Docstoc

Presentation Welcome to Catalysis Database eprints

Document Sample
Presentation Welcome to Catalysis Database eprints Powered By Docstoc
					     PHOTOCATALYSIS -
CHALLENGES AND POTENTIALS




           Prof. B. Viswanathan
         Department of chemistry
  Indian Institute of Technology -Madras
                   catalyst
 Photocatalysis              reaction assisted by photons


 Conventional redox reaction


 Oxidizing agent should have more positive potential


 Photocatalysis - simultaneous oxidation and reduction


 The redox couple capable of promoting both the
reactions can act as photocatalyst


 Metals, Semiconductors and Insulators


                                                             2
WHY SEMICONDUCTOR ?

Metals
                                                   CB
No band gap
Only reduction or oxidation                             CB
                                       H+/H2
Depends on the band            CB
position
                               VB                            E
                                       H2O/O2           VB
Insulators                                              SC
                                                   VB
High band gap                 Metals            Insulators
High energy requirement



                                                                 3
Concepts –Why semiconductors are chosen as
            photo-catalysts?
 For conventional redox reactions, one is interested in either
 reduction or oxidation of a substrate.

 For example consider that one were interested in the
 oxidation of Fe2+ ions to Fe 3+ ions then the oxidizing agent
 that can carry out this oxidation is chosen from the relative
 potentials of the oxidizing agent with respect to the redox
 potential of Fe2+/Fe3+ redox couple.


 The oxidizing agent chosen should have more positive potential
 with respect to Fe3+/Fe2+ couple so as to affect the
 oxidation, while the oxidizing agent undergoes reduction
 spontaneously. This situation throws open a number of possible
 oxidizing agents from which one of them can be easily chosen.


                                                                  4
Water splitting - carry out both the redox reactions simultaneously
- reduction of hydrogen ions (2H+ + 2e- → H2) as well as (2OH-
+ 2h+ → H2O + 1/2O2 ) oxygen evolution from the hydroxyl ions.

The system that can promote both these reactions simultaneously
is essential.

 Since in the case of metals the top of the valence band (measure
of the oxidizing power) and bottom of the conduction band
(measure of the reducing power) are almost identical they cannot
be expected to promote a pair redox reactions separated by a
potential of nearly 1.23 V.

where the top of the valence band and bottom of the conduction
band are separated at least by 1.23V in addition to the condition
that the potential corresponding to the bottom of the conduction
band has to be more negative with respect to be more negative
with respect to while the potential of the top of the valence band
has to be more positive to the oxidation potential of the reaction
2OH- + 2h+ → H2O + ½ O2.

                                                                      5
 This situation is obtainable with semiconductors as
well as in insulators.

Insulators are not appropriate due to the high value
of the band gap which demands high energy photons
to create the appropriate excitons for promoting both
the reactions. The available photon sources for this
energy gap are expensive and again require energy
intensive methods.     Hence insulators cannot be
employed for the purpose of water splitting reaction.


Therefore, it is clear that semiconductors are alone
suitable materials for the promotion of water splitting
reaction.

                                                          6
 Criterion one has to use for the selection of the
semiconductor materials and also how one can fine
   tune the material thus chosen for the water
                 splitting reaction.



Essentially for photo-catalytic splitting of water,
the band edges (the top of valence band and
bottom of the conduction band or the oxidizing
power and reducing power respectively) have to be
sifted in opposite directions so that the reduction
reaction and the oxidation reactions are facile.




                                                      7
Ionic solids as the ionicity of the M-O bond increases, the top
of the valence band (mainly contributed by the p- orbitals of
oxide ions) becomes less and less positive (since the binding
energy of the p orbitals will be decreased due to negative
charge on the oxide ions) and the bottom of the conduction band
will be stabilized to higher binding energy values due to the
positive charge on the metal ions which is not favourable for the
hydrogen reduction reaction.

