Effect of carbon surface functional groups on the synthesis of Ru/C catalysts for supercritical water gasification

Article abstract of DOI:10.1039/c5cy00352k

A carbon support was treated with HNO₃ to create surface functional groups (e.g. -COOH, -OH), which were then characterized by TGA, TPD, CNS elemental analysis, and Boehm titration. HNO₃ modified the carbon surface properties by adding a high amount of carboxylic groups, improved the thermal stability of the carbon support, and reduced ca. 50% of the ash. The thermal pre-treatment (723 K under He) following the HNO₃ pre-treatment successfully removed the carboxylic groups. 4% Ru/C catalysts were synthesized using the surface-modified carbon supports and characterized by H₂-TPR, CO pulse chemisorption, N₂-physisorption and HAADF-STEM. The Ru dispersion was increased in the presence of the carboxylic groups. Catalytic supercritical water gasification (CSCWG) of 10 wt.% isopropanol over the 4% Ru/C catalysts was carried out at 723 K and 30 MPa for 50 hours to assess the performance of the catalysts. It was found that the Ru/C catalyst prepared involving a pre-treatment with HNO₃ did not exhibit a higher catalytic activity than the catalyst whose carbon support was not pre-treated with HNO₃. Hence, the activity and the selectivity during CSCWG were not influenced by the pre-treatment of the catalyst support with HNO₃.

Full text of DOI:10.1039/c5cy00352k

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Effect of carbon surface functional groups on the
synthesis of Ru/C catalysts for supercritical water
gasification
Cite this: Catal. Sci. Technol., 2015,
5, 3658
ad
Gaël Peng,a Fabian Gramm,b Christian Ludwigac and Frédéric Vogel
*
A carbon support was treated with HNO3 to create surface functional groups (e.g. –COOH, –OH), which
were then characterized by TGA, TPD, CNS elemental analysis, and Boehm titration. HNO3 modified the
carbon surface properties by adding a high amount of carboxylic groups, improved the thermal stability of
the carbon support, and reduced ca. 50% of the ash. The thermal pre-treatment (723 K under He) following
the HNO3 pre-treatment successfully removed the carboxylic groups. 4% Ru/C catalysts were synthesized
using the surface-modified carbon supports and characterized by H2-TPR, CO pulse chemisorption,
N2-physisorption and HAADF-STEM. The Ru dispersion was increased in the presence of the carboxylic
groups. Catalytic supercritical water gasification (CSCWG) of 10 wt.% isopropanol over the 4% Ru/C catalysts
was carried out at 723 K and 30 MPa for 50 hours to assess the performance of the catalysts. It was found
that the Ru/C catalyst prepared involving a pre-treatment with HNO3 did not exhibit a higher catalytic activity
than the catalyst whose carbon support was not pre-treated with HNO3. Hence, the activity and the selec-
tivity during CSCWG were not influenced by the pre-treatment of the catalyst support with HNO3.
Received 9th March 2015,
Accepted 11th May 2015
DOI: 10.1039/c5cy00352k
www.rsc.org/catalysis
catalysts are the most suitable catalysts for CH4 production due
to the high activity and high CH4 selectivity of Ru.3–7 For instance,
1. Introduction
There is a strong motivation for developing renewable alterna-
tives to conventional fossil fuels. Due to the worldwide avail-
ability and sustainability of biomass, biomethane production
from biomass appears to be an attractive option. The current
technologies for biomethane production are conventional gas-
ification/methanation and anaerobic digestion. The conven-
tional gasification/methanation is only able to process dried
biomass (water content < 15 wt.%) requiring a drying step with
a high energy demand. Although anaerobic digestion is able to
treat wet biomass (e.g. microalgae, sewage sludge, biomass res-
idues) avoiding the drying step, it has a low thermal efficiency
(25–35%) and requires long residence times (20–33 days).1 Cat-
alytic supercritical water gasification (CSCWG) is an alternative
since wet biomass (water content > 60 wt.%) can be processed
with a high thermal efficiency (60–70%) avoiding the biomass
drying step and offering short residence times (<30 min). For
achieving full biomass conversion and a high CH4 selectivity, a
catalyst is needed for decomposing the large organic molecules
by C–C bond cleavage.2 It has been reported that supported Ru
Osada et al.7 investigated the catalytic performance of various
supported metal catalysts (e.g. Ru, Rh, Pt, Pd, and Ni) during
CSCWG of lignin at 673 K and 37.1 MPa. The catalytic activity
was in the following order: Ru > Pt > Rh > Pd > Ni demon-
strating the better catalytic performance of supported Ru cata-
lysts. Elliott et al.8 studied and reported the long-term stabil-
ity of Ru/rutile-TiO2 (19 weeks), Ru/C (6 weeks) and Ru/ZrO2
(3 weeks) catalysts during catalytic hydrothermal water gasifica-
tion of 10 wt.% phenol at 623 K and 21 MPa. Waldner et al.6
have compared the catalytic performance of a skeletal nickel
