Synthesis of heterometallic metal-organic frameworks and their performance as electrocatalyst for CO2 reduction

Article abstract of DOI:10.1039/c8ra02676a

Herein we report the solventless synthesis and doping of the benchmark HKUST-1(Cu) as a facile route to afford heterometallic metal-organic frameworks (MOFs) having proficient behavior as electrocatalytic materials in the reduction of carbon dioxide. Zn(i

Full text of DOI:10.1039/c8ra02676a

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PAPER
Synthesis of heterometallic metal–organic
frameworks and their performance as
Cite this: RSC Adv., 2018, 8, 21092
electrocatalyst for CO2 reduction†
a
Jonathan Albo, b Garikoitz Beobide,
a Oscar Castillo,a
*
*
Maite Perfecto-Irigaray,
Angel Irabienb and Sonia Perez-Yanez
ac
´
´˜
Herein we report the solventless synthesis and doping of the benchmark HKUST-1(Cu) as a facile route to
afford heterometallic metal–organic frameworks (MOFs) having proficient behavior as electrocatalytic
materials in the reduction of carbon dioxide. Zn(II), Ru(III) and Pd(II) were selected as doping metals (MD)
with the aim of partially replacing the Cu(II) atoms of the pristine structure to afford HKUST-1(Cu,MD)
type materials. Apart from the high yield and good crystallinity of the obtained materials, the extremely
high reagent concentration that the reaction conditions imply makes it feasible to control dopant
loading in all cases. Prepared samples were processed as electrodes and assembled in a continuous flow
filter-press electrochemical cell. Faraday efficiency to methanol and ethanol at Ru(III)-based electrodes
resulted in activity as high as 47.2%, although the activity of the material decayed with time. The interplay
of the dopant metal and copper(II), and the long-term performance are also discussed.
Received 27th March 2018
Accepted 29th May 2018
DOI: 10.1039/c8ra02676a
rsc.li/rsc-advances
anthropogenic CO2 emissions, which has become a critical
challenge for sustainable development. Among the different
Introduction
approaches to solve this issue, CO2 capture and subsequent
storage (CCS) technologies stand out, in which MOFs appear to
be appealing materials due to their high surface area-to-weight
ratio and ability to systematically modulate the pore dimen-
sions and surface chemistry.4 More recently, a related alterna-
tive, carbon capture and utilization (CCU), has started to attract
attention worldwide because it can turn waste CO2 emissions
into valuable products such as chemicals and fuels, while at the
same time contributing to climate change mitigation.5
Regarding the different methodologies employed for CCU, CO2
electrochemical conversion appears to be a promising strategy,
providing products which are strongly dependent on catalyst
materials and the reaction medium.6,7
In this sense, transition metals and their compounds, such
as metal complexes, have been widely evaluated for the elec-
troreduction of CO2. This is probably because these metals have
vacant orbitals and active d electrons, which are believed to be
able to energetically facilitate the bonding between the metal
and CO2 (i.e. adduct formation) and the desorption of the
reduction products.8 Copper has been demonstrated to be one
of the most promising transition metals for obtaining hydro-
carbons and alcohols at high reaction rates. However, elemental
copper generates a range of reaction products, and the selec-
tivity of each product tends to be low.9 For this reason, other Cu-
based materials have also been tested to electrocatalytically
reduce CO2, among which copper oxide based electrodes have
exhibited the most prominent performance yielding methanol
with high selectivity and current efficiency.10,11 It is also
Porous coordination polymers (PCPs), most commonly known
as metal–organic frameworks (MOFs), have led to a myriad of
research works, which up to now have been in the spotlight of
the scientic communities researching chemistry, physics,
materials science and other bordering research areas.1 MOFs
are hybrid crystalline materials containing inorganic nodes,
isolated metals or metal clusters and organic linkers that form
metal–organic coordination networks with potentially acces-
sible voids. The nature and porosity of these compounds,
together with their high specic surface areas (up to ca. 7000 m2
gꢀ1),2 make them ideal materials for a wide range of potential
uses including gas storage, separation and catalysis. In partic-
ular, applications in energy technologies such as fuel cells,
supercapacitors and catalytic conversion have made them
objects of extensive study, industrial-scale production and
application in the last few years.3
Moreover, the excessive atmospheric concentration of
greenhouse gases means it is mandatory to reduce the
a
´
´
´
Departamento de Quımica Inorganica, Facultad de Ciencia y Tecnologıa, Universidad
´
del Paıs Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, Apartado 644, E-48080
Bilbao, Spain. E-mail: garikoitz.beobide@ehu.eus
bDepartment of Chemical & Biomolecular Engineering, University of Cantabria (UC),
Avda. Los Castros, 39005, Santander, Spain. E-mail: jonathan.albo@unican.es
c
´
´
´
Departamento de Quımica Inorganica, Facultad de Farmacia, Universidad del Paıs
Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, E-01006 Vitoria-Gasteiz, Spain
† Electronic supplementary information (ESI) available: Synthesis data, powder
diffraction patterns, and quantitative data on CO2 electroreduction. See DOI:
10.1039/c8ra02676a
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Table
1(Cu,MD)
1
Cu : MD atomic ratio of synthesized bimetallic HKUST-
remarkable that the combination of copper with other electro-
active metals improves the selectivity of the process.
