Micro-/Mesoporous Platinum–SiCN Nanocomposite Catalysts (Pt@SiCN): From Design to Catalytic Applications

Article abstract of DOI:10.1002/chem.201603266

The synthesis, characterization, and catalytic studies of platinum (Pt) nanoparticles (NPs) supported by a polymer-derived SiCN matrix are reported. In the first step and under mild conditions (110 °C), a block copolymer (BCP) based on hydroxyl-group-terminated linear polyethylene (PEOH) and a commercially available polysilazane (PSZ: HTT 1800) were synthesized. Afterwards, the BCP was microphase separated, modified with an aminopyridinato (Ap) ligand-stabilized Pt complex, and cross-linked. The green bodies thus obtained were pyrolyzed at 1000 °C under nitrogen and provided porous Pt@SiCN nanocomposite via decomposition of the PEOH block while Pt nanoparticles grew in situ within the SiCN matrix. Powder X-ray diffraction (PXRD) studies confirmed the presence of the cubic Pt phase in the amorphous SiCN matrix whereas transmission electron microscopy (TEM) measurements revealed homogeneously distributed Pt nanoparticles in the size of 0.9 to 1.9 nm. N₂sorption studies indicated the presence of micro- and mesopores. Pt@SiCN appears to be an active and robust catalyst in the hydrolysis of sodium borohydride under harsh conditions.

Full text of DOI:10.1002/chem.201603266

DOI: 10.1002/chem.201603266
Full Paper
&
Self-Assembly
Micro-/Mesoporous Platinum–SiCN Nanocomposite Catalysts
(Pt@SiCN): From Design to Catalytic Applications
Sabrina M. Sachau,[a] Muhammad Zaheer,*[b] Abhijeet Lale,[c] Martin Friedrich,[a]
Christine E. Denner,[a] Umit B. Demirci,[c] Samuel Bernard,[c] Gꢀnter Motz,[d] and
Rhett Kempe*[a]
Abstract: The synthesis, characterization, and catalytic stud-
ies of platinum (Pt) nanoparticles (NPs) supported by a poly-
mer-derived SiCN matrix are reported. In the first step and
under mild conditions (1108C), a block copolymer (BCP)
based on hydroxyl-group-terminated linear polyethylene
(PEOH) and a commercially available polysilazane (PSZ: HTT
1800) were synthesized. Afterwards, the BCP was microphase
separated, modified with an aminopyridinato (Ap) ligand-sta-
bilized Pt complex, and cross-linked. The green bodies thus
obtained were pyrolyzed at 10008C under nitrogen and pro-
vided porous Pt@SiCN nanocomposite via decomposition of
the PEOH block while Pt nanoparticles grew in situ within
the SiCN matrix. Powder X-ray diffraction (PXRD) studies con-
firmed the presence of the cubic Pt phase in the amorphous
SiCN matrix whereas transmission electron microscopy (TEM)
measurements revealed homogeneously distributed Pt
nanoparticles in the size of 0.9 to 1.9 nm. N2 sorption studies
indicated the presence of micro- and mesopores. Pt@SiCN
appears to be an active and robust catalyst in the hydrolysis
of sodium borohydride under harsh conditions.
Introduction
help minimize the loss of metal during successive catalytic
runs.[3] For instance, our group has reported the synthesis of
dipalladium silicide (Pd2Si) and iridium (Ir) NPs over SiCN as re-
usable catalysts for the hydrogenation of ketones[4] and for the
synthesis of pyrroles, respectively.[5] The SiCN support, howev-
er, possesses very low surface area and, to avoid adverse mass
transport constraints during catalysis, its porosity needs to be
developed. Kamperman et al. reported on a bottom-up ap-
proach towards a porous SiCN material structured over multi-
ple length scale.[6] They combined micromolding and two-com-
ponent colloidal self-assembly with cooperative assembly of
a five-component precursor system to obtain the desired ma-
terials with Pt NPs (1–2 nm) and specific surface areas up to
44 m2 gÀ1. Despite the highly impressive structuring, the ob-
served surface was rather low and the block copolymer (BCP)
used for microphase separation was oxygen rich and might
alter the nature of the ceramic material. Our group had report-
ed the synthesis of porous materials by the fine tuning of py-
rolysis temperature; thus, a nickel modified polysilazane (PSZ)
at 6008C provided microporous Ni@SiCN hybrid materials with
a specific surface area up to 380 m2 gÀ1.[7] Due to the low pyrol-
ysis temperature, a hybrid material was generated but it was
not as stable as SiCN ceramics pyrolyzed at 10008C or higher.
