New insights into the effects of Mn and Li on the mechanistic pathway for CO hydrogenation on Rh-Mn-Li/SiO2 catalysts

Article abstract of DOI:10.1016/j.molcata.2016.06.018

It has been reported widely that Rh-Mn-Li/SiO₂ catalysts can exhibit high selectivity to C₂⁺ oxygenates during CO hydrogenation, with promoters of Mn and Li playing an important role in this behavior. In this study, a series catalysts of Rh-Mn-Li/SiO₂ with different amounts of Mn and Li were prepared, and the new insights into the effects of Mn and Li on the mechanistic pathway for C₂⁺ oxygenates synthesis from syngas were investigated. The XPS analysis showed that Rh existed mainly as metallic Rh after reduction, however partially positively charged Rh^δ+ atoms appeared on the surface of Mn-containing catalyst due to the interaction of Rh-Mn. The results of H₂-TPR indicated that both of Li and Mn can inhibit the reduction of Rh₂O₃. With the increase in the ratio of Mn/Rh, the most effective interaction, associated to the presence of two reduction centers of Rh₂O₃, was obtained when the ratio of Mn/Rh?=?1. In situ-FTIR was used to probe the effects of Mn and Li on CO absorption and hydrogenation. With regards to the CO adsorption, the doping of Mn can enhance the CO adsorption ability of Rh and weaken the CO-Rh bond strength very effectively when the amount of Mn reached 1.5?wt.%. Improved capacity of CO adsorption is conducive to increase of CO conversion, while the weakening of CO-Rh bond is beneficial to the CO insertion reaction, thus contributing to the generation of C₂⁺ oxygenates. Moreover, the low amount of Li (≤0.075?wt.%) can also enhance the CO adsorption, resulting in the improvement of reactivity. On the other hand, Mn and Li promoted dissociation of H₂, which is favorable to an increase in the rate of hydrogenation. But an opposite effect appeared at high content of Mn (>1.5?wt.%). As the optimized results, when Rh, Mn, Li content was 1.5?wt.%, 1.5?wt.% and 0.075?wt.%, the catalyst for CO hydrogenation of C₂⁺ oxygenates achieved the best performance of C₂⁺ oxygenates synthesis.

Full text of DOI:10.1016/j.molcata.2016.06.018

Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
New insights into the effects of Mn and Li on the mechanistic pathway
for CO hydrogenation on Rh-Mn-Li/SiO catalysts
2
a
a,∗
a
a
a,b,∗
Jun Yu , Dongsen Mao , Dan Ding , Xiaoming Guo , Guanzhong Lu
a
Research Institute of Applied Catalysis, School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
Key Laboratory for Advanced Materials and Research Institute of Industrial catalysis, East China University of Science and Technology, Shanghai 200237,
b
China
a r t i c l e i n f o
a b s t r a c t
It has been reported widely that Rh-Mn-Li/SiO2 catalysts can exhibit high selectivity to C + oxygenates
Article history:
2
Received 14 April 2016
Received in revised form 2 June 2016
Accepted 20 June 2016
during CO hydrogenation, with promoters of Mn and Li playing an important role in this behavior. In
this study, a series catalysts of Rh-Mn-Li/SiO2 with different amounts of Mn and Li were prepared, and
+
the new insights into the effects of Mn and Li on the mechanistic pathway for C2 oxygenates synthesis
Available online 21 June 2016
from syngas were investigated. The XPS analysis showed that Rh existed mainly as metallic Rh after
reduction, however partially positively charged Rh␦ atoms appeared on the surface of Mn-containing
catalyst due to the interaction of Rh-Mn. The results of H2-TPR indicated that both of Li and Mn can inhibit
the reduction of Rh2O3. With the increase in the ratio of Mn/Rh, the most effective interaction, associated
to the presence of two reduction centers of Rh2O3, was obtained when the ratio of Mn/Rh = 1. In situ-FTIR
was used to probe the effects of Mn and Li on CO absorption and hydrogenation. With regards to the CO
adsorption, the doping of Mn can enhance the CO adsorption ability of Rh and weaken the CO-Rh bond
strength very effectively when the amount of Mn reached 1.5 wt.%. Improved capacity of CO adsorption
is conducive to increase of CO conversion, while the weakening of CO-Rh bond is beneficial to the CO
+
Keywords:
Rh-Mn-Li/SiO2 catalysts
CO hydrogenation
+
C2 oxygenates synthesis
Mechanistic pathway
+
insertion reaction, thus contributing to the generation of C2 oxygenates. Moreover, the low amount
of Li (≤0.075 wt.%) can also enhance the CO adsorption, resulting in the improvement of reactivity. On
the other hand, Mn and Li promoted dissociation of H2, which is favorable to an increase in the rate
of hydrogenation. But an opposite effect appeared at high content of Mn (>1.5 wt.%). As the optimized
results, when Rh, Mn, Li content was 1.5 wt.%, 1.5 wt.% and 0.075 wt.%, the catalyst for CO hydrogenation
+
+
of C2 oxygenates achieved the best performance of C2 oxygenates synthesis.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
Over the last thirty years, Rh-based catalysts have been found to
display unique efficiency and selectivity in C2 oxygenates synthe-
sis from syngas [5–9]. Since single Rh component exhibits a poor
activity and mainly leads to formation of hydrocarbon products,
with methanol being the primary oxygenate, efforts have been cen-
tered on improving the dispersion of Rh and modification of Rh with
additives and supports in order to selectively synthesize C2 oxy-
The search for processes to provide alternative feedstock for
fuels has been promoted by the increasing concerns about global
climate change, depletion of fossil fuel resources, and rising crude
oil prices. Ethanol has attracted increasing attention as a clean fuel
or an additive to gasoline [1,2]. Direct catalytic synthesis of C oxy-
2
genates (e.g., ethanol, acetaldehyde and acetic acid) from syngas,
which can be derived from biomass, coal, or natural gas, is one
of the most promising technologies [3,4]. However, developing an
genates [10–12]. So far, the catalyst of Rh-Mn-Li/SiO was found to
2
give an excellent activity and C2 oxygenates selectivity [13], with
promoters of Mn and Li playing an important role in this behavior.
Considerable investigations heretofore have been reported to
interpret the role of Mn and Li. Sachtler et al. [14] concluded that
the improved activity of Mn promoted Rh/SiO2 catalysts was due
to the formation of a tilted CO adsorption mode on the Rh surface.
The tilted adsorbed CO dissociated more easily, thereby increas-
ing the CO hydrogenation activity. Wang et al. [15] proposed that
the interaction between Rh and Mn formed a new active site of
efficient and selective catalyst for C oxygenates synthesis has been
2
a major challenge.
