Effect of H2S on carbon-catalyzed methane decomposition and CO2 reforming reactions

The effect of H₂S on catalytic processing of methane is of a great practical importance. In this work, the effect of small quantities (0.5–1.0 vol.%) of H₂S present in the feedstock on the methane decomposition and CO₂ reforming reactions over carbon and metal based catalysts was investigated. Activated carbon (FY5), an in-house prepared alumina-supported Ni catalyst (NiA) and the mixture of both (FY5 + NiA) were used as catalysts in this study. It was found that CH₄ and CO₂ conversions were noticeably increased when H₂S was added to the reacting mixture, which points to (i) the tolerance of carbon catalyst to H₂S and (ii) the catalytic effect of H₂S on carbon-catalyzed decomposition and dry reforming of methane. In contrast, NiA catalyst and the mixture FY5 + NiA were deactivated in the presence of H₂S in both reactions. The effect of the heating system (i.e., conventional electric resistance vs microwave heating) on the products yield of the dry reforming reaction in the presence of H₂S is also discussed in this paper.     

Hierarchical porous carbon catalyst for simultaneous preparation of hydrogen and fibrous carbon by catalytic methane decomposition

Direct coal liquefaction residue was used as the precursor for preparing hierarchical micro-/macro-mesoporous carbon by KOH activation with addition of Al₂O₃, and the resultant carbon AlRC was used as the catalyst for catalytic methane decomposition. The results indicate that the carbon AlRC shows excellent methane conversion, up to 61% after 10 h. Besides hydrogen production from methane decomposition, fibrous carbons were formed on the AlRC catalyst, which is different from other carbon catalysts. The investigations of the formation and growth of the fibrous carbon on the carbon catalyst and its catalytic performance indicated that the formed fibrous carbon contribute to the high methane conversion of AlRC catalyst.     

Effect of coexistence of siloxane on production of hydrogen and nanocarbon by methane decomposition using Fe catalyst

Biogas derived from sewage sludge contains CO₂, siloxane, and methane. In this study, the effect of coexistence of siloxane on the production of hydrogen and carbon nanofiber by methane decomposition using iron oxide-alumina catalyst was investigated. The catalyst was reduced by heating in a flow of methane. Siloxane addition to methane caused a catalytic activity at lower temperatures, shortened the induction period prior to the activity, and accelerated catalytic deactivation. Thermal decomposition of siloxane can occur at a lower temperature compared to that of methane. Carbon species formed by the siloxane decomposition may have a higher reducibility than methane does. The reactivity may lead to a carbon deposition at a lower temperature. Coexistence of CO₂ and siloxane can prolong a catalytic lifetime because CO₂ may inhibit the carbon deposition on catalyst to some extent.     

Hydroxyl-promoter on hydrated Ni-(Mg,Si) attapulgite with high metal sintering resistance for biomass derived gas reforming

As hydrated magnesium-aluminum-silicate crystals, attapulgite and HNO₃/NaOH pretreated attapulgite were used as support to prepare nickel-based catalysts via ultrasonic-assisted impregnation method. The as-prepared catalysts were employed in the biomass derived gas (especially CO₂ and CH₄) reforming with a considerable catalytic performance achieved (CH₄ conversion: 75.26%, CO₂ conversion: 85.75%) over HNO₃-attapulgite (10% Ni) at 700 °C and GHSV of 36000 mL/g.h during 600 min, demonstrating the potential of modified attapulgite as support applied in catalytic reforming. According to the characterization results obtained from BET/FT-IR/H₂-TPR/XRD/SEM/TPO, it was found that the formation of (Ni, Mg) containing phyllosilicate improved metal sintering resistance by the confinement effect. Besides, FT-IR results illustrated the existence of hydroxyl in the catalyst structure, which was beneficial for inhibiting the Boudouard side reaction, further enhancing the carbon resistance of catalysts. Moreover, TPO results showed that the deposited carbon on modified attapulgite was mainly fibrous carbon which can be removed easily, thus maintaining the catalytic performance. Due to its unique structure and high metal sintering resistance, it is believed that the attapulgite supported catalyst can be used in any other catalytic reforming process such as steam reforming of methane.     

