Electrical conductivity & Catalysis
Dr.K.R.Krishnamurthy
NCCR,IITM
Catalytic activity & Electronic states
Catalysis
Chemisorption
Catalyst ↔ Reactant/ Product
Charge transfer
(Adsorption/Desorption)
Electronic levels in catalyst
Electrical conductivity
Electronic structure of solids
Electronic states /Structure-Metals
Band Theory of solids
Electronic states- Non metals/Oxides
Band structure & Concept of charge transport
Charge carriers- Electrons/Holes
Energy band diagram of solids
Electrons in outer most orbitals interact to form energy bands
Reactivity determined by the energy levels
Conduction band
Conduction band
Energy gap
Valance band
Metal
Continuous energy levels
Partially occupied
Free movement of electrons
Semi-conductor
Filled valence band
Empty conduction band
Forbidden gap
Energy needed to
transport electrons
Thermal, doping
Valance band
Insulator
Filled valence band
Empty conduction band
Band gap very high
Typical > 4.0eV
Doping
Fermi level- Concept & Significance
At T= 0K, energy levels up to Fermi level (EF) are
occupied & levels above EF are empty
At T1& T2 (T2>T1>0 K) due to thermal excitation
some levels above EF are filled
EF represents thermodynamic chemical potential
for electrons in solids
When two metals or semi-conductors or metal – semiconductor are in electrical contact, electrons will flow
from the solid with higher EF to the one with lower EF
till the Ee in both are the same
For intrinsic semi-conductors, Ef represents a state
where the probability of finding electron is ½
Location of EF may change depending up on n- or
p- type semi-conductivity
Fermi level- Significance
For pure semiconductor Fermi level falls in the
Middle of band gap
E-EF = Eg
n = e(-Eg/2kT) – n= No of charge carriers excited
Emergy levels in solids
Bending of energy levels
Bending of energy levels
Bending of energy levels
Conductors- Classification
Energy levels- Semiconductors
Conduction band
Conduction band
Donor
levels
-----------------------------
Acceptor levels
---------------------------------Valance band
Valance band
Intrinsic semiconductors- No impurity levels; No.of electrons = No.of holes
Extrinsic semi conductors- Conductivity due to doping
Donor levels – Electrons as charge carriers –n-typeAcceptor levels – Holes as charge carriers – p-type
Type of semi-conductors- Examples
n-Type
Doping of P with 5 electrons in Si with 4 electrons
Creation of donor levels below CB
Electrons from donor level can be excited to CB
Charge carriers -Electrons
p-Type
Doping of Boron with Si with 4 valence electronsCreation of Acceptor levels above VB.
Electrons transferred to acceptor level leaving holes in VB
Charge carriers- Holes
Dopants
M(n+m)+
n-type
Mn+
Matrix
M(n-m)+
p-type
Electrical conductivity
Electrical conductivity σ = neμe + peμp
n &p- No of electrons & holes
e- Charge of an electron,1.602 x 10-19 coloumbs
μe & μp –Mobilities of electrons & holes expressed in cm2 /Volt-Sec ;
Electrons 2.8 times more mobile than holes
For intrinsic semi conductors n= p
Hence, σ = ne(μe + μp )
In the case of extrinsic semi-conductors
Both carriers contribute towards conductivity
Concentration of majority type carriers determine the type of
conductivity- ie., n- or p-type
This will also be reflected in Hall and Seebeck coefficients
Electrical conductivity- Temperature effect
Lnσ
↑
Intrinsic
region
Exhaustion
region
Extrinsic region
1/T →
Electrical conductivity σ = σ0 e(-Eg/2kT) ; Eg= EC –EV
Eg- Band gap; Slope in intrinsic region= Eg/2k
Chemisorption & Electronic states
Chemisorption- Charge transfer between catalyst & reactants
Reactant R on accepting an electron
RCharge transfer may not be complete, but partial- Chemisorptive bond
With R- on surface, induces + charges below the surface
Emergence of + vely charged layer retards further flow of electrons to the
surface- R- species- Depletive chemisorption
ExamplesOxygen adsorption on ZnO- n-type semiconductor
Oxygen adsorption on p-type semiconductor- NiO-Cu2O- Cumulative
chemisorption
H2 chemisorption on ZnO- Cumulative
These charge transfers can be followed by measurement of electrical
conductivity of solids
EUROCAT- V2O5-WO3-TiO2-In-situ Electrical
conductivity studies
Conductivity of Oxide catalysts
Semi-conducting characteristics- TiO2- n-type semiconductor
Electrical conductivity is a function of:
Surface structural defects
Adsorbed species
Reduced/Oxidized species/states
Dissolved ionic impurities
Supported metal oxides- Support & Active phase- Which one contributes?
