Pt nanoparticles on Ti3C2T: X -based MXenes as efficient catalysts for the selective hydrogenation of nitroaromatic compounds to amines

Article abstract of DOI:10.1039/d0dt02594a

The development of Pt nanocatalysts for the selective hydrogenation of nitroaromatic compounds to the corresponding amines is of great significance to solve the drawbacks associated with a low reserve of Pt. Herein, we develop a protocol for the preparation of a Pt/titanium carbide-based MXene heterostructure for the selective reduction of nitroaromatic compounds. In the heterostructure, well-defined and nano-sized metallic Pt crystallites are uniformly decorated on Ti3C2Tx nanosheets using a mild reducing agent of ammonia borane without additional stabilizing agents. The selective hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) was employed as a model reaction to investigate the catalytic performance of the as-synthesized heterostructure, denoted as Pt/Ti3C2Tx-D-AB. Notably, this catalyst can catalyze the complete conversion of p-CNB to p-CAN with 99.5% selectivity, superior to that of Pt/Ti3C2Tx-D-SB synthesized with sodium borohydride. The high performance of the present catalytic system can be ascribed to the well-dispersed Pt nanoparticles, the abundant surface electron-efficient Pt(0), and the synergistic catalysis between Pt/Ti3C2Tx-D-AB and water. This catalyst also shows generality toward the selective hydrogenation of a series of nitroaromatic compounds to the corresponding amines with high efficiency. The present study provides a strategy to synthesize efficient catalysts for catalytic applications.

Full text of DOI:10.1039/d0dt02594a

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DOI: 10.1039/D0DT02594A
ARTICLE
Pt nanoparticles on Ti3C2Tx-based MXene as efficient catalysts for
selective hydrogenation of nitroaromatic compounds to amines
Qian Chen,a Weidong Jiang b and Guangyin Fan a,*
Received 00th January 20xx,
Accepted 00th January 20xx
Development of Pt nanocatalysts for selective hydrogenation of nitroaromatic compounds to the corresponding amines is
of important significance to solve the drawbacks associated with low reserve of Pt. Herein, we develop a protocol for
preparation of Pt/Titanium carbide-based MXene heterostructure for selective reduction of nitroaromatic compounds. In
the heterostructure, well-defined and nano-sized metallic Pt crystallites are uniformly decorated on Ti3C2Tx nanosheets using
a mild reducing agent of ammonia borane without additional stabilizing agents. Selective hydrogenation of p-
chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) was employed as a model reaction to investigate the catalytic
performance of as-synthesized heterostructure, denoted as Pt/Ti3C2Tx-D-AB. Notably, this catalyst can catalyze the complete
conversion of p-CNB to p-CAN with 99.5% selectivity, superior to that of Pt/Ti3C2Tx-D-SB synthesized with sodium
borohydride. The high performance of present catalysis system can be ascribed to the well-dispersed Pt nanoparticles,
abundant surface electron-efficient Pt(0), and the synergistic catalysis between Pt/Ti3C2Tx-D-AB and water. This catalyst also
show generality toward the selective hydrogenation of a series of nitroaromatic compounds to the corresponding amines
with high efficiency. The present study provides a strategy to synthesize efficient catalysts for catalytic applications.
DOI: 10.1039/x0xx00000x
Among these catalysts, Pt nanoparticle catalysts have exhibited
excellent catalytic activity toward the hydrogenation reaction,
whereas the selectivity to the corresponding amines is not
Introduction
Selective hydrogenation of nitroaromatic compounds to
the corresponding amines is of scientific importance and
industrial relevance because of wide application of the products
as chemical intermediates for the manufacture of dyes,
preservatives, herbicides, pesticides, etc. The main challenge
for selective hydrogenation of nitroaromatic compounds to the
corresponding amines is to overcome the adverse side
reactions, e.g., dehalogenation reaction, which considerably
reduce the selectivity toward the target products. The
development of catalysts with high catalytic activity and
selectivity for the selective hydrogenation of nitroaromatic
compounds is one of the most popular research directions.
