Synthesis of C2 oxygenates from syngas over Rh-Mn-Li/SiO2 catalysts: Effect of supports prepared using different ammonia concentrations

Article abstract of DOI:10.1039/c6ra16404h

The effects of surface properties of the monodispersed SiO₂ prepared using different ammonia concentrations on the catalytic performance of Rh-Mn-Li/SiO₂ for CO hydrogenation to C₂ oxygenates were investigated. The investigation based on the catalytic performance and characterizations of the catalysts suggested that the catalytic performance was relatively stable on the Rh-Mn-Li catalyst supported on the SiO₂ prepared using high ammonia concentration, which is attributed to the steady state of the metals supported on it. However, the rough surface of SiO₂ prepared using a low ammonia concentration resulted in easy agglomeration of the metals. Correspondingly, the size of Rh particles increased and the isolated Rh⁺ sites decreased in the reaction process, leading to the decrease of C₂ oxygenate selectivity of the catalyst as a function of time.

Full text of DOI:10.1039/c6ra16404h

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PAPER
Synthesis of C2 oxygenates from syngas over Rh–
Mn–Li/SiO2 catalysts: effect of supports prepared
using different ammonia concentrations
Cite this: RSC Adv., 2016, 6, 76736
a
a
a
ab
*
*
Jun Yu, Dongsen Mao, Dan Ding and Guanzhong Lu
The effects of surface properties of the monodispersed SiO2 prepared using different ammonia
concentrations on the catalytic performance of Rh–Mn–Li/SiO2 for CO hydrogenation to C2 oxygenates
were investigated. The investigation based on the catalytic performance and characterizations of the
catalysts suggested that the catalytic performance was relatively stable on the Rh–Mn–Li catalyst
supported on the SiO2 prepared using high ammonia concentration, which is attributed to the steady
state of the metals supported on it. However, the rough surface of SiO2 prepared using a low ammonia
concentration resulted in easy agglomeration of the metals. Correspondingly, the size of Rh particles
increased and the isolated Rh+ sites decreased in the reaction process, leading to the decrease of C2
oxygenate selectivity of the catalyst as a function of time.
Received 27th June 2016
Accepted 9th August 2016
DOI: 10.1039/c6ra16404h
www.rsc.org/advances
catalytic performance. Solymosi et al.11 proposed that surface
hydroxyl groups on silica play an important role in the change
in the oxidation state of the metal, and the active sites of Rh+
1. Introduction
One of the largest social challenges today is the quest for
alternative fuels, which would reduce greenhouse gas emissions
as well as the heavy dependence on depleting fossil fuels.1 The
synthesis of C2 oxygenates (e.g., ethanol, acetaldehyde and
acetic acid) from syngas, which can be produced from non-
petroleum carbon resources including natural gas, coal, and
biomass, constitutes an alternative pathway for the production
of clean fuels and chemicals.2–5 So far, Rh-based/SiO2 catalysts
have been shown to be active for C2 oxygenate synthesis from
syngas,6–8 however, the CO conversion and selectivity of C2
oxygenates are still not enough for practical use. In order to
further improve the catalytic properties of Rh-based/SiO2 cata-
lysts, the modication of additives and SiO2 supports has been
investigated continuously in the past decades.9–17
can be formed via an oxidation of the Rh0 clusters by the surface
OH groups of SiO2. Chen et al.20 observed that the Rh particles
were more homogeneously dispersed when 14–20 mesh silica
was used instead of 20–40 mesh as a support for Rh–Mn–Li
+
catalyst. As a result, the space time yield and selectivity of C2
oxygenates were signicantly increased from 338.6 g (kgꢀ1 hꢀ1
)
and 49.2% to 618.4 g (kgꢀ1 hꢀ1) and 54.6%, respectively (T ¼ 300
ꢁC, P ¼ 3 MPa, SV ¼ 12 500 hꢀ1). Bao and co-workers found that
the different electronic properties of inside and outer carbon
nanotubes surface can change the Rh–Mn interaction, which
resulted in the different CO adsorption behavior and catalytic
performance.21
In fact, the morphology and surface properties of SiO2
¨
prepared by the Stober method can be modied by the
In our previous studies, it has been found that the catalytic
performance of the Rh–Mn–Li/SiO2 catalyst for the synthesis of
C2 oxygenates from CO hydrogenation was enhanced greatly
ammonia concentration, which may inuence on the supported
metal components. Thus, the effects of surface properties of the
monodispersed SiO2 prepared by different ammonia concen-
tration on the catalytic performance of Rh–Mn–Li/SiO2 catalysts
were investigated in this work. Then, CO hydrogenation
performance of the catalysts was correlated with the interaction
extents among Rh, Mn and SiO2.
