The Nitrogen Cycle
If you have ever been to a nitrogen ice cream shop or seen a demonstration of someone freezing something with liquid nitrogen, you might have thought “cool!” but then never really gave it a second thought. While liquid nitrogen undeniably makes some of the smoothest ice cream on the planet, nitrogen is far more important than you might realize. Nitrogen is so ubiquitous, in fact, that it’s in everything, from the core of your DNA to the majority of the air you breathe.
The diversity of uses for nitrogen-containing small molecules, like ammonia (NH3) in fertilizers and detergents, underscores their importance. Manufacturing nitrogen compounds is so important that the adoption of industrial scale ammonia production can be directly tied to a revolution in agriculture, termed the “green revolution,” that has helped feed millions of people since the mid-twentieth century.Nitrogen is one of the simplest molecules, but it is relatively unreactive due to the strong triple bond that nitrogen atoms form with other nitrogen atoms. Because of nitrogen’s (N2) strong triple bond, converting nitrogen from its gaseous state to more energetically rich molecules, such as ammonia (NH3), is very difficult. Nevertheless, nitrogen provides a powerful potential method for storing energy.
Currently, we use the Haber-Bosch process to fix nitrogen by reacting gaseous nitrogen with gaseous hydrogen at extremely high temperatures and pressures. Not only is this process incredibly energy intensive, consuming almost 1% of the global energy supply, but hydrogen is often sourced from natural gas, contributing greatly to greenhouse gas emissions. If nitrogen is so essential to modern society, and we currently have a method to transform gaseous nitrogen into other nitrogen-containing compounds, then you might be wondering why we need to reinvent the wheel.
Given the importance of nitrogen-containing compounds and the energy benefits of harnessing the power and diversity of nitrogen under milder conditions, researchers at the Center for Molecular Electrocatalysis (CME) are developing catalysts capable of transforming ammonia into gaseous nitrogen to release stored energy, as well as converting nitrogen to ammonia under mild conditions to store energy.
So, what is a catalyst and why would this be considered such an advancement in the field? Simply put, a catalyst is a molecule or material that can lower the activation barrier to a difficult chemical transformation and is not consumed by the reaction. A catalyst that draws its energy and electrons directly from electricity is known as an electrocatalyst and would prove invaluable in creating a more sustainable nitrogen cycle.
Driving nitrogen chemistry with electrocatalysts means that we can store energy from renewable sources in nitrogen-derived fuel cells and/or drastically cut carbon emissions inherent in the Haber-Bosch cycle by reducing the need for molecular hydrogen while continuing to meet society’s demand for nitrogen-containing compounds.
Electrocatalysts draw considerable attention from researchers because the electricity that drives these systems can be derived from several different and more renewable sources, such as solar, nuclear, and wind. EFRC researchers at the Alliance for Molecular Photoelectrode Design (AMPED) have discovered new materials and systems capable of using light to drive electrocatalytic processes.
Light’s Exciting Role in the Nitrogen Cycle
Think back to high school chemistry… Other than that one memorable experience where your lab partner spilled hot water all over you, how much do you remember? In order to understand how light can be used to drive catalysis, we need to consider some of the basic properties of light and how it interacts with matter.
Both light and tiny particles, like electrons, can act as waves, meaning they can interact with one another to excite changes in the electron’s energy state. Atoms have energy levels that are quantized, meaning they must absorb a certain amount of energy before any change will take place. If an atom absorbs a photon of equal or greater energy to this quantized barrier, then an electron will jump from a lower energy state to a higher energy state. In these higher energy “excited states,” other molecules and/or chemical processes can intercept these high-energy electrons to create electricity. The energy from these so called dye-sensitized solar cells (DSSCs) could also be used to drive the catalytic reactions of interest in nitrogen chemistry once suitable electrocatalysts are developed.
Researchers at AMPED have been designing DSSCs and other technologies capable of transforming light into useable electricity. A dye-sensitized solar cell takes advantage of a molecular dye molecule that is capable of absorbing a wide array of photons with different energies. Once a photon hits these dye molecules, one of their electrons jumps to a higher energy excited state. In a DSSC, these dye molecules are often attached to surfaces that can either accept the high energy electron or donate one of its own electrons, creating an electron vacancy (hole) on the surface. In either case, electricity is created. The electricity created can then be used as power and might, one day, reduce nitrogen or oxidize ammonia by being coupled to an electrocatalytic system.
Towards a Better Understanding of the Nitrogen Cycle
Because higher temperatures and pressures are usually required to break nitrogen’s triple bond, researchers at CME have employed a more creative strategy that may enable us to perform nitrogen reduction and its reverse process, ammonia oxidation, under relatively mild conditions. These researchers have turned to metal-based catalysis to tackle this complicated problem head on!
