Influence of noble metals (Pd, Pt) on the performance of Ru/Al2O3 based catalysts for toluene hydrogenation in liquid phase

Article abstract of DOI:10.1016/j.apcata.2016.06.038

Catalytic hydrogenation of aromatic compounds is of great interest due to environmental aspects and the wide range of industrial processes involving such reaction. In this context, the present work aims to study the influence of Pd or Pt addition on the performance of Ru/Al₂O₃ based catalysts for toluene hydrogenation in liquid phase. For this, catalysts were prepared by wet impregnation from chlorinated precursors and reduced in liquid phase by formaldehyde (H₂CO). After impregnation, a part of the catalysts were activated ex situ at 573?K or in situ at 523?K under H₂. The studied solids were characterized by N₂ physisorption, SEM?+?EDX, TEM, XPS and TPR techniques. Catalytic tests were conducted in a slurry Parr reactor at 373?K under constant H₂ pressure of 5?MPa. Results show that solids reduction by H₂CO led to metallic species, while the activation treatments form oxides and decrease the catalytic activity. The initial reaction rate of non-activated monometallic catalysts follows the order: Ru/Al₂O₃???Pd/Al₂O₃?≈?Pt/Al₂O₃. A synergistic effect on the activity of Ru/Al₂O₃ based catalysts is induced by the Pt addition.

Full text of DOI:10.1016/j.apcata.2016.06.038

Applied Catalysis A: General 525 (2016) 41–49
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Influence of noble metals (Pd, Pt) on the performance of Ru/Al O3
2
based catalysts for toluene hydrogenation in liquid phase
a,∗
b
a
Raphael Soeiro Suppino , Richard Landers , Antonio José Gomez Cobo
a
Laboratory for Development of Catalytic Processes, Department of Chemical Systems Engineering, School of Chemical Engineering, University of Campinas,
Cidade Universitária Zaferino Vaz, CEP 13083-852, Campinas, SP, Brazil
Laboratory of Surface Physics, Department of Applied Physics, “Gleb Wataghin” Institute of Physics, University of Campinas, CEP 13083-859, Campinas, SP,
b
Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 April 2016
Received in revised form 10 June 2016
Accepted 30 June 2016
Available online 1 July 2016
Catalytic hydrogenation of aromatic compounds is of great interest due to environmental aspects and
the wide range of industrial processes involving such reaction. In this context, the present work aims
to study the influence of Pd or Pt addition on the performance of Ru/Al2O3 based catalysts for toluene
hydrogenation in liquid phase. For this, catalysts were prepared by wet impregnation from chlorinated
precursors and reduced in liquid phase by formaldehyde (H2CO). After impregnation, a part of the cata-
lysts were activated ex situ at 573 K or in situ at 523 K under H2. The studied solids were characterized by
N2 physisorption, SEM + EDX, TEM, XPS and TPR techniques. Catalytic tests were conducted in a slurry Parr
reactor at 373 K under constant H2 pressure of 5 MPa. Results show that solids reduction by H2CO led to
metallic species, while the activation treatments form oxides and decrease the catalytic activity. The initial
reaction rate of non-activated monometallic catalysts follows the order: Ru/Al2O3 ꢀ Pd/Al2O3 ≈ Pt/Al2O3.
A synergistic effect on the activity of Ru/Al2O3 based catalysts is induced by the Pt addition.
Keywords:
Noble metal
Alumina
Catalyst activation
Formaldehyde
Toluene hydrogenation
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
benzene due to its chemical similarities and lower toxicity [13–15].
In addition, the toluene hydrogenation is often used as probe reac-
tion to test the performance of metal catalysts since it is considered
insensitive with respect to surface structures [16].
Catalytic hydrogenation of aromatic compounds is of great
interest due to environmental aspects and the wide range of
industrial processes involving these reactions. It is an important
route to obtain several chemical intermediates as well as to elim-
inate toxic aromatic compounds present in fuels [1,2]. Indeed, the
hydrodearomatization is one of the most important processes in a
petroleum refinery [3], having great influence on the final quality
of diesel [4,5].
Many catalysts have been tested on the hydrogenation of aro-
matics, amongst which noble metals such as Pd, Pt and Ru, usually
supported on Al O , have shown higher activities and stability
2
3
[3,13]. It is noteworthy that Ru based catalysts have been increas-
ingly employed, since higher yields of the intermediate product are
obtained in presence of this metal [11,17–19].
Most of the work found in the specialized literature focuses on
the hydrogenation of benzene, notably aiming to obtain the par-
tially hydrogenated product, cyclohexene [6–11]. In a recent paper,
Foppa and Dupont [12] presented a thorough review regarding the
main advances achieved for the partial hydrogenation of benzene
in the last four decades.
However, recent efforts have been made to reduce the use of
benzene in researches, since this compound is known to be highly
carcinogenic. Hence, toluene has been employed as a substitute of
Although the incipient impregnation is a common method to
prepare supported catalysts [12,20], the wet impregnation has been
acknowledged as a method that leads to solids with higher metal-
lic dispersion and consequently higher activity [8,11,21–23]. The
characteristics of the solids prepared by wet impregnation may
be affected by variables present in this method [24], such as the
suspension temperature and pH. However, the reducing procedure
appears to induce effects that are more important since it involves
the formation of the active phase of these solids [23].
In wet impregnation, the catalyst reduction procedure is usu-
ally carried out in liquid-phase employing reducing agents such
as sodium borohydride (NaBH ) [25], hydrazine (N H ) [24] or
4
2
4
∗ Corresponding author.
E-mail addresses: rsuppino@feq.unicamp.br, raphaelsuppino@gmail.com
formaldehyde (H CO) [11], under mild conditions. However, high
2
(
R.S. Suppino).
http://dx.doi.org/10.1016/j.apcata.2016.06.038
926-860X/© 2016 Elsevier B.V. All rights reserved.
0

4
2
R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
temperature reduction may also be conducted ex situ under H flow
pension. After this reduction in liquid phase, the suspension was
filtered in a Büchner funnel and the remaining solid was thoroughly
washed with deionized water in order to remove residual chlo-
rine, sodium and formaldehyde. During the washing procedure, the
chlorine and sodium eliminations were respectively observed by
AgNO3 and flame tests.
2
[
8,26,27].
Suppino et al. [11] studied the influence of the reduction method
on the performance of Ru/Al O catalysts prepared by wet impreg-
2
3
nation for partial hydrogenation of benzene in liquid phase. Catalyst
reduction by H CO led to a higher activity and selectivity of cyclo-
2
hexene than the reduction ex situ under H2 flow.
Afterwards, the solids were dried in an oven at 358 K for 24 h.
After drying, a part of the obtained catalysts were submitted to ex
situ or in situ activation. In the ex situ activation, the catalyst was
placed in a Pyrex glass cell and heated at 10 K/min under H2 flow
of 40 mL/min from the room temperature until 573 K, remaining at
this temperature for 3 h. Catalysts submitted to ex situ activation
have the E abbreviation on its denominations.
