CO Disproportionation Research Articles

The effect of the cobalt loading on the growth of single wall carbon nanotubes by CO disproportionation on Co-MCM-41 catalysts

Highly ordered MCM-41 mesoporous molecular sieves in which silicon was isomorphously substituted with 0.5–4 wt.% cobalt were synthesized using an alkyl template with a 16 carbon atoms alkyl chain length. These materials were used as catalysts for the synthesis of uniform diameter single wall carbon nanotubes (SWNT) by CO disproportionation (Boudouard reaction). The SWNT synthesis conditions were identical for all Co-MCM-41 samples, and consisted of pre-reduction of the Co-MCM-41 catalyst in hydrogen at 500 °C for 30 min followed by reaction with pure CO at 800 °C and 6 atm for 1 h (conditions previously optimized for 1 wt.% Co-MCM-41). The SWNT grown in the Co-MCM-41 catalysts were characterized by TGA, multi-excitation energy Raman spectroscopy and TEM. The state of the catalyst and the size of the metallic cobalt clusters formed in Co-MCM-41 during the SWNT synthesis were characterized by X-ray absorption spectroscopy. The mechanism controlling the diameter distribution of the SWNT produced is related to the size uniformity of the cobalt clusters nucleated in the Co-MCM-41 catalytic template: the SWNT growth selectivity and size uniformity is influenced by the cobalt concentration in the framework. If the cobalt is not initially strongly stabilized in the MCM-41 framework during template synthesis, the catalyst produces SWNT with a wider diameter distribution. Co-MCM-41 catalysts with up to 3 wt.% cobalt can be used to grow SWNT with a diameter distribution similar to that obtained with 1 wt.% Co-MCM-41, but at yields greater by a factor of approximately 2.4.

Synthesis and Characterization of Highly Ordered Ni-MCM-41 Mesoporous Molecular Sieves

Highly ordered Ni-MCM-41 samples with nearly atomically dispersed nickel ions were prepared reproducibly and characterized. Similar to the Co-MCM-41 samples, the pore diameter and porosity can be precisely controlled by changing the synthesis surfactant chain length. Nickel was incorporated by isomorphous substitution of silicon in the MCM-41 silica framework, which makes the Ni-MCM-41 a physically stable catalyst in harsh reaction conditions such as CO disproportionation to single wall carbon nanotubes or CO2 methanation. X-ray absorption spectroscopy results indicate that the overall local environment of nickel in Ni-MCM-41 was a tetrahedral or distorted tetrahedral coordination with surrounding oxygen anions. Hydrogen TPR revealed that our Ni-MCM-41 samples have high stability against reduction; however, compared to Co-MCM-41, the Ni-MCM-41 has a lower reduction temperature, and both the H2-TPR and in situ XANES TPR reveal that the reducibility of nickel is not clearly correlated with the pore radius of curvature, as in the case of Co-MCM-41. This is probably a result of nickel being thermodynamically more easily reduced than cobalt. The stability of the structural order of Ni-MCM-41 has been investigated under SWNT synthesis and CO2 methanation reaction conditions as both require catalyst exposure to reducing environments leading to formation of metallic Ni clusters. Nitrogen physisorption and XRD results show that structural order was maintained under both SWNT synthesis and CO2 methanation reaction conditions. EXAFS results demonstrate that the nickel particle size can be controlled by different prereduction temperatures but not by the pore radius of curvature as in the case of Co-MCM-41.

