Hydrogenation of the α,β-unsaturated aldehydes acrolein, crotonaldehyde, and prenal over Pt single crystals: A kinetic and sum-frequency generation vibrational spectroscopy study

Article abstract of DOI:10.1021/ja8092532

Sum-frequency generation vibrational spectroscopy (SFG-VS) and kinetic measurements using gas chromatography have been used to study the surface reaction intermediates during the hydrogenation of three α,β- unsaturated aldehydes, acrolein, crotonaldehyde,

Full text of DOI:10.1021/ja8092532

Published on Web 07/06/2009
Hydrogenation of the r,ꢀ-Unsaturated Aldehydes Acrolein,
Crotonaldehyde, and Prenal over Pt Single Crystals: A Kinetic
and Sum-Frequency Generation Vibrational Spectroscopy
Study
Christopher J. Kliewer, Marco Bieri, and Gabor A. Somorjai*
Department of Chemistry, UniVersity of California, Berkeley, California 94720, and Materials
Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received November 26, 2008; E-mail: somorjai@socrates.berkeley.edu
Abstract: Sum-frequency generation vibrational spectroscopy (SFG-VS) and kinetic measurements using
gas chromatography have been used to study the surface reaction intermediates during the hydrogenation
of three R,ꢀ-unsaturated aldehydes, acrolein, crotonaldehyde, and prenal, over Pt(111) at Torr pressures
(1 Torr of aldehyde, 100 Torr of hydrogen) in the temperature range of 295-415 K. SFG-VS data showed
that acrolein has mixed adsorption species of η2-di-σ(CC)-trans, η2-di-σ(CC)-cis as well as highly coordinated
η3 or η4 species. Crotonaldehyde adsorbed to Pt(111) as η2 surface intermediates. SFG-VS during prenal
hydrogenation also suggested the presence of the η2 adsorption species and became more highly
coordinated as the temperature was raised to 415 K, in agreement with its enhanced CdO hydrogenation.
The effect of catalyst surface structure was clarified by carrying out the hydrogenation of crotonaldehyde
over both Pt(111) and Pt(100) single crystals while acquiring the SFG-VS spectra in situ. Both the kinetics
and SFG-VS showed little structure sensitivity. Pt(100) generated more decarbonylation “cracking” product
while Pt(111) had a higher selectivity for the formation of the desired unsaturated alcohol, crotyl alcohol.
Scheme 1. Hydrogenation Pathways for Acrolein
1. Introduction
The selective hydrogenation of an unsaturated CdC bond in
the presence of a CdO carbonyl group of the same molecule
has been the subject of much research in recent years. Both
experimental and theoretical work has been devoted to this topic
and comprehensive reviews of the subject have been published.1-4
One product of this reaction, the unsaturated alcohol, is an
important intermediate in both the pharmaceutical and fragrance
industries. However, it is quite difficult to achieve a high
selectivity and activity for the unsaturated alcohol product
because the hydrogenation of the CdC is thermodynamically
preferred by roughly 35 kJ/mol2.
The platinum group metals are the catalysts most frequently
employed during this reaction, but the selectivity for the desired
unsaturated alcohol is relatively low for these catalysts. Over
platinum, the selectivity for unsaturated alcohol has been shown
to improve upon alloying with another metal,5 for instance Fe6
or Sn7, as well as upon the addition of various promoters,3,8
such as potassium on the surface. Catalyst structure, such as
the (111) or (100) crystal face of a metal, has also been
suggested to play a role in the reaction selectivity.9
Also effecting the selectivity in the hydrogenation of R,ꢀ-
unsaturated aldehydes is the amount of steric hindrance to
adsorption added to the CdC bond by the placement of bulky
groups, such as phenyl or methyl groups, on this side of the
molecule. Hoang-Van et al.10 reported on the hydrogenation of
acrolein over various platinum catalysts. Each of the widely
used Pt based catalysts (Pt black, Pt-Al, Pt-Si) showed greater
than 99% selectivity for the hydrogenation of the CdC to yield
propionaldehyde (Scheme 1). In comparison, for the hydrogena-
tion of prenal, the same molecule as acrolein with two methyl
groups added to the CdC bond, Birchem et al.7,11 show at 353
K the dominant product to be the unsaturated alcohol, a complete
reversal of the selectivity seen for acrolein hydrogenation.
In this work, the hydrogenation of acrolein, crotonaldehyde,
and prenal (Schemes 1-3) was carried out over Pt(111) while
(1) Gallezot, P.; Richard, D. Catal. ReV.sSci. Eng. 1998, 40, 81–126.
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2005, 292, 1–49.
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(4) Claus, P. Top. Catal. 1998, 5, 51–62.
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121, 69–80.
(6) Beccat, P.; Bertolini, J. C.; Gauthier, Y.; Massardier, J.; Ruiz, P. J.
Catal. 1990, 126, 451–456.
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(10) Hoang-Van, C.; Zegaoui, O. Appl. Catal., A 1997, 164, 91–103.
(11) Birchem, T.; Pradier, C. M.; Berthier, Y.; Cordier, G. J. Catal. 1994,
146, 503–510.
