A highly stable Ru/LaCO3OH catalyst consisting of support-coated Ru nanoparticles in aqueous-phase hydrogenolysis reactions

Article abstract of DOI:10.1039/c7gc02414b

Hydrothermal reduction under aqueous conditions is widely used to convert biomass into more valuable products. However, the harsh conditions inherent in the process can irreversibly alter the intrinsic structure of the support, as well as dissolve the metal ions into the aqueous solution. In this work, for the first time we have synthesized a new highly hydrothermally stable Ru/LaCO₃OH catalyst mostly consisting of Ru nanoparticles partially encapsulated by the LaCO₃OH support with a strong metal-support interaction (SMSI), which confers high stability and activity to the catalyst under hydrothermal reduction conditions in the hydrogenolysis of the biomass model molecules guaiacol and glycerol. During impregnation, the RuCl₃·3H₂O precursor initially reacts with LaCO₃OH to form a LaRu(CO₃)₂Cl₂ complex and LaOCl. XPS demonstrated that Ru was present in the oxidized state, TEM and XRD showed the absence of Ru⁰, and the XRD pattern showed the presence of the characteristic lattice fringe of LaOCl. While the LaRu(CO₃)₂Cl₂ complex was resistant to H₂ reduction at 350 °C, the complex underwent facile reduction to Ru⁰ under hydrothermal conditions at 240 °C. In the subsequent process, LaRu(CO₃)₂Cl₂ and LaOCl underwent hydrolysis, forming crystalline LaCO₃OH (confirmed by Ag⁺ titration and XRD patterns), Ru(OH)₃, and HCl. The Ru(OH)₃ was reduced in situ to Ru⁰ nanoparticles, as revealed by XPS and TEM analysis. The simultaneous hydrothermal reduction of Ruⁿ⁺ species and the formation of crystalline LaCO₃OH result in the formation of Ru nanoparticles encapsulated by a protective LaCO₃OH layer, as evidenced by HRTEM and DRIFTS CO adsorption measurements. The preparation of catalysts with this unique structure comprising metal nanoparticles protected by the support itself, which confers additional stability, is a novel strategy to prepare hydrothermally stable catalysts.

Full text of DOI:10.1039/c7gc02414b

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ARTICLE
Highly Stable Ru/LaCO OH Catalyst Consisting of Support-coated
3
Ru Nanoparticles in Aqueous-Phase Hydrogenolysis Reactions
Received 00th xx 2017,
Accepted 00th xx 2017
a
Bolong Li, Lulu Li, Chen Zhao*
DOI: 10.1039/x0xx00000x
Hydrothermal reduction under aqueous conditions is widely used to convert biomass into more valuable products.
However, the harsh conditions inherent in the process can irreversibly alter the intrinsic structure of the support, as well
as dissolve the metal ions into the aqueous solution. In this contribution, for the fisrt time we have synthesized a new
www.rsc.org/
highly hydrothermally stable Ru/LaCO
LaCO OH support with strong metal-support interaction (SMSI), which confers high stability and activity to the catalyst
under hydrothermal reduction conditions in the hydrogenolysis of biomass model molecules guaiacol and glycerol. During
3
OH catalyst mostly consisting of Ru nanoparticles partially encapsulated by the
3
impregnation, the RuCl
3
·3H
2
O precursor initially reacts with LaCO
3
OH to form a LaRu(CO
)
3 2
Cl
2
complex and LaOCl. XPS
0
demonstrated that Ru was present in an oxidized state, TEM and XRD showed the absence of Ru , and the XRD pattern
3 2 2 2
illuminated the presence of the characteristic lattice fringe of LaOCl. While the LaRu(CO ) Cl complex was resistant to H
0
reduction at 350 °C, the complex underwent facile reduction to Ru under hydrothermal conditions at 240 °C. In the
+
subsequent process, LaRu(CO
3
)
2
Cl
2
3
and LaOCl underwent hydrolysis, forming crystalline LaCO OH (confirmed by Ag
0
titration and XRD patterns), Ru(OH)
3
, and HCl. The Ru(OH)
3
was reduced in situ to Ru nanoparticles, as revealed by XPS
n+
and TEM analysis. The simultaneous hydrothermal reduction of Ru species and formation of crystalline LaCO
3
OH results
in the formation of Ru nanoparticles encapsulated by a protective LaCO
3
OH layer, as evidenced by HRTEM and DRIFTS CO
adsorption measurements. The preparation of catalysts with this unique structure comprising protected metal
nanoparticles by the support itself which confer additional stability, is a novel strategy to prepare hydrothermal-stable
catalysts.
Introduction
products, such as hydrogenolysis of C-C and C-O bonds,
hydrogenation, aldol condensation, isomerization, selective
1
2-16
Bio-oils derived from the fast pyrolysis or liquefaction of sustainable oxidation, and aqueous phase reforming reactions.
biomasses such as cellulose, lignin, chitin, and lipid have attracted
great attention as partial replacements of traditional non- highly dependent on temperature , and hence a reaction catalyzed
It should be noted that the ionization constant of water (K ) is
w
1
7
1
-9
renewable fossil fuels.
However, these bio-oils are highly with a supported metal catalyst at elevated temperatures leads to
+
−
oxygenated due to the high oxygen contents of the precursor enhanced H and OH concentrations which can degrade the
macromolecular biomass, and hence cannot be used directly as fuel catalyst support structure. Thus, the stability of catalysts at
10, 11
sources without prior reduction with metal catalysts.
