lecture ds 1c
TRANSCRIPT
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CatalysisA catalyst,
speeds up a reaction lowers Eaby providing an
alternative path for athermodynamically viable reaction
only small amounts are required
regenerated during the reaction often reaction specific
widely employed in industrial &laboratory syntheses >$900B USD in 2009 alone
Two Types Heterogeneous (different
phase)
Homogeneous (same phase)
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Number of Collisions
Ea(uncatalyzed )
Effective
collisions
(uncatalyzed)
Effective
collisions
(catalyzed)
Ea(catalyzed )
(a)(b)
Number
ofcollisions
withagivenenergy
Numbe
rofcollisions
withagivenenergy
Energy Energy
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Hydrogenation of EtheneCH2=CH2 + H2 CH3CH3
catalysed by Ni surface
Catalyst is in a different phaseto the reactants generally solid
eg. Pt
Steps involved
Adsorption & activation ofthe reactants
Migration of the adsorbedreactants on the surface
Reaction of the adsorbedsubstances
Escape, or desorption, ofthe products
Heterogeneous Catalysis
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AdsorptionTwo possibilities
Physisorption
van der Waals force holds adsorbate to surface
~20 kJ mol-1
adsorbate retains its identity Chemisorption
adsorbate is chemically bonded to the surface
~200 kJ mol-1
adsorbate may be torn apart
exothermic (G = H - TS)
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A2 Adsorption Energies
Not activated Activated
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Langmuir Adsorption Isotherm Limit to the amount of gas that can adsorb
assumed to be at monolayer coverage
adsorption unaffected by neighbour occupancy all sites equivalent
At equilibrium, evaporation rate = adsorption
rate
kdN = kaN(1-)pwhere is the fraction of surface covered, p is the gas
pressure and N is the number of sites
the Langmuir Adsorption Isotherm follows,
where K = ka/kd
Kp
Kp
pkk
pk
ad
a
+=
+=
1
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Rate of Adsorption
Large number of collisions with
surface per time, 1023 collisions cm-2 s-1 at STP
Small number of atoms on anatomically flat surface,
1015
atoms cm-2
Therefore each surface atom is hit108 times per second
Adsorption depends on success ofsticking, S
S = rate of adsorptionrate of collision
Activated process (adsorbate
in potential well)
First order process,Arrhenius form:
where Ea is the activation energyof desorption
Rate of Desorption
RTEaAek /=
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Catalytic Adsorption(1) Very little adsorption,
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eg. Hydrogenation of ethene
on Ni catalyst
H2 weakly adsorbed
Ethene strongly adsorbed
Rate determining step is
the adsorption of H2
Mechanism
Adsorbed molecule often
held by 2 adjacent sites,
can distort (activate)towards reaction
Different catalysts host
adsorbates in differentways,
activation of different
bonds
reaction may be
specific to a certain
catalyst
)(
)(
42
2HCp
Hprate
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H2O2 decomposition in H2Ocatalysed by MnO2
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Catalytic Converters
Engine CO(g) k1
CO(g) + O2(g) CO2(g) k2, slow, Ea~80 kJ/mol
Pt/Pd/Rd deposited as thin film onto a high surface area ceramic substrate
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Sulfuric Acid Contact ProcessMaking the sulfur dioxide
Either by burning sulfur in an excess of air
S(s) + O2(g) SO2(g) or by heating sulfide ores like pyrite in an excess of air
4FeS(s) + 7O2(g)
2Fe2O3(s) + 4SO2(g) In either case, an excess of air is used so that the SO2produced is already mixed withoxygen for the next stage
Converting the sulfur dioxide into sulfur trioxide
This is a reversible reaction, and the formation of the SO3 is exothermic2SO2(g) + O2(g) 2SO3(g) H = -196 kJ/mol
Converting the sulfur trioxide into sulfuric acid
Can't be done by simply adding water to the SO3
