Supported tantalum oxide catalysts: Synthesis, physical characterization, and methanol oxidation chemical probe reaction

Article abstract of DOI:10.1021/jp0276451

Supported tantalum oxide catalysts on Al₂O₃, TiO₂, ZrO₂, and SiO₂ supports were prepared by the incipient wetness impregnation method. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray absorption near edge structure (XANES) under hydrated and dehydrated conditions. The Al₂O₃-, TiO₂-, and ZrO₂-supported tantalum oxide catalysts possess similar surface TaO_x molecular structures, consisting primarily of polymerized surface TaO₅/TaO₆ species at high surface coverage. The surface Ta atom density at monolayer coverage are also similar and were found to be 4.5, 6.6, and 6.3 Ta atoms/nm² on the Al₂O₃, TiO₂, and ZrO₂ supports, respectively. The SiO₂-supported tantalum oxide catalyst is very different and consists of isolated TaO₄ species with a much lower maximum surface Ta density, 0.7 Ta atoms/nm², due to the lower concentration and reactivity of the silica hydroxyls. The catalytic properties of the surface TaO_x species were chemically probed with the methanol oxidation reaction. The Al₂O₃-, TiO₂-, and ZrO₂-supported tantalum oxide catalysts, possessing monolayer surface coverage of surface TaO_x species, were found to exhibit 100% dimethyl ether originating from surface acidic sites. The TOF_acidic values varied by almost 2 orders of magnitude, reflecting the influence of the specific oxide support on the surface TaO_x acidic sites through the bridging Ta-O-support bond. In contrast, the surface TaO_x species on SiO₂ were found to possess redox characteristics rather than acidic characteristics. Thus, the specific reactivity and selectivity of the surface TaO_x species strongly depend on the specific oxide support ligand and its effect on the bridging Ta-O-support bond.

Full text of DOI:10.1021/jp0276451

J. Phys. Chem. B 2003, 107, 5243-5250
5243
Supported Tantalum Oxide Catalysts: Synthesis, Physical Characterization, and Methanol
Oxidation Chemical Probe Reaction
†
‡
§
,†
Yongsheng Chen, Jose L. G. Fierro, Tsunehiro Tanaka, and Israel E. Wachs*
In Situ Molecular Characterization & Catalysis Laboratory, Department of Chemical Engineering,
Lehigh UniVersity, Bethlehem, PennsylVania 18015, Instituto de Catalisis y Petroleoquimica, CSIC,
Cantoblanco, 28049 Madrid, Spain, and Department of Molecular Engineering, Graduate School of
Engineering, Kyoto UniVersity, Kyoto, 606-8501, Japan
ReceiVed: December 6, 2002; In Final Form: March 27, 2003
2 3 2 2 2
Supported tantalum oxide catalysts on Al O , TiO , ZrO , and SiO supports were prepared by the incipient
wetness impregnation method. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS),
Raman spectroscopy, and X-ray absorption near edge structure (XANES) under hydrated and dehydrated
conditions. The Al
molecular structures, consisting primarily of polymerized surface TaO
The surface Ta atom density at monolayer coverage are also similar and were found to be 4.5, 6.6, and 6.3
O
2 3
-, TiO
2
-, and ZrO
2
-supported tantalum oxide catalysts possess similar surface TaO
x
5
/TaO species at high surface coverage.
6
2
Ta atoms/nm on the Al
2
O
3
, TiO
2
, and ZrO
2
supports, respectively. The SiO
species with a much lower maximum surface Ta density, 0.7
Ta atoms/nm , due to the lower concentration and reactivity of the silica hydroxyls. The catalytic properties
of the surface TaO species were chemically probed with the methanol oxidation reaction. The Al -, TiO -,
and ZrO -supported tantalum oxide catalysts, possessing monolayer surface coverage of surface TaO species,
were found to exhibit 100% dimethyl ether originating from surface acidic sites. The TOFacidic values varied
by almost 2 orders of magnitude, reflecting the influence of the specific oxide support on the surface TaO
acidic sites through the bridging Ta-O-support bond. In contrast, the surface TaO species on SiO were
found to possess redox characteristics rather than acidic characteristics. Thus, the specific reactivity and
selectivity of the surface TaO species strongly depend on the specific oxide support ligand and its effect on
the bridging Ta-O-support bond.
2
-supported tantalum oxide catalyst
is very different and consists of isolated TaO
4
2
x
2
O
3
2
2
x
x
x
2
x
Introduction
supported Ta2O5/TiO2 is able to synthesize methanethiol (CH3-
SH) from mixtures of H2S and CO.
1
2
The group 5 (V, Nb, and Ta) metal oxide catalysts, both as
bulk and supported metal oxides, have received much attention
in recent years.
There are two investigations in the literature about the
molecular structures of the surface tantalum oxide species. An
in situ X-ray absorption near edge structure (XANES) study of
SiO2-supported tantalum oxide found that the dehydrated surface
TaOx species possesses TaO4 coordination with a single terminal
1
-7
Vanadia catalysts are employed in many
industrial processes, such as selective oxidation of hydrocarbons
to chemical intermediates and the selective catalytic reduction
of NOx with NH3. Fundamental understanding of the surface
vanadia molecular structures and their relationships with the
catalytic reaction mechanisms have been achieved in recent
9
TadO bond. UV-vis DRS and FT-Raman studies similarly
concluded that for dehydrated SiO2-supported tantalum oxide,
surface TaO4 species exists at low tantalum oxide coverage,
whereas surface TaO6 species also appears at high surface
coverage due to the presence of Ta2O5‚nH2O microcrystals.13
The UV-vis DRS and FT-Raman investigations also concluded
that both dehydrated surface TaO4 and TaO6 species exist on
Al2O3 and ZrO2 supports at all surface coverages. Furthermore,
the surface tantalum oxide species were found to possess a
higher number of Lewis acidic sites than those of the corre-
sponding surface vanadia species.
