The effect of Cu loading on Ni/carbon nanotubes catalysts for hydrodeoxygenation of guaiacol

Article abstract of DOI:10.1039/c6ra00041j

Commercial carbon nanotubes (CNT), were used as supports to prepare Ni/CNT catalysts with a 15 wt% Ni loading and NiCu_x/CNT catalysts with Cu loadings of x: 1.5, 2.25, 3.0 and 3.75 wt% were prepared. The catalysts were characterized by N₂ physisorption, H₂-temperature programmed reduction (TPR), transmission electron microscopy (TEM), STEM-HAADF and X-ray photoelectron spectra (XPS), and they were evaluated in the conversion of guaiacol at 573 K and 5 MPa H₂ pressure for 4 h in a batch reactor. The characterization showed that Ni-Cu alloys were formed which existed along with Ni, Cu and NiO entities. The Ni metallic particle size was lower upon increasing Cu content, however the increased proportion of NiO species and the coverage of Ni sites with Cu diminished the intrinsic activity. The addition of Cu also decreased the hydrogenolysis and deoxygenation ability for the catalysts up to 3 wt% of Cu, which can be related to the lower size of Ni ensembles. Beyond that percentage, the selectivity was modified and it was attributed to changes in the Ni dispersion.

Full text of DOI:10.1039/c6ra00041j

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Effect of Cu loading on Ni/carbon nanotubes catalyst for
hydrodeoxygenation of guaiacol
AB.Dongila*, B. BachillerꢀBaezab, I. RodríguezꢀRamosb, J.L.G. Fierrob, N. Escalonac,d**
aUniversidad de Concepción, Facultad de Ciencias Químicas, Casilla 160C,
Concepción, Chile.
b Instituto de Catálisis y Petroquímica, CSIC, Cantoblanco, 28049 Madrid, Spain.
c Departamento de Ingeniería Química y Bioprocesos, Escuela de Ingeniería, Pontificia
Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, Macul, Santiago,
Chile.
d Facultad de Químicas, Pontificia Universidad Católica de Chile.
Abstract
Commercial carbon nanotubes (CNT), were used as supports to prepare Ni/CNT
catalysts with a 15 wt. % Ni loading and NiCux/CNT catalysts with Cu loadings of x:
1.5, 2.25, 3.0 and 3.75 wt.% were prepared. The catalysts were characterized by N2
physisorption, H2ꢀtemperature programmed reduction (TPR), Transmission electron
microscopy (TEM), STEMꢀHAADF and Xꢀray photoelectron spectra (XPS), and they
were evaluated in the conversion of guaiacol at 573 K and 5 MPa H2 pressure for 4 h in
a batch reactor. The characterization showed that NiꢀCu alloys were formed which
existed along with Ni, Cu and NiO entities. The Ni metallic particle size was lower
upon increasing Cu content, however the increased proportion of NiO species and the
coverage of Ni sites with Cu diminished the intrinsic activity. The addition of Cu also
decreased the hydrogenolysis and deoxygenation ability for the catalysts up to 3 wt. %
of Cu, which can be related to the lower size of Ni ensembles. Beyond that percentage,
the selectivity was modified and it was attributed to changes in the Ni dispersion.
Keywords: Hydrodeoxygenation, Nickel, copper, Carbon nanotubes, guaiacol.
To whom correspondence should be addressed
*Dr. A.B.Dongil., eꢀmail: adongil@udec.cl, Tel: (56ꢀ41) 2207236.
**Dr. Néstor Escalona B., eꢀmail: neescalona@ing.puc.cl, Tel: (56) 2 23544927

