Interface engineering of palladium and zinc oxide nanorods with strong metal-support interactions for enhanced hydrogen production from base-free formaldehyde solution

Article abstract of DOI:10.1039/c8ta12019f

Catalytic hydrogen production from formaldehyde solution provides a promising strategy for future hydrogen-based energy system, while almost all of the present catalytic reactions are carried out in highly alkaline medium. Here, we show the first evidence of the strong metal-support interaction (SMSI) effect between Pd nanoparticles and ZnO nanorods, resulting in the formation of a unique Pd@PdO_x/ZnO core-shell structured catalyst as reversible electron transfer occurs between Pd and ZnO. The special synergic effect realizes efficient H₂ production from base-free formaldehyde solution. Experimental and theoretical observations indicate that the excellent performance can be attributed to the core-shell catalyst, where the Pd⁰ shell facilitates Cannizzaro intermediate production, the PdO_x site attacks the anionic intermediates, and ZnO activates water splitting. Importantly, the SMSI between Pd and ZnO is able to change the electronic distribution of the catalyst and enhances its Lewis basicity, thereby realizing H₂ production from HCHO solution without any base additives. The SMSI effect can be further optimized either by depositing an extra thin ZnO overlayer or by alloying Pd with additional Pt metal to form bimetallic structures that can significantly reduce the activation energy and ultimately improve the catalytic performance. This finding opens a new window to design and develop high performance supported catalysts for catalytic hydrogen production from base-free chemical hydrogen storage chemicals on the basis of precise interface engineering between the metal and the support.

Full text of DOI:10.1039/c8ta12019f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Interface Engineering of Palladium and Zinc Oxide Nanorods with
Strong Metal-Support Interaction for Enhanced Hydrogen
Production from Base-free Formaldehyde Solution
1Received 00th January 20xx,
Accepted 00th January 20xx
Leilei Dua, Kaicheng Qiana, Xiaohui Zhua, Xiaoqing Yan,b Hisayoshi Kobayashic,*, Zhiqi Liua, Yupeng
Loua, Renhong Lia,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Catalytic hydrogen production from formaldehyde solution provides a promising strategy for future hydrogen-based energy
system, while almost all of the present catalytic systems are carried out in highly alkaline medium. Here, we show the first
evidence of the strong metal-support interaction (SMSI) effect between Pd nanoparticles and ZnO nanorods, resulting in the
formation of a unique Pd@PdOx/ZnO core-shell structured catalyst as the reversible electron transfer occurs between Pd
and ZnO. The special synergic effect realizes efficient H2 production from base-free formaldehyde solution. Experimental
and theoretical observations indicate the excellent performance can be attributed to the core-shell catalyst, where the Pd0
shell facilitates Cannizzaro intermediate production, the PdOx site attacks the anionic intermediates, and ZnO activates water
splitting. Importantly, the SMSI between Pd and ZnO is able to change the electronic distribution of the catalyst and
enhances its Lewis basicity, thereby realizing H2 production from HCHO solution without any base addatives. The SMSI effect
can be further optimized either by depositing an extra thin ZnO over layer or by alloying Pd with additional Pt metals to form
bimetallic structures that can significantly reduce the activation energy and ultimately improve the catalytic performance.
This finding opens a new window to design and develop highly performance supported catalysts for catalytic hydrogen
production from base-free chemical hydrogen storage chemicals on the basis of precise interface engineering between metal
and support.
support interaction (SMSI).10 Since then, enormous efforts have
been devoted to the effects of VIII group metals and oxide
supports in SMSI on the basis of the advancement of catalysis
1. Introduction
Supported metal catalysis is one of the most investigated fields
in heterogeneous catalysis because of the outstanding catalytic
behavior presented by such catalysts in response to rising
and surface science.11, 12
The classic route to form SMSI is that the sample is reduced
at high-temperature H2 to activate the surface of reducible
support, which causes the change in the electronic structure at
the interface between metal and support facilitating the
crawling of support on metal surface to form an encapsulation
layer. The sample is then treated with an O2 atmosphere and
the wrapped metal is exposed again.13-15 The SMSI, which
induces the charge redistribution and mass transport processes
at the interface by the geometric and/or electronic modification
between metal nanoparticle and support, is able to modify the
catalysts chemical state, influence the adsorption behavior of
the reactants and the subsequent conversion of the reaction
intermediates, and finally change the reaction path and
catalytic performance.16-18 While in some cases (e.g. CO
oxidation on Pt/TiO2) the SMSI effect was found to increase the
catalytic activity, in other cases the catalytic activity is
considerably reduced by diminishing the number of active sites
through encapsulation of metal particles by support (e.g.
toluene hydrogenation on Pt/TiO2).19, 20 Therefore, even though
the potential of SMSI in catalysis is obvious, its rational
modulation in a given direction to enhance the catalyst
environmental and energy challenges.1, Previous studies
2
demonstrated that the catalytic performance of supported
metal catalysts is crucially influenced by the metal particle as
well as the support.3-6 The principal role of support was initially
ascribed to stabilization of the active component and/or
increase in the metal dispersion and surface area.7-9 The metal-
support interaction effect was first reported in 1978 by Tauster
et al., who found the adsorption of small molecules (such as CO
and H2) was dramatically suppressed on TiO2-supported VIII
group metals after high-temperature reduction in H2. This
phenomenon was subsequently nominated as the strong metal-
a. Department of Materials Engineering, College of Material Textiles, Zhejiang Sci-
Tech University, Hangzhou 310018, China. *E-mail: lirenhong@zstu.edu.cn
b. Department of Chemistry, College of Science, Zhejiang Sci-Tech University,
Hangzhou 310018, China
c. Department of Chemistry and Materials Technology, Kyoto Institute of
Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. *E-mail:
hisabbit@yahoo.co.jp
Electronic Supplementary Information (ESI) available: Reaction kinetic factors,
characterizations and detailed information on the energetics and structures of the
elementary reaction steps based on the DFT calculations. See
DOI: 10.1039/x0xx00000x
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performance is still of great significance for screening optimal In a typical synthesis of ZnO nanorods, solutions oVfiezwinAcrtiaclceeOtnalitnee
catalytic systems.
