Selective modification of the acid-base properties of ceria by supported Au

Article abstract of DOI:10.1002/chem.201002011

Au supported on CeO₂ prepared by deposition-precipitation with urea leads to a basic catalyst. Au acts in two ways as surface modifier. First, Au selectively interacts with Ce⁴⁺ cations by either blocking access to or reducing Ce⁴⁺ to Ce³⁺. Second, the resulting Au atoms (presumably as Au⁺ ions) act as soft, weak Lewis acid sites stabilizing carbanion intermediates and enhancing hydride abstraction in the dehydrogenation of alcohols. In consequence, the thus-synthesized basic catalyst catalyzes the dehydrogenation of propan-2-ol to acetone with high efficiency and without notable deactivation. Additionally, the dehydration pathway of propan-2-ol is eliminated, as Au also quantitatively blocks access to strongly acidic Ce⁴⁺ ions or reduces them to Ce³⁺. Copyright

Full text of DOI:10.1002/chem.201002011

FULL PAPER
DOI: 10.1002/chem.201002011
Selective Modification of the Acid–Base Properties of Ceria by Supported Au
[
a]
[a]
[b]
[b]
Manuela C. I. Bezen, Cornelia Breitkopf, Nadia El Kolli, Jean-Marc Krafft,
[
b]
[a]
Catherine Louis, and Johannes A. Lercher*
Abstract: Au supported on CeO pre-
stabilizing carbanion intermediates and
enhancing hydride abstraction in the
dehydrogenation of alcohols. In conse-
quence, the thus-synthesized basic cata-
lyst catalyzes the dehydrogenation of
propan-2-ol to acetone with high effi-
ciency and without notable deactiva-
tion. Additionally, the dehydration
pathway of propan-2-ol is eliminated,
as Au also quantitatively blocks access
2
pared by deposition–precipitation with
urea leads to a basic catalyst. Au acts
in two ways as surface modifier. First,
4
+
Au selectively interacts with Ce cat-
ions by either blocking access to or re-
Keywords: cerium · gold · hetero-
geneous catalysis · IR spectroscopy ·
supported catalysts
4
+
3+
4+
ducing Ce to Ce . Second, the re-
to strongly acidic Ce ions or reduces
+
3+
sulting Au atoms (presumably as Au
them to Ce .
ions) act as soft, weak Lewis acid sites
4
+
Introduction
Ce in nanostructured ceria thin films decreased after dep-
osition of Au and observed simultaneously a significant con-
centration of oxidized Au species. The deposition of Au ap-
pears, therefore, to markedly alter the acid–base properties
of ceria. In both cases, deposition of Au appears to either
block or alter Lewis acid or base sites of model ceria surfa-
ces, and this suggests a significant modification of its acid–
base properties. Thus, we decided to explore whether Au
can be used to selectively modify the acid–base properties
of real powder ceria catalysts and whether such a modifica-
tion would be sufficient to manifest itself in catalytic trans-
formations.
Ceria is a material of high catalytic relevance and has there-
[1,2]
fore been extensively studied.
as catalyst in alcohol conversion on the basis of its unique
For example, ceria is used
[3]
combination of acid–base properties. The redox properties
[4]
of ceria make it an interesting support for gold catalysts.
Gold supported on ceria has received particular attention
due to its high activity at low temperatures for a number of
[5]
reactions including oxidation of CO, preferential oxidation
[6]
of CO in the presence of H , oxidation of unsaturated ke-
2
[
7]
[8]
tones, hydrogenation of nitro compounds, reduction of
[
9]
NO ,
and the low-temperature water-gas shift reac-
The unique properties of ceria-supported Au cata-
While characterization of the acid properties is well estab-
lished, characterization of the basic properties by probe
molecules is not straightforward, because their application is
x
[
10–14]
tion.
lysts depend on a variety of factors, such as the dispersion of
[
15–17]
[24]
the nanoscale particles,
the oxidation state of the Au
limited to a certain range of base strength. Therefore, we
[18]
[19–21]
particles, and the type of ceria used.
decided to use the well-established probe reaction of conver-
sion of propan-2-ol. Depending on the nature and strength
of active sites, propan-2-ol is dehydrated to propene or di-
There is an on-going discussion about the nature of the
active gold species in ceria-based catalysts. Several surface
science and theoretical studies addressed in detail the
nature and mechanism of Au on ceria. Density functional
[25,26]
A
H
U
G
R
N
N
or dehydrogenated to
[
25,27,28,29]
However, it
[22]
studies on CeO2 ACHTUNGTRENNUNG( 111) slab models by Castellani et al. fa-
[30]
vored deposition of Au on oxygen atoms in different facets
of ceria surfaces resulting in either neutral or oxidized Au
propan-2-ol dehydrogenation.
High-temperature calcina-
tion of ceria (above 8008C), for example, decreased the
strength of Lewis acid sites on the surface and enhanced the
formation of basic sites suppressing propan-2-ol dehydra-
species. Using STM and XPS studies on CeO ACHTUNGTERNNUN(G 111) Baron
2
[23]
et al., on the other hand, argue that the concentration of
[31]
tion.
The main objective of the current study was therefore to
explore the influence of Au on the acid–base properties of
ceria. Acid–base properties and metal state were character-
ized by IR spectroscopy with adsorption of probe molecules
such as pyridine and CO. The catalytic properties with
regard to propan-2-ol conversion were followed in a flow re-
actor at different temperatures and for different activation
procedures as well as by in situ IR spectroscopy during the
reaction.
