A rational design of a Pd-based catalyst with a metal-metal oxide interface influencing molecular oxygen in the aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c9gc00116f

In a green process for selective oxidation of alcohols, the utilization of molecular oxygen as a primary oxidant is the most critical step. Although many palladium (Pd)-based catalysts have shown potential, the role of different Pd-species in the aerobic oxidation reaction is still a matter of discussion. There have been diverse reports, which describe either Pd⁰ or Pd²⁺ as the individual species responsible for the aerobic oxidation of alcohols. Herein we demonstrate that the presence of both Pd⁰ and Pd²⁺ species with a Pd-PdO interface stabilized on the surface of reduced graphene oxide (rGO) is important for the oxidation of alcohols. With an optimum Pd²⁺/Pd⁰ ratio, the catalyst catalyzes the oxidation of benzyl alcohol in water with oxygen, resulting in a turnover frequency (TOF) of up to 18 000 h^-1 with 98% selectivity towards the aldehyde. It is proposed that both metallic Pd and its oxide domains, when co-existing with a phase boundary between them, promote the activation of oxygen. On the other hand, rGO provides surface functionalities for the formation and stabilization of Pd-PdO nanoclusters enabling the catalyst to be both stable and reusable. Using histidine as a scavenger for singlet oxygen, we have also determined the importance of oxygen-activation in the reaction. Furthermore, the catalyst is capable of converting various other alcohols into the corresponding carbonyl compounds. Comparison of various catalysts shows that the Pd-PdO@rGO catalyst is the most efficient in terms of TOF, conversion and selectivity for the oxidation of benzyl alcohol using oxygen compared to the reported Pd-based catalysts, particularly when performed under milder reaction conditions. Therefore, the result on Pd-catalyst designing is believed to be of significance for the further developments in the environmentally benign oxidation processes involving molecular oxygen as the oxidant.

Full text of DOI:10.1039/c9gc00116f

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Journal Name
ARTICLE
A Rational Design of Pd-based Catalyst with Metal-Metal Oxide
Interface Influencing the Molecular Oxygen in Aerobic Oxidation
of Alcohols†
Received 00th January 20xx,
Accepted 00th January 20xx
Songhita Meher,a,b Rohit Kumar Rana*,a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
In a green process for selective oxidation of alcohols, the utilization of molecular oxygen as primary oxidant is the most
critical step. Although many palladium (Pd)-based catalysts have shown potential, the role of different Pd-species in the
0
2+
aerobic oxidation reaction is still a matter of discussion. There have been diverse reports, which describe either Pd or Pd
as individual species responsible for the aerobic oxidation of alcohols. Herein we demonstrate that the presence of both
0
2+
Pd and Pd species with Pd-PdO interface stabilized on the surface of reduced graphene oxide (rGO) is important for the
2
+
0
oxidation of alcohols. With an optimum Pd /Pd ratio, the catalyst catalyzes oxidation of benzyl alcohol in water with
oxygen, resulting in a turnover frequency (TOF) of up to 18000 h-1 with 98% selectivity towards the aldehyde. It is
proposed that both metallic Pd and its oxide domains, when co-exist with a phase boundary between them, they promote
the activation of oxygen. On the other hand, the rGO provides surface functionalities for the formation and stabilization of
Pd-PdO nanoclusters enabling the catalyst to be both stable and reusable. Using histidine as a scavenger for singlet
oxygen, we have also determined the importance of oxygen-activation in the reaction. Furthermore, the catalyst is capable
of converting various other alcohols into the corresponding carbonyl compounds. Comparison of various catalysts shows
that the Pd-PdO@rGO catalyst is the most efficient in terms of TOF, conversion and selectivity for the oxidation of benzyl
alcohol using oxygen than the reported Pd-based catalysts, particularly when performed under milder reaction conditions.
Therefore, the result on Pd-catalyst designing is believed to be of significance for the further developments in the
environmentally benign oxidation processes involving molecular oxygen as the oxidant.
these reactions besides the usage of water as the solvent is
considered extremely critical. Amongst the various catalysts
explored, the palladium (Pd)-based nanomaterials have shown
Introduction
Selective oxidation of alcohols to their corresponding carbonyl
compounds is one of the important organic reactions in the
synthesis of fine chemicals and intermediates. Normally, the
classical oxidation methods used for such reactions involve
stoichiometric amount of inorganic oxidants, such as,
permanganate or chromate reagents producing large amounts
of toxic waste.2
environmental concerns, it is now paramount that alternative
oxidation processes are developed, which would be
environmental friendly apart from being efficient. In particular,
the utilization of molecular oxygen as the primary oxidant in
immense potential in enabling oxygen as the oxidant in the
reaction. It includes a wide variety of reactions, such as,
epoxidation of alkenes, oxidation of CO, hydrocarbon, alcohol,
1
4
-12
glucose, etc.
In the Pd-catalysts, the main focus has been to tailor their
particle size, surface structure, morphology and the supporting
matrix, so as to improve their activities in the oxidation
reactions. The various materials used for the support include
,3
Considering the ever increasing
2 3
hydroxyapatite, Al O , MgO, mesoporous silica, carbon, and
graphene for dispersing and stabilizing the Pd nanoparticles on
9
,13-16
their surface.
Conventionally high surface area and
porous materials are used as the support. Although the role of
these supports is mainly to prevent agglomeration of the
metallic nanoparticles, there are other effects which can
influence the catalytic activity. For example, the use of some of
the metal oxides as support has demonstrated the importance
a. Nanomaterials Laboratory, CSIR-Indian Institute of Chemical Technology,
Hyderabad-500007, India.
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India.
CSIR-IICT Communication Number: IICT/pubs/2018/336
E-mail: rkrana@iict.res.in
b.
