Selective ring opening of methylcyclopentane over surface-decorated Ir-Co bimetallic catalysts synthesized by galvanic replacement reaction

Article abstract of DOI:10.1039/c6ra22153j

Surface-decorated Ir-Co bimetallic catalysts were synthesized by a modified galvanic replacement reaction and evaluated by the selective ring opening (SRO) of methylcyclopentane (MCP). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was utilized to characterize the catalyst synthesis process, and H₂-temperature-programmed reduction (H₂-TPR) was performed to characterize the reduction behavior of the calcined catalysts and oxidation resistance of the reduced catalysts. Transmission electron microscopy (TEM) analysis was used to characterize metallic particle size distribution. The reactor evaluation results show quite different behaviors in conversion and product selectivity of the surface-decorated Ir-Co/SiO₂ and monometallic Ir/SiO₂. At optimized Ir content, MCP conversion and mass-specific rate of SRO are significantly higher when catalyzed by surface-decorated Ir-Co/SiO₂ than by monometallic Ir/SiO₂. Moreover, compared with Ir/SiO₂, Ir-Co/SiO₂ has decreased 2-methylpentane selectivity but improved n-hexane selectivity. These catalytic behaviors were found to correlate with the surface-decorated bimetallic structure consisting of Co/SiO₂ decorated with surface Ir atoms.

Full text of DOI:10.1039/c6ra22153j

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Selective ring opening of methylcyclopentane over
surface-decorated Ir–Co bimetallic catalysts
synthesized by galvanic replacement reaction
Cite this: RSC Adv., 2016, 6, 105063
Zhen Wang,† Aiguo Zheng,† Renyang Zheng,* Mingfeng Li, Huifeng Li, Guangtong Xu
and Guofu Xia*
Surface-decorated Ir–Co bimetallic catalysts were synthesized by a modified galvanic replacement reaction
and evaluated by the selective ring opening (SRO) of methylcyclopentane (MCP). Inductively coupled
plasma atomic emission spectroscopy (ICP-AES) was utilized to characterize the catalyst synthesis
2 2
process, and H -temperature-programmed reduction (H -TPR) was performed to characterize the
reduction behavior of the calcined catalysts and oxidation resistance of the reduced catalysts.
Transmission electron microscopy (TEM) analysis was used to characterize metallic particle size
distribution. The reactor evaluation results show quite different behaviors in conversion and product
selectivity of the surface-decorated Ir–Co/SiO
conversion and mass-specific rate of SRO are significantly higher when catalyzed by surface-decorated
Ir–Co/SiO than by monometallic Ir/SiO . Moreover, compared with Ir/SiO , Ir–Co/SiO has decreased
-methylpentane selectivity but improved n-hexane selectivity. These catalytic behaviors were found to
2 2
and monometallic Ir/SiO . At optimized Ir content, MCP
Received 4th September 2016
Accepted 22nd October 2016
2
2
2
2
2
DOI: 10.1039/c6ra22153j
2
correlate with the surface-decorated bimetallic structure consisting of Co/SiO decorated with surface Ir
www.rsc.org/advances
atoms.
15–19
reaction of naphthenes through the dicarbene mechanism.
However, Ir is one of the rarest and most valuable elements on
1
. Introduction
The selective ring-opening (SRO) process of naphthenic earth. Therefore, many attempts have been made to improve the
compounds, during which only an endocyclic C–C bond is catalytic performance of Ir and reduce the catalyst cost.
20
cleaved, leaving the number of carbon atoms in the reaction Gonz ´a lez-Cort ´e s et al. reported a modied Ir-containing cata-
products unchanged, has great importance for improving the lyst with the addition of potassium to improve the ring-opening
1
–4
21
cetane number of diesel fuel. Numerous studies have shown selectivity for 1,3-dimethylcyclohexane. Shen et al. synthesized
good catalytic performance for SRO by using the noble metal bimetallic Pd–Ru catalyst for SRO of indane, which displayed
5
–9
catalysts (e.g. Ir, Pt, Ru, Rh). There are three main mecha- the same high single-cleavage selectivity as Ir and was envi-
10,11
nisms
for the ring-opening process of naphthenes catalyzed sioned as a viable alternative to monometallic Ir.
