Polysulfide chalcogels with ion-exchange properties and highly efficient mercury vapor sorption

Article abstract of DOI:10.1021/ja3061535

We report the synthesis of metal-chalcogenide aerogels from Pt²⁺ and polysulfide clusters ([S_x]^2-, x = 3-6). The cross-linking reaction of these ionic building blocks in formamide solution results in spontaneous gelation and eventually forms a monolithic dark brown gel. The wet gel is transformed into a highly porous aerogel by solvent exchanging and subsequent supercritical drying with CO₂. The resulting platinum polysulfide aerogels possess a highly porous and amorphous structure with an intact polysulfide backbone. These chalcogels feature an anionic network that is charged balanced with potassium cations, and hosts highly accessible S-S bonding sites, which allows for reversible cation exchange and mercury vapor capture that is superior to any known material.

Full text of DOI:10.1021/ja3061535

Article
pubs.acs.org/JACS
Polysulfide Chalcogels with Ion-Exchange Properties and Highly
Efficient Mercury Vapor Sorption
Youngtak Oh, Collin D. Morris, and Mercouri G. Kanatzidis*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
*
S Supporting Information
ABSTRACT: We report the synthesis of metal−chalcogenide
2
+
2−
aerogels from Pt and polysulfide clusters ([S ] , x = 3−6).
x
The cross-linking reaction of these ionic building blocks in
formamide solution results in spontaneous gelation and
eventually forms a monolithic dark brown gel. The wet gel is
transformed into a highly porous aerogel by solvent
exchanging and subsequent supercritical drying with CO2.
The resulting platinum polysulfide aerogels possess a highly
porous and amorphous structure with an intact polysulfide
backbone. These chalcogels feature an anionic network that is charged balanced with potassium cations, and hosts highly
accessible S−S bonding sites, which allows for reversible cation exchange and mercury vapor capture that is superior to any
known material.
INTRODUCTION
■
Aerogels are three-dimensional random networks with highly
accessible nanoscale pores and very low densities. Metal−oxide
aerogels are the dominant examples in this class and useful in
applications such as catalysis, thermal insulation, adsorbents,
1
and sensors. A novel kind of emerging aerogel features metal−
2
−5
chalcogenide networks instead of metal oxide.
The metal−
chalcogenide aerogels, dubbed chalcogels, exhibit a unique
combination of porous structures with visible and infrared light
absorption, surface polarizability, and chemical selectivity.
These chalcogels show promise in areas such as solar-fuel
6
7
8
catalysis, photoluminescence, selective gas separation,
9
10
catalysis, and heavy metal aqueous waste remediation.
There are three main methods to producing metal−
1
1−13
chalcogenide aerogels: thiolysis,
and condensation,
nanoparticle oxidation
2,3,7,14,15
6,8−10,16
and sol−gel metathesis.
Among these, the metathesis route shows wide applicability
in the synthesis of chalcogels because of the simplicity of the
reaction and diversity in the choice of building blocks. Using
this approach, our group has demonstrated the synthesis of
chalcogenide aerogels using primary building blocks such as
Figure 1. (A) Building blocks of chalcogel for metathesis route: (i)
tetrahedral cluster, (ii) dimeric tetrahedral cluster, and (iii)
adamantane cluster. (B) Photographic illustration of metathesis
process: (iv) precursor solutions containing inorganic building blocks,
(v) mixture of precursor solution undergoing gelation (cross-linking
reaction), and (vi) completed metathesis reaction yielding monolithic
wet gel.
4−
4−
tetrahedral clusters ([MQ ] and [M Q ] , M = Mo, Sn, Ge/
4
2
6
9
,10,16
4−
Q = S, Se),
and adamantine clusters ([M Q ] , M = Sn,
4 10
1
6
Ge/Q = S, Se), Figure 1A. These versatile choices of building
blocks incorporate chemical diversity into enormously porous
structures to produce materials with broad functionalities that
can surpass those of conventional metal−oxide aerogels.
One such outstanding functionality of chalcogels is selective
tested for the remediation of gas-phase pollutants such as
elemental mercury. As a major toxic pollutant from flue gases
0
(
coal combustion exhaust gas), elemental mercury (Hg ) poses
a great challenge to its removal because conventional air
pollution control techniques such as fabric filtering (FF),
electrostatic precipitation (ESP), and flue-gas desulfurization
16
adsorption of ions and molecules. Platinum chalcogels and
10
zinc tin sulfide chalcogels (e.g., ZTS-cg1) are representative
examples that illustrate how these soft Lewis basic surfaces are
effective in selectively removing heavy metal ions (e.g., Hg )
2+
Received: June 24, 2012
Published: August 21, 2012
from aqueous solutions. To date, no chalcogels have been
©
2012 American Chemical Society
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Journal of the American Chemical Society
Article
1
7,18
(FGD) are not effective.
