Supported palladium membrane reactor architecture for electrocatalytic hydrogenation

Article abstract of DOI:10.1039/c9ta07957b

Electrolytic palladium membrane reactors offer a means to perform hydrogenation chemistry utilizing electrolytically produced hydrogen derived from water instead of hydrogen gas. While previous embodiments of these reactors employed thick (≥25 μm) palladium foil membranes, we report here that the amount of palladium can be reduced by depositing a thin (1-2 μm) layer of palladium onto a porous polytetrafluoroethylene (PTFE) support. The supported palladium membrane can be designed to ensure the fast diffusion of reagent and hydrogen to the palladium layer. The hydrogenation of 1-hexyne, for example, shows that the supported Pd/PTFE membrane can achieve reaction rates (e.g., 0.71 mmol h^-1) which are comparable to 0.92 mmol h^-1 measured for palladium membranes with a high-surface area palladium electrocatalyst layer. The root cause of these comparable rates is that the high porosity of PTFE enables a 12-fold increase in electrocatalytic surface area compared to planar palladium foil membranes. These results provide a pathway for designing a cost-effective and potentially scalable electrolytic palladium membrane reactor.

Full text of DOI:10.1039/c9ta07957b

Journal of
Materials Chemistry A
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PAPER
Supported palladium membrane reactor
architecture for electrocatalytic hydrogenation†
Cite this: J. Mater. Chem. A, 2019, 7,
6586
2
ab
c
b
Roxanna S. Delima,
Rebecca S. Sherbo,
aDbacdvid J. Dvorak,
c
Aiko Kurimoto
and Curtis P. Berlinguette
*
Electrolytic palladium membrane reactors offer a means to perform hydrogenation chemistry utilizing
electrolytically produced hydrogen derived from water instead of hydrogen gas. While previous
embodiments of these reactors employed thick ($25 mm) palladium foil membranes, we report here that
the amount of palladium can be reduced by depositing a thin (1–2 mm) layer of palladium onto a porous
polytetrafluoroethylene (PTFE) support. The supported palladium membrane can be designed to ensure
the fast diffusion of reagent and hydrogen to the palladium layer. The hydrogenation of 1-hexyne, for
ꢀ1
example, shows that the supported Pd/PTFE membrane can achieve reaction rates (e.g., 0.71 mmol h
)
ꢀ1
which are comparable to 0.92 mmol h measured for palladium membranes with a high-surface area
palladium electrocatalyst layer. The root cause of these comparable rates is that the high porosity of
PTFE enables a 12-fold increase in electrocatalytic surface area compared to planar palladium foil
membranes. These results provide a pathway for designing a cost-effective and potentially scalable
electrolytic palladium membrane reactor.
Received 22nd July 2019
Accepted 29th October 2019
DOI: 10.1039/c9ta07957b
rsc.li/materials-a
hydride at a cathode. Neither of these methods necessarily
provide a solution for using hydrogen as an energy storage
Introduction
The electrolysis of water provides a means of converting medium, but both enable hydrogenation chemistry to be per-
1–3
renewable electricity into dihydrogen fuels. The formation of formed using electricity and water.
energy dense dihydrogen is particularly appealing for seasonal
Electrocatalytic hydrogenation (Fig. 1a) is analogous to
energy storage and the heavy duty transportation sector, yet the conventional chemical hydrogenation (Fig. 1b), but hydrogen is
difficulties associated with dihydrogen gas storage and derived from an aqueous or protic electrolyte (e.g., water) rather
9
handling continue to constrain the broader deployment of than dihydrogen gas. Chemical hydrogenation activates dihy-
4,5
hydrogen fuels.
drogen gas at a metal surface to create reactive metal hydride
16
These challenges prompted us to explore alternative ways to that is available for further chemistry. Within an ECH reactor,
utilize cheap and abundant renewable electricity to electrolyze protons electrolytically produced at an anode migrate to
water while bypassing the issues surrounding the management a cathode, where they are reduced to form surface-adsorbed
of dihydrogen gas. Our search for such methods drew our hydrogen. This hydrogen-rich surface is then poised for reac-
6
–12
attention to electrocatalytic hydrogenation (ECH)
trolytic palladium membrane reactor (PMR) hydrogenation.
Both methods use electricity to produce protons at an anode nation chemistry at ambient pressures and temperatures,
and elec- tion with unsaturated bonds of organic molecules dissolved in
13–15
the electrolyte. ECH therefore uses electricity to drive hydroge-
11,12
sourced from water that are then converted into reactive metal without ever requiring gaseous hydrogen.
Moreover, the
hydrogen fugacity can be controlled by the applied electro-
a
chemical potential, and a strikingly small applied potential
Department of Chemical and Biological Engineering, The University of British
(
ꢁ0.2–0.3 V) can yield an effective hydrogen pressure of 1000–
Columbia, 2360 East Mall, Vancouver, British Columbia, V6T 1Z3, Canada. E-mail:
17
cberling@chem.ubc.ca
b
10 000 atm. While these features are powerful levers for
Stewart Blusson Quantum Matter Institute, The University of British Columbia, 2355 controlling hydrogenation chemistry, ECH must be performed
East Mall, Vancouver, British Columbia, V6T 1Z4, Canada
in a protic electrolyte, which fundamentally limits the scope of
organic substrates available for chemistry. Conning the
organic reactants, products, and electrolyte in the same elec-
trochemical cell also complicates product purication and
results in a high solution resistance.
c
Department of Chemistry, The University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, V6T 1Z1, Canada
9
d
Canadian Institute for Advanced Research (CIFAR), 661 University Avenue, Toronto,
Ontario, M5G 1M1, Canada
†
Electronic supplementary information (ESI) available: Additional images and
data showing membrane preparation and characterization, electrochemical
characterization, and product quantication including Fig. S1–S19 and Table
S1. See DOI: 10.1039/c9ta07957b
Hydrogenation in a palladium membrane reactor (Fig. 1c)
overcomes several limitations of ECH by separating proton
26586 | J. Mater. Chem. A, 2019, 7, 26586–26595
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Journal of Materials Chemistry A
23,24
20
high surface areas
and inexpensive supports to reduce
palladium content. On this basis, the high catalyst/membrane
cost needs to be addressed in order to fully leverage the 4-fold
25
faster reaction rates achieved for the PMR.
The palladium content can be reduced in gas-fed palladium
membrane reactors by deposition of thin palladium lms onto
2
6–33
rigid, porous supports.