catalyst and a Ru/C catalyst during CSCWG of synthetic lique-
fied wood at 673 K and 30 MPa. They observed that the skeletal
nickel catalyst sintered rapidly while the Ru/C catalyst was sta-
ble for more than 220 hours. Zöhrer et al.9 tested the catalytic
performance of some metal oxides supported Ru catalysts such
as Ru/ZrO2 and Ru/TiO2 during CSCWG of glycerol at 673 K
and 28.5 MPa. Although the tested catalysts exhibited good
catalytic performance, the catalyst deactivation caused by coke
deposits was found to be a serious issue. De Vlieger et al.10
found out that a carbon nanotube supported Ru catalyst was
highly effective during reforming of acetic acid at 543 K and
23 MPa and also at 673 K and 25 MPa. However, when working
in the subcritical region (573–613 K), the high ion product of
subcritical water caused a catalyst deactivation by over-oxidation
of Ru. Recently, we reported on the gasification of 10 wt.%
isopropanol at 723 K and 30 MPa over a period of 96 hours with
a General Energy Research Department, Paul Scherrer Institut (PSI), 5232 Villigen
PSI, Switzerland. E-mail: frederic.vogel@psi.ch; Tel: +41 563102135
b ScopeM, Swiss Federal Institute of Technology Zürich (ETHZ), 8093 Zürich,
Switzerland
c ENAC-IIE, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne,
Switzerland
d Fachhochschule Nordwestschweiz (FHNW), 5210 Windisch, Switzerland
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a Ru/C catalyst at a weight hourly space velocity (WHSVgRu
)
Although the Pt dispersion was improved for the catalyst pre-
pared with HNO3, according to them, the lower catalytic
activity was related to the polarity change of the support
which caused more adsorption competition on the Pt surface
between water and ethylene glycol.
−1
of 1228 gOrg gRu h−1.11 By working at a higher WHSVgRu
(5202 gOrg gRu h−1), we observed that the catalyst lifetime
−1
was affected by the decomposition of isopropanol to solid
carbon (coke) and hydrogen on the carbon surface. Up to
now only few studies12 focused on the catalyst design for
CSCWG. There is still a lack of knowledge about the inter-
dependence between the catalyst formulation, its structure,
and its catalytic performance. In order to further improve the
performance of the Ru/C catalysts, there is a necessity to
assess the effect of the most relevant properties of carbon
(e.g. surface functional groups, degree of graphitization,
impurities) as well as the catalyst preparation method. In a
previous study,11 we reported that a higher Ru dispersion
and the use of a chloride-free ruthenium salt precursor
improved significantly the catalytic activity during CSCWG of
isopropanol (723 K, 30 MPa). In this work, we studied
the effect of the surface acidity of carbon on the synthesis of
Ru/C catalysts by adding different surface functional groups
(e.g. –COOH, –OH groups) with HNO3 prior to active phase
impregnation, and tested them during CSCWG with 10 wt.%
isopropanol in water. It is well known that introducing sur-
face functional groups by acid treatment is beneficial for the
metal dispersion of many carbon supported noble metal cata-
lysts.13 The surface functional groups are known to act as
“anchoring sites” during the catalyst preparation favoring a
better metal dispersion. It was reported that the pre-
treatment of the carbon support with HNO3 was able to
improve the Pt dispersion from 13% up to 92% compared to
the fresh Pt/C catalyst.14 Wang et al.15 reported also the bene-
fit of the HNO3 pre-treatment on the Pt dispersion for multi-
walled carbon nanotubes (MWCNT) supported Pt catalysts.
Numerous studies16,17 have reported on the benefit of
the HNO3 pre-treatment for Ru/C catalysts. For instance,
Li et al.16 found that the HNO3 pre-treatment was able to
enhance the Ru dispersion from 17% up to 38% in compari-
son to the fresh catalyst. Zhu et al.17 reported an increase of
the Ru dispersion from 24% to 60% after the HNO3 pre-
treatment. Gallegos-Suarez et al.18 have recently studied the
effect of the surface functional groups of Ru/C catalysts dur-
ing hydrogenolysis of glycerol and they observed that the
HNO3 pre-treatment of the carbon support led to a smaller
Ru dispersion. For the fresh 4% Ru/C catalyst they found a
Ru dispersion of 19%, whereas the dispersion was 15% for
the catalyst prepared with HNO3. According to them, the sur-
face functional groups were responsible for the Ru nanoparti-
cle (NPs) sintering during the thermal treatment. These con-
tradictory observations might be explained by the nature of
carbon which is a versatile material containing a variety of
impurities as well as exhibiting different surface and physical
structure properties. In relation to gaseous fuels production,
Wang et al.15,19 studied the effect of the surface functional
groups during aqueous phase reforming of ethylene glycol
over single-walled carbon nanotubes (SWCNT) and MWCNT
supported Pt catalysts. They observed that the surface func-
tional groups negatively affected the catalytic activity.
