SBA-15 catalyst for syngas production: Influence of feed composition

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Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition Sharanjit Singh a, Mahadi B. Bahari a, Bawadi Abdullah b, Pham T.T. Phuong c, Quang Duc Truong d,e, Dai-Viet N. Vo a,f,*, Adesoji A. Adesina g a

Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang, Kuantan, Pahang, Malaysia b Chemical Engineering Department, Universiti Teknologi PETRONAS, 31750, Tronoh, Perak, Malaysia c Institute of Chemical Technology, Vietnam Academy of Science and Technology, 1 Mac Dinh Chi Str., Dist.1, Ho Chi Minh City, Viet Nam d Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-Ku, Sendai 980-8577, Japan e Ceramics and Biomaterials Research Group, Advanced Institute of Materials Science & Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam f Nguyen Tat Thanh University, 300A Nguyen Tat Thanh Street, Ward 13, District 4, Ho Chi Minh City, Viet Nam g ATODATECH LLC, Brentwood, CA 94513, USA

article info

abstract

Article history:

Bi-reforming of methane (BRM) was evaluated for Ni catalyst dispersed on SBA-15 support

Received 13 February 2018

prepared by hydrothermal technique. BRM reactions were conducted under atmospheric

Received in revised form

condition with varying reactant partial pressure in the range of 10e45 kPa and 1073 K in

17 July 2018

fixed-bed reactor. The ordered hexagonal mesoporous SBA-15 support possessing large

Accepted 22 July 2018

specific surface area of 669.5 m2 g
1 was well preserved with NiO addition during incipient

Available online xxx

wetness impregnation. Additionally, NiO species with mean crystallite dimension of 14.5 nm were randomly distributed over SBA-15 support surface and inside its mesoporous

Keywords:

channels. Thus, these particles were reduced at various temperatures depending on

Methane bi-reforming

different degrees of metal-support interaction. At stoichiometric condition and 1073 K, CH4

Ni/SBA-15

and CO2 conversions were about 61.6% and 58.9%, respectively whilst H2/CO ratio of 2.14

Mesoporous silica

slightly superior to theoretical value for BRM would suggest the predominance of methane

Syngas

steam reforming. H2 and CO yields were significantly enhanced with increasing CO2/ (CH4 þ H2O) ratio due to growing CO2 gasification rate of partially dehydrogenated species from CH4 decomposition. Additionally, a considerable decline of H2 to CO ratio from 2.14 to 1.83 was detected with reducing H2O/(CH4 þ CO2) ratio due to dominant reverse water-gas shift side reaction at H2O-deficient feedstock. Interestingly, 10%Ni/SBA-15 catalyst was resistant to graphitic carbon formation in the co-occurrence of H2O and CO2 oxidizing agents and the mesoporous catalyst structure was still maintained after BRM. A strong

* Corresponding author. Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang, Kuantan, Pahang, Malaysia. E-mail addresses: [email protected], [email protected] (D.-V.N. Vo). https://doi.org/10.1016/j.ijhydene.2018.07.136 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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correlation between formation of carbonaceous species and catalytic activity was observed. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction

Methane dry reforming:

Fossil fuels, namely, crude oil, natural gas and coal are currently being exploited at an unprecedented rate to meet the demand of rising global population. In fact, approximately 80% of worldwide energy consumption is derived from petroleum-based resources [1]. The increasing depletion of crude oil resource and growing anthropogenic greenhouse gas emissions have spurred an initiative for exploring an alternative and eco-friendly energy to substitute the nonrenewable fossil fuels. Methane has emerged as a promising energy source since it is highly available and can be obtained through various resources, namely, natural gas (including methane hydrate and shale gas) and biogases (viz., landfill gas and sewage gas) obtained from livestock excretion and fermented wastes [2,3]. The conventional approach for the upgrading of methane is to transform it into synthetic gas or syngas (CO and H2 mixture) followed by Fischer-Tropsch synthesis (FTS) to generate environmentally-friendly synfuel [4,5] for replacing crude oil. Although the industrial syngas production currently employs steam reforming of methane (see Eq. (1)), SRM, this approach emits undesirable CO2 greenhouse gas and yields H2/CO ratio greater than 3 unsuitable for downstream production of long-chain hydrocarbons via FTS [6e8]. In order to produce syngas possessing ideal H2/CO ratio ¼ 2 preferred for FTS, partial oxidation of methane, POM (cf. Eq. (2)) has been implemented [9]. However, several drawbacks such as highly exothermic nature, difficult control due to hot-spot formation and risk of explosions are reportedly associated with POM [9,10]. In this regard, methane dry reforming (MDR) has lately gained substantial interest from industrial and academic research since MDR process not only alleviates emission of CO2 but also convert it to valuable products (see Eq. (3)) [10,11], this process yields a low H2/CO ratio (less than 1) unsuitable for FTS owing to concomitant reverse water-gas shift (RWGS) side reaction [12]. However, several drawbacks are associated with catalytic stability in MDR. Indeed, carbon formation is commonly reported as one of the leading causes for catalyst deactivation [13] followed by sintering of metal particles at elevated temperature [14].

