katkat07.pdf

Catalysis and Catalysts - Physical Adsorption

Physical Adsorption

Texture and morphology pore size pore shape pore-size distribution (same size or various

sizes?) pore volume specific surface area of catalyst


Internal Diffusion

Types of diffusion Molecular Knudsen Surface (also called configurational diffusion)

Knudsen number: Kn = /l = molecular free path lengthl = characteristic pore diameterKn > 1 Knudsen diffusion


Pore Diameters, Shapes??

Pore diameters micropores (< 2 nm) mesopores (2 - 50 nm) macropores (> 50 nm)

Experimental techniques capillary condensation Hg intrusion microscopy

Shapes cylinder, slit, ink-bottle, wedge, ...


Pore Size and Diffusion Regimes

Configurational diffusion

Surface migration


Pore Diameters and Measurement Techniques

1 10 100 1000 10000

Pore diameter (nm)

Micro Meso Macro2 50

N2 capillary condensation

Hg porosimetry


Shape Selectivity

Reactant selectivity

+

Product selectivity

CH3OH +

Restricted transition-state selectivity


Pore Shapes

Slit

Ink-bottle

Cylindrical

Wedge


Pore Structures of Zeolites

a b

ZSM-5 Mordenite


Barometric Adsorption Measurement

N2 (77.3 K)

Ar, He, CH4, CO2, Kr

adsorbate

adsorbent

pressuregauge

P V1

V2

high vacuum

Conditions??


Adsorption Isotherms

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

nad

(mm

ol/g

) 1

Adsorption

Desorption

Interpretation??

AluminaN2, 77 K


Adsorption Isotherms

I

n ad

p/p0 p/p

III

n ad

p/p0

VI

n ad

p/p0

V

n ad

p/p0

II

n ad

0

B

IV

n ad

p/p0B


Langmuir Adsorption Isotherm (Type I)

I

nad

p /p 0

Assumptions:

homogeneous surface

(all adsorption sites energetically identical)

monolayer adsorption (so no multilayer adsorption)

no interaction between adsorbed molecules

pKpKnnn mmad +

==1


Type II and IV Isotherms

II

n ad

0

B

IV

n ad

p/p0B

Multilayer adsorption (starting at B)

Common for pore-free materials

same with

pore condensation at high p


Type III and V Isotherms

III

n ad

p/p0

V

n ad

p/p0

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

Similar to III at low p

Pore condensation at high p


Surface Area & Monolayer Capacity

S = nmAmN

monolayercapacity (mol/g)

specific surface area (m2/g)

area occupied by one molecule (m2/molecule)

Avogadros number (molecules/mol)

BET model: SBET

t model: St


Properties of Adsorbates for Physisorption Measurements

Adsorbate Boiling Point (K) Am (nm2/molecule)

N2 77.3 0.162

Ar 87.4 0.142

CO2 194.5 0.17

Kr 120.8 0.152


N2 Adsorption Isotherm in ZSM-5

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1

p/p0

nad (

mm

ol/g

) 1

Langmuir Adsorption?

No:

at low p strong adsorption due to condensation in micropores

at higher p saturation due to finite (micro)pore volume


BET (Brunauer, Emmett, Teller) Method

Based on Langmuir isotherm Monolayer and multilayer adsorption Layers of adsorbed molecules divided in:

First layer with heat of adsorption Had,1 Second and subsequent layers with Had,2 = Hcond

BET isotherm:

BET equation does not fit entire adsorption isotherm different mechanisms play a role at low and at high p

( ) 0mm0ad11

pp

CnC

Cnppnp

+=

=

RTHHC condadexp


BET Model

reality model54

32 1

0

( )...321 210mad +++== nni

For every layer Langmuir model

RTH

RTH

RTH

KKK

KKcondn

ads

ee

e

0,n0,nn

0,11

=

=

( )

+

=

0

0

0m

ad

111ppC

pp

pp

Cnn

RTHH

Ccondads

e

=with

00 1

0 1 1 0 1 01a

a dd

kk p k p K pk

= = =1st layer

nth layer0

1n-1 n n n-1 n n-11

n n aa d

d

kk p k p K pk

= = =

Assume


Non-Porous Silica and Alumina

p/p0

n ad/n

m

(B) (A)

Low p/p0:

filling of micropores

favoured adsorption at most reactive sites (heterogeneity)

High p/p0:

capillary condensation

Range 0.05 < p/p0 < 0.3 is used to determine SBET

BET equation


Texture Data of Commercial Catalysts

Material Mean dp (nm) SBET (m2/g)

