Highly Active Binder-Free Catalytic Coatings for Heterogeneous Catalysis and Electrocatalysis: Pd on Mesoporous Carbon and Its Application in Butadiene Hydrogenation and Hydrogen Evolution

Article abstract of DOI:10.1021/acscatal.6b02240

Heterogeneous catalysis performed in wall-coated reactors and electrocatalysis require homogeneous catalytic coatings with high surface area and good accessibility of the active sites. Conventional coating methods necessitate the use of binder components that often block pores and active sites, which limits catalytic efficiency, and utilization of expensive active metals. We report an approach for the direct and binder-free synthesis of chemically, mechanically, and thermally stable catalytic coatings based on ordered mesoporous carbon films employed as catalyst support. The synthesis relies on the codeposition of a structure-directing agent and small clusters of polymeric carbon precursors along with ionic metal species on a substrate. A sequence of thermal treatments converts the polymer into partly graphitized carbon, decomposes the structure-directing agent, and converts the metal precursor into highly active nanoparticles. Syntheses and catalytic applications are exemplarily demonstrated for palladium on carbon, a system widely used in heterogeneous catalysis and electrocatalysis. The obtained catalysts provide significantly higher space-time yields in the selective gas-phase hydrogenation of butadiene than all reported Pd/C catalysts while at the same time retaining isothermal reactor conditions. Moreover, when they were tested in the electrocatalytic hydrogen evolution reaction (HER), the catalysts outperformed reported Pd/C catalysts by a factor of 3, which underlines the benefits of the developed binder-free catalyst system. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.6b02240

Research Article
pubs.acs.org/acscatalysis
Highly Active Binder-Free Catalytic Coatings for Heterogeneous
Catalysis and Electrocatalysis: Pd on Mesoporous Carbon and Its
Application in Butadiene Hydrogenation and Hydrogen Evolution
Denis Bernsmeier,† Laemthong Chuenchom,†,‡ Benjamin Paul,† Stefan Rummler,§ Bernd Smarsly,‡
̈
and Ralph Kraehnert*,†
†Department of Chemistry, Technische Universitat Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany
̈
‡Institute of Physical Chemistry, Justus Liebig Universitat, Heinrich-Buff-Ring 58, 35392 Giessen, Germany
̈
§Institute of Chemistry, Martin-Luther-Universitat Halle-Wittenberg, von-Danckelmann-Platz 4, 06120 Halle, Germany
̈
S
* Supporting Information
ABSTRACT: Heterogeneous catalysis performed in wall-coated
reactors and electrocatalysis require homogeneous catalytic
coatings with high surface area and good accessibility of the
active sites. Conventional coating methods necessitate the use of
binder components that often block pores and active sites, which
limits catalytic efficiency, and utilization of expensive active metals.
We report an approach for the direct and binder-free synthesis of
chemically, mechanically, and thermally stable catalytic coatings
based on ordered mesoporous carbon films employed as catalyst
support. The synthesis relies on the codeposition of a structure-
directing agent and small clusters of polymeric carbon precursors
along with ionic metal species on a substrate. A sequence of
thermal treatments converts the polymer into partly graphitized
carbon, decomposes the structure-directing agent, and converts the metal precursor into highly active nanoparticles. Syntheses
and catalytic applications are exemplarily demonstrated for palladium on carbon, a system widely used in heterogeneous catalysis
and electrocatalysis. The obtained catalysts provide significantly higher space−time yields in the selective gas-phase
hydrogenation of butadiene than all reported Pd/C catalysts while at the same time retaining isothermal reactor conditions.
Moreover, when they were tested in the electrocatalytic hydrogen evolution reaction (HER), the catalysts outperformed reported
Pd/C catalysts by a factor of 3, which underlines the benefits of the developed binder-free catalyst system.
KEYWORDS: catalyst synthesis, Nafion, carbon, palladium, HER, butadiene hydrogenation
ensure the rapid transport of electrons toward the active sites
but should also provide good accessibility and short diffusion
paths for electrolyte and product species.9
INTRODUCTION
■
Finely divided noble metals such as palladium are excellent
catalysts for a wide range of chemical reactions. In
heterogeneous catalysis supported Pd particles are employed,
for example, in automotive catalytic converters1 as well as
numerous hydrogenation2 and dehydrogenation3 catalysts. In
electrochemistry Pd catalyzes the hydrogen evolution reaction,4
the oxygen reduction reaction,5 and oxidation reactions.6 Most
of these catalysts use carbon as a support material due to its
high abundance, stability, and electrical conductivity.7
Fast and highly exothermic Pd-catalyzed reactions in
heterogeneous catalysis typically require the catalyst to be
present in the form of a wall coating. Such coatings can
transport the heat of reaction efficiently from the catalyst, thus
avoiding hot-spot formation.2c Moreover, nanostructured
porous coatings can provide a rapid mass transfer, which is
required for efficient catalyst utilization.8 Catalysts prepared on
electrode plates in the form of surface coatings are also
essentially required in electrocatalysis. These coatings need to
The most prominent technique to produce catalytic coatings
for heterogeneous catalysis is so-called wash-coating.10
Common synthesis methods employ a slurry prepared from
fine-grained catalyst powders, a binder material, and a solvent
for deposition of the coating. Typical oxide-based catalysts
include micrometer-sized (Pt/Al2O3, Rh/Al2O3,11 Al2O312) or
ball-milled (Al2O3,13 Pd/Na-Al-Si-Ox14) powders of noble
metals supported on oxides. Smaller sized particles are obtained
from sols (Al2O3,15,16 SiO2,15 TiO2,15 CeO217) with addition of
active metal species prior to16 or after deposition,17 followed by
calcination and reduction.
Received: August 5, 2016
Revised: October 24, 2016
© XXXX American Chemical Society
8255
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
a
Scheme 1. Illustration of the Synthesis Approach to Mesoporous Pd/OMC Catalyst Films
a
The structure-directing triblock copolymer F127 (PEO-PPO-PEO) and resorcinol were dissolved in ethanol. After addition of hydrochloric acid,
formaldehyde solution was added to start a polycondensation reaction. The derived polymeric resin was separated by centrifuging and washed. The
polymeric compound was redissolved in a Pd(acac)2-containing THF solution. Under controlled conditions a film was deposited via dip-coating to
obtain a homogeneous mesophase film. After thermal treatment at 100 °C in air and a subsequent carbonization under an inert atmosphere (N2) at
600 °C a mesoporous Pd/OMC film was obtained.
The most common method for the preparation of electrode
coatings is the mixing of carbon-based catalyst powders and an
ionomer binder (e.g., Nafion) with a solvent to form an ink.
