The induction period of dehydrogenation and aromatization of methane over Mo/HZSM 5 was studied by real time resonance enhanced two photon ionization (RE2PI) technique. It is found that the formation of benzene evidently involves three different stages during the induction period. This probably means that the active sites of the catalysts undergo a transformation from Mo 6+ species to lower valence Mo species, and the Mo 6+ species also has feeble catalytic activity.