dihydrate (Zn(Ac)2·2H2O) in ethanol, andDsOoI:lu10t.i1o0n3s9/oCf8TsAo1d20iu19mF
Catalytic hydrogen (H2) production from formaldehyde hydroxide (NaOH) in ethanol were prepared as stock solutions
(HCHO) has drawn intense interest in recent years (i.e., HCHO + in advance. 10 mL of 0.1 M of zinc acetate stock solution in
H2O → H2 + CO2), as its solution has high theoretical hydrogen ethanol was mixed with 20 mL of 0.5 M NaOH solution in
weight percentage (8.4 wt%) as well as provides a potential ethanol to get a clear solution which was transferred to a
approach to decontamination treatment of wastewater Teflon-lined stainless steel autoclave and heated at 150 °C for
containing formaldehyde impurities.21-24 However, almost all of 24 hours. A white precipitate was collected, washed with water
the catalytic systems so far are carried out in highly alkaline and ethanol several times, and dried in air at ambient
26
medium (e.g., NaOH).25,
As
a
promoter, NaOH can temperature.
In a typical synthesis of PdNPs, 150 mg of Pd(acac)2 was
deprotonate the hydrated HCHO (CH2(OH)2) to form the active
Cannizzaro intermediate (CH2(O-)2), which is a key step in dissolved in 30 mL oleylamine at 60 oC for 10 mins. The mixture
dehydrogenation of HCHO via a hydride ion donor reaction with was heated to 90 oC and kept for 1 h after the addition of 2 mL
a water molecule to produce formate and hydrogen (CH2(O−)2 + oleylamine containing 260 mg BTBC as the reducing agent. After
H2O → HCOO− + H2 + OH−).27, 28 Undoubtedly, the utilization of cooling to room temperature, PdNPs were precipitated from
NaOH owns unavoidable defects: (1) HCHO easily undergoes the mixture by addition of 60 mL anhydrous ethanol, and then
disproportionation reactions (Cannizzaro reaction) in highly collected by centrifuge and ethanol washing, and drying in
basic media by converting into the corresponding alcohols and vacuum (60 oC) over night.
carboxylates, which seriously retards the hydrogen production
PdNPs were loaded onto ZnO nanorods (or other supports) by
efficiency, and (2) alkaline solution could cause secondary a colloid deposition method. In a typical preparation, 10 mg
pollution to the water environment if not handled properly. PdNPs were dissolved in 20 mL chloroform. To this solution, 1 g
Exploring an effective way to the success of catalytic hydrogen ZnO nanorods was added. After 30 min stirring, the solid
production from base-free HCHO solution offers a key to the product was centrifuged and dried. The supported PdNPs were
future development of sustainable technology.
calcined in N2 at 450 oC for 3 h to obtain Pd@PdOx/ZnO
We report here that a typical SMSI effect exists between Pd nanocatalysts. The same procedure was applied for the
nanoparticles (PdNPs) and ZnO nanorods, and more preparation of PdO/ZnO or Pd/ZnO nanocatalysts except for
importantly the SMSI realizes efficient H2 generation from replacing the calcination atmosphere by O2 or H2/Ar,
HCHO solution without any base additives. The optimal catalytic respectively.
performance is achieved by a unique Pd@PdOx/ZnO composite
For the synthesis of ZnO/Pd@PdOx/ZnO nanocatalysts, an
catalyst, in which PdNPs are semi-encapsulated by a zinc oxide aqueous solution of Zn(NO3)2 (5 mM, 5 mL) was adjusted to pH
support and the charge redistribution induced by SMSI enables = 2 by HNO3. Pd@PdOx/ZnO was immersed into the solution,
the formation of the special Pd@PdOx core-shell structure. In and deaerated by N2 bubbling for 5 min. The sample was
the hybrid Pd@PdOx/ZnO catalyst composed of Pd0, PdOx and illuminated by using UV-LED lamp at 25 oC. The resulting sample
ZnO, the Pd0 shell could facilitate CH2(OH)2 adsorption and was washed with distilled water and ethanol, and dried in air to
deprotonation to produce Cannizzaro intermediates, the PdOx obtain Zn/Pd@PdOx/ZnO. The deposition amount of Zn on
sites subsequently attack the anionic intermediates due to their Pd@PdOx/ZnO is adjusted by changing the irradiation time (tp)
strong electrostatic affinity to the locally positively charged Pdx+ of UV light. Finally, the sample was further calcined in air at 450
species, and ZnO activates water splitting by formation of H and °C to obtain ZnO/Pd@PdOx/ZnO.
OH species on O and Zn sites, respectively. This synergistic
For the synthesis of PdM@PdOx/ZnO nanocatalysts,
effect among Pd0, PdOx and ZnO enables base-free HCHO PdM@PdOx/ZnO catalyst was prepared by UV-photoreduction
dehydrogenation under ambient conditions. In principle, the method using 500 W high pressure Hg lamps. In a typical
SMSI effect can be strengthened by support modification or by synthesis, 50 mg Pd@PdOx/ZnO was introduced to 5 mL
tuning the catalytically active metals.29-30, 31 As a consequence, methanol-water (1:9/v:v) solution with the corresponding
the H2 production efficiency of Pd@PdOx/ZnO is further metal precursors (H2PtCl6, HAuCl4, CoCl2, RuCl3•xH2O and
enhanced by depositing an extra thin zinc oxide over layer on RhCl3•3H2O). The suspension was purged with N2 atmosphere
Pd surface to tune the charge redistribution property or alloying to remove dissolved oxygen and then photoirradiated for 2h
Pd with additional Pt metals to form bimetallic structures. This with continuously stirring. Then, the resulting dispersion was
study allows us to obtain a deep understanding of the nature centrifuged and the PdM@PdOx/ZnO were obtained.
and mechanism of SMSI between Pd and ZnO, and furthermore Furthermore, we varied the mole ratios of metal sources in the
it may unlock new design strategies for the development and precursor solutions to prepare PdM@PdOx/ZnO with various
operation of supported metal catalysts with high activity compositions.
through precise engineering of the interface between metal and 2.2 Characterizations
support.