†
Electronic Supplementary Information (ESI) available: Experimental details of the of charge transfer between metal and oxide in the oxidation
sample preparation; Details of the equipments used for characterization and
analyses; FE-SEM, HR-TEM, EDS, XRD, XPS data of various samples;
Characterization of Pd nanoparticles; Comparison with the catalytic activities
reported for other catalysts. See DOI: 10.1039/x0xx00000x
4
2
reaction under O . Metal organic framework (MOF) structures
have also been utilized as support for metal nanomaterials,
wherein it is believed to allow generation of a large number of
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active sites in the framework with a controllable space for activity and selectivity in the aerobic oxidation reactions, while
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1
7
DOI: 10.1039/C9GC00116F
chemical reactions to take place. In another approach to bringing out the role of various Pd species in the reaction. In a
provide anchoring sites for the formation and immobilization somewhat similar context, as known in case of the automotive
of metal nanoparticles, organic materials like mesoporous exhaust catalysis, which is done under gas-phase requiring
1
8
ionic copolymer are used as the supporting matrix. Similarly, high reaction temperature and oxygen pressure, the
it has been reported that doped nitrogen atoms in carbon as composition of the Pd-based catalysts play an important role
support can facilitate enhancement of the electron density of in the reaction. It has been proposed that both metallic Pd
2
4
0
14
Pd resulting in increased reaction rate.
and its oxide domains, when co-exist in a polycrystalline
From a rational catalyst-design point of view, the Pd based particle with a phase boundary between them, they can
materials should enable the oxygen activation, which is the key promote the activation of molecular oxygen. From the
4
for the catalytic performance. The oxygen is activated from its thermodynamics point of view, it has also been predicted that
ground state (triplet) to a high energy state (singlet) via Pd–O
interaction, which in turn enables it to participate in the stable interface. During the catalytic reactions, the Pd-PdO
2
the adhesion energy for the Pd and PdO can favourably form a
2
5
oxidation reaction. As this interaction requires electron interface is believed to allow the surface reduction of PdO via
0
26
2
transfer from Pd surface to O , the variation in Pd surface its interaction with Pd to form oxygen vacancy (Pd-O*) sites.
charge density can play an influential role. Therefore, use of a This Pd-O* sites then facilitate the adsorption and activation of
support matrix that can suitably polarise the Pd particles at molecular oxygen.
their interfaces would be beneficial. In this context, the
graphene-based materials with their high electronic
conductivity along with the effective surface area and good
mechanical strength, can be apt for the designing of Pd-based
catalysts.19
Recent reports on graphene based compositions as the
support for Pd have revealed their potential in the aerobic
oxidation of alcohols. Luque et. al. used a surfactant-mediated
Scheme 1. Schematic representation illustrating the Pd-based catalyst designed to have
method to prepare Pd particles of ~40 nm sizes deposited on
exposed Pd-PdO interfaces stabilized on the surface of rGO.
2
0
graphene oxide (GO). Thus obtained catalyst exhibited a yield
of 98% corresponding to a TOF of 175 h− in the aerobic
oxidation of benzyl alcohol to benzaldehyde. Li’s group
demonstrated deposition of Pd nanoparticles of ~4.1 nm sizes
on reduced graphene oxide (rGO) via an impregnation method
1
Therefore, in the present study, to design and understand the
role of the active Pd species in the reaction under oxygen, we
prepared catalysts based on various Pd-PdO compositions. The
formation and stability of Pd-PdO interface is also a challenge
as the inter-convertibility of the metal-metal oxide during the
reaction can concomitantly lead to loss in catalytic activity.
Herein, we utilize the surface functionalities present on
partially reduced exfoliated graphene oxide (rGO) sheets to
effectively stabilize Pd-PdO nanoparticles (Scheme 1). The
catalyst preparation is done under mild reaction conditions
without using any stabilizing agent or surfactant, in which the
GO is first partially reduced in order to provide optimum
oxygen functionalities as anchoring sites for a better dispersion
of the palladium precursor. Then, an in-situ reduction process
leads to the formation of Pd-PdO composition on the rGO
surface. The rGO is used not only to stabilize and promote the
Pd-PdO nanoparticles in the catalytic reaction, but also to
provide a hydrophobic−hydrophilic balance desirable for
efficient interaction of the organic substrate with the catalyst
o
followed by two separate heat treatments at 300 C under He
/He flow. The catalyst showed a conversion
of 73% in the aerobic oxidation of benzyl alcohol at 110 C.
Although, there was a partial oxidation of the Pd to Pd as
observed in the used catalyst, the catalytic activity was
attributed to Pd only, wherein the presence of graphene was
thought to promote the reaction via efficient adsorption of the
alcohol and oxygen. In a similar approach, Wang’s group could
prepare sub-nanometer sized Pd particles on rGO via
impregnation and then heat-treatments in presence of H
But, contrasting to previous result, herein the observed
catalytic activity with a TOF of 1960 h was attributed to the
presence of Pd species coordinated with Cl ions rather than
2
1
flow and 10% H
2
o
0
2+
0
2
2
2
.
−1
2
+
-
0
the Pd species.
Although, there have been investigations to find out the
nature of the Pd species, their role in the aerobic oxidation
reaction is still a matter of discussion. It should be noted that
the Pd-based catalysts, which are mainly reported for the
liquid-phase aerobic oxidation of alcohols, have been prepared
with the major emphasis on the formation of Pd in its metallic
2
7
surface.
Experimental
Materials
0
2+ 4,11,13,22,23
state (Pd ) except a few on Pd .
However, while
4 2 4 2 2 2 3
Graphite flakes (25 μm), KMnO , N H .H O, Na PdCl4, K CO ,
Benzyl alcohol and dialysis sacks (12000 Da MWCO) were
purchased from Sigma-Aldrich and used as received. DI
designing the catalysts, the importance of the composition and
the oxidation state of palladium desirable for activation of
molecular oxygen has rarely been given the due consideration.
