Search

Abstract In this study, nickel supported on various metal oxides were prepared by simple impregnation and the performance for CO2 methanation was tested. The oxide supports were all prepared by thermal decomposition of metal salts to provide comparable oxide properties such as surface area. Among the investigated oxides, nickel supported on CeO2 and Y2O3 showed the highest CO2 conversion of 90% at 320 °C with highest CH4 selectivity of 99%. The order of catalyst activity (XCO2@320°C) was reported: Ni/CeO2 ~ Ni/Y2O3 > > Ni/La2O3 > Ni/ZrO2 > Ni/Al2O3 > Ni/MgO > Ni/CaO > > Ni/MnO. The physicochemical properties of the catalysts were analyzed by TEM, BET, XRD, ICP, H2-TPR, CO2-TPD, H2 chemisorption, TGA, Raman, and XPS. From the characterization results, the catalyst activity was independent to specific surface area of catalyst and crystallite size of Ni. The amount of oxygen vacancies and weak-to-medium basic sites exhibited major roles for enhancing catalyst activity. The CeO2 and Y2O3 as reducible oxide supports not only provided abundant oxygen vacancies / basic sites, but also promoted Ni dispersion with appropriate interaction between metal and support, resulting in higher reducibility at low temperature. The reduction of catalyst at high temperature can significantly improve the performance of Ni supported on non-reducible MgO. However, the Ni/CeO2 and Ni/Y2O3 reduced at high temperature suffered from coalescence of CeO2 and Y2O3, though Ni crystallite sizes are well preserved from sintering.

Abstract In this study, the improvement of heat transfer was applied to eliminate hotspots of a highly exothermic reaction, Fischer–Tropsch synthesis (FTS), by means of two facile methods: (I) adding high thermal conductive materials media diluted in catalysts (SiC and Al chips), and (II) using structured reactors equipped with well-designed structured catalysts with advantages of heat dissipation/removal. The 20%Co/SiO2 catalyst powder prepared by simple impregnation was employed for constructing structured catalysts and granular packed bed catalysts. The structured catalyst was prepared by coating method of Co/SiO2 slurry on an aluminum spiral and plate substrate. The catalytic performance of as-prepared catalysts was then tested for FTS in a fixed-bed reactor at 210–230 °C, 20 bar. Both gaseous and liquid products were collected and analyzed. The heat transfer improvement of packed bed catalytic system and structured catalytic system were compared and discussed. As a result, the structured catalytic system with spiral structured catalyst can provide the best improvement of heat/mass transfer, resulting in enhanced diesel selectivity, though the oil production rate was unsatisfactory. Meanwhile, among the packed bed catalytic systems, SiC media possessed the best heat removal material, producing the highest oil yield. In addition, the fresh and spent catalysts were analyzed by several techniques including TEM, SEM, XRD, BET, ICP-OES, H2–TPR, and TGA to relate the physicochemical properties of the prepared catalysts and its FTS performance.

Abstract Direct conversion of biogas via the integrative process of dry reforming of methane (DRM) and catalytic methane decomposition (CDM) has received a great attention as a promising green catalytic process for simultaneous production of syngas and carbon nanotubes (CNTs). In this work, the effects of reaction temperature of 700–1100 °C and CH4/CO2 ratio of biogas were investigated over NiMo/MgO catalyst in a fixed bed reactor under industrial feed condition of pure biogas. The reaction at 700 °C showed a rapid catalyst deactivation within 3 h due to the formation of amorphous carbon on catalyst surface. At higher temperature of 800–900 °C, the catalyst can perform the excellent performance for producing syngas and carbon nanotubes. Interestingly, the smallest diameter and the highest graphitization of CNTs was obtained at high temperature of 1000 °C, while elevating temperature to 1100 °C leads to agglomeration of Ni particles, resulting in a larger size of CNTs. The reaction temperature exhibits optimum at 800 °C, providing the highest CNTs yield with high graphitization, high syngas purity up to 90.04% with H2/CO ratio of 1.1, and high biogas conversion (XCH4 = 86.44%, XCO2 = 95.62%) with stable performance over 3 h. The typical composition biogas (CH4/CO2 = 1.5) is favorable for the integration process, while the CO2 rich biogas caused a larger grain size of catalyst and a formation of molybdenum oxide nanorods (MoO3). The long-term stability of NiMo/MgO catalyst at 800 °C showed a stable trend (> 20 h). The experimental findings confirm that NiMo/MgO can perform the excellent activity and high stability at the optimum condition, allowing the process to be more promising for practical applications.

