Effect of the preparation methods on Mn promoted Co/γ

Maninder
Kumar,
Gaurav
Rattan51, 1, 2016, 63-72
Journal of Chemical
Technology
and
Metallurgy,
EFFECT OF THE PREPARATION METHODS ON Mn PROMOTED Co/γ-Al2O3
CATALYSTS FOR TOTAL OXIDATION OF METHANE
Maninder Kumar, Gaurav Rattan
Dr. S.S. Bhatnagar University
Institute of Chemical Engineering & Technology
Panjab University, Chandigarh-160014, India
E-mail: [email protected], [email protected]
Received 10 September 2015
Accepted 04 November 2015
ABSTRACT
The effect of the preparation method on the behavior of five catalysts (Mn-Co/γ-Al2O3) for CH4 oxidation containing
different Mn loading (within the range of 4 mass % - 16 mass %) is studied. Incipient wetness, wet impregnation, deposition precipitation and citric acid sol-gel method are examined. The catalytic response is studied on the ground of a fixed
weight (500 mg) of the catalyst and an air flow containing 1.5 % of CH4 introduced to the reactor at a total feed rate of
150 mL min-1. The best catalyst from each production series is characterized by XRD, TGA/DSC and SEM. The preparation methods applied line according to the catalytic activity obtained as follows: Sol-gel (7.21 % of Mn) > Deposition
precipitation (10.44 % of Mn) > Wet Impregnation (13.45 % of Mn) > Incipient Wetness (10.44 % of Mn ). It is found that
the catalyst containing 7.21 mass % of Mn loaded on 18 mass % Co/γ-Al2O3 prepared by sol-gel method is the most active
one - it decreases the temperature required for methane total oxidation by 50oC.
Keywords: methane oxidation, catalysts, cobalt, alumina, oxidation, combustion, manganese.
INTRODUCTION
The catalytic oxidation/combustion of methane
(CH4) is a straightforward reaction (eq. 1) with significant environmental advantages.
CH 4 [ g ] + 2O 2 [ g ] → CO 2 [ g ] + 2H 2 O [ g ] + heat ( 890 kJ / mol )
(1)
It is so because CH4 is a potent green house gas and
its Global Warming Potential (GWP) is 21. The latter
means that CH4 will cause 21 times as much warming
as an equivalent mass of CO2. Therefore the abatement
of CH4 emission from CNG vehicular exhaust is of
paramount importance, as CH4 emissions from CNG
fuelled vehicles are much more in comparison to diesel
and gasoline fuelled vehicles.
The catalytic abatement of CH4 emissions has been
studied by numerous researchers [1 - 5] since the envi-
ronmental legislations were adopted. Great efforts [6]
have been done to develop and explore such catalysts
which can oxidize or combust CH4 at as low temperature
as possible. The noble metals are found very effective
but their less availability and high cost require their
substitution with some low cost and easily available
material. The transition base metals are good candidates
in this regard [9 - 10]. Transition metal oxides, such
as Mn, Cu, Cr, Fe, and Co oxides, become appealing
due to their lower cost and relative abundant resource.
Cobalt and manganese based catalysts has been best
studied and explored among all the transition metals
catalysts [11 - 15]. Various parameters concerning the
preparation methods of the catalysts, the type of the
reactor used, the amount of the catalyst used and the
flow conditions are tailored in the literature in order to
optimize the reaction conditions. It is worth noting that
the minimum temperature for 100 % conversion of CH4
63
Journal of Chemical Technology and Metallurgy, 51, 1, 2016
is 300oC [16], whereas the maximum one is ca 800oC
[17]. The detailed literature survey reveals also that most
of the work reported pertains to unsupported catalysts.
Few report studies carried out on alumina supported
catalysts [18 - 19]. Alumina is cheaper in comparison
to other transition base catalysts as the cost of 1 kg of
cobalt is almost 100 times the cost of the same amount
of alumina. The intense literature survey reveals also
that the combination of cobalt and manganese supported
on alumina has not been reported for CH4 oxidation/
combustion reaction. Thus, the present work is devoted
to alumina supported manganese-cobalt based catalyst
for total oxidation of CH4 and the effect of the preparation method on the catalytic activity observed. Table 1
describes the step wise methodology of the present work.
EXPERIMENTAL
Catalysts synthesis
All the chemicals used were of analytical grade
(AR). Mn-Co/γ-Al 2O3 samples were prepared using
Co(NO3)2.6H2O (HiMedia Laboratories), C4H6MnO4.4H2O
(S D fine chemicals, Mumbai, India) and Al2O3 (S D
fine chemicals, Mumbai, India). The compositions are
expressed in mass percent throughout the paper. Only
managanese loading is varied on 18 mass % Co/Al2O3.
