Download Based Nanomaterials: A Platform to Produce Reactive Oxygen

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Ran Long, Hao Huang, Yaping Li, Li Song, and Yujie Xiong*
tion reactions in Pd-based nanomaterials
have attracted tremendous attention from
the research community among various
Pd-catalyzed reactions.[20–25] In the presence of molecular oxygen (O2), Pd nanomaterials can be used to catalyze a wide
variety of oxidation reactions including
polyol,[26] CO,[27] alcohol[28,29] and glucose
oxidations,[30] and oxidation couplings.[31]
In terms of studies of mechanism, one
has to investigate the interactions of reactive molecules with catalyst surfaces at
the molecular and electronic levels. In the
case of Pd-catalyzed oxidation reactions,
O2 activation is the central theme of mechanism studies, despite the involvement
of different oxidized molecules in various
reactions. In any oxidation reaction, the
reaction activity and selectivity of oxidized
molecules are always maneuvered by the
state of the oxygen species adsorbed on or
generated from the surface of Pd-based
nanomaterials, regardless of atmospheric
or high pressure. It is known that the
ground state of oxygen is a triplet form (3O2). For this reason,
reactions occurring between organic molecules (mainly in a
singlet state) and ground-state O2 to produce new singlet compounds are generally forbidden by the Wigners spin selection
rule.[32,33] Intuitively, we can deduce a hypothesis that groundstate O2 has to be activated to a high-energy state and in turn
participate in oxidation reactions.
A question would then naturally arise from this hypothesis:
what is the nature of high-energy oxygen species produced
from the Pd surface in terms of physical parameters (e.g.,
magnetic moment, binding energy) and chemical reactivity?
Unraveling this mystery, which relies on investigations of interactions of O2 with Pd nanomaterials from the viewpoint of fundamentals, would enable better understanding of the origins
of Pd catalytic activities in oxidation reactions. As a matter of
fact, many studies on O2 activation have led to important findings in the past years, all of which highlight the importance
of Pd–O2 interactions in O2 activation. For instance, Hwang
et al. found that singlet oxygen can be produced through sensitization by Ag, Au, and Pt under incident light.[34] However,
in the case of Pd-catalyzed oxidation reactions, incident light is
not demanded as an indispensable energy source according to
previous reports.[16–27] Thus there must be fundamental differences between Pd and other metals in the process of oxygen
activation, which causes the production of highly active oxygen
species for catalytic oxidation reactions.
Oxidation reactions by molecular oxygen (O2) over palladium (Pd)-based
nanomaterials are a series of processes crucial to the synthesis of fine
chemicals. In the past decades, investigations of related catalytic materials
have mainly been focused on the synthesis of Pd-based nanomaterials from
the angle of tailoring their surface structures, compositions and supporting
materials, in efforts to improve their activities in organic reactions. From the
perspective of rational materials design, it is imperative to address the fundamental issues associated with catalyst performance, one of which should
be oxygen activation by Pd-based nanomaterials. Here, the fundamentals
that account for the transformation from O2 to reactive oxygen species over
Pd, with a focus on singlet O2 and its analogue, are introduced. Methods for
detecting and differentiating species are also presented to facilitate future
fundamental research. Key factors for tuning the oxygen activation efficiencies of catalytic materials are then outlined, and recent developments in
Pd-catalyzed oxygen-related organic reactions are summarized in alignment
with each key factor. To close, we discuss the challenges and opportunities
for photocatalysis research at this unique intersection as well as the potential
impact on other research fields.
1. Introduction
Metallic nanostructures are fundamentally important for future
science and technology.[1] For example, gold (Au)- and silver
(Ag)-based nanomaterials can be used in optical-related technologies for their surface plasmon properties, and platinum
(Pt)-based nanomaterials are known as excellent electrocatalysts for fuel cells.[2–5] Meanwhile, recent years have witnessed a
growing interest in palladium (Pd)-based nanomaterials due to
their fascinating properties and potential applications in catalysts.[1,6–19] Given that the reactivity and selectivity of a catalyst
are highly dependent on specific pathways of reactions, it is
imperative to decode the mechanism behind each reaction in
an effort to achieve rational catalyst design. In particular, oxidaDr. R. Long, H. Huang, Y. Li,
Prof. L. Song, Prof. Y. Xiong
Hefei National Laboratory for
Physical Sciences at the Microscale
iChEM (Collaborative Innovation Center of
Chemistry for Energy Materials)
School of Chemistry and Materials Science
and National Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei, Anhui 230026, P. R. China
E-mail: [email protected]
DOI: 10.1002/adma.201502068
Adv. Mater. 2015, 27, 7025–7042
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Palladium-Based Nanomaterials: A Platform to Produce
Reactive Oxygen Species for Catalyzing Oxidation Reactions
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To provide a contextual backdrop, we begin with a brief introduction to ground-state oxygen and reactive oxygen species
(ROS), as well as adsorbed oxygen species, followed by a summary of detection methods that may open up possibilities to
differentiate oxygen species activated on nanomaterial surfaces
from free singlet oxygen. This discussion of detection methods
includes the common agents used to detect singlet oxygen as
well as an overview of their working mechanisms. Adsorbed
oxygen is a species that behaves like singlet oxygen both chemically and physically. From this discussion, readers may understand why adsorbed oxygen exhibits outcomes similar to or different from singlet oxygen in various detection processes. To
close, we outline the key parameters of Pd-based nanomaterials
that hold the key to activating oxygen, along with specific application examples in catalytic oxidation reactions.
2. Reactive Oxygen Species
2.1. Conventional Reactive Oxygen Species
As the names suggest, ROS should present higher reactivity
than ground-state molecular oxygen. By definition, ROS can be
divided into two types: radicals (e.g., O2− and OH•) and nonradicals (e.g., H2O2 and 1O2).
In the ground state, 3O2 has two unpaired electrons in its
antibonding orbital (Figure 1a).[35] The O2 molecule is at its
lowest energy level when the electrons are unpaired, occupying
separate orbitals with their spins in the same direction. In contrast, there are two types of excited-state oxygen species: a (1Δg)
with the electrons paired on the same orbital, and b (1Σg+) with
the electrons in separate orbitals and their spins in different
directions (Figure 1b). Given its high energy level, O2 (1Σg+) is
extremely short-lived and quickly relaxes to the O2 (1Δg) state.
For this reason, the O2 (1Δg) state is commonly referred to as
singlet oxygen (1O2).
In principle, another class of reactive oxygen species is that
of free radicals, including but not limited to superoxide (O2•−)
(Figure 1b), hydroperoxyl radical (HO2•) and hydroxyl radicals
(OH•), which have also played important roles in chemical
reactions and biological systems. Given the presence of single
unpaired electrons, the radicals are generally less stable than
non-radicals.
2.2. 1O2 Analogue on Pd Surfaces
It is well known that Pd nanomaterials exhibit excellent catalytic
performance in various oxidation reactions.[20–25,28,29] From the
viewpoint of heterogeneous catalysis, we can naturally recognize that this reaction activity should be ascribed to molecular
adsorption on the surface of Pd nanomaterials. In terms of O2
activation, we have to examine the direct interactions of O2 with
Pd surfaces. One of our recent reports indicates that the surface
facets have a huge impact on their efficiency in O2 activation,
using single-faceted Pd nanocrystals as a model system.[36] It
turns out that the activated oxygen species can be formed on
the surface of Pd nanomaterials. This species is termed “1O2
analogue” for the following reasons:
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Ran Long was born in Anhui,
China, in 1987. She received her
BS in chemistry in 2009, and
her PhD in inorganic chemistry
under the tutelage of Professor
Yujie Xiong in 2014, both from
the University of Science and
Technology of China (USTC).
She is currently a postdoctoral
fellow working with Professors
Yujie Xiong and Li Song at the
USTC. Her research interests
focus on controlled synthesis and catalytic applications of metal
nanocrystals.
Hao Huang was born in
Jiangxi, China, in 1994. He
received his BS in chemistry
in 2013 from the University
of Science and Technology of
China (USTC). He is currently a PhD candidate under
the tutelage of Professor
Yujie Xiong at the USTC.
His research interests focus
on the controlled synthesis
and biomedical and catalytic
applications of metal nanostructures.
