHOTOCATALYTIC PROCESSES 4

 high surface area  presence of more number of surface states  wide band gap  position of the VB & CB edge CdS – appropriate choice for the hydrogen production eV ADVANTAGES OF SEMICONDUCTOR NANOPARTICLES 5

PREPARATION, CHARACTERIZATION AND PHOTOCATALYTIC HYDROGEN PRODUCTION BY CdS NANOPARTICLES 6 CHAPTER - 3

PREPARATION OF CdS NANOPARTICLES 1 g of Zeolite (HY, H , HZSM-5) 1 M Cd(NO 3 ) 2, stirred for 24 h, washed with water Cd / Zeolite 1 M Na 2 S solution, stirred for 12 h, washed with water CdS / Zeolite 48 % HF, washed with water CdS Nanoparticles 7

XRD PATTERN OF CdS 8 M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy, 31 (2006) 891

Debye Scherrer Equation d  SPACING AND CRYSTALLITE SIZE  = diffraction angle T = Crystallite size = wave length  = FWHM 9 d-spacing (Å) Catalyst (0 0 2)(1 0 1)(1 1 2) Crystallite Size(nm) CdS (bulk) CdS (bulk) (HF treated) CdS-Y CdS-  CdS-Z

UV –VISIBLE SPECTRA OF CdS SAMPLES Samples Band Gap (eV) CdS – Z CdS – Y CdS -  Bulk CdS M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy, 31 (2006) 891

PHOTOCATALYTIC PRODUCTION OF HYDROGEN 35ml of 0.24 M Na 2 S and 0.35 M Na 2 SO 3 in Quartz cell 0.1 g CdS 400 W Hg lamp N 2 gas purged before the reaction and constant stirring Hydrogen gas was collected over water in the gas burette 11

AMOUNT OF HYDROGEN EVOLVED BY CdS PHOTOCATALYST 12 M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy, 31 (2006) 891

SCANNING ELECTRON MICROGRAPHS 13 CdS-Z CdS-Y CdS-  CdS- bulk

Activity of the catalyst is directly proportional to work function of the metal and M-H bond strength. PHOTOCATALYTIC HYDROGEN EVOLUTION OVER METAL LOADED CdS NANOPARTICLES 14

Metal Redox potential (E 0 ) Metal- hydrogen bond energy (K cal mol -1 ) Work function (eV) Hydrogen evolution rate* (µmol h g -1 ) Pt Pd Rh Ru HYDROGEN PRODUCTION ACTIVITY OF METAL LOADED CdS PREPARED FROM H-ZSM-5 *1 wt% metal loaded on CdS-Z sample. The reaction data is presented after 6 h under reaction condition. 15 M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy, 31 (2006) 891

250 ml of 5 mM Na 2 S solution 250 ml of 1 mM Cd(NO 3 ) 2 Rate of addition 20 ml / h Ultrasonic waves = 20 kHz The resulting precipitate was washed with distilled water until the filtrate was free from S 2- ions PREPARATION OF MESOPOROUS CdS NANOPARTICLE BY ULTRASONIC MEDIATED PRECIPITATION 16

 The particle size is calculated using Debye Scherrer Equation  The average particle size of as- prepared CdS is 4-6 nm UV-VISIBLE SPECTRA & X- RAY DIFFRACTION PATTERN M. Sathish and R. P. Viswanath. Chemistry letters, 36 (2007) The absorption on set of CdS-U shows blue shift compared to bulk CdS particles

 The specific surface area and pore volume are 94 m 2 /g and cm 3 /g respectively  The adsorption - desorption isotherm – Type IV (mesoporous nature)  Mesopores are in the range of 30 to 80 Å size  The maximum pore volume is contributed by 45 Å size pores N 2 ADSORPTION - DESORPTION ISOTHERM 18 M. Sathish and R. P. Viswanath. Chemistry letters, 36 (2007) 948

 The growth of fine spongy particles of CdS-U is observed on the surface of the CdS-U  The CdS-bulk surface is found with large outgrowth of CdS particles  The fine mesoporous CdS particles are in the nanosize range  The dispersed and agglomerated forms are clearly observed for the as-prepared CdS-U CdS - Bulk TEMSEM ELECTRON MICROGRAPHS 19 CdS-U

MetalCdS-UCdS-ZCdS bulk Literature* - Rh Pd Pt (32 ml) wt % Metal loaded CdS – U is 2-3 times more active than the CdS-Z PHOTOCATALYTIC HYDROGEN PRODUCTION Na 2 S and Na 2 SO 3 mixture used as sacrificial agent Amount of hydrogen (µM/0.1 g/h) 20 M. Sathish and R. P. Viswanath. Catalysis Today, 129 (2007) 421

