Supported nickel catalysts for anisole hydrodeoxygenation: Increase in the selectivity to cyclohexane

Article abstract of DOI:10.1016/j.cattod.2018.07.057

The hydrodeoxygenation (HDO) of anisole was performed with Ni catalysts supported on SBA-15 silica and protonated titanate nanotubes (PTN), in order to determine the effect of the support's nature and the method of incorporation of Ni species on the catalytic activity and selectivity in HDO. Catalysts with 10 wt.percent of Ni were prepared by two methods: deposition-precipitation with urea (DP) and the classical incipient wetness impregnation (IM). Characterization of the catalysts was performed by nitrogen physisorption, temperature programmed reduction, temperature programmed desorption of ammonia, high resolution transmission electron microscopy, scanning electron microscopy, powder X-ray diffraction, XPS and O₂ chemisorption. The obtained results show that the support's nature and the preparation method had a strong influence on the dispersion of the supported Ni species and their HDO performance. Catalysts with larger metallic Ni particles showed higher ability for hydrogenation of anisole and higher intrinsic activity (TOF) than those with smaller Ni particles. On the other side, the use of the PTN support and the DP with urea preparation method resulted in catalysts with increased acidity and improved selectivity for the formation of deoxygenated products (cyclohexane).

Full text of DOI:10.1016/j.cattod.2018.07.057

Accepted Manuscript
Title: Supported nickel catalysts for anisole
hydrodeoxygenation: Increase in the selectivity to cyclohexane
Authors: H. Vargas-Villagr a´ n, M.A. Flores-Villeda, I.
Puente-Lee, D.A. Sol ´ı s-Casados, A. G o´ mez-Cort e´ s, G.
D ´ı az-Guerrero, T.E. Klimova
PII:
S0920-5861(18)31337-3
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.07.057
CATTOD 11602
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
20-12-2017
2-7-2018
31-7-2018
Please cite this article as: Vargas-Villagr a´ n H, Flores-Villeda MA, Puente-Lee I, Sol ´ı s-
Casados DA, G o´ mez-Cort e´ s A, D ´ı az-Guerrero G, Klimova TE, Supported nickel
catalysts for anisole hydrodeoxygenation: Increase in the selectivity to cyclohexane,
Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.07.057
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1
Supported nickel catalysts for anisole hydrodeoxygenation: Increase in
the selectivity to cyclohexane
1
1
1
2
H. Vargas-Villagrán , M.A. Flores-Villeda , I. Puente-Lee , D.A. Solís-Casados , A.
3
3
1*
Gómez-Cortés , G. Díaz-Guerrero , T.E. Klimova
1
Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd.
Universitaria, Coyoacán, Ciudad de México, C.P. 04510, México
2
Centro Conjunto de Investigación en Química Sustentable, Universidad Autónoma del
Estado de México-Universidad Nacional Autónoma de México (UAEM-UNAM), km 14.5
Carretera Toluca-Atlacomulco, Unidad San Cayetano, Toluca, Estado de México 50200,
México
3
Instituto de Física, Universidad Nacional Autónoma de México (UNAM), Apartado Postal
2
0-364, Ciudad de México 01000, México
Corresponding author: klimova@unam.mx

2
Graphical abstract
Highlights





Ni catalysts supported on SBA-15 and titanate nanotubes for hydrodeoxygenation
HDO).
(
Effect of the support’s nature and the preparation method on HDO activity was
studied.
Ni/SBA-15 catalysts were more active for HDO of anisole than Ni/titanate
nanotubes.
Catalysts prepared by DP with urea method showed higher dispersion of Ni
nanoparticles.
Ni catalysts prepared by the DP method were more selective to cyclohexane
formation.

3
Abstract
The hydrodeoxygenation (HDO) of anisole was performed with Ni catalysts supported on
SBA-15 silica and protonated titanate nanotubes (PTN), in order to determine the effect of
the support’s nature and the method of incorporation of Ni species on the catalytic activity
and selectivity in HDO. Catalysts with 10 wt.% of Ni were prepared by two methods:
deposition-precipitation with urea (DP) and the classical incipient wetness impregnation
(
IM). Characterization of the catalysts was performed by nitrogen physisorption,
temperature programmed reduction, temperature programmed desorption of ammonia, high
resolution transmission electron microscopy, scanning electron microscopy, powder X-ray
diffraction, XPS and O chemisorption. The obtained results show that the support’s nature
2
and the preparation method had a strong influence on the dispersion of the supported Ni
species and their HDO performance. Catalysts with larger metallic Ni particles showed
higher ability for hydrogenation of anisole and higher intrinsic activity (TOF) than those
with smaller Ni particles. On the other side, the use of the PTN support and the DP with
urea preparation method resulted in catalysts with increased acidity and improved
selectivity for the formation of deoxygenated products (cyclohexane).
Keywords: Hydrodeoxygenation; Anisole; Nickel catalysts; Activity; Selectivity
1
. Introduction

4
Almost 95% of the world's transportation energy comes from petroleum-derived
fuels (gasoline and diesel) [1]. The excessive use of fossil fuels, in addition to their low
quality, increases the concentration of greenhouse gases (GHG) in the atmosphere and
produces pollution, affecting the environment and human health [2]. Biofuels represent an
alternative for fossil fuels. However, the actual production of first generation biofuels
(biodiesel and bioethanol) is not sufficient to satisfy the demand [3,4]. Nowadays, the
production of second generation of biofuels is attracting attention. These biofuels are
sustainable and renewable, since they are obtained from high-yielding biomass crops or
from lignocellulosic biomass (plant dry matter) from agricultural and forestry waste
feedstocks [5-9].
The transformation of biomass to chemicals and fuels can be performed by three
thermochemical methods, including aqueous sugar production by hydrolysis, bio-oil
production by liquefaction or pyrolysis and syngas formation by gasification [10]. Among
these routes, pyrolysis integrated with an upgrading is the simplest and most cost-effective
option. Pyrolysis consists in the thermal decomposition of lignocellulose in the absence of
oxygen at high temperatures (350 to 600 °C), where a large number of reactions take place:
dehydration, depolymerization, isomerization, aromatization, decarboxylation and charring
[
11-13]. The main difference between pyrolized bio-oil and crude oil is the high oxygen
content (up to 47 %) in bio-oils, leading to some unwanted characteristics such as high
acidity, high viscosity, immiscibility with hydrocarbon fuels, corrosiveness, etc. [4].
Hydrodeoxygenation (HDO) reactions can be used to reduce oxygen content in bio-oil,
enhancing its stability and combustion properties. Nowadays, the development of novel,
effective and cheap catalysts for HDO is an actual task.