Yujie Xiong is a Professor of
Chemistry at the University
of Science and Technology
of China (USTC), and
Principal Investigator of the
Hefei National Laboratory
for Physical Sciences at the
Microscale. He received his
BS in chemical physics in
2000 and PhD in inorganic
chemistry in 2004, both from
USTC. After this, he joined
the National Nanotechnology Infrastructure Network
(NSF-NNIN), and served as Principle Scientist and Lab
Manager at Washington University in St. Louis. His
research interests include the synthesis, fabrication and
assembly of inorganic materials for energy and environmental applications.
1) As revealed simulation results,[36] an electron transfer process occurs from the Pd surface to the adsorbed oxygen,
which induces higher electron density in the oxygen
species.
2) The spin state of the adsorbed oxygen behaves physically
like 1O2, according to the nearly zero magnetic moment.
However, the spin reduction on the adsorbed O2 molecules
is caused by the Pd→O2 transferred electrons that occupy
the anti-bonding π* orbital, as indicated by the comparison
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Figure 1. a) Molecular orbital diagrams for a ground-state triplet O2 molecule, two types of singlet O2, superoxide O2− and hydrogen peroxide. b) Possible radiative transitions between three types of O2. Reproduced with permission.[35] 2003, American Chemical Society. The projected density of states
(PDOS) diagrams of c) free molecular O2 and d) O2 adsorbed on the surface of Pd{100}. Reproduced with permission.[36] Copyright 2013, American
Chemical Society.
of the projected density of states (PDOS) between free 3O2
(Figure 1c) and the adsorbed species (Figure 1d).[36] This
mechanism is absolutely different from the case of free
1O where the low spin originates from the electron pairing
2
through a spin-flip process.
3) The activity of the adsorbed oxygen can be quenched by specific free 1O2 scavengers.[36] This feature indicates that the
adsorbed oxygen behaves chemically like 1O2. This reactivity
of the adsorbed oxygen should be ascribed to its high electron density enabled by the Pd→O2 electron transfer.
Overall, adsorbed oxygen on the Pd surface should possess
reactivity that resembles free 1O2, which in turn brings about
important catalytic applications. This newly added member
in the family of reactive oxygen species, however, is a species
that is fixed on metal surfaces with a higher electron density,
thereby causing different behavior in certain systems than
free 1O2.
Adv. Mater. 2015, 27, 7025–7042
3. Detection Techniques: Free 1O2 versus 1O2
Analogue
As mentioned above, Pd nanomaterials can activate molecular
O2 to a high-energy state enabling oxidation reactions. As the
focus of this work is to highlight 1O2 analogue, this section
mainly outlines the detection methods for 1O2 in which free
1O and 1O analogue may exhibit analogous or differentiable
2
2
behavior. Based on the performance of 1O2 analogue in various
detection tests, its chemical and physical properties can be
revealed and understood.
3.1. Phosphorescence
The transition of a → X (i.e., transition 4 in Figure 1b) attributed to 1O2 (1Δg) is the most experimentally studied transition in oxygen species. Krasnovsky et al. first observed the
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a → X emission (i.e., phosphorescence) in solution with photosensitization experiments.[37] Further investigations revealed
that phosphorescence accompanied with the a → X transition
was sensitive to the hydrogen isotopes in aqueous solutions
(D2O versus H2O). Specifically, H2O substantially quenches the
phosphorescence of 1O2,[38] so the photoluminescence characterization can only be performed in D2O. The quantum yield of
a → X emission turned out to be about 1.2 × 10−7 in D2O.[38] As
such, the phosphorescence observable in D2O provides direct
evidence to identify the production of free 1O2. Compared with
other characterization techniques, this phosphorescence measurement requires only a routine instrument and does not rely
on additional probing molecules. In addition, the fundamental
physical mechanism for the origin of this phosphorescence is
relatively clear. As a result, the method has been widely used
to detect 1O2 photosensitized by various materials including
semiconductors and metals.[34,39,40] For instance, such phosphorescence has been observed on Ag, Au and Pt, which indicates
that these metals can photosensitize the formation of 1O2.[34]
However, in the case of Pd, we have not obtained any phosphorescence signal from O2 adsorbed on Pd nanomaterials. This
finding suggests that 1O2 analogue indeed possesses a different
electronic structure from free 1O2. The 1O2 analogue adsorbed
on Pd surfaces, whose nearly zero magnetic moment originates
from its increased electron density, is not in an excited state like
the free 1O2.
3.2. Fluorescence Probes
From the analyses above, it is not difficult to recognize that the
phosphorescence of 1O2 is extremely sensitive to solvents and
often suffers from weak signals given its low quantum yield. For
this reason, various fluorescence probes have been developed
to trap 1O2. Specifically, the fluorescence probes are designed
to offer good sensitivity and selectivity toward 1O2, and do not
generate noticeable responses toward other types of ROS such
as OH• and O2−. Given the extensive research in related fields,
it seems impractical to include all the fluorescence probes discussed here. In order to highlight the key facts, here we mainly
outline two commonly used fluorescence probes. Most of the
1
O2 fluorescence probes with good sensitivity and selectivity
work through the formation of endoperoxides, a stable endoperoxidation product (Figure 2a and 2b).[41,42]
9,10–Dimethylanthracene (DMA) is a fluorescent compound
that can react with 1O2 to strongly quench the fluorescence in
various solvents including water (see the reaction mechanism
shown in Figure 2a).[41,43] This probing molecule possesses a high
reaction rate and good selectivity. Our experimental results have
revealed that the strong fluorescence of DMA can be quenched
by the addition of Pd nanocubes (see Figure 2c). This phenomenon indicates that the 1O2 analogue on the surface of Pd nanomaterials can undergo endoperoxidation similarly to free 1O2.
Similar endoperoxidation occurs at other fluorescence
probes such as 9-[2-(3-carboxy-9,10-dimethyl)anthryl]-6-hydroxy3H-xanthen-3-one (DMAX),[42] 9-[2-(3-carboxy-9,10-diphenyl)
anthryl]-6-hydroxy-3H-xanthen-3-ones (DPAX)[44] and singlet
oxygen sensor green (SOSG), a commonly used probe in recent
years.[34,45] Although these probes have the same reaction
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centers as DMA, their working mechanisms are quite different,
as indicated by a representative example of SOSG in Figure 2b.
Although the chemical structure of SOSG has not been disclosed, it is believed to be an anthracene-fluorescein derivative.
The conjugating structure enables a π-bond intramolecular
electron transfer (ET) from anthracene groups to neighboring
functional groups, which in fact quenches its fluorescence.[34,45]
Upon reaction with 1O2 to form an endoperoxide, the ET process can be precluded by breaking the conjugating structure
of the anthracene group, enabling the generation of fluorescence. The detection of 1O2 by DMAX, DPAX and SOSG thus
works via a process of weak to strong fluorescence, whose success relies on the suppression of intramolecular ET by endoperoxidation. Apparently, such an intramolecular ET process is
affected by the electron density of O–O bonded with conjugated
structures, so 1O2 analogue may exhibit different behavior.
Figure 2d displays typical fluorescence spectra of SOSG in
the presence of Pd nanocubes under different light irradiation
conditions. In a control experiment, SOSG alone displayed
quite weak fluorescence, whose intensity could be enhanced by
light irradiation. This fluorescence enhancement is caused by
the immediate product formed between SOSG and 1O2 which
can also serve as an efficient 1O2 photosensitizer.[45] When the
SOSG was mixed with Pd nanocubes, the fluorescence intensities were reduced both in dark and under light irradiation. Two
facts may inhibit the generation of fluorescence from the SOSG
molecule. First, 1O2 analogue possesses additional electron
charges, which favors ET from the endoperoxidized anthracene
group to its neighbor inside an endoperoxide molecule. This
enhanced ET process quenches the fluorescence. Second, 1O2
analogue is a species adsorbed on Pd surfaces, so the endoperoxidation has to take place at solid surfaces. In this case, the
rigid structure of the large molecule SOSG would induce a
steric hindrance effect on the surface reaction, formulating the
obstacle to produce the endoperoxide structure.
In this section, the nature of the adsorbed oxygen species
(i.e., 1O2 analogue) has been examined by fluorescence probes
including DMA and SOSG. Characterization has revealed that
1
O2 analogue is a species as chemically active as free 1O2, paving
the way to reactions with organic molecules such as endoperoxidation. However, the efficiency of organic reactions by 1O2
analogue depends on the steric structures of organic molecules,
given that the molecules have to react with oxygen on a catalyst surface. On the other hand, one has to keep in mind that
1O analogue possesses a higher electron density, which affects
2
intramolecular or intermolecular charge transfer.
