The Study Of In-Co-based Catalysts For CO₂ Hydrogenation

Nowadays,the excessive CO₂ emission makes greenhouse climate effect.CO₂hydrogenation technology can not only promote economic and energy production,but also retard greenhouse effect.Therefore,there are more deep studies on high product selectivity catalysts for CO₂hydrogenation.In-Co catalyst was repoeted that it have great methanol performance at rigor react conditions（300℃,5 MPa）.So,it is still challenging to enhance the selectivity of specific products under mild reaction condition.In this paper In-Co-based catalysts for arbon dioxide hydrogenation were study.The main results are as follows:（1）The effect on the products selectivity over In-Co catalyst for CO₂hydrogenation by the different supports was studied.The methanol selectivity over In-Co/Ce catalyst was 74%,and the space-time yield of methanol was 276.8mgh^-1g^-1_In-Co which was 4.8 time than that of In-Co catalyst.The CO selectivity over In-Co/Ti catalyst was 80%,and the space-time yield of CO was 487.8 mgh^-1g^-1_In-Cowhich is 11 time than that over In-Co catalyst.The oxygen vacancy induced by H₂(O_V1)and induced by thermal O_V2 may be the active sites for methanol and CO,respectively.The selectivity of products from CO₂ hydrogenation could be determined by the supports influnencing the amount of ratio of O_V1 and O_V2 and the reaction intermediates.Ti O₂ supports not only increase the amount of O_V2,but also inhibit the reduction of Co₃O₄ particles.Meanwhile In₂O₃ can not get enough electrons to formate defect oxygen of O_V1.Lack of metal cobalt and O_V1 made carbonate and bicarbonate prefer to CO.The Ce O₂ support could enhance the reduction of Co₃O₄.And Co/Ce O_2-x structure may contribute to formation of O_V1 on Ce O₂.More importantly,formate on Ce O₂ support might participate in methanol synthesis via In-Co-Ce interface.In addition,In-Co support on Ce O₂and Ti O₂inhibit the formation of methane.And methane selectivity was reduced form 14%to 1%-2%.（2）It was investigated that the activity of In-Co/Ce catalyst for methanol synthesis via CO₂ hydrogenation was affected by loading of In-Co.When the loading rate reached 40%,the carbon dioxide conversion,methanol selectivity and methanol yield was 13.3%,68.4%and 9.1%respectively.When the loading rate is from 20%to40%,the specific surface area and the amount of oxygen vacancies on the sample was increased,which improves the conversion of carbon dio xide.In this range,the selectivity of the product is linearly related to the ratio of O_V1and O_V2.With more In-Co dispersed on the Ce O₂surface,the interaction between Co and Ce was weakened,due to growing particle size of Co.So the low proportion of O_V1 made the methanol selectivity decreased.When loading was 50%,In-Co particle tend to aggregate.And In₂O₃-Ce O₂ sites extremely increase,improveing the CO selectivity on In-Co/Ce（50%）catalyst.As a result,the proportion of O_V1 and O_V2 had lost their dominant role in controlling products selectivity.（3）On the surface of In-Co/Ce 40%catalyst,the In-rich sites or Co-rich sites were formed via changing the In/Co ratio,which make the interaction between In and Co faded.And the catalyst activity was reduced.In addition,decreasing the size of In₂O₃ particles can significantly improve the In-Co interaction.However,the Co₃O₄particles have low degree of reduction when In/CO ratio abate to 1/3.The catalyst activity was decreased because of lack of electron transformation form Co⁰ to In₂O₃.Therefore,In-Co/Ce 40%catalyst with In-Co/Ce has the best activity.Moreover,when the space velocity=12000 mlh^-1g^-1_In-Co,In-Co/Ce catalyst had the best methanol space-time yield,1009 mgh^-1g^-1_In-Co,and methanol selectivity was 78%.（4）DFT calculation was used to investigate the reaction path for CO₂hydrogenation at the In-Co-Ce interface supported on Ce（111）surface.DFT calculation shows that there are two kinds of structure of HCOO*species with close Gibbs free energy,including HCOO*-In and HCOO*-Ce.The intermediates formated form HCOO*-Ce have lower Gibbs free energy and are more stable than those form HCOO*-In.For CH₄ reaction path,HCOO*on Ce O₂ is easier to be hydrogenated to H₂OO*which makes it difficult to produce methane.In conclusion,the effect of Ce O₂on intermediates leads to the enhancement of methanol selectivity.