Palladium nanoparticles encapsulated in core-shell silica: A structured hydrogenation catalyst with enhanced activity for reduction of oxyanion water pollutants

Article abstract of DOI:10.1021/cs500971r

Noble metal nanoparticles have been applied to mediate catalytic removal of toxic oxyanions and halogenated hydrocarbons in contaminated water using H₂ as a clean and sustainable reductant. However, activity loss by nanoparticle aggregation and

Full text of DOI:10.1021/cs500971r

Research Article
pubs.acs.org/acscatalysis
Palladium Nanoparticles Encapsulated in Core−Shell Silica: A
Structured Hydrogenation Catalyst with Enhanced Activity for
Reduction of Oxyanion Water Pollutants
†
,‡,∥
Jinyong Liu, Peng Wang, Charles J. Werth, and Timothy J. Strathmann*,†
†,‡
§
†,⊥
Yin Wang,
†
Department of Civil and Environmental Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United
States
§
Water Desalination and Reuse Center, Biological and Environmental Sciences and Engineering Division, King Abdullah University of
Science and Technology, Thuwal, Saudi Arabia
*
S Supporting Information
ABSTRACT: Noble metal nanoparticles have been applied to mediate catalytic removal
of toxic oxyanions and halogenated hydrocarbons in contaminated water using H as a
2
clean and sustainable reductant. However, activity loss by nanoparticle aggregation and
difficulty of nanoparticle recovery are two major challenges to widespread technology
adoption. Herein, we report the synthesis of a core−shell-structured catalyst with
encapsulated Pd nanoparticles and its enhanced catalytic activity in reduction of bromate
−
(
BrO ), a regulated carcinogenic oxyanion produced during drinking water disinfection
3
process, using 1 atm H at room temperature. The catalyst material consists of a
2
nonporous silica core decorated with preformed octahedral Pd nanoparticles that were
further encapsulated within an ordered mesoporous silica shell (i.e., SiO @Pd@mSiO ).
2
2
Well-defined mesopores (2.3 nm) provide a physical barrier to prevent Pd nanoparticle
(
∼6 nm) movement, aggregation, and detachment from the support into water.
Compared to freely suspended Pd nanoparticles and SiO @Pd, encapsulation in the
2
mesoporous silica shell significantly enhanced Pd catalytic activity (by a factor of 10)
under circumneutral pH conditions that are most relevant to water purification applications. Mechanistic investigation of material
surface properties combined with Langmuir−Hinshelwood modeling of kinetic data suggest that mesoporous silica shell
−
enhances activity by promoting BrO adsorption near the Pd active sites. The dual function of the mesoporous shell, enhancing
3
Pd catalyst activity and preventing aggregation of active nanoparticles, suggests a promising general strategy of using metal
nanoparticle catalysts for water purification and related aqueous-phase applications.
KEYWORDS: palladium nanoparticle, mesoporous silica, core−shell, bromate, oxyanion, water purification
1
8
1
. INTRODUCTION
ceutical micropollutant diatrizoate, all of which are highly
resistant to treatment by conventional drinking water treatment
technologies. However, a major challenge to the application of
Pd NPs for water purification is the susceptibility of NPs that
lack surface stabilizers to aggregate in water to form large bulk
Pd precipitates, and particle aggregation negatively affects
Palladium-based materials have wide application in the field of
catalysis, because of their ability to facilitate a variety of
reactions.
catalysts in water purification and remediation processes for the
reductive transformation of recalcitrants and emerging classes
of aquatic contaminants, including halogenated organics,
oxyanions, nitrosamines, and pharmaceutical and personal
care products (PPCPs).
science and technology have enabled the synthesis of Pd
nanoparticles (NPs) with controllable shapes and sizes.
NPs and Pd-based bimetallic NPs have been applied in
catalyzing various aqueous reactions,
potential as catalysts for water treatment and purification
application, because of their unique properties, such as high
surface area to volume ratio and quantum size effects.
1
−3
One emerging application is to use Pd-based
19−21
reactivity of the NPs.
In addition, Pd NPs are difficult to
recover from water following application, because of their small
size, and there are growing concerns about the release of such
NPs into aquatic environments and associated ecological and
3
−8
Recent developments in material
22
9
−13
public health risks.
Pd
A variety of immobilization strategies are being investigated
to overcome limitations and concerns associated with applying
Pd NPs in aquatic systems. Core−shell support materials
decorated with noble-metal NPs have drawn increased
attention in heterogeneous catalysis, because of their unique
1
4,15
and have shown great
16−18
Shuai et al. synthesized Pd NPs with various shapes and sizes
and demonstrated their ability to catalytically reduce the
Received: July 8, 2014
Revised: September 1, 2014
−
oxyanion nitrite (NO ), the disinfection byproduct N-
2
nitrosodimethylamine (NDMA), and the halogenated pharma-
©
XXXX American Chemical Society
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Scheme 1. Preparation Procedure of the Core−Shell-Structured Silica Materials with Encapsulated Pd NPs
electronic features that affect catalytic activity, lack of
a NaOH-mediated hydrolysis approach (i.e., SiO @Pd@
2
aggregation in aqueous solution, and prevention of sintering
mSiO ). The well-designed pore structure has a small pore
2
23−25
at elevated temperatures (e.g., >350 °C).
