Mechanism Of Carbon Formation Research Articles

Gas–solid carbonation as a possible source of carbonates in cold planetary environments

Carbonates are abundant sedimentary minerals at the surface and sub-surface of the Earth and they have been proposed as tracers of liquid water in extraterrestrial environments. Their formation mechanism is since generally associated with aqueous alteration processes. Recently, carbonate minerals have been discovered on Mars' surface by different orbitals or rover missions. In particular, the phoenix mission has measured from 1% to 5% of calcium carbonate (calcite type) within the soil (Smith et al., 2009). These occurrences have been reported in area where the relative humidity is significantly high (Boynton et al., 2009). The small concentration of carbonates suggests an alternative process on mineral grain surfaces (as suggested by Shaheen et al., 2010) than carbonation in aqueous conditions. Such an observation could rather point toward a possible formation mechanism by dust–gas reaction under current Martian conditions. To understand the mechanism of carbonate formation under conditions relevant to current Martian atmosphere and surface, we designed an experimental setup consisting of an infrared microscope coupled to a cryogenic reaction cell (IR-CryoCell setup). Three different mineral precursors of carbonates (Ca and Mg hydroxides, and a hydrated Ca silicate formed from Ca2SiO4), low temperature (from −10 to +30°C), and reduced CO2 pressure (from 100 to 2000mbar) were utilized to investigate the mechanism of gas–solid carbonation at mineral surfaces. These mineral materials are crucial precursors to form Ca and Mg carbonates in humid environments (0%<relative humidity<100%) at dust–CO2 or dust–water ice–CO2 interfaces. Our results reveal a significant and fast carbonation process for Ca hydroxide and hydrated Ca silicate. Conversely, only a moderate carbonation is observed for the Mg hydroxide. These results suggest that gas–solid carbonation process or carbonate formation at the dust–water ice–CO2 interfaces could be a currently active Mars' surface process. To the best of our knowledge, we report for the first time that calcium carbonate can be formed at a negative temperature (−10°C) via gas–solid carbonation of Ca hydroxide. We note that the carbonation process at low temperature (<0°C) described in the present study could also have important implications on the dust–water ice–CO2 interactions in cold terrestrial environments (e.g. Antarctic).

Biomarkers from individual carbonate phases of an Oligocene cold-seep deposit, Washington State, USA

An Oligocene cold-seep limestone (Lincoln Creek Formation, Washington State, USA) was studied for its lipid biomarker inventory. Biomarker analysis on minute amounts of sample (tens of mg) and complementary 13Ccarbonate analyses allowed us to link biogeochemical processes with individual, closely intertwined carbonate phases. The ancient seep deposit exhibits four major carbonate phases, according to the paragenetic sequence of (I) micrite, (II) yellow aragonite, (III) clear aragonite and (IV) equant calcite spar. For the micrite, varying but significant amounts of archaea-derived isoprenoids clearly indicate that the precipitation of this phase was induced by the microbial anaerobic oxidation of methane (AOM). However, water column-derived lipids present in this carbonate phase reflect the incorporation of organic matter from background sediment cemented by authigenic micrite. Yellow aragonite made up only a minor rock component (<10% vol.), but contained a major portion of lipid biomarkers indicative of AOM. Along with low δ13Ccarbonate values (less than −30‰ Pee Dee Belemnite), this points to an intimate spatial association of AOM consortia with the precipitation of yellow aragonite. Clear aragonite showed similar δ13Ccarbonate values but much lower, if any, contents of AOM biomarkers. This suggests that AOM-derived carbonate ions diffused over a greater distance to the site of precipitation compared with yellow aragonite. The latest phase, equant calcite spar, did not yield appreciable biomarkers, but showed a notable 13Ccarbonate-enrichment that is most likely caused by methanogenesis that prevailed in the sediments after AOM activity had ceased. A comparison of the ancient seep carbonates with modern counterparts from Hydrate Ridge (offshore Oregon, USA) revealed a remarkable coincidence of the respective mineral phases and their biomarker patterns. This suggests that the mechanisms of carbonate formation and the associated biogeochemical processes remained unchanged over geological times. □Biomarkers, carbonates, cold seeps, Hydrate Ridge, isotopes, Oligocene, Washington.