Preparation of activated carbon supported Fe–Al2O3 catalyst and its application for hydrogen production by catalytic methane decomposition

Activated carbon (AC) supported Fe–Al₂O₃ catalysts were prepared by impregnation method and used for catalytic methane decomposition to hydrogen. The XRD and H₂-TPR results showed that ferric nitrate on AC support was directly reduced to Fe metal by the reducibility of carbon at 870 °C. The loading amount and Fe/Al₂O₃ weight ratio affect the textural properties and catalytic methane decomposition. The surface area and pore volume of the catalyst decrease with the loading of Fe and Al₂O₃. Mesopores with size of about 4.5 nm can be formed at the loading of 20–60% and promote the catalytic activity and stability. The mesopores formation is thought that Fe accelerates burning off of carbon wall and enlarging pore sizes during the pretreatment. When the Fe/Al₂O₃ ratio is 16/24 to 24/16 at the loading of 40%, the resultant catalysts show narrow mesopore distributions and relative high methane conversion. Al₂O₃ as the promoter can improve catalytic activity and shorten transitional period of AC supported Fe catalyst.     

Microwave-enhanced methane cracking for clean hydrogen production in shale rocks

Steam methane reforming (SMR) generates about 95% of hydrogen (H₂) in the U.S. using natural gas as a main feedstock. However, this technology also generates a large amount of carbon dioxide (CO₂), a major greenhouse gas causing global warming. Carbon capture and storage (CCS) technique is required, but the cost and safety of storing CO₂ underground are a concern. Here we propose a new approach using microwave/electromagnetic irradiation to produce clean hydrogen from unrecovered hydrocarbons within petroleum reservoirs. Solid carbon or CO₂ produced during this process will be simultaneously sequestrated underground without involving CCS. In this paper, we perform a series of experiments to investigate the in-situ hydrogen production from shale gas (methane) conversion by passing a methane stream through a packed shale rock sample heated by microwave. We found that methane conversion was significantly enhanced in the presence of Fe and Fe₃O₄ particles as catalysts, with a conversion of 40.5% and 100% at reaction temperature of 500 °C and 600 °C, respectively. Methane conversion is promoted at a lower reaction temperature by the catalytic effect of minerals in shale. Additionally, the influences of catalysts, shale rock, and methane flow rate are characterized.     

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.     

Enhanced hydrogen production by the catalytic alkaline thermal gasification of cellulose with Ni/Fe dual-functional CaO based catalysts

A two-stage system involving alkaline thermal gasification of cellulose with Ca(OH)₂ sorbent and catalytic reforming with Ni/Fe dual-functional CaO based catalysts is proposed and applied to enhance H₂ production and in-situ CO₂ capture. The results show that the H₂ concentration is maximized at a considerably lower temperature (500 °C) than commercialized biomass gasification processes, reducing energy consumption. Sol-gel method is deemed better than impregnation method for its lower cost and higher-concentration H₂ production. Among the prepared catalysts, sol-NiCa catalyst exhibits the best performance in CO₂ absorption, resistance to carbon deposition, and cyclic stability, creating maximum H₂ concentration (79.22 vol%), H₂ yield (27.36 mmol g⁻¹ cellulose), and H₂ conversion (57.61%). Introduction of Ni rather than Fe on the CaO based catalyst promotes steam methane reforming at moderate temperature range of 400–600 °C, generating low contents of CH₄ (5.38 vol%), CO₂ (4.82 vol%), and CO (10.58 vol%).     

Combined dry reforming and partial oxidation of methane to synthesis gas on noble metal catalysts

In this paper CO₂ reforming of methane combined with partial oxidation of methane to syngas over noble metal catalysts (Rh, Ru, Pt, Pd, Ir) supported on alumina-stabilized magnesia has been studied. The catalysts were characterized by using BET, XRD, SEM, TEM, TPR, TPH and H₂S chemisorption techniques. The H₂S chemisorption analysis showed an active metal crystallite size in the range of 1.8-4.24 nm for the prepared catalysts. The obtained results revealed that the Rh and Ru catalysts showed the highest activity in combined reforming and both the dry reforming and partial oxidation of methane. The obtained results also showed a high catalytic stability without any decrease in methane conversion up to 50 h of reaction. In addition, the H₂/CO ratio was around 2 and 0.7 over different catalysts for catalytic partial oxidation and dry reforming, respectively.     