TEM images were recorded on a JEOL JEM-1230 operated at 100
kV. Wide-angle XRD patterns were recorded on a Bruker D8
diffractometer using Cu Kα radiation. XPS was performed in a
VG Scientific ESCALAB Mark II spectrometer. All binding
2. Experimental
2.1 Synthesis of materials
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energies were referenced to the C 1s peak at 284.8 eV of the evolution amount, including H2, O2, CO2 and CO. TVhieewTAOrticFlevOanlluinee
surface adventitious carbon to correct the shift caused by is calculated by the following DeOqIu: 1a0t.i1o0n39:/C8TTOA1F2019F
=
charge effect. 1H liquid state NMR spectrum was recorded with nH2/(nPd×t)×(d/1.3), where nH2 and nPd represent the molar
a Bruker Ascend 400 (400 MHz) and Bruker Avance III HD 400M amounts of evolved H2 within t h and Pd catalyst, respectively,
spectrometer by using adamantane (δ = 38.5 ppm) as an and d is the average particle size of Pd.
internal standard. The samples were analyzed by FT-IR 2.4 DFT calculations
spectroscopy, using a Thermo Fisher iS10 spectrometer. Spectra
were recorded in the range of wave numbers from 4000 to 400
cm−1, using sample/KBr pellets prepared as described
DFT calculations were carried out using a plane-wave-based
program, Castep.32, 33 The Perdew−Burke−Ernzerhof functional
was used together with the ultrasoft-core potentials.34, 35 The
basis set cutoff energies were set to 340 eV. Explicitly
elsewhere.
The harmonics of a nanosecond Nd:YAG laser and their
considered electrons are H: 1s1, C: 2s22p2, O: 2s22p4, Zn: 3d104s2,
hydrogen Raman shifted laser lines were used to generate the
and Pd: 4d10. The lattice constants are a = 11.2558 Å, b = 40 Å
(including the vacuum region), c = 10.4109 Å, and α = β = γ =
416 nm excitation wavelength employed in the resonance
Raman experiments. The excitation laser beam was loosely
90o. The cut-off energies are 300 and 340 eV for geometry
focused to the quartz tube involving formaldehyde solution
optimization and energy re-evaluation. Semi-Empirical
and/or solid catalyst as a function of reaction time. A heating
Dispersion Correction presented by McNellis and Meyer was
equipment was used to maintain the reaction mixture at 50 oC.
employed.36 A cluster model composed of Pd11(ZnO)26 was
A backscattering geometry was used for sample excitation and
used, which was first optimized before adsorption. In
for collection of the Raman scattered light by reflective optics
adsorption of H2CO and H2O, and in the successive surface
that imaged the Raman scattered light through a polarizer and
reactions, the structure of Pd11(ZnO)26 was fixed. Although this
entrance slit of a 0.5 m spectrograph. The grating of the
constraint reduces the flexibility of reaction system, it was
spectrograph dispersed the light onto a liquid nitrogen cooled
necessary to characterize transition state structures.
charge coupled device (CCD) mounted on the exit of the
3. Results and discussion
3.1 Characterizations of the Pd@PdOx/ZnO nanorods
spectrograph, and the CCD acquired the Raman signal for about
90-150 s before being read out to an interfaced personal
computer. About 10-30 of these scans were summed to obtain
ZnO nanorods are synthesized via a facile hydrothermal
the resonance Raman spectrum. The Raman shifts of the
approach. Pd@PdOx/ZnO catalyst is prepared by a colloid
resonance Raman spectra were calibrated using the known
deposition method, followed by calcination in nitrogen at 450
vibrational frequencies of the dichloromethane solvent Raman
oC (see Experimental section). The average diameter and length
bands. The spectra of an intensity calibrated deuterium lamp
of ZnO nanorods are around 40 and 400 nm, respectively. PdNPs
were used to correct the resonance Raman spectrum intensities
with average diameters of 8.4 nm are homogenously dispersed
for the variation in detection efficiency as a function of
on ZnO surface, as verified by the transmission electron
wavelength, and portions of the resonance Raman spectra were
microscope (TEM) image (Fig. 1a and Fig. S1, ESI). Lattice fringes
fitted to a baseline plus a sum of Lorentzian bands to find the
with an interplanar spacing of 0.26 nm, corresponding to the
integrated areas of the Raman bands.
ZnO (002) atomic planes are revealed in the HR-TEM image of
X-band EPR signals were recorded on a Bruker EPR A-300
Pd@PdOx/ZnO nanorods (Fig. 1b). It is clear that PdNPs are
spectrometer. The settings for the EPR spectrometer were as
hemispherically attached to the surface of ZnO nanorod and
follows: center field, 3511.39 G; sweep width, 100 G;
encapsulated by a thin layer of ZnO. Surprisingly, the innermost
microwave frequency, 9.86 G; modulation frequency, 100 kHz;
PdNPs are covered by a layer of reduced contrast metallic Pd
power, 101 mW; conversion time, 10 ms. The solid EPR analysis
species (inset of Fig. 1b shows the schematic illustration of the
was carried out by packing sample powders into a glass capillary
Pd@PdOx/ZnO core-shell structure) which appears slightly
tube, which was placed in the EPR chamber for testing. The
more intense in contrast compared to the outermost ZnO semi-
location and the intensity of the g factors were determined by
encapsulated layer, indicating the formation of Pd@PdOx
Bruker’s WINEPR program based on hν = gβH, where h is
Planck’s constant, H is the applied magnetic field, and β is the
structure. The encapsulation of metal NPs by oxides is a classic
feature of SMSI, preliminarily demonstrating the presence of
Bohr magneton.
