PHOTO CATALYTIC FIXATION OF DINITROGEN

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PHOTO CATALYTIC FIXATION OF DINITROGEN
Ph.D. Seminar I G. Magesh 9-5-06
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Contents

• Importance of fixation of dinitrogen
• Properties of dinitrogen
• Various methods for fixation of dinitrogen
• Shortcomings in available methods
• Merits of photo catalytic fixation of dinitrogen
• Fundamentals of photo catalysis
• Challenges in photo catalytic route
• Ways of overcoming them

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Importance of fixation of dinitrogen

• Nitrogen - necessary for functioning of
  biomolecules and plant growth
• Important component of fertilizers and medicines
• Present in dyes, explosives and resins
• Ammonia - starting material for nitrogen
  containing chemicals

Usage of ammonia in various industries
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Nitrogen cycle
Various processes involved in nitrogen cycle
Encyclopaedia Britannica, Encyclopaedia
Britannica (1998)
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Sources of fixed nitrogen

• Haber process
• Fixed nitrogen by bacteria and algae
• Chile salt petre (Sodium nitrate)
• Destructive distillation of decayed vegetable and
  animal matter
• Reduction of nitrous acid and nitrites with
  nascent hydrogen
• Decomposition of ammonium salts by alkaline
  hydroxides or quicklime
• Mg3N2 6 H2O 3 Mg(OH)2 2 NH3

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Fixed nitrogen before and after Haber process
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Properties of dinitrogen which makes it inert

• Dinitrogen - two N atoms connected by triple bond
• Breaking the N?N bond is difficult - high
  dissociation energy of 942 kJ mol-1
• Breaking first ? bond requires 540 kJ mol-1
• Very weak base no interaction with even strong
  acids
• Non-polar

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Other important properties

• High ionization potential and low electron
  affinity - difficult to reduce and oxidize
• Solubility very less - reactions in solution
  phase - difficult

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Activation of dinitrogen
Molecular orbitals diagram of N2 molecule

• Very difficult to activate dinitrogen using
  light, heat and potential
• HOMO very low w.r.to e- acceptors
• LUMO very high w.r.to e- donors

T.A.Bazhenova and A.E.Shilov, Coord. Chem. Rev.
144 (1995) 69-145
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Stepwise redox potentials
Redox potential dependence on the number of
electrons transferred

• Initial two electron transfer requires higher
  potential
• NH3 formation - six electron process - less
  probable

Chatt J, Camara L M P, Richards R L, New Trends
in the Chemistry of Nitrogen Fixation, Academic
Press, (1980)
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Thermodynamics of fixation of N2 to ammonia
N2 3H2 2NH3 ?H -36 kJ mol-1

• Change in entropy, ?S - ve
• II law of thermodynamics - Natural processes tend
  to increase the entropy
• Formation of ammonia by this route cannot be a
  natural process
• Spontaneous reaction ?G ve
• ?G negative at very low temperatures

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Available methods of fixing dinitrogen

• Haber process
• N2 3H2 2NH3

Fe based catalyst
400C, 200 atm
Water gas shift reaction
Various steps in Haber process
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Limitations with the Haber process

• Forward reaction - reduction in number of
  molecules
• Le Chatelier principle - high pressure forward
  reaction
• Not desired in industries - accidents and
  increased cost
• Forward reaction - exothermic
• Temperature must be minimum - Le Chatelier
  principle
• To achieve high rates in industries - temperature
  at 400C
• Conversion of 15

• Hydrogen
• Obtained from fossil fuels a limited resource
• Production requires major part of plant and cost
• Releases green house gases like CO2 and CO

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Biological fixation of dinitrogen

• Enzyme nitrogenase
• Present in soil bacteria, root nodules and algae
• Two decades of research - mechanism not
  established
• Enzyme contains Mo and Fe
• Proposed mechanism - complexation of N2 to metal
  ions
• Reduces bond strength - breaking 1st ? bond easier

• Limitations with biological route
• Nitrogenase - sensitive to O2 requires O2 free
  environment
• Sensitive to environmental conditions -
  temperature, pH
• Cannot be used for large scale N2 fixation

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Fixation of dinitrogen by metal-nitrogen complexes

• Fe, Ti, Zr, Mo - high affinity for N2
• Electron rich ligands TMS, phosphine
• Perturbing N2 donates e- to LUMO of N2

Structure of
(TMS2N)2Ti2-(N2)2- complex
Limitation N2 evolution during reduction
Fryzeuk M D, Johnson S A, Coord. Chem. Rev., 200
(2000) 379
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Alternatives

• Haber process
• Dissociative adsorption of N2 High temperature
  and pressure

• Metal complex based reduction
• Binding N2 Perturb e- acceptor orbital (wave
  function)
• e- donation LUMO of N2
• Limited success

