Catalysis: General Concepts

Question 1

What are some requirements to make sustainable heterogeneous catalysts?

Answer 1

1) To replace rare metals with more abundant or;
2) To reduce their use to the absolute minimum required or;
3) Increase their durability

Question 2

When considering a chemical reaction, one must ask whether the reaction will…

Answer 2

1. Go forwards spontaneously (thermodynamics)
2. And how fast the reaction will go (kinetics)

Question 3

ΔG is determined by?

Answer 3

Question 4

If ΔG < 0 =

Answer 4

spontaneous

Question 5

If ΔG > 0 =

Answer 5

non-spontaneous

Question 6

What is rate of reaction?

Answer 6

Is the change in the concentration of a substance that takes part in a chemical reaction (Kinetics)

Question 7

Average rate is worked out through

Answer 7

Question 8

Instantaneous rate is worked out through

Answer 8

Question 9

Looking at the following question
1. Does the rate change?
2. Rate = k[N₂O₅]ₓ?

Answer 9

Question 10

what does the following zero order graph look like for Ln(conc) against time

Answer 10

Question 11

What does the following first order graph look like for rate against time

Answer 11

Question 12

What does the following 2nd order graph look like for Ln(conc) against time and 1/(conc) against time

Answer 12

Question 13

Work out the overall equation from the following information

Answer 13

NO₃ is an intermediate - links to potential mechanism
Elementary step 1 is slow and is the rate determining step

Question 14

Work out the rate equation from the following information

Answer 14

Question 15

What is a catalyst?

Answer 15

A compound that takes part in a chemical reaction and speeds it up but does not undergo any permanent change itself

Question 16

How does this diagram change for when a catalyst is added?

Answer 16

Catalyst acts by reducing the Ea barrier
Catalysts stabilise the transition state also known as the active complex

Question 17

Difference between an intermediate and a transistion state?

Answer 17

• Can can find out what theoretically is the transition state but you cannot measure it
• You ususally can measure the intermediate

Question 18

Catalysts have no effect on…

Answer 18

thermodynamics of a reaction
Free energy, ΔG, is a state function, independent of path

Question 19

Draw an energy diagram for a catalysed reaction with two transition states? Define the rate determining step and explain reasoning?

Answer 19

RDS at TS#1

Question 20

Define Arrhenius equation

Answer 20

Question 21

What is a Homogeneous reaction?

Answer 21

Reagents and catalyst are all in the same phase; typically in solution
(usually only one metal centre)

Question 22

What is a Heterogeneous reaction?

Answer 22

Reagents are in a different phase from the catalysts, typically reagents are gases or liquids that are passed over a solid catalysts
(usually only multiple metal centres)

Question 23

What are the disadvantages of Homogeneous catalysts

Answer 23

• Difficult to recycle/reuse (same phase)
• How stability (hence low temp)
• Expensive

Question 24

What are the advantages of Homogeneous catalysts

Answer 24

• High activity & selectivity
• Mechanisms ofen known

Question 25

What are the advantages of Heterogeneous catalysts

Answer 25

* Recyclability/Reuse * High stability * Cheap

Question 26

What are the disadvantages of Heterogeneous catalysts

Answer 26

* Low activity & selectivity (many metal centres with different reactivities - which is the most active?) * Poor mechanistic understanding (environment is much less controlled)

Question 27

Heterogeneous catalysts are used in...

Answer 27

refining/bulk chemical syntheses much more than in pharamaceuticals (which tend to use homogeneous catalysis)

Question 28

A new era of catalysis tried to combine features of homogenous and heterogenous catalysts It allows for high activity & selectivity due to its small size due to larger coverage What are the challenges of this?

Answer 28

* Stability - due to small size and the tendency to want to form nanoparticles * Scalability

Question 29

What is the action of a heterogeneous catalyst broken down into 5 step?

Answer 29

* Diffusion of the reactants towards the surface * Physisorption/chemisorption between the catalyst surface and reactants * Surface reaction * Desorption of the product from the catalyst surface * Diffusion of the product away from the catalyst surface

Question 30

What is Physisorption? Is this an exo or endothermic process?

