Heterogenous Catalysts Flashcards
What does ‘1 wt% Au’
It means the % of the metal on the support
i.e. the % of Au on TiO₂ support
What is the differences between bimetallic and monometallic catalysts
The properties of bimetallic catalysts are significantly different from their monometallic analogues because of what is often termed as “synergistic” effects between two metals
What factors might you consider when choosing a catalyst from the selection shown?
- Both platinum and gold are rare and expensive
- Don’t want to choose a catalysts with too high rates as it appears more stable (i.e. purple)
- There is a competition of activity because there is lower activity the higher the ratio of gold is
- Bimetallic catalysis is finding a balance between a catalyst being more active but also more selective and stable
What are 3 factors to consider when choosing a catalysts?
- Size, composition, and shape of a metal catalyst
- Suface chemistry of a metal catalyst
- Catalytic activity and selectivity of a metal catalyst
What does work function mean in terms of catalysis?
- In catalysis, the work function refers to the minimum energy required to remove an electron from the surface of a catalyst into the vacuum
- The work function affects how molecules absorb onto the catalyst surface. A lower work function can enhance electron donation the the reactants, facilitating activation
How does the size of a metal catayst cluster effect the catalyst properties?
- A catalyst which works in say a low size i.e. Au38, the work function will change a lot
- This is due to the geometry that exists - affect HOMO and LUMO
- After the point of Au75, the work function here is constant, meaning the electronics of the catalyst will not change anymore
If you work with large catalyst particles…
The support that you use doesn’t really matter
BUT if you work with a small catalyst then it does matter and will result in a change of the reactivity and electronics of the metal catalyst
Large catalysts also have a crystal structures
How does reactivity change through the crystal structure?
- The high coordinated sites are less active than the low-coordinated sites due to being highly saturated
- (Don’t want the sites to be too reactive where there is poor selctivity + multiple products)
- In which of the surface below, stepped or planar surface, will show a higher CO dissociation rate?
- Draw and label a hypothetical Lennard-Jones potential energy diagram for CO dissociation on stepped and planar surfaces
- Stepped surface due to lower activation energy, therefore higher rates than planar surfaces as by the Arrhenius equation
- Due to the higher number of coordinated sites of the stepped surfacces when compared to flat surfaces, which usually are more reactive
What can the support of a metal catalysts affect?
- Sometimes the support is inert to the reaction
- Sometimes the support can change the electronics of the metal catalyst (depends on the support chosen)
There are 3 classes of supports being…
- Metal oxides - Zeolites, Ceria, Alumina, Silica, Titania
- Carbon based mateirals
- Nitride based supports
(most used are metal oxides)
What are some general features of metal oxides
- Thermally stable
- Chemically inert
- High surface area (zeolite, silica)
- TiO₂ is UV active
Properties of a porous metal oxide include that they are constrained to an internal space and diffusion limitations
The size/dimension of porous channels are important, how do we define them by name?
What is the benefit of a large porous vs small porous channel in a metal oxide?
large - Because it gives a very high surface area so you can increase the dispersion of the metal catalyst
Small - because it gives a confinement on these pores which can increase the selectivity of the reaction
Zeolites are used as molecular sieves due to their well defined pore channel systems
Since the 1960s have been used as shape and size selective catalsts
Why are Zeolites species?
Zeolites can be used as support for metal catalysts and as a catalysts itself due to their acid/basic sites
How can the properties (like the cavity and pore opening) of Zeolites be adjusted?
By different ratios between aluminium and silicon
What is the general formual of a zeolite?
where cations M of valance n neuralise the negatively charged zeolite framework
What is a Bronsted acid?
A Bronsted acid is a substance that donates a proton (H⁺) in a chemical reaction
e.g. HCl → H⁺ +Cl⁻
What is a Lewis acid?
Is a substance that accepts an electron pair to form a covalent bond
What would be the difference in properties if more silicon/aliminium is added?
- If more aluminium is added (+3), can receive one electron so can have an acid site there
- Adding acid can result in a Bronstead acid forming (hydrogen donor)
- If more silicon is added (+4), it is all bonded to oxygen - inert
Substituting [SiO₄]⁴⁻ by [AlO₄]⁵⁻ on a zeolite creates…
an excess of negative charge in the zeolite
Neutrality is provided by an exchangeable cation
Explain the following zeolite structures
(qH - refers to the charge on an absrobed hydrogen atom on a catalyst surface - describes e- transfer and reactivity)
- Increasing the ratio of aluminium, you start to make a super acid - due to the interchange of Bronsted and Lewis acid (hydrogen donor + electron acceptor)
- If this number gets smaller, it means your ratio of silicon/aluminium is small (more Al in structure) AND results in much more reactive zeolite
Explain how this zeolite is show to be reactant selective?
