Bimetallic Ni-Cu Catalysts for the Low-Temperature Ethanol Steam Reforming: Importance of Metal-Support Interactions

Article abstract of DOI:10.1007/s10562-014-1414-2

The activity of bimetallic Ni-Cu catalysts in ethanol steam reforming was evaluated and compared to the activity of the corresponding monometallic Ni catalyst. Copper addition positively affected the catalytic activity only if the proper metal-support int

Full text of DOI:10.1007/s10562-014-1414-2

Catal Lett (2015) 145:549–558
DOI 10.1007/s10562-014-1414-2
Bimetallic Ni–Cu Catalysts for the Low-Temperature Ethanol
Steam Reforming: Importance of Metal–Support Interactions
Valentina Nichele
Matteo Compagnoni
•
Michela Signoretto
Ilenia Rossetti
•
Francesco Pinna
Giuseppe Cruciani
•
Elena Ghedini
Alessandro Di Michele
•
•
•
•
Received: 16 September 2014 / Accepted: 28 October 2014 / Published online: 9 November 2014
Ó Springer Science+Business Media New York 2014
Abstract The activity of bimetallic Ni–Cu catalysts in
ethanol steam reforming was evaluated and compared to
the activity of the corresponding monometallic Ni catalyst.
Copper addition positively affected the catalytic activity
only if the proper metal–support interactions, as well as the
proper ratio between the different reducible species, were
maintained. This target can be pursued by tuning the
synthesis conditions.
reasons, hydrogen should be produced from renewable
sources. Ethanol, which is produced by biomass fermen-
tation and is expected to be available in the near future
from second generation biomass, attracted more and more
attention as a possible source of hydrogen because it has
high hydrogen content, it is renewable, easy to store,
handle and transport because of its low volatility and non
toxicity.
Hydrogen can be produced by ethanol steam reforming
(ESR), an endothermic process which takes place accord-
ing to the following stoichiometric reaction, Eq. 1:
Keywords Hydrogen ꢀ Ethanol steam reforming ꢀ Ni–Cu
catalysts ꢀ Synthesis conditions ꢀ Metal–support interaction
CH3CH2OH þ 3H2O ꢀ 6H2 þ 2CO2;
including the water–gas shift of the intermediate CO
ð1Þ
1
Introduction
(Eq. 2), which further increases the hydrogen yield:
Hydrogen is considered the future energy vector that can
meet the ever growing world energy demand in a clean and
sustainable way. In order to reduce the dependence from
fossil fuels for environmental, economic and political
CO þ H2O ꢀ CO2 þ H2:
ð2Þ
However several reaction pathways can take place; the
most important are ethanol dehydrogenation, dehydration
or decomposition, that lead to the formation of acetalde-
hyde, ethylene and methane respectively [1, 2]. The overall
process depends on the reaction conditions but also, of
course, on the catalyst. Noble metals (Pt, Rh, Ir) show high
activity and selectivity in ESR [3–5], but their high cost is a
serious drawback. Nickel is as active and selective as noble
metals, but it is by far cheaper and more available. Nickel
is highly effective in breaking C–C bonds, that is a fun-
damental requirement for this reaction [6, 7]. Nevertheless,
hydrogen yield is usually lowered by the formation of a
significant amount of methane [8] if ESR is carried out at
conveniently low temperature to limit the heat input to the
reactor (i.e. 500 °C). Moreover, Ni catalysts may suffer
from severe deactivation due to sintering and coking
V. Nichele ꢀ M. Signoretto (&) ꢀ F. Pinna ꢀ E. Ghedini
Molecular Sciences and Nanosystems Department, Ca’ Foscari
University, INSTM Unit, Calle Larga Santa Marta 2137,
30123 Venice, Italy
e-mail: miky@unive.it
M. Compagnoni ꢀ I. Rossetti
Chemistry Department, ISTM-CNR, INSTM Unit, Universita`
degli Studi di Milano, via C. Golgi, 19, 20133 Milan, Italy
G. Cruciani
Physics and Earth Sciences Department, University of Ferrara,
via Saragat 1, 44122 Ferrara, Italy
A. Di Michele
Physics and Geology Department, Universit a` degli Studi di
Perugia, Via Pascoli, 06123 Perugia, Italy
[
9–11]. To control sintering phenomena, Ni particles
should be stabilized through strong interactions with the
123

5
50
V. Nichele et al.
support. This is important also to limit coke formation,
since it is well established that coking occurs more
promptly over large Ni particles and aggregates than over
very dispersed Ni crystallites [12–15]. Another possibility
is to add a second metal to the Ni-based catalyst. Copper
alone is not so efficient for the promotion of ESR because
of its low activity in C–C bonds cleavage [16]. Neverthe-
less, it was reported that copper can segregate the active Ni
particles through the formation of Ni–Cu alloys, thus
inhibiting both Ni sintering and coking phenomena [8, 17].
