Structure Sensitivity in Hydrogenation Reactions on Pt/C in Aqueous-phase

Article abstract of DOI:10.1002/cctc.201801344

Hydrogenation of phenol and of the benzaldehyde carbonyl group catalyzed by Pt are structure sensitive in aqueous phase. The intrinsic reaction rates are directly proportional to the average size of the Pt particles. This trend is indifferent of the genes

Full text of DOI:10.1002/cctc.201801344

Full Papers
DOI: 10.1002/cctc.201801344
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Structure Sensitivity in Hydrogenation Reactions on Pt/C in
Aqueous-phase
Udishnu Sanyal+,[a] Yang Song+,[b] Nirala Singh,[a] John L. Fulton,[a] Juan Herranz,[c]
Andreas Jentys,[b] Oliver Y. Gutiérrez,*[a] and Johannes A. Lercher*[a, b]
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Hydrogenation of phenol and of the benzaldehyde carbonyl
group catalyzed by Pt are structure sensitive in aqueous phase.
The intrinsic reaction rates are directly proportional to the
average size of the Pt particles. This trend is indifferent of the
genesis of the reduction equivalents, i.e., dissociation of H2 or
proton reduction under cathodic potential. It is concluded that
the structure sensitivity is caused by the ensemble size
sensitivity of the adsorption of the aromatic molecules, which is
favored on (100) and (111) surfaces of larger particles. For
electrocatalytic reduction, this structure sensitivity implies that
hydrogen evolution increases in rate relative to hydrogen
addition to reacting substrates as the size of Pt particles
decreases.
Introduction
repulsive interactions of the polar group in the molecule with
the solvent stabilize the adsorbed state(s) and the activated
complex to different degrees. The resulting weaker interaction
may induce sensitivity to specific adsorption structures or an
overall nonlinear impact of the competitive adsorption.
We wish to probe, therefore, the generality of sensitivity to
particle size for hydrogenation in aqueous phase. In particular,
we would like to probe, whether and how such particle size
effect impacts reductive electrocatalytic conversions.
Electrochemical hydrogenation (ECH) conceptually allows
low-temperature hydrogenation with the cathodic potential
providing the reduction equivalents.[15,16,17,18,19,2021] The potential
problem with this approach is that the H2 evolution reaction
(HER) competes with ECH.[22] It has been shown that the former
reaction was not structure or size sensitive.[23] If the ECH would
be structure sensitive, the size of the active metal particles at
the cathode could be used to control the relative rates of H2
evolution and catalytic hydrogenation.
We have chosen the hydrogenation of phenol and
benzaldehyde, because the two molecules present the possi-
bility to compare structure sensitivity for a carbonyl compound
versus an aromatic ring. The size of the catalyst particles is
varied indirectly by varying the concentration of Pt on a carbon
support. The kinetics of reduction is explored either by
providing the reduction equivalents with cathodic potential or
with molecular H2 in an open circuit potential mode.
Hydrogenation of hydrocarbons in gas phase is commonly
accepted to be a structure-insensitive reaction for a wide range
[1],[2]
of metals and reaction conditions.
