Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH₄ over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH₄, as well as to the formation of Mo₂C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH₄ pressure, indicating that the dissociation of CH₄ is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH₄.     

Catalytic decomposition of methane and methane/CO2 mixtures to produce synthesis gas and nanostructured carbonaceous material

Methane and CO₂ are the main components of biogas; therefore its direct conversion into a higher added value gas as syn-gas (mixture of CO and H₂) is a very interesting alternative for the valorisation of such renewable resource. In this work, firstly a thermodynamic analysis of the decomposition of CH₄:CO₂ mixtures at different temperatures and CH₄:CO₂ ratios simulating the biogas composition, has been carried out. Secondly, the decomposition of a mixture with a molar ratio of 1:1 has been studied in a fixed-bed reactor by using a Ni/Al₂O₃ based catalyst, at the temperature range in which according to the thermodynamic study, carbon formation is favoured. Results obtained have been compared to those of methane decomposition carried out under the same experimental conditions. Co-feeding of CO₂ and CH₄ avoids catalyst deactivation substantially, allowing to obtain a syn-gas with H₂:CO ratio close to 1. Moreover, the carbon obtained from mixtures of CH₄ and CO₂ is deposited as fishbone carbon nanofibres at 600 °C and ribbon carbon nanofibers at 700 °C, both being materials with high added value which can be used in multiple applications.     

Decomposition of methane and its reaction with CO2 over Rh/ZSM-5 catalyst

The decomposition of methane, its conversion into higher hydrocarbons and the reaction between CH₄ and CO₂ have been investigated on Rh/ZSM-5 in a fixed bed continuous-flow reactor. Independently of the temperature at 523–973 K, the decomposition of methane gave hydrogen, surface carbon and a small amount of ethane: ethylene and benzene were not detected. The reactivity of surface carbon formed at different temperatures has been examined toward H₂, O₂ and CO₂. The carbon exhibited less reactivity toward CO₂. The reaction between CH₄ and CO₂ occurred rapidly above 673 K to give CO and H₂ with a ratio of 1.3–1.6. Very little carbon was deposited during the reaction. It is concluded that the facile reactions between CH_x and CO₂ are responsible for the lack of carbon deposition. However, a significant amount of carbon deposition and the deactivation of the catalyst occurred when more than 4–5% of ethane was added to the reacting gas mixture. The extent of deactivation can be decreased by using a large excess of CO₂. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon deposition and catalytic deactivation during CO2 reforming of CH4 over Co/MgO catalyst

The deactivation mechanism of Co/MgO catalyst for the reforming of methane with carbon dioxide was investigated. The conversion of CH₄ displayed a significant decrease in the initial stage caused by carbon deposition. There were two types of cokes, carbon nanotubes (CNTs) and carbon nano-onions (CNOs). The number of the CNO layers that coated on the surface of Co nanoparticles (NPs) increased rapidly in the initial reforming time, which was responsible for the deactivation of the Co/MgO catalyst. The deposition of CNOs was attributed to the oxidation of Co NPs. Therefore, the deactivation of the Co/MgO catalyst was originated from the first oxidization of the Co NPs into Co₃O₄ by O species (OH intermediate, CO₂, H₂O) during the reforming reaction, which accelerates the formation of coke that blocked the active metal, thus led to catalyst deactivation.     

Studies on the Stability of a La0.8Pr0.2NiAl11O19 Catalyst for Syngas Production by CO2 Reforming of Methane

CO₂ reforming of CH₄ was studied over a magnetoplumbite-type hexaaluminate La_0.8Pr_0.2NiAl₁₁O₁₉ catalyst, which showed very high activity for over 300 h without deactivation at 1023 K. This catalyst showed good resistance to carbon deposition, which in this reaction and in CH₄ decomposition was investigated by means of XPS and TEM. It is suggested that nano-tube-like carbon is an intermediate in this reaction and a spillover of carbon from crystalline Ni onto the hexaaluminate oxide occurred during the reaction.     

Stable Hydrogen Production from Ethanol through Steam Reforming Reaction over Nickel-Containing Smectite-Derived Catalyst

Hydrogen production through steam reforming of ethanol was investigated with conventional supported nickel catalysts and a Ni-containing smectite-derived catalyst. The former is initially active, but significant catalyst deactivation occurs during the reaction due to carbon deposition. Side reactions of the decomposition of CO and CH₄ are the main reason for the catalyst deactivation, and these reactions can relatively be suppressed by the use of the Ni-containing smectite. The Ni-containing smectite-derived catalyst contains, after H₂ reduction, stable and active Ni nanocrystallites, and as a result, it shows a stable and high catalytic performance for the steam reforming of ethanol, producing H₂.     

