Adsorption Calorimetry:
Basics and Applications
in Heterogeneous Catalysis
Sabine Wrabetz
Fritz-Haber-Institut of the Max Planck Society
Department of Inorganic Chemistry, 14195 Berlin, Germany,
[email protected]
Beijing - November 11, 2010
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Contents
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat
&
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Introduction
 Calor (Latin):
 Metron (Greek):
heat, warmth
measure
Johan C. Wilcke (1732-1796)
Antoine L. Lavoisier (1743-1794)
and Marie P. Lavoisier
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Introduction
▒ adsorption steps, surface reaction processes, and desorption steps
▒ the energetics of these surface physical-chemical events play an important role
in the determination of the catalytic properties of the surface
▒ direct method to determine number, strength and energy distribution of the adsorption sites
 Adsorption Isothermal Micrcalorimetry
▒ allows measurement of the differential heats (ads. enthalpy) evolved
when known amounts of gas probe molecules are adsorbed on the catalyst surface
▒ the evolved heat is related to the energy of the bonds formed between the adsorbed species
and the adsorbent and hence to the nature of the bonds and to the chemical reactivity of the surface
▒ key to the effective use of adsorptive microcalorimetry is the careful choice of
probe molecules and the adsorption temperature
▒ The data obtained are of substantial importance for comparing theoretical
and experimental hypotheses about reaction pathways.
 Analyzing of the catalytic data with respect to the surface processes occurring
on the catalyst material during adsorptive microcalorimetry.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Motivation: surface sites
▒
careful choice of probe molecules and the adsorption temperature
▒
Brønsted acid sites:
transfer of H+
from OH to
Adsorbate
metal oxide catalysts provides acid/base properties
Lewis acid sites:
coordination to an electrondeficient metal atom
n+
cusMe
strong basic NH3 at RT
oxygen
O2-
vacancy:
metall cations
under
participation of
oxygen: MexO-y
acidic CO2 at RT
weak basic CO at 77 K
▒
use of probe molecules such as educt, intermediate, product or molecules
closely related to the reactants is an elegant method to study the surface
sites relevant for catalytic reaction
Tadsorption < Treaction  study of the pure ads. processes
Tadsorption = Treaction  study of the surface chemical events during the reaction
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
OH-
Contents
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat
&
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Equipment
at the Fritz-Haber-Institute
Volumetric system
Calorimeter
Reference cell
(probe molecule dosing system)
Adsorptive
Microcalorimetry
Sample cell
Sample cell chamber
Isolation
Electronics
catalyst
Sample cell
Heating
Heat flow
Calorimetric
Element
Thermocouple
Calorimetric Element
Body
sample holder
L = 70 mm
= 15 mm
MS 70 Tian-Calvet calorimeter
of SETARAM combined with a
custom-designed high vacuum
and gas dosing apparatus.
Karge, H.G. etal., J. Phys. Chem. 98, 1994, 8053.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
The Calorimetric Element
 The sample cell is placed into a
calorimeter element
 The cell is surrounded by a thermopile
made of more than 400 conductive
thermocouples in series
 Thermopile has 2 functions:
1. heat transfer
2. signal generation
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Heat and Heat Flow
The heat produced by the
adsorption/reaction of a dosed probe
molecule with the catalyst surface
is consumed by 2 processes
1. Increase of the temperature of the
sample cell
2. Once there is a temperature gradient
between cell and surrounding block,
heat flow through the thermopile
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Reference Cell
Reference
Cell
Sample
Cell
The calorimetric block consists of
a sample cell and a reference
cell.
The reference cell compensate
external temperature fluctuations
and it provides a good stability of
the baseline.
Measurement of the temperature
difference Δ θ
Δθ
Setup according to
Tian and Calvet
The heat-flow detector gives a
signal “U” which is propotional to
the heat transferred per time unit.
