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Journal of Engineering Science and Technology Special Issue on SOMCHE 2014 & RSCE 2014 Conference, January (2015) 79 - 89 © School of Engineering, Taylor’s University

79

SYNTHESIS AND CHARACTERISATION OF La-Co/MgO CATALYST FOR METHANE DRY REFORMING

C. H. ONG, H. J. CHAN, C. K. CHENG*

Rare Earth Research Centre, Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak,

26300 Gambang, Kuantan, Pahang, Malaysia *Corresponding Author: [email protected]

Abstract

In the present work, different loadings of lanthanum cobalt supported on magnesia catalysts were prepared for the methane dry reforming. The BET specific surface area has declined with the increment of La loading although pore volume increased significantly beyond 1 wt% La doping. The 5 wt% La-Co/MgO catalyst has BET specific surface area of 31.67 m2/g and 0.30 cm3/g of pore volume. From FESEM imaging, an addition of lanthanum promoter has created solid with smaller particle size. This is consistent with XRD analysis where crystallite size significantly dropped when the percentage of La was higher. XRD showed that the peaks representing CO3O4 occurred at 2θ = 37.0° and MgO or Co/MgO at 2θ = 42.9°, 62.3°, 74.7° and 78.6°. As lanthanum loading increased, the crystallite size dropped from 24.5 to 17.9 nm at peak 2θ = 42.9° while the crystallite size decreased from 23.5 to 17.1 nm at peak 2θ = 62.3°. Reaction studies have found that 5wt% La catalyst gave the highest rate formation of product yield (H2 and CO). However, the composition of product yields was quite low at about 1.6% (differential reactor). It was also found that the rate was optimum at CO2:CH4 feed ratio of 0.67.

Keywords: Carbon deposition, Dry reforming, Lanthanum, Methane, Syngas.

1. Introduction

Synthesis gas is a fuel gas mixture consists of hydrogen (H2) and carbon monoxide (CO). It is a valuable feedstock for downstream petrochemical industries, i.e., productions of ammonia, methanol, gasoline etc. Industrially, synthesis gas can be produced from variety of sources such as natural gas, petroleum, biomass and coal. There are some notable chemical routes to produce

80 C. L. Ong et al.

Journal of Engineering Science and Technology Special Issue 3 1/2015

synthesis gas whereby the most established process is steam reforming. However, dry reforming of methane is growingly accepted as alternative to steam reforming [1-3]. It is a process where methane (CH4) and carbon dioxide (CO2) is converted into synthesis gas over a selected catalyst. Significantly, dry reforming is gaining popularity as the H2:CO product ratio is close to unity [4].

Catalyst is comprised of a promoter, active metal and a support. Numerous studies and reviews have been published on the Ni based catalysts albeit with different support materials. Nickel (Ni) catalyst is normally employed as reforming catalyst due to the low cost and high effectiveness at elevated reaction temperatures although sintering of Ni also can take place at reaction temperature above 1073 K [5]. In terms of support, the active metals are usually dispersed on various oxide systems i.e. Al2O3, MgAl2O4, CeO2, ZrO2 etc. It has been suggested that a high dispersion of metal species over the support can reduce the coke formation. Oxide support can be used to control the metal particle sizes [6]. The previous catalytic works are mostly confined to Ni dispersed on the Al2O3 support with significantly less work devoted to magnesia (MgO), a basic material; hence the motivation behind the current work. In addition, cobalt (Co) based catalysts have been used in reforming reaction where it generally showed better coking resistance and higher stability under the extreme reaction conditions [7]. The Co/TiO2 catalyst was reported to be stable at high pressure with a small amount of carbon deposition. An effective way of improving the catalytic performance is through the addition of promoter. Lanthanum (La) promoted the formation of new active sites and increased the dispersion of Co metal [8]. Furthermore, studies showed that rare earth promoter has indeed improved the methanation activity [9]. In the early studies, the addition cerium (Ce), a component of rare earth metal has enhanced the metal dispersion degree and lowered the reduction temperature of cobalt oxides over the pillared montmorillonite supported cobalt catalysts [10]. Therefore, it is believed that the presence of rare earth promoter may significantly improve the performance of catalyst in the methane dry reforming reaction.

Therefore, this research work was carried out to synthesise magnesium oxide supported cobalt catalyst and promoted with various loadings of lanthanum metal, a rare earth element for reaction study of methane dry reforming.