Highly effective microwave catalytic direct decomposition of H2S over carbon encapsulated Mo2C–Co2C/SiC composite

The direct decomposition of H₂S into CO_x-free H₂ and S is an attractive approach in respect of both environmental and energy advantages. However, to achieve the highly effective direct H₂S decomposition reaction is a critical scientific challenge, owning to the serious thermal equilibrium limitation in this reaction. Herein, we develop carbon encapsulated Mo₂C–Co₂C/SiC composite (Mo₂C–Co₂C/SiC@C) via in-situ microwave carbonization, as a novel microwave catalyst for highly effective direct decomposition of H₂S into CO_x-free H₂ and S by microwave catalysis. Significantly, the H₂S conversion is impressively up to 90.3% at 750 °C over Mo₂C–Co₂C/SiC@C microwave catalyst in the microwave catalytic reaction mode, which greatly surpasses the corresponding H₂S equilibrium conversion in the conventional reaction mode. Importantly, Mo₂C–Co₂C/SiC@C exhibits outstanding stability under microwave irradiation. It is found that microwave catalysis can break the chemical equilibrium limitation of the H₂S decomposition reaction, which displays a significant microwave selective catalytic effect. This work provides a novel route for high value utilization of toxic and abundant H₂S resources, and thus opens a new approach for the design of high active and stable microwave catalysts in microwave catalytic reactions.     

Catalytic activity of SBA-15 supported Ni catalyst in CH4 dry reforming: Effect of Al,Zr, and Ti co-impregnation and Al incorporation to SBA-15

In this study, the catalytic activity of the mesoporous SBA-15 supported Ni–Al, Ni–Zr, and Ni–Ti catalysts prepared by an impregnation method were investigated in dry reforming of methane. In addition, Al incorporated SBA-15 (Al–SBA-15) materials used as catalyst support were synthesized following a one-pot hydrothermal route in three different conditions: synthesis in the presence of only HCl, only NaCl, and both HCl and NaCl (denoted as A, S, and B, respectively). All catalysts were characterized by XRD, N₂ adsorption-desorption isotherms, ICP-OES, DRIFTS, SEM, TEM-EDX and TGA techniques before and/or after reaction tests. Among Al, Zr, and Ti impregnated catalysts, Ni–Al impregnated catalyst showed the highest activity in dry reforming of methane. According to activity test results, Al–SBA-15 supported Ni catalyst prepared by the one-pot hydrothermal route in the presence of both HCl and NaCl showed the best catalytic activity with high methane (81%) and carbon dioxide conversion (88%) values at 750 °C. The highest H₂ and CO selectivity values were obtained with the same catalyst with an H₂/CO molar ratio of 0.80. Therefore, these results showed that partial Al (0.11%) incorporated into the structure of SBA-15 was sufficient to improve the catalytic activity of the catalyst in dry reforming of methane.     

Hydrogen production by methane decomposition using bimetallic Ni–Fe catalysts

The catalytic methane decomposition is the leading method for CO_x-free hydrogen and carbon nanomaterial production. In the present study, calcium-silicate based bimetallic Ni–Fe catalysts have been prepared and used to decompose the methane content of the ‘product gas’ obtained in the biomass gasification process for increasing total hydrogen production. Al₂O₃ was used as secondary support on calcium silicate based support material where Ni or Ni–Fe were doped by co-impregnation technique. The activity of catalysts was examined for diluted 6% methane-nitrogen mixture in a tubular reactor at different temperatures between 600 °C and 800 °C under atmospheric pressure, and data were collected using a quadrupole mass spectrometer. Catalysts were characterized by XRD, SEM/EDS, TEM, XPS, ICP-MS, BET, TPR, and TGA techniques. The relation between structural and textural properties of catalysts and their catalytic activity has been investigated. Even though the crystal structure of catalysts had a significant effect on the activity, a direct relation between the BET surface area and the activity was not observed. The methane conversion increased by increasing temperature up to 700 °C. The highest methane conversion has been obtained as 69% at 700 °C with F3 catalyst which has the highest Fe addition, and the addition of Fe improved the stability of catalysts. Moreover, carbon nanotubes with different diameter were formed during methane decomposition reaction, and the addition of Fe increased the formation tendency.     

Investigation of effects of sulfur on dry reforming of biogas over nickel–iron based catalysts