3.3. Electron Spin Resonance
Electron spin resonance (ESR) spectroscopy is another common
way to determine types of ROS. With specific radical trapping
agents, different ROS can show characteristic ESR signals. For
example, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) is particularly
useful for identifying O2− and OH•;[46] while 2,2,6,6-tetramethyl4-piperidone hydrochloride (4-oxo-TMP) has commonly been
used to detect 1O2 that produces stable nitroxide radical 4-oxoTEMPO (see Figure 3a). Specifically, the 4-oxo-TEMPO displays a
1:1:1 triplet signal in ESR spectroscopy, with g = 2.0055.[47]
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Figure 2. Fluorescence probes for the detection of 1O2: a) 9,10-DMA and b) SOSG. a) Reproduced with permission.[41] Copyright 2005, Elsevier.
b) Reproduced with permission.[45] Copyright 2011, Wiley-VCH. c) Fluorescence spectra of DMA in D2O in the presence and absence of Pd nanocubes.
d) Fluorescence spectra of SOSG in D2O in the presence and absence of Pd nanocubes, with or without irradiation of light.
As the species of 1O2 analogue can chemically react with
organic molecules such as free 1O2, the solution of 4-oxo-TMP
clearly showed a similar triplet signal when mixed with Pd
nanocrystals. This finding suggests that molecular oxygen can
be activated in 1O2 analogue on the surface of Pd nanocrystals.
It is worth pointing out that the similarity of chemical reactivity
between 1O2 analogue and free 1O2 has been confirmed by control experiments using 1O2-specific agents. It has been well established that D2O can efficiently prolong the lifetime of 1O2 while
carotene is a specific scavenger that inhibits the generation of
1
O2.[34,48] Our measurements demonstrated that 4-oxo-TEMPO
ESR signals were enhanced by D2O and quenched by carotene
(see Figure 3b), indicating that the 1O2 analogue species does
indeed behave chemically as free 1O2. Another important finding
is that ESR signal intensities are highly dependent on the surface
facets of Pd nanocrystals, as they have a significant influence on
O2 adsorption configurations.[36] 1O2 analogue specifically is preferentially formed on Pd{100} rather than Pd{111} facets.
Adv. Mater. 2015, 27, 7025–7042
Despite confirmation of the chemical similarity between 1O2
analogue and free 1O2, it naturally remains a concern whether
the oxygen species adsorbed on Pd surfaces possess properties analogous to other types of ROS. To ease this concern,
DMPO was also employed as a probing molecule to examine
the system. Interestingly, the solution displayed a 1:1:1 triplet signal instead of DMPO/·OH (1:2:2:1 quartet signal) and
DMPO/·O2H (1:1:1:1 quartet signal) signals.[49] According to
the literature, this signal should be generated from the nitro
products through the pyrroline ring opening of DMPO by 1O2like species (see Figure 3c,d).[46]
3.4. Specific Scavengers
The phosphorescence measurement, as well as fluorescence
and ESR characterizations aided with probe molecules, has
revealed the physical and chemical features of 1O2 analogue
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Figure 3. a) Reaction pathway for the oxidation of 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP). b) ESR spectra of 4-oxo-TMP solution mixed with
Pd nanocrystals or other agents. Reproduced with permission.[36] Copyright 2013, American Chemical Society. c) Possible mechanism for DMPO ring
opening reaction, and d) ESR spectra of the ring opening product. Reproduced with permission.[49] Copyright 1996, American Chemical Society.
(see Table 1). Based on the acquired data, we can conclude that
adsorbed oxygen has a similar magnetic moment and chemical
reactivity to free 1O2. For this reason, the 1O2 analogue species
can react with organic molecules such as free 1O2, via endoperoxidation (except for the case with large steric hindrance),
oxidation of secondary amine, and pyrroline ring opening,
all of which have been tracked by fluorescence and ESR. On
the other hand, 1O2 analogue possesses a different electronic
structure from real 1O2. 1O2 analogue is not in an excited
state but has higher electron density caused by Pd→O2 electron transfer. As reflected by the absence of phosphorescence,
no energy relaxation transition is anticipated. The extra electrons accumulated at O–O may have an impact on intramolecular charge transfer processes when the O–O is involved in
organic molecular structures, as indicated in the case of SOSG
fluorescence.
Upon acquiring the nature of 1O2 analogue, the next question would be: how stable is this species in different chemical
environments? It is known that radical ROS can be transformed into each other in different environments, such as the
Haber-Weiss reaction (1) and the Fenton reaction (2), amongst
others (3–5).[50,51]
O2− + H2O2 → 1O2 + ⋅ OH + OH−
(1)
Fe 2+ + H2O2 → Fe3+ + ⋅OH + OH−
(2)
O2− + 2H+ →1O2 + H2O2
(3)
2H2O2 → 2H2O + 1O2 (alkaline environment)
(4)
2O2− + 2H+ → H2O2 + 3O2
(5)
As a matter of fact, the system of ROS in different chemical
and biological environments is rather complicated. To examine
the behavior of 1O2 analogue in different media, one has to
develop a system in which the probing molecules or techniques
are not sensitive to pH changes. Note that all the detection tests
described above were acquired under neutral conditions, while
the molecules used were highly sensitive to the environmental
Table 1. Comparison of singlet oxygen with singlet oxygen analogue in terms of physical and chemical features.
Features
Physical
Chemical
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Free 1O2
Parameters
1O
2
analogue
≈0
Magnetic moment
0
Molecular orbitals
Two paired electrons in one π* orbital
Phosphorescence
λem ≈ 1270 nm ascribed to a → X
Fluorescence DMA
Quenched fluorescence by endoperoxidation[41,43]
Quenched fluorescence by endoperoxidation
Fluorescence SOSG, etc.
Enhanced fluorescence by suppressing intermolecular ET via
endoperoxidation[34,45]
No enhanced fluorescence, due to high electron density on O–O
and low endoperoxidation probability (steric hindrance effect)
ESR 4-oxo-TMP
1:1:1 triplet characteristic for 4-oxo-TEMPO; quenched by 1O2
scavenger; enhanced by D2O[47]
1:1:1 triplet characteristic for 4-oxo-TEMPO; quenched by 1O2
scavenger; enhanced by D2O
ESR DMPO
1:1:1 triplet characteristic for pyrroline ring opening of DMPO[46]
1:1:1 triplet characteristic for pyrroline ring opening of DMPO
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transition[33,34,39]
Addition electrons from Pd to occupy O2 anti-bonding π* orbital
Not detected
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As TMB can only be oxidized in an acidic solution, we
have employed a second method to monitor the status of the
absorbed oxygen under alkaline conditions. The second method
is based on luminol oxidation which can generate chemoluminescence (see Figure 4b). In this method, 50 µL of Pd nanocubes (0.1 mg mL−1) were added to 1 mL of luminol solution
(10 × 10−3 M luminol in 0.1 M NaOH solution), from which
a strong chemoluminescence signal could be observed without
the need of adding other oxidants. By adding various scavengers
into this reaction system, we have identified that luminol oxidation can be quenched by 1O2 and O2− scavengers. This finding
indicates that oxygen adsorbed on Pd surfaces can still behave
like 1O2 in an alkaline solution, but part of the species is transformed into O2−. It is worth mentioning that this chemoluminescence-based luminol oxidation characterization also helps
exclude the role of light in oxygen activation, as the detection
process does not need to bring in incident light unlike other
characterization methods such as UV–vis and Raman spectroscopy. Overall, the oxygen species adsorbed on Pd materials has
chemical properties analogous to 1O2 regardless of acidic, neutral or alkaline conditions; however, the 1O2 analogue can be
partially converted to O2− in an alkaline solution.
4. Key Factors for Tuning Oxygen Activation
Efficiency
4.1. Surface Facets of Pd Nanocrystals
Figure 4. a) Curves showing the absorbance of TMB oxidation product
at 450 nm versus reaction time in the presence of various specific scavenger molecules in acidic environments. Reproduced with permission.[36]
Copyright 2013, American Chemical Society. b) Chemoluminescence of
luminol with Pd nanocubes in the presence of various specific scavenger
molecules in alkaline environment.
pH. Thus it is imperative to employ some other methods to
track the status of 1O2 analogue in different chemical environments. As these methods are not 1O2-specific, the use of ROSspecific scavengers becomes critical to the success in their
evaluations.
