PVP-stabilized mono- and bimetallic Ru nanoparticles for selective ring opening

Article abstract of DOI:10.1039/c2cy20443f

Selective ring opening of naphthenic molecules in oil upgrading should result in no loss in molecular weight. Benzocyclopentane (indan) ring opening was studied under hydrogen atmospheric pressure at 609 K over poly-(vinylpyrrolidone)-stabilized Ru, Ir and Pd monometallic and Ru-Ir and Ru-Pd bimetallic nanocatalysts prepared by simultaneous reductions. The particle size (mostly within 2 nm) and their monodispersity were confirmed by transmission electron microscopy, while X-ray photoelectron spectroscopy indicated the bimetallics' alloy structure. Pd catalysts displayed the lowest activity in the ring opening; Ru showed the highest formation of undesired o-xylene and lights. Monometallic Ir displayed the highest activity and selectivity toward 2-ethyltoluene and n-propylbenzene. In bimetallic structures, higher Ir content led to improved catalytic performance. Next to the monometallic Ir catalyst, the newly developed Ru1Ir4/γ-Al ₂O₃ catalyst (with 1 : 4 molar ratio of Ru to Ir) displayed superior single cleavage selectivity as well as lower cracking activity compared to industrial Pt-Ir catalysts at a comparable indan conversion. The study can pave the way in the development of Pt-free Ru-containing catalysts with narrow size distributions for selective ring opening, especially taking into consideration a possibility of their higher S-resistance as compared to the Pt catalysts. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20443f

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PVP-stabilized mono- and bimetallic Ru nanoparticles
for selective ring opening
Jing Shen,a Xing Yin,a Dimitre Karpuzovb and Natalia Semagina*a
Cite this: Catal. Sci. Technol.,
2013, 3, 208
Selective ring opening of naphthenic molecules in oil upgrading should result in no loss in molecular
weight. Benzocyclopentane (indan) ring opening was studied under hydrogen atmospheric pressure at
609 K over poly-(vinylpyrrolidone)-stabilized Ru, Ir and Pd monometallic and Ru–Ir and Ru–Pd bimetallic
nanocatalysts prepared by simultaneous reductions. The particle size (mostly within 2 nm) and their
monodispersity were confirmed by transmission electron microscopy, while X-ray photoelectron
spectroscopy indicated the bimetallics’ alloy structure. Pd catalysts displayed the lowest activity in the
ring opening; Ru showed the highest formation of undesired o-xylene and lights. Monometallic Ir
displayed the highest activity and selectivity toward 2-ethyltoluene and n-propylbenzene. In bimetallic
structures, higher Ir content led to improved catalytic performance. Next to the monometallic Ir catalyst,
the newly developed Ru1Ir4/g-Al2O3 catalyst (with 1 : 4 molar ratio of Ru to Ir) displayed superior
single cleavage selectivity as well as lower cracking activity compared to industrial Pt–Ir catalysts at a
comparable indan conversion. The study can pave the way in the development of Pt-free Ru-containing
catalysts with narrow size distributions for selective ring opening, especially taking into consideration a
possibility of their higher S-resistance as compared to the Pt catalysts.
Received 27th June 2012,
Accepted 4th September 2012
DOI: 10.1039/c2cy20443f
www.rsc.org/catalysis
on improving sulfur tolerance by alloying the active component
with another metal. In the 1970s, Exxon introduced Pt–Ir
1. Introduction
Exhaust gas emissions from vehicles and engines contribute to
decreased air quality, and lead to negative environmental and
health effects. In Canada, since May 2006 the sulfur content of
diesel fuel produced or imported has been required to be less
than 15 ppm. At the same time, the quality of crude oil has
decreased, which poses significant challenges for the refining
industry. A catalytic ring opening (RO) of naphthenes, serving
as one of the upgrading steps during hydroprocessing of heavy
crude oils, is a preferred reaction for improving the cetane
number of fuels. During the selective ring opening (SRO), the
naphthenic ring is only cleaved once, maintaining the same
number of carbon atoms, as contrary to hydrogenolysis and
cracking.
catalysts in reforming units, which were several-fold more
active and stable than Pt.2 Ir ensures high RO activity and low
coke formation; the addition of Pt tempers the undesirable
excessive cracking by Ir, and increases its sulfur tolerance and
resistance to agglomeration.3 The conventional Pt–Ir catalysts are
prepared using traditional methods via support impregnation/
co-precipitation with the Pt and Ir metal precursors. Such methods
typically result in polydispersed metal nanoparticles without control
over their size or structure, which often consume expensive metals
in catalytically unfavorable size and structure modes.
Recent advances in nanotechnology and colloidal chemistry
techniques allow the production of metal nanoparticles with
controlled size and structures. The use of various stabilizing
agents, such as surfactants, polymers, and electrostatic stabi-
lizers, to prepare metallic nanoparticles has been actively
exploited in hundreds of research papers and patents. Although
the studies have involved the same basic principle of controlled
metal nanoparticle formation from molecular precursors in the
presence of steric and/or electrostatic protective agents, the
final nanoparticle structure and properties are known to
depend strongly on the synthesis conditions and metal nature.
Typically, the polymer-protected nanoparticles are highly stable
for years.3 The preparation of noble metal nanoparticles using
Platinum group metals are known to selectively catalyze the
ring opening of naphthenes. However, the most significant
drawback of the Pt catalysts for fuel hydrotreating applications
is their very low sulfur tolerance.1 Many studies have focused
a Department of Chemical and Materials Engineering, University of Alberta,
9107–116 Str. Edmonton, Alberta T6G 2V4, Canada.
E-mail: semagina@ualberta.ca; Fax: +1-(780)-492-2881; Tel: +1-(780)-492-2293
b Alberta Center for Surface Engineering and Science, University of Alberta,
Edmonton T6G 2V4, Canada
c
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‘‘magic polymer’’ poly-(vinylpyrrolidone) (PVP) is widely
described for various nanoparticles with a controlled size.
PVP-stabilized bimetallic nanoparticles as catalysts have
recently received special attention for various catalytic reac-
The importance of size control of metal nanoparticles in tions. The addition of the second metal may result in improved
catalysis is far from the traditional concept that the decrease of catalytic activity, selectivity, and stability (sulfur tolerance).
nanoparticle size leading to a higher surface-to-volume ratio Electron and geometric effects as well as the occurrence of
leads to the reaction rate increase. On the contrary, there are mixed sites are claimed to be responsible for synergism
numerous reactions that occur on quite large nanoparticles between two metallic components.18 As with monometallic
(above 3 nm) with much higher rates and selectivities than on particles, the described methods for the nanoparticle prepara-
smaller particles.4 The reason is in the surface configuration: tion originate from well-known colloidal chemistry techniques,
the proportion of atoms on edges and vertices decreases with but each catalytic application requires its own most optimal
the particle size increase, while the proportion of terrace atoms combination of metals and nanoparticle structure modes.