Abstract Biogas has been widely regarded as a promising source of renewable energy. Recently, the direct conversion of biogas over heterogeneous catalysts for the simultaneous production of syngas and carbon nanotubes exhibits a high potential for full utilization of biogas with great benefits. Involving the combined dry reforming of methane and catalytic decomposition of methane, the efficiency of process is strongly depended on the catalyst activity/stability, mainly caused by carbon deposition. In this study, Ni–Mo catalyst is engineered to provide a life-long performance and perform high activity in the combined process. The surface modification of catalysts by a controlled carburization pretreatment is proposed for the first time to produce a carbide catalyst along with improving the catalyst stability as well as the reactivity for direct conversion of biogas. The performance of as-prepared carbide catalysts is investigated with comparison to the oxide and metallic ones. As a result, the Ni–Mo2C catalyst exhibited superior activity and stability over its counterparts, even though the condensed nanocarbon was largely grown and covered on the surface. In addition, up to 82% of CH4 conversion and 93% of CO2 conversion could remain almost constant at 800 °C throughout the entire test period of 3 h under a high flowrate inlet stream of pure biogas at 48,000 cm3 g−1 h−1. The XPS spectra of catalysts confirmed that the presence of Mo2C species on the catalyst surface could promote the stability and reactivity of the catalyst, resulting in higher productivity of carbon nanotubes over a longer time.

Abstract The upgradation of methane in biogas by hydrogenation of CO2 has been currently recognized as a promising route for efficient full utilization of renewable biogas with potential benefits for storage of renewable hydrogen energy and abatement of greenhouse gas emission. As a main constituent of biogas, CO2 can act as a backbone for the formation of additional CH4 by hydrogenation, then producing higher amounts of biomethane. In this work, the upgradation process was investigated in a prototype reactor of double pass operation with vertical alignment using an optimized Ni-Ce/Al-MCM-41 catalyst. The experimental results show that the double pass operation that removes water vapor during the run can significantly increase CO2 conversion, resulting in higher CH4 production yield. As a result, the purity of biomethane increased by 15% higher than a single pass operation. In addition, search for optimum condition of the process was carried out within an investigated range of conditions including flowrate (77–1108 ml min−1), pressure (1 atm–20 bar), and temperature (200–500 °C). The durability test for 458 h was performed using the obtained optimum condition, and it shows that the optimized catalyst can perform excellent stability with negligible influence by the observed change in catalyst properties. The comprehensive characterization on physicochemical properties of fresh and spent catalysts was performed, and the results were discussed.

Auto-methanation (AM), which autonomously progresses at room temperature (approximately 25 °C), is an effective reaction for converting CO2 to CH4 and can thus reduce the processing costs. In this study, the AM reaction over various Ru-based catalysts with different support materials positively proceeded by selecting reaction conditions such as the O2 concentration and gas flow rate. The progression of AM was the best over the Ru/CeO2 catalyst, followed by the Ru/ZrO2 and Ru/Al2O3 catalysts. The progression of AM over the Ru/SiO2 catalyst was slow. The rapid progress of AM over the Ru/CeO2 catalyst was attributed to the presence of oxygen defects on the CeO2 support that accelerates the hydrogen–oxygen combustion as well as to the differences in the CO2 and H2O adsorption characteristics between the CeO2 support and other supports. Even when the Ru-based catalyst underwent H2 reduction at low temperature, AM could proceed over each catalyst. The Ru/CeO2 catalyst was advantageous over other catalysts because AM proceeds only when H2 gas is supplied at room temperature without external heating. X-ray photoelectron spectroscopy indicated that the Ru species on the Ru/CeO2 catalyst are in a metallic state owing to H2 reduction at room temperature, which is associated with AM progression. Moreover, AM proceeded via H2 reduction at 75 °C over the Ru/ZrO2 catalyst and at 150 °C over the Ru/Al2O3 catalyst. The progression of methanation through such low-temperature reduction can strengthen the CO2 conversion process.

