The effects of PVP-modified SiO2 on the catalytic performance of CO hydrogenation over Rh-Mn-Li/SiO2 catalysts

Article abstract of DOI:10.1039/c7ra07011j

Rh-Mn-Li catalysts supported on SiO₂ prepared by PVP-modified St?ber method were used for the synthesis of C₂⁺ oxygenates from CO hydrogenation. The catalysts were characterized by TG, XRD, N₂-adsorption-desorpt

Full text of DOI:10.1039/c7ra07011j

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The effects of PVP-modified SiO on the catalytic
2
performance of CO hydrogenation over Rh–Mn–
Cite this: RSC Adv., 2017, 7, 48420
Li/SiO catalysts
2
Dan Ding, Jun Yu, * Qiangsheng Guo, Xiaoming Guo, Xiuzhen Xiao,
Dongsen Mao * and Guanzhong Lu
Rh–Mn–Li catalysts supported on SiO
2
prepared by PVP-modified St o¨ ber method were used for the
+
2
synthesis of C
oxygenates from CO hydrogenation. The catalysts were characterized by TG, XRD, N
2
-
2
adsorption–desorption, TEM, H -TPR, in situ FT-IR, TPSR, and XPS. Activity testing results showed that
2
the Rh–Mn–Li catalyst supported on the SiO modified by 1 g PVP exhibited the highest CO conversion
+
2
and selectivity of C
oxygenates compared with other catalysts. Characterization results indicated that
the addition of an appropriate amount of PVP is beneficial to the formation of weakly H-bonded
hydroxyl groups on the surface of SiO , which promotes Rh dispersion and weakens the Rh–Mn
2
Received 23rd June 2017
Accepted 28th September 2017
interaction. Furthermore, the higher Rh dispersion and the weaker Rh–Mn interaction promote CO
absorption, enhance the CO dissociation ability and restrain the hydrogenation activity, which are
DOI: 10.1039/c7ra07011j
X
favorable for the CO insertion into the metal–CH band, finally resulting in excellent catalytic
+
2
rsc.li/rsc-advances
performance for C
oxygenates synthesis.
observed that the Rh particles were more homogeneously
dispersed when 14–20 mesh silica was used instead of 20–40
1
. Introduction
The search for processes to provide an alternative feedstock for mesh as a support for Rh–Mn–Li catalyst. As a result, the space
+
chemicals and fuels has been promoted by the increasing time yield and selectivity of C
oxygenates were signicantly
2
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
concern about depletion of fossil fuel resources. Ethanol and increased from 338.6 g kg
h
and 49.2% to 618.4 g kg
ꢁ
h
other C2 oxygenates (e.g. acetaldehyde and acetic acid) are and 54.6%, respectively (T ¼ 300 C, P ¼ 3 MPa, SV ¼ 12 500
ꢀ1
important raw chemicals and, among other possibilities, can be
h
). In our previous studies, it was found that the activity of the
used as fuel additives. Thus, direct catalytic synthesis of C2 Rh–Mn–Li/SiO catalyst for the synthesis of C oxygenates from
oxygenates from syngas, which can be conveniently manufac- syngas was enhanced greatly when a commercial SiO was
tured from natural gas, coal and biomass, is one of the most replaced by a monodispersed SiO
prepared by the St ¨o ber
1,2
2
2
2
2
3–5
22
promising technologies for societal development. The Rh- method. It was also proved that the hydroxyl–metal interaction
based catalysts have been found to display unique efficiency over the Rh–Mn–Li catalyst supported on different silica resul-
and selectivity in C
2
oxygenates synthesis from CO hydrogena- ted in a change in the CO adsorption on Rh particles, leading to
6
–10
23
tion.
mance and primarily leads to formation of hydrocarbon
However, single Rh component exhibits a poor perfor- signicantly different catalytic activities.
24
Latterly, Li et al. designed a PVP-modied St ¨o ber method
products. Therefore, main efforts center on improving the for SiO synthesis, and the porosity of the SiO was expected to
efficiency of Rh and oxygenated compound selectivity by the change. Based on such a strategy, they further prepared core–
11
2
2
7,12–18
modication of Rh with promoters and supports.
