A Pd/Monolayer Titanate Nanosheet with Surface Synergetic Effects for Precise Synthesis of Cyclohexanones

Article abstract of DOI:10.1021/acscatal.7b03463

A catalyst composed of monolayer nonstoichiometric titanate nanosheets (denoted as TN) and Pd clusters is constructed for precise synthesis of cyclohexanone from phenol hydrogenation with high conversion (>99%) and selectivity (>99%) in aqueous media under light irradiation. Experimental and DFT calculation results reveal that the surface exposed acid and basic sites on TN could interact with phenol molecules in a nonplanar fashion via a hexahydroxy hydrogen-bonding ring to form a surface coordination species. This greatly facilitates the adsorption and activation of phenol molecules and suppresses the further hydrogenation of cyclohexanone. Moreover, the surface Pd clusters serve as the active sites for the adsorption and dissociation of hydrogen molecules to provide active H atoms. The synergistic effect of the surface coordination species, TN and Pd clusters remarkably facilitate the high yield of cyclohexanone in photocatalysis. Finally, the possible thermo/photocatalytic mechanisms on Pd/TN are proposed. This work not only highlights the great potential for monolayer nonstoichiometric composition nanosheets in the construction of catalysts for precise organic synthesis but also provides insight into the inherent catalytic behavior at a molecular level.

Full text of DOI:10.1021/acscatal.7b03463

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Article
A Pd/monolayer titanate nanosheet with surface
synergetic effects for precise synthesis of cyclohexanones
Yujie Song, Hao Wang, Xiaomei Gao, feng yingxin, Shijing
Liang, Jinhong Bi, Sen Lin, Xianzhi Fu, and Ling Wu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03463 • Publication Date (Web): 08 Nov 2017
Downloaded from http://pubs.acs.org on November 8, 2017
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ABSTRACT
A catalyst composed of monolayer nonstoichiometric titanate nanosheets (denoted as TN) and
Pd clusters is constructed for precise synthesis of cyclohexanone from phenol hydrogenation
with high conversion (>99%) and selectivity (>99%) in aqueous media under light irradiation.
Experimental and DFT calculation results reveal that the surface exposed acid and basic sites on
TN could interact with phenol molecules in a non-planar fashion via a hexahydroxy hydrogen-
bonding ring to form a surface coordination species. This greatly facilitates the adsorption and
activation of phenol molecules and suppresses the further hydrogenation of cyclohexanone.
Moreover, the surface Pd clusters serve as the active sites for the adsorption and dissociation of
hydrogen molecules to provide active H atoms. The synergistic effect of the surface coordination
species, TN and Pd clusters remarkably facilitate the high yield of cyclohexanone in
photocatalysis. Finally, the possible thermo/photocatalytic mechanisms on Pd/TN are proposed.
This work highlights not only the great potential for monolayer nonstoichiometric composition
nanosheets in the construction of catalysts for precise organic synthesis but also provides insight
into inherent catalytic behavior at a molecule level.
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KEYWORDS: monolayer titanate nanosheet, hydrogenation of phenol, precision synthesis,
green photocatalysis.
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. Introduction
As a key chemical intermediate, cyclohexanone can be used as precursor for the production
of caprolactam for nylon-6 and adipic acid for nylon-66.1-4 Traditionally, cyclohexanone is
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produced on a commercial scale either by the oxidation of cyclohexane or hydrogenation of
phenol. The oxidation of cyclohexane is far from satisfactory since harsh reaction conditions.5
This reaction also suffered from low product yields. An attractive route for the production of
cyclohexanone is selective hydrogenation of phenol in liquid phase by one-step since the
catalytic system offers atom-efficiency and cost/energy savings.6, 7 It was found that phenol
hydrogenation could be catalyzed efficiently by Pd catalysts. Numerous Pd catalytic systems
have been developed such as Pd/C, Pd/MgO, Pd/Al
2
O , Pd/mesoporous silica, Pt/H-ZSM-5, and
3
Pd/MOFs (such as MIL-101 and MIL-53).8-11However, these catalytic systems need high
temperature, high H
2
pressure and toxic regents. Han et al. introduced Lewis acid AlCl into the
3
Pd/C catalysis system to achieve the excellent conversion of phenol hydrogenation. 12a The
additives such as AlCl may restrict the application of these hosts in hydrogenation reaction and
3
against the product purity and reaction conditions. So far, despite a lot of works on phenol
hydrogenation, these works mainly focused on the thermocatalysis and the search for suitable
Lewis acid additives. More importantly, the heterogeneous catalytic mechanism of phenol
hydrogenation is still unclarified at the molecular level. Therefore, developing a versatile green
and new approach for selective hydrogenation of phenol is desirable. Additionally, how to
deeply upstanding the catalytic process are still a huge and promising challenge.
