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Hydrogen production from formic acid

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decomposition in the liquid phase using Pd

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Cite this: DOI: 10.1039/c8se00338f

nanoparticles supported on CNFs with different
surface properties†
Felipe Sanchez,a Mohammad Hayal Alotaibi,b Davide Motta,a
Carine Edith Chan-Thaw,c Andrianelison Rakotomahevitra,c Tommaso Tabanelli, d

Alberto Roldan, *a Ceri Hammond, a Qian He,a Tom Davies,a Alberto Villa*e
and Nikolaos Dimitratos *a

The development of safe and efficient H2 generation/storage materials toward a fuel-cell-based H2

economy as a long-term solution has recently received much attention. Herein we report the
development of preformed Pd nanoparticles supported on carbon nanofibers (CNFs) via sol-
immobilisation and impregnation techniques as efficient catalysts for the liquid phase decomposition of
formic acid to H2. We used CNFs as the preferred choice of support and treated at three different
temperatures for the deposition of Pd nanoparticles. They were thoroughly characterised using XRD,
XPS, SEM-EDX, TEM, Raman spectroscopy and BET. We observed that the Pd particle size, metal
exposure and CNF graphitisation grade play an important role in catalytic performance. We found that
Pd/CNFs prepared by the sol-immobilisation method displayed higher catalytic performance than those
prepared by the impregnation method, due to the smaller Pd particles and high Pd exposure of the
Received 10th July 2018
Accepted 30th August 2018
catalysts prepared by the first method. Moreover, we have shown that the best results have been
obtained using CNFs with a high graphitisation degree (HHT). DFT studies have been performed to gain
DOI: 10.1039/c8se00338f
insights into the reactivity and decomposition of formic acid along two-reaction pathways on Pd(111),
rsc.li/sustainable-energy Pd(011) and Pd(001) surfaces.

is being focused on exploring and nding storage materials that

Introduction are able to full these requirements.
To meet the increasing energy demand without further damage The physical storage of hydrogen has been demonstrated
to the environment, alternative energy sources have been within porous networks such as carbon materials,2 metal–
considered. Amongst them, hydrogen is becoming one of the organic frameworks,3 zeolites,4 clathrate hydrates,5 and organic
most promising energy sources for the future. Its versatility lies polymers.6 However, hydrogen can also be part of a compound
in the possibility to convert it to electricity or heat through i.e. it can be chemically stored, requiring a decomposition
electrochemical and catalytic processes.1 However, due to process to release it from compounds such as, ammonia
technical obstacles for controlling both the storage and release borane,7 amines,8 sodium borohydride,9 alcohols,10 hydrous
of hydrogen, general utilisation is limited. Therefore, research hydrazine11 and formic acid.12
Formic acid is a safe and convenient liquid storage material
a
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, capable of releasing hydrogen under mild conditions. It may be
Park Place, Cardiff, CF10 3AT, UK. E-mail: DimitratosN@cardiff.ac.uk; produced during biomass upgrading, such as the hydration of
RoldanMartinezA@cardiff.ac.uk 5-HMF, as well as by direct hydrogenation of CO2. Several
b
Joint Center of Excellence in Integrated Nano-Systems, King Abdulaziz City for Science properties such as its high stability in the absence of catalysts,
and Technology, P. O. Box 6086, Riyadh 11442, Saudi Arabia
c
being a liquid under ambient conditions, its high volumetric
Institut pour la Maı̂trise de l’Énergie, Université d’Antananarivo BP 566, 101
hydrogen content (4.4 wt%), its environmentally benign nature
Antananarivo, Madagascar
d
Dipartimento di Chimica Industriale “Toso Montanari”, Alma Mater Studiorum
and nontoxicity, have promoted its importance in the eld,
Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy becoming one of the most studied and promising liquid
e
Dipartimento di Chimica, Universitá degli studi di Milano, via Golgi 19, 20133, hydrogen carriers according to the U.S. Department of Energy.
Milano, Italy. E-mail: Alberto.Villa@unimi.it The decomposition of formic acid can proceed following two
† Electronic supplementary information (ESI) available. See DOI: pathways, namely dehydrogenation (HCOOH / CO2 +H2, DG ¼
10.1039/c8se00338f

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48.4 kJ mol1) and dehydration (HCOOH / CO + H2O, DG ¼ the catalytic performance in the case of the liquid phase
28.5 kJ mol1). Acidity or high temperature during the reac- decomposition of formic acid under mild reaction conditions.
tion usually promotes the dehydration pathway. Ultrapure The CNFs used are: (1) pyrolytically stripped (PS-CNF), (2) low-
hydrogen is necessary for the generation of energy with fuel cell heat treated (LHT-CNF) and (3) high-heat treated (HHT-CNF).
devices, and since they are poisoned by CO, inlet CO concen- The choice of the described CNFs allowed us to study the
tration should remain below 20 ppm, otherwise a loss of long- effect of the degree of CNF graphitisation. Furthermore, we
term performance can occur. Mild conditions are also of key studied the effect of these CNFs on the supported Pd particles'
importance since these fuel cells are expected to supply energy morphology prepared by a sol immobilisation method (using
to portable devices with a low heat management prole. polyvinyl alcohol (PVA) as a stabiliser and NaBH4 as a reducing
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agent) and impregnation method – both commonly used