[
a] M. C. I. Bezen, Dr. C. Breitkopf, Prof. Dr. J. A. Lercher
Department Chemie
Technische Universitꢀt Mꢁnchen
Lichtenbergstr. 4, 85748 Garching (Germany)
Fax : (+49)89-289-13544
E-mail: johannes.lercher@ch.tum.de
[
b] N. El Kolli, J.-M. Krafft, Dr. C. Louis
Laboratoire de Rꢂactivitꢂ de Surface, UMR 7197 CNRS
Universitꢂ Pierre et Marie Curie, UPMC
4
place Jussieu, 75252 Paris cedex 05 (France)
Chem. Eur. J. 2011, 17, 7095 – 7104
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7095

supporting wafers and activated in situ at 3008C for 2 h in vacuum. The
Experimental Section
À1
spectra were recorded in transmission mode at a resolution of 4 cm
À1
over the wavenumber range 4000–800 cm . After recording the catalyst
Materials: Commercial ceria with a specific surface area of approximate-
ly 6 m g was obtained from Merck and pretreated in synthetic air at
background spectrum, pyridine was adsorbed at 0.1 mbar and 508C.
2
À1
Weakly adsorbed pyridine was removed by increasing the temperature
À1
1
00 mLmin by increasing the temperature from ambient to 6008C with
À5
from 50 to 2008C for 0.5 h in vacuum (10 mbar).
À1
an increment of 108Cmin for 8 h. The sample is referred to oxidized
The IR spectra of adsorbed CO for the gold-containing samples were re-
ceria. For comparison ceria was also pretreated under hydrogen atmos-
À1
corded in transmission mode at liquid-N
2
temperature by using a Bruker
phere at 3008C (58Cmin ) for 2 h. This sample is referred to reduced
À1
Vector 22 spectrometer with a spectral resolution of 2 cm and accumu-
lation of 64 scans. Self-supporting pellets were prepared from the as-pre-
pared sample powders and treated in situ in the IR cell. The latter was
connected to a vacuum-adsorption apparatus with a residual pressure
ceria.
The ceria-supported Au catalysts were prepared from oxidized ceria by
the deposition–precipitation (DP) method with urea, as described previ-
[
19]
ously by Delannoy et al. The Au precursor was generated by dissolving
HAuCl ·3H (1 g, 2.54 mmol) in 100 mL of deionized water
0.0254 molL ). The pretreated ceria (3 g) was suspended in 300 mL of
À6
below 10 mbar. Prior to the IR experiments, the samples were activated
4
2
O
À1
À1
in hydrogen (100 mLmin ) from RT to 3008C (28Cmin ), maintained
at that temperature for 2 h, and then evacuated for 30 min. The adsorp-
tion experiments were performed at liquid-nitrogen temperature to char-
acterize specific adsorption sites on the metal as well as on metal oxides
that interact weakly with CO.
À1
(
water and heated at 808C. Two Au loadings (0.1 and 1.0 wt%) were gen-
erated by adding the appropriate amount of the precursor solution fol-
lowed by adding the related amount of urea immediately (see Table 1).
The suspensions were stirred for 16 h at 808C. Afterwards, the materials
The conversion of propan-2-ol was fol-
lowed by IR spectroscopy in transmis-
sion mode on a Bruker IFS 88 spec-
trometer. The self-supporting pellets
2
Table 1. Characteristics of ceria and Au/CeO samples: details of preparation, gold loadings, particle sizes,
rates at 2258C to main products propene or acetone. Rates were measured at 2258C in a flow of 40 mLmin
He saturated with propan-2-ol at 138C.
À1
were heated in situ in
20 mLmin of synthetic air (oxidized
a flow of
À1
Sample
Au
precursor
Urea
addition
[g]
Au
loading
Particle
size
[nm]
Rate to
propene
[mmolg
Rate to
acetone
[mmolg
Selectivity
to acetone
[%]
2
ceria) or H (reduced ceria and Au/
À1 À1
À1 À1
[mL]
A
H
U
G
R
N
U
G
s
]
s
]
ceria) from ambient temperature to
À1
3
2
1
008C (58Cmin ), then at 3008C for
[
a]
CeO
CeO
2
2
(oxidized)
(reduced)
Au/CeO
Au/CeO
–
–
–
–
–
–
1.4
35
19
0
0.5
0
1
0
h, and were subsequently cooled to
508C in a flow of 20 mLmin He.
[
b]
–
–
–
À1
[
b]
2
(0.1)
(1.0)
0.6
6.0
0.09
0.90
0.16
0.84
308
847
100
100
The spectra were recorded in transmis-
[
b]
2
0
À1
sion mode with a resolution of 4 cm
[
a] The sample was pretreated at 5008C in He for 1 h. [b] The sample was pretreated at 5008C in H
2
for 1 h.
over the wavenumber range 4000–
À1
8
00 cm
.
After activation
a
back-
ground spectrum was taken at 1508C.
were washed with deionized water. Washing was repeated (with centrifu-
À1
A flow of 10 mLmin He was saturated with propan-2-ol at 138C
(ppropan-2-ol =25 mbar) and led over the sample. Spectra were recorded
every 2 min for 10 min before the temperature was increased stepwise
gation after each washing step) until the pH of the solution was 7 and
À
neither Au nor Cl was detected by testing with NaBH
4
and AgNO
3
, re-
spectively. Both samples were dried in vacuum at ambient temperature
for 16 h. Before use in IR characterization or reaction, the samples were
(
DT=258C) to 2258C.
reduced in H
2
at the given temperature. The samples are referred to Au/
(0.1).
For all adsorption experiments with probe molecules (pyridine, CO,
propan-2-ol), the IR spectra reported in the paper are difference spectra
obtained after subtraction of the background spectrum.
CeO (1.0) and Au/CeO ACHTUNGTRENNUNG
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
Usually, thermal reduction or calcination treatments were performed
À1
from ambient temperature to 3008C (58Cmin
)
in
a
flow of
Catalytic tests: The as-prepared samples (0.02 g diluted in 0.13 g SiC)
À1
1
00 mLmin
H
2
or synthetic air, and a plateau at 3008C was maintained
were fixed with quartz wool in a quartz-tube reactor of 4 mm diameter.
