C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
443
area-weighted mean diameter (dp) was calculated by the equation:
ꢀ
inidi3
inidi2
ꢀ
dp
=
where ni is the number of particles of diameter di.
Chemical analysis was performed by inductively coupled
plasma-mass spectrometry on a PerkinElmer 3300 ICP-MS Spec-
trometer. Samples were solubilized in a nitric/hydrochloric acid
solution and homogenized in microwave oven.
X-ray photoelectron spectra (XPS) of the in-situ reduced (723 K
for 2 h) catalysts were recorded using an Escalab 200R spectrometer
provided with a hemispherical analyzer, operated in a constant pass
energy mode and Mg K␣ X-ray radiation (h = 1253.6 eV) operated
at 10 mA and 12 kV. Charging effects of samples were corrected by
fixing the binding energy (BE) of the C1s core-level of adventitious
carbon at 284.8 eV (accuracy = 0.1 eV).
2.3. Catalytic activity in liquid phase
Fig. 1. Diffraction patterns for (a) TiO2, (b) 5%Ni/TiO2, (c) 10%Ni/TiO2 and (d)
15%Ni/TiO2. Reference patterns (*) Ni0 (JCPDS 00-004-0850); (᭹) TiO2-Anatase
(JCPDS 00-021-1272).
The catalytic assays of maleic anhydride hydrogenation were
performed in a stainless steel (150 mL) Parr-type semi-batch reac-
tor using a substrate/metal ratio of 100, stirring rate of 650 rpm,
50 mL of tetrahydrofuran (THF) as the solvent and 0.100 g of cat-
alyst. Before reaction, the x%Ni/TiO2 catalysts were activated by
reduction with H2 at 723 K for 2 h. Maleic anhydride hydrogena-
tion kinetic experiments were carried out between 323 and 393 K
and a total hydrogen pressure of 4.0 MPa. No mass transfer resis-
tance were found in a catalytic test established using the Madon
and Boudart approach [27]. No catalytic activity for pure TiO2 was
confirmed in a blank test. Pseudo-first-order kinetic constants (k)
were calculated as reported in previous studies [28]. Analytical
quantifications for reactants and products were analyzed by gas
chromatography using a GC instrument, HP-4890 with a semi-
capillary column HP-5, FID detector and N2 as the carrier gas.
Temperature and reusability studies were performed with the best
catalytic system. The recycling assays were performed by centrifu-
gation of the catalyst from the reaction medium, washed four times
consecutively with ethanol (40 mL × 4) to clean the surface and
removed all organic matter, dried under vacuum at 323 K for 12 h
and finally reduced at 473 K for 1 h under H2 flow [29].
Table 2
Structural and morphological analysis of TiO2-anatase support and Ni-TiO2 sup-
ported catalysts.
Sample
SBETa(m2 g−1
)
dp (nm)b
Vpc(cm3 g−1
)
TiO2
156
107
92
8.5
9.7
10.3
9.7
0.37
0.30
0.27
0.23
5%Ni/TiO2
10%Ni/TiO2
15%Ni/TiO2
87
a
Specific surface area, determined by the BET method from N2 physisorption
measurements.
b
Pore diameter, determined from quantity of N2 adsorbed at a relative pres-
sure = 0.99.
c
Pore volume, calculated from the maximum in the BJH pore size distribution.
sity might indicate an increase in the crystallinity of metallic Ni
particles whereas the decrease in their average crystallite size could
be deduced from an increase of the full width at half maximum of
the peaks. For all catalysts, no peaks due to crystalline NixOy-type
species were observed. This observation suggests that their crystal
size could be below detection limit of XRD technique (<4 nm) indi-
cating the high Ni species dispersion on the support surface [18]
tems were analyzed by N2 desorption- adsorption isotherms at
77 K. As shown in Fig. 2, all the systems give type IV isotherms
according to the IUPAC classification, corresponding to mesoporous
that the pore distribution is bimodal for all systems, centered at 7.0
and 11.0 nm approximately. This distribution is due to interparticle
porosity which is characteristic of TiO2 based supports [36]. Table 2
shows the values associated with the textural properties of the sup-
port and catalysts. As shown in Table 2 no significant variations in
the surface area, diameter, and pore volume were observed, when
the values associated to the support and catalysts were compared.
This could indicate that the morphological properties of the mate-
rial remain unchanged after the incorporation of metal despite the
small pore blockage, which is reflected in the small decrease of the
2.4. Ni-leaching tests
The screening catalytic assays of residual filtered catalytic reac-
tion was studied in the cyclohexene hydrogenation and performed
in a stainless steel (150 mL) Parr-type semi-batch reactor at a
substrate/metal ratio of 250, assumed from ICP-AES post-reaction
characterization, at 650 rpm under 4.0 MPa of H2 pressure and
373 K of temperature and reaction time of 4 h.
Fig. 1 presents XRD profiles of the TiO2-anatase and Ni catalysts.
In all of the patterns the diffraction planes associated at 2 angles of
25.4◦, 37.9◦ and 48.2◦ of typical TiO2-anatase were observed (JCPDS
after deposition of the metal phase. In all three catalysts signals
at 2 angles of 44.5◦ and 52.0◦ are observed, which are associated
with diffraction planes (111) and (200) of the metallic nickel (Ni0)
[33,34]. In addition, an increase in the intensity of the diffraction
peak associated to Ni0 (2 = 44.5◦) was observed with the increase
of nominal metal loading. The observed increase in the signal inten-
The morphology of the pure TiO2 substrate and x%Ni/TiO2 cat-
alysts was examined by HRTEM. HR-TEM images of x%Ni/TiO2
catalysts together with that of pure TiO2 anatase support are shown
in Fig. 3.