Chemoselective Hydrogenation Catalysts
A R T I C L E S
conditions and has been assigned to Pt-dicarbonyl species.47 The
presence of the last band in the Pt/CeO2-SiO2 sample reduced
at 773 K, which is not present in the sample reduced at 473 K
is observed in the XPS spectra of the other elements. In this
sense, the presence of two components at BE 72.2 and 71.0 eV
(fwhm 2.4 and 2.5 eV, respectively) can be deduced. Finally
the Pt 4f7/2 core-level spectrum of the reduced sample at 773 K
shows the presence of only one component at BE 71.2 eV (fwhm
4
4
and in previous FTIR studies of adsorbed CO over Pt/CeO2,
can be related to the high amount of low-coordinated, highly
unsaturated Pt sites. This assignation will be in agreement with
of 2.3 eV). While binding energies about 70.5-70.7 eV are
-
1
52-54
the fact that the band at 1853 cm , assigned to bridge CO
characteristic of metallic platinum,
the shift of the Pt 4f
species, was not observed in our case.48
binding energy to higher values (71.2 eV) has been attributed
to final state effects due to reduced screening of the photo hole
Finally, the FTIR spectra of adsorbed CO over a Pt/CeO2
catalyst submitted to a reduction treatment at 773 K has been
studied (spectra not shown). Prior to CO adsorption, IR bands
5
5
after electron emission in small particles. In fact, we have
observed the presence of small particles by TEM and IR spectra
-
1
2+
0
2+
at 3747 cm , referred to OH hydroxyl groups, and IR bands
of adsorbed CO. The presence of both Pt and Pt species (Pt /
-
1
0
at 1526, 1470, 1397, and 1368 cm due to carbonate com-
Pt atomic ratio ) 0.7) in the 473 K reduced sample does not
-
1
pounds are observed. No bands in the 2300-1700 cm region
are present. After CO adsorption, intense IR bands attributed
to linear adsorbed CO and Pt-dicarbonyl species are observed
agree with the TPR results, which shows a high reducibility of
the Pt but can be explained by diffusional problems in the
reduction process in the XPS microreactor.
(
spectra not shown), followed with the appearance of new bands
The XPS spectra of the Ce 3d core level have been fitted
into several components in accordance with Burroughs and co-
workers. The Ce /Ce atomic ratio has been obtained from
the area of the peaks obtained by the deconvolution procedure.
-
1
in the 1600-1000 cm region, due to carbonate formation.
Carbonate formation due to decomposition of adsorbed CO to
CO2 and carbon has been reported in the literature at high
adsorption temperatures.49 On the other hand, CO2 formation
from adsorbed CO with the participation of lattice oxygen from
the support in the interface between Pt and CeO2 can also be
considered. In this case, a reaction mechanism in which CO
dosed from the gas phase adsorbs on the Pt terrace, equilibrates
among more stable corner or step sites, and migrates to the
interface where it picks up a lattice oxygen and forms CO2,
has been proposed.50
5
6
4+
3+
3+
4+
3+
In this way a Ce /(Ce + Ce ) ratio of 0.1 has been obtained
3+
on the calcined sample, while the amount of Ce species
increases appreciably after high reduction temperature [Ce /
Ce + Ce ) atomic ratio of 0.54].
3+
4+
3+
(
The O1s XPS spectrum shows the presence of two oxygen
components at BE 532.9 and 530.0 eV, corresponding to SiO2
and CeO2, respectively. These indicate that oxygen arising from
carbonate impurities also appears around 532 eV, which could
lead to some errors if quantitative measurements are required.
However, it can be said on a qualitative bases that the oxygen
component at BE 530.3 eV decreases by increasing reduction
temperature, something that could be assigned to oxygen loss
by reduction of the CeO2 support.
Worth mentioning is the absence of the IR band at 2026 cm-1
(observed on Pt/CeO2-SiO2 samples), which has been previously
attributed to linear Pt-CO species partially distorted by defect
sites at the metal-support interface. Probably, the blockage of
defect sites at the Pt-CeO2 interface by CO2 formation can be
responsible for the observed behavior.
Thus, from the XPS spectra of the Pt/CeO2-SiO2 sample the
According to our results, the presence or absence of the low-
0
presence of small Pt particles and a high reduction degree
-
1
frequency IR band at 2026 cm is related to the presence or
not of special Pt sites at the contact perimeter of the metal
particle with the support. These sites show a promoting effect
in the activation of the CO bond and therefore can act as possible
active sites in the chemoselective hydrogenation of crotonal-
dehyde.
4+
3+
(
Ce f Ce ) of the CeO2 support is inferred. On the other
hand, a substantial decrease in the peak area of the Ce 3d/Si 2s
core levels in the Pt/CeO2-SiO2 sample compared to the pure
CeO2-SiO2 support has been seen. This decrease in the peak
areas can be attributed to a preferential localization of the Pt
particles on the ceria nanoparticles, in agreement with the results
obtained by HREM.
XPS Characterization. XPS spectra of the Pt 4f, O 1s, Si
s, and Ce 3d core levels of calcined and in situ reduced (at
73 and 773 K) samples have been collected and shown in
2
4
Nanostructured Pt/CeO2 catalyst has also been studied by
XPS, and large differences with respect to the Pt/CeO2-SiO2
sample have been observed. The Pt 4f XPS spectrum shows
Figure 8 for the Pt/CeO2-SiO2 (1 wt % Pt) sample. It can be
seen there that the Pt 4f core-level XPS spectrum is shifted to
lower binding energies by reduction of the sample and a
broadening of the spectrum is clearly observed after low-
temperature reduction (473 K). Deconvolution of the Pt 4f7/2
peak has revealed only one component in the calcined sample
at binding energy 72.8 eV (fwhm 2.3 eV), which has been
the presence of Pt species on the calcined sample, while Pt0
is observed on the sample reduced at 773 K. However, a lower
BE of the Pt 4f7/2 core level is observed (70.9 eV), which is
2+
0
characteristic of metallic Pt particles. In this sense, higher
particle sizes can be inferred in the Pt/CeO2 sample in contrast
to the small particle size observed in the Pt/CeO2-SiO2 sample.
On the other hand, the Ce 3d XPS spectra shows a lower
reduction degree of the Ce ions after in situ reduction of the
2
+
51
attributed to Pt species in PtO. In the reduced sample at
73 K the broadening of the Pt 4f core-level spectrum is due to
the presence of more than one component, since no broadening
4
3
+
4+
3+
Pt/CeO2 sample in H2 at 773 K [Ce /(Ce + Ce ) ) 0.27].
(
47) Jin, T.; Zhou, Y.; Mains, G. J.; White, J. M. J. Phys. Chem. 1987, 91,
931.
48) Dulaurent, O.; Chandes, K.; Bouly, C.; Bianchi, D. J. Catal. 1999, 188,
5
(52) Huidobro, A.; Sepulveda-Escribano, A.; Rodriguez-Reinoso, F. J. Catal.
2002, 212, 94.
(53) Duekers, K.; Bonzel, H. P. Surf. Sci. 1989, 213, 25.
(54) Mullins, D. R.; Zhang, K. Z. Surf. Sci. 2002, 513, 163.
(55) Henry, C. R. Surf. Sci. Rep. 1998, 31, 231.
(56) Burroughs, P.; Hammett, A.; Orchard, A. F.; Thomton, G. J. J. Chem. Soc.,
Dalton Trans. 1976, 17, 1686.
(
2
37.
(
(
(
49) Bourane, A.; Dulaurent, O.; Bianchi, D. J. Catal. 2003, 195, 6.
50) Jin, T. J. Phys. Chem. 1987, 91, 5931.
51) Drawdy, J. E.; Hoflund, G. B.; Gardner, S. D.; Yngvadottir, E.; Schryer,
D. R. Surf. Interface Anal. 1990, 16, 369.
J. AM. CHEM. SOC.
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