1. Catalysts Deactivation
Loss of activity with time on stream
Time
Activity
rigeneration

2. Catalysts Deactivation
Loss of activity with time on stream
Poisoning
Coking
Sintering
Phase Transformation

3. 1. Poisoning
Loss of activity due to the strong chemisorption on the active sites of impurities present in the feed stream
Geometric effect: blocking an active site
Electronic effect: alter the adsorptivity of other species
Chemical effect: alter the chemical nature of the active site
Reconstruction: formation of new compounds

4. 1. Poisoning
Reversible: not too strongly adsorbed
Irreversible: irreversible damages
Selective: contribution of the characteristics of the active site
Non-Selective: poison occurs in a uniform manner
Affected
Selectivity
Not Affected (Selective Poisoning)

5. 1b. Preventing Poisoning
Best method:
Decrease to acceptable levels the poison
content in the feed stream
HDS + H2S absorption
Methanation of COx in NH3 synthesis
Another approch:
Choose proper catalyst formulation and design

6. Catalytic reactions involving HC or CO
2. Coking
Catalytic reactions involving HC or CO
Formation of carbonaceous residues
Coke
Carbon or
Coking or condensation
of HC
CHx: 0.5 < x < 1
CO disproportionation
2 CO C + CO2
Physically cover the active sites
Coke or
Carbon
Pore blocking

7. 2. Coking
Mechanism of coke formation
Olefin polymerization
Olefin cyclization to substituted benzenes
Formation of polynuclear aromatics from benzenes
via carbonium ions intermediates
Analytical techniques IR
EPR
TG-DTA
UV-vis
13C-NMR
Evolution of combustion products
(CO2 and H2O)

8. 2. Coking
Coke deposition
Preferencially on the exterior of the particles
Resistance to the transport of reactants and products in the pores
Pore blocking

9. Appropriate combination of process conditions
2b. Preventing Coking
Appropriate combination of process conditions
The gas mixture composition is kept as far as possible from conditions under which carbon formation is thermodynamically favored
High H2 pressure in catalytic reforming
Low HC / H2O ratios in steam reforming over Ni catalysts
Optimal catalyst composition
Promoters or additives that enhance the rate of gasification adsorben carbon or coke precursors and / or depress the carbon - forming reactions.
K in Ni - based steam - reforming catalysts
Pt - Re / Al2O3

10. Metal catalysts (supported or unsupported)
3. Sintering
Loss of active surface area via structural modificaion of the catalyst.
Metal catalysts (supported or unsupported)
Agglomeration and coalescence of small metal crystallites into larger ones with low surface - to - volume ratio.
Crystalline
migration
Atomic
migration
Result:
Particle size growth
Low
Temperature
High
Temperature

11. Coalescence and growth of bulk oxide cristallites
3. Sintering
Oxide catalysts
Coalescence and growth of bulk oxide cristallites
1250°C
1400°C
1600°C
1550°C
Result:
Increase in crystallite dimension
Increase in grain dimensions
Decrease of surface area and porosity
SEM images of CeO2 sintered at different temperatures
Zhang T. et al. Materials Letters 57 (2002),

12. Key factors: Temperature Reaction atmosphere 3. Sintering
Sinterisation is a thermic activated process
Reaction atmosphere
Water vapor accelerates crystallization and structural change in oxide catalysts.

13. 4. Solid Phase Transformations
Transformation of one phase into a different one
Metal supported catalysts
Incorporation of metal into the support
Ni/Al2O3
active for reforming
Rh/Al2O3
Three - Way catalyst
NiAl2O4
inactive for reforming
Rh2Al2O4
Lean conditions

14. 4. Solid Phase Transformations
Metal oxide catalysts
Transformation of one crystalline phase into a different one with a step - wise decrease in the internal surface area
Limited by rate of nucleation
Foreign compounds in the bulk or even on the surface can influence this process:
Na2O enhances sintering of g - Al2O3
BaO, CeO2, La2O3, SiO2, ZrO2 improve the stability of g - Al2O3

15. Ammonia Synthesis A: Steam reforming CH4 + H2O = CO + 3H2 NiO/Al2O3
B: High temperature water-gas shift CO + H2O = CO2 + H2 NiO/Al2O3/ CaO
C: Low temperature water-gas shift Zn/Cu
D: CO2 absorption basic solution / homogeneous catalyst
E: Methanation (CO poison) CO + 3H2 = CH4 + H2O Ni/Al2O3
F: Ammonia synthesis N H2 = 2NH3 Fe/Al2O3/K2O/CaO
G: NH3 separation.

