Ab initio model study on a water molecule between graphite layers

Interactions between a water molecule and one- and two-layer graphite were calculated by ab initio Hartree-Fock (HF) method. Eclipsed coronene dimer (C₂₄H₁₂)₂ was used as the two-layer graphite model, to study the behavior of a water molecule in the pressure between two graphite layers. Three one-layer models, C₁₆H₁₀, C₄₂H₁₆, and C₈₀H₂₂, were used as reference to investigate the influence of the adsorption site, the orientation of the water molecule, and the size of the model. Only minor differences were found in the three adsorption sites. The HF interaction was weaker with larger models and stronger with larger basis set (6-31G*).     

Electronic structure methods applied to gas-carbon reactions

A review is given on the fundamental studies of gas-carbon reactions using electronic structure methods in the last several decades. The three types of electronic structure methods including semi-empirical, ab initio and density functional theory methods are briefly introduced first, followed by the studies on carbon reactions with hydrogen and oxygen-containing gases (non-catalysed and catalysed). The problems yet to solve and possible promising directions are discussed.     

CO2 adsorption on carbonaceous surfaces: a combined experimental and theoretical study

We present an experimental and theoretical study to provide further insight into the mechanism of CO₂ chemisorption on carbonaceous surfaces. The differential heat of CO₂ adsorption at low and high coverages was determined in the temperature range 553-593 K. We found that the heat profile has two distinct energetic zones that suggest two different adsorption processes. In the low-coverage region, the heat of adsorption decreases rapidly from 75 to 24 kcal/mol, suggesting a broad spectrum of binding sites. In the high-coverage region, the heat becomes nearly independent of the loading, from 9 to 5 kcal/mol. A systematic molecular modeling study of CO₂ chemisorption on carbonaceous surfaces was performed. Several of the carbon-oxygen complexes that have been proposed in the literature were identified and characterized. The calculated adsorption energies are within the experimental uncertainty of the heat of adsorption at low coverage. Pre-adsorbed oxygen groups decrease the exothermicity of CO₂ adsorption. In the high-coverage region, our theoretical results suggest that CO₂ molecules are likely to adsorb on surface oxygen complexes and on graphene planes.     

Kinetics of the oxidation of carbon black by NO2: Influence of the presence of water and oxygen

In a fixed bed reactor, the rate of carbon black oxidation by NO₂ is significant for temperatures above 300°C, leading to NO, CO and CO₂ formation. The presence of O₂ in the feed gas increases the rate of oxidation, as well as the presence of water. A cumulative effect is observed when both water and oxygen are present. An oxygen balance shows that oxygen atoms of water molecules are not consumed. Water acts as a catalyst for the C-NO₂ reaction. A kinetic mechanism in which intermediate nitro-oxygenated species are formed in the presence of NO₂ during an initial step is in agreement with all these observations. Oxygen and NO₂ are able to react with these species at 300°C. A parametric study of the effects of the temperature, NO₂, O₂ and H₂O concentrations was performed. With a one-dimensional model of NO₂ consumption along the thickness of the carbon black bed, kinetic constants were derived and a phenomenological law was proposed, accounting for the effect of the presence of oxygen and water.     

Adsorption on carbonaceous surfaces: cost-effective computational strategies for quantum chemistry studies of aromatic systems

We present a systematic analysis of the accuracy and efficiency of several computational quantum chemistry models for studying reactions involving aromatic systems. In particular, we have examined different multi-layer ONIOM models in which the whole system is divided into subsystems that can be treated at different levels of theory. The carbonaceous surface is modeled by a graphene layer that has unsaturated carbon atoms to represent active sites and has different shapes to simulate the local environment of the active sites of a carbonized material. We emphasized the model performance in predicting geometrical parameters, interaction energies and infrared spectra of carbon-oxygen complexes. We found that any attempt to partition the graphene layer into subsystems for employing different levels of theory yields considerable errors. However, it is possible to obtain reasonable accuracy by using the same level of theory for the whole system at different basis sets. This computational strategy can predict accurate geometrical parameters, interaction energies and infrared spectra of common oxygen complexes at lower computational cost.     

Ab initio molecular orbital study of the mechanism of SO2 oxidation catalyzed by carbon

Ab initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO₂ by O₂/H₂O to form sulfuric acid. Both zigzag and armchair edge sites of graphite, with and without surface oxide, were considered as the possible active sites. For the sites with oxide, both isolated and twin oxides were included. MO calculations at the B3LYP/6-31G(d)//HF/3-21G(d) level were used for calculating the energies of SO₂ adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and all three took place on the zigzag edge sites. Hence the armchair sites were not viable sites. On the bare surface, the only possible pathway involved the formation of a sulfurous acid intermediate. Thus, SO₂ was first adsorbed on the bare zigzag sites, followed by reaction with H₂O to form H₂SO₃, which was further oxidized by O₂ to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO₃ or H₂SO₃ as the intermediate are possible. Chemisorption on the edge sites containing twin oxides was not viable. This latter result explains the seemingly conflicting results in the literature regarding the dependence of SO₂ adsorption (and oxidation) on the amount of surface oxygen.     