1
-3
years.
Comparative studies have also been carried out on
bulk and supported niobia catalysts and much is currently known
about the molecular structure-reactivity/selectivity relationships
of niobia-based catalysts.3 In contrast, the current fundamental
understanding of tantala-based catalytic materials is rather
-7
3
,4
limited.
However, quite a few applications of supported tantalum
oxide catalysts have been reported in recent years. For example,
supported Ta2O5/SiO2 is found to be useful for vapor-phase
decomposition of methyl tert-butyl ether (MTBE) to isobutene
In this paper, supported tantalum oxide catalysts on four
different oxide supports (Al2O3, SiO2, TiO2, and ZrO2) were
prepared via the incipient wetness impregnation of tantalum
ethoxide and physically characterized with different physical
characterization techniques (XPS, Raman, and XANES) under
hydrated and dehydrated conditions. The catalytic properties of
the surface TaOx species were chemically probed with the
methanol oxidation reaction, and the relationship between the
surface TaOx molecular structure and the activity/selectivity was
investigated. In addition, an empirical relationship between
Ta-O bond length and the Raman frequency shifts was
8
and methanol, vapor-phase Beckmann rearrangement of cy-
clohexanone oxime to caprolactam,8 and as a photoactive
catalyst for the oxidation of CO to CO2 and ethanol to diethyl
9
acetal; supported Ta2O5/Al2O3 catalyzes liquid-phase oxidation
1
0,11
of unsaturated fatty acids and hydrocarbon cracking;
and
*
To whom correspondence should be addressed. E-mail: iew0@
lehigh.edu.
†
Lehigh University.
Instituto de Catalisis y Petroleoquimica.
Kyoto University.
‡
§
1
0.1021/jp0276451 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/09/2003

5
244 J. Phys. Chem. B, Vol. 107, No. 22, 2003
Chen et al.
developed and applied to predict the bond length of Ta-O bonds
with different bond order. The surface Ta density of polymerized
species was further predicted.
pressure. A mixture of helium and oxygen from two mass flow
controllers (Brooks 5850) was bubbled through a methanol
saturator cooled by flowing water from a cooler (Neslab RTE
1
10) to obtain a 6/13/81 (mol %) mixture of methanol/oxygen/
Experimental Section
helium at a flow rate of ∼100 sccm. The reactor was vertically
held and made of 6-mm o.d. Pyrex glass, and the flow direction
was downward. The catalysts were placed at the middle of the
tube between two layers of quartz wool and about 20-70 mg
of catalysts was used depending on the catalyst reactivity. Before
the methanol oxidation reaction, the catalysts were pretreated
at 400 °C for 30 min with flowing oxygen and helium in order
to remove adsorbed moisture and any carbonaceous residues.
The outlet line of the reactor to the GC was heated to ∼120-
140 °C to avoid condensation of methanol and its products. The
reaction products were analyzed by an online GC (HP 5840)
equipped with TCD and FID detectors, and two separation
columns (Carboxene-1000 packed column and CP-sil 5CB
capillary column) connected in parallel. By increasing the
temperature of the catalyst bed, the reaction would initiate at
some point with a detectable amount of products and the
temperature was subsequently raised by ∼10 °C until the total
conversion of methanol reached ∼10%. For each supported
tantalum oxide catalyst, the conversion/selectivity as a function
of temperature was measured and the Arrhenius activation
energy was also determined. Furthermore, the turnover fre-
quency (TOFoverall), the number of methanol molecules converted
x
1
. Catalyst Preparation. The oxide supports used for this
2
study were Al2O3 (Engelhard, SBET ) 222 m /g), SiO2 (Cabosil
2
2
EH-5, SBET ) 332 m /g), TiO2 (Degussa P-25, SBET ) 55 m /
2
g), and ZrO2 (Degussa, SBET ) 39 m /g). The supported tantalum
oxide catalysts were prepared by the incipient wetness impreg-
nation method with a tantalum ethoxide precursor (H. C. Starck,
9
9.99%). The supports were impregnated with ethanol solutions
of tantalum ethoxide and dried at room temperature under
flowing N2 in a glovebox. After they were transferred to a
furnace, the samples were initially heated to 120 °C for 2 h
under flowing dry air and then calcined at 500 °C for 5 h. Bulk
2
Ta2O5(L) (SBET ) 4 m /g) and tantalum oxyhydrate,
2
Ta2O5‚nH2O (SBET ) 69 m /g), were also obtained from H. C.
Starck. The Ta2O5‚nH2O was further calcined at 500 °C for 2
2
h, which resulted in a BET surface area of 23 m /g.
2
. Catalyst Characterization. 2.1. X-ray Photoelectron
Spectroscopy (XPS). XPS spectra were collected with a Fisons
ESCALAB 200R electron spectrometer equipped with a hemi-
spherical electron analyzer and a Mg KR X-ray source (hν )
1
253.6 eV) powered at 120 W. A PDP 11/05 computer from
DEC was used for collecting and analyzing the spectra. The
samples were placed in small copper cylinders and mounted
on a transfer rod placed in the pretreatment chamber of the
instrument. All samples were outgassed at 500 °C to remove
adsorbed moisture before the XPS analysis. The binding energies
per surface TaO species per second, was also calculated since
below monolayer coverage of the surface TaO species the
x
tantalum oxide dispersion would be 100%.
(
0
BE) were referenced to Si 2p at 103.4 eV with an accuracy of
.2 eV. The Ta 4f , Al 2p, Ti 2p , Si 2p, and Zr 3d electron
Results
7/2
3/2
5/2
intensities were also used for quantitative analysis. The atomic
concentration ratios were calculated by correcting the intensity
ratios with the theoretical sensitivity factors proposed by the
manufacturer.
