Hi Marian,
Have you tried what I suggested in the sections you quote below? With a
ruthenium metal catalyst, you can probably see some second and perhaps third
shell neighbors. The ratio between the coordination numbers for the shells is a
measure of particle size that is independent of S02, and thus might (depending
on data quality) provide a more reliable estimate of particle size.
--Scott Calvin
Sarah Lawrence College
On Oct 11, 2011, at 8:54 AM, Dreher Marian wrote:
Dear Ifeffit Community,
I have a question regarding the signal damping I mentioned in my last post (I
searched the archive but didn't find an answer. If this topic has been
discussed before, please just send me the link).
As I mentioned before, I'm trying to determine the particle size of a Ru
catalyst. I have 3 different reference spectra from Ru that I can use to
determine the s0^2 for bulk Ru. From that I can calculate the Ru-Ru CN in my
catalyst.
Now, two of my references give a very similar EXAFS signal in terms of
amplitude. When fitting the first Ru-Ru shell I get a s0^2 of 0.6 to 0.8,
depending on k-weight. One of the reference spectra was obtained from a pellet
containing Ru powder ("Ru pellet Sigma Aldrich"), the other from a thick Ru
filter ("Ru filter foil").
I also have a reference spectrum from an unknown Ru sample. The amplitude of
the EXAFS is much higher compared to the other two spectra. However, the edge
step is considerably smaller. Here, a Ru-Ru first shell fit gives me a s0^2 of
0.9 to 1.1, depending on k-weight. Even before fitting, the experimental data
is very close to the theoretical spectrum.
Obviously, the CN I calculate for my Ru catalyst depends on these reference
values for s0^2.
I can see how poor sample preparation leads to signal damping and thus to a low
amplitude in the EXAFS, but I can't imagine how it could cause an amplitude
which is too high. Should I therefore take the unknown reference since it has a
higher amplitude?
I attached an Athena project file with all 3 spectra.
Many thanks,
Marian.
________________________________
From:
ifeffit-boun...@millenia.cars.aps.anl.gov<mailto:ifeffit-boun...@millenia.cars.aps.anl.gov>
[mailto:ifeffit-boun...@millenia.cars.aps.anl.gov] On Behalf Of Dreher Marian
Sent: Mittwoch, 21. September 2011 21:07
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] particle size and coordination number in Runanoparticles
on carbon
Thanks everyone for your help, now everything is working fine. My Ru reference
was simply way too thick which caused a lot of dampening in the signal.
From:
ifeffit-boun...@millenia.cars.aps.anl.gov<mailto:ifeffit-boun...@millenia.cars.aps.anl.gov>
[mailto:ifeffit-boun...@millenia.cars.aps.anl.gov] On Behalf Of Wayne W Lukens
Jr
Sent: Dienstag, 20. September 2011 20:26
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] particle size and coordination number in Ru
nanoparticles on carbon
Hi Marian,
There is a very nice paper by Scott Calvin that describes this in some detail:
J. Appl. Phys. 94:778–83 (2003)
Sincerely,
Wayne Lukens
On Tue, Sep 20, 2011 at 11:04 AM, Scott Calvin
<scal...@sarahlawrence.edu<mailto:scal...@sarahlawrence.edu>> wrote:
Hi Marian,
My big tip is to look at the second nearest-neighbor CN as well, and even
further out if you can get it. It's sometimes hard to pin down the
near-neighbor coordination number because of issues like the one you describe.
But the ratio of higher CNs to near-neighbor CN is quite diagnostic of small
nanoparticles. (Actually, for reasons too lengthy to get in to at the moment,
this method does tend to produce results a bit biased to the small side, so
it's best to try the identical fitting model on a bulk standard for comparison.)
--Scott Calvin
Sarah Lawrence College
On Sep 20, 2011, at 1:50 PM, Dreher Marian wrote:
Dear Ifeffit Community,
I am currently trying to extract the particle size of Ru supported on carbon
from EXAFS data. I also recorded spectra of a Ru foil as a bulk reference
sample.
So, my approach is to fit the first coordination shell of my bulk reference,
setting CN to 12. From that I get a value for s02. I'm then using this value in
the first shell fit for the Ru nanoparticles in order to get the coordination
number.
However, the fit suggests that CN is still pretty much 12. What is changing
drastically, though, is sigma^2 which usually doubles in value compared to the
bulk sample. That makes sense, I guess, since there might be higher disorder in
the nanoparticles.
Obviously, there is a chance that my nanoparticles are rather big, in which
case CN would be close to 12. However, STEM and HRTEM pictures suggest that the
Ru particles are between 1 and 2 nm in diameter.
I'm just starting to get into EXAFS analysis and pretty clueless right now
about how to approach this.
Happy about any input,
many thanks,
Marian.
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