Ultra-thin TiO2 nanosheets decorated with Pd quantum dots for high-efficiency hydrogen production from aldehyde solution

Article abstract of DOI:10.1039/c5ta08720a

Herein, we demonstrate that quantum-sized Pd dot-decorated ultra-thin anatase TiO₂ nanosheets with exposed (001) facets (2 wt%) exhibit a highly efficient catalytic activity for hydrogen generation from formaldehyde solution at room temperature

Full text of DOI:10.1039/c5ta08720a

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Ultra-thin TiO2 nanosheets decorated with Pd quantum dots for
high-efficiency hydrogen production from aldehyde solution
Received 00th January 20xx,
Accepted 00th January 20xx
Shaopeng Li, Hongyan Hu, Yingpu Bi*
DOI:
10.1039/x0xx00000xwww.rsc.org/
Herein, we demonstrate that quantum-sized Pd dots decorated ultra-thin this phenomena had received very little attention and only
anatase TiO2 nanosheets with exposed (001) facets (2 wt %) exhibit the considered as the undesired byproducts during the past one
highly efficient catalytic activity for hydrogen generation from hundred years.17-21 Recently, Ashby et al.22-25 have studied the
formaldehyde solution at room temperature, which is much higher than hydrogen evolution mechanism from alkaline formaldehyde
that of traditional Pd/TiO2 (Degussa P25) and pure Pd nanoparticles. By solutions by the isotopic composition analysis, demonstrating
further optimizing the reaction parameters, the hydrogen generation rates that one hydrogen atom originates in water and another in the
could reach up to 250 mL g-1 min-1 and keep consistent for ten hours. organic moiety. Compared with present hydrogen production
Owing to its high efficiency and stability, this hydrogen production technologies, it provides some significant advantages: it
reaction may serve as an alternate technique for supplying hydrogen in utilizes organic reagents with low costs to generate hydrogen
practical applications.
at room temperature;22 the one hydrogen atom of H2O
molecule can also be utilized; the by-products such as CO and
CO2 are not contained. However, note that the efficiency of
hydrogen generation is very low, producing only about 2 mL of
hydrogen gas even at high temperature.26 Thereby, such low
efficiency has greatly limited its practical applications.
Hydrogen energy has attracted considerable attention
due to the high energy efficiency and environmental benefits
compared with the conventional fuels such as petroleum,
natural gas, and coal, et al. However, most hydrogen
production procedures available to date cannot satisfy fully
the criteria of cost, safety, and purity in the future practical
utilizations, especially for the mobile devices. More
specifically, trace amounts of carbon monoxide presented in
the commercial scale hydrogen from steam reforming of
methane will make the platinum-based catalysts deactivation
and result in the decrease of catalytic efficiency.1-3 On the
other hand, this hydrogen generation system requires external
heat supply and high temperature, which restricts its
miniaturization, simplification for on-line applications.4-6
Recently, hydrogen generation from hydrolysis of chemical
hydrides (i.e., NaBH4) or hydrazine hydrate (N2H4.H2O) under
alkaline conditions has been extensively reported.7-11 Although
this process has many merits compared with other methods
for room temperature hydrogen generation, the high cost is
the significant barriers for its mass application. Therefore, it is
highly desirable to develop a low-cost as well as high efficient
hydrogen generation procedures.
Herein, we demonstrate that ultra-thin anatase TiO2
nanosheets(NS) with exposed (001) facets modified with only a
2 wt % of quantum-sized Pd dots exhibit the extremely high
catalytic activity and stability for the hydrogen generation
from alkaline formaldehyde at room temperature. By further
optimizing the reaction parameters such as sodium hydroxide
concentrations, formaldehyde concentrations, temperature,
the hydrogen generation rates could be further increased up
to 250 mL g-1 min-1, while only 0.3 mg palladium amount has
been practically utilized.27 Therefore, it has been considered
that this Pd/TiO2 based hydrogen generation method may
serve as an alternate hydrogen supply candidate for practical
application.
