Inorganic Chemistry
Article
means to produce specific active sites.42−49 Nevertheless, Sn-
MOF-templated carbon with a specific exposed Sn facet has
not yet been explored although it has high potential to
selectively produce formate from CO2RR.
Here, by means of density functional theory (DFT)
calculations, we first demonstrate that the Sn(101) crystal
plane is energetically more favorable for CO2RR. Then, we
synthesized a series of Sn/SnO2/C composite catalysts with
exposed Sn(101) facets through the carbonization of Sn-MOF
(Sn3O(1,4-BDC)2) at different temperatures in an argon
atmosphere. The high selectivity of electroreduction CO2
toward formate was obtained and achieved a faradaic efficiency
(FE) up to 93.3% using Sn(101)/SnO2/C-500 (carbonization
of Sn-MOF at 500 °C) as the catalyst, where it is higher than
most of the reported Sn-based catalysts, such as Sn-MOF,
SnO2, and Sn. The powder X-ray diffraction and catalytic
experiments confirmed that the intensity of the Sn(101)
diffraction peak is highly relevant to the electrocatalytic activity
and selectivity, which could be further enhanced with the
optimization of carbonization time and acid etching. These
results revealed the fundamental significance of Sn(101) from
carbonized Sn-MOF for highly selective production of formate.
Figure 1. DFT simulation of the CO2RR process on Sn(101) and
Sn(200) planes. (a) Calculated free-energy diagrams for HCOO−,
CO formation; optimized geometric structure of (b) Sn(101) and (c)
Sn(200).
second H+/e− can be added to *OCHO to form *HCOOH
with no free-energy barrier to conquer. However, the
coordination unsaturated Sn atoms on two planes bind
HCOOH strongly, limiting the desorption process potential.
A 0.42 V potential is needed to drive the HCOO− evolution
for Sn(101), while another 0.15 V is required for the Sn(200)
surface. Therefore, the Sn(101) is a more favorable plane for
CO2RR. The side hydrogen evolution reaction (HER) was also
considered. According to our calculations, the adsorption free
energies of H* on both Sn(101) (0.23 eV) and Sn(200) (0.51
eV) are much more positive than those of *OCHO, indicating
that the side HER can be significantly overwhelmed on the
surfaces of Sn. It was worth noting that Sn was more
thermodynamically stable at highly reduced working poten-
tials.15 The p-band center of Sn(101) was much closer to the
Fermi-level than that of SnO(101),55 which indicated the
higher CO2 reduction activities of Sn(101) (Supporting
3.2. Synthesis and Characterization of Materials.
Inspired by DFT calculations, we successfully synthesized a
catalyst with an enhanced Sn(101) plane through Sn-MOF
carbonization and acidic etching. Sn-MOF was fabricated by a
hydrothermal method and was well characterized (Supporting
carbonized in a flow of ultrapure argon at 500 °C for 120
min.53,54 The material was then washed with 0.01 mol/L
hydrochloric acid in order to etch Sn and expose the Sn(101)
plane. Subsequent characterization showed that the X-ray
diffraction (XRD) pattern of the obtained catalyst is well
matched with the Sn and SnO2 mixture (Figure 2a),19,56,57
while the intensity of the Sn(101) diffraction peak is
significantly high possibly due to the selective corrosion of
the Sn crystal surface by HCl. The BET surface area was 137.6
m2/g (Supporting Information Figure S4). For comparison,
HCl-etched Sn and SnO2 and Sn/SnO2 composites showed no
preference in Sn(101), confirming the significance of Sn-MOF
in the production of Sn(101) crystal facets (Supporting
Information Figure S5). Furthermore, an XRD peak occurred
at 2θ = 26.5o, which matched with the graphitic carbon.34,58
This could also be verified by Raman spectroscopy (Figure
2b); there were clearly two characteristic peaks at 1325 cm−1
(D band) and 1588 cm−1 (G band). The relative intensity ratio
of the D/G band (ID/IG) of Sn(101)/SnO2/C-500 was ∼0.50.
Therefore, the material was denoted as Sn(101)/SnO2/C-500.
The further characterization by scanning electron microscopy
and high resolution transmission electron microscopy
(HRTEM) showed the structural difference between
2. EXPERIMENTAL METHODS
2.1. Synthesis of Sn-MOF. Zinc sulfate (0.5 mmol) (SnSO4), 0.5
mmol 1,4-benzedicarboyxlic acid, and 1 mmol KOH were added to 10
mL of water and ultrasonicated until fully dissolved Afterward, the
mixture was transferred into a Tefion-lined container, which would be
put in a steel autoclave, and the reaction temperature was set at 180
°C for 3 days. Water, DMF, and ethanol were used to wash the
colorless crystal powder, which dried in a vacuum at 60 °C.50−52
2.2. Synthesis of Sn(101)/SnO2/C-400 and Sn(101)/SnO2/C-
500. The ligands of as-prepared Sn-MOF were removed by pyrolysis
in an ultrapure Ar environment at two temperatures (400 and 500
°C) for 2 h with a 5 °C/ min rate for heating. The final obtained
products, which were washed with 0.01 mol/L hydrochloric acid and
ultrapure water in turn, were dried in the vacuum drying chamber.
The samples were denoted as Sn(101)/SnO2/C-400 and Sn(101)/
SnO2/C-500 according to their pyrolysis temperatures.53,54
2.3. Electrochemical Measurement. A typical H-type cell was
selected as the electrochemical reactor. With the assistance of an
electrochemical workstation, the electrocatalytic data of the working
electrode whose surface was coated with the synthesized catalyst
could be collected and analyzed.
3. RESULTS AND DISCUSSION
3.1. DFT Calculation. The origin of formate formation
with high selectivity observed on the Sn surface was first
analyzed by DFT calculations. The simulations were
performed on the basis of Sn (101) and (200) planes since
they were the predominantly exposed crystal planes.
Optimized geometric structures are depicted in Figure 1b,c,
while the corresponding energy profiles along the CO2RR
pathway are detailed in Figure 1a. The CO2RR initiates with a
proton-coupled electron transfer to CO2, leading to the
protonation of C or O atoms. Here, we found that the
protonation of the O atom to the *COOH is highly
endothermic on both (101) and (200) slabs with adsorption
free energies of 0.34 and 0.71 eV, respectively. In contrast, the
CO2 activation to *OCHO with the proton added on C atoms
proceeds spontaneously thermodynamically. Compared to flat
Sn(200) with rather weak *OCHO binding (−0.08 eV), the
fold Sn(101) with more exposed top-layer Sn atoms can
anchor intermediates more strongly (−0.28 eV), which is more
beneficial for the second electron-coupled proton transfer. The
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Inorg. Chem. 2021, 60, 9653−9659