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The homogeneous or heterogeneous catalytic decomposition

of formic acid to hydrogen has thus been the topic of much preparative methodologies in academic and industrial groups.
investigation for the last ten years. However, issues such as the The characterisation of these catalysts was thoroughly per-
use of organic solvents, alongside separation problems, prevent formed by means of X-ray diffraction (XRD), X-ray photoelectron
the use of homogeneous catalysts in the device fabrication.13,14 spectroscopy (XPS), transmission electron microscopy (TEM),
Accordingly, heterogeneous catalysts have received much scanning electron microscopy (SEM) with energy dispersive X-
attention in the past few years. In this context, Pd, Au or Ag and ray (EDX), temperature-programmed desorption of ammonia
their alloys have been widely studied.3,13,15–26 Numerous types of (NH3-TPD), Raman spectroscopy and BET (Brunauer–Emmett–
materials have been used as catalyst supports for formic acid Teller) surface area analysis. The catalyst performance study
dehydrogenation, i.e. activated carbon,23–25 zeolites,26 macro- towards aqueous formic acid decomposition was carried out in
reticular resins,15 amines16 and MOFs.3,17,18 Carbon nanobers a batch reactor. Finally, periodic density functional theory (DFT)
(CNFs) and carbon nanotubes (CNTs) have also been success- calculations were employed to gain insights into the energy
fully used as supports for the synthesis of supported metal proles along different decomposition pathways on Pd(001),
nanoparticles for a wide range of important catalytic applica- Pd(011) and Pd(111) surfaces.
tions such as alcohol oxidation,27,28 nitrite reduction,29 oxygen
reduction reactions,30,31 and hydrogen generation.32,33 When
compared with other supports, CNFs present advantages such Experimental
as the possibility to tailor the microstructures of CNFs by Materials and chemicals
selecting growth techniques, the control of the surface chem- Formic acid ($95%, Cat. W248703) and succinic acid (99%, Cat.
istry by surface modication and the facile recovery of the metal S3674-100G) were obtained from Sigma Aldrich. Deionised
by burning off the support.32 Moreover, carbon nanobers water was used as the reaction solvent. CNFs PR24-PS, PR24-
present high mechanical strength, high thermal and electrical LHT and PR24-HHT were purchased from Applied Science
conductivity and good chemical stability in aggressive media.34 Company. The CNFs consist of tubular bres with an average
The carbon nanobers used in this work are produced on diameter of 80  30 nm and a specic surface area of around 50
a large scale and have applications in industry.35 m2 g1. Schlögl and co-workers previously performed thorough
In the present work, we report the effect of different types of characterisation of these materials.35 PS grade carbon nano-
CNFs as potential supports for depositing Pd nanoparticles and bers are produced by pyrolytically stripping the CNFs to
remove polyaromatic hydrocarbons covering the outer CNF
surface. LHT grade carbon nanobers are produced by treating
the CNFs at 1500  C which carbonises any chemical vapour
Dr Nikolaos Dimitratos (ND) is a Chancellor's Research Fellow at deposited carbon from the surface of the CNFs. HHT grade
the Cardiff Catalysis Institute (CCI) and leads the Advanced carbon nanobers are produced by treating the CNFs at 3000  C
Nanoparticulate Materials research group, which specialises in the creating the most graphitic carbon nanobers.
development of nanoparticulate materials for a range of chemical
applications. ND has extensive expertise in the synthesis, charac-
terisation and catalytic testing of a range of materials for gas and Preparation of the Pd/CNF catalyst series
liquid phase reactions. He has been involved in the development Immobilisation method of Pd sol. The following experi-
and management of several leading research projects (Auricat mental protocol was used for the synthesis of Pd supported
(ITN), Glycerol Challenge (TSB) and Dow Methane Challenge), nanoparticles. Na2PdCl4$2H2O (Pd: 0.094 mmol) and freshly
having spent several years in the UK (at Liverpool, Cardiff and UCL) prepared PVA solution (1 wt%) were added (PVA/Pd (wt/wt) ¼
and Italy (Milan). He is currently a principal investigator in a range 0.25) to 100 mL of H2O. Aer 3 minutes, a freshly prepared
of projects at the UK Catalysis Hub (Nanoparticle Design) and a Co- aqueous solution of NaBH4 (0.1 M, NaBH4/Pd (mol mol1) ¼ 8)
Investigator on several EPSRC research grants totalling £4.5 M. was added to the yellow-brown solution under vigorous
To date, ND has authored >100 research articles in top-tier journals magnetic stirring. A brown Pd0 sol was immediately formed.
including Science, Nature Chemistry and Angewandte Chemie, ACS The UV-visible spectrum of the palladium sol was recorded for
Nano, as well as having four international patents, and an H-index ensuring the complete reduction of the PdII precursor. Within
of 38 with more than 5000 citations and has given >40 presenta- a few minutes from its generation, the suspension was acidied
tions (12 invited, two plenaries). at pH 2 using sulfuric acid, and the support (1 g) was added

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under vigorous stirring. Aer one hour, the catalyst was ltered a Hitachi TM3030PLUS equipped with a Quantax70 energy-
and washed several times with distilled water (2 L) to remove dispersive X-ray spectroscope (EDX) to study morphology and
species, such as Na+ and Cl, and to reach neutral pH. The determine the palladium content of the samples (fresh and
samples were dried in an oven at 80  C for two hours under used). The BET surface area was determined from the N2
static air. The amount of support added was calculated to adsorption–desorption isotherms in liquid nitrogen at 77 K
obtain a nal nominal metal loading of 1 wt%. The obtained using a Quantachrome NOVA 2200e instrument. The samples
catalysts were labelled PdSI/CNF-HHT, PdSI/CNF-LHT and PdSI/ were outgassed for 3 h under vacuum at 227  C. The total
CNF-PS. surface area was determined using the BET (Brunauer–Emmett–
Impregnation followed by chemical reduction with sodium Teller) equation and the multi-point method.
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borohydride. Na2PdCl4$2H2O (Pd: 0.094 mmol) diluted in

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Catalytic decomposition of formic acid and analytical

100 mL of H2O was added to the support (1 g) and stirred methods. Liquid-phase formic acid decomposition was per-
vigorously. Aer 6 hours, a freshly prepared aqueous solution of formed in a two-necked 100 mL round-bottom ask placed in an
NaBH4 (0.1 M, NaBH4/Pd (mol mol1) ¼ 8) was added and oil bath with a reux condenser and a magnetic stirrer at
stirred for six more hours. Aer one hour, the catalyst was a preset temperature of 30  C. 10 mL of aqueous HCOOH
ltered and washed several times to ensure the removal of the solution (0.5 M) was placed in the reactor. Upon reaching the
material arising from the reduction treatment. The samples temperature, the desired amount of the catalyst was added, and
were dried at 80  C for two hours under static air. The amount of the reaction was initiated by stirring. Each reaction was per-
support was calculated to obtain a nal nominal metal loading formed at least twice or thrice to ensure reproducibility. The
of 1 wt%. The obtained catalysts were labelled PdIMP/CNF-HHT, TOF (turnover frequency number: moles of reactant converted
PdIMP/CNF-LHT and PdIMP/CNF-PS. per mol of metal per time unit) has been calculated for a suit-
Characterisation of the Pd/CNF catalyst series. X-Ray able comparison of the initial reaction rates of different cata-
diffraction (XRD) data were collected at ambient temperature lysts aer 5 minutes of reaction. The volume of gas evolved was
with a PANanalytical X'PertPRO X-ray diffractometer using Cu calculated using the ideal gas law equation:
Ka radiation and operated at 40 kV and 30 mA. X-ray diffraction
patterns were recorded between 10 and 80 2q at a step size of pV ¼ nRT.
0.017 . X-ray photoelectron spectroscopy (XPS) was performed
on a Thermo Scientic K-alpha+ spectrometer. Samples were
analysed using a monochromatic Al X-ray source operating at The initial rate is expressed as the volume produced within
72 W (6 mA  12 kV), with the signal averaged over an oval- the rst 5 minutes of reaction.
shaped area of approximately 600  400 microns. Data were An approximation of the reaction order was achieved by
recorded at pass energies of 150 eV for survey scans and 40 eV representing the rate of gas formation versus the initial
for high-resolution scans with a 1 eV and 0.1 eV step size concentration of formic acid, and tting the data to a power-law
respectively. Charge neutralisation of the sample was achieved model equation:
using a combination of both low energy electrons and argon
r ¼ kCn
ions (less than 1 eV) which gave a C(1s) binding energy of
284.8 eV. The envelopes were tted aer subtraction of a Shirley where r is the reaction rate, k is the kinetic coefficient, C is the
background36 using CasaXPS (v2.3.17 PR1.1) with Scoeld initial formic acid concentration, and n is the reaction order.
sensitivity factors and an energy exponent of 0.6. Raman Product analysis. HPLC (High Performance Liquid Chro-
spectroscopy was performed with a Renishaw inVia Raman matography) was used to calculate the concentration of formic
microscope for analysing the graphitisation degree of the acid and therefore the conversion of formic acid during reaction
carbon nanobers. Bare supports, alongside fresh and used progress. Liquid samples of the reaction mixture were periodi-
catalysts, were analysed. Typically, a sample of approximately cally withdrawn, diluted to a 1 : 100 volumetric ratio with
0.01 g was placed on a metal slide inside the spectrometer. The deionised water and analysed by using a HPLC model Agilent
powder was analysed under an Ar+ laser (514 nm) with an 1220 Innity LC using a column MetaCarb 87H 250  4.6 mm,
incident laser power of 25 mW. The sample was scanned at an Agilent, at 60  C and a ow rate of 0.4 mL min1. The instru-
attenuation time of 22 seconds, and 10 scans were carried out to ment is equipped with a Variable Wavelength (VW) Detector
obtain a spectrum. Particle size distributions and mean particle pre-set at 210 nm. The eluent was an aqueous solution of
sizes were obtained using transmission electron microscopy phosphoric acid (0.1 wt%). Succinic acid was used as an external
(TEM) using a JEOL JEM 2100 TEM operating at 200 kV. Samples standard for the quantication of formic acid.
for examination were prepared by dispersing the catalyst in Gas analysis. Using a gas burette as in the water displace-
high purity ethanol. A drop of the suspension was allowed to ment method, the gaseous effluent evolved from formic acid
evaporate on a holey carbon lm supported by a 300-mesh decomposition was collected. Analysis for the detection of H2
copper TEM grid. The samples were subjected to bright eld- was carried out by using a GC-TCD (Gas Chromatography-
diffraction contrast imaging experiments. The mean particle Thermal Conductivity Detector CP-3380) equipped with a Por-
sizes and particle size distributions were determined by apak Q 6 m  1/800 2.0 mm 80/100 SS column, and using Ar as
measuring the size of over 200 particles from different areas. a carrier gas. CO and CO2 were quantied using a Varian 450-GC
Scanning electron microscope (SEM) images were taken on

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tted with a CP-Sil 5CB capillary column (50 m length, 0.32 mm surface properties of the material. The most visible feature in
diameter, and carrier gas: He), a methanator unit and both FID the X-ray diffraction patterns of the fresh series of catalysts is
and TCD detectors with a detection limit of CO below 5 ppm. the diffraction peak at approximately 26 , assigned to the
The gases were quantied using calibration curves created from presence of the (002) plane of graphitic carbon (Fig. 1).35 The
commercial standards (BOC gases). intensity of the corresponding diffraction peak increases during
Computational methods. Periodic plane-wave DFT calcula- the heat treatment process since the utilisation of high-
tions were performed using the Vienna ab-initio simulation temperature post-treatment signicantly enhances the
package (VASP),37,38 the Perdew–Burke–Ernzerhof functional graphitic character of carbon materials and therefore, the
revised for solids39 and a kinetic energy of 500 eV to expand the absence of amorphous carbon. The intensity of this character-
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plane-waves of the Kohn–Sham valence states.40 The inner istic diffraction peak is higher for the CNF-HHT samples.
electrons were represented by the projector-augmented wave In Fig. 2, the XRD patterns of the series of catalysts are
(PAW) pseudopotentials also considering non-spherical contri- presented in more detail. Between 42 and 46 , there is a broad
butions from the gradient corrections.41 All the calculations diffraction peak attributed to either (100) or (101) planes of C.
include the long-range dispersion correction approach by However since both hexagonal and rhombohedral graphite
Grimme (D3),42,43 which is an improvement on pure DFT when peaks are present in this region, it is difficult to relate speci-
considering large polarisable atoms.44–49 We included a self- cally both species and diffraction peaks.35
consistent aqueous implicit solvation model.50,51 The optimi- The two catalysts supported on CNF-HHT present intense
sation thresholds were 105 eV and 0.03 eV Å1 for electronic and sharp diffraction peaks at 54 and 78 . They are also
and ionic relaxation, respectively. The Brillouin zone was observed in the other catalysts albeit to a much lesser extent.
sampled with a G-centred k-point mesh generated through The diffraction peaks at 78 correspond to the graphite (110)
a Monkhorst–Pack grid of 5  5  1 k-points, which ensures the plane conrming the presence of rhombohedral graphite.35 The
electronic and ionic convergence.52 In order to improve the assignment of the diffraction peak at 54 is not straightforward
convergence of the Brillouin-zone integrations, the partial since both graphite (004) and PdO (112) planes could be
occupancies were determined using the rst order Methfessel– assigned to the same position.35,56 To clarify this argument, the
Paxton method corrections smearing with a set width for all XRD patterns of the bare supports were analysed, as shown in
calculations of 0.2 eV. Open shell calculations were tested Fig. S1.† It conrms the presence of the graphite (004) plane;
leading to close shell results.
The Pd bulk lattice parameter is 3.893 Å (ref. 53) which is in
very good agreement with the one resulting from our cell opti-
misation (3.836 Å). We have modelled low-Miller index surfaces,
where due to crystal symmetry, we have reduced the number of
surfaces to the {111}, {011} (consisting of the equivalent (011),
(101), and (110) faces), and {001} (which includes the equivalent
(001), (010), and (100) faces). All have a coordination number of
12. We believe that this analysis will provide a deeper under-
standing since most of the literature focus on Pd(111) only.54,55 Fig. 1 XRD patterns of fresh Pd/CNFs. (A) Catalysts synthesised by
These surfaces are simulated by using a slab model containing 5 impregnation. (B) Catalysts synthesised by sol-immobilisation. (a)
atomic layers; the two uppermost layers were relaxed without CNF-HHT, (b) CNF-LHT and (c) CNF-PS.
symmetry restrictions, and the bottom ones were frozen at the
bulk lattice parameter. The slab contains 45 atoms per unit cell
exposing an area of 66.217, 93.645 and 66.217 Å2 for (111), (011)
and (001) respectively. We added a vacuum width of 15 Å
between vertically repeated slabs, to avoid the interaction
between them.
We dened the binding energy (EB) as the difference between
the combined system and the isolated species, and the reaction
energy (ER) of each step is calculated as the total energy differ-
ence between the nal state (product(s)) and the initial state
(reactant(s)). Thus, negative values of EB and ER indicate
favourable adsorption and reaction respectively.

Results and discussion

Catalyst characterisation
The catalysts were initially characterised by X-ray diffraction Fig. 2 XRD patterns of fresh Pd/CNF: (a) PdIMP/CNF-HHT, (b) PdIMP/
(XRD), Raman spectroscopy and X-ray photoelectron spectros- CNF-LHT, (c) PdIMP/CNF-PS, (d) PdSI/CNF-HHT, (e) PdSI/CNF-LHT, and
copy (XPS) to determine the graphitisation degree and thus the (f) PdSI/CNF-PS.

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however, this peak shows a lower intensity compared with the Table 2 Atomic content of sp2 and sp3 carbon and sp2/sp3 ratio from
XRD pattern of the Pd/CNFs. Therefore, we can suppose that the XPS and ID/IG ratio from Raman for the bare supports and the catalysts
subjected to different temperature heat treatments
PdO(112) plane is overlaid by the graphite (004) plane and these
results indicate the presence of PdO species. The characteristic ID/IG
planes (111), (200) and (220) of the face-centered cubic structure
of Pd56,57 are assigned to the reections at 2q ¼ 40.4 , 44.9 and Catalyst C sp2 (%) C sp3 (%) sp2/sp3 Fresh Used
68.3 (grey lines in Fig. 2) and are present only in some catalysts
CNF-HHT — — — 0.11 —
as is evident from the XRD patterns. It is well known that one of CNF-LHT — — — 0.71 —
the limitations of the XRD technique is its sensitivity to small CNF-PS — — — 0.75 —
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crystallite sizes, with crystallite sizes lower than 5 nm being PdIMP/CNF-HHT 82.10 7.50 10.95 0.26 0.21
below the detection limit of the apparatus.33 Therefore, the PdIMP/CNF-LHT 72.01 17.43 4.13 0.78 0.73
PdIMP/CNF-PS 66.73 22.37 2.98 0.67 0.57
absence of these patterns for the catalysts synthesised by sol-
PdSI/CNF-HHT 81.87 8.11 10.09 0.08 0.23
immobilisation could be attributed to a particle size below 5 PdSI/CNF-LHT 70.44 19.05 3.70 0.80 0.90
mm. The crystallinity will be subsequently studied by means of PdSI/CNF-PS 65.80 23.12 2.85 0.78 0.71
TEM. On the other hand, for the catalysts synthesised by
impregnation, it is easier to observe the presence of metallic Pd
and PdO species, which means that the particles could be
larger. The presence and percentage of metallic Pd and PdII
species have been studied by XPS. Note that the diffraction peak
at 36 only appears for the catalyst whose support has been
treated at the lowest temperature; therefore, the most plausible
reason is the presence of an unidentied contaminant. The
XRD patterns of the used samples are displayed in Fig. S2.† The
appearance and increment of the reections at 2q ¼ 40.4 and
44.9 ascribed to the (111) and (200) planes of metallic Pd
indicate (i) the progressive reduction of PdO to metallic Pd due
to H2 generation and (ii) the increase in particle size due to Fig. 3 XPS spectra of fresh Pd/CNF. (A) Catalysts synthesised by
agglomeration. impregnation. (B) Catalysts synthesised by sol-immobilisation. (a)
XPS analysis of the as-synthesised and selected used samples CNF-HHT, (b) CNF-LHT and (c) CNF-PS.
was carried out to determine the electronic states of the samples
(Tables 1 and 2). The XPS spectra of Pd 3d of the as-synthesised
fresh catalysts are presented in Fig. 3 for impregnation and sol- content of Pd0 species indicating that the presence of the PVA
immobilisation methods, respectively. Pd and O content and ligand may inhibit the oxidation of the metallic Pd surface in
the percentage of Pd0 for both fresh and used series derived ambient air. This feature has a signicant impact on the catalyst
from XPS are shown in Table 1. activity as will be discussed later.
The Pd 3d5/2 component at 335 eV approximately was No apparent trend can be extracted for the variations in Pd
assigned to metallic Pd58 and the component at approximately surface content or the percentage of Pd0 according to modi-
337 eV to PdII mainly present as PdO.59 As presented in Table 1, cations in the morphology through temperature treatment. XPS
the impregnated samples display, in general, a lower Pd atomic analyses of the used catalysts are presented in Fig. S3.† Table 1
percentage (0.40–0.71%) in contrast with the sol- shows a reduction in the atomic Pd content on the CNF surface.
immobilisation prepared samples (0.90–1.44%). From Fig. 3 Since XPS is surface sensitive, the decrease of Pd content for the
and Table 1 it is evident that the majority of the catalysts used catalysts could be explained by (i) Pd leaching during the
prepared by the sol-immobilisation method display a higher reaction, (ii) migration of the nanoparticles to the inner wall of
the nanobers and (iii) increase of the Pd mean particle size.
Table 1 displays the atomic percentage of Pd0 of the used
Table 1 Palladium content, (%) Pd0 on the surface and O content from
catalysts, and it is clear that there is a signicant increase of
XPS data for the different catalysts studied, A: Fresh, and B: Used
metallic Pd aer the reaction, suggesting that the hydrogen
Pd content (%) Pd0 on the O content released during the decomposition of HCOOH can facilitate the
(at%) surface (at%) reduction of PdII to Pd0 species. These results are in agreement
with the appearance or increase of the intensity of Pd0 diffrac-
Catalyst Fresh Used Fresh Used Fresh Used
tion peaks in the XRD patterns of the used catalysts (Fig. S2†).
PdIMP/CNF-HHT 0.71 0.52 30.1 74.1 0.94 1.40 XPS has been used to measure the relative concentration of
PdIMP/CNF-LHT 0.57 0.49 55.0 73.7 2.60 2.94 sp3 and sp2 hybridisation types from the deconvolution of the C
PdIMP/CNF-PS 0.40 0.23 45.1 61.2 2.94 6.34 1s region. The C 1s XPS spectra of the fresh and used series of
PdSI/CNF-HHT 0.93 0.72 51.9 73.1 2.72 4.05
catalysts are presented in Fig. S4.† Table 2 displays the
PdSI/CNF-LHT 1.44 0.98 53.1 69.8 4.49 3.49
PdSI/CNF-PS 0.90 0.04 53.7 84.9 5.07 18.23 concentration of sp3 and sp2 hybridisation and their ratio (sp2/
sp3) since it determines the structural properties of carbon

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materials.35 The component appearing at approximately 284 eV

is attributed to the presence of sp2-hybridised carbon species,
whereas the component at 285 eV to the presence of sp3-
hybridised carbon species.60 We observe that both the sp2
content and the ratio of sp2/sp3 increase with increasing
annealing temperature as proof of surface graphitisation in
agreement with previous reports.60 For instance, in the case of
Pd/CNF-PS this ratio is 2.85–2.98; Pd/CNF-LHT, 3.70–4.13; and
for Pd/CNF-HHT, 10.10–10.95, indicating that in CNF-HHT the
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sp2 bond is the most abundant. In agreement with this obser-

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vation, the content of sp3-hybridised carbon species decreases

as expected with increasing annealing temperature and the
deposition of Pd metal nanoparticles has not signicantly
inuenced the observed trend. Fig. 4 Raman spectra of the bare supports: CNF-HHT (black curve),
CNF-LHT (red curve) and CNF-PS (blue curve).
Oxygen has been analysed by the O 1s emission (Fig. S5†). O
1s peaks are well described by a Gaussian–Lorentzian curve
aer Shirley background subtraction. The assignment of the
components in the O 1s spectrum is not straightforward.
However, there is a certain degree of agreement in the position
range of the peaks.
The carbon–oxygen double bond is usually located in the
range of 530.0–531.5 eV; regarding the assignment of C–O–H
and C–O–C, in the literature several contrasting opinions have
been expressed. However, since carbon is slightly more elec-
tronegative than hydrogen, carbon–oxygen single bonds in
hydroxyl groups should appear at a slightly lower energy, in our
Fig. 5 Raman spectra of the fresh samples. (A) Catalysts synthesised
case: 531.5–531.8 eV and carbon–oxygen ether-like single bonds
by impregnation: PdIMP/CNF-HHT (black curve), PdIMP/CNF-LHT (red
at 533.0–533.2 eV; carboxylic groups are usually located in the curve) and PdIMP/CNF-PS (blue curve). (B) Catalysts synthesised by sol-
range of 534.5–535.0 eV and even though the assignment is not immobilisation. PdSI/CNF-HHT (black curve), PdSI/CNF-LHT (red
clear, the peak at 536.7–537.1 eV might be assigned to adsorbed curve) and PdSI/CNF-PS (blue curve).
water.61–63
A clear trend when comparing the preparation method is
observed for the intensity of the peak attributed to the carbon– disorder in sp2-hybridised carbon and the G band is due to the
oxygen single bonds in hydroxyl groups (C–O–H). The sol- stretching of the C–C bonds in sp2 systems leading to graphi-
immobilisation method leads to a higher presence of C–O–H tisation. The relative intensity between these two bands (ID/IG) is
compared to C–O–C due to the presence of PVA and its hydroxyl related to the structural disorder and subsequently to the size of
groups as observed in Fig. S5.† As expected, carbon–oxygen graphitic domains.64 All the Pd/CNFs studied in this work
ether-like single bonds decrease when increasing the annealing present both D and G Raman bands. For the bare supports, as
temperature due to the graphitisation of the surface as previ- expected, the ID/IG ratio decreases with increasing annealing
ously presented. temperature, presenting a more signicant drop when the
In summary, XPS analyses have shown a higher concentra- support is treated at the highest temperature. This should be
tion of sp2 hybridisation as a result of increasing the annealing the trend observed for the catalysts as well. However, we observe
temperature and consequently the graphitisation of the a lower than expected ID/IG ratio for the catalysts supported on
support. The XPS results also indicated that there are two types CNF/PS (Fig. S6†). This fact gave us a hypothesis to consider:
of Pd species present in the fresh samples. Catalysts prepared during the preparation method, the catalyst was washed to
by the sol-immobilisation technique exhibit both a higher remove the remaining stabiliser or metal precursor impurities
atomic Pd content and a higher percentage of Pd0, since the PVA and this could have affected the amorphous phase of the
ligand probably tends to inhibit the oxidation of the Pd surface. carbon, being washed away to some extent and subsequently
Raman spectroscopy was performed to analyse the structure showing this low ID/IG ratio.
and graphitisation degree of the carbon nanobers to a better In order to prove this hypothesis, the bare support was
extent. Raman spectra were measured in the range of treated with water for 1 hour under stirring and dried aer
900–1900 cm1. Fig. 4 and 5 display the Raman spectra of the ltration under the same conditions as the previous samples.
bare supports and the catalysts subjected to different temper- The Raman spectrum of the dried sample showed a further
ature treatments respectively and Table 2 shows the intensity of decrease in ID/IG: 0.61 for PdIMP/CNF-PS and 0.67 for PdSI/CNF-
the peaks and the ID/IG ratio. The main peaks appear at 1348 PS, conrming the fact that the amorphous carbon was
and 1572 cm1 and are attributed to the D and G bands of sp2 removed to a certain extent from the surface when preparing the
clusters respectively. The D band is caused by the presence of catalyst. The difference observable between the concentrations