Steam reforming of methane
 CH4 þ H2 O/CO þ 3H2 DG0 ¼ 222:89
0:25T kJ mol
1

(1)

Partial oxidation of methane:
 1 CH4 þ O2 /CO þ 2H2 DG0 ¼
23:85
0:20T kJ mol
1 2

(2)


 CH4 þ CO2 /2CO þ 2H2 DG0 ¼ 259:27
0:28T kJ mol
1

(3)

As a result, an integrated process of CO2 and steam reforming of methane (also known as bi-reforming of methane, BRM given in Eq. (4)) has emerged as an alluring reforming process among conventional techniques due to greenhouse gas (CO2 and CH4) mitigation, high catalytic stability using two oxidizing agents (such as CO2 and H2O) [15] and desired H2/CO ratio easily achieved via manipulation of feedstock composition [16]. The BRM also offers a direct reaction pathway to produce syngas having H2/CO ratio of 2 (known as metgas) without requiring purification and auxiliary separation of oxidative by-product [15]. Bi-reforming of methane:
 3CH4 þ 2H2 O þ CO2 /8H2 þ 4CO DG0 ¼ 705:08
0:78T kJ mol
1 (4) However, the bibliographic reports on BRM are relatively limited in literature owing to reaction complexity involving SRM and MDR reactions as well as numerous non-coke (see Eq. (5)) and coke forming (see Eqs. (6)e(8)) parallel reactions [17]. Reverse water-gas shift reaction: 
CO2 þ H2 %CO þ H2 O DG0 ¼ 36:37
0:03T kJ mol
1

(5)

Methane decomposition:
 CH4 /2H2 þ C DG0 ¼ 87:32
0:11T kJ mol
1

(6)

Boudouard reaction:
 2CO%C þ CO2 DG0 ¼
171:95 þ 0:18T kJ mol
1

(7)

Beggs reaction:
 H2 þ CO%C þ H2 O DG0 ¼
135:56 þ 0:14T kJ mol
1

(8)

In BRM, nickel metal supported on various semiconductor oxides (such as Al2O3 [18] and ZrO2 [19] supports) or mixed metal oxides (including MgOeAl2O3 [20], CeO2eZrO2 [21], and NiOeB2O3 [22,23]) has been intensively studied owing to its lower cost and great catalytic activity relatively similar to noble metals, namely, Rh [24] and Ru [25]. Olah et al. observed that 15%Ni/MgO catalyst achieved high CO2 and CH4 conversions of above 70% for BRM at 7 atm, 1103 K and CH4/CO2/H2O molar ratio as 3.0/1.2/2.4 [15]. However, supported Ni catalysts are reportedly susceptible to rapid catalytic deterioration owing to metal sintering and coke formation [18e20]. In BRM study over 10%Ni/a-Al2O3 catalyst at CH4/CO2/H2O of 0.8/1.0/ 0.4, Roh et al. observed catalyst deactivation within 6 h onstream owing to severe carbon deposited on catalyst surface

Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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[21]. Although the utilization of excess steam feeding stock could effectively eliminate carbon formation during BRM, this approach would adversely increase operating cost and yield an unfavorable H2/CO ratio for FTS [26]. Recently, two different strategies were introduced for preventing carbon formation and enhancing catalytic performance for nickel-based catalysts during high temperature reforming processes [27]. The first approach implies that coke formation can be efficiently inhibited by preparing catalysts with high oxygen storage capacity or oxygen mobility on catalyst surface [28,29]. Therefore, rare-earth oxides such as La2O3 and CeO2 have been widely employed for the oxidative removal of surface carbon species [30e32]. Despite its efficacy, the reforming reactions still suffer from carbon-induced deactivation during longevity tests. Consequently, the second strategy indicates that both coke formation and sintering can also be prevented by providing better metal dispersion, smaller particle size (<2 nm) and high metal-support interaction [33]. In this regard, the highly stable mesoporous silica scaffold such as MCM-41 and SBA-15 is one of the prevalent dispersants for active nickel particles. In addition, the implementation of mesoporous SBA-15 support for suppressing carbon formation in SRM [7,34] and MDR [11,35] reactions has received significant interest owing to its high silanol group density, high surface area and uniformity of pores, enhancing active metal dispersion with small crystallite size [32]. Recently, various advanced modification techniques, namely, core-shell, skeletal structure and phyllosilicates nanotubes were used to provide nanoconfinements for nickel nanoparticles [36e38]. However, a facile and industrially recognized incipient wetness impregnation technique is used in this work due to its simple execution and low-waste streams [39]. To the best of our knowledge, there is no reported investigation discussing about the effect of partial pressure for each reactant (i.e., CH4, CO2 and H2O) on the catalytic activity of SBA-15 supported nickel catalyst during BRM reaction in literature. Additionally, the influence of reactant partial pressure is an essential factor for understanding BRM kinetics and reactor design. Furthermore, the relationship between feed composition and the resulting amount of deposited carbon is an important aspect for ensuring catalytic stability but it has not been examined before. Therefore, a 10%Ni/SBA-15 catalyst prepared via incipient wetness impregnation technique was characterized and tested for BRM at different feed compositions in this work.

