New Architectures for Designed Catalysts: Selective Oxidation using AgAu Nanoparticles on Colloid-Templated Silica

Article abstract of DOI:10.1002/chem.201704552

A highly modular synthesis of designed catalysts with controlled bimetallic nanoparticle size and composition and a well-defined structural hierarchy is demonstrated. Exemplary catalysts—bimetallic dilute Ag-in-Au nanoparticles partially embedded in a porous SiO₂ matrix (SiO₂–Ag_xAu_y)—were synthesized by the decoration of polymeric colloids with the bimetallic nanoparticles followed by assembly into a colloidal crystal backfilled with the matrix precursor and subsequent removal of the polymeric template. This work reports that these new catalyst architectures are significantly better than nanoporous dilute AgAu alloy catalysts (nanoporous Ag₃Au₉₇) while retaining a clear predictive relationship between their surface reactivity with that of single-crystal Au surfaces. This paves the way for broadening the range of new catalyst architectures required for translating the designed principles developed under controlled conditions to designed catalysts under operating conditions for highly selective coupling of alcohols to form esters. Excellent catalytic performance of the porous SiO₂–Ag_xAu_y structure for selective oxidation of both methanol and ethanol to produce esters with high conversion efficiency, selectivity, and stability was demonstrated, illustrating the ability to translate design principles developed for support-free materials to the colloid-templated structures. The synthetic methodology reported is customizable for the design of a wide range of robust catalytic systems inspired by design principles derived from model studies. Fine control over the composition, morphology, size, distribution, and availability of the supported nanoparticles was demonstrated.

Full text of DOI:10.1002/chem.201704552

A Journal of
Accepted Article
Title: New architectures for designed catalysts: Selective oxidation
using AgAu nanoparticles on colloid-templated silica
Authors: Cynthia M. Friend, Tanya Shirman, Judith Lattimer, Mathilde
Luneau, Elijah Shirman, Christian Reece, Michael Aizenberg,
Robert Madix, and Joanna Aizenberg
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201704552
Link to VoR: http://dx.doi.org/10.1002/chem.201704552
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Chemistry - A European Journal
10.1002/chem.201704552
New architectures for designed catalysts: Selective oxidation using AgAu
nanoparticles on colloid-templated silica
Tanya Shirman,§1 Judith Lattimer,§ Mathilde Luneau, Elijah Shirman, Christian
,2
3
3
1,2
3
2
1
1,2,3
Reece, Michael Aizenberg, Robert J. Madix, Joanna Aizenberg,*
and Cynthia M.
1,3
Friend*
1
John A. Paulson School of Engineering and Applied Sciences, Harvard University,
2
Cambridge, MA 02138, USA. Wyss Institute for Biologically Inspired Engineering,
Harvard University, Cambridge, MA 02138, USA. Department of Chemistry and
3
Chemical Biology, Harvard University, Cambridge, MA 02138, USA.
*
Correspondence and requests for materials should be addressed to J.A. (email:
jaiz@seas.harvard.edu) and C.M.F. (friend@fas.harvard.edu).
§These two authors contributed equally to this paper.
Abstract: A highly modular synthesis of designed catalysts with controlled bimetallic
nanoparticle size and composition and a well-defined structural hierarchy is
demonstrated. Exemplary catalysts—bimetallic dilute Ag-in-Au nanoparticles partially
embedded in a porous SiO
2
matrix (SiO
2
-Ag
x
Au )—were synthesized by the decoration
y
of polymeric colloids with the bimetallic nanoparticles followed by assembly into a
colloidal crystal backfilled with the matrix precursor and subsequent removal of the
polymeric template. We show that these new catalysts architectures are significantly
better than nanoporous dilute AgAu alloy catalysts (nanoporous Ag0.03Au0.97) while
retaining a clear predictive relationship between their surface reactivity with that of single
crystal Au surfaces. This paves the way for broadening the range of new catalyst
architectures required for translating the designed principles developed under controlled
conditions to designed catalysts under operating conditions for highly selective coupling
of alcohols to form esters. Excellent catalytic performance of the porous SiO
2
-Ag
x
Au
y
structure for selective oxidation of both methanol and ethanol to produce esters with high
conversion efficiency, selectivity, and stability was demonstrated, illustrating the ability to
translate design principles developed for support-free materials to the colloid-templated
structures. The synthetic methodology reported is customizable for the design of a wide
range of robust catalytic systems inspired by design principles derived from model
studies. Fine control over the composition, morphology, size, distribution and availability
of the supported nanoparticles was demonstrated.
