How (Else) to Feed the World
Written by Yilin Xie '26
Edited by Anusha Srinivasan '24
Nitrogen: 78% of our atmosphere, 3.3% of our bodies by mass, absolutely essential for life . Yet, despite its abundance in the environment, nitrogen in consumable form for organic life is not easy to come by. One such usable form of nitrogen is ammonia (NH3), widely used as plant fertilisers, naturally found in organic decays and in certain mineral deposits . As the global population grew, though, the demand for food exceeded the fertiliser output from natural sources of ammonia. How, then, can we get ammonia to feed the world?
To chemists in the early 20th century, this was the golden problem. The first one to find a reasonably sufficient process was Fritz Haber, while Carl Bosch, working for the chemical company BASF, engineered the process for industrial-scale ammonia production. The process would be named for the two: Haber–Bosch process. To this day, we use this process extensively to produce ammonia. It is estimated that about 50% of nitrogen in the human in a developed country came from the Haber-Bosch process ; safe to say, we cannot sustain the global population without ammonia synthesis.
However, the Haber-Bosch process must be conducted at high temperature and pressure, which all come with an energy cost. In addition, it uses methane gas as the source of its hydrogen, with carbon dioxide as a byproduct. Single-handedly, the Haber-Bosch process consumes up to 5% of the global annual natural gas production and about 2% of the global annual energy production, while contributing about 1.8% of the global carbon emission . Since ammonia production has such a high energy and carbon footprint, any adjustments to make the process more sustainable would reflect as a significant global change.
A proposed alternative to the Haber-Bosch process is nitrogen reduction reaction (NRR), powered by electricity from renewable sources, operating under ambient conditions, and bypassing the necessity of intense carbon emission by using water as the hydrogen source instead of methane . However, in terms of ammonia production rate and efficiency, NRR is far from competitive, nevermind applicable on an industrial scale.
One promising step for optimising NRR is to develop better catalysts. According to Liu et al., “the development of heterogeneous electrocatalysts with substantially enhanced activity, high selectivity and good durability is undoubtedly the key to promote the development of this technology. ” This is also the angle that Zhang et al. chose to investigate in their study published in Nature: the researchers sought to controllably engineer single atom catalysts with a high density of active sites .
Why are we interested in single atom catalysts? Turns out, when particle size is in nanorange, new properties and reactivities emerge. Not only do catalysts exhibit highly desirable characteristics in small size, there are also more catalytic sites. Imagine a ball of catalytic material. Only the surface is exposed to the environment, and thus the only usable catalytic sites are the ones on the surface. If we take the same ball but divide it into smaller particles—smaller balls—though, it’s easy to see that the sum of the surface area becomes larger, meaning that the same amount of catalyst induces a higher level of catalytic activity. In combination, nano-size catalysts, like single atom catalysts, have both higher numbers of catalytic sites and higher reactivities at each site .
When particles are on the nanoscale, though, they have high surface energies and want to reform clumps to lower their surface energy. If this happens, the aforementioned benefits of small particle size are lost. To prevent catalyst particles from clumping together again, they are dispersed on support material, which holds the individual particles in place .
One highlight in Zhang et al.’s approach is their utilisation of bacterial cellulose (BC) as the basis of their atomic support structure. To insert single atoms into its support, BC is placed in solution with Fe3+ and Co2+ and adsorbs the ions into its structure. Then, [process of carbonisation], BC becomes graphitic carbon while the ions are reduced, becoming embedded catalytic atoms.
To control the ratio of Fe3+ and Co2+ ions, researchers experimented with soaking solutions where the total concentration of Fe3+ and Co2+ is always 20 mM, but the ratio between the ions are 15/5, 5/15, et cetera. They found that the ion ratios in the solutions are directly proportional to the ion ratios in the post-adsorption BC and to the atom ratios in the post-thermal treatment BC: the ratio of catalysts can be directly controlled by altering the soaking solution.
The performance of this atomically dispersed electrocatalyst when applied to NRR is remarkable: the ammonia yield rate is 579.2 ± 27.8 μg h−1 mgcat.−1, while the Faradaic efficiency is 79.0 ± 3.8%. The researchers claimed to have achieved the highest known ammonia yield rate and Faradaic efficiency in NRR using single atom catalysts. Additionally, the engineered catalyst structure is confirmed to be stable when the ammonia yield rate and Faradaic efficiency over 72 hours were 93.2% and 94.7% of the values obtained over 2 hours. Finally, Zhang et al. determined that compared to unimetallic Fe SAs and Co SAs, bimetallic Fe-Co SAs performed better, with near-1:1 Fe:Co ratio performing the best. This suggests that bimetallic Fe-Co sites are exceptional in catalysing NRRs, which can be a point of further investigation.
As a method to tailor SACs, Zhang et al.’s research can substantially improve the efficiency of NRR, making this greener ammonia production technique more industrially feasible. Not to mention, this approach to producing catalysts can be used to design catalysts for other reactions, from the types of catalysts to their ratios. Simply, this could be our way forward to more cleanly and efficiently feed the world, with vast untapped potential beyond.
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