when a commercial SiO2 was replaced by a monodispersed SiO2
18,19
¨
prepared by the Stober method.
The excellent performance
of this catalyst can be mainly attributed to the special surface
properties of the monodispersed SiO2. It is widely accepted that
the properties of supports have a considerable inuence on the
Rh-based catalysts, which nally leads to disparities in the
2. Experimental section
2.1. Catalyst preparation
aResearch Institute of Applied Catalysis, School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai 201418, P. R. China.
E-mail: dsmao@sit.edu.cn; gzhlu@ecust.edu.cn; Fax: +86-21-60873625; +86-21-
64253824
22
¨
The monodispersed SiO2 was prepared by the Stober method.
In a typical synthesis, the mixture solution of 21 ml tetraethy-
lorthosilicate (TEOS, 99.5%, SCRC) and 50 ml anhydrous
ethanol (99.7%, SCRC) was added slowly into the mixed
bKey Laboratory for Advanced Materials and Research Institute of Industrial Catalysis,
East China University of Science and Technology, Shanghai 200237, P. R. China
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solution of 76 ml ammonia (26 vol%, SCRC) and 200 ml anhy-
H2 temperature-programmed reduction (H2-TPR) was
drous ethanol. Then, this synthesis solution was aged for 4 h carried out in a quartz micro-reactor. 0.1 g of the sample was
and separated centrifugally at 7000 r.p.m. Finally, the collected rst pretreated at 350 ꢁC in O2/N2 (20% v/v) for 1 h prior to a TPR
product was washed with de-ionized water and dried at 70 ꢁC for measurement. During the TPR experiment, H2/N2 (10% v/v) was
12 h, which was denoted as SiO2(H). The SiO2(L) was synthe- used at 50 ml minꢀ1 and the temperature was rꢀa1mped from
sized by the same steps as above, but the amount of ammonia room temperature (RT) to 650 ꢁC at 10 ꢁC min while the
was changed by 21 ꢁml. Before being used, the SiO2 was calcined effluent gas was analyzed with a TCD detector.
in static air at 350 C for 4 h.
CO adsorption was studied using a Nicolet 6700 FTIR spec-
RhCl3 hydrate (Rh ꢂ36 wt%, Fluka), Mn(NO3)2$6H2O trometer equipped with a DRIFT (diffuse reectance infrared
(99.99%, SCRC), Li2CO3 (99.5%, SCRC), and SiO2 mentioned Fourier transform) cell with CaF2 windows. The sample in the
above were used in catalyst preparations. Catalysts were cell was pretreated in H2/N2 (10% v/v) at 400 ꢁC for 2 h, and then
prepared by co-impregnation to incipient wetness of the the temperature was dropped to RT. Aer the cell was outgassed
prepared SiO2 (1.0 g) with an aqueous solution of RhCl3 hydrate in vacuum to <10ꢀ3 Pa, the background was scanned. Aer
and precursors of the promoters (Mn and Li), followed by drying introducing CO into the IR cell (pCO ¼ 8.0 ꢃ 103 Pa), the IR
at 90 ꢁC for 4 h, andꢁthen at 120 ꢁC overnight before being spectrum of CO adsorbed on the catalyst was recorded. The
calcined in air at 350 C for 4 h. For both of the catalysts, Rh concentration of CO was higher than 99.97%, and it was pre-
loading was 1.5 wt% and the weight ratio of Rh : Mn : Li ¼ treated by dehydration and deoxygenization before being used.
