Bimetallic Ru/Ni supported catalysts for the gas phase hydrogenation of acetonitrile

Article abstract of DOI:10.1016/j.apcata.2010.03.061

A family of bimetallic Ni-Ru catalysts supported on a mesoporous SBA-15 silica was prepared by conventional impregnation method, with constant metal molar loadings, but varying Ni/(Ni + Ru) atomic ratios. The corresponding Ni and Ru monometallic catalysts were also prepared for comparison. These catalysts were characterized by XRD, N₂ adsorption-desorption at -196 °C, TEM, XPS, H₂-TPR, chemisorption of H₂ at r.t., H₂-TPD and NH₃-TPD techniques. Finally, they were also tested in the hydrogenation of acetonitrile reaction, in the gas phase and at atmospheric pressure. Acetonitrile conversion values depended on the Ni/(Ni + Ru) composition of the bimetallic catalysts. Ru-rich bimetallic catalysts exhibited acetonitrile conversion values higher than that of pure Ni one; thus, although selectivity patterns remained almost unchanged, primary amine yields were increased. These higher conversion values resulted as a consequence of enhanced specific activity of Ni⁰ atoms, attributable to a strong interaction between both metals, Ni and Ru, likely because NiRu alloy nanoparticles were formed.

Full text of DOI:10.1016/j.apcata.2010.03.061

Applied Catalysis A: General 381 (2010) 132–144
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Bimetallic Ru/Ni supported catalysts for the gas phase hydrogenation
of acetonitrile
P. Braos-García, C. García-Sancho, A. Infantes-Molina, E. Rodríguez-Castellón, A. Jiménez-López∗
Departamento de Química Inorgánica, Cristalografía y Mineralogía (Unidad Asociada al ICP-CSIC), Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos,
29071 Málaga, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A family of bimetallic Ni-Ru catalysts supported on a mesoporous SBA-15 silica was prepared by conven-
tional impregnation method, with constant metal molar loadings, but varying Ni/(Ni + Ru) atomic ratios.
The corresponding Ni and Ru monometallic catalysts were also prepared for comparison. These catalysts
were characterized by XRD, N2 adsorption–desorption at −196 ◦C, TEM, XPS, H2-TPR, chemisorption of
H2 at r.t., H2-TPD and NH3-TPD techniques. Finally, they were also tested in the hydrogenation of ace-
tonitrile reaction, in the gas phase and at atmospheric pressure. Acetonitrile conversion values depended
on the Ni/(Ni + Ru) composition of the bimetallic catalysts. Ru-rich bimetallic catalysts exhibited ace-
tonitrile conversion values higher than that of pure Ni one; thus, although selectivity patterns remained
almost unchanged, primary amine yields were increased. These higher conversion values resulted as a
consequence of enhanced specific activity of Ni0 atoms, attributable to a strong interaction between both
metals, Ni and Ru, likely because NiRu alloy nanoparticles were formed.
Received 21 December 2009
Received in revised form 16 March 2010
Accepted 30 March 2010
Available online 4 April 2010
Keywords:
Gas phase acetonitrile hydrogenation
Supported nickel-ruthenium bimetallic
catalysts
Mesoporous silica
SBA-15
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
determined by the propensity of the metals to form multiple bonds:
after deuteration of acetonitrile experiments they have proposed
Lower alkylamines ranging from C2 to C5 are used as solvents,
agrochemicals, surfactants, water treatment chemicals, and phar-
maceuticals [1]. One important industrial route to amines is based
in the hydrogenation of nitriles. The reaction selectivity is of great
importance, particularly for the production of primary amines,
often the most industrially desired products. Due to the high reac-
tivity of partially hydrogenated reaction intermediates, i.e., imines
or Shiff bases, a conventional hydrogenation process leads to a mix-
ture that also contains secondary and tertiary amines [2]. The metal
employed as catalyst is the most important factor in determining
the selectivity for the hydrogenation of nitriles. For the production
of primary amines, the nickel-based catalysts are commonly used
[3–5]. In the case of nickel supported catalysts, also the support
nature has been proved to exert a strong influence on the selectivity
[6–9].
that intermolecular hydrogen transfer from the ␣-methylene group
of one adsorbed nitrile to the strong metal N bond of another
adsorbed species is responsible for amine formation [11,12]. On
this basis, they propose that, among the transition metal catalysts,
Ru displays the highest selectivity to primary amine, while over Pd
and Pt, secondary and tertiary amines are preferentially formed;
the selectivity of Ni and Rh is between these extremes [5].
In any case, the adsorbed nitriles and their intermediates are
active species that react with each other and controlling their inter-
actions is key to govern the overall selectivity of the reaction. In
this sense, a number of studies have dealt with the performance of
bimetallic catalysts [13–15], since the range of properties of metal-
lic systems can be greatly extended by taking mixtures of elements
to generate compounds and alloys. Our group has been working
with nickel catalysts in the hydrogenation of acetonitrile in the gas
phase, finding highly active catalysts with high selectivity towards
ethylamine when the metal is supported on a silica carrier [9]. Thus,
given that Ru catalysts have been proposed to be even more selec-
tive towards the primary amine than Ni ones, although much less
active, the aim of the present work is to test the performance of
bimetallic Ni-Ru catalysts supported on high surface mesoporous
silica. In this sense, nanoalloys can be generated in a variety of
media, such as immobilized on surfaces, or inside pores. One of
the major reasons for interest in alloy nanoparticles is the fact that
their chemical and physical properties may be tuned by varying the
The knowledge of the mechanism of the hydrogenation reaction
is a key point when attempting to rationalize the formation of unde-
sired products, in order to eliminate them, or, at least, reduce them.
Dallons et al. proposed that all reactions leading to amines occur
exclusively on the catalyst’s surface [10] and Huang and Sachtler
have shown that the selectivity to a particular amine is dominantly
∗
Corresponding author. Tel.: +34 952131876; fax: +34 952137534.
E-mail address: ajimenezl@uma.es (A. Jiménez-López).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.03.061

