Synergistic Pd Single Atoms, Clusters, and Oxygen Vacancies on TiO2 for Photocatalytic Hydrogen Evolution Coupled with Selective Organic Oxidation

Article abstract of DOI:10.1002/smll.202006255

Developing efficient photocatalysts for synchronously producing H₂ and high value-added chemicals holds great promise to enhance solar energy conversion. Herein, a facile strategy of simultaneously engineering Pd cocatalyst and oxygen vacancies (V_Os) on TiO₂ to promote H₂ production coupled with selective oxidation of benzylamine is demonstrated. The optimized Pd_SA+C/TiO₂-V_O photocatalyst containing Pd single atoms (SAs), clusters (C), and V_Os exhibits much superior performance to those of TiO₂-V_O and Pd_SA/TiO₂-V_O counterparts. The production rates of H₂ and N-benzylidenebenzylamine over Pd_SA+C/TiO₂-V_O are 52.7 and 1.5?times those over TiO₂-V_O, respectively. Both experimental and theoretical studies have elucidated the synergistic effect of Pd SAs, clusters, and V_Os on TiO₂ in boosting the photocatalytic reaction. The presence of Pd SAs facilitates the generation and stabilization of abundant V_Os by the formation of Pd-O-Ti³⁺ atomic interface, while Pd clusters promote the photogenerated charge separation and afford the optimum active sites for H₂ evolution. Surface V_Os of TiO₂ guarantee the efficient adsorption and dissociation/activation of reactant molecules. This study reveals the effect of active-site engineering on the photocatalysis and is expected to shed substantial light on future structure design and modulation of semiconductor photocatalysts.

Full text of DOI:10.1002/smll.202006255

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Synergistic Pd Single Atoms, Clusters, and Oxygen
Vacancies on TiO for Photocatalytic Hydrogen Evolution
2
Coupled with Selective Organic Oxidation
Ting Wang, Xueqin Tao, Xiaoli Li, Kun Zhang, Shoujie Liu,* and Benxia Li*
in pure water is quite low due to the easy
Developing efficient photocatalysts for synchronously producing H and
2
recombination of photoexcited electron–
hole pairs, high water-oxidation over-
potential, and sluggish water-oxidation
high value-added chemicals holds great promise to enhance solar energy
conversion. Herein, a facile strategy of simultaneously engineering Pd
cocatalyst and oxygen vacancies (V s) on TiO to promote H production
[
3,4]
kinetics.
Various hole scavengers are
O
2
2
usually employed as sacrificial regents to
coupled with selective oxidation of benzylamine is demonstrated. The
optimized PdSA+C/TiO -V photocatalyst containing Pd single atoms (SAs),
improve the photocatalytic H -evolution
2
[5–7]
2
O
efficiency,
which increases the produc-
clusters (C), and V s exhibits much superior performance to those of
tion cost and generate more waste, against
the energy-efficient and environmental
friendly demands. In contrast, the integra-
tion of photocatalytic water splitting into
O
TiO -V and Pd /TiO -V counterparts. The production rates of H and
2
O
SA
2
O
2
N-benzylidenebenzylamine over PdSA+C/TiO -V are 52.7 and 1.5 times
2
O
those over TiO -V , respectively. Both experimental and theoretical
2
O
H with the selective oxidation of organics
2
studies have elucidated the synergistic effect of Pd SAs, clusters, and
V s on TiO in boosting the photocatalytic reaction. The presence of Pd
represents a more efficient solar energy
conversion system that can achieve the
simultaneous productions of solar fuel
and value-added organic chemicals, where
the products are naturally separated in
two phases of gas and liquid, avoiding the
O
2
SAs facilitates the generation and stabilization of abundant V s by the
O
3
+
formation of PdOTi atomic interface, while Pd clusters promote the
photogenerated charge separation and afford the optimum active sites
for H evolution. Surface V s of TiO guarantee the efficient adsorption
2
O
2
additional procedure and cost for separa-
and dissociation/activation of reactant molecules. This study reveals the
effect of active-site engineering on the photocatalysis and is expected
to shed substantial light on future structure design and modulation of
semiconductor photocatalysts.
[8–12]
tion.
However, compared with those
for the photocatalytic H2 evolution with
sacrificial regents, the available high-
efficiency photocatalysts for synchronous
productions of H and high value-added
2
chemicals as well as understanding their
structure-performance relationships are still in short supply,
requiring more efforts to explore.
1. Introduction
Solar-driven water splitting into H via semiconductor photo-
As a relatively cheap platinum group metal, palladium (Pd)
2
catalysis has been considered as a promising and sustain-
able strategy to address the global energy and environmental
cocatalyst is attracting much interest in photocatalytic H evolu-
tion because it can boost charge transfer/separation and pro-
2
issues.[ However, the efficiency of photocatalytic H evolution
1,2]
vide active sites for H evolution owing to their suitable Fermi
2
2
level for trapping the photogenerated electrons and low poten-
[
13,14]
tial for proton reduction.
