C.M.Janet

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Why are the concepts in ammonia synthesis still
dynamic?
Ph.D seminar I
C.M.Janet CY03D020
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Contents

• Introduction
• Why is N2 activation difficult ?
• Development of ammonia synthesis
• Why transition metals?
• Thermodynamics and kinetics
• Factors affecting NH3 synthesis
• Lechatlier principle and its significance
• Biological nitrogen fixation
• Synthetic analogue approach
• Concluding remarks

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NH3 - A flash back
Ammonia was first isolated by Priestley in 1774.
Only gaseous base in the atmosphere.
Ammonia is produced by the anaerobic decay of
organic materials. Ammonia was the first
complex molecule to be identified in interstellar
space and solid NH3 makes up the rings on
Saturn. Prior to the 20th century, ammonia was
made by destructive distillation of animal parts
such as hoofs, horns, etc. 
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Historic importance of ammonia
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In warfare
In agriculture
Nitrogen Food or Flames
Nitrogen compounds can be used to feed or kill
with equal ease. This ironic nature of nitrogen
was tragically demonstrated in the Oklahoma City
Bombing when fertilizer was used as an explosive,
killing hundreds. Origin of green revolution
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NH3 in different dimensions
NH3
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70
10
Fertilizer
Other uses
HNO3
lt10
lt10
Nylon production
Chemical reagents.,etc
lt10
Ammonium salts
Ammonia refrigeration
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High explosives
NH4NO3
NH4OH
Fertilizer
Explosives
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Deposition of excretion of sea birds called
"Guano" accumulated and became several feet
thick.
Available sources of NH3

• Chile salt peter ( Sodium nitrate)
• Nitrate minerals
• Dry distillation of decayed vegetable and animal
  matter
• Fixed Nitrogen by bacteria
• By the reduction of nitrous acid and nitrites
  with nascent hydrogen
• By the decomposition of ammonium salts by
  alkaline hydroxides or
• by unslaked lime (quicklime)

Mg3N2 6 H2O ? 3 Mg(OH)2 2NH3
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N2 cycle and related phenomena
Lightning
Fixation of N2 Converts it to compounds useful
to plants
Rain
Legumes
Plants and animal wastes
Nitrogenase in legumes converts N2 to NH3
To ground water
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Physico-chemical properties of N2

• Very weak base- No interaction with strongest
  acids
• Hydrogenation is highly endothermic for N2

T.A.Bazhenova and A.E.Shilov, Coord. Chem. Rev.
144 (1995) 69-145
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Molecular orbitals of N2 molecule
3?u (Vacant)
1?g 7.3 eV (degenerate vacant)
LUMO
22.9 eV
3?g - 15.6 eV 2 electrons
HOMO
1?u -17.1eV (degenerate 4 electrons)
2?u 18.7 eV 2 electrons
2?g 39.5 eV 2 electrons
T.A.Bazhenova and A.E.Shilov, Coord. Chem. Rev.
144 (1995) 69-145
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Standard reduction potential dependence on the
number of electrons transferred to N2
.N2H
-3 -2 -1
1e-
E (V)
1H
N2H2
2e-
2H
N2H4
4H
4e-
N2
2NH3
6e-
6H
0 1 2 3 4
5 6
No. of electrons supplied
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Thermodynamics of NH3 synthesis
H - ve S - ve G - ve
G H T S
Spontaneous at low temperatures.

Need to specify T   GR  Keq (T)  
N2 (g) 3 H2 (g) 2 NH3 (g)
?H lt 0
H f (NH3) -46 kJ/mol   
S N2   192 J/mol K S H2   130 J/mol K S NH3
193 J/mol K 
Thermodynamics tells us whether a reaction can
occur
N2(g) 3 H2(g) 2 NH3(g),     H
-92 kJ / mol     S -199 J/mol K
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At last ..
Fritz Haber (December 9, 1868 January 29 1934)
From 1894 until 1911 Haber and Carl Bosch
developed the Haber process, which is the
catalytic formation of ammonia from hydrogen and
atmospheric nitrogen under conditions of high
temperature and high pressure using iron as the
catalyst. N2(g) 3 H2(g) 2
NH3(g)    In 1918 he received the Nobel Prize in
Chemistry for this work.
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Development through years !