P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
133
composition and atomic ordering as well as the size of clusters. In
fact, nanoalloys may display magic compositions, i.e., compositions
at which the alloy nanoclusters present a special stability. They may
also display properties which are distinct from the corresponding
bulk alloys due to finite size effects, e.g., there are examples of pairs
of elements which are immiscible in the bulk but readily mix in
finite clusters, such as iron and silver [16]. In the case of Ni and Ru
system, although it has a positive heat of formation of +1 kJ/mol
and is essentially equilibrium immiscible (in fact, in its equilibrium
phase diagram, there is no any alloy phase over the whole compo-
sition range) [17], many authors have described Ni-Ru bimetallic
clusters or alloys [18,19]. Finally, it is well known that acidic oxide
supports originate strong metal–support interactions leading to
metal segregation on bimetallic catalysts [20,21]. To avoid such
effects, as well as secondary reactions leading to higher amines
different from ethylamine, which are favoured by the surface acid-
ity of supports, we have selected a high surface area, but very low
acidity mesoporous silica. Thus, our group has recently published
the synthesis and characterization of a well ordered mesoporous
silica, of the SBA-15 type, with such desired characteristics [22]. It
was therefore the aim of this study to investigate the influence of
Ru atoms on the catalytic performance of Ni supported on a silica
carrier, in the hydrogenation of acetonitrile in the gas phase, by pro-
gressively substituting Ni atoms by Ru ones, that is, by preparing
Ni-Ru bimetallic catalysts.
The catalysts were obtained by treating the precursors with an
He flow (60 ml min−1) at 100 ◦C, for 30 min, and lately cooled down
to room temperature; to be subsequently reduced in flowing H2
(60 ml min−1) from room temperature to and kept at 400 ◦C for
30 min (10 ◦C min−1 of heating rate).
The catalysts will be referred to as Ni and Ru, for monometal-
lic nickel and ruthenium, respectively, and NiRux, for bimetallic
ones, where x is the Ni/(Ni + Ru) molar ratio. The corresponding
compositions are compiled in Table 1.
2.2. Characterization of catalysts
X-ray photoelectron spectra were collected using
a
Physical Electronics PHI 5700 spectrometer with either non-
monochromatic Mg K␣ (1253.6 eV) or Al K␣ radiation (1486.6 eV)
and with a multi-channel detector Electronics 80-365B. Core level
spectra of samples were recorded in the constant pass energy
mode at 29.35 eV, using a 720 ␮m diameter analysis area. All
the binding energies (B.E.) were referenced to the C 1s peak of
adventitious carbon at 284.8 eV, with an uncertainty of 0.1 eV.
A PHI ACCESS ESCA-V6.0 F software package was used for data
acquisition and analysis. A Shirley-type background was sub-
tracted from the signals. Recorded spectra were always fitted using
Gaussian–Lorentzian functions in order to determine the binding
energy of the different element core levels more accurately.
TEM micrographs of the catalysts were obtained with a Philips
CM 200 Supertwin-DX4 high resolution transmission electron
microscope. Once the samples were reduced, they were kept in
cyclohexane. For sample preparation, a drop of these suspensions
was dispersed on a Cu grid.
2. Experimental
2.1. Materials
Powder X-ray diffraction patterns were collected by using a
Phillips Xpert PRO MPD diffractometer, using Cu K␣1 radiation, pro-
vided with a Ge(1 1 1) monochromator.
Nitrogen physisorption measurements were performed at
−196 ◦C with a Micromeritics ASAP 2020 apparatus. Prior to analysis,
the samples were outgassed at 200 ◦C and 10−2 Pa overnight. Sur-
face areas were determined by using the Brunauer–Emmet–Teller
equation [24] and a nitrogen molecule cross-section of 13.5 Å2
[25,26].
Hydrogen temperature-programmed reduction (H2-TPR)
experiments were carried out to 0.080 g of catalyst precursor,
previously treated with a He flow (35 ml min−1) at 100 ◦C for
30 min. After cooling to room temperature, the H2 consumption
was studied between this temperature and 800 ◦C, by using an
Ar/H2 flow (48 ml min−1, 10 vol% of H2) and under a heating
rate of 10 ◦C min−1. Water and/or hydrogen chloride formed in
the reduction reaction were removed by passing the outcoming
flow through a solid CaO trap and a cold finger (−80 ◦C). The H2
consumption was measured with an on-line gas chromatograph
(Shimadzu GC-14A) provided with a TCD.
The mesoporous silica SBA-15 material was prepared by fol-
lowing the method reported by Gómez-Cazalilla et al. [22]. The
resulting solid after calcination at 550 ◦C for 6 h (10 ◦C min−1 heat-
ing rate), was pelletized so that a sieve fraction 0.3–0.4 mm was
used afterwards.
A series of bimetallic supported Ni-Ru catalysts was pre-
pared using the incipient wetness impregnation method of
pelletized support. Aqueous solutions of Ni and Ru, of required
concentrations, were employed in such a way that a constant
loading of metallic moles per gram of solid, 500 ␮mol M g−1
(where M = Ni + Ru), resulted with varying Ni/(Ni + Ru) molar ratios.
Towards this end, the support was impregnated with an aque-
ous solution of Ni(II) citrate [23], air dried overnight at 60 ◦C, and
calcined to 450 ◦C for 2 h (1 ◦C min−1 heating rate), in order to elim-
inate the organic ligands. Afterwards, ruthenium was added by
a subsequent impregnation process with an aqueous solution of
RuCl3·nH2O (∼41 wt% Ru, from Fluka). The resulting solid was air
dried at 60 ◦C for 1 day, giving rise to the corresponding catalyst
precursor. Monometallic Ni and Ru precursors were also prepared
in analogous manner, with 3 wt% of Ni (500 ␮mol g−1), and 1.7 wt%
of Ru (167 ␮mol g−1), respectively.
Temperature-programmed desorption of H2 (H2-TPD) was car-
ried out placing 0.300 g of catalyst precursor in U tube reactor
Table 1
Composition, textural and acidic properties (from NH3-TPD experiments) of Ni, Ru, and NiRux bimetallic catalysts., as well as those corresponding to the support.
a
Catalyst
Ni (wt%)
Ru (wt%)
␮mol Ni g−1
␮mol Ru g−1
SBET (m2 g−1
)
Vp (cm3 g−1
)
dp (nm)b
␮mol NH3 g−1
SBA-15 (pelletized)
Ni
310
257
309
296
302
270
327
307
0.34
0.28
0.34
0.32
0.32
0.29
0.34
0.33
4.3
4.4
4.4
4.3
4.2
4.3
4.2
4.3
99
366
275
262
738
575
883
504
3.0
2.6
2.3
1.9
1.4
0.9
0
0
508
446
399
327
242
159
0
0
55
99
163
241
318
165
NiRu0.89
NiRu0.80
NiRu0.67
NiRu0.50
NiRu0.33
Ru
0.6
1.0
1.7
2.4
3.2
1.7
a
Vp mesoporous volume estimated from the N2 adsorption isotherm at P/P0 = 0.95.
By 4 Vp/A method, where A is the BET surface area.
b

