Carbon Sites Research Articles

Structural features of residue carbon formed by gasification of different coal macerals

Gasification fine slag contains a high content of carbon, which causes waste of resources and environmental hazards. In essence, fine slag is a comprehensive reflection of the separate gasification of various macerals in coal. In this study, different macerals concentrates were firstly prepared from Yangchangwan (YCW) and Meihuajing coal (MHJ). The structure and pyrolysis characteristics of different macerals as well as the morphology and molecular structure of gasification residue carbon at different conversion levels were then investigated by a series of characterization techniques. The results show that the aromatic degree of inertinite concentrates of both coals is greater than that of vitrinite. During the active pyrolysis stage of 200–600 °C, the weight loss rate of vitrinite is higher than that of inertinite, and the weight loss rate reaches maximum at about 445 °C. Maximum thermal weight loss rate of YCW and MHJ vitrinite is 1.39 and 1.43 times higher than that of inertinite. The shrinkage ability of inertinite char is stronger than that of vitrinite and the surface of inertinite residue carbon is rougher and rich in porous structures during gasification, resulting in a higher final gasification carbon conversion level. The fresh vitrinite char obtained from pyrolysis has lower order degree and more active sites. As the gasification proceeds, the content of active sites in vitrinite gasification residue carbon decreases rapidly, while inertinite remains relatively stable. This research innovatively links the properties of macerals with the structure of residue carbon, which is helpful to deepen the understanding of the correlation between coal microstructure and reactivity.

Secondary organic aerosol formation from photooxidation of γ-butyro and γ-valero-lactone: A combined experimental and theoretical study

The formation of secondary organic aerosol (SOA) generated by OH-photooxidation of γ-butyrolactone (GBL) and γ-valerolactone (GVL) in the presence of OH, NO, H2O vapour and aerosol seeds ((NH4)2SO4 and CaCl2) has been investigated for the first time. Experiments were conducted in a smog chamber at 298 K and atmospheric pressure. Lactone's decay was followed by gas chromatography with a mass spectrometric detector (GC-MS), and the temporal evolution of the SOA was monitored using a fast mobility particle sizer (FMPS). The SOA yield increases slightly when the initial GVL concentration increases. On the other hand, SOA formation is minor for GBL, and it rises slightly at low concentrations. In the case of GVL, the organic aerosol formation can be tentatively expressed by a one-product gas/particle partitioning absorption model. The particle number concentration, mass and yield decrease in the presence of NO but increase at higher relative humidity (RH) and seed surface area within the studied range. Acetic acid and succinic anhydride for GVL, and succinic anhydride for GBL were experimentally identified as oxidation products both in gas and particle phase using thermal desorption followed by GC-MS. Moreover, the theoretical studies carried out to complement the experimental results indicate that H-abstraction at the carbon site adjacent to the heterocyclic oxygen is the predominant pathway, being even more favourable when H2O was added. Also, under atmospheric conditions, the opening of the ring of the corresponding oxy-radicals and their subsequent fragmentation could be the possible explanation for the formation of detected products and the higher yield of SOA in the presence of H2O and absence of NO. Considering these results, the formation of SOA from the use of both lactones as potential biofuels is few significative, although SOA generation may be enhancement by particle-phase heterogeneous reactions catalyzed by humidity and the acidity of seed aerosols present in real atmosphere.

