Anatomy of a catalyst and research models

Question 1

What is a Miller index?

Answer 1

Miller index of a surface found by taking reciprocal of points at which the surface crosses the x, y and z axis, followed by scaling to give the lowest set of whole numbers.
Use Miller indices to define a surface.

Question 2

Why is the electron escape depth important?

Answer 2

The mean free path of an electron in a solid limits the escape depth, varies as a function of kinetic energy. Can measure the kinetic energy to understand depth of electron, see how far from the surface spectroscopic/analytical techniques are probing.

Question 3

What is chemisorption?

Answer 3

A strong “chemical bonding” interaction with the surface in a specific way, resulting in monolayers. Surface structure is critical for these interactions.

Question 4

What is physisorption?

Answer 4

Weak interaction similar to melting/condensing on surface, allowing formation of multilayers.

Question 5

What does the Langmuir isotherm look like and what information does it give us?

Answer 5

Plot of Kp/(1+Kp) against pressure. Initially increases and then plateaus about 1. Langmuir isotherm gives the expected behaviour for monolayer adsorption, with initial quick adsorption until there is no space left when it plateaus.

Question 6

What are some key catalytic reactions in industry?

Answer 6

Ammonia synthesis: N2 + 3H2 <=> 2NH3; ΔH = -92 kJ/mol, iron catalysed with Fe2O3 precursor and K promoter
Methanol synthesis: CO + 2H2 <=> CH3OH; ΔH = -91 kJ/mol, copper catalysed with a Cu/ZnO/Al2O3 catalyst
Fischer Tropsch: CO + H2 <=> alkanes + alkenes; ΔH = -165 kJ/mol per mole of CH2. Fe or Co catalysed.

Question 7

What are the components of the anatomy of a catalyst?

Answer 7

Catalytic surface - metal atoms, catalytically active particles on a support, shaped catalyst particles, catalyst bed in a reactor

Question 8

What are the types of heterogeneous catalysts?

Answer 8

Supported metal catalysts (Pt/Al2O3 type), supported single site catalysts (Mo or W polymerisation, LHS of periodic table), acid-base oxide catalysts (e.g. clays and alumina), zeolites

Question 9

What is the purpose of catalyst supports and what are some key properties that enable these purposes?

Answer 9

Prevent thermal aggregation of metal, high surface areas.
Prevent inactivation of catalysts: thermally and chemically stable under reaction condition, mechanically resistant when required

Question 10

What are the pros and cons of SiO2 supports?

Answer 10

Pros: high control over surface structure and fine-ness due to synthetic preparation and significant processing steps
Cons: Pores may collapse at high temperature and digestion can occur at high pH.

Question 11

What are the pros and cons of Al2O3 supports?

Answer 11

Pros: More varied and complicated surface chemistry, more thermally stable than SiO2
Cons: Less predictable than synthetic silica due to impurities and more complex surface

Question 12

What are the pros and cons of carbon supports?

Answer 12

Pros: Precious metal recovery simplified by ability to burn off carbon
Cons: Does not pH buffer like SiO2 or Al2O3 due to lack of surface hydroxyls

Question 13

Why are catalyst pellets used instead of fine powder?

Answer 13

Powder can result in large pressure drops or pore mixing. By using pressed pellets, possible to enhance macroscopic diffusion of reagents and get a higher TOF.

Question 14

What methods can be used to incorporate metals into solid supports?

Answer 14

Co-precipitation and loading, generally followed by some post-treatment

Question 15

How can coprecipitation be used to incorporate metals into solid supports?

Answer 15

Change in conditions used to induce precipitation. Controlled manner required in order to avoid thermal gradients and generate one homogeneous phase. No intrinsic restriction on active metal loading.

Question 16

How can dry (incipient wetness) impregnation be used to incorporate metals into solid supports?

Answer 16

Start with the dry solid support with completely unfilled pores. Metal precursor solution induced by capillary action on a solution volume the same as the pore volume. Evaporation then removes the solvent, leaving the metal crystallised on the solid support. NB. total metal loading restricted by pore volume and metal precursor solubility

Question 17

How can deposition precipitation be used to incorporate metals into solid supports?

