Catalyst and catalysis institute of engineering.ppt

“A substance which changes the reaction rate without
affecting the overall energetic of the reaction is termed as
a catalyst and the phenomenon is known as catalysis.”
Types of Catalysis
1.Homogeneous catalysis
When the reactants and the catalyst are in the same
phase (i.e.liquid or gas phase), the catalysis is said to
be homogeneous. The following are some of the
examples of homogeneous catalysis.
Examples:
a. Homogeneous Catalysis in Gas phase: Oxidation of
sulphur dioxide to sulphur trioxide in presence of NO as
catalyst.
2

b. Homogeneous catalysis in liquid phase:
3

Photocatalysis and Electrocatalysis (Water
splitting)
• Photocatalysis and electrocatalysis are two important processes in
the field of renewable energy and environmental sustainability,
particularly in the context of water splitting for hydrogen
production.
• They offer clean and efficient pathways for hydrogen production
without relying on fossil fuels.
• These processes use abundant resources like sunlight or electricity
to split water molecules into hydrogen and oxygen gases without
emitting harmful greenhouse gases.
4

• Photocatalysis and electrocatalysis play a critical role in
advancing renewable energy technologies, decreasing
reliance on fossil fuels, and combating climate change by
utilizing renewable energy sources and minimizing
environmental impacts.
5

Photocatalysis
 Photocatalysis covers both photocatalytic and photosynthesis
reactions.
 A photocatalyst is a substance which absorbs photon and utilizes
the energy (hυ) to initiate photophysical and photochemical
processes.
 The catalytic process which is accelerated by the absorption of
photon is known as Photocatalysis.
 One of the widely investigated photocatalytic reaction is the
photocatalytic water splitting into H2 and O2 using light energy.
(Theortical potential energy required for water splitting is 1.23 V)
6

Photocatalytic water splitting
 The photocatalytic process splits H2O into H2 and O2 in the
presence of light energy (photon). It is an artificial photosynthesis
process.
 In a photocatalytic water splitting reaction, phototocatalyst plays
a crucial role .
 The most common photocatalysts are semiconductors such as
titanium dioxide (TiO2), zinc oxide (ZnO), and cadmium sulfide
(CdS) etc.
 Photocatalytic water splitting comprises a sequence of
photophysical and electrolytic processes which are as follows (Six
gears for overall water splitting)
7

a. Absorption of photons
b. Separation of excitons
c. Carrier diffusion
d. Carrier transport
e. Catalytic efficiency
f. Mass transfer of reactants and products
The six gear concept illustrates the sequential water splitting process
across different time scale.
Water splitting is a redox process where protons undergo reduction
to form H2, while water undergoes oxidation to produce O2 as
shown in the figure below. Photocatalytic water splitting can also
be viewed as a form of electrolytic water splitting.
Photocatalysis ultimately results the generation of an electromotive force (emf).
This emf is responsible for the hydrogen evolution reaction (HER) and oxygen
evolution reaction (OER) in aqueous medium.
8

Eg (band gap)
Photocatalyst
Fig: Photocatalytic water splitting by photoexcitation systems 9

At valence band (Oxidation)
2H2O O2 + 4H+
+ 4e-
E0
= 1.23 V Vs NHE
At conduction band (Reduction)
4H+
+ 4e-
2H2 E0
= 0.00 V Vs NHE
Overall equation: 2H2O 2H2 + O2
sunlight
10

Semiconductor,
Insulator and conductor
11

[All the materials can be classified as conductors, semiconductors or
insulators- based on band gap energy. All the materials have conduction
band and valence band and there is a band gap, if this band gap is very
small or nearly equal to zero, then those materials are conductors because
valence band and conduction band will be very near to each other i.e.
electrons in valence band can easily transfer to the conduction band.
If there is sufficient band gap then those are semiconductors and if this
band gap is too large – electrons from valence band cannot transfer into
conduction band- those materials are insulators.
The electrons present in valence shell- valence electrons- have own energy
range known as valence band.
Photocatalysis ultimately results the generation of an emf due to electron-
hole transition between valence band and conduction band. This emf is
responsible for the hydrogen evolution reaction (HER) and oxygen
evolution reaction (OER) in aqueous medium.
Lots of semiconductor have been worked like Pt, Ti, Zn, Bi etc. ]
12

