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that the C=O vibration in molecularly adsorbed acrolein, in
acrolein via the C=O bond and the addition of one H atom at
the C next to O. Only one additional step—the insertion of the
second H atom into the Pd···O bond—is required to form pro-
penol. Importantly, it was observed that the propenoxy reac-
tion intermediate appears only after a densely packed overlay-
er of oxopropyl species was formed. This densely packed oxo-
propyl-overlayer creates a geometrical constraint for acrolein
adsorption through the C=C bond and forces the adsorption
via the C=O bond thus inducing the desired selectivity.
which the C=O bond is conjugated to the C=C double bond,
À1 [13b,c]
lies around 1660 cm .
The significantly higher frequency
À1
band at 1755 cm suggests that the corresponding surface
species contain C=O bonds, which are not conjugated to the
[
14]
C=C bonds. The appearance of this vibration points to the
formation of an oxopropyl surface species, resulting from the
partial hydrogenation of acrolein molecules with only one H
atom attached to the C=C bond.
À1
Figure 3 show a possible structure
The third prominent band appears at 1330 cm during the
of the oxopropyl species. Please
period of highest reactivity and steadily grows in intensity, re-
maining intense even after the complete stop of the reaction.
This band was previously related to formation of ethylidyne
note that there are two feasible
structures of oxopropyl species:
[15]
bonded to the surface via the C2-
and ethylidyne-like species. This species can be considered
as the second type of spectator or a surface poison.
carbon atom as shown in Figure 3
Figure 3. Possible structure of
the oxopropyl spectator spe-
and bonded through the terminal
In the following sections, we show that formation of this ox-
opropyl species is necessary, but not sufficient, for propenol
formation over Fe O -supported Pd nanoparticles.
cies that is formed from par-
tial hydrogenation of the C=C
bond in acrolein and is re-
sponsible for activating the
Pd(111) surface for C=O bond
hydrogenation in acrolein.
C3-carbon atom. Based on the
available IRAS data, it is impossible
to differentiate between these
3
4
structures. Remarkably, the band at
Partial hydrogenation of acrolein over Pd/Fe O at 270 K
À1
3
4
1755 cm
already appears at
a very early stage of the reaction,
grows in intensity, and remains in-
Although the surfaces of the Fe O -supported Pd nanoparticles
3 4
[9]
are composed of ~80% (111) facets, Figure 1 demonstrates
that their activity and selectivity in the partial hydrogenation
of acrolein is vastly different than that of Pd(111). Pd nanoparti-
cles exhibit rather low activity and are 100% selective towards
hydrogenation of the C=C bond, whereas Pd(111) is nearly
100% selective towards hydrogenation of the C=O bond. To
understand why the Fe O -supported Pd nanoparticles behave
tense, even after the reaction rate recorded in the gas phase
vanishes. This observation strongly suggests that this species is
not the reaction intermediate leading to the final gas-phase
product propenol, but is merely a spectator.
The second prominent band is the very intense vibration at
À1
1
120 cm . Note that this frequency is present neither in ad-
3
4
[
13a]
sorbed intact acrolein on Pd nor in acrolein ice
and there-
so much differently than Pd(111), the mechanisms of acrolein
fore cannot be related to any prominent vibration of the mo-
lecularly adsorbed acrolein. Further, this band appears only
partial hydrogenation over Pd/Fe O4 were investigated by
3
monitoring the formation of surface species and gas-phase
products simultaneously. We begin by analyzing the mecha-
nism of acrolein hydrogenation over 12 nm Pd particles at the
sample temperature (270 K), which is optimal for propenol pro-
duction over Pd(111).
under reaction conditions: in the presence of H in the temper-
2
ature range 220–270 K. The most striking observation of this
study is that the evolution of this vibrational band shows
strong correlation with the evolution of propenol in the gas
phase. Indeed, this band starts to appear in region 1, which
comprises the induction period and the region of growing re-
action rate (Figure 2a); it then grows in intensity in regions of
the highest reactivity (region 2). Consecutively, the intensity of
this band strongly decreases in region 3 accompanied by the
decrease of the propenol formation rate in the gas phase and
completely disappears when reactivity strongly decreases. A
Figure 4a shows the formation rates of propanal and prope-
nol over 12 nm Pd particles at 270 K during continuous expo-
sure of acrolein and H . As expected from the results of pulsed
2
reactivity experiments (Figure 1), there was no significant pro-
duction of propenol over the entire course of the reaction. In
contrast to the induction period observed in the propenol pro-
duction over Pd(111), the rate of propanal production increases
from the beginning of the reaction to a maximum at ~30 sec-
onds and then decreases to zero after ~90 seconds.
few other IR bands in the region of CH stretching and bend-
x
ing vibrations may also be correlated to the gas-phase forma-
[
5a]
tion rate of propenol.
The IRAS spectra labeled ‘1’ through ‘6’ in Figure 4b were
collected during the corresponding regions labeled in Fig-
ure 4a. All of the bands observed during acrolein hydrogena-
tion over Pd(111) at 270 K (Figure 2b) are absent from the
spectra collected during acrolein hydrogenation over 12 nm
Pd particles under the same conditions (Figure 4b). Instead,
By recording IR spectra with higher time resolution, we were
able to show that there is a strong correlation between the
gas-phase formation rate of propenol and the evolution of the
À1
vibrational band at 1120 cm . This observation implies that
the corresponding surface species is the surface intermediate,
directly involved in the selective hydrogenation of the C=O
bond. Taking into account all vibrational bands associated with
this reaction intermediate, it is possible to identify it as a prope-
À1
there are bands in the 1800–1960 cm region, which become
more intense with increasing reaction time. These bands are
characteristic of CO molecules adsorbed on 12 nm Pd parti-
[9]
noxy-group CH =CHÀCHÀO···Pd, in which the CÀO entity is at-
cles, which are likely formed as a product of acrolein decar-
bonylation. To demonstrate that the IR spectra displayed in
Figure 4b are the result of CO adsorbed on Pd nanoparticles,
2
tached to Pd through the O atom to form a single CÀO···Pd
bond. This intermediate can be formed though adsorption of
&
&
Chem. Eur. J. 2016, 22, 1 – 9
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!