High temperature iron-based catalysts for hydrogen and nanostructured carbon production by methane decomposition

The production of hydrogen and filamentous carbon by means of methane decomposition was investigated in a fixed-bed reactor using iron-based catalysts. The effect of the textural promoter and the addition of Mo as a dopant affects the catalysts performance substantially: iron catalyst prepared with Al₂O₃ showed slightly higher catalytic performance as compared to those prepared with MgO; Mo addition was found to improve the catalytic performance of the catalyst prepared with MgO, whereas in the catalyst prepared with Al₂O₃ displayed similar or slightly poorer results. Additionally, the influence of the catalyst reduction temperature, the reaction temperature and the space velocity on the hydrogen yield was thoroughly investigated. The study reveals that iron catalysts allow achieving high methane conversions at operating temperatures higher than 800 °C, yielding simultaneously carbon nanofilaments with interesting properties. Thus, at 900 °C reaction temperature and 1 l g⁻¹_cat h⁻¹ space velocity, ca. 93 vol% hydrogen concentration was obtained, which corresponds to a methane conversion of 87%. Additionally, it was found that at temperatures higher than 700 °C, carbon co-product is deposited mainly as multi walled carbon nanotubes. The textural and structural properties of the carbonaceous structures obtained are also presented.     

Metallic and carbonaceous –based catalysts performance in the solar catalytic decomposition of methane for hydrogen and carbon production

Solar catalytic decomposition of methane (SCDM) was investigated in a solar furnace facility with different catalysts. The aim of this exploratory study was to investigate the potential of the catalytic methane decomposition approach providing the reaction heat via solar energy at different experimental conditions. All experiments conducted pointed out to the simultaneous production of a gas phase composed only by hydrogen and un-reacted methane with a solid product deposited into the catalyst particles varying upon the catalysts used: nanostructured carbons either in form of carbon nanofibers (CNF) or multi-walled carbon nanotubes (MWCNT) were obtained with the metallic catalyst whereas amorphous carbon was produced using a carbonaceous catalyst. The use of catalysts in the solar assisted methane decomposition present some advantages as compared to the high temperature non-catalytic solar methane decomposition route, mainly derived from the use of lower temperatures (600–950 °C): SCDM yields higher reaction rates, provides an enhancement in process efficiency, avoids the formation of other hydrocarbons (100% selectivity to H₂) and increases the quality of the carbonaceous product obtained, when compared to the non-catalytic route.     

Thermocatalytic decomposition of methane for hydrogen production using activated carbon catalyst: Regeneration and characterization studies

A series of experiments was conducted to study the deactivation and regeneration of activated carbon catalyst used for methane thermocatalytic decomposition to produce hydrogen. The catalyst becomes deactivated due to carbon deposition and six decomposition cycles of methane at temperatures of 850 and 950 °C, and five cycles of regeneration by using CO₂ at temperatures of 900, 950 and 1000 °C were carried out to evaluate the stability of the catalyst. The experiment was conducted by using a thermobalance by monitoring the mass gain during decomposition or the mass lost during the regeneration with time. The initial activity and the ultimate mass gain of the catalyst decreased after each regeneration cycle at both reaction temperatures of 850 and 950 °C, but the amount is smaller under the more severe regenerating conditions. For the reaction at 950 °C, comparison between the first and sixth reaction cycles shows that the initial activity decreased by 69, 51 and 42%, while the ultimate mass gain decreased by 62%, 36% and 16% when CO₂ gasification carried out at 900, 950 and 1000 °C respectively. Temperature -programmed oxidation profiles for the deactivated catalyst at reaction temperature of 950 °C and after several cycles showed two peaks which are attributed to different carbon characteristics, while one peak was obtained when the experiment was carried out at 850 °C. In conclusion, conducting methane decomposition at 950 °C and regeneration at 1000 °C showed the lowest decrease in the mass gain with reaction cycles.     