Gas-fed PMRs typically employ rigid
27
26,30–32
materials such as porous glass, alumina,
or stainless
34
steel. Kikuchi, Uemiya, and coworkers have demonstrated that
high hydrogen permselectivity is achievable under high
temperature and pressure ow conditions using thick (>1 mm),
small pore size (<0.1 mm) supports that provide high mechan-
26–28
ical strength and thermal resistance.
These supports also
provide good adhesion for the palladium lm and enable
a pinhole-free palladium membrane. While there have been
several studies on supported palladium membranes for gas-fed
Fig. 1 Types of hydrogenation reactions according to their corre-
sponding hydrogen source and the role of palladium. (a) Electro-
catalytic hydrogenation uses protons as a hydrogen source and
35
hydrogenation occurs in protic electrolyte on the same side of the systems, they may not be amenable to electrolytic PMRs, where
palladium cathode. (b) Chemical hydrogenation uses dihydrogen gas solvent and/or electrolyte diffusion to the palladium layer is
as a hydrogen source and the hydrogenation reaction occurs in any
solvent on the same side of the palladium. (c) Electrolytic palladium
We report here that a thin layer of palladium sputter-deposited
membrane reactor hydrogenation uses protons from protic electro-
crucial for reactor success.
on a thin, porous polytetrauoroethylene (PTFE) support is
lyte as a hydrogen source and hydrogenation occurs on the opposing
side of the palladium cathode/membrane in any solvent.
capable of matching the performance of a palladium foil in
electrolytic palladium membrane reactors (Fig. 2). We demon-
strate that a dense, pinhole-free, 1–2 mm thick palladium layer
reduction in an electrochemical chamber from hydrogenation adheres to the PTFE membrane, and that fast non-polar solvent
13
in a chemical chamber. This architecture is possible because diffusion occurs through the thin (<100 mm) PTFE support, but
the palladium foil separating the two compartments is selec- does not occur through a thick (<1 mm) porous alumina support.
18
tively permeable to hydrogen atoms. A potential applied to In a proof-of-concept hydrogenation reaction, we show that the
electrodes in the electrochemical chamber produces protons (at Pd/PTFE membrane reduces the mass of palladium used by 20-
the anode) that then migrate to the palladium cathode, where fold compared to a palladium foil membrane, while maintaining
ꢀ1
they are reduced to surface-adsorbed hydrogen and then similar 1-hexyne consumption rates of 0.72 mmol h (compared
ꢀ
1
absorbed into the bulk palladium lattice. These hydrogen atoms to 0.91 mmol h ). We also resolve the effects of support prop-
permeate the palladium membrane to hydrogenate unsaturated erties (e.g., pore size, thickness, hydrophobicity), catalytic surface
organic reactants at the other side of the palladium. The PMR area, and solvent polarity on the membrane design and report on
conguration enables palladium to act as a cathode for proton a cost-effective solution for designing supported palladium
reduction on one side, a catalyst for hydrogenation on the other, membranes for electrolytic PMRs.
and a membrane for permeating hydrogen atoms (Fig. 1c).
Unlike ECH, electrolytic palladium membrane hydrogenation
can be performed in any solvent (including protic and organic
solvents) and therefore substrate scope is not limited by solu-
bility in the reaction medium. Moreover, separating hydroge- Pd 2 target (99.95%) was purchased from ACI Alloys. Kapton
nation from proton reduction with a palladium membrane (500 HN) substrates were purchased from American Duralm.
bypasses challenges of product purication from protic elec- Tetratex Microltration PTFE membranes (TX1301, TX1302,
Experimental section
Materials
0
0
0
0
00
trolyte, and results in signicant energy savings due to lower TX1325; 8.5
ꢂ
11 ) were purchased from Donaldson
Membranes. EpoxySet Resin and EpoxySet Hardener were
A key shortcoming of an electrolytic PMR, however, is the purchased from Allied High Tech Products. A 1 oz wafer bar of
15
solution resistance.
13–15
high cost of the palladium foil.
While this problem can be Pd (99.95%) was purchased from Silver Gold Bull. PdCl (99.9%)
2
addressed by decreasing foil thickness, the mechanical integrity was purchased from Strem Chemicals. Porous alumina discs
cannot be maintained for membranes with thicknesses <20 (0.1 mm pore size, 1 mm thickness) were purchased from
19
mm. This thickness is still too expensive to be economically Coorstek. 1-Hexyne (>97%) was purchased from TCI Chemicals.
feasible according to technoeconomic assessments that have Pentane ($99%), DCM (HPLC grade, $99.8%), CD OD ($99.8%
3
shown that a thickness of <15 mm is required for related gas-fed D), dimethylsulfone (quantitative NMR standard, TraceCERT),
2
0–22
PMR technologies
(where hydrogen is removed through MeOH ($99.8%), HCl (37%), H SO (95ꢀ98%), CH CN
2
4
3
a palladium membrane) to be cost-competitive with packed-bed (>99.8%), tetrabutylammonium hexauorophosphate (>99%),
reactor dehydrogenation catalysts. Indeed, the membrane and H O solution (30 wt% in H O) were purchased from Sigma
2
2
2
reactor catalyst costs >60-fold more than that of the packed-bed Aldrich. Pt gauze (52 mesh, 99.9%), Pt wire (0.5 mm, 99.95%), 6-
reactor, which uses pellets or supported nanoparticles with chloro-1-hexyne (98%) were purchased from Alfa Aesar. Nitric
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torr. Pd deposition was carried out at 100 W aer pre-sputtering
for 3.5 minutes. The target–substrate distance was 10 cm. The
ꢀ
1
deposition rate was 2.5–3.0 A s and resulted in lms with
thicknesses ranging from 1000–3500 nm. Pd mass was quanti-

ed using a Sartorius analytical balance (ꢄ0.001 resolution)
before and aer deposition.
Pd foil preparation. A 1 oz Pd wafer bar was used to roll 1 mm
ꢃ
Pd strips that were then annealed at 850 C for 1.5 hours. The
1
mm strips of Pd were rolled into 25 mm Pd foils and again
ꢃ
annealed at 850 C for 1.5 hours. Thickness of Pd foils were
determined by a Mitutoyo digital micrometer. Pd foils were
cleaned using a 0.5 : 0.5 : 1 concentration of HNO : H O : H O
3
2
2
2
solution mixture for ꢁ45 min or until vigorous bubbling
subsided. Solution mixture changed from clear to yellow during
the cleaning procedure.
