Carbon Sites Research Articles

Enhanced dual atomic Fe-Ni sites in N-doped carbon for bifunctional oxygen electrocatalysis

Atomically dispersed catalysts with high electrocatalytic performance are emerging as promising electrocatalysts for energy conversion and storage devices. Nevertheless, achieving superior bifunctional catalytic activity with single-atom catalysts toward reactions involving multi-intermediates is still facing great challenges. Herein, dual-atomic Fe-Ni pairs dispersed in hierarchical porous nitrogen-doped carbon (FeNi-HPNC) catalysts were successfully synthesized using a facile mechanochemical strategy. By virtue of the engineered electronic structure of Fe coordinated with Ni, the as-synthesized FeNi-HPNC with atomically dispersed dual-metal active sites and pore-rich structure exhibits remarkable bifunctional activities. A high half-wave potential of 0.868 V for oxygen reduction reaction and a low potential of 1.59 V at 10 mA/cm2 for oxygen evolution reaction have been obtained for FeNi-HPNC, which are superior to the single-atom catalysts of Fe-HPNC and Ni-HPNC, respectively, and are even greater than the precious metal catalysts. Combined experimental and theoretical results have revealed that the enhanced bifunctional catalytic activity of FeNi-HPNC is ascribed to the electronic interaction of Fe-Ni sites, which decreases the adsorption energy of oxygen intermediates during oxygen reduction reaction/oxygen evolution reaction. Furthermore, the practical application of FeNi-HPNC catalysts in Zn-air batteries has been demonstrated; a high peak power density and long-term durability are delivered.

On the electrocatalytically active sites in graphene-based vanadium redox flow batteries

There is too much controversy and too little interdisciplinary analysis of the role of carbon electrodes in a wide range of electrochemical and electrocatalytic processes. Here we focus on vanadium redox flow batteries (VRFB), which play a central role in the transition to renewable energy sources in the electricity sector of the global economy. We used density functional theory (DFT) to determine the relationship between chemically and electrocatalytically active sites on the positive electrode and the effects of chemical surface modification that introduces a variety of oxygen functional groups. Carefully selected aromatic (Ar) model clusters were analyzed to assess the extent of the electron density accumulation on and around the free carbon sites at graphene edges. The results reveal trends that help to resolve the controversies surrounding the role of phenolic or carboxyl groups in the redox mechanism. We conclude, in agreement with other chemical and electrocatalytic reactions that involve oxygen transfer (e.g., CO2 gasification or oxygen reduction reaction), that carbene-type edge carbon atoms are responsible for VO2+ adsorption and reduction of V5+ to V4+ in VO2+. The presence of Ar-OH and Ar-COOH groups can actually inhibit the redox process as a consequence of hydrogen and/or oxygen migration to the adjacent active sites. The presence of ArO groups, while affecting the electron density at the active sites, is confirmed to have a positive effect of stabilizing the free zigzag carbon sites in a triplet ground state.

Influence of CO2 on Nanoconfined Water in a Clay Mineral

Developing new technologies for carbon sequestration and long-term carbon storage is important. Clay minerals are interesting in this context as they are low-cost, naturally abundant, can adsorb considerable amounts of CO2, and are present in storage sites for anthropogenic carbon. Here, to better understand the intercalation mechanisms of CO2 in dehydrated and hydrated synthetic Na-fluorohectorite clay, we have combined powder X-ray diffraction, inelastic and quasi-elastic neutron scattering, and density functional theory calculations. For dehydrated Na-fluorohectorite, we observe no crystalline swelling or spectroscopic changes in response to CO2, whereas for the hydrated case, damping of the librational modes related to the intercalated water was clearly observed. These findings suggest the formation of a more disordered water coordination in the interlayer associated with highly confined water molecules. From the simulations, we conclude that intercalated water molecules decrease the layer–layer cohesion energy and create physical space for CO2 intercalation. Furthermore, we confirm that interlayer confinement reduces the Na+ hydration number when compared to that in bulk aqueous water, which may allow for proton transfer and hydroxide formation followed by CO2 adsorption in the form of carbonates. The experimental results are discussed in the context of previous and present observations on, a similar smectite, Ni-fluorohectorite, for which it is established that CO2 attaches to the edge of nickel hydroxide islands present in the interlayer.

