CO Disproportionation Research Articles

Effect of hydrogen on the synthesis of carbon nanofibers by CO disproportionation on ultrafine Fe3O4

AbstractCarbon nanofibers (CNFs) are grown by catalytic CO disproportionation over ultrafine Fe3O4 catalyst at a hydrogen concentration of 0–29.17%, and the time‐depending rates of CNFs growth are continuously monitored and the morphologies of the as‐synthesized CNFs are analyzed. Increasing H2 concentration will lower CO dissociation energy and assist catalyst reconstruction so as to shorten the induction period and increase the growth rate of CNFs, but it will also increase the rate of catalyst deactivation because carbon hydrogasification is not possible and carbon diffusion in the catalyst particle is rate limiting. As a result of H2‐induced catalyst reconstruction and carbon deposition, the morphology of the CNFs changes from twisty to helical and to straight and becomes less entangled when the H2 concentration is increased. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

CO dissociation and CO+O reactions on a nanosized iron cluster

Catalysis over metal nanoparticles is essential for the growth of carbon nanotubes and all the properties of the resulting nanotube, such as diameter and chirality, are affected by the metal particle. Thus, it is very important to understand the carbon chemistry taking place on nanometer size metal particles. Here we present the first ab initio computational study of chemical reactions on a nanosized iron cluster. The clusters have reaction sites, such as edges and vertexes between the facets, Which have not been studied before. First principles electronic structure calculations, fully incorporating the effects of spin polarization and non-collinear magnetic moments, have been used to investigate CO disproportionation on an isolated Fe55 cluster. After CO dissociation, O atoms remain on the surface while C atoms move into the cluster, presumably as the initial step toward carbide formation. Here we show that the lowest CO dissociation barrier found on the cluster (0.77 eV) is lower than on most previously studied Fe surfaces. This dissociation occurs on a vertex between the facets. Several possible paths for CO2 formation were identified. The calculated lowest reaction barrier is 1.08 eV, which is comparable to the barrier of 0.65 eV obtained by experiment.

Kinetically controlled synthesis of carbon nanofibers with different morphologies by catalytic CO disproportionation over iron catalyst

Carbon nanofibers (CNFs) were synthesized by CO disproportionation on iron catalyst at CO concentration between 58.3% and 75.0%, H 2 concentration between 8.3% and 25.0% and reaction temperature between 833 and 913 K. The time-depending rate of CNFs growth as a function of time was determined by an on-line mass spectrometer and the morphologies of all CNFs products were observed by electronic microscopy. Not only the CNFs growth rate but also the morphology of the grown CNFs were shown to vary with the three operating variables. SEM and TEM images showed that the three-dimensional morphologies of the CNFs were twist, helical or straight and an interesting relationship between the maximal growth rate and the morphology was observed. When the growth rate was between 0.8 and 0.9 mmol/(s gcat), the CNFs were twist. As the growth rate increased to 1.0 mmol/(s gcat), more helical nanofibers appeared. Straight nanofibers were produced when the growth rate reached the level of 1.2 mmol/(s gcat). Finally when the rate of CNFs growth was high at 1.8 mmol/(s gcat), the absolute majority of the solid products was amorphous carbon coexisting with some short and thick nanofibers. Under different operating conditions, the crystal faces of the catalysts had different anisotropy properties for carbon deposition, thus producing CNFs with different morphologies.

CO Disproportionation on a Nanosized Iron Cluster

First-principles electronic structure calculations, fully incorporating the effects of spin polarization and noncollinear magnetic moments, have been used to investigate CO disproportionation on an isolated Fe cluster. After CO dissociation, which occurs on a vertex between the facets, O atoms remain on the surface while C atoms move into the cluster as the initial step toward carbide formation. The lowest CO dissociation barrier found (0.77 eV) is lower than that on most of the studied Fe surfaces. Several possible paths for CO2 formation were identified. The lowest reaction barrier was 1.08 eV.