Due to the elevated temperatures is an important factor that should be taken
polar and hydrophilic nature of bio-liquids which contain multiple into consideration for the conversion of biomass. For example,
oxygen-containing functional groups, and meanwhile water is Ravenelle et al. reported that the γ-Al O support gradually
2
3
ubiquitous in biomass utilization, therefore, water is an attractive hydrated to böhmite when Pt/γ-Al O was treated in aqueous
2
3
solvent for the conversion of bio-liquids into useful biofuels. In conditions at 225 °C, resulting in a dramatic decrease in the surface
addition, the acidic or basic properties of water can be adjusted by area, and an increase in the size of the Pt nanoparticles from 4.3 nm
18-19
controlling the pH. Indeed, there are many reports involving metal to 7.8 nm.
Similarly, the surface area of SiO2 was shown to
catalyzed reactions conducted in the aqueous phase for the decrease after treatment under aqueous conditions at 200 °C for 12
3
−1
3
−1 20
conversion of substrates obtained from biomass into more useful h (from 280 cm ‧g to 70 cm ‧g ). With respect to mesoporous
SBA-15 with highly ordered structures, the specific surface area
3
−1
3
−1
decreased from 740 cm ‧g to 30 cm ‧g after treatment under
similar conditions due to the high temperature induced alteration
of the mesoporous structure of the catalyst support in aqueous
2
0
phase. These destructive effect of the catalyst structure caused by
hydrothermal condition greatly decrease the catalytic activities.
Hence, there is a strong need to synthesize catalysts with high
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hydrothermal stabilities, which would in particular increase the the high temperatures involved during air calcination and hydrogen
efficiency of biofuels or biochemicals production under aqueous reduction processes led to a decrease in the BET surface areas of
2
−1
conditions.
the catalysts to 160.0, 69.4, and 3.5 m ·g , respectively (as shown
Great efforts have been made recently in order to enhance the in Fig. S1). The XRD diffractograms of the reduced samples are
0
hydrothermal stability of supported metal catalysts. To protect the shown in Fig. 1. While the characteristic Ru peaks (JCPDS 06-0663)
hydrothermally-unstable catalysts, a strategy involving coating a appeared on the Ru/SiO catalyst, these peaks were absent in the
2
carbon layer onto the metal surface of the support by introducing Ru/ZrO
2
and Ru/LaCO
smaller Ru nanoparticles on the surface of the ZrO
In addition, the same group reported a chemical vapor deposition supports. Notably, in the Ru/LaCO OH catalyst apart from the
3
OH catalysts, probably due to the presence of
2
0,
glucose as a carbonization agent was developed by Datye’s group.
2
and LaCO OH
3
2
1
3
(
CVD) method that formed graphitic carbon coated Pt/γ-Al
2
O
3
,
3
intense peaks assigned to LaCO OH (JCPDS 26-0815), two broad
which showed excellent stability for reforming and hydrogenation peaks appeared at 25.2 ° and 33.9 ° which were assigned to
reactions in the liquid phase. In addition, the atomic layer characteristic LaOCl species (JCPDS 08-0477). The formation of
deposition (ALD) method has been used to prevent lanthanide oxychloride (LnOCl, Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Dy,
leaching/reoxidation of non-noble metal catalysts, thereby and Er) in lanthanide-supported catalysts has been previously
2
2, 23
enhancing the stability of catalysts for aqueous phase reactions
.
reported in the literature when chloride salts have been used as the
26
These methods enhance the hydrothermal stability of the metal precursors. For example, Basinska et al detected the
supported metal catalysts by introducing an inert outer layer coat presence of tetragonal LaOCl phase after conducting the water-gas
on the support surface.
2 3
shift reaction at 350 °C in a Ru/La O catalyst prepared with a
The rare earth element lanthanum oxides, such as La O , chlorine containing precursor and the corresponding rare earth
2
3
2
7
LaO
2 3 3
CO and LaCO OH have been frequently used as supports in oxide. Similarly, Normand et al found that using a palladium
metal catalyzed ammonia synthesis, dry reforming, and oxidation chloride salt as the metal precursor and rare earth oxides (La
2 3
O ,
24,25
reactions
used during its preparation demonstrate that LaCO
high hydrothermal stability. Hence, LaCO OH could be an the LaOCl phase was formed from the RuCl ‧3H O precursor and
. Importantly, the rather harsh aqueous conditions Pr O , and Tb O ) during the impregnation step led to the formation
2 3 2 3
3
OH exhibits of a stable oxychloride phase (LaOCl, PrOCl, and TbOCl). In our case,
3
3
2
appropriate hydrothermally stable support for the metal catalyzed the LaCO OH support after sequential impregnation and calcination
3
hydrogenolysis of C-O bonds of biomass derived material under procedures. On the Ru/LaCO OH sample, the intensity of the peaks
3
aqueous conditions.
corresponding to LaCO OH decreased in intensity, while new broad
3
In our former work, Ru/C was found to be more selective than peaks corresponding to the LaOCl crystalline phase appeared. This
Pd/C and Pt/C for hydrogenolysis of C–O bond of lignin model suggests that LaOCl forms on the external surface of the support as
13,14
compounds in the aqueous phase.
