Applied Catalysis A: General 238 (2003) 41–54
Cu-Ni-K/␥-Al2 O3 supported catalysts for ethanol steam reforming
Formation of hydrotalcite-type compounds as a result of
metal–support interaction
Fernando Mariño a , Graciela Baronetti a , Matı́as Jobbagy b , Miguel Laborde a,∗
a
Departamento de Ingenierı́a Quı́mica (FI), Universidad de Buenos Aires, Pabellón de Industrias,
Ciudad Universitaria, 1428 Buenos Aires, Argentina
b INQUIMAE, FCEyN, Universidad de Buenos Aires, Buenos Aires, Argentina
Received 29 August 2001; received in revised form 14 February 2002; accepted 19 February 2002
Abstract
The interaction of Cu2+ , Ni2+ and Al3+ ions during the impregnation step of K/␥-Al2 O3 support was studied. Cu-Ni
catalyst precursors (“just impregnated solids”), just reduced precursors (H2 , 300 ◦ C), and calcined precursors in the range of
400–800 ◦ C were characterised by X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) measurements. In addition, the catalytic behaviour of reduced precursors and calcined catalysts was
analysed in the ethanol steam reforming reaction at 300 ◦ C and atmospheric pressure.
XRD results of different precursors indicated that, after the impregnation step, copper is present in two different phases: a
copper basic nitrate and a CuAl and/or CuNiAl hydrotalcite-type compound (HT). In the sample containing only nickel, this
metal is present as a NiAl-HT compound. At constant copper content of 6 wt.%, the ratio between the copper phases depends
on the nickel content. Adding nickel favours the formation of HT compounds. While calcination of Cu-Ni precursors in the
range of 400–800 ◦ C produces a CuO segregated phase and/or a phase of copper called “surface spinel”, nickel is always
found as a nickel aluminate after the calcination treatment. The catalytic behaviour of the samples strongly depends on the
conditions of the thermal treatments. Thus, the increase in the calcination temperature of the precursors produces a strong
interaction between nickel and aluminium, decreasing nickel reducibility and selectivity to C1 compounds. On the other hand,
the just reduced precursors showed to have the best catalytic performance for the ethanol steam reforming reaction at 300 ◦ C.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: CuNiAl hydrotalcite-type compounds; Cu-Ni catalysts; Ethanol steam reforming
1. Introduction
Since conventional methods for hydrogen production are based on gasoline, natural gas, and methanol
steam reforming, ethanol steam reforming—based on
a renewable raw material—could be an attractive pro∗ Corresponding author.
E-mail address: [email protected] (M. Laborde).
cess for Latin American countries having extensive
plantations of sugar cane and corn [1–3]. In addition,
it is easier to reform ethanol than gasoline. Nevertheless, the hydrogen obtained from ethanol and meant to
be used in fuel cells requires a complete gasification
of this alcohol.
Scarce research has been carried out as regards
ethanol steam reforming. At low temperature (300 ◦ C)
and at atmospheric pressure, the reaction network for
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 1 1 3 - 8
42
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
this reaction should consist of the following reactions
[3]:
C2 H5 OH = CO + CH4 + H2
C2 H5 OH = C2 H4 O + H2
C2 H5 OH + H2 O = C2 H4 O2 + 2H2
C2 H5 OH = (C2 H5 )2 O + H2 O
CO + H2 O = CO2 + H2
It should be borne in mind that methane, acetaldehyde,
acetic acid, and diethyl ether are undesirable products
because they compete with H2 for the hydrogen atoms.
Therefore, it is necessary to adjust the formulation of
the catalyst so as to obtain the highest ethanol gasification with the highest H2 production.
In previous studies [1–3] copper-nickel-potassium
supported catalysts on ␥-Al2 O3 for ethanol steam
reforming at low temperatures (300 ◦ C) have been
studied. In this system, metallic copper produces a
fast ethanol dehydrogenation to acetaldehyde; nickel
mainly favours carbon–carbon bond rupture of acetaldehyde in order to produce methane and carbon
monoxide. Potassium is added to neutralise acidic
sites of the support, thus avoiding the dehydration
reactions that lead to products such as ethylene or
diethylether.
