Designed Precursor for the Controlled Synthesis of Highly Active Atomic and Sub-nanometric Platinum Catalysts on Mesoporous Silica

Article abstract of DOI:10.1002/asia.201800125

The development of new methods to synthesize nanometric metal catalysts has always been an important and prerequisite step in advanced catalysis. Herein, we design a stable nitrogen ligated Pt complex for the straightforward synthesis by carbonization of uniformly sized atomic and sub-nanometric Pt catalysts supported on mesoporous silica. During the carbonization of the Pt precursor into active Pt species, the nitrogen-containing ligand directed the decomposition in a controlled fashion to maintain uniform sizes of the Pt species. The nitrogen ligand had a key role to stabilize the single Pt atoms on a weak anchoring support like silica. The Pt catalysts exhibited remarkable activities in the hydrogenation of common organic functional groups with turnover frequencies higher than in previous studies. By a simple post-synthetic treatment, we could selectively remove the Pt nanoparticles to obtain a mixture of single atoms and nanoclusters, extending the applicability of the present method.

Full text of DOI:10.1002/asia.201800125

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Designed precursor for the controlled synthesis of highly active
atomic and sub-nanometric platinum catalysts on mesoporous
silica
Authors: Sudipta De, Maria Babak, Max Hülsey, Wee Han Ang, and
Ning Yan
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10.1002/asia.201800125
Chemistry - An Asian Journal
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Designed precursor for the controlled synthesis of highly active
atomic and sub-nanometric platinum catalysts on mesoporous
silica
Sudipta De,[a] Maria V. Babak,[b] Max J. Hülsey,[a] Wee Han Ang,*[b] Ning Yan*[a]
Abstract: Development of new methods to synthesize
nanometric metal catalysts has always been an important and
prerequisite step in advanced catalysis. Herein, we design a
stable nitrogen ligated Pt complex for the straightforward
synthesis by carbonization of uniformly sized atomic and sub-
nanometric Pt catalysts supported on mesoporous silica. During
the carbonization of the Pt precursor into active Pt species, the
nitrogen containing ligand directed the decomposition in a
controlled fashion to maintain uniform size of the Pt species. The
nitrogen ligand showed key role to stabilize the single Pt atoms
on a weak anchoring support like silica. The Pt catalysts exhibited
remarkable activities in the hydrogenation of common organic
functional groups with turnover frequencies higher than in
previous studies. By a simple post-synthetic treatment, we could
selectively remove the Pt nanoparticles to obtain a mixture of
single atoms and nanoclusters, extending the applicability of the
present method.
catalytic activity.[2] A plausible way to address this problem is the
employment of stable metal complexes as the catalyst precursor,
since these compounds have higher thermal stabilities than those
of simple metal salts, and would gradually decompose upon
increasing the temperature, allowing uniform formation of small
metal NPs. Indeed, several organometallic complexes with
unique properties have been employed as metal precursors to
form structured metal NPs with controlled size.[3] Mononuclear
organometallic complexes have also been demonstrated as
promising precursors to produce catalytic active sites as metal
clusters.[4]
Nitrogen ligand containing transition metal (e.g. Fe, Co, Ni)
complexes were recently employed as precursors that could
effectively produce single atom active sites after pyrolysis over
suitable supports.[5] In all cases, the M–N–C (M typically refers to
transition metals) structure was retained in the catalysts due to
the strong M–N bond. As a result, high metal loading (up to 4 wt%)
could be obtained without considerable agglomeration of single
active sites. Similarly, metal organic framework (MOF) approach
