Electrode palladium

A potential pH diagram for the Pd-HjO-Cl system is presented and described whilst the rest potentials of palladium electrodes in oxygen-saturated solutions of different pH values have been determined . From the latter, dEr/dpH = — 65 mV in acid and —35 mV in alkaline solutions, and dEr/dlgPoj = 30 mV for both pH ranges. 

Palladium electrodes are also very active for formic acid oxidation, with higher current densities than platinum electrodes [Capon and Parsons, 1973c]. Oxidation occurs almost exclusively through the active intermediate path, without poison formation. The reaction is also very sensitive to the surface stmcture, and the activity of the... 

Lu G-Q, Crown A, Wieckowski A. 1999. Formic acid decomposition on polycrystalline platinum and palladium electrodes. J Phys Chem B 103 9700-9711. 

Leung L-WH, Weaver MJ. 1990. Influence of adsorbed carbon monoxide on the electrocatalytic oxidation of simple organic molecules at platinum and palladium electrodes in acidic solution A survey using real-time FITR spectroscopy. Langmuir 6 323-333. 

The interfacial chemistry of palladium electrodes (polycrystalline and single crystal) was surveyed by Soriaga etal. recently [103]. According to the authors, the remarkably rich interfacial chemistry of palladium may have its origin in the anomalously weak inter-metallic palladium-palladium bond, a circumstance that is expected not only to enhance lateral surface mobihties but also to facilitate the disruption of substrate-substrate bonds and/or the formation of substrate-adsorbate bonds. 

In a survey dealing with palladium electrode, it is not possible to avoid mentioning the cold fusion controversy, started in 1989, where Pd electrode played and plays a central role. Hundreds of papers relating to palladium electrode are dealing exclusively with cold fusion and related subjects. The status of cold fusion was summarized in [104], recently. 

In [119], the hydrogen adsorption and desorption reactions in thin palladium electrodes were studied using the potential step method in order to analyze the mechanism of phase transformation. Transient current responses were recorded at the onset of the potential step for 47 pm thick Pd electrodes in 1 mol dm H2SO4 at ambient temperature. A model based on a moving boundary mechanism was proposed to account for the experimental i-t curves. It was found that the hydrogen adsorption reaction shows interfacial kinetic limitations and only numerical solutions can be obtained. Such kinetic limitations were not found for the desorption reaction and a semianalytical solution that satisfactorily fits the experimental data was proposed. 

The EQCM method is used to evaluate the processes that occur in/on the palladium electrode in acid and basic solutions. It was concluded that hydrogen electrosorption in palladium is accompanied by an additional frequency shift of... 

This effect created significant problems with the objective estimation of the amount of sorbed hydrogen inside the palladium electrode using the EQCM method. 

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. 

M. P. Soriaga, Y.-G. Kim, J. E. Soto, Interfacial chemistry of palladium electrodes in Interfacial Electrochemistry, Theory, Experiment, and Applications (Ed. A. Wieckowski), Marcel Dekker, New York, 1999. 

No doubt Chadwick and Rutherford would have been quick to pronounce similarly on the experiments of Pons and Fleischmann, a who announced on 23 March 1989 that they had observed I sustained nuclear fusion from the electrolysis of heavy water using palladium electrodes. Deuterium is absorbed by palladium in the same way as hydrogen, but its fusion into helium does not require such extreme conditions (see page 109). All the same, these conditions have long proved impossible to sustain in physicists attempts to harness nuclear fusion for energy generation. Now two chemists were claiming that these massively expensive fusion projects could be abandoned all you needed was a test tube and two strips of palladium. 

In a much-publicized study in 1989, Pons and Fleischmann claimed to have observed cold fusion of nuclei of deuterium (heavy hydrogen, D) within palladium electrodes that were being used to electrolyze D2O. Had this been the case, what other electrode materials might also have shown the same phenomenon ... 

Equivalent electrical circuit for the electrochemical cell described in section 9.2.2 with palladium electrodes of 314mm2, electrically in contact with each other through an electrolyte solution over a distance of 112mm at T=298.0K. 

Similar to the experiments described in the previous section, but this time with various surface areas of the palladium electrodes and at each elec-... 

Similar to the experiments carried out at palladium electrodes and described in Chapter3, the concentration of electrolyte (c), the electrode surface area (A) and the distance between the electrodes (d) will be studied as a function of type of textile structure. In this work, three structures will be studied knitted, woven and non-woven textile structures, all obtained from stainless-steel fibres. To complete the data of this work, palladium sheets will also be inserted in the study as a fourth set of electrodes. Therefore, for palladium electrodes, the work described in section 9.2 will actually be repeated here in order to have a direct comparison between results obtained with palladium electrodes and textile electrodes. Of course, correlation with the data obtained in section 9.2 will be verified. 

Nyquist plots recorded at an electrochemical cell with palladium electrodes or woven, non-woven or knitted textile electrodes with /4=180mm2 and d=103mm for a NaCI concentration of (1) 1x10 2, (2) 1 x10 3 and (3) 1 x10 4mol I- at T=298.0K. Part b is an enlargement of part a. 

Logarithmic plot of the impedance at zero phase-angle shift as a function of electrolyte concentration obtained from the electrochemical cell with palladium electrodes, woven, knitted and non-woven electrodes for > =180mm2, d=103mm and 7=298.0 K. 

Scheme showing the positioning of a palladium electrode (a) and a woven (b) and knitted (c) textile electrode in the cell, and the influence on the configuration of the electrodes due to positioning. (1) Rubber fittings, (2) part of the PVC tubing filled with electrolyte and (3) electrolyte solution. 

It should be taken into account that this is only valid for the textile electrode investigated in this work, because this parameter is also dependent on the roughness of the surface. The roughness is definitely the cause of this edge effect, because it is absent when using smooth palladium electrodes. 

In this section, the distance between the electrodes is studied for different electrolyte concentrations and distances between the electrodes at a constant electrode surface area of A = 180 mm2. The obtained impedances are plotted logarithmically against the distance between the electrodes (d) as shown in Fig. 9.14. Relationships obtained for the textile electrodes are identical to those for the palladium electrodes if the smallest distance between the electrodes is not taken into account. Additionally in this case, the roughness of the textile electrodes is responsible for this effect and can be neglected for distances longer than d=40mm - an effect that increases with decreasing distance between the electrodes. Of course, also in this case,... 

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