SMSI between PdNPs and ZnO nanorods. Fig. 1c shows the X-
2.3 Catalytic dehydrogenation of HCHO solution
ray diffraction (XRD) patterns of the ZnO nanorods and
Catalytic H2 production from HCHO/H2O solution was carried
Pd@PdOx/ZnO. The diffraction peaks of wurtzite structure of
out with 20 mg of catalysts suspended in 5 mL of aqueous HCHO
ZnO (PDF card No. 36-1451) dominate the catalysts, and only a
solution (1.0 M) in a 55 mL Pyrex test tube under stirring (400 ±
weak peak attributed to the (111) peak of the metallic Pd phase
10 rpm). A water bath was used to maintain the reaction
is observed at 40.1° of Pd@PdOx/ZnO spectrum. No peaks
temperature at 50 ± 0.5 oC. The mixture was degassed by
corresponding to PdO are detected, probably because the two
bubbling with pure N2 gas, and the tubes were finally sealed
strong reflections (101) and (002) at 34.5° for PdO (PDF card No.
with silica gel stoppers. Gas volumes of 400 μL were extracted
04-005-4390) are overlapped with the main (002) reflection of
from the test tubes using a microliter syringe at regular
ZnO.
intervals, and GC-TCD was employed for evaluating the gas
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HCHO solution it has the higher concentration ofVmieweAthrtiaclneeOdnliinoel
DOI: 10.1039/C8TA12019F
(n = 2). Thus it can be inferred that methanediol acts as the real
hydrogen source, and more importantly hydrated HCHO makes
water accessible for dehydrogenation in aqueous solution.
To compare the influence of the catalysts structure and
component to catalytic performance of hydrogen production,
different component materials (Pd@PdOx/ZnO, Pd/ZnO and
PdO/ZnO) are prepared (see Experimental section), and their
catalytic properties are also compared. We primarily employ
XPS spectrum to analyze the chemical state and composition of
Pd in the catalysts (Fig. 2c). Pd@PdOx/ZnO including metallic Pd,
PdO and PdO2 can be rationalized by a core-shell structure with
a thin Pd layer shell at the surface and a saturated PdOx at the
core. The Pd species in Pd/ZnO and PdO/ZnO samples are
completely metallic and oxidized, respectively. Pd@PdOx/ZnO
exhibits the optimal catalytic performance for dehydrogenation
of HCHO solution among the three catalysts, while PdO/ZnO is
essentially inactive and Pd/ZnO only shows a poor activity (Fig.
2d). It means that neither metallic nor completely oxidized Pd
species is able to efficiently promote the hydrogen production
from HCHO solution. Therefore, we tentatively propose that the
superior catalytic performance of the Pd@PdOx/ZnO could be
ascribed to the existence of the active center of multiple
synergistic active sites of metallic Pd and PdOx induced by the
SMSI effect.
Fig. 1 (a) TEM and (b) HR-TEM images of Pd@PdOx/ZnO nanorods where black arrows
indicate single PdNP (the inset shows the schematic illustration of the Pd@PdOx/ZnO
structure); (c) XRD patterns of ZnO nanorods and Pd@PdOx/ZnO, and (d) Pd 3d XPS
spectrum of Pd@PdOx/ZnO.
To determine the surface chemical composition of
Pd@PdOx/ZnO, X-ray photoelectron spectroscopy (XPS) analysis
of Pd 3d, Zn 2p and O 1s are performed (Fig. 1d and S2, ESI). The
Pd 3d5/2 exhibits three peaks at 334.3, 334.9, and 336.1 eV,
which are ascribed to the metallic Pd, PdO and PdO2,
respectively; the ratios for each part are approximately 39%,
30%, and 31%.37, 38 The origin of the PdOx species is due to the
high oxygen affinity of PdNPs, showing a tendency to be
oxidized by surface terminated oxygen in ZnO in order to
decrease the total surface energy.39 The cationic charged Pd
mainly locate at the interface between Pd and ZnO surface,
while the metallic Pd species is concentrated on the top surface
of PdNPs and is slightly positively charged when calcined at high
temperature. The ZnO thin layer is observed on the top of
PdNPs, providing a strong evidence for the presence of SMSI
between Pd and ZnO.
In addition, CO2-TPD experiment was conducted to
investigate the effect of basicity on the catalytic performance.
The CO2-TPD profiles of the ZnO support and Pd@PdOx/ZnO are
shown in Fig. S3 (ESI). The TPD result of ZnO shows a strong peak
o
at temperatures above 450 C.40 In the case of Pd@PdOx/ZnO,
the TPD profile shows three broad peaks at 410, 630 and 730
oC, which are characteristic of CO2 desorption from strong-basic
sites. This result indicates that the SMSI between Pd and ZnO
changes the electronic distribution of the catalyst and enhances
its Lewis basicity, thereby realizing hydrogen production from
HCHO solution in base-free medium.
3.2 Catalytic performances of the Pd@PdOx/ZnO
Pd@PdOx/ZnO exhibits an excellent hydrogen evolution activity
in HCHO/H2O solution in an inert gas environment without any
form of base additives. Fig. 2a shows the H2 production
catalyzed by PdNPs supported on different supports (ZnO,
Al2O3, TiO2 and MgO) or different metals (Pd, Pt, Au, Ag and Cu)
loading on ZnO nanorods. In contrast to the superior catalytic
performance of Pd@PdOx/ZnO, other catalysts show poor
catalytic activity under identical reaction conditions. It
demonstrates that the SMSI effect between Pd and ZnO is
advantageous for dehydrogenation reaction. In principal, it is
known that HCHO in water is solely present in the hydrated
form as methanediol or even oligomerized to form PF (i.e., CH2O
+ nH2O → HO(CH2O)nH, n > 1). We find that Pd@PdOx/ZnO also
effectively catalyzes PF solution into H2 (Fig. 2b). The lower rate
of formation of H2 from aqueous PF might be due to its higher
oligomeric mixtures of HO(CH2O)nH oligomer (n > 1), while in
Fig. 2 (a) Pd supported on different supports (ZnO, Al2O3, TiO2 and MgO) or loading
different metals (Pd, Pt, Au, Ag and Cu) on ZnO nanorods for dehydrogenation of HCHO
solution (t = 50 oC); (b) Pd@PdOx/ZnO catalyzed H2 evolution from HCHO solution and
paraformaldehyde solution (PF = 1 M, t = 65 oC); (c) XPS analysis of Pd 3d and (d) catalytic
performances of formaldehyde solution dehydrogenation (t = 40 oC) of Pd@PdOx/ZnO,
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Pd/ZnO and PdO/ZnO. All catalytic H2 production experiments are carried out in N2
atmosphere.
