Hydrogenation/oxidation triggered highly efficient reversible color switching of organic molecules

Article abstract of DOI:10.1039/c6cy02585d

Catalytic hydrogenation and oxidative dehydrogenation reactions are fundamental and significant processes in organic transformation, and reversible color switching of organic redox dyes finds potential applications in rewritable paper, sensing devices, data recording and security feature technologies. In this study, we report an interesting result of reversible hydrogenation and oxidative dehydrogenation of a redox dye over a Pd-ZnO_1-x hybrid nanocatalyst under ambient conditions. Thionine (TH⁺) is used as a model compound to evaluate the catalytic performance. The reversible color switching between purple thionine (TH⁺) and colorless leuco-thionine (LTH) depends on the reducing or oxidizing environments. Our newly developed Pd-ZnO_1-x nanocatalyst exhibits high catalytic activity for the hydrogenation of TH⁺ with a turnover frequency (TOF) as high as 397 h^-1 under H₂ (1 bar). The oxidative dehydrogenation of LTH is performed under 1 bar O₂ flow in the same reaction system. The Pd-ZnO_1-x nanocatalyst readily adsorbs and subsequently dissociates O₂ to oxidize LTH to the original purple colored TH⁺ with higher efficiency. The abundant oxygen vacancies on ZnO_1-x nanorods and strong metal-support interaction (SMSI) promote the adsorption and subsequent dissociation of molecular hydrogen and oxygen leading to high catalytic activity. This novel reversible color switching of organic dyes can be performed successively for more than 10 cycles in a one pot-fashion using a Pd-ZnO_1-x nanocatalyst with a small loss in performance. The highly efficient reversible color switching of TH⁺/LTH over the Pd-ZnO_1-x nanocatalyst provides a state-of-the-art protocol to find practical applications as printing inks for rewritable paper and in sensing and security feature devices.

Full text of DOI:10.1039/c6cy02585d

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Hydrogenation/oxidation triggered highly efficient
reversible color switching of organic molecules†
Cite this: DOI: 10.1039/c6cy02585d
Xiao Zhou, Yi-Fan Jiang, Hong-Li Guo, Xin Wang, Ya-Nan Liu,
*
M. Imran and An-Wu Xu
Catalytic hydrogenation and oxidative dehydrogenation reactions are fundamental and significant pro-
cesses in organic transformation, and reversible color switching of organic redox dyes finds potential appli-
cations in rewritable paper, sensing devices, data recording and security feature technologies. In this study,
we report an interesting result of reversible hydrogenation and oxidative dehydrogenation of a redox dye
over a Pd–ZnO1−x hybrid nanocatalyst under ambient conditions. Thionine (TH+) is used as a model com-
pound to evaluate the catalytic performance. The reversible color switching between purple thionine (TH+)
and colorless leuco-thionine (LTH) depends on the reducing or oxidizing environments. Our newly devel-
oped Pd–ZnO1−x nanocatalyst exhibits high catalytic activity for the hydrogenation of TH+ with a turnover
frequency (TOF) as high as 397 h−1 under H2 (1 bar). The oxidative dehydrogenation of LTH is performed
under 1 bar O2 flow in the same reaction system. The Pd–ZnO1−x nanocatalyst readily adsorbs and subse-
quently dissociates O2 to oxidize LTH to the original purple colored TH+ with higher efficiency. The abun-
dant oxygen vacancies on ZnO1−x nanorods and strong metal–support interaction (SMSI) promote the ad-
sorption and subsequent dissociation of molecular hydrogen and oxygen leading to high catalytic activity.
This novel reversible color switching of organic dyes can be performed successively for more than 10 cy-
cles in a one pot-fashion using a Pd–ZnO1−x nanocatalyst with a small loss in performance. The highly effi-
cient reversible color switching of TH+/LTH over the Pd–ZnO1−x nanocatalyst provides a state-of-the-art
protocol to find practical applications as printing inks for rewritable paper and in sensing and security fea-
ture devices.
