Catalysis Speeding up the approach to equilibrium.
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Transcript of Catalysis Speeding up the approach to equilibrium.
Catalysis
Speeding up the approach to equilibrium
History• Kirchoff in 1814 noted that acids aid hydrolysis of starch to glucose• Faraday (and Davy) studied oxidation catalysts in the 1820’s• Catalyst defined by Berzelius in 1836
A compound, which increases the rate of a chemical reaction, but which is not consumed by the reaction
• Deacon, Messel, Mond, Ostwald, Sebatier processes (HCl, SO2 oxidation, water gas shift, ammonia oxidation, ethene hydrogenation)
• 20th C: ammonia production, cracking reactions, hydrocarbon production, catalytic converters etc.
• Catalysis science developed by Langmuir, Emmett, Rideal and others.
http://dept.chem.polimi.it/~citterio/SilsisMI/Introduction.pdf
Catalysis
When we consider a catalytic reaction, we may imagine that the reaction mechanism consists of many different steps. Catalyst must be a reactant in one of the first steps in the mechanism and a product in one of the last steps.
Heterogeneous catalysis
Chemisorption and catalysis Diffusion of reactants Adsorption Surface diffusion Reaction Desorption Diffusion of products
2 main mechanisms
• Langmuir-Hinshelwood
Reaction between adsorbates
• Eley-Rideal
Reaction between adsorbate and incoming molecule
LH model for unimolecular reaction
B(ads)A (g)A Decomposition occurs uniformly across the surface. Products are weakly bound and rapidly desorbed. The rate determining step (rds) is the surface decomposition step.
Rate = k A
For Langmuir adsorption
p
pk
K1
KRate
pA
fastfast
RDS
khet
A B
LH model for unimolecular reaction
Two limiting cases
High pressures/
Strong binding
Kp>>1
Rate ≈ k
Rate independent of gas pressure
Zero order reaction
Surface coverage almost unity
Low pressures/
Weak binding
Kp<<1
Rate ≈ kKp
Rate linearly dependent on gas pressure
First order reaction
Surface coverage very low
LH model for bimolecular reaction
Langmuir-Hinshelwood reaction with surface reaction as rds
(g) AB (ads) AB (ads) B (ads)A
(ads) B (g) B
(ads)A (g)A
fast
rds
Rate = k AB
pA
fastfastRDS
khet
A
AB
B
pB
Langmuir adsorption of mixed components
BdB
BABaB
AdA
BAAaA
surface
d
a
surfaceg
surface
d
a
surfaceg
kdt
d
pkdt
d
kdt
d
pkdt
d
SBk
kSB
SAk
kSA
rate desorption
)1(B of rate adsorption
rate desorption
)1(A of rate adsorption
Langmuir adsorption of mixed components
Bd
BaSBB
BBdSBBa
Ad
AaSAA
AAdSAAa
k
kp
kpk
k
kp
kpk
BB
AA
K ,K
K ,K
mequilibriuAt
1
1
BAS
BAS
Langmuir adsorption of mixed components
BBAA
BBB
BBAA
AAA
BBAAS
BBAAS
SBBSAAS
pKpK
pK
pKpK
pK
pKpK
pKpK
pKpK
1
1
1
1
11
1
LH model for bimolecular reaction
Rate = k AB
21Rate
BBAA
BBAA
pKpK
pKpkK
LH model for bimolecular reaction
pA
rate
For constant PB
Rate limited bysurface concentration of A
Rate limited bysurface concentration of B
Eley-Ridealbimolecular surface reactions
pA
fast
RDS
khet
AAB
BpBAn adsorbed molecule may
react directly with an impinging gas molecule by a
collisional mechanism
Eley-Ridealbimolecular surface reactions
rate = k pB k KApA pB / (1+KApA)
= 1
pA
rate
For constant PB
Low pressureWeak binding
KApA << 1
rate = khet KA pA pB …….. first order in A
kexp
High pressureStrong binding
KApA >> 1
rate = k pB …….. zero order in A
kexp
Note: For constant pA, the rate is always first order
wrt pB
Diagnosis of mechanism
If we measure the reaction rate as a function of the coverage by A, the rate will initially increase for both mechanisms. Eley-Rideal: rate increases until surface is covered by A. Langmuir-Hinshelwood: rate passes a maximum and ends up at zero, when surface covered by A.
