Ag@Pd nanoparticles immobilized on a nitrogen-doped graphene carbon nanotube aerogel as a superb catalyst for the dehydrogenation of formic acid

Article abstract of DOI:10.1039/c7nj01108c

A nitrogen-doped graphene carbon nanotube aerogel was successfully synthesized through a hydrothermal reaction and employed as an excellent support for immobilizing silver core palladium shell nanoparticles. The catalytic activity of the as-obtained material was examined for the dehydrogenation of formic acid as one of the most important catalytic reactions due to the critical global need for green energy and great features of formic acid as a hydrogen carrier. The effect of the support and the metal composition in the catalyst structure were studied for formic acid decomposition. The turnover frequency (TOF) over the Ag@Pd/N-GCNT aerogel was measured to be 413 h^-1 at 298 K, which showed the superior activity of the presented catalyst for formic acid dehydrogenation without any additives. The activation energy of the reaction was calculated from the Arrhenius plot of lnTOF versus 1/T for the catalyst and found to be 29.28 kJ mol^-1 which is lower than most of the reported values. In addition, the recyclability tests exhibit no significant change in catalytic efficiency after four runs.

Full text of DOI:10.1039/c7nj01108c

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C7NJ01108C
Ag@Pd nanoparticles immobilized on nitrogen doped graphene carbon
nanotube aerogel as a superb catalyst for dehydrogenation of formic acid
Mohammad Reza Nabidꢀ, Yasamin Bide, Bahare Etemadi
Department of Polymer & Material Chemistry, Faculty of Chemistry & Petroleum Science,
Shahid Beheshti University, G.C., P.O. Box 1983969411 Tehran, Iran.
Abstract
Nitrogen doped graphene carbon nanotube aerogel was successfully synthesized through a
hydrothermal reaction and employed as an excellent support for immobilizing silver core
palladium shell nanoparticles. The catalytic activity of the as obtained material was examined for
dehydrogenation of formic acid as one of the most important catalytic reactions due to the
critical global need for green energy and great features of formic acid as a hydrogen carrier. The
effect of the support as well as the metal composition in the catalyst structure were studied for
formic acid decomposition. The turnover frequency (TOF) over Ag@Pd/NꢀGCNT aerogel was
measured to be 413 hꢀ1 at 298 K which showed the superior activity of the presented catalyst for
formic acid dehydrogenation without any additives. The activation energy of the reaction was
calculated by the Arrhenius plot of ln TOF versus 1/T for the catalyst to be 29.28 kJ/mol which
is lower than most of the reported values. In addition, the recyclability tests exhibit no significant
change in catalytic efficiency after four runs.
ꢀ Corresponding author: Fax: +98 21 22431661. Tel: +98 21 29903102.
Eꢀmail address: mꢀnabid@sbu.ac.ir (M. R. Nabid).
1

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Keywords: Graphene carbon nanotube aerogel; Doping; Coreꢀshell; Catalysis; Dehydrogenation;
Formic acid.
1. Introduction
Hydrogen (H2) is expected to play an important role in future renewable energy technologies
although effective hydrogen storage technologies still need to be developed.1ꢀ3 The high density
of hydrogen storage and direct conversion to electrical energy in fuel cells are some advantages
of chemical storage over physical storage.4 The usage of bioꢀrenewable formic acid (FA) as a
hydrogen carrier is attracting more and more attention owing to the several important features
including renewable chemical synthesis, nontoxicity, high stability as a liquid at room
temperature, high hydrogen content (4.4 wt%) and sustainable energy production.5, 6 But, in the
absence of suitable catalyst, the selectivity for FA decomposition to CO2 and H2 is poor due to
the formation of H2O and CO. Although the first reaction is thermodynamically favored but the
energy barrier is high. So far, a large number of both heterogeneous and homogeneous catalysts
have been studied for FA dehydrogenation.7ꢀ10 Heterogeneous catalysts have attracted so much
attention due to their advantages of operating at lower temperatures as well as facile catalyst
12
separation and recycling.11,
Among different heterogeneous catalyst systems, metal
nanoparticle (MNP) catalysts have been extensively investigated but usually suffer from severe
aggregation and limited catalytic activities.13, 14 Among various active metals, Pd,15, 16 Pt17 and
Au18, 19 based catalysts are main catalytic metals. But, using Pt without surface modification or
alloying cause to severe CO poisoning17 and for Au, subnanometric clusters are necessary to give
high
catalytic
activity20
which
limits
their
scale
up
application.
