Resolving Interparticle Heterogeneities in Composition and Hydrogenation Performance between Individual Supported Silver on Silica Catalysts

Article abstract of DOI:10.1021/acscatal.5b02119

Supported metal nanoparticle catalysts are commonly obtained through deposition of metal precursors onto the support using incipient wetness impregnation. Typically, empirical relations between metal nanoparticle structure and catalytic performance are inferred from ensemble averaged data in combination with high-resolution electron microscopy. This approach clearly underestimates the importance of heterogeneities present in a supported metal catalyst batch. Here we show for the first time how incipient wetness impregnation leads to 10-fold variations in silver loading between individual submillimeter-sized silica support granules. This heterogeneity has a profound impact on the catalytic performance, with 100-fold variations in hydrogenation performance at the same level. In a straightforward fashion, optical microscopy interlinks single support particle level catalytic measurements to structural and compositional information. These detailed correlations reveal the optimal silver loading. A thorough consideration of catalyst heterogeneity and the impact thereof on the catalytic performance is indispensable in the development of catalysts.

Full text of DOI:10.1021/acscatal.5b02119

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Research Article
pubs.acs.org/acscatalysis
Resolving Interparticle Heterogeneities in Composition and
Hydrogenation Performance between Individual Supported Silver on
Silica Catalysts
†
†
†
‡
‡
Eva Plessers, Ivo Stassen, Sreeprasanth Pulinthanathu Sree, Kris P. F. Janssen, Haifeng Yuan,
†
‡
†
,†
Johan Martens, Johan Hofkens, Dirk De Vos, and Maarten B. J. Roeffaers*
†Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
‡Department of Chemistry, KU Leuven, Celestijnenlaan 200 F, 3001 Heverlee, Belgium
*
S Supporting Information
ABSTRACT: Supported metal nanoparticle catalysts are
commonly obtained through deposition of metal precursors
onto the support using incipient wetness impregnation.
Typically, empirical relations between metal nanoparticle
structure and catalytic performance are inferred from ensemble
averaged data in combination with high-resolution electron
microscopy. This approach clearly underestimates the
importance of heterogeneities present in a supported metal catalyst batch. Here we show for the first time how incipient
wetness impregnation leads to 10-fold variations in silver loading between individual submillimeter-sized silica support granules.
This heterogeneity has a profound impact on the catalytic performance, with 100-fold variations in hydrogenation performance at
the same level. In a straightforward fashion, optical microscopy interlinks single support particle level catalytic measurements to
structural and compositional information. These detailed correlations reveal the optimal silver loading. A thorough consideration
of catalyst heterogeneity and the impact thereof on the catalytic performance is indispensable in the development of catalysts.
KEYWORDS: interparticle heterogeneity, optical microscopy, selective catalytic hydrogenation, single-particle catalysis,
supported metal catalysts
INTRODUCTION
substrate. As often, these experimental results on the structure
sensitivity of chemoselective hydrogenation with silver nano-
particles are seemingly contradictory, making clear-cut
structure−activity assessments far from trivial. Supported-
metal-catalyzed reactions are inherently chemically complex,
and subtle changes in catalyst structure and properties can
■
Supported platinum group metal (PGM) nanoparticles are
heavily used as hydrogenation catalysts; however, their price
and future availability call for alternatives. Even though silver is
mostly known as an oxidation catalyst, e.g. in the industrial
production of ethylene oxide and methanol, various research
groups have shown that supported silver nanoparticles can also
chemoselectively catalyze the hydrogenation of unsaturated
aldehydes, esters, and nitro compounds; for the last example
silver offers the unique possibility to chemoselectively reduce
functional nitroaromatics to the corresponding anilines, which
1
3,14
clearly have an important impact on the outcome.
Rationalization of bulk catalytic performances, even when
supported by molecular simulations, often oversimplify the
1
−5
15
inherent complexity of the supported metal catalyst itself. In
addition to nanoparticle size and crystal facets also variations in
loading, distribution, and accessibility could play an important
role in the overall catalytic performance. Essential when
experimentally establishing correct structure−activity relation-
ships is the generation of nanoparticles of well-controlled sizes.