the reaction is so uncontrollable that it creates a fog of sulfuric acid. Instead, the SO3 is first dissolved in concentrated sulfuric acid
H2SO4(l) + SO3(g) H2S2O7(l) The product is known asfuming sulfuric acidoroleum
This can then be reacted safely with water to produce concentrated sulfuric acid - twice
as much as you originally used to make the fuming sulfuric acidH2S2O7(l) + H2O(l) 2H2SO4(l)
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2SO2(g) + O2(g) 2SO3(g)
The gases actually react with the surface of the catalyst,temporarily changing it
It is a good example of the ability of transition metals & theircompounds to act as catalysts because of their ability to change
their oxidation state Sulfur dioxide is oxidised to sulfur trioxide by thevanadium(V) oxide. In the process, the vanadium(V) oxide isreduced to vanadium(IV) oxide
SO2(g) + V2O5(s) SO3(g) + V2O4(s)
Vanadium(IV) oxide is then re-oxidised by the oxygen2V2O4(s) + O2(g) 2V2O5(s)
This is a good example of the way that a catalyst can bechanged during the course of a reaction. At the end of thereaction, though, it will be chemically the same as it started
Catalysed by V2O5
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Solution Phase ReactionsExample 1. Os(VIII) catalysis of Ce(IV)/As(III)
In acidic solution with excess Ce(IV),
v = k[Os(VIII)][As(III)]
Proposed mechanism,Os(VIII) + As(III) Os(VI) + As(V) r.d.s.
2Ce(IV) + Os(VI) Os(VIII) + 2Ce(III)
Overall,
2Ce(IV) + As(III) 2Ce(III) + As(V)
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Example 2. Fe2+
catalysis of S2O82-
+ 2I-
2SO42-
+ I2In aqueous solution this requires the collision of two anions,
S2O82- + I- SO4
- + SO42- + I slow (r.d.s)
SO4
- + I- SO4
2- + I fast
2I I2 fast
has the following rate law,
Small amounts of Fe2+ or Cu2+ catalyse the otherwise electrostaticallyopposed reaction,
S2
O8
2- + Fe2+ SO4
- + SO4
2- + Fe3+
SO4- + Fe2+ SO4
2- + Fe3+
2Fe3+ + 2I- 2Fe2+ + I2
]].[[
][
2
][ 2
82
2
== IOSdt
Id
dt
Id
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Acid-Base Catalysis Common
Catalyst is either a Brnsted acid or base Both an acid & a base must be present
Water often plays a key role (acid or base)
H3O+ abbreviated to H+
Each acid-base species contributes to k:
k = k0 + kH+[H+] + kOH-[OH-] + kHA[HA] + kA-[A-]
for a weak acid catalyst, HA
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General Acid-Base Catalysis Reaction between substrate, S & acid, HA:
HA + S SH+ + A- kf& kbSH+ + H2O products k2
Rate,
Steady state approximation,
d[SH+ ]
dt= kf[HA][S] kb[SH
+][A
] k2[SH
+] = 0
d[P]dt
= k2 [SH+ ]
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Rearranging,
Two limiting cases:
(a) k2 >> kb[A-]
ie. general acid catalysis
[SH+ ] =kf[HA][S]
kb [A ] + k2
[SH+
] =k
f
[HA][S]
k2
d[P]
dt
[HA]
(b) k2
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Enzyme Catalysis
Enzyme Characteristics
Proteins with mw 104-106
Several orders of magnitude more effective than other catalysts, effective at low [enzyme] under mild conditions:
[enzyme] ~ 10-8-10-10 mol/L compared with [substrate] ~ 10-6 mol/L
Can be extracted & used under laboratory or industrial conditions
Denature at high T & extreme pH
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Enzyme Kinetics
Rate often [enzyme]
at low [substrate], rate [substrate]
Michaelis-Menten mechanism:
E + S ES kf& kbES E + products k2
steady state:d[ES]
dt= kf[E][S] kb [ES] k2 [ES]= 0
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Kinetic Pathway
Energ
y
Progress of reaction
Activation
energy
Activation
energy
S E + SES
E + PP
Progress of reaction
Energ
y
A B
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Total enzyme concentration,
[E]0 = [ES] + [E]
substituting in,
Now reaction rate,
Michaelis constant:
[ES] =k
f
[E]0
[S]
kb + k2 + kf[S]
v = k2[ES] =
k2kf[E]0[S]
kb + k2 + kf[S]
=k2[E]0[S]
Km + [S]Km =
kb + k2kf
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Rearranging,
linear plot 1/v vs. 1/[S]
Turnover number
k2 = vmax/[E]0
The number of substratemolecules converted per
active site per unit time.