Structural characteristics and thermal conversion performance of ash and slag from circulating fluidized bed gasifier

Based on the circulating fluidized bed (CFB), Xinjiang Zhundong coal (ZDC) gasification ash (FA: fly ash; BA: bottom slag) was analyzed by industrial analysis, ultimate analysis and Fourier infrared spectroscopy to determine the basic properties and functional group species. The results show that BA contains up to 99.30% in ash, while FA shows high fixed carbon and C content of 69.30% and 73.78% respectively. Furthermore, the carbonaceous forms and surface morphology of ZDC and FA were characterized by Raman, XRPES and SEM, and the pyrolysis, combustion and gasification characteristics of ZDC and FA were studied with TG-DTG methods. XRPES show that C content of the FA surface is 89.42%, and exists primarily as >C–C< and >C–H, while O was in the form of >C=O. Alkaline earth metals Ca bound to the above-mentioned carbon-involved functional groups cause high disorder in FA. SEM observed that the rough and porous FA surface occurs due to spherical particles of molten mineral attached and embedded surface and pore channels. The thermal conversion characteristics show that the maximum weight loss rate peak temperature of pyrolysis and combustion of FA is significantly higher than that of ZDC, indicating that the pyrolysis and combustion performance of FA is reduced. However, 100% carbon conversion of FA uses about half the time compared with ZDC and the gasification performance has improved significantly since it has well-developed pore structures, more amorphous carbon and abundant active sites, enhancing diffusion of CO2 from gasifiers. Briefly, FA has the potential and ability to be recycled for direct utilization in CFB as gasification feed.

Comparative Adsorption Performance of Carbon-containing Hydroxyapatite Derived Tenggiri (Scomberomorini) and Belida (Chitala) Fish Bone for Methylene Blue

The utilization of fishbone as the carbon source for methylene blue adsorption has been successfully studied. Fishbone was prepared from two kinds of fish such as marine fisheries (ex. Tenggiri) and freshwater fisheries (ex. Belida). The carbons were prepared by carbonation of fishbone powder at 500 °C for 2 h. Physical properties of carbons were characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), wavelength dispersive X-ray fluorescence (WDXRF), Scanning Electron Microscope (SEM), and hydrophobicity. The carbons were utilized as the adsorbent for removing methylene blue by varying the contact time, initial dye concentration, and temperature. It is concluded that both carbons can very good adsorb the methylene blue. The adsorption performance of carbon (TFC) from Tenggiri fish is better than carbon (BFC) from Belida fish. The adsorption was well fitted with the Langmuir adsorption model (R2 ~ 0.998) and the pseudo-second-order model. This indicated that the dye molecules were adsorbed on the surface-active site of carbon via chemical binding, forming an adsorbate monolayer. Thermodynamic parameters, including the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), indicated that the adsorption of methylene blue onto the carbon from fishbone was spontaneous. Thus, carbon from fishbone can be applied as a low-cost adsorbent to treat industrial effluents contaminated with methylene blue. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Identification of ORR activity of random graphene-based systems using the general descriptor and predictive model equation

Carbon based electrocatalysts are well known promising candidates for the oxygen reduction reaction (ORR), but the random approach to find the best catalyst using experimental method delayed the screening process and it leads to a huge cost with less proficiency. Using Quantum Mechanics followed by Machine Learning (QM/ML) approach, we can predict the best catalyst faster way and the origin of the cause can be identified for further development of carbon-based catalyst. Using the π electronic descriptor unveiled using density functional theory, we applied the analytical simple fit method and six different machine learning algorithms to develop a highly effective predictive model to estimate ΔGOH. Furthermore, structural relations of ZGNR and AGNR are demonstrated to estimate the Dπ(EF), R-Oπ, and ΔGOH of different widths of ribbon that reduces additional DFT calculations. By applying both SVR predictive model and structural relations, we predicted the ORR performance of 2500 sites of GNRs and listed a few most ideal active carbon sites with lower overpotential (η < 0.5V). To validate our study, we predicted the ORR performance of different sites in 0D, 1D, 2D doped graphene systems using SVR model and confirmed the values with the DFT computed results.

