Showing papers on "Ammonia published in 2021"

Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst.

Abstract: Electrochemically converting nitrate, a widespread water pollutant, back to valuable ammonia is a green and delocalized route for ammonia synthesis, and can be an appealing and supplementary alternative to the Haber-Bosch process. However, as there are other nitrate reduction pathways present, selectively guiding the reaction pathway towards ammonia is currently challenged by the lack of efficient catalysts. Here we report a selective and active nitrate reduction to ammonia on Fe single atom catalyst, with a maximal ammonia Faradaic efficiency of ~ 75% and a yield rate of up to ~ 20,000 μg h−1 mgcat.−1 (0.46 mmol h−1 cm−2). Our Fe single atom catalyst can effectively prevent the N-N coupling step required for N2 due to the lack of neighboring metal sites, promoting ammonia product selectivity. Density functional theory calculations reveal the reaction mechanisms and the potential limiting steps for nitrate reduction on atomically dispersed Fe sites. Developing green and delocalized routes for ammonia synthesis is highly important but still very challenging. Here the authors report an efficient ammonia synthesis process via nitrate reduction to ammonia on Fe single atom catalyst.

Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle.

Abstract: Ammonia (NH3) is a globally important commodity for fertilizer production, but its synthesis by the Haber-Bosch process causes substantial emissions of carbon dioxide. Alternative, zero-carbon emission NH3 synthesis methods being explored include the promising electrochemical lithium-mediated nitrogen reduction reaction, which has nonetheless required sacrificial sources of protons. In this study, a phosphonium salt is introduced as a proton shuttle to help resolve this limitation. The salt also provides additional ionic conductivity, enabling high NH3 production rates of 53 ± 1 nanomoles per second per square centimeter at 69 ± 1% faradaic efficiency in 20-hour experiments under 0.5-bar hydrogen and 19.5-bar nitrogen. Continuous operation for more than 3 days is demonstrated.

An experimental and modeling study of ammonia with enriched oxygen content and ammonia/hydrogen laminar flame speed at elevated pressure and temperature

Abstract: Laminar flame speeds of ammonia with oxygen-enriched air (oxygen content varying from 21 to 30 vol.%) and ammonia-hydrogen-air mixtures (fuel hydrogen content varying from 0 to 30 vol.%) at elevated pressure (1–10 bar) and temperature (298–473 K) were determined experimentally using a constant volume combustion chamber. Moreover, ammonia laminar flame speeds with helium as an inert were measured for the first time. Using these experimental data along with published ones, we have developed a newly compiled kinetic model for the prediction of the oxidation of ammonia and ammonia-hydrogen blends in freely propagating and burner stabilized premixed flames, as well as in shock tubes, rapid compression machines and a jet-stirred reactor. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. The experimental results from the present work and the literature are interpreted with the help of the kinetic model derived here. The experiments show that increasing the initial temperature, fuel hydrogen content, or oxidizer oxygen content causes the laminar flame speed to increase, while it decreases when increasing the initial pressure. The proposed kinetic model predicts the same trends than experiments and a good agreement is found with measurements for a wide range of conditions. The model suggests that under rich conditions the N2H2 formation path is favored compared to stoichiometric condition. The most important reactions under rich conditions are: NH2+NH=N2H2+H, NH2+NH2=N2H2+H2, N2H2+H=NNH+H2 and N2H2+M=NNH+H+M. These reactions were also found to be among the most sensitive reactions for predicting the laminar flame speed for all the cases investigated.

