CHBE 452 Lecture 28Mechanism of Catalyst Action

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Last Time We Introduced Catalysis

Homogeneous catalysts

Acids and bases Metal salts Enzymes Radical initiators Solvents

Heterogeneous catalysts

Supported metals Metal oxides,

nitrides, sulfides Solid acids and

bases Polymer bound

species

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Increase rates by 1020 or more

Today: Mechanisms Of Catalyst Action

Catalysts can be designed to help initiate reactions.

Catalysts can be designed to stabilize the intermediates of a reaction.

Catalysts can be designed to hold the reactants in close proximity.

Catalysts can be designed to hold the reactants in the right configuration to react.

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Mechanism Of Catalysts, Continued

Catalysts can be designed to block side reactions.

Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break.

Catalysts can be designed to donate and accept electrons.

Catalysts can be designed to act as efficient means for energy transfer.

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Mechanism Of Catalysis Continued

Is is also important to realize that:

One needs a catalytic cycle to get reactions to happen.

Mass transfer limitations are more important when a catalyst is present.

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Today: The Role Of Catalysts In Initiating Reactions, Stabilizing

Intermediates Catalysts initiate reactions by help

creating active centers (i.e. a radical or ion). Active center could be catalyst itself Could be a radical R-O.

Catalysts stabilize intermediates. Catalyst binds to intermediate, lowering the

free energy of the reactive intermediates. Raises intermediate concentration. Intermediates less reactive.

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How Could Catalysts Change H2+Br22HBr

MechanismBr2→2Br

Br+H2→HBr+H

H+Br2→HBr+Br

2Br→Br2

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Fre

e en

ergy

, kca

l/mol

e of

bro

min

e at

oms

-50

0

50

1/2Br2

Br

H+HBr

Br+2HBr

1/2 Br2+2HBr

Reaction Progress

Modify Intrinsic Barriers

+H2

+Br2

-50

0

50

1/2Br2

Br

H+HBr

Br+2HBr

1/2 Br2+2HBr

Reaction Progress

Initiate Reaction

+H2

+Br2

-50

0

50

1/2Br2

Br

H+HBr

Br+2HBr

1/2 Br2+2HBr

Reaction Progress

Stabilize Intermediates

+H2

+Br2

-50

0

50

1/2Br2

Br

H+HBr

Br+2HBr

1/2 Br2+2HBr

Reaction Progress

Gas Phase

+H2

+Br2

B

D

A

C

Key Principles Of Catalytic Mechanisms

Catalysts bind intermediates at distinct sites.

Mechanism often the same in gas phase & on catalyst.

Initiation much faster – in effect do not need initiation reacion.

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Example Of Catalysts Initiating Reactions

C2H6C2H4+H2

(12.41)

C2H6 2CH3

(12.42)

CH3+C2H6C2H5+CH4

(12.43)

C2H5C2H4+H

(12.44)

H+C2H6C2H5+H2

(12.45)

2CH3C2H6

(12.46)

2c2H5C4H10

(12.47)

CH3C2H5 C3H8

(12.48)

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Gas phase mechanism

Catalysis By NO2

 

Consider adding NO2

(12.50) Catalysts can initiate reactions. The mechanisms

are similar to the mechanisms without a catalyst, but the initiation process is much faster with the catalyst.

 Effect 109 (small for catalysis)

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NO2 C2H6 C2H5 HNO2

Some Examples Of Reactions Initiated By Catalysts

Reaction Catalyst Mechanism of Initiation

CH3 CH3 C2H4 +H2 NO2 NO2 + CH3CH3 HNO2 + CH3CH2

X+I2 2I+X

I + CH3COH HI + CH3CO

ROOR 2RO

RO+CH2=CH2ROCH2CH2

H2 + Br2 2HBr metalic platinum

Br2 + 2S 2Brad

Propylene

PolypropyleneTi+ Tl

+ + propylene CH3CHTiCH2+

C2H5OH + H+ [C2H5OH2]+

[C2H5OH3]+ [C2H5]

+ + H2O

[C2H5]+ C2H4 + H+

2O3 O2 Cl O3 + Cl O2 + ClO

C2H5OH C2H4 + H2O H+

CH3COH CH4 + CO I2

Ethylene polyethylene ROOR

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Initiation Often Not Enough. Also Need To Stabilize Intermediates

