1. Catalyst Selectivity Synthesis gas applications
CH4
CH3OH
CnH2n+2
CnH2n
CnH2n+1OH
(n = 1 - 6)
H2 / CO Ni Cu
Cu + Co
Fe, Co
Catalysis and Catalysts - Activity, Selectivity and Stability

2. Examples of Catalyst Deactivation
Time (h)
1.0
0.8
0.6
0.4
0.2
0.0
r (rel)
Time (h)
Methanol Yield (gcm-3h-1)
p GHSV T
= 70 bar
= h-1
= 515 K b
CO H CH3OH c
FCC
Methanol Synthesis
HDS a
S-344 (660 K) 5
k1.85 (gcm-3h-1%S-0.85)
S-324 (655 K)
Catalysis and Catalysts - Activity, Selectivity and Stability
Time (h)

3. Catalytic Reforming (Gasoline Production) 30 20 10
Conversion (% olefins/initial paraffins)
Time (h)
+ 0.17% W
+ 0.17% Re
+ 0.04% Ru
+ 0.04% Ir
Pt only pH
pHC
LHSV T
= 1.35 bar
= 0.10 bar
= 1 h-1
= 745 K 2
C12H26 C12H H2
Catalyst Pt (0.2%) / Al2O3 d
Deactivation due to coke deposition
Alloying quite successful
Catalysis and Catalysts - Activity, Selectivity and Stability

4. Time-Scale of Deactivation10 -1 1 2 3 4 5 6 7 8
Hydrocracking
HDS
Catalytic reforming EO
Hydrogenations
Aldehydes
Acetylene
Oxychlorination MA
Formaldehyde NH
oxidation
SCR
Fat hardening
Time / seconds
TWC
1 year
1 day
1 hour C
dehydrogenation
FCC
Most bulk
processes
year
Batch
processes
hrs-days
Catalysis and Catalysts - Activity, Selectivity and Stability

5. Deactivation of catalysts irreversible loss of activity
Types of deactivation:
Poisoning: strong chemisorption of impurity in feed (Inhibition: competitive adsorption, reversible)
Fouling: secondary reactions of reactants or products,
‘coke’ formation
Thermal degradation: sintering (loss of surface area), evaporation
Mechanical damage
Corrosion/leaching
Fouling or ‘self-poisoning’ often cause of deactivation
Catalysis and Catalysts - Activity, Selectivity and Stability

6. Types of Deactivation
Catalysis and Catalysts - Activity, Selectivity and Stability

7. What are poisons? Examples High M.W. product producer Strong
chemisorber
Surface active
metal or ion
Sintering
accelerator
Bases
H2S on Ni
NH3 on Si-Al
‘Toxic compounds’
(free electron pair)
Cu on Ni
Ni on Pt
Pb or Ca on Co3O4
Pb on Fe3O4
Fe on Cu
Fe on Si-Al
from pipes
acetylenes
dienes
H2O (Al2O3)
Cl2 (Cu)
from feed
or product
Catalysis and Catalysts - Activity, Selectivity and Stability

8. Typical Stability Profiles in Hydrotreating
Initially high rate of deactivation
mainly due to coke deposition
Subsequently coke in equilibrium
metal deposition continues
Time-on-Stream
Amount of poisoning
activity
coke
metals
Catalytic activity I
III II
Catalysis and Catalysts - Activity, Selectivity and Stability

9. Influence of Pore Size on Vanadium Deposition Hydrotreating of Heavy Feedstock
Catalysis and Catalysts - Activity, Selectivity and Stability

10. Carbon Formation on Supported Metal Catalyst
Catalysis and Catalysts - Activity, Selectivity and Stability

11. Carbon Filaments due to CH4 Decompostion 873 K, Ni/CaO catalyst
Catalysis and Catalysts - Activity, Selectivity and Stability

12. Sintering of Alumina upon Heating
Tcalc (K)
SBET (m2/g)
Sintering
Reduction of surface area
Catalysis and Catalysts - Activity, Selectivity and Stability

