Conte - Phys. chem. of het.cat. Flashcards
Pros and Cons of heterogeneous catalysis
Pros:
- Easily removed from the reaction medium
- Often thermally stable
- favourite for industry
Cons:
- Reaction rate limited by diffusion
- Difficult to identify the active site of the reaction
What are the two ways in which catalysts increase rates
(1) Reduce the activation energy
(2) Increase the frequency factor (A) of the Arrhenius equation
How dies a catalyst increase the frequency factor?
Increasing the reactive cross-section (sigma*) by increasing P, the steric factor by making new reactio n pathways accessible
Define the reactive cross-section
The corresponding area (and volume) for a chemical change to occur at collision
What are the steps of the heterogeneous catalysis cycle
(1) Diffusion
(2) Adsorption
(3) Chemical reaction at surface
(4) Desorption
(5) Diffusion
What are the Langmuir Isotherm assumptions?
(1) Adsorption cannot proceed beyond monolayer coverage
(2) All adsorption sites are equivalent, and the surface is uniform (perfectly flat atomically)
(3) The ability of a molecule to adsorb at a given site is independent of the occupation of neighbouring sites
- -> The rate of change of surface coverage due to adsorption is proportional to:
- Partial pressure of A (P_A)
- Number of vacant sites
Explain the Langmuir-Hinshelwood Mechanism
Both reactants A and B are adsorbed over the surface before coming together to react and release product C
Explain the Eyley-Rideal mechanism
One reactant is adsorbed over the surface, the other(s) are in the fluid phase and react with the adsorbed species to release product
Implications of Langmuir-Hinshelwood
Rate law expected to be 2nd order wrt surface coverage
(1) Rate limited by surface conc of A
(2) Rate limited by surface conc of B
(3) Max rate for intermediate coverage of A and B
- -> gives “bell-shaped” curve where cases 1 and 2 are the minima of either side of the curve and case 3 is the maximum rate
Implications of Eley-Rideal
Rate will be a function of surface coverage of A and partial pressure of B
(1) Rate limited by surface conc of A
(2) Rate limited by excess of A
- -> Case 1 is minimum of curve; case 2 is plateau maximum
What are the 7 diffusion steps in heterogeneous catalysis?
k1: Mass transfer to surface
k2: Diffusion to surface
k3: Adsorption processes
k4: Reaction at surface
k5: Desorption of product
k6: Diffusion of product
k7: Mass transfer away from surface
- -> Generally assumed that overall rate, k = k4
What does Negligible diffusion imply?
Implies rate of reaction is determined by chemical rate
- Reaction kinetics is the same as observed kinetics, only if diffusion effects are negligible
Explain Important diffusion
Rate Limiting step is diffusion rate of reactant from fluid to surface
Catalyst becomes less efficient (eta) by diffusion effects
How to increase efficiency of a catalyst when diffusion is important
- in solid/liquid: Increase stirring
- In gas/solids: Increase vortexes
- in gas/liquid/solid: increase pressure of gas that dissolves into liquid and stirring
What are the criteria for a reaction to occur on a surface?
