Adsorption on platinum

The mode of 2,6-dimethylphenyl isocyanide (DMPI) adsorption on Pt electrodes [43] has been studied using potential-dependent SERS. The Pt electrodes were obtained by depositing Pt nanoparticles ( 17nm) on a polycrystalline Pt film. Three bands appear, at 2166, 2124 and 1997cm , when DMPI is adsorbed on the 

the mode of isocyanide adsorption depends on the nature of the Pt substrate and perhaps on the nature of the isocyanide. On Pt(lll), CH3NC adsorbs by both T (low coverage) and p,2-T h (high coverage) modes. On Pt nanoparticles, only T adsorption is observed for n-dodecyl isocyanide, but on Pt nanoparticle electrodes evidence suggests that DMPI adsorbs by T (on-top), X2-T t (two-fold bridge) and X2-tl T T (three-fold hollow) modes. 

---

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

Figure A3.10.24 UPS data for CO adsorption on Pd(l 10). (a) Clean surface, (b) CO-dosed surface, (c) Difference spectrum (b-a). This spectrum is representative of molecular CO adsorption on platinum metals [M].

KitamuraF, Takahashi M, Ito M. 1989. Carbon monoxide adsorption on platinum (111) singlecrystal electrode surface studied by infrared reflection - absorption spectroscoy. Surf Sci 223 493-508. 

Kazarinov VE, Tysyachnaya GY, Andreev VN. 1975. On the reasons for the discrepancies in the data on methanol adsorption on platinum. J Electroanal Chem 65 391-400. 

Perez JM, Munoz E, Moralldn E, Cases F, Vazquez JL, Aldaz A. 1994. Formation of CO during adsorption on platinum electrodes of methanol, formaldehyde, ethanol and acetaldehyde in carbonate medium. J Electroanal Chem 368 285-291. 

Sobkowski J, Wieckowski A. 1972. A new approach to the radiometric study of methanol adsorption on platinum. J Electroanal Chem 34 185-189. 

These results seemed to indicate that CO adsorption on platinum is an irreversible process, i.e. there is no equilibrium of the form Pt-COad COsol + Pt. The purpose of the following experiment was to check this hypothesis. 

Fig. 4.4. Potential dependence of (a) the coverage degree of platinum by tin and (b) charge transferred during tin adsorption on platinum (according to Eq. 4.1). Tin was adsorbed from Sn(S04)2 ( ) and from SnS04 (O).

As we have seen in Section 2.1.4, depending upon the concentration, methanol adsorbate seems to consist of variable amounts of COH and CO species. Oxidation to C02 requires the splitting of HzO molecules which could deliver an oxygen atom to form C02. The stability of H20 makes the oxidation process difficult. Its weak adsorption on platinum does not contribute to improve the situation. 

Hydrogen adsorption on platinum electrodes in perchloric acid solutions... 

Vibrational Spectroscopic Studies of Adsorbate Competition During Carbon Monoxide Adsorption on Platinum Electrodes... 

The situation is quite different in the case of an acetic acid-water system. The energy of acetic acid adsorption on platinum is low and therefore the voltammetric curves taken in the absence and in the presence of acetic acid in the supporting electrolyte are nearly the same. However, radiometric data show that C-labeled acetic acid is adsorbed on the electrode surface. Most likely the acetic acid molecules are adsorbed on the top of the water molecules populating the electrode surface. Simultaneously recorded voltammetric and counting rate data are shown in Fig. 8. 

Furuya, N., and Koide, S. (1989). Hydrogen adsorption on platinum single-crystal surfaces. Surf. Sci. 220, 18-28. 

C-Propene adsorption on platinum—alumina and platinum—silica [66] differs from ethylene adsorption insofar as a fraction of the initially retained 14C-propene is relatively easily exchanged or removed by hydrogen treatment. This suggests less extensive dissociation of the adsorbed propene and a 7T-allyl species (structure F) has been proposed in this case, viz. 

Finally, two papers have been concerned with a study of adsorption on platinum electrodes in aqueous solution. One was concerned with the adsorption of CO (55) and the other with I2 (56), and although again very weak signals were detected [v(PtC0) at 2096 and 2081 cm-l, and v(I-I) at 174 cm"l], the experiments do illustrate one of the major advantages of the Raman technique, viz, the ability to study adsorption from aqueous media. 

---