The surface properties of supports are one of the most important factors in selective
hydrogenation of phenol. More Lewis acid sites on the catalysts could enhance the chemisorption
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of phenol and promote the conversion rate of phenol hydrogenation, while the selectivity of
cyclohexanone may be suppressed.13-15 More basic sites would improve the selectivity towards
cyclohexanone, but the transformation efficiency of phenol may be restricted.16-18 Therefore, it is
essential to construct a versatile catalyst with the appropriate acid-basic sites for the precise
synthesis of cyclohexanone in high yield under a mild and eco-friendly condition.
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Recently, two dimensional (2D) oxide nanosheets have attracted great attention in many
fields.19-22 In our previous work, niobate nanosheets with abundant surface acid sites showed
higher photocatalytic activities for benzyl alcohols than their layered counterparts because of
their ultrathin structure with high external surface area and an extremely high percentage of
surficial atoms.23-28 Similarly, the Lepidocrocite titanates nanosheets (denoted as TN):
H1.07Ti1.73
O
4
•H O, has an orthorhombic structure consisting of two species oxygen atoms: the 2-
2
coordinated oxygen (μ2-O, denoted as O1) and 4-coordinated oxygen (μ4-O, denoted as O2) 29-31
The abundant Ti defects may lead to unsaturated O2 atoms that serve as the weak Lewis base
4
+
+
sites. The O1 atoms shared by two nearby Ti provide strong Lewis base sites. The H bounded
4
+
with an O1 atom acts as a strong Brønsted acid sites. Furthermore, the exposed Ti ions act as
Lewis acid sites. Therefore, the TN with abundant surface acid and base sites may be an ideal
material to construct a catalyst for the precise synthesis of cyclohexanone.
Herein, on the basis of the characterizations for TN and the adsorption behaviors of phenol
molecules, we designed a catalyst for the precise synthesis of cyclohexanone. It is composed of
Pd clusters (~3 nm) and monolayer H1.07Ti1.73
O
4
•H O nanosheets (denoted as Pd/TN). The
2
catalyst exhibits high phenol conversion (>99%) and high cyclohexanone selectivity (>99%)
under ambient temperature and hydrogen atmosphere using water as solvent. It is much better
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than the layered counterparts. The surface properties and interaction with phenol molecules on
the catalyst were investigated by in situ molecular spectroscopic techniques and theoretical DFT
calculations. The photo-promoted surface catalytic reaction process was discussed based on the
experimental results. Finally, a probable photo-promoted surface synergetic catalytic mechanism
has been revealed at a molecular level.
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. Results and discussion
The TN sample was prepared by a cation-exchange/exfoliation process similar to a previous
report using layered K0.8Ti1.73Li0.67
O
4
as the precursor (Fig. 1). 29 The XRD patterns confirm that
•H O (LT) and TN with pure phase have
been prepared (Fig. S1). After a proton-exchange process, the characteristic peak (020) shifted to
a lower 2θ angle suggesting a distensible interlayer distance of 0.72 nm for K0.8Ti1.73Li0.67 to
.89 nm for LT. This swelling process is beneficial to the intercalation-exfoliation process for the
the layered K0.8Ti1.73Li0.67 , the layered H1.07Ti1.73
O
4
O
4
2
O
4
0
formation of TN. After exfoliation, the disappearance of the characteristic peak (020) of TN
indicates the loss of the periodic layered structure of LT.26
Fig.1. Exfoliation process of layered H1.07Ti1.73
O •H
4 2
O into H1.07Ti1.73
O
4
•H
2
O nanosheets.
SEM images show that the K0.8Ti1.73Li0.67 and LT have a typical layered structure stacked
O
4
layer-by-layer (Fig. 2a, b); the as-prepared TN sample shows a loose and lamellar structure (Fig.
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c). The TEM images of TN further confirm the morphologies of the tissue-like ultrathin 2D
nanosheets (Fig. 3). These results agree well with the XRD results. The selected-area electron
diffraction (SAED) pattern shows the single crystalline nature of the ultrathin layer TN. The
clear lattice fringes demonstrate that these nanosheets retain their high crystallinity. The AFM
image shows that the thickness of the TN is about 1.1 nm, suggesting that the as-prepared TN
sample is only one monolayer thick (1 nm) (Fig. S2). 32 This confirmed that the monolayer TN
sample was successfully prepared. The photo-absorption performances of the samples have been
analyzed by UV-Vis DRS (Fig. S3). It is obvious that the photoabsorption performance of TN is
much higher than that of LT. This may be due to the large photoabsorption area and the surface
electron structure of the 2D nanosheets.
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Fig.2. SEM images for layered K0.8Ti1.73Li0.67
4
O (a), LT (b), and TN (c).
