The US Department of Energy (DOE) is currently supporting six fundamental research projects that aim to develop “novel catalysts and mechanisms for nitrogen activation,” which the DOE hopes will lead to future sustainable ammonia synthesis technologies.
These projects, announced in August 2016 and administered by the Office of Basic Energy Sciences, aim “to investigate some of the outstanding scientific questions in the synthesis of ammonia (NH3) from nitrogen (N2) using processes that do not generate greenhouse gases.”
There are two substantial differences between these awards and the other recent DOE funding for sustainable ammonia technologies, announced in December 2016 and administered through ARPA-E’s “REFUEL” program.
First, the Office of Basic Energy Sciences is only looking at the existing market for ammonia, with the aim of reducing the energy intensity and environmental cost of fertilizer production. “Although [the Haber-Bosch] process has been optimized over the years, it alone consumes 1-2% of the world’s energy supply and about 3% of the natural gas production. It is also responsible for about 3% of all CO2 emissions.”
ARPA-E, on the other hand, is looking explicitly at ammonia as a fuel and a medium for power and/or hydrogen storage and delivery. We’ve previously written about ARPA-E’s vision for carbon neutral liquid fuels, as well as introducing the projects funded under the REFUEL program in both the ammonia fuel-use category and the renewable ammonia synthesis category, where the deliverable is a prototype ammonia synthesis reactor.
Second, these DOE awards do not aim to demonstrate any finished technology or to deliver bench-scale demonstrations. They aim instead to fill gaps in the scientific knowledge, by developing the basic research necessary “to discover novel catalysts that are active, selective, scalable, and stable while carrying out energy-efficient, carbon-neutral and low-pressure ammonia synthesis.” This is vital research because “no heterogeneous, homogeneous or enzyme catalyst currently exists that fulfills all of these requirements. No thermal redox processes in solution, solid or melt, nor electrochemical or photochemical surface processes, have shown practical viability to date.”
This was the “overarching grand challenge,” identified during the DOE’s Roundtable on Sustainable Ammonia Synthesis in February 2016, which clearly informed the design of this funding opportunity.
Of interest is molecular level research that will provide the scientific basis for novel catalysts and mechanisms for nitrogen activation. Ideally, this research should produce fundamental knowledge that will lead to future catalytic processes for ammonia synthesis that are energy efficient, use renewable sources of energy, and do not produce greenhouse gases.
US DOE: Sustainable Ammonia Synthesis (DE-FOA-0001569), 04/12/2016
The projects are actually supported under two separate DOE funding opportunities, although they share identical text except for an additional expectation, in the second FOA, that successful projects should build “scientific research collaborations” with the National Laboratories, through the DOE’s Experimental Program to Stimulate Competitive Research (EPSCoR). This second FOA is Building EPSCoR-State/National Laboratory Partnerships: Sustainable Ammonia Synthesis (DE-FOA-0001572).
The funded projects, which I introduce below, represent diverse fields of academic enquiry, starting with biochemistry and encompassing a range of homogeneous, heterogeneous, plasma, and electro- catalysis. This diversity reflects the challenges, enumerated in the FOA, regarding new and sustainable ammonia synthesis technologies.
“Although various alternatives to the classical Haber-Bosch process have been proposed and studied to date, all of them have significant limitations in regards to their potential to enable long-term, sustainable and energy-efficient ammonia production.” The DOE’s FOA identifies the set of alternative technologies and their challenges as follows:
- Biological systems: “Nitrogenase functions at room temperature and ambient pressure albeit at low turnover frequencies (about 2 NH3 s-1). Understanding how the nitrogenase and its ancillary components gate and control the flow of electrons to the catalytic site could open up intriguing possibilities for the delivery of electrons from alternate sources.”
- Homogeneous catalysis: Fe complexes and Mo complexes (“partly inspired by the metalloclusters of nitrogenases”) have “proven feasibility” for nitrogen fixation under ambient conditions. However, “these complexes, like nitrogenases, produce molecular hydrogen as a byproduct, which accelerates catalyst deactivation. Therefore, stability becomes a problem for sustained and large-scale industrial use.”
