Decomposition of methane over Ni catalysts supported on carbon fibers formed from different hydrocarbons

In order to get pure hydrogen without CO and CO₂, the decomposition of methane into hydrogen and carbon fibers (CF) over Ni/carbon catalysts has been investigated. The reason for the use of carbon materials as supports is to avoid a costly elimination treatment of the catalyst from the formed CF. The Ni/carbon catalysts prepared by the impregnation of various carbon materials with Ni(NO₃)₂ dissolved in acetone, followed by reduction in hydrogen at 573 K, showed better catalytic performance in the decomposition of methane than those prepared by the impregnation with aqueous Ni(NO₃)₂. The Ni(40 wt%)/CF(from 1-C₄H₈) showed the highest catalytic performance giving a C/Ni value (moles of deposited carbon per mole of Ni on the catalyst) of 1920 until complete deactivation of the catalyst. SEM and TEM images of the CF formed from methane indicated their thickness to be ≈10-150 nm with the same size of Ni particles at their tips at the early stage of the decomposition of methane, but the thickness changed to ≈40-100 nm at the final stage of decomposition. An estimate of the average size of Ni crystallites from XRD measurement suggested that the carbons deposited from methane on various Ni/CF would modify the size of Ni crystallites during the reaction. It is suggested that ≈20 nm Ni crystallites are most active for the growth of carbon nanofibers.     

Catalytic filamentous carbon: Structural and textural properties

Catalytic filamentous carbon (CFC) synthesized by the decomposition of methane over iron subgroup metal catalysts (Ni, Co, Fe or their alloys) is a new family of mesoporous carbon materials possessing the unique structural and textural properties. Microstructural properties of CFC (arrangement of the graphite planes in filaments) are shown to depend on the nature of catalyst for methane decomposition. These properties widely vary for different catalysts: the angle between graphite planes and the filament axis can be 0° (Fe-Co-Al₂O₃), 15° (Co-Al₂O₃), 45° (Ni-Al₂O₃), 90° (Ni-Cu-Al₂O₃). The textural properties of CFC depend both on the catalyst nature and the conditions of methane decomposition (T, °C). The micropore volume in CFC is very low, 0.001-0.022 cm³ g⁻¹ at the total pore volume of 0.26-0.59 cm³ g⁻¹. Nevertheless, the BET surface area may reach 318 m² g⁻¹. Results of the TEM (HRTEM), XRD, Raman spectroscopic, SEM and adsorption studies of the structural and textural properties of CFC are discussed.     

Nickel catalysts supported on carbon nanofibers: structure and activity in methane decomposition

Structures of Ni catalysts supported on filamentous carbon (CFC) produced by methane decomposition over coprecipitated Ni and Ni-Cu/alumina catalysts were studied by EXAFS and TEM. Thermal pre-treatment in N₂ at 350°C of samples impregnated by nickel nitrate precursor was found to produce either NiO or nickel carbide, Ni₃C, phase. This was explained by different reducing properties of the carbon nanofibers which depend on the surface structure. High stability of the Ni/C catalysts in methane decomposition reaction at 550°C was found with those prepared from only nickel chloride precursor, due to the formation of large (30-70 nm) Ni particles further leading to new carbon filaments growth. Data implies a common mechanism of the filamentous carbon deposition in all Ni-based catalysts, independent of the support (silica, alumina, carbon) being used. However, accumulation of filamentous carbon is strongly influenced by morphology and texture of the support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Benzene hydrogenation over Ni-Cu/SiO2 catalysts prepared by aqueous hydrazine reduction

Non conventional nickel (1%) and nickel (1%)-copper (0.2%-0.75%) catalysts supported on silica have been prepared by aqueous hydrazine reduction of nickel acetate at 70 °C. They were characterized by TEM, H₂-adsorption, H₂-TPD and tested in the gas phase hydrogenation of benzene at atmospheric pressure in the temperature range 75 °C-230 °C. The obtained results show that nickel is in a whisker-like shape or as a film of low density for the Ni/SiO₂ and Ni-Cu/SiO₂ catalysts respectively. Copper is in the shape of facetted particles in the mono or bimetallic systems with a mean particle size varying from 25 to 100 nm. The presence of copper decreased the nickel phase dispersion as well as the conversion whereas it increased carbon deposit in benzene hydrogenation. The results obtained are ascribed to nickel phase shape changes and Ni-Cu interactions. A kinetic reaction mechanism model is proposed. The comparative study of a pair of classical/non-classical Ni/SiO₂ catalysts showed much higher surface and catalytic properties of the hydrazine catalyst.     