Cyclohexene Dehydrogenation to Produce Benzene on nAu/Pt(100) and nPt/Au(100) (n = 0, 1, 2) Surfaces From a First-Principles Study

Bimetallic alloys such as Au/Pt are known as an efficient catalyst for promoting cyclohexene dehydrogenation to benzene. In this work, we try to understand the unique high reactivity of the Au/Pt surface by the first-principles density functional theory (DFT) calculations. Our DFT results and microkinetic model analysis show that the model modified by carbon and hydrogen was significantly improved in the explanation of the experimental results, and the barriers of rate-determining step (rds) are 1.37, 1.08, and 1.05 eV on Pt(100), 2Pt/Au(100), and Au/Pt (100), respectively. This is in general agreement with the order of the benzene formation rate: Pt(100) < 2Pt/Au(100) < Au/Pt(100). Additionally, the possible reaction mechanism of carbon formation on the Pt surface has been gained in this work for the first time, that is, C6H6 → C6H5 → C6H4 → C6H3 → C + C5H3, and the rate-determining step is the first dehydrogenation step with the energy barrier of ca. 1.80 eV.

A density functional theory investigation on the mechanism and kinetics of dimethyl carbonate formation on Cu2O catalyst

A theoretical analysis about the mechanism and kinetics of dimethyl carbonate (DMC) formation via oxidative carbonylation of methanol on Cu(2)O catalyst is explored using periodic density functional calculations, both in gas phase and in solvent. The effect of solvent is taken into account using the conductor-like screening model. The calculated results show that CO insertion to methoxide species to produce monomethyl carbonate species is the rate-determining step, the corresponding activation barrier is 161.9 kJ mol(-1). Then, monomethyl carbonate species reacts with additional methoxide to form DMC with an activation barrier of 98.8 kJ mol(-1), above reaction pathway mainly contributes to the formation of DMC. CO insertion to dimethoxide species to form DMC is also considered and analyzed, the corresponding activation barrier is 308.5 kJ mol(-1), suggesting that CO insertion to dimethoxide species is not competitive in dynamics in comparison with CO insertion to methoxide species. The solvent effects on CO insertion to methoxide species involving the activation barriers suggest that the rate-determining step can be significantly affected by the solvent, 70.2 kJ mol(-1) in methanol and 63.9 kJ mol(-1) in water, which means that solvent effect can reduce the activation barrier of CO insertion to methoxide species and make the reaction of CO insertion to methoxide in solvents much easier than that in gas phase. Above calculated results can provide good theoretical guidance for the mechanism and kinetics of DMC formation and suggest that solvent effect can well improve the performance of DMC formation on Cu(2)O catalyst in a liquid-phase slurry.

Black perspectives for a green future: hydrothermal carbons for environment protection and energy storage

This perspective review paper provides an overview on recently developed carbon material technology synthesised from the hydrothermal carbonisation (HTC) approach, with a particular focus on the carbon formation mechanism, perspectives on large scale production, nanostructuring, functionalisation and applications. Perceptions on how this technology will be developed especially with regard to application fields where the use of HTC-derived materials could be extended will also be introduced and discussed.