(7) Birchem, T.; Pradier, C. M.; Berthier, Y.; Cordier, G. J. Catal. 1996,
161, 68–77.
(8) Marinelli, T.; Ponec, V. J. Catal. 1995, 156, 51–59.
9
9958 J. AM. CHEM. SOC. 2009, 131, 9958–9966
10.1021/ja8092532 CCC: $40.75  2009 American Chemical Society

Hydrogenation of R,ꢀ-Unsaturated Aldehydes over Pt
A R T I C L E S
Scheme 2. Hydrogenation Pathways for Crotonaldehyde
verified by AES and the crystallographic structure verified with
low-energy electron diffraction (LEED). The samples were then
transferred into the HP cell for SFG and kinetic studies.
Sum-Frequency Generation Vibrational Spectroscopy. For
SFG measurements, an active/passive mode-locked Nd:YAG laser
(Leopard D-20, Continuum) with a pulse width of 20 ps and a
repetition rate of 20 Hz was used. The fundamental output at 1064
nm was sent through an optical parametric generation/amplification
(OPA/OPG) stage where a tunable IR (2300-4000 cm-1) and a
second harmonic vis (532 nm) beam were created. The IR (150
µJ) and vis (200 µJ) beams were spatially and temporally overlapped
on the crystal surface at angles of incidence of 55° and 60°,
respectively, with respect to the surface normal. The generated SFG
beam was collected and sent through a motorized monochromator
equipped with a photomultiplier tube to detect the SFG signal
intensity. The signal-to-noise ratio was further increased by using
a gated integrator while the IR beam was scanned through the
spectral region of interest. The SFG signal, ISFG, is related to the
incoming visible (Ivis) and infrared (IIR) beam intensities, and
second-order susceptibility of the media (ꢁ(2)), according to eq 1:
Scheme 3. Hydrogenation Pathways for Prenal
SFG
I
ω
∝ |ꢁ(2)|2IvisωvisIIRωIR
(1)
SFG
ꢁ
(2) is enhanced when ωIR is at resonance with a vibrational mode
of the molecules, q, according to eq 2:
using sum-frequency generation vibrational spectroscopy (SFG-
VS) to monitor the surface reaction intermediates. These spectra
are then compared to published gas-phase IR spectra, HREEL
spectra, and DFT calculations to assist in the interpretation.
Crotonaldehyde, with a reaction selectivity that is between that
of prenal hydrogenation and acrolein hydrogenation, was chosen
to elucidate the effect that catalyst structure has on this reaction.
Reaction kinetics using gas chromatography and SFG-VS
spectra were taken for crotonaldehyde hydrogenation over
Pt(111) and Pt(100) in the temperature range from 150 to 415
K. Finally, prenal hydrogenation was studied kinetically over
Pt(111) in the temperature range from 308 to 403 K.
Aq
ISFG ∝ ꢁNR(2)eiφ
+
eiγ
(2)
2
q
NR
∑
|
|
ωIR - ωq + iΓq
q
where ꢁNR(2) is the nonresonant nonlinear susceptibility, eiφ is the
phase associated with the nonresonant background, Aq is the strength
of the qth vibrational mode, ωIR is the frequency of the incident IR
laser beam, ωq is the frequency of the qth vibrational mode, Γq is
NR
the natural line width of the qth vibrational transition, and eiγ is
q
the phase associated with the qth vibrational transition. All SFG-
VS spectra reported are data fit to a form of eq 2.
Since the gas phase in the HP cell absorbs some small amount
of the incoming IR radiation, the SFG signal was then normalized
by using the following expression (eq 3):
2. Experimental Section
ISFG
ISFG,norm
)
(3)
Materials. Crotonaldehyde (99.5%, Fluka) and prenal (97%,
Sigma-Aldrich Inc.) were subjected to several freeze-pump-thaw
cycles prior to use and their purities were checked by means of
gas chromatography and quadrupole mass spectrometry. Acrolein
was obtained in gas phase from Airgas with the balance being
research grade argon and the purity was verified with quadrupole
mass spectrometry to be 99%.
The High-Pressure/Ultrahigh Vacuum System. All experi-
ments reported here were carried out in a high-pressure/ultrahigh
vacuum (HP/UHV) system. The UHV chamber is operated at a
base pressure of 2 × 10-10 Torr and is isolated from the HP cell
by a gate valve. The UHV system is equipped with an Auger
electron spectrometer (AES), a quadrupole mass spectrometer
(Stanford Research Systems), and an ion bombardment gun
(Eurovac). The HP cell consists of two CaF2 conflat windows that
allow transmission of infrared (IR), visible (vis), and sum-frequency
radiation for sum-frequency generation (SFG) experiments. The
product gases in the HP cell are constantly mixed via a recirculation
pump, and kinetic data is acquired by periodically sampling the
reaction mixture and analyzing the relative gas phase composition
in a flame ionization detector (FID) of a gas chromatograph
(Hewlett-Packard HP 5890 on a 5% Carbowax 20 M packed
column).
