The Nd-modified Nickel-based Catalysts And Catalytic Performance Study On CO₂ Reforming Of Methane

Dry reforming of methane（DRM）is a promising way to reduce CO₂ emission and moderate greenhouse effect.This technology can produce syngas（H₂+CO）in a suitable ratio,which can be used for Fischer-Tropsch synthesis to produce long chain hydrocarbons and oxygen-containing chemicals with industrial value.Supported nickel-based catalysts have attracted much attention in the field of DRM research due to their high activity,easy availability source and low price.However,nickel-based catalysts are prone to sintering and carbon deposition in DRM reaction,which is the main reason hindering their commercial application.Therefore,to improve the sintering-resistance and carbon-resistance of catalysts and to develop efficient as well as stable nickel-based catalysts is the direction to which the majority of researchers are committed.Due to the rare earth element Neodymium（Nd）has the characteristics of active chemical properties,easy water absorption and carbon dioxide,this paper aims to synthesize a variety of neodymium modified nickel-based catalysts for DRM reaction with high efficiency and stability.By adjusting the catalyst structure and carrier type,the physical and chemical properties of each catalyst and the difference of catalytic DRM activity were studied in detail.The influence of catalyst surface structure,acid and alkali on DRM activity and stability was studied as follows:（1）The Ni/Nd composite oxide catalysts were synthesized by co-precipitation method.The compositions were Ni/Nd=1:9,3:7,5:5,7:3 and 9:1.The phase of nickel in the catalyst can be controlled by adjusting Nd and Ni loading.When the Ni/Nd mole ratio is 3/7～7/3,Ni Nd₂O₄ mainly exists on the surface of catalyst.When the Ni/Nd mole ratio exceeds this range,the surface of Ni species is covered with thick amorphous Nd₂O₃.DRM activity tests at 550-800°C and 750°C showed that the amount of carbon deposition increased with the increase of nickel loading,but the resulting carbon deposition did not block the porous structure of Ni₅Nd₅ and Ni₇Nd₃ catalysts after the reaction.These two catalysts have the highest activity（46.2%,42.4%）and the most stable catalytic performance（methane decay rate:0.24%/h,0.15%/h）.The activity in DRM of the catalysts is closely related to the specific surface area,grain size and structural reconstruction of the catalysts.（2）Neodymium modified hydrotalcite derived nickel-based catalyst（Nix Nd Mg Al O）with Nd content（x）in the range of 2.5-10%was synthesized by co-precipitation method.The sequence of CH₄ initial conversion was Ni7.5Nd Mg Al O>Ni5Nd Mg Al O>Ni Mg Al O>Ni2.5Nd Mg Al O>Ni10Nd Mg Al O.When the Nd content is 7.5 wt%,Ni7.5Nd Mg Al O catalyst has the smallest nickel crystal size（8.1 nm）,the highest metal dispersion（11.8%）,and the largest number of surface active sites（6.3 m²/g）.In addition,Ni7.5Ndg Al O（736.6μmol/g-cat）with the largest CO₂ desorption capacity can effectively oxidize carbon deposition on the catalyst surface.The catalyst shows a good carbon resistance（2.4%）,and the conversion of CH₄（55.8%）and CO₂（59.8%）are the highest in DRM reaction.The kinetic results show that the CO₂ activation energy of Ni Mg Al O catalyst is 15%lower than that of the Ni Mg Al O catalyst with Nd content is 7.5 wt%.When the Nd content is too high,such as in Ni10Nd Mg Al O,there will be a large nickel grain size（10.5 nm）,and the nickel particles will be wrapped by a large amount of carbon deposition during the reaction process,resulting in obvious deactivation of the catalyst.（3）The Ni-x Nd/Si O₂ catalysts supported by Nd-modified mesoporous silica were prepared by one-pot method.The content of neodymium in the catalyst affects the structure,metal-support interaction and oxygen storage/release capacity of the catalyst,as well as the activity of the catalyst and the type of carbon deposition produced.The structure of Nd-modified catalyst consists of two parts.Firstly,the mixture of Nd₂O₃ and Si O₂ is located at the center of catalyst particles.Nickel and nickel phyllosilicate are at the periphery of the catalyst particles.Ni-2Nd/Si O₂ catalysts with 5.25 wt%Nd doping exhibits the strongest metal-support interaction and oxygen storage/release capacity,and has the highest CH₄ and CO₂ conversion.After 24 h reaction,Ni-2Nd/Si O₂ catalyst has the smallest increment in Ni particle size and the lowest amount of inert carbon deposition,thus it shows the best resistance to carbon deposition and sintering.However,for other catalysts with Nd content,larger Ni particle size and more inert carbon deposition lead to the deactivation of these catalysts.The differences in catalytic activity of these catalysts are mainly due to their different metal-support interactions and oxygen storage/release abilities.（4）A series of Ni-based catalysts（Ni-x Nd/Si O₂-AE）supported by neodymium modified silica with low nickel loading（3-4 wt%）were prepared by ammonia evaporation method using silica sol as silicon source.The catalysts were applied to the dry reforming of methane with severe reaction conditions（the flow ratio of CH₄/CO₂ is 1.2:1）.Adding Nd₂O₃ to the catalyst can improve the activity and stability of the catalyst.In addition,the Nd content in the catalyst affects the physical and chemical properties of the catalyst and the DRM activity.When the actual content of Nd is 2.52 wt%,that is,the Ni-2Nd/Si O₂-AE catalyst shows the highest number of alkaline sites and active species content,meanwhile,the DRM performance is the best.The initial CH₄ conversion rate is 63%,and only decreases 6%after 24 hours.The vatiation of specific surface area and pore structure for Ni-2Nd/Si O₂-AE catalyst is the least,and the metal dispersion does not change significantly.The above results indicated that when the Nd content is optimal,the surface deformation of the catalyst was less during the reaction,the active metal site are not easily covered or sintered,and the catalyst exhibits a good resistance to carbon deposition.