11432 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Claus et al.
Table 1. Designation and Texture Properties of the Titania- and
Zirconia-Supported Gold Catalysts Used in This Study
Crystallinity and crystal structure of the samples were evaluated from
selected area electron diffraction (SAED) patterns as well as by high-
resolution electron microscopy (HREM) done at a JEM 4000EX
operating at 400 kV. For electron microscopy examination, the catalyst
samples were dissolved in 2-propanol, dispersed carefully in an
ultrasonic bath, and then deposited on carbon coated copper grids. Image
processing for, for example, contrast enhancement and image evaluation,
was done at digitized electron micrographs by means of the programs
Digital Micrograph (Gatan) and NIH Image.25
catalyst
designation
preparation
method
gold content surface area pore vol
(wt %)
(m2/g)
42
(mL/g)
Au/TiO2-DP deposition-
1.7
0.35
precipitation
Au/TiO2-I
Au/TiO2-SG sol-gel
Au/ZrO2-F coprecipitation
impregnation
2.9
4.8
1.0
42
117
151
0.38
0.17
0.37
Electron Paramagnetic Resonance (EPR). EPR spectra of samples
Au/TiO2-SG, Au/TiO2-DP, Au/TiO2-I, Au/ZrO2-F, and TiO2 were
recorded with the cw spectrometer Elexsys 500-10/12 (Bruker) in
X-band (ν ) 9.515 GHz) at 77 and 293 K using a finger dewar. The
magnetic field was measured with reference to a standard of 2,2-
diphenyl-1-picrylhydrazyl hydrate (DPPH). Reduction of the catalysts
was performed in flowing hydrogen (45 mL min-1) at 723 K (samples
Au/TiO2-SG and TiO2) and 573 (sample Au/ZrO2), respectively, with
an EPR flow cell consisting of two coaxial tubes similar to that
described by Mesaros and Dybowski.26 EPR spectra were recorded after
treatment in H2 without contact to ambient atmosphere.
ammonia. After stirring for 6 h and aging for 3 h, the slurry was filtered
and washed with distilled water until it was chloride-free (proved by
addition of silver nitrate). Drying (373 K, 12 h), calcination in flowing
air (573 K) for 8 h, and reduction in flowing hydrogen at 573 K for 3
h gave the final catalyst, Au/TiO2-DP.
In the case of the impregnated catalyst, Au/TiO2-I, the gold precursor
solution consisted of AuCl3 dissolved in distilled water (0.25 mol L-1
)
which was added dropwise to the titania (0.6 mL/g of TiO2). For this
catalyst, the same treatment conditions in drying, calcination, and
reduction as described above for Au/TiO2-DP were applied.
UV/VIS-DRS. UV/VIS-DRS measurements were performed by a
Cary 400 spectrometer (Varian) equipped with a praying mantis diffuse
reflectance accessory (Harrick). All samples were diluted with BaSO4
white standard (Merck) in 1:50 ratio.
The sol-gel-derived titania-supported gold catalyst, Au/TiO2-SG,
was prepared by using tetrabutoxytitanium(IV) (TBOT, purchased from
Aldrich), gold acetate (99.9%, from Alfa), methanol (HPLC grade),
and distilled water as starting components in the following procedure.
At first, according to Schneider et al.,23 a solution of TBOT (32 g) in
methanol (120 mL) was mixed with a second solution consisting of 30
mL of methanol, 6.78 mL of water, and 0.5 mL of HNO3 (65 wt %).
At room temperature and under vigorous stirring, a lucid gel was
obtained within 20-30 min. Then, after aging for 2 h, the titania gel
was redispersed in 250 mL of methanol to give a clear solution (titania
precursor sol). Gold acetate (770 mg) was dissolved in a mixture of
125 mL of methanol and 12.6 of water and sonicated (0.5 W cm-2, 35
kHz) for 5 min (Au precursor solution). By sonochemical preparation,
stable colloidal metal dispersions and nanoparticles with narrow particle
size distributions have been formed.24 Subsequently, the Au precursor
solution was added dropwise to the titania precursor sol, and this
mixture was vigorously stirred for 2 h. To evaporate the solvent prior
to drying, the viscous solution was stirred in a rotary evaporator at
323 K until gelation occurred. The resulting solid was deep purple. It
was dried at 453 K for 12 h in an oven, followed by calcination in
flowing air (90 mL min -1) at 673 K (4 h) and reduction in flowing
hydrogen (90 mL min -1) at 723 K (3 h) to give the catalyst Au/TiO2-
SG. For comparison, pure sol-gel-derived TiO2 was prepared by the
same procedure but without introducing the gold precursor solution.
