Highly selective transformation of glycerol to dihydroxyacetone without using oxidants by a PtSb/C-catalyzed electrooxidation process

Article abstract of DOI:10.1039/c5gc02865e

We demonstrate an electrocatalytic reactor system for the partial oxidation of glycerol in an acidic solution to produce value-added chemicals, such as dihydroxyacetone (DHA), glyceraldehyde (GAD), glyceric acid (GLA), and glycolic acid (GCA). Under optimized conditions, the carbon-supported bimetallic PtSb (PtSb/C) catalyst was identified as a highly active catalyst for the selective oxidation of glycerol in the electrocatalytic reactor. The product selectivity can be strongly controlled as a function of the applied electrode potential and the catalyst surface composition. The main product from the electrocatalytic oxidation of glycerol was DHA, with a yield of 61.4% of DHA at a glycerol conversion of 90.3%, which can be achieved even without using any oxidants over the PtSb/C catalyst at 0.797 V (vs. SHE, standard hydrogen electrode). The electrocatalytic oxidation of biomass-derived glycerol represents a promising method of chemical transformation to produce value-added molecules.

Full text of DOI:10.1039/c5gc02865e

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Highly selective transformation of glycerol to dihydroxyacetone
without using oxidants by PtSb/C-catalyzed electrooxidation
process
Received 00th January 20xx,
Accepted 00th January 20xx
Seonhwa Lee,a Hyung Ju Kim,b Eun Ja Lim,c Youngmin Kim,d,e Yuseong Noh,d,e George W. Huber*f
and Won Bae Kim*a,d,e
DOI: 10.1039/x0xx00000x
www.rsc.org/
We demonstrate an electrocatalytic reactor system for the partial oxidation of glycerol in an acidic solution to produce
value-added chemicals, such as dihydroxyacetone (DHA), glyceraldehyde (GAD), glyceric acid (GLA), and glycolic acid
(GCA). Under optimized conditions, the carbon-supported bimetallic PtSb (PtSb/C) catalyst was identified as a highly active
catalyst for the selective oxidation of glycerol in the electrocatalytic reactor. The product selectivity can be strongly
controlled as a function of the applied electrode potential and catalyst surface composition. The main product from the
electrocatalytic oxidation of glycerol was DHA, with a yield of 61.4% of DHA at a glycerol conversion of 90.3%, which can
be achieved even without using any oxidants over the PtSb/C catalyst at 0.797 V (vs. SHE, standard hydrogen electrode).
The electrocatalytic oxidation of biomass-derived glycerol represents a promising method of chemical transformation to
produce value-added molecules.
The oxidation of glycerol to valued chemicals or
intermediates has been previously investigated using
heterogeneous catalysis via conventional catalytic processes
Introduction
Biomass feedstocks are alternative sources of valued chemicals
in place of fossil fuels.1–3 Glycerol can be obtained as a
byproduct during the production of biodiesel from biomass.
Biomass-derived glycerol is an attractive source of valuable
chemicals and chemical building blocks for the synthesis of fine
chemicals from renewable sources.3–5 The oxidation of glycerol
produces various chemicals, such as dihydroxyacetone (DHA),
glyceraldehyde (GAD), glyceric acid (GLA), hydroxypyruvic acid
(HPA), tartronic acid (TTA), glycolic acid (GCA), and oxalic acid
(OXA).6–9 In particular, dehydrogenative oxidation of glycerol
can lead to the production of DHA, which is a potentially
important chemical that is used as a main ingredient in the
cosmetic industry and as building blocks of new degradable
polymers.10–13
with various catalysts, such as Pd,13–16 Au,13–15,17,18 Pt,15,16,19
PtBi,11,20–23 PtPd,24 PtAu,25,26 AuPd,27,28 Au-Pt/H-mordenite,29
and Pt/MWCNTS.30 The electrochemical conversion process by
tuning the applied potential has also been used to convert
intermediate products using the electrocatalytic oxidation of
glycerol.31–35 The electrocatalytic oxidation process can control
the oxidizing conditions by variation in the electrode potential,
which can alter the reaction rate and product selectivity.34,36
Scheme
1 represents the dehydrogenative oxidation of
biomass-derived glycerol in an electrocatalytic reactor system.
The dehydrogenation of glycerol generates DHA, protons, and
electrons over the anode electrode (i.e., working electrode,
WE) in this electrocatalytic reactor. At the cathode (i.e.,
counter electrode, CE), the protons in the electrolyte solution
combine with the electrons transported from the anode to
form dihydrogen gas. The reactions involved in this process are
as follows:
Anode reaction (working electrode): C3H8O3 → C3H6O3 + 2H+(aq) + 2e-
Cathode reaction (counter electrode): 2H+(aq) + 2e- → H2
Overall reaction: C3H8O3 → C3H6O3 + H2
A recent study reported the selective electrocatalytic glycerol
oxidation of GLA (45%) and TTA (38%) at a glycerol conversion
of 20.4% on
a Pt/C catalyst using an anion-exchange
membrane fuel cell reactor.32 Wang et al. demonstrated the
high selectivity of glycerol electrocatalytic oxidation to glyceric
acid over gold-based catalysts supported on poly(4-
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pretreated with 6 M hydrochloric acid (HCl, 37%) prior to use.
Tetraethylene glycol (C4H10O3, Sigma-Aldrich) was used as the
reductant and stabilizer to produce PtBi and PtSb nanoparticle
catalysts. The chemicals used in the reactant solutions and
standards included glycerol (C3H8O3, 99.5% Junsei),
dihydroxyacetone (C3H6O3, 97% Aldrich), glyceraldehyde
(C3H6O3, 99.5% Fisher), glyceric acid (C3H6O4, 99.5% Fisher),
hydroxypyruvic acid (C3H4O4, 95% Sigma-Aldrich), tartronic
acid (C3H4O5, 97% Fluka), glycolic acid (C2H4O3, 97% Fluka), and
oxalic acid (C2H2O4, 98% Aldrich).
