T.T.N. Nguyen et al.
AppliedCatalysisA,General549(2018)170–178
ETEM G2 80-300 KV equipped with an objective Cs aberration cor-
rector. Elemental chemical analyses were achieved using an energy-
dispersive X-ray (EDX) analyzer (SDD X-Max 80 mm2 from Oxford
Instruments™). The samples were dispersed in ethanol using a soni-
cator. A drop of the suspension was dripped onto a carbon film sup-
ported on a copper grid; EDX studies were carried out using a 15 nm
probe to analyze the particles.
Table 2
Physico-chemical characteristics of the bulk catalysts: SSA: specific surface area.
Catalysts SSABET
(m2 g−1
ICP analysis
(wt%)
XRD Data
)
Cell par. (nm) Cryst. size
(nm)
Ag
Ag-Au
3.0
11.9
Ag: 100.0
Ag: 3.14 Au: 96.15 Zr:
0.71
0.40869(2)
0.40807(2)
30
16
XPS measurements were performed on the self-supported catalysts
with a Kratos Axis Ultra DLD spectrometer. All the data were acquired
using monochromatic Al Kα radiation Ehν (Al Kα) = 1486.6 eV (10 mA,
15 kV) as the photon source. The analyzer was operated with a hybrid
mode (combined electrostatic and magnetic lens) under ultra-high va-
cuum (5 × 10−9 torr), a 160 eV pass energy for acquisition of survey
and a 20 eV pass energy for acquisition of high-resolution core-level
spectra of Au4f, Ag3d, Cu2p, O1s, C1s. Charge neutralization was used
to compensate charges effects on samples and the area analyzed was
700 μm × 300 μm. The binding energies were calibrated to the C-(C, H)
components of the C1s band fixed at 284.6 eV. The experimental pre-
cision on the quantitative measurements was considered to be around
10%. In order to determine quantitatively the top surface of the alloys,
the low-energy ion scattering (LEIS) technique was used. Experiments
were conducted in the same apparatus than that used for XPS experi-
ments. The analysis was performed with 1-keV 4He+ ions at 25 °C at a
scattering angle of 135°. The primary 4He+ beam intensity was 30 nA,
focused on an impact spot of about 0.5 mm diameter. The relative
sensitivity factors for Cu and Ag, SCu/SAg has been taken equal to 0.23
[25,28] and for Au and Ag, SAg/SAu has been taken equal to 1.1 [29].
The Temperature-Programmed Desorption experiments (TPD) were
achieved with BELCAT-M apparatus [30]. 100–200 mg of sample was
heated to 350 °C under He flow (50 ml min−1) and exposed to the same
oxygen flow for 30 min. The catalyst was then cooled to room tem-
perature under oxygen flow. The tubing was flushed with He, bypassing
the reactor, until no oxygen was detected in the effluent Temperature-
programmed desorption of adsorbed oxygen was then recorded by
heating the sample with heating rate of 8 °C min−1 under He flow
(50 ml min−1). The oxygen desorbing from the sample was con-
tinuously monitored by gas chromatography. The output was then
plotted against temperature, to determine the TPD profile. It has been
checked that no CO2 or H2O were formed during the TPD experiments.
