Isolated single-atom Pt sites for highly selective electrocatalytic hydrogenation of formaldehyde to methanol

Article abstract of DOI:10.1039/d0ta00190b

The direct electrochemical conversion of noxious formaldehyde into value-added chemicals is a quite promising technique for resolving the increasingly serious environmental issue arising from industrial formaldehyde-containing wastewater. However, thus far, it has not been examined, to the best of our knowledge. This study reports the electrocatalytic hydrogenation of formaldehyde to methanol in a neutral aqueous medium under ambient conditions over a Pt single-atom catalyst, and this catalyst exhibits a favorable methanol yield rate and a high faradaic efficiency of 30.7 mg h^-1 mg_cat.^-1 and 95.8% at -0.8 V versus the reversible hydrogen electrode in 0.1 M Na₂SO₄. Also, it exhibits excellent durability. Atomic-scale structural characterization and theoretical calculations revealed that the above-mentioned efficient performance is related to atomically dispersed Pt-O sites, which could lower the free-energy change for the chemisorption of formaldehyde and activate the C-H bond.

Full text of DOI:10.1039/d0ta00190b

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DOI: 10.1039/D0TA00190B
ARTICLE
Isolated single-atom Pt sites for highly selective electrocatalytic
hydrogenation of formaldehyde to methanol
a
a
b
b
b
c
Shunzheng Zhao, Yanfeng Wen, Xianyun Peng, Yuying Mi, Xijun Liu,* Yifan Liu,* Longchao
Received 00th January 20xx,
Accepted 00th January 20xx
d
e
b
a
Zhuo, Guangzhi Hu,* Jun Luo, and Xiaolong Tang*
DOI: 10.1039/x0xx00000x
The direct electrochemical conversion of noxious formaldehyde into value-added chemicals is a quite promising technology
to resolve the increasingly serious environmental issue arising from industrial formaldehyde-containing wastewater.
However, thus far, it has not been examined, to the best of our knowledge. This study reports the electrocatalytic
hydrogenation of formaldehyde to methanol in a neutral aqueous medium under ambient conditions over a Pt single-atom
catalyst, and this catalyst exhibits a favorable methanol yield rate and a high faradaic efficiency of 30.7 mg h–1 mgcat. and
–1
9
2 4
5.8% at –0.8 V versus the reversible hydrogen electrode in 0.1 M Na SO . Also, it exhibits excellent durability. Atomic-scale
structural characterization and theoretical calculations revealed that the above-mentioned efficient performance is related
to atomically dispersed Pt–O sites, which could lower the free-energy change for the chemisorption of formaldehyde and
activate the C–H bond.
The electrochemical conversion method has been considered as a
powerful tool and has attracted considerable interest from the
scientific and engineering viewpoints due to its environmentally
friendly nature, operational simplicity, ease of scalability, reaction
tenability, and economic efficiency because of the use of cost-
1
. Introduction
Formaldehyde (CH
2
O), one of the most versatile basic chemicals, is
widely produced and utilized in various fields (such as: synthetic resin
production, aquaculture industry, adhesive and glue manufacturing,
disinfectant and preservative production, chemical and
effective and/or recyclable electrodes.1
013
In this regard, the direct
petrochemical industry, textile processing, paper manufacturing and electrochemical conversion of CH
2
O into valued-added chemicals
wood processing, etc.). As a result, a large amount of wastewater with high selectivity and efficiency is clearly one of the most
containing variable concentrations of CH
O, ranging from a few to promising strategies for the removal of CH O. Thus far, the
hundreds of milligrams per liter, is produced along with these electrocatalytic hydrogenation of carbon dioxide, benzaldehyde,
2
2
1
4
series of strategies such as aldehyde, and alkynes have been widely investigated.14,15 In contrast,
industries.
Thus far,
a
physical/chemical adsorption, photocatalysis, biological/botanical
filtration, plasma technology, and catalytic oxidation have been
a similar study on CH
2
O has been hitherto unconfirmed.
Single-atom (SA) catalysts,1
utilization and unique electronic properties, have emerged as a
619
which exhibit high atom
5
9
extensively developed for CH
2
O
removal.
