Preparation of Si3N4 porous ceramics via foam-gelcasting and microwave-nitridation method

Si₃N₄ porous ceramics with improved mechanical strength were fabricated for the first time by a combined foam-gelcasting and microwave-nitridation method at 1273–1373?K. The Si₃N₄ porous samples prepared at 1373?K/20?min with the porosity of 68.9% had respectively flexural and compressive strength as high as 8.1 and 20.8?MPa, which values were comparable or even superior to those of Si₃N₄ porous ceramics prepared previously by the conventional heating technique at a much higher temperature of 1773–1973?K, indicating that present preparation strategy is feasible to prepare high quality Si₃N₄ porous ceramic at a much milder condition. Moreover, the thermal conductivity of as-prepared Si₃N₄ porous ceramics at 1073?K was as low as 1.697?W/(m?K), suggesting it could be a potentially good heat insulating material for aluminum electrolyte cells.     

Improving CH4/N2 selectivity within isomeric Al-based MOFs for the highly selective capture of coal-mine methane

Selectively separating CH₄ from N₂ in coal-mine methane is significantly important in the chemical industry, but challenging and energy-intensive. Using porous materials as adsorbents can separate CH₄/N₂ mixtures with low energy consumption, but most adsorbents encounter the problem of poor separation selectivity. Here, we propose a strategy for improving CH₄/N₂ selectivity by controlling pore wall environment in two isomeric Al-based metal–organic frameworks (MOFs) with four highly symmetric polar sites for strengthened adsorption affinity toward CH₄ over N₂. At 298 K and 100 kPa, CAU-21-BPDC with four highly symmetric polar sites in the pore walls exhibits 2.4 times higher CH₄/N₂ selectivity than CAU-8-BPDC without four highly symmetric polar sites. Gas adsorption isotherms, CH₄/N₂ selectivity calculations, Q_st of CH₄, interaction energy calculations, adsorption density distributions of CH₄ and N₂, and breakthrough curves reveal that CAU-21-BPDC is a potential candidate for selective capture coal-mine methane.     

How Does Crack Bridging Change at Cryogenic Temperatures?

The crack bridging behaviors of silicon nitride (Si₃N₄) ceramics based on the advancing cracks have been evaluated at cryogenic temperatures in contrast with those happened at ambient temperatures. R‐Curve behavior of Si₃N₄ ceramics was determined to compare the cumulative effect of bridging at 77 and 293 K. The detailed changes in crack morphologies and crack opening displacement profiles at different temperatures were investigated. The bridging stress maps around a bridging ligament were recorded by in situ Raman spectroscopy. It was found that the highest tensile stress measured at 77 K (~1.0 GPa) is much higher than that at 293 K (~0.7 GPa). The experimental results suggested that Si₃N₄ ceramics at cryogenic temperatures possessed a higher probability of crack bridging with a higher closing force, which could make them a promising candidate for cryogenic applications.     

Synthesis,characterization, and magnetic properties of α-MnS nanocrystals

MnS nanocrystals have been prepared by a colloidal synthesis route through the reaction of MnCl₂ and S[Si(CH₃)₃]₂ in trioctylphosphineoxide. The nanocrystals were characterized using X-ray diffraction and transmission electron microscopy. The magnetic properties were studied with SQUID magnetometry. X-ray diffraction shows that the nanocrystals are of the thermodynamically stable α-MnS (alabandite) structure. Size control was achieved by changing the concentration of the precursors. Nanocrystal sizes were measured by transmission electron microscopy, and three samples of average diameters 20, 40, and 80 nm were obtained, with narrow size distribution (σ˜9%). The zero field cooled magnetization curves for the 80-, 40-, and 20-nm samples showed a cusp at 116 K, 97 K, and 50 K respectively, all smaller than the antiferromagnetic transition temperature, T_N = 130 K, of bulk α-MnS. Below T_N the magnetization exhibits a paramagnetic behavior unlike typical antiferromagnetic materials. These results indicate that there is a mixture of paramagnetic and antiferromagnetic phases in the nanocrystals. The size dependence shows a general trend of decrease of T_N with reduced particle size, indicating size dependent magnetic ordering.     