Catalyst supports

Silica gel 10 200

6 400

4 800

-Al2O3 10 150

5 500

Zeolite 0.6-2 400-800

Activated carbon 2 700-1200

TiO2 400-800 2-50

Aerosil SiO2 - 50-200

Catalysts

MeOH synthesis (Cu/ZnO/Al2O3) 20 80

NH3 synthesis (Fe/Al2O3/K2O) 100 10

Reforming (Pt/Re/Al2O3) 5 250

Epoxidation (Ag/-Al2O3) 200 0.5


Adsorption at Pore Wall

Cylindrical pore

Ink-bottle pore Pore with shape of interstice between close-packed particles

Adsorbed layer


Kelvin Equation for Nitrogen

Rel

ativ

e pr

essu

re, p

/p0

rm (nm)

1.0

0.5

0.00.1 10 100 1000 10000

m0

12lnrRT

Vpp L

=

micro meso macro

VL = 34.6810-6 m3/mol

= 8.88 mN/m


Hysteresis Loops

HI

n ad

p/p0

H3

n ad

p/p0

H2

n ad

p/p0

Information on pore shape


t-method

nm354.0m

ad=

nnt t m mS n A N=

nad

t

Proportional to St

Note:

nad is experimental result

t is calculated from correlation t versus p

9 6ad adt m t0.354 10 5.73 10

n nS A Nt t

= =


t-method

BET only valid in small pressure interval interpretation not very easy

thickness (t) of adsorbed layer can be calculated

plot of t versus p for non-porous materials is the same (has been checked experimentally)

t-plot helps in interpretation

0.354 nm


t-curves

p/p0

Thic

knes

s of

ads

orbe

d la

yer t

(nm

)

a

b

Halsey

Harkins-Jura-de Boer

( )333.0

0/ln00.5354.0

=

ppt

( )5.0

0/log034.099.131.0

=

ppt


t-plot of -alumina

0

2

4

6

8

10

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

n ad (

mm

ol/g

)

St,micro= 0 m2/g

V t,micro = 0 ml/g

St = 200 m2/g

mesopores

macropores


Shape of t-plots

nm354.0m

ad=

nnt

t

nad

t

nad

t

nad

Non-porous Microporous Micro- and mesoporous

St

Smesopores

p

nad

Adsorption isotherm

t = f(p)


t-plot of N2 Physisorption on ZSM-5

0

6

0.0 0.2 0.4 0.6 0.8

t ( nm)

nad

(m

mol

/g)

n 1

n 2

n 1 = liquid N2n 2 = solid N2


Pore-Size Distribution of -Alumina

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000d pore (nm)

dV/d

d (m

l/g/n

m)


Mercury Intrusion Porosimetry

pd 14860=

Convenient method for determining pore volume versus pore size

Hg does not wet surfaces; pressure is needed to force intrusion

nm

bar

From a force balance:


Mercury Intrusion Curve of -Alumina

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (m

l/g)


Surface Areas - SHg and SBETAdsorbent SHg SBET

m2/g m2/g deg

Iron Oxide 14.3 13.3 130

Tungsten Oxide 0.11 0.10 130

Anatase 15.1 10.3 130

Hydroxy Apatite 55.2 55.0 130

Carbon Black (Spheron-6) 107.8 110.0 130

0.5 % Ru/-Al2O3 237.0 229.0 140

0.5 % Pd/-Al2O3 115.0 112.0 140

TiO2 Powder 31.0 25.0 140

Sintered Silica Pellets 20.5 5.0 140

Zeolite H-ZSM-5 39.0 375.0 140

Norit Active Carbon R1 Extra 112.0 915.0 140


Discrepancy SHg and SBETfor Microporous Materials

Hg cannot penetrate small (micro)pores, N2 can Uncertainty of contact angle and surface tension values Cracking or deforming of samples


Texture Properties

N2-physisorption Hg-porosimetry

SBET St Vp dp SHg Vp dpm2/g m2/g ml/g nm m2/g ml/g nm

Wide Pore Silica 78 52 0.91 47 80 0.92 54

-Alumina 196 202 0.49 10 163 0.49 10

-Alumina 9 8 0.12 112 12 0.48 150

Active Carbon 1057a 28 0.51 2 0.6 0.46 106

Raney Ni 76 - 0.14 5.80 - - -

ZSM-5 345 344 0.19 0.58 11 1.1 820b

a p/p0 range of 0.01-0.1 was used in the calculation.b intraparticle voids.


