Glycerol Oxidation Catalyzed by High-valency Ruthenium Species at Electrochemical Interfaces

Article abstract of DOI:10.1246/cl.200056

Herein we report high-valency ruthenium (Ru) species as a new class of electrocatalyst for glycerol oxidation. In this study, Ru-modified covalent triazine framework (Ru-CTF) and ruthenium oxide supported on carbon (RuO₂/C) were used as model materials with high-valency Ru species. The results of this work show that the deep oxidation reactions of glycerol involving C-C bond cleavage can be effectively suppressed by increasing the glycerol concentration relative to the number of active Ru atoms for both catalysts.

Full text of DOI:10.1246/cl.200056

Received: January 24, 2020 | Accepted: February 28, 2020 | Web Released: March 5, 2020
CL-200056
Glycerol Oxidation Catalyzed by High-valency Ruthenium Species at Electrochemical Interfaces
1
1
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3
4
Hiro Tabata, Shintaro Kato, Shingi Yamaguchi, Takashi Harada, Kazuyuki Iwase,
1
,3
1,3
Kazuhide Kamiya,* and Shuji Nakanishi*
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
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2
3
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
4
Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
E-mail: kamiya@chem.es.osaka-u.ac.jp (K. Kamiya), nakanishi@chem.es.osaka-u.ac.jp (S. Nakanishi)
Herein we report high-valency ruthenium (Ru) species as a
new class of electrocatalyst for glycerol oxidation. In this study,
Ru-modified covalent triazine framework (Ru-CTF) and ruthe-
Koper’s group reported that high selectivity for glyceraldehyde
was obtained using Pt catalysts at potentials lower than 0.8 V vs.
RHE in 0.5 M H2SO4. At higher potentials, deep oxidation
6
nium oxide supported on carbon (RuO /C) were used as model
involving C-C bond breaking proceeded, resulting in reduced
selectivity for the desired product. Then, they subsequently
reported that this effect of potential could be changed by
2
materials with high-valency Ru species. The results of this work
show that the deep oxidation reactions of glycerol involving C-C
bond cleavage can be effectively suppressed by increasing the
glycerol concentration relative to the number of active Ru atoms
for both catalysts.
9
modifying the electrode surface on the atomic scale. Many
previous studies have used noble metals such as Pt, Pd and Au
as electrocatalysts, and a large body of knowledge has been
accumulated regarding the glycerol oxidation reaction with
1
0­12
Keywords: Glycerol
|
Electrocatalyst
|
Ruthenium
catalysis by these materials.
The present work investigated heterogeneous Ru-based
materials containing high-valency Ru with the oxidation number
of 4 or more as a new class of electrocatalysts for electrochem-
ical glycerol oxidation. Although the Ru species have been
shown to serve as electrocatalytic sites for alcohol oxidation
Biodiesel fuel (BDF) has attracted attention as a carbon-
neutral energy source. However, an attendant issue is the lack of
uses for the large quantity of glycerol that is generated as a by-
product of BDF production.1 Therefore, there is a significant
demand for the development of partial oxidation processes that
upgrade glycerol to useful chemicals (Figure 1). To date, various
catalytic processes have been devised that have successfully
converted glycerol to energy and/or valuable compounds,
,2
1
3­15
reactions,
no studies have been conducted regarding the
application of these sites to the partial oxidation of glycerol.
In the work reported herein, the electrocatalytic properties of
ruthenium oxide (RuO /C) and a Ru-modified covalent triazine
2
3
­5
including aldehydes, ketones, carboxylic acids, and so on.
framework (Ru-CTF) were investigated and compared, focusing
on the depth of the resulting oxidation.
However, the selective conversion of glycerol to C3 compounds
with higher economic value remains a challenge, although
electrochemical technology is a promising approach to achiev-
ing the synthesis of partially oxidized compounds. Previous
studies have shown that various factors, including electrode
potential, electrolyte pH and atomic orientation of electro-
catalysts, can determine the extent of oxidation and hence the
selectivity for various reaction products.6 As an example,
Ru-CTF is an electrocatalyst in the metal-doped covalent
organic framework (Me-COF) category. These materials are
known to exhibit various electrocatalytic functions depending
1
6­20
on the metal with which they are doped.
Examples include
Cu-COF, which shows electrocatalytic activity during nitrate
1
6,17
reduction in acid and the oxygen reduction reaction,
and
­8
Ni-COF and Co-COF, both of which can serve as efficient
1
8
electrocatalysts for carbon dioxide reduction. Our own group
has also established that Ru-CTF catalyzes the electrochemical
1
9
oxidation of benzyl alcohol to benzoic acid via benzaldehyde.
Importantly, although conventional Ru-based organometallic
complexes can also catalyze the same reaction, Ru-CTF exhibits
1
9
greater resistance to oxidative decomposition. Since the
composition of the products is expected to change with the
progress of deep oxidation of glycerol, it is not appropriate to
use organometallic Ru-complexes that are less resistant against
oxidative decomposition. This is the reason for adopting
Ru-CTF with high durability as a model electrocatalyst for
comparison with RuO2/C in the present work.
Detailed protocols and characterization results for the Ru-
CTF and RuO /C can be found in the Supporting Information
2
associated with this paper (Figure S1). In contrast to RuO2/C,
Ru atoms were expected to be loaded into the CTF and held
in stable configurations by forming coordination bonds with
nitrogen in the framework. Analyses by transmission electron
Figure 1. Reaction pathways for glycerol oxidation.
Chem. Lett. 2020, 49, 513–516 | doi:10.1246/cl.200056
© 2020 The Chemical Society of Japan | 513