À1
for 2 h. The samples were cooled to room temperature in a flow of
The samples were first pretreated in a flow of hydrogen of 20 mLmin
À1
À1
by heating from ambient to 3008C (108Cmin ) then at 3008C for 2 h to
1
00 mLmin He.
activate the sample. Alternatively, the samples were activated in flowing
Characterization: Chemical analysis was performed by inductively cou-
pled plasma emission spectroscopy at the CNRS Center of Chemical
Analysis (Vernaison, France).
He at 5008C (oxidized ceria). Then, the samples were cooled to reaction
À1
temperature in an He flow of 20 mLmin . For reaction, a flow of helium
À1
with 40 mLmin was saturated with propan-2-ol at 138C (ppropan-2-ol
=
The specific surface areas and pore volumes were determined by physi-
sorption of N at 77 K. Measurements were carried out in a PMI auto-
2
matic BET-Sorptometer. Evaluation was done according to the BET
theory.
2
5 mbar) and led over the fixed-bed reactor. The dehydrogenation/dehy-
dration reaction of propan-2-ol was studied in inert atmosphere in the
temperature range from 150 to 2258C with steps of 258C. Every reaction
temperature was tested for 80 min. Unconverted propan-2-ol and dehy-
drogenation/dehydration products were analyzed by gas chromatography
on a Hewlett Packard 5890 (Series II) GC equipped with a flame ioniza-
tion detector. The various components were separated on a Q-column.
The crystalline structure was analyzed by XRD on a Philips XꢄPert Pro
System (CuKa radiation l=0.154 nm) at 40 kV/40 mA. Measurements
were carried out on a spinner with a 1/6-inch slit in the range 2q=20–708
with a step size of 0.058min
À1
.
For transmission electron microscopy (TEM), the sample was ground,
suspended in ethanol, and ultrasonically dispersed. An appropriate
amount of the as-prepared dispersion was dropped on a copper-grid sup-
ported carbon film. Micrographs were recorded on a JEM-2010 Jeol
transmission microscope operating at 120 kV. The average metal particle
sizes dAu were determined from measurements on about 150 particles and
Results
Characterization: The specific surface areas of the commer-
cial ceria supported samples were found to be 5–6 m g .
The final gold loadings were close to the nominal contents
2
À1
were expressed as the arithmetic mean: dAu =ꢀn
number of particles of diameter d
i i i i
d /ꢀn , where n is the
i
.
(
see Table 1).
IR spectroscopy of adsorbed pyridine was carried out on a Thermo Nico-
let spectrometer coupled with an MCT detector. For IR spectroscopic
measurements in transmission mode, the samples were pressed into self-
The particle size distribution was determined by TEM.
Size and dispersion were identified, however, only for the
7096
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Chem. Eur. J. 2011, 17, 7095 – 7104

Selective Modification of Ceria
FULL PAPER
Infrared spectroscopy with ad-
sorbed pyridine: Pyridine ad-
sorption on oxidized and re-
duced ceria surfaces was carried
out to identify the nature of
acid sites. While for oxidized
4
+
ceria mainly Ce is expected,
3
+
for reduced ceria Ce was ex-
pected to be the dominant spe-
cies. Before pyridine was ad-
sorbed at 0.1 mbar and 508C,
2
Figure 1. TEM image and particle size distribution of Au/CeO ACHTUNGTERNNUNG( 1.0).
the pretreated samples were ac-
tivated in situ in vacuum at
sample with the highest Au loading. Determination of the
particle size distribution was based on 150 particles. The
nanosized particles were well dispersed on the ceria surface
3008C to remove carbonates and water. Well-resolved bands
were obtained after subtraction of the background (spec-
trum of the activated sample) for all samples. The relevant
part of the spectra between 1650 and 1400 cm is shown in
Figure 3.
À1
(
(
see Figure 1). The main average particle size for Au/CeO2
1.0) was 1.4 nm.
X-ray diffractograms of ceria (oxidized and reduced) and
reduced Au/CeO (1.0) samples are shown in Figure 2. Re-
2
flections at 2q=28.6, 33.1, 47.5, 56.4, 59.1, and 69. 58 corre-
spond to cubic fluorite-type ceria. The characteristic reflec-
tion of crystalline Au at 2q=388 was not observed, which is
consistent with the presence of the very small gold particles
detected by TEM. Changes of the ceria phase after gold
deposition have not been detected.
Figure 3. IR difference spectra obtained after pyridine adsorption at
5
08C (A) and after evacuation at 2008C for one hour (B). a) Oxidized
ceria; b) reduced ceria; c) Au/CeO (1.0). The samples were activated in
2
ACHTUNGTRENNUNG
vacuum at 3008C after the respective pretreatment (oxidation, reduc-
tion).
For oxidized ceria, the bands of pyridine ring vibrations
appeared at 1444 (n19b), 1489 (n19a), 1575 (n8b), and 1600/
À1
1
628 cm (n8a). The occurrence of the n8a band at two dif-
[1]
ferent wavenumbers was ascribed by Zaki et al. to Lewis
acid sites with different strength. The large width at half-
height of the band at 1443 cm after evacuation suggests
À1
that indeed this band is composed of two contributions (see
arrow in spectrum a in Figure 3B). The band at 1557 cm is
Figure 2. X-ray diffractograms of a) oxidized ceria, b) reduced ceria, and
c) Au/CeO ACHTUNGTRENNUNG( 1.0) (pretreated in H
2 2
at 3008C) (fl Au diffraction peaks; *
À1
ceria fluorite diffraction peaks).
tentatively attributed to pyridinium ions (pyridine chemisor-
Chem. Eur. J. 2011, 17, 7095 – 7104
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7097

J. A. Lercher et al.
bed at Brønsted acid sites), though this band is usually ob-
[31]
served at somewhat lower wavenumbers.
