I. Kaskow, et al.
Molecular Catalysis xxx (xxxx) xxxx
bimetallic catalysts [10]. The activity of bimetallic AuAg/TiO
2
samples
needed to get a final 1 wt. % of Ag). The final materials were obtained
increased as a result of modification of the support with donor additives
by drying at 373 K and calcination at 773 K (MgO and ZnO based cat-
(
La, Mg), while acceptor additives (Ce, Fe) decreased it. Moreover, it
alysts) or at 673 K (Nb
2
O supported catalysts) for 4 h
5
has been proved that the presence of La in the support inhibits the
formation of Au-Ag alloy. Thanks to the stabilization of the active
centres in their optimal electronic state, the high activity and stability
2.2. Characterization of the materials
of AuAg/La/TiO
2
catalyst in n-octanol oxidation was obtained.
The XRD patterns of the supports and calcined catalysts were ob-
The aim of this work was to identify the main common feature that
could explain all these observed effects of the support nature and the
preactivation conditions on the surface and catalytic properties of na-
nometal catalysts for n-octanol oxidation in the liquid phase under mild
conditions, and more specifically of bimetallic AuAg catalysts. To attain
that goal, at a variance of previously published papers focused on the
surface modification of one specific support, in this paper we in-
vestigated three very different single oxide supports, looking for an
homogeneous metal-support interaction for each catalyst.
tained on a D8 Advance diffractometer (Bruker) using CuK radiation
α
(λ = 0.154 nm), with a step size of 0.05° in the 2Θ = 6 - 60° range.
The N
2
adsorption/desorption isotherms were obtained at 77 K
using a Micromeritics ASAP 2020 Physisorption Analyzer. The samples
were pre-treated in situ under vacuum at 573 K. The surface area was
calculated by the BET method, whereas the pore volume and the dia-
meter were estimated according to BJH method from the adsorption
branch of the isotherm.
The actual metal loadings in the samples were measured by in-
ductively coupled plasma atomic emission spectroscopy (ICP-OES) with
a Perkin Elmer ICP-OES Optima 3300 DV spectrometer.
Thus, three metal oxides (MgO, ZnO and Nb
2
O ), representative of
5
different types (basic, amphoteric and acidic, respectively), were used
in the present work as model supports for gold and silver to evaluate the
role of surface properties of the supports on the effectiveness of gold-
silver catalysts in n-octanol oxidation with oxygen.
UV-vis spectra of the catalysts calcined and pre-treated in hydrogen
flow (samples denoted with the suffix –H) were recorded using a
Varian-Cary 300 Scan UV–vis spectrophotometer. Powdered samples
were placed in a cell equipped with a quartz window. The spectra were
recorded in the range from 800 to 190 nm (the wavelength resolution
was 0.4 nm). Spectralon was used as a reference material.
2
. Experimental
2.1. Catalysts preparation
X-ray photoelectron spectroscopy (XPS) was performed in a SPECS
(
Germany) photoelectron spectrometer equipped with a monochro-
2
.1.1. Materials
matic microfocused Al K X-ray source (1486.6 eV). The binding energy
of gold species in monometallic samples were corrected to C1s, with the
known binding energy of 284.6 eV.
The chemical compounds used in this study are: magnesium oxide
(
Aldrich, ≥ 99%), niobium pentoxide (hydrated, CBMM-Brazil),
pluronic P123 ((poly(ethylene glycol)-block-poly(propylene glycol)-
block-poly-(ethylene glycol)-block), zinc acetate (anhydrous,
Aldrich, > 99.0%), oxalic acid (Aldrich, > 99,999%), distilled water,
methanol, (3-aminopropyl)trimethoxysilane (APTMS, Aldrich, 95%),
For transmission electron microscopy (TEM) measurements a JEOL
2000 electron microscope operating at 80 kV was used. The materials
calcined and pre-treated in hydrogen flow (samples denoted with the
suffix –H) were deposited on a carbon-coated grid and transferred to the
above-mentioned microscope. The sizes of metal crystallites were esti-
mated from TEM images by measuring about 100 particles on the
images and plotting crystallite sizes distribution. The standard devia-
tion of these measurements was calculated from the formula:
dry toluene (POCH, ≥99.8%), sodium borohydride (NaBH
4
, Aldrich,
O, Aldrich, 99,995%) and silver
, Aldrich, ≥99%). The chemicals were not purified be-
≥
98%), chloroauric acid (HAuCl
4
∙ 4H
2
nitrate (AgNO
3
fore use.