N. C. Cardona-Martinez and J.A. Dumesic, Advances in Catalysis 38 150-243.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Physisorption and Chemisorption
Physisorption
Chemisorption
van der Waals forces
10 – 20 kJ/mol
noble gases, CH4, N2
chemical bonding,
electron transfer
80 – 500 kJ/mol
Dipole-dipole
20 – 50 kJ/mol
water on oxides
CO on metals
Reversibility
reversible
reversible or irreversible
Speed
fast
can be slow
(e.g. activated
adsorption)
Coverage
multilayers possible
monolayer only
Type of interaction and
heat of adsorption
(negative enthalpy of
adsorption)
dissociative adsorption
(O2, H2 on Pt, H2O on
oxides)
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Physisorption and Chemisorption
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Contents
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat
&
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Power Balance of Tian-Calvet Calorimeter
& Adsorption Heat Signal
The power P [W] necessary to heat the cell by d is
proportional to the heat capacity C [J/K] of the cell
P C
The heat flow
[power] is proportional to the
temperature gradient
between cell and block and to
the thermal conductivity G [W/K]
G(
Total thermal power of cell
Ptotal
The electrical signal is proportional to the temperature
difference; (proportionality factor g=f (number and type of the thermocouple))
The relation between power and electrical signal is then
G [W/K] is constant and if C [J/K] can be considered
constant, then C/G is a constant with units of time
cell
C
d
dt
block
d
dt
)
G
G
U g
Ptotal
C dU G
U
g dt
g
C
G
G
dU
U
g
dt
The Tian equation shows that the power is not proportional to
Ptotal
the temperature difference, the power is delayed with respect to
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
the signal U produced
by the cell
Evolved Heat &
Differential Heats of Adsorption
If heat is released in the cell for a limited period of time,
e.g. through adsorption, then a electrical signal U with an
exponential decrease is obtained .
The integral under the curve is proportional to the evolved
heat
G
A: area under curve [Vs]
Q int
g
U dt
f A
The calorimeter can be calibrated
by using an Ohm resistance which
produces a certain amount of heat.
Differential heats of adsorption as a
function of coverage can be determined
f: calibration factor [J/(Vs)]
Q = U*I*t
f = (U*I*t) / Aohm resistance [Ws/Vs]
qdiff
( Qint / n)T , A
Calculation of Adsorbed Amount
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Contents
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat
&
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Volumetric-Barometric System
pressure gauge dosing system
 VDos
p
DOSING VOLUME VDos
vacuum
CALIBRATION
VOLUME Vcal
probe molecule
CELL VOLUME
vacuum
p
Calibration of the volumetric system

Vcal = 31.8 ml
nads
sample cell
+ catalyst
pressure gauge sample cell
reference cell
T = constant
pCal
pCal
pCal Dos
VCal = 134.4 ml
pDos
Dos
 Wall adsorption isotherm reflects
number of molecules in the
gas phase and on the inner walls.
nSC,w
g ,i
a( pSC,i ) b( pSC,i ) 2 c( pSC,i )3 d ( pSC,i ) 4 ...
Amount of adsorbed molecules
 Determination of the dosed amount
nint,i
( pDos ,bef
pDos ,aft )VDos
RT
 The number of molecules adsorbed
in the (i+1)th step is then
nads,i

1
nint,i
1
nSC ,w
g ,i
nSC ,w
g ,i 1
The total number of ads. molecules
in a given steps is
nads,tot,i
1
nads,tot,i
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
nads,i
1
registered Raw Data
Raw data
equilibrium pressure and thermosignal (Qint.)
3.0
2.0
DOSING
VOLUME
8
1.5
7
1.0
6
0.5
5
4
CELL VOLUME
3
0
2
4
20 22 24 26 28 30 32 34
Zeit / h
0.0
2.5
1.2
2.0
1.0
1.5
0.8
1.0
0.6
0.5
0.4
0.0
0.2
20 21 22 23 24 25 26 27 28 29 30
Time / h
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Equilibrium pressure / hPa
9
1.4
Thermosignal / V
10
Pressure in sample cell / hPa
Pressure in dosing volume / hPa
Raw data
equilibrium pressure
Contents
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat
&
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
 Adsorption isotherm
6. Applications
in heterogeneous
 qdiff of microcalorimetry
strength of surface
sites
 qdiff = f (nads)
distribution of surface sites
catalysis
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
obtained results
Adsorption Isotherm
nads = f ( pequ.)