2
microscopy (TEM) and energy-dispersive X-ray (EDX) spec-
troscopy (Figure S1b) demonstrated a lack of Ru aggregates in
the Ru-CTF. (TEM images for RuO2/C catalyst are also shown
in Figure S1a). Figure S1c shows the Fourier transform of the
loading was 5.1 ¯g/cm . An oxidation current can be observed
for both catalysts near the potential at which high-valency Ru
are formed, suggesting that the Ru-species was the active species
for glycerol oxidation, similar to the oxidation of benzyl alcohol
3
19
k -weighted extended X-ray absorption fine structure (EXAFS)
on Ru-based materials.
Ru-K edge oscillations for the synthesized Ru-CTF. The Ru-
CTF did not generate a peak corresponding to Ru­Ru bonds,
whereas peaks assignable to Ru­N and Ru­Cl bonds were
observed at 1.6 and 1.9 ¡, respectively. These results demon-
strate that atomically-dispersed Ru was present in the Ru-CTF,
as expected. The surface concentration of each element was also
determined by semi-quantitative X-ray photoelectron spectros-
copy (XPS), from which it was determined that the atomic Ru/N
ratios in the Ru-CTF and RuO2/C were 0.24 and 0.40, respec-
tively (Figure S1d). The Ru-CTF and RuO2/C particles were
separately loaded onto glassy carbon substrates by drop casting
to fabricate the electrodes.
In the case of the Ru-CTF, the glycerol oxidation current
increased beginning at 1.0 V and plateaued at approximately
1.2 V. The oxidation current was not observed for CTF without
Ru-loading (Figure S2), further supporting that Ru was the
active species for the glycerol oxidation. In contrast, the onset
potential for glycerol oxidation over the RuO2/C was approx-
imately 1.2 V and the associated current density was much lower
than that produced by the Ru-CTF. Figures 2c and d show the
chronoamperometry data acquired at 1.4 V in electrolytes with
various glycerol concentrations. When using the Ru-CTF, the
current increased as the glycerol concentration was increased
from 10 mM to 1 M (Figure 2c), while the current generated by
the RuO2/C plateaued at glycerol concentrations greater than
100 mM (Figure 2d).
The black curve in Figure 2a is the cyclic voltammogram
obtained in 0.1 M HClO4 for the Ru-CTF electrode, and exhibits
a pair of redox peaks at approximately 1.2 V. A previous study
demonstrated that high-valency Ru species were formed at
this potential in association with the oxidation of coordinated
It is known that this oxidation proceeds via the formation of
3
glyceraldehyde or dihydroxyacetone (Figure 1). Herein, C3
compounds resulting from the two- or four-electron oxidation of
glycerol (i.e., glyceraldehyde, glyceric acid, dihydroxyacetone
and hydroxypyruvaldehyde) are referred to as partial oxidation
products. Similarly, products obtained from additional down-
stream reactions are defined as deep oxidation products. In this
work, the partial oxidation products were analyzed by high
performance liquid chromatography (HPLC) (Figure S3) while
liquid chromatography-mass spectrometry (LC/MS) and differ-
ential electrochemical mass spectrometry (DEMS) were used
to verify the generation of specific deep oxidation products
(Figures S4 and S5). Among the partial oxidation products,
only glyceraldehyde and glyceric acid could be detected at
quantifiable levels, and the formation of dihydroxyacetone was
not confirmed. Among the potential deep oxidation products,
only glycolic acid and carbon dioxide were detected while the
amounts of tartronic acid, hydroxypyruvic acid, oxalic acid, and
formic acid were all below the detection limit of the method.
Figure 3a plots typical time courses of the accumulated
amounts of the partial oxidation products obtained using the
Ru-CTF. With increases in the Coulombic charge (Q), it is
evident that glyceraldehyde (blue) and glyceric acid (orange)
were gradually accumulated. The faradaic efficiency (FE) values
calculated from the data in Figure 3a are summarized in
Figure 3b. It should be noted that the contributions of catalysts
decomposition and oxygen evolution reactions are excluded
when calculating FE (please find details in the Supporting
Information). In the case of the Ru-CTF, the overall FE value for
the partial oxidation products was approximately 20%, while the
remaining FE was associated with the deeper oxidation products
including CO2 (Figure S5). It is also apparent that the FE
associated with the production of glyceraldehyde slightly
decreased (from 10% to 5%) with increases in Q. In addition,
the selectivity for glyceraldehyde generation was significantly
higher when employing the RuO2/C (Figures 3c and d), such
that the overall FE for the partial oxidation products reached
approximately 60%. As shown in Figures 2a and b, the onset
potential for glycerol oxidation over the Ru-CTF was approx-
2
1
water molecules. Redox peaks assignable to the oxidation of
coordinated water (i.e., the formation of high-valency Ru) were
also identified in the data obtained for the RuO2/C electrode
(Figure 2b, black). However, in contrast to the Ru-CTF,
repeating Ru­O­Ru sites were evidently generated on the
RuO2/C, leading to competition with water oxidation via O­O
bond formation at neighboring Ru sites.19 In fact, compared to
Ru-CTF, a large current increase due to oxygen evolution has
been observed when employing RuO2/C at potentials more
positive than 1.4 V.
Current density ( j) versus potential (U ) curves for the
Ru-CTF and RuO2/C electrodes obtained in 0.1 M HClO4
solutions containing 1 M glycerol are presented as red curves
in Figures 2a and 2b, respectively. In both catalysts, the Ru
Figure 2. (a) (b) Cyclic voltammetry of Ru-CTF (a) and
RuO /C (b) electrodes in 0.1 M HClO , without (black line)
2
4
¹
1
and with (red line) 1 M glycerol. Scan rate: 10 mVs . (c) (d)
Chronoamperograms at 1.4 V vs. RHE for Ru-CTF (c) and
RuO /C (d) electrodes in 0.1 M HClO , without (- - - - - - - - - -)
imately 0.2 V more negative than that when using the RuO /C.
2
4
2
and with 1 M (1), 100 mM (2), and 10 mM (3) glycerol.
Therefore, an analysis of the reaction products generated by the
514 | Chem. Lett. 2020, 49, 513–516 | doi:10.1246/cl.200056
© 2020 The Chemical Society of Japan