1. X-ray Photoelectron Spectroscopy (XPS). XPS was
employed in this work to determine the monolayer surface
coverages of tantalum oxide on the different oxide supports.
The XPS atomic ratios of Ta to the different support cations
are presented in Figure 1 as a function of the loading of Ta2O5.
As a common observation, the ratio initially increases linearly
with the tantalum oxide loading and then deviates from the linear
relationship with a significantly reduced slope at a certain
loading. The “break” in the curve is due to the transition from
2D surface TaOx species to the 3D microcrystals, with the latter
2
.2. Raman Spectroscopy. Raman spectra were obtained with
+
the 514.5 nm line of an Ar ion laser (Spectra Physics, model
1
64). The exciting laser power was measured at the sample to
be about 10-50 mW. The scattered radiation from the sample
passed through a SPEX triple-mate monochromator (model
1877) and detected by an OMA III (Princeton Applied Research,
16
model 1463) optical multichannel analyzer with a photodiode
array cooled thermoelectrically to -35 °C. The catalyst samples
were pressed into self-supporting wafers. The hydrated samples
were rotated at 2000 rpm to minimize local heating and to avoid
dehydration by the laser beam. The Raman spectra of the
dehydrated samples were recorded at room temperature and were
obtained after heating the sample in flowing O2 at 400-450
having a low XPS sensitivity. Two straight lines can be drawn,
as shown in the figures, and their intercept determines monolayer
surface coverage. The monolayer values for Al2O3, TiO2, ZrO2,
and SiO2 supports were determined to be ∼24%, ∼11%, ∼8%,
and ∼12% Ta2O5, respectively. These values correspond to
maximum surface densities of 4.5, 6.6, 6.3, and 0.7 Ta atoms/
2
nm for tantalum oxide supported on Al2O3, TiO2, ZrO2, and
°
C for 1 h in a stationary quartz cell, which has been previously
SiO2, respectively.
1
4
described.
2. XANES. For the Ta L1-edge, the energy resolution is
intrinsically poor due to the broad bandwidth of the L1 (2s)
level (∆E ) 5.58 eV full width at half-maximum). Therefore,
preedge peaks appear broad and it is not possible to determine
the TaOx surface molecular structure on the basis of peak
positions (the shift of which is thought to be less than 2 eV).
The preedge peak area (integrated intensity), however, can be
used to determine the molecular structure. The preedge peak
was extracted by subtracting the background, which was created
by fitting a fourth-order polynomial to the XANES spectrum
in the 20-23 and 38-40 eV regions as shown in Figure 2. The
peak area was calculated by the Simpson method. TaO4
structures have different peak area values than TaO6 or TaO5
structures. The TaO4 structures possess values greater than 0.7,
2.3. X-ray Absorption Near Edge Structure (XANES). XANES
analysis was performed with the XAFS (X-ray absorption fine
structure) spectrometer installed on the BL01B1 beam line13
with a Si(111) two-crystal monochromator at SPring-8 at the
Japan Synchrotron Radiation Research Institute (JASRI) located
at Hyogo, Japan. Before the XANES measurement, the dehy-
drated samples were heated to 673 K in vacuo, treated under a
6
0 Torr oxygen atmosphere, pressed into a disk under dry
nitrogen, and sealed with a polypropylene film. The hydrated
samples were kept at high humidity >90% at room temperature
for more than 10 days. Details of the data processing are
described elsewhere.15
3
. Catalysis. The methanol oxidation reaction was carried
9
out in a fixed-bed differential reactor operating at atmospheric
and the TaO6/TaO5 structures possess values less than 0.5. The

Supported Tantalum Oxide Catalysts
J. Phys. Chem. B, Vol. 107, No. 22, 2003 5245
2 5 2 5 2 3
Figure 1. The XPS surface atomic ratio of Ta/support cation vs Ta O loading for the supported tantalum oxide catalysts, (a) Ta O /Al O ,
2 5 2 2 5 2 2 5 2
(b) Ta O /SiO , (c) Ta O /TiO , and (d) Ta O /ZrO .
Figure 2. Determination of the XANES preedge areas and the Ta
coordination, (a) original spectrum, (b) fourth-order polynomial back-
ground, and (c) background subtracted spectrum.
TABLE 1: XANES Results of Ta Coordination of Surface
TaO
x
Species on Different Oxide Supports under Hydrated
and Dehydrated Conditions
2 5
Figure 3. Raman spectra of bulk Ta O and tantalum oxyhydrate before
and after different heat treatments.
hydrated
coordination
dehydrated
coordination
catalyst
area
area
3. Raman Spectroscopy. The Raman spectra of bulk
tantalum oxyhydrate, Ta2O5‚nH2O, before and after exposure
to elevated temperatures in air for 2 h are presented in Figure
3. For comparison, the spectrum of the low-temperature phase
of crystalline Ta2O5(L) is also presented in Figure 3. Tantalum
oxyhydrate is amorphous with a major characteristic broad
Ta
2
Ta
2
Ta
2
Ta
2
O
5
O
5
O
5
O
5
/Al
/SiO
/TiO
/ZrO
2
O
3
0.57
0.44
0.44
0.46
TaO
TaO
TaO
TaO
6
6
6
6
/TaO
/TaO
/TaO
/TaO
5
5
5
5
0.50
0.77
0.48
0.46
TaO
TaO
TaO
TaO
6
4
6
6
/TaO
5
2
2
/TaO
/TaO
5
2
5
-
1
peak area values between 0.5 and 0.7 correspond to mixture of
TaO6/TaO5 and a small amount of TaO4. The XANES results
are presented in Table 1. Essentially all the hydrated supported
tantalum oxide catalysts possess TaO6/TaO5 coordination and
the hydrated Ta2O5/Al2O3 also contains a small amount of some
TaO4 species. For the dehydrated supported tantalum oxide
catalysts, TaO6/TaO5 is the dominant coordination with the
exception of Ta2O5/SiO2, which only possesses a TaO4 coor-
dination.
