Highly monodisperse supported metal nanoparticles by basic ammonium functionalization of mesopore walls for industrially relevant catalysis

Article abstract of DOI:10.1039/c7cc00479f

A strategy for a high dispersion of metal/metal oxide nanoparticles in a mono-modal fashion is developed. The key is the functionalization of mesopore walls with basic-C₃H₆-N⁺(Me)₃(OH)^? groups. The supported metal catalysts obtained in the present work exhibit high catalytic activities for CO₂ methanation at low temperature and CO oxidation.

Full text of DOI:10.1039/c7cc00479f

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Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC00479F
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Highly monodisperse supported metal nanoparticles by basic
ammonium functionalization of mesopore walls for industrially
relevant catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Jangkeun Cho,ab Leilei Xu,a Changbum Jo,*a and Ryong Ryoo*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
A strategy for high dispersion of metal/metal oxide nanoparticles conditions for post-treatment. Therefore, high metal loading in
in mono-modal fashion is developed. The key is the the form of tiny nanoparticles still remains a great challenge.
a
functionalization of mesopore walls with basic –C3H6-N+(Me)3(OH)- Perhaps a strategy to direct the nucleation process specifically
groups. The supported metal catalysts obtained in the present work onto pore walls rather than in the solution would be necessary
exhibit high catalytic activities for CO2 methanation at low to solve this problem.
temperature and CO oxidation.
Supported metal nanoparticles present many advantages as
catalysts.1,2 Supported non-noble metals (e.g., Co, Ni, Cu, etc.)
are attracting increased attention due to their low costs and the
potential to replace noble-metal based catalysts in industrially
relevant catalytic applications.3,4 However, their catalytic
activity is normally much lower than that of noble-metal
catalysts.5,6 To compete with noble metals, the non-noble
metals have to be supported at a high loading (typically, 10 - 30
wt%).7 At such a high level of loading, metal dispersion becomes
very poor because of agglomeration of metal particles. Hence,
high loading with high dispersion is an important issue for non-
noble metal catalysts.
Deposition-precipitation processes are widely used to achieve
high metal dispersion at high loading.8,9 The method induces
precipitation of one or more metal precursors using a base. The
key point of this method is how to cause the precipitation of
Scheme 1 Selective precipitation of the metal nitrate precursors on the mesopore walls
can be realized by functionalization of mesopore walls with -C3H6-N+(Me)3(OH)- groups.
Soluble metal precursors can be converted to insoluble metal hydroxyl species.
metal precursors selectively within the pores of the support. In
Herein, we present a strategy to solve the poor dispersion
problem of non-noble metal catalysts at high loadings ( 25
wt%). The strategy is to induce a regioselective nucleation of
inorganic precursors on the pore walls via surface
functionalization with basic groups. One representative basic
most cases, to achieve this, the base component is slowly added
into a precursor solution to control nucleation and growth
processes as homogeneously as possible.10,11 Bezemer et al.
used urea as a slow precipitating agent, which was gradually
decomposed into ammonia at 90 °C.12 However, the precursor
precipitation is still very sensitive to other details, such as the
intrinsic nature of metals, surface nature of supports, and
∼
group
is
quaternary
ammonium,
Si(OC2H5)3-(CH2)3-
N+(CH3)3(OH)-, which can be grafted onto the surface silanols
through a simple silylation reaction. The ammonium group
increases the basicity of the porous environment, causing
nucleation or local precipitation of metal hydroxide even at a
low concentration of the precursor species (Scheme 1). The
precipitated metal hydroxide is converted to oxide (e.g., NiO,
CuO or SnO2) nanoparticles in situ inside the pores upon air
calcination at high temperatures. These nanoparticles can be
a.Center for Nanomaterials and Chemical Reactions, Institute for Basic Science
(IBS), Daejeon 34141, Republic of Korea. E-mail: jochangbum@kaist.ac.kr,
rryoo@kaist.ac.kr
b.Department of Chemistry, KAIST, Daejeon 34141, Republic of Korea.
