Controlling Reaction Selectivity through the Surface Termination of Perovskite Catalysts

Article abstract of DOI:10.1002/anie.201704656

Although perovskites have been widely used in catalysis, tuning of their surface termination to control reaction selectivity has not been well established. In this study, we employed multiple surface-sensitive techniques to characterize the surface termination (one aspect of surface reconstruction) of SrTiO₃ (STO) after thermal pretreatment (Sr enrichment) and chemical etching (Ti enrichment). We show, by using the conversion of 2-propanol as a probe reaction, that the surface termination of STO can be controlled to greatly tune catalytic acid/base properties and consequently the reaction selectivity over a wide range, which is not possible with single-metal oxides, either SrO or TiO₂. Density functional theory (DFT) calculations explain well the selectivity tuning and reaction mechanism on STO with different surface termination. Similar catalytic tunability was also observed on BaZrO₃, thus highlighting the generality of the findings of this study.

Full text of DOI:10.1002/anie.201704656

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Accepted Article
Title: Controlling Reaction Selectivity via Surface Termination of
Perovskite Catalysts
Authors: Felipe Polo-Garzon, Shi-Ze Yang, Victor Fung, Guo Shiou
Foo, Elizabeth Erin Bickel, Matthew F. Chisholm, De-en
Jiang, and Zili Wu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201704656
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10.1002/anie.201704656
Angewandte Chemie International Edition
COMMUNICATION
Controlling Reaction Selectivity via Surface Termination of
Perovskite Catalysts
Felipe Polo-Garzon[1], Shi-Ze Yang[2], Victor Fung[3], Guo Shiou Foo[1], Elizabeth E. Bickel[4], Matthew F.
Chisholm[2], De-en Jiang[3], Zili Wu*[1]
of important thin films, and its use as an insulating layer for
potential field effect device applications and fundamental
research[5].
Abstract: Although perovskites have been widely used in catalysis,
tuning their surface terminations to control reaction selectivities has
not been well established. In this work, we employ multiple surface
sensitive techniques to characterize the surface termination (one
aspect of surface reconstruction) of SrTiO3 (STO) after thermal
pretreatment (Sr-enrichment) and chemical etching (Ti-enrichment).
We show, using the conversion of 2-propanol as a probe reaction,
that the surface termination of STO can be controlled to greatly tune
catalytic acid/base properties and consequently the reaction
selectivities in a wide range, which are inaccessible using single
metal oxides, either SrO or TiO2. Density functional theory (DFT)
calculations well explain the selectivity tuning and reaction
mechanism on different surface terminations of STO. Similar
catalytic tunability is also observed on BaZrO3, highlighting the
generality of the finding from this work.
The surface reconstruction of STO is found to be quite
complex, depending on treatment temperature, environment and
time. Also, these reconstructions have shown to be reversible
under certain conditions[6]. Druce et al.[7] and Ngai et al.[8] found
A-cation enrichment at the surface after annealing perovskites in
oxygen at 1000 °C for 12 h and at 1300 °C for 30 min,
respectively. Dagdeviren et al.[6] and Nishimura et al. [9] reported
Sr migration to the surface in STO and oxygen depletion during
UHV annealing. Contradictorily, Jiang and Zegenhagen[10]
concluded that the SrO layer is less stable at high temperature
(950 – 1100 °C) both in UHV and in oxygen. Erdman et al.
studied the reconstruction of SrTiO3 (001) and reported single
Ti-rich overlayer arranged as TiO6-x polyhedra, in contrast to
TiO6 polyhedra in the bulk, after annealing under oxygen up to
1000 °C.[11] However, ab initio computational work by Heifets et
al.[12] does not support the (2×1) double-layer (DL) TiO2-
terminated surfaces observed by Erdman et al.[11b] This
discrepancy reflects the complex dependence of surface
structure on treatment conditions; furthermore, observed
terminations could be due to kinetic processes far from
thermodynamic equilibrium. Additionally, recent work[13] reported
the thin-film-like structure of octahedral titania that the surface of
STO can adopt, highlighting the complexity of STO surface
reconstruction.
