Work-function change

Fig. XVni-8. (a) Work function change for Pt(lU) as a function of oxygen adatom coverage. From Ref. 82. b) Same, for potassium. The corresponding sequence of LEED structures is indicated. [Reprinted with permission from R. G. Windham, M. E. Bartram, and B. E. Koel, J. Phys. Chem., 92, 2862 (1988) (Ref. 83). Copyright 1988, American Chemical Society.]...

Figure 2.4. Work function changes, A

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 2.6. Effect of alkali coverage on (a) the alkali adatom dipole moment and alkali desorption energy (b) for Na, K and Cs adsorbed on Ru (0001) and corresponding effect of work function change AO on the alkali desorption energy (c).26 Reprinted with permission from Elsevier Science.

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.

Upon replotting the data of Fig. 2.15a in terms of the work function change, AO, induced by the presence of the alkali (Na) promoter one obtains Fig. 2.15b which shows a linear decrease in AH 01 with increasing and conforms nicely to Eq. (2.23)... 

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 2.16. Work function changes versus CO exposure for clean and K-covered Pt(l 11) at 300 K measured from the onset of the electron emission of He I UPS spectra.42 Reprinted with permission from Elsevier Science.

Figure 2,19. NO induced work function changes, AO, vs 0NO for clean and K-covered Pt(l 11) at 120 K. The dashed line indicates the work function changes after heating the NO saturated K-covered Pt( 111) to various temperatures up to 500 K. Reprinted with permission from...

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.

Figure 4.29. Electrophilic behaviour Effect of catalyst potential and work function change AO on the rate of C2H4 oxidation on a Pt film deposited on CaZr0 9Ino 03.a which is a H+ conductor.104 Reprinted with permission from the Institute for Ionics.

Figure 5.11. Effect of applied current on induced work function change on Pt/p"-Al203 26 dashed line catalyst labeled26 Cl, T = 291°C, pQ2 = 5 kPa, pc2H4 2.1xl0 2 kPa solid lines catalyst labeled26 C2, T = 240°C, p02 = 21 kPa, Inset Effect of applied current on computed initial dipole moment of Na on Pt ( ) I>0, (A) I<0.26 Reprinted with permission from Elsevier Science.

Figure 5.13. Effect of catalyst overpotential, AUWR, on catalytic rate and on catalyst work function changes, AO, during ethylene oxidation on Pt/YSZ at 400°C.34Reprinted with permission from Elsevier Science.

With both types of electrodes Imbihl and coworkers28,29 found good agreement with the work function-change potential-change equality... 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

Figure 6.3. Examples for the four types of global electrochemical promotion behaviour (a) electrophobic, (b) electrophilic, (c) volcano-type, (d) inverted volcano-type, (a) Effect of catalyst potential and work function change (vs I = 0) for high (20 1) and (40 1) CH4 to 02 feed ratios, Pt/YSZH (b) Effect of catalyst potential on the rate enhancement ratio for the rate of NO reduction by C2H4 consumption on Pt/YSZ15 (c) NEMCA generated volcano plots during CO oxidation on Pt/YSZ16 (d) Effect of dimensionless catalyst potential on the rate constant of H2CO formation, Pt/YSZ.17 n=FUWR/RT (=A(D/kbT).

Figure 6.5. Example of rule G1 (electrophobic behaviour) Effect of Na coverage and concomitant work function change on the rate of C6H6 hydrogenation on Pt deposited on P"-A1203 at 130°C. Note that the rate is positive order in C6H6 (D). It is also near zero order in H2.24,25...

Figure 8.8. Effect of catalyst potential (Jwr and corresponding work-function change A on the activation energy of C2H4 oxidation on Rh.13 Po2=l -3 kPa, Pc 2H4=7-4 kPa. Reprinted with permission from Academic Press.

Figure 8.15. Time dependence of the work function change, AO, the reaction rate, r, and the catalyst potential, Uwr, following galvanostatic steps during C2H4 oxidation on RuCVYSZ.20,21 Catalyst Ru02 (m=0.4 mg A=0.5 cm2), 1=50 pA, Pc2H4=1 14 Pa, po2=17.7 kPa, Fy=175 cm3 STP/min, T = 380°C.25...

Figure 8.21. (a) Effect of the rate, I/2F, of electrochemical oxygen ion removal (I<0) on the induced increase in the rate of propylene oxidation on Pt/YSZ.28 (b) Effect of catalyst potential and work function change on the rate enhancement ratio p (=r/r0) at a fixed gaseous composition. Reprinted with permission from Academic Press. 

Figure 8.24. Effect of catalyst potential Uwr and work function change (vs 1=0) on the activation energy E and preexponential factor K° of the kinetic constant K of CH4 oxidation to C02 an average T value of 948 K is used in the rhs ordinate p°H4 =p°2 =2kPa, 29Reprinted with permission from Academic Press.

Figure 9.6. Effect of catalyst potential Uwr corresponding work-function change AO, and linearized Na coverage 0n3 on the rate of CO oxidation on Pt/p"-Al203. Conditions T=350°C, po2=6 kPa , pco=5.3 kPa, O, Pco=2.8 kPa. Reprinted with permission from Academic Press.11...

The change in work function is due to the electrochemically controlled ion backspillover, which is at the same time responsible for the rate enhancement, i.e. for NEMCA. Thus work function change and NEMCA are both caused by the same phenomenon and one should not consider one as the cause for the other. 

Why does the work function change when an atom or molecule adsorbs on a surface ... 

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]. 

While the above XPS results give the impression, that the electrochemical interface and the metal vacuum interface behave similarly, fundamental differences become evident when work function changes during metal deposition are considered. During metal deposition at the metal vacuum interface the work function of the sample surface usually shifts from that of the bare substrate to that of the bulk deposit. In the case of Cu deposition onto Pt(l 11) a work function reduction from 5.5 eV to 4.3 eV is observed during deposition of one monolayer of copper [96], Although a reduction of work function with UPD metal coverage is also observed at the electrochemical interface, the absolute values are totally different. For Ag deposition on Pt (see Fig. 31)... 

Fig. 31. Work function change, referred to the clean Pt electrode surface, as a function of electrode potential for Ag underpotential deposition onto Pt. The work function of bulk Cu would correspond... 

The interpretation of XPS data is not always straightforward as is exemplified by different conclusions drawn by different investigators for the same electrode reaction. These discrepancies can be overcome if certain standards for electrode preparation, emersion and transfer processes are developed. The effects of the relative complexity of the emersed electrochemical interface on XPS and UPS data analysis in terms of (electro)chemical shifts and work function changes have to be considered. 

When polar or polarizable species are adsorbed on a metal, the work function changes. This is partly due to gs(dip) but is also due to the change in xM and other effects.2 As for the interfacial potential in the electrochemical cell, the contributions of adsorbate and metal cannot be separated. Usually, the latter gets ignored. It is precisely this term that interests us here. 

Non-uniformity of composition sometimes can be exploited to advantage, e.g., in observing work function changes at a series of compositions, a sliding cathode photo tube (41), Fig- 4, was used. A curved substrate, either cylindrical or spherical, can also be used in preparing specimens with a broad range of concentrations (42) ... 

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