5
Hydrodeoxygenation is a catalytic process for the removal of oxygen, which,
similarly to the hydrodesulfurization of petroleum fractions, is generally performed at high
temperature and hydrogen pressure [11, 14-16]. Conventional sulfide NiMo and CoMo
catalysts have been widely studied for HDO processes [17-20]. However, their use
generates some problems, such as contamination of the final products with sulfur, catalysts’
deactivation and coke accumulation. Carbides [21-24], nitrides [21,25,26] or phosphides
[
27-29] of different transition metals (Ni, Mo, W, Co, Fe, etc.) have also shown promising
HDO results. Among them, Ni phosphide has received special attention due to its activity
and selectivity attributed to both, metallic and acidic groups [30-33]. Reduced metal
catalysts have also been tested as catalysts for HDO. To date, it is known that HDO
catalysts should be bifunctional, by including an oxygen activator and a hydrogen donator
[
3]. These characteristics strongly depend on the selected active phase, on the nature of the
support, on metal-support interactions (MSI) and on the reaction conditions. Deepa and
Dhepe [34] demonstrated the effects of supports (acidic, neutral or basic) and noble metals
(Pd, Pt, Ru) on the HDO of phenol, guaiacol and eugenol. They found that both factors
(support’s acidity and metal nature) affected the activity of the catalysts and the ratio of the
obtained partially hydrogenated and oxygen-free products. Evaluation of the combinations
of metals and supports showed that metals deposited on a highly acidic SiO -Al support
2
2
O
3
were very selective for the formation of O-free hydrogenated products indicating a real
hydrodeoxygenation, whereas on a mildly acidic support (γ-alumina), complicated product
distributions were obtained. Therefore, it was demonstrated that acidic supports play an
important role in the C-O bond cleavage. Based on these results, it was proposed that the
first step: hydrogenation of the aromatic ring of guaiacol or hydrogenation of the double-
bond of eugenol, is a metal-driven reaction; while subsequent reactions are mostly

6
governed by the support, where the metal acts as a hydrogen supplier. In other works, it
was also reported that an increase in the oxophilicity of the active metal phase or the
presence of oxophilic sites provided by the support, enhanced the C-O cleavage of HDO
model molecules such as anisole [35], m-cresol [36], phenol [37], furfural [38] and guaiacol
[
39,40]. When metal catalysts were supported on acidic materials, HDO performance of the
catalysts, especially their selectivity to oxygen-free hydrogenated products, was improved,
which was attributed to a synergetic effect between the active metal species and the nearby-
located acidic sites (mainly the Brönsted ones) [41-45]. Finally, it should be mentioned that
in the majority of the above-mentioned works, a good HDO performance was obtained with
noble metal catalysts, being Pt, Pd, Ru and Rh the most studied metals [46,47]. However,
the high cost of noble metals limits the possibility of their industrial implementation.
In work [48], Ni, Pd, Pt and Ru catalysts supported on different materials were
tested in the HDO of phenol, and it was demonstrated that Ni-based catalysts can be a
promising alternative to noble metal catalysts. It was found that the order of activity of the
catalysts was: Ni/ZrO
2
> Ni-V
2
O
5
/ZrO
2
> Ni-V
2
O
5
/SiO
2
> Ru/C > Ni/Al
2
O
3
> Ni/SiO ≫
2
Pd/C > Pt/C, being the Ni/ZrO
2
catalyst even more active than the carbon-supported noble
metal catalysts. This was attributed to the participation of the zirconia support in the
activation of phenol, facilitating its hydrogenation. In a review paper [49], Ni-based
catalysts were also suggested for the HDO process because of their ability for hydrogen
activation under mild conditions. In work [50], Ni catalysts (20 wt %. of Ni) supported on
SBA-15, Al-SBA-15, -Al
2
O
3
, microporous carbon, TiO
2
and CeO were prepared to study
2
the effect of the metal-support interaction on the HDO of anisole. It was found that the
support's nature affected Ni dispersion, as well as the activity and selectivity of the

7
catalysts. Thus, the Ni catalysts supported on SBA-15, Al-SBA-15 and alumina were
highly active (100 % of anisole conversion) and selective for the formation of
deoxygenated hydrocarbon products (cyclohexane and n-hexane). In contrast, the other
catalysts, such as Ni/CeO
hydrodeoxygenated aromatics (mainly benzene). In another work [51], nickel catalysts (10
wt. % of Ni) supported on activated carbon, SBA-15, SiO and -Al were also evaluated
in the HDO of anisole at different temperatures (180–220 °C) and H pressures (0.5–3.0
2
, Ni/TiO
2
and Ni/C showed a remarkable selectivity towards
2
2
O
3
2
MPa). It was observed that the reaction conditions have an influence on the activity and
selectivity of the catalysts, but also the support exhibited a significant influence on the
obtained products. Nickel catalysts supported on silica materials showed a better
deoxygenation activity with selectivity to deoxygenated products of 70 % for Ni/SBA-15
and 95% for Ni/SiO . This was attributed to the combination of acidic sites of the supports,
2
which may contribute to the cleavage of C-O bonds, and good dispersion of the metal. In
work [52], the synergistic effect of metal Ni and acidic support was also observed in HDO
of methyl palmitate. Among Ni catalysts (7 wt.% of Ni) supported on SiO
2
, -Al
2
3
O ,
SAPO-11, HZSM-5 and HY, the Ni/SAPO-11 catalyst with weak and medium acidity
showed superior activity to the other catalysts (99.8 % conversion of methyl palmitate) with
high selectivity to C15 and C16 alkanes.
On the other hand, the effect of the support's pore structure has received less
attention. In work [53], Ni catalysts supported on carbon nanotubes were prepared, in
which metallic nanoparticles were loaded inside or outside the nanotubular support.
Activity and selectivity tests in the HDO of guaiacol showed that the selectivity to
cyclohexene was enhanced when the Ni nanoparticles were loaded inside the carbon

8
nanotubes, whereas Ni nanoparticles located outside the nanotubes favored the formation of
cyclohexane. This result was attributed to the different adsorption modes of the guaiacol
molecule inside and outside the nanotubes [54].
In the present work, we inquired into the effect of the support's nature, as well as of
the catalyst's preparation method on the catalytic performance of supported nickel catalysts
in HDO. For this, nickel catalysts (10 wt. % of Ni) supported on SBA-15 silica and
protonated titanate nanotubes (PTN) were prepared by two methods: deposition-
precipitation with urea (DP) and incipient wetness impregnation (IM). The prepared
catalysts were characterized and tested in the HDO of anisole.
2
. Experimental
2
.1. Synthesis of supports
The SBA-15 support was synthesized by the hydrothermal method described by
-1
Zhao et al. [55]. First, 4 g of PEO20-PPO70-PEO20 copolymer (Mw = 5800 g∙mol , Sigma
Aldrich) were dissolved in 120 mL of a 2 M HCl (J. T. Baker, 36 %) solution. The
temperature was set to 35 °C. Afterwards tetraethyl orthosilicate (TEOS, Sigma-Aldrich)
was added dropwise. The formed gel was aged at 80 °C for 48 h without stirring in a closed
Teflon-lined autoclave. The resultant white product was separated by filtration, washed
several times with deionized water, dried at room temperature and calcined at 550 °C for 6
h for the organic template removal.
For the synthesis of the PTN support, the procedure of Kasuga et al. [56] was
followed. In a typical synthesis, 10 g of TiO (anatase nanopowder, Sigma-Aldrich) was
2

