Platinum, crystal faces

Cyclohexane forms a (9 X 9) surface structure on the Ag(l 11) crystal face and a (j j) surface structure on the Pt(l 11) crystal face at around 200 K. This latter surface structure corresponds to the (001) surface orientation of the monoclinic bulk crystal structure of the molecule. On heating the platinum crystal face to 450 K a ( ) surface structure forms that is identical to the surface structure formed by cyclohexene monolayers at the same temperature. It appears that cyclohexane dehydrogenates at elevated temperatures on platinum to form the same species or that of cyclohexene. 

A. The Atomic Surface Structure of the Clean (111) Platinum Crystal Face. 8... 

Some measurements with Pt single-crystal faces have been published recently.140,210,773 Iwasita and Xia210 prepared platinum single crystals according to the method of Clavilier et a/.186 773 After flame annealing and cooling in an H2 + Ar mixture, the electrode was protected... 

It was quickly seen from studies on platinum single crystals that voltammograms for hydrogen adsorption and desorption differ somewhat among the different faces and between the single-crystal faces and polycrystalline platinum. Despite these differences, though, they have common traits as weU. The areas under these curves,... 

The differences between faces usually are small. The reaction rates observed at the different faces as a rule are of the same order of magnitude and differ by no more than a factor of 3 to 5. Significant catalytic effects where one of the faces is tens of times more (or fess) active than the other single-crystal faces of the same metal are rare. One of the few examples is the reduction of CO2 on platinum which occurs with the formation of a strongly bound chemisorbed product (called reduced CO2). At the... 

Van Hove MA, Koestner RJ, Stair PC, Biberian IP Kesmodel LL, Bartos I, Somorjai GA. 1981. The surface reconstructions of the (100) crystal faces of iridium, platinum and gold, 1. Experimental-observations and possible structural models. Surf Sci 103 189-217. 

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.)... 

Cyclic voltammetry studies of single-crystal platinum electrodes in acidic aqueous electrolytes showed that the two characteristic peaks of hydrogen adsorption/desorption on platinum (see Fig. 5.40) correspond in fact to reactions at two different crystal faces the peak at lower potential to Pt(100) and the other one to Pt(lll). 

The electrocatalytic oxidation of methanol was discussed on page 364. The extensively studied oxidation of simple organic substances is markedly dependent on the type of crystal face of the electrode material, as indicated in Fig. 5.56 for the oxidation of formic acid at a platinum electrode. 

Figure 1.3 The arrangement of atoms on the reconstructed (100) crystal faces of gold, iridium and platinum. Side and top views are illustrated. From G.A. Somorjai. Chemistry in Two Dimensions, Cornell University Press, London, 1981, p. 145. Used by permission of Cornell...

It has also been shown [173, 233, 234] that the catalytic activity of various crystal faces of platinum for the reduction of N03 ions is very different. This means that similar effects should be expected in the case of platinized systems with various preferred crystallographic orientations [235]. 

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. 

The adsorption of cyclohexadiene on the Pt(l 11) surface produces the same two surface structures that were found during the adsorption of benzene on this crystal face Thus, this molecule readily dehydrogenates on this platinum surface to benzene at 300 K. 

It is interesting that benzene and naphthalene form monolayer surface structures on the Pt(l 11) crystal face at 300 K and higher temperatures while monolayer surface structures form only at low temperatures ( 200 K) on the Ag(l 11) crystal face While these aromatic molecules are held by strong chemical bonds to the platinum, their heats of adsorption must not be greater than the heats of sublimation... 

So far, only a very few adsorbed molecular structures have been analyzed by surface crystallography. The first system studied in detail was acetylene adsorbed on the (111) crystal face of platinum. We shall discuss the complex adsorption and structural characteristics of this small organic molecule in some detail as it reveals the unique surface bonding arrangements that are possible and points to the importance of the use of additional techniques to complement the diffraction information. 

Fig. 7.79. Current-voltage sweep curves on platinum single-crystal faces (100), (111), and (110) at 25 °C. Sweep range 50-1550 mV, sweep rate 0.1 V/s. (Reprinted from F. G. Will, J. Electrochem. Soc. 112(4) 451, 1965. Reproduced by permission of The Electrochemical Society, Inc.)...

Parts (a) and (d) of Fig. 9.16 are the 100 surface of tungsten and the 111 surface of platinum, respectively. The symmetry of these patterns characterizes the cubic and hexagonal packing of the crystal faces. 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

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