Thereinto supported metal catalysts have captured much
attention in this reaction because of their typical features, such
as high chemical and structural stability as well as the strong
interaction between the metals and supports, which can
prohibit the unavoidable side reaction occurred during the
hydrogenation process, significantly improving selectivity to the
desired product. Recently, metal nanoparticles (NPs) including
Ru, Rh, Pt and Pd have been widely used as catalysts in the
satisfied enough due to serious side reactions. In addition, in
most cases the hydrogenation processes require high
temperature with extra energy consumption and toxic
solutions. For instance, Chen et al. reported the selective
hydrogenation of p-CNB to p-CAN over Pt/AC catalyst with an
excellent catalytic activity under mild reaction conditions,
whereas the selectivity to p-CAN was only 84% 3. Recently,
Milan et al. synthesized Pt/SiO2 as a catalyst to produce p-CAN
from p-CNB with a selectivity of 89% in methanol and diethyl
ether mixture 4. Besides, Xu’s group enhanced the selectivity of
5
p-CAN (98.6 %) by high reaction temperature (353 K)
.
Therefore, there is an urgent need but it is still a significant
challenge to achieve high-performance Pt-based catalysts
toward the selective hydrogenation of nitroaromatic
compounds, especially halonitroaromatic compounds.
Attributing to the superior properties of large specific surface
areas, abundant functional groups and space confinement
effects, various 2D layered materials such as graphene and
graphitic carbon nitride have excellent properties as catalyst
6,
carriers for metal nanoparticle depositions 7. Recently, Ti-
1,
2
selective hydrogenation of nitroaromatic compounds
.
based MXenes have attracted extensive research interest
8, 9
because of its superior chemical and physical properties
.
Compared with the complicated synthetic procedure involved
in preparing graphene, Ti3C2Tx MXene with similar layered
structure can be conveniently synthesized through the etching
of Al layers in Ti3AlC2 10. This process also endows the as-formed
Ti3C2Tx MXene flakes with abundant surface oxygenated groups.
Therefore, extensive efforts have been exerted to explore the
a College of Chemistry and Materials Science, Sichuan Normal University,
Chengdu 610068, China.
b School of Chemistry and Environmental Engineering, Sichuan University of
Science & Engineering, Zigong, Sichuan 643000, China.
* To whom correspondence should be addressed. Tel.: (+86) 28-84760802; E-mail:
fanguangyin@sicnu.edu.cn (G.Y Fan).
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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use of Ti3C2Tx MXene as matrixes to confine metal NPs and ethanol, and 2.0 mL water) were transferred into a 50 mL
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single atoms for hydrogen evolution and electrochemical CO2 stainless-steel autoclave equipped withDaOIt:h10e.r1m03o9c/Do0uDpTle02. 5T9h4Ae
11,
reduction
12. Despite these advances, the use of Ti3C2Tx reaction temperature and hydrogen pressure were maintained
MXene as a NP support to prepare catalytic materials that can at 30 ℃ and 1.0 MPa H2. The hydrogenation reaction was
be used as an efficient catalyst for selective hydrogenation is of performed in a reaction time of 1.0 h. Then conversion and
considerable importance but remains challenging.
selectivity of the selective hydrogenation were analyzed by gas
Herein, we developed a method to prepare highly dispersed chromatography (Agilent 7890A). To study the generality of
Pt NPs on the Ti3C2Tx-D surface (Pt/Ti3C2Tx-D-AB) with the use of Pt/Ti3C2Tx-D-AB toward the selective hydrogenation of
ammonia borane as the reducing agent. The key to this nitroaromatic compounds to the corresponding amines, a
successful synthesis is the use of ammonia borane as a milder variety of substrates were applied under identical conditions.
