Atomic-layer-deposited SnO2on Pt/C prevents sintering of Pt nanoparticles and affects the reaction chemistry for the electrocatalytic glycerol oxidation reaction

Article abstract of DOI:10.1039/d0ta02509g

Atomic layer deposition (ALD) is an efficient technique that allows atomic-level surface control of metal catalysts for the design and development of electrocatalytic materials. Herein, we report a strategy for efficient catalyst design using a particle ALD method to enhance the electrocatalytic glycerol-oxidation-reaction (GOR) performance. Atomically controlled thin SnO2 layers were deposited on a carbon-supported Pt nanoparticle (Pt/C) surface using the ALD technique. The resulting SnO2 overcoated Pt/C (ALD(SnO2)-Pt/C) was then heat-treated at 400 °C under a N2 atmosphere. The onset potential as a kinetic parameter decreased with ALD (SnO2) coatings. The turnover frequency (TOF) for the GOR showed similar values for the tested samples (TOF of Pt/C: 74.86 h-1 and TOF of ALD(SnO2)-Pt/C: 91.29 h-1). Interestingly, interactions between the ALD SnO2 overcoating and Pt nanoparticles improved the catalytic stability for the GOR, preventing sintering of Pt nanoparticle catalysts. This demonstrates that an ALD SnO2 coating on defect sites of Pt can diminish Pt sintering for the GOR. From the GOR in an electrochemical batch reactor, the ALD(SnO2)-Pt/C catalyst also generated more glyceraldehyde (GAD) product than uncoated Pt/C at a similar glycerol conversion level. The density functional theory (DFT) calculations suggest that the binding energies of glycerol and reaction intermediates change at the interface of the SnO2-coated Pt surface compared to those at the Pt surface only, thus affecting the reaction chemistry for the electrocatalytic GOR. This work highlights how we can control reaction performance measures such as catalytic stability and product selectivity by using the particle ALD technique for electrocatalytic reactions such as glycerol oxidation. This journal is

Full text of DOI:10.1039/d0ta02509g

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DOI: 10.1039/D0TA02509G
Atomic-Layer-Deposited SnO2 on Pt/C Prevents Sintering of Pt
Nanoparticles and Affects Reaction Chemistry for Electrocatalytic
Glycerol Oxidation Reaction
Daewon Lee,‡a Youngmin Kim,a Hyunsu Han,b Won Bae Kim,b Hyunju Chang,c Taek-Mo Chung,d
Jeong Hwan Han,*de Hyun Woo Kim,*c Hyung Ju Kim*af
a
Chemical & Process Technology Division, Korea Research Institute of Chemical Technology
(KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, South Korea. E-mail: hjkim@krict.re.kr
b
Department of Chemical Engineering, Pohang University of Science and Technology
(POSTECH), 77 Cheongam-Ro, Nam-gu, Pohang-si, Gyeongsangbuk-do 37673, South Korea.
c
Chemical Data-Driven Research Center, Korea Research Institute of Chemical Technology
(KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, South Korea. E-mail: ahwk@krict.re.kr
d Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141
Gajeong-ro, Yuseong-gu, Daejeon 34114, South Korea.
e
Department of Materials Science and Engineering, Seoul National University of Science and
Technology (SEOULTECH), 232 Gongneung-ro, Nowon-gu, Seoul 01811, South Korea. E-mail:
jhan@seoultech.ac.kr
f
Advanced Materials and Chemical Engineering, University of Science and Technology (UST),
217 Gajeong-ro, Yuseong-gu, Daejeon 34113, South Korea.
† Electronic supplementary information (ESI) available: [Additional HAADF-STEM and SEM
images along with corresponding EDS elemental mapping, TEM results with particle size
distribution analysis, XRD spectra, EXAFS fitting results, electrochemical surface area
measurements, electrochemical performance analyses, DFT calulation results, electrocatalytic
GOR technology scheme, and comparison with the recent results (Figures S1-S12, Scheme S1,
Tables S1-S3)]. See DOI: 10.1039/x0xx00000x
‡ Present address: University of California at Berkeley and Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, United States
1

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Abstract
Atomic layer deposition (ALD) is an efficient technique that allows atomic-level surface
control of metal catalysts for the design and development of electrocatalytic materials. Herein, we
report a strategy for efficient catalyst design using a particle ALD method to enhance the
electrocatalytic glycerol-oxidation-reaction (GOR) performance. Atomically controlled thin SnO2
layers were deposited onto a carbon-supported Pt nanoparticle (Pt/C) surface using the ALD
technique. The resulting SnO2 overcoated Pt/C (ALD(SnO2)-Pt/C) was then heat-treated at 400 °C
under a N2 atmosphere. Onset potential as a kinetic parameter was decreased with ALD (SnO2)
coatings. Turnover frequency (TOF) for GOR showed similar values for the tested samples (TOF
of Pt/C: 74.86 h-1, TOF of ALD(SnO2)-Pt/C: 91.29 h-1). Interestingly, interactions between ALD
overcoated SnO2 and Pt nanoparticles improved the catalytic stability for GOR, preventing
sintering of Pt nanoparticle catalysts. This demonstrates that an ALD SnO2 coating on defect sites
of Pt can diminish Pt sintering for the GOR. From the GOR in an electrochemical batch reactor,
the ALD(SnO2)-Pt/C catalyst also generated more glyceraldehyde (GAD) product than uncoated
Pt/C at a similar glycerol conversion level. Our density functional theory (DFT) calculations
suggest that binding energies of glycerol and reaction intermediates change at the interface of
SnO2-coated Pt surface compared to the Pt surface only, thus affecting the reaction chemistry for
the electrocatalytic GOR. This work highlights how we can control reaction performance measures
such as catalytic stability and product selectivity by using the particle ALD technique for
electrocatalytic reactions such as glycerol oxidation.
