Lezioni 2017 -5.ppt PPT PPT PPTPPPPPTPPPTPPPTT

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1. Thermoanalytical techniques
measurement of a physical
or chemical property of a
solid as the temperature of
the solid is varied in a
predetermined manner
temperature programmed
Catalyst Characterization by
Temperature Programmed Methods

2. Thermoanalytical techniques
Techniques dependent on
dimensional changes
Techniques dependent on
evolved gases
Techniques dependent on
gas analysis from
chemical reaction
DILATOMETRY
Techniques dependent on
weight changes
Thermogravimetry
Techniques dependent on
energy changes
Diff. Thermal analysis
Diff. Scanning calorim.
Temp. prog. desorption
Temp. prog. reaction
Temp. prog. reduction
TPD, TPDE,
TPSR
TPRE, TP...
TPR/TPO

3. temperature programmed desorption/decomposition
time
coverage
temperature
rate
•Pretreatment of the catalyst
•Exposure to reactant gas
•Desorption of physisorbed fraction
•Heating of sample in an inert gas stream
•Analysis of desorbed components

4. temperature programmed reduction/oxidation
time
Degree of
reduction
temperature
reduction
rate
•Pretreatment of the catalyst
•Heating of sample in the presence
of reducing or oxidizing mixture
•Analysis of reductant or oxidant
consumption
MO(s) + H
2(g) M(s) + H
2O(g)
M(s) + O
2(g) MO(s)

5. temperature programmed reaction
Coadsorption of two gases and heating in inert carrier
Adsorption of one component and heating in reactive carrier gas
Heating in reactive atmosphere containing reagents
Temperature programmed methanation,
hydrogenation, sulphidation, combustion…….

6. COCO CO
CO
CO
CO
CO
2CO
2
CO
2
OO
CO
CO
COCO CO
CO
2CO
2
CO
2
TPD
TPDE
TPRE
desorption vs decomposition vs reaction

7. •Characterization of reducibility of catalysts
•Determination of binding energy of adsorbed molecules
•Acidity
•Kinetic of catalytic reaction (combustion, oxidation, methanation….)
•Characterization of surface carbon deposits
•Physical parameters (surface area, dispersion…)

information that can be obtained…

8. Experimental:
apparatus for TP studies
introduction
of reactants
furnace and
reactor
detector
data acquisition
time/temp.
c
o
n
c
e
n
t
r
a
t
i
o
n

9. Experimental: Detector and data acquisition
QMS
GC
Thermal conductivity detector
 Good for TPO/TPR
 Non specific gas analysis
 Concentration can be monitored continuously
Mass spectrometer
 Concentration can be monitored continuously
 Specific gas analysis
 High cost
Micro GC
 Concentration cannot be monitored continuously
(delay 1-2 min.)
 Complex gas analysis
 Accurate quantitative evaluation

10. Experimental: 5. Practical consideration
Gas flow rate
High enaugh to avoid time delay
between desorption/ reaction and
detection
Low flow rate might cause
diffusion problems
Sample mass
Sample particle size
Geometry of the bed
Heating rate
Leaks and carrier-gas
impurities
Mass of sample should be kept to a
minimum to avoid backpressure
problems and temperature gradients
within the bed
smaller particles decrease the
possibility of intraparticle diffusional
limitations and allow better
thermocouple contact
small particles can create pressure
drop or even fall through the reactor
support
Too thin layer results in an irregular
bed
Too deep results in back pressure and
flow changes
Probably best arrangement is to have
roughly equal depth and width
High heating rates result in better
defined peaks, less time per run and
less time for changes in flow rate and
baseline
Programmer must be able to maintain
a linear profile, too high heating rates
can result in diffusional limitations
Because of large carrier gas flow
rate relative to the quantity of
adsorbed gas, extreme care must
be taken in gas purification

11. Temperature Programmed Desorption
Determining the strength of an adsorbate bond to the surface.
TPD-title

12. Lennard-Jones Potential
associative & un-activated adsorption
z
p
o
t
e
n
t
i
a
l
e
n
e
r
g
y
H
ads E
des
H
ads = -E
des
DESORPTION
TPD-un activated

13. Lennard-Jones Potential
dissociative & un-activated
z
p
o
t
e
n
t
i
a
l
e
n
e
r
g
y
H
ads
E
des
H
ads
= -E
des
DESORPTION
TPD-un-activated

14. Lennard-Jones Potential
dissociative & activated adsorption
z
p
o
t
e
n
t
i
a
l
e
n
e
r
g
y
H
ads
E
des
H
ads
=/= -E
des
E
ads
DESORPTION
TPD-activated

15. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt
p

Temperature Programmed Desorption
fixed volume
TPD-fixed volume 1

16. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt
p

Temperature Programmed Desorption
fixed volume
TPD-fixed volume 2

17. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

p
Temperature Programmed Desorption
fixed volume
TPD-fixed volume 3

18. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

p
Temperature Programmed Desorption
fixed volume
TPD-fixed volume 4

19. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

p
Temperature Programmed Desorption
fixed volume
TPD-fixed volume 5

20. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

p

-ddt
Temperature Programmed Desorption
pumped volume
TPD-pumped volume 1

21. 
p
Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

-ddt
Temperature Programmed Desorption
pumped volume
TPD-pumped volume 2

22. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

-ddt

p
Temperature Programmed Desorption
pumped volume
TPD-pumped volume 3

23. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

-ddt

p
Temperature Programmed Desorption
pumped volume
TPD-pumped volume 4

24. Time (t) / s
T
e
m
p
e
r
a
t
u
r
e
(
T
)

/

K
Time (t) / s
P
r
e
s
s
u
r
e
(
p
)

