Platinum work function

Work k u iLCtions of Pure Tungsten and Platinum Work Functions at Optimum Covering Nn, in Alkali Atoms per Square Centimeter of Metal Surface... 

E.R. Koetz, H. Neff, and K. Mueller, A UPS, XPS and work function study ofemersed silver, platinum and gold electrodes, /. Electroanal. Chem. 215, 331-344 (1986). 

Figure 8.29. NEMCA-generated volcano plots obtained by increasing the catalyst work function above its open-circuit value during CO oxidation on Pt pCo=0.2 kPa, Po2=t 1 kPa, , T=560°C, r0= 1.5x1 O 9 mol O/s O, T=538°C ro=0.9xl0 9 mol O/s.36 Reprinted by permission of Platinum Metals Review.

Gomez R, Climent V, Feliu JM, Weaver MJ. 2000. Dependence of the potential of zero charge of stepped platinum (111) electrodes on the oriented step-edge density Electrochemical implications and comparison with work function behavior. J Phys Chem B 104 597-605. 

Platinum electrodes are made usually from poly crystalline metal the crystal planes at the surface include both the (111) and (100) faces in approximately equal proportions. The electrochemical properties of Pt(lll) and Pt(100) faces are not identical. (Generally, the physical properties of individual metal crystal faces, such as work function, catalytic activity, etc., are different.)... 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

The Pt-Au films were not used in catalysis, but the chemisorption of CO was studied. The work function of Pt was only raised by 0.03 eV and there was no change with the alloys after short exposures to CO. It was therefore not possible to titrate the Pt content of the surface with CO in the same way as hydrogen was used with Cu-Ni alloy films (2). Long-term exposure of the films to 10 5-10-4 Torr CO at 20°C for periods up to four days caused the work function of the alloys to increase slowly Fig. 31. After 16 hr this increase was more evident in the Pt-rich region, but the effect was observed on the Au-rich regions after longer exposures. The effect was accelerated if the films were maintained at 100°C. These results were cited as direct evidence for the enrichment of the surface with platinum... 

An interesting correlation exists between the work function of a metal and its pzc in a particular solvent. Consider a metal M at the pzc in contact with a solution of an inert, nonadsorbing electrolyte containing a standard platinum/hydrogen reference electrode. We connect a platinum wire (label I) to the metal, and label the platinum reference electrode with II. This setup is very similar to that considered in Section 2.4, but this time the metal-solution interface is not in electronic equilibrium. The derivation is simplified if we assume that the two platinum wires have the same work function, so that their surface potentials are equal. The electrode potential is then ... 

These results are remarkable Coupled with other results for silver and platinum (19J they show that the emersed electrode work function cam be independent of electrode material (even oxide coated) and electrolyte. The tracks < g one-to-one over a large potential region, even after placement in UHV. The apparatus used allowed for emersion and placement in UHV without exposure to air at any time. 

R. Bouwman and W. H. M. Sachtler, Photoelectric determination of the work function of gold-platinum alloys, J. Catal. 19, 127 (1970). 

Fig. 6-34. Adsorption coverage of hydroxyl radicals on, and work function of, a platinum (111) surface plane observed as functions of coverage of potassium atoms coadsorbed with water molecules adsorption of water vapor takes place on a potassium-adsorbed surface of platimun at 305 K. 6k = coverage of adsorbed potassium atoms 6oh = coverage of hydroxyl radicals adsorbed by partial dissociation of water molecules A

Benzene forms a rotationally disordered structure on the reconstructed (100) platinum surface. However, the work function changes with increasing surface coverage are similar to that of benzene on the (111) crystal face. 

However, electrons are the same independently of the type of material. Thus, how can we understand that different metals behave in different ways How can we understand, for example, that the work function of different metals—say platinum and silver—are different (see Fig. 6.46) Or how to explain the different behavior of different crystallographic surfaces of a given single crystal, say, Ag(l 11) and Ag(100) ... 

Schwab and co-workers (5-7) found a parallel between the electron concentration of different phases of certain alloys and the activation energies observed for the decomposition of formic acid into H2 and CO2, with these alloys as catalysts. Suhrmann and Sachtler (8,9,58) found a relation between the work function of gold and platinum and the energy of activation necessary for the decomposition of nitrous oxide on these metals. C. Wagner (10) found a relation between the electrical conductivity of semiconducting oxide catalysts and their activity in the decomposition of N2O. 

Between these limiting cases, intermediate states are possible The same molecule can give off electrons to a metal with high electronic work function (e.g., H atoms to platinum) or receive electrons from a metal with low electronic work function (formation of hydrides between H atoms and alkali-metal surfaces). 

At room temperature H2 molecules striking those crystallites of a platinum surface which have the highest work function (I) decompose into atoms. If a pure platinum surface is contacted by hydrogen, crystallites I will be covered by H atoms, those with a low work function, II, by H2 molecules, because the decomposition of H2 molecules on crystallites II requires a higher energy of activation than on crystallites I. The work function of crystallites I is lowered by the polarized H atoms that of crystallites II will not be changed essentially. All effects combined, the photoelectric emission therefore increases if hydrogen is adsorbed on a pure platinum surface. 

When such a hydrogen-covered surface is bombarded with electrons of low energy (10 amp., 20 to 300 volts) (2a), both types of crystallites will be struck, The work function of crystallites II islowered, because their H2 molecules decompose into atoms under bombardment that of crystallites I is increased, because their H atoms are shot off. If the first effect predominates, the photoelectric emission of the platinum surface increases if the second effect prevails it decreases. The combination of both effects is seen in the results shown in Fig. 21. By adsorption of hydrogen, the sensitivity is increased D E). The electron bombardment causes at the beginning a further increase (F — F), because the H2 molecules decompose into atoms the later bombardment causes the photoemission to decrease again (F — G), because H atoms are shot off. The work functions, in volts, in the different states are... 

On a platinum surface covered by H atoms, additionally adsorbed hydrogen will not find any crystallites on which to dissociate into atoms. It is adsorbed molecularly and hinders the passage of electrons (see Fig. 22). The original state A is marked by a high sensitivity and a low work function, = 4,05 volts. The influence of molecular hydrogen makes the photoelectric yield decrease at 302.2 mu, i.e. near the thresh-... 

If a platinum foil electrolytically furnished with hydrogen is flashed in vacuo, it gives off one part of the adsorbed gas. In this case after cooling, the H atoms will be situated on crystallites with a high work function, the H2 molecules on those with a low one. The measured work function and also the constant M are therefore low. The admission of hydrogen does in this case not essentially change the sensitivity (Fig. 23, 4 —> B), because... 

---