Structure sensitivity in
heterogeneous catalysis is generally attributed to stabilization
of adsorbed or transition states of substrates. That is, the
substrate will adsorb and be activated only on specific
ensembles of atoms.[3] The fraction of such catalytically active
sites may vary with the particle size or the chemical
composition.[4,5,6] If single atoms act as active sites, the reaction
rate normalized to the concentration of active metal will be
insensitive to changes in metal particle size, i.e., it is structure-
insensitive.[7]
Early work on hydrogenation of unsaturated hydrocarbons
suggested that it is also structure insensitive in liquid phase,
albeit with lower rates than in gas phase reactions.[8,9] The lower
rates were attributed to the competition of solvent and
substrate for adsorption. Few examples, however, of significant
particle size effects for the hydrogenation of polar molecules in
liquid phase were reported.[10,11,12,13,14] Strong attractive or
[a] Dr. U. Sanyal,+ Dr. N. Singh, Dr. J. L. Fulton, Dr. O. Y. Gutiérrez,
Prof. J. A. Lercher
Institute for Integrated Catalysis
Pacific Northwest National Laboratory
Richland WA-99352 (USA)
E-mail: oliver.gutierrez@pnnl.gov
johannes.lercher@pnnl.gov
[b] Dr. Y. Song,+ Dr. A. Jentys, Prof. J. A. Lercher
Department of Chemistry and Catalysis Research Center
Technische Universität München
Garching 85747 (Germany)
Results and Discussion
Structural Characterization of Pt/C Catalysts and Quantifying
the Concentration of Exposed Pt
[c] Dr. J. Herranz
Electrochemistry Laboratory
Paul Scherrer Institute
Villegen CH-5232 (Switzerland)
Pt/C catalysts with Pt loadings of 1, 3, 5 and 10 wt.% were used
in the present study. The average Pt particle size was evaluated
by TEM analysis. Representative TEM images and corresponding
particle size distributions are shown in Figure 1.
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201801344
This manuscript is part of the Anniversary Issue in celebration of 10 years of
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The fractional exposure of Pt varied from 0.37 to 0.11 while
the metal loading of the Pt/C catalysts varied from 1 wt.% to
10 wt.% (Table 1). Pt fractional exposures and average particle
size were also determined by H2 chemisorption. The average
particle diameters obtained by H2 chemisorption were in good
agreement with those estimated by TEM analysis as shown in
Table 1. The fractional exposure estimated by TEM and H2
chemisorption for calculating intrinsic rates has also been
verified by in situ titration with poisons.
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In order to titrate the metal sites accessible under reaction
conditions, the hydrogenation of benzaldehyde was performed
while adding small aliquots of butanethiol with concentrations
increasing up to the concentrations of available metal sites as
determined by H2 chemisorption. We hypothesize that butane-
thiol reacts stoichiometrically with the metal sites according to
the following reaction:
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PtðsÞ þ C4H9-SH ! PtSðsÞ þ C4H10 DG� � À 103 kJ=mol
ðiÞ
Figure 1. Representative TEM images and the corresponding size distribution
histograms of different metal wt.% Pt/C catalysts: a) Pt/C-1; b) Pt/C-3; c) Pt/C-
5; d) Pt/C-10
The intrinsic activity of the catalysts decreased proportion-
ally to the amount of butanethiol added to the reaction
medium. It blocked catalytic activity nearly completely, once
the amount of butanethiol equaled the amount of surface
metal atoms measured by H2 chemisorption (Figure 2). The
same proportional deactivation was observed, when the
titration with thiol was carried out under cathodic potential
(Figure S1 and Table S1).
Table 1. Metal loading and fractional exposure of Pt particle size of Pt
catalysts
Catalyst Pt
Fractional
Particle
size [nm]
[b]
Particle
Exposed metal
[wt.%] exposure
[a]
size [nm] surface [μmol·g-
1
]
[c]
Pt/C-1
Pt/C-3
Pt/C-5
1
3
5
0.37
0.34
0.26
0.11
3.1
3.5
4.5
12.1
2.9
3.2
4.3
10
18
50
65
53
The small residual activity is attributed either to an under-
estimation of the exposed metal surface by H2 chemisorption or
to a less than complete adsorption of the butanethiol aliquots
added. These results confirm the fraction of surface metal atoms
determined by H2 chemisorption are identical to the fraction of
Pt/C-10 10
[a] Determined from H2 chemisorption; [b] The average Pt particle size
calculated based on the fractional exposure determined by H2 chemisorp-
tion and assuming a cuboctahedron shape of the particles; [c] The average
Pt particle size calculated from the TEM micrograph.
Figure 2. Rates of benzaldehyde hydrogenation (turnover frequency, TOF) on different Pt/C catalysts in the presence of varying concentrations of butanethiol
in the reaction media. -SH/Ptsurface denotes the molar ratio of thiol added in the reaction to surface Pt determined by H2 chemisorption. The reactions were
performed under 1 bar flowing H2 at room temperature. TOF values are calculated against the surface Pt determined by H2 chemisorption
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available metal under both experimental conditions in aqueous
phase.
rates increased with increasing cathodic potentials on all Pt/C
catalysts as reported for Rh/C.[24] However, varying cathodic
potential did not influence the product selectivity (Figure S4).