In addition to thermal treatment, chemical treatment under
an acidic environment has been reported.[9, 14] It has also been
reported that Sr-O, Ti-O and mixed terminations of STO
nanoparticles depend upon the synthesis procedure;[15] however,
their stability under reaction conditions for heterogeneous
catalysis was not reported.
Studies on the interaction of select adsorbates (H2O, NO,
CO, CO2, H2, O)[16] with specific terminations of STO and other
perovskites have been examined. However, to the best of our
knowledge, a comprehensive study on tuning reaction selectivity
via controlling the surface termination of perovskite catalysts is
not present and is reported for the first time in this work.
The present work successfully couples the observed
surface terminations via top-surface sensitive characterization
techniques with ab initio simulation and catalytic performance of
Perovskites are metal oxides with the general formula
ABO3, where A represents a lanthanide, alkali or alkaline earth
metal and B represents a transition metal. The cations A and B
can have a variety of oxidation states (A+2B+4O3, A+3B+3O3, and
A+1B+5O3). Also, the oxidation states can differ from the ideal
structure, ABO3, when the perovskite is oxygen deficient or rich
[1]. These materials have shown high oxygen mobility, high
tolerance for metal substitutions into the lattice structure,
excellent thermal stability (up to 1000 °C) and resistance to
sintering of substituted metals. These attributes have driven
interest toward perovskite materials, in particular for redox
catalysis (e.g. methane reforming, CO oxidation, NO oxidation),
whereas acid-base catalysis is yet to be extensively studied[1-2]
.
In the past five decades, researchers have unsuccessfully
attempted to relate catalytic properties of bulk mixed oxides to
bulk properties of the crystal structure, such as the short metal-
oxygen bond. This is due to the fact that the catalytic “stage”,
i.e., the surface of a complex oxide can be different from the
bulk in both composition and structure, which has highlighted the
need for surface sensitive characterization of these materials to
comprehend their catalytic behavior.[3] This is also true for
perovskites where surface reconstruction has been extensively
observed in surface science studies of single crystal or thin film
forms. SrTiO3 (STO) is among the most studied perovskites due
to its applications in catalysis[4], its extensive use for the growth
STO
for
dehydrogenation/dehydration
(acetone/propene
production, respectively) of 2-propanol. Thermal and chemical
pretreatments were performed on the samples while conserving
their crystal structure as shown via X-ray Diffraction (XRD) (see
Supporting Information, Figure S1).
[1]
[2]
F. Polo-Garzon, G. S. Foo, Z. Wu
Chemical Sciences Division and Center for Nanophase Materials
Sciences, Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831, United States
E-mail: wuz1@ornl.gov
S. Yang, M. F. Chisholm
Materials Science and Technology Division, Oak Ridge National
Laboratory
Oak Ridge, Tennessee 37831, United States
V. Fung, D. Jiang
Department of Chemistry, University of California
Riverside, California 92521, United States
E.E. Bickel
Department of Chemical Engineering, Tennessee Technological
University
Cookeville, Tennessee 38505, United States
Commercially obtained STO was thermally pretreated in-
situ in a plug-flow reactor at 550 °C under 50 mL/min 5%O2/He
for different time periods. After each pretreatment, the
conversion of 2-propanol at 303 °C was carried out in a plug-
flow reactor (conversion ≤ 13%). As observed in Figure 1a,
longer pretreatment times greatly increased the rate of acetone
production (dehydrogenation) from 0.15 to ~0.60 µmol/m2/min,
decreased the rate of propene production (dehydration) from
~0.32 to ~0.19 µmol/m2/min; and therefore, decreased the
selectivity toward propene. The possible role of different
amounts of residual carbonates on STO after different
pretreatment durations is excluded as co-feeding CO2 with 2-
propanol does not change the catalytic performance of STO
pretreated for 5 hours (see Figure S2a). It is thus hypothesized
that longer pretreatment times favor the exposure of Sr-atoms,
[3]
[4]
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10.1002/anie.201704656
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since basic sites (predominant on a SrO surface termination as
shown in Figure S3a-b) favor the dehydrogenation product,
performance does not change. Thus, it is hypothesized that an
increase in the pretreatment temperature exposes more Sr
atoms at the surface but reaches a maximum for pretreatment
temperatures above ~500 °C. When the thermally pretreated
catalyst is held at room temperature for an extended period of
time (at least for more than 2 weeks), the effect of the thermal
pretreatment is reversed, i.e., propene dominates over acetone
for 2-propanol reaction over such a sample. It appears Sr-
exposure decreases upon storage of STO at room temperature.
