CO₂ Catalytic Hydrogenation Reduction To CO

With the continuous reduction of oil reserves,the emission of greenhouse gas carbon dioxide is increasing day by day.The resulting El Nino effect aggravates global warming and seriously threatens people’s normal life.China’s carbon dioxide emissions have been ranked first in the world for nearly ten years,so we must reduce carbon dioxide emissions.Many countries in the world have done a lot of research on carbon dioxide emission reduction and carbon dioxide collection and utilization.Among them,the hydrogenation of carbon dioxide to produce high value-added chemicals(such as olefins,aromatics,methanol,diesel oil,etc.)is an effective way to make use of carbon dioxide to turn waste into treasure.In this paper,ZnO and In2O3/ZnO-ZrO2(In/Zn-Zr)catalysts were studied for the selective hydrogenation of carbon dioxide to CO and methanol:(1)A series of ZnO catalysts were synthesized by the coprecipitation method.The crystallineline size of ZnO catalysts was controlled by different calcined temperatures.The crystallineline size was between 10-42.5 nm.The structure and properties of the catalysts were characterized by XRD,BET,XPS,PL,TGA,CO2-TPD-MS,H2-TPR,H2-TPD-MS and temperature-programmed surface reaction(TPSR).The catalytic performance was evaluated by a fixed bed microreactor.The experimental results show that ZnO has a good catalytic activity for selective hydrogenation of CO2 to CO,and the CO selectivity is close to 100%.However,the stability of ZnO catalyst is poor.After a long time of reaction,the catalyst surface is easy to accumulate carbon and generate CH4 by-products.The catalyst treated at 200℃ has the best catalytic activity,the selectivity of CO is close to 100%,and the conversion of CO2 is close to the theoretical value of thermodynamics when the reaction temperature is higher than 390℃.This is because the catalyst for low-temperature calcined at 200℃ is Zn5(CO3)2(OH)6.The carbonate contained in the catalyst will generate CO2 and leave oxygen defects on the surface of the catalyst in the process of catalyst pretreatment and activation.The oxygen defects on the surface of the catalyst can absorb CO2 and make it react.The results of XRD show that the lower the temperature of ZnO catalyst is,the smaller the grain size is;the ability of ZnO catalyst to adsorb CO2 and H2 is also related to the grain size,the smaller the particle size is,the stronger the ability to adsorb CO2 and H2 is.(2)In based catalysts have very strong catalytic activity for CO2 hydrogenation.To further explore their properties,In2O3 catalysts and In/Zn-Zr ternary catalysts were prepared by co-deposition.The performance of these catalysts for CO2 hydrogenation was evaluated by a fixed bed microreactor.The structural properties of these catalysts were characterized by XRD,CO2-TPD-MS,and H2-TPR.The results show that the In2O3 catalyst has a strong activity of reverse water coal vapor shift reaction(RWGS).In2O3 has different crystalline structures on the surface of In/Zn-Zr,such as c-In2O3 and h-In2O3.In the In/Zn-Zr catalyst,the interaction between metal oxides is weaker with the increase of Zr content.The interaction between strong metal oxides is beneficial to the formation of c-In2O3 crystalline on its surface,on the contrary,it is beneficial to the formation of h-In2O3 crystalline.ZrO2 can improve the adsorption performance of CO2.c-In2O3 can make CO2 adsorbed on the catalyst surface for hydrogenation and has strong selectivity of methanol products.When the content of ZnO increases,the adsorption capacity of the catalyst for CO2 decreases,but the activation capacity for H2 increases,and the CO2 adsorbed on the catalyst surface is hydrogenated to generate CO directly.It is found that the selectivity of catalysts can be controlled by adjusting the content of each component of catalyst.Besides,it is found that in supported high Zr catalyst can selectively control its product by adjusting its reaction temperature.The main product of the In/70Zr-30Zn catalyst in methanol at 300℃ and its selectivity is as high as 83%.When the reaction temperature is higher than 420℃,the main product of In/70Zr-30Zn catalyst is CO and its selectivity is close to 100%,which makes it a good RWGS catalyst.