Metal oxide surface, dissociative

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,... 

In contrast to the minimal activity in infrared reflection studies the technique of inelastic electron tunneling spectroscopy (IETS) recently has contributed a large amount of information on monolayer adsorption of organic molecules on smooth metal oxide surfaces,Q),aluminum oxide layers on evaporated aluminum. These results indicate that a variety of organic molecules with acidic hydrogens, such as carboxylic acids and phenols chemisorb on aluminum Oxide overlayers by proton dissociation - 1 — and that monolayer coverage can be attained quite repro-ducibly by solution doping techniques. - The IETS technique is sensitive to both infrared and Raman modes. — However, almost no examples exist in which Raman il and or infrared spectra have been taken for an adsorbate/substrate system for which IETS spectra have been observed. 

The surfaces of mtile Ti02 have been the subject of intense research because of their photo-catalytic properties for the dissociation of water. The hydroxylation rate on the surface and the kinetics of the reaction were shown to depend strongly on the surface stoichiometry and detailed atomic structure. In addition, like the two above surfaces of sapphire and magnesium oxide, rutile titanium dioxide surfaces stand as model metal oxide surfaces. Their atomic structure is thus of fundamental interest. 

Infrared spectroscopic studies (185-188) indicate that nitric oxide is adsorbed onto metal surfaces, not only as the familiar nitrosyl ligand but also as [N202] (hyponitrite) units. On metal oxide surfaces, chelating nitrite has also been detected, and such species are believed to be intermediates in oxygen-exchange reactions between N 0 and bulk oxide in NiO or Fo203. The rate of this process is measurable under conditions (e.g., room temperature) in which gas-phase dissociation is minimal. Spectroscopic studies also suggest that nitric oxide reacts... 

It is generally known that metal oxide surface is covered with hydroxyl groups when oxide is placed in water. The presence of two free electron pairs of oxygen atom and possibility of hydrogen ion dissociation is the evidence of amphoteric character of these groups. On account of this, the most useful parameter in description of the water/metal oxide interface is pH of the solution being in contact with the surface. Adsorption of H" " or OH ions causes protonization or deprotonization of the surface according to the Eqs. (31a) and (31b). 

Anodier example of dissociative chemisorption is the heterolytic cleavage of hydrogen on metal oxide surfaces. The reaction of hydrogen wifli a zinc oxide surface produces a zinc-hydride bond and a proton boimd to an oxygen center (Eq. 5-25) [T3 9]. 

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. 

We note in passing that for dissociation of many gases the adsorption of molecules from the gas phase directly upon the surface of catalyst nanoparticles is not necessary. Tsu and Boudart (1961) and others (Henry et al. 1991 Bowker 1996) showed that the molecules can first adsorb onto the oxide support and diffuse to a catalyst particle. This means that the effective capture radius of a catalyst nanoparticle (Pd, Pt, etc.) can be much greater than the nanoparticle s physical radius. As with the spillover zones, the molecule-collection zones (Tsu and Boudart 1961) overlap when the coverage of catalyst nanoparticles exceeds some threshold value, effectively converting the entire surface of the nanostructure into a molecular delivery system for the metal catalyst nanoparticles. This so-called back-spillover effect further increases the likelihood of molecular dissociation and ionosorption on the metal oxide surface. 

The relative ease with which hydrogen chemisorbs on the surface of a metal oxide surface mainly depends on the chemical nature of the oxide and on the O-vacancies. Thus, hydrogen adsorbs dissociatively on a perfect titanium oxide surface [10,11]. The energetically most favorable mode for the adsorption of atomic hydrogen is the adsorption on the outermost O atom, accompanied by the reduction of a Ti atom. In this mode, protons are formally adsorbed while an equivalent amount of Ti(IV) atoms are reduced to Ti(III). Theoretical calculations have demonstrated that H adsorption is less favorable on a defective surface than on a perfect surface. However, the best adsorption mode for the atomic chemisorption on a defective surface is heterolytic adsorption, which involves two different adsorption sites one H+/0= and one H on the surface. This adsorption mode is best on irreducible oxides such as MgO however, it is less favorable than adsorption on the perfect Ti02 surface [10]. The heat of atomic adsorption in all cases is very weak and dissociation onto the surface is unlikely. The molecular adsorption (physisorption), thus, remains the most stable system. 

Figure 3.11a schematically shows the reactions involved in the oxidation of CO to CO2 on a metal oxide surface, such as that of ZnO. The first step is the adsorption of CO in the form of a surface carbonate, as observable in infrared spectroscopy. Next CO2 desorbs from the surface, leaving two oxygen vacancies behind. The cycle is completed by dissociative adsorption of oxygen, which produces oxygen atoms to fill the vacancies. According to Boreskov, such a stepwise mechanism prevails at higher temperatures (i.e., above 525 K on CuO), where the rates of reduction and reoxidation of the oxide surface are comparable to the overall rate of the CO oxidation. At lower temperatures, however, reduction and reoxidation slow down, whereas the rate of reaction decreases considerably. This is why desorption of CO2 and reoxidation of the surface are believed to occur simultaneously in a concerted reaction step. 

Water molecules will be adsorbed on metal oxide surfaces to compensate the oxygen defects in the lattice. Afterwards, water molecules will dissociate and the surface of mthenium oxide is covered with hydroxide groups. When this layer is exposed to an electrolyte a Helmholtz double layer is formed at the electrodeelectrolyte interface which is responsible for the electrochemical potential of the electrode attributed to the pH of the solution. 

From the positive results of these experiments we concluded that the behaviour of atomic and molecular particles of silver with respect to their influence on electrophysical properties of oxide films is similar to that of atoms and molecules of nonmetals, with the only difference that metal-atom interstitials behave similar to hydrogen-like donors of electrons, independent of the kind of a metal. As to metal molecules, at low temperatures of a semiconductor film, when their surface dissociation does not occur, they do not reveal considerable activity with respect to electrophysical properties of the film. 

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