Received 11th December 2016,
Accepted 18th February 2017
DOI: 10.1039/c6cy02585d
rsc.li/catalysis
dox dye, can be reduced to a colorless leuco form (LTH) in a
reducing environment.8
Introduction
Catalytic hydrogenation and oxidative dehydrogenation reac-
tions have been extensively utilized in the chemical industry
for the production of pharmaceuticals and fine chemicals.1–3
Organic redox dyes are capable of undergoing a reversible
color change under suitable conditions, which has attracted
much attention for potential applications in rewritable paper,
data recording, sensing devices, and security feature
technologies.4–6 Recently, a photoreversible color switching
system that integrates the photocatalytic properties of TiO2
nanocrystals into the redox-driven color change of organic
dyes for the development of rewritable paper has been
reported.7 Thionine dye (TH+, thionine acetate), a purple re-
The past decade has witnessed increased interest in palla-
dium (Pd) based catalysts due to their fascinating properties
and practical applications in organic reactions.9–12 Metal ox-
ides have long been used as a support for noble metals to im-
prove the activity of hydrogenation, dehydrogenation, redox
and acid-catalyzed reactions. Among different supports, non-
stoichiometric supports could endow noble metal nano-
particles new physicochemical properties due to electron
communication and strong metal–support interaction
(SMSI).13 Moreover, defects often dominate the physical and
chemical properties of semiconducting materials. Oxygen va-
cancies in metal oxides are particularly important because
they generally serve as electron donors.14 The increased car-
rier density resulting from oxygen vacancies improves the
electrical conductivity and facilitates charge transfer between
the metal oxide and the substrate at the interface. SMSI is
closely linked to the catalytic properties of the metal in-
volved, playing an important role in enhancing the catalytic
activity in reactions such as hydrogenation.15,16 The ability of
SMSI using metal oxide supports to mediate the catalytic
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical
Sciences at Microscale Department, Department of Materials Science and
Engineering, University of Science and Technology of China, Hefei 230026, P.R.
China. E-mail: anwuxu@ustc.edu.cn
† Electronic supplementary information (ESI) available: Additional TEM, EPR
spectra, and UV-vis spectra. See DOI: 10.1039/c6cy02585d
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behavior of supported metal nanoparticles is of great impor-
tance in the catalytic field with both scientific and commer-
cial significance.17 Hydrogenation of organic dyes is often
used for the evaluation of catalytic activity, because of their
obvious color change and convenient measurements by UV-
vis spectroscopy.18,19 Metal oxide supported Pd nanoparticles
have shown excellent catalytic performance for hydrogena-
tion. This type of hybrid nanocatalyst is also employed for
the activation of molecular oxygen (O2) in various oxidation
reactions, including oxidative dehydrogenation reactions,20
CO oxidation,21 glucose oxidation,22 and oxidative coupling.23
Some model studies have shown that the adsorption and dis-
sociation of H2 and O2 on the surfaces of metal oxide
supported Pd nanoparticles have much influence on the cata-
lytic activities.24–26
stainless steel autoclave. The autoclave was sealed and heated
at 180 °C for 15 h in an oven. After air-cooling to room tem-
perature, the gray-colored ZnO1−x was collected by centrifuga-
tion and washed with distilled water and ethanol three times.
The obtained product was dried at 60 °C in a vacuum oven.
The Pd–ZnO1−x nanocatalyst was obtained by mixing 10 mL
of 1.0 mg mL−1 ZnO1−x NR aqueous suspension with 0.1 mL
of 10 mM Na2PdCl4 in a quartz tube. The nominal weight
content of Pd in the composite is 1.0 wt%. The above solu-
tion was sonicated for 3 min and then irradiated for 60 min
with a 260 W mercury lamp (SHG-200, Mejiro Precision Inc.,
Japan). A color change from grey to dark grey was observed,
indicating the formation of Pd–ZnO1−x hybrid nanostructures.
Finally, the product was collected by centrifugation, washed
three times with distilled water, and dried at 60 °C in a vac-
uum oven for further experiments.