To control elemental mercury
vapor from flue gas, highly adsorptive materials such as
activated carbon have been employed, but display low Hg
capture capacity at elevated temperatures, and suffer from
interference of adsorption by other flue gas chemicals such as
SO . Functionalization of activated carbon with functional
3
19
20
21
groups such as chloride, iodide, and sulfide shows
2
0
improvements in mercury capture efficiency, but contami-
nation of reusable combustion products (fly ash) by activated
carbon remains problematic. In contrast, sulfur-functionalized
noncarbon materials such as Co-doped silica, Co-doped iron
nanoparticles, porous silica, and silica−titania nanocompo-
2
2
2
3
24
2
5
sites possess high mercury capture capacities without the
problems of carbon-based materials. These studies showed
higher mercury capture capacity on samples with more sulfur
content; thus it would be interesting in this regard to study the
Hg absorption properties of purely sulfidic porous networks
such as those of chalcogels.
Figure 2. A proposed structure and potential mercury binding
chemisorption sites of tetra-polysulfide chalcogel.
In this study, we present the synthesis of a novel class of
chalcogels with high surface areas featuring different poly-
−
26,27
[
Se ]2 ligands.
These polychalcogenide ligand structures
sulfides and platinum (K−Pt−S ). These porous metal−
x
x
are consistent with the experimental results presented here such
as gas and Hg absorption and ion-exchange capability of the
chalcogels (see below).
Scanning electron microscope (SEM) images indicate a
sponge-like character containing randomly oriented pores
throughout the micrometer-sized specimens shown in Figure
chalcogenide compounds have ditopic linear polysulfide ligands
−
(
i.e., Sx2 , x = 3, 4, 5, 6). The framework motif is similar to the
crystalline compounds of (Ph P)[M(Se ) ](M = Ga, In, Tl),
4
6 2
which are an exceptional example of an open framework with a
26
polyselenide backbone; however, they lack accessible pores.
We show that the networks in these new K−Pt−S chalcogels
x
3
A and 1S (Supporting Information). To confirm the presence
are anionically charged and potassium cations balance the
charge. Therefore, these materials possess dual functionality:
(
a) ion-exchange properties and (b) S−S bonds capable of
o
undergoing reductive cleavage with, for example, Hg . We
demonstrate here that these materials are the first examples of
chalcogels featuring both ion-exchange properties and
elemental mercury capture abilities. We characterize the
chemical and physical identity of the polysulfide chalcogels
and show the pivotal role of the S−S bridging sites of the
polysulfide ligands in mercury capture experiments. Finally, we
show the ion-exchange functionality of our platinum polysulfide
chalcogels for aqueous environmental waste remediation.
RESULT AND DISCUSSION
■
The reaction between Pt and Sx2− precursors at room
temperature leads to a polymeric three-dimensional inorganic
framework. A formamide solution containing K S (x = 3−6)
2+
2
x
was slowly added to a formamide solution with an equivalent
molar amount of K PtCl resulting in a spontaneous cross-
2
4
2
+
2‑
Figure 3. (A) Scanning electron microscopy (SEM) image of K−Pt−
linking reaction between Pt and S units. Upon mixing of the
x
S chalcogel. (B) Transmission electron microscope (TEM) image of
x
two precursor solutions, a clear deep red colloidal solution
appeared. As the reaction progressed, the viscosity of the
solution increased, and eventually formed a monolithic dark
brown gel after 2 d. To process this wet gel into an aerogel, a
K−Pt−S chalcogel. (C) Powder X-ray diffraction pattern of
x
polysulfide chalcogels. (D) Typical nitrogen adsorption isotherm of
K−Pt−S chalcogels.
