Highly efficient dehydrogenation of formic acid over a palladium- nanoparticle-based mott-schottky photocatalyst

Article abstract of DOI:10.1002/anie.201304652

Exploiting useful contacts: The exceptional catalytic performance of a photocatalyst composed of Pd nanoparticles and mesoporous carbon nitride for the dehydrogenation of formic acid in water at room temperature to produce H ₂ gas (see picture) is due to enhanced electron enrichment of the Pd nanoparticles through charge transfer at the interface of the Mott-Schottky contact. Copyright

Full text of DOI:10.1002/anie.201304652

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DOI: 10.1002/anie.201304652
Hydrogen Generation
Highly Efficient Dehydrogenation of Formic Acid over a Palladium-
Nanoparticle-Based Mott–Schottky Photocatalyst**
Yi-Yu Cai, Xin-Hao Li,* Ya-Nan Zhang, Xiao Wei, Kai-Xue Wang, and Jie-Sheng Chen*
The high energy density of formic acid (FA) along with its low
toxicity, low cost, and high stability make it a promising
hydrogen carrier for safe hydrogen storage and hydrogen-
based green-energy applications.[1–8] FA has been obtained in
large quantities by the hydrogenation of waste carbon dioxide
from industry, biomass processing, and artificial photosyn-
thesis.[1] Thus, the catalytic dehydrogenation of FA with the
release of hydrogen gas under mild conditions can meet the
increasing demand for alternative technologies for the gen-
eration of hydrogen in a more sustainable manner. The
development of efficient catalysts remains the main hurdle for
practical applications of this approach. The dehydrogenation
of FA can be catalyzed by homogeneous metal complexes or
clusters with high hydrogen-evolution rates by the addition of
a base and/or organic ligands.[2,8] Alternatively, heterogene-
ous catalysts based on metal nanoparticles may be used. Such
catalysts are generally more recyclable and stable but less
active than their homogeneous counterparts. The ideal
sustainable catalyst should be recyclable, inexpensive, and
efficient for the dehydrogenation of FA in pure water or
without a solvent in the absence of any other additives at
ambient temperature.
In the past, much effort has been focused on the improve-
ment of catalysts based on palladium nanoparticles (Pd
NPs).[3–7] Pd NPs are the most active single-component
nanocatalysts reported so far and have the lowest effective
barrier for the dehydrogenation of FA without additives.[4]
Recently, there have been many attempts to rationally design
bimetallic[5] and trimetallic[6] nanoparticles with controlled
sizes, core–shell structures,[7] and/or optimized content of
alloying metals. Some of the bimetallic and trimetallic
catalysts exhibited much higher initial turnover frequencies
(TOFs) at relatively low temperatures. However, their
fabrication processes were rather complicated, which limited
their further practical application. Approaches for the
significant enhancement of the room-temperature activity of
single-component Pd-NP-based catalysts for the dehydrogen-
ation of FA without sacrificial additives are still rare.
Support effects can be exploited to improve the catalytic
performance of metal NPs in various catalytic reactions to
a great extent.[9] Typical catalyst supports or stabilizers,
including metal oxides,[8] activated carbon materials,[3] and
metal–organic frameworks,[10] have been used to stabilize
ultrafine Pd NPs to obtain more effective heterogeneous
catalysts for the dehydrogenation of FA. However, the
activities of previously described catalysts based on single-
component Pd NPs are not yet sufficient. In the case of
homogeneous metal-complex catalysts, the introduction of
particular organic ligands enriches the electron density of
metal active sites and thus enhances their activity for hydro-
gen generation.[2] In principle, it is also possible to enhance
the catalytic activity of the nanoparticles in heterogeneous
catalysts by the electron enrichment of Pd NPs. Nevertheless,
effective approaches for tuning the electron density of Pd
particles at the nanoscale have rarely been reported. On the
other hand, noble-metal NPs have been widely used as
cocatalysts to collect photogenerated electrons from photo-
catalysts (usually semiconductors) and enable the reduction
of water with an increased rate of H2 evolution.[11] However,
current efforts are mainly focused on electron- and hole-
induced redox reactions.[12] It is thus of great interest to
investigate the possibility of applying Mott–Schottky catalysis
to enhance the catalytic activity of electron-enriched Pd NPs
for other important organic reactions, for example, the
dehydrogenation of FA.
