CO₂ Utilization-Co Based Catalysts Used For CO₂ Methanation

The sharp increase of CO₂ concentration in atmosphere,caused by the intensive use of fossil fuels,leads to the serious global energy shortage and environmental problems.Thus,the chemical transformation and utilization of CO₂ have received more attentions.CO₂ methanation technology?Sabatier Reaction?,as an effective chemiacl method for CO₂ utilization,can directly catalytic conversion of CO₂ to chemicals and fuels CH₄,and then the transformation of inorganic carbon to organic carbon is achieved.CO₂ methanation technology can not only effectively reduce the concentration of CO₂ in the atmosphere,but also promote the recycling of carbon resources.As the core of CO₂ methanation technology,the catalyst has attracted the attention of the researchers.In this paper,the Co/KIT-6 catalysts were prepared by impregnation method.The physicochemical properties of the Co based catalysts were characterized by Scanning electronmicroscopy?SEM?,Transmissionelectronmicroscopy?TEM?,Brunauer-Emmett-Teller?BET?,hydrogentemperature-programmedreduction?H₂-TPR?,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,H₂-temperature programmed desorption?H₂-TPD?and CO₂-temperature programmed desorption?CO₂-TPD?.The effects of Co loading,reduction temperature,and crystallization temperature used for preparing mesoporous SiO₂ support on the catalytic performance of the Co based catalysts were investigated.And some main conclusions were drawn as follows:1.The crystal size of Co species in Co/KIT-6 catalysts prepared by impregnation method increases with the increasing Co content and the dispersion of Co species reveals an opposite trend.The adsorption capacities of H₂ and/or CO₂ molecules achieve a maximum value at the Co loading of 25 wt%.The CO₂ reaction rate and CH₄selectivity are in the following order:Co15<Co20<Co30<Co25.With the increasing of Co loading,the CO₂ reaction rate increases from 2.21×10^-55 mol/g_cat/s?CO₂conversion of 29.8%?for the Co15 catalyst to 3.91×10^-55 mol/g_cat/s?CO₂ conversion of52.6%?for the Co25 catalyst,and the CH₄ selectivity improves from 67.0%for the Co15 catalyst to 94.5%for the Co25 catalyst at 360^oC,while the other product is CO.The catalytic performance and products distribution of the Co/KIT-6 catalysts for CO₂hydrogenation are sensitive to the crystal size of Co species which can be adjusted by changing the Co content in Co/KIT-6 catalysts.2.Reduction temperature has a significant impact on the specific surface area,crystallinity and content of Co⁰ species,and CO₂ adsorption performance of the prepared Co/KIT-6 catalysts.The low reduction temperature is not conduct to the reduction of CoO_x,while the high reduction temperature lead to the increase of Co⁰species crystallinity and the formation of Co₂SiO₄ and/or Co-O-Si species which are unfavorable to the form Co⁰species.The RT400 catalyst exhibits the highest Co⁰species content and the best CO₂ adsorption performance.Both the CO₂ hydrogenation catalytic activity and CH₄ selectivity of the Co/KIT-6 catalysts follow the order:RT400>RT350>RT450>RT500>RT550,with a small amount of by-product CO.At the reaction temperature of 340oC,the CO₂ reaction rate of the RT400 catalyst can reach2.98×10^-55 mol/g_cat/s?CO₂ conversion of 40%?,the CH₄ and CO selectivity are 86.7%and 13.3%,respectively.3.The pore size,specific surface area,and the Co⁰ species crystallinity of the Co/Si O₂ catalyst prepared by grind-impregnation method increase with increasing crystallization temperature of preparing SiO₂ support.The mesoporous Co/SiO₂ catalyst has the best CO₂ adsorption property when SiO₂ support is prepared at the crystallization temperature of 100oC.The CO₂ reaction rate?CO₂ conversion?of the Co/Si O₂ catalysts is as follows:CK100>CK080>CK120>CK060>CK140>CK040,and the CH₄ selectivity follows the order:CK100>CK080>CK120>CK140>CK060>CK040,accompanied by a certain amount of by-product CO molecules.At 360oC,the CO₂ reaction rate of the CK100 catalyst is up to 3.29×10^-5mol/g_cat/s?CO₂ conversion of 44.3%?,the CH₄ and CO selectivity are 86.5%and 13.5%,respectively.