Study On The Performance And Mechanism Of Mn/Ni Supported On Kaolinite/Diatomite For Single Carbon Conversion

As an essential mineral resource,some of non-metallic minerals exhibit good prospects as active components or catalyst carriers due to their excellent adsorption properties,good chemical stability,and natural nano properties.Kaolinite and diatomite,as important components,have broad application prospects as carrier materials due to their unique lamellar morphology,abundant highly reactive surface groups,and excellent chemical stability.In addition,CO is a common toxic gas that poses a severe threat to human health,and excessive emission of CO₂ usually causes severe environmental problems.Therefore,the catalytic conversion of CO/CO₂ into harmless substances or chemicals with industrial value has become a hot research topic.In this thesis,active metal pieces have been loaded on kaolinite and diatomite surfaces to construct metal oxide/mineral matrix composites by taking full advantage of the characteristic of these materials.The activity and selectivity of the products in CO/CO₂ catalytic oxidation/reduction reactions were investigated and the mechanism of single carbon conversion of the materials was resolved.The main research results obtained are as follows:（1）To investigate the intrinsic relationship between MnO₂morphology and the catalytic activity of CO,MnO₂ with different morphological structures was synthesized using a co-precipitation method.It was found that the ultrathin MnO₂ nanosheet,with more surface defect sites,exhibits excellent catalytic oxidation of CO,which can achieve approximately 100%CO conversion at 150℃.After further introducing various metal species into MnO₂,the CO oxidation results reveal that Cu doping provides an outstanding advantage to MnO₂ in CO oxidation performance,achieving ca.100%CO conversion at as low as 125℃.Mechanistic studies have revealed that in the presence of the Cu species,CO preferentially adsorbs on the Cu site and then reacts with the adjacent oxygen to produce CO₂,revealing the Cu/Mn interface as the main active site for CO oxidation.（2）To further enhance the CO oxidation activity of Mn-based catalysts and address their agglomeration and stacking problems,copper oxide@manganese dioxide/kaolinite（KM@0.10Cu O-NO₃）composites with three-dimensional structure were prepared based on the lamellar morphology and active groups of kaolinite（Kln）.The composite exhibits excellent CO oxidation activity,achieving ca.100%CO conversion at125℃,with high long-term stability,water resistance,and high temperature resistance.Mechanistic studies revealed that the surface charge and active hydroxyl groups of Kln create favorable conditions for the adsorption of MnO^4-,which eventually converts to MnO₂ on the surface of Kln.The introduction of the Cu O species not only promoted the breaking of the Mn-O bond and increased the mobility of oxygen species on the surface of the composites,but also played a leading role in the adsorption and activation of CO during the oxidation process,avoiding the competitive adsorption of CO and O₂,and ultimately improving the activity of CO oxidation of the composites.（3）To further improve the CO oxidation activity of MnO₂,diatomite（Dia）with a large specific surface area and abundant surface hydroxyl groups was selected as the substrate on which Co Mn nanoflowers with ultrathin structures were optimally constructed.Compared to pure Co Mn materials,Co Mn/Dia composites effectively overcome the stacking of Co Mn binary oxides and increase the specific surface area.The effect of Co content on the material morphology and CO oxidation activity was analyzed comparatively.The results show that 0.10Co Mn/Dia has the best CO oxidation activity,achieving ca.100%CO conversion at 175°C.The DFT results show that the energy required for the formation of oxygen vacancies in MnO₂ gradually increases from the surface layer to the interior of the material,and the introduction of Co elements promotes the breaking of the Mn-O bond,resulting in a significant decrease in the energy required for the formation in oxygen vacancies.（4）In order to further resource utilization of the CO₂ generated by CO oxidation and in the environment,kaolinite was introduced into the（Ni-Ce）-based catalyst preparation system to overcome the common problems of sintering and poor stability of the current nickel-cerium（Ni-Ce）-based catalysts.The Ni-Ce/Kaolinite（Ni-Ce/Kln）composites were constructed based on the lamellar morphology and abundant hydroxyl groups of Kln.The effect of citric acid as a dispersant on the dispersion of Ni particles and the effect of Ni elemental content on the CO₂ methanation activity of the materials were also investigated.The results show that 0.10 Ni-Ce/Kln-4obtained about 90%CO₂ conversion at 350℃and 3 bar atmospheric pressure when the citric acid/Ni molar ratio was 4 and the elemental Ni content was 10 wt.%.Finally,in situ DRIFTS revealed that the CO₂methanation followed the formate and CO pathways.（5）Based on the research in the previous section,in order to further enhance the activity and stability of（Ni-Ce）-based catalysts for CO₂methanation at atmospheric pressure.Ni-YCe/Dia composites have been constructed using yttrium（Y）species as additives and cylindrical Dia with a large specific surface area and hydroxyl groups as carriers.It is shown that the hydroxyl groups on the Dia surface provide additional active sites for CO₂ adsorption and activation,and the Y species not only inhibits the sintering agglomeration of the Ni species at high temperatures and reduces the size of the Ni particles,but also provides additional alkaline sites for CO₂ adsorption and activation.The 0.025Ni-YCe/Dia achieved ca.90%CO₂ conversion and 99%methane selectivity at 350°C and maintained stable catalytic activity under more severe stability tests（150 h）and high temperature tests（450℃）.In situ DRIFTS revealed that the CO₂methanation process followed the reaction pathway of formate and CO.