Thus far we have employed two methods to perform evaluations. The first one is based on 3,5,3’,5’-tetramethyl-benzidine
(TMB), a molecule that can be oxidized to products with characteristic colors under acidic conditions (i.e., HAc/NaAc bu er
solution). The reaction process can be easily monitored by UV–
vis spectroscopy (see Figure 4a), as the oxidized products have
characteristic absorbance. It has been well established that 1O2
can be quenched by carotene, NaN3 or histidine, while mannite, catalase, and superoxide dismutase (SOD) are specific
scavengers for OH·, H2O2, and O2− species, respectively.[52,53]
Our experimental results reveal that TMB molecules were oxidized in the presence of Pd nanocubes and air, and that this
oxidation reaction can only be quenched by the 1O2 scavenger −
carotene. This finding indicates that oxygen species adsorbed
on Pd surfaces behave chemically as singlet oxygen in acidic
solutions.
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It has been commonly recognized in catalysis and surface science that molecular adsorption is a process depending on the
atomic arrangement and charge state of catalyst surfaces.[54–57]
Our research group has compared the adsorption of molecular
O2 on two different facets − Pd{100} and Pd{111}.[36] As indicated by theoretical simulations (Figure 5a,b), molecular O2
has better activation on Pd{100} due to the larger O–O bond
length (1.402 Å on {100} versus 1.324 Å on {111}), which has
been verified by synchrotron radiation-based near-edge X-ray
absorption fine structure spectroscopy (NEXAFS). In addition to the difference in bond lengths, the surface facets have
a significant impact on electron transfer from the Pd surface
to O2. The spin charge density analyses (Figure 5c,d) revealed
that about 0.7 electrons could transfer from the Pd{100} surface to the adsorbed oxygen, while the charge transfer in the
case of Pd{111} was only ca. 0.4 electron charges. The different
amounts of electrons transferring into the anti-bonding π*
orbital of O2 led to a reduction in magnetic moment, to a varied
extent (0.017 µB for Pd{100} versus 0.549 µB for Pd{111}).[58]
This finding suggests that the physical behavior of the adsorbed
oxygen species on Pd{100} is more analogous to 1O2 than
that on Pd{111}, which would make {100} an ideal facet for
designing Pd nanocrystals toward oxygen-related catalytic reactions. The performance of Pd{100} can be further enhanced
by increasing its electron density (Figure 5e, discussed in
Section 4.2.1).
Some research groups have developed a few approaches
to Pd nanocrystals with high-index facets such as the Pd concave nanocubes by Xia et al. They achieved the synthesis of
Pd concave nanocrystals using a seeding process.[59] Concave
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Figure 5. The most favorable adsorption configurations of O2 on the
a) Pd{100} facet, and b) Pd{111} facet. The spin charge densities of O2
on the c) Pd{100} facet, and d) Pd{111} facet. Reproduced with permission.[36] Copyright 2013, American Chemical Society. e) Curves showing
the changes in O–O distance and O2 magnetic moment by introducing
additional electrons or holes into the Pd{100} system. Reproduced with
permission.[58] Copyright 2014, Wiley-VCH.
nanocrystals exhibited 1.9 times greater activity than conventional nanocubes in electrocatalytic formic acid oxidation,
owing to a higher density of low-coordinated atomic steps on
their {730} facets. It is anticipated that future research can
reveal the effect of high-index facets on oxygen activation.
4.2. Hybrid Structures Loading or Hosting Pd Nanomaterials
Hybrid structures, in which Pd nanomaterials are loaded on
or hosted in other materials, have been widely used for catalytic oxidation reactions.[60–62] Despite their wide applications,
it often puzzles us why these hybrid structures may lead to
improved catalytic performance in various catalytic reactions.
To understand the specific roles of component materials, we
have to analyze the working mechanisms from the viewpoint
of their interactions with Pd nanomaterials or synergistic
functions. Although most cases in literature have not specifically investigated the behavior of oxygen activation in Pd-based
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hybrid structures, we can find some clues about how other
components in such structures may impact on the efficiency of
oxygen activation.[63,64] Possible major effects from the hybrid
components are listed in Figure 6a.
As the catalysts are fixed on supports, the agglomeration of
nanoparticles can largely be prevented to ensure stable catalytic performance (Figure 6b). This stabilization effect, which
does not require chemically active support materials for catalytic reactions, has been recognized by the catalysis community.[65–67] In addition to the stabilization effect, support materials may play other important roles in reactions through
interactions with Pd nanomaterials. 1) Surface polarization
at interfaces (Figure 6c):[68] when a Pd layer is formed on the
surface of a support, their different work functions will cause
polarization at the Pd-support interface. As long as the Pd layer
is thin enough, the polarized charges can travel to the top surface of the Pd layer. Depending on whether the work function
of the support is lower or higher than that of Pd, this polarization process will accumulate a significant amount of negative
or positive charges on the Pd surface. As oxygen activation is a
process that electrons transfer from Pd surface to O2, the variation in Pd surface charge density is anticipated to enable tuning
of the oxygen activation efficiency (see Figure 5e). 2) Steric hindrance (Figure 6d):[69,70] as a Pd nanocrystal is loaded on a support, their surfaces will form a junction area whose angle will
affect molecular bonding stretching, bending, rocking, wagging
or twisting. Similarly, molecular behavior will be spatially influenced by pore sizes when Pd nanoparticles are hosted in the
pores of a support. Note that O2 is a small molecule that does
not have significant steric hindrance with the supports. However, the reactivity of oxidized reactants (e.g., organic molecules)
can be limited by this steric effect, which in turn maneuver
the desorption kinetics of oxygen species from Pd surfaces
with respect to reactant-oxygen reactions. 3) Synergistic effect
(Figure 6e):[71,72] hybrid structures may provide additional active
sites for reactions. Taking metal-oxide hybrid structures as an
example, one can see that three reaction mechanisms might
be involved in oxidation reactions. In mode 1, oxygen and oxidized reactant are both adsorbed on metal surface. In this case,
the supports do not directly participate in the reaction process.
However, the electron transfer between metal and oxide would
become a major factor that might alter the surface charge state
of Pd catalysts and thus the O2 activation efficiency. In mode
2, the oxidized reactant and oxygen are activated on the surfaces of metal and oxide, respectively, or vice versa. This interfacial synergistic effect has brought about enhanced activity in
catalytic reactions such as in the cases of Au/Fe3O4- and Au/
CeO2-catalyzed CO oxidation.[71,72] It is worth mentioning that
if the oxygen activation occurs on the oxide, in the presence of
light the process may undergo a different pathway from that
on Pd. Raman spectroscopy has shown that oxygen can be activated into superoxide and peroxide species on the surface of
CeO2.[72] In mode 3, the lattice oxygen and oxygen vacancy in
oxide supports are involved in oxidation reactions such as CO
oxidation.[73] The participation of supports in reactions directs
reactions to a different pathway, which no longer relies on the
conventional O2 activation process.
As light is coupled into the reaction system, the situation
becomes more complicated. For instance, when Pd nanocrystals
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Figure 6. a) The main physical mechanisms that may be involved in hybrid structure loading or hosting of Pd nanocatalysts. b) Fixation of nanocatalysts
on supports, preventing the agglomeration of nanoparticles. c) Polarization at the interface of a metal and a semiconductor or another metal, accumulating negative or positive charges on the top metallic surface. d) Steric hindrance brought about by the surface or porous structures of supports,
limiting the molecular bonding stretching, bending, rocking, wagging or twisting needed for reactions. e) Possible reaction pathways for the oxidation
reaction occurring at supported metal nanocatalysts. f) Schottky junction between metal and n-type semiconductor, trapping electrons on the metal
under light illumination. g–i) Effects of plasmonic metal supports on catalytic metal under light illumination: g) generation and injection of plasmonic
hot electrons into the catalysts, h) photothermal conversion inducing a temperature increase, and i) local enhancement of electric field.
are supported on semiconductors, establishing a Schottky junction will trap electrons or holes at Pd in the case of n-type or
p-type semiconductor, respectively (Figure 6f).[63] In a different
design, Pd nanocrystals are integrated with plasmonic metals
such as Au, in which plasmonic hot electrons can be injected
into Pd nanocrystals (Figure 6g).[74] Both cases can tailor the
surface charge density of Pd nanocrystals, providing possibilities to tune the O2 activation efficiency. Certainly, plasmonic
metals may induce multiple effects in addition to hot electrons: photothermal conversion induces high local temperature (Figure 6h) and the enhanced electric field affects bond
strength or electron spin (Figure 6i).[63] Overall, reaction conditions and support materials should be cautiously chosen toward
further catalytic applications in consideration of these effects.