Other attempts include the use of polystyrene nanospheres as
soft template for the synthesis of porous SiCN ceramics[8] and
the fabrication of SiCN nanofibers using self-assembly of block
polymer of PSZ and polyethylene hydroxide (PEOH).[9] Thus the
fabrication of porous SiCN materials may provide robust cata-
lyst supports by coupling our approach to produce and stabi-
lize very small metal NPs with a BCP route. Here, we report on
a one-pot design of porous Pt-SiCN nanocomposites (labeled
Polymer-derived silicon-based ceramics combine high thermal
stability and chemical inertness with a molecular and highly
versatile synthesis pathway.[1] These materials can be prepared
as single-phase, multi-phase, and nanocomposite structures to
provide a large variety of properties and functions. In the cate-
gory of nanocomposites, those made of late transition metals
with SiCN (M@SiCN) are promising catalysts[2] owing to the
presence of nitrogen atoms in the amorphous SiCN matrix,
which allow the synthesis of rather small metal nanoparticles
(NPs). These N-atoms could strongly bind with metal NPs and
[a] S. M. Sachau, M. Friedrich, Dr. C. E. Denner, Prof. Dr. R. Kempe
Lehrstuhl Anorganische Chemie II
Universitꢀt Bayreuth
Universitꢀtsstrasse 30, 95440 Bayreuth (Germany)
E-mail: kempe@uni-bayreuth.de
[b] Dr. M. Zaheer
Department of Chemistry
SBA School of Science and Engineering,
Lahore University of Management Sciences (LUMS)
54792 Lahore (Pakistan)
E-mail: muhammad.zaheer@lums.edu.pk
[c] A. Lale, Prof. U. B. Demirci, Dr. S. Bernard
IEM (Institut Europeen des Membranes)
UMR 5635 (CNRS-ENSCM-UM), Universite Montpellier
Place E. Bataillon, 34095, Montpellier (France)
[d] Dr. G. Motz
Lehrstuhl Keramische Werkstoffe
Universitꢀt Bayreuth
Ludwig-Thoma-Straße 36b, 95447 Bayreuth (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603266.
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Pt@SiCN) with integrated small Pt NPs in a range
from 0.9–1.9 nm and surface areas up to 260 m2 gÀ1
using the self-assembly of block copolymers. The
beauty of the method lies in the fact that the ceram-
ic matrix (SiCN), porosity, and metal NPs are all gen-
erated in a single step. The as-obtained nanocompo-
sites were successfully used as heterogeneous cata-
lysts in the hydrolysis of sodium borohydride.
Figure 2. Schematic synthesis of porous Pt@SiCN. In step (I) BCP (PE-O-b-HTT 1800) is
formed in the presence of complex 1 where inorganic block gets modified with PtII ions.
(II) Microphase separation; (III) pyrolysis under inert atmosphere.
Results and Discussion
tion with the nitrogen functions.[2] During pyrolysis (under ni-
An aminopyridinato (Ap) ligand-stabilized Pt com-
trogen) BCP was removed and PtII was reduced to Pt0 due to
the in situ generation of methane, ammonia, and hydrogen. A
series of four samples labeled Pt@SiCN10, Pt@SiCN20,
Pt@SiCN40, and Pt@SiCN60 with different Pt:Si ratios from
1:10 (Pt@SiCN10) to 1:60 (Pt@SiCN60) have been synthesized
(also see Table T1 in the Supporting Information) according to
the procedure described in Figure 2. The final temperature of
10008C was fixed, based on FT-IR experiments (Figure S1 in
the Supporting Information). The absence of characteristic
stretching vibrations of NÀH (3300–3500 cmÀ1), SiÀH
(2150 cmÀ1), and vinylic (2998–3084 cmÀ1) bonds confirmed
the completion of the ceramization. The broad band observed
at 800 to 1260 cmÀ1 was assigned to SiÀN and SiÀC stretching
vibrations.