This prompted us to focus on a rational designing of the Pd
based nanomaterials in order to achieve efficient catalytic
(Deionized) water (18.2 MΩ) was used for all the solution
preparation.
2
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Synthesis of Pd-PdO@rGO
An aqueous dispersion of graphene oxide (80 mL, 1 mg/mL) GO, the bands at 1750 cm and 1630 cm corresponding to
see ESI†) was prepared via sonication using bath sonicator for C=O stretching of carboxylic and carbonyl functional groups,
0 min. For partial reduction of GO, 0.1 mL of hydrazine respectively were observed. The presence of other oxygen
present in GO using FT-IR spectroscopy (Fig. 1(a)). In case of
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−1
DOI: 10.1039/C9GC00116F
-1
(
3
hydrate (80 v/v % in water) was added drop-wise to the above containing functionalities in GO, such as the alkoxy and
suspension and stirred for 1 h. In a typical synthesis for Pd epoxide groups were revealed from the C-O stretching
-1
−1
2 4
loading on rGO, 0.940 mL of Na PdCl
(10 mM) was added vibrations at 1100 cm and 1240 cm , respectively. After the
slowly to the above suspension followed by the addition of GO was partially reduced, there was a decrease in the
hydrazine hydrate (0.1 mL, 80 v/v % in water). Then it was intensities of the vibrational bands corresponding to the above
exposed to high-intensity ultrasonic irradiation (20 KHz, 40% oxygen containing functional groups confirming the conversion
intensity, Sonic Vibro cell, USA) under ambient air for 30 min at of GO to its reduced form rGO.
o
2
5 C. The resulted black precipitate was centrifuged followed

Pd
PdO
rGO
(a)
Pd-PdO@rGO
rGO
(b)

by washing with DI water for five times to obtain Pd-
PdO@rGO. Thus obtained material was re-dispersed in DI
water to prepare a stock solution of 10 mg/mL. As confirmed
from ICP-OES analysis, there was no Pd-precursor remained in
the filtrate. The Pd-loading was varied from (0.625 to 5.0








5
.0 Wt.%
3
.75 Wt.%
GO
1.25 Wt.%
Wt.%) by taking different amounts of Na
other parameters same as above. The catalysts are denoted as
2
PdCl
4
and keeping all
1000
2000
3000
4000
20
40
60
80
-
1
Wavenumber (cm
)
2Theta (Degree)
Pd-PdO@rGO-x, where x represents Pd-loadings in Wt.%. As Fig. 1
(a) FT-IR spectra of GO, rGO and Pd-PdO@rGO-1.25; (b) XRD patterns of Pd-
found from the ICP-OES analysis, the total Pd-loading in Pd- PdO@rGO samples with different Pd-loadings (Wt.%).
PdO@rGO varied from 0.625 to 5.0 Wt.% (Table S1).
Catalytic oxidation of aromatic alcohols under O
Oxidation of aromatic alcohols over various catalysts was
performed under O atmosphere and water as the solvent. In
each experiment, 20 mg of the catalyst was used in 5 mL of DI
water and mixed with 0.3 mol of K CO under stirring at 800
2
(a)
(b)
2
2
3
rpm/min. Then at a desirable reaction temperature, 0.1 mol of
the reactant was added into the reaction mixture and the
reaction was carried out using oxygen balloon. The products
formed at various time intervals were analyzed by gas
chromatography (GC) equipped with ZB-5 column.
Reusability of the catalyst
Median: 11.2 nm
Mean : 11.6 nm
Median: 12.5 nm
Mean : 13.2 nm
(c)
(d)
30
20
40
20
10
To check the reusability of the catalyst, the catalyst after a
fixed duration of the reaction was separated and washed with
ethyl acetate and then with water for 2-3 times. Thus
separated catalyst was then used in the next cycle for benzyl
alcohol oxidation keeping the reaction conditions similar.
0
0
10
11
12
13
14
15
10-12
12-14
14-16
16-18
18-20
Particle Size (nm)
Particle Size (nm)
(
e)
(f)
Results and Discussion
Designing the Catalyst
During the preparation of the catalyst, we observed that the
reduction of GO to rGO was a critical step, which influenced
the dispersion of Pd-PdO particles on its surface. When the
reduction of both GO and Pd ions was carried out
simultaneously, it resulted in non-uniform and agglomerated
Pd-PdO particles on the reduced GO (Fig. S1a). Since the
oxygenated functionalities present on GO provide anchoring
sites for the Pd2+ species to interact with the support, the
amount or the localized concentration of such functionalities Fig. 2
(
g)
(h)
2
+
FE-SEM images of Pd-PdO@rGO with Pd-loadings of (a) 0.625 Wt.% and (b)
.25 Wt.% and the corresponding (c, d) particle-size distribution plot (from a count of
50 particles), respectively; (e, f) EDS of Pd-PdO@rGO-1.25 and Pd-PdO@rGO-0.625;
g), HRTEM image and (h) the corresponding HRTEM image of the highlighted (red
circle) part depicting the lattice fringes of Pd and PdO crystallites in Pd-PdO@rGO-1.25.