2. Experimental
2.1 Catalyst preparation and characterization
A selected carbon support originating from coconut shells
(denoted here as Org10_CO from Desotec) was sieved to yield
a fraction of 0.3–0.8 mm, and then pre-treated with HNO3
(30 vol.%) under reflux (363 K) for 5 hours. After filtration,
the carbon support pre-treated with HNO3 (CHNO3) was
washed with deionized water until neutralization of the fil-
trate was reached and finally dried at 363 K overnight in an
oven. CHNO3 was then treated under He at 723 K for 4 hours
in order to remove the less thermally stable surface func-
tional groups. The thermally pre-treated CHNO3 support is
denoted as CHT. The samples were impregnated in a pure
acetone solution of RuCl3·xH2O (99.9%, Alfa Aesar) during
24 hours, followed by solvent evaporation in a rotary evapora-
tor and washed with 200 mL of pure water during filtration.
Finally, the samples were dried in an oven at 363 K overnight.
Prior to each experiment, the fresh catalysts were reduced
under flowing H2/Ar (10 : 90, 20 mL min−1) at 723 K for
4 hours in order to clean the catalyst from chloride and other
carbon deposits. The Ru loading was determined by ICP-OES
(Liberty 110, Varian) by measuring the Ru concentration in
the spent impregnation solution by taking into account the
Ru loss during the washing. The surface functional groups
were characterized qualitatively by temperature-programmed
oxidation (TPO) and by temperature-programmed desorption
(TPD) coupled to a FTIR detector for CO2 and CO detection
(NETZSCH STA 449 C). For the TPO, 10 mg of sample was
loaded and heated from room temperature (RT) to 383 K
under Ar atmosphere for 30 min and then heated up to 1173 K
at 10 K min−1 under flowing O2/Ar (10 : 90, 10 mL min−1). The
ash content of the carbon support was determined by TPO.
For the TPD, 50 mg of sample was weighted and measured
from RT to 1173 K with a ramp of 10 K min−1 under flowing
Ar (20 mL min−1). The surface functional groups quantifica-
tion was performed by Boehm titration following the stan-
dardization procedure proposed by Goertzen et al.20,21 This
method relies on the existence of oxygen surface groups hav-
ing different acidities that can be neutralized by bases having
different strengths. NaHCO3 (pKa = 6.37), Na2CO3 (pKa
=
10.25) and NaOH (pKa = 15.74) were used in this work. As the
weakest base, NaHCO3 neutralizes only the carboxylic groups,
Na2CO3 neutralizes the lactonic and carboxylic groups, and
NaOH neutralizes the phenolic, lactonic, and carboxylic
groups. The molar amount of each surface functional group
was estimated by difference. 1.5 g of the carbon support was
added to 50 mL of one of the three bases: 0.05 M NaHCO3;
0.05 M Na2CO3 and 0.05 M NaOH. Then, the samples were
sealed and shaken for 24 hours at RT by a linear shaker. After
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filtration, an aliquot of 10 mL from the sample solution was
taken and neutralized by an excess of 0.05 M HCl and then
back-titrated with 0.05 M NaOH. Note that the titration was
performed under inert atmosphere (Ar) to avoid dissolution
of CO2 from the atmosphere. The endpoint was determined
by phenolphthalein. The determination of the moles of
carbon surface functionalities (nCSF) was calculated by the
following equation according to the back-titration method:
volume was determined with the t-plot method. Prior to N2-
physisorption degassing under He at 573 K for 6 hours was
carried out for all the samples. The Ru NPs were characterized
by Scanning Transmission Electron Microscopy (STEM). The
measurements were performed with a spherical aberration
corrected STEM microscope (Hitachi HD-2700) with a cold
field emission source, operated at an acceleration voltage of
200 kV and equipped with a high angle annular dark field
(HAADF) detector. For each sample, different areas were care-
fully selected in order to have a reliable representation of the
average Ru NPs size. The average Ru NPs size (dp,STEM) and
the Ru dispersion (DSTEM) were calculated as:
n
VB
Va
nCSF

HCl [B]VB  ([HCl]VHCl [NaOH]VNaOH
nB
)
(1)
where [B] and VB correspond to the concentration and the
volume, respectively, of the reaction base mixed with the car-
bon support. [HCl] and VHCl are the concentration and the
volume of the acid added to the aliquot (Va) previously taken
from VB. [NaOH] and VNaOH are related to the concentration
and to the volume used in the back-titration that neutralizes
the remaining moles of acid. Finally, nHCl/nB is the molar
ratio of acid to base allowing to distinguish between mono-
protic vs. diprotic reaction bases.
n d 3

i
i
i
dp,STEM

(4)
(5)
n d 2

i
i
i
dat 3.32
dp,STEM
DSTEM  1.23
for 0.2  DSTEM  0.92
CNS elemental analysis (Vario EL cube, Elementar) was
performed to quantify the carbon, nitrogen and sulfur content.
