Spectroscopic and Structural Characterization of Low-Level Alkali Doping Effects on Mo/Silica−Titania Catalysts

Abstract

The structural characteristics of supported MoO x species inherent over a high TiO 2 content silica-titania mixed-oxide support (1:1 molar ratio), combined with the structural effects that are induced by an alkali promoter, are the primary focus of this investigation. The catalysts were prepared using a "one-pot" modified sol-gel/coprecipitation technique that distributes the molybdenum precursor throughout the Si/Ti support network as it forms during gelation. The Si/Ti 1:1 mixed-oxide support is in a state of nanodispersed titania (anatase) over silica. With the introduction of molybdenum and successive levels of alkali promoter, the dispersion of titania decreases as does the interaction of TiO x species with silica. Concurrent with this change is the observation of an enhanced interaction of the MoO x species with silica. Correlation is found among the propane ODH reactivity, the surface coverage of oxidic molybdenum species, and the nature of the Si/Ti support. 1 H-29 Si CP-MAS NMR data and DRIFT spectra taken in the hydroxyl stretching region (3000-3900 cm -1 ) suggest that, with the addition of alkali, the three dimensionality of the silica support is changing to accommodate the change in MoO x surface structure. The presence of potassium significantly alters the dispersion of titania and the electronic structure of the surface MoO x domains, even before the formation of K-molybdate species is observed at higher K/Mo molar ratios. This electronic interaction is observed readily in the ESR spectrum of Mo(V) that is present in the dehydrated samples. Data suggest that surface-supported species, present as distorted octahedral MoO x , become the most distorted at low alkali levels (K/Mo ) 0.07). The MoO x species at this level of alkali promotion, which experience a decrease in Lewis acidity, could tend to be more reactive toward electronegative Si-Osupport ligands on the Si/Ti 1:1 support. The observed increase in propane ODH reactivity at low K/Mo molar ratios may be ascribed to a highly distorted structure sharing both titania and silica ligands.

Correlation Between Oxygen Reduction Reaction and Oxidative Dehydrogenation Activities Over Nanostructured Carbon Catalysts

“…Additionally, the washed samples likely show higher activity since the presumably inactive alumina phase (70 wt% of the unwashed sample) is removed, leading to a higher active site density. Although halides have been shown to improve ODH reaction rate [31][32][33][34][35][36], fluorine is not detected in XPS, Fig. 2 Correlation between propylene formation rate and ORR activity where ORR activity is indicated by the onset of activity and the propylene formation rate is measured at a 250°C and b 325°C so removal of alumina is a more probable explanation.…”

Structure and photochromic properties of molybdenumphosphoric acid/TiO2 composite films

“…Generally, Mo-O d vibration can be considered as a pure stretching one, whose vibration wavenumber is an increasing function of anion-anion interaction. After adding PMoA into [13,14] .…”

Spectroscopic characterization of Cl-modified Mo/Si:Ti catalysts for oxidative dehydrogenation of propane

“…In our previous work, we have shown Mo/ Si:Ti = 1:1 catalysts to be active and selective for the ODH of lower alkanes [34][35][36][37]. By doping with potassium and varying K/Mo molar ratio, a maximum in selectivity and yield of propylene was obtained for propane ODH.…”