Groppo et al. [28] prepared Pd/Al O3 catalysts by wet impreg-
2
nation using the following reducing agents: sodium formate
(
HCO Na), NaBH and H . According to the authors, the reducing
2 4 2
agents decrease the metal dispersion, which was related to a pos-
sible sintering of the Pd. The occurrence of Pd sintering was also
suggested by other authors [29,30], according to whom the metallic
particles may experience some degree of mobility even in relatively
low temperatures, especially when the catalyst reduction is carried
out under H2 atmosphere.
The use of bimetallic catalysts for hydrogenation reactions has
been subject of many studies. Bimetallic catalysts often present
superior properties than the respective monometallic solids, such
as higher tolerance against sulfur poisoning and thermal stability
In situ activation took place in the reactor itself, where the cat-
alyst reduced by H CO was submitted to the H2 pressure of 3 MPa
2
at 523 K during 1 h. Catalysts submitted to in situ activation have
the I abbreviation on its denominations.
Bimetallic catalysts were prepared by co-impregnation,
employing the wet impregnation procedure described above. It
is noteworthy that only the ex situ activation was studied for
bimetallic catalysts because of the little effect induced by in situ
activation on the active phase formation of monometallic solids,
as discussed hereafter.
[
19,31–34].
Romanenko et al. [31] studied the addition of Ru to Pd/C catalysts
in order to prevent the Pd sintering by the solid reduction under H2
flow. According to the authors, the Ru addition led to an increase
of the metallic dispersion on the catalyst. Moreover, the bimetallic
Pd-Ru/C catalyst has proven to be more resistant to sintering. Such
effect was related to the increase of the potential energy barrier for
the mobility of Pd species promoted by the presence of Ru.
2.2. Support and catalysts characterization
The support ␥-Al O was characterized by potentiometric titra-
2
3
tion in order to determine its isoelectric point, an important
parameter for the wet impregnation procedure. A digital micro-
processed pH meter (Marconi, model MA522) was used to take
pH measurements according to the procedure of Strelko and Malik
More recently, Chen et al. [35] evaluated the effects of the
addition of Pd to Ru based catalysts prepared by the wet co-
impregnation method and reduced by NaBH . According to the
4
authors, bimetallic Ru-Pd catalysts presented superior catalytic
activity over monometallic ones as well as smaller nanoparticle
size with narrower distribution.
[
36].
The specific surface area (Sg) of the solids was determined
through N2 physisorption (B.E.T. method). A sample of 1.00 g of
each solid was previously dried at 473 K under vacuum and the
physisorption was conducted at 77 K in a Tristar Micromeritics
ASAP 2010 equipment.
In this context, the present work aims to study the influence of
Pd or Pt addition on the performance of Ru/Al O based catalysts
2
3
for toluene hydrogenation in liquid phase.
Scanning Electronic Microscopy (SEM) coupled with spectro-
metric X-ray analysis (SEM + EDX) was used mainly with the
purpose of evaluating the chemical composition of the catalysts.
The analyses were conducted in a LEO 440i Leica equipment. Before
insertion in the SEM, all samples were covered with a fine layer of
gold atoms using a 3 mA current for 180 s in order to obtain a gold
film thickness of 92 Å.
Transmission Electronic Microscopy (TEM) analyses were car-
ried out on a Libra 120 Zeiss microscope with Cantega 2k/Olympus
CCD camera and iTEM data acquisition platform. The samples were
gently grinded and then dispersed in water. The dispersion was
placed on ultrasound for 10 min and then left to rest for another
2
. Experimental
2.1. Catalysts preparations
Alumina (Al O ) of commercial grade was used as received as
2
3
catalysts support. According to the manufacturer (Alfa Aesar), the
solid (99.9 wt%) is in the gamma phase, with an average particle
diameter of 40 ␮m.
Mono and bimetallic catalysts were prepared from the precur-
sors RuCl ·xH O, PdCl (Aldrich Chemical Co.) and PtCl (Santa Cruz
3
2
2
2
Biotechnology), all with 99.9 wt% of purity, in order to obtain a total
metal mass fraction of 5 wt%.
1
0 min. A drop of the solution was placed on a 300 mesh cupper
grid coated with parlodium and carbon. The grids were dried at
ambient temperature and examined at 80 kV using the energy fil-
ter at zero loss, 25 eV, 30 eV or 50 eV. The energy positions of 25 eV
and 50 eV correspond to the first and second plasmon.
Wet impregnation was used for the preparation of monometallic
catalysts. In the procedure, deionized water was added to the sup-
port resulting in a suspension continuously agitated by a magnetic
stirrer at room temperature.
X-ray Photoelectrons Spectroscopy (XPS) was employed in
order to study the chemical compounds on the catalysts surfaces.
A spherical analyzer VSWHA-100 with aluminum anode (Al K␣,
hv = 1486.6 eV) was used. The pressure during the analyses was
Since both the Pd and Pt chlorides are insoluble in water, the
precursors of such metals were previously dissolved in aqua regia
(
1 HCl:1 HNO ). The resultant solution was then heated under
3
constant stirring until its complete vaporization, thus remaining
a metal salt of Pd or Pt that was dissolved in water.
The aqueous solution of the metal (Pd, Pt or Ru) was then slowly
added to the support suspension. Afterwards, the resultant suspen-
sion was heated until 353 K and then its pH was adjusted to 10 by
adding a 2 M aqueous solution of NaOH.
−
12
lower than 2.10
MPa. To correct binding energies, the line Al
2
p with binding energy of 74.0 eV was used as reference.
The formation of the catalysts active phases was studied through
temperature programmed reduction (TPR). A Micromeritics Auto
Chem 2910 equipment was used to obtain the TPR profiles. In these
analyses, a sample of 50 mg of each solid was heated at 10 K/min
In sequence, an aqueous solution of formaldehyde (H CO,
2
from 298 to 573 K under 50 mL/min flow of a 10% H in N mixture.
Merck, 37 wt%), used as reducing agent, was added to the sus-
2
2

R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
43
Table 1
Specific surface area (Sg) and elementary chemical composition of solids.
2
Solid
Sg (m /g) Mass fraction (%) by M/Ru atomic ratio
EDX
Al
O
Ru
M
nominal by EDX by XPS
Al2O3
97.2
98.7
99.5
44 56 0.0 0.0
40 55 0.0 4.9
45 51 0.0 4.5
43 53 4.3 0.0 0.00
45 53 2.3 0.5 0.24
44 51 3.2 1.8 0.63
46 51 2.6 0.6 0.13
41 53 2.9 2.3 0.35
42 53 1.5 3.1 0.78
–
–
–
–
–
–
0.00
0.22
0.55
0.23
0.41
1.1
–
–
–
0.00
0.30
1.00
0.25
0.42
0.67
Pd/Al2O3
Pt/Al2O3
Ru/Al2O3
105
4
3
4
3
2
Ru-1Pd/Al2O3 105
Ru-2Pd/Al2O3 102
Ru-1Pt/Al2O3
Ru-2Pt/Al2O3
Ru-3Pt/Al2O3
94.3
98.0
101
M: Pd or Pt.
Fig. 1. Proton affinity curves of the ␥-Al2O3 support.
the present study is slightly below this range. Such difference may
be due to an inaccuracy of the potentiometric titration, since this
technique is sensitive to the volume of electrolytes added during
the procedure.