Silica-based
size and narrow size distribution, and it makes the material
highly suitable as a model catalyst for probing aqueous
core−shell materials have received special attention, because of
their high thermal stability and tolerance of acidic con-
reactions. The catalyst was applied to remove bromate
2
6,27
−
ditions.
Successful synthesis of several types of core−shell
(BrO
3
) as a model drinking water contaminant. Bromate is
silica materials with immobilized noble-metal NPs has been
reported.
nanocatalyst consisting of a single platinum (Pt) nanoparticle
core coated with a mesoporous silica shell (i.e., Pt@mSiO2).
an EPA-regulated byproduct of drinking water disinfection
processes, because of its nephrotoxicity and potential
2
8−31
Joo et al. developed a core−shell-structured
3
5−37
carcinogenicity.
To the best of our knowledge, this is
@Pd@mSiO with a
23
the first report of the construction of SiO
2
2
They found that the presence of the mesoporous shell
prevented the aggregation and Pt NP sintering at high
temperature, thereby enhancing catalytic CO oxidation.
Similarly, synthesis of a Pd-based core−shell-structured silica
was reported with a single Pd NP core encapsulated in a
mesoporous silica shell. Although these materials have
promise in gas-phase applications, their small size (i.e., <50
nm) prevents easy separation and recovery during water
purification applications.
well-defined pore structure with Pd NPs constrained to the
interface between the silica core and the mesoporous shell.
More importantly, this work demonstrates, for the first time,
that the presence of mesoporous silica shell enhances Pd
catalyst reactivity with contaminants under aqueous conditions
relevant to water purification applications.
32
2. RESULTS AND DISCUSSION
2
.1. Synthesis and Characterization of SiO @Pd@
2
An alternative approach to increase the size of the material
involves immobilization of a large number of metal NPs on a
single silica core structure that is subsequently coated with a
mSiO . SiO @Pd@mSiO microspheres were prepared in
2
2
2
three steps (Scheme 1): (1) separate synthesis of shape-
controlled Pd NPs and surface-functionalized nonporous silica
microspheres, (2) immobilization of the preformed Pd NPs on
mesoporous shell to encapsulate the NPs (i.e., SiO @M@
2
2
8,33
mSiO2).
The mesoporous shell allows substrate access to
the reactive metal NPs while enhancing thermal stability. The
mesoporous shell can be developed by a surfactant templating
the surface of the silica microspheres (SiO @Pd), and (3)
2
26
growth of a surfactant-templated mesoporous silica shell on the
SiO @Pd microspheres. A combination of techniques,
2
2
8,33
approach or by selective etching of a nonporous shell.
described individually in detail in the Experimental Section,
were used in the synthesis process, including sol−gel
processing, interfacial deposition, and surfactant templating.
Pd NPs were synthesized by reducing Na PdCl with citric
Recently, Wang et al. reported using the latter approach to
synthesize core−shell silica with Pd NPs sandwiched between a
3
4
nonporous silica core and a mesoporous silica shell. NaOH
was used to etch the nonporous silica shell, and the resulting
material was characterized by a broad pore size distribution
with a relatively large average pore size (i.e., >15 nm). In
designing core−shell-structured supports for water purification
applications, ideally the mesoporous shell will be highly porous,
to allow the transport of reactants to encapsulated metal NPs,
but individual pore sizes will be narrower than the size of metal
NPs, to effectively prevent NP aggregation or release from the
support material.
2
4
13
acid, as reported previously. Polyvinylpyrrolidone (PVP) was
added to serve as a stabilizer to control the growth and prevent
the aggregation of Pd NPs. As observed by transmission
electron microscopy (TEM), the Pd NPs were mainly in an
octahedral shape (>75%) with an edge length of ∼6 nm (see
Figures 1A and 1B). The X-ray diffraction (XRD) pattern of the
Pd NPs matched well with the face-centered cubic (fcc) Pd(0)
crystal reference pattern (Figure 2). No crystalline phases other
than Pd(0) were observed from XRD. The average Pd NP size
was calculated as 6.2 nm from peak widths, according to the
Debye−Scherrer equation, which is consistent with the
observation from TEM.
The primary objective of this contribution was to develop an
advanced silica-based structure to enhance the suitability of Pd
NPs in environmental catalysis application. We report the
design, synthesis, and aqueous reactivity of a silica-based core−
shell-structured material with encapsulation of shaped Pd NPs.
A nonporous silica core was decorated with preformed
octahedral Pd NPs that were further encapsulated within a
surfactant-templated mesoporous silica shell developed through
̈
Silica microspheres were synthesized using the Stober
38
method. As shown in the scanning electron microscopy
(SEM) image (Figure 3A), monodisperse silica microspheres
with a diameter of ∼400 nm were obtained. Ammonia was
added to provide basic conditions to facilitate tetraethylortho-
3
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Figure 3. SEM images of (A) nonporous SiO microspheres, (B)
2
−
NH functionalized SiO microspheres, (C, D) SiO @Pd micro-
2
2
2
spheres, and (E, F) SiO @Pd@mSiO microspheres.