Raman band at ∼660 cm due to Ta-O vibration. The heat
treatment at 600 °C does not affect the main Raman bands of
tantalum oxyhydrate, but a new band begins to emerge at ∼244
-
1
cm . However, further heat treatment at 700 °C transforms
the amorphous tantalum oxyhydrate phase into well crystallized
Ta2O5(L) structure. The Raman results are in agreement with
previous thermogravimetric analysis (TGA) and XRD studies
of tantalum oxyhydrate by Gonz a´ lez and co-workers,17 which
concluded that at 600 °C the tantalum oxyhydrate lost little water

5246 J. Phys. Chem. B, Vol. 107, No. 22, 2003
Chen et al.
and remained amorphous and that at 800 °C and above most of
the water of hydration was lost and the tantalum oxyhydrate
crystallized into different bulk structures of tantalum oxide (at
least two phases with a reversible phase transition occurring at
18
about 1360 °C). The strongest Ta2O5(L) band appears at ∼100
-
1
-1
cm and the second strongest at ∼251 cm . The characteristic
-
1
broad band at ∼660 cm for the amorphous tantalum hydrate
splits into at least two peaks at 621 and 705 cm- when it
crystallizes to Ta2O5(L). Other weak Raman bands are also
observed at approximately 200, 340, 490, and 843 cm and
can be assigned as follows: L-Ta2O5 lattice photon at 100 cm ;
1
-
1
-1
-
1
3
40 cm due to TaO6 symmetric bending; Ta-O symmetric
-
1
stretching at 621 cm ; higher order of Ta-O symmetric
-
1
stretching at 843 cm ; and bridging Ta-O-Ta bending,
symmetric, and antisymmetric stretching vibrations at 251, 490,
-
1
and 705 cm , respectively.
The main features of the Raman spectra of the hydrated
Ta2O5/Al2O3 catalysts change at loadings between 20% and 25%
Ta2O5 as shown in Figure 4a. Below 20% Ta2O5/Al2O3, there
-
1
is only a single major Raman band at ∼910 cm . For 25%
Ta2O5/Al2O3 and higher loadings, another Raman band appears
-
1
-1
at ∼660 cm and its relative intensity to that of ∼910 cm
band increases with the increasing tantalum oxide loading. Thus,
bulk tantalum oxyhydrate formed above monolayer coverage
-
1
of surface TaOx (Raman band at 660 cm ) and the hydrated
surface TaOx species exhibits one main Raman band at ∼910
-1
cm . Monolayer coverage of the surface TaOx on Al2O3 occurs
between 20% and 25% Ta2O5/Al2O3 because bulk tantalum
oxyhydrate begins to form in this region. The supported bulk
Ta2O5‚nH2O phase is more difficult to crystallize during elevated
heat treatment than unsupported pure Ta2O5‚nH2O since Raman
spectroscopy reveals that even up to 800 °C the main features
of the spectra remain without the appearance of the bands
characteristic of crystalline Ta2O5(L).
The main Raman features of the surface TaOx species on
Al2O3 change significantly upon dehydration (compare parts a
and b of Figure 4). There are three main Raman bands for the
dehydrated surface TaOx species at ∼940, ∼740, and ∼610
-
1
-1
cm . The Raman band at ∼940 cm corresponds to the
symmetric stretching mode of the dehydrated terminal TadO
-
1
bond, and the bands at ∼740 and ∼610 cm are assigned to
the antisymmetric and symmetric stretching modes of the
dehydrated bridging Ta-O-Ta bond, respectively. The presence
of bridging Ta-O-Ta bands for dehydrated Ta2O5/Al2O3
reveals that polymerized surface TaOx species is present on the
Al2O3 surface. The weak band at ∼400 cm is assigned to the
bending mode of the dehydrated surface TaOx species. “Ghost
peaks” due to strong Raleigh scattering are observed for Al2O3-
and SiO2-supported Ta2O5 catalysts; the physical “filtering” of
Figure 4. Raman spectra of supported Ta
2 5
O /Al
O
2 3
oxide catalysts:
(a) hydrated and (b) dehydrated.
-
1
-
1
oxyhydrate phase at ∼685 cm appears for 6% Ta O /SiO
2
5
2
and higher tantalum oxide loadings, suggesting that maximum
coverage of surface TaO on SiO occurs between 4% and 6%
x
2
-
1
the spectra below Raman shift of ∼250 cm avoids the effect;
2 5 2 2 5 2
Ta O /SiO . Dehydration of the supported Ta O /SiO catalysts
does not significantly alter the features of the hydrated Ta O /
2 5
-
1
hence, only the region above 300 cm is presented.
The Raman spectra of the hydrated Ta2O5/SiO2 catalysts are
presented in Figure 5. Similar to the hydrated Ta2O5/Al2O3
catalysts, the hydrated surface TaOx species exhibit a Raman
SiO2 Raman spectra (not shown) because of the relatively weak
surface tantalum oxide signals and the much stronger bands due
to the SiO2 support in the same region.