Effect of carbon deposition over carbonaceous catalysts on CH4 decomposition and CH4-CO2 reforming

An investigation was made using a continuous fixed bed reactor to understand the influence of carbon deposition obtained under different conditions on CH₄-CO₂ reforming. Thermogravimetry (TG) and X-ray diffraction (XRD) were employed to study the characteristics of carbon deposition. It was found that the carbonaceous catalyst is an efficient catalyst in methane decomposition and CH₄-CO₂ reforming. The trend of methane decomposition at lower temperatures is similar to that at higher temperatures. The methane conversion is high during the initial of stage of the reaction, and then decays to a relatively fixed value after about 30 min. With temperature increase, the methane decomposition rate increases quickly. The reaction temperature has significant influence on methane decomposition, whereas the carbon deposition does not affect methane decomposition significantly. Different types of carbon deposition were formed at different methane decomposition reaction temperatures. The carbon deposition Type I generated at 900°C has a minor effect on CH₄-CO₂ reforming and it easily reacts with carbon dioxide, but the carbon deposition Type II generated at 1000°C and 1100°C clearly inhibits CH₄-CO₂ reforming and it is difficult to react with carbon dioxide. The results of XRD showed that some graphite structures were found in carbon deposition Type II.     

Effect of steam and carbon dioxide pretreatments on methane decomposition and carbon gasification over doped-ceria supported nickel catalyst

Effects of steam (H₂O) and carbon dioxide (CO₂) pretreatments on methane (CH₄) decomposition and carbon gasification over doped-ceria supported nickel catalysts have been studied from 400 to 500 °C. The doped ceria employed were gadolinia-doped ceria and samaria-doped ceria. Results indicate that a drastic increase of both H₂O and CO₂ dissociation activities occurs as the temperature increases from 450 to 500 °C. The formation of the surface hydroxyl species during H₂O treatment inhibits the followed CH₄ decomposition. CO but no CO₂ was formed during CH₄ reaction after H₂O treatment. Carbon deposition during CH₄ decomposition is quite large but can be removed via gasification with afterward CO₂ treatment. However, some of the deposited carbon species is in a form which can not be removed with CO₂ treatment but can be removed with O₂ treatment. And, higher values of the oxygen-ion conductivity and the density of the surface oxygen vacancies lead to higher activities for all dissociation and decomposition reactions.     

Effect of carbon deposition over carbonaceous catalysts on CH<Subscript>4</Subscript> decomposition and CH<Subscript>4</Subscript>-CO<Subscript>2</Subscript> reforming

An investigation was made using a continuous fixed bed reactor to understand the influence of carbon deposition obtained under different conditions on CH₄-CO₂ reforming. Thermogravimetry (TG) and X-ray diffraction (XRD) were employed to study the characteristics of carbon deposition. It was found that the carbonaceous catalyst is an efficient catalyst in methane decomposition and CH₄-CO₂ reforming. The trend of methane decomposition at lower temperatures is similar to that at higher temperatures. The methane conversion is high during the initial of stage of the reaction, and then decays to a relatively fixed value after about 30 min. With temperature increase, the methane decomposition rate increases quickly. The reaction temperature has significant influence on methane decomposition, whereas the carbon deposition does not affect methane decomposition significantly. Different types of carbon deposition were formed at different methane decomposition reaction temperatures. The carbon deposition Type I generated at 900°C has a minor effect on CH₄-CO₂ reforming and it easily reacts with carbon dioxide, but the carbon deposition Type II generated at 1000°C and 1100°C clearly inhibits CH₄-CO₂ reforming and it is difficult to react with carbon dioxide. The results of XRD showed that some graphite structures were found in carbon deposition Type II.     

TEM evidence for factors affecting the genesis of carbon species on bare and K-promoted Ni/MgO catalysts during the dry reforming of methane

The structure and morphology of carbon species generated under dry reforming of methane (DRM) at 650 and 800°C on ‘bare’ and ‘K-doped’ Ni/MgO catalysts have been comparatively investigated by Transmission Electron Microscopy (TEM) analyses of ‘used’ samples. K-addition (K_at/Ni_at, 0.125) strongly improves the resistance of the Ni/MgO catalyst to coking and sintering phenomena at any temperature. At 650°C, an extensive formation of filamentous (whisker carbon) carbon species on bare Ni/MgO catalyst causes the detachment of a large number of Ni particles from the support with a consequent destruction of the structure and remarkable sintering phenomena of the active phase. Considerably lower amounts of carbon deposits with a shell-like (encapsulating carbon) morphology, forming at 800°C on both catalysts, point to the Bouduard reaction as the main route of carbon deposition on Ni-based catalysts during DRM. The electronic effect induced by potassium on the active phase of the Ni/MgO system, timely monitored by a rise in E_app of DRM from 50 to 70 kJ/mol, markedly hinders the rate of coking also affecting the morphology of carbon whiskers, by inhibiting the processes of C diffusion and nucleation across Ni particles under steady-state conditions.     

