Platinum surface, reconstruction

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

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. 

Some metal surfaces reconstruct either in the clean state or in the presence of adsorbed gases. Platinum, iridium, and gold (100) surfaces, which have square symmetry, all reconstruct to hexagonal close-packed (111) surfaces... 

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. 

In summary, a variety of LEED patterns have been observed for oxygen adsorption on the platinum metals. However, their interpretation is complicated by the uncertainty in the oxygen coverage. A second complication is the uncertainty introduced by oxygen-induced surface reconstruction. The stability of 22 high index Miller planes upon exposure to oxygen has recently been investigated by Blakely and Somorjai (160). 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

The kinetics of chemical reactions on surfaces is described using a microscopic approach based on a master equation. This approach is essential to correctly include the effects of surface reconstruction and island formation on the overall rate of surface reactions. The solution of the master equation using Monte Carlo methods is discussed. The methods are applied to the oxidation of CO on a platinum single crystal surface. This system shows oscillatory behavior and spatio-temporal pattern formation in various forms. 

An adsorbed molecule would experience different adsorption potential when attached to a terrace, step or kink. On the other hand, on some metals (platinum as an example) the heats of adsorption of simple gases (ethylene, CO, H2) vary little with surface structure. Lee, et al. [84] examined the effect of surface reconstruction and relaxation on the electronic coordination numbers of the surface atoms. They found that highly stepped surfaces relax into a configuration where the surface atoms have about the same electronic coordination number as Pt(l 11) surface atoms. It means that they lack the coordinative unsaturation of the atoms, which would take place on unaltered stepped surfaces. 

Although such effects are well known for hydrocarbon reactions over platinum and other metals,it is unlikely to apply to the present situation where considerable surface reconstruction, corresponding to the formation of an oxide film, is likely. The properties of such a layer would change with particle size simply because of the increase in surface energy with decreasing size. The smaller particles would tend more towards the composition Ag20 than the larger ones. Since it is known that silver(i) oxide is not a selective catalyst for ethylene oxidation such a model could explain these size effects. 

In some cases they are, but for platinum, iridium, and gold a reconstruction of the first outermost layer of atoms was observed for some faces of simple indices. Of these three metals, only gold is within the scope of this chapter. Numerous papers were written about the surface reconstruction of gold which, however, happens only under certain conditions of cleanliness and temperature. 

Most recently, Fleischmann and Mao ° ° have investigated the reconstruction of platinum surfaces in the presence of adsorbates. They note that the presence of CO or weakly adsorbed hydrogen eauses a reeonstruction with an enhaneement of the reflection due to the (110) faces. Surprisingly, however, the presence of strongly adsorbed hydrogen did not result in a surface reconstruction. 

The interaction of iodine with palladium and platinum electrodes was studied in different electrolyte solutions and on single crystals with electrochemical techniques and UHV spectroscopic ex situ measurements [111]. The chemisorption of atomic iodine on palladium researched extensively because of its ability to protect the surface from air and water interactions. Moreover, it is able to induce a surface reconstruction from a stepped surface to a (1 x 1) unreconstructed one. 

Results obtained at 10 torr (see Figure 22A) imply that upon exposure to oxygen the intensity of the band at 2070 cm characteristic for the adsorption of CO on bimetallic nanoparticles, decreases, while that of the band at 2086 cm, characteristic for the adsorption of CO on pure platinum, increases. Parallel to the above changes, a very pronounced increase was also observed in the intensities of the (vCO)br band at 1840 cm". However, the difference spectra indicate that the oxygen induced surface reconstruction is relatively fast as there is no measurable difference between these spectra. 

M.A. Van Hove, R.J. Koestner, P.C. Stair, J.P. Biberian, L.L. Kesmodel, I. Bartos, and G.A. Somoijai. The Surface Reconstructions of the (100) Crystal Faces of Iridium, Platinum, and Gold. I. Experimental Observations and Possible Structural Models. Surf. Sci. 103 189 (1981). 

Time-resolved measurements can be made at storage rings with high flux insertion devices that use a quick-scanning mode of operation of the monochromator [574]. In a reported study, products of Mo corrosion in KOH solution could be identified and quantified [578]. Application of time-resolved dispersive high-energy X-ray absorption fine structure (DXAFS) measurements on platinum nanoparticles in a fuel electrode have been described [575]. Results indicate severe surface reconstruction of the nanoparticle surface, showing at least three types of Pt-0 bonds (adsorbed OH, adsorbed atomic O and amorphous PtOx) under oxidative conditions. 

The application of time-resolved high energy X-ray diffraction on platinum nanoparticles in fuel electrodes has been described [28]. Results indicate severe surface reconstruction of the nanoparticle surface showing at least three types of Pt-0 bonds (adsorbed OH, adsorbed atomic O and amorphous PtO ) under oxidative conditions. 

Typical examples include studies of the underpotential deposition of various metals on metallic substrates. The structure of the upd-layer [33, 34], the position of adsorbed anions and water molecules on top of the upd-layer and the respective bond angles and lengths could be elucidated [35, 36]. Surface reconstruction caused by weakly adsorbed hydrogen [37], surface expansion effects of low-index platinum and gold surfaces correlated with adsorption/desorption of solution species [38] and... 

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