- Heterogeneous catalysis: the FOA notes that some catalysts, “for instance Ru and Co-Mo alloys,” are more active than the traditional iron-based catalysts but that their high cost makes them uneconomic. “It may be possible to design catalysts that follow substantially lower energy pathways with consequently much higher activity, compensating for their cost, particularly if the hydrogen is derived from renewable sources and the process conditions are significantly milder than for Haber-Bosch.”
- Electrochemical synthesis: “Direct electrochemical reduction of nitrogen in water at low temperature has also been reported, albeit with low current efficiency. Much higher current efficiency has been obtained using proton conductive membranes (both ceramic at high temperature and polymeric at low temperature), but they possess poor durability. Reproducibility and reliability of results have also been problematic.”
- Photocatalysis: although ammonia synthesis “has been achieved with oxide-doped and noble metal-doped TiO2, some coated with photoactive polymer layers, under illumination with UV light,” unfortunately “the current density attained is typically orders of magnitude below that of electrochemical methods, and the reproducibility and reliability have been problematic, with the mechanisms remaining undefined.”
- Thermochemical synthesis: This involves the “reduction of N2 at very high temperature (2000°C) driven by solar heating … using a metal (e.g. Mn) or alloy intermediate in a redox cycle. The metal is subject to nitridation in one step, oxidation with water in a second step and reduction with carbon compounds in a third step.” Unfortunately, “some of the drawbacks to this approach include materials degradation and energy handling at very high temperature, as well the use of carbon-containing reducing agents.”
The following six projects are currently supported by the DOE under these FOAs:
Engineering a Functional Equivalent of Nitrogenase for Mechanistic Investigations of Ammonia Synthesis
This project is led by Yilin Hu from the Department of Molecular Biology & Biochemistry at the University of California, Irvine, with collaboration from Stanford University. It addresses the challenge that, “despite major efforts in the past decades, the catalytic mechanism of nitrogenase has not been fully deciphered.” It aims to use genetic methods to construct and study “a functional MoFe protein equivalent,” using “a synthetic biology approach.”
NifEN, a scaffold protein that hosts the biosynthesis of nitrogenase cofactor, is a functional homolog of MoFe protein (the catalytic component of nitrogenase). Compared to MoFe protein, NifEN has a lower enzymatic activity and a narrower substrate profile, making it a perfect mutational platform for (re)constructing a functional MoFe protein …
The outcome of the proposed activities could prove instrumental in decoding the nitrogenase mechanism and enable development of energy-efficient strategies for sustainable ammonia synthesis.
DOE funded project abstract: Engineering a Functional Equivalent of Nitrogenase for Mechanistic Investigations of Ammonia Synthesis, 08/29/2016
Novel Homogeneous Electrocatalysts for the Nitrogen Reduction Reaction
Led by John F. Berry, from the Department of Chemistry at University of Wisconsin-Madison, this project is “inspired by the nitrogenase class of enzymes,” and aims to develop homogenous catalysts that would enable a technology “to produce ammonia from cheap acids and electricity.”
The goal of this research project is to explore the feasibility of the elementary steps necessary for the N2RR [nitrogen reduction reaction] using a set of novel compounds based on transition metals that are stable in the presence of protons, electrons, and ammonia, and can undergo several successive redox transformations. Metal-metal bonds imbue these catalysts with strong Lewis acidic character but kinetic lability, ideal for fast and reversible substrate binding necessary for fast catalytic turnover. Aside from the chemical challenges, electrocatalytic N2RR will be a major engineering challenge; screening for successful electrochemical conditions and design of electrochemical cells will be performed to generate useful knowledge of successful reaction conditions for any potential class of catalysts for the N2RR.
DOE funded project abstract: Novel Homogeneous Electrocatalysts for the Nitrogen Reduction Reaction, 09/08/2016
Step Catalysis to Synthesize Fossil-Free Ammonia at Atmospheric Pressure
This is the only one of the projects that is led by a participant in the DOE’s original Roundtable on Sustainable Ammonia Synthesis: Peter Pfromm, from the Chemical Engineering department at Kansas State University. It aims to develop catalysts “with a high nitrogen storage capacity that still readily release nitrogen to form ammonia,” which could be used to produce ammonia in “a relatively simple, atmospheric pressure process.”
