Determination of the loading and stability of Pd in an arborescent copolymer in ethanol by microplasma-optical emission spectrometry

Article abstract of DOI:10.1039/c3ra46232c

We report, for the first time, the utilization of a microplasma-optical emission spectrometry system for the determination, without sample digestion, of the concentration of Pd loaded in a dendritic graft (arborescent) copolymer dissolved in ethanol. The preparation of polymer-stabilized colloidal Pd particles was achieved by adding palladium acetate to a solution of the copolymer, viz. arborescent polystyrene-graft-poly(2-vinylpyridine), in ethanol. No acid digestion was needed prior to the analysis, and only micro-amounts (μL) of sample were required. Calibration curves obtained for Pd in ethanol were linear in the concentration range of interest, and the percent relative standard deviation (%RSD) ranged from 7.4 to 0.1%. The Pd detection limit was 28 pg (absolute) or 3 ng mL^-1 (when using 10 μL samples). The average Pd loading per mole of 2-vinylpyridine unit was determined to be 99.5 mol%. The kinetics of aggregation of the metallic species to Pd black were also determined. The Pd concentration in ethanol without polymer was found to abate to about one third of its initial value after 5 days. In the presence of the copolymer, however, the concentration of Pd in solution remained constant for at least 10 days. The low electric power and gas consumption of the microplasma device, its low operating cost and detection limit, compatibility with organic solvents, and the small sample amount required make this system a greener and cheaper alternative to the inductively coupled plasma (ICP) spectrometry commonly used for Pd quantification.

Full text of DOI:10.1039/c3ra46232c

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Determination of the loading and stability of Pd in
an arborescent copolymer in ethanol by
Cite this: RSC Adv., 2014, 4, 8978
microplasma-optical emission spectrometry†
*
Olivier Nguon, Mario Gauthier and Vassili Karanassios
We report, for the first time, the utilization of a microplasma-optical emission spectrometry system for the
determination, without sample digestion, of the concentration of Pd loaded in a dendritic graft
(arborescent) copolymer dissolved in ethanol. The preparation of polymer-stabilized colloidal Pd
particles was achieved by adding palladium acetate to a solution of the copolymer, viz. arborescent
polystyrene-graft-poly(2-vinylpyridine), in ethanol. No acid digestion was needed prior to the analysis,
and only micro-amounts (mL) of sample were required. Calibration curves obtained for Pd in ethanol
were linear in the concentration range of interest, and the percent relative standard deviation (%RSD)
ranged from 7.4 to 0.1%. The Pd detection limit was 28 pg (absolute) or 3 ng mLꢀ1 (when using 10 mL
samples). The average Pd loading per mole of 2-vinylpyridine unit was determined to be 99.5 mol%. The
kinetics of aggregation of the metallic species to Pd black were also determined. The Pd concentration
in ethanol without polymer was found to abate to about one third of its initial value after 5 days. In the
presence of the copolymer, however, the concentration of Pd in solution remained constant for at least
10 days. The low electric power and gas consumption of the microplasma device, its low operating cost
and detection limit, compatibility with organic solvents, and the small sample amount required make this
system a greener and cheaper alternative to the inductively coupled plasma (ICP) spectrometry
commonly used for Pd quantification.
Received 29th October 2013
Accepted 20th January 2014
DOI: 10.1039/c3ra46232c
www.rsc.org/advances
Gauthier et al. rather synthesized arborescent (dendritic gra)
copolymers incorporating a branched polystyrene (PS) core and a
corona of P2VP chains16,17 to serve as templates for the
Introduction
The unique and size-dependent properties of metallic nano-
particles (NPs) present great opportunities for a wide array of
applications ranging from sensing to optoelectronics, medi-
cine, and catalysis.1,2 Since the rst report by Faraday in 1857,3
various methods have been developed for the preparation of
colloidal metallic particles.4
One of the most commonly used strategies involves the
reduction of a salt precursor in the presence of a stabilizer in
solution. Hirai et al. showed that alcohols could serve as
reducing agents,5 and that in the presence of a polymeric
stabilizer Pd NPs could be prepared.6 Using a similar strategy,
poly(2-vinylpyridine) (P2VP) in the form of linear homopoly-
mers,7–10 block copolymers,11–13 and nanospheres14 was shown
to complex with palladium species, even at high temperatures
and pressures,15 and to lead to the formation of Pd NPs aer
reduction of the metallic species.
