C. Encarnación-Gómez, et al.
AppliedCatalysisA,General600(2020)117631
chemical methods are frequently employed for the incorporation of
particles on the support surface, these methods include ion exchange,
co-precipitation, sol-gel and wet impregnation [39–41]. Nevertheless,
the gas-phase methods have received increasing attention as an alter-
native for the preparation of supported metal nanoparticles because of
its significant advantages over the traditional methods [42,43]. With
respect to precursor, the metal β−diketonates have characteristics that
make a good candidate for use in vapor processes due to ability for pass
into gas phase without decomposition involving the deposition of metal
particles [44]. Also, the decomposition products of the precursor in-
clude the breakdown in ketonic groups which are then decomposed
releasing CO2 gases that are evacuated off the reaction system. For the
Pt supported nanoparticles, the Pt(acac)2 has been widely used as
precursor in CVD process [45–48]. A comparison between the most
used precursor in aqueous solution, H2PtCl6 and the Pt(acac)2 was re-
ported by Reyes et al. [49]. In that study, the use of Pt(acac)2 as pre-
cursor for alumina supported Pt catalyst provides an effective method
to produce a highly dispersed metallic phase, predominantly Pt0. Not-
withstanding, the H2PtCl6 precursor promotes the formation of Pt oxi-
dized species [50] obtain high conversion (96% at 200 °C), in the
naphthalene hydrogenation with catalysts of Pt group from β-diketo-
nates, supported on mesoporous aluminosilicates prepared by aqueous
solution using liquid crystal templates. Yao et al. [51] developed a
vacuum evaporation-impregnation (VEI) method to disperse Pt into
mesoporous MCM-41 using different platinum precursors such as acetyl
acetonate Pt(acac)2, tetraamine platinum (II) nitrate Pt(NH3)4(NO3)2,
and hexacloroplatinic acid (IV) hydrate H2Cl6Pt%6H2O. Higher metal
dispersion at high metal loading (0.5–2 wt. %) was produced by this
method with Pt(NH3)4(NO3)2 solution at pH = 5. As a comparison,
equilibrium adsorption (EA) and ion exchange (IE) methods, where
high or low pH are required, result in the destruction of support’s
mesoporous structure. Jiao et al. [52] proposed a strong electrostatic
adsorption (SEA) as a simple and cheap method, to disperse Pt, Pd, Cu,
Co, and Ru ammine precursors into the matrix of amorphous silica and
mesoporous structure SBA-15. Homogeneously dispersed Pt and Pd
nanoparticles (1.3–2.0 nm) deposited on the internal surface area of
SBA-15 were attained using a low temperature reducing treatment. A
higher metal dispersion was obtained when using the same components
dispersed by a dry impregnation method. Acordingly, the work of
Womes [53] reported the synthesis of Pt nanoparticles with controlled
particle size by the selective decomposition of Pt(acac)2 on reduced Pt
particles. This unusual method allowed to distinguish three different
types of surface sites on alumina supports on the basis of their specific
interaction mechanisms with Pt(acac)2. The occurrence of these various
sites is correlated with the degree of surface hydroxylation and there-
fore with the treatment that the support has undergone prior to im-
pregnation, thus creating changes in the interaction between OH sur-
face sites and Pt precursor resulting in greater or llower reactivity.
As previously mentioned, the precursor nature, the support type and
synthesis method are key determinants that influence the dispersion,
size and distribution of metal nanoparticles. In the present contribution,
we describe the synthesis of Pt nanoparticles using Pt diketonate as
precursor by a methodology denominated vapor phase impregnation
(VPI). This novel process allows the incorporation of finely divided
metal particles on porous supports without the need of liquid solvents
and their drying and reduction steps which may promote undesirable
changes of the active particle size. This can be attained by controlling
the conditions of metal incorporation during the synthesis, as tem-
perature, pressure of system, career gas flow, reaction time and clearly
the precursor chemical nature. The Pt nanoparticles were incorporated
with high metal load on TiO2 nanotubes (TNT), a support of high sur-
face area and were characterized by techniques that led to relate the
physicochemical properties with the catalytic activity in naphthalene
hydrogenation.
