Efficient removal of organic ligands from supported nanocrystals by fast thermal annealing enables catalytic studies on well-defined active phases

Article abstract of DOI:10.1021/jacs.5b03333

A simple yet efficient method to remove organic ligands from supported nanocrystals is reported for activating uniform catalysts prepared by colloidal synthesis procedures. The method relies on a fast thermal treatment in which ligands are quickly removed in air, before sintering can cause changes in the size and shape of the supported nanocrystals. A short treatment at high temperatures is found to be sufficient for activating the systems for catalytic reactions. We show that this method is widely applicable to nanostructures of different sizes, shapes, and compositions. Being rapid and effective, this procedure allows the production of monodisperse heterogeneous catalysts for studying a variety of structure-activity relationships. We show here results on methane steam reforming, where the particle size controls the CO/CO₂ ratio on alumina-supported Pd, demonstrating the potential applications of the method in catalysis.

Full text of DOI:10.1021/jacs.5b03333

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Article
Efficient Removal of Organic Ligands From Supported Nanocrystals by Fast
Thermal Annealing Enables Catalytic Studies on Well Defined Active Phases
Matteo Cargnello, Chen Chen, Benjamin T. Diroll, Vicky V. T.
Doan-Nguyen, Raymond J. Gorte, and Christopher B. Murray
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 May 2015
Downloaded from http://pubs.acs.org on May 12, 2015
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Journal of the American Chemical Society
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Efficient Removal of Organic Ligands From Supported
Nanocrystals by Fast Thermal Annealing Enables Catalytic
Studies on Well-Defined Active Phases
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Matteo Cargnello,1,† Chen Chen,2 Benjamin T. Diroll,1 Vicky V. T. DoanꢀNguyen,3 Raymond J. Gorte2
and Christopher B. Murray1,3,*
1Department of Chemistry, 2Department of Chemical and Biomolecular Engineering, and 3Department of Materials Science
and Engineering, University of Pennsylvania, Philadelphia, PA 19104 (USA)
ABSTRACT: A simple yet efficient method to remove organic ligands from supported nanocrystals is reported for activating
uniform catalysts prepared by colloidal synthesis procedures. The method relies on a fast thermal treatment in which ligands are
quickly removed in air, before sintering can cause changes in the size and shape of the supported nanocrystals. A short treatment at
high temperatures is found to be sufficient for activating the systems for catalytic reactions. We show that this method is widely
applicable to nanostructures of different sizes, shapes and compositions. Being rapid and effective, this procedure allows the
production of monodisperse heterogeneous catalysts for studying a variety of structureꢀactivity relationships. We show here results
on methane steam reforming, where the particle size controls the CO/CO2 ratio on aluminaꢀsupported Pd, demonstrating the
potential applications of the method in catalysis.
1. INTRODUCTION
variety of uniform nanocrystals (NCs) and
treatments to strip the ligands off the particles,11-13 methods
involving plasma,14 or UVꢀozone cleaning procedures.15,16 All
these methods can remove organic byproducts and activate
catalysts to some extent. The problem lies in the fact that more
gentle approaches may leave residues that block the active
sites, and harsher treatments can cause unwanted changes in
the morphology of the systems and sintering of the
clusters/particles that compromise their uniformity.
Furthermore, some of these methods require long treatments to
be effective.17
A
heterostructured materials with different sizes, shapes and
compositions can be prepared with nanometer precision using
surfactantꢀassisted organicꢀ or aqueousꢀbased techniques.1-3
This nanoscale control over a wide variety of different
crystallographic surfaces and compositions is very appealing
for catalytic studies because it allows accurate structureꢀ
activity relationships and mechanisms to be made.4 Studying
finelyꢀcontrolled NC systems is important for answering
fundamental questions that could allow the preparation of
more active catalysts.
In previous work, we found that the minimum temperature
required for removal of organic ligands and activation of
group VIII metal catalysts by thermal treatments was 300 °C.18
Despite having narrow particleꢀsize distributions, the slow
heating ramps and long annealing times led to a change in the
initial particle shapes and a restructuring of the nanocrystals
onto the support.18 The reconstruction was particularly
noticeable with metal nanoparticles on CeO2 (ceria) support
and can be explained by the propensity of metals to wet the
ceria surface.19 Reconstruction must be avoided to study size
and shape effects on catalytic activity. UVꢀozone is one of the
best methods because it is relatively mild and it works at room
temperature. However, there are reports indicating that
catalysts are not completely activated following this process,
likely due to incomplete removal of surfaceꢀbound
surfactants.17
The major drawback of colloidal synthetic methods is that
they often require surfaceꢀbound surfactants to achieve
colloidal stability and to control size and shape. Unfortunately,
catalysis relies (in most although not all cases5) on having
clean surfaces where reactants can bind to surface atoms and
undergo chemical transformations into the desired products.
