Surface structures of supported molybdenum oxide catalysts under ambient conditions

Journal Article · Sat Aug 01 04:00:00 EDT 1992 · Journal of Catalysis; (United States)

DOI:https://doi.org/10.1016/0021-9517(92)90084-U· OSTI ID:7253038

Kim, D S ^[1]; Segawa, Koichi; Soeya, Tomotsune; Wachs, I E ^[2]

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1. Lehigh Univ., Bethlehem, PA (United States)
2. Sophia Univ., Tokyo (Japan)

Two sets of supported molybdenum oxide catalysts, wet (dried at room temperature) and calcined (calcined at 773 K) samples, were prepared by an equilibrium adsorption method at different pH values of the impregnating solution. The adsorbed amounts of molybdenum oxide species onto the oxide support are strongly dependent on the pH of the impregnating solution and increase with decreasing pH. The Raman spectroscopic studies reveal that the surface molybdenum oxide species under ambient conditions, wet and calcined, are hydrated and essentially in an aqueous medium. Furthermore, the surface structures of molybdenum oxide species on the oxide support were found to depend on the net surface pH at point of zero change (PZC) under ambient conditions. The net surface pH at PZC under ambient conditions is determined by the specific oxide support and surface molybdenum oxide coverage. The surface molybdenum oxide structures in the wet, uncalcined, samples are not only dependent on the net surface pH at PZC but also on the number of NH{sub 4}{sup +} cations which coordinate to the surface molybdenum oxide species for compensation of net charge: Mo{sub 7}O{sub 24}{sup 6{minus}} species in NH{sub 4}{sup +}-rich concentrations (high pH region) favor formation of (NH{sub 4}){sub 6}Mo{sub 7}O{sub 24} {center dot} 4H{sub 2}O. Upon calcination, the NH{sub 4}{sup +} ions are removed and the surface molybdenum oxide species become rehydrated upon exposure to air by adsorbing moisture. Consequently, the structures of surface molybdenum oxide species in the calcined samples which have been exposed to ambient are also dependent on the net surface pH at PZC.

• OSTI ID: 7253038
• Journal Information: Journal of Catalysis; (United States), Journal Name: Journal of Catalysis; (United States) Vol. 136:2; ISSN 0021-9517; ISSN JCTLA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ADSORPTION
AMMONIUM COMPOUNDS
CALCINATION
CATALYST SUPPORTS
CATALYSTS
CHALCOGENIDES
CHEMICAL PREPARATION
CHEMICAL REACTIONS
CRYSTAL STRUCTURE
DECOMPOSITION
HYDRATES
IMPREGNATION
LASER SPECTROSCOPY
MICROSTRUCTURE
MOLYBDENUM COMPOUNDS
MOLYBDENUM OXIDES
OXIDES
OXYGEN COMPOUNDS
PH VALUE
PHASE STUDIES
POTENTIALS
PYROLYSIS
RAMAN SPECTROSCOPY
REFRACTORY METAL COMPOUNDS
SORPTION
SPECTROSCOPY
SURFACE POTENTIAL
SURFACE PROPERTIES
SYNTHESIS
THERMOCHEMICAL PROCESSES
TRANSITION ELEMENT COMPOUNDS