Water coadsorption

For this reason, the emphasis in this article is directed more towards the simulation of specific adsorption and, in particular, the recent encouraging comparison of electrochemical and UHV data for the interaction of bromine and chlorine with Ag 110 /7, 8/. A brief outline of the conclusions emerging from alkali-water coadsorption experiments is given to illustrate basic modes of ion-solvent interaction on metal surfaces and to discuss future directions of this research. 

Takita, Y Tashiro, T Saito, Y Hori, F. The effects of water coadsorption on the adsorption of oxygen over metal oxides I. Temperatime-programmed desorption study of C03O4. J. Catal, 1986, Volume 97, Issue 1, 25-35. 

Exposure of K/Ag(lll) to 0.5 L of oxygen on K/Ag(lll) prior co COj led to only peak at a slightly higher temperature (see Figure 3.8). In contrast to the water coadsorption experiments, a new m/e - 44 peak with an area an order of magnitude smaller than that of peak with a corresponding Oj component could be observed close to 590 K, a temperature much lower than at which... 

Anion + water coadsorption, formation of surface oxide + weak anion coadsorption. 

Dawson R, Stevens LA, Drage TC, Snajre CE, Smith MW, Adams DJ, Cooper Al (2012) Impact of water coadsorption for carbon dioxide capture in microporous polymer sorbents. J... 

A strong adsorption of the major part of a biocide on the soil clay minerals reduces its bioavailability and must be compensated by addition of increased amounts. On the other hand, dangerous biocide cations adsorbed on the clay minerals constitute a long-term health hazard when slowly released to the ground water. Coadsorption of the pesticide together with a non-phjrtotoxic compound or displacement of the pesticide (for instance diquat by 4-pyridyl pyridinium chloride [32]) reduce this threat and increase the bioavailability. As shown by Narine and Guy [20], the divalent cations paraquat and diquat are more easily displaced by washing with salt solutions than monovalent cations such as methylene blue and thionine. 

Secondly, absorbent particles such as charcoal and soot are intrinsically inert but have surfaces or infrastructures that adsorb SO, and by either coadsorption of water vapour or condensation of water within the structure, catalyse the formation of a corrosive acid electrolyte solution. Dirt with soot assists the formation of patinae on copper and its alloys by retaining soluble corrosion products long enough for them to be converted to protective, insoluble basic salts. 

Figure 6. Effect of coadsorption of water with other species on Cu(110). (a) Coadsorption...

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

Ataka, K. and Osawa, M. (1998) In situ infrared study of water-sulfate coadsorption on gold(l 11) in sulfuric acid solutions. Langmuir, 14, 951-959. 

Yoshimi K, Song MB, Ito M. 1996. Carbon monoxide oxidation on a Pt(lll) electrode studied by in-situ IRAS and STM Coadsorption of CO with water on Pt(lll). Surf Sci 368 389-395. 

Owing to their strong bond on Ru(OOOl), mixed COa 0.55 V, the shift of the equilibrium between water and adsorbed OHad/Oad towards the latter increases the density of the respective species in the intermixed adlayer, which increases the repulsions between the adsorbed species and hence leads to more weakly bound OHad/Oad and COad species. These latter species are less stable against COOHad or CO2 formation, because of the reduced reaction barrier ( Brpnsted-Polanyi-Evans relation [Bronstedt, 1928]), and can support a reaction via (14.9) or (14.12), respectively, at low rates. (Note that the total density of the adlayer does not need to remain constant, although also this is possible.)... 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

Although quite a few studies of the coadsorption of water and alkalis on metal surfaces in UHV. have been reported /19-21/ the possibility of complete hydration of the alkali adsorbate has not been considered in most cases The reason is probably that, as yet, all the experimental evidence suggests that the alkali 10ns are "specifically" adsorbed in such gas-phase simulation experiments, even when an excess of water (several multilayers) is made available. This result is not yet understood, although one should again keep in mind that the simulation experiments are typically performed 150 K below room temperature. 

The second stage of modeling is the introduction of solvated ionic species into the model double layer. Coadsorption of HF and water yields adsorbed HgO ions the solvation stoichiometries of ions in the first monolayer and in subsequent layers are determined. The third stage of modeling is establishment of potential control in UHV. Hydrogen coadsorption is used to deflect the effective potential of the water monolayer below the potential of zero charge. The unique ways in which UHV models can contribute to an improved molecular-scale understanding of electrochemical interfaces are discussed. 

