sections proportional to the number of electrons transferred and
largely insensitive to the identity of the reduced species (e.g.,
vacancies, OH, alkoxides).2
Spectra were referenced to MgO and converted to difference
spectra by dividing by RH for stoichiometric oxides (Eqn. (1)).
Normalized reflectances were converted to absorbance using
the Kubelka–Munk formalism (Eqn. (2)), where K and S are
proportional to the absorption and scattering coefficients,
respectively.17,18
This relation between measured reflectances and sample
absorbance can lead to inaccurate extents of reduction from
linear interpolations between stoichiometric V3+ sub-oxides,
especially for minority reduced centers, because of the strong
ligand-to-metal transitions and their weak dependence on d-
electron densities. These factors lead to lower values of RH to
larger values of F(RH), and to much poorer signal to noise ratios
than obtainable in the pre-edge region.
Fig. 2 Extent of reduction as a function of C3H8:O2 reactant ratio, (a) 10
wt% (8.0 V nm22), (b) 3.5 wt% (2.3 V nm22) V2O5/Al2O3 [603 K, filled
symbols: 1.0–16 kPa C3H8, 4.0 kPa O2; open symbols: 1.0–16 kPa O2, 4.0
kPa C3H8; dashed lines represent (C3H8/O2)0.5 dependence].
Transient experiments showed that 60–70% of reduced
centers formed during contact with C3H8–O2 were neither
reversible during reaction nor catalytically relevant. These
spectator reduced species appear to form via conversion of
minority V2O5 clusters to stable suboxides, which reoxidize
slowly by V2O5 nucleation at reaction conditions. These
irreversible absorption changes include a fast initial transient
upon contact with reactants ( ~ 60 s), which is not fully reversed
even after 4 h in 4 kPa O2 at 603 K. Reversible and catalytically-
relevant changes in pre-edge intensity were measured during
exposure to 0–16 kPa C3H8 or O2 at constant O2 or C3H8
pressure (4 kPa) for 300 s. This time interval was chosen
because absorption transients were accurately described as a
single first-order relaxation process. Absorbances for these
catalytically-relevant reduced centers were converted to extents
of reduction using the data in Fig. 1 and the results are shown in
Fig. 2 and Table 1. We note that total pre-edge absorption
changes give much higher extents of reduction because they
include irreversible reduction processes unrelated to catalytic
redox turnovers, e.g., total absorption changes give 0.060 and
0.31 e2 V21 for the 2.3 and 8.0 V nm22 catalysts, respectively,
at the conditions listed in Table 1.
Table 1 Extent of vanadium reduction during propane ODHa
C3H6 Rate [*1023
Catalyst (wt%
V2O5 on Al2O3) (V nm22
V surface density e2 transferred mol s21 (g-atom
)
per active V
V)21
]
3.5
10
2.3
8.0
0.020
0.12
0.33
1.0
a 603 K, 14 kPa C3H8, 1.7 kPa O2, balance He.
We show here that the use of the pre-edge region in UV-
visible spectra, the rigorous calibration of the intensity of these
features, and the isolation of the dynamic processes relevant to
catalysis from those of spectator species lead to accurate
measurements of the number of reduced centers present at very
low concentrations during alkane ODH catalysis.
Notes and references
1 K. D. Chen, A. Khodakov, J. Yang, A. T. Bell and E. Iglesia, J. Catal.,
1999, 186, 325; M. D. Argyle, K. D. Chen, A. T. Bell and E. Iglesia, J.
Catal., 2002, 208, 139; A. Khodakov, B. Olthof, A. T. Bell and E.
Iglesia, J. Catal., 1999, 181, 205; B. Olthof, A. Khodakov, A. T. Bell
and E. Iglesia, J. Phys. Chem. B, 2000, 104, 1516.
2 K. D. Chen, A. T. Bell and E. Iglesia, J. Phys. Chem. B, 2000, 104, 1292;
K. D. Chen, E. Iglesia and A. T. Bell, J. Catal., 2001, 192, 197.
3 G. W. Coulston, S. R. Bare, H. Kung, K. Birkeland, G. K. Bethke, R.
Harlow, N. Herron and P. L. Lee, Science, 1997, 275, 191.
