AdvMat:JKB Flashcards
Adv Mat
Why does the synthesis method influence the performance
of a catalyst?
structure of the active sites homogeneity morphology particle size distribution Porosity pore structure alloy/core-shell surface area
Why do metal surfaces sometimes have different structures to
the bulk metal?
Surfaces are high energy
Reduce energy of the system
Strengthen surface later – contraction in layer spacing
Increase coordination number
Why are metal catalysts usually dispersed on a support.
Increase dispersion Stabilise against sintering Small nanoparticles - increase surface area - influence properties
How do the drying conditions influence the structure of
catalysts synthesized using impregnation methods?
Slow drying – naoparticles round mouth of pore
Fast drying – more homogeneous ditribution in pore
What is the most appropriate method to produce core-shell
bimetallic catalysts?
Sol immobilisation
What factors influence the ions in solution during deposition/precipitation?
pH
charge on ion
Size of ion
What are the disadvantages of using co-precipitation to
produce a Pt/SiO2 catalyst?
Metal and support precipitated together
Pt not all on surface
Explain the term net point of zero charge.
pH at which surface is not charged
Lower pH – positive net charge
Higher pH – negative net charge
What is the key difference between microporous supports
such as ALPOs and those synthesized using sol-gel
methods?
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
What method would be most appropriate for synthesizing a
zirconia support for use in liquid phase reactions?
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
Why are metal nanoparticle catalysts prone to sintering?
Surfaces are high energy.
larger particles minimise surface atoms.
Reduce energy of the system
Why does antisolvent precipitation form amorphous
materials?
Fast precipitation process
Nucleation dominates over crystal growth
What is a key advantage of hydrothermal/solvothermal
synthesis compared with other synthesis techniques?
Metastable products rather than thermodynamic products
e.g. zeolites rather than Al2O3 and SiO2
What is the key difference between wet impregnation and
incipient wetness synthesis methods?
Wet impregnation
•excess solution
•can produce high loadings
Incipient wetness
•just enough solvent to for paste
•good for low loadings
Why is the net point of zero charge of the support important
during the synthesis of supported catalys?
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
Which synthesis method would you use if you wanted a
narrow particle size distribution of metal nanoparticles?
Sol immobilisation
Pre form stabilised nanoparticles in solution
What is the difference between hard and soft templating?
Soft – ions, micelles – good for amorphous silicas
Hard – SiO2, C, etc – good for range of metal oxides
What method would be most appropriate for synthesizing a
zirconia support for use in liquid phase reactions?
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
What is the advantage of using macroporous materials over
microporous materials as catalyst supports for liquid phase
reactions?
improved diffusion
Why is adsorption generally favourable and spontaneous??
Minimise surface energy by forming bonds at the surface
e.g. metal surface form oxide layers
Suitable method for disordered metal oxide support and describe method
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
Why zeolites are produced using hydrothermal methods and how the autogenous pressure is controlled?
- Forms metastable products rather than thermodynamic
- Increased solubility of products
- Pressure controlled by varying the volume of solvent in autoclave.
Co-precipitation Method?
- Aqueous solutions of metal precursors mixed together.
- Base added simultaneously
- Precipitate recovered, washed and dried (400C, 16h)
- Calcined in static air (415C,2h)
SAS precipitation
- 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
Wet impregnation influence on particle size distribution
- Average size of metal particle depends on conc of sol.
- Variation in pore volume leads to broad PSD
How low surface area influences PSD?
Low SA=less dispersion=larger particles
How sol-gel can increase SA of support?
- 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
coprecipitation vs deposition
- 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
Synthesis of microporous supports
- By slow hydrothermal crystallisation from a gel
- SiO2 Solution added to autoclave
- Long crystallisation time
Synthesis of Macroporous supports
Use large templates (such as micelles or SiO2 structures
What reaction conditions would macroporous be prefered over microporous
Liquid Phase reactions.
Diffusion limitation can be a problem for microporous
Describe how the metal nanoparticle character can be controlled in supported bimetallic catalysts
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