Efficient reduction of nitrobenzene to aniline by metal-free B-doped graphdiyne

Aniline is an important chemical, which can be obtained by nitrobenzene electrocatalytic reduction (NBER), while searching for efficient electrocatalysts is still a challenge. In this work, the reaction mechanism of B-doped graphdiyne (GDY) with different doped sites for NBER was investigated by density functional theory (DFT) methods. In comparison with pristine GDY, the conductivity of B-doped GDY is increased which is beneficial to the electrocatalytic reaction. Ph-NO2 can be activated by B-doped GDY that the B atom is doped at the acetylenic carbon site. Particularly, B doping at sp-sp2 hybridized acetylenic site of GDY has high catalytic performance for NBER with low limiting potential of − 0.30 V, and the rate-determining step is Ph-NO2* hydrogenation to Ph-NOOH*. This work demonstrates that B-doping is an effective strategy to improve the performance of catalysts and advance the development of metal-free electrocatalysts for Ph-NH2 production.

Theoretical study of H-abstraction and addition reactions of cyclohexene with H, OH and HO2

Reactions with small radicals are of essential significance for alkene oxidation. In this work, the potential energy surfaces and rate constants of cyclohexene plus H, OH, HO2 radicals were thoroughly investigated. The molecular geometries and vibrational frequencies were calculated for all stationary points at BHandHLYP/6-311++G(d,p) level, and then the single point energies were refined at DLPNO-CCSD(T)/CBS level. It contained secondary alkyl, allylic, vinyl H-abstraction and addition channels. The axial and equatorial carbon sites were calculated separately. The reactant and product complexes for allylic H-abstraction and addition channels by OH and HO2 were located with lower energies than the path entrance or exit. In contrast, those complexes for pathways with H-atom had higher energies than reactants or products. The barrierless formation process of reactant complex for cyclohexene plus OH/HO2 was consider to evaluate the influences on rate constants. Rate constants were determined using canonical variational transition state theory in consideration of quantum transmission effects. Notable tunneling effects are observed for H-abstraction reactions with H and HO2 radicals, of which low-temperature rate constants are enhanced, whereas variational and recrossing coefficients of channels with OH deviate far from unity, rendering lower rate constants than those obtained at conventional transition state theory. Comparison of rate constants with similar reactions published in literatures showed that fairly agreements were achieved for H-abstraction and addition channels with H and HO2, but great gaps are noted for those with OH radical. Branching ratio analysis exhibited that addition reactions dominated at T < 1000 K and allylic H-abstraction reactions turned more significant as temperature increased.

Etching-directed nitrogen doping strategy to construct efficient defective sites in carbon for electrocatalytic oxygen reduction

Construction of synergistic coupling sites of intrinsic carbon defects and N dopants in N-doped carbon is of great significance to improve the catalytic activities of metal-free carbon-based materials toward oxygen reduction reaction (ORR). However, the carbon-defect construction and N doping by existing strategies are usually independent processes, resulting in a random arrangement of the intrinsic carbon defects and N-dopants (i.e., hardly forming the coupling sites) in the resultant carbon materials. Herein, an etching-directed nitrogen doping strategy for constructing carbon defect/N dopant coupling sites is developed, in which nitrogen is directionally doped at the freshly created intrinsic carbon defects during the synthesis of N-doped carbon. The as-prepared N-doped carbon catalyst enriched with such coupling sites renders a remarkable ORR catalytic activity with an ORR half-wave potential of 0.891 V vs. RHE and an excellent long-term durability (3% current loss after 15,000 s at 0.6 V vs. RHE). Experimental characterizations and theoretical calculations reveal the formation mechanism of the coupling sites as well as the origin of their high catalytic activities toward ORR. This etching-directed nitrogen doping strategy shows great universality for the controllable construction of active sites in carbon-based materials derived from different precursors for electrochemical energy storage and conversion applications.