Production of COX Free Hydrogen by Catalytic Decomposition of Methane Over Ni/HY Catalysts

In this present paper, we report catalytic decomposition of methane over Ni/HY catalysts, with varying Ni loading at 550 °C and atmospheric pressure. The relationships between catalyst performance and characterization of the fresh and used form of catalysts are discussed from the data obtained by scanning electron microscopy, X-ray diffraction analysis, temperature programmed reduction, O₂ pulse chemisorption and carbon elemental analyses. It is observed that, the catalytic activity of Ni/HY catalysts is high at initial stages and gradually decreased with time and finally deactivated completely. The yield of hydrogen and carbon nanofibers is strongly dependent on Ni loading. It is found that 20 wt% Ni/HY catalyst showed higher hydrogen yield over the other loadings.     

Hydrogen production by catalytic decomposition of methane over carbon catalysts in a fluidized bed

A fluidized bed reactor made of quartz tube with an I.D. of 0.055 m and a height of 1.0 m was employed for the thermocatalytic decomposition of methane to produce CO₂ — free hydrogen. The fluidized bed was used for continuous withdrawal of the carbon products from the reactor. Two kinds of carbon catalysts — activated carbon and carbon black — were employed in order to compare their catalytic activities for the decomposition of methane in the fluidized bed. The thermocatalytic decomposition of methane was carried out in a temperature range of 800–925°C, using a methane gas velocity of 1.0–3.0 U _mf and an operating pressure of 1.0 atm. Distinctive difference was observed in the catalytic activities of two carbon catalysts. The activated carbon catalyst exhibited higher initial activity which decreased significantly with time. However, the carbon black catalyst exhibited somewhat lower initial activity compared to the activated carbon catalyst, but its activity quickly reached a quasi-steady state and was sustained over time. Surfaces of the carbon catalysts before and after the reaction were observed by SEM. The effect of various operating parameters such as the reaction temperature and the gas velocity on the reaction rate was investigated.     

Coprecipitated iron-containing catalysts (Fe-Al2O3, Fe-Co-Al2O3, Fe-Ni-Al2O3) for methane decomposition at moderate temperatures: Part II. Evolution of the catalysts in reaction

Coprecipitated Fe-Al₂O₃, Fe-Co-Al₂O₃ and Fe-Ni-Al₂O₃ catalysts is shown to be very efficient in carbon deposition during methane decomposition at moderate temperatures (600–650 °C). The carbon capacity of the most efficient bimetallic catalysts containing 50–65 wt.% Fe, 5–10 wt.% Co (or Ni) and 25–40 wt.% Al₂O₃ is found to reach 145 g/g_cat. Most likely, their high efficiency is due to specific crystal structures of the metal particles and formation of optimum particle size distribution. According to the TEM data, catalytic filamentous carbon (CFC) is formed on them as multiwall carbon nanotubes (MWNTs). The phase composition of the catalysts during methane decomposition is studied using a complex of physicochemical methods (XRD, REDD, Mössbauer spectroscopy and EXAFS). Possible mechanisms of the catalyst deactivation are discussed.     