Hydrogen production from carbon dioxide reforming of methane over Ni-Co/MgO-ZrO2 catalyst: Process optimization

Greenhouse gases, carbon dioxide and methane are utilized in the production of hydrogen through carbon dioxide reforming of methane catalyzed by Ni-Co/MgO-ZrO₂ catalyst. Design of Experiments (DOE) was used to study the effects of process variables such as, carbon dioxide to methane ratios (1-5), gas hourly space velocity (8400-200,000 mL/g/h), oxygen concentration in the feed (3-8 mol%) and reaction temperature (700-800 °C) over methane conversion and yield of hydrogen. The ANOVA analysis indicated that the effect of each process variable was significant to its respective responses in the proposed quadratic model. The response surface methodology (RSM) was used to find the optimum value of the process variables by maximizing the hydrogen yield in the process model. The optimum space velocity as 145,190 mL/g/h at reaction temperature 749 °C with carbon dioxide to methane ratio of 3 and 7 mol% of oxygen in the feed gave 88 mol% of CH₄ conversion and 86 mol% of hydrogen yield, respectively. The experiments were run at the optimum condition gave 87.7 mol% methane conversion and 85.5 mol% of hydrogen yield, which were in good agreement with the simulated values obtained from the model. The catalyst stability and its regeneration characteristics were studied at the optimum condition by monitoring methane conversion and hydrogen yield with time on stream.     

Methane catalytic decomposition over ordered mesoporous carbons: A promising route for hydrogen production

Methane decomposition offers an interesting route for the CO₂-free hydrogen production. The use of carbon catalysts, in addition to lowering the reaction temperature, presents a number of advantages, such as low cost, possibility of operating under autocatalytic conditions and feasibility of using the produced carbons in non-energy applications. In this work, a novel class of carbonaceous materials, having an ordered mesoporous structure (CMK-3 and CMK-5), has been checked as catalysts for methane decomposition, the results obtained being compared to those corresponding to a carbon black sample (CB-bp) and two activated carbons, presenting micro- (AC-mic) and mesoporosity (AC-mes), respectively. Ordered mesoporous carbons, and especially CMK-5, possess a remarkable activity and stability for the hydrogen production through that reaction. Under both temperature programmed and isothermal experiments, CMK-5 has shown to be a superior catalyst for methane decomposition than the AC-mic and CB-bp materials. Likewise, the catalytic activity of CMK-5 is superior to that of AC-mes in spite of the presence of mesoporosity and a high surface area in the latter. The remarkable stability of the CMK-5 catalyst is demonstrated by the high amount of carbon deposits that can be formed on this sample. This result has been assigned to the growth of the carbon deposits from methane decomposition towards the outer part of the catalyst particles, avoiding the blockage of the uniform mesopores present in CMK-5. Thus, up to 25 g of carbon deposits have been formed per gram of CMK-5, while the latter still retains a significant catalytic activity.     

Nanocomposite Ni/ZrO2: Highly active and stable catalyst for H2 production via cyclic stepwise methane reforming reactions

This work investigates the catalytic performance of nanocomposite Ni/ZrO₂-AN catalyst consisting of comparably sized Ni (10–15 nm) and ZrO₂ (15–25 nm) particles for hydrogen production from the cyclic stepwise methane reforming reaction with either steam (H₂O) or CO₂ at 500–650 °C, in comparison with a conventional Ni/ZrO₂-CP catalyst featuring Ni particles supported by large and widely sized ZrO₂ particles (20–400 nm). Though both catalysts exhibited similar activity and stability during the reactions at 500 and 550 °C, they showed remarkably different catalytic stabilities at higher temperatures. The Ni/ZrO₂-CP catalyst featured a significant deactivation even during the methane decomposition step in the first cycle of the reactions at ≥600 °C, but the Ni/ZrO₂-AN catalyst showed a very stable activity during at least 17 consecutive cycles in the cyclic reaction with steam. Changes in the catalyst beds at varying stages of the reactions were characterized with TEM, XRD and TPO–DTG and were correlated with the amount and nature of the carbon deposits. The Ni particles in Ni/ZrO₂-AN became stabilized at the sizes of around 20 nm but those in Ni/ZrO₂-CP kept on growing in the methane decomposition steps of the cyclic reaction. The small and narrowly sized Ni particles in the nanocomposite Ni/ZrO₂-AN catalyst led to a selective formation of filamentous carbons whereas the larger Ni particles in the Ni/ZrO₂-CP catalyst a preferred formation of graphitic encapsulating carbons. The filamentous carbons were favorably volatilized in the steam treatment step but the CO₂ treatment selectively volatilized the encapsulating carbons. These results identify that the nature but not the amount of carbon deposits is the key to the stability of Ni/ZrO₂ catalyst and that the nanocomposite Ni/ZrO₂-AN would be a promising catalyst for hydrogen production via cyclic stepwise methane reforming reactions.     