• 1880 Haber found the reaction path way
• 1904-1907 Ostwald, Nernst and Haber studied the
  equilibrium
• relationship of this system
• 1909 Haber built a 80 g/h small pilot using Os as
  the catalyst
• 1911 Haber -Bosch process (BASF) using iron as
  the catalyst
• 1913 30 tons /day commercial plant was built at
  Oppau, Germany
• 1918 Haber won the Nobel Prize

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Important criteria of the catalyst
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• high and stable activity
• high and stable selectivity
• controlled surface area and porosity
• resistance to poisons
• resistance to high temperatures and temperature
  fluctuations.
• high mechanical strength
• no uncontrollable hazards
• The parameters to be considered after the
  catalyst preparation
• Whether the catalyst should be supported or
  unsupported ?
• The shape of the catalyst pellets.
• The size of the catalyst pellets.

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Ammonia synthesis Reaction set up
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1N2(g)
3H2(g)
Compressor
Unreacted N2 and H2
Catalyst
Condenser
NH3
Schematic representation
Storage
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Ammonia synthesis ( Before 50s)
Catalyst container
Tube sheet
Heat exchange tubes
Catalyst support
Haber's ammonia production apparatus
Tube sheet
Haber-Bosch reactor
Paul C.J.Kamer and Gadi Rothenberg,Course on
Theory and Applications,January
2002 http//www.elmhurst.edu/chm/onlcourse/chm110
/outlines/topic5.html
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Ammonia synthesis(After 50s)
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A Steam reformingB High temperature water-gas
shiftC Low temperature water-gas shiftD CO2
absorptionE MethanationF Ammonia synthesisG
NH3 separation.
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Ammonia synthesis - reactants
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• Steam reforming
• CH4(g) H2O(g) ? CO (g) 3 H2(g)
• 15-40 NiO/low SiO2/Al2O3 catalyst (760-816 ?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)

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Hydrogenation of CO
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Hydrogenation of CO is thermodynamically
favorable
CO3H2 ? CH4 H2O ( ?G (298 K) -140 kJ/mol)
Eg Methanation catalyzed by nickel reported
by Sabatier and Senderens in 1902
Fischer and Tropsch process Products Linear
alkenes and alkanes (as well as some
oxygenates) 200300C and atmospheric pressure
over Co or Fe catalysts
nCO (2n1)H2 ? CnH2n2 n H2O 2nCO(n1)H2 ?
CnH2n2 n CO2
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Ammonia Synthesis
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Fe/K catalyst exothermic
Possible reversible pathway
NH3
M
2 NH3
N2 3H2
H
H
H
N
()
NH2(ads) H(ads) ? NH(ads) 2H(ads) ? N(ads)
3H(ads)

M
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NH3 Synthesis Catalyst

• A typical composition of an industrial
  ammonia-synthesis catalyst
• 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
• Porosity 40-50

Use of Fe represents a compromise of
(i) Surface nitride formation (ii) Permit rapid
desorption of NH3
Poisons O2 , S , As , P , Cl2 etc.
CaO , SiO2 , MgO - act as structural promoter
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Potential energy curves for dissociative
adsorption of N2 on an iron (100)
Effect of Potassium
K increases intrinsic activity ( increasing
desorption rate) Potassium increases the binding
energy of the molecular N2 precursor on Fe and
thereby assists the formation of atomic N. K
acts as an electron donor enhancing N2
reduction. K on Fe surface prevents the S
poisoning on Fe K enhances the reduction through
an amide intermediate formation Promoter
oxides of aluminum (3) and potassium (1)
prevents sintering
2N
E
a
b
Potential energy
A
Potential energy
949
13
B
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Distance between N2 and surface
Distance between N2 and surface
230
Curve a N2 Fe(100) Curve b N2 K/Fe(100)
Christmann J.K. and G. Ertl J. Mole. Catal., 25
(1-3 ) (1984) 31-49
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Specific surface of a Fe-Al2O3 catalyst as a
function of the amount of Al2O3
Al3 dissolves into Fe lattice (result is a
higher surface area )
Surface area (m2/g)
Wt of Al2O3
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Ammonia synthesis since 1913
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World Ammonia production
150
World
100
Million tones/ year
50
Western Europe
0
1900
1900
1913
1950
1970
1987
2000
Year
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Classification of the metals and semi-metals
according to the chemical
reactivity of their surfaces
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
J.Chatt, General L.M. da Camara Pina and
R.L.Richards, New trends in the chemistry of
Nitrogen fixation ( Academic press) London,
(1980) Chapter 1.
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Chemisorption of gases by metals
3 Unactivated adsorption, 2 activated
adsorption, 1 activated adsorption at lower
temp., 0 No adsorption ? unknown
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Langmuir-Hinshelwood model for bimolecular
reaction
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Kinetics of NH3 synthesis
The basis of the commercial synthesis of ammonia
rests upon an efficient catalyst to speed up the
reaction rate to an economical degree.