Herein, we report a new the RuCl ‧3H O precursor reacts with the LaCO OH support.
3 2 3
Ru/LaCO OH hydrothermally stable catalyst formed as a result of
3
successive procedures consisting of incipient impregnation, air-
calcination, and hydrothermal reduction, which forms protected Ru
♦

ꢂ
Ru
M type ZrO
T type ZrO
ꢁ
ꢀ
LaCO OH
3
LaOCl
2
2
nanoparticles encapsulated by LaCO OH with strong metal-support
3
Ru/SiO
2
interaction (SMSI), and this catalyst is highly stable and leads to
high hydrogenolysis rates for the conversion of the model bio-
molecules guaiacol and glycerol to the corresponding reduced
products. In order to understand the relationship between catalytic
activities and structural properties, extensive characterization of
the catalyst and studies on the hydrogenolysis reaction were
carried out; in addition, the two processes involved into
♦
♦ ♦
♦
♦
♦
ꢂ


ꢂ
Ru/ZrO
2


ꢂ




ꢁ
Ru/LaCO OH
3
ꢁ
ꢀ
ꢁ
ꢁ
-
impregnation of the Cl containing Ru precursor to the support and
ꢀ
ꢁ ꢁ
ꢁ ꢁ ꢁ
ꢁ
ꢁ
hydrothermal reduction of as-formed Ru complex were investigated
in-depth.
1
0
20
30
2
40
50 60
70
80
Theta (degree)
Figure 1. XRD patterns of Ru/SiO
2
, Ru/ZrO
2
, and Ru/LaCO OH
3
Results and discussion
catalysts.
Three separate catalysts were prepared to study and compare
their morphology and catalytic properties, and were initially
prepared by loading a Ru metal precursor with metal contents of 5
wt%, as determined by inductively coupled plasma spectroscopy
Transmission electron microscopy (TEM) was used to
characterize the particle information of the catalysts (Ru/SiO
2
,
Ru/ZrO , and Ru/LaCO OH) after H -reduction of the calcined
2
3
2
samples at 350 °C (Fig. 2). In Ru/SiO
2
, the catalyst has Ru particles
(
ICP-AES), via the impregnation method to different supports (SiO
ZrO and LaCO
OH) with respective Brunner−Emmet−Teller (BET)
surface areas of 231.5, 114.0, and 8.2 m ·g . After impregnation,
2
,
with a mean size of 7.0 nm (Fig. 2a). The TEM image of the Ru/ZrO
catalyst showed little contrast between Ru and ZrO , and only a few
2
2
3
2
2
−1
2
Ru particles (d = ca. 3.2 nm) can be seen at the edge of the ZrO (Fig.
2
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Figure 2. TEM images of (a) Ru/SiO
2
, (b) Ru/ZrO
2
, (c) Ru/LaCO
3
OH, and (d) HRTEM image of Ru/LaCO
3
OH samples with a fringe lattice of
LaOCl after hydrogen reduction at 350 °C.
2
b). The LaCO
3 3
OH support in the Ru/LaCO OH catalyst consisted of and the C 1s peak at 284.8 eV partially overlap, and the binding
large crystals with irregular shapes (Fig. 2c). In addition, the HRTEM energies were calculated taking as reference the C 1s (284.8 eV)
2
8
image shows that while LaOCl (001) with visible lattice fringes (0.68 peak of carbon contamination. In the XPS spectra of Ru/SiO and
2
0
nm) was present (Fig. 4d), no Ru particles were found in this Ru/ZrO
2
catalysts, clearly visible asymmetric peaks corresponding to
catalyst, which is consistent with the XRD result (Fig. 1). It is Ru 3d5/2 were present at 280.2 eV and 280.1 eV respectively, while
−
suggested that Cl ions from RuCl may help to disperse Ru on the the Ru 3d3/2 peaks were observed at 284.3 eV and 284.2 eV
3
LaCO
with the LaCO
highly dispersed on the external surface of the support. The energy reduction at 350 °C.
dispersive spectroscopy (EDS) mapping on the Ru/LaCO OH catalyst With respect to the Ru/LaCO
demonstrated that Ru, Cl, La were evenly dispersed throughout the a C=O signal from the CO
3
OH support, or that RuCl
3
in the metal precursor may react respectively. The presence of the typical peaks corresponding to
0
3
OH support to form a new Ru complex species that is Ru indicates that the SiO and ZrO catalysts undergo facile
2 2
29
3
3
OH catalyst, after deconvolution
group was able to be observed at 289.3
support (Fig. 3a-3c). The high angle annular dark field-scanning TEM eV. In the calcined Ru/LaCO OH catalyst, a Ru 3d signal is
2
-
3
2
8
3
5/2
(
HAADF-STEM) showed that no Ru nanoparticles were present on observed at 282.7 eV (Fig. 4b), which can be attributed to Ru (III)
the detected regions of the catalyst (Fig. 3d).
species; this value is very close to that measured by Mazzieri et al.
3
0
for the Ru/Al O catalyst (282.9 eV). In the hydrogen reduced
2
3
catalyst, a Ru 3d5/2 signal appeared at 281.6 eV corresponding to an
n+
electron deficient ruthenium species (Ru ), which was assigned to
2
9,30
ruthenium (II) with chloride counterions (Fig. 4b).