134
P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
where it was firstly reduced in situ. The obtained catalyst was
purged with a He flow (50 ml min−1) at the reduction temperature
for 15 min, cooled down to 135 ◦C (chemisorption temperature),
and then put in contact with flowing H2 (30 ml min−1) for 1 h, and
lately cooled to r.t. under the same H2 flow. After cleaning with Ar
3. Results and discussion
3.1. Support
The characteristics of the SBA-15 support employed have been
described elsewhere [22]. It is a silica formed by hexagonal array
of mesopores of ca. 40 Å in diameter, with pore walls thickness
of ∼53 Å. These parameters have been further confirmed by XRD
patterns and TEM images of reduced NiRux bimetallic catalysts,
corroborating the high framework stability of this solid during cat-
alysts preparation. With a BET surface area of 310 m2 g−1, and a
pore volume of 0.34 cm3 g−1 (see Table 1), this pelletized meso-
porous silica exhibits very low acidity, as measured in NH3-TPD
experiments (Table 1).
(35 ml min−1), the H2-TPD was performed by heating (15 ◦C min−1
)
from r.t. to 800 ◦C. The evolved H2 was analysed by an on-line chro-
matograph (Shimadzu GC-14A) provided with a TCD. A cold finger
(−80 ◦C) was employed as water tramp. The hydrogen consumed
(TPR) or desorbed (TPD) was quantified by calibration with pure
CuO as reference compound (Aldrich), assuming a total reduction
of CuO to Cu0.
Hydrogen chemisorption measurements at r.t. were carried out
in a Micromeritics ASAP 2010C equipment, under static volumet-
ric conditions. 0.200 g of catalyst precursor was reduced in situ.
After reduction, the sample was evacuated for 12 h at the reduction
temperature and cooled under vacuum (1.3 × 10−2 Pa) to the anal-
ysis temperature, 35 ◦C. The reported hydrogen uptake values were
taken from the volume difference between two hydrogen adsorp-
tion isotherms extrapolated to zero pressure (strongly adsorbed
hydrogen), the second one measured after an evacuation at the
analysis temperature. A H:M (M = Ni and/or Ru) atomic ratio of 1
was used for calculations.
3.2. XRD
X-ray diffractograms at high angles of the catalyst precursors
display only very weak and broad reflections at 2ꢀ = 37.3◦, 43.3◦
and 62.8◦ (Fig. 1a), corresponding to very small NiO particles, that
become less intense as the nickel content decreases. Since hydrated
ruthenium chlorides and/or oxychlorides are amorphous species,
no signal can be seen that could be attributable to any Ru com-
pound [27]. After reduction (Fig. 1b) a broad but more intense peak
appears at 2ꢀ = 44.5◦, which is attributed to the presence of Ni0
particles, and indicates that formation of the metallic phase has
occurred in some extend. So, when compared to the pure Ni cata-
lyst, it seems that the addition of small amounts of Ru has led to a
higher amount of Ni0 crystallites and/or a decrease in the Ni0 dis-
persion (as will be discussed later in Section 3.5); this phenomenon
being especially important for catalyst NiRu0.80. On the other hand,
peaks corresponding to Ru0 are more difficult to appreciate: they
would be expected to appear at 2ꢀ = 38.8◦, 42.2◦ and 44.0◦. Given the
low intensity and broadness of the signals, the most intense peak,
at 44.0◦, would overlap with that of Ni0 at 44.5◦. Only for samples
with Ni/(Ni + Ru) equal or lower than 0.67, two small reflection lines
seem to appear at 38.4◦ and 42.2◦, indicating the presence of very
small and dispersed Ru0 particles. For the pure Ru catalyst, no peaks
can be detected, what can be due to a very low amount of Ru0 (low
degree of reduction for this catalyst, as it will be discussed later)
and/or the very small size of the metallic particles.
Ammonia thermoprogrammed desorption (NH3-TPD) profiles
of catalysts were obtained by placing 0.080 g of catalyst precursor
into a tubular reactor, where it was firstly reduced in situ. After
cleaning with He (35 ml min−1) from r.t. to 400 ◦C (10 min), and
cooling to 100 ◦C, the adsorption of ammonia (5 min) at this tem-
perature was performed. An He flow (35 ml min−1) was next passed
to eliminate the physisorbed ammonia. Thermoprogrammed des-
orption was then carried out by heating the samples from 100 ◦C to
the reduction temperature, 400 ◦C, at a heating rate of 10 ◦C min−1
.
The evolved ammonia was analysed by on-line gas chromatograph
(Simadzu GC-14A) provided with a TCD. In order to quantify the
amount of ammonia desorbed, the equipment was previously cal-
ibrated by measuring the corresponding signals of the thermal
decomposition of known amounts of hexaamminenickel(II) chlo-
ride [Ni(NH3)6]Cl2, supplied by Aldrich.
2.3. Catalytic test
The gas phase hydrogenation of acetonitrile was performed in
a flow system operating at atmospheric pressure. A tubular Pyrex
reactor (27 cm length, 7 mm o.d. and 3.6 mm i.d.) was used. Prior
to any measurement, 0.030 g of catalyst precursor were reduced
in situ. Afterwards, a hydrogen flow, previously bubbled through a
saturator containing acetonitrile (Aldrich, HPLC grade), was intro-
duced into the reactor. The resulting feed contained 4.8 mol% of
acetonitrile (total flow, 146 ml min−1; H2:CH3CN molar ratio = 20).
The reactant and the products were analysed by means of an on-line
chromatograph (Shimadzu GC-14A) equipped with a TRB-14 column
and a FID. Tests were performed at 135 ◦C.
3.3. Textural parameters
N2 adsorption–desorption isotherms of the support hardly
changed upon Ni and Ru metals incorporation and reduction
(Fig. 2), that is to say, neither a shift of the capillary condensation
nor a decrease of the average pore size occurred. Only the values
of surface area slightly reduced for pure Ni and NiRu0.50 catalysts
(Table 1). So, some pore blocking must occurred in these two cat-
alysts, probably because a greater portion of metal loadings has
been introduced in the channels of the support or is covering the
entrance of the pores.
The conversion is defined as
(CH3CNin − CH3CNout
)
3.4. TEM
Conversion(mol%) = 100 ×
CH3CNin
TEM analysis of the pure Ni catalyst shows that no large
nickel particles have been formed; indeed, no particles larger than
∼3.5 nm can be visualized, being homogenously distributed in the
silica matrix (Fig. 3a). It is known that the nickel citrate precur-
sor leads, after calcination, to the formation of small nickel oxide
particles with high dispersion [23]. The metal–support interaction
prevents Ni0 particles agglomeration upon reduction treatment.
The small particle sizes observed are in agreement with the fact
that they are limited by the dimensions of the tubular pores where
they have been formed.
The selectivities have been calculated from peak areas by con-
sidering the different sensitivity factors in the flame ionisation
detector. The selectivity for the product i is defined as
(corrected area)i
Selectivityi(mol%) = 100 ×
sum of all corrected areas
A series of preliminary experiments was previously carried out
in order to rule out the existence of diffusional limitations under
the experimental conditions used.