Incipient wetness method
The required manganese and cobalt precursor salts
were dissolved in doubly distilled water ensuring that
the formed volume should not be greater than the total
pore volume of the alumina particles. This solution
was poured onto the alumina support and the ageing
proceeded for 5 h at 30˚C. The impregnates were dried
overnight in an oven at 110˚C. The samples were further
calcined for 4h in a muffle furnace at 550˚C.
Wet impregnation method
The stoichiometric amounts of manganese acetate as
well as the alumina and cobalt nitrate salts were dissolved
in doubly distilled water. The solution formed was heated
at 80°C with constant stirring until dryness appears. The
samples formed were further dried in an oven overnight at
110 °C and calcined for 4 h in a muffle furnace at 550°C.
Table 1. Step wise methodology adopted for the screening of the good catalyst for methane oxidation.
Step1
Initially catalysts having Mn 0, 4.06, 7.21, 10.44, 13.45, 16.27 mass % on 18 mass %.
Co/ γ-Al2O3 were prepared by deposition precipitation method and their catalytic activity
was tested for methane oxidation. It was found that a 10.44 mass % catalyst is the best
catalyst for total oxidation of methane. It showed complete oxidation at 450oC i.e. T100%
= 450oC. However the catalyst with 0 % Mn showed T100% = 480oC.
Step 2
Further, in order to examine the effect of preparation method; all catalysts were prepared
by Incipient wetness, wet impregnation and sol gel method and tested for methane
oxidation. It results were as follows:
Preparation method
Incipient Wetness
Wet Impregnation
Deposition Precipitation
Citric acid Sol Gel
Manganese (mass %)
10.44
13.45
10.44
07.21
T100% (oC)
450
450
450
430
Further, the best catalyst from each preparation method was characterized by
SEM, XRD, TGA/DSC
Step 3
The ranking order in terms of preparation methods follows
Citric acid Sol-gel (7.21 % Mn) > Deposition precipitation (10.44 % Mn) > Wet
Impregnation (13.45 % Mn) > Incipient Wetness (10.44 Mn %). It can be concluded that
on addition of 7.21 mass % Mn as a promoter to Co/γ-Al2O3 there is a 50oC decrease in
temperature for complete conversion/oxidation of methane.
64
Maninder Kumar, Gaurav Rattan
Deposition precipitation method
The required amount of alumina (Al2O3) support was
added to an aqueous mixture of manganese acetate and
cobalt nitrate. This solution was then added drop wise
to 0.1 mole/liter aqueous sodium carbonate solution
with constant stirring at 100˚C maintaining pH of 9.
The resulting precipitate was aged for 1 h, washed with
distilled water two times and filtered using Whatman
filter paper (Ashless). The resulting sample was dried
overnight in an oven at 110˚C. The solid formed was
calcined in a muffle furnace for 4h at 550˚C.
Citric acid sol-gel method
The metal salts were dissolved in distilled water
according to the stoichiometric amount required and
citric acid was added as a complexing agent with of acid
to metals ions 1.3:1 ratio. The amount of polyethylene
glycol added was 10 % of the citric acid used above.
The blended solution was mixed with continuous stirring at 80°C till a gel was formed. The latter was dried
overnight in oven at 110°C and calcined in the muffle
furnace for 4 h at 550°C.
towards CH4 oxidation were measured in a compact scale
fixed bed down flow tubular reactor. The reactor was
placed vertically in a split open furnace. 500 mg of the
catalyst was diluted with 1 mL of alumina and placed in
the reactor. The reactant gas mixture consisted of CH4
(1.5 mL min-1) and air (148.5 mL min-1) maintaining a
total flow rate of 150 mL min-1. The reaction was carried
out in the range between the ambient temperature and
that of 100 % CH4 conversion. It was insured that the
gas did not contain any moisture or CO2 by passing it
through CaO and KOH pellet drying towers. Digital flow
meters were used for measuring the flow rate of both CH4
and air. The catalytic experiments were carried out under
steady state conditions. The steady state temperature
was controlled by a microprocessor based temperature
controller with precision of 0.5oC. The products and
reactants were analyzed by an online gas chromatograph
(Nucon 5765) using a Porapak Q-column, Methaniser
and FID detector for the concentration of CH4 and CO2.
The fractional conversion of methane was calculated on
the basis of the values of its concentration in the product
stream using the following formula (eq. 2)
Catalytic Activity measurements
The catalytic activities of Mn-Co/γ-Al2O3 catalysts
X CH 4 =
{C
CH 4 in
− CCH 4
CCH 4
}
out i
(2)
in
Table 2. Influence of Mn loading on Light off temperatures of different catalysts
prepared by different methods.
Preparation method
Incipient Wetness
Wet Impregnation
Deposition Precipitation
Citric acid Sol Gel
Manganese
(mass %)
4.06
7.21
10.44
13.45
16.27
4.06
7.21
10.44
13.45
16.27
4.06
7.21
10.44
13.45
16.27
4.06
7.21
10.44
13.45
16.27
Co:Mn
mass %
5:1
5:2
5:3
5:4
5:5
5:1
5:2
5:3
5:4
5:5
5:1
5:2
5:3
5:4
5:5
5:1
5:2
5:3
5:4
5:5
Temperature (oC)
T10% T50% T100%
310 355 480
300 320 475
280 315 450
305 360 460
305 340 470
300 390 490
310 365 470
290 345 465
275 320 450
310 340 480
320 380 480
295 320 465
270 340 450
345 395 470
340 375 470
312 345 460
275 320 430
300 375 445
275 460 470
320 395 475
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Journal of Chemical Technology and Metallurgy, 51, 1, 2016
where the change in CH4 concentration due to oxidation
at any instant is proportional to the decrease in the area
of the chromatogram of CH4 at that instant (eq. 3).
{C
CH 4 in
− CCH 4
}
(3)
out i
Catalyst Characterization
The characterization of the best catalysts from each
preparation series was done by XRD (X-Ray Diffraction), TGA/DSC (Thermo Gravimetric Analysis) and
SEM (Scanning Electron Microscopy). The XRD patterns were recorded by X’PERT PRO diffractometer
using CuKα radiation source with an operating current
and operation voltage of 40 mA and 45 KV, respectively.
The scanning range 2Ɵ was 5.00º - 99.98º with divergence slit of 0.8709º. The continuous scanning was done
with a step size of 0.0170º and step time of 30.36 s. The
TGA/DSC analysis was carried out in air on Thermo
Gravimetric Analyzer. The temperature of the cycle was
programmed from 40˚C to 700˚C increasing at a rate of
10˚C per minute. SEM (Scanning Electron Microscopy)
micrographs were obtained by using a JEOL JSM-6700F
instrument.
Fig. 2. Effect of Mn (mass %) loading on 18 mass % Co/γAl2O3 prepared by citric acid sol gel method.
RESULTS and DISCUSSION
Catalytic activity: effect of the manganese loading & the preparation method
The activity of catalysts having different contents of
Fig. 3. Effect of Mn (mass %) loading on 18 mass % Co/γAl2O3 prepared by wet incipient wetness method.
Fig. 1. Effect of Mn (mass %) loading on 18 mass % Co/γAl2O3 prepared by wet impregnation method.
66
Fig. 4. Effect of Mn (mass %) loading on 18 mass %
Co/γ-Al2O3 prepared by deposition precipitation method.
Maninder Kumar, Gaurav Rattan
Table 3. Best selected catalysts from each preparation method.
Preparation method
Incipient Wetness
Wet Impregnation
Deposition Precipitation
Citric acid Sol Gel
Manganese
(mass %)
10.44
13.45
10.44
0
07.21
10.44
Mn (4.06, 7.21, 10.44, 13.45, 16.27 mass %) supported
on 18 mass % Co/ γ-Al2O3 in respect to CH4 oxidation temperature is outlined in Table 2 and Figs. 1 - 4.
It should be added that the dispersion of manganese
depends upon the method of catalyst preparation. The
catalyst containing 10.44 % of Mn exhibits good results
when prepared by the incipient wetness and the deposition precipitation methods. The same composition but
prepared by the other two methods does not give good
results. Besides, all catalysts compositions studied provide different T100% (the latter denotes the temperature
of 100 % conversion of CH4). It is found that method
of preparation affects the catalytic activity. In case 7.21
mass % of Mn is loaded on 18 mass % of Co/Al2O3 by
citric acid sol gel method, the catalyst activity is greatly
enhanced, i.e. T100% = 430oC. The latter is the minimum
temperature of CH4 conversion observed in this study.
The catalyst containing 4.06 mass % of Mn loaded on
Co/Al2O3 by wet impregnation shows the worst results.
In this case T100% = 490oC. This may be due to manga-
Temperature (oC)
T10% T50% T100%
280 315 450
275 320 450
270 340 450
190 350 480
275 320 430
300 375 445
Catalyst ID
IWa
WIa
DPa
DPb
SGa
SGb
nese poor dispersion. Figs. 1 - 4 illustrating the catalytic
activity show that the reaction occurs with lower activation energy when Mn is absent in the lower region of
conversion. Further, the sharp increase of the catalytic
activity observed in Mn presence at higher temperatures
is attributed to higher activation energy which decreases
the oxidation temperature. The best catalysts screened
from each preparation series are shown in Table 3. Each
catalyst there is assigned a unique catalyst ID.
X-Ray diffractograms (XRD)
X-ray diffraction study is carried out to identify
the phases and oxidation states present in the catalysts
prepared. Fig. 5 displays the XRD pattern of the best
catalyst (IWa, WIa, DPa, SGa, SGb) selected from
each preparation series. The peaks at 32.2622, 34.3736,
35.259 correspond to Mn and Co oxidation states and
alumina. However no strong peak is reflected in case of
WIa which is indicative of its amorphous nature. The
XRD patterns of DPb having 0 % Mn is shown in Fig.
Fig. 5. XRD patterns of the best screened catalysts from each method of preparation.
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Journal of Chemical Technology and Metallurgy, 51, 1, 2016
Fig. 6. XRD patterns of 18 % Co/ γ-Al2O3 catalyst (DPb).
Fig. 7. TGA/DSC analysis of SGa catalyst.
Fig. 8. TGA/DSC analysis of SGb catalyst.
6. It shows the peaks of cobalt, its oxides and alumina.
The narrow peaks indicate large particles whereas the
broad peaks correspond to small particles.
Thermo Gravimetric Analysis (TGA/DSC)
TGA/DSC analysis of SGa is shown in Fig. 7. The
results suggest that the weight loss occurs in various
temperature intervals. There is negligible weight loss
68
between 40oC and 250oC and this might be due to loss
of moisture adsorbed by the catalyst. There is a constant
weight fall with the temperature in the range from 250oC
to 490oC. This is attributed to the loss of nitrate and acetate ions. The weight loss is almost negligible between
500oC to 600oC. A strong exothermic peak is observed
at 500oC which is attributed to catalyst formation. No
significant weight loss is found above 600˚C which
Maninder Kumar, Gaurav Rattan
Fig. 9. SEM micrographs of SGa catalyst.
Fig. 10. SEM micrographs of WIa catalyst.
means that the anhydrous salt is completely decomposed.
There is 20 % weight reduction of the catalyst shown in
Fig.7. The TGA/DSC analysis of catalyst SGb is shown
in Fig. 8. Total weight loss is found approximately equal
to 25 %. There is no significant weight loss (approx. 3
%) up to 300˚C due to adsorbed moisture. After attaining a temperature of 300oC there is sharp decrease in the
weight of the catalyst (approx. 17 %) upto 530oC and
6 % weight is lost in the interval from 550oC to 650oC.
No strong exothermic peak is detected.
SEM analysis
The SEM micrographs of the catalyst prepared by
sol-gel (SGa) are shown in Fig 9. The three micrographs
have different resolutions. It can be seen that cobalt and
manganese are highly dispersed on the alumina. Larger
surface blocks with voids can be seen. The dispersion is
not constant through the particles. The SEM micrographs
provide to conclude that the sol-gel method ensures a
good tendency of particles agglomeration. This is not
observed in case of the methods application. The SEM
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Journal of Chemical Technology and Metallurgy, 51, 1, 2016
Fig. 11. SEM micrographs of DPa catalyst.
Fig. 12. SEM micrographs of IWa catalyst.
micrographs of IWa prepared by the incipient wetness
method is shown in Fig. 10. It clearly shows impregnated
particles. The structure is not uniform as in case of Fig. 9.
However, they consist of cobalt and manganese oxides.
Blocks of larger as well as smaller particles can be seen
which consist of cobalt and manganese. The dispersion
is also not uniform in Fig. 11 which refers to DPa. Both
large as well as small particles can be seen. Fig. 12 displays the SEM micrographs of IWa prepared by the incipient wetness method. Only a little amount of particles are
dispersed. Oxides of cobalt and manganese are present.
70
CONCLUSIONS
20 catalysts of different manganese loading (4 mass
% - 16 mass %) on 18 mass % of Co/ γ-Al2O3 were
prepared by the incipient wetness, wet impregnation,
deposition precipitation and citric acid sol-gel method
and tested for CH4 oxidation. It is found that the catalytic
performance in respect to the reaction pointed above and
Maninder Kumar, Gaurav Rattan
the morphology of the catalysts depend strongly upon the
manganese loading and the catalyst preparation method.
The reaction occurs at lower activation energy in Mn
absence, while the sharp catalytic activity increase at
higher temperatures in Mn presence is attributed to activation energy increase. The catalyst having 7.21 mass
% of manganese loaded on cobalt-alumina (18 mass %
of Co/ γ-Al2O3) shows the best catalytic performance due
to the uniform dispersion of manganese and cobalt on
alumina. The catalyst having 4.06 mass % of Mn on 18
mass % of Co/ γ-Al2O3 shows the worst results in terms
of catalytic activity. The catalyst activity in respect to
methane total oxidation decreasing the corresponding
oxidation temperature by 50oC The catalytic activity in
terms of T100% is lined in accordance with: sol-gel (7.21
% of Mn) > deposition precipitation (10.44 % of Mn) >
wet impregnation (13.45 % of Mn) > incipient wetness
(10.44 % of Mn).
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