increases.5 Such atoms possess different electronic and geo- There are numerous recent reports on improved catalytic
metric properties, affecting the chemisorption strength and activity of bimetallic catalysts synthesized in a controlled
mode of reaction substrates. For example, alkyne adsorbs too environment.3 For example, the enhancement in activity was
strongly on the edge atoms of Pd nanoparticles and does not observed for Au–Pd particles stabilized by PVP as compared to
react, while chemisorption on the terrace atoms favors alkyne those prepared by impregnation for hydrogenation of toluene
hydrogenation, so the reaction rates on larger particles with and naphthalene in the presence of dibenzothiophene.19 PVP-
higher proportions of terrace atoms are higher.4 With the stabilized Pt–Au nanoparticles exhibited higher activity and
increase in size, the reactions demanding large ensembles were more stable than the monometallic particles in naphthalene
of atoms as active sites are affected most. The concept of hydrogenation.20 Pd–Pt and Pd–Au bimetallic nanoparticles with
structure-sensitive reactions was introduced in 1960s.6,7 Since surfaces fully covered by Pd atoms showed the highest activity,
then, they have attracted considerable attention, both in funda- which was greater than Pd nanoparticles themselves.21 It is very
mental and applied catalytic science. For example, an increase important that the addition of the second metal does not simply
of Pd nanoparticle size from 1.7 to 4.2 nm leads to a 15 fold ‘‘add’’ its catalytic activity but often leads to synergism between
increase of the turnover frequency in 1-butyne hydrogenation;8 the two metals. For example, in selective aerobic oxidation of
8 nm Pd nanoparticles show 14% higher selectivity to alkenes crotyl alcohol, PVP-stabilized Au–Pd bimetallic nanoparticles with
in alkyne hydrogenations than 2 nm nanoparticles.9 Mean- precisely controlled structure, i.e., Pd-rich surfaces, showed
while, 2.0 nm Pt particles exhibit 90% selectivity to the RO significantly enhanced activities and higher selectivity than
product n-butylamine vs. 70% selectivity of 1.5 nm particles in co-reduced Au–Pd and their monometallic forms.22
the ring opening of pyrrole.10 In the ring opening of methyl-
Catalytic selective ring opening in heavy oil upgrading has
cyclopentane on Pt, for the particles below 2 nm n-hexane, been studied using both single-ring compound models like
2-methylpentane, and 3-methylpentane are formed as 2 : 2 : 1, methylcyclopentane (MCP) and methylcyclohexane, and multi-
while on larger particles no n-hexane formation took place.11 ple-ring compound models such as indan, decalin, tetralin,
The brutal rupture corresponds to a cubooctahedron with four naphthalene, phenanthrene, etc. These compounds were found
atoms on the edge, the smallest structure allowing the for- in diesel after different levels of hydrogenation.23 Among them,
mation of a reactive species diadsorbed on two contiguous- the ring opening of MCP over supported metal catalysts has
edge.12 A study on the methylcyclohexane ring opening over Ir been most extensively studied. In the study on the conversion
showed that RO selectivity is 5% for 1 nm Ir nanoparticles and of MCP over Pt, Ir and Pt–Ir catalysts, researchers have found
40% for larger particles.13 Different crystal surfaces of Ir show that Ir/g-Al2O3 is the most active catalyst, while MoO2 support
activities in the cyclopropane ring opening that differ by a ratio provides the highest selectivity.24,25 Some noble metal free
of up to ten.14 As seen, the fine-tuning of active metal nano- catalysts, such as Mo, Fe, and molecular sieves, were also found
particles size (i.e., its surface structure) can lead to dramatic to be active in the MCP conversion.26,27 As shown by Gilson
changes in the catalytic behavior of the very same metal.
et al., monofunctional acid and metal catalysts were applied in
Special attention in the preparation of size-controlled parti- the conversion of methylcyclohexane (MCH), another simple
cles is paid to bimetallic systems.15 Toshima et al. have made a mono-naphthenic molecule and it was found that the mono-
great contribution to the synthesis of bimetallic nanoparticles functional metal (Ir) catalysts are better suited than acid solids for
with precisely controlled composition and size using PVP.3,16,17 LCO upgrading in terms of CN improvement.28 The metal–acid
Theoretically, bimetallic particles may form a random alloy, balance is the key parameter for optimum performance in the
cluster-in-cluster, core–shell, and inverted core–shell structures conversion of MCH. This bifunctional catalyst may lead to more
with hetero- and homo-bonds.3 Electronic and geometric prop- effective Ir based catalysts for the RO of other model compounds,
erties of the surface atoms will be strongly affected by the such as decalin, tetralin, and alkyl-naphthalenes.29,30
structure mode and nanoparticle size. Such properties underlie
Although many studies used catalytic RO of MCP as the
the chemisorption strength and mode of catalytic reaction model compound, its structure is not close enough to real
substrates and deposits (sulfur, coke); thus, by changing precisely industrial feed molecules; thus, its use as a probe molecule to
the atom position and its surroundings, one can control their study the heavy oil upgrading can be questioned.28 A multiple-ring
catalytic properties on an atomic level.
compound model, indan (benzocyclopentane), has a structure
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and properties that are closer to the petroleum real feed the five-member rings. However, acid catalysts always favor
molecules than the commonly used MCP.31 The desired ring excessive cracking reactions and are not good options for
opening products are 2-ethyltoluene and n-propylbenzene, selective ring opening.38,39 In the 1980’s Gault has summarized
where the naphthenic ring has been cleaved only once, leaving that RO reactions could be proceeded over metal catalysts via
the molecular weight unchanged. Among them, the formation three different mechanisms: dicarbene, p-adsorbed olefin, and
of n-propylbenzene is preferable from the viewpoint of the metallocyclobutane reaction paths. Dicarbene reaction path
cetane number improvement. Due to the consecutive dealkyla- results in the cleavage of unsubstituted secondary–secondary
tion, further stripping of hydrocarbon fragments occurs irre- C–C bonds; thus, producing highly branched isoparaffins with
vocably and products such as o-xylene, ethylbenzene, toluene, low CNs (high ONs). Both p-adsorbed olefin and metallacyclo-
benzene, and lights are formed.31 The ring opening of indan butane reaction pathways result in C–C cleavage at substituted
over bimetallic Pt–Ir catalysts at 325 1C and atmospheric positions; therefore, the latter two paths are desirable for
pressure was previously studied by Boutonnet et al. Pt–Ir enhancing CNs, as the main consequence of cleaving a sub-
bimetallic catalysts were synthesized from a microemulsion stituted C–C bond is the elimination of molecular branching.11
system. Their catalysts showed superior activities for Ir and Pt–Ir Metal nature, nanoparticle size and support could affect the
bimetallic catalysts as compared to Pt catalyst. Although Pt–Ir ring opening path. For example Gault et al. have reported that
catalysts showed fast deactivation at atmospheric pressure, a highly dispersed Pt catalysts favored C–C bond cleavage via a
better operating stability with no deactivation at high pressure p-adsorbed olefin, whereas Pt catalysts of low dispersion follow
(40 bar) was obtained. The indan conversion increased as the Ir the dicarbene path.11 p-adsorbed olefin mode requires flat
content increased in the bimetallic catalysts.32 Later, Boutonnet adsorption of three neighboring atoms, while dicarbene mode
et al. also studied the RO of indan over Pt–Ir bimetallic catalysts requires metal–carbon bonding on two contiguous metal
supported on seven different materials, and found that 2 wt% atoms, with the molecule adsorbed perpendicular to the
Pt5Ir95/CeO2 is the best catalyst for selective RO of indan.31 surface.23 Ir catalyst did not show the same size effect as Pt
However, no size or structure control was achieved, as the catalyst did, but strong support effect has been reported by
catalysts were prepared via traditional impregnation of the Resasco et al. When Ir was supported on SiO2, the reaction
supports with metal precursors followed by reduction.