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1. Introduction
The depletion of fossil fuels and environmental concerns have prompted the research to
look for alternative sources of energy and chemicals. In this sense, nonꢀedible biomass
as lignocellulosic residues has received considerable interest through the fast pyrolysis
technology to produce bioꢀoil. However, the produced bioꢀoil holds a high O/C ratio,
this leading to a high viscosity, low heating value, corrosivity, incomplete volatility and
thermal instability. The catalytic hydrotreatment of the bioꢀoil, i.e. hydrodeoxygenation
(HDO), has been widely employed to either lower (to be used as a chemical feedstock)
or completely eliminate the oxygen composition (to be used as a fuel component)1,2.
HDO of model compounds is usually performed at high temperature (175ꢀ400 ºC) and
high hydrogen pressure (100ꢀ300 bar) employing a solid catalyst. The most studied
catalysts are hydrodesulfurization (HDS) catalysts such as sulfided CoMo or NiMo
oxides and group 10 type catalysts. The former are less expensive and are capable of
removing the oxygen while minimizing hydrogen consumption, but require activation
by a sulfidation treatment. This might lead to sulfur leaching, which deactivates the
catalyst and results in sulfur contamination of the products3. Noble metal catalysts
demonstrated excellent activity however their selectivity to aromatics deoxygenated
compounds at the higher temperatures required lead to relatively high amounts of
methane and coke formation causing deactivation of the catalysts. Additional
disadvantages of noble metals are their high cost and the excessive hydrogen
consumption stemming from their high hydrogenation activity4. For these reasons,
several alternatives to these catalysts have been explored with special interest on
catalysts that uses less expensive and earthꢀabundant elements. Nickel catalysts, have
been studied by several groups for HDO reactions with promising results compared to
other nonꢀnoble metals. 5ꢀ8 The main drawback of Ni catalysts is their low selectivity
5
,6,7,8

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towards dehydrogenation which can be improved by using bimetallic catalysts. 9 In this
sense, it has been reported that the addition of Cu to Ni catalysts may improve the
catalyst performance for the hydrotreatment of bioꢀoil and its model compounds10.
Some authors studied the effect of adding a certain amount of Cu to Ni supported over
7
Al2O3 11,12, SiO2 13, SiO2ꢀZrO2 14, CeO2 and ZrO2 . In general the hydrogenation yield
was higher when Cu was added, however the trend in the HDO yield varied depending
on the support. Other authors studied the effect of Cu loading to Ni catalysts supported
16
on SiO2 15, Al2O3 and ZrO2 17. Overall, the activity was enhanced by adding small
amounts of Cu but then, it decreased upon increasing Cu loading, however the HDO
yield followed different trend: a volcanoꢀtype dependency for ZrO2 17, a continuous
decrease for Al2O3 upon increasing Cu loading 16 and it was independent for SiO2 except
at low (5%) and high (85%) Cu loading where the HDO yield was higher 15. Besides
influencing the selectivity and activity, copper can prevent the formation of carbon
deposits and the sintering of the Ni nanoparticles as well as decreasing the reduction
temperature of the NiO. 15ꢀ17
The support seems to play an important role on the catalytic performance as it
influences the acidity of the catalysts. In order to eliminate the effect of the support and
study the influence of Cu loading to Ni catalysts we propose to employ carbon
nanotubes (CNT). Besides their high thermal conductivity, accessibility of the active
phase, good chemical stability in aggressive media, and the absence of any
microporosity, they possess a hydrophobic surface that promotes water desorption,
which may favor the HDO reaction. 18 However, carbon nanostructures have scarcely
been employed in the HDO of bioꢀoil model compounds and few examples can be
found.19 We have recently proved the better activity on the HDO of guaiacol of Ni/CNT
catalyst compared to Ni/activated carbon. 20

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Thus, the purpose of the present study was to investigate the effect of Cu content on
Ni/CNT catalysts on the activity and selectivity of the hydrotreatment of guaiacol.
2. Experimental
2.1 Catalysts
Carbon nanotubes Nanocyl 3100 (95% purity) CNT were used as parent support. The
catalysts were prepared by wet impregnation in acetone using the corresponding amount
of Ni (NO3)2 (99.9%, Sigma–Aldrich) to obtain 15 wt.% Ni, with 10 mL of solution per
gram of CNT. After stirring for 12 h, the solvent was removed under vacuum. Finally,
the materials were treated at 623 K for 5 h under flowing He (50 mL/min). The samples
were previously reduced under H2 (50 ml/min) at 733 K for 4 h and transferred carefully
to the reactor limiting the exposition to air. Then, the corresponding amount of
Cu(NO3)2 3H2O (99.9%, Sigma–Aldrich) to obtain 1.5; 2.25; 3.0 and 3.75 wt. % of Cu
corresponding to Cu/Ni atomic ratios of 0.009, 0.014, 0.018, 0.023 was added.
According to this procedure six catalysts were prepared which are labeled as Ni/CNT,
NiCu1.5/CNT, NiCu2.25/CNT, NiCu30/CNT, NiCu3.75/CNT and Cu/CNT.
2.2 Characterization
The specific surface area (SBET) and pore volume (Vp) of the catalysts were determined
from nitrogen isotherms at 77 K using a Micromeritics ASAP 2010 equipment. Prior to
the measurements, the samples were degassed at 473 K for 2 h.
The crystalline phases present in these samples were determined from the Xꢀray
diffraction patterns (XRD). The diffractograms were recorded on a Polycristal X´Pert
Pro PANalytical apparatus using Niꢀfiltered Cu Kα radiation (λ=0.15406 nm) and a
graphite monochromator. For each sample, Bragg’s angles between 4° and 70° were
scanned at a rate of 0.04 deg/sec.

