Catalysts Flashcards
What is a property and examples of unsupported catalysts
-Not common as has a small metal surface area
Examples: wires, gauzes, powders
What is a property of of supported catalyst and an example?
Give the 3 different types of supported catalyst structure too
-High surface area
-Support inhibits sintering of small particles to large particles
-Transition metal support structure such as zeolite
-3 types:
Monoliths, pellets, foam solid catalyst
Describe a monolith catalyst
-A block of catalyst which takes up the entire cross section of a reactor
-Many small channels that run through the block to allow reactants to pass through
-The straight channels give anti-fouling properties and low pressure drops
What are monoliths commonly used in?
-Flue gas removal
-Catalytic converters
What properties do pellet catalysts have?
-increase s.a
-decrease catalyst pore diffusion pathway
-minimise reactor pressure drop
Where are pellet catalysts used
-Packed beds
What properties do foam solid catalysts have?
-Solid catalyst with many pores and high voidage - reduces pressure drop and fouling
-High macro porosity - increased transfer
-High s.a
What criteria do catalysts in fixed beds need to fit?
-High mechanical strength
-High porosity
-Low pressure drop
-Minimum diffusion pathway into catalyst pores
-Long life (slow deactivation)
What criteria do catalysts in fluidised bed reactors need to fit?
-Minimum diffusion pathway into catalyst pores
-High attrition resistance
(attrition is degradation due to wear and tear, deactivates the catalysts)
-Spalling resistance
(spalling is where flakes of the catalyst break away)
-Can handle rapid deactivation
How are pellet catalysts prepared?
-Catalytic materials are added as particles to a liquid creating a slurry with a consistency similar to toothpaste
-Slurry is extruded into a thin cylindrical shape which is then cut radially to create small pellets
-The pellets are then dried to harden them
-Organic materials such as starch may be used to create the pellet, this can then be dissolved or burnt out leaving the pellet with pores to increase catalytic porosity
How are monoliths prepared?
-Produced via extrusion with special moulds to form the axial channels
-Monoliths are usually a support material on which a catalyst layer is washed onto the surface of.
Why may egg yolk catalyst impregnation be done?
To control the reaction rate in exothermic reactions
How do you get uniform catalyst impregnation
Support dipped evenly into a solution containing the active catalyst species
How to get egg shell catalyst impregnation
Support is sprayed with the active catalyst species
How do you get egg yolk catalyst impregnation?
Catalyst species is impregnated deep within the support structure