Electrocatalyst and electrocatalysis
 Electrocatalysis are typically heterogeneous catalysis, which
implies that the reactions occur on the surfaces of the catalysts.
 An electrocatalyst is a catalyst that participates in
electrochemical reactions and modified itself to increase or
decrease the rate of a chemical reaction without being
consumed in the process.
 The electrocatalytic splitting of water for the hydrogen evolution
reaction (HER) involves using electrical energy to drive the
conversion of water into hydrogen gas (H2) and oxygen gas (O2)
at the cathode and anode, respectively, of an electrochemical
cell. This process is essential for renewable hydrogen production
and energy storage. Here's how it typically works:
14

 The electrochemical cell consists of an electrolyte solution, an
anode, and a cathode. The anode and cathode are typically made
of conductive materials such as metals or carbon-based
materials.
Catalyst materials are essential for promoting the water splitting
reactions efficiently. Different catalysts are used for the anode
and cathode. For the HER at the cathode, catalysts are chosen
based on their ability to facilitate the reduction of protons (H+
) to
form hydrogen gas (H2). Common catalysts include precious
metals like platinum (Pt) and non-precious metal catalysts like
transition metal dichalcogenides (e.g., MoS2, WS2), metal oxides
(e.g., nickel-iron oxides), and carbon-based materials. 15

16
Fig: Schematic representation of conventional water electrolysis

• Catalyst: Design criteria
(Designing catalysts to satisfy process needs is like writing
prescriptions to cure illnesses.)
Proper design of catalyst for particular reaction is very
important for industrial use. Only the specific catalyst
can be used for target reaction for better result. The
distinct steps in designing catalyst are
18

Fig: Steps in the catalyst design 19

Target reaction:
Designing of catalyst should be done in accordance to
target reaction. It should be done in such a way show
that there will be optimum yield of product from target
reaction.
20

Stoichiometric analysis:
In this step, three tasks are undertaken. Initially, all potential
stoichiometric chemical equations are compiled and organized
logically, including primary reactants, reactant self-interactions,
reactant cross-interactions, reactant-products interaction, and
product-products interaction, with a focus on chemically stable
compounds. Subsequently, thermodynamics for each reaction
are computed, with emphasis on the most useful property being
the Gibbs free energy change (G) at a specified temperature.
Lastly, the identification of chemical bond changes is conducted
for each reaction, constituting the three integral tasks in this
process. These reactions ought to be categorized using a specific
scheme, such as Dehydrogenation (DH), Hydrogenation (H),
Oxidation (O), Oxygen insertion (OI), Dehydration (DW), and
Group addition (A).
21

Thermodynamic analysis:
The goals in this stage involve evaluating the thermodynamic
feasibility of each stoichiometric equation and categorizing them
into groups with similar chemical functions. To enhance clarity,
numerous intermediate reactions have been excluded from the
rearrangements. Reactions with Gibbs free energy change (G)
values exceeding 10 kcal/mol are not considered here as
reaction becomes unfavorable to proceed.
Molecular mechanism: The aim of this stage is to conceptualize the
molecular processes. In the current instances the pathway is
relatively shaightforward. However, in other cases numerous
molecular mechanisms may be proposed, each serving as a
starting point for subsequent analyses. Discernment is required
to possibilities. distinguish among various competing.
22

Proposed surface mechanism:
This marks the stage where the designer's past
experience and some understanding of existing
surface chemistry may not be helpful. While the
proposed surface mechanism is essentially an
educated guessing capacity. If time permits,
exploration of several schemes is feasible
Otherwise, priorities must be established based
on available information.
23

Reaction path identification: The required reaction pathways have been
defined at this point.
Necessary catalyst properties: This stage essentially involves redefining the
surface mechanism by focusing on properties that aid in identifying suitable
materials. The catalyst must possess:
i.Sites for oxygen adsorption,
ii. Sites for mild dehydrogenation
iii. Proximity of sites to facilitate the final dehydrogenation step.
Search for appropriate materials: The requirement for gentle
dehydrogenation and low oxidation rules out most metals, narrowing the
search to focus on oxides. A typical activity pattern for dehydrogenation, with
high activity concentrated around ions possessing d4
and d6
configurations.
Oxides containing ions such as Cu2+
Ni2+
Fe3+
Mn2+
V3+ V5+
and Ti4+
exhibit mild
dehydrogenation activity.
Proposed catalyst: on the basis of above analysis suitable catalyst is
proposed.
24

