CO₂ Methanation Mechanism On Cubic-ZrO₂ And MgO Supported Ni Catalysts:A Combined Theoretical And Experimental Study

CO₂ catalytic hydrogenation to methane is considered as one of the promising solutions for reducing CO₂ emissions and converting CO₂ into useful low-carbon fuels,which is of great importance in environmental protection and energy source.Deeply understanding the reaction mechanisms of CO₂ methanation processes is critical to develop the heterogeneous catalysts with high activity and selectivity.In this thesis,the mechanism of CO₂ methanation has been investigated via combined experiments and DFT calculation on the ZrO₂ and Y-doped ZrO₂ supported Ni catalysts with low Ni content and the MgO supported Ni catalysts with high Ni content,which were simulated by Ni/ZrO₂,Ni/Y₁Zr₉O_x and MgO/Ni interface.CO₂ adsorption and activation,the roles of Ni,the oxide supports and the Ni-oxide interfaces,and the mechanisms of CO₂ methanation on the above catalysts have been elucidated.The main results are as follows.?1?DFT calculations revealed that Ni/ZrO₂ interface is the most favorable site for theadsorption and hydrogenation of CO₂.We analyzed the potential energy of three typical pathways:CO₂ direct dissociation pathway,the formate pathway?HCOO?and the carboxyl pathway?COOH?.The HCOO*pathway is demonstrated to be most favorable,with the lowest apparent activation energy.The rate determining step is HCOO*hydrogenation to H2COO*with the activation energy barrier of 1.01 eV.In-situ FTIR results display that CO₂ methanation on Ni/ZrO₂ catalysts follows the formate?HCOO*?pathway.In addition,CO*is resulted from the hydrogenation of bridged carbonate species,which cannot be further hydrogenated to CH4 but as a byproduct during CO₂ methanation.Combining in-situ FTIR experiment with DFT calculations,we proposed the optimal pathway of CO₂ methanation on Ni/ZrO₂catalysts CO₂*?HCOO*?H₂COO*?H₂COOH*?H₂CO*?CH₂*?CH₃*?CH₄*.?2?In comparison with Ni/ZrO₂,the Ni/Y₁Zr₉O_x catalyst possesses more amounts of medium-strength basic sites,and the improved the CO₂ adsorption capacity,as a result of better CO₂ activation on Ni/Y₁Zr₉O_x,as testified by CO₂-TPD results.DFT calculations and in-situ FTIR results evidenced that the formation of HCOO*is easier with stronger adsorption energy on Ni/Y₁Zr₉O_x than on Ni/ZrO₂,which even did not be desorbed at high temperature.The activation energy of the rate determining step is effectively reduced on Ni/Y₁Zr₉O_x catalyst.As a result,the Ni/Y₁Zr₉O_x catalyst exhibits higher activity and selectivity for CO₂ methanation.?3?DFT calculations revealed that on the MgO/Ni catalyst the roles of Ni for CO₂methanation are to dissociate H2 and adsorb CHx*species,which is further hydrogenated to CH4.The role of MgO is to adsorb and activate CO₂.The MgO/Ni interface is bifunctional,in which both the Ni and the O^2-sites of MgO is observed to adsorb and activate the key reaction intermediates?COH*,HCOH*?.We proposed the optimal pathway of CO₂ methanation on the MgO/Ni catalyst is CO₂*?COOH*?CO*+O*?COH*?HCOH*?CH*?CH₂*?CH₃*?CH₄*.The rate determining step is the dissociation of COOH*to CO*+OH*with the activation energy of 1.28 eV.?4?In-situ FTIR results demonstrated that on pure MgO surface CO₂ can be effectively adsorbed and activated by forming carbonate or bicarbonate species,but on pure Ni surface CO₂ is hardly adsorbed,which is consistent with the DFT results.Using combined DFT calculations and experimental results,in comparison with pure Ni catalyst,on the MgO/Ni catalyst,the new adsorption sites for CO₂ are provided by MgO and the strong interaction between MgO and Ni is introduced,thus resulting in the high activity and selectivity of the MgO/Ni catalyst for CO₂ methanation.