Dehydrated surface metal oxide species

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. 

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... 

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. ... 

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... 

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. 

The vanadium oxide species is formed on the surface of the oxide support during the preparation of supported vanadium oxide catalysts. This is evident by the consumption of surface hydroxyls (OH) [5] and the structural transformation of the supported metal oxide phase that takes place during hydration-dehydration studies and chemisorption of reactant gas molecules [6]. Recently, a number of studies have shown that the structure of the surface vanadium oxide species depends on the specific conditions that they are observed under. For example, under ambient conditions the surface of the oxide supports possesses a thin layer of moisture which provides an aqueous environment of a certain pH at point of zero charge (pH at pzc) for the surface vanadium oxide species and controls the structure of the vanadium oxide phase [7]. Under reaction conditions (300-500 C), moisture desorbs from the surface of the oxide support and the vanadium oxide species is forced to directly interact with the oxide support which results in a different structure [8]. These structural... 

To investigate the effect of the synthesis method on the structure-reactivity relationship of the supported metal oxide catalysts, a series of V205/Ti02 catalysts were synthesized by equilibrium adsorption, vanadium oxalate, vanadium alkoxides and vanadium oxychloride grafting [14]. The dehydrated Raman spectra of all these catalysts exhibit a sharp band at 1030 cm characteristic of the isolated surface vanadium oxide species described previously. Reactivity studies with... 

The nature of the supported metal oxide species depends upon a number of factors the preparation method (wet chemical synthesis plus calcination), chemical interactions between the support and surface layers, and surface density (surface oxide weight loading and specific surface area of the support oxide) [5]. Figure 11.1 schematically demonstrates the various dehydrated surface structures commonly observed for a mono-oxo metal oxide isolated, oligomeric, polymeric, and crystalline species. Several reviews comprehensively catalog the expected surface structures for transition metal mono- and polyoxoanions in four-, five-, and sixfold coordination under nonreaction conditions [3, 23-25],... 

CO specpure from either Matheson or Praxair was used as a molecular probe in order to assess the Lewis acidic properties of coordinatively unsaturated (cus) cations either located in the dehydrated zeolite nanocavities as charge-balancing cations, or exposed at the dehydrated surface of oxidic materials. CO is capable of interacting with the cus cations leading to the formation of adducts of different stability according to the chemical nature of the cation. Weak electrostatic adducts are formed on alkaline metal cations, u-coordinated species of intermediate stability on non d/d metal cations, whereas high-stability carbonyl-like species originated by a a-coordination -l-7r-back donation of d electrons are formed on d block metal cations. 

CatalyticaHy Active Species. The most common catalyticaHy active materials are metals, metal oxides, and metal sulfides. OccasionaHy, these are used in pure form examples are Raney nickel, used for fat hydrogenation, and y-Al O, used for ethanol dehydration. More often the catalyticaHy active component is highly dispersed on the surface of a support and may constitute no more than about 1% of the total catalyst. The main reason for dispersing the catalytic species is the expense. The expensive material must be accessible to reactants, and this requires that most of the catalytic material be present at a surface. This is possible only if the material is dispersed as minute particles, as smaH as 1 nm in diameter and even less. It is not practical to use minute... 

Where the rate is first-order in catalyst,an interpretation of the rate data is possible but always subject to uncertainties in our knowledge of the sorption isotherms of all species involved in the mechanism. Let us consider, for example, the decomposition of a single molecular species on a catalyst surface. An example might be the dehydration of alcohols on the surfaces of metal oxides such as alumina gel at 300 C to give an olefin plus water. This endothermic reaction, which is thermodynamically favored in the gas phase by an entropy increase of about 36 e.u., can be written as... 

The surface reaction of impregnated mixed metal cluster complexes may be analogous to that of homometallic clusters on hydrated and dehydrated metal oxides as described in Sections III and IV. Bimetallic clusters are converted to anionic surface species by simple deprotonation via 0 on dehydrated MgO or AI2O3 surfaces these species have been characterized by IR spectroscopy (119). The ionic interaction with surface cations such as AF and Mg is demonstrated by IR and NMR measurements. The surface polynuclear carbonyl anions are stable up to about 373 K. If heated in vacuo at higher temperature, extensive decomposition takes place to give a mixture of Ru (or Os) metal particles and Fe oxides, accompanied by the evolution of H2, CO, and CO2. 

As for many solid state reactions, the properties of any particular oxide preparation may be influenced by its method of synthesis [11]. Oxides are often products of thermal treatment. Such heating may influence the surface area, impurity content (e.g. strongly-retained traces of water from hydroxide dehydration, oxidized species retained from the decomposition of a nitrate, carbonate etc.) and concentrations and distribution of defects (e.g. vacancies and non-stoichiometry arising during oxidation of a metal). Thus the preparative method exerts significant control over the numbers... 

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