9
dissolved in 300 mL of a 10 M NaOH (Sigma-Aldrich) solution, then the temperature was
raised to 140 °C. The mixture was maintained at this temperature for 20 h upon constant
magnetic stirring. The obtained white solid formed by hydrated sodium trititanate
nanotubes was recovered by filtration and washed several times with deionized water. To
transform the sodium trititanate into the protonated trititanate nanotubes, a cationic
exchange with a 0.1 M HCl (J. T. Baker, 36 %) solution was performed. Finally, the
resulting PTN solid was dried at 120 °C for 6 h.
2
.2. Preparation of Ni catalysts
Catalysts with a theoretical loading of 10 wt. % of Ni were prepared by two
different methods: conventional incipient wetness impregnation (IM) and deposition-
precipitation with urea (DP). In both cases, nickel nitrate (Ni(NO ∙6H O, Aldrich) was
3
)
2
2
used as a precursor. The Ni/SBA15-IM catalyst was prepared by the impregnation of the
prepared SBA-15 support with an aqueous solution of nickel nitrate. After the
impregnation, the catalyst was dried at room temperature for 12 h. The Ni/SBA15-DP
catalyst was prepared by the DP method adapted from the work of Gómez et al. [57], in
which the nickel phyllosilicate phase was precipitated onto the SBA-15 surface. In this
procedure, 1 g of the prepared SBA-15 was added to 125 mL of an aqueous solution of
Ni(NO
3
)
2
∙6H
2
O (0.14 M), HNO
3
(0.02 M). Urea (Mallinckrodt) was used as the
precipitating agent. Then the suspension was heated to 90 °C and maintained at this
temperature for 2.8 h. Finally, the catalyst was filtered, washed with deionized water and
o
dried at room temperature for 12 h. Both Ni/SBA15 catalysts were then dried at 100 C for
1
2 h and then calcined at 500 °C for 2 h.

10
The Ni/PTN-IM catalyst was prepared by incipient wetness impregnation of the
protonated titanate nanotubes with an aqueous solution of nickel nitrate containing the
required amount of nickel precursor to obtain 10 wt.% Ni loading. The preparation of the
Ni/PTN-DP catalyst was adapted from [58]. Initially, 125 mL of an aqueous solution of
Ni(NO
3
)
2
∙6H O (0.07 M) was prepared. A small portion of this solution (20 mL) was used
2
to dissolve the corresponding amount of urea. A molar ratio of urea:Ni = 6:1 was used. The
PTN support (0.95 g) was added first to the rest of the solution and, afterwards, the nickel-
urea solution was also added. The resulting suspension was heated to 90 °C and maintained
at this temperature for 5.8 h. Finally, the catalyst was recovered by filtration, washed with
deionized water and dried at room temperature for 12 h. Both Ni/PTN catalysts were then
dried at 120 °C for 6 h and calcined at 350 °C for 2 h.
2
.3. Characterization of supports and catalysts
The textural properties of the SBA-15 and PTN supports and the corresponding Ni
catalysts were determined with a Micromeritics ASAP 2020 automatic analyzer at liquid N
2
o
temperature (-197.5 C). Previous to the measurements, the samples were degassed at 270
°
C (SBA-15 support and Ni/SBA-15 catalysts) or at 250 °C (PTN and Ni/PTN samples) for
-1
6
h (P < 10 Pa). N adsorption-desorption isotherms were obtained and specific surface
2
areas were calculated by the BET method (SBET). The total pore volume (Vp) was
determined by nitrogen adsorption at a relative pressure of 0.98. Pore size distributions
were obtained from the adsorption and desorption isotherms by the BJH method. The
mesopore diameters reported in the present work (Dads and Ddes) correspond to the maxima
of the pore size distributions obtained from adsorption and desorption branches of the

11
isotherms, respectively. The micropore area (Sμ) was estimated using the correlation of t-
Harkins & Jura (t-plot method).
The XRD characterization was performed on a Siemens D5000 diffractometer,
using CuKα radiation (λ=1.5406 Å) and a goniometer speed of 1 °(2) per minute. The
diffraction patterns were recorded in the 3 ° ≤ 2 ≤ 80 ° interval.
The temperature programmed reduction (TPR) and temperature programmed
desorption of ammonia (NH -TPD) experiments were carried out in a Micromeritics
3
AutoChem II 2920 automatic analyzer equipped with a thermal conductivity detector. Prior
to the TPR characterization, the samples (50 mg) were pretreated in situ at 400 °C for 2 h in
an air flow and then cooled to room temperature in an Ar stream. Then, the reduction of the
o
samples was performed from room temperature to 1000 °C with a heating rate of 10 C/min
under a stream of a H
2
/Ar mixture (10:90 mol/mol, 50 mL/min flow). The consumption of
H
2
at different temperatures was recorded. For the ammonia TPD experiments, first the
samples (50 mg) were pretreated in situ at 500 °C for 30 min in a helium flow to remove
water and other contaminants. Second, the samples were cooled to 120 °C and contacted
with a NH
pure He (20 mL/min flow) for 30 min. The desorption step was performed in a He stream
50 mL/min) from 120 °C to 500 °C with a heating rate of 10 °C/min, keeping the final
3
/He mixture (10:90 mol/mol, 20 mL/min flow) for 30 min and then purged with
(
temperature until the trace reached the baseline.
XPS measurements were performed with a JEOL JPS-9200 instrument operated at a
pass energy of 10 eV with an X-ray power of 300 W. A Mg Kα radiation source (1253.6
eV) was used for the analysis of the reference nickel samples (Ni(NO
3
)
2
∙6H O and NiO)
2
and Ni/SBA-15 catalysts, whereas a Al Kα radiation source (1483 eV) was employed for

