Mark
G.
White,
Ph.
D.
Dave
C.
Swalm
School
of
Chemical
Engineering
Mississippi
State
University
Mississippi
State,
MS
39762‐9595
[email protected]
Outline
Description
of
heterogeneous
catalysts
and
fundamentals
Characterizing
total
surface
area
of
materials
Non‐porous
Porous
Pores
characterized
by
two‐dimension
(cylindrical
pores)
Pores
characterized
by
thee‐dimensions
(slit‐shaped
pores)
Molecular
Scale
adsorption
experiments
Characterizing
the
active
surface
area
of
supported
metal
catalysts
Supported
Ru
and
Supported
Pt
H2
titration
Supported
Cu
NO
titration
H2/O2
redox
Outline,
Continued
Characterizing
the
active
surface
area
of
supported
metal
oxide
catalysts
Acid
catalysts
Brønsted
acidity
Pyridine
IR
Hexamethyldisilazane
reaction
with
surface
protons
Titrating
strength
of
acid
sites
Temperature
programmed
desorption
of
a
base
molecule
Base
catalysts
Titration
with
carbon
dioxide—hydrotalcite
Microcalorimetry
‐
TGA
molecule
adsorption
Titration
with
sulfur
dioxide—supported
MgO/alumina
Zeolite
catalysts
Hoffmann
elimination
reaction
in
zeolites
to
determine
the
framework
SiO2/Al2O3
ratio.
Description
of
Heterogeneous
Catalysts
Description
of
heterogeneous
catalysts
A
heterogeneous
reaction
occurs
at
the
interface
of
two
phases.
The
catalyst
is
usually
the
solid
phase
and
the
reactants/products/solvent
are
in
the
fluid
phase(s).
The
fluid
could
be
a
liquid
and/or
gas.
The
interfacial
surface
area
influences
the
catalyst
activity.
Description
of
heterogeneous
catalysts
Some
of
the
first
catalysts
were
minerals
(e.
g.,
a
clay)
showing
internal
pore
volume.
The
positive
attributes
of
these
naturally‐ occurring
materials
were
captured
in
synthetic
solids.
Through
modern
design
techniques,
it
become
appropriate
to
synthesize
the
catalyst
support
apart
from
the
catalytic
agent.
Description
of
heterogeneous
catalysts
Techniques
were
developed
to
produce
the
porous
support
material
with
control
of
the
pore
sizes
and
volume.
Two
oxides
often
used
as
supports
are
silica
and
alumina.
These
materials
can
be
synthesized
in
high
purity
and
easily
dissolved
to
be
precipitated
in
a
controlled
fashion
so
as
to
develop
the
desired
pore
structures
and
surface
chemistry.
Fundamentals
–
Non
Selective
Probes
The
use
of
probe
molecules
to
characterize
catalysts
is
based
upon
simple
physico‐chemical
principles.
The
total
surface
area
of
a
solid
can
be
interrogated
with
a
non‐selective
molecule
that
interacts
with
the
surface
by
van
der
Waals
forces.
These
probe
molecules
include
the
inert
gases
(Kr,
Ar)
and
diatomic
gases,
such
as
N2.
The
experiments
are
conducted
near
the
normal
B.P.’s
of
the
adsorbates
and
pressures
below
the
saturation
pressures
(ca.,
P/Po
~
0.05
0.35).
Fundamentals
–
Selective
Adsorbates
These
molecular
probes
selectively
interact
with
certain
parts
of
the
surface
with
chemisorption
forces.
The
types
of
interactions
are
as
follows:
Acid/Base
.
The
choice
of
adsorbate
and
conditions
are
determined
by
the
strength
of
the
surface
sites.
Metal/Ligand.
Certain
transition
metals
are
known
to
form
coordination
bonds
with
ligands.
A
knowledge
of
the
electron
configurations
of
the
metal
will
suggest
the
appropriate
ligand.
Fundamentals
–
Stoichiometric
Reactive
Probes
These
molecules
react
with
certain
sites
in
a
stoichiometric
fashion
so
as
to
change
the
site.
As
such
they
can
be
used
to
count
sites.
Hexamethyldisilazane
is
a
strong
base
that
reacts
with
weakly
acidic
surface
protons
(e.
g.,
silanols).
(CH3)3SiNHSi(CH3)3+2SiOH
2SiOSi(CH3)3
+
NH3
Estimating
Total
Surface
Area
of
Solids
using
non‐Selective
Probe
Molecules
Characterizing
total
surface
area
of
using
non‐selective
probes
The
total
surface
area
of
porous
&
non‐porous
solids
can
be
estimated
by
a
simple
adsorption
experiment
and
a
model
of
the
adsorption
physics
to
the
surface.
One
of
the
first
successful
methods
was
reported
by
researchers
in
the
US.
