A Broader-scope Analysis of the Catalytic Reduction of Nitrophenols and Azo Dyes with Noble Metal Nanoparticles

Article abstract of DOI:10.1002/cctc.201900260

In addition to the broad environmental implications associated with the removal of nitroaromatics from industrial effluent, the catalytic reduction of 4-nitrophenol (4NP) has emerged as a benchmark model for quantifying catalytic activity of metal nanoparticles. Here we present a series of noble metal nanoparticles immobilized on amorphous carbon (Au@C, Ag@C, Pt@C and Pd@C). All materials show competitive catalytic activity over 4NP, amino-substituted nitrophenols (ANPs) and azo dyes. Overall, Pd@C exhibits superior activity that increases further when exposed to recycling protocol. Moreover, testing all materials synthesized over a broader substrate scope with added functionalities reveals inconsistencies in the prognosticating ability of the ubiquitous 4NP model reaction. By incorporating variably substituted ANPs into the substrate scope and averaging performance, the resulting rank of catalyst activity more accurately reflects activity trends when applied to other reducible functionalities, such as -N=N- groups in azo dyes.

Full text of DOI:10.1002/cctc.201900260

Accepted Article
Title: A broader-scope analysis of the catalytic reduction of
nitrophenols and azo dyes with noble metal nanoparticles
Authors: Titel Jurca, Lorianne R. Shultz, Lin Hu, Konstantin Preradovic,
Melanie J. Beazley, and Xiaofeng Feng
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201900260
Link to VoR: http://dx.doi.org/10.1002/cctc.201900260
A Journal of
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A broader-scope analysis of the catalytic reduction of
nitrophenols and azo dyes with noble metal nanoparticles
Lorianne R. Shultz,[a] Lin Hu,[b] Konstantin Preradovic,[a] Melanie J. Beazley,*[a] Xiaofeng Feng,*[b][c][d]
Titel Jurca*[a][d]
Abstract: In addition to the broad environmental implications
associated with the removal of nitroaromatics from industrial effluent,
the catalytic reduction of 4-nitrophenol (4NP) has emerged as a
benchmark model for quantifying catalytic activity of metal
nanoparticles. Here we present a series of noble metal nanoparticles
immobilized on amorphous carbon (Au@C, Ag@C, Pt@C and
Pd@C). All materials show competitive catalytic activity over 4NP,
amino-substituted nitrophenols (ANPs) and azo dyes. Overall, Pd@C
exhibits superior activity that increases further when exposed to
recycling protocol. Moreover, testing all materials synthesized over a
broader substrate scope with added functionalities reveals
inconsistencies in the prognosticating ability of the ubiquitous 4NP
model reaction. By incorporating variably substituted ANPs into the
substrate scope and averaging performance, the resulting rank of
catalyst activity more accurately reflects activity trends when applied
to other reducible functionalities, such as -N=N- groups in azo dyes.
activity of metal nanoparticles.[5] The majority of this work to date,
and the mechanistic understanding derived from it, is based on
noble metal nanoparticles (e.g. Pd, Pt, Ag, and Au).[6] However, in
the past decade, efforts have expanded to include systems based
on other transition metals, and even metal-free systems.[7]
When conducted in the presence of excess NaBH4, as is
common practice, the reduction of 4NP to 4AP (Scheme 1) is
typically described in terms of the Langmuir-Hinshelwood
model.[5a,8] Therein, reactants 4NP* and NaBH4-derived surface
hydrogen species are first adsorbed onto the catalyst surface in a
rapid, reversible process. The reaction proceeds in a pseudo-first
order manner via
a series of intermediates including 4-
nitrosophenol and 4-hydroxylaminophenol (Scheme S1)[9]
ultimately yielding 4AP*, which rapidly desorbs from the surface,
thus having little impact on reaction kinetics (Schemes 1 and S1).
As a result, the rate-limiting step is described as the reaction of
adsorbed 4NP and hydrogen species on the catalyst surface.
