Surfaces anion structures

For instance, in situ Fourier transform infrared (FTIR) spectroscopy has been used by Faguy etal. [176] to study the potential-dependent changes in anion structure and composition at the surface of Pt(lll) electrodes in H 804 -containing solutions. From the infrared differential normalized relative reflectance data, the maximum rate of intensity changes for three infrared bands can be obtained. Two modes associated with the adsorbed anion... 

Surfaces of real crystals never adopt the bulk-truncated structures shown in Fig. 4.5. They reconstruct or relax (inwards or outward movement of the atoms) to minimize their surface energy [42]. Known surface structures of zincblende and wurtzite structure semiconductors are summarized in [43]. Nonpolar surfaces of wurtzite (1120) and (1010) surfaces show no lateral surface reconstructions and are supposed to have a structure similar to the well-known zincblende (110) surface, which is characterized by an inward relaxation of the surface cations and partial electron transfer from the surface cation dangling bond to the surface anion dangling bond [42,43]. 

Immersion of Pt(lll) and Ag(lll) surfaces into aqueous ionic solutions at controlled pH and electrode potential results in the formation of a highly ordered chemisorbed layer, an adlattice [25, 26], For example, the Pt(lll) surface was examined by LEED and Auger spectroscopy after immersion into aqueous KBr and CaBr2 solutions [27], The pH and electrode potentials are both important variables in regard to surface layer structure and composition pH was controlled at 4, 6, 8 and 10 potential was controlled at 50 mV increments from the negative limit due to H + reduction to the positive limit of 02 evolution. Contrary to the traditional suppositions [28], the adsorbed particles are present primarily in the form of neutral atoms (Br) rather than as anions (Br ). The adsorption process is... 

Photoluminescence techniques will be applied to a broader range of systems, particularly oxide-supported sulfides (because of their important role in hydrotreating catalysis) as well as unsupported or oxide-supported (oxi)carbides or (oxi)nitrides (because of their growing importance as substitutes for noble metals and because they have metallic and acidic functions). Moreover, improved procedures for preparing catalytic materials will enable the design of tailored oxides with better defined characteristics, such as size, composition, and structure. The accumulation of data concerning the behavior of surface anions will also lead to a more refined view of the coordination chemistry of anions of nontransition elements. 

Similar observations, regarding the adsorption sequences of cations and anions, have also been made for other oxides (a-Fe203, ZnO) and have led to a description of such oxide surfaces as structure promoting This representation constitutes an extension of the Gurney s interpretation of ion-ion interactions in solution to the ion-surface interactions. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

The intent of this chapter is not to survey noninvasive surface spectroscopy but to illustrate briefly how it is applied to resolve the Stummian issue of whether inner-sphere surface complexes form. For this purpose, the application of electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) spectroscopies to elucidate metal cation speciation and the use of extended X-ray absorption fine structure (EXAFS) spectroscopy to detect surface anion species will be described. Emphasis will be on the interpretation of spectra. Sample preparation and instrumentation details were reviewed in recent volumes edited by Hawthorne (55) and Perry (27). Because the constant capacitance model was developed in the context of adsorption by hydrous oxides, these... 

Tn 1922 Adam (I) published the third paper in his extraordinary series on surface film structure. He observed that fatty acid monolayers greatly expanded on alkaline subphases. He also suggested that fatty acid anions desorbed or dissolved from the monolayer into the alkaline subphase. In 1933 he and Miller (2) showed that the composition of the subphase buffer significantly affected the monolayer thus palmitic and stearic acid monolayers were more condensed on 2N sodium hydroxide than on 2N potassium hydroxide. The expansion, desorption, and cation selectivity of ionizing monolayers are the subjects of this investigation. 

On the other hand, in a sulfuric acid solution, a ( /3 x /3) R30° copper bisulfate structure can be observed for Pt(l 11) surfaces. This structure consists of 0.22 ML of anions and 0.66 ML of copper in the unit cell [108]. The same features were demonstrated on Au(lll) [109]. In all of these models, a potential window exists, where it is possible to co-adsorb products of water and proton discharge, causing an electrocatalytic activity toward oxidation or reduction, respectively. 

The key properties of oxide surface are the chemical bonding feature, coordination environment, oxidation state and acidic or redox properties of surface cations, and the basicity of surface anions. The longer term challenge to oxide surface chemistry is to address important issues in selectivity in catalytic oxidation and acid-base reactions, in particular the principle of tuning of metal reactivity by oxide ligands. In the development of new catalysts, new chemical concepts regarding structure or composition are conceived. The requirements... 

---