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Transmission electron microscopy (TEM) and Scanning Transmission Electron
microscopy (Highꢀangle annular darkꢀfield (STEMꢀHAADF) images were measured
using a JEOL JEM 2100F field emission electron gun microscope operated at 200 kV
and equipped with an EnergyꢀDispersive XꢀRay detector. The samples were ground and
a small amount was suspended in ethanol solution using an ultrasonic bath. Some drops
were added to the gold grid (Aname, Lacey carbon 200 mesh) and the ethanol was
evaporated at room temperature before introduce in the microscope. The Scanning
Transmission Electron Microscopy (STEM) was done using a spot size of 1 nm. Xꢀray
energy dispersive spectroscopic (XEDS) mapping analysis was carried out with the help
of an Oxford Instruments Inca software package. Around 200 particles were measure to
estimate the metal particle size distribution.
Prior to the characterization by XRD and TEM the catalysts were stabilized in N2
atmosphere after their reduction at 723 K to prevent the oxidation of reduced metallic
species.
Xꢀray Photoelectron Spectra (XPS) of the catalysts were obtained on a
MicrotechMultilb 3000 spectrometer, equipped with a hemispherical electron analyzer
and MgKα (hν = 1253.6 eV) photon source. Prior to the analysis, the catalyst samples
were in situ reduced under the same conditions employed before the catalytic tests. The
binding energies (BE) were referenced to the Al 2p level of the alumina support at
74.4 eV. An estimated error of ±0.1 eV can be assumed for all measurements.
Intensities of the peaks were calculated from the respective peak areas after background
subtraction and spectrum fitting by a combination of Gaussian/Lorentzian functions.
The relative surface atomic ratios were determined from the corresponding peak
intensities, corrected with tabulated sensitivity factors, with a precision of ±7%.

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Temperature programmed reduction (TPR) studies were carried out in a quartz cell on a
conventional system equipped with a thermal conductivity detector. In each experiment,
25 mg of the sample was heated under 5% H2/Ar with a flow of 50 cm3min−1. The
sample was heated at a rate of 10 °C min−1 from 298 to 1323 K.
2.3 Hydrodeoxygenation reaction
Catalytic test
Conversion of guaiacol was carried out in a 0.25 L batch reactor equipped with a
hollowꢀshaft 6ꢀbladed mechanically driven turbine with four baffles on the wall of the
reactor to prevent vortex formation (model Parr 4841). The liquid reactant feed,
consisting of guaiacol (0.232 mol Lꢀ1) in nꢀdodecane (80 mL) with hexadecane (0.0341
mol Lꢀ1) as an internal standard, was introduced into the reactor. Then approximately
0.200 g of catalyst previously reduced under H2 (50 ml/min) at 723 K for 4 h were
transferred carefully to the reactor limiting the exposition to air. The system was closed
and, to avoid any air contamination, N2 was bubbled through the solution for 10 min.
Still under N2 the reactor was heated to the reaction temperature of 573 K under stirring.
The pressure was adjusted to 5 MPa by H2 introduction into the reactor and kept
constant during the course of the experiment. Aliquots of around 0.5 ml were taken
periodically during the reaction. The reaction products were identified by Gas
Chromatography (Perkin Elmer ꢀ Clarus 680) coupled to GCMSꢀSQ8T and quantified
by gas chromatograph (PerkinꢀElmer ꢀ Clarus 400) using a Flame Ionization Detector
(FID) and a CPꢀSil 5 column (Agilent, 30 m x 0.53 mm x 1.0 ꢁm film thickness). The
specific rate for the total conversion of guaiacol was deduced from the initial slope of
the conversion as a function of time plot according to the following equation:
[
b × n
m
]
rs =
Eq. (1)