Rem o´ n et al. [18] reported that the major carbon preventing
effect of copper is to block the step sites on nickel particles,
preventing the creation of nucleation sites responsible for
graphite formation. Moreover, copper addition to nickel
enhances the selectivity to hydrogen thanks to the ability of
copper to change the affinity for carbon of nickel particles,
which inhibits coke formation and improves stability [19,
(Sigma-Aldrich, purity C98.5 %), Cu(NO ) ꢀ3H O (Sigma-
3
2
2
Aldrich, puriss. p.a.) and Ca(NO ) ꢀ4H O (Fluka, purity
3
2
2
C99 %) as precursors. Ni, Cu and CaO amounts were kept
constant (Ni: 10 wt%; Cu: 2 wt%; CaO: 9 wt%).
A reference sample was prepared by adding Ni- and
CaO-precursors to Zr(OH) simultaneously and calcining
4
at 500 °C for 4 h in flowing air (50 mL/min STP). This
sample is labelled ZCaNi.
In order to study the effect of copper addition, Ni-, CaO-
and Cu-precursors were simultaneously impregnated on
Zr(OH) . Part of the sample was calcined at 500 °C for 4 h
4
(ZCaNiCu), while the rest was calcined at 800 °C for 4 h
(ZCaNiCu800) to verify the effect of calcination temperature.
Two other samples were prepared by adding Ni and Cu
in two separate and sequential steps. In ZCaNi Cu Ni- and
1
2
CaO-precursors were added simultaneously to Zr(OH) , the
4
sample was calcined at 500 °C for 4 h, then Cu-precursor
was added and the catalyst was calcined again at 500 °C
for 4 h. In ZCaCu Ni the order of addition of Ni and Cu
2
0]. Copper is considered to increase the selectivity to
hydrogen also because of its high activity for water–gas
shift reaction [21].
1
2
was the opposite.
In this work we studied the effect of copper addition to a
Ni/ZrO catalyst. We recently found that this catalyst is
2
2.2 Catalysts Characterization
highly active in ESR [22–25] and that coking, partly
related to the presence of Lewis acid sites on the surface of
the support, can be efficiently inhibited by doping with a
proper amount (9 wt%) of CaO [26]. To further improve
the performance of this catalyst, in particular in terms of
hydrogen productivity, we decided to modify it by adding
copper. As we previously discussed, the effect of copper
addition is already quite well documented in the literature.
The novelty of this work lies in the study of the effect of
copper addition modality to a Ni-based catalyst. In par-
ticular, we evaluated the effect of both the calcination
temperature and the order of metals addition.
Temperature programmed reduction (TPR) measurements
were carried out by placing the catalyst in a quartz reactor
and heating in a 5 % H /Ar mixed gas stream flowing at
2
40 mL/min at a heating rate of 10 °C/min from 25 to
800 °C. H2 consumption was monitored by a TCD
detector.
X-ray powder diffraction (XRD) patterns were measured
by a Bruker D8 Advance diffractometer equipped with a
Si(Li) solid state detector (SOL-X) and a sealed tube provid-
ing Cu Ka radiation. The samples were reduced in H flow for
2
1 h at 500 °C (750 °C for ZCaNiCu800) before the analysis.
The Rietveld refinement method as implemented in the Bru-
ker TOPAS program was used to obtain the refined unit cell
parameters, crystal size, and the quantitative phase analysis
2
Experimental Section
(QPA) for the ZrO support and metal phases in the samples.
2
2
.1 Catalysts Preparation
The crystal size determination is achieved by the integral
breadth based calculation of volume weighted mean crystal-
lize sizes assuming intermediate crystallite size broadening
modelled by a Voigt function. The amorphous fraction was
estimated by fixing in the QPA the nickel fraction to the
nominalcontentof10 wt%.Inthiswaytheminimalcontentof
the amorphous phase was calculated. The results of Rietveld
refinements are reported in Table 1. Very large uncertainties
were obtained for samples containing a poorly crystalline
zirconia phase (e.g. ZCaNiCu and ZCaNi Cu ). Nevertheless
Zr(OH) was prepared by a precipitation method [27] at a
4
constant pH of 10. ZrOCl ꢀ8H O (Sigma-Aldrich, purity
2
2
C99.5 %) was dissolved in distilled water and added with a
peristaltic pump under vigorous stirring to an ammonia
(
33 %, Riedel-de Ha e¨ n) solution. During the precipitation,
the pH value was kept constant at 10.0 ± 0.1 by the con-
tinuous addition of a 33 % ammonia solution. The
hydroxide suspension was aged for 20 h at 90 °C, then
filtered and washed with warm distilled water until it was
1
2
someusefulinformationwereobtainedeveninthepresenceof
an estimated large amorphous fraction.
free from chloride ions (AgNO test) and finally dried
3
overnight at 110 °C.