Although previous studies have
T. Wang, X. Tao, X. Li, Dr. K. Zhang, Prof. B. Li
Department of Chemistry
identified the activity of Pd cocatalyst in photocatalytic reac-
tions, its utilization efficiency is still unsatisfactory in light of
its intrinsic activity, which is restricted by the poor dispersion,
limited active sites and high loading content. In recent years,
researchers are devoted to reducing the size of Pd nanoparticles
College of Science
Zhejiang Sci-Tech University
Hangzhou 310018, P. R. China
E-mail: libx@mail.ustc.edu.cn
[15]
Dr. S. Liu
(
NPs) to single-atom levels for acquiring high activity at low
College of Chemistry and Materials Science
The Key Laboratory of Functional Molecular Solids
Ministry of Education
[
16,17]
Pd loading.
However, the isolated metal atoms which are
stabilized via electron transfer from metal to support usually
present electron-deficient characters and may be unfavorable
Anhui Normal University
Wuhu 241002, P. R. China
[18]
for the proton reduction. In most of the previous reports,
E-mail: jiesliu@ahnu.edu.cn
the cocatalytic activities of metal single atoms (SAs) have been
usually compared with those of the NPs, rarely considering the
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202006255.
[
6,19]
presence of subnanometric clusters.
As a transition state
DOI: 10.1002/smll.202006255
between NPs and SAs, Pd clusters present different geometric
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and electronic structures from those of the other two entities,
optimal coexistence of Pd SAs, clusters, and V s, the PdSA+C/
O
[20]
and thus an exceptional activity is expected. Therefore, tai-
loring Pd cocatalyst at atomic precision as well as revealing the
sensitivity of hydrogen evolution to Pd dispersion is of great
TiO -V photocatalyst prepared by thermal reduction of 30 min
2
O
exhibits the superior performance for H production coupled
2
with selective oxidation of benzylamine. The presence of Pd
significance for boosting photocatalytic H production.
SAs facilitates the generation and stabilization of V s in adja-
2
O
In addition, the controllable construction of oxygen vacan-
cent TiO due to the electron transfer from Pd atoms to TiO ,
2
2
cies (V s) on metal oxide surfaces has been proved to be an
where the V s act as active sites for the adsorption and activa-
O
O
effective strategy to modulate the band structure and surface
atomic arrangement of photocatalysts, affording abundant
tion of reactant molecules. Pd clusters promote the acceptance
of electrons and protons for rapid H evolution. The interme-
2
active sites for promoting reactant (e.g. H O and organic mole-
diates of the photocatalytic reaction are identified by in situ
spectroscopy measurements, providing the direct evidence to
understand the photocatalytic mechanism. The active site engi-
neering strategy by synchronously tailoring metal SAs, clusters,
2
cules) adsorption and lowering the activation energy barriers
for challenging chemical transformations.[
21–23]
However, the
surface V s may gradually vanish during photocatalytic process,
O
resulting in a declined activity; moreover, the excessive subsur-
and V s on oxide semiconductor provides a new choice for the
O
face V s would block the charge separation and transfer to pho-
tocatalyst surfaces due to the destroyed crystalline structures.
precise synthesis of high-efficiency photocatalysts.
O
[24]
It has been verified that the introduction of metal heteroatoms
could induce the generation of V s and maintain their sustaina-
O
2
. Results and Discussion
bility in photocatalytic process, providing a means of addressing
the above problems.[
25,26]
Therefore, synchronously engineering
The synthesis of PdSA+C/TiO -V photocatalyst with a very low
2 O
metal and VO sites on oxide semiconductors is a promising
strategy to improve the photocatalytic performance for hydrogen
evolution coupled with selective oxidation of organics.
Pd content (0.39 wt% determined by ICP-OES) is accomplished
by the procedure illustrated in Figure 1a, consisting of impreg-
nation adsorption, pyrolysis in air and thermal reduction in
H /Ar atmosphere. MIL-125-NH contains abundant dangling
In this work, the Pd-SA modified TiO (denoted as Pd /
2
SA
2
2
2+
TiO -V ) photocatalyst is synthesized by a facile pyrolysis of
−NH groups which can efficiently stabilized Pd cations via
2
O
2
2
+
2+
the Pd -incorporated MIL-125-NH (denoted as Pd /MIL-
the strong coordination and prevent the formation of PdPd
2
2+
1
25-NH ). The dispersion states of Pd species are subsequently
bonds during the pyrolysis of Pd /MIL-125-NH precursor
2
2
modulated by thermal reduction of Pd /TiO -V in H /Ar
mixed gas at 200 °C for a controlled time. Benefiting from the
at high temperature. Meanwhile, carbon metabolism can
create abundant V s on the MIL-125-NH -derived TiO , which
SA
2
O
2
O
2
2
Figure 1. a) Schematic diagram for the preparations of PdSA/TiO -V and PdSA+C/TiO -V photocatalysts. HRTEM image, AC-HAADF-STEM image,
2
O
2
O
HAADF-STEM image and corresponding EDS element mappings of b–d) PdSA/TiO -V and e–g) PdSA+C/TiO -V .