12
the Ni/PTN samples. The base pressure in the analyzing chamber was maintained in the
-8
order of 1×10 mbar. The binding energy was determined using the carbon C (1s) line as a
reference with a binding energy of 284 eV. Quantification and deconvolution were
performed using the Gaussian functions of the Origin 8.1 software.
Ni dispersion in the reduced catalysts was determined by O chemisorption. The
2
experiments were performed in a quartz tubular reactor equipped with a furnace with a
thermocouple and a system for controlling temperature and valves for gas pulses. Prior to
o
the O
2
chemisorption measurements, the samples (0.1 g) were reduced at 400 C for 2h in a
o
hydrogen flow of 60 mL/min followed by cooling down to 25 C in an argon flow. The O
2
o
chemisorption was measured at 25 C by introducing 1 mL pulses of O
2
(5%)/He mixture
was measured.
on Ni was determined with a HIDEN Analytical HPR-20
mass spectrometer following the signal of O (m/z =32). The amount of nickel on the metal
surface was calculated from the O uptake by assuming an O/Niatom stoichiometry of 1;
meaning one O molecule was chemisorbed with dissociation on two Ni surface atoms. The
dispersion of metallic nickel (D) was calculated as a ratio of the amount of Ni on the
into the argon flow. The pulses were continued until no further uptake of O
The amount of chemisorbed O
2
2
2
2
2
surface measures by O
by SEM-EDX (Eq. 1).
2
chemisorption to the total amount of Ni in the sample determined
Surface Ni (mol/ gꢀꢁt
)
퐷 =
× 100%
)
(Eq. 1)
Toꢂal Ni (mol/ gꢀꢁt
The chemical composition of the catalysts was determined by scanning electron
microscopy (SEM-EDX), using a JEOL 5900 LV microscope with an OXFORD ISIS
equipment.

13
The high-resolution transmission electron microscopy (HRTEM) characterization of
the samples was performed using a JEOL 2010 microscope (resolving power 1.9 Å). The
solids were ultrasonically dispersed in heptane and the suspension was collected on carbon
coated grids.
2
.4. Catalytic activity tests
Prior to the catalytic activity tests, the catalysts were activated (reduced) ex situ in a
U-shaped glass flow reactor at 400 °C for 4 h, under a stream of H /Ar (30:70 mol/mol), at
2
atmospheric pressure. The hydrodeoxygenation (HDO) of anisole was performed in a 300
mL stainless steel batch reactor (Parr), with a 0.36 M solution of anisole (Aldrich, 99 %) in
hexadecane (Aldrich, 99.5 %). In each activity test, 50 mL of anisole solution and 100 mg
of the reduced Ni catalyst were used. The HDO reactions were carried out at 280 °C for 6 h
upon constant mechanical stirring (500 rpm). The reactor was filled with 3.5 MPa of
hydrogen at room temperature that gave 4.8 MPa total pressure at the reaction temperature
o
(
280 C). Small aliquots of the reaction mixture (about 0.5 mL) were taken every 15 min
during the first hour of the reaction and then hourly. The course of the reaction was
followed by analyzing the taken aliquots in an Agilent 6890 gas chromatograph equipped
with a flame ionization detector and a non-polar methyl siloxane capillary column HP-1 (50
m × 0.32 mm inner diameter and 0.52 m film thickness). Product identification was
performed on an Agilent 7890A GC system equipped with 5975C MS detector.
Catalytic activity was determined by measuring anisole concentrations at different
reaction times. The fractional conversion of anisole (XAN) was calculated as shown in Eq.

14
-1
2
, where C
0
is the initial anisole concentration (mol∙L ) in the reaction mixture and C is the
-1
concentration of anisole (mol∙L ) at different reaction times (t, h). The rate constant (k
1
)
was determined assuming that the anisole hydrodeoxygenation reaction is of the pseudo-
first order for anisole (Eq. 3) and pseudo-cero order for hydrogen (since hydrogen was
taken in a large excess). The initial reaction rate (r ) was calculated at zero reaction time as
0
the product of the rate constant and the initial concentration of anisole, and then normalized
by the volume (V) of the reaction mixture per weight (mcat) of the catalyst used (Eq. 4).
Finally, turnover frequency (TOF) was calculated by normalizing the initial rates with the
amount of accessible surface Ni atoms.
Cꢃ−C
XAN
=
(Eq. 2)
Cꢃ
−
ln(ꢄ−ꢅꢆꢇ
ꢂ
)
kꢄ =
ꢈꢉ =
(Eq. 3)
(Eq. 4)
ꢊ ∙C ∙V
ꢋ
ꢃ
mꢀꢁt
3
. Results and discussion
3
.1. Characterization of supports and calcined catalysts
The chemical composition of the prepared Ni catalysts was determined by the SEM-
EDX method (Table 1). All the catalysts showed the real Ni loadings close to the
theoretically expected one (10 wt. % of Ni). Namely, the Ni/SBA15 catalysts prepared by
the IM and DP methods had 10.3 and 10.6 wt. % of Ni, respectively. For the corresponding
Ni/PTN-IM and Ni/PTN-DP catalysts, Ni loadings of 9.7 and 9.8 wt. % were found.

15
The SBA-15 support exhibited a type IV nitrogen adsorption-desorption isotherm
Fig. 1A) corresponding to mesoporous solid materials, with an H1 hysteresis loop
(
associated with the presence of uniform cylindrical pore channels. In the case of the PTN
support (Fig. 1B), it also exhibited a type IV isotherm, but with a combination of hysteresis
loops of H1 and H3 types, which can be related to the coexistence of uniform cylindrical
pores and voids between aggregates of particles forming slit-shaped pores of non-uniform
size [59,60]. These observations are in accordance with the TEM micrographs of the
supports (insets in Figs. 1A and 1B), indicating the presence of mesopores with the above-
described characteristics. Table 1 summarizes the textural characteristics of the prepared
supports. The SBA-15 material is known to exhibit high surface areas [61]. In our case, the
2
synthesized SBA-15 support presented a BET surface area of 850 m /g, while the PTN
2
support had SBET of 230 nm /g. In the last case, the surface area of the nanotubular titanate
material (PTN) was remarkably larger than that of the nanopowder TiO precursor used for
2
2
its synthesis (58 m /g).
All prepared nickel catalysts preserved the characteristic shape of the isotherm of
their corresponding supports (Figs. 1A and 1B). However, the textural characteristics of the
Ni catalysts were smaller than those of the corresponding supports. The Ni/SBA15-IM and
Ni/PTN-IM catalysts prepared by the incipient wetness impregnation method showed a
decrease in the BET surface area of 21 and 17 %, respectively. This can be due to the
incorporation of nickel species increasing the materials’ density. It can be mentioned that
the Ni content in the catalysts was around 10 wt. %, which corresponds to about 13 wt. %
of NiO in the calcined samples. Regarding the catalysts prepared by the DP method, the
Ni/SBA15-DP catalyst showed a large decrease in the BET surface area (more than 40 %)
and micropore area (around 70 %) when compared to the SBA-15 support. This can be

16
attributed to a partial dissolution of the silica material at a high pH values and the formation
of a new nickel phyllosilicate phase precipitated on the surface of the starting SBA-15
support [57]. A similar behavior was not observed for the Ni/PTN-DP catalyst due to the
different chemical composition of the nanotubular support, its stability at basic pH values
and a slower pH increment [62].
Regarding pore characteristics of the supports and catalysts (Table 1), SBA-15 had a
narrow pore size distribution centered at 7.7 nm, whereas the PTN material had a bimodal
pore distribution (not shown) with the first maximum corresponding to the inner diameter
of the nanotubes (5.4 – 7.5 nm) and the second one attributed to larger free spaces between
aggregates of nanotubes (50 - 80 nm) [63]. The prepared Ni catalysts showed only small
variations in the adsorption and desorption pore diameters, except for the Ni/SBA15-DP
catalyst, where some smaller pores (3.3. nm) emerged owing to the laminar structure of the
nickel phyllosilicate.
Fig. 2 shows the X-ray diffraction patterns of the supports and catalysts between 10
and 80° (2). The diffraction patterns of the SBA-15 support and oxide Ni/SBA15 catalysts
(Fig. 2A) exhibited a broad signal between 15 and 30° (2) characteristic of amorphous
silica. In addition, the Ni/SBA15-IM catalyst presented five reflections at 37.2, 43.3, 62.9,
7
0
5.4 and 79.3° (2), that can be ascribed to the crystalline NiO phase (JCPDS-ICDD card
1-071-1179). For the Ni/SBA15-DP catalyst, two reflections were clearly observed at 34.3
and 60.6° (2) corresponding, as expected, to the nickel phyllosilicate phase
Ni Si (OH) , JCPDS-ICDD card 00-049-1859). In addition, a broad low-intensity signal
(
3
2
O
5
4
was observed at 43.3° (2) indicating the possibility of the formation of some small crystals
of the NiO phase. A comparison among the intensity and the width of the signals observed

17
in the XRD patterns of the two Ni/SBA15 catalysts, suggests that the DP method resulted
in a better dispersion of the deposited Ni-containing crystalline species than the classical
incipient wetness impregnation method.
On the contrary, the diffraction patterns of the PTN support and the corresponding
oxide Ni/PTN catalysts (Fig. 2B) exhibited only the reflections associated with the
protonated trititanate phase (H
2
Ti
3
7
O , JCPDS-ICDD card 00-047-0561). No evidence of the
formation of a nickel oxide or other nickel-containing crystalline phase was detected in the
XRD patterns of the PTN-supported Ni catalysts, pointing out to a good dispersion of the
Ni oxide species. It can also be noted that the crystalline structure of the nanotubular
support was almost not affected by the preparation method used. The only difference
between the diffraction patterns shown in Fig. 2B is a slight decrease in the intensity of the
signal located at about 11.2 ° (2) in the patterns of the Ni/PTN catalysts. This peak, clearly
observed for the PTN support, corresponds to the (2 0 0) crystal plane of monoclinic
protonated trititanate phase and represents its interlayer distance, equal to 0.78 nm. A lower
intensity of this signal after the incorporation of Ni(II) species, by the impregnation or the
DP with urea methods, can be explained by some distortion in the interlayer distances of
+
the Ni catalysts. This is probably due to the cationic exchange taking place between H ions
and Ni2+ cations during the preparation of Ni catalysts using aqueous nickel(II) nitrate
solutions as in the present work. The possibility of the ion-exchange between the protons of
2
+
2+
the protonated titanate nanotubes and the Ni ions, as well as the adsorption of the Ni
cations on the negatively-charged inner, and the outer, surfaces of the titanate nanotubes
have been reported previously [64-66]. Some of the authors [64] considered that due to the
2
+
+
2+
larger size of the Ni cations in comparison with the H ions, it may be difficult for Ni to

18
replace protons in the interlayer spaces of the trititanate structure. On the other hand, other
authors [65,66] suggested the possibility of the intercalation of some transition-metal ions
2
+
2+
2+
(
Co , Ni , Zn , etc.) between the layers of the multilayered titanante nanotubes.
However, the substitution of the protons by Ni2+ ions should not affect strongly the
interlayer distance because of the rigidity of the titanate framework [64,66]. Nevertheless,
the above cationic exchange would result in a lower total amount of metal cations in the
interlayer space since one Ni2+ cation substitutes two protons, as well as in different
degrees of ion exchange along the nanotubular structure. The above would produce some
irregularities in the interlayer spacing of the multilayered trititanate structure decreasing the
intensity of the (2 0 0) reflection.
TPR profiles of the calcined Ni catalysts are shown in Fig. 3. Nickel oxide species
are reduced to metallic nickel in a single step (NiO + H
the Ni/SBA15-IM catalyst presents two principal reduction signals with maxima at 343 and
41 °C and a small shoulder at 268°C (Fig. 3A). The presence of different overlapped
2
→ Ni + H O). The TPR profile of
2
4
reduction peaks in the reduction profile of this sample indicate the presence of nickel
species of different characteristics. A comparison of the TPR profile of the Ni/SBA-15
catalyst prepared by a classical incipient impregnation with that of unsupported bulk NiO
o
(
one reduction peak with maximum at 329 C) shows that low-temperature hydrogen
o
consumption (signal with the maximum at 343 C) of the Ni/SBA15-IM catalyst could be
ascribed to the reduction of the crystalline NiO phase located on the external surface of the
SBA-15 support and detected in this catalyst by XRD (Fig. 2A). On the other side, the
o
signal at 441 C corresponds to the reduction of NiO in stronger interaction with the silica
support or nickel oxide species located in the interior of the mesoporous structure of SBA-
o
1
5 [67]. Finally, the small shoulder at 268 C could evidence the presence of some traces of

19
o
nickel nitrate remaining after calcination of the Ni/SBA15-IM catalyst at 500 C for 2 h
[
68]. In contrast, the Ni/SBA15-DP catalyst showed only one well-defined peak with the
o
maximum at 552 °C and a small shoulder at about 441 C. This result indicates that the use
of the DP with urea method resulted in a more homogeneous distribution of reducible
nickel species of a similar degree of agglomeration, which are in stronger interaction with
the silica support than those deposited by the classical impregnation method. A significant
2
+
increase in the temperature of reduction of the Ni species in the Ni/SBA15-DP catalyst
can be attributed to the formation of the nickel phyllosilicate crystalline phase, where nickel
is embedded in the silica structure forming Ni-O-Si linkages [69]. On the other side, the
shoulder observed at 441 °C can be ascribed to the reduction of the nickel oxide species
located inside the SBA-15 mesopores coexisting with the nickel phyllosilicate phase [67].
In contrast with the above results obtained for the Ni/SBA-15 catalysts, in the
reduction patterns of the two Ni catalysts supported on the protonated titanate nanotubes,
only one, almost symmetrical reduction peak was observed (Fig. 3B). This can indicate that
the nickel species in the PTN-supported catalysts had similar characteristics, regardless of
the preparation method used (IM or DP). Only a small difference was observed in the
position of the reduction peaks, namely, Ni species in the Ni/PTN-DP catalyst were able to
o
be reduced at a slightly lower temperature (375 C) than those in the Ni/PTN-IM analog
o
(
381 C). The above TPR results clearly illustrate the effect of the support's nature and the
preparation method on the characteristics of the deposited Ni oxide species in the prepared
catalysts.
The degree of reduction of Ni2+ in each catalyst (α, Table 2) was calculated as a
ratio of the experimentally determined hydrogen consumption by the catalyst to the
theoretically estimated value based on the real Ni content in the sample determined by