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where rs is the specific rate expressed in mol g−1s−1, in which b represents the initial
slope of conversion of guaiacol (τ/t), n is the initial number moles of guaiacol, and m
the amount of catalyst (g). The deoxygenation (HDO) rate was calculated from guaiacol
conversion to Oꢀfree compounds (benzene, cyclohexane, cyclohexene and analogs). On
the other hand, the selectivities (%) were determined at 50% of guaiacol conversion,
according to Eq. (2).
Xi
S% =
×100
Eq. (2)
XT
where Xi is the percentage of product formation i, and XT is the guaiacol conversion.
Reaction blank tests were performed with the bare support, CNT, and no activity was
observed during the 4 h reaction time.
3. Results
3.1 Characterization
The nitrogen adsorption/desorption isotherms at 77 K of the catalysts were
performed to study their textural properties and the results are shown in Table 1. The
estimated values of BET surface areas, Vmicro and Vmeso show that the catalysts own
mostly mesopores. For the NiCux/CNT serie, it can be observed a continuous decrease
of the BET surface area for the catalysts as the amount of copper increases, and the
same trend is observed for both the microporous and the mesoporous volumes.
The XRD diffraction patterns of the catalysts are shown in Fig. 1. All the
catalysts displayed the diffraction peak at 26.1º corresponding to the hkl diffraction

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(002) of graphite. The monometallic catalyst Ni/CNT also displayed the characteristic
Ni0 peaks at 44.4º, which overlaps with that of the (100) diffraction of graphite at 43.3º;
at 51.5º and 76.4º corresponding to the diffractions in the (111), (200) and (220)
crystalline planes of the fcc phase, (JCPDS 04ꢀ0850). For the bimetallic catalysts some
differences can be observed: the diffraction corresponding to Ni0 is slightly shifted to
lower angles, i.e. 44.3º, and the diffraction peaks are wider compared to the
monometallic catalyst. These differences have been explained by the formation of Niꢀ
Cu alloys (JCPDS 47ꢀ1406) 12,21 or, at least, as will be later confirmed by H2ꢀTPR and
XPS, an intimate contact between Ni and Cu. 22 The presence of individuals Cu0 entities
whose diffraction peaks corresponding to the (111) and (200) planes appear at 43.3º and
50.4º (JCPDS 04ꢀ0836) cannot be disregarded, though the small percentage of Cu
compared to Ni and the absence of the diffraction peak of the (220) plane at 74.1º
suggest that the contribution of Cu0 in the observed peaks is negligible. Moreover a
small peak at 37.1º (JCPDS 04ꢀ0835) ascribed to NiO is insinuated in the bimetallic
samples. In addition, the low intensity of the diffraction peak at 76.4º ascribed to Ni0 for
the bimetallic catalysts with Cu loading of 1.5ꢀ2.25 wt. % which clearly increases for
NiCu3.75/CNT seem to indicate that the Ni particle size decreases by adding copper until
a certain loading. The particle size of Ni/CNT and the bimetallic catalysts were
estimated by the Scherrer equation using the FWHM values of the (200) diffraction to
eliminate the contribution of graphite. The values are shown in Table 1 and confirmed
that the particle size decreases by adding Cu, though the size is similar for the bimetallic
catalysts, except for NiCu3.75/CNT which clearly displays a larger size in agreement
with the qualitative observations.
To further identify the interaction between Ni and Cu and the distribution of the
nanoparticles over the support, STEMꢀEDS of the catalyst NiCu3.0/CNT as illustrative