2
O
2
O
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facilitates to capture Pd atoms and generate the Pd /TiO -V
the partially oxidized Pd species in the Pd /TiO -V and
SA
2
O
SA
2
O
material.[ The optimal photocatalyst of PdSA+C/TiO -V is
27]
PdSA+C/TiO -V photocatalysts. In comparison, Pd /TiO -V
2
O
2 O SA 2 O
obtained by controlling the thermal reduction of Pd /TiO -V
presents the higher oxidation state of Pd than that in
PdSA+C/TiO -V . The XANES spectrum of Pd /TiO -V is very
SA
2
O
sample for 30 min. X-ray diffraction patterns (Figure S1, Sup-
porting Information) of Pd /TiO -V and PdSA+C/TiO -V are
2
O
SA
2
O
close to the reference spectrum of PdO, suggesting that the
oxidation state of Pd in Pd /TiO -V sample approximates to
SA
2
O
2
O
assigned to the anatase phase of TiO (JCPDS No. 71–1166),
2
SA
2
O
without any diffraction peak of Pd species. Scanning electron
microscopy (Figure S2a,d, Supporting Information) demon-
+2. Whereas, the absorption of PdSA+C/TiO -V shifts obviously
2 O
toward lower energy relative to the PdO reference, meaning
the increased electron density of Pd in Pd /TiO -V
O
strates that both Pd /TiO -V and PdSA+C/TiO -VO samples
SA
2
O
2
SA+C
2
[
32]
consist of the well-dispersed hexahedrons with edge lengths
sample.
The coordination configurations of Pd atoms in
of ≈300 nm and thickness of ≈150 nm, which are well inher-
Pd /TiO -V and PdSA+C/TiO -V are further identified by
SA
2
O
2
O
ited from the MIL-125-NH precursor except the rough surface
Fourier-transformed extended X-ray absorption fine structure
(FT-EXAFS) (Figure 2b). The Pd /TiO -V sample shows only
2
[28]
and some volume shrinkage. Transmission electron micros-
copy (TEM) images (Figure S2b,c,e,f, Supporting Information)
SA
2
O
one peak at 1.53 Å associated with PdO shell of PdO refer-
ence, without PdPd coordination, confirming the formation
of isolated Pd SAs with PdO coordination in Pd /TiO -V .
show that these hexahedrons are constructed by TiO nanocrys-
2
tals with sizes of ≈8.0 nm. The mesoporosity originated from
SA
2
O
granular stack is confirmed by N adsorption-desorption meas-
Whereas, the Pd
/TiO -V sample presents two peaks
SA+C 2 O
2
urements (Figure S3, Supporting Information). The PdSA+C
/
located at 1.53 and 2.43 Å, corresponding to the PdO shell
in PdO reference and the PdPd shell in Pd foil, respectively.
This result confirms the coexistence of isolated Pd atoms and
metallic Pd° clusters in PdSA+C/TiO -V photocatalyst. The
TiO -V sample exhibits larger surface area and pore volume
2
O
than those of Pd /TiO -V and TiO samples (Table S1, Sup-
SA
2
O
2
porting Information), which probably result from the more
2
O
surface oxygen defects of PdSA+C/TiO -VO sample.[
26,29]
In the
high-resolution TEM images of Pd /TiO -V (Figure 1b) and
structure parameters derived from the least square fitting
(Figure S4, Supporting Information) on Pd K-edge EXAFS data
provide the detailed coordination configurations of Pd species
(Table S2, Supporting Information). Two coordination modes of
PdO shell with average bond length of 2.00 ± ±0.01 Å and PdPd
shell with average bond length of 2.72 ± ±0.01 Å coexisting in the
PdSA+C/TiO -V sample can be assigned to the isolated Pd atoms
2
SA
2
O
PdSA+C/TiO -V (Figure 1e), the lattice fringes with spacing of
2
O
0
.351 nm belong to the (101) lattice plane of crystalline TiO2
with anatase phase. The highly dispersed Pd species in both
the samples are identified by aberration-corrected high angle
annular dark field scanning TEM (AC-HAADF-STEM) images
2
O
[
33]
(Figure 1c,f), where the brighter dots (marked by the red cir-
and the metallic Pd° clusters, respectively. Compared with
Pd foil, the PdPd coordination number in Pd
cles) correspond to Pd atoms which have the larger atomic
number than Ti atoms. The individually isolated brighter dots
in Figure 1c indicate the presence of highly dispersed Pd SAs
in Pd /TiO -V sample. In contrast, it is observed that Pd
/TiO -V
SA+C
2
O
sample is greatly decreased to 1.8 ± ±0.3, due to the formation
of subnanometric Pd clusters. Wavelet transform extended
X-ray absorption fine structure (WT-EXAFS) analysis is further
performed to probe the Pd species. As shown in Figure 2c,
Pd /TiO -V sample presents a single intensity maximum at
SA
2
O
clusters less than 1 nm coexist with the isolated Pd SAs in
PdSA+C/TiO -V sample (Figure 1f). The STEM-coupled energy-
2
O
SA
2
O
−
1
dispersive spectroscopy (EDS) element mappings further demon-
strate the presence of Ti, O, and Pd elements in both PdSA/
TiO -V (Figure 1d) and PdSA+C/TiO -VO (Figure 1g) as well as
around 5.8 Å which can be assigned to the PdO contribution
of the first nearest-neighbor coordination shell, indicating that
Pd SAs are anchored on TiO by PdO bonding. In contrast,
2
O
2
2
the uniform distribution of Pd element throughout the hexa-
hedron-shaped structures. The above results demonstrate that
the atomically dispersed Pd sites have been in situ formed in
the WT-EXAFS for Pd
/TiO -V sample shows two intensity
SA+C 2 O
−
1
maxima near 5.8 and 8.6 Å , which can be attributed to the
PdO and PdPd scatterings, respectively, by reference to the
results from Pd foil and PdO. Combined with the FT-EXAFS
and WT-EXAFS analyses, we can confirm the coexistence of Pd
SAs and clusters in the PdSA+C/TiO -V photocatalyst. The Pd
2+
porous TiO2 during the pyrolysis of Pd /MIL-125-NH pre-
2
cursor. As the TiO supported Pd SAs are exposed to H /Ar
2
2
atmosphere at 200 °C, the interaction between Pd atoms and
2
O
TiO support will be weakened, resulting in the formation of Pd
sites in Pd /TiO -V and PdSA+C/TiO -V are further probed
2
SA 2 O 2 O
clusters due to the partial aggregation of Pd atoms.
