(Invited) Nitrogen Activation by Reduction and Oxidation: A Primer for Rigorous and Reproducible Measurements

Abstract

Nitrogen is essential for plant growth, and specifically activated nitrogen in the form of ammonia is used as a synthetic fertilizer (~80%), thereby providing sustenance for roughly half of the global population. The century-old industrial Haber-Bosch process for the production of ammonia is therefore crucial for modern society, but the process unfortunately has significant drawbacks. Nitrogen is extremely inert, and the process is highly energy intensive, not sustainable due to substantial CO2 emissions, and requires large-centralized facilities due to the inherent challenge of activating N2 via high temperatures and pressures. New strategies which would enable sustainable and decentralized production for N2 fixation, such as low-temperature thermochemical catalysis, electrocatalysis, and photo(electro)catalysis, have been pursued over the past few decades. Unfortunately, efforts particularly in electron and photon-assisted N2 fixation have been filled with controversies and contradictory results, while progress has been hindered by the lack of rigor and reproducibility in the collection and analysis of results. This is due to ammonia and other N-containing contaminants being ubiquitous in the environment, which can easily lead to contamination, inflation of reported catalytic performance, and thereby reporting of false positives.In this work, we provide a holistic step-by-step protocol, applicable to all nitrogen-transformation reactions, focused on verifying genuine N2 activation by accounting for all possible contamination sources. The possible sources of contamination, denoted as the system mass, include aspects such as flow gas impurities, impurities in the catalyst/substrate, electrolyte contaminants (for electrolytic systems), and impurities in the absorber material (for photocatalytic systems), among other sources. If the amount of product measured is less than the system mass, scientists need to include quantifiable isotope labelled experimentation coupled with proper gas cleaning to elucidate the source of the activated nitrogen. We focus primarily on electrochemical, photo(electro)chemical, and thermochemical systems due to the size of interest in the fields, but the protocol finds wider applicability to other modes of N2 activation, such as plasma and mechanochemical N2 fixation. Using the protocol’s framework, state-of-the-art results from different catalytic reactions are quantitatively assessed which reveal a number of important insights: 1) to explain why obtaining reliable results for electrochemical and photo(electro)chemical systems are inherently more challenging than thermochemical systems, leaving quantitative isotope experiments as the only cost-effective ways to verify genuine N2 activation. 2) to flag the potential problems related to low-temperature (<250 oC) thermochemical catalysis results due to the alarmingly low product formed in relation to their system size from which contaminants may emerge. 3) to provide an updated and realistic picture of the N2 activation fields in the context of the possible contamination sources. Moreover, Density Functional Theory has been utilized to complement experimental work with a “theory confirms experiment” mindset, but pitfalls in the creation of free energy diagrams, assessment of active site stability and interpretation of the limiting step are commonplace and will be discussed. We conclude, by covering the recommended benchmarks and reporting metrics, best practices to improve reproducibility and rigor, and cost-efficient ways to carry out rigorous experimentation. The future of nitrogen catalysis will require an increase in rigorous experimentation and standardization to prevent false positives from appearing in the literature, and help the field advance towards practical technologies for the activation of N2. Figure 1