Fast Proton NMR Detection of Aqueous Ammonia with Relaxation Agent and Nitrogen Decoupling

Abstract

The Haber–Bosch process, which synthesizes ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂), consumes approximately 2% of the global energy supply. A sustainable alternative is the direct electrochemical conversion of N₂ to NH₃. The selectivity and activity of the electrocatalysts for this process are assessed by quantifying the NH₃ present in the electrolyte. Compared with other analytical methods, ¹H NMR offers a straightforward approach for detecting NH₃ (by analyzing NH₄⁺). ¹H NMR method can also definitely confirm that the detected ammonia originates from the electroreduction of N₂ by comparing results obtained from isotopically labeled ¹⁵N₂ and regular ¹⁴N₂ gases. This capability is unique to the ¹H NMR method, as no alternative approaches offer this level of specificity. However, this method suffers from low sensitivity when measuring NH₄⁺ of low concentration of such as at μM or lower. To address this issue, we developed a novel approach that improves sensitivity by ∼3-fold through the introduction of ¹⁴N decoupling during the ¹H NMR data acquisition. Recently [Kolen, M. ACS Omega 2021, 6, 5698–5704], demonstrated a ∼3.5-fold increase in sensitivity by using a 1 mM concentration of the paramagnetic relaxation agent Gd³⁺. By combining our ¹⁴N decoupling technique with the relaxation agent Gd³⁺, we achieved a synergistic enhancement in sensitivity, resulting in an overall ∼10.9-fold sensitivity increase for the ¹H NMR detection of ¹⁴NH₄⁺. This translates to a reduction in NMR detection time by a factor of ∼119 (10.9²). This significant advancement enables the fast detection of ammonia at μM concentration or lower. ¹H NMR of ¹⁵NH₄⁺ with ¹⁵N decoupling was also demonstrated.