Oxidative conversion of thiourea and N-substituted thioureas into formamidine disulfides with acidified chloramine-T: a kinetic and mechanistic approach

The kinetics of oxidation of thiourea and N-substituted thioureas namely: N-methylthiourea, N-allylthiourea N-phenylthiourea and N-tolylthiourea to the corresponding formamidine disulfides by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in the presence of HClO₄ has been investigated at 278 K. The reactions follow identical kinetics for all thioureas, being first order each with respect to [CAT]_o, [Thiourea]_o and [H⁺]. Ionic strength of the medium and addition of p-toluenesulfonamide or halide ions have negligible influence on the rate. The solvent isotope effect has been studied using D₂O in the case of the oxidation of thiourea. Decrease in the dielectric constant of the medium by adding methanol decreases the rate. The reactions were studied at different temperatures, and the composite activation parameters have been computed. An isokinetic relationship was observed with β=314 K, indicating that enthalpy factors control the reaction rate. Under comparable experimental conditions, the rate of oxidation of thioureas increases in the order: N-allylthiourea>N-phenylthiourea>N-methylthiourea>thiourea>N-tolylthiourea. A mechanism involving the interaction of conjugate acid (CH₃C₆H₄SO₂NHCl) and substrate giving an intermediate complex, in a slow step, has been suggested. The derived rate law is in agreement with the observed kinetics.     

Synthesis, structural characterization and ethylene polymerization behavior of complex [Ph4P][CrCl3{HB(pz)3}] [HB(pz)3 = hydrotris(1-pyrazolyl)borate]

Reaction of CrCl₃(THF)₃ with K[HB(pz)₃] in THF leads to the formation of the complex K[CrCl₃{HB(pz)₃}] (1). The salt metathesis of complex 1 with [Ph₄P]Br in CH₂Cl₂ yields the complex [Ph₄P][CrCl₃{HB(pz)₃}](2). The structure of complex 2 · CHCl₃ has been determined by single crystal X-ray diffraction. In the anion the metal centre shows a distorted octahedral geometry with the hydrotris(1-pyrazolyl)borate bonded as N,N′,N″-donor tripod ligand and three chloride atoms completing the co-ordination sphere. Complex 2 in the presence of MAO leads to the formation of an active catalyst for the polymerization of ethylene.     

Carbon-supported hafnium oxynitride as cathode catalyst for polymer electrolyte membrane fuel cells

Highly stable carbon-supported hafnium oxynitride (HfO_xN_y-C) was synthesized by heating carbon-supported hafnium oxide, prepared using an impregnation method, under NH₃ gas in various conditions. X-ray diffraction patterns, X-ray photoelectron spectra, and field-emission transmission electron microscope images confirmed that HfO_xN_y nanoparticles were dispersed onto commercial carbon black, Vulcan XC-72. The stability of HfO_xN_y-C in 0.1 mol dm⁻³ H₂SO₄ at 303 K was evaluated by measuring the mass ratio of dissolved hafnium to immersed HfO_xN_y-C using inductively coupled plasma atomic emission spectroscopy. It saturated at a low level of 0.8–4.0 mg g⁻¹ with increasing immersion time up to ∼24 h. The oxygen reduction reaction (ORR) activity and rate were evaluated by obtaining cyclic voltammograms and rotating disk electrode voltammograms, respectively. The HfO_xN_y-C exhibited higher ORR activity and a lower Tafel slope than NH₃-treated C under identical conditions, demonstrating that HfO_xN_y is active toward ORR. The ORR activity most depended on the heating temperature. The ORR rate increased with increasing the heating time at 1223 K which could be due to the increased y in HfO_xN_y-C. The maximum onset potential for ORR was 0.78 V vs. standard hydrogen electrode, which is 0.18 V lower than that of carbon-supported platinum.     