2

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1. Introduction
Atomic layer deposition (ALD) is a technique by employing a self-limiting surface
reaction through sequential injections of a metal precursor and reactant.1-5 The ALD has attracted
a great deal of attention as one of the most important processes for the miniaturization of
semiconductor devices due to its precise thickness control and pin-hole free and conformal film
properties.6 Also, ALD has found emerging applications especially in energy, catalysis, and
environment fields by the uniform coating of an ultrathin film on high surface area particles rather
than a silicon substrate for surface functionalization, surface protection, and interface
engineering.7-18 In addition, ALD provides the ability to prepare atomically controlled nanoparticle
materials that can be used to elucidate fundamental phenomena on the modified nanoparticle
surface by ALD.
Recently, the ALD technique has been used to synthesize new types of atomically
controlled nanoparticle catalysts.8,10,12,13,19,20 Previous studies have demonstrated that an Al2O3 or
TiO2 overcoat by ALD can inhibit leaching of base metal nanoparticles such as Cu or Co for
aqueous phase reaction.10,19 In addition, the ALD technique can be used to synthesize new
bifunctional catalysts by adding an acidic oxide layer for liquid-phase hydrogenation reactions.13,20
It has also been reported that the overcoat of metal oxides by ALD can enhance the stability of
semiconductor photoelectrodes and increase the activity in photoelectrochemical reactions.7,9,21,22
Some groups reported that metal oxides or metal nitrides overcoated on a Pt surface by ALD
improved the catalytic activity and stability for fuel cell reactions such as the electrochemical
oxygen-reduction-reaction (ORR).23-26 We recently reported an ALD TiO2 coating on Ni and Au
catalysts increases electrocatalytic glycerol oxidation performance by forming interfacial sites
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between metal and coated TiO2.27,28 However, the mechanisms underlying why the thin metal
oxide layer coated on the metal surface by ALD enhances the catalytic activity and stability for
catalytic reactions are not fully understood. In addition, a fundamental understanding of how the
reaction chemistry for catalytic reactions changes with the ALD coating samples has yet to be
attained.
Here we report that a particle ALD coating technology can be used to enhance the
electrocatalytic stability of a commercial Pt/C catalyst and to control the reaction chemistry for the
electrochemical glycerol-oxidation-reaction (GOR). We first prepared the ALD tin oxide (SnO2)-
coated Pt/C catalyst (ALD(SnO2)-Pt/C) with increasing the ALD(SnO2) coating cycle number
from 75 to 300 by the ALD technique. Then the ALD(SnO2)-Pt/C was heat-treated at 400 °C under
N2 gas to investigate the effect of the SnO2 overcoat on the Pt/C catalyst for the GOR. As we will
demonstrate in this paper, the heat-treated ALD(SnO2)-Pt/C (ALD(SnO2)-Pt/C (HT)) shows
higher catalytic GOR performance than the uncoated Pt/C catalyst after an electrocatalytic stability
test. This suggests that the SnO2 overcoat on the Pt/C surface by ALD prevents Pt sintering due to
the strong Pt-SnO2 interaction. Interestingly, the glyceraldehyde selectivity for ALD(SnO2)-Pt/C
(HT) from the electrocatalytic GOR was higher than that for Pt/C. This means that the creation of
Pt-SnO2 interfacial sites by the SnO2 overcoating on Pt/C can also change the reaction chemistry
for the electrocatalytic GOR. This study demonstrates how the ALD technique can be used to
improve the catalytic stability of Pt nanoparticle catalysts and to change the product selectivity for
the electrochemical GOR. With various experimental and theoretical investigations, detailed
analytical results were discussed and explored to understand why the ALD(SnO2)-Pt/C (HT)
catalyst shows enhanced catalytic GOR performance, as reported in the following section.
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2. Experimental
2.1. Preparation of ALD(SnO2)-Pt/C and their subsequent heat treatment
Commercial 20 wt.% Pt/C (Premetek) was coated with a SnO2 layer by thermal ALD in a
tube-type fluidized bed reactor (FBR). ALD of SnO2 was performed at 393K using bis(1-
dimethylamino-2-methyl-2propoxy)tin and hydrogen peroxide solution (H2O2, 50 wt.% in H2O)
as a Sn-precursor and reactant, respectively. The Sn-precursor was kept in a bubbler type canister
at 60 ℃ and the introduction of the Sn-precursor into the reactor was aided by Ar carrier gas (100
sccm). The H2O2 reactant was kept at 25 ℃ and delivered without a carrier gas. Sn-precursor and
H2O2 delivery lines were maintained at 80 ℃ to prevent the condensation. The Ar gas flow rate of
140 sccm was applied to fluidize the Pt/C powder in the FBR. For each SnO2 ALD process, 0.3 g
of Pt/C was loaded in the FBR. One cycle of ALD SnO2 consists of sequential doses of Sn-
precursor for 5 s, Ar purge gas for 15 s, H2O2 for 2 s, and Ar purge gas for 30 s. The ALD SnO2
film thickness was controlled by changing the number of ALD cycles from 75 to 300 cycles. After
obtaining ALD(SnO2)-Pt/C samples, they were subsequently heat-treated under flowing nitrogen
(99.9992% purity, Air Products and Chemicals) at 673 K (ramping rate: 2 K/min) for 3 h, followed
by overnight cooling to ambient temperature under flowing nitrogen.