This result
OH
and the XRD data together demonstrate that the Ru/LaCO
3
catalyst is incompletely reduced even after hydrogen reduction,
perhaps resulting from the influence of residual chloride in the
RuCl ‧3H O precursor. Indeed, the XPS experiment determined that
3
2
chloride was present as evidenced by the XPS signals with a binding
energy of 199.58 eV and 197.98 eV, assigned to 2p1/2 and 2p
3/2
31
respectively of Cl in LaOCl species (Fig. 4c and Fig. S2). The
intermediate Ru (II) oxidation state is likely to be highly stable in the
−
presence of a LaCO OH supports and residual Cl ions. Moreover, it
3
has previously been reported that chloride containing precursors
3
2
30
may induce incomplete hydrogen reduction of Ru. Mazzieri et al.
reported that the hydrogen reduction of a Ru/Al O catalyst using a
2
3
RuCl precursor led to partial reduction, with signals at 281.5 eV
3
0
assigned to oxidized Ru, and a signal at 280.0 eV assigned to Ru
also present. To reduce the sample in hydrogen at a higher
2
+
Figure 3. TEM-EDS mapping for the elements in Ru/LaCO
3
OH (a) La,
temperature of 500 °C for 4 h, the in-completely reduced Ru still
−
(
b) Cl, (c) Ru, and (d) HAADF-STEM image.
existed with the Cl residue, as confirmed by the signal at 281.5 eV
in XPS spectrum (Fig. S3). This is in accordance with the finding of
33
2+
X-ray photoelectron spectroscopy (XPS) was carried out to Faroldi et al. that Ru was the dominant species with the typical
characterize the Ru 3d oxidation state of the three Ru catalysts Ru 3d5/2 signal at 281.8 eV from XPS spectrum, when reducing of
after hydrogen reduction at 350 °C (Figure 4a). The Ru 3d3/2 peak Ru/La O with H at 550 °C using RuCl ‧3H O as precursor.
2
3
2
3
2
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0
34
particle sized RuO
x
to Ru metal, suggested by Yan et al . In
a
C 1s
84.8
Ru/SiO
comparison, Ru/ZrO
2
showed one reduction peak at 134 °C,
2
2
assigned to the reduction of RuO to Ru metal. The slightly lower
2
0
Ru 3d5/2
2
reduction temperature is attributed to the smaller Ru particle sizes
80.2
and the higher dispersion of Ru on ZrO
XRD pattern and the TEM image results as well. Surprisingly,
Ru/LaCO OH exhibited a broad reduction peak from 260 to 420 °C,
2
, which is supported by the
Ru/ZrO
2
3
0
Ru 3d5/2
with a smaller peak at 341 °C, and the main peak at 382 °C. Taking
into consideration the XPS results which confirm the presence of Ru
in the +2 and +3 oxidation states, and the fact that less energy is
required to reduce the higher oxidation state, the lower reduction
peak at 341 °C can be ascribed to the reduction of (Ru (III)) to an
intermediate oxidation state (Ru (II)), while the peak at 382 °C may
be ascribed to the reduction of Ru (II) species to Ru metal. The
2
80.1
Ru/LaCO OH
3
2
+
Ru 3d5/2
2
81.6
reduction temperatures of the Ru/LaCO
3
OH catalyst were far higher
2
88 286 284 282 280 278
than those of the Ru/ZrO and Ru/SiO
2
2
samples, indicating that Ru
Binding Energy (eV)
formed very stable complex species rather than discrete RuO
particles after air calcination.
x
C 1s
b
2
84.8
Calcined
3
+
Ru 3d5/2
190 °C
149 °C
Ru/SiO
2
82.7
2
1
34 °C
Ru/ZrO
2
Reduced
350°C, H2)
382 °C
3
41 °C
(
Ru/LaCO OH
3
2
+
Ru 3d5/2
1
66 °C
2
81.6
1
56
°C
Ru/LaCO OH chloride free
3
2
92 290 288 286 284 282 280 278
1
00
200
300
400
C)
500
Binding Energy (eV)
Temperature (
°
c
Cl 2p
3/2
Figure 5. H -TPR profiles of air-calcined Ru/SiO , Ru/ZrO , and
1
97.98
2
2
2
Ru/LaCO
3
OH samples.
Cl 2p
1/2
1
99.58
Considering the observed experimental data, we suggest that
−
during impregnation Cl ions from the RuCl ‧3H O precursor
3
2
participate in a surface reaction with the LaCO OH support to form
3
crystalline LaOCl (as detected by XRD and TEM experiments), and
3
+
the Ru ions react strongly with the support to form a complex-like
2
02 200 198 196 194 192
Binding Energy (eV)
species. According to the TEM, XPS, and H -TPR results, ruthenium
2
was found to be in a complex chemical environment (Scheme 1),
Figure 4. (a) XPS spectra of Ru/SiO
2
, Ru/ZrO
2
and Ru/LaCO
3
OH after and based on the stoichiometry of the surface reaction, the
hydrogen reduction at 350 °C with H2; (b) XPS spectra of ruthenium complex species is inferred to be LaRu(CO ) Cl
3
2
2
Ru/LaCO
c) XPS spectra of Cl in Ru/LaCO
hydrogen-reduction at 350 °C.