∗ Corresponding authors at: Research Institute of Applied Catalysis, School of
Chemical and Environmental Engineering, Shanghai Institute of Technology, Shang-
hai 201418, China
E-mail addresses: dsmao@sit.edu.cn (D. Mao), gzhlu@ecust.edu.cn (G. Lu).
http://dx.doi.org/10.1016/j.molcata.2016.06.018
1
381-1169/© 2016 Elsevier B.V. All rights reserved.

1
52
J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
0
+
n+
◦
(
Rhx Rhy )-O-M , wherein a portion of the Rh was present in the
bed. Prior to reaction, the catalyst was heated to 400 C (heat-
+
n+
◦
Rh oxidation state, the promoter Mn (M ) was in close contact
with these Rh species, and the formation of this active site was
conducive to the C2 oxygenates synthesis. With respect to Li, it is
believed that addition of Li can restrain CO dissociation, thereby
improving the selectivity of oxygenates [16]. Chuang et al. [17]
reported that the effect of Li promoter was attributed to electron-
donation, which inhibited the hydrogenation ability of the catalyst
and hence promoted the formation of oxygenates. Although a lot of
experiments and reasoning were focused on the interaction mech-
anism among Rh, Mn, and Li, the synergistic promoting roles of Mn
and Li cannot be confined to one of the theories mentioned above
due to the varied experimental conditions and complex reaction
scheme.
In the present study, to further probe the promoting mecha-
nisms of Mn and Li, the catalytic activities of Rh/SiO2 catalysts
promoted with various amounts of Mn and Li for CO hydrogena-
tion were compared. Considering that the catalytic performance
of the Rh-Mn-Li/SiO2 catalyst for the synthesis of C2 oxygenates
from CO hydrogenation was enhanced greatly when a commer-
cial SiO2 was replaced by a monodispersed SiO2 prepared by the
Stöber method [18,19], the monodispersed SiO2 was employed in
serving as the support for Rh-based catalysts. The techniques of XPS
ing rate ∼3 C/min) and reduced with 10% H2/N2 (total flow
rate = 50 mL/min) for 2 h at atmospheric pressure. The catalyst was
◦
then cooled down to 300 C and the reaction started as gas flow was
switched to a H2/CO mixture (molar ratio of H2/CO = 2, total flow
rate = 50 mL/min) at 3 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 (flame ionization
detector) and a TDX-01 column with a TCD (thermal conductivity
detector). The conversion of CO was calculated based on the frac-
tion of CO that formed carbon-containing products according to:
ꢀ
%Conversion = ( niMi/MCO)·100, where ni is the number of car-
bon atom in product i, Mi is the percentage of product i detected,
and MCO is the percentage of carbon monoxide in the syngas feed.
The selectivity of a certain product was calculated based on carbon
ꢀ
efficiency using the formula niCi/ niCi, where ni and Ci are the
carbon atom 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 CuK␣
(ꢀ = 0.15418 nm). The specific surface area (SBET), pore volume (Vp),
and pore diameter (Dp) of sample were obtained by N2 adsorp-
and H -TPR were used to relate to the structure-activity relation-
2
ships of the catalysts. Furthermore, new insights into the effects
of Mn and Li on the mechanistic pathway for CO hydrogenation
were demonstrated by diffuse reflectance infrared Fourier trans-
form spectroscopy (DRIFTS).
◦
tion at −196 C on a Micromeritics ASAP 2020 apparatus. The metal
loadings of the catalysts were determined by ICP-OES (PerkinElmer
Optima 7000DV).
The amount of hydrogen adsorption of various catalysts was
calculated on the basis of H -TPD profiles. For H -TPD measure-
2
. Experimental
2
2
◦
ments, the catalyst (0.1 g) was reduced in-situ for 2 h at 400 C in
2.1. Catalyst preparation
◦
1
0% H /N , and then was held at 400 C for another 30 min before
2
2
being cooled down to room temperature in He flow. The next step
was H2 adsorption at room temperature for 0.5 h, and then the
gas was swept again with He for 3 h. Subsequently, the sample
SiO2 was prepared by the Stöber method [20] as follows. The
mixture solution of 21 mL tetraethylorthosilicate (TEOS) (99.5%,
SCRC) and 50 mL anhydrous ethanol (99.7%, SCRC) was added
◦
was heated in a flowing He stream (50 mL/min) up to ∼500 C at
slowly into the solution of 76 mL NH ·H O (26 vol%, SCRC) and
3
2
◦
a rate of 10 C/min, while the desorbed species was detected with a
2
00 mL anhydrous ethanol. Then, this synthesized solution was
TCD detector. The uptake of H2 was used to calculate Rh metal dis-
persion and particle size, assuming that each surface metal atom
adsorbs one H atom, i.e. H/Rhsurface = 1.
aged for 4 h and separated centrifugally at 7000 rpm. Finally, the
collected product was washed with de-ionized water three times
◦
and dried at 70 C for 12 h. Before used, it was calcined in static air
Photoelectron spectra (XPS) were acquired with an ESCALAB
◦
at 350 C for 4 h.
2
50Xi spectrometer in the pulse-count mode at a pass energy
RhCl3 hydrate (Rh ∼36 wt.%, Fluka), Mn(NO ) ·6H O (99.99%,
3
2
2
of 20 eV using an Al K␣ (hꢁ = 1486.6 eV) X-ray source. Kinetic
energies of photoelectrons were measured using a hemispheri-
cal electron analyzer working in the constant pass energy mode.
SCRC), Li CO (99.5%, SCRC), and SiO2 mentioned above were
2
3
used in catalyst preparations. Catalysts were prepared by co-
impregnation to incipient wetness of silica (1.0 g) with an aqueous
The background pressure in the analysis chamber was kept below
solution of RhCl hydrate and aqueous solutions of precursors of the
3
−9
7
× 10 mbar during data acquisition. The powder samples were
◦
◦
promoters, followed by drying at 90 C for 4 h, and then at 120 C
overnight before being calcined in air at 350 C for 4 h. The specific
pressed into copper holders and then mounted on a support rod
placed in the pretreatment chamber. Samples were reduced in situ
◦
content of various metals are listed in related tables and figures.
For example, the catalyst referred to as 1.5Rh-1.5Mn-0.075Li/SiO2
indicated that the weight percents of Rh, Mn and Li were 1.5 wt.%,
◦
at 400 C for 1 h under 200 mbar H2 pressure. The binding energies
were calibrated relative to the C 1s peak from carbon contamination
of the samples at 284.9 eV to correct for contact potential differ-
ences between the sample and the spectrometer. The XPS data were
signal averaged for 20 scans and were taken in increments of 0.1 eV
with dwell times of 50 ms.
1
.5 wt.%, and 0.075 wt.%, respectively. Elemental analysis by induc-
tively coupled plasma (ICP) revealed good agreement between the
expected and experimental values. In addition, the possible chlo-
ride composition in the catalyst after calcination was also detected
by ICP, and the absence of chloride was confirmed.