The ink is cast onto the electrode and dried. Unfortunately,
binders such as Nafion can block pores and active sites and
therefore degrade the coating performance.18 Other challenges
include the homogeneity of ink composition and coating, which
critically influences the resulting electrical conductivity of the
deposited film and makes the accurate and reproducible
synthesis of electrode coatings rather challenging.19
Binder-free coatings of carbon have been reported in the
form of so-called “ordered mesoporous carbon” (OMC), a
carbon material with templated mesoporosity which is
accessible via soft templating with micelles of block
copolymers.20 Parameters such as porosity and degree of
graphitization can be finely tuned.21 Furthermore, heteroatom
doping of the carbon network and surface modifications are
accessible.20,22 OMC films combine a number of properties
desired in catalysis: i.e., high surface area, tunable pore size,
high porosity, chemical inertness, and electrical conductivity.9,23
These attributes make OMC films ideal candidates as catalyst
support materials for electrodes, membranes, and wall-coated
reactors in heterogeneous catalysis. However, suitable ways of
introducing active sites such as particles of noble metals are
required.
naphthalene23c together with formaldehyde23b,31 can be used as
precursors.
The synthesis of templated OMC films has at least three
basic requirements,24 all of which could be significantly
influenced by the additional presence of metal species. (1)
The template polymer needs to assemble with the precursor
clusters to form an ordered mesophase. (2) The template needs
to decompose at a temperature lower than that for the cross-
linked precursor. (3) The formed pore structure needs to be
retained despite a massive loss of volatile species25 and
contraction of the system32 during the final pyrolysis step. A
catalytically active metal could be incorporated into the film
synthesis in two different ways, either as a dissolved metal salt
or as preformed metal nanoparticles dispersed in the solution
that is used for film casting. Major challenges result from a
severe aggregation and sintering of dissolved metal species
during carbonization (>600 °C), which produces large particles
and a broad size distribution.33 Moreover, stabilizing agents that
protect colloidal metal particles from aggregation often also
prevent the self-assembly of the polymeric precursor and pore
template.33 In addition, the interaction between metal and
carbon species during carbonization and pore formation
remains so far largely unexplored. A direct synthesis of
catalytically active noble-metal-containing OMC films has
therefore not been reported so far.
Common methods for the synthesis of films of ordered
mesoporous carbon employ micelles of amphiphilic molecules
deposited along with small clusters of a polymeric resin.24
Interactions between the thermoplastic resin and a thermally
decomposable copolymer during evaporation-induced self-
assembly (EISA) result in formation of an ordered nano-
composite film. Subsequent heat treatments stabilize the
polymeric framework, decompose the template molecules,
and finally transform the remaining polymer into a porous
carbon material.9 A variety of precursors, solvents, and
structure-directing agents can be used from the film synthesis.
Dai and co-workers25 synthesized a hexagonally ordered
mesoporous film employing PS-P4VP as a template. Tanaka
et al. reported hexagonally ordered carbon films (COU-1)
using Pluronic F127 (PEO-b-PPO-b-PEO) as template.26 Other
templates include Brij5823a and Pluronic P123.27 Resorci-
nol,25,26,28 phloroglucinol,29 phenol,27,30 and 1,5-dihydroxy-
We report the first direct synthesis route to mechanically,
chemically, and thermally stable OMC films that incorporate
exemplarily catalytically active palladium particles. The general
synthesis procedure is illustrated in Scheme 1. The synthesis is
based on the redissolution of a prepolymerized RF resin that
already contains a structure-directing agent (Pluronic F127) in
THF along with dissolved metal precursors. The use of metal
salts that are soluble in THF is a key requirement of the
synthesis. After film deposition of the combined solution via
dip-coating, thermal cross-linking, and carbonization, an
electrically conductive mesoporous carbon film is obtained
that contains small and accessible metal particles.
The following sections present the synthesis details and the
obtained morphology, crystallinity, and porosity of carbonized
Pd/OMC film as well as its catalytic activity studied exemplarily
in heterogeneous catalysis (butadiene hydrogenation) and
electrocatalysis (hydrogen evolution reaction (HER)). The
8256
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
Figure 1. Physicochemical analysis of a F127-templated ordered mesoporous carbon film, containing 3 wt % Pd, carbonized at 600 °C. Cross-section
SEM image in secondary electron mode with FFT inset (a) and in backscattered electron mode (b). 2D-SAXS patterns with incident angles of β =
90° (c) and β = 7° (d). The anisotropic ring in (c) can be attributed to regular pore lattice distances of 16.0 nm in first order and 9.4 nm in second
order. The scattering spots in (d) are indexed according to a face-centered-cubic pore lattice (space group Im3m) with (110) orientation in respect
̅
to the substrate surface. TEM micrograph of a scraped off film segment (e) showing homogeneously distributed PdNP (dark spots) in an ordered
mesoporous carbon matrix. XRD analysis (f) of a film carbonized at 600 °C in comparison to a film additionally treated at 400 °C in air. Assigned
(hkl) indices correspond to cubic Pd metal (PDF 46-1043) and cubic PdO (PDF 41-1107). A broad reflection (24°) can be attributed to stacking of
graphene.35
comparison to conventional catalysts reported in the literature
illustrates the superior performance of the binder-free Pd/
OMC in both types of applications.
distributed throughout the whole volume of the mesoporous
carbon film.
N2 sorption analysis of a corresponding Pd/OMC powder
sample confirms a narrow size distribution of the formed
mesopores around 5 nm (see QS-DFT evaluation of a full N2
isotherm, Figure S3 in the Supporting Information). This
observation is consistent with the pore sizes observed from
SEM images. The deduced total pore volume amounts to 0.46
cm3/g. A total of 40% of this pore volume can be assigned to
micropores. The accessible surface area of a Pd/OMC film
carbonized at 600 °C was obtained via BET analysis of a Kr
physisorption measurement and amounts to 590 m2 per cm3
film volume.