Coadsorption on Rh(lll) also modifies the HREELS features due to water.. As shown in Figure 3, a substantial sharpening and shift to 610 cm-1 of the pure D 0 band at 580 cm-1 (due to the superimposed D-0 IibraJion and Rh-0 stretch) correlates with the growth of the 650 cm-1 CO mode. The fact that the vibrational spectra of both species are modified by coadsorption suggests adsorption of water and CO onto adjacent sites, a form of hydrophilic coadsorption. 

On Pt(lll) the HREELS features due to water are unchanged by the presence of CO. These observations indicate that water and CO adsorb onto separate patches on the surface, in a form of hydrophobic coadsorption. Water condenses into hydrogen-bonded islands, as indicated by the low 0-H stretching frequency. CO spreads to cover the rest of surface, giving a phase similar to that for CO alone, but with a coverage normalized to the water-free, not total, surface area. COCO repulsions, which have been well documented on Pt(lll) (10), produce a surface pressure within the CO patches which bears upon the edges of the water islands. It is this lateral pressure which causes water to desorb from Pt(lll) at lower temperatures in the presence of coadsorbed CO. 

HydrophiIic versus hydrophobic coadsorption. The contrast between the hydrophilic and hydrophobic coadsorption seen on Rh(111) and Pt(lll), if confirmed under normal electrochemical conditions, might be of electrocatalytic importance. On Rh(lll), where net attractive CO-HgO interactions produce a mixed phase in which CO is displaced to a three-fold binding site which is not occupied in the absence of water, CO and water appear to occupy adjacent binding sites. Such thorough mixing of the oxygen source (water) and the intermediate [or poison] (CO) should improve electrooxidation rates for C 0 H fuels (11). On Pt(lll), where net repulsions cause condensation of CO and water into separate patches, reaction between the adsorbed species could occur only at the boundaries between patches, and one would expect slower kinetics. 

All electrochemical techniques measure charge transferred across an interface. Since charge is the measurable quantity, it is not surprising that electrochemical theory has been founded on an electrostatic basis, with chemical effects added as a perturbation. In the electrostatic limit ions are treated as fully charged species with some level of solvation. If we are to use UHV models to test theories of the double layer, we must be able to study in UHV the weakly-adsorbing systems where these ideal "electrostatic" ions could be present and where we would expect the effects of water to be most dominant. To this end, and to allow application of UHV spectroscopic methods to the pH effects which control so much of aqueous interfacial chemistry, we have studied the coadsorption of water and anhydrous HF on Pt(lll) in UHV (3). Surface spectroscopies have allowed us to follow the ionization of the acid and to determine the extent of solvation both in the layer adjacent to the metal and in subsequent layers. 

Coadsorption of HF and water has no effect on the water desorption peaks, but stabilizes part or all of the HF to higher temperatures, as shown by Figure 5. As long as at least 5 molecules of water per HF molecule are added to the surface (up to monolayer coverage, or 8 H-O/HF for subsequent layers) no HF desorbs until water starts to leave the surface around 170 K, peaking at 180 K. As long as at least 1 molecule of water is initially present per HF, no HF desorption will occur until 150 K, peaking at 162 K. If more HF than H 0 molecules are present initially, some HF will desorb in a peak at 136 K, near the temperature at which HF alone desorbs. Coadsorption thus can yield HF desorption at three peaks, one not stabilized vs. HF alone, one stabilized by 30 K, and one stabilized by 50 K, i.e., to the water desorption temperature. 

HREELS of the H 0 + HF system. The nature of the interaction stabi-iizing HF on thS surface is made clear by the HREELS spectra of Figure 6. As the concentration of HF in the water layer is increased a new peak around 1150 on (and several smaller peaks) first increases and then, as the HF/H-0 ratio exceeds 1, decreases in intensity. By analogy to vibrationaf spectra of acid hydrates of known structure (13-16), this peak is identified as the symmetric bending mode of the pyramidal H 0 ion. We have observed the same peak upon coadsorption of water ana other, stronger, mineral acids. The reaction... 

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