H2 temperature-programmed reduction (TPR) was carried out
in a quartz microreactor. 0.1 g of the as-prepared sample was first
◦
pretreated at 350 C in O /N (molar ratio of O /N = 1/4) for 1 h
2
2
2
2
2.2. Reaction
prior to a TPR measurement. During the TPR experiment, H /N2
2
(
molar ratio of H /N = 1/9) was used at 50 mL/min and the temper-
2 2
◦
◦
CO hydrogenation was performed in a fixed-bed micro-reactor
ature was ramped from room temperature to 500 C at 10 C/min
while the effluent gas was analyzed with a TCD.
with length ∼350 mm and internal diameter ∼5 mm. The catalyst
(
0.3 g) diluted with inert ␣-alumina (1.2 g) was loaded between
CO adsorption was studied using a Nicolet 6700 FT-IR spectrom-
eter equipped with a Harrick diffuse reflectance infrared Fourier
quartz wool and axially centered in the reactor tube, with the
temperature monitored by a thermocouple close to the catalyst
transform (DRIFT) cell with CaF windows. Prior to exposure to the
2

J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
153
Table 1
a
Catalytic activities of non-promoted and promoted Rh/SiO2 catalysts .
Catalystb
CO conv. (C%)
TOFc (s−1
)
Selectivity of products (C%)
STY(C2+ Oxy) (g/(kg h))
CO2
CH4
MeOH
AcH
EtOH
C2+ Oxyd
C2+ HCe
1
1
1
1
.5Rh/SiO2
5.7
6.1
11.5
18.9
0.075
0.076
0.125
0.194
21.2
19.1
14.7
1.1
15.2
11.5
14.4
12.1
15.1
10.4
8.2
5.2
8.3
10.8
25.4
8.1
8.8
12.4
27.1
13.3
19.4
24.4
54.3
35.3
39.6
38.3
30.2
24.4
38.9
91.6
.5Rh-0.075Li/SiO2
.5Rh-1.5Mn/SiO2
.5Rh-1.5Mn-0.075Li/SiO2
2.3
309.1
a
b
c
Reaction conditions: P = 3 MPa, Catalyst: 0.3 g, and flow rate = 50 mL/min (H2/CO = 2), data taken after 15 h when steady state reached. Experimental error: ± 5%.
The numbers in the parenthesis indicate the weight percentage relative to the initial weight of the support material.
TOF based on CO conversion and H2 chemisorption.
d
e
+
C2 Oxy denotes oxygenates containing two and more carbon atoms.
+
C2 HC denotes hydrocarbons containing two and more carbon atoms.
Table 2
Effect of Mn loading on CO hydrogenation over Rh-Mn-Li/SiO2 catalyst .
a
Rh/Mnb
CO conv. (C%)
TOF (s
−1
)
Selectivity of products (C%)
STY (C2+ Oxy) (g/(kg h))
CO2
CH4
MeOH
AcH
EtOH
C2+ Oxy
C2+ HC
1.5:0.5
1.5:1.0
1.5:1.5
1.5:2.5
12.0
12.8
18.9
10.1
0.142
0.139
0.194
0.126
3.6
3.6
1.1
8.8
10.1
12.1
12.1
2.6
3.2
2.3
2.3
36.8
30.0
25.4
25.0
17.8
23.2
27.1
15.6
58.5
56.4
54.3
43.5
35.5
37.0
30.2
44.2
203.4
215.1
309.1
123.1
10.1
a
Reaction conditions as above. Experimental error: ± 5%.
b
Weight ratio of Rh/Mn, while the weight ratio of Rh/Li in all the samples = 1.5:0.075.
Table 3
Effect of Li loading on CO hydrogenation over Rh-Mn-Li/SiO2 catalyst .
a
Rh/Lib
CO conv. (C%)
TOF (s
−1
)
Selectivity of products (C%)
STY (C2+ Oxy) (g/(kg h))
CO2
CH4
MeOH
AcH
EtOH
C2+ Oxy
C2+ HC
0
11.5
12.9
18.9
12.5
8.2
0.125
0.132
0.194
0.132
0.087
14.7
6.4
1.1
8.1
8.9
14.4
12.6
12.1
12.3
12.5
8.2
2.5
2.3
2.1
2.5
10.8
20.8
25.4
24.1
23.7
12.4
23.6
27.1
18.7
16.2
24.4
47.1
54.3
43.2
41.9
38.3
31.4
30.2
34.3
34.2
91.6
180.7
309.1
158.6
102.6
1
1
1
1
.5:0.05
.5:0.075
.5:0.15
.5:0.45
a
Reaction conditions as above. Experimental error: ± 5%.
b
Weight ratio of Rh/Li, while the weight ratio of Rh/Mn in all the samples = 1.5:1.5.
reaction gas, the sample in the cell was pretreated in 10% H /N
Mn promoted Rh/SiO2 catalyst (1.5Rh-1.5Mn/SiO ) showed sig-
2
2
2
◦
(
50 mL/min) at 400 C for 2 h, followed by N2 (50 mL/min) flushing
nificant suppression of the formation of CO2 and methanol, but
the selectivities towards ethanol and acetaldehyde were higher
than that of 1.5Rh/SiO2 catalyst. Compared to Rh/SiO2 promoted
singly by Li or Mn, the doubly promoted catalyst 1.5Rh-1.5Mn-
0.075Li/SiO2 combined the positive promoting effects of both Li
at this temperature for 30 min. During cooling down to the room
temperature in N , a series of background spectra were taken at dif-
2
ferent temperatures. Then, 1% CO/N2 (50 mL/min) was introduced
into the cell and the IR spectra at the desired temperatures were
◦
+
recorded. After heating up to 300 C in CO/N2 mixture for 60 min,
and Mn, resulting in the highest CO conversion and C2 oxy-
a H2 flow (1 mL/min) was added into the flowing CO/N , and the
genates selectivity, along with lower selectivities for CO , CH4 and
2
2
IR spectra were recorded as a function of time. Ultrahigh-purity
N , H and CO used in the IR investigations were further purified
methanol. Moreover, the trend in turnover frequency (TOF) values
of CO conversion over the catalysts was consistent with that of CO
conversion.
2
2
by dehydration and deoxygenization. The spectral resolution was
−
1
4
cm with 64 interferograms being added to obtain a satisfactory
The effect of different amounts of manganese promoter on
the catalytic performance of Rh-Mn-Li/SiO2 catalysts is shown in
Table 2. As observed, with the increase of Mn loading, the selec-
tivity to methanol did not change obviously, while the selectivity
of methane increased. Interestingly, although the selectivity of
acetaldehyde decreased with the increase of Mn loading, the CO
conversion and selectivity to ethanol showed a maximum value at
about Rh/Mn ratios of 1.0. It is clear that there exists an optimum
Mn loading at which oxygenate formation will be maximum.