reducing agent compared with sodium borohydride. By using
selective hydrogenation of p-CNB to p-CAN as a model reaction,
Pt/Ti3C2Tx-D-AB synthesized with ammonia borane exhibited
Results and Discussion
much higher catalytic activity and selectivity in comparison with
Pt/Ti3C2Tx-D-SB prepared by sodium borohydride. Furthermore,
the results suggested that the synergistic catalysis between
hydrophilic Pt/Ti3C2Tx-D-AB and water also enhanced the
catalytic performance toward the selective hydrogenation. This
catalyst also exhibited good generality toward the selective
hydrogenation of other nitroaromatic compounds. This work
Scheme 1. The preparation process of samples.
offers a method to improve the selective hydrogenation of
nitroaromatic compounds to the corresponding amines without
Scheme 1 exhibited the schematic preparation process of
any additives.
Pt/Ti3C2Tx-D samples with different reducing agents. First, the
Ti3AlC2 precursor was treated with HF to removal of the Al
layers, leading to the formation of Ti3C2Tx (Tx=F, O, OH). Then,
DMSO was used for Ti3C2Tx exfoliation to yield Ti3C2Tx-D with a
large number of oxygenated functional groups, which can be
Experimental
Synthesis of Pt/Ti3C2Tx
Ti3C2Tx was synthesized according to the following
procedures: 4.0 g Ti3AlC2 was slowly added into a 250 plastic
beaker containing 40 mL concentrated hydrofluoric acid (HF)
aqueous solution. The suspension was stirred at 25 ℃ for 24 h.
Then, the black solid was collected by centrifugation and
washed with water and ethanol for several times. The resulting
product was dried under vacuum at 60 ℃ for 12 h. Afterward,
0.3 g Ti3C2Tx was mixed with 5.0 mL DMSO and the mixture was
stirred for another 24 h. Then, the collected solid, denoted as
Ti3C2Tx-D, was dispersed into water of 150 mL and treated by
ultrasonication for 6 h. The resulting solid was washed with
water for several times and dried at 60 ℃ for 12 h.
To prepare Pt/Ti3C2Tx-D samples, different reducing agents
were used. For Pt/Ti3C2Tx-D-AB, 50.0 mg Ti3C2Tx-D was added
into 25 mL water, and then 95 μL H2PtCl6 solution (Pt
concentration: 5.51 mg mL-1) was introduced. The mixture was
stirred at room temperature for 1 h. And then, 5.0 mL ammonia
borane aqueous solution (2.0 mg mL-1) was added dropwise.
After the reduction, the black solid was collected by
centrifugation and treated by washing cycles with water and
ethanol for three times. Finally, the sample was dried under
vacuum at 60 ℃ for 12 h. For comparison, the Pt/Ti3C2Tx-D-SB
sample with the same Pt loading was synthesized by using
sodium borohydride as the reducing agent.
effectively used to anchor metal NPs. Finally, the metal
precursor of H2PtCl6 was introduced and followed by a
reduction process in the presence of ammonia borane. The
metal loading determined by ICP-OES was 1.0 wt%. For
comparison, the control sample of Pt/Ti3C2Tx-D-SB with the
same loading was synthesized under identical conditions except
for using sodium borohydride as reductant.
The microstructure and morphology of Pt/Ti3C2Tx-D samples
were firstly observed by TEM. As shown in Fig. 1, it was seen
that the Pt NPs were clearly distributed on the Ti3C2Tx-D surface.
However, The NP’ size and dispersity had a significant difference
in the samples obtained by two reduction method. Note that
the average size of Pt NPs in Pt/Ti3C2Tx-D-AB was 2.2 nm (Figs.