More ionic the M-O bond of the semiconductor is, the less
suitable the material is for the photo-catalytic splitting of
water.      The bond polarity can be estimated from the
expression
                                              (  A   B )2
Percentage ionic character (%) =            
                                   (1  e            4
                                                               )100


                                                                       8
The percentage ionic character of the M-O bond
        for some of the semiconductors
     Semiconductor   M-O       Percentage ionic
                               character
     TiO2            Ti-O      59.5
     SrTiO3          Ti-O-Sr   68.5
     Fe2-O3          Fe-O      47.3
     ZnO             Zn-O      55.5
     WO3             W-O       57.5
     CdS             Cd-S      17.6
     CdSe            Cd-S      16.5
     LaRhO3          La-O-Rh   53.0
     LaRuO3          La-O-Ru   53.5
     PbO             Pb-O      26.5
     ZnTe            Zn-Te      5.0
     ZnAs            Zn-As     6.8
     ZnSe            Zn-Se     18.4
     ZnS             Zn-S      19.5
     GaP             Ga-P      3.5
     CuSe            Cu-Se     10.0
     BaTiO3          Ba-O-Ti   70.8
     MoS2            Mo-S      4.3
     FeTiO3-         Fe-O-Ti   53.5
     KTaO3           K-O-Ti    72.7
     MnTiO3          Mn-O-Ti   59.0
     SnO2            Sn-O      42.2
     Bi2O3           Bi-O      39.6
                                                  9
The oxide semiconductors though - suitable for the
photo-catalytic water splitting reaction in terms of
the band gap value which is greater than the water
decomposition potential of 1.23 V.

Most of these semiconductors have bond character
more than 50-60 % and hence modulating them will
only lead to increased ionic character and hence the
photo-catalytic efficiency of the system may not be
increased as per the postulates developed

Therefore from the model developed in this
presentation the following postulates have been
evolved.


                                                       10
The photo-catalytic semiconductors are often
used with addition of metals or with other hole
trapping agents so that the life time of the
excitons created can be increased.

This situation is to increase the life time of the
excited electron and holes at suitable traps so that
the recombination is effectively reduced.

In this mode, the positions of the energy bands of
the semiconductor and that of the metal overlap
appropriately and hence the alteration can be either
way and also in this sense only the electrons are
trapped at the metal sites and only reduction
reaction is enhanced.

                                                       11
•Hence we need stoichiometrically both oxidation
and reduction for the water splitting and this
reaction will not be achieved by one of the
trapping agents namely that is used for electrons
or holes.


•Even if one were to use the trapping agents for
both holes and electrons, the relative positions of
the edge of the valence band and bottom of the
conducting band may not be adjusted in such a way
to promote both the reactions simultaneously




                                                      12
Normally the semiconductors used in photo-catalytic
processes are substituted in the cationic positions so as
to alter the band gap value.

Even though it may be suitable for using the available
solar radiation in the low energy region, it is not possible
to use semiconductors whose band gap is less than 1.23 V
and any thing higher than this may be favourable if both
the valence band is depressed and the conduction band is
destabilized with respect to the unsubstituted system.


 Since this situation is not obtainable in many of the
available semiconductors by substitution at the cationic
positions, this method has not also been successful.



                                                               13
In addition the dissolution potential of the
substituted systems may be more favourbale than
the water oxidation reaction and hence this will be
the preferred path way.



These substituted systems or even the bare
semiconductors which favour the dissolution
reaction will undergo only preferential photo-
corrosion and hence cannot be exploited for photo-
catalytic pathway. In this case ZnO is a typical
example.



                                                      14
Very low value of the ionic character also is not
suitable since these semiconductors do not have
the necessary band gap value of 1.23 V. - the
search for utilizing lower end of the visible region
is not possible for direct water splitting reaction.

If one were to use visible region of the
spectrum, then only one of the photo-redox
reactions in water splitting may be preferentially
promoted and probably this accounts for the
frequent observation that non-stiochiometric
amounts of oxygen and hydrogen were evolved in
the photo-assisted splitting of water.



                                                       15
Therefore it is deduced that the systems which
has ionic bond character of about 20-30% with
suitable positions of the valence and conduction
band edges may be appropriate for the water
splitting reaction.



This rationalization has given one a handle to
select the appropriate systems for examining as
photo-catalysts for water splitting reaction.