was applied to synthesize single site catalysts, where
carbonization at high temperature produced porous N-doped
carbon support with anchoring groups to stabilize the metal
atoms.[6] It was proven that the M–N–C catalysts had comparative
electrochemical performances with that of commercial Pt/C
catalyst due to the presence of conductive M–Nx coordination
spheres. N-doped metal catalysts have also shown improved
activities in different organic transformation. For example, Co–N–
C system showed superior catalytic activity in coupling reactions
due to electron-rich metal sites adjacent to the N atoms.[5a] It was
recently reported that the M–N species offered favourable
adsorption of reactants on the active sites, facilitating product
formation.[7]
Introduction
Precise control of active catalytic sites plays a crucial role in
designing highly effective heterogeneous catalytic systems. In
particular, the size of the metal species, such as single-sites,
metal nanoparticles (NPs), or bulk state, directly affects their
catalytic activities.[1] Development of synthetic strategies that can
control the nature of the active catalytic centres, switching from
single-sites to nanostructure ensembles, is therefore both an
important and yet challenging task. Conventional methods, such
as sol-gel and wet impregnation, are widely accepted due to their
inherent simplicity in employing simple metal salt precursors,
such as nitrates, chlorides, and acetates. However, these
methods do not normally provide satisfactory control over particle
size. It is often observed that the thermal treatment during
syntheses or catalytic processes induce aggregation of the active
species to form large NPs (>5 nm), resulting in a decrease of
Several metal oxides including Fe2O3,[8] Al2O3,[9] CeO2,[10] TiO2,[11]
[13]
ZnO[12] and nitrides such as C3N4
and TiN[14], as well as
heteropoly acids[15] have been used to stabilize single metal
atoms and sub-nanoclusters due to the presence of anchoring
sites or lattice defects. However, in spite of high surface area, it is
still challenging to stabilize SACs and sub-nanoclusters on silica,
presumably due to its weak metal-support interaction. Herein, we
report a stable mononuclear Pt complex based on a pyridine-
based tridentate ligand (tris(t-butyl)terpyridine or TBTP), which
produced uniformly sized nanometric Pt species along with single
Pt atoms over mesoporous SBA-15 silica. The bulky TBTP ligand
is designed to protect the Pt species and provides high thermal
stability due to the presence of strong Pt–N bonds. As a result,
the decomposition of the Pt complex occurred in a well-controlled
fashion to yield increased population of catalytically active single
Pt atoms.
[a]
[b]
Dr. S. De, M. J. Hülsey, Prof. Dr. N. Yan
Department of Chemical and Biomolecular Engineering
National University of Singapore
4 Engineering Drive 4, 117585, Singapore
E-mail: ning.yan@nus.edu.sg
Dr. M. V. Babak, Prof. Dr. W. H. Ang
Department of Chemistry
National University of Singapore
3 Science Drive 2, 117543 Singapore
E-mail: ang.weehan@nus.edu.sg
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
heterogeneity of Pt species. The metallic character also increased
at higher temperatures, which could possibly decrease the
percentage of single atom species. However, NPs were hardly
observed for any of these samples from TEM (Fig. 5(c) and S4(a,
b)). When the loading was increased to 0.3 wt%, the IR peak at
2090 cm−1 was totally abrogated and the peak at 2076 cm−1
became broader and unsymmetrical (Fig. 3(c)), which indicated
the presence of a mixture of clusters and NPs. When the metal
loading was increased to 1.0 wt%, the IR peak at 2074 cm−1
became much broader (Fig. 3(d)), indicative of NPs formation.
Further evidence was provided by TEM analysis where NPs with
an average diameter of ~3 nm were detected (Fig. S4(e, f)). XRD
analysis of the sample showed a small peak at 39.7° (Fig. 5(a))
which was not clearly observed in other two samples at lower
loading.