2
.3. Catalytic tests
Catalytic tests for hydrogenation of toluene (Sigma Aldrich, 99%)
were performed in a slurry Parr reactor at 373 K, under constant H2
pressure of 5 MPa and at a stirring rate of 1000 rpm.
The pH of 10, much higher than the isoelectric point of the sup-
port, was adopted in this study for the wet impregnation in order
to assure an efficient fixation of the noble metals. Particularly, since
the aqua regia treatment of Pd and Pt precursors possibly formed
These tests were conducted with 25 mL of toluene, 30 mL of dis-
tilled water and 5 mL of n-heptane (Sigma Aldrich, 99%) used as
internal standard for the chromatographic method. The presence
of water is paramount in order to obtain partially hydrogenated
products [12,37]. The n-heptane was considered to be a compound
suitable for the analytical purposes, because it is very soluble in
the organic phase (where the reaction products are) and easily
separated from the reaction products during the chromatographic
analysis, besides being chemically inert under the reaction condi-
tions.
Samples of the organic phase were collected during the reac-
tion and analyzed using an HP 5890 series II gas chromatograph
equipped with a flame ionization detector and a 0.25 mm by 25 m
capillary column of dimethylsyloxane phase.
Conversion of toluene (X) and yield of methylcyclohexene (Y)
were calculated according to Eqs. (1) and (2), respectively. The
experimental values were used to adjust the Y versus X graph
according to the reaction mass balance.
[PdCl4]2 and [PtCl6] , the use of high pH condition is recom-
mended by other researchers [40–42], according to whom it favors
the formation of hydroxide species of these metals which are then
deposited on alumina.
−
2−
Specific surface areas of the studied solids are presented in
Table 1. The results indicate an increase of the specific surface area
for almost all catalysts with respect to the support. As suggested by
Kawi et al. [43], such increase may be related to the formation of
metal hydroxides by the addition of NaOH during the wet impreg-
nation procedure. The formation of such hydroxides was observed
in a previous study of our research group [11].
3.2. Chemical composition
Elementary chemical composition of studied solids, obtained
through EDX analysis, is presented in Table 1, where M stands for
metals (Pd or Pt) other than Ru.
For monometallic catalysts the respective metal load is very
close to each other and to the desired amount (5 wt%). In turn, for
the bimetallic catalysts the metal loads diverged from the nominal
values, especially for low content of added metal (1 wt%). Increasing
moles of reacted toluene
X =
Y =
(1)
(2)
initial moles of toluene
moles of methylcyclohexene formed
initial moles of toluene
the amount of the added metal (e.g. 3Ru-2Pd/Al O ), the metal load
2
3
In order to determine the initial reaction rate, instantaneous
reaction rates were calculated from experimental data of toluene
concentration versus reaction time. Through a linear fit of these
rates, preferably for the values close to the beginning of the reac-
tion, it was made an extrapolation to zero reaction time.
measured by EDX becomes closer to nominal values. Such effect can
be due to the competitive adsorption of the metals on the support
surface, which favors the Ru for low concentration of added metal.
As seen on Table 1, the M/Ru atomic ratios obtained by EDX
are near to the respective nominal values. These results indicate
that, despite deviation on the absolute amounts, both metals were
fixated on the support in the desired proportion.
3
. Results and Discussion
Nevertheless, the 2Ru-3Pt/Al O3 catalyst presented a Pt/Ru
2
3
.1. Proton affinity of the support and specific surface areas of
atomic ratio much higher than expected. This result may indicate
a competitive adsorption of the metals, in which Pt cations are
favored over those of Ru. The XPS analysis also allowed the evalu-
ation of the relative M/Ru amounts on the surface of the catalysts,
indicating an accordance to the EDX results for most of the solids.
solids
Since the ␥-Al O support is known to exhibit an amphoteric
2
3
behavior, the determination of its isoelectric point is essential for
a better fixation of the metal cations by the wet impregnation
method [38]. The Fig. 1 presents the proton affinity curves obtained
for the ␥-Al O , from which it was determined that the isoelectric
The M/Ru atomic ratio of the 3Ru-2Pd/Al O3 catalyst was much
2
higher when obtained by XPS than by EDX, which indicate that Pd
is closer to the catalyst surface than Ru. This close proximity may
be due to the coverage of Ru species by those of Pd.
2
3
point of this support is 6.2.
According to Kosmulski [39], the isoelectric point of ␥-Al O is
In contrast, the 2Ru-3Pt/Al O3 catalyst presented a lower M/Ru
2
3
2
located between the pH of 7 and 8, while the value obtained in
ratio by XPS, suggesting that Ru is closer to the catalyst surface

4
4
R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
Fig. 2. Normalized XPS spectra of mono and bimetallic catalysts: Chart (I) Pd 3d, Chart II Al 2p + Pt 4f, Chart (III) C 1s + Ru 3d.
than Pt. Thus, this close proximity may be due to the coverage of Pt
species by those of Ru.
energies corresponding to phases PtO (73.6 eV) or Pt(OH) (72.1 eV)
were obtained (Fig. 2II.a).
2
No sodium or chlorine content was detected either by EDX or
XPS in any prepared catalyst, which indicates an efficient removal
of both ions by the washing of the solids.
In Fig. 2 (Chart II) one may observe that the ex situ activation
treatment had little effect for Pt based catalysts. The ex situ acti-
vated Pt/Al O -E catalyst (Fig. 2II.b), presented the same binding
2
3
XPS technique was also employed with the purpose of identify-
ing the probable species of noble metals on the catalysts surfaces.
The Fig. 2 presents the XPS spectra of Pd, Pt and Ru for selected
energy of 70.7 eV as the non-activated solid, which seems to cor-
respond to irreducible forms of Pt, as the TPR results indicate. It is
noteworthy that only Pt 4f7 binding energies were considered for
this analysis, since the spectra of Pt 4f overlap with the one of Al
2p (74 eV).
/2
5/2
7/2
5/2
mono and bimetallic catalysts. Binding energies of Pd 3d , Pt 4f
5
/2
and Ru 3d for the analyzed solids are shown in Table 2.
The identification of the probable metal species was based on
XPS patterns obtained from the National Institute of Standards
and Technology (NIST) and the La Surface2 databases. No catalyst
submitted to in situ activation was analyzed by XPS since other
techniques indicated no significant effect on the formed species, as
discussed hereafter.
A non-reduced monometallic Ru catalyst (Ru/Al O -NR) was
2 3
prepared with the purpose of being a baseline on the study about
1
5/2
the role of the agent H CO in the wet impregnation. The Ru 3d
2
binding energy of 281.6 eV may be related to the presence of
Ru(OH)3 formed by NaOH addition during the wet impregnation.
The positive identification of Ru(OH)3 via XPS is challenging and
only recently Morgan [44] obtained an XPS pattern for this specie
with Ru 3d5 binding energy of 282.3 eV. Hence, since no chlo-
rine from the RuCl3 precursor was found on the Ru/Al O -NR solid
In general, the XPS results show that the catalyst reduction in
liquid phase led to the formation of metallic species, indicating that
/2
the reducing agent H CO is able to reduce noble metal precursors
2
2
3
in mild conditions. Nevertheless, the procedures for the catalyst
activation have different influence on the formation of the metals
species, notably depending on their nature.
either by EDX or XPS techniques, the formation of Ru hydroxides
seems a plausible assumption.