The hydrogenation of phenol on the metal is hypothesized
to proceed as a Langmuir Hinshelwood type mechanism with
the reaction of surface hydrogen atoms with the aromatic ring
being the rate determining step (Scheme 1).[25] Cyclohexanone
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Hydrogenation of Phenol on Pt/C Catalysts with Varying Pt
Particle Size
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The rate normalized to the number of surface Pt atoms, i.e., the
turnover frequency (TOF), for phenol hydrogenation decreased
with increasing fractional exposure of Pt. The same result was
observed, when the reaction was performed in the presence of
H2 (thermal catalytic hydrogenation, TCH) and when a cathodic
potential (electrocatalytic hydrogenation, ECH) was used to
generate the reductive environment (Figure 3a, mass-normal-
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Scheme 1. Elementary reaction steps towards phenol hydrogenation and H2
evolution reaction.
is the first observed hydrogenated product in measurable
equilibrium with the mobile phase (Reactions 3 and 4 in
Scheme 1 and desorption Reaction 5). The elementary steps for
hydrogenation are concluded to be identical in the presence or
absence of an external electric field.[26] When the hydrogenation
is performed under cathodic potential, the H2 evolution reaction
(HER) is the major competing pathway (Reactions 6 and 7 in
Scheme 1). The current efficiency is defined here as the fraction
of the current utilized to hydrogenate the organic compounds
relative to the total current passed. As shown in Figure 3b and
Table S2, Pt particle size also impacts the efficiency of the
overall electrocatalytic reaction. The larger particles with higher
hydrogenation TOFs had a higher selectivity for ECH than for
HER. The current efficiencies were not affected by the external
potential in the relatively narrow range explored, i.e., ECH and
HER were enhanced by the cathodic potential to the same
extent for a given dispersion of Pt.
The apparent activation energies did not change signifi-
cantly across different Pt/C catalyst particle sizes, which
indicates that the mechanism and rate determining step of
phenol hydrogenation are not influenced by the Pt particle size.
Figures 4a and 4b show the Arrhenius plots for the hydro-
genation of phenol on Pt/C-5 and Pt/C-10. The Arrhenius plots
for Pt/C-1 and Pt/C-3 are shown in Figures S5a and S5b. The
corresponding apparent activation energies are summarized in
Figure 3. (a) Rates of phenol hydrogenation (turnover frequency, TOF) at
different cathodic potentials (vs Ag/AgCl) under 1 bar N2 (ECH) or 1 bar H2
(TCH) along with the fractional exposures of the Pt/C catalysts. (b) Added
current efficiencies for cyclohexanone and cyclohexanol observed during
electrocatalytic hydrogenation of phenol at different cathodic potentials
along with the fractional exposure of Pt. The reactions were performed at
room temperature. The particle size corresponding to fractional exposure
represented in bottom x-axis are shown at the top of the figure.
ized rates are presented in Figure S2 and Table S2). Cyclo-
hexanone and cyclohexanol were the primary and secondary
products, respectively (Figures S3 shows selected concentration
profiles). Thus, the dependence of TOF on fractional exposure
shows a marked particle size effect on aromatic ring hydro-
genation under the conditions explored. The hydrogenation
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Hydrogenation of Benzaldehyde on Pt/C Catalysts with
Varying Pt Particle Size
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For benzaldehyde only the carbonyl group is selectively hydro-
genated to benzyl alcohol. The TOFs for benzaldehyde hydro-
genation were higher than those of phenol hydrogenation,
indicating that the reactivity of the carbonyl group was higher
than that of aromatic rings, even when considering that only
1/2 of the hydrogen is needed for the first step of the reaction.
