Catalysis: General Concepts Flashcards
What are some requirements to make sustainable heterogeneous catalysts?
1) To replace rare metals with more abundant or;
2) To reduce their use to the absolute minimum required or;
3) Increase their durability
When considering a chemical reaction, one must ask whether the reaction will…
- Go forwards spontaneously (thermodynamics)
- And how fast the reaction will go (kinetics)
ΔG is determined by?
If ΔG < 0 =
spontaneous
If ΔG > 0 =
non-spontaneous
What is rate of reaction?
Is the change in the concentration of a substance that takes part in a chemical reaction (Kinetics)
Average rate is worked out through
Instantaneous rate is worked out through
Looking at the following question
1. Does the rate change?
2. Rate = k[N₂O₅]ₓ?
what does the following zero order graph look like for Ln(conc) against time
What does the following first order graph look like for rate against time
What does the following 2nd order graph look like for Ln(conc) against time and 1/(conc) against time
Work out the overall equation from the following information
NO₃ is an intermediate - links to potential mechanism
Elementary step 1 is slow and is the rate determining step
Work out the rate equation from the following information
What is a catalyst?
A compound that takes part in a chemical reaction and speeds it up but does not undergo any permanent change itself
How does this diagram change for when a catalyst is added?
Catalyst acts by reducing the Ea barrier
Catalysts stabilise the transition state also known as the active complex
Difference between an intermediate and a transistion state?
- Can can find out what theoretically is the transition state but you cannot measure it
- You ususally can measure the intermediate
Catalysts have no effect on…
thermodynamics of a reaction
Free energy, ΔG, is a state function, independent of path
Draw an energy diagram for a catalysed reaction with two transition states? Define the rate determining step and explain reasoning?
RDS at TS#1
Define Arrhenius equation
What is a Homogeneous reaction?
Reagents and catalyst are all in the same phase; typically in solution
(usually only one metal centre)
What is a Heterogeneous reaction?
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)
What are the disadvantages of Homogeneous catalysts
- Difficult to recycle/reuse (same phase)
- How stability (hence low temp)
- Expensive
What are the advantages of Homogeneous catalysts
- High activity & selectivity
- Mechanisms ofen known
What are the advantages of Heterogeneous catalysts
- Recyclability/Reuse
- High stability
- Cheap
What are the disadvantages of Heterogeneous catalysts
- Low activity & selectivity (many metal centres with different reactivities - which is the most active?)
- Poor mechanistic understanding (environment is much less controlled)
Heterogeneous catalysts are used in…
refining/bulk chemical syntheses much more than in pharamaceuticals (which tend to use homogeneous catalysis)
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?
- Stability - due to small size and the tendency to want to form nanoparticles
- Scalability
What is the action of a heterogeneous catalyst broken down into 5 step?
- 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
What is Physisorption?
Is this an exo or endothermic process?
- 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
Is Physisorption a spontaneous process?
- 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
What is Chemisorption?
- 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)
Is chemisorption exo or endothermic?
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
What are some important parameter of adsorption of molecules in heterogenous catalysis:
- Concentration/coverage of reactants on catalyst surface
- “Bond strength” of molecules on catalyst surface
- Mean lifetime of molecules on catalyst surface
How do you work out rate from the following equation
How can you work the rate constant from the following equation
What is the equation to work out the coverage on a catalysts surface
How can we re-write the rate equation to include coverage?
(catalysis can only happen on the surface)
The following graph shows how increasing pressure of a system will increase the coverage of the catalyst
What are the assumptions of this?
- All sites are equivalent (surface is homogenous)
- One molecule per site/single site occupany
- No interaction between adsorbed molecules
What is the derivation of the Langmuir adsorption isotherm which you need to know?
θ = coverage
b = adsorpton constant
Pa = partial pressure of adsorbate (A)
Using the Langmuir equation, describe the dependency of θ at low and high pressure
- 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
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?
Describe what this graph shows
- 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
Draw the Lenard-Jone potential diagram for N₂ on Cr surface vs Au surface using the information in the table below
- 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
What is Sabatier’s principle?
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
What is a way to improve the catalytic abilities of Ag, Co, or Ni in the diagram?
- Create an alloy by putting two catalysts together then modifying the electronics of the catalyst
Frenkel’s equation gives us the time the catalyst will spend on the surface of the molecule
What is it?
Calculate the mean residence time for nitrogen on W surface using the Frenkel equation
Explain the results obtained by constrastng with ΔHads values
For a low adsorption energy (Δ𝐻ads= 5 kJ/mol⁻¹), the residence time is extremely short because the molecule is weakly bound to the surface and escapes quickly.