In those papers, the effect of copper and nickel
loading on the catalytic behaviour for catalysts prepared by coimpregnation method were studied. It
was concluded that the higher ethanol gasification
was achieved with catalyst with the highest Cu-Ni
contents (6 wt.% of each metal). Both the catalytic
activity and the product distribution were strongly
influenced by the catalyst preparation and the thermal
treatments carried out before the reaction.
It is known that metal–support interaction plays an
important role during the impregnation step and the
thermal treatments, since both of them can define the
properties of final catalysts: reducibility, resistance to
the thermal sintering of the active sites, or metallic
dispersion. de Bokx et al. [4], studying the interaction
of Ni2+ ions with a ␥-Al2 O3 support, reported that
the formation of surface compounds occurs during the
impregnation step, and they suggested the existence
of a Ni-Al mixed hydroxide-type precursor for dried
but uncalcined samples. More recently, Paulhiac and
Clause [5] studied the surface coprecipitation of Co2+ ,
Ni2+ and Zn2+ ions with Al3+ during the impregnation step. Supported by techniques such as X-ray
diffraction (XRD) and EXAFS, they concluded that
the alumina support should not be considered as inert
during the impregnation with divalent metal ions in
aqueous solution, even under mild conditions (neutral
pH and room temperature). They found the formation
of Me2+ /Al3+ hydrotalcite-type (HT) coprecipitated
in the samples just impregnated or after drying at low
temperature.
Using EXAFS and IR experiments, d’Espinose de
la Caillerie et al. [6] demonstrated the existence of coprecipitates having a hydrotalcite-type structure when
alumina was impregnated with nickel nitrate solutions,
even at low nickel contents (2 wt.%) and low contact
times. Moreover, by means of dialysis experiments,
these authors observed hydrotalcite crystallites, and
they suggested that a dissolution–precipitation mechanism could be involved. The presence of adsorbed
Ni2+ ions could promote the alumina dissolution.
On the other hand, there are several studies regarding copper catalysts prepared by ␥-Al2 O3 impregnation [7–9]. Nevertheless, these authors characterised
these samples after a calcination treatment at high temperatures (400–900 ◦ C). Hierl et al. [10] studied the
effect of nickel addition to impregnated copper supported catalysts though, once again, they characterised
the samples after calcination at 800 ◦ C.
It is known that all the divalent metals—from Mg2+
to Mn2+ —are able to form hydrotalcite-like compounds [11]. These compounds, also called layered
double hydroxides (LDHs), are an important type of
lamellar solids, with a general formula:
Men 2+ Mem 3+ (OH− )2(n+m) Am/x · yH2 O
These layered materials consist of positively charged
brucite (Mg(OH)2 )-like sheets where a partial Me2+ /
Me3+ substitution has taken place, and the positive
charge excess has been counterbalanced by anions that
are present in the interlayer, such as CO3 − , NO3 − ,
among others, together with water molecules. The insertion of copper ions into the brucite sheets is unfavoured because of the structure distortion caused by
the unpaired electron of Cu2+ ion (which is known as
Jahn–Teller effect). However, the presence of other divalent cations such as Co2+ , Mg2+ and Ni2+ favours
the hydrotalcite formation [12].
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
Taking into account the aforementioned reports
and our own results, this work is concerned with
the interaction of Cu2+ , Ni2+ and Al3+ ions, during
the impregnation step of K/␥-Al2 O3 support with
aqueous solution of copper and nickel nitrates. For
this purpose, Cu-Ni precursors (“just impregnated
solids”), just reduced precursors, and calcined precursors in the range of 400–800 ◦ C were characterised
by XRD, temperature programmed reduction (TPR)
and X-ray photoelectron spectroscopy (XPS) measurements. In addition, it was analysed the catalytic
behaviour of just reduced precursors and calcined
precursors in the ethanol steam reforming reaction
at 300 ◦ C.
2. Experimental
␥-Al2 O3 spheres from Rhône-Poulenc (diameter:
3 mm, specific area: 200 m2 /g, and pore volume:
0.44 ml/g) were used as support. Before impregnation with Cu-Ni solutions, the ␥-Al2 O3 spheres were
doped with a KOH solution to obtain 0.15 wt.% of
the alkali on the support. Subsequently, this support
was dried overnight at room temperature, and then
calcined at 550 ◦ C for 2 h. The samples were prepared
by coimpregnation of K/␥-Al2 O3 , using an adequate
aqueous solution of copper nitrate (Cu(NO3 )2 ·3H2 O)
and nickel nitrate (Ni(NO3 )2 ·6H2 O) [1]. After impregnation, the samples were dried overnight at room
temperature (solids called precursors).