Binding energies for non-activated Ru/Al O catalyst (280.5
2
3
Binding energies obtained for the non-activated Pd/Al O cata-
and 282.1 eV) indicate the coexistence of Ru0 (280.0 eV) and RuO3
2
3
0
lyst (335.8 and 338.0 eV) indicate the coexistence of the following
(282.3 eV), which may have been formed from Ru phase during the
0
phases: Pd (335.3 eV) and PdO2 (337.7 eV). In addition, the possi-
exposure of the solid to the atmospheric air. However, the presence
of Ru hydroxides cannot be discounted in this case, given the results
of Morgan [44]. Nevertheless, these results indicate that similarly
to the case of the Pd/Al O3 and Pt/Al O3 catalysts, Ru hydroxides
ble presence of Pd(OH)4 (338.5 eV) suggests that H CO could not
2
0
reduce completely this compound to form Pd , whereas the PdO
specie may have been formed from oxidation of Pd during the solid
2
0
2
2
exposure to the atmospheric air.
can be reduced by H CO in mild conditions.
2
Monometallic Pd catalyst activated ex situ (Pd/Al O -E) present
The Fig. 2 (Chart III) shows that with the ex situ activation
2
3
5/2
5/2
a very low binding energy (334.0 eV) for Pd 3d , not observed
in the spectrum of non-activated Pd/Al O catalyst. Such low
both Ru 3d
peaks are shifted towards higher binding energies
(Fig. 2III.a and III.b). Indeed, XPS binding energies for the ex situ acti-
vated Ru/Al O -E catalyst (281.1 and 282.8 eV) are possibly related
2
3
energy may be associated to the formation of aluminate phases like
PdAl O , which may have been formed by interaction between Pd
2
3
to oxides species RuO2 (280.7 eV) and RuO3 (282.3 eV). Along with
2
4
0
and Al O support. In addition, it has a binding energy of 335.7 eV
the absence of Ru , it seems that the ex situ activation induce an
2
3
0
corresponding to Pd (335.3 eV) and possibly also to PdO (336.1 eV).
The effect of the ex situ activation treatment for this solid may be
observed in Fig. 2 (Chart I). For both solids (Fig. 2I.a and I.b) two
distinct peaks are observed, however the ex situ activation led to a
shift of these peaks towards lower binding energies for Pd 3d.
intense Ru oxidation when the catalyst is exposed to the contact
with atmospheric air.
Given the XPS results obtained for monometallic catalysts, only
the bimetallic solids reduced by H CO and non-activated were
2
characterized by XPS, in order to evidence the effect of the metal
(Pd or Pt) loading to the bimetallic catalysts.
7
/2
In turn, the binding energy of Pt 4f
for the non-activated
0
Pt/Al O catalyst (70.7 eV) is probably related to Pt (71.1 eV) as
On both Ru-Pd catalysts (4Ru-1Pd/Al O3 and 3Ru-2Pd/Al O )
2
3
2
2
3
well as to a reducible form of Pt (PtOx-Al O ), as suggested by
the TPR results to be discussed posteriorly (Table 2). No binding
the binding energies of Ru were considerably close (280.0 and
279.7 eV, respectively) and possibly related to the existence of Ru0
species (280.0 eV). Whilst Ru remain on its reduced phase, the bind-
ing energies obtained for Pd on these solids (337.5 and 336.8 eV,
respectively) suggest the existence of oxidized species such as
2
3
PdO (337.7 eV) and PdO (336.1 eV). Metallic Pd was also identified
2
1
Data available at: http://srdata.nist.gov/XPS.
Data available at: http://lasurface.com/database/elementxps.php.
(
335.0 eV), notably on the 3Ru-2Pd/Al O3 solid. Moreover, there is
2
2

R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
45
Table 2
Physical and chemical characteristics of the catalysts.
Catalyst
TEM analysis
XPS analysis
Binding energies (eV)
TPR analysis
Probable surface speciesa Temperature (K) H2 consumption
(␮molH2/mgmetal)
dp (nm) ␴ (nm) Metal (Pd 3d5/2 or Pt 4f7/2
)
Ru 3d5/2
Per peak Total
0
Pd/Al2O3
Pd/Al2O3-E
Pd/Al2O3-I
Pt/Al2O3
6.1
29
10
1.9
13
2.0
0.72
335.8; 338.0
334.0; 335.7
n.a.
–
–
Pd ; PdO2; Pd(OH)4
343
339
343
437
4.2
6.7
5.7
3.0
3.5
0.70
0.30
5.0
13
4.2
6.7
5.7
6.5
0
Pd-Al-O; Pd ; PdO
–
n.a.
–
0
3.3
70.7
Pt ; PtO -Al; Pt-Al-O
x
648
0
Pt/Al2O3-E
Pt/Al2O3-I
n.a.
n.a.
n.a.
n.a.
70.7
n.a.
–
–
Pt ;Pt-Al-O
648
351
0.70
5.3
n.a.
648
Ru/Al2O3-NR
Ru/Al2O3
Ru/Al2O3-E
n.a.
6.2
n.a.
n.a.
0.75
n.a.
–
281.6
280.5; 282.1
281.1; 282.8
Ru(OH)3
347
353
340
13
3.4
17
0
–
Ru ; RuO3
3.4
4.0
13
–
RuO ; RuO
3
2
524
Ru/Al2O3-I
n.a.
n.a.
n.a.
2.1
5.3
n.a.
1.6
n.a.
n.a.
n.a.
0.33
1.5
n.a.
334.2; 337.5
n.a.
335.0; 336.8
71.2
n.a.
–
n.a.
333
340
333
328
358
352
329
7.0
4.3
2.6
4.0
4.6
4.8
0.20
1.3
2.9
2.0
7.0
4.3
2.6
4.0
4.6
4.8
1.5
0
0
4
4
3
4
4
3
Ru-1Pd/Al2O3
Ru-1Pd/Al2O3-E
Ru-2Pd/Al2O3
Ru-1Pt/Al2O3
Ru-1Pt/Al2O3-E
Ru-2Pt/Al2O3
280
n.a.
279.7
280
n.a.
280
Ru ; Pd ; PdO2
n.a.
0
0
Ru ; Pd ; PdO
0
0
Ru ; Pt
n.a.
0.43
n.a.
71.0
Ru ; RuO ; Pt
0
0
0
2
3
57
356
33
2.8
0.85
70.8
279.2; 281.0
0
2
Ru ; RuO ; Pt
4.9
2
Ru-3Pt/Al2O3
5
n.a.: solid not analyzed; dp: mean metallic particle diameter; ␴: standard deviation of particle diameter.
a
binding energies patterns available at http://lasurface.com/database/elementxps.php and http://srdata.nist.gov/XPS.
no evidence of Pd(OH) for Ru-Pd/Al O catalysts, in contrast to the
activation temperatures respectively of 523 K and 573 K for in situ
and ex situ activations.
4
2
3
monometallic Pd/Al O3 solid.