-
1
band at 940 cm , seen as a weaker shoulder on the Si-OH
The TiO and ZrO supports give rise to very strong Raman
2
2
-1
-1
vibration at ∼975 cm , and the bulk tantalum oxyhydrate phase
bands below 800 cm , and thus, only Raman bands above 800
cm can be detected for the TiO - and ZrO -supported tantalum
-
1
-1
displays a band at 685 cm (the apparent shift from 660 to
2
2
-
1
6
85 cm of this phase is most likely due to the strong
oxide catalysts, as shown in Figures 6 and 7. Compared to the
background of the SiO2 vibrations in this region). The Raman
signals for the Ta2O5/Al2O3 catalysts are much stronger than
those of the Ta2O5/SiO2 because of the higher concentration of
surface TaOx species, and with the presence of relatively strong
background SiO2 vibrations, the band positions of the latter are
not as well resolved. The Raman band of the bulk tantalum
Raman bands of the surface TaO species, the strong oxide
x
-1
-1
support bands at ∼790 cm for TiO2 and ∼756 cm for ZrO2
dominate the spectra. For the dehydrated 9% Ta2O5/TiO2
sample, there is only some indication of the existence of
vibrations from a surface tantalum oxide species at ∼980, ∼940,
-
1
and ∼855 cm , after subtraction of the TiO2 background, but

Supported Tantalum Oxide Catalysts
J. Phys. Chem. B, Vol. 107, No. 22, 2003 5247
Figure 7. Raman spectra of (a) dehydrated 6% Ta
and (b) ZrO support; spectrum c is the difference spectrum between
a) and (b).
2 5 2
O /ZrO catalyst
2 5 2
Figure 5. Raman spectra of hydrated Ta O /SiO catalysts.
2
(
Figure 8. Schematic drawings of molecular structures of surface TaO
x
species under dehydrated condition.
4
. Methanol Oxidation. The methanol oxidation reaction
chemically probes the nature of the catalyst surface sites via its
reaction products: dimethyl ether (DME) results from surface
acidic sites, formaldehyde (HCHO), methyl formate (MF), and
dimethoxy methane (DMM) originate from surface redox sites
19
and CO/CO2 are produced on surface basic sites. Bulk Ta2O5-
L) and Ta2O5‚nH2O are solid acid catalysts, and the catalytic
(
data for CH3OH oxidation are in agreement with this conclusion
since only DME is formed. The methanol oxidation selectivities
and activities of the different supported tantalum oxide catalysts
as well as those of the bulk Ta2O5(L), Ta2O5‚nH2O, and the
pure oxide supports are presented in Table 2. The supported
surface TaOx monolayers on Al2O3, TiO2, and ZrO2 also possess
100% acidic surface sites since essentially only DME is formed
during methanol oxidation. A significant portion of COx was
observed for the lower tantalum oxide loaded Ta2O5/ZrO2
catalysts due to the exposure of the surface zirconia sites below
monolayer coverage, ∼8% Ta2O5/ZrO2. For low Ta2O5/TiO2
loadings, the exposed surface titania sites produced traces of
HCHO byproducts below monolayer coverage, ∼11% Ta2O5/
TiO2. The selectivity of the SiO2-supported tantalum oxide
depends on the Ta2O5/SiO2 loading. At low Ta2O5/SiO2 load-
ings, the redox product HCHO and the basic product CO2 are
formed. The Ta2O5/SiO2 appears to contain both surface redox
and basic sites since SiO2 is not catalytically active at this
reaction temperature. At 5% Ta2O5/SiO2 and higher tantalum
oxide loadings, the main product is MF from surface redox sites,
and the minor DME originates from the presence of surface
acidic sites present on the bulk Ta2O5‚nH2O phase.
Figure 6. Raman spectra of (a) dehydrated 9% Ta
and (b) TiO support; spectrum c is the difference spectrum between
a) and (b).
2 5 2
O /TiO catalyst
2
(
the surface TaOx bands are so weak and broad that these band
positions are just a rough estimate. The Raman bands in the
-
1
9
40-980 cm region are associated with a terminal TadO
-
1
bond, and the 855 cm band is consistent with the antisym-
metric stretch of bridging Ta-O-Ta bonds. For the dehydrated
6% Ta2O5/ZrO2 sample, the surface TaOx bands are only slightly
better resolved in Figure 7. Two broad Raman bands, at ∼976
-
1
and 932 cm , after subtraction of the ZrO2 background,
corresponding to the terminal TadO bond. The bands at ∼822
-1
and ∼725 cm are assigned to antisymmetric stretching modes
of the bridging Ta-O-Ta bonds. Thus, polymerized surface
TaOx species appear to be present on the TiO2 and ZrO2
supports. The hydrated Raman spectra of the Ta2O5/TiO2 and
Ta2O5/ZrO2 catalysts did not give rise to apparent Raman bands
of the surface TaOx species because hydration results in even
broader Raman bands.
The catalytic activities are expressed as the number of moles
of methanol converted per hour per gram of catalyst and are

5
248 J. Phys. Chem. B, Vol. 107, No. 22, 2003
Chen et al.
