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

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

3. temperature programmed desorption/decomposition
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
coverage
temperature
time

4. temperature programmed reduction/oxidation
MO(s) + H2(g) M(s) + H2O(g)
M(s) + O2(g) MO(s)
reduction
rate
Pretreatment of the catalyst
Heating of sample in the presence of reducing or oxidizing mixture
Analysis of reductant or oxidant consumption
Degree of
reduction
temperature
time

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. desorption vs decomposition vs reaction
TPD CO CO
CO2
CO2
CO2 O O
TPDE CO CO CO
CO2
CO2
CO2
TPRE

7. information that can be obtained…
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…)

8. apparatus for TP studies
Experimental:
apparatus for TP studies
introduction of reactants
furnace and reactor
detector
concentration
data acquisition
time/temp.

9. Experimental: Detector and data acquisition
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
Sample mass
High enaugh to avoid time delay between desorption/ reaction and detection
Low flow rate might cause diffusion problems
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
Mass of sample should be kept to a minimum to avoid backpressure problems and temperature gradients within the bed
Because of large carrier gas flow rate relative to the quantity of adsorbed gas, extreme care must be taken in gas purification
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
Sample particle size
Geometry of the bed
Heating rate
Leaks and carrier-gas
impurities

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

12. Lennard-Jones Potential
TPD-un activated
DESORPTION
Lennard-Jones Potential
associative & un-activated adsorption
potential energy
DHads = -DEdes
DEdes z
DHads

13. Lennard-Jones Potential
TPD-un-activated
DESORPTION
Lennard-Jones Potential
dissociative & un-activated
DHads = -DEdes
potential energy
DEdes z
DHads

14. Lennard-Jones Potential
TPD-activated
DESORPTION
Lennard-Jones Potential
dissociative & activated adsorption
DHads =/= -DEdes
potential energy
DEdes
DEads z
DHads

15. Temperature Programmed Desorption fixed volume
TPD-fixed volume 1
Temperature Programmed Desorption fixed volume
Temperature (T) / K
gradient = dT/dt
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q
Coverage (Q)
Time (t) / s

16. Temperature Programmed Desorption fixed volume
TPD-fixed volume 2
Temperature Programmed Desorption fixed volume
Temperature (T) / K
gradient = dT/dt
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q
Coverage (Q)
Time (t) / s

17. Temperature Programmed Desorption fixed volume
TPD-fixed volume 3
Temperature Programmed Desorption fixed volume
Temperature (T) / K
gradient = dT/dt
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q
Coverage (Q)
Time (t) / s

18. Temperature Programmed Desorption fixed volume
TPD-fixed volume 4
Temperature Programmed Desorption fixed volume
Temperature (T) / K
gradient = dT/dt
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q
Coverage (Q)
Time (t) / s

19. Temperature Programmed Desorption fixed volume
TPD-fixed volume 5
Temperature Programmed Desorption fixed volume
Temperature (T) / K
gradient = dT/dt
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q
Coverage (Q)
Time (t) / s

20. Temperature Programmed Desorption pumped volume
TPD-pumped volume 1
Temperature Programmed Desorption pumped volume
gradient = dT/dt
Temperature (T) / K
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q Q
Coverage (Q)
-dQ/dt
Time (t) / s

21. Temperature Programmed Desorption pumped volume
TPD-pumped volume 2
Temperature Programmed Desorption pumped volume
gradient = dT/dt
Temperature (T) / K
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q Q
Coverage (Q)
-dQ/dt
Time (t) / s

22. Temperature Programmed Desorption pumped volume
TPD-pumped volume 3
Temperature Programmed Desorption pumped volume
gradient = dT/dt
Temperature (T) / K
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q Q
Coverage (Q)
-dQ/dt
Time (t) / s

23. Temperature Programmed Desorption pumped volume
TPD-pumped volume 4
Temperature Programmed Desorption pumped volume
gradient = dT/dt
Temperature (T) / K
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q Q
Coverage (Q)
-dQ/dt
Time (t) / s

24. Temperature Programmed Desorption pumped volume
TPD-pumped volume 5
Temperature Programmed Desorption pumped volume
gradient = dT/dt
Temperature (T) / K
Time (t) / s p
Pressure (p) / mbar
Time (t) / s Q Q
Coverage (Q)
-dQ/dt
Time (t) / s

