Promotion oxidation adsorption energy

There is clear evidence that adsorbate and alloyed metal atoms on platinum surface promote CO electro-oxidation. The reduced overpotential is primarily a result of the promotion of the activation of water. The subsequent kinetics are determined by the details of a Langmuir-Hinshelwood reaction between the adsorbed oxidant (OH) and adsorbed CO. Evidence is also presented that relates this promotion (or poisoning) of CO electro-oxidation to tolerate CO in hydrogen feeds in the hydrogen electro-oxidation reaction. An alternative mechanism that may operate at low potentials [79,113] may be that the reduction in CO adsorption energy on platinum induced by Ru [86,113,114] results in a higher equilibrium concentration of nonpoisoned sites. The relative importance of these mechanisms is a function... 

An even more pronounced difference in light-off performance between pre-oxidation and pre-reduction is seen for C0/AI2O3 (see below). The reason that pre-reduced Pd/Co/AbOs and Pt/Co/Ab03 start to react at lower temperatures than the pre-oxidised samples (see Figure 5) is most likely due to that the initial reaction on cobalt sites starts at lower temperatures for the pre-reduced samples than for the pre-oxidised catalysts. The pre-reduction may also induce alloy formation with lower adsorption energy of CO compared with the noble metal [18]. It is also possible that 0-vacancies form on the cobalt oxide which promote the dissociative adsorption of O2. Reaction with CO will then proceed on the noble metal or at the interface between Pt (or Pd) and cobalt oxide [9-10]. 

Cu, Zn, Ge, Zr, and Rh have high D> c and E for water dissociation, and thus are unsuitable as secondary metals. The enhanced promoting effect of Re, W, or Mo toward H2O dissociation are due to strong M-OH adsorption, and large Eads(OH) does not promote oxidative removal of CO. For M = Re, the activation energy for the COads + OHads combination reaction is the highest of all, and thus the reaction becomes the rate-determining step in CO removal. 

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