However, it is known that common catalyst synthesis strategies
6
,7
is not possible with PGM nanoparticles.
The available
information indicates that selective hydrogenation on sup-
ported silver catalysts is a structure-sensitive reaction in which,
in addition to the structure of the substrate, also nanoparticle
1
,2,8−10
size and support material play an important role.
In the
16−18
preclude variability in nanoparticle size,
making validation
selective hydrogenation of the unsaturated aldehydes croto-
naldehyde and acrolein an increased reactivity and selectivity to
the desired alcohol was found with increasing silver nano-
of the nanoparticle size indispensable. In addition to bulk
techniques such as X-ray diffraction, chemisorption, and
EXAFS that generally yield average nanoparticle diameters
1
,9
particle size. This observation was related to the larger
fraction of Ag(111) surfaces. In contrast, smaller silver
nanoparticles (0.9−3 nm) were found to be superior in the
9
within a supported catalyst, direct imaging using transmission
electron microscopy (TEM) allows precise determination of
19
1
1
12
nanoparticle dimensions and dispersion. As a result of the
selective hydrogenation of 4-nitrostyrene and acrolein in
the high-pressure range. These observations can be rationalized
by the rate-limiting dissociation of H2 at coordinatively
unsaturated silver sites and the adsorption geometry of the
Received: September 22, 2015
©
XXXX American Chemical Society
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atomic resolution that can be achieved via TEM imaging, this
approach is often the only suitable technique to study the size
and dispersion of supported nanoparticles at the nanometer
scale. Unfortunately, no direct link between these offline
measurements and the catalytic performance can be made; thus,
the catalytically active nanoparticles cannot be discriminated
this, the multiwell could be filled with the reaction solution
using a micropipette and placed in the reactor. Since thermal
hydrogenation of 4-nitrostyrene results in the formation of the
unwanted 4-ethylnitrobenzene and little 4-vinylaniline, in each
run several wells were not filled with a catalyst particle to
account for this blank conversion and some wells were only
filled with solvent to ensure that no cross contamination had
occurred. To lower solvent evaporation as much as possible, the
high-boiling N,N-dimethylacetamide (DMA) was used as a
solvent and n-hexadecane as an internal standard for
quantitative GC analysis. Optimization of reaction conditions
led to the use of 18 μL of a 33 mM 4-NSt solution in each
microwell, with hydrogenation performed under 20 bar of H2
on heating to 110 °C for 2.5 h in a Parr reactor filled with 3 mL
of DMA. Analysis of the reaction products was carried out
using a gas chromatograph (Shimadzu, CP-Sil 5, FID detector)
after rinsing the wells two times with pure DMA.
20,21
from spectator species.
In this study we report that typical incipient wetness
impregnation results in an unexpected 10-fold variation in
silver loading between individual silica gel support granules,
leading to intersupport granule variations in number and size of
the silver nanocatalysts. To validate the impact thereof on the
catalytic performance, the selective reduction of 4-nitrostyrene
to 4-vinylaniline was chosen. The aforementioned interparticle
heterogeneity in silver loading leads to 100-fold variations in
hydrogenation performance, and by using optical microscopy, it
is possible to identify the optimal silver loading of the best-
performing supported metal catalyst granules.
RESULTS AND DISCUSSION
■
EXPERIMENTAL SECTION
Compositional Heterogeneities at the Micro- and
Nanoscale. Supported silver catalysts (5−6 wt % Ag) on silica
gel were synthesized via standard incipient wetness impregna-
■
Catalyst Preparation. An aqueous AgNO3 solution,
equaling the total pore volume of the support material, was
added dropwise to dried silica gel (Sigma-Aldrich, Fluka 60752)
until a slurry formed. After equilibration at room temperature
the impregnated samples were dried overnight in air in an oven
at 100 °C and finally calcined at 500 °C for 2 h. Control
4
,17
tion.
During calcination of the white silver nitrate
impregnated silica powder, silver oxide was formed and
subsequently at temperatures above 400 °C completely
22,23
decomposed into metallic silver.
The resulting silica-
supported silver nanoparticle catalyst powder has a typical
yellowish appearance and looks seemingly homogeneous.
