Electrocatalysis is a promising approach to convert waste nitrate to ammonia and help close the nitrogen cycle. This renewably powered ammonia production process sources hydrogen from water (as opposed to methane in the thermal Haber−Bosch process) but requires a delicate balance between a catalyst's activity for the hydrogen evolution reaction (HER) and the nitrate reduction reaction (NO 3 RR), influencing the Faradaic efficiency (FE) and selectivity to ammonia/ammonium over other nitrogencontaining products. We measure ammonium FEs ranging from 3.6 ± 6.6% (on Ag) to 93.7 ± 0.9% (on Co) across a range of transition metals (TMs; Ti, Fe, Co, Ni, Ni 0.68 Cu 0.32 , Cu, and Ag) in buffered neutral media. To better understand these competing reaction kinetics, we develop a microkinetic model that captures the voltage-dependent nitrate rate order and illustrates its origin as competitive adsorption between nitrate and hydrogen adatoms (H*). NO 3 RR FE can be described via competition for electrons with the HER, decreasing sharply for TMs with a high work function and a correspondingly high HER activity (e.g., Ni). Ammonium selectivity nominally increases as the TM d-band center energy (E d ) approaches and overcomes the Fermi level (E F ), but is exceptionally high for Co compared to materials with similar E d . Density functional theory (DFT) calculations indicate Co maximizes ammonium selectivity via (1) strong nitrite binding enabling subsequent reduction and (2) promotion of nitric oxide dissociation, leading to selective reduction of the nitrogen adatom (N*) to ammonium.

Prompted by X-ray detections from multiple surveys, we investigated the A-type star HD 63021 and found that it is a double-lined spectroscopic binary with highly variable emission associated with the primary star. Analysis of our multiepoch spectroscopic observations, the majority of which were carried out on small-aperture telescopes, indicates a very short orbital period of just 2.9 days and a mass ratio M 2/M 1 of 0.23. The A1 V star is a slow rotator, with a rotational speed of ∼34 km s−1. Assuming that its mass is 2.3 M ⊙, the present-day secondary is an evolved star of ∼0.5 M ⊙ that nearly fills its Roche lobe. This secondary star rotates comparatively rapidly at ∼44 km s−1, and we see evidence that it is chromospherically active. Analysis of a photometric light curve from TESS reveals two strong periods, one at the orbital period for the system and another at half the orbital period. These findings suggest that HD 63021 is a close binary system undergoing mass transfer from the secondary star onto the primary star—in all ways like an Algol eclipsing binary system, except without the eclipse. We discuss the system’s mass transfer, which is not steady but seems to run in fits and bursts, and infer the system’s basic physical properties from an orbital parameter study, the Roche lobe geometry, and its extant X-ray emission.

Molten hydroxide scrubbing of off-gas vapors is a potential process to improve safety during the operation of generation IV molten salt nuclear reactors (MSRs). MSRs produce off-gases that can be vented by the reactor core and treated via off-gas scrubbers. Molten hydroxide scrubbers focus on capturing volatile iodine radionuclides, and they can also be used to capture aerosols and particulates and to neutralize acidic species. The performance of these scrubbers depends on the chemical interactions of the scrubbing medium with the off-gas species. Knowledge of the concentration and speciation of scrubbed or target species, as well as process and environmental interferents, can enable advanced operation of MSR off-gas treatment systems. Optical online monitoring is an excellent technology to provide this information in real time, while limiting the need for operators to interact with radioactive samples through hands-on interrogation. Raman spectroscopy can provide crucial chemical information on the state of the molten eutectic during treatment in the molten phase, as well as the gas phase. In this work, Raman spectroscopy is used to detect iodine species, specifically iodate, in the molten phase of a NaOH−KOH eutectic and to construct a calibration curve of the Raman signal of those species. Additionally, a carbonate interferent is followed from the gas phase to the liquid phase as a basis for reaching a Raman-aided mass balance of the molten hydroxide eutectic scrubber system.

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Electrocatalytic reduction of waste nitrate to ammonium provides a circular process with reduced carbon dioxide emissions compared to current nitrate treatment and ammonia production processes. However, electrocatalysts require a delicate balance between a surfaces’ activity for the competing hydrogen evolution (HER) and nitrate reduction reactions (NO3RR). We measure ammonium Faradaic efficiencies (FEs) of several transition metals (TMs) ranging from 3.6±6.6% (on Ag) to 93.7±0.9% (on Co) in neutral buffered media. A microkinetic model identifies competitive adsorption between nitrate and hydrogen adatoms (H*) as the origin of voltage-dependent nitrate rate order. NO3RR FE is described via competition for electrons with the HER, decreasing sharply for TMs with high work function or hydrogen adsorption energy. Density functional theory calculations indicate Co maximizes ammonium selectivity by: (1) binding intermediate nitrite strongly to enable subsequent reduction; and (2) promoting subsequent nitric oxide dissociation, leading to selective reduction of nitrogen adatoms (N*) to ammonium.