乙烯和乙烷在CeO_2上温和条件下表面氧化反应的原位FT-IR研究

SURFACE OXIDATION OF ETHYLENE AND ETHANE ON CERIUM OXIDE AT MILD TEMPERATURES STUDIED BY FT-IR SPECTROSCOPY

本文用原位FT-IR研究了CeO_2上O_2、C_2H_4和C_2H_6的吸附以及C_2H_4和C_2H_6的氧化反应。室温下吸附O_2后在CeO_2表面上形成超氧物种(O_2^-),吸附C_2H_4后在990cm^(-1)处检测到红外峰,归属为吸附C_2H_4与表面晶格氧形成的C-O键的振动频率。这些吸附物种在高于100℃时消失。室温下C_2H_6在CeO_2表面上未形成吸附态。C_2H_4在CeO_2表面上60℃即可发生氧化反应。在300℃以下,C_2H_4氧化后表面上主要形成甲酸根物种。当高于300℃时表面上主要是深度氧化的产物碳酸基物种。C_2H_6在CeO_2表面上200℃始被氧化为甲酸根物种,高于300℃时,表面上也主要形成碳酸基物种。在气相O_2存在下也得到与上述相同的结果。讨论了C_2H_4和C_2H_6的表面氧化机理以及表面氧物种的作用,认为在低于300℃时,参与C_2H_4和C_2H_6氧化的氧物种主要是表面晶格氧,超氧吸附物种可能在高于300℃时才参与反应。

The surface oxidation of ethylene and ethane on cerium oxide at mild temperatures has been studied in situ using Fourier-transform in-frared(FT-IR)spectroscopy.Cerium oxide used in this study was prepared by decomposing cerium hydroxide gel at 500 ℃. The sample was pressed into a self-supporting wafer and was out gassed in a quartz i. r. cell at 700 ℃ in order to remove the surface contaminant species prior to the adsorption or reaction experiment. The surface oxidation of ethylene takes place even at 60℃ on cerium oxide either in the presence of or in the absence of gaseous O2, and the reaction becomes more pronounced with the elevation of temperature. The major surface species derived from the reaction was formate having i. r. bands at 2933, 2842, 1544, 1371, 1355cm-1, in addition, weak i. r. bands due to acetate ( 1563, 1450 cm-1) and carbonate (1454, 1355, 854 cm-1) species were also detected. Ethane oxidation on cerium oxide is initiated at 200 ℃ independent of the presence of gaseous oxygen, and the surface species formed from the ethane oxidation were almost the same as those formed from ethylene oxidation at temperatures below 300℃. Both the reactions at 300℃. produce mainly formate and carbonate species on cerium oxide, and the carbonate species became dominant at temperatures higher than 300 ℃. Adsorbed superoxide species (O2-,1126cm-1) were definitely identified on cerium oxide at temperatures below 100℃ when gaseous oxygen was present. However, isotopic experiment confirmed that the super-oxide was not active enough to react with ethylene or ethane at temperatures lower than 200℃. The adsorbed ethylene species giving an i.r. band at 990 cm-1 was supposed to be a precursor of oxidized products of ethylene, such as formate and acetate species. But no i. r. band due to adsorbed ethane species was observed after introducing ethane to the cerium oxide surface at room temperature. It was concluded that the principal oxygen species paticipating in the oxidation of both ethylene and ethane at temperatures below 300℃ were surface lattice oxygen ions which are coordinatively unsaturated oxygen species created by the outgassing at high temperatures.