Oxidation of CO in H 2–CO mixtures catalyzed by platinum: alkali effects on rates and selectivity

Turnover rates and selectivities for CO oxidation in H 2–CO–O 2 reactant mixtures are much greater on Pt clusters supported on Cs-modified SiO 2 than on clusters of similar size dispersed on SiO 2 or Al 2O 3 supports. Selectivity effects reflect the inhibition of spillover-mediated H 2 oxidation pathways on support surfaces. Infrared spectra showed that Cs, Rb, or Na titrates support hydroxyl groups required for these unselective pathways. These H 2 oxidation pathways are unaffected by competitive CO co-adsorption and can lead to significant selectivity losses during preferential CO oxidation in H 2-rich streams. CO oxidation selectivities on Pt are above 90%, even at H 2/CO ratios of ∼100; much lower selectivities reported previously reflect contributions from spillover-mediated H 2 oxidation pathways on supports, which are not inhibited by CO co-reactants. Cs and Rb also increased turnover rates for monofunctional oxidation of CO on Pt cluster surfaces, apparently by disrupting the growth of chemisorbed CO monolayers, which prevent activation of O 2 co-reactants. The addition of 1.6 Cs/nm 2 to SiO 2 led to CO oxidation turnover rates more than 10 times higher than those on Pt/SiO 2 catalysts with similar Pt dispersion. CO monolayer growth processes are evident from a gradual decrease in CO oxidation rates during the initial stages of H 2–CO–O 2 reactions, which occurs simultaneously with an increase in intensity for chemisorbed CO infrared bands measured during reaction. This densification of chemisorbed CO monolayers is consistent with the recovery of initial rates by thermal treatment in inert streams and with the observed monotonic increase in CO oxidation selectivity as catalysts gradually approach steady-state CO coverages during reaction. Alkali appears to disrupt CO monolayer growth by promoting the formation of unreactive chemisorbed carbon via CO dissociation and disproportionation reactions, the rates of which increase when alkali is added to Pt surfaces. Chemisorbed carbon blocks some active Pt surface atoms, but also introduces obstacles that inhibit the formation of dense CO monolayers, thus retaining exposed Pt atoms required for the activation of O 2 co-reactants. Rb and Cs cations, which are more electropositive than Na cations, increase CO oxidation turnover rates more strongly, consistent with the proposed mechanisms for the electronic promotion effects of alkali on CO dissociation.

Single-wall carbon nanotube synthesis by CO disproportionation on nickel-incorporatedMCM-41

Both Ni- and Co-MCM-41 may be used for the synthesis of single-wall carbon nanotubes(SWNT). We present a comparative investigation that demonstrates that smaller diameterSWNT with a narrower distribution of diameters are produced using Co-MCM-41.Temperature-programmed reduction and x-ray absorption spectroscopy were used tomeasure the reducibility and metal cluster growth of Ni- and Co-MCM-41 in He,H2 and under CO disproportionation reaction. The differences between these two catalysts canbe attributed to a greater reducibility of and a greater CO affinity for Ni relative to Co.

On-line detection of single-walled carbon nanotube formation during aerosol synthesis methods

Differential electrical mobility (DMA) method for the on-line detection of single-walled carbon nanotubes (SWCNTs) formation was used for the first time. Three different gas-phase synthesis processes were used to produce SWCNTs via CO disproportionation in the presence of catalyst nanoparticles formed either by a hot wire generator method or via thermal decomposition of ferrocene or iron pentacarbonyl. The typical product measured with the DMA method was bundles of SWCNTs, which further agglomerated prior to the measurement. Despite the different product morphology and concentration, the on-line measurement was able to distinguish SWCNT formation in each experimental set-up as an increase in the geometric mean particle diameter and as a decrease in the total particle number concentration. Furthermore, information regarding the relative SWCNT concentration can also be obtained from the DMA measurement. A theoretical approach to the mobility of nonspherical particles in the electric field was successfully developed in order to convert the electrical mobility size of the high aspect ratio SWCNTs measured with DMA to the physical size of the product. Size-selected SWCNTs were studied with transmission electron microscopy in order to find the correlation between the on-line DMA measurement data and the SWCNT morphology.

