Copper (II) hydroxide Ionic HCl Acid Dinitrogen pentoxide Covalent Sodium chloride Ionic.
PHOTO CATALYTIC FIXATION OF DINITROGEN
-
Upload
bryce-hawkins -
Category
Documents
-
view
224 -
download
0
description
Transcript of PHOTO CATALYTIC FIXATION OF DINITROGEN
1
PHOTO CATALYTIC FIXATION OF DINITROGEN
Ph.D. Seminar I
G. Magesh
9-5-06
2
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
3
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
Importance of fixation of dinitrogen
Usage of ammonia in various industries
4
Nitrogen cycle
Various processes involved in nitrogen cycleEncyclopaedia Britannica, Encyclopaedia Britannica (1998)
5
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
Sources of fixed nitrogen
6
Fixed nitrogen before and after Haber process
Fixed Nitrogen Production (1000 tons)1913 1934
World 843.5 1972.0Chile 476.7 (56.5%) 141.8 (7.2%)
Germany 131.6 462.5Great Britain 99.5 175.0United States 39.5 256.7
Norway 22.0 65.5France 18.9 187.6Canada 12.7 41.1Belgium 11.0 109.8
Italy 6.3 98.6Japan 3.9 208.0Russia 3.2 45.0
7
Properties of dinitrogen which makes it inert
Dinitrogen - two N atoms connected by triple bond
Breaking the NN 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
Initial hydrogenation is highly endothermic for N2
N2 + H2 N2H2 H = 213.5 kJ mol-1
2 C + H2 C2H2 H = -175.8 kJ mol-1
8
Gas Property
Nitrogen Carbon Oxygen Argon
Ionization potential (eV) 14.3 11.256 13.614 15.755
Electron affinity (eV) 0.073 1.595 1.461 0
Solubility in water (mole/cm3) 0.083 Insoluble 0.153 0.140
Other important properties
High ionization potential and low electron affinity - difficult to reduce and oxidize
Solubility very less - reactions in solution phase - difficult
9
LUMO
HOMO
22.9 eV
Activation of dinitrogen
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
Molecular orbitals diagram of N2 molecule
T.A.Bazhenova and A.E.Shilov, Coord. Chem. Rev. 144 (1995) 69-145
10
Redox potential dependence on the number of electrons transferred
Initial two electron transfer requires higher potential
NH3 formation - six electron process - less probable
Stepwise redox potentials
Chatt J, Camara L M P, Richards R L, New Trends in the Chemistry of Nitrogen Fixation, Academic Press, (1980)
11
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
12
Available methods of fixing dinitrogenHaber process
N2 + 3H2 2NH3400°C, 200 atm
Fe based catalyst
Various steps in Haber process
Water gas shift reaction
13
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 400°C
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
14
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
15
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)2Ti]2-(N2)2-
complex
Compound N-N bond length(Å
)N2 gas 1.0975
H2NNH2 1.460
[(TMS2N)2Ti]2-(N2)2- 1.379
Limitation: N2 evolution during reduction
Fryzeuk M D, Johnson S A, Coord. Chem. Rev., 200 (2000) 379
16
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
Alternatives
17
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
18
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
Photo catalysis
Light induced excitation processes in a photo catalyst
19
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
Choice of materials as photo catalystChoices – Metals, semiconductors, insulators
Catalyst - absorb light in UV or visible region - easily available
Energy
Band gap of available materials
CBCB
CB
VB
VB
VB
Metal
Semiconductor
Insulator
20
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
Energy
-ve
+ve
Band positions of various types of semiconductors
Potential
Reduction (A / A-•)
Oxidation (D/D+•)
Types of semiconductors
21
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
Requirements of photo catalyst for fixation of N2
Band positions of semiconductors w.r.to reactions
eVN2/NH3
22
Photocorrosion
Oxidation potentials of catalysts w.r.to band positions
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
h+ = hole
23
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
Semiconductor M-X Percentage ionic
characterTiO2
SrTiO3
Fe2O3
ZnOWO3
ZnS CdS
CdSe
Ti-OTi-O-Sr
Fe-OZn-OW-OZn-S Cd-S
Cd-Se
59.568.547.355.557.518.0 17.616.5
Selection criterion for dopant ions in semiconductor
Viswanathan B, Bull. Catal. Soc. India, 2 (2003) 71
24
Photo catalytic fixation of dinitrogen
Schrauzer G N and Guth T D, J. Am. Chem. Soc., 99 (1977) 7189
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
25
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
26
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)
VB (Fe2Ti2O7)
N2/NH3
Band positions of Fe2Ti2O7
eV
1.6
Rusina O et al, Chem. Eur. J., 9 (2) (2003) 561
CB (TiO2)
27
Fe2Ti2O7 exhibits more activity - presence of ethanol
Exhibits photocurrent doubling in presence of ethanol
Following mechanism explains above two observations
Mechanism
SC + h SC (h+, e-)
SC (h+, e-) + H2O SC (h+) + Had + OH-
SC (h+) + CH3CH2OH SC + CH3HC•OH + H+
SC + CH3HC•OH SC (e-) + CH3CHO + H+
SC (e-) + H2O SC + Had + OH-