An eco-friendly acidic catalyst phosphorus-doped graphitic carbon nitride for efficient conversion of fructose to 5-Hydroxymethylfurfural

An eco-friendly and stable metal-free acidic catalyst phosphorus-doped graphitic carbon nitride (P–UCN) synthesized for the highly efficient dehydration of fructose to 5-Hydroxymethylfurfural (HMF) was presented. The acidic sites (P sites) were introduced into the g-C3N4 structure via a simple thermal polycondensation of urea. A 91.7% HMF yield was achieved with optimal P–UCN catalyst under mild reaction conditions (160 °C, 3 h). XPS and NMR characterizations of P–UCN catalysts suggested that the P atoms may replace the corner and bay carbon sites, which could provide stable acidic sites for fructose dehydration into HMF. NH3-TPD analysis indicated that a moderate amount of P doped would provide higher catalytic activity for the dehydration reaction. Furthermore, the 1.0P–UCN (real content 1.83 wt%) catalyst was demonstrated high stability in the dehydration system and retained high reactivity after being recycled 5 times. The characterization results of recovered 1.0P–UCN further confirmed that the morphology and structure of the catalyst remained well after the dehydration reaction.

Covalent Modification of Nucleobases using Water-Soluble Palladium Catalysts.

Nucleosides represent one of the key building blocks of biochemistry. There is significant interest in the synthesis of nucleoside-derived materials for applications as probes, biochemical models, and pharmaceuticals. Palladium-catalyzed cross-coupling reactions are effective methods for making covalent modification of carbon and nitrogen sites on nucleobases under mild conditions. Water-soluble catalysts derived from palladium and hydrophilic ligands, such as tris(3-sulfonatophenyl)phosphine trisodium (TPPTS), are efficient catalysts for a range of coupling reactions of unprotected halonucleosides. Over the past two decades, these methods have been extended to direct functionalization of halonucleotides, as well as RNA and DNA oligonucleotides (ONs) containing halogenated bases. These methods can be run under biocompatible conditions, including examples of Suzuki coupling of modified DNA in whole cells and tissue samples. In this account, development of this methodology by our group and others is highlighted along with the extension of these catalyst systems to modification of nucleotides and ONs.

The Seasonal Dynamics of Organic and Inorganic Carbon along the Tropical Usumacinta River Basin (Mexico)

Rivers are important sites for carbon (C) transport and critical components of the global C cycle that is currently not well constrained. However, little is known about C species’ longitudinal and temporal changes in large tropical rivers. The Usumacinta River is Mexico’s main lotic system and the tenth largest in North America. Being a tropical river, it has a strong climatic seasonality. This study aims to evaluate how organic (DOC and POC) and inorganic (DIC and PIC) carbon change spatially and seasonally along the Usumacinta River (medium and lower basin) in rainy (RS-2017) and dry (DS-2018) seasons and to estimate C fluxes into the southern Gulf of Mexico. Concentrations of DOC, POC, DIC, and PIC ranged from 0.88 to 7.11 mg L−1, 0.21 to 3.78 mg L−1, 15.59 to 48.27 mg L−1, and 0.05 to 1.51 mg L−1, respectively. DOC was the dominant organic species (DOC/POC &gt; 1). It was ~doubled in RS and showed a longitudinal increase, probably through exchange with wetlands and floodplains. Particulate carbon showed a positive relationship with the total suspended solids, suggesting that in RS, it derived from surface erosion and runoff in the watershed. DIC is reported for the first time as the highest concentration measured in tropical rivers in America. It was higher in the dry season without a longitudinal trend. The C mass inflow–outflow balance in the RS suggested net retention (DOC and POC sink) in floodplains. In contrast, in the DS, the balance suggested that floodplains supply (C source) autochthonous DOC and POC. The lower Usumacinta River basin is a sink for DIC in both seasons. Finally, the estimated annual C export for the Usumacinta-Grijalva River was 2.88 (2.65 to 3.14) Tg yr−1, of which DIC was the largest transported fraction (85%), followed by DOC (10%), POC (4%), and PIC (&lt;1%). This investigation is the first to present the C loads in a Mexican river.