RESULTS AND DISCUSSION
■
Physicochemical properties of Pd/OMC Films. Figure 1
presents for a typical Pd/OMC catalyst film carbonized for 3 h
at 600 °C in nitrogen flow SEM cross-section images in (a)
secondary electron mode and (b) backscattered electron mode,
2D-SAXS patterns recorded with incident angles of (c) β = 90°
and (d) β = 7°, (e) a TEM micrograph of a film segment, and
(f) diffraction patterns obtained by XRD. The film contains 3
wt % Pd (from WDX analysis and StrataGem evaluation).34
The SEM cross-section image (Figure 1a) recorded in
secondary electron image mode indicates that the Pd/OMC
films exhibit a homogeneous and locally ordered pore structure
throughout the film without cracks, aggregates, or other
irregularities. The film thickness amounts to ca. 430 nm. The
templated mesopores feature dimensions of 8 nm parallel and 6
nm perpendicular to the substrate. The FFT obtained from the
SEM image (inset in Figure 1a) features an ellipsoidal pattern
with bright spots that can be attributed to periodic pore
distances of 14 nm parallel and 9 nm perpendicular to the
substrate. The diffraction spots can be assigned to the (110)
and (101) planes of a cubic closed packing of pores and
therefore indicate a high degree of ordering. The ellipsoidal
pore shape originates from a uniaxial film contraction during
template removal, which is in line with the ellipsoidal nature of
the pores.8b
Small angle X-ray scattering (SAXS) patterns of Pd/OMC
films carbonized at 600 °C show a high degree of pore ordering
(Figure 1c,d). The pattern recorded at 90° beam incident angle
with respect to the film surface plane shows two isotropic rings
that can be attributed to d spacing values of 16.0 and 9.4 nm of
the mesopore lattice (Figure 1c). SAXS measured in gracing
incidence (Figure 1d) shows an ellipsoidal shape that indicates
uniaxial film shrinkage perpendicular to the substrate. The
bright scattering spots can be assigned to the lattice planes
(110) as well as (101) of a contracted cubic pore system.36 The
̅
d spacing value perpendicular to the substrate amounts to 8 nm,
which indicates a film contraction of ca. 50% during template
removal and carbonization. Comparing Pd/OMC catalysts
containing 3 wt % Pd to Pd-free OCM samples indicates that
the added metal does not have a significant influence on the
obtained degree of pore ordering. However, additional tests
with platinum concentrations exceeding 3 wt % Pt suggest that
for high metal loadings a lower degree of pore ordering can be
expected.
SEM images recorded with backscattered electrons on the
same film position (Figure 1b) indicate the location of Pd-
containing particles as bright spots caused by higher metal z
contrast. The formed Pd particles appear to be separate and
8257
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
Figure 2. Catalytic performance of mesoporous Pd/OMC catalytic coatings in the selective gas-phase hydrogenation reaction of 1,3-butadiene. (a)
Butadiene conversion vs temperature. The Pd/OMC catalyst with 3 wt % Pd loading treated for 5 min at 400 °C in air is compared to a nontreated
Pd/OMC catalyst. (b) Selectivity to 1-butene and total butene selectivity vs butadiene conversion. (c) Butadiene conversion vs temperature of 5
coated catalyst plates compared to a scaled-up approach with 10 plates. Both catalyst sets were pretreated for 5 min at 400 °C in air. The reaction
feed contained 5% butadiene and 10% hydrogen in nitrogen at a constant flow rate of 30 mL/min. The composition of the product gas flow was
studied at seven different temperatures between 35 and 80 °C.
A TEM micrograph of a Pd/OMC film segment (Figure 1e)
confirms the homogeneous distribution of Pd particles and a
particle size of about 14.3 3.2 nm with particles being located
inside a well-ordered mesoporous carbon framework. The
majority of the particles located inside the film remain small.
XRD patterns (Figure 1f) were obtained for Pd/OMC films
carbonized at 600 °C in nitrogen and for films treated
additionally for 15 min at 400 °C in air in order to remove
carbon species from the surface of Pd particles (see also Figure
S4 in the Supporting Information). Clear reflections can be
observed at 40.1, 46.7, and 68.1°, which correspond well with
the (111), (200), and (220) lattice planes of metallic palladium
(PDF 46-1043). SAED analysis (not shown) confirms that Pd
is present in a metallic state. The XRD signal at 56° results
from the substrate (Si wafer) employed for the Pd/OMC film
samples studied in XRD. Reflections corresponding to
palladium oxide were not observed, indicating that no Pd
bulk oxide phase is formed. The crystallite size estimated via the
Scherrer equation amounts to ca. 26 nm. A broad reflection
around 24° can be attributed to the (002) lattice plane of
disordered small graphene stacks.35
XP spectra were recorded in the Pd3d and C1s region in order
to assess the near-surface composition of the material (Figure
S5 in the Supporting Information). The Pd3d spectrum (Figure
S5a) is dominated by a signal that corresponds to metallic Pd
(335.0 eV)37 and smaller contributions of PdO and PdO2 at
336.2 eV38 and 337.6 eV,39 respectively. The oxide contribu-
tions are attributed to a surface oxidation of Pd when the film
sample is exposed to ambient air during sample transfer and
storage. An intense peak in the C1s region (Figure S5b)
confirms the dominating presence of graphitic carbon.40
Partial graphitization is an essential requirement of the film
synthesis in order to produce materials with an electrical
conductivity high enough to allow electrocatalytic reactions to
occur on the formed Pd particles. The electrical conductivity
measured for Pd/OMC films carbonized at 600 °C amounts to
0.03 S/cm. Adhesion of a Pd/OMC film to a steel substrate was
tested by the tape test.41 As depicted in Figure S1 in the
Supporting Information, no carbon film was peeled off after
pulling off a tape. This behavior indicates good mechanical
stability and strong adhesion of the carbon film to the substrate.
The developed synthesis approach thus produces conductive
carbon films with well-ordered mesopores throughout the film
volume with a high surface area. The formed metallic Pd
nanoparticles are well dispersed, and the presence of Pd does
not disturb the formation of the open-porous carbon network.
The carbon shows a small degree of graphitization and good
electrical conductivity.
Catalytic Performance in Butadiene Hydrogenation.
The developed Pd/OMC catalytic coatings were tested in a
microstructured reactor designed to accommodate 5 and 10
catalyst plates with minimal channel width (1625 and 830 μm,
respectively). The reactor design (see Figure S2b in the
Supporting Information) maximizes the contact area between
the catalytic coating and thermally conductive reactor material
to ensure that very fast and exothermic reactions such as the
hydrogenation of butadiene can be studied under isothermal
conditions.
Figure 2 displays catalytic results obtained with a stack
containing five reactor plates coated on both sides with Pd/
OMC, carbonized at 600 °C under an N2 atmosphere, and
additionally treated for 5 min at 400 °C in air to remove carbon
species. Shown are (a) butadiene conversion vs temperature
with and without the initial air treatment, (b) selectivity vs
conversion, and (c) a comparison of butadiene conversion
between reactors comprising five and ten stacked catalyst
plates.
Figure 2a indicates a poor activity for the film treated only in
N2, with a butadiene conversion reaching less than 11% at 80
°C. After additional treatment of a corresponding Pd/OMC
film in air (5 min, 400 °C) a drastically higher activity can be
observed that reaches 20% already at 35 °C and 83% at 80 °C.
The strongly enhanced performance suggests that the Pd
particles contained in the “as carbonized” Pd/OMC film might
be covered by a thin carbon shell, which can be removed by the
air treatment and results in a highly active hydrogenation
catalyst (see Figure S4 in the Supporting Information for more
information).
