Oxide, multiple cation/anion

Excess electrons e" can be either easily accepted by Ce + cations to form Ce3+ cations or provided by the latter ones respectively. High vacancy concentrations (Vq) shift this reaction to the left, resulting in the stabilization of Ce + ions in the catalyst. Since the total oxidation of methane which occurs over a pure Ce02 catalyst is at least partly due to the accessible multiple cationic oxidation states [16, 17], it seems rather reasonable to assume that the presence of anionic vacancies which are created by incorporating calcium cations into the Ce02 matrix could favor the reduction of the non-selective surface oxidation through the stabilization of the Ce + state. 

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

Rai and Serne, 1977 Amacher and Baker, 1982). Both elements can exist in multiple oxidation states in aqueous environments. Chromium can exist in trivalent and hexavelent states, while Pu can occur in trivalent, quadrivalent, pentavalent, and hexavalent states. Additionally, both elements can exist as cationic or anionic species in aqueous systems. Trivalent Cr exists as the cation Cr3+ and its hydrolysis products, or as the anion CrO at very low concentrations. Hexavalent Cr occurs as the dichromate Cr207 or chromate HCrO or CrO anions, depending on pH. Plutonium exists in cationic states such as Pu3+ and Pu02 and anionic forms such as Pu02(C030H ). 

What about when the ionic charges aren t equal to each other The most mathematically sophisticated way to determine this ratio is to seek the least common multiple between the absolute values of the charges on the cation and anion. Most beginning students, however, simply use the criss-cross method, as shown in Figure 2.9. That is, the number of cations in the formula is the absolute value of the charge on the anion. The number of anions in the formula is the absolute value of the charge on the cation. For example, as shown above, the formula of the ionic compound containing aluminum ions and oxide ions is... 

It is interesting that oxygen rich species, Cofif and 030 , exhibited O3 loss in CID experiments and underwent oxygen atom transfer as the major reaction with CO. No association of CO onto anionic cobalt oxide clusters was observed. For cationic cobalt oxide clusters, association of CO followed by loss of O3, was the major reaction channel observed, as shown for CoO " in Fig. 14.6. Association of multiple CO molecules accompanied by loss of O3 subunits was also observed for the C0O3 / and Co O cluster species indicating that these cationic centers may become quickly saturated, and therefore, poisoned at high concentrations of CO. 

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