Production of hydrogen by methane catalytic decomposition over Ni–Cu–Fe/Al2O3 catalyst

Catalysts with high nickel concentrations 75%Ni–12%Cu/Al₂O₃, 70%Ni–10%Cu–10%Fe/Al₂O₃ were prepared by mechanochemical activation and their catalytic properties were studied in methane decomposition. It was shown that modification of the 75%Ni–12%Cu/Al₂O₃ catalyst with iron made it possible to increase optimal operating temperatures to 700–750 °C while maintaining excellent catalyst stability. The formation of finely dispersed Ni–Cu–Fe alloy particles makes the catalysts stable and capable of operating at 700–750 °C in methane decomposition to hydrogen and carbon nanofibers. The yield of carbon nanofibers on the modified 70%Ni–10%Cu–10%Fe/Al₂O₃ catalyst at 700–750 °C was 150–160 g/g. The developed hydrogen production method is also efficient when natural gas is used as the feedstock. An installation with a rotating reactor was developed for production of hydrogen and carbon nanofibers from natural gas. It was shown that the 70%Ni–10%Cu–10%Fe/Al₂O₃ catalyst could operate in this installation for a prolonged period of time. The hydrogen concentration at the reactor outlet exceeded 70 mol%.     

Oleic acid gasification over supported metal catalysts in supercritical water: Hydrogen production and product distribution

Oleic acid was examined as a model compound for lipids, which was gasified in supercritical water (SCW) using a batch reactor from 400 to 500 °C at 28 MPa. The influence of operating temperature and several commercial catalysts on the gasification efficiency, hydrogen gas yield, and residual liquid product quality was examined and discussed. The main gaseous components measured were carbon dioxide (CO₂), hydrogen (H₂), methane (CH₄), and traces of carbon monoxide (CO). The residual liquid after reaction was characterized by analyzing the chemical oxygen demand (COD), total organic carbon (TOC), volatile fatty acids (VFAs), and the long chain fatty acids (LCFAs), namely, palmitic, myristic, stearic, linoleic, and oleic acids. The results showed that an increase of temperature coupled with the use of catalyst enhanced the gas yield dramatically. The H₂ yield was 15 mol/mol oleic acid converted using both the pelletized Ru/Al₂O₃ and powder Ni/Silica-alumina catalysts which gave 4 times higher than the equilibrium yield. The COD reduction efficiency ranged from 31% at 400 °C without catalyst to 96 % at 500 °C in the presence of Ni/Silica-alumina catalyst. The composition of residual liquid products was studied using gas chromatography/mass spectrometry (GC-MS), with a generalized reaction pathway for oleic acid decomposition in SCW reported.     

Ni–SiO2 and Ni–Fe–SiO2 catalysts for methane decomposition to prepare hydrogen and carbon filaments

Active and stable Ni–Fe–SiO₂ catalysts prepared by sol–gel method were employed for direct decomposition of undiluted methane to produce hydrogen and carbon filaments at 823 K and 923 K. The results indicated that the lifetime of Ni–Fe–SiO₂ catalysts was much longer than Ni–SiO₂ catalyst at a higher reaction temperature such as 923 K, however, a reverse trend was shown when methane decomposition took place at a lower reaction temperature such as 823 K. XRD studies suggested that iron atoms had entered into the Ni lattice and Ni–Fe alloy was formed in Ni–Fe–SiO₂ catalysts. The structure of the carbon filaments generated over Ni–SiO₂ and Ni–Fe–SiO₂ was quite different. TEM studies showed that “multi-walled” carbon filaments were formed over 75%Ni–25%SiO₂ catalyst, while “bamboo-shaped” carbon filaments generated over 35%Ni–40%Fe–25%SiO₂ catalysts at 923 K. Raman spectra of the generated carbons demonstrated that the graphitic order of the “multi-walled” carbon filaments was lower than that of the “bamboo-shaped” carbon filaments.     