On reduced ceria, the formation of a band characteristic
of pyridinium ions was not observed. Only bands of pyridine
coordinatively adsorbed on Lewis acid sites were noted. In
comparison to oxidized ceria, the n19b and n8a bands were
shifted to lower wavenumbers, indicating lower acid-site
À1
strength. The shoulder at the n8a band (1601 cm ) was not
observed. We speculate that after reduction of ceria in hy-
4
+
drogen the largest fraction of accessible Ce cations was re-
3
+
duced to Ce , so that the remaining bands after pyridine
À1
adsorption at 1630, 1601, 1490, and 1442 cm can be as-
3
+
signed to Ce in different coordination states. The width at
À1
half-height of the band at 1444 cm was much smaller in
this sample after evacuation.
Weakly bound and physisorbed pyridine was removed
after evacuation at 2008C. While the intensity of the n19a
À1
band at 1490 cm decreased drastically for reduced ceria, it
Figure 4. IR difference spectra after CO adsorption at À1708C on Au/
remained with distinct intensity in the case of oxidized ceria.
Thus, the rather strong acidity of the latter sample is con-
firmed. The downshift of the n8a band from 1612 cm for
oxidized ceria to 1602 cm for reduced ceria emphasizes
clearly the lower Lewis acid strength of Ce compared to
Ce induced by reduction in H2.
For reduced Au/CeO similar IR spectra as for reduced
ceria were obtained after pyridine adsorption. Bands of ad-
CeO
2
A
H
U
G
R
N
N
(1.0) activated under
H
2
at 3008C. Inset: IR spectrum of
À7
Au/CeO
2
A
H
U
G
R
N
N
(1.0) after activation in H
2
at 3008C. a) 4ꢅ10 mol (CO);
À7
À7
À1
b) 13ꢅ10 mol (CO); c) 22ꢅ10 mol (CO); d) 1.1 mbar (eq. CO)
À1
3
+
4
+
À1 [40,41]
2150 cm ).
The low-wavenumber band (2150–
À1
2043 cm ) shifting to lower wavenumbers with increasing
2
[23,42]
pressure is assigned to physisorbed CO.
Note that the
sorbed pyridine were found at 1441, 1486, 1573, 1598, and
IR spectrum (insert Figure 4) recorded after reductive acti-
À1
À1
1625 cm . All bands are attributed to pyridine coordinated
vation of Au/CeO showed a band at 2120 cm characteris-
2
3
+ [43]
to Lewis acid sites. Subtle shifts in the wavenumbers, such
tic of the forbidden transition of Ce
The experiment performed with Au/CeO2
shown) exhibited similar bands at identical wavenumbers
.
À1
as from 1442 to 1441 cm for the n19b band and from 1601
ACHTUNGTRENUNNG( 0.1) (not
À1
to 1598 cm for the n8a band, indicate weakly bound, that
[
32]
is, mainly physisorbed pyridine.
under reduced pressure the same bands as for reduced ceria
After heating to 2008C
and intensities to those observed with Au/CeO
A
H
U
G
R
N
U
G
2
À1
for the band at 2090 cm , which was very weak because of
the very low gold loading. This experiment strongly indi-
cates that the high-wavenumber bands located at 2170 and
3
+
remained, indicating the dominance of Ce for this sample.
À1
Infrared spectroscopy of adsorbed CO: Lewis acid sites
were additionally characterized by adsorption of CO on re-
2150 cm are to be assigned to CO adsorbed on Ce and
4
+
Ce , as well as physisorbed CO. The band at 2100–
À1
0
duced Au/CeO at liquid-nitrogen temperature) followed by
2092 cm is attributed to Au , while the presence of contri-
2
+
IR spectroscopy. Figure 4 shows the difference spectra after
butions from Au would be masked by the stronger contri-
adsorption of CO at different pressures for Au/CeO
Introduction of successive small volumes of CO at low pres-
A
H
U
G
R
N
N
(1.0).
butions from CO adsorbed on Ce cations and from physisor-
bed CO.
2
sure in pulses led first to the appearance of a band at
Adsorption of CO also induces changes in the high-wave-
number range of the spectrum of Au/CeO AHCTUNGTERNNUNG( 1.0) correspond-
2
À1
2
100 cm , which is assigned to CO adsorbed on metallic
0
[33,34]
gold (Au ).
Another band of low intensity was also ob-
ing to OH vibrations (Figure 5). Three OH bands were ini-
À1
À1
served at 2170 cm . With increasing CO pressure, the bands
at 2100 and 2170 cm increased in intensity and shifted to-
wards lower wavenumbers (2092 and 2160 cm ). A third
band (2150 cm ) visible first as a shoulder of the high-wave-
tially observed at 3722, 3678, and 3637 cm (Figure 5, curve
À1
À1
a) while the band at 3450 cm was hardly observed. Note,
À1
that the wavenumbers are shifted compared to those ob-
served in Figure 4. This arises from the lower temperature
at which the spectrum was recorded and underlines the an-
À1
number band, grew gradually in intensity and became the
most intense band at high CO pressures. The two bands at
high wavenumbers are attributed to CO stretching vibra-
tions of CO linearly adsorbed on surface metal cat-
À1
harmonicity of the vibrations. The band at 3722 cm is as-
[
35,44]
signed to OH groups coordinated to one metal cation.
À1
The two bands at 3678 and 3637 cm are assigned to two
types of doubly bridging OH groups, while a broad band
at 3450 cm (hardly observed in the present case) is usually
or to hydrogen-
The intensities of the bands at
3722, 3678, and 3637 cm decreased markedly when CO
[
35,36,37,38]
[45]
ions.
In accordance with literature, the band at high
À1
À1
wavenumber (2170 cm ) is assigned to CO coordinated to
weak Lewis acid sites such as Ce ions.
3
+
[39,40,41]
[46]
However, we
assigned to triply bridging OH species
[45,47]
cannot rule out minor contributions to these bands resulting
bonded OH groups.