catalyst by applying an improved photodeposition technique
(Scheme 1). First, Zn is deposited on the Pd@PdOx/ZnO
View Article Online
DOI: 10.1039/C8TA12019F
nanocatalyst by a photochemical underpotential-deposition
method to form an ultrathin layer at monolayer level.41 Then,
The effects of HCHO concentration, Pd loading amount and
reaction temperature on the hydrogen generation rate are
shown in Fig. S4a-c (ESI). The H2 generation rate over
Pd@PdOx/ZnO catalyst greatly depends on the HCHO
concentration. As the HCHO concentration increases from 0.1
to 1 M, the TOF increases rapidly from 1.3 to 106.0 h-1. Further
increasing the concentration of HCHO results in a decrease of
H2 generation rate, indicating that excessive HCHO is
disadvantageous for the dehydrogenation reaction. The effect
of Pd loading amount is investigated in the range of 0.1 to 5
wt%. As expected, higher loadings for Pd@PdOx/ZnO result in a
better performance for H2 production. However, the mass
activity of catalysts reaches the maximum value when the metal
loading weight overtakes 0.1 wt% due to partial covered active
sites. The effects of reaction temperature on the rates of
Zn/Pd@PdOx/ZnO
is
calcined
in
air
to
obtain
ZnO/Pd@PdOx/ZnO. In order to obtain a clear observation and
deep understanding of the influence of the zinc oxide deposit
layer, we select the sample with tp = 7h to study their
encapsulation morphology. As expected, the catalyst displays a
thin layered ZnO surrounding the Pd nanoparticles architecture
which is a typical encapsulation structure in the SMSI effect (Fig.
3a and Fig. S7, ESI). As shown in Fig. 3b, the diffraction peaks of
ZnO dominate all of the catalysts and no peaks of Pd species are
detected. XPS measurement is also carried out to follow the
electronic structure change of the Pd@PdOx/ZnO after ZnO
deposition (Fig. 3c). The XPS Pd 3d spectrum shows three
doublets attributed to Pd0, PdO and PdO2, respectively.
Meanwhile, no obvious shift is observed for the Pd 3d, O 1s and
Zn 2p XPS spectra as compared with pure Pd@PdOx/ZnO. All of
these observations manifest that ZnO over layer have been
successfully deposited on the Pd@PdOx/ZnO surface.
o
hydrogen generation are measured within 30−70 C. The TOF
values of hydrogen generation increases rapidly from 13.6 to
657.1 h−1 with the increase of reaction temperature, evidently
revealing that high temperature facilitates the hydrogen
generation reaction. The Arrhenius plot fitted on the basis of
the TOF values at different reaction temperatures gives an
activation energy (Ea) of 79.8 kJ/mol (Fig. S4d, ESI). A previous
study showed that the optimal TOF value of Pd nanotubes
catalyzing hydrogen generation from alkaline formaldehyde
solution at room temperature is 63.4 h-1,25 while the TOF of our
base-free medium catalytic system is more than 10 times higher
(657 h-1) at elevated reaction temperature. In addition, we
examined the stability of Pd@PdOx/ZnO catalyst by HR-TEM and
XPS characterizations of the used catalysts. It can be seen from
the HR-TEM image that the special Pd@PdOx core-shell
structure feature of the catalyst are well maintained after the
reaction (Fig. S5a, ESI). XPS spectrum of Pd also verified that Pd
species in the catalyst is still composed of metal palladium, PdO
and PdO2 (Fig. S5b, ESI). Furthermore, no significant loss in
activity is observed over 4 cycles (Fig. S6, ESI), indicating that
the Pd@PdOx/ZnO catalyst has high stability under the current
HCHO solution dehydrogenation conditions.
Fig. 3 (a) TEM image for ZnO/Pd@PdOx/ZnO (tp = 7 h), (b) XRD pattern and (c) Pd 3d, Zn
2p and O 1s-XPS analyses for ZnO/Pd@PdOx/ZnO (tp = 3.5 h), (d) the H2 evolution rate vs.
irradiation time of UV light for the dehydrogenation of HCHO catalyzed by the
ZnO/Pd@PdOx/ZnO, and the catalytic H2 production experiments were carried out in N2
atmosphere.
3.3 Characterizations and catalytic performances of
ZnO/Pd@PdOx/ZnO catalyst
Fig. 3d shows the catalytic dehydrogenation activity of a
series
of
ZnO/Pd@PdOx/ZnO
upon
photochemical
underpotential-deposition. The deposition amount of ZnO on
Pd@PdOx/ZnO is adjusted by changing the UV irradiation time
(tp). When tp is very short, the H2 production rate is slightly
lower than that of pure Pd@PdOx/ZnO. This is possibly
attributed to the fact that a large amount of Pd atoms are not
encapsulated by deposited Zn and are oxidized by calcination in
air. With an increase in the tp up to 7 h, the H2 evolution initially
increases and reaches the maximum value of 285.6 h-1 at 3.5 h,
the TOF is around 1.5 times higher than Pd@PdOx/ZnO (TOF =
193.8 h-1), which can be understood as all Pd atoms on the
Scheme 1. Schematic diagram of the synthesis scheme of ZnO/Pd@PdOx/ZnO.