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Fig.3.TEM (a), SAED pattern (b), HRTEM (c) images of TN
The Pd/TN samples were synthesized by a photo-reduction process. The Pd/TN shows a
very similar XRD pattern with the parent TN, indicating that the loading of Pd clusters has a
negligible influence on the crystal structure of TN (Fig. 4). A characteristic peak at 40.0°
assigned to the (111) crystal planes of Pd clusters can be clearly observed indicating the
formation of Pd clusters on the TN. Certainly, the prepared Pd/TN powders were easily
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aggregated after collected and dried at 60 C (Fig. S4a). Because of the electrostatic repulsion of
the polyanion layers and the sustaining of Pd clusters, these re-stacked nanosheets have the large
distance between the two layers. Thus, the obtained Pd/TN powder is easy to re-disperse into the
monolayer nanosheet in water (Fig. S4b). As shown in Fig. 4b, the re-stacked Pd/TN has
dispersed into ultrathin nanosheets. The Pd clusters with the mean sizes of 3 nm are well-
dispersed on the surface of the ultrathin nanosheets. HRTEM shows that the lattice fringe of
0.224 nm matches the interplanar spacing of the (111) planes of metallic Pd.33 The loading
concentration of Pd in Pd/TN was determined to be 2 mol % by ICP-OES.
Fig.4. (a) XRD patterns of TN and Pd/TN; (b) TEM and HRTEM of Pd/TN; (c) Size distribution
of Pd clusters on TN.
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Fig.5. Schematic structure of H1.07Ti1.73
O
4
•H O nanosheets, Atoms are color labeled: Ti (blue),
2
H (white), and O1 (red) and O2 (green).
The surface properties of TN are further studied in detail. From the crystal structure of TN,
we expect strong superficial acidity and basicity on TN (Fig. 5). These superficial properties are
investigated by in situ FTIR spectra of pyridine adsorption. Fig. 6 shows that pyridine is
-1
adsorbed on TN. The strong peaks located at 1485 and 1540 cm are attributed to pyridine on the
-1
strong Brønsted acid sites. Furthermore, a strong and sharp peak at 1450 cm is observed, which
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is assigned to pyridine on the Lewis acid. These peaks remain even after subsequent evacuation
indicating the chemisorption of pyridine and the strong acidity in the surface of TN. However,
there is no noticeable peak over the LT sample. It should be noted that the LT and TN have the
same amount of the acidic sites per unit mass. The preparation method of TN, an intercalation-
exfoliation process from the parent LT, making the sites exposed. Because of the very small
distance between two adjacent layers in LT, the adsorption of pyridine molecules on the
interlayer acidic sites would be restricted. Thus, the in situ pyridine adsorbed FTIR showed that
the amount of acidic sites exposed on the surface of LT is much lower than that on TN. 35
Therefore, the amount of Brønsted acid and Lewis acid sites exposed on the surface of LT is
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much lower than that on TN, and the surface acidity of LT is very weak. These results also
confirm that the acid sites in the titanate sample and they are greatly enhanced when it is
prepared into a 2D nanosheet.
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Fig.6. In situ FTIR spectra of the TN and LT for pyridine-adsorbed. (a) After degassing at 150 ℃
for 4 h. (b) Adsorption for 30 min at RT (physisorption+chemisorption). (c) Further evacuation
of excess probe molecules at 150 ℃ for 5 min (chemisorption).
Next, we performed the solid-state 1H NMR spectroscopy to gain more insights into the
properties of hydrogen species on TN. Fig. 7 shows the 1H NMR for TN before and after
dehydration. Before the dehydration, three peaks are observed at the chemical shift values of
+
about 10.3, 6.3 and 1.3 ppm. These are assigned to the strong acidic OH groups, H
3
O species
and the isolated Ti-OH respectively. After dehydration, the peaks at 10.2, 7.4, 4.6 and 1.3 ppm
are assigned to the strong acidic OH groups, Ti (OH) Ti, Ti-OH with hydrogen-bonds and Ti-OH
groups without hydrogen bonds, respectively. The Ti (OH) Ti of bridging hydroxyl groups may
function as the strong Brønsted acid sites.36-37
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Fig.7. H magic angle spinning NMR spectra for TN before (a) and after dehydration (b).
The surface basicity of TN and LT were determined by CO
Fig. 8. The TN sample clearly adsorbed abundant CO molecules via the chemisorption, but
there is no distinct adsorption of CO on LT. These results demonstrate that the superficial
2
-TPD experiments. As shown in
2
2
basicity of TN is much higher than that of LT, which may be resulted by the unique structural
features of TN. The 2D configuration provides an extremely high percentage of superficial
exposed unsaturated atoms. The CO
2
molecules could not adsorb on the interlayer of the LT
because of the steric inhibition of the adjacent layers. This would results in the low concentration
2
-1
of basic sites. Moreover, the BET specific surface area of TN is about 176.2 m g , which is
2
-1
about 21.5 times larger than that of LT (8.2 m g ) (Fig. S5). Because that the interlayered
confinement effect in LT has been overcome efficiently for TN. Thus, the measured surface area
of TN is much higher than that of LT. Furthermore, the steric inhibition effect also limits the
utilization of the active sites on the interlayer of LT, although the concentration of the active
sites of LT may be similar with that of TN. The high amount of the exposed active sites could
absorb more phenol molecules to enhance the mass transfer. The photocatalytic activity would
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also be increased. Additionally, the CO
2
-TPD curve of TN shows two obvious desorption peaks
at about 153 and 280°C. The former peak may be ascribed to desorption of CO
weak basic sites of TN. The latter one may be ascribed to desorption of CO
strong basic sites of TN. It is worth noting that the CO
2
adsorbed on the
adsorbed on the
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desorption peak area of the strong basic
sites is much higher than the weak basic sites. That is, the concentration of the strong basic sites
is much higher than that of the weak basic sites. When combined with the crystal structure of TN,
it is possible that the weak basic site may be the O2 atoms shared by for Ti atoms and the strong
basic site may be the surface unsaturated O1 atoms of TN.