The electronic structures and local coordination environment
by CO-absorbed diffuse reflectance infrared Fourier transform
spectroscopy (CO-DRIFTS). As shown in Figure 2d, only one
−
1
of Pd atoms in Pd /TiO -V and Pd
/TiO -V photocatalysts
narrow and quasi-symmetrical peak at2163 cm appears in
the DRIFT spectrum of Pd /TiO -V , which is associated with
SA
2
O
SA+C
2
O
are corroborated by X-ray absorption fine structure (XAFS)
analysis. Figure 2a shows their normalized Pd K-edge X-ray
absorption near edge structure (XANES) spectra, with com-
parison to the reference spectra of Pd foil and PdO. The white-
line intensity, a strong adsorption associated with the transition
of Pd 1s core-electron to unoccupied 5p states can reflect the
density of unoccupied states in the Pd 4d-5p hybrid orbitals. Pd
SA
2
O
the stretching frequency of linear-adsorbed CO on the isolated
[
34]
Pd-SA sites. Whereas, the PdSA+C/TiO -V sample exhibits
2
O
−
1
two distinct peaks centered at 2163 and 2114 cm , respectively.
The broad peak at 2114 cm can be ascribed to the linear-
adsorbed CO on Pd clusters. The above results demonstrate
that the thermal reduction of Pd /TiO -V in H /Ar atmos-
−
1
[
35]
SA
2
O
2
species with higher oxidation states corresponds to the stronger
phere results in the coexistence of Pd SAs and clusters in the
PdSA+C/TiO -V photocatalyst. Since both the introduction of
white-line adsorption.[
30,31]
In Figure 2a, the white-line intensi-
2
O
ties of Pd /TiO -V and PdSA+C/TiO -VO are located between
Pd heteroatoms and the thermal H reduction can induce to
SA
2
O
2
2
those of the reference samples (Pd foil and PdO), indicating
generate abundant V s on TiO , electron spin resonance (ESR)
O
2
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Figure 2. a) Normalized Pd K-edge XANES spectra, b) FT-EXAFS spectra, and c) WT EXAFS of PdSA/TiO -V and PdSA+C/TiO -V in reference to Pd
2
O
2
O
foil and bulk PdO. d) CO-DRIFTS spectra and e) ESR spectra of PdSA/TiO -V and PdSA+C/TiO -V .
2
O
2
O
spectra of Pd /TiO -V and PdSA+C/TiO -VO exhibit a distinct
is consistent with the results from DRS studies (Figure S6a–c,
Supporting Information). For the coexistence of Pd SA (Pd1),
Pd cluster (simplified as Pd ) and V , the CB tail state is fur-
SA
2
O
2
[
36]
signal at g = 2.01 related with V s of TiO (Figure 2e). The
O
2
enhanced signal intensity confirms the higher VO concentra-
tion in PdSA+C/TiO -V than that in Pd /TiO -V . Upon being
6
O
ther enhanced and the density of states at valence band maxima
(VBM) also increases, accounting for the higher visible-light
photoabsorption of PdSA+C/TiO -V photocatalyst. According
2
O
SA
2
O
treated in H /Ar atmosphere, the isolated Pd atoms in Pd /
2
SA
TiO -V can efficiently dissociate H and allow it to spill over
2
O
2
2
O
onto TiO support, leading to the formation of more V s on
to the Mott−Schottky (M–S) plots (Figure 3b), all of the photo-
catalysts show positive slopes, indicating the n-type character of
2
O
[37]
TiO in the resulted Pd
/TiO -V sample.
2
SA+C
2 O
The photoabsorption properties of pure TiO , Pd /TiO -
their electronic band structures. The smallest slope of PdSA+C
/
2
SA
2
V , and Pd
/TiO -VO samples are determined by UV–vis-
TiO -V reflects that the coexistence of Pd SAs, clusters and
O
SA+C
2
2
O
ible diffuse reflectance spectroscopy (UV–vis DRS). The three
samples present different colors and photoabsorption fea-
tures (Figure 3a). Pd /TiO -V (brown) and PdSA+C/TiO -V
V s is beneficial for increasing the concentration of free car-
O
[38]
riers.