2.2. Catalysts characterization
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Double Cs-corrected transmission electron microscopy (TEM) (Titan Cubed G2 60-300,
FEI) was used at an acceleration voltage of 300 kV, in order to obtain high-angle annular dark-
field scanning TEM (HAADF-STEM) images along with corresponding energy dispersive X-ray
spectroscopy (EDS) elemental mapping images. We also exploited a field-emission TEM (Tecnai
G2 F30 S-TWIN, FEI) with an acceleration voltage of 300 kV to acquire bright-field TEM images.
Scanning electron microscopy (SEM) (TESCAN MIRA3, TESCAN) was utilized to collect SEM
images, and record EDS spectra using an equipped energy dispersive X-ray spectrometer
(QUANTAX 200, Bruker). We employed a high-resolution thermogravimetric analyzer (TG209
F1 Libra, NETZSCH), and thereby recorded thermogravimetric analysis (TGA) curves under an
air atmosphere with a heating rate of 5 K/min. Energy dispersive X-ray fluorescence (EDXRF)
measurement was performed using an energy dispersive X-ray fluorescence spectrometer (ARL
QUANT’X, Thermo Fisher Scientific). To examine X-ray diffraction (XRD) characteristics, we
used a multipurpose X-ray diffractometer (Ultima IV, Rigaku) equipped with a Cu Kα Source. A
multi-purpose X-ray photoelectron spectrometer (Sigma Probe, Thermo Scientific) using an Al Kα
source was operated to collect X-ray photoelectron spectroscopy (XPS) spectra. In the XPS data
processing, all of the obtained XPS binding energy values were corrected on the basis of the C 1s
reference line at a value of 284.5 eV. On the 7D beamline of the Pohang Accelerator Laboratory
(PAL) with a Si (111) double crystal monochromator (PLS-II, 3.0ꢀGeV, South Korea), we
performed a synchrotron-based X‐ray absorption spectroscopy (XAS) analysis, consisting of X-
ray absorption near the edge structure (XANES) and an extended X-ray absorption fine structure
(EXAFS), at the Pt L3-edge (11,564 eV). The XANES and EXAFS data were processed and
analyzed using the IFEFFIT software package of Athena and Aretemis.29
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2.3. Electrocatalytic performance measurement in an electrochemical half-cell system
For an electrochemical half-cell system, we adapted a three-electrode cell system coupled
with a potentiostat (ZIVE MP2A, WonATech) at room temperature in order to measure
electrocatalytic properties; the working, counter, and reference electrodes were a catalyst-coated
glassy carbon electrode, Pt wire, and Ag/AgCl/1 M KCl, respectively. In order to prepare the
working electrode, we made a catalyst ink as follows: pristine Pt/C, heat-treated ALD(SnO2)-Pt/C
with various ALD cycles, and ALD(SnO2)-C were suspended by sonication for 1 h in a mixture of
distilled water, ethanol (95.0%, Samchun Pure Chemical), and 5 wt% Nafion® ionomer solution
(D521, DuPont). The fabricated catalyst ink was transferred onto the glassy carbon electrode (with
a geometric area of ca. 0.07 cm2), and a subsequent drying process of the ink resulted in a deposited
thin layer of catalyst on the surface of the glassy carbon electrode (with a loading amount of ca.
0.22 mg/cm2). To measure electrochemical properties, cyclic voltammetry (CV) was conducted in
0.5 M H2SO4 (Samchun Pure Chemical) or 2 M glycerol (≥99.0%, Junsei Chemical)/0.5 M H2SO4
solutions: a potential range of 0.036 to 1.236 V (vs. SHE) and a scan rate of 50 mV/s.
2.4. Electrocatalytic performance measurement in an electrochemical batch reactor system
We also utilized the same three-electrode cell system, which was connected to a
potentiostat, except for a catalyst-coated carbon paper as a working electrode, to operate an
electrochemical batch reactor system. The first step for fabricating the catalyst-coated carbon
papers was to prepare catalyst inks; through sonication for 1 h, we dispersed pristine Pt/C and
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150ALD(SnO2)-Pt/C (HT) catalysts in a solution mixture of distilled water, 2-propanol (99.5 %,
Samchun Pure Chemical), and the 5 wt% Nafion® ionomer solution. This catalyst ink was spray-
coated on a carbon paper (TGP-H-090, Toray) (with a geometric area of ca. 4 cm2), and
°
successively dried in a convection oven at 100 C for 1 h. Finally, we obtained the resultant
working electrode consisting of a thin catalyst layer coated on the carbon paper (with a loading
amount of ca. 1.25 mg/cm2). We implemented a chronoamperometry technique, operated at an
anode potential of 1.136 V (vs. SHE), to examine electrochemical properties in 0.1 M glycerol/0.5
M H2SO4 solution of 7 mL at 60 °C. After the chronoamperometry, in order to analyze the collected
liquid reaction products, we injected an obtained liquid product of 10 μL into high-performance
liquid chromatography (HPLC) equipment. The HPLC consisted of a refractive index (RI) detector
(2414, Waters), a binary HPLC pump (1525, Waters), and an ion-exchange ligand exchange
column (Hi-Plex H, Agilent Technologies) using 0.005 M H2SO4 aqueous solution (Samchun Pure
Chemical) as an eluent, and we operated the HPLC with an eluent flow-rate of 0.3 mL/min and a
column oven temperature of 60 °C. Please note that we determined glycerol conversion, mass
balance, and product selectivity on the basis of an external calibration method for the identified
liquid reactions products.
