Deactivation of palm shell-based activated carbon catalyst used for hydrogen production by thermocatalytic decomposition of methane

Thermocatalytic decomposition of methane over activated carbon acting as a catalyst is proposed as a potential alternative for hydrogen production. However, over a certain duration catalyst becomes deactivated due to intensive carbon deposition.     

Thermocatalytic decomposition of methane using activated carbon: Studying the influence of process parameters using factorial design

The thermocatalytic decomposition of methane over activated carbon (AC) is proposed as a potential alternative for the production of hydrogen. The experiments were divided into two parts; the first part was conducted using thermogravimetric analyzer (TGA) while the second part was conducted in a bench-scale unit. For the first part, the research objective is to study the main and interaction effects of decomposition temperature (800-950 °C) and methane partial pressure (0.03-0.63 atm) on the initial specific rate of carbon formation by using statistical method. The experiments were carried out as a general full factorial design consisting of 20 experiments. Quadratic model was developed for initial specific rate of carbon formation in term of temperature and methane partial pressure using response surface methodology. The model’s results show that not only the effects of the main parameters are important, but also the interaction effects between them are significant. For the second part, the main effects of decomposition temperature (775-850 °C) and AC weight (20-120 g) on the initial rate of methane decomposition by using the analysis of variance (ANOVA) were investigated. The results showed that AC weight has higher mean effects than decomposition temperature on the initial rate of methane decomposition.     

Hydrogen production by thermocatalytic decomposition of methane using a fixed bed activated carbon in a pilot scale unit: Apparent kinetic,deactivation and diffusional limitation studies

Thermocatalytic decomposition (TCD) of methane is a promising method to produce hydrogen. A series of experiments was conducted to study the apparent kinetic, catalyst deactivation and effect of mass diffusion for methane TCD to produce hydrogen using palm-shell carbon based activated carbon (ACPS) as a catalyst in a fixed bed reactor. The experiment was carried out under atmospheric pressure at 775–850 °C, and different methane residence times calculated based on changing the ACPS weight at a constant methane flow rate or changing the methane flow rate at a constant ACPS weight. A reaction order as found substantially differs from that found in literature. A deactivation order of 0.5 and deactivation energy of 177 KJ mol⁻¹ is obtained and the results fitted well with a simple developed model. Mass diffusion transfers are accounted for by calculating change of Weisz modulus with time on-stream using different weights of ACPS or different flow rates of methane. The result showed that Weisz modulus decrease with time and it is attributed to the deposition of carbon produced from methane decomposition. Surface properties measurements of the virgin and deactivated ACPSs indicated that methane decomposition occurs mainly within AC micropores.     

Production of hydrogen by catalytic methane decomposition using biochar and activated char produced from biosolids pyrolysis

Catalytic methane decomposition (CMD) was studied by employing biochar and activated char of biosolids’ origin under different reaction temperatures and methane concentrations. Higher reaction temperatures and lower inlet methane concentrations were found to be favourable for achieving higher methane conversion. A maximum initial methane conversion of 71.0 ± 2.5 and 65.2 ± 2.3% was observed for activated char and biochar, respectively at 900 °C and for 10% CH₄ in N₂ within the first 0.5 h of experiment. Active sites from oxygen containing carboxylic acid functional groups and smaller pore volume and pore diameter were attributed to assist in higher initial methane conversion for biochar and activated char respectively. However, rapid blockages of active sites and surfaces of biochar and activated char due to carbon formation have caused a rapid decline in methane conversion values in the first 0.5 h. Later on, crystalline nature of the newly formed carbon deposits due to their higher catalytic activity have stabilised methane conversion values for an extended experimental period of 6 h for both biochar and activated char. The final conversion values at the end of 6 h experiment with biochar and activated char at 900 °C and for 10% CH₄ in N₂, were found to be 40 ± 1.9 and 35 ± 1.6% respectively. Analysing carbon deposits in detail revealed that carbon nanofiber type structures were observed at 700 °C while nanospheres of carbon were found at 900 °C.     

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.     

Deep regeneration of activated carbon catalyst and autothermal analysis for chemical looping methane thermo-catalytic decomposition process

The application of a chemical looping process to methane thermo-catalytic decomposition using activated carbon (AC) as a catalyst has been recognized as a promising technology for continuous high-purity H₂ production in a carbon constrained world. However, it usually needs an external heat supply for the endothermic decomposition reactions. By taking advantage of the chemical looping combustion (CLC) technology, this study proposed a deep regeneration approach using H₂O and O₂ as regeneration agents to overcome the issues with maintaining catalytic activity and producing the heat needed for the endothermic reactions of H₂ production from methane. TG-DTA and bench scale fluidized bed experimental results indicate that a deep regeneration degree of 30% or above could completely reactivate the spent AC catalyst and simultaneously generate sufficient heat than required in the methane decomposition reaction. Characterization study implies that the deep regenerated AC catalyst could maintain its physical properties within a certain number of cycles. Based on the experimental results, the chemical looping methane thermo-catalytic decomposition process was further optimized and assessed by Aspen Plus^® thermodynamic simulation. The results indicate that heat and mass balances could be attained, and the circulation of the AC catalyst with a temperature difference of 262 °C between the decomposer and the regenerator enabling the process to run autothermally.     