This is confirmed by the LEIS analysis as shown in Figure 2a,
where the Sr-to-Ti ratio increases after treatment at 500°C.
However, this reverse process is not due to exposure to CO2 or
H2O in the air, as confirmed experimentally (see Figure S2). The
kinetics of the reverse process at room temperature, once the
thermal pretreatment is performed, are interesting and warrant
further investigation.
acetone[17]
.
a)
b)
80
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
60
40
20
0.5
0.4
0.3
0.2
0.1
0.0
60
40
20
0
0
0
1
2
3
4
5
100 300 500 700 900
Pretreat.
Pretreat.
No
Time (h)
Temp. (°C)
pretreat.
Propene Selectivity Conversion Propene Rate Acetone Rate
1083
a)
b)
TiO2-disk400 C
STO(HNO3),400 C
TiO2-disk400 C
STO(HNO3),400 C
STO400 C
STO550 C
SrO400 C
1066
SrO
STO400 C
STO550 C
Catalyst
STO(HNO3)
Sr/Ti
1160
1138
1108
c)
d)
0.39
SrO400 C
TiO2-disk
10000
1000
100
100
80
60
40
20
0
STO(HNO3),500°C 0.48
STO
STO
0.67
1.44
270
265
260
255 C
250 C
C
C
C
STO(HNO3)
STO500°C
1200
1100
1000
)
Wavenumbers (cm-1
Figure 2 (a) Top surface Sr/Ti cation intensity ratio of the pretreated
STO and STO(HNO3) catalysts measured using LEIS. The subscript
next to catalyst name indicates the temperature at which the
materials were pretreated in-situ before LEIS analysis. (b) FTIR
spectra of methanol adsorption on SrO, TiO2-disk, STO and
STO(HNO3) catalysts at 25 °C. All samples were pretreated at 550 °C
under oxygen.
10
0.0
0.5
1.0
0.0
0.5
1.0
Sr/(Sr+Ti)
Sr/(Sr+Ti)
Figure 1 Steady-state conversion of 2-propanol (a) at 303 ± 1 °C
over STO after different pretreatment times at 550 °C under 50
mL/min 5%O2/He, and, (b) at 300 ± 1 °C after different pretreatment
temperatures under 50 mL/min 5%O2/He for 5 h (1h at 985 °C). (c)
Propene selectivity, and (d) 2-propanol consumption rate (log scale)
To promote the exposure of the Ti-terminated surface, ex-
situ pretreatment in 0.2 M HNO3 was performed on STO
(STO(HNO3)) to remove the outmost SrO layer, as performed by
Peng et al.[14a] on La0.5Sr0.5CoO3. X-ray photoelectron
spectroscopy (XPS) performed on such STO(HNO3) showed no
remaining nitrogen (see Figure S4). Additionally, LEIS
characterization (Figure 2a and Figure S5-S6) confirmed further
exposure of Ti-atoms after treatment with HNO3 (Sr/Ti = 0.4),
and it was found that thermal pretreatment of the washed
sample, STO(HNO3),500°C, achieved minor exposure of the Sr
atoms (Sr/Ti = 0.5), far from the Sr-exposure of the non-washed
thermally treated sample, STO500°C (Sr/Ti = 1.4).
High-angle annular dark-field (HAADF) scanning
transmission electron microscopy (STEM) was performed on
these differently treated STO samples to directly visualize the
atomic structure of the surfaces. STO(HNO3) was imaged after in-
situ heating at 400°C under vacuum and after ex-situ thermal
pretreatment under N2 at 550°C for 5 h. STO was imaged before
and after ex-situ thermal pretreatment under N2 at 550°C for 5 h
(see Figure 3). It is clearly observed that the surface of
STO(HNO3) is predominantly enriched with single and double
layers of Ti. Also, heat treatment at 550 °C did not significantly
affect the surface segregation of Ti for the chemically etched
sample, which is in good agreement with LEIS results. The STO
sample without heat treatment shows similar surface
composition dominated with Ti but with minor presence of Sr.