Answer 17

Metal precipitated onto the support by increasing pH to reduce solubility. Heterogeneous nucleation is thermodynamically preferable to homogeneous as surface energy term much smaller. NB. really good mixing required, so add urea to control mixing, and can then decompose thermally once finished.

Question 18

How can adsorption be used to incorporate metals into solid supports?

Answer 18

Use surface oxide hydroxyls at high or low pH to form a charged surface. Add a charged metal precursor, which will form a monolayer via electrostatic interactions. NB. total metal loading restricted by surface area on which metal ions can be adsorbed.

Question 19

What are single crystal catalyst models?

Answer 19

A single plane of metal atoms, cut and polished to a specific Miller index. Mounted and thermal coupled near to monitor environment.

Question 20

How can we use single crystal catalyst models to investigate catalyst anatomy?

Answer 20

Single crystal can undergo catalysis reactions as standard catalyst. Requires handling in vacuum to reduce bombardment of surface by air molecules and cleaning important due to tiny surface area.

Question 21

Do rates occur differently on different geometric structures of the same metal catalyst and what evidence is there for this?

Answer 21

Yes! By working on different faces of singlet crystal models, the TOF was found to depend on the surface crystalline anisotropy. Different Miller indices mimic particles from different parts of the crystal, showing selectivity.

Question 22

Why is structure sensitivity important?

Answer 22

Arrhenius: k = A exp(-Ea/RT) - exponential relation between Ea and rate. This relationship mean that specific sites on a catalyst that are more active quickly dominate all of the rate.

Question 23

How can structure sensitivity be applied to ammonia synthesis?

Answer 23

Ammonia synthesis is a classic example of a structure sensitive reactive. It was found that small particles of Fe which were not close-packed led to better yields of reaction.

Question 24

What spectroscopy can be done on single crystal models and what does this information give us?

Answer 24

RAIRS: can obtain orientational information from metal surface selection rule
XPS: allows surface competition and site geometry to be probed, identification of atop, hollow and bridging sites.

Question 25

What are the limitations of using single crystal models?

Answer 25

Cannot use single crystals to study metal-support interaction, particle size effects or TOF at high pressures. Leading to pressure and materials gap.
Key example: Cobalt FTS single crystal studies get chain growth probability very different from real catalyst behaviour.

Question 26

What is a deposition based nanostructure and what are the limitations of these studies?

Answer 26

Layer of metal, layer of oxide, metals/oxides, addition of multiple components and poisons/promoters. Limited by difficulty of experiments, with high costs in money and time associated with them.

Question 27

How has a deposition based nanostructure been used to investigate size behaviour of FTS catalysts?

Answer 27

Use of 2D nanostructure. Variation of size of Co particle. Found that there was a sudden jump in TOF for about 3 nm, with above a threshold < 5nm, size not inducing a significant change in rate.

Question 28

How does lithography work?

Answer 28

Formation of 40 nm particles. Lace substrate with a polymer coating. Use electron exposure to develop holes in the coating. Allow development and etching to improve these holes. Metal deposition. Then lift off polymer coating with metal particles in holes remaining attached to the substrate.

Question 29

What are the pros and cons of lithography?

Answer 29

Pros: good technique to explore substrate source-support interactions
Cons: Particles still quite large (4x average catalyst size) and still suffers issues from 2D models with low TON cf. working catalysts

Question 30

How are colloidal nanoparticles synthesised?

Answer 30

Supersaturate solution by inducing a sudden change in solubility promoting sudden growth by forming nuclei. Allow growth to occur in presence of a capping agent to control surface growth and prevent agglomeration.

Question 31

What are the advantages and disadvantages of using colloidal nanoparticles generated via “wet chemistry”?

Answer 31

Advantages: easy to prepare compared to complex vacuum methods, can be used to prepare larger quantities of catalyst than existing methods, can be deposited on any substrate
Disadvantages: residual synthetic or capping agents can impact rate, residual synthetic agents may mask other adsorbates when using spectroscopic techniques, development of synthetic method can be challenging