Kinetics Of Carbon Deposition Research Articles

Chemical Supercritical Fluid Infiltration of Pyrocarbon with Thermal Gradients: Deposition Kinetics and Multiphysics Modeling

The chemical supercritical fluid infiltration process is a recent variation of the chemical vapor infiltration (CVI) process that allows rapid and efficient manufacturing of ceramic-matrix composites (CMCs), albeit still needing optimization. This article proposes a quantitative assessment of the process dynamics through experiments and modeling. The kinetics of carbon deposition were determined through two sets of experiments: CVD on a single filament at pressures between 10 and 50 bar and infiltration at pressures ranging between 50 and 120 bar. The CVI experiments were conducted under important thermal gradients and were interpreted using a model-based reconstitution of these gradients. We found that (i) the kinetic law has to incorporate the potential effect of the reverse reaction (i.e., etching of C by H2); (ii) the activation energy and pre-exponential factor both decrease with pressure up to 50 bar, then remain roughly constant, and (iii) although the apparent activation energy is modest, a favorable situation occurs in which an infiltration front builds up and travels from the hottest to the coldest part of the preform due to the presence of sufficient heat flux. A numerical simulation of the process, based on the solution of momentum, heat, and mass balance equations, fed with appropriate laws for the effective transfer properties of the porous medium and their evolution with infiltration progress, was performed and validated by comparing the simulated and actual infiltration profiles.

Growth of carbon nanotubes inside porous anodic alumina membranes: Simulation and experiment

Porous anodic alumina (PAA) membranes represent a widely used and extensively studied template for production of carbon nanotubes (CNT). The PAA–CNT membranes possess a number of unique properties, such as controllable nanotube geometry, size– and chemically–based selectivity as well as high water permeability. In this work, we first propose a combination of gas phase and surface reaction models to quantitatively describe the growth of carbon nanotubes in PAA membranes in a commercial CVD reactor. A complimentary experimental study of CNT formation from ethanol precursor with argon as a carrier gas is performed. A new method for characterizing carbon nanotubes geometry by SEM and TEM image processing of membrane cross–sections is proposed. The simulations show that the carbon growth rate (in nm/min) averaged over the membrane remains constant during the deposition process until the pore diameter becomes relatively small, and rapidly falls to zero after that. The carbon nanotube thickness near the membrane surface is slightly higher than that in the membrane center. The carbon growth rate increases with synthesis temperature and pressure, while it decreases with the argon flow rate. The dependence of carbon growth rate on the ethanol/water flow rate reaches maximum at some intermediate value. These results are supported by the experimental data obtained from SEM/TEM image processing. It is found that the SEM data provide overestimated values of nanotube diameter and thickness in comparison with the TEM data. The obtained results provide new insights into the CNT growth kinetics in nanoporous media, and develop quantitative guidelines for synthesis of CNT–PAA membranes with precisely controlled nanopore geometry. It also validates the combined homogenous / heterogeneous reaction model by comparison with carbon deposition kinetics on a nanometer scale.

Carbon deposition in porous nickel/yttria-stabilized zirconia anode under methane atmosphere

A commercial solid oxide fuel cell with a Ni/YSZ anode was characterized under a pure methane atmosphere. The amount of deposited carbon increased with an increase in temperature but decreased when the temperature exceeded 700°C. The reactivity of carbon decreased with increasing deposition temperature. Filamentous carbon was deposited from 400 to 600°C, whereas flake carbon was deposited at 700 and 800°C. With increasing temperature, the intensity ratio of the D band over the sum of the G and D bands was constant at the beginning and then decreased with the transformation of the carbon morphology. The crystallite size increased from 2.9 to 13 nm with increasing temperature. The results also indicated that the structure of the deposited carbon was better ordered with increasing deposition temperature. In comparison with pure Ni powders, the interaction between the YSZ substrate and Ni particles could not only modify the carbon deposition kinetics but also reduce the temperature effect on the structure and reactivity variation of carbon.