Different precursors were prepared: precursors with
copper nominal content of 6 wt.% and nickel contents
of 0, 1 and 6 wt.%—which will be hereafter called
P-C6N0, P-C6N1 and P-C6N6, respectively—and a
nickel monometallic precursor (P-C0N6). In addition,
four calcined precursors were prepared, obtained by
calcination of P-C6N6 at 450, 550, 650 and 800 ◦ C
(called C6N6-T, T being the calcination temperature).
Besides, precursors P-C6N0 and P-C6N6, which had
been just reduced in a H2 /N2 stream at the conditions
previous to the steam reforming reaction, were studied. These samples were called R-C6N0 and R-C6N6,
respectively.
Copper and nickel contents were determined by
atomic absorption analysis in a Varian-Techtron equipment Model AA-5, prior to alumina dissolution as reported elsewhere [1]. Precursors, just reduced precur-
43
sors, and calcined precursors were characterised by
XRD, TPR, and XPS.
XRD patterns were recorded for 2θ values, ranging
from 5 to 75◦ , with a Siemens diffractometer Model
D5000 using Cu K␣ radiation, Ni filter and 40 kV. Additional spectra at higher angles (56–70◦ ) were performed using a step size = 0.02 and a step time =
10 s in order to measure the characteristic doublet
(d(1 1 0)–d(1 1 3)) of LDH compounds.
The reducibility of different species was studied by
TPR. It was performed at atmospheric pressure using a
300 mg of a sample previously ground (125–177 ␮m)
which was placed in the reactor and heating it under
a reducing gas mixture (5% H2 in N2 ) from 50 to
800 ◦ C at a programmed rate of 15 ◦ C/min. The detection was carried out with a thermal conductivity cell
and a molecular sieve was used as water trap. Further
details are provided elsewhere [1].
XPS spectra were obtained in a Shimadzu
ESCA 750 Electron Spectrometer, using an Al K␣
(1486.6 eV) as radiation source. Surface charging was
observed for all the samples, and binding energies of
Cu 2p, Ni 2p, and Al 2p were referred to C 1s line at
284.6 eV.
Ethanol steam reforming was used as the test reaction. Catalytic activity was measured in terms of
ethanol conversion, defined as:
x=
OUT
IN
Fethanol
− Fethanol
IN
Fethanol
Product distribution was evaluated through their selectivity. Selectivity of specie i was defined as:
Si =
FiOUT
OUT
IN
Fethanol
− Fethanol
where F is the molar flow.
The reaction was carried out in a glass fixed bed
reactor at 300 ◦ C and 1 atm. Prior to the measurement,
samples were reduced in situ by a H2 /N2 stream at
300 ◦ C as reported elsewhere [1,2].
Two sets of experiments were conducted. On the one
hand, the tests performed to compare the behaviour of
just reduced precursor R-C6N6 with that of calcined
precursor (C6N6-T) at several temperatures were carried out using catalyst particles (diameter = 3 mm) in
a 2.5 g fixed bed. On the other hand, the experiments
conducted to compare monometallic and bimetallic
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F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
just reduced precursors were performed using ground
catalyst particles (diameter = 350 ␮m) in a 0.25 g
fixed bed.
In both cases, a liquid water–ethanol mixture (molar
ratio = 2.5:1) was fed into the reactor at 0.12 ml/min
liquid flow rate. This stream was vaporised on a bed
of glass spheres before entering the catalytic bed. The
reactor effluent was partially condensed, and both liquid and gaseous products were analysed by gas chromatography. Further details can be found elsewhere
[1,2].
3. Results and discussion
3.1. Precursors and just reduced precursors
3.1.1. Chemical composition
Elemental composition of the samples used in this
work is presented in Table 1. Metallic concentrations
of the impregnation solutions were chosen to obtain
the elemental composition referred to as “nominal
content” in the same table. As it can be seen from
Table 1, the achieved compositions are in good agreement with the desired ones.
