Denitrification In Environments Research Articles

Polystyrene spheres-templated mesoporous carbonous frameworks implanted with cobalt nanoparticles for highly efficient electrochemical nitrate reduction to ammonia

Electrocatalytic nitrate reduction reaction (eNO3RR) to ammonia (NH3) provides an intriguing approach for both environmental denitrification and sustainable NH3 synthesis. Herein, we report the fabrication of hierarchically mesoporous Co nanoparticles decorated N-doped carbon composites as a highly efficient electrocatalyst for eNO3RR by using monodispersed polystyrene spheres (PS) as sacrificial templates (MR Co-NC). By taking advantage of the large specific area of the pore-rich structures and the high intrinsic activity of metallic Co, the MR Co-NC shows remarkable activity toward eNO3RR, which achieves a partial current density of 268 mA cm−2 with a Faradaic efficiency (FE) of 95.35 ± 1.75 % and a generation rate of 1.25 ± 0.023 mmol h−1 cm−2 for NH3 production under the optimal conditions. The porous carbon skeleton was found to play a dual role by simultaneously protecting the active sites from oxidation and facilitating long-range charge and mass transfer. Theoretical calculations reveal a lower energy barrier of the rate-determining step (*NO2 + H2O + 2e- → *NO + 2OH-) on the metallic Co of MR Co-NC over β-Co(OH)2 formed by the reconstruction of carbon-free Co nanoparticles.

Governing Interlayer Strain in Bismuth Nanocrystals for Efficient Ammonia Electrosynthesis from Nitrate Reduction

Electrochemical ammonia (NH3) synthesis from nitrate (NO3-) reduction offers an intriguing approach for both sustainable ammonia synthesis and environmental denitrification, yet it remains hindered by a complicated reaction pathway with various intermediates. Here we present that the interlayer strain compression in bismuth (Bi) nanocrystals can contribute to both activity and selectivity improvement toward NH3 electrosynthesis from NO3- reduction. By virtue of comprehensive spectroscopic studies and theoretical calculations, we untangle that the interlayer lattice compression shortens Bi-Bi bond to broaden the 6p bandwidth for electron delocalization, promoting the chemical affinities of nitrogen intermediates. Such a manipulation facilitates NO3- activation to reduce the energy barrier for activity improvement, and also alleviates *NO2 desorption to suppress nitrite generation. As a result, a strain-compressive Bi electrocatalyst yields a maximal Faradaic efficiency of 90.6% and high generation rate of 46.5 g h-1 gcat-1 with industrially scalable partial current density up to 300 mA cm-2 for NH3 product at the optimized conditions, respectively.

Higher diversity and abundance of denitrifying microorganisms in environments than considered previously.

Denitrification is an important process in the global nitrogen cycle. The genes encoding NirK and NirS (nirK and nirS), which catalyze the reduction of nitrite to nitric oxide, have been used as marker genes to study the ecological behavior of denitrifiers in environments. However, conventional polymerase chain reaction (PCR) primers can only detect a limited range of the phylogenetically diverse nirK and nirS. Thus, we developed new PCR primers covering the diverse nirK and nirS. Clone library and qPCR analysis using the primers showed that nirK and nirS in terrestrial environments are more phylogenetically diverse and 2-6 times more abundant than those revealed with the conventional primers. RNA- and culture-based analyses using a cropland soil also suggested that microorganisms with previously unconsidered nirK or nirS are responsible for denitrification in the soil. PCR techniques still have a greater capacity for the deep analysis of target genes than PCR-independent methods including metagenome analysis, although efforts are needed to minimize the PCR biases. The methodology and the insights obtained here should allow us to achieve a more precise understanding of the ecological behavior of denitrifiers and facilitate more precise estimate of denitrification in environments.