Formation Of Carbon Oxides Research Articles

Nanopatterning of carbonaceous structures by field-induced carbon dioxide splitting with a force microscope

We report a tip-based nanofabrication method to generate carbon nanopatterns. The process uses the field-induced transformation of carbon dioxide gas into a solid material. It requires the application of low-to-moderate voltages ∼10–40 V. The method allow us to fabricated sub-25 nm dots and it can be up scaled to pattern square centimeter areas. Photoemission spectroscopy shows that the carbon is the dominating atomic species of the fabricated structures. The formation of carbon nanostructures and oxides by atomic force microscope nanolithography expands its potential by providing patterns on the same sample with different chemical composition.

Mechanism of the formation of carbon oxides under conditions of the oxidative chlorination of methane: III. Kinetics of the reaction of CCl4 with oxygen on copper-containing salt catalysts

The kinetics of the oxidative dechlorination of CCl4 on two supported catalysts for methane oxychlorination (CuCl2-KCl and CuCl2-KCl-LaCl3) at 350–425°C was studied using a gradientless method. Although these catalysts exhibited different activities, the rate of reaction on them was described by one exponential equation at the same activation energy of E ≈ 26.3 kcal/mol. A two-path reaction mechanism was proposed, which involved the dissociative adsorption of initial reactants and chlorine with the formation of phosgene and the oxidation of phosgene to CO2. The occurrence of two types of active sites on the catalyst surface was assumed.

Mechanism of the formation of carbon oxides under conditions of the oxidative chlorination of methane: IV. Kinetics of the reaction of CCl4 with oxygen on copper-containing salt catalysts for methane oxychlorination at reduced partial pressures of Cl2 in the reaction mixture

The kinetics of the oxidative dechlorination of CCl4 on two supported salt catalysts for methane oxychlorination (CuCl2-KCl and CuCl2-KCl-LaCl3) in the presence of methane at 350–450°C was studied using a gradientless method. It was found that, at \( P_{Cl_2 } \) > 0.15 kPa, the presence of methane in the reaction mixture had no effect on the kinetics of CCl4 oxidation but decreased the activation energy E to ∼22 kcal/mol. At \( P_{Cl_2 } \) < 0.15 kPa, the kinetic orders with respect to oxygen and chlorine changed and E decreased to ∼17 kcal/mol. The rate of formation of CO2 as a by-product of the reaction of methane oxychlorination was described by an exponential equation in combination with rate equations for the oxidation of other chloromethanes and methane. An equation consistent with the observed rate laws of this reaction in the presence of methane was derived from the previously proposed reaction scheme for the mechanism of the oxidative dechlorination of CCl4.

A comparative kinetic study on the oxidative coupling of methane over LSCF perovskite-type catalyst

A gas-phase heterogeneous kinetics is described over perovskite with formula La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) for the oxidative coupling of methane under differential conversion conditions in a microcatalytic fixed-bed reactor. The ethane (C2H6) and carbon oxides (COX) formation and methane conversion rates were obtained as a function of methane and oxygen partial pressure under experimental conditions of: 0.20<PCH4<0.82atm, 0.04<PO2<0.15atm, and 1073 < T < 1173 K. The different kinetic models were examined and two Eley–Rideal mechanisms (a and b) were successfully performed to fit the experimental data into the theoretical relations from which two mechanisms having a first step of reversible adsorption of molecular oxygen on the catalyst surface with a subsequent reaction of methane molecule with the adsorbed oxygen were selected for data analysis. The parity graph of the ethane formation rate and the F-test statistical analysis were indicated mechanism “a” involving the formation of CH3O2 (methylperoxy) intermediate species can be suggested as the favorable (main) model for the analysis of kinetic parameters of OCM reaction over LSCF perovskite catalyst. Furthermore, the kinetic rate constants and activation energies were estimated for the selected models. Eventually the best model was introduced in course of conceptual and statistical analysis.