Pd catalyst electrodeposition. Pd black catalyst was electro-
deposited on the Pd foil (Pd/Pd foil) using a three-compartment
2
cell (vide infra). A Pd foil (geometric surface area ¼ 1.22 cm ) was
clamped into the cell as the working electrode and measured
against a Ag/AgCl reference electrode. The middle electro-
2
chemical compartment was lled with 15.9 mM PdCl in 1 M
HCl deposition solution and a Pt mesh was placed in the same
compartment as the working electrode. The other two
compartments were le empty. A potential of ꢀ0.2 V versus Ag/
Fig. 2 (a) Cross-sectional and (b) top-view SEM images of a fabricated
Pd/PTFE membrane. SEM images show a ꢁ1.9 mm layer of Pd sputtered
on a PTFE support. The PTFE support has a 0.05 mm average pore size,
2
ꢀ1
2
5.4 mm thickness, and 104.7 m g BET surface area. (c) Schematic AgCl was applied to the working electrode until 9 C of charge
ꢀ
2
diagram of the electrolytic Pd membrane reactor using a supported (7.38 C cm ) had been passed (ꢁ5 mg material). Pd foil mass
Pd/PTFE membrane as a cathode to perform hydrogenation chem-
was quantied using Sartorius analytical balance (ꢄ0.001
istry. A current is applied to the Pd/PTFE cathode (1) and water is
resolution) before and aer electrodeposition.
oxidized at the Pt anode to form protons (2). Protons are reduced to
surface-adsorbed hydrogen at the Pd surface (3), which diffuse
through the Pd layer to the chemical compartment (4). In the chemical Liquid diffusion measurements
compartment, the organic reactant diffuses through the porous PTFE
Liquid diffusion experiments were carried out in a cell with two
support and reacts with surface-adsorbed hydrogen at the Pd–PTFE
compartments: one with a solvent inlet and the other with an
interface to form hydrogenated products (5).
outlet (Fig. S2†). Prepared PTFE supports and porous alumina
supports were sandwiched between the two compartments.
acid (68–70%) was purchased from VWR. Naon 117
membranes were purchased from Fuel Cell Store. Ag/AgCl
reference electrodes (RE5B) were purchased from BASi. Viton
Viton foam gaskets were used to seal the support and prevent
leaking. The inlet compartment was lled with 5 mL of solvent
and measurements of solvent diffusion were made by marking
the inlet compartment at regular time intervals. Interval length
depended on the solvent and support. Markings for alumina
1
00
foam gasket ( thickness) was purchased from McMaster Carr.
8
2
with all solvents, and PTFE with MeOH and H O were made
every 1–2 hours for 8 hours and at 24 hours. Markings for PTFE
with pentane were made every 15 minutes and with DCM every
Materials preparation
PTFE support preparation. A Kapton substrate was cut into
00
two circular 4 diameter masks each with 16 circular cut outs
using a Cricut (Cut Smart 20). A 1 : 1 concentration of EpoxySet
Resin and EpoxySet Hardener was combined to make epoxy that
was spread on one Kapton mask. Tetratex PTFE was carefully
30 minutes until the inlet compartment was empty.
Scanning electron microscopy (SEM)
ꢃ
Images were acquired with an FEI Helios NanoLab 650 dual
beam SEM at 1 kV and 50 pA using a through-lens detector in
secondary electron mode. Cross-sectional images were taken at
placed on one of the Kapton masks and cured at 120 C for 24
hours. Aer 24 hours, epoxy was spread onto the second Kapton
mask and the mask was attached to the opposite side of the
Tetratex PTFE for mechanical support. The complete PTFE (with
Kapton) support (Fig. S1†) was cured at 120 C for 24 hours and
cleaned by sonication in DI water, acetone, and IPA for 1 minute
each and then blow-dried with nitrogen gas.
ꢃ
an angle of 52 aer a focused ion beam was used to evacuate
ꢃ
a pit and expose a cross-sectional area. A horizontal eld width
(HFW) of 5.97 mm was exposed in both cases.
Brunauer–Emmett–Teller (BET) analysis
Pd sputter-deposition. Palladium was deposited onto
prepared PTFE supports by D.C. magnetron sputtering. The The BET surface area of the PTFE membrane was determined
ꢀ
ꢀ3
6
process chamber had a base pressure of 3 ꢂ 10 torr and an Ar from N
2
adsorption–desorption on a Micrometrics ASAP 2020
instrument measured at 70 K. Prior to analysis, the PTFE was
ꢀ
3
ꢀ2
gas working pressure of 1 ꢂ 10 torr, 2 ꢂ 10 torr, or 2 ꢂ 10
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pretreated at 373 K under a vacuum pressure of 40 Pa for 8 h to was placed with the PTFE facing the chemical compartment. Both
remove any adsorbed moisture from the pores.
the chemical and middle electrochemical compartments were
attached to the atmospheric mass spectrometer. Both compart-
ments were closed to air and stirred at a consistent rate. The ow
Electrochemical measurements
ꢀ1
rate into the instrument was 10 mL min . The potentiostat was
A Metrohm Autolab PGSTAT302N potentiostat and electrolytic used to apply a 100 mA reductive current and an ESS CatalySys
palladium membrane reactor were used for electrochemical atmospheric mass spectrometer was used to measure 2 m/z
experiments. The electrolytic palladium membrane reactor used current ratio over time, switching between the chemical and
has a three-compartment cell conguration consisting of two electrochemical compartment every 2 s with a 5 s purge to detect
electrochemical compartments and one chemical compartment. hydrogen gas evolution on both sides of the cell. The equilibrated
All three compartments were lled with 35 mL solution volume. ion current values were used to calculate the ratio of chem-
The chemical and middle electrochemical compartments were ical : electrochemical hydrogen gas evolution.
separated by Pd/PTFE (or Pd foil) cathode/working electrode. A
Naon membrane was used between the two electrochemical
compartments. Viton foam gaskets were used to seal both the
palladium membrane and the Naon membrane in place and
prevent leaking. A Ag/AgCl electrode (3.0 M NaCl) was used as
a reference electrode and placed in the middle electrochemical
compartment. A 1 cm Pt mesh was used as an anode/counter
electrode and placed in the outer electrochemical compart-
ment. The thickness of the Pd layer of the Pd/PTFE cathode was
Product quantication
Gas chromatography-mass spectrometry (GC-MS). GC-MS
was used to quantify products for the hydrogenation of 1-hex-
yne in pentane and 6-chloro-1-hexyne in pentane and DCM.
