Revisiting the Electrochemical Nitrogen Reduction on Molybdenum and Iron Carbides: Promising Catalysts or False Positives?

Abstract

The electrochemical dinitrogen reduction reaction (NRR) has recently gained much interest as it can potentially produce ammonia from renewable intermittent electricity and replace the Haber−Bosch process. Previous literature studies report Fe-and Mo-carbides as promising electrocatalysts for the NRR with activities higher than other metals. However, recent understanding of extraneous ammonia and nitrogen oxide contaminations have challenged previously published results. Here, we critically assess the NRR performance of several Fe-and Mo-carbides reported as promising by implementing a strict experimental protocol to minimize the effect of impurities. The successful synthesis of α-Mo 2 C decorated carbon nanosheets, α-Mo 2 C nanoparticles, θ-Fe 3 C nanoparticles, and χ-Fe 5 C 2 nanoparticles was confirmed by X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron and Mossbauer spectroscopy. After performing NRR chronoamperometric tests with the synthesized materials, the ammonia concentrations varied between 37 and 124 ppb and are in close proximity with the estimated ammonia background level. Notwithstanding the impracticality of these extremely low ammonia yields, the observed ammonia did not originate from the electrochemical nitrogen reduction but from unavoidable extraneous ammonia and NO x impurities. These findings are in contradiction with earlier literature studies and show that these carbide materials are not active for the NRR under the employed conditions. This further emphasizes the importance of a strict protocol in order to distinguish between a promising NRR catalyst and a false positive.

Low-temperature electrochemical ammonia synthesis: measurement reliability and comparison to Haber-Bosch in terms of energy efficiency

“…Izellar et al 22 critically assessed the ENRR activity of molybdenum and iron carbides, trying to reproduce peer-reviewed works that had reported a superior or excellent catalytic performance. 23,24 They found that the ammonia did not originate from the ENRR but from unavoidable extraneous ammonia and NOx impurities.…”

Effect of Temperature and H Flux on the NH3 Synthesis via Electrochemical Hydrogen Permeation

“…[12,56] This additional purification step was also applied to 15 N 2 (99 atom % 15 N, Sigma-Aldrich) used for isotope labelled experiments, as 15 N 2 might also contain traces of extraneous N species. [56,57]…”

A High-Throughput Screening toward Efficient Nitrogen Fixation: Transition Metal Single-Atom Catalysts Anchored on an Emerging π–π Conjugated Graphitic Carbon Nitride (g-C10N3) Substrate with Dirac Dispersion

“…Currently, industrial-scale NH 3 synthesis has mainly been achieved through the capital-and energy-intensive Haber-Bosch (H−B) approach 8 usually necessitating severe temperature (300−600 °C) and ultrahigh pressure (150−300 atm), 9 which is unsustainable and disadvantageous. 10 This process has a large carbon footprint, accounting for 1−2% of the global total energy consumption 11 and meanwhile discharging a large amount of greenhouse gas annually. 12 Considering such unwilling costs in energy and environment, it is highly desirable to find a cost-effective and sustainable technology to achieve highly efficient synthesis for N 2 reduction.…”

“…Gaseous N 2 needs extremely large cleavage energy to fracture the strong NN triple bond due to its ultrahigh bonding strength, absence of permanent polarity, and low chemical activity resulting from the shared charges in π/2σ orbitals, which results in problematic NH 3 production at ambient conditions. Currently, industrial-scale NH 3 synthesis has mainly been achieved through the capital- and energy-intensive Haber-Bosch (H–B) approach usually necessitating severe temperature (300–600 °C) and ultrahigh pressure (150–300 atm), which is unsustainable and disadvantageous . This process has a large carbon footprint, accounting for 1–2% of the global total energy consumption and meanwhile discharging a large amount of greenhouse gas annually .…”