Catalytic gasification of coal using eutectic salts: identification of eutectics

Different eutectic salt mixture catalysts for the gasification of Illinois No. 6 coal were identified and various impregnation or catalyst addition methods to improve catalyst dispersion were evaluated in this study. In addition, the effects of major process variables such as temperature, pressure, and steam/carbon ratio were investigated in a thermogravimetric analyzer (TGA) and fixed-bed bench scale reactor systems. The TGA studies showed that the eutectic catalysts increased CO₂ gasification rate significantly. The methods of catalyst preparation and addition had significant effect on the catalytic activity and coal gasification. Based on the TGA studies of several eutectic systems, the 43.5% Li₂CO₃-31.5% Na₂CO₃-25% K₂CO₃ and 39% Li₂CO₃-38.5% Na₂CO₃-22.5% Rb₂CO₃ ternary eutectics, the 29% Na₂CO₃-71% K₂CO₃ binary eutectic and the K₂CO₃ single salt catalysts were selected for the fixed-bed studies. The catalyst loading increased the gasification rate and almost complete conversion of carbon was observed when 10 wt.% of catalyst was added to the coal. Upon further increasing the catalyst amount to 20 wt.% and above, there was no significant rise in gasification rate.     

Structural ordering of coal char during heat treatment and its impact on reactivity

The effect of heat treatment on the structure of an Australian semi-anthracite char was studied in detail in the 850-1150°C temperature range using XRD, HRTEM, and electrical resistivity techniques. It was found that the carbon crystallite size in the char does not change significantly during heat treatment in the temperature range studied, for both the raw coal and its ash-free derivative obtained by acid treatment. However, the fraction of the organized carbon in the raw coal chars, determined by XRD, increased with increase of heat treatment time and temperature, while that for the ash-free coal chars remained almost unchanged. This suggests the occurrence of catalytic ordering during heat treatment, supported by the observation that the electrical resistivity of the raw coal chars decreased with heat treatment, while that of the ash-free coal chars did not vary significantly. Further confirmatory evidence was provided by high resolution transmission electron micrographs depicting well-organized carbon layers surrounding iron particles. It is also found that the fraction of organized carbon does not reach unity, but attains an apparent equilibrium value that increases with increase in temperature, providing an apparent heat of ordering of 71.7 kJ mol⁻¹ in the temperature range studied. Good temperature-independent correlation was found between the electrical resistivity and the organized carbon fraction, indicating that electrical resistivity is indeed structure sensitive. Good correlation was also found between the electrical resistivity and the reactivity of coal char. All these results strongly suggest that the thermal deactivation is the result of a crystallite-perfecting process, which is effectively catalyzed by the inorganic matter in the coal char. Based on kinetic interpretation of the data it is concluded that the process is diffusion controlled, most likely involving transport of iron in the inter-crystallite nanospaces in the temperature range studied. The activation energy of this transport process is found to be very low, at about 11.8 kJ mol⁻¹, which is corroborated by model-free correlation of the temporal variation of organized carbon fraction as well as electrical resistivity data using the superposition method, and is suggestive of surface transport of iron.     

Steam-pyrolysis activation of wood char for superior odorant removal

A pre-charred, wood-based carbonaceous precursor was activated using a two-step process (steam-pyrolysis activation) to investigate the potential for optimizing an activation protocol for the production of powdered activated carbon (PAC) tailored to effectively remove the taste- and odor-causing compound 2-methylisoborneol (MIB). Activation temperature, activation time, and steam-to-carbon ratio were the activation parameters modified to find the best operating conditions. The success of the tailored PACs was judged on performance in batch experiments, which were designed to simulate actual water treatment plant conditions and used natural water spiked with ¹⁴C-MIB. Activation temperature had the greatest impact on PAC performance, with 1123 K providing the best operating point. The pore size distribution data revealed a strong correlation between an increase in mesoporosity and MIB removal in the raw waters. In addition, the tailored PAC with the optimized activation protocol outperformed existing commercially available PAC, indicating the plausibility of tailoring activation protocols for superior MIB removal from natural waters.     

A systematic approach for modelling the affinity coefficient in the Dubinin-Radushkevich equation

An unbiased evaluation of predictive models for the affinity coefficient of the Dubinin-Radushkevich equation was performed. The first step involved the selection of a minimum number of representative and chemically diverse organic compounds, the training set. This set, isopropylamine, heptane, dichloromethane, 2-chloro-2-methylpropane, 2-butanone, 1-chloropentane, acetonitrile, and benzene, covering five compounds classes, was selected with the help of PCA and statistical design. Secondly, experimental affinity coefficients of the training set compounds were determined from adsorption isotherms on Norit R1 activated carbon. In a third step, 45 physico-chemical properties were assembled for the training set compounds. A model was developed, based on PLS analysis, which correlates the measured affinity coefficients and the physico-chemical properties. Finally the model was validated by comparing model predictions of the affinity coefficients with literature data for an external validation set of 40 compounds. It was found that the predictive power of this model (RMS error=0.090) is better than using traditional methods based on parachor, molar polarizability or molar volume. The proposed new model for the affinity coefficient is based on three parameters only, the molecular weight and VdW volume of the compound and the calculated energy of interaction between the compound and a graphite model surface.     