+
À1
from CO adsorption on Au (exposed at a shoulder around
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Chem. Eur. J. 2011, 17, 7095 – 7104

Selective Modification of Ceria
FULL PAPER
2 2
Figure 5. IR spectra of Au/CeO ACHTUNGTERNNUG( 1.0) a) after activation in H at 3008C
and b) after adsorption of CO (1 Torr eq.); c) difference spectrum (bÀa).
was introduced (Figure 5, curve b), and at least two new
À1
bands appeared at 3586 and 3650 cm . Both are more visi-
ble in the difference spectrum (Figure 5, curve c). They arise
from OH vibrations perturbed by CO adsorbed on OH. The
Figure 6. IR difference spectra of ceria and Au/CeO
of a flow of 25 mbar propan-2-ol in He at 2258C after one hour time on
stream. a) Oxidized ceria; b) reduced ceria; c) Au/CeO ACHNUTRTGENNUNG( 1.0).
2
2
ACHTUNGERTNNUNG( 1.0) in the presence
À1
À1
band at 3637 cm is downward-shifted by around 50 cm
À1
(
band at 3586 cm ), which is a small shift compared to that
À1
of 300 cm observed, for instance, in acidic zeolite and indi-
cates a lower acid strength of ceria.
À1
were observed at 2961, 2928, and 2882 cm and are more
likely associated with adsorbed acetone or acetone precur-
sors (see below) than with isopropoxyl groups.
Infrared spectroscopy during propan-2-ol conversion: Con-
version of propan-2-ol was also followed by in situ IR spec-
troscopy. The samples were oxidized in synthetic air or re-
Characteristic carbonyl bands between 1750 and
À1
duced in H at 3008C before passing a helium/propan-2-ol
1200 cm were observed for all samples (see Figures 6 and
2
flow over them. Under these conditions, propene and ace-
tone may be formed from propan-2-ol over acid or basic
sites, respectively. Figure 6 shows the difference spectra on
admission of propan-2-ol on oxidized ceria at 2258C (curve
7), and support the existence of acetone precursors and/or
acetone coordinated to relatively strong Lewis acid sites to-
À1
gether with bands of carboxylates (1500 cm ) and hydroxy-
À1
carbonates or carbonates (ca. 1300 cm ). In detail, the dif-
a), reduced ceria (curve b), and reduced Au/CeO
A
H
U
G
R
N
N
(1.0)
ference spectra of ceria and Au loaded ceria varied marked-
ly in the range of 1680–1750 cm , indicating formation of
2
À1
(
curve c), that is, the IR spectra after that procedure minus
the IR spectra after activation.
Let us first address evidence that propan-2-ol was ad-
sorbed dissociatively. Two weak bands characteristic of
cerium alkoxides were observed at 1157 and 1114 cm on
oxidized ceria. The cerium alkoxide bands are shifted to
various surface precursors (see Figure 6). For oxidized ceria,
a band was observed at 1687 cm indicating the presence of
À1
an acetone precursor. Note that the nC=O band of weakly
À1
À1
coordinated acetone appears between 1720 and 1710 cm
[48]
on moderately Lewis acidic oxides.
A small band at
1620 cm is attributed to the nC=C stretching band and in-
À1
À1
lower wavenumbers (1153/1101 cm ) on reduced ceria,
which is in agreement with the lower acid-site strength of
cerium cations. On adding Au to ceria, the adsorption bands
of cerium alkoxide were observed at 1158/1131 cm . There-
dicates formation of propene by dehydration of propan-2-
[49]
À1
ol. The broad band at approximately 3500 cm suggests
the presence of hydrogen-bonded OH groups or of bound
water molecules.
À1
fore, propan-2-ol was concluded to be dissociatively ad-
[47]
sorbed on all samples.
After reduction of ceria, the carbonyl band of the ad-
À1
Three bands in the CÀH stretching region were found at
sorbed acetone precursor shifted to 1690 cm . Furthermore,
À1
2
963, 2929, and 2851 for oxidized ceria. These bands togeth-
a distinct shoulder appeared at 1736 cm , which is attribut-
À1
er with bands around 1157/1114 cm are characteristic of
ed to acetone with a very weakly coordinating C=O group.
[26]
isopropoxyl groups. In case of reduced ceria similar bands
In the case of reduced Au/CeO
served at 1740 and 1715 cm , attributed to two types of
weakly adsorbed acetone. The band at 1620 cm character-
2A
H
U
G
R
N
U
G
À1
were observed, but slightly shifted to higher wavenumbers
À1
(
2979, 2933, 2898 cm ). In the presence of Au, these bands
Chem. Eur. J. 2011, 17, 7095 – 7104
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7099

J. A. Lercher et al.
À1
3
688 cm disappeared, indicating the removal of physisor-
bed propan-2-ol (see Figure 7, curve a). At the same time,
À1
the bands at 3625 (negative), 1683 (positive), and 1374 cm
(
negative) were still observed, suggesting that the chemisor-
bed propan-2-ol intermediate still interacts with surface hy-
droxycarbonates. With further temperature increase to
À1
3
008C, the remaining carbonyl band at 1683 cm almost
disappeared, while the negative bands hardly changed (see
Figure 8, curve a). This indicates that part of the hydroxycar-
bonate must have also eliminated water and transformed
into bidendate carbonates.
On flushing reduced ceria with helium at 2258C (Fig-
À1
ure 7b), a carbonyl bond located at 1681 cm remained
with distinct intensity together with two negative bands at
À1
3
650 and 1380 cm indicating relatively strong adsorption
of the acetone precursor. Raising the temperature to 3008C
caused a distinct decrease of this carbonyl band and forma-
À1
tion of carbonates (1399–1229 cm , Figure 8b).
Treating Au/CeO CAHTUNGTERNN(UNG 1.0) with propan-2-ol at 2258C causes
2
no variations of the intensity of the (negative) C=O bands
À1
characteristic of hydroxycarbonates at 1370 cm
(Fig-
ure 6c), but two strong bands appeared at 1557 and
À1
[51]
1
428 cm and are attributed to surface carboxyl groups.