Ultra-thin anatase TiO2 nanosheets with high percentage
of the exposed (001) facets were fabricated by a modified
hydrothermal reaction. Fig. 1a shows the typical transmission
electron microscopy (TEM) images of as-prepared TiO2
nanoproducts, indicating that well-defined rectangular sheet-
structures with an average side length of ca. 50-80 nm and
thickness of about 5 nm have been obtained. The high-
resolution TEM (HRTEM) image (Fig. 1b) clearly indicates that
the lattice spacing parallel to the top and bottom facets is
0.235 nm, corresponding to the (001) planes of anatase TiO2
crystals. Furthermore, the exposure proportion of (001) planes
It is well known that during the disproportionation of
aldehydes into corresponding alcohols and carboxylic acid in
strongly alkaline medium (Cannizzaro reaction),12,13 a small
quantity of gaseous hydrogen could be produced,14-16 while
State Key Laboratory for Oxo Synthesis & Selective Oxidation, and National
Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou
Institute of Chemical Physics, CAS, Lanzhou 730000, China. E-mail:
yingpubi@licp.cas.cn.
† Electronic Supplementary Information (ESI) available: Experimental procedure,
and additional Figures. See DOI: 10.1039/x0xx00000x
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as well as morphology of TiO2 nanoproducts could be further
performed and presented in Fig. 2A. The diffraction peaks
could be indexed to anatase-phase TiO2 (JCPDS No. 78-2486),
indicating that the as synthesized products were pure anatase
TiO2. It is noteworthy that with increasing amount of HF, the
XRD peak intensities of the samples steadily increase and the
(001) peaks of the TiO2 nanosheets become enhanced. As
compared with pure TiO2 nanoplates, all the diffraction peaks
of Pd/TiO2 samples have no evident changes after the
modification process, indicating that the Pd quantum dot
deposition did not cause any damage to the crystalline TiO2
nanoplates. Moreover, note that no obvious diffraction peaks
of metallic Pd have been observed in the Pd/TiO2 nanosheets,
which may due to its low loading (2 wt%), quantum-sizes, and
well dispersion. Thereby, X-ray photoelectron spectroscopy
(XPS) was further applied to study the surface composition and
shown in Fig. 2B. All the peaks corresponding to Ti, O, Pd and C
element (in which the C element was used to calibration) can
be detected. The high-resolution Pd 3d spectra of the Pd/TiO2
samples (Fig. 2B inset) show two peaks at ca. 334.9 eV and
340.1 eV, which could be assigned to Pd 3d5/2 and Pd 3d3/2 of
metallic Pd, respectively. Furthermore, Fig. S5 shows the
energy dispersive X-ray (EDX) result of Pd/TiO2 samples. Except
for the elements of Cu and C from the fundus, all the Ti, O, and
Pd elements have been clearly observed, indicating that Pd
nanoparticles have been successfully deposited on the TiO2
nanoplates.
Fig. 1 (a) TEM images of TiO2 nanosheets, (b) HRTEM image of
TiO2 nanosheets, (c) TEM images of Pd/TiO2 nanosheets, (d)
HRTEM images of Pd nanoparticles.
rationally tailored by simply adjusting the amount of HF. As
shown in the Fig. S1b, when the amount of HF has been
reduced down to 2 mL, the thickness of as-prepared TiO2
nanosheets has been increased. Further decreasing the HF
amount to 1 mL, only octahedral bipyramid TiO2 structures
with an average side length of ca. 100 nm and width of ca. 80
nm have been synthesized. In contrast, with increasing the
amount of HF up to 6 mL, the ultra-thin TiO2 nanosheets with
an irregular morphology have been prepared (Fig.S1d). Fig. 1c
and Fig. S2 show the TEM images of the Pd quantum dots
modified ultra-thin anatase TiO2 nanosheets that were
obtained by directly reducing the H2PdCl4 with NaBH4 at room
temperature. It can be clearly seen that the Pd nanoparticles
with diameters of 2-5 nm have been successfully grown on the
ultra-thin TiO2 nanosheets. The HRTEM image (Fig. 1d) clearly
reveals that the lattice spacing of the quantum-sized Pd
nanoparticles is ca. 0.223 nm, which is consistent with the
lattice spacing of (111) plane of metallic Pd. For comparison,
the TiO2-P25 supported Pd samples and pure Pd nanoparticles
have also been prepared by the same reduction process and
shown in Fig. S3 and Fig. S4.
Fig. 3 (A) Schematic illustration of the hydrogen production
process form formaldehyde over Pd/TiO2-NS; (B) Hydrogen
production over Pd/TiO2 and Pd nanoparticles, NaOH: 1 mol/L,
HCHO: 0.6 mol/L, catalyst: 15 mg, reaction temperature: 25 oC.