                                                   16
There are some other aspects of photo-
catalysts on which some remarks may be
appropriate.

Though they have been derived from the solid
state point of view like flat band potential , band
bending, Fermi level pinning, these parameters
also can be understood in terms of the bond
character and the redox chemical aspects by
which the water splitting reaction is dealt.




                                                      17
PROCESSES ON THE PHOTO-EXCITED SEMICONDUCTOR
SURFACE AND BULK




  A. Millis and S. L. Hunte J. Photochem. Photobiol. A: Chem 180 (1997) 1
                                                                            18
  TYPICAL PHOTOCATALYTIC PROCESS



 Photodecomposition of water


 Photocatalytic formation of fuel


 Photocatalysis in pollution abatement




                                          19
          HYDROGEN PRODUCTION
    There are various methods and technologies that
    have been developed and a few of them have
    already been practiced. These technologies can
    be broadly classified as:
   Thermo-chemical routes for hydrogen production

   Electrolytic generation of hydrogen

   Photolytic means of hydrogen formation

   Biochemical pathways for hydrogen evolution and

   Chemical (steam ) reformation of naphtha


                                                      20
Photo electrolysis of Water-Holy Grail of
            Electrochemistry

Historically,   the   discovery    of   photo-
electrolysis of water directly into oxygen at
a TiO2 electrode and hydrogen at a Pt
electrode by the illumination of light greater
than the band gap of TiO2 [3.1 eV] is
attributed to Fujishima and Honda though
photo catalysis by ZnO and TiO2 has been
reported much earlier by Markham in 1955



                                                 21
CHALLENGES IN PHOTODECOMPOSITION OF WATER


  2H + + 2e-                     H2            0.00V
  O2 + 4e- + 4H+                 2H2O          1.229V



 The band edges of the electrode must overlap with the
   acceptor and donor states – Minimum band gap 1.23 eV

 Charge transfer from the surface of the semiconductor
   must be fast - prevent photo corrosion

 Shift of the band edges resulting in loss of photon
energy


                                                          22
PHOTO-ELECTROCHEMICAL CELL FOR THE
PHOTO CLEAVAGE OF WATER




                                     23
 TYPES OF SEMICONDUCTORS BASED ON WATER
 ELECTOLYSIS – CHOICE OF MATERIALS
                                                          NH
  OR Type – Oxidation & Reduction                          E
                                H+/H2                     0.00
  R Type   – Reduction
                                    eV
  O Type   – Oxidation
                                H2O/O2                    1.23
  X type   - None


 Semiconductor materials that satisfy the band gap
   requirement (~1.4 eV) - susceptible for photo corrosion.


 Stable materials with a wider band gap absorb light only in
   the UV region.

                                                               24
    Conditions for photo electrolysis of water
    For the direct photo electrochemical decomposition of
    water to occur, several key criteria have to be met
    with.    These can be stated at the first level as
    follows:

   The band edges of the electrode must overlap with the
    acceptor and donor states of water decomposition
    reaction, thus necessitating that the electrodes should
    at least have a band gap of 1.23 V, the reversible
    thermodynamic decomposition potential of water. This
    situation   necessarily    means    that    appropriate
    semiconductors alone are acceptable as electrode
    materials for water decomposition.

   The charge transfer from the surface of the
    semiconductor must be fast enough to prevent photo
    corrosion and shift of the band edges resulting in loss
    of photon energy.

                                                              25
                    What modifications?
   various conceptual principles have been incorporated into
    typical TiO2 system so as to make this system responsive
    to longer wavelength radiations.     These efforts can be
    classified as follows:
   Dye sensitization
   Surface modification of the semiconductor to improve the
    stability
   Multi layer systems (coupled semiconductors)
   Doping of wide band gap semiconductors like TiO2 by
    nitrogen, carbon and Sulphur
   New semiconductors with metal 3d valence band instead of
    Oxide 2p contribution
   Sensitization by doping.
   All these attempts can be understood in terms of some
    kind sensitization and hence the route of charge transfer
    has been extended and hence the efficiency could not be
    increased considerably.    In spite of these options being
    elucidated, success appears to be eluding the researchers.

