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 S_o 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