Surface metallic oxide

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

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-... 

A substance (usually liquid) employed to remove surface metal oxides in preparation for soldering, brazing or other metal fusion techniques. Also, the rate of energy transfer across a given surface area. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

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

Micro-Raman spectroscopy Crystal phase structure, crystal size, surface metal oxide structure and coverage Trained Free... 

In addition to the inorganic hydroxyl groups which are exposed on many mineral surfaces (metal oxides, phyllo-silicates and amorphous silicate minerals) we need to consider the particular features relating to charge on the silica surfaces of layer silicates. 

A related but little studied area of adhesion and corrosion protection involves the chemical effects of metal substrates on coatings and other polymeric materials and conversely of polymeric materials on metals. In the curing of certain air-oxidizing coatings on steel, for example, reduction of ferric to ferrous species in the surface metal oxide, substantial thinning of the oxide, and oxidation of the coating material have been reported to occur in the interfacial... 

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... 

Influence of Surface Metal Oxide Additives upon Oxidation Reactions... 

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. 

Upon exposure to oxygen, all metals form surface metal oxide layers which vary in thickness and structure, depending on the identity of the base metal and the oxide formation conditions. Mercury and noble metals generally form very thin oxide films. On the other hand, most metals of primary commercial importance (i.e. aluminum, iron, zinc, etc.), tend to form oxide layers which are thick enough (40-80 A or more), so that the underlying metal atoms do not contribute in an appreciable way to the adhesion forces in metal/polymer systems U). 

The shorter-term exposure experiments show that some portion of the organically-bound chlorine, such as trichlorethane or its decomposition products, remains absorbed on a 304 stainless steel surface, even after heating at 35-40°C in a high vacuum. Conversion to an ionic species begins after a short contact period and can be detected using XPS. Formation of the ionic chloride is likely the result of hydrolysis by water also absorbed on the surface, and is perhaps catalyzed by the surface metal oxides. Further atmospheric exposure up to a few months increases the relative amount of the ionic form of chlorine. The composition of the surface oxide layer was altered, with chromium oxide replacing iron oxide as the major species. There was further evidence that chlorine was present as iron chloride, perhaps up to 5% of the surface film. The conditions under which oxidation of such surfaces occurred are quite comparable to those which could occur on steel surfaces in industrial usage. 

The actual metal surface that takes part in the bonding is illustrated in Fig. 16.1. Adhesives recommended for metal bonding are in reality used for metal-oxide bonding. They must be compatible with the firmly bound layer of water attached to surface metal-oxide crystals. Even materials such as stainless steel and nickel or chromium are coated with transparent metal oxides that tenaciously bind at least one layer of water. 

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. 

The composition of the surface may also depend on gas pressure, for example, a surface may change from that of a metal with adsorbed oxygen to a surface metal oxide (JJ-JP) or to a metastable (subsurface) oxide that cannot be identified in UHV or by other analysis (60,61). It is apparent that such pressure effects have a strong impact on the catalytic properties and that measurements under elevated pressure are desirable. 

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... 

---