Hi all,
Thank you very much Scott for your not slow
answer and sorry for my very slow one.
I think my system is a little bit complicated. I
have bimetallic catalysts (PtSn)
and I did fits supposing two different
structures but I do not know the relative
proportion of these two structures. In this
sense and as there are paths (from the
two structures) with the same Reff I can not
calculate coordination numbers for all
the paths at the same time, I do not have enough
number of independent points. I
also tried a mulplied shell fit but the signals
in FT decreases considerably in the
outer shells (confusing with noise) and it is difficult to perform a good fit.
As an example, the fit of the first shell of a
PtSn catalyst with Pt fcc and Pt3Sn
structures:
I suppose S02 = 0.85, Nip = 6, 3 variables and
three different single scattering
paths are included in the fit:
Reff = 2.82 Pt-Pt (Pt3Sn)
Reff = 2.82 Pt-Sn (Pt3Sn)
Reff = 2.77 Pt-Pt (Pt fcc)
I can not assume an average coordination number
because each path have a different
degeneracy, so I have 3 new variables in the fit and 6 in all.
If I will perform a multiplied shell fit the number of varibles (specially
coordination numbers) also will increase and
there will not be enough number of
independen points either. Could I perform a
multiplied shell fit considering only
CN of the first shell? I mean, a multiplied
shell fit to increase the number of
independent points and only calculate CN for the first shell.
Sorry for this long e-mail and this crazy questions.
Thank you very much,
Best regards,
JA
Scott Calvin ha escrito:
> Hi Juan,
>
> Sorry for the slow response; the end of the term gets busy for me!
>
> I think there are still some loose ends in this
> discussion that are worth trying to tie up:
>
> At 03:03 PM 12/11/2006, you wrote:
>
> >First of all, I would like to thanks Anatoly
for his file and everybody for
> >useful comments. I have analysed Anatoly's data
> >and I have obtained a good value
> >for S02 = 0.85 or 0.82 (depending on the number
> >of variables used). So, my data
> >is the problem, and it is not my analysis, but
> >maybe my measurements need a more
> >accurate analysis with Athena as Scott suggested.
> >I was not at synchrotron measuring platinum
samples and I only know that are
> >measured in fluorescent mode. As Bruce said,
I have no beamtime now for more
> >measurements.
> >
> >I have more questions, related and not
related to the last subject, but I am
> >still thinking about them:
> >The first one is easy, it is about the
Nyquist theorem. I read in a paper that
> >the formula is 2·deltak·deltaR/pi + 2. The
last "+2" is new for me and I am
> >afraid that Artemis does not consider it. I
am sure that it is a silly thing.
>
> For a while, arguing over this +2 (or +1 or +0)
> was a popular topic in the EXAFS community.
> Eventually it was realized that there isn't
> really as much information as implied by the
> Nyquist criterion anyway. Crudely, the Nyquist
> criterion assumes you have someone trying to
> convey as much information as possible in a
> signal. Nature isn't so obliging. So it's
> becoming more common to leave the +2 off, and
> even that is not conservative. If you're running
> out of independent points, introducing more
> constraints, extending the k-range, or extending
> the r-range can be better ways of getting
> yourself out of trouble than invoking the +2.
>
> >I will try to correct again Ptfoil considering self-absorption in order to
> >obtain a spectrum similar to Anatoly's or
Bruce's one. And then, I will apply
> >the same correction for supported platinum catalysts, right?
>
> Since your samples have a low concentration of
> Pt, the self-absorption correction should not be
> necessary for them. You've talked about changing
> the variable from S02 to N in subsequent fits to
> obtain both variables...if I understand you
> correctly, that won't accomplish anything. If it
> were that "easy," Ifeffit would include it in its
> fitting algorithm! S02 and N for a single-shell
> single-sample fit are 100% correlated and values
> cannot be obtained for both no matter what you
> do. I think the best you can do is fix S02 at
> some plausible value (0.85, say, or the result of
> a FEFF calculation, or whatever), and then
> realize, and explicitly note in publications,
> that this assumption introduces an uncertainty of
> perhaps 10% in N, in addition to whatever
uncertainties are found by Ifeffit.
>
> >I also observed that Anatoly's Pt foil shows
good signal even for large k (20
> >A-1). Nevertheless, obviously platinum
catalysts spectra possess lower signal
> >and specially for high values of k where the
noise is big. The question is,
> >despite Pt foil has a good signal until 20
A-1, it is usually used a smaller
> >k-range (i.e. 3-12 A-1), right? I normally use a k-range of 3-12.
>
> As Matt said, use the data to guide you when
> choosing k-range; not some arbitrarily chosen
> range. I also find it useful to try varying my
> k-range a bit after the fit is done to check that
> the results are stable. Of course, if you are
> visually comparing Fourier transforms of
> different samples, rather than performing fits,
> you want to compare over the same k-range.
>
> Finally, at the risk of repeating myself, I'm
> going to suggest that your system sounds like it
> would benefit from a multiple-shell fit. That's
> pretty easy to do for a metallic cluster like
> platinum. And it reduces some of your problems.
> It's hard, as you've noticed, to determine N for
> a single shell, in part because you have to know
> S02. But since S02 is the same for all shells,
> it's easier to determine the ratio between N for
> the first shell and N for the second shell.
> That's still a little dicey because sigma2 is
> likely to be far different for the first two
> shells, and sigma2 correlates to N (but not 100%
> correlation; the effect of sigma2 depends on k,
> and N does not). If you get to three shells,
> though, then the ratios of N3 to N2 to N1 start
> to get teased out from the other effects, and you
> can start to determine things like crystallite
> size, which it sounds like is the thing you're after.
>
> That's the principle that both Anatoly and I have
> used in the past to find the size of
> nanoparticles or nanocrystals. Our methods differ
> in detail--Anatoly's is better for good data and
> highly uncertain morphology, because it assumes
> less; mine is probably better for iffy data and
> roughly known (e.g. "spherical") morphology,
> because mine has fewer free parameters. A search
> of the literature will reveal several articles by
> each of us detailing how to do this kind of
> analysis, including the APL I mentioned earlier
> on platinum nanoparticles and a JACS article of
> Anatoly's on platinum-ruthenium nanoparticles.
>
> --Scott Calvin
> Sarah Lawrence College
>
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