The Research Of Cu/ZrO₂/CNTs Catalyst For Methanol Synthesis From Carbon Dioxide Hydrogenation

As a greenhouse gas, CO₂emission has been recognized to be one of the most seriousenvironmental problems in the form of greenhouse effect and ozone depletion. Many effortshave been made in order to reduce CO₂emission. Catalytic hydrogenation of carbon dioxideto produce various kinds of chemicals and fuels has received much attention. Methanol, beinga clean liquid fuel, could provide convenient storage of energy for fuel cell applications,particularly in transportation and mobile devices. When used as a fuel, it is a cleaner energycompared with most other sources. In addition, methanol is an important chemical feedstock.Thus CO₂hydrogenation to methanol has been recognized as one of the most interestingtarget product. This will not only alleviate environmental problems, but also pave way forclean energy development.It has been widely reported that the Cu catalysts supported on zirconia exhibit highcatalytic activity for methanol synthesis from CO₂hydrogenation. Unlike synthesis gas,equimolar amount of water will generate for CO₂hydrogenation to methanol. Watergeneration not only reduce the yield of methanol, but also accelerate the crystallization ofcopper in the catalyst and ZrO₂crystallization and phase transition, which rapidly result incatalyst aggregation and inactivation. Carbon material, having a strong hydrophobicity, isbeneficial to the rapid desorption of the generated water, which can effectively reduce theinhibitory effect of water on the catalytic reaction. Carbon nanotubes （CNTs） have been to beas excellent catalyst supports based on their unique electronic properties and structure, such ashigh mechanical strength, thermol-chemical stability, field limitation, and well conductivity,etc. Apart from the support effect, the influence of promoter on the performance ofCu/ZrO₂-based catalysts were also explored.A series of Cu/ZrO₂/CNTs catalysts were prepared for methanol synthesis from CO₂hydrogenation by a co-precipitation method. The effect of synthesis conditions on thecatalytic properties were investigated. Furthermore, the effects of the addition of chromiumon the catalytic structures and performance of Cu/ZrO₂/CNTs catalysts were Studied. Theresults were shown below. Firstly, the activity tests were carried out in a fixed-bed plug flow reactor for CO₂hydrogenation to methanol. The effects of CNTs loading、different solvents and the PH valueon the properties of the catalysts were investigated. The results showed that low content ofCNTs is helpful to the increase of CO₂conversion, while the high content of CNTs contributeto the improvement of methanol selectivity. The catalyst of10Cu/40ZrO₂/CNTs presented thehighest catalytic activity. the surface basicity of carrier in the catalytic system play animportant role, which may influence the interaction between the carrier and active component,the degree of dispersion of the active components, and even the surface species.Secondly, The effect of different solvents on the texture and morphology of the catalystwere ascribed to their surface tension, viscosity, and dielectric constant, which may make thecatalyst have different surface, volume and the dispersion of the active sites. Additionally, thePH value plays an important role in determining the composition of the precipitates. When theprecipitation was conducted at PH=7⁸, the catalysts were more active. So, when CNTs-NH2as the carrier, water as solvent, and the PH value of control in the7⁸, the highest catalyticactivity was obtained.At last, the effects of the addition of chromium on the catalytic performance ofCu/ZrO₂/CNTs catalysts were investigated in order to improve the activity of theCu/ZrO₂/CNTs catalyst. When the amount of chromium loading is1%of the total amount ofCu2+and Zr4+,the methanol yield goes through a maximum. Under reaction conditions of5.0MPa,260°C, V（H2）:V（CO₂）:V（N2）=69:23:8and GHSV=3600ml/（h·gcat）, the methanol yield ofCO₂hydrogenation reaches8.5%. The catalysts were characterized by BET, XRD, H2-TPR,TG/DTA and XPS techniques. The results indicate that the introduction of chromium enhancesthe dispersion of the Cu species, the specific surface area and improves the thermal stability ofthe catalyst. Combining with the catalytic performance, it reveals that CO₂conversion andmethanol selectivity have a close relation to the specific surface area, Cu species dispersionand the surface amount of Cu, Zr and Cu/Zr molar ratio.