Carbon driven oxygen retention in uranium oxycarbide (UCO) fluorite phase

The behavior of oxygen in uranium oxycarbide (UCO) has been investigated using density functional theory. To assess the role of carbon on the stability of oxygen point defects, we first determined the formation energies of oxygen vacancy and interstitial for different carbon configurations. Subsequently, we evaluated the barriers for migration of various oxygen defects in the UCO matrix. Our results show that carbon atoms create strong attraction centers for oxygen atoms, as a consequence the spontaneous formation of oxygen Frenkel pairs in the fluorite structure is promoted. Moreover, the strong affinity of oxygen atoms for sites in the first coordination shell of a carbon center leads to the formation of CO, and thus reducing the mobility of oxygen atoms in the UCO fuel. Upon formation, the CO remains fixed at the carbon site due to significant hybridization with uranium atoms, therefore behaving as trap sites for oxygen atoms. Our findings indicate that the formation of CO molecules increases the retention of oxygen atoms inside the UCO matrix. Consequently, carbon atoms in the UCO matrix contribute to mitigating the deleterious effects that oxygen release from the fuel kernel has on the structural integrity of the silicon carbide layer in tristructural isotropic (TRISO) particles.

Coupling of N-Doped Mesoporous Carbon and N-Ti3 C2 in 2D Sandwiched Heterostructure for Enhanced Oxygen Electroreduction.

2D heterostructures provide a competitive platform to tailor electrical property through control of layer structure and constituents. However, despite the diverse integration of 2D materials and their application flexibility, tailoring synergistic interlayer interactions between 2D materials that form electronically coupled heterostructures remains a grand challenge. Here, the rational design and optimized synthesis of electronically coupled N-doped mesoporous defective carbon and nitrogen modified titanium carbide (Ti3 C2 ) in a 2D sandwiched heterostructure, is reported. First, a F127-polydopamine single-micelle-directed interfacial assembly strategy guarantees the construction of two surrounding mesoporous N-doped carbon monolayers assembled on both sides of Ti3 C2 nanosheets. Second, the followed ammonia post-treatment successfully introduces N elements into Ti3 C2 structure and more defective sites in N-doped mesoporous carbon. Finally, the oxygen reduction reaction (ORR) and theoretical calculation prove the synergistic coupled electronic effect between N-Ti3 C2 and defective N-doped carbon active sites in the 2D sandwiched heterostructure. Compared with the control 2D samples (0.87-0.88V, 4.90-5.15 mA cm-2 ), the coupled 2D heterostructure possesses the best onset potential of 0.90V and limited density current of 5.50 mA cm-2 . Meanwhile, this catalyst exhibits superior methanol tolerance and cyclic durability. This design philosophy opens up a new thought for tailoring synergistic interlayer interactions between 2D materials.

Triphasic Metal Oxide Photocatalyst for Reaction Site‐Specific Production of Hydrogen Peroxide from Oxygen Reduction and Water Oxidation

AbstractThe search for photocatalysts allowing the highly active, selective, and stable conversion of molecular oxygen into hydrogen peroxide is of worldwide interest. Here, the authors report the efficient conversion of O2 into H2O2 with ≈100% selectivity and stable cycle stability by a triphasic metal oxide photocatalyst with a cobalt hydroxide carbonate nanosheet phase for water oxidation as well as iron oxide and titanium oxide phases of a core‐shell morphology for charge transfer and oxygen reduction, denoted as CFT. The different surface energies of 0.78 (anatase) and 0.93 J m‐2 (rutile) for titanium oxide and 1.39 J m‐2 for iron oxide result in a core‐shell morphology. The band gaps for iron oxide (2.02 eV), titanium oxide (≈3 eV), and cobalt hydroxide carbonate (3.80 eV) sites reveal that the CFT photocatalyst allows visible‐to‐UV light absorption. The 18O2 isotope‐labeling experiments prove that the core‐shell structure promotes hole transfer toward the water oxidation site. Additionally, the hole‐induced H2O2 decomposition at the oxygen reduction site is efficiently hindered. Moreover, the photogenerated electrons transfer toward the oxygen reduction site to produce H2O2 from O2 with ≈10‐fold higher activity than those by conventional single‐ or dual‐phase photocatalysts, while giving robust cycle stability.