complexation of Au(III) salts,18 but also for the preparation and
the stabilization of Pd NPs in ethanol.19 Polymer-stabilized Pd
nanocatalysts were shown to be useful for a wide range of organic
reactions such as carbonylation, hydrogenation, oxidation,
reduction, and carbon–carbon cross-coupling reactions.20–23
An important way to assess the ability of a polymer to
sequester a metal is by determining its loading capacity; that is,
by determining the amount of metal loaded per unit amount of
stabilizer. This is a key gure of merit for the evaluation of a
catalyst system, which makes its precise and accurate determi-
nation cardinal. In cross-coupling reactions for instance, the Pd
nanocatalyst concentrations used are typically in the low mg
mLꢀ1 (part per million, or ppm) range, to even the ng mLꢀ1
(parts per billion, or ppb) range.24–26 However, the determina-
tion of such low Pd concentrations in polymers dissolved in
organic solvents has been reported to be challenging.26–29
Many analytical techniques have been applied to the deter-
mination of Pd concentrations in colloidal systems. This
includes cyclic voltammetry,30 UV-visible spectrophotometry,31
neutron activation analysis (NAA),32 X-ray uorescence (XRF) –
such as energy-dispersive (EDX)33,34 or wavelength-dispersive
(WDX)35 X-ray spectroscopy, atomic absorption spectroscopy
Waterloo Institute for Nanotechnology, Department of Chemistry, University of
Waterloo, Waterloo, ON, N2L 3G1, Canada. E-mail: gauthier@uwaterloo.ca; Fax:
+1 519 746-0435; Tel: +1 519 888-4567 ext. 35205
† Electronic supplementary information (ESI) available: Emission signals,
calibration curves, and kinetic plots of the various Pd species. See DOI:
10.1039/c3ra46232c
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(AAS),13,29,36 and inductively coupled plasma-optical emission diluted using either ethanol (undenatured grade, anhydrous,
spectrometry (ICP-OES).33,34,37–39
Commercial Alcohols Inc., Brampton, ON, Canada) or Milli-Q
Among these, ICP-OES is most widely used due to its desir- water (18.2 MU cm, EMD Millipore Systems, Billerica, MA, USA).
able analytical performance characteristics, such as limits of The ethanol was distilled in a polytetrauoroethylene (PTFE)
detection (LODs) in the low-ppb to sub-ppb range for many apparatus, to eliminate the possibility of metal contamination
elements. Despite their applicability, ICP systems are expensive from glassware. All the samples were stored in acid-washed low
to operate and have a relatively large carbon footprint. For density polyethylene (LDPE, Nalgene®) bottles. All the poly-
example, a typical ICP instrument consumes about 20 L minꢀ1 propylene micropipette tips (Bevel Point, 1–20 mL, VWR,
of Ar gas and 1–2 kW of electric power.40 In many cases the high Mississauga, ON, Canada), LDPE bottles and vials used were acid-
cost-per-analysis prohibits the characterization of a large washed by soaking for at least 48 h in a 5% (w/v) nitric acid
number of samples, as it would be essential for instance for the solution, and then rinsed with Milli-Q water. Drying was subse-
systematic “evaluation of catalysts and recycling systems before quently performed in a ventilated dust-free enclosure at room
and aer reaction and continuous monitoring of changes during temperature for a minimum of 48 h. A Mettler-Toledo XS205
26
´
reactions”, as recommended by Molnar.
