white line intensity v. edge position in determining oxidation state of Re
Hi everyone, I am trying to determine the oxidation state of Re in PtRe nanoparticles catalysts supported on carbon. The ReLIII edge was measured before and after an in-situ reduction in hydrogen. Can anyone suggest Which characteristic of the edge is more related to oxidation state ? Is it the integrated white-line intensity or the position of the edge(point of maximum derivative). The edge position of the ReLIII edge of my particles is the same as that of Re foil, however the white line instensity is significanty higher. For the non-reduced PtRe particles, the edge position is similar to that of ReO2 (with a larger white line intensity). Also, does anyone have any Data on Re-foil, HReO4 and ReO2 so that i can compare it to the data i measured for my standards ? Thank you very much for your help -Edward Kunkes
On Thursday 17 May 2007, Edward L. Kunkes wrote:
I am trying to determine the oxidation state of Re in PtRe nanoparticles catalysts supported on carbon. The ReLIII edge was measured before and after an in-situ reduction in hydrogen. Can anyone suggest Which characteristic of the edge is more related to oxidation state ? Is it the integrated white-line intensity or the position of the edge(point of maximum derivative). The edge position of the ReLIII edge of my particles is the same as that of Re foil, however the white line instensity is significanty higher. For the non-reduced PtRe particles, the edge position is similar to that of ReO2 (with a larger white line intensity).
I see that no one has responded to this, so I'll take a stab. Presumably you know what the end members -- Re metal and Re oxide -- look like. Can you do linear combination fitting of the intermediate spectra to determine the average oxidation? B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced 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/
Edward Another method to get information on oxidation states from near-edges was used relatively commonly in the 1980s and early 1990s, but has gone out of fashion since (can someone tell me why? Is the underlying physics flawed?). You determine the ionisation potential (IP) by fitting the arctan step background function simultaneously with Gauss/Lorentz lines for all the near-edge resonances. The inflection point of the arctan corresponds to the IP. Variations in the IP can be interpreted using the same considerations as for binding energy shifts in XPS... For example, for Au there is an almost perfectly linear relationship between IP and oxidation state, from Au(-1) via Au(0) and Au(+1) to Au(+3) ! In this case the correlation with oxidation state is a lot stronger than for white line intensities and/or the now commonly used 'edge inflection points'... Sven -- Sven L.M. Schroeder (mailto:s.schroeder@manchester.ac.uk) School of Chemical Engineering and Analytical Science (CEAS) & School of Chemistry The University of Manchester PO Box 88 Sackville Street Manchester M60 1QD United Kingdom http://www.slmslab.info Tel +44 (161) 306 4502 Lab +44 (161) 306 4486 Fax +44 (161) 306 4399 Offices: School of CEAS: Room C17 (Jackson's Mill) School of Chemistry: Room E3 (Faraday Undergraduate Block) DISCLAIMER The views expressed within this message are those of the sender, not those of The University of Manchester or one of its Departments. While all emails and attachments are scanned for viruses before sending, we cannot accept any responsibility for viruses, so please scan all emails and attachments. This email is intended for the addressee only. If you are not the intended recipient, please notify the sender and delete this email immediately. END OF MESSAGE
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel Sent: 17 May 2007 21:36 To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] white line intensity v. edge position in determiningoxidation state of Re
I am trying to determine the oxidation state of Re in PtRe nanoparticles catalysts supported on carbon. The ReLIII edge was measured before and after an in-situ reduction in hydrogen. Can anyone suggest Which characteristic of the edge is more related to oxidation state ? Is it the integrated white-line intensity or the
the edge(point of maximum derivative). The edge position of
On Thursday 17 May 2007, Edward L. Kunkes wrote: position of the ReLIII
edge of my particles is the same as that of Re foil, however the white line instensity is significanty higher. For the non-reduced PtRe particles, the edge position is similar to that of ReO2 (with a larger white line intensity).
I see that no one has responded to this, so I'll take a stab.
Presumably you know what the end members -- Re metal and Re oxide -- look like. Can you do linear combination fitting of the intermediate spectra to determine the average oxidation?
