DNA-directed growth of ultrafine CoAuPd nanoparticles on graphene as efficient catalysts for formic acid dehydrogenation

Article abstract of DOI:10.1039/c3cc49821b

Ultrafine and well dispersed CoAuPd nanoparticles grown on a DNA-reduced-graphene-oxide (DNA-rGO) composite have been successfully synthesized using a DNA-directed method. The resultant CoAuPd/DNA-rGO composite exhibits high activity and 100% H₂ selectivity toward the dehydrogenation of formic acid without any additive at 298 K.

Full text of DOI:10.1039/c3cc49821b

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DNA-directed growth of ultrafine CoAuPd
nanoparticles on graphene as efficient catalysts
for formic acid dehydrogenation†
Cite this: Chem. Commun., 2014,
50, 2732
Received 29th December 2013,
Accepted 16th January 2014
Zhi-Li Wang, Hong-Li Wang, Jun-Min Yan,* Yun Ping, Song-Il O, Si-Jia Li and
Qing Jiang
DOI: 10.1039/c3cc49821b
www.rsc.org/chemcomm
Ultrafine and well dispersed CoAuPd nanoparticles grown on a DNA– metal catalysts has greatly hindered their practical applications.4,5
reduced-graphene-oxide (DNA–rGO) composite have been successfully Bi/tri-metallic NPs consisting of noble metals and first-row transition
synthesized using a DNA-directed method. The resultant CoAuPd/DNA– metals may offer opportunities for reducing noble metal usage and
rGO composite exhibits high activity and 100% H2 selectivity toward the the high cost of catalysts.6 In our previous study, we first demon-
dehydrogenation of formic acid without any additive at 298 K.
strated that CoAuPd/C can be used for the dehydrogenation of FA,
while the activity was still very low to satisfy the practical require-
The safe and efficient storage and release of hydrogen (H2) are the ment of FA.6a Thus, the synthesis of a CoAuPd composite with a
widely known technological challenges in the fuel cell based H2 high activity is still challenging but very attractive.
economy.1 Recently, formic acid (HCOOH, FA), a product of biomass
Besides the catalyst composition, the support material is also an
processing and catalytic CO2 reduction, has been considered as a important factor for the catalytic properties of the catalyst.5d As a
potential H2 storage material attributed to its high gravimetric novel support material, reduced graphene oxide (rGO), a derivative
energy density, non-toxicity, and excellent stability.2 The decomposi- of graphene,7 might further boost the catalytic performance of
tion of FA proceeds by two possible reaction routes:2
CoAuPd NPs, thanks to the electronic interaction between CoAuPd
NPs and rGO. However, the poor dispersion of rGO severely limits
its applications.8a To solve this problem, biomolecules, such as
DNA, are employed to increase the dispersion of rGO-based
materials.8a Moreover, biomolecules could effectively control the
size and morphology of the NPs.8 Hence, DNA-functionalized rGO
as a catalyst support for growing CoAuPd NPs would lead to an
enhanced catalytic activity for FA dehydrogenation.
Herein we report a DNA-directed and facile approach to
synthesize the CoAuPd/DNA–rGO composite that exhibits high
activity and 100% H2 selectivity toward the FA dehydrogenation
without any additive at 298 K. The key to obtaining the excellent
catalytic properties of the CoAuPd/DNA–rGO composite is the
use of DNA to functionalize graphene oxide (GO) and further to
direct the growth of CoAuPd NPs on the rGO surface.
A two-step method is used for preparing the CoAuPd/DNA–rGO
composite, as schematically described in Scheme S1.† In the first
step, 10.0 mg of DNA is added into 30.0 mL of homogeneous GO
aqueous dispersion (1.5 mg mLÀ1) and sonicated for 2 h, followed
by heating at 368 K for 10 min. During this heating process, the
single-stranded DNA is derived from double-stranded DNA.8b The
successful synthesis of the DNA–GO composite is confirmed by
Raman spectroscopy (Fig. 1a): a G band located at 1604 cmÀ1 for GO
shifted to 1598 cmÀ1 for the DNA–GO composite,8b suggesting some
charge transfer from DNA to GO, which can be attributed to the
strong noncovalent p–p conjugation between the nucleic bases of
HCOOH (l) - H2 (g) + CO2 (g) DG298K = À35.0 KJ molÀ1
(1)
HCOOH (l) - H2O (l) + CO (g) DG298K = À14.9 KJ molÀ1
(2)
Reaction (1) is considered to be a promising CO-free H2 generation
process whereas reaction (2) producing the CO impurity, which is
toxic to fuel cell catalysts, should be strictly controlled.2 Recent studies
have demonstrated that H2 stored in FA can be released using
homogeneous catalysts.3 Compared to homogeneous catalysts,
heterogeneous catalysts are facile to be separated, controlled, and
recycled, and thus have attracted tremendous research interest.4
Although significant progress has been made in the develop-
ment of solid catalysts, such as Au, Pd, and Au/Pd-based bimetallic
nanoparticles (NPs), for FA dehydrogenation,4,5 their catalytic
activities at room temperature, especially in the absence of an
additive that may lead to severe difficulties in device fabrication,4a
are still very low. Moreover, the relatively high cost of these noble
Key Laboratory of Automobile Materials, Ministry of Education,
Department of Materials Science and Engineering, Jilin University,
Changchun 130022, China. E-mail: junminyan@jlu.edu.cn
† Electronic supplementary information (ESI) available: Experimental proce-
dures; XPS, TEM, GC, and EDX data; and the results of H2 generation from FA
experiments. See DOI: 10.1039/c3cc49821b
2732 | Chem. Commun., 2014, 50, 2732--2734
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Fig. 1 (a) Raman spectra for GO, DNA–GO, and CoAuPd/DNA–rGO; high
resolution XPS spectra of C 1s for (b) DNA–GO and (c) CoAuPd/DNA–rGO,
and (d) the high resolution XPS spectrum of N 1s for CoAuPd/DNA–rGO.