SiO
is one of the most extensively used supports for Rh- of methane to syngas. It was found that the catalytic activity was
based catalysts, and its properties play an important role in essentially determined by the size of the Ni cores, and also
shell structured Ni@SiO catalysts and used in partial oxidation
2
2
14,15,19
20
determining the catalytic properties.
Solymosi et al.
somewhat by the porosity of SiO
2
shells. Based on these results,
proposed that surface hydroxyl groups on silica play an it is supposed that the properties of SiO
2
can be changed by the
important role in the change in the oxidation state of the metal, PVP-modied St ¨o ber method, which may inuence on the
+
and the active sites of Rh can be formed via an oxidation of the catalytic activity of Rh–Mn–Li catalysts supported on PVP-
0
21
Rh clusters by the surface OH groups of SiO
2
. Chen et al.
2
modied SiO for CO hydrogenation. Thus, the effects of
properties of the monodispersed SiO2 prepared by the PVP-
modied St ¨o ber method on the catalytic performance of Rh–
Research Institute of Applied Catalysis, School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai 201418, P. R. China.
E-mail: yujun@sit.edu.cn; dsmao@sit.edu.cn; Fax: +86-21-60877231
Mn–Li/SiO catalysts were investigated in this work. Then, CO
2
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hydrogenation performance of the catalysts was correlated with 2.3. Catalyst characterization
the interaction extents among Rh, Mn and SiO2.
The thermogravimetric (TG) analysis was tested on the thermal
ꢁ
analyzer (STA 499 F3, NETZSCH) heated at a rate of 10 per
ꢀ
1
minute under a continuous ow of air (50 ml min ).
2. Experimental section
The X-ray powder diffraction (XRD) experiments were carried
out with a PANalytical X'Pert diffractometer operated at 40 kV
and 40 mA using Ni b-ltered Cu Ka radiation. Two theta angles
2.1. Catalyst preparation
SiO with different contents of polyvinyl pyrrolidone (PVP) was
2
ꢁ
ꢁ
ranged from 10 to 80 with a speed of 6 per minute. Textural
25
prepared by a modied St ¨o ber method. In a typical synthesis,
the solution A was prepared by mixing 21 ml tetraethylortho-
silicate (TEOS) (99.5%, SCRC) with 50 ml anhydrous ethanol
properties were obtained using N adsorption–desorption at
2
ꢁ
ꢀ
196 C on a Micromeritics ASAP 2020 HD88 adsorption
apparatus. Before the adsorption/desorption measurements, all
samples were degassed at 200 C for 10 h. The specic surface
(
(
99.7%, SCRC); the solution B was a mixture of 76 ml NH $H O
3 2
26 vol%, SCRC) and 200 ml anhydrous ethanol and weighed
ꢁ
area and pore volume were calculated by BET and BJH models,
respectively. Transmission electron microscopy (TEM) images
were obtained by TECNAI T20 operated at 200 kV. The metal
loadings of the catalysts were determined by ICP-OES (Perki-
nElmer Optima 7000DV).
PVP (99.7%, SCRC). Secondly, the solution A was added slowly
into the solution B in a ask under rapid stirring at 25 C and
ꢁ
reacted for 4 h. The white solid product was separated centrif-
ugally at 7000 rpm for 5 minutes, washed with ethanol for three
ꢁ
times and then dried at 90 C for 12 h. The weight of PVP dis-
The amount of hydrogen adsorption of various catalysts was
solved in solution B was 0 g, 0.5 g, 1.0 g and 3.0 g, respectively,
and the corresponding products were denoted as SiO2,
SiO (0.5), SiO (1) and SiO (3). Before being used, they were
calculated on the basis of H
2 2
-TPD proles. For H -TPD
measurements, the catalyst (0.1 g) was reduced in situ for 2 h at
2
2
2
ꢁ
ꢁ
2 2
/N , and then was held at 400 C for another
4
3
00 C in 10% H
0 min before being cooled down to room temperature in He
ow. The next step was H adsorption at room temperature for
.5 h, and then the gas was swept again with He for 3 h.
Subsequently, the sample was heated in a owing He stream
ꢁ
calcined in air at 600 C for 4 h.