Heterogeneous catalysis is of paramount importance for contemporary chemical and
energy industries. Hence, the goal of “catalysis by design” has emerged as an important
research field that derives inspiration from model studies.[1] There are reasons for high
interest in designing bimetallic catalytic materials with specific compositions and
structural motifs. First, bimetallic nanoparticles often have chemical properties that are
distinct from either individual metal; thus, presenting tremendous potential for creating
1
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Chemistry - A European Journal
10.1002/chem.201704552
new, efficient catalysts.[2] Second, they are known to provide a clear predictive
relationship between their surface reactivity with that of single crystal Au surfaces
enabling the ability to translate design principles from controlled conditions to operating
[
3]
conditions.
Herein, we demonstrate a modular and versatile approach for the design of an efficient
bimetallic catalyst system based on the synthesis of a porous framework of silica with
embedded bimetallic AgAu nanoparticles of uniform size and composition. This specific
system establishes that catalytic activity and selectivity observed in support-free
[
1a, 3-4]
nanoporous Ag
3
Au97 alloys
translates to the embedded nanoparticles; thus,
demonstrating that the modular synthesis process can be used to more generally test
design principles. Indeed, the AgAu nanoparticles embedded in a colloid-templated
porous (CTP) SiO
alcohols, similar to nanoporous Ag
2
is an active, selective, and robust catalyst for oxidative coupling of
Au97. The synthetic procedure is a generalizable
3
platform for the fabrication of a tailored catalysts that enables an optimal design of its
composition and morphology.
The fabrication employs modification of colloids with metal nanoparticles, assembly of
these “raspberry” colloids into a colloidal crystal monolith and backfilling of the
interstitials with a metal oxide matrix precursor material, followed by heat treatment to
remove the polymeric template (Scheme 1). This method enables the fabrication of
highly porous structures with contiguous, interconnected pores, uniformly decorated with
metal nanoparticles. Due to their high modularity, CTP materials can be designed with
any size and composition of nanoparticles and customized to have specific pore sizes
[
5]
defined by the size of the sacrificial colloids. This approach leads to improved
accessibility and stability of the nanoparticles, which are partially grafted into the matrix
surface, preventing their sintering and surface diffusion.
2
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10.1002/chem.201704552
(i)
(ii)
+
Assembly
Matrix precursor
Template
colloid
Metal
nanoparticles
“
Raspberry”
colloid
Template
removal
(iii)
metal
oxide
pore
ü Activity
ü Selectivity
ü Stability
pore
nano-
particle
ü Mass transport
nanoparticles
SiO2
2 nm
CTP metal oxide-nanoparticles
Scheme 1. Schematic of the synthesis of metal nanoparticles embedded in colloid-templated
porous materials, with emphasis on the fully accessible metal nanoparticles placed at the
pore/matrix interface and summarizing the advantageous properties provided by this type of
material for the design of an efficient heterogeneous catalyst. Inset in the bottom shows a TEM
image of a nanoparticle partially entrenched into the silica matrix.
The AgAu bimetallic nanoparticles were synthesized via galvanic replacement using a
[
6]
modification of published procedures. The average size of the Ag
x
Au nanoparticles
y
increased with the increasing Au content, as reported previously (Figure 1A,B, S1; Table
S1).[
6b, 7]
The composition of the nanoparticles was determined using inductively coupled
plasma mass spectrometry (ICP-MS). Silver is concentrated on or near the surface of
the nanoparticles based on Scanning Transmission Electron Microscopy-Energy
Dispersive X-ray Spectroscopy (STEM-EDS) measurements (Figure 1C, S2); the
centers of individual AgAu nanoparticles are enriched in Au, based on the cross-section
scan (Figure S3).
The AgAu nanoparticles were attached to the surface of polystyrene colloids (385 nm) to
form composite “raspberry” colloids, which were used as sacrificial templates in the
synthesis of CTP silica catalyst (Scheme 1, Figures 1D and S4). The material resulting
from the CTP synthesis has AgAu nanoparticles incorporated into a macroporous silica
matrix with 375±5 nm voids interconnected by ~80 nm windows. The AgAu nanoparticles
were homogeneously distributed at the interface (Figure 1E, S5, S6). The remarkable
robustness and high-temperature stability of the material are demonstrated by the fact
that the size and distribution of the metal nanoparticles was preserved after calcination
at 500 °C in air (Figure 1F, S6, Table S1). In contrast, nanoparticles covalently
immobilized on the surface of preformed CTP SiO
2
structures rapidly sinter upon
3
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Chemistry - A European Journal
10.1002/chem.201704552
calcination of the structure leading to an increase in the nanoparticle diameter from
.2±0.4 to 19.3±5.8 nm (Figure S7). Together, these results demonstrate that partial
4
embedding of the nanoparticles in the silica impart a high degree of thermal stability to
the catalyst structure.