 The CdS nanoparticles show higher photocatalytic activity than the bulk particles  The size, surface area and morphology of the particles play an important role on photocatalytic activity  Pt loading on photocatalyst enhances the hydrogen production activity due to its unique properties  Pt loaded mesoporous CdS nanoparticle - promising catalyst for photocatalytic hydrogen production using sunlight SUMMARY 21

PREPARATION, CHARACTERIZATION OF VISIBLE LIGHT ACTIVE N-DOPED AND N, S CO-DOPED TiO 2 22 CHAPTER - 4

DOPING CATIONSANIONS Photocorrosion of doped element Increases carrier recombination center Introduced oxygen vacancy leads to the formation of lower energy levels Decrease in the electron mobility in the bulk due to localization Orbital overlapping between the doped element and oxygen alters the valence and conduction band position Formation of energy levels closer to the VB and CB No photocorrosion E.g., N, S, P & B Limitations Advantages 23

EFFECTS OF NITROGEN DOPING IN TiO 2 Addition of nitrogen increases the size of the bond orbitals, decreasing the energy bandgap Energy TiO 2 Bond Orbitals TiO 2-x N x Bond Orbitals Conduction Band Ti d + (O2p) Ti d + O2p +N2p) Valence Band N2p + O2p (O 2P +Ti d)+ (Ti d) Ti d O 2p Ti d N2p O2p E g = 3.2 eV E g = 2.5 eV 24

PREPARATION OF N - DOPED TiO 2 Ti 2 S 3 (NH 4 ) X TiS X pH was adjusted to 8.5 by slow addition of ~10 ml liq NH 3 Calcined for 4 h in air at 400, 500 and 600 ºC 50 ml of 15 % TiCl ml of 0.5 M Na 2 S Method - I 25

 Only Anatase phase upto 600 o C  No change in crystal lattice  ~ 120 nm red shift in onset absorption for N - doped TiO 2 UV-VISIBLE ABSORPTION SPECTRA AND X-RAY DIFFRACTION PATTERN 26 M. Sathish, B. Viswanathan, R.P. Viswanath and C.S. Gopinath, Chem. Mater., 17 (2005) 6349

XPS SPECTRA OF N−TiO 2 AND TiO 2  Shift in the Ti 2p 3/2 binding energy to lower energy due to the N- doping on TiO 2 lattice  Lower electronegativity of N than O, reduce the positive charge on Ti in the TiO 2 lattice - Covalency increased N 1S peak at eV shows – low negative charge eV 530 eV eV Nitrogen Binding energy (eV) N-TiO TiN (or) chemisorbed  397 NO or NO 2 > M. Sathish, B. Viswanathan, R.P. Viswanath and C.S. Gopinath, Chem. Mater., 17 (2005) 6349

 N replaces the oxygen in the TiO 2 lattice, which results O-Ti-N environment  Peak at 530 eV due to lattice Oxygen in TiO 2  Peak at eV due to the oxygen present in the O-Ti-N environment  Due to the more covalent nature - O-Ti-N environment compare to O-Ti-O environment, this additional peak appears at higher energy region 28

PREPARATION OF N - DOPED TiO 2 Titanium−Salen Complex Vacuum heating at 400 ºC, 6 h Calcination in N 2 atmosphere for ºC then in air for ºC N−TiO 2 Ti-Salen complex Method- II 29

 Presence of Anatase phase and peak broadening observed  No change in crystal lattice  ~50 nm shift in the onset absorption for N - doped TiO 2 X-RAY DIFFRACTION PATTERN AND UV-VISIBLE ABSORPTION SPECTRA 30 M. Sathish, B. Viswanathan and R. P. Viswanath. Int. J. Nanoscience, 6 (2007) 137

 Shift in the Ti 2p 3/2 binding energy to lower energy due to the N- doping on TiO 2 lattice  Lower electronegativity of N than O, reduce the positive charge on Ti in TiO 2 lattice  Peak at 400 eV for N 1s N in neutral or slight negative charge 400 eV 530 eV XPS SPECTRA OF N - TiO 2 AND TiO 2 31 M. Sathish, B. Viswanathan and R. P. Viswanath. Int. J. Nanoscience, 6 (2007) 137

Average particle size of the N−doped TiO 2 = 14 nm Peak at eV corresponds to the Oxygen 1s in the TiO 2 lattice Peak at eV shows the presence of O in O-Ti-N environment 32 M. Sathish, B. Viswanathan and R. P. Viswanath. Int. J. Nanoscience, 6 (2007) 137

