Catalysisand Catalyst

example: catalyst surface, activated carbon, alumina Adsorbate: molecules or atoms that adsorb onto the substrate. example: nitrogen, hydrogen, carbon monoxide, water Adsorption: the process by which a molecule or atom adsorb onto a surface of substrate. Coverage: a measure of the extent of adsorption of a specie onto a surface
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Eley-Rideal mechanism
Example The reaction A2 + 2B = 2AB may have the following Eley-Rideal mechanism A2 + * = A2* A2* + * = 2A* A* + B = AB + * where the last step is the direct reaction between the adsorbed molecule A* and the gas-molecule B.
CO
physisorption
physisorption/ desorption chemisorption
atomic chemisorption
d
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Adsorption and Catalysis
Adsorbent: surface onto which adsorption can occur.
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Eley-Rideal or Langmuir-Hinshelwood?
For the Eley-Rideal mechanism: the rate will increase with increasing coverage until the surface is completely covered by A*. For the Langmuir-Hinshelwood mechanism: the rate will go through a maximum and end up at zero, when the surface is completely covered by A*. This happens because the step B + * = B*
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Adsorption
For Chemisorption 1. The adsorption energy is comparable to the energy of a chemical bond. 2. The molecule may chemisorp intact (left) or it may dissociate (right). 3. The chemisorption energy is 30-70 kJ/mol for molecules and 100-400 kJ/mol for atoms.
Mechanisms of Catalytic Reactions and Characterization of Catalysts
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What is a Catalyst ? Catalyst is a substance that increases the rate
of the reaction at which a chemical system approaches equilibrium , without being substantially consumed in the process.
Reaction path for conversion of A + B into AB
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Activation Energy
Activation Energy : The energy required to overcome the reaction barrier. Usually given a symbol Ea or ∆G≠
A reaction may have a large, negative ∆Grxn, but the rate may be so slow that there is no evidence of it occurring. Conversion of graphite to diamonds is a thermodynamic favor process (∆G -ve ). ∆
Thermodynamics says NOTHING about the rate of a reaction. Thermodynamics : Will a reaction occur ? Kinetics : If so, how fast ?
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Kinetic Vs. Thermodynamic
Catalyst affects only the rate of the
reaction,i.e.Kinetics. It changes neither the thermodynamics of the reaction nor the equilibrium composition.
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Chemical Reaction
The Activation Energy (Ea) determines how fast a reaction occurs, the higher Activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction
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Activation Energy
Catalyst lowers the activation energy for both forward and reverse reactions.
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Activation Energy
This means , the catalyst changes the reaction path by lowering its activation energy and consequently the catalyst increases the rate of reaction.
cannot proceed when A* blocks all sites. The trick is that the step B + * = B* requires a free site.
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Catalyst Preparation
(1) Unsupported Catalyst Usually very active catalyst that do not require high surface area e.g., Iron catalyst for ammonia production (Haber process)
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Langmuir-Hinshelwood mechanisms
Example The Reaction A2 + 2B = 2AB
may have the following mechanism A2 + * = A2* A2* + * = 2A* B + * = B* A* + B* = AB* + * AB* = AB + *
C (graphite)
C (diamond)
Kinetics makes this reaction nearly impossible
(Requires a very high pressure and temperature over long time)
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Kinetic Vs. Thermodynamic
H H H H H H H H H
adsorbate
coverage θ = fraction of surface sites occupied
H H H H H
adsorbent
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Adsorption Mechanisms
Langmuir-Hinshelwood mechanisms: 1. Adsorption from the gas-phase 2. Desorption to the gas-phase 3. Dissociation of molecules at the surface 4. Reactions between adsorbed molecules Two Questions: • Is the reaction has a Langmuir-Hinshelwood mechanism? • What is the precise nature of the reaction steps? Cannot be solved without experimental or computational studies
H H HH H H H H H H H H H H H H H H
H2 adsorption on palladium
H2 absorption → palladium hydride
Surface process
bulk process
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Adsorption
In physisorption 1. The bond is a van der Waals interaction 2. adsorption energy is typically 5-10 kJ/mol. ( much weaker than a typical chemical bond ) 3. many layers of adsorbed molecules may be formed.