Metal oxide surfaces, oxidation

Perhaps the simplest case of reaction of a solid surface is that where the reaction product is continuously removed, as in the dissolving of a soluble salt in water or that of a metal or metal oxide in an acidic solution. This situation is discussed in Section XVII-2 in connection with surface area determination. 

The silanization reaction has been used for some time to alter the wetting characteristics of glass, metal oxides, and metals [44]. While it is known that trichlorosilanes polymerize in solution, only very recent work has elucidated the mechanism for surface reaction. A novel FTIR approach allowed Tripp and Hair to prove that octadecyl trichlorosilane (OTS) does not react with dry silica. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

Chemisoq)tion bonding to metal and metal oxide surfaces has been treated extensively by quantum-mechanical methods. Somoijai and Bent [153] give a general discussion of the surface chemical bond, and some specific theoretical treatments are found in Refs. 154-157 see also a review by Hoffman [158]. One approach uses the variation method (see physical chemistry textbooks) ... 

Goodman D W 1996 Chemical and spectroscopic studies of metal oxide surfaces J. Vac. Sc/. Technol. A 14 1526... 

Wu M-C, Estrada C A, Corneille J S and Goodman D W 1996 Model surface studies of metal oxides adsorption of water and methanol on ultrathin MgO films on Mo(IOO) J. Chem. Phys. 96 3892... 

Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

The passive state of a metal can, under certain circumstances, be prone to localized instabilities. Most investigated is the case of localized dissolution events on oxide-passivated surfaces [51, 106, 107, 108, 109, 110, ill, 112, 113, 114, 115, 116, 117 and 118]. The essence of localized corrosion is that distinct anodic sites on the surface can be identified where the metal oxidation reaction (e.g. Fe —> Fe + 2e ) dominates, surrounded by a cathodic zone where the reduction reaction takes place (e.g. 2Fi + 2e —> Fi2). The result is the fonnation of an active pit in the metal, an example of which is illustrated in figure C2.8.6(a) and (b). 

Surface heterogeneity is difficult to remove from crystalline inorganic substances, such as metal oxides, without causing large loss of surface areas by sintering. Thus in Fig. 2.14 in which the adsorbent was rutile (TiO ) all three adsorbates show a continuous diminution in the heat of adsorption as the surface coverage increases, but with an accelerated rate of fall as monolayer completion is approached. 

Transmission Fourier Transform Infrared Spectroscopy. The most straightforward method for the acquisition of in spectra of surface layers is standard transmission spectroscopy (35,36). This approach can only be used for samples which are partially in transparent or which can be diluted with an in transparent medium such as KBr and pressed into a transmissive pellet. The extent to which the in spectral region (typically ca 600 4000 cm ) is available for study depends on the in absorption characteristics of the soHd support material. Transmission ftir spectroscopy is most often used to study surface species on metal oxides. These soHds leave reasonably large spectral windows within which the spectral behavior of the surface species can be viewed. 

The molten salts quickly dissolve the metal oxides at high temperatures to form a clean metal surface. Other uses are as catalysts and in fire-retardant formulations (see Flame retardants). 

They readily dissolve metal oxides and are effective metal surface cleaners and fluxes (see Metal surface treatments). They also have bactericidal and fungicidal properties (74). However, the main commercial appHcation among monofluorophosphates is of sodium monofluorophosphate ia dentifrices. 

Inorganic membranes (29,36) are generaUy more stable than their polymeric counterparts. Mechanical property data have not been definitive for good comparisons. IndustriaUy, tube bundle and honeycomb constmctions predominate with surface areas 20 to 200 m. Cross-flow is generaUy the preferred mode of operation. Packing densities are greater than 1000 /m. Porous ceramics, sintered metal, and metal oxides on porous carbon support... 

Among toxic pollutants that may enter the environment, hydraziae is one of the less persistent because it reacts with oxygen and ozone, particularly in the presence of catalytic surfaces such as metals, oxides, etc. The final products of these reactions are innocuous nitrogen and water. 

HCl gas reacts with metal oxides to form chlorides, oxychlorides, and water. Therefore, all the steel equipment should be pickled to remove the oxide scales before it is put in service. Because oxidi2ing agents in the HCl gas such as oxygen or chlorine significantly affect the corrosion rate, it is essential that the operating temperature of the steel equipment be kept below the temperature (316°C) at which ferric chloride is vapori2ed from the metal surface. 

Aluminum, the most common material used for contacts, is easy to use, has low resistivity, and reduces surface Si02 to form interfacial metal-oxide bonds that promote adhesion to the substrate. However, as designs reach submicrometer dimensions, aluminum, Al, has been found to be a poor choice for metallization of contacts and via holes. Al has relatively poor step coverage, which is nonuniform layer thickness when deposited over right-angled geometric features. This leads to keyhole void formation when spaces between features are smaller than 0.7 p.m. New collimated sputtering techniques can extend the lower limit of Al use to 0.5-p.m appHcations. 

Iridium Oxide. Iridium dioxide [12030 9-8] coatings, typically used in combination with valve metal oxides, are quite similar in stmcture to those of mthenium dioxide coatings. X-ray diffraction shows the mtile crystal stmcture of the iridium dioxide scanning electron micrographs show the micro-cracked surface typical of these thermally prepared oxide coatings. 

In some cases, a pigment s thermal and chemical resistance can be improved by the encapsulation of the pigment particles by an iasoluble, colorless layer of metal oxide or oxide—hydroxide, eg, siUca, Si02. The function of such a shell is to prevent direct contact and reaction between the pigment surface and the organic matrix ia which the pigment is dispersed (11). 

The atoms and molecules at the interface between a Hquid (or soHd) and a vacuum are attracted more strongly toward the interior than toward the vacuum. The material parameter used to characterize this imbalance is the interfacial energy density y, usually called surface tension. It is highest for metals (<1 J/m ) (1 J/m = N/m), moderate for metal oxides (<0.1 J/m ), and lowest for hydrocarbons and fluorocarbons (0.02 J /m minimum) (4). The International Standards Organization describes weU-estabHshed methods for determining surface tension, eg, ISO 304 for Hquids containing surfactants and ISO 6889 for two-Hquid systems containing surfactants. 

Hydrolysis. The surfaces of metal oxides and hydroxides can take up or release or OH ions and become charged. Potentials as high as 100 mV may be sustained ia aqueous solutions. For aqueous solutions this is a function of the pH the zeta potential for the particle is positive if the solution pH is below the particle s isoelectric pH (pH ), and negative if the pH is above pH Isoelectric poiats for metal oxides are presented ia several pubheations (22,23). Reactions of hydroxyl groups at a surface, Q, with acid and base may be written as follows ... 

Metal oxides found in RO feed streams typically originate from corroded pipes found in the RO process. These metal oxides can deposit on the membrane surface and decrease the membrane flux. This type of fouling can be prevented by using the proper materials of constmction in the piping system to prevent corrosion. 

Rhenium exhibits a greater resistance than tungsten to the water cycle effect, in which lamps and electron tubes become blackened by deposition of metal. This phenomenon involves catalysis by small quantities of water that react with the metal in a hot filament to produce a volatile metal oxide and hydrogen. The oxide condenses on the surface of the bulb and is reduced back to the metal by hydrogen. 

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