[Ifeffit] Differences between fluorescence and transmission of the same sample

Hello ifeffit users, I am working in the area of In-situ XAS analysis of Fuel cell catalyst. The heart of the fuel cell, MEA(membrane electrode assembly) consist of nafion membrane and the catalyst sprayed on both sides which act as Cathode and Anode(the whole max thickness 1mm). I have a system where Platinum is a common metal catalyst for both anode and cathode, with anode made of alloy of Pt and Ru(nano) ,and Cathode pure Pt(both 2mg/cm2 metal loading). An In-situ transmission XAS measurements on such a system gives no information as the transmission signal will contain both Pt signal from anode and cathode and two different process taking place oxidation and reduction. So we deal with the situation by removing the catalyst from one side. But when we did our measurements in fluorescence(without removing any catalyst) we got two different signals when we turned the cell 180 degree. If the anode was facing the fluo detector we got characteristic signal for Pt/Ru alloy(dampening in FT) and when cathode was facing, that of pure Pt. Is this a solution to separate the signals from anode and cathode without removing any catalyst?. I have attached the file for the fluorescence measurements for my MEA. Thanks in Advance, Ditty On Wed, May 5, 2010 at 5:43 PM, wrote:

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Today's Topics:

1. Re: Differences between fluorescence and transmission of the same sample (Bruce Ravel) 2. Re: Ifeffit Digest, Vol 87, Issue 4 (Bruce Ravel) 3. Differences between fluorescence and transmission of the same sample (Andrew Campos) 4. Re: Differences between fluorescence and transmission of the same sample (Ditty Dixon)

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Message: 1 Date: Wed, 5 May 2010 10:30:18 -0400 From: Bruce Ravel To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Differences between fluorescence and transmission of the same sample Message-ID: <201005051030.18579.bravel@bnl.gov> Content-Type: Text/Plain; charset="iso-8859-15"

Andrew,

I suspect that you are seeing what Grant refers to in his tutorials as "leakage". That is, your transmission data appears to my eye to be what happens when you have a wide spread in sample thickness under the beam, ranging from gaps to very thick particles.

If you go to

http://gbxafs.iit.edu/training/tutorials.html

And follow the link to the "Sample preparation" presentation, the effect of leakage is shown on slide 16.

On the following slide, Grant points out that this thickness effect always reduces amplitudes. That is the case above the edge for your data. And if you normalize data distorted in that way, you end up with an artificially enhanced pre-edge peak, again as in your data.

My US$0.02, B

On Wednesday 05 May 2010 09:55:16 am Andrew Campos wrote:

...

Hi everyone,

My lab mates are running in-situ H2 reduction of Cobalt/SiO2 catalysts using the Co K-edge and are obtaining differences between the fluorescence and transmission signals. The in-situ cell does have quite a bit of sample thickness (2-3 mm approx) and I was wondering if the fluorescence signal only has a penetration depth of a few micrometers. If this is the case that would explain it where the side that is further away from the heating element is more oxidized than the rest of the sample.

Has anyone run into this problem before? We're using the basic furnace/cryostat unit (in-situ cell manufactured by the exafsco) and a 13 element fluorescence detector (germanium diode).

I appreciate any insight into the issue, I have attached a fluorescence and transmission spectrum collected at the same time where the transmission and fluorescence are in the .prj file.

Thanks so much! Andrew

--

Bruce Ravel ------------------------------------ bravel@bnl.gov

National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973

My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/http://cars9.uchicago.edu/%7Eravel/software/exafs/

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Message: 2 Date: Wed, 5 May 2010 10:39:14 -0400 From: Bruce Ravel To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Ifeffit Digest, Vol 87, Issue 4 Message-ID: <201005051039.14501.bravel@bnl.gov> Content-Type: Text/Plain; charset="iso-8859-1"

On Wednesday 05 May 2010 07:56:47 am baran yildirim wrote:

...

My problem is similar; I am trying to simulate a doped crystal (different than chp 10 in Atemis doc, this time impurity atom is subjected to XAS) and the host is similar to inverse-spinel type: include both tetrahedral and octahedral sites in its structure. The dopant may settle in tetrahedral or octahedral sites.

Indeed. Similar enough that you also should read the paper by Scott Calvin that I suggested to Maria Elena.

...

1- I managed to simulate the doped structure as changing the FEFF inp file manually (first I run Atoms as if impurity ions settle in all sites and then keep it in 0 potential and change the 1st potential with the original atom) and did it for both sites, thus i have 2 batch of paths and then fit it. It seems it works (Question: Do you think what i am doing is logical? or any other suggestion? )

I think that's a good approach.

Question 15 at

http://cars9.uchicago.edu/ifeffit/FAQ/FeffitModeling

gives a bit more information. The link to the page authored by Scott is quite useful.

...

2- But problems do not end :) Another problem is impurity ion may take different valences: it can be trivalent or divalent. In FEFF 9.0 i found kinda solution using ION command, it doesnt do what i exactly want but i can get some idea on what happens if the charge distribution changes. But unfortunately, i cant try it in ARTEMIS since i can not install FEFF 9.0 in ARTEMIS. As far as i understood the program structure is changed completely. But I didnt give up :)I generated the chi file in FEFF 9.0 and load it in ARTEMIS and continue to analyze but the results are not promising. (Question: Is there any way to run it together? and any suggestion in changing the valance? )

Artemis does not support Feff9 at this time.

I actually have not played around with Feff9 yet, myself, so I won't be able to help with the rest. Perhaps one of the Seattlites can help ....

B

--

Bruce Ravel ------------------------------------ bravel@bnl.gov

National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973

My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/http://cars9.uchicago.edu/%7Eravel/software/exafs/

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Message: 3 Date: Wed, 5 May 2010 11:24:16 -0400 From: Andrew Campos To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Differences between fluorescence and transmission of the same sample Message-ID: Content-Type: text/plain; charset="iso-8859-1"

Dr. Ravel,

Thanks so much for the link and the advice! I appreciate it greatly. I will advise my lab mates as such and may have to only use the fluorescence data if that is indeed the case.

I also included the file where the lower temperature is included and you might come to the same conclusion. The samples that I ran were pre-sieved, and the ones included in the .prj file aren't so that should be pursued prior to running the experiment. If they crush the particle and sieve the sample, I think that we can be more certain that this is not the case. This was very helpful!

Andrew