The Study Of The Promoter Effects In CO₂ Hhydrogenation To Methanol And Ethanol

Nowadays,the environment and energy problems are drawing great attention in the world.With the drastic increase in the consumption of fossil fuels,CO₂ emissions have increased continuously,causing the greenhouse effect and other serious climate problems.CO₂ hydrogenation is an effective way of the reduction and utilization of CO₂.In recent years,it has received extensive attention and made great progress.However,CO₂ hydrogenation is still confronted with problems such as low conversion,difficulty in controlling product selectivity,and the stability is not satisfying.In the first part of this thesis,the role of Al promoter in Pd/Zn O is studied systematically for the enhanced CO₂ activation in CO₂ hydrogenation to methanol.When the Al content is below 3.93 wt.%,the CO₂ conversion and methanol yield increase with more of Al content.Compared to Pd/Zn O,CO₂ conversion and methanol yield for the optimized Pd/Zn O-3.93Al are increased by 2.5 times and 1.7 times,respectively.Further increase in Al content will decrease the CO₂ conversion and methanol yield.Different Al species are investigated in detail.It is found that the Al doped in the Zn O could accelerate the methanol synthesis while the amorphous Al₂O₃would do harm to the methanol synthesis.Combined with the structural analysis,CO₂-TPD and DFT calculation,we propose that the doping of Al into Pd/Zn O will enhance the CO₂ activation,resulting in the improved CO₂ conversion and methanol yield.In situ DRIFTS suggests that the CO₂ hydrogenation over Pd/Zn O-x Al tends to follow the*COOH mechanism.The second part of this thesis focuses on the control of product selectivity in CO₂hydrogenation to ethanol,including the role of Fe promoter and the mechanism of CO₂hydrogenation to ethanol over Rh/Ti O₂.At present,the selectivity to ethanol is unsatisfied in CO₂ hydrogenation.Fe is an effective promoter in ethanol synthesis from CO₂ or CO hydrogenation,but the role of Fe promoter and mechanism of ethanol synthesis are still unclear.The introduction of Fe often leads to the change in the particle size of the Rh nanoparticles,while the structural sensitivity of Rh is very obvious for ethanol synthesis,thus making it hard to study the individual role of Fe.To avoid the influence of the structural sensitivity and study the mechanism,the Fe promoter on the Rh surface is adjusted by the stepwise impregnation method.The particle size of Rh is controlled to be similar with and without Fe promoter.It is found that with the increasing coverage of Rh on the surface by the iron promoter,the content of the Gem-CO species becomes higher,and the ability to break the C=O bond is weaker.As a result,the formation of methane is inhibited and the selectivity of ethanol is promoted.