Synthesis of carbon-supported PtBi and PtSb catalysts
The carbon-supported 40 wt% PtBi and PtSb catalysts were
synthesized using a colloidal method that was modified from a
previously reported method.39 The carbon support was
activated in an acid solution consisting of 6 M hydrochloric
acid (HCl, 37%) for 12 h. Then, the pretreated carbon was
washed several times with deionized (DI) water until the
solution pH was approximately 5.5 followed by filtering and
drying for 5 h in air at 393 K. The activated carbon was
ultrasonically dispersed in 40 ml of tetraethylene glycol with a
sodium hydroxide solution (NaOH, 50% solution in water) until
the solution pH was approximately 9. In a typical synthesis
reaction, the calculated amount of Pt salt was completely
dissolved in the carbon-suspended solution with vigorous
stirring. The solutions were heat-treated to produce Pt colloids
by heating at 433 K for 2 h with stirring under reflux condition
followed by cooling to room temperature. The resulting
slurries were filtered and rinsed thoroughly with DI water and
then vacuum-dried via a freeze-drying method. To prepare the
PtBi/C and PtSb/C catalysts, the precalculated amounts of Bi or
Sb salt were added to the Pt/C-suspended tetraethylene glycol
solution at a pH of 9. Then, the solution was heated at 523 K
for 2 h with stirring using a reflux system. The final solution
was filtered, washed, and dried under vacuum via a freeze-
drying method to minimize particle agglomeration for the
highly dispersed nanoparticle catalysts. Finally, the dried Pt/C,
PtBi/C, and PtSb/C catalysts were heat-treated under an H2
atmosphere at 373 K for 1 h.
Scheme 1 Schematic illustration of the dehydrogenative oxidation of biomass-
derived glycerol in an electrocatalytic reactor system.
vinylpyridine)-functionalized graphene (Au-P4P/G).33 Ciriminna
et al. reported electrocatalytic glycerol oxidation mediated by
the TEMPO (2,2,6,6-Tetramethylpiperidine 1-oxyl) oxidant with
DHA and HPA yields of 30% and 35%, respectively after 200
h.12 Moreover, the highly selective formation of DHA can be
achieved by electrooxidation of glycerol using a Pt/C electrode
in the presence of bismuth dissolved in an electrolytic
solution.35 Currently, Pt-Bi nanoparticles are considered to be
a good candidate for electrocatalysts for DHA formation, which
requires oxidative dehydrogenation of glycerol. However, this
process exhibited relatively lower selectivity and glycerol
conversion. The PtSb nanoparticles can enhance the oxidative
dehydrogenation of glycerol and hinder further oxidation of
intermediates.37,38 Despite of these advantages, PtSb catalyst
systems using electrochemical methods have not been
previously explored with thorough characterization of the
selective oxidation of glycerol under acidic conditions.
In this study, carbon-supported bimetallic PtSb (PtSb/C)
was applied as an active and stable electrocatalyst for the
dehydrogenation of glycerol in an electrocatalytic reactor to
investigate the electrode potential-dependent product
distribution and yield. A high yield of DHA (61.4% at 90.3%
conversion) can be obtained over the PtSb/C catalyst at 0.797
V
(vs. SHE, standard hydrogen electrode) under acidic
Physicochemical characterizations of the PtBi/C and PtSb/C
catalysts
condition. The synergistic effect of the alloyed Pt–Sb catalyst
for DHA production was demonstrated using electrochemical
methods and discussed in terms of the proposed reaction
pathway and physicochemical properties of the bimetallic
nanoparticles.
The prepared catalysts were characterized by various
physicochemical analyses, such as transmission electron
microscopy (TEM) and X-ray diffraction (XRD) to investigate
the structural modification, energy dispersive X-ray
spectroscopy (EDS) and inductively coupled plasma atomic
emission spectroscopy (ICP-AES) to determine the chemical
properties, and X-ray photoelectron spectroscopy (XPS) and X-
Experimental
Materials
ray
absorption-near-edge
spectroscopy
(XANES)
to
Hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6•xH2O,
Aldrich), bismuth(III) chloride (BiCl3, Aldrich), and antimony(III)
chloride (SbCl3, Aldrich) were used as the metal precursors for
the catalysts. Vulcan XC-72R carbon powder (Cabot Corp., SBET
= 236.8 m2 g-1) was used as the supporting material for the PtBi
and PtSb nanoparticles, and the carbon powder was
characterize the electronic features of the bimetallic catalysts.