In the following subsections we will highlight a few specific
cases existing in the literature to demonstrate the effects from
Pd-based hybrid structures.
state of the Pd surface should thus have a huge impact on
O2 activation efficiency. As described above, a metal-semiconductor junction is a widely used strategy for maneuvering the
charge state of metal. Using a Pd-TiO2 hybrid configuration as
an example, our research group has demonstrated that photoexcited electrons can migrate to and become trapped on the
Pd{100} surface, driven by the Schottky junction and increasing
the Pd electron density.[58] According to theoretical simulations
(Figure 5e), a Pd{100} surface with additional electrons possesses an enhanced O2 activation performance.[58] With additional electrons introduced to the Pd surface, the magnetic
moment and bond length of the adsorbed oxygen can be further reduced to 0.001 µB and increased to 1.415 Å, respectively,
making it more active in oxidation reactions. Oxide supports
have in fact been widely used in organic oxidations; thus highlighting the importance of charge transfer between metal and
oxide for O2 activation.
4.2.1. Supporting Metallic Pd on Semiconductors
4.2.2. Loading Metallic Pd in Metal–Organic Frameworks
Electrons donated from Pd surfaces to adsorbed oxygen are
responsible for the production of 1O2 analogue. The charge
Metal–organic frameworks (MOFs) are a new class of porous
materials with a promising future in various applications.[75,76]
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Table 2. Examples of Pd-based nanomaterials used in oxidation reactions with molecular O2.
Catalystsa)
Nanostructures
Ref.
Pd or Pd-based bimetallic Pd on supports
nanostructures loaded on
supports
PdM on supports
(M = Au, Ni, etc.)
[18,83]
Alloys
[24,28,81,82,86,87]
Core–shell
nanostructures
Pd or Pd-based bimetallic
nanostructures without
supports
Pd
[28,80]
to Pd so as to equilibrate the electron Fermi distribution at
the their interface. This interfacial polarization will increase
the electron density on Pd. On the contrary, integration with
a metal with a higher work function should disfavor oxidation reactions from the viewpoint of oxygen activation, as it
causes a reduction in Pd electron density. Pd-based alloys or
bimetallic hybrid structures have commonly been used as catalysts in oxidation reactions.[85–87] It is anticipated that future
research focused on this aspect will help decode the mechanisms behind each oxidation reaction model.
[27,78]
PdM (M = Au, etc.) Alloys
[79]
5. Recent Developments in Pd-Based
Nanomaterials for Oxygen-Related Catalytic
Reactions
a)
Note that incident light was not identified as a necessary reaction condition in
most cases.
Some MOF structures have been employed to support metal
nanomaterials for efficient chemical catalysis.[77] The MOFs
may make multiple contributions towards improving the efficiency of catalytic reactions: 1) a large number of active sites
provided by MOFs, such as Lewis or Brønsted acidity sites; 2)
a controllable space for chemical reactions offered by the uniform channels of MOFs, which maneuvers selectivity over different products; 3) selective adsorption capability for specific
molecules, which locally concentrates reactants. Integrated with
the advantages provided by MOFs, Pd catalysts are expected to
exhibit improved performance in oxidation reactions.
4.2.3. Integrating Metallic Pd with Other Metals
Pd nanomaterials can be combined with other metals by
forming bimetallic alloys or core–shell structures, which may
be further loaded on supports as outlined in Table 2.[78–83]
Undoubtedly, when compared with bare Pd, the Pd-based
bimetallic nanostructures should possess different features
such as shapes and surface structures, allowing for tuning of
their surface charge state and molecular reactant adsorption.
Here we specifically highlight the tuning of surface charge
state by interfacial polarization, which has not been previously
investigated for oxygen activation. It is known that the metals
have different work functions (e.g., Pd ≈5.12 eV, Pt ≈5.65 eV,
Ag ≈4.26 eV, Au ≈5.1 eV). As a result, interfacial polarization
should be induced at the metal–metal interface, as indicated
by differential charge density simulations and electrocatalysis
characterizations.[84] The surface charge state of Pd can be
altered by a second metal as long as the Pd layer is sufficiently
thin in core–shell structures. In the case of Pd-based alloys, Pd
atoms are uniformly hosted in another metal matrix so that the
polarization between the two different metals can more easily
accumulate polarized charges on Pd atoms. Given the key
role of surface charges in O2 activation, it is quite reasonable
to assume that oxygen adsorbed on polarized Pd can receive
increased or reduced amounts of electrons, maneuvering the
activation efficiency. When Pd is in contact with a metal (M)
with a lower work function, electrons prefer to flow from M
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During the past decade, numerous effective Pd-catalyzed systems have been developed.[12,17,18,88,89] Other than homogeneous Pd catalysts, Pd nanomaterials with well-defined shapes
and surface structures have shown high efficiency and can
be easily recycled for long-term use. As discussed in the last
section, Pd-based nanomaterials exhibit unique properties
in oxygen activation. As a result of these properties, we envision that Pd-based nanomaterials will find wide applications in
the scope of oxygen-related catalytic reactions. In this section,
recent developments in heterogeneous Pd-based catalysts for
oxygen-related reactions are outlined and discussed. Given the
important role of oxygen activation in oxidation reactions, Pdbased nanomaterials favoring oxygen activation are expected to
show high catalytic efficiency in this type of reaction. On the
other hand, the adsorption of oxygen on catalyst surfaces might
hamper certain types of reaction. In this case, nanomaterials
at which oxygen species are weakly adsorbed and moderately
activated may exhibit higher catalytic efficiency.
There are a number of factors that might affect the overall
performance of catalysts in chemical reactions, such as temperatures, pressures, reactant molecules and solvents. As the
scope of this work is to highlight the process of oxygen activation based on Pd nanomaterials, this section will be focused on
the correlation of absorbed oxygen species with oxygen-related
catalytic reactions, which may provide a unique perspective for
catalyst design. Specifically, progress on Pd-catalyzed oxygenrelated reactions will be introduced, in alignment with the fundamental conclusions for oxygen activation in the last section,
to present the structure-property relationship of Pd-based materials toward catalyst design.
5.1. Oxidation Reactions
Oxidation reactions are important chemical transformations
involved in organic chemistry and industrial chemistry.[90] For
instance, the Wacker process has been used in the industrial
production of acetaldehyde for more than 40 years.[91] When
compared with other reaction types outlined below, oxidation
reactions can be considered as the chemical applications in
which Pd-based nanomaterials are directly employed owing to
their capabilities in oxygen activation. As a matter of fact, the
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5.1.1. CO Oxidation
From the perspective of fundamental studies, CO oxidation
reactions are a frequently used probe reaction for investigating
reaction mechanisms.[92] For this reason, we first introduce the
progress on this reaction, followed by discussions on catalytic
organic reactions.
Examples of Facet Effects: Using CO oxidation as a model reaction, Hu et al. found that the reaction barrier on the Pd{100}
facet (0.78 eV) is lower than that on the {111} facet (1.05 eV)
at 1/4 monolayer coverage by simulating reaction pathways in
which oxygen activation is involved.[93] A similar conclusion,
that the Pd{100} facet has a lower barrier than the {111} or
{110} facets, has been reached by other groups.[92,94,95] Here we
have to point out that oxygen activation is not the only key step
to CO oxidation. As commonly recognized by the research community,[96] both CO and O2 activation are crucial to achieving
high-efficiency CO oxidation. The behavior of CO activation,
which also depends on the surface facets, may alter the overall
CO oxidation performance.