Despite the different functionality being reduced, the TOF for
benzaldehyde hydrogenation decreased linearly with decreas-
ing Pt particle size for both ECH and TCH (Figure 5a), just as
observed for phenol. Larger Pt particles showed also higher
selectivity towards benzaldehyde hydrogenation than towards
HER. In parallel, the current efficiency increased from 8% to
50% (Figure 5b). Thus, the hydrogenation of both the aromatic
carbonyl group and aromatic ring of phenol are concluded to
depend on an active site that varies in concentration with the
particle size. Details of benzaldehyde hydrogenation rates (mass
specific rate and TOF), current, current efficiencies obtained
under both electrochemical and thermal condition are compiled
in Table S4.
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Structure-Activity Correlations
Figure 4. Arrhenius plots for the hydrogenation of phenol on Pt/C-5 (a) or
Pt/C-10 (b). The reactions were performed at 1 bar of flowing N2 (ECH) or H2
(TCH).
Particle size effects are usually correlated with the fractions of
surface atoms with varying coordination.[28] The fraction of
metal atoms with low coordination (located at edge and corner
sites) is over-proportionally reduced with increasing particle
size and the surface becomes dominated by low index surfaces
sites. According to cuboctahedra based geometric models
Table 2 (Pt/C-5 and Pt/C-10) and Table S3 (Pt/C-1 and Pt/C-3).
Invariant activation energies allow us to conclude that only the
concentrations of active sites have changed with the particle
size. At the external potential for which the rate of TCH and
ECH were identical (À 0.7 V vs. Ag/AgCl or 1 bar H2), the
apparent activation barriers for phenol hydrogenation are both
approximately 35 kJmolÀ 1. The activation energies for the
electrochemical hydrogenation decreased from 45 kJmolÀ 1 to
26 kJmolÀ 1as the potential varied from À 0.6 V to À 0.8 V vs. Ag/
AgCl. Activation energies measured for Pt/C-5 and Pt/C-10 at
different cathodic potentials are shown in Table 2. We hypothe-
size that this decrease originates from an increase in the
interaction strength of hydrogen on the metal.[27] Conversely,
the interaction of phenol with the surface does not seem to be
affected by potential. The concentration profiles for ECH and
TCH suggest a low reaction order (Figure S6a and Figure S6b).
Rates at varying initial phenol concentrations indicate reaction
orders in phenol of 0.1 - 0.3 in case of TCH and ECH at À 0.7 V
(Figure S7). This indicates that the coverage of phenol is
relatively high.
[29,30,31,32]
of the metal particles used in this study, the fraction of
surface atoms in low index plane sites (such as (111) and (100)
planes) increases from 0.63 at 3 nm particles to 0.9 for 10 nm
particles (for details see supporting information). The rates of
phenol and benzaldehyde hydrogenation scaled linearly with
the calculated fraction of the sum of (111) and (100) planes of
the different catalysts (Figure 6). As we have concluded that
varying particle size does not affect the reaction mechanism or
rate limiting step, we attribute the observed particle size effects
to ensemble requirements towards the adsorption and hydro-
genation of aromatic oxygenates. The increasing fraction of
planar sites with increasing particle size is concluded to lead to
a higher fraction of surface sites preferring the multiple
Table 2. Activation energies obtained from the Arrhenius plots corre-
sponding to phenol hydrogenation on Pt/C-5 and Pt/C-10 at varying
potentials.
Pt/C-5
Cathodic Po-
tential
(V vs Ag/AgCl)
Pt/C-10
Cathodic Po-
Activation Energy
[kJ·mol-1]
Activation En-
ergy
tential
(V vs Ag/AgCl) [kJ·mol-1]
À 0.6
À 0.7
À 0.8
TCH
47
37
26
35
À 0.6
À 0.7
À 0.8
TCH
44
35
26
33
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large ensemble of metal atoms would suffice to serve as active
site, one would tend to observe a nonlinear decay of catalytic
activity upon increasing exposure to butanethiol, as the
probability to reduce large ensemble by stochastic irreversible
adsorption of a thiol molecule is higher than for a single site at
corners and edges of particles.[33,34,35]
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The linear decay upon fractional poisoning could be,
however, caused by two effects. The active sites could decrease
in strict proportion to the fraction of exposed metal atoms. This
could, for example, be the case if the low index surfaces
dominate the particles. It could also be observed, if the
adsorption of thiol was not irreversible, but equilibrated in
aqueous phase.[36]
The extrapolations of the hydrogenation TOFs to zero show
that different thresholds of sizes exist for the Pt particles to be
active towards phenol and benzaldehyde hydrogenation (Fig-
ure 6). The minimum requirement of planar site is higher for
phenol hydrogenation (55%) than for benzaldehyde hydro-
genation (40%), i.e., phenol hydrogenation benefits more from
larger Pt particles than benzaldehyde hydrogenation.
