Titania Morphology-Dependent Gold-Titania Interaction, Structure, and Catalytic Performance of Gold/Titania Catalysts

Article abstract of DOI:10.1002/cctc.201500599

Employing anatase TiO₂ nanocrystals with predominantly {001} facets, anatase TiO₂ nanocrystals with predominantly {100} facets, and TiO₂ P25 with predominantly {101} facets as supports, we have comprehensively studied the morphology effect of TiO₂ on the Au-TiO₂ interaction, structure, and catalytic performance of Au/TiO₂ catalysts in C₃H₆ epoxidation with H₂ and O₂, C₃H₆ oxidation with O₂, and H₂ oxidation. A strong morphology-dependent interplay between the Au-TiO₂ interaction and the catalyst structure was observed. Only Au nanoparticles were present in the Au/TiO₂ catalysts and the Au^δ- species was the largest in Au/TiO₂{001} due to the creation of surface O vacancies of TiO₂{001} upon Au loading, whereas the fraction of Au^δ+ species was largest in Au/TiO₂{100} due to the preserved surface stoichiometry of TiO₂{100} upon Au loading. In H₂ oxidation, Au/TiO₂{100} with the largest fraction of Au^δ+ species was the most active but least selective toward H₂O₂, whereas Au/TiO₂{001} with the largest fraction of Au^δ- species was the most selective toward H₂O₂. In C₃H₆ oxidation with O₂, tiny C₃H₆ conversions with the formation of partial oxidation products were observed at low temperatures, whereas C₃H₆ combustion occurred at high temperatures. In C₃H₆ epoxidation with O₂ and H₂, the ensemble consisting of closely connected Au^δ- and Ti⁴⁺ on anatase TiO₂{001} and {101} facets with weak adsorption ability was the active structure and the Au/TiO₂{001} catalyst containing the largest amount of this ensemble was the most active. These results demonstrated morphological engineering of oxides as an effective strategy to optimize the catalytic performance and understand the fundamentals of catalysis involving oxides. Titania mania: The Au-TiO₂ interaction, structure and catalytic performance of Au/TiO₂ catalysts in C₃H₆ epoxidation with H₂ and O₂, C₃H₆ oxidation, and H₂ oxidation strongly depend on the TiO₂ morphology. In the epoxidation, the ensemble of closely connected Au^δ- and Ti⁴⁺ on anatase TiO₂{001} and {101} facets with weak adsorption ability is the active structure and the Au/TiO₂{001} catalyst containing the largest amount of the Au^δ--Ti⁴⁺ ensemble is the most active.

Full text of DOI:10.1002/cctc.201500599

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DOI: 10.1002/cctc.201500599
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Titania Morphology-Dependent Gold–Titania Interaction,
Structure, and Catalytic Performance of Gold/Titania
Catalysts
Shilong Chen,[a] Bingsen Zhang,[b] Dangsheng Su,[b] and Weixin Huang*[a]
Employing anatase TiO2 nanocrystals with predominantly {001}
facets, anatase TiO2 nanocrystals with predominantly {100}
facets, and TiO2 P25 with predominantly {101} facets as sup-
ports, we have comprehensively studied the morphology
effect of TiO2 on the Au-TiO2 interaction, structure, and catalyt-
ic performance of Au/TiO2 catalysts in C3H6 epoxidation with H2
and O2, C3H6 oxidation with O2, and H2 oxidation. A strong
morphology-dependent interplay between the Au-TiO2 interac-
tion and the catalyst structure was observed. Only Au nanopar-
ticles were present in the Au/TiO2 catalysts and the AudÀ spe-
cies was the largest in Au/TiO2{001} due to the creation of sur-
face O vacancies of TiO2{001} upon Au loading, whereas the
fraction of Aud+ species was largest in Au/TiO2{100} due to
the preserved surface stoichiometry of TiO2{100} upon Au
loading. In H2 oxidation, Au/TiO2{100} with the largest fraction
of Aud+ species was the most active but least selective toward
H2O2, whereas Au/TiO2{001} with the largest fraction of AudÀ
species was the most selective toward H2O2. In C3H6 oxidation
with O2, tiny C3H6 conversions with the formation of partial oxi-
dation products were observed at low temperatures, whereas
C3H6 combustion occurred at high temperatures. In C3H6 epoxi-
dation with O2 and H2, the ensemble consisting of closely con-
nected AudÀ and Ti4+ on anatase TiO2{001} and {101} facets
with weak adsorption ability was the active structure and the
Au/TiO2{001} catalyst containing the largest amount of this en-
semble was the most active. These results demonstrated mor-
phological engineering of oxides as an effective strategy to op-
timize the catalytic performance and understand the funda-
mentals of catalysis involving oxides.
