Sulfur macroscopic forms

Figure XVIII-2 shows how a surface reaction may be followed by STM, in this case the reaction on a Ni(llO) surface O(surface) + H2S(g) = H20(g) + S(surface). Figure XVIII-2a shows the oxygen atom covered surface before any reaction, and Fig. XVIII-2h, the surface after exposure to 3 of H2S during which Ni islands and troughs have formed on which sulfur chemisorbs. The technique is powerful in the wealth of detail provided on the other hand, there is so much detail that it is difficult to relate it to macroscopic observation (such as the kinetics of the reaction).

At 635 °C and in the presence of 1000 ppm H2S, all three alloys exhibit H2 flux that approaches the flux in the absence of H2S - true sulfur tolerance. However, the absence of a scale layer does not guarantee high rates of H2 transport. At lower temperatures, H2S suppresses H2 transport, even in cases where no macroscopic scale layer forms. For example, as illustrated in Figure 7, in l(X)0ppm H2S at 350 C, H2 flux through a Pd47Cu53 membrane H2 drops to zero immediately upon addition of H2S [26]. In cases such as this, H2S suppresses transport by reducing rates of H2 dissociation at the membrane surface. 

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