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PHOTO CATALYTIC FIXATION OF DINITROGEN

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

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PHOTO CATALYTIC FIXATION OF DINITROGEN

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  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. 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

  4. Nitrogen cycle Various processes involved in nitrogen cycle Encyclopaedia Britannica, Encyclopaedia Britannica (1998)

  5. 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

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

  9. Activation of dinitrogen LUMO 22.9 eV HOMO 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

  10. 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)

  11. Thermodynamics of fixationof 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 dinitrogen Haber process N2 + 3H2 2NH3 Fe based catalyst 400°C, 200 atm Water gas shift reaction Various steps in Haber process

  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 Compound N-N bond length(Å) Structure of [(TMS2N)2Ti]2-(N2)2- complex 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. 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

  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 • 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 catalyst Choices – Metals, semiconductors, insulators Catalyst - absorb light in UV or visible region - easily available CB • 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 CB CB Energy VB Metal VB Semiconductor VB Insulator Band gap of available materials

  20. 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 -ve Potential Reduction (A / A-•) Energy Oxidation (D/D+•) +ve Band positions of various types of semiconductors

  21. 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 N2/NH3 eV Band positions of semiconductors w.r.to reactions

  22. 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

  23. 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 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 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

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

  27. 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 + CH3HC•OH + H+ SC + CH3HC•OH SC (e-) + CH3CHO + H+ SC (e-) + H2O SC + Had + OH- N2 + HadN2H2 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 Metal Electron affinity (eV) e- 2Had 2H+ + 2e- 2NH3 N2+ 6Had Ru 1.050 Rh 1.136 Pd 0.557 Pt 2.127 Fe 0.163 Ti 0.079 Trapping of electrons by noble metals Ranjit K T et al, J. Photochem. Photobiol. A: Chem., 96 (1996) 181

  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 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) 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

  31. 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 > 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 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 > methanol > ethanol Tan T et al, J. Photochem. Photobiol. A: Chem., 159 (2003) 273

  33. 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+ Potential (eV) vs NHE N2/NH3 • Supply electrons to conduction band • Capable of reducing reactant by themselves Redox potentials of reaction species

  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 • 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 solvents Property Solvent Viscosity (g cm-1 s-1) Refractive index Polarity High viscosity: Low diffusion coefficient High refractive index: Less penetration of light 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 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)]- 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

  37. 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

  38. Influence of Ti3+ sites on fixation of N2 • Adsorption of N2 - essential for e-transferleading 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

  39. Yields of ammonia – Not sufficient • Reasons • 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 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. 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)

  42. Biological N-Fixation 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. Most nitrogen is fixed by micro-organisms in the soil which include bacteria and cyanobacteria.

  43. 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 --> 2NH3 + CaCl2 + H2O

  44. Structure of RuEDTAN2 complex N2

  45. According to Stoke’s –Einstein equation, Diffusion coefficient, D = kT/6  r  Where r - radius of species  - viscosity of solvent

  46. N2 N2H5+ E = - 0.23 V N2H5+ NH4+ E = + 1.275 V N2 NH4+ E = + 0.275 V

  47. Structures of polymers

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