Science Focus (Issue 23)

Haber Process : More Than J ust N i trogen and Ammon i a 哈柏法: 改變世界的 化學反應 By Lambert Leung 梁卓霖 Have you ever doubted how the agricultural supply manages to feed the exponentially growing wor ld population? One of the keys is to provide nitrogen, an essential nutr ient for good yields, through the use of nitrogenous fertilizers. The question then becomes: Where does all that nitrogen come from? Haber Process T he an s we r i s t he Habe r process – it should ring a bell to some of you, especially those tak ing chemi st r y classes. To recap briefly, it was discovered by the German chemist Fr itz Haber, who was later awarded the Nobel Prize in Chemistry in 1918 . Un l i ke i t s s ubopt i ma l predecessors, the Haber process was more energyefficient and scalable in converting nitrogen into ammonia, which could be further processed into nitrogenous fertilizers like ammonium nitrate (NH4NO3) and urea ((NH2)2CO) [1]. Such an improvement can be attributed to the Le Chatelier’s principle (footnote 1), where chemists maximize yields by casting some magic on chemical equilibrium and kinetics – in industrial practice, the Haber process operates at both high temperature (around 450 ℃) and high pressure (around 200 atm) [2]. The above is only a general picture of the process, if not a tip of the iceberg (most probably you have seen it in textbooks!). Let’s dig deeper into the steps behind the short equation. Nitrogen Fixation It may be tempting to think that since about 78% of atmospheric air is made up of nitrogen, the source for fertilizers should be more than abundant. Yet, atmospheric nitrogen exists as inert diatomic molecules (N2) held together by extremely strong N– N triple covalent bonds. This is why nitrogen won’t react with hydrogen to form N–H bond under normal conditions; moreover, it also prevents plants from converting N2 molecules to other useful forms by themselves, at least not without the help of nitrogenfixing soil bacteria. It simply had posed a challenge for scientists. For this reason, early fertilizers were highly limited to natural sources such as manures and niter mines (for KNO3). To address the i ssue, scient i sts have made attempts, both chemical ly and biological ly, in “cracking” nitrogen. In Haber process, the role of nitrogen-fixing bacteria is chemically substituted by high temperature, pressure, and an iron catalyst. This breaks the nitrogen molecules into atoms for forming ammonia, which will be converted into nitric acid through the Ostwald process as the feedstock of useful fertilizers like urea and ammonium nitrates. Scientists are still searching for ways to fix nitrogen under milder conditions. Inspired by the symbiotic relationship between legumes and soil bacteria, a molecular biologist named Frederick Ausubel once tried to transfer nitrogen-fixing genes from soil bacteria to cereals crops (not legumes) in 1970s to benefit farmers who cannot afford fer ti l izers, N2(g) + 3H2(g) ⇌ 2NH3(g)