Active Sites of Mixed-Metal Core-Shell Oxygen Evolution Reaction Catalysts: FeO4 Sites on Ni Cores or NiN4 Sites in C Shells?

Water electrolysis for clean hydrogen production requires high-activity, high-stability, and low-cost catalysts for its particularly sluggish half-reaction, the oxygen evolution reaction (OER). Currently, the most promising of such catalysts working in alkaline conditions is a core-shell nanostructure, NiFe@NC, whose Fe-doped Ni (NiFe) nanoparticles are encapsulated and interconnected by N-doped graphitic carbon (NC) layers, but the exact OER mechanism of these catalysts is still unclear, and even the location of the OER active site, either on the core side or on the shell side, is still debated. Therefore, we herein derive a plausible active-site model for each side based on various experimental evidence and density functional theory calculations and then build OER free-energy diagrams on both sides to determine the active-site location. The core-side model is an FeO4-type (rather than NiO4-type) active site where an Fe atom sits on Ni oxide layers grown on top of the core surface during catalyst activation, whose facile dissolution provides an explanation for the activity loss of such catalysts directly exposed to the electrolyte. The shell-side model is a NiN4-type (rather than FeN4-type) active site where a Ni atom is intercalated into the porphyrin-like N4C site of the NC shell during catalyst synthesis. Their OER free-energy diagrams indicate that both sites require similar amounts of overpotentials, despite a complete shift in their potential-determining steps, i.e., the final O2 evolution from the oxophilic Fe on the core and the initial OH adsorption to the hydrophobic shell. We conclude that the major active sites are located on the core, but the NC shell not only protects the vulnerable FeO4 active sites on the core from the electrolyte but also provides independent active sites, owing to the N doping.

Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo2C/MoS2 Hybrid Structure via Chemical Conversion of Molybdenum Disulfide.

The epitaxial synthesis of molybdenum carbide (Mo2C, a 2D MXene material) via chemical conversion of molybdenum disulfide (MoS2) with thermal annealing under CH4 and H2 is reported. The experimental results show that adjusting the thermal annealing period provides a fully converted metallic Mo2C from MoS2 and an atomically sharp metallic/semiconducting hybrid structure via partial conversion of the semiconducting 2D material. Mo2C/MoS2 hybrid junctions display a low contact resistance (1.2 kΩ·µm) and low Schottky barrier height (26 meV), indicating the material's potential utility as a critical hybrid structural building block in future device applications. Density functional theory calculations are used to model the mechanisms by which Mo2C grows and forms a Mo2C/MoS2 hybrid structure. The results show that Mo2C conversion is initiated at the MoS2 edge and undergoes sequential hydrodesulfurization and carbide conversion steps, and an atomically sharp interface with MoS2 forms through epitaxial growth of Mo2C. This work provides the area-controllable synthesis of a manufacturable MXene from a transition metal dichalcogenide material and the formation of a metal/semiconductor junction structure. The present results will be of critical importance for future 2D heterojunction structures and functional device applications.

Nickel Supported on Mesoporous Alumina for Dry Reforming of Methane: Combustion Method.

Ni catalysts supported on ordered mesoporous alumina (OMA) were prepared by EISA method and calcined under air and Ar atmospheres. Both catalysts showed stable performance for the dry reforming of methane for 24 h, however the catalytic activity of Ar calcined catalyst was relatively lower than that in the air calcined one. It was found that the carbon (C ß ) layer around nickel particles was observed for the Ar calcined catalyst after dry reforming of methane. The encapsulating carbon species in the Ar calcined catalyst lowering the mass transfer rate of feeds led to lower performance, but no whisker carbon was observed. In the case of the air calcined catalyst, whisker carbon (C(v)) which is inactive during dry reforming was accumulated on the catalyst, and it resulted in catalyst breakdown and pressure drop during the reaction.

The Effect of Fe in Perovskite Catalysts for Steam CO2 Reforming of Methane.

In this work, La0.95Sr0.05Ni(1-x)Fe(x)O3 catalysts were prepared by modified EDTA-cellulose method and the catalysts were characterized by various techniques such as N2 physisorption, TPR, XRD, SEM, TEM-EDS and TG analysis. La00.95Sr0.05Ni0.5Fe0.5O3 catalyst showed better catalytic performance under the reaction conditions of 900 degrees C, 21 bar and feed molar ratio of CH4:CO2:H20 = 1:0.7:1.5. It is considered that the dilution effect on nickel prevented the formation of large monometallic ensembles that favour the carbon deposition in reforming reactions, and the mean metallic particle size of Ni decreased with increasing substitution rate in B site. Therefore, partial substitution of Fe in B site enhances the dilution effect and induces a reaction between CO2 and La2O3, thereby resisting the carbon deposition and increasing CO2 conversion.

Anesthetic management of a patient with undiagnosed paraganglioma -a case report-.

Retroperitoneal paragangliomas are uncommon neuroendocrine tumors which are derived from extra-adrenal paraganglioma with various clinical signs and symptoms. Although most extra-adrenal paragangliomas are histologically benign, some tumors can synthesize and secrete excess catecholamine from the tumor. Excessive production of catecholamine causes numerous cardiovascular manifestations such as severe hypertension, cardiomyopathy, cardiac arrhythmias, and even multiorgan failure. It can lead to high risks of morbidity and mortality, especially in patients who are unrecognized or not adequately prepared. We present a female patient who was preoperatively undiagnosed of secreting retroperitoneal paraganglioma that caused cardiac tachyarrhythmia and severe intraopertive hypertension not controlled by usual antihypertensive agents. A secreting extra-adrenal paraganglioma should be included in differential diagnosis for patient who have incidentaloma and show wide range of hypertension with hemodynamic instability that is not well controlled by common antihypertensive drugs.