Catalyst performance is quantified in terms of the apparent rate
constant kapp (eq. 1), which is derived from the slope of ln(C/C0)
vs time, where C/C0 is obtained from the recorded absorbance
intensity for 4NP* at λmax = 400 nm (A/A0). The details governing
this widely accepted and applied approximation have been
discussed in detail in numerous previous publications.[5]
Para-nitrophenol (4NP), and more broadly nitroaromatic
compounds are widely used in industrial processes for the
manufacture of pharmaceuticals, pesticides, pigments, dyes, and
explosives.[1] It is thus not surprising that nitroaromatic
compounds are one of the most common anthropogenic water
pollutants in industrial effluent. Many of these compounds are
acutely toxic, mutagenic and either suspected or established
carcinogens.[2] However, within the context of 4NP, the reduction
product is 4-aminophenol (4AP), a significantly less toxic and
important chemical intermediate in the synthesis of
agrochemicals,
dyes,
photographic
developers,
and
Scheme 1. Catalytic reduction of 4-nitrophenol (4NP) to 4-aminophenolate
(4AP*) by metal nanoparticles under aqueous conditions with excess NaBH4.
pharmaceuticals (e.g. paracetamol).[3] Thus, access to 4AP, and
similarly derived amino-phenols via mild, aqueous phase catalytic
reduction of 4NP has emerged as an important area of research.[4]
Conversely, due to its reliable simplicity, the reduction of 4-
nitrophenol (4NP) with NaBH4 under aqueous conditions has
become a benchmark model reaction for quantifying the catalytic
−푘푎푝푝푡 = 푙푛 ( 퐴 ) = 푙푛 (퐶퐶 )
(1)
퐴0
0
Here we report a series of materials featuring <10 nm noble metal
nanoparticles supported on amorphous carbon black (Au@C,
Ag@C, Pt@C and Pd@C). Immobilization of nanoparticles offers
several advantages including enhanced stability via inhibition of
nanoparticle aggregation, ease of separability and recyclability.[10]
These materials were tested for the reduction of 4NP, and other
amino-functionalized nitrophenols (ANPs). The latter are also
highly toxic, and used as raw materials or synthetic intermediates
in chemical manufacturing.[11] ANPs can also be viewed as
intermediates in the catalytic reduction of dinitrophenols (high
toxicity chemicals)[11], thus providing information on functional
group (-NH2) effects that may impact the rate of transformation of
multiple –NO2 sites. To both expand the substrate scope of our
catalysts, and validate the prognosticating ability of the ubiquitous
4NP test for selection of the most active species, we explored the
[a]
[b]
L. R. Shultz, K. Preradovic, Prof. M. J. Beazley, Prof. T. Jurca
Department of Chemistry, University of Central Florida
Orlando, Florida, 32816, USA
E-mail: Titel.Jurca@ucf.edu
L. Hu, Prof. X. Feng
Department of Materials Science and Engineering, University of
Central Florida
Orlando, Florida, 32816, USA
Prof. X. Feng,
Department of Physics, University of Central Florida
Orlando, Florida, 32816, USA
Prof. X. Feng, Prof. T. Jurca
Renewable Energy and Chemical Transformations Cluster,
University of Central Florida
Orlando, Florida, 32816, USA
Supporting information for this article is given via a link at the end of
the document.
[c]
[d]
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10.1002/cctc.201900260
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reduction of common azo dyes methyl red (MR) and methyl
orange (MO). Such species can be found in effluent from the
industrial manufacture of textiles, paint, plastics, cosmetics, etc.
and pose a risk to both human and aquatic health.[12] Thus, their
degradation is also of significant interest. Moreover, they offer the
opportunity to compare reactivity of -N=N- moieties, vs. -NO2.
While all four materials (Au@C, Ag@C, Pt@C and Pd@C)
displayed excellent activity across the entire substrate scope,
Pd@C proved to be both remarkably active and highly reusable.
Notably, by testing all materials over a broader substrate scope,
it becomes evident that the commonly used 4NP reaction does
not adequately rank broader catalyst activity. While variance in
selectivity across different catalysts is not surprising, it does raise
the question of whether performing a single test with 4NP is
adequate to select a catalyst to promote to a broader substrate
scope.