Aliquots of 300 mL were taken every 2–4 hours during the reac-
tion and diluted in 700 mL pentane or DCM. GC-MS samples
were further diluted with 300 mL of dilute reaction mixture with
2
700 mL pentane or DCM. GC-MS experiments were conducted on
1
–2 mm (or 25 mm for the Pd foil cathode) and the geometric
an Agilent GC-MS using a HP–5ms column and electron ioni-
zation. The prepared samples were run on an autosampler with
a 1 mL injection volume and a split ratio of 20 : 1. Reactions in
2
surface area of the cathode was 1.22 cm on both sides. The PTFE
support was 25.4 mm thick with 0.05 mm pore size. Experiments
were chronopotentiometric where a reductive current was
applied by the potentiostat to the Pd/PTFE (or Pd foil) working
electrode (cathode) and the potential was measured between the
Pd cathode and the reference electrode.
ꢃ
pentane: the oven temperature began at 30 C for 2 min and
ꢃ
ꢃ
ꢀ1
ꢃ
ꢃ
ꢀ1
ramped to 40 C at 2 C min then to 200 C at 40 C min . A
solvent delay of 2.15 min was employed. Peaks for 1-hexyne, 1-
hexene, n-hexane, 6-chloro-1-hexyne, 6-chloro-1-hexene, and 6-
chlorohexane were identied by searching the National Insti-
tute of Standards and Technology (NIST) database for matching
mass spectra. Reactions in DCM: the oven temperature began at
Electrochemically active surface area (ECSA). Electrochemi-
cally active surface area (ECSA) measurements of the Pd/PTFE,
bare Pd foil, and Pd/Pd foil were performed by running cyclic
ꢀ
1
voltammograms at varying scan rates (10 to 100 mV s ) and
plotting current versus scan rate. The middle electrochemical
compartment was lled with 0.15 M tetrabutylammonium
ꢃ
ꢃ
ꢃ
ꢀ1
7
1
0 C for 1 min and ramped to 80 C at 5 C min and held for
ꢃ
ꢃ
ꢀ1
min. The temperature was then ramped to 81 C at 1 C min
ꢃ
ꢃ
ꢀ1
then to 200 C at 50 C min . A solvent delay of 2.80 min was
employed. Peaks for 6-chloro-1-hexyne, 6-chloro-1-hexene, and
6 3
hexauorophosphate (TBA-PF ) in CH CN with the other two
compartments le empty. A Ag/AgCl reference electrode and Pt
mesh counter electrode were used. For Pd/PTFE, double-layer
capacitance measurements were done on both sides of the
membrane to study the electrochemical surface area at the Pd–
PTFE interface and Pd interface. The liquid diffusion of the
organic electrolyte through the PTFE layer was conrmed by
a relatively low uncompensated resistance of ꢁ95–98 U
6-chlorohexane were identied by searching the NIST database
for matching mass spectra.
Nuclear magnetic resonance (NMR). NMR was used for
product quantication of 6-chloro-1-hexyne hydrogenation in
MeOH in the chemical compartment. 500 mL of the reaction
mixture (0.1 M), 500 mL of methanol-d4 with 0.1 M dime-
1
thylsulfone internal standard were added to an NMR tube. H
(
0
compared to ꢁ74–76 U for Pd foil). A potential range of 0.05 to
NMR spectra were acquired on a Bruker Avance 400inv spec-
trometer at 298 K. Relative concentrations were determined by
comparing methylene signals (2H) of 6-chloro-1-hexyne (ꢁ1.83
ppm) and 6-chloro-1-hexene (ꢁ2.11 ppm) and methyl signal
.25 V versus Ag/AgCl with the plotted current taken at 0.15 V
versus Ag/AgCl (the open circuit voltage) was used for all three
cases. The slope of the plot was used to measure double-layer
capacitance. Specic capacitance was calculated using bare Pd
foil because it embodies an atomically smooth planar Pd
(
3H) of 6-chlorohexane (ꢁ0.90 ppm). In each case the internal
standard was integrated to 6.
36
surface. ECSA was calculated by dividing double-layer capaci-
tance by specic capacitance of Pd to quantify chemical surface
area available for reaction.
Palladium leaching quantication
Atmospheric mass spectrometry (atm-MS). Hydrogen evolu-
Inductively coupled plasma-optical emission spectroscopy
tion measurements were made using the three-compartment cell (ICP-OES). ICP-OES studies were performed with the Pd/PTFE
with no substrate in the chemical compartment. 1 M H SO was membrane reactor to determine whether any palladium content
2
4
added to both electrochemical compartments with a Naon was leached from the membrane aer multiple hydrogenation
membrane in between and the solvent in the chemical compart- cycles. The Pd/PTFE membrane was used for three hydrogenation
ment was changed for each experiment. The Pd/PTFE membrane reaction cycles (8 hours per cycle) with 1-hexyne in pentane in the
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2
6,30–32
chemical compartment under an applied current of 50 mA. The 1 mm thickness), commonly used in gas-fed PMRs,
yiel-
ꢀ1
sample was prepared by removing the nal 30 mL reaction ded rates of <0.15 mL h for all solvents tested. These data
mixture from the chemical compartment aer each hydrogena- show that alumina exhibits slower diffusion rates than PTFE in
tion cycle. Substrate and solvent were removed using a rotary each case, with the exception of H O.