semi-micro balance with a 0.01 mg display was used for sample
Furthermore, when using the most widely employed method preparation. Pd-containing microsamples were pipetted with a
to introduce samples into an ICP, viz. a pneumatic nebulizer, the Corning Lambda™ micropipette (2–20 mL, accuracy ꢂ 5.0%, and
total volume of sample required per analysis ranges from one to a percent relative standard deviation (%RSD) # 1.5%). The
a few milliliters. The sample introduction efficiency of a nebu- carrier gas, also used as the microplasma support gas, was a
lizer is low (1–5%); therefore over 95% of a sample must be mixture of 97% Ar–3% H2, v/v, (Praxair Canada Inc., Mississauga,
collected and disposed of properly. Other issues also arise when ON, Canada). Palladium(II) acetate (Pd(OAc)2, min. 98%, Strem
using a nebulizer to introduce nanoparticles or polymers directly Chemicals Inc., Newburyport, MA, USA) was the source of Pd.
into an ICP. For instance clogging of the nebulizer by nano-
particles, and sample-to-sample carry-over from polymer
Copolymer synthesis
adhering to the walls of the spray chamber or on the tubing
A rst-generation polystyrene-gra-poly(2-vinylpyridine) (G0PS-
(memory effects) have been reported.41 To overcome these
g-P2VP) arborescent copolymer was synthesized by Gauthier
and Munam, by anionic polymerization and graing tech-
issues, polymer-stabilized nanocatalysts must be digested (or
dissolved), typically with an acid. Although effective, acid
niques, according to a reported procedure.17 The PS core of the
digestion increases the risks of both analyte loss during sample
copolymer was a comb-branched (or generation 0, G0) polymer
processing and contamination from the digestion reagents.42
Furthermore, if organic solvents are introduced into an ICP, the
use of a mixed-gas Ar–O2 plasma is required to eliminate plasma
instability and to prevent carbon deposits from the solvents.43
Such procedures further increase the complexity, cost-per-anal-
ysis, as well as the carbon footprint of the analytical procedure.
prepared by graing randomly about 17 PS side chains (each
with Mn ¼ 5500 g molꢀ1) onto a linear PS substrate (Mn ¼ 5200 g
molꢀ1). The comb-branched polymer was further graed with
182 P2VP side chains (Mn ¼ 5500 g molꢀ1), corresponding to a
2-vinylpyridine (2VP) unit content of 91 mol% in the copolymer
obtained. The molar-mass dispersity (Mw/Mn) of the sample was
1.08. The structure of the copolymer obtained, referred to as G1
(overall generation 1), is depicted in Scheme 1.
Weagant and Karanassios developed a low-cost and greener
`
analytical method (vis-a-vis ICP) using microplasmas (16 ꢁ 2 ꢁ 9
mm3, length ꢁ width ꢁ height) that have a low gas ow rate
(0.23 L minꢀ1) and a low power consumption (<15 W).44 The
Loading of Pd in the copolymer
same group demonstrated its applicability to solid, liquid, and
gaseous samples;45 and so far eleven elements have been char-
Loading of the Pd catalyst was achieved by co-dissolution of
acterized by that technique, primarily using dry residues
derived from aqueous microsamples.40 The LODs achieved
ranged from 5 to 650 picograms (pg).
Pd(OAc)2 with the arborescent copolymer in ethanol solutions
as follows: the polymer was dissolved overnight in ethanol
In this work, we demonstrate for the rst time that this
microplasma-based analytical method can be used for the direct
determination (i.e., without digestion) of Pd loading in arbo-
rescent copolymers dissolved in organic solvents. We also apply
this method to determine the kinetics of aggregation and the
stability of palladium acetate, and of arborescent polymer-
stabilized Pd nanoparticles in ethanol.
Experimental section
Materials and methods
ICP-grade Pd standard solutions (Pd-Std) in 10% HCl, 1000 mg
mLꢀ1 ꢂ 0.5% (PlasmaCAL, ICP-AES/MS standard, SCP Science,
Scheme 1 Loading of the G1 arborescent copolymer (G0PS-g-P2VP)
Baie d'Urfe, QC, Canada) were used. The solutions were freshly with Pd(OAc)2 in ethanol.
