Study On The Modification Of Cu-Zn-Al Hydrotalcite Catalyst For CO₂ Hydrogenation To Methanol

Excessive emission of CO₂ will cause great harm to human health,environment and ecosystem.Converting CO₂ into valuable chemicals and fuels,such as methanol（CH₃OH）,is currently an effective method to solve the excessive emission of CO₂.For CO₂ hydrogenation to CH₃OH,the activity of the catalyst will directly affect the CO₂ conversion rate(X_CO2)and CH₃OH selectivity(S_CH3OH).Commonly used catalysts are mainly copper-based catalysts,but simple copper-based catalysts have the disadvantages of small specific surface area,poor dispersion of active components,and easy deactivation at high temperatures.Hydrotalcite（LDH）has the advantages of small crystal size,controllable surface characteristics of acids and bases,and is not easy to be sintered at high temperatures.It is widely used in chemical reactions that require an alkaline environment in reaction systems such as hydrogenation and polymerization.However,the specific surface area of LDH is generally relatively small(approximately 5-20 m²·g^-1),and the active center of the composite metal oxide（LDO）formed after roasting is easily aggregated,which affects its catalytic performance.In this study,CuZnAl-LDH（CZA-LDH）was mainly prepared by co-precipitation method,and CuZnAl-LDO（CZA-LDO）was obtained by high-temperature calcination;polyethylene glycol（PEG）,polyvinylpyrrolidone（PVP）and three ethanolamine（TEA）modified the prepared CZA-LDH sample,calcined at 500°C to obtain CZA-LDO;introduced graphite oxide（GO）,prepared CZA-LDH containing reduced graphene oxide（RGO）.After calcination,CZA-LDO/RGO is obtained.XRD,FT-IR,SEM,TEM and XPS were used to characterize the structure and morphology of the catalyst;CO₂-TPD and H₂-TPD were used to test the chemical adsorption performance of the sample;The performance was evaluated.The result shows:（1）Cu（NO₃）₂·3H₂O,Zn（NO₃）₂·6H₂O,and Al（NO₃）₃·9H₂O were used as raw materials,and NaOH and Na₂CO₃ were used as precipitants to prepare CZA-LDH by co-precipitation method.CZA-LDH was successfully prepared by co-precipitation method（Cu:Zn:Al molar ratios are 1:1:0.2,1:1:0.4,1:1:0.6 and 1:1:0.8）.It is pure and free from other impurities.The shape is complete and hexagonal sheet layer,and the dispersion is relatively uniform.Studies have shown that when the molar ratio of Cu:Zn:Al is 1:1:0.6,its catalytic performance is optimal,X_CO2 and S_CH3OHH3OH were 12.1%and 42.3%respectively.This is because the proper amount of Al³⁺can not only improve the specific surface area of the catalyst and the dispersibility of the active components,but also can adjust the acidity and alkalinity of the catalyst surface,which helps to increase the amount of CO₂ adsorbed by the CZA-LDO catalyst.（2）Using Cu（NO₃）₂·3H₂O,Zn（NO₃）₂·6H₂O,and Al（NO₃）₃·9H₂O as raw materials,and NaOH and Na₂CO₃ as precipitants,CZA-LDH was prepared by co-precipitation method,PEG,PVP and TEA are introduced,and the modified catalyst is obtained after calcination.The regular crystal structure of the modified catalyst is distorted to a certain extent,a large number of flocs cover the surface of the crystal grains,and the surface layer of the catalyst becomes loose.The CZA-LDO/TEA obtained by introducing TEA showed the best performance.When temperature,pressure and space velocity are 250°C,3 MPa and 3000 h^-1,X_CO2 and S_CH3OH were 17.2%and54.3%,respectively,an increase of 5.1%and 12%compared to CZA-LDO.This is because TEA can destroy the surface crystal structure,promote the reduction of the CuO component in the catalyst,increase the number of active components on the catalyst surface,and increase the chemical adsorption capacity of CO₂ and H₂,thereby improving the catalytic performance of the catalyst.（3）Using Cu（NO₃）₂·3H₂O,Zn（NO₃）₂·6H₂O,and Al（NO₃）₃·9H₂O as raw materials,CZA-LDH/RGO was prepared by hydrothermal method,and CZA-LDO/RGO was obtained by calcination.The CZA-LDO/RGO catalyst is composed of hexagonal CZA-LDO graded nanosheets with a size of 100 nm and dispersed on the tulle-shaped RGO.The amount of H₂ and CO₂ adsorbed increases first and then decreases with the increase of RGO.When the RGO content is 4%,the CZA-LDO/RGO catalyst has the most optimized performance.When the temperature,pressure and space velocity are 250°C,3 MPa,and 3000 h^-1,X_CO2 and S_CH3OH were 22.1%and56.8%respectively,which were 4.9%and 3.5%higher than CZA-LDO.This is due to the synergistic effect of metal ions and RGO,so that CZA-LDO has good dispersion on RGO,which increases the exposed area of Cu and accelerates the diffusion of surface solutes.At the same time,the presence of RGO improves the H₂ adsorption capacity of the catalyst,provides a good reducing atmosphere,and prevents the catalyst from being deactivated during high-temperature reactions.