More
significantly, compared to Pt and Au, Pd is cheaper. So, Pd based catalysts are most attractive
metal based systems for hydrogen generation from FA.21ꢀ,23 It was found that bimetallic
2

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nanoparticles enhanced the activity and selectivity compared to their single component
counterparts for the dehydrogenation of FA.24 To increase the CO resistance and catalytic
performance, Ag was chosen as the secondary metal to modify Pd, owing to more significant
electronic promotion effect than other noble metals because of the largest difference in work
function between Pd (5.6 eV) and Ag (4.7 eV) as well as the charge transfer from Ag to Pd.25
The support plays a critical role on the metal particles size and dispersion and consequently their
catalytic activity.26 In 2016, Bi et al. reported the synthesis of carbonꢀsupported Pd nanoparticles
as an effective way to boost efficiency for dehydrogenation of FA due to prominent surface
electronic modulation.27 In this work, nitrogenꢀdoped graphene carbon nanotube (NꢀGCNT)
aerogel was chosen as the support to immobilize Ag@Pd core shell nanoparticles. Graphene and
carbon nanotube (CNT) have attracted much attention due to their high surface areas, enhanced
mobility of charge carriers, and high stability.28, 29 Moreover, CNT bridges the defects for
electron transfer and inhibit the restacking of graphene sheets while graphene can simultaneously
inhibit the aggregation of CNT.30 The integration of graphene and CNT into a hybrid aerogel
may enhance their dispersion and collect the fascinating properties of both materials as well as
the unique characteristics of aerogels. Furthermore, nitrogen doping can induce unique optical,
electrical and especially catalytic properties to the support.31
2. Experimental
2.1. Materials
Graphite powder (325 mesh), potassium peroxodisulfate (K2S2O8), diꢀphosphorus pentoxide
(P2O5), dicyandiamide and palladium chloride (PdCl2) were purchased from Merck Chem.
MWCNTs (dia.= 110–170 nm, length = 5–9 micron) and Silver nitrate (AgNO3) were prepared
3

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from Aldrich company. All other chemicals were purchased from Aldrich or Merck companies
and used as received without any further purification.
2.2. Instruments and characterization
Transmission electron microscopy (TEM) was performed by LEO 912AB electron microscope.
Xꢀray powder diffraction (XRD) data were collected on an XDꢀ3A diffractometer using Cu Kα
radiation. Xꢀray photoelectron spectroscopy (XPS) was performed using a VG multilab 2000
spectrometer (ThermoVG scientific) in an ultrahigh vacuum. Scanning electron microscope
(SEM) was performed on a Zeiss Supra 55 VP SEM instrument. The specific surface area was
calculated using the Brunauer–Emmett–Teller (BET) equation using a PHSꢀ1020(PHSCHINA)
device. Pore size distribution was determined by Barrett–Joyner–Halenda (BJH) method. The
sample was degassed at 150 oC for 20 h before measurements. AAꢀ680 Shimadzu (Kyoto, Japan)
flame atomic absorption spectrometer (AAS) with a deuterium background corrector was used
for determination of the metals.
2.3. Synthesis of nitrogen doped graphene carbon nanotube aerogel
Graphene oxide (GO) was prepared through a modified Hummers method.32, 33 In a typical
procedure, a certain amount of CNTs (75 mg) and 0.75 mg polyvinylpyrrolidone (PVP) were
added into 30 mL ethanol and the resulting mixture was sonicated for 5 h to form a dispersion
(2.5 mg/mL). GO and CNTs solution with mass ratio of 3:1 with total volume of 40 mL were
mixed and stirred for 1 h to form a uniform GOꢀCNTs dispersion. After that, 1.85 g (0.6 mmol)
dicyandiamide was added and this dispersion was transferred into a Teflonꢀlined stainless steel
autoclave and heated at 120 oC for 12 h to prepare grapheneꢀCNTs hybrid hydrogel. The resulted
o
hydrogel was washed with highꢀpurity milliꢀQ water for several times and freezeꢀdried (ꢀ80 C
preꢀcooling) 48 h to obtain the NꢀGCNT aerogel.