The catalysts were characterized by H2-temperature-programmed
reduction (H2-TPR) and CO pulse chemisorption in a fully
automated instrument (TPD/R/O 1100, Thermo) connected to
a TCD. For each H2-TPR measurement, 100 mg of sample
was weighted and heated (10 K min−1) from RT to 723 K
under flowing H2/Ar (10 : 90, 20 mL min−1). For the CO pulse
chemisorption, the samples were reduced under H2/Ar (10 : 90,
20 mL min−1) at 723 K for 4 hours in order to clean the ruthe-
nium surface from any deposited carbon species. Then it was
flushed by pure He at 723 K for 1.5 hours to remove adsorbed
H2 from the catalyst and finally cooled down to RT. The CO
pulses were carried out with CO/He (20 : 80, 40 mL min−1) at
RT. The dispersion was calculated by assuming 1 as the stoi-
chiometric factor for CO : Ru. The following formula was used
for determining the dispersion:
22
where ni is the number of particles with diameter di.
2.2 Catalytic tests
The CSCWG experiments were carried out in a fixed-bed plug
flow (PF) reactor described in detail elsewhere.11 Isopropanol
was purchased from VWR BDH Prolabo (99.8%). The mass
flow rate (F) was kept at 3 g min−1. The amount of Ru/C
catalyst added in the reactor was ca. 0.2 g. WHSVgRu was
−1
1972 gOrg gRu h−1 for all experiments. The reactor was first
heated up with water to 723 K under 30 MPa prior to
switching to isopropanol (10 wt.% in water). The residence
time in the catalyst bed was ca. 0.6 s (pure water density at
reaction conditions is 148 kg m−3). The experiments were all
carried out for 50 hours.
2.3 Analytical methods for gaseous and liquid effluents
The gas phase was analyzed offline with a gas chromatograph
(HP 6890, columns: HP-Plot Q 30 m × 0.53 mm × 40 μm and
HP-Plot Molecular Sieve 5A, 30 m × 0.53 mm × 40 μm) with
helium as the carrier gas using a Thermal Conductivity
Detector (TCD) to detect CO2, CH4, CO and H2 and a Flame
Ionization Detector (FID) for higher hydrocarbons IJC2H6 and
C3Hx). The liquid samples were collected regularly manually
and the total organic carbon (TOC) was measured with a TOC
analyzer (Vario TOC cube, Elementar).
Nads  F 10 Mmet
s
DCO

(2)
wmet
where Nads is the amount of gas adsorbed in pulse chemisorp-
tion (mmole g−1); Fs corresponds to the stoichiometric factor
(metal mole per gas mole); Mmet is the metal atomic weight
(g mole−1) and wmet is the metal loading of the catalyst (wt.%).
The average metal particle sizes were calculated as:
dat 5.01
DCO
2.4 Chemical equilibrium composition
dp,CO

for DCO  0.2
(3)
The thermodynamic chemical equilibrium calculation was
performed using the Aspen Plus® 2006 simulation package
by using the Peng–Robinson equation of state. The values for
the dry gas composition at the thermodynamic chemical
equilibrium (723 K, 30 MPa and 10 wt.% isopropanol)
are: CO2 = 24.7 vol.%; CH4 = 65.6 vol.%; H2 = 8.7 vol.%; CO =
0.7 vol.%.
where dat is the atomic diameter of Ru (dat = 2.6 Å).22 The
N2-physisorption measurements were performed with an
Autosorb-1 (Quantachrome Instruments) for determining the
BET specific surface area and the porosity. The total pore
volume was measured at p/p0 = 0.99 and the mesopore
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2.5 Terms and definitions
In Table 2, CHONS elemental analysis results show a
decrease of the carbon content after the HNO3 pre-treatment
due to the increase of the oxygen content, whereas the nitro-
gen and the hydrogen concentration increased after the oxi-
dation pre-treatment. HNO3 is able to provide a nitronium
For comparing the catalytic performances based on the Ru
amount, the weight hourly space velocity normalized to one
gram of Ru (WHSVgRu) is used:
+
ion IJNO2 ) to an aromatic ring (nitration reaction).25,27 After
ꢀ
mOrg
WHSVgRu

(6)
mcat  wRu
the thermal pre-treatment, a fraction of the nitrogen was
released probably by desorption of NO2 that is reported to
decompose around 553 K.27 The new increase of the carbon
concentration after the thermal pre-treatment reflects the
loss of oxygen. Indeed, a considerable fraction of the acidic
oxygen-containing functional groups (e.g. carboxylic groups)
are removed during the thermal treatment.25 The sulfur
concentration was found to decrease slightly after the HNO3
pre-treatment.
