Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate heterostructures for O-removal of dibenzofurane

Article abstract of DOI:10.1016/j.cattod.2015.12.006

Bifunctional PdNb catalysts were studied in the hydrodesoxygenation (HDO) reaction of dibenzofuran (DBF) at 275?°C and 15?bar of H₂ pressure. The influence of both the support employed (silica and zirconium phosphate heterostructure (PPH)) and the catalyst preparation procedure were evaluated in the catalytic response in the HDO reaction. The catalysts were prepared by incipient wetness impregnation by using two synthetic routes. The catalysts were characterized by means of X-ray diffraction (XRD), N₂ adsorption-desorption, thermoprogrammed desorption of NH₃ (TPD-NH₃), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and elemental analysis. The results show that silica supported catalysts are much more active than those supported on PPH. While the characterization results point to a higher dispersion of the supported catalysts on PPH and better textural and acidic properties, the PdCl₂ precursor salt remains on these catalysts even after calcination and catalytic tests, explaining the lower catalytic performance presented by these systems: fewer active centers and more residues of carbon. With respect to the preparation method, regardless the support employed, the catalysts synthesized by incorporating Pd after Nb incorporation and calcination, are more active, most probably due to a better phase dispersion and therefore to a higher amount of active centers.

Full text of DOI:10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
Catalysis Today xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Pd-Nb binfunctional catalysts supported on silica and zirconium
phosphate heterostructures for O-removal of dibenzofurane
A. Infantes-Molinaa,∗∗, E. Morettib, E. Segoviaa, A. Lenardaa, E. Rodríguez-Castellóna,∗
a Departamento de Química Inorgánica, Cristalografía y Mineralogía (Unidad Asociada al ICP-CSIC), Facultad de Ciencias, Universidad de Málaga, Campus
de Teatinos, 29071 Málaga, Spain
b Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Via Torino 155/B, 30172 Mestre Venezia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 July 2015
Received in revised form 28 October 2015
Accepted 7 December 2015
Available online xxx
Bifunctional PdNb catalysts were studied in the hydrodesoxygenation (HDO) reaction of dibenzofuran
(DBF) at 275 ◦C and 15 bar of H2 pressure. The influence of both the support employed (silica and zirconium
phosphate heterostructure (PPH)) and the catalyst preparation procedure were evaluated in the catalytic
response in the HDO reaction. The catalysts were prepared by incipient wetness impregnation by using
two synthetic routes. The catalysts were characterized by means of X-ray diffraction (XRD), N2 adsorption-
desorption, thermoprogrammed desorption of NH3 (TPD-NH3), X-ray photoelectron spectroscopy (XPS),
transmission electron microscopy (TEM) and elemental analysis. The results show that silica supported
catalysts are much more active than those supported on PPH. While the characterization results point to
a higher dispersion of the supported catalysts on PPH and better textural and acidic properties, the PdCl2
precursor salt remains on these catalysts even after calcination and catalytic tests, explaining the lower
catalytic performance presented by these systems: fewer active centers and more residues of carbon.
With respect to the preparation method, regardless the support employed, the catalysts synthesized by
incorporating Pd after Nb incorporation and calcination, are more active, most probably due to a better
phase dispersion and therefore to a higher amount of active centers.
Keywords:
HDO
DBF
Noble metals
Palladium
Niobium
SiO2
Phosphate heterostructures
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
weight of oxygen in the fast pyrolysis bio-oil. Therefore, it is nec-
essary to reduce the levels of oxygenates to use it as a starting
Biomass has been described as the renewable energy source
with the greatest potential to contribute to the energy needs of
modern society. Biomass can be defined as any organic material of
plant or animal origin, including materials from natural or artificial
transformation [1]. The use of biomass as an energy source instead
of fossil fuels involves a number of environmental benefits: reduc-
tion of sulfur emissions; reduced particulate emissions; reduction
of pollutants such as CO, HC and NOx; CO2 neutral cycle; no con-
tribution to greenhouse effect; and use of agricultural waste. These
advantages make the biomass one of the potential sources of energy
to be employed in the future, being a very important element of
territorial balance, especially in rural areas.
point in the synthesis of biofuels. That is why there is a consid-
erable interest in investigating deoxygenation processes to reach
such a goal [2]. Hydrodeoxygenation (HDO) is considered one of
the most promising methods for this purpose. As a result of the
HDO, a hydrocarbon mixture comparable to those derived from
petroleum fractions can be obtained. The main purpose of this reac-
tion is to partially remove oxygen, under moderate temperatures
(250–600 ◦C) with high pressure of hydrogen in the presence of
a heterogeneous bifunctional catalyst. The ideal catalyst for the
“upgrading” of this bio-oil, should possess the following charac-
teristics: (1) high deoxygenation activity to minimize the size of
the reactor and to obtain the desired performance; (2) ability to
withstand large amounts of coke and/or minimize its formation,
as the bio-oil tends to form more coke than the typical petroleum
fractions; (3) ability to regenerate through a simple process to min-
imize capital expenditures; (4) high tolerance to poisons from the
viewpoint of increasing the life of the overall catalyst charged; and
(5) high tolerance to water since the catalysts are expected to be
exposed to large amounts of water. Previous studies indicated that
the presence of water can have a detrimental effect on the perfor-
mance of the catalyst [3].
Lignocellulosic biomass from the plant cell walls is mainly
composed of three biopolymers: cellulose (40–50%), hemicellulose
(25–35%) and lignin (16–33%). Their structures contain a high per-
centage of oxygen, which results in the presence of 30–60% by
∗
Corresponding author. Fax: +34 952131870.
Corresponding author.
