Fangna Gu

A thermodynamic analysis of methanation reactions of carbon oxides for the production of synthetic natural gas

Synthetic natural gas (SNG) can be obtained via methanation of synthesis gas (syngas). Many thermodynamic reaction details involved in this process are not yet fully understood. In this paper, a comprehensive thermodynamic analysis of reactions occurring in the methanation of carbon oxides (CO and CO2) is conducted using the Gibbs free energy minimization method. The equilibrium constants of eight reactions involved in the methanation reactions were calculated at different temperatures. The effects of temperature, pressure, ratio of H-2/CO (and H-2/CO2), and the addition of other compounds (H2O, O-2, CH4, and C2H4) in the feed gas (syngas) on the conversion of CO and CO2, CH4 selectivity and yield, as well as carbon deposition, were carefully investigated. In addition, experimental data obtained on commercial Ni-based catalysts for CO methanation and three cases adopted from the literature were compared with the thermodynamic calculations. It is found that low temperature, high pressure, and a large H-2/CO (and H-2/CO2) ratio are favourable for the methanation reactions. Adding steam into the feed gas could alleviate the carbon deposition to a large extent. Trace amounts of O-2 in syngas is unfavourable for SNG generation although it can lower carbon deposition. Additional CH4 in the feed gas almost has no influence on the CO conversion and CH4 yield, but it leads to the increase of carbon formed. Introduction of a small amount of C2H4, a representative of hydrocarbons in syngas, results in low CH4 yield and serious carbon deposition although it does not affect CO conversion. CO is relatively easy to hydrogenated compared to CO2 at the same reaction conditions. The comparison of thermodynamic calculations with experimental results demonstrated that the Gibbs free energy minimization method is significantly effective for understanding the reactions occurring in methanation and helpful for the development of catalysts and processes for the production of SNG.

Recent advances in methanation catalysts for the production of synthetic natural gas

Methanation of coal-or biomass-derived carbon oxides for production of synthetic natural gas (SNG) is gaining considerable interest due to energy issues and the opportunity of reducing greenhouse gases by carbon dioxide conversion. The key component of the methanation process is the catalyst design. Ideally, the catalyst should show high activity at low temperatures (200-300 degrees C) and high stability at high temperatures (600-700 degrees C). In the past decades, various methanation catalysts have been investigated, among which transition metals including Ni, Fe, Co, Ru, Mo, etc. dispersed on metal oxide supports such as Al2O3, SiO2, TiO2, ZrO2, CeO2 etc. have received great attention due to their relatively high catalytic activity and selectivity. Furthermore, over the past few years, great efforts have been made both in methanation catalysts development and reaction mechanism investigation. Here we provide a comprehensive review to these most advancements, covering the reaction thermodynamics, mechanism and kinetics, the effects of catalyst active components, supports, promoters and preparation methods, hoping to outline the pathways for the future methanation catalysts design and development for SNG production.

Highly active and stable Ni/γ-Al2O3 catalysts selectively deposited with CeO2 for CO methanation

We report the preparation of highly active, coking-and sintering-resistant CeO2-decorated Ni/gamma-Al2O3 catalysts by an impregnation method followed by a modified deposition-precipitation (DP) of CeO2. The samples were characterized by nitrogen adsorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, H-2 temperature-programmed reduction, H2 temperature-programmed desorption and zeta potential analysis. The results revealed that compared with the Ni catalysts with a NiO loading of 40 wt% prepared by co-impregnation (CI) and sequential impregnation (SI) methods, the Ni catalyst synthesized by DP method showed enhanced catalytic performance for CO methanation at atmospheric pressure and an extremely high weight hourly space velocity (WHSV) of 240 000 mL g(-)1 h(-1). In a 50 h high-pressure life-test, this catalyst showed a high resistance to both coking and sintering. It was found that CeO2 nanoparticles were selectively deposited on the surface of NiO rather than on Al2O3 due to the electrostatic interaction during the DP process, effectively preventing Ni particles from sintering during the reduction and reaction at high temperatures, and inhibiting coke formation by increasing the supply of active oxygen species on the nickel surface. As a result, the formed CeO2-decorated Ni/Al2O3 catalyst exhibited excellent catalytic performance and stability in CO methanation. This work demonstrated that catalytic properties could be much improved for a usual catalyst with a well-designed structure.