Herein, we synthesize oxygen deficient ZnO1−x nanorods
as a support for loading noble metal nanoparticles to evalu-
ate the catalytic performance using redox thionine dye as a
probe. The activated hydrogen over Pd–ZnO1−x hybrid nano-
structures can rapidly bleach the original purple color of TH+
by the hydrogenation process. Notably, the formed colorless
leuco-thionine (LTH) can be quickly switched back to purple
colored TH+ through the oxidative dehydrogenation process
with activated oxygen over the Pd–ZnO1−x nanocatalyst. This
strategically tailored catalyst not only shows high catalytic re-
duction of thionine to leuco-thionine but also exhibits effi-
cient activation of molecular oxygen for oxidative dehydroge-
nation in a one-pot reaction system. The Pd–ZnO1−x hybrid
nanocatalyst proves to be a promising candidate for highly ef-
ficient, multifunctional hydrogenation/oxidative dehydroge-
nation reactions, which may find applications in data record-
ing, data security, rewritable paper and organic
transformations.
Characterization
The X-ray powder diffraction (XRD) patterns of the samples
were collected using a Rigaku/Max-3A X-ray diffractometer
with Cu Kα radiation (λ = 1.54178 Å); the operation voltage
and current were maintained at 40 kV and 200 mA, respec-
tively. The morphologies of nanocrystals were examined by
transmission electron microscopy (TEM), using
a JEOL
JEM12010 high resolution transmission electron microscope
operated at 200 kV. High resolution TEM (HRTEM) images
were obtained using a JEOL JEM12100F field emission high
resolution transmission electron microscope operated at 200
kV. X-ray photoelectron spectroscopy (XPS) was performed
using a Perkin-Elmer RBD upgraded PHI-5000C ESCA system.
The actual content of Pd was measured using a Thermo Sci-
entific Plasma Quad 3 inductively-coupled plasma mass
spectrometer (ICP-MS) after dissolving the sample with a mix-
ture of HCl and HNO3 (3 : 1, volume ratio). The electron para-
magnetic resonance (EPR) spectra were recorded on a JEOL
JES-FA200 EPR spectrometer (140 K, 9064 MHz, 0.998 mW, X-
band).
Experimental section
Materials
Na2PdCl4 was purchased from Alfa Aesar. Thionine acetate
and hydroxyethyl cellulose (HEC) were purchased from
Sigma-Aldrich. Zinc chloride (ZnCl2), tin tetrachloride
(SnCl4), ammonium chloride (NH4Cl), zinc powder, tin pow-
der and potassium hydroxide (KOH) were obtained from
Sinopharm Chemical Reagent Co., Ltd. All chemicals were
used as received without further purification. Double distilled
water was used in all experiments.
Reversible hydrogenation/oxidative dehydrogenation over the
Pd–ZnO1−x nanocatalyst
Typically, 0.25 mg of 1 wt% Pd–ZnO1−x hybrid catalyst and
0.1 g of HEC were dispersed in 9.3 mL of distilled water in a
25 mL quartz cell with vigorous stirring for 5 min; then 0.7
mL of TH+ (1 mM) was added. To remove the oxygen from
the system, the quartz cell was sealed with a rubber cap and
then continually purged with nitrogen for at least 30 min be-
fore reaction, and an adsorption/desorption equilibrium be-
tween the catalyst and TH+ could be guaranteed. The system
was sequentially bubbled with hydrogen and then oxygen at a
flow rate of 10 mL min−1 measured using a mass flow con-
troller (D07-19B, Sevenstar Electronics Co., Ltd., Beijing). The
concentration of TH+ at different time intervals was moni-
tored by the absorption peak at λ = 600 nm using a U-3900/
3900H UV-vis spectrophotometer (Hitachi).
Synthesis of the ZnO1−x nanorods and the Pd–ZnO1−x
nanocatalyst
In a typical synthesis, 1 mmol of ZnCl2 and 0.5 mmol of
SnCl4 were dissolved in distilled water (15 mL) with vigorous
stirring for 15 min; then, the metallic zinc powder (1 mmol)
and tin powder (0.5 mmol) were added into the aqueous solu-
tion. After stirring for another 15 min, 2 mmol of NH4Cl and
0.05 mol of KOH were added, and then the mixture was
stirred for 0.5 h to obtain a homogeneous suspension. The
mixed solution was transferred into a 25 mL Teflon-lined
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Reversible color switching of the spotted pattern
A TH+/HEC/Pd–ZnO1−x mixed solution was prepared using the
same procedure as described above. 30 μL of TH+/HEC/Pd–
ZnO1−x solution was dropped onto a hydrophobic Teflon
plate/film to obtain a spotted pattern. Then, the spotted pat-
tern was transferred to a container charged with 1 bar H2/O2
for reversible color switching.