Despite MOFs being extensively studied, only a handful of
research works have assessed the electrocatalytic potential of
MOFs to transform CO2 into added value chemicals, such as
formic acid, CO + H2, and oxalic acid.12 In this regard, in
a previous work,13 we concluded that Cu-based metal–organic
porous materials with unsaturated coordination positions
exposed on the pore surface are preferred to enhance the
performance of the CO2 electroreduction to alcohols. Speci-
cally, among the different tested metal–organic materials the
ubiquitous HKUST-1 microporous MOF, supported in gas
diffusion electrodes (GDEs), rendered the best results in terms
of reaction rates and Faraday efficiencies.
MD
MD : Cu ratio
Sample code
Yield (%)
—
Zn
0 : 100
5 : 95
8 : 92
19 : 81
3 : 97
7 : 93
10 : 90
3 : 97
5 : 95
11 : 89
H_Cu
90
70
88
75
86
89
83
85
79
77
H_Zn5
H_Zn8
H_Zn19
H_Ru3
H_Ru7
H_Ru10
H_Pd3
H_Pd5
H_Pd11
Ru
Pd
In the present work, we rst set the objective of assessing the
aptness of a solventless synthesis route to afford heterometallic
MOFs by doping of the benchmark HKUST-1(Cu), and secondly
to evaluate the performance of these materials into the elec-
trocatalytic conversion of CO2. Zn(II), Ru(III) and Pd(II) have been
selected as dopant metals (MD), due to both their compatibility
to suit the metal site and their recognized catalytic activity
(Ru(III), Pd(II)) or stabilizing properties in long-term runs
(Zn(II)).8 The powdered material was thoroughly characterized
to study the doping process and then each doped-MOF was
processed as a gas diffusion electrode. These MOF-GDEs were
preliminary characterized through cyclic-voltammetry analyses,
and then tested in the electrocatalytic reduction of CO2 using
a continuous lter-press electrochemical cell.
ꢁ
˚
radiation, l ¼ 1.5418 A) over the range 5 < 2q < 70 with a step
size of 0.02ꢁ, a variable automatic divergence slit and an
acquisition time of 2.5 s per step at 293 K. Indexation of the
diffraction proles was made by means of the FULLPROF
program (pattern-matching analysis)15 on the basis of the space
group and the cell parameters found in the Cambridge Struc-
tural Database (CSD)16 for the single crystal X-ray structure of
HKUST-1 (CSD entry: UVIPIZ). Further details can be found in
the ESI (Fig. S1†).
X-ray uorescence (XRF) measurements were made using the
XDRL Fischerscope X-ray system at 50 KeV power, under
a nickel lter and with a 0.1 dm collimator. Ruthenium con-
taining samples were further analyzed using a SPECS X-ray
photoemission spectrometer (XPS).
Scanning electron microscopy (SEM) studies were carried out
on a JEOL JSM-7000F microscope operated at 10–20 kV and
coupled with an energy dispersive X-ray spectrometer (EDX).
