Molecular Structures of Surface Metal Oxides

The Raman spectrum for the 4% Cr03/Si02 sample is very different from that obtained for 4% C1O3/AI2O3 and 4% Cr03yTi02 and reflects the presence of a more 

Rgune 20 Raman spectra of 4% chromium oxide supported on alumina, titania, and silica. 

Under dehydrated conditions, the adsorbed moisture is removed and the in situ Raman spectra of the surface metal oxides differ markedly showing that the structures of the dehydrated species are very different from those of their hydrated counterparts (see references above in Section 6.2.2). These changes are not surprising since the influence of the net zero surface charge of the oxide support can only be exerted in an aqueous medium. For the dehydrated surface metal oxides, however, essentially the same molecular structures are seen on all the oxide supports for each supported metal oxide.  

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Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

Predicting molecular structures of surface metal oxide species on oxide supports under ambient conditions. Journal of Physical Chemistry, 95 (15), 5889-95. 

I.E. Wachs, Molecular structures of surface metal oxide species Nature of catalytic active sites in mixed metal oxides, in Metal oxides Chemistry and applications, Taylor Francis Group, LLC Boca Raton, FL, pp. 1-30, 2006. 

Titanium oxide monolayer on y-AljOj is a potential support for noble metals [1-4]. Many studies have shown that two-dimensional transition metal oxide overlayers are formed when one metal oxide (Vj05, Nb205, MoOj, etc.) is deposited on an oxide support (AljOj, TiO, etc.) [5-7]. The influence of the molecular structures of surface metal oxide species on the catalytic properties of supported metal oxide catalyst has been examined [8-9]. It has been demonstrated that the formation and location of the surface metal oxide species are controlled by the surface hydroxyl chemistry. Moreover, thin-layer oxide catalysts have been synthesized on alumina by impregnation technique with alkoxide precursor [10]. It has been found for titanium oxide, by using Raman spectroscopy, that a monolayer structure is formed for titanium contents below 17% and that polymeric titanium oxide surface species only posses Ti-O-Ti bonds and not Ti=0 bonds. Titanium is typically ionic in its oxy-compounds, and while it can exist in lower oxidation states, the ionic form TF is generally observed in octahedral coordination [11-12]. However, there is no information available about the Ti coordination and structure of this oxide in a supported monolayer. In this work we have studied the structural evolution of the titanium oxy-hydroxide overlayer obtained from alkoxide precursor, during calcination. 

Molecular Structures of Surface Metal Oxide Species Nature of Catalytic Active Sites in Mixed Metal Oxides... 

Molecular Structures of Surface Metal Oxide Species 3... 

The finding that only one parameter, the net pH at PZC, controls the hydrated molecular structures of surface metal oxide species also has very... 

The electronic and molecular structures of surface metal oxide species present on oxide supports have received enormous attention over the past three decades... 

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