The 100-year old Haber-Bosch process for the production of ammonia is an energy guzzler and also a big contributor towards global warming. More sustainable alternatives using biocatalysts, photocatalysts and electrocatalysts are in early stages of development. They hold out the promise of producing ammonia from water and air using only renewable energy like solar and wind.
The Haber-Bosch process for synthesis of ammonia is widely regarded as the most significant, if not the greatest, invention of the 20th Century. It ensured food security for the masses and is even believed to be responsible for the explosive population growth in the last century. More than half the nitrogen in an average human is estimated to have been produced by this process. But viewed through the lens of climate emergency, the Haber-Bosch process leaves much to be desired. One percent of the all the energy consumed in the world goes into the manufacture of ammonia. Ammonia production accounts for 1.4 percent of the global emission of carbon dioxide. For every ton of ammonia produced, 1.9 tons of carbon dioxide is released into the atmosphere.The carbon footprint of the Haber-Bosch process is elephantine and a more sustainable alternative is the need of the hour.
Reversible Chemistry
The chemistry of ammonia synthesis is a de rigueur example to illustrate the Le Chatelier’s Principle in undergraduate classrooms. The equilibrium of the reversible chemical reaction is favoured by high pressure and low temperature. A catalyst is required to achieve commercially feasible kinetics. Even with the best catalyst, temperatures in excess of 400 degrees celsius and pressures north of 80 bar are required for a decent degree of equilibrium conversion. The catalyst used in the process is iron promoted with alumina and potash and hasn’t changed much from the one first used by Karl Bosch at BASF in 1909. Considerable amount of time and money has been spent on studying and understanding the catalyst. There have been many improvements, but only marginal. Ruthenium has been proposed as a more efficient, though expensive, catalyst. The optimisation of the ammonia process has been largely related to energy integration and no new catalysts have emerged in the last hundred years to challenge the dominance of the iron-based catalysts.
Biocatalysis
Before the advent of the Haber-Bosch process, plants relied entirely on bacteria for their daily nitrogen fix. Nitrogenase, the bacterial enzyme responsible for reducing nitrogen to ammonia, was isolated and purified half a century ago and has been one of the most intensely studied alternatives for ammonia synthesis. Three types of nitrogenase have been isolated from the bacteria -iron (Fe), molybdenum-iron (MoFe) and vanadium-iron (VFe). MoFenitrogenase is the most widely investigated and understood among the three. The nitrogenase is made up of two oxygen sensitive protein units which function in tandem, one serving as a catalyst and the other serving as a reducing agent. Nitrogen binds to the catalytic MoFeprotein and is reduced to ammonia. The other protein hydrolyses energy carrying Adenosine Triphosphate (ATP) molecules to Adenosine Diphosphate (ADP) and transfers electrons to the catalytic protein for the reduction reaction. Despite the great advances in elucidating the molecular structure of the MoFenitrogenase, the mechanism of the nitrogenase-mediated cleavage of the nitrogen – nitrogen bond continues to be a mystery. Immobilising nitrogenase on electrodes will open up avenues for utilising renewable energy to carry out a biological process.
Photocatalysis
Another alternative approach is the photocatalytic route in which sunlight is harvested to synthesise ammonia from nitrogen and water. Photosynthesis of ammonia draws inspiration from the bacterial process occurring in nature. Sunlight hitting a semiconductor excites the electrons into the conduction band where the nitrogen is reduced to ammonia with the holes in the valence band oxidising water and liberating the protons required for synthesis. The pioneers of the photocatalytic synthesis of ammonia are Schrauzer and Guth, who in 1977 demonstrated the ability of titanium dioxide to reduce nitrogen to ammonia in the presence of water and sunlight. This spurred a series of studies using various metal oxides like iron oxide, zinc oxide, tungsten oxide, gallium oxide, etc. Layers of conductive polymers have been added to the light absorbing surfaces to raise the energy levels of the conduction band. Biomimetic photocatalysts have also been developed. Two interlinked reactions take place on the surface of the photocatalyst – water splitting and nitrogen reduction. While the Haber-Bosch process relies on the dissociative pathway requiring extremely high energy to cleave the N-N triple bonds, the photocatalytic reduction uses the associative pathway in which nitrogen molecules on the catalyst are hydrogenated.
Electrocatalysts
Electrocatalytic reduction of nitrogen to ammonia is attracting a great deal of curiosity because of the mild operating conditions and zero carbon dioxide emission. The first electrochemical synthesis of ammonia was reported in 1985. Since then, a variety of catalyst systems have been investigated, but the yield of ammonia continues to be very poor and nowhere near commercialisation.The yields are presently so low that the researchers find it difficult to establish beyond a shadow of doubt that the ammonia has actually come from nitrogen reduction and not emerged through contamination. Some of the more promising electrocatalysts are based on iron, molybdenum, rhenium and ruthenium. Electrolytes ranging from polymers at room temperatures to ceramics at high temperatures have been studied. The challenge is to develop high-performance catalysts, which can give a higher faradaic efficiency by supressing the competing water splitting reaction resulting in hydrogen. Nanoclusters with various customised morphologies are being actively examined. If and when the electrochemical technology becomes successful, ammonia production can be coupled to a renewable source of power like solar or wind.
Afterword
Nitrogen molecule is the most stable diatomic molecule with the triple bond between the two nitrogen atoms being one of the strongest chemical bonds having a disassociation enthalpy of 226 kcal/mol. The catalysts for reducing nitrogen to ammonia must be able to activate the extremely docile nitrogen molecules and promote the thermodynamically uphill reaction to occur at reasonable rates. The key to a more sustainable alternative for ammonia synthesis lies in developing a high-performance catalyst that is active, selective and scalable. The alternative routes for ammonia synthesis offered by biocatalysis, photocatalysis and electrocatalysis are presently in a stage of infancy. A great deal of fundamental work in understanding the reaction mechanisms needs to be done before these technologies mature and can be launched in the market. An additional important advantage of these alternative technologies is that ammonia production can be distributed and located closer to consumers.
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