x
solvent exchange was conducted with EtOH:H O (4:1)
solution for 5 d, followed by EtOH for 5 d, and subsequently
2
of incorporated inorganic building blocks (Pt , Sx2−) and to
estimate the relative atomic composition of each element, we
analyzed the series of chalcogels formed from different
2+
supercritically dried with CO . These procedures were
2
necessary to extract liquid components out of gel structure
without destroying the porous network. With this treatment,
the monolithic wet gel was transformed into a fluffy dark brown
aerogel retaining the shape and volume of the original wet-
polysulfide chains (S32 , S4 , S5 , S6 ) with energy dispersive
X-ray spectroscopy (EDS). On the basis of EDS analysis, it was
confirmed that not only are platinum and the polysulfide
species present, but also potassium was incorporated as part of
the chalcogel. The EDS results from several different samples
showed the presence of potassium in all platinum polysulfide
chalcogels. This is similar to the cobalt polysulfide chalcogels
−
2−
2−
2−
6
,9,10,16
gel.
A schematic of a proposed structure adopted by
these chalcogels is shown in Figure 2. These schematic model
of the K−Pt−S chalcogel structure was inspired by the
x
previously reported open framework crystal structures of
K Pd(Se ) , and (Ph P)In(Se ) featuring polyselenide
28
reported previously. The average K/Pt/S ratio of each
2
x
2
4
6 2
1
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Journal of the American Chemical Society
Article
2
−
chalcogel was 0.37/0.81/3 for the trisulfide (S ), 0.38/0.84/4
point (P/P > 0.8), a small hysteresis was observed for each
3
o
2−
2−
for the tetrasulfide (S4 ), 0.36/0.87/5 for pentasulfide (S ),
chalcogel. This adsorption−desorption hysteresis loop
5
2
−
and 0.30/0.87/6 for the hexasulfide (S6 ).
(ADHL) is indicative of a percolation effect seen in disordered
30
The K−Pt−S chalcogels possess excellent ion-exchange
porous structures with large pores (d > 50 nm). A gel sample
x
properties associated with the potassium sites. When the K−
dried under ambient nitrogen conditions (called a xerogel)
2
Pt−S chalcogel was placed in 3 mM CsCl aqueous solution,
showed very low surface area (∼5 m /g), indicating the
3
we observed complete exchange of potassium ions with cesium
importance of supercritical point drying to preserve the
porosities of the chalcogels.
+
ions as shown in Figure 2S. Cs -exchanged polysulfide
chalcogels were examined by EDS analysis and showed the
Raman spectra of these chalcogels are consistent with the
presence of S−S vibrations of polysulfide ligands, shown in
Figure 4S. All chalcogels exhibited broad shifts in the region
+
absence of any detectable K . The reverse ion-exchange
+
reaction was also possible when the Cs -exchanged chalcogels
−1
were submerged with a 5 mM solution of aqueous KCl. The
between 430 and 500 cm . These broad peaks correspond to a
+
reverse-exchange yielded the complete replacement of Cs with
number of symmetric and asymmetric stretching modes of S−S
+
31
K . During the ion-exchange process, the polysulfide chalcogels
bonds in the polysulfide ligands. Broadening of these bands is
retained their frameworks as shown from SEM images.
consistent with the amorphous nature of chalcogels, and similar
Transmission electron microscope (TEM) images of the
polysulfide chalcogels revealed their nanoscale porous nature as
shown in Figure 3B and Figure 3S. Because of the absence of
structural directing agents during synthesis, the porous
structures of these chalcogels showed no regularities in shape,
size, or orientation. Electron diffraction of all samples showed
only diffuse scattering indicating a random aperiodic structure.
As shown in Figure 3C, this result is in agreement with the
broadening was observed in other glassy transition metal−
31−33
polysulfide compounds.
Prominent shifts of K−Pt−S
6
chalcogel from 300 to 400 cm− match with S−S stretching
1
peak of polycrystalline K S indicating the K S inclusion in the
2
6
2 6
gel matrix. Infrared spectra of these chalcogels show a similar
broadening of the peaks, Figure 5S. The broad absorption band
between 400 and 500 cm− corresponds to the S−S stretching
1
−1
mode, whereas that between 300 and 400 cm most likely
powder X-ray diffraction patterns of the K−Pt−S chalcogels,
arises from the presence of various platinum polysulfide
x
34
which show broad diffuse peaks and the absence of any
crystalline phase.
stretching modes converged into one broad band. The
−1
absorption at 500−520 cm is in the region of S−Pt−S
35
On the basis of the nitrogen adsorption isotherm and
Brunauer−Emmett−Teller (BET) model, these supercritically
stretching modes from other platinum sulfide compounds.