However, surface enrichment with Sr is clearly observed when
heat treatment at 550 °C is performed. For all STO samples, the
(100) plane is confirmed as the main plane exposed at the
surface with minor (110) truncation at the corners (see complete
set of images in Figure S7); therefore, our DFT calculations are
performed upon this major plane.
for conversion of 2-propanol at 250-270 °C over STO400°C, STO550°C
,
STO(HNO3),400°C
, TiO2-disk400°C and SrO400°C catalysts. Reaction
conditions: 50 mL/min Ar, 30 mg of catalyst, WHSV = 0.8 h-1. The
subscript next to catalyst name indicates the pretreatment
temperature under 50 ccm 5%O2/He for 5 h before kinetic data were
collected.
To test this hypothesis, low energy ion scattering (LEIS)
characterization was performed to determine the composition of
the top atomic monolayer of material (~0.3 nm)[18] before and
after thermal pretreatment. At the surface, STO presented a Sr-
to-Ti ratio around 0.7 before thermal pretreatment and 1.4 after
thermal pretreatment at 500 °C in O2 for 30 min (see Figure 2a),
confirming the exposure of more Sr atoms upon thermal
treatment at 500 °C under oxygen. In addition, it is observed that
after 4 to 5 hours of thermal treatment, the catalytic performance
of STO does not change considerably. These results are in good
agreement with the results reported by Bachelet et al.[19], where
the SrO termination of STO substrates can be varied from 0% to
100% when annealing at 1300 °C under air for different periods
of time (2 – 72 h).
Conversion of 2-propanol was also evaluated on STO after in-
situ pretreatment at different temperatures for 5 h under 50
mL/min of 5%O2/He. After each pretreatment, the conversion of
2-propanol at 300 °C was carried out and the results are shown
in Figure 1b. For pretreatment temperatures between 450 and
500 °C, the selectivity toward propene decreases significantly
from 54% at 450 °C to 31% at 500 °C. However, for
pretreatment temperatures above 500 °C the catalytic
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To study the relation between the exposure of Sr/Ti atoms
and the selectivity toward dehydrogenation/dehydration, SrO
(obtained commercially) and anatase TiO2-disk (terminated by
large percentage with the (100) plane)[20] were compared with
pretreated STO samples. To compare the types of sites
encountered in the strontium titanate samples with the sites in
SrO and TiO2-disk, methanol adsorption followed by FTIR
spectroscopy was performed (Figure 2b). Vibrational spectra of
adsorbed methanol on both STO550°C and STO(HNO3),550°C
samples reveal spectral features resembling those present on
strength of 2-propanol is similar on Ti and Sr sites, except when
more than one SrO layer is stacked at the surface, which is the
case of the pure SrO catalyst (where CO2 adsorption may
involve reaction to form SrCO3). DFT calculations revealed
dissociate adsorption of 2-propanol on the Sr-terminated surface
and chemisorption that easily leads to dissociation on the Ti-
terminated surface. On the Ti-terminated surface, the reaction
energy (∆Hrxn
)
for dissociation of 2-propanol and the
corresponding activation barrier (∆Eact) are -0.33 and 0.18 eV,
respectively. The calculated adsorption energies for dissociated
Sr Ti
Ti
Ti+Ti
a) STO(HNO3),400 C
Ti+Ti
Ti
Sr
Ti
Ti+Ti
Ti+Ti
Ti
5 nm
b) STO(HNO3),400 C c) STO(HNO3),550 C
5 nm
Ti
Sr
Ti Sr
Ti
(110) surface
Sr vacancy
5 nm
5 nm
(010)
d) STO
e) STO550 C
Figure 3 HAADF STEM images of STO(HNO3) after heat treatment at (a)(b)400°C and (c) 550°C; as well as images of STO (d) before and (e)
after heat treatment at 550°C.