Experimental Catalyst preparation Mesoporous silica (SBA-15) support was prepared using a facile hydrothermal route. About 3.13 g of non-ionic triblock poly (ethylene glycol)-block-poly (propylene glycol)-blockpoly (ethylene glycol) referred as Pluronic® P-123 (EO20PO70EO20, Sigma-Aldrich Chemicals) was dissolved in 110 ml of fuming HCl (1.6 M) solution (supplied by Merck Millipore) with pH of about 1. The mixture was subsequently stirred for 2 h at 313 K to ensure the complete dissolution of the P-123 templating agent. Thereafter, tetraethyl

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orthosilicate (TEOS also procured from Merck Millipore) was added to the former transparent solution and then stirred vigorously for 24 h at 313 K. The resulting solution was further transferred to a Teflon-lined stainless steel autoclave and heated at 373 K for 24 h. The white solid powder was subsequently filtered off and washed with a copious amount of deionized water followed by drying overnight in an oven at 333 K and air-calcination at 823 K for 5 h with ramping rate of 2 K min
1. The as-synthesized SBA-15 support was mixed with a precisely calculated amount of Ni(NO3)2$6H2O metal precursor solution (purchased from Sigma-Aldrich Chemicals) using incipient wetness impregnation technique. The resulting ¨ CHI slurry was mixed vigorously in a rotary evaporator (BU Rotavapor R-200) for 2 h at 333 K under vacuum condition. The obtained greenish mixture was then dried overnight in the oven at 333 K and subjected to calcination in air at 1073 K with a heating rate of 2 K min
1 for 5 h.

Catalyst characterization The specific Brunauer-Emmett-Teller (BET) surface area of SBA15 support and 10%Ni/SBA-15 catalyst was calculated from N2 adsorption-desorption isotherms obtained in Micromeritics ASAP-2010 instrument at 77 K. Total pore volume and average pore diameter were determined from the desorption branch of isotherms using the Barrett-Joyner-Halenda (BJH) desorption method at P/P0 ¼ 0.99. Prior to each measurement, the samples were properly degassed at 573 K for 1 h under N2 atmosphere. Xray diffraction experiments were carried out for SBA-15 support, fresh and spent 10%Ni/SBA-15 catalysts on a Rigaku Miniflex II system using Cu monochromatic X-ray radiation with the wavelength, l ¼ 1.5418  A and the system was operated at 30 kV and 15 mA in the angular 2q range of 3 e80 . The low scan speed of 1 min
1 and small stride size of 0.02 were also used to ensure high pattern resolution during the scanning. The average crystallite size was estimated using Scherrer equation [40]. The surface chemistry of SBA-15 support, fresh and spent 10%Ni/SBA-15 catalysts was studied by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy in a Thermo Fisher Scientific Nicolet iS5 spectrometer fitted with an iD7 ATR accessory. All measurements were carried out in the range of 400e4000 cm
1 with the spectral resolution of 4 cm
1 and the number of scans of 100. Spectra were collected and interpreted using OMNIC™ Series software version 8.0. H2 temperature-programmed reduction (H2-TPR) was measured on an AutoChem II-2920 system. The sample was pre-heated in He flow of 50 ml min
1 for 30 min at 373 K for removing moisture and volatile compounds. It was then reduced in 50 ml min
1 of 10%H2/Ar mixture from 373 K to 1173 K with a heating rate of 10 K min
1. High-resolution transmission electron microscope (HR-TEM) images were captured for the fresh 10%Ni/SBA-15 catalyst in TOPCOM EM-002B unit operated at 200 kV. All spent 10%Ni/SBA-15 catalysts obtained through BRM reactions were subjected to temperature-programmed oxidation (TPO) measurements to determine the carbonaceous type and quantify the amount of carbon deposited on catalyst surface. The spent catalysts were firstly dehydrated at 373 K for 30 min in flowing N2 of 100 ml min
1 before being heated under 100 ml min
1 flow of 20%O2/N2 mixture to

Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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1023 K. Specimens were then kept isothermally at the final temperature for 30 min in the same gaseous mixture.