Apart from the above methods, another desirable bimetallic
by noble metals: the dicarbene mechanism, multiplet mecha-
nism and metallocyclobutane mechanism. When SRO reaction catalyst structure, with active Ir atoms dispersed on the nano-
occurs according to the dicarbene mechanism, naphthenic particle surfaces of the inexpensive metal (e.g. Co, Ni), will allow
compounds adsorb to the catalyst active sites perpendicularly more Ir atoms to be exposed to the reactants and promote the
22
via two carbon atoms of the naphthenic ring, which preferen- SRO reaction. Ziaei-Azad et al. synthesized bimetallic core–
tially induces the breakage of C–C bond at the unsubstituted shell structured nanoparticles with Ni atoms in the particle core
12,13
position of the ring.
However, if the C–C bond is cleaved at and Ir atoms on the surface by hydrogen-sacricial technique,
the substituted position of the ring by the multiplet mechanism and then loaded the nanoparticles onto the support for the SRO
or the metallocyclobutane mechanism, the cetane number can reaction of indane, signicantly improving the catalytic activity
1
0,14
be improved.
Among the various noble metals, Ir is proven to be the most of MCP on supported metal catalysts has been extensively
compared to the monometallic Ir. As a model reaction, the SRO
23–25
active and selective ring-opening catalyst, promoting the SRO studied.
Since ring opening of MCP on Ni catalyst is usually
accompanied by extensive secondary cracking of the primary C
6
-
1
alkane hydrogenolysis products to C –C paraffins, in the
1
5
Research Institute of Petroleum Processing (RIPP), SINOPEC, 18 Xueyuan Road,
Haidian District, Beijing 100083, China. E-mail: zhengrenyang.ripp@sinopec.com;
xiaguofu.ripp@sinopec.com
current work, we selected Co nanoparticles to support the
admetal Ir in order to avoid the excessive hydrogenolysis. Thus,
the above leading works motivate us to explore surface-
†
These authors contributed equally to this work.
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ꢀ
ꢀ
ꢁ1
decorated Ir–Co bimetallic catalysts synthesized via a modied room temperature to 700 C at the rate of 10 C min . A
galvanic replacement,
26,27
an improved method based on the thermal conductivity detector (TCD) was used to detect the
traditional impregnation method. Several characterization amount of hydrogen consumption. Furthermore, to compare
methods, including inductively coupled plasma atomic emis- the surface compositions of the catalyst, oxidation resistance
sion spectroscopy (ICP-AES),
reduction (H -TPR) and transmission electron microscopy catalysts were reduced, cooled down to room temperature,
TEM), were employed to verify the structures and compositions exposed to the air, and then used for the TPR experiment.
2
H -temperature-programmed experiments via a modied TPR process were performed. The
2
(
of the catalysts and understand their catalytic behavior in the
SRO reaction of MCP.
2.2.3. Transmission electron microscopy (TEM). TEM
analysis was conducted on pre-reduced catalysts using a JEOL
ARM 200F equipped with a cold-eld emission gun operated at
2
0
00 keV and a Probe Cs corrector providing a point resolution of
2. Experimental methods
˚
.8 A at scanning transmission electron microscopy (STEM)
2
.1. Catalyst preparation
mode. STEM imaging of bimetallic catalysts were measured on
a JEOL ARM 200F equipped with a STEM-EDX (energy-
dispersive X-ray spectroscopy) analysis system at a voltage of
The surface-decorated Ir–Co bimetallic catalysts were synthe-
sized by a modied galvanic replacement in this study. The
precursor Co(NO ) $6H O was dissolved in deionized water
200 kV. Spot size 4 with the spatial resolution of a few
3
2
2
angstroms was used to obtain compositional line prole across
the bimetallic catalysts. TEM samples were prepared by
grinding and suspending reduced catalysts in ethanol, and then
a few droplets of this solution were placed onto a carbon-coated
copper grid. The samples were pretreated by ion cleaning and
heating before being placed into the ARM 200F to obtain the
atomic resolution images in STEM mode.