20
SEM-EDX (Table 1). It was observed that the Ni catalysts supported on PTN had higher
degrees of reduction than the SBA-15-supported ones. The Ni/PTN-IM catalyst showed the
highest degree of reduction (α = 0.96), whereas the Ni/SBA15-IM catalyst showed the
lowest one (α = 0.72). No clear influence of the preparation method used was observed on
the degree of reduction of the deposited nickel oxide species.
The chemical nature of the surface Ni oxide species in calcined catalysts was further
studied by XPS. Fig. 4 shows the Ni 2p region of the prepared catalysts. The spectra of
unsupported nickel nitrate and nickel oxide are also shown for comparison purposes. The
two main chemical spin states of Ni 2p3/2 and Ni 2p1/2 were observed in the spectrum of
2
+
Ni(NO
3
) at a binding energy of 856.5 and 874.7 eV, representing the Ni state (Fig. 4a).
2
In addition, the two shake-up satellite peaks were found at 861.5 and 881.1 eV,
respectively, for the emission lines of Ni 2p. The XP spectrum of the unsupported NiO
reference had a similar shape to that described above for nickel nitrate, but the positions of
all signals were shifted to lower binding energies. Thus, the principal contributions were
observed at 853.8 and 871.6 eV and the corresponding satellite signals at 859.8 and 878.1
eV (Fig.4b). In both cases, each component of the doublet (Ni 2p3/2 and Ni 2p1/2) of Ni2+
compounds exhibits a main peak and strong satellite located on the high binding energy
side that is characteristic for all transition metal compounds with unfilled 3d level
[
67,70,71]. Ni 2p3/2 binding energies have been considered to be particularly useful for the
characterization of supported nickel catalysts. According to literature [70-72], the position
of the main Ni 2p3/2 peak, the shape of the satellite and the relative intensities of the main
peak and the satellite depend on the nature of the ligands and on the ionic or covalent
2
+
character of the chemical bonds with the Ni cations. The above explains the difference in
the positions of the main Ni 2p3/2 peaks in the spectra of the reference compounds shown in

21
Fig. 4, namely the shift in the position of the Ni 2p3/2 peak of Ni(NO
3
2
) to higher binding
2
+
2-
energy than in the unsupported NiO, could indicate the presence of Ni -O bonds less
covalent than in nickel oxide. The spectra of the supported Ni catalysts in their oxide form
are shown in Fig. 4 (c-f). The Ni/SBA15-IM and Ni/PTN-IM catalysts prepared by the
impregnation method showed the main Ni 2p3/2 contributions at 854.6 and 853.6 eV,
respectively, attributable to the nickel oxide phase [70,73,74]. This peak was wider in the
case of the Ni catalyst supported on SBA-15 that can be due to the presence of NiO species
of different degrees of agglomeration; which is in line with the TPR results obtained for
this catalyst (Fig.3A). The Ni/PTN-IM catalyst showed the Ni 2p3/2 signal almost at the
same position as in the unsupported NiO reference (Fig 4b). This indicates the formation of
NiO particles almost unaffected by the presence of the PTN support. On the contrast, in the
spectrum of the Ni/SBA15-DP catalyst, the main Ni 2p3/2 peak was located at 858.1 eV.
2
+
This suggests a strong change in the chemical environment around the Ni cations in this
sample and can be attributed to the formation of a nickel phyllosilicate phase detected by
XRD (Fig. 2A). Regarding the XP spectrum of the Ni/PTN-DP catalyst, the main peak was
observed at 855.4 eV. A similar signal at 855.5 eV observed previously for Ni/TiO
Ni/CeO samples was assigned to very small NiO particles deposited on titania or ceria
70]. On the other hand, an increase in the binding energy of the nickel oxide phase in the
2
and
2
[
2+
Ni/PTN-DP catalyst, compared to unsupported NiO, can also be ascribed to Ni cations in
more intimate interaction with the titanate support, probably to some surface Ni2+ ions
introduced into the titanate structure by the ion exchange mechanism.
Acid properties of the prepared supports and Ni catalysts were characterized by the
ammonia temperature programmed desorption (NH
3
-TPD) method. Obtained NH -TPD
3
results are presented in Fig. 4 and Table 3. The SBA-15 support showed a much lower total

22
amount of acid sites than the PTN material (58 μmol/g vs. 420 μmol/g, respectively, Table
). The addition of nickel to the SBA-15 and PTN supports produced different effects on
3
the samples’ acidity. Thus, for the catalysts of the Ni/SBA-15 series, an increase in the
amount of acid sites of different strengths (weak, medium and strong) was observed after
the addition of Ni to the SBA-15 support. However, the magnitude of this increase
depended on the method used for the preparation of Ni catalyst (IM or DP with urea, Fig.
4
A). Thus, the total amount of acid sites of the Ni/SBA15-IM catalyst was higher than that
of the starting SBA-15 support and un-supported NiO phase that could be attributed to a
sum of the acid sites of the SBA-15 material and the deposited NiO species (Table 3). In
comparison, the Ni/SBA15-DP catalyst showed a total amount of acid sites threefold higher
than that of the Ni/SBA15-IM sample (Fig. 4A and Table 3). This result clearly points out
to the appearance of new acid sites in the former catalyst after deposition of Ni species on
the SBA-15 support by the DP with urea method (Table 3). According to the previous
reports [71,75], these new acid sites could arise from the formation of a layered nickel
phyllosilicate phase detected in the Ni/SBA15-DP catalyst by powder XRD (Fig. 2A). This
conclusion was made based on the comparison of the acidity of nickel phyllosilicate with
that of the Ni/SiO
amount of acid sites in the nickel phyllosilicate sample [75]. Further characterization of
calcined and reduced nickel phyllosilicate samples by FT-IR and NH -TPD methods
2
catalysts prepared by impregnation observing a significantly larger
3
confirmed this supposition. The origin of acid sites of nickel phyllosilicate was related to
the presence of coordinatively unsaturated (cus) Ni(II) sites near the edges/surfaces of
phyllosilicate and surface OH groups. The IR spectra of pyridine (Py-IR) adsorbed on
nickel phyllosilicate catalysts confirmed the presence of Lewis acid sites related to cus-
-1
Ni(II) sites, meanwhile the absence of a band at 1540 cm ruled out Brönsted acidity