Methane steam reforming for producing hydrogen in an atmospheric-pressure microwave plasma reactor

A methane steam reforming process for producing mainly hydrogen in an atmospheric-pressure microwave plasma reactor is demonstrated. Nano carbon powders, CO_x, C₂H₂, C₂H₄, and HCN were also formed. Intermediates such as OH, NH, CH, and active N₂ were identified using optical emission spectroscopy. The selectivity of H₂ was greater than 92.7% at inlet H₂O/CH₄ molar ratio (R) ≧ 0.5, and was higher than that obtained using methane plasmalysis because steam inhibited the formation of C₂H₂. The highest methane conversion was obtained at R = 1, reaching 91.6%, with the lowest specific energy consumption of H₂ formation at [CH₄]_in = 5%, 1.0 kW, and 12 slpm. The plasma-assisted catalysis process, which packed Ni/Al₂O₃ catalysts in the discharge zone and supplied heat using hot effluents, was used to elevate the methane conversion and hydrogen selectivity. However, large amounts of 40–70 nm carbon powder, which is electrically conductive, were produced, resulting in rapid catalyst deactivation due to carbon being deposited on the surface and in the pores of catalysts.     

Parametric study of the decomposition of methane using a NiCu/Al2O3 catalyst in a fluidized bed reactor

CO₂-free production of hydrogen via catalytic decomposition of methane (CDM) was studied in a fluidized bed reactor (FBR) using a NiCu/Al₂O₃ catalyst. A parametric study of the effects of some process variables, including catalyst particle size, reaction temperature, space velocity and the ratio of gas flow velocity to the minimum fluidization velocity (u_o/u_mf), was undertaken. A mean particle size of 150 μm allows optimization of results in terms of hydrogen production without agglomeration problems. The operating conditions strongly affect the catalyst performance: hydrogen production was enhanced by increasing operating temperature and lowering space velocity. However, increases in operating temperature, space velocity and the ratio u_o/u_mf provoked increases in the catalyst deactivation rate. At 700 °C, carbon was deposited as carbon nanofibers, while higher temperatures promoted the formation of encapsulating carbon, which led to rapid catalyst deactivation.     

Autothermal reforming of methane for producing high-purity hydrogen in a Pd/Ag membrane reactor

Autothermal reforming of methane includes steam reforming and partial oxidizing methane. Theoretically, the required endothermic heat of steam reforming of methane could be provided by adding oxygen to partially oxidize the methane. Therefore, combining the steam reforming of methane with partial oxidation may help in achieving a heat balance that can obtain better heat efficacy. Membrane reactors offer the possibility of overcoming the equilibrium conversion through selectively removing one of the products from the reaction zone. For instance, only can hydrogen products permeate through a palladium membrane, which shifts the equilibrium toward conversions that are higher than the thermodynamic equilibrium. In this study, autothermal reforming of methane was carried out in a traditional reactor and a Pd/Ag membrane reactor, which were packed with an appropriate amount of commercial Ni/MgO/Al₂O₃ catalyst. A power analyzer was employed to measure the power consumption and to check the autothermicity. The average dense Pd/Ag membrane thickness is 24.3 μm, which was coated on a porous stainless steel tube via the electroless palladium/silver plating procedure. The experimental operating conditions had temperatures that were between 350 °C and 470 °C, pressures that were between 3 atm and 7 atm, and O₂/CH₄ = 0–0.5. The effects of the operating conditions on methane conversion, permeance of hydrogen, H₂/CO, selectivities of CO_x, amount of power supply, and the carbon deposition of the catalyst after the reaction is thoroughly discussed in this paper. The experimental results indicate that an optimum methane conversion of 95%, with a hydrogen production rate of 0.093 mol/m². S, can be obtained from the autothermal reforming of methane at H₂O/CH₄ = 1.3 and O₂/CH₄ near 0.4, at which the reaction does not consume power, and the catalysts are not subject to any carbon deposition.     