RhCl
hydrate (Rh ꢂ 39 wt%, Fluka), Mn(NO
99.99%, SCRC), LiNO (99.95%, SCRC) and the supports
3
3 2 2
) $6H O

2
(
3
0
mentioned above were used in catalyst preparation. All the
catalysts were prepared by the incipient wetness impregnation
method, and Rh loading was 1.5 wt% based on the weight of
support, and the weight ratio of Rh : Mn : Li ¼ 1.5 : 1.5 : 0.07.
ꢀ1
ꢁ
ꢁ
ꢀ1
(
50 ml min ) up to ꢂ500 C at a rate of 10 C min , while the
desorbed species was detected with a TCD detector. The uptake
of H was used to calculate Rh metal dispersion and particle
ꢁ
2
Impregnated catalysts were dried at 90 C for 4 h, and then at
size, assuming that each surface metal atom adsorbs one H
ꢁ
ꢁ
1
10 C overnight before calcined in air at 350 C for 4 h. The
obtained catalysts were denoted as RML/SiO , RML/SiO (0.5),
RML/SiO (1), and RML/SiO (3), respectively. Elemental analysis
atom, i.e. H/Resurface ¼ 1.
2
2
2
H temperature-programmed reduction (TPR) of the cata-
2
2
lysts were carried out in a quartz micro-reactor. 0.1 g of the
by inductively coupled plasma (ICP) revealed good agreement
between the expected and experimental values.
ꢁ
sample was rst pretreated at 350 C in O /N (molar ratio of O /
2
2
2
N ¼ 1/4) for 1 h prior to a TPR measurement. During the TPR
2
experiment, H
2
/N
2
(molar ratio of H
2
/N
2
¼ 1/9) mixture was
ꢀ
1
used at 50 ml min and the reduction temperature ramped
2.2. Testing of the catalytic activity
ꢁ
ꢁ
ꢁ
ꢀ1
from 50 C to 500 C (10 C min ) while the effluent gas was
analyzed with a thermal conductivity detector (TCD).
CO hydrogenation was performed in a xed-bed micro-reactor
with length ꢂ350 mm and internal diameter ꢂ5 mm. The
catalyst (0.3 g) diluted with inert a-alumina (1.2 g) was loaded
between quartz wool and axially centered in the reactor tube,
with the temperature monitored by a thermocouple close to the
CO adsorption was studied using a Nicolet 6700 FTIR spec-
trometer equipped with a diffuse reectance infrared Fourier
transform (DRIFT) cell with CaF windows. The sample in the
2
cell was pretreated in H /N (molar ratio of H /N ¼ 1/9) at
2
2
2
2
ꢁ
catalyst bed. Prior to reaction, the catalyst was heated to 400 C
ꢁ
ꢀ1
ꢁ
4
00 C for 2 h, followed by N (50 ml min ) ushing at 400 C
2
ꢁ
ꢀ1
(
heating rate ꢂ3 C min ) and reduced with H
2 2
/N (molar ratio
2
for 0.5 h. During cooling down to the room temperature in N ,
ꢀ1
of H
/N
2 2
¼ 1/9, total ow rate ¼ 50 ml min ) for 2 h at atmo-
a series of background spectra were taken at different temper-
ꢁ
spheric pressure. The catalyst was then cooled down to 300 C
ꢀ1
atures. Then, 1% CO/N
2
(50 ml min ) was introduced into the
and the reaction started as gas ow was switched to a H /CO
2
cell and the IR spectra at the desired temperatures were recor-
ꢀ
ꢁ
1
2
mixture (molar ratio of H /CO ¼ 2, total ow rate ¼ 50 ml min
)
ꢁ
ded. Aer heating up to 300 C in CO/N mixture for 60 min,
2
at 3 MPa. All post-reactor lines and valves were heated to 150 C
to prevent product condensation. The products were analyzed
for both hydrocarbons and oxygenates on-line (FL GC 9720)
using a HP-PLOT/Q column (30 m, 0.32 mm ID) with detection
with a ame ionization detector (FID) and a TDX-01 column with
a TCD. The CO conversion was calculated based on the fraction
of CO that formed carbon-containing products and the selec-
tivity of a certain product was calculated based on carbon
ꢀ1
a H ow (1 ml min ) was added into the owing CO/N , and
2
2
the IR spectra were recorded as a function of time. Ultrahigh-
purity N , H and CO used in the IR investigations were
2
2
further puried by dehydration and deoxygenization. The
ꢀ1
spectral resolution was 4 cm with 64 interferograms being
added to obtain a satisfactory signal-to-noise ratio.