• Langmuir-Hinshelwood reaction mechanism with
  surface reaction

A (g) ? A (Ads) B (g) ? B (Ads) A (g) B (
ads) AB (Ads) AB
(g)
rds
fast
Rate k QA QB
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LH model for bimolecular reaction
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Diagnosis of mechanism
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A detailed mechanism for the catalysis leading to
ammonia is as follows
N2 2Fe 2 Fe - N ads
H2 2Fe 2 Fe - H ads
N ads H ads NH ads
NH ads H ads NH2 ads
NH2 ads H ads NH3 ads
NH3 ads NH3 desorb
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Potential energy diagram for ammonia synthesis


Un catalyzed
Potential Energy
catalyzed by an iron surface
Ertl.G,Catal.Rev.-Sci. Eng.,21 (2) (1980), 201-223
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LeChateliers Principle
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• If a system at equilibrium is disturbed,
• the system will adjust so that the change is
  partially offset.

If a reactant is added, more products will be
formed. If a product is added, more reactants
will be formed
Reactions of Gases
If the volume of the container is decreased, the
reaction shifts towards the side of the reaction
that has fewer molecules.
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LeChateliers Principle - Continued
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But, changing the volume of the container has no
effect on the equilibrium if n gas 0.
CO H2O CO2 H2 H2 I2
2HI
Adding a non-reacting gas (He, Ar) has no effect
because doing so has no effect on the partial
pressures of the reacting gases. Raising the
temperature favors the direction of the reaction
that is endothermic.
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The rate is dependent on surface structure
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Atoms prefer high coordination
Strongin D.R., J. Carrazza, S.R Bare and G.A.
Somorjai, J. Catal. 103 (1987) 213
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Case Study Synthesis of NH3

• Haber Process N2 3H2
  2NH3
• Temperature should be reasonably high enough for
  N2 dissociation.
• Increase in pressure leads to ammonia formation
• ? n lt 0 , hence addition of inert gas will reduce
  ammonia formation.
• Run at high pressures of N2 and H2.
• Remove NH3 as it is formed.
• Use a catalyst which is capable of dissociating
  N2 and H2

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Yield of ammonia
Ru catalyst based ammonia synthesis
Mole percent
Temperature (?C)
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Effect of pressure, Temperature and Inert gas on
equilibrium NH3 concentration
Inlet H/N 31
NH3 at equilibrium mol
Inlet H/N 31 With 7 CH4 and 3 Ar
Pressure MPa
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Ammonia synthesis over Ru
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Ru(0001)
step
Rod, Logadottir, Nørskov, J.Chem.Phys., 112
(2000) 5343
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Principle of Sabatier
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When different metals are used to catalyze the
same reaction, it is generally observed that the
reaction rate can be correlated with the position
of the metal in the periodic table
Variation of rates on ammonia synthesis
NH3 activity/ arbitrary units
A volcano curve
d-band occupancy/
Ozaki, A. and K. Aika, Catalytic Activation of
Dinitrogen, in Catalysis Science and Technology,
J.R. Anderson and M. Boudart, Editors. 1981,
Springer Verlag New York. p. 87-158.
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Calculated ammonia synthesis rates400 ?C, 50
bar, H2N231, 5 NH3
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Jacobsen etal., J. Catal. 205, (2002) 382-387
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The Brønsted-Evans-Polanyi relation
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Logatottir, Rod, Nørskov, Hammer, Dahl and
Jacobsen, J. Catal. 197 (2001) 229
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Interpolation in the periodic table
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Jacobsen, Dahl, Clausen, Bahn, Logadottir,
Nørskov, J.Am.Chem.Soc. 123 (2001) 8404.
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Measured ammonia synthesis rates 400 ?C, 50 bar,
H2N231
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Jacobsen, Dahl, Clausen, Bahn, Logadottir,
Nørskov, J.Am.Chem.Soc. 123 (2001) 8404.
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Lessons from biology
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• Catalysis at ambient temperature and pressure
• Extreme selectivity
• Direct coupling of energy into the important
  reaction coordinate (non-thermal catalysis)
• Highly specific
• Polynuclear cluster complexes

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• Biological nitrogen fixation is being done by
• Free living Nitrogen fixing bacteria
• Eg Clostridium pasteurianum
• Klebsiella, a close relative of E. coli.
• photosynthetic bacteria, e.g.
  Rhodobacter
• Azotobacter

Since nitrogenase is inactivated by O2, the
fixation of N2 must occur under conditions which
are anaerobic