Effect of hydrogen additive on methane decomposition to hydrogen and carbon over activated carbon catalyst

The effect of H₂ addition on CH₄ decomposition over activated carbon (AC) catalyst was investigated. The results show that the addition of H₂ to CH₄ changes both methane conversion over AC and the properties of carbon deposits produced from methane decomposition. The initial methane conversion declines from 6.6% to 3.3% with the increasing H₂ flowrate from 0 to 25 mL/min, while the methane conversion in steady stage increases first and then decreases with the flowrate of H₂, and when the H₂ flowrate is 10 mL/min, i.e. quarter flowrate of methane, the methane conversion over AC in steady stage is four times more than that without hydrogen addition. It seems that the activity and stability of catalyst are improved by the introduction of H₂ to CH₄ and the catalyst deactivation is restrained. Filamentous carbon is obtained when H₂ is introduced into CH₄ reaction gas compared with the agglomerate carbon without H₂ addition. The formation of filamentous carbon on the surface of AC and slower decrease rate of surface area and pores volume may cause the stable activity of AC during methane decomposition.     

Hydrogen production by propylene-assisted decomposition of methane over activated carbon catalysts

Catalytic decomposition of methane (CDM) permits obtaining hydrogen in high yields and – what is essential – it does not lead to release of CO₂. Unfortunately, most of the catalysts used in this process undergo fast deactivation. Their possible regeneration, consisting in the removal of pore blocking carbonaceous deposit of low catalytic activity, leads to generation of undesirable carbon dioxide. An alternative solution for maintaining high catalyst activity in the CDM reaction can be generation of the catalytically active carbonaceous deposit on its surface. Such a deposit can be obtained by decomposition of different organic substances. This paper reports on methane decomposition carried out in the presence of propylene (used in the concentration of 10 or 20%). The reaction was performed at three temperatures of 750 °C, 850 °C or 950 °C. Three types of activated carbon were tested as catalysts: the first one was obtained by activation of pine wood biomass with Na₂CO₃, whereas the second and third ones were commercial carbons (WG-12 and Norit RX3 Extra). According to the results, the addition of propylene to the CDM system effectively reduces deactivation of the activated carbon catalysts and permits fast stabilisation of their catalytic activity at a high level.     

Production of hydrogen and nanofibrous carbon by selective catalytic decomposition of propane

The process of production of highly concentrated CO_x-free hydrogen and nanofibrous carbon (NFC) by catalytic propane decomposition on Ni and Ni–Cu catalysts (different in active phase composition) at relatively low temperatures (400–700 °C) was investigated. The bimetallic Ni–Cu catalysts showed significantly higher propane conversion and longer lifetime than monometallic Ni catalyst. The Ni (50 wt.%)–Cu (40 wt.%)/SiO₂ catalyst exhibited the best activity and selectivity at 600 °C. Total hydrogen yield of 60.8 mol H₂/g_cat (during 24 h time on stream) and the total H₂:CH₄ ratio of 8.4 were obtained during propane decomposition under these optimal conditions. The possible reaction scheme of propane decomposition over Ni-based catalysts and the reasons of increasing the selectivity of hydrogen are discussed.     

Evidence of composition deviation of metal particles of a Ni–Cu/Al2O3 catalyst during methane decomposition to COx-free hydrogen

A coprecipitated Ni–Cu/Al₂O₃ catalyst was examined in a fluidized bed reactor for the decomposition of methane to CO_x-free hydrogen and carbon nanofibers using two different reaction temperature schemes, i.e. constant temperature and pre-induction operation. At constant temperatures, the catalyst showed quasi-stable activity below 913 K for a notable time period, while its performance decayed fast above this temperature and a quick deactivation was observed at 1013 K. However, for reaction at 1013 K, the durability was improved if the catalyst was induced for 10 min at 823 K in the reaction atmosphere. A detailed examination of the metal particles after reaction with EDS revealed that the composition of the metal particle depended strongly on the reaction temperature and the induction scheme. The metal particles in the reduced catalyst showed a composition deviation also. The HRTEM micrographs of the carbon–metal interface and the EDS results support the surface migration and ensemble mechanism for carbon formation and give evidence for the reconstruction of the metal particles during the induction period.     