both SrO and TiO2-disk, with STO(HNO3),550°C closer to TiO2-disk
and STO550°C closer to SrO, further supporting the surface
enrichment as analyzed by LEIS and STEM. The observation of
different methanol species on both STO samples suggests a
synergistic effect rising from the coexistence of Sr and Ti at the
surface (details in Supporting Information, section S 2.7). To
quantify the concentration and strength of basic and acid sites,
adsorption microcalorimetry measurements were performed with
CO2 and NH3, respectively. In general, the result (Figure S3a-b)
showed that STO550°C has more basic sites but fewer acidic sites
when compared with STO(HNO3),550°C, which is consistent with the
higher Sr/Ti ratio on the former STO sample. Nonetheless, the
strength (Figure S3a-b) of the basic or acid sites approaching
zero surface coverage does not directly correlate with the
density of the sites (µmol/m2) (Table S3) or with the fraction of Sr
at the outermost layer [Sr/(Sr+Ti)] (Table S3). This synergistic
effect can be explained due to the presence of Sr-O or Ti-O
sublayers that together tune the basic/acid properties of the
surface (see further details in Supporting Information, section S
2.3).
2-propanol on Sr and Ti-terminated surfaces of STO were 135
and 112 kJ/mol, respectively (see Figure S8 – S12), setting the
boundary for the strongest adsorption energy (at coverage
approaching zero) of 2-propanol on the STO catalyst where both
Sr and Ti-terminated surfaces are present. This range (between
135 and 112 kJ/mol) is in good agreement with the experimental
values ranging from 110 to 103 kJ/mol (Figure S3c).
To further understand our experimental observation of the
selectivity changes upon different conditioning of STO, DFT was
employed to probe the reaction pathways of 2-propanol on the
Ti- and Sr-terminated STO (100) surfaces. These surfaces are a
simplified version of the more complicated real surfaces;
therefore, they are used to shed light on reactivity trends and
comparisons to experimental results are rather qualitative. The
results (see section S 2.8 in Supporting Information) suggest
that both dehydrogenation and dehydration of 2-propanol involve
initial deprotonation to generate the 2-propanoxy intermediate;
then, depending upon the basicity of the adjacent surface
oxygen, either the Cβ-H or Cα-H bond is cleaved to produce
propene or acetone, respectively. This reaction mechanism is
denoted as the E1cB pathway and it is expected from the weak
acidity of the surface sites in STO[21]. As shown in Figure 4, the
rate-determining step (RDS) for acetone formation is the
cleavage of the Cα-H bond and for propene formation is the
The heat of adsorption of 2-propanol was also measured.
Noticeably, the adsorption strength of 2-propanol does not vary
significantly amongst three of the samples studied (STO,
STO(HNO3), and TiO2-disk), suggesting that the adsorption
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concerted breaking of the Cβ-H and C-O bonds. Calculations
show that the Ti-terminated surface of STO favors the
production of propene (ΔEa,propene = 145 kJ/mol, ΔEa,acetone = 155
kJ/mol) and the Sr-terminated surface favors the production of
acetone (ΔEa,propene = 235 kJ/mol, ΔEa,acetone = 149 kJ/mol), which
agrees well with our experimental observations.
perovskite catalysts allow to control the ratio of dehydrogenation
and dehydration rates due to the synergy between acid and
base sites as observed through FTIR spectroscopy and
adsorption microcalorimetry experiments.
In conclusion, we show, using the conversion of 2-
propanol as
a probe reaction, that altering the surface
Apparent activation energies were calculated by fitting the
Arrhenius equation to kinetic data (Figure S13, Table S4)
collected at differential conditions (conversion ≤ 13%)[21] and
used to compare reactivity data at the same temperature for the
five samples: TiO2-disk400°C, SrO400°C, STO(HNO3),400°C, STO400°C
termination of SrTiO3 allows tuning its acid/base catalytic
properties, providing selectivities inaccessible using single metal
oxides, namely, SrO and TiO2. Controlled enrichment of Sr or Ti
at the surface of SrTiO3, attained via thermal and chemical
treatments was revealed via LEIS and HAADF-STEM. Methanol
adsorption followed by FTIR spectroscopy along with adsorption
microcalorimetry measurements revealed the synergistic nature
of the surface Sr and Ti sites for 2-propanol conversion. DFT
calculations were in good agreement with experimental data and
showed that both the dehydrogenation and dehydration
pathways proceed via the 2-propoxy intermediate. Furthermore,
the work expanded to BaZrO3 (Figure S15) suggests that the
potential of utilizing the induced surface termination of
perovskites for controlling catalytic selectivity is general. The
finding of this work has significant implication for catalysis by
mixed oxides where the surface and bulk compositions can be
different depending on treatment and reaction conditions.