plex (1) (See Scheme 1 in the Supporting Information for its
synthesis) was used to chemically modify the PSZ and as a pre-
cursor for the generation of Pt NPs. Such complexes possess
both good solubility in aprotic solvents and contain the same
elements as the ceramic precursor. Moreover, Ap complexes
are easy to synthesize on a multigram scale and are available
for many different transition metals.[10]
Figure 1 shows the crystal structure of the synthesized Pt
complex 1, which crystallizes in a monoclinic crystal system
with P21/c space group. The coordination of Pt center is planar
with N-Pt-N angles of 64.72(11) and 115.28(11)8. The very small
N-Pt-N angle indicates a strained binding of Ap ligand which
might be beneficial for metal-transfer reactions. The PtÀN dis-
tances are PtÀN1: 2.021(2) and PtÀN2: 2.061(3) ꢂ, which indi-
cate an “aminopyridinato” ligand mode. CCDC 1045617 con-
tains the supplementary crystallographic data for this paper.
These data are provided free of charge by The Cambridge
Crystallographic Data Centre.
X-ray powder diffraction (XRD) and TEM observations were
carried out on the well-representative Pt@SiCN10 sample. The
XRD pattern of Pt@SiCN10 is provided in Figure S2 (SI). In
Pt@SiCN10 (the highest metal loading), peaks at 2q values
39.88, 46.28, and 67.58 could be correlated to the reflections of
the (111), (220), and (322) planes of cubic Pt. The existence of
very broad peaks is most probably due to the very small size
of the Pt NPs, as calculated by the Scherrer equation (1.6 nm).
The microstructure of Pt@SiCN10 was analyzed using trans-
mission electron microscopy (TEM) and scanning electron mi-
croscopy (SEM). The TEM micrographs (Figure 3a and b) show
homogeneously distributed NPs all over the surface. The size
of these particles was calculated to be in the range of 0.9 to
1.9 nm (Figure S3 in the Supporting Information), which is in
close agreement with that calculated from powder XRD data
using Scherrer formula. However the presence of regular pore
Figure 2 summarizes the stepwise process of fabrication of
porous Pt@SiCN ceramics based on the complex 1. In the first
step the PSZ is reacted with PEOH to synthesize PEO-b-HTT
1800 polymer, followed by the addition of complex 1 (details
are provided in the Supporting Information). Upon self-assem-
bly, the block copolymers (BCP) generate nanodomains in a va-
riety of morphologies depending on the volume ratio of the
two blocks.[11] Upon the addition of the complex 1, PSZ block
gets modified with PtII ions most likely due to their coordina-
Figure 1. Molecular structure of complex 1 determined by single-crystal X-
ray diffraction structure analysis. Hydrogen atoms are omitted for clarity. Se-
lected bond lengths [ꢂ] and angles [8]: Pt1ÀN1 2.021(2), Pt1ÀN2 2.061(3),
N1-Pt1-N2 64.72(11)
Figure 3. TEM images of Pt@SiCN10. Inset in (b) shows a ceramic piece with
the highlighted area magnified in (a) and (b).
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structure as was expected could not be verified by TEM analy-
sis either due to their small size or non-ordered nature. SEM
images (Figure S4 in the Supporting Information), however,
suggest bicontinuous pore morphology in the fabricated mate-
rials. To investigate the properties of these materials at meso-
scopic length, we investigated N2 sorption.
Table 1. BET surface area of the Pt@SiCN ceramics.
