Manganese-promoted Rh supported on a modified SBA-15 molecular sieve for ethanol synthesis from syngas. Effect of manganese loading

Article abstract of DOI:10.1016/j.crci.2010.03.025

A modified mesoporous molecular sieve SBA-15 possessing larger pore diameter, pore volume, and high hydrothermal stability was developed. Loaded with RhCl₃ and Mn(NO₃)₂, the as-prepared SBA-15 performed excellently in catalyzing direct conversion of syngas to ethanol. The highest perimeter of Rh-MnO interface and ethanol yield were reached at the Mn/Rh weight ratio of about 1. High CO conversion, more than 90% in the first three hours, was obtained over the catalyst 7 wt% Rh-7 wt% Mn/SBA-15. Improved catalytic performance is attributed to the increase in the amount of reducible oxide, which resulted in an enhancement of the perimeter of the Rh-MnO interface when the Mn/Rh weight ratio is about 1, and the enhanced ability of rhodium to dissociate and to hydrogenate the carbon monoxide molecule by the addition of manganese.

Full text of DOI:10.1016/j.crci.2010.03.025

C. R. Chimie 13 (2010) 1384–1390
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´
Full paper/Memoire
Manganese-promoted Rh supported on a modified SBA-15 molecular
sieve for ethanol synthesis from syngas. Effect of manganese loading
*
Guochang Chen, Xiaohui Zhang, Cun-Yue Guo, Guoqing Yuan
Beijing National Laboratory for Molecular Sciences, Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A modified mesoporous molecular sieve SBA-15 possessing larger pore diameter, pore
volume, and high hydrothermal stability was developed. Loaded with RhCl3 and
Mn(NO3)2, the as-prepared SBA-15 performed excellently in catalyzing direct conversion
of syngas to ethanol. The highest perimeter of Rh-MnO interface and ethanol yield were
reached at the Mn/Rh weight ratio of about 1. High CO conversion, more than 90% in the
first three hours, was obtained over the catalyst 7 wt% Rh-7 wt% Mn/SBA-15. Improved
catalytic performance is attributed to the increase in the amount of reducible oxide, which
resulted in an enhancement of the perimeter of the Rh-MnO interface when the Mn/Rh
weight ratio is about 1, and the enhanced ability of rhodium to dissociate and to
hydrogenate the carbon monoxide molecule by the addition of manganese.
Received 5 November 2009
Accepted after revision 25 March 2010
Available online 15 May 2010
Keywords:
Rhodium
Manganese
CO hydrogenation
C2 oxygenates
Ethanol
´
ß 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
SBA-15
Syngas
1. Introduction
Developing new types of catalyst to synthesize ethanol
from synthesis gas is very meaningful from both practical
Ethanol has found widespread use as both an energy
carrier and fuel additive. viable route for the
manufacture of ethanol is the direct conversion of syngas
(Eq. (1)), which can be obtained by the gasification of coal
or a renewable resource like biomass.
and theoretical points of view. A growing consensus
regarding ethanol synthesis from syngas is that supported
Rh has a great potential for the reaction, but suitable
supports and promoters are needed to enhance the
reactivity of Rh [1].
a
A
Most previous works have been performed over SiO2-
supported systems [2] and examples of other supports
studied include Al2O3 [3], TiO2 [4], NaY [5], ZrO2 [6] and
CeO2 [7]. The mesoporous molecular sieve SBA-15 with
two-dimensional hexagonally ordered arrays of channels,
discovered by Zhao et al. [8], has attracted much attention
for potential applications as versatile catalysts and catalyst
supports [9], template [10] and separation materials [11]
because of its appealing textural properties and high
surface area, and appreciable thermal and hydrothermal
stability. However, to our knowledge, no detailed study of
the effect of Mn loading on Rh/SBA-15 catalysts on the
activity of ethanol synthesis from syngas has been
reported.
2COðgÞ þ 4H2ðgÞ ! C2H5OHðgÞ þ H2OðgÞ
(1)
D
D
Hꢀ298 ¼ ꢁ253:6 kJ=mol of ethanol
Gꢀ298 ¼ ꢁ221:1 kJ=mol of ethanol
However, methane is the most thermodynamically
favored product (Eq. (2)), and is generally undesirable
economically.