1
>
(
must be affecting the Pd-PdO dispersion on the surface of the
graphene sheets. Therefore, in a control experiment we first
allowed the GO to be partially reduced prior to their
2
+
interaction with the Pd precursor. The reduction process was
monitored by analyzing the oxygenated functional groups
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Thus prepared rGO was then used along with the Pd2+ corroborates well with the observation made froVmiewXARrtiDcledOantlinae.
precursor and subjected to the reduction process again for the We further analysed the sample by higDhO-aIn: 1g0l.e10a39n/nCu9lGaCr0d0a11r6kF-
synthesis of Pd-PdO@rGO. This resulted in further decrease in field scanning transmission electron microscopy (HAADF
the intensity or disappearance of the FT-IR bands due to the STEM) and elemental mapping (Figure S4). The elemental
oxygen containing functionalities (Fig. 1(a)).
mapping for O and Pd in the STEM image shows that there are
The as-synthesized materials were further characterized by X- regions having both Pd and O elements indicative of PdO
ray diffraction technique. The XRD pattern of the prepared GO content in the sample. The other regions, wherein Pd is
sample showed a typical weak, but broad diffraction centred predominant represent the presence of Pd component in the
at 26.4° corresponding to the graphitic (002) plane, while the sample. Therefore, the result further supports the formation
known interlayer graphitic (100) diffraction peak at 11.7° was Pd-PdO interface in the synthesized materials. It further
absent (Fig. S1(b)). This indicated that the insertion of oxygen reveals that the nano-sized metallic Pd and PdO co-exist with
functionalities had successfully led to exfoliation of the an interface between them. Importantly, these Pd-PdO
graphitic sheets.2 The XRD pattern also remained similar for interfaces are seen to remain exposed at their surface and
the rGO prepared via the reduction of GO (Fig. S1(b)). In case hence should be accessible to the reactants as desirable for
of Pd-PdO@rGO-x samples (“x” indicates Pd-loadings in terms the catalytic oxidation reaction under oxygen atmosphere. We
of total Pd-content in Wt.% ), the broad diffraction seen at could rule out the possibility of any core–shell morphology or
8,29
2
6.4° clearly indicated the presence of exfoliated graphene an oxide layer formation on the Pd metal surface as known in
sheets (Fig. 1(b)). The diffraction peaks observed at 40.4, 45.5 case of the catalyst prepared at elevated temperatures, which
and 79o were assigned to the (111), (200) and (311) planes, would otherwise obstruct the Pd-PdO interface to be exposed
2
3
respectively, of metallic Pd with a face-centered cubic lattice. to the reactants.
In addition, we observed the diffraction peaks at 35.5, 54.3, In order to obtain information on the structure and quality of
4.6 and 71.8o, which could be indexed to the (101), (112), the graphene-based materials and their reduction during the
103) and (202) planes, respectively, of PdO with a tetragonal synthesis of Pd-PdO@rGO, we analyzed the as-synthesized
6
(
3
0
lattice. Thus the XRD data revealed the formation of both Pd materials by confocal micro-Raman spectroscopy. The Raman
and PdO in the as-synthesized Pd-PdO@rGO. The crystallite spectra of Pd-PdO@rGO, rGO and GO samples are shown in
sizes of these particles were then estimated using the Debye- Fig. 3. Graphene-based materials generally exhibit two
1
9
Scherrer equation. For Pd and PdO, the crystallite sizes were in characteristic modes (G and D) of vibration. While the D-
the range of 2.4-2.78 and 0.54-0.65 nm, respectively (Table band corresponds to out-of-plane vibrations attributed to the
S2). It was also observed that the intensity of the presence of structural defects, the G-band is a result of in-
corresponding XRD peaks due to Pd and PdO increased with plane vibrations of carbon atoms in the graphene structure. In
increase in the Pd-loading.
case of GO and rGO-containing samples (rGO and Pd-
The morphological characterization of Pd-PdO@rGO was PdO@rGO), the D and G bands appear within a range of 1298-
-1
−1
carried out using FE-SEM and HR-TEM. The FE-SEM images and 1330 cm and 1533-1590 cm , respectively. The intensity ratio
the corresponding particle-size distribution plots for Pd- of the D and G bands (I /I ) as a measure of the reduction
D
G
PdO@rGO samples with different Pd-loadings showed that process, was seen to increase in the following order, GO (1.28)
particles of sizes 10-15 nm having distinctly high contrast were < rGO (1.46) < Pd-PdO@rGO (1.59). This gradual increase in
present on sheet-like structures of rGO (Fig. 2(a-d)). As the ratio with reduction suggests that more graphitic domains
determined from the EDS analysis, we found the presence of are being formed as a result of the effective reduction of the
3
1
Pd, C (carbon) and O (oxygen) in these samples (Fig. 2(e, f), oxygenated functionalities present on GO. Moreover, the
Table S2), while the particles mainly consisted of Pd (Fig. S2). shift in the G band position towards lower wavenumber in
Further analysis by EDS for elemental mapping showed a case of Pd-PdO@rGO also reflects a more ordered graphitic
homogeneous distribution of the Pd along with the other structure in this material than that in rGO and GO samples.32
elements. The average size for the Pd-containing particles as The chemical composition and surface functional groups
estimated from the size-distribution plot was ~11 nm for the present in Pd-PdO@rGO was investigated by X-ray
Pd-PdO@rGO sample with a Pd-loading of 0.625 Wt.% (Fig. photoelectron spectroscopy. Especially in case of GO or rGO,
2
c). As we increased the Pd-loading the sizes of these the binding energy of C1s electron is useful in identifying the
nanoparticles increased reaching to a value of ~40 nm for the various types of carbon attached with oxygen and therefore
sample with 5.0 Wt.% loading (Fig. S2 (d,e)). can provide information on the GO-reduction process. The
The HRTEM analysis of Pd-PdO@rGO sample also showed the deconvoluted C1s spectrum for GO revealed five different
presence of Pd-containing particles with dark contrast and peaks with binding energies of 284.4, 285.3, 286.2, 286.7, and
their sizes were in the range of 10-15 nm (Fig. 2(g)). At a higher 288.2 eV (Fig. 4(a)). These binding energies correspond to
magnification, the HRTEM image displayed the lattice fringes graphitic carbon C=C, C-C, and carbon bound to oxygen species
for these particles (Fig. 2(h), Fig. S3). The lattice fringe with an (C-O (hydroxyl and epoxy), C=O (carbonyl), and O-C=O
3
3
interplanar distance (d-spacing) of 0.226 nm could be indexed (carboxyl) groups), respectively.
to the (111) plane of Pd metal, while the d-spacing of 0.267 nm
corresponded to the (101) plane of PdO with a tetragonal
lattice. The presence of both Pd and PdO in the sample also
4
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Pd/PdO@rGO Therefore, the decrease in the oxygen
View Article Online
DOI: 10.1039/C9GC00116F
containing
functionalities
in
Pd-PdO@rGO
further
Pd-PdO@rGO
substantiates the data obtained from Raman analysis on the
formation of rGO.