A study on sulphidation and regeneration of Z-Sorb III sorbent for H 2S removal from simulated ELCOGAS IGCC syngas

The sulphidation and regeneration of the sorbent Z-Sorb III has been investigated at bench scale in a high-temperature, high-pressure fixed-bed unit, over a range of conditions relevant to the Puertollano IGCC plant (Spain). The effect of gas hourly space velocity (1000– 5000 h - 1 ), temperature (673–773 K) and pressure (1–2 MPa) on sulphidation is studied. As predicted by thermodynamics, the sorbent is capable of reducing the H 2S concentration from 10,000 ppmv to near 0 ppmv. The analysis of the experimental results shows that gas residence time has a strong effect on sorbent utilization. Below 2000 h - 1 , the sorbent seems to promote the Boudouard reaction. As a result of this, CO content in the gas stream increases with simultaneous carbon formation in the solid phase. At higher gas velocities ( 3500 h - 1 ) , CO disproportionation does not occur and the sulphidation reaction proceeds isothermally to complete conversion of ZnO. Further increase in space velocity to 5000 h - 1 leads to lower sorbent utilization, which is typical of diffusion-limited gas–solid heterogeneous reactions. On the other hand, sulphidation temperature has very little effect on sulphur loading uptake which is slightly higher at 773 K than at 673 K. Regarding the effect of pressure, the sorbent is most effective at 2 MPa, almost reaching the theoretical saturation capacity. The study of the durability of the sorbent is investigated through multiple sulphidation–regeneration cycles. Oxidative dry mild regeneration using an oxygen concentration of 2% v/v results in complete regeneration of the sulphided sorbent without severe structural property changes or activity loss. The primary reaction during regeneration of the sulphided sorbent is direct oxidation of the metal sulphide with SO 2 release.

Performance of Ni/MgO–AN catalyst in high pressure CO2 reforming of methane

The catalytic activity of 8.8 wt Ni/MgO–AN prepared from alcogel derived MgO was studied for the dry reforming of methane under high pressure (1.5 MPa). The catalyst showed a self-stabilization process during the reaction that lasted for 50 h, in which the catalytic activity decreased with increasing the reaction time on stream (TOS) up to 12 h, and then became stabilized thereafter. The activity decline during the initial 12 h of the reaction was found closely related to an increase in the amount of carbon deposits on the catalyst, which also became stabilized after the catalyst had served the reaction for 12 h. Comprehensive characterizations of the coked catalyst with Temprature programmed hydrogenation (TPH), X-ray photoelectron spectroscopy (XPS) and X-ray diffractometer (XRD) techniques revealed two kinds of carbon deposits (α-carbon and β-carbon) on the used catalyst. The α-carbon deposits were found to be produced from CH 4 decomposition while the β-carbon deposits from CO disproportionation. It was revealed that the accumulation of β-carbon deposits was a key cause for the activity decline and the self-stabilized catalysis during the initial 12 h of the high-pressure reaction. Moreover, it was also observed that an unavoidable sintering of metallic Ni particles from 6.5 to 11 nm, which happened within the very first hour of the reaction, was not directly related to the catalyst stability.

Coke Formation over a Nickel Catalyst under Methane Dry Reforming Conditions: Thermodynamic and Kinetic Models

The CO2 reforming of methane is studied over a 20 wt % Ni/USY-zeolite, and more specifically, a thermodynamic analysis of the formation of coke is used as a basis for the kinetic modeling of coke phenomena that exist under dry reforming conditions. Two thermodynamic parameters, α and β, are compared to the equilibrium constants for the CH4 decomposition and the CO disproportionation reactions and defined to determine whether coke formation is favored. This thermodynamic analysis elucidates the significance of the CO disproportionation reaction on the amount of coke deposited over the catalyst under consideration. A kinetic model with negative overall order of one, with respect to the partial pressure of carbon monoxide, is found as the most accurate prediction of the rate of coke formation. This type of kinetics strongly suggests the requirement of three adjacent free catalyst sites for the coking reaction to proceed under allowable thermodynamic conditions.

Effect of catalyst preparation on the carbon nanotube growth rate

A series of silica supported Fe catalysts were prepared by different methods in order to obtain varying Fe particle sizes. The catalysts were characterized by XRD, TPR, BET, and TEM. The CNT growth from CO disproportionation was studied in order to establish a relationship between the CNT growth rate and the particle size. We found that there is an optimum catalyst particle size at around 13–15 nm which will lead to the maximum growth rate. The influence of the metal loading on the growth rate was also investigated. A CNT growth model has been formulated to explain the experimental results.