2
evaporator under vacuum and the vial was treated with 500 mL of
0% HNO for 72 h. Subsequently, 6 mL of dH O was added to on PTFE supports with different pore sizes and under different
yield a <5% HNO concentration matrix mixture. A calibration sputtering pressures. The amount of palladium required to
curve was prepared using a 1000 mg L Pd in 5% HNO
standard solution. The calibration curve ranged from 0 to 2.5 ppm on pore size, surface roughness, and surface chemistry of the
We then tested the effect of depositing a thin palladium lm
7
3
2
3
ꢀ
1
3
stock achieve a pinhole-free, continuous palladium lm is dependent
33
with 3 emission lines (340 nm, 361 nm, and 324 nm). ICP-OES porous support layer. A 1 mm lm of palladium was sputter-
measurements of the samples yielded results of 0.021 ꢄ deposited onto PTFE supports with 0.05 mm, 0.1 mm, and 0.2
0
.007 ppm for the rst cycle, 0.011 ꢄ 0.003 ppm for the second mm pore sizes and palladium surface coverage on each support
cycle, and 0.014 ꢄ 0.005 ppm for the third cycle in the initial was examined by SEM (Fig. S4†). SEM images conrmed that
30 mL reaction mixtures.
a reduction in support pore size resulted in higher palladium
surface coverage, and a palladium layer with no visible pinholes
>
100 nm was obtained using the 0.05 mm pore-sized PTFE
Results
support (Fig. 2b and S4a†). A ꢁ1.5 mm lm of palladium was
ꢀ3
Design and fabrication of Pd/PTFE membranes
then sputtered using three working pressures: 1 ꢂ 10 torr, 2
ꢀ3 ꢀ2
ꢂ 10 torr, and 2 ꢂ 10 torr to study the effect of argon
pressure on palladium surface morphology. SEM images
showed that reducing sputtering pressure resulted in smaller
Pd crystals and a more continuous lm (Fig. S5†). A pressure of
The supported palladium membranes used in this study were
fabricated by sputter-deposition of 1–2 mm of dense palladium
onto porous PTFE supports (Fig. 2a and b). PTFE supports were
prepared by sealing a PTFE membrane (0.05 mm average pore
size, 25.4 mm thickness, 104.7 m g BET surface area) between
two Kapton masks with circular cutouts with a geometric
ꢀ
3
2
ꢀ1
1 ꢂ 10 torr resulted in large cracks across the membrane
ꢀ2
while a pressure of 2 ꢂ 10 torr did not provide full surface
ꢀ
3
2
coverage. A sputtering pressure of 2 ꢂ 10 torr was used for all
Pd/PTFE membranes hereaer since it provided a continuous
palladium layer without any cracks, which is consistent with
observations made for sputter-deposition of thin Pd lms on
surface area of 1.22 cm (Fig. S1†). The Pd/PTFE membranes
13
were tested in a three-compartment membrane reactor, and
used to separate the two electrochemical compartments from
the chemical compartment (Fig. 2c). The Pd/PTFE membrane
was congured such that the PTFE support faced the chemical
compartment to match the hydrophobicity of the membrane to
that of the organic solvents used in the chemical compartment
37
supports in gas-phase systems.
We next performed electrochemical experiments on these
dense Pd/PTFE membranes to demonstrate that the hydro-
phobic PTFE support should be oriented toward the chemical
compartment and the palladium toward the electrochemical
compartment. The membrane was rst tested with the PTFE
support facing the electrochemical compartment, and both
compartments were lled with 1 M H SO (Fig. S6a†). A 100 mA
(vide infra). The chemical compartment was lled with 35 mL of
solvent with or without reactant. The two electrochemical
compartments each contained 35 mL of 1 M H SO as the
electrolyte. A proton-conducting Naon membrane was used to
2
4
2
4
separate the outer electrochemical compartment containing a 1
2
current was applied and the voltage immediately dropped to the
10 V limit of the instrument, suggesting that the electrolyte
cm platinum mesh anode from the middle electrochemical
ꢀ
compartment containing a Ag/AgCl reference electrode and the
Pd (on PTFE) cathode. In each experiment, a reductive current
was applied to the system and protons formed at the Pt anode
from water oxidation passed through the Naon membrane to
be reduced to surface-adsorbed hydrogen on the Pd cathode.
was not able to diffuse through the PTFE and make contact with
the palladium layer. The Pd/PTFE was then reversed such that
the PTFE layer faced the chemical side (Fig. S6b†) and a poten-
tial of ꢁ ꢀ0.6 V was recorded over 1 hour (Fig. S6c†). In all
experiments hereaer, the Pd/PTFE membrane was orientated
such that the PTFE layer faced the chemical side to ensure ionic
contact between the electrolyte and the palladium cathode and
Wettability and liquid diffusion experiments
We tested the wettability and liquid diffusion of bare PTFE to ensure PTFE wettability by organic solvents.
supports with different pore sizes and thicknesses (pore size,
thickness ¼ 0.05 mm, 25.4 mm; 0.1 mm, 74 mm; 0.2 mm, 66 mm)
Hydrogen permeation measurements
and found that lower-polarity solvents yielded faster liquid
diffusion in all cases. Solvent droplet images show a trend in Hydrogen evolution was measured to conrm that the Pd/PTFE
wettability consistent with the polarities of the solvents, with membrane was selectively permeable to hydrogen atoms and
non-polar solvents fully wetting the support (Fig. S3†). Liquid blocked passage of solvents. For these tests, the electrochemical
diffusion experiments showed the same trend, where pentane compartment was lled with 1 M H SO , the solvent was varied in
2
4
ꢀ1
had the fastest diffusion rates of $5.5 mL h and no H O the chemical compartment, and a 100 mA current was applied
2
diffused through any of the PTFE supports tested over the 24 (Fig. 3a). An atmospheric-mass spectrometer (atm-MS) was used
hour period (Fig. S2 and Table S1†). Liquid diffusion to measure the relative production of hydrogen gas evolved on
measurements on porous alumina supports (0.1 mm pore size, the chemical and electrochemical sides of the membrane
26590 | J. Mater. Chem. A, 2019, 7, 26586–26595
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Fig. S7†), with hydrogen ux being dened as the amount of
Journal of Materials Chemistry A
(
hydrogen gas evolved on chemical side per unit time. Experi-
ments performed with pentane, DCM, MeOH, and H SO in the
2
4
chemical compartment (Fig. 3b and S7†) resulted in 55%, 80%,
8%, and 96%, respectively, of hydrogen gas released on the
8
chemical side, indicating successful hydrogen permeation.
Solution resistances did not change before and aer the experi-
ments (ꢁ4–6 U), suggesting that the organic solvent did not pass
through the palladium layer in to the electrochemical compart-
ment. These results conrm that only hydrogen (and not solu-
tion) permeates the Pd/PTFE membrane in all solvents tested,
and that this hydrogen is available for hydrogenation reactions.
Hydrogen permeation measurements were also performed
on the Pd/PTFE membranes to study the effect of Pd layer
thickness on hydrogen ux. A palladium layer of 1 mm, 1.5 mm, 2
mm, and 3.5 mm was sputter-deposited onto the 0.05 pore-sized
PTFE membrane. All Pd thicknesses resulted in a hydrogen ux
ꢀ
2
ꢀ1
of ꢁ1.7 mmol cm
h , under 100 mA applied current with 1 M
H SO in both compartments (Fig. S8†). These data suggest that
2
4
hydrogen ux is independent of thickness in our Pd/PTFE
membrane reactor.
