Anomalous surface compositions of stoichiometric mixed oxide compounds

Article abstract of DOI:10.1002/anie.201001804

Coated: Surface analytical studies (including low-energy ion scattering (LEIS)) show that the outer surface of bulk, stoichiometric mixed vanadates and molybdates can be strongly enriched with VO_x or MoO_x species. Such surface recons

Full text of DOI:10.1002/anie.201001804

Angewandte
Chemie
DOI: 10.1002/anie.201001804
Surface Chemistry
Anomalous Surface Compositions of Stoichiometric Mixed Oxide
Compounds**
Sergiy V. Merzlikin, Nikolay N. Tolkachev, Laura E. Briand, Thomas Strunskus, Christof Wꢀll,
Israel E. Wachs, and Wolfgang Grꢁnert*
Surface-oxide films are present in many types of oxide-
known about their outermost surface composition. Models
based on surfaces derived from a truncation of the bulk
structure have dominated discussion on catalytic reaction
mechanisms and active sites (reviewed, for example, in
Ref. [6]). This view has been questioned by several recent
studies reporting the surface enrichment and depletion
[1]
containing materials, such as grain boundaries in ceramics,
interfaces in ceramic-ceramic and metal-oxide systems,
[
2]
[3]
and affect their materials and transport properties. In
heterogeneous catalysis, the properties of the outermost
surface layer are of prime importance because they control
the catalytic performance.
[
7]
phenomena in solid-oxide solutions (e.g., Co Ni O ), the
x
1Àx
Although bulk mixed-metal oxide catalysts are widely
used in industrial selective oxidation processes, not much is
identification of TiO -rich overlayers on reconstructed
2
[
4,5]
[8]
SrTiO (001) model surfaces, and evidence for the formation
3
of amorphous oxide overlayers in which there is surface
enrichment of one of the components under selective
[
+]
[
*] Dr. S. V. Merzlikin, Prof. Dr. W. Grꢀnert
Lehrstuhl fꢀr Technische Chemie, Ruhr-Universitꢁt Bochum
Postfach 102148, 44780 Bochum (Germany)
Fax: (+49)234-32-14115
[
9,10]
oxidation reaction conditions.
However, the development
of realistic concepts on reactant activation, surface reaction
mechanisms, and the design of advanced catalytic materials
are still hampered by the lack of detailed knowledge of the
surface composition and structure of bulk mixed-metal
oxides.
E-mail: w.gruenert@techem.rub.de
Homepage: http://www.techem.rub.de
[
#]
Dr. N. N. Tolkachev
N. D. Zelinsky Institute of Organic Chemistry
Russian Academy of Sciences, Moscow (Russia)
For such studies, X-ray photoelectron spectroscopy (XPS)
with laboratory sources is of limited value because its average
sampling depth of 1–3 nm results in a signal where the
outermost surface layer only contributes on the order of 30%.
Synchrotron radiation allows for increasing the surface
sensitivity of XPS by decreasing excitation and, hence,
photoelectron kinetic energies. Exclusive information on the
outermost surface layer, however, is only given by low-energy
ion scattering (LEIS) because ions penetrating below the
Dr. L. E. Briand
Centro de Investigaciꢂn y Desarrollo en Ciencias Aplicadas
UNLP, CONICET, Buenos Aires (Argentina)
[
$]
[++]
Dr. T. Strunskus, Prof. Dr. C. Wꢃll
Lehrstuhl Physikalische Chemie I
Ruhr-Universitꢁt Bochum (Germany)
Prof. I. E. Wachs
Operando Molecular Spectroscopy and Catalysis Laboratory
Department of Chemical Engineering, Lehigh University
Bethlehem, PA 18015 (USA)
[
11]
surface become largely neutralized.
The surfaces of stoichiometric bulk mixed-metal molyb-
dates and vanadates have also been characterized through
+
+
$
[
] Present address: Max-Planck Institute of Iron Research
Dꢀsseldorf (Germany)
their interactions with probe molecules, for example,
+
[12–15]
[
] Present address: KIT, Institut fꢀr funktionelle Grenzflꢁchen
Karlsruhe (Germany)
CH OH,
which allows CH O* and intact CH OH*
3 3
3
intermediates on different surface cations to be discriminated
by IR spectroscopy. For such materials, combined methanol
[
] Present address: Christian-Albrechts-Universitꢁt Kiel (Germany)
#
[
] Present address: Rosnano Inc.