Ni, Cu and CaO were added to Zr(OH) by means of the
TEM images have been obtained using a Philips 208
Transmission Electron Microscope. The samples were
prepared by putting one drop of an ethanol dispersion of
4
incipient wetness impregnation technique using Ni(NO ) ꢀ6H O
3
2
2
1
23

Bimetallic Ni–Cu Catalysts for the Low-Temperature Ethanol Steam Reforming
551
the catalysts on a copper grid pre-coated with a Formvar
film and dried in air.
and ca. 0.5 g were loaded into the reactor after dilution 1:3
(vol/vol) with SiC of the same particle size. The void part
of the reactor over and below the catalyst bed has been
filled with quartz beads (0.5–1 mm size). The catalysts
SEM images have been obtained using a Field Emission
Gun Electron Scanning Microscopy LEO 1,525, after
metallization with Cr. Elemental composition was deter-
mined using Bruker Quantax EDS.
3
were activated in 50 cm /min of a 20 vol% H /N gas
2
2
mixture, while heating by 10 °C/min up to 500 °C (750 °C
for ZCaNiCu800), then kept for 1 h. During activity testing
Micro-Raman sampling was made by an OLYMPUS
microscope (model BX40) connected to an ISA Jobin–
Yvon model TRIAX320 single monochromator, with a
3
0.017 cm /min of a 3:1 (mol/mol) H O:CH CH OH liquid
2
3
2
mixture were fed to the reactor by means of a Hitachi, mod.
3
L7100, HPLC pump. 56 cm /min of N , used as internal
-
1
resolution of 1 cm . The source of excitation was a Melles
Griot 25LHP925 He–Ne laser that was used in single line
excitation mode at l = 632.8 nm. The power focused on the
samples was always less than 2 mW. The scattered Raman
photons were detected by a liquid-nitrogen cooled charge
coupled device (CCD, Jobin–Yvon mod. Spectrum One).
Temperature programmed oxidation (TPO) of the spent
catalysts was carried out by feeding 40 mL/min of a
2
3
standard, and 174 cm /min of He were also added. Such
dilution of the feed stream was calibrated so to keep the
reactants mixture in the vapour phase even at zero con-
version at the reactor outlet. This allows the direct quan-
tification of all the reactants and products in a single step,
with more reliable quantification of carbon balance, con-
version and selectivity. Internal and external mass transfer
limitations were found negligible during system set up,
given the high linear velocity of the gas mixture and the
selected particle size.
1
0 vol% O /He gas mixture while heating by 10 °C/min
2
from 25 to 650 °C.
2
.3 Catalytic Tests: Ethanol Steam Reforming
The activity tests were carried out at atmospheric pres-
-
1
sure, GHSV = 2,500 h (referred to the ethanol ? water
gaseous mixture) at 500 °C for the samples calcined at
500 °C, at 750 and 500 °C for the sample calcined at 800 °C.
The analysis of the out-flowing gas was carried out by a
gas chromatograph (Agilent, mod. 7980) equipped with
two columns connected in series (MS and Poraplot Q) with
Activity tests were performed by means of a continuous
micropilot plant including an Incoloy 800 downflow reac-
tor (i.e. 0.9 cm, length 40 cm) heated by an electric oven
connected to an Eurotherm 3204 TIC. The catalysts were
pressed, ground and sieved into 0.15–0.25 mm particles
Table 1 Results from Rietveld
refinements of catalysts
˚
a (A)
˚
c (A)
˚
3
Phase
V
cell (A )
Crystal size (nm)
Phase fraction (wt%)
Sample ZCaNi
t-ZrO
2
3.622 (2)
3.528 (2)
5.124 (7)
5.1 (8)
67.2 (1)
43.9 (1)
13 (1)
5 (1)
89.8 (5)
10.2 (5)
Ni
Sample ZCaNiCu
t-ZrO
2
3.6 (3)
66 (15)
44 (1)
1 (7)
3 (5)
8 (2)
82 (24)/57 (17)
b
17 (20)/10
Ni
3.53 (1)
a
3.615
a
Cu
47.2
3 (4)/2 (3)
–/31 (96)
Amorphous
Sample ZCaNi
t-ZrO2
t-ZrO
zirconia’ (s.g. P4
2
stands for ‘tetragonal
/nmc); Ni(Cu)
1
Cu
2
2
3.63 (1)
5.07 (1)
67 (1)
6 (1)
5 (3)
8 (1)
39 (3)/8 (1)
b
50 (2)/10
stands for Ni–Cu alloy
Ni
3.533 (4)
a
3.615
44.1 (2)
a
47.2
Estimated standard deviations in
parentheses refer to last
significant digit
Cu
10 (1)/2 (1)
–/80 (1)
Amorphous
a
1 2
Sample ZCaCu Ni
The cell parameter of metal
Cu was always close to the
tabulated one (PDF card no.