2
2
Figure 1. TEM images of (A, B) Pd NPs, (C, D) SiO @Pd
2
Based on the Debye−Scherrer equation, the Pd particle size on
microspheres, and (E, F) SiO @Pd@mSiO microspheres.
2
2
SiO @Pd was calculated as 6.5 nm, which is in close agreement
2
with measurements of the free Pd NPs. Results suggest that
both the size and the crystal structure of the Pd NPs were
preserved during immobilization on the silica core. For the
SiO @Pd, the Pd NPs were strongly attached to the surface of
2
the −NH -functionalized silica sphere. In addition to the
2
interaction between Pd NP and −NH group, PVP that is
2
coated on the Pd NP surface might also facilitate the
immobilization of Pd NPs by forming hydrogen bonds with
−
NH groups on the silica surface or by modifying surface
2
42
charge to enhance the electrostatic interactions. Attempts to
immobilize the Pd NPs on unfunctionalized and −SH-
functionalized silica microspheres, which were previously
Figure 2. XRD pattern of the unimmobilized Pd NPs and Pd NPs
2
9,43
deposited on the silica microspheres (SiO @Pd). The reference
reported to promote Au NP immobilization,
unsuccessful.
were
2
pattern for fcc Pd(0) crystal is included for comparison.
The mass loading of Pd NPs immobilized on the silica
surface can be tuned by adjusting the relative amounts of Pd
silicate (TEOS) hydrolysis and was also used to control the
shape of the silica acquired. The synthesized silica microspheres
had a spherical shape with smooth surface, as observed by SEM
NPs and −NH -functionalized silica microspheres (see Figure
2
S1 in the Supporting Information (SI)). Almost all the Pd NPs
in aqueous suspension were immobilized when their amount
(
Figure 3A). As previously reported, irregularly shaped silica
38
particles were obtained if ammonia was not added. To
support immobilization of Pd NPs, the silica microsphere
surfaces were further functionalized with amino groups using 3-
aminopropyl triethoxysilane (APTES), because of the strong
was <3 wt % of the −NH functionalized silica microspheres,
2
whereas further increases in the suspension concentration of Pd
NPs did not yield higher Pd loadings on the silica microspheres.
Elemental analysis confirmed that the maximum Pd concen-
chemical interaction between the NPs and the −NH
tration of SiO @Pd is 3.0 wt %. For such material, the surface
2
2
3
9−41
group.
An oxygen-free environment was employed to
was uniformly covered by a single layer of Pd NPs, as shown by
SEM (Figure 3D). Note that the particle size as well as the
solvent might affect the uniformity of metal nanoparticle
deposition. Westcott and Halas reported that increasing
ethanol-to-water ratios promote Au NP aggregation and cluster
prevent oxidation of the amino groups. As shown in Figure 3B,
the size and the surface roughness of the silica microspheres
were unchanged by −NH functionalization.
2
SiO @Pd was obtained by mixing PVP-stabilized Pd NPs
2
29
with −NH functionalized silica microspheres in water. Pd NPs
formation on the surface of silica. Wang et al. reported
nonuniform attachment and low surface coverage of Pd NPs
deposited on silica microspheres in water, which is probably
due to the larger Pd NP particle size (∼20 nm) used in their
2
evenly covered the surfaces of silica microspheres in the
resulting material (Figures 1C and 1D and 3C and 3D). The
XRD pattern of SiO @Pd was similar to that of the free Pd
2
34
NPs, and no Pd(II) crystalline phases were detected (Figure 2).
study. In the present work, the smaller size of the Pd NPs
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3
3
combined with the use of water as the solvent yielded a more
uniform distribution of Pd NPs on silica microsphere surfaces.
A surfactant templating method was used to grow
average pore size of 2.2 nm. Wang et al. synthesized SiO @
2
Pd@mSiO2 using a NaOH etching method without the
34
addition of a surfactant template. This resulted in a less-
uniform pore size distribution and much larger average pore
size (>15 nm) than that reported here. It is worth noting that
the pore size obtained here is significantly smaller than the size
of the preformed Pd NPs, thereby providing a physical barrier
to prevent release of the NPs into solution. In addition, it is
worth mentioning that the surfactant templating method may
also result in a highly ordered pore orientation that is
perpendicular to the core, since this preferred alignment of
surfactant micelles and TEOS minimizes the overall interfacial
mesoporous silica shells on the SiO @Pd microspheres.
2
TEOS was hydrolyzed in the presence of SiO @Pd and
2
hexadecyltrimethylammonium bromide (CTAB), a structure
directing agent that promotes formation of a mesostructured
CTAB/silica shell on the surface of the SiO @Pd micro-
2
4
4
spheres. The CTAB then was removed by extraction in a
mixture of ethanol and concentrated HCl to obtain SiO @Pd@
2
45
mSiO microspheres. The thickness of the mesoporous shell
2
was ∼60 nm and the Pd content of the SiO @Pd@mSiO was
2
2
33
33,46,47
determined to be 1.3 wt %. Instead of NH ·H O, dilute
energy of the system.