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In situ XANES Characterization
As outlined above, the fact that fractions of the Pt-surface exist
and that the organic compounds cannot successfully access
implies that the ratio of adsorbed molecules to metal surface
sites change with particle size. In specific, it is hypothesized that
small particles accommodate less organic molecules at satura-
tion coverage than large particles, because the latter have
higher fractions of surface available for hydrogenation. The
hypothesis that larger metal particles accommodate a higher
density of organic molecules at the surface than small particles
was tested by measuring the XANES of the Pt L3 edge of
selected Pt/C catalysts in aqueous phase during the application
of cathodic potential.
Despite XANES being a bulk technique (the spectra being
an average of all Pt atoms), for small particles (<3 nm), the
fraction of surface sites is sufficient to observe surface species
such as adsorbed H.[37] Upon adsorption of H2 the intensity
above the Pt L3 edge at 11572 eV increased and the difference
spectra of a Pt catalyst with and without adsorbed hydrogen
shows this increase clearly.[38,39,40] Figure 7 shows the difference
in situ XANES spectra of two Pt/C catalysts synthesized by
strong electrostatic adsorption (Pt/C Darco, 3 nm and Pt/C
Vulcan 1.5 nm).[41] When a 0.1 M phenol solution was passed
through the cell (at À 0.55 V vs Ag/AgCl), this peak decreased.
This is attributed to adsorption of phenol and its competition
with H for adsorption sites. The decrease of intensity of the
PtÀ H signal is shown in Figure 7a for a ~3 nm Pt/C catalyst. For
the smaller particles, ~1.5 nm, there was little or no decrease in
the PtÀ H signal as phenol was admitted (Figure 7b). This
difference between small and larger particles points to the fact
that on smaller particle size phenol blocks the adsorption of H2
less effectively than on larger particles. We conclude, therefore,
that on the smaller Pt/C particles a smaller fraction of surface
Figure 5. Intrinsic activity (TOF) for benzaldehyde hydrogenation as a
function of fractional exposure under electrochemical hydrogenation (ECH)
and catalytic thermal hydrogenation (TCH) (a); Current efficiency obtained
during ECH of benzaldehyde as a function of Pt fractional exposure (b). ECH
was performed at À 0.7 V vs Ag/AgCl. All reactions were performed at
ambient temperature under 1 bar of flowing N2 (ECH) or H2 (TCH). The
particle size corresponding to fractional exposure represented in bottom x-
axis are shown at the top of the figure.
Figure 6. Correlation of rates of phenol and benzaldehyde hydrogenation
with the fraction of terrace sites, determined from the calculated number of
atoms in the (100) and (111) planes, using a geometric model for the catalyst
particles. TOF indicated herein corresponds to ECH reaction performed at
À 0.7 V vs Ag/AgCl.
adsorption of the aromatic compounds. This enhances TCH and
ECH rates, while leaving the energetics during the catalytic
process unchanged.
