Study On The Surface Oxygen Vacancy Of Cerium Based Catalyst And Its Catalytic Synthesis Of Dimethyl Carbonate From CO₂

The increasing atmospheric CO₂ concentration has led to climate change and serious natural disasters.Chemical fixtion CO₂ to the useful chemicals and materials was considered as one of the important routes to reduce the concentration of atmospheric CO₂.It is difficult to activate CO₂ due to its stable structure and strong chemical inertia.CO₂ molecule can be activated and transformed by transition metal and rare earth metal atoms due to their special electronic outer structure,which is benifical to interact with CO₂.Among them,CeO₂ has a special surface structure,and the oxygen vacancies can adsorb and activate CO₂,which formed by electron transfer in the surface of CeO₂.Thus,CeO₂ has an excellent catalytic effect on the formation of dimethyl carbonate（CH₃OCOOCH₃,DMC）from CO₂ and CH₃OH.In this work,the surface oxygen vacancy over the catalysts was modulated by controlling the morphology of CeO₂ catalyst and metal ions doping,the direct synthesis of dimethyl carbonate（DMC）from CO₂ and methanol was used as the target reaction.The influence of morphology of CeO₂,kinds and doping proportion of doped metal ions on the structure of CeO₂ catalysts,surface oxygen vacancy formation and content,CO₂ adsorption properties and catalytic performance were investigated well.The catalytic mechanism of DMC synthesis from CO₂ and CH₃OH for the synthesis of DMC is studied by in situ infrared technology.The main conclusions of this study include the following aspects:1.For calcium-doped CeO₂ catalysts,the synergistic effect between Ca ions and Ce ions promoted the concentration of the surface oxygen vacancy of the CeO₂ catalyst.While a further increase in Ca content,the concentration of the surface oxygen vacancy of the catalysts decreased.When the mass fraction of Ca is 1.5%,the catalyst had the largest concentration of the surface oxygen vacancy,the adsorption capacity of CO₂ reaches 0.40 mmol/g.The catalyst had the best activity.The amount of DMC per unit catalyst reached the maximum,which the value was 2.47 mmol/g.The activity of calcium-doped CeO₂ catalysts is increased by 20%compared to the pure CeO₂ catalysts.2.In the series of CeO₂ catalysts with different morphologies,the nanorods catalysts are mainly exposed to（111）and（110）crystal surfaces,the nanocubes are mainly exposed to（100）crystal surface,and the nanoparticles are mainly exposed to（111）crystal surface.The concentration of the surface oxygen vacancy of the three different morphologies follows the order:nanorod>nanoparticle>nanoscale.The CO₂ adsorption amount and the activity of the Zr doped CeO₂catalysts is higher than the corresponding pure CeO₂ catalysts.The activity evaluation results indicated that the Zr doped CeO₂ nanorod catalyst had the best catalytic performance,which the value is 13.3 mmol/g.This is due to it has the higher concentration of the surface oxygen vacancy than other catalysts.The amount of CO₂ adsorbed on the catalyst surface and the activity of the catalysts increased with the increasing of the surface oxygen vacancy concentration.It shows that increasing the concentration of the surface oxygen vacancy is beneficial to the rasing of the CO₂ adsorption capacity and catalytic performance of the catalysts.3.Metal-doped in CeO₂ nanorods enhanced the content of Ce³⁺and the surface oxygen vacancy of the catalyst.Among them,Zr-doped CeO₂ produced more oxygen vacancies on the surface of the catalyst,and the concentration of the surface oxygen vacancy is 9.4%.This is because the crystal structure of ZrO₂ is similar to that of CeO₂,both of which are cubic fluorite structure.Zr⁴⁺can form a homogeneous solid solution with CeO₂ easily compared with other metal ions,which promoted the increase of the surface oxygen vacancy content.The Zr doped CeO₂ nanorod has the highest CO₂ adsorption and the best activity,which the value is 0.89 mmol/g and 13.3 mmol/g,respectively.4.Further studies showed that doping Zr into CeO₂ nanorods can promote the reduction of Ce⁴⁺to Ce³⁺on the surface of the catalyst,which benifical to increase the surface oxygen vacancy content.When the mole ratio of Zr to Ce was0.1,the catalyst had the highest surface oxygen vacancy concentration（10.4%）and the DMC yield reached 14.2 mmol/g.However,with the increasing of the Zr content,the Zr⁴⁺began to accumulate on the catalyst surface.The accumulation of Zr⁴⁺over the surface of the catalyst covered the oxygen vacancy sites on the catalyst surface,which resulting in the decrease of the surface oxygen vacancy concentration.The concentrations of the surface oxygen vacancy gradually decreased during the reation,which was the main cause of the decrease in the activity of the catalyst.5.In situ DRIFT spectra study of the reaction of CO₂ and CH₃OH catalyzed by Zr doped CeO₂ nanorods showed that the CO₂ molecule adsorbed on the surface oxygen vacancies of the catalyst through O atom to form bidentate carbonate（?）,and the methanol molecules can react with the surface hydroxyl groups to produce the adsorbed methoxyl species（-OCH₃）over the surface of the catalyst.First,the surface methoxyl species can react with the bidentate carbonate to form methyl carbonate（?）.Another methanol molecule adsorbs on the surface of the catalysts through interaction of the hydroxyl oxygen of the methanol with the Lewis acid center Ce on the surface of the catalysts react with the methyl carbonate to form DMC.The oxygen vacancy and hydroxyl groups on the catalyst surface regenerated after the DMC molecule desorbed from the surfaces of the catalyst.The formation of bidentate carbonate by adsorption of CO₂ on the surface oxygen vacancies was an important process for the synthesis of DMC from CO₂ and methanol.