I'll let Stanislav answer some of the other questions but I'll try these. I am sure that Stanislav will correct me if necessary. The particles are in a reducing environment and therefore we did not expect to see any Ru-O bonds at all. In the case of Pt, a model which only includes Pt-Pt and Pt-Ru neighbors works extremely well. The same model is used to fit the metal neighbors of the Ru and for all data that we have thus far, there is consistency in the combined fits. When we take the same particles and measure them at room temperature outside the fuel cell, it is clear that there are Pt-O (or other light element) bonds as well as Ru-O while the metal bonds are the same as in the rest of the data. Therefore, I am quite confident that under operating conditions, there is no Pt-O contribution. The hypothesis is that the Ru in the particles promotes the oxidation of the methanol fuel. It is therefore reasonable to see Ru-O bonds at the surface of the particle. What appears as partial oxidation is simply the ratio of surface Ru to total Ru in the 3.6nm particle. The picture is appealing and we are fairly confident because the near edge data also seems to be a linear combination of metallic Ru and partially oxidized Ru (Ru oxide hydrate fits best) but we have been vexed by the negative Debye Waller factor of the Ru-O bond. I suppose that it is possible that it is not Ru-O at all but Ru-C but certainly there is a light atom there. What Stanislav was trying to do, and we are not sure that it is a valid procedure, is to separate the real Ru-O contribution from what might be just background leakage. The data is taken in transmission and since we must take the electrode the way it is made for real fuel cells, the edge jump is relatively small. This limits the range in k-space that we can use and I think it leads to some background subtraction issues. Probably more than anyone wanted to hear about this... Carlo On Thu, 15 Apr 2004, Matt Newville wrote:
Related to that: Is there a good understanding of why the Ru would partially oxidize while the Pt appears to not oxidize? This seems perfectly reasonable to me, but I'll ask anyway: you see Ru-O but not Pt-O right?
If you see that the Pt edge background is OK and there's no need to add oxygen to the Pt model, that does make the need to include a 'background+oxygen peak' at the Ru edge more believable. But I'd still suggest looking at the different fit contributions in both k- and R-space.
Hope that helps, even though you've probably tried all these things and more....
--Matt
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