The XRD measurements were performed with a Rigaku
Rotalflex (RU-200B) diffractometer using a Cu Kα (λ = 1.5418 Å)
source and a Ni filter to characterize the structural changes in
the Pt-based crystallites. The source was operated at 40 kV
and 100 mA. The 2θ angular region between 10° and 80° was
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explored using a fixed time mode (0.025°/3 s). The Pt, PtBi and 0.2 mg cm-2 metal loading) using a spray method followed by
PtSb nanoparticle morphologies on the carbon support were drying in an oven at 343 K for 1 h to prepare a thin film of
examined using a TECHNAI- F20 TEM operated at 200 kV. All of catalyst impregnated on the vitreous carbon electrode for use
the TEM samples were prepared by ultrasonically dispersing as the working electrode. Oxidation of glycerol was carried out
the catalyst particles in an ethanol solution. Drops of the in a 10 ml mixture consisting of 0.1 M glycerol in a 0.5 M H2SO4
suspension were placed onto a standard Cu grid (200-mesh) solution with stirring at 400 rpm. The temperature was
that was covered with a carbon film and dried for 30 min in a controlled by directly heating the electrochemical reactor
convection oven to evaporate the ethanol and leave the using a temperature regulator. Prior to any electrochemical
catalyst particles dispersed on the grid prior to being inserted measurements, the mixture was completely purged with pure
into the microscope. ICP-AES analyses were performed using N2 to eliminate the dissolved dioxygen gas. The pH values of
an OPTIMA 4300 DV (PerkinElmer, USA) to estimate the actual the mixture were nearly the same during the reaction and
metal loading and the chemical composition of the carbon invariant under all of the conditions with a pH variation of 0.4–
supported Pt, PtBi and PtSb catalysts. An EDS analysis was also 0.6. The turnover frequency (TOF) was determined at
performed to determine the purity and chemical dispersion of conversions of less than 10%. From the cyclic voltammograms
the PtBi and PtSb nanoparticles at 200 kV using an INCA-X (CVs) (See Fig. S1 in ESI†), the electrochemical active surface
(Oxford Instruments) in conjunction with TEM. The surface areas (EAS) values were estimated from the oxidation charges
oxidation states of the Pt, PtBi, and PtSb catalyst were of the surface Pt-H species (210 μC cm-2).39 EAS of the Pt/C,
analyzed by XPS (ESCALAB 250, UK) using a monochromic Al Kα PtBi/C, and PtSb/C electrodes was measured as 43.9, 30.5,
X-ray source (E = 1486.6 eV). The background was corrected 31.3 m2 g-Pt-1, respectively. These measurements were taken
using the Shirley method, and the binding energy (BE) of the C using the cyclic voltammetry data recorded in the 0–1.2 V
1s peak from the support at 284.5 eV was used as an internal potential range (vs. SHE) at a 50 mV s-1 scan rate in 0.5 M
standard. Data processing was performed with the XPSPEAK H2SO4.
software program. The Pt:Bi (or Sb) surface atomic ratios were
The reaction products formed during glycerol
calculated from peak areas that were normalized by the electrooxidation were collected and examined by analyzing
published atomic sensitivity factor of the corresponding each phase of the reaction mixture using high-performance
element.40 XANES was performed at beam lines 3C1 and 7C at liquid chromatography (HPLC, Waters 2535) with a RI detector
the Pohang Accelerator Laboratory (PAL; 2.5 GeV with the and UV–Vis detector (220 nm). A Biorad Aminex HPX-87H
stored currents of 130–180 mA), Korea. A Si (1 1 1) double sugar column was used with 0.005 M H2SO4 (at a flow rate of
crystal monochromator was used to monochromatize the X- 0.6 ml min-1) as the eluent at 65 °C. 1 μl liquid samples were
ray photon energy. The spectra were obtained at room injected after filtration through a 0.2 μm filter. The amount of
temperature in transmission mode for the L3-edge of Pt consumed glycerol and the reaction product yields were
(11,564 eV) under ambient conditions. Higher-order harmonic quantified with an external calibration method. The glycerol
contamination
was
eliminated
by
detuning
the conversion (XGly), product selectivity (Sp), and reaction rate (R)
monochromator to reduce the incident X-ray intensity by were calculated according to the C2 and C3 products present in
approximately 30%. Energy calibration was performed using a the liquid phase, where n is the moles of compound and in/out
standard metal foil. XANES was analyzed using the IFEFFIT is the reactor inlet/outlet. All of the reaction rates were
software program.41,42 Then, the resulting elemental calculated at glycerol conversions of less than 10%.
absorption was normalized using the atomic-like absorption at
ꢄꢁꢂꢃ,ꢅꢆ − ꢄꢁꢂꢃ,ꢇꢈꢉ
the edge. The white line (WL) areas of the Pt L3-edge were
ꢀ
ꢁꢂꢃ (%) =
× 100
ꢄꢁꢂꢃ,ꢅꢆ
calculated by adopting an arctangent step and fitting a
Lorentzian function to the WL curve in the region from 50 to
100 eV.
ꢄꢋ,ꢇꢈꢉ
ꢊ
ꢋ (%) =
× 100
ꢄꢁꢂꢃ,ꢅꢆ − ꢄꢁꢂꢃ,ꢇꢈꢉ
Oxidation reaction of glycerol in an electrocatalytic reactor
ꢄꢁꢂꢃ,ꢅꢆ − ꢄꢁꢂꢃ,ꢇꢈꢉ
ꢄꢐꢉ × ꢔ
Electrooxidation of glycerol was carried out in an
electrocatalytic three-electrode reactor using a potentiostat
(Solartron Analytical 1400, AMETEK) to control the electrode
potential. A Pt wire and Ag/AgCl (in 3 M KCl) were used as the
counter and reference electrodes, respectively. A carbon
paper (Toray, TGP-H090) was used as the working electrode
for the glycerol electrooxidation reaction. For the catalyst-
coated carbon paper, the prepared Pt/C, PtBi/C, and PtSb/C
catalysts were suspended in isopropyl alcohol, which
contained a 5 wt% Nafion solution (DuPont), for 3 h by
sonication. Next, catalyst ink was deposited on the surface of
the carbon paper electrodes (with a 2 cm2 geometric area and
ꢌ (
ꢍꢎꢏꢁꢂꢃꢍꢎꢏꢐꢉꢑꢒꢍꢓꢄꢑꢒ) =
Results and discussion
Characterization of the PtBi/C and PtSb/C catalysts
Fig. 1 shows representative TEM images with particle size
distributions for the Pt/C, PtBi/C, and PtSb/C catalysts. All of
the prepared metal nanoparticles were highly dispersed and
uniformly deposited on the carbon support. The mathematical
mean particle diameter (d) was calculated using the following
equation:
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Table 1 Quantitative composition and structural parameters for the Pt/C, PtBi/C,
and PtSb/C catalysts measured using ICP-AES, TEM, and XRD
Metal loadinga
(wt%)
Mean particle size
(nm)
Lattice
parametere
(Å)
d
2θmax
Catalysts Pt
Bi, Sb
-
TEMb
XRDc
(°)
Pt/C
39.1
35.4
35.9
3.6 ± 0.9 4.3
4.3 ± 1.2 4.8
4.2 ± 0.9 4.6
67.53
67.07
67.17
3.920
PtBi/C
PtSb/C
4.26
2.47
3.944
3.938
a
Metal loading amounts on the carbon support estimated from ICP-AES
analysis. b Mean particle diameter of the catalysts from TEM images using at
least 200 visible particles. c Crystallite sizes of the catalysts calculated by line
d
broadening of powder XRD peak. The angular position of Pt (2 2 0)
reflection peak. e Lattice parameter calculated from XRD analysis.
