Infrared Spectroscopic Studies of Adsorbed Methane on Oxide Surfaces at Low Temperatures
Methane activation has long been a challenging task in catalysis. There has been much theoretical and experimental work done for methane activation on metal and oxide surfaces,1–5 and especially significant achievements have been made in oxidative coupling of methane6–8 since 1982. But at a fundamental level, the mechanism of methane activation including how methane interacts with surface and how the C-H bond cleaves, is still not well understood. It is difficult to obtain experimental information on methane activation at catalyst surfaces because the surface reaction of methane normally takes place at high temperatures where methane collides on the surface and subsequently, surface-generated radical fragments or methane itself leave the surface instantaneously. For example, a general conclusion from methane oxidative coupling is that the reaction involves generation of methyl radicals at the catalyst surface which is then followed by radical coupling in the gas phase at high temperatures.9,10 From the catalysis principle, the adsorption of methane on catalyst is a necessary step for methane activation, although the residence time of methane at the surface is very short at high temperatures.11 However, methane is the most inert molecule among hydrocarbons and it is difficult to adsorb on catalyst surface even at room temperature. An expedient measure to approach the activation mechanism is to study the adsorption of methane at low temperatures where methane may be adsorbed on the catalyst surface.
KeywordsCatalyst Surface Lewis Acid Site Active Oxygen Species Adsorbed State Adsorbed Methane
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