Furthermore, the as-prepared Pd/TiO2 samples as well as
pure Pd nanoparticles were utilized as the catalysts in the
hydrogen production reaction from alkaline formaldehyde
solution at room temperature, and the schematic illustration
has been shown in Fig. 3A. It can be seen that in the presence
of pure TiO2 nanoplates, almost no any hydrogen could be
detected. Surprisingly, when Pd/TiO2-NS catalysts were
introduced in this reaction system, the catalytic reaction of
hydrogen production started immediately without any
induction period. It can be clearly seen from Fig. 3B that
Pd/TiO2-HF4 exhibit highest catalytic activities than pure Pd
nanoparticles and other Pd/TiO2 (Pd/TiO2-HF6, Pd/TiO2-HF2,
Pd/TiO2-HF1, Pd/TiO2-P25) samples. Moreover, as shown in
Fig. S6, the 2 wt % weight ratios between Pd to TiO2 exhibit
the best catalytic performance. Herein, it was considered that
the improved activity of Pd/TiO2-HF4 might be due to the high
specific surface area (Table S1) and high surface energies of
Fig. 2 (A) XRD patterns of TiO2 nanosheets and Pd/TiO2
nanosheets; (B) XPS spectra of Pd/TiO2 nanosheets.
To further confirm the compositions of the hybrid
nanostructure, the X-ray diffraction (XRD) pattern has been
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Fig. 5 The effect of reaction temperature on H2 production,
Pd/TiO2 catalyst: 15 mg, HCHO: 0.6 mol/L, NaOH: 1 mol/L; (B)
The calculation of activation energy for Pd/TiO2.
TiO2 (001) facets. More specifically, the unique plate-like
structure could effectively provide more absorption sites in
comparison to conventional solid particles. In contrast, the
rapid aggregation of pure Pd nanoparticles during the catalytic
reactions generally result in the decrease of the catalytic
activities.28 The above results clearly demonstrate that the
rational construction of Pd/TiO2 catalysts could serve as an
effective approach for enhancing their catalytic performances.
The effects of reaction temperature on H2 generation
rates were shown in Fig. 5A. As the temperature increased
from 10 to 25 oC
，the hydrogen generation rate increased
rapidly from 132.7 to 217.8 mL min-1 g-1, suggesting that the
increasing temperature could facilitate this hydrogen
generation reaction. Furthermore, it can be seen that the
amount of hydrogen generated is linearly dependent on the
reaction time at each temperature, which demonstrates that
this hydrogen generation reaction could be considered zero
order reaction. Thereby, the reaction rate equation can be
written as follows:
k = A exp(-E/RT), Eq.(1)
where E is the activation energy, R is the gas constant,
and T is the absolute temperature, k is the rate constant.
Following Eq.(1), lnk versus 1/T, which was plotted in Fig. 5B
from the experimental data shown in Fig. 5A. Hence, form the
slop of Fig. 5B, the calculated activation energy for the
hydrogen generation reaction over Pd/TiO2 catalysts was 18.2
kJ mol-1, which was lower than the previously reported value,
65 kJ mol-1, for non-catalytic cases.
Fig. 4 (A) The effect of NaOH concentrations on H2 production,
Pd/TiO2 catalyst: 15 mg, HCHO: 0.6 mol/L, temperature: 25 oC;
(B) The effect of HCHO concentrations on H2 production,
Pd/TiO2 catalyst: 15 mg, NaOH: 1 mol/L, temperature
：
25 oC.
Fig. 4A shows the effect of NaOH concentrations on
hydrogen generation rates. As can be seen from Fig. 4A, no
hydrogen can be produced in the absence of NaOH, while
quantitative hydrogen was production immediately when only
a small quantity of NaOH was introduced into the reaction
system, indicating that the alkaline condition is indispensable
for this catalytic process. As NaOH concentrations increased
from 0.5 to 1.0 mol L-1, the average rates of hydrogen
production obviously increased. However, further increasing
NaOH concentration up to 3 mol L-1 and 5 mol L-1, the rate of
hydrogen production decreased obviously, which may be due
to the competition with the Cannizzaro reaction for
transforming formaldehyde into the corresponding methanol
and formic acid under highly alkaline conditions. Furthermore,
the effects of HCHO concentrations on hydrogen generation
have also been studied and shown in Fig. 4B. It can be clearly
seen that the HCHO concentrations play a crucial role in
determining the hydrogen production rates, and the highest
rate of hydrogen generation was obtained at 0.6 mol L-1.
However, with increasing the HCHO concentrations (0.9 or 1.2
mol L-1), the rate of hydrogen generation slightly decreased.