Pt:Sb, which are nearly consistent with the nominal atomic
ratio (90:10). This result is also confirmed by the results from
the EDS analysis (i.e., 87:13 for Pt:Bi, 92:8 for Pt:Sb) that are
shown in Fig. 1. No impurities, such as Cl–, were observed in
the synthesized catalysts based on EDS.
The XRD patterns of the Pt/C, PtBi/C, and PtSb/C catalysts
are shown in Fig. 2(a). The first broad peak located at
approximately 25° was due to the (0 0 2) plane in the
hexagonal structure of the Vulcan XC-72R carbon black
support.43 The main characteristic peaks at approximately 39°,
46°, and 67°, which appeared in all of the catalysts, correspond
to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively, of a
crystalline face-centered cubic (fcc) Pt phase. It is important to
note that any noticeable characteristic peaks from the Bi and
Sb metal phases, their metal oxides/hydroxides, or other
compounds were not detected in the XRD measurements,
which implies that these secondary phases may only be
present in very small amounts or in amorphous forms in the
prepared materials.37,44 The lattice parameters and mean
particle size of the prepared catalysts were calculated
according to the angular position (2θmax) of the (2 2 0) peaks
and the full width at half maximum (FWHM), as summarized in
Fig. 1 TEM images of the Pt/C, PtBi/C, and PtSb/C catalysts with EDS spectra and
particle size distributions for the metal nanoparticles.
∑
ꢄꢅꢕꢅ
ꢄꢅ
d =
∑
where ꢄꢅ is the frequency of the catalyst particles with a
diameter of size ꢅ. The mean particle size of the PtBi/C and
ꢕ
PtSb/C catalysts was approximately 4.2–4.3 nm, which would
be slightly larger than that of Pt/C (ca. 3.6 ± 0.9 nm), as listed
in Table 1.
Table 1. The shifts in the 2θ position from the Pt crystallites
The metal loadings of Pt, PtBi, and PtSb over the carbon
supports determined by ICP-AES were ca. 39.1, 39.7 and 38.4
wt%, respectively, as summarized in Table 1. These values are
close to the pre-calculated value of 40 wt% during the
preparation. The results from the ICP-AES indicate that the
bulk atomic ratios are 89.9:10.1 for Pt:Bi and 90.1:9.9 for
and corresponding changes in their lattice parameters reflect
the incorporation of either Bi or Sb atoms into the Pt crystal
lattice. In this study, the Pt (2 2 0) reflection peak from PtBi/C
and PtSb/C appears to be slightly shifted toward a lower 2
θ
value compared to that from Pt/C, indicating increases in the
lattice parameter with lattice dilation when either Sb or Bi
Table 2 Electronic parameters and chemical composition of the catalysts characterized by XPS, XANES, and ICP-AES measurements
Binding energy (eV)
Pt 4f7/2
Pt/M (atomic ratio)
From ICPd
XANES
Catalysts
Pt foil
Bi 4f7/2, Sb 3d5/2
From XPSe
E0 (eV)f
11564.3
11564.7
WL intensity (a.u.)g
-
1.22
1.39
Pt/C
71.27 (63)a
72.44 (37)a
71.60 (66)
72.79 (34)
71.62 (72)
72.90 (28)
-
-
-
-
PtBi/C
PtSb/C
158.1 (65)b
159.3 (35)b
529.5 (73)c
530.5 (27)c
8.9
9.1
3.1
4.5
11564.4
11564.0
1.44
1.41
a Relative % of the Pt0 and Pt2+ species. b Relative % of the Bi0 and Bi2+ species. c Relative % of the Sb0 and Sb3+ species. d Chemical composition determined by ICP-
e
f
g
AES analysis. Surface atomic ratio calculated from XPS by using peak areas normalized on the basis of sensitivity factors. Pt L3-edge energy. White line
intensity.
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Fig. 2 (a) X-ray diffraction patterns, (b) X-ray photoelectron spectra of Pt 4f, Bi 4f, and Sb 3d, and (c) Pt L3 edge XANES spectra along with Pt foil as the reference for the Pt/C,
PtBi/C, and PtSb/C catalysts.
atoms are incorporated into the fcc lattice of the Pt deconvoluted into two pairs of peaks at 158.1 eV, which were
crystallites.37,45,46 The calculated crystallite sizes for the Pt, assigned to Bi0 (65%) and 159.3 eV assigned to Bi2+ (35%). For
PtBi, and PtSb particles on carbon were approximately 4.3, 4.8, Sb 3
d
XPS, the Sb 3d5/2 spectra were also deconvoluted into
and 4.6 nm, respectively.