4 J. G. Eon, R. Olier and J. C. Volta, J. Catal., 1994, 145, 318.
5 M. A. Vuurman and I. E. Wachs, J. Phys. Chem., 1992, 96, 5008.
6 X. Gao, J. M. Jehng and I. E. Wachs, J. Catal., 2002, 209, 43.
7 R. L. Puurunen, B. G. Beheydt and B. M. Weckhuysen, J. Catal., 2001,
204, 253.
8 X. Gao, S. R. Bare, J. L. G. Fierro and I. E. Wachs, J. Phys. Chem. B.,
1999, 103, 618.
9 X. Gao, M. A. Banares and I. E. Wachs, J. Catal., 1999, 188, 325.
10 X. Gao, S. R. Bare, B. M. Weckhuysen and I. E. Wachs, J. Phys. Chem.
B., 1998, 102, 10842.
11 D. Wei, H. Wang, X. Feng, W. Chueh, P. Ravikovitch, M. Lyubovsky,
C. Li, T. Takeguchi and G. L. Haller, J. Phys. Chem. B, 1999, 103,
2113.
12 G. Grubert, J. Rathousky, G. Schulz-Ekloff, M. Wark and A. Zukal,
Microporous Mesoporous Mat., 1998, 22, 225.
The extent of reduction during steady-state propane ODH is
much lower than for stoichiometric reduction of V5+ to V4+ or
V3+. These data confirm the conclusion from kinetic and
isotopic studies2 that lattice oxygen atoms are the most
abundant reactive intermediates during propane ODH on VOx
domains, even at relatively high C3H8/O2 reactant ratios. The
extent of reduction was lower on the 2.3 V nm22 sample than on
the 8.0 V nm22 sample, suggesting that the larger VOx domains
prevalent in the latter sample undergo faster redox cyles than
isolated VOx species,1,2 consistent with the observed increase in
ODH rates (per V or Mo) with increasing VOx or MoOx domain
size1,19 (Table 1). These higher rates parallel the measured
higher extents of reduction because turnover rates are limited by
reduction steps involving activation of methylene C–H bonds in
propane using lattice oxygen atoms.2 The extent of reduction
increases with increasing C3H8 or decreasing O2 pressure (Fig.
2), consistent with proposed ODH mechanistic steps. The data
in Fig. 2 are in quantitative agreement with the expected
dependence of reduced centers on C3H8:O2 (dashed curves)
when oxygen vacancies are the prevalent reduced centers.2
The extents of reduction reported here for catalytically
relevant species on VOx/Al2O3 are much lower than reported
previously on VOx/ZrO2 for similar conditions and surface
densities, but using total absorption edge intensity changes (e.g.,
13 J. Melsheimer, S. S. Mahmoud, G. Mestl and R. Schlogl, Catal. Lett.,
1999, 60, 103.
14 B. M. Weckhuysen, Chem. Commun., 2002, 97.
15 B. M. Weckhuysen, A. A. Verberckmoes, J. Debaere, K. Ooms, I.
Langhans and R. A. Schoonheydt, J. Mol. Catal. A, 2000, 151, 115.
16 A. Brückner, Chem. Commun., 2001, 2122.
17 P. Kubelka and F. Munk, Z. Tech. Phys., 1931, 12, 593.
18 G. Kortüm, Reflectance Spectroscopy, Springer-Verlag, Berlin, 1969.
19 K. D. Chen, A. T. Bell and E. Iglesia, J. Catal., 2002, 209, 35.
0.10 e2 V21 on VOx/Al2O3; 0.60 e2 V21 on VOx/ZrO2 6:1
6,9
C3H8/O2). Spectator reduced centers, perhaps in part on ZrO2
supports,9 may be responsible for these differences and also
account for the higher COx selectivities on ZrO2-supported
samples.1,6
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