1a and 1b). High-resolution TEM (HRTEM) was used to measure
the crystal structure of Pt NPs in Pt/Ti3C2Tx-D-AB, in which the
d-spacing lattices of 0.23 nm were observed, corresponding to
the Pt (111) plane (Fig. 1b inset). Compared with Pt/Ti3C2Tx-D-
AB, Pt/Ti3C2Tx-D-SB had a relatively large average size of Pt NPs
(2.7 nm), which means a relatively low dispersion of Pt NPs on
the Ti3C2Tx-D surface (Figs. 1c and 1d). To confirm this
conclusion, the metal dispersion of Pt NPs in the two samples
was further calculated by CO chemisorption. The result shows
that the Pt dispersion for Pt/Ti3C2Tx-D-AB (7.9%) is much higher
than that of Pt/Ti3C2Tx-D-SB (4.1%). These results highlighted
Activity tests
The catalytic activity of the as-prepared samples was evaluated
using the selective hydrogenation of p-CNB to p-CAN as a model
reaction. Typically, reaction reagents including Pt/Ti3C2Tx-D-AB
catalyst (5.0 mg), p-CNB (1.0 mmol), and solvent (5.0 mL
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intensities of Ti3C2Tx-D is stronger than that of Ti3C2Tx, indicating
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DOI: 10.1039/D0DT02594A
the presence of more oxygenated surface functional groups,
which contributes to successful anchoring of Pt NPs 19. No
obvious differences were observed between the two Pt/Ti3C2Tx-
D samples, indicating that the attachment of Pt NPs did not
affect the structure of Ti3C2Tx-D.
Fig. 2 (a) XRD patterns of Ti3AlC2, Ti3C2X2, Ti3C2Tx-D and Pt/Ti3C2Tx-D-AB. (b) The
FTIR characterization of Ti3C2X2, Ti3C2Tx-D, Pt/Ti3C2Tx-D-AB and Pt/Ti3C2Tx-D-SB.
The composition and chemical states of the samples were
further analyzed by XPS. The survey spectra evidenced the
coexistence of F, O, Ti, C, and Pt elements, signifying that Pt had
been successfully anchored on the Ti3C2Tx-D surface in all
samples (Fig. S2). Considering the identical metal loadings
determined by ICP, it can be concluded that the surface content
of Pt in Pt/Ti3C2Tx-D-AB was much higher than that of Pt/Ti3C2Tx-
D-SB owing the relatively small size and high dispersity of Pt NPs
in Pt/Ti3C2Tx-D-AB. This conclusion can be confirmed by the
surface composition of the two samples listed in Fig. S2 inset.
Moreover, the F and O elements introduced during HF etching
Fig. 1 TEM images for (a, b) Pt/Ti3C2Tx-D-AB and (c, d) Pt/Ti3C2Tx-D-SB. Insets of (a, c) are
the size distributions and inset (b) is the HRTEM image of Pt NPs in Pt/Ti3C2Tx-D-AB.
that the reducing agent strongly affected the particle size and
distribution of Pt NPs on the Ti3C2Tx-D surface. The reasons are
as follows: (1) the sodium borohydride acted as a strong
reductant with the high reduction rate and rapidly forming
cluster, however, those unstable clusters easily aggregated and
led to the further growth of nanoparticles with reduction
process existed as surface terminators on the Ti3C2Tx surface 20
.
13,
continues
14. Compared with sodium borohydride, the
Ti 2p spectrum could be deconvoluted into six peaks at 454.8,
456.4, 458.7, 460.8, 462.5 and 464.4 eV, which can be ascribed
ammonia borane has a relatively low reducibility, which
decreased the rate of particle formation and controlled the
aggregation of nanoparticles. Therefore, ammonia borane was
inferred as a feasible reductant to supply activation hydrogen
to facilely reduce metal precursors and from highly distributed
metal NPs anchored on Ti3C2Tx-D.
to the Ti-C(2p3/2), Ti3+(2p3/2), Ti-O(2p3/2), Ti-C(2p1/2), Ti3+(2p1/2
)
and Ti-O(2p1/2) groups 21-24 (Figs. 3a and Fig. S3a). C 1s spectrum
could be deconvoluted to four peaks with binding energies of
288.11, 286.42, 284.8 and 281.39 eV, which should be
attributed to O=C-OH, C-OH, C-C and Ti-C species, respectively
(Fig. 3b and Fig. S3b) 25-27. The peaks with binding energies of
530.1, 531.7 and 532.9 eV can be assigned to the Ti-O, the
chemisorbed or physically absorbed oxygen and Ti-OH species,
suggesting the presence of abundant surface oxygenated
groups on the surface of Ti3C2Tx-D (Fig. 3c and Fig. S3c) 28-30. Figs.