An experimental investigation into the CO2 gasification of deactivated activated-carbon catalyst used for methane decomposition to produce hydrogen

A series of experiments was conducted to study the CO₂ gasification of a deactivated palm-shell-based activated-carbon (ACPS) catalyst used for the thermocatalytic decomposition of methane to produce hydrogen. This catalyst becomes deactivated due to the accumulation of carbon deposits during the methane-decomposition process. The CO₂ gasification was carried out at 850, 900, 950 or 1000 °C to study the deactivated ACPS, which was used at methane-decomposition temperatures of 850 or 950 °C. A series of six methane-decomposition cycles at 950 °C alternating with five gasification cycles using CO₂ at 900, 950 and 1000 °C was also carried out to evaluate the stability of the catalyst. The experiments were conducted using a thermobalance by monitoring the change in mass of the catalyst with time, i.e., the mass gain during methane decomposition or the mass loss during CO₂ gasification. Gasification of the virgin and deactivated ACPS showed strong temperature dependence, with the half and complete gasification times having an exponential dependence on temperature. The gasification reactivity at different conversions was higher for the virgin ACPS and increased with increases in the decomposition temperatures used for deactivation of the ACPS. The activation energies of virgin ACPS and ACPS deactivated at a decomposition temperature of 850 °C decreased with an increase in conversion, while they increased for the ACPS deactivated at a decomposition temperature of 950 °C; the activation energies varied between 81 and 163 kJ/mol. The gasification reactivity changed with methane conversion, showing maximum values for both the virgin and deactivated ACPS at a decomposition temperature of 950 °C. The initial gasification reactivity of the catalyst decreased after three gasification cycles at 1000 °C, while no significant change was observed with gasification cycles at 950 or 900 °C.     

Microwave-assisted catalytic decomposition of methane over activated carbon for -free hydrogen production

The aim of this work was to combine microwave heating with the use of low-cost granular activated carbon as a catalyst for the production of CO₂-free hydrogen by methane decomposition in a fixed bed quartz-tube flow reactor. In order to compare the results achieved, conventional heating was also applied to the catalytic decomposition reaction of methane over the activated carbon. It was found that methane conversions were higher under microwave conditions than with conventional heating when the temperature measured was lower than or equal to . However, when the temperature was increased, the difference between the conversions under microwave and conventional heating was reduced. The influence of volumetric hourly space velocity (VHSV) on the conversion tests using both microwave and conventional heating was also studied. In general, there is a substantial initial conversion, which declines sharply during the first stages of the reaction but tends to stabilise with time. An increase in the VHSV has a negative effect on CH₄ conversion, and even more so in the case of microwave heating. Nevertheless, the conversions obtained in the microwave device at the beginning of the experiments are, in general, better than the conversions reported in other works which also use a carbonaceous-based catalyst. Additionally, the formation of carbon nanofibres in one of the microwave experiments is also reported.     

Decomposition of hydrogen iodide via wood-based activated carbon catalysts for hydrogen production

In this study, the catalytic activity of wood-based catalysts produced by different activation methods was evaluated for the decomposition of hydrogen iodide (HI) as part of the sulfur-iodine hydrogen production process. The wood-based activated carbon catalysts showed strong improvement in the HI conversion compared to a blank, especially for carbon catalysts activated using H₃PO₄. Proximate analysis and ultimate analysis, XRD, BET, SEM, Boehm titration, TPD-MS, XPS were carried out to examine the characteristics of the catalysts. High carbon content (C_ad) seemed to favor high catalytic activity, while high ash content (A_ad) reduced catalytic activity of samples likely due to displacement of catalytically active material. Oxygen-containing groups were not directly responsible for catalytic activity. HI conversion increased as the surface area and pore diameter increased. Unsaturated carbon atoms maybe the main active constituent, therefore, low area density of oxygen [O] that was closely related to unsaturated carbon atoms was beneficial to HI conversion.     