To investigate the catalytic performance of Au@C, Ag@C, Pt@C
and Pd@C, reductions of the ubiquitous 4-nitrophenol (4NP),
variably substituted aminonitrophenols 2-amino,5-nitrophenol
(2A5NP), 4-amino,3-nitrophenol (4A3NP), 4-amino,2-nitrophenol
(2A5NP), and common azo dyes methyl orange (MO) and methyl
red (MR) were performed (Scheme 2). Reactions were conducted
in-situ (1 cm path length quartz UV cell) in deionized water (DIW),
with excess NaBH4 (pH 10.46); see experimental section. In each
case (MO excluded), introduction of NaBH4 to the substrate
resulted in brighter colors and red-shifted absorbance due to
formation of respective phenolates (4NP*, 2A5NP*, 4A3NP*, and
4A2NP*) or carboxylate (MR*) denoted by “*“(Figure S3). After a
t0 absorbance was recorded, 1 mg of catalyst was added and
stirred gently for ~2 s. The reactions were allowed to proceed
without further intervention and monitored at fixed intervals by the
decreasing absorbance at (4NP*) λmax = 400 nm, (2A5NP*) λmax
=
Carbon-supported metal nanoparticles were synthesized by
the ascorbic acid-assisted polyol reduction method [13]. The metal
precursors used for the synthesis of Au, Ag, Pt, and Pd
nanoparticles were HAuCl4, AgNO3, H2PtCl6, and K2PdCl4,
respectively. Further details are provided in the experimental
section. Characterization using transmission electron microscopy
(TEM) revealed a relatively uniform dispersion of particles, with a
diameter, on average, of < 10 nm; Au@C, 6.79 ± 1.16 nm; Ag@C,
8.18 ± 2.47 nm; Pt@C, 4.44 ± 0.85 nm; Pd@C, 7.85 ± 1.41 nm
(Figures 1, S2). Although the target metal content was 30% w/w,
analysis by ICP-MS confirmed values of Pd = 13.5 ± 1.9 %, Pt =
27.8 ± 1.5 %, Au = 6.8 ± 0.9 %, and Ag = 23.5 ± 0.4 %,
460 nm, (4A3NP*) λmax = 510 nm, (4A2NP*) λmax = 480 nm, (MO)
λmax = 466 nm, and (MR*) λmax = 430 nm. All reactions proceeded
rapidly with a noted decoloration and appearance of lower
intensity absorbances commensurate with the formation of the
respective aminophenolates and anilines at (4AP*) λmax = 310 nm,
(2,5AP*) λmax = 308 nm, (3,4AP*) λmax = 306 nm, (2,4AP*) λmax
=
304 nm, (DMPD) expected at λmax = ~245 nm and ~305 nm[14] but
not observed due to overlap of 4ABS and ABA peaks , (4ABS)
λmax = 254 nm[15], and (ABA) λmax = 310 nm and 240 nm[16]
(Scheme 2, Figures S4-S27). Notably, all catalysts promote
hydrolysis of NaBH4, which leads to a turbid solution, resulting in
artificially increased baseline absorbance (up to ~1 a.u.). To
extract data under these conditions, the recorded spectra were
normalized to either isosbestic points or the baseline. Reactions
of Au@C, Ag@C, Pt@C and Pd@C with 4NP, MO, and MR in
the absence of NaBH4 resulted in no shift in absorbance or loss
of intensity indicating negligible leaching of metal ions (Figures
S30 and S31).
respectively.
Analysis
by
UV-Vis
spectroscopy
and
thermogravimetric analysis (TGA) is described in the supporting
information (Figure S1).
Scheme 2. Substrate scope for catalytic reduction utilizing Au@C, Ag@C,
Pt@C and Pd@C.
Figure 1. TEM images with particle size distributions in the inset for (A) Au@C,
6.79 ± 1.16 nm; (B) Ag@C, 8.18 ± 2.47 nm; (C) Pt@C, 4.44 ± 0.85 nm; (D)
Pd@C, 7.85 ± 1.41 nm.