However, close inspection using optical microscopy revealed
an unexpected variability in color between different support
granules (Figure 1A−C); to our knowledge this interparticle
samples were obtained by calcining commercial AgNO on
3
silica gel (Sigma-Aldrich 248762) in air at 500 °C for 2 h. In
this work this catalyst is referred to as “commercial Ag/SiO2”.
Optical Microscopy. Images were obtained via the
eyepieces using an adapter from Micro-Tech-Lab (Austria) to
connect a Canon EOS5D color camera to an Olympus BX51
Upright microscope with a standard mercury lamp, equipped
with infinity corrected air objectives 4× (0.16 N.A.) and 20×
(0.40 N.A.). Color sorting of individual supported silver catalyst
granules was performed on a stereomicroscope (Leica
M165FC).
Scanning Electron Microscopy. High-resolution SEM
images were obtained with a Nova NanoSEM 450 instrument
(FEI). SEM-EDX was conducted using a FEI XL30FEG
electron microscope equipped with an EDAX detector. Spectral
analysis and quantification were performed with Genesis 4.61
software. Samples were mounted onto a copper TEM grid (300
mesh, Agar Scientific) fixed on a gold-coated cover slide which
was then immobilized on to an aluminum stub using carbon
sticker. These were imaged without any further sample
modification.
Figure 1. Interparticle heterogeneity in Ag/SiO at the support
2
granule level revealed by optical microscopy: (A−C) silver on silica gel
obtained via typical incipient wetness impregnation; (D−F)
commercial AgNO on silica gel after calcination on the macroscale
3
Catalytic Performance Testing at the Bulk Level. Bulk
hydrogenation reactions were performed in high-pressure 15
mL TOP reactors and a 100 mL Parr reactor (2 h, 110 °C, 20
(A, D) and microscale (B, E) (C, F) Interparticle heterogeneity (n =
2
50) illustrated with pie diagrams. The colors represent the red color
index of individual Ag/SiO granules (for determination see the text in
2
bar of H , 0.35 mol % of Ag, 70 mM 4-nitrostyrene in DMA,
2
the Supporting Information and Figure S1).
and 500 rpm unless stated otherwise). Analysis of the reaction
products was carried out using a gas chromatograph (Shimadzu,
CP-Sil 5, FID detector), and n-tetradecane was added as
internal standard for quantitative GC analysis. Identification of
the compounds was carried out using GC-MS.
Catalytic Performance Testing at the Single Support
Particle Level. Individual single-granule hydrogenation
reactions were performed by use of a multiwell placed in a
color heterogeneity has not been reported so far for supported
metal catalysts. An even more pronounced interparticle color
heterogeneity, ranging from transparent to yellow to red-
brown, was observed in a supported silver catalyst (6 wt % Ag)
made by calcining commercial AgNO on silica gel obtained
3
from Sigma-Aldrich (Figure 1D−F). Strikingly, within one large
support granule of about 100 μm in diameter no significant
color variation was observed.
Since pure silica powder is optically transparent, also after a
similar heat treatment, the observed color formation must be
1
00 mL Parr reactor, enabling 21 reactions in parallel. Prior to
the catalytic reaction, single support particles were carefully
placed one by one in the different wells via an eyelash
manipulator and a stereomicroscope (Leica M165FC). After
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Figure 2. Correlation of the optical appearance of single Ag/SiO granules to silver nanoparticle size (A−C) and silver loading (D−F). (A) Optical
2
micrograph of single Ag/SiO granules. (B) HR-SEM micrograph showing the overview of the same area. (C) HR-SEM micrograph of a dark
2
support granule showing small nanoparticles (<10 nm, center) and very large silver nanoparticles (700−1000 nm, left lower corner). (D) Optical
micrograph of single Ag/SiO granules. (E) Silver loading of the numbered single Ag/SiO granules measured via energy dispersive X-ray analysis.
2
2
(
F) Link between red color index of individual Ag/SiO granules and their silver loading. The results shown here were obtained from the commercial
2
6
wt % Ag/SiO catalyst.
