When we think of energy-intensive processes and greenhouse gas emissions, our minds probably jump to gas-guzzling cars and coal-fired power plants. But agriculture is another culprit, largely due to a single industrial process that accounts for 1 percent of the world’s energy use just to convert nitrogen (N2) molecules to ammonia (NH3) for fertilizer! Luckily, new research in the Journal of the American Chemical Society suggests a promising new path to make ammonia that only requires light and chemical molecules made of cheap, earth-abundant metals.
First, let’s understand why we need this process in the first place. Plants require nitrogen in the soil to grow. They also need carbon and oxygen, but these are abundant in the atmosphere in a form that readily reacts to be available in the air and soil. Nitrogen is the problem child because it’s so happy with its normal state of being as an N2 molecule. N2 is everywhere, making up 78% of the atmosphere, but it’s extremely inert. Three strong, covalent bonds keep the the two nitrogen atoms snugly together so that the molecule is extremely happy how it is. In chemical terms, this means that a large input of energy is required to break the bonds between the nitrogen atoms.
Before World War I, farmers would grow as many crops as they had nitrogen to feed into the soil, the major limitation in agricultural as well as population growth. But in the early 20th century, Fritz Haber and Carl Bosch, two German chemists, found a way to reduce N2 with hydrogen to make ammonia (NH3), a much more reactive molecule that can be manufactured and imbued into the soil to provide its nitrogen needs, a process known as fixation. This basically provided farmers with an unlimited supply of nitrogen that allowed agriculture expansion to take off and take us to the overcrowded planet we have today.
This process, though, never really got around the fact that N2 is so stable and requires large energies to break up. The method, known as the Fritz-Haber process, uses high temperatures up to 600 C and extremely large pressures between 150-250 bar (atmospheric pressure is 1 bar!) – this requires a lot of input energy to reach these conditions! The figure below is a large-scale manufacturing example of the process – N2 in on the left, NH3 out on the right. It’s in German, but you don’t need to read it to see that it’s a complex, energy-intensive, Rube-Goldberg-like method!
And, of course, where do we get most of that required input energy? From fossil fuels! Since more available nitrogen means crops, and more crops means faster population growth, this is a strong positive feedback cycle for more and more nitrogen fixation into ammonia. Thus, ammonia production for fertilizer has become a major contributor to both total energy use (1%) and total CO2 emissions (around 2%).
Many researchers have been looking into catalysts for the N2 -> NH3 reduction process (catalysts are molecules or compounds that, when in the vicinity of N2, facilitate reduction to NH3 while requiring much lower energies). To this end, Banerjee et al from Northwestern University have come across a very promising solution. Taking inspiration from Nature, they noticed that many biological enzymes in nature use molecular clusters made of metal and sulfur as a catalyst to break down nitrogen. Mimicking this, the researchers mixed an iron-molybdenum-sulfur cluster (FeMoS) into a sponge-like material called a chalcogel made of tin (Sn) and sulfur:The FeMoS cluster (top left) is combined with the chalcogel (top right) to form a compound of a network of the metal clusters connected by the chalcogel. This leads to several ideal properties for catalysis and nitrogen fixation. First, the compound is formed at ambient temperature and pressure, removing the need for high-energy processes required in the Fritz-Haber process. Second, the chalcogel is extremely porous, giving it a large surface area with many sites for nitrogen to fly in and chemically interact with the metal clusters. Third, as seen on the right, the compound is black, which means it readily absorbs light, the critical last part of the process…
Under ambient pressure and temperature, the researchers flowed nitrogen into the solution containing the FeMoS-chalcogel network and then shone ultraviolet to visible light onto it. The solar energy provides enough energy to overcome the energy barrier to produce NH3 from N2 in the presence of the metal cluster catalyst (which drastically reduces the barrier). Sure enough, ammonia began to appear in the solution as N2 reacted at metal cluster sites. You can see this below, where ammonia shows up as blue in the solution over time:This is a great step toward a solar-energy-based, environmentally-friendly way to produce ammonia, a central compound to sustain modern agriculture and our population. Next steps will be understanding how the metal cluster operates as a catalyst and if other metals would work even better by providing a smaller energy barrier and increasing reaction rates. But the combination of the chalcogel to provide large surface area for N2 and the metal cluster to lower the energy barrier provides a great framework for further N2 reduction methods!
While this is quite a promising step to reduce emissions from one of the most ubiquitous industrial processes on the planet, there’s still the issue of environmental effects of nitrogen-based fertilizers. Farmers often over-saturate soil with more nitrogen (in the form of ammonia) than the plants can absorb, causing huge amounts of runoff. This leads to environmental calamities such as the Chesapeake Bay and the EPA’s ongoing restoration initiative. Or, if the nitrogen reaches the atmosphere, it can react with oxygen to form N2O, nitrous oxide, which is a potent greenhouse gas. So there are still over-reaching concerns about moving agriculture away from heavy fertilizer use, but this method does seem promising to drastically reduce the energy required and emissions released during the ammonia production process. This will be every more critical as we try to reduce greenhouse gas emissions with an ever-rising global population.
Banerjee, A., Yuhas, B., Margulies, E., Zhang, Y., Shim, Y., Wasielewski, M., & Kanatzidis, M. (2015). Photochemical Nitrogen Conversion to Ammonia in Ambient Conditions with FeMoS-Chalcogels Journal of the American Chemical Society DOI: 10.1021/ja512491v