Results and discussion
Oxygen vacancy abundant ZnO1−x nanorods (NRs) were first
prepared by hydrothermal treatment; then, Pd–ZnO1−x hybrid
nanostructures were synthesized by a photo-reduction deposi-
tion method, in which photo-induced electrons from ZnO1−x
were employed to reduce PdCl42− (Pd2+) on the ZnO1−x surface
(see the ESI†). The surface of Pd nanoparticles (NPs) is clean
since no additional surfactants were added. The crystal struc-
ture of the as-prepared oxygen vacancy-rich ZnO1−x support
and Pd–ZnO1−x nanocatalyst was investigated using the X-ray
diffraction (XRD) pattern. As shown in Fig. 1, the diffraction
peaks from the oxygen deficient ZnO1−x sample can be readily
indexed to the hexagonal wurtzite structure of zinc oxide crys-
tals (JCPDS no. 36-1451). After the deposition of Pd NPs, the
diffraction peaks for Pd were not observed likely due to the
ultralow Pd (nominal content: 1 wt%) loading.
Fig. 2a and b display the transmission electron micros-
copy (TEM) images of the bare ZnO1−x NRs and Pd–ZnO1−x
sample. The high resolution TEM (HRTEM) images further
reveal the formation of the Pd–ZnO1−x hybrid nanocatalyst
(Fig. 2c and d). Ultrasmall, uniform and highly dispersed Pd
NPs (<4.5 nm) are clearly observed on the surface of ZnO1−x
NRs (the inset in Fig. 2c shows the size distribution of Pd
particles). The as-synthesized ZnO1−x sample displays a rod-
like shape with an average diameter of 125 nm (Fig. 2a and
S1†). The HRTEM image clearly shows that Pd NPs and
ZnO1−x NRs have a single crystalline structure (Fig. 2d). The
Fig. 2 TEM images of a ZnO1−x NR (a) and a Pd–ZnO1−x hybrid catalyst
(b and c). HRTEM image of the Pd–ZnO1−x nanocatalyst (d). The inset in
(c) shows a size distribution of the Pd nanoparticles supported on
ZnO1−x NRs.
clear lattice spacing of 0.224 nm corresponds to the planar
distance of the Pd {111} plane. The lattice fringe spacing of
0.258 nm is assigned to the hexagonal ZnO1−x {002} plane.
X-ray photoelectron spectroscopy (XPS) analysis was
performed to reveal the chemical states and composition of
the Pd–ZnO1−x catalyst. The results from the XPS analysis
confirm the presence of zinc, palladium and oxygen (Fig. 3a)
in the Pd–ZnO1−x sample. Fig. 3b shows the high resolution
XPS spectra of Pd 3d; the peaks at 335.6 and 340.9 eV are at-
tributed to the 3d5/2 and 3d3/2 binding energies of the zero-
valent state of metallic Pd, respectively. The corresponding
binding energy of Pd 3d has a positive shift by about 0.7 eV
compared to the standard peak position,27 indicating the
SMSI between the Pd NPs and the ZnO1−x NRs.28 The peaks
that appeared at 337.2 and 342.5 eV correspond to the 3d5/2
Fig. 1 XRD patterns of the as-prepared ZnO1−x NRs and Pd–ZnO1−x
nanocatalyst. The PDF standard card for the hexagonal zinc oxide crys-
tal and cubic Pd is shown at the bottom.
Fig. 3 XPS spectra of the Pd–ZnO1−x nanocatalyst: (a) the survey
spectrum, (b) Pd 3d spectra, (c) Zn 2p spectrum and (d) O1s spectrum.
