Key words: mixed oxides, synthesis, water fluid, rare-earth elements, aluminates, catalytic oxidation, oxidative coupling of methane
The effect of the synthesis method of mixed Ln—Al (Ln — La, Ce, Pr) oxide systems with the atomic ratio Ln : Al = 1 : 1 on the formation of their phase composition and catalytic properties in methane oxidation were studied. The precursors were prepared by impregnation of ashless filter paper with mixed solutions of nitrates of the corresponding metals according to the water absorption capacity, followed by drying and combustion of the obtained mass in air. Further processing was carried out by combining calcination at 600 and 900 °C with treatment in water or water-ammonia fluid (WF, WAF) media. The patterns of the amorphous precursor of Pr-Al oxides transformation in the WF and WAF media and during high-temperature synthesis are similar to those obtained for the La—Al system. In both cases, the amorphous precursor in the water-containing fluid transforms into LnAlO3 with a cubic perovskite structure with an admixture of AlO(OH) (boehmite) and basic REE carbonate phases. The subsequent treatment at 900 °C in air leads to the formation of mixture containing aluminates LnAlO3 and free oxides La2O3 or PrO2. Single-phase samples containing only La and Pr aluminates were obtained by heating the amorphous precursors at 900 °C in air. During the treatment in the watercontaining fluid medium of the Ce—Al system, well-crystallized oxide CeO2 instead of aluminate or Ce-containing hydroxides is formed, and the Al-containing component remains X-ray amorphous. CeAlO3 was obtained by treating the mixture of Ce and Al oxide precursors in a hydrogen stream. It has been established that due to the differences in the values of the 4th ionization potential (IP 4) of La, Ce and Pr atoms (49.9, 36.7 and 39.0 eV, respectively), very different synthesis conditions are required for the formation of perovskite-type aluminates of the LnAlO3 formula containing the REE in the (3+) oxidation state. The catalytic properties of the synthesized samples in methane oxidation have been studied. The efficiency and stability of isostructural aluminates LnAlO3 in the oxidative coupling of methane (OCM) decreases in the La > Pr ≫ Ce series. While PrAlO3 is the most active among them, LaAlO3 shows the highest selectivity in the formation of the OCM products (ethane + ethylene).
doi:10.1134/S1990793124030023