TABLE 3: Activation Energies of Supported Tantalum
TABLE 2: Selectivity and Activity of Supported Tantalum
Oxide Catalysts during CH OH Oxidation
a) Supported Ta /Al at 200 °C
3
Oxide Catalysts at Monolayer Surface Coverage and Pure
(
2
O
5
2
O
3
3
Bulk Oxides for CH OH Oxidation
selectivity (%)
activation
energy
(kcal/mol)
activity
(10-2 mol/g h)
temperature
range
reaction
products
catalyst
(L)
DME
CO
x
DMM
catalyst
Ta
Al
2
O
5
100
100
100
100
100
100
100
100
100
100
100
0.072
13.3
9.2
2
O
3
Ta
Ta
2
O
O
5
(L)
‚nH
220-260
200-260
150-190
200-350
300-350
300-350
170-210
290-340
270-330
280-330
13.7
15.0
20.2
DME
2
5
1
1
2
2
3
3
4
% Ta
% Ta
2
O
O
5
/Al
/Al
2
2
O
3
3
O
2
O
5
2
O (500 °C)
DME
8.6
Al
SiO
TiO
ZrO
2
3
DME
2
5
0% Ta
5% Ta
0% Ta
5% Ta
0% Ta
5% Ta
0% Ta
2
2
2
2
2
2
2
O
O
O
O
O
O
O
5
/Al
5
/Al
5
/Al
5
/Al
5
/Al
5
/Al
5
/Al
2
O
2
O
2
O
2
O
2
O
2
O
2
O
3
3
3
3
3
3
3
6.2
2
not active
DME
4.5
2
19.8
36.4
18.6
16.3
17.7
13.2
4.1
2
CO
x
2.9
Ta
Ta
Ta
Ta
O
2 5
O
2 5
O
2 5
O
2 5
/Al
/TiO
/ZrO
/SiO
2
O
2
3
DME
DME
DME
MF
2.8
2.3
2
2.1
2
(
b) Supported Ta
2
O
5
/SiO
2
at 300 °C
energy of ∼20 kcal/mol, and the basic ZrO2 support exhibits a
higher value of ∼36 kcal/mol. The SiO2 support was found to
be inactive for CH3OH oxidation in this temperature range and
no activation energy could be determined. The acidic supported
tantalum oxide species, yielding only DME, possess an activa-
tion energy of ∼16-19 kcal/mol, which is intermediate between
the acidic supports and bulk Ta2O5(L) and Ta2O5‚nH2O. Sup-
ported Ta2O5/SiO2 exhibits a lower activation energy, but this
is for the production of MF and cannot be compared to the acidic
surface sites found in the other catalysts. The apparent activation
energy Eapp is an overall parameter describing the catalytic
processes over the catalysts and is related to Eact + ∆H, where
selectivity (%)
activity
MF DME (10- mol/g h)
2
catalyst
HCHO CO
2
SiO
2
not active
6.0
17.4
6.3
6.5
6.7
2
4
5
6
8
% Ta
O
2 5
O
2 5
O
2 5
O
2 5
O
2 5
/SiO
/SiO
/SiO
/SiO
/SiO
2
74
66
26
34
% Ta
% Ta
% Ta
% Ta
2
2
2
2
100
95
84
trace
5
16
(
c) Supported Ta
2
O
5
/TiO
2
at 300 °C
selectivity (%)
activity
(10- mol/g h)
2
catalyst
DME
CO
x
HCHO
TiO
2
100
100
100
100
100
100
100
trace
trace
trace
trace
0.34
0.42
0.47
0.56
0.62
1.09
1.14
∆
H is a negative value and the exothermic heat of adsorption
3
6
9
1
1
1
% Ta
% Ta
% Ta
2 5
O /TiO
2 5
O /TiO
2 5
O /TiO
2
2
2
of methanol. Nevertheless, Eapp allows extrapolation of the
methanol oxidation activities of the catalysts to nearby temper-
atures (especially for the very active Ta2O5/Al2O3 system, which
required lower temperatures to ensure differential reaction
conditions).
2% Ta
5% Ta
8% Ta
O
2 5
O
2 5
O
2 5
/TiO
/TiO
/TiO
2
2
2
(
d) Supported Ta
selectivity (%)
CO HCHO
2 5 2
O /ZrO at 300 °C
Discussion
activity
(10- mol/g h)
2
catalyst
DME
x
1
. Surface TaOx Density at Monolayer Coverage. The
ZrO
2
trace
69
92
100
31
8
0.32
0.72
0.93
1.16
1.18
1.61
1.48
monolayer loadings of the surface TaOx species on the different
oxide supports were determined from the XPS measurements
to be 23.8% Ta2O5/Al2O3, 10.7% Ta2O5/TiO2, and 7.9% Ta2O5/
ZrO2. Raman reveals that the bulk Ta2O5‚nH2O phase starts to
appear on the Al2O3 support between 20% and 25% Ta2O5/
Al2O3 (see Figure 4a). Raman is not able to detect the incipient
formation of bulk Ta2O5‚nH2O on the TiO2 and ZrO2 supports
due to the strong Raman vibrations of the supports below 800
3
6
9
1
1
1
% Ta
% Ta
% Ta
O
2 5
O
2 5
O
2 5
/ZrO
/ZrO
/ZrO
2
2
2
100
100
100
100
2% Ta
5% Ta
8% Ta
2
2
2
O
5
O
5
O
5
/ZrO
/ZrO
/ZrO
2
2
2
presented in Table 2. The activities of Ta2O5/Al2O3 catalysts
continuously decrease with Ta2O5 loading, which suggests that
the surface TaOx acidic species on Al2O3 is less active than the
surface acidic sites of the Al2O3 support but more active than
the bulk Ta2O5‚nH2O phase (possessing a relatively small
number of exposed surface tantalum oxide sites). In contrast,
the activities of the supported Ta2O5/TiO2 and Ta2O5/ZrO2
catalysts change with tantalum oxide loading: increasing up to
monolayer coverage and continuing to increase after monolayer
coverage. This activity pattern suggests that the surface TaOx
species on TiO2 and ZrO2 is more active than the corresponding
oxide supports but less active than the bulk Ta2O5‚nH2O phase.
The selectivity of the supported Ta2O5/SiO2 catalysts dramati-
cally changes with tantalum oxide loading due to the varying
nature of the surface tantala sites with surface coverage (See
Raman section above).
-
1
cm , which overshadows the characteristic band of the bulk
-
1
Ta2O5‚nH2O phase at ∼660 cm . However, the methanol
oxidation reaction provides information about the completion
of the surface TaOx monolayers on the oxide supports since
exposed support sites give rise to side reactions (see Table 2).
The TiO2 support gives rise to trace amounts of HCHO, which
disappears above 12% Ta2O5/TiO2. The ZrO2 support is very
active in oxidizing methanol to COx, but COx is not detected
above 9% Ta2O5/ZrO2. These results suggest that the monolayer
loading is between 9%-12% Ta2O5/TiO2 and between 6%-
9% Ta2O5/ZrO2, which is in agreement with the XPS measure-
ment results. Thus, for tantalum oxide supported on Al2O3, TiO2,
and ZrO2, the monolayer surface coverage determined by XPS,
Raman, and CH3OH oxidation are in agreement.