Figure 1 TEM images of: (A) Ag50Au50 nanoparticles and (B) Ag10Au90 nanoparticles. (C) STEM-
EDS map of Ag10Au90 nanoparticles. D) TEM image of raspberry colloids PS@Ag10Au90. (E) SEM
image of CTP SiO
2
-Ag10Au90 (white dots are metal nanoparticles). (F) STEM-EDS map of CTP
SiO -Ag10Au90, showing uniform size and metal distribution after calcination at 500 °C in air.
2
The CTP SiO
2
-Ag
x
Au
y
catalysts required activation by ozone followed by flowing
reactants prior to driving the oxidative coupling of methanol in a process similar to that
[
1a]
for free-standing nanoporous Ag
3
Au97 (npAu).
As-prepared catalysts had very low
activity (Figure 2A); however, once activated, the catalysts selectively transformed
methanol to form methyl formate (Figure 2B, Table 1). Typically, the methyl formate yield
increased smoothly over ~7 h following ozone treatment, and was stable thereafter for
over 40 h, the end of the testing period (Figure 2B). For example, CTP SiO
stabilized at 35% conversion of methanol with a selectivity of 90% for methyl formate
Figure 2B). The only other product detected was CO . Conversion and selectivity
2
-Ag10Au90
(
2
calculations can be found in the SI. Understanding the catalyst activation will require
further study. Similar activation of npAu leads to enrichment of the surface in Ag to form
[
8]
an alloy that triggers the dissociation of O
2
.
Further study is required to fully
understand the underlying reasons for the induction period.
4
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A
B
2
1
1
0
5
0
5
0
1
00
80
100
80
selectivity
ozone treated
CO2
60
60
preozone
afterozone
MF
selectivity
methyl formate
40
40
as prepared
20
20
CO2
0
0
0
5
10
15
0
10
20
30
40
Time on stream (h)
Time on stream (h)
Figure 2 (A) Conversion of methanol to methyl formate using CTP SiO
wt. 4.4 mg) as prepared (closed squares) and after ozone treatment (open squares). (B)
Conversion of methanol to methyl formate (MF, black squares) and carbon dioxide (CO , black
circles) and selectivity for methyl formate (blue triangles) using CTP SiO -Ag10Au90 (nanoparticle
wt. 3.3 mg) immediately following ozone treatment. Reaction conditions: 6 mol% methanol and 20
mol% O in He at flow rate 50 ml/min at 423 K.
2
-Ag10Au90 (nanoparticle
2
2
2
There was no conversion of methanol detected for the CTP SiO matrix without metal
2
nanoparticles (Figure S8). The most active catalyst composition among those screened
exhibited nearly five times the activity per unit weight of metal compared to the best-
performing npAu under the same conditions, albeit with slightly lower selectivity (Table 1,
Figure S9). The lower selectivity may be due to the higher Ag content in the CTP SiO -
2
[
1a]
Ag
x
Au
y
catalysts (10 mol% in the AgAu nanoparticles vs 1-3 mol% in npAu). Previous
(Table S2).[4]
work on npAu with 10 mol% Ag showed only complete combustion to CO
2
The generality of the oxidation catalysis driven by CTP-Ag10Au90 was demonstrated by
the activity for oxidative coupling of methanol and ethanol (Figure 3, Table 1). Only
esters and acetaldehyde were produced; no CO
2
was detected.
The product
distributions measured as a function of methanol mole fraction showed that excess
methanol (~80%) is required for maximum production of methyl acetate, similar to
[
9]
npAg0.03Au0.97 under similar conditions.
The dependence of the selectivity for
acetaldehyde production decreased both as the total alcohol concentration increased
from 2 to 10% and as the temperature was increased up to 240 °C (Figure S10), also
[
9]
analogous to npAg0.03Au0.97. There are, though, differences between the catalysts: The
CTP-Ag10Au90 produced significantly more acetaldehyde[
9]
and the rate of ethanol
-1
-1
conversion was 3-4 times higher for the CTP SiO -Ag10Au90 catalyst (0.018 mmol g s )
2
-1
-1
than for npAu under the same conditions (0.004 mmol g s , Table S2). Further study is
required to understand these differences in detail, although the compositional difference
most likely plays a role.