For the construction of effective Mott–Schottky catalysts
based on Pd NPs, a rectifying contact between the Pd NPs and
the semiconductive support is required; this contact depends
on the band structure of the semiconducting material.[13]
Among the semiconductors available for the embedding of
metal NPs, nanostructured carbon nitride (g-C3N4, also
abbreviated as CN)[14] with a band gap of 2.7 eV has proved
to be an ideal candidate, as the work function of most noble
metals is located between the conduction band and the
valence band of g-C3N4.[11a,12d] As shown in Figure 1A, metal
NPs (exemplified by Pd herein) with higher work functions
can give rise to an elevated Schottky barrier and thus enhance
the charge separation at the contacted interface of the NP and
g-C3N4. Moreover, cheap, abundant, and stable nanostruc-
tured g-C3N4 with rich in-built amino surface functional
groups outperforms its inorganic semiconductive counter-
parts in stabilizing various metal NPs.[12d]
[*] Y. Y. Cai, Prof. X. H. Li, Y. N. Zhang, Dr. X. Wei, Prof. K. X. Wang,
Prof. J. S. Chen
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
Shanghai 200240 (P. R. China)
E-mail: xinhaoli@sjtu.edu.cn
Herein, we report the application of the Mott–Schottky
catalyst based on Pd NPs and g-C3N4 (Pd@CN) in the room-
temperature dehydrogenation of FA in water. Simply through
the construction of the rectifying contact between Pd NPs and
graphitic carbon nitride, the rate of evolution of hydrogen gas
over the catalyst (Pd@CN) at room temperature is elevated
chemcj@sjtu.edu.cn
[**] This research was supported by the National Basic Research
Program of China (2013CB934102, 2011CB808703) and the
National Natural Science Foundation of China.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304652.
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Figure 1. A) Work functions of typical metals and carbon, and band
structures of carbon nitride and N-doped carbon (NC). B) Schematic
view of a Mott–Schottky-type Pd@CN contact (EF: work function; EC:
conduction band; EV: valence band).
Figure 2. Decomposition of FA over different catalysts (CN: carbon
nitride; CB: carbon black; N-LC: layered carbon; hn: irradiation with
visible light (lꢀ400 nm)). Typical conditions: 1m aqueous FA solution
(10 mL), Pd@CN-8% (20 mg), 288 K.
markedly as compared with that in the presence of the
benchmark catalyst, carbon-supported Pd NPs. The activity of
Pd@CN as a photocatalyst can be further enhanced through
irradiation with visible light.
TOF value is more than six times higher than those (2.3 and
6.7) of the carbon-supported Pd catalysts (Pd@N-LC and
Pd@CB) under the same conditions and is comparable to
those of the best catalysts at higher temperatures (usually
> 350 K). It is also distinctly higher than those of previously
reported single-component Pd-NP-based catalysts under
similar conditions (see Table S2). As the surface area, types
of nitrogen functional groups,[15] sizes of the Pd NPs, and Pd
loadings (8 wt%) of Pd@CN and Pd@N-LC were similar (see
Table S1), the highly enhanced catalytic activity of Pd@CN is
attributed to the support effect, that is, the Mott–Schottky
effect.