À1
A smaller band at 1382 cm is attributed to CÀH vibrations
of the methyl groups of these carboxylate species or to ad-
Figure 7. IR difference spectra of ceria and Au/CeO
flushing in He-flow at 2258C exposing the samples before to a flow of
5 mbar propan-2-ol in He for one hour. a) Oxidized ceria; b) reduced
ceria; c) Au/CeO (1.0).
2
ACHTUNGERTNNUNG( 1.0) samples after
[49]
sorbed acetone.
2
On flushing reduced ceria with He, a carbonyl band locat-
À1
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ed at 1681 cm remained with distinct intensity together
À1
with two negative bands at 3650 and 1380 cm indicating
istic of propene was neither ob-
served with reduced ceria nor
with Au/CeO2 ACHTUNGTRENNUNG( 1.0).
The interaction of propan-2-
ol (undissociated) with hydroxyl
groups of oxidized and reduced
ceria caused a negative band
À1
around 3650 cm
associated
with the appearance of a broad
À1
weak band around 3500 cm
(
Figure 6a and b). These
changes come together with the
strong negative band at
370 cm attributed to the in-
À1
1
teraction of propan-2-ol with
hydroxycarbonates (C=O hy-
droxycarbonates:
1317–
À1 [50]
1230 cm ). The intensity var-
À1
iation of the band at 1370 cm
is much stronger for oxidized
than for reduced ceria, whereas
the broad weak band around
À1
3500 cm
is stronger for re-
duced ceria.
After flushing the oxidized
ceria sample with helium at
2
Figure 8. IR difference spectra of ceria and Au/CeO AHCTUNGTERNNUNG
2258C, the negative band at 3008C in He-flow. a) Oxidized ceria; b) reduced ceria; c) Au/CeO
2
ACHTUNGTRENNUNG
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Selective Modification of Ceria
FULL PAPER
2
Figure 9. Arrhenius plots and rates of product formation for propan-2-ol conversion from 150 to 2508C for various pretreated ceria and Au/CeO sam-
ples. a) Formation of propene; b) formation of acetone.
relatively strong adsorption of the acetone precursor. Rais-
For the reduced Au/CeO AHCTUNGTRENNUNG( 0.1) sample, conversion of
2
ing the temperature to 3008C caused a distinct decrease of
propan-2-ol was observed already at 1508C. At all investi-
this carbonyl band and the formation of carbonates (1399–
gated temperatures solely acetone was formed. The activity
À1
À1 À1
1229 cm ) on reduced ceria. In contrast, three bands in the
increased from 18 mmolg
s
(1% conversion) at 1508C to
À1 À1
nCÀH vibration region remained in case of Au/CeO
A
T
N
T
E
U
G
308 mmolg
s
(10% conversion) at 2258C.
For the reduced Au/CeO ACHTUNGRNET(NUNG 1.0) sample, conversion of
2
2
The carbonyl bands associated with weakly adsorbed ace-
À1
tone at 1740 and 1715 cm were removed after switching to
propan-2-ol was also observed already at 1508C. At all in-
pure He flow.
vestigated temperatures solely acetone was formed. The ac-
À1 À1
tivity increased from 26 mmolg
s
(1% conversion) at
À1 À1
Catalytic conversion of propan-2-ol: The rates of formation
of propene and acetone for all investigated samples are
summarized in Figure 9. At temperatures below 2008C,
propan-2-ol conversion was not observed for oxidized ceria.
At 2008C, propene was formed with a rate of 16 mmolg
1508C to 847 mmolg
contrast to ceria, Au/CeO2ACHTUNGTRENNUNG
s
(28% conversion) at 2258C. In
(1.0) showed stable catalytic ac-
tivity even at the highest reaction temperatures.
À1 À1
While the rate of dehydration was 35 mmolg
s
with oxi-
À1 À1
À1 À1
s
dized ceria and 19 mmolg
s
with reduced ceria (at
(
6% conversion) and 98% selectivity. After increasing the
2258C), the deposition of even the smallest concentration of
gold on ceria suppressed the formation of propene. While
for pure ceria the rates of dehydrogenation were hardly
temperature to 2258C, conversion almost doubled to
3
dized ceria underwent 21% deactivation at 2258C during
À1 À1
5 mmolg
s
(14%) with 99% selectivity to propene. Oxi-
À1 À1
measurable (<0.5 mmolg s ) at 2258C, they were dramati-
80 min of propan-2-ol reaction.
cally higher for the Au/CeO2 catalysts; for example, the
For reduced ceria, the rate of propan-2-ol dehydration at
rates increased with increasing concentration of Au from
À1 À1
À1 À1
2
008C was 6 mmolg
s
(0.3% conversion), and propene
308 to 847 mmolg
s
for 0.1 and 1.0 wt% Au at 2258C.
was formed with 100% selectivity. Propene was still formed
The apparent activation energies (see Arrhenius plots in
Figure 9) for dehydration were determined to 65 and
87 kJmol for oxidized and reduced ceria, respectively. The
with 100% selectivity at 2258C, but again only with low ac-
À1 À1
À1
tivity (19 mmolg s , 0.6% conversion). Reduced ceria un-
derwent 25% deactivation at 2258C during 80 min of
propan-2-ol reaction.
apparent activation energy for dehydration of acetone was
À1
À1
47 kJmol for oxidized ceria, 66 kJmol for Au/CeO2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(0.1),
À1
and 81 kJmol for Au/CeO
A
H
U
G
R
N
U
G
2
Chem. Eur. J. 2011, 17, 7095 – 7104
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7101

J. A. Lercher et al.
À1
Discussion
3500 cm in the form of a relatively broad and structured
feature in the difference spectrum (IR spectrum of CeO2
after that procedure minus the IR spectrum of the activated
Pure CeO2 is generally described as a moderately strong
[2]
solid acid. Accordingly, IR spectroscopy on adsorbed pyri-
dine indicates the presence of different types of Lewis acid
sites and, only in case of oxidized ceria, of a low concentra-
tion of Brønsted acid sites. In consequence, after evacuation
CeO ), as well as by two negative bands at 3625 and
2
À1
1370 cm (see Figure 6a) These two bands are identified as
characteristic of surface hydroxycarbonates (OH and C=O
vibrations).
thermal decomposition of cerium carbonate in He or H
[36]
This is in accordance with the fact that the
À1
of oxidized ceria at 2008C bands at 1443 and 1547 cm are
2
[53,54]
assigned to pyridine coordinated to Lewis acid sites of mod-
starts at 1908C and is completed at 5008C.