Nametkina Street, 12A, 117420 Moscow (Russia)
chemisorption and oxidation kinetic studies suggested a
[12,14,15]
strong surface enrichment of MoO or VO .
In meth-
x
x
[
**] Financial support from the German Science Foundation (grant no.
Gr 1447/9) is gratefully acknowledged. We also thank the Federal
Ministry of Education and Research for the supporting travel grants
anol oxidation studies, similar catalytic turnover frequencies
were found over bulk mixed-metal oxides and related
supported metal oxides (e.g., Fe (MoO ) and MoO /Fe O ),
2
4
3
3
2
3
(
reg. no. 05 ES3XBA/5), the staff of BESSY II for continuous
which supports the idea of surface MoO enrichment of the
support, Dr. O. Shekhah for participating in the synchrotron
measurements, and Dr. O. P. Tkachenko for participating in the
LEIS measurements. I.E.W. acknowledges the support of Depart-
ment of Energy (DOE)-Basic Energy Sciences (grant DEF-G02-
x
[
16–19]
bulk phases.
These observations, however, are qualitative
as exposed metal oxide ions of low catalytic activity would not
be detected by the test reaction. Thus, we have undertaken a
study of the outermost surface compositions of such com-
pounds by LEIS and excitation-energy resolved XPS
9
3ER14350) for financial support. L.E.B. acknowledges the CONI-
CET (USA–Argentina) collaboration (res. no. 0060)) and Agencia
Nacional de Promociꢂn Cientꢄfica (project PICTRedes 729/06). The
authors gratefully acknowledge Prof. D. Buttrey, University of
Delaware, for providing the K-free bismuth molybdate single-crystal
materials.
(ERXPS). The LEIS was applied in sputter series taking
advantage of its destructive character, the ERXPS is a version
[
20]
utilizing information from different sampling depths.
LEIS sputter series from stoichiometric bulk mixed oxides
and related supported metal oxides are given in Figure 1 and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001804.
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Figure 2: ZrV O and a V O /ZrO catalyst of near-mono-
Ref. [19]) whereas for the submonolayer MoO /CeO catalyst
3 2
2
7
2
5
2
layer surface VOx coverage are shown in Figure 1, and
Ce Mo O and MoO /CeO2 (Mo content ca. 80% of
theoretical MoOx monolayer capacity) are shown in
Figure 2. In all cases, the initial V and Mo signals were
strong whereas the initial counterion signals were low and
increased as the surface was sputtered by the He ions
a finite initial Ce/Mo ratio was found (Figure 2d), that was
much larger than for the bulk molybdate (Figure 2b).
Diverging trends can be seen only at longer sputter times:
For the Zr/V mixed oxides, the Zr/V intensity ratio leveled off
for bulk ZrV O but went on increasing for the supported
8
12 49
3
2
7
V O /ZrO catalyst. The difference arises from the underlying
2
5
2
(
Figure 1a, 2a are from bulk phases, Figure 1c, and 2c are
compositions uncovered by the sputter process. The asymp-
totic behavior seen with bulk ZrV O (Figure 1b) is due to the
presence of V in the bulk phase whereas the increasing Zr/V
ratio in the supported system, Figure 1c,d, is related to the
from supported systems). The Zr/V and Ce/Mo intensity
ratios extrapolate to very small values at zero sputtering time
for the bulk phases (Figure 1b, 2b) which reflects surface
enrichment by Vand Mo, respectively. In the supported V O /
2
7
absence of V in the ZrO support. With the Ce/Mo mixed
2
5
2
ZrO catalyst, Zr was not initially exposed (Figure 1d, cf.
oxides, the trends are similar in the LEIS spectra though their
reflection in the numer-
2
ical Ce/Mo ratios is less
clear (Figure 2).