Decomposition of metal carbides as an elementary step of carbon nanotube synthesis

The role of catalyst components in catalysts containing molybdenum, Mo/M/MgO (MNi, Co, and Fe), as well as Mo-free catalysts, M/MgO (MNi, Co, and Fe), for carbon nanotube (CNT) synthesis have been investigated by TEM, XRD, and Raman spectroscopy. CNT synthesis by the catalytic decomposition of CH₄ over M/MgO catalysts can proceed at reaction temperatures higher than the decomposition temperature of the metal carbides (Ni₃C, Co₂C, and Fe₃C), which indicates that carbon in the CNT originates from the graphitic carbon formed on the catalyst surface by the decomposition of metal carbides. For all catalysts containing Mo, thin CNT formation starts at an identical temperature of 923 K, corresponding to the decomposition temperature of MoC_1−x into Mo₂C. The significant effect of the addition of Mo is concerned with the formation of Mo₂C in a catalyst particle during CNT synthesis at high reaction temperatures. The presence of a stable Mo₂C phase leads to the formation of thin CNT with better crystallinity at high reaction temperatures. The role of Ni, Co, and Fe in the Mo/M/MgO catalysts is ascribed to the dissociation of CH₄.     

Hydrogen production via catalytic steam reforming of fast pyrolysis bio-oil in a two-stage fixed bed reactor system

Hydrogen production was prepared via catalytic steam reforming of fast pyrolysis bio-oil in a two-stage fixed bed reactor system. Low-cost catalyst dolomite was chosen for the primary steam reforming of bio-oil in consideration of the unavoidable deactivation caused by direct contact of metal catalyst and bio-oil itself. Nickel-based catalyst Ni/MgO was used in the second stage to increase the purity and the yield of desirable gas product further. Influential parameters such as temperature, steam to carbon ratio (S/C, S/CH₄), and material space velocity (W_BHSV, GHSV) both for the first and the second reaction stages on gas product yield, carbon selectivity of gas product, CH₄ conversion as well as purity of desirable gas product were investigated. High temperature (> 850 °C) and high S/C (> 12) are necessary for efficient conversion of bio-oil to desirable gas product in the first steam reforming stage. Low W_BHSV favors the increase of any gas product yield at any selected temperature and the overall conversion of bio-oil to gas product increases accordingly. Nickel-based catalyst Ni/MgO is effective in purification stage and 100% conversion of CH₄ can be obtained under the conditions of S/CH₄ no less than 2 and temperature no less than 800 °C. Low GHSV favors the CH₄ conversion and the maximum CH₄ conversion 100%, desirable gas product purity 100%, and potential hydrogen yield 81.1% can be obtained at 800 °C provided that GHSV is no more than 3600 h^− 1. Carbon deposition behaviors in one-stage reactor prove that the steam reforming of crude bio-oil in a two-stage fixed bed reaction system is necessary and significant.     

Suppression of carbon deposition in CO2-reforming of methane on metal sulfide catalysts

The catalytic performance of metal sulfides of Mo and W was studied for the CO₂-reforming of methane by comparing with that of Ni/SiO₂. The sulfide catalysts have lower activity than the Ni/SiO₂ catalyst for this reaction, however, no deactivation due to carbon deposition was observed on the sulfide catalysts. The activity for direct decomposition of CH₄ was much smaller on the sulfides than on supported Ni. The rate equation suggested that, during steady-state reaction, the surface was abundant in adsorbed CO₂ on sulfides, by which direct decomposition of CH₄ should be retarded in addition to their lower activity for this reaction.     

Hydrogen production from catalytic decomposition of methane over ordered mesoporous carbons (CMK-3) and carbide-derived carbon (DUT-19)

This paper presents a thermogravimetric analysis of catalytic methane decomposition using ordered mesoporous carbon nanorods (CMK-3) and ordered mesoporous carbide-derived carbon (DUT-19) as catalysts. X-ray diffraction and N₂ physisorption analyses were performed for both fresh catalysts. Threshold temperatures for methane decomposition with DUT-19 and CMK-3 were estimated by three different methods found in literature. Carbon formation rate and carbon weight gain as a function of time at various temperatures and methane partial pressures were studied, and the kinetics of CMK-3 and DUT-19 as catalysts for methane decomposition were investigated. Arrhenius energy values of 187 kJ/mol for CMK-3 and 196 kJ/mol for DUT-19 with a reaction order of 0.5 were obtained for both catalysts. Results show that carbon deposition on the catalyst during the reaction lead to catalyst deactivation with significant surface modification. Scanning electron microscope studies of fresh and deactivated catalyst samples show the blocking of catalyst pores and the formation of agglomerates on the outer surface of the catalyst during the course of reaction. DUT-19 catalytically outperforms CMK-3 because of a lower threshold temperature, higher surface area, and higher pore volume. These results show that ordered mesoporous carbons are promising catalysts for methane decomposition.     