Evaluation of activated carbons based on olive stones as catalysts during hydrogen production by thermocatalytic decomposition of methane

Catalytic decomposition of methane over carbon materials has been intensively studied as an environmental approach for CO₂-free hydrogen production without further by-products except hydrogen and valuable carbon. In this work, we will investigate the catalytic activity of activated carbons based on olive stones prepared by two different processes. Additionally, the effect of three major operational parameters: temperature, weight of catalyst and flow rate of methane, was determined. Therefore, a series of experiments were conducted in a horizontal-flow fixed bed reactor. The outflow gases were analysed using a mass spectrometer. The textural, structural and surface chemistry properties of both fresh and used activated carbons were determined respectively by N₂ gas adsorption, X-Ray Diffraction and Raman and Temperature Programmed Desorption. The results reveal that methane decomposition rate increases with temperature and methane flow however it decreases with catalyst weight. The two carbon samples exhibit a high initial activity followed by a rapid decay. Textural characterization of the deactivated carbon presents a dramatic drop of surface area, pore and micropore volumes against an increase of average pore diameter confirming that methane decomposition occurs mainly in micropores. XRD characterization shows a turbostratic structure of fresh samples with more graphitization in deposed carbon explaining the lowest activity at the end of reaction. Raman spectra reveal the domination of the two bands G and D which varying intensities affirm that the different carbons tend to organise in aromatic rings. Finally the surface chemistry qualitatively changes greatly after methane dissociation for CAGOC unlike CAGOP but quantitatively a small difference is observed which indicates that these functionalities may have a role in this heterogeneous reaction but cannot be totally responsible. Among the two catalysts tested, CAGOC has the highest initial methane decomposition rate but CAGOP is the most stable one.     

Production of hydrogen and carbon nanofibers through the decomposition of methane over activated carbon supported Pd catalysts

Hydrogen has been produced by decomposing methane thermocatalytically at 1123 K in the presence of activated carbon supported Pd catalysts (Samples coded as Pd5 and Pd10 respectively) procured from SRL Chemicals, India. The studies indicated that the Pd10 catalyst has higher catalytic activity and life for methane decomposition reaction at 1123 K and volume hourly space velocity (VHSV) of 1.62 L/hr?g. An average methane conversion of 50 mol % has been obtained for Pd10 catalyst at the above reaction conditions. SEM and TEM-EDXA images of Pd10 catalyst after methane decomposition showed formation of carbon nanofibers. XRD of the above catalyst revealed, moderately crystalline peaks of Pd which may be responsible for the increase in the catalytic life and the formation of carbon nanofibers.     

Effect of N-doping on the catalytic decomposition of hydrogen iodide over activated carbon: Experimental and DFT studies

A series of N-doped activated carbon (NAC) with varying N contents were prepared to study the effect of N-doping on hydrogen iodide (HI) decomposition over activated carbon in sulfur-iodine (SI) cycle for hydrogen production. Element analysis, XPS and N₂ adsorption were performed for samples’ characterization. The HI conversion was proportional to the N content and the sample with 6.006% N content gave the highest HI conversion of 22.84%. Density functional theory (DFT) calculations confirmed the experimental results and elucidated the influence mechanism. The increase of N contents was favor to the decomposition of HI by enhancing the chemisorption of HI molecule on carbon structure. The role of N-doping was to remodel the local electronic density and charge distribution on the carbon surface. In addition, the effect of types of N-containing functional groups on HI decomposition was calculated. Results showed that the N-6 structure was conducive to the chemisorption of HI but N-5 and N-Q structures had a reverse effect. However, when the three N-containing functional groups exist simultaneously, the overall chemisorption of HI was enhanced, which, in turn, enhanced the decomposition of HI. This study can provide underlying guidance for preparing highly efficient NAC for HI decomposition.     

Natural Fe-based catalysts for the production of hydrogen and carbon nanomaterials via methane decomposition

Tierga and Ilmenite Fe-based ores are studied for the first time in the catalytic decomposition of methane (CDM) for the production of carbon dioxide-free hydrogen and carbon nanomaterials. Tierga exhibits superior catalytic performance at 800 °C. The effect of the reaction temperature, space velocity and reducing atmosphere in the catalytic decomposition of methane is evaluated using Tierga. The highest stability and activity (70 vol% hydrogen concentration) is obtained at 850 °C using methane as a reducing agent. Reduction with methane causes the fragmentation of the iron active phase and inhibits the formation of iron carbide, improving its activity and stability in the CDM. Hybrid nanomaterials composed of graphite sheets and carbon nanotubes with a high degree of graphitization are obtained. Considering its catalytic activity, the carbon quality, and the low cost of the material, Tierga has a competitive performance against synthetic iron-catalysts for carbon dioxide-free hydrogen and solid carbon generation.     