The obtained kinetic data for the reductions were interpreted
according to the Langmuir-Hinshelwood kinetic model, and the
data for apparent rate (kapp) reported according to eq. 1 (Figure
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2A).[5a,8] Due to the variance in metal content, and consequent
weight % across the four materials, kapp is not adequately
representative of the activity of the metal nanoparticles, thus
hindering a meaningful comparison. Accounting for the metal
weight % derived from ICP-MS leads to normalized apparent rate
K (min-1g-1; g of metal). The results are graphically interpreted in
Figure 2B and detailed in Table 1. While this renders the results
self-consistent within the context of this study, direct comparison
to previously reported catalysts is complicated by the dependence
of kapp on a multitude of factors including NaBH4 concentration,
reaction volume, influence of mass transport, temperature and
mass of catalyst. Nonetheless, it is accepted that kapp as a function
of catalyst mass and reaction volume, dubbed “activity parameter”
nonetheless reproducible (Figures S10, S14, and S18). Overall,
all catalysts remained effective in the presence of amino
functionality, completing the reduction in < 8 min. Switching the
reducible functionality from nitro (-NO2) to azo (-N=N-) with MO
and MR as substrates resulted in similarly high activity governed
by pseudo-first order kinetics (Figures 3, S20-S27, Table 1). As
with the amino-nitrophenol substrates, reactions with Pt@C were
more turbid owing to faster H2 evolution, which ultimately led to
greater noise, but nonetheless reproducible data (Figures S22,
S26). To rule out possible contribution by carbon black, catalytic
degradation of all substrates was conducted under identical
conditions (vide supra), with 1 mg carbon black as the active
species. Although the reactions progressed under similar pseudo-
κ (s-1g-1L) can facilitate a useful comparison.[17] Within that context, first order kinetics, the rates obtained, compared to the slowest
focusing on the broadly studied 4NP system, Pd@C, which is
unequivocally our most active species (Figure 2, Table 1), exhibits
a κ value of 1.67 s-1g-1L. This is comparable to some of the most
active Pd-catalysts reported to date, which are consequently
among the most active metal species for 4NP conversion. Select
examples include but are not limited to: κ = Pd@Fe3O4/dextran
supported nanoparticle catalyst, were at least 60 x slower
(Figures 2B, S28, S29, Table 1).
A common practice with the 4NP4AP reaction is for the
selection of top catalyst candidates to be studied with an
expanded substrate scope. As can be clearly ascertained from
Figure 2B, for 4NP, the order of performance is
Pd@C>>Au@C>>Ag@C≥Pt@C. The simple addition of an
amino functionality (2A5NP, 4A3NP, 4A2NP) dramatically alters
the ranking established by the model system. While it is not
surprising that different catalysts exhibit differing selectivities, it
does call into question the prognosticating ability, and suitability
of the ubiquitous 4NP reaction for catalyst selection. By averaging
normalized apparent rate K across 4NP and 2A5NP, 4A3NP,
4A2NP we obtain Figure 3 (error bars = std. dev.). Now, the order
of performance becomes Pd@C>Au@C≥Pt@C>>Ag@C, and
the error bars indicate variance of performance across different
functionalities. By expanding the screening to several
functionalized substrates, we gain a better understanding of the
catalyst performance, with a very minimal penalty in effort. While
Pd@C is still the most active species overall, the disparity in
performance compared to Au@C and Pt@C is dramatically
reduced. Additionally, the large comparative error bars inform us
that Pd@C will display significantly different rates in the presence
of various amino functionalities, whereas Au@C, Pt@C, and
Ag@C will remain largely consistent. Notably, the averaged rates
more closely resemble the order of performance for azo dyes MO
and MR, with Pd@C displaying the largest variance across the
two substrates, as predicted. (Figure 3 inset).
