catalysis and catalysts - physical adsorption physical adsorption texture and morphology –pore...

Catalysis and Catalysts - Physical Adsorption

Physical AdsorptionPhysical Adsorption

Texture and morphology– pore size– pore shape– pore-size distribution (same size or various sizes?)– pore volume– specific surface area of catalyst


Internal DiffusionInternal Diffusion

Types of diffusion– Molecular– Knudsen– Surface (also called “configurational diffusion)”

Knudsen number: Kn = /l

= molecular free path length

l = characteristic pore diameter

Kn << 1 bulk diffusion

Kn >> 1 Knudsen diffusion


Pore Diameters, Shapes??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 RegimesPore Size and Diffusion Regimes

Configurational diffusion

Surface migration

1000 100 10 1 0.1

10-4

10-8

10-12

10-16

1000 100 10 1 0.1

100

50

0

Ea (kJ/mol)

D (m2/s)

Pore diameter (nm) Pore diameter (nm)

Molecular diffusion Knudsen

diffusion

Surface migration


Pore Diameters and Measurement TechniquesPore Diameters and Measurement Techniques

1 10 100 1000 10000

Pore diameter (nm)

Micro Meso Macro2 50

N2 capillary condensation

Hg porosimetry


Shape SelectivityShape Selectivity

Reactant selectivity

+

Product selectivity

CH3OH +

Restricted transition-state selectivity


Pore ShapesPore Shapes

Slit

Ink-bottle

Cylindrical

Wedge

a b

dc


Pore Structures of ZeolitesPore Structures of Zeolites

a b

ZSM-5 Mordenite


Volumetric Adsorption MeasurementVolumetric Adsorption Measurement

N2 (77.3 K) or

Ar, He, CH4, CO2, Kr

adsorbate

adsorbent

pressuregauge

P V1

V2

high vacuum


Adsorption IsothermsAdsorption 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 IsothermsAdsorption 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


Langmuir Adsorption Isotherm (Type I)Langmuir Adsorption Isotherm (Type I)

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 II and IV IsothermsType II and IV Isotherms

Multilayer adsorption (starting at B)

Common for pore-free materials

Similar to II at low p

Pore condensation at high p

p/p

II

nad

0

B

IV

n ad

p/p0

B


Type III and V IsothermsType III and V Isotherms

III

nad

p/p0

V

nad

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 CapacitySurface Area & Monolayer Capacity

S = nmAmN

monolayer capacity (mol/g)

specific surface area (m2/g)

area occupied by one molecule (m2/molecule)

Avogadro’s number (molecules/mol)

BET model: SBET

t model: St


Properties of Adsorbates for Physisorption MeasurementsProperties of Adsorbates for Physisorption Measurements


N2 Adsorption Isotherm in ZSM-5N2 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

(m

mol

/g) 1

Langmuir Adsorption?

No:

strong adsorption at low p due to condensation in micropores

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


BET (Brunauer, Emmett, Teller) MethodBET (Brunauer, Emmett, Teller) Method

Modification of Langmuir isotherm Both 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

0mm

0ad

11

p

p

Cn

C

Cnppn

p

RT

HHC condadexp


BET ModelBET Model

reality model5

43

21

0

...321 210mad nni

1-nn1-n1

0

nn1-n1

0101

0

111

00

pKpk

kkpk

pKpk

kkpk

d

and

na

d

ada

1st layer

nth layer

For every layer Langmuir model

Assume

RT

H

RT

H

RT

H

KKK

KK

condn

ads

ee

e

0,n0,nn

0,11

0

0

0m

ad

111pp

C

pp

pp

C

n

nRT

HH

Ccondads

e

with


Non-Porous Silica and AluminaNon-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 CatalystsTexture 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 WallAdsorption at Pore Wall

Cylindrical pore

Ink-bottle pore

Pore with shape of interstice between close-packed particles

Adsorbed layertdpdm


Kelvin Equation for NitrogenKelvin Equation for Nitrogen

m0

12ln

rRT

V

p

p L

micro meso macro

VL = 34.6810-6 m3/mol

= 8.88 mN/m

dm (nm)

Rel

ativ

e pr

essu

re p

/p0


Hysteresis LoopsHysteresis Loops

HI

n ad

p/p0

H2

n ad

p/p0

H3

n ad

p/p0

Information on pore shape


t-methodt-method

nm354.0m

ad n

nt

t

nS

NAt

nS

NAnS

ad6t

m9ad

t

mmt

1073.5

10354.0

nad

t

Proportional to St

Note:

nad is experimental result

t is calculated from correlation t versus p


t-methodt-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


p/p0

Ads

orb

ed-la

yer

thic

knes

s t

(nm

)

t-curvest-curves

a

b

Halsey

Harkins-Jura-de Boer

333.0

0/ln

00.5354.0

ppt

5.0

0/log034.0

99.131.0

pp

t


t-plot of -aluminat-plot of -alumina

0

2

4

6

8

10

0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)

na

d (m

mol

/g)