Proton-filtering covalent organic frameworks with superior nitrogen penetration flux promote ambient ammonia synthesis

Abstract: The simultaneous achievement of both high ammonia yield and Faradaic efficiency in electrochemical nitrogen reduction is a long-sought-after goal. However, due to the strong competing hydrogen evolution and extremely low solubility of N2 in aqueous systems, thermodynamic modulation at the catalyst level is insufficient, leaving the current performance still far from practical application. Here, we rationally control the diffusion of the reactants to obtain suppressed proton supply and greatly enhanced nitrogen flux using proton-filtering covalent organic frameworks, forcing a highly selective and active nitrogen reduction. In this proof-of-concept system, we achieved a high performance in the electrochemical ammonia synthesis (ammonia yield rate 287.2 ± 10.0 μg h−1 $${\rm{mg}}_{\rm{cat.}}^{-1}$$ , Faradaic efficiency 54.5 ± 1.1%) using a traditional carbon-based catalyst. The proposed strategy successfully optimizes the mass transfer that greatly facilitates nitrogen reduction, providing powerful guidelines for achieving green ammonia production at a more practical level. The simultaneous achievement of both high ammonia yield and Faradaic efficiency in electrochemical nitrogen reduction is a challenging goal. Now, the diffusion of reactants to the catalyst surface is controlled using a covalent organic framework, which results in high-performance electrochemical ammonia synthesis.

Mechanochemistry for ammonia synthesis under mild conditions.

Abstract: Ammonia, one of the most important synthetic feedstocks, is mainly produced by the Haber–Bosch process at 400–500 °C and above 100 bar. The process cannot be performed under ambient conditions for kinetic reasons. Here, we demonstrate that ammonia can be synthesized at 45 °C and 1 bar via a mechanochemical method using an iron-based catalyst. With this process the ammonia final concentration reached 82.5 vol%, which is higher than state-of-the-art ammonia synthesis under high temperature and pressure (25 vol%, 450 °C, 200 bar). The mechanochemically induced high defect density and violent impact on the iron catalyst were responsible for the mild synthesis conditions. The ammonia was synthesized under ambient conditions via a mechanochemical method, reaching a final concentration of 82.5 vol%.

Excessive ammonium assimilation by plastidic glutamine synthetase causes ammonium toxicity in Arabidopsis thaliana.

Abstract: Plants use nitrate, ammonium, and organic nitrogen in the soil as nitrogen sources. Since the elevated CO2 environment predicted for the near future will reduce nitrate utilization by C3 species, ammonium is attracting great interest. However, abundant ammonium nutrition impairs growth, i.e., ammonium toxicity, the primary cause of which remains to be determined. Here, we show that ammonium assimilation by GLUTAMINE SYNTHETASE 2 (GLN2) localized in the plastid rather than ammonium accumulation is a primary cause for toxicity, which challenges the textbook knowledge. With exposure to toxic levels of ammonium, the shoot GLN2 reaction produced an abundance of protons within cells, thereby elevating shoot acidity and stimulating expression of acidic stress-responsive genes. Application of an alkaline ammonia solution to the ammonium medium efficiently alleviated the ammonium toxicity with a concomitant reduction in shoot acidity. Consequently, we conclude that a primary cause of ammonium toxicity is acidic stress.

Recent Advances on Photocatalytic and Electrochemical Oxidation for Ammonia Treatment from Water/Wastewater

Abstract: Heterogeneous photocatalytic (PC) and electrochemical (EC) oxidation of ammonia/ammonium pollutants in water/wastewater have been thoroughly investigated for ammonia abatement from aqueous streams,...

Catalytic ammonia decomposition over Ni-Ru supported on CeO2 for hydrogen production: Effect of metal loading and kinetic analysis