completely and then impregnated onto the SiO support. Aer
2
ꢀ
the impregnation process, the product was dried at 120 C for
ꢀ
4
h and then calcined in air at 300 C for 3 h to produce the
calcined Co/SiO
cooled down to room temperature, and subsequently put into
the aqueous solution of IrCl $xH O for continuous shaking. It
2 2 2
. The calcined Co/SiO was reduced by H ,
3
2
should be pointed out that the reduced catalyst was put directly
into IrCl $xH O solution, and the treatment of the reduced
3
2
catalyst by ushing N2 to remove the adsorbed H2 has no
obvious inuence on the catalytic performance. Then, the
2.3. Catalytic evaluation
supernatant was removed, and the resultant catalysts were Studies of SRO reaction for MCP were performed in a contin-
ꢀ
washed and dried to obtain the surface-decorated bimetallic uous ow, xed-bed microreactor under 280 C and 2.0 MPa.
catalysts x%Ir–10%Co/SiO
weight ratio of Ir to the catalyst (x ¼ 0.1, 0.3 and 1.0). The Co was reduced by H
content of the bimetallic samples is 10 wt%, with the amount of for 1 h in the microreactor. Through the reduction process, the
metallic Co replaced by Ir being neglected.
catalyst was completely reduced. During the reaction, the
2
, where x stands for the nominal Before the reaction, 0.15 g of catalyst mixed with quartz powder
ꢀ ꢀ
2
at 350 C (450 C for monometallic Co/SiO )
2
ꢁ
1
For comparison, a traditional co-impregnation (CI) method feedstock of MCP, with a liquid ow rate of 0.1 mL min , was
was used to synthesize 1%Ir–10%Co/SiO -CI catalyst. The carried to the reactor by a constant ow pump. The gas ow of
2
ꢁ
1
precursors of IrCl
deionized water completely and impregnated onto the SiO
support. Aer the impregnation process, the catalyst was dried (FID).
3
$xH
2
O and Co(NO
3
)
2
$6H
2
O were dissolved in
H
2
was 400 mL min . The products were analyzed by an online
Agilent 7890 GC equipped with a ame ionization detector
2
ꢀ
ꢀ
at 120 C for 4 h and then calcined in air at 300 C for 3 h to
produce calcined 1%Ir–10%Co/SiO -CI. The corresponding
2
monometallic Ir/SiO and Co/SiO catalysts were also synthe- 3. Results and discussion
2
2
sized by impregnation method to serve as control samples.
3.1. Characterization of synthesis process by ICP-AES
During the synthesis of surface-decorated Ir–Co/SiO
2
, the
different reduction potentials (1.16 V for the Ir /Ir pair and
2
.2. Catalyst characterization
3+
2
+
2
.2.1. Inductively coupled plasma atomic emission spec- ꢁ0.28 V for the Co /Co pair) produce the driving force for the
3
+
28,29
troscopy (ICP-AES). The concentration of Ir during the galvanic replacement reaction.
When reduced Co/SiO was
2
3
+
galvanic replacement procedure was determined by ICP-AES. put into the Ir solution, it was assumed that a galvanic
The experiments were carried out using a PerkinElmer replacement reaction occurred between Ir and Co, leading to
Optima 7300V spectrometer with the radio-frequency power of the preferential deposition of Ir on the surfaces of Co nano-
3
+
ꢁ1
1
300 W and plasma gas ow of 15 L min
.2.2. -temperature-programmed reduction (H
TPR experiments were performed using a Micromeritics replacement during the synthesis procedure was measured
.