1.5 : 1.5 : 0.07. The obtained catalysts were denoted as RML/ The spectral resolution was 4 cmꢀ1 and the scan times were 64.
SiO2(H) and RML/SiO2(L). In addition, the used catalysts were
denoted as RML/SiO2(H)-UD and RML/SiO2(L)-UD.
The temperature-programmed surface reaction (TPSR)
experiment was carried out as follows: aer the catalyst was
reduced at 400 ꢁC in H2/N2 (10% v/v) for 2 h, it was cooled down
to RT and CO was introduced for adsorption for 0.5 h; aer-
ward, the H2/N2 was swept again, and the temperature was
increased at the rate of 10 ꢁC minꢀ1 with the QMS as the
detector to monitor the signal of CH4 (m/z ¼ 15).
2.2. Testing of the catalytic activity
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
catalyst bed. Priorꢁto reaction, the catalyst was heated to 400 ꢁC
(heating rate ꢂ 3 C minꢀ1) and reduced with H2/N2 (10% v/v,
total ow rate ¼ 50 ml minꢀ1) for 2 h at atmospheric pres-
sure. The catalyst was then cooled down to 300 ꢁC and the
reaction started as gas ow was switched to a H2/CO mixture
(molar ratio of H2/CO ¼ 2, total ow rate ¼ 50 ml minꢀ1) at 3
3. Results and discussion
3.1. Catalytic activities
Fig. 1 and Table 1 show the catalytic performances of the two
catalysts for CO hydrogenation. It can be seen that the catalytic
performance of RML/SiO2(L) and RML/SiO2(H) exhibited
ꢁ
MPa. All post-reactor lines and valves were heated to 150 C to
prevent product condensation. The products were analyzed on-
line (Agilent GC 6820) using a HP-PLOT/Q column (30 m, 0.32
mm ID) with detection with an FID (ame ionization detector)
and a TDX-01 column with a TCD (thermal conductivity
detector). The conversion of CO was calculated based on the
fraction of CO that formed carbon-containing products
P
according to: % conversion ¼ ( niMi/MCO) ꢃ 100%, where ni is
the number of carbon atoms in product i; Mi is the percentage of
product i detected, and MCO is the percentage of CO in the feed.
The selectivity of a certain product was calculated based on
P
carbon efficiency using the formula niCi/ niCi, where ni and Ci
are the carbon number and molar concentration of the ith
product, respectively.
2.3. Catalyst characterization
The X-ray powder diffraction (XRD) spectra of samples were
obtained on a Rigaku D/MAX-IIIA X-ray diffractometer with
CuKa (l ¼ 0.15418 nm). The specic surface area (SBET), pore
volume (Vp), and pore diameter (Dp) of samples were obtained
by N2 adsorption at ꢀ196 ꢁC on a Micromeritics ASAP 2020
apparatus. Transmission electron microscopy (TEM) images
were obtained by JEM-2100 operated at 200 kV.
Fig. 1 The catalytic performance vs. time-on-stream for the catalysts
of (a) RML/SiO2(L) and (b) RML/SiO2(H): CO conversion (D), selectivity
of C2 oxygenates (>), selectivity of hydrocarbon (,), STY of C2 Oxy
(+), STY of hydrocarbons (C).