´
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(0.2 mg mLꢀ1) in a LDPE vial. Immediately aer the dissolution
of Pd(OAc)2 in ethanol (0.25 mg mLꢀ1) and sonication for
2 minutes, the desired amount of catalyst solution (either
0.25 or 1.5 molar equiv. of Pd per 2VP unit) was added to the
polymer solution to obtain a yellow translucent solution. The
separation of free Pd from the polymer-bound Pd was achieved
by transferring 15 mL of the polymer–catalyst solution to a
dialysis membrane (1000 molecular weight cut-off Spectra/Por®
7 regenerated cellulose), and stirring for 6 h in 200 mL of
ethanol while changing the solvent thrice. The polymer–catalyst
solution was then recovered from the dialysis membrane and
diluted to obtain a 2VP unit concentration of 0.7 mg gꢀ1
(0.7 ppm).
Transmission electron microscopy imaging
Imaging by transmission electron microscopy (TEM) was
performed in the bright-eld mode on a Philips CM10 electron
microscope operated at an accelerating voltage of 60 kV. The
samples were prepared by depositing two drops of Pd-contain-
ing solution (0.07 mg mLꢀ1) onto a 300-mesh Formvar®
carbon-coated copper TEM grid (Electron Microscopy Sciences,
FCF300-Cu).
Instrumentation and operating conditions
A block diagram of the instrumentation used for the quantita-
tive determination of Pd is shown in Fig. 1a. Conceptually, the
instrumentation consists of 5 parts: (1) a microsample intro-
duction system; (2) a microplasma device; (3) a scanning
monochromator (Heath, 0.35 m Czerny–Turner design equip-
ped with a 1200 grooves per mm grating); (4) a photomultiplier
tube (PMT) detector (Hamamatsu model R928 tted inside a
Heath EU-701-30 PMT module); and (5) an amplier (SRS 570)
with a data acquisition sub-system (National Instruments
DAQCard 1200) including a computer running a locally devel-
oped LabVIEW program for data acquisition.
Fig. 1 (a) Schematic of the instrumentation used (illustration not to
scale) and (b) representative signal.
The electrothermal vaporization microsample introduction
system (Fig. 1a) consisted of a vaporization chamber, and a for 30 s. When the copolymer was present in the sample, an
cylindrical ceramic support equipped with a rhenium coiled additional step was performed to char the dried polymer
lament at one end. Cables running through conduits in the remaining on the coil. This was accomplished by applying
ceramic support connected the lament to an external electric progressively higher electric power levels; for example, 0.27 W
power supply. Deposition of the microsamples was performed for 15 s, then 0.44 W for 15 s, and subsequently 0.72 W for 30 s.
by retracting the ceramic support from the vaporization Progressive heating was used to avert possible bursting of
chamber, and pipetting a few microliters (3.0–10.0 mL) of solu- sample microdroplets from the rapidly heated coil, which
tion onto the coil. The ceramic support, along with the sample- would have resulted in analyte loss. Progressive heating during
carrying coil, was then re-inserted into the vaporization charring was also used to avoid potential bursting of the poly-
chamber.
mer matrix. Aer sample drying (and charring if required) the
Solvent removal from the sample prior to the analysis was coil was allowed to cool for 60 s, with the Pd-containing residue
found essential to avoid microplasma instability. In the remaining on the coil.
presence of ethanol, the plasma took on a purple color and
The microplasma was subsequently ignited and the visually
displayed an erratic background emission. It is well known that stable, blue-colored microplasma was allowed to thermally
plasmas (regardless of their size), when unaided by mixing equilibrate for 60 s (so that stable microplasma background
oxygen gas, do not tolerate organic solvent vapors. The sample emission was obtained). A higher electric power was then
drying procedure used for such samples was the following: the applied to the coil (e.g., 44.8 W, corresponding to ca. 2500 ꢃC) to
electric power applied to the coil was rst set to 0 W for 1 min vaporize the Pd-containing residue. This temperature was
(to allow for the bulk of the volatile solvent to vaporize at room found sufficient to vaporize the residues from the nely
temperature). The electric power was then increased to 0.12 W dispersed Pd samples (vide infra), which are expected to display
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a lower vaporization temperature than their bulk counter- lower for the aqueous solutions, and for larger injection
parts.46 The gas-phase metal atoms exited the vaporization volumes. Solvent blanks (i.e., without any Pd added) did not
chamber and were transported to the microplasma by the show any emission signals.