2. Materials and Methods

The catalyst employed in the current experiment was La-Co/MgO catalyst. All the materials required for preparing the catalyst viz. lanthanum nitrate (La(NO3)3), cobalt nitrate (Co(NO3)2) and magnesium oxide (MgO) with purity greater than 98.0% were of Sigma-Aldrich brand. In addition, gaseous reactants that required in the current work were comprised of methane (CH4) and also carbon dioxide (CO2) with purity of at least 98% supplied by the Air Products.

All the catalysts were prepared using wet-impregnation method. The basic MgO was employed as the catalyst support and doped with two separate solutions containing cobalt (10 wt% metal basis) and lanthanum nitrate salts. Promotion with La was carried out for weight loadings of 0, 0.5, 1, 3 and 5 wt%. The catalysts after impregnation were oven-dried for overnight followed by calcination at 1023 K for 3 hours.

Synthesis and Characterisation of La-Co/MgO Catalyst for Methane Dry . . . . 81


The specific surface area, pore volume and pore size distribution of catalysts were determined based on the N2 adsorption-desorption isotherm post-measurement using Thermo Surfer instrument. The N2 adsorption-desorption isotherms were measured in order to examine the porous properties of sample’s catalyst. The phase identification of calcined catalysts was determined via XRD technique using CuKα radiation from 2θ = 20° to 90° at a speed of 1°/min (Rigaku miniflex II (λ = 1.542 Å) at 30 kV and 15 mA). In addition, the morphology of the fresh and spent catalyst was examined using field emission scanning electron microscope (FESEM). The measurement was carried out in JSM-7800F unit instrument. The thermal characteristic of the catalysts was carried out using Q500 TGA unit to monitor the calcination profiles of dried catalysts under air-blanket using three different heating ramps viz. 10, 15 and 20 K/min. For used catalysts (collected after reaction), the amount of deposited carbon was determined by measuring the weight change using oxygen as gasifying agent at heating ramp of 10 K/min up to 1173 K.

The catalytic evaluation was carried out by transferring 0.20 g of catalyst into the stainless-steel fixed-bed reactor (ID: 10 mm) supported by two layers of quartz wool. The CO2 and CH4 were subsequently metered into the reactor using digital mass flow controller (Alicat Scientific Model: MC-500SCCM-D). Catalyst screening was carried out using feed comprised of CH4 and CO2 mixture at equal proportion and reaction temperature at 1073 K. The composition of syngas product was determined using gas chromatography (GC-Agilent 6890 N series) equipped with thermal conductivity detector (TCD). Two packed columns were used viz. Supelco Molecular Sieve 13x and Agilent HayeSep DB. Separation and quantification of gas analytes viz. hydrogen (H2), methane (CH4) and carbon dioxide (CO2) were performed using HayeSep DB column whilst CO was analysed using the Molecular Sieve 13x column. He gas (20.0 ml/min) was employed as the carrier gas whilst detector temperature was always kept at 423 K. The catalytic performance was evaluated in terms of:

100(%) ×−

=inin

outoutinin

Fy

FyFyX (1)

catalyst

outoutininCH

W

FyFyr

−=

4 (2)

catalyst

outoutproduct

W

Fyr = (3)

whereby X (%) is the reactant conversion, 4CHr is the CH4 consumption rate, ri is

the product formation rate, y is the molar composition, F is the molar flowrate and Wcatalyst is the weight of catalyst.

3. Results and Discussion

3.1. Fresh catalyst characterisation

Figure 1 shows the XRD spectrum of all La-Co/MgO catalyst samples of different lanthanum loadings. From Fig. 1, the peaks representing CO3O4 occurred at 2θ = 36.94° and MgO or Co/MgO at 42.94°, 62.32°, 74.74° and 78.64°. Indeed the obtained results were very much similar to the previous XRD analysis of

82 C. L. Ong et al.


Co/MgO catalyst [11]. Significantly, at around 2θ = 42.94°, these peaks have sharp intensity indicating that all La-Co/MgO has good crystallinity (cf. Fig. 1). In contrast, amorphous phase was non-existent. In addition, La phases were not detected. This may be attributed to the well dispersion of La species on the Co/MgO support. Moreover, since the percentage of La employed was comparatively low, therefore the formed crystallites could be well below the limit of XRD detection. Even so, the intensity of the peaks decreased while the width broadened with La incorporation. Significantly, this has indicated that the La can tailor the size of the crystals and made it smaller (cf. Table 1). This can be attributed to the “spacer” role by the La which possesses larger molecule size compared to the other species and prevented agglomeration.

Fig. 1. XRD of La-Co/MgO catalyst samples at different loading of La.

Table 1. Crystallite size decrement with La (wt%) loadings.

Sample

(La wt%) 2θθθθ = 42.94 = 42.94 = 42.94 = 42.94

((((nm))))

2θ θ θ θ = 62.32= 62.32= 62.32= 62.32

((((nm))))

0 24.5 23.5 0.5 25.3 24.5 1 24.4 24.6 3 22.2 21.2 5 17.9 17.1

The textural properties of the fresh La-Co/MgO catalysts were characterised using liquid N2 physisorption technique. The hysteresis pattern has conclusively shown that the catalysts are mesoporous materials (isotherm not shown). Table 2 shows that the entire samples exhibited relatively large BET specific surface area, in the range of 28.0 to 38.0 m2/g and substantially high pore volumes at up to 0.30 cm3/g. The average pore diameters of the catalysts were in the range of 20 to 39 nm. The BET specific surface area has declined with the increment of La loading symptomatic of pore blocking. Moreover, pore volumes values were averaging 0.20 cm3/g when the La was present in small quantity (< 1.0 wt%). However, pore volume increased significantly beyond 1.0 wt% La doping, in particular at 3.0 and 5.0 wt% La loadings. In addition, the increase of pore volume was in tandem with the enlargement of the pore diameter.



Table 2. BET specific surface area and pore volume of sample.

Sample

(La wt%)

BET surface

area (m2/g)

Pore volume

(cm3/g)

Pore diameter

(nm)

0 38.20 0.22 20.0 0.5 33.10 0.23 26.9 1 29.54 0.19 26.2 3 28.62 0.29 39.1 5 31.67 0.30 33.6

3.2. Dry reforming reaction studies

3.2.1. Catalyst screening

For the catalyst screening process, the feed comprised of CH4 and CO2 mixture at equal proportion was employed and the reaction temperature was set at 1073 K. Figure 2 shows the influence of La loading on the rate of formation of product yield. For all the runs, it can be seen from Fig. 2 that 5 wt% La catalyst yielded the highest formation rates of H2 and CO. For all the runs, the ratio between H2 and CO was always unity as also indicated by the stoichiometric reaction of methane dry reforming (CH4 + CO2 → 2CO + 2H2).

Fig. 2. Rate of formation for both H2 and CO.

The CH4 conversion over the Co/MgO catalysts with various lanthanum loadings was also determined. As can be seen from Fig. 3, an increment in La loading increased CH4 conversion and hence the CO2 conversions although generally still lower than unpromoted catalyst with the exception of 5 wt% La which showed the highest CH4 conversion. This can be explained by unpromoted catalyst which has the largest BET specific surface area, hence larger active area for CH4 dry reforming reaction. Nevertheless, the advantage of having largest BET surface area was offset by the increasing influence of superior physicochemical property endowed by the addition of La, specifically at the highest La metal loading (5 wt%). This has led to the highest conversion achieved by 5 wt% La catalyst. Significantly, the inclusion of La offers alternative pathway to gasify any deposited carbon species attributed to its excellent redox property, thus ensuring sustainable active area for methane dry reforming. Consequently, higher yield of CO was obtained (cf. Fig. 2) and higher conversion was achieved.

84 C. L. Ong et al.


Nevertheless, the conversion was low (X < 5%) probably due to the C-H bonding in CH4 species which was not easily cracked on the metallic cobalt surface. In addition, the low conversion can also be associated with differential reactor conditions employed in the current studies.

Fig. 3. CH4 conversion with various La loadings.

3.2.2. Reaction kinetics studies

From the catalyst screening study, 5 wt% La-Co/MgO catalyst has been determined as the best performing catalyst. Subsequently, this catalyst was employed for further reaction studies. The influence of parameters such as molar feed ratio and temperature on the reaction rate over 5 wt% La catalyst was further investigated. Figure 4 shows the rate of CO and CH4 rates over CO2 to CH4 feed molar ratios from zero to four. As can be visualised, the increase in the molar feed ratios resulted in increase of the reaction rate at the lower region of feed ratio, attaining an optimum at 0.67 before declining at over the stoichiometric ratios.

Fig. 4. Rate of formation of CO and rate of consumption of CH4 with various feed ratios.



In accordance to the principle of Arrhenius, when reforming temperature was increased, the reaction rate also increased in exponential manner. Figure 5(a) shows the relationship between rates of formation of CO with temperature whilst Fig. 5(b) shows the rate of consumption of methane with temperature.

Fig. 5. (a) Rate of formation of CO with temperature, (b) Rate of consumption of CH4 with temperature.

To gauge the stability of the 5wt% La-Co/MgO catalyst, a longevity study was conducted for 12 hours at 1073 K. Figure 6 shows the percentage of CO formed during 12 hours, indirectly also representing H2 as H2:CO was always at unity ratio. Significantly, it shows that the percentage of CO peaked at around 2.4 mol% which was recorded at the 2nd hours and gradually decreased to a stable value of 1.5 mol% at the 8th hours until the end of the 12th hours. Therefore, it can be inferred that 5 wt% La-Co/MgO catalyst showed some initial deactivation trend followed by a period of stability. This is typical of deactivation caused by carbon laydown. The result has demonstrated the excellent stability of 5 wt% La catalyst at 1073 K. Previous result by Chen and co-workers [12] also showed that

(a)

(b)

86 C. L. Ong et al.


cobalt has good activity for soot oxidation, which helped in promoting the carbon resistance of the catalyst in reforming reaction.

Fig. 6. Composition of gaseous products during 12 hours reaction at 1073 K.

3.2.3. Spent catalyst characterisation

Figure 7 shows FESEM image structure of the coke formed at the spent catalyst. From this figure, it can be clearly seen that whisker carbon formed around catalyst. In addition, TGA analysis in the presence of O2 showed (at 10 K/min ramping rate) that all the deposited carbon can be fully removed at temperatures beyond 970 K.

Fig. 7. SEM image of spent 5 wt% La Co/ MgO catalyst.



The non-isothermal gasification profile was further subjected to Coats-Redfern kinetics modelling employing Avrami-Erofeev and Geometrical Contracting models. Figure 8 shows the resulting modelling exercise. The three-points modelling employed, was a limitation of the current analysis. Therefore, the results should be construed as an estimation of the gasification kinetics parameters. Notwithstanding, the data-points at similar gasification temperature were chosen and fitted for the various models, hence the validity of method was not compromised and therefore acceptable.

Fig. 8. Coats-Redfern modelling of coke gasification.

Table 3 summarises the obtained results. Significantly, the best models were A1 and R3 based on the combinations of producing regression coefficient of greater than 0.98 and also yielding physically-meaningful activation energy (averaging 15.0 kJ/mol).

Table 3. Activation energy obtained from various model fittings.

Model Ea (J/mol ) R²

A1 14491.30 0.99

A2 6580.69 0.99

A3 2624.65 0.99

R2 8294.63 0.90

R3 15505.61 0.98

4. Conclusions

The BET specific surface area has declined with the increment of La loading. The 5 wt% La-Co/MgO catalyst has BET specific surface area of 31.67 m2/g and 0.30 cm3/g of pore volume. XRD analysis reported that crystallite size significantly dropped when the percentage of La was higher. The peaks representing CO3O4

88 C. L. Ong et al.


can be found at 2θ = 37.0° and MgO or Co/MgO at 2θ = 42.9°, 62.3°, 74.7° and 78.6°. Reaction studies have found that the 5 wt% La catalyst yielded the highest rate formation of product (H2 and CO) at equimolar feed mixture (CH4 and CO2) and reaction temperature of 1073 K. This can be explained by addition of La that provides an alternative pathway to gasify deposited carbon species attributed to its excellent redox property and hence retained its nascent active area for methane dry reforming reaction to occur. This is consistent with the highest conversion achieved by the 5 wt% La catalyst. However, the composition of products yield was quite low. It was about 1.6%. The generally low CO yield among all the catalyst is due to the fact that CH4 which was not easily cracked on the metallic cobalt surface. The 5 wt% La-Co/MgO catalyst was employed for further reaction studies. The increase in the molar feed ratio results in increase of the reaction rate. The rate was optimum at CO2:CH4 feed ratio of 0.67. On the other hand, when reforming temperature was increased, the reaction rate also increased. This is in agreement with the Arrhenius principle. Longevity study was carried out to determine the stability of the catalyst. Results showed that 5 wt% La Co-MgO catalyst did not exhibit any significant deactivation for 12 hours of reaction at 1073 K. This may be due to the fact that cobalt metal which helps in carbon resistance of the catalyst in reforming reaction. Thus, a longer stability was achieved. From TGA analysis conducted at 10 K/min, all the deposited carbon/ coke can be fully removed at temperatures beyond 970 K. The activation energy obtained was 15.0 kJ/mol.

Acknowledgement

We would like to acknowledge the Ministry of Science, Technology and Innovation, Malaysia for funding this research via RDU130501.

References

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