Figure 2b plots for the air-treated Pd/OMC catalyst the
selectivity toward 1-butene and toward the sum of all butenes
vs the butadiene conversion. The catalyst performs very
selectively. At low conversion selectivities amount to about
54% (1-butene) and 92% (sum of all butenes). The observed 1-
butene selectivity agrees well with typical Pd/C and Pd/TiO2
catalysts reported in the literature.8b,34,42 More remarkably, the
high selectivity to 1-butene is retained also well above 80%
8258
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
Figure 3. Electrocatalytic performance of Pd/OMC, Pd/C/Nafion, and Pd-free OMC films in the HER: (a) current response at a potential of −200
mV (vs RHE) plotted vs cycle number derived from the first 50 cathodic sweeps during CV testing; (b) geometric current density plotted vs
potential for the 50th cycle of CV testing; (c) Tafel evaluation (potential vs log(current density) for the 50th cycle. The first 15 points in the HER
regime were employed for linear fitting. All CVs were recorded in 0.5 M sulfuric acid with a scan rate of 20 mV/s. The graphs compare a mesoporous
3 wt % Pd/OMC catalyst with a Pd-free OMC coating (390 nm thick film) and a conventional Pd/C/Nafion reference catalyst film based on a
Nafion ink and commercial Pd/C (5 wt % on activated carbon). Both Pd-containing catalysts feature the same geometric Pd loading of 1.15 μgPd/
cm2.
butadiene conversion, whereas typical catalysts reported in the
literature show a significant decrease in selectivity starting
already at butadiene conversion levels of about 50% (Pd/TiO2
in ref 2c) to 60% (Pd/C in ref 42b, Pd/TiO2 in refs 8b and
42a).
(Pd/TiO2, 68 kJ/mol45 and 62 kJ/mol;8b,42a Pd/Al2O3, 48−66
kJ/mol2d) as well as carbon-supported Pd (Pd/C, 58 kJ/mol;
PdNP/graphene, 65 kJ/mol; both calculated from data
reported in ref 42b). An apparent energy of activation which
amounts to about half the value of the true activation energy is
a typical indicator of transport limitations by pore diffusion.46
Physisorption measurements on the Pd/OMC catalysts indicate
that about 40% of their total pore volume can be assigned to
micropores (see Figure S3 in the Supporting Information). The
observed apparent energy of activation of 31.5 kJ/mol could
thus indicate that a major fraction of the active Pd could be
located inside micropores. However, Pd/OMC still outper-
forms all comparable carbon-supported Pd catalysts reported in
the literature in terms of butene formation rates.
The space−time yield of butene formed per mass of Pd
contained in the catalyst can be used to compare the
performance of different reported Pd/C catalysts directly. Yan
et al.42b reported performance data obtained under conditions
comparable to those of our study (48 °C, cH2:cBD = 2.5:1, 110
μg of active Pd, 1.01 bar). The reported catalysts include a
commercial Pd/C, Pd nanoparticles supported on graphene,
and single-site Pd catalysts supported on graphene. The
obtained STYs for those catalysts amount to 2.5, 2.5, and 3.0
molbutene s−1 kgPd−1, respectively. In contrast, the Pd/OMC
One specific advantage of the developed synthesis route is
the simple and facile scale-up of the catalyst and reactor
production via “numbering up” of the number of catalyst plates
fitted into the reactor housing. A photo of a microstructured
reactor fitted with 5 and 10 catalyst plates is shown in Figure S2
in the Supporting Information. The corresponding perform-
ance of both catalyst stacks is compared in Figure 2c in terms of
butadiene conversion plotted vs temperature. At 35 °C the
conversion amounts to 20% (five plates) and 40% (10 plates):
i.e., twice as high. Moreover, the observed space−time yield
−1
−1
catalyst presented in Figure 2a produces 3.8 mol
s
kgPd ,
butene
which indicates a 25−50% higher mass-specific catalytic activity
toward butene formation.
Different factors might be contributing to the observed high
activity. Silvestre et al. and Dal Santo et al. reported that
butadiene hydrogenation is a highly structure sensitive
reaction.43 The hydrogenation reaction occurs on the Pd
(111) surface sites for supported Pd particles that exceed a
particle size of 4 nm.43a A closer look at the shape of the
particles in this work reveals that the predominant particle
shape is octahedral with some minor fractions of truncated
octahedra. The facets of octahedra are (111). Thus, hydro-
genation on (111) surfaces is likely to be the dominating
reaction. Moreover, Schauermann, Freund, and co-workers
reported in a series of publications that the presence of
subsurface carbon atoms on Pd surfaces drastically lowers the
activation barrier for subsurface migration of H atoms on Pd
(111) facets, which critically influenced the corresponding
activity and selectivity of olefin conversions in their studies.44
During the high-temperature treatment of our Pd/OMC
catalysts, carbon species were abundantly present, which
makes the migration of carbon into the subsurface region of
Pd particles likely and could explain the high hydrogenation
reactivity. However, such subsurface carbon species are very
difficult to analyze and thus are beyond the scope of this work.
Further insights can be gained from the Arrhenius plots. The
activation energy derived for Pd/OMC between 35 and 56 °C
amounts to 31.5 kJ/mol. This value is about 50% lower than
activation energies reported for oxide-supported Pd catalysts
−1
amounts to 2.2 molbutene s−1 kgPd for both configurations,
illustrating perfect scale-up behavior. The data illustrate the
simplicity of the scale-up of catalyst and reactor and confirms
that excellent temperature control and close to isothermal
conditions can be retained despite the doubled amount of heat
produced by the fast and highly exothermic reaction.
Electrocatalytic Performance in the Hydrogen Evolu-
tion Reaction. The HER performance of Pd/OMC catalyst
films deposited on glassy carbon was tested by cyclic
voltammetry (CV) performed in acidic medium (0.5 M
H2SO4). The catalyst films were carbonized at 600 °C and
then treated for 5 min at 400 °C in air (oxidative cleaning) and
15 min in H2/Ar atmosphere at 400 °C (reduction). Figure 3
plots (a) the current density recorded at −200 mV during the
first 50 cycles, (b) the 50th recorded CVs as current density vs
applied potential, and (c) the corresponding Tafel plots for the
Pd/OMC catalyst as well as a conventional Pd/C/Nafion
catalyst shown for reference. Both catalysts feature the same Pd
loading (1.15 μg/cm2). Figure 3b shows in addition the activity
of Pd-free OMC.
