Hi people, I fitted several supported platinum based catalysts by Artemis. I obtained good R-factors < 0.02 but stranges values of SO2 (<0.30) and I do not know if these low values of SO2 are related with a low coordination number for Pt. I know that particle sizes of these catalysts are very small and then the coordination number is smaller than 12. Thank you very much. JA
Hi JA, You are correct. Anatoly -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Juan Antonio Maciá Agulló Sent: Thursday, October 20, 2005 12:13 PM To: XAFS Analysis using Ifeffit Subject: [Ifeffit] S02 Hi people, I fitted several supported platinum based catalysts by Artemis. I obtained good R-factors < 0.02 but stranges values of SO2 (<0.30) and I do not know if these low values of SO2 are related with a low coordination number for Pt. I know that particle sizes of these catalysts are very small and then the coordination number is smaller than 12. Thank you very much. JA
Thank you very much Anatoly for your help. These type of catalysts are complicated and when I obtain stranges values I always think that something is wrong, maybe they are too complicated for a beginner like me. JA
On Thursday 20 October 2005 11:13, Juan Antonio Maciá Agulló wrote:
Hi people,
I fitted several supported platinum based catalysts by Artemis. I obtained good R-factors < 0.02 but stranges values of SO2 (<0.30) and I do not know if these low values of SO2 are related with a low coordination number for Pt. I know that particle sizes of these catalysts are very small and then the coordination number is smaller than 12.
Without knowing anyting else about the system, I am inclined to agree with Anatoly and you that the low S02 is indicative of small particle size and the effective reduction of coordination. But it seems prudent to seize upon your question as an opportunity to remind everyone that interpreting the S02 parameter is one of the hardest parts of doing exafs analysis. As Juan Antonio points out, S02 is heavily correlated with coordination. It is also heavily correlated with empirical problems such as sample inhomogeneity. S02 can also be quite highly correlated with sigma^2. Often times a suspiciously small S02 is coincident with a suspiciously small sigma^2. There are even situations where you might be suspicious of Feff's energy model. In any case you should always evaluate a fitting result in the context of what you know about the sample. In the case of a supported platinum catalyst, you have good reason to suspect the coordination number is effectively reduce compared to the bulk. One should always be suspicious of sample preparation protocols and do whatever is reasonable within the context of the experiment to make the sample a homogenous as possible, remembering that an absorption length in your material might be only a few microns. Finally, one should always be mindful of correlations with other variables -- often a suspicious value for a parameter is Artemis's way of telling you that you need to re-evaluate the decisions you made when constructing your fitting model. But, if you can explain the suspicious value you get for a parameter in the context of what you know about the sample, then you might be justified in running with it. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advance Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
Thank you very much Bruce for your explanation. As you have commented, I have high correlations between S02 and sigma^2, I can not eliminate them by changing kw, so... what can I do? JA
On Thursday 20 October 2005 12:03, Juan Antonio Maciá Agulló wrote:
Thank you very much Bruce for your explanation. As you have commented, I have high correlations between S02 and sigma^2, I can not eliminate them by changing kw, so... what can I do?
The answer might very well be that you have to live with it. These correlations will be reflected in the size of the uncertainty. In many cases it is possible to change some kind of extrinsic parameter and invoke some knowledge of how your parameters change when thatr extrinsic parameter is changed. An example of an extrinsic parameter is temperature. We presume that S02 is constant in temperature and that sigma^2 behaves like a single-frequency oscillator with temperature. If you are able to make a sequence of measurements as a function of temperature, the variations in the data combined with your understanding of how the data should behave can be very useful in determining parameters with more confidence. Of course, there are many situations where it is not possible to take measurements as a function of temperature (or presure or whatever). In that case we have to do the best we can and acknowledge that there is some systematic uncertainty in the system. In that case, one must be honest about the uncertainties when reporting results. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advance Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
Ok, thank you very much Bruce. I will try do measurements at different temperatures next time. JA
Hi Juan, Another suggestion in the mean time (until you can do a temperature series) is to fit multiple coordination shells--there are some excellent papers by Anatoly that do that with systems very much like yours. The reasoning is that for very small samples, the coordination number of shells at higher Reff will be suppressed MORE than the nearest neighbor coordination number, with the ratio determined by the size and shape of the catalyst particles. This helps some in separating coordination number effects from other effects which have been mentioned (e.g. correlation with sigma2, sample homogeneity, etc.). If the suppression in S02 is due to one of the latter factors, I'd expect it to affect all coordination shells pretty much equally, but if it's due to small crystallite size, it should be highly influenced by including more shells in your fit. Combining this with, e.g., a temperature series might well provide you even more confidence that you understand what's going on. --Scott Calvin Sarah Lawrence College
Hi Scott Calvin, Firstly sorry for my delayed answer (I was out) and thank you very much for your interesting answer. Frequently only the first or the first and second shells have an appreciable intensity in my samples and it is difficult to fit the subsequent shells with low intensity. Anyway, I will have a look at outer shells. JA
participants (4)
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Anatoly Frenkel -
Bruce Ravel -
Juan Antonio Maciá Agulló -
Scott Calvin