Introduction
Oxides are widely used in heterogeneous catalysis as catalysts
and catalyst supports. Crystal planes exposed on oxide nano-
particles that determine the surface compositions and surface
structures are among the most important structural parame-
ters affecting the surface chemistry and catalysis. Recent prog-
ress in nanotechnology has realized the controlled synthesis of
several oxide nanocrystals with uniform and well-defined mor-
phologies that selectively expose one or two types of crystal
planes, and these oxide nanocrystals have exhibited strong
morphology-dependent catalytic performances if used as cata-
lysts and catalyst supports.[1–7] Control of the oxide morpholo-
gy is thus emerging as a new approach to optimize per-
formance in catalysis involving oxides.
TiO2 is one of the most important oxide catalysts and sup-
ports. Uniform anatase TiO2 nanocrystals with different mor-
phologies that predominantly expose the {001} and {100}
facets have been synthesized[8–12] and used as photocatalysts
and catalyst supports.[8–18] Rich morphological effects were ob-
served on catalytic performances of TiO2 nanocrystals. The var-
iations in Au species in Au/TiO2 catalysts subjected to reduc-
tion and oxidation treatments also depended on the TiO2 mor-
phology and were related to the Au-TiO2 interfacial charge-
transfer processes.[18]
Of the reactions catalyzed by TiO2-containing catalysts, C3H6
epoxidation with O2 and H2 catalyzed by Au/TiO2 is of particu-
lar importance. Propylene epoxide (PO) is an essential industri-
al chemical intermediate but its current manufacturing pro-
cesses such as the chlorohydrin and Halcon processes are envi-
ronmentally unfriendly.[19] Hayashi et al. reported that Au nano-
particles highly dispersed on TiO2 catalyzed C3H6 epoxidation
with O2 and H2 to produce PO with selectivities of over 90%[20]
and this system has since been studied extensively.[21–30] The
most appropriate size of supported Au particles was claimed
to be 2–5 nm[20,25,26,31] but the best Ti-containing support did
not remain undoubted.[22,27–30] However, the TiO2 morphology
effect on the catalytic performance of Au/TiO2 catalysts in the
important C3H6 epoxidation with O2 and H2 had not been ex-
plored. For this paper, we prepared Au/TiO2 catalysts employ-
ing TiO2 nanocrystals with different morphologies as the sup-
ports and studied their structures and catalytic performances
[a] S. Chen, Prof. Dr. W. Huang
Hefei National Laboratory for Physical Sciences at Microscale
CAS Key Laboratory of Materials for Energy Conversion
Department of Chemical Physics
University of Science and Technology of China
Jinzhai Road 96, Hefei 230026 (P.R. China)
E-mail: huangwx@ustc.edu.cn
[b] Prof. Dr. B. Zhang, Prof. Dr. D. Su
Shenyang National Laboratory for Materials Science
Institute of Metal Research
Chinese Academy of Sciences
Wenhua Road 72, Shenyang 110016 (P.R. China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500599.
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in C3H6 epoxidation with O2 and
H2 and the related C3H6 oxida-
tion with O2, C3H6 hydrogena-
tion, and H2 oxidation. Strong
morphology effects were ob-
served on the Au-TiO2 interac-
tion, structure, and catalytic per-
formance of Au/TiO2 catalysts,
and the active structures of Au/
TiO2 catalysts in the catalytic re-
actions studied were identified.