23
caused by surface OH groups. Nevertheless, it was observed that surface OH groups were
able to interact with pyridine via H-bonds formation [75]. In work [71], Ni/MCM-41
catalysts were prepared by a template ion exchange approach leading to the formation of
nickel silicate with layered structure. It was also observed that introduction of nickel to the
MCM-41 support resulted in the formation of two additional types of acid sites with weak
and moderate acidity.
On the contrary, the PTN support and the corresponding Ni/PTN catalysts, prepared
by the two methods, showed similar ammonia TPD profiles with the presence of mostly
weak and medium-strength acid sites (Fig. 4B). The total amount of acid sites in both Ni
catalysts was smaller than in the starting PTN material (Table 3). In this case, the amount of
acid sites was proportional to the sample’s surface area, as it can be seen in Table 3, where
2
similar values of the total amount of acid sites (about 2 μmol/m ) were obtained for the
PTN support and two Ni/PTN catalysts. No effect of the preparation method used (IM or
DP) was observed. The above results indicate that the acidity of the Ni/PTN catalysts is
mainly due to the protonated trititanate support. Previously it was shown that this material
behaves like a weak acid [66], and in a more recent work [76] it was demonstrated that
protonated titanate nanotubes possess both Brönsted and Lewis acid sites, whose
-1
concentrations were estimated to be 0.10 and 0.25 mmol g , respectively. This result is
close to the total amount of acid sites in the PTN support determined in the present work
-1
(
0.42 mmol g , Table 3). Finally, it can be mentioned that, in general, the total amount of
acid sites was larger in the Ni/PTN catalysts than in the corresponding Ni/SBA-15 samples,
and that the Ni/SBA15-DP catalyst was the one with the largest amount of strong acid sites
among all the prepared Ni catalysts.

24
3
.2. Characterization of reduced catalysts
Prior to the catalytic activity tests, the prepared nickel catalysts were reduced at 400
C for 4 h. The dispersion of the metallic Ni in the reduced catalysts was characterized by
powder XRD, HRTEM and O chemisorption, while the chemical state of nickel species in
°
2
the reduced catalysts was determined by XPS. Fig. 2 (C and D) shows X-ray diffraction
patterns of reduced Ni catalysts supported on SBA-15 and PTN, respectively. The XRD
pattern of the reduced Ni/SBA15-IM catalyst prepared by the classical incipient
impregnation method showed the presence of two crystalline phases. Thus, low intensity
signals at 37.2°, 43.3° and 62.8° (2θ) corresponded to the NiO crystalline phase (JCPDS-
ICDD card 01-071-1179), while new reflections at 44.7°, 52.1° and 76.8° (2θ) were
0
attributed to the reduced metallic Ni crystals (JCPDS-ICDD card 01-071-4655). According
o
to the work [67], the presence of oxidized nickel phase after reduction at 400 C for 4 h can
be related to small NiO particles located inside the mesoporous channels of the SBA-15
support that makes their reduction more difficult compared to the NiO phase deposited at
the outer surface of SBA-15. The crystal size of the metallic Ni nanoparticles in the reduced
Ni/SBA15-IM catalyst was estimated with the Scherrer equation using the (111) reflection
0
of Ni (the most intense signal at 44.7° (2θ)) giving a value around 17 nm (Table 4).
Regarding the diffraction pattern of the reduced Ni/SBA15-DP catalyst, it showed the
presence of two signals at 34.3 and 60.6° (2) corresponding to the nickel phyllosilicate
phase (Ni
3
Si
2
O
5
(OH) , JCPDS-ICDD card 00-049-1859) and one very low intensity signal
4
at 44.7° (2) which could indicate the presence of some reduced Ni phase. However, the
low intensity and broadness of the last signal did not allow estimation of the size of the

25
reduced Ni species. The above results point out to the incomplete reduction of Ni species in
both samples and to the very small crystalline size of the reduced metallic Ni particles in
the Ni/SBA15-DP one. In comparison with the Ni/SBA-15 samples, the Ni/PTN catalysts
suffered stronger changes after the reduction treatment (Fig. 2D). In both reduced Ni/PTN-
IM and Ni/PTN-DP samples, a mixture of two crystalline phases was observed: titania
anatase (TiO
2
, JCPDS-ICDD card 84-1285) and metallic Ni (JCPDS-ICDD card 01-071-
Ti ) were detected in the
4
655). No reflections of the protonated trititanate support (H
2
3
O
7
patterns of the reduced samples. Therefore, the reduction of these catalysts resulted in the
phase transformation of the hydrogen trititanate support into titania anatase and the
agglomeration of nickel species with the formation of metallic Ni particles detectable by
XRD. The size of the metallic Ni was estimated to be around 10 nm in the reduced Ni/PTN-
DP sample and about 15 nm in the Ni/PTN-IM one (Table 4). Previously, a similar phase
transformation of the titanate into the TiO
2
anatase was observed upon reduction of sodium
o
titanate nanotubes impregnated with nickel chloride solution at 400 C for 2 h [65]. It was
2
+
attributed to the intercalation of Ni ions into the interlayer spaces of the titanate support
what promoted titanate phase transformation upon reduction and growth of metal Ni
particles. The change of nickel chloride to other precursors (nickel nitrate or acetate)
resulted in a lower amount of intercalated Ni2+ ions and less pronounced phase
transformation. Nevertheless, in the present work, we observed the phase transformation of
o
the hydrogen trititanate support into anatase phase at similar reduction conditions (400 C,
4
h) with nickel nitrate as a precursor. This can be due to the lower thermal stability of
hydrogen trititanate nanotubes if compared with the sodium titanate ones or because Ni
loading in our samples was larger than in work [65] (10 wt. % vs. 1.5 wt. %, respectively).