Figure 3. The amount of partial glycerol oxidation products
for Ru-CTF (a) and RuO2/C (c) electrodes, and faradic effi-
ciency (FE) of Ru-CTF (b) and RuO2/C (d) electrodes at 1.4 V
vs. RHE in 0.1 M HClO4 with 1 M glycerol. Blue line and bar
show glyceraldehyde, and orange ones show glyceric acid.
Figure 4. FEs for glycerol oxidation under various conditions
at 1.4 V vs. RHE in 0.1 M HClO4. (a) (b) glycerol concentration
dependences for Ru-CTF (a) and RuO2/C (b). (c) (d) The
amount of loaded Ru atoms dependences for Ru-CTF (c) and
RuO2/C (d). Blue bar shows glyceraldehyde, and orange one
shows glyceric acid.
Ru-CTF was also conducted at 1.2 V so as to have the same
over-potential for both catalytic materials. Little difference can
be seen in the distribution of the reaction products (Figure S6),
indicating that the over-potential was not a critical factor
determining the reaction selectivity. In addition, both the Ru-
CTF and RuO2/C appeared to catalyze glyceraldehyde oxidation
for the experiments shown in Figures 4, S8, and S9 because
the contribution of side reactions (i.e., oxygen evolution and
oxidative decomposition of the electrode) to the total flow of
charge varied in a concentration-dependent manner. Even so, the
contribution of such side reactions to the contribution to the
charge flow was only approximately 10% and thus did not affect
the qualitative trends described above.
(Figure S7). These results suggest that the higher selectivity for
glyceraldehyde generation exhibited by the RuO2/C was not due
to lower activity.
The effect of the glycerol concentration on the reaction
products was subsequently examined. As shown in Figures 4a
and b, for both the Ru-CTF and RuO2/C, the overall FE for the
partial oxidation products became smaller with decreases in the
concentration of glycerol. Furthermore, the amount of glyceric
acid (the four-electron oxidation product) obtained relative to the
amount of glyceraldehyde (the two-electron oxidation product)
increased with lower concentration of glycerol.
As shown in Figure 2, for both the Ru-CTF and RuO /C,
2
the oxidation of glycerol was catalyzed by high-valency Ru sites
formed during the electrochemical oxidation of water coordi-
nated to Ru atoms. The number of such sites increased along
with the positive potential shift, which eventually produced
increases in the current. Because these Ru-species promoted the
reaction of glycerol and of the partial oxidation products, these
various organic substances all competed for the limited number
of the Ru sites. In principle, the degree of this competition
would depend on the relative concentration of active Ru sites.
That is, a higher glycerol concentration and/or lower concen-
tration of Ru sites would reduce the likelihood of further
oxidation of the partial oxidation products, resulting in the
suppression of deep oxidation. In good agreement with this
expectation, when changing the concentration of the substrate
or Ru loading by orders of magnitude, the overall FE for the
partial oxidation products increased and the relative ratio of
glyceraldehyde was also increased (Figure 4). Interestingly,
the same qualitative trends were observed when using both the
Ru-CTF and RuO2/C, whereas the effect of the Ru loading
Thus, deeper oxidation proceeded on both the Ru-CTF and
RuO2/C when the glycerol concentration was reduced. Consid-
ering that the RuO /C was composed of clusters while the Ru
2
in the CTF was atomically dispersed, the effective number of
Ru atoms functioning as catalytically active sites would have
been greater in the Ru-CTF.20 Therefore, the effect of the Ru
loading amount on the deep oxidation level was examined in an
electrolyte containing 1 M glycerol. The j vs. time (t) curves
and the FE values for the reaction products obtained using the
Ru-CTF are shown in Figure S8a and Figure 4c, respectively.
2
Upon reducing the Ru loading to 0.51 ¯g/cm , the FE for
glyceraldehyde synthesis was increased from 10% to approx-
imately 40%, while the overall FE for the partial oxidation
products, including glyceric acid, was increased from 20% to
approximately 60%. However, when employing the RuO2/C
catalyst, the selectivity for the partial oxidation products
was increased only slightly by reducing the amount of Ru
was lower in the case of the RuO /C. This could have occurred
2
because the effective ratio of glycerol molecules to high-valency
Ru sites was sufficiently in excess even for the RuO2/C, for
¹
2
which the Ru loading was 5.1 ¯g cm . In fact, it was also con-
firmed that the extent of deep oxidation suppressed by reducing
¹
2
(Figures S8b and 4d). Table S1 summarizes the amounts of
the Ru loading by an order of magnitude to 0.51 ¯g cm when
the effective glycerol concentration was reduced from 1 M to
100 mM (Figure S9).
compounds produced at various glycerol concentrations and Ru
loadings. It should be noted that the same Q could not be used
Chem. Lett. 2020, 49, 513–516 | doi:10.1246/cl.200056
© 2020 The Chemical Society of Japan | 515