Catalytic reduction of N2O by H2 over well-characterized Pt surfaces

A 0.65% Pt/SiO₂ catalyst has been prepared using an ion exchange technique and extensively characterized prior to being used for continuous catalytic N₂O reduction by H₂ at very low temperatures, such as 363 K. The supported Pt with a high dispersion of 92% gave no presence of O atoms remaining on an H-covered Pt _s , based on in situ DRIFTS spectra of CO adsorbed on Pt _s after either N₂O decomposition at 363 K or subsequent exposure to H₂ for more than 1 h; thus the residual uptake gravimetrically observed even after the hydrogen titration on an O-covered surface is associated with H₂O produced by introducing H₂ at 363 K onto the oxidized Pt _s . Dissociative N₂O adsorption at 363 K on Pt _s was not inhibited by the H₂O_(ad) on the silica surface but not on Pt _s , as acquired by IR peaks at 3,437 and 1,641 cm⁻¹, in very consistent with the same hydrogen coverage, established via H₂-N₂O titration on a reduced Pt _s , as that revealed upon the titration reaction with a fully wet surface on which all bands and their position in IR spectra for CO are very similar to that obtained after H2 titration on a reduced Pt _s . Based on the characterization using chemisorption and in situ DRIFTS and TPD measurements, the complete loss in the rate of N₂O decomposition at 363 K after a certain on-stream hour, depending significantly on N₂O concentrations used, is due to self-poisoning by the strong chemisorption of O atoms on Pt _s , while the presence of H₂ as a reductant could readily catalyze continuous N₂O reduction at 363 K that is a greatly lower temperature than that reported earlier in the literature.     

Cu3P and Ni2P co-modified g-C3N4 nanosheet with excellent photocatalytic H2 evolution activities

Copper-nickel phosphides/ graphite-like phase carbon nitride (Cu₃P-Ni₂P/g-C₃N₄) composites were obtained through a facile one-pot in situ solvothermal approach. The coexistence of Cu₃P and Ni₂P plays an important role in enhancing the catalytic activity of g-C₃N₄. The 7 wt% Cu₃P-Ni₂P/g-C₃N₄ bimetallic phosphide photocatalyst demonstrates the best photocatalytic hydrogen (H₂) evolution rate of 6529.8 μmol g⁻¹ h⁻¹, which is 80.7-fold higher than that of g-C₃N₄. The apparent quantum yield (AQE) was determined to be 18.5% at 400 nm over the 7% Cu₃P-Ni₂P/g-C₃N₄. This in situ growth strategy produced intimate contact interfaces, leading to a significantly promoted separation of charge carriers, and hence strengthened the photocatalytic H₂ production. Moreover, the coexistence of Cu₃P and Ni₂P reduced the overpotential of H₂ during the evolution process, further benefiting H₂ production. Finally, the photocatalytic enhancement mechanism was proposed and verified by fluorescence and electrochemical analysis. This work provides a low-cost strategy to synthesize nonprecious bimetallic phosphides/carbon nitride photocatalyst with outstanding H₂ production activity. © 2020 Society of Chemical Industry     

A new approach to quantify the microporosity of activated carbons by analysing the N2/77 K and CO2/273 K adsorption data by the simplex flexible method

A mathematical model has been applied to N₂/77 K and CO₂/273 K adsorption isotherms for a series of activated carbons prepared by carbonising olive stones in N₂ and then activating them in CO₂ to six different levels of burn-off in the range 8–80%. Narrow and wide micropore volumes of activated carbons were calculated from the Dubinin-Radushkevich and Dubinin-Astakhov equations considering one, two and three micropore size distributions in each sample, and allowing a variation of the micropore volume and characteristic energy of each distribution with the burn-off. The flexible simplex method was applied to obtain the parameters of each distribution in the mathematical model. Generally, it was found that increasing the number of micropore size distributions above two did not significantly improve fits. Each isotherm was fitted using six parameters at most. However, various constraints were imposed, and the parameters were estimated from each isotherm using non-linear, least-squares regression analysis. The results obtained confirm the valuable use of CO₂/273 K adsorption to quantify the narrow microporosity of activated carbons. Differences between N₂/77 K and CO₂/273 K adsorption in microporous activated carbons were due to the wide microporosity. An agreement between micropore volumes obtained from CO₂/273 K adsorption and that corresponding to one of the two distributions of micropores obtained from N₂/77 K adsorption was obtained. The Dubinin-Radushkevich equation was more successful than the Dubinin-Astakhov equation in the quantification of the microporosity with N₂/77 K and CO₂/273 K. On the other hand, the exponent n of the Dubinin-Astakhov equation was better correlated with the burn-off of the carbons than with the parameter B.     