†Electronic Supplementary Information (ESI) available: [Experimental procedures,
XRD, sorption, NMR, STEM, chemisorption, catalytic stability results.] See
DOI: 10.1039/x0xx00000x
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used as highly dispersed oxide catalysts, or metal catalysts after reflections corresponding to nickel oxide hydroxiVdieew[APrtDiclFe Ocnalirnde
proper reduction with H2. In the present work, we demonstrate No. 000130229]15 and nickel hydroxDiOdeI: 10[.P10D3F9/Cc7CarCd004N79oF.
that the strategy can be applied to various mesoporous 010768988].16 We confirmed that the Ni species were located
supports that are crystalline or amorphous, such as mesoporous within the porous AF-SZN support even at 25 wt% Ni loading,
MFI zeolite nanosponge, MCM-48 mesoporous silica, and using scanning transmission electron microscopy (STEM)
mesoporous γ-alumina.
images. On the other hand, when the pristine (i.e., un-
functionalized) SZN zeolite was impregnated with the same Ni
precursor, no XRD peaks corresponding to nickel hydroxides
appeared. Hence, we believe that the Ni precursor was
impregnated as an amorphous phase. STEM images of the
sample exhibited Ni species impregnated outside the porous
zeolite matrix. As this result shows, ammonium
functionalization was very effective in controlling the
impregnation into the porous matrix. The loading of 25 wt% Ni
is too high to explain by simple surface adsorption phenomena
that can be induced by electrostatic or van der Waals
interactions. As we proposed above, the base functional groups
could cause a sharp increase in the local pH. As a result, the
deposition-precipitation of nickel hydroxide could selectively
occur inside the pores.17 In fact, a pH increase because of
ammonium was confirmed by the purple colour of the
functionalized zeolite when phenolphthalein solution was used
as an indicator (Fig. S4).
After the incipient wetness impregnation, the Ni precursor on
zeolite was converted to Ni oxide by calcination in air.
Subsequently, the Ni oxide was reduced to Ni metal
nanoparticles using H2. The resultant sample is denoted as x-Ni-
AF-SZN or x-Ni-SZN, where x indicates the Ni wt%. The metal
loading was determined by inductively coupled plasma atomic
emission spectroscopy. Fig. 1 shows the XRD patterns and STEM
images of 15-Ni-AF-SZN and 15-Ni-SZN. Both samples exhibited
XRD peaks corresponding to the structure of the MFI zeolite
Fig. 1 XRD pattern (A, bottom), STEM image (B), and SAED pattern (D) for the 15 wt% Ni-
supported AF-SZN sample. The XRD pattern (A, top) and STEM image (C) for the 15 wt%
Ni-supported SZN sample are displayed for comparison. The XRD peaks of metallic Ni are
marked with an inverted triangle.
A siliceous (aluminium-free) form of MFI zeolite nanosponge
(SZN) was chosen as a representative support.13,14 The SZN
sample was base-functionalized by the reaction with Si(OC2H5)3-
(CH2)3-N+(CH3)3(OH)- (see Section S1 in Electronic
Supplementary Information (ESI†) for experimental details).
nanosponge (Fig. 1) and two additional peaks centred at 45
52 . These peaks could be assigned to the (111) and (200) peaks
° and
°
of metallic Ni. The Ni metal peaks in the case of 15-Ni-AF-SZN
were much lower in intensity than those for 15-Ni-SZN. This
result indicated a much better Ni dispersion into the tiny metal
nanoparticles in the case of the ammonium-functionalized
zeolite support. High dispersion was further clarified by the
scanning transmission electron microscopy (STEM) images (Figs.
1B and 1C). In the case of 15-Ni-AF-SZN, the STEM image
showed highly dispersed Ni nanoparticles with a narrow size
distribution centred at 2 nm (Fig. 1B). The selected area
diffraction pattern (SAED) image indicated that the Ni
nanoparticles were crystalline (Fig. 1D). The particle diameters
were much larger than the zeolite micropore diameters (0.6
nm). Considering that the mesopore diameters were
approximately 4.5 nm, it was reasonable that the Ni
nanoparticles were located inside the zeolite mesopores. This
was also consistent with a significant decrease of the mesopore
volume from 0.44 cm3 g-1 to 0.33 cm3 g-1 (Table S1).
1
The organic functionalization was confirmed by solid-state H-
NMR spectroscopy (Fig. S1). The maximal functionalization level
was 1.0 mmol g-1, based on elemental analysis (EA). The
ammonium-functionalized SZN, which is denoted as AF-SZN,
exhibited no significant changes in the pore diameter and
volume (Fig. S2). In this step, although the zeolite was
functionalized with ammonium hydroxide, the hydroxyl content
was not high enough to complete the precipitation of nickel
hydroxide when all the Ni(NO3)2 was added at once. Hence,
prior to the addition, the Ni(NO3)2 solution was added with a
proper amount of NaOH solution, so that it could increase pH to
a point just below where precipitation began. The amount of
NaOH solution was determined by a precipitation titration
(experimental details in Section S1 of ESI†). After the pH
adjustment, the solution was impregnated into AF-SZN by the
incipient-wetness method.