∗∗
E-mail addresses: ainfantes@uma.es (A. Infantes-Molina), castellon@uma.es,
ainfantes@uma.es (E. Rodríguez-Castellón).
http://dx.doi.org/10.1016/j.cattod.2015.12.006
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
2
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
Both hydrogenation (HYD) and acid sites are needed for the HDO
pathway, and this bifunctionality is a critical step in the catalyst
design process when developing HDO catalysts [4]. To this end,
great strides have been made in finding catalysts that are suffi-
ciently active and/or selective. Carbides, nitrides and phosphides
of transition metals were investigated due to their good properties
for hydrotreating reactions and because they are catalytically active
for many reactions involving hydrogen transfer, since they behave
as metals from groups 8, 9 and 10 of the periodic Table for most
reactions. They possess hydrogenating properties similar to those
of noble metals and exhibit high tolerance to streams with high
sulfur content. These materials have been proven to be more effi-
cient in hydrotreating reactions than conventional catalysts [5]. In
particular, transition metal phosphides presented very promising
properties for HDO reactions [6].
Other investigations are focused on the study of active phases
based on noble metals such as Ru, Rh, Pd and Pt for HDO pro-
cesses [7–13]. Wildschut et al. [14] reported the promising activity
of supported Ru, Pd, and Pt catalysts for the hydrotreating of whole
bio-oils compared to traditional transition metal sulfides catalysts.
Supported noble metals are an attractive alternative because they
can activate molecular hydrogen, no sulfur feed is required as in
the case of traditional sulfide phases; they are not disabled so
easily in the presence of water and show good yields with long
lifetimes of the catalyst. With mild operation conditions, these cat-
alysts have much better performance than conventional catalysts,
since they are very efficient in the activation of molecular hydro-
gen. Unfortunately, the main drawback of theses catalysts is their
high cost [4]. Another approach is the preparation of bimetallic cat-
alysts coupling a noble metal with a non noble metal, that not only
increases the activity in hydrotreating processes, but it is also an
opportunity to reduce the cost of the catalyst. Moreover an acidic
function is incorporated into the resultant catalyst. With this cata-
lyst formulation, hydrogen is activated and is easily split over the
interface or surface to react with other reagents, and oxygenates
can be adsorbed and activated at different sites, mainly acid sites,
or metal-support interface [7,13,15,16].
Niobium species show a promoting effect when added in small
amounts to known catalysts, improving the catalytic activity and
selectivity and prolonging the catalyst life. Niobium oxides show
a great variety of functions in catalysis, such as promoter, support,
redox and acid properties, and they have been proposed as effective
catalysts or catalyst promoters for many reactions [17,18]. More-
over, niobium oxide is highly acidic, especially in amorphous form.
The hydrated niobium oxide, known as niobic acid (Nb2O5·nH2O)
shows high acidity. It also presents a high catalytic activity, selectiv-
ity and stability for acid catalyzed reactions where water molecules
are released [19].
The main goal of this paper is the preparation of bifunctional cat-
alysts, where a noble metal and a transition metal oxide presenting
acidic properties, such is the case of niobium oxide, are combined.
PdNb based catalysts, supported on two different porous supports,
SiO2 and PPH, presenting different textural and acidic properties,
were prepared and studied in the O-removal of a model molecule
present in biomass derived bio-oil.
dure described by Jiménez-Jimenez et al. [20]. Thus Pd and Nb were
added by a two step impregnation process following two synthetic
routes.
2.1.1. Route 1
The desired concentration of niobium(V) oxalate was added
to the incipient volume of deionized water and the impregnated
support. After drying at 60 ◦C overnight, the palladium aqueous
solution with the desired concentration of palladium(II) chloride
was incorporated into the Nb containing solid, dried at 60 ◦C
overnight and finally calcined at 450 ◦C for 2 h.
2.1.2. Route 2
The incorporation of Pd took place after the calcination of the
Nb-containing solid at 450 ◦C for 2 h. All other steps of the prepa-
ration procedure remained the same.
Thus, four catalysts containing a total metal loading of 2 wt%
(1.07 wt% Pd and 0.93 wt% Nb) and a Pd/Nb atomic ratio of 1 were
prepared and denoted as PdNbc/SiO2, PdNb/SiO2, PdNbc/PPH y
PdNb/PPH, where c indicates that Pd was incorporated after cal-
cination of Nb containing solid.
2.2. Characterization of catalysts
BET specific surface areas were determined from N2 adsorption-
desorption isotherms at −196 ◦C by using a Micrometric ASAP
2020 apparatus. Prior to analysis, all the samples were outgassed
at 200 ◦C (10 h). Pore size distributions were analyzed by the
BJH method applied to the desorption branch of the isotherm.
X-ray diffraction patterns were obtained with a Philips X’pert
PRO MPD diffractrometer, using CuK␣1 (ꢀ = 1.5406 Å) radiation
and X’Celerator RTMS (Real Time Multiple Strip) detector. The
acidity of pure supports and prepared catalysts was determined
by temperature-programmed desorption (TPD) of ammonia mea-
surements, carried out in a Shimadzu GC-14A chromatograph as
described elsewhere [21]. HRTEM micrographs were measured
using a Philips CCCM 200 Supertwin-DX4 microscope. X-ray pho-
toelectrom spectra were recorded with a Physical Electronics PHI
5700 spectrometer with non-monochromatic Mg K␣ radiation
(300 W, 15 kV, and 1486.6 eV) provided with a multi-channel detec-
tor. Spectra were recorded in the constant pass energy mode
at 29.35 eV, using a 720 ␮m diameter analysis area. Charge ref-
erencing was measured against adventitious carbon (C 1s at
284.8 eV). A PHI ACCESS ESCA-V6.0 F software package was used
for acquisition and data analysis. A Shirley-type background was
subtracted from the signals. Recorded spectra were always fitted
using Gaussian–Lorentzian curves in order to determine the bind-
ing energies of the different element core levels more accurately.
The amount of coke in the used catalysts was determined be ele-
mental analysis with a PerkinElmet 240C equipment.