3.1.2. XRD measurements
Fig. 1 shows the XRD patterns of the different
precursors, namely P-C6N0, P-C6N1, P-C6N6, and
P-C0N6. The patterns of all the precursors show signals of ␥-Al2 O3 support at 2θ = 37.06, 45.86, and
67.024. In the case of the samples containing copper, the precursor patterns clearly show the characteristic lines of a basic nitrate phase (Cu2 (OH)3 NO3 ,
gerhardite), the main reflection being at 2θ = 12.804
(the reflections of gerhardite are indicated with symbol (䊏) in Fig. 1).
Table 1
Chemical composition of sample tested
Sample
C6N0
C6N1
C6N6
C0N6
Nominal content (wt.%)
Actual content (wt.%)
Cu
Ni
Cu
Ni
6
6
6
0
0
1
6
6
6.04
6.72
6.59
0
0
1.16
5.96
6.07
As mentioned, the preparation of this type of catalysts involves interaction of Cu2+ and Ni2+ ions,
which were originally present in the metal solutions with alumina support during the impregnation
step. Thus, this interaction can produce the formation of coprecipitates with a HT structure [6]. The
insertion of Cu2+ ions into the brucite-like layers
is unfavoured due to the Jahn–Teller effect of Cu2+
ions; in addition, depending on the Cu2+ /Me2+ ratio,
the CuAl-HT is mixed with other pure copper phases
as malachite or gerhardite [12]. Cavani et al. [11]
reported that when the Cu2+ /Me2+ ratio is higher
than 1, Cu2+ ions can be situated in a near lying
octahedral, and the formation of a segregated copper
compound is energetically preferred to that of a HT
compound. In contrast, when the Cu2+ /Me2+ ratio
is lower or equal to 1, a HT compound formation
is favoured [11]. However, Rives and Kannan [13]
have recently reported the synthesis of a pure CuNiAl
hydrotalcite-type compound, for Cu2+ /Ni2+ ratio up
to 4.
As regards the Cu-Ni precursors used in this study,
Fig. 1 shows that the intensity of the characteristic
gerhardite lines decreases as the nickel content in the
samples increases (the intensity of ␥-Al2 O3 lines being
used as reference).
There is a strong similarity between the basic salt
and the hydrotalcite-type structure [6]. It is rather
difficult to identify the characteristic reflections of
(0 0 3), (0 0 6) and (0 0 9) LDH planes by XRD if
a well-crystallised gerhardite is present, particularly,
considering that the hydrotalcite-like phase could be
poorly crystallised for supported samples [6]. Consequently, XRD patterns with more precision in the zone
where the gerhardite does not present characteristic reflections were carried out. Fig. 2a shows the zone close
to 2θ = 60–62◦ for P-C6N0, P-C0N6 and ␥-Al2 O3 . In
that range of 2θ angles, the LDHs have the typical doublet of d(1 1 0)–d(1 1 3) planes. This doublet is clearly
observed in the sample containing copper (P-C6N0) at
2θ = 60.9 and 62.6 for the d(1 1 0)–d(1 1 3) planes, respectively (the alumina line at 67.024 is used as reference in this spectrum). These peaks are slightly shifted
to lower distance according to those reported in the
International Diffraction Data, file 37-0630. This fact
could be attributed to a lattice contraction since the
samples used in this work are richer in aluminium. A
shift to lower interatomic distances around 0.04 Å was
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
45
Fig. 1. XRD patterns of the different precursors.
also observed in a family of NiAl LDH compounds
having variable Ni/Al ratio [14].
The typical LDH doublet in 2θ close to 60◦ was
also observed in the Cu-Ni precursor (Fig. 2b). This
fact evidences that there is a LDH phase containing copper, nickel and aluminium. In Cu-Ni precursors, the formation of LDH compounds is favoured by
the presence of Ni2+ ions in the impregnating solution, which can promote the breaking of Al–O bonds
[6]. For the Cu-Ni precursors used in this work, the
gerhardite phase decreases as the nickel content increases (see Fig. 1), which suggests a higher LDH
formation for the precursor with the highest nickel
content.
In contrast, as revealed in Fig. 1, peaks corresponding to nickel segregated phases were not detected for
the sample containing only nickel (P-C0N6). It is
known that Ni2+ ions strongly interact with alumina
support, leading to NiAl-HT [4,6]. In Fig. 2a, the
presence of a shoulder in the doublet LDH zone could
indicate the existence of a poorly crystallised NiAl
hydrotalcite in sample P-C0N6. In fact, d’Espinose
de la Caillerie et al. [6] reported that, for just impregnated Ni/␥-Al2 O3 , it is necessary to have a nickel content higher than 10 wt.% to detect a well-crystallised
NiAl-HT by XRD.