Fig. 2 TEM images of (a) CoAuPd/rGO and (b) CoAuPd/DNA–rGO; (c) the
HRTEM image of CoAuPd/DNA–rGO; (d) the XRD pattern of CoAuPd/DNA–rGO.
pattern of the CoAuPd/DNA–rGO composite (Fig. 2d) shows that all
diffraction peaks can be assigned to (111), (200), (220), and (311)
planes of the fcc structure.6a In addition, the diffraction peaks
between the fcc diffraction peaks of Au and Pd further indicate that
CoAuPd is formed as an alloy and not as a core–shell structure.5j,6a
Such a XRD phenomenon has also been observed for the other
trimetallic alloy NPs.10 The metal loading for the CoAuPd/rGO and
CoAuPd/DNA–rGO composites is determined by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES) to be 14.2 wt% and
17.5 wt%, and the atomic ratios of Co : Au : Pd are 0.22 : 0.33 : 0.45 and
0.23 : 0.32 : 0.45, respectively.
The catalytic performance of the CoAuPd/DNA–rGO composite
toward the dehydrogenation of FA is evaluated in a typical water-filled
graduated buret system6a and compared with the CoAuPd/rGO
composite and CoAuPd NPs (Fig. 3a). Clearly, the CoAuPd/DNA–
rGO composite exhibits the highest activity, with which 234 mL of gas
can be released within 210 min without any additive at 298 K, which
corresponds to a conversion of 96%. In contrast, the CoAuPd/rGO
composite shows much lower activity, with which 124 mL of gas is
generated within 400 min, and CoAuPd NPs release only 70 mL of gas
within 400 min. In addition, no gas is generated from aqueous FA
solution in the presence of DNA or the DNA–GO composite (Fig. S8†),
suggesting that DNA and DNA–GO serve as a template and support,
respectively, for the growth of CoAuPd NPs, and not as catalysts, for
DNA chains and GO.8b The X-ray photoelectron spectroscopic (XPS,
Fig. S1†) and ultraviolet-visible (Fig. S2†) results also prove the
formation of the DNA–GO composite.8b In the second step, 5.0 mL
of aqueous solution containing CoCl2 (5.0 mM), HAuCl4 (5.0 mM),
and Na2PdCl4 (5.0 mM) is mixed with the above DNA–GO suspension,
and then an aqueous NaBH4 solution (5.0 mL, 300.0 mM) is added
into the above mixture under magnetic stirring for 90 min. During
this step, GO is reduced to rGO, which is confirmed by the Raman
(Fig. 1a) and XPS results (Fig. 1b and c).9 The binding energies of
Co 2p3/2, Au 4f7/2, and Pd 3d5/2 are measured to be 778.6, 83.6, and
335.3 eV (Fig. S3†), respectively, corresponding to the metallic state of
Co, Au, and Pd.6a In addition, the C–N band (285.7 eV)8b and the
pronounced N peak (Fig. 1d) are observed in the CoAuPd/DNA–rGO
composite, suggesting the successful DNA decorations of rGO.8a–c
The morphologies of the as-prepared samples are characterized by
transmission electron microscopy (TEM). Without the use of DNA, the
NPs in the CoAuPd/rGO composite have an average of about 11.2 nm
with serious aggregation (Fig. 2a and Fig. S4a†). In strong contrast, in
the presence of DNA, well dispersed CoAuPd NPs with an average size
as small as 3.1 nm are obtained (Fig. 2b and Fig. S4b†). The drastic
size and distribution difference highlights the important role of DNA
chains in directing the growth of CoAuPd NPs, which can be
explained as follows: the PO43À groups regularly arranged on the
sugar-phosphate backbone of DNA in DNA–GO (Fig. S5†) are highly
negatively charged,8a which can attract Co2+, Au3+, and Pd2+ ions
(ClÀ ions coordinated to the Au3+ and Pd2+ ion are labile and thus are
easily removed)8b onto the surface of the DNA–GO as the initial
nucleation sites.8a–c Upon reduction by NaBH4, the DNA–GO can
anchor CoAuPd NPs and control their size and distribution during the
nucleation and growth process.8a–c In addition, the use of DNA can
promote the good dispersion of the CoAuPd/DNA–rGO composite in
water (Fig. S6†).8a The corresponding energy-dispersive X-ray (EDX)
spectrum reveals the existence of Co, Au, and Pd elements (Fig. S7†).