Obtaining clean surfaces free of extraneous species in these
uniform systems is therefore essential to prepare active, stable
and selective catalytic materials for numerous applications. It
has been clearly shown that the removal of the ligands is
essential for observing full activity. Even small amounts of
ligands left on the surface might be extremely detrimental for
catalysis.6
Several methods have been reported in the literature to
remove organic surfactants and stabilize the surfaces of
nanoparticles, small clusters or even single atoms for
catalysis.7 The most popular ways include thermal treatments
at low temperatures,8,9 solvent extraction,10 chemical
Here we report on a simple yet effective thermal treatment
to remove organic ligands from catalysts prepared by colloidal
methods. We show that the uniformity of the particles is
preserved while the systems are activated for catalytic
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Page 2 of 8
reactions. We expect this method to be valuable to other Preparation of catalyst powders. A dispersion of 1 g of
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materials and applications. As a demonstration of the
usefulness of this approach, we present studies on methane
activation through steam reforming on sizeꢀselected Pd NCs.
either Al2O3 (TH100ꢀ150, Sasol, calcined at 900 °C for 24h
before use) or CeO2 (prepared by precipitation of Ce(NO3)3
with NH4OH21 and calcined at 700 °C for 5h) in toluene (50
mL) was sonicated for 5 minutes and then stirred at high
speed. A solution containing an appropriate volume of Pd NCs
(to give a final catalyst loading of 0.5 wt. %) was prepared by
diluting the NCs to 10 mL with tetrahydrofuran. This solution
was then quickly added to the support dispersion in toluene
and left under fast stirring for 5 minutes. The support became
dark and the solution was colorless at this point. The powder
was recovered by centrifugation (8000 rpm, 3 minutes),
washed once with tetrahydrofuran and recovered by
centrifugation again (8000 rpm, 3 minutes). The sample was
finally dried in an oven at 110 °C overnight before use. This
sample is considered untreated.
2. EXPERIMENTAL SECTION
2.1 Synthesis of Pd nanocrystals. The synthesis was
performed using standard airꢀfree Schlenk techniques.
Reaction conditions for appropriate sizes are reported in Table
1. 76 mg of palladium acetylacetonate (0.25 mmol, 35% Pd,
Acros Organics) were dissolved in 10 mL of either
trioctylamine (TOA, 97%, Acros Organics) or benzyl ether
(98%, SigmaꢀAldrich) together with oleylamine (OLAM, 80ꢀ
90%, Acros Organics) and oleic acid (OLAC, 90%, Sigmaꢀ
Aldrich), if needed. The mixture was evacuated at RT
(pressure <2 mTorr) for 5 minutes, then trioctylphosphine
(TOP, 97%, SigmaꢀAldrich) was injected and the mixture kept
under evacuation and heated to 100 °C for 30 minutes. The
flask was then switched to nitrogen and the yellow solution
quickly (~50 °C minꢀ1) heated to the appropriate temperature
and kept for 15 minutes. The reaction was cooled to <50 °C
and open to atmosphere for purification. Particles were
washed with isopropanol and separated by centrifugation
(8000 rpm, 3 minutes) for three times, with the addition of 10
µL of oleylamine after each step except after the final wash.
Finally, particles were dissolved in toluene producing a deep
black solution at a concentration of ~25 mg Pd mLꢀ1.
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Rapid thermal treatment of catalyst powders. The untreated
sample was placed in an alumina crucible. A square furnace
(Thermolyne, Thermo Scientific) was preꢀheated to an
appropriate temperature (300, 500 or 700 °C) and the sample
swiftly introduced using a pair of long, heat resistant tongs
(ATTENTION: the high temperature in the furnace can
cause burns and the introduction and withdrawal of the
samples must be performed carefully). The treatment was
performed in static air. After the appropriate time interval, the
sample was again quickly removed from the furnace and left
cooling on a bench top.
Characterization techniques. TEM was performed on a
JEOL JEM 1400 operating at 120 kV, and HRTEM on a JEOL
JEM 2100 operating at 200 kV. Samples were prepared by
dropꢀcasting isopropanol dispersions of the samples onto 300
mesh carbonꢀcoated Cu grids (Electron Microscopy Sciences).
