Lecture 23 Thursday 4/1/08

OCNB ExplosionNoble Prize 2007Catalytic Steps

pWWpBBpAA

rxA

CNCNCN

DHVrTTUA

dt

dT

00

0 ))(()(

p

rg

NC

QQ

dt

dT

9:55t = 0

10:40 10:50 midnight 12:18

Isothermal Operation

Cooling RestoredQr = 0

200

175

400

Tem

pera

ture

o C

Temperature-time trajectory

fuse

Catalysts and Catalysis

A Catalyst is a substance that affects the rate of chemical reaction but emerges from the process unchanged.

Catalysis is the occurrence, study, and use of catalysts and catalytic processes.

Approximately 1/3 of the GNP of materials produced in the U.S. involves a catalytic process.

Catalysts affect both Selectivity and Yeild

Steps in a Catalytic Reaction

Active Site

• Reactions are not catalyzed over the entire surface but only at certain active sites or centers that result from unsaturated atoms in the surface.

• An active site is a point on the surface that can form strong chemical bonds with an adsorbed atom or molecule.

The Adsorption Step

The Surface Reaction Step

The Rate Limiting Step:Which Step Has the Largest

Resistance?

Octane Numbers

Catalytic Reforming

Normal Pentane Octane Number = 62

Iso-Pentane Octane Number = 95

Intro to Catalysis

Reforming catalyst –

Algorithm

1) Postulate a mechanism

2) Postulate a rate-limiting step

3) Express Ci·s in terms of partial pressures

4) Site balance (Total number of sites constant)

5) Derive rate law

6) Evaluate parameters

1252105

105105

2105125

C-cC

step) (slow CC-n

CC-n

pentane-c pentane-n

52

HHHc

HcH

HHnH

Pt

OAl

Pt

Pt (sputtered on)

Al2O5 (scaffold)

CV=concentration of vacant sites

Postulate a mechanism: (single site)

In 75% of heterogeneous catalysis, surface reaction is the slow step

SISI

SISN

SNSN

VIISID

DIVISIDDN

SSISNSSN

NANNSNVVNADN

SNNVNNN

CPkCk

kCPCkrr

kCCkrr

kkkkCCPkr

CkCPkr

/

/

/ /

'

'

SISI

SISN

SNSN

VIISIDD

SSISNSS

NSNVNNAD

CPKCkr

kCCkr

kCCPkr

/

/

VIISIVIISIDD

VNNSNNSNVNNAD

CkPCCPkCkr

CkPCkCCPkr

0/

/0/

VDINNSS

VSIDNNSSN

CkPPkkr

CkPkPkkrr

/

/

Site Balance:

Is this consistent with the data?

Linear when PNi0 is low

Low slope when PNi0 is large

0

00 1

0@Ni

NiNiI kP

kPrP

Consistent!!

PNi0

rNi0

IINN

PIN

k

TSNSN

IINNVT

SISNVT

PkPk

kPPCkkrr

PKPkCC

CCCC

1

/

1

'

IINN

TV PkPk

CC

1