Water Vapor with Metal Oxide Surfaces

Interaction of Water Vapor with Metal Oxide Surfaces 

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The reaction of water vapor with metal oxide surfaces - surface science and theoretical studies of simplified model systems illustrating effects of defect density and adsorbate cooperative effects... 

There have been a number of applications of SR-based XPS to materials of relevance to environmental science and low temperature geochemistry including studies of the interaction of water vapor with metal oxide surfaces (e.g., Liu et al. 1998a,b,c,d Kendelewicz et al. 1999 2000b), reaction of CO2 gas with metal oxide surfaces (Carrier et al. 1999 Doyle et al. 1999b), and oxidation of metal sulfide surfaces (e.g., Schaufuss et al. 1998 Nesbitt et al. 2000, 2002). 

Another example of the interaction of water with a relatively simple metal oxide surface is provided by the water vapor/a-Al203(0001) system (Figure 7.9(a)). Oxygen Is synchrotron radiation photoemission results indicate that significant dissociative chemisorption of water molecules does not occur below 1 torr p(H20) [149]. However, following exposure of the alumina (0001) surface to water vapor above this threshold p(H20) , a low kinetic energy feature in the Is spectrum grows quickly,... 

The formation of OH, HO2 species and of H2O2 for semiconductor oxides in contact with aqueous solutions or exposed to water vapor has often been proposed (22,23, 40-42). As metal oxide surfaces carry OH groups and as the oxTdation of organic compounds produces water, the formation of the above species cannot be excluded even in the absence of added water. 

A different technique was used by Durand Keklikian and Partch [16] to generate particles with a surface coating. Previously, a more volatile substance was coated on a less volatile particle. For their case, oil droplets coated with metal oxide were generated. This was accomplished by nebulizing solutions of titanium or aluminum alkoxides in oil. Hydrolysis of the alkoxide to the oxide occurred in the presence of water vapor, forming a solid shell encapsulating the oil droplet. 

Following the reaction of water or water vapor with a metal oxide surface, as discussed above, one might expect the surface structure to be different than a simple termination of the bulk structure for normally anhydrous minerals. This is in fact what... 

Surface Hydroxylation When the metal is exposed to the atmosphere, there is an instant reaction of water vapor with the metal oxide surface. In most cases, the water molecule dissociates upon reaction and forms metal-oxygen or metal-hydroxyl bonds. The probability for the water molecule to dissociate, rather than to bond to the substrate in molecular form, increases with the number of lattice defects in the metal oxide. As a result, surface hydroxyl groups are formed, which act as adsorption sites for other water molecules. Surface hydroxylation is a rapid process that occurs on a timescale far shorter than a second. 

Start with the reaction of metal oxide surfaces with water vapor and then discuss investigations of different kinds of catalytic systems before ending with an example of the application of APXPS to electrochemistry. 

There are many methods for application of organosilanes on metals and metal oxide surfaces. Dip coating in aqueous solutions or alcohol solutions containing controlled amount of water, vapor deposition in atmosphere with controlled humidity, and spraying for large application areas are among the mostly utilized methods. 

The corrosion behavior of plutonium metal has been summarized (60,61). a-Plutonium oxidizes very slowly in dry air, typically <10 mm/yr. The rate is accelerated by water vapor. Thus, a bright metal surface tarnishes rapidly in normal environments and a powdery surface soon forms. Eventually green PUO2 [12059-95-9] covers the surface. Plutonium is similar to uranium with respect to corrosion characteristics. The stabilization of 5-Pu confers substantial corrosion resistance to Pu in the same way that stabilization of y-U yields a more corrosion-resistant metal. The reaction of Pu metal with Hquid water produces both oxides and oxide-hydrides (62). The reaction with water vapor above 100°C also produces oxides and hydride (63). 

Zirconium is a highly active metal which, like aluminum, seems quite passive because of its stable, cohesive, protective oxide film which is always present in air or water. Massive zirconium does not bum in air, but oxidizes rapidly above 600°C in air. Clean zirconium plate ignites spontaneously in oxygen of ca 2 MPa (300 psi) the autoignition pressure drops as the metal thickness decreases. Zirconium powder ignites quite easily. Powder (<44 fim or—325 mesh) prepared in an inert atmosphere by the hydride—dehydride process ignites spontaneously upon contact with air unless its surface has been conditioned, ie, preoxidized by slow addition of air to the inert atmosphere. Heated zirconium is readily oxidized by carbon dioxide, sulfur dioxide, or water vapor. 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

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