4

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2.4. Synthesis of Ag@Pd nanoparticles immobilized on nitrogen doped graphene carbon
nanotube aerogel (Ag@Pd/N-GCNT aerogel)
Typically, for the preparation of Ag@Pd 1:1 ratio, in a 100 mL round bottom flask, 0.025g (0.23
mmol) of AgNO3 and 0.34 g of NꢀGCNT aerogel were dissolved in 30 mL of ethylene glycol
o
under stirring. The solution was slowly heated (7 C/min) to 120oC under N2 atmosphere for half
an hour. A solution containing 0.06 g (0.23 mmol) of PdCl2 and 30 mL of ethylene glycol were
added to the above mixture. The resulting solution was heated (7oC/min) to 90oC for 2 h under
N2 atmosphere. The obtained Ag1@Pd1/NꢀGCNT aerogel was washed with acetone twice and
dried under N2 atmosphere. The preparations of Ag1@Pd2/NꢀGCNT aerogel, Ag2@Pd1/NꢀGCNT
aerogel, Ag/NꢀGCNT aerogel, and Pd/NꢀGCNT aerogel catalysts were synthesized with the
above procedure except that the molar ratios of Ag and Pd were 1:2, 2:1, 1:0 and 0:1,
respectively. The contents of Ag and Pd in the as obtained material were determined by AAS as
shown in Table 1.
Table 1. AAS results of Ag@Pd/NꢀGCNT aerogel catalysts.
Catalysts
Ag (wt%) Pd
(wt%)
Initial
ratio
Ag:Pd Final
ratio
Ag:Pd
7.21
13.69
9.68
6.98
1:2
1:1
2:1
1:1.90
1:0.94
2.01:1
Ag1@Pd2/N-GCNT
aerogel
10.34
14.05
Ag1@Pd1/N-GCNT
aerogel
Ag2@Pd1/N-GCNT
aerogel
2.5. Hydrogen generation from formic acid aqueous solution
An aqueous suspension containing the asꢀprepared catalyst was placed in a twoꢀneck roundꢀ
bottom flask, which was placed in a water bath under ambient atmosphere. A gas burette filled
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with water was connected to the reaction flask to measure the volume of released gas. Firstly, 3
mg catalyst and 2 mL distilled water were placed in the round bottomed flask. The reaction was
started when 1.5 mL of 1 mmol/mL aqueous solution of FA was injected into the sealed flask.
The volume of the evolved gas was monitored by recording the displacement of water in the gas
burette.
3. Results and discussion
3. 1. Synthesis and recognition of the catalyst
Morphological structure of the resulting Ag@Pd/NꢀGCNT aerogel has been verified by SEM
and TEM as shown in Figure 1. From SEM image, it is observed that CNTs can exist on
graphene wall, between two graphene sheets or in the middle of graphene sheets. Therefore,
several forms between CNTs and graphene sheets induce the surface roughness of the sheets and
consequently a cross linked structure between layers. The SEM image shows the porosity of the
structure which in accordance with BET results. In addition, the numerous nanoparticles are
observed in SEM image, but for further investigation TEM was necessary. The TEM image
indicates that hybrid aerogel is composed of thin graphene sheets and tortuous CNTs. The
Ag@Pd NPs are well dispersed and immobilized on the NꢀGCNT aerogel with particle size of 3ꢀ
12 nm. According to TEM image, the support can prevent the Ag@Pd NPs from aggregation,
which will benefit their catalytic performance. Because of almost no difference in contrast
between the Ag as core and Pd as shell elements (Ag and Pd are next to each other in the
periodic table), direct imaging of the core–shell by highꢀresolution TEM was failed.
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Fig. 1. SEM (A) and TEM (B and C) images of Ag@Pd/NꢀGCNT aerogel.
Xꢀray powder diffraction (XRD) has been used to determine the phase and structure of the
synthesized materials as shown in Figure 2. The XRD pattern of CNT shows a broad peak at
25.88o related to the (0 0 2) plane of the graphite like structure of the MWCNTs. In the XRD
pattern of GO, the peak at 10.32o is associated to the (0 0 2) plane of GO. In addition, a weak
peak is observed at 2θ=20.3° indicating the existence of oxygenꢀbased functional groups on the
surface of the graphite.34 After the synthesis of NꢀGCNT aerogel in the hydrothermal process,
the diffraction peak at 25.48o appeared demonstrating the development of graphitic structures.
