AdvMat:JKB

Question 1

Why does the synthesis method influence the performance

of a catalyst?

Answer 1

structure of the active sites
homogeneity
morphology 
particle size distribution
Porosity 
pore structure
alloy/core-shell 
surface area

Question 2

Why do metal surfaces sometimes have different structures to

the bulk metal?

Answer 2

Surfaces are high energy
Reduce energy of the system
Strengthen surface later – contraction in layer spacing
Increase coordination number

Question 3

Why are metal catalysts usually dispersed on a support.

Answer 3

Increase dispersion
Stabilise against sintering
Small nanoparticles 
- increase surface area
- influence properties

Question 4

How do the drying conditions influence the structure of

catalysts synthesized using impregnation methods?

Answer 4

Slow drying – naoparticles round mouth of pore

Fast drying – more homogeneous ditribution in pore

Question 5

What is the most appropriate method to produce core-shell

bimetallic catalysts?

Answer 5

Sol immobilisation

Question 6

What factors influence the ions in solution during deposition/precipitation?

Answer 6

pH
charge on ion
Size of ion

Question 7

What are the disadvantages of using co-precipitation to

produce a Pt/SiO2 catalyst?

Answer 7

Metal and support precipitated together

Pt not all on surface

Question 8

Explain the term net point of zero charge.

Answer 8

pH at which surface is not charged
Lower pH – positive net charge
Higher pH – negative net charge

Question 9

What is the key difference between microporous supports
such as ALPOs and those synthesized using sol-gel
methods?

Answer 9

ALPOs like zeolites but with Al and P instead of Al and Si
Crystalline – well defined pore size

Sol gel – not well defined pore sizes

Question 10

What method would be most appropriate for synthesizing a

zirconia support for use in liquid phase reactions?

Answer 10

Want macropores to improve diffusion in liquid phase.
Soft templating with micelles doesn’t work well with ZrO2.
Need to use hard templating with SiO2 or carbon template

Question 11

Why are metal nanoparticle catalysts prone to sintering?

Answer 11

Surfaces are high energy.
larger particles minimise surface atoms.
Reduce energy of the system

Question 12

Why does antisolvent precipitation form amorphous

materials?

Answer 12

Fast precipitation process

Nucleation dominates over crystal growth

Question 13

What is a key advantage of hydrothermal/solvothermal

synthesis compared with other synthesis techniques?

Answer 13

Metastable products rather than thermodynamic products

e.g. zeolites rather than Al2O3 and SiO2

Question 14

What is the key difference between wet impregnation and

incipient wetness synthesis methods?

Answer 14

Wet impregnation
•excess solution
•can produce high loadings

Incipient wetness
•just enough solvent to for paste
•good for low loadings

Question 15

Why is the net point of zero charge of the support important

during the synthesis of supported catalys?

Answer 15

Determines the charge on the surface at different pH.
Add metal using cations or anions as appropriate for
electrostatic interactions.
Can control particle size and loading

Question 16

Which synthesis method would you use if you wanted a

narrow particle size distribution of metal nanoparticles?

Answer 16

Sol immobilisation

Pre form stabilised nanoparticles in solution

Question 17

What is the difference between hard and soft templating?

Answer 17

Soft – ions, micelles – good for amorphous silicas

Hard – SiO2, C, etc – good for range of metal oxides

Question 18

What method would be most appropriate for synthesizing a

zirconia support for use in liquid phase reactions?

Answer 18

Want macropores to improve diffusion in liquid phase.
Soft templating with micelles doesn’t work well with ZrO2.
Need to use hard templating with SiO2 or carbon template

Question 19

What is the advantage of using macroporous materials over
microporous materials as catalyst supports for liquid phase
reactions?

Answer 19

improved diffusion

Question 20

Why is adsorption generally favourable and spontaneous??

Answer 20

Minimise surface energy by forming bonds at the surface

e.g. metal surface form oxide layers

Question 21

Suitable method for disordered metal oxide support and describe method

Answer 21

Sol-Gel synthesis

• Metal Salt solution undergoes hydrolysis and condensation (polymerisation) to form 3D structures in liquid phase
• Dried and calcined to remove excess solvent

Question 22

Why zeolites are produced using hydrothermal methods and how the autogenous pressure is controlled?

Answer 22

• Forms metastable products rather than thermodynamic
• Increased solubility of products
• Pressure controlled by varying the volume of solvent in autoclave.

Question 23

Co-precipitation Method?

Answer 23

• Aqueous solutions of metal precursors mixed together.
• Base added simultaneously
• Precipitate recovered, washed and dried (400C, 16h)
• Calcined in static air (415C,2h)

Question 24

SAS precipitation

Answer 24

• Solution of metal/support pumped through capilary.
• CO2 pumped coaxial
• solution sprayed into SC-CO@
• Droplet and CO2 rapidly diffuse into each other
• metal/support precipitated rapidly (homogeneous structure)
• Nucleation dominates over crystal growth

Question 25

Wet impregnation influence on particle size distribution

Answer 25

• Average size of metal particle depends on conc of sol.

- Variation in pore volume leads to broad PSD

Question 26

How low surface area influences PSD?

Answer 26

Low SA=less dispersion=larger particles

Question 27

How sol-gel can increase SA of support?

Answer 27

• Metal salts are hydrolysed and polymerised to form colloidal suspension
• Forms 3D structure in liquid
• Dried and calcined to remove solvent
• Fast drying porosity maintained
• Slow drying structure collapse

Question 28

coprecipitation vs deposition

Answer 28

• Deposition has all metal ions adsorbed onto the surface
• coprecipitation, metal not all on the surface
• Coprecipitation is a simple exp where support and metal ion precipitate together, some metal therefore in bulk.
• Deposition, metal precipitated onto support

Question 29

Synthesis of microporous supports

Answer 29

• By slow hydrothermal crystallisation from a gel
• SiO2 Solution added to autoclave
• Long crystallisation time

Question 30

Synthesis of Macroporous supports

Answer 30

Use large templates (such as micelles or SiO2 structures

Question 31

What reaction conditions would macroporous be prefered over microporous

Answer 31

Liquid Phase reactions.

Diffusion limitation can be a problem for microporous

Question 32

Describe how the metal nanoparticle character can be controlled in supported bimetallic catalysts

Answer 32

Bimetallic catalysts can produce alloys or core/shell structures based on ordering in Sol-immobilisation method
-Properties can be tuned by the amount of each metal used