Ni/Al2O3 catalysts for CO methanation: Effect of Al2O3 supports calcined at different temperatures

The correlation between phase structures and surface acidity of Al2O3 supports calcined at different temperatures and the catalytic performance of Ni/Al2O3 catalysts in the production of synthetic natural gas (SNG) via CO methanation was systematically investigated. A series of 10 wt% NiO/Al2O3 catalysts were prepared by the conventional impregnation method, and the phase structures and surface acidity of Al2O3 supports were adjusted by calcining the commercial gamma-Al2O3 at different temperatures (600-1200 degrees C). CO methanation reaction was carried out in the temperature range of 300-600 degrees C at different weight hourly space velocities (WHSV = 30000 and 120000 mL.g(-1).h(-1)) and pressures (0.1 and 3.0 MPa). It was found that high calcination temperature not only led to the growth in Ni particle size, but also weakened the interaction between Ni nanoparticles and Al2O3 supports due to the rapid decrease of the specific surface area and acidity of Al2O3 supports. Interestingly, Ni catalysts supported on Al2O3 calcined at 1200 degrees C (Ni/Al2O3-1200) exhibited the best catalytic activity for CO methanation under different reaction conditions. Lifetime reaction tests also indicated that Ni/Al2O3-1200 was the most active and stable catalyst compared with the other three catalysts, whose supports were calcined at lower temperatures (600, 800 and 1000 degrees C). These findings would therefore be helpful to develop Ni/Al2O3 methanation catalyst for SNG production.

Nickel catalysts supported on calcium titanate for enhanced CO methanation

Nickel catalysts supported on the perovskite oxide CaTiO3 (CTO) were prepared by an impregnation method for CO methanation to produce synthetic natural gas (SNG). X-Ray diffraction, nitrogen adsorption, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, H2-temperature programmed reduction and desorption, and X-Ray photoelectron spectroscopy were employed for the characterization of samples. The results revealed that the Ni/CTO catalysts showed a better performance than Ni/Al2O3 for CO methanation at various reaction conditions. The life time test at 600 °C and 3.0 MPa indicates that Ni/CTO is also more active, thermally stable and resistant to carbon deposition. This is because of the relatively weak Ni–CTO support interaction, highly stable CTO support, the absence of acidic sites on the surface of CTO and the proper Ni particle size of about 20–30 nm. The work is important for the development of effective methanation catalysts for SNG production.

Effect of nickel nanoparticle size in Ni/α-Al2O3 on CO methanation reaction for the production of synthetic natural gas

A series of α-Al2O3-supported Ni catalysts with different Ni particle sizes (5–10, 10–20, and 20–35 nm) were prepared and applied in the CO methanation reaction for the production of synthetic natural gas (SNG). The catalytic tests showed that the Ni nanoparticles influenced the catalytic performance in the CO methanation, and the catalyst with a Ni nanoparticle size of 10–20 nm showed the highest CO conversion, CH4 yield, and turnover frequency, and the lowest carbon deposition, demonstrating the possibility of improving the Ni/α-Al2O3 catalysts in the CO methanation for SNG production by controlling their Ni particle size.

Coking-resistant Ni-ZrO2/Al2O3 catalyst for CO methanation

Highly coke-resisting ZrO2-decorated Ni/Al2O3 catalysts for CO methanation were prepared by a two-step process. The support was first loaded with NiO by impregnating method and then modified with ZrO2 by deposition-precipitation method (IM-DP). Nitrogen adsorption-desorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, H-2 temperature-programmed reduction and desorption, NH3 temperature-programmed desorption, and zeta potential analysis were employed to characterize the samples. The results revealed that, compared with the catalysts with the same composition prepared by co-impregnation (CI) and sequential impregnation (SI) methods, the Ni/Al2O3 catalyst prepared by IM-DP showed much enhanced catalytic performance for syngas methanation under the condition of atmospheric pressure and a high weight hourly space velocity of 120000 mL.g(-1).h(-1). In a 80 h life time test under the condition of 300-600 degrees C and 3.0 MPa, this catalyst showed high stability and resistance to coking, and the amount of deposited carbon was only 0.4 wt%. On the contrary, the deposited carbon over the catalyst without ZrO2 reached 1.5 wt% after a 60 h life time test. The improved catalytic performance was attributed to the selective deposition of ZrO2 nanoparticles on the surface of NiO rather than Al2O3, which could be well controlled via changing the electrostatic interaction in the DP procedure. This unique structure could enhance the dissociation of CO2 and generate surface oxygen intermediates, thus preventing carbon deposition on the Ni particles in syngas methanation.