Electronic Band Structure of Coiled Carbon Nanotubes

More than fifteen years have passed since the first report of experimental evidence of regularly coiled carbon nanotubes, but, the structure, formation mechanism and theoretical aspects of these nanotubes still remain unresolved. We propose model of hexagonal, helically coiled single wall carbon nanotubes, determine their line group symmetry and calculate electronic band structure of the relaxed configurations by means of fully symmetry adopted density functional tight binding method implemented into the POLSym code. Electrical properties of the straight and coiled carbon nanotubes of dierent chiralities are compared and analyzed. PACS: 61.48.De, 73.22.‐f, 81.07.‐b

Aerobic biomineralization of Mg-rich carbonates: Implications for natural environments

We studied the formation of Mg-rich carbonate in culture experiments using different aerobic bacterial strains and aqueous Mg/Ca ratios (2 to 11.5) at Earth surface conditions. These bacteria promoted the formation of microenvironments that facilitate the precipitation of mineral phases (dolomite, huntite, high Mg-calcite and hydromagnesite) that were undersaturated in the bulk solution or kinetically inhibited. Dolomite, huntite, high Mg-calcite, hydromagnesite and struvite precipitated in different proportions and at different times, depending on the composition of the medium. The Mg content of dolomite and calcite decreased with an increasing Ca concentration in the medium. The stable carbon isotope composition of the Mg-rich carbonate precipitates reflected the isotope composition of the organic compounds present in the media, suggesting that microbial metabolism strongly influenced the carbon isotope composition of biomediated carbonates. We observed that Ca-enriched carbonate precipitates have relatively low carbon isotope composition. These results provide insights into the mechanism(s) of carbonate formation in natural systems, and they are of fundamental importance for understanding modern environments in which carbonate minerals form as a window into the geologic past.

Carbonate Formation from CO2 via Oxo versus Oxalate Pathway: Theoretical Investigations into the Mechanism of Uranium-Mediated Carbonate Formation

We report theoretical investigations of the reaction of [((MeArO)3mes)U] with CO2 in order to support previously reported experimental data. Experimentally, the reaction in toluene leads to the imm...

Gas flux and carbonate occurrence at a shallow seep of thermogenic natural gas

The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ∼200 m2 area known as Brian Seep, located in 10 m water depth. A single comprehensive survey of gas flux at Brian Seep yielded a methane release rate of ∼450 moles of CH4 per day, originating from 68 persistent gas vents and 23 intermittent vents, with gas flux among persistent vents displaying a log normal frequency distribution. A subsequent series of 33 repeat surveys conducted over a period of 6 months tracked eight persistent vents, and revealed substantial temporal variability in gas venting, with flux from each individual vent varying by more than a factor of 4. During wintertime surveys sediment was largely absent from the site, and carbonate concretions were exposed at the seafloor. The presence of the carbonates was unexpected, as the thermogenic seep gas contains 6.7% CO2, which should act to dissolve carbonates. The average δ13C of the carbonates was −29.2 ± 2.8‰ VPDB, compared to a range of −1.0 to +7.8‰ for CO2 in the seep gas, indicating that CO2 from the seep gas is quantitatively not as important as 13C-depleted bicarbonate derived from methane oxidation. Methane, with a δ13C of approximately −43‰, is oxidized and the resulting inorganic carbon precipitates as high-magnesium calcite and other carbonate minerals. This finding is supported by 13C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ13C ranged from −84 to −25‰). The inconsistency in δ13C between the carbonates and the seeping CO2 was resolved by discovering pockets of gas trapped near the base of the sediment column with δ13C-CO2 values ranging from −26.9 to −11.6‰. A mechanism of carbonate formation is proposed in which carbonates form near the sediment–bedrock interface during times of sufficient sediment coverage, in which anaerobic oxidation of methane is favored. Precipitation occurs at a sufficient distance from active venting for the molecular and isotopic composition of seep gas to be masked by the generation of carbonate alkalinity from anaerobic methane oxidation.