N2 Adsorption Isotherms & Pore Volume Distributions

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

n ad (

mm

ol/g

) 1

wide-pore silica -alumina

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

n ad (

mm

ol/g

) 1

0.00

0.02

0.04

0.06

0.08

0.10

1 10 100 1000d pore (nm)

dV/d

d (m

l/g/n

m)

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000d pore (nm)

dV/d

d (m

l/g/n

m)


N2 Adsorption Isotherms & Pore Volume Distributions-alumina activated carbon

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

n ad (

mm

ol/g

) 1

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

n ad (

mm

ol/g

) 1

0.000

0.002

0.004

0.006

0.008

0.010

1 10 100 1000d pore (nm)

dV/d

d (m

l/g/n

m)

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000d pore (nm)

dV/d

d (m

l/g/n

m)

} Tensile strength effect


N2 Adsorption Isotherms & Pore Volume DistributionsRaney Ni ZSM-5

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

n ad (

mm

ol/g

) 1

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p 0

n ad (

mm

ol/g

) 1

0.00

0.02

0.04

0.06

0.08

0.10

1 10 100 1000d pore (nm)

dV/d

d (m

l/g/n

m)

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0d pore (nm)

dV/d

d (m

l/g/n

m)


Hg Intrusion Curves & Pore Volume Distributionswide-pore silica -alumina

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (m

l/g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (m

l/g)

0

0.02

0.04

0.06

0.08

1 10 100 1000 10000

d pore (nm)

dV/d

d (m

l/g/n

m)

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000 10000d pore (nm)

dV/d

d (m

l/g/n

m)


Hg Intrusion Curves & Pore Volume Distributions-alumina activated carbon

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (m

l/g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (m

l/g)

0.000

0.001

0.002

0.003

0.004

0.005

1 10 100 1000 10000d pore (nm)

dV/d

d (m

l/g/n

m)

0.000

0.002

0.004

0.006

0.008

0.010

1 10 100 1000 10000d pore (nm)

dV/d

d (m

l/g/n

m)


Hg Intrusion Curves & Pore Volume DistributionsRaney Ni ZSM-5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000

p (MPa)

V (m

l/g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (m

l/g)

0.00

0.02

0.04

0.06

0.08

0.10

1 10 100 1000 10000

d pore (nm)

dV/d

d (m

l/g/n

m)

0

0.001

0.002

0.003

0.004

0.005

1 10 100 1000 10000 100000d pore (nm)

dV/d

d (m

l/g/n

m)


BET- & t-plotswide-pore silica -alumina

0.0

0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0

p/[

nad

(p0 -

p)]

(g/

mm

ol)

S BET = 78 m2/g

C = 146

0.0

0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0

p/[n

ad(p

0 -p

)] (

g/m

mol

)

S BET = 196 m2/g

C = 97

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

nad

(mm

ol/g

)

S t,micro=28 m2/g

V t,micro = 0.013 ml/g

0

2

4

6

8

10

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

nad

(mm

ol/g

)

S t,micro= 0 m2/g

V t,micro = 0 ml/g


BET- & t-plots-alumina activated carbon

0.0

0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0

p/[n

ad(p

0 -p)

] (g

/mm

ol)

S BET = 9.3 m2/g

C = 142

0.0

0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0

p/[n

ad(p

0 -p)]

(g/

mm

ol)

S BET = 1057 m2/g

C = 1057p/p 0 = 0.01 - 0.1

0.00

0.05

0.10

0.15

0.20

0.25

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

nad

(mm

ol/g

)

S t, micro= 1.4 m2/g

V t,mcro = 0.001 ml/g

0

5

10

15

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

nad

(mm

ol/g

)

S t,micro = 856 m2/g

V t,micro = 0.42 ml/g


BET- & t-plotsRaney Ni ZSM-5

0.0

0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0

p/[n

ad(p

0 -p

)] (

g/m

mol

)

S BET = 76 m2/g

C = 46

0.0

0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0

p/[n

ad(p

0 -p)]

(g/

mm

ol)

S BET = 345 m2/g

C = -245

p/p 0 : 0.01 -0.1

0

1

2

3

4

5

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

n ad

(mm

ol/g

)

St,micro = 0 m2/g

Vt,micro = 0 ml/g

0

2

4

6

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

n ad (

mm

ol/g

)

St ,micro= 344 m2/g

Vt,micro = 0.18 ml/g

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