According to these results, in P-C6N0, P-C6N1 and
P-C6N6, copper is present in two different phases: a
copper basic nitrate and a hydrotalcite-type compound
of CuAl and/or CuNiAl. For the sample without copper, P-C0N6, nickel would be present as NiAl-HT
compound.
On the other hand, the sample called R-C6N6
(P-C6N6 just reduced precursor) was also characterised by XRD. The XRD pattern of this sample
(Fig. 3) clearly shows the diffraction lines corresponding to a metallic copper phase, though no signals of
metallic nickel were detected. Even if Fig. 3 does
not show signals corresponding to any nickel-reduced
phase, our catalytic results (discussed in next section)
proved that a fraction of the nickel was reduced. Ross
et al. [15] reported that the reduction of the surface
nickel aluminate species in impregnated Ni/Al2 O3
leads to isolated (monodispersed) nickel atoms closely
associated with the alumina structure. More recently,
it has been proposed [16] for NiAl hydrotalcite a
“decorated” three-phase model in which Al-doped
NiO, Ni-doped alumina and an aluminate-type phase
would coexist. However, regardless the existing
nickel species before reduction, they are less reducible as the calcination temperature increases
[3,17].
46
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
Fig. 2. XRD patterns of: (a) monometallic precursors and ␥-Al2 O3 support; (b) bimetallic precursor (P-C6N6); both of them in the zone
of 2θ angles corresponding to doublet of d(1 1 0)–d(1 1 3) LDH planes.
3.1.3. TPR measurements
It must be considered that the reduction of these
uncalcined samples implies a decomposition of the
different structures while the layer cations are being
reduced; however, this technique has been applied
to the aforementioned compounds [17]. As we have
already discussed, the gerhardite phase and a HT
compound coexist in these precursors.
TPR profiles of P-C6N0, P-C6N6 and P-C0N6 are
shown in Fig. 4. The profiles of samples containing
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
47
Fig. 3. XRD pattern of just reduced precursor, R-C6N6.
copper show a sharp peak, the maximum being at
272 ◦ C. Several authors working with the HT system
[12,17] have monitored the evolved gases during the
TPR experiments by a quadruple mass spectrometer.
They have reported that in TPR conditions the compensating anions (in our case NO3 − ) could be reduced
together with the Cu and/or Ni cations.
On the one hand, for P-C6N0 and P-C6N6, the peak
at 272 ◦ C can be attributed to the reduction of Cu2+ to
Cu0 together with NO3 − . In fact, the XRD patterns of
P-C6N6 sample clearly show the presence of metallic
copper after the reduction treatment at 300 ◦ C (Fig. 3).
These results are in agreement with those reported by
Rives and Kannan [13], who found that, for CuNiAl
hydrotalcite-type compounds, the copper reduction is
complete at 300 ◦ C, irrespective of nickel content.
On the other hand, the precursor that contains only
nickel (P-C0N6) shows a TPR profile with two zones
that are shifted to higher temperatures. According to
the results reported by Tichit et al. [17], the first maximum (at 308 ◦ C) corresponds to the NO3 − reduction, whereas the broad zone could correspond to the
nickel reduction. These authors also reported that the
maximum of the broad zone is shifted to higher temperatures as the calcination temperature increases. In
fact, in a previous work [1], we have reported that
the nickel reduction for calcined Ni/Al2 O3 catalysts
occurs at 800 ◦ C, a higher temperature than that observed for the uncalcined sample. In addition, for the
bimetallic precursor (P-C6N6), this broad zone could
also include the reduction of Ni2+ species. Therefore,
it is necessary to reduce the precursors at temperatures
higher than 450 ◦ C in order to achieve a complete reduction of Ni2+ species.
3.1.4. XPS measurements
XPS surface analysis of just reduced precursors
R-C6N0 and R-C6N6 are shown in Table 2. It must
be noted that, while nickel addition might favour the
appearance of a HT copper phase, this fact does not
modify the surface copper distribution. Besides, it can
be observed that the Cu/Al surface ratio is higher than
the Ni/Al one for similar contents of both metals.