We followed a simple two-step procedure to synthesize the
mononuclear Pt complex ([Pt(tBu3tpy)Cl]Cl or Pt(TBTP)) (see
supporting information), and the product was characterized using
ESI-MS analyses (Fig. 1(a)) which showed a strong molecular ion
peak at m/z 632.2. Thermogravimetric analyses (Fig. 1(b))
revealed that the Pt complex was stable up to 350 °C before
starting to decompose and the weight of the residue became
stable from 450 °C onwards. Supported Pt catalysts (designated
as xPt-SBA-15-T, where x is Pt loading and T is treatment
temperature) with different metal loading were prepared by wet
impregnation method. To obtain information on the formation of
Pt species, temperature programmed desorption-mass
spectrometry (TPD-MS), elemental analysis (EA) and X-ray
photoelectron spectroscopy (XPS) studies were conducted with
1.5 wt% Pt(TBTP) complex supported on SBA-15. TPD-MS
profile (Fig. 2) showed the loss of carbon species in the form of
CO and CO2 at different temperatures. Presumably, the silica
support provided trace amount of oxygen for carbon oxidation. Up
to 600 °C, significant amount of carbon was retained and a very
little amount of nitrogen loss was observed. XPS results (Fig. S1)
showed that the oxidation state of Pt remained the same as in the
Pt precursor after treatment at 400 °C. For treatment temperature
at 500 °C however, Pt species exhibited different oxidation states
due to partial removal of ligand and formation of clusters/NPs.
Nitrogen content also decreased upon increasing treatment
temperature from 400 °C to 500 °C. In both samples, nitrogen
species were observed in the form of imine bonding modes (C=N–
C). Combination of TPD-MS, EA and XPS studies reveal that both
carbon and nitrogen species were partly retained on the support,
which stabilized the positively-charged Pt single atoms and close
to zero-valent, small clusters (vide infra).
Figure 2. TPD-MS profile of Pt-complex supported on SBA-15.
Figure 1. (a) ESI-MS spectrum and (b) TGA-DTA profile of Pt(TBTP)Cl2
complex.
Initial characterization results showed that the heat treatment of
supported Pt sample (0.2 wt% Pt) at 500 °C produces a mixture
of single atoms and small clusters (or NPs) as observed from the
IR spectra (Fig. 3(a)). Two characteristic peaks located at 2090
cm−1 and 2075 cm−1 can be assigned as linearly adsorbed CO on
Pt single atoms and clusters (or NPs) respectively. XPS results
also supported this observation since Pt moieties existed in
partially oxidized state (Fig. S2(a)). When the temperature was
increased to 600 °C, the IR peak at 2090 cm−1 shifted to 2087
cm−1 with concomitant peak-broadening, presumably due to the
combination of the two peaks (Fig. 3(b)), indicating the higher
Figure 3. IR spectra of CO adsorbed on different samples: (a) 0.2Pt-SBA-15-
500, (b) 0.2Pt-SBA-15-600, (c) 0.3Pt-SBA-15-500, (d) 1.0Pt-SBA-15-500.
Spectra were taken under N2 flow after CO adsorption.
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To provide direct evidence on the local Pt coordination structure,
X-ray absorption studies were performed. Extended X-ray
absorption fine structure (EXAFS) spectra (Fig. 4(a)) revealed
that both Pt–Pt and Pt–O(or N) contributions in the 0.2Pt-SBA-15-
500 sample, indicative for significant Pt–Pt contribution. At
decreased Pt loading (0.1 wt%), the sample showed higher
amounts of Pt–O(or N) contribution. The weaker peak at 2.75 Å
could correspond to the higher shell of Pt–O(or N) contribution as
observed in the L3-edge k-space spectra (Fig. S3).[16] Fig. 2(b)
showed the normalized X-ray absorption near-edge structure
(XANES) spectra of the two samples, and the reference spectra
of Pt foil and PtO2. The white-line intensities in the spectra
reflected the positive oxidation state of Pt in two samples with
different loading. The Pt species in 0.1Pt-SBA-15-500 sample
carried higher positive charges, suggesting the dominance of Pt–
O(or N) contribution. At higher loading the white-line intensity was
closer to that of the Pt foil, which suggested that the sample
contained Pt clusters or NPs.
adopted to determine the surface atom fraction (Fig. S7).
Assuming that each CS2 molecule blocked two surface Pt atoms,
we determined the surface atom fraction to be 20% and 1.5% for
0.2Pt-SBA-15-500
and
0.2Pt-SBA-15-500-C
catalysts,
respectively, which was in accordance with the results obtained
from H2 pulse titration method.