Hydrothermal preparation of carbon microspheres from mono-saccharides and phenolic compounds

Isolated carbon microspheres in a diameter range of 1–4μm were prepared by hydrothermal treatment of mono-saccharides (xylose for pentose and fructose for hexose) with phenolic compounds–phenol, resorcinol, and phloroglucinol. It was found that addition of these compounds, particularly phloroglucinol, into a sugar solution led to a substantial increase (roughly 20-fold) of the carbon yield. Mono-saccharides are dehydrated into furan compounds during hydrothermal carbonization, and these compounds subsequently react with phloroglucinol, as revealed by examination of high-performance liquid chromatograms. Through elemental analysis and spectroscopic studies, it was found that several oxygen functionalities including hydroxyl, carbonyl, carboxyl, ether, quinone, and ester groups were located throughout the carbon spheres derived from sugars and phloroglucinol. On the basis of the carbon formation mechanism, the C–O–C bond formed by linkage of OH groups of phenolic compounds preferentially exists at the core of carbon spheres whereas the outer carbon surface contains reactive oxygen functionalities such as hydroxyl, carbonyl, carboxylic, and ester groups, corresponding with results obtained with sugar-derived carbon materials.

Insights into the mechanism of carbonate formation through reductive cleavage of carbon dioxide with low-valent uranium centers

The low-valent U(III) complexes [((t-BuArO)3mes)U] and [((AdArO)3N)U] react with CO2 to form the bridging carbonate complexes [{((t-BuArO)3mes)U}2(mu-kappa2:kappa2-CO3)] and [{((AdArO)3N)U}2(mu-eta1:kappa2-CO3)]. Uranium(IV) bridging oxo complexes have been determined to be the intermediate in these transformations.

Carbon dioxide reforming of methane under periodic operation

The carbon dioxide reforming of methane under periodic operation over a commercial Ni/SiO2·MgO catalyst was investigated at two different temperatures, 923 and 1,023 K. According to this operation, pure methane and carbon dioxide were alternately fed to the catalyst bed where methane cracking and the reverse Boudouard reaction took place, respectively. Therefore, hydrogen and carbon monoxide products appeared separately in different product streams. The performance of this operation was compared to that of the steady state operation with simultaneous feed of both carbon dioxide and methane. At 1,023 K, the methane conversion and hydrogen yield from the periodic operation initially decreased with time on stream and eventually leveled off at values about half of those obtained in the steady state operation with co-feed of both reactants. The decreased catalytic activity was due to the accumulation of carbonaceous deposit and loss of metal active sites. However, a different trend was observed at 923 K. The methane conversion and hydrogen yield were almost constant over the time on stream, although more carbonaceous deposit was progressively accumulated on the catalyst bed during the reaction course. At this temperature, the periodic operation offered the equivalent hydrogen yield to the steady state operation. The observed behavior could be due to the different mechanisms of carbon formation over the catalyst. Finally, it was found that cycle period and cycle split did not influence the reaction performance within the ranges of this study.

Carbon deposition on Ni/YSZ composites exposed to humidified methane

Abstract Carbon formation on Ni/YSZ cermets in high-temperature processes involving hydrocarbons can be a severe problem. Understanding the mechanisms of carbon formation is necessary for developing strategies to avoid or minimize the problem. In this study, Ni/YSZ pellets (70/30 NiO/YSZ) have been exposed to humidified methane at temperatures between 773 K and 1073 K. The carbon formed on these pellets and/or any structural changes in the pellets was studied with X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO). At temperatures below 873 K, carbon fibres dominated. At higher temperatures, the majority of the carbon dissolved into the Ni particles. After TPO and removal of the carbon, the Ni structure was damaged to various extents depending on the exposure temperature. The amount of carbon deposited was significantly reduced by placing zirconia-doped ceria pellets on each side of the Ni/YSZ pellet. The addition of a ceria layer did not completely eliminate the carbon formation but the carbon that was formed was weakly attached to the Ni/YSZ such that the structure was not irreversibly changed. The role of the ceria is beyond simply reforming the methane.

