The Influence Of Synergistic Effect Between Rare Earth And Transition Metal Based Promoters And Ni On The Mechanism Of Dry Reforming Of Methane

CO₂ reforming of CH₄,i.e.,the dry reforming of methane(DRM),can not only recycle the greenhouse gas of CH₄ and CO₂,but also produce the syngas(CO and H2)for the downstream industrial synthesis.Thus,the study on DRM is important in both fundamental science and potential industrial applications.Since CH₄ and CO₂ are very stable small gas molecules,their reaction should be activated by a highly efficient catalyst.In this aspect,3d transition metal Ni has attracted much attention for DRM due to the high activity and low price.The problem for Ni is that it can be deactivated rapidly by the carbon deposition during the reaction,which limits its application in industry.Thus,it is necessary to find effective methods to keep the activity of Ni,while at the same time to reduce the carbon deposition.Experimental studies show that rare earth and transition metal based promoters can improve the activity of Ni and reduce the carbon deposition.However,the performance of promoters depends highly on their structures and the proposed reaction mechanisms are also inconsistent.Considering the small size and flexible structures of the promoters,it is difficult to directly observe the reaction mechanism from the experiments.Thus,in this thesis,by employing the density functional method and microkinetic model,the reaction mechanism of DRM is investigated by modifying Ni(111)surface through rare earth and transition metal based promoters.Based on the analysis of reaction mechanism,the influence of the promoter's structure and composition on the activity and selectivity in DRM is presented.The obtained results are as follows:(1)The influence of La and La₂O₃ promoters on the reaction mechanism is investigated.The results indicated that after the promoters are introduced,the adsorption energies of the reactants and intermediates are strengthened obviously,particularly on La₂O₃ modified Ni(111)surface.The strong adsorption of CO₂ on La₂O₃ modified Ni(111)surface results in a dramatic structure change of La₂O₃,and the complex intermediate CO₂(La2O₂-O)is formed.It is this complex intermediate that refrains the coke formation.The reaction mechanism is CH₄+CO₂+La₂O₃?CH+CO₂(La2O₂-O)+3H?CHO+CO₂(La2O₂)+3H?2CO+2H2+La₂O₃.The rate determining step is CH₄?CH3+H.For La modified Ni(111),its reaction mechanism is the same as pure Ni.Microkinetic analysis revealed that in the temperature range of 500-700?,the reaction rate is high for promoter modified Ni(111),compared with pure Ni(111).The study on the coverage of the intermediates suggested that the main intermediate is CH for La modified Ni,while it is CH3 for La₂O₃ modified Ni.La₂O₃ modified Ni catalyst is sensitive to the change of the temperatures.(2)CO₂ activation mechanisms on RE₂O₃/Ni(111)(RE=Ce,Pr,Nd,Sm,Eu and Gd)in dry reforming of methane are investigated by first principles calculations.Our results indicated that RE₂O₃ promoters enhance CO₂ adsorption and activation on Ni(111).The activity follows trend of Ce₂O₃/Ni>Eu₂O₃/Ni>Sm₂O₃/Ni?Gd₂O₃/Ni?Pr2o3/Ni>Nd₂O₃/Ni.For Ce₂O₃/Ni,Eu₂O₃/Ni and Sm₂O₃/Ni,direct activation is favorable,while H induced activation is favorable on Pr₂O₃/Ni.For Nd₂O₃/Ni and Gd₂O₃/Ni,both direct activation and H induced activation are competitive with the former slightly favored.Moreover,O atoms from both CO₂ and Ce₂O₃ or Gd₂O₃ show high activity for CH,which is different from the other RE₂O₃.(3)The DRM on the Mn doped(Mn-Ni)and Mn adsorbed Ni(111)surface(Mn@Ni)has been investigated by using the density functional method.The results indicated that the doping Mn structure Mn-Ni does not change the reaction mechanism for DRM,i.e.,it has the same mechanism as pure Ni:CH₄+CO₂?CH+O+CO+3H?CHO+CO+ 3H?2CO+2H2.The rate determining step is CH+O?CHO.For adsorption structure Mn@Ni,however,the order of CH₄ dissociation is changed,i.e.,the energy brrier for the dehydrogenation of CH₄ is lower than that for the dehydrogenation of CH3 and CH2.Meanwhile,the main intermediate is changed from CH to CH2.The most favorable reaction pathway is CH₄+CO₂?CH2+O+CO+2H?CH2O+CO+2H?CHO+CO+3H?2CO+2H2.The rate determining step is CO₂ CO+O.The resistant for the coke formation is enhaced due to the decrease of CH.Our study also showed that the reaction temperature can be decreased by increasing the CO₂/CH₄ ratios.