However, the
aforementioned methods typically exhibit disadvantages associated
with limited adsorption capacity, byproduct formation, poor activity,
and high costs. Therefore, it is urgent to further develop a more
efficient and environmentally-friendly approach for the removal of
2
022
frontier for electrocatalysis (such as hydrogen evolution,
1
4, 15, 23
24
nitrogen reduction,
and water oxidation ). Owing to the
benefits of heterogeneous and homogeneous catalysts, SA catalysts
exhibit notably enhanced electrochemical activities. MXene, as a
unique family of two-dimensional (2D) metal carbides, has excellent
electronic conductivity, catalytically active basal planes with exposed
metal sites, hydrophilic surface functionalities and graphene-like
2
noxious CH O.
2
5, 26
unique layered structure.
In addition, MXene not only have high
a. Department of Environmental Engineering, University of Science and Technology
surface area but also show unique features of a high reducing
capability and suitable surface defects. These unique features of
MXene make it is expected to be an ideal candidate to support and
stabilize Pt single atoms. However, it is still challenging to develop
robust electrocatalysts with well-defined active SA sites to boost the
Beijing, Beijing 100083, China.
E-mail: txiaolong@126.com
Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional Porous
19
b.
Materials, Institute for New Energy Materials & Low-Carbon Technologies, School
of Materials Science and Engineering, Tianjin University of Technology, Tianjin
3
00384, China.
Email: xjliu@tjut.edu.cn
hydrogenation performance of CH
Inspired by these studies, a heterogeneous catalyst comprising
Pt SAs anchored on ultrathin Ti MXene (T refers to surface
functional groups) nanosheets (Pt-SA/Ti ) for the selective
production of methanol (CH OH) from the electrocatalytic
2
O.
c.
Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and
Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen
University, Shenzhen 518060, China.
3
C
2
T
x
x
E-mail: liuyifan@mpi-hd.mpg.de
School of Materials Science and Engineering, Xi'an University of Technology, Xi'an
3 2 x
C T
d.
3
7
10048, China.
e.
School of Chemical Science and Technology, Yunnan University, Kunming 650091,
China.
E-mail: guangzhihu@ynu.edu.cn
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI:
1
0.1039/x0xx00000x
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Journal Name
hydrogenation of CH
2
O under ambient conditions is examined, deionized water. Finally, the Pt-NP/Ti
3
C
2
T
x
composite was obtained
View Article Online
DOI: 10.1039/D0TA00190B
affording a faradaic efficiency (FE) and a yield rate of 95.8% and 30.7 by freeze-drying.
–1
–1
mg h mgcat. , respectively. Notably, the obtained yield rate is
greater than those of catalysts reported previously for the
Physical characterization of materials
3
electrosynthesis of CH OH (please check Table S1 in the Electronic
Supplementary Information for details). Moreover, the FE, yield rate, The crystalline phases were identified by Rigaku D/max 2500 X-ray
and reduction current density remain stable during the consecutive diffractometer (XRD) using Cu Kα radiation with a wavelength of
–
recycling and long-term electrolysis. Electrochemical measurements 0.154598 nm (40 kV voltage, 40 mA current) at a scan rate of 10° min
1
and density functional theory (DFT) calculations revealed that the Pt–
O active moieties in Pt-SA/Ti
limiting adsorption of CH
O* while simultaneously promoting the electron microscopy (FESEM, Verios 460L) and transmission electron
within 2θ range of 3° – 60°. The size and morphology of the
3 2 x
C T
strongly facilitate the potential- prepared samples were characterized by field-emission scanning
2
activation of the C–O bond significantly. The proposed microscopy (TEM, Talos F200X) equipped with energy-dispersive X-
electrochemical conversion method in this study not only ray spectroscopy (EDS). High-angle annular dark-field scanning
demonstrates the potential capacity to extend the scope of indirect transmission electron microscopy (HAADF-STEM) images were taken
routes for the production of methanol from CO
sustainable approach for the uptake of formaldehyde.
2
but also provides a using an FEI Titan Cubed Themis G2 300 STEM with a probe corrector
at 200 kV. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi,
Thermo Scientific) was employed for elemental mapping using a
monochromatic Al Kα radiation source. The binding energy
calibration was performed by referencing the C 1s main peak at 284.8
2
. Experimental
2
eV. N adsorption-desorption isotherms were measured on a Tristar
Synthesis of Ti
Typically, 1 g of lithium fluoride (LiF) powder was added to 10 mL of
M HCl solution. This mixture was stirred for 5 min with a magnetic
Teflon stir bar to dissolve the salt. 1 g of Ti AlC powder (300 mesh)
was carefully added over the course of 15 min to avoid initial
overheating of the solution as a result of the exothermic nature of
the reactions. The reaction mixture was held at 35 °C for 24 h, after
which the mixture was washed with DI water several times by
centrifugation until the pH of the supernatant reached
3 2 x
C T
II 3020 Micromeritics analyzer. Pore volumes and sizes were
computed by a quenched solid density functional theory method and
specific surface areas were discerned by the Brunauer–Emmett–
Teller (BET) method. Atomic force microscopy (AFM) images were
collected in tapping mode under ambient atmosphere using a Bruker
microscope (Dimension icon) and processed with NanoScope
Analysis software.