t-ZrO
2
3.624 (9)
3.534 (2)
5.129 (3)
5.123 (5)
67.4 (5)
44.1 (1)
19 (2)
11 (1)
8 (3)
70 (5)/24 (2)
b
29 (4)/10
Ni
0
fixed to this value
4-0836) and therefore kept
a
a
Cu
3.615
47.2
2 (1)/1 (2)
–/65 (5)
b
Amorphous
Sample ZCaNiCu800
Ni fraction was kept fixed to
nominal content when refining
10 wt%. This allowed
[
t-ZrO2
Ni(Cu)
3.623 (2)
3.5344 (3)
67.2 (8)
12 (1)
15 (1)
90.3 (2)
9.7 (2)
estimation on the minimal
amorphous fraction
44.15 (1)
123

5
52
V. Nichele et al.
Table 2 Results of the activity tests for ethanol steam reforming at 500 °C (in the last column, at 750 °C for ZCaNiCu800). Activation at
5
00 °C for 1 h for every sample except ZCaNiCu800, activated at 750 °C for 1 h. Time-on-stream: 8 h, data averaged out 4–8 h-on-stream
Blank test
ZCaNi
ZCaNiCu
ZCaNi
1
Cu
2
ZCaCu Ni
1
2
ZCaNiCu800
ZCaNiCu800
50 °C
7
EtOH conversion/%
13 ± 5
100 ± 0
63 ± 5
0.86 ± 0.05
13 ± 1
–
72 ± 9
47 ± 2
100 ± 0
68 ± 2
100 ± 0
52 ± 6
48 ± 11
47 ± 6
–
100 ± 0
2
H O conversion/%
60 ± 2
H
2
productivity/mol/min kgcat
–
–
–
–
0.45 ± 0.10
5 ± 1
1.04 ± 0.04
12 ± 2
–
0.65 ± 0.08
4 ± 1
1.13 ± 0.02
CH
CH
4
3
selectivity/%
11 ± 2
38 ± 9
19 ± 2
–
–
–
CHO selectivity/%
36 ± 4
7 ± 3
2
C H
4
selectivity/%
–
19 ± 5
2 ± 2
14 ± 3
Fig. 1 Ethanol conversion (filled circle) and C balance (empty circle) trends for: a ZCaNi, b ZCaNiCu
a thermal conductivity detector (TCD), properly calibrated
for the detection of ethanol, acetaldehyde, acetic acid,
acetone, water, ethylene, CH , CO, CO , H . Material
Contemporarily, the selectivity to acetaldehyde and
ethylene, initially nil, started increasing after ca. 3 h-on-
stream, reaching the steady state value reported in Table 2.
Although it may be correlated to many factors, i.e. the
formation of different species, the carbon balance can be
conveniently taken as index of possible catalyst coking. In
particular, low values of C balance can denote coke for-
mation. In the case of ZCaNiCu, the low C balance
(average value: 69 %) in the first 3 h of the test seems to
suggest that a certain accumulation of coke took place
4
2
2
balance on C-containing products was checked to quantify
coke deposition. Repeated analyses of the effluent gas were
carried out every hour and the whole duration of every test
was ca. 8 h. The raw data, expressed as mol/min of each
species outflowing from the reactor, have been elaborated
as detailed in Refs. [23, 25].
(vide infra), leading to deactivation of the catalyst and a
3
Results and Discussion
fast decrease of ethanol conversion and increase in the
selectivity of by-products.
3
.1 Effect of Copper Addition
This result can be related to the fact that ZCaNiCu
shows a certain selectivity to C2 by-products, that were
absent with ZCaNi. The presence of these by-products can
be ascribed to copper, which is not active in C–C bonds
cleavage, as already reported in the Introduction (Sect. 1).
It seems that the presence of copper favoured mainly the
secondary reactions leading to by-products, such as acet-
aldehyde and ethylene, which are precursors of coke. The
formation of coke, suggested by the initial low C balance,
led to the deactivation of the Ni active sites of the catalyst,
indicated by the decrease of ethanol conversion. As a
consequence of the formation of significant amounts of
by-products, a strong decrease of hydrogen productivity
was also observed.
In Table 2 the results of ESR tests are reported. Data
reported are averaged out 4–8 h-on-stream.
By analyzing the results related to ZCaNiCu it is pos-
sible to evaluate the effect of copper addition, with respect
to the activity of the reference sample ZCaNi. Contrary to
what expected, Cu doping has made the catalytic perfor-
mance worse. Ethanol conversion is much lower with
ZCaNiCu than with ZCaNi. In particular, ethanol conver-
sion is complete and stable with ZCaNi (Fig. 1a), while in
the case of ZCaNiCu it decreases sharply after about
3
h-on-stream, in correspondence to an increase of the
carbon balance (Fig. 1b).
1
23

Bimetallic Ni–Cu Catalysts for the Low-Temperature Ethanol Steam Reforming
553
works [22–25] we demonstrated that the interactions
between the active phase and the support play a key role in
determining the performance of a catalysts. However, it also
appears a non negligible contribution of highly reducible Ni
species. TEM characterization (Fig. 3) helps throwing light
on the attribution of these reduction features.