13. Sintering of Supported Catalysts
monomer dispersion
2-D cluster
3-D particle
particles migrate
coalesce
vapour
surface
interparticle transport
metastable
migrating
stable
Dependent on:
carrier properties
temperature
composition of bulk fluid ….
Predictable?
Catalysis and Catalysts - Activity, Selectivity and Stability

14. THüttig and TTamman Sintering is related to melting
THüttig : defects become mobile
Ttamman: bulk atoms become mobile
Tmelting THüttig Ttamman
Al2O Cu
CuO
CuCl
Catalysis and Catalysts - Activity, Selectivity and Stability

15. Deactivation due to Mechanical Damage
during transport, storage, packing, use
loading in barrels, unloading, packing of reactor
in reactor: weight of column of particles
attrition in moving systems (fluid beds, moving beds)
during start-up, shut-down
temperature variations (thermal shocks)
chemical transformations
sulphiding, reduction
regeneration: high T, steam
Catalysis and Catalysts - Activity, Selectivity and Stability

16. Corrosion / Leaching - Examples
Alumina
dissolves at pH > 12 and pH < 3,
so close to these pH-values corrosion and leaching
use carbon instead at very low or very high pH
Sulphiding of oxides in the presence of H2S
Liquid-phase catalysis
in heterogenisation of homogeneous catalysts activity was due to the leached compounds rather than the solid phase
in solid-catalysed fat hydrogenation traces of the Ni catalyst appear in the product; with Palladium this is not the case
Catalysis and Catalysts - Activity, Selectivity and Stability

17. Influence of Deactivation on Reaction Rate
conversion or
kobs
process time
initial level
‘constant’
‘variable
variable
loss of surface area
loss of active sites
blocking of pores
Fouling
Sintering
Catalysis and Catalysts - Activity, Selectivity and Stability
Poisoning

18. Deactivation - depends on?
Catalysis and Catalysts - Activity, Selectivity and Stability

19. Stability too low; What to do?
Understand the cause of deactivation
Take logical measures
at catalyst level
sound reactor and process design
good engineering practice
Catalysis and Catalysts - Activity, Selectivity and Stability

20. Catalyst Level improvement of active phase or support
e.g. use titania instead of alumina in SCR
optimisation of texture
use wide-pore catalyst in HDM to prevent pore blocking
profiling of active phase
e.g. egg-yolk profile will protect active sites against poisoning and fouling if these are diffusion-limited and the reaction is not
reduce sintering by structural promoters or stabilisers
make catalyst more attrition resistant
encapsulation of active material in porous silica shell increases attrition resistance without influencing activity
Catalysis and Catalysts - Activity, Selectivity and Stability

21. Tailored Reactor and Process Design
Relation between time-scale of deactivation and reactor type
Time scale Typical reactor/process type
years fixed-bed reactor;
no regeneration
months fixed-bed reactor;
regeneration while reactor is off-line
weeks fixed-bed reactors in swing mode, moving-bed reactor
minutes - days fluidised-bed reactor, slurry reactor;
continuous regeneration
seconds entrained-flow reactor with continuous regeneration
Catalysis and Catalysts - Activity, Selectivity and Stability

22. Different Engineering Solutions allowing for Regeneration
Propane dehydrogenation - deactivation by coke formation
Catalysis and Catalysts - Activity, Selectivity and Stability

23. Good Engineering Practice
Feed purification for removal of poisons
upstream reactor
poison trap inside reactor on top of catalyst (if flow is downward)
overdesign of reactor if catalyst itself is poison trap
Optimisation of reaction conditions
use of excess steam in steam reforming decreases coke deposition
catalyst deactivation in selective hydrogenation of CCl2F2 strongly increases above 500 K  operate below 510 K
Optimisation of conditions as function of time-on-stream
compensate for activity loss by increasing T with time
Catalysis and Catalysts - Activity, Selectivity and Stability

24. Examples
Catalysis and Catalysts - Activity, Selectivity and Stability