9
3
2
3 2 x
approximately 6. To obtain the thin Ti C T
flakes, the freshly XAFS measurements
produced powder was bath sonicated in 50 mL of DI water for 1 h
under a flowing Ar atmosphere in an ice bath to minimize oxidation.
Then, the solution was centrifuged at 3500 rpm for 1 h and the
supernatant was collected. Finally, the Ti C T was obtained by
3 2 x
freezing the resulted colloidal solution at -80 °C for 24 h and then
freeze-drying.
XAFS spectra were obtained at the 1W1B station in BSRF (Beijing
Synchrotron Radiation Facility, China) operating at 2.5 GeV with a
maximum current of 250 mA. XAFS measurements at the Pt L -edge
3
were performed in the fluorescence mode using a Lytle detector.
Athena and Artemis codes were used to extract data and fit profiles.
Preparation of electrode
Synthesis of Pt-SA/Ti
Pt-SA/Ti was prepared by a similar one-step etching method as
described above except that 20 μL of freshly prepared chloroplatinic
acid hexahydrate (H PtCl ·6H O) solution with a concentration of 1
3 2 x
C T
For the preparation of the working electrode, the catalyst ink was
firstly prepared by mixing 6 mg of catalyst powder, 0.5 mL of Nafion
3 2 x
C T
(0.05 wt.%) solution and 0.5 mL of absolute ethyl alcohol to form
2
6
2
homogeneous ink. A following high-power sonication (500 W) was
employed to achieve a fine dispersion. After 60 min, the resultant
suspension was then air-brushed onto a 0.5 x 2 cm Sigracet 29 BC
M was added to the mixture 9 M HCl solution of LiF and Ti
vigorous stirring.
3 2
AlC under
-2
gas-diffusion layer (GDL, Fuel Cell Store) using nitrogen as the carrier
gas. The loading amount of the catalyst was determined to be 0.2 mg
cm by the GDL weight gain after the air-brushing. The electrode was
dried in ambient air for the subsequent electrochemical testing
experiments.
Synthesis of Pt-NP/Ti
3 2 x
C T
−2
For the synthesis of Pt-NP/Ti
dissolved in 50 mL of water in a round-bottom flask, then sonicated
for 30 min to obtain a uniform dispersion. Then, 20 μL of 1 M
3 2 x 3 2 x
C T , 50 mg of Ti C T was firstly
H
2
PtCl
solution gently, with electromagnetic stirring for 20 min. Then, 24 mg
of sodium borohydride (NaBH ) dissolved in 0.5 mL of 2 M NaOH All electrochemical CH O electrolysis was performed in a flow cell
6 2 3 2 x
·6H O aqueous solution was added into the above Ti C T
2
Flow-cell CH O electrolysis
4
2
solution was quickly added into the above resultant mixture under configuration using a CHI 760E electrochemical workstation (CHI
vigorous stirring for 2 h at 0 °C using an ice bath to maintain the Instruments, Inc.) at room temperature (25 °C). The GDE with the
temperature for prevention of Pt nanoparticle aggregations. deposited catalyst, Ag/AgCl electrode (with saturated KCl as the
Subsequently, the resulting solution was centrifuged and washed by filling solution), and nickel foam were applied as the working
2
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electrode, reference electrode, and counter electrode, respectively. solution. The potential window was between 0.22 V and 0.32 V
View Article Online
DOI:– 11 0.1039/D0TA 0– 10 190B
The catholyte and anolyte compartments were separated by a versus RHE. The scan rates ranged from 5 mV s to 25 mV s with
–1
proton exchange membrane (Nafion 117), which was protonated by an interval of 5 mV s . Cdl was estimated by plotting the △j = (j
a
–
heating in a 5% H aqueous solution at 80°C for 1 h and in DI water )/2 versus RHE (where j and j are the anodic and cathodic current
2
O
2
j
c
a
c
for another 1 h. The working electrode, proton exchange membrane, densities, respectively) against the scan rate. The Cdl value was
and counter electrode were positioned and clamped together using derived from the slope. Electrochemical impedance spectroscopy
polytetrafluoroethylene spacers such that the catholyte and anolyte (EIS) was performed in the frequency range from 100 kHz to 10 mHz
could be introduced into the chambers between the anode and with an AC amplitude of 5 mV.
