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/