Influence of the surface area/volume ratio on the chemistry of carbon deposition from methane

The chemistry of carbon deposition from methane as a function of methane pressure was studied at a temperature of 1100 °C and surface area/volume ratios of 0.8 and 3.2 mm⁻¹ by analysis of both gaseous and condensing, i.e. aromatic reaction products. Conversion of methane as well as the yields of the hydrocarbons formed increase with increasing pressure. The surface area/volume ratio has a significant influence on the formation of aromatic hydrocarbons showing much higher yields at the lower ratio. This result, expected from preceding studies of deposition rates, confirms that a change of this ratio leads to a change of the deposition chemistry of carbon.     

Reactivities of Guaiacyl and Syringyl Lignin Model Phenols Towards Oxidation with Oxygen-Alkali

A range of guaiacyl and syringyl lignin model phenols was treated with oxygen in 1M potassium hydroxide solution at 70°C. The reactions were monitored by high performance liquid chromatography and gas chromatography-mass spectrometry. The reactions of the phenols, which followed pseudo-first-order kinetics, were faster for syringyl than for guaiacyl phenols. For the various 4-substituted syringols the reactivities were in decreasing order CH₂-syringyl > CHOH[sbnd]CH₃ π C₃H₇ ⁿ > CH₂OH > COOH > CHO > CO[sbnd]CH₃. Reaction of 1-guaiacylpropane in 1M potassium hydroxide with oxygen gave products of oxidative scission of the aromatic ring and no dehydrodimer, whereas at pH 11.5 some dehydrodimer was among the reaction products. Vanillyl alcohol and syringyl alcohol yielded vanillin and syringaldehyde, respectively, as minor oxidation products. However, the reaction sites for the series of phenols were generally the aromatic rings rather than the side-chains. Oxidation of alkaline solutions of the phenols with oxygen at 1.0 MPa pressure and 110 and 150°C gave similar mixtures of acids and hydroxyacids.     

A highly active silica(silicon)-supported vanadia catalyst for C1 oxygenates and hydrocarbon production from partial oxidation of methane

A very low surface area silica-silicon substrate has been used as a support for vanadium oxide and has been tested in the partial oxidation of methane. Use of a reactor with variable dead volume ahead of the bed of the catalyst allows determining the relevance of gas phase reactions in initiating methane conversion. Experimental evidence supports that at atmospheric pressure C₁ oxygenates are essentially produced on the catalyst surface rather than in the gas phase. Comparison with a high surface area silica-supported vanadium oxide catalyst clearly highlights the double role of surface area in promoting catalytic activity, but also in promoting non-selective further oxidation of reaction products. It is shown that a reaction system combining dead volume upstream the bed of the catalyst and a very low surface area is very promising to activate methane conversion to C₁ oxygenates and C₂₊ hydrocarbons at remarkable TOF number preventing further non-selective oxidation. In addition, production of C₂₊ hydrocarbons is observed at temperatures as low as 750 K.     

Modelling the growth of carbon nanotubes produced by chemical vapor deposition

An improved model of C₂H₂ deposition for the growth of carbon nanotubes (CNT) in a horizontal tube reactor has been developed. This includes detailed gas-phase reactions of acetylene pyrolysis, and surface catalytic reactions for CNT growth. Based on this model, the mechanism of CNT growth has been studied by analyzing the change of the CNT growth rate for different growth conditions such as pressure, temperature, number of catalyst nanoparticles per unit area, and diameters of catalyst nanoparticles. The influence of gas-phase reactions and their products on CNT growth has also been evaluated. It is found that although C₂H₂ is the main contributor to the growth of CNTs, the contribution from the gas-phase products could not be ignored, especially at high temperature.     

Chemistry and kinetics of chemical vapour deposition of pyrocarbon: VII. Confirmation of the influence of the substrate surface area/reactor volume ratio

Carbon deposition from a methane–hydrogen mixture (p_CH4=17.5 kPa, p_H2=2.5 kPa) was studied at an ambient pressure of about 100 kPa and a temperature of 1100°C, using deposition arrangements with surface area/reactor volume ratios, [A_S/V_R], of 10, 20, 40 and 80 cm⁻¹. Steady-state deposition rates and corresponding compositions of the gas phase as a function of residence were determined. The deposition rates in mol/h increase with increasing [A_S/V_R] ratio at all investigated residence times up to 1 s. However, surface-related deposition rates in mol/m²h decreased. As the same results have been obtained in a preceding study using pure methane at a partial pressure of 10 kPa, it has been confirmed that all the kinetics can be determined by changing the [A_S/V_R] ratio.     