Figure 2. A) XRD patterns, B) Au4f XPS, and C) Au LIII-edge XANES spectra of a) Au/P25, b) Au/TiO2{001}, and
c) Au/TiO2{100} catalysts.
Results and Discussion
The predicted morphology of
anatase nanocrystals under equi-
and {100} facets in TiO2{100} nanocrystals were approximately
80%. Surface structures of anatase TiO2{101}, {001}, and {100}
crystal planes without structural optimization are shown in Fig-
ure 1A3–C3, respectively. In the bulk anatase TiO2, Ti was six-
fold (Ti6c) and O threefold-coordinated (O3c). The {101} facets
had twofold-coordinated O (O2c) on the topmost layer and five-
fold-coordinated Ti (Ti5c) and O3c on the second layer; the
{001} facets had O2c on the topmost layer and Ti5c on the
second layer; and the {100} facets had O2c, O3c, and Ti5c on the
topmost layer and O3c and Ti6c on the second layer.[33–35]
librium conditions following the Wulff construction is a slightly
truncated tetragonal bipyramid enclosed by more than 94%
{101} and fewer than 6% {001} facets.[12,32] Thus, the surface
of spherical or irregular anatase TiO2 particles, such as commer-
cial P25, is assumed to be predominantly composed of {101}
facets. Anatase TiO2 nanocrystals with dominant {001} and
2À
{100} facets were prepared by employing FÀ and SO4 as the
capping ligands, respectively. We took great effort to remove
the capping ligands on the as-synthesized anatase TiO2 nano-
crystals thoroughly by repeated cycles of washing. As a result,
2À
no FÀ or SO4 could be detected by using X-ray photoelectron
The compositions of Au/TiO2 catalysts were determined to
be 0.97 wt%-Au/P25, 0.95 wt%-Au/TiO2{001}, and 1.05 wt%-
Au/TiO2{100}. As shown in Figure 2A, Au/TiO2{001} and Au/
TiO2{100} displayed diffraction patterns of anatase TiO2 (JCPDS
card No. 89-4921), whereas Au/P25 displayed diffraction pat-
terns evidencing dominant anatase TiO2 and minor rutile TiO2
(JCPDS card No. 89-4920). No Au diffraction peaks were ob-
served for any of the catalysts, indicating high dispersions of
Au particles on P25, TiO2{001}, and TiO2{100} despite their dif-
ferent specific surface areas. The Au structures in Au/TiO2 cata-
lysts were also characterized by using XPS and X-ray absorp-
tion spectroscopy (Figure 2B and C). All Au/TiO2 catalysts ex-
hibited a single Au4f component with the Au4f7/2 binding
energy at 83.2 eV, a typical value of metallic Au supported on
TiO2.[36] In addition, all Au/TiO2 catalysts exhibited Au LIII-edge
X-ray absorption near-edge structure (XANES) spectra similar
to that of Au foil. Both results proved that Au in our Au/TiO2
catalysts was metallic.
