Adsorption of methane on zeolite studied by FT-IR spectroscopy

摘要

The activation of methane is regarded as one of the challenges in catalysis and chemistry. Much effort has been made for the selective oxidation and oxidative coupling of methane in catalysis; however, at fundamental level the activation of methane oncatalyst surface is still poorly understood. Experimentally, it is difficult to get insights into the interaction mechanism between methane and catalyst because methane is chemically most a stable molecule which cannot be easily captured on the surfaceof catalyst. To explore the interaction and activation of methane on catalyst, a quartz-made IR cell combined with a reaction system was made specially for IR study in a wide temperature range from 130 to 1 000 K. In our previous work the adsorptions ofmethane on CeO_2 and MgO at 173 K were studied by FT-IR spectroscopy. It was found that several types of adsorbed methane could be formed on these catalysts and the formation of adsorbed methane strongly depends on the kind and the surface state of catalyst. Weakly adsorbed methane gives two IR peaks of v_3 and v_4 which are similar to those of gas phase except for a slight red shift in frequency. But for strongly adsorbed methane, not only the v_3 and v_4 peaks were observed but also the infrared-forbidden vibration of v_1 appeared and shifted to lower frequency. In the present study, methane adsorption on zeolites has been investigated with an intention to understand the interaction of methane with an acidic surface. This note shows that methane interacts strongly with surface oxygen anion, hydroxyls and Lewis acidic sites in forming the adsorbed state.