167 Search Results

Ammonia (NH₃) is a carbon-free energy carrier with an infrastructure for production, storage, and distribution. There is interest in direct NH₃ combustion, but managing pollutant emissions is a key challenge, particularly nitric oxides (NO_x) due to the fuel-bound nitrogen atom, nitrous oxide (N₂O), which is a potent greenhouse gas, and unburned NH₃, which is harmful to humans and the environment. Rich staged combustor concepts with extended primary residence times (τ_res,primary), like Rich-Relax- Quick-mix-Lean (RRQL), offer a viable pathway for direct NH₃ combustion with low levels of NO_x formation. However, minimizing secondary emissions such as N₂O and unburned NH₃ and hydrogenmore » (H₂) remains a critical challenge. Prior atmospheric-pressure studies have demonstrated that RRQL operation with sufficiently long τ_res,primary enables substantial NO_x relaxation and promotes NH₃ cracking to H₂, if heat losses from the relaxation stage are limited. However, the combined influence of elevated pressure and long residence time on RRQL performance has not been explored. The present work examines RRQL operation at pressures up to 5 bar and elevated τ_res,primary. Exhaust measurements of NO_x, NH₃, and N₂O are used to quantify the extent of NO_x relaxation and NH₃ cracking under nonadiabatic conditions. To contextualize and quantify the effects of heat losses in the experimental data, chemical reactor networks (CRNs) incorporating prescribed heat loss rates are employed to assess the sensitivity of emissions to thermal losses in the relaxation stage. Collectively, the results demonstrate that management and quantification of heat losses are essential to preserve NO_x relaxation and limit NH₃ and N₂O emissions.« less

Photocatalytic ammonia (NH₃) synthesis offers a carbon-neutral alternative to the Haber−Bosch process, which generates 42 million metric tons of CO₂ equivalent emissions annually. However, solar-to-ammonia conversion with contemporary photocatalysts remains far from practical requirements, and understanding the limiting factors in systems with well-defined active sites is crucial. Here, we show how the μ₃-oxo-centered trinuclear Fe cluster in MIL-101(Fe) functions as the catalytic motif for N₂-to-NH₃ conversion through combined experimental and computational investigations. Comparative studies with a molecular analogue demonstrate that the cluster is stabilized within the MOF framework, sustaining redox cycling and maintaining high catalytic activity. We systematically functionalized themore » dicarboxylate ligands of MIL-101(Fe) with −NH₂, −Br, −NO₂, −F, and −CF₃ to probe how ligand chemistry modulates Fe electron density, N₂ adsorption capacity, and proton availability, correlating these properties with catalytic performance using spectroscopic and surface characterization techniques alongside timeresolved infrared to assess excited-state lifetimes. F-functionalization optimally balances N₂ activation, proton availability at Fe active sites, and excited-state lifetimes, boosting NH₃ production by ∼ 60% relative to unmodified MIL-101(Fe). This study of ligandfunctionalized MIL-101(Fe) MOFs uncovers the underlying structure-activity relationships and advances design principles for solardriven NH₃ synthesis.« less

Here, this Letter reports on a vertical, bulk-conducting photoconductive semiconductor switch (PCSS) fabricated on an intrinsic Type IIa single-crystal diamond substrate. Under near-bandgap excitation at 225 nm with a 20 μJ pulse, a strong photocurrent response of 17.1 A at 1 kV DC bias magnitude is obtained by (i) tuning the optical trigger wavelength to the “matched-absorption” window (224–235 nm) near the band edge, where the optical penetration depth becomes comparable to the 500 μm substrate thickness, and (ii) choosing the bias polarity ensuring electron-dominant conduction, given that electrons have a higher mobility than holes in diamond. The PCSS hasmore » an area-normalized responsivity of 54.2 mA W⁻¹ cm⁻² and an effective on-resistance of 8.48 Ω, with a fast 90%–10% transient fall time of 25 ns, attributed to carrier sweep-out. These results support vertical, bulk-conducting intrinsic diamond PCSS as a promising platform for high-power optical switching and provide new insight into intrinsic photoconductivity in diamond.« less