particles instead of SiO
-TPR). assumption, the concentration of Ir before and aer galvanic
2
support (Scheme 1). To verify the above
3
+
2
H
2
2
3
+
ꢁ1
AutoChem II 2920 to determine the reduction behavior of the (Table 1). Only trace amount of Ir (<15 mg L ) remains in the
calcined catalysts. For each experiment, 0.10 g of calcined supernatants aer the galvanic replacement reaction, and the
3
+
catalyst with 100–200 mesh was put into the U-shaped tubular calculated loading ratio of Ir on the x%Ir–10%Co/SiO catalyst
2
3
+
quartz reactor. Then, the catalyst was exposed to the mixed gas is more than 90%, implying that most of the Ir in the aqueous
consisting of 10.0 vol% H in Ar with a temperature ramp from precursor solution is loaded onto the catalyst.
2
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Scheme 1 Illustration of the modified galvanic replacement from Co/
SiO to Ir–Co/SiO .
2 2
Furthermore, a comparative experiment was performed to
distinguish the distribution and state of the Ir species loaded
3
+
onto the x%Ir–10%Co/SiO catalyst, whether in the form of Ir
2
via adsorption or metallic Ir via galvanic replacement reaction.
The calcined 10% Co/SiO , without the reduction process, was
2
3 2
directly put into the aqueous precursor solution of IrCl $xH O
with continuous shaking, then the supernatants were collected
for ICP-AES analysis. The data in the second and last rows of
3
+
Table 1 show little difference in Ir concentration between the
supernatants and aqueous precursor solution, indicating that
3
+
only trace amount of Ir is loaded onto the catalyst via
adsorption. Therefore, it can be deduced that by the galvanic
3
+
replacement reaction, most of the Ir in the aqueous precursor
solution is preferentially deposited onto the surfaces of Co
3
+
nanoparticles in the form of metallic Ir instead of Ir adsorbed
by SiO support.
2
3
.2. H -TPR results
2
The reduction behavior of the calcined catalysts by TPR is
shown in Fig. 1(a). The Co/SiO shows two main reduction
2
ꢀ
ꢀ
peaks at about 285 C and 320 C, which attribute to the
Fig. 1 TPR profiles of (a) the calcined catalysts and (b) the “reduced-
successive reduction of Co O to CoO and CoO to metallic Co, oxidized” catalysts. The signal of 1%Ir/SiO in (a) has been magnified 5
3
4
2
3
0,31
times. The “reduced-oxidized” catalysts were analyzed by a modified
respectively.
1
As shown in Fig. 1(a), Ir can be reduced at about
-CI, hydrogen
ꢀ
TPR process: the catalysts were reduced, cooled down to room
temperature, exposed to the air, and then used for the TPR
experiment.
63 C, lower than that of Co. Thus for Ir–Co/SiO
2
that dissociates on the Ir surface could migrate to the surface of
Co and support, resulting in Co being reduced at much
lower temperatures. These results also suggest that the catalysts
3 4
O
ꢀ
ꢀ
were completely reduced by H at 350 C (450 C for mono- oxidized)” shows an almost-at prole, indicating that reduced Ir
2
metallic Co/SiO2).
is nearly not oxidized in the air atmosphere at room temperature.
To further verify the surface compositions of the catalysts, For “10%Co/SiO
oxidation resistance experiments for the reduced catalysts were around 180 C can be attributed to the reduction of CoO, the
2
(reduced-oxidized),” the intense reduction peak
ꢀ
performed via a modied TPR process, the results of which are oxidation state of Co at room temperature. Specially, for the three
shown in Fig. 1(b). The H
2
-TPR analysis of “Ir/SiO
2
(reduced- curves of “x%Ir–10%Co/SiO
2
(reduced-oxidized)” (x ¼ 0.1, 0.3 and
3+
Table 1 Ir concentration during the catalyst synthesis procedure
a
a
a
Items
0.1%Ir
0.3%Ir
1%Ir
3
+
ꢁ1
Ir concentration in aqueous precursor solution (before galvanic replacement)/mg L
55.7
<1
99.9%
32.0
167
562
14.2
97.5%
569
3+
b
ꢁ1
Ir concentration in the supernatants (aer galvanic replacement) /mg L
14.5
91.3%
154
3+
b
Loading ratio of Ir on the catalyst
3+
c
ꢁ1
Ir concentration in the supernatants (comparative samples) /mg L
a
b
For x%Ir (x ¼ 0.1, 0.3 and 1), x stands for the nominal weight ratio of Ir to Ir–Co/SiO
2
.
Catalyst synthesis via the galvanic replacement reaction.
RSC Adv., 2016, 6, 105063–105069 | 105065
c
2
Comparative synthesis without the Co/SiO reduction procedure.
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1.0), the intensity of reduction peaks for “CoO/Co” decreases (Table 2). No detectable conversion of MCP is observed over
compared to that for “10%Co/SiO2 (reduced-oxidized).” These monometallic Co/SiO . For the monometallic Ir/SiO catalysts,
2
2
indicate that for the surface-decorated Ir–Co/SiO2 catalysts, Ir the increased Ir content from 0.1 wt% to 1 wt% gives a signi-
atoms decorated on the surfaces of Co nanoparticles via galvanic cant improvement of MCP conversion. On the other hand, for
replacement suppress the oxidation of Co at room temperature.
2
the surface-decorated x%Ir–10%Co/SiO , with the increase of Ir
content, the MCP conversion displays a volcano trend and
reaches its maximum value at 0.3 wt%. It can be inferred that
during the synthesis of x%Ir–10%Co/SiO , the increased Ir
2
3.3. TEM results
3
+
High-angle annular dark eld (HAADF) TEM images and particle
size distributions are shown in Fig. 2. Particle size distributions
of the supported bimetallic nanoparticles, assuming a spherical
shape, were obtained from the measurement of about 300
particles found in several arbitrarily chosen areas of enlarged
micrographs. The dominant particle size in each catalyst appears
to range in diameter from approximately 2 nm to 5 nm. The
average diameter is 3.4, 3.2 and 2.9 nm for 0.3%Ir–10%Co/SiO2,
concentration in aqueous precursor solution accelerates the
reaction rate of galvanic replacement, leading to the non-
uniform deposition of Ir atoms on the Co nanoparticle
surfaces. Thus, the non-uniform deposition of Ir atoms
restrains the catalytic behavior of 1%Ir–10%Co/SiO
Fig. 4 displays the mass-specic rate for SRO of MCP as
a function of Ir content over monometallic Ir/SiO and surface-
decorated bimetallic Ir–Co/SiO . Increasing Ir content improves
the mass-specic rate for monometallic Ir/SiO but decreases it
2
. Moreover,
2
2
1
%Ir–10%Co/SiO
Furthermore, STEM-EDX was used to measure the distribu-
tions of Ir and Co in the 1%Ir–10%Co/SiO catalyst, as shown in
2 2
and 1%Ir–10%Co/SiO -CI, respectively.
2
for Ir–Co/SiO bimetallic catalysts. These suggest that SRO is
2
2
sensitive to Ir content and the surface structure of bimetallic
catalysts.
Fig. 3. The images obtained by three scanning modes, including
a selected point, selected line scan and selected area scan
For 0.1 wt% and 0.3 wt% Ir content, MCP conversion (Table
(Fig. 3(a–c)), show that all the bright nanoparticles are
2
) and mass-specic rate of SRO (Fig. 4) are much higher when
catalyzed by surface-decorated Ir–Co/SiO than by mono-
metallic Ir/SiO . It could be explained that for Ir–Co/SiO , the
composed of Co atoms, while some brighter nanoparticles are
composed of both Co and Ir atoms. Combining the STEM and
HRTEM images (Fig. 3(d)), the distribution of Ir atoms is clearly
identied as single atoms or clusters (<1 nm) situated at
surfaces of Co nanoparticles.