Fig.8 Temperature-programmed desorption profiles of TN and LT.
It has been generally accepted that the surface properties have tremendous effects on the
catalytic properties of the nanomaterials. Thus, to understand the effect of the distinctive surface
structure of TN on the catalytic performance, the photocatalytic hydrogenation of phenol to
cyclohexanone was carried out over the samples under the simulated solar light (750 nm ≥ λ ≥
3
00 nm) irradiation at 298 K with hydrogen atmosphere using water as the solvent (Table 1). The
thermostat bath was used to control the given temperature (Fig. S6). An almost quantitative
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transformation of phenol (>99%) with high yield (>99%) to cyclohexanone is achieved over the
Pd/TN under irradiating for 10 h. No byproducts such as cyclohexanol and cyclohexane were
detected, indicating that the precise synthesis of cyclohexanone can be realized over the Pd/TN.
It is worth mentioning that the experiment can be scaled up without an obvious decrease in
performance. Efficient conversion of phenol (>92%) is observed with high selectivity to
cyclohexanone (>99%) over Pd/TN under simulated solar light irradiation for 10 h when the
concentration of phenol is scaled up by a factor of 10. Furthermore, the catalytic activities are
negligible in the absence of Pd/TN or Pd clusters. In addition, when the reactions are carried out
in the dark, only 21% of the phenol is catalyzed to cyclohexanone at 298 K. When the reaction
temperature increased to 333 K, the phenol conversion can increase to 51% in the dark. These
results reveal that the hydrogenation of phenol is driven by Pd clusters with the assistance of TN,
and the catalytic activities of Pd/TN can be greatly enhanced under the simulated solar light
irradiations. Thus, the Pd/TN composite would make full use of the thermal and light energy to
more efficiently drive the phenol hydrogenation reaction at room temperature. The reusability
and stability of the catalyst are also studied (Fig. S7 and S8). The recycling of the catalyst for
four runs shows no obvious decrease of catalytic activity. After the reaction, the separation of the
catalyst from the heterocatalytic system is very facile, and the catalyst can be reused directly.
The XRD patterns and XPS spectra of Pd/TN are almost unchanged after the photocatalytic
reactions demonstrating the high stability of the Pd/TN photocatalyst (Fig. S8).
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To shade light on the role of TN, a set of controlled experiments were performed (Table
S1). Because TN could not be excited by visible light (Fig. S3), a greatly decreased
photocatalytic activity is obtained over the Pd/TN under the visible light irradiation (750
nm≥λ≥420 nm, 25% Conv.), suggesting that the photoexcited TN can efficiently facilitate the
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transformation of phenol. Compared with Pd/TN, it is obvious that the Pd/LT sample exhibits a
much lower conversion rate with or without light irradiation (Table 1, entry 6-7). Moreover,
when the common photocatalyst anatase TiO
2
used as a support (Table S1, entries 2, 3),
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Pd/anatase TiO shows the much lower photocatalytic activities (<2% Conv. in the dark and 5%
2
Conv. under the visible light irradiation) than those of Pd/TN (21% Conv. and 25% Conv.,
respectively, Table S1, entry 1). In addition, negligible cyclohexanone is observed, when the
Pd/Al
S1, entries 4-6). For a better comparison of the TN and Al
carried out the TEM images of the Pd/Al sample for the characterization of Pd clusters (Fig.
S9). It can be observed that the average size of the Pd clusters on the Al is 3 nm, similar with
that on the TN. Moreover, the XPS spectra for Pd/Al were also characterized to examine the
surface chemical state of Pd clusters. As shown in Fig. S10a, the 334.9 and 340.2 eV are
2
O
3
as a catalyst at 1 atm H
2
under the room temperature or visible light irradiation (Table
2
O
3
-supported catalyst, we have
2
O
3
2
O
3
2
O
3
0
1, 8-9
assigned for the Pd 3d5/2 and Pd 3d3/2 of Pd , respectively.