2.5. Computational details
All density functional theory (DFT) calculations were performed with the Vienna ab initio
simulation package (VASP).30,31 Following our previous studies,27,32 we employed optB86b-vdW
density functional33,34 to include noncovalent interactions. Solvation effects in aqueous media were
implicitly taken into account by VASPsol.35 The Pt(111) surface was prepared with 96 Pt atoms
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in four layers, which were well-placed in a unit cell with a size of 9.74 × 16.87 × 30.0 Å3. To
investigate the effect of SnO2, we put a portion of SnO2(110) surface on our Pt(111) surface model,
as shown in the ESI (see Fig. S11†). We optimized each equilibrium structure by relaxing all atoms
except Pt atoms in two bottom layers. For all DFT calculations, we set the kinetic energy cutoff to
450 eV and sampled the Brillouin zone with 3 × 3 × 1 Monkhorst-pack k-point grids. All structures
were optimized until all forces become smaller than 0.03 eV Å-1.
3. Results and discussion
3.1. Structural and morphological analyses
Fig. 1a illustrates the preparation procedure of the ALD(SnO2)-Pt/C (HT) catalysts. First,
SnO2 layers were deposited on the commercial Pt/C catalyst through ALD with various ALD
coating cycles of 75, 150, and 300, and subsequent heat treatment was applied. Figs. 1b-c and Figs.
S1a†-S2a† showHAADF-STEM images of the Pt/C and ALD coated Pt/C samples before heat
treatment and their corresponding elemental mapping images. There was no noticeable evidence
for the presence of SnO2 from all the ALD(SnO2)-Pt/C specimens, as also indicated by the SEM
results (Fig. S3a†). In terms of the EDS mapping results, this can be described on the basis of a
limitation of X-ray analysis: when element A is present throughout a sample with a large amount,
we cannot obtain unambiguous EDS mapping results showing the presence of small amounts of
the element A in a specific region from the sample.36 Thus, we believe that very thin layers of
SnO2 covered the entire surface of ALD(SnO2)-Pt/C, including Pt nanoparticles and the carbon
support, and this made it difficult for us to identify the deposited SnO2 layers from SEM and
HAADF-STEM images. The EDS spectra (Fig. S3b†) extracted from the SEM analysis confirm
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the existence of Sn in the ALD coated samples and the signal corresponding to a Sn peak gradually
increased according to the number of ALD SnO2 cycles. The loading amount of the coated SnO2
layers based on the recorded TGA curves (Fig. 2) corresponds to the number of ALD cycles: 3.19,
6.60, and 11.97 wt % for 75, 150, and 300 ALD cycles, respectively (Table 1). The deposition
behavior according to the precursor supply time for ALD SnO2 deposition on the powder was
further examined. The deposition was conducted with varying Sn precursor pulse time from 1 s to
15 s at 120 ℃ for 50 ALD cycles with Pt/C powder of 0.3 g. The Sn atomic fraction (%) for SnO2
coated Pt/C powder was examined using EDXRF. As shown in Fig. 3, the Sn atomic fraction
rapidly increased between 1-5 s of Sn(dmamp)2 precursor supply time, and thereafter the
increasing tendency declined, which indicates that powder ALD SnO2 (5 s-15 s-2 s-30 s) was
conducted in a semi-saturated ALD condition. Although the deposition conditions in this
experiment are not in the perfect saturation window, they showed a rather weaker Sn pulse time
dependency in the semi-saturation regime. Furthermore, the growth of SnO2 thin film showed a
very linear increase with cycle number, as can be seen from the TGA, EDS results in Fig. 2 and
Fig. S3b†, respectively. This suggests that precise thickness control of SnO2 film, which is one of
the representative advantages of the ALD technique, was well applied in this work. The following
heat treatment on the ALD(SnO2)-Pt/C at 673 K for 3 h under a nitrogen atmosphere induced
significant structural modifications in the ALD(SnO2)-Pt/C, especially for the coated SnO2 layers.
Even though there were no significant changes in SEM images and EDS spectra (Fig. S3c† and
S3d†), HAADF-STEM and EDS elemental mapping results for ALD(SnO2)-Pt/C (HT) indicated
that most of the atomic-layer-deposited SnO2 layers migrate onto Pt nanoparticles from the carbon
support during heat treatment (Fig. 1d, S1b†, and S2b†). In detail, as clearly displayed in magnified
HAADF-STEM and corresponding EDS elemental mapping images for 300ALD(SnO2)-Pt/C
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(HT) (Fig. 4), the migrated SnO2 layers mostly covered Pt nanoparticles (please refer to the
illustration at the rightmost from Fig. 1a), eventually creating numerous interfacial sites between
the Pt nanoparticles and ALD-overcoated SnO2 layers. Note that these covering SnO2 layers were
in the amorphous phase without any observable lattice fringes, in contrast to the lattice fringes
observed from Pt nanoparticles. We ascribe this phenomenon after the heat treatment to the strong
chemical interaction between Pt and atomic-layer-deposited SnO2, which is similar with strong
metal-support interaction (SMSI) in Pt nanoparticles supported on SnO2 support.37
We investigated the effects of the ALD coating and the heat treatment on the Pt
nanoparticles using a morphological analysis based on TEM (Figs S4a† and S4c†). We exploited
120 different Pt nanoparticles from the recorded TEM images for each specimen, in order to
calculate the mean Pt particle size, and to present the Pt particle size distribution using a histogram.