The temperature-programmed surface reaction (TPSR)
experiments were carried out as follows: aer the catalyst was
26
efficiency.
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ꢁ
reduced at 400 C in H
2
/N
2
(molar ratio of H
2
/N
2
¼ 1/9) for 2 h, it
was cooled down to room temperature and CO was introduced
for adsorption for 0.5 h; aerwards, the H /N mixture was
2
2
swept again, and the temperature was increased at the rate of
ꢁ
ꢀ1
1
0
C min
Balzers OmniStar 200) as the detector to monitor the signal of
CH
with a quadrupole mass spectrometer (QMS,
4
(m/z ¼ 15).
XPS measurements were performed using a Kratos Axis Ultra
DLD spectrometer equipped with an Al Ka (1486.6 eV) X-ray
source. In these experiments, samples were reduced in situ at
ꢁ
4
00 C for 1 h under 200 mbar H pressure. The binding ener-
2
gies were calibrated relative to the C 1s peak from carbon
contamination of the samples at 284.9 eV to correct for contact
potential differences between the sample and the spectrometer.
+
Fig. 1 CO conversion/C
2
oxygenates selectivity vs. time-on-stream
for the RML/SiO (1) catalyst.
2
3. Results and discussion
3.2. TG analysis
3
.1. CO hydrogenation performance of the catalysts
Fig. 2 shows the TG patterns of PVP and the corresponding
supports before calcination. For the supports before calcina-
tion, it is clear that a continuous weight loss occurred from
The addition of different amounts of PVP modied the catalytic
properties of the Rh–Mn–Li/SiO catalysts, as shown in Table 1.
As observed, the CO conversion and selectivity of C2 oxygenates
2
+
ꢁ
room temperature to about 600 C. The process of weight loss
increased rst with the amounts of PVP doping, then reached
a maximum at PVP amount of ca. 1.0 g, and decreased when the
PVP addition reached 3.0 g. Moreover, the trend in turnover
frequency (TOF) values of CO conversion over the catalysts was
consistent with that of CO conversion. Interestingly, it is
obvious that the selectivity of ethanol did not change obviously
as the addition of PVP, while it indeed could accelerate the
formation of acetaldehyde. On the other hand, the selectivity of
can be divided mainly into two steps. The weight loss observed
from room temperature to 150 C should be attribute to the
ꢁ
desorption of adsorbed water. Combined with the TG pattern of
PVP (Fig. 2b), the second step of weight loss started from
ꢁ
ꢁ
ꢂ250 C to ꢂ600 C, and it can be attributed to the combined
effect from both of the PVP decomposition and partial de-
hydroxylation being converted into oxides on the surface.
Compared with the precursor of SiO , the precursor of SiO (1)
2
2
hydrocarbons decreased noticeably over RML/SiO (0.5) and
RML/SiO (1) catalysts, which indicated that the ability of
2
hydrogenation can be inhibited on the catalysts modied by
appropriate amount of PVP. Overall, it can be seen that the
2
lost more than 2.5% weight during the heating process, which
suggested that the PVP was successfully doped into SiO (1).
However, it is curious that the TG pattern of SiO (3) was similar
to that of SiO , which inferred that the PVP cannot be doped into
2
2
2
RML/SiO
selectivity of C
reached the maximum of 290.4 g kg
2
(1) catalyst obtained the highest CO conversion and
the support if the amount of PVP provided is too high in the
preparation process. Moreover, it is indicated that when the
+
+
2
oxygenates, and the yield of C
2
oxygenates
ꢀ
1
ꢀ1
ꢁ
h
.
temperature > 600 C, the PVP can be completely decomposed.
+
ꢁ
Typical time dependent changes of CO conversion and C2
Thus, the supports were designed to calcine at 600 C.
oxygenates selectivity over the representative RML/SiO (1)
2
catalyst are shown in Fig. 1. It can be seen that both the CO
+
3.3. Structural and textural properties of catalysts
conversion and C
2
oxygenates selectivity decreased distinctly
during the rst 15 h on stream. However, the catalyst kept good XRD patterns of silica with different PVP contents (not shown)
stability aer 15 h, and the catalytic activity was hardly reduced and the corresponding catalysts (Fig. 3) showed no crystalline
until 300 h.
phases, indicating that the SiO
2
were XRD-amorphous and the
a
Table 1 CO hydrogenation performance on different catalysts
Selectivity of products (C%)
+
CO conv.