2
associated with the silver nanoparticles. It is well-known that
the optical appearance of silver nanoparticles is related to the
surface plasmon resonance. Next to nanoparticle shape and
refractive index of the environment, plasmon absorbance and
hence selective (visible) light absorption is largely determined
deposition of the granules on a coverslip with a marked copper
grid. Then, HR-SEM was performed to probe the outer surface
of the same support granules, of which the optical color is
known. Marks on the copper grid and the irregular shape of the
support granules make the correlation of the optical images and
SEM micrographs highly reliable (Figure 2A,B). By specifical
probing of the outer surface of transparent, yellow, and red
granules, three distinct silver nanoparticle size ranges were
noticed: 1−10, 20−50, and >400 nm (Figure S3 in the
Supporting Information). The relative contribution of the
larger 20−50 nm nanoparticles increases with increasing
coloration, and the extremely large silver crystals (>400 nm)
were only visible on the dark red silica granules (Figure 2C and
Figure S3E). These electron micrographs thus evidence that
there is a clear relation between color targeted via optical
imaging and silver nanoparticle size at the outer surface of the
support granules.
24
by the nanoparticle size. For silver nanoparticles in silica
below about 10 nm diameter the surface plasmon resonance
peaks at around 420 nm, resulting in a yellow appearance upon
white light illumination. Increasing the size to around 100 nm
diameter results in a strong red shift of about 100 nm, which
25
causes red coloration. Nitrogen physisorption measurements
of the used silica gel provide a BJH desorption average pore
width of 60 Å (Figure S2 in the Supporting Information).
Although the rather broad pore size distribution would give rise
to polydisperse silver nanoparticles formed inside the support
pores, their size is restricted to below 10 nm, resulting in an
overall yellowish appearance. The presence of larger,
unconfined silver nanoparticles at the outer surface of the
support granules could give rise to color variations. However,
several other factors will influence the optical appearance such
as the local nanoparticle concentration and the absolute
support granule size, which is directly linked to the optical
However, these findings do not exclude the possibility that
the optical heterogeneity can also be induced by a difference in
silver nanoparticle concentration, analogous to the observed
19
heterogeneity in Pt/zeolite Y catalysts. Therefore, we adopted
energy dispersive X-ray (EDX) spectroscopy to probe the silver
content of individual silica support granules, again directly
correlated to the optical appearance of the exact same granules
(Figure 2D). Figure 2E shows that typical supported silver
catalysts, obtained from a commercial supplier or via standard
impregnation methods, display at least a 10-fold variation in
silver loading between different silica support granules.
Comparison of the EDX results and color indexing of the
optical images led to a clear trend between optical appearance
and silver content (Figure 2F). Since the silver content
determined via SEM-EDX is limited to the first few
25
path length and hence the resulting light absorption. Since no
relation was found between optical appearance and the absolute
support granule size, the latter can be excluded and the color
heterogeneity between individual silica granules must be sought
at the level of the supported silver nanoparticles.
In order to investigate the color heterogeneity at the silver
nanoparticle level, we resorted to high-resolution scanning
electron microscopy (HR-SEM) correlated with optical
microscopy. First, the optical appearance of the silica-supported
silver catalyst was examined via optical microscopy after
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Figure 3. Schematic representation of a multiwell catalytic measurement at the single support particle level. In a typical run around 13 catalyst
granules were tested together with four blank tests and two wells were only filled with solvent (18 μL of a 33 mM 4-NSt solution in each microwell,
2
0 bar of H , 110 °C, 2.5 h, 100 mL Parr reactor filled with 3 mL of DMA).
2
micrometers below the outer surface, this approach could lead
to a misinterpretation of the total silver loading of this support
granule because a silver gradient might exist along the cross
in 18 μL of N,N-dimethylacetamide (DMA) solvent was added,
the multiwell plate was loaded in the Parr hydrogenation
reactor at 20 bar of H and heated to 110 °C; under these
26
2
section of the support granule. To further validate the SEM-
EDX measurements, focused ion beam (FIB) milling was used
to section a yellow granule in the middle. A clear silver gradient
can be observed, with silver concentrations decreasing from the
outer surface of the granule toward the center; even in the
center significant silver amounts could still be detected (Figure
S4 in the Supporting Information). Furthermore, on these FIB
sections no large silver nanoparticles were observed in the
interior of the support granules with high-resolution SEM.