The XRD pattern of Ag@Pd nanoparticles immobilized on NꢀGCNT aerogel has been presented
in Fig. 2. To clarify the structure of the catalyst, the XRD patterns of Ag@Pd, Ag and Pd
nanoparticles immobilized on NꢀGCNT aerogel were provided (See ESI, Fig. S1). In the XRD
pattern of Ag@Pd/NꢀGCNT aerogel, besides the peak of NꢀGCNT aerogel, a series of diffraction
peaks at 39.73, 46.17, 64.37, 67.47, and 76.67° are existed. These peaks can be attributed to the
combination of diffraction peaks of Pd/NꢀGCNT aerogel at 40.28, 46.82, and 68.18°
corresponded to Pd (111), (200), and (220), respectively (Fig. S1) (JCPDS 04e0802) and Ag/Nꢀ
GCNT aerogel at 38.29, 44.66, 64.66 and 77.71 assigned to Ag (111), (200), (220) and (311),
respectively (Fig. S1) (JCPDS 04e0783) with a faceꢀcentered cubic structure.35, 36 The XRD
pattern shows a Pdꢀlike pattern suggesting the Ag core Pd shell structure (Fig. S1).
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Fig. 2. XRD patterns of CNT, GO, NꢀGCNT aerogel, and Ag@Pd/NꢀGCNT aerogel.
In catalytic applications, the surface area is one of the most vital factors for a support. The Nꢀ
GCNT aerogel displays a BET surface area of 381.8 m2/g. The high porosity in the NꢀGCNT
aerogel can be attributed to the fact that some CNTs bridges between graphene sheets preventing
them from the restacking aggregation. Thus, the as obtained structure provides more voids and
cavities available for N2 adsorption. BET testing also showed a mesoporous structure (i.e., the
pore size between 2 and 50 nm, defined by the IUPAC classification) with the average pore sizes
of 15.4 nm from analysis of the adsorption branch by the Barrett–Joyner–Halenda (BJH) method.
Because of the mesoporous structure of the aerogel, the BJH method was adopted to analyze the
mesopore size distribution37 (the inset of Figure 3).
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Fig. 3. Nitrogen adsorption isotherm of NꢀGCNT aerogel. The inset shows the poreꢀsize
distribution obtained from adsorption branch using the BJH method.
To investigate the electronic structure and composition of the asꢀprepared Ag@Pd/NꢀGCNT
aerogel in detail, the XPS study was carried out. The XPS survey scan shows five elements, C,
O, N, Ag and Pd, in the sample (Fig. 4A). The presence of nitrogen with the atom percentage of
2.8% confirms the successful doping of nitrogen into graphene carbon nanotube aerogel. The
high resolution XPS C 1s spectrum (Fig. 4B) can be deconvoluted into four subpeaks, indicating
the existence of four types of carbon. The peaks at 284.6, 285.3, 285.9 and 287.3 eV are assigned
to graphiteꢀlike sp2 C (CꢀC linkage), CꢀN, CꢀO and C=O linkages, respectively. The presence of
the peak at 285.3 eV attributed to C−N confirms the substitution of the nitrogen atoms within
graphene carbon nanotube aerogel. It is noteworthy that the lower intensity of CꢀO and C=O
linkages peaks means the effective removal of oxygen containing groups after the hydrothermal
process.38 The high resolution peaks of N 1s, Pd 3d and Ag 3d are shown in Fig. S2. In the N 1s
XPS core level spectrum, four peaks are observed at 397.7, 400.0, 401.2, and 403.6 eV attributed
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to the pyridinic N, pyrrolic N, quaternary N, and oxidized N, respectively.39 XPS peaks of Pd
3d5/2 and Pd 3d3/2 are observable at 335.2 and 340.0 eV, respectively, which are consistent with
metallic Pd.40 In addition, two shoulders were also existed at binding energies of 341.8 and 336.7
eV related to oxidized Pd species. It also confirms the formation of Ag core Pd shell because
only Pd shell exposes to the air during the sample preparation. Ag 3d5/2 and Ag 3d3/2 peaks are
located at 367.6 and 374.1 eV, respectively which are in accordance with metallic Ag.40
Fig. 4. XPS spectrum of Ag@Pd/NꢀGCNT aerogel (A), and high resolution XPS spectrum of C
1s (B).
