Surface cations

Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

Steady-state molecular beam studies of the reaction of methylacetylene on reduced Ti02 (001) surfaces were undertaken to determine whether this reaction could be performed catalytically under UHV conditions. A representative experiment is presented in Figure 1. Prior to each experiment, the surface was sputtered and annealed to a temperature between 400 K and 550 K surfaces prepared in this manner have the highest fraction of Ti(+2) sites (ca. 30% of all surface cations) of any surface we have been able to create by initial sputtering [3]. Thus these are the surfaces most active for cyclotrimerization in TPD experiments [1]. Steady-state production of trimethylbenzene (as indicated by the m/e 105 signal detected by the mass spectrometer) was characterized by behavior typical of more traditional catalysts a jump in activity upon initial exposure of the crystal to the molecular beam, followed by a decay to a lower, constant level of activity over a longer time scale. Experiments of up to 6 hours in duration showed... 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

Emersion resulting in substantial amounts of electrolyte remaining on the (hydrophilic) electrode is much more commonly observed. When the solvent evaporates from the surface, the electrolyte is left behind as small crystallites. These can distort LEED and RHEED patterns and, more importantly, can render quantitative evaluation of the surface cation and anion concentrations by ESCA impossible. 

Fig. 2-30. Surface dangling states and surface ion-induced states (a) surface dangling donor (DL-B) and acceptor (DL-AB) leveb on covalent bonding semiconductors, (b) surface cation-induced acceptor (SCL) and surface anion-induced donor (SAL) levels on ionic bonding semiconductors.

Lattice defects in ionic crystals are interstitial ions and ion vacancies. In crystalline sodium chloride NaCl a cation vacancy Vn - is formed by producing a surface cation NaJ, (Nal - NaJ + Vua ) this is called the Schottky defect. On the other hand, in crystalline silver chloride AgCl a pair of cation vacancy Va,. and interstitial cation Ag is formed, (Ag - Agj + ) this is called the Frenkel... 

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Fig. 9-13. Reaction rate of simultaneous dissolution of surface cations and anions from a semiconductor electrode of ionic compound as a iimction of potential of a compact layer 4 )=potmitial of acorn-...

It appears that the concentration of surface cations increases with increasing cathodic polarization (decreasing 4< >h ) whereas, the concentration of surface anions increases with increasing anodic polarization (increasing 4h). The dependence of the concentration of surface constituents on suggests that the dissolution rate of MX is determined by the transfer of cations at less anodic potentials and by the transfer of anions at more anodic potentials. 

Semiconductor electrodes of ionic compounds can also dissolve with the oxidation of surface anions or with the reduction of surface cations as shown schematically in Fig. 9-15. 

Equation 9—49 is the anodic transfer of surface cation into aqueous solution (cation dissolution) and Eqn. 9-60 is the anodic oxidation (hole capture) of surface anion producing molecules ofX2, i (e.g. gaseous oxygen molecules irom oxide ions). Electric neutrality requires that the rate of cation dissolution equals the rate of anion oxidation hence, the rate of the oxidative dissolution of semiconductor electrode can be represented by the anodic hole current for the oxidation of surface anions. 

Fig. 9-15. Oxidative and reductive dissolution reactions of semiconductor electrodes of ionic compounds (a) cation dissolution coupled with anodic hole oxidation of surface anions, (b) anion dissolution coupled with cathodic electron reduction of surface cations.

Equation 9-54 indicates that when tiie electrode interface is in the state of band edge level pinning is constant), the concentration of surface cations increases with increasing anodic polarization. 

In the state of band edge level pinning where all the change in electrode potential occurs in the space diarge layer, Mec, the anodic polarization curve of the oxidative dissolution follows Eqn. 9-53. As anodic polarization increases, the electrode interface enters a state of Fermi level pinning, in which all the change in electrode potential occurs in the compact layer, A ir, and the concentration of surface cations in Eqns. 9-54 then decreases with increasrng anodic polarization. 

Next, we discuss the reductive dissolution of ionic compoxmds shown in Fig. 9-15(b). The reductive dissolution is composed of a cathodic reduction of surface cations and a cathodic transfer of surface anions as expressed in Eqns. 9-55 and 9-66 ... 

The soluble complexes formed by activators will desorb cation from the lime depressed pyrite surface, which will expose a fresh pyrite surface and activate pyrite flotation. Therefore, the moderately strong acids such as oxalic acid and phosphoric acid exhibit a strong activation action on lime-depressed pyrite because of their ability to decrease pulp pH and to form soluble complexes with hydrated surface cations. 

The Bonding of the Activator Polar Group with Surface Cation... 

Basic supports induce a surface disproportionation reaction on the carbonyl, giving homo- or heteronuclear ion-pairing species with Co or a surface cation as counter-cations [139, 150] ... 

Where naturally occurring isotopes of the cation are not suitable, then enrichment of the surface cations with nonzero-nuclear-spin isotopes is a powerful technique, e.g., 95Mo03 on A1203, Si02, or MgO (108). Often, the presence of a superhyperfine interaction leads to spectra of low resolution and it is useful to increase both the intensity of the spectra and the resolution. Second or even higher derivative spectra can be used to enhance the resolution and in some cases secondary reactions (121) have been shown to increase the intensity. 

Dowden (27) considers the active centers for carbonium ion formation to be associated with surface cation vacancies. A proton, derived from water contained in the catalyst, is attracted to the anions surrounding the vacancy. A hydrocarbon molecule is assumed to be held by polarization forces above this lattice defect and the proton will be distributed between the hydrocarbon and the anions, forming a carbonium ion of a definite lifetime. 

The release of cations is interpreted to have resulted chiefly from two processes an initial release caused by rapid exchange of surface cations for hydrogen followed by a slow release due to structural attack and disintegration of the aluminosilicate lattice. Other processes which could complicate the form of the dissolution curves are adsorption of cations released by structural breakdown, ion exchange on interlayer sites of cations released by structural breakdown and surface exchange (shale only), precipitation of amorphous or crystalline material, and dissolution rate differences among the various crystalline phases. 

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