Graphitic carbon nitride, g-C3N4, can be made by polymerization of cyanamide, dicyandiamide or melamine. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities are obtained. The firstly formed polymeric C3N4 structure, melon, with pendant amino groups, is a highly ordered polymer. Further reaction leads to more condensed and less defective C3N4 species, based on tri-s-triazine (C6N7) units as elementary building blocks. High resolution transmission electron microscopy proves the extended two-dimensional character of the condensation motif. Due to the polymerization-type synthesis from a liquid precursor, a variety of material nanostructures such as nanoparticles or mesoporous powders can be accessed. Those nanostructures also allow fine tuning of properties, the ability for intercalation, as well as the possibility to give surface-rich materials for heterogeneous reactions. Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions, such as for the activation of benzene, trimerization reactions, and also the activation of carbon dioxide. Model calculations are presented to explain this unusual case of heterogeneous, metal-free catalysis. Carbon nitride can also act as a heterogeneous reactant, and a new family of metal nitride nanostructures can be accessed from the corresponding oxides.

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Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts

Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3 This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction

Spinels with the formula of AB2O4 (where A and B are metal ions) and the properties of magnetism, optics, electricity, and catalysis have taken significant roles in applications of data storage, biotechnology, electronics, laser, sensor, conversion reaction, and energy storage/conversion, which largely depend on their precise structures and compositions. In this review, various spinels with controlled preparations and their applications in oxygen reduction/evolution reaction (ORR/OER) and beyond are summarized. First, the composition and structure of spinels are introduced. Then, recent advances in the preparation of spinels with solid-, solution-, and vapor-phase methods are summarized, and new methods are particularly highlighted. The physicochemical characteristics of spinels such as their compositions, structures, morphologies, defects, and substrates have been rationally regulated through various approaches. This regulation can yield spinels with improved ORR/OER catalytic activities, which can furth...

Spinels: Controlled Preparation, Oxygen Reduction/Evolution Reaction Application, and Beyond

Ionic conduction in space charge regions

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

The standard Gibbs energy of formation of LaMnO3 has been measured in the temperature range from 900 to 1400 K using a solid-state cell incorporating a buffer electrode. The oxygen potential corresponding to the three-phase mixture MnO + La2O3
+ LaMnO3 has been measured using pure oxygen gas at 0.1 MPa as the reference electrode, and yttria-stabilized zirconia as the solid electrolyte. The buffer electrode prevented polarization of the three-phase electrode and ensured accurate data. The measured oxygen chemical potential corresponding to the decomposition of LaMnO3−δ to MnO, La2O3 and O2 gas is in fair agreement (±6 kJ mol−1) with most data available in the literature and can be represented by the equation,ΔμO2/J mol−1
(±520)
=
−583160 + 170.55 (T/K)Combining this with available information on oxygen potential variation with the non-stoichiometric parameter δ, the standard Gibbs free energy of formation of LaMnO3 from MnO, La2O3 and O2 was evaluated. The correction for non-stoichiometry was ∼1.6(±0.7) kJ mol−1. The heat capacity of LaMnO3 has been measured by a differential scanning calorimeter from 400 to 1060 K. Information on low-temperature heat capacity available in the literature was joined smoothly with the high-temperature data to evaluate the standard entropy of LaMnO3 at 298.15 K as 116.68 J mol−1 K−1. The “third-law” analysis of the Gibbs energy values obtained in this study yields a value of −155.93 (±0.8) kJ mol−1 for the standard enthalpy of formation of LaMnO3 from MnO, La2O3 and O2 at 298.15 K. This value provides a critical test of calorimetric data on enthalpy of formation of LaMnO3. A consistent set of thermodynamic data for LaMnO3 is presented from 298.15 to 1400 K based on the results obtained in this study and other supporting information in the literature.

http://repository.ias.ac.in/12905/1/342.pdf

Refinement of thermodynamic data for LaMnO3

The Gibbs' energy change for the reaction, 3CoO (r.s.)+1/2O2(g)→Co3O4(sp), has been measured between 730 and 1250 K using a solid state galvanic cell: Pt, CuO+Cu2O|(CaO)ZrO2|CoO+Co3O4,Pt. The emf of this cell varies nonlinearly with temperature between 1075 and 1150 K, indicating a second or higher order phase transition in Co3O4around 1120 (±20) K, associated with an entropy change of ∼43 Jmol-1K-1. The phase transition is accompanied by an anomalous increase in lattice parameter and electrical conductivity. The cubic spinel structure is retained during the transition, which is caused by the change in CO+3 ions from low spin to high spin state. The octahedral site preference energy of CO+3 ion in the high spin state has been evaluated as -24.8 kJ mol-1. This is more positive than the value for CO+2 ion (-32.9 kJ mol-1). The cation distribution therefore changes from normal to inverse side during the phase transition. The transformation is unique, coupling spin unpairing in CO+3 ion with cation rearrangement on the spinel lattice, DTA in pure oxygen revealed a small peak corresponding to the transition, which could be differentiated from the large peak due to decomposition. TGA showed that the stoichiometry of oxide is not significantly altered during the transition. The Gibbs' energy of formation of Co3O4 from CoO and O2 below and above phase transition can be represented by the equations:ΔG0=-205,685+170.79T(±200) J mol-1(730-1080 K)
and ΔG0=-157,235+127.53T(±200) J mol-1(1150-1250 K).

