80
M. Kuhn et al. / Catalysis Communications 57 (2014) 78–82
emission spectrometry (ICP-OES) with a Vista-MPX (Varian). X-ray
powder diffraction patterns of all samples were obtained by STOE-
STAD-IP diffractometer using Cu-Kα1-radiation (1.5405 Å, 40 kV,
40 mA) and a Ge(111)-monochromator.
nanoparticles containing various metal compositions (0:100, 30:70,
50:50, 70:30 and 100:0 molar ratios of [Pd(NO3)2]:H[AuCl4]) were com-
pared with the spectra of respective monometallic particle dispersions
(Fig. 1). The molar ratio of stabilizer 2 to metal salts was maintained
constant at 1:100. Fig. 1a shows a series of UV–vis spectra for different
ratios of Pd and Au monometallic nanoparticles. With all samples that
contain monometallic Au nanoparticles a characteristic surface plasma
adsorption at approximately 540 nm was observed [30]. The adsorption
peak increased with decreasing Pd/Au ratio. The sample of pure mono-
metallic Pd nanoparticles revealed as expected no adsorption between
350 and 800 nm [30]. The UV–vis spectra for the bimetallic Pd/Au nano-
particles, as illustrated in Fig. 1b, exhibit a different absorption behavior
but no surface plasma adsorption is found. These investigations indicate
that the dispersions of nanoparticles prepared by co-precipitation of
Pd2+ and Au3+ do not contain monometallic Au particles, but nanopar-
ticles with bimetallic structure. This is consistent with spectra reported
for bimetallic Pd/Au nanoparticles [31,32].
TEM images were used to determine the individual particle size, size
distribution and morphology of the appropriate dendrimer-stabilized
Pd/Au nanoparticles (Fig. 2). In all cases almost regular spherical shaped
nanoparticles were formed. Moreover, the alloyed structure of the re-
ceived nanoparticles could be confirmed by electron diffraction patterns
and XRPD (for more information see supplementary data S1). The elec-
tron diffraction ring pattern affiliated by TEM verifies a face centered
cubic (fcc) structure and exhibits a lattice constant of 4.0 Å, which lies
between the lattice constants of pure Au (4.08 Å) and Pd (3.89 Å) [33].
In general, Pd/Au nanoparticles with a particle size between 6.0
2.3. Fixed bed reactor and the reaction conditions
All experiments for catalyst performance in VAM synthesis were car-
ried out in a continuous flow system. The reactor consisting of a
0.25 inch inner diameter and 10.25 inch length stainless steel tube filled
with 0.4 g catalyst and 8.0 g SiC was impregnated with 4.0 wt.% KOAc.
The feed mixture contains 50.0 vol.% ethylene, 6.5 vol.% oxygen,
18.0 vol.% acetic acid, as well as 1.5 vol.% cyclohexane as internal stan-
dard and nitrogen as diluting agent. Acetic acid and cyclohexane were
vaporized in the presence of ethylene and nitrogen in an evaporator at
190 °C. All tubes were heated to 160 °C to prevent a condensation of
the reactants and products. The gas hourly space velocity (GHSV) was
−1
−1
varied from 15 Nm3 kg h−1 to 20 Nm3 kg h−1 and the reactor tem-
cat
cat
perature from 150 °C to 170 °C at an excess pressure of 8 bar. The mix-
ture leaving the reactor was analyzed by gas chromatography (CP-3800,
Varian, FID). After condensing volatile compounds at −9 °C, the
amounts of oxygen (Oxynos 100, Rosemount) as well as CO and CO2
(Binos 1001, Rosemount) were determined continuously in the exhaust
gas. All experiments were performed for 5 h at each temperature and
GHSV, respectively. Furthermore a pre-run period of 5 h at 150 °C and
−1
20 Nm3 kg h−1 was upstreamed.
cat
(
1.2) and 10.4 ( 1.7) nm were obtained. The size of the gained
3. Results and discussion
dendrimer-stabilized Pd/Au nanoparticles thereby depends on the
type of dendrimer and the correspondent dendrimer-to-metal ratio.
From Fig. 2 it can be seen that with a constant molar dendrimer-to-
metal ratio of 1:100 and a constant Pd:Au ratio of 70:30 the 2-Pd/Au
nanoparticles (Ø 8.1 1.5 nm) are smaller than the 1-Pd/Au nanopar-
ticles (Ø 9.6 1.4 nm). Obviously, dendrimer 2 is a better stabilizer be-
cause of its higher number of terminal ethylene glycol moieties,
preventing further particle growth.
To confirm a bimetallic structure for the Pd/Au nanoparticles UV–vis
absorbance spectroscopy was used. Therefore the spectra of bimetallic
Table 1 shows that the particle size can also be controlled by molar
dendrimer-to-metal ratio. As expected, an increasing stabilizer ratio
leads with both dendrimers to decreasing nanoparticle sizes. The mor-
phology of dendrimer-stabilized Pd/Au nanoparticles is not affected by
the type of stabilizer or dendrimer-to-metal ratio.
The respective dendrimer-stabilized Pd/Au nanoparticles were used
as metal precursors for the preparation of silica-supported Pd/Au cata-
lysts and tested for their productivity in VAM synthesis. Therefore the
metallodendrimers were immobilized onto a silica support by liquid
phase reduction with hydrazine and calcined. All catalysts were pre-
pared with a constant molar dendrimer to metal ratio of 1:100, a con-
stant Pd:Au ratio of 70:30 and a nominal Pd loading of 2.5 wt.%. The
catalyst loadings were determined by ICP-OES. The results approve a
complete fixation of the noble metals onto the supporting material
(for more information see Supplementary data S2).
The XRPD patterns of the calcined catalysts (Fig. 3) show the expect-
ed reflexes of a Pd/Au alloy at 39°, 46° and 67°. Additionally there are re-
flexes caused by silica (60°, 71°) and by the formation of PdO (34°, 55°)
[34]. The crystallite size of the Pd/Au alloy calculated by Scherrer rises
with increasing calcination temperature from 5 nm at 300 °C up to
24 nm at 550 °C (for more information see Supplementary data S3).
This observation indicates the sintering of single crystallites to larger ag-
glomerated particles at higher temperatures.
Furthermore, Fig. 3 reveals an increase of crystallinity and an adjust-
ment of the Pd/Au reflexes to smaller angles with increasing calcination
temperature. This adjustment is caused by the formation of PdO at high
temperatures, which exhibits another crystal structure than Pd and Au
[35].
Fig. 1. UV–vis spectra of (a) monometallic 2-Pd and 2-Au nanoparticles and (b) bimetallic
2-Pd/Au nanoparticles with various molar metal ratios and constant dendrimer-to-metal
ratio of 1:100.
Due to economic reasons, in order to receive small crystallite sizes
and to inhibit the formation of PdO, a low calcination temperature