St,micro= 0 m2/g

V t,micro = 0 ml/g

mesopores

macropores

St,micro = 0 m2/g

Vt,micro = 0 ml/g

St = 200 m2/g


Shape of t-plotsShape of t-plots

nm354.0m

ad n

nt

t

nad

t

nad

t

nad

Non-porous MicroporousMicro- and

mesoporous

St

Smesopores

p

nad

Adsorption isotherm

t = f(p)


t-plot of N2 Physisorption on ZSM-5t-plot of N2 Physisorption on ZSM-5

t (nm) 0 0.5 1

n2

n1

n1 = liquid N2

n2 = solid N2

n ad

(mm

ol/g

)6

3

0


Pore-Size Distribution of -AluminaPore-Size Distribution of -Alumina

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000

dp (nm)

dV/d

d (m

l/g/n

m)


Mercury Intrusion PorosimetryMercury Intrusion Porosimetry

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

From a force balance:

(d in nm, p in bar)

Convenient method for determining pore volume versus pore size

pd

14860p


Mercury Intrusion Curve of -Alumina 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 (

ml/g

)


Surface Area’s - SHg and SBETSurface Area’s - SHg and SBET

Adsorbent 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 SBET for Microporous Materials

Discrepancy SHg and SBET for Microporous Materials

Hg cannot penetrate small (micro)pores, N2 can

Uncertainty of contact angle and surface tension values Cracking or deforming of samples


Texture PropertiesTexture Properties

N2-physisorption Hg-porosimetry

SBET St Vp dp SHg Vp dp

m2/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 DistributionsN2 Adsorption Isotherms & Pore Volume Distributions

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p

0

nad

(m

mol

/g) 1

wide-pore silica -alumina

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p

0

nad

(m

mol

/g) 1

0.00

0.02

0.04

0.06

0.08

0.10

1 10 100 1000d pore (nm)

dV/d

d (

ml/

g/nm

)

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000d pore (nm)

dV/d

d (

ml/

g/nm

)



-alumina activated carbon

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p

0

nad

(m

mol

/g)

1

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p

0

nad

(m

mol

/g) 1

0.000

0.002

0.004

0.006

0.008

0.010

1 10 100 1000d pore (nm)

dV/d

d (

ml/

g/nm

)

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000d pore (nm)

dV/d

d (

ml/

g/nm

)} Tensile strength effect



Raney Ni ZSM-5

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p

0

nad

(m

mol

/g) 1

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1p/p

0

nad

(m

mol

/g) 1

0.00

0.02

0.04

0.06

0.08

0.10

1 10 100 1000d pore (nm)

dV/d

d (

ml/

g/nm

)

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0d pore (nm)

dV/d

d (

ml/

g/nm

)


Hg Intrusion Curves & Pore Volume DistributionsHg Intrusion Curves & Pore Volume Distributions


0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (

ml/

g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000

p (MPa)

V (

ml/

g)

0

0.02

0.04

0.06

0.08

1 10 100 1000 10000

d pore (nm)

dV/d

d (

ml/

g/nm

)

0.0

0.1

0.2

0.3

0.4

0.5

1 10 100 1000 10000d pore (nm)

dV/d

d (

ml/

g/nm

)




0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (

ml/

g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (

ml/

g)

0.000

0.001

0.002

0.003

0.004

0.005

1 10 100 1000 10000d pore (nm)

dV/d

d (

ml/

g/nm

)

0.000

0.002

0.004

0.006

0.008

0.010

1 10 100 1000 10000d pore (nm)

dV/d

d (

ml/

g/nm

)



Raney 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 (

ml/

g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000p (MPa)

V (

ml/

g)

0.00

0.02

0.04

0.06

0.08

0.10

1 10 100 1000 10000

d pore (nm)

dV/d

d (

ml/

g/nm

)

0

0.001

0.002

0.003

0.004

0.005

1 10 100 1000 10000 100000d pore (nm)

dV/d

d (

ml/

g/nm

)


BET- & t-plotsBET- & t-plots


0.0

0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30

p/p 0

p/[

nad

(p0 -p

)] (

g/m

mol

)

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/[

nad

(p0 -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




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/m

mol

)

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 0.30

p/p0

p/[

nad

(p0 -p

)] (g

/mm

ol)

SBET = 1057 m2/gC = 1057

p/p0 = 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

(m

mol

/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

(m

mol

/g)

S t,micro = 856 m2/g

V t,micro = 0.42 ml/g



Raney 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 0.30

p/p 0

p/[

nad

(p0 -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/[

nad

(p0 -p

)] (

g/m

mol

)

SBET = 345 m2/g

C = -245

p/p0 : 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

catalysis and catalysts - physical adsorption physical adsorption texture and morphology –pore...

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