2
The XPS spectra for Pd and Ru obtained for the 3Ru-2Pd/Al O
According to Toebes et al. [30], even though Pd sintering temper-
ature is elevated (ca. 1,050 K), this metal may experience mobility
at temperatures as low as 423 K, mainly when exposed to H2.
Therefore, TEM results indicate that the activation of Pd cata-
2
3
catalyst are presented in Fig. 2 (I.c and III.c spectra, respectively).
Comparing the bimetallic with the monometallic spectra one may
observe that both Ru and Pd are probably more reduced on the
Ru-Pd/Al O solid. These results may be interpreted as a synergis-
lysts under H leads to a strong decrease of the metallic dispersion,
2
3
2
tic interaction between the metals in the sense that Ru seems to
promote the reduction of Pd, whereas Pd inhibits the oxidation of
Ru.
which can consequently cause a decrease of the catalytic activity
as presented hereafter. On the other hand, non-activated Pd/Al O3
2
catalyst reduced by H CO in liquid phase under mild conditions,
2
In turn, the XPS analyses of 3Ru-2Pt/Al O catalysts suggest that
leads to small metal particles and high dispersion, as observed by
Groppo et al. [28].
2
3
the addition of Pt also inhibits the oxidation of Ru (Fig. 2II.d and III.d,
respectively). However this effect may be related to the amount of
Pt added to the solid. Both catalysts with lower Pt amount (4Ru-
Non-activated Pt/Al O3 catalyst presents the smallest particles
2
amongst the studied monometallic solids along with a very narrow
1
2
Pt/Al O and 3Ru-2Pt/Al O ) presented Ru binding energies of
80.0 eV, evidencing that on these solids Ru is in its metallic phase.
size distribution (3.3 ± 0.72 nm). In turn, results in Table 2 show that
2
3
2
3
non-activated Ru/Al O catalyst presents a mean metallic particle
2 3
In contrast, for the 2Ru-3Pt/Al O catalyst, a second binding energy
diameter of 6.2 nm, which is very close to that for non-activated
2
3
for Ru is observed (281.0 eV), which may be related to a RuO specie
Pd/Al O3 solid (6.1 nm), although Ru size distribution is much nar-
2
2
(
280.7 eV). It is noteworthy that, similarly to monometallic cata-
rower (standard deviation of 0.75 nm).
0
lysts, only Pt binding energies were obtained on bimetallic Ru-Pt
solids.
Hence, the combined results of XPS and TEM suggest that the
wet impregnation with reduction by H CO is an efficient method to
2
obtain well reduced catalysts with elevated metallic dispersion and
narrow particle size distribution, notably for Ru and Pt. According to
Irmak et al. [23], formaldehyde is a strong reducing agent, thus rapid
nucleation could be realized by a fast reduction rate, which could
explain the smaller metallic particles observed when this reducing
agent is employed.
3
.3. Metallic particle size
TEM technique was employed in order to investigate the effect of
activation treatments on the size of metallic particles. Mean metal-
lic particle diameters as well as their standard deviation are shown
in Table 2.
TEM results for non-activated bimetallic solids show that the
addition of Pd or Pt to Ru/Al O catalysts decreases the mean metal-
2
3
As seen in Fig. 3 and Table 2, Pd particles with mean diameter
lic particle diameter and improves size uniformity (Fig. 4). In the
case of Pd addition (Fig. 4b), the mean metallic particle diameter
of 6.1 nm are on the surface of the non-activated Pd/Al O catalyst
2
3
(
(
Fig. 3b), for which the standard deviation of particles diameter
1.9 nm) is relatively low, indicating a narrow size distribution.
The particle size of Pd increased substantially after in situ acti-
of 3Ru-2Pd/Al O3 catalyst (2.1 ± 0.33 nm) is smaller than the one
2
obtained for both monometallic Pd/Al O (6.1 nm) and Ru/Al O
3
2
3
2
(6.2 nm) solids (Figs. 3 b and 4 a, respectively). This effect was
vation (10 ± 2.0 nm) and even more significantly for the ex situ
activation (29 ± 13 nm), as seen on Fig. 3d and c, respectively. The
Pd/Al O -E solid also presents a broader particle size distribution.
also observed by Romanenko et al. [31] for bimetallic Pd-Ru/C cat-
alysts, which were considered to be more resistant to sintering
than monometallic solids. Similar effects were also observed by
2
3
These results suggest the occurrence of Pd sintering, due to high

4
6
R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
Fig. 3. TEM images: effect of the activation treatment on the morphology of the catalysts (a) ␥-Al2O3, (b) Pd/Al2O3, (c) Pd/Al2O3-E and (d) Pd/Al2O3-I.
Fig. 4. TEM images: effect of noble metal (Pd, Pt) addition on the morphology of the catalysts (a) Ru/Al2O3, (b) 3Ru-2Pd/Al2O3, (c) 4Ru-1Pt/Al2O3 and (d) 3Ru-2Pt/Al2O3.
Chen et al. [35], according to whom there is a contribution of Pd
toward morphological features including the smaller nanoparticle
size, narrower distribution and prevented particle aggregation.
Similarly to the bimetallic Pd-Ru system, the addition of Pt to
Ru/Al O catalyst leads to a decrease of the mean metallic particle
2
3
diameter, which passes through a minimum value (1.6 ± 0.43 nm)
for the 3Ru-2Pt/Al O catalyst (Fig. 4d). The metallic particle sizes
2
3
of Ru-Pt catalysts are close to what Liu et al. [34] obtained in their
research (2–3 nm), also employing liquid-phase reduction proce-
dures.
3.4. Active phase formation
The temperatures of the H2 consumption peaks and the quan-
tities of H2 consumption obtained from catalysts TPR profiles are
presented in Table 2, whereas TPR profiles of non-activated mono
and bimetallic catalysts are presented in Fig. 5.
Pd/Al O catalysts presented similar TPR profiles with a sin-
2
3
gle H2 consumption peak at ca. 340 K (e.g. Fig. 5a), in which the
corresponding H2 consumptions are very close to each other (ca.
Fig. 5. TPR profiles of non-activated mono and bimetallic catalysts.
5
.5 ± 1.0 ␮mol of H /mgPd). These H2 consumptions may be due to
2
the reduction of Pd oxides and hydroxide.
For non-reduced Ru/Al O -NR solid, the obtained H consump-
2
3
2
Considering the stoichiometric values for the complete reduc-
tion (13 ␮mol of H /mgRu) is very close to the stoichiometric value
2
tion of the PdO2 (19 ␮mol of H /mg ), PdO (9.4 ␮mol of H /mgPd)
required for the complete reduction of the Ru(OH)3 (15 ␮mol of
2
Pd
2
and Pd(OH)4 (19 ␮mol of H /mgPd), the low H2 consumptions for
H /mgRu). Such H consumption is much higher than that obtained
for the reduced Ru/Al O3 catalyst (3.4 ␮mol of H /mgRu). These
2 2
2
2 2
0
Pd/Al O catalysts reinforces the presence of irreducible Pd , as
2
3
suggested by XPS results.
results suggest that the agent H CO reduces the precursor Ru(OH)3
2
TPR profile of non-activated Pt/Al O catalyst (Fig. 5b) show two
in liquid phase, which is consistent with the energies obtained by
XPS.