This study aims to clarify the effect of twist angle in the spiral-type catalyst on CO2 auto-methanation (AM) performance. The spiral-type catalysts with different twist angles of 360°, 540°, 720°, and 1080° were constructed and investigated for their methanation properties. By increasing the twist angle, high methanation performance was obtained at the lower temperature range. Furthermore, as a result of the calculation of heat transfer coefficient (U) and Nusselt number (Nu) on the spiral-type catalysts based on their catalytic properties, the U and Nu were enhanced with an increment of twist angles, which would produce a high methanation performance of the spiral-type catalyst. A double-tube reactor with a spiral-type catalyst was constructed to improve catalytic performance in order to avoid loss of generated heat to the outside of the reactor. This system exhibited an excellent AM performance with CO2 conversion of about 66% and methane selectivity of 100% at a severe feed rate of 1600 mL/min due to efficient utilization of heat energy generated from H2 combustion and CO2 methanation.

Abstract Selecting a suitable catalyst for implementing the simultaneous production of hydrogen-rich syngas and multi-walled carbon nanotubes through the integration of dry reforming and methane decomposition reactions has recently gained great interests. In this study, a series of bimetallic (NiMo/MgO) and trimetallic (CoNiMo/MgO, FeNiMo/MgO, CoFeMo/MgO) catalysts was prepared and evaluated for a catalytic activity of CH4 and CO2 conversions of biogas in a fixed bed reactor at 800 °C and atmospheric pressure. Among the investigated catalysts, the bimetallic NiMo/MgO catalyst showed the outstanding catalytic performance with 86.4% CH4 conversion and 95.6% CO2 conversion as well as producing the highest syngas purity of 90.0% with H2/CO ratio = 1.1. Moreover, the characterization of the synthesized solid products proved that the well-aligned structured morphology, high purity, and excellent textural properties of CNTs were obtained by using NiMo/MgO catalyst. On the other hand, using trimetallic catalysts which have the composition of Co and Fe leads to the severe deactivation. This could be attributed the catalyst oxidation with CO2 in biogas, resulting in the transformation of metals into large metal oxides. The integrative process with NiMo/MgO catalyst is regarded as a promising pathway, which has a high potential for directly converting biogas into the high value-added products and providing a green approach for managing the enormous amounts of wastes.

Technologies for the recovery of carbon from greenhouse gases such as carbon dioxide and methane in the solid state will lead to the development of a low-carbon society. For the purpose of realizing such technology, we constructed a catalytic reaction system that continuously captures solid carbon while producing synthesis gas by dry reforming of CH4 (DRM). The constructed reaction system exhibited high DRM activity without catalyst deterioration, with a maximum rate of 20.3% for capturing solid carbon. The use of an iron-based wall-type catalyst and the control of the capturing temperature were effective in improving the carbon capturing rate. We have succeeded in developing a novel catalytic reaction system that can collect solid carbon at a much lower temperature than that by methane decomposition. It was considered that carbon deposition from synthesis gas proceeds via a reaction mechanism which includes formation of cementite (Fe3C). The quality of the collected carbon varied depending on the capturing conditions. In particular, it was confirmed that carbon subjected to higher temperatures contained carbon-nanotubes (CNTs) with high crystallinity and elongation.

OSDA/solvent-free synthesis of zeolites is suitable for practical commercialization due to low environmental damage and low cost. However, only a few types of zeolites have been successfully synthesized by this method. In this study, ANA, CAN, GIS, SOD, FAU and LTA type zeolites were synthesized by the OSDA/solvent-free synthesis method by changing the Si/Al ratio and crystallization temperature in detail. At higher Si/Al ratios, FAU, GIS and ANA type zeolites were obtained with an increase of crystallization temperature. Meanwhile, at lower Si/Al ratios, LTA, SOD and CAN type zeolites were obtained with an increase of crystallization temperature. It was found that zeolites with high T atom density were synthesized with an increase of the crystallization temperature; zeolite with a high T atomic density is more stable. This work reveals the correlation between the Si / Al ratio and the crystallization temperature in OSDA/solvent-free synthesis of zeolites, which provides a clear guideline for the OSDA/solvent-free synthesis of zeolites. [ABSTRACT FROM AUTHOR]