Figure 5. XRD spectra of Pt-SBA-15 materials at different loading prepared
from (a) TBTP complex and (b) H2PtCl6 precursors. TEM images of (c) 0.2Pt-
SBA-15-500 and (d) 0.2Pt-SBA-15-500-C.
Figure 4. (a) EXAFS and (b) XANES spectra of Pt-SBA-15 materials at two
different Pt loading.
The activities of the catalysts were further verified for the
hydrogenation of -NO2 group in nitrobenzene (Table S5). The
main purpose of selecting the hydrogenation of -NO2 group is to
differentiate the nature of Pt species based on the product
selectivities. Since -NO2 group is very easy to be reduced; its
hydrogenation depends on many factors including both catalyst
features and reaction conditions. The hydrogenation of
nitrobenzene follows sequential steps to give aniline as final
product and produces different intermediates during the reaction.
In a fixed reaction condition, the product selectivity is observed to
be dependent on nature of the metal, oxidation state of the metal
and acid-base property of the catalyst support. Since in the
present case, we use only Pt metal as catalyst and the silica
support is neutral, the product selectivity is expected to be
dependent on the oxidation state of Pt species only. Very high
reaction rates were observed for the Pt-SBA-15 catalysts
compared to the controlled catalysts (Pt-SBA-15-C). Notably, the
product distribution was drastically different. Over Pt-SBA-15
catalysts, four different products were detected, including aniline,
nitrosobenzene, azobenzene, and azoxybenzene with aniline as
the major product (Table S5). In sharp contrast, aniline was the
only product obtained over Pt-SBA-15-C. This could be attributed
to the presence of both positively charged single atoms as well as
reduced NPs in Pt-SBA-15 which gave different intermediate
products along with targeted aniline. In case of the catalyst (A)
derived from Pt(TBTP) precursor, the Pt species are slightly
oxidized due to the presence of single atoms and nanoclusters,
and therefore produced significant amount of intermediate
products (such as nitrosobenzene and azoxybenzene in this
To validate the effectiveness of our present methodology,
controlled catalysts (designated as xPt-SBA-15-T-C) using
H2PtCl6 precursor were prepared using the same method. IR
spectra of controlled catalysts at a loading of 0.3 and 1.0 wt%
showed the characteristic peaks at 2063 and 2062 cm−1 (Fig. S5(a
and b)), designated as the linearly adsorbed CO on NPs. The
controlled sample of 0.2 wt% Pt loading did not show any CO
adsorption. XRD analysis revealed obvious peaks at 39.7°, 46.2°
and 67.4° corresponding to the crystalline plane (111), (200) and
(220) of Pt NPs for these samples (Fig. 5(b)).[17] The peak
intensities increased with Pt loading, implying increased particle
sizes, which was further confirmed by TEM analysis (Fig. S4(g,
h)). H2 pulse titration experiment provided the dispersion of Pt
over SBA-15 in both cases. Pt dispersion was observed to be
much higher in the samples derived from Pt(TBTP) precursor
compared to H2PtCl6 (Fig. S6).
The catalytic performance of the catalysts was tested in the
hydrogenation of different functional groups. Catalysts prepared
from the Pt(TBTP) complex showed very high activities as
compared to the catalysts prepared from H2PtCl6. Using a very
low Pt/substrate ratio (1:4000), almost quantitative conversion
could be achieved in the hydrogenation of phenylacetylene (Table
S4). At longer reaction times, the catalysts treated at 500 °C gave
higher selectivity for semi-hydrogenation product as compared to
controlled catalysts. To obtain the intrinsic hydrogenation
activities of the catalysts, CS2 poison titration method was
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case). However in case of controlled catalyst (A-C), the Pt species
are highly reduced due to the formation of nanoparticles, and
therefore gave the fully hydrogenation product, i.e. aniline. We
also compared the turnover frequency (TOF) of our catalysts with
previously reported nanometric and single atom Pt catalysts for
the hydrogenation of -C≡C, -NO2 and -C=O groups (Table 1 and
2). Several-fold increments of activities were observed as
compared to the previously reported Pt catalysts. In particular, the
TOFs for -NO2 hydrogenation were exceptionally high, which was
presumably due to the presence of Pt–N species, favouring the
dissociative adsorption of the reactant nitrobenzene and
facilitated the desorption of the product aniline.[7]
After studying the hydrogenation of -C≡C and -NO2 functionalities,
we sought to find out the activities of our catalysts in -C=O bond
hydrogenation. The experiments were carried out at high stirring
speed to rule out mass transfer limitations (see Fig. S8 for the
effect of stirring speed). Effects of different parameters (reaction
time, temperature and H2 pressure) were studied (Fig. S9, S10,
S11), which reveal that Pt-SBA-15 performed considerably better
than Pt-SBA-15-C. Kinetic studies were also performed (Fig. S12).
