M. Huchede et al.
Applied Catalysis A, General 617 (2021) 118016
Sb3d3/2 at 541 eV was preferred to Sb3d5/2, due to the superposition of
the 1 s oxygen signal at 531ꢀ 532 eV [25] (Fig. S2). A study of the
Sb3d3/2 signal produced by mixed-valence compounds has shown that
very small changes in ionization energy, of the order of 0.5 eV, are
associated with the Sb3+ and Sb5+ oxidation states [26]. In addition, by
studying the decomposition of the Sb3d3/2 signal of mixed valence
antimony oxides, it has been shown that the Sb5+ and Sb3+ signals are
difficult to separate, and that it is challenging to determine the indi-
vidual atomic concentrations of each of these two species [27,28]. On
the other hand, a study of the Sb4d level at 34ꢀ 35 eV shows that it is
straightforward to distinguish between the signals corresponding to its
two oxidation states. For all of these reasons, the Sb4d signal was
initially studied to determine whether Sb3+ and Sb5+ coexist in the same
sample. If only one oxidation state was detected, it would be preferable
to use the Sb3d signal for the quantification of this element, due to the
improved accuracy that could be expected from its considerably higher
(20x) intensity. As an example, the presence of a single oxidation state is
characterized by a "spin-orbit" of 1.2–1.3 eV, with an area ratio of 0.66
between the two signals.
just one graphite sheet was inserted between the front plate and the
central element. However, as these sheets absorb a high proportion of
X-ray photons at the L1-edge of Sb, it was not possible to record the
spectra of antimony under these conditions. For this reason, all of the
graphite sheets were perforated in their center, in order to increase the
number of transmitted photons, and a 50 μm thick Kapton window was
added on each side of the central element, to keep the sample inside the
(5 × 8 mm) cell. As Kapton has a low thermal resistance, the spectra
were only recorded beyond 250 ◦C. These modifications made it possible
to carry out a in situ study of the oxidation state of antimony at the
L1-edge, as previously reported [32]. Two to three mg of the catalyst was
diluted in boron nitride (10 % by mass), then placed in the reactor and
heated to 250 ◦C under air flow (7 mL.minꢀ 1, 5 ◦C.minꢀ 1), prior to
vaporization of the ethyl lactate. The composition of the reaction flow
was almost identical to that used for the evaluation of catalytic prop-
erties: EtLA/ inert/ O2: 12.3/ 66.4/ 19.3. spectra were recorded for a
period of 15 min at each edge, leading to a total test duration ranging
between 30 min and 2 h. Measurements were also carried out at room
temperature, prior to heating and after cooling.
Iron edge is characterized by two peaks: the 2p level, located at
709ꢀ 711 eV for Fe2p3/2, and at 722ꢀ 724 eV for Fe2p1/2. The various
oxidation states of iron are characterized mainly by the presence of a
signal at 709.3 eV, corresponding to the Fe2+ ion, and a second signal at
711.4 eV, corresponding to the Fe3+ ion. XPS spectral analysis thus al-
lows each of these species to be distinguished and quantified. The sat-
ellite identified at 717.6–717.7 eV is characteristic of the Fe2+ ion,
whereas the satellite identified at 718.8–719.9 eV is characteristic of the
Fe3+ ion [29]. In the case of compounds with a mixed valence, con-
taining both Fe2 + and Fe3+ ions, the signals corresponding to the sat-
ellites of each of these species are superimposed, thus decreasing the
accuracy of their quantification.
2.3. Catalysts testing
The catalytic properties of the synthesized compounds were evalu-
ated using the same setup as that described in previous studies [8].