Comparative in Situ DRIFTS-MS Study of12CO- and13CO-TPR on CuO/CeO2Catalyst

A ceria-supported copper oxide catalyst has been examined with respect to interaction with CO by means of CO-TPR tests employing a DRIFTS cell as reactor. Parallel analysis of isotopic shifts in the DRIFTS spectra obtained upon interaction of the catalysts with either 12CO or 13CO allows unambiguous assignment of the various carbonyl and carbonate-type species formed. The study is complemented by separate examination of the stability of carbonyl species focused to get hints on the nature of CO adsorption centers at the catalyst surface. Joint analysis of gases evolution during the CO-TPR tests and DRIFTS spectra is employed to determine the nature of processes taking place during such tests. Additionally, analysis of the various CO2 isotopes evolving during the course of a TPO run subsequent to 13CO-TPR test is employed to discard the occurrence of CO disproportionation upon interaction of the catalyst with CO up to 400 °C.

Sol−Gel Synthesis and Characterization of Co−Mo/Silica Catalysts for Single-Walled Carbon Nanotube Production

A series of silica-supported Co−Mo samples prepared by the sol−gel method has been compared as catalysts for the synthesis of single-walled carbon nanotubes (SWNT). The concentration ratio of ammonium hydroxide to the silica precursor tetraethoxysilane (TEOS) has an important effect on the resulting morphology of the silica support and, consequently, on the nature of the Co−Mo catalytic species. In turn, these morphology changes have significant effects on carbon yield, quality, and type of the single-walled carbon nanotubes obtained by the disproportionation of CO at 750 °C. In addition, a catalyst with an open microscale structure has been prepared by using carbon fibers as burnable sacrificial templates. This open structure results in several-fold enhanced carbon yield, while keeping the same nanotube quality as those obtained on conventional powder catalysts.

Kinetic Modeling of the SWNT Growth by CO Disproportionation on CoMo Catalysts

A kinetic model has been developed to describe the growth of single-walled carbon nanotubes (SWNT) in the CoMoCAT method, which is based on the disproportionation of CO on supported CoMo catalysts. The model attempts to capture mathematically the different stages involved in this method: (i) catalyst activation or in-situ creation of active sites, i.e., reduced Co clusters by transformation of CoMoOx precursor species, or oxidized sites; (ii) CO decomposition over active sites, which increases the surface fugacity of carbon until reaching a certain threshold; (iii) nucleation of ordered forms of carbon; (iv) C diffusion (both across the surface and into the metal particle); (v) SWNT growth; (vi) termination, by either deactivation of the catalyst active sites or by increase in the carbon concentration at the metal/SWNT interface, approaching that of the metal/gas interface and eliminating the driving force for diffusion. Previous investigations have only explained the growth termination by the former. Here, we emphasize the possible contribution of the later and propose a novel "hindrance factor" to quantify the effect of nanotube interaction with its surroundings on the growth termination. To test the kinetic model and obtain typical values of the physical parameters, experiments have been conducted on a CoMo/SiO2 catalyst in a laboratory flow reactor, in which the rate of carbon deposition was continuously evaluated by the direct measurement of the CO2 evolution as a function of time. The experimental data are fitted very well with model.

Reaction swing approach for hydrogen production from carbonaceous fuels

The paper presents the results of the investigations on a reaction swing approach for the production of high purity hydrogen from syngas exploiting the Boudouard at temperatures ranging from 550 to 700 ∘ C and pressures ranging from 0 to 150 psig. The concept of the process is the sequential use of a catalyst, that enhances the CO disproportionation reaction in conjunction with the water gas shift reaction to completely eliminate the CO in the product stream, and a CO 2 removal agent for in situ capture of the produced CO 2 . The thermodynamic evaluation of the above system shows the potential of achieving greater than 99% hydrogen purity values in the product stream even at atmospheric pressures. The effects of temperature, pressure and steam addition on the hydrogen enrichment are reported. Experiments were also conducted to evaluate the effect of catalyst and CO 2 removal material loadings on the product gases. Increasing the pressure improved the purity and the duration of the enrichment cycle. The separation efficiency was maximized at a temperature of 650 ∘ C . A 1:2 ratio of the catalyst to CO 2 removal material was found to be sufficient to produce nearly carbon-free hydrogen for over an hour before regeneration. The use of 25% steam was found to increase the hydrogen yield by nearly 50% (due to the contribution of the water gas shift). The results from simultaneous coal gasification and hydrogen enrichment experiments in a single reactor are also presented. It was found that for a Fe 2 O 3 : coal ratio of 22:1 for effective gasification and over 99% pure hydrogen stream.

Morphology control of Fe2O3 nanocrystals and their application in catalysis

We synthesized four iron oxide catalysts with different morphologies and testedtheir activities in CO disproportionation. The four iron oxides were mesoporousoxide, two different sized iron oxide nano spheres and silica supported iron oxide.Hypothetical equivalent average particle size (EAPS), which was calculated from thesurface area and unit cell parameter of the particles, was used to evaluate thecatalytic activities of the iron oxides. A size effect (EAPS effect) was observedin these iron oxides. The CO disproportionation test results showed that silicasupported iron oxide was the most active due to it having the smallest EAPS.

Residue catalyst support removal and purification of carbon nanotubes by NaOH leaching and froth flotation

The synthesis of single-walled carbon nanotubes (SWNTs) by the catalytic decomposition of gaseous carbon-containing molecules has been considered to be the most promising process for large-scale production with a relatively low cost. However, the as-prepared SWNTs from this process always contain significant amounts of impurities such as metallic catalysts, catalyst support, and other unwanted forms of carbon. For a variety of potential applications, the purification of SWNTs is considered to be an important step. In this research, NaOH solution was utilized to dissolve the silica support, a major impurity, from the SWNTs produced by the CO disproportionation over a CoMo/SiO 2 catalyst. Subsequently, froth flotation was used to further recover and concentrate the total carbon after the NaOH pretreatment. To achieve a high purity of total carbon by the proposed purification techniques, the effects of operating parameters were investigated in order to minimize the entrainment of undissolved silica with the froth. At the optimum operating conditions: 3 h of sonication time, 30 mg/l of surfactant dosage, 1.0 g/l of pulp density, 100 ml/min of air flow rate, and 22 cm of froth height, the purity of total carbon was found to increase to 30% after the first step of NaOH treatment and to 78% after the second step of froth flotation as compared to the initial carbon content of only 4% of the as-prepared SWNTs. From the temperature-programmed oxidation (TPO) results, the Raman spectra and the scanning electron microscope (SEM) images, the physical and chemical structures of SWNTs are not damaged by NaOH treatment and froth flotation.

The dynamic surface chemistry during the interaction of CO with ceria captured by Raman spectroscopy

The interaction of CO with ceria under conditions typically used to measure the oxygen storage capacity (OSC) of automotive three way catalysts (TWC) has been investigated by in situ Raman spectroscopy. During exposure of the ceria to CO at 623 K vibrational bands at 1582–1600 and 1331–1340 cm−1 appeared; these bands increased with increasing time of exposure to CO. The band positions are consistent with phonon modes of carbon; however, assignment to carboxylate species or carbonate species cannot be excluded. Subsequent exposure to O2 at room temperature resulted in a decrease in the intensities of the 1582–1600 and 1331–1340 cm−1 bands by more than 90%. As well, exposure to O2 at room temperature also resulted in the appearance of Raman modes characteristic of formate and peroxide surface species. The mechanism by which formate forms upon room temperature O2 exposure is discussed in the context of the assignment of the 1582–1600 and 1331–1340 cm−1 bands to carbon phonon modes which result from the disproportionation of CO on reduced ceria.

Pd nanoparticles with highly defined structure on MgO as model catalysts: An FTIR study of the interaction with CO, O 2, and H 2 under ambient conditions

Model catalytic studies under ambient conditions require materials with well-defined structures for the establishment of clear relationships between structural and catalytic properties. In the present work, such a system was prepared by controlled decomposition of organometallic Pd precursors on a MgO support. The resulting Pd nanoparticles of highly defined morphology were characterized by high-resolution transmission electron microscopy and investigated with respect to their interaction with CO, O 2, and H 2 by IR spectroscopy (DRIFTS). Temperature-dependent CO adsorption experiments revealed the available adsorption sites on the Pd particles and indicated CO disproportionation on the Pd particles. We report in particular on the interplay between metal and support in these processes, which was analyzed by a thorough comparison of Pd/MgO with the pure MgO support. Investigations of the interaction with CO and H 2 suggest that Pd facilitates the formation of formates on MgO via dissociation of hydrogen on the Pd particles. Furthermore, CO oxidation was studied as a function of the CO/O 2 ratio in the temperature range 100–150 °C.