Both catalysts followed first order kinetics up to a certain
concentration of benzaldehyde in presence of a fixed amount of
catalyst. However, the reaction rate of Pt-SBA-15 was much
higher than the Pt-SBA-15-C in all experiments. The reusability of
the catalyst was examined in the hydrogenation of benzaldehyde
at an optimized condition; the activity was retained for up to seven
consecutive cycles (Fig. S13).
Based on the earlier studies made on gold and iron,[23] we propose
that the Pt NPs in the catalyst could be selectively dissolved in
aqua regia, leaving positively-charged single atoms and clusters
on the support because of their stronger metal-support interaction.
Indeed, Pt NPs could be selectively removed after optimization of
the concentration of aqua regia. After treatment of the catalyst
with 1 M aqua regia, DRIFTS results showed that the peak at
2057 cm−1 disappeared, corresponding to the removal of Pt NPs
(Fig. S14). The new peak at 2075 cm−1 could be attributed to a
Table 1. Hydrogenation of common functional groups over Pt catalysts.
TOF (h−1
)
Entry
1b
Substrate
PhAc
Catalyst
A
Major products (% selectivity)
S.A.a (molsub/molPt h)
Styrene, 94
3888
315
16336
15517
Ethylbenzene, 6
Styrene, 85
2b
PhAc
A-C
Ethylbenzene, 15
Nitrosobenzene, 25
Aniline, 47
3b
Nitrobenzene
A
7552
31731
Azoxybenzene, 28
Aniline, 100
4b
5c
6c
Nitrobenzene
Benzaldehyde
Benzaldehyde
A-C
A
208
391
23
10246
1643
1133
Benzyl alcohol, 100
Benzyl alcohol, 100
A-C
a S.A. stands for specific activity. Reaction conditions: 2 mL methanol, 10 bar H2. b substrate/Pt = 4000, room temperature, 15 min. c substrate/Pt =
500, 50 °C, 1 h. A: 0.2Pt-SBA-15-500, A-C: 0.2Pt-SBA-15-500-C.
Table 2. TOF values for the hydrogenation of different functional groups over Pt-based catalysts reported in earlier literatures.
Group
Substrate
Product
Catalyst
T (°C)/H2 (bar)
RT/10
RT/10
RT/10
RT/10
50/5
Substrate/Pt
1950
1950
1000
1220
500
TOF (h−1
366
)
Ref
-C≡C
PhAc
styrene
Pt/KCC-1
Pt/KCC-1
Pt-PMA/AC
Pt-CNF-P
Pt-Q@IL
[18]
[18]
[15a]
[19]
[20]
[16a]
[21]
[18]
[15a]
[22]
[22]
PhAc
ethylbenzene
styrene
73
PhAc
492
-NO2
4-chloronitrobenzene
3-nitrostyrene
3-nitrostyrene
nitrobenzene
nitrobenzene
nitrobenzene
benzaldehyde
4-hydroxybenzaldehyde
4-chloroaniline
3-vinylaniline
3-vinylaniline
aniline
610
1300
1500
4900
325
Pt/FeOx
40/3
1250
2000
1950
2000
590
Pt-NHCs
30/1
aniline
Pt/KCC-1
Pt-PMA/AC
Pt/MgAl2O4
Pt/MgAl2O4
RT/10
RT/10
40/1
aniline
774
-CHO
benzyl alcohol
56
4-hydroxybenzyl
alcohol
40/1
590
22
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mixture of single Pt atoms and clusters in oxidized state. After
hydrogen reduction of the sample at 200 °C, the peak shifted
toward a higher value at 2085 cm−1 with a shoulder at 2076 cm−1,
suggesting the presence of single Pt atoms and clusters in
reduced state. After the selective leaching of Pt NPs, the catalyst
showed a small decrease in specific activities, but the selectivity
for the semihydrogenation product was increased (Table S7 and
S8), characteristic of single atom catalysts based on earlier
studies.[13, 24] The decrease in activity of the acid-leached samples
could be attributed to the higher oxidation state of Pt species (Fig.