3d shows the Pt 4f spectrum of Pt/Ti3C2Tx-D-AB, including four
sub-peaks with binding energies of 70.91 and 74.36 eV
corresponding to metallic Pt (0), and the other two small peaks
with binding energies of 72.16 and 75.61 eV could be assigned
to 4f7/2 and 4f5/2 of Pt (Ⅱ) oxidation state. The results suggest
that the Pt mainly exists as metallic Pt (0) with a small amount
of Pt (Ⅱ) species, attributing to the incompletely reduction of
Pt precursor due to the interaction between the metal
precursor and Ti3C2Tx-D, which the oxidation state Pt2+ can form
Ti-Pt and C-Pt bond 25. Compared with Pt/Ti3C2Tx-D-SB (Fig. S3d),
the negative shift of Pt 4f peaks in Pt/Ti3C2Tx-D-AB can be
ascribed to the electron transfer from Ti3C2Tx-D to Pt NPs,
indicating the stronger interaction between Pt NPs and Ti3C2Tx-
D in the Pt/Ti3C2Tx-D-AB sample 31, 32. This result can be ascribed
to the strong metal-support interactions,
XRD was explored to investigate the phase structure of the
catalysts and the results are displayed in Fig. 2a. The sharp and
strong diffraction peaks were observed from the pristine
Ti3AlC2, indicating the high purity of the raw material 15. After
etching by HF, the characteristic peaks of Ti3AlC2 disappeared
and new peaks corresponding to Ti3C2Tx were detected,
implying the successful synthesis 16. When the achieved Ti3C2Tx
was treated in DMSO solution, its (002) peak shifted to low
angle direction, indicating the exfoliation of Ti3C2Tx-D 17. Note
that the adsorption of Pt ions and followed by reduction on the
Ti3C2Tx-D surface led to the (002) peak further shifted as the
same direction, which was probably ascribed to the
intercalation of Pt NPs. Interestingly, no obvious diffraction
peaks assigned to the Pt species (JCPDS 88-2343) in the XRD
patterns of Pt/Ti3C2Tx-D-AB and Pt/Ti3C2Tx-D-SB were observed,
attributing to the low loading and/or small size of Pt NPs, which
was consistent well with the TEM results (Fig. S1). Moreover,
the FTIR peaks of Ti3C2Tx and Ti3C2Tx-D show the presence of -
OH and C=O species at 3435 and 1630 cm-1 after the HF acid
treatment and DMSO exfoliation 18 (Fig. 2b). Notably, the peak
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These results demonstrate that Pt/Ti3C2Tx-D-AB is VainewaAcrtticivleeOanlninde
selective catalyst toward p-CNB selectiveDhOyId: 1r0o.1g0e3n9a/Dti0oDnT.02594A
The effect of water on the catalytic performance of
Pt/Ti3C2Tx-D-AB was investigated for the hydrogenation of p-
CNB to p-CAN with varied water content from 0 to 56 % (volume
percent) at 30 ℃ and 1.0 MPa H2 and the results were
illustrated in Table 1. The p-CNB conversion of 72.2% and the p-
CAN selectivity of 91.1% were detected using ethanol as the
sole solvent (entry 2). With the increasing water content to
14%, the p-CNB conversion and the p-CAN selectivity increased
to 94.0% and 92.4% respectively (entry 5). The p-CNB
conversion of 100% and the highest p-CAN selectivity of 99.5%
were obtained when the content of water reached 28% by using
Pt/Ti3C2Tx-D-AB (entry 3), which is significantly superior to most
Table 1. Effect of solvent on catalytic hydrogenation of p-CNB to p-CAN catalyzed
by Pt/Ti3C2Tx-D-AB a.