2.3. Catalytic activity
The prepared catalysts were evaluated in the HDO of dibenzo-
furan (DBF) (2 wt% in decaline) as a model molecule. The catalytic
test was carried out in a high-pressure fixed-bed catalytic reactor
operating in the down flow mode. The organic feed was supplied by
means of a Gilson 307SC piston pump (model 10SC). For the activ-
ity tests, 0.25 g of catalyst were used (particle size 0.85–1.00 mm)
and were diluted with quartz sand to 3 cm3. Catalytic activities
were measured at 275 ◦C under 1.5 MPa of H2, at a flow rate of
30 mL min−1 and feed flow of 0.18 mL min−1 (WHSV 0.8 h−1 and ␶
2. Experimental
2.1. Preparation of catalysts
PdNb supported catalysts on silica and porous zirconium phos-
phate heterostructure (PPH) materials were prepared by following
the incipient wetness impregnation method. Commercial silica
(fumed silica from Sigma–Aldrich) was used as received, on the
contrary the PPH support was synthesized according to the proce-
6 s−1
)
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
3
The activity is described in terms of the HDO conversion accord-
ing to Eq. (1):
3. Results and discussion
3.1. Activity of the catalysts
The catalytic reaction was performed at medium hydrogen
pressure (15 bar) and at intermediate temperature of 275 ◦C. The
reaction conditions are milder and less favorable than those
reported in literature [7,8,22]. for noble metal based catalysts in
HDO of DBF reaction.
noh − ꢁnoxi
XHOD
=
× 100
(1)
noh
where n0h is the initial amount of DBF and noxi are the amount of
oxygenated compounds in the pool.
HDO conversion
Total conversion
HDO conversion
PdNbc/SiO2
PdNb/SiO2
Total conversion
100
100
80
60
40
20
0
80
60
40
20
0
1
2
3
4
5
6
1
2
3
4
5
6
Time (h)
Time (h)
HDO conversion
Total conversion
HDO conversion
Total conversion
PdNbc/PPH
PdNb/PPH
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
4
5
6
1
2
3
4
5
6
Time (h)
Time (h)
Fig. 1. Evolution of the conversion as a function of T.o.S. for PdNb catalysts. Reaction conditions: T = 275 ◦C, P = 15 bar, WHSV = 0.8 h−1; ꢂ = 6 h−1
.
Scheme 1. Reaction mechanism of HDO of DBF on PdNb catalysts.
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
4
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
100
CHB
Total Conversion
70
BCH
HDO Conversion
CH
80
60
40
20
0
THDBF
HHDBF
BCH-3-en-2-ol
60
50
40
30
20
10
0
PdNbc/SiO2 PdNb/SiO2 PdNbc/PPH PdNb/PPH
PdNbcSiO2 PdNbSiO2 PdNbcPPH PdNbPPH
Fig. 2. Comparison of total and HDO conversion at T.o.S. = 6 h. Reaction conditions:
T = 275 ◦C, P = 15 bar, WHSV = 0.8 h−1; ꢂ = 6 h−1
Fig. 3. Selectivity to the reaction products in the HDO of DBF reaction. Reaction
.
conditions: T = 275 ◦C, P = 15 bar, WHSV = 0.8 h−1; ꢂ = 6 h−1
.
Fig. 1 compares the total conversion vs the HDO conversion for
the four studied catalysts. The former shows the fraction of reagent
that is converted, while the latter (HDO conversion) refers to the
percentage of reagent converted into free oxygen products, which
is of greater interest.
Non-oxygenated
Oxygentated
75
60
45
30
15
0
It can be seen that, regardless of the type of catalyst used in the
catalytic reaction, the total conversion appears to be higher than
the HDO conversion and both of them decrease during the course
of the reaction. The silica supported catalysts exhibit higher HDO
conversion values, between 80–50%, compared to PPH supported
catalysts, 40–20%. Therefore, it can be stated that the support plays
an important role. Furthermore, the synthetic route used seems to
be also very important, since the catalysts synthesized by route
2 are always more active than those prepared by route 1. This
difference is most significant in the catalysts supported on silica:
PdNbc/SiO2 seems to be reaching a constant value of conversion,
while the catalyst PdNb/SiO2 suffers a sharp drop in activity after
the first hour of reaction. The histogram of Fig. 2 shows more clearly
the comparative study of total conversion versus HDO conversion
for all catalysts after 6 h of reaction.
PdNbcSiO2 PdNbSiO2 PdNbcPPH PdNbPPH
Fig. 4. Yield of non-oxygenated and oxygenated products in the HDO of DBF reac-
tion. Reaction conditions: T = 275 ◦C, P = 15 bar, WHSV = 0.8 h−1; ꢂ = 6 h−1
.
It is clearly seen how after 6 h of reaction all catalysts give total
conversion values higher than the HDO conversion ones, the latter
is higher for silica supported catalysts. In addition, the catalysts
prepared by route 2 are more active than those prepared by route
1, mainly the catalyst PdNbc/SiO2.
ples because of their lower activity in this reaction. In both cases,
the main deoxygenated product is BCH, but with much lower selec-
tivity values (20–25%). It should also be noted that CHB and CH were
detected for these samples. With respect to the oxygenated prod-
ucts, after 6 h of reaction, HHDBF is the main oxygen-containing
product for PdNbc/PPH, as for silica catalysts; but for PdNb/PPH
the main product is THDBF, i.e., the catalyst PdNbc/PPH appears to
have better hydrogenating properties than that of PdNb/PPH one.
The observed trend is that as the reaction proceeds, the selectiv-
ity to BCH and deoxygenated products decreases; the selectivity to
HHDBF decreases meanwhile the selectivity to THDBF raises, i.e.,
not only decreases the catalyst activity but also its hydrogenating
capacity. The reaction is held at earlier stages of the HDO process
increasing the content of HHDBF, THDBF and BCH -3 -en- 2-ol in
the pool.