Decomposition of methane in the presence of carbon dioxide over Ni catalysts

Methane decomposition was carried out in the presence of CO₂ over the nickel catalysts. Spherical alumina and glycothermally synthesized zirconia were used as the catalyst supports. In the presence of CO₂, CH₄ was decomposed in the same fashion as pure methane decomposition, and fibrous carbons were formed. However, the formation of hydrogen, carbon monoxide, and water continued even after the apparent carbon formation ceased, and this phenomenon was observed irrespective of the support materials. These results showed a sharp contrast against the results for the pure methane decomposition where the catalyst was completely deactivated when the carbon formation ceased. Further carbon formation was observed when the feed gas containing CO₂ was replaced with pure CH₄. Mechanisms for these phenomena are discussed from the thermodynamical point of view.     

The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils

Carbon nanofibers (CNFs) were deposited on metal foils including nickel (Ni), iron (Fe), cobalt (Co), stainless steel (Fe:Ni; 70:11 wt.%) and mumetal (Ni:Fe; 77:14 wt.%) by the decomposition of C₂H₄ at 600 °C. The effect of pretreatment and the addition of H₂ on the rate of carbon formation, as well the morphology and attachment of the resulting carbon layer were explored. Ni and mumetal show higher carbon deposition rates than the other metals, with stainless steel and Fe the least active. Pretreatment including an oxidation step normally leads to higher deposition rates, especially for Ni and mumetal. Enhanced formation of small Ni particles by in situ reduction of NiO, compared to formation using a Ni carbide, is probably responsible for higher carbon deposition rates after oxidation pretreatment. The addition of H₂ during the CNF growth leads to higher carbon deposition rates, especially for oxidized Ni and mumetal, thus enhancing the effect of the reduction of NiO. The diameters of CNFs grown on metal alloys are generally larger compared to those grown on pure metals. Homogenously deposited and well-attached layers of nanotubes are formed when the carbon deposition rate is as low as 0.1-1 mg cm⁻² h⁻¹, as mainly occurs on stainless steel.     

Potassium-enhanced stability of Ni/MgO catalysts in the dry-reforming of methane

The catalytic behaviour of the bare and K-promoted 19 wt% Ni/MgO catalyst in the CO₂-reforming of CH₄ at 650°C has been investigated. The effects of the K loading (1.5–2.5 wt%) on the catalytic activity, stability and coking rate have been addressed. Both sintering and formation of large amounts of whisker carbon lead to a marked deactivation of the bare Ni/MgO system. K addition depresses the reactivity of the catalyst strongly improving its resistance to both coking and sintering processes. A change in the electronic and geometric properties of the active phase, monitored by a rise in E_app from 50 to 70 kJ mol⁻¹, accounts for the effects of K addition on the catalytic behaviour of the Ni/MgO catalyst in the dry-reforming of CH₄.     

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.     

EFFECTS OF CARBON DEPOSITS ON THE SPECIFIC ACTIVITY OF NICKEL AND NICKEL BIMETALLIC CATALYSTS

The effects of carbon formation on methanation activity of nickel and nickel bimetallic catalysts were investigated. Carbon was deposited on these catalysts at 675-700 K, 1 atm, H₂/CO = 2 and space velocities of 80,000 to 200,000 h^?1 over a period of 6-24 hours. Specific methanation activities were measured before and after carbon depositing treatments at 500-575 K, 1 atm H₂/CO = 4 and space velocities of 100,000 h^?1. The results show that Ni/Al₂O₃ loses 20-60% of its initial activity within 10-15 hours of treatment. Platinum and cobalt promoted nickel are significantly more resistant to deactivation by carbon. However, Ni-MoO₂ is highly susceptible to deactivation, losing essentially all of its activity within a few hours. Data showing the effects of reaction conditions, metal concentration and catalyst composition on the extent of deactivation and the effects of deactivation on catalyst strength are presented and discussed     

Catalytic dry-reforming on Ni-zeolite supported catalyst

In the present work, the catalytic behaviour in the dry reforming of methane on Ni-based Silicalite-1 type catalyst was studied. The Silicalite-1 support has been synthesized in order to check the role of the silanol groups on the overall catalyst performance: methane and carbon dioxide conversion, hydrogen/carbon monoxide ratio and coke deposition. The population of defect groups on the Silicalite-1 surface was modified by aging of the gel, thermal treatment, ionic exchange and silylation procedure. Among these treatments, the silylation of the support surface leads to the formation of smaller and more reducible Ni-oxide species that not only improve the CH₄ and CO₂ conversion but also reduce the deactivation of the catalyst due to coke deposition and the obtained H₂/CO value is 1.04.