carrier gas (230 mL minꢀ1, Ar–H2). Argon mixed with hydrogen
(3% v/v) was used to prevent oxidation of the Re coil by the low-
ppm levels of water and oxygen typically present in commercial
compressed gas cylinders.47 Interaction of the vaporized sample
residue with the microplasma led to the atomic emission from
Pd I at 340.458 nm which was measured by the PMT detector
(Fig. 1a). The output of the detector was amplied, digitized,
and stored onto a computer system. An example of signal so
obtained is shown in Fig. 1b.
Although the emitted signals lasted for only about 0.5 s, data
were acquired for 5 s to monitor microplasma background
emission during the pre-vaporization and post-vaporization
time intervals (Fig. 1b). Furthermore, when a polymer was
present, the electric power was applied to the coil for an addi-
tional 5 s to remove any carbonaceous material potentially
Copolymer-stabilized Pd solutions in ethanol
The LOD (using the 3s criterion) for Pd was estimated from the
standard deviation of the background before the emission peak
when using the same number of data points as for the peak.48
The average LOD estimated using Pd-loaded polymers diluted
in ethanol was 28 pg (expressed in absolute amount, Table S1†).
When using 10 mL of solution, these LODs correspond to 3 ng
mLꢀ1 (or 3 ppb, in relative concentration units). Such a limit of
detection is ca. 15 times lower, for instance, than the concen-
tration at which Pd impurities were found to be catalytically
active (50 ppb).49 This is also well below some of the “homeo-
pathic” concentrations used (>500 ppb) in a variety of Heck and
Suzuki cross-coupling reactions.36,50–52
remaining on the coil. At the end of this sequence the electric
power was turned off, and the coil was allowed to cool for 60 s Quantication of Pd loading in the arborescent copolymer
before a subsequent run. For each sample or standard solution,
To determine the maximum amount of Pd that can be loaded in
3.0, 6.0 and 10.0 mL injections volumes were used in at least
the arborescent copolymer described earlier (G0PS-g-P2VP,
triplicate runs (Tables S1–S3†). There was therefore a minimum
Scheme 1) an excess of Pd atoms (1.5 molar equiv.) with respect
to the 2VP units was added. Dialysis of the polymer–Pd solution
in ethanol was then used to remove any unbound metal. The
of 9 runs per sample or standard solution. It is worth noting
that the above steps (drying, charring, and coil-cleaning
procedure when applicable) were repeated for more than 1700
residual Pd content in the polymer was measured and quanti-
vaporization cycles (i.e. analytical runs) without evidence of
ed based on a calibration curve constructed from non-dialyzed
carbon deposition, fouling of the microplasma electrodes, or
polymer–Pd solutions diluted in ethanol (R2 ¼ 0.9920). The
coil alteration. The microplasma background emission was also
calibration curve obtained and the amount of Pd measured are
stable, with a %RSD in background variations determined to be
shown in Fig. 2. The average Pd content per 2VP unit was found
less than 0.6% over 10 h of continuous operation. An example of
to be 99.5 mol%, with a %RSD below 3.3% (Table S2†). In the
remaining of the discussion, this Pd-loaded polymer sample
will be referred to as G1-Pd[100 mol%].
this is shown in Fig. S1.† It is estimated that the operating cost
of the microplasma is roughly 100 times lower than for that of a
current ICP-OES instrument.
The Pd concentration was also determined in a polymer-
stabilized Pd sample containing 0.25 equiv. of Pd per 2VP unit.
Results and discussion
Pd standard solutions in water and ethanol
The amount of Pd was selected to insure complete loading of
the micelles, and has been shown to lead to the formation of
stable Pd(II)–polymer hybrid systems.19 Aer dialysis, the overall
Pd content measured using a minimum of 9 runs was 23.7 mol
%, with a %RSD # 5.4% (Fig. S3 and Table S3†). This corre-
sponds to 93.3 mol% of the Pd added prior to dialysis. This
Both aqueous and ethanolic solutions, prepared from a Pd
standard solution, were analyzed for the rst time using the
microplasma set-up shown in Fig. 1a. A typical optical emission
signal and stable microplasma background emission obtained
during the pre- and post-vaporization time intervals are also
displayed in Fig. 1b. Calibration curves, obtained using the
integrated area of the optical emission peaks, were linear and
comparable in the concentration range of interest (with typi-
cally R2 ¼ 0.99998 in ethanol) (Fig. S2†).
