Dispersity and reducibility of the molybdenum oxide phase in SiO2, SiO2−Al2O3 and γ-Al2O3 supported molybdena catalysts

Comparison of Silica-Supported MoO3and V2O5Catalysts in the Selective Partial Oxidation of Methane

“…The higher-loaded (1.3-3.5 Mo/nm 2 ) MoO 3 /SiO 2 catalysts exhibited two distinct reduction stages: an initial rapid weight loss during the first few minutes (equivalent to that of the low-loaded samples) and then a slow process which reached equilibrium after approximately 3.5 h at 823 K. The appearance of the second reduction stage is associated with a decrease in the dispersion of molybdenum oxide species on silica, consistent with the appearance of surface polymolybdate species and crystalline orthorhombic molybdenum oxide features observed in Raman spectra of the dehydrated catalysts with concentrations of 1.3 Mo/nm 2 and above. This longer induction periods in isothermal reductions has been observed by Valyon et al (39). Previous TPR studies (40) show that the longer time required to reduce the aggregated molybdenum oxide species appears to result in part to mass transfer limitations.…”

“…The reduction isotherms show that reduction occurs in two stages for the xMo series at concentrations above those showing a maximum in R 0 . This reduction behavior is characteristic of bulk crystalline oxide (39). At 1.3 Mo/nm 2 molybdate species are formed while at higher loadings the formation of crystalline orthorhombic α-MoO 3 occurs.…”

Synthesis, characterization, and catalytic activity of sulfided silico-alumino-titanate (Si−Al−Ti) mixed oxides xerogels supported Ni−Mo catalyst

“…These results imply that the higher surface area and higher ion-exchange capacity of the acidified support is more important for preparing active catalysts than the support's thermal stability. However, for catalysts synthesized with Si/Ti mole ratio of 0.5 (catalysts #6, 15,20), there is no remarkable difference in the activity when difllerent pretreatment methods are used.…”