Abstract: Ceria-supported Ni-Ru bimetallic catalysts with different metal loadings have been prepared by co-impregnation, characterized and tested in the production of hydrogen from the catalytic decomposition of ammonia. The bimetallic catalysts showed an excellent catalytic performance in long-term stability tests with respect to monometallic Ru/CeO2 and Ni/CeO2 and in multicycle tests under pure ammonia. The best catalytic performance has been obtained over catalysts with 2.4−5 wt.% Ni, 0.4−0.6 wt.% Ru, and a Ni/Ru wt.% ratio of ca. 7. TOFH2 values exceeding 2 s−1 have been obtained, which are among the highest reported for ammonia decomposition at 400 °C. Raman spectroscopy, XRD, HRTEM, XPS, TPR and H2 chemisorption have revealed the existence of an intimate contact between Ni and Ru and CeO2, which is considered the reason of the excellent catalytic activity and stability observed. A kinetic model has been developed using the Langmuir-Hinshelwood-Hougen-Watson approach for the decomposition of ammonia in a fixed bed reactor. The reaction rate expression of the ammonia decomposition on Ni-Ru bimetallics supported on ceria suggests that the dehydrogenation of the ammonia adsorbed on the surface of the catalyst is the limiting step of the reaction and that ammonia decomposition is inhibited by the presence of H2.

Solar-driven electrochemical synthesis of ammonia using nitrate with 11% solar-to-fuel efficiency at ambient conditions

Abstract: Ammonia is an essential commodity chemical used in the manufacture of fertilizers, pharmaceuticals, ammunition, and plastics, and is a promising alternative fuel source and carrier. Today most ammonia is manufactured by the century-old Haber–Bosch process, which accounts for 1–2% of worldwide energy production and a substantial fraction of global greenhouse gas emissions. Solar-driven electrochemical synthesis of ammonia using nitrates presents a sustainable pathway to produce renewable fuels utilizing wastewater. Previous efforts in solar-driven electrosynthesis of ammonia have been seriously affected by lower specific activity (<10 mA cm−2) of electrochemical nitrate reduction reaction (NiRR) and thereby lower solar-to-fuel (STF) efficiency (<1%). Here, we show oxide-derived Co as an efficient NiRR catalyst with the highest specific activity (∼14.56 mA cm−2 at −0.8 V vs. RHE) and selectivity. The oxide-derived Co offers a maximum faradaic efficiency of 92.37 ± 6.7% and ammonia current density of 565.26 mA cm−2 at −0.8 V vs. RHE. Integrating this catalyst in a PV-electrolyzer cell yields an unprecedented STF efficiency of 11% for ammonia, which is an order of magnitude higher than state-of-the-art systems.

Robust Nitritation Sustained by Acid-Tolerant Ammonia-Oxidizing Bacteria.

Abstract: Oxidation of ammonium to nitrite rather than nitrate, i.e., nitritation, is critical for autotrophic nitrogen removal. This study demonstrates a robust nitritation process in treating low-strength wastewater, obtained from a mixture of real mainstream sewage with sidestream anaerobic digestion liquor. This is achieved through cultivating acid-tolerant ammonia-oxidizing bacteria (AOB) in a laboratory nitrifying bioreactor at pH 4.5-5.0. It was shown that nitrite accumulation with a high NO2-/(NO2- + NO3-) ratio of 95 ± 5% was stably maintained for more than 300 days, and the obtained volumetric NH4+ removal rate (i.e., 188 ± 14 mg N L-1 d-1) was practically useful. 16S rRNA gene sequencing analyses indicated the dominance of new AOB, "Candidatus Nitrosoglobus," in the nitrifying guild (i.e., 1.90 ± 0.08% in the total community), with the disappearance of typical activated sludge nitrifying microorganisms, including Nitrosomonas, Nitrospira, and Nitrobacter. This is the first identification of Ca. Nitrosoglobus as key ammonia oxidizers in a wastewater treatment system. It was found that Ca. Nitrosoglobus can tolerate low pH (<5.0), and free nitrous acid (FNA) at levels that inhibit AOB and nitrite-oxidizing bacteria (NOB) commonly found in wastewater treatment processes. The in situ inhibition of NOB leads to accumulation of nitrite (NO2-), which along with protons (H+) also produced in ammonium oxidation generates and sustains FNA at 3.0 ± 1.4 mg HNO2-N L-1. As such, robust PN was achieved under acidic conditions, with a complete absence of NOB. Compared to previous nitritation systems, this acidic nitritation process is featured by a higher nitric oxide (NO) but a lower nitrous oxide (N2O) emission level, with the emission factors estimated at 1.57 ± 0.08 and 0.57 ± 0.03%, respectively, of influent ammonium nitrogen load.