3
2
NaOH (1.7 mM) was used as a base during the surfactant-
templating procedure to catalyze TEOS hydrolysis and preserve
the sphere shape of the mesoporous shell. With the use of
NaOH, sandwich-structured SiO @Pd@mSiO microspheres
The thickness of the mesoporous shell can be controlled by
adjusting the amounts of TEOS and CTAB. For example,
increasing the TEOS concentration 2.4-fold during synthesis
resulted in an increase of the mesoporous shell thickness from
60 nm to 120 nm (see Figures S3 and S4 in the SI).
Interestingly, further increasing the TEOS concentration by
another 50% resulted in a nonuniform coating of mesoporous
silica shell, and SiO @Pd@mSiO microspheres with multiple
2
2
were obtained where Pd NPs were well-constrained at the
interface between the silica core and shell materials (Figure 1E
and 1F). In contrast, when using NH ·H O as a base the
3
2
majority of the Pd NPs were liberated from the surface of the
inner silica core and distributed randomly throughout the
emplaced mesoporous silica shell (see Figure S2 in the SI). The
loss of Pd NPs from the core surface may be due to competing
2
2
sizes were obtained (see Figure S4 in the SI).
2.2. Catalytic Reduction of Bromate. The activity of
SiO @Pd@mSiO in catalyzing the reduction of bromate
2
2
−
Pd interactions with −NH groups on the silica surface and
(BrO ) was measured and compared with that of free Pd NPs
2
3
dissolved NH . Using NaOH to form the mesoporous shell also
and SiO @Pd in H -saturated water at pH 7 and ambient
3
2
2
resulted in uniform silica spheres (see Figure 1E). Joo et al. also
temperature (20 ± 1 °C). The overall reaction can be described
reported uniform core−shell-structured Pt@mSiO spheres
as
2
when adding diluted NaOH to catalyze surfactant-templated
23
Pd
−
−
TEOS hydrolysis in the presence of free Pt NPs. It is worth
noting that previous studies report that higher NaOH
concentrations (>0.5 M) may react directly with silica, etching
BrO + 3H → Br + 3H O
(1)
3
2
2
For all three materials, BrO3− consumption was accompanied
2
8,34
the mesoporous silica structure and changing the pore size.
The porous nature of the SiO @Pd@mSiO microspheres
by Br− production, and the sum of BrO3− and Br−
concentrations remained very close to the initial BrO3−
2
2
was confirmed by measuring N2 adsorption−desorption
isotherms (Figure 4). The Brunauer−Emmett−Teller (BET)
concentration (see Figure S5 in the SI), confirming that the
observed BrO3− loss from solution resulted from reduction of
the oxyanion rather than adsorption to the high surface area
−
material. None of the oxyanion intermediates (BrO and
2
−
BrO ) were detected, indicating that the reduction of such
−
intermediates is much faster than that of BrO .
3
Comparing the time courses for BrO3− reduction observed
with the three materials under neutral pH conditions (Figure
5
A) reveals that the reaction with SiO @Pd@mSiO proceeded
2 2
much faster than with Pd NPs and SiO @Pd. Kinetics followed
2
a pseudo-first-order rate law (Figure 5A), and the resulting rate
constants (Pd mass normalized) were determined for all the
experiments (see Table 1). Pseudo-first-order rate constants are
typically used to describe the kinetics of different catalytic
5
,8,48−51
Similar rate constants for BrO3−
2
reduction experiments.
reduction with Pd NPs and SiO @Pd were measured at pH 7
(
Figure 5B). Thus, immobilization of Pd NPs onto silica
supports did not decrease the apparent catalytic activity. Under
these same pH conditions, which is highly relevant to water
treatment applications, the rate constant for SiO @Pd@mSiO
Figure 4. N2 adsorption−desorption isotherms and pore size
distribution (inset) of SiO @Pd@mSiO microspheres prepared
2
2
with an ∼60-nm-thick mesoporous shell.
2
2
was more than an order-of-magnitude larger than those
2
surface area was measured to be 480 m /g, and the total pore
volume was estimated as 0.27 cm /g. Application of the
obtained for Pd NPs and SiO @Pd (Figure 5B). Initial
2
3
turnover frequency (TOF ) values were also determined
0
Barrett−Joyner−Halenda (BJH) model confirmed that the
surfactant templating approach yielded a highly uniform pore
size distribution in the mesoporous shell with an average
diameter of 2.3 nm (see Figure 4). The value is similar to that
reported by Deng et al., who also used CTAB to form core−
shell-structured Fe O @SiO −Au@SiO microspheres with an
(Table 1); they followed the same trend as the pseudo-first-
order rate constants. To the best of our knowledge, this is the
first report of mesoporous-shell-enhanced activity of metal NPs
in catalysis applications at ambient temperature. These results
indicate that, in addition to the active metal sites, the structure
of the solid supports play an important role in determining the
3
4
2
2
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Figure 5. Reduction of 100 μM BrO3 by 2 mgPd L−1 loading of Pd NPs, SiO @Pd, and SiO @Pd@mSiO in water at pH 7 (20 ± 1 °C, P = 1
−
2
2
2
H
2
−
atm): (A) Time courses for BrO3 reduction (dashed lines represent pseudo-first-order kinetics model fits); (B) Pd mass-normalized rate constants
for BrO3 reduction (error bars represent one standard deviation determined from triplicate experiments).
