C. Li et al.
MolecularCatalysis455(2018)78–87
and carbon support.
The textural properties of supported Pt samples were disclosed by
low-temperature N2 adsorption-desorption experiments. As shown in
Fig. 5, all samples present typical type IV adsorption−desorption iso-
therms with a H1-type hysteresis loop, indicative of the character of
large pores formed between the interconnected large particles. Mean-
while, samples show the broad pore size distributions in the ranges of
2–100 nm (inset in Fig. 5). As listed in Table 1, the specific surface area
of supported samples gradually decreases with the increasing of Co
content. However, three cobalt oxide-decorated Pt samples still possess
quite high surface areas (> 317 m2/g). In this work, the small size of Pt
NPs in samples should be attributable to uniform arrangement of Pt
species in the brucite-like layers of LDH precursors, as well as the high
specific surface areas of hybrid supports composed of plate-like MMOs
and amorphous carbon.
To investigate surface electronic structure of Pt species on samples,
XPS characterization was conducted (Fig. 6). Because of quite low Co
content, however, Co 2p signals do not appear in all Co-containing
samples. Considering that the Al 2p region lies at the binding energy of
around 84.3 eV, fine Pt 4f spectra are fitted with five contributions.
Besides the Al 2p region, two large components at about 71.2 eV and
Fig. 7. The normalized intensity of Pt–L3 EXAFS for Co-Pt/C-0.6, Pt/C, Pt foil,
and PtO2 samples.
74.4 eV are assigned to Pt 4f
and Pt 4f
core levels, which is
7/2
5/2
characteristic of metallic Pt0 species [21,39]. Meanwhile, another two
small components appearing at about 72.2 eV and 75.4 eV are assigned
to cationic Pt2+ species. As shown in Table 1, the relatively high in-
tensity ratios of Pt0 species to total Pt species determined by integrated
areas demonstrate the reduction of most cationic Pt4+ species. Spe-
cially, compared with Co-free one, CoOx-decorated samples provide
increased proportions of surface Pt0 species, probably due to the elec-
tron transfer from Co to Pt according to lower electronegativity of Co
atom (1.9) than that of Pt atom (2.2). Such charge transfer is beneficial
for the generation of electron-rich Pt0 and CoOx species, thereby
probably influencing catalytic behavior to some extent [40,41].
To further determine the interactions between Pt NPs and CoOx
species, EXAFS characterization was conducted (Fig. 7), where the
photoenergy indicates the electronic structure of the Pt center. It can be
clearly recognized that the Pt–L3 edge energies of two Pt/C and Co-Pt/
C-0.6 are higher than that for Pt foil, indicative of the character of
cationic Pt species. Meanwhile, compared with Pt/C, Co-Pt/C-0.6
slightly shifts to the lower energy, illustrating that Pt sites in the Co-Pt/
C-0.6 tend to be more negatively charged, because the strong interac-
tions between Pt NPs and CoOx species facilitate the electron transfer
from Co to Pt.
In addition, the O 1s core level of four supported Pt samples was
analyzed. As shown in Fig. 8. In the case of Pt/C sample, there is only a
large peak (OI) at about 530.2 eV, which is assignable to lattice oxygen
species on the surface in MMOs. It is interestingly noted that besides the
large OI peak, a small peak (OII) appears at about 532.2 eV. Such a peak
with the higher binding energy is associated with defect-oxide or hy-
droxyl species [42,43]. Notably, the proportion of OII species in the
total oxygen species, which may determine the relative amount of
surface defects in the lattice (e.g. oxygen vacancies, Ov), increases
gradually from zero for Pt/C to 9.0% for Co-Pt/C-0.3, 12.8% for Co-Pt/
C-0.6 and 18.1% for Co-Pt/C-0.9, indicative of the formation of more
surface defects due to the increased amount of surface CoOx species.
According to the above characterization results, we speculate that
the structure of CoOx-decorated Pt samples is probably as follows: Pt
species are mainly in the metallic state, while Co species can form CoOx
clusters that cover the partial surface of Pt NPs. Correspondingly, the
electron transfer may occur between Pt and CoOx species, generating
electron-rich Pt0 species in CoOx-decorated Pt samples.
Fig. 8. XPS of the O 1s core level for Pt/C (a), Co-Pt/C-0.3 (b), Co-Pt/C-0.6 (c)
and Co-Pt/C-0.9 (d) samples.
of Pt0 phase, in good accordance with the above XRD results. Further-
more, the existence of relatively thin Pt NPs is an indicator of a strong
surface interaction between metallic Pt phase and carbon support [38].
In view of TEM images for all supported Pt samples, it is concluded that
uniform and small-sized Pt NPs can be governed effectively by the in-
situ LDH-C assembly and resulting structural transformation.
Further, the distributions of Pt and Co elements were characterized
unambiguously by STEM measurements. Fig. 4 presents the STEM
image and resulting Pt and Co elemental mappings for Co-Pt/C-0.6
sample. Obviously, no aggregates of NPs can be observed on the sur-
face, while high-intensity of Pt and Co with relatively low intensity are
uniformly distributed over the surface of an individual NP, indicating
the successful modification of Pt NPs by Co species, although it is not
easy to distinguish between Pt and Co-containing species in the STEM
image. Therefore, the aforementioned STEM result distinctly illustrates
the intimate contact of Pt NPs with Co-containing species on a single
NP, suggestive of the strong interactions between them. Meanwhile, the
enhanced Pt dispersion degree from 9.6% for Pt/C to 16.5% for Co-Pt/
C-0.9 (Table 1) further demonstrates the successful decoration of Pt NPs
by Co-containing species. In this case, amorphous carbon support
combined with MMOs phase as dispersant agents can enhance the
dispersion of formed Pt NPs and inhibit the aggregation and growth of
Pt NPs and Co-containing species, thereby being beneficial to the for-
mation of strong interactions between Pt NPs or Co-containing species
3.2. Catalytic hydrogenation performance of catalysts
During the hydrogenation of CAL molecule concomitantly con-
taining olefin and conjugated carbonyl groups, there exists several-step
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