• Look for
• Perturb orbital (wave function) of e- donor and
  acceptor
• e- donation to LUMO N2 activation
• Very strong N2 adsorption
• Hydrogen addition without interruption

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Merits of photo catalytic fixation of dinitrogen

• Utilizes light and efforts are on to use sunlight
  - a renewable source
• H2 for reduction obtained from water - a widely
  available source
• No pollution associated with the process
• Process of photo catalysis is well understood
• Carried out at atmospheric pressure and room
  temperatures
• Methods to perturb catalyst orbitals transfer
  e- to LUMO

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Photo catalysis

• Photo catalysis - 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

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Choice of materials as photo catalyst
Choices Metals, semiconductors,
insulators Catalyst - absorb light in UV or
visible region - easily available

• Metal
• No band gap
• Only reduction or oxidation band position
• Semiconductor
• Optimum band gap
• UV or Visible light
• Insulator
• High band gap
• Requires light - higher energy than UV light

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Types of semiconductors
For reduction Conduction band potential - more
negative than potential of reduction reaction For
oxidation Valence band potential - more positive
than potential of oxidation reaction OR Type
Oxidation and Reduction R Type Reduction O
Type Oxidation X type - None
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Requirements of photo catalyst for fixation of N2

• N2/NH3 0.059 eV
• H/H2 0.000 eV
• Conduction band potential - more negative than
  above potentials
• H2O/O2 1.229 eV
• Valence band potential - more positive than above
  potential
• Very strong N2 adsorption
• No photocorrosion
• Good light absorption
• Chemically inert

Band positions of semiconductors w.r.to reactions
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Photocorrosion

• CdS, ZnS, ZnO undergo photocorrosion
• Activity decrease as the time increases
• Catalyst gets oxidised
• Oxidation potential of catalyst More -ve
  than desired oxidation reaction potential
• S deposition on catalyst - reduce light
  absorption

Oxidation potentials of catalysts w.r.to band
positions
h hole
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Selection criterion for dopant ions in
semiconductor

• Doping cations and anions altering band
  positions
• Increase in ionic character of M-X bond - band
  gap decreases and vice versa
• Ionic Character ( 1 - exp - (XM - XX)2 / 4
  ) x 100 X- electronegativity

Viswanathan B, Bull. Catal. Soc. India, 2 (2003)
71
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Photo catalytic fixation of dinitrogen

• First reported - Schrauzer and Guth in 1977 with
  moist TiO2 using UV light
• Transfer of e- from CB to N2 directly or
  indirectly
• Potential requirement - N2 reduction and
  photo-splitting of water - similar
• Activation barrier in N2 reduction is high

Reduction of one mole of N2 N2 6H 6e- 2
NH3 3H2O 6h 3/2 O2 6H (requires 6
electrons) Photo-splitting of water 2H
2e- H2 H2O 2h 2H 1/2
O2 (requires 2 electrons)
h hole
Schrauzer G N and Guth T D, J. Am. Chem. Soc., 99
(1977) 7189
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Problems associated with photo catalytic fixation
of N2

• Oxidation of NH3 formed to nitrites and nitrates
• Recombination of excited electrons
• Simultaneous H2 evolution leading to its lesser
  availability
• Less ve conduction band potential of available
  catalysts
• Oxidation reactions by the holes
• Lesser adsorption of N2 on catalyst surface

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Fixation of N2 by iron based catalysts

• Fixation of N2 by iron TiO2 based catalysts -
  reported in 1977
• Compound responsible - not established
• Fe2Ti2O7 responsible
• Has a bandgap of 2 eV
• Fe2Ti2O7 Conduction band at 0.4 eV compared
  to TiO2 (0.2 eV) high reduction potential
• Valence band at 1.6 eV

CB (Fe2Ti2O7)
CB (TiO2)
N2/NH3
eV
VB (Fe2Ti2O7)
1.6
Band positions of Fe2Ti2O7
Rusina O et al, Chem. Eur. J., 9 (2) (2003) 561
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Mechanism

• Fe2Ti2O7 exhibits more activity - presence of
  ethanol
• Exhibits photocurrent doubling in presence of
  ethanol
• Following mechanism explains above two
  observations

SC h? SC (h, e-) SC (h, e-) H2O
SC (h) Had OH- SC (h) CH3CH2OH
SC CH3HCOH H SC CH3HCOH
SC (e-) CH3CHO H SC (e-)
H2O SC Had OH- N2
Had N2H2 or
NH3
Photocurrent doubling
h hole SC Semiconductor
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Effect of noble metal dispersion