2
2
2
catalytically active Ir atoms are highly dispersed on the Co
nanoparticle surfaces via the galvanic replacement reaction,
thus signicantly improving the dispersion and utilization of Ir
compared to Ir/SiO . A similar trend has also been observed on
2
3
.4. Catalytic evaluation
the SRO of indane over Ir–Ni bimetallic catalysts reported by
22
Ziaei-Azad et al. With the increase of Ir content to 1 wt%, the
%Ir–10%Co/SiO catalyst does not have an advantage
Flow reactor studies of SRO reaction for MCP were employed to
compare the catalytic performance of different catalysts
1
2
2 2 2
Fig. 2 TEM images and particle size distributions for (a) 0.3%Ir–10%Co/SiO , (b) 1%Ir–10%Co/SiO and (c) 1%Ir–10%Co/SiO -CI.
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Fig. 3 Representative STEM images in (a) a selected point, (b) selected line scan mode, (c) selected area scan mode (mapping), and (d) HRTEM
images of 1%Ir–10%Co/SiO catalyst. The green signals stand for Co and blue signals for Ir in (b) and (c).
2
compared to 1%Ir/SiO . At the same time, compared with impregnation synthesis procedure and not exposed on the
2
another two catalysts with 1 wt% Ir (1%Ir–10%Co/SiO and 1% catalyst surfaces.
2
Ir/SiO
2
), 1%Ir–10%Co/SiO
2
-CI shows the least catalytic activity
The SRO reaction of MCP produces n-hexane (n-H), 2-meth-
for SRO of MCP. The content of Co in 1%Ir–10%Co/SiO
2
-CI is ylpentane (2MP) and 3-methylpentane (3MP) (Scheme 2).
much higher than that of Ir, with the Co/Ir atomic ratio of 33 : 1. Previous research studies have demonstrated that SRO reaction
Therefore, one possible reason is that most Ir atoms in 1%Ir– occurs according to a multiplet mechanism, a dicarbene
2
10%Co/SiO -CI are covered by Co atoms through the co-
Table 2 Catalytic performance of Ir–Co bimetallic and monometallic
a
catalysts
b
RO selectivity (%)
Conversion
Catalysts
(%)
SUM 2MP 3MP n-H 2MP/3MP
0
0
0
0
1
1
1
1
.1%Ir/SiO
.1%Ir–10%Co/SiO
.3%Ir/SiO
.3%Ir–10%Co/SiO
%Ir/SiO
2
2.8
18.3
10.5
21.9
75.0
12.7
99.8 70.5 28.9 0.4 2.4
77.2 46.0 28.1 3.1 1.6
98.6 70.3 28.1 0.2 2.5
76.9 46.8 27.1 3.0 1.7
99.1 71.7 27.1 0.3 2.6
79.8 47.8 28.8 3.2 1.7
100 69.0 29.5 1.5 2.3
2
2
2
2
%Ir–10%Co/SiO
%Ir–10%Co/SiO
2
2
-CI 0.4
0
0%Co/SiO
2
—
—
—
—
—
a
Reaction conditions: catalyst 0.15 g, pressure 2.0 MPa, temperature
80 C. 2MP: 2-methylpentane; 3MP: 3-methylpentane; n-H: n-
ꢀ
b
2
hexane; SUM: the sum of 2MP, 3MP and n-H.
Fig. 4 Mass-specific rate as a function of Ir content for monometallic
Ir/SiO and surface decorated bimetallic Ir–Co/SiO
2
2
.
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3
p levels makes it very difficult to determine the exact binding
43
energy shi. More detailed density functional theory (DFT)
calculations would be needed to further understand the elec-
tronic properties.
4. Conclusions
Scheme 2 Ring-opening products of MCP.
We explored the utilization of surface-decorated Ir–Co bime-
tallic catalysts synthesized via galvanic replacement for the
mechanism and a metallocyclobutane mechanism over different selective ring-opening of MCP. Results from ICP-AES show that
1,10,11,32
3+
noble metals.