• Symbiotic nitrogen fixing bacteria Symbiotic
  bacteria are protected
• from oxygen by inhabiting a plant host.
• Eg Rhizobium and Bradyrhizobium inhabit the
  root nodules of leguminous plants
  (e.g. peas, beans, clover, alfalfa, Soya beans)

Anabaena azollae - used to enrich rice paddies
with organic nitrogen
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Enzyme responsible for N2 fixation is nitrogenase
Nitrogenase
Fe protein ? ? ? MoFe protein
Electrons
Fe4S4-type Fe/S cluster between two units of a
homodimer.
P cluster (Fe8S7)
FeMo-cofactor
(R-homocitrate-MoFe7S9)
Dimitri Coucouvanis etal. J. Am. Chem. Soc. 124
(2) (2002) 216
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Nitrogenase
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nitrogenase
ATP
complex formation
Fe protein

4Fe-4S cluster
MoFe protein
P-cluster
nucleotide replacement
ATP cleavage electron transfer
FeMo cofactor
Fe protein
reduction

complex dissociation
Burgess, Lowe, Chem. Rev. 96, 2983
(1996) Schindelin, Kisker, Schlessman, Howard,
Rees, Nature 387, 370 (1997)
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Fe Protein cycle
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E
MoFe protein
Fe protein
ATP
1)
4Fe-4S cluster
FeMoco
P-cluster
2)
E
3)
ADP
E
4)
Spee, Arendsen, Wassnik, Marrit, Hagen, Haaker,
FEBS Lett. 432, (1998) 55
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Nitrogenase N2 fixing mechanism
Three types Mo- Fe,
V-Fe, Fe- Fe
http//www.science.siu.edu/microbiology/micr425/42
5Notes/12-NitrFix.html
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Biological N2 fixation Vs Haber process
2NH3H216MgADP16Pi
N2 8 H 8 e- 16 MgATP
?H ? -46.2kJmol-1 ?S ? -99Jmol-1/K-1
Fe or Ru catalyst 100-300atm 400-500 ?C
N2(g) 3H2(g)
2NH3
N2 3H2 2NH3 ?G? -8
kcal/mole
N2 H2
N2H2 ?G 50 kcal/mole ( 33.44 kJ/ mol )




(approximately) N2 2e- 2H
N2H2 Eo 1200 mV (approximately) ?G
231.6 kJ/ mole
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N2 hydrogenation on FeMoco
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Rod and Nørskov, J.Am.Chem.Soc.,122 (2000) 12751
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Comparing the FeMoco and Ru (0001)
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Rod, Logadottir, Nørskov, J.Chem.Phys., 112
(2000) 5343
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Turning point ?
Structure of the FeMo cofactor including an
interstitial nitride
Interstitial six coordinate N2 atom occupies the
cavity bridging the six coordinatively
unsaturated Fe atoms which are now understood as
tetra coordinate ESEEMS/ENDOR Study established
that the central atom does not exchange with
dinitrogen derived atoms during catalysis
Mackay B.A. and M.D. Fryzuk, Chem. Rev., 104
(2004) 385-401
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Synthetic analogue approach to
Metallobiomolecule active sites
Spectroscopy Magnetism XAS(EXAFS,XANES) Crystallog
raphy
Types of metal sites
Physico chemical investigations
Site analogue Composition, Stereochemistry
Deduction, Formulation
Oxidation states, electronic features, Ligand
binding, Stereo chemistry
Stoichiometric/ catalytic action
Synthetic analogue
Convergent
structural
Functional
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Dinitrogen bonding modes in monometallic and
bimetallic Complexes Activation schemes
Favourable binding modes for N2 FeMo cofactor
given an interstitial nitride
Weak activation
Strong activation
End-on mono metallic
End on Bimetallic
Side on Bimetallic
Side-on End on Bimetallic
Venkateswara Rao P and R.H. Holm Chem.Rev., 104
(2004) 385-401
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Controversies in Nitrogen fixation
Hidai process
Chatt mechanism
N2
H2
Fryzuk mechanism
N2
H2
Michael . d. Fryzuk, Nature 427 ( 2004 ) 498-499
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Chemical mechanism of N2 fixation in
biological and model
systems
J.Chatt, General L.M. da Camara Pina and
R.L.Richards, New trends in the chemistry of
Nitrogen fixation ( Academic press) London,
(1980) Chapter 1.
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Concluding remarks
Inspite of the enormous knowledge that have been
generated with respect to ammonia synthesis, it
appears that we are still at the learning stage ,
since we are yet to understand and replicate the
natures way of fixing nitrogen.