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example was also performed. The STEMꢀEDS mapping images, in Figure 2a, showed
the presence of NiꢀCu alloy with a homogenous distribution of Ni and Cu in the single
alloy particle, along with pure Ni nanoparticles in agreement with the XRD patterns.
The STEMꢀEDS lineꢀscanning spectra, also showed individual Cu nanoparticles.
Therefore the microscopy identified the presence of Ni, Cu and NiꢀCu alloys. The
average particle sizes were estimated from the TEM images and the values, in Table 1,
agree quite well with those obtained by XRD.
The H2ꢀTPR profiles of the catalysts are shown in Fig. 3. It can be observed that
the monometallic catalysts Ni/CNT and Cu/CNT displayed a low temperature peak at
595 and 510 K respectively. The profiles of the bimetallic catalysts also showed a
reduction peak at low temperature at around 530 K which, for the samples NiCu3.0/CNT
and NiCu3.75/CNT, comes along with an additional contribution at 560ꢀ574 K. In
literature, the reduction of bulk NiO has been observed at 638 K 23,24 which is shifted to
different temperatures depending on the support, particle size and/or location of the
25,26
particles.
For the Ni/CNT and bimetallic catalysts, the first reduction peak would
correspond to the reduction of NiO and the temperature at which it appears is in the
27, 28
range of previous reported values.
As expected, considering the lower reduction
temperature of copper oxide compared to nickel oxide, the reduction of NiO takes
places at lower temperature for the Cuꢀcontaining catalysts, this confirming the
formation of the NiꢀCu alloy. 15, 29 Nonetheless, the appearance of a second contribution
in this peak for the samples NiCu3.0/CNT and NiCu3.75/CNT suggest a different effect of
Cu to the Ni catalysts by further increasing the Cu content. An additional reduction peak
with different intensities is observed at medium temperatures, 710ꢀ740 K, for Cuꢀ
containing samples 1.5ꢀ3 wt. %, which has been ascribed to the reduction of free NiO
clusters with weak interaction with the support and formed during the thermal

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treatments. 30,31 Moreover, the catalyst Ni/CNT displayed a high temperature reduction
peak at 780 K which is shifted to higher temperatures for the bimetallic samples.
Besides the reduction of NiO with different interaction with the support, this peak
would include the H2 consumption corresponding to the methanation of the support by
the nickel nanoparticles.23,32 The shift to higher temperature upon increasing the Cu
content can be an indication of the lower ability of the Ni nanoparticles to catalyze the
methanation when NiꢀCu alloys are formed. 33 Therefore, the H2ꢀTPR results confirmed
the interaction between Ni and Cu in agreement with the XRD and TEM analyses.
Analyses of the XPS spectra of the catalysts were performed. In Fig. 4 the Ni 2p spectra
of the catalysts are shown. All the catalysts displayed a doublet centered at 853.0 and
870.3 eV corresponding to the Ni 2p3/2 and Ni 2p1/2 transition respectively, with lowꢀ
intensity satellites. 14, 16 The Ni 2p3/2 region could be deconvoluted in two components
for all the samples with binding energies in the ranges 852.8ꢀ853.1 eV and 855.0ꢀ855.6
eV, and the proportion of each component is included in brackets. The component at the
lower binding energy corresponds to Ni0 and the component at higher binding energy to
34,35
Ni+2 and it is assigned to NiO.
These results indicate that under the reduction
conditions employed a certain proportion of NiO was still present. Interestingly, the
bimetallic catalysts displayed a higher proportion of NiO compared to the monometallic
Ni/CNT but its proportion is similar for the NiCux/CNT catalysts. It is known that NiO
is reduced at lower temperature in NiꢀCu alloys 16, 22 as we have confirmed by H2ꢀTPR.
However, as also the performed H2ꢀTPR showed, in the bimetallic catalysts took place
the formation of NiO clusters with weak interaction with the support whose reduction
temperature is similar to bulk NiO. The formation of these entities during the hydrogen
thermal treatment was somehow favored when Cu was added, this leading to a lower
reduction degree of the bimetallic catalysts compared to the monometallic Ni/CNT

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which is apparently independent of the Cu content. The discrepancies found between
the XPS analyses and XRD profiles, which showed barely no diffractions ascribed to
metal oxides, could be explained by a small particle size of the oxides which would be
below the detection limit of the XRD, i.e. 4 nm. Furthermore the maximum of Ni 2p3/2
region was shifted to lower binding energies in the Cuꢀcontaining samples compared to
Ni/CNT except for NiCu3.75/CNT. This slight change is significant as it can be related to
15,36
the formation of NiꢀCu alloys.
On the other hand, the Cu 2p3/2 region displayed
only one contribution at 932.7ꢀ932.8 eV. This contribution is in the range of the values
37
reported for Cu0 and Cu+1 whose binding energies are too close. To elucidate the
chemical state of copper, the modified Auger parameter, α, can be used. Therefore, the
Auger parameter of the NiCu1.5/CNT catalyst was evaluated as reference and the
spectrum region is shown in the supplementary information. The obtained value was
1852.3 eV, which corresponds to that of metallic Cu, in line with the reduction
temperature observed for copper in the H2ꢀTPR profile 13,15, 38
.
The relative amounts of the different species were calculated from the corresponding
peak areas divided by the sensitivity factors, and the most relevant data are summarized
in Table 2. It can be observed that, despite the same nominal loading of Ni, the Ni/C
ratio increased upon increasing Cu content for all the catalysts except for NiCu3.75CNT
and only slightly for NiCu3.0CNT . While this increment can be explained by the
stabilization of smaller Ni/NiO nanoparticles by Cu, as previously reported for other Niꢀ
Cu catalysts 16, the lower Ni/C ratio for NiCu3.75CNT can be attributed to the formation
of aggregates as the XRD showed.
In summary, the characterization showed that by adding Cu to Ni/CNT catalyst, NiꢀCu
alloys are formed. The particle size decreases upon increasing Cu content up to a 3 wt.
% Cu loading. Moreover, a higher proportion of NiO was observed in the bimetallic