3.2. Catalytic activity
The catalytic activity of the as synthesized Ag@Pd/NꢀGCNT aerogel was investigated for
dehydrogenation of FA. We started our investigation by testing the different amounts of the
catalyst. To this purpose, 0.002, 0.003, and 0.004 g Ag@Pd/NꢀGCNT aerogels were employed
under the similar conditions. Increasing the amount of the catalyst from 2 mg to 3 mg results in
decreasing the reaction time from 48 min to 35 min with approximately complete conversion but
more increasing of the catalyst amount does not enhance the reaction significantly (Fig. 5). So, 3
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mg catalyst was chosen as the optimized catalyst amount at 25 oC for 1.5 mmol FA generating 66
mL gas in 35 min giving a TOF as high as 413 hꢀ1 (See ESI for TOF calculations).
Fig. 5. The effect of catalyst amount on gas generation (25 oC and 3 mg catalyst).
The catalytic activity of Ag@Pd nanoparticles without support were examined to evaluate the
effect of the support on the catalyst efficiency (Fig. 6). The reaction proceed until releasing 35
mL gas in 20 min indicating the decreased catalytic efficiency compared to Ag@Pd on the Nꢀ
GCNT aerogel as the support. Actually, nitrogen doping promoted the electrical conductivity and
metal immobilization by strengthening the metalꢀsupport interactions. In addition, the structure
of graphene carbon nanotube aerogel can afford additional anchoring points for the metal
nanoparticles. So, NꢀGCNT aerogel as the support enhance the catalytic activity for FA
decomposition probably due to immobilization of the nanoparticles and inhibit them from
aggregation. In addition, weakly basic amino groups may facilitate the cleavage of OH bond of
FA which is related to the rateꢀdetermining cleavage of CꢀH bond from the HCOOꢀ intermediate
to release H2.41, 42
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The catalytic activity of Ag/NꢀGCNT aerogel and Pd/NꢀGCNT aerogel were tested for
dehydrogenation of FA. According to the results, Ag@Pd showed much better activity compared
to singe metals counterparts because of the synergistic effect of two metals. As mentioned
before, adding Ag to Pd enhance the catalytic performance because of electronic promotion
effect and the charge transfer from Ag to Pd. The Ag/Pd ratio was also optimized to obtain the
best metal composition in the catalyst. The composition of Ag/Pd in Ag@Pd/NꢀGCNT aerogel
was modified by changing the designed molar ratio of AgNO3 and PdCl2, and measured using
AAS, as shown in Table 1. Fig. 6 shows the gas generation from the FA system in the presence
of Ag1@Pd2/NꢀGCNT, Ag1@Pd1/NꢀGCNT, and Ag2@Pd1/NꢀGCNT aerogels which the best
result was attributed to Ag1@Pd1.
Fig. 6. Volume of the generated gas versus time for the dehydrogenation of FA over the different
catalysts at 25 oC.
The catalytic dehydrogenation of FA was also carried out at different temperatures to obtain the
activation energy (Ea) of this reaction (Fig. 7). Increasing the reaction temperature form 25 to 45
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and 60 oC enhance TOF of the reaction from 413 hꢀ1 to 803 and 1446 hꢀ1, respectively. According
to the Arrhenius plot of ln TOF vs. 1/T for this catalyst (Fig. 7), Ea was calculated to be 29.28 kJ
molꢀ1 which is lower than most of the previously report for FA dehydrogenation (Table 2).
Fig. 7. Volume of the generated gas (CO2 + H2) versus time, Arrhenius plot (ln(TOF) vs. 1/T).
Another pathway for FA decomposition is a dehydration reaction releasing CO. To investigate
the composition of generated gas, a trap containing 10 M NaOH solution was used to complete
absorption of CO2 from the as formed gas of the catalytic reaction.43, 44 The volume of gas
reduced to half the original value suggesting a volume ratio of CO2 and H2 to be 1:1. Moreover,
the amount of CO was measured by GC spectrum which demonstrated no CO from the evolved
gas. These results exhibited the generation of only CO2 and H2 without releasing CO. Therefore,
the Ag@Pd/NꢀGCNT aerogel catalyst exhibited excellent H2 selectivity for the decomposition of
FA.