                                                                 26
            Conditions to be satisfied?
   The band edges of the electrode must overlap with the
    acceptor and donor states of water decomposition
    reaction, thus necessitating that the electrodes should
    at least have a band gap of 1.23 V, the reversible
    thermodynamic decomposition potential of water. This
    situation   necessarily    means    that    appropriate
    semiconductors alone are acceptable as electrode
    materials for water

   The charge transfer from the surface of the
    semiconductor must be fast enough to prevent photo
    corrosion and shift of the band edges resulting in loss
    of photon energy.




                                                              27
 ENGINEERING THE SEMICONDUCTOR
     ELECTRONIC STRUCTURES

 without deterioration of the stability


 should increase charge transfer processes at
the interface


 should improvements in the efficiency




                                                 28
Positions of bands of semiconductors relative to the
standard potentials of several redox couples




                                                       29
     THE AVAILABLE OPPORTUNITIES
 Identifying and designing new semiconductor materials
with considerable conversion efficiency and stability


 Constructing multilayer systems or using sensitizing
dyes - increase absorption of solar radiation


 Formulating multi-junction systems or coupled
systems - optimize and utilize the possible regions of
solar radiation


 Developing nanosize systems - efficiently dissociate
water

                                                          30
 ADVANTAGES OF SEMICONDUCTOR NANOPARTICLES
 high surface area

 morphology

 presence of surface states
                                 eV

 wide band gap

 position of the VB & CB edge


 CdS – appropriate choice for
 the hydrogen production




                                        31
                    The opportunities

   The opportunities that are obviously available
    as such now include the following:

       Identifying and designing new semiconductor
        materials with considerable conversion efficiency and
        stability
       Constructing multilayer systems or using sensitizing
        dyes so as to increase absorption of solar radiation.
       Formulating multi-junction systems or coupled
        systems so as to optimize and utilize the possible
        regions of solar radiation.
       Developing catalytic systems which can efficiently
        dissociate water.


                                                                32
                Opportunities evolved
   Deposition techniques have been considerably
    perfected and hence can be exploited in various
    other applications like in thin film technology
    especially for various devices and sensory
    applications.
   The knowledge of the defect chemistry has been
    considerably improved and developed.
   Optical collectors, mirrors and all optical analysis
    capability have increased which can be exploited in
    many other future optical devices.
   The understanding of the electronic structure of
    materials has been advanced and this has helped
    to our background in materials chemistry.
   Many electrodes have been developed, which can
    be a useful for all other kinds of electrochemical
                          devices.

                                                           33
          Limited success – Why?
The main reasons for this limited success in all these directions
  are due to:
 The electronic structure of the semiconductor controls the
  reaction and engineering these electronic structures without
  deterioration of the stability of the resulting system appears
  to be a difficult proposition.
 The most obvious thermodynamic barriers to the reaction and
  the thermodynamic balances that can be achieved in these
  processes give little scope for remarkable improvements in
  the efficiency of the systems as they have been conceived
  and operated. Totally new formulations which can still satisfy
  the existing thermodynamic barriers have to be devised.
 The charge transfer processes at the interface, even though
  a well studied subject in electrochemistry has to be
  understood more explicitly, in terms of interfacial energetics
  as well as kinetics. Till such an explicit knowledge is available,
  designing systems will have to be based on trial and error
  rather than based on sound logical scientific reasoning.


                                                                   34
   Nanocrystalline (mainly oxides like TiO2, ZnO, SnO and
    Nb2O5    or    chalcogenides    like  CdSe)    mesoscopic
    semiconductor materials with high internal surface area
    If a dye were to be adsorbed as a monolayer, enough
    can be retained on a given area of the electrode so as to
    absorb the entire incident light.

   Since the particle sizes involved are small, there is no
    significant local electric field and hence the photo-
    response is mainly contributed by the charge transfer
    with the redox couple.

   Two factors essentially contribute to the photo-voltage
    observed, namely, the contact between the nano
    crystalline oxide and the back contact of these materials
    as well as the Fermi level shift of the semiconductor as
    a result of electron injection from the semiconductor.