Specimens were mounted on conductive carbon adhesive tabs
and imaged aer chromium sputter coating of 5 nm to make
them conductive.
The permanent porosity was studied by means of the
measurements of N2 adsorption isotherms at 77 K using
a Quantachrome Autosorb-iQ-MP analyzer. All samples were
dried under vacuum at 150 ꢁC over 6 h to eliminate solvent guest
molecules prior to measurements. The surface area values were
obtained by the ttings of the adsorption data to the Brunauer–
Emmett–Teller (BET) equation. In order to choose the appro-
priate pressure range and to avoid ambiguity when reporting
the BET surface area of MOFs, we used the three consistency
criteria proposed by Walton and Snurr: (1) the pressure range
selected should have values of V(Po–P) increasing with P/Po. (2)
The points used to calculate the BET surface area must be linear
with an upward slope. (3) The line they form must have a posi-
tive y-intercept.17
Experimental
Synthesis of Cu/M/BTC MOFs
All of the chemicals were of reagent grade and were used as
commercially obtained.
Doped HKUST-1(Cu,MD) samples were prepared by modi-
fying a synthetic procedure described for the solventless
synthesis of pristine homometallic HKUST-1(Cu).14 In the
general procedure, corresponding amounts of Cu(OAc)2$H2O
(OAc: acetate), dopant metal (MD) source (Zn(OAc)2$2H2O,
RuCl3$xH2O, Pd(OAc)2) and trimesic acid (H3BTC: benzene-
1,3,5-tricarboxilic acid, 0.40 mmol) to satisfy the stoichiometry
of [(Cu1ꢀxMDx)3(BTC)2]n (x ¼ 0, 0.05, 0.10 and 0.20) were hand-
grinded thoroughly to obtain a homogeneous mixture (see
Table S1 of ESI†). The nal mixture was sealed in a 2 mL glaꢀss1
ampule and heated to 120 ꢁC using a heating rate of 2 ꢁC min
in a conventional oven for 48 h. The products were washed with
water and ethanol to remove unreacted reagents. Table 1 shows
the sample coding and their respective MD : Cu molar ratios. It
should be noted that, in general, MD : Cu ratios of the products
are below those set in the aforementioned reaction mixture
(5 : 95, 10 : 90, 20 : 80). This is further discussed in the results
and discussion section.
Preparation of MOF-GDEs
The GDEs (A ¼ 10 cm2) were prepared by airbrushing a catalytic
ink onto a porous carbon paper type TGP-H-60 (Toray Inc.) as
described in our previous study.18 The catalyst loading in the
GDEs was kept at 1 mg cmꢀ2. The catalytic ink was formed by
Physical measurements
Powder X-ray diffraction (PXRD) measurements were performed a mixture of the synthesized HKUST-1(Cu,MD) as the electro-
on a Phillips X'PERT diffractometer (equipped with Cu-Ka catalysts, Naon® dispersion 5 wt% as the binder and
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isopropanol (IPA) as the vehicle, with a 70 : 30 catalyst/Naon
The performance of the electrochemical process was evalu-
mass ratio and a 3% solids (catalyst + Naon) percentage. The ated by productivity, r (i.e. product obtained per unit of cathode
mixture was sonicated for 15 min before airbrushing onto the area and time), and the Faraday efficiency, FE (i.e. selectivity of
carbon papers and the resulting MOF-GDEs were dried and the reaction for the formation of the different products).
rinsed with deionised water before use.
Results and discussion
Cyclic voltammetry characterization
This section addresses, rstly, the chemical and microstructural
characterization of HKUST-1(Cu,MD) doped with Zn(II), Ru(III)
and Pd(II), describing the dopant loadings, their distribution
and their effect in the crystallinity and microporosity. There-
aer, the performance of HKUST-1(Cu,MD) type MOFs as
cathode material is assessed by means of cyclic voltammetry
analyses and continuous reaction in a lter-press electro-
chemical cell. Throughout the paper samples have been coded
as H_MDX, in which MD corresponds to the dopant metal (Zn,
Ru or Pd) and X affords the incorporated dopant content.
The electrochemical behavior of the materials was evaluated by
cyclic voltammetries with a MSTAT4 (Arbin Instruments)
employing a conventional three electrode electrochemical cell.
The current–voltage curves were obtained with a scan rate of
50 mV sꢀ1 at potentials ranging from 0 to ꢀ2 V versus Ag/AgCl in
a CO2 saturated 0.5 M KHCO3 aqueous solution as electrolyte.
Portions of the MOF-based GDEs were used as working elec-
trodes, while a glassy carbon and Ag/AgCl (sat. KCl) were used as
the counter and reference electrode, respectively. Current
density is expressed as the total current divided by the
geometric surface area of the electrodes.
Characterization of bimetallic Cu/M/BTC samples
Electrochemical cell and experimental conditions for CO2
reduction
Synthesized MOF samples were initially characterized by XRF
and PXRD to analyze the dopant content and its inuence in the
The prepared MOF-GDEs were evaluated for the continuous HKUST-1 crystalline phase. Table S1† collates the amount of
electrocatalytic reduction of CO2 using a lter-press electro- dopant (MD) added as a reagent in the synthesis mixture and the
chemical cell (Micro Flow Cell, ElectroCell A/S) system as amount incorporated into the synthesis product. In this respect,
described elsewhere.18 A Naon 117 cation exchange membrane the relative agreement between dopant content targets and
was used to separate the cathode and anode compartments. The amounts found in the HKUST-1 samples varies with dopant
MOF-GDEs were employed together with a platinized titanium element. In the case of zinc, whose size and atomic number are
electrode as the anode and a Ag/AgCl (sat. KCl) reference elec- the closest to copper, the amount of dopant incorporated is
trode was assembled close to the cathode. The cathode side of almost the same as that added in the reagent mixture. Ruthe-
the reactor was fed with CO2 gas (99.99%) with a ow/area of Qg/ nium and palladium are less efficiently assimilated during the
A ¼ 10 mL minꢀ1 cmꢀ2. A 0.5 M KHCO3 aqueous solution was synthesis process, giving rise to dopant contents somewhat
used as both the catholyte and anolyte, with a ow rate of Qe/A ¼ below the target values (Table 1). XPS measurements taken on
2 mL minꢀ1 cmꢀ2. In this study, the lter-press electrochemical H_Ru samples revealed the presence of chloride ions (Cl 2p
system possesses three inputs (catholyte, anolyte and CO2 199.5 eV) which in turn conrms the presence of ruthenium(III).
separately) and two outputs (catholyte-CO2 and anolyte), which It should be noted that in this case the chloride ion is required
makes the formation of a gas–solid–liquid interface possible for to counteract the additional positive charge that implies the use
the electrocatalytic reduction of CO2 in the gas phase.18
of a trivalent metal. In addition, previous works have reported
All the experiments were performed at galvanostatic condi- the solvothermal synthesis of mixed valence HKUST-1(RuII,R-
tions (j ¼ 20 mA cmꢀ2), using an AutoLab PGSTAT 302N uIII) in which chloride acts as a counterion to balance the
potentiostat (Metrohm, Autolab B.V.).
network charge.19 With regard to the reaction yields, they do not
These Qg/A, Qe/A and j conditions were previously found to be seem to be inuenced by the type of dopant, reaching in all
optimal for the CO2 reduction using copper(I) and copper(II) cases values comparable to those obtained for undoped HKUST-
type electrocatalysts.18
1 (H_Cu).
As mentioned above, PXRD results conrmed the mainte-
The experimental time was 90 min, during which pseudo-
stable conditions are ensured, according to our previous anal- nance of the crystalline phase corresponding to HKUST-1, as the
yses.10,18 Liquid samples were taken every 15 min from the PXRD patterns of the doped samples are in good agreement
catholyte tank. To quantify the concentration of each product in with that of pristine HKUST-1. For comparative purposes, Fig. 1
the liquid phase, the samples were analyzed in duplicate in and Table 2 gather the results of the prole ttings carried out
a headspace gas chromatograph (GCMS-QP2010, Ultra Shi- on PXRD patterns of H_Cu and of samples with dopant content
madzu) equipped with a ame ionization detector (FID). close to 10% (H_Zn8, H_Ru10 and H_Pd11).