By making a comparative analysis of product selectivity after 6 h
on stream, (Fig. 3), it is observed that the silica-supported catalysts
are more selective to BCH than those supported on PPH. BCH is the
main reaction product for the catalysts supported on silica; instead
the main product obtained for PPH supported ones is an oxygenated
intermediate, THDBF or HHDBF, depending on the synthetic route
employed. Notably, despite the lower activity of the PPH based cat-
alysts, these show the formation of CH, produced from the breakup
of the BCH molecule and probably associated with an increased
acidity of these catalysts.
The removal of O from DBF molecule can be carried out by dif-
ferent pathways [21]. The reaction will proceed by one way or
another according to the reaction conditions, such as tempera-
ture, pressure of H2 and the initial concentration of DBF [23]. With
these catalysts the main reaction products detected were tetrahy-
drodidibenzofuran (THDBF), hexahydrodibenzofurane (HHDBF), 2-
cyclohexylphenol (2-CHP), 1-1ꢀbi(cyclohexan)-3-en-2-ol (BCHol);
bicyclohexane (BCH); cyclohexane (CH); cyclohexylbenzene (CHB);
cyclopentylcyclohexane (C-PE-CH); and cyclopentylmethylcyclo-
hexane (C-PE-ME-CH). Considering the products identified in this
study, pathway 1 (hydrogenation) could be the main way for DBF
HDO reaction on these catalysts, i.e., the hydrogenation of the aro-
matic rings prior to the O-removal. Thus, the main reaction product
is BCH (see Scheme 1).
Both silica supported catalysts show a very similar pattern of
selectivity, with close values (not shown). The main intermediate
product is HHDBF and the main reaction product is BCH. The selec-
tivity reached after 6 h of reaction was close to 60%. BCH selectivity
decreased with reaction time, while HHDBF selectivity increased.
CHB was also detected, but with selectivity values lower than 10%.
On the other hand, catalysts supported on PPH show selectivity
patterns different from those observed with silica supported sam-
Fig. 4 reports the product distribution in terms of yield, adding
oxygenated on one hand and deoxygenated on the other one. A
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
Table 1
5
100
Total
Textural properties of the PdNb catalysts and the bare support.
Oxygenated
Sample
SBET (m2 g−1
)
Vp (cm3 g−1
)
dp (nm)
80
60
40
SiO2
218
191
191
534
181
219
0.52
0.85
0.73
0.65
0.15
0.15
14.8
17.1
16.8
12.3
5.0
PdNbc/SiO2
PdNb/SiO2
PPH
PdNbc/PPH
PdNb/PPH
4.2
Table 2
Acidic properties of the as-prepared catalysts.
Non-Oxygenated
20
0
HDO
−1
Acidity (␮molNH3 gcat
)
Acidity (␮molNH3 m−2
)
Weak
Medium
Strong
Total
Conversion (%)
Selectivity (%)
PdNbc/SiO2
PdNb/SiO2
PPH
PdNbc/PPH
PdNb/PPH
54
50
494
467
291
9
15
285
261
110
113
143
39
96
49
117
133
818
768
412
0.6
0.7
1.5
4.2
1.8
Fig. 5. Yield of non-oxygenated and oxygenated products in the HDO of DBF reac-
tion for Pd/SiO2 catalyst. Reaction conditions: T = 275 ◦C, P = 15 bar, WHSV: 0.8 h−1
ꢂ = 6 h−1
;
.
Weak: ammonia desorbed between 100–300 ◦C.
Medium: ammonia desorbed between 300–500 ◦C.
Strong: ammonia desorbed between 500–800 ◦C.
clear difference in yield values of deoxygenated products for silica
catalysts with regard to PPH ones was observed.
After 6 h of reaction, the catalysts supported on fumed silica
show a yield of deoxygenated products close to 70%, while those
supported on PPH do not reach 30%. Therefore, the silica supported
catalysts seem to be better performing in the achievement of the
objective of the HDO reaction.
In order to highlight the important role of Nb on these systems, a
monometallic Pd/SiO2 catalyst has been prepared and tested under
the same experimental conditions. The obtained results after 6 h
on stream are shown in Fig. 5, in terms of Total and HDO con-
version, as well as selectivity to O-containing and O-free products,
respectively.
As seen from this figure, the monometallic sample presents
a high conversion but mainly to O-containing intermediates. In
this case, the main O-intermediate is BCH-3-en-2-ol, with a selec-
tivity value of 64%. On the other hand, the observed O-free
compounds have been C-PE-ME-CH and C-PE-CH. Therefore, Nb
addition improves the activity of the Pd base system.
that could partially destroy the PPH structure. On the contrary,
silica supported samples are hardly affected, maintaining the tex-
tural properties of the bare support: SBET value slightly decreases,
meanwhile the total pore volume and pore diameter increase, with
PdNbc/SiO2 catalyst showing the highest values.
The characterization of the samples by means of X-ray diffrac-
tion showed in all cases that both supports are of amorphous
nature, presenting a broad band between 20–30◦ of 2ꢃ. As expected,
the catalysts do not show well defined diffraction lines due to
the low metal loading (2 wt%). The diffractograms of silica sup-
ported catalysts (Fig. 7A) only show a main diffraction peak located
at 2ꢃ = 34◦ corresponding to palladium(II) oxide (JCPD N◦ 00-041-
1107). For PdNbc/SiO2, this signal is less intense, suggesting that the
attained dispersion is better if Pd is added after Nb/SiO2 calcination.
Considering PPH samples (Fig. 7B), it is observed both the diffrac-
tion signal of PdO and a diffraction peak at 2ꢃ = 40.1◦ that could be
ascribed to metallic palladium (JCPD N◦ 03-065-6174), again more
intense for PdNb/PPH. The formation of Pd◦ during calcination can
occur since the thermal decomposition of oxalate ions liberated CO
that act as a reducing agent [25]. In general, if the two prepara-
tion routes are compared, it can be clearly observed that regardless
the support employed, the addition of Pd after the calcination of
Nb species leads to a much better catalytic dispersion. Moreover,
the usage of PPH instead of SiO2 as catalytic support improves the
dispersion, as expected due to its better textural properties. By
applying the Scherrer equation to the main diffraction line of PdO
phase for PdNb/SiO2 sample, a particle size of 7.7 nm is calculated.