To improve the statistical condence, three different injec-
tion volumes were used (viz. 3.0, 6.0 and 10.0 mL) for each
standard solution or sample. In absolute units, the amount of
Pd injected ranged between 3.0 and 10.0 ng. The precision,
expressed in %RSD, was determined from at least triplicate runs
for each of the injection volumes used (Tables S1–S3†). The %
RSD obtained for the solution of Pd-Std in water was below
1.7%, and for the Pd-Std solution in ethanol it ranged between
7.4 and 0.7%. For the samples of Pd and polymer dissolved in
Fig. 2 Calibration curve for Pd in a solution of G1-Pd(OAc)2 [1.5 equiv.]
in ethanol (blue diamonds), and analysis of the dialyzed sample
G1-Pd(OAc)2 using the following injection volumes: 3.0 mL (red
ethanol, the %RSD was below 3.8%. In general, the %RSD was square), 6.0 mL (green triangle) and 10.0 mL (purple circle).
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sample will be referred to as G1-Pd[24 mol%] in the rest of the water, these solutions being more easily prepared for rapid
discussion.
Imaging by TEM of the polymer–Pd solutions conrmed the
analysis (vide infra).
From the results shown in Fig. 4, it can be concluded that
presence of Pd in the copolymer templates, displaying an even for a Pd concentration as low as 2.4 ꢁ 10ꢀ6 M (0.25 mg
overall diameter ca. 18 nm, as shown in Fig. 3. Pd NPs 2–7 nm in mLꢀ1), there is a rapid decrease in the amount of Pd remaining
diameter, presumably formed through an Ostwald ripening dispersed in solution. Aer 5 days, for instance, only about one
process, are also visible within G1-Pd[100 mol%].
third of the initial Pd content was le in solution. Assuming a
It was thus concluded that the arborescent copolymer can rst-order rate process, one can write the aggregation rate as:
successfully complex with the Pd(II) species present in solution. r ¼ ꢀd[Pd]/dt. The apparent rate constant kapp is then obtained
It also appears that all the 2VP units in the G1 arborescent from ln([Pd]/[Pd]0) ¼ ꢀkappt, where [Pd]0 and [Pd] represent the
copolymer are accessible to the metallic ions and can contribute initial concentration of Pd in solution and the concentration at
to forming stable colloidal dispersions. These conclusions are time t, respectively. The graphical method seems to conrm the
in accordance with earlier work reporting the fast coordination validity of the rst order assumption, and the apparent rate
of Pd2+ species,53 and strong interactions of Pd with the lone constant calculated for Pd aggregation from Pd(OAc)2 in
electron pair of the nitrogen atom in the 2-vinylpyridine units in ethanol was (kapp EtOH
)
¼ 9.89 ꢁ 10ꢀ3 hꢀ1 (Fig. S4†).
aqueous media,10,13,14 or even at high pressures and tempera-
In more concentrated solutions and in the absence of a
tures in organic solvents.15 However the exact nature of the polymeric stabilizer, a deposit of Pd black was observed aer
complex formed still requires further investigation. Bekturov only two hours, as shown in Fig. 5b. The reduction of Pd(II) to
et al. suggested a model for the complexation of PdCl2 by P2VP Pd(0) in ethanol according to Scheme 2 is well-known; in fact
which accounts for a 1 : 1 molar ratio of 2VP units and Pd.7 this was taken advantage of in the preparation of various poly-
However, more recently Fernandez et al. reported that up to 1/3 mer-stabilized Pd nanoparticles.4,6,14,34
of the Pd in commercial Pd(OAc)2 could be in the form of Pd(0).
It should be noted that aqueous solutions prepared from a
These reduced species accounted for an excess of Pd measured Pd standard solution (by dilution with Milli-Q water to 1 mg
in polymer brushes decorated with dipyridylamine ligands, mLꢀ1 or 9.40 mM in Pd, and pH 2.7) displayed a similar decrease
when compared to the stoichiometric complexation expected in concentration, but a plateau was reached aer 2 days at about
with Pd(II) species.54 The results obtained in the current study 72 weight% (wt%) of the initial Pd concentration (Fig. S5†). The
are consistent with the formation of a 1 : 1 complex between the rst-order apparent rate constant of aggregation, determined
2VP units and Pd as suggested by Bekturov et al.
before reaching the plateau, was (kapp water
(Fig. S4†). The diluted solutions remained yellow and
)
¼ 6.41 ꢁ 10ꢀ3 hꢀ1
Stability and kinetic studies
Stability of Pd(II) in ethanol and water, and kinetics of
aggregation. A solution of Pd(OAc)2 in ethanol was prepared
and its Pd concentration was determined daily over 6 consec-
utive days. The vial containing Pd(OAc)2 in ethanol was stored
in the dark, since light exposure has been reported to enhance
the reduction rate of Pd(II).55 The change in Pd concentration in
the ethanolic solution was determined by calibrating the
instrument with freshly prepared solutions of a Pd standard in
Fig. 4 Evolution of the mass concentration of Pd in ethanolic solutions
of Pd(OAc)2 (red squares), and G1-Pd(OAc)2 [0.25 equiv.] (green trian-
gles) as measured by microplasma-optical emission spectrometry.
Fig. 3 TEM images obtained from ethanol solutions of G1-Pd[24 mol
%] (left), and G1-Pd[100 mol%] (right). The brightness and contrast
were adjusted for better visualization (the scale bars represent 50 nm).
Inset: magnification over a single micelle, with a dotted circle added to Fig. 5 Appearance of (a) a Pd standard solution in water, and ethanolic
help visualization; the scale bars represent 20 nm.
solutions of (b) Pd(OAc)2, (c) G1-Pd[24 mol%], and (d) G1-Pd[100 mol%].
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copolymer, however, the Pd concentration in ethanol remained
stable for at least 10 days.
Overall, the microplasma-based approach described above
will help address the need for greener and cheaper quantitative
analytical methods, and thus facilitate more widespread use of
such methods in catalysis, as articulated by Molnar.26 Work is in
´
Scheme 2 Reduction of Pd(II) to Pd(0) in ethanol.
progress to evaluate other organic solvents (e.g., THF) and
aqueous systems, with the aim of making the overall catalytic
process greener and cheaper.
translucent, as shown in Fig. 5a. The formation of a chlorohy-
droxypalladium(II) precipitate has indeed been reported for Pd2+
at concentrations above 1 ppm, but this was thought to be less
predominant at lower Pd concentrations.56–58 Clearly, the solu-
tions used for calibration purposes should be freshly prepared
before use.
Acknowledgements
The authors gratefully thank the Natural Sciences and Engi-
neering Research Council of Canada (NSERC), and GreenCentre
Canada for their support of the work.
Stability of Pd(OAc)2 in the arborescent copolymer in
ethanol. The analysis of G1-Pd[24 mol%] in ethanol revealed
that the Pd concentration remained constant for at least 10 days
(Fig. 4). In the presence of the polymer, the solutions remained
yellow (G1-Pd[24 mol%]) or brown (G1-Pd[100 mol%]) and
translucent, with no indication of precipitate formation, as
shown in Fig. 5c and d. This contrasts with the rapid drop in Pd
concentration and the formation of Pd black observed in etha-
nolic solutions without polymer. Hoogsteen and Fokkink made
a similar observation with linear P2VP-stabilized Pd, and
showed that the polymer delayed the reduction of Pd(II) in
water.10 The arborescent P2VP template obviously also acts as a
stabilizer to prevent the reduction and aggregation of the Pd(II)
species in ethanol.
Notes and references
1 G. Schmid, in Nanoparticles, Wiley-VCH, Weinheim, 2nd
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