The negatively charged tin-vacancy center in diamond (SnV⁻) is an excellent solid state qubit with optically-addressable transitions and a long electron spin coherence time at elevated (∼1.7 K). However, implementing scalable quantum nodes with high-fidelity optical readout of the electron spin state requires efficient photon emission and collection from the system. Here, in this manuscript, we report a quantum photonic interface for SnV−centers based on one-dimensional photonic crystal cavities fabricated in diamond thin films. Furthermore, we provide a rigorous description of the spontaneous emission dynamics of our system, taking into account individual contributions from both the C and D transitionsmore » of the emitter. This allows for determination of Purcell factors per transition and, by extension, the C/D branching ratio SnV− zero phonon line. We observe quality factors up to ~6000 across this sample, and measure up to a 12-fold lifetime reduction, which translates into a Purcell factor of 𝐹_𝐶=26.2±1.5 for a targeted C transition. By considering the cavity mode polarization alignment with the C and D transition dipole moments, we validate the C/D branching ratio to be 𝜂_BR=0.75±0.01, in line with previous theoretical and experimental findings.« less

Reduction of (depe)₂CrCl₂ (depe = 1,2-bis- (diethylphosphino)ethane) and (dep-benz)₂CrCl₂ (dep-benz = 1,2-bis(diethylphosphino)benzene) under 1 atm of N₂ furnished the dinitrogen complexes (depe)₂Cr(N₂)₂ and (dep-benz)₂Cr(N₂)₂, respectively. One-electron oxidation of these products with FcBAr^F ₄ (Fc = ferrocenium, BAr^F ₄ = B(3,5-(CF₃)₂C₆H₃)₄) yielded the unusual, high-spin tetrahedral complexes [(depe)₂Cr][BAr^F ₄] and [(dep-benz)₂Cr][BAr^F ₄] with concomitant loss of dinitrogen. Reaction of the chromium(I) derivatives with Ph₃CN₃ furnished rare examples of chromium(IV) nitrides as confirmed spectroscopically and by X-ray crystallography. While [(depe)₂Cr(≡N)][BAr^F ₄] underwent association of isocyanides accompanied by partial ligand dissociation, neither chromium nitride was reactive toward H₂ or diphenylsilane under thermal ormore » photochemical conditions. These results distinguish the unique properties of the chromium(IV) nitrides as compared to heavier group 6 congeners and demonstrate both the feasibility of nitride synthesis and the limitations of dinitrogen cleavage and subsequent N−H bond formation.« less

Cyanobacterial production of a biofertilizer shows promise as an environmentally benign alternative to conventional nitrogen fertilizers, reducing environmental and energy burdens through light-driven nitrogen and carbon fixation. One route to realizing the potential for a nitrogen-rich, slow-releasing biofertilizer involves the genetic engineering of cyanobacteria to produce guanidine. Recent advances have demonstrated enzymatic guanidine production in cyanobacteria, but an understanding of cyanobacterial guanidine metabolism is still limited. This Perspective highlights strategies and opportunities for cyanobacterial guanidine production in a Design−Build−Test−Learn cycle. Exploring new guanidine-producing enzymes via phylogenetics could expand candidate enzymes, while understanding the metabolism of substrates can identify constraints andmore » opportunities in substrate utilization. Additionally, guanidine sensing and export are crucial areas of study to enable continuous fertilizer production and stable nitrogen flux. These strategies will guide the development of advanced nitrogen biofertilizer strategies for the agricultural sector.« less

MXenes have become one of the most versatile families of two-dimensional (2D) materials due to their high conductivity, hydrophilicity, and remarkable electrochemical performance. This has stimulated intense efforts to design and synthesize MXenes, including structurally unique in-plane ordered 2D MXenes called i-MXenes. Here, we have synthesized the quaternary rare earth (RE)-based i-MAX phase (Mo_2/3Er_1/3)₂AlC using an arc melting method, and the corresponding 2D i-MXene was then obtained through a LiF/HCl soft etching process. Literature studies have shown that Al and the RE element are etched out during the etching process, leading to the formation of pure vacancy-ordered Mo1.33C 2D i-MXene.more » However, our investigation reveals that upon exposure to a fluorine solution, the i-MAX phase forms RE fluoride impurities, which are challenging to remove through HCl−DI water washing and persist in the final product, resulting in impure Mo_1.33C@Er i-MXene. These results were confirmed by various characterizations such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning transmission electron microscopy. Although the Mo_1.33C@Er electrode showed a 24-fold increase in specific capacitance compared to its parent i-MAX phase, it still exhibited a high charge-transfer resistance arising from the insulating nature of RE fluoride byproducts, which adversely influence the overall capacitance behavior of the synthesized 2D Mo_1.33C@Er i-MXenes. This study contributes to identifying pathways for the preparation of pure 2D i-MXenes from RE-based i-MAX phases and developing improved synthesis methods. With additional process optimization, the 2D i-MXene holds a strong potential for electrochemical energy storage applications. Additionally, the electronic structures of Mo_1.33C were theoretically studied using first-principles density functional theory calculations, which revealed that pristine Mo_1.33C is metallic, and this metallic nature is preserved even with −O, −F, and mixed functionalization.« less