Study of Methane Decomposition on Fe/MgO-Based Catalyst Modified by Ni,Co, and Mn Additives

Catalytic decomposition of methane (CDM) generates clean hydrogen and carbon nanomaterials. In this study, methane decomposition to hydrogen and carbon was investigated over Ni-, Co-, or Mn-doped Fe/MgO catalysts. The doping effect of different metals, varying from 3 to 10?wt%, was investigated. The catalytic performance of the obtained materials (noted 15%Fe+x%metal/MgO) revealed that the doping effect of Ni, Co, and Mn significantly improved the activity of Fe/MgO. Among the Ni-doped catalyst series, the 15%Fe+3%Ni/MgO catalyst performed better than the rest of the Ni catalysts. The 6%Co-containing catalyst remained the best in terms of activity in the Co-doped catalyst series and the 15%Fe+6%Mn/MgO solid showed better methane conversion for the Mn-doped series. Overall, 3%Ni-containing catalyst displayed the best catalytic performance among all Ni-, Co-, and Mn-doped catalysts. XRD, N₂ sorption, and H₂ temperature-programmed reduction (TPR), Laser–Raman spectroscopy, thermogravimetric analysis (TGA) under air, and temperature-programmed oxidation (TPO) were used for catalyst characterization. The results revealed that all the doped catalysts exhibited better metallic active site distribution than 15%Fe/MgO and proved that metal doping played a crucial role in catalytic performance.     

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.     

Preparation of carbon micro-coils involving the decomposition of hydrocarbons using PACT (plasma and catalyst technology) reactor

Carbon micro-coils as well as carbon fibers with various morphologies were prepared by the decomposition of hydrocarbons, such as acetylene, methane, propane, ethylene, etc., at 770°C using a PACT (plasma and catalyst technology) reactor. The preparation conditions, growth mechanism and morphology of the carbon micro-coils were examined. The Ni electrode of the PACT reactor was used as the catalyst as well as a plasma source electrode. It was found that hydrocarbons, such as methane, propane and ethylene, decomposed under the plasma and catalyst atmosphere to form acetylene as the main decomposition product, and then this acetylene was further decomposed to form carbon micro-coils. Using a Ni powder catalyst dispersed on the substrate, the carbon micro-coils with a double helix structure, in which two pieces of carbon coils entwine each other in the same coiling direction, grew among the single straight carbon fibers and paired straight fibers. On the other hand, the carbon micro-coils with a single helix structure and wide coil pitch were obtained by the indirect decomposition of acetylene using the N₂ plasma formed by the PACT reactor.     

A model for the structure and growth of carbon nanofibers synthesized by the CVD method using nickel as a catalyst

Carbon nanofibers were synthesized at 450-800 °C by the catalytic CVD method using alumina plate-supported nickel as catalyst and acetylene as carbon source. It was found that Ni/alumina catalyst exhibited a large catalytic effect on the growth of carbon nanofibers at the temperatures between 550 and 700 °C. TEM observation revealed that most of the carbon nanofibers synthesized at 550 °C had a coil-like shape, and many thick platelet nanofibers were found in the product at 700 °C. A growth model was proposed to explain the structural diversity of the carbon nanofibers. Although the carbon nanofibers showed low crystallinity, they can be easily graphitized at 2500 °C.     

HI decomposition over active carbon supported binary Ni–Pd catalysts prepared by electroless plating

A series of binary Ni–Pd catalysts supported on active carbon was prepared by electroless plating method. For comparison, active carbon supported monometallic Pd and Ni catalysts were also prepared by liquid-phase reduction. Among the Pd, Ni and binary Ni–Pd catalysts, the catalyst of 1%Ni–1%Pd/C showed the best catalytic performance for the decomposition of HI. Furthermore, characterizations by BET and XRD revealed that the binary Ni–Pd catalyst had the higher stability in specific surface area and structure than monometallic Pd catalyst during HI decomposition.     

Catalytic test of supported Ni catalysts with core/shell structure for dry reforming of methane

Supported nickel catalysts with core/shell structures of Ni/Al₂O₃ and Ni/MgO-Al₂O₃ were synthesized under multi-bubble sonoluminescence (MBSL) conditions and tested for dry reforming of methane (DRM) to produce hydrogen and carbon monoxide. A supported Ni catalyst made of 10% Ni loading on Al₂O₃ and MgO-Al₂O₃, which performed best in the steam reforming of methane (97% methane conversion at 750 °C) and in the partial oxidation of methane (96% methane conversion at 800 °C), showed also good performance in DRM and excellent thermal stability for the first 150 h. The supported Ni catalysts Ni/Al₂O₃ and Ni/MgO-Al₂O₃ yielded methane conversions of 92% and 92.5%, respectively and CO₂ conversions of 95.0% and 91.8%, respectively, at a reaction temperature of 800 °C with a molar ratio of CH₄/CO₂ = 1. Those were near thermodynamic equilibrium values.     

