Hydrogen adsorption on Pt

The voltammetry curve for the Ru(ioio) surface in 0.05 M H2SO4 solution (Fig. 4a) reveals a remarkable difference between the oxidation processes forRu(OOOl) andRu(10l 0). The oxidation of this face is more facile than that of Ru(OOOl), as indicated by the onset of the reaction at lower potentials and by increase of the charge with each potential cycle. This difference most likely is the consequence of the more open structure of the Ru(1010). A pair of peaks at 0.12 and 0.3 V is reminiscent of hydrogen adsorption on Pt metals. However, CO displacement showed a negative charge of -354 pC cm . Thus, the peaks probably represent partial Ru oxidation to RuOH, wherein OH is the predominant adsorbed species, perhaps with some co-adsorption of bisulfate. 

Following this strategy, the standard rate constant for hydrogen adsorption on Pt(100) and Pt(llO) have also been evaluated, concluding that A 10 " and 10" s", respectively. Similarly, the kinetics of OH adsorption on Pt(l 11) have also been characterized. 

Dttring the same period in situ IR spectroscopy started its active development. Nowadays the multivarious versions of this technique are widely applied in routine electrocatalytic studies, with predominating attention to adsorption of organic species. However Bewick s IR studies of hydrogen adsorption on Pt poly and on single crystals of several platinmn metals should not be forgotten. 

Ogasawara, H. and Ito, M. (1994) Hydrogen adsorption on Pt(100), Pt(110), Pt(lll) and Pt(llll) electrode surfaces studied by in-situ infrared reflection-absorption spectroscopy. Chemical Physics Letters, 221, 213-218. 

FIGURE 28. The most probable model for hydrogen adsorption on Pt in 1 M H2SO4, involving a water dimer/ ... 

The charge density corresponding to hydrogen adsorption on the free Pt sites is... 

Gomez R, Femandez-Vega A, Feliu JM, Aldaz A. 1993. Hydrogen evolution on Pt singlecrystal surfaces—Effects of Irreversibly adsorbed bismuth and antimony on hydrogen adsorption and evolution on Pt(lOO). J Phys Chem 97 4769-4776. 

In this reaction scheme, hydrogen adsorption (on an empty site ) can occur only after OH removal. Since the adsorption energy of hydrogen is gained only in the second step, this may shift the onset of the reaction to more cathodic potentials. We wUI see later that on a surface covered by Pt monolayer islands that allow an easy formation of Had (see below), a homolytic reaction according to... 

There is a negligible effect of adsorbate-adsorbate interaction on step surfaces. Some lateral repulsion of hydrogen adsorbed on Pt(lll) could be inferred. A strong adsorption of bisulphate and sulphate anions on the (111) oriented terraces and step sites considerably affects both reactions. These data show that each crystallographic orientation of the electrode surfaces gives a different electrochemical entity. 

Hydrogen Adsorption on the Low-Index Surfaces. The data on the hydrogen electrosorption on the low-index planes of Pt differ in some detail, as will be discussed below with the help of results for stepped surfaces. 

Voltammetry curves for all three low-index surfaces are given in Fig. 1. Hydrogen adsorption at Pt(lll), the process at -0.25 < E < -0.05 V in Fig. 1, is not affected by the nature of the anion (such as SO 2-, CIO.- or F-) (12). The lack of a well defined peak, in the drawn-out curve of Fig. 1 clearly indicates a strong lateral repulsion between adsorbed hydrogen adatoms. This is probably a consequence of a partially charge on the adsorbed hydrogen adatoms which, in turn, does not allow the... 

The interpretation of voltammetry curve for the Pt(100) surface poses some problems, e.g. the origin of the peak at E=—0.15 V (Fig. 1). Markovifi et al. (12) ascribed this peak to hydrogen adsorption on particular surface imperfections, the (111)-oriented step sites. The height of this peak varies from one set of data to another, indicating a lack of control of the surface structure. Further support of this view will be shown below with the data for stepped surfaces. 

In any of the schemes presented, hydrogen dissociative adsorption on Pt is possible after Sn addition, as was checked by hydrogen chemisorption. From these results, it is possible to think of a scheme to represent the main reaction pathway during the hydrogenation of a,(3-unsaturated aldehydes. Depending on the catalyst used, such a scheme is shown in Figure 6.12, which summarizes the results from the characterizations and catalytic tests performed in this work [47]. 

Ogura et al. [322] have studied hydrogen adsorption on Ag (and Au) monolayers grown on Pt(lll). 

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