Arrhenius relationships were obtained for CH3OH oxidation
over the supported tantalum oxide catalysts as well as the pure
oxides and the apparent activation are presented in Table 3.
Pure Ta2O5(L) and Ta2O5‚nH2O possess the lowest activation
energies of 13.7 and 15.0 kcal/mol, respectively. The acidic
Al2O3 and TiO2 supports exhibit a slightly higher activation
For supported Ta2O5/SiO2 catalysts, Raman reveals that the
bulk of Ta2O5‚nH2O phase already exists in the catalysts with
loadings of 6% Ta2O5/SiO2 and above (see Figure 5), whereas
XPS suggests a monolayer coverage of 11.8% Ta2O5/SiO2. The
methanol oxidation reaction reveals that the reaction selectivity
changes dramatically at a loading of 5% Ta2O5/SiO2 (see Table

Supported Tantalum Oxide Catalysts
J. Phys. Chem. B, Vol. 107, No. 22, 2003 5249
TABLE 4: Surface Ta Densities at Monolayer Surface
Coverage on Different Oxide Supports
TABLE 5: Methanol Oxidation Turnover Frequency (TOF)
Values at 300 °C for Supported Tantalum Oxide Catalysts at
2 5
Monolayer Coverage and Bulk Ta O
2
BET surface area (m /g)
TOF (10- s-1
3
)
tantalum
oxide
monolayer
coverage
XPS
monolayer
loading
(wt %)
Ta surface
density
(Ta/nm )
catalyst
DME (acidic)
MF (redox)
2
(L)a
/Al
/TiO
/ZrO
/SiO
support
Al
original
Ta
2
Ta
2
Ta
2
Ta
2
Ta
2
O
5
O
5
O
5
O
5
O
5
90
246
3.3
2
O
3
2
2
O
3
222
332
55
190
210
49
23.8
4.5 a
(0.7 )
6.6
a
SiO
2
11.8 (5.0 )
2
8.4
TiO
2
10.7
7.9
2
78
ZrO
2
39
37
6.3
a
a
From ref 28.
From Raman and methanol oxidation.
identical with a “double bond” of valence 1.72. For a surface
TaOx unit to occupy the maximum area, a square pyramidal
structure is assumed, which means that there are four bridging
Ta-O bonds to the oxide support or adjacent surface TaOx sites
with each bond having valence of 0.82, which, from eq 3,
corresponds to a Ta-O bond length of 1.98 Å. For bridging
Ta-O-Ta bonds with an angle of 180°, a maximum distance
between Ta atoms of 3.97 Å is obtained. Hence, the maximum
area that a polymerized surface TaOx unit can occupy is 15.8
2
), which is in agreement with the Raman results. The XPS
measurement most probably resulted in a higher value for
monolayer surface coverage for the Ta2O5/SiO2 catalyst system
because the weak interaction between SiO2 and the surface TaOx
species resulted in an exterior surface composition, line-of-sight,
that may not have been representative of the entire sample. In
contrast, both Raman and the methanol oxidation probe the
entire sample and are not limited to only the line-of-sight
external composition. Thus, monolayer loadings of surface TaOx
on the different oxide supports correspond to 23.8% Ta2O5/
Al2O3, 10.7% Ta2O5/TiO2, 7.9% Ta2O5/ZrO2, and 5.0% Ta2O5/
SiO2, respectively.
A BET measurement of the supported Ta2O5 catalysts near
monolayer coverage shows shrinkage of the oxide supports after
impregnation and calcination. The BET surface areas of 25%
Ta2O5/Al2O3, 12% Ta2O5/TiO2, 6% Ta2O5/ZrO2, and 5% Ta2O5/
SiO2 are 143, 43, 35, and 200 m /g, respectively, so the surface
areas of the supports at monolayer coverage are 190, 49, 37,
2
Å and its reciprocal gives the minimum surface Ta density of
a monolayer of polymerized surface TaOx species, 6.3 Ta atoms/
2
nm . This value matches the experimental monolayer surface
densities for Ta2O5/TiO2 and Ta2O5/ZrO2. The slightly lower
surface TaOx density on Al2O3 may be due to the presence of
a minor amount of isolated surface TaO4 units, which is detected
by the XANES measurements.
2
. Molecular Structures of the Surface TaOx Species.
2
Under hydrated conditions, the surfaces of the supported
tantalum oxide catalysts possess a thin film of moisture and
the aqueous solution chemistry controls the hydrated surface
TaOx species, which is determined by the pH value of the
2
and 210 m /g, accordingly. Thus, the surface densities of Ta
atoms at monolayer coverage on the different oxide supports
can be determined and are listed in Table 4. The monolayer
22
aqueous film and the concentration of TaOx. The oxide support
2
surface TaOx densities are 4.5, 6.6, 6.3, and 0.7 Ta atoms/nm
influences the final pH value at the PZC theory (point of zero
charge). The Al2O3, TiO2, ZrO2, and SiO2 supports have net
pH at PZC of 8.9, ∼6.2, ∼6.0, and 3.9, respectively. The
presence of the acidic surface TaOx overlayer will further lower
the net pH at PZC since the Ta2O5 possesses a net pH at PZC
on Al2O3, TiO2, ZrO2, and SiO2 supports, respectively.
Additional estimates of monolayer surface TaOx coverage can
also be determined from knowledge of the structures of surface
TaOx phases. Hardcastle and Wachs developed an empirical
relationship between the Nb-O bond length and its stretching
22,23
24
of 2.9.
