Adsorbed Poisoning Species

The mechanism of the electro-oxidation of methanol on platinum was thoroughly established, mainly after the identification of both reactive intermediates and adsorbed poisoning species [4]. In the first step, methanol is dissociatively adsorbed at Pt-based catalysts by cleavage of C - H bonds, leading to the so-called formyl-like species -(CHO)ads- From this species, different steps can occur, but with platinum, the dissociation of -(CHO)ads gives rapidly adsorbed CO, which is responsible for the electrode poisoning. This is the explanation of the rather poor performance of Pt catalysts, due to the relatively high potential necessary to oxidize such CO species. 

As in the development of CO-tolerant catalysts for PEFC anodes, the main challenge for the development of catalysts for the oxidation of alcohols is to reduce or to avoid the formation of strongly adsorbed poisoning species (i.e., CO) or to favor their oxidation at low overpotentials. 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

Another very interesting result obtained from these FURS measurements is the difference between adsorbed CO obtained from dissolved CO and that from the dissociation of adsorbed methanol. The shift in wave number is more important with dissolved CO. These shifts may also be correlated with the superficial composition of the alloys, and it was observed that the optimized composition for the oxidation of CO (about 50 at.% Ru) is different from that for the oxidation of methanol (about 15 at.% Ru). FTIR spectra also revealed that the amount of adsorbed CO formed from methanol dissociation is considerably higher on R than on Pt-Ru. For a Ptog-Ru-o i alloy, the amount of linearly adsorbed CO is very small (Fig. 8), suggesting a low coverage in the poisoning species. Moreover, by observing the potentials at which the COj IR absorption band appears, it is possible to conclude that the oxidation of both (CHO)ads and (CO)acis species occurs at much lower potentials on a R-Ru alloy electrode than on pure Pt. 

Intrinsic Activity Poisons. These poisons decrease the activity of the catalyst for the primary chemical reaction by virtue of their direct electronic or chemical influence on the catalyst surface or active sites. The mechanism appears to be one that involves coverage of the active sites by poison molecules, removing the possibility that these sites can subsequently adsorb reactant species. Common examples of this type of poisoning are the actions of compounds of elements of the groups Vb and VIb (N, P, As, Sb, O, S, Se, Te) on metallic catalysts. 

With Pt-Ru catalysts, it appears clearly from the literature, and this was fully confirmed by IR reflectance spectroscopic studies, that the presence of adsorbed OH on ratheninm sites leads to the oxidation of adsorbed CO at potentials much lower than those on pure platinum. It is also probable that CHOads can be oxidized directly to carbon dioxide, without the formation of adsorbed CO poisoning species. ... 

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