Specifically, for the mean Pt particle size, we adopted the length-number mean diameter (dLN),
which is expressed by Equation (1),
Σnidi
Σni
dLN
=
(1)
where ni is the number of Pt particles having a diameter of di.38 As depicted in Fig. S4b†, estimated
dLN values for 75ALD(SnO2)-Pt/C, 150ALD(SnO2)-Pt/C, and 300ALD(SnO2)-Pt/C were ca. 2.6,
2.7, and 2.9 nm, respectively, which means that the size of the Pt nanoparticle increases with the
increase of the ALD cycles. This might be due to the deposition of SnO2 on Pt/C. This was
supported by the XRD spectra showing no noticeable sharpening of Pt diffraction peaks according
to increasing ALD cycles (Fig. S5a†). After the subsequent heat treatment at 673 K, we clearly
found identified sintering of Pt nanoparticles from the TEM images for ALD(SnO2)-Pt/C (HT)
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catalysts (Fig. S4c† and S4d†). In detail, their dLN values were about 3.6, 3.5, and 3.2 nm for 75,
150, and 300 ALD cycles, respectively, which are larger than those of ALD(SnO2)-Pt/C specimens:
these dLN values correspond to the degree of peak broadening in respective XRD spectra for
ALD(SnO2)-Pt/C (HT) catalysts (Fig. S5b†).39 As tabulated in Table S1†, it is worthwhile to note
the changes in dLN values before and after the heat treatment: 1.0, 0.8, and 0.3 nm for 75, 150, and
300 cycles, respectively. With the XRD data shown in Fig. S5†, all of the measured samples
displayed no apparent XRD patterns for the SnO2, reflecting the amorphous nature of the SnO2.
We also could not find any shifts in the pronounced XRD peaks corresponding to face-centered
cubic (fcc) Pt (JCPDS No. 04-0802), indicating there is no alloy formation between Pt and other
elements such as Sn.39
3.2. Chemical and electronic state analyses
The surface chemical state of the Pt/C after the coating of SnO2 with different ALD cycles
and the subsequent heat treatment in N2 at 673 K were investigated by XPS. The pristine Pt/C
showed two Pt 4f peaks, including Pt 4f5/2 and Pt 4f7/2 at 71.3 eV and 74.6 eV, respectively (Fig.
5a). Each peak was deconvoluted into two different Pt oxidation states of metallic Pt0 and oxidized
Pt2+. The fraction of these two Pt species was obtained from the relative peak areas of their
deconvoluted peaks, as summarized in Table 2. A decrease in the Pt 4f binding energy was
observed when SnO2 was coated onto Pt/C by ALD. The Pt 4f7/2 peak shifted to 71.2 eV for
75ALD(SnO2)-Pt/C, 70.7 eV for 150ALD(SnO2)-Pt/C and 70.8 eV for 300ALD(SnO2)-Pt/C. The
negative shifts of binding energy imply electron transfer from Sn to Pt due to the electronic
interaction of Pt nanoparticles with the over-coated SnO2.39 Fig. 5b shows that the binding energies
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shift to slightly higher energies by 0.1-0.3 eV after heat treatment, which may be caused by the
preferential Sn-rich layer formation over Pt particles as observed in the above HAADF-STEM
analysis. It is also found that the amount of metallic Pt species increased after ALD SnO2 coating
and the subsequent heat treatment. The XPS spectra of SnO2 show two main peaks at ca. 487 eV
and ca. 495 eV in ALD(SnO2)-Pt/C catalysts that corresponds to Sn 3d5/2 and Sn 3d3/2, confirming
the presence of tin in the oxidation state (IV).40 Table 2 summarizes the Pt 4f and Sn 3d peaks
including peak assignment, binding energy, and relative area of the deconvoluted peaks for all the
samples before and after heat treatment.
The ALD(SnO2)-Pt/C sample was further studied by XAS in order to determine the
electronic and local structure around Pt atoms. Fig. 6a presents the Pt L3-edge XANES spectra for
the 150ALD(SnO2)-Pt/C before and after heat treatment and for the Pt/C. From the XANES
spectra, it can be clearly seen that the white line (WL) peak, which corresponds to the density of
vacant 5d electronic states in Pt,32 of the 150ALD(SnO2)-Pt/C was smaller than that for the Pt/C.
The decreased WL intensity can be attributed to partial filling of the unoccupied Pt 5d orbital by
electrons that are donated by adjacent Sn.39 Further reduction in the WL intensity was observed
after heat treatment of 150ALD(SnO2)-Pt/C. It may be correlated with the above mentioned XPS
results that metallic Pt species appeared to increase with the heat treatment of ALD coated samples.
The quantitative values of WL and the edge energy from XANES analysis are listed in Table 3.