ZCaNiCu
ZCaNi
The fresh ZCaNi sample (Fig. 3a) is characterized by a
uniform array of 5–10 nm Ni particles, whereas sample
ZCaNiCu (Fig. 3b) is constituted by poorly uniform metal
particles, where much bigger particles coexist with smaller
ones. This confirms the attribution of the low temperature
TPR peak to sintered Ni particles.
1
00
200
300
400
500
600
700
800
The results here reported seem to suggest that also the
ratio between the different reducible species is important
and correlated with metal particle size.
Temperature / °C
Fig. 2 TPR profiles of ZCaNi and ZCaNiCu
For a further understanding of the role of crystallo-
graphic nature of metal/support phases on the catalytic
performances, XRD patterns of ZCaNi and ZCaNiCu were
taken as reported in Fig. 4.
In conclusion, copper addition was not satisfactory for
ESR. Several characterization measurements were carried
out in order to explain these results.
The TPR profiles of ZCaNi and ZCaNiCu are reported
in Fig. 2.
As for ZCaNi, zirconia is present only in the tetragonal
phase (the most intense peak at 2h & 30°). In our previous
4
?
2?
For both samples several reduction peaks, in a broad
region between 250 and 550 °C, can be detected. As for
0
study [26] we showed that Zr substitution by Ca takes
place; this replacement does not affect the nature of the
2
?
ZCaNi, since it is known that Ni
is reduced to Ni
ZrO polymorph of the support, but it modifies Ni reduc-
2
without any intermediate oxide [28], the presence of sev-
eral peaks suggests the existence of NiO species differently
interacting with the support [22, 29]. The first peak at
ibility, in accordance with the TPR results, because oxygen
vacancies are generated. Nickel appears only in the
metallic form (peak at 2h = 44.5°), suggesting that the
reduction treatment at 500 °C substantially reduced all
360 °C can be assigned to poorly interacting NiO, which is
more easily reducible, since unsupported NiO reduces at
2
?
0
Ni to Ni .
about 280 °C [30, 31]. The second and third peaks, at about
With regard to ZCaNiCu, a substantial difference can be
detected: zirconia in this sample appears very poorly
crystalline and a very small crystal size were obtained from
Rietveld refinement for the Ni and Cu crystallites. This
could be due to the synthetic approach we adopted, in
which all metals have been added to the support simulta-
neously. It is known that dopant addition can modify the
crystallization process [35, 36].
4
20 and 510 °C, respectively, can be ascribed to NiO
progressively more strongly interacting with the support
2
32, 33]. No peak related to the reduction of Ca can be
?
[
detected, but CaO modifies nickel reducibility. As we
previously reported [26] and according to the literature [29,
2
?
4?
3
4], in ZCaNi Ca
replaces Zr
in the lattice, thus
generating oxygen vacancies which weaken the Ni–O bond
and increase nickel reducibility (i.e. the intensity of the first
peak) with respect to the undoped sample.
In conclusion, besides the different ratio between the
NiO species on the support, another important difference
between ZCaNi and ZCaNiCu can be highlighted, that is
the crystallographic nature of zirconia, which is crystalline
in ZCaNi, while it is very poorly crystalline in ZCaNiCu.
Since the catalytic behaviour of the two samples is
completely different, we decided to check the role of both
the ratio between the various NiO species on the support
and the crystallographic nature of zirconia. To do this, one
possible way is to modify the synthesis procedure.
With regard to ZCaNiCu, the same three peaks related to
2
?
Ni reduction can be noticed, plus one peak at 280 °C
ascribable to the reduction of CuO. A lower reduction tem-
perature can be ascribed to lower metal-support interaction
strength and/or to bigger particle size of NiO. Nevertheless, a
substantial difference in the relative intensity of the three Ni
peaks must be underlined, that is the increase of the intensity
of the third peak (the fraction of NiO with strong interactions
with zirconia) at the expense of the first two (NiO weakly and
non-interacting). Even if the TPR analysis is not used here as
a quantitative technique, the intensity of a peak is roughly
proportional to the amount of reduced species. We can
conclude that copper addition seems to have increased the
fraction of strongly interacting Ni species. In our previous
3.2 Effect of the Order of Metals Addition
The results have shown that the contemporary addition of
all the metals alters the crystallization process, so we
decided to add Ni and Cu in two separate and sequential
123

5
54
V. Nichele et al.
1 2 1 2
Fig. 3 TEM micrographs of fresh catalysts. a ZCaNi, b, c ZCaNiCu, d ZCaNi Cu , e ZCaCu Ni , f ZCaNiCu800. Marker size 100 nm
ZCaNiCu
t-ZrO
2
ZCaCu Ni
1
2
ZCaNi Cu
1
2
t-ZrO
2
0
Ni
0
Ni
ZCaNi
ZCaNi
2
0
30
40
50
60
2
0
30
40
50
60
2
Θ
2
Θ
Fig. 5 XRD powder patterns of reduced ZCaNi, ZCaNi
ZCaCu Ni
1 2
Cu and
Fig. 4 XRD powder patterns of reduced ZCaNi and ZCaNiCu
1
2
steps (samples ZCaNi Cu and ZCaCu Ni described in
2
with ZCaNiCu, also in this case the presence of copper
seems to negatively affect the conversion of by-products,
acetaldehyde and ethylene, which are not completely
converted over the active phase. However, the observed
selectivity to acetaldehyde and ethylene is lower when Cu
is added before Ni with respect to their contemporary
addition. Accordingly, the selectivity to ethylene and
acetaldehyde, initially nil, started increasing after 4 h-on-
stream to the steady state value reported in Table 2. Thus
we can conclude that the addition of Cu modifies the Ni
active sites, partially limiting their activity in C–C bond
cleavage, depending likely on its surface exposure.
