Synthetic fertilizers have been vital for sustaining a booming world population. Without them, we would not have about one-third of the food that is produced today. They are produced using a century-old industrial process that is used to make nearly 500 million tons of fertilizer every year. However, this process is energy-intensive and consumes almost two percent of the world’s energy supply.
We live in a sea of nitrogen. All living things require nitrogen for survival yet our bodies can't access it from the air. The world depends on only two known processes to break nitrogen's ultra-strong bonds to allow conversion to a form that humans, animals and plants can consume. One is a natural, bacterial process on which farmers have relied since the dawn of agriculture. The other is the century-old Haber-Bösch process, which revolutionized fertilizer production and spurred the unprecedented growth of the global food supply.
Researchers have recently announced a light-driven process that could, once again, revolutionize farming. This process could also reduce the world food supply's dependence on fossil fuels and relieve Haber-Bösch's heavy carbon footprint. They have demonstrated that photochemical energy can replace adenosine triphosphate. This is typically used to convert dinitrogen, the form of nitrogen found in the air, to ammonia, the main ingredient of commercially produced fertilizers.
The researchers demonstrated that cadmium sulfide (CdS) nanocrystals can be used to harvest light, allowing the energy from that light to energize electrons with sufficient potential to propel the reduction of N2 into ammonia, which takes place within the nitrogenase molybdenum-iron (MoFe) protein. The new method replaces the ATP hydrolysis-dependent enzymatic process with CdS nanorod light-harvesting and energy conversion.
Reducing N2 to ammonia is usually a very energy-intensive process. In the long-standing Haber-Bosch process used by industry, N2 reduction is accomplished by using high temperatures and high pressure. In biology, the reduction is catalyzed by nitrogenase in a reaction that requires ATP to act as the energy source. This constrains how fast the reaction can take place. Both the biological and industrial processes require a high energy input, and both result in the emission of high levels of carbon dioxide. The new process requires far less energy and emits no carbon dioxide.
The new research is expected to inspire alternative concepts for meeting the demand for ammonia, but in a more energy-efficient and sustainable manner, with a lower impact on the environment than current commercial processes. Because this study is the first to demonstrate the new process, the findings are important to help future studies evaluate the technological impact on a practical system.