These correlated microscopy data link the optical appearance
of a support granule to both silver concentration and size of
nonpore confined silver nanoparticles at the outer surface. The
increasing coloration observed in optical microscopy is related
to both variations in silver loading and the presence of larger
silver nanoparticles at the outer surface of the support granules.
These calibration data can now be used to quantify the amount
of silver in every support granule by simply using optical
images. Optical microscopy is easily accessible without lengthy
sample preparation and allows rapid determination of
interparticle heterogeneity on the support granule level. Metal
loading quantifications based on optical images rely on
correlations with other analytical methods such as EDX, HR-
SEM, etc.
conditions no distillation of the reactant or its products was
observed. After 2.5 h the reaction mixture of every microwell
was analyzed via gas chromatography. Figure 4A shows the
normalized catalytic performance of 47 individual supported
catalyst granules as a function of their total silver loading (more
details are given in the Supporting Information).
Linking Hydrogenation Performance to Catalyst
Composition via Catalytic Measurements at the Single
Support Particle Level. Because of the large variability in
silver loading and nanoparticle size it is not straightforward to
unequivocally link performance to catalyst properties, certainly
not from typical ensemble averaged measurements using at
least milligrams of powdered catalyst. In this study we
measured for the first time the catalytic performance at the
level of the individual silica support granule. In order to
minimize variations in experimental conditions, multiwell plates
were used enabling 21 reactions in parallel in the same high-
pressure Parr reactor; the multiwell catalytic measurements at
the single support particle are shown schematically in Figure 3.
These multiwell plates also allow recording of optical
transmission images of the individual supported catalyst
granules placed in each reaction well; from these optical
images the exact silver content in every microwell is
determined, which is critical for normalizing catalyst perform-
ance. Subsequently, after 590 nmol of 4-nitrostryrene (4-NSt)
Figure 4. (A) Single-granule catalytic hydrogenation of 4-nitrostyrene
with commercial Ag/SiO catalyst: results of 47 individual silver
2
supported silica granules. Estimation of silver loading was based on red
color index and SEM-EDX, complemented by ICP-AES measurements
of color-sorted granules. More details can be found in the text and
Figure S5 of the Supporting Information. (B) Bulk hydrogenation
reactions with color-sorted commercial Ag/SiO2 samples (column
bars), displaying up to a 2.5-fold increase of yield with respect to
unsorted commercial catalyst (filled area) (110 °C, 20 bar of H , 2.5
2
h).
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These single support particle data undoubtedly reveal an
optimal silver loading. Granules with about 6 wt % of silver
show up to 100-fold higher normalized 4-vinylaniline yield in
comparison to catalyst granules from the same batch containing
the multiwell experiments, a similar performance trend was
observed (Figure 5). In addition, the results of the commercial
Ag/SiO sample (6 wt %) fit perfectly within the results of the
2
self-synthesized samples. The outcome of the single support
particle experiments can thus undoubtedly be extrapolated to
the bulk level. Although the normalized yield of the commercial
sample increased by 38% upon increasing the overall silver
loading to 13 wt %, a considerably higher improvement must be
possible on the basis of the observed heterogeneity. Ideally one
would rationally synthesize a batch which consists of only
yellow supported silver catalyst granules; however, none of the
typical impregnation approaches gave a satisfactory result. In an
attempt to obtain a more homogeneous catalyst sample, the
commercial silver on silica catalyst was manually sorted into
three different fractions (Figure S7 in the Supporting
Information). As can be expected from the single support
particle experiments, the batch with yellow support granules
outperforms the batches with transparent or orange and red
granules to the same degree as could be expected from the
single support particle studies (Figure 4B).