(C%)
STY(C
(g kg
2
Oxy)
h )
d
ꢀ1
+
b
+
c
ꢀ1 ꢀ1
Catalyst
TOF (s
)
CO
2
CH
4
MeOH
AcH
EtOH
C
2
HC
C
2
Oxy
RML/SiO
RML/SiO
RML/SiO
RML/SiO
2
2
2
2
18.9
22.3
24.8
19.8
0.152
0.169
0.183
0.154
3.8
3.4
3.6
2.9
17.1
13.8
14.1
14.4
4.3
3.0
2.7
2.5
18.6
22.1
21.5
21.0
14.4
14.4
14.7
13.6
37.6
36.4
36.1
39.3
37.2
43.4
43.5
40.9
192.3
262.5
290.4
222.2
(0.5)
(1)
(3)
a
ꢁ
ꢀ1
Reaction conditions: T ¼ 300 C, P ¼ 3 MPa, catalyst: 0.3 g, and ow rate ¼ 50 ml min (H
2
/CO ¼ 2), data taken aer 15 h when steady state
b
+
c
+
reached. Experimental error: ꢃ5%.
C
2
HC denotes hydrocarbons containing two and more carbon atoms.
C
2
Oxy denotes oxygenates
d
2
containing two and more carbon atoms. TOF based on CO conversion and H chemisorption.
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in Table 2, the specic surface area of supports were measured
2
ꢀ1
to be ca. 8.0 m g . Similar surface areas of SiO prepared by
2
25
the St ¨o ber method have been reported by Szekeres et al. and
Hsu et al. Upon being loaded with metal components, there
27
were a slight decrease in the surface area. With respect to the
pore size, the pore diameter of SiO
2
was 7.9 nm. Compared with
(0.5) and SiO (1)
the support of SiO , the pore diameter of SiO
2
2
2
increased slightly, which suggested that an appropriate amount
of PVP can be doped into the silica during syntheses, and the
decomposition of PVP in calcination slightly enlarged the pore
size. Moreover, the amount of hydrogen adsorption of various
catalysts calculated on the basis of their H -TPD proles is
2
summarized in Table 2. It can be seen that the amount of
hydrogen adsorption slightly increased rst with the amounts
of PVP doping, but decreased when the PVP addition reached
3.0 g.
In comparison, no signicant difference was observed in the
pore sizes aer loaded probably due to the fact that the
concentrations of Rh and promoters were relatively low in all
the catalysts. Combined with the catalytic activity, it is supposed
that the specic surface area and pore size of the catalyst are not
directly related to the catalytic performance of the catalyst.
Fig. 4 shows the TEM micrographs and the corresponding
Rh particle size distributions of catalysts. As shown in Fig. 4A, C
and E, the particles of supports were monodispersed spherical
with a mean size of 500 nm. According to the TEM micrographs
of catalysts (Fig. 4B, D and F), it can be seen that the Rh particles
loading on the support of SiO (1) were more uniform, the mean
2
size of Rh particles was 17 nm. However, compared with the
RML/SiO
on SiO and SiO
became larger.
2
(1) catalyst, the distributions of Rh particles loading
2
2
(3) were wide, and the sizes of Rh particles
Fig. 2 TG patterns for the precursors of supports (a) and PVP (b).
Combined with the catalytic results, it is conceivable that the
size of Rh particles has a remarkable inuence on the activity
and selectivity of Rh–Mn–Li/SiO2 for CO hydrogenation. The
results showed that the Rh particles loading on the SiO2
modied by a moderate amount of PVP were smaller and more
uniform, which, in turn, was reected by the excellent catalytic
+
activity along with the higher C
catalyst of RML/SiO (1).
2
oxygenates selectivity over the
2
The IR spectra of supports and the corresponding catalysts
ꢁ
in N atmosphere at 300 C are presented in Fig. 5. In these IR
2
spectra, there are two main absorption regions: the rst band at
ꢀ
1
3
672 cm is assigned to the weakly H-bonded OH groups; and
ꢀ
1
the second region in the range 3500–2750 cm
a maximum at 3465 cm originates from the absorption of H O
2
and strongly H-bonded OH groups.
with
ꢀ
1
19,28
As shown in Fig. 5, there were different intensities of the
hydroxyl groups on the different silica surface. Almost weakly H-
Fig. 3 XRD patterns of the catalysts: (a) RML/SiO
c) RML/SiO (1), (d) RML/SiO (3).
2
, (b) RML/SiO
2
(0.5), bonded hydroxyls covered on the surface of SiO (0.5) and
2
(
2
2
SiO (1), whereas the surface of SiO and SiO (3) exhibited more
2
2
2
strongly H-bonded hydroxyls accompanied by partial weakly H-
bonded hydroxyls. It is indicated that the surface property of
metal particles were highly dispersed on the supports or below SiO
the detection limit of XRD due to the small content. added with a high amount (3 g) cannot be doped into silica
adsorption–desorption was used to characterize the during the preparation, which is consistent with the result of TG.
texture of supports and the corresponding catalysts. As shown Furthermore, the intensity of the hydroxyl groups on supports
2 2
and SiO (3) were almost the same, suggesting that the PVP
N
2
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P
Table 2 Specific surface areas (SBET), pore volume (V ) and pore
diameter (D
P
) from N
2
adsorption–desorption and H
2
chemisorption
S
BET
V
P
D
(nm)
P
H
2
chemisorption
2
ꢀ1
3
ꢀ1
ꢀ1
Sample
(m g
)
(cm g
)
(mmol g
)
SiO
SiO
SiO
SiO
RML/SiO
RML/SiO
RML/SiO
RML/SiO
2
2
2
2
8.0
8.2
8.5
7.9
4.5
4.6
4.4
4.3
0.050
0.061
0.062
0.054
0.037
0.041
0.038
0.042
7.9
9.0
8.9
8.1
7.4
8.4
8.5
7.7
—
—
—
—
23.1
24.6
25.2
23.9
(0.5)
(1)
(3)
2
2
2
2
(0.5)
(1)
(3)
changed obviously aer supporting metal components. It can be
seen that the weakly H-bonded hydroxyls decreased distinctly
and the strongly H-bonded hydroxyls were almost unchanged,
which can be attributed mainly to a strong interaction only
occurred between the metal components and weakly H-bonded
hydroxyls. Since the surface of SiO (0.5) and SiO (1) covered
Fig. 5 FT-IR spectra of (a) SiO
SiO (0.5) (straight) and RML/SiO
RML/SiO
2
(straight) and RML/SiO
(0.5) (dash), (c) SiO (1) (straight) and
(3) (straight) and RML/SiO (3) (dash) in N
2
(dash), (b)
2
2
2
2
(1) (dash), (d) SiO
2
2
2
ꢁ
flow at 300 C.
2
2
more weakly H-bonded hydroxyl groups compared with that of
By combining the activities of catalysts and the result of
TEM, it can be further inferred that more hydroxyl–metal
interaction occurred on the surface of SiO (0.5) and SiO (1),
SiO2 and SiO (3), more metal components can be interacted
2
during the course of catalyst preparation, resulting that more
decrease in the absorption intensity of hydroxyl groups of
SiO (0.5) and SiO (1) aer supporting.
2 2
2
2
which is more conducive to the dispersion of Rh, nally
Fig. 4 TEM images of SiO
2
(A), RML/SiO
2
2 2 2 2
(B), SiO (1) (C), RML/SiO (1) (D), SiO (3) (E), RML/SiO (3) (F) and Rh particle size distributions of different
catalysts.
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2
resulting in the better catalytic performances of RML/SiO (0.5)
and RML/SiO (1).
2
3.4. The reducibility of the catalysts
Fig. 6 shows the temperature-programmed reduction (TPR)
proles of the catalysts with different PVP doping. There were
three distinct reduction peaks in the TPR prole of RML/SiO .
2
7
,29
According to the literatures,
1
the low temperature peak at
ꢁ
ꢁ
30 C and the shoulder at 160 C are ascribed to the reduction
of Rh
2
O
3
not intimately contacting with Mn species (denoted as
intimately contacting with Mn
Rh(I) species) and of Rh
2 3
O
species (denoted as Rh(II) species), respectively. The high
ꢁ
temperature peak at 200 C can be ascribed to the reduction of
17,30
ꢁ
MnO species.