Hydrogen production via propane dry reforming: Carbon deposition and reaction-deactivation study

Carbon deposition during carbon dioxide reforming reaction of C3H8 has been studied over alumina-supported bimetallic Mo/CoNi catalysts. To better understand the carbon-induced deactivation during the reforming reaction, changes in catalyst morphology and carbon deposition kinetics were examined. Different characterization techniques were used for both fresh and rejuvenated catalysts including liquid nitrogen adsorption/desorption, chemisorption via hydrogen, ammonia and CO2 desorption, and thermogravimetric measurement of the coked catalysts. The time dependant reaction rate profiles indicated that MoNi catalyst has higher syngas (H2/CO) formation rates with lower CO2 rate of consumption compared to CoNi catalyst. However, the H2:CO ratio values were almost the same for both catalysts suggesting similarity in the product formation pathway. Conversion-time analysis showed that MoNi catalyst was more stable and active during a 72-h run while CoNi suffered noticeable deactivation after 30 h on-stream. Reaction-deactivation models implicated a higher deactivation coefficient (kd) with activation energy of Ed = 78.1 kJ mol−1 for the cobalt-containing Ni catalyst, while the Ni catalyst with molybdenum had a lower deactivation coefficient with smaller activation energy of just under 70 kJ mol−1. Post-mortem analysis (TPR-TPO dual cycle and TOC) of spent catalysts confirmed that the surface of CoNi catalyst has more carbon residue than the MoNi sample which was consistent with the higher deactivation of CoNi.

Measuring the Unmeasurable by IR Spectroscopy: Carbon Deposition Kinetics in Dry Reforming of Methane.

Dry reforming of methane converts two greenhouse gases to syngas, and Ni catalysts are commonly used for this reaction. A major catalyst deactivation mechanism is carbon deposition. Although numerous kinetic modelling works have been performed on carbon formation, there have been only scarce attempts to measure carbon deposition kinetics under relevant (but not real) conditions, owing to technical difficulties. Here, we report the first successful measurements of the kinetics under real reaction conditions. This was made possible by using a novel algorithm that we have developed. We use IR to measure the molar fractions of unreacted CH4 and CO2 , and reaction products, CO and H2 O, in the effluent from the reactor. By applying the general mass balance principle and the relevant reaction stoichiometries, the carbon deposition rate as well as the flow rates of all these gases in the effluent, including H2 , are calculated. Compared to the dominant GC-based approach for catalyst performance evaluation, this method has much higher time resolution and much smaller measurement errors.

Effects of specific surface area of metallic nickel particles on carbon deposition kinetics

Carbon deposition on nickel powders in methane involves three stages in different reaction temperature ranges. Temperature programing oxidation test and Raman spectrum results indicated the formation of complex and ordered carbon structures at high deposition temperatures. The values of I(D)/I(G) of the deposited carbon reached 1.86, 1.30, and 1.22 in the first, second, and third stages, respectively. The structure of carbon in the second stage was similar to that in the third stage. Carbon deposited in the first stage rarely contained homogeneous pyrolytic deposit layers. A kinetic model was developed to analyze the carbon deposition behavior in the first stage. The rate-determining step of the first stage is supposed to be interfacial reaction. Based on the investigation of carbon deposition kinetics on nickel powders from different resources, carbon deposition rate is suggested to have a linear relation with the square of specific surface area of nickel particles.

Comparative study of 1,2-dichlorethane decomposition over Ni-based catalysts with formation of filamentous carbon

Catalytic chemical vapor deposition of 1,2-dichlorethane over Ni-based catalysts into carbon nanostructured materials was studied. The catalysts were prepared by mechanochemical activation and by metal dusting of bulk nickel-containing alloy precursors. Model Ni-M alloys, where M is Co, Cu, and Fe, were obtained by coprecipitation technique. Loading of M in the samples was varied in a range of 1–5at.%. Pure nickel was used a reference. The kinetics of carbon deposition was investigated using flow reactor equipped with McBain balances. The samples of carbon product were characterized by nitrogen adsorption, scanning and transmission electron microscopies. The hydrogen addition into reaction mixture was shown to have opposite effect on both catalytic behavior and carbon yield depending on catalyst’s nature. Segmented structure of carbon filaments formed specifies its developed surface area. Both bulk chlorination of nickel particles and its blockage by dense carbon deposits in the case of mechanochemically prepared samples were suggested to be responsible for rapid deactivation of the catalyst.

Thermodynamics and kinetics of carbon deposits on cobalt: a combined density functional theory and kinetic Monte Carlo study.

