Adsorption site blocker blocking effect

As a conclusion, the promoting effects of adsorption site blockers on H entry can be understood not by considering the effective diminution of 0 the overall coverage of OPD H (determined by integration of adsorption pseudocapacitance vs. potential curves [117-119]), but by considering the variation of x the local OPD H coverage in the sites not blocked [obtained by normahzation of 0 or from the Tafel and permeation slopes (see Tables 1 and 2)]. If the surfece-bulk transfer step is in equilibrium, a significant increase of the bulk H eoncentration can be induced by ASB surface effects only for the HER mechanism where the steps of electroadsorption and chemical combination are coupled. This analysis provides a quantitative explanation of the effects of promotion of H absorption into iron and ferrous alloys. 

Figure 12 Theoretical variations, for a coupled electroadsorption-chemical combination HER mechanism, of the overall H coverage 0jj (fiill line), the local H coverage Xjj (dotted line), the HER current (dashed line), and the bulk H fraction for charging (dashed line) with the coverage of adsorption site blocker (ASB). The represented case is for Langmuir type H adsorption and a geometric blocking effect, where one ASB atom blocks one H site, with random ASB adsorption 6h asb)% her cc asb) h Above a certain value of 0 gg...

The effects of adsorption site blockers on the rates of the surface reactions involving adsorbed hydrogen may be modeled simply by taking into account simultaneously but separately the blocking effect that reduces the number of sites for H adsorption and the effect on the bond in the sites not blocked, without any a priori assumption on the... 

An alternative explanation for the promoter effects exists for nonnoble metals, such as Fe or Ni. The HER rarely proceeds on a bare surface of these metals because, except at high overpotentials, they are covered by corrosion products or passive oxide films [55,102]. Adsorption site blockers such as sulfur block sites for O or OH adsorption and thus impede or inhibit surface oxidation. In the gas phase, it has been shown that at low oxygen pressure and room temperature, Ni oxidation is inhibited when the surface is covered by a complete... 

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