ID/IG= 1.59
The spectrum corresponding to Pd 3d electron for Pd-
PdO@rGO consisted of two sets of peaks indicating that the
palladium exists in two different oxidation states (Fig. 4(d)).
The one with binding energies at 335.87 and 341.14 eV was
ID/IG= 1.46
ID/IG= 1.28
rGO
GO
1000
1250
1500
1750
-1
)
Wavenumber (cm
0
assigned to Pd3d5/2 and Pd3d3/2 electrons in Pd metal (Pd ).
Fig. 3
Confocal micro-Raman spectra of GO, rGO and Pd-PdO@rGO-1.25.
The other set having Pd3d5/2 and Pd3d3/2 peaks at 337.62 and
2
+
34
2
+
0
342.88 eV revealed the presence of Pd state as PdO.
A
Table 1 The C:O and Pd :Pd ratio obtained from the XPS analysis of various samples
quantitative analysis of the XPS peaks shows that in case of Pd-
PdO@rGO, the metallic Pd content is more than that of the
2
+
0
Sample
Oxidized carbon to
graphitic carbon ratio
C1S/O1S
ratio
Pd /Pd
ratio
2
+
oxide form (Pd ) as present on the surface of the catalyst.
Moreover, with increase in the Pd-loading in the catalyst, the
(C1S)
2
+
0
GO
1.14
0.88
0.92
0.90
2.07
2.89
3.02
2.98
-
Pd : Pd ratio was found to increase from a value of 0.21 to
0.96 (Table 1, Fig. S5). Similarly, the estimation of Pd and PdO
content by the XRD analysis also showed an increase in the
PdO content as the Pd-loading was increased (Table S3,
Supporting Information). Moreover, the percentage of Pd
content obtained from XRD analysis was found to be higher
than that was estimated from the XPS analysis. This result
indicates that the percentage of PdO is more on the surface
compared with that in the bulk of the sample. The presence of
both Pd and PdO also supports the observation made from
HRTEM analyses. The XPS for O1s also showed the presence of
a peak at 530.16 eV corresponding to PdO besides three other
Pd-PdO@rGO-0.625
Pd-PdO@rGO-1.25
Pd-PdO@rGO-5
0.21
0.33
0.96
C1s
O1s
C=C
(b)
C=O
(
a)
4
000
000
0
3000
2000
1000
0
C-OH
C-C
C=O
-
2
COO
-
COO
C-OH
534
2
90
288
286
284
282
536
532
530
528
Binding Energy (eV)
Binding Energy (eV)
components
functionalities (Fig. 4(e), Fig. S5).
assignable
to
the
oxygen-containing
6
000
000
000
0
C1s
C=C
6
4
2
000 (c)
(d)
Pd3d
5/2
0
Pd
(a)
(b)
4
2
Pd3d
3/2
100
100
80
60
40
20
000
000
0
0
C-C
Pd
2+
2+
80
60
Pd
-
C-OH
C=O
COO
Pd
40
2
90
288
286
284
282
344 342 340 338 336 334
Binding Energy (eV)
Binding Energy (eV)
20
2
4
6
8
10
1
000
Time (h)
O1s
(e)
(c)
(d)
C=O
-
COO
1.0
0.8
10000
5
00
C-OH
8000
0.6
0.4
0.2
Pd-O
6000
4000
0
536
534
532
530
528
Binding Energy (eV)
1
2
3
4
5
Pd-Loading (Wt.%)
Fig. 4
XPS analysis showing (a) C1s and (b) O1s spectra for GO sample; (c) C1s, (d)
Fig. 5
(a) Time-dependent reaction progress for the catalytic conversion of benzyl
O1s and (e) Pd 3d spectra for Pd-PdO@rGO-1.25 sample.
alcohol and the selectivity towards benzaldehyde; (b) Conversion and selectivity with
respect to different Pd-loading in Pd-PdO@rGO as catalyst; (c) Plot representing the
catalytic activity (TOF in sec ) and the variation in PdO/Pd ratio with respect to
The O1s spectrum for GO also showed the presence of three
-1
oxygen-containing functionalities with binding energies of different Pd-loading in Pd-PdO@rGO as catalyst; (d) Conversion and selectivity with
-
respect to different catalysts. Reaction conditions: Catalyst (2.35μmol of Pd-content),
.1 mol benzyl alcohol, 0.3 mol K CO , 5 mL water, O2 (~1atm), 80 °C, 4h.
5
4
31.1 eV (COO ), 532.36 eV (C=O) and 533.43 eV (C-OH) (Fig.