IR,beforeIIR,after
√
I
where IIR,before and IIR,after define the IR beam intensities measured
before and after the HP cell to correct for any infrared radiation
absorbed between the HP cell window and the sample, which is
mounted exactly halfway between the HP cell entrance and exit
window. More information on the HP/UHV system and SFG
measurement can be found elsewhere.12-16
3. Results and Discussion
3.1. Sum-Frequency Generation Vibrational Spectroscopy
of Acrolein, Crotonaldehyde, and Prenal Hydrogenation over
Pt(111). 3.1.1. Acrolein Hydrogenation on Pt(111)sSFG-VS
Results. Acrolein is the simplest R,ꢀ-unsaturated aldehyde,
which makes it attractive for surface science study. As can be
seen in Scheme 1, it contains only the conjugated carbonyl and
alkene groups with no side groups. Using density functional
theory analysis, Loffreda et al.17 examined possible adsorption
(12) Bratlie, K. M.; Flores, L. D.; Somorjai, G. A. Surf. Sci. 2005, 599,
93–106.
Sample Preparation. Prior to each experiment, the Pt(111) and
Pt(100) crystal surfaces were cleaned in the UHV chamber by Ar+
(1 keV) sputtering for 20 min at approximately 3 × 10-5 Torr of
Ar. After sputtering, the crystals were heated to 1103 K in the
presence of O2 of 5 × 10-7 Torr and annealed at the same
temperature for 2 min. The cleanliness of the crystal surfaces was
(13) Kung, K. Y.; Chen, P.; Wei, F.; Rupprechter, G.; Shen, Y. R.;
Somorjai, G. A. ReV. Sci. Instrum. 2001, 72, 1806–1809.
(14) Shen, Y. R. Annu. ReV. Phys. Chem. 1989, 40, 327–350.
(15) Shen, Y. R. Nature 1989, 337, 519–525.
(16) Yang, M. C.; Tang, D. C.; Somorjai, G. A. ReV. Sci. Instrum. 2003,
74, 4554–4557.
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A R T I C L E S
Kliewer et al.
Figure 1. SFG vibrational spectrum of 1 Torr of acrolein in the absence of hydrogen over Pt(111) at 295 K. The aldehyde C-H modes at 2785 and 2850
cm-1 clearly identify the presence of η2-trans and η2-cis adsorption modes, respectively.
Table 1. Summary of Experimental Acrolein Vibrational Modes
(given in cm-1) for Gas Phase Acrolein,18 Acrolein Adsorbed onto
modes of acrolein on Pt(111) at various coverages and deter-
mined adsorption energies and predicted a vibrational spectrum
for each of the possible stable structures. Experimental assign-
ments of the gas-phase modes of acrolein have been previously
published.18 Further, Murillo et al.19 examined the adsorption
of acrolein to the Pt(111) surface at 110 K using HREELS. Our
spectra obtained in situ during the catalytic hydrogenation
reactions are compared to the published data.
Pt(111) at 110
Vibrational Modes for 1 Torr of Acrolein on Pt(111) at 298 K
K
Using HREELS,19 and the Observed SFG
assignment
IR gas phase
HREELS at 110 K
SFG-VS at 298 K
ν
asym(dCH2)
ν(ꢀ)(CH)
3103
3069
2998
2800
3098
2997
2908
2854
3060
3020
2930
2850/2785
ν
sym(dCH2)
aldehyde(CH)
ν
There are seven stable adsorption modes presented in the
theoretical work mentioned above. First, the η1 adsorption mode
has the oxygen atom as the only part of the acrolein molecule
that interacts with the Pt(111) surface. The η2-cis and η2-trans
modes involve a di-σ bond to two Pt atoms originating from
the CdC of the acrolein. Clearly, the cis conformer is the one
with the oxygen of the carbonyl pointing back toward the
acrolein molecule, while in the trans form it points away. The
η3-cis and η3-trans are the same as the η2 adsorption modes
with the exception that the oxygen atom of the carbonyl is
pointing down to and bonding with the metal surface. Finally,
the η4-cis and η4-trans adsorption modes are bound to the surface
via two di-σ bonds, one originating from the CdC and one from
the CdO.
Figure 1 shows the SFG vibrational spectrum of 1 Torr of
acrolein with no hydrogen added over a Pt(111) single crystal
at 295 K. Five vibrational resonances are seen in this C-H
region. The vibrational resonance furthest to the red is present
at 2785 cm-1, which can be assigned to the aldehyde C-H
stretch. The aldehyde C-H in gas-phase acrolein is at 2800
cm-1, so a 15 cm-1 red-shift of this clearly identifiable mode
occurs upon adsorption. Table 1 gives a direct comparison
between the observed SFG-VS vibrational modes and those
previously published for acrolein in the literature.
near this value is the η2-trans species. The rest of the resonances
are seen at 2850, 2930, 3020, and 3060 cm-1. There is a possible
shoulder at ∼2865 cm-1 as well. The η1 adsorption species,
interacting with the Pt surface solely through the oxygen atom,
may be ruled out due to the absence of any modes above 3100
cm-1 in the SFG-VS spectrum; likewise, the η4-cis can be
eliminated as an option. The mode observed at 2850 cm-1
corresponds best with the η2-cis C-H aldehyde vibrational
mode. The two lower frequency aldehyde peaks at 2850 and
2785 cm-1 are quite indicative of the η2 di-σ(CC) adsorption
mode, as none of the more highly coordinated surface species
have C-H resonances that far to the red.