Interestingly, Wang et al. revealed that in CO oxidation Pd
octahedrons covered by {111} facets had superior activity to
Pd nanocubes covered by {100} facets.[97] This indicates that a
certain factor other than oxygen activation changes the overall
performance of CO oxidation. It is known that Pd surfaces can
be readily poisoned by strongly adsorbed CO,[98] and as such,
O2 activation may also be influenced by CO molecules. Thus
the involvement of CO molecules significantly impacts on the
efficiency of CO oxidation.
On the other hand, we also notice some inconsistent findings on CO oxidation such as size-dependent performance
from different reports. In the system of Pd–SiO2–nanospheres
demonstrated by Wang et al., no remarkable size effect on the
CO oxidation efficiency was observed.[97] In a different report,
however, Xia et al. found that Pd nanocubes with sizes <10 nm
exhibited excellent activity against the samples at large sizes.[27]
Given that these works used different experimental conditions
(e.g., O2 content, flow speed, synthetic methods for nanocrystals, and support materials), it seems infeasible to systematically compare their findings and make solid conclusions.
Nevertheless, we can still analyze the origins for these different
outcomes. Given the role of CO in poisoning Pd surfaces, variations in gas flow and supports may cause different surface
conditions, leading to inconsistent CO oxidation performance.
Another factor related to surface chemistry is the capping agent
that is commonly involved in the synthesis of Pd nanocrystals
and left on Pd surfaces.[1] Tuning the surface facets and particle
sizes of Pd nanocrystals inevitably makes a difference to the
type or coverage of capping agents on their surface, which thus
has a huge influence on the molecular adsorption and activation process.
Adv. Mater. 2015, 27, 7025–7042
Examples of Support Effects: Peterson and Datye reported on
a nanostructure in which isolated Pd atoms are loaded in alumina, which showed an onset of catalytic activity at 40 °C for
CO oxidation.[99] This conversion temperature (40 °C) is dramatically lower than that for bare Pd nanocrystals (150–300 °C).
Furthermore, the addition of lanthanum oxide improved the
stability of alumina and isolated Pd atoms. The performance
improvement originates from two facts. Firstly, isolated Pd
atoms are not poisoned by the adsorbed CO, so the reaction
temperature can be substantially reduced, enhancing the catalyst stability. Secondly, the oxygen activation undergoes a different mechanism in alumina. As illustrated by mode 3 in
Figure 6e, the reaction proceeds via two steps: 1) CO reacts
with lattice oxygen (Olat), forming CO2 and leaving an oxygen
vacancy (VO) (CO + Olat → CO2 + VO); 2) molecular oxygen is
adsorbed on the oxygen vacancy, recovering the lattice oxygen
for the next catalytic cycle (O2 + VO → Olat). As a result, both
CO and O2 activation can be facilitated.
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applications of Pd-based nanomaterials in oxidation reactions
have a long history. Yet investigations over the oxygen activation
process on Pd surfaces have not been systematically conducted
until recent years . With the chemical and physical features of
adsorbed oxygen in mind, it would be more straightforward to
understand the behaviors of Pd catalysts in oxidation reactions.
5.1.2. Alcohol Oxidations
Examples of Facet Effects: Alcohol oxidations are a class of typical organic oxidation, which provide an excellent platform for
evaluating the facet effect of oxygen activation in catalytic oxidation reactions. For instance, glucose oxidation can be effectively
quenched by the scavenger for 1O2 analogue species − carotene – indicating that 1O2 analogue is responsible for the oxidation reaction.[36] As such, the glucose oxidation efficiency can
be well correlated with the oxygen activation performance. The
experiments showed that Pd nanocubes covered by {100} facets
had remarkably higher activity than Pd octahedrons by those
covered by {111} facets (see Figure 7a),[36] in agreement with
the conclusion in Section 4.1 that adsorbed oxygen is better
activated on Pd{100}. Other than {100} and {111} facets, Wang
et al. also investigated the {110} facets of Pd nanomaterials
which exhibited superior activity to other low-index facets in the
solvent-free oxidation of alcohols such as DL-sec-phenethylalcohol.[100] The turnover frequency (TOF) of Pd{110} facets (i.e.,
the number of reactant molecules that can convert to products
per unit of time, a key parameter to evaluate the performance
of catalysts) turned out to be roughly twice that of Pd{111}
(see Figure 7b). In the case of nanomaterials, it still remains a
great challenge to obtain Pd nanocrystals that are fully covered
by {110} facets.[1] Thus the performance of the Pd {110} facet
needs to be further assessed both theoretically and experimentally. Furthermore, in order to identify high-efficiency catalysts
for oxidation reactions, it is necessary to evaluate the behavior
of high-index facets (which are known to have higher activities
in other types of catalytic reactions) in oxygen activation.
Examples of Support Effects: In addition to the facet effect,
Pd-based hybrid nanomaterials have shown fascinating properties in oxidation reactions owing to their unique interfaces. For
instance, Li and Ding demonstrated that Pd/FeOx hybrid structures loaded on SiO2 exhibited enhanced catalytic activity in the
oxidation of benzyl alcohol (see Figure 7c and d).[62] According to
their proposed mechanism, the hybrid structures offered perfect
interfaces to enable synergistic effects on the reaction: 1) benzyl
alcohol was adsorbed and activated on the Pd surface; 2) lattice
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Figure 7. Examples of Pd-catalyzed oxidation reactions in the presence of O2. a) Oxidation of glucose by Pd nanocubes with {100} facets and Pd
octahedrons with {111} facets using molecular O2. The data are collected from the work by Lang et al.[36] b) Oxidation of DL-sec-phenethylalcohol
by Pd{110} and Pd{111} facets. The data are collected from the work by Wang et al.[100] c) Schematic illustrating the oxidation of benzyl alcohol on
FeOx/Pd/SiO2. d) Catalytic activities of benzyl alcohol oxidation by Pd/SiO2 and FeOx/Pd/SiO2. Reproduced with permission.[62] Copyright 2015, Royal
Society of Chemistry. e) Catalytic activities of benzyl alcohol oxidation by Pd/TiO2, Pt/TiO2 and PdPt/TiO2 using molecular O2. Reproduced with
permission.[103] Copyright 2014, American Chemical Society. f) Summary of alloy effects on toluene oxidation using molecular O2. g) Catalytic activities of toluene oxidation by Pd/C, Au/C and AuPd/C in the absence of solvent using O2 (160 °C, 0.1 MPa O2, 20 mL of toluene, 1 wt% Au1Pd1.85/C
catalyst, 7 hours). The data are collected from the work by Kesavan et al.[86] h) Catalytic activities of toluene oxidation by Pd/TiO2, Pt/TiO2 and PdPt/
TiO2 (160 °C, 150 psi O2, 10–40 mL of toluene, 2.5 wt% Pt1Pd1/TiO2 catalyst). Reproduced with permission.[103] Copyright 2014, American Chemical
Society. i) Schematic illustrating the one-step conversion of benzene to phenol with a Pd membrane reactor. Reproduced with permission.[105] Copyright 2010, Elsevier B.V.
oxygen was formed on the surface of FeOx (similar to mode 3
in Figure 6e); 3) the adsorbed benzyl alcohol reacted with the
lattice oxygen, leaving an oxygen vacancy; 4) molecular O2 was
activated on the FeOx or Pd-FeOx interface, recovering the lattice
oxygen to accomplish a catalytic cycle. From the mechanism,
one can see that the use of such hybrid structures designates
oxides or Pd-oxide interfaces instead of Pd surfaces as sites for
oxygen activation, which apparently undergo a different mechanism from the activation process by Pd→O2 electron transfer.