0
2
3
(b)). In case of the Pd-PdO@rGO samples, we observed a
decrease in the intensity of the C1s peaks particularly for the
carbon bound to oxygen functionalities (Fig. 4(c)). When it was
Catalytic activity of Pd-PdO@rGO in the oxidation of alcohols
compared with that of GO, clearly there was a decrease in the Catalytic activity of the prepared catalysts was tested in
intensity ratio of the oxygen-containing carbon with respect to oxidation of both aromatic and aliphatic alcohols using
the graphitic carbon in Pd-PdO@rGO (Table 1). In addition, as molecular oxygen as the oxidant and water as the solvent. In
estimated from the total C1s and O1s (oxygen attached with order to optimise the reaction conditions, we varied the
carbon) peak areas, the ratio of carbon to oxygen (C/O) was experimental parameters, such as, temperature, catalyst
found to increase from a value of 2.07 in GO to 3.02 in amount and reaction duration using benzyl alcohol as the
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model substrate (Table S4). When the reaction temperature hydration equilibrium is shifted towards the geminal diol
View Article Online
5,3 60 0116F
was increased from 25 C to 100 C, though there was a catalyzed by strong bases and at higher teDmOIp: 1e0r.a10tu39re/Cs9.
o
o
3
GC
gradual increase in the conversion, the highest selectivity We also studied other solvent systems in the reaction keeping
o
(
98.2%) towards benzaldehyde was observed at a temperature the temperature fixed at 80 C. Amongst the various solvents
of 80°C (Table S4). The reaction is known to proceed via a two- used, water was found to be the most suitable in achieving
st
step mechanism. The 1 step, the base in the reaction both higher conversion and selectivity (Table S4). Besides the
promotes deprotonation of the Pd-bound alcohol and then the solubility of oxygen, water is also reported to provide
β-H elimination reaction in presence of oxygen results in appropriate wetness of the surface facilitating chemisorption
benzaldehyde.23 In the 2 step, the aldehyde undergoes of O
nd
as required for good activity.
37,38
The variation in the
2
hydration to form geminal diol followed by its catalyst (Pd-PdO@rGO-1.25) amount showed that with
dehydrogenation to the corresponding carboxylic acid. increase in its amount from 10 mg to 40 mg, there was an
Therefore, the decrease in selectivity observed at increase in the conversion from 83.0% to 98.5%, but, the
o
temperatures above 80 C could be due to the facilitation of selectivity towards benzaldehyde decreased from 98% to 61%
the 2nd step of the reaction. Higher rates of aldehyde as the higher amount of catalyst allowed over-oxidation of the
oxidation have also been observed in cases where the product.
Table 2 Effect of composition, surface area and particle-size on the catalytic activity of various catalysts in the oxidation of benzyl alcohola
Catalyst
Pd-loading
Wt.%)
Pd-PdO Particle-size
(nm)
Surface area
(m /g)
Conv.b
(%)
Select. c
(%)
TOF (h-1)
2
(
Pd-PdO@rGO-0.625
Pd-PdO@rGO-1.25
Pd-PdO@rGO-2.5
Pd-PdO@rGO-3.75
Pd-PdO@rGO-5
0.625
1.25
2.5
10-15
10-20
20-25
_
260
211
199
101
98
69
93.6
98.26
96
7343
9801
92.1
78
1
8304d
8301
4363
3150
3.75
5.0
41
92.6
87
40-50
29.6
a
(~1atm), 80 oC, 4h. b Conversion determined by GC, c
, 5.0 mL water, O
2
Reaction conditions: Catalyst (2.35 μmol of Pd-content), 0.1 mol benzyl alcohol, 0.3 mol K
2
CO
3
d
Selectivity towards Benzaldehyde, TOF estimated for 1h of the reaction.
The optimum catalyst amount was 20 mg, which resulted in case of the catalyst with lowest Pd-content (0.625 Wt.%),
2.1% conversion and 98.2% selectivity for benzaldehyde despite having the textural properties similar to that for Pd-
9
within 4h of reaction. The time profile of the reaction involving PdO@rGO-1.25, the catalytic activity was low. This further
the Pd-PdO@rGO catalyst in the conversion of benzyl alcohol strengthens the importance of PdO:Pd ratio in the activity of
and the selectivity to benzaldehyde in water under oxygen the catalyst. The Pd-PdO@rGO with 1.25 Wt.% Pd-loading was
atmosphere is shown in Fig. 5(a). The conversion increased found to be the optimum composition with PdO:Pd of 0.33 for
with time and reached the maximum at 4 h. After 4h there was obtaining the highest activity within 4 h of reaction time and
no obvious change seen in the conversion, but, further the major product obtained was benzaldehyde with a
oxidation of the benzaldehyde led to decrease in its selectivity. selectivity of 98.2%.
The other products were identified as benzoic acid and benzyl The effect of both Pd and PdO in the catalytic reaction was
benzoate.
further investigated. In two separate controlled conditions, we
It was observed that either in absence of any catalyst or rGO as prepared catalysts with either Pd or PdO nanoparticles
the catalyst, there was no or only a negligible amount of (experimental section). The Pd nanoparticles with sizes of ~15
product formed under similar reaction conditions (Table S5). nm, prepared via a reduction process (Fig. S7), showed 46%
This clearly indicates that the presence of Pd-PdO conversion with 22% selectivity towards benzaldehyde (Fig.
nanoparticles deposited on rGO is responsible for achieving 5(d), Table S5). When the Pd nanoparticles were physically
the higher conversions in the oxidation of benzyl alcohol. The mixed with rGO, it still displayed an almost similar conversion
catalytic conversion, selectivity and turnover frequency (TOF) as that of unsupported Pd nanoparticles, but, with a slightly
for all the Pd-PdO@rGO-x catalysts are compared in Table 2 better (40%) selectivity. The better selectivity observed in the
(Fig. 5(b), Fig. S6). With increase in the Pd-loading from 0.625 presence of rGO could be due to its hydrophobicity preventing
to 1.25 Wt.%, there was an increase in the activity, but further the possibility of hydration of the benzaldehyde and its further
2
7
increase in loading led to decrease in the activity. Apart from oxidation. The selectivity towards benzaldehyde could be
the differences in Pd-PdO particle-size and surface area, the improved further by an in-situ deposition of Pd nanoparticles
increased PdO:Pd ratio could be one of the reasons for the on rGO (Pd@rGO) (Fig. 5(d), Fig. S8, Table S5). However there
lower activity seen at higher Pd-loadings (Table 2, Fig. 5(c)). In was no further improvement in the conversion for the
6
| J. Name., 2012, 00, 1-3
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Pd@rGO catalyst. Under the same reaction conditions, the species on the catalyst surface, plausibly in a similarity to the
View Article Online
commercially available Pd/C (Pd-loading of 10 Wt.% on singlet oxygen, are quenched by the histidDiOnIe: 1.