<
4 wt % or >8 wt % of silver. From the detailed
physicochemical characterization (vide supra) the highest
hydrogenation performance can be attributed to support
granules with yellow appearance (red color index 0.12−0.20):
i.e., 5−7 wt % silver. These yellow granules contain in addition
to the 6 nm pore confined silver nanoparticles also a reasonable
amount of larger, 20−50 nm silver nanoparticles on the surface
of the support granule. When a whole support granule is
considered, the pore confined silver nanoparticles represent
over 99.9% of the total number of nanoparticles. Silver in these
yellow granules is up to 100 times more efficiently used as in
highly loaded (>8 wt % Ag) support granules, with silver
nanoparticles larger than 400 nm on the support’s outer
surface, and as in transparent granules (<4 wt % Ag) which only
contain the pore confined 6 nm silver nanoparticles. Hence, it
can be concluded that in this sample support granules with 6 wt
%
silver and the highest relative contribution of 20−50 nm
CONCLUSIONS
silver nanoparticles are the most active in the selective 4-
nitrostyrene reduction, contradicting earlier reports based on
ensemble-averaged hydrogenation data.
■
We have shown that typical incipient wetness impregnation
brings about severe heterogeneity in metal loading at the
support particle level. Specifically, 10-fold variations in silver
metal loading between individual silica support granules are no
exception within one catalyst batch. These differences in metal
loading severely affect the catalytic performance on measure-
ment at the same scale. Here optical microscopy proved to be a
convenient tool to directly interlink the physicochemical
properties and hydrogenation performance of individual
support particles. Following this approach, we could resolve
100-fold variations in normalized catalytic performance for the
selective 4-nitrostyrene reduction and determine the optimal
silver loading for this reaction. More specifically, these single-
particle experiments indicate that support granules which have
the relative highest contribution of silver nanoparticles of about
11
Rationally Improving the Hydrogenation Perform-
ance of Supported Silver Catalysts. In order to validate
these single support particle results, namely that the catalyst
granule’s optical appearance is linked to the silver content and
its catalytic performance, we carried out bulk catalytic
hydrogenation reactions. Ag/SiO samples with four different
2
silver contents ranging from 4 to 19 wt % at the bulk level were
synthesized. As expected, all of these materials showed a
significant interparticle heterogeneity in optical appearance
(Figure S6 in the Supporting Information). Using the average
performance determined for every color of catalyst granule via
single support particle experiments, the estimated theoretical 4-
vinylaniline yield of each of these catalyst samples was
estimated (Figure 5; the calculation is explained in the
Supporting Information). These calculations predict that the
sample with a bulk loading of 13 wt % in silver should show the
best selective hydrogenation performance. Indeed, in a typical
hydrogenation reaction with 0.35 mol % silver and with the
same relative amounts of substrate and solvent as were used in
2
0−50 nm show the highest 4-vinylaniline (4-VAn) yield.
Following traditional catalyst impregnation optimization based
on ensemble averaged characterization and catalytic perform-
ance measurements it would be far from trivial to find this
optimal catalyst composition. The proposed optical screening
method is widely applicable to supported metal catalysts: e.g.,
similar heterogeneity in color and thus loading were also
observed in Pt/SiO (Figure S8 in the Supporting Informa-
2
tion).
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02119.
Additional information regarding the experimental
execution and data analysis and FIB-SEM, N -phys-
2
isorption, and Pt/SiO data (PDF)
2
AUTHOR INFORMATION
Figure 5. Bulk hydrogenation reactions of 4-nitrostyrene with self-
■
*
synthesized (4 to 10, 13, and 19 wt % Ag) and commercial 6 wt % Ag/
Corresponding Author
E-mail for M.B.J.R.: maarten.roeffaers@biw.kuleuven.be.
SiO catalysts (■) and estimated theoretical yield based on optical
2
appearance (□) (for the determination see the Supporting
Information, silver loading based on ICP-AES) (110 °C, 20 bar of
Notes
H , 2 h).
2
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
Research Article
■
The authors thank the Research Foundation-Flanders (FWO)
Grant G.0990.11 and G.0962.13, Ph.D. scholarships to E.P.
(
and I.S. and postdoctoral fellowships to K.P.F.J. and H.Y.),
Belspo (IAP-VII/05), and the Flemish government (Long term
structural funding-Methusalem funding CASAS METH/08/
0
4). M.B.J.R. acknowledges the European Research Council for
financial support (ERC Starting Grant LIGHT 307523).
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DOI: 10.1021/acscatal.5b02119
ACS Catal. 2015, 5, 6690−6695