Production of COx-free hydrogen by the thermal decomposition of methane over activated carbon: Catalyst deactivation

Hydrogen, an environment-friendly energy source, is deemed to become strongly in demand over the next decades. In this work, CO_x-free hydrogen was produced by the thermal catalytic decomposition (TCD) of methane by a carbon catalyst. Deactivated catalysts at four-stage of progressive were characterized by nitrogen sorption and scanning electron microscopy. TCD of methane at 820 and 940 °C was about 13- and 8-folds higher than non-catalytic decomposition, respectively. High temperatures positively affected the kinetics of hydrogen production but negatively influenced the total amount of hydrogen and carbon products. The total pore volume was a good indicator of the total amount of hydrogen product. Catalyst activity was decreased because of the changes in the catalyst's textural properties within three ranges of relative time, that is, 0 to 45, 0.45 to 0.65, and 0.65 to 1. Models for specific surface area and total pore volume as functions of catalyst deactivation kinetics were developed.     

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.     

Decomposition of methane in the presence of ethanol over activated carbon catalyst

The interest in hydrogen as a potential fuel of the future has stimulated development of new technologies of its production. The main method of hydrogen production is based on the process of steam reforming of methane, but recently increasing attention has been paid to the catalytic decomposition of methane (CDM) whose advantage is its pro-ecological character. This reaction, besides hydrogen, produces also catalytically low-active carbonaceous deposit which settles on the surface of the catalyst and leads to its deactivation. The study reported is an attempt at suppressing the catalyst deactivation by developing a method leading to formation of carbonaceous deposit potentially active in CDM process. For this purpose, it was proposed that the reaction system would contain methane and ethanol. Simultaneous decomposition of these two substances was performed in parallel at three temperatures of 750, 850 or 950 °C. The catalyst was activated carbon obtained from the hazelnut shells. The addition of ethanol was found to have a positive effect on the course of CDM, leading to an increase in the amount of hydrogen produced and to stabilisation of the catalyst activity at a high level.     

Natural sand as a non-conventional catalyst for hydrogen production by methane thermo-catalytic decomposition

The thermo-catalytic decomposition of methane is considered a promising process for H₂ production in the carbon constrained world. A durable and cost-effective catalyst is required for practical methane decomposition processes within industrial applications; unfortunately, most catalysts suffer from extensive deactivation because of carbon deposition. To address this issue, this study assessed a low-cost, widely-available material - natural sand - as a non-conventional catalyst with the realization that it contained impurities such as iron oxides which may impart reaction activity. Its interesting performance in the methane decomposition reaction is reported herein and assessed relative to a potential cause of increasing catalytic activity with longer reaction times. One result of possible significance is the development of tubular carbon structures on the sand's surface that grew significantly in diameter and length with longer reaction times. High Resolution Transmission Electron Microscopy (HRTEM) imaging showed that this tubular carbon contained extensive humps on the external surface of the tube walls which grew in prominence with longer reaction times. The humps did not contain iron particles, in contrast to the heads of the tubes, and consisted of highly disordered graphitic layers. Previous research has pointed to the existence of free radicals or unsaturated bonding in these types of disordered layers, which can provide sites for catalytic reactions. Hence, it is proposed that the increasing prominence of the humps as the reaction time was increased, and by extension an increasing number of surface free radicals, was a possible cause for an increasing catalytic activity after the iron particles on the sand surface were covered with carbon and tube growth was initiated. These data are seen as potentially useful for devising alternative approaches to diminish catalytic deactivation during methane conversion to H₂.     

Methane decomposition over Ni loaded activated carbon for hydrogen production and the formation of filamentous carbon

Methane decomposition over Ni loaded activated carbon (AC) was investigated in a fixed-bed reactor, and the results were compared with those of the individual methane decomposition over AC. XRD results show that there is no NiO observed, and only Ni metal crystallite is found in the catalyst even if it is calcined in Ar, which eliminates the inevitable reduction step with other supports. When Ni is loaded on AC, the Ni/AC catalyst shows higher activity in methane decomposition than the original carbon. Ni crystal size increasing and the new crystallite Ni₃C formation during the process lead to the deactivation of the catalysts. Filamentous carbon formation is observed and interlaced with the deactivated catalyst surface at moderate condition with low amount of Ni loaded. Temperature has great effect on the catalytic performance of Ni/AC catalyst and the formation as well as the characterization of the filamentous carbons.     

Production of hydrogen from methane decomposition using nanosized carbon black as catalyst in a fluidized-bed reactor

Nanosized carbon black (NCB) was employed as catalyst for methane decomposition to produce hydrogen in a fluidized-bed reactor. The carbon atoms of the surface defects of NCB act as active sites in this reaction. The activity of NCB is improved after more defects in the surface of NCB are generated after the treatment in nitric acid and calcination in nitrogen gas. The loading of small amounts of Ni and Co can obviously increase the initial activity of NCB, however, their activity deceases very quickly after the reaction begins due to the encapsulation of the corresponding metal particles inside amorphous carbon produced from methane decomposition. After reaction, the formed carbon was found to grow into carbon flakes and cover the surface of NCB. The investigation with TEM and SEM indicates that they may form from a new carbon crystallite, not build upon the existing hexagon layer in the surface defects of NCB.