A comparison study on carbon dioxide reforming of methane over Ni

A comparison study on carbon dioxide reforming of
methane over Ni catalysts supported on mesoporous
SBA-15, MCM-41, KIT-6 and -Al2O3
Mohamad Hassan Amin, James Tardio, Suresh K. Bhargava
Centre for Advanced Materials & Industrial Chemistry, School of Applied Sciences,
RMIT University, Melbourne, VIC 3001, Australia
Email: [email protected]
Abstract— The activity of Ni supported on mesoporous
SBA-15, MCM-41, KIT-6, and a sol-gel prepared Ni/Al2O3, for catalysing methane dry reforming was
investigated. The chemical and physical characteristics of
the catalysts before and after catalytic testing were
investigated using X-Ray diffraction, X-ray Photoemission
Spectroscopy, Transmission Electron Microscopy,
Scanning Electron Microscopy / Energy Dispersive X-ray
analysis, Thermogravimeteric Analysis, Temperature
Programmed Reduction techniques and N2 adsorptiondesorption isotherms. Support type was found to have a
significant influence on catalytic activity and stability.
Among all the supported catalysts tested, the Ni/SBA-15
exhibited excellent catalytic performance in terms of
conversion and long-term stability. The activity for the
silica framework catalysts correlated strongly with the
surface area and pore diameter of these materials with the
degree of CH4 and CO2 conversions observed increasing
with decreasing surface area and increasing pore
diameter.
Keywords- CO2 reforming of methane; Ni/SBA-15; Ni/Al2O3; Ni/KIT-6; Ni/ MCM-41.
I.
INTRODUCTION
Catalytic reforming of methane with carbon dioxide, also
known as dry reforming (CH4(g) + CO2(g) ↔ 2CO(g) + 2H2(g) ),
is a promising process for simultaneous chemical
transformation of two major greenhouse gases, CO 2 and CH4,
to syngas, which can be used in a variety of downstream
processes such as methanol production, Fischer-Tropsch
synthesis processes and in carbonylation, hydrogenation, and
hydroformylation processes [1, 2]. In recent years the main
aim of research on CO2 reforming of methane has been the
development of improved materials for catalysing this
reaction. Different types of catalysts have been employed for
dry reforming but from an industrial standpoint, Ni based
catalysts are considered to be the most promising as they
exhibit high catalytic activity [4-7], are readily available, and
cost effective.
Ni/SiO2 and Ni/Al2O3 have been two of the most often
investigated catalysts for dry reforming of methane since 1979
[8] and 1980 [9] respectively. This interest has been mostly
due to SiO2 and γ-Al2O3 (with melting points of 1973C and
2318C [10] respectively) having high mechanical strength
and relatively low cost. Recently, three types of siliceous
materials with different structural features have been
investigated as support materials for Ni dry reforming
catalysts.These materials, MCM-41, SBA-15 and KIT-6 are
silica-based mesoporous materials with different pore
diameter and surface area. In 1992, C. T. Kresge and
coworkers [11] synthesized MCM-41 mesoporous molecular
sieves by a liquid-crystal template mechanism. These
materials presented an opportunity for the design of
catalytically active sites inside uniform channels with
controllable nano-size pore diameters. In 2009, Liu et al.
employed MCM-41 as support and prepared a Ni/MCM-41
catalyst for dry reforming of methane [12]. They reported that
the catalyst had good catalytic activity, high stability and
produced syngas with reasonable H2/CO ratios.
In 1998, B. F. Chmelka and G. D. Stucky et al. [13]
reported the use of amphiphilic triblock copolymers to direct
the organization of polymerizing silica species resulted in the
preparation of well-ordered hexagonal mesoporous silica
structures (SBA-15). SBA-15 is a type of ordered mesoporous
silica with large pore diameter, thick wall, and good
hydrothermal stability. In 2006, JI Shengfu and LI Chengyue
et al. [14] investigated dry reforming of methane using a
Ni/SBA-15 catalyst and reported that the 12.5% Ni/SBA-15
catalyst showed high activity and good stability at 800C.. In
2012, W. Yang and Z. Liu et al. [19] used a kind of Ni
containing mesoporous silica, Ni-KIT-6 as a dry reforming
catalyst. .
In this work, a comparison study on methane dry reforming
with carbon dioxide over Ni supported on mesoporous SBA15, MCM-41, KIT-6 and sol-gel prepared -Al2O3 has been
investigated. This involved evaluation of CH4 and CO2
conversions using a custom built catalysis rig. Activity and
stability results were compared and are discussed with regards
to the characterisation data obtained.
II.
EXPERIMENTALS
A. Catalyst preparation
Ni/-Al2O3 containing 15wt% Ni was prepared using a solgel method as reported earlier [1]. SBA-15 and KIT-6 were
prepared using tetraethyl orthosilicate (TEOS) as silica
sources and triblock copolymer P123 (Pluronic P123) as
surfactant for both support materials. MCM-41 was prepared
using hexadecyl trimethylammonium bromide
C16H33(CH3)3NBr (CTMABr) as surfactant and sodium silicate
solution (27 wt.% SiO2, Aldrich) as the silicate source. The
support materials were collected after calcining at 540 °C for
6 h in flowing air.
The Ni(15 wt%)/silica based catalysts were prepared via
wetness impregnation. .The SBA-15, MCM-41 and KIT-6
supports were impregnated with aqueous solutions containing
appropriate amount of nickel nitrate at room temperature
overnight, followed by drying at 110C for 12 h and
calcination at 550C for 5 h.
B. Characterization
Materials were characterised using the following techniques
/ instruments – surface area Micromeritics ASAP 2010 surface
area analyser and thermogravimeteric Analysis (TGA). The
temperature-programmed reduction (TPR) of the calcined
catalysts was undertaken in a quartz reactor (I.D. 4.5 mm),
packed with 50 mg catalyst in a flow (20 mL.min-1) of H2-Ar
mixture (4.14 mol% H2) from 250 oC to 880 oC at a linear
heating rate of 10oC.min-1. The H2 consumed in the TPR study
was measured quantitatively by TCD. Before the TPR study,
the catalyst was pretreated at 500 oC for 1h under a flow (20
cm3 min-1) of argon.
C.
Catalytic activity testing
Catalytic CO2-reforming of methane was carried out in a
fixed-bed continuous flow quartz reactor at atmospheric
pressure at a gas hourly space velocity (GHSV) of 52,000
mLg−1h−1. In the current study, the activity and stability runs
were performed at 700 C. All the catalysts were pre-reduced
in situ in a mixed flow of H2 and He (10:40 mL min-1) at
700C for 2h. Then the reactant gases (CO2 and CH4) were fed
(CO2: CH4: He = 1:1:2) into the reactor. The effluent mixed
gases were cooled in an ice-water trap to remove gaseous
water generated via the reverse water gas shift (RWGS)
reaction. The product gas mixture was analysed by on-line gas
chromatography (Shimadzu GC, 17A) equipped with a silica
packed column and a TCD. The time-on-stream activity and
durability of the catalysts were carried out at 700 C for 20 h.
The catalytic properties have been evaluated in term of the
conversion of CO2 and CH4. After the durability test, the
catalyst was removed and thermogravimeteric Analysis (TGA)
(Perkin-Elmer TGA 7) was used to calculate the amount of
deposited carbon.
The effects of residence time (feed flow rate (gas film
thickness)) and particle size were determined experimentally
with a feed mixture composed of CO2: CH4: He (=1:1:2 vol. %)
at 700 C. It was found that methane conversion was
independent of average particle size, within a particle size
range of 500-710 microns. Therefore mass transfer limitations
did not occur under the aforementioned conditions. The
potential influence of external diffusion limitations was also
investigated by measuring the activity as a function of the
catalyst loading at constant W/F ratio (W= catalyst mass, F =
total flow of feed gas). It was found that conversion was not
affected by varying catalyst load and the flow, i.e., at constant
contact time. Thus, external mass transfer limitations do not
occur under the conditions applied.
III.
RESULTS AND DISCUSSIONS
A. Characterization
The textural characteristics of the prepared mesoporous
materials are summarized in Table 1. All samples exhibited
type IV nitrogen adsorption-desorption isotherms with H1
shaped hysteresis loops (not shown), which are characteristic of
materials having uniform “cylindrical shaped” pores in the
framework [20].Three distinct regions can be discerned in the
isotherms for these mesoporous materials: (1) monolayer–
multilayer adsorption of nitrogen on the pore walls at low
pressures, (2) a sudden steep increase of the adsorbed nitrogen
at intermediate pressures due to capillary condensation, and (3)
multilayer adsorption on the external surface of the particles.
The existence of capillary condensation at higher relative
pressures in SBA-15 relative to KIT-6 and MCM-41 samples is
consistent with the larger pore diameters of the SBA-15. The
broad hysteresis loop observed for the SBA-15 was most likely
caused by the presence of long mesopores connected by
smaller micropores, thus hampering the filling and emptying of
the accessible volume [15]. After introducing Ni, an obvious
decrease in the BET surface area, pore volume and pore
diameter is observed. The introduction of Ni causes local
blockage of pore channels by depositing Ni species at the pore
entrance and/or partial structural degradation [15].
The reducibility of the nickel in the prepared catalysts was
studied using temperature-programmed reduction (TPR).
Figure 1 shows the TPR profiles of the catalysts and a nonsupported NiO for comparison.
Table 1. Textural properties of different samples
Nickel content
Surface area (m2/g)
Pore volume (cm3/g)
(wt.%)
0
142
0.25
0
165
0.53
0
900
1.22
0
1355
1.14
15
124
0.24
15
161
0.40
15
463
0.38
15
1392
0.64
Support
-Al2O3
SBA-15
KIT-6
MCM-41
-Al2O3
SBA-15
KIT-6
MCM-41
Pore diameter (nm)
7.81
13.95
6.05
4.16
8.17
10.5
4.04
2.96
Reference sample of NiO
TCD signal (a.u.)
a
200
250
300
350
400
450
500 550 600 650
Temperature (C)
700
750
800
850
TCD signal (a.u.)
b
Ni/-Al2O3
Ni/SBA-15
Ni/MCM-41
Ni/KIT-6
200
250
300
350
400
450
500 550 600
Temperature (C)
650
700
750
800
Figure 1. TPR profile of 15 wt.% Ni catalysts on different support.
850
900
The TPR profile for NiO shows that reduction of the Ni(II)
started at ~240 C and reached a maximum rate at ~400C. The
TPR profile of the Ni/-Al2O3 catalyst consisted of two clear
reduction temperature zones. The low temperature zone (370
– 570oC) and a high temperature zone (600 – 900oC) represent
the reduction of NiO and non-stoichiometric nickel aluminate
in Ni/-Al2O3 respectively [1]. Based on the amount of H2
consumed in the two temperature zones the majority of the
oxidised Ni in Ni/-Al2O3 catalyst presents in the form of a
non-stoichiometric nickel aluminate. All the SiO2 based
catalysts exhibit similar hydrogen reduction profiles showing a
reduction peak at the low temperature zone which implies the
presence of NiO particles. However, the reduction of Ni(II) in
the Ni/KIT-6 and Ni/MCM-41 catalysts was slightly higher
than that observed for the Ni/SBA-15 catalyst. This is most
likely due to the arrangement of Si–OH on the inner surface is
relatively regular for Ni/KIT-6 and Ni/MCM-41 while the pore
surface of Ni/SBA-15 is rough with a substantial amount of
Si(OH)2 groups associated with surface defects [15].
B. Catalytic activity for dry reforming of methane
The ability of the supported nickel materials to catalyse the
dry reforming of methane was investigated at 700 °C using a
GHSV of 5.2 × 104 mLg−1h−1 for a period of 20 h. The results,
in terms of CH4 and CO2 conversion, from the aforementioned
tests are presented in Figures 2a and b. From the results
presented in Figures 2a and b it can seen that the Ni/SBA-15
catalyst was clearly more active in terms of CH4 and CO2
conversion than all of the other materials tested. This catalyst
was able to catalyse CH4 conversions between 84.5 and 85.1
% and a CO2 conversion of ~92.4 % throughout the entire
testing period studied. The CH4 conversion approached the
conversion expected for the system at thermodynamic
equilibrium which is 91.5% [17] (CH4/CO2=1, atmospheric
pressure and 700C). The CO2 conversion was higher than
that expected if the system was at thermodynamic equilibrium
(66.3%) [21], due to the influence of the reverse water gas
shift (RWGS) reaction, which has been reported to occur
under the experimental conditions used in this study [22].
CO2 + H2  CO + H2O
(RWGS)
Of the other catalysts tested (Ni/-Al2O3, NiKIT-6 and
Ni/SBA-15) the Ni/MCM-41 was clearly the least active
material. After 20 h time on stream, the activity sequence in
terms of CH4 conversion was as following: Ni/SBA-15
(84%) > Ni/KIT-6 (77%) > Ni/-Al2O3 (67%) > Ni/MCM-41
(51%).The trends observed in activity for the SiO2 based
catalysts correlated strongly with the surface area and pore
diameter of these materials (Figure 3a and b) with the degree
of CH4 and CO2 conversions observed increasing with
decreasing surface area and increasing pore diameter. Similar
behavior was observed for PdNi/MCM-41 [23], Ni(5wt%)/
SBA-15 and Ni(5wt%)/MCM-41 catalysts [24] where
catalysts exhibiting higher surface area were less active.
Based on these results it can be concluded that catalyst
activity in these materials is also strongly influence by other
factors, such as pore diameter active metal environment and /
or morphology [23-27].
The H2/CO ratios obtained at 20h are shown in Table 2.
From the data given in Table 2 it can be seen that the H2/CO
ratio did not equal one (as predicted based on the
stoichiometry of the overall dry reforming reaction). This
however was expected as thermodynamics predicts a H2/CO
ratio of unity only at temperatures above 800C [28]. The
H2/CO ratios that were obtained have a similar pattern to the
activity trends observed for the different supports. The
Ni/MCM-41 catalyst produced a product gas with a low
H2/CO ratio (0.83) which suggests that the reverse water gas
shift (RWGS) easily occurs on this catalyst, which results
from the presence of more unreacted CO2 under low catalytic
activity [15], whilst the Ni/SBA-15 catalyst produced
synthesis gas with the highest H2/CO ratios of 0.97. This high
H2/CO ratio in the Ni/SBA-15 catalyst was most likely due to
reduced water gas shift reaction occurring which may have
been due to more rapid conversion of CO2 on Ni/SBA-15. In
terms of stability all of the catalysts were relatively stable over
the time frame tested (20 h) except for the Ni/KIT-6 catalyst
which had a gradually reduced activity during the reaction.
The amount of carbon deposited on the spent catalysts
(which is expressed in Table 2 in terms of carbon deposition
rate) was determined using TGA. Carbon deposition over the
spent catalysts after 20 h time on stream was quantified by TG
analysis. As shown in Table 2, the rate of carbon deposition on
the catalysts was as follows: Ni/KIT-6 (0.0002) < Ni/ MCM41 (0.0003) < Ni/SBA-15 (0.0005) < Ni /-Al2O3 (0.0011). It
is evident that the carbon deposition rate was lowest when
using the Ni/KIT-6 catalyst and highest when using the Ni /Al2O3 catalyst. No clear trend in carbon deposition rate was
observed for the materials tested. Interestingly the amount of
carbon deposited on the Ni/KIT-6 catalyst, which underwent a
gradual loss in activity in terms of CH4 and CO2 conversion,
was lower than the amount deposited on the other catalysts
which displayed stable activity up to 20 h of testing. This
result indicates that the amount of carbon deposited was either
most likely not solely responsible for the significant loss of
activity observed and that it may have been due to the type(s)
of carbon deposited and / or the location of the carbon
deposition [1].
Table 2. H2/CO ratios and rate of carbon deposition on spent 15 wt.% Ni catalysts on different support.
Support
SBA-15
KIT-6
MCM-41
-Al2O3
Carbon deposition rate (gc.g-1cat. h-1)
0.0011
0.0005
0.0002
0.0003
H2/CO ratios
0.90
0.97
0.93
0.83
90
100
Ni/KIT-6
70
Ni/-Al2O3
60
50
Ni/MCM-41
CO2 conversion (%)
CH4 conversion (%)
Ni/SBA-15
Ni/SBA-15
80
90
Ni/KIT-6
80
Ni/-Al2O3
70
60
Ni/MCM-41
50
40
0
2
4
0
6 8 10 12 14 16 18 20
Time on stream (h)
2
(a)
4
6
8 10 12 14 16 18 20
Time on stream (h)
(b)
Figure 2. The effect of support on CH4 (a) and CO2 (b) conversion over Ni catalysts at 700 C and at GHSV of 52,000 mL h-1g-1.
100
90
Conversion (%)
Conversion (%)
100
80
70
60
80
70
60
50
50
40
40
2
4
6
8
10
Pore diameter (nm)
90
500
1000
Surface area (m2/g)
1500
100
CO2 conversion % at 20 h time on stream
CH4 conversion (%)
(a)
0
12
100
CH4 conversion (%)
90
(b)
90
CH4 conversion % at 20 h time on stream
80
80
Figure 3. CH4 and CO2 conversions (%) versus (a) pore diameter and (b) surface area of SiO2 supported Ni.
70
60
CONCLUSIONS
Three different mesoporous molecular sieves SBA-15,
50 MCM-41 and -Al2O3 were used to support Ni and
KIT-6,
40
70
catalyse CO2 reforming of CH4. Based on the results obtained
60
the catalytic conversion of CH4 using supported Ni is highly
dependent on the nature of support. Among all the supported
50
catalysts tested, Ni/SBA-15 exhibited excellent catalytic
40
2
4
6
8
10
Pore diameter (nm)
(a)
12
0
500
1000
Surface area (m2/g)
(b)
1500
performance in terms of catalytic conversion and long-term
stability. The activity for the SiO2 based catalysts correlated
strongly with the surface area and pore diameter of these
materials with the degree of CH4 and CO2 conversions
observed increasing with decreasing surface area and
increasing pore diameter. In summary, the advantages of good
structural stability and the unique pore structural properties of
SBA-15 make this a promising catalytic support material for
Ni for use in the dry reforming of methane.
ACKNOWLEDGMENT
The authors thank Dr Selvakannan Periasamy for his
helpful discussion, Dr Jarrod Newnham and Dr. Samuel
Ippolito for their help in automating the reactor system.
The authors gratefully acknowledge the facilities, and the
scientific and technical assistance, of the Australian
Microscopy & Microanalysis Research Facility at the RMIT
University.
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