Catalysis in industry
activation energy without catalyst
1
2N2(g)
activation energy with catalyst
+ 11 2 H2(g)
H 1
2N2(ads)
= – 46 kJ mol-1
2
NH3(g)
+ 11 2 H2(g)
NH3(ads)
Energy NH2(ads) + H(ads)
This potential energy diagram illustrates the synthesis of ammonia using a catalyst. Each step is shown as a simple energy diagram, with its own activations energy and enthalpy change of reaction. The estimated activation energy for the reaction without a catalyst is very high.
NH(ads) + 2H(ads)
When a catalyst is used, the activation energy barriers for individual steps are much lower, as shown in this diagram.
Overall equation: 1 2 N2(g)
+ 112 H2(g)
NH3(g)
N(ads) + 3H(ads) Progress of reaction
Figure 3 The activation energy barriers for the reactions occurring during the catalytic synthesis of ammonia.
General requirements for a heterogeneous catalyst To be successful the catalyst must allow the reaction
palladium. Thus all traces of sulfur compounds must be removed from the petrol used in cars fitted with catalytic converters.
to proceed at a suitable rate under conditions that are
Further, solid catalysts are much more effective if they
economically desirable, at as low a temperature and
are finely divided as this increases the surface area.
pressure as possible. It must also be long lasting. Some reactions lead to undesirable side products. For example in the cracking of gas oil (Unit 4), carbon is formed which is deposited on the surface of the catalyst, a zeolite, and leads to a rapid deterioration of its effectiveness. Many catalysts are prone to poisoning which occurs when an impurity attaches itself to the surface of the catalyst and prevents adsorption of the reactants. Minute traces of such a substance can ruin the process, One example is sulfur dioxide, which poisons the surface of platinum and
Figures 4 and 5 Two ways by which the surface area of a catalyst can be increased. In Figure 4, vandium(V) oxide (used in the manufacture of sulfuric acid (Unit 50)) has been produced in a ‘daisy’ shape. In Figure 5, above, the platinumrhodium alloy (used in the manufacture of nitric acid (Unit 40)) is in the form of very fine wire that has been woven to construct a gauze.
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