H2 production from methane pyrolysis over commercial carbon catalysts: Kinetic and deactivation study

Abstract Hydrogen production from catalytic methane decomposition (DeCH4) is a simple process to produce high purity hydrogen with no formation of carbon oxides (CO or CO2). However, to completely avoid those emissions, the catalyst must not be regenerated. Therefore, it is necessary to use inexpensive catalysts, which show low deactivation during the process. Use of carbon materials as catalysts fulfils these requirements. Methane decomposition catalysed by a number of commercial carbons has been studied in this work using both constant and variable temperature experiments. The results obtained showed that the most active catalyst at short reaction times was activated carbon, but it underwent a fast deactivation due to the deposition of the carbon formed from methane cracking. On the contrary, carbon blacks, and especially the CB-bp sample, present high reaction rates for methane decomposition at both short and long reaction times. Carbon nanotubes exhibit a relatively low activity in spite of containing significant amounts of metals. The initial loss of activity observed with the different catalysts is attributed mainly to the blockage of their micropores due to the deposition of the carbon formed during the reaction.

On the formation of higher carbon oxides in extreme environments

Due to the importance of higher carbon oxides of the general formula CO x ( x > 2) in atmospheric chemistry, isotopic enrichment processes, low-temperature ices in the interstellar medium and in the outer solar system, as well as potential implications to high-energy materials, an overview on higher carbon oxides CO x ( x = 3–6) is presented. This article reviews recent developments on these transient species. Future challenges and directions of this research field are highlighted.

Kinetic study of the UO 2/C interaction by high-temperature mass spectrometry

For very high-temperature reactors (V)-HTR, one of Generation IV future systems, the high-level operating temperature of the fuel materials in normal and accidental conditions requires the prediction of the possible chemical interactions between the fuel component (UO 2± x ) and the structural materials (C, SiC). To predict the thermo-mechanical behaviour of the TRi-ISOtropic (TRISO) particle, it is necessary to better understand the gaseous carbon oxides formation at the fuel–buffer interface that leads to the build up of the internal pressure. High equilibrium CO (g) pressures resulting from the UO 2± x /C reaction are predicted using thermodynamic calculations. The kinetic mechanisms involved in this reaction that limit this pressure increase have to be determined by convenient experiments and associated models. Some of the reported data on the kinetics of CO (g) formation due to the UO 2± x and carbon interaction have been reviewed. The discrepancies between the reaction mechanisms can be explained ( i) by the different geometries and sample types and ( ii) by the oxide stoichiometry and the flowing gas used during the experiments. Depending on these characteristics, the phenomena involved in CO (g) formation can be of three different origins: interface, surface or diffusion. Using high-temperature mass spectrometry (HTMS), kinetic measurements of the CO (g) and CO 2(g) species evolved during the interaction between UO 2± x and carbon were performed. The samples are pressed pellets consisting of a mixture of UO 2± x (60% molar) and carbon black (40% molar) powders. CO (g) is the major product above 1200 K. Rates of the CO (g) formation have been established taking into account the oxygen composition of the non-stoichiometric uranium dioxide and temperature. Results underline the upmost importance of kinetic factors for studying the CO (g) pressure variation inside the TRISO particle.

Removal of carbon layers by oxygen glow discharges in TEXTOR

Glow discharge cleaning (GDC) in mixtures of O2 and He was applied in TEXTOR for about 4h to study the removal of redeposited carbon layers. The behaviour of the formation of volatile carbon oxides (CO and CO2) and its dependence on the O2 injection rate and on the addition of He into the O2 GDC are described. Carbon (5.22g) was removed from TEXTOR in about 4h GDC corresponding to a carbon removal rate of ∼2.1×1019C/s. Plasma recovery was obtained after wall cleaning in H2 GDC and finally by boronisation.

Dual-Bed Catalytic System for Direct Conversion of Methane to Liquid Hydrocarbons