B + S B-Scannot proceed when A blocks all sites.
Transition State Model of Catalyst Activity
reactants
products
Ehom
pote
ntia
l ene
rgy
Eads
Edes
Ehet
reaction co-ordinate
Langmuir-Hinshelwood KineticsAdsorption of reactants and desorption of products
are very fast. Eads and Edes very small.
Surface Reaction is RDS: Ehet
transition state#hom
adsorbed reactants
adsorbed products
#het
Principle of SabatierWhen different metals are used to catalyse the same reaction, it is generally observed that the reaction rate can be correlated with the position of the metal in the periodic table:
A “volcano” curve
Catalyst PreparationFor a catalyst the desired properties are
• high and stable activity • high and stable selectivity • controlled surface area and porosity • good resistance to poisons • good resistance to high temperatures and temperature fluctuations. • high mechanical strength • no uncontrollable hazards
Once a catalyst system has been identified, the parameters in the manufacture of the catalyst are
• If the catalyst should be supported or unsupported. • The shape of the catalyst pellets. The shape (cylinders, rings, spheres,
monoliths) influence the void fraction, the flow and diffusion phenomena and the mechanical strength.
• The size of the catalyst pellets. For a given shape the size influences only the flow and diffusion phenomena, but small pellets are often much easier to prepare.
• Catalyst based on oxides are usually activated by reduction in H2 in the reactor.
Case studies
• Ammonia synthesis (Haber-Bosch)• Hydrogenation of CO (Fischer-Tropsch)
http://www.uyseg.org/greener_industry/index.htm
Ammonia synthesis
A: Steam reformingB: High temperature water-gas shiftC: Low temperature water-gas shiftD: CO2 absorptionE: MethanationF: Ammonia synthesisG: NH3 separation.
Ammonia reactantsSteam 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 shiftCO(g) + H2O(g) CO2(g) + H2(g)
Cr2O3 and Fe2O3 as catalystcarbon dioxide removed by passing through sodium hydroxide.
CO2(g) + 2 OH-(aq) CO32-(aq) + H2O(l)
Ammonia Synthesis
Fe/K catalyst
exothermic
Mechanism
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*Step 2 is generally rate-limiting. Volcano curve is therefore apparent with d-block metals as catalysts.
Ru and Os are more active catalysts, but iron is used.
Hydrogenation of COHydrogenation of CO is thermodynamically favourable; the first example, methanation catalysed by nickel was reported by Sabatier and Senderens in 1902
CO+3H2CH4+H2O ( G298, -140 kJ/mol)In their classic 1926 papers Fischer and Tropsch showed that linear alkenes and alkanes (as well as some oxygenates) are formed at 200–300°C and atmospheric pressure over Co or Fe catalysts
nCO+(2n+1)H2CnH2n+2+nH2O
2nCO+(n+1)H2=CnH2n+2+nCO2
Since syngas (CO + H2) is readily available from a variety of fossil fuels, including coal, the Fischer–Tropsch process became industrially important for economies which had good supplies of cheap coal but which lacked oil
Fischer-Tropsch
Iron catalysts give mainly linear alkenes and oxygenates, while cobalt gives mostly linear alkanes. Ruthenium, one of the most active catalysts but one which, owing to its expense is little used industrially, can give high molecular weight hydrocarbons; rhodium catalysts make significant amounts of oxygenates in addition to hydrocarbons, while nickel gives mainly methane. Catalyst can be immobilised on Kieselguhr (diatomaceous silicate earth), alumina, active carbon, clays and zeolites.
FT mechanism
adsorption and cleavage of CO and the stepwise hydrogenation of surface carbide giving methylene and other species
Maitlis, P. M.; Quyoum, R.; Long, H. C.; Turner, M. L. Appl. Catal. A: General 1999, 186, 363-374. Towards a Chemical Understanding of the Fischer-Tropsch Reaction: Alkene Formation
Other mechanisms?
• Boudouard reaction. Important in methanation (over Nickel).
2CO C + CO2
Some evidence that hydrogenation of adsorbed carbon leads to formation of hydrocarbons.
Also an important side (undesired) reaction in some hydrocarbon conversion reactions (coking)