8259
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
Table 1. Comparison of the HER Performance in Acidic Solution of the Herein Presented Catalysts Pd/OMC and Pd/C/Nafion
with State of the Art Catalysts Reported in the Literature
Pd loading (μg/
j at −200 mV
jmass‑based at −200 mV
(mA/μgPd)
Tafel slope b
author
this study
catalyst
Pd/OMC
cm2)
electrolyte
(mA/cm2)
(mV/dec)
1.15
1.15
43
0.5 M H2SO4 (N2)
0.5 M H2SO4 (N2)
0.5 M H2SO4 (N2)
0.5 M H2SO4 (Ar)
0.5 M H2SO4 (Ar)
0.01 M HCl
−3.7
−2.9
−16
−11
−50
−3.2
−2.5
−0.37
−0.24
−1.1
111
155
121
85
this study
Pd/C/Nafion
Pd/C/Nafion
PdNP/Nafion
PdNP/rGO/Nafion
pc Pda
Bhowmik et al.49
Bai et al.50
46
Bai et al.50
46
Pentland et al.48
107
Pd/OMC shows throughout the first 50 cycles a significantly
higher HER current than the Pd/C/Nafion reference coating
(Figure 3a). Moreover, both catalysts show decreasing currents
in the first 20 cycles followed by a regime of stable
performance. The initial activity follows a similar pattern for
both catalysts and is therefore not specific to Pd/OMC. The
decrease could be related to an initial saturation of the
electrolyte with the formed hydrogen gas that hinders
subsequent hydrogen desorption.
The CVs depicted in Figure 3b for the 50th cycle indicate the
HER activity as a function of potential. No oxidation or
reduction features other than HER are observed. Both Pd/
OMC and Pd/C/Nafion contain the same amount of Pd. The
Nafion-free Pd/OMC catalysts show higher HER currents
particularly at a more negative applied potential: i.e., higher
current densities. Ordered mesoporous carbon films without
palladium (Figure 3b “OMC”) show no signs of HER activity
or any other reductive currents.
Tafel evaluation plots can provide insights into mechanistic
aspects and transport limitations in the HER.47 Figure 3c
displays Tafel plots for Pd/OMC and Pd/C/Nafion after 50
HER cycles. The observed Tafel slopes amount to 111 mV/dec
for Pd/OMC and 155 mV/dec for Pd/C/Nafion. Pentland et
al. derived a Tafel slope of about 99−107 mV/dec for
unsupported polycrystalline Pd.48 Bhowmik et al. reported
similar values around 121 mV/dec for Pd/C/Nafion catalysts.49
Bai et al. observed 85 mV/dec for preformed Pd cubes (d = 10
nm) fixed with Nafion on a working electrode.50 Hence, Pd/
OMC exhibits Tafel slope values that are well within the range
of literature values reported for Pd. Values around 120 mV/dec
were reported to be indicative for the discharge of a proton on
the metal surface to be rate determining (Volmer reaction: H+
+ e− + M ⇌ MHads).47
The intrinsic activity of different Pd-based catalysts can be
compared on the basis of the mass-normalized current density
recorded at the same overpotential. Table 1 compares for
different catalysts the current density measured at −200 mV vs
RHE, the corresponding Pd loading, the mass-normalized
activity, and the respective reported Tafel slopes. The new Pd/
OMC catalysts achieve a mass-normalized HER current of −3.2
mA per μg Pd after 100 cycles, whereas the Pd/C/Nafion
measured for comparison reaches only −2.5 mA/μgPd. Carbon-
supported catalysts reported in the literature show lower
activities in general. Bhowmik et al. report −0.37 mA/μgPd for a
Pd/C/Nafion catalyst.49 Catalysts reported by Bai et al.
perform with −0.24 mA/μgPd (Pd nanocubes/Nafion) and
−1.15 mA/μgPd (Pd nanocubes/reduced graphene oxide
(rGO)/Nafion).50 Hence, the Nafion-free Pd/OMC catalysts
outperform the Pd/carbon catalysts reported in the literature
by at least a factor of 3.
CONCLUSION
■
Highly active binder-free carbon-based supported catalysts can
be synthesized in the form of wall coatings by a synthesis route
that employs the codeposition of a structure-directing agent
and small clusters of polymeric carbon precursors along with
ionic metal species on a substrate. A sequence of thermal
treatments converts the polymer into partly graphitized carbon,
decomposes the structure-directing agent, and converts the
metal precursor into highly active nanoparticles. The resulting
Pd/OMC films contain small Pd particles well distributed in a
mesoporous carbon matrix with high surface area, chemical,
mechanical, and thermal stability, and electrical conductivity.
The presented concept omits the necessity of a binder (e.g.,
Nafion) in the synthesis of the catalytic coating.
The binder-free Pd/OMC films prove to be highly active in
the gas-phase hydrogenation of butadiene as well as the
electrocatalytic hydrogen evolution reaction. In butadiene
hydrogenation the obtained Pd/OMC coatings provide
significantly higher space−time yields than all reported Pd/C
catalysts while at the same time enabling isothermal reactor
conditions. The presented concept provides an easy path for
the scale-up of synthesis and reaction while preserving ideal
operating conditions. Moreover, when tested in the electro-
catalytic hydrogen evolution reaction (HER), the catalysts
outperform reported Pd/C catalysts by a factor of 3, which
underlines the benefits of the developed binder-free catalyst
system.
The presented concept paves the way to the controlled
synthesis of a wide range of highly active catalytic coatings that
employ carbon as a support for other active metals such as Pt,
Ru, and Rh and the opportunity to study these catalysts under
very defined reaction conditions. Moreover, the catalyst
properties can be easily tuned and enable the investigation of
parameters such as pore size, film thickness, and conductivity,
and in particular of the role of Nafion in electrocatalysis.
EXPERIMENTAL SECTION
■
Chemicals. Resorcinol, formaldehyde (37 wt % in water),
and Pluronic F127 (PEO106-b-PPO70-b-PEO106, Mw = 12600)
were purchased from Sigma-Aldrich. Milli-Q water was used for
all synthesis procedures. Ethanol (EtOH, >99.9%, absolute)
and tetrahydrofuran (THF, >99.9%, absolute) were purchased
from VWR. Hydrochloric acid (HCl, 3 M) was prepared from
12 M HCl (Alfa Aesar). Palladium(II) acetylacetonate
(Pd(C5H7O2)2 or Pd(acac)2, 98%) was purchased from Alfa
Aesar. A 5% Nafion solution, isopropyl alcohol, and Pd on
activated carbon (Cat. No. 20.568-0, 5 wt %) were obtained
from Sigma-Aldrich. All chemicals were used without further
purification.
Substrate Pretreatment. Si wafer substrates were cleaned
with ethanol and calcined in air (2 h, 600 °C) prior to film
8260
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
deposition. Stainless steel plates (1.4301) were cut into 33 × 27
mm substrates. The steel substrates were ground, washed, and
calcined as described earlier.8b Glassy-carbon (Sigradur G)
substrates were purchased from HTW. All glassy-carbon (GC)
substrates were polished with a 0.05 μm diamond dispersion
and cleaned thoroughly with ethanol and water. SiO2 glass
substrates for conductivity measurements were purchased from
UniversityWafer and cleaned with ethanol.
The SAXS data were processed employing the software FIT2D
Version V12.077. XRD was measured on a Bruker D8 Advance
instrument (Cu Kα radiation) with a gracing incident beam
(1°). XP spectra were recorded with an Omicron DAR 400 X-
ray source with Al Kα excitation. Electrons were detected with
an EA 125X Hemispherical Energy Analyzer.