It should be noted at this point that the total activity
followed linearly the exposure to butanethiol. If a relatively
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Figure 7. XANES difference spectra of Pt L3 edge for Pt/C with Pt particles of: a) ~3 nm and; b) ~1.5 nm as determined by EXAFS. The spectra were recorded in
the presence and the absence of 0.1 M phenol in acetate buffer, pH 4.6, room temperature and atmospheric pressure. The cathodic potential applied herein
was À 0.55 V vs Ag/AgCl. The data shown in (b) are reproduced from reference.[37]
99.9%), KCl (�99.9%), and butanethiol (98%). Water was purified
atoms exists that can accommodate aromatic rings, while H2
can dissociatively adsorb on all metal sites.
with a Milli-Q system up to a resistivity of 18.2 MΩ·cm. H2 (Air
Liquide, >99.99%) was used for catalytic thermal hydrogenation,
Ar (Air Liquide, >99.99%) was used to remove O2 from the
electrolyte before ECH and N2 (Air Liquide, >99.99%) was used to
adjust the partial pressure of H2.
Conclusions
Catalyst preparation. Pt/C catalysts with different loadings (1, 3, 5
and 10 wt.%) were purchased from Sigma-Aldrich. Two catalysts
were prepared for XANES experiments. These catalysts were
synthesized following the strong electrostatic adsorption procedure
reported by Regalbuto et al.[41] Aqueous solutions of tetraamine
platinum nitrate were mixed with samples of XC-72 C (Vulcan) and
Darco carbons allowing ion-exchange for 12 h. After the metal
deposition, the catalysts were dried in air and reduced in H2 at
Aqueous phase hydrogenation of both phenol and benzalde-
hyde on Pt/C is structure-sensitive. The intrinsic reaction rates
are inversely proportional to Pt fractional exposure, i.e., the
averaged hydrogenation activity of accessible Pt increases with
increasing particle size. This trend is identical for hydrogenation
of aromatic rings and carbonyl groups. It is indifferent of the
genesis of the reduction equivalents, i.e., dissociation of H2 or
proton reduction under cathodic potential.
°
375 C for 4 h. The catalysts were designated as Pt/C Vulcan and Pt/
C Darco, respectively.
Catalyst characterization. The metal loadings of the catalysts were
verified by atomic absorption spectroscopy (AAS) carried out on an
UNICAM 939 AA-Spectrometer equipped with a GF 95 graphite
furnace. Specific surface areas and pore diameters of the catalysts
were derived, according to BET and BJH models, from N2
physisorption isotherms measured at 77 K on a PMI automated BET
sorptometer. The samples were outgassed before measurements at
523 K for 2 h.
The structure sensitivity is concluded to be caused by the
ensemble size sensitivity of the adsorption of the aromatic
molecule. Thus, the increasing intrinsic rates with increasing
metal particle size are attributed to increasing fractions of (100)
and (111) planes, which permit adsorption of the aromatic ring.
This conclusion is supported by in situ XANES that showed a
higher fraction of sites capable of phenol adsorption on larger
particles. It is notable that the hydrogenation of the carbonyl
group in benzaldehyde requires a smaller ensemble than the
hydrogenation of the aromatic ring.
Overall, the existence of the structure sensitivity allows one
to guide the design of Pt based electrocatalysts that can have a
precisely tailored selectivity to hydrogenation (of polar organic
molecules) compared to H2 evolution. It is to be explored,
however, whether such selectivity changes are specific to Pt
based catalysts or a much broader phenomenon, as well as the
contribution of solvation effects on the appearance of structure
sensitivity.
For H2 chemisorption measurements, the materials were initially
reduced at 573 K for 1 h under H2 atmosphere followed by
treatment in vacuum at 573 K for 1 h. The samples were then
cooled to 313 K. A first set of adsorption isotherms were measured
from 1 to 40 kPa. Afterwards, the samples were outgassed at 313 K
for 1 h and a second set of isotherms were measured, which
corresponded to physisorbed H2. The concentrations of chem-
isorbed H2 were determined by extrapolating the difference
isotherms to zero H2 pressure. The fractional exposures of the
supported metals were estimated from the concentration of
chemisorbed hydrogen assuming a stoichiometry of 1:1 surface
metal to hydrogen atoms.