spectroscopy (XPS) on the anatase TiO2 nanocrystals (Figure S1
in the Supporting Information). The BET specific surface areas
of P25, TiO2{001}, and TiO2{100} were measured to be 45, 102,
and 88 m2 gÀ1, respectively. As shown in Figure 1, P25 nanopar-
ticles were of sizes between 20 and 40 nm, TiO2{001} nano-
crystals between 40 and 60 nm, and TiO2{100} nanocrystals of
a width distribution of 10–15 nm and a length distribution of
20–50 nm. According to the previously reported procedure,[13]
we estimated that the percentages of {001} facets in TiO2{001}
Representative high-angle annular dark-field (HAADF)-STEM
images of various Au/TiO2 catalysts are shown in Figure 3. Au
nanoparticles mainly took a spherical form. Au particles in Au/
P25 had a uniform size distribution of 3.1Æ0.6 nm in which
approximately 95% of Au particles were between 2 and 5 nm
in size. Au particles in Au/TiO2{100} were also quite uniform
with a size distribution of 2.7Æ0.6 nm and approximately 85%
of Au particles were between 2 and 5 nm in size. However, Au
particles in Au/TiO2{001} exhibited a bimodal size distribution
of 1.4Æ0.6 and 4.6Æ1.4 nm and approximately 50% of Au
particles were less than 2 nm in size with approximately 37%
of Au particles between 2 and 5 nm. The fraction of Au parti-
cles in Au/TiO2 catalysts below 2 nm followed the order Au/
Figure 1. TEM and HRTEM images of A1,A2) P25, B1,B2) TiO2{001},
C1,C2) TiO2{100}, and surface structure of anatase TiO2 A3) {101}, B3) {001},
and C3) {100} crystal planes. The insets schematically illustrate the predomi-
nantly exposed facets on P25, TiO2{001}, and TiO2{100}. *: O3c, *: O2c,
*: Ti6c, and *: Ti5c atoms.
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order TiO2{100}>TiO2{001}>P25, although the specific sur-
face area of TiO2{100} was lower than of TiO2{001}. This could
be associated with the surface structure of anatase TiO2{101},
{001}, and {100} facets exposed predominantly on P25,
TiO2{001}, and TiO2{100} nanocrystals, respectively (Fig-
ure 1A3–C3). Ti5c atoms were located on the topmost layer of
{100} crystal planes but on the second layer of {101} and
{001} crystal planes, thus, Ti5c atoms of {100} crystal planes
were more easily accessible to CO. In the zoomed-in DRIFTS
spectra, tiny peaks for COa at defective Ti4c sites could be ob-
served at 2217–2220 cmÀ1 and varied with TiO2 morphology in
a similar trend to those of COa at Ti5c sites. In addition, 13COa at
the Ti5c sites, in proportion with the 13C natural abundance, ap-
peared at 2133, 2132, and 2130 cmÀ1 on TiO2{100}, TiO2{001},
and P25, respectively.[37–39]
The loading of Au particles on P25 significantly enhanced
the vibrational features of COa at the Ti5c and Ti4c sites by fac-
tors of approximately 17 and 9, respectively. The COa feature at
the Ti5c sites did not shift but that at the Ti4c sites red-shifted
slightly to 2214 cmÀ1. The loading of Au particles on TiO2{001}
also enhanced the vibrational features of COa at the Ti5c and
Ti4c sites by factors of approximately 2.5 and 1.8, respectively,
and the COa features at the Ti5c and Ti4c sites red-shifted slight-
ly to 2178 and 2214 cmÀ1, respectively. However, the loading
of Au particles on TiO2{100} reduced the vibrational feature of
COa slightly at the Ti5c sites but enhanced it at the Ti4c sites by
a factor of approximately 1.6, with neither feature shifting.
These results demonstrated that the loading of Au particles
made the Ti5c sites on the second layer of TiO2{101} (P25) and
{001} crystal planes much more accessible for CO adsorption
but slightly blocked the Ti5c sites on the topmost layer of
TiO2{100} crystal planes. In addition, the loading of Au particles
gave the Ti5c and Ti4c sites of Au/P25 and Au/TiO2{001} a similar
binding strength to COa that was weaker than that of Au/
TiO2{100}.
Figure 3. Representative HAADF-STEM images and Au particle-size distribu-
tions of A1,A2) Au/P25, B1,B2) Au/TiO2{001}, and C1,C2) Au/TiO2{100} cata-
lysts.
TiO2{001}@Au/TiO2{100}>Au/P25, which agreed with the
order of specific surface area of TiO2 supports, TiO2{001}>
TiO2{100}@P25.