S14(b vs. d)). However, the activity could be recovered after
treating the sample with hydrogen as the Pt species returned to
their initial oxidation state. This highlighted that our method is
applicable to single atom and sub-nanocluster catalyst on an inert
support with higher metal loading, by providing a simple post-
leaching treatment. In order to gain further understanding on the
mechanism of the here investigated hydrogenation reactions, we
plan to conduct parahydrogen-induced polarisation experiments.
They have been previously proven to provide insight into the
pairwise hydrogen addition route on atomically dispersed[25] and
nanoparticle[26] catalysts. Different reaction pathways between
the platinum single-atom and subnanometric catalysts are
possible.
with water and ethanol thoroughly, and dried in air for overnight.
The product was then calcined at 873 K for 6 h to obtain SBA-15
with the BET surface area of 582 m2 g−1.
Preparation of [Pt(tBu3tpy)Cl]Cl complex
The total synthesis was carried out in two steps using previously
reported literature.[29] To prepare the Pt(COD)Cl2 complex,
potassium tetrachloroplatinate (K2PtCl4, 2.00 g, 4.8 mmol) was
dissolved in a solution of 53.0 mL of water and 63.0 mL of acetic
acid. To the light red solution was added 1.6 mL (1.36 g, 12.6
mmol) of 1,5-cyclooctadiene. The reaction mixture was stirred
rapidly and heated to 110 °C. Over 3 h the solution became pale
yellow and a white precipitate was formed. The volume of the
solution was reduced to about 20 mL by evaporation under
reduced pressure. The precipitate was collected and washed in
succession with small portions of water, ethanol, and diethyl ether.
The product was dried in vacuum to give white needles (1.71 g,
95% yield).
H2O/Acetic acid
K2PtCl4
+
Pt
Step 1
Cl
Cl
In the following step, a portion of Pt(COD)Cl2 (286 mg, 0.76 mmol)
and tBu3tpy (317 mg, 0.79 mmol) were stirred in 40 mL H2O, 15
mL MeOH and 2 mL acetone in air at 80 °C. A yellow solution and
undissolved white reactant were evident after 12 h of stirring. After
~48 h of stirring the solution was filtered over Celite to remove a
minute amount of undissolved starting materials and the methanol
and acetone were removed in vacuum. The product was extracted
into CH2Cl2 to produce a yellow organic layer and a cloudy white
aqueous layer. The CH2Cl2 was removed in vacuum to produce a
yellow powder. The powder was washed with diethyl ether to give
a yield of 93%. The product [Pt(tBu3tpy)Cl]Cl was characterized
Conclusions
In summary, we have presented an effective method to
synthesize single atom and sub-nanometric Pt catalysts over an
inert silica support. High thermal stability of designed Pt(TBTP)
complex, and its ability to partially retain carbon and nitrogen after
decomposition, were key to achieving well-dispersed, uniform Pt
species on SBA-15. The advantage of the synthetic protocol was
clearly proven by comparing the dispersion and catalytic activities
of the new catalyst with the ones derived from conventional simple
salt precursor. The catalysts showed exceptional activities and
high selectivities in the hydrogenation of different functional
groups, indicating that the present method may potentially be
extended to the synthesis of a series of highly active catalysts for
a wide range of applications in catalysis. Future research
endeavours may also be directed to reveal the correlations
between surface property and the single-atom/sub-nano-cluster
catalytic activity. For instance, the effect of the
hydrophobicity/hydrophilicity of the support, which plays a critical
role in nanoparticle catalysts[27], remain to be explored in the
current system.
