Refining Universal Procedures for Ammonium Quantification via Rapid ¹H NMR Analysis for Dinitrogen Reduction Studies

Abstract

As research on sustainable ammonia synthesis via electrochemical and photochemical N 2 reduction progresses to include a wider variety of aqueous and aprotic electrolytes, 1 H NMR spectroscopy is increasingly adopted as a means for ammonium quantification. However, this method is highly sensitive to experimental parameters, as demonstrated herein using a highly versatile and robust NMR pulse program. We demonstrate the sensitivity of the measurement to the final pH of the analyzed solution and identify a [H + ] concentration range enabling robust quantification. We compare direct quantification versus calibration approaches to show that the former is highly sensitive to spin relaxation effects and identify the latter as the most reliable approach. This method, when optimized, enables direct, rapid quantification of both 14 NH 4 + and 15 NH 4 + within 12−22 min. The limit of detection of 5−10 μM, depending on the solvent, which meets the needs of current electrochemical and photochemical N 2 reduction research.

Identification and elimination of false positives in electrochemical nitrogen reduction studies

“…no quantitative comparison is attempted with the main 14 N 2 data), which is entirely inconclusive in the NRR context. More recently emerging problems are associated with the misuse of the highly convenient method for the quantitative analysis of 15 NH 4 + -proton nuclear magnetic resonance ( 1 H NMR) 10 . To observe a clear ammonium triplet for 14 N 2 and doublet for 15 N 2 , complete ammonia protonation must be achieved and be precisely controlled 10 , which is very rarely done.…”

“…More recently emerging problems are associated with the misuse of the highly convenient method for the quantitative analysis of 15 NH 4 + -proton nuclear magnetic resonance ( 1 H NMR) 10 . To observe a clear ammonium triplet for 14 N 2 and doublet for 15 N 2 , complete ammonia protonation must be achieved and be precisely controlled 10 , which is very rarely done. The most common fundamental mistake is quantitative analysis of spectra that are subject to a very significant and uncontrolled level of hydrogen/deuterium (H/D) exchange.…”

“…The same approach to NO x compound removal and quantification needs to be applied. The amount of 15 NH 3 and 14 NH 3 in 15 NRR experiments should be quantified using reliable procedures 10,20 . As 14 N 2 can be difficult to completely remove from these experiments it is to be expected that both 15 NH 3 and 14 NH 3 can be present and both should be quantified with their relevant calibration curves.…”

Recent Progress on Electrocatalytic Synthesis of Ammonia Under Amibent Conditions

“…为得到相对可靠的结果, 除显色法和离子色谱外, 还需要使用 15 N 同位素-核磁共振 NMR 对实验结果进行 验证. 通过同位素标定可直接对生成氨的来源进行追 踪, 以确定催化剂活性, 其检测范围为 5～10 μmol/L [21] . 15 [22] .…”

Across the Board: Federico Bella on Electrochemical Nitrogen Reduction

“…1 H NMR spectroscopy with a solid calibration approach and based on a solvent‐suppression methods was later proposed by Hodgetts et al. as a means for ammonium quantification . The optimized method led to a rapid quantification of both 14 NH 4 + and 15 NH 4 + within 12–22 min, targeting a detection limit of 5‐10 μ m .…”