Table 2
Cu/Al and Ni/Al surface ratios for precursors R-C6N0 and R-C6N6
Cu/Al surface molar ratio (XPS)
Cu/Al actual molar ratio (calculated)
Ni/Al surface molar ratio (XPS)
Ni/Al actual molar ratio (calculated)
R-C6N0
R-C6N6
0.059
0.052
0.059
0.063
0.047
0.061
48
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
Fig. 4. TPR profiles for the different precursors.
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
49
Fig. 5. Average ethanol conversion achieved in the steam reforming reaction using just reduced precursors. Ethanol conversion at t = 90 min
for sample R-C0N6.
3.1.5. Activity and selectivity measurements
The catalytic behaviour of just reduced precursors
R-C6N0, R-C6N6 and R-C0N6 were studied. Fig. 5
shows the average ethanol conversion achieved with
the different tested samples. It can be observed that the
activity is clearly increased by the nickel addition, considering the samples R-C6N0 in relation to R-C6N6.
Nevertheless, when a precursor containing only nickel
(R-C0N6) was tested, a low ethanol conversion was
achieved. Besides, this last sample showed a marked
deactivation unlike the rest of the samples.
Fig. 6 clarifies the effect of nickel on product distribution for the steam reforming reaction. The presence
of nickel in sample R-C6N6 strongly increases ethanol
gasification given the existence of significant amounts
of CH4 and CO together with a 50% increase in hydrogen production compared to the R-C6N0 sample.
On the other hand, the just reduced precursor without copper (R-C0N6) hardly produces gaseous compounds. Furthermore, Table 3 indicates that nickel
addition to the precursor containing 6 wt.% of Cu
causes a diminution in C2 compounds selectivity (acetaldehyde and acetic acid) and the appearance of C1
compounds (methane and carbon monoxide).
Another important issue related to the catalytic
activity of just reduced precursors is their possi-
ble deactivation during the reaction. Thus, sample
R-C0N6 exhibits a different performance with respect to copper-containing samples. As it can be
observed in Fig. 7, just reduced precursors R-C6N0
and R-C6N6 show an acceptable stability, similar to
that of calcined precursors, which will be presented in
the next section. On the contrary, R-C0N6 completely
deactivates after 2 h of operation.
3.2. Calcined precursors
3.2.1. XRD measurements
The effect of calcination temperature for the
C6N6-T sample was analysed in the range of
450–800 ◦ C. XRD patterns are shown in Fig. 8. While
it can be observed that the segregated CuO phase is
Table 3
Effect of nickel on product distribution of the steam reforming
reaction
Sample
R-C6N0
R-C6N6
Selectivity
H2
CH4
CO
CH3 CHO
CH3 COOH
0.86
1.07
0.00
0.53
0.00
0.53
0.67
0.35
0.04
0.03
50
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
Fig. 6. Average gasification and product distribution of the steam reforming reaction using just reduced precursors. Data at t = 90 min for
sample R-C0N6.
present for all temperatures, no characteristic peaks
corresponding to the NiO segregated phase were detected for any temperature. However, it must be noted
that the area of the central wide reflection correspond-
ing to the aluminate zone increases as the temperature
increases. It was verified that the ␥-Al2 O3 support
calcined at the same temperatures does not change its
XRD pattern [1].
Fig. 7. Catalytic stability of just reduced precursors.
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
51
Fig. 8. XRD patterns of C6N6-T sample as a function of calcination temperature.
In order to explain these results, it must be considered that the thermal decomposition of the copper basic nitrate phase produces a simple oxide phase with
a poor dispersion [12]. In contrast, depending on the
temperature of the thermal treatments, the decomposition of HT compounds leads to a complex family of
well-dispersed phases, such as simple oxides, mixed
oxides, and/or “non-stoichiometric spinel” structures
[11,17].
After calcination of CuNiAl hydrotalcite-type compounds Rives and Kannan [13] detected, by XRD, the
presence of divalent metal oxide at low temperatures
(400–500 ◦ C); in turn, at higher temperatures (850 ◦ C),
they found CuO, NiO and/or NiAl2 O4 spinel in relative amounts, depending on the relative contents of
the different metal cations.
