Kinetics and particle size effects in ethene hydrogenation over supported palladium catalysts at atmospheric pressure

Article abstract of DOI:10.1016/j.jcat.2009.09.013

Palladium catalysts were synthesized in a highly controlled chemical vapor deposition process, giving narrowly distributed Pd nanoparticles with median sizes ranging from 1.3 to 5 nm on a SiO₂ or TiO₂ support. Unsupported Pd nanopart

Full text of DOI:10.1016/j.jcat.2009.09.013

Journal of Catalysis 268 (2009) 150–155
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Kinetics and particle size effects in ethene hydrogenation over supported
palladium catalysts at atmospheric pressure
a
b
a
Axel Binder a, , Martin Seipenbusch , Martin Muhler , Gerhard Kasper
*
a Institut für Mechanische Verfahrenstechnik und Mechanik, Karlsruher Institut für Technologie (KIT), Straße am Forum 8, D-76131 Karlsruhe, Germany
b Technische Chemie, Ruhr-Universität Bochum, Universitätsstr. 150, D-44801 Bochum, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2009
Revised 15 September 2009
Accepted 15 September 2009
Available online 13 October 2009
Palladium catalysts were synthesized in a highly controlled chemical vapor deposition process, giving
narrowly distributed Pd nanoparticles with median sizes ranging from 1.3 to 5 nm on a SiO2 or TiO2 sup-
port. Unsupported Pd nanoparticles with median sizes between 3 and 9 nm were also generated by spark
discharge.
The influence of Pd particle size and support on the hydrogenation of ethene to ethane was investigated
in a fixed-bed flow reactor at atmospheric pressure. The TOF was found to peak at 3 to 4 nm with a weak
dependence on the support material. Metal support interactions were generally weak, indicated by clo-
sely matching activation energies of 20 kJ molÀ1 for unsupported Pd, and 28 kJ molÀ1 for titania and silica
supported catalysts. Peak TOF values varied systematically with H2 partial pressure, indicating a pro-
nounced volume effect of the Pd particles on the reactivity.
Keywords:
Ethene hydrogenation
Particle size effect
Structure sensitivity
Supported palladium catalyst
Chemical vapor deposition (CVD)
Chemical vapor synthesis (CVS)
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
Pd/SiO2 catalyst in the particle size range of 1–30 nm [8]. Other
studies showed that the TOF in the hydrogenation of ethyne in
Palladium catalysts are predominantly used in hydrogenation
and dehydrogenation reactions of hydrocarbons. A large number
of these reactions are known to be structure sensitive [1], a phe-
nomenon which has been intensively investigated for supported
metal catalysts [2,3]. The hydrogenation of ethene is extensively
studied and a general reaction mechanism was proposed by
Horiuti and Polanyi [4], in which ethene is hydrogenated by
dissociated hydrogen atoms being sequentially added to the ad-
sorbed alkene intermediates. In the literature, the structural sen-
sitivity of the hydrogenation of ethene is under debate and is
believed to be nonexistent by some authors [2]. Kinetic studies
of Schlatter and Boduard for instance indicated that the rate at
which ethene hydrogenates is independent of the particle size
[5]. Additionally, Shaikhutdinov and Rupprechter reported that
the activity of ethene hydrogenation is independent of the
ethyne–ethene mixtures on Pd/Al2O3 catalysts rises with increas-
ing particle size [9–11]. In addition to the experimental findings,
Monte-Carlo simulations demonstrated a strong increase of turn-
over frequency (TOF) for the ethene hydrogenation on Pd/SiO2
catalysts in size range of 0–40 nm [12].
A possible reason for these apparent discrepancies in the litera-
ture may be found in the variation of experimental conditions and
materials. Most experimental studies were conducted on ideal sys-
tems such as metal foils and ideal conditions such as high vacuum
and in excess H2. The transfer of these results to real process con-
ditions is therefore questionable. Furthermore, to our knowledge
experimental data for Pd supported on different materials at com-
parable conditions are unavailable, hindering the assessment of
catalyst support interactions.