Pd clusters are in a metallic form on the Al . The difference in activities between Pd/TN and
Pd/Al could be attributed to the distinctive properties of the supports. Therefore, it is
The spectra of Pd 3d suggest that
2
O
3
2
O
3
reasonable to deduce that the phenol molecules can directly interact with the TN for the further
activation and transformation. The enhanced catalytic activity may be attributed to the distinctive
surface structure and unique electronic properties of 2D titanate nanosheets. 34
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Table 1. Hydrogenation of phenol molecules over the samples with and without stimulated
irradiation.
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Entry
Catalyst
Pd/TN
Pd/TN
hν
+
Temp. (K)
298
Conv. (%)
＞99
C=O (%)
C-O (%)
1
2
＞99
＜1
_
298
21
＞99
99
＜1
1
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6
Pd/TN
-
_
+
+
+
333
298
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298
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TN
0
0
0
Pd/LT
15
＞99
＜1
7
Pd/LT
-
298
10
＞99
＜1
Reaction conditions: simulated sunlight (750 nm≥λ≥300 nm), phenol (0.1 mmol), catalyst
(15 mg, 2mol% Pd,), water (2 mL), H (1 atm) unless otherwise noted.
2
The interaction between TN and the phenol molecules was revealed by in situ FTIR
spectroscopy (Fig. 9A). It is obvious that phenol molecules could chemisorb on the surface of
-1
TN even with the evacuation at 150°C. Three peaks at about 1620, 1495 and 1250 cm was
3
8
observed. These peaks can be assigned to the σ(C=C) and σ(C-O) respectively. However, the
cyclohexanone molecules can only physisorb on the surface of TN (Fig. 9B), resulting in the
easy released products from the catalyst. Moreover, the adsorption sites were further revealed by
theoretical density functional theory (DFT) calculations. As shown in Fig. 10 and 11, the results
suggest that the phenol molecules anchor on the surface of TN by the strong Ar-OH•••O2 and
Ar-O•••HO2 interactions via a hydrogen-bonding ring (Fig. 10) forming the surface coordination
species.36 The adsorption energy of phenol and cyclohexanone molecules on TN in a stable
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adsorption configuration is estimated to be 1.25 eV. However, the adsorption energy of
cyclohexanone is only 0.91 eV. These results agree with that of in situ FTIR. The strong
chemisorption of phenol molecules on the TN with a surface coordination species could facilitate
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the activation and transformation of phenol molecules. Additionally, the easy desorption of
cyclohexanone in the surface active sites could efficiently inhibit the over hydrogenation of
cyclohexanone. Thus, the hydrogenation of phenol would be catalyzed at a high conversion rate
with the high selectivity that we observed.
Fig.9. In situ FTIR spectra of the monolayer TN for phenol (a) and cyclohexanone adsorbed (b).
(a) After degassing at 150 ℃ for 4 h. (b) Adsorption for 30 min at RT
(physisorption+chemisorption). (c) Further evacuation of excess probe molecules at 150 ℃ for 5
min (chemisorption).
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Fig.10.Top view (left) and side (right) of the most stable adsorption configuration of phenol.
Atoms are color labeled: Ti (light grey), H (white), C (grey), O atoms (red) in TN, O atoms (blue)
in phenol and O (green) atoms interacted with the phenol molecule.
Fig.11. Top view (left) and side (right) of the most stable adsorption configuration of
cyclohexanone. Atoms are color labeled: Ti (light grey), H (white), C (grey), O atoms (red) in
TN, O atoms (blue) in cyclohexanone and O atoms (green) interacted with the cyclohexanone
molecule.
To find out where the reaction takes place exactly, the adsorption of phenol molecules on
the Pd clusters should be also calculated via the theoretical density functional theory (DFT)
calculations. Minimum size of Pd clusters is about 0.6 nm characterized by TEM images (Fig. 4)
in this work. Thus, the Pd13 with a similar size was selected as a catalyst model for the phenol
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adsorption. The adsorption energies of phenol on Pd13 cluster/TN interface, pure Pd13 cluster
were calculated and then they are compared with the value on TN. As shown in Fig. 12, the
adsorption energies of phenol on Pd13 cluster/TN interface and pure Pd13 cluster are about 0.89
and 0.93eV, respectively. While for phenol on the TN, the adsorption energy is 1.25eV,
suggesting that the phenol molecules prefer to adsorb on the TN. These results are in good
agreement with the in-situ FTIR for phenol adsorption on the TN. The phenol molecules would
adsorb on the TN via the formation of surface coordination species, resulting in the activation of
phenol molecules. This reduces the potential energy of phenol to cyclohexanone, increasing the
catalytic transformation of phenol molecules. Moreover, a set of control experiments also
suggested that the conversion efficiency of phenol enormously depends on the supports of Pd
clusters. Therefore, the roles of Pd clusters are not responsible for the adsorption of phenol
molecules. However, the Pd clusters are indispensable for the phenol hydrogenation because
negligible cyclohexanone was obtained in the absence of Pd clusters. Thus, the Pd clusters could
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be the main active sites for the dissociation of H
to H atoms as the literatures reported.12, 13, 40
2
Moreover, the hydro-oxygen titration for Pd/TN and TN were carried out to further elucidate the
o
dissociation of H
2
molecules on Pd clusters. Fig. S12 shows the obvious signal of H
2
O at 60 C
o
over Pd/TN. Meanwhile, a later peak is observed at 275 C over Pd/TN sample attributed to the
4
1
dissociative adsorption of hydrogen on Pd clusters. However, TN exhibits no obvious signals
for H O and dissociation of hydrogen, indicating that the TN cannot adsorb and activate the H
2
2
molecules. Therefore, the Pd clusters are the main active site for the dissociation of H
H atoms.