followed the dicarbene mode and resulted in low CN; whereas
RO reactions of model compounds with two fused rings, on Ir/Al2O3 catalysts, the preferred reaction path was metallo-
such as decalin, have been mostly reported for acid catalysts, cyclobutane mode that cleaves C–C bonds at substituted posi-
mainly zeolites, and follow the mechanism comprising proto- tions for products of high CN.38 Moreover, the addition of a
lytic dehydrogenation, skeletal isomerization, b-scission, and second metal could also lead to different RO mechanisms than
hydride transfer. The resultant products over acid catalysts are the existing metal. Resasco’s group has found that both Ni and
branched and low in molecular weight because of excessive K are potential promoters to inhibit the secondary hydrogenolysis
cracking. Thus, these are not potential catalysts for selective on Ir/Al2O3 catalysts.33 A variety of factors affect the ring opening
ring opening and cetane number improvement.28,33,34 Murzin positions, i.e., substituted C–C bonds (high CNs) or unsubstituted
et al. also summarized the drawbacks of monofunctional C–C bonds (high ONs), such as the type of catalyst used (acid or
catalysts. High Brønsted acidity favored excessively cracking metal or bifunctional), metal particle size, type of support, as well
reactions regardless of the zeolite structure.35 The zeolites were as the model compound that has been studied.28,38
modified with noble metals like Pt and Ir; the interactions
A variety of transition metals were evaluated in metal-
between Pt and Brønsted acid sites reduced the strength of the catalyzed RO reactions. As an example, one of the previous
Brønsted acid and less cracking products were formed.36 Later, studies used 11 different metals to test the ring opening of
they reported that the high selectivity to RO products could be MCP. Among the monometallic catalysts tested, Ir was the most
obtained only on mildly acidic Ir-modified zeolite catalyst. The active and selective catalyst.40 In order to obtain bimetallic
´
importance of the metal–acid balance was also stressed by catalysts leading to selective RO, Marecot et al. studied the RO
Daage et al. in the study of a variety of model compounds ring of MCP over several Pt-based bimetallic catalysts. Although Pt
opening over metal catalysts deposited on the supports with was the least active catalyst for the RO of MCP as compared to
different acidity. Ir was also reported as the most active and other monometallic catalysts, Pt–Rh bimetallic catalyst allowed
selective catalyst to cleave unsubstituted bonds, while Pt was increasing RO activity and selectivity, which are similar to those
able to break substituted C–C ring bonds.37
of Ir catalysts.41 In another example, Ru–Pt catalyst with Ru
In terms of the cetane number CN improvement for diesel, excess on the surface resulted in a synergism factor of 4 in a
only selective RO at substituted C–C bonds will lead to the CN ring opening reaction.42 Moreover, catalytic activity also
increase. When unsubstituted bonds are cleaved, highly depends on the metal composition of the bimetallic nano-
branched molecules are produced, which increase octane particles. It has been found that Pt(core)Pd(shell) bimetallic
number ON.38 As seen, six-member rings of single- or multi- catalyst with a Pt/Pd molar ratio of 1/4 was the most active
ple-ring compounds are more difficult to rupture than a five- catalyst for the hydrogenation of 1,3-cyclooctadiene to cyclo-
member ring. Acid function is required to isomerise the octene.16 From the sulfur resistance point of view, greater
six-member structures to five-member rings (ring shrinkage) resistance of Pt–Pd bimetallic catalyst (as compared to their
before opening; and then either acid or metal catalyst can open monometallic counterparts) to poisoning by sulfur compounds
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was confirmed by the study involving the hydrogenation of At room temperature, Ru(NO)(NO3)3 and PVP (MW 10 000) were
toluene and naphthalene in the presence of 1200 ppm dissolved in 100 mL EG in a 250 mL 3-neck flask under stirring.
dibenzothiophene.43
The concentration of metal ions was 1.12 mM, and the molar
This study is focused on the development of Ru-containing ratio of PVP/metal was 10/1. The mixture was stirred and
catalysts without expensive and unstable Pt component found refluxed under air. The reduction temperature was increased
in industrial catalysts for SRO. The special interest in ruthe- from room temperature to the reflux point of ethylene glycol
nium stems from the fact that RuS2 is the most active hydro- (198 1C), and then maintained at 198 1C for 3 hours. After the
desulfurization catalyst44 (an order-of-magnitude more active reactions, transparent dark-brown colloidal dispersions of Ru
than conventional sulfided Mo), so its high activity in the sulfur nanoparticles were obtained without any precipitates.
feed is anticipated. The current study is a preliminary investi-
PVP-stabilized Pd nanoparticles were synthesized by
gation of Ru ring opening activity and selectivity in hydro- Miyake’s one-step alcohol (ethanol/water system) reducing
genolysis of model compound indan in a sulfur-free feed method.46 To synthesize PVP-stabilized Pd nanoparticles,
aiming to get insight into Ru SRO performance, as well as to 50 mL of 2.0 mM PdCl2 aqueous solution was prepared by
find some bimetallic compositions with improved catalytic diluting PdCl2 solution (5% w/v) with milliQ water (resistivity
properties. The main distinctive feature of our work as com- 18.2 MO cm). A mixture of 50 mL 2.0 mM PdCl2 solution, 167 mL
pared to other studies on Ru performance in ring opening ethanol/water solution ([ethanol] = 40 vol.%), and 0.111 g of PVP
reactions42 is the precise control over the nanoparticles’ size (MW 40000) (molar ratio of PVP/Pd = 10) was stirred and refluxed
using a PVP stabilizer. Contrary to the polydispersed samples in a 250 mL 3-neck round bottom flask for 3 hours under air.
with mean sizes, as reported in the literature, the ability to Transparent dark-brown colloidal dispersions of Pd metal nano-
produce and evaluate monodispersed nanoparticles may pro- particles were obtained without any precipitate.
vide unique information on the performance of Ru-containing
The size control of PVP-stabilized Ir nanoparticles has been
nanoparticles of a definite size, which is especially important barely studied. Therefore, Miyake’s alcohol reduction method
for structure-sensitive reactions. As follows from the literature for Pd nanoparticles (as described above) was applied to
review above, particle size no less than 1 nm is preferable synthesize PVP-stabilized Ir nanoparticles.46 In order to get a
to ensure high selectivity. We also evaluated Ir and Pd as better understanding of the size control of bimetallic Ru–Ir and
co-metals in bimetallic compositions with Ru, as Ir is known Ru–Pd nanoparticles, monometallic Ir and Pd nanoparticles
to have high RO activity and Pd is an excellent hydrogenation were also prepared by Li’s EG reduction method used for Ru
catalyst. The majority of the reported catalysts are within a nanoparticles as described above, which would be applied for
2 nm range with narrow size distributions; thus, their catalytic the syntheses of Ru-based bimetallic nanoparticles later.
behavior can be directly correlated to the metal nature without
PVP-STABILIZED RU-BASED BIMETALLIC NANOPARTICLES. Ru–Ir and
any size effects. The study may pave the way to the development Ru–Pd bimetallic nanoparticles with so-called alloy structures
of Pt-free Ru-based catalysts with high stability in sulfur feed were synthesized by a simultaneous reduction of both Ru and Ir
for ultra-deep hydrodesulphurization.