12

-50

0

50

1/2Br2

Ent

halp

y, k

cal/m

ole

of b

rom

ine

atom

s

Br

H+HBr

Br+2HBr

1/2 Br2+2HBr

Reaction Progress

Gas Phase

+H2

+Br2

-50

0

50

+Br2

Ent

halp

y, k

cal/m

ole

of b

rom

ine

atom

s

Br(ad)

H(ad)+HBr

Br(ad)+2HBr

1/2 Br2+2HBr

Reaction Progress

1/2Br2

Surface

+H2

Termination

Figure 12.7 The enthalpy changes during the gas phase reaction H2 + Br2 2 HBr assuming that the reaction terminates after one cycle

Figure 12.8 The enthalpy changes during the Rideal-Eley surface reaction H2 + Br2 2 HBr on Pt(111) assuming that the reaction terminates after one cycle

Stabilization Of Ionic Intermediates

R R RRHC C H C CHH

(12.63)

Possible gas phase mechanism

X + HC C H HC CH XR R R R (12.64)

R R R R R HC C H X 2HCC H X

(12.65)

R R2HCCH X 2C CH H X

(12.66)

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Note big barrier to first step

Acid Catalyzed Reaction

R R R RHC C H H HC C HH (12.67)

R R RRHC C HH HC CHH (12.68)

RR RRHC CHH C CHH H (12.69)

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Stabilization Of Intermediates. Can We Have Too Much of a Good Thing?

When we stabilize intermediates we increase the intermediate concentration. We also decrease the reactivity of the intermediates.

Which wins?

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Experimental Evidence HCOOHH2+CO2

Heat Of Formation Of Formate

Tem

pera

ture

For

50

% C

onve

rsio

n

Au

Ag

Pt

Pd

Ir

Rh

Ru

Cu

Co

Ni Fe

W

50 60 70 80 90 100 110 120

350

400

450

500

550

600

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HCOOH HCOO(ad) Had

H(ad) HCOOad CO2 H2

(12.75)

Sabatier’s Principle

The best catalysts are substances which bind the reactants strongly, but not too strongly.

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Consider H2+Br2HBr

Rideal Eley mechanism Complicated derivation (see text)

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Br2 2S 2BradBrad H2 HBr

Had Br2 HBr Br

Had

(12.76)

rHBr 2 kBrexp p,2 Had / BT [H2][S][Br2]12 ( )1

kBr 2k20 Ea,2

0p,2 HO T Sad BT

exp /

(12.85)

(12.86)

For Unlimited Sites

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-40 -20 0 20 40 60 801E+6

1E+11

1E+16

1E+21

1E+26

1E+31

Heat Of Formation Of Intermediate, Kcal/mole

2R

ate,

Mol

ecul

es/C

m

Sec

Figure 12.10 The rate of HBr formation as calculated from Equation (12.85), with [S] = 1e14/cm2 and p= 0.5, T = 500K, .P P atmH Br2 2

1

For Finite Number Of Sites

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[S]So

1 KBr2 PBr2 KH2 PH2

Heat Of Formation Of Intermediate, Kcal/mole

2R

ate,

Mol

ecul

es/C

m

Sec

-40 -20 0 20 40 60 801E+0

1E+2

1E+4

1E+6

1E+8

1E+10

1E+12

1E+14

1E+16

Figure 12.11 The rate of HBr formation calculated from Equation (12.85), with [S] from Equation (12.87) and p= 0.5, T = 500K, .P P atmH Br2 2

1

(12.87)

Common Plots In Literature

Sachtler-Frahenfort plots:

Use heat of oxidation per mole of oxygen as surrogate for heat of formation of product.

Tanaka-Tamaru plots:

Use heat of oxidation per mole of metal as surrogate for heat of formation of product.

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Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru

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Pt

Pd Ir Rh

Ru

Cu

Co

Ni

Fe

W Ta

-300-250

-200-150

-100-50

01E+10

1E+12

1E+14

1E+16

1E+18

1E+20

Heat of formation of oxideper mole of metal, Kcal/mol

Pt Ir Rh

Ru

Cu

Co

Ni

Fe

W Ta

-110-100

-90-80

-70-60

-50-40

-30-20

-100

1E+10

1E+12

1E+14

1E+16

1E+18

1E+20

Heat of formation of oxideper mole of oxygen, Kcal/mol

Tanaka-TamaruSachtler-Frahrenfort

Pd

Rat

e, M

olec

ules

/cm

/se

c2

Rat

e, M

olec

ules

/cm

/se

c2

Figure 12.13 A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.

