An In-situ FTIR Study On CeO₂Catalysts For Direct Synthesis Of Dimethyl Carbonate From Carbon Dioxide And Methanol

As a green chemical, dimethyl carbonate (DMC) is widely applied in fueladditives, organic chemical reaction and lithium ion battery solvent. More attentionhas been paid on direct formation of DMC from carbon dioxide (CO2) and methanoldue to the high atomic economy and cheap resources. What is more, CO2as thefeedstock is a good way to alleviate the climate change impact caused by excessemission of CO2. Our previous reasearch indicates that cerium oxide exposingdifferent planes exhibit distinct activity in DMC formation. But the mechanism ofDMC production over ceria catayst is still ambiguous. In this paper, we combinein-situ FTIR and other characterization to investigate the reaction mechanism of DMCformation from CO2and methanol on ceria catalyst surface.Three ceria catalysts including nanorods (110), nanocubes (100) and nanoctahedron(111) were perpared by hydrothermal method. These samples were used to in-situmethanol adsorption, CO2adsorption and DMC adsorption. Incorporation theadsorption of CO2, methanol and DMC with catalytic activity, we deduced thatmonodentate methyl carbonate was not the intermediate in DMC formation over ceriacatalyst and speculated that the arising peak at1290cm-1represents the intermediate-carbomethoxide. This peak was unique on ceria surface compared with zirconiumoxide. Based on the above results, we figured out the mechanism of DMC formationfrom CO2and methanol over ceria catalyst. The adsorbed CO2reacted with adsorbedmethanol to produce carbomethoxide and hydroxyl on catalyst surface. DMC wasformed by the reaction between carbomethoxide with the methoxy of anothermethanol. The remaining H atom combined with the hydroxyl to produce H2O.Ce3+and Ce4+coexisted on ceria surface. The ratio of Ce3+and Ce4+on ceriasurface can be modified by controlling the particle size of cerium oxide throughdifferent calcination temperatures. A certain ratio of Ce3+and Ce4+exhibited a betterperformance in DMC formation. Catalysts with a higher or lower percent of Ce4+didnot play well in catalytic test. The ceria surface experienced reduction and oxidationduring the reaction process depending on the results of methanol and CO2adsorption.It was also concluded that the reactive methoxy was the II bridging methoxy adsorbedon Ce3+instead of the on-top methoxy.