Catalysts Flashcards

1
Q

What is a property and examples of unsupported catalysts

A

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

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2
Q

What is a property of of supported catalyst and an example?

Give the 3 different types of supported catalyst structure too

A

-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

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3
Q

Describe a monolith catalyst

A

-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

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4
Q

What are monoliths commonly used in?

A

-Flue gas removal
-Catalytic converters

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5
Q

What properties do pellet catalysts have?

A

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

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6
Q

Where are pellet catalysts used

A

-Packed beds

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7
Q

What properties do foam solid catalysts have?

A

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

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8
Q

What criteria do catalysts in fixed beds need to fit?

A

-High mechanical strength
-High porosity
-Low pressure drop
-Minimum diffusion pathway into catalyst pores
-Long life (slow deactivation)

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9
Q

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

A

-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

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10
Q

How are pellet catalysts prepared?

A

-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

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11
Q

How are monoliths prepared?

A

-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.

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12
Q

Why may egg yolk catalyst impregnation be done?

A

To control the reaction rate in exothermic reactions

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13
Q

How do you get uniform catalyst impregnation

A

Support dipped evenly into a solution containing the active catalyst species

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14
Q

How to get egg shell catalyst impregnation

A

Support is sprayed with the active catalyst species

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15
Q

How do you get egg yolk catalyst impregnation?

A

Catalyst species is impregnated deep within the support structure

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16
Q

What other ways could a catalyst species be obtained?

A

-Catalyst 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

17
Q

Why and how might catalysts be activated?

A

-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

18
Q

What is sintering?

A

-Method of catalyst deactivation where catalyst materials are subject to high temps.

-Effect can be:
Catalyst particle agglomeration reducing total available surface area
Changes in structure of catalyst particles such as closing of catalyst pores

-Rate of decay y sintering modelled as a second order process

19
Q

What are the 3 methods of catalyst degradation

A

-Sintering (changes structure directly or through agglomeration)
-Coking/fouling (build up of material clogging pores/active sites)
-Poisoning (molecules become irreversibly chemisorbed on active sites, reducing no. of sites for reaction)

20
Q

What chemical and physical properties do support catalysts have?

A

Physical:
-mechanical strength, optimises catalyst porosity, optimal bulk density

Chemical:
-Inert to undesired reactions, reduces catalyst poisoning effects, stabilises against sintering