Therefore, the other samples show particle sizes lower than this
value.
Considering the used samples in the HDO reaction (not shown
here), the diffractograms of silica supported samples do not show
the diffraction peak of PdO, while a new peak assigned to metal-
lic palladium appears at 2ꢃ = 40.1◦ assigned to the reduction of
palladium oxide under the reaction conditions employed. On the
contrary, the diffractograms of PPH samples do not show the
appearance of new diffraction peaks.
More information related to the distribution of the active phase
on the material support was obtained from TEM analysis (Fig. 8),
even if it is not possible to distinguish among Nb and Pd parti-
cles due to the similar electronic density of the metals. Figs. 7A
and B show representative micrographs of the catalysts supported
on silica. In both cases the coexistence of small particles (<5 nm
3.2. Catalyst characterization
The textural properties of the investigated samples were
evaluated by nitrogen physisorption at −196 ◦C. The N2 adsorption-
desorption isotherms of the bare supports and the final catalysts are
presented in Fig. 6A-B.
As to SiO2 based catalysts (Fig. 5A), the isotherms are of type
IIb according to the IUPAC classification [24], characteristic of
solids with low porosity, presenting meso and macroporosity. By
comparing the isotherms of the two catalysts with those of the
corresponding bare support, the main detectable difference is the
sharp increase in the adsorbed volume at high pressures due to
the presence of larger pores. It should be noted that the fumed
SiO2 porosity is generally attributed to interparticles voids, there-
fore the active phase should be located in these voids increasing so
the interparticle volume and diameter. Zirconium phosphate het-
erostructure (PPH) based samples show isotherms of type IV and
characteristic of mesoporous materials. In this case, after Pd and Nb
incorporation, a sharp decrease can be clearly observed in the textu-
ral properties of the corresponding catalysts compared to the bare
support. It points to a partial collapse of the mesoporous structure
after Pd and Nb incorporation and thermal treatment.
The values included in Table 1 clearly show that, after active
phase incorporation, the textural properties decrease, mainly for
PPH based catalysts. This could be ascribed to the fact that, dur-
ing the impregnation procedure, an acidic solution was employed,
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
6
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
600
500
400
300
200
100
0
B
A
PdNbc/SiO2
PdNb/SiO2
400
300
200
100
0
PPH
PdNb/PPH
SiO2
PdNbc/PPH
0.8 1.0
0.0
0.2
0.4
0.6
P/P0
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 6. N2 adsorption-desorption isotherms of PdNb catalysts: (A) Supported on SiO2; and (B) Supported on PPH.
A
B
200 a.u.
500 u.a.
PdO
PdO
Pd
PdNbc/PPH
PdNb/PPH
PdNbc/SiO2
PdNb/SiO2
20
30
40
50
60
70
20
30
40
50
60
70
2 Theta (º)
2 Theta (º)
Fig. 7. Wide angles X-ray diffraction patterns of PdNb catalysts: (A) Supported on SiO2; and (B) Supported on PPH.
Table 3
Pd 3d and Nb 3d binding energy values and catalyst surface quantitative analysis of the as prepared and used.
Binding energy (eV)
Surface atomic ratio
Pd 3d5/2
Nb 3d5/2
Nb5+
207.6
207.7
207.7
207.0
Pd/Nb
Pd/Sup
Nb/Supa
FRESH
Pd0
335.0
334.8
–
PdO
PdCl
–
–
338.4
338.5
Cl (%)b
–
0.15
0.45
0.68
PdNbc/SiO2
PdNb/SiO2
PdNbc/PPH
PdNb/PPH
USED
336.3
336.5
336.4
336.8
0.63
0.40
1.92
0.91
0.0014
0.0017
0.0071
0.0032
0.0023
0.0043
0.0037
0.0035
–
PdNbc/SiO2
PdNb/SiO2
PdNbc/PPH
PdNb/PPH
335.2
335.0
335.1
335.3
336.5
337.0
–
–
–
207.7
207.8
–
0.54
0.41
0.73
0.75
0.0020
0.0020
0.0034
0.0061
0.0038
0.0049
0.0046
0.0082
–
–
0.47
0.6
336.9
337.5
–
–
a
Sup: Si for SiO2 and (Si + P + Zr) for PPH.
Surface atomic concentration of Cl.
b
of size) and some larger aggregates are clearly observed. During
the micrographs acquisition, the presence of aggregates was more
evident in the PdNb/SiO2 catalyst, consistent with the lower dis-
persion observed by XRD. The micrographs corresponding to the
catalysts supported on PPH (Figs. 7C and D) show a very homoge-
neous dispersion of the active phase on the surface of the catalysts
in the form of small particles. These data corroborate the informa-
tion obtained from the XRD analysis, namely a better dispersion of
the active phase on this support.
Moreover, and regardless of the support employed, catalysts
prepared from route 2 show a higher dispersion of the active phase.
The acid properties of the prepared catalysts were evaluated by
means of NH3-TPD measureents and the corresponding values in
terms of ␮molNH3 g−1 are included in Table 2.
The acid properties are totally different depending on the sup-
port employed. SiO2 support hardly adsorbed ammonia; on the
contrary, PPH samples presented a great amount of acid sites. As
expected, the catalysts supported on PPH are much more acidic,
although the acidic sites are of weak nature. In a previous paper
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
7
Fig. 8. TEM micrograph of PdNb based catalysts: (A) PdNbc/SiO2, (B) PdNb/SiO2, (C) PdNbc/PPH and (D) PdNb/PPH.