On the contrary, the Ni nanoparticles supported on the SZN
samples exhibited a poor dispersity with a broad distribution of
particle size ranging from 3 to 15 nm (Fig. 1C). Among the Ni
nanoparticles, the particles larger than the mesopore diameter
The AF-SZN sample after nitrate impregnation exhibited new
peaks in the X-ray powder diffraction (XRD) analysis, in addition
to peaks corresponding to the structure of the MFI zeolite
nanosponge (Fig. S3). These peaks could be assigned to
(∼
4.5 nm) in size are expected to be mostly located on the
particle surface of the SZN supports.
2 | J. Name., 2012, 00, 1-3
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The Ni loading on AF-SZN could be increased to 25 wt%
without a critical increase in the particle size or metal dispersion.
STEM images of the 25 wt% sample (i.e., 25-Ni-AF-SZN) still
exhibited a narrow distribution of Ni particle diameters
although the mode value increased to 3.5 nm. At 30 wt%,
however, much larger Ni particles than 5 nm were detected (Fig.
S5). The XRD peaks corresponding to the metallic Ni phase also
became quite a distinct feature (Fig. S6). This result indicates
that a progressively increasing amount of Ni precursor was
deposited outside the zeolite mesopores as the loading was
increased beyond 25 wt%. This phenomenon can be attributed
to blocking if an exceedingly large amount of Ni precursor is
added to the zeolite even when the pore walls are
functionalized with ammonium. The particle size trend
depending on the Ni content and functionalization was further
confirmed by CO and hydrogen chemisorption (detailed
discussion in Sections S3 of ESI†).
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DOI: 10.1039/C7CC00479F
Fig. 3 STEM images of 20 wt% Cu supported AF-MCM-48 (A) and AF-γ-Al2O3 (B).
The present protocol can be used on other supports like
amorphous mesoporous silica (e.g., MCM-48) and mesoporous
γ-alumina (PURALOX® SBa 200, Sasol). The supports were
treated with Si(OC2H5)3-(CH2)3-N+(CH3)3(OH)- in anhydrous
ethanol for basic ammonium functionalization. The
functionalized-ammonium group content was determined to be
1 mmol g-1 for MCM-48, and 0.3 mmol g-1 for γ-Al2O3. The
organosilane could be covalently linked to the mesopore walls
through the formation of Si-O-M (M = Si, Al) bonds.19 The
resultant materials are denoted as AF-MCM-48 and AF-γ-Al2O3,
respectively. The procedure for loading Cu nanoparticles was
the same as that for 20-Cu-AF-SZN. Figs. 3A and 3B show the
representative STEM images of 20-Cu-AF-MCM-48 and 20-Cu-
AF-γ-Al2O3. The results clearly showed that Cu nanoparticles
were homogeneously supported on AF-MCM-48 and AF-γ-Al2O3.
It is noteworthy that the particle size distribution of the
supported Cu nanoparticles was influenced very little by the
type of supporting material (Fig. S7). In particular, the textural
properties (e.g., mesopore size distribution, specific surface
area, and pore volume) of SZN, MCM-48, and γ-Al2O3 samples
were very different (Fig. S8). Thus, the high dispersion of metal
nanoparticles with similar size distributions cannot be directly
linked to the mesopore diameter of the supports. In contrast to
the ammonium-functionalized supports, pristine MCM-48 and
mesoporous γ-alumina exhibited poor dispersion of Cu
nanoparticles at 20 wt% metal loading (Fig. S9).
Fig. 2 (A,B) XRD patterns (bottom) and (C, D) STEM images of 20 wt% Cu and 25 wt%
SnO2 supported AF-SZN samples. (A,B) XRD patterns (top) of 20 wt% Cu and 25 wt% SnO2
supported SZN samples are also displayed for comparison. The XRD peaks of metallic Cu
and SnO2 are marked with an inverted triangle and a triangle.
