CO₂ Utilization

Large-scale consumption of fossil fuels has resulted in the sharp increment of CO₂ in the atmosphere,which has triggered a series of serious environmental problems and posed a huge threat to the survival of human beings.It is urgent to reduce CO₂ emissions.CO₂ hydrogenation synthesizes more active CO via CO₂ reverse water gas shift?RWGS?reaction and follows by the F-T reaction to produce the long-chain hydrocarbons and other valuable chemicals,which can effectively achieve CO₂ utilization to reduce CO₂ emissions.CeO₂ has attracted an increasing attention in the research area of green catalysis because of its rich resources,no secondary pollution to the environment and the superior resistance to poisoning.In this paper,a series of Ce-based catalysts were prepared.The effects of preparation methods and introducing transition metal elements on the catalytic performance of the corresponding catalyst in the RWGS reaction were investigated.The H₂-TPR,XRD,BET,TEM,in-situ XPS,AAS,CO₂-TPD,and H₂-TPD characterizations are conducted to understand the relationship of the physical and chemical properties of the corresponding catalyst with catalytic performance.The main conclusions are as follows:1.The preparation method of CeO₂ catalyst has a significant effect on the catalytic RWGS reaction performance of the corresponding catalyst.The prepared CeO₂ catalyst using a hard template method has the highest RWGS reaction performance.The formed surface oxygen vacancies on the surface of CeO₂ catalyst are considered as main active centers of the RWGS reaction.CeO₂ catalyst has the CO₂ conversion rate of 0.24 mmol·gcat^-1·min^-1and a 100% CO selectivity at 400 °C.2.The transition metal elements?M = Ni,Cu,Co,Fe,and Mn?have significant effects on the RWGS reaction activity and selectivity of the prepared mesoporous CeM catalysts using the hard template method.Mn+can replace the Ce⁴⁺of CeO₂ to form a Ce-O-M solid solution.The prepared CeM catalysts have the developed mesoporous structure and the large specific surface areas exceeding 120.0m2/g.The adsorption ability of the catalysts for reactants CO₂ and H₂ molecules can be improved to different extents because transition metals are affected by the d-orbital holes.Consequently,the RWGS reaction performance of the corresponding catalyst is affected.CO₂ conversions are ranked as follows: Ni-CeO₂ > Cu-CeO₂ > Co-CeO₂ >Fe-CeO₂ ? Mn-CeO₂,and CO selectivity of corresponding catalyst is 28.0%,100%,63.0%,100% and 100%.3.The CexCuy catalysts prepared by the hard template method have an excellent RWGS reaction performance.The Ce/Cu mole ratio has a significant effect on the reaction performance of the corresponding catalyst.Cu species can replace the Ce⁴⁺ of CeO₂ to form a solid solution.The introduction of Cu species can effectively improve the reducibility of Ce_xCu_yO precursor.The surface oxygen vacancies and active Cu0 species can be formed in the CeCu catalysts by the H₂ reduction.The electronic effect between Ce and Cu species can effectively boost the adsorption and activation performances of the reactant CO₂ and H₂ molecules on the corresponding CeCu catalysts.The synergistic interaction between the active species significantly improved the RWGS reaction performance of the corresponding catalyst.The CexCuy catalyst has 100% CO selectivity and CO₂ conversion are ranked as follows:Ce₁.1Cu₁ > Ce1.2Cu1 > Ce1Cu1.1 > Ce1Cu1.2.The Ce₁.1Cu₁ catalyst demonstrated the highest CO₂ conversion rate,reaching 1.38 mmol·gcat^-1·min^-1 at 400 °C.Its excellent catalytic performance in the RWGS reaction was related to the complete synergistic interaction between the active species.Meanwhile,the Ce₁.1Cu₁ catalyst has a high stability in the RWGS reaction and low activation energy of 42.1 kJ/mol.