Fig. 3 High resolution XPS spectrum of Ti 2p (a), C 1s (b), O 1s (c) and Pt 4f for
Pt/Ti3C2Tx-D-AB, respectively.
Selectivity (%)
VH2O
(%)
Conversion
(%)
Entries
1
p-
CAN
p-
AN
-
others
-
CNSB
specially the electron transfer triggered by the difference of
work function between Pt (5.6 eV) and the reduced MXene
0b
-
-
-
33
or, alternatively, the difference in the electrochemical
potential. Compared with that of Pt/Ti3C2Tx-D-SB, the electron
transfer is more obvious for small Pt NPs in Pt/Ti3C2Tx-D-AB,
owing to the low coordination number of surface Pt atoms and
the closer contact between Pt NPs and MXene 34. Moreover, the
ratio of Pt (0) and Pt (Ⅱ) is obviously difference. As for Pt (0),
the Pt/Ti3C2Tx-D-D-AB (67.4%) is significantly higher than
Pt/Ti3C2Tx-D-SB (55.1%) (Table S1), suggesting that the reducing
agents can efficiently influence the reduction degree of H2PtCl6
and the corresponding surface composition of samples.
Catalytic property of Pt/Ti3C2Tx-D-AB was investigated for the
selective hydrogenation of p-CNB to p-CAN at 30 oC and 1.0 MPa
hydrogen pressure. Blank test or using Ti3C2Tx-D as a catalyst
were firstly conducted and the results showed no conversion of
p-CNB to p-CAN (entry 1). The hydrogenation of p-CNB was
initiated when Pt/Ti3C2Tx-D-AB was introduced to the reactor,
indicating that Pt species was the active sites for the
transformation of p-CNB to p-CAN (entry 2). Regarding the
product distributions, the main product was p-CAN and the by-
products were p-chloronitrosobenzene (p-CNSB), AN, and a
small quantity of other products (4,4′-dichloroazoxybenzene
and 4,4′-dichloroazobenzene). It should be pointed out that the
p-CNSB intermediate can be further hydrogenated to p-CAN
with the increase of reaction time as shown in Table S2. The
effects of reaction temperature and hydrogen pressure on
hydrogenation of p-CNB to p-CAN over Pt/Ti3C2Tx-D-AB was
investigated and the results were displayed in Tables S3 and S4.
A p-CNB conversion of 100% and p-CAN selectivity of 99.5%
were achieved within 1 h at 30 ℃ and 1.0 MPa H2 in the
presence of ethanol/water mixture with a volume percent of
28%, which are obviously higher than that of Pt/Ti3C2Tx-D-SB
(conversion of 40.4% and p-CAN selectivity of 84.3%) (entry 3
and 4), indicating Pt/Ti3C2Tx-D-AB with high content Pt(0) and
high dispersed Pt NPs can efficiently improve catalytic activity.
2
3
0
72.2
100.0
40.4
94.0
99.8
96.9
2.6
91.1
99.5
84.3
92.4
95.2
92.9
67.0
95.0
90.8
83.3
8.2
0.4
0.4
0.1
2.2
0.5
1.0
0.9
0
0.3
0
28
4
28c
14
13.5
7.1
0
5
0
6
42
3.7
0.1
0.1
7.9
0.1
0
7
56
6.1
8
100
28d
28e
28f
25.1
4.6
9
98.2
78.1
39.3
0.3
0
10
11
9.2
16.7
0
0
a Reaction conditions: catalyst, 5.0 mg; solvent (ethanol/water, total volume:
7.0 mL); p-CNB: 1 mmol; time: 1 h; temperature: 30 ℃; the pressure of
hydrogen: 1MPa.
b Ti3C2Tx-D was used as a catalyst.
c Ti3C2Tx-D-SB was used as a catalyst.
d methanol/water mixture.
e propanol/water mixture.
f 1,4-dioxane/water mixture.
of the reported catalysts, such as Pt@Cu(0.23)/TiO2 2, Pt/CMK-
37
3
35, Pt–NiO@mSiO2 36, PtNi/CNTs
(Table S5). Further
increasing the content of water resulted in a decrease of the p-
CNB conversion and the p-CAN selectivity (entries 6-7). When
the volume percent of water in ethanol was increased to 56%,
the hydrogenation reaction did not bring to an obvious
enhancement due to the decrease of mass transfer efficiency.