Particle size distributions were calculated using ImageJ
software to measure particle diameter. FTꢀIR was performed
on films of Pd NCs prepared by dropꢀcasting of concentrated
solutions onto quartz substrates; spectra were collected using a
ThermoꢀFisher Continuµm FTꢀIR system in transmission
mode. Inductively coupled plasma optical emission
spectrometry (ICPꢀOES) was performed on a SPECTRO
GENESIS ICP spectrometer.
Table 1. Conditions for the synthesis of Pd particles used
in this work.
Average
Pd Size
TOP
(mmol)
Temp.
(°C)
Solvent
OLAM (mmol)
2.3
3.5
4.4
5.5
7.7
TOA
TOA
TOA
TOA
BE
1.25
0.5
2.5
290
290
290
290
250
1.25
5
1.25
2.5
5
2.5 + 0.25 OLAC
1.25
Catalytic characterization. CO oxidation experiments were
conducted at overall atmospheric pressure. Measurements
were performed in a tubular quartz reactor (length ~400 mm,
internal diameter of 4 mm). The catalyst (20 mg) was sieved
below 150 ꢁm of grain size and loaded into the reactor to give
a bed length of about 2 mm, between two layers of quartz
wool used both for preventing displacement of the catalyst
powder and for preꢀheating the reagents. The reactor was
heated by a Carbolite tubular furnace and the temperature
monitored with a Kꢀtype thermocouple placed inside the
furnace but on the external side of the quartz reactor. No
appreciable conversion was detected when only quartz wool or
the bare supports (ceria and alumina) were placed in the
reactor, in the range of temperatures used for this study (under
our conditions, bare ceria starts to give >1 % CO conversion
only above 150 °C). The reactant mixture composition,
CO(1%) + O2(1%), was controlled by varying the flow rates of
CO, O2 and He while the total flow rate was kept constant at
30 mL minꢀ1. The conditions corresponded to a Gas Hourly
Space Velocity (GHSV) of 45000 mL gꢀ1 hꢀ1. The composition
of the effluent gases was monitored onꢀline using a Gas
Chromatograph (GC) (Buck Scientific 910) equipped with a
Synthesis of tetrahedral Pt nanocrystals. The synthesis was
performed using standard airꢀfree Schlenk techniques. 78 mg
of platinum acetylacetonate (98%, Acros Organics) were
dissolved in 10 mL of trioctylamine (97%, Acros Organics)
together with 0.64 mL of oleylamine (80ꢀ90%, Acros
Organics) and 2.5 mL of oleic acid (90%, SigmaꢀAldrich). The
mixture was evacuated at RT (pressure <2 mTorr) for 5
minutes, then 45 µL of trioctylphosphine (97%, Sigmaꢀ
Aldrich) were injected and the mixture kept under evacuation
and heated to 100 °C for 30 minutes. The flask was then
switched to nitrogen and the yellow solution quickly (~50 °C
minꢀ1) heated to 250 °C and kept at this temperature for 30
minutes. The reaction was cooled to <50 °C and open to
atmosphere for purification. Particles were washed with
isopropanol and separated by centrifugation (8000 rpm, 3
minutes) for three times, with the addition of 10 µL of
oleylamine after each step except after the final wash. Finally,
particles were dissolved in toluene producing a deep black
solution at a concentration of ~25 mg Pd mLꢀ1.
Synthesis of Au@FexOy core-shell structures. The synthesis
followed literature procedures.20
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Thermal Conductivity Detector (TCD) and He as the carrier
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gas. Prior to measuring rates, each catalyst was reduced under
a flowing mixture of H2 (5%) in He at 40 mL minꢀ1 at 150 °C
for 15 minutes. After cooling the sample to room temperature,
the reactant mixture was then introduced, and the reactor was
heated up to the desired temperature at 3 °C minꢀ1.
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The methane steam reforming (MSR) and waterꢀgas shift
(WGS) experiments were performed in a ¼ꢀinch, quartz,
tubular reactor, using 100 mg of catalyst. Water was
introduced to the reactor by saturation of a He carrier gas
flowing through deionized water. For MSR, the reactant
partial pressures at the inlet were fixed at 35 torr CH4 and 70
torr H2O with a total flow rate of 120 ml minꢀ1. For WGS
reaction, the reactant partial pressures were fixed at 25 torr CO
and 25 torr H2O with a total flow rate of 120 ml minꢀ1. In
determining reaction rates, conversions of CH4 or CO were
kept well below 10% so that differential conditions could be
assumed. Products were analyzed using an onꢀline gas
chromatograph (SRI8610C) equipped with a Hayesep Q
column and a TCD detector. Prior to measurements, each
catalyst was pressed into thin wafers and then cleaned at 300
°C in a flowing mixture of 20% O2 and 80% He for 30 min.
