Effect of phase transformation of La2Zr2O7 catalysts on catalytic performance for the oxidative coupling of methane
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摘要: 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O22-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。Abstract: 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 O22- 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.
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Key words:
- La2Zr2O7 /
- structural change /
- surface base /
- oxygen species /
- oxidative coupling of methane
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图 1 催化剂的SEM-mapping元素分布
Figure 1 SEM-mapping element distribution of the La2Zr2O7 catalysts
(a): LZO-CP-700; (b): LZO-CP-800; (c): LZO-CP-1000; (d): LZO-CP-1200
图 7 催化剂的反应性能
Figure 7 Reaction performances of the La2Zr2O7 catalysts
(a): oxygen conversion; (b): methane conversion; (c): C2+ selectivity; (d): C2+ yield reaction conditions: 0.4g, CH4:O2:N2=3:1:1, GHSV=20000 mL/(h ·gcat)
图 8 催化剂表面氧物种相对含量与750 ℃时C2+烃选择性之间的关系
Figure 8 Relationship between the relative amount of oxygen species on the catalyst surface and the C2+ selectivity at 750 ℃
图 9 催化剂焙烧温度与750 ℃下C2+烃选择性之间的关系
Figure 9 Relationship between the calcination and the C2+ selectivity at 750 ℃
图 10 催化剂LZO-CP-700在750 ℃反应条件下的稳定性能
Figure 10 Long-term stability test of LZO-CP-700 catalyst at 750 ℃
图 11 催化剂LZO-CP-700的XRD谱图(a)和拉曼光谱谱图(b)
Figure 11 XRD patterns (a) and Raman spectra (b) of the LZO-CP-700 catalysts
表 1 La2Zr2O7催化剂的BET表征
Table 1 BET results of the La2Zr2O7 catalysts
Sample SBET /(m2·g-1) LZO-CP-700 2.3 LZO-CP-800 1.8 LZO-CP-1000 1.5 LZO-CP-1200 0.7 
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表 2 催化剂CO2-TPD的量化
Table 2 Quantified CO2-TPD results of the La2Zr2O7 catalysts
Sample Weak basic sites amount /(μmol·g-1) Intermediate basic sites amount /(μmol·g-1) Total basic sites amount/(μmol·g-1) LZO-CP-700 47 251 298 LZO-CP-800 30 194 224 LZO-CP-900 18 113 131 LZO-CP-1000 15 48 63 LZO-CP-1100 12 15 27 LZO-CP-1200 6 6 12
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表 3 La2Zr2O7催化剂的XPS拟合
Table 3 XPS O 1s peaks fitting of the La2Zr2O7 catalysts
Sample Relative amount /%a (O2-+ O22-)/O2- O2- CO32- O22- O2- LZO-CP-700 11.1 22.4 37.4 29.0 1.67 LZO-CP-800 25.4 24.9 18.8 30.9 1.44 LZO-CP-1000 26.0 28.0 15.3 30.7 1.35 LZO-CP-1200 7.9 24.5 19.0 48.5 0.55 a: percentage of oxygen species on the surface of the catalyst
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表 4 La2Zr2O7催化剂的O2-TPD定量
Table 4 Quantified O2-TPD results of the La2Zr2O7 catalysts
Sample Oxygen amount of 100-200 ℃ Oxygen amount of 200-400 ℃ Oxygen amount of above 400 ℃ Total oxygen amount /(a.u.) LZO-CP-700 2 75 23 100 LZO-CP-800 1 36 20 57 LZO-CP-1000 1 15 14 30 LZO-CP-1200 1 10 0 11 note: relative value of total oxygen species on the surface of LZO-CP-700 catalyst is 100 for calculation and comparison
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