Adsorption
4
+
3+
erate strength (i.e., Ce and Ce ) and to Brønsted acid
of propan-2-ol occurs with the simultaneous appearance of
OH stretching bands associated with ceria and a decrease in
the concentration of surface hydroxycarbonates. We specu-
sites, respectively. The minor shoulder in the peak at
À1
1
443 cm at higher wavenumber is tentatively attributed to
4
+
À1
pyridine adsorbed on Ce , and hence the main band (Fig-
late that the appearance of the OH band at 3500 cm ac-
3
+
ure 3a) to pyridine adsorbed on Ce
.
companied by formation of an adsorption band at 1650 and
À1
The nature and concentration of acid sites was changed
by reduction of ceria. The lack of the characteristic bands of
pyridinium ions indicates the absence of Brønsted acid sites.
Compared to oxidized ceria, the lower wavenumber of the
n8a band (1602 cm ) indicates that the strength of Lewis
acid sites was much lower. Thus, we conclude that at least a
major fraction of the coordinatively unsaturated Ce was
reduced to Ce . Note that the shoulder at the band at
444 cm indeed has lost drastically in intensity, and the
1620 cm indicates that propan-2-ol is dehydrated to propene
[26]
and that water and propene are adsorbed to some extent.
Acetone interacts as electron-pair donor with the Lewis
acid sites (electron-pair acceptor sites) of metal oxides. The
strength of this interaction increases with increasing Lewis
acidity of the site, and as a result the lower the wavenumber
the C=O group will be. Thus, the C=O band observed at
À1
4
+
3
+
À1
4+
1687 cm in the case of oxidized ceria (presence of Ce )
À1
1
indicates the strongest interaction with the C=O group. A
shift to 1736 cm was observed in the case of reduced ceria
À1
overall band has become narrower. This tendency was even
more pronounced with Au/CeO (1.0), as not only pyridine
3
+
A
H
U
G
R
N
U
G
(Ce ). Consequently, for Au/ceria the band observed at
2
4
+
À1
adsorbed on the stronger Lewis acidic Ce disappeared,
but also the concentration of accessible Ce
after reduction in the presence of Au.
1740 cm can be attributed to very weak Lewis acid sites
3
+
3+
+
À1
decreased
such as Ce or Au ions, whereas that at 1715 cm is at-
tributed to Ce at the Au/ceria interface having a slightly
higher Lewis acid strength.
3
+
Besides supporting these interpretations, the IR spectra of
adsorbed CO on Au/CeO (1.0) also provide information on
À1
A
H
N
R
N
U
The presence of the band at 1687 cm suggests that the
2
the nature of the Au species. A band assigned to the forbid-
CÀO bond of chemisorbed propan-2-ol resembles more a
keto group than an alcoholate. The simultaneous disappear-
ance of vibrations of hydroxycarbonates (3625 and
3
+
À1
den transition of Ce at 2120 cm appeared in the spec-
trum of Au/CeO (1.0) after activation in hydrogen at 3008C
ACHTUNGTRENNUNG
2
[44]
À1
(
(
see inset in Figure 4). The sites binding CO the strongest
1370 cm ) led us to speculate that the OH group in the hy-
À1
2170 and 2100 cm ) are attributed to adsorption of CO on
droxycarbonate interacts with the carbon atom of this
moiety (see Figure 6). It is remarkable that this complex in-
teraction seems to be reversible for propan-2-ol, as further
increase of the temperature under He flow led to desorption
of propan-2-ol (Figure 7a). If the evacuation temperature
3
+
0
Ce and on Au . The latter assignment is derived by com-
parison with different Au loadings on CeO2.
With increasing coverage, a small band and later shoulder
À1
+
is found at 2150 cm attributed to CO adsorption on Au
À1
species acting as relatively weak and soft Lewis acid sites.
was raised to 3008C, the intensity of the band at 1683 cm
À1
The intense band at 2150–2140 cm is characteristic for ad-
remaining was marginal, but the concentration of hydroxyl
À1
À1
sorption of CO on OH groups. The band at 2100 cm in-
carbonate (3620 cm ) has still been reduced compared to
creased continuously with increasing surface coverage and
partial pressure and led us to conclude that the main frac-
tion of accessible Au is in the metallic state. The fact that
the maximum of the band shifts to lower wavenumbers is
puzzling, as it indicates that sites of increasing backdonation
to CO are occupied. It is unclear at present, however,
whether this effect is induced by the adsorbed CO or the Au
the situation before the adsorption of propan-2-ol (see Fig-
ure 8a). This suggests that the decrease in intensity of the
bands of hydroxycarbonate stems not only from the reversi-
ble interaction with surface bound propan-2-ol, but also
from conversion from a hydroxycarbonate to a bidendate
carbonate. This conclusion is supported by the fact that the
bands of hydroxycarbonate do not reappear after desorption
of propan-2-ol, that is, the corresponding negative band
does not lose intensity despite desorption of all surface spe-
cies derived from propan-2-ol.
[52]
particles rearrange in the presence of CO.