The specific aims are to synthesize and characterize micro to nanoscale particles of Mn and related transition-metal doped alloys that strike a balance between dinitrogen activation/nitrogen storage and ammonia formation. The experimental data will be used to arrive at atom-level models to rationalize the data and direct future work.
DOE funded project abstract: Step Catalysis to Synthesize Fossil-Free Ammonia at Atmospheric Pressure, 08/10/2016
Advancing Sustainable Ammonia Synthesis through Plasma-Assisted Catalysis
This project, led by Jason Hicks at the University of Notre Dame, investigates the use of plasmas to facilitate ammonia synthesis in ambient conditions.
Plasmas (or gas discharges) can input electrical energy into the reactant gas mixture (N2/H2) to create reactive intermediates to enhance the yield of NH3 and preclude the need for high pressures or temperatures, leading to overall better energy efficiency. Plasmas can be efficiently generated, influence reaction chemistry, and offer a number of controllable design parameters, especially in the presence of a catalyst. Despite the potential merits, plasma catalysis has not received close attention from the catalysis science community and has not benefited from the concerted coupling of synthesis, measurement, and theory-driven modeling that has been so successful in the design of thermal catalytic systems.
DOE funded project abstract: Advancing Sustainable Ammonia Synthesis through Plasma-Assisted Catalysis, 08/22/2016
Peptide Control of Electrocatalyst Surface Environment and Catalyst Structure: A Design Platform to Enable Mechanistic Understanding and Synthesis of Active and Selective N2 Reduction Catalysts
This project is led by Lauren Greenlee, from the Department of Chemical Engineering at the University of Arkansas, with collaboration from Case Western Reserve University and Pennsylvania State University. This team is looking at electrochemical ammonia synthesis and aims to address the challenge that “current electrocatalysts are plagued by extremely low selectivity, and thus efficiency.”
The premise of this project is that control of the local surface environment of these electrocatalysts may enable enhanced selectivity for N2 reduction over H2 evolution. In this project, small-chain peptides comprised of specific amino acid sequences will be explored as a design platform for both controlling local catalyst electrochemical environment and understanding the key steps of the N2 electroreduction reaction mechanism. In particular, amino acids will be chosen to control hydrophobicity and proton shuttling at the catalyst surface. Fundamental measurements of N2 adsorption, water vapor adsorption, electrochemical performance, and peptide stability will be coordinated with theoretical model development with the goal of establishing initial predictors for ligand/catalyst environments that enable selective and active N2 reduction electrocatalysts.
DOE funded project abstract: Peptide Control of Electrocatalyst Surface Environment and Catalyst Structure, 08/25/2016
Low temperature, ambient pressure electrochemical ammonia synthesis in alkaline media Mechanistic studies and catalyst design
This project, led by Bingjun Xu at the University of Delaware, also aims to develop new catalysts that would enable efficient electrochemical synthesis, “at or close to ambient conditions,” with a focus on enabling modular and distributed ammonia production.
The central hypothesis of the proposed research is that ENRR [electrochemical nitrogen reduction reaction] in alkaline media with hydroxide exchange membranes and metal/nitride bifunctional electrocatalysts could simultaneously increase the ENRR activity and suppress the competing hydrogen evolution reaction …
The proposed concept of ENRR in alkaline media represents a paradigm shift from the existing PEM-based devices in that it not only eliminates the possibility of the produced ammonia reacting with the acidic PEM, but also retards the competing hydrogen evolution reaction. The systematic mapping of ENRR activities on monometallic catalysts will provide experimental verification for computational predictions. Moreover, quantification of ammonia production rates and selectivities at well-defined potentials, along with identification of adsorbed reaction intermediates with in-situ surface enhanced spectroscopies, will provide mechanistic information that could enable rational catalyst design.
DOE funded project abstract: Low temperature, ambient pressure electrochemical ammonia synthesis in alkaline media, 08/22/2016