2
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Fig 12. Optimized structures of the adsorption of phenol on Pd13/TN (a) and pure Pd13 (b).
Atoms are color labeled: Ti (light grey), H (white), O (red), and C (grey), O atom (blue) in
phenol and cyclohexanone, Pd (grayish blue).
Considering that the phenol hydrogenation could be performed with a relatively high activity
over the Pd/TN in the dark and no catalytic activity over TN, Pd clusters play a key role in the
catalytic reactions. The surface chemical states of Pd, O, and Ti elements on Pd/TN were
disclosed by X ray photoelectron spectroscopy (XPS). Fig. 13 shows the high resolution XPS
spectra of O 1s, Ti 2p and Pd 3d for TN and Pd/TN. The curve of O 1s on TN are deconvoluted
into three peaks at about 530.2, 531.2 and 532.6 eV assigned to the O1, O2 and absorbed oxygen
species respectively.28-31 The Ti 2p spectrum of TN can be deconvoluted into two peaks at 458.9
and 459.6 eV for Ti 2p3/2, representing two type of Ti species (Fig 13b). The former Ti species
(
Ti1) is origin from the perfect octahedral TiO and the latter (Ti2) is attributed to the Ti species
6
nearby the Ti vacancies (Fig 13c). It is obvious that the binding energies of O1 and O2 are
shifted to the higher values, and the binding energy of Ti1 is shifted to the lower value after the
Pd loaded on the TN via an in-situ photoreduction method. Moreover, The curve of Pd 3d is
fitted into two peaks at 335.2 and 340.4 eV for Pd 3d5/2 and Pd 3d3/2, respectively, suggesting
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1, 8-9, 42
that the loaded Pd is metallic Pd (Fig 13d).
These results also suggest that the surface Pd
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clusters are locate at the perfect octahedral TiO via the electrons transfer from lattice oxygen to
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Fig. 13. The XPS spectra of O1s (a), Ti 2p (b) for TN and Pd/TN. The structure of TN (c) and
the XPS spectrum of Pd 3d for surface Pd clusters.
How the surface coordination species facilitate the activation and transformation of phenol
molecules? The in situ electron spin resonance (ESR) and diffuse reflectance UV-visible
spectroscopy (UV-vis DRS) techniques were used to reveal the role of the surface coordination
species in phenol hydrogenation. The in situ ESR spectroscopy (Fig. 14-15) was carried out to
elucidate the changes of electron structures in surface catalytic reaction. Neither Pd/TN nor
Pd/LT initial catalysts show ESR signals. However, when phenol molecules adsorbed on the
surface of catalyst, an obvious ESR signal of oxygen vacancies (V
observed, and this is further enhanced under the light irradiation. Moreover, as shown in Fig.
