Mild-condition nitrogen fixation using "green" solar energy, merely requiring a solar-to-NH3 (STA) efficiency of 0.1% for potential use, is a promising alternative to the Haber-Bosch process but remains a great challenge. The bottleneck lies in the ultra-low efficiency originating from the sluggish surface reaction involving 6e(-)/6H(+), specifically, the rate-limiting reductive activation of nitrogen. To meet this challenge, a common approach adopted by natural nitrogenase or artificial catalysts is to craft an active center (transition metal) of electron-rich nature, thus enriching the LUMO (lowest unoccupied molecular orbital) electron density of N-2 via the e(-) -> pi*-orbital (N) transition to facilitate the molecular nitrogen activation. Recent progress suggests that anion vacancies such as O, C, N, and S vacancies, which inherently bear excess electrons and coordinatively unsaturated metal ions, could be explored in promoting the molecular nitrogen activation in photocatalysis. In this review, we summarize the in depth anion vacancy-dominated nitrogen photo-fixation systems, and then focus on the mechanical understanding of how an anion vacancy kinetically affects the nitrogen reductive activation including N-N triple bond weakening, N-2 adsorption and activation energies, and kinetic pathways, and finally propose the future challenges and prospects of this emerging area.
We describe an infinite two-parameter subfamily of theories of class S where dialing one of the parameters interpolates between Gaiotto's T-N theory and a theory of N-2 free hypermultiplets. After using the reduced superconformal index to study the operator content, we use these theories to construct new N =3D 1 SCFTs and then examine the flows between them.
Tomina, N. N.; Solmanov, P. C.; Maksimov, N. M.; Pimerzin, A. A.
The dependence of the catalytic activity of nickel-molybdenum-tungsten sulfide catalysts on the Mo/W molar ratio is studied in order to develop an effective catalytic composition for the hydrorefining of petroleum. Three-metallic catalysts of Ni-6-PMonW12-n/Al2O3 composition, where n =3D 0-12 (Ni-6-PMonW12-n(S)/Al2O3 after sulfurizing), are synthesized. The morphology of the active-phase particles of sulfide samples is investigated using high-resolution transmission electron microscopy. The average length of NiMo(W)S phase layers, the average number of layers in a crystallite, the number of Mo(W) atoms in a layer, and the fraction of Mo(W) atoms in the rib and angular positions are calculated. The effect of the Mo/W molar ratio on the catalytic activity of the prepared samples is studied during the dibenzothiophene hydrogenolysis, hydrodesulfurization (HDS), and hydrogenation of olephins and aromatic compounds when hydrotreating diesel fractions and vacuum gas oil. A sample with molar ratio Mo : W =3D 1 : 1 was found to be the one most active in the reactions of HDS and hydrogenation.
Auroral breakup at the onset of substorm expansion is sometimes preceded by auroral forms known as quiet arcs and N‐S arcs. Observations have shown that both the auroral forms tend to move equatorward, and the initial brightening takes place in or near one of the quiet arcs. The auroral forms attract great attention, but generation of auroral forms and their association with the initial brightening are poorly understood. Recent global magnetohydrodynamic simulations are capable of producing upward field‐aligned currents (FACs) that resemble the auroral forms in both shape and temporal evolution. Based on the simulation results, we propose the following scenarios: (1) When the convection electric field is weak (northward interplanetary magnetic field (IMF)), the high‐pressure region is elongated from the plasma sheet toward higher latitudes and is structured by a coupling between the magnetosphere and the ionosphere (interchange‐like instabilities). (2) When the convection electric field is strong (southward IMF), the structured high‐pressure region moves equatorward (toward the plasma sheet). Upward currents are generated around it, which can be observed as arcs in the ionosphere. The upward current can be tentatively intensified in the course of the equatorward movement before the formation of a near‐Earth neutral line (NENL). (3) The NENL releases magnetic tension and results in the enhancement of plasma pressure at off‐equator in the near‐Earth region. Sudden formation of the off‐equatorial high‐pressure region generates the onset current system that manifests initial brightening. Our scenario can explain the observational fact that poleward arcs remained undisturbed at the onset.
We give a decomposition formula for tensor powers of the defining representation of S-n and apply it to bound the mixing time of a Markov chain on S-n.