Conductivity- related to the support
Percolation threshold- 40 % min- for active phase to contribute
Active-phase-support interactions could play key role- Modify support
conductivity
V2O5,WO3 & TiO2- n-type semi-conductors
6NO + 4NH3 5N2 + 6H2O 333.5 kcal=mol – Reducing atmosphere
4NO + 4NH3 + O2
4N2 + 6H2O 291.4 kcal=mol- Oxidizing atmosphere
In-situ Electrical conductivity measurement
Conductivity σ = (1/R)*(t/S)
R- Resistance in Ohms
T- Thickness of pellet ,cm
S- Cross sectional area, cm2
M.Breyesse et.al, J. Catalysis. 27, 275,1972
EUROCAT- V2O5-WO3-TiO2-In-situ Electrical
conductivity studies
Factors responsible for activity
the stability of surface vanadium oxide phase on titania
The structure of the deposited vanadium oxide phase
the strength and the number of V=O bonds
The acidity of the surface vanadium oxide and
the ease of reduction of the supported vanadium oxide catalyst.
Electrical conductivity measurements could be used to study many of the
factors above, besides the behaviour of the catalyst under
in-situ reaction conditions
Oxidation/reduction conditions
Establishing surface transformations & reaction mechanism
Titania- Origin of conductivity
Dissolved V & W ions lead to formation
of delocalized electrons responsible
for electrical conductivity
Ionic size critical for substitution in
lattice
Conductivity in different atmospheres
Introduction of 230 mg of catalyst in the cell & evacuation at room temp.
Introduction of 400 Torr of oxygen
Heating at 5°C/min up to 300°C and attaining of the steady-state conductivity
Evacuation of oxygen and introduction of 1.52 Torr; NO which corresponds
to the partial pressure of 2000 ppm NO
Evacuation of NO and introduction of 2000 ppm NH3
Evacuation of NH3 and re-admission of NO to check the reversibility of the
electrical conductivity under NO atmosphere;
Evacuation of NO and admission of the reaction mixture (NO+NH3) 2000
ppm each to follow in- situ the redox processes during reaction;
Evacuation of the reaction mixture and introduction of 400 Torr O2 to follow
the reoxidation of the catalyst;
Evacuation of O2 and introduction of a second reaction mixture (NO+NH3) to
follow σ variations under reaction mixture.
Changes in electrical conductivity
Ionosorption of NO with electron
Capture
NO(g) + e−
NO−(ads)
V0+ + NON0 + N0
Fig. 2. First exposure to 2000 ppm NO of
WO3/TiO2 and V2O5–WO3/TiO2 in an oxidized
state and after a prompt outgassing, recorded
at 573 K.
N0 +O2-s
N2
Electrons drawn from titania are
delocalized
Electrons are transferred from the active phase
to the support
Increase in conductivity is due to the interaction between
adsorbed NH species and NO- A resultant increase in conductivity
Fig. 6. Exposure to the reaction mixture (2000 ppm NO ~2000 ppm NH3 ~500
ppm O2) of WO3 /TiO2 and V2O5–WO3/TiO2 (fresh and used) catalysts in an
oxidized state, recorded at 573 K.
Surface transformations-Summary
NO alone and in the absence of oxygen has an electron acceptor character
since δσ/δPNO <0, forming NO- ionosorbate and/or filling anionic vacancies.
