Structures of Molybdenum Species in Silica-Supported Molybdenum Oxide and Alkali-Ion-Modified Silica-Supported Molybdenum Oxide

Abstract

Silica-supported molybdenum oxide (MoO3/SiO2) and alkali-ion-modified MoO3/SiO2 samples were characterized by Mo K-edge XAFS and Raman spectroscopies. The structure of the molybdenum species in MoO3/SiO2 was confirmed to be monomeric and tetrahedral, (OMoO)O2. In alkali-ion-modified MoO3/SiO2, the structure of the molybdenum species varied with the kinds and/or the amounts of added alkali ions. In Na+-modified MoO3/SiO2 with sodium ions of the molar ratio Na/Mo < 1.0, the molybdenum species were mainly Na2Mo2O7 besides (OMoO)O2 which was present in MoO3/SiO2, and the addition of sodium ions of molar ratio Na/Mo > 1.0 caused the formation of Na2MoO4 at the expense of Na2Mo2O7. In the case of K+- and Rb+-modified MoO3/SiO2, the addition of alkali ions equimolar with the molybdenum atoms in the samples brought about the formation of Mo2O7 species, and by the addition of alkali ions of molar ratio (alkali ion/Mo) > 1.0, MoO4 species was formed on the silica support. In the molybdenum species in K-MS and Rb-MS with alkali ions of molar ratio (alkali ion/Mo) < 1.0, the proportion of an octahedral MoO6 unit to all the molybdenum species in the samples was higher than that in K-MS and Rb-MS with alkali ions of the molar ratio > 1.0.

An XAS study of molybdenum speciation in hydrothermal chloride solutions from 25–385°C and 600bar

“…This feature has been assigned to the 1s-4d transition, and the intensity is considered to be proportional to the number of Mo@O double bonds per molybdenum atom (e.g., Cramer et al, 1978;Yokoi et al, 1987). Takenaka et al (1998) also used intensity and integrated area of this pre-edge feature to estimate the asymmetry of the species. The tetrahedral moiety [MoO 4 ] 2À in Na 2 MoO 4(s) displays a more intense pre-edge than MoO 3(s) , which has a distorted octahedral [MoO 6 ] structure (Takenaka et al, 1998;Leisegang et al, 2005; Table 2).…”

“…Takenaka et al (1998) also used intensity and integrated area of this pre-edge feature to estimate the asymmetry of the species. The tetrahedral moiety [MoO 4 ] 2À in Na 2 MoO 4(s) displays a more intense pre-edge than MoO 3(s) , which has a distorted octahedral [MoO 6 ] structure (Takenaka et al, 1998;Leisegang et al, 2005; Table 2). Such a distorted octahedral structure is typical of Mo(VI) compounds containing Mo in octahedral coordination; usually, Mo is bonded to 2 oxygen atoms at short distance ($1.60 Å ), 2 at intermediate distance, and 2 at a long distance ($2.3-2.4 Å ; Table 2).…”

Preparation of Cr–Ti Binary Oxide Anchored Mesoporous Silica by CVD Method and Their Photocatalytic Activities

“…-, Mo 6? -oxide) containing silica-based materials such as zeolites and mesoporous silicas show the unique properties as efficient catalysts for various photocatalytic reactions, i.e., the decomposition of NO x , partial oxidation of hydrocarbons as well as reduction of CO 2 with H 2 O [1][2][3][4][5][6]. The transition metal oxide in these catalysts is considered to be highly dispersed state at the atomic level and exist in the well-defined structure on these surfaces or within the frameworks.…”

In situ characterization of the highly dispersed Mo6+-oxide species supported onto various oxides and their photocatalytic reactivities

“…It is known that supported Mo 6+ -oxide catalysts exhibit unique and high photocatalytic activity for various significant reactions such as the decomposition of NO in the presence of reductant gasses such as CO [1][2][3][4] or hydrocarbons [5], the partial oxidation of hydrocarbons into oxygen-containing compounds [6,7] and olefin metathesis [8][9][10][11]. It has, thus far, been revealed that the charge transfer excited triplet state of the highly dispersed tetrahedral Mo 6+ -oxide species plays a significant role in these photocatalytic reactions [12,13].…”