two distinguishable peaks with different intensities, and these
Based on the XRD and TEM measurements, no significant peaks correspond to Sb0 and Sb3+. Approximately 73% and 27%
differences were observed in the metal particle size under our of surface antimony exist as Sb0 and Sb3+, respectively, in the
preparation conditions. Therefore, the particle size effects PtSb/C catalyst. The Sb
does not influence the electrocatalytic activity and selectivity overlapping peak with high binding energy, which is the O 1
over PtBi/C and PtSb/C.
oxygen peak at 531.9 eV.37,48 The results from the XPS
3d5/2 spectrum contained an
s
XPS measurements were carried out to elucidate the investigation suggest that the dominant surface phases for Bi
surface oxidation states of the synthesized catalysts. Fig. 2(b) and Sb in the PtBi/C and PtSb/C consist of zero-valent species.
shows the XPS spectra for Pt 4
supported Pt, PtBi, and PtSb catalysts. The binding energy (eV) catalysts measured by XPS were lower than those measured
of the core electrons and relative chemical states for Pt 4 , Bi using ICP-AES, which suggests that either Bi or Sb was
, and Sb 3 were estimated from the relative intensities of relatively enriched at the surface layer of the PtBi/C and
their deconvoluted peaks (Table 2). The Pt 4 XPS spectra PtSb/C catalysts, which is consistent with results that were
contained two peaks corresponding to the Pt 4f7/2 and 4f5/2 reported elsewhere.22,23
f, Bi 4f, and Sb 3d in the carbon In Table 2, the Pt/M atomic ratios for the PtBi/C or PtSb/C
f
4f
d
f
states from the spin-orbital splitting, and each peak can be
For the electronic structure analysis of these catalysts, the
deconvoluted to two different Pt oxidation states (i.e., Pt0 and XANES spectra for the Pt L3 edges of the prepared catalysts
Pt2+).37,38 The two peaks located near 71 eV and 72 eV may be were obtained and are shown in Fig. 2(c), and the fitted
due to zero-valent Pt and Pt2+, respectively, and the latter peak parameters are listed in Table 2. Overall, the XANES shapes of
may be produced from the surface layer formation with Pt– PtBi/C and PtSb/C are similar to those of Pt foil and Pt/C
O(ad) bonds.47,48 Pt0 is a predominant species (60~70% in except for the edge absorption intensity. The strong peak
amount) in all of the prepared catalysts with relatively small above the edge energy position (see the magnified region in
amounts of oxidized Pt species (30~40%). The Pt 4
f
binding the inset figure) is closely related to the density of the vacant
electronic states of Pt.49 This white line (WL) peak is
energies for PtBi/C (71.60 eV) and PtSb/C (71.62 eV) were
5
d
shifted to higher levels compared to that of the Pt/C (71.27 eV) frequently recognized as an important parameter for catalytic
catalyst, which implies that the electronic state of Pt was activity.50 The WL intensities of both the PtBi/C and PtSb/C
modified by the addition of Bi or Sb.37,44 The Bi 4f spectra was catalysts are slightly higher than that of Pt/C, which may be
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due to a decrease in 5d orbital filling in the Pt alloys due to
oxidation and hybridization. With respect to the Sb content,
the relationship between the physicochemical properties and
the electrocatalytic activities will be explored and discussed for
glycerol electrooxidation.
Selective oxidation of glycerol via an electrochemical reaction
Glycerol oxidation was performed over the Pt-based catalyst in
an electrocatalytic reactor using a solution consisting of 0.1 M
glycerol in a 0.5 M H2SO4 electrolyte at 60 °C at a constant
potential of 0.797 V (vs. SHE). The oxidation of glycerol
produced several products including DHA, GAD, GLA, HPA,
TTA, GCA, and OXA.6–9 Fig. 3(a) shows the DHA, GAD, and GLA
product distributions at a glycerol conversion of 50% on the (a)
Pt/C, (b) PtBi/C, and (c) PtSb/C catalysts. For Pt/C, GAD and
GLA were primarily observed during glycerol oxidation, while
the selectivity to DHA was only 2% or less. For PtBi/C, DHA
formation was more favorable. However, a smaller amount of
GAD was observed compared to the results for Pt/C. For
PtSb/C, the selectivity to DHA was as high as 80% with a
DHA/GAD selectivity ratio of approximately 8. The difference
in the product distributions may be attributed to the different
structural and electronic properties of the Pt/C, PtSb/C, and
PtBi/C catalysts. The negligible activity for glycerol oxidation
over Bi/C and Sb/C (See Fig. S2 in ESI†) indicates that Bi or Sb
alone was not capable of activating the chemical reactions
with glycerol. The product selectivity is reported as a function
of glycerol conversion for the PtSb/C catalyst in Fig. 3(b). When
Fig. 4 Comparison of the catalytic performance of glycerol electrooxidation over
the prepared catalysts. [Reaction conditions: 0.1 M glycerol, 0.797 V, 60 °C, 10 h]
the glycerol conversion was 10%, DHA was the main product
with nearly 100% selectivity, while GAD and GLA were barely
observed. As the glycerol conversion increased, the DHA
selectivity slightly decreased, which most likely indicates that
DHA was an intermediate product during glycerol oxidation.
When the glycerol conversion increased to 90%, the selectivity
to GAD and GLA increased to 18.7% and 7.9%, respectively,
and the selectivity to DHA decreased to 68.1%. However, DHA
remained the predominant product with high selectivity at all
glycerol conversions.
