Catalysts are vitally important to industry across the world. The annual value of the catalyst industry worldwide is measured in billions of dollars.
In the Haber process hydrogen reacts with nitrogen to produce ammonia. Without a catalyst the process would proceed so slowly as to be economically unviable. One of the reasons for this is because of the nature of the reaction:
The ‘obvious’ route is for the products to be built up bit by bit. One route is as follows:
However breaking open the nitrogen triple bond to form N2H2 is very difficult and the resulting compound is highly unstable and is likely to dissociate almost as soon as it forms. The next intermediate product N2H4 is hydrazine, this is used as a rocket monopropellant for satellites, when it decomposes it does so very exothermically and so its formation is highly endothermic. Clearly these steps are not going to occur easily. In practice four separate molecules have to be brought together in the same space at the same time and this is a very rare occurrence.
The catalyst works by providing both a ‘work bench’ to hold on to the reactants while they are interacting and the ‘tools’ to achieve the reaction. The actual route is set out here:
Nitrogen and hydrogen molecules are adsorbed on to the metallic iron surface. The hydrogen almost immediately splits into its component atoms by sharing or exchanging electrons with the catalyst surface.
The nitrogen molecules split into their component atoms and they lock in to the surface of the iron catalyst. This is the slowest step and so limits the rate of the entire process.
The hydrogen atoms migrate across the surface of the catalyst…
and react, in stages to produce NH3 molecules that are still bound to the surface.
The assembled ammonia molecules are desorbed from the surface.
If you have never come across this detailed a description before don’t be too surprised with your teacher. The research that went in to working it out won Gerhard Ertl the 2007 Nobel Prize for Chemistry! He even managed to work out why iron is such a good catalyst for the reaction but adding a little potassium to the iron increases the rate of reaction even more. The iron is needed because the spacing between the iron atoms just allows the nitrogen atoms to slide in between them, this helps split the nitrogen molecules into individual atoms. The potassium is able to increase the number of free electrons available and so this makes breaking the nitrogen bond even easier. Since this is the rate limiting step the whole reaction runs faster.
It is important to remember that the creation of ammonia is a reversible reaction and so the catalyst also provides a speedy and efficient way to break ammonia down in to hydrogen and nitrogen. For this reason ammonia is removed as quickly as is practicable by cooling the gasses and allowing the ammonia vapour to condense to liquid.
The iron catalyst used has another secret that is not often mentioned; it is porous which dramatically increases the surface area. The starting material is magnetite (Fe3O4) which is then ‘leached’ by exposing it to hot hydrogen. The hydrogen reacts with the oxygen which is removed in the form of water, leaving the catalyst as largely pure iron, at least near the surface. Because the overall volume of the catalyst is not greatly changed the result is a highly porous structure with a high surface area.
Just to convince you that you can’t keep a good catalyst down you might like to know that iron is not the only catalyst used in the production of ammonia. The original Haber process catalyst was osmium and he discovered that uranium would also work but these were either too expensive or too unstable for commercial use. It took Haber and his team of researchers over two years to identify the currently used iron catalyst together with its potassium promoter. Even today another catalyst is required for the full industrial process; the hydrogen feed stock for the Haber process is obtained from methane (natural gas) and steam by passing them over a nickel oxide catalyst.
This is used in the production of sulphuric acid. The catalyst used is vanadium(V) oxide (V2O5 this is not a hair product, honestly). Unlike the Haber process (where nitrogen and hydrogen are adsorbed onto the surface of the iron catalyst, forming temporary bonds) the contact process involves a temporary chemical change in the catalyst.
The first step of the contact process does not require a catalyst as it simply involves the burning of clean sulphur in air to produce sulphur dioxide:
The next step is to feed the sulphur dioxide into a converter in order to produce sulphur trioxide in the reaction:
However this step makes use of the catalytic reaction:
The conditions of this reaction are a little curious. Since the overall reaction is very exothermic Le Chatelier’s principle suggests that to increase the yield the temperature should be kept as low as possible. However the catalytic reaction becomes inactive below about 380 °C and so the temperature needs to be kept at about 420 °C. Also the net reaction goes from three moles of reactants to two moles of product and so Le Chatelier suggests that we should make use of a high pressure chemical plant as with the Haber process. However the addition of the equilibrium position lies so far to the right hand side of the reaction that there is no need (also the sulphur dioxide could be liquidised at a high enough pressure, reducing the surface area available for the reaction to take place. As a result the working pressure is usually around two atmospheres, just enough to forces the gasses around the converter.
The effect of the final step is to dissolve the sulphur trioxide in to water to create sulphuric acid. Unfortunately this reaction is so exothermic any direct attempt would produce a fine and unmanageable mist. Instead the gas is passed though previously made, concentrated sulphuric acid in order to make ‘fuming sulphuric acid’ or oleum (H2S2O7) in the reaction:
This is then diluted to make the more manageable (though still highly concentrated) sulphuric acid: