A PdAg-CeO2 nanocomposite anchored on mesoporous carbon: A highly efficient catalyst for hydrogen production from formic acid at room temperature

Article abstract of DOI:10.1039/c9ta06987a

The efficient and selective dehydrogenation of formic acid by a robust solid catalyst at room temperature is highly attractive for a fuel cell-based hydrogen economy but still very challenging. Although significant progress has been achieved in the development of heterogeneous catalysts, their catalytic performance remains inadequate. Herein, we report a facile and surfactant-free method for the anchoring of the PdAg-CeO₂ nanocomposite (3.6 nm in diameter) onto mesoporous carbon (denoted as PdAg-CeO₂/MC). The optimized PdAg-CeO₂/MC catalyst exhibited an exceedingly high catalytic activity (turnover frequency, 2272.8 h^-1 at 303 K and 5275.5 h^-1 at 333 K) with a 100% hydrogen selectivity, among the highest catalytic performances reported to date for all heterogeneous catalysts for formic acid dehydrogenation. Systematic studies indicated that strong synergistic interactions between PdAg-CeO₂ nanocomposites and the MC host were realized due to the presence of amorphous CeO₂ with abundant oxygen vacancies for coordination; the excellent catalytic results indicated more possibilities for the effective application of formic acid as a promising hydrogen storage material.

Full text of DOI:10.1039/c9ta06987a

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Zhenhai Wen et al.
An electrochemically neutralized energy-assisted low-cost
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PdAg-CeO2 nanocomposite anchored on mesoporous carbon:
Highly efficient catalyst for hydrogen production from formic acid
at room temperature
Received 00th January 20xx,
Accepted 00th January 20xx
Zhujun Zhang,‡a Yixing Luo,‡a Shiwen Liu,b Qilu Yao,a Shaojun Qing,c and Zhang-Hui Lu*a
DOI: 10.1039/x0xx00000x
Efficient and selective dehydrogenation of formic acid by a robust solid catalyst at room temperature is highly attractive for
a fuel-cell-based hydrogen economy but still very challenging. Although great progress has been achieved in the
development of heterogeneous catalysts, their catalytic performance remains inadequate. Here, we report a facile and
surfactant-free method for the anchoring of PdAg-CeO2 nanocomposite (3.6 nm in diameter) on mesoporous carbon
(denoted as PdAg-CeO2/MC). The optimized PdAg-CeO2/MC catalyst exhibit exceedingly high catalytic activity (turnover
frequency, 2272.8 h-1 at 303 K and 5275.5 h-1 at 333 K) with a 100% hydrogen selectivity, among the highest catalytic
performance ever reported for all heterogeneous catalysts for formic acid dehydrogenation. Systematic studies implicated
that strong synergistic interaction between PdAg-CeO2 nanocomposites and MC host was realized due to the consist of
amorphous CeO2 with abundant oxygen vacancy for coordinations. The excellent catalytic results provide more possibilities
for the effective application of formic acid as a promising hydrogen storage material.
with promising catalytic performance for FA dehydrogenation under
ambient conditions is still urgently required.22
Introduction
HCOOH(l) → H2(g) + CO2(g)
HCOOH(l) → H2O(I) + CO(g)
(1)
(2)
Due to the benefits of high energy capacity and efficiency without
the environmental damage, hydrogen is generally considered as a
promising energy carrier for satisfying the ever-increasing demand
for clean and sustainable energy supply.1-3 Storing hydrogen safely
and efficiently is one of the most important issues for the emerging
hydrogen-based economy but full of challenging.4-6 Liquid hydrogen
storage materials have attracted considerable attention due to their
convenience in recharge and transport.7-10 Formic acid (FA, HCOOH),
a common liquid biomass product and contains gravimetric and
volumetric H2 capacities of 4.4 wt% and 53.4 g/L, respectively, can be
recognized as a nontoxic and inexpensive liquid hydrogen storage
material for fuel cell, specially designed for portable use.11-15 FA can
be catalytically decomposed to H2 and CO2 through dehydrogenation
(Eq. 1) or CO and H2O by dehydration (Eq. 2), which is largely
dependent on the catalysts used and catalytic conditions.16-20
Recently, selective dehydrogenation of FA for H2 generation has been
achieved via homogeneous catalysts with high activity.21
Nevertheless, the recycling of homogeneous catalysts requires the
use of special additive and organic solvents and may restrict their
practical application. Accordingly, heterogeneous catalysts, which
can be easily recycled from reaction solutions, could effectively
circumvent these problems. However, ideal heterogeneous catalysts
FA dehydration is favored on acidic oxides while the FA
dehydrogenation is preferred on basic metal oxides.23,24 As a typical
basic metallic oxide, ceria (CeO2) has attracted much attention as
catalyst host for different catalytic processes, such as CO oxidation,25
water-gas shift reactions,26 as well as dehydrogenation reactions.27
The catalytic performance improved by metal/CeO2 is often assigned
to interface properties optimization by the strong coordination
between metal and CeO2 host.28-30 However, the relatively low
surface area of the CeO2 host might limit the catalytic activity due to
the poor mass transfer efficiency. To this end, mesoporous carbon
(MC) materials with high surface area and pore structures would be
an ideal candidate to stabilize active nanoparticles and improve the
mass transfer processes.31,32 Based on the above consideration, PdAg
alloy nanoparticles,22,33-37 one of the most active catalysts for FA
dehydrogenation, modified by CeO2 and supported on MC would be
an effective way to realize optimized interface properties and mass
transfer efficiency simultaneously.
aInstitute of Advanced Materials, College of Chemistry and Chemical Engineering,
Jiangxi Normal University, Nanchang 330022, China. bJiangxi Province Center for
Disease Control and Prevention, Nanchang 330029, China. cInstitute of Coal
Chemistry, Chinese Academy of Sciences, Taiyuan 030000, China.
‡These authors contributed equally to this work. *Email: luzh@jxnu.edu.cn
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000
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Herein, we report a facile and surfactant-free method for prepare
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the amorphous CeO2-modified PdAg alloy nanocomposites on
mesoporous carbon (PdAg-CeO2/MC) as a highly active catalyst for
FA dehydrogenation. The PdAg-CeO2 nanocomposites with a mean
size of 3.6 nm were highly dispersed and well stabilized by the MC
host. Further study indicates that the abundant and strong
coordination sites provided by the small amount of amorphous CeO2
would strongly improve the synergistic effect between MC and PdAg.
As a result, the representative PdAg-CeO2/MC catalyst exhibited
exceedingly high activity for FA dehydrogenation with 100%
hydrogen selectivity and high turnover frequency (TOF) value up to
of 2272.8 h-1 at 303 K and 5275.5 h-1 at 333 K, respectively, which are
among the state-of-art values ever reported for all heterogeneous
catalysts.
Scheme 1. Schematic illustration of the synthesis of PdAg-CeO2/MC
nanocatalyst and its catalytic dehydrogenation of FA for H2 evolution.
microscope (LabRAM HR). N2 adsorption/desorption isotherms at
liquid nitrogen temperature (77 K) after degassing under vacuum at
180 °C for 8 h were measured by using BELSORP-mini II. The
morphologies, as well as compositions of the samples, were
determined by using a transmission electron microscope (TEM,
Tecnai G2 F20) equipped with an energy dispersed X-ray detector
(EDX) for elemental analysis. One or two droplets of the synthesized
catalyst suspension were deposited on a carbon-coated copper grid
for the TEM characterization. X-ray photoelectron spectra (XPS) were
measured by a Thermo scientific ESCALAB 250 multifunctional
imaging electron spectrometer calibrated by C1s at 284.6 eV.
Inductively coupled plasma atomic emission spectroscopic (ICP-AES,
725-ES, and Agilent 720ES) was adopted to detect the detail
components of the samples. Fourier-transform infrared
spectroscopy (FTIR) analysis of the samples was obtained by a
Thermo Nicolet 870 instrument. The evolved gas from FA
decomposition was analyzed by gas chromatography (Shimadzu,
GC2014C) with thermal conductivity detector (TCD) and flame
ionization detector (FID)-Methanator.
Experimental
Materials
All chemicals were commercially available and used without need
any further purification. Carbon powder (C, Strem Chemicals, 99+%),
sodium borohydride (NaBH4, J&KCHEMICAL®, 98%), cerium (III)
nitrate hexahydrate (Ce(NO3)3·6H2O, J&KCHEMICAL®, 99.5%), formic
acid (FA, HCOOH, Aladdin, 99%), sodium formate (SF, HCOONa,
J&KCHEMICAL®, 99%), hexadecyltrimethylammonium bromide-aided
(CTAB, CH3(CH2)15N(Br)(CH3)3, Fluka, ≥96.0%), silver nitrate (AgNO3,
Xiya Reagent®, 99%), and sodium tetrachloropalladate (Na2PdCl4,
Aladdin, 98%) were used as obtained. We use ordinary distilled water
as the reaction solvent.
Synthesis of catalysts
The PdAg-CeO2/MC catalysts were facilely synthesized via a
surfactant-free co-reduction of well-dispersed carbon and the metal
precursors using NaBH4 as a reducing agent. Typically, 10 mg of
carbon powder was dispersed in 3 mL H2O and under sonicated at
hot water for 1 h to fully exfoliate the carbon. Then, Na2PdCl4 (0.080
mmol), AgNO3 (0.020 mmol), and Ce(NO3)3·6H2O (0.025 mmol) were
dissolved in the above suspension. After rapid magnetic stirred for 3
h, NaBH4 (0.5 M, 2 mL) was quickly added and keep stirring for
another 1 h, resulting in the generation of Pd0.8Ag0.2-CeO2/MC
catalyst as a black suspension. The catalyst was collected by
centrifugation and washed by distilled water for three times. The
monometallic Ag- and Pd-CeO2/MC were synthesized by changing
the type of metal precursors. The PdAg-CeO2/MC with various
Catalytic decomposition of formic acid
The catalytic activity of all the synthesized catalysts toward
dehydrogenation of FA for H2 evolution was tested on a water-filled
graduated burette system. In general,
a two-necked round-
bottomed flask reactor (25 mL) contains a mixture of as-synthesized
catalyst and distilled water (6 mL) was placed in a water bath at a
certain temperature (303-353 K) under ambient atmosphere. The
released gas was measured by an exactly scaled gas burette filled
with water and connected with the reaction flask. Before the
catalytic reaction, SF was dissolved in the above solution. The
catalytic reaction is started by the injection of FA (0.11 mL) into the
above catalyst suspension. For comparison, all the catalytic reactions
were performed with a fixed molar ratio of PdAg/FA (0.033). The
volume of generated gas was measured through the gas burette and
monitored by recording the volume of displaced water in the gas
burette.
bimetallic compositions (Pd0.9Ag0.1
, Pd0.7Ag0.3, Pd0.4Ag0.4, and
Pd0.5Ag0.5) were synthesized by adjusting the initial molar ratio of Pd
and Ag precursors. The Pd0.8Ag0.2-CeO2/MC catalyst with different
compositions of CeO2 and carbon were also synthesized by changing
the initial amounts of Ce(NO3)3·6H2O and carbon. Besides, the
referenced Pd0.8Ag0.2/MC catalyst was synthesized without use
Reusability test
Ce(NO3)3·6H2O as metal precursors. The Pd0.8Ag0.2 and Pd0.8Ag0.2
-
The reusability of the Pd0.8Ag0.2-CeO2/MC catalyst was tested.
Typically, the catalyst was recollected via centrifugation and washed
with distilled H2O for three times after the first cycle of the catalytic
reaction was completed. Then, the catalyst was reused for the same
catalytic reactions. The reactions were repeated for five runs under
the same conditions as the first cycle. Then, the catalyst was
CeO2 catalysts without carbon support were synthesized via a similar
co-reduction method with the introduction of CTAB.
Characterization
Powder X-ray diffraction (XRD) patterns were collected on a Rigaku
RINT-2200 X-ray diffractometer using Cu-Kα radiation at 40 kV and
40 mA. Raman spectra were analyzed on a confocal Raman
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collected from the reaction solution by centrifugation, washed by around 3.6 nm are highly dispersed on the MC. The mesoporous
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water and dried under vacuum for further use.
structure is observed from the typical TEM images of the original MC
(Fig. S1). The high-resolution TEM image shows that the metallic
nanoclusters display a crystal lattice space of d = 0.238 nm (Fig. 1b),
which is assigned to PdAg alloy nanostructure. Amorphous species
Results and Discussion
Fig. 1. (a) TEM (inset: the corresponding SAED image and particle size histogram); (b) HRTEM; (d) HADDF-STEM; and (e) the corresponding
elemental mapping images of the optimized PdAg-CeO2/MC catalyst. TEM (inset: the corresponding SAED image and particle size
histogram) of the reference catalysts: (c) PdAg/MC; and (f) PdAg-CeO2.
The novel PdAg-CeO2/MC catalysts were synthesized via a facile can be observed in the area adjacent to the crystal. HAADF-STEM
and surfactant-free co-reduction process (Scheme 1), i.e. directly image (Fig. 1d) shows that the surface amorphous species has a
reducing the metal precursor salts, typically Na2PdCl4, AgNO3, and relatively low contrast as compared to that of the PdAg alloy
Ce(NO3)3·6H2O of various molar ratio, in an aqueous solution particles. To determine their detail compositions, elemental
contains a well-dispersed catalyst host, MC, at room temperature. mapping of the selected region from the green square in Fig. 1d was
During magnetic stirring process, metal ions can be adsorbed on the analyzed. As Fig. 1e shows, the entire catalyst surface is composed of
defects of MC due to its high surface area and hydrophilic properties. carbon and boron (Fig. 1e). Most of the boron species are oxidized
After adding of NaBH4 into the solution, the amorphous CeO2 and boron deposited on the surface of mesoporous carbon which can be
PdAg alloy formed simultaneously and served as the in situ seeds for removed during the catalytic and wash process, while only a small
the growth of PdAg-CeO2 nanocomposites on MC. Here, NaBH4 was amount of metal-boron species (<0.1 wt%) was formed in the
utilized as a reducing agent for the formation of PdAg alloy catalyst (vide infra). The Pd and Ag elements are strictly distributed
nanoparticles, and the hydrolysate (NaB(OH)4) of NaBH4 provide an in the regions where the two metallic particles are located, while the
alkaline environment for the in-situ formation of CeO2.30 Finally, a distribution and location of Ce ions are well-matched with the
very small amount of the boron ions might react with the metallic regions with low contrast on the surface of PdAg nanoparticles, thus
alloy to form metal-B species (vide infra).38,39
implying the amorphous species is CeO2. The EDX spectrum confirms
The synthesized catalysts were collected by centrifugation, the existence of Pd, Ag, Ce, O and B elements in the PdAg-CeO2/MC
washed by water and dried in a vacuum oven for further catalyst (Fig. S2). The atomic ratio for Pd:Ag:Ce was further
characterization experiments. The microstructures and compositions determined by ICP-AES analysis (Table S1), and the values are quite
of the PdAg-CeO2/MC catalyst were determined by TEM close to the concentrations of initial metal precursors. The growth of
measurements. As can be seen from the TEM and high angle annular CeO2 on the surface of PdAg alloy nanoclusters did not influence the
dark-field scanning TEM (HAADF-STEM) images of PdAg-CeO2/MC particle size and distribution of PdAg nanoparticles on MC host. (Fig.
catalyst (Fig. 1a, b, and d), the metallic nanoclusters with size of 1a and c). The PdAg-CeO2 nanocomposites synthesized without the
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MC, however, were aggregated to larger clusters (Fig. 1f). Therefore, PdAg/MC), further confirms the low crystallinity property of PdAg by
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the MC host plays an important role in controlling the particle size the modification of amorphous CeO2.
and distribution of PdAg-CeO2 nanocomposites. As determined by
In the Raman spectrum (Fig. 2b), the peaks centered at 1334 and
the corresponding selected area electron diffraction (SAED) images, 1587 cm-1 are attributed to the characteristic D- and G-band of
the CeO2-free sample (PdAg/MC) exhibits strong and bright crystal carbonaceous materials. The D-band represents the vibration modes
diffraction rings and spots, while the CeO2 doped samples (PdAg-
of carbon rings close to defects or edges. Thus, the higher intensity
Fig. 2. (a) Powder XRD patterns of the obtained MC, Pd-CeO2/MC, PdAg, PdAg/MC, PdAg-CeO2, PdAg-CeO2/MC and Pd-CeO2/MC
catalysts; (b) Raman spectrum of the MC and PdAg-CeO2/MC catalyst; (c) N2 sorption isotherms and (d) the corresponding pore size
distribution curves of the MC and PdAg-CeO2/MC catalyst; (e) Structural illumination of the obtained PdAg-CeO2/MC.
CeO2/MC and PdAg-CeO2) only exhibit weak and broad diffraction of D band of both MC and PdAg-CeO2/MC indicates the high degree
rings (Fig.1a and f), thus implying the amorphous CeO2 modifier of disorder or high concentration of defect sites.41 The abundant
would effectively decrease the crystallinity of PdAg alloy surface defects can effectively stabilize the PdAg-CeO2
nanoparticle (NPs). Due to the consists of abundant surface nanocomposites as revealed by TEM results (Fig. 1). Besides the
distortions and oxygen vacancies in the amorphous CeO2,40 the PdAg carbon signatures (D and G band), a new band at around 446 cm-1
atoms can strongly bind with these defect sites and therefore disturb was detected for the PdAg-CeO2/MC sample, which is originated
their long-range order by the ligand and strain effect around these from CeO2.42 The band value of the amorphous CeO2 is shifted to
sites.
lower energy than that of the typical first-order F2g peak of CeO2
The XRD patterns of all the synthesized catalysts and the MC host (464 cm-1), suggesting the existence of electron transfer from MC
are shown in Fig. 2a. The original MC has a broad and weak peak at host to the CeO2 dopant.42 Owing to the consist of abundant
approximately 2θ = 24°, which is associated with amorphous coordination sits in the amorphous CeO2, it can strongly coordinate
carbon.31 The characteristic diffraction peaks observed from the with PdAg alloy NPs and the defects of MC to form intimate
monometallic Ag- and Pd-CeO2/MC reference samples are assigned interfaces between them as observed from the HRTEM image (Fig.1
to the (111), (200) and (220) face of metal Ag (PDF#65-8428) and Pd c). Such intimate interfaces might further contribute to the electronic
(PDF#65-6174), respectively. All the bimetallic samples exhibit interaction between the MC host and PdAg alloy NPs.
crystalline peaks centered between the diffraction peaks of Ag and
Both MC and PdAg-CeO2/MC exhibit combined type I and type IV
Pd, thus further confirms the formation of PdAg alloy nanostructure. N2 sorption isotherms with parallel and horizontal branch loops of
The cerium component was not detected in all the CeO2-modified H4-type (Fig. 2c), indicating that they possess similar well-developed
samples due to the amorphous state as verified by TEM images (Fig. small pores with open slit-shaped pore structure. The pore size
1b and f). In particular, the diffraction peaks of PdAg alloy for the distribution curve (Fig. 2d) further demonstrated that the MC and
CeO2-modified samples (PdAg-CeO2 and PdAg-CeO2/MC) are weak PdAg-CeO2/MC catalyst both contain pores with similar pore sizes of
and broad as compared to the CeO2-free samples (PdAg and approximately 1.1 and 4.0 nm. The surface area (486 m2/g) and pore
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volume (0.48 cm3/g) of the PdAg-CeO2/MC catalyst are lower than
Surface compositions and electronic valance states of the catalysts
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that of the MC (1651 m2/g and 1.26 cm3/g), revealing that a part of were determined by XPS analyses (Fig. 3). The XPS survey spectrum
the defects or pores were occupied by the PdAg-CeO2 shows the co-existence of Pd, Ag, Ce, C, O and B in PdAg-CeO2/MC
nanocomposites as shown in the schematic diagram (Fig. 2e). This (Fig. S3), and their atomic contents calculated according to the XPS
PdAg-CeO2/MC catalyst with high surface area and abundant peak areas are 7.40, 2.70, 7.48, 19.03, 50.99 and 12.41 At.%,
mesopores are typically beneficial for mass transfer processes, which respectively. Peaks with binding energies (BEs) of 335.7 and 340.8 eV
can largely increase the reaction dynamics.
for Pd 3d5/2 and 3d3/2 of PdAg-CeO2/MC, respectively (Fig. 3a), can be
attributed to Pd0 state. And peaks centered at BEs of 367.9 and 373.9
eV in Ag 3d5/2 and 3d3/2 orbits, respectively, were assigned to Ag0
state (Fig. 3b). The XPS spectrum of PdAg-CeO2/MC catalyst shows
that there is a negative shift in BEs of Pd 3d and Ag 3d as compared
to those of PdAg/MC and PdAg-CeO2, thus implying the electron
transfer from the MC host and CeO2 to PdAg surface in the PdAg-
CeO2/MC catalyst. As the key step of the C-H bond cleavage in the FA
dehydrogenation reaction can be easier catalyzed by the modulated
Pd NPs with surface electron-rich properties.43,44 Such strong
interaction will effectively optimize the surface electronic state of
PdAg and improve its catalytic activity for FA dehydrogenation.18,45
The valance state of Ce was analyzed by XPS Ce 3d sand O 1s
spectrum (Fig. 3c,d). The peaks centered at 883.1, 901.6, and 917.0
eV are attributed to Ce4+, and the peaks at 885.5 eV, 898.7 eV, and
904.2 eV are originated from Ce3+, indicating the existence of a mixed
Ce valence state in CeO2. The consist of Ce3+ ions indicates the
formation of oxygen vacancies (OV) in CeO2 lattice, which can serve
as defects to coordinate with metal atoms as well as the MC host.46
In the XPS O 1s spectrum (Fig. 3d), the peaks centered at the BE of
532.0 and 529.9 eV are assigned to the OV and lattice oxygen (OL),
respectively. The higher peak intensity of OV further proves the
existence of abundant surface defects in CeO2 dopant.47 Such a high
concentration of defects in CeO2 would highly improve the
coordination with PdAg and MC, and finally, increase the electronic
interaction between them. The C1s XPS spectrum (Fig. S4a) can be
well-fitted with four peaks of the dominant carbon species based on
the Shirley algorithm. The main peak with BE of approximately 192.3
eV in the XPS B1s spectra is assigned to oxidized boron (B-O),48 while
another peak with BE of approximately 188.9 eV was attributed to
the metal-B species (Fig. S4B).48,49 According to XPS B 1s depth profile
(Fig. S4c), ICP-AES (TableS1), and FTIR analysis result,50 the oxidized
boron species at surface can be removed during the catalytic and
wash processes, while a very small amount of metal-B (<0.1 wt%)
remains in the catalyst, which can be neglected in contribution of the
catalytic activity.
Fig.3. XPS spectrum of (a) Pd 3d, (b) Ag 3d for the synthesized
PdAg-CeO2/MC, PdAg/MC, and PdAg-CeO2 catalysts. XPS
spectrum of (c) Ce 3d, and (d) O 1s of the synthesized PdAg-
CeO2/MC catalyst.
Hydrogen generation from FA dehydrogenation over the as-
synthesized catalysts was evaluated based on volumetric
measurements of the gases released during the reaction. Fig. 4a
shows the catalytic performance of PdAg, PdAg-CeO2, PdAg/MC and
PdAg-CeO2/MC catalysts for H2 generation from an FA-SF mixture
aqueous solution (nFA:nSF = 1:6, n(Pd +Ag)/nFA=0.033) at room
temperature (303 K). The PdAg alloy NPs exhibited a relative poor
catalytic activity with 87 mL gas (H2+CO2) released in 45 min and a
TOF value of 274.4 h-1 (Fig. 4b). The PdAg-CeO2 shows an improved
activity with 105 mL gas (H2 + CO2) released in 43 min, and reach a
TOF value of 322.0 h-1, which is 1.18 times that of pristine PdAg (Fig.
4b). As revealed by TEM and XRD results (Fig. 1 and Fig. 2a), the
crystallinity of PdAg alloy NPs can be decreased by the amorphous
CeO2 modifying, which implies that more active sites can be exposed
to improve the catalytic activity. The PdAg/MC catalyst without CeO2
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doping shows largely enhanced catalytic activity with 141 mL gas (H2
The catalytic performance of PdAg-CeO2/MC catalyst for FA
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+ CO2) generated in 18.0 min (Fig. 4a) and the corresponding TOF dehydrogenation is also influenced by the molar ratio of Pd/Ag as
value is 593 h-1, which is 2.16 times that of the pristine PdAg catalyst shown in Fig. 4c and Fig. S8. The monometallic Pd-CeO2/MC catalyst
(Fig. 4b). This enhanced catalytic is typically due to the synergistic exhibited 100% hydrogen conversion for FA dehydrogenation in 11.4
effect between MC and PdAg.51-53 In addition, the electron transfer min with a TOF value of 852 h-1, while the monometallic Ag-CeO2/MC
from MC and CeO2 to PdAg as revealed by XPS results (Fig. 3) would catalyst was almost inactive. After alloyed with a small amount of Ag,
effectively increase the catalytic activity for FA dehydrogenation. the catalytic activity of PdAg-CeO2/MC catalysts can be greatly
After introducing a small amount of CeO2, the optimized PdAg- improved. With a decrease of the Pd/Ag molar ratio from 9/1 to 8/2,
CeO2/MC catalyst exhibited greatly enhanced catalytic activity for FA the catalytic activity of the PdAg-CeO2/MC catalysts was increased
dehydrogenation (Fig. 4a and b). The reaction was completed in only with TOF from 1694.0 to 2272.8 h-1, and a further decrease of Pd/Ag
4.0 min with 142 mL of gas (H2 + CO2) generated. The TOF for H2 molar ratio from 8/2 to 5/5 resulting the decrease of activity with
evolution is calculated to be 2272.8 h-1, which to our knowledge, is TOF value from 2272.8 to 681 h-1. This synergistic effect between the
one of the state-of-art values among all the reported heterogeneous different components of Pd and Ag is ascribed to the strain and ligand
catalysts for FA dehydrogenation at 303 K (Table S2).9,11,15,18,38,43,54-63 effects induced surface electronic structure and catalytic
Interesting, the TOF value is 8.30 times that of the pristine PdAg (Fig. performance modification in alloy nanostructures.15 Additionally, the
4b), and much higher than that the sum of PdAg-CeO2 (1.17 times) contents of Ce dopant and the MC host also affect the catalytic
and PdAg/MC (2.16 times). The generated gas was only the mixture activity of the PdAg-CeO2/MC catalysts. When the PdAg-CeO2/MC
of H2 and CO2 according to the gas chromatography (GC) analysis catalyst with a Ce content of 25.2 wt% and a PdAg-CeO2 loading
results (Fig. S6 and Fig. S7), indicating the 100% H2 selectivity of the amount of 31.0 wt%, the optimized catalytic activity for FA
PdAg-CeO2/MC for FA dehydrogenation. Based on the above TEM, dehydrogenation can be realized (Fig. S9 and Fig. S10).
XRD, Raman, and XPS analysis results, the much improved catalytic
Furthermore, dehydrogenation of FA by adding SF has been
activity would be contributed by the strongly increased synergistic typically served as an effective strategy to increase H2 evolution
effect between MC and PdAg via the modification of amorphous kinetics. In this study, we have thoroughly studied the effects of SF
CeO2.
concentration on catalytic activity. Fig. 4d and Fig. S11 shows the
Fig. 4. (a) Volume of the generated gas (H2 + CO2) versus time and (b) the corresponding TOF values for FA dehydrogenation in a FA-SF
solution at 303 K over PdAg, PdAg-CeO2, PdAg/MC and PdAg-CeO2/MC catalysts, respectively (nFA/nSF = 1/6; n(Pd+Ag):nFA = 0.033); (c)
Catalytic performance of the PdAg-CeO2/MC catalysts with different Pd/Ag molar ratios for FA dehydrogenation at 303 K in a FA-SF
solution (nFA/nSF = 1/6; n(Pd+Ag):nFA = 0.033); (d) Catalytic performance of the optimized Pd0.8Ag0.2-CeO2/MC catalyst for FA
dehydrogenation at 303 K in FA-SF solutions with different FA/SF molar ratios.
catalytic performance of the optimized PdAg-CeO2/MC for FA
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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ARTICLE
dehydrogenation with different FA/SF molar ratio at 303 K. Without S13c). The corresponding activation energy can be furtVhieewr AdreticclreeOansleinde
adding SF in the system, the catalyst realized a TOF value of 431.3 h- to Ea3=23.9 kJ/mol (Fig. S13f), which is the lowest value among all the
DOI: 10.1039/C9TA06987A
1
with 105 mL gas (H2+CO2) generated in 12.8 min. After the reported heterogeneous catalysts (Table S2). The correlation
introduction of SF in the system, the catalytic activity was largely between the activation energy, FA/SF molar ratio, and TOF is
improved. The optimized FA/SF molar ratio was found to be 1/6 in summarized in Fig. 5. It is clear that the activation energy based on
the present conditions (Fig. 4d). The high surface area, abundant reaction temperature can be successfully decreased by the increase
defects and mesopores provided by the MC can promote the of SF concentration, and thus the kinetics for FA dehydrogenation
adsorption of HCOO- on the surface of the catalyst. It was reported over PdAg-CeO2/MC catalyst can be largely improved by a relative
that the adsorbed HCOO- on catalyst can induce a favourable higher SF concentration as revealed by the increased TOF values. It
adsorption orientation of FA (H-down) on the active sites to boost should be noted that the optimized SF/FA molar ratio (6/1) is
the FA dehydrogenation process or act as the FA dehydrogenation relatively lower than reported catalytic systems (3/1), which might
intermediate.55 Therefore, a relative high SF concentration is due to the abundant mesoporous structure of MC can effective
beneficial for the FA dehydrogenation over the PdAg-CeO2/MC adsorb of the SF molecules and thus further improve the FA
catalyst. The H2 generation from
concentration of pure SF solution was compared with that from an
a
relatively high or low dehydrogenation kinetics.