nads (total) : overall adsorbed amount under an equilibrium pressure of 95 mbar
nads (irrev.) : chemisorbed amount
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
evaluation criteria
Analysis of the adsorption isotherm
higher order Langmuir model,
of propane on 10%V/SBA15 catalyst
active in oxidation of propane
0,14
Freundlich model
; R2
Analysis
0,12
The enthalpy of adsorption ΔaH (qdiff )
per site is constant with coverage Θ
0,10
0,08
T activation
0,06
Nads - coverage with certain equilibrium pressure
Nmono - monolayer coverage
p
- equilibrium pressure
n
- adsorption order
K/A - adsorption equilibrium constant
R2 - correlation coefficient; goodness of fit
S
- stoichiometry
0,04
0,02
0,00
0
10
20
30
40
50
60
The enthalpy of adsorption ΔaH (qdiff )
per site decreases with coverage Θ
0.14
0.12
N / mmol*g
amount of adsorbed propane [mmol / g]
Adsorption Isotherm
0.10
Higher order Langmuir model
st
1 order Langmuir model
Freundlich model
R2=0.99983
R2=0.99853
R2=0.99973
0.08
0.06
0.04
0.02
equilibrium pressure [mbar]
0.00
0
10
20
30
40
px,i / mbar
50
60
Specific surface area propane = N mono * S * cross-section area propane,T * Avogadro constant
n = 1 non-dissociative ads.
n > 1 dissociative ads. ;
activated ads.
10%V/SBA15
dehydration
temperature
Nmono
µmol *g-1
n
373 K
0.9 (2)
1.20 (2)
573 K
1.3 (4)
673 K
1.2 (3)
R2
Spropane
BET
m2*g-1
SN2
m2*g-1
0.99983
226 (10)
329 (4)
1.22 (2)
0.99982
304 (10)
1.22 (2)
0.99905
290 (10)
higher order Langmuir
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
obtained results
Differential heat versus the nads
Differential heat
Interaction between
adsorbed molecules
strong
Lewis
acid sites
Brønsted
acid sites
Heterogeneous
acid site
Homogeneous acid strength
Enthalpy of condensation
Adsorbed volume
A. Auroux, Lecture Oct. 23,2009
Initial differential heat
Slope: heterogeneously distributed and
energetically different adsorption
sites
Plateau: homogeneously distributed and
energetically uniform adsorption sites
Completion
of monolayer
Differential heat of adsorbed
propane / kJ/mol
Classical calorimetric curve
Amount of adsorbed propane on phase-pure MoVTeNb#6059 / mmol/g
Differential heat of adsorbed
/ kJ/mol
oxygen
ads. [kJ/mol]
of oxygen
heatatom
Differential
Classical Calorimetric Curve
High diff. heats & heat oscillation :
dynamic ads. process
chemisorption-oxidation-reaction
800
700
600
most active
500
400
300
less active
200
100
0
0.0
0.1
0.2
0.3
5
10 15 20 25 30
Amount of ads. O on Pd/N-CNF
at Treact. / µmol/g
[ mol/g]
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the
MPG,
Germany
amount
of ads.
