Study On The Active Species And Structure-activity Relationship Of Co₃O₄-based Catalyst For Methane Combustion

The catalytic combustion of methane not only increases energy efficiency,avoids energy wastage,but also solves the environmental pollution problem caused by the traditional combustion of methane.The key to the catalytic combustion of methane is preparation the catalysts with high activity and thermal stability.Co₃O₄ with spinel structure is one of the most highly active transition metal oxides.It is well known that Co belongs to the variable valence element,and Co₃O₄ catalyst surface has two main types of Co ions,namely Co²⁺ and Co³⁺.According to the literature,Co²⁺ occupies tetrahedral sites,while Co³⁺ ion occupies octahedral sites.However,the correlation between the surface cobalt valence states and the catalytic activity remains controversial.In our work,several series of cobalt-based catalysts were prepared for methane catalytic combustion,the effects of preparation conditions and promoter modification on the catalytic performances were investigated.These samples were characterized using XRD?XRF?BET?SEM?TEM?XPS?Raman and H₂-TPR technologies.The structure-activity relationship and the active sites for methane combustion were discussed in depth.?1?A series of Co₃O₄ catalysts were prepared by a facile precipitation method just changing the aging time and tested for methane combustion.It was found that the activity for the reaction increased firstly and then decreased with increasing aging time in the form of a volcano curve.The Co₃O₄ aged for 8 h?Co₃O₄-8?exhibited the best catalytic performance with the specific reaction rate?Rs?of 25.91 nmol·s-1m-2 at 340 ?,which was 29.5 times than Co₃O₄-96 sample,although the Co₃O₄-8 catalyst showed the minimum BET surface area and the largest particle size.The XPS and Raman results indicated that the C03O4-8 catalyst possessed the highest ratio of ATetrahearal/Aoctahedral at the surface of the catalyst.H₂-TPR and in situ XRD results also confirmed the Co₃O₄-8 catalyst behaved with excellent high-temperature reduction ability.In combination with the activity performance,the C03O4-8 catalyst had the best performance of methane combustion due to abundant active tetrahedral Co²⁺cationic species.The long-term stability tests demonstrated that the step of aging in the process of preparation can improve water tolerance of Co304 catalyst for methane combustion.?2?In order to prepare the Co₃O₄ catalysts with excellent performance for methane catalytic combustion,the preparation parameters were ajusted and then the influence of the preparation parameters on microstructure of Co₃O₄ catalyst were examined.It was found that the preparation temperature,pH values at titration end point and the stirring time after precipitation have significant influence on microstructure of Co304 catalyst and catalytic activity.According to the regulation of preparation parameters,we determine the optimization of conditions is 30?,pH=9.0 and without further stirring after precipitation for preparation Co304 catalysts.This method saved time,manpower and material resources and could improve the feasibility of industrial application.The catalytic activity and long-term stability under vapor conditions of Co304 catalyst prepared by the improved method are comparable to that of Co₃O₄-8 catalyst.?3?A series of MO₂-Co₃O₄?M=Ce?Zr?Mn?Sn?mixed oxides were prepared by the improved method.It was found that catalytic activity of these catalysts followed the order of Zr02-Co304>Ce02-Co304>Co₃O₄>MnO₂-Co₃O₄>SnO₂-CO3O4.In addition,the doping of other element has slightly influence on the microstructure and the concentration of surface tetrahedron coordination Co²⁺ ions,but it can effectively increase the specific surface area and reduce the crystalline size of the catalyst.The results of XPS showed that although the ratio of ATetranedral/AOctahedrai for the catalysts doped Ce,Zr,Mn and Sn was not significant,but the ratio of the surface adsorption oxygen to the lattice oxygen?Oads/OLatt?was significantly different.Combined with the catalytic activity,we infered that the concentration of surface tetrahedral sites Co²⁺ and the surface active oxygen species can promote the catalytic activity of methane combustion.The concentration of surface Co²⁺ and oxygen species in the composite oxides of CeO₂-Co₃O₄ and ZrO₂-Co₃O₄ were higher than that of pure Co₃O₄,thus exihibited excellent catalytic activity for methane combustion.And for MnO₂-Co₃O₄,SnO₂-Co₃O₄ composite oxide,though the concentration of surface Co²⁺ were similar to that of pure Co₃O₄,but its surface active oxygen species decreased significantly,as a result,the doping of Mn and Sn could not improve the catalytic activity for methane cobustion.?4?In order to further improve the concentration of surface tetrahedral sites Co²⁺ and active oxygen species and obtain higher catalytic performance for methane combustion,the Zr02?x?-Co304 composite oxide catalyst of different ZrO₂ content was prepared.The microstructure of the Zr02?x?-Co304 catalyst was investigated by various characterization methods.XPS and Raman analyses show that the substitute of Zr species for Co³⁺ at octahedral sites in Co₃O₄ spinel structure with small amount can increase Co²⁺ concentration.For the catalysts with Zr02 content higher than 2%,considering that the octahedral sites are fully occupied by Zr4+,the adding Zr element will then occupy the tetrahedral sites,thus the concentration of surface Co²⁺ decreased.Moreover,the content of Zr02 has significant influence on the surface active oxygen species?Oads?OLatt)of Zr02-Co₃O₄ catalyst.This is directly related to the catalytic performance of catalyst.The activity for methane combustion of ZrO₂?x?-Co₃O₄ catalysts increases with Co²⁺ concentration and the surface ratios of Oads/Olatt.ZrO₂-Co₃O₄ composite oxide with 2%Zro2 performs best in the lean methane combustion with T90 of 335? and it might be a potential non-noble catalyst in practical application.The characterization results suggest that the excellent performance of Zr02?x?-Co3o4 catalyst in the lean methane combustion is related to the surface area,the ratios of Oads/olatt and ATetrahedral/Aoctahedral.Furthermore,these catalysts possessed superior stability;and no deactivation phenomenon was observed during the catalytic combustion of methane for 60 h.Therefore,the role of surface oxygen species and the concentration of curface Co²⁺ ion in methane catalytic combustion were further clarified by means of analysis the structure-activity relationship of catalysts.