A dual-bed catalytic system is proposed for the direct conversion of methane to liquid hydrocarbons. In this system, methane is converted in the first stage to oxidative coupling of methane (OCM) products by selective catalytic oxidation with oxygen over La-supported MgO catalyst. The second bed, comprising of the HZSM-5 zeolite catalyst, is used for the oligomerization of OCM light hydrocarbon products to liquid hydrocarbons. The effects of temperature (650–800 °), methane to oxygen ratio (4–10), and SiO 2/Al 2O 3 ratio of the HZSM-5 zeolite catalyst on the process are studied. At higher reaction temperatures, there is considerable dealumination of HZSM-5, and thus its catalytic performance is reduced. The acidity of HZSM-5 in the second bed is responsible for the oligomerization reaction that leads to the formation of liquid hydrocarbons. The activities of the oligomerization sites were unequivocally affected by the SiO 2/Al 2O 3 ratio. The relation between the acidity and the activity of HZSM-5 is studied by means of TPD-NH 3 techniques. The rise in oxygen concentration is not beneficial for the C 5+ selectivity, where the combustion reaction of intermediate hydrocarbon products that leads to the formation of carbon oxide (CO+CO 2) products is more dominant than the oligomerization reaction. The dual-bed catalytic system is highly potential for directly converting methane to liquid fuels.

Kinetics of Oxidative Coupling of Methane over Na/BaTiO3/MgO Catalyst: Mechanistic Aspects

The kinetics of oxidative coupling of methane reaction over a Na/BaTiO 3 /MgO catalyst was studied under differential conditions in a fixed bed flow reactor. The experiments were carried out at 725, 775 and 825°C. Methane and oxygen partial pressures in the reactor were varied from 0.05 to 0.92 and 0.05 to 0.17 atm respectively with the total pressure of 1 atm for all reactions. Using molecular point of view the semi-mechanistic reaction rate equations for C 2 H 6 (ethane) formation and CH 4 (methane) conversion rates have been proposed. The approximate total carbon oxides formation rates have also been deduced intuitively. The parameter 'n' was introduced that shows the portion of the reaction, which produces carbon oxides. After investigating some mechanistic approaches one of them was selected as the best mechanism.

Kinetics of Oxidative Coupling of Methane over Na/BaTiO&lt;sub&gt;3&lt;/sub&gt;/MgO Catalyst: Mechanistic Aspects

The kinetics of oxidative coupling of methane reaction over a Na/BaTiO3/MgO catalyst was studied under differential conditions in a fixed bed flow reactor. The experiments were carried out at 725, 775 and 825°C. Methane and oxygen partial pressures in the reactor were varied from 0.05 to 0.92 and 0.05 to 0.17 atm respectively with the total pressure of 1 atm for all reactions. Using molecular point of view the semi-mechanistic reaction rate equations for C2H6 (ethane) formation and CH4 (methane) conversion rates have been proposed. The approximate total carbon oxides formation rates have also been deduced intuitively. The parameter 'n' was introduced that shows the portion of the reaction, which produces carbon oxides. After investigating some mechanistic approaches one of them was selected as the best mechanism.

Effect of vanadium substitution in the cesium salts of Keggin-type heteropolyacids on propane partial oxidation

The catalytic behavior of the cesium salts of heteropolyacids with vanadium species substituted at the primary and/or secondary structure of Keggin oxoanions was systematically studied for propane partial oxidation. The physicochemical properties of the samples were explored by FTIR, UV–visible diffuse reflectance spectroscopy, XRD, and TPR techniques. The introduction of vanadium species to the primary structure of heteropoly compounds (HPCs) resulted in enhanced catalytic activity; with time on stream, a small fraction of vanadium species could be segregated from the oxoanion. When vanadium species was introduced to the secondary structure of HPCs, activity and selectivity to acrylic acid (AA) were notably enhanced. It is deduced that a small amount of vanadium species at the secondary structure of Keggin-type HPCs could interact synergistically with the oxoanion and cesium entities, inducing a positive effect on propane oxidation to AA as a result. The highest AA yield was observed over the H 0.1Cs 2.5(VO) 0.2PMo 12O 40 catalyst. The effect of operation variables on catalyst performance was also investigated, and it was found that the H 0.1Cs 2.5(VO) 0.2PMo 12O 40 catalyst was efficient in both oxygen-rich and hydrocarbon-rich atmospheres, and that the presence of steam can considerably suppress the formation of carbon oxides.