(or Pd) precursors using the synthesis method of monometallic
Ru nanoparticles (PVP/metals molar ratio of 10/1).45 For each
kind of bimetallic structure, three different metal molar ratios
were synthesized: Ru4Ir1 (Ru/Ir molar ratio = 4/1), Ru2Ir1,
Ru1Ir4, Ru4Pd1, Ru1Pd1, and Ru1Pd2. The reduction tempera-
2. Experimental section
2.1. Materials
Ruthenium(III) nitrosylnitrate (Ru(NO)(NO3)3, Alfa Aesar), ture for Ru1Pd2 was 160 1C instead of 198 1C, while other
hydrogen hexachloroiridate(IV) hydrate (H2IrCl6, 99.98%, conditions were kept the same. After reactions, transparent
Sigma-Aldrich), palladium(II) chloride solution (PdCl2, 5% w/v, dark-brown homogeneous colloidal solutions of Ru–Ir and
Acros), poly-(vinylpyrrolidone) (PVP) (MW: 10000 and 40 000, Ru–Pd nanoparticles were obtained without any precipitate.
Sigma-Aldrich), ethylene glycol (EG, 99.8%, Sigma-Aldrich),
NANOPARTICLE DEPOSITION. PVP-stabilized monometallic Ru, Ir
gamma aluminum oxide (g-Al2O3, 150 mesh 58 Å pore size, and Pd, and bimetallic Ru–Ir and Ru–Pd colloidal solutions
Sigma-Aldrich), reagent alcohol (ethanol, 95 vol.%, Fisher Scien- were dialyzed against 3000 mL milliQ water in a 28 mm dialysis
tific), and acetone (99.7%, Fisher Scientific) were used as tube (3500 MW cut-off) twice (48 hours each time) to remove EG
received. Argon and hydrogen of ultra-high purity 5.0 and unreduced metal salts. Synthesized mono- and bimetallic
(99.999%) were purchased from Praxair. MilliQ water was used nanoparticles were deposited on the support for the future
throughout the work. Pt–Ir/Al2O3 is an industrial catalyst; due to catalytic reactions. Nanocatalyst support, g-Al2O3, was dried in
data confidentiality, no physico-chemical characteristics are an oven at 120 1C for more than 12 hours. PVP-stabilized
presented for this catalyst, apart from its catalytic activity and nanoparticles were deposited on g-Al2O3 by incipient impreg-
selectivity.
nation. Finally, all the catalysts were dried in a fumehood.
2.2. Preparation of catalysts
2.3. Colloids and catalyst characterization
PVP-STABILIZED MONOMETALLIC RU, IR, AND PD NANOPARTICLES. PVP-
UV-VIS SPECTROSCOPY (UV-VIS). Formation of metal nanoparticles
stabilized monometallic Ru nanoparticles were synthesized in their colloidal dispersions was confirmed with a Varian Cary 50
by Li’s one-step reduction method in ethylene glycol (EG).45 Scan UV-vis spectroscopy using a quartz cell (1 cm).
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TRANSMISSION ELECTRON MICROSCOPY (TEM). As-prepared metal nanoparticles at a lower temperature than free PVP decomposi-
nanoparticles were characterized by TEM using a JEOL 2100 tion according to a Somorjai et al.’s study. In their study, the
transmission electron microscope operating at 200 kV. Samples PVP-containing catalysts were calcined in 20% O2/He for 1 h at
for TEM were prepared by placing a drop of the colloidal 200 1C, purged with helium for 0.5 h at 200 1C, and treated
dispersion of metal nanoparticles onto a carbon-coated copper in H2 for 1 h at 200 1C. Somorjai et al. also confirmed that
grid, followed by evaporating the solvent at room temperature. ethylene hydrogenation activity was maximized with an in situ
Samples were prepared immediately after nanoparticle synth- oxidation–reduction cycle at 200 1C.47
esis. Mean diameters (d) and standard deviations (s) of over
200 particles were determined using ImageJ software.
EXPERIMENTAL SETUP AND CONDITIONS. Ring opening of indan was
carried out in a fixed bed reactor, which is a 1600 long stainless
X-RAY PHOTOELECTRON SPECTROSCOPY (XPS). X-ray photoelectron steel tube with an inner diameter of 1/200, was packed with the
spectra were obtained using a Kratos Axis 165 X-ray photo- catalyst corresponding to 1.2 mg active metal(s) (diluted with
electron spectrometer with a mono Al Ka source operated at 2 g 150 mesh SiC) and encircled by a furnace. The reactant,
15 mA and 14 kV. The survey spectrum and the high resolution indan, was fed into the catalytic system by bubbling
spectrum were scanned with pass energies of 160 eV and 20 eV, 50 mL minꢀ1 H2 through indan at a constant temperature bath
respectively. During the acquisition of a spectrum, charge at 10 1C. Indan flow rate of (2.0 ꢁ 0.2) ꢂ 10ꢀ7 mol minꢀ1 was
neutralization was applied to compensate the insulating confirmed using GC, which was calibrated with an HPLC pump
problem of the sample. CASAXPS software was used for the with a known indan flow rate. A high H2-to-indan molar ratio
data processing. The samples were as-prepared catalysts after (4800–11 600) was used to avoid coke formation. The 99.999%
calcination-hydrogenation pretreatment as described below. pure Ar and H2 flows were controlled by calibrated mass flow
Binding energies were referenced to C (1s) (284.7 eV).
controllers (Sierra Instruments). The catalytic reactions were
ION SCATTERING SPECTROSCOPY (ISS). Ion scattering spectra were carried out at an internal temperature of 336 1C and atmo-
obtained using a Kratos Axis 165 X-ray photoelectron spectro- spheric pressure. Selective ring opening was studied at low
meter equipped with an ion scattering spectrometer, operated pressure, as low pressure promotes the RO route vs. the hydro-
using noble gas ion beams (He+) at 1 keV and scattering angle genation route at high pressure (40 bar).31 The reactor up- and
of 135 degrees. ISS of metallic silver foil was used to reference down- streamlines were heated to 220 1C to preheat reactants
the energy scale. Catalysts were prepared with expected metal and avoid product condensation. The outgoing stream was
loading of 1 wt%, as ISS detector requires relatively high analyzed online with a Varian 430-GC-FID every 30 minutes
metal loading for powdered samples. Prior to ISS analysis, after the reaction was started. The GC capillary column is a
samples underwent the calcination-hydrogenation pretreatment WCOT fused silica column, 50 m length ꢂ 0.32 mm inside
as described below.