−
Table 1. Results of Kinetics Experiments Measuring
a
Catalytic Reduction of Bromate
b
rate constant, k
initial turnover frequency,
b
−1
−
1
−1
catalyst
Pd NP
pH
(L h gPd
)
TOF0 (h )
0
0
0
1
2
3
1
0.30 (±0.16) × 100
0.21 (±0.05) × 10
0.47 (±0.09) × 100
7
8
7
6
4
2
8
6.88 (±3.69) × 10
2.47 (±0.53) × 10
5.45 (±1.03) × 10
1.56 (±0.55) × 10
5.07 (±0.66) × 10
2.62 (±0.65) × 10
2.89 (±0.56) × 10
0
SiO @Pd
2
SiO @Pd
2
0
SiO @Pd
1.34 (±0.47) × 10
2
SiO @Pd
4.35 (±0.57) × 101
2
2
SiO @Pd
2.25 (±0.56) × 10
2
SiO @Pd@
3.23 (±0.63) × 100
7.40 (±1.89) × 100
1.52 (±0.41) × 101
5.56 (±0.33) × 101
3.20 (±0.41) × 102
2
mSiO2
SiO @Pd@
7
6
4
2
6.60 (±1.69) × 101
9.39 (±2.55) × 101
4.96 (±0.29) × 102
2.86 (±0.37) × 103
2
mSiO2
SiO @Pd@
2
mSiO2
Figure 6. Influence of pH on rate constants for reduction of 100 μM
−
−1
SiO @Pd@
BrO
3
by 2 mgPd
L
loading of SiO @Pd and SiO @Pd@mSiO in
2 2 2
2
mSiO2
water (T = 20 ± 1 °C, PH2 = 1 atm). Error bars represent one standard
deviation of replicate experiments.
SiO @Pd@
2
mSiO2
a
−
All experiments were conducted at [BrO ] = 100 μM, T = 20 ± 1
3
0
49
−1
catalyst. They proposed that acidic condition promoted
°
C, PH2 = 1 atm, and catalyst loading equivalent to yield 2 mgPd L .
−
hydrogen bonding between ClO4 and the immobilized Re
b
Uncertainty represents one standard deviation derived from at least
metal species, thereby accelerating oxygen atom transfer in the
proposed reaction mechanism. Chen et al. found that the
reduction rate of bromate increased with decreasing pH with an
alumina supported Pd catalyst, and they attributed the pH-
dependent activity to both the redox potential of bromate
reduction and the surface properties of the catalyst. The zeta
potentials of SiO @Pd and SiO @Pd@mSiO were measured
duplicate experiments.
activity of heterogeneous catalysts. The mechanism of enhance-
ment is examined in a subsection below.
The effect of pH on reaction kinetics was determined for the
two silica-supported catalysts (SiO @Pd and SiO @Pd@
53
2
2
2
2
2
−
mSiO ). BrO3 reduction rates increased with decreasing pH
in H -saturated aqueous solutions as a function of pH in this
2
2
for both SiO @Pd and SiO @Pd@mSiO (Figure 6). SiO @
work (Figure 7). The surfaces of both SiO @Pd and SiO @
2
2
2
2
2
2
Pd@mSiO was found to be much more active than SiO @Pd
Pd@mSiO were less negatively charged with decreasing pH.
2
2
2
for pH 6−8, but activities for the two materials converged
under more acidic conditions. Compared to metal- or metal-
oxide-based supports, including alumina and iron, silica is not
only stable under mild basic conditions (pH <12), but also is
Similar to others, we propose that less negative surface charge
at lower pH increases electrostatic attractive interactions
between BrO3− and the catalyst surfaces and increases the
apparent reaction rate. It is worth noting that the presence of
52
well-known for its high stability under acidic conditions.
dissolved H markedly affects the zeta potential of Pd-based
2
Therefore, silica may potentially serve as an ideal candidate for
a Pd NP support for catalytic applications in a wide pH range,
including acidic media.
catalyst surfaces, and here the addition of H (g) shifted the zeta
2
potential of both SiO @Pd and SiO @Pd@mSiO to a more
2
2
2
negative values at a given pH (see Figure S6 in the SI).
Solution pH may affect both the interaction between active
Pd(0) sites and bromate, and the surface properties of the silica
supports. Strong pH-dependent activities have been reported
for catalytic reduction of oxyanions including bromate, nitrate,
Similarly, Choe et al. reported negative shifts in the surface
54
charge of Pd/C catalysts in the presence of H2. The negative
shift of zeta potential may be attributed to the formation of
activated H species, which may adsorb on the surface of Pd or
react with support surface functional groups, leading to the
4
9,51,53
and perchlorate.