3.65 s-1g-1L[17b], Pd@cellulose nanocrystals 1.34 x 10-1 s-1g-1L[18]
,
Pd@Fe3O4/SiO2 4.02 x 10-1 s-1g-1L[19], Pd@Camorphous 3.46 x 10-2 s-
1g-1L[20], Pd@organic-frameworks 2.49 s-1g-1L[21], Pd@TiO2 80.6 s-
1g-1L[22], Pdnanowires 0.19 s-1g-1L (performed at 27 °C)[23], Pd@RGO
13.04 s-1g-1L[24]
.
Figure 2. (A) Observed apparent rate (kapp) and (B) apparent rate normalized to
weight% of metal (K) for the catalytic reduction of substrates presented in
Scheme 2 by Au@C, Ag@C, Pt@C and Pd@C. (error bars: A = high/low range,
B = % error)
Reduction of amino-substituted nitrophenols 2A5NP, 4A3NP, and
4A2NP proceeded in a similar fashion (Figure 2, Table 1).
However, in-situ data obtained for Pt@C, in all instances, was
subject to a great deal of noise owing to the high turbidity of the
solution as a result of rapid H2 evolution; measurements were
Figure 3. Normalized apparent rate (K) averaged for the catalytic reduction of
4NP, 2A5NP, 4A3NP, and 4A2NP by Au@C, Ag@C, Pt@C and Pd@C; inset
comparison to K for MO, MR and 4NP. (error bars = standard deviation)
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Table 1. Observed apparent rate (kapp) and apparent rate normalized to
weight% of metal (K) for the catalytic reduction of substrates presented in
Scheme 2 by Au@C, Ag@C, Pt@C Pd@C and C.
to workup and exposure to a reducing environment, adsorption of
amine products to surface which may block active sites, and
potential leaching. Remarkably, Pd@C, which accounted for the
highest rates for 4NP reduction, exhibited
a
dramatic
Substrate
4NP
Catalyst
C
kapp (min -1
)
K (min-1g-1)
enhancement in catalytic rate under the recycling protocol
(300%+). At the fifth trial, kapp = 21.283 min-1, corresponding to κ
= 7.84 s-1g-1L. Repeating a truncated trial with 2A5NP (3 trials)
also revealed modest enhancement of activity for Pd@C, and
different levels of decline for the remaining catalysts (Figure S40).
We postulate that this results from a combination of factors,
including exposure to the reducing environment (NaBH4) which
removes surface bound contaminants (e.g. residual ascorbic acid
from synthesis, Figure S1) from Pd@C, thus enhancing catalytic
rate.[25] While the other catalysts are also exposed to a similar
protocol, the combination of contaminant removal, and
introduction of new species such as aminophenols and boron-
based byproducts post reduction has a net deleterious effect.
0.0053
5.3
4NP
Au@C
Ag@C
Pt@C
Pd@C
C
0.9981 ± 0.0689
0.8503 ± 0.0887
0.8950 ± 0.0651
4.5394 ± 0.1688
0.0037
14648.86 ± 1011.42
3608.06 ± 376.19
3219.08 ± 233.99
33451.73 ± 1244.75
3.7
4NP
4NP
4NP
2A5NP
2A5NP
2A5NP
2A5NP
2A5NP
4A3NP
4A3NP
4A3NP
4A3NP
4A3NP
4A2NP
4A2NP
4A2NP
4A2NP
4A2NP
MO
Au@C
Ag@C
Pt@C
Pd@C
C
0.4037 ± 0.1079
0.4666 ± 0.0076
2.6586 ± 0.3243
0.8583 ± 0.0603
0.0042
5924.27 ± 1582.91
1979.82 ± 32.04
9562.65 ± 1166.32
6324.61 ± 444.08
4.2
Au@C
Ag@C
Pt@C
Pd@C
C
0.2645 ± 0.0024
0.2505 ± 0.0322
1.1412 ± 0.3111
0.8997 ± 0.0742
0.0041
3881.27 ± 34.88
1063.00 ± 136.64
4104.65 ± 1118.83
6629.70 ± 546.50
4.1
Au@C
Ag@C
Pt@C
Pd@C
C
0.4492 ± 0.0745
0.5179 ± 0.0696
2.7612 ± 0.3688
0.5389 ± 0.1442
0.0071
6592.06 ± 1092.72
2197.51 ± 295.14
9931.69 ± 1326.38
3970.89 ± 1062.28
7.1
MO
Au@C
Ag@C
Pt@C
Pd@C
C
0.4962 ± 0.0586
0.5448 ± 0.0534
1.3117 ± 0.0403
3.6840 ± 0.2680
0.0042
7282.60 ± 860.11
2311.87 ± 226.60
4718.11 ± 144.98
27147.75 ± 1974.92
4.2
MO
MO
MO
MR
MR
Au@C
Ag@C
Pt@C
Pd@C
0.4517 ± 0.0084
0.5386 ± 0.1176
1.7625 ± 0.1344
1.1350 ± 0.0289
6628.75 ± 122.88
2285.35 ± 498.83
6339.61 ± 483.44
8363.67 ± 212.91
Figure 4. Percent apparent rate (kapp) for the catalytic reduction of 4NP by
Au@C, Ag@C, Pt@C and Pd@C over five recycling trials.