2
3
H2 consumption peaks at 437 and 648 K. Authors [45–47] associ-
ated the existence of these peaks to the reduction of Pt species
respectively in weak and strong interaction with the support.
Weak interaction may be associated with Pt oxides highly dis-
persed on the Al O support (PtOx-Al). In turn, aluminate species
The TPR profile of the Ru catalyst submitted to ex situ activa-
tion, Ru/Al O -E, presents two peaks of H consumption at low
2
3
2
(340 K) and high (524 K) temperature. In this case, XPS results indi-
cate a strong oxidation of the Ru with the coexistence of both RuO2
and RuO3 species, which is in agreement with the high overall H2
2
3
(
Pt-O-Al) would be in such a strong interaction with the support
that would hardly be reducible, as suggested by XPS results and
low H consumption (0.70 ␮mol of H /mgPt) of the ex situ activated
consumption obtained (17 ␮mol of H /mgRu).
2
As observed for the Pd/Al O3 and Pt/Al O3 solids, TPR results
2
2
2
2
Pt/Al O -E catalyst.
for the Ru/Al O3 catalysts seem to indicate that the in situ acti-
2
3
2

R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
47
Table 3
vation has less effect than the ex situ activation on the active
Catalytic performances for toluene hydrogenation in liquid phase.
phase formation. TPR profile for the activated in situ catalyst,
Ru/Al O -I, presents only one peak of H consumption at low
ar0
X (%)
Ymax (%)
Xmax (%)
2
3
2
Catalyst
temperature (333 K), similar to the profile obtained for the non-
activated Ru/Al O catalyst (Fig. 5c). However, the Ru/Al O -I
Pd/Al2O3
5.3
1.7
2.0
5.3
2.7
1.3
211
212
104
57
213
131
69
224
167
226
57
7.0
2.4
3.6
7.0
4.2
1.5
100
100
100
100
100
96
77
100
95
100
89
0.0
0.0
0.15
1.0
0.16
0.13
5.0
6.3
3.1
6.1
2.3
3.6
6.4
2.8
5.2
4.4
5.8
–
–
2
3
2
3
Pd/Al O -E
2
3
catalyst presents a higher H2 consumption, which suggests a
stronger oxidation of the metal in comparison to the non-activated
Ru/Al O catalyst.
Pd/Al2O3-I
Pt/Al2O3
2.2
7.0
2.7
1.1
45
79
84
99
44
68
77
83
92
82
89
Pt/Al2O3-E
Pt/Al2O3-I
Ru/Al O -NR
2
3
TPR results for bimetallic Ru-Pd/Al O3 and Ru-Pt/Al O3 cata-
2
2
2
3
lysts reinforces the possible existence of synergistic interactions
between the metals, as reported by Romanenko et al. [31].
Ru/Al2O3
Ru/Al2O3-E
Ru/Al2O3-I
The addition of Pd to the non-activated Ru/Al O catalysts
2
3
4
4
Ru-1Pd/Al2O3
Ru-1Pd/Al2O3-E
decreases the temperature of the single peak of H2 consump-
tion (Fig. 5d and e). This parameter (353 K for the monometallic
Ru/Al O ) decreases to 340 K (for the 4Ru-1Pd/Al O solid) and to
3Ru-2Pd/Al O
4Ru-1Pt/Al2O3
2
3
2
3
2
3
4
3
2
Ru-1Pt/Al2O3-E
Ru-2Pt/Al2O3
Ru-3Pt/Al2O3
3
28 K (for the 3Ru-2Pd/Al O3 catalyst), thus being possibly related
2
to the increase of the Pd content. In turn, the H2 consumption
slightly increases when Pd is present in the non-activated Ru/Al O3
2
ar0 in (mmoltoluene L−1 min gcat ): initial reaction rate.
X: conversion of toluene after 6 h of reaction.
Ymax: maximum yield of methylcyclohexene.
Xmax: conversion of toluene at the maximum yield.
−1
−1
catalyst.
Such results suggest that the presence of Pd promotes the cata-
lyst reduction, which may be due to easier H2 activation or higher
dispersion of the metal. According to TEM results (Table 2), the size
of metallic particle decreases for the bimetallic 3Ru-2Pd/Al O sys-
2
3
electron-deficient character of Ru, which could explain the higher
activity observed in the present study regarding Ru/Al O catalysts.
As seen on Table 3, the non-reduced Ru/Al O -NR catalyst
presents practically the same activity of the Ru/Al O solid reduced
in liquid phase by H CO. However, the reduced catalyst is more
selective to methylcyclohexene at a higher conversion of toluene.
XPS and TPR results suggest the presence of Ru(OH) on the sur-
face of the non-reduced Ru/Al O -NR catalyst. Therefore, it seems
reasonable to suppose that this hydroxide may be reduced in the
reaction medium, where the reactant H2 is present. The hydroxide
reduction could lead to the Ru formation and, consequently, to the
coexistence of both metallic and cationic species, which is a neces-
sary condition to the catalytic activity for aromatics hydrogenation
20,48].
tem with respect to their monometallic catalysts. This promotion
effect was also observed by Chen et al. [35], whose results indicate
that increasing the amount of Pd on the Ru based solid leads to
lower reduction temperature peak and H2 consumption.
2
3
2
3
2
3
The Pd presence in the ex situ activated Ru/Al O -E catalyst
2
2
3
eliminates the peak of H consumption at high temperature (524 K)
2
and decrease intensely the H consumption. Hence, the Pd addition
3
2
seems to prevent the intense metal oxidation induced by ex situ
activation.
2
3
Results show that the Pt addition to the Ru/Al O catalyst practi-
2
3
0
cally does not modify the temperature of the H consumption peak
2
(
353 K) attributed to the reduction of RuO2 phase (Fig. 5f–g).
On the other hand, the presence of Ru seems to promote the
[
reduction of Pt species in the bimetallic catalysts, notably in the
Ru-2Pt/Al O system, to which the H2 consumption (1.5 ␮mol of
3
2
3
H /mgmetal) is the lowest. These results indicate that most of the Ru
2
3.5.1. Effects of the catalyst activation
and Pt in these solids are in metallic form, which is corroborated by
the XPS analyses. Nevertheless, TPR profiles suggest the presence
of small amounts of metal oxides in the Ru-Pt/Al O systems.
Results presented in Table 3 indicate that both ex situ and in situ
activations decrease the initial activity of the studied mono and
bimetallic catalysts.
2
3
As observed in the case of Pd addition, the Pt presence in ex
situ activated Ru/Al O -E catalyst eliminates the peak of H con-
As seen by TEM analysis, both ex situ and in situ activa-
tions increase the metallic particles size of Pd/Al O3 catalyst. Such
2
3
2
2
sumption at high temperature (524 K) and decrease intensely the
H2 consumption. The association Ru-Pt also eliminates the peak
of H2 consumption at high temperature (623 K) observed on the
decrease of the metal dispersion is possibly due to the occurrence
of sintering, which is an effect of catalysts activation according to
previous studies [28–30].