5a, V is also produced when the phenol molecule is adsorbed on the monolayer TN, although
O
, g = 2.003) could be
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the intensity is weaker than that on Pd/TN. This result suggests that V
O
derives from the
chemical interaction between phenol and monolayer TN. The surface coordination species can
induce the formation of V
•H
O
by the surface charge distribution. Combining with the crystal
O, the existence of Ti defects may induce the transformation of
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structure of H1.07Ti1.73
O
4
2
superficial O1 atoms into the terminal oxygen atoms (denoted as T-O). Due to the strong
chemical adsorption for phenol molecules (Fig. 14) as well as the influence of adjacent Pd
clusters, this T-O atom may be removed from the initial position to form V . Furthermore, the
O
4+
resulting V may cause the exposition of intimal Ti and the adsorbed phenol molecules might
O
4
+
interact with these Ti in a non-planar fashion. Moreover, the adjacent deposited Pd clusters can
enhance the concentration of V . The intensity of the ESR signal for TN is much higher than that
O
for LT (Fig. 15b) because of the higher percent of superficial exposed O2 atoms and the higher
concentration of phenol molecules in TN. The influence of the surface coordination species on
the optical absorption of TN was analyzed by UV-Vis DRS. As shown in Fig. 16, the TN can
only respond to the UV-light (λ≤ 350 nm). After the absorption of phenol, the TN shows the
visible light absorption property. Accordingly the color of the monolayer TN sample changes
from white to yellow after the adsorption of phenol molecules. This result suggests that a ligand-
to-metal charge transfer (LMCT) takes place in the surface coordination species, resulting in the
activation of phenol molecules under visible light irradiation.43 Therefore, the photocatalytic
activity of Pd/TN improved from 21% conv. to 25% conv (Table 1, entry 1). Under visible light
irradiation, this may be attributed to the enhanced photoabsorption performance and the charge
transfer from phenol molecules to TN in the surface coordination species.42
To further elucidate the roles of the generated V in this phenol hydrogenation reaction, DFT
O
calculations were carried out to study the adsorption behaviors of phenol on the V . Considering
O
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the Ti vacancies, there are two types of defect oxygen atoms in TN: the defect oxygen adjacent
to Ti vacancy (denoted as VO1) and the defect oxygen neighboring perfect TiO
denoted as VO2). Two computational models of the TN were proposed: VO1 and VO2. As shown
in Fig. S13, the adsorption energy of phenol and cyclohexanone molecules on the VO1 and VO2
have been calculated. Once the phenol molecules absorb on the V the H atom in –OH of phenol
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(
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will leave to the nearby O atoms in TN, resulting in the in-situ formation of C=O. Moreover, the
adsorption energy of cyclohexanone on the VO1 and VO2 are 2.98 and 1.55eV respectively. In
these cases, it is difficult for cyclohexanone molecules to desorb from the TN inhibiting the
adsorption cycle of phenol molecules. In addition, in situ ESR experiments showed that the
signal of V
in TN is not covered by adsorbing the phenol or cyclohexanone molecules. Moreover, the
high selectivity of cyclohexanone over the Pd/TN may also suggest that the generated V does
not adsorb the phenol molecules. Based on the above ESR and DFT calculations results, it can be
speculated that these generated V during this reaction might not further adsorb the phenol
molecules. However, it is difficult to construct an accurate model of V in experiment.
O
was observed throughout the whole catalytic process (Fig. 14), suggesting that the
V
O
O
O
O
Therefore, the mechanism of the generation for Vo and the role of Vo plays in this reaction is
still not fully understood. A further study on Vo will be carried out in the future.
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Fig.14. ESR spectra of Pd/TN
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Fig.15. ESR spectra of TN (a) and LT (b).
Fig.16. UV-Vis DRS of TN and TN/phenol.
Based on the experimental results, the possible catalytic mechanisms for the hydrogenation
of phenol to the cyclohexanone over Pd/TN are proposed (Scheme 1 and 2). Due to the abundant
superficial acid and basic sites in the as-prepared TN, phenol molecules would bind with TN in a
non-planar fashion via a hydrogen-bonding ring forming the surface coordination species. The
strong interactions result in the formation of V
O
, promoting the exposure of Ti4+ (Lewis acid
sites). These Lewis acid sites are beneficial to the further activation and hydrogenation of phenol.
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Under the hydrogen atmosphere, the metallic Pd dissociates hydrogen molecules to form
hydrogen atom. And then, the activated phenol molecules are hydrogenated by the hydrogen
atoms to enol molecules and further transformed to cyclohexanone via a thermocatalytic process
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(
Scheme 1).1, 12a The cyclohexanone is very easily desorbed from the reaction active site on the
surface of TN. That is essential for the continuous reaction and avoids further hydrogenation of
cyclohexanone to cyclohexanol and cyclohexane.
When the catalytic reaction was performed under the simulated solar light irradiation
(Scheme 2), the electron transfer takes place in the surface coordination species from phenol
molecules to Pd/TN resulting in the activation of phenol molecules to produce a carbonium ion.
Simultaneously, TN would be excited to generate the electron-hole pairs. Subsequently, the
photogenerated electrons transfer to Pd clusters to form electron-rich Pd species and promote the
dissociation of molecular H into active H atoms. The photogenerated holes would recombine the
2
photoinduced electron from the surface coordination species. These approaches may be the
important reasons for the efficient photocatalytic conversion of phenol over Pd/TN under
ambient temperature and pressure. The synergistic effect of the surface coordination species, TN
and Pd clusters remarkably facilitate the high yield of cyclohexanone in photocatalysis. Finally,
the formed carbonium ion is hydrogenated by the nearby active H in Pd clusters forming an enol
intermediate and isomerizes rapidly to cyclohexanone, which is similar to the thermocatalytic
process.
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Scheme 1. Possible Reaction Mechanism of phenol hydrogenation over Pd/TN via the
thermocatalytic process. Atoms are color labeled: Ti (blue), H (white), and O1 (red) and O2
(
yellow).
Scheme 2. Possible Photo-promoted Reaction Mechanism of phenol hydrogenation over
Pd/TN under the stimulated irradiation.
