Hydrogen voltammetric oxidation

As the pH is lowered, the overall rates decrease, that is, the reaction becomes less reversible, as evidenced by an increase in the peak separation in voltammetric curves recorded in quiescent solutions. Close inspection of the data (not shown here) indicates that this effect is due to a shift in the peak ascribed to hydrogen peroxide oxidation toward more positive values and not to a shift in E°n2set. 

Solid metal electrodes are usually polished mechanically and are sometimes etched with nitric acid or aqua regia. Purification of platinum group metal electrodes is effectively achieved also by means of high-frequency plasma treatment. However, electrochemical preparation of the electrode immediately prior to the measurement is generally most effective. The simplest procedure is to polarize the electrode with a series of cyclic voltammetric pulses in the potential range from the formation of the oxide layer (or from the evolution of molecular oxygen) to the potential of hydrogen evolution (Fig. 5.18F). 

The behaviour of hydrogen peroxide alone (Fig. 5.42, curve 3) is in agreement with this explanation the catalytic reduction obeys Eq. (5.7.8) at potentials more positive than the non-catalytic oxidation. The voltammetric curve obtained is characterized by a continuous transition from the anodic to the cathodic region. The process occurring at negative potentials is then... 

Figure 1. Solid lines - cyclic voltammetric curves for hydrogen and oxygen adsorption at the three Pt single crystal planes investigated determined from the pure (CO free) 0.1 M HC10, solution. Dotted lines - cyclic voltammetric curves of adsorbed CO oxidation (note the sensitivity for the CO oxidation curves has been ten fold attenuated. Sweep rate 50 mv/s.

The study of hydrogen and deuterium electrosorption in palladium limited volume electrodes (LVE) was carried out by the same group in both acidic and basic solutions [124,130,134]. It was found that the hydrogen capacity, H (D)/Pd, measured electrochemically, depends significantly on sweep rate in cyclic voltammetric experiments and also on the thickness of the LVE. Two different mechanisms of hydrogen desorption, that is, the electrochemical oxidation and the nonelectrochemical recombination step, which take place in parallel within the Pd—LVE, have been postulated. 

Since hydrogen peroxide can decompose faster under the influence of catalysts, this so-called catalyst decomposition was also studied by means of voltammetric methods applied to the oxidation and reduction reactions... 

Molecular hydrogen (H2 AHDBE, 435 kJ mol-1) is resistant to electrochemical oxidation at inert electrodes (glassy carbon or passivated metals Ni, Au, Hg, Cu). At passivated Pt and Pd dissolved H2 exhibits only broad, diffuse, anodic voltammetric peaks with irreproducible peak currents that are not proportional to the partial pressure of dissolved H2 (PH2). However, with freshly preanodized Pt and Pd electrodes well-defined oxidation peaks for H2 are obtained, which have peak currents that are proportional to P (Figure 8.3).14 The surface... 

Figures 11.10 (a) and (b) show that the voltammetry of these couples in a range of RTILs is nearly electrochemically reversible. Note however that, unlike the ferrocene- and cobaltocenium-based couples, the reduction potentials are likely to vary significantly from one RTIL to another. In experimental practice it is also important to verify that the calibration molecules do not interfere chemically with the voltammetric process under study. For example, we have investigated the oxidation of molecular hydrogen in the presence of TMPD and observed a reaction of the two species, as noted by the disappearance of the reverse-peak of the first redox couple (see Figure 11.11). This implies that the peak potentials ofTMPD +/TMPD are no longer obvious, and that this redox couple cannot be used as an internal reference in this type of experiment.

In order to reveal the nature of deactivation, the potential of the catalyst slurry was continuously measured during the partial oxidation of alcohols. Cyclic voltammetric measurements [16] were also performed in the same aqueous alkaline solution with model (unsupported) catalysts for the interpretation of the potential values. The experiments revealed that the oxidation of alcohols may be divided into three groups. The basis of classifying is the oxidation state of proroot-ed catalyst and its surface coverage with hydrogen or oxygen (OH) during reaction. 

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