Preparation Of The Copper-Based Catalysts And Its Catalysts Performance In CO₂ Hydrogenation To Methanol

The demand for global energy keeps growing in a row with the increase of world population and energy consumption.Rapid consumption of fossil fuels resulted in large amounts of greenhouse gas（GHG）emissions,leading to serious environmental problems,such as global warming and climate change.Although CO₂ emissions are the major contributions to GHG,they can become valuable carbon resources for sustainable development.CO₂ hydrogenation to produce high value-added chemicals（methanol,syngas,and dimethyl ether）is considered as a promising way for CO₂utilization.Methanol is an important chemical feedstock and excellent alternative fuel,which can also be further transformed into chemicals,including formaldehyde,acetic acid,dimethyl ether,and various petrochemicals.However,the reactions of CO₂hydrogenation are thermodynamically limited due the high stability and low reactivity of CO₂.Therefore,it is necessary to develop efficient catalysts to enhance CO₂adsorption and activation.Among the various catalysts,Cu-based catalysts have attracted wide attention due to their low price and high catalytic activity.In this paper,the structure-activity relationships of several different Cu-based catalysts are analyzed.The results are as follows:A series of Cu-ZnO-ASep catalysts were prepared by the co-precipitation method.The effects of the content of ASep（0-40%）on the physicochemical properties of the catalysts and their catalytic activity of CO₂ hydrogenation were investigated.As compared to the Cu-ZnO catalyst,the methanol synthesis activity increases from 2.8mmol _MeOH/(g_cat·h)to 3.4 mmol _MeOH/(g_cat·h)with the addition of 10%ASep.Moreover,the selectivity to CH₃OH increases with the addition of ASep.The selectivity to methanol increases from 36.5%to 65.5%when the content of ASep increases to 30%.The characterization results indicate that the content of ASep plays a critical role on the physicochemical properties of the catalysts and their catalytic performance.ASep promotes the formation of the Cu-ZnO solid solution,and the Cu atoms are extracted from the solid solution to form highly dispersed metallic Cu crystallites during the reduction procedure,thus increasing the interaction with the surface of the ZnO particles.In addition,the specific surface area of catalysts increases with the increases of the content of ASep,while it decreases when the loading of ASep is above 30%.The high selectivity to methanol may be related to the presence of Cu⁰ and Cu⁺sites,and the interaction of Cu and ZnO promotes Cu⁺site formation.The selectivity to CH₃OH is linear with Cu⁺/（Cu⁰+Cu⁺）,indicating that Cu⁺is the key active site for CO₂ hydrogenation to methanol.Three Cu/ZnO/Al₂O₃ catalysts were synthesized using hydrothermal,Na₂CO₃coprecipitation,and（Na₂CO₃,Na OH）coprecipitation method,and denoted as CZA-HM,CZA-DP and CZA-HT,respectively.The results show that CZA-HM exhibits the best catalytic performance(11.4%yield of methanol and 4.6 mmol _MeOH/(g_cat·h)STY_MeOH).The physicochemical properties of the catalysts were determined using X-ray diffraction（XRD）,N₂ physisorption,thermogravimetric analysis（TGA）,temperature programmed reduction（H₂-TPR）,temperature programmed desorption（CO₂-TPD and H₂-TPD）,transmission electron microscopy（TEM）,and X-ray photo-spectroscopy（XPS）techniques.The characterization results indicate that the preparation method determines the phase distribution of the Cu O/ZnO/Al₂O₃precursors.Compared with the other Cu O/ZnO/Al₂O₃ precursors,the CZA-HM precursor exhibits better Cu O dispersion and reducibility,more oxygen vacancies,and more basic sites,thus showing the best catalytic performance.Moreover,CZA-HT shows the lowest catalytic performance.The reason may be explained by the unfavorable microstructure of the Cu/ZnAl₂O₄ catalyst and the Cu particles are embedded in a compact oxide matrix（ZnAl₂O₄）.Furthermore,the results reveal that the selectivity to CH₃OH and catalytic performance can be greatly enhanced by the remarkable increase of the proportion of basic sites.Cu/ZnO/ZrO₂ catalysts with flower,plate,and rod-like ZnO morphologies were prepared by urea hydrolysis method and used for CO₂ hydrogenation to methanol.Among these catalysts,Cu/ZnO/ZrO₂ with flower-like ZnO（CZZ-flower）exhibits the best catalytic performance(10.7%yield of methanol and 4.3 mmol _MeOH/(g_cat·h)STY_MeOH).The characterization results show that the CCZ-flower possesses the largest amount of oxygen vacancies.The experimental results reveal that the yield of methanol increases linearly with the increment of oxygen vacancy concentration,suggesting that oxygen vacancy plays an important role in CO₂ hydrogenation to methanol.Additionally,the amount of oxygen vacancy can be modulated by changing reduction temperature,reduction time,and reducing gas concentration.It can be seen from DFT（density functional theory）calculations that oxygen vacancy is able to promote CO₂ and active hydrogen（H^*）adsorption,activate reaction intermediate and reduce the whole energy barriers.Three Metal-organic framework materials（MOF-5,Ui O-66,and Ce-BTC）were prepared and used as the precursors of Cu/ZnO@C,Cu/ZrO₂@C,and Cu/CeO₂@C catalysts,the results show that the MOFs support has a large effect on the activity of the CO₂ hydrogenation.MOF-5 has a high BET surface area and a small pore size,and the Cu/ZnO@C derived from MOF-5 shows 9.6%CO₂ conversion,53.5%selectivity to methanol,and 2.1 mmol _MeOH/(g_cat·h)STY_MeOH,attributing to the strong metal support interaction between Cu and ZnO.Moreover,the Cu-CeO₂/ZnO@C、Cu-In₂O₃/ZnO@C and Cu-ZrO₂/ZnO@C catalysts were further synthesized by adding the CeO₂,In₂O₃,and ZrO₂ additives.The characterization results show that Cu-CeO₂/ZnO@C exhibits stronger interaction between ZnO and Cu compared with Cu-In₂O₃/ZnO@C and Cu-ZrO₂/ZnO@C.The reason may be due to the addition of CeO₂increases the surface basicity catalysts so as to increases the CO₂ adsorption and further facilitate the methanol synthesis.Among these catalysts,Cu-CeO₂/ZnO@C possesses the largest amount of oxygen vacancies and exhibits the best catalytic performance(STY_MeOH=3.6 mmol _MeOH/(g_cat·h)).Additionally,the Cu-CeO₂/ZnO@CN containing abundant nitrogen（N）dopants was fabricated by pyrolyzing the mixture of Cu-CeO₂/ZnO@C and melamine.Compared with the Cu-CeO₂/ZnO@C,Cu-CeO₂/ZnO@CN shows the highest selectivity to methanol（96.4%）and space-time yield of methanol(STY_MeOH=4.3 mmol _MeOH/(g_cat·h))in CO₂ hydrogenation to CH₃OH.The characterization results indicate that the presence of pyridinic nitrogen promotes the interaction of metal and support,thus increasing metallic Cu dispersion and Cu⁺species,and enhancing CO₂ adsorption.