Compounds were separated on a DB-Wax 30 m ꢂ 0.25 mm ꢂ
The introduction of a doping element implies a slight
0.25 mm column, with an injection and detector temperature of increase of the cell parameter with respect to the homometallic
250 ꢁC and 270 ꢁC, respectively. Helium was used as a carrier gas H_Cu. This fact is attributable to the occurrence of longer M–
at a ow rate of 50 mL minꢀ1. The product concentration was Ocarboxylate coordination bond distances that implies the pres-
averaged from at least three replicates (standard deviations ence of the dopant in the paddle-wheel shaped dinuclear
below 12.8%).
[M2(OOC)4] secondary building unit (SBU). In the case of Ru(III)
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the samples show polycrystalline samples (Fig. 2) with heaped
sub-micrometric crystals. Such small sized particles can benet
the performance of the MOF-GDEs, as a greater external surface
area of the dispersed catalytic material might favor the reaction
kinetics.
Furthermore, to assess the microstructural homogeneity of
the samples, EDX spectra and backscattered electron images
(BSE) were also collected during the SEM analysis. EDX data
conrmed that mostly all measured samples (Fig. S3†) show
a homogeneous distribution of the dopant metal throughout all
of the material and only in the case of the H_Ru10 sample were
some Ru(III) enriched areas observed. Accordingly, when BSE
images of Pd(II) and Ru(III) samples with the highest dopant
content (Fig. 3) are compared, the absence of contrast in
H_Pd11 indicates a homogeneous distribution of Cu(II) and
Pd(II), while lighter spots and darker broad particles in H_Ru10
correspond to more and less concentrated Ru(III) zones,
respectively.
For the microstructural analysis, N2 adsorption isotherms
were measured at 77 K (Fig. 4). All of them can be dened,
mainly, as type I isotherms according to the IUPAC classica-
tion22 with a marked knee (B-point) at P/Po values < 0.12, which
is characteristic of the adsorptive behavior of microporous
compounds. At intermediate pressures, the adsorption presents
a certain monotonic rise followed by a sudden increase at high
relative pressures (P/Po > 0.9). Both phenomena are attributable,
respectively, to multilayer adsorption and capillary condensa-
tion occurring in the intergranular space of the aforementioned
submicrometric MOF particles.
The data obtained from the numerical analysis of the
isotherms are gathered in Table 3. In concordance to the height
of the plateau of the isotherms at intermediate pressures,
homometallic H_Cu shows the greatest surface area (SBET),
while the inclusion of a second transition metal produces, in all
cases, materials with smaller surface areas. This reduction is
not only related to the higher atomic weight of the doping
metal, as the heaviest of them (Pd) gives greater area values than
the lighter ones (Zn and Ru). A possible explanation relies on
the inuence of the dopant reagent on the crystallinity of the
material (considering that a greater number of defects produces
a detrimental effect on the porosity). The subtraction of the
microporous area from the total surface area leads to external
surface area values (area attributable to the outer surface of the
MOF particles) ranging from 45 to 284 m2 gꢀ1. In a rough
Fig. 1 PXRD pattern-matching refinement plots for H_Cu, H_Zn8,
H_Ru10, and H_Pd11 (green ticks: calculated reflections; red circles:
experimental pattern; black line: simulated pattern; blue line: differ-
ence between patterns).
and Pd(II), which are elements of the second transition series,
their greater ionic radii values are consistent with an increase in
coordination bond distances and cell parameters. On the
contrary, Zn(II) exhibits a rather similar ionic radius to that of
Cu(II), but the Jahn–Teller effect of Cu(II) (d9) implies shorter
equatorial coordination distances than those found in analo-
gous paddle-wheel shaped Zn(II) complexes.16 Despite the
details described above, this indicates that the dopant is
incorporated into the SBU of the MOF, but we cannot disregard
that part of the dopant might also be appended as a defect in
the surface of the framework.20 Accordingly, a reduction in the
surface area and micropore volume might be expected, but
a similar effect can also be attributed to the slight modications
of the synthetic conditions (such as a change in the starting
reagents). When considering the PXRD analysis, it must be
pointed out that despite the fact that solventless synthesis has
been demonstrated to be a suitable procedure to incorporate
dopants into MOFs,21 the method is quite sensitive to the hand-
milling and heating procedure and as a result, it can produce
some batches with some minor crystalline impurities in the
products (Fig. S2 of ESI†).
The homogeneous nature of the samples and their crystallite
size have been analyzed by means of SEM. SEM micrographs of
Table 2 Cell parameters and agreement factors obtained in the pattern-matching performed on H_Cu, H_Zn8, H_Ru10, and H_Pd11a
H_Cu
H_Zn8
H_Ru10
H_Pd11
˚
a (A)
26.283 (3)
18 156 (4)
1.11
31.2
2.27
26.322 (4)
18 238 (5)
1.92
31.3
2.33
26.345 (2)
18 285 (2)
13.9
49.3
1.45
26.413 (3)
18 428 (4)
82.6
32.0
1.77
3
˚
V (A )
Rb
Rp
c2
!,
!
ꢀ
ꢂꢃꢀ
ꢂ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
P
P
P
P
2
a
Rb ¼ 100
Iobs;h ꢀ Ical;h
Iobs;h ; Rp ¼ 100
Yi ꢀ Yc;i
yi ; c2 ¼ ½Rwp=Rexpꢃ .
h
h
i¼l;n
i¼l;n
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Fig. 2 SEM micrographs (SEI mode) at 25 000 magnifications of
H_Cu, H_Zn8, H_Ru10, and H_Pd11.
calculation, using the spherical particle approach and corrected
crystallographic densities, these external area values imply
mean particle sizes of 79, 24, 149, and 67 nm for H_Cu, H_Zn8,
H_Ru10 and H_Pd11, respectively. Both the external surface
(particle size) and the microporous one (crystal structure of the
MOF) are of special relevance in the functionality of the herein
studied materials, being benecial, in general, to achieving
high values of both for their use as GDEs.
Fig. 4 N2 adsorption isotherms at 77 K for H_Cu, H_Zn8, H_Ru10 and
H_Pd11. Filled and empty symbols correspond to adsorption and
desorption branches, respectively.
CO2 to liquid products. These results can be taken as indica-
tive of the activity, and a deeper insight is presented when
describing below the results obtained in the reactor cell.
Cyclic voltammetry characterization
Prior to analysis of all the H_MD samples in the electrochemical
cell, a preliminary test by cyclic voltammetry was performed in
a CO2-saturated (0.5 M KHCO3) aqueous solution in order to
qualitatively assess if the doping exerts any inuence on the
electrochemical behavior. Fig. 5 gathers the voltammograms
yielded by representative MOF-GDEs aer 5 scans (remaining
samples exhibit a similar trend). The current densities (j) are
normalized to the geometric area of the MOF-GDEs.
Continuous electroreduction of CO2
The analysis of the liquid fraction shows that the electro-
catalytic reduction of CO2 using the GDEs modied by HKUST-
1(Cu,MD) type materials produces methanol (CH3OH) and
ethanol (C2H5OH) as major products. It should be noted that
neat GDEs (i.e. carbon paper without MOFs) did not generate
any measurable liquid product.
All voltammetric curves show a reduction process starting
at ca. ꢀ1 V versus Ag/AgCl. This reduction peak may be initially
assigned to the reduction of CO2. The reduction wave is more
pronounced for Zn- and Ru-doped samples than for H_Cu,
which indicates their notable activity as a cathode material. It
is noteworthy that within the applied voltage, the Ru(III) doped
sample appears to be the most promising candidate, while the
incorporation of Pd(II) hinders the electroreduction process of
As representative cases of continuous CO2 reduction capa-
bility, Fig. 6 depicts the time-dependence of the total Faraday
efficiency (FET) provided by H_Zn8, H_Ru10 and H_Pd11
materials in comparison with H_Cu at xed current density (j ¼
20 mA cmꢀ2), gas ow (Qg ¼ 10 mL minꢀ1 cmꢀ2) and electrolyte
ow (Qe/A ¼ 2 mL minꢀ1 cmꢀ2). The FE values are calculated
considering a 6-electron-pathway for CO2 reduction to CH3OH
and a 12-electron-pathway to C2H5OH. It is noteworthy that the
outstanding efficiency of the Ru(III) doped sample (FE ¼ 47.2%)
is comparable to those relatively stable values provided by Cu2O
nanoparticles-based electrodes (FE ¼ 54.8%).18 However, the
activity of the Ru(III) doped sample falls abruptly aer 60 min of
operation to reach a stable plateau close to the 10%. FE values of
H_Cu and H_Zn8 show a similar trend, but they start from more
moderate values (17–19%) and fall monotonically until stabi-
lizing at values within 12–13%. On the contrary, Pd(II) doped
samples show markedly lower FE values (close to the 5%) within
the rst 2 hours of operation, but fall to a negligible value of 1–
2% at longer operation times. Previous works have reported the
formation of the less reduced CO as a major product when Pd-
Fig.
