By Jonathan Cohen
Atmospheric CO2 generated from human sources, called anthropogenic CO2, include many industrial and consumer level sources but one large source is the combustion of methane for energy. One is tempted to believe that if methane was no longer required as a source of fuel for energy then this enormous source of anthropogenic CO2 would be completely eliminated but this would be a simplistic assumption. Methane serves as an important precursor for the generation of hydrogen gas used in the chemical industry. One key example is in the production of ammonia from nitrogen and hydrogen, and subsequently the production of ammonium nitrate from ammonia for use as fertilizer. In fact, the world’s population growth over the last century is inextricably linked to the availability of methane for the production of ammonia. However, research into alternative methods for producing ammonia that does not require fossil fuel precursors is an area of very promising research.
At the start of the 20th century the world was approaching a crisis in food security. Farmlands, especially in the developed world, were becoming depleted in ammonia, necessary for the formation of proteins in plants. Modern techniques of crop rotation, application of nitrogen-fixing organisms to convert nitrogen to ammonia in soil, and the application of organic fertilizers were already well understood by this time but could not keep up with the need for higher crop yields on an increasingly finite amount of arable land. All that changed in the early years of the 20th century when the German chemist Fritz Haber developed a small laboratory reactor to synthesize ammonia directly from nitrogen and hydrogen and he would be awarded the Nobel Prize in Chemistry for this work in 1918. Later, another German chemical engineer named Carl Bosch and others would develop the equipment and methods to ramp up the reaction to industrial scale and he’d get his Nobel in 1931. Today, this is known jointly as the Haber-Bosch process and by the year 2000 the world would collectively make more than 109 million metric tons of ammonia per year, making ammonia one of the most synthesized molecules in the world. It is estimated that as much as 3-5 per cent of the world’s methane is consumed in the production of ammonium nitrate fertilizer.
Nitrogen gas is an extremely stable molecule that is notoriously unreactive. As a result, the Haber-Bosch process will only produce useful amounts of ammonia at temperatures approaching 300-550 degrees Celsius and pressures up to 110 atmospheres on a metal surface acting as a catalyst. Heating and pressurizing the hydrogen and nitrogen does not come free. That energy needs to be generated by a separate power plant. More importantly, while nitrogen can be obtained directly from air, hydrogen gas must be synthesized. Hydrogen used in the Haber-Bosch process today is generated on-site by heating methane with steam, again at very high temperatures and pressures, to create hydrogen gas and generating CO2 as a waste product.
There is hope that researchers can eventually develop a better recipe for making ammonia. Last month, John Anderson, Jonathan Rittle, and Jonas Peters, at the California Institute of Technology, published a paper in the British journal Nature that represents a significant breakthrough in the field of ammonia synthesis from nitrogen. Scientists have known for many years that certain micro-organisms have evolved nitrogen-fixing enzymes, biological catalysts that can convert nitrogen to ammonia at room temperature and pressure using hydrogen ions in solution and electrons from other proteins rather than hydrogen gas. Like the Haber-Bosch process, these enzymes rely on metal atoms at the site of catalysis but can convert nitrogen to ammonia at room temperature and pressure. Anderson, Rittle and Peters synthesized a small, iron-based catalyst capable of generating small amounts of ammonia from nitrogen using hydrogen ions obtained from acids and electrons from other donor molecules under milder temperatures and pressures than Haber-Bosch.
The reaction requires a lot of improvement before it will be ready to ramp up for industrial use. However, the goal of their work was not to develop a plug-and-play replacement for the Haber-Bosch process but rather to provide more insight into the physics and chemistry required to convert N2 to NH3 catalytically and under conditions that more closely resemble biological nitrogen fixation.
This line of research should be encouraged. If an alternative catalytic system can be developed to convert nitrogen to ammonia, one that does not require fossil fuel based reactants or extreme reaction conditions, this will go a long way to eliminating a significant source of anthropogenic CO2 while simultaneously reducing the overall energy demands of the planet. Those 109 million metric tons of ammonia produced in the year 2000 required 46.7 million tons of methane and resulted in the production of 128 million tons of anthropogenic CO2 that year alone. That’s equivalent to approximately 0.5 per cent of the global CO2 production that year. While 0.5 per cent seems like a small number the effects of added CO2 are largely cumulative, and in a world where even a fraction of a percent reduction in greenhouse gas emissions by any country seems a herculean political task, the prospect of a 0.5 per cent reduction is significant.
Many environmentally conscious readers might be led to question the value of this research if nitrogen-fixing microorganisms can do the job so much better. Unfortunately, there is tremendous resistance to the introduction of foreign genes into food crops, and the genetic expression and regulation of nitrogen fixing genes in bacteria remain under study. Even if a crop plant containing nitrogen fixing genes from another organism could be produced, the sociopolitical barriers to introduction would likely take decades if ever to overcome. The introduction of nitrogen-fixing bacteria in soil and the wider use of organic fertilizers should continue but these efforts alone will not be enough to meet the coming food demand of the up to 10 billion people expected to occupy the planet by the end of the 21st century. The resources of the developed world must continue to pursue all methods available to reduce greenhouse gas emissions regardless of the source. This work will require more than the search for alternatives to fossil fuels for energy but also alternative ingredients and recipes critical to ensuring global food security.
(Photo Credit: Robert Barossi)
Jonathan Cohen received his Ph.D. in Chemistry from the Oregon Graduate Institute, Oregon Health and Science University. His work studying nitrogen molecules bound to inorganic metal complexes have been published in the Journal of the American Chemical Society and the Journal of Inorganic Chemistry.