Catalysts
Introduced| English | Meaning |
|---|---|
| heterogeneous catalyst/ˌhetrəˈdʒiːnɪəs ˈkætəlɪst/ | A catalyst in a different physical phase from the reactants. |
| adsorption/ədˈsɔːpʃn/ | The sticking of molecules onto a surface, the first step of heterogeneous catalysis. |
| desorption/dɪˈsɔːpʃn/ | The release of product molecules from a catalyst surface. |
| surface area/ˈsɜːfɪs ˈeərɪə/ | The total area of the outer surfaces of a three-dimensional object. |
| homogeneous catalyst/həˈməʊdʒnɪəs ˈkætəlɪst/ | A catalyst in the same physical phase as the reactants. |
Speeding things up, again
- Catalysts are homogeneous or heterogeneous.
- Each works in a characteristic way.
- Both come back unchanged at the end.
A heterogeneous catalyst is in a different physical state from the reactants.
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e.g. a solid catalyst with gaseous reactants.
Heterogeneous catalysts 多相催化剂
- In a different physical state from the reactants (usually a solid with gases). It works in three stages:
- adsorption 吸附: reactant molecules stick to the surface.
- their bonds are weakened, so they react more easily.
- desorption 脱附: the products leave the surface.
- Examples: iron in the Haber process; Pt/Pd/Rh in a catalytic converter.
A heterogeneous catalyst works by adsorption, reaction, then desorption on its surface
Industrial heterogeneous catalysts are made as small shaped pellets, rings and discs to give a large surface area 表面积 for the reactants to stick to
How a catalyst speeds a reaction
Turn the catalyst on and watch the rate jump — it gives more successful collisions per second by offering a lower-energy path.
A heterogeneous catalyst:
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Heterogeneous = different state (e.g. solid + gases); reactants adsorb, react, then products desorb.
Put the stages of heterogeneous catalysis in order.
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Reactants adsorb, their weakened bonds react, then the products desorb.
Homogeneous catalysts 均相催化剂
- In the same physical state as the reactants.
- It is used up in one step and reformed in a later step, so it returns unchanged.
- Examples: NOₓ helping oxidise atmospheric $\text{SO}_2$; $\text{Fe}^{2+}/\text{Fe}^{3+}$ speeding the $\text{I}^-$ + $\text{S}_2\text{O}_8^{2-}$ reaction.
A homogeneous catalyst:
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Homogeneous = same state; it reacts in one step and is regenerated in a later step, so it returns unchanged.
Which is a homogeneous catalyst?
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Aqueous Fe²⁺/Fe³⁺ ions are in the same state as the reacting ions — homogeneous; solid Fe and Pt are heterogeneous.
Two kinds of catalyst
- Heterogeneous catalysts (different phase) work by adsorbing reactants on their surface.
- Homogeneous catalysts (same phase) work via an intermediate.
You've got it
- heterogeneous (different state): adsorption → weakened bonds → desorption (e.g. Fe in Haber, Pt/Pd/Rh converter)
- homogeneous (same state): used up then reformed, so unchanged (e.g. $\text{Fe}^{2+}/\text{Fe}^{3+}$)