关键词: 钙钛矿, 表面结构, Au纳米颗粒, CO氧化, 酸刻蚀

Abstract:

Perovskite is widely used as catalyst supports because of its flexible composition, good redox performance, and excellent thermal stability. However, the use of perovskite oxides as catalyst supports has two disadvantages: low surface area due to synthesizing the perovskite structure at high temperatures, and native perovskite surfaces preferentially have A-sites instead of catalytically active sites. On the other hand, interaction between the support and metal affects the size and valence state of noble metals. Therefore, perovskite oxides with different structures were prepared and were used to support Au catalysts, in order to obtain excellent catalytic activity and high stability. Specifically, stoichiometric LaMnO₃ and nonstoichiometric LaMn_1.2O₃ perovskites were prepared by the ethylene glycol sol-gel method, and then the LaMnO₃-AE oxide was prepared by treating LaMnO₃ perovskite with dilute nitric acid. The perovskite-supported Au catalyst was prepared by the deposition precipitation method and its catalytic activity for CO oxidation was evaluated. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and H₂ temperature-programmed reduction (H₂-TPR), it was found that LaMnO₃ and LaMn_1.2O₃ perovskite carriers were beneficial for the dispersion of Au; however, the Au nanoparticle size significantly increased with increasing calcination temperature, indicating poor Au thermal stability. In contrast, LaMnO₃ perovskite (LaMnO₃-AE) etched by nitric acid is not conducive to dispersion of Au, but it is beneficial for improving the thermal stability of Au. Au was always maintained in the zero-valence state after calcination at different temperature. H₂-TPR results revealed that the reducibility of the catalysts changed largely after thermal treatment at high temperatures, and was mainly influenced by the agglomeration of Au nanoparticles. Although the reducibility of the Au/LaMnO₃-AE catalyst calcined at 250 ℃ is lower than that of Au/LaMn_1.2O₃ and Au/LaMnO₃ catalysts calcined at the same temperature, the former exhibited higher reducibility when the catalyst was calcined at high temperatures (500 and 900 ℃). In the CO oxidation reaction, the catalytic activity of all the prepared catalysts decreased when the calcination temperature was increased from 250 to 500, 700, and 900 ℃. However, the catalytic activity of the Au/LaMn_1.2O₃ catalyst was significantly higher than those of LaMnO₃- and LaMnO₃-AE-supported Au catalyst, when calcination temperature was below 500 ℃, while the activity of the Au/LaMnO₃-AE catalyst was significantly higher than those of the Au/LaMnO₃ and Au/LaMn_1.2O₃ catalysts when the calcination temperature was more than 700 ℃. As shown in characterization results, after the catalyst was calcined at high temperatures (700 and 900 ℃), the Au nanoparticle size on the Au/LaMnO₃-AE catalyst was lower than those on Au/LaMnO₃ and Au/LaMn_1.2O₃ catalysts, leading to high reducibility and catalytic activity of the Au/LaMnO₃-AE catalyst. The Au/LaMnO₃-AE catalyst also exhibited high stability in CO oxidation. The catalytic activity of the Au/LaMnO₃-AE catalyst can be maintained for 20 h at 130 ℃.

Key words: Perovskite, Surface structure, Au nanoparticle, CO oxidation, Acid etching