Formation of sulfur oxide groups by SO2 and their roles in mercury adsorption on carbon-based materials

The presence of SO2 display significant effect on the mercury (Hg) adsorption ability of carbon-based sorbent. Yet the adsorption and oxidation of SO2 on carbon with oxygen group, as well as the roles of different sulfur oxide groups in Hg adsorption have heretofore been unclear. The formation of sulfur oxide groups by SO2 and their effects on Hg adsorption on carbon was detailed examined by the density functional theory. The results show that SO2 can be oxidized into SO3 by oxygen group on carbon surface. Both C-SO2 and C-SO3 can improve Hg adsorption on carbon site, while the promotive effect of C-SO2 is stronger than C-SO3. Electron density difference analyses reveal that sulfur oxide groups enhance the charge transfer ability of surface unsaturated carbon atom, thereby improving Hg adsorption. The experimental results confirm that surface active groups formed by SO2 adsorption is more active for Hg adsorption than the groups generated by SO3.

Graphene quantum dot inlaid carbon nanofibers: Revealing the edge activity for ultrahigh rate pseudocapacitive energy storage

The edge sites of carbon show high electrochemical activity because of the unique electronic structure and functionalization. But revealing their energy storage ability and the underlying mechanism are still challenging. Herein, we prepare a novel edge-enriched carbon nanofiber fabric by exposing the embedded graphene quantum dots (GQDs) through partial hydrolysis of the electrospun GQDs/polyacrylonitrile fabric. The edges of inlaid GQDs are mainly terminated by various oxygen-bearing functional groups even after high temperature treatment, simultaneously contributing to abundant surface redox active sites and good electrolyte wettability. Further facilitated by the entire conductive networks, the edge sites are proved to show an ultrafast pseudocapacitive performance (200 F g−1 at 1 A g−1, 130 F g−1 at 100 A g−1) because of the robust electrochemical kinetics. Moreover, at an ultra-high mass loading of 25.5 mg cm−2 (thickness: 1.4 mm), the multilayer-folded fabric performs a remarkable areal capacitance (3.95 F cm−2 at 25.5 mA cm−2), outstanding rate performance (2.04 F cm−2 at 510 mA cm−2), and good cycle stability (98% capacitance retention after 10,000 times), demonstrating the great potential of edge structure design for high power devices.

Optimal Eco-Compensation for Forest-Based Carbon Sequestration Programs: A Case Study of Larch Carbon Sink Plantations in Gansu, Northwest China

This study investigated the potential financial benefits that private forest famers can derive when participating in a larch carbon sink plantation project in the northwestern Chinese province of Gansu. A decision matrix was developed to help forest farmers justify participation in forest carbon sink projects relative to the traditional land-use goal of timber production under various carbon trading prices, site conditions, and contract terms. The results showed that when the carbon trading price is at the theoretically optimal carbon price (CNY 110/tCO2e, equivalent to USD 17/tCO2e), Chinese business entities are willing to pay for forest carbon credits under the current global carbon emissions level, and forest farmers who participate in a 25-year forest carbon sink plantation project on high-productivity sites would generate the greatest financial benefit compared with the net income from pure timber production forests. Thus, the government does not need to provide a carbon sink subsidy for participating tree farmers. However, at the current average carbon trading price (CNY 19.8/tCO2e or USD 3/tCO2e) in the domestic market, a minimum additional subsidy of CNY 735/ha (USD 113/ha) is required upfront to motivate forest farmers to convert timber production forests into carbon sink forests. The results of this study can help policymakers and forest managers formulate optimal eco-compensation strategies for enrollment in forest-based carbon sequestration programs.

Theoretical Investigation on H-Abstraction Reactions of Silanes with H and CH3 Attacking: A Comparative Study with Alkane Counterparts.

