Platinum-rhodium electrodes

Given the results obtained on platinum electrodes discussed in some detail in the previous section, it is clearly of fundamental interest to study the mechanism of CO oxidation on other transition metal electrodes, and to compare the results with platinum. Rhodium has been the electrode material that has been studied in greatest detail after platinum, and results obtained with rhodium have provided some very significant insights into some of the general issues about the CO oxidation mechanism. 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

Bond and coworkers [521] have described Au(III) electroreduction and Au(0) oxidation stripping processes in dilute aqua regia utilizing platinum, rhodium, iridium, gold, and glassy carbon electrodes. Sorption of tetrachloroaureate ions on carbon paste electrode modified with montmorillonite has been performed as a preconcentration step in the determination of gold in pharmaceutical preparation [522]. 

It is further important to note that all the current/voltage characteristics depicted in Fig. 6 are unchanged by the presence of liquid fuels such as methanol, formaldehyde, formic acid, or hydrazine. The phthalocyanine electrode remains completely inert toward such substances. For this reason, no mixed potential can be formed at a phthalocyanine electrode, as for example can occur at a platinum electrode, when it is used as cathode in a methanol cell containing sulfuric acid. This is shown by a comparison (see Fig. 7) of the stationary characteristics of the platinum alloy we found to be the most active in the presence of methanol, namely a Raney ruthenium—rhodium electrode, with an iron phthalocyanine electrode, both measured in 4.5 N H2SO4+2M CH3OH. 

FIGURE 4.42 Inside view of the specimen holder for TDA. 1, electric conductors to the dielectric sensor 2, ceramic specimen holder for a TDA 3, nickel made concentric cylindrical electrodes 4, void where is located a platinum-rhodium thermocouple 5, hollow place where is located the samples under test in the form of powders (about 1 g of sample). 

Hori and Murata [174], working at pH 6.8 in a phosphoric buffer electrolyte, proposed the electrooxidation of the protecting layer of the adsorbed CO to C02 followed by the electrooxidation of the Ni electrode. This was deduced from the values of the potential of the CO electrooxidation on the platinum and the rhodium electrodes and the standard equilibrium potential of the CO electrooxidation reaction. For solutions at pH values between 1 and 3 with sulfuric and perchloric electrolytes [175,176], it was proposed that the CO removal consisted of the stripping of an adsorption layer that includes the CO that was competitively adsorbed with water. This suggestion was because the Ni oxidation peak in the CO-containing acid solutions appeared only to be shifted in comparison with that in the pure acid solutions [167]. 

Thus, EMIRS provided an elegant insight into the structure of the doublelayer at platinum or rhodium electrodes. These papers are also of interest in that the authors also reported employing a modulated (cf. the later work of Kunimatsu [67, 68]) form of linear sweep voltammetry to study the reflectivity (ARjR) change of an electrode at a particular wavenumber as a function of potential. 

Many studies on carbon monoxide adsorbed on polycrystalline and single crystal Pt, Pd, and Rh electrodes have been carried out during recent years by means of electrochemical methods and IR spectroscopy (EMIRS, SNIFTIRS, IRRAS, etc.), potential-modulated reflectance spectroscopy and other methods.Electrochemical results show that the number of Pt adsorption sites per CO molecule is changing from 2 to 1 with increasing coverage in acidic solution. There is, however, a discussion in the literature about the evaluation of absolute saturation coverage on ordered low-index platinum (and rhodium) electrodes with particular reference to Pt(l 1... 

Such behavior leads to the supposition that the catlytic activity of the rhodium electrode towards the mixture CO/NO2 is about the same as that of the platinum, while the catalytic activity of platinum towards carbon monoxide remains higher than that of the rhodium. 

When the fact of hydrogen adsorption on platinum metals was well documented, a search for self-consistent models started, aimed to description of surface hydrogen coverage dependence on electrode potential and temperature. Breiter was the first who studied the temperature dependence of hydrogen adsorption on smooth platinum, rhodium and iridium. He demonstrated a possibility to present experimental dependences typical for polycrystalline platinum by combination of two Fmmkin isotherms (see below in Section IV. 5). 

Woods R. Hydrogen adsorption on platinum, iridium and rhodium electrodes at reduced temperatures and the determination of real surfaee area. J Electroanal Chem 1974 49(2) 217-26. 

Weaver MJ, Chang SC, Leung LWH, Jiang X, Rubel M, Szklarczyk M, et al. Evaluation of absolute saturation coverages of carbon monoxide on ordered low-index platinum and rhodium electrodes. J Electroanal Chem 1992 327 247-60. 

The electrocatalytic oxidation of D-glucose and related polyols on large platinum and gold electrodes was discussed in a symposium report, and the oxidation of glucose and polyols on nickel electrodes and on platinum, rhodium, and iridium electrodes under alkaline... 

A. Wieckowski, J. Sobrowski, P. Zelenay, K. Franaszczuk, Adsorption of acetic add on platinum, gold and rhodium electrodes, Electrochim. Acta 26 (1981) 1111-1119. 

A comparison of the amount of hydrogen formed during dehydrogenation of methanol and the amount of organic compound adsorbed was made on platinum- ruthenium [54] and rhodium [55] electrodes. Since these values agreed to a first approximation, it was proposed that particles with composition HCO were chemisorbed on both electrodes. Evolution of methane was observed at rhodium and platinum- rathenium electrodes [54, 55] at 20°C and at a Pt/Pt electrode at 80°C [56] in concentrated acidic solutions of methanol, which indicates that the methanol is hydrogenated by adsorbed hydrogen. 

Chemisorption of Formic Acid at Platinum, Rhodium, and Gold Electrodes... 

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