Ion-Induced Nucleation in Nanoparticle Synthesis by Ionization Chemical Vapor Deposition

Particle formation mechanisms in nanoparticle synthesis by ionization chemical vapor deposition (CVD), in which source gases are ionized before being fed into the reactor, were investigated experimentally. First, nonagglomerated nanoparticles were formed from a tetraethylorthosilicate (TEOS)/O ₂ mixture ionized with a sonic-jet ionizer at various furnace temperatures, T _f, and TEOS concentrations, N _TEOS. Then, the electrical mobility of the nanoparticles and ions were measured using a differential mobility analyzer combined with a Faraday cup electrometer, and the particle number concentration by a condensation nucleus counter. Measurements at 293 ≤ T _f ≤ 1173 K and 0 ≤ N _TEOS ≤ 1.38 × 10 ^?5 mol/l showed that (a) negative TEOS and TEOS cluster ions were formed at T _f = 293 K, (b) gas-phase intermediates forming nanoparticles were formed at T _f ≥ 673 K, (c) for T _f = 873 K condensation of the intermediates onto ions (i.e., ion-induced nucleation) started at N _TEOS ≥ 4.48 × 10 ^?7 mol/l, and (d) the growth from ions to nanoparticles was discontinuous for this change in N _TEOS.     

Structure and properties of N-octadecylated poly(p-phenylene terephthalamide)

N-octadecylated PPTA's with various molecular weights were synthesized from PPTA and n-octadecyl bromide via the metalation reaction. The polymer exhibited side-chain crystallization at 314 K due to the long alkyl side chains and both crystallinity and the perfection of the crystal increase with increasing in the molecular weight. The results of IR spectra and the wide angle X-ray indicate that the polymer crystallizes in hexagonal form. The mechanical and the dynamic mechanical properties of N-octadecylated PPTA's were measured. Two tan δ peaks were observed in a temperature range of 310–350 K and at 260 K, respectively, which can be designated α_c and α_a relaxation, respectively. The concentrated solutions of N-octadecylated PPTA's in tetrahydrofuran, dichloroethane, tetrachloroethane, and bromoform showed liquid crystalline behavior of lyotropic type.     

Efficient separation of N2 and he at low temperature using MFI membranes

Ultra‐thin MFI membranes were evaluated for N₂/He separation over the temperature range of 85–260 K for the first time. The membranes were rather nitrogen selective at all the conditions investigated. A highest N₂/He selectivity of 75.7 with a high N₂ flux of 83 kg/m²/h was observed at 124 K. The separation was attributed to adsorption selectivity to N₂, effectively hindering the transport of He in the zeolite pores. The exceedingly high permeance even at low temperatures was ascribed to the ultrathin (<1μm) membrane used. As the pressure ratios increased, a better separation performance was obtained. A mathematical model showed the largest difference of adsorbed loading over the film at ca. 120 K was the main reason for the observed maximum selectivity. Further, the modelling indicated the selectivity would increase 2–3 times by reducing the influence of defects, concentration polarization, and pressure drop over the support. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2833–2842, 2016     

Supercritical fluid extraction of fungal oil using CO2, N2O,CHF3 and SF6

The extraction of oil from fungi (Mortierella ramanniana var.angulispora) was studied using carbon dioxide (CO₂), nitrous oxide (N₂O), trifluoromethane (CHF₃) and sulfur hexafluoride (SF₆) under supercritical conditions. The oil solubility was highest in SC-N₂O followed by SC-CO₂, while both SC-CHF₃ and SC-SF₆ showed poorer solvent power. The recorded oil solubilities at 333 K and 24.5 MPa were 2.3 wt% in N₂O, 0.48 wt% in CO₂, 0.0099 wt% in CHF₃ and 0.0012 wt% in SF₆. The oil solubilities in SC-N₂O and SC-CO₂ were measured over the pressure range 15.7–29.4 MPa and at temperatures ranging from 313–353 K. N₂O always showed greater solvent power than did CO₂ at the same temperature and pressure. The solvent power of a supercritical fluid increases with density at a given temperature, and increases with temperature at constant density. The change in neutral lipid composition of the extracted oil with the extraction ratio was measured. Free fatty acids or diglycerides were extracted more easily than triglycerides or sterol esters. The change in fatty acid composition was also measured. The proportion of γ-linolenic acid in the extract remained constant throughout the extraction.     