The K−Pt−S chalcogels were analyzed by thermal
x
2
dried chalcogels possess high surface areas (74−230 m /g),
gravimetric analysis (TGA) to investigate weight changes
caused by an increase in temperature. Under both nitrogen and
3
large pore volumes (0.29−0.52 cm /g), and 9−15 nm average
pore diameters, Table 1 and Figure 3D. These chalcogels show
air, the K−Pt−S chalcogels did not show any weight loss up to
x
2
00 °C, shown in Figure 6S. These results suggest that the
Table 1. Nitrogen Porosimetry Results of Potassium
Platinum Polysulfide Chalcogels (K−Pt−Sx)
chalcogels do not decompose at temperatures relevant to those
of flue gas emission (140 °C). TGA analysis and other
characterization results prompted us to test for possible Hg
vapor capture. Each S−S bonding site in the chalcogel structure
will be readily accessible to incoming Hg atoms and is expected
to be potential mercury vapor chemisorption sites according to
the equation:
a
avg. pore
c
elemental
comp. (EDS)
S
volume
avg. pore
diameter [nm]
BET
b
2
3
chalcogel
[m /g]
[cm /g]
K−Pt−S3
K−Pt−S4
K−Pt−S5
K−Pt−S6
K0.37Pt0.81S3
K0.38Pt0.84S4
K0.36Pt0.87S5
K0.30Pt0.87S6
115−230
183−200
74−171
0.38−0.47
0.42−0.52
0.29−0.48
0.37−0.48
8−13
8−14
10−22
9−14
−S − S − S − + 2Hg → − S − Hg − S − Hg − S−
128−195
a
In addition to chemisorption, there is also the possibility of
Three different samples of each chalcogel were used to evaluate these
b
physisorption of Hg on the surfaces of the chalcogel. The K−
results. Composition normalized per one polysulfide chain.
c
Pt−S chalcogels were tested for mercury vapor capture in a
Brunauer−Emmett−Teller (BET) surface area.
x
closed vial setup. An amount of ∼20 mg of each chalcogel was
placed in a glass vial containing ∼200 mg of elemental mercury,
and the vial was kept in a heated sand bath (∼140 °C) for 24 h.
minor adsorption at low relative pressures (0 < P/P < 0.5),
o
o
indicating that only a small fraction of the adsorption occurs in
The chalcogel was supported above the Hg by conical-shaped
micropores (d ≤ 2 nm). The majority of the adsorption occurs
filter paper to avoid a direct contact with elemental mercury. A
scheme of the experimental set up is shown in Figure 7S.
The glass vial was cooled to room temperature, and the
polysulfide chalcogel was retrieved for further analysis. To
at higher relative pressures (P/P > 0.5) where meso- (2 < d <
o
5
0 nm) and macropores (d > 50 nm) are the main
29
contributors. At relative pressures close to the saturation
a
Table 2. Mercury Vapor Capture Results for K−Pt−S Chalcogels
x
c
temp
[C°]
adsorption capacity (ICP)
adsorption capacity (EDS)
theoretical chemisorption capacity
atomic ratio of Hg-laden chalcogels
(EDS)
b
Chalcogel
[μg/g]
[μg/g]
[μg/g]
6
6
1.47 × 106
1.90 × 106
2.31 × 10
2.70 × 106
K−Pt−S3
K−Pt−S4
K−Pt−S5
K−Pt−S6
140
140
140
140
(1.56−3.00) × 10
(0.63−4.29) × 10
(3.20−5.45) × 10
(0.43−3.8) × 10
(0.81−1.01) × 10
(0.76−1.65) × 10
(0.49−1.79) × 10
(1.62−2.05) × 10
Hg1.4K0.21Pt0.86S3
Hg2.6K0.27Pt0.89S4
Hg3.1K0.29Pt0.82S5
Hg3.8K0.26Pt0.86S6
6
6
6
6
6
6
6
a
b
Three samples of each chalcogel were used to evaluate these results. Micrograms of mercury captured per gram of adsorbent (chalcogel) used.
c
Based on the assumption that each S−S bond captures one mercury atom.
