Revisiting the Electrochemical Nitrogen Reduction on Molybdenum and Iron Carbides

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

Izelaar, B. (TU Delft Large Scale Energy Storage)
Ripepi, D. (TU Delft ChemE/Materials for Energy Conversion and Storage)
Asperti, S. (TU Delft Large Scale Energy Storage)
Dugulan, A.I. (TU Delft RID/TS/Instrumenten groep; TU Delft RST/Fundamental Aspects of Materials and Energy)
Hendrikx, R.W.A. (TU Delft Team Amarante Bottger)
Bottger, A.J. (TU Delft Team Amarante Bottger)
Mulder, F.M. (TU Delft ChemE/Materials for Energy Conversion and Storage)
Kortlever, R. (TU Delft Large Scale Energy Storage)

2023

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-Mo₂C decorated carbon nanosheets, α-Mo₂C nanoparticles, θ-Fe₃C nanoparticles, and χ-Fe₅C₂ nanoparticles was confirmed by X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron and Mössbauer 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.

ammonia
carbide
electrocatalysis
impurities
iron
molybdenum
nitrogen reduction reaction

http://resolver.tudelft.nl/uuid:57a266a7-77d4-49db-87d9-c4ddd7028bf9

https://doi.org/10.1021/acscatal.2c04491

2155-5435

ACS Catalysis, 13 (3), 1649-1661

Institutional Repository

journal article

© 2023 B. Izelaar, D. Ripepi, S. Asperti, A.I. Dugulan, R.W.A. Hendrikx, A.J. Bottger, F.M. Mulder, R. Kortlever