L12 CRE II Heterogeneous Catalysis

Prof. K.K.PantDepartment of Chemical Engineering

IIT Delhi.kkpant@chemical.iitd.ac.in

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• How to find the rate data ??

•How can we calculate the weight of the catalyst needed for obtaining the given conversion ??

D

BAA

eq

BCAAS

A

Kp

pK

Kpp

pKk

r

1

)(

Ax

AAA rdxFW0

0 //

• Express the partial pressures in terms of xA

AA

A

A

A

x

x

p

p

1

1

0 AA

AR

A

R

x

xarM

p

p

1

)/(

0

)( AA xfr Use numerical / graphical integration

3

Finding a Mechanism consistent with the experimental dataDeducing Rate law from experimental data (Example 10.2 , 10.3: Example : Hydrodemethylation of Toluene

4

Use of Multiple regression technique (Rate or CONVERSION Data from Differential reactor identical to CSTR)

Y = a0 + a1 X1J + a2 X2J

A0

, a1

… are the parameters of the model.

Use regression methods / Polymath, MATLAB, etc.

k, KB, KT are the parameters of the equation

Evaluation of Rate law Parameters

Isotherms

Assumptions:

• homogeneous surface

(all adsorption sites energetically identical)

• monolayer adsorption (so no multilayer adsorption)

• no interaction between adsorbed molecules

pK

pKnnn mmad

1

I

n ad

p/p0

Type I Langmuir Adsorption Isotherm

Isotherms

Multilayer adsorption (starting at B)

Common for pore-free materials

p / p

nad

0

B

Type II

Type IV

Similar to II at low P

Pore condensation at high P

n ad

p /p 0

B

Isotherms

pK1pK

nθnn mmad

I

nad

p / p 0

Type I Langmuir Adsorption Isotherm

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Assumptions:

• homogeneous surface

(all adsorption sites energetically identical)

• monolayer adsorption (so no multilayer

adsorption)

• no interaction between adsorbed molecules

Isotherms

Multilayer Physical adsorption (starting at B)

Common for many porous materials

p / p

nad

0

B

Type II

Type IV

Similar to Type II at low P,

Pore condensation at high P

n ad

p /p 0

B

IsothermsType III

Type IV

nad

p / p 0

Strong cohesion force between adsorbed molecules, e.g. when water adsorbs on hydrophobic activated carbon

n ad

p / p 0

Similar to III at low P

Pore condensation at high P. III and v are rare.

N2 PhysisorptionAdsorption and Desorption Isotherms

Langmuir Adsorption? •strong adsorption at low P due to condensation in micropores

•at higher P saturation due to finite (micro)pore volume

PhysisorptionDifferent Adsorbates Used in Physisorption Studies

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NAME ISOTHERM EQUATION

APPLICABILITY

Chemisorption and Physical adsorption

Henry Chemisorption and Physical adsorptionAt low coverages

Freundlich Chemisorption and Physical adsorptionAt low coverages

LangmuirV bp

=θ=V 1+bpn

'V=kp

1nV=kp n>1

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Temkin Chemisorption

Brunauer – Emmett-Teller(BET)

Multilayer ,physical adsorption

Polanyi(c)

physical adsorption

V

= =AlnBpVm

p 1 c 1 p

= + ×V po-p V c V c pom m

ε=RTln po/p

Determination of Surface Area

• Physisorb an inert gas such as argon or

nitrogen and determine how many

molecules are needed to form a complete

monolayer

• For example, the N2 molecule occupies

0.162 nm2 at 77 K, the total surface area

follows directly

Determination of Surface Area

• Physisorb an inert gas such as argon or nitrogen and determine how many molecules are needed to form a complete monolayer.

• For example, the N2 molecule occupies 0.162 nm2 at 77 K, the total surface area follows directly.

• Although this sounds straightforward, in practice molecules may adsorb beyond the monolayer to form multilayers.

• In addition, the molecules may condense in

small pores. The narrower the pores, the easier

N2 will condense in them.

• This phenomenon of capillary pore

condensation, as described by the Kelvin

equation

• Phenomenon can be used to determine the

types of pores and their size distribution inside

a system

N2 PhysisorptionAdsorption and Desorption Isotherms

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

na

d (m

mo

l/g) 1

Adsorption

Desorption

Adsorption and Desorption Isotherms

III

nad

p/p0

VI

n ad

p/p0

V

n ad

p/p0

I

n ad

p/p0 p/p

II

nad

0

B

IV

n ad

p/p0

B

N2 Physisorption

Pore Size and ShapeWhy is it important?•it dictates the diffusion process through the material.

Why is it important?

directly affect the selectivity of the catalytic reaction.

Pore Size and Shape

Pore Diameter

– micropores (< 2 nm)– mesopores (2 – 50 nm)– macropores (> 50 nm)

Pore Shape– cylinder– slit– ink-bottle– wedge

vv

Pore Size and ShapeMeasurement Techniques

1 10 100 1000 10000

Pore diameter (nm)

Micro Meso Macro2 50

N2 capillary condensation

Hg porosimetry

Pore volume determination ( Helium -Mercury Method)

• The pore volume of the catalyst can be determined by the helium-

Mercury method.

• The volume of Mercury and Helium displaced by the catalyst is used

to measured the pore volume of the catalyst.

• Since mercury cannot pass through the pores of the catalyst , the

difference in the volume gives the pore volume.

• Vmercury => extrenal volume of solid, VHe = pore vol+soild vol.

• Pore volume Vg = (Vmercury – VHelium)/(Mass of catalyst)

• Porosity= e= 1/ρp - 1/ρS = ρp Vg

Pore size distribution

• An important property of catalysts is the distribution of pores across the

inner and outer surfaces. The most widely used method for

determining the pore distribution in solids is mercury porosimetry and

BET method.

Mercury Porosimetry:

• The pore size distribution is determined by measuring the volume of

mercury that enters the pores under pressure.

•

• σ is surface tension of Hg = 0.425 N/m

• Pressures of 0.1 to 200 MPa allow pore sizes in the range 20–7500 nm

to be determined.