Answer 30

* It does not involve any bond breaking or bond formation in the adsorbate Enthalpy of adsorption is quite low **(<30 kJ mol⁻¹)** * Exothermic process (ΔH) - enthalpy decreases due to gas molecules having less freedom

Question 31

Is Physisorption a spontaneous process?

Answer 31

* Exothermic process (ΔH) - enthalpy decreases due to gas molecules having less freedom * And TΔS will be positive * Meaning overall ΔG is negative and the reaction is spontaneous

Question 32

What is Chemisorption?

Answer 32

* It involves bond breaking or bond weakening in the adsorbate; accompanied by formation of new bonds between the adsorbate and the surface * Enthalpy of chemisorption is relatively high (>30 kJ mol⁻¹)

Question 33

Is chemisorption exo or endothermic?

Answer 33

In chemisorption, it is very difficult to generalise whether chemisorption will be exothermic or endothermic process; as there are many parameters to be considered * There are a few ways a molecule can be adsorbed on the surface, i.e. H₂ and N₂ will adsorp side on due to high electron density in the middle

Question 34

What are some important parameter of adsorption of molecules in heterogenous catalysis:

Answer 34

1. Concentration/coverage of reactants on catalyst surface 2. "Bond strength" of molecules on catalyst surface 3. Mean lifetime of molecules on catalyst surface

Question 35

How do you work out rate from the following equation

Answer 35

Question 36

How can you work the rate constant from the following equation

Answer 36

Question 37

What is the equation to work out the coverage on a catalysts surface

Answer 37

Question 38

How can we re-write the rate equation to include coverage?

Answer 38

(catalysis can only happen on the surface)

Question 39

The following graph shows how increasing pressure of a system will increase the coverage of the catalyst What are the assumptions of this?

Answer 39

* All sites are equivalent (surface is homogenous) * One molecule per site/single site occupany * No interaction between adsorbed molecules

Question 40

What is the derivation of the Langmuir adsorption isotherm which you need to know?

Answer 40

θ = coverage b = adsorpton constant Pa = partial pressure of adsorbate (A)

Question 41

Using the Langmuir equation, describe the dependency of θ at low and high pressure

Answer 41

* If Pa is very small (i.e low pressure) - denominator = 1 and nominator = 0 * So any increase on the pressure, will have an increase on the coverage * If Pa is very large (i.e. high pressure) - there will be no increase on the coverage

Question 42

The following diagram is a Lennard-Jones Pontential diagram - for the process of absorption What does Ea, ΔHc and ΔHp stand for on the diagram?

Answer 42

Question 43

Describe what this graph shows

Answer 43

* hydrogen approaches the catalyst (nickel) and the process of physisorption happens * Vann der Walls interactions - electrons of the molecules star to fuse * The closer the to catalyst, chemisorption starts to compete with the physisorption process * Where the two lines intersect is the activation energy - the higher this is, the harder it is to activate the molecule * Informs us about the electronics of the catalyst

Question 44

Draw the Lenard-Jone potential diagram for N₂ on Cr surface vs Au surface using the information in the table below

Answer 44

* Table shows strong chemisorption occurs for N₂ for Cr but unobserable/weak for Au * Therefore the activation energy for the dissociation of the dinitrogen is too high for Au - wouldn't work as a catalyst

Question 45

What is Sabatier's principle?

Answer 45

The necessity of binding the adsorbate either too strong nor too weak for a good catalyst (TOF = higher TOF, higher activity & ΔH relates to absorption) This is what the volcano plot shows: a good catalyst needs to be not too strong or weak

Question 46

What is a way to improve the catalytic abilities of Ag, Co, or Ni in the diagram?

Answer 46

* Create an alloy by putting two catalysts together then modifying the electronics of the catalyst

Question 47

Frenkel's equation gives us the time the catalyst will spend on the surface of the molecule What is it?

Answer 47

Question 48

Calculate the mean residence time for nitrogen on W surface using the Frenkel equation Explain the results obtained by constrastng with ΔH*ads* values

Answer 48

For a low adsorption energy (Δ𝐻*ads*= 5 kJ/mol⁻¹), the residence time is extremely short because the molecule is weakly bound to the surface and escapes quickly.