Oxidation cation, chemisorption

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... 

With the oxide semiconductors, anionic chemisorption would take place over the metal cations, and the interaction problem would be between the orbitals on the foreign atom and the cation band (the 3d band in CU2O, for example). The discussion in this section is relevant if this is the highest filled band. 

With the oxide semiconductors, cationic chemisorption should occur over the lattice anions, and we would expect therefore to have an interaction problem involving the anion bands as well as the cation band. This makes the whole problem much more complicated and because the cation-anion band model is not adequate for the transition metal oxides, we shall not discuss this problem here. 

With the semiconducting oxides, we expect anionic chemisorption to occur over the lattice cations, and our simple molecular orbital theory will be adequate if the conduction band is associated mainly with the cation lattice. This is certainly the case with AI2O3, where there is direct evidence in the soft X-ray emission spectra that the highest filled band is the oxygen 2p band 16). 

Therefore it is evident that Cr2O3 will catalyze neither step (37) nor steps (39a) or (40a). Likewise, the catalytical activity cannot be changed by additions of foreign oxides with cations of different valencies (see Fig. 11). In the case of CuO, the chemisorption is predominant at lower temperatures with formation of boundary layers, accompanied by polarization effects, especially at the start and during the steady state of chemisorption. At higher temperatures, however, we must expect a noticeable... 

Oxide electrodes have been observed to be almost immune from poisoning effects due to traces of metallic impurities in solution [99]. This is undoubtedly due primarily to the extended surface area. It can be anticipated that the calcination temperature must have a sizable effect. But in addition, a different mechanism of electrodeposition must be operative. Chemisorption on wet oxides is usually weak because metal cations are covered by OH groups [479]. As a consequence, underpotential deposition of metals is not observed on Ru02, although metal electrodeposition does takes place. However, electrodeposited metals give rise to clusters or islands and not to a monomolecular layer like on Pt. Therefore, the oxide active surface remains largely uncovered even if metallic impurities are deposited [168]. Thus, the weak tendency of oxides to adsorb ions, and its dependence on the pH of the solution is linked to their favorable behavior observed as cathodes in the presence of metallic impurities. 

The crystal structure of a-Cr203 is made up by a hexagonal close-packed lattice of oxide ions (sequence ABAB ) Two-thirds of the octahedral sites are occupied by Cr3+ ions. Possible idealized surface structures, based on the (001), (100), and (101) planes and the creation of surface sites in the form of coordi-natively unsaturated cations and anions on dehydroxylation of the surface, have been discussed by Burwell et al. (21) and by Stone (144). The (001) face is the most likely crystal plane to predominate in the external surface of well-crystallized a-Cr203 (145). A possible surface model that maintains the overall as well as the local electrical neutrality, as proposed by Zecchina et al. (145) for the dehydroxylated (001) face, is shown in Fig. 2a. It can clearly be seen that equal numbers of four- and five-coordinate Cr3+ ions are to be expected on this idealized surface. Dissociative chemisorption of water would lead to the formation of surface OH groups, as shown in Fig. 2b, for a partially hydroxylated model surface. In fact, on adsorption of D20, Zecchina et al. (145) observed OD-stretching fundamental bands at 2700 and 2675 cm-1, which were narrow and isolated. As evidenced by the appearance of a H20 bending band at 1590... 

In spite of numerous investigations of metals in the SMSI state (cf. bibliography of Ref. 153), the exact nature of the phenomenon is still controversial (131), although there exists a fairly compelling relation to support reducibility (14, 71, 128, 153, 288,290). Changes in chemisorptive, catalytic, and structural properties in the SMSI state strongly suggest an electronic interaction at the metal-oxide interface with (whole or partial) electron transfer between a subjacent cation and a supported metal. Since the SMSI state apparently encompasses both a structural and an electronic... 

The surface chemistry of zinc oxide is of particular interest in relation to its catalytic and photocatalytic properties. For example the (0001) hexagonal crystal plane appears to have a special role in the catalytic methanol-synthesis reaction (Bowker et al., 1983). The chemisorption of CO and dissociative chemisorption of H2 occur on the exposed Zn2+ cations Bolis et al. (1986) have found that the relative magnitude of this active area of ZnO was highly dependent on the nature of the precursor (oxalate, carbonate of Zn). Similar conclusions can be drawn from the infrared spectroscopic measurements of Chauvin et al. (1986). 

Nanosized ceria-zirconia materials with improved thermal stability can be prepared by using the surfactant-assisted method. Structural refinements confirm that the nanocrystals contain structural microstrain and cationic lattice defects. Zirconium addition to ceria supresses the crystal sintering and imporves the thermal stability but leads to structure distortion. Both catalytic tests and CO-chemisorption show that Pd supported ceria-zirconia nanoparticles are active for CO oxidation. 

Since the surfaces of metal oxides contain both negatively charged anions and positively charged cations — often with more than one cation valence state present —many avenues of adsorption are available. The various chemisorption and physisorption possibilities on metal oxides are discussed at length in Ref. 1 they will only be briefly summarized here. 

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

In the past, there have been two main types of interpretations of chemisorption and catalysis on chromia. One is more or less of the type given in this chapter (for example, 39, 56, 63a, 66). The other is based upon theories of semiconductors (for example, 68-70). Chromia is a semiconductor at high temperatures (20). No complete theory of chemisorption on chromia is possible at present one can only use approximate treatments. However, in our opinion, the first type of approximation (which is related to coordination chemistry and cry.stal field theory) is much more useful than the second type for reactions in reducing atmospheres at lower temperatures, say below 300°. Morin (70a) has given an analysis of transition metal oxides which indicates that the 3d band in a-Ci 203 is so narrow as to correspond to 3d charge carriers localized on the cations. 

Removal of lattice oxygen from the surface of nickel oxide in vcumo at 250° or incorporation of gallium ions at the same temperature [Eq. (14)] causes the reduction of surface nickel ions into metal atoms. Nucleation of nickel crystallites leaves cationic vacancies in the surface layer of the oxide lattice. The existence of these metal crystallites was demonstrated by magnetic susceptibility measurements (33). Cationic vacancies should thus exist on the surface of all samples prepared in vacuo at 250°. However, since incorporation of lithium ions at 250° creates anionic vacancies, the probability of formation of vacancy pairs (anion and cation) increases and consequently, the number of free cationic vacancies should be low on the surface of lithiated nickel oxides. Carbon monoxide is liable to be adsorbed at room temperature on cationic vacancies and the differences in the chemisorption of this gas are related to the different number of isolated cationic vacancies on the surface of the different samples. 

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