Catalysts

Question 1

When are unsupported catalysts used?

Answer 1

-Not common as has a small metal surface area
Examples: wires, gauzes, powders

Question 2

When are supported metals used?

Answer 2

-Aim to improve dispersion of the metal species, and therefore increase the activity of the catalyst
-Can be used to best utilise expensive metals such as platinum or to stabilise metals such as nickel
-Most common supports: zeolites, Al2O3, SiO2,, activated carbons

Question 3

How are precipitated catalysts formed?

Answer 3

-Formed from a liquid solution of metal and support components
-Key to control crystallite size during precipitation and ensure adequate mixing
-Support may be added as a finely divided powder or prepared in the same solution from soluble salts e.g. aluminium salts converted to alumina

Question 4

What are impregnated catalysts?

Answer 4

-A support which needs the active catalyst to be applied to it for the reaction to be catalysed
-The way the catalyst is added (impregnation) can create catalyst types

Question 5

What is calcination?

Answer 5

-Heat treatment at a temp slightly above the expected operating temp of the catalyst
-Aims to stabilise physical, chemical and catalytic properties
-Thermally unstable compounds are decomposed during the process (e.g. nitrates and carbonates)
-Crystalline and pore structures are modified

Question 6

What is a monolith?

Answer 6

-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

Question 7

What are monoliths commonly used in?

Answer 7

-Flue gas removal
-Catalytic converters

Question 8

What properties do pellet catalysts have?

Answer 8

-increase s.a
-decrease catalyst pore diffusion pathway
-minimise reactor pressure drop

Question 9

Where are pellet catalysts used

Answer 9

-Packed beds

Question 10

What properties do foam solid catalysts have?

Answer 10

-Solid catalyst with many pores and high voidage - reduces pressure drop and fouling
-High macro porosity - increased transfer
-High s.a

Question 11

What criteria do catalysts in fixed beds need to fit?

Answer 11

-High mechanical strength
-High porosity
-Low pressure drop
-Minimum diffusion pathway into catalyst pores

Question 12

What criteria do catalysts in fluidised bed reactors need to fit?

Answer 12

-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)

Question 13

How are pellet catalysts prepared?

Answer 13

-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

Question 14

How are monoliths prepared?

Answer 14

-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.

Question 15

Why may egg yolk catalyst impregnation be done?

Answer 15

To control the reaction rate in exothermic reactions

Question 16

What are the different types of catalyst impregnation?

Answer 16

-Uniform catalyst impregnation is done by dipping the support evenly in a solution containing the active catalyst species
-Egg shell catalyst impregnation is done by spraying the support with the active catalyst species
-Egg yolk catalyst impregnation is done by inserting the catalyst species deep into the support structure
-Catalyst could also be precipitated onto the support, e.g. dissolving the catalyst in warm solution, the lowering the temp of the solution to super saturate it causing the catalyst to precipitate
-Catalyst species could be exposed to the support and coats onto the support via ion adsorption

Question 17

Why and how might catalysts be activated?

Answer 17

-If impregnation is done with a metal oxide (for ease of move onto the support) it needs to be reduced back to the pure metal
-The reduction is the activation step
-Hydrogen gas is often used to reduce the metal oxide at an appropriate temp for that catalyst