0.1039/C9GC00116F
43
carbon) exhibited 81% conversion with 51% selectivity towards The performance of the Pd-PdO@rGO-1.25 catalyst was then
benzaldehyde (Fig. 5(d), Table S5). On the other hand, when investigated in the oxidation of other aromatic and aliphatic
the metal oxide was deposited on rGO (PdO@rGO, Fig. S8), alcohols (Table 3). In case of the para-substituted benzyl
both the conversion and selectivity for the catalyst still alcohols, the catalytic activity was found to follow the
remained in the lower side (Fig. 5(d), Table S5). Therefore, the Hammett rule. Accordingly, substitution with electron
3 3
higher activities observed in Pd-PdO@rGO could be due to the donating groups like -OCH and -CH showed better catalytic
presence of Pd-PdO composition in the catalyst. Especially, the activity compared with that for electron withdrawing groups (-
generation of Pd-PdO interface is the crucial step in influencing F). However, owing to the stereic effect of these substituents,
the catalytic activity. It provides the active sites suitable for the overall activity was found to be lower than that for un-
achieving better conversion and selectivity in the oxidation substituted benzyl alcohol. The catalyst was also effective in
reactions involving molecular oxygen as the oxidant.
the oxidation of heterocyclic alcohols, such as furfuryl alcohol
and thiophene-2-methanol (Table 3, entries 5 and 6). For the
non-aromatic alcohols, both cyclic and linear forms could be
selectively oxidized to the corresponding aldehydes using the
Pd-PdO@rGO-1.25 as catalyst (Table 3, entries 7-11).
Moreover, in case of an unsaturated linear compound (crotyl
alcohol, entry 12), the observed selective oxidation of the
alcohol group further expands the scope of this catalyst. It was
noted that similar selectivity via oxidative dehydrogenation by
Pd-based catalysts were reported earlier, but the presence of
surface oxide rather than the metallic Pd was attributed for
Table 3 Oxidation of various alcohols using Pd-PdO@rGO-1.25 as the catalysta
Entry
Substrate
Conv.
%)
80
69
41
79
82
59
73
Select. (%
aldehyde)
100
73.9
73.4
95.1
97.5
85.6
94.5
TOF
(h )
-1
(
1
2
3
4
5
6
7
8
9
4-methoxy benzyl alcohol
3-methyl benzyl alcohol
4-fluoro benzyl alcohol
1-phenyl ethanol
Furfuryl alcohol
2-thiophene methanol
Cyclooctanol
Cyclohexanol
8524
7343
4363
8407
8726
6279
7769
8301
7237
8886
5693
8620
1
4,44
the catalytic activity.
The presence of sulphur in the
78
68
100
87.8
± Menthol
compound (entry 6 and 11) resulted in lower activity indicating
that the unshared electron pairs present on sulphur keeps
these compounds strongly bound to the metal surface thereby
1
1
1
0
1
2
1-heptanol
6-mercapto1-hexanol
Crotyl alcohol
83.5
53.5
81
85
76.6
96.3
45
preventing their oxidation.
a
Catalytic stability and reusability of Pd-PdO@rGO
Reaction conditions: 20 mg catalyst (Pd-PdO@rGO-1.25), 0.1 mol alcohol, 0.3
o
mol K
2
CO
3
, 5 mL water, O
2
(~1atm), 80 C, 4h.
In order to gain insight into the stability and reusability, we
first investigated the heterogeneous nature of the Pd-
PdO@rGO catalyst. This was done by separating the catalyst
from the reaction mixture in the middle of the catalytic
reaction (at 2 hr) at the reaction temperature of 80 oC. The
reaction was then continued in absence of the solid catalyst.
As shown in Fig. 6(a), the reaction initially progressed with
increase in the conversion till 2h, but, after that as the catalyst
was removed there was no further increase in the conversion.
Analysis of the reaction mixture by ICP-OES did not show any
presence of leached palladium. Therefore, the observed
conversion of benzyl alcohol in the oxidation reaction is clearly
associated with the Pd-PdO@rGO catalyst, in which the Pd-
PdO particles are well stabilized on the rGO, and as a result it
exhibits a typical heterogeneous catalytic behaviour.
A significant advantage of heterogeneous catalysts is the ease
of their separation from the reaction mixture and their reuse
in subsequent reactions. Therefore, to test the reusability of
the catalyst, it was separated from the reaction mixture and
washed for further use in the next reaction cycles. As tested
for 5 cycles, the Pd@rGO-1.25 catalyst could retain its activity
without any significant loss in both conversion and selectivity
As known in metal-metal oxide systems, the interface between
Pd and PdO can play an important role in O activation.
2
3
9
Particularly, the work functions of the metal and oxide that
allows the charge transfer at their interface is ascribed as one
of the major factors responsible for their catalytic activity and
selectivity. As discussed earlier, the electronic interaction at
Pd-PdO interface can result in oxygen vacancies, which then
allows adsorption and activation of the molecular oxygen as
2
8
required in the catalytic oxidation reactions. Moreover, it is
also known that the surface charge density of metals can be
influenced by the graphene support, which in turn can
2
1,40
contribute to the oxygen activation.