1
v=
1
vmax+
Km
vmax[S]
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E + I EI
Necessary adjustments,
[E]0 = [ES] + [E] + [EI]
k2
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therefore,
which rearranges to,
or,
[E]0 = [ES] + Km[ES][S]
+ Km[ES][I]KI[S]
[ES] =[S][E]0
[S] + Km 1 +
[I]
KI
then,
v = k2[ES]= k2[S][E]0
[S]+ Km 1+[I]
KI
1
v=
1
vmax
+
1+ [I]KI
Km
vmax
[S]
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Noncompetitive Inhibition
ES + I IES
Necessary adjustments,
[E]0
= [ES] + [E] + [IE] + [IES]
with only ES giving rise to products
KI & Km as before
Assume KI = KI*because the binding sites aredistant
KI* =
[ES][I]
[IES]
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so,
or,
[E]0 = [ES] + Km[ES][S] +Km[ES][I]
KI[S]+ [ES][I]KI
=
[ES]
[S] [S]+ Km +
Km[I]
KI +
[S][I]
KI
=[ES]
[S]
1 +[I]
KI
[S]+ Km( )
thus,
v = k2 [ES]=k
2
[S][E]0
1+[I]
KI
[S]+ Km( )
1
v=
1+[I]
KI
vmax+
1+[I]
KI
Km
vmax[S]
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Inhibition?? Plot 1/v vs 1/[S]
1/v
1/vmax
Slope= Km/vmax
Noncompetitive
inhibition CompetitiveinhibitionSlope = (Km/vmax).(1+[I]/KI)
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Application: Biofuel Production
Barnard, Casanueva, Tuffin, & Cowan, 'Extremophiles in biofuel synthesis',Environmental Technology, 31:8, 871 888 (2010)
From 2006 to 2008, total biofuel consumption in the EU increased from 5625
Mtoe (million tons of oil equivalent) to 10064 Mtoe
>90%
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Example: Bioethanol Production
http://biofuelsandclimate.wordpress.com/background-info/
E l Bi di l P d i
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Example: Biodiesel Production
Biodiesel is a mixture of methyl esters of long-chain fatty acids
Biodiesel is a better lubricant than petrodiesel, but is currently disadvantaged by higher viscosity,
lower energy content, higher NOx emissions, lower engine compatibility, speed & power, $
Full life-cycle analysis must be considered for production to be viable
Production Chemistry
Oil or fat reacts in a transesterification reactions with (m)ethanol in the presence of a strong base catalyst
(KOH, NaOH) to yield biodiesel: (m)ethyl esters and glycerol
The triglyceride may be an edible oil or an inedible oil such as Jatropha oil in which e.g. the Indian government is
investing in huge plantations in wasteland regions
Disadvantage is treatment of alkaline wastewater & removel of glycerol Alternatively lipase can catalyse the biodiesel production although currently not commercially competitive
Milder conditions (pH, T)
Slower rate
Lower yield
Lipase is currently too expensive
Demirbas, Progress and recent trends in biodiesel fuels',Energy Conversion & Management, 50, 14-34 (2009).
Kumari, Mahapatra, Garlapati & Banerjee, Enzymatic transesterification of Jatropha oil,Biotechnology for Biofuels, 2:1 (2009).