We have built a lattice gas model for cobalt-carbon interaction to investigate the thermodynamics and kinetics of carbon deposition on Co(0001) surfaces. The formation of carbon structures on cobalt is considered to be one of the causes of deactivation of a cobalt Fischer-Tropsch (FT) catalyst. The formation of graphene - the most thermodynamically stable phase under FT conditions - is kinetically inhibited during the first 30 hours of exposure of the surface to carbon, while the build-up of surface carbide is the fastest reaction. Our simulations clearly show that the kinetics of carbon deposition is the result of two competing effects: a fast subsurface diffusion and a slower surface diffusion to form a carbon-carbon bond.

Carbon deposition on bimetallic Co–Ni/Al 2O 3 catalyst during steam reforming of glycerol

This paper reports the nature and kinetics of carbon deposition on bimetallic Co–Ni/Al 2O 3 catalyst during the steam reforming of glycerol. Reaction runs were carried out under stoichiometrically excess steam conditions between 773 and 823 K. Total organic carbon content of used catalysts varied between 18 and 30 wt.%. Carbon deposition occurred on the calcined Al 2O 3 support probably due to the presence of weak acid centres. Thermogravimetric analysis revealed two types of carbonaceous pools on catalyst surface—a relatively smaller deposit reactive with both H 2 and O 2 and a larger reservoir that could only be gasified with O 2. Gas composition analysis showed that only CH 4 and H 2O were produced during H 2-TPR while CO 2 and H 2O were produced during TPO. FTIR characterisation confirmed the presence of C–O, C–C, C–H, O–H and C C bonds in the carbon residue. Carbon deposition rate has an order of 0.55 with respect to glycerol partial pressure and order of −0.22 for steam. Indeed, a Langmuir–Hinshelwood kinetic model involving dissociative adsorption of glycerol and molecular chemisorption of steam adequately captured the carbon deposition rate behaviour. Catalyst regeneration using a TPO–TPR–TPO–TPR scheme restored the physicochemical properties to the same level as the fresh catalyst.

Gas products and carbon deposition kinetics in chemical vapor deposition from propylene

Gas products and carbon deposition kinetics in chemical vapor deposition from propylene

Gas products and carbon deposition kinetics in chemical vapor deposition from propylene

Gas products and chemical vapor deposition kinetics of pyrocarbon (PyC) formed during pyrolysis of a propylene/argon mixture in the range 700–1200 °C were analyzed by out-line gas chromatography coupled with mass spectrometry and by an in-situ magnetic suspension balance, respectively. More than 30 components were identified. Benzene was the main product, substituting for naphthalene as the temperature increased. PyC was formed with an apparent activation energy of 137 ± 25 kJ/mol, assumed to be controlled by the acetylene-forming reaction from 800 °C to 1000 °C. Whereas, above 1000 °C, the deposition behavior was representative of a combination between gas phase diffusion from the bulk flow to the solid surface and gas phase nucleation. Reaction models describing homogeneous gas phase PyC deposition reactions were derived. On the basis of these results, a reaction path scheme of PyC formation from propylene was proposed.

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.

Thermodynamics and Kinetics of Carbon Deposition from Mixtures of Hydrogen and Carbon Tetrachloride

The thermodynamics and kinetics of pyrolytic carbon deposition via hydrogen reduction of CCl4 were studied. Thermodynamic analysis of the C–Cl–H system was used to determine the compositions of the gas and condensed phases at CCl4 : H2 = 1 : 4 to 1 : 90, temperatures from 700 to 1400 K, and a pressure of 105 Pa. The reactions leading to the formation of solid carbon in the H2–CCl4 system were considered, and the deposition parameters were optimized in terms of carbon yield.

The carbon depositing behavior and its kinetic research in benzene alkylation process over high silicate ZSM-5 zeolite

In our invention, FCC (fluid catalytic cracking) dry gas could be used to react with benzene without any special purification, and more than 90% ethylene was converted to ethylbenzene. The phenomenon of carbon deposition over catalyst surface was obvious and leads to a deactivation of catalyst, so it is important to study the behavior of carbon deposition of catalyst during alkylation of benzene. The influence of several factors such as temperature, reaction time, reactant concentration of the amount and the kinetics of carbon deposition were investigated, during which carbon depositing rate equations were obtained for different reactant.

