A Study of Heterogeneous Catalysis by Nanoparticle-Embedded Paper-Spray Ionization Mass Spectrometry

Article abstract of DOI:10.1002/anie.201607204

We have developed nanoparticle-embedded paper-spray mass spectrometry for studying three types of heterogeneously catalyzed reactions: 1) Palladium-nanoparticle-catalyzed Suzuki cross-coupling reactions, 2) palladium- or silver-nanoparticle-catalyzed 4-nitrophenol reduction, and 3) gold-nanoparticle-catalyzed glucose oxidation. These reactions were almost instantaneous on the nanocatalyst-embedded paper, which subsequently transferred the transient intermediates and products to a mass spectrometer for their detection. This in situ method of capturing transient intermediates and products from heterogeneous catalysis is highly promising for investigating the mechanism of catalysis and rapidly screening catalytic activity under ambient conditions.

Full text of DOI:10.1002/anie.201607204

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607204
German Edition: DOI: 10.1002/ange.201607204
Heterogeneous Catalysis
A Study of Heterogeneous Catalysis by Nanoparticle-Embedded
Paper-Spray Ionization Mass Spectrometry
Shibdas Banerjee, Chanbasha Basheer, and Richard N. Zare*
Abstract: We have developed nanoparticle-embedded paper-
spray mass spectrometry for studying three types of heteroge-
neously catalyzed reactions: 1) Palladium-nanoparticle-cata-
lyzed Suzuki cross-coupling reactions, 2) palladium- or silver-
nanoparticle-catalyzed 4-nitrophenol reduction, and 3) gold-
nanoparticle-catalyzed glucose oxidation. These reactions were
almost instantaneous on the nanocatalyst-embedded paper,
which subsequently transferred the transient intermediates and
products to a mass spectrometer for their detection. This in situ
method of capturing transient intermediates and products from
heterogeneous catalysis is highly promising for investigating
the mechanism of catalysis and rapidly screening catalytic
activity under ambient conditions.
Figure 1. a) Workflow for studying heterogeneous catalysis using
nanoparticle-embedded paper-spray mass spectrometry. b) SEM
images of normal filter paper and the different metal-nanoparticle-
doped filter papers used in the experiment.
P
aper-spray ionization is a direct sampling ionization
method in open air, recently introduced for the mass
spectrometric analysis of complex chemical mixtures.[1] By
attaching nanoparticles (NPs) to the paper support, we are
able to use paper spray as a chemical reactor for studying
heterogeneous catalysis. The high surface-to-volume ratio
and highly active surface atoms make NP catalysts capable of
responding to mild reaction conditions with a high turnover
number.[2] Catalytically active NPs are being studied inten-
sively in coupling reactions,[3] reductions,[4] oxidations,[5] and
electrochemical reactions in fuel cells.[6] Herein, we inves-
tigated the feasibility of paper-spray mass spectrometry (PS-
MS) to drive the reaction on the surface of solid NP catalysts
embedded in the paper and thereby directly transfer the
reactants, isolated intermediates (if any), and products to the
mass spectrometer by paper spray, allowing their detection
immediately after starting the reaction on the nanocatalyst-
embedded paper support (Figure 1).
nanoparticles (Au-NPs). Our results suggest that this rapid
and convenient method of in situ observation of heteroge-
neous catalysis under microscale and ambient conditions
(room temperature and atmospheric pressure) is highly
promising for probing the reaction mechanism, evaluating
the activity, and performing high throughput screening of
nanocatalysts for characterizing new reactions.
The palladium-catalyzed carbon–carbon bond-forming
Suzuki cross-coupling reaction is a popular gateway to the
synthesis pharmaceuticals, molecular electronics, conjugated
polymers, and natural products.[7] Recently, the use of Pd-NPs
in Suzuki cross-coupling reactions has been tailored to
provide more efficient, environmentally benign, and ligand-
free syntheses.[3b,8] However, much controversy exists con-
cerning the nature of the mechanism for this NP-catalyzed
reaction. It has been debated whether the catalysis originates
from metal leached out of the NP[9] or from the surface
activity of the NP.[3b,10] To this end, we performed this reaction
under the Pd-NP-embedded paper-spray condition.
