Preparation Of Cu-,Ce-,Co-Doped Nickel Oxide Catalysts And Their Performance In Methane Combustion Reaction

The sulfides and nitrogen oxides produced by the combustion of coal and oil can cause serious environmental pollution.Natural gas has gradually become an alternative energy due to its low price,cleanliness,abundant reserves and high thermal efficiency,which can effectively alleviate the problem of energy shortage.The main component of natural gas is methane,and the greenhouse effect of unburned methane emitted into the air is 21 times greater than that of CO₂.Catalytic combustion is the most effective way to deal with incompletely converted methane.Although the most widely studied noble metal-based catalysts demonstrate excellent activity at low temperature,their high cost and the sintering behavior at high temperature restrict their practical application.Therefore,more researches have been devoted to developing metal oxides with lower cost and better thermal stability in recent years.The p-type semiconductor nickel oxide tends to form defects such as cation vacancies and electron holes,and it is established that nickel has good catalytic activity for methane oxidation at temperature below 500℃,but their low-temperature activity is not ideal.Based on the above characteristics of nickel oxide,exploring effective methods to further optimize its structure and properties is expected to design and synthesize nickel oxide-based catalysts with excellent structural properties,good low-temperature activity,and high stability for methane combustion.In this paper,taking the NiO system as the research object,Cu,Ce and Co elements with d electronic structure are respectively doped into the NiO lattice by co-precipitation method to adjust the types and the distribution of active nickel species and active oxygen species in nickel oxide.In this way,high efficient and stable NiO-based mixed oxide catalysts for methane oxidation will be fabricated.The difference in the influences of the introduction of different metals on the structure properties of NiO was investigated.Besides,The effects of different metals on the number and types of reactive oxygen species in the NiO lattice were studied,and the effects of interaction of added metal with Ni,redox properties and surface chemical composition of composite oxide catalysts on catalytic combustion performance of methane were explored.The main research contents and results are as follows:（1）Taking advantage of the similarity in electronegativity and ionic radius of Cu and Ni,and the feature of that Cu can easily insert into the NiO lattice,NiCucomposite oxide catalysts were synthesized by co-precipitation method and their catalytic performance for methane combustion was studied.The effects of the interaction between Ni and Cu,and the resulting NiCuO_xsolid solution on surface chemical composition,surface acid-base performance,redox performance and catalytic activity of catalysts were explored.Results showed that the formation of NiCuO_xsolid solution weakened the strength of Ni-O bonds.In addition,the number of unsaturated coordination Ni atoms and the molar ratio of Ni³⁺/Ni²⁺was increased,which could be due to the partial substitution of Ni²⁺by Cu²⁺in the NiO matrix.On the other hand,the incorporation of Cu gave richer lattice defects of NiO,leading to more oxygen vacancies,which helped to adsorb more reactive oxygen species and thus endowed the catalyst with enhanced methane oxidation.Among them,the 9Ni1Cu catalyst demonstrated the larger number of acid-base sites and higher oxygen mobility,giving better catalytic activity(T₉₉=430℃).（2）NiCe composite oxide catalysts were synthesized by introducing Ce into NiO system,and their structural properties and performance in methane combustion were investigated.The catalyst 9Ni1Ce provided the highest activity(T₉₉=415℃),which was75℃ lower than that of pure NiO.Appropriate Ni/Ce molar ratio could adjust the particle size of CeO₂on the catalyst surface.The interaction of Ni with Ce weakened the Ce-O binding strength and generated more Ce³⁺species,resulting in more surface oxygen vacancies.Furthermore,the addition of Ce considerably not only reduced the temperature of O-release,but also improved the mobility of surface oxygen species.The methane conversion on 9Ni1Ce just fluctuated within 5%in the 50-hour stability test at both T₅₀（365℃）and T₉₀（400℃）,indicating its superior activity stability.（3）Making use of the characteristics of that Co possesses similar electronegativity and ionic radius with Ni,Co₃O₄owns weaker metal-O bond strength,both nickel and cobalt oxides have lattice defects,and they can enter into each other’s lattice structures and substitute some lattice position,NiCo composite oxide catalysts with different Ni/Co molar ratios were synthesized by co-precipitation method.Results exhibited that NiCo composite oxides afforded higher activity towards methane combustion as compared to single NiO and Co₃O₄.Appropriate Ni/Co molar ratio was conducive to the formation of NiCo₂O₄structures that were favorable for methane combustion.The interaction of Ni and Co weakened the Co-O bond,generating more active oxygen species,lowering the temperature of oxygen release,and improving the reduction properties of NiCo composite oxides.The sample 5Ni5Co showed the most excellent activity in methane combustion(T₉₉=390℃),and excellent long-term stability in the 50-hour stability test under both wet and dry conditions.