Reactions were carried out at temperatures ranging between 250 and
400 ◦C, with a gas mixture composition corresponding to EtLA/O2 /inert
= 12.3/ 18.4/ 69.3, with mcat =250 mgꢀ 1, PPH =0.1 h and Vtotal =65
mL.minꢀ 1. After being diluted in SiC alpha (mSiC/ mcat = 2), the catalyst
remained stable for longer than 22 h, under reaction conditions. The
condensable reaction products were trapped in acetonitrile at 0ꢀ 2 ◦C
and analyzed on a Shimadzu GC-2014 chromatograph, equipped with a
In the case of vanadium, its main line corresponds to the 2p level,
which is characterized by binding energies between 515 and 517 eV for
V2p3/2, and between 522 and 524 eV for V2p1/2. It can be difficult to
analyze vanadium on the surface of iron and in vanadium antimonates,
because it can be present in three different oxidation states. High-
resolution analysis of the V2p signal can be used to quantify each of
these species. Two separate regions are thus defined for V2p3/2 and
V2p1/2. As it is not straightforward to analyze the continuous back-
ground of the V2p1/2 signal at 522 eV, which is strongly affected by the
presence of the O1 s signal at 531 eV, only the structure of level V2p3/2
at 515ꢀ 517 eV is presented here. XPS analysis of the V2p3/2 signal
produced by vanadium oxide compounds of different valences reveals
significant changes in bond energy. When several oxidation states
coexist in the same compound, small changes in position are neverthe-
less observed, with respect to the pure compound [30].
Nukol type (30 m x 0.53 mm x0.5
μ
m) polar capillary column. Tetra-
hydrofuran (THF) was used as an external standard. The volatiles were
characterized on line with a
μGC-TCD analyzer (SRA R3000 chromato-
graph equipped with a molecular sieve, a Porapak Q and a TCD detec-
tor). Nitrogen was used as an internal standard. The carbon balances
were greater than 98 %, and a blank test showed no conversion of ethyl
lactate up to 350 ◦C.
3. Results
3.1. Characterization of the synthesized catalysts
The measured molar ratios and surface areas of the synthesized
VFeSbO catalysts are provided in Table 1. The chemical analysis of these
solids is found to be in good agreement with the theoretical formulas
proposed in the first published studies of these catalysts [15,19,22].
Following the introduction of iron, the V4+ content decrease continu-
ously until V3+ cations only were observed, with no remaining cationic
vacancy. The BET surface areas were rather low, and comparable for all
of the solids. They were found to decrease slightly when the temperature
of the final heat treatment was increased.
The XAS measurements described in the present study were carried
out at the SOLEIL synchrotron, on the ROCK beamline (Rocking Optics
for Chemical Kinetics), at the K-edge of iron (7112.0 eV) and vanadium
(5465.1 eV), and at the L1-edge of antimony (4698.3 eV). All analyses
were performed in the transmission mode, in Quick-EXAFS (QEXAFS),
allowing the acquisition of 4 spectra per second. For the ex-situ analyses,
the samples were first diluted in 10%–15% of BN, and then pelletized. In
order to obtain the best possible response from each element, the
quantity of catalyst was optimized and 2 pellets were made for each
sample (one for the analysis of iron, and the other for that of vanadium
and antimony). In-situ studies were also carried out, in order to monitor
any change in oxidation state in the bulk of the catalyst under catalysis
conditions. By optimizing the detectors, it was possible to simulta-
neously analyze iron, vanadium and antimony in the same sample. The
in-situ cell could be used for studies at atmospheric pressure and at
temperatures as high as 600 ◦C [31]. The sample holder comprises a
central element, as well as front and rear plates with small apertures
designed to permit transmission of the X-ray beam. In order to ensure the
The X-ray diffraction patterns of the solids revealed just one, well-
crystallized rutile-type phase for all prepared samples (Figs. 3 and 4).
The influence of calcination temperature on the structure was studied
Table 1
Results of chemical analyses and specific surface area measurements.
Catalyst
Molar ratio
SSA (m2. gꢀ 1
)
V/(V + Fe)
(V + Fe/Sb)
ht 700 ◦
C
ht 900 ◦
C
VSbO4
1.00
0.80
0.59
0.34
0.13
0.91
0.95
0.92
0.95
0.96
13
12
18
10
10
11
11
11
V
V
V
V
0.8Fe0.2SbO4
0.6Fe0.4SbO4
0.35Fe0.65SbO4
0.15Fe085SbO4
air-tightness of the cell under reaction flow, two 500
μm thick graphite
sheets were inserted between the rear plate and the central element, and
3