An in-situ reduction/oxidation XAS study on the EL10V8 VO
x
/
TiO
2
(anatase) powder catalyst
Geert Silversmit
a,
*, Hilde Poelman
a
, Isabelle Sack
b
, Guy Buyle
a
, Guy B. Marin
b
, and Roger De Gryse
a
a
Department Solid State Sciences, Ghent University, Krijgslaan 281 S1, B-9000 Gent, Belgium
b
Laboratorium voor Petrochemische Techniek, Ghent University, Krijgslaan 281 S5, B-9000 Gent, Belgium
Received 4 November 2005; accepted 24 November 2005
The structural changes of the supported vanadium oxide in the V
2
O
5
/TiO
2
(anatase) EUROCAT EL10V8 powder catalyst
during reduction and oxidation at 420 and 490 °C were studied with in-situ X-ray absorption spectroscopy (XAS). The Vanadium
K-edge XAS results are compared with pure bulk V
2
O
5
. For the reduction–oxidation cycle at 420 °C, similar structural changes as
for bulk V
2
O
5
were observed for the supported vanadium oxide: a reduction to the VO
2
structure and re-oxidation back to V
2
O
5
.
After reduction at 490 °C however, a different structure was obtained: very regular ‘‘VO
6
’’ octahedra with a V
2.8+
valence. This
may point to a structural support effect.
KEY WORDS: EUROCAT EL10V8; supported vanadium oxide; absorption spectroscopy; VO
x
/TiO
2
(anatase); in-situ XAS;
V
2
O
5
.
1. Introduction
The European EUROCAT programme [1] consisted
of a detailed study on two V
2
O
5
/TiO
2
(anatase) powder
catalysts with different V
2
O
5
loadings (1 and 8 wt%).
These catalysts were characterized with XRD, BET
surface area, TPR, DTA, H
2
reduction and subsequent
O
2
adsorption, vibration spectroscopies, NMR, EPR,
XPS, SIMS, electric conductivity, chemical test reac-
tions (partial oxidation of o-xylene to phthalic anhy-
dride, decomposition of isopropanol),.... The two
powders are therefore often used as reference catalyst
systems.
X-ray absorption spectroscopy (XAS) was not
applied in this EUROCAT study. As XAS can be per-
formed in-situ, valuable structural information can be
obtained with this technique. The present paper
describes an in-situ VK-edge XAS study on the EU-
ROCAT EL10V8 V
2
O
5
/TiO
2
(anatase) powder catalyst
(hereafter named V8) during two subsequent reduction/
oxidation cycles at 420 and 490 °C. The XAS results are
compared with pure unsupported V
2
O
5
subjected to
identical reduction/oxidation cycles at 420 °C.
An XAS experiment implies the registration of the
total absorption of the sample as function of incident X-
ray energy from $200–100 eV before an absorption
edge up to 1000 eV after the edge. The region around
the absorption edge up to about 50 eV above it is called
the XANES region (X-ray Absorption Near-Edge
Structure) and the region beyond 50 eV above the edge
the EXAFS region (Extended X-ray Absorption Edge
Structure). This distinction is made as both regions
contain different information: electronic information
from the XANES region and structural from the EX-
AFS.
2. Experimental
The transmission Vanadium K-edge X-ray absorp-
tion spectroscopy (XAS) measurements were performed
at the DUBBLE CRG beamline (BM26A) of the ESRF
synchrotron during a 2*1/3 filling mode, giving a storage
ring current of 200–160 mA. The XAS spectra were
recorded with a Si(111) double crystal monochromator
and ionisation chambers filled with Ar/He mixtures. The
higher harmonics of the bending magnet synchrotron
radiation that are also being transmitted by the mono-
chromator were suppressed with the vertical focussing
mirror after the monochromator.
Typically 5–6 mg of catalyst powder was mixed with
40 mg of BN, an inert and weakly absorbing binder, and
pressed into the rectangular hole of a stainless steel
sample holder. The weight of the sample was chosen to
give a total absorption (lx=ln(I
0
/I
t
)) of 2.5 at the
Vanadium K-absorption edge in which I
0
is the intensity
of the incident X-ray beam and I
t
of the transmitted
beam. For the in-situ measurements, a stainless steel
XAS chemical cell designed and built at the Ghent
University was used. The XAS cell has Kapton foils
(25lm) as X-ray transparent windows. At the reaction
temperatures, up to 6 spectra were averaged for the V8
* To whom correspondence should be addressed.
Catalysis Letters Vol. 107, Nos. 1–2, February 2006 (Ó 2006) 61
DOI: 10.1007/s10562-005-9732-z
1011-372X/06/0200–0061/0
Ó
2006 Springer Science+Business Media, Inc.