Observed volatilization behavior of silicon carbide in flowing hydrogen above 2000 K

The intrinsic compatibility of silicon carbide (SiC) and hydrogen (H₂) at high temperatures (2000-2473 K) and pressure near one atmosphere was evaluated through a combination of thermodynamic calculations and hot hydrogen exposure testing. Thermodynamic calculations predict the decomposition of SiC in a hydrogen environment to form free silicon (Si) and free carbon (C). Free Si is predicted to vaporize from the surface as a volatile species, while free C may interact with H₂ to form the hydrocarbons CH₄ (T < 2100 K) or C₂H₂ (T > 2100 K). Coupons of high purity chemical vapor deposition (CVD) β-SiC were exposed to slowly flowing hydrogen at temperatures ranging between 2000 and 2473 K. SiC experienced active attack as the result of H₂ exposure, exhibiting linear weight loss kinetics and an Arrhenius dependence of weight loss on exposure temperature. The linear volatilization constant was experimentally evaluated to correspond with an activation energy of 370 ± 18 kJ/mol. Due to the dependence of observed corrosion rates on gas velocity, corrosion of SiC in flowing H₂ was determined to be governed by external mass transfer of volatile Si species through the boundary layer. Experimentally derived mass losses were in good agreement with mass losses predicted by a boundary layer limited gas diffusion model.     

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.     

Kinetic model of an oxygen‐free methane conversion on a platinum catalyst

In many metal‐catalyzed conversion processes of hydrocarbons at atmospheric pressure a carbonaceous overlayer quickly builds up at the catalyst covering nearly the whole surface. However, the metal still remains catalytically active. Several models have been proposed over the years to explain the crucial role of the carbonaceous overlayer during the conversion of hydrocarbons. The model presented here contemplates adsorbate effects, which means that surface carbon modifies the dehydrogenation activity of Pt. A hydrocarbon reaction mechanism on platinum, including C₁ and C₂ species, is established. The mechanism is based on elementary reactions offering the opportunity of using the same mechanism for a wide range of applications. It is also applied to extended simulations of higher pressures and smaller flow velocities revealing increased C₂H₆ yields under these conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Steam reforming of LPG over Ni and Rh supported on Gd-CeO2 and Al2O3: Effect of support and feed composition

The steam reforming of liquefied petroleum gas (LPG) over Ni- and Rh-based catalysts supported on Gd-CeO₂ (CGO) and Al₂O₃ was studied at 750-900 °C. The order of activity was found to be Rh/CGO > Ni/CGO ∼ Rh/Al₂O₃ > Ni/Al₂O₃; we indicated that the comparable activity of Ni/CGO to precious metal Rh/Al₂O₃ is due to the occurring of gas-solid reactions between hydrocarbons and lattice oxygen () on CGO surface along with the reaction taking place on the active site of Ni, which helps preventing the carbon deposition and promoting the steam reforming of LPG.The effects of O₂ (as oxidative steam reforming) and H₂ adding were further studied over Ni/CGO and Ni/Al₂O₃. It was found that the additional of these compounds significantly reduced the amount of carbon deposition and promoted the conversion of hydrocarbons (i.e., LPG as well as CH₄, C₂H₄ and C₂H₆ occurred from the thermal decomposition of LPG) to CO and H₂. Nevertheless, the addition of too high O₂ oppositely decreased H₂ yield due to the oxidizing of Ni particle and the possible combusting of H₂ generated from the reaction, while the addition of too high H₂ also negatively affect the catalyst activity due to the occurring of catalyst active site competition and the inhibition of gas-solid reactions between the gaseous hydrocarbon compounds and on the surface of CGO (for the case of Ni/CGO).     