Synthesis of Size‐Controlled Acid‐Resistant Hybrid Calcium Carbonate Microparticles as Templates for Fabricating “Micelles‐Enhanced” Polyelectrolyte Capsules by the LBL Technique

Size-controlled, low-dispersed calcium carbonate microparticles were synthesized in the presence of the amphiphilic block copolymer polystyrene-b-poly(acrylic acid) (PS-b-PAA) by modulating the concentration of block copolymer in the reactive system. This type of hybrid microparticles have acid-resistant properties. By investigating the aggregation behaviors of PS-b-PAA micelles by transmission electron microscopy (TEM), the mechanism of hybrid calcium carbonate formation illustrated that the block copolymer served not only as "pseudonuclei" for the growth of calcium carbonate nanocrystals, but also forms the supramicelle congeries, a spherical framework, as templates for calcium carbonate nanocrystal growth into hybrid CaCO(3) particles. Moreover, this pilot study shows that the hybrid microparticle is a novel candidate as a template for fabricating multilayer polyelectrolyte capsules, in which the block copolymer is retained within the capsule interior after core removal under soft conditions. This not only facilitates the encapsulation of special materials, but also provides "micelles-enhanced" polyelectrolyte capsules.

Adsorbed Carbon Formation and Carbon Hydrogenation for CO2Methanation on the Ni(111) Surface: ASED-MO Study

Using the ASED-MO (Atom Superposition and Electron Delocalization-Molecular Orbital) theory, we investigated carbon formation and carbon hydrogenation for <TEX>$CO_2$</TEX> methanation on the Ni (111) surface. For carbon formation mechanism, we calculated the following activation energies, 1.27 eV for <TEX>$CO_2$</TEX> dissociation, 2.97 eV for the CO, 1.93 eV for 2CO dissociation, respectively. For carbon methanation mechanism, we also calculated the following activation energies, 0.72 eV for methylidyne, 0.52 eV for methylene and 0.50 eV for methane, respectively. We found that the calculated activation energy of CO dissociation is higher than that of 2CO dissociation on the clean surface and base on these results that the CO dissociation step are the ratedetermining of the process. The C-H bond lengths of <TEX>$CH_4$</TEX> the intermediate complex are 1.21 <TEX>$\AA$</TEX>, 1.31 <TEX>$\AA$</TEX> for the C<TEX>${\cdot}{\cdot}{\cdot}H_{(1)}$</TEX>, and 2.82 <TEX>$\AA$</TEX> for the height, with angles of 105<TEX>${^{\circ}}$</TEX> for ∠ <TEX>$H_{(1)}$</TEX>CH and 98<TEX>${^{\circ}}$</TEX> for <TEX>$H_{(1)} CH _{(1)}$</TEX>.

Effects of acidity and a metabolic inhibitor on incorporation of calcium and inorganic carbon into endolymph and otoliths in salmon Oncorhynchus masou

: To investigate the mechanisms of calcium carbonate formation of teleost otoliths, using the double labeling technique of 45Ca2+ and H14CO3–, the effects of tissue elimination, a metabolic inhibitor (cyanide) and acidity on the incorporation of calcium and inorganic carbon into endolymph and otoliths were examined. Incorporation of calcium and inorganic carbon into otoliths was decreased to 36% and 45%, respectively, by removing the sacculus. In contrast, cyanide inhibited incorporation of inorganic carbon and calcium into endolymph and otoliths in a concentration-dependent manner. Acidity in the incubation medium decreased the accumulation of inorganic carbon within the endolymph and otoliths. Calcium incorporation was not affected by variation of pH in the medium. These results indicate that calcium and bicarbonate are actively transported to endolymph via the sacculus, and it is suggested that the transport is involved in the oversaturation state for calcium carbonate in the endolymph and regulates subsequent calcification of otoliths.