Results of CO hydrogenation over Rh-Mn-Li/SiO2 catalysts with
various loadings of Li are shown in Table 3. As observed, the CO
conversion and selectivity of C2+ oxygenates increased first with
the Li loading. When 0.075 wt.% Li was added into the 1.5Rh-
signal-to-noise ratio.
3
. Results and discussions
3.1. Catalytic performances
Table 1 compares the catalytic activities of the non-promoted
and Mn and/or Li promoted Rh/SiO catalysts for CO hydrogena-
2
◦
tion at 300 C. It can be seen that the 1.5Rh/SiO catalyst showed a
poor catalytic performance for C2 oxygenate synthesis from syn-
2
+
gas. Addition of Mn and Li promoters modified both CO conversion
and product selectivities. As listed in Table 1, adding 0.075 wt.% Li to
1
1
.5Mn/SiO2 catalyst, the selectivity to CO2 decreased largely from
the 1.5Rh/SiO2 catalyst, namely 1.5Rh-0.075Li/SiO , restrained the
2
4.7 to 1.1%, while the CO conversion and selectivity towards
+
formation of CH4 and increased the selectivities of C2 oxygenates
+
C2 oxygenates increased from 11.5 and 24.4% to 18.9 and 54.3%,
respectively. However, the CO conversion and selectivity towards
and hydrocarbons, which indicated that Li addition promoted chain
growth probability. Compared to the non-promoted catalyst, the

1
54
J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
3.3. XPS
XPS studies have been performed to gain information about the
chemical states of Rh and Mn on catalyst surfaces after reduction
◦
at 400 C, and the results are presented in Fig. 2. It is observed that
Rh existed as Rh O3 (typical XPS Rh 3d5/2 at 309.2 eV, not shown)
2
in all the fresh samples, and it is severely reduced after in situ
1.5Rh-1.5Mn-0.075Li/SiO2
pre-treatment (∼307.2 eV), evidencing the presence of metallic
0
rhodium (Rh ). The binding energy of Rh 3d
did not change
/2
5
as the Li promoter was added. However, after the introduction
of an appropriate amount of Mn (<1.5 wt.%), the binding energy
of Rh 3d5/2 shifted towards higher values (307.4–307.7 eV), indi-
cating that the electronic density on Rh particles decreased [21].
1
.5Rh/SiO2
SiO2
␦+
This suggested that some partially positively charged Rh species
also co-exist with Rh0 on the surfaces of the reduced catalysts
containing Mn promoter. Probably the presence of such oxidized
Rh species might be indicative of a strong interaction of some
Rh oxide particles with the assistant of Mn, considering that the
electrons of rhodium atoms can be attracted by the electron-
withdrawing property of Mn. Obviously, this Rh-Mn interaction
would be weakened when the ratio of Mn/Rh exceeded 1.0, cor-
responding to that the photoelectrons binding energy of Rh 3d5/2
over 1.5Rh-2.5Mn-0.075Li/SiO2 catalyst shifted back to ∼307.3 eV.
On the other hand, it can be found that the binding energy of the
Mn 2p3/2 peaks were centered at about 642 eV on various Mn-
containing catalysts after reduction. The XPS spectra around the
Mn 2p3/2 peak between 645 eV and 641 eV cannot be deconvoluted
into various Mn components of the signal (from NIST: MnO = 640.3
1
0
20
30
40
50
60
70
80
o
2
Dgree ( )
Fig. 1. The XRD patterns of support and the representative catalysts.
C2+ oxygenates decreased rapidly when the amount of Li exceeded
.075 wt.%.
0
3
.2. Textural characterization
to 642.5 eV, MnO = 641.1 to 643.4 eV, Mn O = 641.2 to 642.8 eV,
2
2
3
XRD patterns of support and the representative catalysts
Mn O = 641.1 to 641.9 eV) with the current spectra. Thus, it is sug-
3
4
showed no crystalline phases (Fig. 1), indicating that the SiO2 is
XRD-amorphous and the metal particles are highly dispersed on
the SiO2 support due to the small content.
2+
gested that Mn existed in the form of Mn or higher valence states
3+
4+
(
Mn and Mn ).
N2 adsorption-desorption was carried out to characterize the
textural properties of support and the corresponding supported
3.4. H -TPR
2
catalysts. As shown in Table 4, the BET surface area of SiO was mea-
2
Fig. 3 shows the TPR profiles of various catalysts. It can be
2
sured to be 11.0 m /g, and the BET surface areas of all the Rh-based
◦
seen that there was only one peak at 115 C in the TPR profile of
.5Rh/SiO catalyst, which corresponded to the reduction of Rh O
2
catalysts were measured to be ca. 10.0 m /g. For the pore distri-
1
2
2
3
bution, SiO2 and the corresponding catalysts had a similar pore in
the range of 7.5–8.5 nm. No significant difference was observed in
the surface areas and pore sizes probably due to the fact that the
concentrations of Rh and promoters were relatively low in all the
catalysts prepared. Moreover, the amount of hydrogen adsorption
particles [22]. The reduction peak of Rh O moved to a higher tem-
2
3
◦
perature (145 C) and peak area increased pronouncedly with the
addition of 0.075 wt.% Li to 1.5Rh/SiO2 catalyst. The addition of Mn
species into 1.5Rh/SiO led to a split of the reduction peak of Rh O ,
2
2
3
while the reduction peak of manganese species appeared at a higher
of various catalysts calculated on the basis of their H -TPD profiles
◦
2
temperature (∼260 C). According to Ding’s assignment, the split
is summarized in Table 4, and the corresponding Rh dispersion and
particle size are also exhibited. It can be seen that the doping of Mn
and Li slightly improved the Rh dispersion, and, correspondingly,
the Rh particle size decreased.
◦
◦
peaks at 105 C and 135 C can be ascribed to the reduction of Rh O
2
3
not intimately contacting with Mn species (denoted as Rh(I)) and of
Rh O intimately contacting with Mn species (denoted as Rh(II)),
2
3
respectively [22,23]. The TPR profile of 1.5Rh-1.5Mn-0.075Li/SiO2
307.2 eV
Rh3d5/2
Rh3d3/2
642 eV
g
f
Mn 2p3/2
Mn 2p1/2
g
f
e
d
e
d
c
c
b
a
b
a
320
315
310
305
660
655
650
645
640
635
Binding energy (eV)
Binding energy (eV)
Fig. 2. XPS spectra of samples: (a) 1.5Rh/SiO2; (b) 1.5Rh-0.075Li/SiO2; (c) 1.5Rh-1.5Mn/SiO2; (d) 1.5Rh-1.5Mn-0.075Li/SiO2; (e) 1.5Rh-0.5Mn-0.075Li/SiO2; (f) 1.5Rh-2.5Mn-
.075Li/SiO2; (g) 1.5Rh-1.5Mn-0.45Li/SiO2.