Conventional XPS
did not detect any sur-
face V enrichment for
ZrV O7
Some
(Table 1).
surface Mo
2
enrichment was found
for Ce Mo O , but a
8
12 49
comparison of the ini-
tial LEIS intensity
ratios in Figures 2b
and 2d suggests that
the Ce exposure in the
external surface layer
of Ce Mo O
was
8
12 49
probably much smaller
than indicated by the
XPS result.
Figure 1. LEIS sputter series and intensity trends measured with Zr/V mixed oxides. a),b) ZrV O E0 =1000 eV,
2
7,
XPS spectra of bulk
ZrV O recorded with
2
c),d) 4 wt% V O /ZrO (V content (7.5 atoms per nm ) near the theoretical monolayer limit, see Table S1 in the
2
5
2
2
7
Supporting Information), E =1000 eV.
0
different
energies
excitation
(ERXPS)
are presented in Fig-
ure 3a. The plot of P
(the V 3p/Zr 4p inten-
sity ratio) versus the
excitation energy (E0;
Figure 3b) was mod-
eled with a variety of
concentration depth
profile functions (see
examples
in
Fig-
ure 3c). Except for
some physically mean-
ingless versions, all
models
acceptable fits (Fig-
ure 3b) involved
providing
a
dense, thin surface
layer of exclusively
vanadium oxide spe-
cies. In the one
shown, a 0.6 nm thin
VOx layer covers
Figure 2. LEIS sputter series and intensity trends measured with Ce/Mo mixed oxides. a),b) Ce Mo O
8
12 49,
2
E =1000 eV, c),d) 2.7 wt% MoO /CeO (Mo content (3.6 atoms per nm ) ca. 80% of the theoretical monolayer
0
3
2
limit, cf. Table S1 in the Supporting information), E =2000 eV.
0
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Table 1: Bulk mixed oxides studied: Preparation routes and conventional XPS analysis.
depleted V content below the sur-
face. Notably, ERXPS overesti-
mates the depth coordinate below
Oxide
Metal
Preparation
route
XPS lines used
Mo/M or V/M ratio
[
a]
[
20]
rough surfaces,
therefore, the
[
b]
bulk
2
XPS
2.0
0
.6 nm thickness of the VO over-
x
ZrV O7
Zr
Al
A
A
B
B
B
B
B
C
V 2p3/2, Zr 3d
V 2p3/2, Al 2p
Mo 3d, Fe 2p
Mo 3d, Co 2p
Mo 3d, Ni 2p
Mo 3d, Mn 2p
Mo 3d, Ce 3d
Mo 3d, Bi 4f
2
layer is rather an upper limit. As the
extent of overestimation is on the
order of 20–60%, except for rough-
AlVO4
1
1.5
1
1
1
1.5
1.5
0.63
1.83
0.93
0.94
0.84
2.5
Fe (MoO )
Fe
Co
Ni
Mn
Ce
Bi
2
4
3
CoMoO4
NiMoO4
[20]
ness profiles with many clefts, the
MnMoO4
Ce Mo O
actual thickness of this overlayer is
probably 0.3–0.4 nm. This value
supports the view that stoichiomet-
ric ZrV O is terminated by a mon-
8
12 49
a-Bi Mo O (K)
1.72
2
3
12
(K:Bi=0.09)
g(H)-Bi MoO (K)
Bi
C
Mo 3d, Bi 4f
0.5
0.33
2
7
2
6
[
14]
(
K:Bi=0.10) omolecular surface VO layer.
x
a-Bi Mo O
Bi
Bi
D
D
Mo 3d, Bi 4f
Mo 3d, Bi 4f
1.5
0.5
1.08
0.86
2
3
12
More examples of stoichiomet-
ric bulk mixed oxides strongly
B: coprecipitation, from enriched with surface VOx or
NH ) Mo O ·4H O and metal nitrates; C: solid-state reaction between a-Bi O and MoO3; D: MoOx species are given in the
2
g(H)-Bi MoO
6
[
12,14]
[a] A: citrate-based route, from NH VO3 and metal nitrates;
4
[26]
(
4
6
7
24
2
2
3
purified phases, see Supporting Information. [b] Photoionization cross sections from Ref. [27] used
together with an empirical correction for dependence of spectrometer sensitivity of the kinetic energy of
the photoelectrons.
