From Pd nanoparticles to single crystals: 1,3-butadiene hydrogenation on well-defined model catalysts

Article abstract of DOI:10.1039/b513030a

Although 1,3-butadiene hydrogenation is known to be a structure-sensitive reaction, correlation of the catalytic activity with the exact Pd particle surface structure shows that the reaction is in fact particle size independent. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b513030a

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From Pd nanoparticles to single crystals: 1,3-butadiene hydrogenation
on well-defined model catalysts{
Joaquin Silvestre-Albero, G u¨ nther Rupprechter* and Hans-Joachim Freund
Received (in Cambridge, UK) 14th September 2005, Accepted 27th October 2005
First published as an Advance Article on the web 18th November 2005
DOI: 10.1039/b513030a
Although 1,3-butadiene hydrogenation is known to be a
structure-sensitive reaction, correlation of the catalytic activity
with the exact Pd particle surface structure shows that the
reaction is in fact particle size independent.
strong dependence of the particle adsorption properties on the
14
synthesis procedure. Furthermore, we are also able to distinguish
between the different butene isomers. Most importantly, only an
exact characterization of the Pd nanoparticle morphology and
surface structure by scanning tunnelling microscopy (STM) and a
comparison of the absolute reaction rates of Pd nanoparticles with
reference measurements on Pd(111) and Pd(110) single crystals
allow us to propose a model which explains the observed apparent
particle size dependence.
The hydrogenation of unsaturated hydrocarbons on Pd catalysts is
among the most frequently applied reactions of heterogeneous
1
catalysis. We have previously examined the adsorption, decom-
position and reaction (hydrogenation) of olefins such as ethene or
pentene with clean and well-defined alumina supported Pd
nanoparticles, employing a number of surface-spectroscopic
Although butadiene hydrogenation is an important and well-
optimized technological process to remove undesired diene
1
impurities from alkene streams, a complete microscopic under-
2–6
techniques under ultrahigh vacuum (UHV). A general observa-
tion was that the properties of disordered small Pd particles may
substantially deviate from those of Pd single crystals (due to the
higher fraction of low-coordinated sites on small Pd particles),
while larger Pd particles start to approach the behaviour of a
Pd(111) single crystal. However, a quantitative comparison in
terms of turnover frequencies (TOFs; molecules reacted per surface
site and second) has not been carried out so far, since this requires
reaction studies at ambient pressure, similar to those of technical
heterogeneous catalysis.
standing has not been obtained yet. Apart from this, butadiene
hydrogenation is also an interesting reaction for model catalysis
because it has four different products (1-butene, trans-2-butene,
cis-2-butene, n-butane) and therefore allows the study of both
activity and selectivity issues. To start with, we first discuss the
reaction kinetics on small Pd nanoparticles (mean size y2 nm) on
Al O at 373 K (Fig. 1a). Fig. 1b and 1c show corresponding
2
3
measurements on Pd particles with a mean size of 8 nm, and on
Pd(111), respectively. All four hydrogenation products were
observed but the kinetic behaviour and the selectivity are
remarkably different for the three model catalysts. As evident
from the consumption of 1,3-butadiene (100% relates to 5 mbar
butadiene), the hydrogenation rate increases with decreasing
butadiene pressure for 2 nm Pd nanoparticles (Fig. 1a), while
the 8 nm particles and the (111) single crystal surface exhibit nearly
zero order reaction kinetics (Fig. 1b, c; i.e. the reaction rate is
(nearly) independent of the actual butadiene pressure). This
indicates that for the 2 nm Pd nanoparticles the rate-limiting
step(s) change with the butadiene surface concentration. The
observed kinetics on small particles somewhat resemble the
transition from diffusion-control to a kinetic regime. At high
butadiene pressure (coverage) the reaction rate is limited by
adsorption/desorption processes, as indicated by the high
Here we report atmospheric pressure kinetic studies of 1,3-
butadiene hydrogenation on Pd–Al O catalysts for a wide range
2
3
of mean particle size, from 2 to 8 nm. By employing well-defined
model catalysts, i.e. Pd nanoparticles grown in UHV on thin
7–9
films,
Al O
2 3
and by determining their absolute activity for
butadiene hydrogenation, we are able to conclusively prove that
the behaviour of large Pd particles, in fact, is identical to that of
Pd(111), while for Pd particles below 4 nm it is not. This is only
feasible because we correlate the microscopic structural information
on the Pd nanoparticles with their macroscopic catalytic properties.