3 SEM micrographs (BSE mode) at 1000 magnifications of
H_Pd11 and H_Ru10.
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Table 3 Surface area values and pore volumes determined for H_Cu, H_Zn8, H_Ru10 and H_Pd11a
Sample
SBET (m2 gꢀ1
)
Smicro (m2 gꢀ1
)
Sext (m2 gꢀ1
)
Vmicro (cm3 gꢀ1
)
VT (cm3 gꢀ1
)
H_Cu
1560
1143
741
1474
859
696
86
284
45
0.574
0.333
0.269
0.515
0.759
0.968
0.338
0.788
H_Zn8
H_Ru10
H_Pd11
1425
1325
100
a
SBET: BET surface area; Smicro: microporous surface area; Sext: external surface area; Vmicro: micropore volume; VT: total pore volume determined at
P/Po: 0.993.
based materials are employed as cathode materials, therefore increase of the dopant content produces a lowering of the
the worsening in the methanol and ethanol yield observed for catalyst performance.
H_Pd11 is probably due to a change in the product selectivity
caused by the dopant.8
Despite ethanol being the major product in the liquid frac-
tion, it seems that the dopant content exerts a certain degree of
To get a deeper insight, the quantitative reduction perfor- control on the product selectivity, in such a way that the
mances (r and FE) at 90 min of operation time for all samples undoped sample and samples containing the smallest dopant
are summarized in Fig. 7. Further details are provided in the ESI loading exhibit lower selectivity values towards the formation of
(Table S2†). Regarding the production rates and Faraday effi- C2H5OH respective to CH3OH (C2H5OH molar selectivity values
ciencies the maximum values are achieved for the smallest and estimated from production rates: 67, 55, 85 and 45% for H_Cu,
intermediate Ru and Zn loadings (r ¼ 1.78–2.22 ꢂ 10ꢀ5 mol mꢀ2 H_Zn5, H_Ru3 and H_Pd3). In all cases, a further increase of
sꢀ1 and FE of ꢄ22%), in comparison to those values provided by the dopant loading promotes an increase of the ethanol selec-
pristine HKUST-1 (2.66 ꢂ 10ꢀ5 mol mꢀ2 sꢀ1 and 12.9%). Further tivity reaching values ranging from 93–100%. This fact might be
attributable to the increase of the permanence time caused by
the establishment of stronger interactions with the dopant
metal, and as a consequence, it would favor a C–C coupling
reaction to ethanol. It has been previously hypothesized that
C2–C3 products occur through an enol-like surface interme-
diate, which desorbs to convert to its alcohol, diol and/or keton
form.23 The C–C bond formation is one of the most critical
factors to be taken into account when designing an efficient
electrocatalyst, and further experimental work is needed to fully
elucidate CO2 reduction steps to form alcohols using Cu-based
GDEs.
Furthermore, to analyze the in-use stability of the MOF-GDEs
and its relationship with activity decay observed until the mid-
term plateau is achieved, PXRD were measured on fresh elec-
trodes and on those subjected to 45 and 300 min of continuous
reaction (Fig. 8a). Whereas fresh electrodes exhibit all the
distinctive peaks of the HKUST-1 crystalline phase, their
intensity falls progressively as the reaction time increases. In
fact, in the most aged electrode (300 min of reaction) only the
most intense reection of the MOF (i.e. 2 2 2) is appreciated.