A novel hybrid carbon material

Both fullerenes and single-walled carbon nanotubes (SWNTs) exhibit many advantageous properties. Despite the similarities between these two forms of carbon, there have been very few attempts to physically merge them. We have discovered a novel hybrid material that combines fullerenes and SWNTs into a single structure in which the fullerenes are covalently bonded to the outer surface of the SWNTs. These fullerene-functionalized SWNTs, which we have termed NanoBuds, were selectively synthesized in two different one-step continuous methods, during which fullerenes were formed on iron-catalyst particles together with SWNTs during CO disproportionation. The field-emission characteristics of NanoBuds suggest that they may possess advantageous properties compared with single-walled nanotubes or fullerenes alone, or in their non-bonded configurations.

Deactivation Studies over Ni−K/CeO2−Al2O3 Catalyst for Dry Reforming of Methane

The activity of 13.5Ni−2K/10CeO2−Al2O3 catalyst was tested for 60 h time on stream (TOS) for the carbon dioxide reforming of methane at three different temperatures (650, 700, and 750 °C). The amount of coke deposited on the catalyst at different time intervals was estimated by thermogravimetric analysis (TGA). Results suggested that both CH4 cracking and CO disproportionation contribute to coke deposition. No appreciable deactivation was observed for the catalysts at all three temperatures for the 60-h run. The used catalysts were characterized by Brunauer−Emmett−Teller (BET) surface area, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis to understand the morphology of the coke deposited on the catalyst. XRD patterns showed that carbon formed on 13.5Ni−2K/10CeO2−Al2O3 after 60 h TOS at 700 and 750 °C dispersed well and could not be observed, while after 60 h TOS at 650 °C, mainly graphitic carbon (peak at 2θ = 26.3°) formed. XPS characterization demonstrated the existence of mainly two kinds of carbon species, graphitic (−C−C−) and oxidized carbons (−C−CO−, CO3-). TGA, XRD, and XPS studies revealed that significant amount of coke was deposited on 13.5Ni−2K/10CeO2−Al2O3 catalyst at 650 °C. However, the amount of accumulated coke with TOS did not affect the high activity of the catalyst up to 60 h. This suggested that some carbon species formed on the surface of the catalyst must be involved in the reaction to produce CO. TEM results indicated that a large part of the graphitic carbon deposited on the catalyst surface was of the filamentous form with nickel on top of these carbon filaments. This form of graphitic carbon is more active in the reforming reaction of methane, probably because of its close interaction with nickel particles. Therefore, the catalyst had high stability at 650 °C despite the coke deposited. The value of the activation energy for coke oxidation for the 13.5Ni−2K/10CeO2−Al2O3 catalyst was estimated to be in the range of 110−160 kJ/mol.

Aqueous route for mesoporous metal oxides using inorganic metal source and their applications

In this paper, we report a synthetic route for mesoporous metal oxides from inorganic metal sources in aqueous media. This synthesis route offers a versatile, low cost and environmental friendly method to produce mesoporous metal oxides that have very high surface areas. As an example, the synthesis of iron oxide is described in detail. Synthesis conditions including aging time, aging temperature and amount of urea were varied to find the optimal synthesis conditions. We found that recycling the Cetyltrimethylammonium Bromide (CTAB) template is possible when the amount of urea is reduced to stoichiometric. The mesoporous metal oxides made under these conditions are self assemblies of leaf-like single crystal sub-units with randomly distributed mesopores embedded into the crystals. As a result of the crystalline nature, these mesoporous metal oxides have high thermal stabilities and their applications as gas sensors and CO disproportionation catalysts indicate promising aspects of these materials.