Structure – Activity Relationship
• Structure-Activity Relationship (SAR) in catalysis refers to the
relationship between the structure of a catalyst and its catalytic
activity. Understanding SAR is crucial in designing catalysts with
improved performance for various chemical reactions.
• Specific surface area, pore size, and pore volume of catalyst play
major role in shaping the properties and effectiveness of catalyst.
• Generally, if the surface area or active sites in the catalyst for
heterogeneous catalysis increases i.e. particle size gets reduced
then overall rate of reaction gets raised.
25

• Pore size plays a crucial role in facilitating the diffusion of
reactant molecules to the active sites within the catalyst
material. Smaller pores may restrict mass transport, leading to
diffusion limitations and reduced catalytic activity, especially for
bulky reactant molecules. On the other hand, larger pores can
enhance mass transport by allowing easier access for reactants
to reach the active sites, thereby promoting catalytic activity.
26

Selection criteria of catalyst
28

1. Activity
• The ability of a catalyst to increase the rate of a chemical
reaction is called its activity.
• Activity is a measure of how fast one or more reactions
proceed in the presence of catalyst.
• The catalyst should possess high activity. (It should facilitate
the desired reaction efficiently at reasonable temperatures
and pressures.) Higher activity allows for faster reaction rates
and improved productivity.
29

• It describes the accelerating power of the catalyst on the rate of reaction.
• Some substances increase the activity of a catalyst. These are known as
promoters. In Haber’s process, molybdenum acts as a promoter by
increasing the activity of iron (catalyst).
• Some substances decrease the activity of a catalyst. These are known as
poisons.
• The activity of a solid catalyst depends on how strongly gas molecules or
atoms form chemical bonds with the solid surface of the catalyst (also
known as chemisorptions). These bonds can be ionic or covalent. The
reactants have to get adsorbed on the surface of the catalyst to get
activated. The bond formed during adsorption between the catalytic
surface and the reactants must not be too strong or too weak.
30

• It must be strong enough to make the catalyst active whereas, not so
strong that the reactant molecules get immobilized on the catalytic
surface leaving no further space for the new reactants to get adsorbed.
• Generally for the hydrogenation reaction, from Group 5 to Group 11
metals, the catalytic activity increases. The catalytic activity is found to be
highest for group 7-9 elements of the periodic table.
Turnover Number
•The turnover number is defined by the number of catalytic cycles that
can be performed by the catalyst before it deteriorates. Commonly used
industrial catalysts have a turnover number ranging from 10 to 105
.
31

Turnover Frequency
• The turnover frequency is the number of times the reaction
takes place per catalyst per unit time.
2. Selectivity
• Selectivity refers to the catalyst's ability to promote the desired
reaction while minimizing side reactions or undesired by-
products.
• A good catalyst selectively promotes the desired reaction
pathway, leading to higher yields and purity of the target
product.
• A catalyst can accelerate a particular reaction selectively and
suppress other side reactions.
• Catalysts are highly selective in nature.
• The reaction with same reactants and different catalyst may
yield different products. E.g.,
32

• The reaction with same reactants and different catalyst may
yield different products. For example,
33

3. Stability
• The catalyst should be stable under the operating conditions of
temperature, pressure, and chemical environment. It should not
undergo significant degradation or deactivation over time.
• The stability of a catalyst is essential for maintaining its
effectiveness and longevity over time. Catalyst stability refers to
its ability to maintain activity, selectivity, and structural integrity
under the conditions of the chemical reaction.
• The catalyst should exhibit a stable activity throughout its
lifetime, or be easily regenerated to a similar activity level.
34

The stability of catalyst can be explained in different terms:
•Thermal Stability: Catalysts must withstand the temperature
conditions of the reaction without undergoing significant
thermal degradation.
• Chemical Stability: Catalysts should resist chemical reactions
with reactants, intermediates, products, or other components
present in the reaction environment. Chemical instability can
lead to catalyst poisoning, fouling, or leaching of active
components, resulting in a loss of catalytic activity.
35

