Haber Process: More Than Just Nitrogen and Ammonia
By Lambert Leung 梁卓霖
Have you ever doubted how the agricultural supply manages to feed the exponentially growing world population? One of the keys is to provide nitrogen, an essential nutrient for good yields, through the use of nitrogenous fertilizers. The question then becomes: Where does all that nitrogen come from?
Haber Process
The answer is the Haber process – it should ring a bell to some of you, especially those taking chemistry classes. To recap briefly, it was discovered by the German chemist Fritz Haber, who was later awarded the Nobel Prize in Chemistry in 1918. Unlike its suboptimal predecessors, the Haber process was more energy-efficient and scalable in converting nitrogen into ammonia, which could be further processed into nitrogenous fertilizers like ammonium nitrate (NH4NO3) and urea ((NH2)2CO) [1].
N2(g) + 3H2(g) ⇌ 2NH3(g)
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 nitrogen-fixing 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 issue, scientists have made attempts, both chemically and biologically, 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 fertilizers, but to no avail [3]. The incompatibility between bacterial genes and plants, as well as conflicting mechanisms of oxygenic photosynthesis and anaerobic nitrogen fixation, posed technical challenges that were too difficult to overcome [3]. Recent research focus has shifted to the use of coordination compounds (footnote 2) and nanoparticles of other transition metals [4].
Hydrogen Generation
Another key ingredient in the Haber process is hydrogen, which naturally exists in the forms of fossil fuels and water. It is typically generated from steam-methane reforming (CH4(g) + H2O(g) → 3H2(g) + CO(g)), then followed by water-gas shift reaction to further produce hydrogen (CO(g) + H2O(g) → H2(g) + CO2(g)) and pressure swing adsorption to extract pure hydrogen from the gaseous mixture [5]. There are several alternatives for hydrogen production. To name a few, electrolysis of water has been known as a promising option besides steam-methane reforming as scientists keep exploring its potential by testing different electrolytes and membranes. Second, dark fermentation offers a pathway to produce hydrogen from biomass, which utilizes groups of anaerobic bacteria to decompose carbohydrates in absence of light and oxygen [6]. Current research aims to reduce the emission of carbon dioxide and uses of fossil fuels (methane as feedstock, others as fuels to achieve high reaction temperature) to better align with green chemistry.
Can We Do Better?
With proper design regarding equilibrium and rate of reaction, the Haber process has expanded food productions by addressing crops’ needs for nitrogen, thus preventing famine. It is amazing that the original Haber process is still relevant and used nowadays. Despite its immense contribution to agriculture, the Haber process is energy demanding and relies on the use of fossil fuels, being responsible for 1.4% of global carbon dioxide emissions plus 1% of global energy consumption [7]. This calls for modern adaptations to optimize conditions and catalysts to ensure the reaction is compatible with the goal of sustainable development.
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The Dark Side of the Haber Process
In many textbooks, the Haber process is commonly introduced as a life-saving contribution to mankind. It really would have been great if the process is used in producing fertilizers only.
Aside from being a nutrient for plants, nitrogen constitutes explosives. As aforementioned, nitrogen tends to exist as a highly stable, triple-bonded diatomic form (N2). However, nitrogen atoms are bound by much weaker N–O or N–H bonds in nitrogen compounds compared to the strong N–N bond in nitrogen gas. When those weak bonds in the nitrogen compounds are broken by ignition, large amounts of energy are released when the triple bonds are reformed to form the more stable nitrogen gas (footnote 3), which can expand rapidly as a gas and create a shock wave. Hence, fertilizers themselves are explosives, leading to the Beirut explosion in 2020 [8].
During the World War I (1914-1918), not only was the process widely used in manufacturing explosives for the German army, but Haber himself also advocated chemical warfare: He proposed and supervised the deployment of extremely toxic chlorine gases in the Second Battle of Ypres, the first use of chemical weapons in war [9]. Shocked and unprepared, the Allies suffered severe casualties and had to retreat. |
1 Le Chatelier’s principle: When changes in conditions of temperature, concentrations, pressure, and volume are applied to a system, the equilibrium position would respond by counteracting those changes.
2 Coordination compound: A central metal atom that is chemically bound to, or surrounded by, other groups of non-metal atoms.
3 Editor’s remark: Recall that the breaking of bonds requires energy (endothermic), while the forming of bonds releases energy (exothermic).
References:
[1] Barona, A., Etxebarria, B., Aleksanyan, A., Gallastegui, G., Rojo, N., & Diaz-Tena, E. (2018). A Unique Historical Case to Understand the Present Sustainable Development. Science and Engineering Ethics, 24(1), 261-274.
[2] BBC. (n.d.). Ammonia and sulfuric acid. Retrieved from https://www.bbc.co.uk/bitesize/guides/z234tyc/revision/2
[3] Ausubel, F. M. (2018). Tracing My Roots: How I Became a Plant Biologist. Annual Review of Genetics, 52, 1-20. doi:10.1146/annurev-genet-120417-031722
[4] Morlanés, N., Almaksoud, W., Rai, R. K., Ould-Chikh, S., Ali, M. M., Vidjayacoumar, B., . . . Basset, J. (2020). Development of catalysts for ammonia synthesis based on metal phthalocyanine materials. Catalysis Science & Technology, 10(3). doi:10.1039/C9CY02326G
[5] U.S. Department of Energy. (n.d.). Hydrogen Production: Natural Gas Reforming. Retrieved from https://www.energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming
[6] Ghavam, S., Vahdati, M., Grant Wilson, I. A., & Styring, P. (2021). Sustainable Ammonia Production Processes. Frontier in Energy Research, 9. doi:10.3389/fenrg.2021.580808
[7] Capdevila-Cortada, M. (2019). Electrifying the Haber–Bosch. Nature Catalysis, 2(1055). doi:10.1038/s41929-019-0414-4
[8] Guglielmi, G. (2020, August 10). Why Beirut’s ammonium nitrate blast was so devastating. Nature. doi:10.1038/d41586-020-02361-x
[9] King, G. (2012, June 6). Fritz Haber’s Experiments in Life and Death. Smithsonian Magazine. Retrieved from https://www.smithsonianmag.com/history/fritz-habers-experiments-in-life-and-death-114161301/