ChE542 Catalyst Characterization

Ch E 542: IntermediateReactor Analysis & Design

Catalyst Characterization

David A. Rockstraw, Ph.D., P.E.

New Mexico State University

Chemical Engineering

Surface Area Measurement

• Standard method is based on physical adsorption of a gas on a solid surface.

• Usually use nitrogen at the normal boiling point (-195.8°C).

• Adsorption isotherm determined based on development by Brunauer-Emmett-Teller.

Surface Area Measurement

• A sample contained in an evacuated sample tube is cooled (typically) to cryogenic temperature, then is exposed to analysis gas at a series of precisely controlled pressures.

• With each incremental pressure increase, the number of gas molecules adsorbed on the surface increases. The pressure at which adsorption equilibrium occurs is measured and the universal gas law is applied to determine the quantity of gas adsorbed.

• As adsorption proceeds, the thickness of the adsorbed film increases. Any micropores in the surface are quickly filled, then the free surface becomes completely covered, and finally larger pores are filled. The process may continue to the point of bulk condensation of the analysis gas.

• Then, the desorption process may begin in which pressure systematically is reduced resulting in liberation of the adsorbed molecules. As with the adsorption process, the changing quantity of gas on the solid surface is quantified. These two sets of data describe the adsorption and desorption isotherms. Analysis of the isotherms yields information about the surface characteristics of the material

MicromeriticsASAP 2010

Surface Area Determination

• Langmuir Isotherm

• Brunauer, Emmett, and Teller Isotherm (BET)

• Plot of vs. is linear with

ommo pc

p1c

c

1

pp

p

mm

p

K

1p

mc

1cslope

c

1intercept

m

pp

p

o op

p


• Solve these slope/intercept equations for the monolayer adsorption volume (m):

bm

1m

mc

1cmslope

c

1bintercept

m


• If is the total surface area covered by one adsorbed molecule, the total surface per gram of solid (Sg) can be found as,where No is Avogadro’s number and V is the volume of gas at conditions of m.

V

NS om

g


• Emmett and Brunauer proposed that is simply the projected area of a molecule on the surface when the molecules are arranged in close 2D packing.

32

oN

M09.1


• Example calculations based on Figure 8-1 of Smith, Chemical Engineering Kinetics.

Void Volume and Solid Density

• The void (pore) volume of a catalyst particle can be estimated by filling the pores with a liquid of known density, followed by weighing of the particle.

• The weight difference (before/after filling) is due to liquid contained in the pores.

• Standard system is helium-mercury.

Void Volume and Solid Density

• Finding the void fraction:

pg

sg

sg

spgp

gp

p

V

1V

V

1mVm

Vm

particle of volumetotal

particle of volumepore

particle porous ofdensity

phase solid ofsity den

particles gramper volumevoidV

particle of massm

orosityp

p

s

g

p

p

samplecatalyst

displaced Hedisplaced Hgg m

VVV

displaced He

samplecatalyst s V

m

Pore-Volume Distribution

• Mercury-penetration method Hg porosimetry is based on the intrusion of mercury into a

porous structure under stringently controlled pressures. From pressure versus intrusion data, volume and size

distributions may be generated. Hg does not wet most substances and will not

spontaneously penetrate pores by capillary action, it must be forced into the pores by the application of external pressure. The required pressure is inversely proportional to the size of the pores, only slight pressure being required to intrude Hg into large macropores, whereas much greater pressures are required to force Hg into micropores.


• Micromeritics AutoPore IV 9500

Able to measure pore diameters from 0.003 to 360 µm

Can achieve pressures to 33,000 psi or 60,000 psi


• Equate force of surface tension with the force of the applied pressure

2

5

2

p

inlbp

1075.8Åa

p

cos2a

cosa2pa

FF

Hg

140°


• Use the data of Table 8-3 (Smith, Chemical Engineering Kinetics) to calculate the pore-volume distribution shown in Figure 8-5.

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ChE542 Catalyst Characterization