MR
MR
MR
In summary, we report a series of noble metal nanoparticles
immobilized on amorphous carbon (Au@C, Ag@C, Pt@C and
Pd@C). All materials display competitive catalytic activity over
paranitrophenol, amino-substituted nitrophenols and azo dyes;
Pd@C exhibits especially remarkable activity, which increases
further when exposed to recycling protocol. By testing all
materials over a broader substrate scope with both added
functionalities, and even different reducible moieties, it becomes
evident that the commonly used 4NP test reaction does not
adequately rank broader catalyst activity. This raises the question,
are potentially highly active or selective catalysts being
overlooked by an oversimplified screening protocol? Within the
narrow scope tested, simply including variably substituted amino-
nitrophenols more accurately reflects catalyst activity trends when
applied to azo functionalities. Our ongoing work focuses on further
expanding the scope with the goal of developing a facile
screening method that is more broadly predictive.
Utilizing identical conditions to the in-situ catalytic trials with 4NP
(vide supra), we sought to determine the recyclability and long-
term stability of Au@C, Ag@C, Pt@C and Pd@C. After each trial,
the turbid solution was allowed to settle overnight, the liquid
component carefully decanted, contents washed with DIW,
decanted, and the remaining catalyst in the cuvette placed under
vacuum overnight. Four subsequent trials were conducted by the
addition of 3 mL of DIW containing 4NP and NaBH4 (Figures S32-
S35). By comparing the percent apparent rate (kapp) over five trials
we can learn more than solely reporting the percent conversion
over an ill-defined time frame; further aiding informed catalyst
selection (Figure 4). Therein, Au@C experiences the least
fluctuation in rate across five trials, signalling that its behaviour is
predictable, reproducible, and the catalyst is easily recycled.
Pt@C and Ag@C experience a substantial loss in rate after the
first trial, which recovers slightly over trials 4 and 5. We attribute
this to a combination of mechanical loss, surface restructuring due
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and stirred gently for ~2 s. The reaction was allowed to proceed without
further intervention and monitored at fixed intervals by the decreasing
absorbance at the respective λmax value. Practically, this denotes a color
change from bright yellow to colorless. Further details are noted in the
Supporting Information.
Experimental Section
General Methods: All reactions were carried out under air, at ambient
temperature (21-23 °C, 45-55% relative humidity). Metal salts HAuCl4,
AgNO3, H2PtCl6, K2PdCl4 and Methyl Orange (MO) were purchased from
Acros Organics. Carbon black (Vulcan XC 72) was purchased from Fuel
Cell Store. Ethylene glycol and L-ascorbic acid were purchased from
Fisher Scientific. 4-nitrophenol (4NP), 4-amino,3-nitrophenol (4A3NP), 4-
amino,2-nitrophenol (4A2NP), 2-amino,5-nitrophenol (2A5NP), and
NaBH4 were purchased from TCI America. Methyl Red (MR) were
purchased from Fisher Scientific. All chemicals were used as received
without further purification. UV-Vis Spectra were collected on an Agilent
Cary 60 spectrophotometer utilizing 1 cm quartz cuvettes purchased from
Spectrocell inc. X-ray photoelectron spectra (XPS) were obtained using a
Thermo Scientific ESCALAB Xi+ X-ray Photoelectron Spectrometer with
an Al Kα X-ray source (1486.67 eV). pH measurements were conducted
with an OHaus 2100 meter with ST210 electrode.
