.
Angewandte
Communications
DOI: 10.1002/anie.201410632
Heterogeneous Catalysis
The Electronic Factor in Alkane Oxidation Catalysis**
Maik Eichelbaum,* Michael Hꢀvecker, Christian Heine, Anna Maria Wernbacher,
Frank Rosowski, Annette Trunschke, and Robert Schlçgl
[
5,8]
Abstract: This article addresses the fundamental question of
whether concepts from semiconductor physics can be applied
to describe the working mode of heterogeneous oxidation
catalysts and whether they can be even used to discriminate
between selective and unselective reaction pathways. Near-
ambient-pressure X-ray photoelectron spectroscopy was
applied to the oxidation of n-butane to maleic anhydride on
the highly selective catalyst vanadyl pyrophosphate and the
transfer across the bulk–surface–adsorbate interface.
This
charge transfer generates a potential gradient and hence
electric field between surface and bulk and can induce
conductive channels with current-rectifying properties like in
a p–n junction diode. The height of this surface potential
barrier, which charge carriers have to overcome to move
between bulk and surface, could have a significant kinetic
impact on the oxidation reaction on the surface and the
activation of oxygen. The surface barrier height under
steady-state reaction conditions could hence be a descriptor
for the catalytic performance of oxidation catalysts. However,
it has not yet been proven that oxidation catalysts act indeed
like semiconducting gas sensors with the formation of a sur-
face potential barrier controlled by the gas phase.
[
9]
[5]
moderately selective MoVTeNbO M1 phase. The catalysts
x
were found to act like semiconducting gas sensors with
a dynamic charge transfer between the bulk and the surface,
as indicated by the gas-phase-dependent response of the work
function, electron affinity, and the surface potential barrier. In
contrast, only a minor influence of the gas phase on the
semiconducting properties and hence no dynamic surface
potential barrier was monitored for the total oxidation catalyst
V O . The surface potential barrier is hence suggested as
Understanding the working mode of vanadyl pyrophos-
phate (VPP), which is the commercial catalyst for the
[
10–14]
oxidation of n-butane to maleic anhydride,
is of general
2
5
descriptor for selective catalysts.
interest, since it is a benchmark system in selective oxidation
[15–21]
catalysis,
representing one of the most important class of
S
ince the middle of the last century, semiconductor physics
heterogeneously catalyzed reactions in the context of the
[22]
concepts have been used to explain the working mode of
anticipated paradigm shift in raw materials.
Due to its
[
1–7]
selective alkane and alkene oxidation catalysts.
The vision
increasing conductivity in air and decreasing conductivity in
n-butane-containing gas mixtures, VPP was identified as a p-
type semiconductor with electron holes as the majority charge
was—and still is—to predict the catalytic activity and
selectivity of materials in different reactions on the basis of
their electronic structure (the so-called “electronic factor”).
In semiconductor theory the difference between the Fermi
potential of the semiconducting catalyst and the redox
potential of adsorbates creates a driving force for charge
[
10–14,23–25]
carriers.
However, the gas-phase-dependent conduc-
tivity response alone is not a sufficient descriptor for
selectivity, since vanadium(V) oxide exhibits a reversible
[26]
conductivity response under reaction conditions but cata-
lyzes only the total oxidation of n-butane to CO and CO2.
Herein we report on the successful application of near-
ambient-pressure X-ray photoelectron spectroscopy (NAP-
XPS) to investigate the influence of the reactive gas phase on
the surface potential barrier of VPP under catalytic n-butane
oxidation conditions with proven production of maleic
anhydride. The results show that the transfer of charge
carriers between the bulk catalyst and the surface can be
explained by—and are thus the first experimental proof for—
the previously only theoretically proposed semiconductor
[
*] Dr. M. Eichelbaum, Dr. M. Hꢀvecker, Dr. C. Heine,
A. M. Wernbacher, Dr. A. Trunschke, Prof. R. Schlçgl
Department of Inorganic Chemistry
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4–6, 14195 Berlin (Germany)
E-mail: me@fhi-berlin.mpg.de
Dr. M. Eichelbaum, Dr. F. Rosowski
BasCat, UniCat BASF JointLab, TU Berlin
Marchstrasse 6, 10587 Berlin (Germany)
[
1]
[2]
[3]
catalyst concepts of Boudart, Schwab, Volkenshtein, and
Morrison.
Dr. M. Hꢀvecker
Helmholtz Centre Berlin/BESSY II, Catalysis for Energy
Albert-Einstein-Strasse 15, 12489 Berlin (Germany)
[4,5]
We compare these results with the electronic
response of the unselective oxidation catalyst V O and the
2
5
Dr. F. Rosowski
moderately selective catalyst MoVTeNbOx (orthorhombic
M1 phase) in order to identify a general concept that can
explain selectivity.
Process Research and Chemical Engineering
Heterogeneous Catalysis, BASF SE
Carl-Bosch-Strasse 38, 67056 Ludwigshafen (Germany)
We investigated the polycrystalline catalyst VPP with
NAP-XPS at 25 Pa and 4008C in 1:10 mixtures of n-butane/
oxygen (C H /O ), helium/oxygen (O ), and n-butane/helium
[
**] This work was conducted in the framework of the BasCat
collaboration between BASF SE, TU Berlin, FHI, and the cluster of
excellence Unicat. We thank the HZB staff for their continual
support of the electron spectroscopy activities of the FHI at BESSY
II.
4
10
2
2
(
C H ), according to the protocol described in the Supporting
4 10
Information. The studied catalyst produces, in a fixed-bed
flow-through reactor at 1 bar, maleic anhydride with a selec-
[
24]
tivity between 70 and 80%.
In semiconductor physics,
2
922
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 2922 –2926