Surface metal oxide species

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Wachs, I.E. (1996) Raman and IR studies of surface metal oxide species on oxide supports Supported metal oxide catalysts, Catal. Today, 27, 437. 

Structural characterization of the surface metal oxide species was obtained by laser Raman spectroscopy under ambient and dehydrated conditions. The laser Raman spectroscope consists of a Spectra Physics Ar" " laser producing 1-100 mW of power measured at the sample. The scattered radiation was focused into a Spex Triplemate spectrometer coupled to a Princeton Applied Research DMA III optical multichannel analyzer. About 100-200 mg of... 

The above discussion demonsi rates that it is possible to molecularly design supported metal oxide catalysts with knowledge of the surface oxide - support interactions made possible by the assistance of characterization methods such as Raman spectroscopy and the methanol oxidation reaction. The formation and location of the surface metal oxide species are controlled by the... 

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

Poisons. - Unlike secondary surface metal oxide additives that indirectly interact with the surface vanadia sites via lateral interactions, poisons are surface metal oxide additives that directly interact with the surface vanadia sites and decrease the TOF. For example, the addition of surface potassium oxide to supported vanadia catalysts results in both a structural change and a reactivity change of the surface metal oxide species.50 This interaction, at submonolayer coverages, reflects the attractive interaction between these two surface metal oxide species. The presence of the surface potassium oxide poison alters the V-O bond lengths and the ratio of polymeric and isolated surface vanadia species (favoring isolated surface vanadia species). The interaction of the surface potassium oxide poison with the surface vanadia species is schematically shown in Figure 5. 

Combination of UV-vis DRS and Raman spectroscopy data has allowed for the quantitative determination of the monomer and polymer concentrations of the surface metal oxide species (Tian et al., 2006). The... 

In contrast to acidic surface metal oxides with cation oxidation states of +5 to +7, which are anchored to the support by surface hydroxyl groups, basic surface metal oxides with cation oxidation states of +1 to +3 are anchored at surface Lewis acid sites (Bredow et al., 1998 Cortez et al., 2003 Diebold, 2003 Jehng and Wachs, 1992 Vuurman et al., 1996). Raman spectra demonstrated that supported basic metal oxides are, in contrast to acidic supported metal oxides, insensitive to moisture. The Raman spectra of basic surface metal oxide species do not show the bands at about 1000 cm 1 that would indicate terminal M = 0 bonds. The spectra typically exhibit Raman bands in the wave number region of 500-700 cm-1, characteristic of M—O bonds (Chan and Wachs, 1987 Tian et al., 2006 Vuurman et al., 1996) similar behavior was observed for TiO, ZrOx, Pt02, and other oxide surface species with cations in the +4 oxidation state. 

Catalytic reaction conditions or the exposure to reducing environments may lead to the formation of reduced surface metal oxide species. It is generally difficult to obtain good Raman signals for reduced supported metal oxide species because of their low Raman cross-sections. On the other hand, many reduced transition metal ions have electronic absorption bands in the visible regime. Hence, the laser frequency may be tuned to these absorption bands, and resonantly enhanced Raman spectra should be obtained. 

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. 

Supported metal oxide catalysts consist of dispersed surface metal oxide species, the catalytic active sites, which are supported on high-surface-area oxides [1-3]. The... 

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. 

Wachs, I. (1996). Raman and IR Studies of Surface Metal Oxide Species on Oxide Supports Supported Metal Oxide Catalysts, Catal. Today, 27, pp. 437 55 Wachs, I. (2005). Recent Conceptual Advances in the Catalysis Science of Mixed Metal Oxide Catalytic Materials, Catal. Today, 100, pp. 79-94. 

In the past few years, in situ Raman spectroscopy studies of supported metal oxide catalysts have focused on the state of the surface metal oxide species during catalytic oxidation reactions (see Table 2). As mentioned earlier, there has been a growing application of supported metal oxide catalysts for oxidation reactions. The influence of different reaction environments upon the surface molybdena species on Si02 was nicely demonstrated in two comparative oxidation reaction studies (see Fig. 4). The dehydrated surface molybdena on silica is composed of isolated species (no Raman bands due to bridging Mo—O—Mo bonds at —250 cm ) with one terminal Mo=0 bond that vibrates at —980 cm" The additional Raman bands present at —800, —600, and 500-300 cm in the dehydrated sample are due to the silica support. During methane oxidation, the surface... 

For most oxidation reactions (especially selective oxidation of hydrocarbons), however, the in situ Raman studies reveal that the surface metal oxide species are relatively... 

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

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