To demonstrate the versatility of our approach, we tested the
same procedure for other kinds of metals or metal oxides. To
this end, we supported 20 wt% Cu and 25 wt% SnO2
nanoparticles on AF-SZN samples (denoted by 20-Cu-AF-SZN
and 25-SnO2-AF-SZN, respectively). Cu(NO3)2·3H2O and
SnCl4·5H2O were used as the metal precursors (experimental
details in Section S1 of ESI†). Figs. 2A and 2B show the XRD
patterns for Cu and SnO2 supported on AF-SZN samples (20-Cu-
AF-SZN and 25-SnO2-AF-SZN). Average particle sizes calculated
from XRD patterns using the Scherrer equation18 were 4.5 nm
for Cu and 2.5 nm for SnO2. STEM images clearly showed that
copper and tin oxide were highly dispersed on AF-SZN supports
with narrow particle size distributions (Figs. 2C and 2D). In
Fig. 4 (A) Conversion of CO2 to methane over Ni supported on various kinds of support
materials (Reaction conditions: GHSV = 17000 h-1, H2:CO2 molar ratio = 4:1, 300 °C, 1
atm). (B) Rate of CO conversion over Cu/CeO2 supported catalyst and Pt/CeO2 supported
catalyst (reaction conditions: 130 °C, 50 mg of catalyst, 73 mL min-1 of gas flow rate with
contrast, the dispersion of Cu and SnO2 nanoparticles over the a molar composition of CO/O2/H2/He = 0.7/0.8/63.0/34.9, 1 atm).
pristine SZN support with identical amounts of loading exhibited
only poor dispersion (Figs. 2A and 2B, top), as confirmed by XRD.
This journal is © The Royal Society of Chemistry 20xx
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In order to investigate the effect of different supports on the interfacial area because two or more inorganic sVaieltwsAcrtoiculelOdnlbinee
catalytic properties of Ni, a series of 15 wt% Ni-based catalysts co-precipitated once they were in close DpOroI:x1i0m.1i0t3y9t/oC7tChCe00b4a7s9iFc
supported on AF-SZN, AF-MCM-48, and AF-γ-Al2O3 were groups.
prepared. All the samples exhibited a similar distribution of
This work was supported by IBS-R004-D1. Dr Jo and Mr. Cho
particle diameters in the STEM images (Fig. S10). The CO thank to Pavla Eliášová and Venkatesan Chithravel for valuable
chemisorption indicated a similar portion of surface Ni particles discussions.
in all the three samples (see Sections S3 of ESI†). We chose the
hydrogenation of CO2 to methane as a probe reaction since Ni
base materials are typical catalysts for this reaction. We used
Notes and references
the CO2 conversion in 1 h under the reactant gas flow at 300 °C.
The CO2 conversion in Fig. 4A shows a remarkable difference
depending on the surface functionalization and supporting
frameworks: 15-Ni-AF-γ-Al2O3 (63%), 15-Ni-AF-SZN (46%), 15-
Ni-AF-MCM-48 (37%) and 15-Ni-SZN (18 %). As this result shows,
the alumina-supported nickel catalyst showed the highest
catalytic conversion, which can be attributed to strong metal-
support interactions. On the other hand, the mesoporous silica
support, which does not have strong metal-support interactions,
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a
low catalytic conversion.20 Compared to the
mesoporous silica, the ammonium-functionalized zeolite
catalyst (15-Ni-AF-SZN) is still in a high-purity silica form.
Nonetheless, the zeolite catalyst exhibited quite a high CO2
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The merit of the ammonium functionalization was also
remarkable when Cu and CeO2 were sequentially impregnated
on porous supports. We supported 20 wt% Cu and 10 wt% CeO2
together on AF-SZN, and measured its catalytic activity for
preferential oxidation of CO (experimental details in Section S1
of ESI†). We used the CO conversion rate in 1 h under the
reactant gas flow at 130 °C. The Cu/CeO2-AF-SZN catalyst
achieved a conversion rate of approximately 15.2 CO mmol g-1
h-1. Compared to the results obtained from a previous report (5
wt% Cu/10 wt% CeO2-Al2O3), this value is five times higher at
the same reaction temperature.21 Even, their catalytic activity is
three times higher than that of 1 wt% Pt/10 wt% CeO2-AF-SZN
(Fig. 4B). Such a high catalytic activity can be attributed to a
large number of surface active sites of high dispersion of
supported Cu.
In conclusion, we demonstrated a high dispersion of supported
metals (and metal oxides) by basic group-functionalization on
various porous materials. The supported non-noble metal
catalysts obtained here demonstrated the potential to replace
noble-metal-based catalysts in the CO oxidation reaction. In
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catalysts. In addition, Gartmann et al. showed that functional
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the spatial distribution of supported metal particles is
controlled by the location of functional groups in porous
materials. Furthermore, the present method allows for the
rational synthesis of multicomponent catalysts with a high
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Table of contents graphics:
A strategy for achieving high dispersion of metal and metal oxide
nanoparticles in a mono-modal fashion is developed.
This journal is © The Royal Society of Chemistry 20xx
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