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and 3d3/2 levels of Pd2+, respectively.29 The atomic ratio of
Pd(0) to PdIJII) is estimated to be about 1 : 0.7. The peaks at
1021.9 and 1044.2 eV are assigned to Zn 2p3/2 and Zn 2p1/2
(Fig. 3c), respectively.30 The O 1s electron binding energy
peak, as shown in Fig. 3d, can be fitted well by two Gaussian
components having centres at 530.4 and 531.8 eV.31 The low
binding energy component accounting for about 48% of the
O 1s spectrum is ascribed to the O2− ions in the wurtzite
structure with hexagonal Zn2+ ions arranged periodically. The
component at the high binding energy region, accounting for
the remaining 52%, is related to the O 1s in the zinc hydrox-
ide species and adsorbed or chemisorbed oxygen species due
to surface hydroxyl groups.32 The atomic O content (44.7%)
calculated from the XPS data is less than the Zn content
(55.3%) in the ZnO1−x (x = 0.19, x stands for the concentra-
tion of oxygen vacancies) sample, which demonstrates the ex-
istence of abundant oxygen vacancies in ZnO1−x NRs. This is
in good agreement with the electron paramagnetic resonance
(EPR) measurements (Fig. S2†). The ZnO1−x sample gives rise
to a very strong EPR signal at g = 2.001, which was previously
identified as electrons trapped on surface oxygen vacan-
cies.33,34 These free electrons provided by the oxygen vacan-
cies in the support can transfer to Pd, promoting its catalytic
performance. The actual Pd content in the Pd–ZnO1−x sample
was determined to be 0.9 wt% by inductively-coupled plasma
mass spectrometry (ICP-MS), which is close to the nominal
content (1 wt%) in the experiment.
Pd-based nanomaterials have been extensively used for hy-
drogenation reactions because of their higher catalytic activi-
ties, but their high-cost and limited availability forces us to
find an alternative option by using promoter-supported
ultralow Pd loadings. The use of a non-stoichiometric sup-
port for noble metal particles could provide new physico-
chemical properties by SMSI (as confirmed by the XPS spec-
tra) to improve the catalytic activities. We synthesized
oxygen-vacancy-rich ZnO1−x NRs as a support and a promoter
for Pd NPs, because it is reported that the zinc oxide itself
can function in the dissociation of molecular hydrogen pro-
ducing hydrogen species capable of participating in hydroge-
nation reactions.35 The hydrogenation of thionine (TH+) was
performed over the Pd–ZnO1−x hybrid catalyst under an initial
hydrogen pressure of 1 bar and at room temperature. The
purple colored TH+ has an absorption maximum at 600 nm
(Fig. 4a). Upon hydrogenation, the color of the aqueous TH+
solution rapidly changed from purple to colorless with in-
creasing reaction time, which corresponds to the reduction
of TH+ to leuco-thionine (LTH), and completely disappeared
after 5 min (Fig. 4a). The hydrogenation activity of the Pd–
ZnO1−x hybrid nanocatalyst was evaluated by the calculation
of turnover frequency (TOF, defined as moles of TH+ mole-
cules per mole of the Pd catalyst per hour), resulting in a
TOF value of 397 h−1. This high TOF value strongly suggests
the enhanced catalytic activity of Pd–ZnO1−x for the hydroge-
nation of thionine. The catalytic hydrogenation over Pd–ZnO
(commercial ZnO without oxygen vacancies) was also
performed for comparison, and the results are shown in Fig.
Fig.
4
Reversible hydrogenation/oxidative dehydrogenation of the
redox thionine dye over the Pd–ZnO1−x catalyst exposed to a hydrogen
atmosphere and an oxygen atmosphere at room temperature. (a) UV-
vis spectra showing the hydrogenation process under 1 bar H2. (b) UV-
vis spectra showing the oxidative dehydrogenation process under 1 bar
O2. (c) UV-vis spectra of the reaction solution after each H2 and O2 ex-
posure for 10 continuous cycles. (d) The absorption intensity at 600
nm recorded after each H2 and O2 exposure for 10 continuous cycles.
S3.† Pd–ZnO1−x with oxygen vacancies exhibits a 4 times
higher activity as compared to Pd–ZnO without oxygen vacan-
cies; this further confirms that ZnO1−x with abundant oxygen
vacancies plays a promoter role in the enhancement of TH+
hydrogenation. Previous report showed that the hydrogena-
tion rate is linearly dependent on the concentration of oxygen
vacancies, as the oxygen deficient surface activates molecular
H2 easily from kinetic and thermodynamic aspects.36 The
presence of oxygen vacancies leads to an increased adsorp-
tion energy of molecular hydrogen and lowers the energy bar-
rier associated with the cleavage of the H–H bond.37 Minute
amounts of Pd NPs loaded on the surface of the oxygen defi-
cient ZnO1−x promoter significantly enhance the catalytic ac-
tivity due to the strong metal support interaction and syner-
gistic effect.