Sample ID
Pt:Si ratio
Surface area [m2 gÀ1
]
Pt@SiCN10
Pt@SiCN20
Pt@SiCN40
Pt@SiCN60
1:10
1:20
1:40
1:60
259
192
153
121
N2 sorption was carried out to determine the specific surface
areas and pore-size distribution of the Pt@SiCN ceramics. Al-
though the type of the isotherms is complex, the representa-
tive sample Pt@SiCN10 shows an adsorption–desorption iso-
therm close to type IV, which is typical of mesoporous materi-
als (Figure 4a). Also, the relatively high adsorption of nitrogen
at P/P0<0.1 suggests the existence of micropores as well. The
presence of micropores was also confirmed by t-plot method
which provided a surface area of 37 m2gÀ1 contributed by mi-
cropores. Also pore size distribution calculated by NLDFT as
presented in Figure 4b provides an average pore size of
1.6 nm. Mesopores however show a bimodel distribution with
maxima at 2.0 and 5.0 nm.
(Table 1) which could be attributed to the organic ligand of
complex 1 acting as a porogen during pyrolysis.[7] Thus, we
can suggest that the results of N2 sorption analysis, in addition
to the TEM observations showing the presence of Pt NPs at
the surface of the nanocomposites, were the driving forces
behind the use of these materials as catalysts. The catalytic ac-
tivity of the nanocomposites was investigated for the hydroly-
sis of sodium borohydride at 808C and in a very alkaline aque-
ous solution (pH> 10). We chose this particular reaction as
studies on similar type of materials are available for a compari-
son of the catalytic activity.[12] Moreover, conventional oxide
supports will simply dissolve under these harsh conditions
(pH 14; 808C). Elsewhere, Pt-free SiCN was found to be inactive
in the same conditions.[13] The hydrogen generation results for
the four samples are presented in Figure 5. All of the samples
are catalytically active. The hydrolysis starts immediately, with
no induction period. This means that Pt is accessible in these
samples and that it is in metallic state.
Table 1 provides the specific surface area values for the four
samples based on the BET isotherms. An increment in the sur-
face area with an increment in metal to silicon ratio (metal
loading) from Pt@SiCN10 to Pt@SiCN60 was observed
Indeed, the occurrence of an induction period is generally
observed with oxidized surfaces.[14] The hydrogen evolution is
fast and, as expected, the nanocomposites with a higher Pt
amount have higher rates of hydrogen generation. The hydro-
gen generation rates (slope of the curves at a conversion
<50%) were determined for each sample (Table 2). Expressed
À1) is mea-
À1
per gram of Pt, a maximum rate of 38.1 Lmin
g
Pt
sured for the sample Pt@SiCN60 which is the best per-
formance ever reported for non-oxide ceramics used as sup-
ports.[12] However, a direct comparison to the literature is
somehow complicated because of the low amount of Pt (less
than 1 wt%) used in these published reports in comparison to
Figure 5. Volume of hydrogen (in mL) measured during the hydrolysis of
sodium borohydride over the same amount (16 mg) of the nanocomposites
at 808C. Under these conditions, the amount of Pt contained by the samples
was different.
Figure 4. Measured N2 adsorption-desorption isotherm (a) and pore size dis-
tribution (b) of Pt@SiCN10. Inset in (b) shows a magnified view of the select-
ed pore sizes with a bimodal distribution.
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Hydrolysis experiments were performed at different temper-
atures, from 50 to 808C, and the apparent activation energies
(AAEs) were calculated for all four systems (Table 2). The AAE is
an important parameter that gives insight into the kinetics of
the hydrolysis reaction. Because hydrolysis of sodium borohy-
dride with Pt-based material as catalyst is a surface-catalyzed
reaction, it can be argued that higher AAE implies that a lesser
amount of Pt is actually accessible for the catalysis of the reac-
tion. However, we observe that the maximum rate of reaction
in terms of mLminÀ1 does not correspond to the sample with
the lowest AAE. The AAEs of Pt@SiCN10, Pt@SiCN20, and
Pt@SiCN60 are comparable; that of Pt@SiCN40 is higher. To
explain these values, we believe that other factors like specific
surface area, structure of the matrix, structure of the Pt parti-
cles, and surface acidity may have some effects on the rates.
