La2Zr2O7催化剂结构相变对甲烷氧化偶联反应性能的影响

采用共沉淀法并通过改变焙烧温度制备了一系列具有不同晶相结构的La₂Zr₂O₇催化剂，在微型固定床反应器上评价其甲烷氧化偶联反应性能，并利用XRD、Raman、CO₂-TPD、XPS等表征手段，探究催化剂的物相结构、表面碱性以及表面氧物种的变化规律。结果表明，随着焙烧温度从700℃逐渐升高到1200℃，La₂Zr₂O₇催化剂结晶度不断提高，晶相发生明显变化，从无定形结构逐渐向缺陷萤石结构过渡，最终转变成烧绿石结构。焙烧温度提高促使La₂Zr₂O₇晶相转变过程中，催化剂表面的碱性强度减弱，中等碱性位数量以及具有催化活性的表面氧物种O₂^2-和O₂^-的相对含量不断减少，致使催化剂的CH₄转化率和C₂₊选择性不断降低。其中，无定形LZO-CP-700催化剂表现出最佳的甲烷氧化偶联反应性能。

Effect of phase transformation of La2Zr2O7 catalysts on catalytic performance for the oxidative coupling of methane

La₂Zr₂O₇ catalyst was prepared using co-precipitation method and then calcined at different temperatures to obtain a series of catalysts with different phase structures. Their catalytic performances for oxidative coupling of methane were evaluated in a fixed bed micro-reactor. Meanwhile, the changes of phase structure, surface base sites and surface oxygen species upon these samples were characterized with XRD, Raman, CO₂-TPD, and XPS. With the increase of the calcination temperature from 700 to 1200℃, the crystallinity of La₂Zr₂O₇ catalysts increased continuously and the crystal phase changed obviously. The structure of the catalysts gradually changed from amorphous to disordered defective fluorite structure, and ultimately to phrochlore structure. Both the number of medium basic sites and the electrophilic oxygen species, such as O₂^2- and O₂^-, on the catalysts decreased with the phase transition caused by the temperature raising, resulting in the decrease of CH₄ conversion and C₂₊ selectivity. The amorphous LZO-CP-700 catalyst showed the best performance in methane oxidation coupling reaction.