5
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A
100
8
0
0
0
0
0
6
4
2
acetaldehyde
esters (total)
0
.0
0.2
0.4
0.6
0.8
1.0
Methanol mole fraction
B
100
8
0
0
0
0
0
6
4
2
methyl formate
methyl acetate
ethyl acetate
0.0
0.2
0.4
0.6
0.8
1.0
Methanol mole fraction
Figure 3. The general ability for the CTP-SiO -Ag10Au90 to catalyze alcohol oxidation is
2
demonstrated by the activity for selective oxidation of mixtures of methanol and ethanol. The
dependence of selectivity on the methanol mole fraction for oxidation of various mixtures of
methanol and ethanol over CTP-SiO
shows that a high methanol mole fraction is required to maximize the formation of the methyl
ester. This behavior is qualitatively similar to the performance of nanoporous Ag Au97. (A) The
2
-Ag10Au90 (nanoparticle wt. 5.7 mg) using O
2
as an oxidant
3
selectivity for formation of the products: acetaldehyde and esters (the sum of methyl formate,
methyl acetate, and ethyl acetate). (B) Relative selectivity for production of the various esters
(methyl formate, ethyl acetate, and methyl acetate). Reaction conditions: 6% total alcohol, 20%
oxygen in He, 50 ml/min flow, 423 K. No CO is detected in any of these experiments.
2
The long-term stability of CTP SiO -Ag10Au90 was demonstrated by assessing its activity
2
by repeated measurements over a period of 6 months (Figure 4A). Although the sample
was occasionally removed from the reactor and exposed to ambient conditions, no
additional ozone activation was required: the sample was only activated once—at the
beginning of the first experiment. After reintroduction into the reactor, selectivity
6
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Chemistry - A European Journal
10.1002/chem.201704552
decreased somewhat, but was largely recovered under reaction conditions after 24 h.
Consistent with this stable catalytic performance, there were no significant morphological
or compositional changes detected by SEM, TEM and ICP-MS after catalysis (Figure
S11, Table S1). The catalytic performance at higher temperatures (up to 240 °C)
showed increased methanol conversion accompanied by a drastic decrease in selectivity
for methyl formate (Figure S12). The fact that the catalyst performance does not change
over long periods of time, without special efforts to store it in a controlled atmosphere
and without the need to reactivate between uses, is in and of itself a very valuable result.
Table 1 Reaction rates and selectivities of npAu and CTP SiO
-AgAu catalysts.a
2
Rate of methanol
Selectivity to methyl
Catalyst compositionb
conversion,
c
formate,
c
-
1
-1
mmol s g
%
npAu
foils
0.055
95
Ag10Au90
0.23
0.07
0.02
90
CTP SiO
2
Ag20Au80
Ag50Au50
100
100
a
Catalysts were treated with flowing ozone and then kept in reaction conditions for at least 24 h to ensure stability, and those
are the rates and selectivities reported here. Rates are calculated on a per gram of metal basis.
b
Composition of CTP-SiO
2
catalysts determined from ICP-MS measured after reaction. Details in SI.
in He; flow rate 50 ml/min; 423 K.
c
Conditions: 6 mol% methanol and 20 mol% O
2
The catalyst architecture is free from diffusional limitations in rate under reaction
conditions based on several analyses. First, controlled variation of catalyst loading (9,
3
0, and 36 mg of CTP SiO
performance consistent with reaction-limited behavior (Figure 4B, squares). Second,
independently measured activities of three CTP SiO -Ag10Au90 catalysts with varied
2
-Ag10Au90 with 15 wt% nanoparticles) shows catalytic
2
nanoparticle content (10, 16, and 21 wt%) were found to increase linearly with increasing
nanoparticle loading (Figure 4B, diamonds). Third, the estimated Weisz modulus and
Carberry number (Tables S2, S4, S5) were at least one order of magnitude lower than
the threshold values characteristic of diffusional limitations.[10] Fourth, the activity of the
catalyst remained constant as the flow rate and volume of catalyst were varied in
concert, maintaining constant space velocity, between 80 ml/min and 50 ml/min. Only
below a flow rate of 50 ml/min was there a decrease in conversion, indicating that there
[
11]
was no external diffusion limitation under reaction conditions. Finally, the activity of the
catalyst did not increase as the catalyst macro-particle size was decreased
systematically, indicating that the reaction rate was not pore-diffusion-limited under
[
11]
these conditions (Table S3, Figure S13).