COa adsorbed at the Au sites of Au/TiO2 catalysts was also
observed and demonstrated morphology-dependent Au spe-
cies. The vibrational peaks at 2081–2089 and 2099–2105 cmÀ1
could be assigned to COa at the AudÀ and Au0 sites, respective-
ly.[42–45] The vibrational feature at 2127 cmÀ1 for Au/P25 and
The surface structures of TiO2 and Au/TiO2 were probed with
CO adsorption at 120 K by using operando diffuse-reflectance
IR Fourier transform spectrosco-
py (DRIFTS). As shown in
Figure 4, CO adsorbed (COa) on
bare
P25,
TiO2{001},
and
TiO2{100} surfaces gave strong
vibrational features at 2179,
2181, and 2183 cmÀ1, respective-
ly, that could be assigned to COa
at the Ti5c sites of TiO2.[37–39] COa
with a larger vibrational wave-
number at Ti5c sites was demon-
strated to bind to Ti5c sites more
strongly,[40,41] thus, Ti5c sites on
TiO2{001} and TiO2{100} surfaces
should bind COa more strongly
than on P25 (TiO2{101}). Their
Figure 4. Operando DRIFTS spectra of CO adsorption on TiO2 and Au/TiO2 samples at 120 K and P=250 Pa. The
peak intensities followed the insets show zoomed-in spectra.
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Au/TiO2{001} should have been contributed largely by 13COa at
the Ti5c sites with vibrational peaks at 2130 and 2129 cmÀ1, re-
spectively, and minorly by COa at the Aud+ sites.[43,44] Addition-
ally, the vibrational feature at 2133 cmÀ1 was stronger for Au/
TiO2{100} than for TiO2{100} but that of COa at the Ti5c sites at
2183 cmÀ1 was weaker for Au/TiO2{100} than for TiO2{100},
demonstrating that the vibrational feature at 2133 cmÀ1 for
Au/TiO2{100} was contributed by both 13COa at the Ti5c sites
and COa at the Aud+ sites. Indicated by the intensities of corre-
sponding COa peaks, the fraction of AudÀ species followed the
order Au/TiO2{001}>Au/P25@Au/TiO2{100}, whereas the frac-
tion of Aud+ species was largest in Au/TiO2{100}. Analysis of
previous experimental and theoretical results of Au/TiO2 model
catalysts established that the charge transfer occurred from re-
duced TiO2 to Au clusters for Au clusters nucleated on the O
vacancies of reduced TiO2 surfaces but from Au clusters to stoi-
chiometric TiO2 surfaces for Au clusters nucleated on stoichio-
metric TiO2 surfaces.[46,47] Thus, the quantity of surface O vacan-
cies on TiO2 followed the order Au/TiO2{001}>Au/P25@Au/
TiO2{100}. These results agreed with the above results from
COa adsorption at the Ti5c sites of Au/TiO2 catalysts in that the
loading of Au particles made the Ti5c sites on the second layer
of TiO2{101} and {001} crystal planes much more accessible
for CO adsorption but blocked the Ti5c sites on the topmost
layer of TiO2{100} crystal planes. The enhanced accessibility of
the Ti5c sites on the second layer of TiO2{101} and {001} crystal
planes upon Au loading suggested the depletion of O2c on the
topmost layer of TiO2{001} and {101} facets and the subse-
quent creation of surface O vacancies that gave the supported
Au particles a partial negative charge. The blocking of the Ti5c
sites on the topmost layer of TiO2{100} crystal planes upon Au
loading suggested the nucleation and growth of Au particles
on stoichiometric TiO2 surfaces, which would make them par-
tially positively charged.
Figure 5. A) C3H6 conversion and selectivity for propylene oxide and B) H2
conversion and efficiency of C3H6 epoxidation with H2 and O2 catalyzed by
various Au/TiO2 catalysts at 508C.
Au/TiO2 catalysts at 120 K that probed the structures of both
Au and TiO2, the TiO2 morphology-dependent Au-TiO2 interac-
tion and Au species depended mainly on the surface reducibili-
ty and surface O vacancy concentration of the TiO2 support.