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of sp2 and sp3 from XPS for the catalysts is in agreement with
the results obtained from Raman; the catalysts annealed at
3000  C present the highest graphitisation and a big difference
with LHT samples (annealed at 1500  C) as observable in the
sp2/sp3 ratio. Between LHT and PS samples there is a small
difference conrmed by both the concentrations of sp2 and sp3
and the ID/IG ratio.
The particle size distributions of the catalyst series for both
impregnated and sol-immobilised samples were assessed from
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the analyses of bright eld TEM micrographs. The samples

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synthesised via the impregnation route (Fig. 6) present

a particle size distribution in the 2.5–10 nm range, with an
average particle size of 5.9 nm, and those catalysts prepared by
the sol-immobilisation method present a narrower particle size
distribution of 2–8 nm (Fig. 7) and a smaller average particle
size of 4.2 nm.
Table 3 shows the mean particle size of the as-synthesised
fresh and used catalysts. Fig. S7 and S8† present the represen-
tative TEM micrographs and particle size distribution for the
used catalysts. No remarkable increment in the particle size has
been observed for the used catalysts. TEM analyses provide
evidence that the Pd nanoparticles were more evenly dispersed
on the catalysts prepared by sol-immobilisation in comparison
with the impregnated samples. Regarding graphitisation, it is
remarkable how the particle size tends to decrease for the

Fig. 7 Bright field TEM micrographs and corresponding histograms of

the particle size distributions for the fresh catalysts prepared by sol-
immobilisation. (A and B) PdSI/CNF-HHT, (C and D) PdSI/CNF-LHT, and
(E and F) PdSI/CNF-PS.

catalysts whose support has been treated at increasing annealing

temperature. The concentration of sp2 could explain this
behaviour and sp3 carbon previously calculated. Carbon with sp2
hybridisation is less reactive than carbon with sp3. The catalysts
supported on CNF-HHT present a high percentage of sp2, which
means that less reactive sites are present on the carbon surface.
During the preparation method, sol-immobilisation or impreg-
nation methods, the nanoparticles bind to the nanober pref-
erentially by the most reactive sites of carbon (sp3). This could
facilitate a smaller particle size for HHT-CNF since the concen-
tration of sp3 sites is lower and supposedly, these sites will be
distributed more spatially within the nanober.
The distribution and dispersion of Pd within the Pd/CNF
catalysts were evaluated with SEM-EDX. Fig. 8 displays
a typical SEM image of the fresh and used PdIMP/CNF-HHT. No
signicant variation is appreciable between the fresh and the
used samples. To deeply characterise the catalysts using this
technique, EDX analysis on a large area during SEM observation
was performed conrming the presence of Pd. Total metal
loading derived from the EDX analysis for both fresh and used
catalysts series is presented in Table 4. The total metal loading
of the as-synthesised catalysts approaches the theoretical value
Fig. 6 Bright field TEM micrographs and corresponding histograms of
the particle size distributions for the fresh catalysts prepared by of 1 wt%, and it is not considerably affected by the preparation
impregnation. (A and B) PdIMP/CNF-HHT, (C and D) PdIMP/CNF-LHT, method used in this work.
and (E and F) PdIMP/CNF-PS.

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Table 3 Statistical mean and standard deviation of particle size distributed in the inner walls of CNFs, besides being deposited
analysis on the external surface.
Table 5 presents the total surface area determined from the
Fresh Used
BET equation. It ranges from approximately 36 m2 g1 to
Mean particle Standard Mean particle Standard 52 m2 g1. This low surface area compared with that of carbon
Catalyst size (nm) deviation size (nm) deviation nanotubes (up to 1200 m2 g1) is caused by the thickness of the
walls (ca. 45 nm). Since BET surface area instruments have
PdIMP/CNF-HHT 5.4 0.9 5.5 0.4
PdIMP/CNF-LHT 5.7 1.3 7.3 0.4 a certain error, in some cases up to 10%, no conclusion can be
PdIMP/CNF-PS 6.9 1.8 6.8 0.3 extracted for the apparent variations in the surface area of these
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PdSI/CNF-HHT 3.9 1.2 4.5 0.3 samples.

PdSI/CNF-LHT 4.2 1.3 6.2 0.4
PdSI/CNF-PS 4.6 1.5 5.4 0.4
Catalytic activity of Pd/CNF catalysts for HCOOH
decomposition
Fig. 9 displays a comparison of the effect of the preparation
methods on the catalytic performance of the Pd/CNFs with
different morphologies (A to C). TOFs obtained are presented in
Table 6 and compared with previously reported data.
As observed from Table 6, the Pd/CNFs prepared by the sol-
immobilisation method are more active for the formic acid
liquid phase decomposition compared with the Pd/CNFs
prepared by the impregnation procedure. This higher activity
can be partially explained by XPS data; for the colloidal method
used there is a higher atomic Pd percentage on the surface and
a higher content of metallic Pd. These two facts can enhance
Fig. 8 (A) SEM image of fresh PdIMP/CNF-HHT, (B) SEM image of used
catalyst activity. Furthermore, the particles' size plays an
PdIMP/CNF-HHT, (C) mapping images of fresh PdIMP/CNF-HHT, and
(D) mapping images of used PdIMP/CNF-HHT.

Table 5 BET surface areas of the as-synthesised catalysts and

A comparison of fresh and used catalysts does not reveal supports. N2 adsorption–desorption in liquid nitrogen at 77 K. Samples
signicant variation in the metal loading during the reaction; outgassed for 3 h under vacuum at 227  C
however, SEM-EDX mapping in Fig. 8 exposes that whereas for
Support Impregnation Sol-immobilisation
the fresh sample Pd is homogeneously distributed in the cata- Catalyst (m2 g1) (m2 g1) (m2 g1)
lyst, areas with some extent of particle agglomeration and
higher Pd particle density are observed in the used catalysts. Pd/CNF-HHT 34 37 36
EDX is a bulk-sensitive technique and we observed a similar Pd/CNF-LHT 32 41 36
Pd/CNF-PS 43 50 47
metal loading for all the catalyst series, whereas, from the XPS
data in Table 1, we obtained a higher Pd atomic percentage on
the CNF surface, in the case of the sol-immobilised samples.
Therefore, we can hypothesise that, with the sol-immobilisation
method, Pd nanoparticles were preferentially distributed on the
surface of the CNFs, whereas with the impregnation method,
a portion of the formed Pd nanoparticles was located and

Table 4 Palladium loading from EDX and AAS data for the different
catalysts studied

Pd loading Pd loading
EDX (wt%) AAS (wt%)

Catalyst Fresh Used Fresh

PdIMP/CNF-HHT 1.03 1.01 0.99

PdIMP/CNF-LHT 1.03 0.99 1.01
PdIMP/CNF-PS 1.04 0.95 1.02
PdSI/CNF-HHT 0.91 1.01 0.94
PdSI/CNF-LHT 1.09 0.96 1.06 Formic acid dehydrogenation reaction on (A) Pd/CNF-HHT, (B)
Fig. 9
PdSI/CNF-PS 1.05 1.05 1.03 Pd/CNF-LHT, and (C) Pd/CNF-PS. Comparison of preparation
methods.

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Table 6 Comparative catalytic activity of various Pd-based catalysts for liquid-phase formic acid dehydrogenation under mild conditions

TOF (h1)
Activation energy
Catalyst T ( C) Reagent Initial 2h (kJ mol1) Ref.

PdIMP/CNF-HHT 30 Formic acid (0.5 M) 563.2 27.5 This work

PdIMP/CNF-LHT 30 527.5
PdIMP/CNF-PS 30 136.3
PdSI/CNF-HHT 30 979.1 26.2
PdSI/CNF-LHT 30 965.2
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PdSI/CNF-PS 30 484.4
Pd/C 21 Formic acid (1.33 M) 18 15a 53.7 25
30 48 28a
Pd/C (citric acid) 25 Formic acid 64b 57
Pd/C 30 Formic acid/sodium formate 1 : 9 228.3 24
Au41Pd59/C 50 Formic acid (1 M) 230 28  2 13
Ag@Pd (1 : 1) 35 Formic acid 156c 30 19
50 252c
Ag/Pd alloy (1 : 1) 20 144c
Ag42Pd58 50 Formic acid (1 M) 382 22  1 65
Pd–MnOx/SiO2–NH2 20 Formic acid (0.265 M) 140 61.9 66
50 1300
Ag0.1Pd0.9/rGO 25 Formic acid 105 67
a b c
TOF calculated aer 50 min. TOF calculated aer 160 min. TOF calculated based on the surface metal sites.

important role regarding the catalytic activity. Analyses by TEM The most common (111), (011) and (001) surfaces have been
conrmed a lower mean particle size for the sol-immobilisation used to simulate each step of formic acid decomposition. The
samples. Therefore, the higher catalytic activity observed could energy prole (Fig. 10) presents the energy requirements of two
be attributed to the smaller Pd mean particle size and as different paths initiated by O–H (1: HCOO formation) and C–H
a consequence the increasing proportion of Pd surface atoms. (2: COOH formation) dissociation.
However, aer 2 hours of reaction, as observed in Fig. 9, the (111) Surface. The HCOO pathway begins with the adsorp-
three pairs of catalysts led to similar nal conversion even tion of trans-HCOOH (EB ¼ 0.77 eV) and the subsequent
though the TOF values differ signicantly during the initial splitting of the O–H bond (ER ¼ 0.32 eV). The separation of the
period. As previously commented, PVA used during the sol- hydrogen atom and breakage of C–H suppose a further stabili-
immobilisation catalyst preparation leads to a higher pres- sation of the system by 0.04 eV. The COOH pathway starts with
ence of C–O–H that might be occupying active sites and leading a reorientation of HCOOH from trans to cis conguration, which
to a slightly faster deactivation than catalysts prepared by the imposes a slight energy increment of 0.02 eV. Aer the
impregnation method. exothermic C–H scission (ER ¼ 1.20 eV), the hydrogen spills
Table S1† presents the H2, CO2 and CO concentrations over the catalyst, which further stabilises the carboxylic species
evolved from formic acid decomposition by the most active by 0.08 eV. From this intermediate, both pathways 1 and 2
catalysts studied in this work. As observed, very low CO either follow route a to produce CO2 or route b to CO. Though
concentration is produced by both preparation methods, with both formate (1a) and carboxylic (2a) intermediates are very
the lowest level of CO production being observed for catalysts close in energy (1.13 and 1.25 eV respectively), HCOO (1a) is
prepared by the colloidal method. the preferable intermediate since C–H scission is largely
unfavourable as previously reported.12 HCOO decomposition
DFT results leads to CO2 and H2 (1a) because the C–O dissociation step (1b)
is highly unfavourable (+0.94 eV from adsorbed HCOO)
The characterisation data of the supported Pd nanoparticles supporting the low ppm level concentration of CO observed
indicate that the majority of the Pd species are metallic and (Table S1†).
there is a variation in the Pd mean particle size in the range of (011) Surface. In this case, the adsorption of trans-HCOOH
4–7 nm. Moreover, the catalytic activity showed a clear depen- supposes a relaxation of EB ¼ 0.94 eV and the splitting of the
dence on the Pd mean particle size. Therefore, in order to O–H bond (ER ¼ 0.36 eV). Both separations of hydrogen and
provide insights into the decomposition of formic acid on breakage of C–H stabilise the system by 0.18 eV. Aer the re-
different sites that are expected to be present on Pd/CNFs with orientation of HCOOH to cis conformation and the C–H scission
different particle sizes, we focused our attention to carry out (ER ¼ 1.52 eV), hydrogen spillover stabilises the carboxylic
DFT calculations. They provided insights into the energetics of species by just 0.01 eV. As within the (111) surface, both path-
the pathways of formic acid decomposition (dehydrogenation ways 1 and 2 split following route a to CO2 and route b to CO
versus dehydration) on the most usual Pd(001), Pd(011) and respectively. The HCOO decomposition also leads to CO2 and
Pd(111) surfaces.