The flat-band potential of TiO in Pd
/TiO -V is
SA+C 2 O
2
estimated to be −0.14 V relative to the reversible hydrogen elec-
trode potential (vs RHE, pH = 7). Therefore, its CBM level is
approximately equal to −0.14 V (vs RHE, pH = 7) and its VBM is
SA
2
O
2
O
(grey brown) show the enhanced absorption in visible region
in comparison with pure TiO (pale yellow). DFT calculation
2
(Figure S5, Supporting Information) indicates that the introduc-
calculated to be 2.66 V according to the band gap (E = 2.80 eV),
g
tion of V creates a band tailing near the conduction band min-
which agrees well with the XPS-measured result (Figure S6d,
Supporting Information). These results suggest that the
O
imum (CBM) of TiO and results in a reduced band gap, which
2
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Figure 3. a) UV–vis DRS, b) M–S plots at frequency of 1.0 kHz, c) PL spectra and d) transient photocurrent with on/off cycles of different samples.
•+
Insets in (a) are the photographs of samples. Inset in (b) shows the band structure of PdSA+C/TiO -V and the redox potentials of BA/BA and ∙OH/
2
O
3
H O. The differential charge density distribution of e) Pd /TiO -Vo and f) Pd +Pd /TiO -V at isosurfaces of 0.003 e/Å .
2
1
2
1
6
2
O
photogenerated electrons have a low potential enough for proton
reduction, and the holes are capable of oxidizing benzylamine
the deaerated conditions and constant reaction temperature of
8 °C. The performance of different photocatalysts are evaluated
•+
into carbon-centered radical (BA ) and H O into ·OH, respec-
by a typical reaction system consisting of 0.5 mL H O, 0.5 mL
2
2
tively.[
12,39]
Moreover, the presence of Pd species remarkably
benzylamine, and 50 mL N,N-dimethylformamide (DMF), as
shown in Figure 4b,c and Table S3, Supporting Information.
reduces the radiative recombination of photogenerated electrons
and holes, as demonstrated by the photoluminescence spectros-
copy (PL, Figure 3c) and the photocurrent response (Figure 3d).
The PdSA+C/TiO -V photocatalyst shows the highest efficiency
The H production increases linearly with the irradiation time
2
(Figure 4b), and the selectivity of producing N-benzyliden-
ebenzylamine is higher than 99%. Among them, the PdSA+C
/
2
O
of photoinduced charge carrier separation, probably because
TiO -V photocatalyst exhibits the highest generation rates
2
O
−
1
−1
the formation of a M–S junction between Pd cluster and TiO
of H2 (585.4 µmol·g ·h ) and N-benzylidenebenzylamine
2
cat
greatly facilitates the charge separation.[
38]
The differential
(257.3 µmol·gcat ·h ). The pure TiO before (denoted as TiO )
−1 −1
2
2
charge density distributions of the simulated Pd /TiO -V and
or after (denoted as TiO -V ) H /Ar thermal reduction per-
1
2
O
2 O 2
Pd +Pd /TiO -V models (Figure 3e,f) provide further informa-
forms poorly in the photocatalytic reaction. The PdNPs/TiO -V
2 O
1
6
2
O
tion about the electron transfer between Pd and TiO support.
photocatalyst, prepared by loading Pd NPs on the pyrolysis-
synthesized TiO2 via a wet reduction method, also exhibits
an inferior performance. The results demonstrate that the in
2
Notably, electrons are transferred from Pd single atom (Pd ) to
1
TiO2 at the PdOTi atomic interface. However, an inverse
electron transfer from TiO to Pd cluster (Pd ) is observed in
situ anchored Pd species in TiO plays a pivotal role in photo-
2
6
2
Pd +Pd /TiO -V due to the enhanced electron capture ability of
catalytic H2 evolution along with selective oxidation of ben-
zylamine, which should be attributed to the highly dispersed
Pd sites and the abundant PdOTi atomic interface. More-
over, it is noticed that the in situ anchored Pd species promote
1
6
2
O
Pd cluster, originating from the increase of the electron affinity
and the downshift of d-band center as the Pd atom number
increases.[ The electron transfer from Pd SAs to TiO con-
18]
2
tributes to the stabilization of V and Ti3 defects through the
+
more efficiently H evolution than N-benzylidenebenzylamine
O
2
3+
[6]
PdOTi atomic interface. Meanwhile, the inverse electron
production (Figure 4c), suggesting that the Pd species have
transfer from TiO to Pd cluster promotes the photogenerated
served as active sites for proton reduction and H evolution.
2
2
charge separation and facilitates to capture electrons on Pd
cluster for the proton reduction. Therefore, the coexistence of
The influence of the thermal H /Ar reduction time (x min)
2
on photocatalytic activity is studied for optimizing the perfor-
mance (Figure S7 and Table S4, Supporting Information). All
the PdSA+C/TiO -V photocatalysts obtained from the H /Ar
Pd SAs, clusters, and V s in Pd
/TiO -VO photocatalyst not
O
SA+C
2
only enhances the photoabsorption and photogenerated charge
separation but also affords multiple active sites for molecular
adsorption/dissociation and proton reduction.
