THIS PRESENTATION IS MODIFICATION OF OTHER PRESENTATION IN MORE SIMPLE WAY FOR READERS AND STUDENTS LEARNING FOR THE FIRST TIME.

2. TYPES OF CATALYSIS
Catalysts can be divided into two main types –
• heterogeneous and
• homogeneous.
• In a heterogeneous reaction, the catalyst is in a different phase from the
reactants.
• In a homogeneous reaction, the catalyst is in the same phase as the reactants.

3. HETEROGENEOUS CATALYSIS
A. Adsorption
B. Surface Reactions
C. Concepts
In heterogeneous catalysis, the reactants diffuse to the catalyst surface and
adsorb onto it, via the formation of chemical bonds. After reaction, the products
desorb from the surface and diffuse away
• For solid heterogeneous catalysts, the surface area of the catalyst is critical since
it determines the availability of catalytic sites.
• The most common approach to maximizing surface area is by the use of
catalyst supports,
• which are the materials over which the catalysts are spread
D. Classes of Heterogeneous Catalysts
Although the majority of heterogeneous catalysts are solids, many variations exist.
Reacting phases: solid + gas, solid + solution, and immiscible liquid phases.

4. How the heterogeneous catalyst works (in general terms)
Most examples of heterogeneous catalysis go through the same stages:
• One or more of the reactants are adsorbed on to the surface of the catalyst
at active sites.
Adsorption is where something sticks to a surface. It isn't the same as
where one substance is taken up within the structure of another.
An active site is a part of the surface which is particularly good at adsorbing
things and helping them to react.
• There is some sort of interaction between the surface of the catalyst and the
reactant molecules which makes them more reactive.
This might involve an actual reaction with the surface, or some weakening of
bonds in the attached molecules.
• The reaction happens.
At this stage, both of the reactant molecules might be attached to the surface,
one might be attached and hit by the other one moving freely in the gas or

5. Langmuir-Hinshelwood mechanism:
The two molecules A and B both adsorb to the surface. While adsorbed to the
surface, the A and B "meet," bond, and then the new molecule A-B desorbs.
Rideal-Eley mechanism:
One of the two molecules, A,adsorbs to the surface. The second molecule, B,
A on the surface, having never adsorbed to the surface, and they react and bind.
Then the newly formed A-B desorbs.
Precursor mechanism:
One of the two molecules, A, is adsorbed on the surface. The second molecule, B,
collides with the surface, forming a mobile precursor state. The molecule B then
collides with A on the surface, they react, bind and the new molecule desorbs.
Various types of Surface Reactions

6. General features:
• Different reaction phases possible: „classic“ gas/solid; liquid/solid or liquid/liquid
systems.
• High industrial relevance (about 85% of all catalytic processes are
catalysed).
• In general wide range of operating conditions (high temperatures/pressures).
• Specialised set of analytic methods required (e.g. X-ray methods, Operando
spectros.).
• Major advantage: Ease of separation of reactants/products/catalysts.

7. Advantages
•There is little difficulty in separating and recycling the catalyst.
Disadvantages
•There is a lower effective concentration of catalyst since the reaction occurs only
on the exposed active surface.

8. Homogeneous
catalysis• In homogeneous catalysis, all the reactants and catalysts are present in a single
fluid phase and usually in the liquid phase.
• Homogeneous catalysts are the simple molecules or ions such as HF, H2SO4,
Mn+2 as well as complex molecules such as organometallic complexes,
macrocyclic compounds and large enzyme molecules.

9. General features:
• Liquid phase reactions dominate the field.
• Industrially less relevant; but complex organic or asymmetric transformations
possible!
• Reaction conditions milder than required for heterogeneous reactions (-78 °C -
~200 °C).
• Investigation of reactions by spectroscopic methods (NMR, MS, IR, UV-Vis)
in solution possible.
• Fine-tuning of catalyst properties using different ligands/additives easy possible.
• Major challenge: Separation of products and catalysts/additives.

10. Advantages
Advantages of homogeneous processes can be summarized as follows:
 In many reactions, homogeneous catalysts are more active and/or selective
compared to heterogeneous catalysts.
 In homogeneous catalysis, the catalysts are molecularly dispersed within the
fluid. Hence, pore diffusion limitations are absent. However, bulk phase mass
transfer limitation may occurs.
Catalytic chemistry and mechanism for homogeneous catalysis are better
and understood. Therefore, it is easier to control and manipulate the process
parameters.

11. Disadvantages
However, homogeneous processes are also associated with some major
disadvantages which result in limited use of these processes.
These disadvantages are summarized below:
– Homogeneous catalysts are stable only in relatively mild conditions which limit
their applicability.
– Since the catalysts are molecularly dispersed in the phase as the reactant,
and solvents, the separation at end of the process is difficult and expensive. In
cases, it is not possible to recover the catalyst.