Volume-area mean Pt particle sizes were calculated from the metal
fractional exposure assuming a cuboctahedron geometry for the
particles. The corresponding equations are [Equation (1) and (2)]:[32]
Experimental Section
Chemicals and catalytic materials. All chemicals were obtained from
Sigma-Aldrich and used as received: Phenol (�99.0%), benzalde-
hyde (�99.0%), acetate buffer solution (pH 4.6), ethyl acetate (�
99.9%, HPLC), NaOH (�99.9%, HPLC), Na2SO4 (�99.9%), NaCl (�
5:01
D
ð1Þ
d ¼
� dat for D < 0:2
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cooling/heating circulator (Julabo F25-ED). All procedures were
performed with an electrochemical workstation VSP-300, Bio Logic
3:32
ð2Þ
1
2
d ¼
� dat for 0:2 � D � 0:92
D1:23
Thermal catalytic hydrogenation (TCH). A glass batch reactor was
used to perform TCH experiments. Typical measurements were
performed at atmospheric pressure with H2 (10 mL·minÀ 1) flowing
through the reactant solution at 296 K with 10 mg of catalyst and
stirring at 500 rpm. The concentration of phenol and benzaldehyde
was 20 mM.
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where d is volume-area mean particle size, D is the dispersion or
fractional exposure obtained from hydrogen chemisorption, and dat
is the atomic diameter of Pt (0.275 nm).
The Pt particle sizes can also be calculated from the metal fractional
exposure using the Equation (3) wherein the spherical geometry
was assumed[43]
Control experiments. Series of control experiments were performed
by varying the amount of catalyst and stirring rates. As shown in
Figure S9a, the conversion of phenol increased proportionally with
the amount of catalyst while the corresponding TOF remained the
same (Figure S9b). The current did not change beyond 400 rpm
(Figure S9c). Thus, all experiments were performed at stirring of
500 rpm. These observations allowed us to discard external mass
transport limitations. The calculated effectiveness factor (1) and
Weisz modulus (0.005) indicated that the measured kinetics were
not limited by internal mass transport (Supporting information,
Figure S10). Disappearance of the reactant or product formation
were not observed in the absence of cathodic potential, H2, or a
material containing Pt.
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6A
1sLD
d ¼
ð3Þ
where d is the mean particle size, D is the fractional exposure, A =
Pt atomic mass (195.1 g/mole), 1=density (21.45×106 g/m3), s =
average surface area occupied by Pt atom (0.81×10À 19 m2), and L is
the Avogrado’s constant
For titration with butanethiol, increasing amounts of thiol were
added to suspensions containing the Pt/C catalysts in acetate
buffer. After stirring for 30 minutes, TCH or ECH experiments were
performed following the protocols described below.
Product analysis. The course of the reaction was followed by
periodically withdrawing aliquots of 1 mL from the cathode
compartment of the electrochemical cell (ECH) or from the batch
reactor (TCH). The organic phase was separated from the aqueous
phase by extraction with ethyl acetate and dried on Na2SO4. A
sample of the dry organic phase was mixed with a solution
containing acetophenone as external standard. Quantitative analy-
ses of the samples were performed by gas chromatography
coupled with mass spectrometry (Shimadzu GCMS-QP2010). The GC
was equipped with a plot Q capillary column (30 m×250 μm) and a
thermal conductivity detector.
Characterization by transmission electron microscopy (TEM) was
performed after the samples of the catalysts were ground and
ultrasonically dispersed in ethanol. Drops of the suspensions were
applied on carbon coated copper grids and the measurements
were carried out in a JEOL JEM-2011 electron microscope with an
accelerating voltage of 120 keV. Statistical treatment of the metal
particle size was done by counting at least 200 particles detected in
several places of the grid. The volume-area mean particle size was
calculated by using the Equation (4):[43]
In-situ XANES measurements. To prepare the working electrode, a
suspension containing the catalyst powder was passed through a
carbon felt (Alfa Aesar, 3.1 mm thick). In this process, more than
95% of the suspended powder was infiltrated into the felt. Finally,
the carbon felt was punched to a diameter suitable for the
electrochemical XAFS cell, which was described in detail else-
where.[37] The XANES and EXAFS of the Pt L3 edge was taken under
flow conditions in the electrochemical cell with applied potential at
Sector 20 of the APS. Prior to the spectra, the Pt/C catalysts were
reduced for ~15 minutes to ensure a metallic Pt (rather than
surface oxide). XANES difference spectra were taken by comparing
the spectra under reaction conditions to a bulk Pt foil (which does
not show any surface effects from adsorbed species). EXAFS fitting
was done in Artemis as described elsewhere.[37]
P
nid3i
nid2i
P
ð4Þ
dTEM
¼
where dTEM is the volume-area mean diameter of the particle, di is
the diameter of the particle measured from TEM images and ni is
the number of particles with diameter di.
