Raman and X-ray absorption studies of Mo species in Mo/H-ZSM5 catalysts for non-oxidative CH{sub 4} reactions

Journal Article · Tue Apr 25 04:00:00 EDT 2000 · Journal of Catalysis

DOI:https://doi.org/10.1006/jcat.1999.2795· OSTI ID:20030520

Li, W; Meitzner, G D; Borry, III, R W; Iglesia, E

The structure of MoO{sub x} and MoC{sub y} species in Mo/H-ZSM5 after exchange and during CH{sub 4} reactions was probed by in situ Raman and X-ray absorption spectroscopy. Raman spectra of physical mixtures of H-ZSM5 and MoO{sub 3} powders initially showed strong Raman bands characteristic of bulk MoO{sub 3}. The intensity of Raman bands for Mo-O-Mo decreased relative to those for Mo{double_bond}O bonds during treatment in air at 773 K, suggesting that MoO{sub 3} spreads as (MoO{sub 3}){sub n} oligomers on external zeolite surfaces. The Raman bands for MoO{sub 3} crystallites became much weaker after treatment in air at 973 K; this occurred concurrently with H{sub 2}O evolution, indicating that dispersed MoO{sub x} species exchanged with acidic OH groups in the zeolite. The X-ray near-edge spectrum of MoO{sub 3}/H-ZSM5 mixtures gradually evolves to one characteristic of tetrahedral Mo{sup 6+} centers in bulk MgMo{sub 2}O{sub 7} as H{sub 2}O forms during treatment at 773--973 K, confirming that exchange occurs in this temperature range. The absorption edge and near-edge features in exchanged MoO{sub x}/H-ZSM5 evolve during CH{sub 4} reactions at 973 K to resemble bulk Mo{sub 2}C without long-range periodicity, with the concurrent evolution of CO, CO{sub 2}, and H{sub 2}O and an increase in the rate of hydrocarbon formation. Multiple scattering analysis of a MoC{sub x} cluster bonded to an O atom leads to a radical structure function in excellent agreement with experiment.

• Research Organization: Lawrence Berkeley National Lab., CA (US)
• Sponsoring Organization: US Department of Energy; National Science Foundation
• DOE Contract Number: AC03-76SF00515; AC03-76SF00098
• OSTI ID: 20030520
• Journal Information: Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 2 Vol. 191; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
10 SYNTHETIC FUELS
CATALYTIC EFFECTS
METHANE
MOLYBDENUM
MOLYBDENUM CARBIDES
MOLYBDENUM OXIDES
MORPHOLOGY
REFORMER PROCESSES
ZEOLITES