C. Liu et al. / Catalysis Communications 67 (2015) 72–77
73
SiO2-N. The resultant mixture was immediately homogenized via ultra-
sonic treatment for 1 h, and the solid was separated by centrifugation
before being dried under vacuum at 333 K for 12 h. Pt/SiO2-N was pre-
pared by the same method [9]. The loading was fixed at 0.2 wt.%.
synthesized Au NP-coated SiO2-N composite particles have a relatively
stable structure (Fig. 1A–F).
3.2. Hydrogenation tests
2.4. Characterization and catalytic hydrogenation reactions
The thermal conductivity coefficients of BD, butene and argon at
100 °C (the TCD temperature) are 0.023, 0.021 and 0.022 W / (m·°C),
respectively, which are almost one order of magnitude lower than
that of hydrogen (0.224 W / (m·°C)). In addition, part of 1-butene grad-
ually transforms mainly into trans-2-butene at any temperature [3].
Consequently, the activity not only is expressed in terms of H2 con-
sumed, but also depends on butene distribution. However, that phe-
nomenon can be neglected because the bands of 2-butenes were not
observed by in situ FT-IR (see Section 3.3). As a result, the consumption
of H2 basically contributes to the TCD signal, and the catalytic activity of
various Au NPs was quantitatively determined by the consumed H2, as
shown in Fig. 2. Between 223 and 233 K, there is a significant consump-
tion peak, especially for the pure support (SiO2-N) in Fig. 2G. The
reasonable explanation may be the inside thermocouple or other
experimental artifacts. So this peak was viewed as the experimental
background.
The BD conversion and butene distribution were reported in Table 1.
The smaller the particle size, the higher the conversion of BD. However,
as for Au NPs with mean diameter of 2.1 nm, the conversion is not high.
The evolution of the selectivities to butenes at low temperature depends
on the particle size. The smaller the particle size, the lower the selectiv-
ity to 1-butene except for the catalyst A. However, 1-butene remains the
main product and the distribution, 1-butene N cis-2-butene N trans-2-
butene, did not change over the Au catalysts. The full hydrogenation
product (n-butane) was not produced.
The textural properties were measured by isothermal adsorption–
desorption of N2 at 77 K with a Micromeritics 3020 apparatus. Chemical
analyses were performed with an X-ray fluorescence (XRF) Spectrome-
ter (AXIOS). TEM analysis was performed using a JEOL JEM-2100EX
microscope. The NPs sizes were determined at least 330 particles. In a
quartz reactor, 0.5 g (0.36 mL) of catalyst was mixed with quartz
wool. In a typical run, the catalyst was pretreated in 50 mL/min of Ar
at 333 K for 1 h. After being cooled to 193 K, the reaction gas mixture
(BD:H2:Ar = 2:8:90, volume ratio) was introduced in 50 mL/min. A
mixture of isopropyl alcohol and liquid nitrogen (LN2) was used to
cool the reactor. The TCD signal was recorded with temperature increas-
ing to 243 K at ca. 0.15 K/min. The effluent was analyzed online using a
MS (OmniStar GSD320) and GC (Bruker 450) equipped with a FID de-
tector. Quantitative H2 consumption calculation was performed on the
same chemisorption analyzer (AutoChem 2950 HP). The value of
0.00685 was calculated by integrating the TCD signal, which corre-
sponds to 2.12 × 10−7 mol of H2.
Turnover frequencies (TOFs, s−1) expressed per surface gold atom
were calculated, thanks to the following equation:
ðS−SbÞ ꢀ 2:12 ꢀ 10−7=0:00685
TOF ¼
nAu ꢀ D ꢀ t
In previous works, the TOF has only a slight increase with decreasing
the particle size at 393 K [2] and gold particle size has not a drastic influ-
ence on the catalytic properties [3]. However, the TOF of Au NPs greatly
increases when the mean diameters are smaller than 5.6 nm in this
work. Therefore BD hydrogenation is size-sensitive at very low temper-
ature. These higher TOFs are close to those of 0.01–0.02 s−1 at 423 K
[11]. However, the Pt catalyst is inactive, contradictory with the com-
mon observations. There might be a strong interaction between hydro-
carbons and the surface Au NPs. Thus supported Au NPs in the 2–3 nm
range and Pt/SiO2-N were characterized by in situ FT-IR spectroscopy.
where S is the integrating area of TCD signal, Sb is the background area,
nAu is the amount of Au atoms (mol), D is the dispersion from TEM re-
sults [10] and t is the reaction time.
2.5. Measurement of IR spectra
FT-IR spectroscopy was performed with a Bruker VERTEX 70 spec-
trometer, with a resolution of 2 cm−1 and 128 scans for each spectrum.
The low temperature reaction cell was evacuated by the turbo pumping
station (HiCube 80 Eco) until the pressure did not decrease
(~10−4 Torr). Then, LN2 was filled into the Dewar. When the temper-
ature decreased to ca. 193 K, the substrate IR spectra was recorded.
Subsequently, BD in Ar was introduced for adsorption experiment.
After 30 min, the cell was outgassed to detect the strong bonded species
until all the bands were not changed. The sample was exposed to H2 to
record IR spectra of products with a LN2 cooled MCT detector. All the
spectra were given by the reflectance as a function of wavenumbers in
the 4000–600 cm−1 region.
3.3. IR measurements
The spectra for BD (see Supporting Information), butenes and bu-
tane are issued from the literature [12]. In Fig. 3a, the following changes
are observed. (a) In the high frequency region, the bands at 3107, 3067
and 3024 cm−1, which correspond to ν(CH2) asym, ν(CH) and ν(CH)
C2v, respectively, become weaker and weaker in intensity (Fig. 3b).
(b) In the 2000–1250 cm−1 region, the bands at 1972, 1493 and
1288 cm−1 also become close to baseline in intensity while other
bands that centered at 1828, 1597 and 1377 cm−1 develop to single
bands. For C_C groups with only σ-type metal substitution, v(C_C)
is observed at lowered frequencies: 1650–1550 cm−1; with π-
bonding only, 1600–1460 cm−1 [13]. So the band at 1597 cm−1 could
be attributed to either π or σ-bonded species. In Fig. 3c, the band at
1473 cm−1 did not disappear with time on stream, which is also prob-
ably due to di-π-adsorbed BD (Fig. 3d) [13]. Under vacuum, only strong
adsorbate can be detected. However, the single π species are less stable
than the di-π species [13]. Thus, two bands at 1597 and 1598 cm−1
should be attributed to σ-bonded species. The corresponding surface
coverages are 0.45 and 0.42, respectively (see Supporting Information).
According to the surface selection rule [14], we interpreted the absence
of the IR bands assigned to the in-plane stretching, τ(CH), ρ(CH2) and
ω(CH2) vibrations as due to a flat orientation of the s-trans adsorbate
on the substrate surface.
3. Results and discussion
3.1. Supported Au catalysts
The textural properties and chemical composition were shown in
Table S1 (see Supporting Information). Although the specific area is
low, the loading of NPs is still satisfying. Fig. 1(a2–f2) shows the
selected-area electron diffraction pattern of particles in the solution.
The diffraction pattern of several NPs reveals a ring pattern that can
be indexed as derived from the (111), (200), (220), (311) and (222) lat-
tice planes of gold. In Fig. 1(d1–f1), the typical fringes were observed
due to Au (111), (200) and (220) facets with 2.35, 2.03 and 1.44 Å
interplanar distances, respectively. In contrast, the crystalline lattice
fringes of Au NPs are not clearly visualized in Fig. 1(a1–c1). After ultra-
sonic treatment, the SiO2-N microspheres were decorated with Au NPs,
except for some small aggregates. The high dispersion indicates that the
Liu, Chenguang