Fig. 4 shows the DHA yield and initial reaction rate at
conversions less than 10% for the oxidation reaction of
glycerol. A high DHA yield (61.4%) and reaction rate (4.6
molglycerol molPt-1 min-1) were achieved with the PtSb/C catalyst
system. The DHA yield decreased in the following order:
PtSb/C > PtBi/C > Pt/C. The PtSb/C catalyst exhibited a higher
DHA yield, which was 38 times higher than that of the Pt/C
catalyst. The initial reaction rate decreased as follows: PtSb/C
> Pt/C ~ PtBi/C. The initial reaction rate of the PtSb/C catalyst
was 63% higher than that of the PtBi/C catalyst for the
oxidation of glycerol. The different reactivities with the
catalysts may be explained by the CV results (see Fig. S3 in
ESI†). The CV curves exhibit two typical anodic peaks due to
the electrooxidation of glycerol molecules. The forward anodic
peak current density (if) was due to the oxidation currents for
breaking the C–H or C–C bonds during glycerol
electrooxidation. The reverse anodic peak current density (ir)
was due to the oxidation currents for removal of the
incompletely oxidized carbonaceous residues on the Pt
surface. The if/ir ratio for the PtSb/C catalyst was larger than
those for Pt/C and PtBi/C, which indicates
a lower
accumulation of intermediate residues on the catalyst during
the glycerol electrooxidation processes. The carbonaceous
intermediates tend to strongly adsorb on the Pt surface, which
can block the active catalyst sites for the next turnover making
the oxidation reactions more sluggish.47,51
Various reaction parameters (i.e., applied potentials,
reaction temperature, and reaction time) were investigated
over the PtSb/C catalyst. The glycerol conversion, DHA
selectivity, and reaction rate as a function of increasing applied
potential are compared in Fig. 5(a). As the applied potential
Fig. 3 (a) Selectivity to DHA, GAD, and GLA at a glycerol conversion of 50% over
the Pt/C, PtBi/C, and PtSb/C catalysts and (b) glycerol conversion as a function of
the product selectivity over the PtSb/C catalysts. [Feed solution: 0.1 M glycerol;
Anode applied potential: 0.797 V; Reaction temperature: 60 °C]
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current density (See Fig. S5 in ESI†) because these parameters
modify the activation energy required to cleave the C–C bond
and oxidatively remove the carbonaceous species adsorbed on
the Pt sites for glycerol oxidation.
The time-dependent glycerol conversion and product
distribution on PtSb/C catalysts are compared in Fig. S6. As
expected in the batch-type reactor, the glycerol conversion
increased gradually with the reaction time. At a reaction time
of 0.5 h, the DHA selectivity reached a maximum and gradually
decreased with reaction time, which may be due to the
subsequent oxidations of DHA under the reaction conditions.
DHA exhibited the highest selectivity at all of the glycerol
conversions compared to the other products. The GAD
selectivity was less than 20% and remained nearly constant
after 2 h. The GAD was the primarily observed product over
Pt/C and PtBi/C compared to that over PtSb/C under these
conditions, indicating that the dehydrogenative oxidation of
glycerol to DHA was more favorable on the PtSb/C surface at
0.797 V. The minor products (i.e., HPA and TTA) were observed
after 8 h. However, the expected C2 intermediates, such as
GCA and OXA, were not observed, which may be due to the
oxidation of the C3 to C2 molecules accompanied by C–C bond
cleavage being hindered over PtSb/C under this reaction
condition.
The influence of the applied potential on the product
distributions for the conversion of glycerol over the Pt/C,
PtBi/C, and PtSb/C catalysts was investigated, and the results
are shown in Fig. 6. On the Pt/C catalyst, as the applied
potential increased from 0.397 V to 1.197 V, the GAD
selectivity decreased, and GLA selectivity gradually increased,
while the selectivity to DHA was less than 5% for all of the
potential ranges. For PtBi/C, DHA formation is more favorable
compared to that on Pt/C, which is in agreement with previous
results using PtBi electrocatalysts by Kimura et al.22,23 It is
important to note that the DHA selectivity decreased
dramatically from 0.597 V because the Bi on the Pt surface
forms Bi oxide/hydroxide species at 0.6–0.7 V, which are not
active for this reaction.35,38,52 For PtSb/C, at lower potentials,
observations similar to PtBi/C were made. As the applied
potential increased, the glycerol conversion increased from
22.5% to 92.7%. The selectivity of DHA increased from 57.6%
to 68.1% with potentials ranging from 0.397 V to 0.797 V.
However, the selectivity of GAD and GLA gradually decreased
from 0.397 V to 0.797 V and then increased to 1.197 V. The
minor products from glycerol oxidation (i.e., HPA, TTA, GCA,
and OXA) appeared to form at high potentials via consecutive
oxidations of GLA, indicating that a higher applied potential
may facilitate C–C bond cleavage as well as C–H and O–H bond
breakage.32,33 These differences in product distribution among
the tested catalysts indicate that the reaction pathway of
glycerol electrocatalytic oxidation is strongly dependent on the
nature of the catalyst and the electrode potential. The carbon
balance of the reaction products was examined on the
detected C2 and C3-type products that were present in the
liquid phase (see Table S1 in ESI†). The carbon balance tended
to decrease as the applied potential increased from 0.397 V to
1.197 V, which indicates that more C2 products (e.g., GCA and
Fig. 5 Performance of the PtSb/C catalysts in the electrooxidation of glycerol as a
function of the (a) applied electrode potential and (b) reaction temperature.
[Reaction conditions: (a) 0.1 M glycerol, 60 °C, 10 h; (b) 0.1 M glycerol, 0.797 V,
10 h]
increased, the glycerol conversion increased to 95.8% up to
0.997 V but then slightly decreased at higher potentials. The
DHA selectivity and reaction rate increased from 0.397 V to
0.797 V but then decreased when the applied potential
increased to 1.197 V. At an applied potential of 0.797 V, the
DHA selectivity increased to 68%., while at 1.197 V, its
selectivity was only 35.4%. At higher potentials, the C–C bond
cleavage reaction dominated, resulting in lower selectivity to
DHA. This result will be further discussed below and supported
by the DHA selectivity being strongly dependent on the applied
potential (Fig. 6). The temperature effect was explored in a
temperature range of 25 °C to 70 °C. The reaction temperature
can change the activation barrier for glycerol oxidation, which
can affect the reaction rate and product selectivity.20 As shown
in Fig. 5(b), the conversion of glycerol increased from 20% to
95.8% from 25 °C to 70 °C, and the reaction rate also
proportionally increased from 1.0 to 5.0 molglycerol molPt-1 min-1.