12. What are the “components” of a heterogeneous catalysts?
 Support; stabilize the catalytic particles
 Catalytic particles; (oxide, metal or sulphide) hold the active sites
 Promoters; enhance the catalytic performance or structural effects

13. Catalyst support
• In chemistry, a catalyst support is the material, usually a solid with a high surface
area, to which a catalyst is affixed.
• The activity of heterogeneous catalysts and nanomaterial-based catalysts occurs
at the surface atoms.
• Consequently, great effort is made to maximize the surface area of a catalyst by
distributing it over the support. The support may be inert or participate in the
catalytic reactions. Typical supports include various kinds of carbon, alumina,
and silica.

14. Preparation of catalyst
The preparation of supported catalysts aims to attach the active phase onto the support 
• Impregnation,
• co-precipitation (controlled pH or not),
• homogeneous deposition,
• deposition of surfactant (organic agent) stabilized metal particles 
The support is either a powder or a pre-shaped solid the most common ones
-Al2O3, -Al2O3, SiO2, TiO2 or carbons

15. Catalyst Deactivation And Regeneration
• Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a
problem of great and continuing concern in the practice of industrial catalytic
processes.
• Costs to industry for catalyst replacement and process shutdown total billions of
dollars per year.
• Time scales for catalyst deactivation vary considerably; for example, in the case
of cracking catalysts, catalyst mortality may be on the order of seconds, while in
ammonia synthesis the iron catalyst may last for 5–10 years.
• However, it is inevitable that all catalysts will decay.

16. Deactivation issues (i.e., extent, rate, and reactivation) greatly impact
• Research
• Development
• design, and
• operation of commercial processes

17. Mechanisms of solid catalyst deactivation
Thus, the mechanisms of solid catalyst deactivation are many; nevertheless, they
be grouped into six intrinsic mechanisms of catalyst decay:
(1) poisoning,
(2) fouling,
(3) thermal degradation,
(4) vapour compound formation and/or leaching accompanied by transport from
the catalyst surface or particle,
(5) vapour–solid and/or solid–solid reactions, and
(6) attrition/crushing.

18. Mechanism Type Brief definition/description
Poisoning Chemical Strong chemisorption of species on catalytic
sites which block sites for catalytic reaction
Fouling Mechanical Physical deposition of species from fluid
onto the catalytic surface and in catalyst
Thermal degradation and
sintering
Thermal
Thermal/chemical
Thermally induced loss of catalytic surface
area, support area, and active phase-support
reactions
Vapour formation Chemical Reaction of gas with catalytic phase to
produce volatile compounds
Vapour – solid and
solid – solid reactions
Chemical Reaction of vapour, support, or promoter with
catalytic phase to produce inactive phase
Attrition/crushing Mechanical Loss of catalytic material due to abrasion; loss
of internalsurface area due to mechanical-
induced crushing of thecatalyst particle

19. POISONING:
Poisoning is the strong chemisorption of reactants, products, or impurities on
sites otherwise available for catalysis. Thus, poisoning has operational meaning;
is, whether a species acts as a poison depends upon its adsorption strength
to the other species competing for catalytic sites.
FOULING:
Fouling is the physical (mechanical) deposition of species from the
phase onto the catalyst surface, which results in activity loss due to
blockage of sites and/or pores. In its advanced stages, it may result in
disintegration of catalyst particles and plugging of the reactor voids.

20. Thermal Deactivation
Thermally induced deactivation of catalysts results from
(i) loss of catalytic surface area due to crystallite growth of the catalytic phase
(ii) loss of support area due to support collapse and of catalytic surface area due to
pore collapse on crystallites of the active phase, and/or
(iii) chemical transformations of catalytic phases to non-catalytic phases.

21. Gas/vapour–solid and solid-state reactions
In addition to poisoning, there are a number of chemical routes leading to catalyst
deactivation:
(1)reactions of the vapour phase with the catalyst surface to produce
(a)inactive bulk and surface phases (rather than strongly adsorbed species) or
(b)Volatile compounds which exit the catalyst and reactor in the vapour phase,
(2) catalytic solid–support or catalytic solid–promoter reactions, and
(3) solid-state transformations of the catalytic phases during reaction

23. Prevention of Catalyst Decay
• It is often easier to prevent rather than cure catalyst deactivation.
• Many poisons and foulants can be removed from feeds using guard beds,
scrubbers, and/or filters.
• Fouling, thermal degradation, and chemical degradation can be minimized
careful control of process conditions, e.g., lowering temperature to lower sintering
rate or adding steam, oxygen, or hydrogen to the feed to gasify carbon or coke-
forming precursors.
• Mechanical degradation can be minimized by careful choice of carrier materials,
coatings, and/or catalyst particle forming methods.