Kinetics of tire derived fuel (TDF) char oxidation and accompanying changes in surface area

The oxidation behavior of tire-derived fuel (TDF) char has recently been studied by several groups. In the present study, TDF char oxidation has been examined between 670 and 825 K, at oxygen partial pressures ranging from 2 to 19.8 kPa. The order of reaction with respect to oxygen varied with burnoff, and was in the range 0.72-0.86. The activation energy of reaction ranged with burnoff from 138 to 150 kJ/mol. The reaction rate does not correlate well with BET surface area, but did correlate well with the surface area in pores ranging in size from 1.2 to roughly 7 nm in width. Pores smaller than 1.2 nm exist in the char, but appear not to be used or developed by the oxidation reaction. Results for chars that have been acid washed to remove some inorganic matter show lowered reactivity, and a distinctly different pattern of pore development with burnoff. This is, in turn, reflected in a very different pattern of reactivity change with burnoff for such materials.     

Prediction of elastic properties for single-walled carbon nanotubes

Based on a link between molecular and solid mechanics, an analytical method was developed for modeling the elastic properties of single-walled carbon nanotubes (SWNTs). A SWNT is regarded as a continuum-shell model which is composed of the discrete molecular structures linked by the carbon-to-carbon bonds. The elastic properties were investigated for the SWNTs as a function of the nanotube size in terms of the chiral vector integers (n,m). The theoretical prediction on elastic properties agreed reasonably with the existing experiment and theoretical results. The present formulas are able to serve as a good approximation of the elastic properties for SWNTs.     

Combustion behavior of xylite/lignite mixtures

The behavior and the kinetics in nitrogen and air of two low-rank coals (lignite and xylite) and their blends, as well as the compatibility of the component coals in the blends were evaluated, in an effort for the rational use of poor coals.The experiments were conducted in a thermobalance system, at non-isothermal heating conditions, with heating rates of 20 and 100 °C/min, in the temperature range of 25-850 °C. Material particle size was −100 μm.A first-order parallel independent reactions model and a power law model fitted successfully the rate data of pyrolysis and combustion, respectively. Activation energy values and reaction orders ranged from 23 to 182 kJ/mol and 0.8 to 2 respectively. The heating rate did not affect the kinetic parameters considerably, however when this was increased the reactions were shifted to higher temperatures and the rates were greater.The pyrolysis kinetics of lignite/xylite blends could be sufficiently predicted, based on the data of the individual fuels. However, this was not true in the case of char combustion. Blending of lignite with xylite, in any proportion, seemed to cause some interactions between the component coals in air.     

Production of active carbons from waste tyres--a review

A review of the production of activated carbons from waste tyres is presented. The effects of various process parameters, particularly, temperature and heating rate, on the pyrolysis stage are reviewed. The influence of activating conditions, physical and chemical, nature of the activation chemicals, on the active carbon properties are discussed. Under certain process conditions several active carbons with BET surface areas over 1000 m²/g have been produced with extensive micropore volumes, over 40% of the total pore volume.A review is carried out of the reaction kinetic modeling applied to the pyrolysis of tyres and the chemical activation of tyres. The models cover one step and two step pyrolysis models, plus more recent models which are based on the actual chemical components such as natural rubber, SBR and other additives.     

Liquid reagent CVD of carbon. II. Kinetic experiments and heat and mass transport analysis

Liquid reagent CVD of carbon was performed where the substrate was immersed in liquid benzene and cyclohexane. A kinetic analysis showed the deposition rates obeyed the Arrhenius relationship with apparent activation energies of 280 and 263 kJ/mole, respectively. Deposition rates were highest for benzene. A simple analysis of heat and mass transport based on film boiling theory was performed to achieve fundamental understanding of the process and to define the rate limiting mechanism. It was demonstrated that the hydrogen inhibition effect can be neglected for the liquid reagent CVD process and a high precursor concentration near the substrate surface is achievable, which permits a high deposition rate.     

Analysis and modeling of mesophase sphere generation, growth and coalescence upon heating of a coal tar pitch

The present study has been undertaken aiming: (1) to provide an experimental procedure for measurement of characteristics of mesophase sphere formation from a coal tar pitch upon heating at 673-723 K in terms of the time-dependent changes in the population density and the radius distribution of the spheres and (2) to prove that the nucleation, growth and coalescence of the spheres are essential for quantitative explanation of the time-dependent changes. In regard to (1), this paper describes the conditions that are required in microscopic analysis of cross-sections of the bulk of heat-treated pitch for the conversion of the population density and the radius distribution of the cross-sectional circles into those of spheres on volume basis. Regarding (2), a model analysis demonstrates that the change in the radius distribution with time, as well as that in the population density of spheres, can quantitatively be predicted. The analysis also reveals that the spheres grow without undergoing coalescence in the early period, and the sphere coalescence commences after termination of sphere generation while the growth continues.