First‐Principles Investigation of Phase Stability in Substoichiometric Zirconium Carbide under High Pressure

AbstractNaCl‐type carbides of the early transition metals can exhibit a substantial sub‐stoichiometry at the carbon site, impacting a host of bulk properties that depend upon carbon concentration including melting points, mechanical and elastic properties, and superconducting transition temperatures. Unfortunately, control over vacancies remains challenging with current preparation methods, motivating the search for new synthetic approaches that will allow for the prescription of specific vacancy configurations. Here, density functional theory is augmented with alloy cluster expansion to examine the structure and zero kelvin enthalpy of millions of structures across composition and pressure space. The results are used to examine how extreme pressures might be used to access novel ordered and disordered phases of ZrC, many of which have been calculated to be thermodynamically stable yet remain synthetically elusive. High pressure is shown to significantly reduce sub‐stoichiometry and drive the system toward fully stoichiometric ZrC. They examine the root of these changes and find that pressure exerts an influence over the distribution and abundance of specific nearest‐neighbor vacancy pairs. These results suggest that pressure is a powerful tool for the control of vacancies, and can offer a new synthetic handle on the bulk properties exhibited in this industrially important class of materials.

Longquan lignite-derived hierarchical porous carbon electrochemical sensor for simultaneous detection of hazardous catechol and hydroquinone in environmental water samples

Catechol (CC) and hydroquinone (HQ) featured with toxicity and wide application are regarded as organic pollutant in wastewater, so the detection of CC and HQ is very important. Besides, direct use of low-rank lignite for combustion is not only wasteful, but also leads to increased greenhouse gas production. Hence, its efficient and clean utilization accords with the process of social development and the concept of carbon neutrality. Based on the above two problems, a method of killing two birds with one stone was proposed to solve the clean utilization of lignite and be used to detect CC and HQ. Herein, a series of porous carbons derived from Longquan lignite with well-developed microporous and mesoporous structure were fabricated by pyrolysis, carbonization, and activation under various activation temperatures (600–900 °C). This combination of microporous and mesoporous structures can increase the surface area (3113 m2/g) and the active site of the porous carbon, improve its electrical conductivity and electron transport efficiency, and make it as a substrate for constructing electrochemical sensor (ECS) with excellent electrocatalytic activity, and successfully be applied in simultaneous detection of CC and HQ. Compared with some reported results, the prepared ECS exhibits satisfactory electrochemical sensing performance with wider linear concentration range and lower detection limits of 0.16 μM for CC and 0.16 μM for HQ, which is comparable to graphene-based sensors. Furthermore, the ECS possesses good stability, reproducibility, selectivity, and anti-interference, and shows practical applicability in environmental water samples.