oxygen
2 Berlin,
Evaluation criteria
of the calorimetric experiment
Integral heat signal of
adsorption and desorption
Background of the thermo signal
during the stepwise adsorption
stepwise adsorption of propane on MoVTeNb oxide at 313K
stepwise n-butane ads. on sulf. ZrO2 at 313SZ
K
0.460
+ qintegral (ads.) = 7892 mJ
thermosignal / V
0.455
0.450
0.445
0.440
0.435
0.430
0
2000
4000
6000
8000
10000
12000
14000
time / s
- qintegral (des.)= 6560 mJ
q int (ads.) > q int (des.)  partially irreversible ads.;
activated ads. process
q int (ads.) < q int (des.)  instability of the catalyst
in the presence of probe molecule
q int (ads.) = q int (des.)  reversible ads. process
Background deviates from the base-line
 Adsorption process is accompanied by
secondary processes
e.g. during n-butane ads. a partial
isomerization of n-butane to isobutane
in the calorimeter cell was observed
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Evaluation criteria
of the calorimetric experiment
Determination of the time constant
y
y0
A exp
x
of the integral heat signal
x0
5.9 µmol/g propylene
adsorbed on MoVTeNb
at 313 K
1.36
0.62
gamma Al2O3, Methanol-Ads, 40grd, 3. Ads.-Step
(A+B*exp(-(x-C)/D),A=0.5438~,B=0.0249~,C=9237.80~,D=722.458~))
R=1kΩ →27.45 mJ
→
1.3
(calorimeter+cell) ~265 s
= 287 s ~ cal+cell
 pure ads. process
1.28
Signal Thermocolumns (V)
Signal Thermocolumns (V)
1.34
1.32
4.8 µmol/g
methanol adsorbed
on Al2O3 at 313 K
= 722 s > cal+cell
 ads. + reaction
0.6
0.58
0.56
0.54
1.26
0.52
1.24
8.8 104
8.85 104
8.9 104
8.95 104
9 104
9.05 104
9.1 104
8000
9000
1 104
1.1 104
1.2 104
1.3 104
1.4 104
Time (s)
Time (s)
Shape of the integral heat signal
O2 adsorption on 2%Pd/N-CNF473K at 353K=Treaction
1.0
0.35
K(t)= 377 s
0.30
0
10
Response20
time / 0.430
h
40
Thermosignal [V]
Thermosignal [V]
A
2 μmol/g oxygen
B
0.8
0.6
K(t)= 998 s
0.4
0.2
300 310 Response
320 330
time340
/ 0.7 h350 360
1.0
Thermosignal [V]
0.039 μmol/g oxygen
0.40
quasi pure dissociative
14 μmol/g oxygen oxygen chemisorption
C on Pd
nads.↑
0.8
0.6
0.4
0.2
200 300
400 500
700
Response
time600
/ 10.3
h 800 900
oxygen chemisorption
combined by
secondary processes
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Contents
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat
&
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Applications
Selected calorimetric measurements on supported metal oxide and mixed metal oxides .
Probe
Catalyst / Activation / Catalytic activity
T adsorption
q initial +
molecule
[K]
[kJ/mol]
n-butane
0.5wt%Pt/H-Mordenite / H2 reduced at 648 K /less active
313
350
n-butane
0.5wt%Pt/H-Mordenite / dehydr. at 723 K / active
313
42
n-butane
3.5wt% VOx-Al2O3 / H2 reduced at 773 K /active
313
63
n-butane
Θ Al2O3 / H2 reduced at 773 K /non-active
313
35
propane
10 wt% VxOy-SBA15 / dehydrated at 373 K / active
573 K
637K
313
45
80
160
propane
3 wt% VxOy-SBA15 / dehydrated at 373 K / active
313
45
propane
SBA15 / dehydrated at 373 K / non-active
313
32
propane
propane
hydrothermal synth. – M1 /dehydr. at 423 K /active, 58*
precipitation – M1 /dehydrated at 423 K /active , 43*
313
313
71
56
propane
SHWVT ** – M1 / dehydrated at 423 K / active, 5*
313
64
CO2
CNFox functionalized by NH3 at 873 K
673 K
473 K
313
150
50
90
CO2
O2
O2 ***
O2 ***
+
Fe-CNT / dehydrated at 373 K
FeIO-XT 24PS-CT / dehydrated at 373 K
313
272
191
2wt%Pd#/N-CNT873K/ dehydrated at 353 K
2wt%Pd#/N-CNT473K/ dehydrated at 353 K
2wt%Pd/CNT/ H2 reduction at 423 K
353
353
313
500-700
500
175
precipitation – M1 / H2 activated at 653 K
precipitation – M1 / propane activated at 653 K
473
218
257
473
248
8wt%VxOy-SBA15 / dehydrated at 673 K
The
method
was
broadly
employed in several projects of
our department and yielded a
surprising spread of energetic
data for the same molecule on
different surfaces .