Supporting Information: AlVO4
compared with model V O /
a
2
5
Al O3 catalyst (Figure S1: LEIS),
2
NiMoO4 (Figure S2: LEIS), and
Fe (MoO ) (Figure S3a,b: LEIS,
2
4 3
another layer that still has some V enrichment (ca. 80% V,
.3 nm thick) before the composition decays to the (fixed)
Figure S3c,d: ERXPS). For the Fe (MoO ) , the best
2 4 3
1
ERXPS model fit suggests a 0.35 nm overlayer of exclusively
Mo oxide species on an extended subsurface phase still
enriched in Mo relative to the bulk (Figure S3c,d). From a
recent STEM study, House et al. reported a near-surface 5–
bulk value of 67% V. The significance of the intermediate
layer may be doubtful, but it may reflect a smooth transition
from the enriched surface layer to the bulk composition.
Models with single surface layers and free bulk concentration
reproduced the data only at the expense of a significantly
[
21]
8 nm Mo enrichment zone in Fe (MoO ) crystals. Accord-
2
4 3
ing to our results, this zone supports an outermost layer
containing almost exclusively Mo oxide species. Conventional
XPS data (Table 1) are in disagreement with these findings for
NiMoO and AlVO while a Mo surface enrichment found for
4
4
Fe (MoO ) seems to track the thicker Mo enrichment zone
2
4 3
[
21]
found by House et al.
Some examples that prevent the tempting generalization
that the surfaces of all stoichiometric bulk molybdates or
vanadates are more or less completely covered by a Mo or V
oxide overlayer are given in Figure 4 and Figure S4 in the
Supporting Information. Neither with CoMoO4 nor with
MnMoO is there any significant change of the signal shapes
4
Figure 3. ERXPS analysis of the ZrV O surface. a) spectra taken at
2
7
different excitation energies (intensities scaled with different factors
for representation in a single Figure, BE scale referenced to O 2s
orbitals=21.0 eV, see Supporting Information), b) dependence of
experimental intensity ratios on excitation energies, modeled on the
basis of different mathematical concentration depth profile types
(
see (c)), c) Optimized depth-profile functions (N=100ꢅ the number
of V atoms divided by the sum of the V and Zr atoms). Note that the
Gauss and powered Gauss results are unrealistic towards the bulk of
the material. Due to unspecified influences of surface roughness, the
actual thickness of the layer(s) may be smaller, see text.
Figure 4. LEIS sputter series of bulk CoMoO , E =1000 eV.
4
0
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Communications
along the LEIS sputter series: Apparently, their surfaces had
The present study of stoichiometric bulk mixed molybdate
and vanadates suggests that their outermost surface layers
may be strongly enriched or almost completely covered with
MoO and VO species. This enrichment region is probably
limited to approximately one atomic layer according to the
ERXPS measurements, which makes the outermost surface
resemble that of the corresponding model supported metal
oxide catalyst. The enrichment is a result of surface recon-
struction rather than preferential exposure. Even if truncation
of bulk mixed-oxide structures was preferentially exposing
one of the component elements at the surface, this would not
give the net concentration gradient normal to the surface as
detected in thus study.
the same compositions as the subsequent layers, which may
still deviate from the bulk compositions. Conventional XPS
indicated a slight Mo depletion in the external surface region
for both compounds (Table 1).
x
x
Trace impurities in the bulk phase that segregate to the
[22]
oxide surface are known to influence surface properties. We
have experienced this with bismuth molybdate phases, which
are ingredients of industrial propene (amm)oxidation cata-
[
4,5]
lysts.