Mott–Schottky type Pd@CN catalysts with mesoporous
carbon nitride as the catalyst support were fabricated by
conventional wet impregnation (see the Supporting Informa-
tion for details). Since soluble polymer molecules and organic
ligands can form an insulating layer between Pd and carbon
nitride that is not favorable for the rectifying contact, organic
stabilizers were not involved in our synthetic process. Control
samples with different catalyst supports, including nitrogen-
doped layered carbon (N-LC, surface area: 190 m2 gÀ1)[15] and
carbon black (CB, surface area: 80 m2 gÀ1), were prepared
under the same conditions as used for Pd@CN; the resulting
materials are represented by Pd@N-LC and Pd@CB, respec-
tively (for detailed information, see Table S1 in the Support-
ing Information). The weight percentage of the Pd NPs
deposited on the mesoporous carbon nitride and the carbon
materials was 8% in all cases. The powder X-ray diffraction
(PXRD) patterns (see Figure S1 in the Supporting Informa-
tion) of Pd@CN and Pd@N-LC exhibited broad Pd (111)
peaks, which indicated the formation of Pd NPs with small
sizes, whereas the sharp peaks of Pd (111), (200), and (220) in
the PXRD pattern of Pd@CB suggested that the Pd NPs in
this material were larger. X-ray photoelectron spectroscopy
(XPS; see Figure S2) of all samples at the Pd 3d level
unambiguously revealed the formation of metallic palladium
on the surface of the catalyst supports. The average size of the
Pd NPs in Pd@CN and Pd@N-LC were estimated to be 3–
5 nm on the basis of analysis by transmission electron
microscopy (TEM; see Figure S3). When graphitic carbon
(exemplified herein by CB) was used as the catalyst support,
the size of Pd NPs formed in situ was as large as 10 nm.
Considering the potential applications of FA as an H2-
storage medium for practical devices (e.g. fuel cells), a sus-
tainable and cost-effective strategy for the release of H2 from
water solution of FA requires room-temperature dehydro-
genation reactions without the involvement of sacrificial
additives. To this end, we tested the activity of the Pd-NP-
based catalysts for the decomposition of formic acid (1m) in
water at room temperature. The Pd@CN sample exhibited the
highest activity for the generation of H2 gas with a TOF of
49.8 molH2 molÀ1 PdhÀ1 at 288 K (Figure 2). This excellent
The Mott–Schottky effect is revealed by the remarkably
decreased photoluminescence intensity (see Figure S4) of
carbon nitride after the introduction of Pd NPs. For a Mott–
Schottky heterojunction to be formed, electrons flow through
the metal–semiconductor interface until the Fermi level
equilibrium is reached, which results in bending of the
conduction band (Figure 1B).[13] Indeed, the conduction-
band position of carbon nitride is clearly lowered by coupling
with Pd NPs, as confirmed by electrochemical analysis (see
Figure S5).[14] Meanwhile, the band gap of carbon nitride
remains nearly the same (see Figure S6). All these observa-
tions demonstrate the formation of a Mott–Schottky hetero-
junction at the interface of carbon nitride and the Pd NPs.
Visible light can be used to further promote the catalytic
performance of Pd@CN for the dehydrogenation of FA
(Figure 2), although the Mott–Schottky effect exists at the
interface of Pd and g-C3N4 regardless of whether there is
irradiation with light or not. The TOF was elevated to
71 molH2 molÀ1 PdhÀ1 at 288 K (reaction time: 2 h) under
photoirradiation (l ꢀ 400 nm). This TOF value surpassed all
those of reported heterogeneous catalysts containing single-
component Pd NPs for the dehydrogenation of FA under
similar conditions (see Table S2). The Mott–Schottky effect
can extend the lifetime of photogenerated charge carriers by
enhancing charge separation at the interface. The decreased
photoluminescence intensity (see Figure S4) and the
increased photovoltage intensity (see Figure S6) of the
Pd@CN sample as compared with those of bare CN unambi-
gously demonstrated the enhanced charge separation at the
metal–semiconductor interface. The dehydrogenation reac-
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tion of formic acid did not consume electrons and holes.