Fig. 6b compares Fourier transforms (FT) of Pt L3-edge k3-weighted EXAFS data of Pt/C,
150ALD(SnO2)-Pt/C, and 150ALD(SnO2)-Pt/C (HT). In the ALD coated catalysts, peaks at 1-2 Å
attributed to the Pt-O neighbors, whereas peaks at 2-3 Å result from Pt-Sn and Pt-Pt neighbors.41
In order to obtain the structural parameters of Pt species including coordination number (N), bond
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distance (R), and Debye-Waller factor (σ2), the curve-fitting analysis was performed and the results
are included in Table 3. Fig. S6†-S8† shows good agreement between the experimental data and
fitted data in the curve-fitting analysis for the EXAFS functions and their corresponding FT and
real and imaginary parts of each sample. Curve-fitting of the FT-EXAFS revealed that the Pt-O
shells were located at about 2.0 Å and the Pt-Sn and Pt-Pt shells were located at about 2.71 Å and
2.76 Å, respectively. The Pt-Pt bond distance of Pt/C was almost unchanged after ALD SnO2
coating and the following heat treatment process while the coordination numbers of Pt-Pt and Pt-
Sn were changed. The increased coordination number of the Pt-Pt bond and the decreased
coordination number of the Pt-Sn bond in 150ALD(SnO2)-Pt/C (HT) as compared to
150ALD(SnO2)-Pt/C may be attributed to the enlarged crystallite size during the heat treatment
process, in agreement with the results from the aforementioned XRD and TEM analyses.
3.3. Electrocatalytic performance analyses
We conducted electrochemical half-cell experiments using ALD(SnO2)-Pt/C (HT)
catalysts and pristine Pt/C in order to investigate the effect of ALD SnO2 coating on Pt/C for the
electrocatalytic GOR (Fig. 7). A three-electrode cell system with a glassy carbon working
electrode was used for the electrochemical testing. The catalysts were deposited with the active
geometric area (ca. 0.07 cm2) of the glassy carbon electrode. As shown in Fig. 7a, the CV in 0.5
M H2SO4 electrolyte at room temperature was used to examine the H2 adsorption/desorption
behavior of pristine Pt/C and ALD(SnO2)-Pt/C (HT) with various ALD cycles (75, 150, 300
cycles). Based on the area of the H2 adsorption/desorption peaks below ~0.3 V (vs. SHE), we
measured electrochemical surface area of Pt from Pt/C and ALD(SnO2)-Pt/C (HT) catalysts (Table
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S2†).42 In Table S2†, the measured electrochemical Pt surface area values for 75ALD(SnO2)-Pt/C
(HT), 150ALD(SnO2)-Pt/C (HT), and 300ALD(SnO2)-Pt/C (HT) were 1.98, 2.04, and 2.20 cm2 ,
Pt
respectively. This trend is attributed to the decreased degree of Pt nanoparticle sintering throughout
the heat treatment, as the amount of the overcoated SnO2 on Pt/C surface increases (Table S1†).
The electrocatalytic GOR activities over Pt/C and ALD(SnO2)-Pt/C (HT) catalysts were
analyzed via the CV technique in 2 M glycerol + 0.5 M H2SO4 electrolyte at room temperature
(Fig. 7b, Table 4, and Fig. S9†). As presented in Fig. S9†, the current density values toward the
electrocatalytic GOR decrease in the order of Pt/C, 75ALD(SnO2)-Pt/C (HT), 150ALD(SnO2)-
Pt/C (HT), and 300ALD(SnO2)-Pt/C (HT). This could be because the Pt amounts loaded on glassy
carbon electrodes were different for the ALD(SnO2)-coated samples and uncoated sample, and the
Pt active sites were blocked by coating the thin SnO2 layers. We therefore mainly focus on
measuring the specific activity (mA/cm2 ), which is the normalized current values on the basis of
Pt
the electrochemical Pt surface area. We also evaluated the electrocatalytic performance in terms
of the onset potential (as summarized in Table 4) to confirm the activity enhancement by the effect
of ALD-overcoated SnO2 on the GOR.32,43 The 150ALD(SnO2)-Pt/C (HT) and 75ALD(SnO2)-
Pt/C (HT) catalysts exhibited their onset of GOR at 0.555 and 0.556 V (vs. SHE), respectively.
The onset potential value for uncoated Pt/C was 0.586 V (vs. SHE). This suggests that the
interactions between the Pt nanoparticles and ALD-coated SnO2 layers may result in improved
catalytic activity toward the electrocatalytic GOR. As presented in the CV curves in Fig. 7b, the
specific activity of 75ALD(SnO2)-Pt/C (HT) (0.428 mA/cm2 ) was increased compared to that of
Pt
Pt/C (0.388 mA/cm2 ) but decreased with increasing ALD SnO2 coating cycles. The
Pt
75ALD(SnO2)-Pt/C(HT) catalyst also showed GOR activity comparable to or even better than the
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previously reported Pt-based catalysts (as shown in Table S3†). This indicates that careful control
of the interactions between Pt nanoparticles and overcoated metal oxide layers may lead to
different capability of surface Pt active sites, thus enhancing the catalytic GOR activity. However,
the overall activity on the ALD SnO2 coated samples was not significantly improved compared to
the uncoated sample in our electrochemical half-cell experiments.
To examine the electrocatalytic stability for the GOR, we performed a CV cycling
durability test (Fig. 7c) in a 2 M glycerol + 0.5 M H2SO4 solution at room temperature, along with
structural analyses such as change of particle size before and after the stability test (Fig. 8 and
Table 5). The current density of each cycle (I) was normalized by the initial current density (I0).
