The Defect Construction Of Non-noble Metal Oxides And Its Application In The Catalytic Combustion Of Methane

The demand for energy is increasing with the development of human society.In order to reduce the influence of coal bed methane generated during the coal mining process and achieve the efficient utilization of natural gas,we need to make efforts to the efficient conversion of CH₄.Considering the ultra-high thermodynamic stability of CH₄,direct combustion of methane requires high temperature and generates NO_x pollutants.Therefore,low-temperature methane combustion with the assistance of catalysts is an advantageous way.Although precious metal-based catalysts have excellent C-H bond activation capabilities,their high cost forces us to turn our attention to non-precious metal catalysts.This paper aimed at the construction of defects on non-precious metal catalysts,mainly involving MnO_x-based and Co₃O₄-based materials,and the relationship between the structure and performance of the catalyst is explored through various characterizations.Defects-richγ-MnO₂ is prepared from LaMnO₃ by a combination of ball-milling and selective atoms removal.The study found that the ball-milling treatment could reduce the size of the catalyst and produce surface defects,while the lattice defects originate from the removal of La atoms.The loss of La atoms has a series of beneficial effects:higher specific surface area and Mn^4+/Mn³⁺ratio,more surface active oxygen,and higher low-temperature redox performance.Under the condition of WHSV=33,000m L g^-11 h^-1,the catalyst showed excellent methane catalytic combustion performance,with T₅₀ of 398 ^oC and T₉₀ of 465 ^oC,which is better than the previously reportedα-MnO₂.Co₃O₄ catalysts with different exposed crystal facets are synthesized and the sample with the best activity for methane catalytic combustion were screened and treated with N₂ plasma.It is found that for samples with different exposed crystal facets,the order of their catalytic activity is Co₃O₄-111<Co₃O₄-100<Co₃O₄-110.The advantages of（110）facet makes the catalyst have the highest defect content and surface oxygen species.At a space velocity of 33,000 mL g^-11 h^-1,the T₅₀ is 363 ^oC and the T₉₀ is 413 ^oC.Compared with Co₃O₄-110,the sample treated with nitrogen plasma has more oxygen vacancies content and disorder of surface structure.The specific surface area is further enlarged and more micropores structures are generated.N atoms replace more oxygen atoms and reduce the activation energy barrier of C-H bonds in methane.N-Co₃O₄-110 has a T₅₀ of302 ^oC at a space velocity of WHSV=33,000 mL g^-11 h^-1,significantly 63 ^oC lower than that of Co₃O₄-110.N-doping enhances the water resistance of the catalyst,and the conversion rate is reduced by 3%in 10 h,running at 525 ^oC with the presence of water.N-Co₃O₄-110 is one of the best non-noble metal catalysts at present and has good application potential.Hexagonal Co₃O₄ nanosheets were prepared and reduced by Na.The results showed that Na reduction significantly enhanced the oxygen vacancis content of Co₃O₄nanosheets,which lead to higher redox ability.The grinding process enabled the catalyst to obtain a richer mesopore distribution and enhanced its surface area and pore volume.Under the condition of WHSV=33,000 mL g^-11 h^-1,the T₅₀ of the Co₃O₄-NSs-Na decreased by 46 ^oC compared with the Co₃O₄-NSs,which was a significantly improved.