[Ifeffit] Re[2]: a question about background refinement

Carlo U. Segre segre at iit.edu
Thu Apr 15 22:55:58 CDT 2004


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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-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Special Projects, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at agni.phys.iit.edu    http://www.iit.edu/~segre


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