In the present work, we report the hydrogenation of ethene
on nanoscale catalyst particles at atmospheric conditions with
special emphasis on the influence of the Pd particle size on
the reactivity to understand structure-function relationships of
this reaction at industrially relevant process conditions. We
studied the reactivity of unique narrowly distributed Pd particles
on inert supports such as silica, on active supports such as tita-
nia and unsupported Pd particles to exclude metal–support
interactions as an obstruction in the determination of structural
sensitivity.
particle size over Pd/Al2O3/NiAl(1 1 0) between
1 and 6 nm,
´
respectively [6,7]. In contrast to these studies Borodzinski
reported a slight increase of the turnover frequency (TOF) of
the hydrogenation of ethyne in ethyne–ethene mixtures on a
* Corresponding author. Fax: +49 721 6086563.
E-mail addresses: axel.binder@kit.edu (A. Binder), martin.seipenbusch@kit.edu
(M. Seipenbusch), muhler@techem.ruhr-uni-bochum.de (M. Muhler), gerhard.
kasper@kit.edu (G. Kasper).
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.09.013

A. Binder et al. / Journal of Catalysis 268 (2009) 150–155
151
2.2. Catalytic activity and kinetic measurements
2. Experimental
For the catalytic experiments the catalyst particles were col-
lected on a polycarbonate membrane filter (diameter 45 mm, pore
2.1. Catalyst preparation
size 0.2 lm) which was placed downstream of the coating reactor.
Supported palladium catalyst particles were generated in the
gas phase by a combined chemical vapor synthesis (CVS) and me-
tal–organic chemical vapor deposition (MOCVD) process, which
was described in detail in previous publications [13,14]. Ultrafine
silica and titania support particles were generated by thermal
decomposition of their metal alkoxide vapors in a CVS process
[15]. For this, the alkoxide tetraethylorthosilicate (Si(OC2H5)4,
TEOS) and titanium tetraisopropoxide (Ti(OC3H7)4, TTIP) were
evaporated at 50 and 75 °C, respectively, and then fed to a tube fur-
nace, where the vapors were mixed with oxygen and a diluting
nitrogen flow. The precursors oxidize into highly super-saturated
oxide phases and particle formation takes place by homogeneous
nucleation. This first step of particle formation is followed by con-
densation of the growth species on the newly formed particles and
subsequent surface reactions. Agglomeration of the highly concen-
trated aerosol particles results in fractal structures with surface
areas up to 350 m2 gÀ1. Downstream, the aerosol was sintered at
1500 °C to obtain spherical particles with well-defined surfaces.
The resulting mean particle sizes were about 80 nm with geomet-
ric standard deviations of 1.3. The particle number concentration
To obtain large palladium particle sizes the catalyst particles were
sintered prior to deposition in the aerosol state in a tube furnace
(residence times of about 30 s) downstream of the coating reactor
and were then collected on a filter.
The set-up of the catalytic experiments is shown in Fig. 1. For
each experiment all tubing, the filter and the IR chamber, which
is a self-made chamber with a length of 224 mm, were flushed
with nitrogen and hydrogen.
A
flow of ethene (1 ml minÀ1
99.9%), and dilution nitrogen
,
99.9%), hydrogen (1 ml minÀ1
,
(50 ml minÀ1, 99.99%) bypassing the reactor was directed to an
FTIR (Bruker Vector 22), where the ethene concentration was de-
tected between wave numbers of 500 and 4000 cmÀ1 with a reso-
lution of 4 cmÀ1. The rates were detected by the decrease in the
ethene concentration when directing the flow through the filter
(Fig. 1, right). For determination of the ethene concentration, the
spectra between wave numbers of 1810 and 1960 cmÀ1 were inte-
grated. This peak was chosen for analysis because there is no over-
laying of the spectra of ethene and ethane (Fig. 1, inset right).
Please note that this band is not easy to be related to a specific
band but should be allocated to the harmonic of the CH2-wagging.