Liquid fuel production from syngas using bifunctional CuO-CoO-Cr2O3 catalyst mixed with MFI zeolite

CuO-CoO-Cr₂O₃ mixed with MFI Zeolite (Si/Al = 35) prepared by co-precipitation was used for synthesis gas conversion to long chain hydrocarbon fuel. CuO-CoO-Cr₂O₃ catalyst was prepared by co-precipitation method using citric acid as complexant with physicochemical characterization by BET, TPR, TGA, XRD, H₂-chemisorptions, SEM and TEM techniques. The conversion experiments were carried out in a fixed bed reactor, with different temperatures (225-325 °C), gas hourly space velocity (457 to 850 h⁻¹) and pressure (28-38 atm). The key products of the reaction were analyzed by gas chromatography mass spectroscopy (GC-MS). Significantly high yields of liquid aromatic hydrocarbon products were obtained over this catalyst. Higher temperature and pressure favored the CO conversion and formation of these liquid (C₅-C₁₅) hydrocarbons. Higher selectivity of C₅ + hydrocarbons observed at lower H₂/CO ratio and GHSV of the feed gas. On the other hand high yields of methane resulted, with a decrease in C₅+ to C₁₁+ fractions at lower GHSV. Addition of MFI Zeolite (Si/Al = 35) to catalyst CuO-CoO-Cr₂O₃ resulted a high conversion of CO-hydrogenation, which may be due to its large surface area and small particle size creating more active sites. The homogeneity of various components was also helpful to enhance the synergistic effect of Co promoters.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

The effect of specific surface area on the activity of nano-scale ceria catalysts for methanol decomposition with and without steam at SOFC operating temperatures

Nano-particulate high surface area CeO₂ was found to have a useful methanol decomposition activity producing H₂, CO, CO₂, and a small amount of CH₄ without the presence of steam being required under solid oxide fuel cell temperatures, 700-1000 °C. The catalyst provides high resistance toward carbon deposition even when no steam is present in the feed. It was observed that the conversion of methanol was close to 100% at 850 °C, and no carbon deposition was detected from the temperature programmed oxidation measurement.The reactivity toward methanol decomposition for CeO₂ is due to the redox property of this material. During the decomposition process, the gas-solid reactions between the gaseous components, which are homogeneously generated from the methanol decomposition (i.e., CH₄, CO₂, CO, H₂O, and H₂), and the lattice oxygen on ceria surface take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen ( can produce synthesis gas (CO and H₂) and also prevent the formation of carbon species from hydrocarbons decomposition reaction (C_nH_m⇔nC+m/2H₂). V_O^·· denotes an oxygen vacancy with an effective charge 2⁺. Moreover, the formation of carbon via Boudouard reaction (2CO⇔CO₂+C) is also reduced by the gas-solid reaction of carbon monoxide with the lattice oxygen .At steady state, the rate of methanol decomposition over high surface area CeO₂ was considerably higher than that over low surface area CeO₂ due to the significantly higher oxygen storage capacity of high surface area CeO₂, which also results in the high resistance toward carbon deposition for this material. In particular, it was observed that the methanol decomposition rate is proportional to the methanol partial pressure but independent of the steam partial pressure at 700-800 °C. The addition of hydrogen to the inlet stream was found to have a significant inhibitory effect on the rate of methanol decomposition.     

Effect of ethanol on the chemistry of formation of precursors of polyaromatic hydrocarbons in a fuel-rich ethylene flame at atmospheric pressure

The effect of the addition of ethanol (EtOH) to the initial combustible mixture on the concentration of various compounds, in particular, those preceding the formation of polyaromatic hydrocarbons in a fuel-rich (equivalence ratio of fuel ? = 1.7) flat premixed ethylene/oxygen/argon flame at atmospheric pressure was studied experimentally and by numerical modeling using a detailed mechanism of chemical reactions. Concentrations of various stable and labile species, including reactants, major combustion products, and intermediates in C₂H₄/O₂/Ar and C₂H₄/EtOH/O₂/Ar flames were measured along the height above the burner using molecular beam mass spectrometry. Experimental mole fraction profiles were compared with those calculated using the previously proposed mechanisms of chemical reactions. This mechanism was analyzed to determine the cause of the ethanol effect on the flame concentration of propargyl, the main precursor of polyaromatic hydrocarbons.     