• Mechanical stability: In heterogeneous catalysis, where the catalyst is
typically a solid material, mechanical stability is important to withstand the
physical stresses encountered during reaction conditions, such as fluid flow,
pressure changes, or mechanical agitation.
• Hydrothermal stability: Many catalytic reactions, particularly those involving
water or steam, are conducted at elevated temperatures and pressures. In
such environments, catalysts may undergo various degradation mechanisms,
leading to decreased performance or even complete deactivation.
Hydrothermal stability is crucial for ensuring the long-term effectiveness and
durability of catalysts in these conditions.
36

4. Compatibility
• The catalyst should be compatible with the reactants,
intermediates, and products involved in the reaction. It should
not react with or adsorb these species excessively, which could
lead to catalyst poisoning or fouling.
• Compatibility is crucial for maintaining the catalyst's activity,
selectivity, and stability over time.
• Incompatible interactions with reactants, intermediates, or
products can hinder the catalyst's active sites or alter its surface
properties, leading to reduced activity. By ensuring compatibility,
the catalyst can maintain its active sites and promote the
reaction efficiently.
• Incompatibility with certain reactants or reaction conditions can
alter the reaction mechanism, leading to changes in selectivity.
Compatible catalysts maintain the desired reaction pathways,
resulting in high selectivity. 37

• Incompatibility with the reaction environment,
such as thermal or chemical instability, can lead
to catalyst poisoning, fouling, or structural
changes that compromise stability. Compatible
catalysts remain structurally and chemically
stable under the operating conditions, ensuring
long-term performance.
38

5. Volumetric Efficiency
• The volumetric efficiency of a catalyst refers to its ability to
utilize the available reactor volume effectively to promote the
desired chemical reaction. It is a measure of how efficiently the
catalyst occupies the reactor space while maximizing the
conversion of reactants to products.
• Volumetric efficiency is crucial for catalytic converters to
effectively reduce emissions, meet regulatory standards,
optimize performance, improve fuel efficiency, and enhance
durability.
39

6. Regeneration
• The regeneration of catalysts is a crucial process in industries
where catalysts are employed, such as petrochemical,
chemical, and environmental sectors.
• The catalyst should be capable of regeneration through
methods such as washing, leaching, mechanical treatment or
oxidation or thermal treatment. This helps to extend the
catalyst's lifespan and reduce operating costs.
• Catalysts can lose their activity over time due to fouling,
deactivation, or the accumulation of reaction by-products.
Regeneration restores the catalyst's activity, allowing it to
continue catalyzing the desired reactions effectively.
40

• For e.g., thiophene – poisoned Ni/SiO2 catalyst can be
regenerated by employing a series of oxidation – reduction
process at low pressure of O2 and 1 atm H2.
7. Cost
• The cost of a catalyst is an important consideration during its
selection process, as it directly impacts the overall economics of
a chemical process or industrial operation.
• The catalyst should be cost-effective, taking into account its
activity, selectivity, and stability.
• The catalyst should be economically viable for large-scale
production. Factors such as material cost, preparation method,
and catalyst lifespan contribute to the overall cost-
effectiveness.
41

• When selecting a catalyst, it's essential to consider not only the
upfront cost but also factors such as performance, longevity, and
overall cost-effectiveness over the lifecycle of the catalyst.
• A thorough economic analysis, taking into account these factors,
can help determine the most cost-effective catalyst option for a
particular application or process.
42

7. Environmental Impact
• When selecting a catalyst, it's crucial to consider its
environmental impact, as various aspects of catalyst design and
usage can affect the environment.
• Considerations such as toxicity, waste generation, and energy
consumption associated with catalyst preparation and usage are
important from an environmental perspective.
• Sustainable catalysts are favored, as they adhere to green
chemistry principles by minimizing the usage of harmful
compounds and lowering pollutant emissions.
• Energy efficient catalysts that promote waste minimization
through high selectivity and low by – product production are
preferred.
43

• Durability and longevity of catalysts are essential
for reducing the need for replacements, and
recyclable and biodegradable catalysts support
acceptable disposal techniques.
• The catalyst complies with relevant
environmental regulations and standards to
maintain environmental sustainability in the
chemical industry.
44

