Silver, coadsorption

K.J. Maynard, and M. Moskovits, A surface enhanced Raman study of carbon dioxide coadsorption with oxygen and alkali metals on silver surfaces,/. Chem. Phys. 90(11), 6668-6679 (1989). 

Here, the dissociative adsorption of hydroi l radicals increases with increasing coadsorption of electropositive potassium atoms on the platinum surface. It has also been reported that coadsorption of electronegative oxygen molecules accelerates the adsorption of hydroxjd radicals on the surface of copper, silver and nickel [Thiel-Madey, 1987]. 

In summary, the results of TDS [13], photoemission [13,45] and scanning tunnelling microscopy [24,45] indicate that at low sulphur coverages the interactions between S and Ag on Ru(OOOl) can be classified as repulsive, in the sense that there is weakening of the Ru-Ag bond and no mixing of the adsorbates. Once the ruthenium substrate becomes saturated with sulphur, then attractive interactions between silver and sulphur are possible and AgS is formed [13,45]. Very similar trends are observed for the coadsorption of sulphur and copper on Ru(OOOl) [13,23]. 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

Another puzzling cases of coadsorption is that observed for adsorption from 300 mM n-CsHnOH (in 50 mM Na2S04) on faces of silver on which a well-controlled density of growth steps exists ... 

C and n-octadecane-1,2-H at a concentration of 0.35 molar in stearic acid and studied the coadsorption of stearic acid and octadecane on polished surfaces of silver, platinum, copper and iron. The films were prepared by retraction from the melt at 40 C (at room temperature the mixture was solid). The proportion of n-octadecane in the film was assayed by differential extraction with cyclohexane. The results of the investigation adequately demonstrate that n-octadecane coadsorbs with stearic acid but not necessarily as a mixed oriented monolayer. Some of the data indicate that more than a single layer is present on the surface. Thus the structure of the long-chain material on the surface may be open to conjecture, but that each constituent adsorbs and in what relative amount is directly determined by radioactive assay. 

In addition, the OH coadsorption significantly influences the adsorption of all other double layer components [55. 56]. For example, in chloride containing media, a significant reduction of the Cl surface concentration was found, while the concentration of cations and surface water was significantly increased compared to the emersion from acidic solutions [55]. Even the adsorption of the strongly binding iodine on silver is pH dependent [31]. The results can be explained by a specific adsorption of OH on silver [55, 56, 59] this interpretation is consistent with the fact that appreciable amounts of adsorbed OH were also found for copper electrodes in alkaline solutions [60]. 

Radioisotopic tracer techniques were applied to study the coadsorption of n-octadecane and stearic acid on a metal surface immersed in a n-octadecane solution of stearic acid. Dual labeling was employed for determining the surface concentrations of both n-octadecane and stearic acid. n-Octadecane was labeled with tritium and stearic acid with carbon-14. The results of half-hour adsorption experiments provide direct proof of coadsorption of polar and nonpolar materials on iron, copper, silver, and platinum surfaces. The films produced on silver and copper by 19-hour adsorption consisted of approximately one molecular layer of stearic acid and two molecular layers of octadecane. A new model is proposed to describe the structure of this thick coadsorbed film. 

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

Investigated systems include various metal deposits, like underpotentially deposited silver on Au(l 11) [69], thallium on a Pt(l 11) surface as a function of solution pH and bisulfate coadsorption [70] and other upd-systems [36, 59]. Evidence of dealloying of Cu3Au(lll) has been reported [71]. Near-neighbor distances between atoms in upd-monolayers of various transition metals deposited on Ag(lll) and Au(lll) surfaces have been measured as a function of electrode potential [72]. Typical results of a study of thallium-upd showing the changing Tl-Tl distance as a function of electrode potential are shown in Fig. 6.9. 

Fleischmann, M., Hendra, P.J., HiU, I.R. and Pemble, M.E. (1981) Enhanced Raman spectra from species formed by the coadsorption of halide ions and water molecules on silver electrodes. Journal of Electroanalytical Chemistry, 117, 243-255. 

Tian, Z.Q., Lian, YZ. and Fleischmann, M. (1990) In-sitn Raman spectroscopic studies on coadsorption of thiourea with anions at silver electrodes. Electrochimica Acta, 35, 879-883. 

To measure binary coadsorption equilibria by tbe volumetric-gravimetric method one proceeds as follows A sorbent sample of 1 g - 3 g and appropriate counterweights, typically lead or silver balls, are placed to the buckets of the microbalance. Then the sorbent is activated by exposing it to helium gas at higher temperatures, i. e. 433 K for activated carbons, 673 K for zeolites and inorganic molecular sieves. After cooling down and evacuation (< 10 Pa) the adsorption chamber is prepared for an adsorption experiment. 

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