The observed activity is defined as the total organic car-
bon conversion (Xc) from the feed to the liquid effluent:
TOCOut
XC (%) 1
100%
(7)
TOCFeed
The carbon gasification efficiency (GEC) is the relation
between the total amount of carbon in the gas phase and the
total amount of carbon in the feed, defined as:
In Fig. 1, the first peak at 490 K for the CHNO3 can be
assigned to the decomposition of the carboxylic groups.28
The CHNO3 started to oxidize at a lower temperature than
the fresh C and the CHT. The fresh C appears to be fully oxi-
dized at 870 K, whereas the temperature was shifted to 900 K
and 940 K for the CHNO3 and CHT, respectively. According to
this observation it seems that the HNO3 pre-treatment
enhanced the thermal resistance of the carbon support.
Moreover, the thermal pre-treatment further increased its
thermal stability. Chiang et al.29 have reported that the ther-
mal stability of the MWCNT was improved after the
H2SO4/HNO3 pre-treatment. According to them, some existing
reactive groups on the support such as CH2 and –CH are
decomposed during the acidic pre-treatment, rendering the
material more thermally resistant. During the TPD experi-
ments the surface functional groups decompose within a spe-
cific temperature range (according to the type of surface func-
tional groups) to produce CO2, CO, and H2O. In Fig. 2, the
TPD results for the fresh C support show only a small CO2
desorption peak at 973 K likely corresponding to carboxylic
anhydride while the CO signal above 1173 K can be related to
carbonyl and/or quinone.30 After the HNO3 pre-treatment the
CO2 desorption peaks at ca. 573 K and 723 K can be assigned
to carboxylic and lactone, respectively. The CO2 and CO
desorption peaks above 973 K are attributed to more ther-
mally stable groups such as carboxylic anhydride, quinone,
and carbonyl. The effect of the thermal pre-treatment under
He carried up to 723 K removed a large quantity of carboxylic
groups, whereas the more thermally stable groups were
preserved.
Total mol CGas
GEC (%) 
100%
(8)
Total mol CFeed
3. Results and discussion
3.1 Characterization of the carbon support
Some structural properties of the carbon support are listed in
Table 1.
The physical structure of the carbon support was affected
by the HNO3 pre-treatment since a loss of the BET SSA and
the total pore volume occurred. This decrease of the porosity
was caused either by a collapse of the physical structure or by
a physical blockage of the pores by the formation of humic
substances during the HNO3 pre-treatment.23–25 After the
thermal pre-treatment, the micropore volume increased by
almost 30% while the mesopore volume decreased by 35%.
Possibly ashes and humic acids trapped in the micropores
were removed and led to the increased microporosity.
The ash content was reduced by 55% after the HNO3 pre-
treatment, showing its efficiency for cleaning the pores as
reported by others.17,26
Table 1 Structural properties of the carbon support
BET SSA Vtotal
Sample (m2 g−1 (cm3 g−1
Vmesop.
Vmicrop.
Ash content
(wt.%)a
)
)
(cm3 g−1
)
(cm3 g−1
)
In Fig. 3, the Boehm titration results reveal a good correla-
tion with the TPD results since no carboxylic groups and only
a small amount of lactonic groups were detected on the fresh
C. After the HNO3 pre-treatment, a considerable increase of
Fresh C 802
0.83
0.63
0.66
0.69
0.45
0.37
0.14
0.18
0.29
11.0
4.9
4.6
CHNO3
CHT
781
999
a
Determined by TPO analysis.
Table 2 CHONS elemental analysis of the carbon support
Sample
C (wt.%)
H (wt.%)
O (wt.%)
N (wt.%)
S (wt.%)
Fresh C
CHNO3
CHT
85.12 0.05
78.58 0.10
81.88 0.05
0.75 0.03
0.91 0.01
0.70 0.02
5.00 0.00
13.00 0.00
10.00 0.00
0.54 0.01
1.26 0.04
1.08 0.06
0.26 0.02
0.17 0.01
0.19 0.01
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surface properties. As expected, the thermal pre-treatment
mainly removed the less thermally stable groups, i.e. the car-
boxylic groups while the lactonic groups were hardly affected.