3
OH after air-calcination and hydrogen-reduction at 350 °C; (Equation I):
(
3
OH after air-calcination and
2LaCO OH + RuCl → LaOCl + LaRu(CO ) Cl + H O (Equation I)
3
3
3 2
2
2
The reducibility of the three air-calcined supported Ru
catalysts was investigated by hydrogen temperature-programmed by the TPR-H2 profile confirms the highly stable nature of the
reduction (H -TPR), as shown in Fig. 5. The H -TPR profile of the LaRu(CO ) Cl complex. However, when the chloride counterions
Ru/SiO catalyst exhibited two reduction peaks at 149 °C and 190 °C. were removed from the LaRu(CO ) Cl complex by washing with
The two temperatures were attributed to the reduction of different ammonia, there was observed a marked decrease in the reduction
The high reduction temperature of ruthenium as determined
2
2
3
2
2
2
3
2
2
4
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3 3 2
Scheme 1. The surface reaction between RuCl ‧ H O precursor and
3
LaCO OH support.
temperature to 166 °C (as shown in Figure 5), further confirming
the hypothesis that chloride participates in the surface reaction
during impregnation to form a highly stable Ru complex species.
0
Hydrothermal reduction of Ru complex on LaCO
presence of H
3
OH to Ru in
2
Thus far, the TPR-H
of reducing the Ru/LaCO
while other experimental data (TEM, XPS, and XRD) suggest that
RuCl reacts with LaCO OH to form a new La species (LaOCl) and a
complex Ru species (LaRu(CO Cl ) on the external surface of
LaCO OH (Scheme 1). Next, we attempted to reduce the calcined
Ru/LaCO OH catalyst under hydrothermal conditions (240 °C, and
at 2 bar pressure for 3 h. During hydrothermal treatment, the pH
2
experiment has determined the difficulty
3
OH catalyst after calcination,
3
3
3
)
2
2
3
3
H
2
value gradually decreased from 6.89 to 4.45 (180 min), suggesting
+
that hydrolysis may release HCl (Fig. 6a). The addition of Ag ions
led to the precipitation of white AgCl (inset of Fig. 6a), showing that
chloride ions are released into the aqueous phase during
hydrothermal treatment. This suggests that the harsh conditions
during hydrothermal treatment induce the hydrolysis of chloride
containing species in the solid phase, releasing chloride ions into
the aqueous phase as HCl (Scheme 2).
In addition, after hydrothermal treatment XRD analysis
3
determined that the Ru/LaCO OH catalyst contained a highly
crystalline LaCO OH phase (JCPDS: 26-0815, Fig. 6b) while the LaOCl
3
lattice disappeared. This indicated that the LaOCl phase was no
longer present after hydrolytic removal of surface chloride species,
Figure 6. (a) The variation of pH values during hydrothermal
3
+
treatment at 240 °C in the presence of H
2
, (b) XRD patterns of
3
and the La was reconstituted and recrystallized into LaCO OH
LaCO OH, Ru/LaCO OH after H -reduction, and Ru/LaCO OH after
under hydrothermal conditions. Notably, no Ru species were
observed in the XRD pattern, perhaps due to overlap with the
3
3
2
3
hydrothermal reduction in the presence of hydrogen, (c) XPS
spectra of Ru/LaCO OH after H reduction, and Ru/LaCO OH after
hydrothermal reduction in the presence of H
strong crystal peak of LaCO
characterize the electronic character of Ru after hydrothermal
treatment of Ru/LaCO OH in the presence of hydrogen (Fig. 6c).
Under hydrothermal conditions, the Ru 3d5/2 signal shifted from
81.6 eV (H2 reduction at 350 °C) to the well-known signal
3
OH. Subsequently, XPS was used to
3
2
3
2
.
3
Therefore, while in the LaRu(CO ) Cl complex the reduction of
2
3 2
0
2
0
0
ruthenium in the presence of H
2
to Ru in flowing H
2
at 350 °C was
indicative of Ru at 280.1 eV Ru . Hence, under hydrothermal
minimal, under hydrothermal conditions the reduction occurs
conditions after elimination of surface chloride species on the
under much milder conditions (240 °C). During the hydrothermal
Ru/LaCO OH catalyst, there is facile reduction of oxidized
3
−
0
process, Cl ions in both LaRu(CO
3
)
2
Cl
2
and LaOCl are removed from
ruthenium in the calcined catalyst to Ru .
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3
Scheme 2. The evolution of Ru/LaCO OH under hydrothermal
treatment in the presence of H
2
.
the catalyst surface into the aqueous solution, and LaCO
3
OH is
+
formed by recrystallization (confirmed by Ag titration and XRD
n+
patterns). Under hydrothermal conditions, the Ru species in the
LaRu(CO
subsequently reduced to Ru nanoparticles (confirmed by XPS). The
overall process for the hydrolysis of LaRu(CO Cl and LaOCl, and
the in situ reduction of Ru(OH) occurring on the surface of the
3 2 2 3
) Cl complex is hydrolyzed to Ru(OH) , which is then
0
3
)
2
2
3
catalyst is summarized by equations II and III:
LaOCl + LaRu(CO ) Cl + 4H O → 2 LaCO OH + 3HCl + Ru(OH)
3
3
2
2
2
3
(
Equation II)
2
Ru(OH) + 3H → Ru + 6H O (Equation III)
3 2 2
n+
0
Since the reduction of Ru
species to Ru and the
recrystallization of LaCO OH occurred simultaneously, it is possible
3
3
that the LaCO OH could be interacting with the hydrothermally
reduced Ru nanoparticles. To confirm the specific nature of the
interaction and the morphology of the hydrothermally reduced
catalyst, HRTEM and diffuse reflectance infrared Fourier Transform
spectroscopy (DRIFTS) of CO adsorption experiments were carried
out. As direct evidence, the TEM and HRTEM images of the
Figure 7. (a) TEM image and particle distribution and (b) the HRTEM
image of hydrothermally H
2
-reduced Ru/LaCO
3
OH, (c) the Ru (100)
OH.