1
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Journal of the American Chemical Society
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6
evaluate the total amount of Hg captured by either
chemisorption or physisorption, we quantified the amount of
mercury captured by each polysulfide chalcogel with two
different methods. First, the relative elemental ratio of Hg-laden
polysulfide chalcogels was used to calculate the amount of
mercury bound to the surface of S−S bonding sites. In the
second method, a diluted aqueous solution containing dissolved
chalcogel was analyzed by ICP to quantify the total amount of
mercury bound to each chalcogel.
adsorption capacity of each chalcogel ((3.0−5.45) × 10 μg/
g), evaluated from ICP analysis, exceeded the theoretical
estimation of full chemisorption capacity ((1.47−2.70) × 10
6
μg/g). This result demonstrates that these chalcogels are highly
capable of adsorbing additional mercury vapor even after all of
the chemisorption sites of the polysulfide surface are occupied.
Unlike the case of activated carbon, where a rise in temperature
lowers the adsorption capacity, none of the polysulfide
chalcogels showed any indication that increasing temperature
interferes with the adsorption process. On the basis of these
results, both chemisorption and physisorption contribute to
mercury vapor capture; chemisorption increases as more S−S
bonds (longer polysulfide chains) are available, whereas
physisorption is increased as larger surface area and more
porosities are available.
The relative atomic ratios of the polysulfide chalcogels based
on EDS analysis are Hg K0.21Pt S , Hg K0.27Pt S ,
1
.4
0.86
3
2.6
0.89 4
Hg K0.29Pt S , and Hg K0.26Pt S6 for trisulfide, tetrasul-
3.1
0.82
5
3.8
0.83
fide, pentasulfide, and hexasulfide chalcogels, respectively,
Figure 8S and Table 2. This corresponds to mercury adsorption
6
6
6
6
capacities of 1.01 × 10 , 1.65 × 10 , 1.79 × 10 , and 2.05 × 10
μg/g for the K−Pt−S , K−Pt−S , K−Pt−S , and K−Pt−S
3
4
5
6
To better understand the effect of chalcogel surface area on
Hg adsorption and further confirm that the presence of
polysulfide ligands is indeed important in the Hg adsorption
chalcogels, respectively, which accounts for 68−86% of the
theoretical chemisorption capacity; the theoretical Hg capacity
for each chalcogel was calculated on the basis of the assumption
that each S−S bridging site in the polysulfide ligand binds one
mercury atom. All mercury-laden chalcogels showed diffraction
peaks that suggest the development of crystalline HgS in the
chalcogels during the Hg capturing process, Figure 9S.
However, many parts of Hg-laden chalcogels retained their
spongy appearance in SEM images, which suggests that major
frameworks of chalcogels remained intact after the mercury
capture. EDS analysis gave an underestimated amount of Hg in
the chalcogels because the high e-beam absorption by Hg did
not allow proper sampling of the bulk. In contrast, ICP
provided a more reliable estimate via complete dissolution of
the samples.
capacity of the K−Pt−S chalcogels, we ran Hg capture
x
experiments using two control groups for comparison: the first
group was a crystalline compound with available S−S bonds,
that is, low surface area K S , and the second group was a zinc
2
3
2
10
tin sulfide chalcogel (ZTS-cg1, 503−520 m /g), which is a
very high surface area material lacking polysulfide functionality.
In these experiments, the K S sample did not show any Hg
capture as determined by EDS analysis. In contrast, the ZTS-cg
sample did show high adsorption capacity ((0.29−1.21) × 10
μg/g), which is more than an order of magnitude higher than
previously reported materials. However, the mercury capture
efficiency of ZTS-cg was still lower relative to the K−Pt−Sx
chalcogels. The results from these two control groups indicate
that both the rich polysulfide sites and the highly accessible
2
3
5
ICP evaluation showed higher mercury adsorption capacity
of the K−Pt−S , K−Pt−S , K−Pt−S , and K−Pt−S chalcogels
3
4
5
6
6
6
6
(
(1.56−3.00) × 10 , (0.63−4.29) × 10 , (3.25−5.45) × 10 ,
porous structure of K−Pt−S chalcogels contribute to the
x
6
and (0.43−3.80) × 10 μg/g, respectively). The equilibrium
adsorption capacity values of the polysulfide chalcogels are
higher than those of any commercially used or academically
record high mercury adsorption efficiency.
CONCLUSION
2
1
■
studied material, including sulfur impregnated coal, phenolic
polymers, sulfur functionalized copper doped Fe nano-
particles, and sulfur functionalized porous silica, Table 3.