Electrocatalytic hydrogenation (ECH). The two-compartment batch
electrolysis cell was used to perform ECH experiments.[22] Cathodic
and anodic compartments were separated by a Nafion 117 proton
exchange membrane (Ion Power, Inc.), which was treated in a H2O2
solution (3 vol.%) and in sulfuric acid (2 M) before reaction. A piece
of carbon felt (Alfa Aesar >99.0%, 3.2 mm thickness), connected to
a graphite rod (Sigma Aldrich, 99.99%), was used as working
electrode in the cathode compartment. The typical size of the
carbon felt used for the electrode preparation was 3 cm X1.5 cm
(geometric surface area 4.5 cm2). A platinum mesh (Alfa Aesar,
99.9%) was used as counter electrode in the anodic compartment.
An Ag/AgCl electrode (saturated KCl) (Ametek) with a double
junction protection was used as reference electrode. The cathode
compartment was filled with 60 mL acetate buffer solution at
pH 4.6 and added with 10 mg of the catalysts. Prior to ECH, stirring
at 500 rpm allowed complete infiltration of the powder into the
carbon felt. Polarization of the catalyst was performed at À 40 mA
for 30 min. Phenol and benzaldehyde were typically added into the
cathode compartment to obtain a final concentration of 20 mM
although the concentration of phenol was varied �25% to
determine reaction orders. The anode compartment contained
acetate buffer (pH 4.6) as the electrolyte. ECH experiments were
performed at fixed potential while a flow of N2 was kept through
the reactant solution. All reactions were performed at atmospheric
pressure and constant potential. Temperature was controlled with a
Acknowledgements
The authors would like to thank the group of Prof. Hubert A.
Gasteiger at the Technische Universität München for advice and
valuable discussions. The authors are grateful to Donald M.
Camaioni, Manuel Wagenhofer and Gary Haller for fruitful
discussions. We are also grateful to Xaver Hecht and Martin
Neukamm for technical support. The research described in this
paper is part of the Chemical Transformation Initiative at Pacific
Northwest National Laboratory (PNNL), conducted under the
Laboratory Directed Research and Development Program at PNNL,
a multiprogram national laboratory operated by Battelle for the
U.S. Department of Energy. This research used resources of the
ChemCatChem 2018, 10, 1–9
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© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Conflict of Interest
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Keywords: Electrocatalysis · aqueous phase hydrogenation ·
phenol · benzaldehyde · Pt/C · particle size effects
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Manuscript received: August 19, 2018
Revised manuscript received: October 10, 2018
Version of record online: ■■■, ■■■■
ChemCatChem 2018, 10, 1–9
www.chemcatchem.org
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© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPERS
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Structure sensitivity: Intrinsic hydro-
genation rates of phenol and benzal-
dehyde in aqueous phase are directly
proportional to average size of Pt
particles with hydrogen being
supplied by both, molecular H2 and
in situ by a cathodic potential. In the
latter case, it is possible to control
the relative rates of hydrogen
Dr. U. Sanyal, Dr. Y. Song, Dr. N. Singh,
Dr. J. L. Fulton, Dr. J. Herranz, Dr. A.
Jentys, Dr. O. Y. Gutiérrez*, Prof. J. A.
Lercher*
1 – 9
Structure Sensitivity in Hydrogena-
tion Reactions on Pt/C in Aqueous-
phase
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addition to organics and of H2
evolution using renewable electricity.