According to the Boehm titration results, no thermally stable
surface functional groups (carbonyl, quinone) were found on
the carbon surface. Therefore the pre-treatment with HNO3
rather produced carboxylic and lactonic groups.
3.2 Catalyst characterization
The characteristics of the Ru/C catalysts are listed in Table 3.
The impregnation with Ru affected the specific surface
area significantly (compare with Table 1). Although the three
Ru/C catalysts have the same Ru loading, their respective spe-
cific surface area differs considerably. According to the Ru
NPs measurements, it seems that larger Ru NPs led to a
reduction of the specific surface area and of the micropore
volume. The blockage of the entrance of some of the micro-
pores by larger Ru NPs may be the reason.
Fig. 1 TPO analysis of the fresh C, CHNO3, and CHT
.
Fig. 4 shows the STEM images of the three Ru/C catalysts
where the bright dots represent the Ru NPs. According to the
histograms of the particle size distribution, the Ru NPs size
distribution of the 4% Ru/C appears to be larger in compari-
son to the 4% Ru/CHNO3 and 4% Ru/CHT since Ru NPs from
2–10 nm can be seen. The main reason is the lack of anchor-
ing sites which help to obtain smaller Ru NPs during the cat-
alyst preparation.13,16 The larger Ru NPs of the 4% Ru/CHT in
comparison to the 4% Ru/CHNO3 reveals that carboxylic
groups play an important role for the Ru dispersion improve-
ment. The CO chemisorption results confirmed that the sur-
face functional groups are needed for the Ru dispersion
improvement. However, the Ru NPs size based on the CO
chemisorption has been overestimated (7–11 nm) for the
three Ru/C catalysts. The presence of residual chloride com-
ing from the Ru salt precursor (RuCl3) is responsible for the
inhibition of the CO adsorption on the Ru surface.11,16,18,31
For instance, Gallegos-Suarez et al.18 have compared the
effect of residual chloride on the Ru NPs size by preparing
4% Ru/C catalysts with a RuCl3·xH2O and a RuIJNO)IJNO3)3.
The Ru NPs size determined with CO chemisorption was
6.8 nm for the catalyst prepared with RuCl3·xH2O vs. 3 nm
for the for the catalyst prepared with RuIJNO)IJNO3)3. Interest-
ingly, the Ru NPs were in the range of 3 nm when deter-
mined with transmission electron microscopy for both cata-
lysts. Therefore, in presence of residual chloride, the STEM
measurements are much more reliable for the determination
of the Ru NPs size. In Fig. 5, the H2-TPR results of the three
Ru/C catalysts are depicted. After integration of the reduction
peaks (see Table 3), the 4% Ru/CHNO3 exhibits the highest H2
consumption, likely due to the reduction of some surface
functional groups in the vicinity of the Ru NPs and/or its
higher Ru dispersion in comparison to the two other
catalysts.
Fig. 2 CO2 and CO-TPD analysis of the fresh C, CHNO3, and CHT
.
Fig. 3 Boehm titration results analysis of the fresh C, CHNO3, and CHT
.
the carboxylic and lactonic groups was observed, and the
total number of acidic sites was ca. 11 times higher. Hence,
the pre-treatment with HNO3 was able to modify the carbon
Two distinct reduction peaks can be seen for the Ru/C cat-
alysts. The reduction peaks at 538 K and 563 K can be both
attributed to the reduction of RuCl3 to Ru0.32,33 Interestingly,
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Table 3 Characteristics of the Ru/C catalysts
BET SSA
Vtotal
Vmicrop.
Ru loading
(wt.%)
H2 consumption
DCO
DSTEM
dp,CO
dp,STEM
Sample
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(μmol g−1
)
(−)a
(−)b
(nm)a
(nm)b
4% Ru/C
4% Ru/CHNO3
4% Ru/CHT
551
728
646
0.67
0.70
0.61
0.08
0.14
0.12
4.2
4.0
4.2
790
964
867
0.09
0.13
0.10
0.26
0.39
0.31
15.3
9.9
12.5
4.5 0.2
2.7 0.1
3.6 0.1
a
Determined by CO pulse chemisorption. b Determined by STEM.
Fig. 4 STEM images of the (a) 4% Ru/C, (b) 4% Ru/CHNO3, and (c) 4% Ru/CHT catalysts.
carboxylic groups which have stabilized Ru against the reduc-
tion. Gallegos-Suarez et al.18 have also observed a similar
shift from 470 K to 520 K when the Ru/C catalyst was pre-
treated with HNO3. For the 4% Ru/CHT, the two distinct
reduction peaks support the suggestion that the carboxylic
groups contribute to the enhancement of the metal–support
interaction.