Effect of Co-MCM-41 Conversion to Cobalt Silicate for Catalytic Growth of Single Wall Carbon Nanotubes

Cobalt silicate was studied as a model catalyst to seek an understanding of why Co-MCM-41 catalyst is not active for single wall carbon nanotubes (SWNT) upon treatment at high temperature. Cobalt orthosilicate was formed during high-temperature pretreatment of Co-MCM-41 (at 900 °C) in an oxidizing environment. Cobalt silicate did not produce any carbon species during the first reaction cycle because the high thermal stability of cobalt silicate did not result in sufficient metallic Co clusters responsible for carbon dissociation in CO disproportionation. With repeated reaction cycles, however, undesirable carbon species, i.e., amorphous carbon and graphite, were produced resulting from larger metallic cobalt clusters formed at the high reduction temperature and without the confinement and/or anchoring of the MCM-41 matrix. These relatively large clusters do not catalyze the growth of SWNT.

Gas‐Phase Production of Single‐Walled Carbon Nanotubes from Carbon Monoxide: A Review of the HiPco Process

AbstractFor Abstract see ChemInform Abstract in Full Text.

Carbon formation on Ni–MgO catalyst during reaction of methane in the presence of CO 2 and CO

Reactions of 13CH 4 and 12CH 4 in the presence of either CO 2 or CO over reduced and oxidized Ni–MgO catalyst were studied to obtain mechanistic information on carbon deposition at low and high pressure. As the reaction time increased the amount of carbon deposited on the catalyst also increased. The carbon deposited on the catalyst during the first hour of reaction appears to be very active and was removed readily by reacting with 2 vol.% oxygen in helium at 660 °C and the catalyst recovered its original activity and selectivity after removal of carbon. During the temperature-programmed oxidation (TPO) of deposited carbon the catalyst consumed a significant amount of oxygen forming nickel oxide, only a fraction of which is consumed by reacting with carbon species on the surface. This indicates that the surface oxygen plays a significant role in oxidizing and removing carbon species from the catalyst surfaces. Pulse reaction and oxidation experiments show that methane is activated on the surfaces of both the oxidized and reduced catalyst whereas the CO 2 activation requires the catalyst to be in reduced state. Carbon deposited on the catalyst is formed from both methane and CO 2, possibly via disproportionation of CO which is produced from both 13CH 4 and 12CO 2.

Adsorption of NO and CO over transition-metal-incorporated mesoporous catalytic materials

Novel high-surface-area mesoporous catalysts of type Al-MCM-41 containing transition metals such as iron, nickel, cobalt, zinc, copper, and cobalt were prepared and characterized using techniques such as BET, FTIR, ICP-MS, XPS, and XRD. The XPS measurements indicated that the transition-metal particles are located in the bulk or pore channels of the Al-MCM-41 materials. A detailed in situ FTIR investigation undertaken on the adsorption and disproportionation of NO and CO over the transition-metal-Al-MCM-41 mesoporous catalysts indicated the formation of various NO/CO species or complexes with active metal sites. The structure and dynamics of the metal activated complex and reactive species formed during the CO/NO reaction together with advantages of these catalysts and the influence of reaction temperature and pressure have been studied. NO adsorption at room temperature leads to the formation of adsorbed N(2)O, NO(2), MNO(2), MNO, and [M(NO)(2)] complexes. CO adsorption at room temperature leads to the formation of physisorbed carbon dioxide and cationic Lewis acid carbonyl moieties as well as transition-metal carbonyl complexes. The copper mesoporous catalysts prepared by different procedures (ion exchanged and as-synthesized) were compared for their interactions with CO and NO probe molecules.

