on Raman and X-Ray Absorption Studies Catalysts for Non-Oxidativ

The structure of MoOx and MoCy species in Mo/H-ZSM5 after exchange and during CH4 reactions was probed by in situ Raman and X-ray absorption spectroscopy. Raman spectra of physical mix-tures of H-ZSM5 and MoO3 powders initially showed strong Raman bands characteristic of bulk MoO3. The intensity of Raman bands for Mo–O–Mo decreased relative to those for Mo==O bonds during treatment in air at 773 K, suggesting that MoO3 spreads as (MoO3)n oligomers on external zeolite surfaces. The Raman bands for MoO3 crystallites became much weaker after treatment in air at 973 K; this occurred concurrently with H2O evolution, indicating that dis-persed MoOx species exchanged with acidic OH groups in the zeolite. Weak bands appeared at 970 and 1045 cm¡1; they were assigned to stretching modes in exchanged MoOx species. The X-ray near-edge spectrum of MoO3/H-ZSM5 mixtures gradually evolves to one characteristic of tetrahedral Mo6C centers in bulk MgMo2O7 as H2