COðgÞ þ 3H2ðgÞ ! CH4ðgÞ þ H2OðgÞ
(2)
D
D
Hꢀ298 ¼ ꢁ205:9 kJ=mol of ethanol
Gꢀ298 ¼ ꢁ141:9 kJ=mol of ethanol
Here, a modified procedure for SBA-15 preparation [12]
giving better reproducibility of the hexagonal porous array
was adopted. The mesoporous SBA-15 was obtained
* Corresponding author.
E-mail address: yuangq@iccas.ac.cn (G. Yuan).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.03.025

G. Chen et al. / C. R. Chimie 13 (2010) 1384–1390
1385
through adding promoter heteropoly acids to the template
and extending crystallization time. Small amounts of
phosphomolybdic acid (HPMo) can significantly enhance
the crystallization process for SBA-15 formation. The reason
for this is that, on the one hand, the interaction between the
octahedral molybdic species and the EO moieties of the
template P123 would be in favor of hydronium ions
associating with them in the low concentration HCl media,
which adds long-range Coulombic interactions to the
coassembly process, thus enhancing the tendency for
mesoscopic ordering to occur. On the other hand, the
assembly of mesoporous SBA-15 may be expected to
proceed through a (S0Moꢁ)I+ synthesis route (where S0 is
the template P123, Moꢁ is the octahedral molybdic species,
and I+ issilicate cation). Consequently, the synthesistime for
the mesoporous SBA-15 formation is shortened and the
corresponding degree of silicate polymerization (i.e., silanol
group condensation) is expected to increase with the same
crystallizationtime, whichwouldresultintheimprovement
of hydrothermal stability [12]. In comparison to the
conventional synthesis method [8], the toxic liquid acids
HCl concentration of reaction mixture solution is only 3.6%
and the total H+ originated from both HCl and heteropoly
acids dissociation is about 4%. The SBA-15 samples prepared
in the modified procedure had a BET surface area of 450–
600 m2/g. In the present research, the effect of manganese
oxide loading on Rh/SBA-15 catalysts for ethanol formation
from syngas is investigated in order to significantly improve
the performance of catalysts for the synthesis of oxygenates
from syngas, and as-prepared catalysts exhibit excellent
activity and selectivity.
for 48 h. The reaction products were filtered, washed, and
dried at 318 K for 48 h. Finally, the samples were calcined
at 823 K in air for 8 h with a slow temperature increase of
1 Kminꢁ1
.
2.1.3. Catalyst synthesis
Catalysts were prepared by impregnating SBA-15 with
an aqueous solution of RhCl3ꢂ3H2O and/or Mn(NO3)2 via
the incipient wetness technique. The impregnates were
subsequently dried in air at 383 K for 2 h. Calcination was
carried out at 773 K for 12 h (heating rate of 10 K minꢁ1).
The rhodium content and the weight percent of manganese
is listed in related tables and figures.
2.2. Catalyst characterization
2.2.1. N2 adsorption-desorption
N2 adsorption-desorption isotherms were obtained at
77 K on an Omnisorp-100CX apparatus (USA). Prior to
analysis, all samples were degassed in high vacuum for 2 h
at 523 K. BET surface areas were calculated from the linear
part of the BET plot. The pore size distribution (PSD) was
calculated by the Barret-Joyner-Halenda model, and the
total pore volumes were estimated from the N2 uptake at P/
P0 = 0.994.
2.2.2. X-ray powder diffraction (XRD)
XRD patterns were recorded on a Rigaku D/max 2550PC
diffractometer (Rigaku, Japan) using Cu Ka radiation,
operating at 40 kV and 300 mA.
2.2.3. Transmission electron microscopy (TEM)
2. Experimental
TEM images were obtained on a JEM-2010HR micro-
scope (JEOL, Japan). Samples for TEM studies were
prepared by dipping a carbon-coated copper grid into a
suspension of mesoporous material in ethanol that was
pre-sonicated for 10 min.
2.1. Materials and catalyst synthesis
2.1.1. Materials
Nonionic triblock copolymer EO20PO70EO20 (Pluronic
P123) was purchased from Aldrich. Phosphomolybdic acid
with principal composition of H3PO4ꢂ12MoO3ꢂ24H2O, was
obtained from J & K Chemical. Tetraethoxysilane (TEOS)
and concentrated hydrochloric acid (c-HCl, 36 wt%) were
provided by Shanghai Chemical Regent Company of China.
Rhodium chloride (RhCl3ꢂ3H2O) was purchased from
Beijing Chemical Agents Plant. Mn(NO3)2 was obtained
from Tianjin Tianhe Chemical Agents Plant.
2.2.4. Temperature programmed reduction (TPR)
TPR experiments were carried out on a Micromeritics
TPD/TPR 2900 apparatus. The catalyst (about 40 mg) was
pretreated under dry air at 383 K for 1 h. The TPR profile
was recorded by heating the sample from room tempera-
ture to 973 K at a rate of 10 K minꢁ1 under a H2/Ar (10%
flow.
v/v)
2.2.5. Temperature programmed surface reaction (TPSR) of
adsorbed CO
2.1.2. Synthesis of SBA-15
A modified procedure for SBA-15 preparation [12]
affording larger pore diameter, pore volume, and higher
hydrothermal stability was adopted. In a typical synthesis
batch, 3.0 g of P123 was dissolved in 78.75 mL of distilled
water with vigorously stirring at room temperature for 3 h,
then 0.48 g of HPMo (the molar ratio of Si/Mo was 12.4)
was added into the solution. After the P123 and HPMo
were dissolved completely, 6.9 mL of TEOS and 0.56 mL of
c-HCl were added dropwise to the above mixture solution.
The chemical composition of the reaction mixture was: 3 g
P123; 0.031 mol TEOS; 0.00021 mol heteropoly acids;
0.0065 mol HCl; 4.37 mol H2O. Subsequently, the mixture
was stirred at 313 K for 24 h and thermally treated at 373 K
TPSR experiments of CO adsorbed were performed
using a U-shaped quartz reactor connected to a Baltzer
Prisma QMS200TM mass spectrometer. The experimental
procedure was as follows: The reactor was loaded with
about 50 mg of sample (0.25–0.30 mm particle size). After
the sample was reduced according to the process described
above, the microreactor was swept with He for 0.5 h and
then cooled down to 323 K. CO was adsorbed by flowing a
CO/He (5% v/v) mixture at 298 K for 30 min. Physisorbed CO
was purged with Ar for 30 min. Finally, the temperature
was linearly increased from 298 to 923 K at a heating rate
of 10 K minꢁ1 under a H2/Ar (10% v/v) flow. The signals of
H2 (m/z = 2), CO (m/z = 28), CH4 (m/z = 16), H2O (m/z = 18),

1386
G. Chen et al. / C. R. Chimie 13 (2010) 1384–1390
and Ar(m/z = 40) were simultaneously recorded with the
mass spectrometer.
also presented the same trend, increasing from 120.6 to a
maximum 508.4 g (kg-cat h)ꢁ1 at Mn/Rh = 1, then decreas-
ing to 182.4 g (kg-cat h)ꢁ1
.
2.3. Catalytic measurements
The effects of flow rate of syngas on the conversion and
selectivity with the 5 wt% Rh-5 wt% Mn/SBA-15 catalyst
are summarized in Table 2. As expected, CO conversion
decreased with increasing flow rate. Obviously, the
product selectivities are also affected. Table 2 illustrates
that increasing syngas flow rate from 60 to 90 cm3 minꢁ1
decreased the average CO conversion from 25.0 to 17.6%
while simultaneously increasing the ethanol selectivity
from 8.7% at a flow rate of 60 cm3 minꢁ1 to a maximum
12.9% at 75 cm3 minꢁ1 and then decreasing to 11.6% at
CO hydrogenation reactions were performed using a
high-pressure fixed-bed microreactor (stainless steel
316#, 200 mm length and 4 mm internal diameter). First,
0.5 g catalyst was reduced in situ at 673 K (heating rate of
1 K minꢁ1) for 16 h in a flow of H2/N2 (1:9) at a rate of
100 cm3 minꢁ1. The reactor was then cooled down and the
gas flow was switched to 75 cm3 minꢁ1 (or 60 cm3 minꢁ1
or 45 cm3 minꢁ1) of syngas (molar ratio H2/CO = 2). The
system was pressurized at 1.0 MPa and heated to the
90 cm3 minꢁ1
.
desired temperatures at a heating ramp of 10 K minꢁ1
.
The effect of Rh loading on the activity and selectivity of
Mn promoted Rh/SBA-15 catalysts (Mn/Rh weight ra-
tio = 1) is shown in Table 3. As expected, CO conversion
over the catalysts increased with increasing Rh loading.
However, ethanol selectivity reached a maximum for
5 wt%Rh-5 wt%Mn/SBA-15, the formation of ethanol being
suppressed at higher Rh loading. Fig. 1 illustrates how the
CO conversion changed with time over a 13-h time
interval. It should be noted that the catalyst 7 wt% Rh-
7 wt% Mn/SBA-15 exhibits excellent activity. CO conver-
sion is more than 90% in the first 3 h, and average CO
conversion is 64.3%, as shown in Table 3. The increased
catalytic performance is attributed to the increase in the
amount of reducible oxide which resulted in an enhance-
ment of the perimeter of the Rh-MnO interface when the
Mn/Rh weight ratio is about 1, and the enhanced ability of
rhodium to dissociate and to hydrogenate the carbon
monoxide molecule by the addition of manganese. This
point will be discussed in following sections.
Temperature was measured with a type-K thermocouple
buried in the catalytic bed. Flow rates were controlled
using a Brooks 5850 TR Series mass flow controllers. All
experimental variables were carefully controlled to ensure
identical reaction conditions when testing catalysts. Some
´
´
recommendations and criteria proposed by Perez-Ramırez
et al. [13] were followed to ensure accurate measurements.
Product analysis was performed online with a gas
chromatograph (GC2014) equipped with TCD and FID
detectors and two in-series fused silica capillary columns:
SPB-5
(60 m ꢃ 0.53 mm)
and
Supel-Q
Plot
(30 m ꢃ 0.53 mm). It was possible to analyze inorganic
gases such as H2, CO and CO2 etc., C1–C20 hydrocarbons,
C1–C10 alcohols, and other oxygenated compounds. The
identification of reaction products and gas chromatograph
calibration were accomplished using gas chromatography-
mass spectrometry (GC-MS) and quantitative standard
solutions from AccuStandard.
3.2. Characterization of the catalysts by XRD
3. Results and discussion
Fig. 2 presents the XRD patterns for the SBA-15-
supported Rh samples with different loading amounts of
manganese. In the case of parent SBA-15, three well
resolved diffraction peaks, which are indexed as the (1 0 0),
(1 1 0) and (2 0 0) reflections, associated with p6 mm
hexagonal symmetry [8], are observed. With increasing
loading of manganese, although peaks (1 1 0) and (2 0 0)
became unobservable, indicating the de-organization at
long range of the mesoporous structure, the peak (1 0 0)
was clearly observed, which indicated that the hexagonal
patterns were still retained on the impregnated catalysts.
3.1. CO hydrogenation results
Manganese loading plays a crucial role in promoting the
yield of oxygenated compounds, as shown in Table 1. Only
a trace of ethanol was produced on the unpromoted Rh/
SBA-15 catalyst, although high CO conversion was
observed. It indicated that a completely promoter-free
rhodium catalyst could not produce ethanol. When the
Mn/Rh ratio increased from 0.5 to 2, the average CO
conversion increased from 8.2 to a maximum 21.9% at Mn/
Rh = 1, then decreasing to 9.3%. The yield of liquid products
Table 1
Effect of Mn loading on Rh-Mn/SBA-15 catalysts for syngas conversion.
Catalyst
Conversion (%)
%Selectivity
Y g/(kg-cat h)
CH4
CO2
CH3OH
C2H5OH
C3H7OH
CH3CHO
C4H9OH
5%Rh/SBA-15
68.9
8.2
36.6
47.2
61.6
53.4
53.3
62.8
26.7
23.8
25.8
20.4
0.4
0.3
1.2
0.8
0.3
0.2
21.5
12.9
19.0
22.2
0.01
4.0
0.4
0.7
3.3
0
0
–
5%Rh-2.5%Mn/SBA-15
5%Rh-5%Mn/SBA-15
5%Rh-7.5%Mn/SBA-15
5%Rh-10%Mn/SBA-15
0.1
0.1
0.2
0.2
0.2
0.1
0.1
0.4
120.6
508.4
368.4
182.4
21.9
19.8
9.3
Nominal conditions are T = 573 K, P = 1 MPa, 0.50 g catalyst, H2:CO = 2:1, syngas flow = 75 cm3(STP)minꢁ1
selectivity = niMi/
.
Conversion (%) =
S
niMi ꢃ 100/MCO and
S
niMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO is the mole percent of
carbon monoxide in the feed. Y implies all the liquid products such as methanol, ethanol, propanol, acetaldehyde, etc. Reaction time: 13 h.

G. Chen et al. / C. R. Chimie 13 (2010) 1384–1390
1387
Table 2
Effects of flow rate on the reactions of syngas over a 5wt% Rh-5 wt% Mn/
SBA-15-HPMo catalyst (reaction time: 13 h).
Condition
1
2
3
T (K)
573
1
573
1
573
1
P (MPa)
H
2 + CO (cm3 minꢁ1
)
60
2:1
75
2:1
90
2:1
H2/CO
Conversion (%)
Selectivities (%)
CH4
25.0
21.9
17.6
58.2
31.5
1
61.6
23.8
1.2
63.2
22.6
1.5
CO2
CH3OH
C2H5OH
C3H7OH
C4H9OH
CH3CHO
8.7
12.9
0.5
11.6
1.0
0.42
0.03
0.1
0.1
0.02
0.1
0.1
3.3. Characterization of the catalysts by N2 adsorption-
desorption isotherms
Fig. 1. CO Conversion over 7 wt% Rh-7 wt% Mn/SBA-15.
Table 4 gives the textural properties of parent SBA-15
and 5 wt%Rh-5 wt%Mn/SBA-15 by the N2 adsorption-
desorption isotherms. Impregnation of SBA-15 with RhCl3
and Mn(NO3)2 produces a decrease in specific surface area
which is attributed to clogging support pores by RhCl3 and
Mn(NO3)2 species that makes them inaccessible for
nitrogen adsorption. This phenomenon was already
observed by Laugel et al. [14]. After RhCl3 and Mn(NO3)2
introduction, the average volume of the catalysts becomes
smaller but the average pore diameter does not change.
These results suggest that the RhCl3 and Mn(NO3)2 species
are probably located inside the mesoporous channels of
SBA-15 rather than dispersing on the wall surface. This is
consistent with the results of Wang et al. [15], who
examined the SBA-15-supported iron phosphate catalyst
for partial oxidation of methane to formaldehyde, and
believed that iron phosphate (FePO4) species with loading
amount lower than 40 wt% were located inside the
mesoporous channels of SBA-15.
Fig. 2. XRD patterns at low diffraction angles: (A) SBA-15, (B) 5 wt%Rh-
0.5 wt%Mn/SBA-15, (C) 5 wt%Rh-2.5 wt%Mn/SBA-15, (D) 5 wt%Rh-
5 wt%Mn/SBA-15 and (E) 5 wt%Rh-7.5 wt%Mn/SBA-15.
3.4. Characterization of the catalysts by TEM
TEM observations further support the encapsulation of
RhCl3 and Mn(NO3)2 species within the mesopores. As
typical example, TEM images of the 5 wt%Rh-5 wt%Mn/
SBA-15 are shown in Fig. 3. The hexagonal array of
mesoporous channels is clearly observed. Moreover, small
clusters encapsulated inside the mesoporous channels can
also be discerned when Figs. 3B and C are compared. No
large particles locating outside the mesopores are ob-
served in the case of 5 wt% Rh-5 wt% Mn/SBA –15 (Figs. 3A
and B).
Table 3
Effect of Rh loading on syngas conversion over Rh-Mn/SBA-15 catalysts (Rh/Mn = 1:1). Reaction time: 13 h.
Catalyst
Conversion (%)
%Selectivity
CH4
CO2
CH3OH
C2H5OH
C3H7OH
1%Rh-1%Mn/SBA-15
3%Rh-3%Mn/SBA-15
5%Rh-5%Mn-/SBA-15
7%Rh-7%Mn/SBA-15
3.4
10.9
21.9
64.2
53.6
74.6
61.6
91.8
43.1
21.8
23.8
3.9
2.0
1.4
1.2
0.2
1.3
2.1
0.2
0.07
0.5
12.9
4.0
0.1
Nominal conditions are T = 573 K, P = 1 MPa, 0.50 g catalyst, H2:CO 2:1, syngas flow = 75 cm3(STP)minꢁ1
selectivity = niMi/
carbon monoxide in the feed.
.
Conversion (%) =
S
niMi ꢃ 100/MCO and
S
niMi where ni is the number of carbon atoms in product i, Mi is the mole percent of product i measured, and MCO is the mole percent of

1388
G. Chen et al. / C. R. Chimie 13 (2010) 1384–1390
Table 4
Porous properties of parent SBA-15 and 5 wt%Rh-5 wt%Mn/SBA-15.
Sample
Surface area (m2 gꢁ1
)
Pore volume (cm3 gꢁ1
)
Pore diameter (nm)
Crystal Structurea
SBA-15
495.2
363.6
1.2
0.9
8.6
8.7
Hexagonal
Hexagonal
5 wt%Rh-5 wt%Mn/SBA-15
a
Crystal structure was confirmed by XRD.
Fig. 3. TEM images of the 5 wt%Rh-5 wt%Mn/SBA-15 (A) and (B), and SBA-15 (C). (A) Taken with the beam parallel to the pore direction and (B) and (C) taken
with the beam perpendicular to the pore direction.
3.5. Characterization of the catalysts by TPR
moted catalyst to 400, 410 and 410 K for the samples with
Mn/Rh = 0.5, 1 and 2 (Fig. 4C, D, E), respectively. At the
same time, a remarkable shift to a lower reduction
temperature of the manganese oxide is observed. The
manganese oxide reduction peaks seem to overlap with
that of the rhodium oxide in the case of 5 wt%Rh-5 wt%Mn/
SBA-15 (Fig. 4C). The ability of a noble metal oxide to
accelerate the reduction of a second surface metal oxide
species with which it is in intimate contact has been
reported previously [18]. Metallic rhodium provides sites
for hydrogen adsorption and dissociation with extremely
low activation energy. If MnOx particles are close enough to
the metal sites, their reduction by atomic hydrogen takes
place at a lower temperature than if molecular hydrogen
has to interact with MnO2, and hence the reducibility of Mn
Fig. 4 shows TPR profiles for the catalysts investigated.
The Rh-free Mn-supported SBA-15 sample exhibits two
small reduction peaks at about 456 and 783 K, which can
be ascribed to the reduction sequence MnO2 ! M-
Mn3O4 ! MnO [16]. This is based on thermodynamic
arguments and experimental evidence from the literature
[17]. The Rh/SBA-15 catalyst without promoter gives a
reduction peak centered at 370 K, which corresponds to
the reduction of Rh3+ to give metallic rhodium. The
addition of the Mn promoter changed the reducibility of Rh
and Mn oxides. Upon increasing the manganese loading,
the reducibility of the rhodium was decreased, shifting the
maximum of reduction peak from 370 K for the unpro-

G. Chen et al. / C. R. Chimie 13 (2010) 1384–1390
1389
Fig. 5. Methane formation spectra during TPSR of adsorbed CO under H2
flow over catalysts: (A) 5 wt%Rh/SBA-15, (B) 5 wt%Rh-2.5 wt.%Mn/SBA-
15, (C) 5 wt% Rh -5wt%Mn/SBA-15, (D) 5 wt%Rh-10 wt%Mn/SBA-15.
Fig. 4. TPR profiles: (A) 5 wt%Mn/SBA-15, (B) 5 wt%Rh/SBA-15, (C) 5 wt
%Rh-2.5 wt%Mn/SBA-15, (D) 5 wt%Rh-5 wt%Mn/SBA-15 and (D) 5 wt%Rh-
10 wt%Mn/SBA-15.
oxides is enhanced by the close proximity of Rh [16]. These
findings clearly support that a close interaction between
Rh and Mn oxides existed in our catalysts.
observation was that the temperature at which methane
started to be formed and the corresponding maximum
were shifted toward lower temperatures with increasing
promoter loading. The methane formation peak shifted
from 490 to 450, 440 and 430 K for Mn/Rh = 0.5, 1 and 2.
This indicates that the addition of manganese increased
the ability of rhodium to dissociate the CO molecules,
which can strongly affect catalytic performance. This is
consistent with the literature [20]. It can be explained by
taking into account electronic effects. According to the
well known model of CO adsorption over metal surfaces
first postulated by Blyholder [21], the carbon monoxide-
to-metal bond is considered to result from a charge
With increasing manganese loading, the H2 consump-
tion increased to maximum at Mn/Rh = 1, and then
decreased. The enhancement of H2 consumption
(Fig. 4D) was related to the increase in the amount of
reducible oxide [19], which resulted in an enhancement of
the perimeter of the Rh-MnO interface responsible for CO
insertion [1]. It was one of reasons for the increased
catalytic performances when the Mn/Rh weight ratio is
about 1. This can be explained by taking into account two
aspects: the amount of promoter added and the size of the
MnO particles deposited. When the Mn content is low, the
MnO particles deposited over the rhodium surface are
rather small, and any increase in promoter loading would
be reflected as an enhancement of the perimeter of the Rh-
MnO interface, thus increasing the number of active CO
insertion sites. However, when the Mn/Rh ratio is higher
than 1, large MnO patches are deposited over the rhodium
surface, and the Rh-MnO interface perimeter is dimin-
ished. So there exists an optimum promoter loading, that is
Mn/Rh (weight ratio) = 1, at which the Rh-MnO interface
has the highest perimeter.
transfer from the HOMO of CO (5
d orbital of the metal and an electron back-donation from
an occupied symmetry d orbital of the metal into the
unoccupied 2 * antibonding orbital of CO. This back-
s) into a free ssymmetry
p
p
donation to the CO antibonding orbital weakens the CO
bond. As an electron acceptor, the oxophilic promoter Mn
ions coadsorbed CO with Rh on the same surface [20],
therefore the back-donation to the antibonding molecular
orbital is enhanced and, in turn, the ability of rhodium to
dissociate the CO molecule is increased. The second
observation was the increase in the methane peak area
from B to D. In principle, it would be expected that
increasing the amount of promoter could partially block
the rhodium surface, thereby diminishing the number of
active sites. Accordingly, the increment in the CO
dissociate would be due to a higher catalytic activity
per rhodium site, i.e., a higher turn over frequency.
Sachtler and Ichikawa [20] believed that on the Rh-
promoter interface the adsorbed CO could be readily
dissociated. This effect became stronger as the Mn/Rh
weight ratio increases.
3.6. Characterization of the catalysts by TPSR
The reactivity of chemisorbed CO under H2 flow
provides useful information about the ability of rhodium
to dissociate and to hydrogenate the carbon monoxide
molecule. Since the hydrogenation of carbon into methane
is very fast on Rh-Mn based catalysts, methane production
can be used as a tool for measuring the CO dissociation over
such catalysts. Fig. 5 shows the production of CH4 when the
hydrogenation of chemisorbed CO is carried out from room
temperature to 900 K.
So the increased ability of rhodium to dissociate the CO
molecules by the addition of manganese was another
reason for the improved catalytic performances of Rh-Mn/
SBA-15.
Two effects were observed when manganese was
added to the unpromoted Rh/SBA-15 catalyst. The first

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G. Chen et al. / C. R. Chimie 13 (2010) 1384–1390
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4. Conclusions
(b) D. Jiang, Y. Ding, Z. Pan, X. Li, G. Jiao, J. Li, W. Chen, H. Luo, Appl.
Catal. A 331 (2007) 70.
The modified SBA-15 molecular sieves, support for
manganese-promoted Rh catalysts (Mn/Rh = 0, 0.5, 1 and 2,
weight ratio), were used in the synthesis of ethanol from
syngas. The optimum Mn/Rh ratio was found to be around
1 for ethanol formation. The CO conversion is more than
90% in the first three hours over the catalyst 7 wt% Rh-
7 wt% Mn/SBA-15. The improved catalytic performance is
attributed to the increase in the amount of reducible oxide,
which resulted in an enhancement of the perimeter of the
Rh-MnO interface when the Mn/Rh weight ratio is about 1,
and the enhanced ability of rhodium to dissociate and to
hydrogenate the carbon monoxide molecule by the
addition of manganese.
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