To further prove the catalytic performance enhancement by
SMSI effect, we prepare sandwich-like ZnO/Pd@PdOx/ZnO
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catalyst participate in the reaction and the deposited ZnO Zn 2p and O 1s signals of PdPt@PdOx/ZnO is observed relative
View Article Online
DOI: 10.1039/C8TA12019F
further enhanced the SMSI effect between Pd and ZnO. A to pure Pd@PdOx/ZnO. Thus, XPS analysis provides a strong
further increase in the tp of UV-light shows a negative effect on evidence that reversible electron transfer occurs between
the dehydrogenation of HCHO solution, mainly because of the PdNPs and ZnO nanorods due to their strong electronic
thick ZnO encapsulation layer suppressing the SMSI effect. The interactions.47 The synergy between Pd and Pt is suggested to
above results indicate that a proper amount of the ZnO support promote the activation of ZnO, causing more ZnO migration
encapsulation layer on the surface of PdNPs leads to a higher onto PdNPs surface and reconstitution of the interface between
number of active sites for water activation and a greater SMSI metal and support, thereby enhancing the SMSI.
effect between Pd and ZnO, thereby regulating the
The solid electron paramagnetic resonance (EPR) spectra
o
dehydrogenation reaction performance. In addition, it is collected at 25 C provides spectroscopic evidence of oxygen
noteworthy that Zn/Pd@PdOx/ZnO with the formation of PdZn vacancies in the catalyst. Fig. S9a (ESI) reveals two signals for all
alloy does not show any hydrogen evolution activity, samples: the signal at g = 2.002 and g = 1.961 can be assigned
demonstrating that the special SMSI between Pd and ZnO to superoxide ions and singly ionized oxygen vacancy in pure
promotes the dehydrogenation performance rather than the ZnO nanorods, respectively.48 The signal intensity of oxygen
alloying effect of PdZn.42-44 Combined with the previous XPS vacancy follows the sequence of Pd@PdOx/ZnO
>
analysis, it can be concluded that further deposition of a proper PdPt@PdOx/ZnO ≈ ZnO/Pd@PdOx/ZnO ≈ ZnO NRs. Moreover,
ZnO layer is able to enhance the catalytic activity of the catalyst the Pd 3d XPS spectra of Pd@PdOx/ZnO, PdPt@PdOx/ZnO and
by strengthening the SMSI effect.
3.4 Characterizations and catalytic performances of
PdM@PdOx/ZnO catalysts
ZnO/Pd@PdOx/ZnO catalysts showed that the ratio of Pd+
species is almost the same, while there is observable difference
in Pd2+ species. We subsequently analyzed the catalytic
performance of these four samples by catalyzing hydrogen
production from HCHO solution, and found that there is no
direct relationship between oxygen vacancy and catalytic
dehydrogenation performance. It suggests that the SMSI
between Pd and ZnO rather than oxygen vacancy plays a vital
role in the dehydrogenation property, and the active center is
most likely related to the multiple synergistic active sites of
metallic Pd and PdO.
Scheme 2. Schematic diagram of the synthesis scheme of PdPt@PdOx/ZnO.
In addition to the support alteration, the modification of
metal has also been proved to be an effective way to enhance
the SMSI.45,46 Therefore,
a series of alloyed catalysts,
PdM@PdOx/ZnO (M = Pt, Au, Co, Ru and Rh) are prepared by a
sequential deposition-UV-photoreduction method to further
reinforce the SMSI between Pd and ZnO (Scheme 2). Typically,
TEM images of PdPt@PdOx/ZnO show that it is covered by a thin
layer of ZnO and preserved its size and morphology after
alloying (Fig. 4a and S8, ESI). Clear lattice fringes are observed
by HR-TEM, indicating that a rather good crystallinity is realized.
The lattice spacing is about 0.23 nm which is ascribed to the
(111) crystal plane of the PdPt alloy. To further understand the
synergistic effect between Pd and Pt, XPS analysis is used to
follow the change of PdPt@PdOx/ZnO (Fig. 4b). With respect to
the Pt 4f XPS data, the spectrum can be deconvoluted into three
doublets: the most intense, with Pt 4f7/2 and 4f5/2 components
centered at 70.8 and 74.1 eV, corresponding to Pt0, and the
other two less doublets peaks can be attributed to the Pt2+ and
Pt4+ oxidation states, respectively. The result manifests that Pt
is mainly present in the metal state on the catalyst. The Pd 3d
spectrum shows two peaks at ca. 334.8 and 336.3 eV, which
could be assigned to the Pd 3d5/2 of metallic Pd and PdO,
respectively. Compared to the Pd@PdOx/ZnO nanocatalyst, the
absence of Pd4+ peak is most likely attributed to the synergy
between Pd and Pt, causing the reduction of PdO2 during
catalyst preparation. Meanwhile, a red shift of ~0.5 eV for the
Fig. 4 (a) HR-TEM image, (b) Pd 3d, Pt 4f, Zn 2p and O 1s-XPS analyses of PdPt@PdOx/ZnO
(1.0 wt% Pd, 0.8 wt% Pt); (c) TOF vs. mass fraction of metal (M = Pt, Au, Co, Ru and Rh)
for the PdM@PdOx/ZnO catalysts (t = 50 oC); (d) the H2 evolution rate vs. reaction
temperature for the dehydrogenation of HCHO catalyzed by the PdPt@PdOx/ZnO (inset
shows the Arrhenius plot). All catalytic H2 production experiments were carried out in N2
atmosphere.
The TOF curves of HCHO dehydrogenation in the presence of
PdM@PdOx/ZnO catalysts are shown in Fig. 4c. The PdM ratio is
controlled by varying the concentration of M while maintaining
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Fig.