Catalytic Methane Decomposition to Hydrogen over a Surface‐Protected Core‐Shell Ni@SiO2 Catalyst

Methane decomposition is a promising method to obtain CO_x‐free hydrogen. The main difficulty of this process is that the produced carbon would deposit on the active phase of the catalyst, leading to catalyst deactivation. In this study, a core‐shell‐structured composite catalyst comprising highly active Ni nanoparticles (NP) as core and mesoporous silica as shell is introduced. The silica shells were synthesized by using cetyltrimethylammonium bromide as template and tetraethyl orthosilicate as precursor. Ni NP and Ni@SiO₂ were examined as catalysts for hydrogen production by methane decomposition at different temperatures and gas hourly space velocities. The results show that the core‐shell catalyst exhibited much better stability in methane decomposition than Ni NP without silica shell and a traditional supported catalyst.     

Mixed and autothermal reforming of methane with supported Ni catalysts with a core/shell structure

Supported nickel catalysts with a core/shell structure of Ni/Al₂O₃ and Ni/MgO-Al₂O₃ synthesized under multi-bubble sonoluminescence (MBSL) conditions were tested for mixed steam and dry (CO₂) reforming and autothermal reforming of methane. In the previous tests, the supported Ni catalysts made of 10% Ni loading on Al₂O₃ or MgO-Al₂O₃ had shown good performances in the steam reforming of methane (methane conversion of 97% at 750 °C), in the partial oxidation of methane (methane conversion of 96% at 800 °C) and in dry reforming of methane (methane conversion of 96% at 850 °C) and showed high thermal stability for the first 50-150 h. In this study, the supported Ni catalysts showed good performance in the mixed and autothermal reforming of methane with their excellent thermal stability for the first 50 h. In addition, very interestingly, there was no appreciable carbon deposition on the surface of the tested catalysts after the reforming reaction.     

Thermogravimetric studies of carbon nanofiber formation from methane at low temperature over Ni-based skeletal catalysts and the effect of substrate pre-carburization

Using thermogravimetry (TG) under conditions that minimize inhibition by the hydrogen produced, the intrinsic catalytic rates of skeletal Ni, pure and alloyed with solute metals Fe, Co, or Cu, were evaluated in methane decomposition to carbon nanofibers. In “standard” tests, i.e., after pre-reduction in H₂ and exposure to CH₄ directly at 450 °C, several catalysts reached stable activities exceeding 4 mg C/mg cat./h, comparable with literature values obtained at 500 °C or above. TG evidence is presented for partial bulk carburization of Ni in CH₄ below 350 °C, which leads to substantially increased coking rates. TEM evidence supports the view that carburization promotes catalyst particle disintegration, thereby inducing faster and more stable nanofiber growth. Irregularities in alloy response to carburization are interpreted in terms of the stability of the respective mixed-metal carbides. TEM also shows that alloying changes the metal nanocrystallite shape (habit), with consequences for the carbon nanofiber structure. Evidence for the easy dissociation of CH₄ is corroborated by direct catalyst activation in the absence of H₂. Reduction begins in pure hydrocarbon around 300 °C and leads to coking activities at 450 °C comparable to those for samples pre-reduced in H₂. Skeletal metal catalysts offer distinct advantages in low-temperature natural gas conversion.     

High yield preparation of tubular carbon nanofibers over supported Co-Mo catalysts

In the search for high yield synthesis of carbon nanotubes (CNTs) at lower temperatures, Co-Mo catalysts on carbon black were investigated with ethylene and CO as carbon sources in catalytic gas-phase pyrolysis in comparison to that on TiO₂. The carbon black support was expected to be advantageous because of the feasibility of a CNT/carbon black composite possibly fabricated for several applications without removal of the support. Depending on the catalyst support, the catalytic activity toward CO and ethylene showed great differences. Co-Mo (9:1) catalysts on titania or carbon black provided a high carbon yield from CO and ethylene at the rather low temperatures of 450-530 °C.     