Two-point sheet conductivity measurements were performed
with a Keithley Model 6517B Electrometer employing a 8 × 8
pin probe head with an altering polarity sequence of the pins.
Kr and N2 adsorption isotherms were measured at 77 K with a
Quantachrome Autosorb-iQ instrument. The samples were
degassed under vacuum at 150 °C for 2 h prior to sorption
analysis. The surface areas were determined by the Brunauer−
Emmett−Teller (BET) method. Pore size evaluation was done
with a QSDFT equilibrium Kernel and a model assuming
cylindrical pores.
Synthesis of Pd/OMC: Pd-Containing Mesoporous
Carbon Films and Powders. For the catalyst synthesis
according to Scheme 1 a 1.1 g amount of of resorcinol was
mixed with 0.3 g of F127 in 4.5 mL of EtOH until a clear
solution was obtained. Then 4.5 mL of 3 M HCl was added.
The mixture was stirred for 5 min. A 1.3 g portion of
formaldehyde (37% in water) was added and thoroughly mixed.
After ca. 4 min the solution became turbid. Ten minutes after
the addition of the formaldehyde solution, a white precipitate
was separated from the mixture by centrifugation at 7500 rpm
for 5 min and the top-layer solution was discarded. The white
precipitate was washed with water and then dissolved in 5 mL
of THF containing 31.5 mg of dissolved Pd(acac)2. A clear
orange dispersion was obtained and used for dip-coating. Films
were deposited via dip-coating at 25 °C and 30% relative
humidity. The coated substrates were thermally treated at 100
°C for 12 h in air, followed by carbonization at 600 °C for 3 h
under a flowing nitrogen atmosphere (heating ramp of 1 K/
min). The corresponding powder samples were prepared by
drying the dip-coating suspension in a crucible followed by an
thermal treatment identical with that applied to the films. For
enhanced catalytic performance, films were additionally treated
in a muffle furnace at 400 °C for 5 min in air to remove residual
carbon from the formed Pd particles. In case of electrocatalytic
testing in the hydrogen evolution reaction the catalyst was in
addition treated for 15 min at 400 °C in a tube furnace under
an atmosphere of 5% hydrogen in argon.
Synthesis of Pd/C/Nafion Reference Catalyst. The
reference catalyst was prepared by dissolving 5 mg of Pd/
active carbon (5 wt %) in 3.98 mL of water. A 20 μL portion of
Nafion solution (5%) was carefully added. After addition of 1
mL of isopropyl alcohol the mixture was sonicated for 15 min
with 6 W output power with a Branson Sonifier. The resulting
ink was immediately employed for film deposition via drop-
casting and subsequent drying at 60 °C. The resulting reference
catalyst film had a geometric Pd loading of 1.15 μg/cm2. In the
following the catalyst will be denoted as Pd/C/Nafion.
Physicochemical Properties. SEM images were collected
on a JEOL 7401F instrument at 10 kV. The ImageJ program,
version 1.39u (http://rsbweb.nih.gov/ij), was employed to
determine the pore diameter, film thickness, and size of Pd
particles and to obtain fast Fourier transformed (FFT) images.
TEM and selected area electron diffraction (SAED) images
were recorded on a FEI Tecnai G2 20 S-TWIN instrument
operated at 200 kV on fragments of film samples scraped off the
substrates and deposited on carbon-coated copper grids. To
determine the amount of the Pd in the film, the mass depth was
calculated using the STRATAGem film analysis software (v
4.8) on the basis of wavelength dispersive X-ray (WDX) spectra
analyzed with a JEOL JXA-8530F electron microprobe at 7 and
at 10 kV.
Adhesion of a carbonized Pd/OMC coating to a steel
substrate was tested by the “tape test” as described in ASTM
D3359 Standard Test Methods for Measuring Adhesion by Tape
Test (http://www.astm.org/Standards/D3359.htm). An X-cut
was made through the coating penetrating the substrate. An
adhesive tape (Scotch tape) was applied over the cut. The tape
was smoothed and then pulled away. Poorly adhered coatings
would peel away with the tape, whereas well-adhered coatings
would be stable. Photographs before and after the test were
taken, employing an Olympus BX40 optical microscope.
Catalytic Testing in Hydrogenation of 1,3-Butadiene.
Pd/OMC films were coated on both sides of steel substrates.
For each catalytic run, 5 or 10 identical steel plates with the
same thickness (0.5 mm for 5 plates and 0.25 mm for 10 plates)
were stacked parallel into the reactor housing. Photographs of a
catalyst stack and the reactor are presented in Figure S2 in the
Supporting Information. A test setup and procedure similar to
that described in refs 8b and 51 was used. A reaction mixture
consisting of 5% butadiene (2.5 purity), 10% hydrogen (5.0
purity), and 85% nitrogen (5.0 purity) was passed through the
reactor at a flow rate of 60 mL/min (STP) at 1.05 bar. The
catalyst was heated to 80 °C under a reactive gas flow and
equilibrated to reaction conditions for several hours. Afterward,
the temperature was gradually decreased stepwise from 80, 72,
64, 56, 48, and 40 °C and finally to 35 °C with a dwell time of
60 min for each temperature set point. Analysis of the gas
products was continuously performed every 7 min by an online
gas chromatograph (Agilent GC 7890 equipped with FID,
TCD, and columns HP Plot Al2O3, Molsieve 5A, HP Plot Q,
and DB FFAP). The space−time yield (STY) was calculated as
moles of butenes produced per second per kilogram of the
catalyst (molbutene s−1 kgcatalyst−1) and kilograms contained
(molbutene s−1 kgPd−1).
Electrocatalytic Testing in Hydrogen Evolution Re-
action in Acidic Medium. All electrocatalytic testing was
performed by using a three-electrode disk setup with a RHE
(Gaskatel, HydroFlex) as a reference and Pt gauze (ChemPur,
1024 mesh cm−2, 0.06 mm wire diameter, 99.9%) as a counter
electrode. All potentials in this work are referenced to the
reversible hydrogen electrode. Small disks with a diameter of 6
mm were cut out from the coated GC substrates. These
homogeneously coated disks were mounted on a rotating disk
shaft and served as working electrodes (n = 2000 rpm) using
0.5 M H2SO4 as the supporting electrolyte (Fixanal, Fluka
Analytical) and a BioLogic SP-200 instrument as the
potentiostat. The electrolyte solution was purged for at least
30 min with nitrogen before the catalytic tests. Prior to testing a
Films coated on thin silicon wafers (50 μm) were used for
2D-SAXS measurements. 2D-SAXS patterns were collected at
the μSpot beamline at the BESSY II synchrotron (Berlin,
Germany) with a calibrated radiation energy of 12.399 keV.