The palladium particle sizes were determined by transmission
electron microscopy (TEM) image analysis before and after reac-
tion. The TEM images were recorded on a Philips CM 12 electron
microscope operating at 120 kV. The particle size distributions
were expressed in terms of log-normal distributions. The total pal-
ladium mass of each catalytic sample was determined by induc-
tively coupled plasma optical emission spectrometry (ICP-OES).
Additional information about the shape of the Pd particles
would be helpful but an alternative determination of the Pd parti-
cle size by H2 or CO chemisorption was not possible because the
total Pd mass of every sample was simply too low. Furthermore,
additional complications would arise, such as hydrogen spill-over
on Pd/TiO2, and the multiple bondings of CO as a function of the
Pd particle size, which would end in misleading results. Thus, we
preferred to derive the TOF based on the well-established TEM im-
age analysis of the Pd particle sizes with sufficiently narrow parti-
cle size distributions.
was 107 cmÀ3 at a total flow rate of 300 ml minÀ1
.
The support particles were coated with palladium directly in
the aerosol state via MOCVD using the metal–organic compound
cyclopentadienyl-allyl-Pd [Cp(allyl)Pd; synthesized by the working
group of Prof. Fischer, Lehrstuhl für Anorganische Chemie II, Ruhr
Universität Bochum]. The palladium precursor concentration was
1.4 Â 10À7 mol lÀ1 at a temperature of 80 °C in the CVD reactor.
The unsupported palladium particles were synthesized using a
spark discharge generator [16], consisting of two palladium-elec-
trodes in parallel with a capacitor charged by a high-voltage source
and nitrogen (1 l minÀ1, 99.99%) as carrier gas. The charging of the
capacitor leads to an increase in the potential difference between
the electrodes until the breakthrough voltage is reached. The en-
ergy stored in the capacitor is then released into a spark plasma,
which provides the energy for the evaporation of the material from
the electrode surfaces but also creates a large quantity of ions
which are accelerated in the decaying electric field, and physically
removed the material from the electrodes by sputtering. The parti-
cle generation subsequently occurs by condensation of the metal
atoms in the cold carrier gas. The particle size depends mainly
on the material and lies in the range between 2 and 6 nm.
All catalytic experiments were performed at atmospheric pres-
sure at a constant temperature of 293 K. The turnover frequencies
were expressed in mol ethane per second and per mol palladium
Fig. 1. Experimental set-up of the catalytic experiments (left) and IR spectra of ethene and ethane (right) and spectra range for conversion measurements (inset).

152
A. Binder et al. / Journal of Catalysis 268 (2009) 150–155
surface atoms. The palladium surfaces were calculated by the total
palladium mass determined by ICP-OES and the mean palladium
diameter determined by TEM image analyses assuming semi-
spherical palladium particles on the surface of the support
particles.
The ethene concentration dependence was measured at a reac-
tion temperature of 293 K and a hydrogen concentration of
3.4 Â 10À9 mol lÀ1 (partial pressure 76.9 mbar) for palladium on a
silica support. The ethene concentration was varied from
1.4 Â 10À9 to 5.5 Â 10À9 mol lÀ1 (partial pressures 30.8–123 mbar).
The hydrogen concentration dependence was also measured at
a reaction temperature of 293 K and an ethene concentration of
4.1 Â 10À9 mol lÀ1 (partial pressure 90.9 mbar). The hydrogen con-
centration was varied from 8.1 Â 10À10 to 2.0 Â 10À8 mol lÀ1 (par-
tial pressures 18.2–455 mbar).
The temperature dependence was measured for palladium sup-
ported on silica and titania and for unsupported palladium parti-
cles generated by spark discharge. The reaction was carried out
in a temperature range between 283 and 313 K. The ethene and
hydrogen concentrations were 7.3 Â 10À10 mol lÀ1 (partial pres-
sures of 16.4 mbar).
To study the influence of the palladium particle size on the TOF
of ethene hydrogenation several samples with different palladium
particle sizes were prepared by variation of the MOCVD conditions
and/or by sintering of the deposited palladium particles. We pre-
pared palladium particles supported on silica and titania in a size
range of 1.8–4.4 and 1.3–5.1 nm, respectively. The reaction was
carried out at 293 K and atmospheric pressure. The ethene and
hydrogen concentrations were set to 7.3 Â 10À10 mol lÀ1 (partial
pressures of 16.4 mbar). We compared these results to unsup-
ported palladium in size range of 3.1–8.7 nm.