Performance studies of various cracking catalysts in the conversion of canola oil to fuels and chemicals in a fluidized-bed reactor

Studies were conducted at atmospheric pressure at temperatures in the range of 400–500°C and fluidizing gas velocities in the range of 0.37–0.58 m/min (at standard temperature and pressure) to evaluate the performance of various cracking catalysts for canola oil conversion in a fluidized-bed reactor. Results show that canola oil conversions were high (in the range of 78–98 wt%) and increased with an increase in both temperature and catalyst acid site density and with a decrease in fluidizing gas velocity. The product distribution mostly consisted of hydrocarbon gases in the C₁–C₅ range, a mixture of aromatic and aliphatic hydrocarbons in the organic liquid product (OLP) and coke. The yields of C₄ hydrocarbons, aromatic hydrocarbons and C₂–C₄ olefins increased with both temperature and catalyst acid site density but decreased with an increase in fluidizing gas velocity. In contrast, the yields of aliphatic and C₅ hydrocarbons followed trends completely opposite to those of C₂–C₄ olefins and aromatic hydrocarbons. A comparison of performance of the catalysts in a fluidized-bed reactor with earlier work in a fixed-bed reactor showed that selectivities for formation of both C₅ and iso-C₄ hydrocarbons in a fluidized-bed reactor were extremely high (maximum of 68.7 and 18 wt% of the gas product) as compared to maximum selectivities of 18 and 16 wt% of the gas product, respectively, in the fixed-bed reactor. Also, selectivity for formation of gas products was higher for runs with the fluidized-bed reactor than for those with the fixed-bed reactor, whereas the selectivity for OLP was higher with the fixed-bed reactor. Furthermore, both temperature and catalyst determined whether the fractions of aromatic hydrocarbons in the OLP were higher in the fluidized-bed or fixed-bed reactor.     

A Mechanistic Study of the Fischer–Tropsch Synthesis Using Transient Isotopic Tracing. Part 2: Model Quantification

A quantitative mechanistic model for the low-pressure Fischer–Tropsch synthesis reaction on a Co/Ru/TiO₂ catalyst is presented. Although the Fischer–Tropsch synthesis is operated at dry conditions, the presence of a physisorbed state is essential in the mechanism. The monolayer coverage of the C₃ to C₂₀ hydrocarbons in the physisorbed state is low at 0.3%. The most abundant chemisorbed surface species are CO_ads and two single-C species, C_α,ads and C_β,ads. The fractional surface coverage of growing hydrocarbon chains is low at 1.4%. With increasing H₂/CO feed ratio, the surface concentrations of H_ads and free sites increase. The rate coefficient for chain initiation is one order of magnitude lower than that for chain growth. The rate coefficient of ethene readsorption is one order of magnitude higher than that for the readsorption of higher 1-olefins. Chain branching and bond shift are important secondary reactions at atmospheric pressure, transforming reactive 1-olefins into unreactive internal and isoolefins and thus decreasing the asymptotic chain growth probability. As is to be expected, the termination to paraffin is represented by a hydrogenation reaction. The termination to olefin, however, appears to be a desorption reaction rather than a hydrogenation or a dehydrogenation reaction. The growing hydrocarbon chain is therefore represented by a C_iH_2i species. The quantitative mechanistic model as presented in this paper in combination with additional assumptions is used to successfully predict the characteristics of the product distribution of the high-pressure Fischer–Tropsch reaction.     

Preparation of molecular sieving carbon from waste resin by chemical vapor deposition

Molecular sieving carbons (MSCs) were prepared from carbonized phenol-formaldehyde resin wastes by the chemical vapor deposition (CVD) of the pyrolyzed carbon from hydrocarbon species. The pore size of the MSCs could be controlled in the range 0.37-0.42 nm by changing the hydrocarbon species pyrolyzed, the pyrolyzing temperature, and the processing time. It is shown that some of the MSCs have an excellent selectivity for separating CO₂ and CH₄, and others for separating C₃H₈ and C₃H₆. As the mechanism for controlling the pore size during CVD processing, we elucidated that the adsorption of hydrocarbon molecules first takes place on the pore surface and then the adsorbed hydrocarbons pyrolyze into carbon. Therefore, the pore size of the MSC can be adjusted by controlling the amount hydrocarbon adsorbed on the phenol-formaldehyde resin char.     

