Revealing the Correlation between Catalytic Selectivity and the Local Coordination Environment of Pt Single Atom

Article abstract of DOI:10.1021/acs.nanolett.0c02940

Single atom catalysts (SACs) have recently attracted great attention in heterogeneous catalysis and have been regarded as ideal models for investigating the strong interaction between metal and support. Despite the huge progress over the past decade, the deep understanding on the structure-performance correlation of SACs at a single atom level still remains to be a great challenge. In this study, we demonstrate that the variation in the coordination number of the Pt single atom can significantly promote the propylene selectivity during propyne semihydrogenation (PSH) for the first time. Specifically, the propylene selectivity greatly increases from 65.4% to 94.1% as the coordination number of Pt-O increases from ～3.4 to ～5, whereas the variation in the coordination number of Pt-O slightly influences the turnover frequency values of SACs. We anticipate that the present work may deepen the understanding on the structure-performance of SACs and also promote the fundamental research in single atom catalysis.

Full text of DOI:10.1021/acs.nanolett.0c02940

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Communication
Revealing the correlation between catalytic selectivity
and the local coordination environment of Pt single atom
Yong Xu, Mingyu Chu, Fangfang Liu, Xuchun Wang, Yu Liu, Muhan
Cao, Jin Gong, Jun Luo, Haiping Lin, Youyong Li, and Qiao Zhang
Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.0c02940 • Publication Date (Web): 10 Aug 2020
Downloaded from pubs.acs.org on August 10, 2020
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Revealing the correlation between catalytic
selectivity and the local coordination environment of
Pt single atom
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Yong Xu,1,2,† Mingyu Chu,2,† Fangfang Liu,2,† Xuchun Wang,2,† Yu Liu,2 Muhan Cao,2 Jin Gong,2
Jun Luo,3 Haiping Lin 2,* Youyong Li,2 Qiao Zhang2,*
1Guangzhou Key Laboratory of Low-Dimensional Materials and Energy Storage Devices,
Collaborative Innovation Center of Advanced Energy Materials, School of Materials and Energy,
Guangdong University of Technology, Guangzhou, 510006, Guangdong, China
2Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-
Based Functional Materials and Devices, Soochow University, Jiangsu, 215123, China
3Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science
and Engineering, Tianjin University of Technology, Tianjin 300384, China
Keywords: Local coordination environment; single atom catalyst; catalytic selectivity; Pt;
propyne semi-hydrogenation.
ABSTRACT: Single atom catalysts (SACs) have recently attracted great attention in
heterogeneous catalysis and been regarded as ideal models for investigating the strong interaction
between metal and support. Despite the huge progress over the past decade, the deep understanding
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on the structure-performance correlation of SACs at a single atom level still remains great
challenges. In this study, we demonstrate that the variation in the coordination number of Pt single
atom can significantly promote the propylene selectivity during propyne semi-hydrogenation
(PSH) for the first time. Specifically, the propylene selectivity greatly increases from 65.4% to
94.1% as the coordination number of Pt–O increased from ~3.4 to ~5, whereas the variation in the
coordination number of Pt–O slightly influences the turnover frequency (TOF) values of SACs.
We anticipate that the present work may deepen the understanding on the structure-performance
of SACs and also promote the fundamental researches in single atom catalysis.
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Introduction
Single atom catalysts (SACs) have recently emerged as a new frontier in heterogeneous
catalysis owing to their unique propertie including maximized utility of metals, low-coordination
environment and strong metal-support interactions.1-7 Since the pioneering work from Zhang et al.
in 2011,8 SACs have been widely used to catalyze vairous reactions, including CO oxidation,8-12
CH4 conversion,13,14 CO2 reduction,15-18 hydrogen evolution reaction,19,20 water gas shift
reaction,21,22 hydrogenation,23-26 etc. Many strategies have been developed to synthesize SACs
over the last decade, and the familiy of SACs has been expanded from noble metals (Pt, Pd, Rh,
etc.) to non-noble metals (Co, Fe, Ni, etc.).18,27-34 With the in-depth research, chemists realize the
enhancement of activity and selectivity on SACs may be attributed to the signifcantly different
electronic properties of the isolated metal atoms in SACs. As a result, trenmendous effort has been
devoted to undestanding the correlation between the structure of single atom and catalytic
performace.
It has recently been reported that the activity of SACs is strongly determined by the local
coordination environment (e.g., coordination atom and number) of each isolated atom. For
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example, Co–N4 SAC displayed much higher activity than Co–N4-xCx SAC toward CO2
electrocatalytic reduction (CO2RR) because Co–N4 was found to promote the kinetics of CO2
activation.35 Gong et al. reported that Ni–N2–C2 SAC exhibited higher activity and selectivity than
Ni–N3–C1 and Ni–N4–C1 for CO2RR.36 Fei et al. claimed that Ni–N–C SAC exhibited dramatically
enhanced oxygen evolution reaction activity (OER).37 In another case, Zhang et al. demonstrated
that doping Fe SACs with S (Fe–N4–S2) significantly promoted the activity of oxygen reduction
reactions (ORR).38 On the other hand, the obtained catalytic performance on SACs may obviously
different with the changes in coordination number. Jakub and coworkers claimed that the CO
adsorption on Ir/Fe3O4 SACs was much stronger than that on Ir nanoparticles.39 Similarly, Wang
et al. revealed that Co-SACs exhibited a promising activity towards CO2RR when the Co–N
coordination number was decreased from 4 to 2.40 Ren et al. indicated that the increase in Pt–O
coordination number (from ~2 to ~4) of Pt/FeOx SACs resulted in a significant improvement in
the activity of CO oxidation.41 However, previous reports mainly focus on studying the synergy
between coordination environment and activity, while the the correlation of coordination
environment and SACs’ selectivity has not been systematically investigated yet. The great
challenges in the regulation of coordiantion enviroment in SACs, especially in oxide-supported
SACs, have seriously impeded their application in catalysis.41
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Benefitting from the strong adsorption of reactants, noble metal atoms are generally
recognized as promising sites for many reactions, however, they suffer the disadvantage of poor
selectivity.14,24,41 The precise control on the selectivity of SACs is therefore of tremendous
significance. Given that the geometric isolation of metal atoms in SACs exihibits promising
selectivity for hydrogenation owing to their variable adsorption abilities for reactants,
intermediates and products,14,24,41 we beleive that the control on the coordination number of single
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atom in SACs can correspondingly tailor the selectivity of different products. Herein, we
systematically investigated the correlation between coordination number and selectivity in
propyne semi-hydrogenation (PSH) over Pt/FeOxHy SACs with tailored Pt–O coordination
numbers from ~3.4 to ~5. Specifically, the propylene selectivity significantly increased from
65.4% to 94.1% when the coordination number of Pt–O was increased from ~3.4 to ~5, however,
the variation in the coordination number of Pt–O slightly influenced the turnover frequency (TOF)
values. Density functional theory (DFT) calculations on the reaction paths depicted that Pt single
atom on Fe2O3 tended to selectively catalyze propyne to propylene, while Pt single atom on
FeOOH favored propyne over-hydrogenation (POH), being well consistent with our experimental
observations.
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Results and Discussion
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Figure 1. Structural analysis of different catalysts. (a) SEM image of S-4, (b) AC-STEM image
of S-4. The isolated Pt atoms are labeled with red cycles, (c) high resolution AC-STEM image of
S-4, (d) Fe 2p XPS spectra of FeOOH and Pt/FeOxHy catalysts, (e) O 1s XPS spectra of FeOOH
and Pt/FeOxHy catalysts, (f) CO-DRIFTS spectra of Pt/FeOxHy SACs, 0.5%Pt-FeOxHy and 1%Pt-
FeOxHy. The CO linear and bridge adsorption are highlighted in purple and blue in (f). Note that
the absence of CO bridge adsorption peak in the CO-DRIFTS spectra suggests Pt atoms are
presented as single state in Pt/FeOxHy SACs, while the presence of CO bridge adsorption and linear
adsorption in the CO-DRIFTS spectra of 0.5%Pt-FeOxHy and 1%Pt-FeOxHy suggests Pt atoms
were presented as cluster or nanoparticle.
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FeOOH was prepared via a simple hydrothermal process, and Pt/FeOxHy SACs were prepared
by depositing a certain amount of Pt(NH3)(NO3)2 on FeOOH (see experimental section). Urchin-
like structure with an average size of ~1 μm was successfully prepared (Figure S1a). After
depositing Pt, no obvious change in morphology or size of FeOOH nanostructures was observed
(Figures 1b, 1c, and S1c). More importantly, the urchin-like shape was maintained even when the
thermal treatment temperature was increased to 400 oC (Figure S1d−f). These calcinated samples
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at 160, 200, 300, and 400 C were simplified as S-1, S-2, S-3, and S-4, respectively. X-ray
diffraction (XRD) patterns indicated that FeOOH gradually evolved into Fe2O3 with the increased
treatment temperature. For instance, characteristic peaks of α-FeOOH (PCPDF No.: 81-0463)
appeared in the XRD pattern of S-1, while intense peaks of α-Fe2O3 (PCPDF No.: 79-0007) were
observed in the XRD pattern of S-4 (Figure S2). No peaks corresponding to Pt appeared in the
XRD patterns of Pt/FeOxHy SACs, which might be attributed to the low loading amount and high
dispersity of Pt in SACs. Moreover, aberration-corrected STEM (AC-STEM) measurement was
employed to characterize the isolated state of Pt atoms in Pt/FeOxHy SACs. We took S-4 as an
example. Pt single atom was observed in the AC-STEM image (red dash cycles in Figures 1b and
S3), suggesting that Pt atoms were presented as single state. The lattice distance of 0.267 nm
corresponded to α-Fe2O3 (104) facet (Figure 1c). In contrast, Pt clusters with a mean size of ~1.2
and ~1.9 nm were observed in the AC-STEM image of 0.5%Pt-FeOxHy and 1%Pt-FeOxHy,
respectively (Figures S4 and S5).
X-ray photoelectron spectroscopy (XPS) measurement was performed to study the surface
properties of those samples. As depicted in Figure 1d, the characteristic peaks at 712.3, 725.1, and
719.2 eV in the XPS spectrum of FeOOH were indexed as Fe 2p3/2, Fe 2p1/2, and Fe satellite,
respectively.42,43 The peaks of Fe 2p3/2 and Fe 2p1/2 appeared at 710.7 and 723.8 eV in the XPS
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spectrum of Fe2O3 (Figure 1d).42,43 For Pt/FeOxHy SACs, Fe was a mixture of FeOOH and Fe2O3,
and the molar ratio of Fe2O3 gradually increased from 9.1% to 91.3% with the increased
temperature from 160 to 400 oC (Table S1). Similarly, O 1s XPS spectra indicated that Pt/FeOxHy
consisted of FeOOH and Fe2O3, in which the peaks at 529.5, 530.2, 531.4, and 532.6 eV
corresponded to O−Fe in FeOOH, O−Fe in Fe2O3, O−H in FeOOH, and O−H in H2O, respectively,
(Figure 1e).42 The absence of Pt peaks in XPS spectra might be attributed to the high dispersity
and low content of Pt in Pt/FeOxHy SACs (Table S2). N2 adsorption-desorption measurement
indicated that the dehydration of FeOOH (2FeOOH→Fe2O3+H2O) would result in the formation
of porous structures (Figure S6 and Table S2). For instance, the surface area of FeOOH was 27.32
m2/g, which significantly increased to 181.62 m2/g in S-4. To further study the dehydration of
FeOOH, thermogravimetric analysis (TGA) measurement was performed in both air and argon in
the temperature range of 30−1000 oC. Both the TGA curves obtained in air and argon displayed
similar features, suggesting that the weight loss during heating was caused by dehydration (Figure
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o
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S7). Specifically, the weight loss at the temperature ranges of <150 C and 150−800 C were
ascribed to the removal of adsorbed H2O and FeOOH dehydration, respectively. Considering the
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fact that the highest treatment temperature for Pt/FeOxHy SACs was 400 C, we concluded that
FeOOH was partially dehydrated in Pt/FeOxHy SACs, which further confirmed the results from
XPS measurement.
To further reveal the structures of Pt atoms, the diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) measurement was performed on different samples using CO as the prober
molecule. A broad peak at 2070−2150 cm-1 was observed in the CO-DRIFTS spectra of Pt/FeOxHy
SACs, which was indexed as the linear adsorption of CO on Pt atoms (Figure 1f).44-46 In contrast,
several peaks appeared at 2070−2150, 2020−2100, and 1850−1900 cm-1 in the CO-DRIFTS
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spectra of 0.5%Pt-FeOxHy and 1%Pt-FeOxHy, which were ascribed to CO linear adsorption on
Ptδ+, CO linear adsorption on Pt0, and CO bridge adsorption on Pt, respectively.44-47 Only the
presence of CO linear adsorption in the CO-DRIFTS spectra of Pt/FeOxHy SACs confirmed the
single state of Pt atoms in Pt/FeOxHy SACs, while the co-existence of CO linear and bridge
adsorption in the CO-DRIFTS spectra of 0.5%Pt-FeOxHy and 1%Pt-FeOxHy suggested that Pt
atoms were presented as small nanoparticles (Figure 1f).
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Figure 2. Structural analysis of Pt/FeOxHy SACs. (A) XANES spectra of Pt/FeOxHy SACs, Pt
foil and PtO2 at Pt L3-edge. (B) EXAFS spectra of Pt/FeOxHy SACs, Pt foil and PtO2 at Pt L3-edge.
Wavelet transform of EXAFS spectra at Pt L3-edge, (C) Pt foil, (D) PtO2, (E) S-1, (F) S-2, (G) S-
3, and (H) S-4.
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Pt L3-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray fine
structure (EXAFS) were measured to study the electronic structures and coordination states of
Pt, and Pt foil and Fe2O3 were used as references. As shown in Figure 2, comparing with the
XANES spectrum of Pt foil, Pt in Pt/FeOxHy SACs were presented as their oxidation state.
However, the lower absorption energies and weaker “white line” intensities in comparison to
PtO2 suggested that Pt atoms in Pt/FeOxHy SACs were low-coordinated (Figure 2a). The
appearance of Fe2O3 features in the Fe K-edge XANES spectra of different Pt/FeOxHy SACs
confirmed the dehydration of FeOOH to Fe2O3 during thermal treatment (Figure S8). The
characteristic peaks of Pt−O and Pt−Pt coordination were observed at 1.7 and 2.4 Å, respectively,
in the EXAFS spectra of PtO2 and Pt foil. Only the peak of Pt−O coordination at 1.6−1.7 Å was
observed in the EXAFS spectra of Pt/FeOxHy SACs, which implied that Pt atoms were presented
as isolated state (Figure 2b). Moreover, wavelet transform of EXAFS spectra at Pt L3-edge further
confirmed the isolated state of Pt atoms in Pt/FeOxHy SACs, as evidenced by the disappearance of
Pt−Pt coordination in the profile (Figures 2c−h). We analyzed the coordination environment of
Pt in Pt/FeOxHy SACs by fitting the EXAFS spectra, and the coordination numbers of Pt−O in S-
1, S-2, S-3, and S-4 were ~3.43, ~4.35, ~4.75, and ~5.04, respectively (Table S3). The
aforementioned results revealed that the coordination number of Pt single atom in Pt/FeOxHy
SACs could be systematically tuned during synthetic process. As a comparison, XANES spectra
of 0.5%Pt-FeOxHy and 1%Pt-FeOxHy at Pt L3-edge were obtained. Pt in 0.5%Pt-FeOxHy and 1%Pt-
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FeOxHy exhibited similar features to Pt foil in XANES spectra. However, the “white line”
intensities of 0.5%Pt-FeOxHy and 1%Pt-FeOxHy were higher than that of Pt foil, which indicated
that the surface of Pt nanoparticles was partially oxidized (Figure S9a). EXAFS spectra analysis
further confirmed the partial oxidation nature of Pt nanoparticles (Figure S9b). Consequently,
results from XANES and EXAFS spectra were consistent with those from AC-STEM images and
CO-DRIFTS spectra (Figure 1), which vividly revealed the isolated state of Pt atoms in Pt/FeOxHy
SACs.
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Figure 3. Catalytic performance of PSH on different catalysts. (A) The TOF values, C3H6
selectivity and C3H8 selectivity of different catalysts. (B) The C3H6 STY of different catalysts. The
columns of Pt SACs and Pt nanoparticles are highlighted in blue and purple, respectively, in (A)
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and (B). Reaction conditions: C3H4/H2/N2 = 3/30/2 mL/min, T = 120 oC, catalyst weight: 100 mg.
(C) In-situ DRIFTS spectra of various catalysts in Ar. (D) In-situ DRIFTS spectra and (E) enlarged
spectra in the range of 3005–3020 cm-1 of various catalysts in C3H4 and H2. C3H4/H2 = 1/10. Color
in (C-E): black (S-1), red (S-1), blue (S-3), pink (S-4), green (0.5%Pt-FeOxHy), and purple (1%Pt-
FeOxHy).
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To further investigate the correlation of Pt−O coordination number and catalyst selectivity,
Pt/FeOxHy SACs were used to catalyze PSH in a fix-bed reactor with an inner diameter of 10 mm.48
As shown in Figure 3a, all Pt/FeOxHy SACs exhibited similar activities. When the coordination
number of Pt−O was increased from ~3.43 to ~5.04, the turnover frequency (TOF) value slightly
increased from 9.2×105 to 9.4×105 h-1, whereas, the propylene selectivity significantly increased
from 65.4% to 94.1%. Consequently, the propylene STY increased from 6.0×103 to 8.9×103
molC3H6/molPt/h (Figure 3b). In contrast, the TOF values of 0.5%Pt-FeOxHy and 1%Pt-FeOxHy
were 3×105 and 1.4×105 h-1, respectively, which were much lower than those of Pt/FeOxHy SACs
(Figure 3a). Moreover, experimental results indicated that 0.5%Pt-FeOxHy and 1%Pt-FeOxHy
favored POH), giving the propane selectivities of 89.8 and 94.1%, respectively. Consequently, the
propylene STY of S-4 (8.9×103 molC3H6/molPt/h) was ~28 and ~100 times higher than those of
0.5%Pt-FeOxHy (305.3 molC3H6/molPt/h) and 1%Pt-FeOxHy (83.6 molC3H6/molPt/h), respectively.
These results indicated that Pt/FeOxHy SACs could be used as highly active and selective catalysts
for PSH. In addition, in situ DRIFTS measurement was performed to monitor the surface species
on catalysts. As shown in Figure 3c, when Ar was introduced into the cell (without C3H4 and H2),
intense peaks at 1641, 3151 and 3449 cm-1 were observed in the spectra of S-1, S-2, and S-3, which
were ascribed to the H2O bending vibration, stretching Fe-OH vibration, and H2O stretching
vibration, respectively,49 in FeOOH (Figure 3c). FeOOH was gradually converted into Fe2O3 with
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the increased treatment temperature, leading to the weakening peak intensity in the DRIFTS
spectra (Figure 3c). When C3H4 and H2 were introduced, the characteristic peaks of C3H4 at
2100−2200 and 3275−3390 cm-1 were observed (Figure 3d), which corresponded to the stretching
vibration of C≡C and C−H associated with C≡C, respectively,48 The shoulder peak at ~3015 cm-1
was indexed as the stretching vibration unsaturated C−H associated with C=C, indicating that
propylene was formed (Figures 3d and 3e). More importantly, the intensity of propylene peak
strongly varied in the DRIFTS spectra of different catalysts, suggesting the propylene selectivities
of Pt/FeOxHy SACs were different (Figure 3e). The gradual increase in the propylene peak
intensity at 3015 cm-1 in the DRIFTS spectra implied that the increase in treatment temperature
favored PSH. For 0.5%Pt-FeOxHy and 1%Pt-FeOxHy Catalysts, the weak peak at 3015 cm-1 but
intense peaks at 2900−2985 cm-1 suggested that most of propyne was converted into propane.48
Moreover, the spent catalysts were characterized by XRD and XPS measurement. Compared to
the XRD patterns of the fresh SACs, no obvious changes in the XRD patterns of the spent SACs,
suggesting the excellent stability of catalyst under the indicated conditions (Figure S10).
Additionally, XPS measurement indicated the ratios of FeOOH/Fe2O3 in the spent SACs were
maintained in the spent catalysts comparing with the fresh catalysts (Figure S11 and Table S4).
Specially, the ratios of FeOOH/Fe2O3 in the fresh and spent S-4 were 91.3/8.7 and 91.4/8.6,
respectively, which further confirmed the excellent stability of SACs under the indicated reaction
conditions (Table S4). Based on the above-mentioned results, we concluded that Pt SACs could
significantly promote the selective hydrogenation of propyne to propylene.
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Figure 4. Energy profiles of propyne hydrogenation over Pt single atom on hydrogenated
FeOOH and Fe2O3. (a) Energy profiles of propyne hydrogenation on 3-coordinated Pt single atom
on hydrogenated FeOOH (black) and 7-coordinated Pt single atom on partially hydrogenated
Fe2O3 (red) via C-1 path, (b) Energy profiles of propyne hydrogenation on 3-coordinated Pt single
atom on hydrogenated FeOOH (black) and 7-coordinated Pt single atom on partially hydrogenated
Fe2O3 (red) via C-2 path. (c) Energy profiles of propyne hydrogenation on 4-coordinated Pt single
atom on partially hydrogenated Fe2O3 via C-1 path, (b) Energy profiles of propyne hydrogenation
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on 4-coordinated Pt single atom on partially hydrogenated Fe2O3 via C-2 path. Note that the POH
is highlighted in purple.
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To further reveal the mechanism, DFT calculations were performed on Pt single atom
supported on FeOOH and Fe2O3. DFT results implied that Pt single atom preferred to replace one
surface H atom on FeOOH (110) surface and coordinate with two surface O atoms and one
sublayer O atom, giving a coordination number of 3 (Figure S12). On Fe2O3 (001) surface, Pt
single atom preferred to occupy the hollow sites of Fe2O3 (001) surface surrounded by six surface
oxygen atoms, leading to a variable coordination number of 4 or 7 due to the structural relaxation
(Figure S13). Taking into considerations that the PSH occurred in hydrogen-rich environment, O
atoms on FeOOH and Fe2O3 surfaces should be terminated with H atoms. Therefore, the
hydrogenated Pt/FeOOH and Pt/Fe2O3 surface were selected to mimic the surfaces. In Horiuti-
Polanyi mechanism, the first H atom might be added to either the terminal carbon (C-1 path) or
middle carbon atom (C-2 path) of propyne and propylene molecules, the energy barriers of
different paths during propyne hydrogenation were calculated. Given that the structural relaxation
of Pt single atom on Fe2O3 (001) surface, DFT calculations were performed on hydrogenated
Pt/FeOOH (110) surface, 7-coordinated and 4-coordinated Pt single atom on partially
hydrogenated Fe2O3 (001) surface, as depicted in Figure 4. The atomic structures of initial state,
transition state and final state were given in Figures S14−19. For C-1 path, the energy barrier for
the first step (*CH3C≡CH+*H→*CH3CCH2) was 1.35 eV on Pt/Fe2O3 surface with a Pt−O
coordination number of 7 (red profile), which was higher than that of C-2 path (1.09 eV, red profile
in Figure 4b), suggesting that propyne hydrogenation to propylene preferred C-2 path on Pt/Fe2O3
surface. In contrast, propyne hydrogenation to propylene preferred C-1 path on Pt/FeOOH surface,
because the energy barrier of C-1 path (1.64 eV, black profile in Figure 4a) was lower than that of
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C-2 path (1.92 eV, black profile in Figure 4b). DFT calculations revealed that Pt/FeOOH favored
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POH comparing with Pt/Fe2O3. Typically, the energy barriers of C3H7 formation on Pt/FeOOH
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via C-1 and C-2 path were 1.28 eV (black profile in Figure 4a) and 1.49 eV (black profile in Figure
4b), respectively, indicating that POH competitively occurred via C-1 and C-2 path on Pt/FeOOH.
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*
Conversely, the energy barrier for the formation of C3H7 (*CH3CHCH2+*H→*CH3CHCH3) on
Pt/Fe2O3 via C-1 path was 1.58 eV (red profile in Figure 4A), which was higher than that via C-2
path (1.00 eV, red profile in Figure 4b), suggesting that C-2 path was more preferential on Pt/Fe2O3.
Interestingly, the energy barrier of C3H7 dehydrogenation (*C3H7→*CH3CHCH2+*H) was only
*
0.22 eV (highlighted by blue dash rectangle), which was much lower than that of *C3H7 formation
from propylene (*CH3CHCH2+*H→*CH3CHCH3), indicating that the formation of C3H7 was
*
thermodynamically suppressed on Pt/Fe2O3. In other words, *C3H7 tended to dehydrogenize once
it was formed on Pt/Fe2O3. Moreover, DFT calculations were performed on Fe2O3 (001) surface
with a 4-coordianted Pt single atom. As shown in Figure 4c−d, similar phenomenon with 7-
*
coordianted Pt single atom on Fe2O3 (001) surface were observed. The energy barrier of C3H7
dehydrogenation (*C3H7→*CH3CHCH2+*H) was only 0.09 eV (highlighted by blue dash rectangle
in Figure 4d), which was much lower than that of *C3H7 formation from propylene (0.88 eV, Figure
4d). Note that Pt/FeOOH exhibited a much stronger propylene adsorption than Pt/Fe2O3 (2.11 vs.
*
*
1.33 eV), suggesting that C3H7 tended to dehydrogenize into C3H6 once it was formed on
Pt/Fe2O3. Based on the aforementioned results from DFT calculations, we concluded that
Pt/FeOxHy SACs with a higher Pt−O coordination number could selectively catalyze PSH to form
propylene, while Pt/FeOOH with a lower Pt−O coordination number favored POH to form propane.
Conclusions
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In conclusion, we synthesized a series of urchin-like FeOxHy nanostructures supported Pt
SACs with tunable Pt−O coordination numbers via a simple thermal treatment. The removal of H
and O atoms from FeOOH in the form of H2O during thermal treatment resulted in a consecutive
tunability of Pt−O coordination numbers from ~3.43 to ~5.04. The structures of Pt SACs were
detailed revealed by AC-STEM, CO-DRIFTS, XANES, and EXAFS measurements. The increase
in the Pt−O coordination numbers led to a significant increase in propylene selectivity but a
negligible influence in activity during PSH. DFT calculations indicated that Pt SACs with high
Pt−O coordination number favored PSH, while Pt SACs with low Pt−O coordination number
favored POH. This study may deepen the understanding on coordination environment-catalytic
performance correlation and also promote the fundamental researches in single atom catalysis.
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ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on the ACS
Publications website site.
Experimental section, Figures S1−S19, Tables S1−S4 (PDF).
Corresponding Author
*hplin@suda.edu.cn, qiaozhang@suda.edu.cn
Author Contributions
†These authors contributed equally to this work.
Notes
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The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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This work is supported by National Natural Science Foundation of China (51922073, 21673150,
and 51802206) and Natural Science Foundation of Jiangsu Province (BK20180097,
BK20180846). We acknowledge the financial support from the 111 Project, Collaborative
Innovation Center of Suzhou Nano Science and Technology (NANO-CIC) and the Priority
Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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