Fig. 7 The infrared spectra of absorbed CO in the He flow at 30 C: (a)
RML/SiO , (b) RML/SiO (0.5), (c) RML/SiO (1), (d) RML/SiO (3).
2 2 2 2
2
As shown in Fig. 6, following the increase of PVP amount, the
reduction peak of MnO moved to the higher temperature in
company with the decrease of the reduction temperature of
Rh . Considering that the width of temperature region
2
are shown in Fig. 7. It can be seen that the IR spectrum was
ꢀ
1
2 3
O
mainly composed by a doublet at 2100 and 2037 cm and
ꢀ
1
31,32
between Rh and Mn oxides reduction peaks can be a judge for
the strength of Rh–Mn interaction, the larger width of temper-
ature region between Rh and Mn oxides reduction peaks indi-
cates a weaker Rh–Mn interaction. It is indicated that the
a band at around 2068 cm . According to the literatures,
ꢀ1
the 2068 cm band can be ascribed to the linear adsorbed CO
CO(l)] and the doublet can be assigned to the symmetric and
[
+
asymmetric carbonyl stretching of the gem-dicarbonyl Rh (CO)2
surface properties of SiO modied with appropriate amount of
2
[CO(gdc)]. It is widely accepted that the CO(gdc) is formed on
+
0
15,33
PVP can weaken the Rh–Mn interaction. Combined with the
hydroxyls information observed by FT-IR and the result of TEM,
it is proposed that there were more weakly H-bonded hydroxyls
the Rh sites and CO(l) is on the Rh sites.
that the intensity of CO adsorption increased in the order of
RML/SiO < RML/SiO (3) < RML/SiO (0.5) < RML/SiO (1), sug-
It is worthy to note
2
2
2
2
2 2
on the surface of SiO (0.5) and SiO (1), and the more interaction
gesting that high Rh dispersion and CO adsorption capacity
were achieved by the assisting of hydroxyl–metal interaction,
which is also consistent with the result of CO conversion.
Fig. 8 shows the infrared spectra of absorbed CO on different
between the metal components and weakly H-bonded hydroxyls
increased the metal dispersion and weakened the Rh–Mn
interaction.
However, the reduction temperatures of Rh O and MnO
ꢁ
catalysts at 300 C and followed by ushing with 10% H /N . It
2 2
2
3
2
species in the TPR prole of RML/SiO
of RML/SiO , indicating that SiO and SiO
surface properties, which is in line with the above IR study.
2
(3) were similar with that
can be seen that all the samples only appeared the peak of CO(l)
at 300 C and the intensity decreased as a function of time,
(3) had the similar
ꢁ
2
2
2
which indicated that the CO(l) species is the mainly reactive
species at the reaction temperature. The rates of decrease in the
band for CO(l) on different catalysts followed the order as
shown in Fig. 8: RML/SiO (1) z RML/SiO (0.5) < RML/SiO z
3.5. FT-IR study
2
2
2
A series of IR spectra of the in situ reduced catalysts with
ꢁ
RML/SiO
It is conceivable that, the more hydroxyl–metal interaction
occurred on the surface of SiO (0.5) and SiO (1) weakened the
2
(3).
different PVP amounts aer CO adsorption at 30 C for 30 min
2
2
Rh–Mn interaction. The weak Rh–Mn interaction might restrain
the hydrogenation activity on Rh particles, then decreased the
consuming of absorbed CO by hydrogenation, resulting in a low
hydrogenation activity of RML/SiO
2 2
(0.5) and RML/SiO (1).
3.6. TPSR
The temperature-programmed surface reaction (TPSR) is one of
the most effective way to study the catalytic activity by the
hydrogenation of adsorbed CO. Fig. 9 displays the TPSR proles
of the samples. It can be seen that on the proles of RML/SiO
2
and RML/SiO
2
(3), the peaks of CH
4
formation were centered at
ꢁ
ꢁ
260–265 C; however, the CH
4
peak shied to ca. 245 C on
RML/SiO (0.5) and RML/SiO (1) catalysts. In addition, the
2
2
amount of CH4 produced by the catalysts were different,
following the order as shown in Table 3: RML/SiO > RML/
SiO (3) > RML/SiO (1) > RML/SiO (0.5).
