On the induction period of methane aromatization over Mo-based catalysts

The behavior of different species during the temperature-programmed surface reaction (TPSR) of methane over various catalysts is traced by an online m ass spectrometer, It is demonstrated that the transformation of MoO3 to mol ybdenum carbide hinders the activation of methane as well as the succeeding aromatization in the TPSR, If this transformation process is done before t he reaction, the temperature needed for methane activation and benzene form ation will be greatly lowered (760 and 847 K, respectively). On the basis o f comparison of the catalytic behavior of molybdenum supported on different zeolites, it is suggested that the initial activation of methane is the ra te-determining step of this reaction. For the cobalt catalysts supported on HMCM-22 or Mo catalysts supported on TiO2, no benzene formation could be o bserved during the TPSR, However, the prohibition of benzene formation is d ifferent in nature over these two catalysts: the former lacks the special p roperties exhibited by molybdenum carbide, which can continuously activate methane even when multiple layers of carbonaceous species are formed on its surface, while the latter cannot accomplish the aromatization reaction sin ce there are no Bronsted acid sites to which the activated intermediates ca n migrate, although the activation of methane can be achieved on it. Only f or the catalysts that possess both of these properties, together with the s pecial channel structure of zeolite, can efficient methane aromatization be accomplished. (C) 2000 Academic Press.