8261
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
(9) Feng, D.; Lv, Y.; Wu, Z.; Dou, Y.; Han, L.; Sun, Z.; Xia, Y.;
Zheng, G.; Zhao, D. J. Am. Chem. Soc. 2011, 133, 15148−15156.
(10) Pennemann, H.; Dobra, M.; Wichert, M.; Kolb, G. Chem. Eng.
Technol. 2013, 36, 1033−1041.
break-in procedure was conducted (50 potential cycles) in the
cathodic regime (50 to −210 mV) followed by a conditioning
time of 2 h at open circuit potential. During break-in treatment
and in the subsequent HER measurement a flow of N2 gas was
maintained over the electrolyte. The activity in the hydrogen
evolution reaction was investigated by cyclic voltammetry (50
cycles) in a potential window of 50 to −210 mV vs RHE with a
scan rate of 20 mV s−1.
(11) Zapf, R.; Kolb, G.; Pennemann, H.; Hessel, V. Chem. Eng.
Technol. 2006, 29, 1509−1512.
̈
(12) Simsek, E.; Avci, A. K.; Onsan, Z. I. Catal. Today 2011, 178,
157−163.
(13) Schmidt, S. A.; Kumar, N.; Zhang, B.; Eranen, K.; Murzin, D. Y.;
̈
Salmi, T. Ind. Eng. Chem. Res. 2012, 51, 4545−4555.
(14) Qiu, H.; Bednarova, L.; Lee, W. Y. Appl. Catal., A 2006, 314,
200−207.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b02240.
■
S
(15) Haas-Santo, K.; Fichtner, M.; Schubert, K. Appl. Catal., A 2001,
220, 79−92.
(16) Chen, H.; Bednarova, L.; Besser, R. S.; Lee, W. Y. Appl. Catal., A
2005, 286, 186−195.
Photographs of the film adhesion test and reactor design
as well as additional physicochemical characterization via
SEM, N2 sorption, and XPS (PDF)
(17) Ates, A.; Pfeifer, P.; Gorke, O. Chem. Ing. Tech. 2013, 85, 664−
̈
672.
(18) (a) Tu, H.-C.; Wang, W.-L.; Wan, C.-C.; Wang, Y.-Y. J. Phys.
Chem. B 2006, 110, 15988−15993. (b) Kocha, S. S.; Zack, J. W.; Alia,
S. M.; Neyerlin, K. C.; Pivovar, B. S. ECS Trans. 2013, 50, 1475−1485.
(c) Shinozaki, K.; Pivovar, B. S.; Kocha, S. S. ECS Trans. 2013, 58, 15−
26.
(19) (a) Ahn, C.-Y.; Cheon, J.-Y.; Joo, S.-H.; Kim, J. J. Power Sources
2013, 222, 477−482. (b) Garsany, Y.; Ge, J.; St-Pierre, J.; Rocheleau,
R.; Swider-Lyons, K. E. J. Electrochem. Soc. 2014, 161, F628−F640.
(20) Tang, J.; Liu, J.; Li, C.; Li, Y.; Tade, M. O.; Dai, S.; Yamauchi, Y.
Angew. Chem., Int. Ed. 2015, 54, 588−593.
(21) Tang, J.; Liu, J.; Torad, N. L.; Kimura, T.; Yamauchi, Y. Nano
Today 2014, 9, 305−323.
(22) Tang, J.; Liu, J.; Salunkhe, R. R.; Wang, T.; Yamauchi, Y. Chem.
Commun. 2016, 52, 505−508.
AUTHOR INFORMATION
Corresponding Author
*E-mail for R.K.: ralph.kraehnert@tu-berlin.de.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Bjorn Eckhardt, Katrin Schulz, Roman
̈
Schmack, and Arno Bergmann from TU Berlin, Eik Koslowski
from Universitat Halle, and Soren Selve and Jorg Nissen from
̈
̈
̈
the ZELMI (TU Berlin) for support in material analytics. We
acknowledge funding from the BMBF (FKZ 03EK3009). R.K.
thanks in particular the Einstein Foundation Berlin for
generous support provided by an Einstein-Junior-Fellowship
(EJF-2011-95). SAXS measurements were conducted at the
μSpot beamline at the BESSY II synchrotron.
(23) (a) Zhang, C.; Geng, Z.; Ma, J. Microporous Mesoporous Mater.
2013, 170, 287−292. (b) Kataoka, S.; Yamamoto, T.; Endo, A.;
Nakaiwa, M.; Ohmori, T. Colloids Surf., A 2009, 347, 142−145.
(c) Simanjuntak, F. H.; Jin, J.; Nishiyama, N.; Egashira, Y.; Ueyama, K.
Carbon 2009, 47, 2531−2533.
(24) Chuenchom, L.; Kraehnert, R.; Smarsly, B. M. Soft Matter 2012,
8, 10801−10812.
REFERENCES
■
(1) Bonet, F.; Grugeon, S.; Herrera Urbina, R.; Tekaia-Elhsissen, K.;
Tarascon, J. M. Solid State Sci. 2002, 4, 665−670.
(25) Liang, C.; Hong, K.; Guiochon, G. A.; Mays, J. W.; Dai, S.
Angew. Chem., Int. Ed. 2004, 43, 5785−5789.
(2) (a) Lee, D. C.; Kim, J. H.; Kim, W. J.; Kang, J. H.; Moon, S. H.
Appl. Catal., A 2003, 244, 83−91. (b) Oosthuizen, R. S.; Nyamori, V.
O. Platinum Met. Rev. 2011, 55, 154−169. (c) Ortel, E.; Polte, J.;
Bernsmeier, D.; Eckhardt, B.; Paul, B.; Bergmann, A.; Strasser, P.;
Emmerling, F.; Kraehnert, R. Appl. Catal., A 2015, 493, 25−32.
(d) Seth, D.; Sarkar, A.; Ng, F. T. T.; Rempel, G. L. Chem. Eng. Sci.
2007, 62, 4544−4557. (e) Silvestre-Albero, J.; Rupprechter, G.;
Freund, H.-J. Chem. Commun. 2006, 80−82.
(26) Tanaka, S.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Chem.
Commun. 2005, 2125−2127.
(27) Song, L.; Feng, D.; Fredin, N. J.; Yager, K. G.; Jones, R. L.; Wu,
Q.; Zhao, D.; Vogt, B. D. ACS Nano 2010, 4, 189−198.
(28) Mitome, T.; Uchida, Y.; Egashira, Y.; Nishiyama, N. Colloids
Surf., A 2014, 449, 51−56.
(29) (a) Tanaka, S.; Katayama, Y.; Tate, M. P.; Hillhouse, H. W.;
Miyake, Y. J. Mater. Chem. 2007, 17, 3639−3645. (b) Liang, C.; Dai, S.
