Conte - Phys. chem. of het.cat.

Question 1

Pros and Cons of heterogeneous catalysis

Answer 1

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

Question 2

What are the two ways in which catalysts increase rates

Answer 2

(1) Reduce the activation energy

(2) Increase the frequency factor (A) of the Arrhenius equation

Question 3

How dies a catalyst increase the frequency factor?

Answer 3

Increasing the reactive cross-section (sigma*) by increasing P, the steric factor by making new reactio n pathways accessible

Question 4

Define the reactive cross-section

Answer 4

The corresponding area (and volume) for a chemical change to occur at collision

Question 5

What are the steps of the heterogeneous catalysis cycle

Answer 5

(1) Diffusion
(2) Adsorption
(3) Chemical reaction at surface
(4) Desorption
(5) Diffusion

Question 6

What are the Langmuir Isotherm assumptions?

Answer 6

(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

Question 7

Explain the Langmuir-Hinshelwood Mechanism

Answer 7

Both reactants A and B are adsorbed over the surface before coming together to react and release product C

Question 8

Explain the Eyley-Rideal mechanism

Answer 8

One reactant is adsorbed over the surface, the other(s) are in the fluid phase and react with the adsorbed species to release product

Question 9

Implications of Langmuir-Hinshelwood

Answer 9

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

Question 10

Implications of Eley-Rideal

Answer 10

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

Question 11

What are the 7 diffusion steps in heterogeneous catalysis?

Answer 11

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

Question 12

What does Negligible diffusion imply?

Answer 12

Implies rate of reaction is determined by chemical rate

- Reaction kinetics is the same as observed kinetics, only if diffusion effects are negligible

Question 13

Explain Important diffusion

Answer 13

Rate Limiting step is diffusion rate of reactant from fluid to surface
Catalyst becomes less efficient (eta) by diffusion effects

Question 14

How to increase efficiency of a catalyst when diffusion is important

Answer 14

• in solid/liquid: Increase stirring
• In gas/solids: Increase vortexes
• in gas/liquid/solid: increase pressure of gas that dissolves into liquid and stirring

Question 15

What are the criteria for a reaction to occur on a surface?

Answer 15

(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)

Question 16

Define an active site

Answer 16

A point of a surface where a reaction takes place

Question 17

Active site implications

Answer 17

• 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&raquo_space; number of active sites hen the catalyst is poisoned

Question 18

Effect of water for ionic metal oxides

Answer 18

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

Question 19

Interaction of water an ionic metal oxides at the surface

Answer 19

• 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

Question 20

Effect of water for covalent metal oxides

Answer 20

Water irreversibly coordinates to the surface

- Lewis basicity disappears completely as metal centres have been saturated by water

Question 21

Lewis acidity to Bronsted Acidity

Answer 21

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

Question 22

Effect of increased surface area

Answer 22

Changing bulk material to smaller components affects the overall surface area and has consequences on the surface-to-bulk ratio of the ‘single’ particles

Question 23

Explain the ‘free movement’ of electrons in bulk materials

Answer 23

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

Question 24

How to particles in the nanoparticle range alter properties?

Answer 24

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

Question 25

Effect of transitioning from bulk to nanopaticle catalyst

Answer 25

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

Question 26

Nanoparticle stability

Answer 26

Np’s are thermodynamically unstable

• Need to stabilise to use as catalysts
• Synthesis only possible under kinetic control

Question 27

Methods to stabilise np’s

Answer 27

• ‘Protect’ by using an appropriate ligand
• Bind np’s to solid surface
• Combination of the two

Question 28

Exceptions to triplet oxygen stability

Answer 28

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

Question 29

What are the two routes to alcohol oxidation?

Answer 29

(1) Activate oxygen to abstract H from substrate

(2) Complete catalytic cycle where oxygen is only present to restore catalyst

Question 30

Ionic and Covalent metal oxides for oxidation of hydrocarbons and alcohols

Answer 30

Ionic metal oxides (MgO, ZnO) –> Good cat for oxidation of alcohols
Covalent metal oxides (MoO3, Fe2O3) –> Good cat for oxidation of hydrocarbons

Question 31

Describe the Mars and Van Krevlen mechanism

Answer 31

Oxygen in products comes from metal oxide lattice
Oxygen from atmosphere restores catalyst by re-insertion into metal oxide lattice
(V2O5-x and V2O5)

Question 32

Key properties of a catalyst support

Answer 32

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

Question 33

Types of catalyst support

Answer 33

1) High mp metal oxides: SiO2, TiO2, Al2O3, CeO2
2) Clays
3) Activated carbon

Question 34

FeOx/MoOx

Answer 34

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

Question 35

Explain the 3 types of shape selective catalysis in zeolites

Answer 35

(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

Question 36

Formation of aci sites on aluminium silicate zeolites

Answer 36

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