Catalysis Flashcards
(33 cards)
What is a catalyst?
A substance that accelerates the rate of reaction but emerges from the process unchanged
Activated complex and activation energy
As reactants approach each other, a transition state is formed, called activated complex, and the energy difference between the activated complex and the reactants is known as activation energy. Some energy is required to overcome the repulsion between the electrons in the reacting molecules. Some energy is required to stretch the bonds that are to be broken.
What is syn gas?
Mixture of H2, CO and CO2 usually obtained from natural gas that can be transformed into different chemicals
Elementary and multi-step reactions
Multi-step reaction mechanisms are characteristic for heterogeneous catalysis. In these mechanisms, Ea of each elementary step is lower than Ea of overall reaction.
Each elementary reaction involves breaking or formation (or both) of chemical bonds. Each elementary reaction has its own transition state and activation energy, which can differ significantly from each other.
What happens to Ea with catalyst?
Ea decreases with a catalyst, a catalyst may provide a new reaction pathway for formation of different products
Arrhenius equation
kT = k0 e ^(-Ea/RT) k0 = initial rate of reaction
Oxidation of ethene over diff catalysts
- Pt
- Ag
- PdCl2, CuCl2, HCl
- No catalyst
- Pt : Ethene + O2 -> 2CO2 + H2O
- Ag : Ethene + O2 -> O(CH2)2 Industrially important
- Cl : CH3CHO
- Complete oxidation
Do catalysts affect equilibrium constants?
No as long as products produced are still the same
What should a catalyst accelerate?
A specific reaction, which can proceed in parallel with other possible reaction within a particular system
Classification of catalysts
- Individual/ Mixtures
- Physical state
- Massive, supported, structured, anchored
- Single component catalysts eg Al2O3, Pt or mixtures such as alloys, mixed oxides
- Gas, liquid, amorphous solid (SiO2), crystalline solid (zeolites)
- Massive: consists only of active component
- Supported: Active component is distributed on support
- Structured: Active sites are located inside the pores of the catalyst
- Anchored: Active sites are anchored to catalytically inert support
What are heterogeneous and homogeneous catalytic processes? Which is most common in industrial chemical processes?
Het: Physical state of reactants different to catalyst
Homo: Physical states of reactants and catalyst same
Most industrial processes involve heterogeneous catalysis
Advantages of hetero and homo?
Hetero: simple and complete separation of the fluid product mixture from solid catalysts
Continuous processes are easier to control and there are no changes from batch to batch (homo batch)
Homo: homo catalysis and catalysts could be more selective in a number of cases
Steps of catalytic reactions on solid surfaces
- Physical adsorption of at least one reactant
- Chemical adsorption of that reactant
- Chemical transformation (Achem,sorb –> Pchem,sorb)
- Change in the adsorption state of the product
- Desorption of the product
What happens when a molecule hits the surface?
The gas molecule may be scattered back into the gas phase
The molecule may lose sufficient energy to the solid to become trapped in a physically adsorbed state
If a chemisorbed state can be formed in the vicinity of the site of incidence, the molecule may pass directly to the chemisorbed state
If the molecule is trapped in the physically adsorbed state, it may become chemisorbed, jump back into the gas phase, or hop to neighboring site
During formation of chemisorbed species, the molecule (or its dissociated fragments) may become localised at the original site, make a limited number of diffusive hops until a sufficient amount of energy is lost or undergo continual migration
Why does a molecule stick to the surface?
Surface atoms posses certain unsaturation which can be balanced by adsorption of molecules which are in the vicinity of the surface. Driving force of adsorbed s energy reduction.
What are the two types of adsorption?
Which does catalysis require?
Physical and Chemical, both are exothermic
Physical: the forces of attraction between the adsorbed molecules and the solid surface are weak
During chemisorption, the adsorbed molecules are held to the surface by valence forces. As a result, the electronic structure of the chemisorbed molecules is perturbed significantly, and this makes these species highly reactive
Catalysis requires chemical
What is a common feature of all catalysts?
How can we develop a solid catalyst with a large surface area?
Is catalysis a thermodynamic or kinetic phenomena? Is thermodynamics important?
Surface area of industrial solid catalysts?
- Active sites accelerating the reaction
- Porous
- Kinetic, but if delta G is positive then cannot expect any physical reaction to occur, v low conversion
- 2 - 500 m^2/g
Conversion of reactant A = ?
XA = (FA0 - FA)/FA0 = (CA0-CA)/CA0
Rate of reaction = ?
RA = ?
no of moles of reactant converted/(amount of catalyst x time)
RA =-(CA0-CA)/(W/F0)
W/F0 = contact time
units of RA: (mol/s)/gcat
RB (product B formation) = ?
CB/(W/F0) (mol/g cat s)
What is turnover frequency?
Comment on possible reasons of observed diff in turnover frequency when same no of active sites?
no of moles of reactant converted/ (no of active sites x time)
Used to compare diff catalysts
Diff effects of diff ligands or result of transport? one zeolite may have much smaller pores? –> slow diffusion
Is catalytic activity greater on larger or smaller particles
smaller
SPi = ?
SPi = CPi/sum CPi (conc of all products).
What does selectivity normally depend on?
Conversion