- only straight n-alkanes are able to enter the pores of the zeolite where they are cracked on the acid sites
- Brached alkanes are too large to entrer the pores of the zeolite
- The zeolite catalyst selectively cracks straight alkanes
Explain how this zeolite is show to be product selective
- All three of the isomers of dimethylbenzene are formed in the cavities of the zeolite - but only the 1, 4 isomer is small enough to escape through the pore openings
- the selected product is 1,4 dimethyl benzene
- The zeolite selects the only product observed - 1,4 dimethylbenzene
Explain how this zeolite is shown to be restricted transition state selective
- The species in the pore cavities are ‘transition state’
- The transition state leading to 1,3,5 trimethylbenzene is too large to fit inside the cavity - so only the 1,2,4 product is selectively obtained
What are some general features of a carbon based support?
- Thermally stable
- Chemically inert
- High surface area (CNTs)
- g-C₃N₄ is UV active
What are the 3 core steps in the preparation of a supported catalyst?
(note the multiple metal centre which can have different reactivity depending on corner/planar etc once catalyst formed)
What would you change here in this system to have more heterolytic activation of hydrogen
- If you break down this catalyst to form a single atom
- It increase the size of the interface, so you have more hydrogen binding to a pollutant and to the support
- So there is more heterolytic activation of hydrogen
What is the difference between the size selectivity of these two catalysts
- Just one molecule of thr Pd catalyst means there isnt complete hydrogenation of the molecule due to a steric factor - there is no space for this second double bond there
- However the larger Pd catayst allows for hydrogenation to these different product
What is the difference between the site selectivity of catalysts
- Change in metal (active site) changes the reaction pathway
- Basically depending on what metal you use, you can have different products - a different method gives different products
- The reagents bind at different strengths to the catalyst element, changes the time spent at the surface and hence the pathway of the reaction mechanism - hence different product
Cerium oxide is a bit off on this plot showing different metal oxide supports affect of the activity of Ruthenium
note: for the reaction N₂(g) + 3H₂(g) → 2NH₃(g)
- This is a gas phase reaction
- Raise the pressure of one of the gases to see if the Ruthenium is being poisoned
- Hydrogen is posioning the catalyst (But for Cerium under hydrogen can lose oxygen - creating vacancies on the oxygen under cerium which will bind to vacant oxygen - results in it performing than it should)
What does Sintering mean?
Refers to the undesired aggregation/coalesence of catalyst particles at high temperatures, leadingt to a loss of active surface area and catalytic performance
This phenomenon is a major cause of catalyst deactivation through reducing surface energy and activity
What are the two mechanisms for sintering of nanoparticles?
- Particle migration & coalescence: involves the mobility of particles in a Brownian-like motion on the support surface, with subsequent coalescence leading to nanoparticle growth
- Ostwald Ripening: involves the migration of adatoms or mobile molecular species, driven by differences in free energy and local adatom concentrations on the support surface
State which of the sintering mechanisms, Particle migration or Ostwald ripening?
- Particle migration and coalescence
- If Ostwald ripening would expect to see some decrease in the size particles in this image before they go into one big one
How can you prevent catalyst deactivation
- Making traps for the particle
- I.e. a porous support like in a metal oxide
- Or a graphitic carbon nitride - nitrogen bind very strongly to support which can stabilise the particle
Why is graphene one of the most effective ways to prevent catalyst deactivation?
- It can be used for any support
- i.e. a metal oxide with a oxygen vacancy which you can bind the carbon to
- Removing carbons atoms can create unsaturates site which have a propensity to bind to something - usually metal
Why are these catalyst difficult to make?
Difficult to make however, as the vacant sites are very reactive and oxygen will usually bind straight away in solution
Pt nanoparticles were impregnated on two different support: boron nitride (BN) and alumina (Al₂O₃), images show are pre-catalysis
After the catalytic process, performed at 350°C, a significant increase on Pt nanoparticle size was observed for one of the samples. In which of the samples above would expect to observed this drastic change in nanoparticle size? Explain your reasoning
- Boron nitride
- Because you have a more smooth surface than alumina which favours the atoms/particles migration thus forming large particles
Catalyst poisons act by blocking sites on the surface and so reduce the number of active sites
How does steam reforming cause this to nickel catalyst?
- The majority of poisoning is caused by sulfur
- At more than 5 ppm, it leadds to rapid Ni catalyst poisoning
- At 20-30 ppb (ppm/1000) it causes a slow Ni catalyst poisoning
- Nickel strongly absorbs onto sulfur
What might we use surface area for to characterise the supported metal catalyst?