Figure 1 illustrates our workflow. We synthesized Pd-NPs
and immobilized them onto filter paper cut in the form of an
isosceles triangle with the apex pointing toward the inlet of
a mass spectrometer (see the Supporting Information, Fig-
ure S1a and the experimental section). Scanning electron
microscopy (SEM) images showed that the Pd-NPs are
densely packed on the paper, and the average size of the
Pd-NPs is around 10 nm (Figure 1 and the Supporting
Information, Figure S2). This NP-doped dry paper was
clipped to a high-voltage power supply (typically À7 kV) in
front of a high-resolution mass spectrometer. A microscale
reaction was initiated by dispensing 7 mL of the reagent
mixture onto the catalyst-doped paper. Within few seconds of
We apply this method to the study of three model
reactions with different substrates and metal nanoparticle
catalysts: 1) Two Suzuki cross-coupling reactions catalyzed by
palladium nanoparticles (Pd-NPs), 2) the reduction of 4-
nitrophenol catalyzed by Pd-NPs or silver nanoparticles (Ag-
NPs), and 3) the oxidation of glucose catalyzed by gold
[*] Dr. S. Banerjee, Prof. R. N. Zare
Stanford University, Department of Chemistry
333 Campus Drive, Stanford, CA 94305-4401 (USA)
E-mail: zare@stanford.edu
Prof. C. Basheer
Department of Chemistry
King Fahd University of Petroleum and Minerals
P.O. 1509, Dhahran 31261 (Saudi Arabia)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201607204.
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the initiation of the reaction, paper-spray ionization occurred,
followed by mass spectrometric detection of the species
formed in the reaction.
Two Suzuki cross-coupling reactions involving phenyl-
boronic acid (1) and 4-bromophenylacetic acid (2a) or 4-
bromophenol (2b) (Figure 2a) were studied under the
intermediates in the corresponding mass spectra (Supporting
Information, Figures S5 and S6) of either of the reactions
(Figure 2), which suggests that metal leaching is not necessary
to drive the catalytic cycle under these relatively gentle
conditions. We conclude that these reactions are catalyzed by
the chemisorption of substrates onto the surface of the
heterogeneous catalyst (Pd-NPs) under the present condi-
tions.
We have analyzed the substrate-specific catalytic activity
of the Pd-NPs by estimating reaction yields (Figure 2). From
standard calibration plots (Supporting Information, Fig-
ure S7) we find an approximately 7% yield from the reaction
of 1 and 2a and an approximately 3% yield from the reaction
of 1 and 2b within 30 s of the reactions (Figure 2) on paper.
À
Indeed, an electron releasing group (for example, OH)
present on the aryl halide substrate (for example, 2b) is
known to suppress the yield of the Suzuki cross-coupling
reaction.[13] Therefore, this method could also be used to
screen the activity of the nanocatalyst on a given set of
substrates. These results also suggest marked acceleration of
these reactions on the Pd-NP-doped paper when compared
with conventional bulk-phase methods, which typically take
several minutes to hours to yield a significant amount of
product even at elevated temperatures.[3a]
The catalytic reduction of 4-nitrophenol by sodium
borohydride is one of the popular model reactions for
evaluating the performance of metal nanocatalysts.[4b,c]
Nitro-aromatic pollutants are stable against chemical and
biological degradation.[14] Much attention has been paid to
converting 4-nitrophenol to the less toxic 4-aminophenol,
which can be used as the precursor for the synthesis of
pharmaceuticals and dyestuffs.[14] Based on absorption spec-
troscopy studies,[15] the NP-catalyzed reduction of 4-nitro-
phenol (4) to 4-amino phenol (8) was proposed previously to
proceed through the stepwise formation of a number of
intermediates including the nitroso compound 6 and hydrox-
ylamine compound 7 (Figure 3a). To our knowledge, no
in situ approach was previously used to capture and detect
individual intermediates (5–7).
We have successfully detected three intermediates (de-
protonated 5–7), along with the reactant (deprotonated 4)
and product (deprotonated 8), using Pd-NP-embedded PS-
MS (Figure 3). The experimental details are given in the
Supporting Information. The time-dependent abundances of
those species are given as extracted ion chromatograms in
Figure 3b–f. These results unambiguously demonstrate that
the reduction of 4 occurs in a number of steps, in which
intermediates (5–7) are able to leave the nanoparticle surface
after each reduction step. The mobility of the desorbed
product in the paper is likely to be faster than that of the
intermediates, some of which might be reabsorbed by the
surface of the nanocatalyst for further reaction. Therefore,
intermediate ion signals are abundant during the late stage of
the paper spray (Figures 3c–e). Furthermore, the differential
ion current profiles of the intermediates might also be
attributed to their different kinetic stability and retention
time on paper during the reaction. It should be noted that the
presence of the isolated intermediate 5 was never confirmed
before, although spectroscopic signatures provided evidence
Figure 2. Pd-NP-catalyzed Suzuki cross-coupling reaction of phenyl-
boronic acid (1) and 4-bromophenol (2a) or 4-bromophenylacetic acid
(2b) under paper-spray conditions. a) General reaction scheme. The
abundances of reactants and products with time (extracted ion
chromatogram) after starting the reaction are shown for the reaction
between b) 1 and 2a and c) 1 and 2b. Insets of (b) and (c) show the
detected product ion signals with high mass accuracy (see the
Supporting Information, Table S1). The full average spectra are given
in the Supporting Information, Figure S3.