Silicon-based organic precursors are widely applied in the vapor-fed flame synthesis of monocrystalline silicon, silicon dioxide, and silicon nitride. Due to the lack of kinetic investigations on reactions of silicon-based organic precursors, rate constants were usually analogized to those of their hydrocarbon counterparts. Investigations on the similarities and differences between the two types of compounds become necessary. This work reports a comparative theoretical investigation on H-abstraction reactions with H and CH3 attacking for silanes and their alkane counterparts, including silane and methane, disilane, methylsilane and ethane, dimethylsilane and propane, trimethylsilane and iso-butane, and tetramethylsilane and neo-pentane at the domain-based local pair natural orbital coupled cluster with perturbative triple excitations (DLPNO-CCSD(T))/cc-pVTZ//M06–2X/cc-pVTZ level. The rate constants were calculated using the conventional transition-state theory coupled with the asymmetric Eckart tunneling corrections over 600–2000 K. The calculated results show that dramatic discrepancies exist between H-abstraction from silicon sites in silanes and equivalent carbon sites in their alkane counterparts with H and CH3 attacking. The H-abstraction reactions from the primary carbon sites in silanes have generally lower barrier energies than the similar reactions in their alkane counterparts, while those in methylsilane and dimethylsilane with H attacking are the only two with higher barrier energies. Electrostatic potential mapped molecular van der Waals surfaces were adopted to provide insight into the calculated trends in barrier energies. The H-abstraction reactions from silicon sites in silanes have much higher rate constants than those from equivalent carbon sites in their alkane counterparts, especially under low-temperature conditions, while the rate constants of H-abstraction reactions from primary carbon sites in silanes and their alkane counterparts show relatively strong analogy.

Soil phosphorus status and P nutrition strategies of European beech forests on carbonate compared to silicate parent material

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochemistry,https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENIP) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-Porg precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition.

Domain‐Confined Etching Strategy to Regulate Defective Sites in Carbon for High‐Efficiency Electrocatalytic Oxygen Reduction

AbstractCoupling sites of nitrogen dopants and intrinsic carbon defects (N/DC) in metal‐free carbon are highly active toward electrocatalytic oxygen reduction reaction (ORR), and yet they are hard to be effectively constructed. Here, a novel synthesis strategy for fabrication of N/DC‐enriched metal‐free carbon from precursors containing metal‐nitrogen moieties (M‐Nx) is reported. During pyrolysis of the precursors, the M‐Nx is in situ converted to endogenous metal oxides (MOx) via a halide‐mediated “bait and switch” mechanism to realize a domain‐confined etching of the carbon matrix around the nitrogen dopants, forming abundant N/DC coupling sites in the final carbon products. The optimized N/DC‐enriched carbon exhibits an outstanding ORR activity and stability with a half‐wave potential of 0.903 V (versus RHE) in 0.1 m KOH solution, outperforming most of the reported metal‐free carbon catalysts. Experimental investigations and theoretical calculations demonstrate an improved electron‐donating ability and optimized binding energies of the N/DC coupling sites toward the ORR intermediates, which are primarily responsible for such an excellent ORR activity. This domain‐confined endogenous MOx etching strategy is also widely applicable to different M‐Nx‐containing precursors as well as different halide mediators, which opens up new possibilities to fabricate metal/nonmetal doped carbon materials enriched with various coupling sites of desired synergistic properties.

Active Cu0–Cuσ+ Sites for the Hydrogenation of Carbon–Oxygen Bonds over Cu/CeO2 Catalysts

CeO2-supported copper species have been reported as an active catalyst for the hydrogenation of carbon–oxygen bonds (CO, CO2, furfural, esters, etc.). However, the identification of active sites remains challenging. Herein, we prepared a series of rod-shaped ceria-supported copper catalysts with different copper sizes (single-atom, 1.4 nm nanoclusters, 3.0 nm and 6.8 nm nanoparticles) and applied them for methyl acetate (MA) hydrogenation. The structure and chemical environment of copper species were detected, and the surface Cu0 and Cuσ+ species and defects (oxygen vacancy and M–[Ox]–Ce solid solution) were quantitatively measured. To identify the active sites for MA hydrogenation, we also prepared contrast samples with increased surface defects or with reduced Cu0–Cuσ+ species. It is demonstrated that the Cu0–Cuσ+ species rather than oxygen vacancies or M–[Ox]–Ce solid solution are the primary active sites for MA hydrogenation. From the results of in situ experiments and various chemisorption and density functional theory calculations, the Cu0–Cuσ+ interface located at the surface Cu deposits is evidenced to play the key role in enhancing the adsorption and activation of MA. The turnover frequency of Cu/CeO2 catalysts for MA hydrogenation is linearly increased with the increase of the Cu0–Cuσ+ interfacial perimeter. This insight into active sites for carbon–oxygen bond hydrogenation may provide guidance for high-performance catalyst design.

