[Ifeffit] particle size and coordination numberin Runanoparticles on carbon

[Dreher Marian]

Hi Scott,

thanks for the suggestion.

I've looked through your recent publications and also read your post
from Fri, 04 Feb 2011, concerning the R dependence of s02. Comparing the
CNs of the 1st and 2nd shell (that are the only ones that are properly
resolved in my spectra) makes a lot of sense. 
I also read the following publication:

Jentys, A. Physical Chemistry Chemical Physics 1999, 1, 4059-4063. 

Basically, I could use their formula to calculate the theoretical ratio
of 1st and 2nd shell CN at a given cluster size and compare that to my
data. Their calculations are based on a fcc structure but it should be a
good enough approximation for my hcp Ru clusters.
Does that sound reasonable?


Regards,

Marian.


Marian Dreher, Dipl. Chem.
PhD Student, Catalytic Process Engineering Group

Paul Scherrer Institut
General Energy Research (ENE)
Laboratory for Bioenergy and Catalysis
Office: OVGA/115
CH-5232 Villigen PSI
Switzerland

Phone: +41 (0)56 310 2937
email:  marian.dreher at psi.ch

-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Scott
Calvin
Sent: Dienstag, 11. Oktober 2011 15:04
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] particle size and coordination numberin
Runanoparticles on carbon

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-bounces at millenia.cars.aps.anl.gov<mailto:ifeffit-bounces at milleni
a.cars.aps.anl.gov> [mailto:ifeffit-bounces at 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-bounces at millenia.cars.aps.anl.gov<mailto:ifeffit-bounces at milleni
a.cars.aps.anl.gov> [mailto:ifeffit-bounces at 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
<scalvin at sarahlawrence.edu<mailto:scalvin at 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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