MOFs are another type of support material for loading Pd
nanomaterials in addition to oxides. Acidic MOF (MIL-101, MIL
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stands for Materials of Institute Lavoisier) loading Pd nanomaterials have been synthesized by Chen and Li, which have
excellent activity and selectivity in alcohol oxidations. In the
oxidation of cinnamyl alcohol, the Pd/MIL-101 exhibited nearly
fourfold greater activity than the Pd nanoparticles on active
carbon, with 99% selectivity for producing benzaldehyde.[101] It
is known that double bonds can be oxidized by O2 in the present of Pd nanocrystals.[102] In this Pd/MOF system, the C=C
double bond in cinnamyl alcohol was fully protected, making
an important contribution to the selectivity of organic reactions. As discussed in Section 4.2.2, uniform MOF channels
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5.1.3. Carbon-Hydrogen Bond Oxidations
Examples of Alloy Effects: The oxidation of carbon-hydrogen
bonds is a highly important topic for the sustainable exploitation of available feedstock. The intermediate and final products
of carbon-hydrogen bond oxidations are all of industrial importance. Kesavan and Hutchings systematically investigated the
reaction of toluene oxidation using Pd-based alloys loaded on
various supports (see Figure 7f). For instance, the AuPd alloy
supported by active carbon showed a high conversion rate and
selectivity (see Figure 7g), which reached a level of 99% conversion and 95% selectivity toward benzylbenzoate, respectively
(reaction conditions: 160 °C, 0.1 MPa O2, 20 mL of toluene,
1 wt% Au1Pd1.85/C catalyst, more than 48 hours).[86] Meanwhile,
control experiments revealed that AuPd alloy supported by TiO2
had lower conversion but similar selectivity (94.3%) toward
benzylbenzoate.[86] In parallel, PtPd alloy supported by TiO2 was
also exploited, which showed different selectivity in the toluene
oxidation (see Figure 7h). The primary product of toluene oxidation by PtPd/TiO2 is benzoic acid (91%), with a total 99% conversion (reaction conditions: 160 °C, 150 psi O2, 10–40 mL of
toluene, 2.5 wt.% Pt1Pd1/TiO2 catalyst).[103] Although different
reaction conditions were used in the two works, this series of
results indicates that synergistic effects by two different metals
(e.g., a metal providing sites for selective bond cleavage and a
metal for bond formation in organic molecules) can improve
the reaction selectivity.
It is worth mentioning that there exists a particularly interesting reaction in the scope of carbon-hydrogen bond oxidations:
one-step conversion of benzene to phenol and cyclohexanone in
the presence of H2 and O2, which was developed by Niwa and
Mizukami.[104] In this system, ROS are formed on Pd surfaces
in the presence of H2 and O2, then reacted with benzene to produce phenol. As Pd is a unique metal that has strong interactions with both H2 and O2,·OOH radicals can be formed in the
reaction system. Later on, Sato et al. synthesized a number of
Pd-based alloys and evaluated their properties in the conversions
of benzene to phenol and cyclohexanone (see Figure 7i).[105]
Examples of Support Effects: As described in Section 4.2, integrating metal catalysts with appropriate supports may induce
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interfacial polarization to enhance oxygen activation (refer to
Figure 6c). This concept has been implemented in catalytic
hydrocarbon oxidations. In a typical example, the activity of
indane oxidation has been significantly improved by using a
Pd@N-doped carbon hybrid structure, as reported by Zhang
and Wang.[106] Such a hybrid structure benefits from doped
nitrogen, which can enhance the adsorption of related reactants
on the catalyst surface. X-ray photoelectron spectroscopy (XPS)
analyses further showed that loaded Pd atoms were preferentially deposited on nitrogen sites. As a result, nitrogen atoms
may increase the electron density on Pd atoms, resulting in an
increased reaction rate.
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may limit the space for chemical reactions (Figure 6d) and be
responsible for this protection of double bonds.
Examples of Alloy Effects: Pd-based alloy nanoparticles loaded
on various supports have played important roles in organic
oxidations. For instance, Forde and Hutchings discovered that
in the oxidation of benzyl alcohol, the TOF value by Pd/TiO2
was about 12 000 h−1, while that of PdPt/TiO2 dropped down
to 6000–9000 h−1.[103] Given that Pd possesses a lower work
function than Pt (Pd ≈5.12 eV versus Pt ≈5.65 eV), the interfacial polarization may induce a flow of electrons from Pd to
Pt and reduce the electron density of Pd atoms (as discussed
in Section 4.2.3). As a result, the reduced Pd electron density would disfavor oxygen activation processes as the activation occurs through electron transfer from Pd to O2,[36] which
we believe could be responsible for the alcohol oxidation rate
reduction. Despite the lower conversion rate, the PdPt alloy
enabled higher selectivity toward benzaldehyde (see Figure 7e).
5.1.4. Alkene and Alkyne Oxidations
Examples of Epoxidation Reactions: The oxidation of alcohols
seems relatively straightforward, producing only a limited
number of oxidation products with predictable molecular
structures. In contrast, the products from the oxidation of
alkene and alkyne are rather complex. For example, styrene
oxidation reactions can generate numerous possible products
including benzoic aldehyde, phenyl acetaldehyde, benzoic acid,
benzeneacetic acid, styrene epoxide and many others. Other
than total conversion rates, selectivity toward epoxidation is
another key parameter for evaluating catalytic performance,
as epoxides are a class of highly important products. Pd-based
nanomaterials such as Pd/SiO2 and Pd/C have shown remarkable activities in epoxidation reactions.[107] For instance, Hikazudani and Takita reported that Pd/TiO2 could catalyze the
epoxidation of propylene at ambient temperature in the presence of H2 (10 mol%) and O2 (20 mol%).[108] Specifically, they
have recognized that ·OOH, one ROS that is generally considered as the protonation species of O2−, can be formed on
Pd surfaces through the reaction between H2 and O2. As a
following step, the alkene can react with ·OOH to form propylene oxide.
It is worth pointing out that the requirement for oxygen
activation by epoxidation is quite different from that by alcohol
oxidations. In the oxidation of alcohols, highly activated oxygen
such as 1O2 analogue formed on Pd{100} surfaces has been
identified as an efficient agent for oxidation. In the case of
epoxidation reactions, however, too strong an oxidation or high
reaction rate may cause epoxide ring opening. In this sense,
it can be predicted that activated oxygen with a suitable reactivity (e.g., 1O2 analogue with relatively low electron density
which can easily be controlled by facets or supports) may offer
higher selectivity toward the formation of epoxidation products.
Existing Pd-catalyzed epoxidation techniques often employ
H2O2 as an oxidant. By establishing a relationship between
oxygen activation and epoxidation, promising Pd-based nanomaterials may be designed for epoxidation reactions by O2 to
replace the expensive H2O2 technique.
Overall, Pd-based nanomaterials have preeminent performance in various oxidation reactions, which has been commonly recognized by the catalysis and organic chemistry
research community. Certainly, the function of Pd nanomaterials in activating oxygen should play an important role in
such oxidation reactions; however, the correlation of oxygen
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activation with various parameters in Pd
nanomaterials (particularly the effects from
supports and alloys) has not been fully discovered. It is envisioned that future research
can be conducted along this line, which will
enable development of Pd-based catalysts
with higher efficiency and selectivity toward
oxidation reactions.
5.2. Oxygen-Related Coupling Reactions
In addition to oxidation reactions, certain
types of organic reactions are oxygen-related
as indicated by reaction mechanisms detailed
in literature. For instance, Pd-based nanomaterials are known as efficient catalysts for
coupling reactions (Figure 8a),[109] among
which some reactions have been found to be
related to oxygen or air. In this case, oxygen
adsorbed on Pd surfaces has to be taken into
account.
Examples of Facet Effects: Holmes and
McGlacken reported that Pd nanocubes with
{100} facets exhibited superior catalytic performance to {111} facets in Suzuki–Miyaura
coupling reactions.[31] With solid evidence
from control experiments and surface characterizations such as XPS, they concluded that
the sensitivity of Suzuki–Miyaura coupling
reactions to catalyst shapes originated from
facet-dependent oxygen adsorption.[31] It is
known that the oxidative addition of haloalkane (R–X) to Pd that yields an intermediate
R–Pd–X complex (i.e., step 2 in Figure 8b)
is often the rate-determining step in the
coupling reactions.[31] They found that the
air condition involving O2 favored high efficiency in Suzuki–Miyaura coupling reactions
(see Figure 8c), while the concentration of
Pd catalysts was a crucial parameter to the Figure 8. a) Reaction models for Suzuki–Miyaura coupling and Heck coupling. b) Mechanism
reactions. Most likely the oxygen species, for the O2-promoted Suzuki–Miyaura coupling reaction. c) Catalytic activities of the Suzuki–
by Pd nanocubes under aerobic and inert conditions. Reproduced
which could be better activated on Pd{100}, Miyaura coupling[31reaction
with permission. ] Copyright 2014, Wiley-VCH. d) Mechanism for the catalytic cycle in the
promoted the formation of R–Pd–X and thus
Heck reaction. Reproduced with permission.[112] Copyright 2000, American Chemical Society.
accelerated coupling reactions. Astruc et al. e) Support effect on Suzuki–Miyaura coupling reaction. The data are collected from the work
have identified that step 3 in Figure 8b limits by Metin et al.[109]
the reaction efficiency of Suzuki–Miyaura
couplings.[110] Thus it can be assumed that the presence of
independent groups.[114] However, Wagner and Muhler revealed
oxygen species (e.g., 1O2 analogue) not only promotes the forthat the Pd species dissolved in solution is indispensible to the
reaction.[115] Given the unclear role of Pd nanomaterials and
mation of R–Pd–X, but also restrains the unfavorable step 3.