membrane as well as between the membrane and cathode. The
2
electrode area was 1 cm and the distance from the electrode to the
membrane was 0.2 cm. Gaseous CH O flowed through the gas
2
Computational details
compartment at the backside of the gas diffusion layer to diffuse into The starting models of pristine MXene Ti
the catholyte present around the catalyst. The CH
O gas flow rate conventional way by removing Al sheets from the parent MAX phase
into the flow cell was controlled at 20 sccm (standard cubic Ti AlC . The 2D material consists of five (Ti ) atomic sheets with a
3 2
C were constructed in a
2
3
2
3 2
C
centimeter per minute) via a mass flow controller. In the catholyte hexagonal-like unit cell. The atomic models of the functionalized
stream, a port drilled into the polytetrafluoroethylene spacer MXenes were designed assuming their complete surface termination
allowed an Ag/AgCl reference electrode to be positioned at a specific by oxygen atoms. The periodic system had a vacuum thickness of 15
distance from the working electrode. The electrolyte (0.5 M Na
2
SO
4
Å, which was used to eliminate spurious interactions between the
aqueous solution, 10 mL for each of the catholyte and anolyte) was adsorbate and the periodic image of the bottom layer of the surface.
circulated through the electrochemical cell with a flow rate of 10 mL All the computations were performed with spin-polarized DFT
−
1
3
27
min by controlling with a peristaltic pump. The reactions lasted for implemented in the DMol code of the Materials Studio.
A
at least 10 min before the liquid and gas products were collected for generalized gradient approximation (GGA) with the Perdew, Burke,
analysis. All measured potentials were referenced to the reversible and Ernzerhof (PBE) exchange-correlation functionals was
2
8
hydrogen electrode (RHE) according to the Nernst equation employed. The double numerical plus polarization (DNP) was used
potential:
as the basis set. Effective core potentials were adopted as the core
treatment to conduct a metal relativistic effect. The geometry
–
5
convergence tolerance for energy change was 2  10 Ha, the max
force was 0.004 Ha/Å, and the max displacement was 0.005 Å. We
chose 4.5 Å to be the real-space global orbital cutoff radius.
Structural optimization was performed without any constraints. The
Gibbs free energy change (ΔG) of each elemental step was calculated
according to the CHE model, which used one half of the chemical
potential of hydrogen as the chemical potential of the proton-
E
RHE(V) = EAg/AgCl(V) + 0.0591 × pH + 0.197 V
Determination of CH
The concentration of produced CH
chromatograph (GC, Agilent 7890) equipped with a Poraplot Q
column and a molecular sieve column. The CH OH production was
analyzed by flame ionization detector (FID) with N as a carrier gas.
In detail, at each GC testing experiment, 1 μL of catholyte was
injected with a syringe after CH O electrolysis at an applied potential
3
OH
3
OH was determined by a gas
3
2
9
electron pairs. ΔG was calculated by the equation: ΔG = ΔE + ΔEZPE
ΔG – TΔS, where ΔE is the electronic energy difference directly
obtained from DFT calculations, ΔEZPE is the zero-point energy, T is
the temperature (T = 298.15 K), ΔS is the change in entropy, and ΔG
2
+
U
2
persisted for 60 min (Agilent, 5190-1475).
U
is the free energy contributions related to the applied electrode
potential U.
Determination of FEs
3
FE of the CH OH was calculated using the following equation:
3
. Results and discussion
FE = znF/Q × 100%
Figure 1a shows the straightforward one-step synthesis of Pt-
SA/Ti (Pt loading of 1.26 wt%). Therein, ultrathin Ti
nanosheets are formed during etching, and isolated Pt atoms are
concomitantly embedded into the surface of the Ti C T nanosheets
3
C
2
T
x
3 2 x
C T
where z is the number of electrons transferred, n is the number of
−1
moles of the product, F is Faraday’s constant (96485 C mol ), Q
represents the total amount of electrons which were supplied to the
system.
3 2 x
due to the rich surface terminal groups T
see details in the Experimental Section). For comparison, pristine
Ti and Pt nanoparticles supported on Ti (denoted as Pt-
NP/Ti ) nanosheets were prepared using individual precursors by
the same protocol (Figs. S1–S4).
x
serving as anchoring sites
(
3
C
2
T
x
3 2 x
C T
3 2 x
C T
Electrochemical characterization
The electrochemically active surface areas of the three catalytic
electrodes were estimated based on double-layer capacitance (Cdl).
The Cdl was determined by measuring the geometric current at a
potential window in which no Faradaic process occurred as a
function of the scan rate of the cyclic voltammetry stripping. Cyclic
voltammetry (CV) was performed in a 0.5 M Na
2 4
SO aqueous
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DOI: 10.1039/D0TA00190B
Fig. 2 XANES and EXAFS characterization results of Pt-SA/Ti
3 2 x
C T . (a)
XANES spectra at the Pt L -edge. PtO and Pt foil are reference
3
2
3 2 x
Fig. 1 Synthesis and characterization of Pt-SA/Ti C T . (a) Schematic
image of the synthesis of Pt-SA/Ti C T . (b) SEM image. (c) TEM
3 2 x
image. (d) HAADF-STEM image, in which some of the Pt SAs are
highlighted by red circles. (e) Line profiles of regions A and B in panel
samples. (b) FT-EXAFS curves in which R and χ(k) denote the radial
distance and EXAFS oscillation function, respectively. (c) WT-EXAFS
plots in which k and (R + α) represent the electron wave vector and
interatomic distance without phase correction, respectively. (d)
(d).
Fitting of the FT-EXAFS curve of Pt-SA/Ti
model of Pt-SA/Ti
3 2 x
C T . (e) Atomic-structure
3 2 x
C T .
The X-ray diffraction (XRD) patterns (Fig. S1) revealed that the
crystalline structure of Pt-SA/Ti is similar to that of the pristine
Ti sample, but characteristic peaks corresponding to Pt chlorides
3 2 x
C T
X-ray absorption near-edge structure (XANES) and extended X-
ray absorption fine structure (EXAFS) spectroscopy at the Pt L -edge
were employed to probe the local electronic structure and
3 2 x
C T
3
or oxides are not observed. Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images revealed that Pt-
1
1, 12
coordination environment of Pt-SA/Ti
The absorption-edge position of Pt-SA/Ti
Pt foil and PtO
3
C
2
T
C
x
at the atomic level.
is located between the
SA/Ti
hundreds of nanometers (Fig. 1b,c). Meanwhile, Pt nanoparticles are
not observed on the Pt-SA/Ti nanosheet surface (Fig. 1c), which
3 2 x
C T nanosheet exhibits a flat surface with a lateral size of
3
2
T
x
4
+
2
, suggesting that the Pt species are reduced from Pt
3 2 x
C T
2
–
in its [PtCl
Figs. 2a and S9). X-ray photoelectron spectroscopy (XPS) analysis
also afforded a similar result (Fig. S10). Figure 2b shows the Fourier
transform (FT) EXAFS spectra at the Pt L -edge. For Pt-SA/Ti
only a strong peak is observed at ca. 1.7 Å, corresponding to the Pt−O
6 3 2 x
] precursor by the highly reductive Ti C T nanosheets
is in agreement with the XRD results. Figure 1d and Figure S5 show
the atomic-resolution high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) images. A high
concentration of bright dots, albeit with random distribution, is
observed, corresponding to the isolated Pt atoms (marked by red
circles). Loose interatomic distances and different intensities of line
profiles (Fig. 1e) further confirmed the absence of Pt dimers or
(
3
3 2 x
C T ,
3
0
bands, and it is clearly different from the results observed for the
Pt foil and PtO . In addition, it can be found a clear peak at 2.88 Å,
which does not belong to either Pt-Pt (2.52 Å) or Pt-Ti (2.2 Å) . The
2
2
5
clusters in Pt-SA/Ti
corresponding elemental mapping (Fig. S6) revealed the
homogeneous distribution of Pt atoms in Pt-SA/Ti nanosheets.
Atomic force microscopy measurements (Fig. S7) revealed that the
average thickness of Ti nanosheets is 2.1 ± 0.2 nm, thereby
3 2 x
C T . Moreover, HAADF-STEM and the
3
1, 32
similar peaks were also found in other works.
Previously study has suggested that Pt single atoms were
stabilized at the sites preferentially occupied by Ti due to strong
3 2 x
C T
3 2 x
metal-carbon (namely M–C) interactions with the Ti C T MXene
3 2 x
C T
2
5
support. Hence, Pt–C and Pt–Ti bonds are observed in FT-EXAFS.