diameter ꢂ 1.2 mm thickness. Initially, the oven temperature
ATOMIC ADSORPTION SPECTROSCOPY (AAS). The actual loadings of was stabilized at 40 1C for 2.5 min; and then it started to
Pd and Ru nanoparticles on g-Al2O3 were determined using increase at a rate of 30 1C minꢀ1 until the temperature reached
a SpectrAA 220FS Atomic Absorption Spectrometer (Varian). 110 1C and maintained at 110 1C for 20 min. FID and injector
Samples for AAS were prepared by dissolving a certain amount temperature were 280 1C. The split ratio was 1. Helium flow rate
of catalysts in hot concentrated nitric acid (or aqua regia when was constant at 25 mL minꢀ1
.
necessary) followed by filtration through a Whatman filter
paper and then quantitative dilution.
REACTION PRODUCT IDENTIFICATION. In the previous study on ring
opening of indan over Pt–Ir catalysts, Boutonnet et al. reported
NEUTRON ACTIVATION ANALYSIS (NAA). The actual loadings of Ir, that the major ring opening products are 2-ethyltoluene,
Ru–Ir, and Ru–Pd on g-Al2O3 were determined by NAA at the n-propylbenzene, o-xylene, ethylbenzene, toluene, and benzene,
Slowpoke facilities at the University of Alberta or Becquerel which were confirmed in our study.31 In order to identify the
Laboratories Inc. in Mississauga, Ontario. Samples were GC peaks in the current study, retention times of major ring
irradiated for 110 s in the Cd shielded, epi-thermal site of the opening products were compared with those of the pure
reactor core (McMaster University). They were counted for reference compounds. Compounds with GC retention times
30 min each on an Aptec CS11–A31C gamma detector, approxi- less than that of benzene were named as ‘‘lights’’. The desired
mately 12 h after irradiation.
products of ring opening of indan are 2-ethyltoluene and
n-propylbenzene, where the naphthenic ring has been cleaved
only once. Further dealkylation, which results in toluene,
2.4. Low-pressure ring opening of indan
CATALYSTS PRETREATMENT. Prior to the catalytic reactions, poly- benzene and lights, is highly undesired.31
mers must be removed, as PVP could block the active sites of
The reactant (indan and impurities), major reaction pro-
the catalysts. PVP-stabilized Ru, Ir, Pd, Ru–Ir, and Ru–Pd ducts, and by-products were confirmed by GC-MS. GC-MS was
catalysts were calcined at 200 1C in an oven under air for performed with an Agilent Technologies 7890 GC coupled with
1 hour. Catalysts were then reduced in situ at 359 1C for 1 hour 5975C MSD. The GC column used is a ZB-50 (Phenomenex)
under hydrogen flow (80 mL minꢀ1) by increasing the reactor column, 30 m length ꢂ25 mm i.d. ꢂ25 mm thickness. Oven
internal temperature from room temperature to 359 1C at a rate temperature was stabilized at 40 1C for 0.5 min, then increased
of 5 1C minꢀ1, and then holding for 1 hour. This oxidation– to 110 1C at 30 1C minꢀ1, and then increased to 280 1C at
hydrogenation cycle is known to remove PVP from metal 50 1C minꢀ1
.
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For the online product analysis, after steady state was achieved 9 moles of methane may be produced per mole of indan. The
(at 150 min time on stream), no more than 5% deviation in the FID detector is a mass-sensitive analyzer that responds to
mass balance was observed (typically, within 2%) as compared to the number of carbon atoms entering per unit of time; thus,
the mass flow of incoming indan. Raw GC results were corrected for each RO opening product, the selectivity was determined
for indan impurities, and all calculations for indan conversion and as the GC area of each product dividing by the difference
product selectivities were based on the corrected GC results. Indan between total GC area and the GC area of indan. For most
purified by distillation contains 0.09% benzene, 0.15% n-propyl- of the catalysts, 8 (or 6) data points were obtained at 150, 180,
benzene, 0.02% 2-ethyltoluene, 0.02% lights, and 2.91% other.
210 (and 240) minutes of time on stream with a duplicate
Selectivities are reported on a mass basis as molar selectivity experiment (unless stated otherwise). One standard deviation
can give a distorted picture of indan utilization because up to never exceeded 10% and was typically within 3%. Within the
Fig. 1 TEM images of PVP-stabilized monometallic Ru, Ir and Pd colloids and corresponding size distribution histograms: (a) Ru (EG), (b) Ir (ethanol/water) and (c) Pd
(ethanol/water).
Fig. 2 TEM images of PVP-stabilized (a) Ru/g-Al2O3, (b) Ir/g-Al2O3, (c) Pd/g-Al2O3 catalysts; and (d) Ru/g-Al2O3 after RO reaction, (e) Ir/g-Al2O3 after RO reaction, and
(f) Pd/g-Al2O3 after RO reaction (336 1C, 6 hours on stream).
c
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indicated times on stream the catalysts did not show noticeable
deactivation.
Fig. 1 shows TEM images of monodispersed PVP-stabilized
Ru, Ir and Pd nanoparticles with mean diameters of 2.3 nm,
1.6 nm, and 2.3 nm, respectively. With the applied synthesis
methods, all three kinds of monometallic nanoparticles show
narrow size distributions. TEM images of supported nanocatalysts
also show that Ru, Ir, and Pd nanoparticles were dispersed evenly
on g-Al2O3 (Fig. 2(a)–(c)). Fresh and used Ru and Ir catalysts (after
RO reaction for 4 hours on stream at 336 1C) showed no
significant sintering, but sintering could be observed on portions
of the used Pd catalysts (Fig. 2(d)–(f)).
Fig. 3 shows TEM images of PVP-stabilized Ru–Ir and Ru–Pd
bimetallic nanoparticles and corresponding size distribution
histograms. With the same synthesis conditions, an average
mean diameter of 2.0 nm was obtained among all Ru–Ir
bimetallic nanoparticles (Fig. 3 (a)–(c)). This result is consistent
with the sizes of monometallic Ru and Ir nanoparticles, which
range from 1.6 nm to 2.3 nm. For Ru–Pd bimetallic nano-
particles, the particle size was affected significantly by the
3. Results and discussions
3.1. Catalyst synthesis and characterization
Monometallic Ru, Ir, Pd and bimetallic Ru–Ir and Ru–Pd
nanoparticles were synthesized by the ethylene glycol reduction
method at the same reduction conditions in the presence
of PVP. Ir and Pd nanoparticles were also prepared by the
alcohol reduction method. Nanoparticles sizes and size distri-
butions were determined by TEM; the formation of metal
clusters was confirmed by UV-visible spectroscopy; the form-
ation of the Ru–Ir and Ru–Pd bimetallic structures was
confirmed by XPS, while with ISS the presence of both metals
was confirmed at the bimetallic nanoparticle surface; and the
loadings of supported nanocatalysts were determined by either
AAS or NAA.