The heterogeneous nature of Ag@Pd/NꢀGCNT aerogel was investigated by removing the
catalyst from the reaction mixture at half FA conversion. Further processing of the filtrate under
the similar conditions did not increase the conversion. AAS analysis of the filtrate showed that
the content of Ag and Pd in the solution were below the detection limit. These results confirmed
the heterogeneous nature of Ag@Pd/NꢀGCNT aerogel.
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Table 2. Comparison of catalytic activities of AgPdꢀbased catalysts for dehydrogenation of
formic acid.
TOF (h-1)
Catalyst
Solvent/medium
Temp. (oC)
25
Activation
energy
(kJ/mol)
Ref.
Ag@Pd/N-GCNT Aqueous
aerogel
413a
1183b,
854.4c,
613.3d
29.28
Present
work
365c
73.6d
333c
22
ꢀ
24
45
46
47
48
49
35
50
51
CꢀAg42Pd58
AgAuPd/rGO
AgPdꢀHs/G
Aqueous
Aqueous
50
25
25
Aqueous
HCOONa
28
43.1
27
51.38
ꢀ
(Co6)Ag0.1Pd0.9/rG
O
Aqueous
HCOONa
50
2739a
453a
848b
25
80
Ag20Pd80@MILꢀ
101
Aqueous
HCOONa
AgPd@ZIFꢀ8
Aqueous
HCOONa
80
25
25
50
580b
Agglomerated Agꢀ
Pd
Aqueous
HCOONa
31.5a
105.2b
893b
Ag0.1ꢀPd0.9/rGO
Aqueous
HCOONa
ꢀ
AgPd/mCND/SB
Aꢀ15
Aqueous
HCOONa
43.2
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43
36
Ag74Pd26/graphene
Aqueous
HCOONa
25
25
572d
110d
27.12
33.9
Co1.6Ag62.2Pd36.2/gr
aphene
Aqueous
HCOONa
a TOF were calculated after the reactions completions.
b Initial TOF were calculated during the first 5 min of the reactions.
c Initial TOF were calculated during the first 10 min of the reactions.
d Initial TOF were calculated during the first 20 min of the reactions.
The sustainable catalysts which decreases the cost and required time for replacing the spent
catalyst with the fresh one are always highly desirable in chemical industries. The presence of Nꢀ
GCNT aerogel provides the possibility for easy isolation of the catalyst form the reaction mixture
which is very important for complete catalyst recovery. To investigate the catalyst recyclability,
after the reaction completion, the catalyst was isolated by filtration or centrifugation, washed and
dried to be used for the next run. The catalyst was employed for four runs without considerable
loss in activity showing the stability of the presented catalyst (Fig. 8). It is worth mentioning that
a little decrease in activity may be attributed to the increasing of the particle size.
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Fig. 8. Stability test on the Ag@Pd/NꢀGCNT aerogel catalyst in the dehydrogenation of FA at 25
oC.
4. Conclusion
In this work, Ag@Pd nanoparticles immobilized on NꢀGCNT aerogel was synthesized and
characterized by various analyses such as TEM, XPS, and BET. The high surface to volume ratio
as well the nitrogen doping made NꢀGCNT aerogel suitable to stabilize metal nanoparticles. The
low catalytic activity of support free Ag@Pd confirmed that NꢀGCNT aerogel was an excellent
support because it causes the high dispersion and relatively small size of nanoparticles.
Moreover, the enhanced catalytic activity for Ag@Pd/NꢀGCNT aerogel was observed compared
to single metals counterparts suggesting the synergistic effect of two metals for FA
dehydrogenation. The catalyst could be easily recycled by filtration or centrifugation and
showing no significant loss in activity over four runs. The mild reaction conditions, no need to an
additive, high activity, possibility of recycling, relatively low cost, and ease of applicability are
some advantages of the Ag@Pd/NꢀGCNT aerogel catalyst for FA decomposition compared to
most of the previously reports. These properties made the presented catalyst unique and suitable
for practical and sustainable applications.
Acknowledgements
The financial support provided by the Iran National Science Foundation (INSF) is gratefully
acknowledged.
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Catalytic dehydrogenation of formic acid by silver
palladium supported on nitrogen doped graphene carbon
nanotube aerogel