Graphite has been used in large volumes as a structural material and neutron moderator since the earliest days of nuclear fission. However, no international consensus exists on the disposal of irradiated graphite, leaving much of the historic radioactive graphite inventory in interim vault or silo storage. With several new graphite-moderated reactors planned or under construction, the issue of graphite waste management is becoming increasingly urgent. This paper reviews and quantifies impurities in both historic and modern nuclear graphite, with emphasis on nitrogen—responsible for much of the ¹⁴C inventory—and chlorine, which plays a critical role in repository performance and design. Modernmore » graphites, benefitting from stringent quality-control measures developed for non-nuclear industries, meet or exceed the ASTM Ultra-High Purity nuclear standards, even without halide purification. Both chlorine and nitrogen concentrations have declined over time. For chlorine, identified as a key impurity influencing U.S. waste repository design, we propose a target of 0.1 appm in as-fabricated billets as a reasonable benchmark. Nitrogen sources are traced throughout the graphite production process, with surface and bulk concentrations characterized for all materials studied. Modern graphites commonly exhibit nitrogen levels below 5 appm, with values approaching 1 appm achievable. Using such reduced-nitrogen grades is critical to keeping graphite-induced radioactivity below the greater-than-Class-C waste threshold, thereby avoiding disposal cost penalties of nearly an order of magnitude.« less

Chemical synthesis of Li₃N through lithium nitridation has potential to advance rechargeable battery and nitrogen fixation technology. However, studies of the conditions for forming Li₃N on the lithium surface via nitrogen gas exposure report contradictory findings, such as the spontaneous reaction of Li with pure N₂, the impossibility of forming Li₃N through pure Li and N₂ interaction, the requirement of trace H₂O to catalyze the reaction, and evidence to the contrary. In this study, ambient pressure X-ray photoelectron spectroscopy (APXPS) was applied to evaluate the in situ chemical evolution of the lithium metal surface under nitrogen gas up to 800more » mTorr. At pressures ≤10 mTorr, no Li₃N was detected. At higher pressures, surface Li₃N rapidly reacts with trace CO₂. Additionally, because metallic lithium is readily oxidized by trace gases, the atomic nitrogen concentration of the lithium surface remains below 2%. When nitridation follows oxidation by O₂ gas, CO₂ gas, or H₂O vapor, surface Li₃N formation is inhibited. These results suggest that nitrogen gas can diffuse through the oxidized lithium metal surface to react with subsurface metallic lithium.« less

Understanding how point defects in the bulk govern redox transformations is essential for advancing hydrogen-based metal production and designing high-performance oxide materials. This study reveals the atomic-scale mechanisms driving hydrogen-induced reduction of γ-Fe₂O₃ to Fe₃O₄, focusing on how bulk vacancy dynamics dictate structural evolution and reaction kinetics. A key finding is the pronounced contrast in defect behavior between the two oxides: in γ-Fe₂O₃, intrinsic Fe vacancies promote oxygen vacancy clustering, destabilizing the local lattice and driving nanopore formation. In contrast, Fe₃O₄ exhibits a higher oxygen vacancy formation energy and lacks intrinsic Fe vacancies, suppressing vacancy aggregation and maintaining a dense,more » pore-free structure. This divergence governs distinct reduction pathways—γ-Fe₂O₃ undergoes an interface-reaction-limited transformation confined to the γ-Fe₂O₃/Fe₃O₄ boundary, while Fe₃O₄ supports a uniform increase in oxygen vacancy concentration, enabling bulk-phase reduction to lower-oxide FeO. Integrated in situ electron microscopy and density functional theory modeling uncover a vacancy-mediated mechanism, where synergistic cation-anion vacancy dynamics steer microstructure evolution and phase progression. These insights highlight the critical role of vacancy dynamics in controlling oxide reactivity and offer a pathway toward vacancy engineering to enhance reduction kinetics in hydrogen metallurgy and to tailor porosity, reactivity, and structural resilience in oxide-based catalysts and energy materials.« less