Acknowledgements
This work was supported by the Department of Chemistry,
Department of Physics, College of Sciences, and the FCI at the
University of Central Florida. L. R. Shultz thanks the UCF Office
of Undergraduate Research for a Student Research Grant Award.
Prof. G. Chen, UCF Chemistry is thanked for helpful discussion.
Authors acknowledge the NSF MRI: XPS: ECCS: 1726636,
hosted in MCF-AMPAC facility, MSE, CECS.
Keywords: heterogeneous catalysis • noble metal nanoparticles
• nitrophenol reduction • aqueous catalysis • green chemistry
Catalyst Synthesis and Characterization: Carbon-supported metal
nanoparticles were synthesized by ascorbic acid-assisted polyol reduction
method. The metal precursors used for the synthesis of Au@C, Ag@C,
Pt@C, and Pd@C nanoparticles were HAuCl4, AgNO3, H2PtCl6, and
K2PdCl4 solutions with a metal concentration of 6 mg mL−1, respectively.
Typically, 35 mg carbon black was dispersed in 25 mL ethylene glycol by
sonicating for 1 h. Then, 2.5 mL of metal precursor solution was added.
After stirring for 1 h, the mixture was heated to 120 °C, and 5 ml 0.5 M L-
ascorbic acid was added dropwise into the solution. The solution was kept
at this temperature for 2 h under vigorous stirring. Finally, the catalyst
slurry was filtered and washed by copious water and ethanol, and then
dried in vacuum at ~21 °C overnight. Actual metal loading was determined
by ICP-MS. Dry metal nanoparticles were each digested in 3:1 v/v nitric
aid to hydrochloric acid (trace metal grade; Fisher Scientific) overnight at
85 °C. The digests were filtered (0.2 µm pore size) and dissolved Au, Ag,
Pt, and Pd quantified using a Thermo Fisher Scientific iCap Qc inductively
coupled plasma mass spectrometer with QCell technology and operated
in kinetic energy discrimination (KED) mode of analysis with helium as the
collision gas. Calibration, internal, and quality control standards (Inorganic
Ventures) were prepared in 2% trace metal grade nitric acid (Fisher
Scientific). Cysteine (0.5% w/v) was added to the standards and rinse to
eliminate adsorption and memory effects. Holmium and bismuth were
used as internal references in both standards and samples. TEM images
were acquired using a JEOL TEM-1011 operated at 100 kV. TEM samples
were prepared by depositing a diluted metal nanoparticle dispersion in
ethanol onto copper grids covered with ultrathin carbon films. The size
distribution of nanoparticles was obtained by counting over 100
nanoparticles for each sample. TGA of catalysts and support were
measured on an ISI TGA-1000 with a 5 cc/min flow of UHP N2, housed
inside a N2 atmosphere glovebox.
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Entry for the Table of Contents (Please choose one layout)
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To pNP or not to pNP? A series of
noble metal nanoparticles immobilized
on amorphous carbon are reported
(Au, Ag, Pt, and Pd). Screening
catalytic activity for the reduction of
the ubiquitous paranitrophenol, amino-
nitrophenols, and azo dyes reveals
competitive activity across the board,
with the Pd species being most active.
More importantly, it highlights the
limitations of using the benchmark
paranitrophenol test as a tool for
catalyst selection.
Lorianne R. Shultz, Lin Hu, Konstantin
Preradovic, Melanie J. Beazley,*
Xiaofeng Feng,* Titel Jurca*
Page No. – Page No.
A broader-scope analysis of the
catalytic reduction of nitrophenols
and azo dyes with noble-metal
nanoparticles
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