P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
135
Fig. 1. XRD patterns at high angle for catalyst precursors (A) and reduced catalysts (B): (a) Ni, (b) NiRu0.89, (c) NiRu0.80, (d) NiRu0.67, (e) NiRu0.50, (f) NiRu0.33, and (g) Ru;
(*) 38.8◦ and 42.2◦.
with increased intensity of Ni0 peaks in XRD patterns, as has been
already discussed, where NiRu0.80 catalyst exhibited the peaks of
Ni0 with maximum intensity.
The same fact can be said about the pure Ru catalyst (Fig. 3b),
where the micrograph even shows that metallic particles are
aligned, being a demonstration that most of these are inside of the
silica mesopores.
By further decreasing the Ni/(Ni + Ru) molar ratio, the same fea-
ture is observed, although the proportion of both kinds of metallic
particles seems to vary. In general, small particles are more abun-
dant while the large ones are more difficult to find (see Fig. 4). So,
this suggests that a higher metallic dispersion has been achieved
on increasing the amount of Ru. For all samples, the size of small
metallic particles is never greater than 5 nm, and again, they seem
to form rows, indicating that they are mainly located inside of the
tubular pores of the support.
By contrast, the situation is very different for bimetallic NiRux
catalysts. In these samples, the metallic particles are distributed
heterogeneously into two manners: (i) those with small sizes, inter-
acting with the support, and mainly located inside the silica pores;
and (ii) those with large metallic particle sizes, forming aggre-
gates, which are situated on the external surface of the support, and
scarcely interacting with this. Furthermore, the proportion of these
two kinds of metallic particles varies from one catalyst to another
depending on the Ni/(Ni + Ru) atomic ratio. Thus, for NiRu0.89 cata-
lyst, we can find zones of support which contain small particles (not
greater than 4 nm in diameter) (Fig. 3c); on the other hand, large
particles or particle aggregates can also easily be found (Fig. 3d).
For NiRu0.80 catalyst, the support hardly contains dispersed
particles (Fig. 3e). Instead, large particles or aggregates are the dom-
inant form for the metallic particles (Fig. 3f). This is in agreement
3.5. H2-TPR
Fig. 5 shows the TPR profiles of supported monometallic and
bimetallic catalyst precursors in flowing hydrogen.
The H2-TPR profile of the pure supported NiO shows a sharp
peak, centred at 316 ◦C, and a very broad one, centred at 550 ◦C. This
high temperature peak originated from the slow reduction of small
nickel oxide particles, strongly interacting with the silica frame-
work, or even Ni2+ ions interacting with Si–O− sites. As regards to
the low temperature peak, it is normally attributed to the presence
of very small NiO particles, well dispersed and scarcely interacting
with the support [23].
As regards to the pure Ru unreduced catalyst, the profile shows
two reduction peaks, centred at 134 ◦C and 300 ◦C, respectively.
For comparison, a TPR experiment was carried out employing the
amount of pure RuCl3·xH2O salt that contains the same amount
of Ru moles as pure Ru catalyst. The corresponding TPR profile is
also shown in Fig. 5, and indicates that the reduction process is
complex, with several narrow peaks occurring between 180 and
278 ◦C, the most intense one centred at 200 ◦C, in accordance with
the results published by others [28]. Furthermore, the H2 consump-
tion is intermediate between the values expected for the reduction
of Ru(III) and Ru(IV). This result is in agreement with the fact that
our starting ruthenium salt, the commercially hydrated ruthenium
trichloride, is actually a mixture of different ruthenium compounds,
presumably mostly in the +IV oxidation state [27] (calculations
indicate that 68% of Ru is in the +IV oxidation state). None of these
peaks are present in our pure Ru catalyst precursor, indicating that
Fig. 2. N2 adsorption (solid symbols) and desorption (open symbols) isotherms for
SBA-15 carrier and bimetallic NiRu0.89 catalyst.

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P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
Fig. 3. TEM micrographs for reduced catalysts (a) Ni, (b) Ru, (c) NiRu0.89, small, well-dispersed metallic particles on the support, (d) NiRu0.89, large particles, (e) NiRu0.80,
support, and (f) NiRu0.80, large particles, segregated from the support.

P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
137
Fig. 4. TEM micrographs for reduced catalysts (a) NiRu0.67, small, well-dispersed metallic particles on the support, (b) NiRu0.67, large particles, (c) NiRu0.50, small, well-
dispersed metallic particles on the support, (d) NiRu0.50, large particles, segregated from the support, and (e) and (f) NiRu0.33, small, well-dispersed metallic particles on
the support.

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P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
whose maxima appear at temperatures between 100 and 290 ◦C.
So, the mixture of the supported solids, NiO and RuCl3, enhances
the reducibilities of both metal cations. This effect has also been
reported by other authors [32], and has been attributed to the inti-
mate contact of Ru species with the Ni ones. Furthermore, exchange
Si–O− sites are possibly occupied by Ni2+ species when the Ru
salt is added. This would explain the low temperature of reduc-
tion of Ru species to form Ru0, and, more importantly, these Ru0
species are capable of chemisorbing hydrogen molecules and sup-
ply the hydrogen active species to nickel and/or other ruthenium
species, by hydrogen spillover, either directly or through the sup-
port. Moreover, this ease of reduction would also explain why we
do not find surface residual chloride species in our catalysts by XPS
analysis, as will be discussed in Section 3.8. On the other hand,
the simultaneous reduction of both metals, as well as the pres-
ence of several overlapping peaks, suggest the formation of one
or more Ni/Ru alloys, as reported by other authors [33,34]. Quan-
tification of these curves has been carried out assuming the total
reduction of ruthenium species. For Ni-rich samples, NiRu0.89 and
NiRu0.80, this hypothesis seems very likely, given that TEM micro-
graphs suggest that the interaction of Ru particles and the support
is negligible. For the Ru-richest sample, NiRu0.33 catalyst, the area
under the TPR curve indicates 100% of reduction for both Ni and Ru
metals. This result may seem surprising when compared with the
difficulty of reduction of Ru cations for pure Ru catalyst, but, we
must keep in mind that probably the strongest acidic sites of the
support are already neutralized by Ni2+ ions in NiRux bimetallic
catalysts, thus favouring the total reduction of Ru species. Results
are compiled in Table 2, where the Ni/(Ni + Ru) molar ratios for
reduced metals, Ni0/(Ni0 + Ru0), are also shown. The values found
for samples NiRu0.89 and NiRu0.80, with very different percentages
of reduced nickel, strongly suggest that these degrees of reduc-
tion of Ni species in both samples are governed by the formation
of some Ni/Ru bimetallic phase with a definite composition. This
phenomenon would provide an explanation about why the very
well-dispersed Ni0 particles in these samples have migrated from
the interior of the channels of the support to the external surface
upon the reduction process, as deduced from TEM micrographs. The
migration of Ni(II) species assisted by Ru0 has also been observed
by Li et al. in periclase type mixed oxides, Mg(Al, Ni)O, when doped
with ruthenium [18]. The same phenomenon has probably occurred
for Ru-rich catalysts, NiRu0.50 and NiRu0.33, which exhibit the
same Ni0/(Ni0 + Ru0) molar ratio.
Fig. 5. H2-TPR profiles of catalyst precursors: (a) Ni, (b) NiRu0.89, (c) NiRu0.80, (d)
NiRu0.67, (e) NiRu0.50, (f) NiRu0.33, (g) Ru, and (h) pure RuCl3·nH2O.
interaction of RuCl3·xH2O with the silica support greatly affects
its reducibility. The low temperature peak has been reported by
a number of authors [29,30], and can be assigned to the presence of
very small well-dispersed particles of the ruthenium salt, which
would be more reducible than unsupported ones. As regards to
the high temperature peak, some authors have attributed it to the
reduction of RuOxCly species, containing Ru(IV) [31]. These species
are present in our materials, as pointed out by XPS data (see later).
Furthermore, this peak can also be due to ruthenium ions strongly
interacting with the support, likely with Si–O− sites, as proposed
by Eliche-Quesada et al. [30]. The percentage of Ru reduction esti-
mated at 400 ◦C is 33% (Table 2). This low reduction percentage
clearly corroborates the strong interactions between ruthenium
species and the support.
The reduction patterns of NiRux bimetallic catalyst precursors
are clearly different from those of the monometallic ones of ruthe-
nium and nickel. The reduction process begins at temperatures
as low as 100 ◦C for all the samples and finishes below 400 ◦C,
showing a very complex pattern, with several overlapping peaks
3.6. H2 chemisorption
Hydrogen chemisorption at 35 ◦C was used to estimate sur-
face composition of the catalysts. It is necessary to point out that
the contribution of Ru metal itself in the H2 uptake on the NiRux
Table 2
Characterization of the metallic phase for Ni, Ru and NiRux bimetallic catalysts.
a
e
Catalyst
%Ni0/Nit
Ni0/(Ni0 + Ru0)a
␮mol H2/gb
␮mol H2/gc
Peak I (Tmax
H/Ni0 molar ratiob
H/Ru0 molar ratioc
Ni0/(Ni0 + Ru0)s
,
◦C)d
Peak II
Peak III
Ni
30.5
45.1
65.4
45.9
51.2
100
33
1.00
0.78
0.72
0.48
0.34
0.33
0.00
35.7
12.2
10.4
11.7
15.7
13.9
–
0.6 (119)
1.3 (88)
1.9 (93)
34.1 (99)
31.5 (114)
74.2 (116)
14.4 (118)
8.8
1.0
1.6
1.5
1.2
2.1
12.7
57.1
69.4
40.0
0.46
0.12
0.08
0.16
0.25
0.18
–
–
1.00
0.90
0.84
0.26
0.33
0.16
0.00
NiRu0.89
NiRu0.80
NiRu0.67
NiRu0.50
NiRu0.33
Ru
0.05
0.04
0.42
0.26
0.47
0.53
2.1
13.7
28.2
1.0
a
From the H2-TPR experiments.
b
c
From volumetric H2 chemisorption at room temperature.
From H2-TPD experiments, considering only peak I signal.
In brackets, temperature of the maximum.
d
e
Surface Ni/(Ni + Ru) molar ratio, from both r.t. and TPD-H2 chemisorption experiments.

