Lecture Ds 1c

8/12/2019 Lecture Ds 1c

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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).

Lecture Ds 1c

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