Valence states

The principal use of Auger spectroscopy is in the determination of surface composition, although peak positions are secondarily sensitive to the valence state of the atom. See Refs. 2, 82, and 83 for reviews. 

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

Fig. XVin-6. Curve-fitted Mo XPS 3d spectra of a 5 wt% Mo/Ti02 catalyst (a) in the oxidic +6 valence state (b) after reduction at 304°C. Doublets A, B, and C refer to Mo oxidation states +6, +5, and +4, respectively [37]. (Reprinted with permission from American Chemical Society copyright 1974.)...

The first reliable energy band theories were based on a powerfiil approximation, call the pseudopotential approximation. Within this approximation, the all-electron potential corresponding to interaction of a valence electron with the iimer, core electrons and the nucleus is replaced by a pseudopotential. The pseudopotential reproduces only the properties of the outer electrons. There are rigorous theorems such as the Phillips-Kleinman cancellation theorem that can be used to justify the pseudopotential model [2, 3, 26]. The Phillips-Kleimnan cancellation theorem states that the orthogonality requirement of the valence states to the core states can be described by an effective repulsive... 

The orthogonality condition assures one that the lowest energy state will not converge to core-like states, but valence states. The wavefimction for the solid can be written as... 

In this equation, the electronegativity of an atom is related to its ionization potential, 1, and its electron affinity, E. Mulhken already pointed out that in this definition the ionization potential, and the electron affinity, E, of valence states have to be used. This idea was further elaborated by Hinze et al. [30, 31], who introduced the concept of orbital electronegativity. 

Table 7-5 gives these coefficients for the various valence states of a variety of atoms. 

This approximation leads to the CNDO/2 scheme, ft remains to explore the valiiesoflf l vvhich are closely related to valence state... 

Values of the coefficients a, b and c were derived for common elements in their usual valence. states (for example, for carbon there are different values for sp, sp Tr and spw valence states). 

The diagonal integrals Za,a, which represent the mutual coulomb repulsions between a pair of electrons in the valence-state orbital labeled a, are calculated in terms of the valence-state IP and EA of that orbital ... 

Cerium is especially intereshng because of its variable electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small amounts of energy are required to change the relahve occupancy of these electronic levels. This gives rise to dual valency states. 

Even ia 1960 a catalytic route was considered the answer to the pollution problem and the by-product sulfate, but nearly ten years elapsed before a process was developed that could be used commercially. Some of the eadier attempts iacluded hydrolysis of acrylonitrile on a sulfonic acid ion-exchange resia (69). Manganese dioxide showed some catalytic activity (70), and copper ions present ia two different valence states were described as catalyticaHy active (71), but copper metal by itself was not active. A variety of catalysts, such as Umshibara or I Jllmann copper and nickel, were used for the hydrolysis of aromatic nitriles, but aUphatic nitriles did not react usiag these catalysts (72). Beginning ia 1971 a series of patents were issued to The Dow Chemical Company (73) describiag the use of copper metal catalysis. Full-scale production was achieved the same year. A solution of acrylonitrile ia water was passed over a fixed bed of copper catalyst at 85°C, which produced a solution of acrylamide ia water with very high conversions and selectivities to acrylamide. 

Replacement of Hydrogen. Three methods of substitution of a hydrogen atom by fluorine are (/) reaction of a G—H bond with elemental fluorine (direct fluorination, (2) reaction of a G—H bond with a high valence state metal fluoride like Agp2 or GoF, and (J) electrochemical fluorination in which the reaction occurs at the anode of a cell containing a source of fluoride, usually HF. 

Higher valence-state metal ions can abstract hydrogen from a hydroperoxide (25) (eq. 35) or from a substrate (eq. 36). 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Iron(II) can be analy2ed by a luminol—air reaction in the absence of hydrogen peroxide (276). Iron in the aqueous sample is reduced to iron(II) by sulfite other metals which might interfere are also reduced to valence states that are inactive in the absence of hydrogen peroxide. The detection limit is 10 ° M. 

The bulk stmcture of the catalyticaHy active phase is not completely known and is under debate in the Hterature (125,131—133). The central point of controversy is whether (Valone or in combination with other phases is the most catalyticaHy active for the conversion of butane to maleic anhydride. The heart of this issue concerns the role of stmctural disorder in the bulk and how it arises in the catalyst (125,134,135). Most researchers agree that the catalysts with the highest activity and selectivity ate composed mainly of (Vthat exhibits a clustered or distorted platelet morphology (125). It is also generaHy acknowledged that during operation of the catalyst, the bulk oxidation state of the vanadium in the catalyst remains very close to the +4 valence state (125). 

As the oxidation state of manganese increases, the basicity declines, eg, from MnO to Mn20y. Oxyanions are more readily formed ia the higher valence states. Another characteristic of higher valence-state manganese chemistry is the abundance of disproportionation reactions. 

Oxidation of manganese dioxide to higher valence states takes place in the fusion process of Mn02 and KOH. A tetravalent manganese salt identified as K MnO [12142-27-7] (63) which disproportionates spontaneously is formed. 

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