As mentioned, the NiO characteristic peak could
not be observed in any of the samples analysed in this
work. In agreement with this, de Bokx et al. [4] reported that the calcination of Ni-Al mixed hydroxidetype precursors—formed during the impregnation step
in similar catalysts—leads to a poorly crystallised, defective nickel aluminate. These authors only detected,
by XRD, a segregated nickel phase for catalysts with
Ni content higher than 15 wt.%. However, for temper-
atures higher than 600 ◦ C the Ni–Al interaction increased and led to a NiAl2 O4 phase.
The high nickel content in sample C6N6 favours
CuNiAl hydrotalcite-type compounds formation. The
subsequent calcination increases Ni–support interaction as it can be inferred from the growth of the central
wide reflection (aluminate zone) in the XRD patterns
of samples calcined at 650 and 800 ◦ C (Fig. 8).
In a previous work [2] we have reported that CuO
signals disappear at temperatures higher than 625 ◦ C
for ␥-Al2 O3 supported catalysts with low nickel content (Fig. 9). In the present work, calcination of the
P-C6N6 sample at temperatures as high as 800 ◦ C
leads to catalysts where CuO lines are still detected by
XRD (Fig. 8). Thus, the strong Ni–support interaction
mentioned could be responsible for copper segregation as CuO, even at calcination temperatures as high
as 800 ◦ C.
3.2.2. XPS
Cu/Al and Ni/Al surface ratios for C6N6-550 and
C6N6-800 were obtained by XPS, and they are presented in Table 4. The amount of copper ions on the
surface does not decrease as the calcination temperature increases. Actually, there is an increase in the
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F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
Fig. 9. XRD patterns, as a function of calcination temperature, of a sample with the following nominal contents: 6 wt.% of Cu and 4 wt.%
of Ni (taken from [2]).
(Cu/Al) ratio with the calcination temperature that
cannot be ignored, even if the experimental errors
associated with the XPS measurements are taken
into account. Previous experiments, using a 6 wt.%
Cu-4 wt.% Ni sample, have already shown this tendency [2]. Lo Jacono et al. [7] and Alejandre et al.
[9] have reported the presence of surface spinels in
systems which are similar to the catalysts presented in
this work; high copper contents and high calcination
temperatures favour the formation of these spinels.
In addition, Hierl et al. [10], working with Cu/␥-Al2 O3
catalysts calcined at temperatures higher than 700 ◦ C,
reported that Ni2+ ions push Cu2+ ions to the surface as Ni2+ ions prefer to occupy subsurface sites.
Paulhiac and Clause [5] reported that the strong inTable 4
Cu/Al and Ni/Al surface ratios for samples C6N6-550 and
C6N6-800
Cu/Al surface molar ratio (XPS)
Cu/Al actual molar ratio (calculated)
Ni/Al surface molar ratio (XPS)
Ni/Al actual molar ratio (calculated)
C6N6-550
C6N6-800
0.063
0.063
0.036
0.061
0.076
0.035
teraction between nickel and ␥-Al2 O3 starts during
the impregnation step. For this reason, once precursors are calcined, a further increment of calcination
temperature does not modify the surface Ni/Al ratio.
3.2.3. Catalytic behaviour
As mentioned, calcination temperature has an important effect on the different phases present in the
calcined precursors. The activity, selectivity and stability of these final catalysts will be analysed in this
section. Table 5 shows the effect of calcination temperature on catalytic activity of C6N6-T samples. As
it can be seen, the average ethanol conversion slightly
decreases as the calcination temperature of the tested
sample increases. The catalytic test using just reduced
precursor R-C6N6 (in the same operation conditions)
confirms this tendency.
All the tested samples, including R-C6N6, presented an acceptable stability during the steam reforming reaction as it is revealed by the conversion
values registered after 90 and 240 min of operation.
Table 6 presents the product distribution obtained
with the different samples in terms of selectivity.