Mechanism of Cobalt Cluster Size Control in Co-MCM-41 during Single-Wall Carbon Nanotubes Synthesis by CO Disproportionation

The mechanisms controlling the reduction of cobalt and the growth of metallic cobalt clusters in Co-MCM-41 catalysts at different stages in the process of single-wall carbon nanotubes (SWNT) synthesis were investigated both by in-situ and ex-situ X-ray absorption spectroscopy. We have found that prereduction of the catalyst in hydrogen at temperatures below 700 °C does not reduce the cobalt ions to metallic cobalt, but removes hydroxyl groups and oxygen ions creating oxygen vacancies and/or a partially reduced cobalt species. The prereduction treatment, however, does increase the density of electrons at the Fermi level weakening the interaction of Co2+ with the silica framework. Subsequent exposure of the catalyst to CO at 750 °C causes CO to strongly interact with the cobalt clusters most likely by the transfer of electrons into the d orbitals of Co. This strong interaction makes the cobalt more mobile at the surface and allows it to nucleate into clusters capable of dissociating CO and initiating the growth of SWNT. Prior to exposure to CO, the reduced cobalt species strongly interacting with the silica framework do not nucleate into larger clusters in the presence of He or H2, preserving near atomic cobalt dispersion, as determined by EXAFS.

Synthesis of uniform diameter single wall carbon nanotubes in Co-MCM-41: effects of CO pressure and reaction time

The influence of the CO pressure and of the duration of catalyst exposure to CO was systematically investigated with respect to the carbon yield, selectivity, and diameter uniformity of single wall carbon nanotubes (SWNT) produced by CO disproportionation over Co-MCM-41 catalysts. The SWNT were characterized by Raman and near infrared (NIR) spectroscopy, while the state of the catalyst and the size of the metallic cobalt clusters were investigated by X-ray absorption spectroscopy (XAS). The experimental results suggest that both the selectivity to SWNT and the uniformity of their diameters are controlled by the relative rates of the following competing processes: cobalt reduction, nucleation of cobalt into clusters, and SWNT growth. It was found that reaction at low CO pressures leads to a wider distribution of diameters and lower SWNT selectivity. The duration of catalyst exposure to CO was found to strongly affect the SWNT selectivity at the early stages of SWNT growth, while it has a minor effect on the SWNT diameter uniformity.

Reduction and desorption with carbon monoxide in Cr/SiO2 catalysts for ethylene polymerization

Abstract Cr/SiO 2 catalysts for ethylene polymerization, prepared with 1 wt.% Cr and different chromium precursors, were characterized by temperature-programmed reduction (TPR) and desorption of carbon monoxide. Complete reduction of Cr 6+ to Cr 2+ species was observed with CO at 623 K, but the reduction profile modified in the presence of Cr 3+ , depending on the chromium precursor. The desorption of CO occurred in relatively low temperature for catalysts without Cr 3+ , independent of the Cr A 2+ /Cr B 2+ active sites ratio. The desorption of CO was observed at higher temperature in the catalysts containing Cr 3+ , suggesting modification of the Cr 2+ –CO adsorption strength in the presence of Cr 3+ . The desorption of CO 2 was detected in all catalysts, associated to the CO disproportionation reaction at lower temperatures or to the decomposition of carbonates and carboxilates species above 500 K. The desorption of CO agreed with polymerization activity results, showing the importance of this technique to characterize active chromium sites.

Investigations on the novel niobium incorporated mesoporous catalytic materials

High surface area novel niobium mesoporous catalysts were synthesised via sol-gel method. The Nb-Al-MCM-41 catalysts characterised by BET, FTIR, ICP-MS, XPS and XRD. The Nb-Al-MCM-41 samples exhibited very large surface area of 2285 m(2)/g (pore volume 2.16 cm(3) g(-1)) and 1550m(2)/g (pore volume 1.83cm(3) g(-1)) with average pore diameter of 37.9 Angstrom (by BET) and 44.1 Angstrom, respectively. The adsorption and disproportionation of NO and CO over Nb-Al-MCM-41 mesoporous catalysts indicated the formation of various NO/CO species or complexes with active metal sites. The structure-dynamics of the metal activated complex and reactive species formed during the CO/NO reaction has been studied. The nitrous oxide, nitrite, mononitrosyl, nitrate and dinitrosyl species formation were observed after the NO adsorption over the Nb-Al-MCM-41 catalysts. CO adsorption at room temperature lead to the formation of up to six bands attributed to physisorbed carbon dioxide and cationic Lewis acid carbonyl moieties as well as transition metal carbonyl complexes.