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which absorption energies increase, having a negative impact

on the catalyst activity. In fact, the last step in pathway 1a, the H
spillover, on the (001) has a potential energy of ER ¼ 1.38 eV,
while on the (111) surface ER ¼ 1.41 eV and on (011) ER ¼
1.49 eV. Hence, the resulting energy of the system is higher for
the (001) surface and therefore, we can conclude that very small
nanoparticles with a large extensive area of the (001) surface
would be detrimental to both catalyst activity and CO evolution.
In our investigation, we have presented the maximum activity
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for the catalysts with a mean particle size of approximately

4 nm. This diameter seems to be large enough to avoid the (001)
surface to a large extent.
Taking into account the observed catalytic and potential
energy surface trends, the catalytic activity increases with the
increase of (111) and (011) surfaces, and furthermore, it is
evident that the cis conguration is the pathway (2a) that leads
to CO formation even at low temperature is particularly
enhanced in the case of the (001) surface. Therefore, it is crucial
to design experimental methodologies to develop supported Pd
nanoparticles exposed with a higher degree of (111) and (011)
surfaces.

Conclusions
A series of monometallic Pd nanoparticles supported on several
carbon nanobers (CNFs) were synthesised. Three CNFs with
different graphitisation grades were used. Sol-immobilisation
and impregnation techniques were selected as model prepara-
tion methods widely used for the deposition of Pd
nanoparticles.
Fig. 10 Potential energy surface for formic acid decomposition on (A) Pd/CNFs prepared by the sol-immobilisation method exhibit
Pd(111), (B) Pd(011) and (C) Pd(001) surfaces. Red and blue lines indi- higher catalytic activity when compared with catalysts prepared
cate HCOO and COOH paths respectively. The solid lines lead to CO2 by the impregnation method due to (i) the higher surface
whereas the dashed line to CO. atomic Pd percentage, (ii) higher percentage of Pd0 and (iii)
smaller Pd particle size. Probably, the sol-immobilisation
method tends to distribute Pd metal nanoparticles on the
H2 (1a) because the C–O dissociation step (1b) is even more surface of the nanobers, whereas the impregnation method
unfavourable (+1.12 eV) for (011) than for the (111) surface and leads to a partial lling of the nanobers and distribution of Pd
thus, CO evolution is largely avoided. nanoparticles in the inner wall, besides the distribution on the
(001) Surface. The adsorption of trans-HCOOH supposes outer CNF surface.
a relaxation of EB ¼ 0.80 eV and the breakage of the O–H bond The heat treatment on CNFs affects catalyst activity. The
(ER ¼ 0.25 eV). The separation of hydrogen and the splitting of catalytic performance of the samples signicantly increases
C–H stabilise the system by 0.30 eV. Aer the re-orientation of with the increase of annealing temperature. A heat treatment at
HCOOH to cis and the C–H scission (ER ¼ 1.55 eV), the 1500  C did not signicantly modify the surface properties of
hydrogen separation destabilises the carboxylic species by CNFs. In contrast, treating CNFs at 3000  C rearranges the
0.04 eV. In this case, the C–O dissociation step (1b) is still structure improving the order and the degree of graphitisation.
unfavourable, but the value is much lower compared with the This increase of activity with annealing temperature has been
other two surfaces (+0.33 eV). Besides this, O–H spillover, the attributed to the presence of smaller Pd nanoparticles formed
last step in 2b is just slightly unfavourable compared as well and improved dispersion. A TOF of 979 h1 was achieved by
with (011) and (111) where it certainly needed more energy. Due PdSI/CNF-HHT, the most active catalyst in this series, with high
to these two facts, (001) is the surface that can lead to a higher selectivity for H2 (>99%) under mild reaction conditions, e.g.
concentration of CO since its evolution is not as unfavourable as 30  C. Finally, DFT studies provide valuable insights into the
in (011) and (111). role of under-coordinated sites in terms of activity and prefer-
Since 1a (via HCOO) is the main pathway followed by formic ence of reaction pathways toward CO2 and H2 evolution against
acid decomposition, according to Fig. 10, it is clear that for both CO formation and provide answers to the important question of
(111) and (011) surfaces, this pathway is exothermic. On the how to avoid/minimise the formation of CO. (001) was found to
other hand, for the (001) surface, pathway 1a nds two steps in be the surface in which CO would evolve at faster rates

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