3. Results and discussion
3.1. Morphology of catalyst particles
The morphology of the supported Pd particles is shown in Fig. 2.
The deposited Pd particles from the MOCVD process were distrib-
uted log-normally with mean diameters in the range of 1.3–3 nm,
depending on Pd precursor concentration, and very small geomet-
ric standard deviations of about 1.1 (for more detailed information
see [13]). Also the deposited and sintered Pd particles could be de-
scribed by log-normal distributions with small geometric standard
deviations of about 1.2. The unsupported Pd particles generated by
spark discharge were also distributed very narrowly so the samples
could be characterized in terms of mean diameters. Because of the
very narrowly distributed Pd particle sizes and their precise deter-
mination this process supplies ideal model catalyst particles.
The purity of the oxide carrier particles as well as the obtained
Pd coatings was analyzed by energy-dispersive X-ray (EDX) analy-
sis up to 10 keV. High-purity particles and coatings were found
which contain no detectable carbon contamination (compare
[13]). Electron diffractions obtained by TEM indicates that the Pd
particles have a crystalline structure, while the support particles
are more or less amorphous.
3.2. Kinetics of ethene hydrogenation at atmospheric conditions
Fig. 3 shows the plot of ln (TOF) versus ln (ethene pressure) for
Pd particles with a mean diameter of 4.2 nm supported on silica,
where the ethene partial pressure is expressed in mbar. A fit to
the data yields the reaction order in ethene pressure of
0.39 0.01. This value differs from reaction orders in ethene
Fig. 2. Top row: TEM images of silica-supported Pd after the coating reactor (left, d), sintered at 873 K (middle, .), and unsupported Pd catalyst particles sintered at 473 K
(right, j). Below: corresponding cumulative distributions of the projected Pd diameters.

A. Binder et al. / Journal of Catalysis 268 (2009) 150–155
153
For silica-supported Pd a value of 0.66 was reported in the liter-
ature for a reaction temperature of 243 K [17], which can be
explained by the temperature dependence of the reaction order
[20]. Additionally, a value of 1 was found for alumina-supported
Pd [21,22] and of 1.03 on a Pd(1 1 1) surface at 300 K [18].
The variation of the reaction temperature was studied for Pd
supported on SiO2 and TiO2, and for unsupported Pd particles.
Fig. 5 shows the results of the temperature dependence plotted
as ln (rate constant) versus inverse temperature. The activation
energies calculated from the slopes are given in Table 1.
Reported activation energies for ethene hydrogenation of sup-
ported Pd and Pt catalysts vary between 25 and 46 kJ molÀ1
[5,17] and are in good agreement with our values which are on
the lower end of the scale. The value of the unsupported Pd is also
in the range of reported activation energies for Pd and Pt single
crystals between 12.6 kJ molÀ1 on the Pd(1 1 1) surface [23] and
45 kJ molÀ1 [18,24–26]. Only the values from Monte-Carlo simula-
tions are slightly higher with 37–50 kJ molÀ1 on the Pd(1 0 0) sur-
face [19].
To conclude, ethene hydrogenation at the investigated condi-
tions is in good agreement with the reported data in the literature
with the exception of the reaction order in ethene, which seems
slightly too high. The activation energies show virtually identical
values for Pd on different supports. A slightly lower value for
unsupported Pd indicates a small influence of the support.
Fig. 3. Plot of ln (TOF) versus ln (ethene partial pressure). Reaction temperature
was 298 K at atmospheric pressure and H2 concentration was set to
3.4 Â 10À9 mol lÀ1
.
reported in the literature which were described to be nearly zero,
for example a value of À0.03 on silica-supported palladium at
243 K [17] and negative like À0.22 for Pd(1 1 1) at 320 K [18]. A
better agreement was found with orders for Pt/SiO2 catalysts of
0.25 at 233 K [17]. Precise comparisons are difficult because this
value clearly depends on reaction conditions such as temperature
and pressure as Hansen and Neurock found in their Monte-Carlo
simulation of the hydrogenation of ethene on a Pd(1 0 0) surface.
