[Ifeffit] Manganese oxide EXAFS modelin

[Ebrahim Rezaei]

Hi everyone,


I am working with bulk and
nano-particles manganese oxides. Basically I have collected EXAFS of Mn K-edge
for my reference material (Mn2O3, bixibyite) and also the
catalyst (Mn supported on alumina). The catalyst shows formation of
nanoparticles of bixibyite on alumina with size around 20 nm. 


My first question is about modeling
the reference material in order to find So which will be used as a
fixed parameter to find oxygen and manganese first and second shell CNs for the
catalyst.  


The difficulty with modeling the
reference material arises due to the rather complex atomic coordinates of Mn2O3
containing oxygen atoms at different but very close distances. Basically Mn2O3
has two sites (25% and 75% occupancy for site 1 and 2 respectively) with the
following coordinates:


Site 1                                                               Site
2 

Atom               R (Å)               CN                  Atom               R (Å)               CN

O                     1.993               6                      O                     1.899               2

Mn                   3.104               6                      O                     1.985               2

Mn                   3.565               6                      O                     2.249               2

                                                                        Mn                   3.115               6

                                                                        Mn                   3.575               6

 

I am aware that in order to fit the
EXAFS equation I should considers paths from both sites and multiply their
amplitudes by their site occupancy. But to make the life easier I simplified
the atom coordinates of Mn2O3 to be able to extract the
CNs from the catalyst. To my understanding one can expect to have 6 oxygen
atoms in average at the first shell (0.25*6 + 0.75*6 = 6), 6 Mn atoms at the
second shell and 6 Mn atoms at the third shell regardless of the site occupancy.



Therefore I ignored the site
occupancy and considered the following atomic coordinates assuming that there
is only one site in the in the model developed for the reference material:

 

Atom               R (Å)               CN

O                     1.985               4

O                     2.249               2                                  

Mn                   3.115               6                                  

Mn                   3.565               6          

 

The model works fairly acceptable
and I am able to get CN from the catalysts as well. But I am not sure if my
assumptions could be acceptable or not. I was wondering if I can have idea of the
EXAFS community experts on this.


My other question is about dealing
with σ2 as a fixed parameter. I have noticed that after fitting the
catalysts, σ2 of paths in the catalysts are almost close to that of
the reference material. Therefore I tried to fix the values of σ2 for
the catalyst and use the ones I have obtained from the reference material. This
increases the degree of freedom of the fit and results in smaller error bars in
CN while χ2 increases by 2.5%. Basically I have always kept number
of variables to almost less than half of the number of the independent points
either with fixing or relaxing σ2. 


I was wondering if estimation of σ2
in the catalysts from the reference material might be a good assumption
in my case or not. I think it should not be a bad idea since size of Mn2O3
nano-particles on alumina is fairly large (around 20 nm) showing formation of
complete crystalline of Mn2O3 on the catalyst.  


I have my project file in hand and I
can send it for further discussion as well.


I appreciate your time and help in
advance.


Best Regards,

Ebrahim Rezaei,

PhD Student, Chemical Engineering
Department,

University of Saskatchewan,
Saskatoon, SK, Canada 
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