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Table 1 CO hydrogenation performance on the catalysts after reaction for 2 ha
Selectivity of productsb (C%)
STY (C2 Oxy)
(g (kgꢀ1 h)ꢀ1
STY (HC)
Catalyst
CO conv. (C%)
CO2
MeOH
AcH
EtOH
C2 Oxyc
HCd
Other Oxye
)
(g (kgꢀ1 hꢀ1))
RML/SiO2(L)
RML/SiO2(H)
15.7
16.6
6.7
3.8
1.7
1.1
35.2
35.7
17.5
18.1
57.7
59.8
31.2
32.1
2.7
3.2
303.1
327.8
87.4
101.1
a
Reaction conditions: T ¼ 300 ꢁC, P ¼ 3 MPa, catalyst: 0.3 g, and ow rate ¼ 50 ml minꢀ1 (H2/CO ¼ 2). Experimental error: ꢄ5%. b Based on carbon
P
efficiency, carbon selectivity ¼ niCi/ niCi. c C2 Oxy denotes oxygenates containing two carbon atoms. d HC denotes all hydrocarbons. e Other Oxy
denotes oxygenates containing more than two carbon atoms.
similar activity in the initial stage of reaction, but showed were XRD-amorphous and the metal particles were highly
different trends as the function of time. As shown in Fig. 1a, the dispersed on the SiO2 support due to the small content.
CO conversion of RML/SiO2(L) increased slowly aer a slight
N2 adsorption–desorption was carried out to characterize the
decline at the initial stage of reaction. But the selectivity of C2 textural properties of the samples. It can be seen from Table 2
oxygenates decreased gradually followed by the increase in that the supports of SiO2(H) and SiO2(L) kept a similar specic
hydrocarbons selectivity with the time, and the selectivity of C2 surface area, average pore diameter, and pore volume. Upon
oxygenates eventually stabilized at ca. 10% when the reaction being loaded with metal components, there was a slight
time exceeded to 15 h. Because of the decrease of C2 oxygenates decrease in the surface area and pore diameter for both of the
selectivity, the space time yield (STY) of C2 oxygenates decreased catalysts. It is supposed that the loading of metal components
from the original 303.1 g (kgꢀ1 hꢀ1) to 87.4 g (kgꢀ1 hꢀ1) aer did not have major inuence on the textural properties of the
reaction for 15 h. On the other hand, the catalytic activities of supports.
the RML/SiO2(H) catalyst were performed quite stably as the
The IR spectra of SiO2 and the corresponding catalysts in N2
ꢁ
function of time. The CO conversion kept at ꢂ16%, and the atmosphere at 300 C are presented in Fig. 3. According to the
selectivity towards to C2 oxygenates also stabilized at ꢂ56% in previous studies,23,24 the region in the range 3500–2750 cmꢀ1
the whole reaction process. Correspondingly, the STY of C2 with a maximum at 3400 cmꢀ1, originates from the absorption
oxygenates was maintained at ca. 300 g (kgꢀ1 hꢀ1).
of H2O and –OH interacted with hydrogen bond; and the band
The reaction results showed that the catalytic performance at 950 cmꢀ1, arises from the absorption of Si–OH. As shown in
was relatively stable on SiO2(H) supported catalyst, however, the Fig. 3, it is obvious that there were more hydroxyl groups on the
reactivity of the RML/SiO2(L) catalyst changed obviously as the surface of SiO2(L) than that of SiO2(H), suggesting that the
function of time. It is suggested that the changing of ammonia surface Si–OH groups on SiO2(H) condensed more signicantly.
concentration played an important role in the modication of Moreover, it can be found that the IR proles of catalysts did not
the properties of SiO2. Correspondingly, the structure and state change obviously compared with the corresponding supports,
of the supported metals could be different on the two supports, but the intensity of the hydroxyl groups on SiO2 decreased aer
resulting in the different catalytic performance.
supporting metal components, suggesting that the metal
components were xed by the hydroxyl–metal interaction.
3.2. Textural characterization
XRD patterns of supports and the corresponding catalysts
showed no crystalline phases (Fig. 2), indicating that the SiO2
3.3. TEM
Fig. 4 shows the TEM micrographs and the corresponding Rh
particle size distributions of catalysts supported on the different
SiO2. Firstly, as shown in the enlarged photographs in Fig. 4a
and b, both of the particles of SiO2(H) and SiO2(L) were mon-
odispersed spherical with a mean size of ꢂ500 nm. However,
compared with the SiO2(H), the spherical surface of SiO2(L) was
more rough, meanwhile many small particles around the
spheres were observed for the SiO2(L). It is obvious that the
Table 2 Specific surface areas (SBET), pore volume (Vp) and pore
diameter (Dp) of supports and the corresponding catalysts
Sample
SBET (m2 gꢀ1
)
Vp (cm3 gꢀ1
)
Dp (nm)
SiO2(H)
RML/SiO2(H)
SiO2(L)
11.6
10.4
20.8
19.3
0.021
0.021
0.036
0.031
7.9
6.7
7.1
6.3
RML/SiO2(L)
Fig. 2 The XRD patterns of supports and the corresponding catalysts.
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Fig. 3 The IR spectra of supports and the corresponding catalysts: (a)
SiO2(H) (the dashed line) and RML/SiO2(H) (the straight line); (b) SiO2(L)
(the dashed line) and RML/SiO2(L) (the straight line).
concentration of ammonia inuenced the morphology of SiO2.
The high concentration of ammonia restrained the hydrolysis
proceeding, thus the continuous condensation of nuclei
supplied the formation of uniform spheres. In the contrast, the
low concentration of ammonia speeded the hydrolysis rate,
which provided more substrate reservoir for condensation,
resulting in the formation of small particles and spheres with
rough surface.
It can be seen that the Rh nanoparticles were highly
dispersed on the fresh catalysts, and the particle size fell sharp
in the range of 1–4 nm. The mean sizes of Rh particles over RML/
SiO2(H) and RML/SiO2(L) were very close, which were 2.6 and 3.1
nm, respectively. Compared with the fresh catalysts, the mean
size of Rh particles in the RML/SiO2(H)-UD catalyst became
slightly larger (3.5 nm), whereas the mean size of Rh particles in
the RML/SiO2(L)-UD increased strikingly to 8.1 nm. It is inferred
that, compared with the smooth convex surface of SiO2(H), the
rough surface of SiO2(L) looked like a huge amounts of hollows
in the surface, which should be more conducive to the
agglomeration of metal particles in these hollows.
Combined with the catalytic results, it is obvious that the size
of Rh particles has a remarkable inuence on the activity and
selectivity of Rh–Mn–Li/SiO2 for CO hydrogenation. It is
conceivable that, because of the similar Rh particle size of the
fresh catalysts, the initial catalytic performance over the two
catalysts were almost the same. Furthermore, the catalytic
activities of the RML/SiO2(H) catalyst were found to be quite
stable with the time, which should be attributed to the stable Rh
particle size on the SiO2(H) support. In contrast, the mean size
of Rh particles supported on SiO2(L) increased remarkably
during the reaction process, which, in turn, was reected by the
gradual decrease of C2 oxygenates selectivity along with the
increase in hydrocarbons selectivity. As reported in the litera-
ture,25–27 the optimal Rh particle size for the formation of C2
oxygenates is 2–6 nm. Meanwhile, Ojeda et al.28 found that as
Rh particle size increased, the higher hydrocarbons formation
was favored at expenses of the oxygenated compounds. These
Fig. 4 TEM images and Rh particle size distributions of different
catalysts: (a) RML/SiO2(H), (b) RML/SiO2(L), (c) RML/SiO2(H)-UD, (d)
RML/SiO2(L)-UD.
conclusions are consistent with our results. Moreover, it is
noteworthy that although the selectivities of RML/SiO2(L)
changed dramatically as the function of time, but the conver-
sion rate was relatively stable, indicating that CO conversion is
little inuenced by the Rh particle size, consistent with the
suggestion reported by Underwood et al.29 that for Rh/SiO2, the
dispersion of Rh had little effect on the total catalyst activity, but
did strongly inuence product selectivity.
3.4. H2-TPR
Fig. 5 shows the H2-TPR proles for both of the fresh and used
catalysts. It can be seen that there were three peaks of H2
consumption in the TPR prole of the RML/SiO2(H) catalyst. As
reported in previous paper,30,31 the high temperature peak
centered around 320 ꢁC was ascribed to the reduction of MnO2,
ꢁ
ꢁ
and the major peak at 140 C and the shoulder peak at 160 C
were assigned to the reduction of Rh2O3 not intimately con-
tacting with Mn species (denoted as Rh(I)) and of Rh2O3
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Fig. 6 FT-IR spectra of CO chemisorbed at 30 ꢁC: (a) fresh catalysts;
(b) used catalysts.
It is widely accepted that the dicarbonyl species can only be
formed on the Rh+ sites which may be highly dispersed and
linear CO be on the Rh0 sites.33,34 As shown in Fig. 6a, both the
peak intensity of adsorbed CO and the peak area ratio of
CO(gdc)/CO(l) (Rh+/Rh0) were almost the same on the two fresh
catalysts, implying that the state of Rh on both of them was
quite similar. Correspondingly, at the beginning of reaction, the
two catalysts showed similar catalytic properties.
The spectra of the adsorbed CO on the used catalysts in
Fig. 6b showed that the intensity of adsorbed CO decreased
severely compared with the fresh catalysts, suggesting that the
active sites of Rh lost part of activity or were covered partly.
Meanwhile, the peak area ratio of CO(gdc)/CO(l) also decreased
aer reaction. In more detail, the peak area ratio of CO(gdc)/
CO(l) on RML/SiO2(L)-UD become much smaller than that of
RML/SiO2(H)-UD. Considering that the CO(gdc) species can
only be formed on the highly dispersed Rh+ sites, it is concluded
that the Rh supported on SiO2(L) is easier to aggregation
compared with those supported on SiO2(H), which is also
consistent with the result of TEM.
Fig. 5 H2-TPR profiles of the catalysts: (A) RML/SiO2(H) and RML/
SiO2(H)-UD, (B) RML/SiO2(L) and RML/SiO2(L)-UD.
intimately contacting with Mn species (denoted as Rh(II)),
respectively. Compared with the catalyst of RML/SiO2(H), the
reduction peak of Rh(I) over RML/SiO2(L) catalyst shied to the
lower temperature (130 ꢁC). While the peak of Rh(II) became
wide along with a severe shi of the reduction peak of MnO2 to
the lower temperature (220 ꢁC). It is inferred that the rough
surface of SiO2(L) is more conducive to the Rh–Mn interaction.
From the patterns of used catalysts, it can be found that the
reduction of Rh2O3 returned to a single wide peak, and the
reduction peak area decreased obviously than that of fresh
catalysts. Simultaneously, the reduction of Rh and Mn became
more isolated. We inferred that, aer reaction, the part of Rh
and Mn clusters became to aggregate by themselves, resulting
in the weakening of Rh–Mn interaction. On the other hand, the
reduction temperature of Rh2O3 over RML/SiO2(L)-UD catalyst
was much higher than that of RML/SiO2(H)-UD. It is further
proposed that, during the reaction, the Rh and Mn particles are
much easier to agglomerate on the surface of SiO2(L) compared
to that of SiO2(H), which is consistent with the result of TEM.
Combined with the reaction data, it is inferred that the
relatively stable ratio of Rh+/Rh0 is correlated to the steady
catalytic performance of RML/SiO2(H) in the reaction process.
Ichikawa and co-workers35 proposed that Rh0 is active for CO
dissociation and Rh+ is active for CO insertion, and, corre-
spondingly, the changing of ratio of Rh+/Rh0 will change the
3.5. FT-IR
The IR spectra of the in situ reduced catalysts interacting with 80 catalytic properties. Chuang and Pien also suggested that the
mbar CO in vacuum at 30 ꢁC for 30 min are shown in Fig. 6. The presence of isolated Rh+ sites may contribute to the increased
IR spectrum was mainly composed by a band at around 2067 selectivity for higher oxygenates.36 According to the above
cmꢀ1 and a doublet at ꢂ2100 and ꢂ2030 cmꢀ1. The 2067 cmꢀ1 propositions, we inferred that, because the ratio of Rh+/Rh0 on
band can be attributed to the linear adsorbed CO [CO(l)] and the RML/SiO2(L) would decrease markedly during the reaction, the
doublet can be assigned to the symmetric and asymmetric hydrocarbons formation was favored at expenses of oxygenated
carbonyl stretching of the gem-dicarbonyl Rh+(CO)2[CO(gdc)].32 compounds as the function of time.
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3.6. TPSR
Fig. 7 displays the TPSR proles of the samples. It can be seen
that there was one main methane peak at 240 ꢁC in the proles
of the fresh catalysts, both the location and peak area for the
two catalyst were almost the same. It is indicated that the
number and property of active sites responsible for methane
production are equivalent on the fresh catalysts, which can also
be reected by the similar selectivity of hydrocarbons at the
beginning of reaction. In addition, the broad peak centered at
520 ꢁC in the pattern of RML/SiO2(H) might be due to the
hydrogenation of carbon species formed at lower temperatures
and transformed to a less active form.37
The TPSR experiments over the used catalysts are also
showed in Fig. 7 in order to compare with the fresh samples.
Compared with the catalyst of RML/SiO2(H), one peak of CH4
ꢁ
formation at about 240 C also appeared on RML/SiO2(H)-UD,
however, the peak area decreased strikingly. It is inferred that
the state of Rh may not change evidently, but the number of Rh
sites on RML/SiO2(H) decreased aer reaction caused by the
coverage of the high boiling products, which is in line with the
result of IR. On the prole of RML/SiO2(L)-UD, the CH4 peak was
centered at around 275 ꢁC, which shied to the higher
temperature compared to that of RML/SiO2(L). The shiing of
the CH4 peak should be attributed to the aggregation of Rh
particles supported on SiO2(L), which is good agreement with
the results of TEM and TPR. By combining the catalytic prop-
erties of the catalysts and above discussions, it can be suggested
that the stable catalytic properties of RML/SiO2(H) is attributed
to the steady state of Rh; on the contrary, the aggregation of Rh
Fig. 8 The TPSR profiles (the straight lines) and temperature proce-
dure experiment profiles in the flow of H2 without CO pro-absorption
(the dash lines): (A) RML/SiO2(H)-UD; (B) RML/SiO2(L)-UD.
particles supported on SiO2(L) is viewed as an important factor
affecting selectivity.
ꢁ
In addition, a wide peak of CH4 at the range of 300–700 C
appeared on both of the used catalysts. Since the peak area was
too large, it should not be formed by the hydrogenation of
adsorbed CO. In order to conrm where it came from, the
temperature procedure experiment over the used catalysts in
the ow of H2 without CO pro-absorption was used to compare
with the TPSR proles, as shown in Fig. 8. It can be seen that the
proles obtained without CO pro-absorption just appeared one
peak at the range of 300–700 ꢁC, which coincided with the
second peak of TPSR proles of the used catalysts. This obser-
vation clearly indicated that the peak at the range of 300–700 ꢁC
should be formed by the hydrogenation of carbon disposition
on the used catalysts.
4. Conclusions
Two Rh–Mn–Li catalysts supported on the monodispersed SiO2
¨
synthesized by the Stober method with different ammonia
concentration were tested for the synthesis of C2 oxygenates via
CO hydrogenation. The reaction results showed that the cata-
lytic performance was relatively stable on SiO2(H) supported
catalyst, however, the reactivity of the RML/SiO2(L) catalyst
changed obviously as the function of time.
The results proved that the changing of ammonia concen-
tration played an important role in the modication of the
properties of SiO2. Correspondingly, the structure and state of
the supported metals could be different on the two supports.
Fig. 7 TPSR profiles of the catalysts for CH4 formation: (a) RML/
SiO2(H) and RML/SiO2(H)-UD, (b) RML/SiO2(L) and RML/SiO2(L)-UD.
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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
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