- To determine the surface area of the supported metal catalyst (usually, it is measured by the combination of the surface area of the support and catalyst)
- if you have high surface area on the support you have high dispersion of your metal catalyst - more space on the surface
- SA is determined using the Brunauer-Emmett-Teller (BET) methods - modification of Langmuir isotherm
What are some general assumptions associated with using surface area to characterise the supported metal catalyst?
- There is only one molecule absorbed per site
- Gas molecules are physically adsorbed “infinitely”
- There is no interactions between absorbed layers
What does this isotherm plot to work out surface area show?
- The isotherm curve which measures the nitrogen being released from the surface
- You know the amount of nitrogen within the chamber, and after a set period start to heat the support in which the nitrogen will be desorbed
- Then measure the pressure increase due to desorption
- If you have high surface area, the difference between final and inital pressure will be high due to high amount of nitrogen on the surface
These equations can be used to work out surface area from a isothermal plot
What is each component
What might we use Temperature Programmed Desorption (TPD) for to characterise the supported metal catalyst?
- To determine the products/molecules by mass spectrometry on the surface of the catalyst
- You undertake reaction on catalyst surface, then remove it and heat it to 1000°C to desorb everything from the catalyst surface
- If you have a poisoning effect, you should see the poison coming out of your catalyst
What conclusions can you take from this plot from temperature programmed desorption (TPD)?
One conclusion is that you could have nitrogen posioning
Known from that atomic nitrogen (intermediate) has very strongly binded and you have high concentration of hydrogen on the surface of the catalyst
Therefore reducing the conversion rate of the catalsyst (graph) could confirm this
Selective adsorption is a type of TDP, where a ‘mode’ which is used to differentiate a metal particle from the support
CO is often used selectively absorb onto the metal, explain how this work?
- We can work out the exact number of available sites on the catalyst by getting a fresh catalyst
- Putting it inside the reating furnace of the TPD with He (carrier gas) and CO
- CO is a good ligand and will absorb on the surface of both the catalyst (chemisorped) and the support (physisorped)
- When the temperature is raised CO on the support will desorb - will produce a peak
- When the temperature is raised again the CO on the catalyst will desorb - produce a second peak - this will be related to the CO which was on the platinum
- If one CO binds to one Pt, you can work out exactly how many active sites there are
What can be deduced from the Temperature programmed desorption graph?
- They all have a strong interaction with the surface hence could have a possibility of playing a role in posioning but we need to…
- We need to look at the reaction profile
- And to determine there is no sintering, look at micrscopy
What might we use ICP-OES for to characterise the supported metal catalyst?
- To determine the concentration of metal deposited on supports
- Involves dissolving the metal in a solution (e.g. nitric acid)
- Preparing the metal standards with different concentrations and running measurements
What might we use X-ray diffraction for to characterise the supported metal catalyst?
- Use to determine the lattice spacing of crystals and particle size - from peak broadening to the Scherrer equation (need a crystal structure to do this)
- The broader the peaks from this, the crystalline structure is not very well defined
Using this equation here you can define the size of your particles from XRD
- Nanoparticles mean size order: a>b>c>d
- As the number of atoms in the nanoparticles decreases, less x-ray diffraction are generated thus a broader peak instead of sharp line is observed
X-ray diffraction can also be used to determine the lattice spacing of crystals and particle size - from peak broadening using the Scherrer equation
What does it tell us
- Low Miller index faces (planes) are: (100) (111) (110)
- The coordination of atoms in each of these planes is different
- Surfaces with lower coordinated surface atoms have;
1. the highest surface free energy
2. the highest reactivity of absorption
3. the strongest binding for the absorbate and therefore high ΔHads
Consider FCC planes, fill the gaps in the table below
Low coordination = highest reactivity
What might we use Microscopy for to characterise the supported metal catalyst?
Use to visualise the metal catalyst dispersion on the support surface and determine the metal catalyst mean diameter
Basically to define the size of the particle of our catalyst
What is the process occuring with catalyst before and after the reaction?
Suggest a deactivation mechanism, explain your reasoning?
Sintering
The mechanism is particle migration as no single atoms and/or particles are observed in the images after the reaction
What might we use XPS for to characterise the supported metal catalyst?
- Is a surface-sensitive technique that can identify the surface elements within a material (elemental composition), their chemicals state, and the overall electronic structure and density of the electronic states in the material
- Electrons are released, and the energy of this electron will relate to the oxidation state of that catalyst type - important for the type of reaction you are undertaking
What is the difference between in-situ and in-operando characterisation?
- In-situ: all the same reaction conditions inside the microscope (gas/temp/pressure) - information about this mechanism
- In-operando: exactly same conditions where you are detecting the product coming out