Hurley and Shapley investigated
catalytic ClO4− reduction by a Pd/Re-based bimetallic
formation of negatively charged species.
54
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reacting with Hads and is affected by the amount of the active
surface Pd(0) sites, K is the equilibrium adsorption constant
ads
−1
−
(
L mmol ) for bromate on the catalyst surface, and [BrO ] is
3
−1
the aqueous bromate concentration (mmol L ).
Equation 2 was fit to the initial rates of BrO3− reduction by
SiO @Pd and SiO @Pd@mSiO measured for varying initial
2
2
2
−
BrO3 concentration (see Figure 8), and parameters derived
Figure 7. Influence of pH on the zeta potential of H -saturated
2
aqueous suspensions of SiO @Pd and SiO @Pd@mSiO (20 ± 1 °C,
2
2
2
−1
1
atm H , 2 mg
L
catalyst loading).
2
Pd
2
.3. Discussion of Activity Enhancement of SiO @Pd@
2
mSiO . Potential reasons for enhanced catalytic activity of
2
SiO @Pd@mSiO , relative to SiO @Pd, may be due to
2
2
2
differences in the amount of active surface Pd(0) sites and
−
the interaction between BrO3 and the catalyst surface. The
Figure 8. Influence of bromate concentration on the initial rate of
amount of active surface Pd(0) sites is influenced by the total
mass of Pd, the size and shape of the Pd NPs, and the
accessibility of Pd NPs within the support matrix and can be
−1
aqueous bromate reduction by 2 mgPd
L
loading of SiO @Pd and
2
SiO @Pd@mSiO (pH 7, 20 ± 1 °C, P = 1 atm). Dashed lines
2
2
H
2
represent fits to the Langmuir−Hinshelwood (L-H) kinetics model.
evaluated by CO(g) chemisorption. For both SiO @Pd and
2
SiO @Pd@mSiO , similar amounts of Pd octahedra with an
2
2
average size of 6.2 nm were immobilized without evidence of
NP aggregate formation. CO(g) chemisorption measurements
from model fitting indicate that the presence of the
mesoporous shell increases reactivity with BrO3− by enhancing
2
yielded active Pd surface areas of 55 and 43 m /g of Pd for
oxyanion adsorption prior to reaction with H . Based on
ads
SiO @Pd and SiO @Pd@mSiO , respectively. This suggests
chemisorption measurements, the amount of the active surface
Pd(0) sites for SiO @Pd and SiO @Pd@mSiO are similar, so
2
2
2
that encapsulation in the mesoporous silica shell slightly
2
2
2
−
reduces the Pd surface area available for reaction with BrO .
a single krxn was assumed for the two materials during fitting.
Simultaneous fitting (method of least-squares errors) of
experimental data for both materials yielded an optimum
3
Thus, enhanced reactive surface area cannot be the reason for
the greater reactivity of SiO @Pd@mSiO .
2
2
value of krxn of 24.9 ± 4.7 mmol gPd−
1
h
−1
for both materials
A Langmuir−Hinshelwood (L-H) kinetic model was applied
−
to provide insights into the interaction between BrO and the
(uncertainty represents 95% confidence limits), but large
3
surface of catalysts. The L-H model has widely been applied to
describe heterogeneous catalytic reaction kinetics.
differences in K . The adsorption constant obtained for
ads
3
,50,55,56
−1
SiO @Pd@mSiO (1.35 ± 0.87 L mmol ) was more than 5-
2
2
According to the L-H model, catalytic hydrogenation reactions
using Pd-based materials can be described by a series of
fold larger than that obtained for SiO @Pd (0.26 ± 0.13 L
2
1
−
mmol ). Although k was assumed to be the same for fits
rxn
3
,50,55
steps,
involving rapid equilibrium adsorption of the target
shown, fits conducted with independent krxn values also yielded
good fits with similar krxn values and large differences in Kads for
the two materials. Thus, fitting indicates that the addition of th−e
mesoporous shell serves to enhance the extent of BrO3
reactant and H onto the surface of the catalyst, H dissociation
2
2
by Pd(0) to form reactive atomic hydrogen (H ), and reaction
ads
of Hads with the target reactant. It is anticipated that (i) catalytic
reduction of bromate by SiO @Pd@mSiO and SiO @Pd
adsorption prior to reaction with H . The model result is
2
2
2
ads
follows these general steps, and (ii) diffusion limitations are
consistent with the experimental observation that the difference
negligible. The latter assumption is valid for the small particle
sizes and rapid stirring rates used in this work. The aqueous
in activity between SiO @Pd and SiO @Pd@mSiO decreased
2
2
2
56
with increasing bromate concentration (see Figure 8) as sites
near the Pd(0) source of Hads become saturated with both
materials. It should be mentioned that, while the adsorption
H concentration was estimated as a constant of ∼0.8 mM
2
throughout the experiments, based on Henry’s law for PH2 = 1
−
52
constant increased by 5-fold for SiO @Pd@mSiO , little BrO
atm, since H (g) was continuously sparged to the reactor.