Thermodynamics of Cobalt (II, III) Oxide (Co3O4): Evidence of Phase Transition

The activity of Cr20~ in Cr20~-A12Oa solid solution has been determined in the temperature range 800~176 from electromotive force measurements on the solid oxide galvanic cell Pt,Cr + Cr2OJY~O~-ThO2/Cr + Cr~A12-xO~,Pt
The activities of Cr203 and A120~ in the solid solution show both positive and negative deviations from Raoult's law. The heat and entropy of mixing of the solid Solution obtained from the temperature dependence of the emf can be
expressed as AH = XCr203XA1203 [31,700Xcrzo3 -}- 37,470XA1203] J mole -I hS = -- 1.8R [Xcr2o3 In Xcr2o3 + XA12o3 In XAaos]The entropy of mixing is 10% lower than that predicted by the Temkin model.The large positive heat of mixing in the Cr2Os-A12Oa solid solution, however,
suggests that this apparent: entropy discrepancy originates with the clustering of positive ions on the cation sublattice. The asymmetric miscibility gap exhibited
in the CrzOa-A12Oa system below 900~ is consistent with the thermodynamic data trends recorded at the more elevated temperatures.

https://iopscience.iop.org/article/10.1149/1.2131408/pdf

Electrochemical Determination of Activities in Cr2 O 3 ‐ Al2 O 3 Solid Solution

Thermodynamic properties of cuprous and cupric yttrates (CuYO 2 and Cu 2 Y 2 O 5 ) and oxygen potentials corresponding to three three-phase fields in the system Cu-Y-O have been determined by using solid-state galvanic cells; Pt, Cu + CuYO 2 + Y 2 O 3 ||(Y 2 O 3 )ZrO 2 ||Cu + Cu 2 O, Pt; Pt, CuYO 2 + Cu 2 Y 2 O 5 ||(Y 2 O 3 )ZrO 2 ||Cu 2 O + CuO, Pt; and Pt, CuYO 2 + Cu 2 Y 2 O 5 + Y 2 O 3 ||(Y 2 O 3 )ZrO 2 ||Cu 2 O + CuO, Pt. Yttria-stabilized zirconia was used as the solid electrolyte in the temperature range 873-1323 K. The compound CuYO 2 was prepared by the reduction of Cu 2 Y 2 O 5 at 1373 K under argon gas with a residual oxygen partial pressure of ~Pa. For the reaction ½ Cu 2 O(s) + ½ Y 2 O 3 (s) → CuYO 2 (s), Δ G° = -5800 + 3.90T (± 30) J mol -1 , and for 2CuO(s) + Y 2 O 3 (s) → Cu 2 Y 2 O 5 (s), Δ G° = 11 210 - 15.072T (± 120) J mol -1 . The oxygen potentials corresponding to the coexistence of phases CuYO 2 + Cu 2 Y 2 O 5 and CuYO 2 + Cu 2 Y 2 O 5 + Y 2 O 3 were found to be the same over the temperature range of measurement, thus indicating negligible solid solubility of Y 2 O 3 in CuYO 2 and Cu 2 Y 2 O 5 . On the basis of the present results and auxiliary thermodynamic data from the literature, phase relations in the Cu-Y-0 system at 723, 950, and 1373 K have been deduced.

Gibbs energies of formation of cuprous and cupric yttrates (CuYO2 and Cu2Y2O5) and phase relations in the system copper-yttrium-oxygen

The stability fields of various sulfide phases that form on Fe-Cr, Fe-Ni, Ni-Cr, and Fe-Cr-Ni alloys have been developed as a function of temperature and the partial pressure of sulfur. The calculated stability fields in the ternary A-B-S system are displayed on plots of log \textpS2 pS2 vs. the conjugate extensive variable (nA/nA–nB), which provides a better framework for following the sulfidation of Fe-Cr-Ni alloys at high temperatures. Experimental and estimated thermodynamic data were used in developing the sulfur potential diagrams. Current models and correlations were employed to estimate the unknown thermodynamic behavior of solid solutions of sulfides and to supplement the incomplete phase-diagram data of geophysical literature. These constructed stability field diagrams are in excellent agreement with the sulfide phases and compositions determined experimentally during the sulfidation of SAE 310 stainless steel. The sulfur potential plots appear to be very useful in predicting and correlating the sulfidation of commercial alloys.

K. T. Jacob

Papers

Refinement of thermodynamic data for LaMnO3

Thermodynamics of Cobalt (II, III) Oxide (Co3O4): Evidence of Phase Transition

Electrochemical Determination of Activities in Cr2 O 3 ‐ Al2 O 3 Solid Solution

Gibbs energies of formation of cuprous and cupric yttrates (CuYO2 and Cu2Y2O5) and phase relations in the system copper-yttrium-oxygen

Phase relations in the Fe-Ni-Cr-S system and the sulfidation of an austenitic stainless steel