Flame‐retardant effect of polyaniline coating deposited on cellulose fibers

AbstractFiltration paper was coated with a thin polyaniline film. The content of conducting polymer was 8.2 and 6.3 wt % for polyaniline hydrochloride and polyaniline base, respectively. After burning, the coated material retains the original fibrilar morphology of cellulose. The polyaniline coating converts to solid carbonaceous products. The resulting structure prevents the formation of gaseous carbon oxides by restricting the access of oxygen to cellulose. While the ash from the uncoated paper after burning is 0.005 wt %, the microtubular residue of polyaniline‐coated paper is 16–24 wt % of the original mass. The flame‐retardant performance of polyaniline and poly(1,4‐phenylenediamine) coatings was comparable both for the protonated forms and the corresponding bases. The conversion of polyaniline‐coated cellulose fibers to solid pyrolytic products was characterized by FTIR and Raman spectroscopies. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2347–2354, 2005

Oxidation of o-xylene on mesoporous Ti-phosphate-supported VO x catalysts and promoter effect of K + on selectivity

The selective oxidation of o-xylene on catalysts based on mesoporous titanium phosphate-supported vanadium oxide has been studied. The catalysts were characterized by different physico-chemical techniques (XRD, XPS, N 2 isotherms, TPD of chemisorbed NH 3 and Raman spectroscopy). The conversion and yield to phthalic anhydride increased with vanadium oxide loading up to one theoretical monolayer. Beyond this point, no substantial improvement was achieved. Moreover, a significant improvement in the yield of phthalic anhydride, coinciding with a decrease in the formation of carbon oxides, was observed when potassium was incorporated to the vanadium catalysts. The enhancement of the phthalic anhydride yield was correlated with the neutralization of the stronger surface acid centers by the K + ions, providing evidence for the hypothesis that stronger acid sites are involved in the formation of total oxidation products (carbon oxides).

The contribution of homogeneous and non-oxidative side reactions in the performance of vanadyl pyrophosphate, catalyst for the oxidation of n-butane to maleic anhydride, under hydrocarbon-rich conditions

The reactivity of vanadyl pyrophosphate, catalyst for the selective oxidation of n-butane to maleic anhydride, was examined under n-butane-rich conditions, simulating a feed in which oxygen is the limiting reactant, and a process in which the unconverted n-butane is recycled. A lower selectivity to maleic anhydride was found with respect to the hydrocarbon-lean conditions, due to the higher selectivity to carbon oxides and to the formation of C 8 by-products: tetrahydrophthalic and phthalic anhydrides. The latter compounds formed by a consecutive reaction of maleic anhydride with the unsaturated C 4 intermediates. This occurred under conditions of total oxygen conversion, due to the decreased catalyst oxidizing property. A relevant contribution of radicalic, homogeneous reactions was also observed, which mainly led to the formation of carbon oxides and olefins. This contribution decreased in the presence of the catalyst, which acted as a radical scavenger, but nevertheless remained important at temperatures higher than 400 °C. When conditions were used under which the conversion of oxygen was not total, olefins generated in the gas phase reacted at the catalyst surface yielding maleic anhydride. This homogeneously initiated heterogeneous process led to an unusual effect, of a relevant increase of maleic anhydride yield over 400 °C.

Inorganic Membrane Reactors for the Gas Phase Partial Oxidation of Toluene

The gas phase oxidation of toluene was studied as an alternative way to obtain benzaldehyde and benzoic acid. A traditional support (TiO 2 ) and a new mixed Al 2 O 3 –TiO 2 support for the active catalyst element, vanadium, was used. The oxidation process was investigated by either using a packed bed reactor or inorganic membrane reactors. Vanadium was deposited on the supports (powder or inorganic membranes) through either ion exchange or impregnation from a solution of ammonium metavanadate. The catalysts were characterized in terms of vanadium content and distribution, morphology, specific surface area and porosity. The reaction runs quantified catalytic activity of the catalysts prepared. The effect of the temperature and of the hydrocarbon=oxygen ratio was investigated. Selectivity to intermediate oxygenated products was higher for vanadium supported by Al 2 O 3 –TiO 2 and when the hydrocarbon to oxygen ratio was higher, the formation of carbon oxides was lowered. Finally inorganic membrane reactors were explored using different reactor configurations (i.e. both the reactants, toluene and oxygen, were flowed inside a inorganic membrane tube with catalytic walls, or were forced through the membrane from the internal side to the outer side).