CATALYSIS FOR ENERGY AND ENVIRONMENTAL APPLICATIONS
Catalyst play vital role for generation of green energy source.
Use of catalyst to generate energy fulfills the increased demand
of energy with less impact on environment.
Catalytic conversion of fossil fuel: Converting fossil fuels into
hydrogen typically involves processes such as steam reforming or
partial oxidation, both of which utilize catalysts to facilitate the
reactions. Here's a brief overview of each process:
45

• A. Steam Reforming:
Process: Steam reforming is the most common method for producing
hydrogen from fossil fuels, especially natural gas (methane). In this
process, methane reacts with steam (H2O) at high temperatures
(typically 700°C to 1000°C) and in the presence of a catalyst to
produce hydrogen gas (H2) and carbon monoxide (CO).
Catalyst: Nickel-based catalysts, often supported on alumina or other
refractory materials, are commonly used for steam reforming.
These catalysts facilitate the methane steam reforming reaction
(CH4 + H2O → CO + 3H2) as well as the water-gas shift reaction (CO
+ H2O → CO2 + H2), which helps increase the hydrogen yield.
• Byproducts: Besides hydrogen, steam reforming also produces
carbon monoxide, carbon dioxide, and small amounts of methane.
46

• B. Partial Oxidation:
Partial oxidation involves reacting a hydrocarbon fuel (such as
methane or liquid hydrocarbons) with oxygen (O2) in a limited
supply, typically at high temperatures (above 1000°C). The
reaction produces hydrogen gas along with carbon monoxide and
carbon dioxide.
Catalyst: Catalysts are not always necessary for partial oxidation,
as the process can occur without a catalyst under certain
conditions. However, catalysts may be used to enhance the
reaction rates and selectivity. Noble metal catalysts like platinum
(Pt) or rhodium (Rh) are sometimes employed.
Byproducts: Besides hydrogen, partial oxidation produces carbon
monoxide and carbon dioxide. The presence of oxygen in the
reaction mixture can lead to higher production of carbon dioxide
compared to steam reforming.
47

Both steam reforming and partial oxidation are
established industrial processes for hydrogen
production from fossil fuels. However, they are
associated with carbon emissions, especially
when the fossil fuel source is not methane.
Therefore, efforts are underway to develop and
scale up more sustainable methods for hydrogen
production, such as electrolysis powered by
renewable energy sources or carbon capture and
storage (CCS) technologies applied to fossil fuel-
based processes.
48

• C. Pyrolysis:
Pyrolysis involves heating a fossil fuel, such as natural gas or
biomass, in the absence of oxygen to produce hydrogen-rich
gases. The process typically occurs at high temperatures (usually
above 500°C) in a reactor.
Catalyst: Catalysts are not always necessary for pyrolysis, but they
can enhance the process by promoting desired reactions and
increasing the yield of hydrogen. Transition metal catalysts like
nickel (Ni), iron (Fe), or cobalt (Co) are commonly used for
pyrolysis of hydrocarbons.
Byproducts: Pyrolysis typically produces hydrogen gas along with
other byproducts such as methane, ethane, propane, and
heavier hydrocarbons, depending on the composition of the
feedstock and process conditions.
49

• D. Autothermal Reforming (ATR):
Autothermal reforming combines partial oxidation and steam
reforming in a single process. It involves reacting a hydrocarbon
fuel (such as natural gas or diesel) with oxygen and steam in the
presence of a catalyst at elevated temperatures. The exothermic
partial oxidation reaction provides heat to drive the endothermic
steam reforming reaction, resulting in a self-sustaining process.
Catalyst: ATR typically employs catalysts to enhance reaction rates
and improve hydrogen yield. Similar to steam reforming, nickel-
based catalysts supported on refractory materials like alumina are
commonly used for ATR.
Byproducts: Like steam reforming, ATR produces hydrogen gas
along with carbon monoxide, carbon dioxide, and small amounts
of methane as byproducts.
50

Both pyrolysis and ATR offer potential pathways for hydrogen
production from fossil fuels. However, they also generate carbon-
containing byproducts, contributing to carbon emissions unless
carbon capture and storage (CCS) technologies are implemented
to mitigate greenhouse gas emissions. Moreover, ATR is often
preferred in industrial settings due to its higher efficiency and the
ability to produce hydrogen-rich gas streams suitable for various
applications. Nonetheless, ongoing research aims to improve the
efficiency and environmental sustainability of both pyrolysis and
ATR processes for hydrogen production.
51