Fig. 4 (a) Hydrogenation reaction of 1-hexyne to 1-hexene and n-
hexane using (b) electrodeposited palladium catalyst on palladium foil
Catalytic hydrogenation measurements
(
Pd/Pd foil) membrane and (c) palladium sputtered on PTFE (Pd/PTFE)
The hydrogenation of 1-hexyne in the chemical compartment of
the PMR (Fig. 4a) was used as a proof-of-concept reaction to
demonstrate that the Pd/PTFE membrane has comparable
membrane in the electrolytic palladium membrane reactor. 1-Hexyne
consumption and product formation using the (d) Pd/Pd foil
membrane and (e) Pd/PTFE membrane over an 8 hour period. Each
reaction rates to a palladium foil with a palladium catalyst layer compartment was filled with 35 mL of solution volume. A 50 mA
(
Pd/Pd foil) (Fig. 4 and S9). We have previously shown that current was applied in both cases. The Pd foil is 25 mm thick and the Pd
layer (of the Pd/PTFE membrane) is 1–2 mm thick.
electrodepositing a layer of palladium black catalyst (ꢁ5 mg)
onto a planar palladium foil can lead to an increase in catalytic
surface area that results in a ꢁ10-fold increase in 1-hexyne
13
similar working electrode voltages for both reactors (Fig. S10†).
Electrochemical surface area (ECSA) measurements were used
as an approximation for the palladium catalytic surface area,
and a comparison between a planar Pd foil membrane, Pd/Pd
foil, and Pd/PTFE (Fig. S11†) indicated that the catalytic
consumption rate of Pd/Pd foil compared to Pd foil. In all
cases, the chemical compartment was lled with 0.1 M 1-hexyne
in pentane, the electrochemical compartment was lled with
1
M H SO , and a 50 mA current was applied (Fig. 4b and c). We
2 4
measured the 1-hexyne to be completely consumed aer 6 and 8
2
2
2
surface areas were 1.22 cm , 64.2 cm , and 14.9 cm for the
three membranes, respectively. These results demonstrate that
a supported palladium membrane is effective for hydrogenation
in an electrolytic palladium membrane reactor.
hours for Pd/Pd foil and Pd/PTFE, respectively (Fig. 4d and e), at
Three successive 1-hexyne hydrogenation experiments were
performed using a single Pd/Pd foil and Pd/PTFE membrane to
measure the stability of the membranes aer multiple reaction
cycles. For a Pd/Pd foil membrane, the starting 1-hexyne
material was consumed aer 6 hours during the rst two cycles,
but took 8 hours to reach completion by the third cycle
(Fig. S12a†). The electrodeposited Pd black on the Pd foil was
replaced every #3 uses to ensure that reaction rates were not
affected by any modications to the catalyst surface. The
removal of Pd black using our cleaning procedure (see Experi-
mental), however, reduced the mechanical stability of the
Fig. 3 (a) Electrolytic Pd/PTFE membrane reactor setup to measure
hydrogen evolution and flux through the membrane. (b) Hydrogen flux
through the palladium layer for solvents with different polarities in the palladium foil and over ꢁ3 cleaning procedures, pinholes
chemical compartment (pentane being the most non-polar and began to form in the foil rendering it unusable (Fig. S12b and
H
2
SO
4
being the most polar). A 100 mA current was applied and an
c†). A Pd foil membrane lasts ꢁ9 hydrogenation cycles before it
must be replaced with a new foil. The reaction rates also
diminished over three hydrogenation cycles for Pd/PTFE
atmospheric mass spectrometer (atm-MS) was used to measure H
evolved on the chemical and electrochemical sides. Each compart-
ment was filled with 35 mL of solution volume.
2
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38
(
Fig. S12d†), and aer 3–5 cycles the membranes form small ow conditions are not necessarily amenable to electrolytic
cracks and must be replaced (Fig. S12e and f†). The reaction PMRs. In the electrolytic PMR, replacing the Pd foil membrane
mixtures aer each hydrogenation cycle with the Pd/PTFE with Pd deposited on a porous support introduces a resistive
membrane were analyzed by ICP-OES to determine residual layer that reduces how quickly the electrolyte or reactant
palladium aer reaction and no detectable quantities of palla- (depending on membrane orientation) can diffuse through to
dium were found in the chemical compartment. For these tests, the palladium catalyst. This extra layer can hinder diffusion
39
3
0 mL of 1-hexyne in pentane was used and a palladium rates, negatively impacting the rate of hydrogenation and
concentration of <21 ꢄ 7 ppb was measured at various stages of performance of the reactor. For these reasons, a support layer
the reaction with the Pd/PTFE membrane (Fig. S13†). that enables fast liquid diffusion while providing good adhe-
Hydrogenation experiments were then performed in various sion is fundamental to the success of the reactor. Gas-fed
solvents to examine how solvent polarity affects reaction rates. studies have shown that sputtering <10 nm of palladium onto
The reactant 6-chloro-1-hexyne (Fig. 5a) was used because it an intermediate PTFE support layer (on porous glass) can
40
could be solubilized in a range of solvents (pentane, DCM, and enable better adhesion than supports without PTFE. More-
MeOH). The electrochemical compartment was lled with 1 M over, the liquid diffusion experiments we performed on porous
H
2
SO
and a 50 mA current was applied (Fig. 5b). Experiments were PTFE supports were faster than with the alumina supports in all
carried out on both Pd/Pd foil and Pd/PTFE to determine if cases, except for H O (Fig. S3 and Table S1†), with a 50-fold
solvent polarity impacted rates of non-supported membranes increase in rate for the most non-polar solvent (pentane). These
Fig. 5c, S14–S17†). Consumption rates of 6-chloro-1-hexyne data demonstrate that the supported Pd membranes developed
were determined by the slope of the initial 3 hours of up to this point are not amenable for electrolytic PMRs and
4
, the solvent was varied in the chemical compartment, alumina supports show that solvent diffusion rates through the
2
(
ꢀ1
consumption (mmol h ). Consumption rates were the highest highlight the importance of developing membranes with thin
for the most non-polar solvent, pentane (0.58 mmol h ) and supports (<200 mm thick) and good liquid diffusion.
