Performance Study Of Copper-Based Catalysts For CO₂ Hydrogenation To Methanol

The combustion of fossil fuels has emitted large amounts of carbon dioxide in recent years,and the greenhouse effect and climate problems are becoming increasingly serious,posing a serious threat to the sustainable development of our society.Therefore,the efficient conversion of CO₂ into high-value chemicals（e.g.methanol,methane,low carbon olefins,etc.）is of great research importance.Among them,methanol is not only an important basic raw material for chemical industry,but also an excellent performance motor fuel and an important energy source.Methanol synthesis by CO₂ hydrogenation is a carbon conversion technology that not only makes effective use of CO₂ resources,but also plays a key role in the sustainable development of society.The CuO-Zn O-Al₂O₃ catalysts usually used in industry can catalyze CO₂hydrogenation for methanol synthesis under harsh reaction conditions（temperature>300°C and pressure>5 MPa）.However,they face a series of problems,such as poor activation of CO₂ molecules,low methanol selectivity,and weak interaction between copper species and supports.Therefore,the design and development of new copper-based catalysts with high efficiency and stability is of great importance and has become a hot research topic at home and abroad.Since manganese cerium and lanthanum cerium solid solutions have excellent redox ability,good thermal stability and can form strong interactions with other metals,copper-manganese cerium and copper-lanthanum cerium catalysts were synthesized in this thesis using manganese cerium and lanthanum cerium solid solutions as supports,respectively.The effects of the change of manganese and lanthanum content on the structure and catalytic activity of the samples were investigated systematically.The effects of oxygen vacancies,medium to strong basic sites and catalyst reduction ability on the catalytic efficiency of CO₂ hydrogenation for methanol synthesis were studied.The reaction pathways of CO₂hydrogenation to methanol in Cu-Mn-Ce and Cu-La-Ce catalysts were investigated using in situ infrared techniques to deepen the understanding of the reaction mechanism of copper-based catalysts.The specific research work in this paper is as follows:（1）A series of Cu-Mn-Ce catalysts with different manganese contents were prepared by co-precipitation method,so that the smaller Mn²⁺replaced part of Ce⁴⁺to form manganese-ceria solid solution,and the metal-supports interactions with different strengths were realized by changing the manganese content in the catalysts,and the influence law on the catalytic activity was explored.It was found that the CuO/Mn_yCeO_x（y=0.1,0.2,0.25,0.3）catalysts had superior catalytic ability than the CuO/CeO₂ catalysts,with the CuO/Mn_0.2CeO_x catalysts having the best catalytic activity.At the reaction temperature of 260°C,the methanol yield over the CuO/Mn_0.2CeO_x catalyst reached 0.250 g _CH3OH·g_cat^-1·h^-1 in empty time,with a methanol selectivity of 82.3%,and exhibited excellent catalytic stability in the 72 h catalytic activity test.A series of characterization and in situ diffuse reflectance infrared Fourier-transform spectroscopy tests confirmed that the strong interactions favor the generation of medium to strong basic sites and oxygen vacancies.The excellent catalytic performance was also found to be associated with the highly dispersed Cu⁰ on the catalyst surface,the abundant oxygen vacancies and the abundance of medium to strong basic sites,and the high methanol yield was achieved by promoting the adsorption and conversion of CO₂ on the catalyst surface and accelerating the hydrogenation of formate intermediates.It was also found that methanol synthesis over CuO/Mn_yCeO_x catalysts was achieved via the formate pathway.（2）CuO/La_yCeO_x（y=0,0.1,0.2,0.25,0.3）catalysts with different lanthanum contents were prepared by co-precipitation method and used in the catalytic synthesis of methanol from CO₂ hydrogenation to investigate the effects of different lanthanum contents in the catalysts on the catalyst structure and catalytic reaction performance.It was found that the introduction of lanthanum in the catalyst promoted the formation of lanthanum-cerium solid solution and enhanced the interaction between copper and lanthanum-cerium solid solution,which made the catalyst rich in medium to strong basic sites and oxygen vacancies and improved Cu dispersion.The adsorption conversion of CO₂ and the accelerated hydrogenation of the formate intermediate were promoted,resulting in a high methanol yield.The CuO/La_0.25CeO_xcatalyst achieved a methanol yield of 0.281 g _CH3OH·g_cat^-1·h^-1 in methanol at a reaction temperature of260°C with a methanol selectivity of 83.3%.