2
O
2
reduction for different times (x = 10, 30, and 60 min) exhibit
the improved activities toward both H evolution and selective
2
The unique surface structure and electronic properties of
PdSA+C/TiO -V make it an attractive photocatalyst to boost
oxidation of benzylamine, compared with the pristine PdSA/
TiO -V photocatalyst. The photocatalytic activity increases
2
O
2
O
hydrogen evolution coupled with selective oxidation of ben-
zylamine (Figure 4a), which is performed on an automatic
on-line trace gas analysis system (Labsolar-6A, Beijing Per-
fectlight) equipped with a specialized quartz reactor to provide
along with the prolonging of reduction time from 10 to 30 min
but falls as the reduction time is further extended to 60 min.
The appropriate presence of subnanometric Pd clusters in the
PdSA+C/TiO -V photocatalyst can afford active sites for more
2
O
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Figure 4. Photocatalytic H production coupled with selective oxidation of benzylamine. a) Schematic diagram of the photocatalytic reaction, b) H2
2
evolution over different photocatalysts, c) production rates of H (red) and N-benzylidenebenzylamine (blue) over different photocatalysts, d) H evolu-
2
2
tion with different dosage of H O and benzylamine over Pd
/TiO -V photocatalyst, e) production rates of H (red) and N-benzylidenebenzylamine
2
SA+C
2
O
2
(
blue) with different dosage of H O and benzylamine over Pd
/TiO -V photocatalyst.
SA+C
2
2
O
efficient H generation. However, the formation of larger Pd
high stability of PdSA+C/TiO -V catalyst for the photocatalytic
2
2
O
clusters reduces the number of Pd active sites, leading to the
depressed photocatalytic activity. As a result, the optimal coex-
reaction. Compared with the reported photocatalysts for the
H evolution coupled with selective oxidation of benzylamine
2
istence of Pd SAs, subnanometric clusters, and V s in the
(Table S6, Supporting Information), the PdSA+C/TiO -V photo-
O
2
O
PdSA+C/TiO -V photocatalyst endows it with the enhanced
photocatalytic performance. The apparent quantum efficiency
catalyst exhibits a higher activity for H production than that of
the Pt/PCN-777 photocatalyst. In addition, the PdSA+C/TiO -V
2 O
2
O
2
[
8]
of the PdSA+C/TiO -VO photocatalyst is determined to be 3.94%
photocatalyst also exhibits a remarkably high H production
2
2
−
1
−1
toward the H production coupled with selective oxidation of
rate of 18196.6 µmol·g ·h from methanol aqueous solution
(20 vol%), as shown in Figure S9 and Table S7, Supporting
Information.
2
benzylamine under the irradiation of 365 nm and the opti-
mized reaction conditions (Supporting Information). Moreover,
the PdSA+C/TiO -V photocatalyst can maintain its high activity
In the present photocatalytic reaction system, the dosages
2
O
in a long-term photocatalytic reaction (Figure S8a and Table S5,
Supporting Information). The loading mass of Pd in the
PdSA+C/TiO -V photocatalyst after durability test is determined
of H O and benzylamine have a notable influence on the pro-
2
duction rates of H and N-benzylidenebenzylamine, due to the
2
competitive adsorption of H O and benzylamine on the surface
2
O
2
to be 0.37 wt% by ICP-OES measurement, which is approxi-
mately equal to that (0.39 wt%) in the original PdSA+C/TiO -V
of photocatalyst. All the H evolutions (Figure 4d) over PdSA+C/
2
TiO -V photocatalyst with different dosages of H O and ben-
2
O
2
O
2
photocatalyst. Besides, the TEM-associated EDS element map-
zylamine present the linear relationship with the irradiation
pings of the PdSA+C/TiO -VO photocatalyst after durability test
time, whereas the corresponding production rates of H and
2
2
are shown in Figure S8b, Supporting Information, where the
element distributions are well consistent with those of the orig-
inal PdSA+C/TiO -V photocatalyst. These results confirm the
N-benzylidenebenzylamine (Figure 4e; and Table S8, Supporting
Information) are quite different. With only 0.5 mL H O in
2
−
1
−1
DMF solvent, the H production rate is 338.11 µmol·g ·h ,
2
O
2
cat
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much lower than that obtained in the coexistence of H O and
the measurements on electrochemical hydrogen evolution
reaction (Figure 5c) demonstrate the more positive onset
potential of PdSA+C/TiO -V than that of PdSA/TiO -V and
2
benzylamine. Compared with the system of 0.5 mL H O and
2
0
.5 mL benzylamine in DMF, increasing the dosage of ben-
2
O
2
O
zylamine to 2 mL without changing the dosage of H O has
pure TiO , confirming the superior activity of Pd
/TiO -V
SA+C 2 O
2
2
little impact on the production rates of both H and N-ben-
for hydrogen evolution.