                                                                35
Another  aspect of thee nano crystalline state is
the alteration of the band gap to larger values as
compared to the bulk material which may facilitate
both the oxidation/reduction reactions that cannot
normally proceed on bulk semiconductors.

The   response of a single crystal anatase can be
compared with that of the meso-porous TiO2 film
sensitized by ruthenium complex (cis RuL2 (SCN)2,
where L is 2-2’bipyridyl-4-4’dicarboxlate).

The    incident photon to current conversion
efficiency (IPCE) is only 0.13% at 530 nm ( the
absorption maximum for the sensitizer) for the
single crystal electrode while in the nano crystalline
state the value is 88% showing nearly 600-700
times higher value.


                                                         36
This  increase is due to better light harvesting
capacity of the dye sensitized nano crystalline
material but also due to mesoscpic film texture
favouring photo-generation and collection of
charge carriers .


It  is clear therefore that the nano crystalline
state in combination with suitable sensitization is
one    another    alternative  which   is    worth
investigating.




                                                      37
   The second option is to promote water splitting in the
    visible range using Tandem ells. In this a thin film of a
    nanocrystalline WO3 or Fe2O3 may serve as top
    electrode absorbing blue part of the solar spectrum.
    The positive holes generated oxidize water to oxygen
   4h+ + 2H2O --- O2 + 4 H+

   The electrons in the conduction band are fed to the
    second photo system consisting of the dye sensitized
    nano crystalline TiO2 and since this is placed below the
    top layer it absorbs the green or red part of the solar
    spectrum that is transmitted through the top electrode.
    The photo voltage generated in the second photo
    system favours hydrogen generation by the reaction
   4H+ + 4e------ --- 2H2

   The overall reaction is the splitting of water utilizing
    visible light.    The situation is similar to what is
    obtained in photosynthesis

                                                                38
   Dye sensitized solid hetero-junctions and extremely
    thin absorber solar cells have also been designed
    with light absorber and charge transport material
    being selected independently so as to optimize solar
    energy harvesting and high photovoltaic output.
    However, the conversion efficiencies of these
    configurations have not been remarkably high.

   Soft junctions, especially organic solar cells, based
    on      interpenetrating      polymer      networks,
    polymer/fullerene blends, halogen doped organic
    crystals and a variety of conducting polymers have
    been examined.      Though the conversion efficiency
    of incident photons is high, the performance of the
    cell declined rapidly. Long term stability will be a
    stumbling block for large scale application of
    polymer solar cells.


                                                            39
                New Opportunities

1.   New semi-conducting materials with conversion
     efficiencies and stability have been identified.
     These are not only simple oxides, sulphides but
     also multi-component oxides based on perovskites
     and spinels.

2.   Multilayer configurations have been proposed for
     absorption of different wavelength regions. In
     these systems the control of the thickness of
     each layer has been mainly focused on.




                                                        40
               New Opportunities

3.   Sensitization by dyes and other anchored
     molecular species has been suggested as an
     alternative to extend the wavelength region of
     absorption.

4.   The coupled systems, thus giving rise to multi-
     junctions is another approach which is being
     pursued in recent times with some success

5.   Activation of semiconductors by suitable
     catalysts for water decomposition has always
     fascinated scientists and this has resulted in
     various metal or metal oxide (catalysts) loaded
     semi conductors being used as photo-anodes

                                                       41
          New opportunities (Contd)
   Recently a combinatorial electrochemical synthesis and
    characterization route has been considered for
    developing tungsten based mixed metal oxides and this
    has thrown open yet another opportunity to quickly
    screen and evaluate the performances of a variety of
    systems and to evolve suitable composition-function
    relationships which can be used to predict appropriate
    compositions for the desired manifestations of the
    functions.
   It has been shown that each of these concepts,
    though has its own merits and innovations, has not
    yielded the desired levels of efficiency. The main
    reason for this failure appears to be that it is still
    not yet possible to modulate the electronic structure
    of the semiconductor in the required directions as well
    as control the electron transfer process in the desired
    direction.