Figure 2 displays the SFG spectra as 100 Torr of hydrogen
is added and the temperature is raised to 415 K. Upon the
addition of hydrogen at room temperature, the spectrum
undergoes two changes. The small aldehyde peak at 2785 cm-1,
corresponding to the η2-trans surface species, disappears as well
as the peak at 3020 cm-1. Due to the fact that those two modes
disappear together, as well as the fact that DFT predicts the
η2-trans adsorption mode to have absorptions in both those
regions, the 3020 cm-1 peak is also assigned to a C-H vibration
off of the di-σ bond of the η2-trans species (Figures 1 and 2),
which agrees with the assignment made in the comparison to
experimental values in Table 1. Upon heating the Pt(111) crystal
to 370 K, a strong new peak appears at 2867 cm-1. The other
resonances become very weak and the only one with appreciable
signal remains at 2930 cm-1. Upon further heating the Pt(111)
crystal to 415 K, the only remaining peak is the 2867 cm-1
resonance corresponding to ethylidyne on the surface resulting
from the decarbonylation of acrolein. The ethylidyne is red-
shifted by about 10 cm-1 from its usual location on a clean
Pt(111) crystal, but the coadsorption of CO and ethylidyne on
Pt(111) has been previously reported to cause a similar red-
shift in the ethylidyne peak.20 The observation of ethylidyne
on Pt(111) after acrolein adsorption was also noted by de Jesus
and Zaera21 using RAIRS.
To gain deeper insight into the true bonding characteristics
of the acrolein molecule to the Pt(111) surface, the vibrational
spectra are compared to those predicted with DFT. Although
there are often several structures that correspond to a given
vibrational frequency, one can only accept structures if the other
peaks in the predicted spectrum also agree with the observed
vibrational modes. In the DFT work of Loffreda et al.,17 the
only chemisorption mode of acrolein with a vibrational mode
(18) Hamada, Y.; Nishimura, Y.; Tsuboi, M. Chem. Phys. 1985, 100, 365–
375.
(17) Loffreda, D.; Jugnet, Y.; Delbecq, F.; Bertolini, J. C.; Sautet, P. J.
Phys. Chem. B 2004, 108, 9085–9093.
(19) Murillo, L. E.; Chen, J. G. Surf. Sci. 2008, 602, 919–931.
9
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Hydrogenation of R,ꢀ-Unsaturated Aldehydes over Pt
A R T I C L E S
Figure 2. SFG-VS spectra of 1 Torr of acrolein over the Pt(111) crystal surface. The bottom spectra was taken in the absence of hydrogen, and then the
subsequent spectra include the addition of 100 Torr of hydrogen and heating the crystal to 415 K. The acrolein peaks vanish and only ethylidyne remains
as the crystal is heated.
According to the Gibbs free adsorption energy curves reported
by Loffreda et al.22 for acrolein adsorption to Pt(111), the
experimental conditions used here (1 Torr of acrolein, 295-415
K) likely crosses the 0 Gibbs free adsorption energy line as the
temperature is raised, thus favoring desorption of the acrolein
molecule. This is in agreement with the SFG-VS spectra, as by
415 K the only species remaining on the surface is ethylidyne.
Lastly, the peak observed at 2930 cm-1 is indicative of a more
highly coordinated η3-trans or η4-trans surface species. In
relation to the DFT calculations, it is most closely matched with
the η3-trans species. Clearly, then, for acrolein adsorption and
hydrogenation over Pt(111) there exists a mixed state of
adsorption mode surface species.
will build up a high concentration of allyl alcohol, but the gas
phase will show a high selectivity for the formation of
propionaldehyde. In this work, the frequency range from 2500
to 4000 cm-1 was studied and there was no evidence found for
the unsaturated alcohol on the surface, as was proposed by
Loffreda et al.,23 although a species with the OH bond perfectly
parallel to the surface cannot be ruled out due to the metal
surface selection rule.24,25
3.1.2. Crotonaldehyde Hydrogenation on Pt(111)sSFG-VS
Results. The hydrogenation of crotonaldehyde (Scheme 2) has
selectivity for the desired unsaturated alcohol between those of
acrolein and prenal hydrogenation (section 3.2). The addition
of the methyl group improves the selectivity for the unsaturated
alcohol, but it also adds complexity to the vibrational spectrum,
especially in the C-H region. Figure 3 shows the spectrum for
a multilayer of crotonaldehyde on Pt(111) at 150 K in the
absence of hydrogen as a reference spectrum, followed by the
temperature-dependent spectra of 1 Torr of crotonaldehyde and
100 Torr of hydrogen reacting over a Pt(111) single crystal from
295 to 415 K. In the multilayer (bottom) at 150 K, four
vibrational resonances are seen. At 2750 cm-1 a strong peak is
seen for the trans aldehyde C-H vibration. The analysis of the
other three peaks at 2830, 2935, and 2995 cm-1 can be made
In a later paper, Loffreda et al.23 study the hydrogenation
reaction pathway with DFT over the Pt(111) surface. Their
conclusion was that the reaction actually favors the attack of
the CdO carbonyl, but the desorption step of the resulting allyl
alcohol has a very high barrier height compared to the reaction
pathway involving the attack of the CdC and desorption of the
propionaldehyde product. Thus, they conclude that the surface
(20) Chen, P.; Westerberg, S.; Kung, K. Y.; Zhu, J.; Grunes, J.; Somorjai,
G. A. Applied Catalysis A: General 2002, 229, 147–154.
(21) de Jesus, J. C.; Zaera, F. Surf. Sci. 1999, 430, 99–115.
(22) Loffreda, D.; Delbecq, F.; Sautet, P. Chem. Phys. Lett. 2005, 405,
434–439.
(24) Dignam, M. J.; Moskovit, M; Stobie, R. W. Trans. Faraday Soc. 1971,
67, 3306.
(23) Loffreda, D.; Delbecq, F.; Vigne, F.; Sautet, P. Angew. Chem., Int.
Ed. 2005, 44, 5279–5282.
(25) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976, 59, 205–217.
9
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A R T I C L E S
Kliewer et al.
Figure 3. SFG-VS spectra of crotonaldehyde hydrogenation over Pt(111). The bottom spectrum was taken at 150 K after exposing the crystal to 15 L of
crotonaldehyde to form a multilayer. The top three spectra were taken with 1 Torr of crotonaldehyde and 100 Torr of hydrogen over the Pt(111) crystal at
the temperatures listed.
Table 2. Summary of Experimental Crotonaldehyde Vibrational
going peak to a negative-going peak due to a change in its
Modes (given in cm-1) for Gas-Phase Crotonaldehyde34 and 15 L
relative phase, as seen in the fit to eq 2. Further, the absence of
of Crotonaldehyde Adsorbed to Pt(111) at 150 K Using SFG-VS
any peak at 2750 cm-1 or below suggests that the η2-di-σ(CC)-
assignment
IR gas phase
SFG-VS at 150 K
E-(s)-trans species is not present in significant surface concen-
tration at 295 K in 100 Torr of H2. Although the trans adsorbed
species disappears as the temperature is raised and hydrogen is
added, this does not necessarily mean it is no longer participating
in the reaction mechanism. The trans species could react more
quickly than the cis surface species, in which case no appreciable
surface concentration of the trans species would build up.
At 295 K, once again in comparison to the DFT-calculated
frequencies,26,27 the frequencies agree quite well with the η2-
di-σ(CC)-E-(s)-cis-crotonaldehyde species (Figure 3). As the
Pt(111) crystal is heated to 415 K, the only notable change is
ν
ν
ν
ν
ν
asym(dCH)
sym(dCH)
asym(CH3)
sym(CH3)
3058
2995
2963
2938
2995
2995
2995
2935
aldehyde(CH)
2805/2728
2830/2750
by comparison to gas-phase IR (Table 2) and DFT calculations26
for interpretation. In this context, the peak at 2830 cm-1
corresponds to the aldehyde C-H stretch of E-(s)-cis-crotonal-
dehyde, the 2935 cm-1 peak corresponds to the symmetric
stretching in the methyl side group, and the 2995 cm-1 peak
corresponds to both the asymmetric methyl stretch and the
vinylic C-H stretches of the CdC bond.
The spectrum recorded at 295 K in the presence of hydrogen
in Figure 3 shows five vibrational resonances much different
from the multilayer condensed onto Pt(111). First, the trans-
aldehyde C-H stretch around 2750 cm-1 disappears by 295 K.
Four more resonances are seen at 2865, 2915, 2965, and 2995
cm-1. The 2995 cm-1 resonance has flipped from a positive-
the disappearance of the negative-going peak at 2995 cm-1
.
3.1.3. Prenal Hydrogenation on Pt(111)sSFG-VS Results.
Figure 4 displays the SFG-VS spectra of 1 Torr of prenal both
with and without 100 Torr of hydrogen over Pt(111). A striking
difference between these spectra and those discussed for acrolein
and crotonaldehyde is the broad -OH hydrogen-bonded peak
centered around 3380 cm-1 present even before hydrogen is
added to the system (made clearer in Figure 5). The hydrogena-
tion of prenal has been shown to be more selective for the
(27) Haubrich, J.; Loffreda, D.; Delbecq, F.; Jugnet, Y.; Sautet, P.; Krupski,
(26) Haubrich, J. Thesis, Universitat Bonn, 2006.
A.; Becker, C.; Wandelt, K. Chem. Phys. Lett. 2006, 433, 188–192.
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Hydrogenation of R,ꢀ-Unsaturated Aldehydes over Pt
A R T I C L E S
unsaturated aldehyde reactant. As reported by Haubrich et al.,29
another possible reaction product in the hydrogenation of prenal
is water in a dehydration to form the 3-methylbutane product.
The -OH stretch of water cannot be eliminated as a possible
source for this observed broad -OH stretch.
Once again, comparing the observed spectra with the DFT-
calculatedspectraforthevariousadsorptionmodesisinformative.26,29,30
For the spectra (Figure 4, bottom and Figure 5, top) taken prior
to the addition of hydrogen, a peak is visible at 2720 cm-1
,
which should clearly correspond to an intact aldehyde C-H.
Other than the -OH peak discussed, vibrational peaks are
observed at 2845, 2915, and 3015 cm-1. Upon the addition of
100 Torr of hydrogen, the 2720 cm-1 peak vanishes and the
3015 cm-1 peak becomes much weaker. The only changes in
the spectra as the crystal is heated to 415 K is that the -OH
band becomes more intense and red-shifted and a new peak
grows in at 2945 cm-1. The peak growing in at 2945 cm-1
agrees well with the growing presence of either the η3-di-σ(CC)-
σ(O)-(s)-cis, corresponding to the asymmetric stretching in one
of the methyl groups or to the aldehyde C-H stretch of the
η4-di-σ(CC)-di-σ(CO)-(s)-trans species on the surface at higher
temperatures. The high temperature spectrum is reversible. After
heating to 415 K and then returning the crystal to 295 K, the
peak at 2945 cm-1 disappears again. This is proof that it is not
a cracking product or poison on the surface, but a reversible
change in surface intermediate.
3.2. Hydrogenation of Crotonaldehyde over Pt(111) and
Pt(100)sKinetic and SFG-VS Results. The hydrogenation of
crotonaldehyde was carried out with 1 Torr of crotonaldehyde
and 100 Torr of hydrogen over both the Pt(111) and Pt(100)
crystal faces to elucidate the effect that catalyst structure plays
in this reaction. The industrially desired product crotyl alcohol,
the unsaturated alcohol, results from the hydrogenation of the
CdO (Scheme 2), while the thermodynamically favored product,
butyraldehyde, results from the hydrogenation of the CdC.
Further, the temperature range from 295 to 415 K was used,
and apparent activation energies were calculated for the reaction
products over both surfaces.
Figure 4. SFG-VS spectra of 1 Torr of prenal over Pt(111) from 295 to
415 K. The bottom one is before the addition of H2, while the rest are with
100 Torr of H2. The -OH peak indicates alcohol on the surface at all
temperatures. The “soft” modes at 2720 and 2845 cm-1 are indicative of
η2 adsorption species. The 2945 cm-1 peak is indicative of more highly
coordinated η4 or possibly η3 adsorbed prenal species.
Figure 6 shows the Arrhenius plots and reaction selectivities
over both metal surfaces, and Table 3 displays the apparent
activation energies for all reaction pathways. The activation
energy for the formation of crotyl alcohol is significantly less
than that for the formation of butyraldehyde. This is evident
also in the reaction selectivities of Figure 5, which show a
significantly enhanced selectivity for the formation of the
unsaturated alcohol at low temperatures (35 °C). At such low
catalytic temperatures, however, the turnover frequencies (TOF,
product molecules/Pt site/s) are very low for all products, on
the order of 10-3, while as the temperature is raised to 130 °C
the TOF for the formation of butyraldehyde approaches ∼100
over both surfaces. It is interesting to note that the apparent
activation energy for the production of crotyl alcohol is lower
than that for the production of butyraldehyde, yet at most
temperatures butyraldehyde is the dominant product. The reason
for this is 2-fold. First, the reported “apparent” activation energy
for crotyl alcohol formation would be underestimated if, for
instance, the cracking product resulted only from molecules that
first formed crotyl alcohol. In fact, it has been shown that the
sticking coefficient for an unsaturated alcohol is significantly
Figure 5. SFG-VS spectra of the -OH region of 1 Torr of prenal on
Pt(111) (top) and 1 Torr of crotonaldehyde on Pt(111) (bottom) in the
absence of excess hydrogen.
formation of the desired unsaturated alcohol both in previous
work6 and in this work (section 3.3). The buildup of -OH on
the surface would indeed indicate that the desorption step of
the alcohol is a rate-limiting step, as was proposed for acrolein
hydrogenation by Loffreda et al.23,28 Upon heating, the -OH
vibration is red-shifted, indicating a more electron-donating
interaction with the surface. This -OH stretch may indicate
either the presence of the 3-methyl-2-buten-1-ol product bound
to the surface or a hydroxyallyl type reaction intermediate, as
proposed by Loffreda et al.,28 to be in equilibrium with the
(29) Haubrich, J.; Loffreda, D.; Delbecq, F.; Sautet, P.; Jugnet, Y.; Krupski,
A.; Becker, C.; Wandelt, K. J. Phys. Chem. C 2008, 112, 3701–3718.
(30) Hirschl, R.; Delbecq, F.; Sautet, P.; Hafner, J. J. Catal. 2003, 217,
354–366.
(28) Loffreda, D.; Delbecq, F.; Vigne, F.; Sautet, P. J. Am. Chem. Soc.
2006, 128, 1316–1323.
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A R T I C L E S
Kliewer et al.
Figure 6. Arrhenius plots (top) and reaction selectivities (bottom) taken once a steady state was reached for the hydrogenation of crotonaldehyde over
Pt(111) (right) and Pt(100) (left). The major reaction products are butyraldehyde (0), crotyl alcohol (O), propylene ([), and butanol (2).
Table 3. Activation Energies in kJ/mol for the Various Products of
Crotonaldehyde Hydrogenation Carried out over Pt(111) and
distribution the crotyl alcohol selectivity has dropped to ∼10%.
Pt(100) with 1 Torr of Crotonaldehyde and 100 Torr of Hydrogen
45%, but by the time the reaction reaches a steady-state product
This change in selectivity may be due to either the selective
Pt(111)
Pt(100)
blocking of active sites by CO during the decarbonylation of
crotonaldehyde or by carbonaceous deposits, both of which have
previously been reported to deactivate this reaction over model
Pt nanoparticle catalysts.32 This gives a further reason why the
reaction appears to shift to favor crotyl alcohol production at
low temperatures. At lower temperatures the deactivation
mechanism which causes the shift in selectivity with time may
be less active. STM experiments are currently underway to help
elucidate this effect.
Figure 8 compares a spectrum taken over Pt(111) to one taken
over Pt(100) under 1 Torr of crotonaldehyde and 100 Torr of
hydrogen at 295 K. The vibrational features between the two
crystal faces were identical here and for all temperatures studied.
This argues that the dominant surface reaction intermediates
are the same for both crystal faces for the hydrogenation of
crotonaldehyde. This is in contrast to theoretical work done by
Delbecq et al.,33 which demonstrated significantly different
adsorption modes for crotonaldehyde between the Pt(111) and
Pt(100) surfaces. However, it is consistent with the reaction
selectivities and kinetics above which demonstrated very similar
activities and selectivities for the two surfaces, the primary
difference simply being in the barrier heights for the various
reaction pathways evidenced by the differing activation energies
calculated allowing for some kinetic control of the product
distribution.
butyraldehyde
crotyl alcohol
propylene
75
35
68
59
90
38
63
79
butanol
higher than its corresponding saturated aldehyde,11 which may
lead to longer residence times and further reactions, such as
decarbonylation. Second, the pre-exponential factors for the two
reaction pathways could conceivably be different. Given that
there may exist more than one adsorption mode of the
crotonaldehyde molecule on the Pt(111) surface, it is reasonable
that one adsorption mode may favor the formation of crotyl
alcohol while another favors the formation of butyraldehyde,
changing their associated reaction entropies.
Pt(111) shows a higher selectivity for the formation of the
unsaturated alcohol, especially at low temperatures, while
Pt(100) shows a higher selectivity for the formation of the
decarbonylation “cracking” product propylene (Figure 6). The
selectivity for the totally saturated product butanol remained
low, less than 1%, on both surfaces at the temperatures studied.
Similar to the results reported by Lawrence and Schreifels,31
the selectivity for the formation of the desired unsaturated
alcohol, crotyl alcohol, varied with time. Figure 7 shows the
time dependence for the hydrogenation of 1 Torr of crotonal-
dehyde with 100 Torr of hydrogen for 2 h at 90 °C. Initially,
the selectivity for the formation of crotyl alcohol is as high as
(32) Grass, M.; Rioux, R.; Somorjai, G. Catal. Lett. 2009, 128, 1–8.
(33) Delbecq, F.; Sautet, P. J. Catal. 1995, 152, 217–236.
(34) Oelichmann, H. J.; Bougeard, D.; Schrader, B. J. Mol. Struct. 1981,
77, 179–194.
(31) Lawrence, S. S.; Schreifels, J. A. J. Catal. 1989, 119, 272–275.
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Hydrogenation of R,ꢀ-Unsaturated Aldehydes over Pt
A R T I C L E S
Figure 7. Product selectivities in the hydrogenation of crotonaldehyde (1 Torr of crotonaldehyde/100 Torr of hydrogen) at 90 °C. The major reaction
products are butyraldehyde (0), crotyl alcohol (O), propylene ([), and butanol (2). A dramatic change in reaction selectivity is seen during the first 20 min
and a steady-state product distribution is reached at around 2 h.
reaction selectivity at 35 °C, the reaction was allowed to proceed
for 14 h. The only major detectable product was the desired
unsaturated alcohol; however, this effect may be somewhat
exaggerated as the reaction may not have yet reached its final
steady-state product distribution.
4. Conclusions
Using a combination of SFG vibrational spectroscopy and
kinetic measurements, along with detailed comparison to
published IR, HREELS, and DFT calculations, the surface
species during the catalytic hydrogenation of acrolein, crotonal-
dehyde, and prenal have been clarified. The effect that catalyst
structure has on the hydrogenation of crotonaldehyde has been
elucidated by comparing both the reaction kinetics and SFG-
VS spectra over Pt(111) and Pt(100) single crystals.
Figure 8. SFG-VS spectra of 1 Torr of crotonaldehyde and 100 Torr of
hydrogen at 295 K over Pt(100) (bottom) and Pt(111) (top). SFG-VS spectra
over Pt(111) and Pt(100) showed identical features at all temperatures from
295 to 415 K.
In the case of acrolein, a mixed adsorption state existed
including η2-cis and η2-trans species, as well as more highly
coordinated η3 or η4 species. However, upon the addition of
hydrogen, the η2-trans surface species vanished. Further, as the
Pt(111) crystal was heated to 415 K, all of the surface
intermediate peaks vanished, except that of ethylidyne, forming
from the decarbonylation of acrolein.
The reaction kinetics for crotonaldehyde hydrogenation
showed a lower activation energy for the formation of the
industrially desired crotyl alcohol than for the formation of the
thermodynamically favored butyraldehyde. Thus, at lower
temperatures the selectivity for the desired product was enhanced.
3.3. Hydrogenation of Prenal over Pt(111)sKinetic
Results. For comparison to the results obtained for the hydro-
genation of crotonaldehyde, the hydrogenation of prenal was
carried out over Pt(111) over the temperature range from 35 to
130 °C under 1 Torr of prenal and 100 Torr of hydrogen. The
addition of the extra methyl group (Scheme 3) to the CdC bond
adds steric hindrance to the adsorption of this moiety, and thus
may increase the selectivity to the unsaturated alcohol product.
Figure 9 displays the time dependence of the reaction selectivity
for prenal hydrogenation over Pt(111) at 70 °C for 2.5 h. The
initial reaction selectivity was 100% for the production of the
desired unsaturated alcohol, but the reaction selectivity reached
a steady state at about 1.5 h and the production of unsaturated
alcohol had fallen to 33% by this time.
As can be seen in Figure 10, the selectivity for the formation
of unsaturated alcohol at 100 °C was 37%. This is more than 5
times higher than the selectivity for the unsaturated alcohol
during crotonaldehyde hydrogenation at this temperature. As
the temperature is raised to 130 °C, however, the onset of
cracking product formation appears to come at the cost of
unsaturated alcohol production. The selectivity for 3-methyl-
2-buten-1-ol drops to ∼10% at this temperature as the selectivity
for the cracking products concomitantly rises. To obtain the
For the adsorption of crotonaldehyde to Pt(111) at 150 K,
both cis and trans species were found with SFG-VS. However,
in the presence of hydrogen at 295 K and hotter, the only surface
species observed was η2-cis. This may be an indication that the
η2-trans species is not stable at the higher temperature or is
simply the fastest one to react and desorb, not allowing a
significant surface concentration to build up. The SFG-VS data,
then, agree nicely with the reaction kinetics, given that the η2
adsorption modes do not have the carbonyl interacting with the
surface and the selectivity for the hydrogenation of the CdO
was low in the case of crotonaldehyde hydrogenation.
Pt(111) and Pt(100) showed little structure sensitivity for this
reaction. The Pt(111) had a somewhat higher selectivity for the
formation of crotyl alcohol, while Pt(100) had a higher
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A R T I C L E S
Kliewer et al.
Figure 9. Product selectivities as a function of time for prenal hydrogenation over Pt(111) using 1 Torr of prenal and 100 Torr of hydrogen. Initial selectivity
was 100% for the unsaturated alcohol, but as the reaction progressed to a steady state after about 2 h, the selectivity for the unsaturated alcohol fell to 33%.
The major reaction products are 3-methylbutyraldehyde (0), 3-methyl-2-buten-1-ol (O), C4 cracking products ([), and 3-methylbutanol (2).
Figure 10. Product selectivities during the hydrogenation of prenal over Pt(111) as a function of temperature. All selectivities were taken once a steady
state had been reached, typically about 2 h into the reaction. At 35 °C, the data points were taken at 14 h. The major reaction products are 3-methylbutyraldehyde
(0), 3-methyl-2-buten-1-ol (O), C4 cracking products ([), and 3-methylbutanol (2).
selectivity for the decarbonylation “cracking” reaction. Further,
the SFG-VS spectra of crotonaldehyde hydrogenation over both
surfaces were identical.
in reaction selectivities. The CdO of prenal interacts with the
surface at higher temperatures, unlike the case for crotonalde-
hyde, and the CdO is hydrogenated significantly more during
prenal hydrogenation than crotonaldehyde hydrogenation.
For the hydrogenation of prenal over Pt(111), the SFG-VS
results indicated the presence of an -OH group, even without
the presence of excess hydrogen. This is consistent with the
flipped selectivity for the hydrogenation of prenal, in which the
dominant product is the unsaturated alcohol. SFG-VS spectra
demonstrated the presence of the η2 surface species definitively.
Further, upon heating the Pt(111) crystal to 415 K, a highly
coordinated η3 or η4 species grows in on the surface. This clear
difference from crotonaldehyde also agrees with the difference
Acknowledgment. This work was supported by the Director,
Office of Energy Research, Office of Basic Energy Sciences, and
Materials Sciences Division of the U.S. Department of Energy under
Contract DE-AC02-05CH11231. M.B. thanks the Swiss National
Science Foundation (SNF) for financial support.
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