Catalyst : N - TiO 2 and commercial TiO 2 (Degussa P25) Experimental condition: 25 mg of catalyst + 25 ml of 110 ppm methylene blue solution Filters: monochromatic 365, 405, 436, 546 nm 400W Hg fixed wavelength for 30 min Method - IMethod - II DECOMPOSITION OF METHYLENE BLUE IN THE VISIBLE REGION 33

PREPARATION OF N- DOPED TiO 2 Melamine ( 2:1 ethanol: water mixture) Ti( i OPr) 4 in ethanol 3:1 molar ratio Ti-Melamine sol-gel Stirred for 24 h, then kept for 4 days Washed with hot water, calcined at 400, 500, 600, 700 o C 34

UV-VISIBLE SPECTRA & X- RAY DIFFRACTION PATTERN  No change in the d values- indicates no change in the crystal lattice due to doping of N in the TiO 2 lattice M. Sathish, B.Viswanathan and R. P. Viswanath, Appl Catal B, 74 (2007)

 Broad peak centered around eV – presence of N-Ti-O environment  A peak around eV shows the presence of Ti- N bonding  eV peak is due to adsorbed nitrogen on TiO 2 & 400 eV is due to adsorbed N-containing organic species in the grain boundary N 1s X- RAY PHOTOELECTRON SPECTROSCOPY 36

 The N-doped TiO 2 particles calcined at 400 o C exhibits spherical and leaves like morphology SCANNING ELECTRON MICROGRAPH Calcination Temperature ( o C) Specific e surface area (m 2 /g) Crystalli te size (nm) Crystalline nature Anatase Anatase & rutile 37

TEM measurement shows that the particle are in the range of 30 nm in size Spherical type particles can also be seen in TEM clearly TEM 50 nm 100 nm 38

VISIBLE LIGHT PHOTOCATALYTIC DECOMPOSITION OF METHYLENE BLUE Experimental Condition  Catalyst = 0.1 g TiO 2 (N- TiO 2 and P25)  Solution: 50 ml of 50 ppm methylene blue solution  Light source : 400 W Hg lamp  Filter : HOYA – L – 42 (UV cutoff filter)  Time : 3 h  The mixture was stirred for 15 min in the dark to attain adsorption equilibrium  The samples were collected every 30 min and UV-Visible absorbance was measured at 660 nm ( max of methylene blue) 39

 Higher photocatalytic activity was observed for N-doped TiO 2 compared to P25 catalyst in the visible region  Highest photocatalytic activity was observed for N-TiO 2 calcined at 500 o C  Above 500 o C, the activity decrease due to loss of N in the N-TiO 2 sample  UV-visible absorbance spectra also supports the above observation 40 M. Sathish, B.Viswanathan and R. P. Viswanath, Appl Catal B, 74 (2007) 308

PREPARATION OF N, S CO-DOPED TiO 2 TB TS TS/TB complexes calcined in vacuum at C for 12h, followed by calcination in N 2 at C for 6h Finally calcined in air at required temperature (between 400 and C) to remove carbon completely. 41

A shift in the on set optical absorption of about 0.21 eV for N, S doped TiO 2 than Pure TiO 2 OPTICAL ABSORPTION SPECTRA OF N, S CO-DOPED TiO 2 42 M. Sathish, R.P. Viswanath and C.S. Gopinath, Chem. Mater., (communicated)

Particle size variation between nm observed TEM OF N, S DOPED TiO 2 43 M. Sathish, R.P. Viswanath and C.S. Gopinath, J Nanoscience and Nanotechnology (Accepted)

N and N,S-co-doped systems show a decrease in Ti 2p BE Oxidation state of S is S 6 + (as in sulfate) and N as in NO Oxidation state of N is different on N-TiO 2 and N,S-TiO 2 STATE OF N AND S ON N,S-CO-DOPED TiO 2 -XPS 44

PHOTOCATALYTIC DECOMPOSITION OF METHYLENE BLUE ON N, S CO-DOPED TiO 2 SURFACE 45 M. Sathish, R.P. Viswanath and C.S. Gopinath, J Nanoscience and Nanotechnology (Accepted)

 N-doping on TiO 2 via chemical process shows more red shift than the decomposition of N containing Ti precursor process  XPS results show, Nitrogen replaces the Oxygen in TiO 2 lattice and formation N-Ti-O environment – also increases the covalent nature Ti–O bond  N-TiO 2 shows higher photocatalytic activity than TiO 2 (degussa P 25) in the visible region  N, S co-doped TiO 2 shows more activity than N-doped TiO 2 SUMMARY 46