TPR profile of the monometallic Pt/Al O -E system, attributed to
XPS and TPR characterizations of the Pt/Al O -E catalyst acti-
2
3
2 3
species of the aluminate type.
vated ex situ suggest a complete reduction of the Pt species and a
strong metal-support interaction. In this case, the decrease in the
catalytic activity induced by the ex situ activation may be related
to a low amount of Pt␦ sites.
+
3.5. Catalytic performance for toluene hydrogenation
Taimoor and Pitault [48] conduced a kinetic study about the
gas phase hydrogenation of toluene on Pt catalysts. The authors
propose that metallic sites as well cationic sites (metal-support
interaction) have both an important role in this reaction. Accord-
Table 3 presents the catalytic performances of the studied solids
for toluene hydrogenation in liquid phase. The reaction time of
60 min was chosen as pattern to compare the catalytic perfor-
3
mances since it was sufficient time to reach total conversion of
toluene for Ru based catalysts.
ing to the authors, H would be adsorbed on metallic sites, whereas
toluene would be adsorbed on cationic sites.
2
The obtained results show that the non-activated Pd/Al O3 and
The activity loss observed for activated Ru/Al O -E and
2
2 3
Pt/Al O catalysts present similar performances, while Ru/Al O3
Ru/Al O -I catalysts may be due to the probable formation of Ru
2 3
2
3
2
solid is much more active and selective to methylcyclohexene.
oxides, as indicated by the XPS results. In turn, TPR analyses sug-
gest that the reduction of such oxides may be more difficult and,
consequently, the Ru /Ru ratio might be affected.
Ru catalysts have been widely employed for aromatic hydro-
genation reactions. Authors suggest that its remarkable activity and
selectivity are due to the singular nature of this metal [11,17,18].
As reported by Kluson and Cerveny [17], toluene is more strongly
adsorbed on Ru than on Pd surfaces. Such effect is associated to an
0
␦+
After 360 min of reaction, the conversion of toluene is practically
total for the Ru based catalysts, due to their high activities, while it
is less than 7% for the monometallic Pd/Al O3 and Pt/Al O3 solids.
2
2

4
8
R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
The addition of Pd to the Ru/Al O catalyst leads to a decrease of
2
3
the yield of methylcyclohexene, as observed for the 4Ru-1Pd/Al O3
2
system, although this does not change the initial reaction rate.
Increasing the Pd load, the 3Ru-2Pd/Al O system presents a higher
2
3
selectivity at the expense of a significant loss of catalytic activity,
being unable to achieve total conversion within 360 min of reaction.
XPS results (Table 2) indicate the presence of PdO2 and PdO
respectively in 4Ru-1Pd/Al O3 and 3Ru-2Pd/Al O3 systems. These
2
2
oxides could cover the particles of Ru, leading to a decrease in
catalytic activity observed when the Pd content increases.
In turn, the addition of Pt to the Ru/Al O3 catalyst increases
2
the initial reaction rate in the case of the 4Ru-1Pt/Al O and 3Ru-
2
3
2
Pt/Al O3 systems, at the expense of their selectivity, a result
2
predicted by Liu et al. [34]. The initial activity decreases remark-
ably with increasing Pt load (2Ru-3Pt/Al O3 catalyst), similarly to
2
that observed for the 3Ru-2Pd/Al O3 system.
2
TEM results (Table 2) indicate that the bimetallic Ru-Pt/Al O3
2
systems present smaller metallic particles (higher metal disper-
sion) than the monometallic Ru/Al O3 catalyst, which may be the
2
Fig. 6. Effect of the reduction method on the yield of methylcyclohexene for the
Ru/Al2O3 catalysts.
cause of increased catalytic activity. As explained by Liu et al. [34],
smaller metal particles can offer higher reactivity in catalysis, due
to their higher surface to volume ratio and thus larger surface areas
exposed to reactants.
XPS analysis shows the presence of RuO2 in the 2Ru-3Pt/Al O3
2
catalyst (Table 2), which may have led to lower activity, as
observed in the cases of the activated monometallic Ru/Al O -E
2
3
and Ru/Al O -I catalysts.
2
3
Such results indicate the existence of a synergistic effect induced
by the Pt addition on the catalytic activity of Ru/Al O3 based cat-
2
alysts for toluene hydrogenation in liquid phase. Similar effects
where observed by Liu et al. [34], according to whom the syn-
ergistic interaction between these metals may be related to the
electronic modification in a bimetallic surface through the forma-
tion or core–shell morphologies.
4. Conclusions
Results of the present study show that the addition of noble
metals Pd and Pt induce remarkable effects on the morphology and
performance of Ru/Al O based catalysts for toluene hydrogenation
in liquid phase.
Fig. 7. Influence of Pd or Pt addition on the initial reaction rate of non-activated
Ru/Al2O3 based catalysts.
2
3
Reduction by formaldehyde during catalysts preparation
through wet impregnation leads to the formation of metallic
species from chlorinated precursors. Subsequent ex situ or in situ
activation under H2 form Pd and Ru oxides and decrease the cat-
alytic activity.
The Fig. 6 presents the evolution of the yield of methylcyclo-
hexene during the reaction course (toluene conversion) for the
monometallic Ru/Al O catalysts.
2
3
Although the non-reduced Ru/Al O -NR catalyst has an initial
2
3
activity comparable to the Ru/Al O3 solid reduced by H CO, the
2
2
Initial reaction rates of non-activated catalysts follow the order:
Ru/Al O ꢀ Pd/Al O ≈ Pt/Al O . A synergistic effect on the cat-
latter leads to higher yields of methylcyclohexene at high toluene
conversion. As observed by Zhou et al. [49], non-reduced Ru cata-
lysts treated with hydroxides display remarkable performance on
the hydrogenation of aromatics. These authors attributed this result
to base electronic promotion, blockade of cyclohexene chemisorp-
2
3
2
3
2
3
alytic activity can be induced by the Pt addition to Ru/Al O based
2
3
catalysts.
Acknowledgements
−
tion sites and OH hydrophilic increment effect.
According to, Mazzieri et al. [20], Taimoor and Pitault [48] and
Zanabaev et al. [50], a reasonable balance between Ru and Ru
species is important for the formation of the intermediate product
of hydrogenation.
To the “National Counsel of Technological and Scientific Devel-
opment” (CNPq) for the financial support and research scholarship
granted to Raphael Soeiro Suppino.
0
␦+
References
3.5.2. Effects of the Pd and Pt load
Bimetallic Ru-Pd/Al O3 and Ru-Pt/Al O3 catalysts containing
[1] K. Sato, M. Aoki, R. Noyori, Science 281 (1998) 1646–1647.
2
2
[
[
[
2] E. Dietzsch, P. Claus, D. Hönicke, Top. Catal. 10 (2000) 99–106.
3] M. Zhou, G. Lin, H. Zhang, Chin. J. Catal. 28 (2007) 210–216.
4] T. Fujikawa, K. Idei, T. Ebihara, H. Mizuguchi, K. Usui, Appl. Catal. A Gen. 192
(2000) 253–261.
different amounts of noble metals were prepared in order to inves-
tigate the effects of the Pd or Pt addition on the performance of
Ru/Al O based catalysts for hydrogenation of toluene. Results of
2
3
[
[
[
5] H. Backman, A.K. Neyestanaki, D.Y. Murzin, J. Catal. 233 (2005) 109–118.