                                                          42
PREPARATION OF CdS NANOPARTICLES
    1 g of Zeolite (HY, H, HZSM-5)

                           1 M Cd(NO3)2 , stirred for
                            24 h, washed with water

                 Cd / Zeolite

                          1 M Na2S solution, stirred for
                             12 h, washed with water

                CdS / Zeolite

                          48 % HF, washed with water

                CdS Nanoparticles

                                                        43
                             XRD PATTERN OF CdS




                                                          CdS- 
         Intensity (a.u.)




                                                              CdS-Z


                                                              CdS-Y


                                                       CdS (bulk)



                        20    30   40    50       60     70           80
                                        2 theta

M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)
                                                                                 44
d SPACING AND CRYSTALLITE SIZE

Debye Scherrer Equation
       0.89            = diffraction angle
    T                  = wave length
                                               T = Crystallite size
                                                = FWHM
        cos 
       d-spacing (Å)                                                  Crystallite
                       (0 0 2)       (1 0 1)          (1 1 2)
 Catalyst                                                             Size(nm)
     CdS (bulk)         1.52          1.79             2.97              21.7

     CdS (bulk)         1.52          1.79             2.93              21.7
    (HF treated)
       CdS-Y            1.53          1.79             2.96              8.8

       CdS-            1.52          1.78             2.93              8.6


       CdS-Z            1.52          1.79             2.97              7.2



                                                                                    45
      UV –VISIBLE SPECTRA OF CdS SAMPLES


                                                     CdS (bulk)
                                                     CdS - 
                                                     CdS - Z             Samples       Band Gap
Absorbance (a.u.)




                                                     CdS - Y                             (eV)
                                                                          CdS – Z         2.38

                                                                          CdS – Y         2.27

                                                                          CdS -          2.21

                                                                         Bulk CdS         2.13



                         500           600           700
                                   Wavelength (nm)

                    M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)
                                                                                                     46
PHOTOCATALYTIC PRODUCTION OF HYDROGEN


              35ml of 0.24 M Na2S and
            0.35 M Na2SO3 in Quartz cell


  N2 gas purged before the         0.1 g CdS
reaction and constant stirring   400 W Hg lamp


               Hydrogen gas was collected over
                  water in the gas burette




                                                 47
AMOUNT OF HYDROGEN EVOLVED BY CdS
         PHOTOCATALYST
 Amount of Hydrogen (micro moles / 0.1g )   700
                                                          CdS - Y
                                            600           CdS - Z
                                                          CdS - 
                                            500           CdS - with HY
                                                          CdS (bulk)
                                            400

                                            300

                                            200

                                            100

                                              0
                                                  0   1       2      3     4   5   6
                                                                    Time (h)


                                                                                       48
TEM IMAGE OF CdS NANOPARTICLES
                             Surface   Rate of hydrogen   CdS-Z
             Particle Size
 Catalyst                      area       production
                 (nm)
                              (m2/g)     (  moles /h)
CdS - Y          8.8           36            102

CdS - Z           6            46            68

CdS -            11           26            67

CdS - Bulk        23           14            45




   CdS-Z                                       CdS- 




                             100 nm                               100 nm
                                                                           49
SCANNING ELECTRON MICROGRAPHS
    CdS-Z             CdS-Y




    CdS-             CdS- bulk




                                  50
PHOTOCATALYSIS ON Pt/TiO2 INTERFACE


                                                              Vacuum level
 Electrons are transferred
  to metal surface
 Reduction of H+ ions               Aq. Sol     Pt   TiO2        Aq. Sol
  takes place at the metal
  surface                          pH = 7             C.B
                                         H+/H2
 The holes move into               pH=0
                                                                      EF
  the other side of
  semiconductor
 The oxidation takes
  place at the                                         V.B
  semiconductor surface


              T.Sakata, et al Chem. Phys.Lett. 88 (1982) 50
                                                                             51
MECHANISM OF RECOMBINATION REDUCTION BY
METAL DOPING


                      Conduction Band
                -
   e-(M) <-- M+ee- e- e- e- e- e- e- e- e- e- e- e-

               Eg    Electron/hole pair          Electron/hole pair
                      recombination                 generation

                           Valence Band
                    h+ h+ h+ h + h+ h+ h+ h+ h+ h +



 Metallic promoter attracts electrons from TiO2 conduction
 band and slows recombination reaction


