Marwan Houalla

Characterization of isopoly metal oxyanions using electrospray time-of-flight mass spectrometry

Electrospray time-of-flight mass spectrometry was used to study dilute (10(-3) M) solutions of tungstate, vanadate and perrhenate oxyanions under acidic and basic conditions. The electrospray mass spectra of tungstate and vanadate solutions indicate the presence of protonated monomeric oxyanions in basic solution (pH approximately 10) and protonated polymeric species in acidic solutions (pH approximately 4). Monomeric perrhenate was the only species detected in both acidic and basic perrhenate solutions. For all three systems, the core metalate species detected by electrospray were consistent with reported speciation in aqueous solutions. Copyright 2000 John Wiley & Sons, Ltd.

Development of the acidity of zirconia-supported niobia catalysts

A series of NbOx/ZrO2 catalysts containing up to 2.67wt Nb (ca. 80 nominal surface coverage) was prepared by incipient wetness impregnation from niobium oxalate and oxalic acid solution. The structure of the catalysts was monitored by X-ray diffraction and Raman spectroscopy. The results indicated the presence of a surface Nb phase. No evidence for the formation of crystalline Nb2O5 species was found. The development of the acidity as a function of Nb loading was monitored by adsorption of a basic probe molecule followed by infrared spectroscopy. The results indicated the appearance of Brnsted acid sites for a threshold of Nb loading. The abundance of Brnsted acid sites correlated well with the isopropanol dehydration activity. The overall behavior was very similar to that reported earlier for the WOx/ZrO2 system.

Characterization of supported Nb catalysts by ToF‐SIMS

A series of Nb2O5/TiO2 catalysts were prepared by incipient wetness impregnation of TiO2 with aqueous solutions of niobium oxalate/oxalic acid. Eight different catalysts with Nb loadings of 0.48–8.5 wt.% Nb were prepared. Analysis of the time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectra of Nb2O5/TiO2 catalysts shows the presence of monomeric, dimeric and trimeric NbxOy clusters at all loadings. This indicates that polymeric Nb oxide species are present even at loadings well below that corresponding to monolayer coverage (∼4.5 wt.% Nb). Moreover, it was found that the degree of aggregation of the niobia active phase increases with Nb loading. The tetrameric Nb oxide cluster was the largest fragment detected for amorphous Nb2O5. For Nb2O5/TiO2 catalysts the tetrameric Nb cluster was first detected in the 2.8 wt.% Nb catalyst. It is concluded that bulk-like Nb2O5 is present in catalysts containing ≥2.8 wt.% Nb (i.e. ≥60% theoretical coverage). Copyright

Development of acidic sites in WOx/ZrO2

Evidence is shown of the appearance of Bronsted acidic sites in WOx/ZrO2 at a given W loading and of a direct correlation between the abundance of ‘strong’ Bronsted acidic sites in these catalysts and propan-2-ol decomposition activity.