Summary So Far

Catalysts work by initiating reactions, stabilizing intermediates.

Leads to 1020 increase in rates – need other effects to get to 1040.

Can stabilize too much.

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Example: Constructing Sachtler-Frauhenfort And Tanaka-Tamaru

Plots

Table 12.E.1 some data for the rate of ethylene hydrogenation on a number of metals 0 C. Next lets construct a Sachtler-Fahrenfort plot of the data.

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Table 12.E.1 The spreadsheet for example 12.E

A B C D E F G 4 Metal rate Hf Of

Oxide Oxygens in oxide

Metal atoms

in oxide

Hf per mole of oxide

Hf per mole of metal

5 Pt 1.0E+16 -9.7 1 1 =$C5/D5 =$C5/E5 6 Pd 3.0E+16 -20.4 1 1 =$C6/D6 =$C6/E6 7 Ir 3.0E+16 -40.1 2 1 =$C7/D7 =$C7/E7 8 Rh 3.0E+17 -21.7 1 1 =$C8/D8 =$C8/E8 9 Ru 9.0E+18 -52.5 2 1 =$C9/D9 =$C9/E9

10 Cu 8.3E+10 -39.8 1 2 =$C10/D10 11 Cu 8.3E+10 -37.1 1 1 =$C11/E11 12 Co 3.0E+16 -57.2 1 1 =$C12/D12 =$C12/E12 13 Ni 1.0E+14 -58.4 1 1 =$C13/D13 =$C13/E13 14 Fe 1.0E+16 -197.5 3 2 =$C14/D14 =$C14/E14 15 Fe 1.0E+16 -63.7 1 1 16 W 3.0E+13 -136 2 1 =$C16/D16 =$C16/E16 17 Ta 3.0E+13 -499.9 5 2 =$C17/D17 =$C17/E17

Solution

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A B C D E F G 4 Metal rate Hf Of

Oxide Oxygens in oxide

Metal atoms

in oxide

Hf per mole of oxide

Hf per mole of metal

5 Pt 1.0E+16 -9.7 1 1 =$C5/D5 =$C5/E5 6 Pd 3.0E+16 -20.4 1 1 =$C6/D6 =$C6/E6 7 Ir 3.0E+16 -40.1 2 1 =$C7/D7 =$C7/E7 8 Rh 3.0E+17 -21.7 1 1 =$C8/D8 =$C8/E8 9 Ru 9.0E+18 -52.5 2 1 =$C9/D9 =$C9/E9

10 Cu 8.3E+10 -39.8 1 2 =$C10/D10 11 Cu 8.3E+10 -37.1 1 1 =$C11/E11 12 Co 3.0E+16 -57.2 1 1 =$C12/D12 =$C12/E12 13 Ni 1.0E+14 -58.4 1 1 =$C13/D13 =$C13/E13 14 Fe 1.0E+16 -197.5 3 2 =$C14/D14 =$C14/E14 15 Fe 1.0E+16 -63.7 1 1 16 W 3.0E+13 -136 2 1 =$C16/D16 =$C16/E16 17 Ta 3.0E+13 -499.9 5 2 =$C17/D17 =$C17/E17

Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru

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Pt

Pd Ir Rh

Ru

Cu

Co

Ni

Fe

W Ta

-300-250

-200-150

-100-50

01E+10

1E+12

1E+14

1E+16

1E+18

1E+20

Heat of formation of oxideper mole of metal, Kcal/mol

Pt Ir Rh

Ru

Cu

Co

Ni

Fe

W Ta

-110-100

-90-80

-70-60

-50-40

-30-20

-100

1E+10

1E+12

1E+14

1E+16

1E+18

1E+20

Heat of formation of oxideper mole of oxygen, Kcal/mol

Tanaka-TamaruSachtler-Frahrenfort

Pd

Rat

e, M

olec

ules

/cm

/se

c2

Rat

e, M

olec

ules

/cm

/se

c2

Figure 12.13 A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.

Stabilizing Intermediates Not Entire Effect

Leads to 1020 increases in rates – -need other effects to get to 1040

Does not lead to selectivity

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Summary

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Catalysts work by initiating reactions, stabilizing intermediates.

Leads to 1020 increase in rates – need other effects to get to 1040.

Can stabilize too much.

Other effects connect selectivity.

Query

What did you learn new in this lecture?

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