Nb 3
d
core level spectra
Pd 3
d
core level spectra
Nb(V)
PdNbc/PPH
PdNbc/PPH
PdCl2
PdO
PdNb/PPH
PdNbc/SiO2
Nb(IV)
PdNb/PPH
PdNb/SiO2
Pd
PdNbc/SiO2
PdNb/SiO2
214
212
210
208
206
204
345
342
339
336
333
330
Binding Energy (eV)
Binding Energy (eV)
Fig. 9. Pd 3d and Nb 3d core level spectra corresponding to the as prepared samples.
related to the synthesis and characterization of these materials
[20], infrared spectra of adsorbed pyridine indicated that the acidity
was mainly of Lewis type associated to Zr, although Bronsted acid-
ity from P-OH groups was also present but in a lesser extent. On
the contrary SiO2 based samples present a lesser concentration of
acid sites but of higher strength and associated to Nb oxide species
where both Bronsted and Lewis acid sites are present [26]. On the
other hand the acidity of the silica catalysts is similar regardless
of the preparation method employed. Instead, on PPH, the catalyst
prepared through route 1 is less acidic. In order to compare the
acidity values due to the different specific surface area of both sup-
ports, the acidity values have been normalized by the BET specific
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
8
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
Table 4
Cl content (%)
Carbon content (wt%) of used catalysts.
0.0
80
0.2
0.4
0.6
Cl content
0.8
C content (wt%)
Sample
1.441
PdNbc/SiO2
1.479
PdNb/SiO2
2.577
PdNbc/PPH
2.707
PdNb/PPH
PdNbc/SiO2
C content
surface area. The normalized acidity also reflected the much higher
acidity of PPH based samples compared to SiO2 ones.
60
40
20
PdNb/SiO2
The surface properties of the catalysts were investigated by
X-ray photoelectron spectroscopy. Table 3 includes the binding
energy values of Pd 3d5/2 Nb 3d5/2, and the surface atomic ratios
of the as prepared samples as well as those of the used catalysts in
the HDO reaction of DBF.
Pd 3d and Nb 3d XPS spectra for SiO2 and PPH supported sam-
ples are included in Fig. 9. For silica supported catalysts similar
spectra are obtained, showing two contributions in the form of
doublets, 3d5/2 (continuous curves) and 3d3/2 (dotted curves). The
3d5/2 component exhibits two contributions, the first one, centered
around 335.0 eV, was assigned in the literature to metallic Pd, and
the second one, located at 336.4 eV, was attributed to the presence
of palladium(II) oxide [27]. The latter is the most important sig-
nal, especially in the case of PdNb/SiO2 catalysts. Regarding PPH
supported catalysts, the Pd 3d core level spectra presents a contri-
bution corresponding to the Zr 3p signal coming from the material
support. This contribution was considered in the corresponding
decomposition of the spectra. In these catalysts spectra, two dou-
blets indicating the presence of two different types of Pd species
were also observed: PdO with the 3d5/2 band located at 336.0 eV;
the contribution to higher binding energy corresponds to the PdCl2
phase [28] in accordance with the detection of significant quanti-
ties of Cl− ions in these samples. However, the presence of metallic
Pd, detected from X-ray diffraction, seems to be masked by the
presence of Zr 3d signal.
PdNb/PPH
PdNbc/PPH
2.8
1.2
1.6
2.0
2.4
C content (%)
Fig. 10. Chloride and carbon content relationship with the HDO conversion.
twice (1.92 vs 0.91). These data indicate the preferential location
of palladium species on the surface with respect to niobium ones,
and are most probably ascribed to the high dispersion of the active
phase in these samples, as observed from TEM and XRD characteri-
zations. Finally, the surface atomic ratios corresponding to the used
catalysts reflect a decrease in the Pd/Nb ratio. This fact is much more
evident for PdNbc/PPH. Considering Pd/Sup and Nb/Sup ratios, in
all cases a raise in the Nb/Sup relationship is observed, meanwhile
that of Pd decreases, suggesting an enrichment of the surface for all
samples during the catalytic test.
After the catalytic test, in the case of silica based catalysts, a
significant increase in the contribution of the metallic phase is
observed (not shown here), as detected by XRD analysis. So the
main phase after catalytic testing is Pd◦, although a small amount
of noble metal remains as PdO. Regarding PPH supported catalysts,
the XPS interpretation is more complex due to the overlapping sig-
nal of Zr 3d. For both PPH catalysts, the signal corresponding to
the chlorine phase remains and the amount of chlorine present in
the used catalyst is similar to that in the fresh one. These data sug-
gest that the chloride phase remains unchanged during the catalytic
process. Furthermore, the proportion of palladium that was initially
as an oxide is now present as metal, in agreement with XRD results.
Furthermore, Nb 3d spectra are also shown in Fig. 9. In the case
of silica supported catalysts, the only contribution (doublet) of the
3d5/2 component is centered on 207.6 eV, corresponding to nio-
bium pentoxide [26]. This signal is more intense for the catalyst
PdNb/SiO2, suggesting that this has a higher surface concentration
of Nb, as indicated by the atomic ratio Nb/Sup shown in Table 4.
In the case of PPH supported catalysts, the PdNbc/PPH samples
exhibits, like the silica ones, one doublet and the main component
at 207.6 eV; but the PdNb/PPH one shows two doublets. One with
the main component at 205.6 eV and the second one at 207.0 eV,
assigned in the literature to Nb(IV) and Nb(V) species respectively
[29]. On the other hand, no important changes in the Nb signal after
testing are observed.
Finally the used catalysts were characterized by means of CNH
elemental analysis in order to evaluate the deposition of carbon
as the main cause of catalyst deactivation, as reported in literature
for HDO catalysts [3]. The carbon content obtained is included in
Table 4.
The results show that silica catalysts exhibit lower values of car-
bon content than PPH based samples, with the latter having close
to twice the carbon content than silica ones. This is directly corre-
lated with the acidity (Table 2), since catalysts supported on PPH
are much more acidic. These data could justify in part the catalytic
results previously described.