CATALYST FOR POLLUTION CONTROL
• Catalysts are used for the reduction of air pollutant gaseous from
the atmosphere and toxic organic pollutants present in water
body. They can convert CO2 gas into alcohol and reduce the CO2
concentration in atmosphere. Similarly, they can degrade toxic
organic molecules present in water as well as in air (VOCs) into
CO2 and H2O.
• Catalytic reduction of greenhouse gas emissions:
Catalytic technologies play a crucial role in mitigating the rise in
tropospheric concentrations of greenhouse gases (GHGs) and
curtailing their impact on global warming. After outlining
potential general applications of catalytic technologies for GHG
reduction, two specific scenarios are examined.
52

a. Reduction of anthropogenic emissions of non-CO2, GHG’s (N2O
and CH4):
For controlling methane emissions, combustion is a primary
option. The utilization of catalytic combustion can offer economic
benefits, given the typically low concentration of methane in
emissions, and it avoids the formation of trace by-products like
formaldehyde, which might be more harmful than methane itself.
The commonly employed methods for the catalytic conversion of
methane to hydrogen are partial oxidation, dry reforming and
steam reforming. The catalysts employed in methane are similar
to those used in the catalytic conversion of fossil fuel to hydrogen.
53

The catalytic control of N₂O emissions is challenging task due to
various emission sources. The control technology covers catalytic
abatement or reuse of N2O from industrial emissions (especially
from adipic and nitric acid production). The alternative treatment
of emissions from power plants or waste combustion is the
catalytic decomposition or reduction. Fe-ZSM-5 catalysts show
good performances in the selective catalytic reduction (SCR) of
nitrogen oxides (NO, N2O, NO2 ) with oxygen and a selective
inorganic or organic reducing agent. There are no any exact
protocols to convert fluorocarbons to inert chemicals. The
catalytic conversion of CFCs to fluorocarbons is implemented best
alternative to control the ozone layer depletion. Metal catalysts
such as palladium on alumina or carbon, aluminum fluoride (AlF3)
and various oxides, exhibit activity in the process of
hydrodechlorination, converting chlorofluorocarbons (CFCs) into
hydrofluorocarbons (HFCs).
54

b. Reduction or conversion of CO2
• It is urgent to reduce to concentration of CO2 from the
atmosphere which is the major cause of global warming and
therefore responsible for climate change. Numerous catalytic
routes exist for utilizing CO2 in organic syntheses to obtain
valuable chemicals and materials. Recent attention has centered
on catalytic conversion of carbon dioxide into fuels, (e.g.,
methanol) and other products.
55

• Catalytic dye degradation involves using catalysts to break
down and remove dyes from wastewater or polluted water bodies. Here are
some methods of catalytic dye degradation:
Photocatalytic Degradation:
Photocatalytic degradation utilizes catalysts activated by light to degrade dyes
through oxidation reactions. The catalyst absorbs light energy and generates
reactive oxygen species (ROS), such as hydroxyl radicals (•OH), which oxidize
and break down dye molecules into smaller, less harmful compounds.
Catalyst: Titanium dioxide (TiO2) is the most commonly used photocatalyst for
dye degradation due to its stability, high catalytic activity, and ability to
absorb UV light. Other semiconductor materials like zinc oxide (ZnO) and
tungsten trioxide (WO3) are also used.
Light Source: UV light is typically used to activate the photocatalyst and initiate
the degradation process, although visible light-responsive catalysts are also
being developed.
56

• Biocatalytic Degradation:
• Process: Biocatalytic degradation utilizes enzymes or microorganisms to
degrade dyes through enzymatic or metabolic reactions. Enzymes like
peroxidases, laccases, and azoreductases are capable of breaking down dye
molecules into simpler, non-toxic compounds.
• Catalyst: Enzymes isolated from microbial sources or produced through
recombinant DNA technology are used as biocatalysts for dye degradation.
Microorganisms such as bacteria and fungi can also be employed for
biodegradation of dyes in wastewater treatment systems.
• Catalytic dye degradation methods offer environmentally friendly and cost-
effective solutions for treating dye-contaminated wastewater and mitigating
the environmental impact of dyeing industries. These approaches are being
continuously researched and developed to improve efficiency, selectivity, and
scalability for industrial applications.
57

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