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samples, although the reduction temperature was lower than for the monometallic
Ni/CNT. These results suggest that, in the studied system, copper loadings above 3 wt.
% cannot be further dissolve in the nickel lattice, this limiting the formation of NiꢀCu
alloys and increasing the particle size.
3.3 Reaction results
Fig. 5 shows the evolution of the conversion of guaiacol and products yield over
Ni/CNT and NiCu(x)/CNT catalysts. The monometallic catalyst Cu/CNT showed very
low conversion and the figure has been omitted. The main reaction products obtained
over all the catalysts were cyclohexanol, cyclohexane, methoxycyclohexanol and
cyclohexene. As side products, the formation of cyclohexanone, BTX (benzene, toluene
and xylene) and lights (mainly methanol) was observed. Moreover, the formation of
cyclohexanone, methoxycyclohexanol and cyclohexanol increased and then decreased
with time, this indicating that these compounds are intermediates products. These
results are consistent with guaiacol conversion schemes proposed by Dongil et al. 20 for
Ni/Carbon catalysts, which was adapted from Laurent and Delmon 39, Bui et al. 40,41 and
42
Nimmanwudipong et al.
and it is shown in Fig.6. On the basis of this reaction
scheme, the guaiacol can be transformed by direct deoxygenation route (DDO) to form
anisole, by direct demethoxylation way (DMO) to form phenol and by hydrogenation
way (HYD) to form methoxyclyclohexanone and methoxycyclohexanol respectively.
The cyclohexanone can be formed from methoxycyclohexanone DMO or by phenol
HYD. The cyclohexanol can be formed from hydrogenation of cyclohexanone or
methoxycyclohexanol DMO. On the other hand, benzene can be formed by DDO of
phenol or by DMO of anisole. The cyclohexene can be formed by HYD of benzene or
dehydration (DHY) of cyclohexanol. Furthermore, the formation of toluene takes place

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through the methylation route (ME) and then DDO of phenol, while xylene can be
formed by ME of toluene.
The products distribution calculated at 50% of guaiacol conversion over Ni/CNT and
NiCu(x)/CNT catalysts is shown in Fig. 7. It can be observed that the addition of Cu
modifies the products distribution: cyclohexanol decreases and methoxycyclohexanol
increases upon increasing the Cu content, this suggesting that Cu decreases the DMO
pathway. As long as the selectivity to HDO products is concerned, the bimetallic
catalysts showed less yield to HDO products than Ni/CNT up to 3 wt. % of Cu loading.
The HDO activity depends on the feasibility to activate CꢀO bonds whose scission takes
place on electron donor sites, where the molecule adsorbs vertically. 43 According to the
characterization the size of the NiꢀCu alloys ensembles is smaller than that of pure Ni
and, thus, the electron donating properties from Ni dꢀorbitals are reduced compared to
monometallic Ni catalysts, this limiting the adsorption strength of the oxygenated
compounds and therefore its activation. 44 The product distribution also showed that the
percentage of light products is lower upon increasing Cu content up to 3 wt. %, and it
increased again for the NiCu3.75CNT catalyst, which indicates that hydrogenolysis of the
CꢀC bond is hampered by the addition of Cu, this reducing the formation of coke and
therefore the catalyst deactivation. It has been reported that the scission of CꢀC bonds is
favored over larger Ni nanoparticles due to the adsorption of the hydrocarbon
intermediates on bigger particles. 14 The characterization performed showed that, in the
studied system, the addition of Cu increased the dispersion of Ni up to 3 wt. % Cu
loading, therefore our results are in quite good agreement with previous literature
results.
The initial rates obtained for the guaiacol hydrotreatment are summarized in Table 3.
The values show that the higher catalytic activity was obtained on the catalyst without