The IR spectra of adsorbed propan-2-ol show important
changes of the sites at the ceria surface induced by reduc-
tion in hydrogen and by deposition of Au particles. On all
À1
The higher intensity of the band at 1690 cm with re-
materials acetone or the surface precursor of acetone is
duced CeO at 2258C suggests that the concentration of the
2
À1
formed ( n˜ C
=
O
=1687 cm ). The presence of this band (after
precursor to acetone or adsorbed acetone is higher than in
the case of oxidized ceria, that is, these species are formed
to a more significant extent.
exposing oxidized CeO to propan-2-ol at 2258C) is accom-
2
panied by the appearance of OH stretching bands around
7102
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Chem. Eur. J. 2011, 17, 7095 – 7104

Selective Modification of Ceria
FULL PAPER
The relatively high frequency of the C=O vibrations at
tone for Au/CeO is attributed to the transformation of
2
À1
4+
3+
4+
1
740/1715 cm for Au/CeO
A
H
U
G
R
N
U
G
Ce to Ce or the blocking of Ce . This demonstrates
clearly that the reaction path to propene via dehydration of
2
tion of acetone with the catalyst surface is relatively weak.
Note that, despite the tenfold higher conversion of propan-
-ol to acetone in the catalytic flow experiment, the surface
4
+
propan-2-ol requires the presence of strong Ce cations.
The rate of dehydrogenation, in contrast, is related to the
2
coverage by acetone or its precursor is quite low, in line
with the weak interaction between acetone and the catalyst
surface.
concentration of Au. We tentatively conclude at present that
+
the formation of Au on the Au/CeO interface creates very
2
weak and soft Lewis acid sites, which enhance hydride ab-
straction. This strongly facilitates the last step in the catalyt-
Overall, dehydrogenation of propan-2-ol is initialized by
dissociative adsorption of the alcohol on an acid–base site,
4
+
ic dehydrogenation. At the same time, elimination of Ce
3
+
4+
that is, on exposed Ce /Ce and oxygen surface atoms.
This is supported by the fact that bands characteristic of
and the presence of Au reduces the retention of acetone by
decreasing the formation of carboxylates and thus stabilizing
the catalytic activity. These carboxylates form from ad-
sorbed acetone on most basic mixed oxides and are charac-
À1
cerium alkoxide (1158/1114 cm ) are observed for all sam-
ples in the situ IR experiments. The low intensity of these
bands points to low surface concentrations of these species.
However, the results also suggest that the interaction of
propan-2-ol varies between oxidized and reduced ceria as
À1
terized by IR bands observed at 1557 and 1428 cm attrib-
À
uted to asymmetric and symmetric nCOO vibrations, re-
[48]
spectively.
well as with Au/CeO2 ACHUTNGRNENUG( 1.0). Under reaction conditions, the
interaction of propan-2-ol with surface OH groups only re-
À1
sulted in a negative band near 3650 cm in the case of re-
Conclusion
duced ceria and Au/CeO ACHTUNGTERNN(UNG 1.0), hardly providing evidence
2
for the formation of new OH bands (Figure 6c). In contrast,
the band at 3500 cm appearing in the case of oxidized and
Deposition–precipitation of Au on a CeO support modifies
2
the surface by in situ formation of Ce and Au at the Au/
À1
3+
+
reduced ceria indicates formation of hydrogen-bonded OH
groups (from which water may be formed) during the dehy-
CeO interface. We conclude that Au interacts selectively
2
4
+
with Ce to eliminate its accessibility and function as a
strong Lewis acid. Simultaneously, it induces formation of
dration of propan-2-ol to propene. Formation of carbonates
À1
+
(
1318–1235 cm ), which was observed for both ceria sam-
very soft Lewis acid sites, presumably by generation of Au
ples, was markedly less pronounced in the presence of Au.
The type of catalytic elimination from propan-2-ol (see
Figure 10) changes from elimination of water (100% selec-
at the Au/CeO interface. The latter site facilitates hydride
abstraction.
The combination of these effects leads to suppression of
the dehydration of propan-2-ol that dominates with the
2
tivity to propene) for CeO to dehydrogenation (100% se-
2
lectivity to acetone) for Au/CeO samples. During the con-
parent oxidized CeO . This reaction route is induced by
2
2
version of propan-2-ol with ceria, the catalysts were marked-
ly deactivated (>20%) with time on stream, while the Au/
strong interaction of the oxygen atom of propan-2-ol with
4
+
Ce
and the presumably simultaneous elimination of a
CeO catalysts were extremely stable.
Combining the acid–base characterization and the surface
chemistry of propan-2-ol, the high selectivity towards ace-
proton at the b-carbon atom. Reduction of CeO lowers the
2
2
4
+
rate of dehydration by reducing the concentration of Ce
,
but it does not eliminate this reaction pathway completely.
3
+
The presence of a high concentration of Ce alone, howev-
er, is apparently not able to catalyze the dehydrogenation of
propan-2-ol. On the surface, acetone or its precursor is
formed, but irreversibly bound to the CeO surface under a
2
wide variety of operating conditions.
The presence of Au eliminates the deactivation of CeO2
associated with the presence of at least moderately strong
4
+
and relatively hard Ce Lewis acid sites. Only dehydrogen-
ation of propan-2-ol is catalyzed, and the dehydration path-
way is completely blocked. In this reaction pathway, the al-
cohol is dissociatively adsorbed, forming an alkoxy species
3
+
at Ce and a hydroxyl group. As the rate of dehydrogena-
tion increases with increasing concentration of Au and the
main path of dehydrogenation of propan-2-ol on the Au par-
ticles can be ruled out (low activity for catalyzing the re-
verse reaction), we suggest that the Au particles provide
weak and extremely soft Lewis acid sites at the support/
metal interface that facilitate the acceptance of hydride ions
and thus accelerate dehydrogenation.
Figure 10. Schematic mechanism of propan-2-ol adsorption on ceria.
a) Formation of acetone over isopropoxide intermediate; b) formation of
propene over carbenium-ion intermediate.
Chem. Eur. J. 2011, 17, 7095 – 7104
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7103

J. A. Lercher et al.
The interplay of selective deposition of Au and subtle
modification of the support has thus led to a uniquely basic
material. The results show indeed that the leads from DFT
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