Selectivity of C1 compounds is clearly unfavoured,
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
53
Table 5
Ethanol conversion achieved with just reduced precursor and calcined precursor containing 6 wt.% of Cu and 6 wt.% of Ni
At t = 90 min
At t = 240 min
Average
R-C6N6
C6N6-450
C6N6-550
C6N6-650
C6N6-800
0.96
0.94
0.95
0.87
0.88
0.87
0.82
0.81
0.82
0.74
0.76
0.74
0.64
0.64
0.64
Table 6
Selectivity (defined as produced moles per each ethanol converted mole) obtained with just reduced precursor and calcined precursor
containing 6 wt.% of Cu and 6 wt.% of Ni
H2
CH4
CO
CH3 CHO
CH3 COOH
R-C6N6
C6N6-450
C6N6-550
C6N6-650
C6N6-800
1.33
0.97
0.88
0.11
0.03
1.35
0.81
0.80
0.29
0.06
1.36
0.53
0.54
0.50
0.09
1.27
0.25
0.26
0.62
0.11
1.31
0.16
0.16
0.74
0.11
whereas selectivity in C2 compounds is enhanced in
experiments with samples calcined at higher temperatures. Hydrogen selectivity is practically insensitive
to the catalyst used. Again, the results obtained for
the R-C6N6 sample confirm the tendency.
4. Analysis of the results
Results obtained by XRD analysis of the precursors have shown that two copper phases coexist: a
pure phase (gerhardite) and a phase born from the
metal–support interaction (CuAl-HT, or CuNiAl-HT
compound when nickel is present).
The ratio between these two copper phases depends
on the nickel content of the sample. In a previous
work, we have proven that this ratio also depends on
the copper content [2]. On the other hand, nickel easily
interacts with ␥-Al2 O3 to produce a HT compound
(including copper, provided it is present).
As reported in the literature [12], thermal decomposition of gerhardite leads to a poorly dispersed
CuO segregated phase. Decomposition of Cu-HT
compounds also originates a CuO phase and another
phase, referred to as “surface spinel” by other authors
[7]; such phase comprises a Cu2+ ions dispersion
on the ␥-Al2 O3 surface. Nickel addition particularly
favours the HT compounds formation, with Cu2+ ,
Ni2+ and Al3+ ions in the cationic layers.
The subsequent calcination enhances Ni–support
interaction and favours copper segregation as CuO.
Hence, XRD patterns of both the C6N6-650 and
the C6N6-800 samples show an increment of the
wide reflection assigned to the aluminates and they
keep the signals of a CuO segregated phase even
at 800 ◦ C (Fig. 8). Nevertheless, CuO signals disappear for catalysts having low nickel content and
calcined at a temperature higher than 625 ◦ C (Fig. 9).
In summary, high nickel contents produce more HT
compounds and favour the segregation of CuO after
calcination.
To elucidate the catalytic behaviour of the samples,
it must be considered that the activity of copper and
nickel depends on both their surface presence and their
reducibility under the operating conditions. Regardless
the copper phase obtained, the (Cu/Al) surface ratio
was neither influenced by the nickel content nor by
the calcination temperature. This fact would explain
why the catalytic activity of the samples is not strongly
affected by the thermal treatment.
In calcined precursors, the selectivity for C1 compounds decreases as the calcination temperature increases. This fact may be the evidence that Ni–support
interaction becomes stronger as calcination temperature increases, decreasing the nickel reducibility.
Considering that metallic nickel is able to slightly
catalyse the ethanol dehydrogenation to acetaldehyde, the decrease in reducibility of this metal as
54
F. Mariño et al. / Applied Catalysis A: General 238 (2003) 41–54
temperature increases could explain the slight reduction in the catalytic activity.
Taking into account the stability of the just reduced
precursor R-C6N6, and its catalytic results of ethanol
conversion and product distribution, this sample shows
the best performance among the samples studied in
this work.
5. Summary
The CuNiAl hydrotalcite-type compounds formation occurs during the impregnation step of Cu-Ni-K/
␥-Al2 O3 catalysts with aqueous solutions of metallic
nitrates. Nickel addition favours the formation of these
compounds.
The increase in the calcination temperature of the
precursors produces a strong interaction between
nickel and aluminium, decreasing nickel reducibility
and the selectivity to C1 compounds.
Calcination of Cu-Ni precursors in the range of
400–800 ◦ C produces a CuO segregated phase and/or
a phase of copper called “surface spinel”.
The just reduced bimetallic precursors show the best
catalytic performance and an acceptable stability, similar to that of calcined precursors, for the ethanol steam
reforming reaction at 300 ◦ C.
Acknowledgements
The authors are grateful to Mr. Roberto Tejeda,
Miss Verónica Mas and Mr. Marcelo Boveri for their
technical assistance. In addition, the authors thank the
Universidad de Buenos Aires for the financial support
(TI-20 project), and the JICA for the donation of the
ESCA spectrometer.
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