STUDIES ON THE ELECTROLYTIC GENERATION OF HYDROGEN – DESIGN OF COMPARTMENTALIZED CELL CHAPTER - 5

Reaction E 0 (V) In acidic medium 2H + + 2e - ⇌ H O 2 + 4e - + 4H + ⇌ 2H 2 O In alkaline medium O 2 + 4e - + 2H 2 O ⇌ 4OH H 2 O + 2e - ⇌ 2OH - + H 2 −0.828 MediumOver potential (V) H2H2 O2O2 Acidic~ 0.05~ 0.5 Alkaline~ ~ 1 For Pt electrodes HYDROGEN AND OXYGEN EVOLUTION POTENTIAL 48

S. No MediumDecomposition potential (V) 1HNO H 2 SO HCl NaOH KOH NH 3 (aq) 1.74 Decomposition potential of water in different media 1. Nature of the electrolyte or medium 2. Temperature 3. pH FACTORS AFFECTING THE DECOMPOSITION POTENTIAL 49

INFLUENCE OF TEMPERATURE ON THE REVERSIBLE POTENTIAL FOR WATER ELECTROLYSIS 50

INFLUENCE OF pH ON THE REVERSIBLE POTENTIAL FOR WATER ELECTROLYSIS The hydrogen evolution potential at pH = 0 and 14 are 0 and V The oxygen evolution potential at pH = 0 and 14 are and V The overall reversible decomposition potential at any given pH will be equal to V Separation of anode and cathode with different (pH) electrolytes will alter the decomposition potential 51 pH −0.828 Potential (V) 0

COMPARTMENTALIZED ELECTROLYTIC CELL 1.Cathode 4. Catholyte 2.Anode 5. Anolyte 3.Chemically treated separator 52

THE COMMON AND COMPARTMENTALIZED ELECTROLYTIC CELL  In compartmentalized electrolytic cell the cell current for an applied potential of 1.2 V is 1.56 mA  In common electrolytic cell the cell current for an applied potential of 1.2 V is 0.01 mA 53 R. P. Viswanath and M. Sathish. Indian Patent Filed 810/Che/2003

The decomposition potential is ~ 1.0 V in the compartmentalized electrolytic cell, whereas ~ 1.8 V for the common electrolytic cell At 1.8 V the compartmentalized cell shows higher cell current than the common electrolytic cell 54 R. P. Viswanath and M. Sathish. Indian Patent Filed 810/Che/2003

EFFECT OF ELECTROLYTE CONCENTRATION ON THE CELL CURRENT Pt / electrolyte / Pt The concentration of the electrolytes play a major role on electrolysis both in acid and alkaline electrolytic cell 55

Optimum concentration of anolyte and catholyte is 1N R. P. Viswanath and M. Sathish. Indian Patent Filed 810/Che/

RATE OF HYDROGEN AN OXYGEN PRODUCTION AT AN APPLIED DC POTENTIAL OF 1V The volume ratio of H 2 and O 2 evolved at the cathode and anode is 2 : 1 The products analyzed using gas chromatography No side reaction or side products observed 57

Co and Ni are deposited over Pt and Ti electrode by electrodeposition method from their corresponding metal salts Co and Ni on Pt and Ti electrode shows higher activity than pure Ti and Pt electrodes – high surface area NATURE OF THE ANODE 58

MULTIPLE ELECTROLYTIC CELL All the cells are connected in parallel and the anolytes and catholytes are passed from one cell to another cell 59

The net cell current is increased when three cells are connected in parallel (-)(+) H2H2 O2O2 Advantages 1.The distance between the anode and cathode is reduced 2.Variety of other separators can be used 3.The diameter of the separators can be varied 4.High cell current can be obtained 60

 Hydrogen and oxygen has been produced at 1.0 V by compartmentalized electrolytic cell  Different electrodes (anode and cathode) can be used  Different electrolytes can also be used in the anodic and cathodic compartments SUMMARY 61

ELECTROCHEMICAL DEGRADATION OF AQUEOUS PHENOL AND REMOVAL OF ARSENIC FROM WATER CHAPTER - 6

Anode : Carbon Cathode : Pt Anolyte : 40 ml of phenol Catholyte : 40 ml of 1 N H 2 SO 4 (200 ppm in 0.1 N NaCl) Potential : 5 V Variation of anode potential and cell current as a function of electrolysis time ( in NaCl medium) Decomposition profile of phenol in the NaCl medium ELECROCHEMICAL REMOVAL OF PHENOL 63