The photocatalytic activities of Pd/TN for the hydrogenation of phenol derivatives with a
set of substituent groups have also been evaluated. The Pd/TN sample is a widely applicable
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catalytic system for the hydrogenation of hydroxylated aromatic compounds (Table 2). In all
cases, the Pd/TN could catalyze the hydrogenation of phenol derivatives into the corresponding
substituted cyclohexanones with high conversion (>99%) and selectivity (>99%). Notably, the
substituent groups have affected the reaction rates. Phenol derivatives with electron-donating
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substituents like -CH and -OH show a decreased conversion rate (Table 2, entries 1-6), while
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phenol derivative with electron-withdrawing group like -Cl exhibits a slightly increased
conversion rate (Table 2, entry 7). Furthermore, the steric hindrance also affects the reaction rate.
The cresol molecules with methyl group (-CH ) at different positions of phenyl ring show the
3
reaction rate in the order of ortho- < meta- < para. Similar electron and steric hindrance effects
and Pd-PANI/CNT.1, 2, 44
were also observed previously over Pd@MMT-1, Pd@mpg-C
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N
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Table 2. Aqueous-Phase Hydrogenation of Hydroxyl Aromatic Derivatives over the Pd/TN.
Entry
Substrate
Time (h)
10
hv
+
Conv. (%)
＞99
Product
Sel.(%)
＞99
98
1
2
12
+
＞99
3
12
+
＞99
98
4
5
6
7
15
15
15
8
+
+
+
+
99
92
85
99
99
97
99
＞99
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4 2
Reaction conditions: simulated light irradiation, phenol (0.1mmol), Pd/ H1.07Ti1.73O •H O 20mg (Pd
1
4 2 2
wt % relative to H1.07Ti1.73O •H O), water solvent (2 mL), H 0.1MPa unless otherwise noted.
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. Conclusions
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In summary, the experiments described here demonstrate the feasibility of direct
construction of a catalyst for precise synthesis of cyclohexanone using H1.07Ti1.73 •H
nanosheets and Pd clusters. This sample exhibits a high conversion (>99%) and high selectivity
>99%) for phenol hydrogenation to cyclohexanone by photocatalysis under ambient temperature
O
4
2
O
(
and hydrogen atmosphere using water as solvent. These performances are much higher than
those of their layered counterparts. The composite catalyst can be also expanded to catalyze the
hydroxylated aromatic compounds with high yield. The abundant Lewis acid and basic sites in
the surface of monolayer TN were revealed by CO -TPD, in situ FTIR and 1H NMR spectra.
2
These active sites greatly promote the adsorption and activation of phenol molecules by forming
the surface coordination species via a hydrogen-bonding ring in a non-planar fashion. Moreover,
the surface highly dispersed negative Pd clusters on the monolayer TN serve as the active sites
for the dissociation of H to H atoms. Thus, the activated phenol molecules would react with the
2
strongly reductive H atoms to form enol molecules and further transformed to cyclohexanone.
Under the simulated solar light irradiation, the photo-excited surface coordination species can
induce the surface charge transfer from phenol molecules to TN forming the active carbonium
ions. Theses surface charges may annihilate the photogenerated holes from TN, promoting the
separation efficiency of photogenerate electron-hole pairs on TN. Simultaneously, the
photogenerate electrons of TN would transfer to the surface Pd clusters for the dissociation of H
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to H atoms. Finally, this surface synergetic catalytic mechanism would contribute to the high
yield of cyclohexanone over Pd/TN. This study not only offers an in-depth fundamental
understanding of the photocatalytic mechanism for hydrogenation of phenol at the molecular
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level, but also suggests that these monolayer nanosheets with inherent defects may have great
potential in the construction of catalysts for precise organic synthesis.
ASSOCIATED CONTENT
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Supporting Information is available free of charge on the ACS Publications website at DOI:
xxxxx.
Experimental procedures, preparation and characterization of catalysts, theoretical calculations
method and cyclic data have been added in Supplementary materials.
AUTHOR INFORMATION
Corresponding Author
*
*
Email: sjliang2011@gmail.com (S. Liang)
Email: wuling@fzu.edu.cn (L. Wu);
* Email: slin@fzu.edu.cn (S. Lin)
Author Contributions
In this manuscript, Y. Song, H. Wang and J. Bi carried out the synthesis and characterizations of
materials and the photocatalysis experiments. S. Lin, X. Feng, and X. Gao did the DFT
calculations. S. Liang, X. Fu and L. Wu conceived the project, supervised the research work and
discussed the results. All authors contributed to the paper writing.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
Thanks Prof. Xianhui Bu (Department of Chemistry and Biochemistry, California State
University Long Beach) for his constructive suggestions on the manuscript polished. This work
was supported by the National Natural Science Foundation of China (21677036, 51672048 and
21673040); S. Liang also thanks the Natural Science Funds for the Distinguished Young Scholar
of Fujian Province (2016J06004), the major science and technology projects of Fujian Province
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(2015YZ0001-1).
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