However, a Pt–Ti bond is not detected in this work. Considering that
2
–1
inducing a large surface area (420.9 m g ; Fig. S8), leading to
accessible active sites and facilitating the mass transport during
electrocatalysis.
the Ti
3

C
2
T
x
MXene surface is rich in oxygen-containing groups (such
2
as OH and O ) and that these functional groups are negatively
4
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charged,19, 26 which can provide anchoring sites for stabilizing metal active than Ti
atoms. Therefore, it is reasonable to speculate that single-atomic Pt electrocatalytic hydrogenation of CH
sites in Pt-SA/Ti are captured by surface oxygen-containing
groups, thereby forming PtO bonds. Furthermore, we also
calculated the formation energy (Eform) for Ti-vacancy in Ti and
adsorption energy (Ead) for Pt atom adsorbed on Ti during the
synthesis process of Pt-SA/Ti . As a result, the value of E is +5.7
, Pt-NP/Ti
, and commercial Pt/C for the
3
C
2
T
x
3
C
2
T
x
View Article Online
DOI: 10.1039/D0TA00190B
2
O.
3 2 x
C T
3 2 x
C T
3 2 x
C T
3
C
2
T
x
f
eV, which is significantly larger than that of Ead (–0.8 eV), indicating
that Pt atoms are more easily to exist in the form of adsorption on
the Ti
between Pt atoms and Ti
energies, the density of states (DOS) of Pt-NPs and Pt-SA/Ti
were calculated (Fig. S11). It can be found that the Pt-SA/Ti
.6 eV) has a more negative d band center than that of Pt-NPs (-1.3
3
C
2
T
x
surface. In order to understand the bonding mechanism
3 2 x
C T
and inherent rules for adsorption
3 2 x
C T
C T (–
3 2 x
1
eV). According to Nørskov’s d-band theory, the downshift of the Pt d
band center could pull more of the antibonding states below the
2
1,
Fermi level, leading to the optimal activation energy of reactants.
3
3
Moreover, the further away from the Fermi level, the more
electrons it has. Therefore, it could be considered that the Ti has
a strong interaction with Pt atoms in the Pt-SA/Ti
To more precisely disclose the atomic dispersion of Pt
throughout the entire Pt-SA/Ti , the wavelet transform (WT) of
Pt L -edge EXAFS oscillations was employed due to its powerful
resolution ability in both k and R spaces. The WT-EXAFS contour
plot of Pt-SA/Ti exhibits only one intensity maximum at ca. 4.95
3 2 x
C T
3 2 x
C T .
3 2 x
C T
3
3
4
3 2 x
C T
–1
Å , corresponding to the Pt–O bonding (Fig. 2c). Moreover, WT-
EXAFS signals corresponding to the Pt–Pt contribution are not
3 2 x
observed in the plot of Pt-SA/Ti C T , but these signals are observed Fig. 3 Electrocatalytic hydrogenation performance. (a) Polarization
in the reference plots. The above results further validated that the Pt curves of Ti C T and Pt-SA/Ti C T in 0.1 M of Na SO fed with CH O
3
2
x
3
2
x
2
4
2
3 2 x
atoms in Pt-SA/Ti C T are atomically dispersed as SAs and that these gas. E and j denote the applied potential and measured current
atoms are bonded to O atoms. A least-squares EXAFS fitting is density, respectively. (b) FEs of CH OH on Ti C T and Pt-SA/Ti C T at
3
3
2
x
3 2 x
employed to determine the quantitative structural parameters of Pt various applied potentials. (c) Comparison of the CH OH yield rates
3
in Pt-SA/Ti
3 2 x
3 2 x 3 2 x
C T (Fig. 2d). The Pt−O coordination number is ~2, and its from Ti C T and Pt-SA/Ti C T under ambient conditions. (d)
bond length is 1.63 Å. The combination of experiments and Chronoamperometry curve of Pt-SA/Ti C T at 0.8 V vs RHE. (e) FEs
3
2 x
simulations suggested that the Pt atoms are dispersed on the and yield rates of CH OH for eight sampling experiments from the
3
3 2 x
3 2 x
surface-adsorbed O of Ti C T (O linked with Ti), as shown in Fig. 2e. catholyte over Pt-SA/Ti C T before and after the long-term
The electrochemical hydrogenation of CH
2
O to CH
3
OH was chronoamperometry experiment in (d).
evaluated under ambient conditions using
a
gas-tight two-
compartment cell separated by a Nafion-117 proton exchange
Next, the electrocatalytic hydrogenation activity of Pt-
SA/Ti C T was systematically examined at different working
1
1, 12, 20
membrane (see the Experimental Section for more details).