P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
139
small quantity of H2 chemisorbed on its metallic phase. This is
in contrast to the large value measured by the room temperature
volumetric method. Nevertheless, the relative large amount of H2
chemisorbed in the metal–support interface agrees with the fact
that a high metallic dispersion of Ni0 particles is achieved in this
sample, as demonstrated by room temperature chemisorption and
TEM. The reason why this sample does not chemisorb the amount
of hydrogen expected is not clear. Perhaps, in our experimental
conditions, fast cooling in flowing hydrogen to room temperature
and subsequent purge with flowing argon, this sample did not have
time enough to chemisorb hydrogen. In this sense, Boudjahem et
al. also found that the amounts of chemisorbed H2 at room tem-
perature on Ni supported catalysts were much higher than those
desorbed in the low temperature peak of the H2-TPD experiments
[41]. Also, Znak and Zielinski reported that, in the case of hydrogen
preadsorption at 20 ◦C, the peak at 127 ◦C was absent in a Ni powder
catalyst, while a high preadsorption temperature and extension of
the time of the interaction to 24 h made it to appear [42].
For Ru catalyst, the amount of H2 chemisorbed corroborates the
very high dispersion of the metallic phase, as indicated by TEM
micrographs. At the same time, it is worthy to note its great capacity
to spillover hydrogen onto the support.
H2-TPD profiles of Ni-richest bimetallic catalysts, NiRu0.89 and
NiRu0.80, are also different from the others. They show a small and
narrow desorption peak centred at ca. 90 ◦C. Furthermore, the three
signals, I, II and III, are very small, which is in agreement with the
lack of metal–support interactions in these catalysts, due to the
agglomeration of the metallic phase and its segregation from the
silica carrier. Nevertheless, the amounts of hydrogen chemisorbed
by the metallic particles are very small when compared to the val-
ues obtained by the volumetric method, as happens with Ni pure
catalyst, so, it seems that nickel does not chemisorb hydrogen in
these catalysts.
In contrast, for the Ru-richest bimetallic catalysts, NiRu0.5 and
NiRu0.33, the amount of hydrogen chemisorbed on their surfaces
are much higher, indicating a better dispersion of the metallic par-
ticles, as well as much greater metal–support interactions. Since
hydrogen spillover is related to the metal–support interaction and
the perimeter of the interface between metal and support, these
catalysts also show a very intense and broad band at high temper-
ature (peak III), as does pure Ru catalyst. Finally, catalyst NiRu0.67,
although with a peak I similar to that of Ru-rich catalysts, seems
to have metal–support interactions lower than them, given the
relative low intensity of its II and III signals.
Fig. 6. H2-TPD chromatograms for reduced catalysts: (a) Ni, (b) NiRu0.89, (c)
NiRu0.80, (d) NiRu0.67, (e) NiRu0.50, (f) NiRu0.33 and (g) Ru.
catalysts must be negligible, since the pure Ru catalyst did not
chemisorb any detectable amount of H2 at this temperature, as
also reported by Miyata et al. [35]. It can be a consequence of the
well-established character of the activated process that exhibits the
hydrogen adsorption on ruthenium [36]. So, it is assumed that at
room temperature, one hydrogen atom is chemisorbed per surface
nickel atom, as reported in the case of Ni/Cu alloys [37,38]. Results
are depicted in Table 2. The amount of chemisorbed H2 decreases
when adding Ru to the pure Ni catalyst. Agglomeration of the metal-
lic phase, for Ni-rich catalysts could explain the low amount of H2
taken up; nevertheless, when the amount of Ru increases, the dis-
persion of the Ni-Ru phases is higher, and the amount of H2 taken
up remains almost constant.
3.7. H2-TPD
From these data, it is clear that the substitution of some Ni atoms
in the 3% Ni supported catalyst by Ru atoms initially agglomerates
the metallic phase, but increasing the Ru loading, progressively
rises the metallic dispersion of the bimetallic catalysts. This is in
accordance with the positions of temperature maxima of peak I,
which are located at a high temperature for Ni and Ru monometallic
catalysts, ca. 118 ◦C, but shift smoothly to lower temperatures as the
amount of Ni increases, suggesting that the agglomeration of the
metallic phase provides less strong chemisorption sites for hydro-
gen. When sintering takes place, the electron density of the surface
metal atoms increases, thus, weakening the metal–hydrogen bond
[43,44].
On the other hand, by considering the amount of H2
chemisorbed at 135 ◦C (peak I), it is clear that, as already discussed,
for pure Ni, NiRu0.89 and NiRu0.80 catalysts, Ni0 does not absorb
hydrogen at this temperature. On the contrary, for the rest of the
NiRux bimetallic catalysts, the amount of H2 chemisorbed at 135 ◦C
(peak I) is much higher than that obtained at r.t., what would
be attributable to chemisorption on Ni0 and Ru0 in a bimetallic
phase. Nevertheless, if H/Ru0 molar ratios are calculated (where
only peak I is considered, see Table 2), we find that these values
are lower than that corresponding to pure Ru catalyst. This find-
Given that hydrogen chemisorption on metallic ruthenium is an
activated process, the measurement of its surface is very commonly
carried out at high temperature, that is, 100 ◦C or above [39]. In
our case, as mentioned in Section 2, the preadsorption of hydro-
gen was performed at 135 ◦C, and prior to the TPD experiment.
Chromatograms are shown in Fig. 6; the corresponding quantifica-
tion is depicted in Table 2. The TPD profiles show two main peaks,
centred at ca. 100 ◦C (peak I) and 480 ◦C (peak III). There is also
a less defined desorption between these two peaks, with maxima
between 225 and 350 ◦C (peak II). The lower temperature peak,
peak I, is assigned to desorption of H2 from metallic centres, while
the high temperature peak, peak III, is attributed to H2 evolved
from very strong chemisorption sites on the surface of the catalyst,
likely hydrogen species on the support, distant from the metallic
particles, as a consequence of a spillover phenomenon. As regards
to the desorption peak II, it is originated by hydrogen desorbed
from chemisorption sites located at the metal–support interface
[40]. The H2-TPD profile of the pure Ni catalyst is quite different
from the others, exhibiting a very broad signal with a maximum at
ca. 268 ◦C, and only a shoulder at ca. 119 ◦C, indicative of a very