1
using H NMR and ESI-MS, and in agreement with literature.[29]
Purity check was carried using elemental analyses and in good
agreement with the predicted values (Table S3).
H2O/MeOH/Acetone
Cl
N
Pt
Cl
+
N
Pt
N
N
N
N
Cl
Cl
Step 2
Preparation of Pt-SBA-15 catalysts from Pt-TBTP
precursor
Required amount of Pt-TBTP complex was dissolved in 5 mL
methanol and pretreated SBA-15 (0.45 g) was added in this
solution. The mixture was sonicated for 10 min and then
vigorously stirred at room temperature for another 30 min. 30 μL
of 0.5 M NaOH solution was added in the solution and stirring was
continued for 1 h. The mixture was washed with methanol and
centrifuged at 7000 rpm to collect the solid product, which was
dried at 50 °C for overnight. The powder sample was then treated
at 500 °C for 2 h at a heating rate of 10 °C/min under N2
Experimental Section
Preparation of mesoporous SBA-15 silica
Silica SBA-15 was prepared according to the method reported in
the previous literature.[28] Pluronic P123 (6 g) was dissolved in 45
g of water and 180 g of 2 M HCl solution with stirring at room
temperature for 30 min. TEOS (12.75 g) was added to the solution
with stirring at 308 K for 20 h. The mixture was aged at 363 K for
24 h. The white powder was recovered through filtration, washed
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atmosphere. The catalyst with x wt% Pt loading was designated
as xPt-SBA-15-500.
Praying Mantis™ high temperature reaction chamber with ZnSe
windows. The CO adsorption on Pt-SBA-1-5 samples was
performed at room temperature. 5% CO/Ar (UHP) was introduced
into the DRIFTS cell at a flow rate of 40 mL min−1. After the CO
saturation, N2 purge at a flow rate of 40 mL min−1 was performed
to remove gas phase CO from the DRIFTS cell and then the
adsorbed CO on Pt. All the spectra were recorded using 32 scans
and a resolution of 4 cm−1.
Preparation of controlled Pt-SBA-15 catalysts from
H2PtCl6 precursor
Required amount of H2PtCl6 was dissolved in 5 mL methanol and
pretreated SBA-15 (0.45 g) was added in this solution. The
mixture was sonicated for 10 min and then vigorously stirred at
room temperature for another 30 min. Minimum amount μL of 0.5
M NaOH solution was added in the solution at stirring condition to
fully adsorb the platinum salt on SBA-15. The mixture was
washed with methanol and centrifuged at 7000 rpm to collect the
solid product, which was dried at 50 °C for overnight. The powder
sample was then treated at 500 °C for 2 h at a heating rate of
10 °C/min under N2 atmosphere. The catalyst with x wt% Pt
loading was designated as xPt-SBA-15-500-C (C denotes
controlled sample).
Transmission electron microscopy
Transmission electron microscopy (TEM) was performed on a
JEM 2100F (JEOL, Japan) microscope operated at 200 kV. X-ray
diffraction (XRD) analysis was carried out using a Bruker D8
Advance XRay Diffractometer, at a scan rate of 3º min−1. It was
operated at 40 kV applying a potential current of 30 mA. X-ray
photoelectron spectra (XPS) were recorded on a VG Escalab
MKII spectrometer, using a mono Al Kα X-ray source (hν =
1486.71 eV, 5 mA, 15 kV), and the calibration was done by setting
the C1s peak at 284.5 eV. Electrospray ionization (ESI) mass
spectrometric analysis was performed on a Bruker microTOF-Q
system. Thermogravimetric analysis was conducted on a DTG-
60A thermogravimetry analyser (Shimadzu) under a nitrogen
atmosphere. Pt content in the catalysts was determined using an
iCAP 6000 series inductively coupled plasma optical emission
spectrometer (ICP-OES). Gas chromatography (GC) analysis
was done using an Agilent 7890C GC equipped with a HP-5
column and a flame ionization detector (FID) detector.