• Recombination of electrons and holes - reduces
  efficiency
• Solution - dispersing noble metals on TiO2
  surface
• Noble metals - high electron affinity - traps
  excited electrons immediately

Ranjit K T et al, J. Photochem. Photobiol. A
Chem., 96 (1996) 181
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Effect of noble metal dispersion

• Another advantage - reduces H2 evolution
• Reduced H should be as Had not evolved as H2
• High H2 evolution Low N2 reduction
• Noble metals - promote adsorption of hydrogen on
  surface

Yield of ammonia (µmol h-1)

• Reduction order Ru gt Rh gt Pd gt Pt
• H2 evolution overpotential and M-H bond strength
  follows same order
• Higher loading of metal - lesser activity than
  TiO2 - hindrance to light absorption

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Fixation of N2 on TiOx- poly-3-methyl
thiophene(P3MeT) composite

• Drawback - Oxidation of ammonia to nitrites and
  nitrates
• Convert to its salts immediately
• A TiOx-conducting polymer doped with ClO4- used
• NH3 formed reacts with ClO4- to form NH4ClO4
  crystals

SEM image of NH4ClO4 crystals on polymer surface
N2 reduction and conversion to NH4ClO4
Hoshino K, Chem. Eur. J., 7 (13) (2001) 2727
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More negative band position

• Less negative conduction band (CB) potential
  Lower rate of reduction
• At TiOx-polymer interface - alteration of
  bandposition - CB at 1.1 eV
• CB TiO2 (-0.2 eV)
• Increases reduction rate at interface

eV
Polyfuran and polycarbazole - active Reactivity
order Carbazole gt Furan gt Thiophene
Band position change at TiO2-polymer interface
Tomohisa O et al, J. Photopolym. Sci. Technol.,
17 (1) (2004) 143
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Role of hole scavengers in photo catalytic
reduction

• Holes in valence band
• Increases recombination
• Involve in oxidation of NH3
• Necessary to quench the holes formed

• Sucrose, acetic acid, salicylic acid, formic
  acid, methanol and ethanol - investigated
  with TiO2
• No improvement for sucrose, acetic acid and
  salicylic acid
• Improvement order formic acid gt methanol gt
  ethanol

Tan T et al, J. Photochem. Photobiol. A Chem.,
159 (2003) 273
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Reduction potential of the radical species
Formic acid, methanol and ethanol form reducing
radicals HCOO- h COO- H RCH2OH
h RCHOH H RCHOH SC RCHO SC (e-) H

• Supply electrons to conduction band
• Capable of reducing reactant by themselves

Redox potentials of reaction species
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Solvent effects on photo catalytic reduction
Effect of various alcohols as solvents on photo
catalytic reduction Activity order Methanol gt
Ethanol gt 1-propanol gt 2-propanol gt 1-butanol gt
(iso-butanol) 2-methyl-propan-1-ol

• Properties of solvents which play a role
• Viscosity
• Refractive index
• Polarity
• Stability of radicals

Brezolva V et al, J. Photochem. Photobiol. A
Chem., 107 (1997) 233
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Properties of the various solvents
High viscosity Low diffusion coefficient High
refractive index Less penetration of light
High polarity More stabilization of the charge
carriers
Stability 2-methyl-propan-1-ol(iso-butanol) gt
1-butanol gt 2-propanol gt 1-propanol gt Ethanol gt
Methanol Stability of radicals - reverse order
of activity
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Fixation of N2 on a CdS/Pt RuII(H-EDTA)(N2)-
system

• High N2 bond strength - cleavage difficult
• Dinitrogen complexation - weakens N-N triple bond
  - reductively cleaved by various means
• Conventionally reduced using LiAlH4, NaBH4, Al
  metal
• Photoexcited electrons used for the reduction

Nageswara Rao N, J. Mol. Catal., 93 (1994) 23
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Mechanism
N2 fixation on a CdS/Pt/RuO2 Ru(H-EDTA)(N2)-
system
EDTA - sacrificial agent enhances rate
Taqui Khan M M and Nageswara Rao N, J. Mol.
Catal., 52 (1989) L5
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Influence of Ti3 sites on fixation of N2

• Adsorption of N2 - essential for e- transfer
  leading to reduction
• Ti3 defect sites
• Increase N2 adsorption
• Responsible for n-type semiconductivity
• Directly gives electrons to N2
• 6 Ti4-OH 6
  Ti3-OH
• 6 Ti3-OH 6 Ti3 3 H2O
  3/2 O2
• 6 Ti3 N2 6 H2O 6 Ti4-OH 2 NH3
• Catalyst with more Ti3 sites - more active for
  N2 reduction
• Doping TiO2 - favorable preparation methods

h?
Ranjit K T and Viswanathan B, Ind. J. Chem., 35A
(1996) 443
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Yields of ammonia Not sufficient