The LEIS measurements of phases containing
potassium originating either from crucible walls or from
commercial precursor compounds, and of samples purified by
recrystallization procedures utilizing zone refinement effects
are shown in Figure 5 and Figures S5–S7 in the Supporting
Information. The alkali-metal-contaminated surfaces are of
practical relevance as commercial bismuth molybdate cata-
As seen in the study with bismuth molybdate phases,
alkali-metal contamination may cause such surface recon-
struction. Competition between potassium and surface BiOx
in their interaction with the molybdate appears to result in
outer surfaces primarily constituted of sur-
[
23,24]
lysts are mostly promoted by alkali-metal ions.
face MoO and KO species. Re-inspection
x
x
or re-measurement of the LEIS spectra of all
the remaining samples demonstrated that
their surfaces were not contaminated by any
alkali-metal or alkaline-earth metal ions that
could be differentiated from the other ele-
ments by LEIS, except for AlVO , where
4
minor surface potassium concentrations
would be difficult to detect between the Al
and V signals. The observed surface enrich-
ment might be explained by differences in
free surface energies. These energies are
lower for V=O and Mo=O terminated sur-
face VOx or MoOx species than for the
metal-OH terminated species of the counter-
[
25]
ions. However it seems to be to early to
make any generalizations because of the
lack of surface enrichment found for Co and
Figure 5. LEIS sputter series of pure and potassium-contaminated bulk bismuth molyb-
dates. a) pure g(H)-Bi MoO and a-Bi Mo O , E =2000 eV, b) Bi/Mo intensity trends
2
6
2
3
12
0
related to a, short scans accounted for according to scan duration, c) K-containing g(H)-
Bi MoO (K) and a-Bi Mo O (K), E =1000 eV, d) intensity trends related to (c). For K/Mo
Mn molybdate.
2
6
2
3
12
0
intensity trends see Supporting Information, Figure S6b,c.
The data reported herein sound a note of
caution regarding the discussion of catalytic
reaction mechanisms on the basis of surface
In the pure phases (Figure 5a, Figure S7 in the Supporting
Information), a clear Bi signal was already present in the first
scan. After a short initial increase, the Bi/Mo intensity ratios
leveled off or decayed (g-(H)-Bi MoO , see also a-Bi Mo O ,
structures obtained by the truncation of the bulk mixed-oxide
structure. Apparently, surface reconstruction is a frequent
phenomenon and can be present even in the initial calcined
stoichiometric mixed-metal oxides. Additional reconstruction
of such overlayers may take place during catalysis (e.g. in
selective hydrocarbon oxidations as suggested by synchro-
2
6
2
3
12
Figure S7 in the Supporting Information). With potassium
present (Figure 5c,d, and Figures S5, S6 in the Supporting
Information), the Bi signal was initially very small and grew
significantly upon sputtering. The Bi/Mo intensity ratios
extrapolated to zero for t = 0 (Figure 5d) except for phases
with large Bi excess (Figure S5, S6 in the Supporting
Information). The strong potassium signal decreased upon
sputtering without disappearing completely (Figure 5, and
Figure S5, S6 in the Supporting Information). The dramatic
surface Mo enrichment of potassium-containing bismuth
molybdates was not found by conventional XPS (Tables 1,
Figure S2 in the Supporting Information): The data do not
correlate with either the surface Mo enrichment or depletion
that is suggested by LEIS for the potassium-containing and
for the pure phases.
[
10]
tron-based in situ XPS studies ). Thus, more sophisticated
surface analysis work is clearly needed to develop realistic
reaction mechanisms for bulk mixed-metal oxide catalysts.
The present study confirms that conventional XPS is of
limited value for this analysis, although its failure to detect the
enrichment phenomena seen by LEIS and ERXPS indeed
confirms that these phenomena are confined to the outermost
surface layer(s). Still, conventional XPS gives valuable
information about deeper-lying enrichment or depletion
zones as suggested by the results with Fe (MoO ) (Table 1,
2
4 3
Figure S3c,d in the Supporting information). As to the
outermost surface layer, further progress will mainly rely on
synchrotron-based XPS as it can be used in situ, unlike LEIS.
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Angewandte
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Quantitative assessment of depth composition profiles inher-
ent in the synchrotron-based XPS intensity data as exempli-
fied in this study (ERXPS) may become a useful complement
to the new synchrotron-based in situ XPS techniques.
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Revised: July 9, 2010
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Keywords: LEIS · mixed oxides · molybdates ·
.
surface composition · vanadates
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Angew. Chem. Int. Ed. 2010, 49, 8037 –8041
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8041