This
method,
known
as
the
BET
method,
takes
it’s
name
from
the
three
researchers,
S.
Brunauer,
P.
Emmett,
and
E.
Teller.1
1S.
Brunauer,
P.
H.
Emmett
and
E.
Teller,
J.
Am.
Chem.
Soc.,
1938,
60,
309.
doi:10.1021/ja01269a023
BET
theory
The
concept
of
the
theory
is
an
extension
of
the
Langmuir
theory,
to
multilayer
adsorption
with
the
following
hypotheses:
(a)
gas
molecules
physically
adsorb
on
a
solid
in
layers
infinitely;
(b)
there
is
no
interaction
between
molecules
in
the
first
adsorption
layer;
and
(c)
the
Langmuir
theory
can
be
applied
to
each
layer.
BET
Equation
The
BET
equation
relates
the
results
from
an
adsorption
experiment:
v,
volume
adsorbed
(STP
cm3)
and
relative
pressure,
(P/Po),
to
the
characteristics
of
the
solid:
Vm,
volume
adsorbed
in
a
monolayer,
and
(E1‐EL),
difference
in
the
heats
of
adsorption
between
first
layer
and
liquid.
___1_______
=
(C‐1)(P/Po)
+
___1__
V[(Po
/P)
‐
1]
VmC
VmC
C
=
exp[(E1‐EL)/RT].
This
equation
is
plotted
versus
f,
P/Po.
The
slope
and
intercept
are:
Slope
=
(C‐1)/(VmC);
Intercept
=
1/VmC
From
these
two
equations
one
can
determine
Vm.
From
Vm,
the
surface
area
is
determined
knowing
how
the
probe
molecule
decorates
the
surface
at
its
characteristic
dimension.
Discussion
of
BET
method
This
technique
gives
adequate
estimates
of
surface
area
when
using
data
developed
on
the
range
of
relative
pressures
from
0.05
to
0.35.
It
works
for
non‐porous
solids
such
as
Cab‐O‐ SilTM
and
amorphous,
porous
solids
having
surface
areas
>
50
m2/g.
Other
versions
of
the
isotherm
must
be
used
for
crystalline,
porous
solids
such
as
zeolites.
Types
of
Isotherms
Five
types
of
isotherms
have
been
identified
as
useful
for
characterizing
porous
solids.
Type
I:
modeled
by
Langmuir
equation,
for
modeling
sub‐monolayer
coverage’s.
Type
II:
BET
isotherm,
multilayer
adsorption
w/o
capillary
condensation.
Type
III:
modeled
by
polynomial
equation1
for
adsorption
of
Kr
on
alkali
metals/oxides
(K2O).
Type
IV
&
V:
models
porous
solids
that
experience
capillary
condensation
in
the
pores
(zeolites).
1Poehlein,
S.
and
M.
G.
White,
"Hyperbaric
Kinetics
for
the
Reaction
of
Potassium
Superoxide
with
Water
and
Carbon
Dioxide,"
Ocean
Engineering
Division‐‐
A.S.M.E.,
Vol.
12,
"Current
Practices
and
New
Technology
in
Ocean
Engineering,"
pp.
55‐61
(1987).
Types
of
Isotherms
The
figures
show:
a)
Type
I
isotherm;
b)
Type
IV
isotherm,
and
c)
Type
II
isotherm.1
Filled
symbols
are
adsorption,
open
symbols
are
desorption.
Figure
(a)
shows
N2
adsorbed
on
0.5
wt%
Pt/Cs2.1H0.9PW12O40
obeying
a
Langmuir
isotherm.
Figure
(b)
shows
N2
adsorbed
on
0.5
wt%
Pt/Cs2.5H0.5PW12O40
obeying
a
Type
IV
isotherm
suggesting
mesoporous
texture.
2
Figure
(c)
shows
adsorption
on
0.5
wt%
Pt/silica
and
these
data
obey
Type
II
isotherm,
the
BET
isotherm.
1Y.
Yoshinaga
&
T.
Okuhara,
J.
Chem.
Soc.
Faraday
Trans.
1998,
94(15),
2235‐40.
2S.
J.
Gregg
&
K.
S.
W.
Sing,
in
Adsorption,
Surface
Area,
&
Porosity,
Academic
Press,
London,
2ND
Edition.
1982.
Use
of
Molecular
Probe
Method
The
size
of
the
pores
can
be
estimated
using
the
Molecular
Probe
Method
(aka
Plug/Gauge
Method).
Molecules
are
chosen
having
a
range
of
characteristic
dimensions
and
shapes
for
the
purpose
of
estimating
pore
sizes
of
the
adsorbent.