2
2
3
2
adsorption was observed in H (g)-free control experiments for
Assuming that bromate and H2 adsorb to the catalyst
noncompetitively and that surface reaction is the rate-limiting
2
both SiO @Pd and SiO @Pd@mSiO , which is consistent with
2
2
2
−
5
5,56
predictions from L-H Kads values that <1% BrO3 adsorbs to
step,
the corresponding L-H kinetic expression can be
both materials. Although the extent of adsorption is low for
described as
both materials (within the uncertainty level of the ion
−
−
⎛
⎞
⎟
K [BrO ]
chromatography measurements for aqueous BrO ), the 5-
ads
3
3
R = krxn⎜
−
fold difference is critical to the net reaction kinetics because
reaction occurs between adsorbed BrO3− and H species
according to the L-H model. Therefore, results support a
mechanism whereby oxyanion uptake by the high surface area
⎝
1 + K [BrO ] ⎠
where R is the rate of bromate reduction (mmol g
(2)
ads
3
−1
−1
h ), k
Pd
rxn
−1
−1
−
is the rate constant (mmol gPd h ) for adsorbed BrO3
3
556
dx.doi.org/10.1021/cs500971r | ACS Catal. 2014, 4, 3551−3559

ACS Catalysis
Research Article
mesoporous shell enhances the overall rate of reduction in
water.
relevant to water purification applications. In addition to
activity enhancement, the presence of the mesoporous shell
may have additional benefits to environmental applications,
including preventing NP aggregation and release, protecting the
active metals from colloidal foulants, and potentially providing
for the selective uptake of target solutes in complex aquatic
matrices (e.g., by size exclusion or pore channel functionaliza-
tion). The high stability of silica materials at low pH also makes
them suitable for treating acidic water and wastewater. These
results suggest that core−shell-structured mesoporous silica
may serve as an ideal support for a wide range of metal
nanomaterials that can potentially be used in different
applications relevant to environmental treatment processes.
Future work will be dedicated to (1) optimize the catalyst
composition (e.g., Pd NP loading, size, and shape) to further
enhance the activity; (2) determine the performance of the
catalysts as well as Pd leaching and release in the presence of a
number of water constituents; (3) assess the capability of the
catalyst to remove a variety of emerging contaminants; (4)
extend the methods demonstrated here to immobilize other
metal or metal oxide nanomaterials with various shapes and
sizes; and (5) evaluate the reusability and longevity of the
catalysts in treatment applications in the field.
We speculate that the increased Kads for SiO @Pd@mSiO2
2
results from the increase in silica surface sites for bromate
adsorption near the source of reactive H . While the specific
ads
2
surface area of the silica core is estimated as only ∼1.4 m /g
based on spherical shape), the presence of the mesoporous
(
shell significantly enhances the specific surface area of SiO @
2
2
Pd@mSiO to be 480 m /g (measured by BET). Theoretically,
2
only bromate adsorbed on the active Pd(0) sites would be
reduced to bromide since H adsorption and activation would
2
only occur on the active Pd(0) sites. However, bromate
adsorbed to silica surface sites close to active Pd(0) sites may
also be reactive, because of the possible spillover of Hads.
Hydrogen spillover is described as the dissociative chem-
isorption of H onto an active metal site, followed by migration
2
5
7,58
of the Hads onto the surface of the support.
Previous studies
suggest that Hads typically only migrates a limited distance (in
the nanometer range) from active metal sites of generation,
56
because of the high energy for migration. Hydrogen spillover
has been widely observed in hydrogenation reactions in which
57−59
carbon and silica are used as support materials.
A recent
study has proposed that hydrogen spillover may enhance the
51
activity of perchlorate reduction on Pd−Re/C. It is possible
that hydrogen spillover may also occur on silica supported Pd
catalysts and reactions between the spilled Hads and bromate
adsorbed on the mesoporous shell results in an enhancement of
4. EXPERIMENTAL SECTION
4.1. Material Synthesis. Details of all the chemicals used in
the present work, and methods of synthesis of Pd NPs,
synthesis and surface modification of silica microspheres, and
synthesis of SiO @Pd and SiO @Pd@mSiO , have been
the catalytic activity of SiO @Pd@mSiO .
2
2
An alternative rationale for the higher reactivity of SiO @
2
2
2
2
Pd@mSiO that was ruled out was a shift in the prevailing
2
provided in the SI.
surface charge of the catalyst to favor uptake of the anionic
4
.2. Catalytic Reduction of Bromate. The kinetics of
−
B
r
O
s
p
e
c
i
e
s
.
Z
e
t
a
p
o
t
e
n
t
i
a
l
m
e
a
s
u
r
e
m
e
n
t
s
c
o
n
d
u
c
t
e
d
i
n
H
-
3
2
bromate reduction by H were measured in continuously stirred
2
saturated suspensions exhibited similar values for both SiO @
2
batch reactors at ambient temperature (20 ± 1 °C). Pd NPs,
Pd and SiO @Pd@mSiO at all the pH values (Figure 7).