These results indicate that an increase in temperature can
directly promote the glycerol electrooxidation reaction. The
reaction rate appears to be significantly affected by
temperature under the studied reaction conditions. No
significant changes in the DHA selectivity were observed for
the studied temperature range. The kinetic information as well
as the apparent activation energy (Ea) from the experimental
results are shown in Fig. S4. Further experimental studies are
required to establish the rate-determining steps (rds) and
calculate the theoretical Ea. The increasing applied potential
and reaction temperature can increase the activity with a high
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Fig.
7 Stability test of the PtSb/C electrode for glycerol oxidation. [Reaction
conditions: 0.1 M glycerol, 0.797 V, 60 °C, 10 h]
control of the activation barrier by tuning the applied
electrode potential.34,35 Another advantage is that a strong
oxidant, such as O2, is not required.
To investigate the electrocatalytic stability of PtSb/C for
selective glycerol oxidation, recycling experiments were
carried out, as shown in the Fig. 7. These results were obtained
from repeated cycles in a mixture of 0.1 M glycerol and 0.5 M
H2SO4 at a constant potential of 0.797 V (vs. SHE). The
electrode was washed several times with DI water and then
dried in an oven at 343 K for 2 h prior to reuse. The glycerol
conversion and selectivity to DHA and GAD were retained with
only ~ 5% variations during the 5 cycles of repeated reactions.
Effects of Sb in the PtSb/C on the selective glycerol oxidation
The atomic ratios of Pt/Sb in the PtSb/C catalysts exhibited
lower values based on measurements using the surface-
specific XPS technique compared to those quantified using ICP
for the bulk compositions (See Table 2). These compositional
analyses indicate that the Sb element is relatively enriched in
the surface layer of the bimetallic Pt alloy catalysts. From the
CVs of the Pt/C, PtBi/C, and PtSb/C catalysts (See Fig. S1 in
ESI†), which were performed in a 0.5 M H2SO4 electrolyte at a
50 mV s-1 scan rate between 0 and 1.2 V (vs. SHE) without
glycerol, Pt/C exhibited a high oxygen-reduction peak at
approximately 0.74 V during the cathodic scan, while the
reduction peak for the PtBi/C and PtSb/C catalysts appeared to
be smaller. Additionally, the adsorption and desorption peaks
with hydrogen were broader and smaller for the PtBi/C and
PtSb/C catalysts but Pt/C exhibited the well-defined fcc-
polycrystalline Pt phase, which may be results from structural
modifications of Pt due to interaction between Pt and the
second metal.53 Several groups have reported that a higher
catalytic performance including a higher TOF requires an
optimal Pt–Pt bond distance for the electrochemical oxidation
of simple alcohols and oxygen reduction reaction.21,38,47 In
addition, Sb incorporation may cause a geometric (blocking)
effect, which may promote the selective formation of the
desired products on the Pt catalysts.22,35,38 The XRD analyses
(see Fig. 2 and Table 1) indicated that the geometric structure
in the bimetallic catalysts was altered by the addition of Sb or
Fig.
6 Product distribution and glycerol conversion with different applied
potentials over the (a) Pt/C, (b) PtBi/C, and (c) PtSb/C catalysts. [Reaction
conditions: 0.1 M glycerol, 60 °C, 10 h]
OXA) were further oxidized into C1 molecules (e.g., formic acid
and carbon dioxide) due to the high electrode potential
facilitating C–C bond cleavage. At 0.797 V, the carbon balance
for Pt/C and PtSb/C was 83.0% and 97.9%, respectively. This
result indicates that more products were oxidized to C1
products (formic acid and carbon dioxide) over Pt/C compared
to that over the PtSb/C catalysts and the bimetallic PtSb/C
system could be advantageous for selective glycerol oxidation
reactions.
Glycerol oxidation to valued chemicals or intermediates
has been studied over various catalysts using O2 as an oxidant.
For example, Nie et al. reported the production of DHA (46.3%
yield) over PtSb alloy catalysts under base-free conditions
using O2 oxidant (glycerol/Pt molar ratio = 557).38 In our
electrocatalytic process, the yield of DHA was 61.4% at a
glycerol conversion of 90.3% (glycerol/Pt molar ratio = 617),
which is approximately 32% higher than that obtained with the
non-electrocatalytic system. The TOF of this electrocatalytic
system (843.0 h-1) was comparable to that of the non-
electrocatalytic system (878.1 h-1). The electrocatalytic
oxidation process of glycerol possesses several key advantages
over the catalytic oxidation processes of glycerol including
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Bi to the Pt site, which was accompanied by an increase in
lattice parameter values compared to those of pure Pt. The
lattice parameters appeared to increase in the following order:
Pt/C < PtSb/C < PtBi/C. In our study, a higher selectivity
(68.0%) to DHA was obtained over the PtSb/C catalyst, which
has a lattice parameter of 3.938 Å. The specific geometric
arrangement due to the addition of Sb to Pt may affect the
properties of dehydrogenative oxidation and C–C bonds
breakage, leading to enhanced DHA production. For the
electronic state, as shown by the changed BE and WL
intensities in the XPS and XANES spectra, the electronic
modification due to changing the Fermi level of the 5d Pt band
states of the PtSb/C catalyst most likely affects C–H bond
dissociation in the glycerol molecules as well as the
adsorption/desorption properties of glycerol and other
reaction products. The CV, XRD, XPS, and XANES results
indicate that the bimetallic PtBi/C and PtSb/C catalysts formed
alloy phases with a fcc Pt structure. PtSb/C exhibited a higher
yield with a high selectivity to DHA for the electrocatalytic
oxidation of glycerol. PtSb/C was highly active for the
dehydrogenative oxidation of glycerol to DHA at all of the
potentials in the electrocatalytic reactor system.