The lattice spacing is measured by high-resolution TEM (HRTEM) to
be 0.229 nm. This value is the distance between the {111} planes of
face-centered cubic (fcc) Au (0.235 nm) and Pd (0.224 nm),6a revealing
Fig. 3 (a) Gas generation by the decomposition of aqueous FA solution (0.5 M,
10.0 mL) versus time over different catalysts at 298 K (nmetal/nFA = 0.012);
(b) initial rates of H2 generation from aqueous FA solution (0.5 M, 10.0 mL) over
the formation of CoAuPd alloy NPs.6a,10 The X-ray diffraction (XRD) CoAuPd/DNA–rGO composites prepared with different amounts of DNA.
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this reaction. Gas chromatography (Fig. S9†) analysis shows that the of FA in aqueous solution without any additive at 298 K. This high
generated gases are H2 and CO2 with the H2 :CO2 molar ratio of performance non-noble metal containing catalyst may strongly
1.0 : 1.0, and no CO has been detected (detection limit for CO: encourage the practical application of FA as a promising H2
10 ppm, Fig. S10†). The initial TOF over the CoAuPd/DNA–rGO storage material.
composite is measured to be 85.0 mol H2 mol catalystÀ1 hÀ1 at
298 K, which is almost 1.9, 6.4, and 2.3 times higher than that of the Foundation of China (grant number 51101070), the National
CoAuPd/rGO composite (45.1 mol H2 mol catalystÀ1 À1), CoAuPd Key Basic Research, Development Program (grant number
NPs (13.3 mol H2 mol catalystÀ1 À1), and CoAuPd/C (36.8 mol H2 2010CB631001), the Program for New Century Excellent Talents
mol catalystÀ1 À1) under the same conditions, respectively. in University of the Ministry of Education of China (grant
This work was supported in part by the National Natural Science
h
h
h
Additionally, this TOF value is higher than those obtained for most number NCET-09-0431), the Jilin Province Science and Tech-
of the reported noble metal catalysts for this reaction without nology Development Program (grant number 201101061), and
additives at room temperature,4a,b,d,6a and even comparable to most the Jilin University Fundamental Research Funds.
of those obtained at elevated temperatures (usually >350 K) or/and
with the use of additives (Table S1†).5a,c,e,6b The initial rate of H2
À1
À1, corresponding
Notes and references
production is determined to be 15.2 L H2 h
g
metal
À1
to a theoretical power density of 20.5 W hÀ1 gmetal for energy
generation.5k,6a Taking an operation efficiency of 60% and a typical
energy requirement value of 0.5–2.0 W h for portable terminals,
0.3–1.2 g of the present the CoAuPd/DNA–rGO composite would be
sufficient to supply H2 for the small PEM fuel cell devices.5k, 6a
The above results clearly indicate that the use of DNA can
significantly improve the activity of the CoAuPd/DNA–rGO composite
toward FA dehydrogenation. The enhanced activity may be attributed
to the formation of the ultrafine and uniformly distributed CoAuPd
NPs on DNA–rGO. On the other hand, the change of the electronic
structure of Pd, the crucial active element for this reaction,5d,h is also
investigated by XPS. As shown in Fig. S11,† the Pd 3d peaks of the
CoAuPd/DNA–rGO composite are shifted to lower binding energies
compared to those of the CoAuPd/rGO composite, suggesting that
some electrons are transferred to Pd atoms when the CoAuPd NPs are
grown on DNA–rGO due to the strong noncovalent p–p conjugation
between the nucleic bases of DNA chains and rGO.8a This more
electron-rich Pd surface favours the rate determining step of splitting
of the C–H bond of the HCOO* intermediate to produce H2 and
CO2.4a,b,11 To investigate the effect of the amount of the DNA on the
catalytic performance, a series of the CoAuPd/DNA–rGO composites
are prepared with different amounts of DNA. It is found that the
activity of the composite increases with the increasing amount of DNA
up to 10.0 mg (Fig. 3b). However, a further increase in the amount of
DNA leads to a reduction of the activity. Moreover, upon replacing
DNA by polyvinylpyrrolidone (PVP) or hexadecyltrimethyl ammonium
bromide (CTAB), the resultant catalysts show much lower activities
than the CoAuPd/DNA–rGO composite (Fig. S12†), highlighting the
role of DNA again.
+
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The stability of the CoAuPd/DNA–rGO composite is examined by
adding an additional aliquot of FA into the reaction vessel after the
completion of the previous run. As a result, no significant loss in
activity is observed after the third run (Fig. S13†). After the catalytic
reaction, there is no significant change in the morphology of NPs,
but the size of the NPs becomes bigger (Fig. S14†). Further work to
enhance the stability of the CoAuPd/DNA–rGO for FA dehydrogena-
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2734 | Chem. Commun., 2014, 50, 2732--2734
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