Interestingly, all of the ALD(SnO2)-Pt/C (HT) catalysts improved the catalytic stability for the
GOR compared to uncoated Pt/C. After 900 CV cycles, 300ALD(SnO2)-Pt/C (HT),
150ALD(SnO2)-Pt/C (HT), and 75ALD(SnO2)-Pt/C (HT) present normalized current density
ratios (I/I0) of 37.17%, 36.42%, and 28.69%, respectively, which are higher than that of Pt/C
(20.13%). Although the 75ALD(SnO2)-Pt/C (HT) exhibited the best performance in terms of the
specific activity among the tested catalysts (Fig. 7b), these durability test results suggest that a
sufficient amount of the SnO2 overcoating should be deposited onto the Pt/C surface in order to
enhance electrocatalytic stability for the GOR. We conducted additional TEM characterizations to
understand why the SnO2 overcoated Pt/C catalyst showed higher stability than the uncoated Pt/C
(Fig. 8 and Table 5). After the electrochemical stability test, the change of Pt particle size for the
Pt/C catalyst was 2.4 nm (dLN of Pt particles for fresh Pt/C = 2.4±0.3 nm and that of used Pt/C =
4.8±1.2 nm). However, after the CV cycling durability test, the changes in the Pt particle size for
all ALD(SnO2)-Pt/C (HT) catalysts were lower than that for uncoated Pt/C (Pt particle size changes
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were 1.1 nm, 1.2 nm, and 1.2 nm for the samples with ALD overcoating cycles of 75, 150, and
300, respectively). Based on these analytical results, we verified that the ALD SnO2 coating
inhibited sintering of Pt for the GOR in the acidic solution. It is also possible that the ALD coated
catalyst affected the prevention of catalyst degradation routes such as electrochemical Ostwald
ripening and carbon corrosion.26 It has been reported that sintering or leaching of metal
nanoparticles occurs at under-coordinated metal atoms located at defect, corners, and edge
sites.10,19,44 The ALD SnO2 layers could be coated on the defect or under-coordinated sites of Pt.
The interfacial interaction between Pt defect sites and overcoated SnO2 thin layer could diminish
the Pt sintering for the GOR, thus leading to enhanced electrocatalytic stability. Future studies
should include in-depth analyses of the deposition characteristics of powder ALD SnO2 on Pt/C
according to various process conditions and the catalytic role of ALD overcoated SnO2 by using
comparative samples such as Pt nanoparticles supported on SnO2 or Pt nanoparticles mixed with
SnO2 nanoparticles.
To evaluate how the reaction chemistry for electrocatalytic GOR changes with
ALD(SnO2)-Pt/C and Pt/C catalysts, we designed and used an electrochemical batch reactor
system that consists of a three-electrode based system featuring a catalyst-coated carbon paper
working electrode (active electrode area: ca. 4 cm2). We used the 150ALD(SnO2)-Pt/C and
uncoated Pt/C catalysts for the electrochemical batch reactor study to determine how the ALD
SnO2 coating affects the electrocatalytic GOR. As plotted in Fig. S10†, the chronoamperometry
method was used to implement the electrochemical batch reaction in 0.1 M glycerol + 0.5 M H2SO4
feed solution at 60 °C by applying an anode potential of 1.136 V (vs. SHE) to the carbon paper
working electrode. It is noteworthy that the electrocatalytic oxidation of biomass-derived glycerol
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presents a promising method of co-producing renewable chemicals and hydrogen in an electrolysis
cell system (see Scheme S1†). The chosen potential of 1.136 V (vs. SHE) in this study was enough
to demonstrate the changed catalytic reaction chemistry toward GOR over ALD(SnO2)-Pt/C and
uncoated Pt/C catalysts. As shown in Table 6, turnover frequencies (TOFs), an indicator of the
kinetic activity of a catalyst, were calculated at a similar glycerol conversion of less than 10% by
using the electrochemical batch reactor with 150ALD(SnO2)-Pt/C and Pt/C catalysts. The TOF of
the ALD-SnO2-coated Pt/C catalyst (91.29 h-1) did not exhibit a noticeable enhancement compared
to the uncoated Pt/C (TOF of 74.86 h-1), verifying that the SnO2 overcoating on the Pt/C surface
does not significantly increase the reaction kinetics for the GOR. On the other hand, we determined
the change of product selectivity in the electrocatalytic GOR over 150ALD(SnO2)-Pt/C and Pt/C.
Glycerol conversion and product selectivity were confirmed via a HPLC analysis of liquid reaction
products created from the electrocatalytic batch reactions using 150ALD(SnO2)-Pt/C and Pt/C
catalysts. As illustrated in Fig. 9a, glycerol is first oxidized to glyceraldehyde (GAD) in the
pathway of primary alcohol oxidation, and is subsequently oxidized to glyceric acid (GLA). GLA
is further oxidized by C-C bond cleavage into glycolic acid (GCA). For both the 150ALD(SnO2)-
Pt/C (HT) and Pt/C catalysts, reaction products generated from the electrocatalytic GOR were
GAD, GLA, and GCA. This indicates that the primary alcohol oxidation pathway was favored by
both 150ALD(SnO2)-Pt/C (HT) and Pt/C catalysts, rather than the secondary alcohol oxidation.
From the glycerol conversion vs. product selectivity plots of Figs. 9 b-c, the GAD selectivity values
for 150ALD(SnO2)-Pt/C (HT) were higher than those for Pt/C. The SnO2-coated Pt/C catalyst
showed lower GLA selectivity values as compared to the Pt/C catalyst. These results demonstrate
that the SnO2 overcoating on the Pt surface can change the glycerol oxidation reaction chemistry
compared to on the uncoated Pt surface, thus affecting the product selectivity.