Furthermore, a very low p-CNB conversion of 2.6 % and p-CAN
selectivity of 67.0 % were discovered when pure water was used
as solvent for the hydrogenation reaction at the same
conditions, which could be assigned to the low solubility of p-
CNB in water (entry 8). In addition, the catalytic activity of
Pt/Ti3C2Tx-D-AB for the selective hydrogenation of p-CNB in
different alcohol/water medium was also studied and the
results are illustrated in Table 1 (entries 9-11). It is noteworthy
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that higher p-CNB conversion and p-CAN selectivity are support and water also considerably promoted the selective
View Article Online
DOI: 10.1039/D0DT02594A
observed in protic alcohol/water mixture than that of aprotic hydrogenation reaction. As a result, the high performance of
solvent such as 1, 4-dioxane. The results confirm the conclusion the present catalysis system for the selective hydrogenation of
that water plays an important role in improving the catalytic p-CNB to p-CAN can be ascribed to the Pt NPs with high
activity and selectivity of supported catalysts for the selective dispersion, the coexistence of electron-efficient Pt(0) species
hydrogenation of nitroaromatic compounds in alcohol/water and the synergistic catalysis between Pt/Ti3C2Tx-D-AB and
mixture 38-43. With regard to the positive effect of water on the water.
selective hydrogenation reaction, previous studies have shown
The selective hydrogenation of other nitroaromatic
that the existence of water is beneficial for accelerating the compounds was further studied to further demonstrate the
surface reaction rates of catalytic reactions, including generality of Pt/Ti3C2Tx-D-AB and the results are displayed in
42, 44-48
hydrogenation, oxidation, electrochemical reaction
.
Table 2. The investigated substrates could be hydrogenated to
Specifically, the hydrophilic catalyst support can facilitate the the corresponding amines at different reaction time. The
transfer of water across its surface for the adsorbed reactants conversions of nitroaromatic compounds could reach 100%
and catalytically active sites, which can probably enhance the with the reaction time ranging from 1 to 3 h and the selectivity
49,
water involved hydrogenation
50. Moreover, as a protic to the target products are higher than 94% in all tests. The
solvent water can accept H atoms to form an H3O+-like results indicate that Pt/Ti3C2Tx-D-AB is a versatile catalyst which
transition state through hydrogen spillover pathway. can be applied to produce a series of aromatic amines.
Afterward, the adsorbed water near the p-CNB can generate
hydrogen bond with nitro group and the H atoms of H3O+
species can be transferred to the nitro groups, facilitating the
Table 2. Catalytic hydrogenation of other nitroaromatic compounds catalyzed by
Pt/Ti3C2Tx-D-AB.
Tim
e (h) n (%)
Conversio
Selectivit
y (%)
Substrates
Products
following hydrogenation. In this work, the abundant
oxygenated functional groups enable Ti3C2Tx-D with strong
hydrophilicity, which can act the similar roles as mentioned
above in accelerating the selective hydrogenation of p-CNB to
p-CAN. Consequently, the Pt/Ti3C2Tx-D-AB exhibited much
higher catalytic performance in the presence of water/alcohol
solvents.