Selective leaching of Pt NPs.
100 mg of catalyst was added to 2 mL 2 M aqua regia and stirred
for 12 h at room temperature. The mixture was centrifuged to
separate the catalyst and the solution was analysed by inductively
coupled plasma optical emission spectrometry (ICP-OES) to
determine the leached Pt amount. The catalyst was washed
several times with distilled water and finally with methanol and
oven dried at 50 °C for overnight.
X-ray absorption spectroscopy
Temperature programmed decomposition
Pt L3-edge X-ray absorption spectra (XAS) of supported catalysts
and references (Pt foil and PtO2) were recorded at the BL01B1
beamline at the Spring-8 (Japan Synchrotron Radiation Research
Institute, Hyogo, Japan) in the transmission mode at ambient
temperature. Data analysis was carried out with Athena and
Artemis included in the Ifeffit and Demeter package. For curve-
fitting analysis on extended X-ray absorption fine structure
(EXAFS) spectra, each theoretical scattering path was generated
with FEFF 6.0 L. The k3-weighted EXAFS oscillation in the range
of 3.0–13 Å−1 was Fourier transformed.
Temperature programmed decomposition of catalyst precursor
was measured on a Micromeritics AutoChemII 2920 instrument.
Typically, 30 mg of sample was pretreated at 50 °C in pure Helium
(15 mL/min) gas flow for 1 h. Subsequently, the temperature was
raised from 50 to 900 °C at a rate of 10 °C/min under Helium flow.
The effluent gases were analyzed by a mass spectrometer.
Hydrogen-Oxygen titration
H2-O2 titration was conducted on a ChemBET Pulsar TPR/TPD
(Quantachrome). Certain amount of catalyst was pretreated with
air at 100 °C for 30 min to remove the hydrogen atoms adsorbed
(if any) on Pt atoms. After that, N2 was used as carrier gas at 80
mL min−1, and the successive doses of H2 gas were subsequently
introduced into N2 stream by means of a calibrated injection valve
(159 μL H2 pulse−1) at 100 °C. The titration will end when the
intensities of three peaks in a row keep constant. In principle, the
titration should follow this equation: PtO (surface) + 3/2 H2 = PtH
(surface) + H2O. Thus the dispersion of Pt (dPt) should be
calculated using this formula: dPt = 2/3 n (H2)/ntotal(Pt).
Hydrogenation of phenylacetylene
Required amount of phenylacetylene, catalyst and solvent were
charged into a 20 mL autoclave reactor. The reactor was then
flushed with purified H2 for three times and finally filled with 10 bar
of H2. The reaction was performed at room temperature with a
stirring speed of 1000 rpm. After completion of the reaction, H2
pressure was released and products were analysed by GC.
Hydrogenation of benzaldehyde
Required amount of benzaldehyde, catalyst and solvent were
charged into a 20 mL autoclave reactor. The reactor was then
flushed with purified H2 for three times and finally filled with 10 bar
of H2. The reaction was performed at desired temperature in a
pre-heated oil-bath with a stirring speed of 1000 rpm. After
completion of reaction, the reactor was cooled down to room
Diffuse reflectance infrared Fourier transform
spectroscopy
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) measurements were performed on a Thermo Scientific
Nicolet iS50 FT-IR spectrometer with a MCT detector, and a
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10.1002/asia.201800125
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Sudipta De, Maria V. Babak, Max J.
Hülsey, Wee Han Ang,* Ning Yan*
Page No. – Page No.
Designed precursor for the controlled
synthesis of highly active atomic and
sub-nanometric platinum catalysts on
mesoporous silica
Stabilizing the instable: A designed platinum precursor facilitates the synthesis of
stable platinum catalysts on an inert support such as silica. A simple post-synthetic
step selectively removes platinum nanoparticles and increases the selectivity toward
several hydrogenation reactions.
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