Structural and piezoelectric properties of textured NLKNS-CZ thick films and their application in planar piezoactuator

0.97(Na_0.5-xLi_xK_0.5)(Nb_0.89Sb_0.11)O₃-0.03CaZrO₃ [(N_0.5-xL_xK)(NS)-CZ] piezoceramic (x = 0.325) has a pseudocubic-tetragonal-orthorhombic (PC-T-O) multi-structure. The PC structure formed in this piezoceramic was identified as the R3m rhombohedral structure. This piezoceramic showed the large piezoelectric charge constant (d₃₃) of 515 pC/N due to the PC-T-O multi-structure. The NaNbO₃ (NN) templates were used to texture the (N_0.5-xL_xK)(NS)-CZ thick films along the (001) direction, and the textured thick film (x = 0.0375) had a large Lotgering factor of 95.6%. The PC-T-O multi-structure was observed in this thick film (x = 0.0375), but the thick film (x = 0.0325) showed a PC-O structure owing to the diffusion of the NN templates into the thick film. The textured thick film (x = 0.0375) exhibited an increased d₃₃ of 625 pC/N because of the PC-T-O multi-structure and the lineup of grains along the [001] direction. A textured thick film (x = 0.0375) was used to fabricate a planar-type actuator to confirm its applicability to electrical devices. This actuator exhibits large acceleration (580.3 G) and displacement (150 μm) at a low electric field of 0.2 kV/mm with a short response time of 3.0 ms. Therefore, the (N_0.5-xL_xK)(NS)-CZ thick films are excellent lead-free piezoceramics.     

An expedient synthesis of methyl vinyl sulfide from dimethyl disulfide and acetylene

Dimethyl disulfide reacts with acetylene under atmospheric pressure in basic reductive systems MOH (M=Na, K)–N₂H₄?·?H₂O–N-methylpyrrolidone and Na₂S?·?9H₂O–N₂H₄?·?H₂O–N-methylpyrrolidone to form methyl vinyl sulfide in up to 79% yield.     

Enantioselective Synthesis of Chiral Tetrahydroisoquinolines by Iridium‐Catalyzed Asymmetric Hydrogenation of Enamines

Chiral iridium complexes based on spiro phosphoramidite ligands are demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of unfunctionalized enamines with an exocyclic double bond. In combination with excess iodine or potassium iodide and under hydrogen pressure, the complex Ir/(S_a,R,R)‐ 3a provides chiral N‐alkyltetrahydroisoquinolines in high yields with up to 98% ee. The L/Ir ratio of 2:1 is crucial for obtaining a high reaction rate and enantioselectivity. A deuterium labeling experiment showed that an inverse isotope effect exists in this reaction. A possible catalytic cycle including an iridium(III) species bearing two monophosphoramidite ligands is also proposed.     

Study on reduction reaction of MoO2 powder with NH3

Reaction mechanism between MoO₂ powder and NH₃ in the temperature range of 1028‐1273 K has been investigated. The results show that reduction products are strongly dependent on the reaction temperature employed. It is found that molybdenum nitride (Mo₂N) can be successfully synthesized when the temperature is below 1078 K, with the morphologies keep the same as that of the raw material MoO₂. Although the temperature is above 1078 K, metal Mo powder will be turned up; at the even higher temperatures, the thermodynamically stable phase will be metal Mo. In addition, MoO_xN_1?x as an intermediate product is formed during the reduction processes.     

Synthesis and Properties of a Cationic Gemini Asphalt Emulsifier

A cationic Gemini surfactant with a benzene ring (abbreviated as C₁₄‐CGB) was synthesized in 2 steps with aniline, epichlorohydrin, and N,N‐dimethyltetradecylamine as starting materials. This product was characterized using mass spectroscopy and nuclear magnetic resonance (¹H NMR). The critical micelle concentration (CMC) and surface tension (γ_cmc) of C₁₄‐CGB were measured from 298 to 313 K and thermodynamic parameters of micellization were calculated. The results showed that the CMC and γ_cmc were 1.269 × 10^?3 mol L^?1 and 38.33 mN m^?1 at 298 K, respectively. Moreover, upon increasing the temperature, the CMC increases, γ_cmc decreases, the maximum surface adsorption capacity (Γ_max) decreases, and the minimum molecular area (A_min) increases. The emulsified asphalt test showed that C₁₄‐CGB is a slow‐breaking asphalt emulsifier exhibiting excellent emulsifying ability.