26
The chemical state of nickel in the reduced catalysts was characterized by XPS (Fig.
). As expected, the presence of metallic Ni was observed in all reduced samples. The
6
signals at 852.0 – 852.8 eV and 869.2 – 869.7 eV were assigned to the binding energy of
0
the Ni 2p3/2 and Ni 2p1/2, respectively, for the Ni [65,67,72]. In addition, the presence of
2
+
some oxidized Ni species was detected in the XP spectra of all reduced catalysts. Thus, in
the spectrum of the reduced Ni/SBA15-IM catalyst (Fig. 6a) the signal at 853.7 eV
accompanied by the satellite at 859.2 eV was attributed to the presence of NiO. The
presence of nickel(II) oxide in this reduced sample was also detected by XRD (Fig. 2C).
However, it was assumed that the small NiO particles remaining unreduced were located
inside the SBA-15 mesopores, and because of this, they should be invisible for the XPS
technique [67]. On the other side, NiO could be formed due to the oxidation of metal Ni
particles under air atmosphere at room temperature [77]. The reduced Ni/SBA15-DP
0
catalyst, in addition to the 2p3/2 signal of Ni (852.8 eV), showed the binding energy of the
unreduced nickel phyllosilicate phase (858.3 eV). The position of this peak was close to
that observed in the XP spectrum of the calcined Ni/SBA15-DP sample (858.1 eV, Fig. 4d).
The presence of unreduced nickel phyllosilicate after reduction of the Ni/SBA15-DP
catalyst was confirmed by powder XRD (Fig. 2C). In the case of the two reduced catalysts
supported on PTN (Figs. 6c and 6d), their XP spectra were not well-defined, having a large
amount of noise that probably can be due to a small amount of surface Ni species in these
catalysts. Nevertheless, the presence of a mixture of both reduced Ni and oxidized Ni2+
species cannot be ruled out. In this case, the binding energies of Ni 2p3/2 were observed at
0
8
55.8 - 856.2 eV and Ni 2p1/2 at 873.9 and 874.1 eV, which can be attributed to the Ni2+
cations intercalated into the titanate support structure or to the nickel species existing in the
form of –TiO–NiO–H state [77].

27
The reduced Ni catalysts were also examined by high-resolution transmission
electron microscopy (HRTEM) in order to obtain the Ni particle size distributions. Fig. 7
shows the HRTEM images and particle size distributions of the prepared Ni catalysts. The
particle size distributions were determined from the measurement of at least 100 Ni
particles from each sample. It can be seen in Fig. 7, that the Ni/SBA15-IM catalyst
exhibited randomly distributed nickel particles with a size that varied in a wide range (from
4
to 36 nm). This result is in line with the TPR observations for this catalyst (Fig. 3A),
where the presence of Ni oxide species of different degrees of agglomeration was detected.
The average size of Ni nanoparticles in the reduced Ni/SBA15-IM catalyst was 16.2 nm. In
contrast, the Ni/SBA15-DP catalyst showed a significantly different image than the above
catalyst, Ni/SBA15-IM. At first glance, the presence of some unreduced nickel
phyllosilicate species (seen as dark lines on the support surface) can be noted in this
catalyst, in line with the XRD and XPS results (Figs. 2C and 6b). A more detailed
inspection of the phyllosilicate zone allowed to observe small metallic nickel nanoparticles
embedded into the phyllosilicate phase. These particles are indicated by white arrows in
Fig. 7. The size of the metallic Ni particles in the reduced Ni/SBA15-DP catalyst varied
between 2 and 7 nm, being the average nickel particle size of 3.2 nm, which explains why
they were not clearly detected by XRD (Fig. 2C).
Regarding the reduced Ni/PTN catalysts, their HRTEM images were very similar.
The presence of metallic Ni nanoparticles of different sizes was found in both of them (Fig.
7
). For the Ni/PTN-IM catalyst, Ni nanoparticles sized between 4 and 20 nm were
observed. In this case, the average particle size was 13.8 nm. For the Ni/PTN-DP catalyst,
the presence of smaller Ni nanoparticles with a size from 2 to 16 nm was detected; being
the average particle size of 9.4 nm. On the other side, it was observed that the characteristic

28
shape of the titanate support was still maintained in the reduced catalysts. This allowed to
assume that not all of the titanate support was transformed into the titania anatase phase. It
is possible that remains of the hydrogen trititanate phase are still present in the support after
the reduction. The above XRD and HRTEM results showed that both: the support's nature
and the preparation method, have an influence on the dispersion of the catalytically active
metal Ni nanoparticles. Both characterization techniques (XRD and HRTEM) showed
similar results (Table 4), namely an increase in the average particle size of metallic Ni in
the reduced catalysts in the following order Ni/SBA15-DP < Ni/PTN-DP < Ni/PTN-IM <
Ni/SBA15-IM, indicating that, in general, the use of the classical IM method resulted in
larger Ni nanoparticles than the DP with urea method.
Taking into account that the above characterization techniques are more sensitive
for relatively large metallic Ni nanoparticles and their application is limited for very small
metal particles, oxygen chemisorption was used to evaluate the dispersion of reduced Ni
particles in the prepared catalysts. The advantage of this method is that O chemisorbs on
2
all surface atoms of the metal particles independently of their size and, therefore, the
0
correct determination of the total amount of Ni atoms on the surface of the reduced
catalysts can be performed. Results from O chemisorption are presented in Table 4. It can
2
be seen that the dispersion of Ni in the prepared catalysts varied in a wide range (from 6.0
to 48.7 %). For the Ni/SBA-15 catalysts, a threefold increase in Ni dispersion was observed
when the method of preparation of the catalyst was changed from impregnation to
deposition-precipitation with urea. On the other side, both PTN-supported Ni catalysts
showed low dispersion of Ni (6.0 – 6.9 %) that was almost not affected by the preparation
method used. This result is in line with the results from the characterization of the reduced
catalysts by XRD and HRTEM described above (Figs. 2D and 7). Therefore, it can be

29
concluded that Ni catalysts supported on SBA-15 and PTN materials behave different upon
reduction. In the first case, the characteristics of the deposited NiO species in the calcined
Ni/SBA-15 catalysts determine the characteristics of the reduced ones. In contrast, well-
0
dispersed NiO in the PTN-supported catalysts transforms into large Ni particles upon
reduction, and this is accompanied by a phase transformation of the titanate support into
titania anatase.
Since the acidic properties of the reduced catalysts are important for their activity
and selectivity in the HDO of anisole, they were characterized by ammonia TPD. Fig. 5 and
Table 3 show the obtained results. It was found that all reduced catalysts showed a slight
increase in the total amount of acid sites (Table 3), and especially in the amount of
medium-strength acid sites (Fig. 5) in comparison with the corresponding calcined samples.
The total amount of acid sites in the reduced catalysts increased in the following order:
Ni/SBA15-IM < Ni/SBA15-DP < Ni/PTN-IM < Ni/PTN-DP. The above changes in the
acidity of the catalysts after reduction could be related to the change in the oxidation state
of nickel and its agglomeration with the formation of metallic Ni nanoparticles.
3
.3. Catalytic performance in hydrodeoxigenation of anisole
Catalytic performance of the prepared Ni catalysts was tested in
hydrodeoxygenation of anisole. This reaction has already been performed with nickel
catalysts supported on different materials (activated carbon, SBA-15, Al-SBA-15, Al
TiO , CeO ) [50,51]. Fig. 8 shows conversions of anisole obtained with the Ni catalysts
2
3
O ,
2
2
prepared in the present work. It can be observed that the Ni catalysts supported on SBA-15
resulted in higher anisole conversions than those supported on PTN. Both catalysts: