Catalytic performance of ceria-supported cobalt catalyst for CO-rich hydrogen production from dry reforming of methane

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Catalytic performance of ceria-supported cobalt catalyst for CO-rich hydrogen production from dry reforming of methane Bamidele V. Ayodele, Maksudur R. Khan, Chin Kui Cheng* Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang Kuantan, Pahang, Malaysia

article info

abstract

Article history:

Dry reforming of methane was studied over ceria-supported cobalt (20wt%) catalyst pre-

Received 16 May 2015

pared via wet-impregnation method. The synthesized catalyst was characterized using

Accepted 12 October 2015

thermogravimetric analysis (TGA), X-ray diffraction (XRD), field emission scanning electron

Available online 21 November 2015

microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), N2 physisorption and Fourier transform infrared spectroscopy (FTIR). The catalytic methane dry reforming was

Keywords:

carried out over the 20wt%Co/80wt%CeO2 catalyst in a fixed-bed reactor. The experiment

Ceria

was performed at atmospheric condition with time-on-stream (TOS) of 4 h, reaction

Cobalt

temperatures of 923e1023 K, and CH4:CO2 feed ratios of 0.1e1.0. The XRD pattern showed

Dry reforming

good dispersion of the cobalt metal on the support. This was corroborated by the FESEM-

Methane

EDX and FTIR spectrum. The N2 physisorption revealed that the BET specific surface area

Syngas

of the calcined catalyst was more than double the ceria support. The conversions of CH4 and CO2, respectively, as well as the H2 and CO yield, were found to increase with reaction temperature and CH4:CO2 feed ratios. The highest conversions for both CO2 and CH4 were 87.6% and 79.5%, respectively, at 1023 K. Moreover, highest yield of 40% was obtained for CO while that of H2 was 37.6%. Syngas ratio of 0.99 was obtained at a feed ratio of 0.9, which has further cemented the suitability of methane dry reforming over ceria-supported cobalt catalyst for production of syngas meant for FischereTropsch synthesis. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The quest for alternative and sustainable source of energy especially for transportation sector, has increased over the decades [1], as a result from the projected depletion in crude oil reserves, as well as the environmental concerns associated with it [2]. The burning of hydrocarbon-based fuels and other human activities such as deforestation, industrial processes etc. have led to emission of noxious gases such as N2O, CH4 and

CO2. These greenhouse gases have been reported to be responsible for “greenhouse effect”, a major causes of global warming [3]. In order to mitigate the effects of these greenhouse gases, several methods have been investigated [4,5]. Carbon capture and storage (CCS) has been extensively investigated as one of the means to reduce the level of CO2 in the air [4,6]. Although, the technology has been proven to have the potential for mitigating CO2 emission, the process is however complicated with high risk and is also capital-intensive [7,8].

* Corresponding author. Tel.: þ60 9 5492896; fax: þ60 9 5492889. E-mail address: [email protected] (C.K. Cheng). http://dx.doi.org/10.1016/j.ijhydene.2015.10.049 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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To reduce the effects brought by the excessive release of CO2 and CH4 into the atmosphere, it has been proposed to employ both gases as the feedstock for production of synthetic gas (syngas), a mixture of H2 and CO [9]. Syngas which can be produced by steam methane reforming [10e12], partial oxidation reforming [12e14] and also dry methane reforming [15e17], is a chemical intermediate for the production of oxygenated fuel via FischereTropsch Synthesis (FTS) [18e20]. Amongst these methods, dry reforming of methane has the advantage of utilizing the two principal components of greenhouse gases, hence helps to reduce their impact on the environment [15]. Moreover, the syngas ratio obtained is suitable for FTS process for production of synthetic fuels [21]. Nevertheless, one of the major challenges common to these reforming processes is the issue of catalyst deactivation via carbon deposition, sintering and poisoning [22]. In order to overcome these challenges, previous researchers have focused on the designing and synthesizing catalysts with potentials of high activity, stability and less prone to deactivation [23]. The use of transition metals such as Ni, Pt, Rh, Ru, Pd, Ir and Co on different supports has been extensively investigated [17e21]. Although Rh and Ru have been shown to exhibit very high stability and activity compared to the other group VIII transition metals, they are however, very expensive which render them infeasible for commercial dry reforming process [24]. From economic standpoint, supported-Ni catalysts are less expensive and readily available. Nevertheless, it is very prone to carbon deposition. Consequently, its time-onstream catalytic performance exhibited transient deterioration [29]. Significantly, supported cobalt catalysts have been touted as the potential alternative to supported nickel catalysts as the latter are highly susceptible to catalyst deactivation, whilst supported Rh and Ru catalysts are expensive [24]. The use of alumina and silica supported cobalt catalysts for dry reforming of methane has been extensively investigated [23e26]. These supported cobalt catalysts have been shown to be very stable and possessed significant catalytic activity [24]. However, one of the major drawbacks is their strong metalsupport interaction which often affects the reducibility of the catalysts. Besides, the acidic nature of the supports makes them unsuitable for activation of CO2, an acidic gas [24]. Ceria has high capacity for oxygen retention, which enhance its properties as a good support and enable the catalysts to be less prone to carbon deposition [33,37]. High surface area ceria synthesized by surfactant-assisted approach has been used as metal oxide catalysts for production of H2 and CO from methane dry reforming for solid oxide fuel cell [22,23]. Moreover, the use of ceria as promoter has been shown to enhance the catalytic activity and stability in dry reforming of methane [21,27e29]. In addition, production of H2 from steam reforming of ethanol over ceria-supported Co, Ir, Ni and Rh has been investigated [30e34]. Unfortunately, existing literature pertaining to the production of H2 and/or CO from dry reforming of methane over Co/CeO2 remains scarce. The only reported work to the best of our knowledge was based on 7.5wt% Co/CeO2 prepared by surfactant assisted co-precipitation method [16]. Hence, the aim of this study is to investigate the catalytic performance of ceria-supported Co catalyst for the production of CO-rich hydrogen in methane dry reforming. The ceria-supported Co

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catalyst was synthesized using wet impregnation method, characterized and the catalytic performance investigated in the dry reforming of methane at different temperature zones and CH4:CO2 feed ratios.

Materials and methods Synthesis of catalyst support The ceria supported cobalt catalyst was synthesized using wetimpregnation method. Cobalt nitrate salt Co(NO3)2.6H2O (99.99% purity, Sigma Aldrich) and cerium nitrate salt Ce(NO3)2.6H2O (99.99% purity, Sigma Aldrich) were used as the precursors for the active metal and ceria support, respectively. The ceria support was prepared by thermal decomposition of Ce(NO3)2.6H2O at 773 K for 2 h [42] and then crushed to powder. The Co/CeO2 (Co wt% ¼ 20) was prepared by impregnating the Ce(NO3)2.6H2O salt solution into the ceria support followed immediately by magnetic stirring for 3 h [35]. The mixture was then dried overnight in the oven at 393 K and then calcined at 873 K for 6 h.

Catalyst characterization The fresh 20wt%Co/80wt%CeO2 catalyst was characterized using several methods. The textural property of the assynthesized catalyst was determined using N2 physisorption method. Prior to the measurements, the synthesized catalyst sample was degassed at 523 K. X-ray diffraction (XRD) was performed on the catalyst to examine its crystallinity. The recording of the XRD diffractogram was carried out using a RIGAKU miniflex II X-ray diffractometer capable of measuring diffraction pattern from 3 to 145 in 2q scanning range. The Xray source was Cu Ka with wavelength (l) of 0.154 nm radiation. The XRD was equipped with the latest version of PDXL, RIGAKU full function powder-diffraction analysis software. The information about the surface morphology of the catalyst and the elemental chemical compositions were determined using JEOL field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy (EDX). The thermal properties of the freshly-prepared catalyst as function of temperature (room temperature to 1173 K) was analysed prior to calcination using thermogravimetric analysis (Thermal analyser instrument Q-500 series). Compressed air (O2/N2: 20%/80%) and heating rates of 10, 15 and 20 K/min were employed. Fourier Transform Infrared Spectroscopy (FTIR) (Perkin Elmer Spectrum 100) was also employed to determine the nature of chemical bonding of the catalyst.

Catalytic activity The catalytic activity of the 20wt%Co/80wt%CeO2 catalyst towards the methane dry reforming, was examined in a tubular fixed-bed quartz reactor (internal diameter: 10 mm and longitudinal: 35 cm). The catalyst, weighing 200 mg, was supported with quartz wool in the fixed-bed reactor mounted vertically inside a split-furnace that was equipped with a Type-K thermocouple and PID temperature controller. Before the commencement of the actual catalytic activity test, initial runs to examine the effects of temperature (873e1173 K) on the

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performance of the catalyst was performed. Subsequently, the activity evaluation was performed at 923e1023 K, with CH4:CO2 feed ratios between 0.1 and 1, at atmospheric pressure and a total flow of 100 ml/min. The flow rate of the inlet gases (CH4, CO2 and N2) was controlled by digital mass flow controllers. The gas hourly space velocity (GHSV) for the reaction was maintained at 30,000 h1 STP. Prior to the reaction, the catalyst was reduced in-situ in a flow of 50 ml/min H2/N2 (1:5) at 973 K for 1 h employing a ramping of 10 K/min. The reaction was performed at time-on-stream (TOS) of 4 h in order to ensure steady-state conditions. The composition of exit gas was determined using gas chromatography instrument (GC-Agilent 6890 N series) equipped with TCD. Two packed columns were used, viz. Supelco Molecular Sieve 13 (10 ft  1/8 in OD  2 mm ID, 60/80 mesh, Stainless Steel) and Agilent Hayesep DB (30 ft  1/8 in OD  2 mm ID, 100/120 mesh, Stainless Steel). Helium gas was used as the carrier with flowrate of 20 ml min1 and operating column temperature of 393 K.

Reaction metrics The reaction metrics for the evaluation of the catalytic performance are represented in Equations (1)e(6) CH4 conversionð%Þ ¼

FCH4in  FCH4out  100 FCH4in

(1)

CO2 conversionð%Þ ¼

FCO2in  FCO2out  100 FCO2in

(2)

H2 mole of H2 produced ¼ Syngas ratio mole of CO produced CO

Fig. 1 e Temperature-programmed calcination of the fresh 20wt% Co/80wt%CeO2 catalyst.

respectively, can be clearly observed at 317 K, 343 K, 381 K and 495 K. The peaks I, II and III are most likely due to the consecutive weight losses of physical water, followed by hydration water from the nitrate salt as the temperatures involved were in the region of water evaporation. The peak IV, at temperature of 495 K, can be linked to the formation of cobalt oxide from the thermal decomposition of Co(NO3)2. Overall, this calcination process can be represented as shown in Equations (7)e(9), respectively [32].

(3)

Co(NO3)2$6H2O/Physical H2O / Co(NO3)2$6H2O þ Physical H2O

(7)

(8)

H2 yield ¼

FH2out  100 2  FCH4in

(4)

Co(NO3)2$6H2O / Co(NO3)2 þ 6H2O

CO yield ¼

FCOout  100 FCH4in þ FCO2in

(5)

Co(NO3)2 / CoO þ N2O5

Carbon deposition per gram catalysts ¼

Molar flow of ðCH4 þ CO2 Þin the feed  molar flow of ðCH4 þ CO2 þ COÞ in the product weight of catalyst

3CoO þ 1/2 O2 / Co3O4 FCO2in ¼ inlet molar flow of CO2; FCO2out ¼ outlet molar flow of CO2; FCH4in ¼ inlet molar flow of CH4; FCH4out ¼ outlet molar flow of CH4.

Results and discussion Fresh catalyst characterization The TGA analysis performed with heating rates of 10e20 K/ min for the uncalcined catalysts is shown in Fig. 1. Four weight losses represented by peaks coded as I, II, III and IV,

(9a)

(6)

(9b)

Certain parameters are essential to evaluate the performance of heterogeneous catalysts. One of such parameters is the specific surface area which can be determined using N2 physisorption. The BET specific surface area, pore volume and pore diameter of the fresh 20wt%Co/80wt%CeO2 catalyst determined from the isotherms of N2 physisorption are summarized in Table 1. Significantly, the BET specific surface area of the 20wt%Co/80wt%CeO2 has doubled than that of the CeO2 support. This enlargement could be attributed to the good dispersion of the active cobalt metals on the surface of the ceria support. However, the pore diameter was reduced as

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Table 1 e N2 physisorption of the fresh 20wt%Co/80wt% CeO2 catalyst. Catalyst

Specific surface area (m2/g)

Pore volume (m3/g)

Pore diameter (nm)

CeO2 20wt%Co/ CeO2

19.72 39.35

0.0008 0.0141

1.47 1.16

evidenced in Table 1. Most likely, the formation of fine particles has led to the blockage/narrowing of the existing tunnels/ pores. The XRD pattern of the calcined 20wt%Co/80wt%CeO2 catalyst is depicted in Fig. 2. The XRD pattern shows the formation of well-structured crystalline phase of the catalyst sample. The diffractogram displays multiple sharp diffraction peaks within the 2q range of 3 e80 symptomatic of the formation of Co3O4 spinel and CeO2. Indeed, species that can be revealed from the XRD pattern are CeO2 (2q ¼ 29.4 , 38.4 , 48.7 and 57.9 ) and Co3O4 (2q ¼ 31.3 , 44.5 , 59.7 , 65.1 , 67.7 and 77.1 ). Furthermore, the diffraction peaks for the CeO2 can be assigned to the crystalline phase of (111), (220), (311), (220) and (422) which represent faced-centre cubic structure as reported in Ref. [44]. The intensity of the peaks for Co3O4 was smaller compared to that of CeO2. The detection of Co3O4 phase signifies high interaction between the active metal (cobalt) and the support (ceria). FESEM images (cf. Fig. 3) show the formation of irregular and bulky surface morphology of the synthesized catalyst. The FESEM image in Fig. 3(a) illustrates the uniformity of particles despite its irregular shape. Furthermore, two types of pores are visible (refers to Fig. 3(a)), and this may likely provide a clue to the different values of pore volume and pore diameter obtained for both CeO2 and 20wt%Co/80wt%CeO2, respectively, as summarized in Table 1. Significantly, the presence of larger pore volume was most likely due to the CeO2 support, whilst the much smaller pore volume (needle-like morphology) can be attributed to the distribution of Co-metal on the CeO2 support [38e40]. Due to the well-dispersion of Co metal on the CeO2 support, the pore volume of 20wt%Co/80wt%CeO2 (0.0141 m3/g) increased compared to the pristine CeO2 (0.0008 m3/g). In contrast, the average pore diameter for 20wt% Co/80wt%CeO2 (1.16 nm) was smaller than the pure CeO2

support (1.47 nm). Once again, this can be attributed to the formation of smaller particles (due to the well dispersion) as clearly visible in Fig. 2 that has blocked some of the pores of CeO2 furthering validating the N2-physisorption results. In addition, energy dispersive X-ray (EDX) scanning was carried out at five different spots of the calcined 20wt%Co/ 80wt%CeO2 catalyst and the averaged elemental composition was obtained. Fig. 4 shows the typical spectrum of the resulting scanning, which exhibited the presence of elements Co, Ce and O only. The elemental compositions depicted in Fig. 5 shows that the ceria support has high retention of oxygen which is one of the features as a good support. This was demonstrated by its atomic weight percentage of more than 60%. In addition, Co-atom accounts for about 20% of the atomic weight, which is consistent with the stipulated 20wt% Co/80wt%CeO2 catalyst formula. Consequently, this result proved good homogeneity of the prepared catalyst and also demonstrated that the Co particles were uniformly dispersed. The FTIR spectrum was collected using a 45 ZnSE IRE (Perkin Elmer Spectrum 100). The collected FTIR spectrum of the calcined 20wt%Co/80wt%CeO2 catalyst is shown in Fig. 6. Absorption bands can be observed in the region of 3417.8, 1583.3, 659 and 554.8 cm1. The peak at 3417.8 cm1 represents the stretching vibration of OH which might be due to the presence of adsorbed water vapour while the peak at 1583.3 cm1 represents the presence of dissolved or atmospheric CO2. Furthermore, transmittance peaks in the region of 659 and 554.8 cm1 portray the presence of vibration metal oxide (MO) bonds in the internal structure of the spinel of Co3O4 and CeO2 in the form of d(CeeOeC) and d(CoeOeC) [43,45].

Catalytic activity tests Theoretically, methane dry reforming will produce syngas with H2:CO ratio equals to unity from the consumption of the two anthropogenic gases (CH4 and CO2) as represented in Equation (10). In the current investigation, the catalytic test for the 20wt%Co/80wt%CeO2 catalyst was performed in order to evaluate its activity considering the effects of temperature and CH4:CO2 feed ratios on the conversion of the reactants, yield, as well as the ratio of the produced syngas. CH4 þ CO2 /2CO þ 2H2

DH1023K ¼ 261 kJ mol

1

(10)

Determination of effective reaction temperature

Fig. 2 e X-ray diffraction pattern of the 20wt%Co/80wt% CeO2 catalyst.

Before conducting a full-scale catalytic activity test, preliminary work was carried out to obtain suitable range of reforming temperatures. In order to achieve this objective, the dry reforming reaction was performed in a wider range of temperature, 873e1173 K, to capture the reaction trend. The effects of temperature on the CH4 and CO2 conversions, H2 and CO yield, as well as the syngas ratio are shown in Figs. 7e9, respectively. It can be seen that the conversions of both CH4 and CO2 gradually increased with the reaction temperatures and almost levelled-off beyond 1123 K, consistent with the Arrhenius behaviour. Significantly, the conversion of CO2 was always higher than the CH4 over the entire range of reaction temperature, with the highest

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Fig. 3 e FESEM pictures of the 20wt%Co/80wt%CeO2 catalyst at (a) £10,000 (b) £50,000.

Fig. 4 e EDX picture and spectrum showing elemental composition of the 20wt%Co/80wt%CeO2 catalyst.

conversion of 96.7% was recorded at 1173 K, compared to 95.3% conversion for CH4. This probably indicates that the CeO2 support has slightly higher affinity towards CO2 adsorption. In addition, the conversions of CH4 (58%) and CO2 (66%) from this study (refers to Fig. 7) were lower compared to the equilibrium conversion obtained by Nikoo and Amin [40] at similar temperature range of 873e923 K. This trend was explained by Istadi and Amin [47] to be as results of parallel reactions (cf. Equations (11) and (12)) that led to the lower CH4 conversion at temperature below 923 K. Nevertheless, a higher conversion was obtained at temperature greater than 1123 K compared to that of equilibrium conversion, an indication of the onset of competing side reactions that have also

Fig. 5 e Element composition of the 20wt%Co/80wt%CeO2 catalyst from EDX.

consumed the CH4. Thermodynamically, dry reforming reaction has been reported to be favoured by temperature greater than 773 K and low pressure [48]. This implies that higher conversion and yield can be attained at temperature greater than 773 K as also found in this study. CO2 þ 4H2 /CH4 þ 2H2 O

DH1023K ¼ 165 kJ mol

1

1

CO þ 3H2 /CH4 þ H2 O DH1023K ¼ 206:2 kJ mol

(11) (12)

In addition, the yield of H2 and yield of CO increased

Fig. 6 e FTIR spectrum of fresh calcined 20wt%Co/80wt% CeO2 catalyst.

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thermodynamically favoured at temperature greater than 1023 K as reported by Yaw and Amin [48]. With this trend, syngas ratio close to unity is favoured between 948 and 1023 K as shown in Fig. 9. Although the syngas ratios increased from 0.73 at 773 K to reach the maximum value of 1.40 at 1123 K, it is preferable to employ reaction temperature from 948 to 1023 K in order to produce syngas ratio suitable as the intermediate in the FTS process.

Effects of feed ratio

Fig. 7 e Effect of temperature of the conversion of CH4 and CO2 (CH4:CO2 ratio ¼ 1:1, GHSV ¼ 30,000 h¡1).

45

CO H

40

Yield (%)

35 30

The effects of varying CH4 and CO2 feed ratios on conversion at temperature range of 923e1023 K are shown in Figs. 10 and 11. The conversion of CH4 increased with increase in feed ratio and temperature from 40.8% at 923 K to reach the maximum value of 79.5% at feed ratio of 0.4 and temperature of 1023 K. Thereafter, slight decrease in the CH4 conversion was observed at feed ratios greater than 0.4 for all the reaction temperatures. The slight decrease observed might be due to parallel reaction which was favoured at CH4:CO2 ratio greater than 0.4 [47]. In agreement with this trend, Sajjadi et al. [49] in their investigation on dry reforming of methane over Co doped Ni/Al2O3 catalyst have reported that lower conversion of CH4 was obtained at CH4:CO2 ratios greater than 0.4. Nikoo and Amin [46] reported that the conversion of methane

25 20 15 10 5 850

900

950

1000

1050

1100

1150

1200

Temperature (K)

Fig. 8 e Effects of temperature on the yield of H2 and CO (CH4:CO2 ratio ¼ 1:1, GHSV ¼ 30,000 h¡1).

exponentially with the increase in temperature as shown in Fig. 8. At the initial stage, between 873 and 1023 K, higher yield of CO was obtained compared to H2. However, at temperature greater than 1023 K, the opposite trend was obtained, with H2 having the highest yield of 42% at 1123 K. This trend can be attributed to the fact that dry reforming reaction is

Fig. 9 e Effects of temperature on the H2 and CO ratio (CH4:CO2 ratio ¼ 1:1, GHSV ¼ 30,000 h¡1).

Fig. 10 e Effects of varying feed ratio on CH4 conversion (GHSV ¼ 30,000 h¡1).

Fig. 11 e Effects of varying feed ratio on CO2 conversion (GHSV ¼ 30,000 h¡1).

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attained maximum at CH4:CO2 ratios between 0.3 and 0.5 for temperature range of 1023e1073 K. In a separate study, Li et al. [50] reported that the methane conversion was favoured by CH4:CO2 ratio of 0.2 at temperature of 1023 K. Their findings were based on the equilibrium calculations using Gibbs free energy minimization technique. However, due to the variation in catalytic activity as well as the experimental conditions, there might be variation in the effects of feed ratios on the conversions of CH4 and CO2, as observed by Lavoie [15]. On the contrary, the conversion of CO2 showed an increased trend with increase in the feed ratios and temperature which is consistent with the previous finding [49]. The observed increment in CO2 conversion might be due to the strong adsorption of the CO2 molecule on the surface of the catalyst [16]. Increasing the feed ratios of CH4 and CO2 exhibited a corresponding increase in the syngas ratio alongside with the increase in temperature (cf. Fig. 12). Syngas gas with ratio close to unity (0.99) was obtained at feed ratio of 0.8 and temperature of 1023 K. On the contrary, Wisniewski et al. [51] obtained syngas ratio of unity at feed ratio of 2.0 at 1073 K for dry reforming of methane over supported iridium catalysts. This difference could be explained in terms of the catalytic performance of the individual catalysts at the specified conditions. Moreover, the increasing trend was also observed for the H2 and CO yield as depicted in Figs. 13 and 14. The yield of H2 and CO increases with increase in both feed ratio and temperature. As expected, CO showed the highest yield of about 40% at feed ratio of 0.9 and temperature of 1023 K. This could be explained in terms of parallel reverse water gas shift reaction shown in Equation (13). CO2 þ H2 /CO þ H2 O DH298K ¼ 41 kJ=mol

(13)

Fig. 13 e Effect of varying feed ratio on H2 yield (GHSV ¼ 30,000 h¡1).

Fig. 14 e Effect of varying feed ratio on CO yield (GHSV ¼ 30,000 h¡1).

At a higher CH4:CO2 ratios the H2 formed during the dissociation of methane reacts with CO2 to form CO and water. The effects of CH4:CO2 ratio on the amount of carbon deposited per gram catalyst is shown in Fig. 15. The carbon deposition was estimated based on the Equation (6). Carbon deposition was observed to increase with increase in the feed ratio. At initial stage, between CH4:CO2 ratios of 0.1e0.5, the carbon deposition increased with corresponding increase in temperature. However, the trend changed above feed ratio of

Fig. 15 e Effects of varying feed ratio on carbon deposition/ g catalyst.

Fig. 12 e Effect of varying feed ratio on H2 and CO ratio (GHSV ¼ 30,000 h¡1).

greater than 0.5. Ginsburg et al. [52] in their studies demonstrated the effect of CH4:CO2 ratios on the carbon deposition on supported nickel-based catalyst. Their findings showed that less carbon deposition was observed for feed ratio CH4:CO2 less than 0.5 which implies a feeding of excess CO2. Li

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Fig. 16 e FESEM pictures of the used 20wt%Co/80wt%CeO2 catalyst at (a) £10,000 (b) £30,000.

Fig. 17 e Temperature-programmed calcination of the used 20wt% Co/80wt%CeO2 catalyst.

Fig. 19 e Element composition of the used 20wt%Co/80wt% CeO2 (above stoichiometric) catalyst from EDX.

et al. [36] confirmed this through their investigations in which they discovered that with excess CO2 in the feed, low amount of carbon deposition was obtained.

viz. CH4:CO2 ratios of above and below stoichiometric. The major changes in the morphology of the used catalyst are represented in FESEM images shown in Fig. 16(a) and (b) at different magnifications. It can be seen that the particle size of the catalysts in the FESEM image grew bigger which might be as result of agglomeration possibly caused by nonhomogenous distribution of the metal precursor [53]. Similarly, this agglomeration phenomenon has been reportedly

Characterization of the used catalysts Post reaction, the catalyst was characterized to examine the possibility of carbon depositions. Two cases were considered

Fig. 18 e EDX picture and spectrum showing elemental composition of the used 20wt% Co/80wt%CeO2 catalyst.

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Acknowledgements The authors would like to acknowledge the Sciencefund RDU130501 granted by the Ministry of Science and Innovation Malaysia (MOSTI). DSS scholarship conferred to BVA by Universiti Malaysia Pahang is gratefully acknowledged.

references

Fig. 20 e FTIR spectrum for the used 20wt%Co/80wt%CeO2 catalyst.

ascribed to temperature effect on the catalyst grains which often leads to deactivation. This can further be explained from the TGA analysis of the used catalyst represented in Fig. 17. The TGA analysis shows that there was no visible weight loss in the used catalyst. Moreover, there was noticeable gain in weight which is a characteristic feature for the possibility of a reaction leading to gasification of the carbonaceous atom. This can be explained in terms of the deposited carbon being combusted in air at temperature of about 533 K. The EDX images and spectra depicted in Fig. 18 shows the depletion in the intensity of the different elemental compositions. Moreover, the high intensity of the carbon peak shows the possibility of carbon deposition (Fig. 19) for CH4/CO2 above stoichiometric. The FTIR spectra in Fig. 20 cannot detect the presence of bond for the cobalt catalyst. This could be as a result of drastic loss in the active catalytic site.

Conclusions Catalytic performance of ceria-supported cobalt catalysts has been investigated in the dry reforming of methane for syngas production by considering the effect of temperature (923e1023 K) on the conversion and yield as well as the effect of varying feed ratios (CH4:CO2) on the conversion, yield, syngas ratios and carbon deposition. The characterization study of the unreduced catalyst revealed the homogeneous dispersion of the cobalt metal on the ceria-support thereby increasing the surface area. The results obtained from the catalytic activity tests shows that the highest conversions of 79.5% and 87.6% were obtained at temperature of 1023 K for CH4 and CO2 respectively. Furthermore, maximum yields of 37.6% and 40% were obtained for H2 and CO respectively. It is clear from this study that the synthesized ceria-supported cobalt catalyst has high activity toward dry reforming which was proven from the high conversion of the reacting gases obtained. The suitability of the syngas production with H2:CO ratio of one from dry reforming of methane over 20wt%Co/ 80wt%CeO2 catalyst was also ascertained. Hence, ceriasupported cobalt catalyst has the potential to be used for production of CO-rich Hydrogen gas for further use in downstream productions of other value added chemicals.

[1] Madadian E, Lefsrud M, Lee CAP, Roy Y. Green energy production: the potential of using biomass gasification. J Green Eng 2014;4(2):101e16. [2] Chaubey R, Sahu S, James OO, Maity S. A review on development of industrial processes and emerging techniques for production of hydrogen from renewable and sustainable sources. Renew Sustain Energy Rev 2013;23:443e62. [3] Guo S, Shao L, Chen H, Li Z, Liu JB, Xu FX, et al. Inventory and inputeoutput analysis of CO2 emissions by fossil fuel consumption in Beijing 2007. Ecol Inf Nov. 2012;12:93e100. [4] Bowen F. Carbon capture and storage as a corporate technology strategy challenge. Energy Policy May 2011;39(5):2256e64. [5] Ang CT, Morad N, Ismail Norhashimah, Tahir Ismail Mohd. Projection of carbon dioxide emissions by energy consumption and transportation in Malaysia: a time series approach. J Energy Technol Policy 2013;3(1). [6] Gibbins J, Chalmers H. Carbon capture and storage. Energy Policy January 2007;36:4317e22. 2008. € gersten S, Colls J, Steven M, Black C. [7] Al-Traboulsi M, Sjo Potential impact of CO2 leakage from Carbon Capture and Storage (CCS) systems on growth and yield in maize. Plant Soil 2013;365:267e81. [8] Braga TP, Santos RCR, Sales BMC, da Silva BR, Pinheiro AN, Leite ER, et al. CO2 mitigation by carbon nanotube formation during dry reforming of methane analyzed by factorial design combined with response surface methodology. Chin J Catal April 2014;35(4):514e23. [9] Samimi A, Zarinabadi S. Reduction of greenhouse gases emission and effect on environment. J Am Sci 2012;8(8):1011e5. [10] Chiodo V, Freni S, Galvagno A, Mondello N, Frusteri F. Catalytic features of Rh and Ni supported catalysts in the steam reforming of glycerol to produce hydrogen. Appl Catal A Gen Jun. 2010;381(1e2):1e7. [11] Liang B, Suzuki T, Hamamoto K, Yamaguchi T, Sumi H, Fujishiro Y, et al. Performance of NieFe/gadolinium-doped CeO2 anode supported tubular solid oxide fuel cells using steam reforming of methane. J Power Sources 2012;202:225e9. [12] Koh ACW, Chen L, Keeleong W, Johnson B, Khimyak T, Lin J. Hydrogen or synthesis gas production via the partial oxidation of methane over supported nickelecobalt catalysts. Int J Hydrogen Energy May 2007;32(6):725e30. [13] Zhou X, Huang H, Liu H. Study of partial oxidation reforming of methane to syngas over self-sustained electrochemical promotion catalyst. Int J Hydrogen Energy May 2013;38(15):6391e6. [14] Cimino S, Lisi L. Impact of sulfur poisoning on the catalytic partial oxidation of methane on rhodium-based catalysts. Ind Eng Chem Res Jun. 2012;51(22):7459e66. [15] Lavoie J-M. Review on dry reforming of methane, a potentially more environmentally-friendly approach to the

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 8 e2 0 7

[16]

[17]

[18]

[19]

[20]

[21] [22]

[23]

[24]

[25]

[26]

[27]

[28]

[29] [30]

[31]

[32]

[33]

[34]

increasing natural gas exploitation. Front Chem November 2014;2:1e17. Luisetto I, Tuti S, Di Bartolomeo E. Co and Ni supported on CeO2 as selective bimetallic catalyst for dry reforming of methane. Int J Hydrogen Energy 2012;37:15992e9. Hadian N, Rezaei M, Mosayebi Z, Meshkani F. CO2 reforming of methane over nickel catalysts supported on nanocrystalline MgAl2O4 with high surface area. J Nat Gas Chem Mar. 2012;21(2):200e6. Xiong H, Moyo M, Motchelaho MA, Tetana ZN, Dube SMA, Jewell LL, et al. FischereTropsch synthesis: iron catalysts supported on N-doped carbon spheres prepared by chemical vapor deposition and hydrothermal approaches. J Catal 2014;311:80e7. Inderwildi OR, Jenkins SJ, King DA. FischereTropsch mechanism revisited: alternative pathways for the production of higher hydrocarbons from synthesis gas. J Phys Chem C 2008;112:1305e7. Reichling JP, Kulacki FA. Comparative analysis of FischereTropsch and integrated gasification combined cycle biomass utilization. Energy November 2011;36(11):6529e35. Er H, Bouallou C, Werkoff F. Dry reforming of methane e review of feasibility studies, vol. 29; 2012. p. 163e8. Tsakoumis NE, Rønning M, Borg Ø, Rytter E, Holmen A. Deactivation of cobalt based FischereTropsch catalysts: a review. Catal Today Sep. 2010;154(3e4):162e82. Zhang J, Wang H, Dalai A. Development of stable bimetallic catalysts for carbon dioxide reforming of methane. J Catal Jul. 2007;249(2):300e10. Budiman AW, Song S-H, Chang T-S, Shin C-H, Choi M-J. Dry reforming of methane over cobalt catalysts: a literature review of catalyst development. Catal Surv Asia September 2012;16(4):183e97. Nagaraja BM, Bulushev DA, Beloshapkin S, Ross JRH. The effect of potassium on the activity and stability of NieMgOeZrO2 catalysts for the dry reforming of methane to give synthesis gas. Catal Today Dec. 2011;178(1):132e6. Souza MMVM, Schmal M. Autothermal reforming of methane over Pt/ZrO2/Al2O3 catalysts. Appl Catal A Gen Mar. 2005;281(1e2):19e24. da Silva AM, de Souza KR, Jacobs G, Graham UM, Davis BH, Mattos LV, et al. Steam and CO2 reforming of ethanol over Rh/CeO2 catalyst. Appl Catal B Environ 2011;102:94e109. Gokon N, Yamawaki Y, Nakazawa D, Kodama T. Kinetics of methane reforming over Ru/g-Al2O3-catalyzed metallic foam at 650e900 C for solar receiver-absorbers. Int J Hydrogen Energy Jan. 2011;36(1):203e15. Guo J, Lou H, Zheng X. The deposition of coke from methane on a Ni/MgAl2O4 catalyst. Carbon NY May 2007;45(6):1314e21. Song S-H, Son J-H, Budiman AW, Choi M-J, Chang T-S, Shin C-H. The influence of calcination temperature on catalytic activities in a Co based catalyst for CO2 dry reforming. Korean J Chem Eng December 2013;31(2):224e9. -Alonso D, Juan-Juan J, Illa  n-Go  mez MJ, Roma  nSan-Jose Martı´nez MC. Ni, Co and bimetallic NieCo catalysts for the dry reforming of methane. Appl Catal A Gen Dec. 2009;371(1e2):54e9. Foo SY, Cheng CK, Nguyen T-H, Adesina AA. Kinetic study of methane CO2 reforming on CodNi/Al2O3 and CedCodNi/ Al2O3 catalysts. Catal Today 2011;164(1):221e6. Yu C, Hu J, Zhou W, Fan Q. Novel Ni/CeO2-Al2O3 composite catalysts synthesized by one-step citric acid complex and their performance in catalytic partial oxidation of methane. J Energy Chem Mar. 2014;23(2):235e43. Laosiripojana N, Assabumrungrat S. Catalytic dry reforming of methane over high surface area ceria. Appl Catal B Environ Sept. 2005;60(1):107e16.

207

[35] Reddy BM, Rao KN, Bharali P. Copper promoted cobalt and nickel catalysts supported on ceria e alumina mixed oxide: structural characterization and CO oxidation activity. Ind Eng Chem Res 2009;48:8478e86. [36] Li K, Wang H, Wei Y, Yan D. Direct conversion of methane to synthesis gas using lattice oxygen of CeO2eFe2O3 complex oxides. Chem Eng J Feb. 2010;156(3):512e8.   P, Pintar A. Strategies to [37] Aw MS, Osojnik Crnivec IG, Djinovic enhance dry reforming of methane: synthesis of ceriazirconia/nickelecobalt catalysts by freeze-drying and NO calcination. Int J Hydrogen Energy Aug. 2014;39(24):12636e47. [38] Wang H, Liu Y, Wang L, Qin YN. Study on the carbon deposition in steam reforming of ethanol over Co/CeO2 catalyst. Chem Eng J 2008;145:25e31. [39] Zhang B, Tang X, Li Y, Cai W, Xu Y, Shen W. Steam reforming of bio-ethanol for the production of hydrogen over ceriasupported Co, Ir and Ni catalysts. Catal Commun 2006;7:367e72. [40] Song H, Ozkan US. The role of impregnation medium on the activity of ceria-supported cobalt catalysts for ethanol steam reforming. J Mol Catal A Chem 2010;318:21e9. [42] Lee SS, Zhu H, Contreras EQ, Prakash A, Puppala HL, Colvin VL. High temperature decomposition of cerium precursors to form ceria nanocrystal libraries for biological applications. Chem Mater 2012;24:424e32. [43] Jagadale AD, Kumbhar VS, Bulakhe RN, Lokhande CD. Influence of electrodeposition modes on the supercapacitive performance of Co3 O4 electrodes. Energy 2014;64(3):234e41. [44] Ketzial JJ, Nesaraj AS. Synthesis of CeO2 nanoparticles by chemical precipitation and the effect of a surfactant on the distribution of particle sizes. J Ceram Process Res 2011;12(1):74e9. [45] Dos Santos ML, Lima RC, Riccardi CS, Tranquilin RL, Bueno PR, Varela JA, et al. Preparation and characterization of ceria nanospheres by microwave-hydrothermal method. Mater Lett 2008;62:4509e11. [46] Nikoo M, Amin NAS. Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation. Fuel Process Technol Mar. 2011;92(3):678e91. [47] Istadi I, Amin NAS. Modelling and optimization of catalyticedielectric barrier discharge plasma reactor for methane and carbon dioxide conversion using hybrid artificial neural networkdgenetic algorithm technique. Chem Eng Sci Dec. 2007;62(23):6568e81. [48] Yaw TC, Aishah NOR, Amin S. Analysis of carbon dioxide reforming of methane via thermodynamic equilibrium approach. J Teknol 2007;43:31e49. [49] Sajjadi SM, Haghighi M, Eslami AA, Rahmani F. Hydrogen production via CO2-reforming of methane over Cu and Co doped Ni/Al2O3 nanocatalyst: impregnation versus sol-gel method and effect of process conditions and promoter. J Sol Gel Sci Technol 2013;67:601e17. [50] Li Y, Wang Y, Zhang X, Mi Z. Thermodynamic analysis of autothermal steam and CO2 reforming of methane. Int J Hydrogen Energy May 2008;33(10):2507e14. ave A, Ge lin P. Catalytic CO2 reforming of [51] Wisniewski M, Bore methane over Ir/Ce0.9Gd0.1O2x. Catal Commun 2005;6:596e600. ~ a J, El Solh T, De Lasa HI. Coke formation [52] Ginsburg JM, Pin over a nickel catalyst under methane dry reforming conditions: thermodynamic and kinetic models. Ind Eng Chem Res 2005;44:4846e54. [53] Arsalanfar M, Mirzaei AA, Bozorgzadeh HR, Atashi H, Shahriari S, Pourdolat A. Structural characteristics of supported cobalt e cerium oxide catalysts used in FischereTropsch synthesis. J Nat Gas Sci Eng 2012;9.