Industrial Catalysis
Lecture for Makrokierunek
Lecture 7
Heterogeneous catalysis
-Introduction
Nikodem Kuźnik
Silesian University of Technology
Gliwice, Poland
Lecture 7
Scope of the lecture
•
•
•
•
•
•
Heterogeneous catalysis steps
Physical and chemical adsorption
Active site
Sabatier Principle of moderate binding
Catalyst shapes
Catalyst composition: active phase, support, promoters,
modifiers,…
• Deactivation of heterogeneous catalyst
An example
Steps of Heterogeneous Catalytic Process
•
The long journey for reactant molecules to
j. travel within gas phase
k. cross gas-liquid phase boundary
l. travel within liquid phase/stagnant layer
m. cross liquid-solid phase boundary
n. reach outer surface of solid
o. diffuse within pore
p. arrive at reaction site
q. be adsorbed on the site and activated
r. react with other reactant molecules, either
being adsorbed on the same/neighbour
sites or approaching from surface above
•
Product molecules must follow the same track
in the reverse direction to return to gas phase
•
Heat transfer follows similar track
gas phase
reactant molecule
j
k
l
gas phase
liquid phase /
stagnant
layer
mn
o
porous
solid
pore
pq r
Thermodynamics of catalytic process
Physical adsorption
• Physical Adsorption:
- Van der Waals forces; BET surface area
• Pore Size distribution (Wheeler, de Boer,
BJH)
• Pore size affects reaction order, temperature
coefficient, selectivity …
Chemisorption
• Kinetic description:
– Langmuir isotherm; A + B → R + S
– Langmuir –Hinshelwood
θi – degree of covrage; k – experiment. const
– Eley- Rideal mechanisms of surface reactions;
Chemisorption
Examples of chemisorption processes
Classification of metals according to their chemisorption properties
Chemisorption - Intermediates
O
C
O
Pt
Pt
Pt
Pt Pt Pt
Pt
Pt Pt Pt
Pt Pt Pt
O
Pt Pt Pt Pt
Pt Pt Pt Pt
Pt
C
Pt
Pt Pt Pt
Pt Pt Pt
Pt Pt Pt
CO oxidation: exhauts gases
CO redution: F-T,
methanol syntesis
Removal of sulphur from organic
compounds - hydrodesulphurization
Chemi- and physical adsorption
The Active Site
• There will be all extremes between the case
in which all the atoms in the surface are
active and that in which relatively few are
so active .
• The amount of surface which is
catalytically active is determined by the
reaction catalyzed.
H.S.Taylor,Proc Roy Soc (London)A108(1925)105
11
Active Sites-Metals:
Structure sensitivity of Catalytic
reactions over metals
• Structure Sensitive - rate changes markedly when
crystallite/particle size is changed; “active site” comprises
ensemble of many metal; example: isomerization,
reforming.
• Structure Insensitive - rate is independent of
crystallite/particle size; each surface metal atom is a
potential active site; example: hydrogenation,
dehydrogenation.
12
Heterolytic adsorption on Ionic
oxide surfaces
Oxide Surface: M+ -O - - M+ - O - - M+
Lewis acid(e- acceptor)
Bronsted base(H
acceptor)
H+ HH2: M+- O - - M+ - O - - M+
H+ OHH2O: M+ -O --M+ - O - - M+ ( B acid and B
base)
C2H5 H
H
OCH3
C2H6 & CH3OH: M+ -O - - M+ - O - - M+
13
The Sabatier Principle
• There is an optimum of the rate of a
catalytic reaction as a function of the heat
of adsorption - Sabatier,1905:
• If the adsorption is too weak, the catalyst has little
effect;
• If too strong, the adsorbates will be unable to
desorb from the surface;
• Hence, the interaction between reactants or
products with surface should be neither too
strong nor too weak.
Sabatier Principle – Volcano plot
Sabatier Principle
next example – NH3 synthesis
Specificity
Cu catalyst - dehydrogenation
Al2O3 catalyst - dehydration
CH3CH2OH → CH3CHO + H2 CH3CH2OH → CH2=CH2 + H2O
Spacing of sites suitable to bond O-H
Spacing of sites suitable to bond C-O
Nature of catalyst
Shape of heterogeneous catalysts
Solid Catalysts
•
Catalyst composition
– Active phase
• Where the reaction occurs (mostly metal/metal oxide)
– Promoter
• Textual promoter (e.g. Al - Fe for NH3 production)
• Electric or Structural modifier
• Poison resistant promoters
Catalyst
– Support / carrier
Support
• Increase mechanical strength
• Increase surface area (98% surface area is supplied within the porous
structure)
• may or may not be catalytically active
Modification of heterogeneous catalyst
Promoters
Modification of heterogeneous catalyst
Support influence
Influence of support materials on the hydrogenation of CO with
rhodium catalysts
Solid Catalysts
• Some common solid support / carrier materials
– Alumina
• Inexpensive
• Surface area: 1 ~ 700 m2/g
• Acidic

Other supports
– Silica
• Inexpensive
• Surface area: 100 ~ 800 m2/g
• Acidic





Active carbon (S.A. up to 1000 m2/g)
Titania (S.A. 10 ~ 50 m2/g)
Zirconia (S.A. 10 ~ 100 m2/g)
Magnesia (S.A. 10 m2/g)
Lanthana (S.A. 10 m2/g)
Active site
– Zeolite
•
•
•
•
mixture of alumina and silica,
often exchanged metal ion present
shape selective
acidic
porous
solid
pore
Classification of Heterogeneous Catalyst
By principle functions
Class
Conductivity type
Metals
conductors
Metal
semi conductors
Oxides, p-type and/or n-type
Sulfides
Functions
Examples
hydrogenation
Fe , Ni,
dehydrogenation Pt , Pd,
hydrogenolysis Cu , Ag
( oxidation )
oxidation
NiO , ZnO
reduction
CuO ,WS2
cyclization
Cr2O3
dehydrogenation
desulfurization
(hydrogenation)
Class
Conductivity type
Metal oxides
insulators
Functions
dehydration
isomerization
Acids
polymerization H3PO4,
isomerization
H2SO4,
cracking
AlCl3 , HF,
alkylation
SiO2-Al2O3
By type of reaction
(i) oxidation – reduction ( homolytic splitting )
Cat• + A : R
A : Cat + R•
or B : Cat + A : R
A : Cat + B : R
(ii) Acid-base ( heterolytic splitting )
A : Cat
A+ + Cat A : Cat
A- + Cat +
Examples
SiO2-MgO,
Al2O3
Deactivation of catalyst
• Some substances may block active sites (i.e. they adsorb
and will not come off).
e.g. S in Haber process
e.g. Pb in catalytic converters
Catalyst deactivation