For the experimental conditions in Figure 3 (a glycerol
Voltage Electron Microscopy, Osaka University, under the
guidance by Dr. T. Sakata. Synchrotron radiation experiments
were performed using the BL01B1 Beam Line of SPring-8
with the approval of the Japan Synchrotron Radiation Research
Institute (JASRI; Proposal Nos. 2018A1349, 2019A1394, and
2019B1159).
¹
2
concentration of 1 M and Ru loading of 5.1 ¯g cm ), the
¹
1
turnover number for partial oxidation exceeded 83 and 23 h for
Ru-CTF and RuO /C, respectively (assuming that all the Ru
2
atoms in the Ru-CTF and RuO2/C were active). It should be
noted that this turnover number is under-estimated, since some of
the Ru atoms would be expected to be inactive. (Details for
estimating the turnover number can be found in the Supporting
Information.) The Ru-CTF and RuO2/C exhibited different
activity levels during the glycerol oxidation, as shown in
Figure 2. The onset potentials of the glycerol oxidation currents
were different, with values of 1.0 and 1.2 V for the two materials,
respectively. This difference may be due to the different valances
of ruthenium. For Ru-CTF, a peak assignable to Ru(III)/Ru(II)
couple can be observed at around 0.6 V vs RHE as shown in
Figure S10a. Based on this fact, it is speculated that the active
center for glycerol oxidation associating a redox peak in more
positive potential region (i.e., 1.0­1.2 V) is Ru(IV) and Ru(V).
In fact, it is known that [Ru(tpy-PO3H2)(OH2)3]2 , which has
a coordination structure similar to Ru-CTF, exhibits catalytic
activity for alcohol oxidation at around 1.0 V vs. RHE.22 On the
other hand, the oxidation number of Ru and hence the catalytic
Supporting Information is available on https://doi.org/
10.1246/cl.200056.
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1
7
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2
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XAFS measurements at SPring-8. TEM inspections were carried
out using a facility in the Research Center for Ultra-High
516 | Chem. Lett. 2020, 49, 513–516 | doi:10.1246/cl.200056
© 2020 The Chemical Society of Japan