140
P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
Table 3
XPS data for the supported NiRux bimetallic materials, before and after reduction.
Sample
Binding energy (eV)
Ni 2p3/2
Ni/(Ni + Ru) molar ratio
Ru 3p3/2
Ru(IV)
Ni(II)
Ni0
RuOxCly
Ru0 (FWHM)
Precursors
Ni
855.9
855.7
855.4
855.9
855.9
856.4
–
–
–
–
–
–
–
–
–
–
–
1.00
0.59
0.50
0.48
0.67
0.53
0.00
NiRu0.89
NiRu0.80
NiRu0.67
NiRu0.5
NiRu0.33
Ru
–
–
–
–
–
–
462.1
462.2
462.2
462.5
463.0
462.7
465.2
465.1
465.3
465.7
465.6
466.0
Catalysts
Ni
855.8
855.9
855.6
855.8
855.8
855.9
–
852.9
852.7
852.7
852.8
852.7
852.7
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.00
0.71
0.62
0.66
0.72
0.60
0.00
NiRu0.89
NiRu0.80
NiRu0.67
NiRu0.5
NiRu0.33
Ru
461.4 (3.3)
461.6 (3.5)
461.8 (3.4)
462.4 (5.0)a
461.9 (3.9)
461.7 (4.3)
a
See text in Section 3.8, and Fig. 7.
ing suggests that, in the present experimental conditions, it is very
likely that the amounts of H2 measured on the metals are due to its
chemisorption on Ru atoms only, for all catalysts. Thus, given that
855.9 eV. This value is higher than that corresponding to pure NiO
(855.0 eV [45]), and has been assigned to NiO present as small parti-
cles inside the mesopores [46]. However, the corresponding signals
for bimetallic precursors are less intense and noisy, likely as a con-
sequence of descending nickel loading. The corresponding binding
energy values (Table 3) support the same idea, as already deduced
from XRD data. After reduction, the Ni 2p3/2 signal becomes asym-
metric, and can be decomposed into two contributions, one centred
at ca. 855.8 eV and another one at ca. 852.8 eV, respectively (Fig. 7).
The peak at 855.8 eV could be associated with the very small crys-
tallites of remaining NiO located on the walls of the mesostructure;
while the other peak at ca. 852.8 eV corresponds to Ni0 species. For
bimetallic NiRux catalysts, the maximum of this peak is slightly
shifted to lower B.E. values, compared to the pure Ni catalyst, what
could be due to differences in the metal–support interactions, but
also as consequence of the interaction of Ni0 and Ru0 atoms. Never-
theless, this last possibility seems unlikely, since the contribution
of such an interaction to the core level shift is minimal, well within
H
2 chemisorption at r.t. is due to surface Ni0 atoms, and that peak I
in H2-TPD profiles is due to surface Ru0 atoms, a surface Ni/(Ni + Ru)
molar ratio can be calculated for these catalysts (see Table 2). The
similarity of Ni/(Ni + Ru) values obtained both from H2-TPR and
chemisorption experiments indicates that this approximation is
right. The results show that, as expected, the surface composition of
the bimetallic catalysts changes with the added Ni/(Ni + Ru) molar
ratios, and strongly suggests that certain Ni-Ru compositions are
specially stable in our experimental conditions.
3.8. XPS
The XPS patterns for Ni 2p core level spectra of precursor sam-
ples show that the pure nickel precursor possesses a Ni 2p3/2
signal which is symmetric and sharp, with a maximum centred at
Fig. 7. Ni 2p and Ru 3p core level spectra for reduced catalysts: (a) Ni, (b) NiRu0.89, (c) NiRu0.80, (d) NiRu0.67, (e) NiRu0.50, (f) NiRu0.33 and (g) Ru.

P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
141
the experimental error [47,48]. The peak areas of Ni0 signals were
much smaller than that of NiO signals. This may indicate that only
a small fraction of NiO was reduced, in contrast to what is deduced
from H2-TPR experiments. We must keep in mind that before XPS
analysis, the reduced samples were shortly exposed to the air while
moved from the reactor to the analysis chamber. So, this low frac-
tion of reduction could be the result of the reaction of dispersed Ni0
with oxygen at room temperature. Furthermore, it is well known
that, when Ni0 are formed, the atoms located in the interior of the
particles, can hardly be detected by this technique [49].
Ru species were analysed by recording the Ru 3p spectrum of
samples. The Ru 3p3/2 signals for catalyst precursors are slightly
asymmetric, and can be decomposed into two contributions: a main
one with its maximum centred at ca. 462.1–463.1 eV, that can be
assigned to the presence of Ru(IV) species, and a small shoulder
at higher binding energies, between 465.1 and 466.0 eV, which is
attributed to the presence of RuOxCly compounds, most likely with
Ru atoms in +IV oxidation state [50], which corroborates that Ru
in our pristine salt is mostly in the +IV oxidation state, as already
mentioned.
Recorded spectra for reduced catalysts are also depicted in
Fig. 7. The Ru 3p3/2 signals are now symmetric, with their max-
ima between 461.4 and 461.9 eV, which, according to the literature
[50], correspond to the binding energy of Ru0. Nevertheless, the
FWHM of the signals of more than 3.3 eV could suggest the pres-
ence of ruthenium atoms in more than one oxidation state. This
is the case for pure Ru catalyst, the signal being the result of
the presence of Ru0 and Run+ species, as deduced from H2-TPR
experiments. Besides, this catalyst shows the broadest signal. On
the other hand, for NiRux bimetallic catalysts, very high metal
reduction degrees are likely, thus these broad signals must be due
to different chemical environments for Ru0 atoms originated by
metal–support interactions, as well as Ni–Ru interactions (note that
these Ru 3p signals are narrower than that for pure Ru catalyst, with
the exception of NiRu0.50 catalyst, with a very low intensity and
noisy signal). Furthermore, before XPS analysis, the reduced sam-
ples were shortly exposed to the air, and so, these broaden signals
could also be the result of some surface RuO2 [51].
Fig. 8. Surface Ni/(Ni + Ru) molar ratio, as measured by XPS analysis, for catalyst
precursors and reduced catalysts, as a function of bulk composition.
to remain on the external surface of the support, thus giving rise
to surface Ni/(Ni + Ru) molar ratio values lower than those of bulk
ones. An important consequence of this may be the fact that prob-
ably a close interaction between the two metal cations, Ni2+ and
Run+, occurs, already before the reduction process, which would be
reflected in the H2-TPR profiles, as discussed.
As regards the Ni/(Ni + Ru) molar ratio values for reduced sam-
ples, also shown in Fig. 8, they are slightly higher than those
corresponding to the non-reduced ones, and again, they do not
follow the bulk values, but oscillate around a mean value of 0.66.
This apparent contradiction can be explained by taking into account
that in low loading-Ni catalysts, Ni atoms are mainly forming the
metallic particles located on the external surface of the support,
which are well detected by XPS technique, and, furthermore, with
the same composition as those present in high loading-Ni catalysts.
This implies that the metallic particles situated within the pores of
the support, not detectable by XPS, must be Ru-enriched, which cor-
roborates the formation of two or more bimetallic phases of special
stability in our series of catalysts.
On the other hand, it is worthy to point out that Ru 3p signals
become less intense and noisier as the Ru content increases. Consid-
eringthatXPS isessentiallyasurfacetechnique, most of the metallic
particles located in the porous structure of the support, with wall
thickness of ca. 50 Å, are not detected [52]. This means that, at least
for Ru-rich samples, NiRu0.33 and NiRu0.50, a great fraction of the
metal has entered into the tubular channels of the support, as also
happens with pure Ru catalyst. On the contrary, NiRu0.80 catalyst,
with still a low percentage of Ru, shows the most intense Ru 3p
signal, which indicates that almost all the metallic particles were
formed on the external surface of the support, as has been already
described.
Concerning the surface chemical composition, in Fig. 8 the
Ni/(Ni + Ru) surface molar ratios for catalyst precursors (from XPS
chemical analysis) have been plotted versus the corresponding bulk
values. Surprisingly, all surface values differ from those of bulk val-
ues in such a way that the former ones are around a constant value
of 0.55. As NiO is a basic oxide, and RuCl3 is an acid salt, and given
that the respective cations sizes are very similar [53], it is likely
that an interaction between both solids has happened during the
impregnation and subsequent drying process, perhaps giving rise
to any kind of mixed compound, and thus, altering the Ru distri-
bution on the support surface, and hence, the Ni/(Ni + Ru) surface
molar ratios. It means that, when the amount of NiO present on the
support before impregnation with the Ru salt is small, as in samples
NiRu0.50 and NiRu0.33, a great fraction of Ru ions has entered into
the mesopores of the support. The opposite must have happened
for Ni-rich precursors, where a higher fraction of Ru cations seems
Finally, it has been reported that Ru catalysts derived from the
reduction of RuCl3 retain residual chlorine after reduction with
hydrogen [31]. However, no XPS peaks for chloride atoms are found
in the reduced materials, indicating that this element has been com-
pletely removed upon the reduction process, in accordance with
results reported by others [54].
3.9. NH3-TPD
The NH3-TPD profile for the SBA-15 support (Fig. 9) exhibits one
small and well-defined, almost symmetric desorption peak, with
its maximum centred at 136 ◦C, which corresponds to weak acid
sites. After nickel incorporation onto the support, this peak shifts
to higher temperature, 151 ◦C, while becomes asymmetric, with a
pronounced tail which extends to high temperature; only when
the temperature ramp is stopped at 400 ◦C, the signal falls to the
base line again. This profile clearly indicates the broad distribu-
tion of acid centres strengths, those in the weak range being the
more abundant. Moreover, the total number of acid sites of the cat-
alyst increases in comparison to that of the support (Table 1). The
nickel modification over the acidity of the support has two different
causes. The nickel species could cover some of the acid sites (both
Bronsted and Lewis acid sites), causing the decrease of the total acid
sites; but, on the other hand, the co-ordinately unsaturated nickel
cations could serve as a kind of new Lewis acid centres, which com-
pensate the original acid sites being covered. In the present case,
the compensating effect of the Ni species overrides the covering