Anode : Carbon Cathode : Pt Anolyte : 40 ml of 200 ppm phenol Catholyte : 40 ml of 1 N H 2 SO 4 Potential : 5 V DECOMPOSITION PROFILE OF PHENOL IN NaOH AND NaOH + NaCl MEDIUM Variation of current with time 64 M. Sathish and R. P. Viswanath. Korean. J. Chem. Engg. 22 (2005) 358

DECOMPOSITION OF PHENOL IN DIFFERENT MEDIA. The decomposition may occur via  Direct oxidation of phenol on the electrode surface  Oxidation by hypochlorite or hypochlorous acid  Chlorination followed by oxidation The rate of decomposition of phenol and 4-chloro phenol are comparable In alkaline condition, the decomposition rate of phenol is less compared to neutral medium 65

S.NoCompounds max (nm) Phenol Phenoxide ion o-Chlorophenol p-Chlorophenol Sample (NaCl medium) (after 5 h) Sample (NaCl medium) (after 15 h)  max shifts from 268 nm to 276 nm during electrolysis  Chlorination followed by oxidation is one of the pathways of the decomposition  Formation of 4-chlorophenol intermediate has been identified by Gas chromatography & IR spectroscopy UV-VISIBLE STUDIES 66

PROPOSED PATHWAY FOR THE DECOMPOSITION OF PHENOL In Alkaline medium Only route II and III are favoured In neutral medium Chlorination followed by decomposition will occur – in presence of NaCl 67 M. Sathish and R. P. Viswanath. Korean. J. Chem. Engg. 22 (2005) 358

S.No Time (h) Phenol concentration (ppm) Phenol COD (ppm) 4-chlorophenol concentration (ppm) 4-chlorophenol COD (ppm) PHENOL’S CONCENTRATION AND COD AS A FUNCTION OF ELECTROLYSIS TIME IN NaCl AS SUPPORTING ELECTROLYTE  The change in concentration of phenols shows complete decomposition of phenol from water  The COD values indicate that – No complete oxidation of phenol into carbon dioxide and water 68

Amount of hydrogen produced in the cathode during the electrolysis in NaCl medium  The chemical reaction for the phenol mineralization is C 6 H 6 O + 7 O 2 → 6 CO H 2 O  Equivalent to 14 moles of hydrogen at the cathode.  The amount of hydrogen generated indicates a current efficiency > 97 %.  Lower amount of decomposition than the calculated value indicates that not all the liberated oxygen is used for phenol oxidation. CURRENT EFFICIENCY M. Sathish and R. P. Viswanath. Korean. J. Chem. Engg. 22 (2005) ACE = 63 % for phenol and 85 % for 4- chlorophenol in the NaCl medium.

The experiments have been carried out in galvanostatic condition (20 mA and 30 mA current ) The standard proposed by WHO is 10  g/L for drinking water Current (mA) Initial concentration  g/L Final concentration  g/L The concentration of arsenic decreased drastically up to 12 h. Concentration profile of arsenic in galvanostatic condition – 30mA ELECTROLYTIC REMOVAL OF ARSENIC FROM WATER 70

 Electrochemical degradation of phenol is faster in NaCl as supporting electrolyte.  Formation of 4-chlorophenol intermediate enhances the decomposition rate of phenol in NaCl medium.  IR studies show that in the alkaline medium a strong coating of phenolic compounds (polymer) on the electrode surface.  Passive coating may be responsible for the slower degradation in the alkaline medium.  Compartmentalized electrolytic cell - efficient for arsenic removal from water. SUMMARY 71

CONCLUSIONS  Photocatalytic activity of CdS for hydrogen generation depends on particle size, surface area, crystalline phase and morphology of the system  Photocatalytic activity of TiO 2 can be achieved in visible region by substitution of hetero atom (N and/or S) in TiO 2 lattice  Hydrogen and Oxygen can be generated at lower applied voltages using compartmentalized electrolytic cell  Water decontamination can also be achieved by employing compartmentalized electrolytic cell 72

Prof. R. P.Viswanath Prof. B. Viswanathan Prof. G. Sundararajan Prof. R. Dhamodharan Dr. G. Ranga Rao Prof. A. Ramesh (ME) Prof. T. Panda (CE) Prof. T.K.Varadarajan, Prof. M.S.Subramanian and Prof. N.Balasubramanian Dr. C.S.Gopinath – NCL pune CGBS, SAIF – IIT Madras Department of Metallurgical and Materials Engineering UGC and DST All My Friends ACKNOWLEDGEMENT