The high performance of the present catalysis system toward
the selective hydrogenation of p-CNB to p-CAN was analyzed on
the basis of the above characterization and experimental
results. First, previous studies have shown that the presence of
abundant oxygenated function groups in reduced graphene
oxide (RGO) could facilitate the formation of metal NPs and
thereby deduced a high catalytic activity in hydrodechlorination
reaction 51, 52. With the similar 2D layered structure and the
existence of oxygenated functional groups, Ti3C2Tx-D can be
acted the similar function as RGO in anchoring Pt NPs and
prohibited their overgrowths on the Ti3C2Tx-D surface, which
can be confirmed by the TEM results. Second, the used of
ammonia borane as a milder reducing agent facilitates the
formation of smaller Pt NPs with high dispersion, as compared
with that of Pt/Ti3C2Tx-D-SB synthesized with the strong
reducing agent of sodium borohydride. The Pt NPs in Pt/Ti3C2Tx-
D-AB enable more Pt atoms are exposed on the Ti3C2Tx surface
NH2
NO2
1.0
1.5
2
100
99.5
96.5
95.8
94.7
98.2
95.7
97.4
94.7
99.2
Cl
Cl
Cl
Cl
NH2
NO2
100
100
100
100
100
100
100
100
Cl
Cl
NH2
NO2
NO2
NO2
NH2
CH3
NO2
CH3
2
NH2
2
H3
C
H3
C
NH2
NO2
3
O
O
NH2
1.5
1.5
1
HO
HO
NH2
NO2
F
NH2
F
NO2
Reaction conditions: catalyst, 5.0 mg; solvent, ethanol/water mixture with a
as confirmed by XPS survey scan results. Third, the strong volume percent of 28% (total volume: 7.0 mL); substrates, 1 mmol; hydrogen
pressure, 1.0 MPa; temperature, 30 ℃.
interaction between Pt and Ti3C2Tx arouses the electron transfer
from Ti3C2Tx to Pt NPs, enabling the generation of electron-
efficient Pt species, which are beneficial for the adsorption of
substrates and the subsequent hydrogenation due to the
electron absorption property of nitro groups in the substrates
42. Furthermore, owing to the superior capability of activating
H2 to form H atoms, zero-valence metal(0) was considered as
the real active sites for hydrogenation reaction 53, 54. Note that
Reusability of Pt/Ti3C2Tx-D-AB was investigated in successive
runs performed using the isolated catalyst after the completion
of the previous run of p-CNB hydrogenation. The catalyst
recycling tests were repeated for six times in the same
conditions. As shown in Fig. 4a, the Pt/Ti3C2Tx-D-AB catalyst can
be recycled for p-CNB hydrogenation at least six times without
the obvious loss of activity. The p-CNB conversion of 98.0% and
Pt/Ti3C2Tx-D-AB has more metallic Pt(0) species than that of the p-CAN selectivity of 92.0% are recorded even after the sixth
run. The results show that the Pt/Ti3C2Tx-D-AB catalyst exhibits
relatively good deactivation resistance toward selective p-CNB
hydrogenation. The TEM image of the isolated catalyst after the
Pt/Ti3C2Tx-D-SB, which can achieve a much higher catalytic
activity toward the selective hydrogenation. Finally, as
discussed above the synergistic effect between hydrophilic
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sixth run shown in Fig. 4b confirms the aggregation of Pt NPs
with an average diameter of 2.7 nm on the Ti3C2Tx surface. The
slight increase in average size of Pt NPs led to the loss of surface-
active sites and thus generation of more p-CNSB intermediate,
thereby slight reducing the selectivity of p-CAN for the selective
hydrogenation of p-CNB. Additionally, the good stability of the
catalyst should be ascribed to the structure stability of the
Ti3C2Tx-D support. The Pt/Ti3C2Tx-D-AB catalyst after reusability
tests was collected and characterized through XRD to check the
structure changes. No obvious structure changes were detected
in the XRD pattern of the reused Pt/Ti3C2Tx-D-AB catalyst,
demonstrating the excellent stability of the matrix during the
reusability tests for p-CNB hydrogenation (Fig. S4). Further work
should be done to improve the reusability of the present
catalyst.
Acknowledgements
View Article Online
We acknowledge the financial supporDtsOIf:r1o0m.103t9h/De0DNTa0t2io59n4aAl
Natural Science Foundation of China (21777109).
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Table of contents entry
Pt/Ti3C2Tx-D-AB synthesized using ammonia borane as a reducing agent showed excellent
performance for the selective hydrogenation of p-CNB to p-CAN.