Our approach has consisted first in
the simulation of the XANES of models
compounds (MoO
3
, (NH
4
)
6
[Mo
7
O
24
],
(NH
4
)
6
[Al(OH)
6
Mo
6
O
18
], (NH
4
)
4
[Mo
8
O
26
],
Na
2
MoO
4
,…) in order to assign every
spectroscopic fingerprint to a given
structural property. We identify then
the same spectroscopic fingerprints for
the references compounds on a supported
catalyst which allows us to formulate
appropriate hypotheses on the active
phase structure that will then be optimized
by DFT calculations.
The preliminary investigation of XANES
spectra of models compounds shows
that XANES spectrum of catalyst exhibits
similar characteristics of polyoxoanions
compounds which indicate that the
supported catalyst phase contains
polymeric octahedral Mo. Furthermore,
the use of XANES, allows us to probes
oxygen atoms located in the second
coordination shell. This feature clearly
could not be possible using EXAFS,
accounting on the low atomic number
of oxygen. Based on these findings
and the characteristics of the catalyst’s
experimental spectrum, rational
structures of Mo species adsorbed
on (101) TiO
2
surface were generated by
DFT calculations and their corresponding
XANES spectra were calculated by XANES
modeling. Comparison of experimental and
calculated spectra (Figure
➊
) provide us
unprecedented insights into the adsorption
modes of Mo species on the titanium
surface which finally bring to the light
that the active site structure is derived
from the Anderson one : {[Mo
14
O
45
, n.
Mo
4
O
12
]
6
-, 6. (HO—TiO
2
)
+
}, (Figure
➊
right
).
The structure of the active site established
during this work favorably interacts with
the surface of the support on account of
the correspondence between the surface
structure of the support and the adsorbed
ion. This result thus showed the important
role played by the oxide support in the
genesis of the oxomolybdate phase, where
the speciation of molybdenum depends
strongly on the structure of the oxide
surface, which directly controls the way
the MoO
6
octahedral are arranged.
Coupling XANES and DFT simulations
for the heterogeneous catalyst
characterization can be very powerful
and that simulation of the XANES spectra
allows a better relationship between
the spectral features and the structure
of the absorbing element.
Results
Conclusion
SAMBA beamline
ASSOCIATED PUBLICATION
Synergy between XANES spectroscopy and
DFT calculation to reveal molecular-scale
structure of heterogeneous catalysts: TiO
2
-
Supported Molybdenum Oxide catalysts case
A. Tougerti, E. Berrier, A.S. Mamede,
C. La Fontaine, V. Briois, Y. Joly, E. Payen,
J-F. Paul, and S. Cristol*
Angewandte Chemie International Edition
52(25) (2013) 6440
REFERENCES
[1] J. L. G. Fierro & J. C. Mol, in Metal Oxides:
Chemistry and Applications; (Eds: Fierro,
J. L. G.), TAYLOR & FRANCIS, Boca Raton.
2006 517; H. Thoulhoat & P. Raybaud
in Catalysis by transitions metal sulfides, IFP
Energies nouvelles publications TECHNIP
2013
[2] V. Briois et al. Proceedings of UVX 2010,
EDP Science 41 (2011)
[3] Y. Joly, Phys. Rev. B . 63 (2001), 125120
*Unité de Catalyse et de Chimie du Solide,
Université Lille 1, UMR CNRS 8181,
Cité Scientifique, 59655 Villeneuve d’Ascq
Cedex, France
CORRESPONDING AUTHOR
49
SYNCHROTRON
HIGHLIGHTS
2013