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Figure 1. FTꢀIR studies of different treatments applied to Pd
nanocrystals. The particles were dropꢀcasted onto a quartz slide,
introduced into a preꢀheated furnace at the indicated temperatures,
and heatꢀtreated for the indicated period of time. The powder
samples were subjected to the same treatment and representative
TEM pictures are reported in insets a (300 °C, 600s), b (500 °C,
60s) and c (700 °C, 30s).
Pd dispersions were quantified by volumetric CO adsorption
measurements at room temperature following appropriate
pretreatments. The calcined samples were placed in the
adsorption apparatus, heated in 200 torr of O2 at 250 °C, and
then reduced at 150 °C in 200 torr of H2. The samples were
then evacuated, cooled to room temperature, and then exposed
to small pulses of CO until no further CO uptake was detected.
We initially demonstrate that our method is effective using
monodisperse 7.5ꢀnm Pd nanocrystals (Figure S1). The
nanocrystals were prepared by thermal decomposition of
organometallic precursors in the presence of surfactants.18 The
particles were dropꢀcast onto a quartz slide and subjected to
thermal treatments using different heating programs. Infrared
spectroscopy was employed to evaluate the degree of removal
of organic ligands by looking at the CꢀH stretching region of
the spectrum (3000ꢀ2700 cmꢀ1), which is free of other signals
in nanocrystals (Figure 1). At 300 °C, about 10 minutes were
needed to bring the CꢀH stretching signal to below 95 % of the
initial intensity. By treating the particles at 500 °C for only 60
seconds, the CꢀH signals disappear, indicating complete
removal of the organic ligands. At a temperature of 700 °C,
the removal is complete in the even shorter period of 30
seconds.
3. RESULTS AND DISCUSSION
The method consists of a rapid thermal treatment of the
sample in air for a very short time with very fast heating and
cooling ramps. This process is realized by introducing the
sample into a furnace which had been preꢀheated at a specific,
relatively high temperature. The sample is quickly removed
from the furnace after the treatment and cooled down on a
benchtop. The heating and cooling ramps that can be obtained
with this method are of the order of several tens of degrees per
second, similar to rapid temperature annealing procedures
used in the semiconductor industry for introducing dopants in
silicon.22 With these high heating and cooling ramps, kinetic
transformations are favored over thermodynamics, preventing
the system from relaxing to its lower energy state (e.g. by
reducing high surface energy facets or surface energy in
general via sintering).
Sizeꢀdistribution histograms of the initial Pd NCs and of the
supported NCs treated at different temperatures are reported in
Figure 2. The average particle size of the initial Pd NCs is
7.8±0.5 nm. After removal of the ligands at 300 °C, 500 °C or
700 °C, the size shows only a very small change to 7.6 nm and
the variation remains below 6%. It should be noted that, when
using rapid thermal annealing procedures but with slower
heating ramps (on the order of 100ꢀ300 °C minꢀ1), particle size
distributions are not as narrow as when using the above
described temperatureꢀshock treatment.
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The removal of the ligands with the present rapid thermal
1
procedure not only maintains the narrow size distribution of
the particles, their shape and composition, but also activates
the systems for catalytic reactions. It is indeed important to
keep in mind that, despite IR suggests the removal of ligands
has taken place, the catalytic activity is the final proof that the
method is effective. We therefore demonstrate that the
removal of the organic ligands leads indeed to the full
activation of catalysts by using CO oxidation. CO oxidation by
Pd/CeO2 occurs at temperatures much below those which have
been found to remove organic ligands18 and therefore provides
a gauge of the efficacy of our annealing strategy. Pd/CeO2 was
chosen because this catalyst shows activity at lower
temperatures compared to other supported systems.18 Lightꢀoff
curves of CO oxidation experiments provide qualitative results
of the effectiveness of the ligand removal (Figure 4). It is
found that the untreated sample is not active for CO oxidation
in the investigated temperature window, but after removal of
the organic ligands by treatment at either 300, 500 or 700 °C,
the catalyst is activated. Surface blockage by the ligands is the
cause for the observed inactivity of untreated sample, given
that even by reducing the catalyst inꢀsitu before reaction did
not produce any appreciable CO conversion activity in the
investigated temperature window.