Moreover, visible-light irradiation was not effective at all in
triggering the decomposition of FA over bare carbon nitride
(squares, Figure 2). The only way that light can change the
activity of Pd@CN is to enhance the electron enrichment of
Pd NPs (Figure 3, inset). The Schottky barrier can prevent the
reverse electron flow from the metal to the semiconductor;
catalytic activity. The volume of gas (CO2 + H2) generated in
the dehydrogenation of a 1m aqueous solution of FA (10 mL)
was plotted against the reaction time over the Pd@CN-x%
catalysts (20 mg) at 288 K with and without photoirradiation
(see Figure S11). The rate of gas evolution generally
increased as the weight percentage of Pd increased up to
8%, as more active sites for the dehydrogenation of FA were
introduced on the surface of g-C3N4. Since the surface active
sites for the construction of rectifying Mott–Schottky contacts
are limited, and too much Pd (> 10%, Figure 4A) might
partially block the active sites, Pd@CN-8% is the optimum
catalyst for maximizing the Mott–Schottky effect in promot-
ing gas evolution.
Figure 3. Dependence of the activity of Pd@CN on the irradiation
wavelength for the photocatalytic decomposition of FA. Reaction
conditions: 1m aqueous FA solution (10 mL), Pd@CN-1% (20 mg),
1 h, 288 K.
the electron enrichment of Pd NPs is thus facilitated when
more electron–hole pairs are separated at the Mott–Schottky
interface under photoirradiation. The effect of increased
electron enrichment of the Pd NPs was further reflected by
the significantly enhanced catalytic activity, whereas the
structure, composition, and surface functional groups of the
catalyst support (g-C3N4) remained the same before and after
photoirradiation. More importantly, the enhanced catalytic
performance of the Mott–Schottky catalyst is wavelength-
dependent (Figure 3). These results directly demonstrate the
key role of the Mott–Schottky effect in promoting the
catalytic activity of Pd@CN.
A dehydration reaction might also occur in the decom-
position of FA. Such a reaction would result in the release of
CO, which is unwanted for practical applications. A base trap
containing 10m NaOH was used to remove CO2 from the as-
formed gas of the catalytic reaction, and a loss of around 50%
in volume was observed (see Figure S7). No CO was detected
in either the as-formed or the base-treated gas phase by IR
spectroscopy (see Figure S8). Analysis by gas chromatogra-
phy (GC; see Figure S9) further confirmed that the gas
treated with the base trap was pure hydrogen with no CO and
thus excluded the possibility of a dehydration reaction over
Pd@CN in the current catalytic system. Therefore, the
decomposition of FA over the Mott–Schottky catalyst pro-
ceeds only through a dehydrogenation pathway.
Besides the support effect, the content of Pd NPs in the
catalyst also affects the catalytic performance. Pd@CN with
weight percentages of Pd from 1 to 17% (Pd@CN-x%, x = 1–
17) were prepared under the same conditions. The sizes of the
Pd NPs in Pd@CN-x% were all around 4 nm (see Figure S10),
which excludes a possible size effect of Pd NPs on their
Figure 4. Effect of A) the Pd loading (conditions: 1m aqueous FA
solution (10 mL), catalyst (20 mg), 4 h, 288 K) and B) the molecular
ratio of FA and Pd (conditions: 1m aqueous FA solution (10 mL),
catalyst (5–123 mg), 4 h, 288 K) on the mean gas-evolution rate in the
dehydrogenation of FA with (spheres) and without photoirradiation
(lꢀ400 nm; diamonds).
To determine the effect of the molecular ratio of FA (nFA
)
to Pd (nPd) on the catalytic performance of Pd@CN, various
amounts of the best catalyst, Pd@CN-8%, were used to
catalyze the dehydrogenation of a 1m aqueous FA solution
(10 mL) at 288 K. For heterogeneous catalytic reactions, the
reaction rate depends principally on mass transfer or diffusion
between the liquid phase and the solid phase (in this case, the
catalyst).[16] Considering the fixed concentration and thus the
unchanged diffusion rate of FA in these experiments, the
amount of the solid catalyst dominates the reaction rate.
When 122 mg of Pd@CN-8% was used, the conversion of FA
reached 100% within 300 min (see Figure S12), although the
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Received: May 29, 2013
Published online: September 23, 2013
Keywords: heterogeneous catalysis · hydrogen generation ·
.
Mott–Schottky catalysts · nanostructures · photocatalysis
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