B
-- Bruce Ravel ---------------------------------------------- bravel@anl.gov
Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced 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/
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
On Monday 21 May 2007, Sven L.M. Schroeder wrote:
Edward
Another method to get information on oxidation states from near-edges was used relatively commonly in the 1980s and early 1990s, but has gone out of fashion since (can someone tell me why? Is the underlying physics flawed?).
You determine the ionisation potential (IP) by fitting the arctan step background function simultaneously with Gauss/Lorentz lines for all the near-edge resonances. The inflection point of the arctan corresponds to the IP. Variations in the IP can be interpreted using the same considerations as for binding energy shifts in XPS...
For example, for Au there is an almost perfectly linear relationship between IP and oxidation state, from Au(-1) via Au(0) and Au(+1) to Au(+3) ! In this case the correlation with oxidation state is a lot stronger than for white line intensities and/or the now commonly used 'edge inflection points'...
Sven
My understanding of the physics behind the assumption that you can fit a XANES spectrum by an arctan and some peak-y functions is that you are assuming that the system is a perturbation from a simple, Drude-like gas of electrons. At zero kelvin, the electron density is a well defined, sharp step function with the step at the Fermi energy. At finite temperature, that sharp edge step is smeared into a Fermi function -- essentially an arctan. As a perturbation, there is some structure to the electron density that is handled by the peak-y functions. To the extent that you can point to the location of the Fermi energy on a XANES spectrum, the underlying physics of using an arctan is well-founded. In the sequence of pure-materials you describe, the arctan method should work well assuming you are talking about pure phase materials, even in the case of Au+3 where the "perturbation" (also known as a rather enormous white line) is not small. However, I don't think the use of one arctan is well suited to a mixed phase situation, which is a possibility for what Edward is seeing in his electrochemistry experiments. That is, at some intermediate voltage, a fraction of the sample might be oxidized and a fraction metallic. In that case, the XANES spectrum needs two arctans mixed in some ratio. Then the fitting of lineshapes has to include a mixing term to describe the amount of each species. The line shape fitting thus inherits some of the systematic uncertainty of the linear combination approach, along with all the rest of its sources of error. In the case of mixed species, tracking a feature in the XANES (inflection, white line position, what have you) can usually be shown to be as accurate as a linear combination solution assuming the correct feature can be identified. And in some systems, fitting a single arctan might work well also -- but that would not be generalizable to all systems. My US$0.02 worth, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced 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/
Edward
Another method to get information on oxidation states from near-edges was used relatively commonly in the 1980s and early 1990s, but has gone out of fashion since (can someone tell me why? Is the underlying physics flawed?).
You determine the ionisation potential (IP) by fitting the arctan step background function simultaneously with Gauss/Lorentz lines for all the near-edge resonances. The inflection point of the arctan corresponds to
I agree with Bruce on this, and would like to add that in nanoparticles the very Fermi energy may be shifting around - as a function of size, metallicity, support-particle interaction, etc. For example, in bulk materials, the change in the oxidation state of an absorber by, say, -1, due to reduction of an oxide, for example, is accompanied by the shift to the lower energies of characteristic XANES features (peak position, max derivatives, or the energy at the half edge step, - these are most popular "reference" points for the chemical shift determination). The reason for this "chemical" shift is screening of the coure hole potential by the "extra" electron and the concomitant decrease in the ionization potential. However, in nanoparticles, the changes in the charge state of the absorber also noticeably change Fermi energy, which a few extra electrons per particle _increase_ Fermi energy compared to the initial sample. That, of course, causes the increase of the ionization potential. Which of this trends prevails is a very interesting question - we have seen both positive and negative shifts in nanoparticles, which means that these effects are at least the same order of magnitude. What is important, however, is that the energ shift in nanoparticles does not scale simply with the formal oxidation state. Anatoly -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov]On Behalf Of Bruce Ravel Sent: Monday, May 21, 2007 6:59 PM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] white line intensity v. edge position indeterminingoxidation state of Re On Monday 21 May 2007, Sven L.M. Schroeder wrote: the
IP. Variations in the IP can be interpreted using the same considerations as for binding energy shifts in XPS...