The catalysts have been tested for the oxidative dehydrogenation of
allyl alcohol between 200 and 400 °C under atmospheric pressure using
Au
Ag-Cu
6.3
3.9
Au: 100.0 Zr < 1
Ag: 97.5 Cu: 2.5
0.40797(1)
0.40810(1)
20
53
3. Results
3.1. Self-supported Ag, Ag0.975Cu0.025, Au and Au0.95Ag0.05 catalysts
The studied catalysts are presented together with their main char-
acteristics in Table 2. In the case of the prepared gold-silver sample,
chemical analyses showed that the silver content was slightly lower
than the pre-set value for the preparation, indicating a slight loss of
silver during the lixiviation process or during the acidic dissolution of
the sample for analysis. The formation of a small fraction of silver
precipitate was indeed observed after dissolution. Specific surface areas
equal to 3.0 and 6.3 m2 g−1 were determined for the Ag and Au, re-
spectively, whereas the specific surface areas of the alloys including Au
and Cu were respectively higher and lower, at 11.9 and 2.2 m2 g−1. All
of the X-ray diffraction patterns were similar. Both Ag and Au crystal-
lized with the same face-centred cubic structure, and the Ag-Au alloy
formed a solid solution over the full range of compositions. At the op-
posite, even if pure Ag and Cu crystallize with the same structure, they
form an immiscible alloy over the entire composition range, at least at
temperature lower than 200 °C [31]. Calculated unit cell parameters
were close to the published values: 4.0855(1) Å for Ag [32], and
4.0796(1) Å for Au [33]). The cell parameters for the Au0.95 Ag0.05 and
Ag0.975 Cu0.025 alloys were closer to those of pure Au and Ag, respec-
tively. The size of the crystallites was estimated from the broadening of
X-ray diffraction peaks, using the Scherrer method to fit the complete
patterns, and was in agreement with the measured values of specific
The pure Ag and Au0.95 Ag0.05 alloy was studied using scanning
electron microscopy, showing that the silver and gold silver catalysts
are micrometric powders composed of grains ranging in size between a
few μm and 200 μm, with a sponge-like morphology composed of a
network of interconnected channels, as shown on the SEM images
a
stainless steel plug-flow micro-reactor. The catalyst mass
(100–200 mg) was chosen to be able to obtain complete or near com-
plete allyl alcohol conversion. Prior to the reaction, the samples were
pre-heated at the reaction temperature for 15 min in flowing air
(75 ml min−1). Allyl alcohol, which was introduced into the reaction
system using a syringe pump, was vaporized in the air and He flow to
the reactor using a homemade vaporization device. Reactor was a fixed
bed reactor, which was placed in a furnace. The pipes were heated to
eliminate condensation of allyl alcohol and liquid products. The allyl
alcohol was always fed at a rate of 0.0215 mol h−1, and the air gas flow
is 20 ml min−1 giving a molar relative composition allylic alcohol/
O2 = 2); the effect of oxygen partial pressure was studied by changing
the N2/O2 ratio and the effect of contact time by changing the catalyst
mass. The gas products (CO and CO2, propene) were analyzed online,
using a gas chromatograph. N2 was used as internal standard. Organic
substrates were be trapped in ethanol first at 0 °C and then at low
temperature (−23 °C) thanks to the cryostat during the reaction and
analyzed off line using gas chromatographs equipped with FID detec-
tors and a Nukol column or a ZBwax plus column. Air, high purity
helium gas and allyl alcohol from Aldrich with the purity higher 99%
were used for the experiments. Ag catalysts, rate of allyl alcohol con-
version was calculated per m2 of alloys and m2 of Ag considering the
silver surface atomic content of the alloys determined from LEIS data.
The surface composition of the Au0.95 Ag0.05 and Ag0.975 Cu0.025
alloys was studied by XPS and LEIS spectroscopy, in order to char-
acterize their uppermost surface layer. In order to check for possible
changes in the surface composition during catalytic testing, alloys were
also characterized by LEIS after catalytic testing. The results of these
tests are presented in Table 3. Quantitative analysis of the Au0.95 Ag0.05
sample revealed the absence of residual Zr and a strong enrichment in
Ag. LEIS revealed that this enrichment reached 35.5% in the topmost
atomic surface layer. Small amounts of impurities such as O were de-
tected. Following catalytic testing, a slight change in surface composi-
tion, associated with a higher Ag content, was detected (38.5%). These
results are in agreement with those published by T. Déronzier et al.
concerning poor Ag alloys (from 2 to 6% silver) [29]. These authors
accounted for the surface Ag enrichment by a lower surface free energy
for silver, and a stronger interaction between silver and oxygen during
Concerning the AgCu alloy and based on thermodynamic para-
meters such as surface tension or size effect, we would expect a strong
silver segregation. Nevertheless, LEIS analysis revealed a strong en-
richment of the Cu content (37%) in the topmost atomic surface layer.
172