• Reasons
• CB of photo catalyst Not matching LUMO of N2
• N2 adsorption Not strong to perturb orbitals

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• The activation of dinitrogen appears to be still
  intriguing. Even though, various methods of
  activation of dinitrogen have been attempted, the
  perturbations of the frontier wave functions of
  dinitrogen with respect to energy and symmetry
  have been considered to be the key.
• However, in photocatalytic routes the frontier
  wave functions of the reacting species (photo
  catalysts) are perturbed so as to be able to
  interact with ground state wave functions of
  dinitrogen. It essentially means that the
  emphasis is shifted from the reacting species
  (i.e. dinitrogen) to the species with which the
  reacting species interacts.
• However, even this shift in the emphasis does not
  seem to have provided the answer.

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Thank you
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Ammonia reactants

• Steam reforming
• CH4(g) H2O(g) ? CO(g) 3 H2(g)
• 15-40 NiO/low SiO2/Al2O3 catalyst (760-816C)
• products often called synthesis gas or syngas
• Water gas shift
• CO(g) H2O(g) ? CO2(g) H2(g)
• Cr2O3 and Fe2O3 as catalyst
• carbon dioxide removed by passing through sodium
  hydroxide.
• CO2(g) 2 OH-(aq) ? CO32-(aq) H2O(l)

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Biological N-Fixation

• Most nitrogen is fixed by micro-organisms in the
  soil which include bacteria and cyanobacteria.

Some plants like legumes and alder trees have
special adaptations on their roots to fix
nitrogen which are called nodules. This is an
example of a symbiotic relationship between the
plant and N-fixing bacteria.
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NH4Cl Ba(OH)2 NH3 H2O BaCl Destructive
distillation The decomposition of wood by
heating out of contact with air, producing
primarily charcoal Magnesium nitride Fomed by
interaction of magnesium with nitrogen in
atmosphere Reaction with quick lime 2NH4Cl
CaO --gt 2NH3 CaCl2 H2O
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Structure of RuEDTAN2 complex
N2
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According to Stokes Einstein equation,
Diffusion coefficient, D kT/6 ? r ? Where r -
radius of species ? - viscosity of solvent
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Structures of polymers
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Li Be
B C Na Mg

D Al Si K Ca (Sc)
Ti (V) Cr Mn Fe Co Ni Cu Zn
Ga Ge Rb Sr Y Zr Nb Mo (Tc) (Ru)
Rh Pd Ag Cd In Sn Cs Ba La (Hf)
Ta W Re (Os) Ir Pt Au Hg Tl Pb
C A B
E
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Structural basis of biological nitrogen
fixation Philosophical Transactions of the Royal
Society A Mathematical, Physical and Engineering
Sciences Volume 363, Number 1829 / April 15,
2005, 971 - 984     Biological nitrogen fixation
is mediated by the nitrogenase enzyme system that
catalyses the ATP dependent reduction of
atmospheric dinitrogen to ammonia. Nitrogenase
consists of two component metalloproteins, the
MoFe-protein with the FeMo-cofactor that provides
the active site for substrate reduction, and the
Fe-protein that couples ATP hydrolysis to
electron transfer. An overview of the nitrogenase
system is presented that emphasizes the
structural organization of the proteins and
associated metalloclusters that have the
remarkable ability to catalyse nitrogen fixation
under ambient conditions. Although the mechanism
of ammonia formation by nitrogenase remains
enigmatic, mechanistic inferences motivated by
recent developments in the areas of nitrogenase
biochemistry, spectroscopy, model chemistry and
computational studies are discussed within this
structural framework.
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Composition in activated form() Fe2O3
1.1 - 1.7 FeO
14.3 - 14.6 Fe
79.7 - 81.6 CaO 0.1
0.2 SiO2 0.1 0.7 MgO
0.3 - 0.6 Al2O3
1.5 2.1 K2O
0.2 0.5
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• Free-living (asymbiotic)
• Cyanobacteria
• Azotobacter
• Associative
• RhizosphereAzospirillum
• Lichenscyanobacteria
• Leaf nodules
• Symbiotic
• Legume-rhizobia
• Actinorhizal-Frankia

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Structural Basis of Biological Nitrogen
Fixation James B. Howard, Douglas C. Rees Chem.
Rev. 1996, 96, 2965-2982
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Why the reduction process is difficult
Physisorption
This is the slow step
N2 3H2 ? 2NH3 DH-36 kJ/mole
Exothermic
As Temperature increases, should drive the
reaction to the left. But, dissociation is only
significant at high temperatures -? Very
inefficient reaction (low reaction probability)
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