An experimental study on Fischer-Tropsch catalysis: Implications for impact phenomena and nebular chemistry

Abstract— Fischer-Tropsch catalysis, by which CO and H2 are converted to CH4 on the surface of transition metals, has been considered to be one of the most important chemical reactions in many planetary processes, such as the formation of the solar and circumplanetary nebulae, the expansion of vapor clouds induced by cometary impacts, and the atmospheric re-entry of vapor condensate due to asteroidal impacts. However, few quantitative experimental studies have been conducted for the catalytic reaction under conditions relevant to these planetary processes. In this study, we conduct Fischer-Tropsch catalytic experiments at low pressures (1.3 times 10−4 bar ≤ P ≤ 5.3 times 10−1 bar) over a wide range of H2/CO ratios (0.25–1000) using pure iron, pure nickel, and iron-nickel alloys. We analyze what gas species are produced and measure the CH4 formation rate. Our results indicate that the CH4 formation rate for iron catalysts strongly depends on both pressure and the H2/CO ratio, and that nickel is a more efficient catalyst at lower pressures and lower H2/CO ratios. This difference in catalytic properties between iron and nickel may come from the reaction steps concerning disproportionation of CO, hydrogenation of surface carbon, and the poisoning of the catalyst. These results suggest that nickel is important in the atmospheric re-entry of impact condensate, while iron is efficient in circumplanetary subnebulae. Our results also indicate that previous numerical models of iron catalysis based on experimental data at 1 bar considerably overestimate CH4 formation efficiency at lower pressures, such as the solar nebula and the atmospheric re-entry of impact condensate.

Structure of the active component and catalytic properties of catalysts prepared by the reduction of layered nickel aluminosilicates

The reduction of Ni-Mg aluminosilicates with the amesite structure was studied using thermogravimetry, high-resolution electron microscopy, XPS, and XRD. It was found that the reduction with hydrogen at 920 K resulted in the formation of nickel particles coated with a difficult-to-reduce amorphous oxide shell. The reduced samples were incapable of chemisorbing oxygen; however, they exhibited a high adsorption capacity for hydrogen. The Ni0 core-oxide shell decorated particles were highly active in steam methane reforming and CO hydrogenation reactions. At the same time, they were inactive in the formation of graphite-like carbon in both methane decomposition and CO disproportionation.

Effect of annealing on the optical absorption spectra of single-walled carbon nanotubes

The thermal stability of initial and purified samples of single-walled carbon nanotubes prepared through gas-phase disproportionation of carbon monoxide CO in the presence of iron particles under high pressure (the HiPCO method) is investigated using optical absorption spectroscopy and thermogravimetry. An analysis of the optical absorption spectra demonstrates that thermal oxidation of the initial material proceeds rather rapidly and uniformly owing to the catalytic effect caused by the presence of iron particles in the sample. The destruction of the carbon nanotubes contained in the as-prepared and purified samples begins at temperatures of ∼250 and ∼300°C, respectively. It is shown that single-walled metallic nanotubes undergo faster oxidation as compared to the single-walled semiconducting nanotubes.

The thermal decomposition of copper(II) oxalate revisited

DSC, TG and TG-FT-IR, and XRPD have been used to examine the effects of supposedly inert atmospheres of argon and nitrogen on the mechanism of the thermal decomposition of copper(II) oxalate. The DSC curves in pure argon at 10 °C min −1 show a broad endotherm with onset at about 280 °C and maximum at about 295 °C. In mixtures of argon and nitrogen, as the proportion of argon gas is decreased, the endothermic character of the decomposition decreases until, when nitrogen is the main component, the decomposition exhibits a complex broad exothermic character. XRPD studies showed that, regardless of the proportions of nitrogen and argon, the DSC residues consisted of mainly copper metal with small amounts of copper(I) oxide (cuprite) and, under some conditions, traces of copper(II) oxide (tenorite). Various explanations for this behaviour are discussed and a possible answer lies in the disproportionation of CO 2(g) to form small quantities of O 2(g) or monatomic oxygen. The possibility exists that the exothermicity in nitrogen could be explained by reaction of the nitrogen with atomic oxygen to form N 2O(g), but this product could not be detected using TG-FT-IR.

Controlling the growth of vertically oriented single-walled carbon nanotubes by varying the density of Co [sbnd]Mo catalyst particles

A simple method to grow vertically aligned arrays of various forms of single-walled carbon nanotubes (V-SWNT) is reported. CO disproportionation on Co Mo bimetallic catalysts has been used in the synthesis. Resonant Raman and transmission electron microscope (TEM) were employed to characterize the as-produced SWNTs, which display a high purity and very low concentration of other carbon forms. Combination of AFM and SEM gives evidence for the crucial role of the surface distribution of the bimetallic (Co/Mo) catalyst particles on the substrate during the growth of SWNT. A novel growth mechanism is discussed.