2
zylidenebenzylamine. Conversely, increasing the dosage of
To further understand the mechanism of the photocatalytic
H O to 2 mL without changing that of benzylamine can pro-
2
H evolution coupled with selective oxidation of benzylamine,
2
mote H generation but remarkably suppress the oxidation of
the adsorptions of H O and benzylamine on different active
2
2
benzylamine into N-benzylidenebenzylamine. The H genera-
sites of the photocatalysts are further studied by DFT calcula-
tions. The optimized configurations and adsorption energies
(E ) are displayed in Figure S11, Supporting Information, and
ad
2
tion rate is further enhanced to 1173.6 µmol·gcat− ·h when the
1
−1
dosages of H O and benzylamine are simultaneously increased
2
to 2 mL, whereas the production of N-benzylidenebenzylamine
Figure 5d, where V -1 and V -2 denote the V sites of Pd1/
O
O
O
decreases a little to 221.25 µmol·gcat− ·h . As the photocatalytic
1
−1
TiO -V and Pd +Pd /TiO -V , respectively. It is noted that as
2
O
1
6
2
O
selective oxidation of benzylamine is performed in the absence
one molecule of H O or benzylamine is put on the Pd site of
2
1
of H O (0.5 mL benzylamine in DMF), the N-benzylideneben-
either Pd /TiO -V or Pd +Pd /TiO -V , the molecule sponta-
1 2 O 1 6 2 O
2
−1
−1
zylamine production rate is 324.77 µmol·gcat ·h , accompa-
neously moves onto the VO site after structural optimization,
meaning that the adsorptions of H O and benzylamine on V
−
1
−1
nied by a very low H production rate of 115.32 µmol·g ·h .
2
cat
2
O
A series of controlled experiments indicate that the presence of
site are more stable than those on Pd site. By contrast, the
1
benzylamine can obviously promote H generation from water
Pd +Pd /TiO -V model presents the stronger interactions
2
1
6
2
O
splitting, while the excessive H O suppresses the oxidation of
with H O and benzylamine molecules than Pd /TiO -V ,
2
2
1
2
O
benzylamine to N-benzylidenebenzylamine possibly because
accounting for the superior activity of PdSA+C/TiO -V in the
2 O
the excessive H O molecules tend to occupy the active sites for
photocatalytic reaction. Specifically, the adsorption of H O
2
2
the adsorption and activation of benzylamine.
on the VO site (V -2; Figure S11b, Supporting Information)
O
To reveal the active sites for H generation, the Gibbs free
of Pd +Pd /TiO -Vo involves the formation of hydrogen bond
2
1
6
2
energy of hydrogen adsorption (ΔGH*) is calculated on the
basis of the different adsorption configurations of Pd +Pd /
with the O atom neighboring Pd cluster, resulting in the most
negative adsorption energy of −1.44 eV and the elongated
1
6
TiO -V (Figure 5a,b; Figure S10, Supporting Information),
OH bond in H O molecule from 0.97 to 1.01 Å, which is
2
O
2
where the ideal ΔG value should be close to 0 eV for rapid
greatly beneficial to the water splitting and the subsequent
H*
H2 evolution.[ In comparison, the Pd -1 site neighboring
40]
H evolution on the optimal Pd site (Figure 5b). Meanwhile,
2
6
TiO (Figure 5b) shows the optimal ΔG value (−0.28 eV).
the V -2 site of Pd +Pd /TiO -V also shows the strongest
2
H*
O
1
6
2
O
The interfacial interaction between Pd cluster and TiO lowers
interaction with the N atom of benzylamine (Ead = −2.42 eV;
2
the d-orbital electron density of Pd -1 site to some extent,
Figure S11e, Supporting Information), which facilitates the
6
endowing the Pd -1 site with a more suitable d-orbital electron
activation and selective oxidation of benzylamine on the hole
6
+
density, which is higher than that of Pd site (Figure S10b, Sup-
(h )-accumulated TiO . The calculations confirm the com-
1
2
porting Information) for a more efficient proton reduction and
petitive adsorption between H O and benzylamine on the
2
lower than that of Pd -2 site (Figure S10c, Supporting Infor-
PdSA+C/TiO -V photocatalyst, as demonstrated by the above
experimental results.
6
2
O
[
41]
mation) for a weaker hydrogen adsorption.
Additionally,
Figure 5. a) Free energy profiles for H adsorption on different sites of Pd +Pd /TiO -V model: O site near V , Pd site, Pd -1 site at the Pd -TiO
2
1
6
2
O
O
1
6
6
interface, Pd -2 site on the top of Pd cluster. b) Optimal H adsorption on Pd -1 site. c) The liner scan voltammetry curves of several catalyst-based
6
6
6
electrodes in N -saturated 1 ꢀ KOH solution. d) Adsorption energies of H O and benzylamine on different sites of Pd /TiO -V and Pd +Pd /TiO -V .
2
2
1
2
O
1
6
2
O
e, f) ESR signals of DMPO-∙OH (♠) and DMPO-carbon-centered radicals (⚫) from the photocatalytic system without or with benzylamine.