Au/TiO2 catalysts were selective in catalyzing C3H6 epoxida-
tion with O2 and H2. The catalytic performances of our Au/TiO2
catalysts in propylene oxidation with O2 and H2 at 508C were
strongly TiO2 morphology-dependent (Figures 5 and 6). As
shown in Figure 5A, Au/P25 achieved the best performance
after 20 min reaction time, giving a C3H6 conversion of 0.50%,
a PO selectivity of 98.4%, and a PO formation rate of
À1
18.1 mmolh
g
À1. These data were comparable to previously
Au
reported typical results of Au/P25.[20] As the reaction proceed-
ed, the C3H6 conversion continued to decrease, whereas the
PO selectivity was stable, demonstrating catalyst deactivation
in agreement with previous observations.[20] Au/TiO2{001} ex-
hibited a similar behavior to Au/P25 but with a much higher
C3H6 conversion, achieving a maximum PO formation rate of
À1
The above operando DRIFTS results of CO adsorption on Au/
TiO2 catalysts at 120 K that probed the structures of both Au
and TiO2 revealed a morphology and crystal plane-dependent
interplay between the Au-TiO2 interaction and the structure of
the Au/TiO2 catalyst. The {101} and {001} surfaces terminated
with O2c facets were more facilely reduced upon Au loading
than the {100} surfaces terminated with O2c, O3c, and Ti5c. In
turn, the AudÀ species formed by the charge transfer from the
surface O vacancies on the partially reduced {101} and {001}
surfaces dominated in Au/TiO2{001} and Au/P25 catalysts,
whereas the Aud+ species dominated in the Au/TiO2{100} cata-
lyst with more stoichiometric TiO2 surfaces. Similar morphology
and crystal plane-dependent interplay between the metal-
oxide interaction and the structure of the oxide-supported cat-
alyst have also been observed for CeO2 nanocrystal-supported
Au, Ag, and Pt catalysts.[48–50] Liu et al. studied Au/TiO2 nano-
crystals catalysts with DRIFTS of CO adsorption at ambient
temperature and observed that the Au-TiO2 interaction, Au
species, and reduction and oxidation pretreatment effects
were dependent on the TiO2 morphology.[18] The authors pro-
posed TiO2 crystal plane-dependent band-structure bending
and electron flows of different Au-TiO2 interfaces. However,
based on our operando DRIFTS results of CO adsorption on
25.4 mmolh
g
À1, more than 40% higher than that of Au/
Au
P25. The PO selectivities of Au/P25 and Au/TiO2{001} increased
from the initial value of approximately 95% to above 98%. Of
the three catalysts, Au/TiO2{100} exhibited the worst catalytic
performance with a maximum C3H6 conversion of 0.17%. Its
PO selectivity increased from the initial 83.3% to approximate-
ly 96%. Another difference of Au/TiO2{100} to Au/P25 and Au/
TiO2{001} was that its C3H6 conversion increased up until
50 min reaction time and then decreased. Therefore, in C3H6
epoxidation with O2 and H2 at 508C, the C3H6 conversion fol-
lowed the order Au/TiO2{001}>Au/P25@Au/TiO2{100} and
the PO selectivity the order Au/TiO2{001}ꢀAu/P25>Au/
TiO2{100}. In turn, the PO formation rate followed the order
Au/TiO2{001}>Au/P25>Au/TiO2{100}. H2 conversions and effi-
ciencies over various Au/TiO2 catalysts are compared in Fig-
ure 5B. The H2 conversions decreased as the reaction proceed-
ed, whereas the H2 efficiencies increased. The H2 conversion
followed the order Au/TiO2{100}>Au/P25>Au/TiO2{001},
whereas the H2 efficiency followed the order Au/TiO2{001}>
Au/P25>Au/TiO2{100}, in consistence with the order of the
PO formation rate. Thus, Au/TiO2{100} was most active in cata-
lyzing the H2 combustion reaction in C3H6 epoxidation with O2
and H2, whereas Au/TiO2{001} was most efficient in catalyzing
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