2
2
SiO @Pd, or SiO @Pd@mSiO was added to a 50-mL round-
2
2
2
Although the surface SiO @Pd was slightly more negative than
2
bottom flask with a solid loading sufficient to yield a
SiO @Pd@mSiO , such a small difference cannot explain the
−1
2
2
concentration of 2 mgPd L . An aliquot of a phosphate stock
1
0-fold higher activity observed for SiO @Pd@mSiO under
2 2
solution was added to provide a phosphate concentration of 1
mM to buffer pH, and HCl or NaOH was used to adjust the
solution pH to the desired value (pH 2−8). The reactor was
then capped with a rubber stopper that contained two 16-gauge
neutral pH conditions.
3
. CONCLUSIONS AND IMPLICATIONS
In the present work, Pd octahedral NPs with an average size of
.2 nm were synthesized and immobilized in core−shell-
structured silica materials (SiO @Pd and SiO @Pd@mSiO ).
stainless steel needles, with one serving as the H gas inlet and
2
6
the other as both the gas outlet and the liquid sampling port.
After sparging the catalyst suspension with H for 30 min, the
2
2
2
2
A novel structured SiO @Pd@mSiO was synthesized using a
reaction was initiated by injecting bromate to yield the desired
initial concentration (0.1−5 mM). Suspension aliquots were
then collected periodically, immediately filtered (0.45 μm
polytetrafluoroethylene (PTFE)) to quench reactions, and
analyzed for BrO3− and Br concentrations. Each experimental
condition was run at least in duplicate, except for those with
initial bromate concentrations higher than 0.1 mM.
2
2
combination of techniques including sol−gel processing,
interfacial deposition, and surfactant templating. Highly
ordered mesopores with a uniform pore size of 2.3 nm were
obtained for SiO @Pd@mSiO . Encapsulation between the
−
2
2
nonporous core and mesoporous shell physically prevents
detachment of the Pd NPs into the aqueous phase being treated
and the open pore structure allows the reactants to reach the
active surface Pd(0) sites. SiO @Pd@mSiO showed much
4.3. Analytical Methods. Catalyst Characterization. The
shape and morphology of the synthesized Pd NPs, SiO @Pd,
2
2
2
higher catalytic activity than unsupported Pd NPs and SiO @
and SiO @Pd@mSiO were determined by TEM and SEM.
TEM images were acquired by a JEOL Model 2010LAB6
microscope operated at 200 kV. SEM images were taken using
2
2
2
Pd for bromate reduction in water at neutral pH and ambient
temperature. An L-H model was developed to describe the
−
kinetics of catalytic BrO3 reduction and the elevated activity of
a Hitachi Model S4700 high-resolution microscope. N (g)
2
SiO @Pd@mSiO , compared to SiO @Pd, was attributed to
enhanced adsorption of BrO3 to the mesoporous shell prior to
reaction of the oxyanion with Hads.
adsorption−desorption isotherms were performed with a
Micromeritics ASAP 2020 surface analyzer to measure the
surface area and pore size distribution of SiO @Pd@mSiO .
2
2
2
−
2
2
Using bromate as a probe reactant, we demonstrate, for the
first time, that encapsulation of shaped Pd NPs in mesoporous
silica materials can significantly enhance the activity of catalyzed
aqueous reactions at circumneutral pH, a condition that is most
Specific surface area was calculated using the BET method, the
pore volume and pore size distribution was obtained via BJH
model, and the total pore volumes were obtained from the
adsorbed quantity at a relative pressure (P/P ) of 0.995. The
0
3
557
dx.doi.org/10.1021/cs500971r | ACS Catal. 2014, 4, 3551−3559

ACS Catalysis
Research Article
metallic Pd surface area of SiO @Pd and SiO @Pd@mSiO
2
was quantified using CO(g) chemisorption (Micromeritics
acquire the XRD patterns and elemental analysis, respectively.
We appreciate the help from Ruiqing Lu (UIUC) with
obtaining the zeta potential measurements, and Dr. Shaoying
Qi (UIUC) for gas adsorption experiments. Technical
Assistance at KAUST was provided by Dr. Hongnan Zhang,
Dr. Zhonghai Zhang, Mr. Rubal Dua and Mr. Guoying Chen.
Four anonymous reviewers provided insightful comments to
help improve the presentation of the results in this paper.
2
2
ASAP 2020C). The samples were heated in H (g) at 350 °C for
2
2
h before performing CO chemisorption analysis at 35 °C.
The crystalline phases of Pd NPs and SiO @Pd were
2
determined by powder XRD measurement performed on a
Bruker General Area Detector Diffraction System using Cu Kα
radiation. The surface zeta potentials of SiO @Pd and SiO @
2
2
Pd@mSiO were measured with a Malvern Zetasizer ZS 90. To
2
measure zeta potential, solids were added to a reactor with a
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AUTHOR INFORMATION
Corresponding Author
Tel.: 217-244-4679. E-mail: strthmnn@illinois.edu.
■
*
(
Present Addresses
(
∥
Y. Wang: Department of Civil and Environmental Engineering
University of Wisconsin-Milwaukee, Milwaukee, WI 53211
wang292@uwm.edu
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