lowest for the most polar solvent, MeOH (0.47 mmol h ) when
Pd/PTFE was used, and 0.6–0.63 mmol h for all solvents when 1-hexyne hydrogenation reaction rates were both affected by the
Pd/Pd foil was used. These data suggest that the hydrophobicity polarity of the solvent in the chemical compartment (Fig. 3b
of the support does affect the hydrogenation reaction and the and 5c). We attribute both of these effects to the high hydro-
high hydrophobicity of the PTFE support enables the fastest phobicity and solvent-dependent wettability of the PTFE
ꢀ
1
ꢀ1
The hydrogen ux through the membrane and the 6-chloro-
ꢀ
1
hydrogenation rates in non-polar solvents.
support (shown schematically in Fig. 6). While 96% of the
hydrogen permeated through the Pd when H SO was contained
in the chemical compartment, merely 55% of hydrogen
permeated when pentane was used. Polar solvents like H SO
or water) cannot diffuse through the PTFE support, and the
2
4
Discussion
2
4
(
Supports used in gas-fed PMRs that are designed to enable fast
PTFE support layer acts effectively as a gas-phase layer (Fig. 6a
and S3†). In contrast, pentane fully wets the PTFE support
hydrogen gas diffusion under high temperature and pressure
(Fig. 6b and S3†). Hydrogen permeation into the liquid-phase is
known to be less favoured than permeation into the gas-phase
because a liquid layer can affect both hydrogen transition
from bulk to surface and hydrogen recombination at the Pd–
41
PTFE interface.
Fig. 6 Schematic depictions of hydrogen flux through the palladium
layer demonstrate the difference between (a) a polar solvent (H SO
Fig.
5 (a) Hydrogenation reactions of 6-chloro-1-hexyne to 6-
2
4
)
chloro-1-hexene and 6-chlorohexane. (b) Cell architecture using the
Pd/PTFE membrane reactor and (c) 6-chloro-1-hexyne consumption
rates in pentane, DCM, and MeOH with the Pd/Pd foil membrane (light
purple) and Pd/PTFE membrane (dark purple). Each compartment was
filled with 35 mL of solution volume. A 50 mA current was applied in
both cases and consumption rates were determined for the first three
hours of reaction.
and (b) a non-polar solvent (pentane) in the chemical compartment.
For a polar solvent, there is no liquid diffusion through the PTFE
support which results in a gas-phase PTFE layer that enables hydrogen
evolution to occur freely at the palladium-PTFE interface. For a non-
polar solvent, liquid diffuses through the PTFE support layer to the
palladium-PTFE interface and can affect the rate of hydrogen
recombination.
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Journal of Materials Chemistry A
Table 1 Comparison of mass of palladium, electrochemically active surface area, and 1-hexyne consumption rates using palladium foil with
palladium catalyst (Pd/Pd foil) and Pd/PTFE membranes in the electrolytic palladium membrane reactor
Pd mass (g)
ꢄ 0.005
Consumption rate
Normalized consumption rate
2
ꢀ1
ꢀ1 ꢀ1
Membrane
ECSA (cm )
(mmol h
)
(mmol gPd
h )
Pd/Pd foil
Pd/PTFE
0.257
0.011
64.2
14.9
0.91
0.72
3.5
65.5
Faster diffusion of non-polar solvents, and therefore the 6-
The comparable consumption rates of 1-hexyne by Pd/PTFE
chloro-1-hexyne reactant, to the catalyst surface also enabled and Pd/Pd foil may be attributed, in part, to the catalytic surface
faster hydrogenation rates. The hydrogenation rates of 6-chloro- area at the Pd–PTFE interface. ECSA measurements recorded on
1-hexyne followed a solvent polarity trend (Fig. 5c and S18†), both sides of the Pd/PTFE membrane show that the catalytic
conrming the effect of PTFE wettability in the chemical surface area of palladium at the Pd–PTFE interface is ꢁ3ꢂ
compartment on hydrogenation rates. These experiments also higher than that at the Pd interface (Fig. S19†). The higher ECSA
demonstrated that, even in polar solvents, substrate and solvent at the Pd–PTFE interface is attributed to the surface structure of
can diffuse through the PTFE support to participate in hydro- the porous PTFE layer, which creates palladium-lled pores (as
genation at the palladium layer. Moreover, these ndings shown by cross-sectional SEM in Fig. 2a) that result in a non-
suggest that the hydrophobicity of the support is an important planar palladium layer. We have previously shown that 1-hex-
factor to consider when designing supported membranes for yne hydrogenation rates are affected by catalytic surface area,
electrolytic PMRs, and that the hydrophobicity of the support and that electrodeposition of a palladium catalyst on a planar
13
needs to be tuned to match the polarity of the solvent/reactant palladium foil leads to faster reaction rates. The catalytic
being studied. surface area of the palladium at the Pd–PTFE interface was
2
2
Hydrogenation of 1-hexyne proceeded at similar rates using measured to be 14.9 cm compared to 1.22 cm for planar Pd
the Pd/PTFE membranes compared to Pd/Pd foil membranes foil. The higher catalytic surface area of Pd at the Pd–PTFE
(Fig. 4d and e), with 20-fold less palladium metal. Mass- interface compared to a planar palladium electrode (Fig. S11†)
independent and -dependent (normalized) consumption rates suggests that the surface area is one factor that contributes to
were calculated using the slope of the initial 3 hours of the performance of the Pd/PTFE membranes in hydrogenation
ꢀ1
consumption (mmol h ) divided by mass of palladium content reactions. These data also indicate that catalytic surface area
ꢀ
1
ꢀ1
(mmol
g
Pd
h ) (Table 1). The normalized 1-hexyne can be increased by deposition of palladium directly onto
consumption rates of the Pd/PTFE and Pd/Pd foil membranes a high-surface area porous support without the need for addi-
ꢀ1
ꢀ1
ꢀ1 ꢀ1
were 65.5 mmol gPd
The 6-chloro-1-hexyne reaction data mirrored these results, with
Fig. S18†) normalized consumption rates of Pd/PTFE >18-fold lifetime and must be replaced aer a few uses (Fig. S12†). For
h
and 3.5 mmol gPd
h
, respectively. tional catalyst.