Carbon states and kinetics of carbon deposition from ethylene at Pt(110) surface

The nature of high temperature carbon states at Pt(110) has been studied by X-Ray Photoelectron Spectroscopy. The formation of diamond-like carbon (DLC) is reported for the first time. The kinetics and temperature-programmed reactions of DLC were numerically simulated.

Kinetics of carbon deposition from ethylene on Pt(110) surface

The formation of carbonaceous deposits on Pt(110) surface due to ethylene decomposition in the temperature range of 770–970 K has been studied. Both kinetic and XPS data show that chemisorbed carbon (with no C−C bonds, but only C−Pt bonds) forms at T=770–870 K, the saturation coverage θs being about 1 ML. At higher temperatures, θs increases by a factor of 1.5, indicating the formation of more densely packed carbon species (DPC) due to C−C bonding. A kinetic model has been proposed to describe the process of carbon conversion.

Carbon deposition on a Ni/α-Al2O3 catalyst

The kinetics of carbon deposition over a Ni/α-Al2O3 commercial catalyst, has been studied using CH4−H2 gas mixtures in the range of 748–873 K. A Hougen-Watson type model gives good agreement with the experimental rates.

The relative coke-inducing tendencies of pyrolysed, gasified and combusted Devonian oil shales

A dual fluidized-bed reactor system was used to examine the isothermal kinetics of carbon deposition when nascent shale oil vapours were passed through beds of pyrolysed, gasified and combusted Devonian shale. Naturally occurring samples of the principal silicate minerals found in Devonian shale (kaolinite, illite and quartz) were also tested. The rate of carbon uptake declined with increasing carbon deposition on the solids. The coke-inducing activities of the processed shales were ranked as: combusted shale gasified shale pyrolysed shale. The effect of coking temperature on carbon deposition was relatively small, which has important implications for solids-recycle oil shale retorts.

Carbon deposition over Fe, Ni, and Co foils from CO-H 2-CH 4-CO 2-H 2O, CO-CO 2, CH 4-H 2, and CO-H 2-H 2O gas mixtures: II. Kinetics

Nonequilibrium C-H-O phase diagrams were used to investigate the effect of gas composition on the kinetics of carbon deposition over Fe, Ni, and Co foils at 900 K and 1 atm. The gasphase carbon activity, which is proportional to the driving force for carbon deposition, was fixed at 10.0 for all experiments. The reduced metal foils were initially exposed to five-component gas mixtures of CO, H 2, CH 4, CO 2, and H 2O, after which the gas composition was switched to a binary CO-CO 2 or CH 4-H 2 gas mixture, or a ternary CO-H 2-H 2O gas mixture. On exposure to either of the binary gas pairs, the rate of carbon deposition over all three metals was found to be approximately the same ( ≈10 ∗ 10 −6 min −1 ), and the rate was not observed to be a function of the partial pressure of the carbonbearing gas alone, either CO or CH 4. In the case of Fe and Co, the rate of carbon deposition from the binary gas pairs dropped two orders of magnitude ( 2500 ∗ 10 −6 to ≈10 ∗ 10 −6 min −1 ) and one order of magnitude ( 500 ∗ 10 −6 to ≈10 ∗ 10 −6 min −1 ), respectively, compared to the rate from the five-component gas mixture. In the case of Ni, the rate of carbon deposition from the binary gas pairs was not observed to change from the rate of the five-component gas mixture ( ≈10 ∗ 10 −6 min −6 ). On exposure of Fe and Co foils to ternary gas mixtures, the rate changed proportionally to the product of the P COP H2. Exposure of Ni foils to the ternary gas mixture had no effect on the rate of carbon deposition.

Kinetics of carbon deposition on nickel metal by decomposition of carbon monoxide.

The kinetics of carbon deposition are reported for the temperature range 773-1023 K. A reaction model composed of adsorption and reaction steps is proposed and compared with experimental data. In this model the rate equation of carbon deposition takes into account the catalytic action of nickel. The calculated results are in good agreement with the experimental ones. The activation energy and the heat of adsorption of CO are 59.3kJ/mol and 61.7kJ/mol, respectively.