Thereby, these demonstrations clearly reveal that in order to
achieve high hydrogen generation rates, both HCHO and NaOH
should be controlled in the appropriate concentrations.
Fig. 6 (A) Stability of the Pd/TiO2 catalyst and Pd nanoparticles
for the H2 production, catalyst: 15 mg, NaOH: 1 mol/L, HCHO:
o
25 C; (B) Continuity of the Pd/TiO2
0.6 mol/L, temperature
：
catalyst for the H2 production, catalyst: 15 mg, NaOH: 1 mol/L,
HCHO: 0.6 mol/L, temperature
25 oC.
：
According to the above discussions, Pd/TiO2 could serve
as a highly efficient catalyst for catalyzing hydrogen production
form formaldehyde solution at room temperature. Besides,
the catalytic stability should also be considered due to their
future practical application. Thus, the catalytic stability has
been investigated under identical reaction conditions for 600
min (shown in Fig. 6A). The average speed of the hydrogen
production has no dramatic decline and kept at about 250
mL-1 min-1 g-1. However, the hydrogen production rates of pure
Pd nanoparticles have been markedly decreased even at 30
min, which should be due to the rapid aggregation of Pd
nanoparticles during the catalytic reactions. In contrast, the
loading Pd nanoparticles on TiO2 nanosheets could efficiently
prevent agglomeration, and TiO2 nanosheets exposed with the
(001) facets could promote formaldehyde molecules adsorbed
to the active site of the catalysts. Thereby, Pd/TiO2 could
catalyze formaldehyde to produce hydrogen with excellent
catalytic activity and stability in alkaline aqueous solutions
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during a long-term reaction process. In order to further verify production system, H2 was the only gaseous product in all catalytic
that the hydrogen evolution from formaldehyde solution at experiments, and other gases such as CO or CO2 were not
room temperature, and the consecutive hydrogen generations generated. Thereby, we consider that owning to the low costs of
were shown in Fig. 6B. Under the first 50 min, a total amount reagents and the high rate of the hydrogen production, this
of 75 mL H2 is produced without noticeable deterioration of hydrogen generation system may serve as an alternate technique
the activity until the completely transformation of 2.5 mL for supplying hydrogen.
formaldehyde. More meaningful, once the formaldehyde is
continued to supply, the hydrogen evolution will start
Acknowledgements
immediately, and the rates of hydrogen production still keep
constant. Thereby, it has been considered that this hydrogen
production system may offer the potential to provide on-line
hydrogen supply.
This work was supported by the “Hundred Talents Program” of
the Chinese Academy of Science and National Natural Science
Foundation of China (21273255, 21303232, 21573264).
Notes and references
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Scheme
1 (A) Schematic illustration of the hydrogen
generation over Pd/TiO2; (B) Reaction sequences for the
hydrogen from formaldehyde aqueous solution.
6
On the basis of the experimental results, we proposed a
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hydrogen generation reaction from formaldehyde and water over
Pd catalytic active-sites could be effectively accelerated, and a high
rate of hydrogen generation has been achieved. More specifically,
Scheme 1B demonstrated the separate steps of the hydrogen
generation reaction. It has been well recognized that in aqueous
solution, formaldehyde is mostly hydrated to methylene glycol
intermediate,22-25,34 which has no relationship with the catalysts.
Subsequently, when Pd/TiO2 catalysts has been introduced in this
system, which facilitate the transform reaction of methylene glycol
intermediate under alkaline condition into hydrogen and formic
acid. However, the exact mechanism for hydrogen generation over
Pd/TiO2 catalysts cannot be completely understood until now.
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In summary, we have demonstrated a facile and efficient
strategy for facilitating the hydrogen production from
formaldehyde aqueous solution by using quantum-sized Pd dots
decorated ultra-thin anatase TiO2 nanosheets as the catalysts at
room temperature. Moreover, by further optimizing the
formaldehyde concentrations, sodium hydroxide concentrations
and reaction temperature, the highly efficient hydrogen generation
over the Pd/TiO2 catalyst could be achieved. In this hydrogen
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Journal of Materials Chemistry A
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Ultra-thin TiO2 nanosheets decorated with Pd quantum dots for
high-efficiency hydrogen production from aldehyde solution
Shaopeng Li, Hongyan Hu, Yingpu Bi*
We demonstrate that quantum-sized Pd dots decorated ultra-thin anatase TiO2 nanosheets with exposed (001)
facets exhibit the highly efficient catalytic activity for hydrogen generation from formaldehyde solution at room
temperature