Both the Pd/Pd foil and Pd/PTFE membranes have a limited
(
larger than Pd/Pd foil for all cases. These data indicate that the lab-scale reactions, multiple Pd/PTFE membranes are produced
Pd/PTFE membranes have a signicantly higher reaction rate at once (Fig. S1a†), and thus are easier to replace at a reduced
per mass of palladium compared to Pd/Pd foil membranes.
cost compared to the Pd foil membranes. The current Pd/PTFE
In spite of the fast normalized hydrogenation rates of Pd/ membranes must be replaced due to cracks formed on the Pd
PTFE, the measurement of hydrogen ux through Pd/PTFE surface (Fig. S12f†). The formation of the cracks may be
membranes with different palladium thicknesses showed that attributed to the expansion and contraction caused by phase
33
hydrogenation rates are independent of palladium thickness. transformations of the palladium lattice at room temperature.
For reactors with the palladium membrane, palladium thick- This situation may be avoided in future studies by alloying
ness is inversely proportional to hydrogen permeation only palladium with metals such as silver, which improves
when diffusion is rate-limiting, and reaction rates are inde- mechanical stability by reducing phase transformations at
26,38,43
pendent of thickness when other processes (e.g., absorption room temperature.
into the palladium or hydrogenation on the opposing side of the
42
membrane) are rate-limiting. The data shown in Fig. S8†
demonstrates that the hydrogen uxes are held nearly at parity
Conclusions
ꢀ
2
ꢀ1
(ꢁ1.7 mmol cm
h ) for palladium membrane thicknesses
We have demonstrated that a supported Pd/PTFE membrane
can reduce the mass of palladium by 20-fold compared to Pd foil
membranes in electrolytic palladium membrane reactors. We
have shown that supports used in gas-fed reactors are incom-
patible with the electrolytic environment, and therefore the
design of electrolytic PMRs are subject to different constraints
compared to gas-fed PMRs. Palladium deposition onto the
porous PTFE support enabled a higher catalytic surface area at
ranging from 1 mm to 3.5 mm. The constant ux at varying
thicknesses, as well as the reduction in reaction rate when
higher polarity solvents are placed in the chemical compart-
ment (Fig. 3b and 5c), suggest that hydrogenation, and not
hydrogen diffusion through palladium, is the rate-limiting step
in our Pd/PTFE membrane system.
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the Pd–PTFE interface compared to planar palladium
membranes and fast liquid diffusion of organic solvents to the
palladium layer. The supported membranes yielded similar
9 J. Lessard, in Encyclopedia of Applied Electrochemistry, ed. G.
Kreysa, K.-I. Ota and R. F. Savinell, Springer New York,
New York, NY, 2014, pp. 443–448.
hydrogenation rates to palladium foil membranes and 10 N. Singh, Y. Song, O. Y. Guti ´e rrez, D. M. Camaioni,
improved reaction rates per mass of catalyst. This study shows
that supported palladium membranes can be designed to
C. T. Campbell and J. A. Lercher, ACS Catal., 2016, 6,
7466–7470.
provide a more cost-effective and potentially scalable palladium 11 L. Coche and J. C. Moutet, J. Am. Chem. Soc., 1987, 109, 6887–
membrane reactor for electrolytic environments.
6889.
1
1
1
1
2 J. M. Chapuzet, A. Lasia and J. Lessard, Electrocatalysis, 1998,
155–159.
Conflicts of interest
3 R. S. Sherbo, R. S. Delima, V. A. Chiykowski, B. P. MacLeod
and C. P. Berlinguette, Nat. Catal., 2018, 1, 501–507.
4 C. Iwakura, Y. Yoshida and H. Inoue, J. Electroanal. Chem.,
There are no conicts to declare.
1997, 431, 43–45.
Acknowledgements
5 R. S. Sherbo, A. Kurimoto, C. M. Brown and
We are grateful to Stewart Blusson Quantum Matter Institute,
C. P. Berlinguette, J. Am. Chem. Soc., 2019, 141, 7815–7821.
the Canadian Natural Science and Engineering Research 16 S. Nishimura, Handbook of Heterogeneous Catalytic
Council (RGPIN 337345-13), Canadian Foundation for Innova-
tion (229288), Canadian Institute for Advanced Research (BSE-
Hydrogenation for Organic Synthesis, Wiley–VCH, New York,
2001.
BERL-162173), and Canada Research Chairs for nancial 17 T. Maoka and M. Enyo, Electrochim. Acta, 1981, 26, 607–614.
support. We thank Brian Ditchburn for fabricating the cells. We 18 M. A. V. Devanathan and Z. Stachurski, Proc. R. Soc. London,
also thank Dr Maria Ezhova and Mark Okon for access to the
Ser. A, 1962, 270, 90–102.
850 MHz spectrometer, Dr Yun Lung for help with GC-MS 19 Y. Kato, K. Inoue, M. Urasaki, S. Tanaka, H. Ninomiya,
experiments, and Majed Alamoudi for help with BET analysis.
ICP-OES measurements were performed by Maureen Soon in
the Pacic Centre for Isotopic and Geochemical Research. SEM
T. Minagawa, A. Sakurai and J. Ryu, Proceedings of the
International Symposium on Innovative Materials for
Processes in Energy Systems 2010, vol. 2010.
measurements were performed in the Centre for High- 20 M. Sheintuch and D. S. A. Simakov, in Membrane Reactors for
Throughput Phenogenomics at the University of British
Columbia, a facility supported by the Canada Foundation for
Innovation, British Columbia Knowledge Development Foun-
Hydrogen Production Processes, ed. M. De De Falco, L.
Marrelli and G. Iaquaniello, Springer London, London,
2011, pp. 183–200.
dation, and the UBC Faculty of Dentistry. Sputter-depositions 21 A. Criscuoli, A. Basile, E. Drioli and O. Loiacono, J. Membr.
were done at the 4D LABS shared facilities supported by the Sci., 2001, 181, 21–27.
Canada Foundation for Innovation (CFI), British Columbia 22 R. D. Dolan and N. C. Dave, Int. J. Hydrogen Energy, 2010, 35,
Knowledge Development Fund (BCKDF), Western Economic
10994–11003.
Diversication Canada (WD), and Simon Fraser University 23 H. Ramsurn and R. B. Gupta, New and Future Developments in
(SFU).
Catalysis: Chapter 15. Hydrogenation by Nanoparticle
Catalysts, Elsevier Inc. Chapters, 2013.
2
4 P. N. Rylander, Organic Syntheses with Noble Metal Catalysts,
Elsevier, 2012.
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