In
addition,
we
observed
significant differences of the
energetic data for the same
molecule on slightly modified
surfaces.
We have adopted the calorimetric sign criterion (positive energetic quantity for an exothermic process).
partial oxidation of propane (POP); Selectivity to acrylic acid [mol-%]
superheated-water vapor treatment (SHWVT)
cooperation with Instituto de Quimica Fisica “Rocasolano", CSIC, Madrid (Spain)
CNFox functionalized by NH3 at 873K or 473K, Pd catalysts are obtained via sol-immobilization on the CNFs
in which Na2PdCl4, NaBH4 and polyvinylalcohol (PVA) were used . Hence, Pd particles are partially covered by PVA.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
## Vapor Growth Commercial Carbon Nanofiber; oxygen-containing nanocarbon was obtained by treating with HNO3 at 373 K.
*
**
***
#
CO adsorption on Pt/Al2O3 at 40°C
Adsorption isotherm of CO
Differential heat of adsorbed CO
Saturation concentration
Saturation
concentration
n ads. (irreversible)
= 20.4 µmol/g
0.035
0.030
Pt-CO = 1:1 = 206 kJ/mol
Pt2-CO = 94 kJ/mol[
amount of adsorbed CO
/ mmol*g
0.025
0.020
Model: higher order Langmuir model
qdiff = 200kJ/mol = constant versus nads
0.015
R = 0.88731
0.010
Nmono = 0.02631 (1strun) - 0.00583(2ndrun)
0.005
K
n
J.Therm. Anal. Cal., 82, (2005) 105
2
= 1662.35201 --> irreversibility
= 1.15679
SCO = Nmono * S * cross-section-area CO,T * Avogadro constant
SCO = 20.4*10-6mol/g * 1* 16.2*10-20m * 6.022*1023mol-1 = 1.99 m2/g
0.000
0.0
0.1
0.2
0.3
0.4
euilibrium pressure of CO / mbar
0.5
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
n-butane (educt) adsorption on
Pt/H-Mordenite catalyst at 40°C
Aim: structure-activity relationship study of Pt/HM for n-butane isomerization
Differential heat of ads. n-butane at 40 C
IR spectra of ads. CO at RT
400
15 kPa n-butane in H2 at 300 oC
2000
1800
fully H2-reduced Pt/HM
catalyst activated in H2 at 350 oC for 2 h
1600
200
not completely H2-reduced Pt/HM
100
catalyst activated in H2 at 300
oC
for 1 h
Diff. heat of
adsorbed n-butane [kJ/mol]
Rate of isomerisation [ mol/g.h]
n-butane isomerisation
3+
Al
0
1
2
3
0
Pt
300
x+
Pt
80
Ptx-CO
2300
2200
2100
2000
1900
Wavenumber / cm-1
60
1800
at 0.7 mbar
200 mol/g
450 mol/g
40
0.0
0
oct.
0.1
0.2
0.3
0.4
0.5
Equilibrium pressure of n-butane [mmol/g]
Time on stream [h]
The active states of Pt/HM are characterized by:
a small amount of strong Lewis acid sites (Al3+oct., extraframework alumina)
well dispersed metallic platinum particles (1-3 nm)
higher number of n-butane adsorption sites
weak interaction of the surface acid sites with n-butane.
The weaker interaction with the alkane is apparently favorable for the catalytic performance; perhaps because of facile product desorption.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Fe/sulf.ZrO2 is active in isomerization of
n-butane to isobutane at low temp. (100°C)
Question:
Does iso-butane (product) reacts with surface species
of Fe/sulf. ZrO2 at low temperature ?
Differential heat of adsorbed iso-butane & neo-pentane at 40 C
Differential heat [kJ/mol]
80
CH 3
70
C
CH3
60
CH3
CH3
The neo-pentane heat profile
follow the classical calorimetric
heat profile - heat decreases
with increasing the coverage
 pure ads.