T
=
273
K;
P/Po
=
0.1;
2,2‐dimethylpropane
0.5
wt%
Pt/silica
0.5
wt%
Pt/Cs2.5H0.5PW12O40
0.5
wt%
Pt/Cs2.1H0.9PW12O40
0.62
nm
1Y.
Yoshinaga
&
T.
Okuhara,
J.
Chem.
Soc.
Farad.
Trans.
1998,
94(15),
2235‐40.
Examples
of
Probe
Molecules
Examples
of
Molecular
Scale
Probe
Molecules
are
shown
in
table
with
the
characteristic
molecular
scales.
These
molecular
scales
characterize
the
“effective
dynamic”
size
of
the
molecules
inside
pores
of
a
certain
diameter,
not
their
van
der
Waals
diameters.
Molecule
Molecular
Scale,
nm
n‐butane
0.43
i‐butane
0.50
benzene
0.59
2,2‐dimethylpropane
0.62
1,3,5‐ trimethylbenzene
0.75
Non‐Cylindrical
Shaped
Pores
Certain
solids
show
non‐circular
shapes
pores
(e.
g.,
clays).
For
these
solids,
another
treatment
of
volume
adsorbed
data
is
useful
to
determine
pore
sizes.
This
technique
is
called
the
“DFT”
method
as
it
makes
use
of
Density
Functional
Theory
that
has
found
favor
in
quantum
mechanics.1
1Tarazona,
Marconi,
&
Evans,
Molecular
Physics,
60,
(1987),
pp
573‐95;
Seaton,
Walton,
and
Quirke,
Carbon,
27(6),
(1989),
pp.
853‐61;
Olivier
&
Conklin;
7TH
Intern.
Conf.
Surf.
Coll.
Scie.,
Compiegne,
France
(1991).
DFT
Method
The
DFT
method
recognizes
that
the
adsorption
of
molecules
in
pores
occurs
in
a
discrete
fashion.
Consider
the
adsorption
of
neo‐pentane
(~0.6
nm)
in
a
1.5
nm
pore.
The
pore
size
permits
only
a
discrete
number
of
molecules
in
a
single
adsorption
layer,
filling
the
pore
from
the
bottom
to
the
top.
Other
arrangements
of
these
molecules
are
possible,
but
only
a
finite
number
of
arrangements
are
possible.
Selective
Probe
Molecules
Supported
Metal
Catalysts
Certain
supported
metals
(e.
g.,
Pt)
can
be
characterized
for
active
surface
area
using
a
chemisorbing
probe
molecule.
These
molecules
must
form
a
strong
bond
with
the
surface
metal
atoms,
and
the
stoichiometry
of
this
chemisorption
must
be
unique
and
known.
The
same
molecule
must
not
form
a
strong
bond
with
the
support.
Supported
Pt,
Supported
Ru
Pt,
as
well
as
Ru,
when
supported
on
certain
supports,
can
be
interrogated
for
metal
surface
area
using
H2.
This
system
forms
Pt‐H
and
Ru‐H
with
a
stoichiometry
of
1
H/1
Pt
or
1
H/1
Ru.
The
catalyst
is
usually
pretreated
to
fully
reduce
the
metal
and
to
remove
volatiles
that
might
cover
the
metal
sites.
Example
of
Ru/g‐Al2O3
140
Total
Sorption
at
20C
Amount
Sorbed,
micro‐mol/g
catalyst
120
Data
of
H2
to
characterize
supported
Ru
on
alumina.1
The
amount
adsorbed
was
100
recorded
in
a
volumetric
apparatus
at
two
temperatures:
20
&
100
C.
80
At
each
temperature,
the
catalyst
was
exposed
to
the
hydrogen
gas,
and
the
sample
60
was
evacuated
between
exposures.
This
procedure
was
repeated
for
40
increasing
pressures.
The
difference
between
total
and
reversibly
sorbed
was
also
20
plotted
versus
pressure
and
fit
with
line.
This
line
was
extrapolated
to
zero
pressure
to
0
find
amount
sorbed:
68
mmol/g.
Total
Sorption
at
100C
1Applied
Catalysis
A:
General
Volume
319,
1
March
2007,
Pages
202‐209
Reversible
Sorption,
100C
Reversible
Sorption,
20C
Difference,
100C
y
=
0.0109x
+
66.727
Difference,
20C
Linear(Differenc e,
100C)
y
=
‐0.0067x
+
70.223
Linear(Differenc e,
20C)
Linear(Differenc e,
20C)
0
50
100
150
200
Pressure,
Torr
250
300
Example:
Ru/alumina
This
amount
adsorbed
is
converted
to
number
of
exposed
Ru
atoms
assuming
that
1
H
titrates
1
Ru
surface
atom:
136
micro
mol
Ru/g
catalyst.
This
number
is
compared
to
the
total
number
of
Ru
atoms
in
the
sample.
For
many
samples,
this
ratio
is