MnOx–CeO2 supported on a three-dimensional and networked SBA-15 monolith for NOx-assisted soot combustion

This paper reports the preparation and characterization of MnOx–CeO2/SBA-15 monolith (MnCe/SM) catalysts for NOx-assisted soot combustion. The SM with a three-dimensional (3D) network structure was synthesized by a sol–gel method, in which the shearing force and the acidity of the solution were finely tuned to direct the formation and assembly of the primary particles. The MnCe/SM catalysts were further prepared by a facile isovolumetric impregnation method. The samples were characterized by nitrogen adsorption, scanning electron microscopy, energy dispersed spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, O2 temperature-programmed desorption, NO temperature-programmed desorption, and NO temperature-programmed oxidation. It is found that the MnOx–CeO2 nanoparticles were dispersed in the channels or/and on the outer surface of the SM support depending on the loading, and had a strong synergistic effect. The MnCe/SM catalysts with an appropriate MnOx–CeO2 loading showed much higher catalytic performance for soot combustion than that of the unsupported MnOx–CeO2 mixture. This is attributed to the combination of the MnOx–CeO2 active component with the 3D network structure of SM. The later not only provides large surface area and high accessibility for the soot particulates to the active sites, but also acts as a particulate filter.

A Co3O4-CeO2 functionalized SBA-15 monolith with a three-dimensional framework improves NOx-assisted soot combustion

In this work, a cobalt and cerium composite oxide functionalized SBA-15 monolith (SM) with an inner three-dimensional (3D) network structure (CoCe/SM) was prepared by isovolumetric impregnation. Its catalytic activities towards NOx-assisted soot combustion were evaluated and the loading and Co/Ce ratio were optimized. X-ray diffraction, scanning electron microscopy, energy dispersed spectroscopy, transmission electron microscopy, N2 adsorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and O2 temperature-programmed desorption were employed to characterize the samples. It was revealed that, compared with unsupported Co3O4–CeO2 particles, the CoCe/SM catalysts with optimized Co3O4–CeO2 loading showed much higher catalytic ability, achieving a complete oxidation of the soot to CO2 below 400 °C in the presence of NOx, which is attributed to the high dispersion of Co3O4–CeO2 on SM and the cross-linked macroporous structure of SM that can house the soot particulates, thus providing closer contact between the soot and catalytically active sites.

Hydrogenolysis of cellulose to C4-C7 alcohols over bi-functional CuO-MO/Al2O3 (M=Ce, Mg, Mn, Ni, Zn) catalysts coupled with methanol reforming reaction.

This work demonstrates the efficient hydrogenolysis of cellulose to C4-C7 alcohols and gas products (reaction 1) by coupling it with the reforming reaction of methanol (reaction 2) over bi-functional CuO-based catalysts. In this process, the CuO-based catalysts catalyze both the reactions 1 and 2, and the in situ regenerated H2 in the reaction 2 is used for the reaction 1. A series of CuO-MO/Al2O3 (M=Ce, Mg, Mn, Ni, Zn) catalysts were prepared by the co-precipitation method. Among these catalysts, CuO-ZnO/Al2O3 exhibited the highest activity to generate a high cellulose conversion of 88% and a high C4-C7 alcohols content above 95% in the liquid products. The CuO-ZnO/Al2O3 catalyst was stable under the reaction conditions and reusable after 4 runs. This work provides a cost-effective route to convert abundant renewable cellulose to liquid fuels.