Engineering Mn atomic sites in multi-dimensional nitrogen-doped carbon for highly efficient oxygen reduction reaction.

Nitrogen-coordinated single-atom manganese in multi-dimensional nitrogen-doped carbon electrocatalysts (Mn-NC) were successfully constructed by combining two-dimensional nanosheets and one-dimensional nanofibers. The Mn-NC exhibited excellent oxygen reduction reaction catalytic activity with a half-wave potential of 0.88 V, which is higher than the 0.85 V of Pt/C.

Boosting the oxygen reduction reaction behaviour of atomic Fe–N4active sites in porous honeycomb-like carbonviaP heteroatom doping

Fe single atoms immobilised on N- and P-codoped porous honeycomb-like carbon (Fe–N4/NP-PHC) with abundant accessible active sites produce impressive electrocatalytic performance towards oxygen reduction reaction.

Site-dependent nuclear dynamics in core-excited butadiene.

Symmetry breaking and competition between electronic decay and nuclear dynamics are major factors determining whether the memory of the initial core-hole localisation in a molecule is retained long enough to affect fragmentation. We investigate the fate of core holes localised at different sites in the free 1,3 trans butadiene molecule by using synchrotron radiation to selectively excite core electrons from different C 1s sites to π* orbitals. Fragmentation involving bonds localised at the site of the core hole provides clear evidence for preferential bond breaking for a core hole located at the terminal carbon site, while the signature of localisation is weak for a vacancy on the central carbon site. The origin of this difference is attributed to out-of-plane vibrations, and statistical evaporation of protons for vacancies located at the central carbon sites.

The adsorption behaviors of N2O on penta-graphene and Ni-doped penta-graphene.

In order to develop the adsorption application of penta-graphene (PG) to N2O gas molecule, we calculated the sensing properties of PG and Ni-doped PG to N2O molecule via first-principles calculations. Based on the calculated adsorption energy, charge transfer, band gap, density of states and partial density of states, we observed that this gas molecule was weakly physically adsorbed on the surface of intrinsic PG, while the adsorption behaviors on the surface of Ni-doped PG were greatly influenced by the doping sites and adsorption orientations. With the Ni atom doped at the sp2 hybridized carbon site, strong chemical adsorption between the gas molecule and the substrate was induced. The adsorption structure of the N2O molecule with its N atom close to the substrate exhibited better stability. Moreover, an external perpendicular electric field could enhance the adsorption performance of the N2O molecule and adjust the charge transfer between the molecule and substrate. Our results broaden the adsorption applications of PG and indicate that Ni-doped PG is a potential candidate for N2O gas sensors.

Electronic properties and surface reactive sites of carbon and oxygen doped porous boron nitride: A DFT study

Metal-free doping is an effective way to modify the electronic band structure and regulate the physical and chemical properties of porous boron nitride (p-BN). Herein, the first principles calculation has been employed on C and O doped p-BN, and their electronic properties and stability have been discussed, respectively. In additionally, p-BN with C replaced B and O replaced N defects (named CB1ON4-p-BN) is proposed for the powerful adsorption ability of carbon monoxide (CO) with adsorption energy of 4.20 eV, which have been demonstrated to be a chemical reaction with partial electronic density of state (PDOS) analysis and the amount of charge transferring (0.190 |e|). The Highest Occupied Molecular Orbital (HOMO) of p-BN suggests the intrinsic reactivity of p-BN for most cases involving defective C sites or the atoms around the defects. The fabricated CB1ON4-p-BN has verified thermodynamically stability by the vibration frequencies, cohesive energy, and integrated crystal orbital Hamilton population (ICOHP). Our results exhibit that the gases sensitivity of p-BN can be promoted by the C and O co-doped strategies, and also will open a new way of designing high-performance CO adsorbents.

