Introduction to catalysis chemistry

1

• Introduction to catalysis chemistry

• Preliminaries
• Definition
• Types of catalysis
• Example

2
Reaction Rate

• Function of reactants (or products)
  concentrations (or pressure), temperature,
  presence of catalyst (or inhibitor)
• a, ß, ... are the orders of the reactants
• k is the rate constant
• Example rate of chemical adsorption
• Rads k P x

3
Activation energy

• Arrhenius Law
• A pre-exponential factor
• Ea Activation energy

• Observations
• Reaction don't start even if it is
  thermodynamically favorable (?G lt 0)
• Reaction rate increase with T
• This increasing is specific for each reaction

4
Definition

• What is a catalyst ?
• A catalyst modifies the mechanism of the
  reaction
• But never change the equilibrium state and
  evolution direction

Catalyst is a chemical species which increase the
rate of a reaction without being used up in the
process
5
Thermodynamic and kinetic

• A catalyst reduces Ea for both forward and back
  reactions
• More steps in the reaction
• Effects of catalyst different for each reactions
• ?H do not change

6
Catalyst Cycle

• Catalyst can be used again and again in theory
• A catalyst has active sites
• But many problems can occur

7
Catalyst deactivation

• Poisoning
• impurities which can bind strongly to the active
  sites on the surface
• Sintering of the active metal particles
• Sintering and solid-solid phase transitions of
  the support (encapsualtion)

8
Catalyst Parameters

• Activity
• Turnover factor is the number of molecules
  converted by each catalytic site in a unit of
  time
• Selectivity
• A selectivity of 99,99 means that the catalyst
  makes one mistake in 10.000 conversions
• Stability
• The number of reactions performed by each active
  site before it decays or becomes inactive

9
The three Types of catalysis

• Homogeneous Catalysis
• The catalyst and reactants are in the same phase,
  usually liquid
• Heterogeneous catalysis
• Catalyst and reactants are in different phases
• Enzymatic Catalysis
• Catalyst is an enzyme (macromolecules made of
  amino acids)

10
Homogeneous Catalysis

• Advantages
• Good contact with reactants
• Greater effective concentration of catalyst than
  with heterogeneous catalysts
• Disadvantages
• Catalysis needs to be separated after reaction
• Catalyst recovery may be difficult because the
  temperature for the distillation can destroy the
  catalyst

11
Heterogeneous Catalysis

• 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
• To maximize the surface area catalysts are spread
  thinly on a cheap and sturdy support

12
Enzymatic Catalysis

• We would not survive without enzymes!
• Not fully understood
• Extremely efficient and highly selective
  accept only one variety of substrates

An Enzyme
With a substrate
13
Example of simple catalysis

• Hydrogenation
• Catalysts used are Ni Pd Pt Ru Rh Co
• unsaturated ? saturated products
• (Vegetable oils ? margarine)

14
Conclusion

• Catalysis only a kinetic improvement
• Never change the thermodynamic of the system
• Three type of catalysis
• A majority of industrial processes use catalysis