1
1
2
Sect. 2.1). The catalytic results reported in Table 2 show
that the two samples have markedly different catalytic
performance, also if compared to the reference catalyst
ZCaNi, confirming that the synthesis procedure can
strongly affect the catalytic behaviour.
In particular, we can observe that ethanol conversion is
complete with both ZCaNi Cu and ZCaCu Ni , but the
1
2
1
2
products distribution is totally different once again.
With regard to ZCaCu Ni , the products distribution is
1
2
quite similar to the ZCaNiCu one. A certain amount of C2
by-products and a low H productivity can be achieved. As
2
1
23

Bimetallic Ni–Cu Catalysts for the Low-Temperature Ethanol Steam Reforming
555
With regard to the second parameter we decided to
investigate, namely the ratio between the different NiO
species on the support, the TPR profiles of the samples are
reported in Fig. 6.
ZCaCu Ni
1
2
Comparing the TPR pattern of ZCaNi Cu with the
1
2
ZCaNi one, it can be noticed that in ZCaNi Cu a CuO
1
2
species reduction peak is located between 180 and 290 °C.
With regard to the region of Ni reduction, the two pro-
ZCaNi Cu
2?
1
2
files are almost identical, in terms of both the position of
the peaks and their relative intensities. On the contrary, the
TPR profile of ZCaCu Ni is completely different. The
ZCaNi
800
1
2
1
00
200
300
400
500
600
700
most evident differences lies in: i) the final reduction
Temperature / °C
temperature of NiO, which was lower of about 200 °C
(
from 650 to 450 °C); ii) the relative distribution of the
Fig. 6 TPR profiles of ZCaNi, ZCaNi Cu and ZCaCu Ni
2 1 2
1
2?
various Ni species. In the literature it is often reported
that the presence of copper lowers the reduction tempera-
ture of nickel [20, 37, 38]. P e´ rez-Hern a´ ndez et al. [39], for
example, reported that Cu probably causes spillover of
hydrogen on nickel, inducing a competitive reduction of
both copper oxide and NiO. Our results show that this
depends also on the synthesis approach. The sequential
impregnation of Ni and Cu precursors (ZCaNi Cu and
On the contrary, the behavior of ZCaNi Cu is in line
1
2
with that of ZCaNi, but the ESR results are even better. In
fact, all other results being equal, hydrogen productivity
increased from 0.86 (reference ZCaNi) to 1.04 mol/
min kg . This result is accompanied by a higher water
cat
conversion with ZCaNi Cu and the absence of C2
1
2
1
2
by-products (with the exception of a very little formation of
ethylene towards the end of the test).
ZCaCu Ni ) leads to better results than the simultaneous
1 2
one (ZCaNiCu). Nevertheless the order of metals impreg-
nation is important as well, as reported also by other
authors [40, 41]. In our case, the catalytic results showed
that ZCaNi Cu is more active than ZCaCu Ni . Maybe
Some characterization measurements have been carried
out to interpret these results.
The XRD patterns of ZCaNi Cu and ZCaCu Ni are
1
2
1
2
1
2
1
2
reported in Fig. 5 and compared with ZCaNi.
this can be ascribed to the fact that in this sample the
proper ratio between NiO species with medium and strong
interactions with the support has been preserved, as in the
reference sample ZCaNi. On the contrary, in ZCaCu Ni
Looking at these patterns along with data in Table 1 it can
be noted that both ZCaNi Cu and ZCaCu Ni samples con-
1
2
1
2
taina major fractionof amorphous zirconiaphase. This causes
an intensity bump which is responsible for the high angle
asymmetry of the most intense zirconia peak. The refined
crystalsizefort-ZrO inZCaCu Ni (seeTable 1)isrelatively
1
2
NiO with low interaction seems to be present.
TEM analysis is reported in Fig. 3 also for these samples.
Substantially uniform, ca. 5 nm, metal nanoparticles may be
observed for sample ZCaNi Cu , whereas a broader particle
2
1
2
large suggesting that a minor component of better crystallized
zirconia coexists with the amorphous phase. Concerning the
NiandCumetalphases, whilethepeakataboutabout44°2his
broad in sample ZCaNi Cu and can be interpreted as the
1
2
size distribution was evident for sample ZCaCu Ni .