Synthesis of uniform diameter single-wall carbon nanotubes in Co-MCM-41: effects of the catalyst prereduction and nanotube growth temperatures

Catalyst pretreatment and CO disproportionation reaction conditions were observed to strongly affect the diameter uniformity of single-wall carbon nanotubes (SWNT) grown on Co-MCM-41 catalysts. The prereduction and CO disproportionation reaction temperatures were varied systematically while the carbon loading and the SWNT diameter uniformity were monitored by TGA, Raman spectroscopy, and TEM. The state of the catalyst during prereduction and the size of the cobalt clusters formed during the SWNT growth process were monitored by in situ XANES during the prereduction of the Co-MCM-41, and ex situ EXAFS of catalyst samples was performed after carbon deposition. These experiments allow development of correlations between the SWNT quality and the state of the catalyst. Control of the cobalt cluster size in the Co-MCM-41 catalyst is critical to the SWNT diameter control. The size of the cobalt cluster changes with both the prereduction and the SWNT synthesis temperatures. SWNT with a very narrow diameter distribution can be grown in Co-MCM-41 by controlling both the prereduction and the reaction temperatures.

Rietveld refinement and activity of CO oxidation over Pd/Ce0.8Zr0.2O2 catalyst prepared via a surfactant-assisted route

A surfactant-assisted preparation method was successfully used to synthesize the Ce0.8Zr0.2O2 nanophases that had homogeneous chemical composition, enhanced thermal stability and improved catalytic activity. Structural refinement by the Rietveld method confirmed that many cationic lattice defects were formed in the ceria-zirconia crystals due to incorporation of the cationic surfactant in the network of the solid. Zirconium dissolving into cubic structure of ceria improved its thermal stability but led to structure distortion. FT-IR in situ characterization showed that CO chemisorption on the palladium loaded ceria-zirconia catalyst formed linear and multibonded CO–metal complexes. It was also evidenced that CO2 was produced in the CO chemisorption procedure from the pathways of CO disproportionation and CO reacting with lattice oxygen of the catalyst. Owing to high dispersion of the loaded metal palladium and defective structure of the ceria-zirconia support, the Pd/Ce0.8Zr0.2O2 catalyst calcined at a temperature range between 873 and 1173 K not only exhibited a more stable catalytic activity for CO oxidation but also performed a lower light-off temperature at cool start below 373 K in comparison with Pd/CeO2.

Gas-phase production of single-walled carbon nanotubes from carbon monoxide: a review of the hipco process.

The latest process for producing large quantities of single-walled carbon nanotubes (SWNTs) to emerge from the Rice University, dubbed HiPco, is living up to its promise. The current production rates approach 450 mg/h (or 10 g/day), and nanotubes typically have no more than 7 mol % of iron impurities. Second-generation HiPco apparatus can run continuously for 7-10 days at a time. In the HiPco process nanotubes grow in high-pressure, high-temperature flowing CO on catalytic clusters of iron. Catalyst is formed in situ by thermal decomposition of iron pentacarbonyl, which is delivered intact within a cold CO flow and then rapidly mixed with hot CO in the reaction zone. Upon heating, the Fe(CO)5 decomposes into atoms that condense into larger clusters. SWNTs nucleate and grow on these particles in the gas phase via CO disproportionation: CO + CO --> CO2 + C (SWNT), catalyzed by the Fe surface. The concentration of CO2 produced in this reaction is equal to that of carbon and can therefore serve as a useful real-time feedback parameter. It was used to study and optimize SWNT production as a function of temperature, pressure, and Fe(CO)5 concentration. The results of the parametric study are in agreement with current understanding of the nanotube formation mechanism.