2. Experimental
2.1. Preparation of titania nanotubes supports
Titania with nanotubular morphology was synthesized by hydro-
thermal treatment of an anatase precursor with crystallite size of 8.0 nm
(as determined by XRD Rietveld Refinement analysis) [29]. Forty-five
grams of TiO2 anatase powder were suspended in 3 L of an aqueous
10 M NaOH solution and the resulting suspension was placed in a 4 L
autoclave. The hydrothermal reaction was conducted at 100 °C, during
18 h under stirring at 200 rpm. Then, the white slurry was filtered and
neutralized with a 1 M HCl solution until the pH was lowered to 3.0.
The resulting suspension was maintained at this pH overnight under
continuous stirring. The material was repeatedly washed with abundant
deionized water until it was chlorine-free, i.e. by testing with silver
nitrate solution. The material was finally dried overnight at 100 °C,
yielding a hydrous titania powder with nanotubular morphology.
Thereafter, the sample was calcined at 400 °C under dynamic nitrogen
flow in a tubular oven for 4 h.
2.2. Catalysts preparation
Platinum nanoparticles were incorporated into TNT support by a
vapor phase impregnation (VPI) method. A finely ground powder
fraction of the TNT was dried in static air to 120 °C for 12 h, then, it was
directly mixed with the platinum salt (bis-acetylacetonate), (Pt(acac)2,
97%) in an amount to have a nominal content of 1, 3, 6, 10, 14 and
20 wt.% of Pt in the final solid. The mixture was mechanically ground
in an agate mortar for 0.5 h. Then, the resulting material was heat-
treated at 180 °C in a dynamic Ar atmosphere at a flow rate of 100
cm3 min−1 for 0.25 h, thereafter, it was placed at the higher tempera-
ture zone at 400 °C for 0.25 h. The total pressure of the system was
maintained at 500 Torr in a horizontal furnace.
For the catalytic test, the samples were reduced by heating at
2 °C min−1 up to 350 °C in a H2 stream for 1 h. Hereinafter, this catalyst
will be referred to as XPt/TNT, where X means the amount of Pt loaded
in wt % (1, 3, 6, 10, 14 and 20 wt%).
2.3. Materials characterization
X-ray diffraction. The structure of the supports and catalysts was
analyzed by X-ray diffraction (XRD) in a Bruker Advance D-8 dif-
fractometer having theta–theta configuration and a graphite secondary-
beam monochromator. The XRD patterns were acquired at room tem-
perature with CuKα (λ = 1.5409 nm) radiation in the 2θ range be-
tween 4° and 80°, with a 2θ step of 0.02 for 2 s per point.
Transmission electron microscopy and scanning transmission electron
microscopy. Transmission electron microscopy (TEM) and scanning
transmission electron microscopy (STEM), were carried out both in a
JEM-2200FS microscope with an accelerating voltage of 200 kV. The
microscope is equipped with a Schottky-type field emission gun, and an
ultra-high resolution configuration (Cs = 0.5 mm; Cc = 1.1 mm; point-
to-point resolution = 0.19 nm) and in-column omega-type energy filter.
The microscope operates with an aberration-corrected device CEOS in
STEM mode producing significantly smaller and brighter electrons
beam, a probe size of 0.1 nm. High angle annular dark field (HAADF)
image was obtained using the HAADF detector in the STEM mode. In
this technique, the detector collects electrons that undergo Rutherford
scattering where their intensities are approximately proportional to Z2
(Z being the atomic number of the scattering atom). Elements with a
high Z show higher intensities and brighter contrast in the image is
displayed giving useful information in catalytic materials where gen-
erally the active phase is a heavier element than support. Particle size
was determined directly on HAADF-images by counting and measuring
around 200 particles in each sample. Chemical mapping was obtained
by using energy dispersive X-ray spectroscopy (EDXS) in a NORAN
2