The presented results seem to indicate that both the prepara-
tion method and the support employed play an important role in
the catalytic activity of the final catalyst. Regarding the prepara-
tion method, it can be said that the catalysts prepared by route
2 (previous calcination before the addition of Pd) are more active.
This difference is more marked in the silica supported catalysts, and
could be associated with a better dispersion of the active phase (as
shown by XRD, TEM and XPS results) and lower residual Cl content
in the fresh catalysts, which decreases the deposition of carbona-
ceous residues. In PPH supported catalysts, the difference in activity
of both catalysts is low, although the catalyst PdNbc/PPH is slightly
more active (route 2) and has better activity toward hydrogenation,
which appears to be associated with greater dispersion of Pd and
therefore a greater number of hydrogenating centers.
If the role of the support material is evaluated, both supports
are totally different in terms of textural and acidity properties. Thus,
SiO2 is commercial porous silica whose porosity comes mainly from
interparticle porosity; instead PPH is a lamellar zirconium phos-
phate pillared with silica, presenting a very high surface area, of
mesoporous nature. In terms of acidity, SiO2 did not show any inter-
action with NH3 as clearly seen from NH3-TPD experiments, while
PPH support desorbed close to 800 ␮molNH3/g.
If the atomic concentrations at the surface are considered, the
Pd/Nb ratio depends on both the support used and the method
of preparation carried out. Thus, this ratio is lower for catalysts
supported on silica, which leads to a surface enrichment of Nb in
respect of Pd, mainly for the catalyst PdNb/SiO2. In fact, the Nb/Si
ratio is almost twice for this catalyst, i.e., there is a greater disper-
sion of Nb on the surface of this sample. With respect to supported
catalysts on PPH, in the catalyst PdNb/PHH the Pd/Nb ratio is close
to the unity, whereas for the catalyst PdNbc/PPH the ratio is almost
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006

G Model
CATTOD-9924; No. of Pages9
ARTICLE IN PRESS
A. Infantes-Molina et al. / Catalysis Today xxx (2015) xxx–xxx
9
Catalytic results indicate that silica supported catalysts are
much more active. Although the metal dispersion is lesser, due to
the smaller surface area thereof facing the PPH, its stability under
acidic conditions, such as those carried out during the impregnation
process, is much larger. It is observed how the supported catalysts
on PPH suffered a drastic reduction of the specific surface area, most
probably ascribed to a partial collapse of the porous structure, as
revealed by a comparison among low angle XRD profiles of bare
PPH and PdNb supported catalysts (not shown). In it, it is seen how
the signals of the two supported catalysts overlap and that what is
observed is not a well-defined peak but a small shoulder.
There is a direct correlation between the conversion of HDO and
the carbon content analyzed by CNH and the amount of surface
chlorine detected by XPS, as shown in Fig. 10, where conversion of
HDO shown at 6 h of reaction for each catalyst is represented as a
function of the carbon content (circles) and chlorine (stars). It can
be noted that the HDO conversion decreases with the increase of
carbon and chlorine content. Therefore, it could be stated that the
carbon formation correlates with acidity (Table 2) and with the Cl
content present in the starting material (Table 3). It is outlined in
the literature that surface Cl− is responsible for carbon formation
and thus for deactivation by coke deposition on the catalyst surface
[10]. PPH supported catalysts have higher carbon content, more
chlorine in its surface and higher acidity and this could justify their
lower activity in the reaction of dibenzofuran HDO. However, the
main factor determining the low activity of these catalysts is that
PdCl2 phase is present both in fresh and used catalysts, therefore a
large fraction of palladium species in these catalysts is inactive in
the HDO reaction.
With regard to literature data, the activity of the prepared cat-
alysts supported on silica are close or even higher than those
reported for noble metal catalysts in the HDO of DBF reaction. Wang
et al. [30]. studied Pd supported on mesoporous silica (COK-12)
and tested at 300 ◦C and 3.0 MPa. The catalyst containing 1 wt.%
of Pd, close to us, reached the maximum total conversion, 70%, at
the highest weight time a selectivity to BCH 35%. In a later work,
these author prepared Pt, Pd and Ru catalysts supported on SBA-
15 [8] and also tested in the HDO of DBF reaction at 280 ◦C and
30 MPa. These authors observed that high pressures were required
to achieve high conversion values, Pt catalyst was the most active in
TOF basis and the selectivity to deoxygenated products was close
to 25% for Pt and Pd catalysts and 35% for Ru one. Therefore, the
results presented by silica based catalysts are quite promising also
considering the much lower reaction pressure employed with the
concomitant energy savings.
a correlation between the amount of carbon in used catalyst and
the amount of chlorine in the catalyst is observed. The less active
catalyst, PdNb/PPH, is the one with higher content of carbon and Cl.
However, the main factor determining the low activity of these cat-
alysts is that PdCl2 is present in the fresh catalysts, and still remains
in the used ones. Therefore, an important fraction of Pd does not
participate in the HDO reaction. PPH supported catalysts possess
fewer active sites (as part of Pd is PdCl2) with a higher amount of
carbonaceous residues, linked to increased acidity and surface Cl−
species. Moreover, regarding the influence of the synthetic method-
ology used, Route 2 gives rise to catalysts that are more active in this
reaction, probably due to the better dispersion of the active phase
reached, besides not presenting surface residual Cl. In conclusion,
the most suitable catalyst for the HDO reaction under these condi-
tions DBF is silica supported and prepared by route 2 (PdNbc/SiO2),
and having high values of activity and outstanding stability with
respect to other catalysts.
Acknowledgements
The authors acknowledge to Ministerio de Economía y Com-
petitividad CTQ2012-37925-C03-03, FEDER funds, and Project of
Excellence P12-RNM 1565 of Junta de Andalucía for financial sup-
port.
References
[1] A. Demirbas, Appl. Energy 88 (2011) 17–28.
[2] M.S. Talmadge, R.M. Baldwin, M.J. Biddy, R.L. McCormick, G.T. Beckham, G.A.
Ferguson, S. Czernik, K.A. Magrini-Bair, T.D. Foust, P.D. Metelski, C. Hetrick,
M.R. Nimlos, Green Chem. 16 (2014) 407–453.
[3] E. Furimsky, Appl. Catal. A 199 (2000) 147–190.
[4] D.A. Ruddy, J.A. Schaidle, J.R. Ferrell Iii, J. Wang, L. Moens, J.E. Hensley, Green
Chem. 16 (2014) 454–490.
[5] H.Y. Zhao, D. Li, P. Bui, S.T. Oyama, Appl. Catal. A 391 (2011) 305–310.
[6] P. Bui, J.A. Cecilia, S.T. Oyama, A. Takagaki, A. Infantes-Molina, H. Zhao, D. Li, E.
Rodríguez-Castellón, A. Jiménez López, J. Catal. 294 (2012) 184–198.
[7] Y. Wang, Y. Fang, T. He, H. Hu, J. Wu, Catal. Commun. 12 (2011) 1201–1205.
[8] L. Wang, C. Li, S. Jin, W. Li, C. Liang, Catal. Lett. 144 (2014) 809–816.
[9] A. Gutierrez, R.K. Kaila, M.L. Honkela, R. Slioor, A.O.I. Krause, Catal. Today 147
(2009) 239–246.
[10] T. Nimmanwudipong, R.C. Runnebaum, D.E. Block, B.C. Gates, Energy Fuels 25
(2011) 3417–3427.
[11] Y.-C. Lin, C.-L. Li, H.-P. Wan, H.-T. Lee, C.-F. Liu, Energy Fuels 25 (2011)
890–896.
[12] L. Nie, D.E. Resasco, J. Catal. 317 (2014) 22–29.
[13] S. Echeandia, B. Pawelec, V.L. Barrio, P.L. Arias, J.F. Cambra, C.V. Loricera, J.L.G.
Fierro, Fuel 117 (2014) 1061–1073.
[14] J. Wildschut, F.H. Mahfud, R.H. Venderbosch, H.J. Heeres, Ind. Eng. Chem. Res.
48 (2009) 10324–10334.
[15] R. Pestman, R.M. Koster, J.A.Z. Pieterse, V. Ponec, J. Catal. 168 (1997) 255–264.
[16] M.J. Mendes, O.A.A. Santos, E. Jordão, A.M. Silva, Appl. Catal. A 217 (2001)
253–262.
[17] K. Tanabe, Catal. Today 78 (2003) 65–77.
[18] M. Ziolek, Catal. Today 78 (2003) 47–64.
4. Conclusions
[19] C. García-Sancho, I. Agirrezabal-Telleria, M.B. Güemez, P. Maireles-Torres,
Appl. Catal. B 152–153 (2014) 1–10.
[20] J. Jiménez-Jiménez, M. Rubio-Alonso, D.E. Quesada, E. Rodríguez-Castellón, A.
Jiménez-López, J. Mater. Chem. 15 (2005) 3466.
Nb-Pd bimetallic catalysts, supported on commercial fumed
silica and zirconium phosphate heterostructure and prepared by
incipient wetness impregnation method, were investigated in the
hydrodeoxygenation of dibenzofuran reaction. This reaction pro-
ceeds through the hydrogenation pathway (HYD), which requires
complete saturation of the benzene rings before the removal of oxy-
gen heteroatom. The silica supported catalysts presented a higher
selectivity to deoxygenated products. The main reaction product
was BCH for these catalysts. Instead, PdNb supported on PPH shows
greater selectivity to O-intermediates, THDBF and HHDBF.
Characterization results show that the addition of Pd and Nb
increases the acidity of the catalysts (TPD-NH3). Besides presenting
better textural and acidic properties, characterization results seem
to indicate a higher dispersion of the active phases on the PPH sup-
port, as observed by XRD and TEM analyses. However, the presence
of PdCl2 and this high acidity, appear to play a crucial role in the
studied reaction, leading to very low HDO conversion values. Thus,
[21] A. Infantes-Molina, E. Gralberg, J.A. Cecilia, E. Finocchio, E.
Rodríguez-Castellón, Catal. Sci. Technol. 5 (2015) 3403–3415.
[22] V. LaVopa, C.N. Satterfield, Energy Fuels 1 (1987) 323–331.
[23] S. Krishnamurthy, S. Panvelker, Y.T. Shah, AIChE J. 27 (1981) 994–1001.
[24] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.
Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619.
[25] P.K. Gallagher, F. Schrey, Thermochim. Acta 1 (1970) 465–476.
[26] C. García-Sancho, J.A. Cecilia, A. Moreno-Ruiz, J.M. Mérida-Robles, J.
Santamaría-González, R. Moreno-Tost, P. Maireles-Torres, Appl. Catal. B 179
(2015) 139–149.
[27] A. Infantes-Molina, J. Mérida-Robles, E. Rodríguez-Castellón, J.L.G. Fierro, A.
Jiménez-López, Appl. Catal. B 73 (2007) 180–192.
[28] D. Briggs, M.P. Seah, Practical Surface Analysis—Auger and X-ray
Photoelectron Spectroscopy, 2nd ed., Wiley Interscience, 1990.
[29] C.O.A. Olsson, D. Landolt, Corrosion Sci. 46 (2004) 213–224.
[30] L. Wang, M. Zhang, M. Zhang, G. Sha, C. Liang, Energy Fuels 27 (2013)
2209–2217.
Please cite this article in press as: A. Infantes-Molina, et al., Pd-Nb binfunctional catalysts supported on silica and zirconium phosphate
heterostructures for O-removal of dibenzofurane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.006