Fig. 3 TEM images of PVP-stabilized bimetallic Ru–Ir and Ru–Pd colloids (the scale bar is 20 nm) and corresponding size distribution histograms: (a) Ru4Ir1, (b) Ru2Ir1,
(c) Ru1Ir4, (d) Ru4Pd1, (e) Ru1Pd1, and (f) Ru1Pd2.
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Fig. 4 UV-vis spectroscopy of PVP-stabilized Ru–Ir and Ru–Pd nanoparticles and the precursors, Ru(NO)(NO3)3, H2IrCl6 and PdCl2.
molar ratio of the two metals, i.e., with the same synthesis to metal ions disappear, as shown in Fig. 4. All these are
conditions (except Ru1Pd2 synthesized at 160 1C) the more the evidence of the formation of PVP-stabilized Ru–Ir and Ru–Pd
Pd in the bimetallic structure, the larger the mean diameter of nanoparticles.46,48
nanoparticles (Fig. 3 (d) and (e)). Fig. 3 also shows that the
Metal loadings for the synthesized supported catalysts deter-
mean diameter of Ru1Pd2 is 5.3 nm; such a large particle size mined by either AAS or NAA are shown in Table 1 (NAA was
could be due to both a higher Pd ratio in the bimetallic applied as Ir cannot be dissolved in aqua regia). The target
structure and a lower reduction temperature (160 1C). Unlike loading was 0.3 wt%. Lower than desired loadings are most
Ru, monometallic Pd nanoparticles synthesized under the likely due to the incomplete reduction of metal precursors that
same experimental conditions as Ru–Pd bimetallic nano- were removed during the catalyst washing procedure.
particles (EG reduction) have a relatively large particle size of
Prior to the catalytic reactions, the active sites of nano-
7.1 nm. Particles sizes of PVP-stabilized Ru–Pd bimetallic particles must be made available for chemisorption, so the
nanoparticles show consistency with their monoforms, i.e., all catalysts were subjected to oxidation–hydrogenation pretreat-
bimetallic nanoparticles are in the range of 2.3 nm (Ru) and ment to remove PVP. According to the XPS results of the
7.1 nm (Pd).
pretreated catalysts (Table 2), the mass ratios of metal-to-
The formation of Ru–Ir and Ru–Pd nanoparticles was con- nitrogen show 93–99% removal of PVP as compared to the
firmed by UV-vis spectroscopy. The solutions containing metal ratio used in the synthetic procedures.
ions turned from yellow to dark brown after reflux was
The binding energies of Ru (3d5/2), Ir (4f7/2), and Pd (3d5/2)
achieved. After a 3 hour reduction, the absorbance from ultra- for the metals in monometallic colloids were 280.2 eV, 61.1 eV,
violet to visible region increases and the peaks corresponding and 335.4 eV, respectively (Table 3, Fig. 5), and were in good
agreement with those for the corresponding pure metals.49 Two
other Ru chemical states, RuO2 and RuO3, can be identified
from the Ru (3d5/2) binding energy at 281.2 and 283.2 eV (for
Table 1 Catalyst loadingsa on g-Al2O3 determined by AAS or NAA
the monometallic Ru sample), respectively; PdO can be identi-
fied from the Pd (3d5/2) binding energy at 336.5 eV (for the
Sample designation Ru loading wt% Ir loading wt% Pd loading wt%
Mono-Ru
Mono-Ir
Mono-Pd
Ru4Ir1
Ru2Ir1
Ru1Ir4
Ru4Pd1
Ru1Pd1
Ru1Pd2
0.16
—
—
0.06
0.04
0.02
0.07
0.08
0.04
—
0.18
—
0.03
0.04
0.16
—
—
—
0.08
—
—
monometallic Pd sample). The binding energies of oxidized Ru
and Pd show good agreement with literature data as well.50–52 It
should be noted that the XPS analyses were performed on the
samples exposed to air (room temperature) for about 2 weeks;
hence Pd and Ru surface oxidation was possible. Previous study
on PVP-Pd nanoparticles showed that Pd0 content decreased
from 82% to 25% after 3 months’ exposure to the air at room
temperature.53
—
0.02
0.07
0.10
—
—
a
Desired loading was 0.3 wt% for all synthesized catalysts.
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Table 2 Catalyst compositions as determined by XPS (in mass concentration)
Ru
5.9
—
—
40.9
43.7
9.3
0.1
0.2
—
Ir
Pd
Ru4Ir1
Ru2Ir1
Ru1Ir4
Ru4Pd1
Ru1Pd1
Ru1Pd2
Ru
Ir
Pd
Al
O
C
Cl
N
—
13.5
—
36.0
38.5
10.8
0.4
0.7
—
—
—
10.3
6.4
—
35.2
38.0
8.9
0.5
0.3
0.5
—
10.8
7.7
—
34.2
36.2
10.0
0.0
0.3
0.4
0.5
1.2
17.6
—
34.5
36.7
8.7
0.8
0.3
0.3
—
13.6
—
2.2
35.3
35.5
11.5
0.3
0.3
0.4
0.8
16.7
—
8.3
29.1
32.0
12.2
0.3
0.5
0.4
0.6
5.4
—
16.6
30.3
37.0
14.0
0.6
1.5
—
9.3
33.9
36.2
14.5
0.4
0.3
0.2
0
Na
F
—
—
—
Table 3 Binding energies of Ru (3d5/2), Ir (4f7/2), and Pd (3d5/2) of mono- and
bimetallic nanoparticles supported on g-A2O3 after oxidation–reduction
pretreatment. All peak positions were referenced to C (1s) peak at 284.7 eV
In Ru–Ir bimetallic systems the binding energy of Ir0 (4f7/2
)
shifts to lower values, suggesting electron transfer from Ru to
Ir, which correlates with the electron affinity values of both
metals: the electron affinity of Ir (112 kJ molꢀ1) is higher than
that of Ru (101 kJ molꢀ1), indicating electron-accepting proper-
ties of Ir. Besides, a higher proportion of Ru is in its zero-
oxidation state as compared to mono-Ru of the same B2 nm
size probably because less Ru is located on the surface of
nanoparticles; thus, lower Ru portion is oxidized (Fig. 6 and
7, Table 3).