J. Am. Chem. Soc. 2006, 128, 5316−5317.
(3) Guo, X.-F.; Jang, D.-Y.; Jang, H.-G.; Kim, G.-J. Catal. Today 2012,
186, 109−114.
(4) (a) Ghasemi, S.; Hosseini, S. R.; Nabipour, S.; Asen, P. Int. J.
Hydrogen Energy 2015, 40, 16184−16191. (b) Darabdhara, G.; Amin,
M. A.; Mersal, G. A. M.; Ahmed, E. M.; Das, M. R.; Zakaria, M. B.;
Malgras, V.; Alshehri, S. M.; Yamauchi, Y.; Szunerits, S.; Boukherroub,
R. J. Mater. Chem. A 2015, 3, 20254.
(30) Kimura, T.; Emre, A. M.; Kato, K.; Hayashi, Y. J. Mater. Chem. A
2013, 1, 15135−15141.
(31) Kataoka, S.; Yamamoto, T.; Inagi, Y.; Endo, A.; Nakaiwa, M.;
Ohmori, T. Carbon 2008, 46, 1358−1360.
(32) Tanaka, S.; Doi, A.; Nakatani, N.; Katayama, Y.; Miyake, Y.
Carbon 2009, 47, 2688−2698.
(5) Zheng, J.-S.; Zhang, X.-S.; Li, P.; Zhu, J.; Zhou, X.-G.; Yuan, W.-
(33) Sun, Z.; Sun, B.; Qiao, M.; Wei, J.; Yue, Q.; Wang, C.; Deng, Y.;
Kaliaguine, S.; Zhao, D. J. Am. Chem. Soc. 2012, 134, 17653−17660.
(34) Ortel, E.; Hertwig, A.; Berger, D.; Esposito, P.; Rossi, A. M.;
Kraehnert, R.; Hodoroaba, V.-D. Anal. Chem. 2016, 88, 7083−7090.
(35) Ruland, W.; Smarsly, B. J. Appl. Crystallogr. 2002, 35, 624−633.
(36) Crepaldi, E. L.; Soler-Illia, G. J. d. A. A.; Grosso, D.; Cagnol, F.;
Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770−9786.
(37) Han, Y. F.; Kumar, D.; Sivadinarayana, C.; Goodman, D. W. J.
Catal. 2004, 224, 60−68.
K. Electrochem. Commun. 2007, 9, 895−900.
(6) Yang, S.; Shen, C.; Lu, X.; Tong, H.; Zhu, J.; Zhang, X.; Gao, H.-j.
Electrochim. Acta 2012, 62, 242−249.
(7) (a) Huang, H.; Wang, X. J. Mater. Chem. A 2014, 2, 6266−6291.
(b) Frackowiak, E.; Beg
(8) (a) Hessel, V.; Hardt, S.; Lowe, H., A Multi-Faceted, Hierarchic
́
uin, F. Carbon 2001, 39, 937−950.
̈
Analysis of Chemical Micro Process Technology: Sections 1.6−1.9. In
Chemical Micro Process Engineering; Wiley-VCH: Weinheim, Germany,
2005; pp 66−124. (b) Ortel, E.; Sokolov, S.; Zielke, C.; Lauermann, I.;
Selve, S.; Weh, K.; Paul, B.; Polte, J.; Kraehnert, R. Chem. Mater. 2012,
24, 3828−3838.
(38) Li, Q.; Liang, W.; Ku Shang, J. Appl. Phys. Lett. 2007, 90,
063109.
8262
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263

ACS Catalysis
Research Article
(39) Aoki, Y.; Urita, K.; Noguchi, D.; Itoh, T.; Kanoh, H.; Ohba, T.;
Yudasaka, M.; Iijima, S.; Kaneko, K. Chem. Phys. Lett. 2009, 482, 269−
273.
(40) Zhao, X.; Wang, A.; Yan, J.; Sun, G.; Sun, L.; Zhang, T. Chem.
Mater. 2010, 22, 5463−5473.
(41) Blackstock, J. J.; Rostami, A. A.; Nowak, A. M.; McCreery, R. L.;
Freeman, M. R.; McDermott, M. T. Anal. Chem. 2004, 76, 2544−
2552.
(42) (a) Bernsmeier, D.; Ortel, E.; Polte, J.; Eckhardt, B.; Nowag, S.;
Haag, R.; Kraehnert, R. J. Mater. Chem. A 2014, 2, 13075−13082.
(b) Yan, H.; Cheng, H.; Yi, H.; Lin, Y.; Yao, T.; Wang, C.; Li, J.; Wei,
S.; Lu, J. J. Am. Chem. Soc. 2015, 137, 10484−10487.
(43) (a) Silvestre-Albero, J.; Rupprechter, G.; Freund, H.-J. J. Catal.
2006, 240, 58−65. (b) Dal Santo, V.; Gallo, A.; Naldoni, A.; Sordelli,
L. Inorg. Chim. Acta 2012, 380, 216−222.
(44) (a) Wilde, M.; Fukutani, K.; Ludwig, W.; Brandt, B.; Fischer, J.-
H.; Schauermann, S.; Freund, H.-J. Angew. Chem., Int. Ed. 2008, 47,
9289−9293. (b) Neyman, K. M.; Schauermann, S. Angew. Chem., Int.
Ed. 2010, 49, 4743−4746. (c) Aleksandrov, H. A.; Vines, F.; Ludwig,
̃
W.; Schauermann, S.; Neyman, K. M. Chem. - Eur. J. 2013, 19, 1335−
1345.
(45) Ortel, E.; Polte, J.; Bernsmeier, D.; Eckhardt, B.; Paul, B.;
Bergmann, A.; Strasser, P.; Emmerling, F.; Kraehnert, R. Appl. Catal., A
2015, 493, 25−32.
(46) Xu, B.; Fan, Y.; Zhang, Y.; Tsubaki, N. AIChE J. 2005, 51,
2068−2076.
(47) Lasia, A., Hydrogen evolution reaction. In Handbook of Fuel
Cells; Wiley: Chichester, U.K., 2010.
(48) Pentland, N.; Bockris, J. O. M.; Sheldon, E. J. Electrochem. Soc.
1957, 104, 182−194.
(49) Bhowmik, T.; Kundu, M. K.; Barman, S. ACS Catal. 2016, 6,
1929−1941.
(50) Bai, S.; Wang, C.; Deng, M.; Gong, M.; Bai, Y.; Jiang, J.; Xiong,
Y. Angew. Chem., Int. Ed. 2014, 53, 12120−12124.
(51) Cukic, T.; Kraehnert, R.; Holena, M.; Herein, D.; Linke, D.;
Dingerdissen, U. Appl. Catal., A 2007, 323, 25−37.
8263
DOI: 10.1021/acscatal.6b02240
ACS Catal. 2016, 6, 8255−8263