The aim of this study is to maintain and increase the activity of the catalyst in the presence of H₂S with the addition of iron to the Ni catalyst. Alumina-supported monometallic iron and bimetallic nickel-iron catalysts with different weight percentages (8% Fe, 3% Ni – 3% Fe and 8% Ni – 8% Fe) were synthesized using the wet impregnation method in this study. Alumina was prepared by the sol-gel method. The activities of these synthesized catalysts in the methane dry reforming reaction were investigated at different H₂S concentrations (0 ppm, 2 ppm, and 50 ppm) with a total flow rate of 60 mL/min containing an equimolar ratio of CH₄, CO₂, and Ar at 750 °C and atmospheric pressure. To investigate the effect of sulfur compounds on the catalytic activity, the catalysts were also exposed to different gas compositions such as the mixture of H₂S + He, H₂S + CO₂ + He, and H₂S + CO₂ + CH₄ + He. In this case, FT-IR with a gas cell was used to determine the components in the gas stream at the reactor outlet. To explain catalytic performance, characterization studies were carried out using XRD, N₂ adsorption/desorption, DRIFT, SEM, TGA, and XPS analysis. All-synthesized materials showed Type-IV isotherm with a hysteresis loop corresponding to an ordered mesoporous structure. The DRIFT analysis showed a decrease in the Lewis acid sites after the addition of iron into the Ni-catalysts. In the activity test carried out in the presence of 50 ppm H₂S, it was observed that the iron-containing 8Ni–8Fe@SGA catalyst increased the sulfur resistance slightly, compared to the monometallic 8Ni@SGA catalyst. TGA analysis showed that Fe addition reduced coke deposition, as the Ni–Fe catalyst had a lower nickel crystal size than the Ni-based catalyst. FTIR analysis with a gas cell showed that sulfur in H₂S transformed to other sulfur compounds such as COS and/or SO₂ during dry reforming of biogas over alumina-supported Ni–Fe catalysts.     

Steam reforming of acetic acid in the presence of Ni coated with SiO2 microsphere catalysts

Steam reforming of acetic acid was investigated in the presence of Ni@SiO₂ microsphere catalysts. The effects of Ni loading, H₂O/AcOH ratio, and temperature on hydrogen selectivity and acetic acid conversion were determined via statistical analysis. Results indicated the dependence of parameters on hydrogen selectivity.The stable activity observed for the reaction conducted at 2.5H₂O/AcOH ratio and 750 °C implied future utilization potential of the catalyst for time on stream experiments despite the inevitable coke formation. Boudard reaction and methane decomposition, known as possible carbon sources, was ruled out due to the opposite trends between CO₂ and CO selectivities and mitigation of methanation reactions. The ambiguous pattern of conversions observed for varying H₂O/AcOH implied the presence of a different reaction path leading to consecutive ketonozation of acetic acid and aldol-condensation of acetone as the primary sources of carbon deposition.     

Joint-use of activated carbon and carbon black to enhance catalytic stability during chemical looping methane decomposition process

Chemical-looping methane decomposition using activated carbon as a catalyst has been considered a potentially promising approach for high-purity H₂ production with low cost and low CO₂ emission. However, activated carbon is known to deactivate fast despite its high initial catalytic activity. Oppositely, carbon black has shown stable catalytic methane decomposition that even increases slowly with time of reaction. Considering these two different activity trends, activated carbon and carbon black are jointly used to prepare catalysts and then test for the decomposition of the methane via chemical looping in this study that aimed to examine catalyst and reaction properties which may combine the high initial activity of activated carbon with the steady-and-increasing activity of carbon black. These mixed catalysts are examined using X-ray diffraction analysis (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), Brunauer-Emmett-Teller (BET) and high-resolution transmission electron microscopy (HRTEM) before and after reaction testing to reveal chemical and physical constituents which contributed to their reactivities, and the mechanism of long catalytic activity has been discussed. The results point to insights and potential directions for modifying carbonaceous catalysts for chemical looping thermo-catalytic decomposition of methane.     

Preferential oxidation of CO in a H2-rich stream over multi-walled carbon nanotubes confined Ru catalysts

Multi-walled carbon nanotubes (MWNTs) confined Ru catalysts were prepared by a modified procedure using ultrasonication-aided capillarity action to deposit Ru nanoparticles onto MWNTs inner surface. The structure properties of MWNTs supports and Ru catalysts were extensively characterized by XRD, TGA, H₂-TPR, XPS, TEM, FTIR and Raman spectra. The catalytic performance in the preferential oxidation of CO in a H₂-rich stream was examined in detail with respect to the influences of Ru loading, MWNTs diameter, various pretreatment conditions, and the presence of CO₂ and H₂O in the feed stream. In contrast with Ru catalysts supported on MWNTs external surface and other carbon materials, the superior activity was observed for the MWNTs-confined Ru catalyst, which was discussed intensively in terms of the confinement effect of carbon nanotubes. The optimized catalyst of 5 wt.% Ru confined in MWNTs with diameter of 8–15 nm can achieve the complete CO conversion in the wider temperature range and the favorable stability at 80 °C under the simulated reformatted gas mixture, which proves a promising catalyst for preferential CO oxidation in H₂-rich stream.     