We also show that the selective hydrogenation of 1,3-butadiene—
1
0
although being structure-sensitive —is particle size independent if
the observed reaction rates are normalized taking into account the
exact size and surface structure of the Pd nanoparticles.
2
1
The Pd catalysts were grown and characterized in UHV and
subsequently transferred (under UHV) to a high-pressure reaction
(apparent) reaction activation energy of 75 kJ mol . With
increasing butadiene consumption the reaction itself becomes rate-
limiting. Similar high activation barriers were reported for highly-
1
1
cell, where reaction kinetics were determined in a recirculated
batch mode by on-line gas chromatography. When compared to
13,15
dispersed Pd–Al O catalysts,
3
2
which indicates that our model
1,10,12,13
previous studies,
our model catalysts enable full control of
catalyst mimics real catalysts quite well. The similarity of the
kinetic behaviour of the larger Pd nanoparticles (mean size = 8 nm)
with that of Pd(111) can be rationalized considering the STM
characterization of large Pd particles. High-resolution STM
particle size and cleanliness. This is crucial because recent studies
on wet-chemically prepared Pd–Al O catalysts have revealed a
2
3
2
,16
Fritz-Haber-Institut, Faradayweg 4-6, 14195 Berlin, Germany.
E-mail: rupprechter@fhi-berlin.mpg.de
images
octahedral shape with sometimes incomplete (111) facets
comprising y80% of the surface) and with (100) side facets
(cf. Fig. 1b, 2b).
indicated that larger particles have a truncated cubo–
{
Electronic supplementary information (ESI) available: Details about
(
experimental setup, catalysts preparation and supplementary experimental
data. See DOI: 10.1039/b513030a
8
0 | Chem. Commun., 2006, 80–82
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2 3
Fig. 2 Turnover frequency (TOF) for the different Pd–Al O /NiAl(110)
catalysts as a function of mean particle size, (a) normalized by the total
number of Pd surface atoms, and (b) normalized by using the number of
Pd atoms on (111) facets, using a truncated cubo-octahedron as the
structural model (shown as inset). TOF values for Pd(111) and Pd(110)
under identical reaction conditions are included (dashed lines).
importance of a critical hydrocarbon coverage. A stronger bonding
of 1,3-butadiene with decreasing particle size (2.8–1.2 nm) was also
1
7
suggested by Bertolini et al. on the basis of core level
photoemission.
Nevertheless, even the 8 nm Pd particles are more selective to
butenes than Pd(111). The adsorption of butadiene is presumably
still stronger than on Pd(111), as evident from the absence of re-
adsorption processes of butenes at low butadiene coverage, i.e. no
butane is formed until butadiene has (nearly) completely
disappeared, resulting in a 100% selectivity to butenes in the
presence of butadiene.
Fig. 1 Product distribution versus reaction time for 1,3-butadiene
hydrogenation at 373 K on (a, b) Pd–Al /NiAl(110) catalysts (mean
particle size 2 nm (a) and 8 nm (b)) and on (c) a Pd(111) single crystal.
2 3
O
2–4
STM images
and a LEED pattern are shown as insets.
Comparing Fig. 1a with 1b and 1c suggests that butadiene is
more strongly bonded to small Pd nanoparticles and hydrogen
adsorption on small Pd particles is hence more efficiently hindered
than on large ones. The catalytic activity of small Pd particles is
thus governed by the rate of hydrogen adsorption on the
hydrocarbon-covered Pd particles. Below a critical butadiene
coverage (y70% for the conditions of Fig. 1a) hydrogen
adsorption is less affected and the reaction slowly switches to a
kinetic regime. In fact, when the experiment whose results are
shown in Fig. 1a was performed with an initial butadiene pressure
of 3.5 mbar (i.e. 70% of the original concentration of Fig. 1a), the
reaction rate increased from the beginning, confirming the
To further explore the kinetic properties we have also measured
the particle size dependence of the reaction rate. Fig. 2 displays the
specific activity (turnover frequency, i.e. molecules of butadiene
reacted per Pd surface atom within 1 hour) as a function of particle
size. In Fig. 2a the total number of Pd surface atoms is used for
normalization (assuming particles of truncated cubo-octahedral
shape; filled and open circles refer to Pd deposition at 90 K and
300 K, respectively). Such a plot is common in heterogeneous
catalysis and shows, for the current case, that the TOF grows
linearly with particle size. This indicates that the activity does not
scale with the number of Pd surface atoms and that larger Pd
particles seem more active than smaller ones. Similar trends were
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1
0,12
reported for Pd particles up to y4 nm
characterization was carried out, preventing an accurate structure–
but no in situ structural
Summarizing, we report for the first time that butadiene
hydrogenation, in spite of being structure-sensitive, is particle size
activity correlation.
2 3
independent for Pd nanoparticles supported on Al O if the
In contrast, our STM measurements provide exact microscopic
information on the shape and surface structure of the Pd
nanoparticles. This allows us to propose a more realistic model
of the Pd nanoparticles including incomplete surface terraces (a
y5 nm cubo-octahedron with incomplete (111) facets is shown in
Fig. 2b as inset; for graphical reasons incomplete layers on side
facets are not displayed). Taking into account that the reaction is
structure sensitive, i.e. that different crystallographic orientations
will have different contributions to the overall catalytic activity,
and considering the aforementioned model, we are able to
calculate a more accurate (‘‘real’’) TOF by dividing the total
hydrogenation activity by the number of surface sites in a specific
crystallographic orientation. Along these lines, the number of Pd
surface atoms present in (111) and (100) terraces as well as at edges
and at the metal–support border were calculated for different Pd
clusters sizes. As a result we find that when the number of Pd
atoms in the incomplete (111) facets is used for normalization, the
TOF of butadiene hydrogenation is clearly particle size independent
catalytic activity is scaled by the number of active sites, i.e. Pd
atoms in incomplete (111) terraces. This is supported by
comparing the catalytic activity of small and large Pd nanopar-
ticles with Pd(111) and Pd(110) single crystals under the same
reaction conditions. With this we bridge the ‘‘materials gap’’
between heterogeneous catalysis on metal nanoparticles and
surface science studies on single crystals, by showing that here
Pd particles larger than 4 nm behave very similar to Pd(111).
Spectroscopic
2–6,11
desorption
measurements
(infrared
and
thermal
) on ethene and pentene have suggested either
p-bonded or di-s bonded species as active intermediates,
depending on the hydrocarbon length. However, the complexity
of butadiene allows for a large number of adsorption configura-
20,21
tions
and future spectroscopic studies on Pd nanoparticles are
certainly required to further explore the reaction mechanism of
diene hydrogenation.
J.S.A. acknowledges support by the Alexander von Humboldt
Foundation. We thank M. Heemeier and M. B a¨ umer for STM
sample characterization.
(Fig. 2b). This suggests that the reaction takes place preferentially
on the (111) facets of the Pd nanoparticles, at least for a mean
particle size above 4 nm (smaller sizes will be discussed below).
To validate this model we have carried out reference measure-
ments on Pd(111) and Pd(110) single crystals. The Pd(110) catalyst
Notes and references
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2
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value of 38 s was obtained (values included in Fig. 2b as dashed
lines). The y5-times lower activity of Pd(111) is probably due to
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2
3
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1
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2 | Chem. Commun., 2006, 80–82
This journal is ß The Royal Society of Chemistry 2006