Cr Modified Effect On Reactivity Of Ru-based Catalysts For CO₂ Methanation

CO₂ methanation reaction is one of the promising means to solve the energy and environmental problems.The in-depth study of this reaction is not only of great significance in energy,environment,and other fields,but also can provide theoretical guidance for CO₂ hydrogenation to produce other low-carbon compounds.The key of this technology is to design catalysts with high low-temperature activity and CH₄selectivity.Meanwhile,studyingthe reaction mechanism of CO₂ methanation is helpful to understand the relationship between active intermediates and sites,which can provide theoretical guidance for catalyst design.In this thesis,Ru/Ti_1-δCrδO_x and Ru/Cr₂O₃/Al₂O₃ catalysts with high low-temperature activity were obtained by Cr-doping and modification.The effects of Cr doping on the structure of support,active components and catalytic performance were studied by multiple characterizations.The effects of Cr doping on the reaction mechanism of CO₂ methanation were also discussed.The main research results are summarized.（1）The Cr³⁺-doped Ti O₂ supported Ru-based catalysts were prepared and used for CO₂ methanation.Compared with the Ru/Ti O₂ catalyst,the Ru/Ti_1-δCrδO_x catalysts have higher catalytic activity for CO₂ methanation.The Ru/Ti_0.95Cr_0.05O_x catalyst with5%Cr³⁺doping has the best catalytic activity,and the turn of frequency value is 7.4times of Ru/r-Ti O₂ at 150 ^oC.Cr³⁺is dissolved into Ti O₂ crystal lattice to form a solid solution structure.Cr³⁺doping inhibits the phase transformation from anatase to rutile and regulates the phase composition of the supports.Cr³⁺doping also inhibites the sintering of Ti O₂,which greatly increases specific surface area and pore volume of the supports,promotes the dispersion of Ru species and significantly reduces the size of Ru particles.Meanwhile,the Cr³⁺doping enhances the interaction between metal and support,and improves H-spillover effect.The in situ FTIR results show that CO₂ methanation reaction follows the CO pathway on Ru/Ti O₂ catalyst.Cr³⁺doping induces the generation of Ru under different chemical environments,thus resulting in the formation of a new linear CO*(2003 cm^-1)intermediate.Compared with the linear-CO*(～1980 cm^-1)on the Ru/Ti O₂ catalyst,the new linear CO*speices possesses much higher activity for hydrogenaton into CH₄at much lower temperatures.Consequently,the CO pathway was remarkably promoted so that the Ru/Ti_1-δCrδO_x catalysts show much higher low-temperature activity for CO₂methanation.（2）The Cr³⁺-modified Al₂O₃ supported Ru-based catalysts were prepared and used for CO₂ methanation.Compared with the Ru/Al₂O₃ catalyst,the activity of Ru/Cr₂O₃/Al₂O₃ catalyst improved significantly.The Ru/7%Cr₂O₃/Al₂O₃ catalyst with the loading of 7%Cr₂O₃ shows the best catalytic activity with CO₂ conversion reaching66%at 280 ^oC,while CO₂ is not converted yet on the Ru/Al₂O₃ catalyst at the temperature.For the fresh Ru O₂/Cr₂O₃/Al₂O₃ catalyst,part of Cr cations on the surface of the support dissolves the Ru O₂ lattice to form a solid solution structure.This structure inhibites the aggregation of Ru O₂ during reduction,which significantly decreases the size of Ru particles and enhances the interaction between metal and support.In situ FTIR results demonstrated that the reaction mechanism and reaction intermediates are significantly affected by Cr modification.In one hand,Cr cation modification helps the dissociation of CO₂to CO,thus promoting CO pathway.In the other hand,Cr₂O₃ species cover the basic sites on the surface of Al₂O₃,so that CO₂species are not easy to combine with basic sites,thus being harmful to the formation of HCOO*species and inhibiting the formate pathway.Therefore,the Ru/Cr₂O₃/Al₂O₃catalysts have higher low-temperature activity for CO₂ methanation.（3）The surface and bulk modification of different supports by Cr cations present different mechanisms,but the Cr modifications enhance the strong metal-support interaction,significantly inhibit the growth of Ru particles,and promote the CO pathway,so that the Ru-based catalysts can achieve higher low-temperature reaction activity for CO₂ methanation.Furthermore,the findings of in situ controling the Ti O₂phase composition by Cr cation doping and the formation of surface solid solution by Cr cations entering Ru O₂ lattice provide new ideas for the rational design of supports and active components in heterogeneous catalysis.