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Figure 2. Histograms of particle size distributions for the initial Pd
NC sample and for the supported systems treated at different
temperatures and times. Lines represent Gaussian fittings of the
distributions.
To demonstrate the wide applicability of our method, we
investigated the effect of the rapid thermal treatment onto
particles with different morphologies and shapes. Au@FexOy
coreꢀshell particles and tetrahedral Pt NCs were heatꢀtreated at
500 °C for 60s to remove the organic surfactants.
Representative TEM pictures (Figure 3) demonstrate that the
geometry and shape of the NCs is maintained. The Au@FexOy
NCs, in particular, show a shrinkage in the shell volume (the
void space between Au and iron oxide shell is drastically
reduced after the treatment), demonstrating that the thermal
treatment was responsible for increasing the density of the
shell surrounding the Au core. In the case of Pt, there is no
appreciable difference in the morphology of the NCs after the
treatment, with sharp edges and round corners present in both
samples. This is important result considering that clean,
tailored facets of a particular crystallographic orientation are
very interesting for fundamental catalytic studies.23 It is also
likely that a thermal treatment does not introduce surface
disorder as is generally the case of UVꢀozone procedures.24
Figure 4. CO conversion as a function of temperature for a
Pd/CeO2 sample that was untreated (organic ligands are still on
the surface of the particles) or rapidly heated to 300, 500 or 700
°C as indicated.
Given that the uniformity of the particles is maintained after
the ligand removal at high temperature, we investigated the
influence of particle size on a reaction that occurs under
conditions at which lowꢀtemperature treatments would not
provide useful data, such as methane steam reforming (MSR).
The activation of methane for this reaction occurs at
temperatures above 300 °C, requiring active and thermally
stable systems.25 Pd NCs in five different sizes ranging from
2.3 to 7.7 nm and narrow particle size distributions were
prepared and deposited on Al2O3 and CeO2 supports (Figure
S2). The ligands were removed and the catalysts activated at
700 °C for 30 s, as described above. CO chemisorption studies
(Table 2) show an increased Pd dispersion at decreasing Pd
particle size in the series, corroborating the TEM studies that
demonstrate that particle size and size distribution are well
maintained. Furthermore, chemisorption data of the aluminaꢀ
supported samples demonstrates that the surface of the Pd NCs
is able to coordinate CO, and therefore free of
Figure 3. Representative TEM images of Au@FexOy coreꢀshell
structures (a, c) and tetrahedral Pt NCs (b, d) before (top) and
after (bottom) fast thermal treatment at 500 °C for 60 s. The NCs
maintain their size and shape; only a modest shrinkage in the shell
volume is observed for Au@FexOy.
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ligands/contaminants, because samples are capable of CO
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uptake. The calculated particle diameter by CO chemisorption,
assuming a spherical geometry,26 is in very good agreement
with TEM studies, except for a slight deviation in the 7.7 nm
sample, for which the small volume of CO adsorbed increases
the error of the measurement. The good agreement suggests
that the spherical geometry is maintained, in contrast to lowꢀ
temperature treatments that tend to promote particle
reconstruction.
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Table 2. TEM and CO chemisorption characterization of
Pd/Al2O3 samples after thermal treatment at 700 °C for 30
s.
Pd particle size
(nm)a
Pd dispersion (%)b
Pd particle size
(nm)b
2.3 ± 0.2
3.5 ± 0.2
4.4 ± 0.3
5.5 ± 0.3
7.7 ± 0.5
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2.3
3.4
4.3
5.5
7.0
26
20.3
16
aCalculated from TEM.
bCalculated from CO chemisorption experiments assuming a
spherical geometry and CO/Pd stoichiometry of 1.
a
Dispersion is defined as the ratio of Pd atoms able to adsorb
CO divided by the total number of Pd atoms.
Figure 5. Kinetic data (a) and CO selectivity (b) for methane
steam reforming reaction on sizeꢀselected Pd NCs on alumina
(empty symbols) and ceria (filled symbols) after thermal
treatment at 700 °C for 30 s. Carbon balance was very close to
100%. Rates were calculated by exposed Pd surface area
measured by CO chemisorption.
We studied methane steam reforming (MSR) reaction at
temperatures below 500 °C, using a steamꢀtoꢀcarbon ratio of 2.