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3.4. Theoretical investigations
To understand how the SnO2 coating on the Pt surface changes the reaction chemistry for
the GOR, we compare interactions between reaction intermediates and Pt(111) surfaces with and
without SnO2 deposition (Fig. 10 & Figs. S11†-12†). As shown in Fig. 9, the glycerol molecule is
primarily oxidized to GAD, which is further oxidized to GLA.45-48 By assuming that surface OH
drives the oxidation reaction from glycerol to GAD to GLA,49-51 we propose a reaction pathway
from glycerol to GAD to GLA (see Fig. 10a) and compare the thermodynamics stabilities of
equilibrium structures along this pathway. In Fig. 10, we observed that reaction intermediates are
more stabilized in the presence of SnO2. As expected, this additional stability comes from
hydrogen bonds between oxygen atoms in SnO2 and hydrogen atoms in hydroxyl groups (see Fig.
10b). Moreover, we observed that oxygen atoms in glycerol or its oxidized forms can directly
interact with Sn atoms. Therefore, SnO2 deposited surfaces hold both reactants and products much
more strongly, which both positively and negatively affects desired reactions based on the Sabatier
principle. In addition, as reactants are strongly absorbed to the catalytic Pt surface in the presence
of SnO2, the initial reaction could be faster with the SnO2 deposited Pt surfaces (Figs. 10c-d).
However, these surfaces interrupt the reorganization of the intermediate molecules for subsequent
reactions. This could be weakly correlated to the selectivity of GAD as SnO2-coated Pt surfaces
may enhance the production of the first intermediate, GAD in this case. From experimental results
of the glycerol conversion vs. product selectivity in Figs. 9 b-c, we found that the ALD SnO2-
coated Pt catalyst produced more GAD than the uncoated Pt catalyst. These results indicate that
the SnO2-coated Pt surface can modulate the reaction chemistry towards the electrocatalytic GOR.
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Although we performed DFT calculations for a specific reaction pathway, our microscopic
explanation mainly relies on noncovalent interactions between SnO2 and reaction intermediates.
Thus, we believe that the general trends between the SnO2-coated Pt(111) and Pt(111) surfaces
will be maintained for different reaction pathways.
4. Conclusions
SnO2-coated Pt/C materials synthesized by the ALD technique were used to study the
effect of SnO2 overcoating on commercial Pt/C for electrocatalytic GOR. Structural and
morphological characterization results demonstrate that an amorphous-phase SnO2 thin layer was
overcoated onto the Pt/C surface by ALD and subsequent heat treatment. Chemical and electronic
state analyses indicated that the Pt physicochemical property was changed by the interaction
between the Pt nanoparticles and ALD coated SnO2. Although the overall activity on the ALD
SnO2 coated Pt/C samples was not significantly enhanced compared to the uncoated Pt/C sample,
all of the ALD(SnO2)-Pt/C (HT) catalysts improved the catalytic stability for the GOR compared
to uncoated Pt/C. Pt particle size measurements before and after the stability tests revealed that the
ALD SnO2 coating prevented sintering of Pt for the GOR in the acidic solution. In addition, the
ALD(SnO2)-Pt/C (HT) catalyst produced more GAD at glycerol conversion levels similar to those
of the Pt/C catalyst. Our DFT calculations confirm that oxygen atoms in glycerol or its oxidized
forms can directly interact with Sn atoms. Therefore, SnO2 coated surfaces maintain both reactants
and products much more strongly. The binding energies of glycerol and reaction intermediates
also change at the interfacial site of the SnO2-deposited Pt surface compared to those of the Pt
surface only, which affect the reaction chemistry for the electrocatalytic GOR. This work shows
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how the physicochemically-modified Pt nanoparticle catalyst prepared using the particle ALD
coating technique can be employed to enhance the catalytic stability and to control the reaction
chemistry for the electrocatalytic GOR. This study also paves the way for future material design
in a variety of electrochemical applications such as electrolyzers and fuel cells.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the core KRICT project [grant number SI2011-30] from the
Korea Research Institute of Chemical Technology (KRICT). The first author thankfully
acknowledges financial support by Kwanjeong Educational Foundation.
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Fig. 1. Schematic illustration and electron microscopy analysis of Pt/C and ALD(SnO2)-Pt/C.
a, Schematic illustration for the ALD SnO2 coating on Pt/C and the subsequent heat treatment on
ALD(SnO2)-Pt/C. b,c,d, HAADF-STEM images (upper) and corresponding EDS elemental
mapping (lower) of Pt for pristine Pt/C (b) and the composite of Pt and Sn for 300ALD(SnO2)-
Pt/C (c) and 300ALD(SnO2)-Pt/C (HT) (d).
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Fig. 2. Thermogravimetric analysis of Pt/C and ALD(SnO2)-Pt/C catalysts to determine the
loading amount of ALD-coated SnO2. TGA curves for Pt/C and ALD(SnO2)-Pt/C with various
ALD cycles, which were obtained by ramping the temperature with a rate of 5 °C/min under an air
atmosphere.
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Fig. 3. Sn atomic fraction obtained from the SnO2 coated Pt/C powders examined using
EDXRF. Sn atomic fraction vs. Sn precursor pulse time data based on the EDXRF results. Please
note that sub-saturation and semi-saturation regimes are divided by a vertical dotted line.
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Fig. 4. Electron microscopy analysis obviously representing the effect of the subsequent heat
treatment on ALD(SnO2)-Pt/C. Enlarged HAADF-STEM image and corresponding EDS
elemental mapping of Pt, Sn, and the composite of Pt and Sn for an ALD(SnO2)-covered Pt particle
from 300ALD(SnO2)-Pt/C (HT).
29