The cathode compartment was continuously supplied with CH
at a constant flow rate of 20 mL min throughout electrolysis. The corresponding FEs of Pt-SA/Ti C T . Strikingly, the highest FE of 95.8%
3
2 x
2
O gas potentials. Figure 3b,c shows the average CH OH yields and
3
−1
3
1
2 x
–
–1
as-produced CH
3
OH was quantitatively determined by gas with a yield rate of 30.7 mg h mgcat. are obtained at 0.8 V vs. RHE.
chromatography (Fig. S12). All potentials were converted to the The FEs and corresponding CH OH yield rates gradually decrease
3
reversible hydrogen electrode (RHE) scale. As a result, the reduction with potentials from 0.8 to 1.0 V vs. RHE, which is mainly caused
current density of Pt-SA/Ti C T is greater than that of Ti C T in a by the rapid increase in the hydrogen evolution activity. To the best
3 2 x 3 2 x
wide potential range (Fig. 3a). To rationally compare the catalytic of our knowledge, this observed selectivity at –0.8 V vs. RHE is the
performance, the catalytic activity was normalized to the "current/Pt highest value reported for the electrosynthesis of CH OH under
3
loading mass". As shown in Fig. S13, the polarization curves for Pt- comparable conditions (Table S1). For comparison, the activities of
NP/Ti
voltammetry (LSV) measurements. Notably, Pt-SA/Ti
much better catalytic activity than Pt-NP/Ti catalysts. Besides, and 25%, respectively, corresponding to about one-fifth and one-
we also test the electrocatalytic hydrogenation performance of the fourth of that of Pt-SA/Ti C T (Figs. 3b,c and S15). This comparison
3 2 x 3 2 x
3 2 x 3 2 x
C T and Pt-SA/Ti C T were recorded from linear sweep pristine Ti C T and Pt-NP/Ti C T were also examined at the same
3 2 x
3 2 x 3 2 x
C T displays potentials. The highest FEs for Ti C T and Pt-NP/Ti C T are 17.5%
3 2 x
C T
3
2 x
commercial Pt/C under the same conditions (Fig. S14). Clearly, the again highlights the uniqueness of Pt-SA/Ti C T .
3
2 x
commercial Pt/C provides more hydrogen evolution reaction and
insignificantly electrocatalytic CH O hydrogenation performances. that the introduction of atomically dispersed Pt atoms in Ti C T
These differences suggested that Pt-SA/Ti
Electrochemical impedance spectroscopy analysis confirmed
2
3
2 x
3
C
2
T
x
is considerably more nanosheets induces a low internal resistance and rapid charge-
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J. Name., 2013, 00, 1-3 | 5
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
Page 6 of 8
ARTICLE
Journal Name
transfer kinetics toward electrocatalytic hydrogenation (Fig. S16).35 electrocatalytic hydrogenation of CH
Furthermore, double-layer capacitance (Cdl), which is widely utilized SA/Ti . (b) Charge density difference of the CH
as an indicator of the electrochemical active surface area, was configuration on Pt-SA/Ti . Green and pink regions in (b) denote
is the electron accumulation and depletion in the space, respectively.
(c) PDOS for Pt-SA/Ti Pt-SA/Ti -CH O, and CH
(55.5 mF cm ), implying additional configurations.
accessible active sites exposed on the Pt-SA/Ti catalyst.
The stability of Pt-SA/Ti for the electrocatalytic
hydrogenation of CH O was verified by consecutive recycling and conducted to reveal the electronic structure of Pt-SA/Ti
potentiostatic electrolysis tests at –0.8 V vs. RHE. As shown in Fig. 3d, seen, the electrons from the Pt atom are transferred to adsorbed
no obvious changes in the total current density during the long-term CH O (Fig. 4b), which increases the C–O bond length to 1.32 Å in
electrolysis were observed. At the same time, before and after the comparison with that of free CH O (1.20 Å). Furthermore, as
continuous electrolysis, the FEs and CH OH yield rates remained confirmed by the PDOS results (Fig. 4c), the antibonding orbital of
stable (Fig. 3e), further suggesting the excellent electrochemical the carbon-oxygen double bond of CH O molecule gets electrons
stability of Pt-SA/Ti for ambient CH OH electrosynthesis. from the d orbital of the Pt atom. These comparative results
2
View Article Online
3 3 2 x
O to CH OH on Ti C T and Pt-
DOI: 10.1039/D0TA00190B
3
C
2
T
x
2
O-adsorbed
3 2 x
C T
1
0
3 2 x
estimated (Fig. S17). Consequently, the Cdl value of Pt-SA/Ti C T
–2
considerably greater (246.3 mF cm ) than those of Pt-NP/Ti
57.3 mF cm ) and Ti C T
3 2 x
3
C
2
T
x
3
C
2
T
x
,
3
C
2
T
x
2
2
O
–2
–2
(
3 2 x
C T
3
C
2
T
x
Nest, the charge density difference and PDOS calculations were
. As
2
3 2 x
C T
2
2
3
2
3
C
2
T
x
3
Additional HAADF-STEM characterization demonstrated that the confirmed that atomically dispersed Pt sites can activate CH
atomically dispersed Pt atoms are still preserved without any molecules and promote the subsequent hydrogenation.