Impact of impurities on biogas valorization through dry reforming of methane reaction

Biogas is mainly composed of methane, carbon dioxide as well as other compounds such as water and volatile organic compounds (VOCs). In this study, the composition of a biogas produced in a landfill is determined using gas chromatography. CH₄ and CO₂ represent 60% of its composition, rendering a valorization via the dry reforming of methane (DRM) reaction very promising. Moreover, H₂O and VOCs represent respectively 1.5% and 1500 ppm of biogas, which may affect the catalytic efficiency. The performance of CoNiMgAl catalysts derived from hydrotalcites is studied in the presence of toluene, water and a combination of both. The presence of toluene causes a progressive increase in the catalytic activity as well as higher carbon deposition. The addition of water decreases CO₂ conversion and carbon formation and increases the H₂/CO to values closer to 1. When both molecules are added, the catalytic activity remains stable, confirming that DRM is possible in the presence of both compounds.     

Effects of structure on the carbon dioxide methanation performance of Co-based catalysts

Mesoporous Co/KIT-6 and Co/meso-SiO₂ catalysts were prepared via hydrogen reduction and were subsequently used in CO₂ catalytic hydrogenation to produce methane. The properties of these catalysts were investigated via low-angle X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, and transmission electron microscopy (TEM). The results indicate that the synthesized Co/KIT-6 and Co/meso-SiO₂ catalysts have mesoporous structures with well-dispersed Co species, as well as high CO₂ catalytic hydrogenation activities. The Co/KIT-6 catalyst has a large specific surface area (368.9 m² g⁻¹) and a highly ordered bicontinuous mesoporous structure. This catalyst exhibits excellent CO₂ catalytic hydrogenation activity and methane product selectivity, which are both higher than those of the Co/meso-SiO₂ catalyst at high reaction temperatures. The CO₂ conversion and methane selectivity of the Co/KIT-6 catalyst at 280 °C are 48.9% and 100%, respectively. The high dispersion of the Co species and the large specific surface area of the prepared Co-based catalysts contribute to the high catalytic activities. In addition, the highly ordered, bicontinuous, mesoporous structure of the Co/KIT-6 catalyst improves the selectivity for the methane product.     

On the effect of yttrium promotion on Ni-layered double hydroxides-derived catalysts for hydrogenation of CO2 to methane

Ni-containing mixed oxides derived from layered double hydroxides with various amounts of yttrium were synthesized by a co-precipitation method at constant pH and then obtained by thermal decomposition. The characterization techniques of XRD, elemental analysis, low-temperature N₂ sorption, H₂-TPR, CO₂-TPD, TGA and TPO were used on the studied catalysts. The catalytic activity of the catalysts was evaluated in the CO₂ methanation reaction performed at atmospheric pressure. The obtained results confirmed the formation of nano-sized mixed oxides after the thermal decomposition of hydrotalcites. The introduction of yttrium to Ni/Mg/Al layered double hydroxides led to a stronger interaction between nickel species and the matrix support and decreased nickel particle size as compared to the yttrium-free catalyst. The modification with Y (0.4 and 2 wt%) had a positive effect on the catalytic performance in the moderate temperature region (250–300 °C), with CO₂ conversion increasing from 16% for MO-0Y to 81% and 40% for MO-0.4Y and MO-2.0Y at 250 °C, respectively. The improved activity may be correlated with the increase of percentage of medium-strength basic sites, the stronger metal-support interaction, as well as decreased crystallite size of metallic nickel. High selectivity towards methane of 99% formation at 250 °C was registered for all the catalysts.     

Hydrogen production from biogas reforming and the effect of H2S on CH4 conversion

Biogas produced during anaerobic decomposition of plant and animal wastes consists of high concentrations of methane (CH₄), carbon dioxide (CO₂) and traces of hydrogen sulfide (H₂S). The primary focus of this research was on investigating the effect of a major impurity (i.e., H₂S) on a commercial methane reforming catalyst during hydrogen production. The effect of temperature on CH₄ and CO₂ conversions was studied at three temperatures (650, 750 and 850 °C) during catalytic biogas reforming. The experimental CH₄ and CO₂ conversions thus obtained were found to follow a trend similar to the simulated conversions predicted using ASPEN plus. The gas compositions at thermodynamic equilibrium were estimated as a function of temperature to understand the intermediate reactions taking place during biogas dry reforming. The exit gas concentrations as a function of temperature during catalytic reforming also followed a trend similar to that predicted by the model. Finally, catalytic reforming experiments were carried out using three different H₂S concentrations (0.5, 1.0 and 1.5 mol%). The study found that even with the introduction of small amount of H₂S (0.5 mol%), the CH₄ and CO₂ conversions dropped to about 20% each as compared to 65% and 85%, respectively in the absence of H₂S.