M.O. Guerrero-Pérez et al.
CatalysisTodayxxx(xxxx)xxx–xxx
and for its ammoxidation [9,57,76,77]. In all cases, the vanadium
oxidation state during reaction determines the behavior of the catalytic
system. XANES is a useful characterization tool to assess it.
2. Experimental section
2.1. Synthesis of catalysts
XANES provides information about the oxidation state of the given
atom, such as vanadium. Unlike XPS, it also delivers structural in-
formation, especially when combined with EXAFS [28–31]. The K-edge
position in XANES reports the oxidation state of vanadium. It is con-
nected to the energy of the core electron which decreases with higher
valence and thus the K-edge shifts to higher energy [32] The pre-edge
of the given spectrum reflects transitions form K-shell to the empty
bound d-levels, is used to determine oxidation states. Additionally, the
intensity of the pre-edge provides information of the coordination of the
analyzed metal. The pre-edge intensity of vanadium species with in-
version symmetry, like regular octahedral is very low. Coordination
environments lacking inversion symmetry, like tetrahedrons, result in
high intensities of the pre-edge peak [33]. However, the intensity of the
pre-edge peak of a distorted octahedral site increases significantly due
to the loss of the inversion symmetry caused by the disorder. In the
present work vanadium coordination and oxidation states will be
monitored using XANES during partial oxidation reactions for three
different V-based mixed oxides catalytic systems. In 2003, one of the
first studied in which XANES was performed under in situ conditions for
the characterization of Cu based complexes inside the zeolite cavities
was published [34]; and in 2005, Payen et al. described the design of a
reaction cell for obtaining operando XANES and EXAFS [35]. Such cell
was used for the characterization of Co and Mo species during oxidative
and reductive pre-treatments of a silica-supported Fischer-Tropsch re-
action. After those first papers, much more studies under reaction
conditions have been performed by different groups, showing that in
situ and operando XANES studies have the potential to study the changes
on the oxidation states of the active phases under different environ-
ments. Most of these studies focus on Ce- [36–42], Pt [43], W- [44], Fe
[45], Mo- [46] and Cu- [47,48] based catalytic systems. XANES has
proven also to be a useful technique for the characterization of V-based
catalysts [49,50]. López-Nieto et al. used operando XANES for char-
acterization of supported vanadium oxide catalysts during the oxidation
of H2S to Sulphur [51], detecting the transformation of crystalline
V2O5, present in fresh catalyst, to V4O9. XANES has been used also for
the characterization of multioxide Mo-V-Nb-O catalytic system by
Millet and coworkers [52–54]. In those works, XANES and other tech-
niques were used to determine the oxidation state of the Te, V, Mo and
Sb elements in several catalysts and in the pure M1 and M2 phases.
Operando-Raman studies over different V-based catalysts performed
by our group [10,55–60] have demonstrated that important structural
changes occur during reaction which directly affect the catalytic per-
formance. In those studies, alumina-supported catalysts were used ra-
ther than conventional bulk catalysts; that facilitated the discrimination
of relevant surface species signals. This approach is not only appro-
priate to study molecularly dispersed oxide species, but also that of
their nanoscaled bulk counterparts. Thus, active phases formed under
reaction conditions have been identified, such as rutile VSbO4 [58], or
bulk Mo-V-O phases [61]. In all the studied V-based catalysts, the im-
portance of vanadium redox capacity was outlined, and the fact that it
is accompanied by the formation of mixed valence compounds, being
the V3+/V4+/V5+ ratios related, directly, with the catalytic properties.
Thus, XANES/EXAFS study under reaction conditions have been per-
formed, in order to complement these operando Raman studies that
have been performed to Mo-V-Nb-Te-O and V-Sb-O catalytic systems,
with the aim of identify the V oxidation states under reaction condi-
tions, as complementary and valuable information to the structural
information given by the Raman spectroscopy in the previous studies.
MoVTeNbOx catalyst were been prepared starting from slurries
obtained by mixing aqueous solutions of the metal components. The
catalyst named as 12Mo5V4Nb0.5Te0.5O-N, with a final Mo/V/Te/Nb
atomic ratio of 0.5–0.6/0.4–0.3/0.05/0.05. The first digit indicates the
total number of atoms supported per nm2 of support. The digits after
the elements indicate their stoichiometry. The synthesis method was via
an aqueous slurry of the corresponding salts, as reported elsewhere
[62,63], and calcined at 600 °C for 2 h in N2 flow. It was prepared in
order to have a total Mo + V + Nb + Te coverage of 12 atoms per nm2
on alumina support, which is roughly two monolayers coverage. A bulk
MoVNbTeO material mainly composed by M1 phase was used as re-
ference material. The synthesis method and the characterization study
of such sample was already described [64].
The Sb–V catalyst was prepared dissolving the required amount of
antimony acetate (Aldrich) in tartaric acid (Sigma) 0.3 M. This solution
was kept under stirring until all antimony dissolves. Then, NH4VO3
(Sigma) and the γ-Al2O3 (Sasol, Puralox SCCa-5/200) support were
added. The solution was dried in a rotatory evaporator at 80 °C and
0.3 atm. The resulting solid was dried at 115 °C for 24 h and then cal-
cined at 400 °C for 4 h in air. The catalyst was prepared so that a total
coverage of V + Sb would correspond to one monolayer, and with a Sb/
V molar ratio of 1. SbVO4 rutile phase and VOx dispersed species were
found on this sample, as was described elsewhere [65].
Commercially available oxides were also used as reference, such as
V2O5 (Sigma-Aldrich), NH4VO3 (Sigma), (VO)2P2O7 (Sigma), VOSO4
(Sigma), VOHPO4 (Sigma).
2.2. XANES in situ study
V K-edge XANES/EXAFS spectra of all the samples (catalysts and
oxides used as reference) were acquired at the BM31 beamline at the
European Synchrotron Radiation Facility (ESRF) in Grenoble using a Si
(111) monochromator, in the 5400–5600 eV range, with an energy step
of 1 eV s−1. A vanadium metal foil was used as references to calibrate
the pre-edge absorption energy of the V spectra to 5468 eV. The reac-
tion cell used for obtaining the XANES/EXAFS spectra under reaction
conditions is shown in Fig. 1, which is a modified commercial (Specac)
Fig. 1. Reaction cell used in the in situ-XANES/EXAFS studies of propane amm(oxida-
tion).
2