However, AAEs of 46–73 are consistent with many of the
values reported so far in the literature.[15]
Table 2. Hydrogen generation rates and apparent activation energy
values. nc=nanocomposites.
Sample ID Rate
[mLminÀ1
Rate
[Lmin
Rate
[Lmin
Apparent
À1
À1
À1
À1
Pt
]
g
]
g
]
activation energy
nc
[kJmolÀ1
]
Pt@SiCN10 45.7
Pt@SiCN20 30.9
Pt@SiCN40 21.1
Pt@SiCN60 13.4
2.8
1.9
1.3
0.8
16.1
23
29.3
38.1
54.2
46.3
73.3
55.8
that of nanocomposites investigated in the present paper. On
the other hand, a wider comparison with other catalytic sys-
tems is irrelevant because of discrepancies in the experimental
conditions. The main point here is that the nanocomposites
display a porous structure allowing the accessibility of the Pt
NPs which offers high catalytic performances. As a second ex-
periment, we performed hydrolyses by varying the weight of
the different nanocomposites so that the amount of Pt is iden-
tical (systematically with 0.35 mg of Pt). We then compared
the samples with each other with the same amount of Pt
(Figure 6). The objective of that was to see how much the
Conclusion
Pt NPs supported by a structured polymer-derived SiCN matrix
were synthesized. The molecular approach utilizing microphase
separation of PEO-block-HTT 1800 polymer and an Ap- ligand-
stabilized Pt complex was used to modify PSZ block with PtII
ions. The Pt modified PEO-HTT1800 provides Pt NPs in the
range of 0.9 to 1.9 nm after a sophisticated pyrolysis procedure
at 10008C under nitrogen. For a 70:30 PEOH/HTT 1800 ratio,
surface area of the fabricated material increased with the
metal content. The four catalysts were considered as potential
catalysts for H2 generation by hydrolysis of NaBH4. All showed
attractive performance with, for example, rates up to
À1), validating their high potential as heteroge-
À1
38.1 Lmin
g
Pt
neous catalysts. Further works are now required to optimize
features like Pt content, surface structure, and accessibility of
all the Pt NPs.
Experimental Section
Figure 6. Volume of hydrogen (in mL) measured during the hydrolysis of
sodium borohydride over the mass of nanocomposites containing the same
amount of Pt (0.35 mg) at 808C. Under these conditions, the amount (in mg)
of nanocomposite used was different.
[Pt(ApTMS)2] (1): To a solution of ligand (ApTMSH) (1.45 g, 8.01 mmol)
in diethylether (30 mL), n-buthyllithium (0.51 g, 8.01 mmol) was
added at 08C. After stirring for 2 h at room temperature, the solu-
tion was added to a suspension of [PtCl2(cod)] (1.50 g, 4.01 mmol)
in diethylether (200 mL) at 08C. The yellow mixture was allowed to
warm to room temperature and was stirred for 24 h. After filtration
and crystallization, yellow crystals were obtained. Yield 85%
(1.92 g, 3.41 mmol). 1H NMR (300 MHz, [D6]benzene, 296 K, TMS):
d=7.64 (d, J(H,H)=5.7 Hz, 2H; NCH), 5.62 (s, 2H; arom. H), 5.60 (d,
J(H,H)=5.4 Hz, 2H; arom. H), 1.74 (s, 6H; CH3), 0.35 ppm (s, 18H;
Si(CH3)3). 13C NMR (75 MHz, C6D6, 296 K, TMS): d=179.06, 148.51,
143.07, 112.17, 108.51, 21.58, 1.99 ppm.
structuring of the nanocomposite influences the accessibility
of the Pt NPs. The most efficient sample is Pt@SiCN60, fol-
lowed by Pt@SiCN40, Pt@SiCN20, and Pt@SiCN10. This is the
reverse trend compared to the results in Figure 6. Given that
the weight of Pt is similar for all of these experiments, this
result suggests that in samples with high Pt loading part of Pt
NPs are not accessible for catalysis. For example, the compari-
son of Pt@SiCN60 and Pt@SiCN10 reveals hydrogen genera-
tion rates of 13.4 and 2.5 mLminÀ1 (knowing that the weight
of Pt is identical). This could also be attributed to the possible
increase in the size of metal NPs (agglomeration) with increas-
ing metal content, thereby decreasing the surface area of the
active Pt NPs and also blocking the porosity due to bigger
sizes.