New network assemblies composed of Pt and Sx2− form
monolithic inorganic gels incorporating K . These monoliths
were successfully transformed into highly porous polysulfide
2+
2
1
+
2
3
22
aerogels upon solvent exchange and supercritical drying. The
Table 3. Mercury Vapor Adsorption Capacity of Various
Absorbent Materials
K−Pt−S chalcogels are thermally stable up to 200 °C under
x
nitrogen or air, demonstrating the durability of these materials
at flue gas temperatures. The polysulfide chalcogels show high
Hg vapor adsorption capacity, which confirms that both
polysulfide functionality and accessible pores contribute to
the mercury vapor capture efficiency. Furthermore, the
potassium sites of the polysulfide chalcogels exhibit reversible
ion-exchange functionality. These preliminary results open up
the possibility of using ion-exchangeable chalcogel materials for
water remediation to remove toxic metal ions via replacement
of potassium ions. Future studies aimed at evaluating Hg
adsorption of these chalcogels in realistic flue gas conditions, in
which O , CO , and SO are continuously flowing with mercury
surface area
adsorption capacity
2
[
m /g]
temp [C°]
[μg/g]
5
K−Pt−S CG
74−230
N/A
140
140
140
140
140
140
140
(0.43−5.45) × 10
x
K2S3
N/A
a
2.94 × 104
1.44 × 103
3.26 × 103
ZTS-cg
503−520
160
b
PILOT5-S40
c
ACF-20-S40
94
d
3
Fe−Cu−S
Si-1
29.6
(0.17−2.73) × 10
e
1.98 × 104
391.3
2+
a
4−
b
Zinc tin sulfide (Zn −[SnS ] ) chalcogel. Sulfur impregnated
4
21
c
21 d
coal. Phenolic polymer.
Sulfur-functionalized copper-doped Fe
23
e
22
2
2
2
nanoparticle. Copper-doped sulfur functionalized porous silica.
vapor, are needed. Of course, the K−Pt−S chalcogels can only
be regarded to be model systems given the presence of
platinum. These results, however, clearly point to the high
potential of chalcogels in environmental application, and we
anticipate usage of other widely available linking metals such as
Because our experimental setting of mercury vapor capture may
be very different from the ones used to measure the value
reported in the literature, the direct comparison of adsorption
capacity should be made with caution. However, these high
values of platinum chalcogels do confirm the mercury vapor
capture functionality of polysulfide ligands. The highest
2+
2+
2+
2+
2+/4+
Ni , Zn , Fe , Cu , Sn
, etc., as alternatives to platinum
that will create practical low cost materials for Hg vapor capture
using the paradigm shown here.
1
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dx.doi.org/10.1021/ja3061535 | J. Am. Chem. Soc. 2012, 134, 14604−14608

Journal of the American Chemical Society
Article
(
9
c) Park, Y. B.; Kanatzidis, M. G. Angew. Chem., Int. Ed. Engl. 1990, 29,
ASSOCIATED CONTENT
Supporting Information
Experimental and characterization details. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
14.
27) Li, J.; Chen, Z.; Wang, R. J.; Proserpio, D. M. Coord. Chem. Rev.
999, 192, 707.
28) (a) Shafaei-Fallah, M.; He, J.; Rothenberger, A.; Kanatzidis, M.
*
S
(
1
(
G. J. Am. Chem. Soc. 2011, 133, 1200. (b) Shafaei-Fallah, M.;
Rothenberger, A.; Katsoulidis, A. P.; He, J.; Malliakas, C. D.;
Kanatzidis, M. G. Adv. Mater. 2011, 23, 4857.
AUTHOR INFORMATION
Corresponding Author
m-kanatzidis@northwestern.edu
■
(29) Rouquerol, F.; Rouquerol, J.; Sing, K. S. W. Adsorption by
Powders and Porous Solids: Principles, Methodology, and Applications;
Academic Press: San Diego, CA, 1999.
(30) Lopez, R. H.; Vidales, A. M.; Zgrablich, G. Langmuir 2000, 16,
Notes
The authors declare no competing financial interest.
6
999.
ACKNOWLEDGMENTS
(31) Janz, G. J.; Coutts, J. W.; Downey, J. R.; Roduner, E. Inorg.
Chem. 1976, 15, 1755.
■
This research was supported by the National Science
Foundation (Grant DMR-1104965). ICP-AES and Raman
spectroscopy measurements were made from Integrated
Molecular Structure Education and Research Center (IM-
SERC) facilities, and electron microscope imaging work (TEM,
STEM) was performed in the EPIC, facility of NUANCE
Center at Northwestern University.
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(
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