For example, for Au there is an almost perfectly linear relationship between IP and oxidation state, from Au(-1) via Au(0) and Au(+1) to Au(+3) ! In this case the correlation with oxidation state is a lot stronger than for white line intensities and/or the now commonly used 'edge inflection points'...
Sven
My understanding of the physics behind the assumption that you can fit a XANES spectrum by an arctan and some peak-y functions is that you are assuming that the system is a perturbation from a simple, Drude-like gas of electrons. At zero kelvin, the electron density is a well defined, sharp step function with the step at the Fermi energy. At finite temperature, that sharp edge step is smeared into a Fermi function -- essentially an arctan. As a perturbation, there is some structure to the electron density that is handled by the peak-y functions. To the extent that you can point to the location of the Fermi energy on a XANES spectrum, the underlying physics of using an arctan is well-founded. In the sequence of pure-materials you describe, the arctan method should work well assuming you are talking about pure phase materials, even in the case of Au+3 where the "perturbation" (also known as a rather enormous white line) is not small. However, I don't think the use of one arctan is well suited to a mixed phase situation, which is a possibility for what Edward is seeing in his electrochemistry experiments. That is, at some intermediate voltage, a fraction of the sample might be oxidized and a fraction metallic. In that case, the XANES spectrum needs two arctans mixed in some ratio. Then the fitting of lineshapes has to include a mixing term to describe the amount of each species. The line shape fitting thus inherits some of the systematic uncertainty of the linear combination approach, along with all the rest of its sources of error. In the case of mixed species, tracking a feature in the XANES (inflection, white line position, what have you) can usually be shown to be as accurate as a linear combination solution assuming the correct feature can be identified. And in some systems, fitting a single arctan might work well also -- but that would not be generalizable to all systems. My US$0.02 worth, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced 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/ _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Edward: For rhenium the examples published in the literature use the first inflection point to define the oxidation state. Indeed for a set of rhenium oxides of formal oxidation state 4, 6 and 7 there is close to a linear relationship of formal oxidation state with edge position. We have collected such data several times. Regarding your other point about the white line intensity of rhenium nanoparticles, this has been the subject of some recent research of ours. We are close to submitting a manuscript where we discuss this point for rhenium supported on alumina. We also observe the same phenomena that you describe: even though the rhenium is formally reduced, the white line intensity is substantially more intense that the equivalent metal reference. Until this manuscript is ready I refer you to an older publication: J. Phys. Chem. 95 (1991) 225-234 by Fung et al. where they also note the same. We have also done some FEFF8 XANES calculations of small Re clusters and there is a clear trend of increasing white line intensity with decreasing cluster size. I would be willing to share our rhenium reference spectra if you contact me directly. Simon R. Bare Senior R&D Associate UOP LLC 25 East Algonquin Road Des Plaines, IL 60017-5017 Phone - 847.391.3171 Cell - 630.842.6890 Fax - 847.391.3719 simon.bare@uop.com www.uop.com -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Edward L. Kunkes Sent: Thursday, May 17, 2007 10:29 AM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] white line intensity v. edge position in determining oxidation state of Re Hi everyone, I am trying to determine the oxidation state of Re in PtRe nanoparticles catalysts supported on carbon. The ReLIII edge was measured before and after an in-situ reduction in hydrogen. Can anyone suggest Which characteristic of the edge is more related to oxidation state ? Is it the integrated white-line intensity or the position of the edge(point of maximum derivative). The edge position of the ReLIII edge of my particles is the same as that of Re foil, however the white line instensity is significanty higher. For the non-reduced PtRe particles, the edge position is similar to that of ReO2 (with a larger white line intensity). Also, does anyone have any Data on Re-foil, HReO4 and ReO2 so that i can compare it to the data i measured for my standards ? Thank you very much for your help -Edward Kunkes _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
participants (5)
-
Anatoly Frenkel -
Bare, Simon R -
Bruce Ravel -
Edward L. Kunkes -
Sven L.M. Schroeder