50
CH 3
40
CH
30
20
0.00
CH
0.01
0.01
0.02
0.02
0.03
0.03
CH
3
3
0.04
Amount of neo-pentane or iso-butane sorbed [mmol/g]
Answer:
0.04
The scattering of the diff.
heats for iso-butane at low
coverage
indicates
the
presence of exothermic and
endothermic processes during
the adsorption.
 ads. + reaction
Yes, iso-butane (product) reacts with surface species of
Sabine Wrabetz, Electronic Structure
and Adsorption
/ Metals,
AC, Fritz
Haber Institute of the MPG, Berlin, Germany
Fe/sulf.
ZrO
low
temperature.
2 atDept.
pure-phase MoVTeNb catalyst active in
direct oxidation of propane to acrylic acid (aa)
study of the post-reaction state of the surface “used catalyst” in
comparison with the prepared state of the surface “fresh catalyst”
/ kJ/mol
Differential heat
Aim:
80
50
MoV
Mo
Nb
Te
O
Differential heat of propane adsorption at 40°C
phase-pure M1, Saa = 53%
Spropane=11.4m2/g
70
MoV oxide, Saa = 1.8%
Spropane=13.4m2/g
oxidized M1, Saa = 37%
Spropane=10.3m2/g
60
40
30
20
10
Saa
0
0.001 0.002 0.003 0.004 0.005
Amount of propane adsorption / mmol/m 2
→ homogeneity
energetically uniform
ads. sites
0
0.001 0.002 0.003 0.004 0.005
Amount of propane adsorption / mmol/m 2
→ strength of interaction between educt and surface sites
0
0.001 0.002 0.003 0.004 0.005
Amount of propane adsorption / mmol/m 2
→ density of propane ads. sites
( which is apparently favorable for catalytic performance;
perhaps because of facile intermediate desorption)
 the prepared state of the surface is different from the post-reaction state of the surface
 dynamic surface during reaction
Sabine Wrabetz, Yury V. Kolen’ko, Jutta Kröhnert, Lenard Csepei, Olaf Timpe, Wei Zhang, Annette Trunschke, and Robert Schlögl,
SabineofWrabetz,
Electronic
Adsorption
/ Metals,
Dept.
AC, Fritz Haber
Institute of the
Germany; in preparation 2010.
Characterization
MoVTeNb
catalyst Structure
in their asand
prepared
and active
state
by adsorption
microcalorimetry
andMPG,
-FTIRBerlin,
spectroscopy
quasi in-situ microcalorimetry
Study of the educt and product interaction with the surface of sulf. ZrO2
if the surface at the state of highest activity in the n-butane isomerization
-1
-1
Rate of Isomerization ( mol h g )
n-butane isomerisation at 378K
in the flow-type calorimetric sample cell
45
40
35
30
stopped
at
TOS=120min
25
20
15
10
5
0
0
20
40
60
80 100 120 140
Time on Stream (min)
n-butane &
iso-butane ads.
at 313 K
after degassing at 378K
Differential heat at 40 C
Differential heat [kJ/mol]
Aim:
80
70
60
50
40
30
20
10
0
0,000
K n-butane= kads./kdesorp. = 0.162
K iso-butane= kads. /kdesorp. = 0.011
0,005
0,010
0,015
0,020
0,025
Amount of adsorbed n-butane & iso-butane [mmol/g]
The state of maximum activity of sulf. ZrO2 is characterized by:
A stronger interaction with n-butane (~70kJ/mol, educt) than with iso-butane (~45kJ/mol, product).
The weak interaction with iso-butane and the equilibrium constant (Kiso-butane=0.011) indicate
an increasing easiness of desorption of iso-butane from the surface sites.
Wrabetz, Sabine;
Yang,Wrabetz,
Xiaobo; Electronic
Tzolova-Müller,
Genka;
Schlögl, Robert;
Jentoft,
C., J. ofInstitute
Catalysis
269MPG,
/2 (2010)
351
- 359.
Sabine
Structure
and Adsorption
/ Metals,
Dept.Friederike
AC, Fritz Haber
of the
Berlin,
Germany
Microcalorimetric titration of basic sites
on N-CNFs by CO2 adsorption at 40°C
Oxidation of VGCNF by HNO3 at 373K for 2h and
amination using NH3 at 473/673/873 K.