Increasing the activity of Fe/N/C catalysts in PEM fuel cell cathodes using carbon blacks with a high-disordered carbon content

Fe/N/C catalysts for the reduction of oxygen in PEM fuel cells were prepared by pyrolyzing three series of iron acetate-impregnated developmental carbon blacks at 950 °C. The carbon supports used were derived from the N234, N330, and N650 commercial furnace grades. In this study, we tried to increase the performance of Fe/N/C-based cathode of PEM fuel cells by using the following two approaches: (1) increasing the number of catalytic sites on the carbon black either by optimizing the structural parameters of the pristine carbon supports or by increasing the initial metal content above 0.2 wt% Fe on the carbon support; (2) increasing the catalyst loading in the cathodic layer of a PEM fuel cell. For (1), we show, on the one hand, that optimizing the structural parameters of the pristine carbon support, in order to increase the number of catalytic sites, has its limits and that these limits have been reached for the present synthesis method of Fe/N/C catalysts. On the other hand, increasing the initial metal content above 0.2 wt% Fe leads to a decrease in catalytic activity. For (2), it is shown that increasing the catalyst loading per cm² of cathode well improves the performance of a cathode based on Fe/N/C catalysts in the kinetic region of the polarization curve. At lower potentials, a large improvement in the performance of these non-precious metal cathodes would occur if the mass transport properties in these electrodes were significantly increased.     

Methane reforming reaction with carbon dioxide over SBA-15 supported Ni-Mo bimetallic catalysts

A series of mesoporous molecular sieves SBA-15 supported Ni-Mo bimetallic catalysts (xMo1Ni, Ni = 12 wt.%, Mo/Ni atomic ratio = x, x = 0, 0.3, 0.5, 0.7) were prepared using co-impregnation method for carbon dioxide reforming of methane. The catalytic performance of these catalysts was investigated at 800 °C, atmospheric pressure, GHSV of 4000 ml·g_cat^− 1·h^− 1 and a V(CH₄)/(CO₂) ratio of 1 without dilute gas. The result indicated that the Ni-Mo bimetallic catalysts had a little lower initial activity compared with Ni monometallic catalyst, but it kept very stable performance under the reaction conditions. In addition, the Ni-Mo bimetallic catalyst with Mo/Ni atomic ratio of 0.5 showed high activity, superior stability and the lowest carbon deposition rate (0.00073g_c·g_cat^− 1·h^− 1) in 600-h time on stream. The catalysts were characterized by power X-ray diffraction, N₂-physisorption, H₂-TPR, CO₂-TPD, TG and TEM. The results indicate that the Ni-Mo bimetallic catalysts have smaller metal particle, higher metal dispersion, stronger basicity, metal-support interaction and Mo₂C species. It is concluded that Mo species in the Ni-Mo bimetallic catalysts play important roles in reducing effectively the amount of carbon deposition, especially the amount of shell-like carbon deposition.     

Carbon dioxide reforming of methane over nickel catalysts supported on mesoporous MgO

In this paper, ordered mesoporous MgO nanocrystals [MgO(M)] were synthesized, and the nickel catalysts supported on MgO(M) were facilely prepared by impregnation method. The obtained Ni/MgO(M) catalysts with advantageous textural properties were investigated as the catalysts for the carbon dioxide reforming of methane reaction. It was found that compared with the Ni/MgO(C) catalyst [MgO(C): commercial MgO], the mesoporous pore structure of MgO(M) could effectively limit the growth of the activity metal, and the Ni/MgO(M) catalysts showed high catalytic activities as well as long catalytic stabilities toward this reaction. The results showed that the conversions of CH₄ and CO₂ were only decreased <5 % after 100 h of reaction at 650 °C. The improved catalytic performance was suggested to be closely associated with both the advantageous structural properties, such as large specific surface area, uniform pore size, and the “confinement effect” of the mesoporous matrixes contributed to stabilize the Ni active sites during the reaction. The carbon species deposited on the spent Ni/MgO(M) catalyst were analysized by TG and Raman, and the results exhibited that the carbon species after 100 h of reaction were mainly active carbon species.