According to the hydrolysis of cations, the Ta(V)
2
0
frequency, referred to as the diatomic approximation,
species may form hexanuclear or mononuclear species at
6
-
different pH values. The Ta6O19 (aq) species exists in very
-1.9238R
ν(Nb-O) ) 26262e
(1)
basic solutions and exhibits major Raman bands at ∼857 and
-
1 25
∼
516 cm . The Ta(OH)5 species exists at pH values between
-
1
where ν is the stretching frequency (cm ) and R is the length
of the Nb-O bond (Å). Due to the known similarity between
Ta-O and Nb-O bonds,21 a similar relationship can be derived
for Ta-O by correcting the above expression with the higher
mass of Ta, which results in
1
and 12. Therefore, Ta(OH)5 is the main hydrated TaOx species
on all the four oxide supports under hydrated conditions, and it
-
1
exhibits its strongest Raman band at ∼910 cm (see Figure
4). Under dehydrated conditions, the surface TaOx species react
with the surface hydroxyls of the oxide support and become
anchored on the support. The extent of linkage among the
surface species depends on a spatial factor: the distance between
Ta atoms from each other and a chemical factorsthe strength
of the Ta atoms interacting with the oxide supports. The stronger
the interaction, the less likely for the neighboring Ta species to
form bridging bonds. Hence, scarce surface hydroxyls and strong
interaction are favorable to the formation of isolated surface
TaOx species, while dense surface hydroxyls and weak interac-
tions would be expected to favor the formation of polymerized
surface TaOx species. The surface Ta density at monolayer
coverage is a reflection of the availability and reactivity of
surface hydroxyls (see Table 4) and silica has less reactive
-
1.9238R
ν(Ta-O) ) 25308e
(2)
Brown and Wu also presented a relationship between the bond
valence s and bond length R for Ta-O:21
-5
R
s(Ta-O) )
(3)
(
)
1
.907
For supported Ta2O5/Al2O3 catalysts under dehydrated condi-
tions, the largest Raman shift observed for the terminal TadO
-
1
bond of surface tantalum oxide species is ∼940 cm , corre-
sponding to a bond length of 1.71 Å and a bond valence of
26
1
.72 from eqs 2 and 3, respectively.
surface hydroxyls than the other three supports. H2-TPR
2
7
The monolayer surface TaOx density for the polymerized
surface TaOx species can be estimated from eq 3. A simplified
structural model is proposed: the surface TaOx species are
studies of supported vanadia catalysts demonstrated that the
strength of V-O-support band decreases in the order of SiO2
> Al2O3 > ZrO2 ∼ TiO2 and that a similar trend could also be

5250 J. Phys. Chem. B, Vol. 107, No. 22, 2003
Chen et al.
TABLE 6: Acidic and Redox TOFs of Pure and Supported
Tantalum Oxide and Niobium Oxide Catalysts29 Containing
stable during methanol oxidation and retains the same molecular
structure as the dehydrated surface TaOx species.
30
Monolayer Surface Coverage during CH
30 °C
3
OH Oxidation at
2
Conclusions
acidic TOF (×10 S1-
-
3
)
redox TOF (×10 S1-
-3
)
The molecular structures and catalytic properties of supported
tantalum oxide catalysts depend on the specific oxide support.
The Al2O3-, TiO2-, and ZrO2-supported tantalum oxide catalysts
consist of polymerized surface TaO5/TaO6 species under dehy-
drated conditions with very similar surface TaOx densities at
monolayer surface coverage and possess 100% acidic sites. The
TOF values vary by almost 2 orders of magnitude revealing a
significant influence of the oxide support through the bridging
Ta-O-support bond (Al . Zr > Ti). The dehydrated SiO2-
supported tantalum oxide catalyst only consists of isolated
surface TaO4 species, before formation of Ta2O5‚nH2O micro-
crystals, with a much lower surface TaOx density at maximum
coverage and possesses mostly redox properties. The specific
oxide support is proposed to be the major factor that determines
the molecular structure, TOF, and selectivity of the supported
surface TaOx species.
oxide support
bulk
Ta O
2 5
Nb
2
O
5
Ta
2
O
5
2 5
Nb O
16.9
25.3
21
200
50
0
0
0
0
0
0
0
0
Al
ZrO
TiO
SiO
2
O
3
2
0.97
2
0.45
0
40
2
0
15.6
62
expected for surface TaOx species. Thus, the silica support
possesses a lower concentration of reactive surface hydroxyls
and a very strong interaction with the surface TaOx species.
Consequently, isolated species are most likely formed. For the
other oxide supports, the polymerized surface TaOx species are
more favorable because of the higher concentration of reactive
surface hydroxyls and somewhat weaker Ta-O-support interac-
tion. This model is supported by the Raman and XANES data
(see Table 1 and Figure 4). Under dehydrated conditions, surface
TaOx species on SiO2 are isolated TaO4 structures and surface
TaOx species on Al2O3, TiO2, and ZrO2 are present as polym-
erized surface TaO5/TaO6 structures as shown schematically in
Figure 8. The TaO4 unit has one TadO double and three Ta-
O-Si bonds. The TaO5/TaO6 unit has two bridging Ta-O-Ta
bonds and two bridging Ta-O-support bonds on Al2O3, TiO2,
and ZrO2. For the surface TaO6 structure, there could be another
Ta-O-support bond at a much longer Ta-O bond length,
which corresponds to an oxygen atom of the oxide support. The
distribution of the number of surface TaOx units in the
polymerized surface TaOx species is not known at present.
Acknowledgment. The financial support of H.C. Starck for
this research is gratefully acknowledged.
References and Notes
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(
(
2) Wachs, I. E.; Weckhuysen, B. M. Appl. Catal. A 1997, 157, 67.
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(
3
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(
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(
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6. Furthermore, the H2-TPR of Ta2O5/Al2O3 shows almost no
(
reduction up to 700 °C, suggesting that surface TaOx species is