Nature of the Mixed-Oxide Interface in Ceria−Titania Catalysts:
Clusters, Chains, and Nanoparticles
Aaron C. Johnston-Peck,
†
Sanjaya D. Senanayake,
‡
Jose ́ J. Plata,
§
Shankhamala Kundu,
‡
Wenqian Xu,
‡
Laura Barrio,
∥
Jesú s Graciani,
§
Javier Fdez. Sanz,
§
Rufino M. Navarro,
∥
Jose ́ L. G. Fierro,
∥
Eric A. Stach,*
,†
and Jose ́ A. Rodriguez*
,‡
†
Center for Functional Nanomaterials and
‡
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000,
United States
§
Department of Physical Chemistry, University of Seville, E-41012 Seville, Spain
∥
CSIC-Institute of Catalysis and Petrochemistry, Cantoblanco, E-28049-Madrid, Spain
* S Supporting Information
ABSTRACT: The ceria−titania mixed metal oxide is an important component of catalysts
active for the production of hydrogen through the water−gas shift reaction (CO + H
2
O → H
2
+ CO
2
) and the photocatalytic splitting of water (H
2
O+ hv → H
2
+ 0.5O
2
). We have found
that ceria−titania catalysts prepared through wet chemical methods have a unique hierarchal
architecture. Atomic resolution imaging by high-angle annular dark field scanning
transmission electron microscopy (HAADF STEM) reveals that ceria supported on titania
exhibits a range of morphologies. One can clearly identify ceria structures involving clusters,
chains, and nanoparticles, which are distributed inhomogeneously on the titania support.
These structures are often below the sensitivity limit of techniques such as X-ray diffraction
(XRD), which in this case identifies the average particle size of the ceria and titania
nanoparticles (via the Debye−Scherer equation) to be 7.5 and 36 nm, respectively. The
fluorite-structured ceria grows epitaxially on the anatase-structured titania, and this epitaxial
growth influences the morphology of the nanoparticles. The presence of defects in the ceria
such as dislocations and surface stepswas routinely observed in HAADF STEM. Density
functional theory (DFT) calculations indicate an energetic preference for the formation of O vacancies and the corresponding
Ce
3+
sites at the ceria−titania interface. Experimental corroboration by soft X-ray absorption spectroscopy (SXAS) does suggest
the presence of Ce
3+
sites at the interface.
■
INTRODUCTION
Metal oxides are an important class of materials used in
industrial processes including energy production (H
2
), storage
(batteries), conversion (fuel cells), remediation of environ-
mental pollutants (automotive emissions), fine chemical
synthesis (Fischer−Tropsch), electronic materials (semicon-
ductors), and photovoltaics (solar cells).
1−3
Of particular
interest are mixed metal oxides generated by the deposition
of nanoparticles of a given oxide on top of the surface of a
second oxide.
4−7
For example, structures of M/CeO
x
/TiO
2
(M
= Au, Cu, or Pt) produced H
2
by the water−gas shift (WGS)
reaction (CO + H
2
O → CO
2
+H
2
) with considerably better
activity than either of the individual oxide counterparts.
6,7
The
origin of this notable enhancement is likely from a combination
of structural and electronic properties unique to the oxide
nanoparticles and the interfacial region between the oxide and
the metal.
8
In principle, these systems could expose reactant
molecules to unique structures.
9−11
Therefore, efforts are
needed to understand the fundamental properties of these
nanoscale oxides to better interpret their catalytic behavior and
to improve the design of new catalysts.
12,13
When considering the CeO
x
/TiO
2
system, the two oxides on
their own have been studied considerably and are prototypical
lanthanide (CeO
2
) and transition metal (TiO
2
) oxides.
Catalytically, both offer excellent redox chemical properties
including reducibility, stable oxidation states (Ce
4+
/Ce
3+
, Ti
4+
/
Ti
3+
), and oxygen storage capacity (OSC).
14,15
However, both
structurally (CeO
2
−fluorite versus TiO
2
−anatase lattice) and
electronically (Ce-4f versus Ti-3d valence levels), there are
substantial differences between these two materials.
In this report, a suite of complementary techniques have
been used to probe the local and global properties of CeO
x
/
TiO
2
. High-resolution transmission electron microscopy
(HRTEM), high-angle annular dark field scanning transmission
electron microscopy (HAADF STEM), and X-ray diffraction
(XRD) were used to characterize the structural and
morphological properties of the ceria−titania system. Soft X-
ray absorption spectroscopy (SXAS) and calculations based on
density functional theory (DFT) were used to study the
Received: December 19, 2012
Revised: June 18, 2013
Published: June 19, 2013
Article
pubs.acs.org/JPCC
© 2013 American Chemical Society 14463 dx.doi.org/10.1021/jp3125268 | J. Phys. Chem. C 2013, 117, 14463−14471