Effect of hydrocarbons precursors on the formation of carbon nanotubes in chemical vapor deposition

High-temperature decomposition of hydrocarbons may lead to the formation of carbon deposits. However in our present studies, we found that the morphology of carbon deposits over MgO supported Fe catalyst during chemical vapor deposition (CVD) process was closely related to the thermodynamic properties and chemical structures of hydrocarbon precursors. Six kinds of hydrocarbons (methane, hexane, cyclohexane, benzene, naphthalene and anthracene) were used as carbon precursors in this study. Methane which has a pretty simple composition and is more chemically stable was favorable for the formation of high-purity single walled carbon nanotubes (SWNTs). For high-molecular weight hydrocarbons, it was found that the chemical structures rather than thermodynamic properties of carbon precursors would play an important role in nanotube formation. Specifically, the CVD processes of aromatic molecules such as benzene, naphthalene and anthracene inclined to the growth of SWNTs. While the cases of aliphatic and cyclic hydrocarbon molecules seemed a little more complicated. Based on different pyrolytic behaviors of carbon precursors and formation mechanism of SWNTs and multi-walled carbon nanotubes (MWNTs), a possible explanation of the difference in CVD products was also proposed.

Regularly coiled carbon nanotubes

Regularly coiled carbon nanotubes, their structure, and formation mechanism are puzzling questions. The first models were based on the very regular incorporation of a small fraction (of the order of 10%) of nonhexagonal (n-Hx) rings: (pentagons and heptagons) in a perfect hexagonal (Hx) lattice. It is difficult to understand by which mechanism takes place such a regular incorporation of isolated n-Hx rings. In this paper, a new family of Haeckelite nanotubes is generated in a systematic way by rolling up a two-dimensional three-fold coordinated carbon network composed of pentagon-heptagon pairs and hexagons in proportion 2 : 3. In this model, the n-Hx rings are treated like regular building blocks of the structure. Cohesion energy calculation shows that the stability of the generated three-dimensional Haeckelite structures falls between that of straight carbon nanotubes and that of C/sub 60/. Electronic density of states of the Haeckelite computed with a tight-binding Hamiltonian that includes the C-/spl pi/ orbitals only shows that the structures are semiconductor. The relation of the structures with experimental observations is discussed.

Consideration of reaction mechanisms leading to pyrolytic carbon of different textures

A distinction between a growth and a nucleation mechanism is not sufficient to draw direct conclusions in relation to the texture of pyrolytic carbon. This is determined by the carbon formation mechanisms, which are analogous or at least similar to the mechanisms of aromatic growth. The latter mechanisms are reviewed in the first part of the paper with special consideration of structural chemical aspects. The relevance of the individual mechanisms is analyzed in the second part based on experimentally determined reaction products. Most important mechanisms are aryl–aryl combination, intramolecular dehydrocyclization and ethine addition reactions. The influence of mechanisms concerning an inhibition of the formation of five-membered rings and a transformation of five- into six-membered rings is difficult to estimate. The results indicate that a high textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic to small linear hydrocarbons (ethine). This model is called the particle-filler model (aromatic hydrocarbons: molecular particles; ethine: molecular filler).

Determination of sulfide in spent caustic solutions from the chemical cleaning of coal

Chemical cleaning of coal by the molten caustic leaching process results in the recovery of a spent caustic stream which contains a complex mixture of inorganic and organic salts. Determination of sulfur forms in this matrix is required to elucidate the mechanisms of sulfur removal and carbonate formation during molten caustic leaching. Since the sulfide present in spent caustic is quickly oxidized to sulfate, the method for sulfide must be rapid in order to yield meaningful results about the mechanisms of caustic desulfurization of coal. Thus, a rapid method has been developed for the determination of sulfide in such a caustic matrix. In this method, samples are acidified and purged with nitrogen gas to isolate hydrogen sulfide. The hydrogen sulfide is then either trapped in a solution of lead acetate and total sulfur determined in the resulting precipitate, or the hydrogen sulfide is trapped by a resin impregnated with lead acetate and packed in a calibrated tube. In the latter variant, sulfur concentration determination is based upon the length of the resin bed in the tube that is blackened by the formation of lead sulfide.