Synthesis of the catalysts: In a glass vial contained in a Schlenk
tube, PEOH (233 mg) was dissolved in toluene (5 mL) at 1108C and
HTT1800 (100 mL) was added (PEOH:HTT 1800=70:30 by weight).
The mixture was held at 1108C under static conditions for one
hour. Afterwards, Pt complex (amounts as given in Table T2 in the
Supporting Information) dissolved in toluene (1 mL) and dicumyl
peroxide (4.5 mg) were added to initiate cross-linking. The mixture
was held at this temperature overnight (12 h) and slowly cooled to
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room temperature. Subsequently, the solvent was removed under
reduced pressure and samples were placed in an oven for pyrolysis
at 10008C under nitrogen. Pyrolysis conditions: The heating rate
was 1 KminÀ1 from room temperature to 10008C and samples
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.
Catalysis: The catalytic ability of the catalysts was assessed for hy-
drogen production by hydrolysis of sodium borohydride and the
hydrogen generation measurement was performed according to
the general procedure we reported elsewhere.[12] In the first of
a series of hydrolysis experiments, the same amount of nanocom-
posite (16 mg) was milled and sieved, and then transferred into
a reactor (glass tube) that was sealed with a silicon septum. The re-
actor was connected to a water-filled inverted burette (water col-
ored in blue), via a cold trap for steam, maintained at 08C. Then,
the reactor was immersed in an oil bath kept at 808C. To start the
hydrogen generation, 1 mL of a stabilized (2 wt% NaOH) aqueous
solution of NaBH4 (120 mg) was injected into the nanocomposite.
Hydrogen started to evolve immediately and rapidly, and the evo-
lution was video-recorded for 2 h, to be computationally analyzed
after the experiment. In a second series of experiments, we com-
pared the nanocomposites while taking into account the same
quantity of Pt (0.35 mg). A third series of experiments was carried
out to determine the apparent activation energy (AAE) of the four
catalytic systems. The hydrolysis was carried out at 50, 60, and
708C for all four systems (in addition to the experiment at 808C
described above). The hydrogen generation rates in the first
30 min for each experiment were calculated from the slope of the
hydrolysis curve. The rates at each temperature were plotted with
the inverse of the temperature and the Arrhenius equation was
used to calculate the AAE.
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Acknowledgements
The authors are grateful to Beate Bojer and Dr. Wolfgang
Milius for powder XRD measurements. Dr. Germund Glatz and
Dr. Awal Noor are acknowledged for single crystal X-ray analy-
sis and structure solution (X-ray). The team at IEM gratefully ac-
knowledges the financial contribution from CEFIPRA agency
(project N85108-1). R.K. acknowledges the support of the Deut-
sche Forschungsgemeinschaft, DFG, SFB 840 B1.
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Keywords: block copolymers
·
ceramics
·
mesoporous
materials · self-assembly · supported catalysts
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[1] a) P. Colombo, G. Mera, R. Riedel, G. D. Sorarꢃ, J. Am. Ceram. Soc. 2010,
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Received: July 9, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
5
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
&
Self-Assembly
S. M. Sachau, M. Zaheer,* A. Lale,
M. Friedrich, C. E. Denner, U. B. Demirci,
S. Bernard, G. Motz, R. Kempe*
&& – &&
Hierarchically porous SiCN ceramics
with surface Pt NPs were fabricated by
the self-assembly and subsequent pyrol-
ysis of an in situ generated block copo-
lymer whose inorganic block (Si-N back-
bone) was modified with PtII ions. The
as-obtained catalysts were tested for
the hydrolysis of alkaline sodium boro-
hydride.
Micro-/Mesoporous Platinum–SiCN
Nanocomposite Catalysts (Pt@SiCN):
From Design to Catalytic Applications
&
&
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
6
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!