Direct determination of trace nitrogen dioxide by atmospheric pressure lonization mass spectrometry (APIMS) without conversion to nitric oxide

Electrochemical Nitrogen Reduction to Ammonia Under Ambient Conditions: Stakes and Challenges

Aqueous electrochemical nitrogen reduction (ENR) to ammonia (NH₃ ) under ambient conditions is considered as an alternative to the energy-intensive Haber-Bosch process for ammonia production. Many metal, non-metal, carbon-based materials along with metal-chalcogenides, metal-nitrides have been explored for their ENR activity. The reported NH₃ production through ENR is still in the micro-gram level and often falls in the range of NH₃ and NO_x contaminations from the surrounding. The quantification of NH₃ at very low concentration possess enormous challenge in this field and thus many reported ENR electrocatalysts suffer from reproducibility issue. This review highlights in detail the challenges associated with ENR in aqueous medium and necessitates standardization of protocols to quantify the low concentration of NH₃ free of false-positives. It concludes the prospects of electrochemical NH₃ production through lithium-mediated N₂ reduction.

Electrical detection of ppb region NO2 using Mg-porphyrin-modified graphene field-effect transistors

The trace detection of NO₂ through small sensors is essential for air quality measurement and the health field; however, small sensors based on electrical devices cannot detect NO₂ with the desired selectivity and quantitativity in the parts per billion (ppb) concentration region. In this study, we fabricated metalloporphyrin-modified graphene field-effect transistors (FETs). Mg-, Ni-, Cu-, and Co-porphyrins were deposited on the graphene FETs, and the transfer characteristics were measured. With the introduction of NO₂ in the ppb concentration region, the FETs of pristine graphene and Ni-, Cu-, and Co-porphyrin-modified graphene showed an insufficient response, whereas the Mg-porphyrin-modified graphene exhibited large voltage shifts in the transport characteristics. This indicates that Mg-porphyrin acts as an adsorption site for NO₂ molecules. An analysis of the Dirac-point voltage shifts with the introduction of NO₂ indicates that the shifts were well-fitted with the Langmuir adsorption isotherm model, and the limit of detection for NO₂ was found to be 0.3 ppb in N₂. The relationship between the mobility and the Dirac-point voltage shift with the NO₂ concentration shows that the complex of NO₂ and Mg-porphyrin behaves as a point-like charge impurity. Moreover, the Mg-porphyrin-modified graphene FETs show less response to other gases (O₂, H₂, acetic acid, trimethylamine, methanol, and hexane), thus indicating high sensitivity for NO₂ detection. Furthermore, we successfully demonstrated the quantitative detection of NO₂ in air, which is near the environmental standards. In conclusion, the results of the Mg-porphyrin-modified graphene FETs enable a rapid, easy, and selective detectability.

Electrochemical Ammonia Generation Directly from Nitrogen and Air Using an Iron-Oxide/Titania-Based Catalyst at Ambient Conditions

Ammonia was produced electrochemically from nitrogen/air in aqueous alkaline electrolytes by using a Fe₂O₃/TiO₂ composite catalyst under room temperature and atmospheric pressure. At an applied potential of 0.023 V versus reversible hydrogen electrode, the rate of ammonia formation was 1.25 × 10^-8 mmol mg^-1 s^-1 at an overpotential of just 34 mV. This rate increased to 2.7 × 10^-7 mmol mg^-1 s^-1 at -0.577 V. The chronoamperometric experiments on Fe₂O₃/TiO₂/C clearly confirmed that Fe₂O₃ along with TiO₂ shows superior nitrogen reduction reaction activity compared to Fe₂O₃ alone. Experimental parameters such as temperature and applied potential have a significant influence on the rate of ammonia formation. The activation energy of nitrogen reduction on the employed catalyst was found to be 25.8 kJ mol^-1. Real-time direct electrochemical mass spectrometry analysis was used to monitor the composition of the evolved gases at different electrode potentials.