Heterogeneous Catalysis Flashcards
What does Coversion mean?
is the total convension of reactant [A] to product [B]
(cannot talk about activity from conversion data)
What does Activity mean?
is the rate of conversion of reactant [A] to product [B]
(Ni is more stable than Pt but Pt is more active)
What is the equation for Turnover number (TON)?
What is the equation for Turnover frequency (TOF)?
What does Selectivity mean?
Is defined as the conversion of the reactant to the desirable product (in moles) divided by the overall conversion of the reactant (in moles)
Using all the information in the table below, calculate TON (0.75 point), and then TOF (s⁻¹) for the reaction below
Consider that all Pd atoms are available for the reaction (all surface atoms), and that no catalyst deactivation or poisoning occuring during the reaction
(CAL = cinnamaldehyde, COH = cinnamyl alcohol, HCAL = hidrocinnamaldehyde, HCOH = hydrocinnamyl alcohol)
What type of reaction occurs when hydrogen adsorbs onto Pt with and without and metal oxide support
Heterolytic splitting without metal oxide catalyst
Homolytic splitting with metal oxide catalyst
What is a steady-state approximation?
It assumes that the concentration of an interemediate species (often the catalyst-substrate complex) remains constant over time, meaing its rate of formation equals its rate of consumption
E.g. the productions of the intermediate C and consumption occurs at the same rate, so the rate of change of [C] with respect to time is zero
What is the equation to work out the concentration of C using steady-state approximation?
What is the equation to work out the rate of reaction using steady-state approximations
(K₋₁ + K₂) because in the steady-state approximations is can go back or forwards
What is a Pre-equilibrium approximation in catalysis?
- It assumes that an early step in a reaction mechanism rapidly reaches equilibrium before the rate determining step occurs
- This means that the forward and reverse rates of an initial step are much faster than the subsequent steps, allowing the intermediate’s concentration to be expressed in terms of equilibrium constant
- E.g. the equilibrium between A+B and C is first established before the consumption of intermediate C to the product D
What is the equation to work out the concentration of C using pre-equilbrium approximations
(K₋₁ - because equilibrium has be established between A, B and C before the reaction can move forward)
What is the equation to work out rate of reaction using pre-equilibrium approximations
If θ is worked out using this equation
How can we work out the rate?
What is the Derivation of the Langmuir adsorption isotherm which you need to know?
b = k₁/k₋₁ (adsorption constant)
What are the parameters that the Langmuir-Hinshelwood Model works within?
- The reaction only takes place on catalyst surface
- Rate determining step (RDS) is the surface reaction
- Fractional coverage of reactant (A) is determined by:
- θ α pressure of (A) and the constant for adsorption of A)
For a Unimolecular reaction (non-competitive)
What does Kₛ and θA stand for?
Kₛ - Rate Constant
θA - Fraction Coverage of A
For a Unimolecular reaction (non-competitive)
Using the rate law below, describe the dependency of the rate at low and high pressure
- Should have a directly porpotional relationship as the pressure increases from the yellow circle and subsequent rate will increase
- At the top the rate is not affected by the pressure (full coverage)
For a unimolecular reaction (competitive) where B is absorbed
what consideration must be taken into account?
For a competitive unimolecular reaction we have to consider the adsorption of molecule B on catalyst surface
The rate for a unimolecular reaction which is competitive, the following equation can be used for rate
What is the driving force?
- The driving force represents the chemical affinity of the overall reaction in reaching thermodynamic equilirbium. It is proportional to the concentration difference of the reactants with respect to their equilibrium concentrations
- It does not contain any parameter associations with the catalyst, consistent with the fact that the catalyst does not affect chemical equilibrium
The rate for a unimolecular reaction which is competitive, the following equation can be used for rate
What is the adsorption term?
- Represents the reduction of the number of available sites duee to adsorption
- The individual terms represent the distribution of the active sites over the different intermediate surface species and vacancies
- It may contain square roots of partial pressures, indicating dissociative adsorption
For a Unimolecular reaction (competitive) where B is adsorbed
What is the rate law equation?
For a Unimolecular reaction (competitive) where C is adsorbed
What is the rate equation?
(will be given in the exam)
C is weakly absorbed to affect is negible
(can use TPD as evidence that C is weakly absorbed, as it should come off a R.T/Low Temp)
Find the rate equation for the conditions below and discuss their implications in catalysis
Find the rate equation for the conditions below and discuss their implications in catalysis