present conditions. We monitored the formation of the
products over time using extracted ion chromatograms as
shown in Figure 2b,c. The results suggest that the reaction is
nearly instantaneous as there is no detectable time delay
between the appearances of reactants and the product ion
signals, although it took nearly 12 s for the spray to start after
dispensing the reagent onto the paper. It should be noted that
we have not used any base (for example, NaOH) in the
reagent mixture, although a basic environment is generally
required in the Suzuki cross-coupling reaction[11] (Supporting
Information, Scheme S1). We have detected the formation of
trihydroxy(phenyl)borate (1’; Supporting information, Fig-
ure S4), a precursor of the transmetallation step in the
catalytic cycle (Supporting Information, Scheme S1), which
indicates the formation of OHÀ ions from the solvent
reduction of water on the paper under the high negative
voltage (À7 kV).[12] We have also inspected the mass spectra
for the isolated Pd-containing intermediates, for example, Ar-
Pd-Br and Ar-Pd-Ph (see the Supporting Information,
Scheme S1), if formed by the metal leaching process in the
reaction. However, we have not detected any of these
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Oxidation of organic molecules by an eco-friendly cata-
lytic method is of growing interest for developing sustainable
chemical processes.[17] Particularly, the oxidation of glucose is
important in biosensor development.[18] The use of gold
catalysts in chemical oxidation is important owing to their
resistance to deactivation by oxygen.[17b] We have also studied
the aerobic oxidation of glucose (9) to gluconic acid (11) by
using Au-NP-doped PS-MS (Figure 4). The average size of
Figure 3. Pd-NP-catalyzed reduction of 4-nitrophenol (4) under the
paper-spray conditions. a) General reaction scheme in which details of
the mechanism are supported by the ion signals of the individual
species (4–8). The extracted ion chromatogram of b) [4ÀH]À,
c) [5ÀH]À, d) [6ÀH]À, e) [7ÀH]À, and f) [8ÀH]À. Insets in (b)–(f)
show the detected ion signals of the corresponding species (reactant/
intermediate/product) with high mass accuracy (see the Supporting
Information, Table S1). The full average spectrum is given in the
Supporting Information, Figure S8.
Figure 4. Aerobic oxidation of d-glucose (9) catalyzed by gold nano-
particles (Au-NPs) under paper-spray conditions. a) Reaction scheme
with details of the mechanism, which is also supported by the ion
signals of the intermediate 10 and product 11. b) The extracted ion
chromatograms of [9ÀH]À (black), 10 (blue), and [11ÀH]À (red). Inset
of (b) shows the detected ion signals of species 10 and 11 with high
mass accuracy (see Table S1, in the Supporting Information). The
intensity of 9 is associated with the left y-axis and that of 10 and
[11ÀH]À with the right y-axis. The full average spectrum is given in the
Supporting Information, Figure S13.
of other intermediates (6 and 7).[15] Herein, we confirm the
presence of 5, from the ion signal at m/z 140.0356 (Figure 3c).
We have also measured the NP-specific catalytic effi-
ciency of 4-nitrophenol reduction by changing the nano-
catalyst from palladium to silver. When Ag-NP-doped paper
(Supporting Information, Figure S9) was used to study this
reaction, we observed similar mass spectral features, which
allowed us to identify the intermediates as shown in the
Supporting Information, Figure S10. The yields of both these
reactions were measured by using standard calibration plots
(Supporting Information, Figure S11a). Yields from the Pd-
NP and the Ag-NP catalysis were circa 42% and circa 10%,
respectively. The higher yield from the Pd-NPs is attributed to
their smaller size (10 nm; Supporting Information, Figure S2)
compared to that of the Ag-NPs (20 nm; Supporting Infor-
mation, Figure S9), which provides more effective surface
area for the reaction to occur. Thus, this method for studying
heterogeneous catalysis could be used to rapidly screen the
activity of different nanocatalysts for a given reaction. Once
again, the approximately 42% yield within 30 s (Figure 3 f)
represents a significantly faster rate of reduction of 4-nitro-
phenol on the Pd-NP-doped paper when compared with
conventional bulk-phase methods, which require minutes to
hours.[16]
the Au-NPs was approximately 25 nm (Supporting Informa-
tion, Figure S12). The experimental details are given in the
Supporting Information. Figure 4a shows the proposed
mechanism of glucose oxidation,[5a] in which glucose is first
hydrated for its adsorption onto the gold surface and then
oxidized by molecular oxygen. The hydration of glucose
under Au-NP-doped paper spray is directly evident by the
formation of ions at m/z 197.0666 corresponding to the
species 10 (Figure 4). The time-dependent abundances of the
reactant (9), isolated intermediate (10), and product (11) are
shown in their extracted ion chromatograms (Figure 4b). This
study confirms the presence of the crucial intermediate 10,
which was previously postulated.[5a,19] Furthermore, we have
not detected gold-atom-bound species corresponding to 10a
and 10b (Supporting Information, Figure S14), which sug-
gests that the reaction is not conducted by the gold leaching
process. From the standard calibration plot (Supporting
Information, Figure S11b), the estimated yield within 30 s
of the reaction was found to be less than 0.1%. This low yield
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is attributed to the requirement of molecular oxygen for this
oxidation process. However, the reaction on the paper was
assisted by the oxygen from the air, and no additional oxygen
source was used to drive the reaction.