7
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A
B
4
0
0
1
00
100
80
60
40
20
0
selectivity
8
6
4
2
0
0
0
0
0
3
methyl formate
CO2
20
10
0
30
60
90
120 150 180
0
1
2
3
4
5
6
7
8
Time since activation (days)
Metal NP weight (mg)
Figure 4 (A) Steady-state conversion of methanol to methyl formate (MF, black squares) and
carbon dioxide (CO , black circles) and selectivity to methyl formate (blue triangles) using CTP
SiO -Ag10Au90 (nanoparticle wt. 5.2 mg). (B) Methanol conversion to methyl formate as a function
of total metal nanoparticle loading in the reactor. The squares correspond to 9, 30, and 36 mg of
CTP SiO -Ag10Au90 prepared with with 15 wt% nanoparticles (☐). The diamonds correspond to 28
mg of CTP SiO -Ag10Au90 prepared with 10, 16, and 21 wt% nanoparticles (). Reaction
conditions: 6 mol% methanol, 20 mol% O in He; flow rate 50 mL/min; 423 K.
2
2
2
2
2
In conclusion, AgAu nanoparticles incorporated into colloid-templated porous materials
are robust catalysts for selective alcohol oxidative coupling, similar to npAu. The
advantage of the colloid template method used here is that it is a highly versatile and
tunable approach for synthesizing a library of bimetallic catalyst materials without mass
transport limitations. This first demonstration of the catalytic activity of CTP SiO
2
-Ag
x
Au
y
for selective oxidative coupling of methanol to form methyl formate provides clear
evidence that controlled incorporation of the bimetallic nanoparticles yields an extremely
robust catalyst—both in catalyst performance and in structural integrity.
Despite many similarities, there are key differences in the catalytic behavior of the CTP
SiO
2
-Ag
x
Au and npAu. Notably, in npAu increasing the overall Ag content to 10% leads
y
[
4]
to complete combustion with low conversion rates (Table S2). In contrast, a higher Ag
content in the nanoparticle catalyst decreases its per gram activity, but the selectivity
remains high. These differences may be due to effects of the support, e.g. incorporation
of Ag into a silicate and/or due to differences in the distribution of the Ag in the near-
surface region.17 Future investigations are in progress to probe the role of Ag in the CTP
[
12]
SiO
2
-Ag
x
Au
y
catalysts. It is noteworthy that while both catalytic systems show excellent
-Ag10Au90 in comparison to
activity at mild conditions, the important feature of CTP SiO
2
npAu is the substantial increase in conversion coupled with significant reduction in metal
content. In addition, the considerably higher stability of the CTP catalysts towards high
temperatures allows for a wider range of reaction conditions, potentially allowing one to
access reaction products or increase selectivities beyond the capability of npAu.
8
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10.1002/chem.201704552
The impact of this work lies in the modularity of the synthesis described here as a
platform for making and testing a plethora of rationally designed catalysts with controlled
bimetallic nanoparticle size and composition and a well-defined structural and
compositional hierarchy. The synthesis provides the ability to systematically vary the
nature of the metal oxide support, the pore size and structure, and the composition, size,
loading and placement of the single or multi-metallic nanoparticles (Figure S14).[
Hence, novel catalyst materials can be produced for a broad range of catalyst
compositions and for a wide variety of reactions. The robustness of these catalysts that
show no sintering or agglomeration of the nanoparticles is an added benefit. This
approach to the design of bimetallic nanoparticle catalysts opens immense possibilities
in terms of tailoring the catalyst composition towards efficient and selective chemical
transformations in a sustainable and material-efficient way.
13]
Acknowledgements
This work was supported as part of the Integrated Mesoscale Architectures for
Sustainable Catalysis, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Basic Energy Sciences under award #DE-
SC0012573. T.S. acknowledges support from the Weizmann Institute of Science –
National Postdoctoral Award Program for Advancing Women in Science. Electron
microscopy were performed at the Center for Nanoscale Systems (CNS) at Harvard
University, supported by the NSF under award number ECS-0335765.
Keywords: nanoparticles, colloid templated porous materials, heterogeneous catalysis,
selective oxidation, gold, silver.
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