5
(a) 1H-NMR analysis of dehydrogenation of formaldehyde catalyzed by
a constant Pd loading of 1.0 wt%. Interestingly, all of the
catalysts display a “volcano-like” catalytic behavior with the
increase of M content. As an example, PdPt@PdOx/ZnO with 0.8
wt% of Pt exhibits the optimal hydrogen evolution efficiency
(TOF = 279.1 h-1). This value is around 3 times higher than the
TOF of the original catalyst PdPt@PdOx/ZnO. The five
PdM@PdOx/ZnO nanocatalysts under investigation exhibit
different activities, following the sequence of PdPt@PdOx/ZnO
> PdCo@PdOx/ZnO > PdAu@PdOx/ZnO > PdRu@PdOx/ZnO ≈
PdRh@PdOx/ZnO, and the last two catalysts show negative
effect as compared with pure Pd@PdOx/ZnO. This order is in
agreement with the order of their work functions (Pt > Pd > Au
> Co > Rh > Ru). The work function is the energy difference
between the Fermi level of a metal and the vacuum energy
level.49 When two metals with different work functions are
connected, charge is transferred between the two metals.
Therefore, Pt with the largest work function (5.65 eV) may
significantly modify the electronic structure of Pd@PdOx/ZnO,
and serve as an excellent electron sink for hot electrons transfer
from the original catalyst. As a result, PdPt@PdOx/ZnO exhibits
the best catalytic activity, even though Pt itself on ZnO is
completely inactive for H2 production from HCHO solution.50
We also consider the effect of reaction temperature on the
dehydrogenation of HCHO solution catalyzed by
PdPt@PdOx/ZnO (Fig. 4d). As the temperature increases from
30 to 70 oC, the H2 generation rate increases rapidly, suggesting
that the elevating temperature could facilitate the hydrogen
evolution reaction. By plotting the logarithmic TOF vs. 1000/T,
we obtained the Arrhenius plot. From the slope of Fig. 4d, the
calculated activation energy for the H2 production over
PdPt@PdOx/ZnO catalysts is only 30.8 kJ/mol. The much lower
value in contrast to Pd@PdOx/ZnO (79.8 kJ/mol) indicates that
the synergy between Pt and Pd alters the reaction pathway for
dehydrogenation of HCHO solution by means of the great
modification of electronic structure of Pd@PdOx/ZnO. It also
suggests that Pt alloyed with PdNPs is able to enhance the
deprotonation of hydrated HCHO through electron-rich catalyst
surfaces, as discussed later.
View Article Online
Pd@PdOx/ZnO; (b) FT−IR spectra of Pd@PdOx/ZnO fresh and collected different reaction
DOI: 10.1039/C8TA12019F
times during HCHO solution dehydrogenation, and (c-d) in-situ Raman spectroscopy of
Pd@PdOx/ZnO catalyzed formaldehyde solution (1 M) in N2 at 50 oC in the wavenumber
range of 350−2000 cm-1.
To get insights into the reaction pathways of HCHO solution
dehydrogenation catalyzed by Pd@PdOx/ZnO under real
reaction conditions, we have undertaken H-NMR spectrum,
1
Fourier transform infrared (FT−IR) spectroscopy and in-situ
resonance Raman spectroscopy. The 1H-NMR spectrum of
aqueous HCHO solution in D2O catalyzed by Pd@PdOx/ZnO is
shown in Fig. 5a. A peak at 2.46 ppm is the protons of the CH in
HCHO, while the broad peaks between 3 and 4 ppm are
originate from the protons of CH2O group in the HO(CH2O)nH
oligomer.51, 52 The hyperfine structure of the broad peaks is
caused by the coupling between the different degrees of
polymerization of the CH2O group and becomes still more
complicated by the different possible configurations of the C−H
group. Fig. 5b shows the FT-IR spectra of fresh Pd@PdOx/ZnO
and samples after different reaction times. An obvious vibration
band at 1505 cm−1 is assigned to the C=O stretching mode,
indicating that HCHO molecules are adsorbed on catalyst
surface. The band at 1046 cm−1 attributed to the C−O bidentate
stretching vibration mode suggests that HO(CH2O)nH oligomer
adsorbs in an η2−configuration in which the carbonyl is parallel
to the surface and both the C and O atoms bond to the Pd.53
Vibrational peaks observed at 945 cm−1 can be contributed the
C−H bending mode from the HCHO or oligomer, while the band
at 738 and 706 cm−1 are assigned to the O−H bending mode.
These experiments provide strong evidence that the hydrated
HCHO actually participates in the reaction. in-situ Raman
spectra of catalytic dehydrogenation of HCHO solution recorded
at 50 °C in the spectral range of 300−2000 cm−1 are shown in
Fig. 5c-d. The band at 908 cm−1 assigned to symmetric C−O
stretching mode corresponds to the presence of the C−O−C
groups within the HO(CH2O)nH oligomer chains.54-56 A band of
high intensity is observed at 1040 cm−1 with two shoulder bands
at 1018 and 1054 cm−1. These bands can be assigned to
antisymmetric C−O stretching modes related to C−O−C groups
within the oligomer chains and C−OH end groups, respectively.
It can be clearly seen that the intensities of bands become
weaker with increasing reaction time, indicative of the gradual
decomposition of HO(CH2O)nH oligomers into H atom and OH,
leaving CH2O group as the Cannizzaro intermediate. The band
observed at 1249 cm−1 corresponds to the OH bending mode
and a diminution of the band intensity owing to decrease of
C−OH end-group abundance and water content. The similar
decrease of the intensities are also observed for the bands
located at 1487 cm−1 and 1634 cm−1, which are assigned to
δ(CH2) mode and δ(OH) mode. It indicates that CH2O further
converts into CO through breaking of C−H bond and the O−H
bond of H2O splits into H, thereby generating H2 and CO2. All of
the above results suggest the hydrated HCHO molecules
undergo direct dehydrogenation over Pd@PdOx/ZnO via
HO(CH2O)nH → (CH2O)n + OH + H process followed by
decomposition of CH2O group, (CH2O)n + OH → CO2 + H2.