0

J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
155
Table 4
Specific surface areas (SBET), pore volume (Vp) and pore diameter (Dp) from N2 adsorption-desorption and H2 chemisorption.
2
3
Samples
SBET (m /g)
Vp (cm /g)
Dp (nm)
H2 chemisorption
H2ads (␮mol/g)
Dispersiona (%)
Particle size (nm)
SiO2
11.0
10.6
10.4
10.1
10.1
10.3
10.2
10.0
10.2
10.2
10.3
0.027
0.027
0.026
0.025
0.025
0.026
0.026
0.024
0.025
0.026
0.025
8.3
8.1
8.0
7.7
7.8
7.9
7.7
7.6
7.8
7.7
7.7
–
–
–
1
1
1
1
1
1
1
1
1
1
.5Rh/SiO2
.5Rh-0.075Li/SiO2
.5Rh-1.5Mn/SiO2
.5Rh-1.5Mn-0.075Li/SiO2
.5Rh-0.5Mn-0.075Li/SiO2
.5Rh-1.0Mn-0.075Li/SiO2
.5Rh-2.5Mn-0.075Li/SiO2
.5Rh-1.5Mn-0.05Li/SiO2
.5Rh-1.5Mn-0.15Li/SiO2
.5Rh-1.5Mn-0.45Li/SiO2
20
21
24
26
22
24
21
26
25
25
27
29
33
35
30
33
29
35
34
34
4.1
3.9
3.4
3.2
3.7
3.4
3.9
3.2
3.3
3.3
a
Assuming H/Rhsurface = 1; experimental error: ± 5%.
Rh(||)
Rh(I)
Mn
Rh(|)
f
Rh(II)
Mn
Mn
e
Rh(|)
Rh(||)
d
c
d
c
b
a
b
a
Rh
100
200
300
400
o
Temperature ( C)
100
200
Temperature
300
400
o
C
Fig. 4. The H2-TPR profiles of the Rh-Mn-Li/SiO2 catalysts with different Mn
and Li amount: (a) 1.5Rh-0.075Li/SiO2; (b) 1.5Rh-0.5Mn-0.075Li/SiO2; (c) 1.5Rh-
Fig. 3. The H2-TPR profiles of the different catalysts: (a) 1.5Rh/SiO2; (b) 1.5Rh-
.075Li/SiO2; (c) 1.5Rh-1.5Mn/SiO2; (d) 1.5Rh-1.5Mn-0.075Li/SiO2.
1Mn-0.075Li/SiO2; (d) 1.5Rh-1.5Mn-0.075Li/SiO2; (e) 1.5Rh-2.5Mn-0.075Li/SiO2; (f)
.5Rh-1.5Mn-0.45Li/SiO2.
0
1
reflected the combined effect of Mn and Li. The TPR profile shape
the 1.5Rh-1.5Mn-0.45Li/SiO2 catalyst, the Rh-Mn interaction was
excessively enhanced, resulting in a clear increase in the peak area
ratio of Rh(II) versus Rh(I) and the movement of the reduction peak
of Mn to lower temperature (215 C) compared to the 1.5Rh-1.5Mn-
0.075Li/SiO2 catalyst.
looked identical to that of 1.5Rh-1.5Mn/SiO , but the reduction
2
peaks shifted to high temperature and the peaks area increased.
These results indicated that the addition of Li can inhibit the
reduction of Rh O , which, in turn, was reflected by the higher
◦
2
3
reduction temperature of Rh O caused by the presence of Li. With
These results suggested that the intensity of Rh-Mn interaction
is related to the Mn/Rh ratios. Increasing the ratio of Mn/Rh, the
Rh-Mn interaction increased firstly, resulting in the presence of
two reduction centers of Rh2O3. However, the Rh-Mn interaction
decreased again when the ratio of Mn/Rh exceeded 1.0, which can
be interpreted by the return of single reduction peak of Rh2O3 in the
TPR profile of 1.5Rh-2.5Mn-0.075Li/SiO2 catalyst. This observation
is also consistent with the result of XPS. On the other hand, it is con-
ceivable that the excess amount of Li (0.45 wt.%) would enhance the
Rh-Mn interaction intensively. By combining the activities of cat-
alysts with TPR, it is proposed that the presence of two reduction
centers of Rh2O3 assisted by the doping of Mn is beneficial for high
CO conversion and ethanol selectivity, but the excessive synergism
of Mn and Rh promoted by excess doping of Li would lead to the
decrease of activity.
2
3
respect to the doping of Mn, the reduction peak of Rh O3 would
2
split into two peaks because of the interaction between Rh and
Mn, suggesting that two reduction centers of Rh O3 would appear
2
by the synergism of Mn and Rh. According to the change of total
area of the profiles, it is obvious that the addition of Mn and Li
can increase the hydrogen consumption of Rh-based/SiO , which
2
suggested that the doping of Mn and Li improves the dispersion of
Rh, resulting in the increase of active centers. Some authors are of
the other opinion [24] that Li can promote the overflow of surface
hydrogen during the reduction process, that is to say, the disso-
ciative hydrogen atoms can be easy to overflow into the support
surface from Rh particles by assist of Li, which increases the H2
consumption.
The TPR profiles of Rh-Mn-Li/SiO2 catalysts with different Mn
and Li loadings are shown in Fig. 4. As the amount of Mn increased
from 0.5 to 1.5 wt.%, the original single reduction peak of Rh split
into two reduction peaks progressively, which have been assigned
to Rh(I) and Rh(II). However, when the loading of Mn reached
3.5. DRIFTS study
Infrared spectroscopy provides an alternative and powerful tool
to study the interaction of CO with catalysts. Firstly, Fig. 5(a)
shows a series of infrared spectra acquired for the in situ reduced
2
.5 wt.%, the reduction of Rh O3 returned to a single peak. More-
2
over, when the Li content was too high (0.45 wt.%), namely in

1
56
J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
Table 5
The frequency and peak area of the adsorbed CO species over different catalysts in Fig. 5(a).
−
Catalysts
Frequencies (cm 1
)
Areas
CO(l)
CO(l)
CO(gdc)
CO(b)
CO(gdc)
CO(b)
1
1
1
1
1
1
.5Rh/SiO2
.5Rh-0.075Li/SiO2
.5Rh-1.5Mn/SiO2
.5Rh-1.5Mn-0.075Li/SiO2
.5Rh-2.5Mn-0.075Li/SiO2
.5Rh-1.5Mn-0.45Li/SiO2
2067
2067
2070
2070
2067
2069
–
–
–
–
–
1850
1815
1850
0.20
0.18
0.22
0.24
0.21
0.21
–
–
0.05
0.21
0.03
0.20
–
–
–
0.23
0.15
0.18
2104,2030
2105,2032
2104,2030
2104,2030
+
gem-dicarbonyl Rh (CO)2 [CO(gdc)] [25]. It is widely accepted that
a
+
the CO(gdc) can be formed on the Rh sites which may be highly dis-
0
persed [27]. This result suggested that two adsorbed sites (Rh and
+
Rh ) appeared by the assisting of Mn, which is consistent with the
0.01
2030
result of H -TPR. On the other hand, the higher carbonyl stretch-
2
2104
0
ing frequency of CO(l) may suggest less back-donation from Rh
6
5
into acceptor ␲*CO orbital, which infers that the Rh0 sites are more
electropositive caused by the electron-withdrawing effect of Mn.
The XPS data characterizing the Mn promoted SiO -supported Rh
2
4
3
2
catalysts also showed the presence of such oxidized Rh species,
consistent with the above inference. Meanwhile, it also can be seen
(Table 5) that the adsorption intensity of CO(l) was enhanced, sug-
gesting that the addition of Mn could improve the dispersion of Rh.
1
Doping of Mn and Li simultaneously (1.5Rh-1.5Mn-0.075Li/SiO ),
2
the intensity of CO(l) and CO(gdc) further increased, and a band at
2200
2100
2000
Wavenumbers(cm )
1900
1800
-1
−1
∼1830 cm appeared, which can be attributed to bridge bonded
CO [CO(b)] [9,28]. However, when the loading of Mn reached
2
.5 wt.%, the position of CO(l) turned back to lower frequency and
the intensity of CO(gdc) decreased clearly, proposing that Rh-Mn
interaction decreased by the excessive loading of Mn, which is
b
in good agreement with the result that the reduction of Rh O3
2
0
.01
returned single peak in the TPR profile of 1.5Rh-2.5Mn-0.075Li/SiO2
catalyst. Moreover, the intensity of CO adsorbed on the 1.5Rh-
6
5
1
1
.5Mn-0.45Li/SiO2 catalyst also decreased compared with that of
.5Rh-1.5Mn-0.075Li/SiO2 catalyst.
Fig. 5(b) shows the IR spectra of adsorbed species on the
◦
in situ reduced catalysts in CO/N2 flow at 300 C. It can be seen
that all the samples only appeared the peaks of gaseous CO and
CO(l), which indicated that the CO(l) species is the mainly reactive
species at the reaction temperature. Compared with the cata-
4
3
2
1
lysts of 1.5Rh/SiO2 and 1.5Rh-0.075Li/SiO , the CO(l) band on
2
the catalysts of 1.5Rh-1.5Mn/SiO , 1.5Rh-1.5Mn-0.075Li/SiO , and
2
2
1.5Rh-1.5Mn-0.45Li/SiO2 was blue shifted. However, when the
amount of Mn reached 2.5 wt.%, the CO(l) band was red shifted
again.
2200
2100
2000
1900
1800
-1
Wavenumbers(cm )
It is obvious that the effect of Mn and Li on the state of Rh parti-
◦
◦
◦
Fig. 5. The infrared spectra of chemisorbed CO at 30 C (a) or 300 C (b) on
1) 1.5Rh/SiO2; (2) 1.5Rh-0.075Li/SiO2; (3) 1.5Rh-1.5Mn/SiO2; (4) 1.5Rh-1.5Mn-
.075Li/SiO2; (5) 1.5Rh-2.5Mn-0.075Li/SiO2; (6) 1.5Rh-1.5Mn-0.45Li/SiO2 after
cles at the reaction temperature is similar to the situation at 30 C.
(
0
Combined with the result of H -TPR, it is suggested that the interac-
2
tion between Rh and Mn changed by the ratio of Rh and Mn. When
the ratio of Mn/Rh reached 1.0, the CO(l) band was shifted to a
higher frequency, owing to the decrease of electron donation of Rh
particles into the ␲*co orbital caused by the electron-withdrawing
effect of Mn. That is to say, the shifted position of adsorbed CO
also can be suggested that the CO-Rh bond strength was weak-
ened by the assisting of Mn. Since the C2+ oxygenates should be
formed by the reaction route-insertion of CO into a metal-CHX
bond [29,30], it is likely that the weakened CO-Rh bond is favor-
able for increased CO insertion, and accordingly the increase of
exposing the in situ reduced catalysts to CO/N2 flow for 30 min.
◦
catalysts in the CO/N2 flow at 30 C. Adsorption of CO on cata-
lyst of 1.5Rh/SiO2 occurred mainly in two distinct modes, phase
CO with characteristic bands at ca. 2180 and 2125 cm , linear
adsorbed CO [CO(l)] with adsorption band at ca. 2067 cm [25].
It is indicated that the catalyst surface are mainly dominated
by Rh0 sites due to the general recognition that CO(l) species is
formed on it [26,27]. The IR spectrum of CO adsorbed on the 1.5Rh-
−
1
−
1
+
C2 oxygenates selectivity, which is good consistent with the cat-
0
.075Li/SiO2 catalyst looked identical to that of CO adsorbed on
the 1.5Rh/SiO2 catalyst. Compared to the catalysts of 1.5Rh/SiO2
and 1.5Rh-0.075Li/SiO , the position of CO(l) adsorbed on 1.5R-
alytic performance. In contrast, the interaction between Rh and Mn
became weakly if the content of Mn exceeded a certain value, then
the CO(l) band was shifted to a lower frequency again. Correspond-
ingly, the C2+ oxygenates selectivity of 1.5Rh-2.5Mn-0.075Li/SiO2
2
1
.5Mn/SiO2 catalyst shifted slightly to higher frequency, and a
−1
doublet at ∼2104 and ∼2030 cm can be noticed, which is assigned
decreased than that of 1.5Rh-1.5Mn-0.075Li/SiO . Moreover, the
2
to the symmetric and asymmetric carbonyl stretching of the

J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
157
1.5Rh/SiO2
2062
1.5Rh-0.075Li/SiO2
2053
o
o
150
2
C
1
00 C
o
2
00 C
o
1
50 C
o
00
3
C
o
o
300 C
3
0
C
o
o
100 C
00
C
o
30
C
0.005
0.005
2200
2150
2100
2050
2000
1950
1900 2200
2150
2100
2050
2000
-1
1950
1900
-1
Vavenumbers (cm )
Wavenumbers (cm )
1.5Rh-1.5Mn-0.075Li/SiO2
1
.5Rh-1.5Mn/SiO2
2065
2064
o
150
C
o
150
C
o
o
100
C
o
200
C
200 C
o
o
1
00
C
o
3
0
C
300 C
o
30
C
o
300
C
0.005
0.005
2200
2150
2100
2050
2000
1950
1900 2200
2150
2100
2050
2000
1950
1900
-1
-1
Wavenumbers (cm )
Wavenumbers (cm )
Fig. 6. The infrared spectra of adsorbed CO as a function of temperature after exposing the in situ reduced catalysts to CO/N2 flow.
excessive doping of Mn and Li can weaken the adsorption ability of
Rh, resulting in the decrease of CO conversion.
order as shown in Fig. 7: 1.5Rh/SiO < 1.5Rh-0.075Li/SiO < 1.5Rh-
2 2
2.5Mn-0.075Li/SiO < 1.5Rh-1.5Mn/SiO < 1.5Rh-1.5Mn-
2
2
A series of infrared spectra of adsorbed CO over the in situ
reduced catalysts in the CO/N2 flow as a function of tempera-
ture are given in Fig. 6. It can be seen that the catalyst surface
has been in the transformation process of adsorption and desorp-
0.45Li/SiO < 1.5Rh-1.5Mn-0.075Li/SiO .
2
2
It is conceivable that, without the promoting effect of promoters,
the dissociation ability of H2 on Rh particles is very weak, result-
ing in a low hydrogenation activity. The addition of Li and/or Mn
was in contact with the Rh particles, which promoted the dissoci-
ation of hydrogen, then increased the consuming of absorbed CO
by hydrogenation. Compared with the 1.5Rh-1.5Mn-0.075Li/SiO2
catalyst, the decreasing rates of CO(l) on the catalysts of 1.5Rh-
2.5Mn-0.075Li/SiO2 were slower. It is suggested that the higher
doping amount of Mn destroyed the fitting Rh-promoter interac-
tion, further restrained the hydrogen dissociation, which is in line
tion. For the catalyst of 1.5Rh/SiO , the amount of adsorbed CO
2
increased as the increase of temperature, then reached a maxi-
◦
mum at 100 C, and decreased by the acceleration of desorption rate
when the temperature further increased. Compared with the non-
promoted catalyst, the desorption rate of adsorbed CO decreased
by the doping of 0.075 wt.% Li, the maximum amount of adsorbed
◦
CO was reached until 200 C. For the 1.5 wt.% Mn-containing cata-
lyst, the desorption rate of adsorbed CO also reduced than that of
with the result of H -TPR. In addition, the higher amount of Li
2
1
.5Rh/SiO catalyst, the maximum peak of adsorbed CO appeared
(0.45 wt.%) in the catalyst can still keep a high rate of hydrogenation.
Based on the IR study, it is concluded that the addition of Mn
and Li have a compositive influence on the CO adsorption and des-
orption, CO insertion, H2 dissociation, and hydrogenation, etc., and
the effects of them change with the doping content.
The results indicated that the doping of Mn can change the Rh
valence and dispersion. With regards to the CO adsorption, these
changes can enhance the CO adsorption ability of Rh and weaken
the CO-Rh bond strength very effectively when the amount of
Mn reached 1.5 wt.%. Improved capacity of CO adsorption is con-
ducive to increase of CO conversion, while the weakening of CO-Rh
bond is beneficial to CO insertion reaction, thus contributing to
the generation of C2+ oxygenates. Moreover, the low amount of
Li (≤0.075 wt.%) can also enhance the CO adsorption, resulting in
the improvement of reactivity. But an opposite effect appeared at
high content of Li (>0.075 wt.%).
2
◦
at 150 C.
It can be inferred that the addition of 0.075 wt.% Li and/or
.5 wt.% Mn restrains the desorption of chemisorbed CO or
1
enhances the adsorption ability of Rh sites at the higher tempera-
ture, resulting in the more chemisorbed CO can join in the reaction
under the reaction temperature. Moreover, according to the inten-
sity of CO(l), compared with the effect of Li, the doping of Mn can
enhance the ability of CO adsorption more obviously, considering
that the addition of Mn could further improve the dispersion of Rh.
As a result, the synergistic promoting effect of Mn and Li can be
revealed on the CO conversion of catalysts as seen in Table 1.
Fig. 7 shows the IR spectra taken after CO reaction with H2 at
◦
3
00 C on the different catalysts. For all the catalysts, the spectra
looked quite similar to the spectra for CO absorbed in the absence
of H2 on the catalytic surfaces, but the intensities and wavenum-
bers of CO(l) decreased with different degrees as a function of time,
ascribable to a decrease in dipole coupling with desorption and the
decrease in surface coverage [31]. It can be seen that the rates of
decrease in the band for CO(l) on different catalysts followed the
On the other hand, Mn and Li promoted dissociation of H , which
2
is favorable to increase the rate of hydrogenation. However, when
the content of Mn exceeded 1.5 wt.%, the promotion effect would
be weakened.

1
58
J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
◦
Fig. 7. The infrared spectra after CO hydrogenation on different catalysts at 300 C.
weak, the catalytic activity should also be restrained. It is usually
considered that the formation of hydrocarbons from CHx hydro-
genation and the formation of C2+ oxygenates from CO insertion
are the couple of competitive reaction, and the relative reaction
rate between them determines the activity and selectivity forwards
+
C2 oxygenates. Thus, considering that the weakening of Rh-CO
Fig. 8. Mechanism for synthesis of C2+ oxygenates from syngas.
band strength is conducive to the reaction of CO insertion, which
should be responsible for the higher C2+ oxygenates selectivity. As
the optimized results, when Rh, Mn, Li content was 1.5 wt.%, 1.5 wt%
and 0.075 wt%, the catalyst for CO hydrogenation of C2+ oxygenates
achieved the best performance.
Generally, the mechanism for synthesis of C2+ oxygenates from
syngas is proposed (see Fig. 8) as follows [29,30]. The adsorbed CO
dissociation and hydrogenation to produce CHx species is likely the
first step, although it remains unclear whether C O bond cleavage
occurs through direct breaking of this bond in an adsorbed CO or
by a process involving hydrogen. The CHx species then undergoes:
4. Conclusion
(
a) formation of C2+ oxygenates precursor of CHxCO by CO inser-
The roles of the loading of Mn and Li promoters in the cat-
alytic performance for the synthesis of C2+ oxygenates from syngas
on Rh-Mn-Li/SiO2 catalysts were explored. The results showed
that, the synergy of Rh-Mn-Li can increase the CO conversion
and C2+ oxygenates selectivity. As the increase of Mn loading, the
tion, or (b) hydrogenation to form methane, or (c) chain growth
to form higher hydrocarbons. According to this mechanism, the
increase of CO adsorption capacity is an important contributor
to CO conversion. Meanwhile, if the hydrogenation capacity is

J. Yu et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 151–159
159
C2+ oxygenates decreased ceaselessly; while the CO conversion
References
had the maximum when the Mn content reached 1.5 wt.% and
decreased rapidly when the amount of Mn exceeded 1.5 wt.%. The
[
[
1] J.J. Spivey, A. Egbebi, Chem. Soc. Rev. 36 (2007) 1514–1528.
2] J.R. Rostrup-Nielsen, Science 308 (2005) 1421–1422.
addition of Li could both improve the CO conversion and C2+
[3] D.H. Mei, R. Rousseau, S.M. Kathmann, V.A. Glezakou, M.H. Engelhard, W.L.
Jiang, C.M. Wang, M.A. Gerber, J.F. White, D.J. Stevens, J. Catal. 271 (2010)
oxygenates selectivity when the doping amount was very low
325–342.
(
0.075 wt.%); and high amount of Li addition (0.45 wt.%) would
ꢀ
[
4] A.E. Farrell, R.J. Plevin, B.T. Turner, A.D. Jones, M. O Hare, D.M. Kammen,
Science 311 (2006) 506.
reverse this effect.
Our results seem to suggest that the changes in catalytic perfor-
mances observed here are due to changes in the catalyst abilities
[5] R.P. Underwood, A.T. Bell, Appl. Catal. 21 (1986) 157–168.
[
6] H. Ehwald, H. Ewald, D. Gutschick, M. Hermann, H. Miessner, G. öhlmann, E.
Schierhorn, Appl. Catal. 76 (1991) 153–169.
of CO adsorption, CO insertion, H dissociation, and hydrogenation,
2
[
7] M. Ojeda, M.L. Granados, S. Rojas, P. Terreros, F.J. García-García, J.L.G. Fierro,
Appl. Catal. A: Gen. 261 (2004) 47–55.
etc., which are mainly controlled by the states of Rh.
The XPS analysis showed that Rh existed mainly as metallic Rh
after reduction. Due to the interaction of Rh-Mn, some partially
positively charged Rh species appeared on the surfaces of the
[8] X.L. Pan, Z.L. Fan, W. Chen, Y.J. Ding, H.Y. Luo, X.H. Bao, Nat. Mater. 6 (2007)
07–511.
9] J. Gao, X. Mo, A.C. Chien, W. Torres, J.G. Goodwin Jr., J. Catal. 262 (2009)
19–126.
[10] S.S.C. Chuang, G. Srinivas, M.A. Brundage, Energy Fuels 10 (1996) 524–530.
5
[
ı+
1
reduced catalysts containing Mn promoter. The results of H -TPR
2
[
[
11] R. Burchand, M.J. Hayes, J. Catal. 165 (1997) 249–261.
12] B.J. Kip, P.A.T. Smeets, J.H.M.C. Van Wolput, H.W. Zandbergen, J. Van
Grondelle, R. Prins, Appl. Catal. 33 (1987) 157–180.
indicated the intensity of Rh-Mn interaction is related to the Mn/Rh
ratios. Increasing the ratio of Mn/Rh, the most effective interaction
of Rh-Mn was reached when the ratio of Mn/Rh = 1.
The results of IR showed that, with a reasonable rate of Rh/Mn,
the addition of Mn can enhance the CO adsorption ability of Rh
and weaken the CO-Rh bond strength very effectively. And the low
doping of Li can not only enhance the CO adsorption capacity, but
[13] P.Z. Lin, D.B. Liang, H.Y. Luo, C.H. Xu, H.W. Zhou, S.Y. Huang, L.W. Lin, Appl.
Catal. A 131 (1995) 207–214.
[
[
14] W.M.H. Sachtler, M. Ichikawa, J. Phys. Chem. 90 (1986) 4752–4758.
15] Y. Wang, H.Y. Luo, D.B. Liang, X.H. Bao, J. Catal. 196 (2000) 46–55.
[16] R. Ugo, Catal. Rev. 11 (1975) 225–297.
[
[
[
17] S.C. Chuang, J.G. Goodwin, I. Wender, J. Catal. 95 (1985) 435–446.
18] J. Yu, D.S. Mao, G.Z. Lu, Q.S. Guo, L.P. Han, Catal. Commun. 24 (2012) 25–29.
19] J. Yu, D.S. Mao, L.P. Han, Q.S. Guo, G.Z. Lu, J. Mol. Catal. A: Chem. 367 (2013)
38–45.
also improve the dissociation of H . The increase of CO adsorption
2
capacity can help to improve the rate of CO conversion, and the
weakened CO-Rh bond strength is beneficial for the CO insertion
[
20] M. Szekeres, O. Kamalin, P.G. Grobet, R.A. Schoonheydt, K. Wostyn, K. Clays, A.
Persoons, I. Dkénáy, Colloids Surf. A: Physicochem. Eng. Aspects 227 (2003)
+
reaction, which promoted the generation of C2 oxygenates. On the
77–83.
other hand, the promotion of H dissociation is favorable to increase
[21] J. Raskó, J. Bontovics, Catal. Lett. 58 (1999) 27–32.
[22] H.M. Yin, Y.J. Ding, H.Y. Luo, H.J. Zhu, D.P. He, J.M. Xiong, L.W. Lin, Appl. Catal.
A: Gen. 243 (2003) 155–164.
2
the rate of hydrogenation. As the optimized results, when Rh, Mn,
Li content was 1.5 wt.%, 1.5 wt.% and 0.075 wt.%, the catalyst for CO
[
23] D.H. Jiang, Y.J. Ding, Z.D. Pan, X.M. Li, G.P. Jiao, J.W. Li, W.M. Chen, H.Y. Luo,
Appl. Catal. A: Gen. 331 (2007) 70–77.
+
hydrogenation of C2 oxygenates achieved the best performance.
[
24] W.M. Chen, Y.J. Ding, D.H. Jiang, G.P. Jiao, H.J. Zhu, Z.D. Pan, H.Y. Luo, Chin. J.
Catal. 27 (2006) 999–1004.
Acknowledgments
[
[
25] S.D. Worley, G.A. Mattson, R. Caudlll, J. Phys. Chem. 87 (1983) 1671–1673.
26] R.R. Cavanagh, J.T. Yates Jr., J. Chem. Phys. 74 (1981) 4150–4155.
The authors gratefully acknowledge financial support from the
Science and Technology Commission of Shanghai Municipality
[27] C.A. Rice, S.D. Worley, C.W. Curtis, J.A. Guin, A.R. Tarrer, J. Chem. Phys. 74
1981) 6487–6497.
(
[
[
28] F. Solymosi, M. Pasztor, J. Phys. Chem. 89 (1985) 4789–4793.
29] M. Ichikawa, Chem. Tech. 12 (1982) 674–680.
(
08520513600), Leading Academic Discipline Project of Shanghai
Education Committee (J51503) and Shanghai Municipal Science
and Technology Commission (13ZR1461900).
[30] Y. Choi, P. Liu, J. Am. Chem. Soc. 131 (2009) 13054–13061.
31] P. Winslow, A.T. Bell, J. Catal. 86 (1984) 158–172.
[