They reported reaction orders in ethene between À0.4 and 0 or
slightly positive at 298 K depending on ethene pressure [19].
The hydrogen pressure dependence was also determined for Pd
particles with a mean diameter of 4.2 nm supported on silica. The
resulting plot of ln (TOF) versus ln (hydrogen pressure), where the
hydrogen partial pressure is expressed in mbar, is shown in Fig. 4.
The reaction order determined from the slope of the best fit to the
data was 1.04 0.07.
3.3. Influence of palladium particle size on the reaction rate
The variation of the Pd particle size was investigated for Pd sup-
ported on silica and titania as well as on unsupported Pd particles.
The resulting TOFs as a function of Pd particle diameter are plotted
in Fig. 6.
Fig. 5. Arrhenius plot for ethene hydrogenation catalyzed by Pd/SiO2 (d), Pd/TiO2
(j), and unsupported Pd (N). Reactions were conducted at atmospheric pressure
with H2 and C2H4 concentrations of 7.3 Â 10À10 mol lÀ1
.
Table 1
Activation energies for the hydrogenation of ethene on Pd/SiO2, Pd/TiO2, and
unsupported Pd at atmospheric pressure with hydrogen and ethene concentrations
of 7.3 Â 10À10 mol lÀ1 and temperature range of 283–313 K.
Sample
Calculated activation
Palladium particle size (nm)
energy (kJ molÀ1
)
Pd/SiO2
Pd/TiO2
Unsupported Pd
27.6 0.27
26.9 1.06
20.0 0.93
4.2
4.2
5.1
Fig. 4. Plot of ln (TOF) versus ln (hydrogen partial pressure). Reaction was
performed at 298 K at atmospheric pressure and C2H4 concentration of
8.1 Â 10À10 mol lÀ1
.

154
A. Binder et al. / Journal of Catalysis 268 (2009) 150–155
Fig. 6. TOF versus Pd particle size for Pd/SiO2 (d), Pd/TiO2 (j), and unsupported Pd
(N) and for the hydrogenation of ethene. Reaction was carried out at atmospheric
pressure at a reaction temperature of 293 K; H2 and C2H4 concentrations were
Fig. 7. TOF versus Pd particle size for Pd/TiO2 (j, H2:C2H4 = 1:1 and h,
H2:C2H4 = 5:1, left) for the hydrogenation of ethene. Reaction was carried out at
atmospheric pressure at reaction temperature of 293 K and ethene concentration of
8.6 Â 10À4 mol lÀ1
.
8.6 Â 10À4 mol lÀ1
.
which will be correlated to the particle volume. In that case other
stoichiometric compositions, i.e. an excess of H2 should influence
the TOF or the maximum activity, respectively. Higher H2 concen-
trations should not influence the activity of the small Pd particles
because of their limited storage capacity but increase the activity
of the larger ones due to an increase in availability of subsurface
hydrogen. Consistently, the reaction order in H2 should be size
dependent and it was indeed shown that it is decreasing with
increasing particle size [34]. Fig. 7 shows the resulting TOF on Pd/
TiO2 as a function of the Pd particle size for an excess of H2
(H2:C2H4 = 5:1, unfilled symbols). Indeed, one can see that the max-
imum activity for Pd/TiO2 shifts from a Pd particle size of 3 nm for
stoichiometric conditions to larger particles sizes of about 4.2 nm
for excess H2.
This influence of H2 partial pressure on the optimal Pd particle
size could also be the reason for the contradictory results reported
by Shaikhutdinov ([Pd/Al2O3/NiAl(1 1 0)], H2:C2H4 = 3:1, À183 °C,
vacuum) and Masson (Pt on amorphous Al2O3 film,
H2:C2H4 = 10:1, 100 °C, 1 atm) who found an independence of
TOF in the Pd particle size range of 1–3 nm and 1.7–2.8 nm, respec-
tively [6,35]. These results are contradictory to the structure sensi-
tivity reported here, but both studies cover very small Pd particle
size ranges, which are in particular lower than the observed max-
imum of the TOF at excess H2 in this study (see Fig. 7).
The plot demonstrates a clear dependence of the TOF on Pd par-
ticle size. For the titania-supported Pd and the unsupported Pd a
maximum TOF is observed at a particle size of 3 nm, while for silica
supported Pd the maximum of the rate seems to be at larger parti-
cle sizes of about 4 nm. This shift in the size with maximum activ-
ity is maybe caused by the influence of the support material and
indicated a small metal–support interaction between Pd and silica
at these conditions. The activity for all catalysts is about 8-fold
higher for the maximum rates in comparison to the lowest values.
One can also see that the support has no strong influence on the
maximum activity with measured values of TOF being very similar
for unsupported Pd (9.4 sÀ1), Pd/SiO2 (8.4 sÀ1), and Pd/TiO2
(7.5 sÀ1). Similar results were reported by Arakawa for the hydro-
formylation of ethene measuring the ethane by-production over a
Rh/SiO2 catalyst finding a maximum of TOF for a particle size of
4 nm [27].
It is commonly believed that the
nates to ethane [21,28,29]. Additionally, the activation of molecu-
lar -bonds requires a unique configuration of several metal atoms
or step-edge sites, respectively, in which the -bond dissociates
p-bonded ethene hydroge-
p
p
through multiple contacts with several surface atoms [30]. Such
step sites can physically not be present below a particular particle
size. The relative probability of such sites is a function of particle
size and shows a maximum which could be an explanation for
the maximum of the TOF [31].
Another possible explanation for the observed size effect is based
on Doyle’s demonstration of the importance of the subsurface H2 as
the active species in the hydrogenation reaction. The authors inves-
tigated the hydrogenation of ethene on both Pd(1 1 1) single crys-
tals and Pd particles supported on an Al2O3/NiAl(1 1 0) film under
low-pressure conditions [32,33]. They reported that on Pd(1 1 1)
no hydrogenation occurs while palladium particles were highly
active at the same conditions. The authors concluded that in the
nanoparticle dimensions the subsurface hydrogen is accessible to
the adsorbed alkene while for the Pd(1 1 1) surface the hydrogen
atoms diffuse deeply into the bulk and are thus not readily available
for the reaction at the surface. The decrease in the TOF for larger par-
ticles could thus also be a result of a volume effect, where the
increasing particle volume decreases the accessibility of the subsur-
face hydrogen. For small particles on the other hand volume can also
provide an explanation for decreasing activity below the optimum
particle size. If subsurface hydrogen is indeed crucial for the reac-
tion the storage capability of the particles is an important factor
4. Conclusion
The hydrogenation of ethene over narrowly distributed sup-
ported palladium nanoparticle catalysts at atmospheric pressure
was investigated. The rate for the hydrogenation of ethene cata-
lyzed by palladium shows a clear size dependence in the size range
of 1–9 nm for Pd supported on SiO2 or TiO2 and for unsupported Pd
with peaks at 3–4 nm. The supports have nearly no influence on
the structure sensitivity behavior, indicating a negligible influence
of the support for vapor deposited metal particles. This was sup-
ported by similar activation energies for the unsupported Pd parti-
cles (20 kJ molÀ1) and for Pd on silica and titania (27–28 kJ molÀ1).
The results clarify a strong volume effect of the Pd where the acces-
sibility of the subsurface H2 seems to be important. The influence
of the H2 concentration on the optimal Pd particle size for the reac-
tion confirms this assumption.
The results underline the influence of process conditions on the
structural sensitivity and thus emphasize on the importance of

A. Binder et al. / Journal of Catalysis 268 (2009) 150–155
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Acknowledgments
This project was partially funded by Deutsche Forschungsgeme-
inschaft (DFG) in SPP 1119. The authors would also like to thank
Prof. Fischer and co-workers (Lehrstuhl für Anorganische Chemie
II, Ruhr Universität Bochum) for synthesizing the precursor and
advising in matters of precursor chemistry.
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