As observed from the
variation in Pd-loading, an optimum ratio of PdO:Pd is
necessary for the best catalytic activity, while at higher
percentage of Pd-loading the increased PdO amount in the
catalyst becomes detrimental to the catalytic activity. In order
to determine the role of the generated reactive oxygen
species, we performed the catalytic reaction in presence of a
1
scavenger for singlet oxygen ( O2). It has been well established
that histidine with its imidazole side chain is susceptible
towards singlet oxygen and hence used as a scavenger or
(Fig. 6(b)). There was also no leaching of the Pd species as
quencher.4
1,42
We observed that the presence of histidine
determined from the ICP-OES analysis of the reaction medium
during different cycles. We further characterized the used
catalyst by FE-SEM, XRD, XPS, micro-Raman spectroscopy and
electron microscopy (Fig. 7, Fig. S9). The presence of both Pd
resulted in a drastic decrease in the catalytic activity of the Pd-
PdO@rGO catalyst in the aerobic oxidation of benzyl alcohol
(Table S6). This finding indicates that the adsorbed oxygen
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metal and PdO in the used catalyst could be seen from the XRD species were present in the catalysts, the catalytic activity was
View Article Online
and XPS analyses (Fig. 7(a,b)). Accordingly, in the HRTEM attributed only to the individual specieDsO. I
: 10.1039/C9GC00116F
14,23
In the present
image, the lattice fringes corresponding to the (111) plane of case, the results demonstrate the importance of both Pd metal
Pd metal and (101) plane of PdO could also be seen in the used and its oxide with Pd-PdO in designing the catalysts for
catalyst (Fig. 7(c,d)). Importantly, the presence of Pd-PdO improved activity and selectivity, but also with required
interface in the used catalyst as desirable for the catalytic stability to be used as a recyclable heterogeneous catalyst in
activity could also be confirmed in the HRTEM images. The the oxidation reactions involving molecular oxygen as the
average Pd-PdO particle-size was in the range of ~12.8 nm (Fig. oxidant.
S9(c)). The graphitic structure of rGO in the used catalyst was
analyzed by the Raman analysis, which showed a I
D G
/I ratio of
Conclusions
1
.54 (Fig. S9(d)). The XPS results further showed a PdO:Pd ratio
of 0.32 and a C:O ratio of 2.93 indicating that the composition
and morphology of the catalyst remained almost similar to
that of the fresh catalyst (Table S7, Fig. S10).
In this work, we demonstrated a rational designing for Pd-
based catalysts in selective oxidation of alcohols using oxygen
desirable for catalytic oxidation processes. It involved an in-
situ method to control the interfacial assembly of Pd-PdO on
the rGO surface without using any external reagents like
surfactants or templates. The method resulted in the
formation of Pd-PdO composite nanoparticles with a distinct
interface between them and well dispersed on the surface of
the graphene sheets. Thus obtained Pd-PdO@rGO exhibited
efficient catalytic activity and selectivity in the oxidation of
(
a)
In presence of the catalyst
Removal of the catalyst at 2 h
(b)
Conversion
Selectivity
(
a)
(b)
100
100
80
60
40
20
75
5
0
5
0
2
60
120
180
240
300
1st
2nd
3rd
4th
5th
Time (min)
No. of cycles
Fig. 6 (a) Comparison of the reaction progress with time wherein the catalyst is
removed after 2h of the reaction to determine the heterogeneous catalytic behaviour
of the catalyst; (b) Recyclability of the catalyst showing catalytic activity at various
reaction cycles. Reaction conditions: 20 mg catalyst (Pd-PdO@rGO-1.25), 0.1mol benzyl
2
benzyl alcohol to benzaldehyde under O atmosphere and in
water as the solvent. The presence of Pd-PdO was found to be
the key in influencing the catalytic reaction involving the
molecular oxygen as the oxidant. Moreover, the hybrid
catalyst could also be used in catalyzing oxidation of varieties
of other alcohols. Therefore, while these findings bring out the
importance of Pd-PdO interface and its stabilization on the
rGO to achieve enhanced catalytic activity in oxidation
o
alcohol, 0.3 mol K
2
CO
3
, 5 mL water, O
2
(~1atm), 80 C.
3
2
2
1
1
000
500
000
500
000

80
Pd3d
0

Pd
PdO
rGO
5/2
(a)
(b)


Pd
60
40
20
0
Pd3d
3/2
0

Pd
+
2+
Pd

2
2
reactions under O as the oxidant, the catalyst designing is
Pd



anticipated to further aid in developing environmentally
benign chemical processes for the oxidation reactions. The
conclusions section should come in this section at the end of
the article, before the acknowledgements.


5
00
20
30 40 50 60 70 80
Theta (Degree)
344
342
340
338
336
334
2
Binding Energy (eV)
(c)
(d)
Conflicts of interest
There are no conflicts to declare.
Fig. 7 Characterization of the used Pd-PdO@rGO-1.25 catalyst; (a) XRD pattern;
b) XPS for Pd 3d electrons; (c) HR-TEM image; (d) HR-TEM image depicting the
Acknowledgements
(
lattice fringes for Pd and PdO crystallites.
The work was supported by CSIR, India (FTT, MLP-0024) and
DST, India (Nanomission, SR/NM/NS-111/2010). S.M.
acknowledges CSIR, India (SRF) for the fellowship. The authors
are grateful to Dr. S.V. Manorama, Dr. Gousia Begum, Y.
Swarnalatha, B. Arun Kumar and Bikash Sharma for helping in
materials characterization.The acknowledgements come at the
end of an article after the conclusions and before the notes
and references.
It should be noted that in comparison to the various reported
Pd-based heterogeneous catalysts, the present catalyst
-1
exhibits activity and selectivity (TOF= 18001 h at 1 h)
comparable to the best ones for the benzyl alcohol oxidation
in presence of oxygen (Table S8). Moreover, to the best of our
knowledge, particularly amongst the reported aerobic
oxidation reactions carried out under mild reaction conditions,
the performance of the present catalyst is the most efficient in
terms of conversion, selectivity and TOF. While this activity Notes and references
could be attributed to the controlled formation of Pd-PdO on
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| J. Name., 2012, 00, 1-3
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