3.6 DFT Calculation
3.5 Catalytic mechanism study
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atoms abstracted from HCHO and H2O combine to form H2
View Article Online
absorbed on Pd site (#8 to #9). ConfiguratiDoOnI#: 1100.1s0h3o9/wC8sTHAC12O0O19HF
absorbed on the catalyst via formation of Pd−O bonds, this
structure is built by (1) H2 desorption and (2) re-orientation of
*OH bond from [#9]. Subsequently, HCOOH is further
decomposed into H2 and CO2 (#11), the activation energy for the
TS is estimated to 263 kJ/mol. This step is considered to be the
rate determining step. Unexpected but reasonable, H2 is
dissociatively adsorbed to Pd atoms (#12). Finally, the
dissociation of adsorbed two H atoms combine to form H2 (#13),
the reaction is monotonically endothermic (−65 kJ/mol). To
conclude, it is suggested that the Pd0 site could facilitate
CH2(OH)2 adsorption and deprotonation to produce Cannizzaro
intermediates, the oxidized Pd site subsequently attacks the
anionic intermediates due to their strong electrostatic affinity
to the locally positively charged Pdx+ species, and ZnO activates
water splitting by formation of H and OH species on O and Zn
sites, respectively. The stoichiometry for the overall HCHO
dehydrogenation reactions over Pd@PdOx/ZnO is thus
suggested as follows:
Fig. 6 (a) The structures of six transition states (TS) based on Pd11(ZnO)26 unit cell
(corresponding #2, 4, 6, 8, 11 and 13, respectively), and (b) Energy changes along the
reactions occurred on Pd11(ZnO)26 unit cell.
HCHO + H2O → HO(CH2O)H
→ HCO* + *OH + 2H
→ HCOOH + H2
→ CO2 + 2H2
Density functional theory (DFT) has become a powerful tool
to predict the elementary reaction steps. In this work, the above
results suggest that ZnO promotes the partial oxidation of Pd
and forms Pd@PdOx core-shell structure as the active phase for
the catalytic reaction, thus we employ Pd11(ZnO)26 unit cell
containing Pd−O and Pd−Pd bonds as the calculation model (Fig.
S10, ESI). Energetics and structures of elementary reaction
steps are shown in Fig. S11 (ESI). In our case, ZnO(11-20) surface
rather than ZnO(0001) is adopted because of the non-polarized
face. Although in aquerous solution, methanediol is the main
form of formaldehyde, it is easily decomposed into
formaldehyde and water through the interactions with the
catalyst. So formaldehyde and water were adopted as the
starting materials of the present calculation. The whole reaction
consists of 14 configurations including 8 local minima and 6 TS
(Fig. 6a), and their optimized structures are shown in the Fig. 6b
with configuration numbers, #1 to #14. The reaction is initiated
by co-adsorption of HCHO and H2O on Pd11(ZnO)26 site. Firstly,
HCHO molecule is adsorbed on the Pd11(ZnO)26 by its O atom
combined with the Pd site and C−H bond is stretched, then
dissociated into HCO* species and H atom (#1 to #2). Following
this, dissociated species H and HCO* are optimized into the
structure shown in the Fig. S11b (ESI), and H2O is adsorbed
molecularly onto this surface complex, the adsorption energy is
calculated to be −295 kJ/mol. Specially, HCO* is adsorbed by the
η2−configuration of C and O atoms bind to two adjacent Pd
atoms (#3), which is consistent with the FT−IR observations. This
parallel configuration of HCO* is unstable. Then H2O
dissociation into *OH species and H atom adsorbed at two
adjacent palladium site (#4 to #5). *OH is combined with HCO*
on Pd11(ZnO)26 catalyst to produce HCOOH (#6), the relative
energy is below zero energy (−46 kJ/mol), and the reaction
proceeds without essential energy barrier. Next step is two H
4. Conclusions
In summary, we show the existence of a classical SMSI between
PdNPs and ZnO nanorods, which leads to the formation of
unique Pd@PdOx/ZnO core-shell structured catalysts that
exhibit superior catalytic performance for dehydrogenation of
HCHO solution in base-free medium. Based on experimental
and theoretical observations, it is found that in the hybrid
Pd@PdOx/ZnO catalyst composed of Pd0, PdOx and ZnO, where
the Pd0 shell promotes CH2(OH)2 adsorption and deprotonation
to produce Cannizzaro intermediates, the PdOx site
subsequently attacks the anionic intermediates, and ZnO
activates water splitting by formation of H and OH species.
Importantly, the SMSI between Pd and ZnO is able to change
the electronic distribution of the catalyst and enhances its Lewis
basicity, thereby realizing H2 production from HCHO solution in
base-free medium. In order to further improve the SMSI,
interface engineering of Pd@PdOx/ZnO is achieved by applying
a modified photochemical underpotential-deposition technique
to form sandwich-like ZnO/Pd@PdOx/ZnO catalyst as well as a
sequential deposition-UV-photoreduction method to form
PdPt@PdOx/ZnO catalyst. The SMSI between Pd and ZnO can be
simply optimized by changing the amount of the ZnO support
encapsulation layer, thereby regulating the dehydrogenation
reaction performance. In addition, Pt with large work function
may modify the electronic structure of Pd@PdOx/ZnO, and
serve as an excellent electron sink for hot electrons transfer
from Pd, thus reduces the reaction activation energy from 79.8
to 30.8 kJ/mol as compared with its counterpart catalyst,
suggesting that the reaction pathway for dehydrogenation of
HCHO solution is altered by alloying Pd with Pt. These
discoveries allow us to obtain new insights into the nature of
SMSI between Pd and ZnO support, and provide new interface
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Acknowledgements
We are grateful for financial support from the National Science
Foundation of China (21872123, 21503189 and 21403197) and
Zhejiang Provincial Natural Science Foundation of China
(LY18B030007 and LY15B030009).
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Interface Engineering of Palladium and Zinc Oxide Nanorods with Strong Metal-Support
Interaction for Enhanced Hydrogen Produc-tion from Base-free Formaldehyde Solution