                                                                      52
                                                                                             (Amount of hydrogen (micro moles/ 0.1g))
PHOTOCATALYTIC                                                                                                                                   3500
                                                                                                                                                            Pt / CdS         H beta
                                                                                                                                                            Pd / CdS
HYDROGEN EVOLUTION                                                                                                                               3000
                                                                                                                                                            Rh / CdS
                                                                                                                                                            CdS (Bulk)
                                                                                                                                                 2500
OVER METAL LOADED                                                                                                                                           Ru / CdS

                                                                                                                                                 2000
CdS NANOPARTICLES
                                                                                                                                                 1500

                                                                                                                                                 1000
Activity of the catalyst is directly
                                                                                                                                                 500
proportional to work function of the
metal and M-H bond strength.                                                                                                                       0
                                                                                                                                                        0   1       2        3        4   5   6
                                                                                                                                                                         Time (h)
                                          4000




                                                                                                      Amount of Hydrogen (micro moles / 0.1g )
Amount of Hydrogen (micro moles / 0.1g)




                                                                           H-ZSM-5                                                               3000                        HY
                                          3500           Pt / CdS
                                                         Pd / CdS                                                                                               Pt / CdS
                                                         Rh / CdS                                                                                2500           Pd / CdS
                                          3000
                                                         CdS (Bulk)                                                                                             CdS (Bulk)
                                                         Ru / CdS                                                                                               Rh / CdS
                                          2500                                                                                                   2000
                                                                                                                                                                Ru / CdS
                                          2000
                                                                                                                                                 1500
                                          1500
                                                                                                                                                 1000
                                          1000
                                                                                                                                                  500
                                          500

                                            0                                                                                                       0
                                                 0   1       2         3         4   5   6                                                              0   1       2        3        4   5   6
                                                                      Time (h)                                                                                            Time (h)

                                                                                                                                                                                                  53
HYDROGEN PRODUCTION ACTIVITY OF METAL
LOADED CdS PREPARED FROM H-ZSM-5



              Redox        Metal- hydrogen         Work         Hydrogen
   Metal     potential      bond energy          function     evolution rate*
               (E0)         (K cal mol-1)          (eV)      (µmol h-1 0.1g-1)
    Pt         1.188             62.8             5.65             600
    Pd         0.951             64.5             5.12             144
    Rh         0.758             65.1             4.98             114
    Ru         0.455             66.6             4.71             54



  *1 wt% metal loaded on CdS-Z sample. The reaction data is presented
                   after 6 h under reaction condition.



 M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)
                                                                                  54
 EFFECT OF METALS ON HYDROGEN EVOLUTION RATE

                                          Pt
                                          Pd
                              1000
 Pt, Pd & Rh show higher                 Rh
  activity                                Au

 High reduction potential.               Cu
                              100
 Hydrogen over voltage                   Ag
  is less for Pt, Pd & Rh
                                          Ni


                              10
                                          Fe


                                          Ru

                                     3%
                                               55
         EFFECT OF SUPPORT ON THE CdS PHOTOCATLYTIC
         ACTIVITY
                        2, 5,10 and 20 wt % CdS on support - by dry impregnation method
                              80
                                                            CdS (ZSM-5)/MgO
                              75
Rate of hydrogen production




                                                             CdS (ZSM-5)/Al2O3
                              70                                                           Alumina & Magnesia
                                                                                           supports enhance
       (µmol h 0.1g )




                                                                CdS (ZSM-5)
                   -1




                              65                                                           photocatalytic activity
                                                              Bulk CdS/MgO
              -1




                              60

                              55
                                                   Bulk CdS/Al2O3                          MgO support has higher
                              50
                                                                                           photocatalytic activity -
                              45                                Bulk CdS                   favourable band position
                              40
                                   0   2   4   6    8   10 12       14   16   18 20   22
                                                    CdS (Wt %)


                                                                                                                       56
                                           Pb2+/ ZnS
 Absorption at 530nm (calcinations at 623-673K)
 Formation of extra energy levels between the band gap by Pb
  6s orbital
 Low activity at 873K is due to PbS formation on the surface
  (Zinc blende to wurtzite)




                                                           Eg




(a) 573 K, (b) 623 K, (c) 673 K, (d) 773 K, and (e) 873K   Band structure of ZnS doped with Pb.