16. Ammonia Synthesis
The catalyst beds in a reactor for production of 1500 t NH3/day are, in round numbers, 100 m3 and contain 250 t of catalyst.
1010 kg / year of NH4NO3
for USA fertiliser and explosives industry
4NH3 + 5O2  4NO + 6H2O Pt or Pt alloy
NO + 1/2 O2  NO2
NO2 + 1/2 H2O  HNO3

17. Ammonia Synthesis Dn = -2 DH773K = -109 KJmol-1 Rate ? T = 400-500 °C
1/2 N /2 H2  NH3
T = 300 °C
T = 400 °C
T = 500 °C
Dn = -2
DH773K = -109 KJmol-1
Rate ?
XNH3
T = °C
P = atm
Pressure

18. Ammonia Synthesis
i =
R = k q q = f (T, flow rate and gas composition, catalyst….)
q = 4/10

19. Heterogeneous reactions
Surface boundary layer
1. Diffusional transport of reactants from bulk phase to external catalyst surface
2. Pore diffusion of reactants to internal surface
3. Adsorption of reactants
4. Chemical reaction
5. Desorption of products
6. Pore diffusion of products to external surface
7. Diffusional transport of products to bulk phase
Bulk phase
reactants
3, 4, 5 1
Catalyst pore 6 7 2
products
3, 4, 5
External catalyst surface

20. Transport phenomena Porous solid Intrareactor transport Intrapellet
Stagnant gas film
Pores
Solid
Porous solid
Intrapellet
transport
Intrareactor transport
Interphase transport
Stagnant gas film
Flowing bulk gas

21. Ammonia Synthesis Molecular route Atomic route
N2* H*  2NH3 * + 4* N* H*  NH3 * + 3*
N2  N2(ads)  2N (ads) N2  N2(ads)  2N (ads)  NH3
 surface saturation
NH3
steady- state [N(ads)] = k
Thermal stability
H2(g) *  2H* Tdes < 200°C
N2(g) *  N2* Tdes <-100°C
N2 * *  2N* Tdes °C
NH3(g) *  NH3* Tdes <100°C
Fe(111) 310 °C in vacuum after reaction

22. Ammonia Synthesis 1 H2(g) + 2*  2H* 2 N2 (g) + *  N2*
3 N2* *  2 N* slow
4 N* H*  NH* *
5 NH* H*  NH2* + *
6 NH2* H*  NH3* + *
7 NH3*  NH3(g) + *

23. Ammonia Synthesis

24. Ammonia Synthesis
Cat. BASF: FeOX / K2O / Al2O3 / CaO
Reduction at 400° C
Fe met / K2O / Al2O3 / CaO
XPS: X-ray Photoelectron Spectroscopy
AES: Auger Electron Spectroscopy

25. Ammonia Synthesis p* s
Promoter (K)
Poison N2 Fe N N Fe K

26. Ammonia Synthesis
The reactor may contain 2 catalyst beds and 2 internal heat exchangers inside a pressure shell:
Upper heat
exchanger
Lower heat
exchanger
First bed
Second bed

27. Ammonia Synthesis
A: The main flow (pale blue) first flows along the pressure shell and cools it. The gas is then heated in the lower heat exchanger and quench gas (green) is added.
B: In the first bed, both the temperature and the concentration increases. At the outlet from the first bed, the state of the gas is very near equilibrium (red curve).
C: The gas is cooled in the upper heat exchanger and the third inlet gas stream (dark blue) is added. This inlet stream was preheated in the upper heat exchanger.
D:In the second bed, both the temperature and the concentration increases.
E: The gas is cooled in the lower heat exchanger and leaves the reactor as the outlet stream (red).