2
Fig. 6 TPR profiles of the catalysts.
2
2
2
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Fig. 9 The TPSR profiles of the different catalysts: (a) RML/SiO
2
, (b)
RML/SiO (0.5), (c) RML/SiO (1), (d) RML/SiO (3).
2
2
2
reect the number of active sites responsible for methane
production. Based on the above results, it can be concluded that
compared with the supports of SiO
2
and SiO
2
(3), the metal
(1) obtained
components supported on SiO (0.5) and SiO
2
2
a weaker Rh–Mn interaction due to the stronger hydroxyl–metal
interaction, which may promote the ability of CO dissociation
and decrease the temperature of CH formation. On the other
4
hand, it is usually considered that the formation of hydrocar-
X 2
bons from CH hydrogenation and the formation of C
oxygenates from CO insertion are the couple of competitive
reaction, and the relative reaction rate between them deter-
+
35
mines the activity and selectivity forwards C
2
oxygenates.
Combined with the reactive performance, it is suggested that
the good ability of CO dissociation in company with moderate
hydrogenation activity exhibited on the catalysts modied by
appropriate amount of PVP (0.5–1 g) are conducive to the
+
formation of C
2
oxygenates.
3.7. XPS
The chemical states of Rh on catalyst surfaces before and aer
ꢁ
reduction at 400 C have been studied by XPS. The 3d spectrum
of Rh in all the fresh catalysts are presented in Fig. 10A, it can be
seen that the binding energy of the Rh 3d5/2 peaks were centered
at 309.8 eV, indicating that Rh species existed as Rh
2 3
O . The Rh
3d core level spectrum of catalysts aer in situ reduction are
displayed in Fig. 10B. It is observed that the binding energy of
the Rh 3d peaks were located at 306.8–307.0, indicating that
5
/2
0
Rh is the major Rh species on all the reduced catalysts.
However, it is obvious that the binding energy of Rh 3d5/2 of
2 2
RML/SiO and RML/SiO (3) were shied to higher values, sug-
3
1
gesting that the electronic density on Rh particles decreased. It
Fig. 8 The infrared spectra of absorbed CO on different catalysts at
ꢁ
d+
3
00 C and followed by hydrogenation with hydrogen: (a) RML/SiO
2
,
is inferred that some partially positively charged Rh species
0
(
2 2 2
b) RML/SiO (0.5), (c) RML/SiO (1), (d) RML/SiO (3).
also co-exist with Rh on the surface of RML/SiO
2
and RML/
SiO (3) catalysts aer reduction. Considering that the electrons
2
34
According to the literature, the temperature of CH4 of rhodium atoms can be attracted by the electron-withdrawing
d+
formation can be used as a tool for determining the ability of property of Mn, the presence of Rh species can be attributed to
CO dissociation, and the peak intensity of CH
4
formation can the stronger Rh–Mn interaction obtained on the surfaces of
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Table 3 The peak area of methane produced during TPSR in different amount of PVP is benecial to the formation of weakly H-
catalyst
bonded hydroxyls groups on the surface of SiO . The result of
2
TEM and H -TPR further suggested that the interaction between
metal components and weakly H-bonded hydroxyls can
promote the Rh dispersion and weaken the Rh–Mn interaction.
2
Sample
A
CH4
a
RML/SiO
RML/SiO
RML/SiO
RML/SiO
2
2
2
2
1
(0.5)
(1)
(3)
0.74 Moreover, based on the result of IR and TPSR, it is conceivable
0.80
0.85
that the higher Rh dispersion and weaker Rh–Mn interaction
promote the CO absorption, enhance the CO dissociation ability
catalyst was and restrain the hydrogenation activity, which are favorable for
a
The peak area of CH
4
formed in TPSR over the RML/SiO
2
quantitative for one unit; experimental error: ꢃ5%.
the CO insertion into metal–CH band, nally resulting in the
X
+
excellent catalytic performance of RML/SiO (1) for C2 oxygen-
2
ates synthesis.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors gratefully acknowledge nancial support from the
Science and Technology Commission of Shanghai Municipality
(
08520513600), Leading Academic Discipline Project of
Shanghai Education Committee (J51503) and Shanghai
Municipal Science and Technology Commission
13ZR1461900).
(
Notes and references
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