Synthesis And Performance Of Highly Efficient And Stable Nickel-based Composite Oxide Catalysts For Methane Catalytic Combustion

As a clean energy source,natural gas has been widely applied in gas turbine,gas power plants and natural gas vehicles（NGVs）because of its high energy efficiency and low pollutant emissions.However,the associated release of unburnt methane could cause serious environmental problems since it is a powerful greenhouse gas with a high global warming potential.Catalytic combustion of methane has been recognized as a promising way to reduce methane emissions in industrial processes.Therefore,it is of great significance to develop efficient and stable catalysts with low cost,abundant reserves and practical application value.Transition metal-based catalysts（e.g.,Mn O_x,Co₃O₄,and Ni O）with abundant oxygen species,showing good catalytic performance for methane combustion.In particular,the active Ni²⁺and oxygen species of Ni O catalyst can effectively activate the C-H bond of methane,but the low-temperature activity and stability still need to be improved.In this paper,based on the understanding of the defect structure of nickel oxide,the distribution of active nickel species and oxygen species in nickel oxide catalysts were adjusted by introducing different electronic structures metal and silicon elements to design and synthesize nickel-based catalysts for methane combustion with excellent structural performance,high efficiency and stability.The major research works manifested in the following several aspects:1.Ni_0.9M_0.1O_2-δ（M=Cu,Mg,Sm,Ce,Nb）composite oxide catalysts were synthesized via co-precipitation method,taking nickel nitrate as nickel source,PEG-400as dispersant,sodium carbonate as precipitant,and copper,magnesium,samarium,cerium and niobium salts were used as precursors,respectively.The effects of different electronic structures metal doping on the structural properties of Ni O were investigated.The differences in the distribution and quantity of Ni²⁺and active oxygen species（O^-）in metal-doped catalysts with different electronic structures were investigated.The performance of the catalysts for methane combustion were evaluated.The results revealed that compared with the undoped Ni O,Ni_0.9M_0.1O_2-δcatalysts provided larger specific surface area,better methane oxidation activity and more excellent sintering-resistance.It was found that the Ni_0.9Ce_0.1O_2-δcatalyst doped with high valence cerium ions could inhibit the conversion of Ni²⁺to Ni³⁺and increase the number of non-stoichiometric oxygen species（O^-）.More Ni²⁺and O^-in the catalysts could activate the C–H bond of methane more effectively.The Ni_0.9Ce_0.1O_2-δcatalyst shown high activity for methane combustion,and its T₉₀was 375℃.2.On the basis of the research in the previous chapter,（i）Zr O₂demonstrated distinctive acid–base properties.（ii）The ionic radii of Zr ions and Ni ions are similar.The introduction of high valence zirconium into nickel oxide to adjust the physical and chemical properties of the catalysts.The effects of Zr doping amount and calcination temperature on the structure and catalytic activity of Ni O catalysts were investigated.The relationship between surface acidity,redox property,active nickel species and nonstoichiometric oxygen species of Ni_1-xZr_xO_2-δcatalysts and catalytic performance were revealed.The water resistance,thermal stability and thermodynamic equilibrium conversion（CO₂selectivity）of the catalysts were studied.The reaction mechanism of methane combustion over Ni_1-xZr_xO_2-δcatalyst was described.The results revealed that the Ni_0.89Zr_0.11O_2-δnanocatalyst exhibited higher specific surface area(142 m²g^-1)and smaller grain size（4–5 nm）,which were conducive to the exposure of active sites and improve the mobility of active oxygen species.In addition,Zr⁴⁺doping promoted the conversion of Ni³⁺to Ni²⁺,resulting in abundant active Ni²⁺sites and oxygen vacancies,and more surface acid-base sites,which thus promoted methane adsorption,dissociation and intermediate product transformation.Consequently,Ni_0.89Zr_0.11O_2-δcatalyst exhibited an excellent low-temperature activity(T₉₀=380℃),high CO₂selectivity,and superior catalytic stability during the reaction.3.Taking advantage of the excellent thermal stability of silicon oxides at high temperature,silicon was introduced into nickel oxide to inhibit the sintering behavior of nickel oxide-based catalysts at high temperature.Based on the above,x Si-Ni catalysts were synthesized using tetraethyl orthosilicate as silicon source precursor and triethylamine as precipitant.The effects of silicon doping on the structure and texture properties of Ni O were investigated.The performance of x Si-Ni catalysts for methane combustion reaction were studied.The relationship between structure and activity of catalysts were revealed.The results revealed that triethylamine as a precipitant could provide OH^-for homogeneous precipitation through the dissociation of amine,which endowed catalysts with small grain size and high specific surface area.It was found that the Si-modified Ni O catalysts had abundant active Ni²⁺sites and oxygen species,thus exhibiting excellent low temperature activity for methane combustion.The interaction between Ni and Si could effectively inhibit the migration,aggregation and sintering of Ni O particles during methane combustion at high temperature,thus bringing forth excellent thermal stability of the catalyst.Among them,10Si-Ni catalyst performed the best performance,giving its T₉₀at 395℃.The catalysts still showed better catalytic activity after reaction at 800℃.