2
1/2
S=[Σ (d
sured, d
i
– dav) /Σ n
i
]
, where: n – number of crystallites mea-
i
2
.1.2. Zinc oxide synthesis
i
– size of the indicated crystallite, dav – mean crystallite size.
Zinc oxide was synthesized by the method described in [17]. Firstly,
The conversion (dehydration and dehydrogenation) of 2-propanol
(POCH) was performed in a microcatalytic pulse reactor inserted be-
tween the sample inlet and the column of an SRI 310C chromatograph.
The catalyst bed (0.1 g with a grain size fraction of 0.5 < Ø < 1 mm)
pluronic P123 (8.6 g) was dissolved in methanol (83 mL) and zinc
acetate (10 g) in distilled water (83 mL). These two solutions were
mixed together on a magnetic stirrer for one hour. Meanwhile a solu-
3
−1
tion of acetic acid (4.9 g) in water-methanol (83 mL of H
2
O, 83 mL of
was first activated at 673 K for 2 h under nitrogen flow (40 cm min ).
The 2-propanol conversion was studied at 473, 523 and 573 K using 3 μl
MeOH) was prepared and slowly added dropwise to the mixture of zinc
acetate and pluronic P123 upon stirring. The white precipitate formed
was filtered and washed with water and methanol. The solid obtained
was dried at 333 K and calcined in 723 K for 4 h to get the final product.
3
−1
pulses of alcohol under nitrogen flow (40 cm min ). The reactant and
reaction products: propene, 2-propanone (acetone) and diisopropyl
ether were analysed using a SRI 310C gas chromatograph on line with
microreactor. The reaction mixture was analysed on 2 m column filled
with Carbowax 400 loaded on Chromosorb W in nitrogen flow (40
2
.1.3. Functionalization of the supports (ZnO, MgO, Nb
The three oxides were grafted with (3-aminopropyl)trimethox-
ysilane to functionalize the supports. Eight grams of ZnO, MgO or
2
O
5
) with APTMS
3
−1
cm min ) and detected by FID.
Nb
2
O
5
were heated at 383 K with a mixture of dry toluene (200 mL) and
2.3. Oxidation of n-octanol
APTMS (20 mL) under reflux for 18 h. Then, the solid was recovered by
filtration and washing with toluene (200 mL), water (100 mL) and
acetonitrile (100 mL). The last step was drying of the solid at 353 K.
Prior to testing the reaction of n-octanol (Merck, 99%, HPLC grade)
oxidation, the calcined catalysts were either dried overnight at 393 K
(
denoted as -D) or reduced in 10 mL/min of H for 1 h in 673 K (denoted
2
2
.1.4. Metal incorporation
as -H). The appropriate amount of pre-treated catalyst (Au/n-octanol
=100 mol/mol) was put into a four-necked round bottom flask and
25 mL of 0.1 M n-octanol solution in n-heptane (Scharlau, 99%, HPLC
grade) was added. The flask was equipped with a thermocouple, oxygen
inlet and septum cap. The reaction was run for 6 h at 353 K under re-
To get monometallic catalysts, adequate amount of the support was
mixed with a water solution of gold or silver precursor (HAuCl or
4
AgNO , in amounts needed to get a final content of 2 wt. % of Au or Ag)
3
for one hour at room temperature. Then it was filtered and washed with
water. The obtained solid was mixed with the water solution of sodium
flux, upon continuous stirring, while O (Air Liquide, 99.99%) was
2
borohydrate (NaBH
4
) for 20 min and then filtered again. To obtain the
bubbled through the reaction mixture. Small amounts of the reaction
mixture were collected at specified run times during the oxidation
process (0.5, 1, 2, 4 and 6 h) to analyse the progress of the reaction. The
bimetallic AuAg catalyst the second step was stirring of the gold con-
taining material with the water solution of silver precursor (AgNO , as
3
2