Following the boosted catalytic activity of the Pd–ZnO1−x
catalyst for the hydrogenation of thionine, we assessed its
performance for oxidative dehydrogenation of leuco-thionine
(LTH) in a one-pot reaction. Pd-based nanomaterials have
been widely used in oxidation reactions,38 as the adsorption
and dissociation of O2 on the surfaces of Pd are well known
to function in a range of selective oxidative cleavages.39 LTH
was oxidized back to thionine (TH+) over the Pd–ZnO1−x
nanocatalyst in the presence of hydroxyethyl cellulose (HEC).
HEC added in the solution can significantly slow down the
oxidation process of LTH through hydrogen bonding between
the –NH2 groups on LTH and the –OH groups on HEC mole-
cules (Fig. S4 and S5†).7 After hydrogenation was terminated,
the system was then exposed to 1 bar oxygen. The absorption
spectra of the colorless aqueous LTH/HEC/Pd–ZnO1−x system
have completely recovered after 6 min under an initial oxygen
pressure of 1 bar at room temperature (Fig. 4b). The colorless
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leuco-thionine was quickly switched back to the purple col-
ored thionine by the activated O2 over Pd–ZnO1−x, inducing
back electron transfer from LTH to oxygen. Meanwhile, the
Pd–ZnO (commercial ZnO without oxygen vacancies) catalyst
exhibits a lower oxidation activity of LTH, and the original
purple color cannot be completely recovered even after 60
min (Fig. S6†). The efficient oxidative dehydrogenation activ-
ity of the Pd–ZnO1−x nanocatalyst can be attributed to the de-
fect promoting effect, as the oxygen vacancies in Pd–ZnO1−x
enhance the oxygen activation, which further facilitates the
activation of a N–H bond of LTH; this is a key step in the
Mars–van Krevelen mechanism and increases the rate of LTH
interaction with the lattice oxygen.40
Fig. 5 Schematic illustration of the reversible hydrogenation/oxidative
dehydrogenation process of TH+/LTH over a highly active Pd–ZnO1−x
hybrid catalyst under ambient conditions. The insets show the
photographs of micro-droplet arrays of ‘USTC’ in response to hydro-
gen and oxygen for erasing and writing information.
To evaluate the stability of the Pd–ZnO1−x nanocatalyst for
potential applications, the hydrogenation/oxidative dehydro-
genation processes were repeated by sequentially exposing
the system to a hydrogen atmosphere and an oxygen atmo-
sphere. As shown in Fig. 4c, the recorded spectra nearly over-
lap, indicating the complete reversible conversion between
LTH and TH+. Fig. 4d plots the intensity of absorption maxi-
mum (600 nm) against cycles of exposure to hydrogen and
oxygen, showing an almost complete reversibility of hydroge-
nation and oxidative dehydrogenation processes. The Pd–
ZnO1−x hybrid catalyst was able to be recycled at least ten
times with a small loss in performance, which is considered
as a prerequisite for practical applications. Based on these re-
sults, it is noted that the reversible color switching property
could be used to develop rewritable paper and sensing and
security feature devices. To realize this possibility, a spotted
pattern was obtained by drop-casting aqueous dispersion of
TH+/HEC/Pd–ZnO1−x on Teflon substrates. Thionine (TH+) ex-
hibits a bright purple color under an oxidizing environment;
the photograph of micro-droplet arrays of ‘USTC’ is vividly
displayed for writing information. Upon changing to a reduc-
ing environment (1 bar H2), purple TH+ was reduced into col-
orless LTH; the pattern completely disappeared for erasing
information (see Video S1 in the ESI†). When the system was
switched to an oxidative environment (1 bar O2), the pattern
that vanished appeared quickly (see Video S2 in the ESI†).
This redox-responsive actuation color switching is readily vi-
sualized for many times, as clearly shown in Fig. 5. The drop-
let size of the TH+/HEC/Pd–ZnO1−x mixture solution affects
the color switching process; a bigger size drop takes a longer
time for both color fading and color recovery processes. In
addition, the concentration of Pd–ZnO1−x has a minor influ-
ence on the switching behavior (see Tables S1 and S2 in the
ESI†).