Fig.
8
Proposed glycerol electrooxidation sequences on Pt-based catalysts in
Reaction pathways for glycerol electrooxidation over the PtSb/C
catalysts
acidic media.
surface of Pt promotes the dehydrogenative oxidation of
glycerol and improves DHA selectivity.
Fig. 8 shows the possible reaction pathway over the Pt-based
catalysts for glycerol electrooxidation in acidic media. The
electrooxidation reaction of glycerol accompanied by breaking
of the C–C, C–H, and O–H bonds can produce intermediate
species on the active sites of Pt.54 As previously shown in Fig.
6, the glycerol molecule is primarily oxidized to DHA and GAD
(according to reactions (1) - (3)), which are further oxidized to
GLA. Reactions (1) and (2) represent the adsorption of glycerol
onto the Pt active site and the electrochemical
dehydrogenative oxidation reaction, respectively. Reaction (3)
represents desorption of surface DHA and/or GAD.
To further explore the origins of the other products,
oxidation reactions with DHA, GAD, or GLA over PtSb/C in the
electrocatalytic reactor with aqueous solutions consisting of
0.5 M H2SO4 were also conducted under similar conditions as
those used for glycerol oxidation (See Table S2 in ESI†). GAD
was efficiently converted, and a higher GLA yield of 51.0% was
achieved at 1.197 V. Interestingly, GLA was observed as a
reaction intermediate from the oxidation of GAD, meanwhile
GLA was not oxidized from DHA on the PtSb catalysts. These
results reveal that GAD and DHA were primarily formed from
the glycerol oxidation, while GLA was a secondary product
from the oxidation of GAD. When DHA was used as the
feedstock for the oxidation reaction, HPA, GCA, and OXA were
obtained with a lower DHA conversion of 53.3% at 0.797 V. As
the potential increased from 0.797 V to 1.197 V, as shown in
Fig. 6(c), the DHA selectivity decreased, and the GCA selectivity
gradually increased, which may indicate that DHA was an
intermediate of glycerol oxidation and GCA was a byproduct
from further oxidation of DHA. For the oxidation of GLA, low
levels of HPA and TTA were observed, indicating that the
oxidation of GLA to GCA under acidic conditions may be
favored by C-C bond cleavage. This observation is consistent
with the reaction mechanism discussed above. Similar
evidence for two competitive pathways has been previously
reported for Pt-based catalysts.11,20,35,38 The presence of both
DHA and GAD intermediates confirms that glycerol oxidation
proceeds via oxidative dehydrogenation of glycerol followed
by the production of GLA from GAD accompanied by further
oxidation of GLA, GCA and other intermediates over Pt-based
catalysts under these conditions.
Pt + C3H8O3 → Pt-C3H8O3
(ad)
Pt-C3H8O3 (ad) → Pt-C3H6O3 (ad) + 2H+
(1)
(2)
(3)
+ 2e-
(aq)
Pt-C3H6O3 (ad) → Pt + C3H6O3
As the oxidation potential increased (see Fig. 6), the adsorbed
DHA and GLA may be further oxidized to HPA, TTA, GCA, and
OXA by cleavages of the C–C bonds.32,34 Based on the results in
Fig. 5 and Fig. 6, the formation of DHA is favored on PtSb/C at
0.797 V, which may due to the dehydrogenative oxidation of
the secondary hydroxyl groups on the glycerol molecules.
These reactions are more likely to occur over PtSb/C than over
other catalysts. Several groups reported that relative
enrichment of Sb (see XPS analysis in Fig. 2 and Table 2) at the
surface is more effective for the production of DHA than GAD
due to a decrease in Pt ensemble sites for the dehydrogenative
oxidation of primary hydroxyl groups.22,37,38 In addition,
despite the further oxidation of DHA being unavoidable, it
occurred relatively slowly, which maintained the high
selectivity to DHA. Therefore, the presence of Sb on the
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Conclusions
In this study, we investigated the highly selective
electrooxidation of glycerol to DHA by controlling the
electrode potential in an electrocatalytic reactor. The
selectivity of the observed products was strongly dependent
on the applied potential and catalyst surface composition.
Under the optimized conditions, PtSb/C served as an efficient
electrocatalyst in terms of a high DHA yield of 61.4% with
90.3% glycerol conversion at 0.797 V (vs. SHE). The structural
and electronic changes of the active Pt sites by adjacent Sb
atoms may lead to improvement in the selectivity to the target
products. Furthermore, we investigated the relationship
between the applied electrode potential and the selectivity for
glycerol oxidation under acidic conditions, and the results
indicated that higher anode potentials promote C–C bond
breaking. The electrocatalytic reactor produced a high TOF and
DHA selectivity compared to a non-electrocatalytic reactor
that uses an O2 oxidant. These results may be applied to the
design of a catalytic system for the efficient transformation of
biomass-derived oxygenated molecules in an electrocatalytic
process in the future. Along with the detailed kinetics of the
reaction, a study of the selectivity of intermediate products
over the catalysts in a continuous flow electrocatalytic system
is currently ongoing.
27 J. Xu, H. Zhang, Y. Zhao, B. Yu, S. Chen, Y. Li, L. Hao and Z. Liu,
Green Chem., 2013, 15, 1520–1525.
Acknowledgements
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,
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
(no. 2014R1A2A1A11052414).
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