Oxide surface metal ions

Schematic representation of the cross section of the surface layer of a metal oxide. , Metal ions O, oxide ions. The metal ions in the surface layer (a) have a reduced coordination number. They thus behave as Lewis acids. In the presence of water the surface metal ions may first tend to coordinate H20 molecules (b). For most of the oxides dissociative chemisorption of water molecules (c) seems energetically favored.

In heterogeneous photoredox reactions not only the solid phase, i.e. the semiconducting mineral, may act as the chromophore (as discussed in Chapter 10.2) but also a surface species (i) a surface complex formed from a surface metal ion of a metal (hydr)oxide and a ligand that is specifically adsorbed at the surface of the solid phase, and (ii) a chromophore that is specifically adsorbed at the surface of a solid phase. In the following these three cases will briefly be discussed. 

The Triple T.aver Model and the Stern Model. The ions most intimately associated with the surface are assigned to the innermost plane where they contribute to the charge Oq and experience the potential tI>q These ions are generally referred to as primary potential determining ions. For oxide surfaces, the ions H+ and 0H are usually assigned to this innermost plane. In Stern s original model, the surface of a metal electrode was considered, and the charge cjq was due to electrons. 

The results of Weiss field calculation on ferric ions at the surface metal ion sites are given in Figure 6 of ref 4, and the values for room temperature are shown in Figure 10. Since both ferric and pentavalent Sb ions can occupy octahedral or distorted octahedral sites with six ligand oxide ions and bulk hematite is considered to accommodate pentavalent Sb—119 ions in the metal ion sites (3 ), we can estimate STHF interactions on tetravalent Sn-119 ions at the surface metal ion sites of hematite. Using the magnetization of surface ferric ions at room temperature, the STHF magnetic fields on tetravalent Sn-119 ions at the surface sites are calculated to be... 

In contrast, the pentavalent Sb-119 ions at the interfaces are weakly bonded to the oxide ion layer of the hematite surfaces in neutral and slightly acidic region, while in the acidic region most of the adsorbed Sb-119 ions are in the zeroth or first metal ion layers of the substrate forming Sb-O-Fe bonds. The pentavalent Sb-119 ions having once been incorporated into the surface metal ion sites retain their chemical form, even when the pH of the aqueous phase is raised above 7. Heating of suspensions at 98°C results in chemical rearrangement of the hematite surfaces to yield pentavalent Sb-119 ions in the second or deeper metal ion layers. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

G. (1999) Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine. Chem. Geol. 158 81-103 Allen, G.C. Kirby, C. Sellers, R.M. (1988) The effect of the low-oxidation-state metal ion reagent tris-picolinatovanadium(II) formate on the surface morphology and composition of crystalline iron oxides. J. Chem. Soc. Faraday Trans. I. 84 355-364... 

Fig. 3. Surface-bound intermediate formed in the ring-opening of isomeric 2-methyl-3-phenyloxiranes on oxides (M= metal ions).

McBride and Wesselink (1988) studied IR spectra of catechol adsorbed onto the oxide surface and found evidence that the compound was chemically altered, indicating that chemisorption was the dominant mechanism. In addition to catechols, phenols are known to adsorb onto metal oxide surfaces. This adsorption is dependent on the number and position of hydroxy substitutions on the benzene ring. Diphenolic compounds adsorb to a greater extent than monophenolic compounds, suggesting the formation of a bidentate bond with the metal oxide. This bidentate bond is formed when the two phenolic ligands coordinate with one or two surface metal ions (McBride and Wesselink, 1988). 

For quite some time, it was not clear whether the catalytic cracking process of these aluminosilicates involves the concerted action of both Lewis and Bronstedt acid sides, or only Lewis sites, or even the dissociation of the adsorbed hydrocarbons by surface metal ions (impurities).16,17,18 19 The need for oxide systems that allow evidence concerning the above catalytic theories was one of the driving forces for the efforts that have been done to coat a silica surface with aluminium compounds. One of the earlier studies, concerning both the reaction mechanisms of the aluminium... 

It is assumed that the organometallic, RME, is oxidized to metal ion ME and radical R which reacts with active anodes ME2 to form RME2. An Sg2-reaction of ME, formed at the anode surface, with RME, may however also... 

Carbon dioxide, C02, is a fairly small molecule with acidic properties, which has frequently been used as a probe molecule for basic surface sites and as a poison in catalytic reactions. As shown in the following, C02 adsorption onto oxide surfaces leads to a variety of surface species such as bicarbonates and carbonates that coordinate to surface metal ions in various ways. The type of the coordination influences the symmetry of these ligands so that different surface species held by distinct surface sites can be distinguished by means of their infrared absorptions (162). The characteristic infrared (and Raman) bands of C02 and possible surface species are summarized in Table VI. The wave-number range below 1000 cm"1 was usually not accessible in studies on adsorbed C02 because of the strong absorption of the oxides at lower wave numbers. 