EPR (Fig. S2†). The transfer of free electrons from ZnO1−x to
Pd enriches the electron density of metallic Pd and acceler-
ates the hydrogenation reaction.42 Hydrogen would undergo
homolytic dissociation on the palladium surface into H
atoms with a partially negative charge (Hδ−, δ represents the
electric charge amount).43 The electron donation from oxygen
vacancies of ZnO1−x leads to an electron-rich Pd surface.
Tuning the electronic structure of nanocatalysts is an effec-
tive strategy to optimize their catalytic activities.44,45 Such an
electronic effect also makes the catalytic surface favour the
desorption of negatively charged
H
atoms (Hδ−) from
electron-rich Pd NPs, resulting in rapid hydrogenation of pur-
ple colored TH+ to colorless LTH. For oxidative dehydrogena-
tion reaction, oxygen activation and dissociation on the Pd–
ZnO1−x nanocatalyst occur when oxygen flows into the reac-
tion system. Oxygen can be easily dissociated on the Pd sur-
face as the dissociation energy barrier is very small.26 In the
first step of oxidative dehydrogenation, O2 adsorbs on the
catalyst surface with a high probability in a weakly bound
state and a Pd–O bond forms. Then, the hydrogen atom of
the N–H bond in LTH combines with one negatively charged
oxygen atom, and finally, thionine (TH+) is reproduced. The
In the hydrogenation process, when hydrogen was intro-
duced into the reaction system, molecular hydrogen is
thought to initially bind to the atoms of Pd nanoparticles
supported on ZnO1−x nanorods via its H–H σ-bond and then
dissociates,41 as depicted in Fig. 6. The abundant oxygen va-
cancies on ZnO1−x provide additional free electrons that can
be transferred to Pd, further promoting the cleavage of the
H–H bond.36,39 Oxygen vacancies carry unpaired electrons
that could serve as effective electron donors, as confirmed by
Fig. 6 Proposed reaction mechanism of one-pot reversible thionine
hydrogenation and leuco-thionine oxidative dehydrogenation over a
Pd–ZnO1−x nanocatalyst.
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reaction of LTH with the activated oxygen regenerates Pd(0)
species, along with the formation of H2O. Taken together,
the strong metal support interaction between Pd and ZnO1−x
with abundant oxygen vacancies plays an important role in
highly efficient reversible hydrogenation/oxidative dehydroge-
nation of redox thionine molecules.
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Conclusions
In this work, we have developed a novel Pd–ZnO1−x hybrid
nanocatalyst which exhibits high catalytic activity for both hy-
drogenation and oxidative dehydrogenation reactions
performed successively in a one pot-fashion. Thionine dye is
used as a probe to evaluate the catalytic performance of Pd–
ZnO1−x by sequential exposure of the reaction solution to oxi-
dizing and reducing environments. The enhanced activities
are attributed to abundant oxygen vacancies in ZnO1−x and
the strong metal–support interaction in Pd–ZnO1−x, which
not only provide free electrons to Pd nanoparticles but also
lower the energy barrier for the adsorption and subsequent
dissociation of H2 and O2. The catalytic performance of Pd-
ZnO without oxygen vacancies is much lower as compared to
that of the Pd–ZnO1−x nanocatalyst, which further confirms
the role played by oxygen vacancies in the promotion of activ-
ities. The highly efficient activities of the hydrogenation and
oxidative dehydrogenation reactions over the Pd–ZnO1−x hy-
brid nanocatalyst may find practical applications for synthe-
sizing pharmaceuticals and fine chemicals in industry. In ad-
dition, we anticipate that this thionine/HEC/Pd–ZnO1−x
reversible color switching system could be used as printing
inks for rewritable paper, sensing devices, data recording and
security feature technology. However, because of the poor the
solubility and diffusion of H2 and O2, there are still some dif-
ficulties to overcome in constructing an actual device from
the solution. Our future work will focus on the practical
application.
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Acknowledgements
The special funding support from the National Natural Sci-
ence Foundation of China (51572253 and 21271165), the Sci-
entific Research Grant of Hefei Science Center of CAS
(2015SRG-HSC048), and the cooperation between the NSFC
and the Netherlands Organization for Scientific Research
(51561135011) is acknowledged.
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