Study of the deactivation mechanism of carbon blacks used in methane decomposition

Carbon blacks have recently gained attention as suitable catalysts for the CO_x-free hydrogen production by thermo-catalytic decomposition of methane (TCD) because of their stability and efficiency. In the present work, several commercial carbon blacks were studied as catalysts for the TCD of methane by varying the temperature and the methane space velocity. The BP2000 carbon black sample, which showed the highest activity in methane decomposition per mass of catalyst, was studied more thoroughly. Despite BP2000 exhibiting stable activity in the TCD of methane during several hours on stream, a long duration run carried out at 950 °C revealed that it finally became deactivated. The changes in the physicochemical properties (textural properties, surface chemistry and crystallinity) of the BP2000 sample at different stages of the catalyst lifetime were measured, and the main results obtained are presented here. The paper also discusses the potential of the production of a wide range of hydrogen–methane mixtures, which can be directly fed to an internal combustion engine, by means of TCD with carbonaceous catalysts.     

COX-free H2 production via catalytic decomposition of CH4 over Ni supported on zeolite catalysts

Catalytic decomposition of methane produces CO_X-free hydrogen, which is necessary for PEM fuel-cell applications. In this paper, hydrogen production by catalytic decomposition of methane at 550 °C over Ni on HY, USY, SiO₂ and SBA-15 supports is examined at atmospheric pressure. The catalytic activities and the life times of the catalysts are evaluated and discussed. The relationships between catalyst performance and characterization of the fresh and used catalysts are discussed with the results obtained from SEM, XRD, TPR, solid acidity and the measured carbon contents generated of the used samples along with their H₂ production rates. Among all the catalysts tested, Ni supported on HY zeolite showed a higher activity of 955 mol H₂ (mol Ni)⁻¹ and a longevity of 720 min at 550 °C.     

Influence of ethylene on carbon-catalysed decomposition of methane

Catalytic decomposition of methane is a much promising pro-ecological method of hydrogen production. However, the drawback of this method is fast deactivation of the catalyst by deposition of a low-active methane-originated carbon on its surface. In this study an attempt has been made to reduce the process of catalyst deactivation by adding admixture of ethylene to methane directed to the reactor. The study has been performed on the activated carbon obtained by Na₂CO₃ activation of pine wood and two commercial types of activated carbons. All the carbon types have been subjected to ultimate analysis, determination of the surface area and pore structure. It has been shown that ethylene also forms a carbonaceous deposit but in contrast to the methane-originated deposit the ethylene-originated one shows good catalytic properties in the reaction of methane decomposition. The addition of 20% ethylene seems to be optimum for ensuring high yield of hydrogen for a long time. The ethylene admixture addition is the more effective the higher the temperature of the process.     

Influence of space velocity and catalyst pretreatment on COx free hydrogen and carbon nanotubes production over CoMo/MgO catalyst

The influence of catalyst pretreatment and space velocity in methane decomposition into COx-free hydrogen and carbon nanotubes were investigated over CoMo/MgO catalyst. The reduction of catalyst before methane decomposition leads to a hydrogen production without significant formation of COx (concentration lower than 5 ppm after 25 min of reaction) suitable for its use in fuel cells. However, a high hydrogen space velocity in the pretreatment increased the rate of catalyst deactivation. The CO and CO₂ formation rates showed a common trend for all conditions tested: there was a high initial rate and, after 2 min of reaction, there was a lower stabilized rate. The increase in methane space velocity increased the hydrogen formation rate and the degree of carbon nanotubes graphitization. However, it strongly decreases methane conversion, as expected. The use of low hydrogen space velocity, 0.25 h⁻¹, in catalyst pretreatment and high methane space velocity, 8 h⁻¹, in reaction step, provided the highest hydrogen yield and well-structured carbon nanotubes.