142
P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
Fig. 10. Conversion and selectivity changes for NiRu0.89 catalyst, as
a func-
tion of time on stream, for the hydrogenation of acetonitrile in the gas phase
at 135 ◦C and atmospheric pressure (F/W = 121 ␮mol CH3CN gcat−1 s−1; H2:CH3CN
molar ratio = 20).
The changes in acetonitrile conversion as well as in selectivities
with time on stream (TOS) for sample NiRu0.89 are shown in Fig. 10.
In all cases, with the exception of Ru catalyst, which was inactive,
the initial conversion of acetonitrile was 100%, but, after a deactiva-
tion period, the steady state was achieved. It is worthy to note that
pure Ru catalysts employed by other authors were active at similar
reaction temperatures, but with very low conversion values [57].
Table 4 depicts the values of acetonitrile conversion and the
selectivities towards the amines for Ni and NiRux bimetallic cata-
lysts in the steady state. Conversion% values are low, between 30%
and 45%, what is consistent with the reaction mechanism proposed
by Huang and Sachtler [12,58], after which, desorption of a product
molecule must be assisted by the chemisorption of an acetonitrile
molecule, giving rise to an overall reaction
Fig. 9. NH3-TPD patterns for reduced catalysts: (a) Ni, (b) NiRu0.89, (c) NiRu0.80,
(d) NiRu0.67, (e) NiRu0.50, (f) NiRu0.33, (g) Ru, and (h) SBA-15 support.
one, which causes an increase of the total acid sites [55]. The low
degree of Ni reduction explains this great increment of the total
acidity in comparison with the support. This TPD pattern is fol-
lowed by the other catalysts, with only slight modifications, which
can be attributed to ruthenium content and the dispersion of the
metallic phase. Thus, for Ni-rich catalysts (NiRu0.89 and NiRu0.80)
the ammonia desorption peak is narrower, and less intense, while
for the rest of catalysts, this peak gets broader and more intense
as the ruthenium content and metallic dispersion increase. These
data are in accordance with those reported by other authors [56],
and are attributed to the higher tendency of Ru to adsorb ammonia
molecules. In fact, a good correlation of the total amount of des-
orbed ammonia and the surface Ru0 content of the bimetallic cata-
lysts (calculated by chemisorption methods) is found, which clearly
indicates that the surface Ru0 is responsible, at least, of the major
part of the ammonia chemisorbed by the bimetallic NiRux catalysts.
2CH3CN + 2H2 → CH3CH2NH2 + CH3CN,
for the primary amine production [59].
The observed conversion% values change as a function of cata-
lysts composition, decreasing upon the addition of small amounts
of Ru, in comparison with the pure Ni catalyst, but increasing when
larger amounts of Ni have been substituted for Ru, giving rise to
higher values than that observed for the pure Ni catalyst. Concern-
ing the selectivity values towards EA, they decrease monotonically
as the amount of Ru in the bimetallic catalysts increases, although,
in all cases, they are higher than that for pure Ni catalyst. As a
result, higher EA yields are achieved for Ru-rich catalysts. These
values are much higher than those reported by Huang et al. [57]
for pure Ru supported on different zeolites: at conversion values
between 2% and 4%, they describe selectivity values towards EA of
ca. 71–81%, when a reaction temperature of 75 ◦C was employed,
but, at conversion values of 8.6%, a selectivity of 46% towards EA was
3.10. Catalytic results
This family of NiRux bimetallic catalysts was evaluated in the
hydrogenation of acetonitrile in the gas phase at atmospheric
pressure. Under our experimental conditions, only the primary,
secondary and tertiary amines have been obtained (referred to as
EA, DEA and TEA, respectively).
Table 4
Catalytic performance of Ni, and bimetallic NiRux catalysts, in the hydrogenation of acetonitrile.
Catalyst
Conversion (mol%)
TOFNi (s−1
)
Selectivity (mol%)
EA
EA yield (%)
DEA
TEA
Ni
38.7
30.6
27.3
36.1
40.2
45.0
0.73 (1.0)a
1.69 (2.3)
1.78 (2.4)
2.08 (2.8)
1.73 (2.3)
2.18 (3.0)
57.0
60.9
60.4
59.4
59.4
58.4
33.9
27.8
28.1
27.8
30.9
30.5
9.1
11.3
11.5
12.8
9.7
22.1
18.6
16.5
21.4
23.9
26.3
NiRu0.89
NiRu0.80
NiRu0.67
NiRu0.50
NiRu0.33
11.1
a
In brackets, relative values, TOFNi/(TOFNi for pure Ni catalyst).