The methanation performance of a Ni/CeO2 structured catalyst having a spiral shape was investigated using a feed material gas containing 111 vol% oxygen. The spiral catalyst exhibited high methane yield for a high oxygen concentration (911 vol%) and fast feed flow rate even under an automethanation condition, which does not require external heating. Such high methane yield was continuously maintained for 60 hours or longer. Under the auto-methanation over the structured catalyst, the reaction field generated high thermal energy, which did not exceed the Tammann temperature of the nickel component, due to selective hydrogen combustion. [ABSTRACT FROM AUTHOR]

CO2 methanation over a Ni/CeO2 catalyst was investigated under the feeding of a raw material gas containing oxygen of 110 vol% concentration. The presence of oxygen greatly improved the methanation activity and provided high methane yield, even with cutting external heating, near room temperature. This positive phenomenon was caused by the fact that hydrogencombustion by oxygen selectively occurred on the Ni/CeO2 catalyst, and a large amount of thermal energy was generated. We found a novel route of CO2 methanation that can be driven without external heating, even near room temperature, which we named auto-methanation. [ABSTRACT FROM AUTHOR]

The aim of this study is to develop a noble-structured catalyst for methane decomposition to produce pure hydrogen. The Ni/Al2O3 structured catalyst, which was prepared by electroless plating with NaBH4 as the reducing agent, showed a higher activity and stability, indicating the advantage for ensuring the flow path. After a steam treatment to remove the deposited carbon, the decomposition performance of the structured catalyst was regenerated. [ABSTRACT FROM AUTHOR]

The objective of this study was to develop an iron oxide structured catalyst by a template-free hydrothermal synthesis method. The formed iron oxide had a unique morphology, i.e. of a microdisk-like shape. The microdisk was randomly deposited on an aluminum plate, which produced a relatively high surface area. The prepared iron oxide structured catalyst displayed a good activity by loading a small amount of palladium and potassium under severe reaction conditions. [ABSTRACT FROM AUTHOR]

The objective of this study was to develop a yttria-stabilized zirconia catalyst-supported nickel (Ni/YSZ) with high tolerance to coke deposition during methane steam reforming (MSR). The Ni/YSZ catalyst prepared by an electroless plating method (Ni(el)/YSZ) exhibited better stability during the MSR than did the catalyst prepared by a conventional impregnation method (Ni(im)/YSZ). Additionally, the amount of coke deposition on the Ni(el)/YSZ catalyst was lower than that on the Ni(im)/ YSZ catalyst. By thermal treating the Ni(el)/YSZ catalyst in a nitrogen atmosphere before the reforming reaction, a further decrease in the deposited coke amount was observed. In particular, the deposited coke was not detected at all over the Ni(el)/YSZ catalyst pretreated at 1073 K for 2h. [ABSTRACT FROM AUTHOR]

A structured Ni/Al2O3 catalyst was prepared by combination of a solgel method for forming an oxide layer and electroless plating for depositing a metallic component on a stainless honeycomb fin. When using a sodium borohydride bath for the plating, the prepared catalyst indicated a high activity for methane-steam reforming, which was almost equal to the equilibrium, under severe conditions. The developed combination method is effective for preparing a structured catalyst and could be used to prepare other kinds of structured catalysts. [ABSTRACT FROM AUTHOR]

In the low-temperature watergas shift reaction, the effect of the catalyst pretreatment conditions on the adsorption state of CO was investigated on a structured copper-based catalyst, which was prepared by electroless plating. The amount of CO adsorbed on the catalyst was higher due to the oxidation pretreatment than that by the reduction treatment. The CO adsorbed on the oxidized catalyst formed many carbonate and carboxylate species, which have a slightly weak adsorption. Such an adsorption state would enhance shift performance. [ABSTRACT FROM AUTHOR]