Effect of Coadsorption

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

Changes in bond strength with variation of coordinative unsaturation of surface atoms are very similar to the effects of coadsorption [11]. When two adsorbate atoms share a bond with the same surface atoms, the surface atom effectively has an increased coordination number. Therefore the adatom bond strength is weaker than when there is no sharing between metal surface atoms. The heat of adsorption generally decreases at high adsorbate surface coverage as a result of this effect. 

Daniels R, Rupprecht H. Effect of coadsorption on sorption and release of surfactant paraben mixtures from silica dispersions. Acta Pharm Technol 1985 31 236-242. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

Similar is the effect of S coadsorption on the CO TPD spectra on Pt(lll) as shown in Figure 2.29. Sulfur coadsorption weakens significantly the chemisorptive bond of CO. 

Broqvist, P., Molina, L.M., Gronbecka, H. and Hammer, B. (2004) Promoting and poisoning effects of Na and Cl coadsorption on CO oxidation over MgO-supported Au nanopartides. Journal of Catalysis, 227, 217-226. 

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. 

The study of thiourea adsorption on an Hg electrode from ethanolic solutions shows that different supporting electrolytes can make a comparison of the adsorption parameters more difficult. The data obtained for various electrolytes (KF, KPFe, LiCl, NH4NO3, and KCNS) suggest that coadsorption, size, and polarizability of ions strongly influence the interfacial behavior of TU, at high concentration of the ions in particular. The effect of the electrolyte on AG° and parameter A in the Frumkin isotherm is illustrated in Table 6. 

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. 

A reexamination of the ammonia-toluene coadsorption shows that ammonia prevents the formation of benzyl-species at room temperature in the presence of ammonia, in fact, toluene only weakly adsorbs at r.t. in a reversible form. This agrees with the strong inhibiting effect of toluene conversion by ammonia due to the competitivity of the two reactants on the same adsorption sites (77,72). The spectrum... 

Indirect experimental evidence implying the presence of a n-bond-containing complexes from ethene on metal catalysts was first produced by Shopov and Palazov and their colleagues (161-163). They pointed out that coadsorption of ethene and CO led to a low-wavenumber shift of the vCO absorption and interpreted this in terms of the effects of -donation by ethene. Subsequently, they suggested that the complex in question was a (di-cr/zr) ethyne surface... 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

Assumptions underlying the adsorption models are not often discussed in the literature, since the exact nature of the relevant surface complexes or phases is difficult to identify. In particular, lateral interactions between adsorbed ions, site heterogeneity as well as phenomena involving the oxide dissolution or rchy-dration are not taken into account systematically. The latter phenomena are discussed in section D.d. Lateral interactions between adsorbed ions (ion coadsorption) have been reported [27, 28] and make questionable the use of mass action equations at interfaces. The effect of surface structure, site heterogeneity and surface composition, in particular on the ZPC value, were also pointed out [29, 30]. 

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