The multiplet mechanism is usually by the galvanic replacement reaction, most of the Ir in the
observed over Pt catalysts, with an equal probability of breaking precursor solution is preferentially deposited onto the surfaces
3
+
endocyclic bonds, producing the statistical ratio of of Co nanoparticles in the form of metallic Ir instead of Ir
10,33
2MP : 3MP : n-H ¼ 2 : 1 : 2.
The dicarbene mechanism oen adsorbed by SiO support. The modied TPR results show that
2
happens over Ir catalysts, which preferentially induces endocyclic Ir atoms decorated on the surfaces of Co nanoparticles via
C–C bond breakage at the unsubstituted position of the ring, galvanic replacement suppress the oxidation of Co at room
leading to the statistical ratio of 2MP : 3MP ¼ 2 : 1 and trace temperature. TEM measurements provide additional informa-
12,13
amounts of n-H.
In addition to these two typical mechanisms tion on the metallic particle size distribution, and the combi-
34,35
for SRO reaction, Gault et al.
proposed an alternative one that nation of STEM-EDX and HRTEM images identies the single
would operate through a metallocyclobutane intermediate con- atoms or clusters of Ir situated at the surface of Co nano-
sisting of a metal atom and three C atoms, which would allow the particles. At optimized Ir content, MCP conversion and mass-
breakage of substituted C–C bonds if an external methyl group specic rate of SRO are signicantly higher when catalyzed by
were involved in the intermediate.
surface-decorated Ir–Co/SiO2 than by monometallic Ir/SiO2.
The ring-opening (RO) selectivity, product distribution and Moreover, compared with Ir/SiO2, Ir–Co/SiO2 decreases
molar ratio of 2MP/3MP are listed in Table 2. The results of Ir/ 2-methylpentane selectivity but improves the n-hexane selec-
SiO
2
in the present work are consistent with previous research tivity, which is attributed to the surface-decorated bimetallic
work showing that the hydrogenolysis of MCP over Ir-based structure consisting of Co/SiO decorated with surface Ir atoms.
2
1,17
catalysts is selective to the formation of 2MP and 3MP.
Results from the current study also identify research opportu-
More interestingly, the surface-decorated Ir–Co/SiO and nities in synthesizing supported bimetallic catalysts by surface
2
monometallic Ir/SiO show quite different behaviors in terms of decoration via galvanic replacement.
2
product selectivity. The RO selectivity over Ir–Co/SiO is about
2
7
6–80%, lower than the 99% over Ir/SiO
on Ir–Co/SiO mainly include methane, butane and 2-methyl-
butane, as analysed by gas chromatography-mass spectrom- This work was supported by the Major State Basic Research
etry (GC-MS). Meanwhile, compared with Ir/SiO , Ir–Co/SiO
Development Program (Grant No. 2012CB224802) and SINOPEC
2
. The cracking products
Acknowledgements
2
2
2
shows a lower selectivity toward 2MP (ꢂ47% vs. 71%), corre- Research Project (Grant No. 115059).
sponding to lower 2MP/3MP ratio (ꢂ1.6 vs. 2.5) but higher
selectivity toward n-H (ꢂ3.1% vs. 0.3%). Therefore, it can be
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2
a different mechanism compared to monometallic Ir/SiO
2
.
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As reported by Chen and other research groups,
bime-
tallic catalysts oen show electronic and chemical properties
that differ distinctly from those of the parent metals. For
example, it has been demonstrated that Pt-terminated Pt–Co–
Pt(111) surface, which represents a subsurface bimetallic
structure with Pt on the top-most surface layer and Co residing
in the subsurface region, shows much higher activity for the
hydrogenation of cyclohexene than Co-terminated Co–Pt–
4
1
Pt(111) and the corresponding monometallic surfaces. The
catalytic hydrogenation pathway on the subsurface bimetallic
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tronic properties of Pt by the subsurface Co atoms. It is
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light on the electronic effects of surface-decorated structure.
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