P. Braos-García et al. / Applied Catalysis A: General 381 (2010) 132–144
143
reported, when the reaction temperature was 110 ◦C. Furthermore,
when Ru was supported on alumina, the catalyst was not active at
75 ◦C; only at 125 ◦C it exhibited an acetonitrile conversion of 15.3%,
with selectivity towards EA of 24.2%. On the other hand, our results
are similar to those reported by other authors [15,60], who also
found an enhanced catalytic activity in Ni-Fe bimetallic catalysts,
although the selectivity pattern remained unchanged.
uality in alloys, as also happens with Ni–Cu alloys [66], the observed
promoting effect might have some other cause.
Taking into account the strong interaction of ammonia
molecules with Ru0 atoms, as shown by NH3-TPD experiments, it
is possible that surface Ru0 atoms adjacent to Ni0 atoms can attract
the electron pair from the non-bonding orbital on the N atom of the
nitrile molecule [15], thus favouring the anchorage of the CN group
to the active Ni0 ones, and hence, its activation. Furthermore, these
Ru0 atoms, could also stabilize some reaction intermediates, such
as the nitrene, CH3–CH2–N , doubly bonded to the surface. It is
known that Ru atoms are better than Ni ones in establishing multi-
ple bonds [67]. This would supply more activated forms for the last
hydrogenation step, before desorption of EA, and/or for subsequent
condensation steps.
Although the pure Ru catalyst was inactive in the hydrogenation
reaction, it is possible that it becomes active when alloyed with Ni
atoms. To explore this possibility, a turnover frequency of surface
Ni0 atoms (referred to as TOFNi), calculated as acetonitrile moles
converted per exposed Ni0 atom and second, has been calculated
(Table 4). These values are very similar for all the bimetallic cata-
lysts, and 2.3–3.0 times higher than that of pure Ni catalyst. This
clearly indicates that the active sites are Ni0 atoms, and that their
specific activity (TOFNi) has been enhanced by direct interaction
with Ru atoms, irrespective of other factors that might also affect,
although in a lesser extend, as the dispersion. Furthermore, these
data rules out any structure demanding character of the hydro-
genation reaction, as proposed by some authors [61]. Since these
TOFNi values remain almost constant for the bimetallic catalysts,
that is, with the surface Ni0 concentration, the dilution of these Ni0
atoms with non-active Ru0 atoms has no effect on the Ni0 specific
catalytic activity, and so, an ensemble effect is dismissed, i.e., the
catalytic centres seem to be monoatomic surface Ni0 sites, as also
reported by others [15]. This enhancement of Ni specific activity
in the hydrogenation of different nitriles by direct interaction with
other metals have already been reported by many authors [62,13];
some of them attributed this finding to electronic effects (the so-
called ligand effect), i.e., to an electron transfer from promoter atoms
to the active Ni0 atoms, thus changing the electronic properties of
nickel atoms.
Lastly, another possibility is that Ru0 atoms can enhance des-
orption of product molecules. Here, it is important to take into
account the reaction mechanism proposed by Huang and Sachtler
[12], in which desorption of a product molecule is proposed to be
the rate-limiting step: the release of a strongly held intermediate
from the surface is assisted by the interaction with another colliding
nitrile molecule, which suffers a dissociative H–C chemisorption,
thus acting simultaneously as hydrogen donor in the last hydro-
genation step of a final product. In this sense, Ru is known to be
a very good catalyst for C–H bond dissociation, as proved by its
catalytic activity in reactions such as methane conversion [68], or
␣,␣-bonding of alkanes [67]. Hence, the amines formed in the Ni0
catalytic centres may be easily transferred to adjacent Ru0 atoms,
given its great affinity for N-containing species, and finally des-
orbed by an enhanced acetonitrile dissociative chemisorption, thus
regenerating the catalytic Ni0 site. The number of such catalytic
sites, exposed adjacent Ni0-Ru0, must be a function of surface com-
position. Besides, if the reaction rate is controlled by desorption of
products from one M0 atom in the way already explained, then, the
catalytic centre responsible for this step would be monoatomic, as
observed experimentally.
To analyse this possibility, it is necessary to briefly review the
chemisorption properties of the acetonitrile molecule. The nitrile
group contains a heteronuclear triple bond with occupied ␴-non-
bonding and doubly degenerated ␲-bonding orbitals and doubly
degenerated ␲*-orbitals. Acetonitrile has two possible adsorption
geometries, either perpendicular to the surface through the nitro-
gen lone pair or parallel to the surface through the ␲ system. When
an end-on coordination via the N atom takes place, an increase
in the strength of C–N bonding is found compared to the gaseous
acetonitrile molecule. In this case, back donation from the metal
orbitals to the ␲* orbitals of the nitrile group is negligible. In con-
trast, in the side-on bonded coordination, a forward donation of a
pair of electrons from the bonding ␲ orbitals of the nitrile group
to the metal atoms takes place, while back donation from metal
atom orbitals to the unoccupied antibonding molecular orbitals ␲*
happens, resulting in a reduction of C–N bond order. Thus, the reac-
tivity of the nitrile group is directly related to the adsorption mode
[61]. According to this, the parallel chemisorption seems to be the
best form for reaction with hydrogen [63].
4. Conclusions
The bimetallic NiRux catalysts exhibited a synergism effect for
the gas phase hydrogenation of acetonitrile, in comparison with
Ni and Ru monometallic catalysts. This synergism seems to be the
result of intimate contact between both metallic atoms, as a con-
sequence of some nanoalloy/s formation. Adjacent Ni0–Ru0 sites
could be responsible for the catalytic performance observed, likely
by a better desorption of hydrogenation products, as proposed in
the literature. Nevertheless, results also show the need for a better
control of the dispersion of the metallic phases, in order to enhance
conversion values. Thus, only well-dispersed metallic phases gave
high catalytic acetonitrile conversions as well as high ethylamine
yields. Given that the specific activity per Ni0 atom has been greatly
enhanced for all Ni/(Ni + Ru) compositions, it seems desirable to
modify the synthetic route to attain well-dispersed Ni-rich Ni–Ru
alloys. On the other hand, selectivity patterns for bimetallic cata-
lysts remained almost equal to that of pure Ni catalyst.
Therefore, increased back donation ability of Ni0 in the Ni-Ru
bimetallic particles would favour the CN bond weakening, thus
leading to higher TOFNi values for NiRux bimetallic catalysts in the
hydrogenation reaction, as experimentally observed.
Nevertheless, literature dealing with Ni electronic properties in
mixed Ni-Ru phases is not clear [64]. On the other hand, experi-
mental techniques employed in the present work suggest that the
electron-richness of the surface of metallic phases increases as the
Ni-content is raised. Therefore, the activation of the acetonitrile
molecule should be easier with increasing amounts of Ni, and thus,
higher TOFNi values would be expected, contrary to what is experi-
mentally found. Therefore, our catalytic results are not explained by
considering electronic effects. So, given that it seems that for Ru–Ni
alloys the electronic structure of Ni atoms varies only marginally by
alloying [65], and that both components also preserve their individ-
Acknowledgements
We gratefully acknowledge the funding of this work by the Junta
de Andalucía (Spain) Excellence Project P06-FQM-01661.
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