Highly active nitrogen – doped carbon nanostructures as electrocatalysts for bromine evolution reaction: A combined experimental and DFT study

Electrocatalytic bromine evolution reaction (BER) is significant for bromine production, energy storage, and wastewater treatment applications. Previously reported electrocatalysts for BER either include unstable graphite anodes or precious metal-containing materials. To overcome their disadvantages, this work has proposed, for the first time, the use of nitrogen-doped carbon nanostructures (CNx) as anode catalysts for BER in acidic medium. In terms of BER activity, CNx outperforms both Vulcan Carbon and commercial 10 % Pt/C at bromide concentration as low as 0.025 M. CNx also exhibits high BER selectivity and remarkable stability at highly oxidizing potentials. Tafel slope was measured to be ∼ 120 mV/dec supporting a first electron transfer limiting step for BER on CNx. The high activity and stability of CNx is attributed to several nitrogen-doped carbon sites in its graphitic matrix, which have been examined using DFT. The inclusion of oxygen evolution reaction (OER) intermediates is critical to identify BER active sites accurately, as the most active sites (zigzag pyridinic, zigzag oxide, and pyrrolic oxide) proceed via a OH*-mediated Volmer-Heyrovsky mechanism. For these sites, DFT predicts that the potential determining step is the first electron transfer to form Br* in agreement with the experimental Tafel slope. Comparison to post-reaction XPS shows good agreement between DFT predicted Br* structures. This work thus provides a direction for the rational design of CNx electrocatalysts for BER.

CoN2O2 sites in carbon nanosheets by template-pyrolysis of COFs for CO2RR

Electrocatalytic CO2 reduction (CO2RR) is critical in addressing CO2 emissions. Single atom catalysts, are attracting considerable attention for CO2RR because of their high atom utilization efficiencies and tailored electron states. However, the catalytic sites remain limited and developing novel catalytic centers is a significant and challenge. Herein, for the first time, we fabricated CoN2O2 sites in 2D carbon for use in CO2RR via template pyrolysis of covalent organic frameworks (COFs). After forming the COF on the surface of Mg/Al-LDH, the obtained catalyst displayed a layered morphology with a thickness of approximately 36 nm and had abundant CoN2O2 sites (4.27 wt% Co). The catalyst showed a remarkable catalytic activity towards CO2RR, with Faradaic efficiencies of 80.2–96.5 % at applied potentials between −0.6 and −1.0 V. Theoretical calculations showed that the CoN2O2 sites favor the stretching and cleavage of COOH* at the Co active centers, accelerating the rate-determining step of COOH* formation.

Single-Step Mechanism for Regioselective Nitration of 9,10-BN-Napthalene with Acetyl Nitrate in the Gas Phase.

The energetics of the regioselective mononitration of 9,10-BN-naphthalene with acetyl nitrate (H3C2NO4) were modeled with ab initio simulations in the gas phase and an acetonitrile solvent. The single-electron-transfer (SET) nitration mechanism leading to a σ-complex and a single-step nitration mechanism were modeled. The energy barrier for the single-step mechanism was lower than that for the SET mechanism in the gas phase. However, the two are much more energetically competitive in the solvent. The σ-complex was found to be unstable in the gas phase owing to the interaction with the counterion. Using the single-step mechanism, the carbon site 1 nearest boron had the lowest activation energy for nitration of 22.6 kcal/mol, while site 3 had the second lowest barrier of 24.6 kcal/mol. Details on the molecular structures at intermediate and transition states as well as charges in different configurations are discussed.

F+ center exchange mechanism and magnetocrystalline anisotropy in Ni-doped 3C-SiC