Differential heat of adsorbed CO2 [kJ/mol]
Different oxygen and nitrogen species in CNFs
Differential heat of adsorbed CO2
N-CNF873K
N-CNF473K N-CNF673K
VGCNF
160
a)
b)
c)
d)
140
depending on the Tamination the N-CNF
surface provides energetically different
basic sites which are heterogeneously
distributed
120
CNF surface chemistry plays a crucial
role in Metal/CNF interaction
100
partially irreversible
adsorbed
80
60
40
20
0
2
4
6
8
0
2
4
6 0
2
4
6 0
Amount of adsorbed CO2 [ mol/g]
1
2
Fe/N-CNF873K is the best catalyst in
electrocatalytic conversion of CO2 .
Pd/N-CNF873K is the best catalyst in
oxidation of benzyl alcohol to
3
benzaldehyde
Arrigo, Rosa; Haevecker, Michael; Wrabetz, Sabine; Blume, Raoul; Lerch, Martin; McGregor, James; Parrott, Edward; Zeitler, J; Gladden,
Lynn; Knop-Gericke, Axel; Schlogl, Robert; Sabine
Su, Dangsheng,
Journal
of theStructure
American and
Chemical
Society/ 132/28
- 9630.
Wrabetz,
Electronic
Adsorption
Metals,(2010)
Dept. 9616
AC, Fritz
Haber Institute of the MPG, Berlin, Germany
Conclusion
Microcalorimetry alone or combined with to other techniques, is applied
for the characterization of catalysts, supports and adsorbents, and to
the study of catalytic reactions (adsorption-desorption phenomena).
very sensitive and hence a selective surface characterization method
but very time consuming ( ~1 week for 1 experiment)
The nature of the reactive surface sites can be studied by
measuring the thermal effects during the reaction.
The knowledge about the energetics of the surface chemical
events helps better to understand the catalytic properties of the
surface and hence the catalytic reaction characteristics.
The calorimetric data obtained are of substantial importance for comparing
theoretical and experimental hypotheses about reaction pathways.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Literature
A. Auroux “Thermal Methods: Calorimetry, Differential Thermal Analysis, and
Thermogravimetry” in “Catalyst characterization: physical techniques for solid
materials”, Eds. B. Imelik, J.C. Vedrine, Plenum Pr., New York 1994
E. Calvet, H. Prat, H.A. Skinner “Recent progress in microcalorimetry”,
Pergamon Pr., Oxford1963
B. E. Handy, S.B. Sharma, B.E. Spiewak and J.A. Dumesic, “A Tian-Calvet
heat-flux microcalorimeter for measurement of differential heats of adsorption”,
Meas. Sci. Technol. 4 (1993) 1350-1356.
N. C. Cardona-Martinez and J.A. Dumesic, “Application of Adsorption
Microcalorimetry to the Study of heterogeneous Catalysis”, Advances in
Catalysis 38 150-243.
Z. Knor, “Static Volumetric Methods for Determination of Adsorbed Amount of
Gases on Clean Solid Surface”, Catalysis Reviews 1 (1) (1968) 257-313.
S. Černý and V. Ponec, „ Determination of Heat of Adsorption on Clean Solid
Surfaces”, Catalysis Reviews 2 (1) (1969) 249-322.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Acknowledgement of financial support
Thanks the European Union for finacial support of the project
“Integrated Design of Nanostructured Catalytic Materials for a
Sustainable Development” .
Financial support for “Brückenschläge zwischen idealen und realen
Systemen in der Heterogenen Katalyse“ through DFG priority
program 1091 (JE 267/1-3) is gratefully acknowledged.
The SFB-project “Struktur, Dynamik und Reaktivität von
Übergangsmetalloxid- Aggregaten“ was sponsered by DFG.
Author thanks the DFG for providing an Emmy Noether
fellowship to the project leader “propane oxidation over
V/SBA15” Prof. C. Hess
The project “Pt-doped H-mordenite is used as a solid acid
catalyst for the isomerization of light alkanes” was financially
supported by BMBF grant 03C0307E.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Thank you for your attention !
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany