Determining Aqueous and Gaseous Product Composition of High-Preforming Titanium for Nitrate Reduction to Ammonia
Authors: Willoughby Wrayzene , Jaramillo Thomas F. 
Published: December 2024

Reactive nitrogen species are a key commodity and serve as fertilizers that sustain the global agricultural system, however, their use poses two significant challenges. First, most reactive nitrogen compounds are synthesized from ammonia (NH3) generated via the Haber-Bosch process. The Haber-Bosch process is energy-intensive, as it accounts for 2% of the world’s annual energy use (19.3 kWh/kg-N).1 Secondly, fields are often times overfertilized, leading to the release of reactive forms of nitrogen, such as nitrate (NO3
-), to the environment. This reactive nitrogen pollution can lead to harmful algal blooms, hypoxic dead zones in open water and other harmful effects to the environment.2

There is a need for new methods and technologies to separate reactive forms of nitrogen in wastewater and reuse these nutrients as an inexpensive fertilizer supply. To meet this need, our research aims to understand, design, and control a multifunctional electrochemical unit process that enables NH3 manufacturing and water purification in tandem. To this end, we must develop electrocatalysts that have high selectivity towards NH3 and understand the associated reaction mechanisms. Previous research from our group has demonstrated promise to transform NO3
-R to NH3 using titanium (Ti) as the electrocatalyst.3 Ti showed an optimal NH3 Faradaic efficiency of 82% and partial current density of -22 mA/cm2 while using an electrolyte containing 0.4 M NO3
- with a pH of 0.77 and applying a potential of -1 V vs the reversible hydrogen electrode (RHE). Despite this promise with Ti, given the high overpotential of -1 V vs RHE and only NH3 production quantified, this leaves an analytical gap of what other products are being produced and how can we quantify them.
Recently, our group has developed an analytical method using ion and gas chromatography (IC and GC), nuclear magnetic resonance (NMR), gas chromatography-mass spectroscopy (GC-MS) and 15N isotope labeling to track all possible NO3
-R products to close the nitrogen mass balance. Thus far we have identified and quantified the aqueous and gaseous products of NO3
-, NO2
-, NOx, NH3, NH4
+, N2, N2O and H2. Furthermore, to understand the NO3
-R mechanism on Ti, we have implemented operando electrochemical mass spectroscopy (EC-MS) to identify and quantify gaseous products at various given potentials. This technique, paired with ex-situ characterization elucidates properties of NO3
-R to NH3 and product production that can inform the experimentation and molecular design of NO3
-R systems.
References

Erisman, J. W.; Sutton, M. A.; Klimont, Z.; Galloway, J.; Winiwarter, W. How a century of ammonia synthesis changed the world; Nature Publishing Group, 2008.

Guest, J. S.; Skerlos, S. J.; Barnard, J. L.; Beck, M. B.; Daigger, G. T.; Hilger, H.; Jackson, S. J.; Karvazy, K.; Kelly, L.; Macpherson, L.; et al. A new planning and design paradigm to achieve sustainable resource recovery from wastewater. Environ Sci Technol 2009, 43 (16), 6126-6130. DOI: 10.1021/es9010515 From NLM Medline.

McEnaney, J. M.; Blair, S. J.; Nielander, A. C.; Schwalbe, J. A.; Koshy, D. M.; Cargnello, M.; Jaramillo, T. F. Electrolyte Engineering for Efficient Electrochemical Nitrate Reduction to Ammonia on a Titanium Electrode. Acs Sustain Chem Eng 2020, 8 (7), 2672-2681. DOI: 10.1021/acssuschemeng.9b05983. Blair, S.J., Doucet, M. et al.
ACS Energy Lett., 7, 6, 1939-1946 (2022).