6] L. Ronchin, L. Toniolo, Appl. Catal. A Gen. 208 (2001) 77–89.
7] J. Wang, Y. Wang, S. Xie, M. Qiao, H. Li, K. Fan, Appl. Catal. A Gen. 272 (2004)
29–36.
the catalytic tests are given in Table 3, whereas in Fig. 7 the initial
reaction rate for the studied catalysts are compared in regards to
the Ru mass fraction of these solids.

R.S. Suppino et al. / Applied Catalysis A: General 525 (2016) 41–49
49
[
8] J. Bu, Y. Pei, P. Guo, M. Qiao, S. Yan, K. Fan, Stud. Surf. Sci. Catal. 165 (2007)
69–772.
9] P.d.C. Zonetti, R. Landers, A.J.G. Cobo, Appl. Surf. Sci. 254 (2008) 6849–6853.
[30] M.L. Toebes, J.A. van Dillen, K.P. de Jong, J. Mol. Catal. A: Chem. 173 (2001)
75–98.
[31] A. Romanenko, E. Tyschishin, E. Moroz, V. Likholobov, V. Zaikovskii, S. Jhung,
Y.-S. Park, Appl. Catal. A Gen. 227 (2002) 117–123.
7
[
[
10] C. Zanutelo, R. Landers, W.A. Carvalho, A.J.G. Cobo, Appl. Catal. A Gen. 409
(
2011) 174–180.
[32] S. Loiha, K. Föttinger, K. Zorn, W. Klysubun, G. Rupprechter, J. Wittayakun, J.
Ind. Eng. Chem. 15 (2009) 819–823.
[33] Y. Yu, O.Y. Gutiérrez, G.L. Haller, R. Colby, B. Kabius, J. Rob van Veen, A. Jentys,
J.A. Lercher, J. Catal. 304 (2013) 135–148.
[
[
[
11] R.S. Suppino, R. Landers, A.J.G. Cobo, Appl. Catal. A Gen. 452 (2013) 9–16.
12] L. Foppa, J. Dupont, Chem. Soc. Rev. 44 (2015) 1886–1897.
13] M. Saeys, M.-F. Reyniers, J.W. Thybaut, M. Neurock, G.B. Marin, J. Catal. 236
(
2005) 129–138.
[34] H. Liu, R. Fang, Z. Li, Y. Li, Chem. Eng. Sci. 122 (2015) 350–359.
[35] J. Chen, X. Liu, F. Zhang, Chem. Eng. J. 259 (2015) 43–52.
[36] V. Strelko, D.J. Malik, J. Colloid Interface Sci. 250 (2002) 213–220.
[37] M.M. Johnson, G.P. Nowack, J. Catal. 38 (1975) 518–521.
[38] J.P. Brunelle, Stud. Surf. Sci. Catal. 3 (1979) 211–232.
[39] M. Kosmulski, Surface Charging and Points of Zero Charge, CRC, Press, Boca
Raton, Florida, United States of America, 2009.
[
[
14] F. Alhumaidan, D. Cresswell, A. Garforth, Energy Fuel 25 (2011) 4217–4234.
15] J.N. Stanley, F. Heinroth, C.C. Weber, A.F. Masters, T. Maschmeyer, Appl. Catal.
A Gen. 454 (2013) 46–52.
16] S. Hu, M. Xue, H. Chen, Y. Sun, J. Shen, Chin. J. Catal. 32 (2011) 917–925.
17] P. Kluson, L. Cerveny, Appl. Catal. A Gen. 128 (1995) 13–31.
18] P. Kluson, J. Had, Z. Belohlav, L. Cerveny, Appl. Catal. A Gen. 149 (1997)
[
[
[
3
31–339.
19] L. Zhu, H. Sun, H. Fu, J. Zheng, N. Zhang, Y. Li, B.H. Chen, Appl. Catal. A Gen. 499
2015) 124–132.
[40] K. Köhler, R.G. Heidenreich, S.S. Soomro, S.S. Pröckl, Adv. Synth. Catal. 350
(2008) 2930–2936.
[41] O.A. Simakova, P.A. Simonov, A.V. Romanenko, I.L. Simakova, React. Kinet.
Catal. Lett. 95 (2008) 3–12.
[42] O. Belskaya, V. Duplyakin, V. Likholobov, Smart Nanocomposites 1 (2011)
99–133.
[43] S. Kawi, S.Y. Liu, S.C. Shen, Catal. Today 68 (2001) 237–244.
[44] D.J. Morgan, Surf. Interface Anal. 47 (2015) 1072–1079.
[45] O.A. Bariås, A. Holmen, E.A. Blekkan, J. Catal. 158 (1996) 1–12.
[46] S. De Miguel, M. Román-Martınez, D. Cazorla-Amorós, E. Jablonski, O. Scelza,
Catal. Today 66 (2001) 289–295.
[47] P.T. Do, M. Chiappero, L.L. Lobban, D.E. Resasco, Catal. Lett. 130 (2009) 9–18.
[48] A.A. Taimoor, I. Pitault, React. Kinet, Mech. Catal. 102 (2011) 263–282.
[49] G. Zhou, X. Tan, Y. Pei, K. Fan, M. Qiao, B. Sun, B. Zong, Chem. Cat. Chem. 5
(2013) 2425–2435.
[
[
[
(
20] V.A. Mazzieri, P.C. L’Argentière, F. Coloma-Pascual, N.S. Figoli, Ind. Eng. Chem.
Res. 42 (2003) 2269–2272.
21] J.-L. Liu, L.-J. Zhu, Y. Pei, J.-H. Zhuang, H. Li, H.-X. Li, M.-H. Qiao, K.-N. Fan, Appl.
Catal. A Gen. 353 (2009) 282–287.
22] P. Mäki-Arvela, D.Y. Murzin, Appl. Catal. A Gen. 451 (2013) 251–281.
23] S. Irmak, B. Meryemoglu, B.K. Ozsel, A. Hasanoglu, O. Erbatur, Int. J. Hydrogen
Energy 40 (2015) 14826–14832.
24] C. Roth, N. Martz, H. Fuess, PCCP 3 (2001) 315–319.
25] G.-Y. Fan, W.-D. Jiang, J.-B. Wang, R.-X. Li, H. Chen, X.-J. Li, Catal. Commun. 10
[
[
[
[
(
2008) 98–102.
26] Á. Szegedi, M. Popova, V. Mavrodinova, C. Minchev, Appl. Catal. A Gen. 338
2008) 44–51.
[
[
[
[
(
27] P.G. Savva, K. Goundani, J. Vakros, K. Bourikas, C. Fountzoula, D. Vattis, A.
Lycourghiotis, C. Kordulis, Appl. Catal. B Environ. 79 (2008) 199–207.
28] E. Groppo, G. Agostini, A. Piovano, N.B. Muddada, G. Leofanti, R. Pellegrini, G.
Portale, A. Longo, C. Lamberti, J. Catal. 287 (2012) 44–54.
[50] B. Zhanabaev, P. Zanozina, B. Utelbaev, Kinet. Catal. 32 (1991) 191–194.
29] R. Pattabiraman, Appl. Catal. A Gen. 153 (1997) 9–20.