1 PHOTO CATALYTIC FIXATION OF DINITROGENPh.D. Seminar I G. Magesh 9-5-06
2 Contents Importance of fixation of dinitrogen Properties of dinitrogenVarious 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 Importance of fixation of dinitrogenNitrogen - 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
4 Nitrogen cycle Various processes involved in nitrogen cycleEncyclopaedia Britannica, Encyclopaedia Britannica (1998)
5 Sources of fixed nitrogenHaber 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 Mg3N H2O Mg(OH) NH3
6 Fixed Nitrogen Production (1000 tons)Fixed nitrogen before and after Haber process Fixed Nitrogen Production (1000 tons) 1913 1934 World 843.5 1972.0 Chile 476.7 (56.5%) 141.8 (7.2%) Germany 131.6 462.5 Great Britain 99.5 175.0 United States 39.5 256.7 Norway 22.0 65.5 France 18.9 187.6 Canada 12.7 41.1 Belgium 11.0 109.8 Italy 6.3 98.6 Japan 3.9 208.0 Russia 3.2 45.0
7 Properties of dinitrogen which makes it inertDinitrogen - 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 + H N2H H = kJ mol-1 2 C + H C2H H = kJ mol-1
8 Other important propertiesGas 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 Solubility in water (mole/cm3) 0.083 Insoluble 0.153 0.140 High ionization potential and low electron affinity - difficult to reduce and oxidize Solubility very less - reactions in solution phase - difficult
9 Activation of dinitrogenLUMO HOMO 22.9 eV 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)
10 Stepwise redox potentialsRedox 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)
11 Thermodynamics of fixation of N2 to ammoniaN2 + 3H2 2NH 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 + 3H NH3 Fe based catalyst 400°C, 200 atm Water gas shift reaction Various steps in Haber process
13 Limitations with the Haber processForward 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 dinitrogenEnzyme 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 Limitation: Fixation of dinitrogen by metal-nitrogen complexesFe, Ti, Zr, Mo - high affinity for N2 Electron rich ligands – TMS, phosphine Perturbing N2 – donates e- to LUMO of N2 Compound N-N bond length(Å) N2 gas 1.0975 H2NNH2 1.460 [(TMS2N)2Ti]2-(N2)2- 1.379 Structure of [(TMS2N)2Ti]2-(N2)2- complex Limitation: N2 evolution during reduction Fryzeuk M D, Johnson S A, Coord. Chem. Rev., 200 (2000) 379
16 Metal complex based reductionAlternatives 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
17 Merits of photo catalytic fixation of dinitrogenUtilizes 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 Light induced excitation processes in a photo catalystPhoto 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 Light induced excitation processes in a photo catalyst
19 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 CB VB Metal Semiconductor Insulator 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
20 Types of semiconductorsFor 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+•)
21 Band positions of semiconductors w.r.to reactionsRequirements of photo catalyst for fixation of N2 N2/NH3 = eV H+/H2 = eV Conduction band potential - more negative than above potentials H2O/O2 = eV Valence band potential - more positive than above potential Very strong N2 adsorption No photocorrosion Good light absorption Chemically inert eV N2/NH3 Band positions of semiconductors w.r.to reactions
22 Oxidation potentials of catalysts w.r.to band positionsPhotocorrosion 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
23 Selection criterion for dopant ions in semiconductorDoping 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 X- electronegativity Semiconductor M-X Percentage ionic character TiO2 SrTiO3 Fe2O3 ZnO WO3 ZnS CdS CdSe Ti-O Ti-O-Sr Fe-O Zn-O W-O Zn-S Cd-S Cd-Se 59.5 68.5 47.3 55.5 57.5 18.0 17.6 16.5 Viswanathan B, Bull. Catal. Soc. India, 2 (2003) 71
24 Photo catalytic fixation of dinitrogenFirst 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 O H+ (requires 6 electrons) Photo-splitting of water 2H e- H2 H2O + 2h+ 2H /2 O2 (requires 2 electrons) h+ = hole Schrauzer G N and Guth T D, J. Am. Chem. Soc., 99 (1977) 7189
25 Problems associated with photo catalytic fixation of N2Oxidation 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 catalystsFixation 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 1.6 VB (Fe2Ti2O7) Band positions of Fe2Ti2O7 Rusina O et al, Chem. Eur. J., 9 (2) (2003) 561
27 Mechanism Fe2Ti2O7 exhibits more activity - presence of ethanolExhibits 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 + CH3HC•OH + H+ SC + CH3HC•OH SC (e-) + CH3CHO + H+ SC (e-) + H2O SC + Had + OH- N Had N2H2 or NH3 Photocurrent doubling h+ = hole SC = Semiconductor
28 Effect of noble metal dispersionRecombination of electrons and holes - reduces efficiency Solution - dispersing noble metals on TiO2 surface Noble metals - high electron affinity - traps excited electrons immediately Metal Electron affinity (eV) Ru 1.050 Rh 1.136 Pd 0.557 Pt 2.127 Fe 0.163 Ti 0.079 Trapping of electrons by noble metals 2H+ + 2e- 2Had e- N2 + 6Had 2NH3 Ranjit K T et al, J. Photochem. Photobiol. A: Chem., 96 (1996) 181
29 Effect of noble metal dispersionAnother 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 > 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
30 Fixation of N2 on TiOx- poly-3-methyl thiophene(P3MeT) compositeDrawback - 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
31 More negative band positionLess 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 > Furan > Thiophene Band position change at TiO2-polymer interface Tomohisa O et al, J. Photopolym. Sci. Technol., 17 (1) (2004) 143
32 Role of hole scavengers in photo catalytic reductionHoles 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 > methanol > ethanol Tan T et al, J. Photochem. Photobiol. A: Chem., 159 (2003) 273
33 Reduction potential of the radical speciesFormic 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 Supply electrons to conduction band Capable of reducing reactant by themselves Redox potentials of reaction species
34 Solvent effects on photo catalytic reductionEffect 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 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
35 Properties of the various solventsProperty Solvent Viscosity (g cm-1 s-1) Refractive index Polarity Methanol 0.544 1.326 0.60 Ethanol 1.074 1.359 0.54 1-Propanol 1.945 1.383 0.52 2-Propanol 2.038 1.375 0.48 1-Butanol 2.544 1.397 0.47 iso-Butanol 4.312 1.394 0.40 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) > 1-butanol > 2-propanol > 1-propanol > Ethanol > Methanol Stability of radicals - reverse order of activity
36 Fixation of N2 on a CdS/Pt – [RuII(H-EDTA)(N2)]- systemHigh 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
37 Mechanism EDTA - sacrificial agent – enhances rateN2 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
38 Influence of Ti3+ sites on fixation of N2Adsorption 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 Ti3+-OH 6 Ti3+-OH Ti 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
39 Yields of ammonia – Not sufficientReasons CB of photo catalyst – Not matching LUMO of N2 N2 adsorption – Not strong to perturb orbitals
40 The activation of dinitrogen appears to be still intriguingThe 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 Thank you
42 Ammonia reactants Steam reforming CH4(g) + H2O(g) CO(g) + 3 H2(g)15-40% NiO/low SiO2/Al2O3 catalyst ( C) 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)
43 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 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 = V N2H5+ NH4+ E = V N2 NH4+ E = V
48 Structures of polymers
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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, 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(%)Fe2O FeO Fe CaO – 0.2 SiO – 0.7 MgO Al2O – 2.1 K2O – 0.5
55 Free-living (asymbiotic)Cyanobacteria Azotobacter Associative Rhizosphere–Azospirillum Lichens–cyanobacteria Leaf nodules Symbiotic Legume-rhizobia Actinorhizal-Frankia
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57 Structural Basis of Biological Nitrogen FixationJames B. Howard, Douglas C. Rees Chem. Rev. 1996, 96,
58 Why the reduction process is difficultPhysisorption 1 N2(g) + * N2* 2 N2* + * 2N* 3 N* + H* NH* + * 4 NH* + H* NH2* + * 5 NH2* + H* NH3* + * 6 NH3* NH3(g) + * 7 H2(g) + 2* 2H* This is the slow step N2 + 3H2 2NH 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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