In the Heck type coupling reactions, Pd catalysts have also
oxygen species, it seems infeasible to correlate the Heck reacexhibited facet-dependent performance;[111] however, the roles
tion performance with oxygen activation at this moment. It is
anticipated that more research on heterogeneous-catalytic Heck
of Pd catalysts and oxygen species are rather complicated. As
reactions will clarify this point in the future.
indicated by homogeneous catalysis, the formation of R–Pd–X
Examples of Support Effects: Metin and Sun have synthesized
complex is also the first step to Heck type coupling reactions
Ni–Pd core–shell nanocrystals which are the most active catalyst
(see Figure 8d).[112] During the reaction process, Pd clusters or
for the Suzuki–Miyaura coupling reaction.[109] As the work funcnanocrystals can be formed as active catalysts for Heck type coupling reactions.[113,114] Reetz et al. called this reaction pathway
tions of Ni and Pd are about 4.6 eV and 5.12 eV, respectively,
the electron density in Pd shells can be increased through the
a “ligand-free” pathway, which has been verified by other
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5.3. Oxygen-Related Dehydrogenation Reactions
Pd is an efficient catalyst for dehydrogenation reactions in the
presence of molecular O2.[116,117] Interestingly, Pd nanoparticles
were produced from Pd(II) in homogeneous catalysts during a
dehydrogenation reaction in the presence of O2 (see Figure 9a
and b).[117] This phenomenon indicates that heterogeneous
catalysis may also play a role in reactions even when homogeneous catalysts are used.
Examples of Alloy Effects: Meyer et al. demonstrated that PdZn
alloys exhibit excellent performance in propane dehydrogenation.[118] Their systematic investigations revealed that the adjacent atomic location of Pd to Zn is indispensible for catalytic
activity, and a 2%Pd–10%Zn catalyst had a stable propylene
selectivity at 98%. Based on this finding, they concluded that
the high selectivity of the 2%Pd–10%Zn catalyst was enabled
by the geometric isolation of active Pd atoms surrounded by Zn
neighbors.
Examples of Support Effects: Lu and Stair designed a hybrid
structure with Pd nanoparticles supported on alumina, which
possessed the capability of effectively convert ethane into ethylene (see Figure 9c).[116] It is known that the formation of coke
during the reaction process is disadvantageous to the dehydrogenation of ethane.[119] In this design, the 2 nm pores in alumina effectively stabilized Pd nanoparticles and thus inhibited
coke formation.
It can be concluded that the intrinsic relationship between
oxygen activation and catalytic reactions is worth exploring.
Research along this line will not only clarify reaction mechanisms, but also become insightful to the design of high-efficiency catalysts. To analyze the role of oxygen activation in
catalytic oxidation reactions, one has to reveal and appreciate
another key process involved in reactions − the adsorption
and activation of related reactant molecules. This issue seems
particularly important in oxygen-related reactions, a class of
reactions which oxygen activation only impacts on indirectly.
In this section, we have named a few examples of recent progress. Although most catalytic results can be well explained
from the viewpoint of oxygen activation, many unsolved issues
still remain that need further investigation. The ultimate goal
of this fundamental study is to obtain catalysts with high efficiency and selectivity for organic reactions.
PROGRESS REPORT
electron accumulation from Ni to Pd, promoting O2 activation
on Pd. They further loaded core–shell nanocrystals on different
supports − carbon and Al2O3 – and found that NiPd/Al2O3 had
superior activity to NiPd/C (see Figure 8e). This performance
enhancement (i.e., the activity increase of NiPd/Al2O3 against
NiPd/C) may result from the synergistic support effect (i.e.,
mode 3 in Figure 6e), similarly to the examples in Section 5.1.2.
6. Conclusion
The nature of catalytic activities brought about by Pd nanomaterials in the presence of molecular oxygen is a fascinating
topic in materials science, with a view toward catalytic applications. With the fundamental principles described above in mind,
it would be more straightforward to understand the behavior
of Pd-based nanomaterials in chemical and biological systems
in terms of O2 activation. In this work, we have emphatically
highlighted recently demonstrated adsorbed
oxygen species that may bring about many
catalytic applications. In the scope of fundamental studies, three basic topics have mainly
been discussed: 1) ground-state O2 can participate in oxidation reactions through activation by Pd nanomaterials; 2) oxygen adsorbed
on the surface of Pd nanocrystals possesses
some unique characteristics in contrast to
conventional triplet O2 and free singlet O2,
which adds a new member into the family of
ROS; 3) the state of the adsorbed oxygen species can be readily tailored by tuning a set of
factors, such as the atomic arrangement (i.e.,
surface facets) and charge state (e.g., supports) of the Pd nanocrystal surface.
In the field of catalytic oxidation reactions,
various efforts have been made to tailor the
size, shape and structure of Pd nanoparticles
toward high catalytic efficiencies. As a
matter of fact, the dependence of catalytic
performance on these parameters has been
widely implemented in catalyst design for
oxidation reactions or oxidation-related couFigure 9. a) Size evolution of Pd nanoparticles observed in 3-methylcyclohexanone dehydropling reactions. However, the correlation of
genation up to 5 min. b) TEM image of Pd nanoparticles observed after 3-methylcyclohexcatalytic performance with oxygen activation
anone dehydrogenation for 0.25 h. Reproduced with permission.[117] Copyright 2013, American
Chemical Society. c) Schematic illustrating the oxidative dehydrogenation of ethane by coated has somehow been ignored in most studies.
Here, we specifically spotlight examples
and uncoated Pd/Al2O3. The data are collected from the work by Lu et al.[116]
Adv. Mater. 2015, 27, 7025–7042
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where Pd-based nanostructures were used to activate groundstate molecular oxygen into reactive oxygen species in various
chemical systems. In most oxidation reactions, chemical conversion can benefit from the higher activity of adsorbed oxygen
species than ground-state O2. In this sense, upon decoding
the reaction mechanisms, we should be capable of rationally
designing surface structures and compositions of Pd-based
nanomaterials to enable superb catalytic activities.
It is well known that ROS are also fundamentally useful species to chemically affect the viability of living matters involved
in life processes. As a result, their generation can enable a
variety of biological functions, and have played a causal role in
cell senescence,[120–122] DNA damage,[123] insulin resistance and
anticancer drugs.[124] It is anticipated that in the future, one can
adopt the concepts of ROS formation on nanostructure surfaces
in chemical systems, and thus facilitate further applications of
nanomaterials in biological systems.
These opportunities indicate that lots of challenges have to
be addressed before the research community can move concepts forward to real applications. For instance, from Section 3,
one can see that the detection of oxygen species mainly relies
on luminescence and ESR characterization based on probing
molecules. The capabilities of these techniques in distinguishing various oxygen species are quite limited, while their
working mechanisms still need further validations. On the
other hand, some key aspects in O2 activation such as Pd–O2
charge transfer and O2 spin state still remain elusive although
theoretical simulations have provided some clues. Intuitively,
some advanced characterization techniques such as ultrafast absorption spectroscopy and synchrotron-radiation X-ray
absorption spectroscopy may reveal these features with high
temporal, spectral and spatial resolutions at molecular and
electronic levels. Fundamental understanding of the processes
of oxygen activation and species production holds the key to
completely resolving the roles of Pd nanomaterials in various
related applications and to better rational design of Pd-based
nanomaterials for improved performance.
Acknowledgements
This work was financially supported by the NSFC (No. 21471141,
21101145), the Recruitment Program of Global Experts, the CAS
Hundred Talent Program, Fundamental Research Funds for the Central
Universities (No. WK2060190025, WK2060190037, WK2310000035), and
the China Postdoctoral Science Foundation (2014M560514).
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