Formation of C m H n (m ≥ 3, n ≤ m) hydrocarbons from C1 and C2 molecules by high‐temperature (≥1000°C), short‐contact‐time (1–10 ms) nozzle beam reactions

A nozzle, fabricated from nickel, molybdenum, iron, palladium, and quartz was utilized to produce longer chain hydrocarbons, C _m H _n (m ≥ 3, n≤ m) from C₂ (ethane, acetylene) and C₁ (methane) reactants at nozzle temperature range 1000–1150°C. The conversion of ethane was close to 100% at T _noz = 1000°C, while that of methane reached 20% at T _noz = 1150°C. The contact time in the nozzle is in the 10^-3–10^-2 s range. The reactions are first and higher order in reactant pressure. The reaction mechanism involves the formation of free radicals at the nozzle surface followed by gas‐phase reactions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diffusion and catalytic reaction in zeolite ZSM-5

Rates of reaction and product-distributions for small and large pellets and for small and large crystals of the same zeolite-preparation (H-ZSM-5) were observed in order to evaluate the influence of diffusion (concentration-gradients) on the conversion of (CH₃)₂O to hydrocarbons with this catalyst. Diffusivities of (CH₃)₂O and C₆H₆ in the zeolite crystals were obtained from sorption kinetics. Macropore-diffusion affects activity and selectivity if pellets with dia > 2 mm are used; intracystalline mass-transfer does not seem to be important with respect to catalyst-activity and selectivity at temperatures below 600 K. The conventional model of diffusion and reaction in porous catalysts can not be applied to the entire reaction-network in the zeolite-crystals, because migration of olefins in the zeolite can not be understood as random-walk diffusion.     

Kinetic study of the pyrolysis of tetrachloroethylene in a hydrogen rich environment

Experiments on pyrolysis of C₂Cl₄ with hydrogen in argon bath gas (C₂Cl₄: H₂: AR=0.5:7:92.5) were performed in a laboratory scale flow reactor, to determine reaction paths and kinetic parameters, plus to observe hydrogen as a source to convert chlorocarbons into hydrocarbons and HCl. The reaction was carried out at 1 atmosphere total pressure in the tubular flow reactor, over temperature ranges from 575 to 850 °C, with average residence times in the range of 0.3 to 1.2 s. The major reaction products were C₂HCl₃, CH₂CCl₂, C₂H₆, C₂H₄ and HCl. Trace intermediates including CH₄, C₂H₂, C₃H₆, C₃H₄, C₄H₈, C₄H₆, C₄H₄, C₂H₃Cl, C₂HCl, trans-CHClCHCl, cis-CHClCHCl C₂Cl₂ and aromatic compounds were found. The equation for overall loss of C₂Cl₄ (k (s⁻¹)) was 1.35×10⁶exp(−27055/RT). This study shows that C₂H₄ became the major product for reaction temperatures higher than 700 °C, and became one of the final products together with HCl.A detailed kinetic mechanism consisting of 202 elementary reactions with 59 species was developed to model the results obtained from the experiments. Sensitivity analyses were performed to rank the significance of each reaction in the mechanism. Modeling and sensitivity analysis revealed that C₂Cl₄+H→C₂HCl₃+Cl, C₂Cl₄+H→C₂Cl₃+Cl, and C₂Cl₄→C₂Cl₃+Cl are the primary reactions for the decomposition of C₂Cl₄.     

Methane conversion to higher hydrocarbons in a dielectric-barrier discharge reactor with Pt/γ-Al2O3 catalyst

Plasma catalytic reactions were applied to the conversion of methane to C₂, C₃ or higher hydrocarbons in a dielectric-barrier discharge (DBD) reactor at atmospheric pressure. Methane conversion was increased with the increase of Pt loading on γ-Al₂O₃. The highest C₂H₆ selectivity was 50.3% when 3 wt% Pt/γ-Al₂O₃ catalyst was calcined at 573 K. Methane conversion was increased with the increase of the catalyst weight in DBD reactor. The major products were C₂H₆ and C₃H₈, which were independent of catalyst weight in the presence of catalyst.