1
2
By considering all these results we may conclude that the
composition and synthesis conditions which allow the for-
mation of stable and uniform small Ni particles, as in the case
of samples ZCaNi and ZCaNi Cu , lead to high activity and
1
2
partial overlap of separate Ni and Cu peaks, in the case of
ZCaCu Ni this interpretation is less straightforward because
1
2
1
2
the appearance is that of a relatively sharp single peak corre-
sponding to relatively large Ni nanocrystals. On this basis and
given the very low refined fraction of Cu, the occurrence of a
Ni–Cu alloy cannot be ruled out altogether. The refined larger
cell parameter of Ni does not provide a clear-cut indication
because of the large standard errors (see Table 1).
stability of the catalyst with time-on-stream. The absence or
limited formation of coke deposits ensures the absence of C2
by-products. The latter are usually found when Ni becomes
more and more covered by coke, which effectively limits the
C–C bond breaking ability of the active site. This is likely
due to the progressively lower availability of vicinal Ni sites
for C–C bond cleavage. Therefore, when big Ni particle size
is present and/or Ni species poorly interacting with the
support which easily sinter during use, a low initial C balance
is usually observed at the beginning of the test, increasing
with time-on-stream accompanied by a decrease in ethanol
conversion and increasing selectivity towards ethylene and
acetaldehyde.
The large amorphous content in ZCaNi Cu and
1
2
ZCaCu Ni samples seem to confirm that the addition of
1
2
both metals hampers the crystallization of the catalyst. On
the other hand, the crystalline phase of the support does not
affect the catalytic activity so deeply, since good catalytic
performance was achieved with the almost amorphous
ZCaNi Cu sample.
1
2
123

5
56
V. Nichele et al.
Upon calcination of ZCaNiCu catalyst at 800 °C a clear
shift to lower 2-theta angle (larger d-spacing) is observed
for the diffraction line of metal (see XRD patterns in
Fig. 7).
ZCaNiCuC800
ZCaNi
Due to the increased crystallinity of sample upon cal-
cinations at 800 °C the Rietveld refined unit cell parameter
is affected by a small error (see Table 1) and is interme-
diate between that of pure Ni and pure Cu, showing that a
Ni–Cu alloy was formed. Furthermore, the TPR analysis
4
2
44
46
2
Θ
ZCaNiCuC800
ZCaNi
(not reported for the sake of brevity) evidenced a much
higher reducibility of Ni in sample ZCaNiCu800 with
respect to sample ZCaNi. In particular, only highly
reducible Ni species survived and the more disperse ones,
strongly interacting with the support, completely disap-
peared. This confirms the tendency to sinter of Ni,
according to our previous results [23, 24, 42], and helps
elucidating why at 500 °C, where coke gasification is not
useful to clean up metal surface, progressive and fast
deactivation takes place.
2
0
30
40
50
60
2
Θ
Fig. 7 XRD patterns of ZCaNi and ZCaNiCu800
3
.3 Effect of the Calcination Temperature
Calcination temperature is another important parameter
that can affect both the interactions between metal and
support and the crystallographic phase of the support. We
recently showed that the calcination temperature affects the
speciation of Ni when supported over oxides which
establish a strong metal support interaction [42, 43]. For
Therefore, preparation conditions which favor the
instauration of a stronger metal support interaction allow to
keep dispersed the active phase and limit the formation of
coke filaments during low temperature operation. The rel-
ative amount of these species may be correlated with cat-
alyst life during testing at 500 °C. Indeed the sample for
which only very reducible species were present was the
least stable. By contrast, samples characterized by a sig-
nificant concentration of less reducible Ni species (higher
dispersion and stronger metal–support interaction) dem-
onstrated higher activity for ethanol (and methane)
reforming and above all higher stability with time-on-
stream. This is ultimately due to a higher resistance
towards coking.
instance, when Ni/TiO is calcined at 500 °C it is com-
2
pletely inactive, whereas when calcined at 800 °C it
exhibits satisfactory catalytic performance. In the present
case a different calcination temperature may also affect the
mutual interaction between the two metals. Therefore, a
sample for comparison has been prepared by contempo-
rarily adding Ni and Cu precursors and calcining at 800 °C
(
sample ZCaNiCu800). This sample was tested at 500 °C
after activation at 750 °C under reducing conditions
Table 2). A further test at 750 °C was also added.
The catalytic performance of sample ZCaNiCu800 at
(
3.4 Spent Samples
5
00 °C was even worse than that achieved after calcination
Finally, the spent samples have been characterized by TEM
(Fig. 8), FE-SEM and Raman spectroscopy (Fig. 9).
TEM confirms limited sintering after use of samples
ZCaNiCu and ZCaNi Cu . This confirms that the presence
at 500 °C, with negligible H2 productivity and lower
activity for both ethanol and methane reforming. Selec-
tivity to C2 by-products and the decrease of ethanol con-
version became evident after the first hour-on stream. It has
been also observed that C balance was rather low at the
beginning of the test (ca. 86 %) and increased with time-
on-stream to reach ca. 100 %. This behavior may be related
to some sintering of the active phase with respect to the
sample calcined at 500 °C, inducing broader formation of
carbon filaments which induce a faster catalyst deactiva-
tion. By contrast, operation at 750 °C gave rise to satis-
factory and stable results, with full ethanol conversion and
good carbon balance, no C2 by-products and no methane.