3.3 CSCWG over Ru/C catalysts
CSCWG of 10 wt.% isopropanol in the absence of a catalyst
(blank experiment) at supercritical conditions (723 K and
30 MPa) showed a very low total organic carbon conversion
(XC = 4%) indicating the inertness of the reactor wall as well
as the stability of isopropanol at these conditions. The cata-
lysts were then tested with 10 wt.% isopropanol at 723 K and
30 MPa. As shown in Fig. 6 and in Table 4, by working at a
Fig. 5 H2-TPR profiles of the 4% Ru/C, 4% Ru/CHNO3, and 4% Ru/CHT
catalysts.
−1
WHSVgRu = 1972 gOrg gRu h−1, all the three catalysts were
able to gasify properly isopropanol to a CH4–rich gas during
50 hours.
The gas composition was close to the thermodynamic
chemical equilibrium indicating the good performance of
Ru/C catalysts for enhancing the methanation reaction.
it seems that the presence of the surface functional groups
favored the reduction of Ru at higher temperature suggesting
a better interaction between Ru and the carbon support. It is
likely that Ru was grafted on the oxygen function of the
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Fig. 6 CSCWG (723 K, 30 MPa) of 10 wt.% isopropanol over the (a) 4% Ru/C, (b) 4% Ru/CHNO3, (c) and 4% Ru/CHT catalysts during 50 hours with
−1
WHSVgRu = 1972 gOrg gRu h−1. (d) Represents the evolution of the total organic carbon conversion (XC) of the three Ru/C catalysts. The dashed
lines denote the calculated thermodynamic equilibrium concentrations.
treated (1273 K under He) the carbon support after the HNO3
pre-treatment and reported an activity ca. 15% lower in com-
parison to the fresh Pt/MWCNT catalyst. They claimed that
the main reason for the lower catalytic activity was a change
of the carbon surface polarity that inhibited the adsorption
of the reactants caused by the carboxylic groups surrounding
the Pt NPs. The Ru NPs size effect might also be responsible
for the different activity observed. Masini et al.34 observed
that the turnover frequency (TOF) was higher for 10 nm Ru
NPs than for 4 nm NPs confirming the structure-sensitivity of
the methanation reaction. According to them, larger Ru NPs
exhibit a higher concentration of under-coordinated sites
(e.g. kinks or steps) which are reported to be needed for the
improvement of the CO bond dissociation since the latter
only takes place on these specific sites.35 The high capability
for the CO bond cleavage is determinant since the latter is
known to be the rate-determining step of the methanation
reaction.35,36
Table 4 Results summary after 50 hours of CSCWG (723 K, 30 MPa) of
10 wt.% isopropanol for the Ru/C catalysts performed at WHSVgRu = 1972
g
Org gRu−1 h−1
Gas composition (vol.%)
Time XC GEC
Sample
(h)
(%) (%)
CH4 CO2 H2 CO C2H6 C3Hx
4% Ru/C
50
96.0 102.0 65.8 24.3 9.9 <0.1 <0.1 <0.1
4% Ru/CHNO3 50
4% Ru/CHT 50
94.5 101.2 65.0 20.6 14.1
95.9 102.0 66.2 22.3 11.2
0.3 <0.1 <0.1
0.4 <0.1 <0.1
Furthermore, the absence of higher hydrocarbons (≥C2+) in
the gaseous product is also a reliable indication confirming
the high activity of the Ru/C catalysts for C–C bond cleavage.
After 50 hours, the activity (XC) of all catalysts dropped from
99% to ca. 95%. This slight decrease was due to a progressive
coke deposition on the Ru/C catalyst as reported in a previ-
ous study.11 Although the Ru dispersion of the 4% Ru/CHNO3
was higher than the two other catalysts, its catalytic activity
was slightly lower (see Fig. 6 (d)). Moreover, the lower CH4
and higher H2 and CO concentration for the 4% Ru/CHNO3
seems to indicate that the methanation reaction was not fully
achieved in comparison to the 4% Ru/C. Wang et al.15 have
observed the same negative effect of the carboxylic groups
during aqueous phase reforming (498 K, 2.7 MPa) of ethylene
glycol over Pt/MWCNT catalysts where the catalytic activity of
the catalyst prepared with HNO3 was ca. 30% lower than the
fresh Pt/MWCNT catalyst. They had also thermally pre-
3.4 Characterization of the spent catalysts
The Ru/C catalysts were characterized by N2-physisorption
after CSCWG (see Table 5). Although some slight changes of
the porosity were noted after 50 hours, the physical structure
of the catalysts was well preserved. These results are relevant
because they confirm the high stability as well as the robust-
ness of the physical structure of the carbon support during
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CSCWG. The micropore volume of the spent 4% Ru/C
increased to twice the value of the fresh catalyst. A similar
result was reported in a previous study.11 Up to now there is
no good explanation for such a large increase but it is likely
that the washing out of some dust from the pores may form
some additional micropores, since the carbon support of the
4% Ru/C was not pre-treated. Although the catalytic activity
slightly decreased from 99% to 95%, the porosity was not
affected by the coke deposits. In a previous study,11 the
porosity of a Ru/C catalyst was also well preserved during
CSCWG of 10 wt.% isopropanol although the catalytic activity
dropped from 100% to 90%. However, when the catalytic
activity dropped up to 10%, most of the porosity was lost due
to coke within the catalyst support.