Fine structure in electronic transitions attributed to nitrogen donor in silicon carbide

Nitrogen in group-IV semiconductors has become a well-established element of qubits capable of room-temperature operation. In silicon carbide, nitrogen can occupy different nonequivalent lattice sites, giving rise to different shallow donor states. We report a triplet fine structure in electronic transitions of nitrogen donors on the quasi-cubic carbon site in 4H silicon carbide with activation enthalpies of around 100 meV. The intensities of triplet components have a prominent dependence on the voltage bias. The activation enthalpies of the transitions exhibit the Poole–Frenkel effect, while no bias dependence is observed for the magnitude of splitting. A tentative explanation of the fine structure involves local symmetry changes due to stacking faults.

Modulation of graphene and graphdiyne by metaln (n = 1–5) adsorption and nucleation and the effect on hydrogen evolution reaction

Comprehending the interaction of metal clusters with two-dimensional (2D) carbon materials at the atomic level is important for the rational design of relative materials for various applications. The geometrical and electronic modulation of graphene and graphdiyne (GDY) by the adsorption and nucleation of Cun, Irn, and Pdn (n = 1–5) and the effect on hydrogen evolution reaction (HER) were studied employing dispersion-corrected density functional theory methods. The bonding strength between the metal Mn (n = 1–5) clusters and the considered 2D materials generally followed the order of Irn-substrate > Pdn-substrate > Cun-substrate. The modified electronic properties induced by the metal decoration affected the H adsorption. According to the Sabatier’s principle, the Mn@graphene present better HER catalytic properties than the pristine graphene, while the Mn@GDY depends. Based on the calculated ΔG0(H) and H diffusion barrier, H can easily diffuse from the C site of graphene to the metal site and show better HER activities. The reverse H diffusion from the metal site to the carbon site of GDY for GDY supported Pd1, Pd5, and Ir5 could happen at room temperature, thereby improving the HER activity at pH = 0. A series of linear relationships are found, revealing the possible structure–activity relations.

Biodiesel soot combustion: Analysis of the soot-catalyst contact in different experimental conditions

To simulate the oxidation of Biodiesel soot in a catalyzed Diesel Particulate Filter, oxidation experiments of undoped or Na-doped model soot were performed in the presence of Pt-Pd/Al2O3 under Temperature Programmed Oxidation and different carbon-catalyst configurations. Loose or tight contact was simulated by mixing soot and catalyst either with a spatula or in a mortar. Samples were characterized by Flame Atomic Adsorption Spectroscopy, Inductively Coupled Plasma-Optical Emission Spectroscopy, N2-physisorption, X-ray Photoelectron Spectroscopy, Raman Spectroscopy and Transmission Electron Microscopy coupled with Energy Dispersive X-Ray. The results showed that reducing the carbon granulometry led to an enhancement of the reactivity under NO2, due to the external combustion mechanism of carbon crystallite. In the absence of catalysts, no impact of the granulometry was observed under a flue gas containing NO and O2. A better catalyst efficiency was obtained when the carbon-catalyst contact was increased. In tight contact, a mechanism where NO2 can react adsorbed on a metallic site with a carbon site was proposed. In the presence of water, a simple addition of the catalytic activity of both water and Pt-Pd/Al2O3 catalyst impact on carbon oxidation was observed. The use of Biodiesel, simulated by Na-doped model soot, led to a significant increase in the catalyst efficiency in loose contact. Thus, a second mechanism was proposed, assuming that the doped sites play the role of NO2-reservoir and carrier between catalyst and carbon sites.