N2 + Had N2H2 or NH3
Photocurrent doubling
h+ = hole
SC = Semiconductor
28
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
Metal Electron affinity
(eV)Ru 1.050Rh 1.136Pd 0.557Pt 2.127Fe 0.163Ti 0.079
Trapping of electrons by noble metals
2H+ + 2e- 2Hade-
N2 + 6Had 2NH3
29
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
Reduction order: Ru > Rh > Pd > Pt
H2 evolution overpotential and M-H bond strength follows same order
Higher loading of metal - lesser activity than TiO2 - hindrance to light absorption
Yie
ld o
f am
mon
ia (µ
mol
h-1)
30
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
N2 reduction and conversion to NH4ClO4
SEM image of NH4ClO4 crystals on polymer surface
Hoshino K, Chem. Eur. J., 7 (13) (2001) 2727
31
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
Polyfuran and polycarbazole - active
Reactivity order:
Carbazole > Furan > ThiopheneBand position change at TiO2-polymer
interface
More negative band position
Tomohisa O et al, J. Photopolym. Sci. Technol., 17 (1) (2004) 143
eV
32
Role of hole scavengers in photo catalytic reduction
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 > methanol > ethanol
Tan T et al, J. Photochem. Photobiol. A: Chem., 159 (2003) 273
Holes in valence band:
Increases recombination
Involve in oxidation of NH3
Necessary to quench the holes formed
33
• Supply electrons to conduction band
• Capable of reducing reactant by themselves
Redox potentials of reaction species
Reduction potential of the radical species
Formic acid, methanol and ethanol form
reducing radicals
HCOO- + h+ •COO- + H+
RCH2OH + h+ R•CHOH + H+
R•CHOH + SC RCHO + SC (e-) + H+ N2/NH3
Potential (eV) vs NHE
34
Solvent effects on photo catalytic reduction
Effect of various alcohols as solvents on photo catalytic reduction
Activity order
Methanol > Ethanol > 1-propanol > 2-propanol > 1-butanol > (iso-butanol) 2-methyl-propan-1-ol
Brezolva V et al, J. Photochem. Photobiol. A: Chem., 107 (1997) 233
Properties of solvents which play a role:
Viscosity
Refractive index
Polarity
Stability of radicals
35
High polarity:
More stabilization of the charge carriers
Property
Solvent
Viscosity (g cm-1 s-1)
Refractive index
Polarity
Methanol 0.544 1.326 0.60Ethanol 1.074 1.359 0.541-Propanol 1.945 1.383 0.522-Propanol 2.038 1.375 0.481-Butanol 2.544 1.397 0.47iso-Butanol 4.312 1.394 0.40
Properties of the various solvents
High viscosity:
Low diffusion coefficient
High refractive index:
Less penetration of light
Stability:
2-methyl-propan-1-ol(iso-butanol) > 1-butanol > 2-propanol > 1-propanol > Ethanol > Methanol
Stability of radicals - reverse order of activity
36
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
Fixation of N2 on a CdS/Pt – [RuII(H-EDTA)(N2)]- system
Nageswara Rao N, J. Mol. Catal., 93 (1994) 23
37
N2 fixation on a CdS/Pt/RuO2 – [Ru(H-EDTA)(N2)]- system
EDTA - sacrificial agent – enhances rate
Mechanism
Taqui Khan M M and Nageswara Rao N, J. Mol. Catal., 52 (1989) L5
38
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 methodsRanjit K T and Viswanathan B, Ind. J. Chem., 35A (1996) 443
h
39
Reasons
CB of photo catalyst – Not matching LUMO of N2
N2 adsorption – Not strong to perturb orbitals
Yields of ammonia – Not sufficient
40
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.
41
42
Ammonia reactants
Steam reformingCH4(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 shiftCO(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) CO3
2-(aq) + H2O(l)
Biological N-FixationMost 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.
44
NH4Cl + Ba(OH)2 = NH3 + H2O + BaClDestructive 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 --> 2NH3 + CaCl2 + H2O
45
N2
Structure of RuEDTAN2 complex
46
According to Stoke’s –Einstein equation, Diffusion coefficient, D = kT/6 r Where r - radius of species
- viscosity of solvent
47
N2 N2H5+ E = - 0.23 V
N2H5+ NH4
+ E = + 1.275 V
N2 NH4+ E = + 0.275 V
48
Structures of polymers
49
50
51
52
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
53
Structural basis of biological nitrogen fixationPhilosophical 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.
54
Composition in activated form(%)Fe2O3 1.1 - 1.7FeO 14.3 - 14.6Fe 79.7 - 81.6CaO 0.1 – 0.2SiO2 0.1 – 0.7MgO 0.3 - 0.6Al2O3 1.5 – 2.1K2O 0.2 – 0.5
55
Free-living (asymbiotic)• Cyanobacteria• Azotobacter
Associative• Rhizosphere–Azospirillum• Lichens–cyanobacteria• Leaf nodules
Symbiotic• Legume-rhizobia• Actinorhizal-Frankia
56
57
Structural Basis of Biological Nitrogen FixationJames B. Howard, Douglas C. ReesChem. Rev. 1996, 96, 29652982
58
As Temperature increases, should drive the reaction to the left.
But, dissociation is only significant at high temperatures
- Very inefficient reaction (low reaction probability)
N2 + 3H2 2NH3 H=-36 kJ/mole Exothermic
1 N2(g) + * N2*
2 N2* + * 2N*
3 N* + H* NH* + *
4 NH* + H* NH2* + *
5NH2* + H*
NH3* + *
6 NH3* NH3(g) + *
7 H2(g) + 2* 2H*
This is the slow step
Physisorption
Why the reduction process is difficult
59
60