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Cu promotion, and that the activity decreases gradually upon increasing the Cu content.
In literature it has been reported that bimetallic catalysts are more active than
monometallic Ni at the lowest investigated Cu loading, but then the activity decreases
15ꢀ17
with increasing Cu content.
The better performance of NiꢀCu catalysts has been
15,16
explained by the exposure of more surface Ni atoms
or by an optimal adsorption
strength of H2 over the NiꢀCu alloy at certain Cu loading. 17 Our results showed that the
NiꢀCu alloy was formed and, according to the XPS results, the proportion of Ni surface
atoms was higher on the bimetallic catalysts. However, the presence of NiO free
particles was promoted by the addition of Cu and these oxides particles are not active in
the reaction, which would result in a lower activity. Moreover, the reaction proceeds via
the hydrogenation of the aromatic ring which takes place in electron acceptor sites
where the reactant adsorbs parallel to the surface. The presence of smaller particles
would be unfavorable to follow this path which may in turn lower the activity43.
However, the activity is also lower for the catalyst with the highest Cu loading, which
displayed a larger particle size than the other bimetallic catalysts. For this sample, it is
plausible that the nonꢀdissolved Cu particles are located on the most active sites of Ni,
this decreasing the activity as it has been reported for other bimetallic systems. 45,46
4. Conclusions
We have prepared a series of bimetallic NiꢀCu catalysts with different loadings of Cu
supported on carbon nanotubes, and we have studied their catalytic performance in the
hydrodeoxygenation of guaiacol, a lignin model compound. The characterization
performed showed that by adding Cu, NiꢀCu alloys were formed which existed along
with Ni and Cu individual entities. The presence of NiO was also observed by XRD and
XPS in a higher proportion for the bimetallic catalysts compared to the monometallic
catalyst. The addition of Cu favored the stabilization of smaller Ni nanoparticles and

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decreased the reduction temperature of Ni. However, NiO free clusters were formed and
these entities were more difficulty reduced as the NiO/Ni ratio obtained by XPS
confirmed.
It was observed that the intrinsic activity of the catalysts decreased upon increasing the
Cu content. This effect can be explained by the loss of metallic Ni sites due to the
increasing proportion of NiO and the coverage by Cu; and by the unfavorable
adsorption of the aromatic ring over smaller nanoparticles. Concerning the selectivity,
the results showed a decrease in the formation of HDO and light products by adding Cu
which could be also explained by the limited activation of the CꢀO and CꢀC bonds in the
smaller Ni nanoparticles. The different selectivity has been observed for the bimetallic
samples up to a 3 wt. % Cu loading. Beyond that Cu percentage, the trend changes, and
the percentage of lights and HDO products increased, which according to the
characterization can be explained by the formation of Ni agglomerates.
Acknowledgments
The authors acknowledge the financial support by CONICYTꢀFONDECYT grant
1140528, Fondecyt Postdoctoral grants 3130483 and PFBꢀ27 grants. Financial support
of the Spanish Ministerio de Economía y Competitividad by project CTQ2014ꢀ52956ꢀ
C3ꢀ3ꢀR is also acknowledge.
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Table 1. Textural properties of the CNT, the catalysts and metal particle size.
b
S BET Vmicro V meso
Vt
Crystallite
dTEM
Sample
(m2/g) (cm3/g) (cm3/g) (cm3/g) size (nm)a
(nm)
-
-
CNT
300
241
0.003
0.006
1.762
1.078
1.765
1.084
Ni/CNT
13.9
5.0
5.4
4.8
7.7
-
14.2
6.8
-
NiCu1.5/CNT
NiCu2.25/CNT
NiCu3.0/CNT
NiCu3.75/CNT
Cu/CNT
260
237
229
0.007
0.006
0.009
1.035
1.062
1.054
1.042
1.068
1.063
6.4
8.9
-
209
262
0.005
0.010
0.971
0.890
0.976
0.900
a Estimated by the (200) reflection using the Scherrer equation.
b Estimated from TEM images.