A zirconia-supported gold catalyst, noted as Au/ZrO2-F, was
prepared by coprecipitation. Two solutions were prepared. The first
solution consisted of 141.1 mg of HAuCl4 dissolved in 10 mL of H2O
and the second of 10 g of ZrOCl2‚8H2O (99.99 wt %, from Aldrich) in
35 mL of H2O. The latter was added into a beaker to the aqueous
solution of HAuCl4 followed by adding 14.2 mL of ammonia (4.98 N)
under stirring. The resulting coprecipitate was filtered, washed with
water, and then dried at 393 K for 8 h. Calcination in flowing air at
573 K for 8 h and reduction in flowing hydrogen at 573 K for 5 h
gave the final catalyst, Au/ZrO2-F.
Selective Hydrogenation of Acrolein. Gas-phase hydrogenation of
acrolein (AC, purchased from Aldrich) was carried out in a computer
controlled fixed-bed microreactor system which has been described in
detail elsewhere.27 This equipment allows the performance of high-
pressure gas-phase hydrogenations of unsaturated organic compounds,
which are usually liquids with low vapor pressures at standard
conditions (STP). The reactor effluents were on-line analyzed by an
HP 5890 gas chromatograph, equipped with a flame ionization detector
and a 30-m J&W DB-WAX capillary column. The gold catalysts were
reduced in situ at the conditions described above. The reaction
conditions of the acrolein hydrogenation were as follows: temperature
range 453 K e T e 593 K, total pressure p ) 2 MPa, molar ratio
H2/AC ) 20, reciprocal space time W/Fo ) 15.3 gcat h mol-1, where
Ac
W is the weight of catalyst (0.23 g; particle size 0.2-0.5 mm) and F
is the molar flow of acrolein. Temperatures in the range of 513-553
K were required for zirconia-supported gold catalysts to be active, and
conversion data up to 10% were obtained. Using Au/TiO2 catalysts,
acrolein hydrogenation to allyl alcohol and propanal started at lower
temperatures around 473 K. For the purpose of comparison, the catalytic
data are reported at 513 K, which is close to a common reaction
temperature for all catalytic runs. Over the titania-based catalysts,
increased formation of 1-propanol due to consecutive hydrogenation
of propanal and allyl alcohol was observed above 553 K. Moreover,
the extent of C2 and C3 hydrocarbon formation also increased at higher
temperatures because of the increased extent of side reactions (decar-
bonylation of acrolein and propanal, dehydratization of allyl alcohol).
All the gold catalysts exhibited good activity maintenance during the
time on stream (catalytic runs over 3 h). Minimal activity loss was
only observed during the first 10 min in the case of Au/TiO2 catalysts.
This could be due to the formation of acrylic acid and formyldihydro-
pyran, which were detected as byproducts. At 553 K, they were formed
with a maximum selectivity of 8%. In contrast to that, acrolein
hydrogenation at temperatures between 513 and 553 K over Au/ZrO2
catalysts gave allyl alcohol and propanal as main products beside traces
of 1-propanol and hydrocarbons. Note, that all given catalytic data
(specific activities, selectivities) are based on steady-state behavior of
the catalysts. The selectivities of reaction products were calculated from
moles of product formed per moles of acrolein converted, and the
catalyst activities were expressed as specific activities (on a gram of
gold basis).
The gold contents of the catalysts were determined by atomic
emission spectroscopy with inductively coupled plasma (AES-ICP,
Perkin-Elmer Optima 3000XL) after dissolving the materials in a
mixture of HF/HNO3 by means of a MDS-2000 microwave unit (CEM).
The designations of the supported gold catalysts used in this study
and their metal contents are compiled in Table 1.
Electron Microscopy (TEM, HREM). Qualitative and quantitative
characterization of the catalysts were carried out using a JEM 100C
operating at 100 kV for TEM in bright-field and dark-field modes.
(23) Schneider, M.; Duff, D. G.; Mallat, T.; Wildberger, M.; Baiker, A.
J. Catal. 1994, 147, 500-514.
(24) (a) Okitsu, K.; Mizukoshi, Y.; Bandow, H.; Maeda, Y.; Yamamoto,
T.; Nagata, Y. Ultrason. Sonochem. 1996, 3, 249-251. (b) Okitsu, K.;
Nagaoka, S.; Tanabe, S.; Matsumoto, H.; Mizukoshi, Y.; Nagata, Y. Chem.
Lett. 1999, 271-272. (c) Salkar, R. A.; Jeevavandam, P.; Aruna, S. T.;
Koltypin, Y.; Gedanken, A. J. Mater. Sci. 1999, 9, 1333-1335.
(25) Rasband, W. NIH Image; public domain software, U.S. National
Institute of Health, FTP: zippy.nimh.nih.gov.
(26) Mesaros, D. V.; Dybowski, C. Appl. Spectrosc. 1987, 41, 610-
612.
(27) Lucas, M.; Claus, P. Chem.-Ing.-Tech. 1995, 67, 773-777.