The reusability test of PdAg-CeO2/MC catalyst for FA
FA and SF mixture solution over the same catalyst (Fig. S12). dehydrogenation at 303 K is shown in Fig. S14. The H2 generation rate
Although a slightly higher volume of H2 generated in a high SF and production are well maintained during the test, indicating the
concentration (3 M) than in a low SF concentration (1 M), the total excellent stability of PdAg-CeO2/MC catalyst. The slightly prolonged
volume of H2 generated from SF only accounts for a very small reaction time after 5 cycles is mostly due to the loss of catalyst during
proportion of that from mixture solution during the recorded the recycling process. The recovered catalyst after the reusability
reaction time (4 min).
test was characterized by TEM (Fig. S15) and XRD (Fig. S14), showing
that there is no obvious variation in the particle size, distributions,
and crystal structure, further confirming the excellent stability of
PdAg-CeO2/MC catalyst for FA dehydrogenation under the present
conditions.
Conclusions
In summary, a facile surfactant-free co-reduction method has
been successfully applied for the synthesis of novel PdAg-CeO2/MC
catalysts with exceedingly high catalytic activity for H2 production
from FA dehydrogenation. According to TEM, XRD, Raman, and XPS
results, amorphous CeO2 modifier with abundant surface defects can
strongly coordinate with PdAg and MC host, and thus create intimate
interfaces between PdAg and MC to improve the electron transfer
from MC and CeO2 to PdAg alloy nanoparticles. As a result, the
synthesized PdAg-CeO2/MC catalyst exhibited exceedingly high
catalytic activity for FA dehydrogenation with 100% hydrogen
selectivity and TOF of 2272.8 h-1 (FA/SF = 1/6) at 303 K and 5275.5 h-1
(SF-free) at 333 K, respectively, which are among the highest values
ever reported for all heterogeneous catalysts for FA
dehydrogenation. The strategy of introducing a metal oxide modifier
to improve the synergistic effect between active metallic
nanoparticles and host is expected to improve the catalytic activity
of other supported catalysts.
Fig. 5. The correlation between activation energy, FA/SF molar
ratio and TOF value.
To understand the influence of SF concentration on the FA
dehydrogenation kinetics, the experiment of temperatures
depended FA dehydrogenation in different FA/SF molar ratios were
carried out. Fig. S13a shows the catalytic performance of the PdAg-
CeO2/MC catalyst in FA dehydrogenation without adding SF at
temperatures ranging from 303 K to 353 K. The results show that the
H2 generation rate and production are increased with the increase of
reaction temperature. The PdAg-CeO2/MC catalyst exhibited the
highest catalytic activity with a TOF value as high as 5244.8 h-1 in the
absence of SF at 353 K (Table S2). The corresponding activation
energy (Ea1) calculated based on the Arrhenius plot fitted by H2
release rates at different reaction temperatures is 40.2 kJ/mol (Fig.
S13d). Fig. S13b shows the catalytic performance of the same catalyst
with a FA/SF molar ratio of 1/3 at temperatures range from 303 K to
343 K. The rates for H2 generation are increased with the increase of
reaction temperatures and the corresponding activation energy (Ea2)
is calculated to be 35.1 kJ/mol (Fig. S13e), which is lower than that in
the case without adding additive (Fig. S13a). The catalytic activity can
be further increased under all the measured temperatures after
further increase the SF concentration (FA/SF molar ratio = 1/6) (Fig.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by National Natural Science
Foundation of China (No. 21763012 and 21463012) and the
Natural Science Foundation of Jiangxi Province of China (No.
20181BAB213005 and 20192BAB203009).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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32 X. Wei, Z. Zhang, M. Zhou, A. Zhang, W. D.VieWwuAr,ticZle. OWnlinue,
Nanoscale, 2018, 10, 16839–16847.
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DOI: 10.1039/C9TA06987A
Table of Contents
CeO2-modified PdAg alloy nanocomposites are anchored on mesoporous carbon, showing
exceedingly high catalytic activity for HCOOH dehydrogenation at room temperature.