Erosion of Carbon in Dual-Beam Experiments: an Overview

Multi-species impact is of high relevance for fusion reactors, where, in addition to the hydrogen fuel, the He ash, impurity species (e.g., C and O), and in some cases injected elements are also present. Combined irradiation of graphite with relatively low-energy H+ (typically below 100 eV) and energetic (above 300 eV) hydrogenic and non-hydrogenic ions leads to an enhancement of the chemical erosion yield compared to the low-energy H+-only case. This is due to the additional energy deposition and associated lattice damage by the added energetic ions. Studies of combined species irradiation also have the potential to enhance our understanding of the mechanisms leading to erosion and hydrogen transport in graphite. Mixed isotope experiments with simultaneous H+–D+ irradiation of graphite led to the conclusion that thermalized H and D move and recombine on internal surfaces within the implantation zone. In contrast, during H+–D+ irradiation, methane molecules are formed at the end of range of the incident ions. Combined H+ and O+ irradiation of graphite leads to the formation of hydrocarbons, carbon oxides and water, with water being formed at the end of the O+ implant range.

Acyl Iodides in Organic Synthesis: IV. Reaction of Acetyl Iodide with Carboxylic Acids

In contrast to acyl chlorides, reactions of acetyl iodide with monocarboxylic acids follow the exchange pattern to give the corresponding acyl iodides and acetic acid. The reaction attracts interest from the preparative viewpoint as a simple and convenient route to acyl iodides. Acetyl iodide reacts with phthalic acid, yielding acetic acid and phthalic anhydride, while the reaction of acetyl iodide with oxalic acid leads to formation of acetic acid, carbon(II) oxide, and molecular iodine.

Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile: Part II. Effect of catalyst composition on catalytic performance

Rutile-type, Cr/V/Sb mixed oxides having different atomic ratio between components were studied as catalysts for propane and propylene ammoxidation to acrylonitrile. Catalysts were more active than Cr/Sb and V/Sb mixed oxides; this was attributed to (i) the higher specific surface area of Cr/Sb/O and Cr/V/Sb/O with respect to V/Sb/O, and (ii) the preferential formation of V 4+ in Cr/V/Sb/O. The nature of the V species and the catalytic performance of Cr/V/Sb/O samples were functions of the (Cr+V)/Sb atomic ratio. When the latter was higher than ≈1, the prevailing species was V 4+; the catalysts were very active but poorly selective to acrylonitrile (selectivity lower than 20%) because of the prevailing formation of carbon oxides and propylene. This was due to the absence of sites able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was between ≈1 and ≈0.5, catalysts reached a selectivity to acrylonitrile between 20 and 30%, and to propylene lower than 10%. In these samples, the presence of intra-crystalline Sb gradients in the rutile lattice provided a Sb surface enrichment and the development of allylic ammoxidation sites, able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was lower than ≈0.5, i.e. in systems having excess Sb, the prevailing species was V 3+; the selectivity to acrylonitrile was higher than 30%, with low formation of carbon oxides and of propylene. In this case additional sites for allylic ammoxidation were provided by excess antimony oxide dispersed over the rutile surface.

Composition and Electrical Properties of Metallic Ru Thin Films Deposited Using Ru(C6H6)(C6H8) Precursor

Metallic ruthenium films were prepared by metalorganic chemical vapor deposition (CVD) using a new precursor named (η6-benzene)(η4-1,3-cyclohexadiene)ruthenium (Ru(C6H6)(C6H8)) in Ar atmosphere, and the absolute composition and electrical properties were investigated. The absolute composition including hydrogen was determined by means of the elastic recoil detection time of flight (ERD-TOF). It was found that carbon contents in the films markedly decreased when tetrahydrofuran (THF) was supplied with the precursor during deposition. The variation in carbon content could be interpreted by the formation of hydrocarbon compounds as well as the formation of carbon oxide, resulting from the reaction between carbon and THF. In particular, Ru films contained hydrogen that originated in the hydrogen atoms in the precursor and was involved in the CVD process due to the catalytic effect of ruthenium on hydrocarbon and hydrogen. It was shown that grain size, among several other factors strongly affected the electrical properties of ruthenium films.