• F + center exchange mechanism has been invoked in order to explain the long range magnetic order in the system. • The Curie temperature is found to be above 350 K for all the doped samples obtained from the modified Bloch’s T 3/2 law. • The dynamic nature of the BMPs sizes have been studied extensively using Hc vs T data. • The spontaneous spin polarization has been induced in the system due to strong hybridization between (C) 2p and (Ni) 3d orbital. • The decrease in magneto crystalline anisotropy constant value with increase in temperature essentially indicates the weakening of the magnetic interaction in the system. • The incomplete quenching of orbital angular momentum and partial removal of orbital degeneracy resulted in reduction of line width and enhancement of integrated intensity as the temperature rises. Towards the development of a magnetic semiconductor suitable for spintronic device applications in extreme environments, we explored the possibility of inducing magnetic interaction in SiC by doping Nickel. The X-ray diffraction and Raman Spectroscopy studies confirm the incorporation of Ni into the host lattice. The magnetic measurements and electron spin resonance studies indicate the presence of room temperature ferromagnetic interaction in the system. The Curie temperature of 1, 3, and 5% Ni-doped samples have been found to be 420 K, 520 K, and 540 K respectively. Electron spin resonance study reveals that the valence state of Ni is 2 + , which implies the creation of vacancies at both Silicon (V Si ) and Carbon (V C ) sites as they are tetravalent. The change in magnetization of the system with an increase in dopant concentration is consistent with the variation in the number of vacancies and free holes. The analysis of magnetization data using the Law of approach to saturation shows that the anisotropic constant decreases with an increase in temperature. The long-range magnetic interaction in the system is explained using the F + center exchange mechanism.

Synthesis of nitrogen-rich porous carbon nanotubes coated Co nanomaterials as efficient ORR electrocatalysts via MOFs as precursor

Transition metal-based nitrogen-doped carbon materials have been proved to be a promising low-cost electrocatalyst for oxygen reduction reaction (ORR), which is a critical step in systems such as fuel batteries and zinc-air batteries. In this paper, transition metal (cobalt) N-doped carbon nanomaterials derived from metal-organic framework (MOFs) were synthesized by calcination of ZIF-67 precursors at high temperature (Co@NCNTs). Nitrogen atoms were introduced into the carbon matrix by capturing volatile CNx from dicyandiamide, and porous carbon nanotube structures with N-doped and uniformly dispersed active sites were obtained. The effects of different carbonization temperatures and molar ratio of Co/2-MI on the catalytic activity of materials for ORR reaction were studied. It was found that the Co@NCNTs prepared at 700 ℃ with the ratio of cobalt metal to dimethylimidazole of 1: 8 shows excellent ORR activity in both acidic and alkaline electrolytes. In alkaline conditions, the half-wave potential E 1/2 = 0.81 V, initial potential E onset = 0.89 V, and limiting current density even reaching 7.4 mA cm −2 . And in acidic electrolytes, E 1/2 = 0.66 V, while the limiting current density is 6.4 mA cm −2 , which is still better than that of Pt/C. Its excellent ORR performance is due to the synergistic effect of the porous carbon nanotube structure, N-doped carbon and Co-Nx active sites. Meanwhile, the optimum catalyst also shows better stability and methanol resistance than Pt/C in both acidic and alkaline electrolytes. • N-doped porous carbon nanotubes coated Co material was used to enhance ORR activity in alkaline and acidic conditions. • Nanotube structure provides sufficient effective contact area and active sites. • Porous structure is conducive to mass diffusion and reduces electron transmission resistance. • N-doped carbon tube, Co-Nx and pyridine nitrogen jointly promote ORR activity.

Oscillation and Stepwise of Hydrocarbon Melting Temperatures as a Marker of their Cluster Structure

The presence of melting temperatures oscillatory and stepwise changes for hydrocarbons four homologous series is demonstrated and analyzed. The oscillating dependence is manifesteds on the principle of «even-odd» molecules with different deviations from linearity. According to the working hypothesis, this is due to the presence of the matter smallest structural unit in the cluster form of with a certain coordination number. The oscillation of melting temperatures in hydrocarbons series is explained by the fact that clustering can occur both at the site of the final carbon in the molecule and at other carbons in the molecule chain, and this fact depends on the «even-odd» effect. Based on the known values of melting temperatures in homologous series, the clusters probable structure is assumed. It is shown that graphs for the calculated values of equivalent lengths of these clusters correlate with corresponding graphs for hydrocarbons melting temperatures. An approximation formula has been developed to predict melting temperatures of hydrocarbons based on the values of the equivalent length and the cluster molecular weight, which operates with an approximation coefficient of 0.997 and a mean deviation of 4.2 K.