fringe lattice of hydrothermally H
2
-reduced Ru/LaCO
3
3
Ru/LaCO OH catalyst after hydrothermal hydrogen reduction are
shown in Figs. 7a and 7b. From these images it is clear that the
lattice fringe for LaOCl and the LaRu(CO ) Cl species both
2
supports under a purge of N at room temperature is shown in
−1
−1
3
2
2
Figures 8a-8c. The strong bands at 2171 cm and 2117 cm on the
three Ru catalysts disappeared after a prolonged purge, and were
attributed to gaseous CO rather than the adsorption of
disappeared after hydrothermal
H
2
reduction, while newly
OH crystal surface.
formed Ru appeared as dark spots on the LaCO
3
Ru particles with crystal facets were observed with a mean
diameter of 2.8 ± 0.3 nm (Fig. 7a). After hydrothermal treatment,
n+ 33
multicarbonyl species on Ru . In the initial stage of the purge,
−1
four bands are detected on the Ru/SiO catalyst at 2171 cm , 2125
2
the newly formed LaCO
3
OH layer partially coated Ru particles with a
−1
−1
−1
cm , 2117 cm and 2065 cm . After purging for 1500 seconds, the
fringe lattice of Ru (100) (d spacing: 0.234 nm), as clearly shown in
−1
−1
bands at 2171 cm and 2117 cm disappeared. The band at 2125
Fig. 7b and 7c. Such structural protection may significantly impede
−1
n+
n+
n
cm is assigned to muticarbonyl species bound to Ru (Ru (CO) ),
the particle growth and enhance the stability of the Ru/LaCO
catalyst under the harsh hydrothermal conditions.
3
OH
−1
which lies in the high-frequency 1 (HF
1
) region (2020-2156 cm ),
−1
while the band at 2065 cm corresponds to CO bound linearly to
n+
It is expected that the presence of a LaCO
3
OH coat surrounding
−1 36
Ru , and lies in the HF
corresponding to bridge-bonded CO was observed in Ru/SiO
CO desorption pattern on the Ru/ZrO catalyst was quite similar to
that of Ru/SiO , except that the bands in the HF
2
region (2060-2110 cm ). No band
Ru particles on the hydrothermally hydrogen-reduced Ru/LaCO
3
OH
2
. The
catalyst would change the surface adsorption characteristics, which
can be confirmed by diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) measurements of CO adsorption. The IR
spectra for CO desorption on the Ru catalysts with three different
2
−1
2
−1
1
(2127 cm ) and
HF
2
(2071 cm ) regions are slightly shifted to higher wavenumbers.
The blue shift of the bands may be due to the electronic deficient
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support layer partially coat around Ru nanoparticles showing a
strong metal-support interaction (SMSI).
2
125
117
2
2
171
a
b
c
2
60s
Combining the results obtained thus far (XRD patterns,
changes in pH, XPS, HRTEM, and DRIFT of adsorbed CO), allows a
determination of the various species formed during the
1
3
1
20s
00s
500s
065
Ru/SiO
hydrothermal reduction of the Ru/LaCO
Under hydrothermal conditions the surface species LaOCl and
LaRu(CO Cl are hydrolyzed (as confirmed by XRD), and the
3
OH calcined catalyst.
2
after reduction
3
)
2
2
with H
2
chloride ions that are released in the process lead to a reduction of
the pH via the formation of HCl, while the lanthanum side-product
LaOCl is recrystallized into LaCO
shown in Scheme 2, hydrolysis of LaRu(CO
formation of the Ru intermediate Ru(OH)
3
3
OH (as determined by XRD). As
Cl also leads to the
, and was sequentially
reduced to Ru nanoparticles in the presence of (as
)
2
2
2
250 2200 2150 2100 2050 2000 1950 1900 1850
3
-
1
Wavelength (cm
)
0
2
171
H
2
2118
2
127
60s
demonstrated by XPS). We propose that during the hydrothermal
reduction, the Ru nanoparticles are partially coated with a support
3
3
1
00s
60s
800s
2071
layer of LaCO OH, since the formation of reduced Ru nanoparticles
3
occurs simultaneously with the release of LaCO
hydrolysis of the initially present LaRu(CO Cl
and DRIFT of adsorbed CO further validated the idea of the
presence of a LaCO OH support partially coating around Ru
3
OH, formed via the
Ru/ZrO
2
)
2
2
. The HRTEM image
after reduction
3
with H
2
3
nanoparticles. This support-wrapped metal catalyst is expected to
exhibit high stability and activity in aqueous phase hydrogenolysis
reactions at high temperatures.