Direct ammonia synthesis from the air via gliding arc plasma integrated with single atom electrocatalysis

Abstract: Industrial ammonia synthesis revolutionized global agriculture and industry, but it consumes significant amounts of energy and releases vast quantities of CO2. One alternative, electrocatalytic nitrogen reduction generally suffers from a low ammonia yield rate and poor selectivity. Here, a tandem "plasma-electrocatalysis" strategy was proposed to harvest ammonia from the air. An ammonia yield rate (~1.43 mgNH3 cm-2 h-1) with almost 100% faradaic efficiency was achieved during over 50 hours of stable operation at -0.33 V vs. RHE. The ammonia yield rate reached up to ~3.0 mgNH3 cm-2 h-1 with a faradaic efficiency of ~62% at -0.63 V vs. RHE. This marked performance is achieved by separating activation of stable nitrogen molecules via non-thermal plasma, followed by selective ammonia synthesis via a cobalt single-atom electrocatalyst. This strategy may rival the Haber-Bosch process and the aspirational electrochemical nitrogen reduction at a distributed small-size ammonia production based on a techno-economic analysis.

Plasma Activated Electrochemical Ammonia Synthesis from Nitrogen and Water

Abstract: Ammonia is an important precursor of fertilizers, as well as a potential carbon-free energy carrier. Nowadays, ammonia is synthesized via the Haber–Bosch process, which is a capital- and energy-intensive process with an immense CO2 footprint. Thus, alternative processes for the sustainable and decentralized ammonia production from N2 and H2O using renewable electricity are required. The key challenges for the realization of such processes are the efficient activation of the N2 bond and selectivity toward NH3. In this contribution, we report an all-electric method for sustainable ammonia production from nitrogen and water using a plasma-activated proton conducting solid oxide electrolyzer. Hydrogen species produced by water oxidation over the anode are transported through the proton conducting membrane to the cathode where they react with the plasma-activated nitrogen toward ammonia. Ammonia production rates and Faradaic efficiencies up to of 26.8 nmol of NH3 s–1 cm–2 and 88%, respectively, were achieved.

Ammonia oxidation at pH 2.5 by a new gammaproteobacterial ammonia-oxidizing bacterium.

Abstract: Ammonia oxidation was considered impossible under highly acidic conditions, as the protonation of ammonia leads to decreased substrate availability and formation of toxic nitrogenous compounds. Recently, some studies described archaeal and bacterial ammonia oxidizers growing at pH as low as 4, while environmental studies observed nitrification at even lower pH values. In this work, we report on the discovery, cultivation, and physiological, genomic, and transcriptomic characterization of a novel gammaproteobacterial ammonia-oxidizing bacterium enriched via continuous bioreactor cultivation from an acidic air biofilter that was able to grow and oxidize ammonia at pH 2.5. This microorganism has a chemolithoautotrophic lifestyle, using ammonia as energy source. The observed growth rate on ammonia was 0.196 day-1, with a doubling time of 3.5 days. The strain also displayed ureolytic activity and cultivation with urea as ammonia source resulted in a growth rate of 0.104 day-1 and a doubling time of 6.7 days. A high ammonia affinity (Km(app) = 147 ± 14 nM) and high tolerance to toxic nitric oxide could represent an adaptation to acidic environments. Electron microscopic analysis showed coccoid cell morphology with a large amount of intracytoplasmic membrane stacks, typical of gammaproteobacterial ammonia oxidizers. Furthermore, genome and transcriptome analysis showed the presence and expression of diagnostic genes for nitrifiers (amoCAB, hao, nor, ure, cbbLS), but no nirK was identified. Phylogenetic analysis revealed that this strain belonged to a novel bacterial genus, for which we propose the name "Candidatus Nitrosacidococcus tergens" sp. RJ19.