NH3 has a reducing character since δσ/δPNH3 >0.
The SCR reaction mixture (1 NO (2000 ppm)+NH3 (2000 ppm)) has also a
reducing character with respect to the surface of V2O5/WO3/TiO2
catalyst,either in a slightly reduced state (Fig. 5) or in an oxygen-containing
atmosphere (Fig. 6).
Therefore, two different cases have to be envisaged
Surface transformations w/o Oxygen
Surface transformations w/o Oxygen
Surface transformations with Oxygen
Surface transformations with Oxygen
Electrical conductivity of Cs doped NiMoO4
α- NiMoO4- n-type
semi-conductor
Cs is available on the surface & does not affect molybdate structure
Increase in conductivity with Cs due to ionic conductivity
At high Cs loading- loss of dispersion due to growth of Cs particles
Conductivity of Cs doped NiMoO4
Conductivity of NiMoO4
Defects in the form of anionic
oxygen vacancies are
responsible for n-type conductivity
Conductivity of NiMoO4 in Oxygen
Seebeck Effect
A temperature difference between two points in a conductor /semiconductor gives rise to a voltage difference between these two points
That is , a temperature gradient in a conductor results in an electric field
This effect is called Seebeck Effect or thermo electric effect
Thermo electric voltage developed per unit temperature difference is called
Seebeck coefficient
Only the net Seebeck voltage between two different metals can be
measured
The principle of thermocouples is based no Seebeck effect
S = dV/dT
Seebeck effect
The thermopower/emf of a material, represented as S, depends on the
material's temperature, and crystal structure
Typically metals have small thermo emf because most have half-filled
bands. Electrons (negative charges) and holes (positive charges) both
contribute to the induced thermoelectric voltage thus canceling each other's
contribution to that voltage and making it small.
In contrast, semiconductors can be doped with an excess amount of
electrons or holes and thus can have large positive or negative values of the
thermopower depending on the charge of the excess carriers.
The sign of the thermopower can determine which charged carriers
dominate the electric transport in both metals and semiconductors.
Superconductors have zero thermopower since the charged carriers carry
no entropy. Equivalently, the thermopower is zero because it is impossible
to have a finite voltage across a superconductor. (For example, by Ohm's
law, V=IR=0, since the resistance, R, is equal to zero in a superconductor.)
Seebeck effect
Seebeck Effect
Seebeck effect
Seebeck effect
Seebeck Effect
Seebeck Coefficient- Measurement
A- Sample held between two metal clamps
Th- Thermocouples attached to the sample
in iso-thermal region
Temperature and potential must be taken
at the same point inside the sample.
Mechanically attached probes cause a
temperature jump between the probe & sample.
Reliable Seebeck measurements require
elimination of the contact potentials
Hall Effect – Edwin Hall (1879)
A conductor/semi-conductor carrying
current I is subjected to a magnetic field B
at right angles, then in a plane mutually
Perpendicular there develops a voltage
known as Hall voltage
Hall effect is observed in metals &
semi-conductors
Hall effect- Illustration
Current I in X direction L to R
Mag. Field B in Z direction
Hall voltage VH in Y direction
Hall effect due to deflection of
Charge carriers in mag. Field
VH = IB/qnd
q- Electron charge
1.602 x10-19 coloumb
n- No.of charge carriers
d- Thickness of sheet
nd= ns- Charge density
Charge carriers
per unit vol.
ns = IB/ q*I VHI
VH negative
VH positive
n-type semi conductor
p-type semi conductor
Hall effect- Illustration
Hall effect
RH – Hall coefficient
RH =1/ ne
n- No. of charge carriers per cm3
Hall effect measurement
Hall voltage measurement
Sample n-type Ge metal strip
Hall voltage across terminal 3
Applied voltage measured across 2.1 & 2.2 or 2.3 ( up to 30 ma current)
Knob 5 for adjusting off-set voltage ( emf when mag. Field is zero)
Plugs 4 –Support; For applying current to heat the crystal
Jacks 7- For thermocouple
Magnetic field applied perpendicular to the metal strip