Fig. 5 Cyclic-voltammetry responses for (a) H_Zn8, H_Ru10 and
H_Pd11 in comparison to H_Cu-based GDEs in a medium saturated
with CO2 and, (b) responses for H_Ru10 in the presence and absence Fig. 6 Time-dependence of FE for H_Zn8, H_Ru10 and H_Pd11
of CO2.
materials, in comparison to H_Cu.
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Fig. 7 FEs for C2H5OH (in red) and CH3OH (in blue) formation and r for C2H5OH (C) and CH3OH (B) in the electrocatalytic reduction of CO2 at
H_Zn, H_Ru and H_Pd in comparison to H_Cu.
This fact might be related to the material leaching caused by the
gas ow at the initial reaction stage. In fact, the comparative
SEM images taken on fresh and used electrodes (Fig. 8b) show
at areas from which MOF particles have been detached.
Despite this, electrode fatigue is compatible with the afore-
mentioned activity decay, a slow but progressive crystallinity
loss of the MOF occurring during operation might also inu-
ence this. Note that the full width at half maximum (FWHM) of
(2 2 2) reection varies from 0.12ꢁ in the fresh electrode to 0.14ꢁ
in the one used for 45 min. In any case, it must be pointed out
that aer 300 min of running, a weak reection sited at 14.1ꢁ
emerged, for which a perusal in the powder diffraction les of
the Inorganic Crystal Structure Database24,25 matches the most
intense reection of copper(II) formate (ICSD: 109965). It should
also be noted that the low sample loading, its dispersion in the
GDE and the Naon matrix, inhibit the observation of further
peaks and as a consequence it precludes the univocal identi-
cation of the formed crystalline compound. Nonetheless, the
formation of copper(II) formate during the reaction would imply
a frozen state of a reaction intermediate, which in turn would be
the key that favors the formation of more reduced products,
such as alcohols. In any case, further research is required to
elucidate this plausible mechanism.
Conclusions
The solventless synthesis was demonstrated to be a suitable
approach and facile route to afford HKUST-1(Cu,MD) type
MOFs, in such a way that the dopant (MD) loading can be
controlled by its ratio in the reagent mixture. Note that the
extremely high reagent concentration implies that the reaction
conditions can be a key issue making the controllable doping of
MOFs feasible. The products are obtained in high yields and
good crystallinity as demonstrated by PXRD analysis and N2
Fig. 8 Comparative (a) PXRD patterns and (b) SEM images taken on
fresh and used H_Ru10-GDEs (t0, t45 and t300 stand for GDEs subjected
adsorption isotherms. The dopant causes a slight increase of
the cell volume attributable to the smaller size of the Cu(II) ion
and to the Jahn–Teller contraction implied by the Cu–Ocarboxylate
to 0, 45 and 300 min).
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bonds. With regard to the microstructure, it must be empha-
sized that all the samples consist of sub-micrometric crystals
which can be considered as an appealing feature since this fact
increases the external surface area and prompts reagent diffu-
sion in catalytic processes.
Regarding the electrocatalytic performance of the herein
prepared samples, Ru(III) doping has a striking inuence on the
alcohol yield, reaching relatively high Faraday efficiencies
(47.2%) comparable to those provided by nanosized inorganic
catalysts. Unfortunately, its activity decays aer 60 min of
continuous reaction until it reaches a similar performance to
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13 J. Albo, D. Vallejo, G. Beobide, O. Castillo, P. Castano and
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Conflicts of interest
´
O. Castillo, J. Cepeda, D. Vallejo-Sanchez and A. Luque,
RSC Adv., 2014, 4, 60409.
There are no conicts to declare.
´
15 J. Rodrıguez-Carvajal, FULLPROF: A Program for Rietveld
Renement and Pattern Matching Analysis. in Abstracts of
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Acknowledgements
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This work has been funded by the Universidad del Paıs Vasco/
Euskal Herriko Unibertsitatea (GIU17/50 and PP617/37) and
´
J. Rodrıguez-Carvajal, FULLPROF 2000, version 2.5d,
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Laboratoire Leon Brillouin (CEA-CNRS), Centre d'Etudes de
Saclay, Gif sur Yvette Cedex, Francia, 2003.
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Ministerio de Economıa y Competitividad (MAT2016-75883-C2-
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