agglomeration (Fig. S18), further suggesting the robustness of Pt-
2
O
SA/Ti
selectivity, productivity, and stability for catalyzing the
hydrogenation of CH O.
3 2 x 3 2 x
C T . To sum up, Pt-SA/Ti C T simultaneously exhibits high
Conclusions
2
3 2 x
In summary, Pt SAs anchored on ultrathin Ti C T nanosheets were
To explore the origin of the efficient electrocatalytic
performance by the participation of Pt SAs, DFT calculations were
carried out. First, the complete electrocatalytic hydrogenation
successfully developed for the highly selective electrocatalytic
hydrogenation of CH O under ambient conditions. This catalyst
2
exhibited not only a high FE of 95.8%, with a high reduction current
reaction pathways on Pt (111), Ti
calculated (Fig. 4a and S19). The results revealed that the initial
chemical adsorption step of CH O is the potential-determining step
PDS). Clearly, the free-energy change of the PDS step for Pt-
SA/Ti decreases to only 0.36 eV, which is much less than those
of Pt (111) (1.28 eV) and Ti (1.72 eV). This result indicated that
the presence of Pt SAs can effectively promote the chemisorption of
CH O, leading to an enhanced hydrogenation activity. This result is in
line with our experimental results that the FE and yield rate for the
production of CH OH over Pt-SA/Ti are greater than those
observed for Pt-NP/Ti and Ti (Figs. 3b,c and S13).
3
C
2
T
x
, and Pt-SA/Ti
3
C
2
T
x
were
–2
–1
density of 27.0 mA cm , but also a high yield rate of 30.7 mg h
–1
mgcat. at 0.8 V vs. RHE for the production of CH
3
OH; these values
2
are 4 and 5 times greater than those obtained for Pt NPs, respectively.
Furthermore, it showed excellent stability during electrolysis. DFT
calculations further revealed that the electronic interactions
(
3 2 x
C T
3
C
2
T
x
between Pt SA and the Ti
3 2 x
C T
support can reduce the energy barrier
2
for the chemisorption of CH O while simultaneously promoting the
2
activation of the C–O bond. This study is the first to experimentally
confirm that the SA catalyst can effectively electrocatalyze the
hydrogenation of CH O to CH OH with high productivity, selectivity,
2 3
and stability altogether. Notably, it paves a new route for the
exploration of other SA catalysts for the conversion of volatile
organic compounds to value-added chemicals.
3
3 2 x
C T
3
C
2
T
x
3 2 x
C T
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by National Key R&D
Program of China (2017YFA0700104), National Natural Science
Foundation of China (51971157, 21601136, 51808037 and
5
1761165012), and Tianjin Science Fund for Distinguished
Young Scholars (19JCJQJC61800). This work made use of the
resources of the Wuxi Research Institute of Applied
Technologies of Tsinghua University. The authors wish to thank
facility support at the 1W1B beamline of Beijing Synchrotron
Radiation Facility (BSRF) and Beijing Super Cloud Computing
Center.
Fig. 4 DFT calculations. (a) DFT-calculated free-energy diagram of the
most energetically favorable reaction pathways for the
6
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
3
2 J. Li, Q. Guan, H. Wu, W. Liu, Y. Lin, Z. Sun, X. YeV,ieXw.AZrhticelenOgn,liHne.
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Journal of Materials Chemistry A
Page 8 of 8
Table of Contents
View Article Online
DOI: 10.1039/D0TA00190B
Pt single atoms anchored on ultrathin Ti C T nanosheets can effectively catalyze the electrochemical
3
2 x
hydrogenation of formaldehyde to methanol with a desirable faradaic efficiency of 95.8 % and a record
–1
–1
high yield rate of 30.7 mg h mg
as well as good stability.
cat.