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Table 2. Binding energies (eV) and atomic surface ratios of the catalysts.
C1s
Cu2p3/2 Ni2p3/2
Cu/Ni Ni/C
Cu/C
Ni/CNT
284.8 (77)
286.2 (21)
287.7 (2)
-
853.1 (66)
855.6 (34)
-
0.0140
-
284.8 (76)
286.2 (21)
287.8 (3)
Cu/CNT
932.7
932.7
932.6
932.7
932.8
-
-
0.058
284.8 (74)
286.2 (22)
287.8 (4)
NiCu1.5/CNT
NiCu2.25/CNT
NiCu3.0/CNT
NiCu3.75/CNT
852.9 (57)
855.1 (43)
0.2381 0.0210 0.005
0.3288 0.0365 0.012
0.3596 0.0445 0.016
1.3291 0.0158 0.021
284.8 (75)
286.2 (20)
287.8 (5)
852.9 (54)
855.2 (46)
284.8 (76)
286.2 (21)
287.8 (3)
852.8 (58)
855.0 (42)
284.8 (78)
286.2 (18)
287.7 (4)
853.0 (59)
855.0 (41)

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Table 3. Specific initial rates for guaiacol conversion.
Specific initial rate
Catalyst
(×106 mol g−1 cat s−1)
Ni/CNT
44.2
25.7
NiCu1.5/CNT
NiCu2.25/CNT
NiCu3.0/CNT
20.1
13.4
NiCu3.75/CNT
9.9
0.5*
Cu/CNT
*Estimated at 10% conversión.

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Figure 1. XRD patterns of the reduced catalysts, inset magnified zone for Ni (111).
Ni (200)
NiCu alloy
Cu (200)
Ni (220)
40
45
NiCu3.75/CNT
NiCu3.0/CNT
NiCu2.25/CNT
NiCu1.5/CNT
Ni/CNT
10
20
30
40
50
60
70
80
2θ (degree)

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Figure 2. a) STEM-EDS mapping images of the selected area (inside the square) and
line Ni(blue), Cu (red); b) and c) TEM images of the reduced NiCu3.0/CNT catalyst.
a)
b)
c)

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Figure 3. H2-TPR profiles of the catalysts.
NiCu /CNT
3.75
NiCu /CNT
3.0
NiCu /CNT
2.25
NiCu /CNT
1.5
Ni/CNT
Cu/CNT
323
423
523
623
723
823
923 1023 1123 1223
Temperature (K)

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Figure 4. XPS spectra of the Ni 2p region for the reduced catalysts a) Ni/CNT, b)
NiCu1.5/CNT c) NiCu2.25/CNT d) NiCu3.0/CNT e) NiCu3.75/CNT.
b)
c)
a)
850
860
870
880
850
860
870
880
850
860
870
880
BE (eV)
BE (eV)
BE (eV)
d)
e)
850
860
870
880
850
860
870
880
BE (eV)
BE (eV)

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Figure 5. Conversion of guaiacol and yield of products vs time, a) Ni/CNT, b)
NiCu1.5/CNT c) NiCu2.25/CNT d)
100
80
60
40
20
0
100
80
60
40
20
0
b)
100
80
60
40
20
0
100
80
60
40
20
0
a)
0
3000
6000
9000
12000
15000
0
3000
6000
9000
12000
15000
Time (s)
Time (s)
c)
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
100
d)
80
60
40
20
0
0
3000
6000
9000
12000
15000
0
3000
6000
9000
12000
15000
Time (s)
Time (s)

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NiCu3.0/CNT e) NiCu3.75/CNT.
e)
100
80
60
40
20
0
100
80
60
40
20
0
0
3000
6000
9000
12000
15000
Time (s)
(BTX+ cyclohexene+ cyclohexano)

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Fig. 6: Reaction scheme for the conversion of guaiacol [39].
CH3
CH3
HYD
CH3
O
OH
O
ME
DMO
OCH3
OCH
3
DMO
HYD
Toluene
Guaiacol
HYD
DDO
Methoxy
cyclohexanone
Cyclohexanone
OH
HO
CH3
CH3
ME
D
HY
CH3
OH
OH
Methylcyclohexane
ME
Phenol
ol
Methylphen
O
DMO
DDO
OCH
3
Cyclohexanol
Methoxycyclohexanol
HYD
HYD
DDO
Anisole
e
Cyclohexen
Benzene
Cyclohexane
DHY
HDO products