H2 gas itself is rapidly sorbed by the transition metals and more slowly by metal oxides and elements such as carbon (graphite) and germanium. On the oxides the sorption frequently leads to the formation of hydroxides, and on heating H2O may be desorbed. Some reversible sorption occurs as well, and it has been suggested that this corresponds to a hydride formation with the surface metal ions. In the case of metals, H2 gas is sorbed rapidly even at 78 K with a heat of sorption which may be of the order of 40 Kcal or more, decreasing slowly with increasing coverage until near saturation, when it can approach zero. A considerable account of evidence supports the view that the sorption on metals is a direct 1 1 stoichio-... 

Illustration of the surface of a metal oxide where small circles are the metal atoms and large circles are oxygen atoms. (A) Surface metal atoms are not totally coordinated. (B) In water, surface metal ions coordinate water molecules. (C) Dissociative chemisorption leads to a hydroxylated surface. From Schindler and Stumm (1987). 

Although this review has dealt with the interactions of metals with reduced oxide surfaces, metal-support interactions are certainly not limited to these. Evidence for metal-support interaction involving non-reduced surfaces exists even in the metal/ titania system. Enhanced hydrogenolysis activities have been found for low-temperature-reduced Rh/titania (7) and Ru/titania (49). These effects presumably involve interaction with Ti4+ ions. 

Reductive dissolution reactions can be described by a three-step reaction sequence (Stone, 1986). The steps are (i) adsorption of the reductant forming either an inner- or outer-sphere surface complex, (ii) electron transfer from a reducing agent to a surface metal ion, and (iii) release of the reduced ion. For Mn(III/IV) oxides the reductive step is complicated by the necessity for Mn(IV) to be reduced first to Mn(III) then to Mn(II). All natural Mn oxides, however, contain both Mn(IV) and Mn(III) (McKenzie, 1989). Even laboratory MnO2 preparations contain some Mn(III). 

The dissolution of oxides and silicates in the presence of O-containing organic ligands that form bidentate complexes with surface metal ions is a function of the concentration of complexation sites on the surface. In some cases, the precursor site for detachment may involve protonation as well as surface complexation. Some data suggest that at high concentrations of ions that form strong surface complexes, the rate-controlling step may involve surface attack. 

As with metal cations, anion chemisorption occurs on soil minerals that possess surface hydroxyl groups. The most important minerals in this regard are noncrystalline aluminosilicates (allophanes) oxides and hydroxides of Fe, Al, and Mn and layer silicate clays (edge sites only). It is the H2O or valence-unsatisfied OH ligands bound to surface metal ions (usually Fe, Al, or Mn) that are the sites of chemisorption. In general terms, the surface reaction can be written... 

Surface-localized redox reactions can be viewed as the transfer of an electron between one particular surface metal ion and an adsorbed molecule, with a change in the oxidation state of the metal ion. This is a reasonable description if electrons have no mobility in the mineral that is, if the mineral is an insulator. However, some minerals are semiconductors or conductors, in which case electron transfer might be better described as insertion of electrons into (or extraction of electrons from) the overlapping electronic orbitals of the solid. The resultant electron excess or deficit is then delocalized over the solid, not associated with one particular metal ion at one surface location. 

In the case of metallic corrosion, the local cell model assumes that corrosion occurs as a combination of anodic metal oxidation and cathodic oxidant reduction. The anodic metal oxidation (dissolution) is a process of metal ion transfer across the metal-solution interface, in which the metal ions transfer from the metallic bonding state into the hydrated state in solution. We note that, before they transfer into the solution, the metal ions are ionized forming surface metal ions free from the metallic bonding electrons. The metal ion transfer is written as follows ... 

Metal atoms on the metal surface, as mentioned earlier, are soft acid, and hence they combine with anions of soft base on the metal surface. Once these metal surface atoms are ionized, they form metal ions such as iron ions and aluminum ions, and the metal surface turns to be hard acid. The metal ions then combine with anions of hard base such as hydroxide ions, OH, oxide ions, 02, and sulfate ions, SO4, to form insoluble metal oxides and salts of ionic bonding character. The two-dimensional concentration of surface metal ions increases with the electrode potential of the metal, and hence the metal surface gradually becomes harder in the Lewis acidity with increasing electrode potential until it combines with anions of hard base such as oxide ions to form a metal oxide film adhering firmly to the metal surface. The passivation potential of a metal is thus regarded as a threshold potential where the metal surface grows hard enough in the Lewis acidity to combine with a hard base of oxide ions. 

Reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes led to very active bimetallic, heterogeneous olefin metathesis catalysts. Surface metal ions might be involved in bonding interactions with the organometallic complex, possibly leading to heterometallic species on inorganic oxides. ... 

Hair is an excellent ion exchange system. Metallic ions may be sorbed to hair in multiple forms such as lipids (e.g., calcium stearate) or as particulates (e.g., metal oxides). Many metallic ions such as copper (-1-2) [11] can adsorb to hair, especially after frequent exposure to swimming pool water. It has been suggested that metallic ions such as chromium, nickel, and cobalt may bind to hair from swimming pool water [11]. Sorption of metallic ions like calcium or magnesium occurs even from low concentrations in the water supply rather than from hair products. However, fatty acids present in hair products enhance the adsorption of most of these metallic ions to the hair surface, as described earlier. Heavy metals such as lead and cadmium have been shown to collect in hair from air pollution [12], and other metals like zinc are available from antidandruff products, from the zinc pyrithione active ingredient. 

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