Advantages are yet to be taken of the surface terminations of
these materials as a unique route to tune the catalytic
performances. It also underscores the importance and necessity
of surface sensitive characterization of bulk mixed oxides (prior
to and post reaction, ideally under reaction conditions) for
unambiguous structure – catalysis correlations.
and STO550°C
production on surface-Sr-rich STO (STO550°C) (163 kJ/mol) and
for propene production on a surface-Ti-rich STO (STO(HNO3),400°C
.
Apparent activation energies for acetone
)
(130 kJ/mol) showed general agreement with the magnitude of
the DFT-calculated activation energies for the RDS; namely, 149
and 145 kJ/mol, respectively. Here we note that although the
good agreement between our DFT barriers of the rate-limiting
steps and the experimental apparent activation energies does
shed light on the reaction mechanisms on the two different
terminations, a proper reaction kinetic analysis is warranted in
the future to firmly establish a relationship between the DFT-
predicted mechanism and the experimental kinetic data.
Propene formation on ‘A’
Acetone formation on ‘A’
Propene formation on ‘B’
Acetone formation on ‘B’
Experimental Section
See Supporting Information, S 1. Experimental Procedures.
Acknowledgements
This research is sponsored by the U.S. Department of Energy, Office
of Science, Office of Basic Energy Sciences, Chemical Sciences,
Geosciences, and Biosciences Division. Part of the work including
FTIR, BET and kinetic measurement was conducted at the Center
for Nanophase Materials Sciences, which is a DOE Office of Science
User Facility. This research used resources of the National Energy
Research Scientific Computing Center, a DOE Office of Science
User Facility supported by the Office of Science of the U.S.
Department of Energy under Contract No. DE-AC02-05CH11231.
Electron microscopy at Oak Ridge National Laboratory (S.Z.Y. and
M. F. C.) was supported by the U.S. Department of Energy, Office of
Science, Basic Energy Sciences, Materials Science and Engineering
Division. The authors thank Henry Luftman (Lehigh University) for
performing the LEIS analysis and Zach Hood (graduate student at
Georgia Institute of Technology) for performing XPS measurements.
Figure 4 Minimum-energy paths for conversion of 2-propanol over
Sr-terminated (A) and Ti-terminated (B) surfaces of STO(100) to
propene and acetone.
The unique tunability of reaction selectivity from induced surface
terminations of STO is evident from the comparison with the
individual single oxides. As seen in Figure 1c, in the range 250-
270 °C, TiO2-disk400C and SrO400°C have around 95% and 15%
selectivity towards propene, respectively. The perovskite
samples enabled to access propene selectivities ranging from
25 to 87 % by tuning their surface composition. Figure 1d
suggests that the coexistence of Sr and Ti atoms at the surface
with composition around 28 – 59 % Sr induces lower rates for
both dehydrogenation and dehydration. Since deprotonation of
2-propoxy is assisted by the nearby surface oxygen; for a mixed
Sr-Ti surface, with different basicities, protonation/deprotonation
processes may simultaneously occur, which reflects on a
reduction on 2-propanol consumption rate for STO catalysts.
Also, adsorption microcalorimetry measurements suggest that
sub-surface layers may tune the acidity and basicity of the
surface (see details in Supporting Information, section S 2.3),
Keywords: surface termination, perovskite, acid/base catalysis,
selectivity, dehydration/dehydrogenation.
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This article is protected by copyright. All rights reserved.

10.1002/anie.201704656
Angewandte Chemie International Edition
COMMUNICATION
Table of Contents
COMMUNICATION
Controlling the surface termination of
SrTiO3, via thermal pretreatment (Sr-
enrichment) and chemical etching (Ti-
enrichment), showed great potential in
tuning catalytic selectivities in a wide
range using the conversion of 2-
propanol as a probe reaction.
Felipe Polo-Garzon, Shi-Ze Yang, Victor
Fung, Guo Shiou Foo, Elizabeth E.
Bickel , Matthew F. Chisholm, De-en
Jiang, Zili Wu*
OH
O
Ti
Sr
Ti
Controlling Reaction Selectivity via
Surface Termination of Perovskite
Catalysts
5 nm
This article is protected by copyright. All rights reserved.