               I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1                      57
PREPARATION OF MESOPOROUS CdS NANOPARTICLE
BY ULTRASONIC MEDIATED PRECIPITATION

                       250 ml of 1 mM
                         Cd(NO3)2

                               Rate of addition
                                  20 ml / h

  Ultrasonic waves
   = 20 kHz




      250 ml of 5 mM   The resulting precipitate was
      Na2S solution     washed with distilled water
                       until the filtrate was free from
                                     S2- ions

                                                          58
N2 ADSORPTION - DESORPTION ISOTHERM

 The specific surface area and pore volume are 94 m2/g and
   0.157 cm3/g respectively
 The adsorption - desorption isotherm – Type IV (mesoporous nature)
                                              140                         8




                                                    Relative volume (%)
                                                                          6
                                              120

 Mesopores are in the


                             Volume (cm /g)
                                                                          4

                                              100


                             3
  range of 30 to 80 Å size                                                2

                                              80
                                                                          0
                                                                              0   20    40     60       80    100
                                                                                       Pore range (A)
                                              60
 The maximum pore
                                              40
  volume is contributed by
  45 Å size pores                             20

                                               0
                                                0.0                               0.2               0.4             0.6   0.8   1.0
                                                                                                             P/Po

                                                                                                                                      59
 X- RAY DIFFRACTION PATTERN
 XRD pattern of as-prepared CdS -U shows the presence of cubic
  phase

 The observed “d” values are 1.75, 2.04 and 3.32 Å corresponding
   to the (3 1 1) (2 2 0) and (1 1 1) planes respectively - cubic

 The peak broadening shows the                               (111)

   formation of nanoparticles



                                      Intensity (a.u.)
The particle size is calculated
                                                                            (220)   (311)
 using Debye Scherrer Equation


 The average particle size of as-
  prepared CdS is 3.5 nm
                                                         20       30   40           50      60   70
                                                                       2 theta

   M. Sathish and R. P. Viswanath Mater. Res. Bull (Communicated)
                                                                                                      60
 ELECTRON MICROGRAPHS
  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
        TEM                                  SEM
CdS-U                  CdS-U                  CdS - Bulk




              100 nm
                                                                     61
PHOTOCATALYTIC HYDROGEN PRODUCTION
                                                                10000




                                   -1
                                                                                Pt / CdS-U




                                   Amount of hydrogen/M 0.1g
                                                                                Pd / CdS-U
                                                                 8000
      Na2S and Na2SO3 mixture                                                   Rh / CdS-U
                                                                                CdS-U
       used as sacrificial agent                                 6000


                                                                 4000
Amount of hydrogen (µM/0.1 g)
                                                                 2000
Metal   CdS-U    CdS-Z    CdS
                          bulk                                     0
                                                                        0   1       2        3      4   5   6
  -       73       68      45                                                            Time (h)


 Rh       320     114      102
                                                                 1 wt % Metal loaded CdS – U
                                                                 is 2-3 times more active than
 Pd       726     144      109                                             the CdS-Z

 Pt      1415     600      275

                                                                                                            62
LIMITED SUCCESS – WHY?


  Difficulties on controlling the semiconductor electronic
   structure without deterioration of the stability


  Little scope on the thermodynamic barriers and the
   thermodynamic balances for remarkable improvements in the
   efficiency


  Incomplete understanding in the interfacial energetic as well
   as in the kinetics




                                                                   63
THE OTHER OPPORTUNITIES EVOLVED

 Deposition techniques -thin film technology, for various devices
  and sensory applications.

 Knowledge of the defect chemistry has been considerably
  improved and developed.

 Optical collectors, mirrors and all optical analysis capability
  have increased

 Understanding of the electronic structure of materials


 Many electrodes have been developed- useful for all other
  kinds of electrochemical devices.


                                                                     64
 Thank you all for
your kind attention

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:119
posted:4/8/2011
language:English
pages:65