Surface metal oxide species structure

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-... 

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... 

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. 

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... 

Metal oxide catalytic materials currently find wide application in the petroleum, chemical, and environmental industries, and their uses have significantly expanded since the mid-20th century (especially in environmental applications) [1,2], Bulk mixed metal oxides are extensively employed by the chemical industries as selective oxidation catalysts in the synthesis of chemical intermediates. Supported metal oxides are also used as selective oxidation catalysts by the chemical industry, as environmental catalysts, to selectively transform undesirable pollutants to nonnox-ious forms, and as components of catalysts employed by the petroleum industry. Zeolite and molecular sieve catalytic materials are employed as solid acid catalysts in the petroleum industry and as aqueous selective oxidation catalysts in the chemical industry, respectively. Zeolites and molecular sieves are also employed as sorbents for separation of gases and to trap toxic impurities that may be present in water supplies. Significant molecular spectroscopic advances in recent years have finally allowed the nature of the active surface sites present in these different metal oxide catalytic materials to be determined in different environments. This chapter examines our current state of knowledge of the molecular structures of the active surface metal oxide species present in metal oxide catalysts and the influence of different environments upon the structures of these catalytic active sites. 

Molecular Structures of Surface Metal Oxide Species 3... 

The molecular structures of the hydrated surface metal oxides on oxide supports have been determined in recent years with various spectroscopic characterization methods (Raman [34,37,40 3], IR [43], UV-Vis [44,45], solid stateNMR [32,33], and EXAFS/XANES [46-51]). These studies found that the surface metal oxide species possess the same molecular strucmres that are present in aqueous solution at the same net pH values. The effects of vanadia surface coverage and the different oxide supports on the hydrated surface vanadia molecular structures are shown in Table 1.2. As the value of the pH at F ZC of the oxide support decreases, the hydrated surface vanadia species become more polymerized and clustered. Similarly, as the surface vanadia coverage increases, which decreases the net pH at PZC, the hydrated surface vanadia species also become more polymerized and clustered. Consequently, only the value of the net pH at PZC of a given hydrated supported metal oxide system is needed to predict the hydrated molecular structure(s) of the surface metal oxide species. 

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 dehydrated surface metal oxide species are not coordinated to water and, therefore, their molecular structures are not related to those present in aqueous solutions. Consequently, the pH at PZC model cannot be employed to predict the dehydrated surface metal oxide structures. The molecular structures of the dehydrated surface metal oxide species, however, possess similarity to the structural inorganic chemistry of bulk metal oxides because of the absence of water ligands in both systems [60-62]. Instead of being solvated by coordinated water in the aqueous solution complexes, the bulk metal oxide structures are coordinated to various cations (e.g., K, Na, Ca, Mg, Fe, Al, Ce, Zr, H, etc.). Prior to discussing the current understanding of the molecular structures of the dehydrated surface metal oxide species, a brief review of the structural inorganic chemistry of bulk metal oxides and their determination methods are presented to highlight the molecular structural similarities, as well as differences, between these two- and three-dimensional metal oxide systems. 

At present, the molecular structures of the dehydrated reduced surface metal oxide species present for supported metal oxide catalysts under reactive environments are not well-known and, hopefully, will receive more attention in the coming years. Fortunately, the fully oxidized surface metal oxide species are the predominant species found to be present under typical reaction conditions employed for redox supported metal oxide catalysts. 

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