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Subsequently, the radical intermediates generated in the
photocatalytic reaction system with PdSA+C/TiO -V cata-
irradiation, these peak intensities gradually decrease with the
extension of irradiation time, suggesting that the photocatalytic
reaction takes place. Furthermore, the band around 1658 cm
2
O
[45]
−1
lyst are identified by in situ ESR detections in the presence of
,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin-trapping rea-
5
increases with the prolonged irradiation time, due to the for-
gent. For comparison, the two cases of photocatalytic systems
without (PdSA+C/TiO -V catalyst and traces of H O in DMF sol-
mation of CN in the PhHCNH intermediate or the final
[46]
product of N-benzylidenebenzylamine.
2
O
2
vent) and with benzylamine (PdSA+C/TiO -VO catalyst, traces of
H O and benzylamine in DMF solvent) are detected, respectively,
2
All the above results and discussions have depicted the photo-
catalytic mechanism (Figure 6b, Figure S12, Supporting Infor-
2
as shown in Figure 5e,f. The radical signal cannot be observed in
dark for either case. When exposed to UV–visible irradiation, the
strong ESR signal of DMPO-∙OH radical (♠) featured with 1:2:2:1
mation) of H generation coupled with the oxidative coupling of
benzylamine, which is boosted by the optimized coexistence of Pd
2
SAs, clusters, and V s in the Pd
/TiO -V photocatalyst. Both
O
SA+C
2 O
[42]
quartet peaks are acquired in both the photocatalytic systems,
suggesting that the hydroxyl radical (·OH) has been generated
H O and benzylamine molecules are preferably adsorbed and
2
activated on the V sites of Pd
/TiO -V photocatalyst. The dis-
2 O
O
SA+C
+
−
+
−
via the h -driven oxidation of OH from H O dissociation on
sociation of H O affords protons (H ) and hydroxyl species (OH
2
2
−
+
PdSA+C/TiO -V photocatalyst. In addition, the carbon-centered
or OH). Under irradiation, electrons (e ) and holes (h ) gener-
ated from the photo-excitation of TiO are separated efficiently by
2
O
α-amine radical is produced and trapped by DMPO to exhibit
2
the ESR signals with 1:1:1:1:1:1 sextet peaks (⚫) in the photocat-
the M–S junction at Pd-cluster/TiO interface, where the electrons
2
[43]
[47]
alytic system with the coexistence of H O and benzylamine,
are enriched on Pd clusters and the holes are reserved in TiO2.
2
confirming that both ·OH and carbon-centered α-amine radi-
Therefore, the proton reduction and H generation are achieved
on Pd clusters. Meanwhile, the benzylamine molecules adsorbed
on the VO sites are preliminarily oxidized into benzylamine rad-
2
cals have been generated during the photocatalytic H evolution
2
coupled with selective oxidation of benzylamine. The intermedi-
ates of this photocatalytic reaction are further probed by in situ
ical cations by the holes in TiO . The benzylamine radical cations
2
−
DRIFTS measurements (Figure 6a). After exposing PdSA+C
/
react with OH via a proton transfer, producing a nitrogen-
TiO -V photocatalyst to the mixed vapor of benzylamine and
centered radical and a H O molecule, where the nitrogen-centered
2
O
2
−1
H O, the bands between 3750 and 3350 cm can be attributed
radical would transform into a more stable carbon-centered rad-
2
[48]
to the stretching vibrations of hydroxyl (OH) and amidogen
ical via the intramolecular hydrogen transfer. In addition, the
[
44]
+
−
(
NH) groups, respectively. The bands for CH symmetric
h -driven oxidation of OH can produce hydroxyl radical (·OH)
which further dehydrogenates the carbon-centered radicals to
generate the primary imine intermediate (Ph−CHNH), along
−1
stretching vibrations of methylene (2953, 2922, and 2850 cm )
and the NH2 rocking (1266 cm ) are also observed. Upon
−1
Figure 6. a) In situ DRIFTS measurement on the co-adsorption of benzylamine and H O vapors on Pd
/TiO -V photocatalyst at varied irradiation
2 O
2
SA+C
times under Ar atmosphere. b) Schematic diagram of the mechanism for photocatalytic H evolution coupled with selective oxidation of benzylamine
2
over PdSA+C/TiO -V photocatalyst.
2
O
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with the release of another H O molecule. Finally, the N-ben- Conflict of Interest
2
zylidenebenzylamine is produced by the condensation reac-
The authors declare no conflict of interest.
tion between Ph−CHNH and benzylamine, accompanied by
[12]
releasing a NH molecule. The presence of excessive H O will
3
2
occupy the V sites for benzylamine adsorption, suppressing the
O
conversion of benzylamine to N-benzylidenebenzylamine. The Keywords
appropriate coexistence of Pd SAs, clusters, and V s in PdSA+C/
O
hydrogen evolution, Pd active sites, photocatalysis, selective oxidation,
surface engineering
TiO -V photocatalyst not only enhances the generation and sepa-
2
O
ration of photo-generated charge carriers, but also affords the
more efficient active sites for molecular adsorption/dissociation
Received: October 8, 2020
Revised: November 29, 2020
Published online:
and proton reduction. Therefore, the PdSA+C/TiO -VO photocata-
2
lyst exhibits superior performance for H evolution coupled with
2
selective oxidation of benzylamine.
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