In Fig. 7, three distinct CO2 desorption peaks were
observed for the three spent catalysts. Although these peaks
look relatively similar, some differences in their intensity and
desorption temperature appear. For instance, the intensity of
the first desorption peak (540–560 K) increases in the follow-
ing order: 4% Ru/CHNO3 > 4% Ru/CHT > 4% Ru/C, while the
opposite is observed for the desorption peak at 730 K. Inter-
estingly, the CO2 desorption peak at high temperature is
shifted to higher temperature for the 4% Ru/CHT (850 K) in
comparison to the two other catalysts (800 K). In Fig. 8, a
similar trend was observed for the 4% Ru/CHT since the CO
desorption started at 850 K while for the other catalysts the
Fig. 8 CO-TPD analysis of the fresh and spent catalysts.
latter began at 800 K. At the moment it is not clear what
this desorption peak corresponds to. It may be either some
CO2/CO species bonded on the Ru surface or even some func-
tional groups in the vicinity of Ru. These peaks might be
related to the CO2/CO desorption from different Ru active
sites. In fact, each catalyst exhibits specific active sites
according to its Ru NPs size distribution.
It is also interesting to discuss the TPD spectra of the
fresh Ru/C catalysts. It seems that the Ru incorporation on
the carbon support followed by the reduction treatment have
both considerably changed the carbon surface functionality
(compare with Fig. 2).
Table 5 Physical structure of the fresh and spent Ru/C catalysts
Time BET SSA Vtotal
Vmesop.
Vmicrop.
Sample
(h)
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(cm3 g−1
)
For instance, for the 4% Ru/C, three CO2 desorption peaks
at 580 K, 640 K, and 780 K are observed, whereas for the
fresh C only one desorption peak at 973 K are detected. The
same observation can be done for the CO desorption peak at
910 K since the CO desorption occurred above 1100 K for the
fresh C. It seems likely that the presence of Ru and the reduc-
tion treatment are both responsible for this shift to lower
temperature. The desorption of the surface functional groups
in the vicinity of the Ru NPs are certainly influenced and
facilitated by the interaction with Ru. The similarity of the
CO2 desorption peaks for the 4% Ru/CHNO3 with the 4% Ru/C
indicates that the carboxylic groups were removed during
the reduction treatment. This is a relevant observation
confirming that prior to CSCWG the carboxylic groups were
fully decomposed. As a consequence, it seems unlikely that
the lower catalytic activity for the 4% Ru/CHNO3 was due to a
change of the carbon surface acidity but rather due to the for-
mation of smaller Ru NPs exhibiting less under-coordinated
sites with the HNO3 pre-treatment.
4% Ru/C
Spent 4% Ru/C
4% Ru/CHNO3
Spent 4% Ru/CHNO3 50
4% Ru/CHT
Spent 4% Ru/CHT
551
712
728
693
646
680
0.67
0.72
0.7
0.72
0.61
0.64
0.59
0.57
0.56
0.6
0.49
0.51
0.08
0.15
0.14
0.12
0.12
0.13
50
50
4. Conclusions
The surface functional groups were needed for the improvement
of the Ru dispersion where the carboxylic groups played a role
as anchoring groups for the Ru salt precursor. As a result, the
Fig. 7 CO2-TPD analysis of the fresh and spent catalysts.
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Ru NPs size was 2.7 0.1 nm for the Ru/C prepared with
HNO3; 3.6 0.1 nm for the Ru/C prepared with HNO3 and
thermally pre-treated (723 K under He); and 4.5 0.2 nm for
the untreated Ru/C. The pre-treatment of the carbon support
with HNO3 was not able to improve the catalytic performance
of Ru/C catalyst during CSCWG of isopropanol. The catalytic
performance of the Ru/C prepared with HNO3 and thermally
pre-treated was similar to the one with the untreated carbon
support.
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Acknowledgements
This work was financially supported in the frame of the
SunCHem project by the Competence Center Energy and
Mobility (CCEM-CH) and swisselectric research. The authors
would like to thank Albert Schuler for the ICP-OES analyses,
Martin Elsener for the CO pulse chemisorption measure-
ments, and Alexander Wokaun for the fruitful discussions.
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