This means that at high temperature coke gasification
occurs and avoids the encapsulation of the active phase
which leads to the deactivation observed at 500 °C.
1
2
of poorly reducible species, i.e. strongly interacting with
the support, helps in keeping Ni dispersed during use.
Some formation of carbon nanotubes is observed for
ZCaNiCu and substantially due to bigger metal particles,
which are well known to be more prone to C accumulation
(Fig. 8c). According to the much bigger Ni particle size,
extensive nanotubes formation was achieved with sample
ZCaNiCu800 (Fig. 8d). In all cases the formed C nano-
tubes showed a multiwalled structure with external diam-
eter of 28–29 nm. Some graphitic C deposition may also be
present due to the presence of some residual Lewis acid
sites on the ZrO support, as evidenced by micro-Raman
2
analysis (Fig. 9).
1
23

Bimetallic Ni–Cu Catalysts for the Low-Temperature Ethanol Steam Reforming
557
Fig. 8 TEM images of spent samples. a ZCaNi
1 2
Cu , b, c ZCaNiCu, d ZCaNiCu800. Marker size 100 nm
D
G
carbon species deposed on the samples, since lower oxi-
dation temperature (T ) may be attributed to amorphous
ox
ZCaNiCu800
carbon, whereas more ordered forms such as nanotubes and
graphitic layers are expected to oxidise at higher temper-
ature. According to this qualitative scale, the coke deposed
ZCaNi Cu2
1
over sample ZCaNiCu, with T \ 450 °C, was predomi-
ox
ZCaNi
nantly amorphous. Tox progressively increased with ZCa-
Ni Cu and ZCaCu Ni and finally a very intense peak
2
1
2
1
with maximum at 589 °C characterised sample ZCaNi-
Cu800, which was abundantly covered by carbon
nanotubes.
ZrO2
2
00
400
600
800 1000 1200 1400 1600 1800 2000
4
Conclusions
-
1
Raman Schift (cm )
The activity of bimetallic Ni–Cu catalysts supported on a
CaO-doped zirconia in ESR was evaluated and compared
to the activity of the corresponding monometallic Ni cat-
alyst. The results showed that big Ni particles and/or Ni
species poorly interacting with the support, which easily
sinter during use, cause a low initial C balance, accompa-
nied by a decrease in ethanol conversion and an increase in
selectivity towards C2 by-products.
Fig. 9 MicroRaman spectra of spent samples
The latter spectroscopy allows to identify two typical
bands ascribable to coke, the D and G contributions. The
latter is usually attributed to ordered graphite layers,
whereas the former is mostly attributed to much disordered
carbon species with respect to graphite layers (amorphous
carbon and nanotubes) and is predominant for sample
ZCaNiCu800.
High activity and stability of the catalyst were reached
by properly tuning the synthesis conditions in order to
obtain the formation of stable and uniform small Ni par-
ticles (i.e. favoring the instauration of a stronger metal
TPO analyses were also carried out on the spent
samples. They may further help elucidating the nature of
123

5
58
V. Nichele et al.
support interaction). The separate and sequential addition
of the metals, as well as the proper calcination temperature,
are compulsory to achieve the best performance.
19. Fierro V, Akdim O, Mirodatos C (2003) Green Chem 5:20
0. Calles JA, Carrero A, Vizca ´ı no AJ (2009) Microporous
Mesoporous Mater 119:200
2
2
1. Tuza PV, Manfro RL, Ribeiro NFP, Souza MMVM (2013)
Renew Energy 50:408
Acknowledgments The authors are indebted to Carlotta Pagan for
the excellent technical assistance. Valentina Nichele acknowledges
MIUR for her PhD scholarship. ‘‘The characterisation was partly
22. Nichele V, Signoretto M, Menegazzo F, Gallo A, Dal Santo V,
Cruciani G, Cerrato G (2012) Appl Catal B 111–112:225
23. Rossetti I, Biffi C, Bianchi CL, Nichele V, Signoretto M, Men-
egazzo F, Finocchio E, Ramis G, Di Michele A (2012) Appl Catal
B 117–118:384
24. Rossetti I, Gallo A, Dal Santo V, Bianchi CL, Nichele V,
Signoretto M, Finocchio E, Ramis G, Di Michele A (2013)
ChemCatChem 5:294
2
supported by H FC European Infrastructure Project (IntegratingEu-
ropean Infrastructure to support science and development ofHydrogen
and Fuel Cell Technologies towards European Strategy for Sustain-
able Competitive and Secure Energy), Project Reference 284522’’.
2
5. Rossetti I, Lasso J, Nichele V, Signoretto M, Finocchio E, Ramis
G, Di Michele A (2014) Appl Catal B 150–151:257
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