Platinum adsorption process

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

The first step of oxide-layer formation is oxygen adsorption (chemisorption). In the case of platinum, the process stops at this stage, and depending on the conditions, an incomplete or complete monolayer of adsorbed oxygen is present on the platinum surface. In the case of other metals, layer formation continues. When its thickness 5 has attained two to three atomic diameters, the layer is converted to an individual surface phase that is crystalline (more seldom, amorphous) and has properties analogous to those of the corresponding bulk oxides. 

The degree of coverage, however, seems to influence the adsorbate composition at low methanol concentrations also. In particular, on smooth platinum, the dependence of surface composition on 6 is observed at concentrations as low as 5x 10-3 M (Fig. 2.7). In this case it could be possible that COH can be formed as long as adjacent sites are available. At high coverage (by all species involved in the adsorption process), the formation of COH should be geometerically prevented. 

These studies are of paramount importance if we are to gain a complete understanding of adsorption processes at electrodes such as platinum since, as we have seen above, we previously were able to determine the number of... 

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

Electrochemical Adsorption at Catalytic Electrodes. A classification of adsorption processes at catalytic electrodes, such as platinum or rhodium, first proposed by Horanyi (24) and further developed by Wieckowski (21,25,26), categorizes adsorption processes into three fundamental groups ... 

Wieckowski, A. The Analysis of Electrochemical Adsorption Processes on Platinum P. W. U., Warsaw, Poland, 1984. 

H. A.-Kozlowska, J. Klinger, and B. E. Conway, /. Electroanal. Chem. 75, 45 (1911). Fukuda and A. Aramata, The kinetic study of adsorption processes of the phosphate species on platinum)111) in aqueous acidic solutions, J. Electroanal. Chem., in press (1997). 

The electrociiemical oxidation of ethanol has been extensively studied at platinum electrodes [22-34]. The first step is the dissociative adsorption of ethanol, either via an 0-adsorption or a C-adsorption process [25, 26], to form acetaldehyde (AAL) according to the following reaction equations. Indeed, it was shown by Hitmi ef al. [34] that AAL was formed at potentials lower than 0.6 V vs RHE. Thus ... 

Similar studies were carried out with benzoic acid on porous palladium electrodes [150]. The objective of this work was to investigate the adsorption processes and the reactivity of benzoic acid on different noble metals, in order to compare these results with those obtained for related aromatic compounds. On-line mass spectroscopy analysis of volatile products revealed that the adsorption of benzoic acid is irreversible at platinum while it is mainly reversible on palladium. Accordingly, different catalytic activity of platinum and palladium was found in the electrooxidation. 

What does Eq. (6.246) mean This equation represents the adsorption process of ions on metallic surfaces. It includes several conditions that are characteristic of the adsorption process of ionic species, namely, surface heterogeneity, solvent displacement, charge transfer, lateral interactions, and ion size. However, is this equation capable of describing the adsorption process of ions In other words, what is the success of the isotherm described in Eq. (6.246) Figure 6.104 shows a comparison of data obtained experimentally for the adsorption of two ions—chloride and bisulfate—on polycrystalline platinum, with that obtained applying Eq. (6.246). The plots indicate that the theory is able to reproduce the experimental results quite satisfactorily. The isotherm may be considered a success in the theory of ionic adsorption. 

What information can be obtained from A/ ds One important parameter involved in the enthalpy of the reaction is the ion-metal bond. However, A7 ds includes all the different interactions involved in the adsorption process, e.g., the breaking of the hydration sheet of the electrode and the ion, lateral interactions, and heterogeneity of the surface. One can subtract all these energy terms from AH ds and obtain in this way the energy (or strength) of the ion-metal bond. For the example of the adsorption of bisulfate ions onpolycrystalline platinum (Fig. 6.105), the ion-metal bond was found to be -214 60 kJ mol-1. 

On the other hand, according to the study of Horanyi et al.[ °] on the oxidation of glyoxal and glyoxylic acid at platinum electrodes, the C-C bond breaking may play a significant role during the adsorption process. 

From these results it appears that the addition of tin to platinum greatly favors the formation of AA eomparatively to AAL. This can be explained by the bifunctionnal mechanism where ethanol is adsorbed dissociatively at platinnm sites, either via an 0-adsorption or a C-adsorption process followed by the oxidation of these adsorbed residues by oxygenated species formed on Sn at lower potentials giving AA. 

In electrocatalysis, the structure sensitivity of an adsorption process at the platinum-electrolyte interface is evidenced by the effect of surface orientation on the distribution of the configuration states in current vs. potential profiles [1-4]. The electrolyte composition at constant pH affects these adsorption states because of the anion-specific adsorption [2],... 

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