/
m
b
a
r
Time (t) / s
C
o
v
e
r
a
g
e




gradient = dT/dt

-ddt

p
Temperature Programmed Desorption
pumped volume
TPD-pumped volume 5

25. Time (t) / s
C
o
v
e
r
a
g
e



 
-ddt
Use the Arrhenius expression for the rate of
desorption R
des
:
Temperature Programmed Desorption
the rate of desorption
R
des
= - d/dt = [reactant] Aexp { - E
des
/R T }
- d/dt =  exp { - E
des
/R T } equation I
Arrhenius constant = frequency factor
Concentration = coverage
TPD-Arrhenius

26. TPD-frequency factor
Temperature Programmed Desorption
the frequency factor
- d/dt =  exp { - E
des
/R T } equation I
Reactive collisions per second = frequency of vibration
For a vibration:- E = h 
The energy in a vibration E = kT
 = kT/h = ca. 10
13
s
-1
at 300K

27. For a linear heating rate:
T = T
0
+ At
where A = dT/dt (the heating rate)
hence d /dt = A d /dT
Temperature Programmed Desorption
the rate of desorption
Time (t) / s
T
e
m
p
e
r
a
t
u
r
e

(
T
)
/
K
T
0
gradient = A = dT/dt
T
t
- d/dT = ( /A)exp { - E
des
/R T } equation II
Therefore from equation I
TPD-linear heating

28. - d/dT = ( A)exp { - E
des
/R T } equation II
At the peak maximum, corresponding to a temperature T
P
- d
2
/dT
2
= 0
Temperature (T) / K
-
d


d
t
T
-ddT
T
P
TPD-peak maximum
Temperature Programmed Desorption
the peak maximum

29. - d/dT = (A) { exp { - E
des
/R T } }
Differentiate - d/dT with respect to T to obtain - d
2
/dT
2
Using d(U*V)/dT = U dV/dT + V dU/dT
A {(E
des
/R T
2
) exp { - E
des
/R T } + d/dT exp { - E
des
/R T } }
- d
2
/dT
2
= 0 =
constant
d/dT = E
des
/R T
P
2
equation III
Therefore since at T=T
P, - d
2
/dT
2
= 0 :
TPD-differentiation
Temperature Programmed Desorption
differentiation

30. d/dT = E
des
/R T
P
2
equation III
- d/dT = ( A)exp { - E
des
/RT
P
} equation II
Combining equation III with equation II to eliminate the term d/dT :
E
des
/R T
P
2
= (A)exp { - E
des
/R T
P
} equation IV
Temperature (T) / K
-
d


d
t
T
-ddT
T
P From a measured value of T
P one can
calculate
E
des
In the case of non-activated
adsorption
E
des
= -
ads
TPD-result
Temperature Programmed Desorption
the equation

31. E
des
/R T
P
2
= (A)exp { - E
des
/R T
P
} equation IV
The easiest way to obtain – E
des
from T
P is to use an iterative
method on a re-arranged form of equation IV:
E
des
/R T
P
2
= (A)exp { - E
des
/R T
P
} equation IV
E
des
= R T
P ln { R T
P
2
/ A E
des
ie.guess a sensible value for E
des
(eg 100 kJ mol
-1
for chem.);
substitute in RHS, calculate E
des
;
Substitute new value in RHS, re-calculate, etc
The solution quickly converges!
TPD-calculation
Temperature Programmed Desorption
the calculation

32. TPD-coverage
Temperature Programmed Desorption
the coverage
Temperature (T) / K
-
d


d
t
T
-ddT
T
P
The area under a TPD peak is proportional to the coverage.
( - d/dT) dT  d
T
1
T
2

33. Ru/Al
2
O
3
Fischer-Tropsch Catalyst
C + H
2O  CO + H
2
CO + H
2
 HC / ROH
CO – TPD
CO preadsorbed at 303 K
Flow of He 30 cc/min

34. CO-TPD from Al
2O
3
CO desorbed from
Al
2
O
3
is 1/3 of that
desorbed from
Ru/Al
2
O
3

35. CO-TPD from Ru/Al
2
O
3
:Effect of gas flow rate
Re-adsorption!!!

36. CO
(ads)
 C
(ads)
+ O
(ads)
slow step
CO
(g)
+ O
(ads)
 CO
2(g)
TPR
coking
Ru single crystal at
low Pco
No CO
2 formation

37. 300 700 1100
H
y
d
r
o
g
e
n

C
o
n
s
u
m
p
t
i
o
n

(
a
.
u
.
)
Temperature (K)
10
20
30
40
50 T
50 C
60
70
80
90
100
Bulk Reduction
% CeO2
Rh2O3
L.S.A. : Surface Area 1 m /g
Temperature Programmed Reduction

38. 300 700 1100
H
y
d
r
o
g
e
n

C
o
n
s
u
m
p
t
i
o
n

(
a
.
u
.
)
Temperature (K)
10
20
30
40
50 T
50 C
60
70
80
90
100
Bulk Reduction
% CeO2
Rh2O3
L.S.A. : Surface Area 1 m /g
Temperature Programmed Reduction
Ce - O = 2.31 A (8)
Zr - O = 2.13 A (4)
= 2.34 A (2)
> 2.62 A (2)
°
°
°
°
Zr
Ce
O