catalyzed glucose oxidation were studied as a proof of concept
of this method. These reactions were almost instantaneous,
transferring the transient isolated intermediates and products
to the mass spectrometer for their subsequent detection. The
data provide compelling evidence that a metal leaching
process is not important to drive these reactions on the paper;
rather, the reactions occurred on the surface of the nano-
catalysts. Direct detection of transient isolated intermediates
provides valuable information on the mechanistic details of
these reactions, as some of the intermediates had been
postulated but not directly observed. Furthermore, estimates
of the reaction yields suggest that the reactions are many fold
faster (typically by a factor of one thousand or more) on
nanoparticle-embedded paper under the spray conditions,
when compared to conventional reactions in the bulk phase.
This technique can serve as a lab-on-paper for engineering
new heterogeneous catalysis reactions, for mechanistic stud-
ies, and for high throughput screening of nanocatalyst
performance, especially because of the simplicity of this
method for conducting reactions under ambient conditions.
For the above three reactions (Figures 2, 3, and 4), we
performed control studies with normal filter paper without
the embedded nanocatalysts. None of the control studies
(data not shown) was able to catalyze any of the above
reactions in the absence of catalytic nanoparticles. We also
performed a voltage-dependent study of those reaction
(Figures 2, 3, and 4). On changing the paper spray voltages
from 0 to À7 kV, the reaction efficiencies (formation of
intermediates and products) increased (Supporting Informa-
tion, Figures S15 and S16), which indicates the requirement of
a basic environment for these reactions, which is provided by
the abundant solvent (water) reduction at elevated negative
voltages. It should be noted that the increased negative spray
voltage can also improve the ionization efficiency and ion
detection sensitivity in mass spectrometry, which can also
contribute to modifying the product-to-reactant ratios[20]
(Supporting Information, Figures S15 and S16). We also
investigated the addition of base to the reaction mixture
(pH ꢀ 11) but found no increase in the reaction yield. This
suggests that water electrolysis is sufficient for providing
a basic environment.
We have evaluated the catalytic response of the nano-
catalysts (Pd-NP, Ag-NP, and Au-NP) for their corresponding
reactions (Figures 2, 3, and 4) from several cycles of paper-
spray ionization. In each cycle (ca. 4-minute interval), 7 mL of
reactant mixture was dispensed onto the NP-doped paper,
which was maintained at a constant voltage (À7 kV) through-
out the experiment, and the corresponding product formation
was monitored by the extracted ion chromatogram of the
product. Figure 5 shows representative data obtained from
Acknowledgements
We thank Lydia-Marie Joubert of the CSIF Beckman Center,
Stanford University, for her help in SEM study. This work was
supported by the Air Force Office of Scientific Research
through Basic Research Initiative grant (AFOSR FA9550-16-
1-0113). C.B. gratefully acknowledges financial support
provided by the Deanship of Scientific Research through
the International Summer Scholarship program (2016–2017)
and the King Fahd University of Petroleum and Minerals.
Keywords: heterogeneous catalysis · intermediates ·
mass spectrometry · nanoparticles · paper-spray ionization
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Received: July 25, 2016
Published online: && &&, &&&&
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Communications
Heterogeneous Catalysis
Reaction monitoring looks good on
paper: Ambient-condition paper-spray
ionization from nanoparticle-doped
paper allows the identification of reaction
mechanisms and enables rapid screening
of catalytic activity.
S. Banerjee, C. Basheer,
R. N. Zare*
&&&& — &&&&
A Study of Heterogeneous Catalysis by
Nanoparticle-Embedded Paper-Spray
Ionization Mass Spectrometry
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