(1) Reactant molecules(s) to be adsorbed onto surface
(2) For an adsorbed molecule to be able to react, we need:
- Change in electron density
- Bond cleavage (or formation)
Define an active site
A point of a surface where a reaction takes place
Active site implications
- an active site is always an adsorption site
- An adsorption site is not always an active site
- If adsorption is very strong - poisoning may occur
- if number of adsorption sites»_space; number of active sites hen the catalyst is poisoned
Effect of water for ionic metal oxides
Water adsorbs dissociatively over ionic solid surfaces
- MgO surface is converted to Mg(OH)2, which is basic
For highly ionic oxides, ions that build the solid can act as acids or bases
Interaction of water an ionic metal oxides at the surface
- Coordinatively unsaturated cations act as Lewis acid sites
- Unsaturated (Oxygen) anions ac as Lewis Basic sites
- Interaction of water converts the surface of the oxide into basic hydroxyl groups
Effect of water for covalent metal oxides
Water irreversibly coordinates to the surface
- Lewis basicity disappears completely as metal centres have been saturated by water
Lewis acidity to Bronsted Acidity
If the induction effect of electron withdrawing from oxygen of water to metal of metal oxide is strong enough, the formation of surface hydroxyl groups is responsible for weak Bronsted acidity
–> Solid MoO3 is basic in bulk but acidic at the surface
Effect of increased surface area
Changing bulk material to smaller components affects the overall surface area and has consequences on the surface-to-bulk ratio of the ‘single’ particles
Explain the ‘free movement’ of electrons in bulk materials
Bad difference in Energy levels is so small that electrons of metals can move freely brom one level to another at Tk_B
–> Use classical physics to describe the behaviour of macroscopic solids
How to particles in the nanoparticle range alter properties?
np’s are intermediates between molecule and bulk solids
np’s can have spatial arrangements of its constituent atoms like bulk solids
BUT
- Has very high number of coordinatively unsaturated atoms at the surface
- Has energy levels where quantization is possible
–> New, altered properties
np’s can affect melting point by changing size of clustered atoms
Effect of transitioning from bulk to nanopaticle catalyst
The transition from bulk to np can reverse the catalytic nature of the material, or create new ones:
- Pd - Reduction catalyst –> Oxidation catalyst
- Pt - Inert/reduction catalyst –> Oxidation catalyst
- Au - Inert –> Oxidation/Chlorination catalyst
Nanoparticle stability
Np’s are thermodynamically unstable
- Need to stabilise to use as catalysts
- Synthesis only possible under kinetic control
Methods to stabilise np’s
- ‘Protect’ by using an appropriate ligand
- Bind np’s to solid surface
- Combination of the two
Exceptions to triplet oxygen stability
Autoxidation and flames Autoxidation - no need for catalyst Flames - convert hydrocrabon to CO2 and H2O --> bot require radical R^. --> spin allowed HC's are easy to burn but hard to react
What are the two routes to alcohol oxidation?
(1) Activate oxygen to abstract H from substrate
(2) Complete catalytic cycle where oxygen is only present to restore catalyst
Ionic and Covalent metal oxides for oxidation of hydrocarbons and alcohols
Ionic metal oxides (MgO, ZnO) –> Good cat for oxidation of alcohols
Covalent metal oxides (MoO3, Fe2O3) –> Good cat for oxidation of hydrocarbons
Describe the Mars and Van Krevlen mechanism
Oxygen in products comes from metal oxide lattice
Oxygen from atmosphere restores catalyst by re-insertion into metal oxide lattice
(V2O5-x and V2O5)
Key properties of a catalyst support
1) Chemically inert towards secondary undesirable reactions
2) Good mechanical properties (resists abrasion and thermal expansion)
3) High surface area
4) High porosity
5) Low cost
Types of catalyst support
1) High mp metal oxides: SiO2, TiO2, Al2O3, CeO2
2) Clays
3) Activated carbon
FeOx/MoOx
Covalent metal oxide behaviour
Can oxidise alcohol via redox pathway –> still not acid/base pathway as in ionic
Metal oxides can be a support and a catalyst
Covalent/ionic distinction is useful but intermediates occur
Explain the 3 types of shape selective catalysis in zeolites
(1) Reactant selectivity - Only reactants which fit in a channel can react
(2) Product selectivity - Only products which fit can leave
(3) Restricted TS Selectivity - Only specific intermediates can be formed
Formation of aci sites on aluminium silicate zeolites
Anhydrous aluminium silicate: Al is 3-coordinate –> introduce Lewis Acidity
Hydrated aluminium silicate: Treat with steam –> Introduce Bronsted acidity
–> functionalise channels with acid catalyst behaviour added to selectivity control