When Pd was co-reduced with Ru for the Ru–Pd catalyst
preparation, no oxidized Pd was found, as in the case of
monometallic Pd nanoparticles, indicating the absence of
monometallic nanoparticles in the bimetallic Ru–Pd structures
Binding energies (eV)
Zero-oxidation state
Catalysts Ru0 (3d5/2
)
Ir0 (4f7/2
)
Pd0 (3d5/2
335.4
)
Ru
Ir
Pd
Mono-Ru 280.2
Mono-Ir
Mono-Pd
31.5%
61.1
100%
58.6%
Ru4Ir1
Ru2Ir1
Ru1Ir4
Ru4Pd1
Ru1Pd1
Ru1Pd2
279.8
279.9
280.0
279.8
279.7
279.8
60.5
60.8
60.7
49.7% 100%
63.1% 100%
56.0% 100%
60.1%
65.3%
68.9%
335.3
335.1
335.1
100%
100%
100%
Fig. 5 X-ray photoelectron spectra of monometallic Ru, Ir, and Pd nanoparticles supported on g-A2O3.
Fig. 6 X-ray photoelectron spectra of the Ru 3d peak for three different Ru–Ir bimetallic nanoparticles supported on g-A2O3 (from left to right: Ru4Ir1, Ru2Ir1, and Ru1Ir4).
c
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Fig. 7 X-ray photoelectron spectra of the Ir 4f peak for three different Ru–Ir bimetallic nanoparticles supported on g-A2O3 (from left to right: Ru4Ir1, Ru2Ir1, and Ru1Ir4).
Fig. 8 X-ray photoelectron spectra of the Pd 3d peak for three different Ru–Pd bimetallic nanoparticles supported on g-A2O3 (from left to right: Ru4Pd1, Ru1Pd1, Ru1Pd2).
Fig. 9 X-ray photoelectron spectra of the Ru 3d peak for three different Ru–Pd bimetallic nanoparticles supported on g-A2O3 (from left to right: Ru4Pd1, Ru1Pd1, Ru1Pd2).
(Fig. 8 and 9 and Table 3). The relative amount of unoxidized alloyed with Pd. These findings confirm the formation of
Ru increased twice, which could be in part due the larger size of bimetallic Ru–Ir or Ru–Pd nanoparticles instead of a physical
Ru1Pd1 and Ru1Pd2 colloids (3.6 and 5.3 nm, respectively) as mixture of monometallic Ru and Ir (or Pd) nanoparticles.
compared to monometallic Ru particles (2.3 nm). The electron
Ion scattering spectroscopy was performed on the mono-
affinities of Ru and Pd are 101 and 54 kJ molꢀ1, respectively, metallic and bimetallic catalysts and confirmed the presence of
indicating the electron transfer from Pd to Ru, which correlates both metals in the top surface layer of the bimetallic formula-
with the XPS data: Ru0 BE is shifted toward lower values when tions (Fig. 10). The results quantification is limited by the
c
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Fig. 10 Ion scattering spectra of Ru–Ir and Ru–Pd bimetallic, as well as mono-Ru, mono-Pd and mono-Ir nanoparticles supported on g-A2O3.
uncertainty of the inelastic losses and the neutralization rate are even less accurate than for Ru–Ir catalysts due to Ru and Pd
depending on ion trajectories; however, we estimated the ratios peaks overlapping, but they still correlate with the observed
of peak areas under the deconvoluted peaks to roughly assess the catalytic activity, when the behavior of Ru4Pd1 catalyst was closer
abundance of each metal on the nanoparticle surface. For the to the Pd catalyst behavior than to that of the monoRu, regardless
Ru4Ir1, Ru2Ir1 and Ru1Ir4 catalysts the area ratios were 3/1, 2/1 of the high bulk Ru content: according to the ISS results, the Ru–
and 1/1, respectively, indicating the abundance of Ru in the Pd catalysts are characterized by the Pd abundance in the
nanoparticle shell, which correlates well with the observed selec- nanoparticle shell. This is also in line with the XPS results, when
tivities discussed below. For Ru4Pd1, Ru1Pd1 and Ru1Pd2 cata- the proportion of unoxidized Ru in the bimetallic catalysts
lysts the area ratios were 1/2, 1/1 and 1/1, respectively. These ratios decreased as compared to the monometallic Ru.
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Fig. 11 Catalytic activities and conversions vs. time on stream for monometallic and bimetallic catalysts in indan ring opening (336 1C; 1 atm; 1.2 mg of total active
metal loading).
3.2. Catalytic behaviour in the ring opening of indan
Pd/Al2O3 catalysts with different palladium contents (thus,
nanoparticle sizes) in the hydrogenolysis of MCP.54
The comparison of activities and conversions among synthe-
sized catalysts and industrial Pt–Ir catalyst is shown in Fig. 11;
Table 4 shows the product selectivities. The desired products
are 2-ethyltoluene and n-propylbenzene, when the C5-ring of
indan is cleaved only once. In terms of cetane number improve-
ment, n-propylbenzene formation is required.
Monometallic Ir nanocatalyst with a mean diameter of
1.6 nm is the most active catalyst, it also showed 1.4-fold higher
activity than the industrial Pt-containing catalyst. The mono-
metallic Pd catalyst showed almost negligible activity, while Ru
took the intermediate position between Ir and Pd. In this study,
palladium nanoparticles sintered under the reaction conditions
(Fig. 2(f)), which also could be the reason for low catalytic
activity. Dieguez et al. reported that deactivation occurred for
The most selective catalysts towards 2-ethyltoluene and n-propyl-
benzene among the monometallic catalysts are 2.3 nm Pd and
1.6 nm Ir. It should be noted that the high selectivity of Pd is
observed at low conversions (Table 4), contrary to B50% conversion
for the Ir and Ru catalysts; however, other catalysts (Ru4Ir1, Ru4Pd1)
showed low selectivities at low conversions. In terms of the light
production, Pd takes an intermediate position between Ir and Ru
catalysts. Hydrocracking products were also found in the hydro-
genolysis of MCP over 1 wt% Pd/Al2O3.55 The iridium catalyst
displays low cracking activity, which may be seen by the relatively
low selectivities to o-xylene and lights. Boutonnet et al. have reported
high selectivity in indan ring opening over 2 wt% Ir/boehmite at
steady state.32 Another work has showed that Ir is the most active
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Table 4 Product selectivities (on mass basis, at 120 min time on stream) for indan ring opening over the synthesized catalysts. Total metal loadings were 1.2 mg of
active metals for all catalysts (except 0.6 mg for mono-Ir catalyst to ensure similar conversions for selectivity comparison). Ru1Pd1 and Ru1Pd2 are omitted as they
displayed less than 1% conversions
Selectivities/%
2-Ethyl- n-Propyl- Ethyl-
n-Propylbenzene
Catalysts
Particle size/nm Conversion/% toluene benzene benzene o-Xylene Benzene Toluene Lights yield, %
Ru
Ir
Pd
Ru4Ir1
Ru2Ir1
Ru1Ir4
Ru4Pd1
2.3
1.6
2.3
2.1
2.1
1.9
2.9
51
50
4
8
63
68
31
26
57
6
1
11
11
2
2
5
3
2
2
3
3
2
2
4
38
13
2
30
39
24
10
22
4
1
1
2
1
0
3
2
20
5
2
12
12
5
26
5
14
20
17
7
0.4
5.7
0.4
0.1
0.3
2.8
0.1
4
7
18
55
9
1
6
7
16
71
12
Industrial Pt–Ir n.a.