Catalysts, especially those that employ transition metals, allow access to high-energy chemical transformations. Researchers at CME have shown that with the correct scaffold around various metal centers, like iron and chromium, they can create favorable conditions for molecular nitrogen to bind directly to the metal center. Upon binding to the metal center, the nitrogen’s triple bond weakens because one of the nitrogen atoms must share some of its electrons with the metal center. As a result, the bound nitrogen molecule becomes more susceptible to chemical transformations. CME researchers have found that adding a source of protons (H+) to their highly reduced N2 bound chromium catalyst results in the production of ammonium (NH4+). While these compounds are still largely in the research and development phase, these advancements in nitrogen reduction chemistry might allow us to store energy within the chemical bonds of ammonium (NH4+) and provide us with a possible alternative to the Haber-Bosch process. Finding electrocatalysts similar to these catalytic systems is a major goal of CME research.
If we spend so much of the world’s energy supply on making ammonia from nitrogen, you might be wondering why there is so much interest in returning ammonia (NH3) back into gaseous nitrogen. When we fix nitrogen into ammonia, we are storing the energy used to form those N-H bonds in the molecule itself. We can then generate fuel in the form of H2 by returning ammonia back into gaseous nitrogen. Since it liquifies easily, ammonia is more compatible with current methods of fuel transport than hydrogen, which improves the economic viability of using it as a fuel source. The released energy might then be used to power a number of devices, including cars or industrial processes, or it could be used to produce electricity for homes among other things. In essence, if catalyst systems can work in tandem to both store and release energy from the nitrogen cycle, then we can create more efficient fuel cell technologies.
For this reason, the oxidation of ammonia back to N2 has garnered significant attention in recent years. Recently, researchers at CME have oxidized ammonia by removing three successive hydrogen atoms. Researchers studied a ruthenium catalyst with a bound ammonia molecule and found that by introducing free ammonia into the reaction along with some additional additives, they were able to liberate nitrogen gas. While these systems do not currently operate as electrocatalysts and the efficiency of these systems is limited, our increased understanding of ammonia oxidation may lead to next-generation catalyst systems that could potentially function as electrocatalysts and be employed in nitrogen-containing fuel cells.
The nitrogen cycle remains an important problem in basic energy research with the potential to drastically alter our current energy landscape. The efforts of multiple EFRCs to find ways to both generate and store more sustainable energy have already led to discoveries in solar energy and nitrogen chemistry that might, one day, provide us with a more elegant method to produce and store energy.
Bhattacharya P, ZM Heiden, GM Chambers, SI Johnson, RM Bullock, and MT Mock. 2019. “Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction.” Angewandte 58(34):11618–11624. DOI:10.1002/anie.201903221.
Johnson SI, SP Heins, CM Klug, ES Wiedner, RM Bullock, and S Raugei. 2019. “Design and Reactivity of Pentapyridyl Metal Complexes for Ammonia Oxidation.” Chemical Communications.35:5083–5086. DOI: 10.1039/c9cc01249d.
Egbert JD, M O’Hagan, ES Wiedner, RM Bullock, NA Piro, WS Kassel, and MT Mock. 2016. “Putting Chromium on the Map for N2 Reduction: Production of Hydrazine and Ammonia. A Study of Cis-M(N2)2 (M = Cr, Mo, W) Bis(Diphosphine) Complexes." Chemical Communications. 52:9343-9346. https://doi.org/10.1039/c6cc03449g.
Flynn, CJ; EE Oh, SM McCullough, RW Call, CL Donley, R Lopez, JF Cahoon. 2014. "Hierarchically-Structured NiO Nanoplatelets as Mesoscale p-Type Photocathodes for Dye-Sensitized Solar Cells." J. Phys. Chem. C 2014, 118 (26), 14177-14184. DOI: 10.1021/jp5027916.
Bhattacharya P, et al. This work was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, and by start-up funding from Montana State University. Mass spectrometry experiments and computational resources were performed at EMSL, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at PNNL. Additional computational resources were provided by the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory and at Washington State University. Pacific Northwest National Laboratory is operated by Battelle for DOE.
Johnson SI, et al. This work was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences. PNNL is operated by Battelle for DOE. Calculations were performed using the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy, Office of Science User Facility (Contract No. DE-AC02-05CH11231) and the Cascade supercomputer at EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research.
Mock MT, et al. This work was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (U. S. DOE), Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U. S. DOE.
Flynn, CJ, et. al. This work was wholly funded by the UNC Energy Frontier Research Center (EFRC) “Center for Solar Fuels”, an EFRC funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001011. E.E.O. acknowledges the J. Thurman Freeze Summer Research Scholarship. We thank A. Kumbhar of CHANL for assistance with TEM imaging.