25. Temperature Programmed Desorption the rate of desorption
TPD-Arrhenius
Time (t) / s
Coverage (Q) Q
-dQ/dt
Temperature Programmed Desorption the rate of desorption
Use the Arrhenius expression for the rate of desorption Rdes :
Rdes = - dQ/dt = [reactant] A exp { - DEdes/R T }
Concentration = coverage
Arrhenius constant = frequency factor
- dQ/dt = Q n exp { - DEdes/R T } equation I

26. Temperature Programmed Desorption the frequency factor
TPD-frequency factor
Temperature Programmed Desorption the frequency factor
- dQ/dt = Q n exp { - DEdes/R T } equation I
Reactive collisions per second = frequency of vibration
For a vibration:- E = h n
The energy in a vibration E = kT
n = kT/h = ca s-1 at 300K

27. Temperature Programmed Desorption the rate of desorption
TPD-linear heating
Temperature Programmed Desorption the rate of desorption
Time (t) / s
Temperature (T) / K T0
gradient = A = dT/dt T t
For a linear heating rate:
T = T0 + At
where A = dT/dt (the heating rate)
hence d1 /dt = A d1 /dT
Therefore from equation I
- dQ/dT = (Q n /A) exp { - DEdes/R T } equation II

28. Temperature Programmed Desorption the peak maximum
TPD-peak maximum
Temperature Programmed Desorption the peak maximum
- dQ/dT = (Q n / A) exp { - DEdes/R T } equation II
At the peak maximum, corresponding to a temperature TP
- d2Q/dT2 = 0
Temperature (T) / K
-dQ/dtT
-dQ/dT TP

29. Temperature Programmed Desorption differentiation
TPD-differentiation
Temperature Programmed Desorption differentiation
Differentiate - dQ/dT with respect to T to obtain - d2Q/dT2
Using d(U*V)/dT = U dV/dT + V dU/dT
- dQ/dT = (n /A) { Q exp { - DEdes/R T } }
constant
- d2Q/dT2 = 0 =
n /A {(Q DEdes/R T2 ) exp { - DEdes/R T } dQ /dT exp { - DEdes/R T } }
Therefore since at T=TP, - d2Q/dT2 = 0 :
dQ /dT = Q DEdes/R TP equation III

30. Temperature Programmed Desorption the equation
TPD-result
Temperature Programmed Desorption the equation
dQ /dT = Q DEdes/R TP equation III
- dQ/dT = (Q n / A) exp { - DEdes/RTP } equation II
Combining equation III with equation II to eliminate the term dQ /dT :
DEdes/R TP2 = (n / A) exp { - DEdes/R TP } equation IV
Temperature (T) / K
-dQ/dtT
-dQ/dT TP
From a measured value of TP one can calculate DEdes
In the case of non-activated adsorption
DEdes = -DHads

31. Temperature Programmed Desorption the calculation
TPD-calculation
Temperature Programmed Desorption the calculation
DEdes/R TP2 = (n / A) exp { - DEdes/R TP } equation IV
The easiest way to obtain – DEdes from TP is to use an iterative method on a re-arranged form of equation IV:
DEdes/R TP2 = (n / A) exp { - DEdes/R TP } equation IV
DEdes = R TP ln {n R TP2 / A DEdes
ie.guess a sensible value for DEdes (eg 100 kJ mol-1 for chem.);
substitute in RHS, calculate )Edes;
Substitute new value in RHS, re-calculate, etc
The solution quickly converges!

32. Temperature Programmed Desorption the coverage
TPD-coverage
Temperature Programmed Desorption the coverage
The area under a TPD peak is proportional to the coverage. T2
( - dQ/dT) dT a dQ T1 TP
-dQ/dtT
-dQ/dT
Temperature (T) / K

33. Ru/Al2O3 Fischer-Tropsch Catalyst
C + H2O  CO + H2
CO + H2  HC / ROH
CO – TPD
CO preadsorbed at 303 K
Flow of He 30 cc/min

34. CO-TPD from Al2O3
CO desorbed from Al2O3 is 1/3 of that desorbed from Ru/Al2O3

35. CO-TPD from Ru/Al2O3 :Effect of gas flow rate
Re-adsorption!!!

36. TPR
CO(ads)  C(ads) + O(ads) slow step
CO(g) + O(ads)  CO2(g)
Ru single crystal at low Pco
No CO2 formation
coking