Palladium diffusion data

Values of kH2 and n at elevated temperatures and pressures were determined from hydrogen flux data. The log-log representation of this data (Eqn. 6) in terms of hydrogen partial pressure is illustrated in Figure 2. The n values were 0.51, 0.53 and 0.55 at 700, 800 and 900°C, respectively. This value of approximately 0.5 is similar to the values reported for diffusion membranes such as palladium [Armor, 1995], The temperature-dependent km value was determined from the intercept of each set of data. The results were then fit to an Arrenhius relation (Eqn. 7),... 

The study of the voltammetric changes of the iodide-copper interaction on palladium allows an examination of the control process during underpotential deposition. The electrochemical desorption of copper produces an increase in the current value with almost no variations in the peak potential. The fitting of the current yields again a diffusion control for Cu2+ desorption from the surface. On the other hand, the cathodic peak not only changes the current intensities but also produces a negative shift in the potential values. Also, a diffusion-control process for copper deposition on palladium is found after fitting the experimental data. 

Constant acceleration spectra were obtained with an Austin Science Associates, Inc. S-600 Mossbauer spectrometer equipped with an electromagnetic Doppler velocity motor. The source was 50 m Ci of Co diffused into a palladium matrix, and it was obtained from New England Nuclear, Inc. The pulses from the proportional counter detector (Reuter Stokes) were amplified, shaped and gated using Austin Science Associates electronics. These shaped pulses were then sent to a Tracor Northern NS-900 multichannel analyzer. The MCA was interfaced directly to a PDP-11 minicomputer, greatly facilitating data storage and analysis. 

A large quantity of experimental data has been obtained for the diffusivity of hydrogen in various metals, particularly palladium, iron, and nickel [49]. The dependence of diffusivity on temperature generally follows an Arrhenius relationship over a wide temperature range, although marked breaks occur for some metals. Palladium is characterized by... 

Table 5 compares values of the preexponential factor Do) and activation energy [Ea] from the dependence of diffusivity on temperature for a-iron [48]. The data were compiled for specimens on which surface effects were reduced by use of a palladium coating on one side. At higher temperatures, the activation energy falls in a narrow range from 6.7 to 7.1 kj raol , but at temperatures of 70 and lower, the activation energy varies between 4.2 and 7.1 kJ mol . This range is consistent with other values of 6.9 kJ mol [38] and 8.0 kJ mol [50]. The diffusivity for... 

Fig. 80. Summary of data on diffusion constants of hydrogen in palladium.

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