Adsorption work function changes

Figure 2.5. Potassium (a) and sodium (b) induced work function changes for adsorption at 100 K (open circles) and after annealing to 350 K or upon alkali adsorption at 350 K (open triangles) on Al single crystals.23 Reprinted with permission from the American Vacuum Society.

Figure 2,11. Work function changes induced by S adsorption on Ni(100) and Pt(lll) surface.6,38 Reprinted from ref. 6 with permission from Elsevier Science.

The last point is confirmed by measuring the work function changes upon CO chemisorption on clean and alkali-promoted metal surfaces. Figures 2.16 and 2.17 show the work function changes induced by CO adsorption on a K/Pt(lll) and on a Na/Ru(1010) surface respectively, for various alkali... 

Figure 2.15. Dependence of the initial heats of CO adsorption. AH 0, on the alkali coverage, as estimated from the CO TPD spectra at very low CO coverages assuming invariable frequency factor45,46 (a) and on the corresponding work function change AO45,46 (b). Reprinted with permission from Elsevier Science.

Figure 2.31. CO induced work function changes during adsorption on Ni(l 11) modified with increasing amounts of oxygen.88 Reprinted with permission from Elsevier Science.

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

In surface science, work function measurements are considered to be rather sensitive towards changes of the sample surface. Work function measurements are used to follow adsorption processes and to determine the dipole established at the surface. During oxygen adsorption and oxide formation the sign of the work function change allows one to distinguish between oxygen atom adsorbed on the surface or sub-surface [30]. 

Despite these arguments and the conceptual attractiveness of the procedure which is sketched in Fig. 1 convincing evidence for the relevance of a particular gas phase adsorption experiment can only be obtained by direct comparison to electrochemical data The electrode potential and the work function change are two measurable quantities which are particularly useful for such a comparison. In both measurements the variation of the electrostatic potential across the interface can be obtained and compared by properly referencing these two values 171. Together with the ionic excess charge in the double layer, which in the UHV experiment would be expressed in terms of coverage of the ionic species, the macroscopic electrical properties of the interracial capacitor can thus be characterized in both environments. 

Fig. 4 Work function changes induced by water adsorption on Pt(l 11) for various predosed Cl coverages.64...

Work Function Changes Following Methanol Adsorption on Clean Metals Monolayer Coverages)... 

Cyclohexene adsorbed on the Pt(l 11) surface produces a (4 j) surface structure at 300 K. The work function change upon adsorption is —1.7 eV. As the temperature is increased to 450 K a new (q °) surface structure appears. 

Boudart (26) suggests that the presence of the electrical double layer produced by the surface dipoles can account for the observed fall in the heat of adsorption and change in work function as the surface coverage is increased. Furthermore, assuming that the dipole interaction is negligible, as will be the case for small surface coverages, the heat of adsorption and work function changes should be related by the equation... 

Another concept is that the electronic work function changes linearly with the amount adsorbed or that the dipole moment is independent of the concentration. The (1 — 0) concept states that the rate of adsorption for a constant arrival rate is proportional to the fraction of the surface which has not yet been covered. The last two concepts permitted Langmuir to derive his famous adsorption isotherm, which has been verified by experiment in many systems (see the discussion in section IV). Langmuir s experimental work for Cs on W convinced him that the (1 — 0) law was not applicable in this system. This work also led to the concept that the energies involved in surface migration were much smaller than the energies involved in evaporation. 

Fig. 11. Adsorption energy as a function of the CO coverage and work function change on Pd(Ill) (87).

The magnitude of the work function change associated with the adsorption of unsaturated hydrocarbons where 7i-electrons make major contributions to the bonding is in the range —1.3-2.0 V. Saturated hydrocarbons that were studied produce much smaller work function changes, in the range... 

Work Function Changes and Structural Information for Adsorption of Organic Compounds on the Pt(111) and Pt(IOO)-(5 x 11 Surfaces... 

V. The largest work function change was observed during the adsorption of pyridine (—2.7 V) and reflects the large contribution of the nitrogen lone electron pair and/or the permanent dipole moment to the charge transfer. 

The work function change on adsorption for most of the molecules studied varies approximately inversely with the first ionization potential of the adsorbate (24). The data are scattered, for while many types of molecules are represented, some in fact have sizable permanent dipole moments. 

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