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room temperature. A prereduction of RuÀ AAOs is performed at
3. Conclusions
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150 C and 50 bar H2 for 24 h.
Anodized aluminum oxides (AAOs) with uniform, tubular pores
were synthesized according to a four-step electrochemical
synthesis procedure. The AAOs are used as catalyst supports
and are impregnated with Ru to create Ru-based hydrogenation
catalyst materials. Using SEM and TEM analysis, AAOs were
characterized and the presence of uniform, ordered nanopores
with pore diameter of 17�4 nm was confirmed. To the best of
our knowledge, the presence of Ru nanoparticles inside the
tubular pores is evidenced for the first time via HAADF-STEM-
EDX. The RuÀ AAO catalysts are employed in hydrogenation
reactions of toluene and butanal as model compounds of
aromatics and aldehydes. High catalytic activity is obtained and
no diffusional limitation is observed. After the reaction, the
pores of the AAOs remain preserved. It can thus be concluded
that the AAOs synthesized in this work exhibit uniform, ordered
nanopores with high catalytic activity, opening up the possi-
bility to make catalyst materials for other reactions.
Catalyst Characterization
To calculate the BET specific surface area, Kr physisorption
isotherms are measured on a Micromeritics 3Flex Surface Analyzer
at 77 K. X-ray diffraction data were recorded on a Malvern
PANalytical Empyrean diffractometer equipped with a PIXcel3D
solid state detector using a Cu anode (Cu Ka1: 1.5406 Å; Cu Ka2:
1.5444 Å). Samples were placed on
a programmable stage
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motorized in the x, y and z directions. Before each measurement,
an iterative scheme was employed to optimize both sample height
(z) and sample tilt (ω). Grazing incidence diffraction patterns were
recorded at room temperature in reflection geometry (Bragg-
°
°
°
Brentano, incident beam angle 0.2 ) within a 5 –70 range using a
°
step size of 0.053 and a counting time of 1000 s per step. On the
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incident beam side, a 1/16 fixed anti-scatter slit was used to limit
the divergence of the beam. FTIR spectra are recorded on a Bruker
Optics IFS 66 V/S spectrometer (128 scans, 4 cmÀ 1 resolution, 500~
4000 cmÀ 1) under vacuum. The surface morphology of the AAOs is
measured on a SEM (FEI-Helios Nanolab G3UC) at an accelerating
voltage of 10 kV. TEM micrographs are obtained on a FEI-Talos
F200x; prior to the analysis, the surface of the AAO sample is coated
with Pt to enhance the conductivity. The Ru content in the catalyst
samples is analyzed by EDX (Oxford X–MaxN 150 SSD) attached to
SEM.
Experimental Section
Materials and Catalyst Synthesis
All chemicals were purchased and used without further purification:
aluminum foil (Alfa Aesar, 0.25 mm thick, annealed, 99.99% (metals
basis)), perchloric acid (HClO4, Acros Organics, for analysis, ca. 70%
solution in water), ethanol (Fischer Chemical, absolute), sulfuric acid
(Fischer Chemical, S.G. 1.83 (>95%)), nitric acid (Sigma Aldrich,
puriss. p.a., reag. ISO, reag. Ph. Eur., for determinations with
dithizone, �65%), acetic acid (VWR Chemicals, 100%), orthophos-
phoric acid (VWR Chemicals, 85%, ACS, Reag. Ph. Eur), ruthenium
(III) chloride hydrate (Alfa Aesar, Premion 99.99% (metals basis)),
levulinic acid (Acros Organics, 98+%), toluene (Acros Organics,
99.8+%, for analysis), methanol (VWR Chemicals), butanal (Acros
Organics, 99%), deuterium oxide (Sigma-Aldrich, 99.9 atom% D)
and n-nonane (Acros Organics, 99% pure).
Catalyst Testing
Hydrogenation reactions are performed in a 15 mL stainless steel
autoclave. A Teflon ring is placed at the bottom of the reactor with
a magnetic stirrer in the center of the ring. The RuÀ AAO is placed
on top of the ring, the solvent and substrate are added and the
reactor is sealed. The magnetic stirrer ensures mixing throughout
the whole reaction mixture. First, the autoclave is purged 3 times
with N2 and subsequently 3 times with H2 and then, the reactor is
pressurized with H2 and the reaction temperature is applied. After
the applied reaction time, the reactor is cooled down on ice and
the catalyst platelet is removed. The reaction mixture is then
analyzed by NMR or GC(-MS).
Reaction products are identified and quantified with 1H-NMR for
hydrogenations performed in H2O. Samples for 1H-NMR are
prepared by adding 300 μl of reaction mixture to 300 μl of D2O. The
Bruker Ascend 400 MHz spectrometer is equipped with a BBO
5 mm atma probe. In order to suppress the water signal at 4.7 ppm,
the following pulse program is used: p1 8 μs; plw1 15 W; plw9
5.7·10À 5; o1P on the resonance signal of water, which is automati-
cally determined and selected.
The synthesis procedure for the AAO platelets used in this study
was optimized starting from procedures described in the
literature.[20–23] A high purity aluminum sheet (99.99%), with a
thickness of 0.25 mm is first cut into platelets of 2×3.3 cm2. The
platelet is degreased with acetone and undergoes a four-step
anodization procedure with platinum gauze as the cathode while
the aluminum platelet is the anode. During the electropolishing
step, the platelet is immersed in a 20 vol% HClO4 in ethanol
solution. Subsequently, a voltage of 12 V and a maximum current
°
For reactions carried out in ethanol, gas chromatography coupled
to a mass spectrometer (GC-MS) is used for product identification.
An Agilent 6890 GC provided with an HP-5 ms column and coupled
to a 5973 MSD mass spectrometer are used. Quantification of these
reaction mixtures is done by GC analysis on a Shimadzu GC-2010
instrument with a 60 m CP SIL 5 CB column and an FID detector.
Nonane is used as an internal standard.
of 1 A are applied for 10 min at 10 C. The first anodization step is
°
carried out at 0 C for 10 min in a 20 wt% H2SO4 solution at 19–21 V
with 1 A as maximum current. Thirdly, the platelet is immersed in a
5 wt% HNO3 (65 wt% in H2O) – 5 wt% acetic acid – 75 wt% H3PO4
°
(85 wt% in H2O) solution at 40 C for 5 min during the chemical
etching step. The last step is again an anodization step in the same
solution and at the same voltage and current, at 0 C for 7 h. After
the anodization procedure, the AAO is stored in H2O until it is dried
°
ICP-OES analysis was used to determine the Ru content of the
reaction mixture of butanal hydrogenation using a Varian 720-ES
equipped with a double-pass glass cyclonic spray chamber, a Sea
Spray concentric glass nebulizer and a high solids torch.
°
at 100 C for 15 min.
In a final step, Ru is deposited onto the AAOs via an impregnation
method. RuCl3 hydrate is dissolved in 5 ml H2O or ethanol with Ru/
substrate=1.5 mol% and the solution is transferred to a watch
glass. Immediately after drying the AAO, the platelet is immersed
into the Ru precursor solution and the solvent is evaporated at
ChemistryOpen 2019, 8, 532–538
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© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA