Transport phenomena, analysis with scaling

Let us take an example of mixing fluorescein with water (D = 3x 10" cm /s). For a microsystem with / 100 pm and 17 30 pm/s, the Peclet number is equal to 10. Thus, td> ra, which is contrary to equation 1.33. This indicates that diffusion phenomenon is much slower than hydrodynamic transport phenomenon, which is contrary to what was suggested based on the scaling analysis. Hence, the scaling laws cannot be used blindly. It provides an estimate of the process, which need to be verified from the exact analysis. 

Microkinetic analysis of H2-D2 exchange experiments conducted over PdCu alloys in the presence of H2S provides context for low-temperature inhibition, revealing that H2S both increases the activation barrier for H2 s dissociative adsorption and blocks a fraction of the lowered activity sites [36]. The increased activation barrier could be related to either formation of a thin (undetectable by XRD) scale layer or by electronic influence of neighboring adsorbed S [26], Site blocking, and not lower inherent dissociation activity, is likely the main contributor to lower fluxes. As temperature increases, H2S desorbs from the surface and more sites become available for H2 dissociation - and H2 transport can proceed. As noted earlier, the phenomenon can be framed in terms of the equilibrium between gas phase and surface H2S (or its reversible decomposition products), which can be described by an equilibrium constant XhiS- HjS depends on alloy composition and temperature. In the specific case of Pd47Cu53, exchange experiments reveal that the fraction of sites blocked by exposure to 1000 ppm at 450 °C is 65% [36]—this is consistent with the 70% flux decline that Pd47Cu53 experiences in the presence of 1000 ppm H2S [17]. At 635 C, the fraction of sites blocked by H2S drops to below 20% and the alloy experiences no flux decline. 

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