62
37
and selective catalyst in the ring opening of MCP.41 Also, in a study A similar result has been reported by Marecot’s group for Ru–Pt
in the ring opening of cyclohexane, 4.6 nm Ir catalyst showed high catalysts. In their study, the ring opening of MCP favored the
conversion, high selectivity to desired n-hexane, and low selectivity production of cracking products, as Ru added onto a parent
toward undesired benzene and lights.55
Pt/Al2O3 catalyst by a surface redox reaction.41 Notably, in our study,
The Ru catalyst promotes dealkylation reactions as evidenced bimetallic Ru–Pd catalysts displayed lower activities and single
by high selectivity to o-xylene and lights. Previous studies on the cleavage selectivities than monometallic Ru and Pd, indicating that
hydrogenolysis of cyclohexane over the Ru/Al2O3 catalyst show the electron transfer between Pd and Ru evidenced by XPS leads to
that Ru was less selective to desired n-hexane formation than Rh, antagonism between these two metals.
and had a high cracking selectivity.56 Kustov et al. have also
In terms of single cleavage selectivity for CN improvement, the
reported that the hydrogenolysis of cyclohexane was unselective formation of n-propylbenzene is favorable, which occurs at
over 1 wt% Ru/Al2O3 as the reaction temperature reached 280 1C, substituted positions via p-adsorbed olefin. At similar conversions
which led to a 100 wt% yield of light products (C1–C3).57 More- (50–60%), only monometallic Ir allowed relatively high n-propyl-
over, the ring opening of MCP shows that C1–C5 cracking benzene yield (Table 4), as well as Ru–Ir bimetallic particle with the
41
products were favored in the presence of 1.5 wt% Ru/SiO2.
highest Ir content. Ru in its mono and bimetallic forms showed
Although both mono-Ru and mono-Ir resulted in high catalytic excessive hydrogenolysis to o-xylene and toluene.
activities in the ring opening of indan, the combinations of these
Fig. 11 and Table 4 also contain results on the industrial
two metals are less active (Fig. 10) than their monoforms unless a Pt–Ir catalyst. The developed Ir catalyst shows both higher
very high Ir content is present (Ru1Ir4: Ru/Ir molar ratio = 1/4). The activity and single cleavage selectivity (sum of selectivities to
higher the Ir content in the alloy structure, the higher the activity of 2-ethyltoluene and n-propylbenzene), as well as higher yield of
the bimetallic Ru–Ir catalysts. A similar trend is noticed in terms of n-propylbenzene. The bimetallic Ru1Ir4 catalyst exhibits higher
selectivity (Table 4): the Ru–Ir alloys showed significantly higher single cleavage selectivity at a comparable conversion as the
selectivity to 2-ethyltoluene and n-propylbenzene than that of the industrial catalyst. Both catalysts do not contain an expensive
mono-Ru. This improvement is due to the presence of Ir metal in platinum component, and the Ru1Ir4 catalyst is believed to be a
the bimetallic structure, as it tempers the undesirable excessive promising alternative to the industrial catalyst, as Ru forms
cracking of Ru. As Ir content decreases in the bimetallic structure, sulfided active sites in the presence of sulfur in the feed,44
Ru2Ir1 and Ru4Ir1 become less selective to the desired RO products contrary to the sulfur-poisoned platinum. The catalyst perfor-
and lead to excessive cracking similar to the monometallic Ru mance should also be evaluated at high hydrogen pressure,
nanocatalyst. Relatively high o-xylene formation by the Ru1Ir4 which promotes hydrogenation route31 affecting the resulting
catalyst is most likely due to the Ru abundance on the nanoparticle cetane number. This study is currently underway.
surface as detected by ISS. It is important that the activities and
selectivities of the Ru–Ir bimetallic catalysts are compared for the sized catalysts in indan ring opening (calculated per mole of
same particle size with a narrow size distribution, i.e., B2 nm. total active metals) is in the order of Ir > Ru1Ir4 > Ru > Ru2Ir1 >
Based on the catalytic test results, the activity of the synthe-
Alloying Ru with Pd resulted in lower activities as compared to Ru4Pd1 > Ru4Ir1 > Pd > Ru1Pd1 > Ru1Pd2. Comparing only
the monoform of Ru which correlated with low activity of mono-Pd. monodispersed catalysts with the sizes of 1.9–2.3 nm and
The activity of the Ru4Pd1 catalyst was closer to the Pd catalyst omitting Pd due to its sintering, the trend is Ru1Ir4 > Ru >
activity than to that of the monoRu, regardless of the high bulk Ru Ru2Ir1 > Ru4Ir1. The selectivity to single cleavage products
content: according to the ISS results, the Ru–Pd catalysts are within the latter trend is Ru1Ir4 > Ru2Ir1 E Ru4Ir1 c Ru, with
characterized by the Pd abundance in the nanoparticle shell. How- the selectivity to lights being the lowest for Ru1Ir4. The
ever, the presence of Ru significantly reduced the selectivity: at importance of these findings is that the activities and selectiv-
similar conversions and particle sizes the Ru4Pd1 catalyst displayed ities are compared for the same particles size with narrow size
higher selectivity to lights as compared to the monometallic Pd. distribution, contrary to the polydispersed catalysts typically
c
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4. Conclusions
Monometallic and bimetallic Ru, Ir, Pd, and Ru–Pd, Ru–Ir
nanoparticles were synthesized in the presence of PVP and
deposited on g-Al2O3 for a two-phase indan ring opening at
atmospheric H2 pressure and 336 1C. TEM images show mono-
dispersed particles ranging from 2 to 5 nm with narrow size
distributions. XPS analysis of mono- and bimetallic structures
suggested that bimetallic particles of Ru–Pd and Ru–Ir were
formed instead of a physical mixture of monometallic particles;
binding energy shifts correlate with the electron affinity of the
corresponding metals. ISS analysis confirmed the presence of
both metals on the nanoparticle surface in bimetallic formula-
tions. Pd nanoparticles sintered under the reaction conditions,
while other catalysts were resistant to agglomeration. High Ir
content in both monometallic and bimetallic catalysts was
responsible for high activity and selectivity to 2-ethyltoluene
and n-propylbenzene, with Ru exhibiting higher selectivity to
o-xylene and lights. Pd showed the lowest activity, which was
improved by the addition of Ru. The study compares catalytic
properties of the monodispersed mono- and bimetallic nano-
particles with similar B2 nm sizes. Next to the monometallic Ir,
the Ru1Ir4/g-Al2O3 catalyst has superior indan ring opening
selectivity at a lower cost as compared to the industrial Pt–Ir
catalyst at a comparable activity. The study can pave the way in
the development of Pt-free Ru-containing catalysts for selective
ring opening, especially taking into consideration a possibility
of their higher S-resistance as compared to the Pt catalysts.
30 S. Lecarpentier, J. v. G. Gestel, K. Thomas, J. P. Gilson and
M. Houalla, J. Catal., 2008, 254, 49.
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¨
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