2
250 2200 2150 2100 2050 2000 1950 1900 1850
-
1
Wavelength (cm
)
3
High stability of hydrothermally reduced Ru/LaCO OH catalyst for
2171
60s
80s
40s
00s
00s
40s
aqueous phase hydrogenolysis reactions
1
2
3
4
5
2117
To test the hydrogenolysis activity and stability of the novel
hydrothermally prepared reduced Ru/LaCO OH catalyst, the
3
reduction of guaiacol and glycerol (substrates widely available from
bio-feedstocks) under aqueous conditions were chosen as model
reactions, and their activities were compared with the
Ru/LaCO OH after
3
hydrothermal treatment
conventionally prepared Ru/SiO
product distributions resulting from guaiacol hydrogenolysis in
aqueous conditions at 240 °C and 0.2 MPa H pressure with the
2 2
and Ru/ZrO catalysts. The
2
2
250 2200 2150 2100 2050 2000 1950 1900 1850
three catalysts are summarized in Fig. 9a. The conversion of
guaiacol reached 38.6%, 95.4%, and 95.6%, while the yield of
-1
Wavelength (cm )
Figure 8. The DRIFT spectra of CO adsorption on (a) H -reduced benzene was 2.4%, 30.6%, and 75.8%, for the Ru/SiO
2
, Ru/ZrO
OH catalysts, respectively. As shown in Fig. 9b, the
kinetic of guaiacol hydrogenolysis on hydrothermal-reduced
Ru/LaCO OH was displayed. Accompanied with the rapid decrease
, which may be induced by the Lewis of the guaiacol reactant, the maximum phenol yield reached 60.0%
2
2
, and
2
Ru/SiO
2
, (b) H
2 2
-reduced Ru/ZrO
, and (c) hydrothermally-reduced Ru/LaCO
3
Ru/LaCO
3
OH in the presence of hydrogen.
3
character of Ru on Ru/ZrO
acidic Zr atoms on ZrO . However, in the Ru/LaCO
3
OH catalyst the at 30min with an initial hydrogenolysis rate of 3.24 g·g ·h , and
-1
-1
2
-1
bands in the region of CO adsorption on Ru (2040-2069 cm ) and then gradually decreased since then. The yield of benzene gradually
the gaseous CO peak rapidly decreased to near baseline levels. It increased to 75.8% at the reaction time of 150 min. This result have
might be caused by the partial exposure of the Ru particles and the suggested that the hydrogenolysis of the methoxy group in guaiacol
weak adsorbed CO on Ru particles, which is in line with the HRTEM occurs as the first step in the reduction process, and is followed by
image (Fig. 7b) showing that Ru nanoparticles are partially coated the hydrogenolysis of phenol to benzene as the second step, which
by a LaCO
for Au/HAP-500, where the IR spectra of CO adsorption confirmed modeling (Fig. S4). This result indicates that the newly developed
3
OH support layer. This result was similar to that observed was determined to be the rate-limiting step according to kinetics
35
that Au nanoparticles were wrapped by hydroxyapatite (HAP).
Therefore, the characterization of the catalyst with both HRTEM hydrogenolysis of guaiacol to benzene under hydrothermal
3
Ru/LaCO OH catalyst shows high activity and selectivity in the
and DRIFT of adsorbed CO both suggest the presence of a LaCO
OH conditions.
3
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ARTICLE
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catalysts when recycling at high temperature water phase.
In order to test the influence of the protector layer towards
the hydrogenolysis reaction, Ru/LaCO OH without a support layer (5
wt% Ru loading, identical to the metal loading of Ru/LaCO OH with
Conversion
Benzene Yield
a100
3
80
60
40
20
0
3
-
−
Cl ) was synthesized subsequently. To obtain the Cl free catalyst,
the RuCl ·3H O metal salt was impregnated with the basic agent of
3
2
NH ·H O solution. Thus, Ru(OH) precursor was formed after
3
2
3
impregnation, and NH
4
Cl stayed in the aqueous phase as confirmed
+
−
by Ag titration. In contrast to the Ru/LaCO
3
OH catalyst with Cl ions,
−
in the Cl free catalyst Ru nanoparticles were not protected by a
support layer, as demonstrated by the HRTEM image in Fig. S7.
Accordingly, the as-formed unprotected Ru nanoparticles were
Ru/SiO
Ru/ZrO
Ru/LaCO OH
3
2
2
comparatively easily reduced with
H
2
at
a
relatively low
b100
Benzene
temperature (166 °C), as shown from the TPR-H
profile in Fig. 5.
2
Cyclohexane
cyclohexanone
Phenol
8
6
4
2
0
0
0
0
0
a
100
Conversion
1,2-Propanediol yield
Guaiacol
80
60
40
20
0
0
20
40 60 80 100 120 140 160
Reaction time (min)
Ru/SiO
Ru/ZrO
2
Ru/LaCO OH
3
2
Figure 9. (a) Hydrogenolysis of guaiacol over Ru/SiO
hydrothermally-reduced Ru/LaCO OH, Conditions: 1.0 g guaiacol,
.75 g catalyst, H O (150 mL), 0.8 MPa N and 0.2 MPa H , 150 min.
b) kinetic for hydrogenolysis of guaiacol on hydrothermally-
reduced Ru/LaCO OH.