Under these conditions, the metal dispersion was stable even
for the smallest particle sizes,as confirmed by CO
chemisorption experiments before and after reactivity tests.
Kinetic data for this reaction is reported in Figure 5.
MSR rates for Pd/CeO2 are ~5 times higher than for
Pd/Al2O3 (Figure 5a) thanks to ceria assisting water activation
and favoring the removal of activated carbon species (C*)
from the Pd surface.27-30 A larger effect on the relative rates
might be expected, but the relatively high calcination
temperature required for stabilizing the ceria support (700 °C
deg) is known to reduce its surface area and alter its structure
in a manner known to strongly suppress the MSR activity of
Pd catalysts.28 The data normalized by mass of accessible Pd
(Pd dispersion, see Table 2) shows that the reaction is sizeꢀ
insensitive on Pd/CeO2 in the size regime of this study, and it
is only weakly sensitive in the case of Pd/Al2O3, with larger
particles delivering ~4 times higher rates than smaller ones.
This result is in accordance with recent studies on Pd31 and
underlines the peculiar behavior of this metal compared to
others, which show a linear dependence of TOFs on exposed
metal atoms.31,32
CO selectivity data (Figure 5b), measured at similar CH4
conversion, show support and sizeꢀdependent results. To
understand the selectivity data it is important to note that MSR
occurs under conditions that allow the waterꢀgas shiftꢀreaction
(WGSR) to occur simultaneously (Scheme 1).
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* cbmurray@sas.upenn.edu
Scheme 1. Methane Steam Reforming (MSR) and water-
gas shift-reaction (WGSR) and associated reaction
enthalpy values at 298 K.
1
Present Addresses
2
3
4
5
†Department of Chemical Engineering and SUNCAT Center for
Interface Science and Catalysis, Stanford University, Stanford,
CA 94305 (USA).
CH4 + H2O
CO + H2 (MSR)
ꢂH0 (298 K) = +206 kJ molꢀ1
r
Author Contributions
6
7
8
9
CO + H2O
CO2 + H2 (WGSR)
ꢂH0 (298 K) = ꢀ41 kJ molꢀ1
The manuscript was written through contributions of all authors.
r
ACKNOWLEDGMENT
CeO2ꢀsupported systems produce CO2 (and H2, not reported)
almost exclusively, and only small traces of CO are measured.
Because of the WGSR equilibrium, CO can be further
oxidized to formation CO2 and additional H2 (Scheme 1).
WGSR is known to be promoted on ceriaꢀsupported systems
compared to alumina.33 The case of the Pd/Al2O3 is radically
different. CO selectivity is ~30 % for 2.3 nm particles and it is
similar in the temperature window of our experiments.
However, with increase in particle size, a steady increase in
CO selectivity is observed, with 7.7 nm particles resulting in
~95 % selectivity for CO with only trace CO2 measured. In
contrast to other transition metals, the high activity of Pd in Cꢀ
H bond activation renders this step for MSR reversible,
whereas H2O activation and WGSR steps remain
irreversible.31 For this reason, WGSR controls the equilibria of
CO, CO2, H2 and H2O, and the rates of formation of all
products. We separately investigated the rates for WGSR on
Pd/Al2O3 (Figure S3) and found that indeed smaller particles
are more active than larger ones. Therefore, we postulate that
smaller Pd particles supported on Al2O3, rich in
undercoordinated metal atoms, promote WGSR and the
production of CO2. Larger particles instead contain a much
lower fraction of undercoordinated sites, thus showing a much
lower WGSR activity. It is important to note that particle size
is only very slightly influenced by the reaction conditions, and
narrow size distributions are still maintained despite the high
temperature of the MSR (Figure S4). In this way, we
demonstrate that it is possible to control activity and
selectivity of the MSR by controlling Pd particle size on an
inert support such as alumina. This result is possible by using
a combination of sizeꢀselected NCs as active phase and the
appropriate activation of the catalysts by ligand removal using
the method developed in this work.
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M.C. acknowledge support from the National Science Foundation
through the Nano/Bio Interface Center at the University of
Pennsylvania, Grant DMR08ꢀ32802. C.C. and R.J.G. were
supported by the Department of Energy, Office of Basic Energy
Sciences, Chemical Sciences, Geosciences and Biosciences
Division, Grant No. DEꢀFG02ꢀ13ER16380. C.B.M. is grateful for
the support of the Richard Perry University Professorship.
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