Part 2 - Catalysts synthesis Flashcards
What determines the internal surface area in the support?
The size and number of pores.
What sizes are desirable for porous support structures?
Small pores sizes and many pores have a higher surfaces area, but too small pores can hinder diffusion.
How does one measure the pore size distribution using BET?
For porous materials with diameters smaller than 100 Å, one can use the same equipment as for pore sizes. But a high relative pressure are used for condensing N2 in the catalyst pores.
The volume adsorbed in either the ascending or descending branches of the BET plot at relative pressures close to 1
How can the pore size be measured by mercury intrusion?
This method is usually preferred for materials with pores larger than 30 nm. Measures the penetration of mercury as a function of applied pressure.
When solutions are used in catalyst preparation, what steps are usually included in the process?
1) Synthesis
2) Drying
3) Calcination (high T, air)
4) Activation (reduction)
Describe the synthesis step of catalyst preparation.
The application of solution chemistry to obtain anchoring of complexes to a surface or precipitation.
Describe the drying step of catalyst preparation.
Removal of excess H2O or other solvents.
Describe the calcination step of catalyst preparation.
Decomposition of the salts at elevated temperatures. (for example NO3 -> NOx)
Describe the activation step of catalyst preparation.
Treatment under non-inert, usually reducing atomsphere.
What are some commonly applied synthesis methods for catalysts?
- Impregnation
- Co-precipitation
- Sol-gel
- Nanoparticle decomposition
- Hydrothermal synthesis
Describe the wet impregnation of metals in catalyst synthesis.
Immerse the support into solution of metal ions, such as RhNO3 or RhCl.
The metal amount and initial particle distribution is determined by:
- The composition of the solution
- The interaction between the support surface and the metal complex
- Transport phenomena inside pore system of support.
- How many times the support is dipped.
Describe the dry impregnation (incipient wetness) of metals in catalyst synthesis.
Drip the amount of metal ion solution onto your dry support to fill the pore system.
The metal amount and initial particle distribution is determiend by:
- Composition of the solution.
- Interaction between the support surface and the metal complex.
- Transport phenomena inside pore system of support.
- Number of cycles (impregnation and drying).
Support material must be in powder form.
Describe the deposition-precipitation method in catalyst synthesis.
Based on nucleation and growth. The support is suspended in precursor solution. Parameter change in order to induce precipitation. This can be:
- pH increase by adding base.
- Reduction by adding reducing agents.
- Ligand removal.
What are the parameters that can be change to induce precipitation in the deposition-precipitation method in catalyst synthesis? What is an other name for this method?
Also called electroless plating.
- pH increase by adding base. Can add urea and mix, and then heat. This causes urea hydrolysis that increases the pH homogeneously.
- Reduction by adding reducing agent. Can be formaldehyde at low temperature, and then increasing the temperature.
What are some issues with electroless plating?
- Thermodynamic instability of solution
- State of the surface (positive charged SiO2)