2
, Ru/ZrO
2
, and
b
70
3
Conversion
6
0
Propylene glycol
Ethylene glycol
Methanol
0
2
2
2
(
50
3
4
3
2
1
0
0
0
0
0
Glycerol hydrogenolysis in the aqueous phase was conducted
over these three Ru catalysts at 200 °C and 3 MPa H pressure. The
2
glycerol conversion attained 10%, 21%, and 62%, while the
selectivity to 1,2-propanediol reached 2%, 5%, and 46% with the
Ru/SiO
A kinetic curve for glycerol conversion (Fig. 10b) over hydrothermal-
reduced Ru/LaCO OH revealed that the primary product was 1,2-
2 2 3
, Ru/ZrO , and Ru/LaCO OH catalysts, respectively (Fig. 10a).
0
20 40 60 80 100 120 140 160 180 200
Reaction time (min)
3
-1
-1
propanediol with an initial hydrogenolysis rate of 2.0 g·g ·h . With Figure 10. (a)hydrogenolysis of glycerol over Ru/SiO
2
, Ru/ZrO2, and
reaction time prolongs, the C-C cleavage products of ethylene glycol hydrothermally-reduced Ru/LaCO OH, conditions: 0.4 g glycerol, 0.1
3
2 2
and methanol slightly increased to 5%, while the yield of 1,2- g catalyst, H O (80 mL), 3 MPa H , 3 h; (b) kinetic for hydrogenolysis
3
propanediol increased to nearly 40%. These results further confirm of glycerol on hydrothermally-reduced Ru/LaCO OH.
the superiority of the Ru/LaCO OH catalyst for achieving high
3
activity and selectivity for aqueous phase hydrogenolysis reactions.
The hydrogenolysis of guaiacol was conducted over four recycling
OH samples. In
Ru/LaCO OH with Cl ions, a constant conversion of 95% and
In the recycling tests for guaiacol hydrogenolysis in water, runs to determine the stability of two Ru/LaCO
3
−
Ru/LaCO OH showed much higher durable capability than Ru/SiO
3
2
3
and Ru/ZrO
maintained high conversion (>95%) and benzene selectivity (> 70%) 11). The leaching of Ru ions into the solution in the recycling tests
even after eight runs, but Ru/ZrO and Ru/SiO only attained was not detected by ICP measurement. Under identical conditions,
2 3
, as displayed in Fig. S5 and Fig. S6. Ru/LaCO OH still benzene yields of 76% were obtained for four recycling runs (Fig.
2
2
−
respective 70% and 10% conversion after four runs with 22% and 1% the hydrogenolysis of guaiacol with the Cl free catalyst took place
benzene selectivity after four runs. A dramatic activity loss was with 96% conversion and a benzene yield of 35% in the first run.
observed on unprotect Ru nanoparticles on Ru/ZrO
2
and Ru/SiO
2
The successive recycling tests led to gradual decreases in activity
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ARTICLE
3
and selectivity, with a conversion of 94% and benzene yield of 12.1% that the Ru nanoparticles were partially encapsulated by LaCO OH
in the fourth run. The slightly higher conversion in the first run of layers.
−
the Cl free catalyst may be due to the presence of greater amounts
The strong interaction between the Ru nanoparticles and the
3
of unprotected Ru active sites. The marked difference in the partially coated LaCO OH support layers impeded the further
capacity to undergo hydrodeoxygenation between these two growth of metal nanoparticles under harsh reduction conditions.
catalysts demonstrates the importance of the protective coating Our developed new methodology, wherein the metal center is
−
around Ru with SMSI, and suggests that the Cl containing species partially protected by support layers itself during an in-situ
may hold great potential as a robust catalyst for reactions hydrothermal-hydrogen reduction process, is a novel strategy that
conducted under hydrothermal conditions.
offers the potentials of synthesizing hydrothermally-stable
catalysts.
-
Conversion (Ru/LaCO OH with Cl )
3
-
Acknowledgements
Selectivity to benzene (Ru/LaCO OH with Cl )
1
1
20
00
3
-
Conversion (Ru/LaCO OH Cl free)
3
-
We acknowledge the financial support from the National Key
Research and Development Program of China (Grant No.
Selectivity to benzene (Ru/LaCO
OH Cl free)
3
2
016YFB0701100), the Recruitment Program of Global Young
Experts in China, National Natural Science Foundation of China
Grant No. 21573075), and the Foundation of Key Laboratory
8
6
4
2
0
0
0
0
0
(
of Low-Carbon Conversion Science & Engineering, Shanghai
Advanced Research Institute, Chinese Academy of Sciences.
Notes and references
1
2
3
4
5
6
7
8
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Run 1
Run 2
Run 3
Run 4
Figure 11. Four-run recycling tests on hydrogenolysis of guaiacol
1
−
over hydrothermally-reduced Ru/LaCO
3
OH with and without Cl
7
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We have synthesized a new highly hydrothermally-stable
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3
7
coated Ru nanoparticles, via sequential incipient impregnation, air-
calcination, and hydrothermal reduction procedures. This catalyst
exhibited superior performance in terms of stability and activity
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Ru ions was negligible.
4
1
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standard (non-aqueous) conditions,
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2
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the presence of the stable RuLa(CO
3 2 2
) Cl complex and LaOCl, which
are formed instead of RuO during the impregnation procedure
x
when the RuCl ·3H O precursor reacts with LaCO OH. However,
3
2
3
upon hydrothermal H
2
reduction at 240 °C, LaOCl and RuLa(CO
3
)
2
Cl
2
0
were hydrolyzed into highly crystalline LaCO
3
OH and Ru particles.
The HRTEM image and DRIFT CO adsorption experiments suggested
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Table of Content:
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