H2 =CO ratio ¼

Bi-reforming of methane tests Methane bi-reforming reactions were performed at atmospheric pressure in a quartz tube (with O.D. ¼ 9.5 mm, I.D. ¼ 7 mm and length ¼ 43 cm) fixed-bed continuous flow reactor. Prior to the catalytic activity test, approximately 100 mg of 10%Ni/SBA-15 catalyst with average particle size of 100e140 mm mounted by quartz wool in the middle of reactor was reduced in-situ with 60 ml min
1 of 50%H2/N2 mixture from 298 K to 973 K with a heating rate of 10 K min
1 and held at this temperature for 2 h. The flow rates of gaseous reactants (such as CH4 and CO2), H2 reducing agent and diluent gas (N2) were precisely controlled by individual mass flow controllers (Alicat mass flow controllers) and the flow rate of water was accurately regulated by the syringe pump (KellyMed KL-602). CH4 and CO2 gaseous reactants previously diluted with N2 gas (acting as a tie component for material balance and ensuring the total flow rate of 60 ml min
1) were mixed with vaporized water before entering the inlet of fixed-bed reactor. BRM reaction was initially carried out at 1073 K and stoichiometric condition with reactant partial pressure, Pi given as; PCH4 ¼ 45 kPa, PH2 O ¼ 30 kPa and PCO2 ¼ 15 kPa (PCH4 :PCO2 PH2 O ¼ 3:1:2). In order to investigate the effect of reactant partial pressure on BRM performance, one reactant was varied within the range of 10e45 kPa whilst the partial pressure of other two reactants was maintained constant at the above values used for stoichiometric investigation. The gas hourly space velocity (GHSV) was also fixed at high
1 for all runs to guarantee the insignificant value of 36 L g
1 cat h internal and external mass transfer resistances. The gaseous products from the outlet of fixed-bed reactor were passed through the cold trap and drierite adsorbent bed to remove the unreacted water from the effluent stream. Additionally, the gaseous composition was analyzed in a gas chromatograph (Agilent 6890 Series GC system) equipped with HP-PLOT Q capillary column (30 m  0.53 mm  40 mm) and TCD detector. In addition, the N2 internal standard was continuously measured during BRM reaction to verify and assure the precise GC analysis. Catalytic assessment is based on reactant conversion, Xi (with i being CH4 or CO2); selectivity, Sj (j: H2 or CO); yield, Yj and product ratio (H2/CO) calculated from Eqs. (9)e(13).

Xi ð%Þ ¼

out Fin i
Fi

Fin i

Sj ð%Þ ¼ P

YCO ð%Þ ¼

YH2 ð%Þ ¼

 100%

Fout j out j¼H2 ;CO Fj

 100%

Fout CO Fin CO2

þ Fin CH4

 100%

2Fout H2 in 4Fin CH4 þ 2FH2 O

 100%

and

(9)

(10)

(11)

(12)

Fout H2 Fout CO

(13)

where Fin and Fout are the corresponding inlet and outlet molar flow rates (mol s
1).

Results and discussion Textural properties The N2 adsorption-desorption isotherms of SBA-15 support and fresh 10%Ni/SBA-15 catalyst are shown in Fig. 1 for confirming the mesoporous characteristic of these metal oxides. Both support and catalyst exhibit a type IV isotherm curve with a noticeable H1-type hysteresis loop at the relative pressure, P/P0 range of 0.58e0.85, which is a typical feature of the well-ordered mesoporous materials with cylindrical channels, according to IUPAC classification [8,41]. In addition, the considerable inflection at P/P0 ¼ 0.58e0.85, identified in the isotherm curves for both SBA-15 support and 10%Ni/SBA-15 catalyst, was attributed to the capillary condensation and N2 desorption within ordered mesopores [42] and hence indicating the presence of large mesopores [43]. Notably, the similarity in isotherm profiles for both SBA-15 support and 10%Ni/SBA-15 catalyst would indicate the insignificant change in the mesoporous structure of SBA-15 support with NiO incorporation during incipient impregnation. However, the pore size distribution (see inset in Fig. 1) shows that the pore diameter ranging from 6.7 to 13.4 nm for SBA-15 support was slightly increased to 14.4 nm with the diffusion of NiO particles into the mesoporous support in agreement with other studies [32]. Although the mesoporous configuration of SBA-15 support was not significantly altered with the incorporation of NiO particles, the textural properties of 10%Ni/SBA-15 catalyst (including BET surface area, total pore volume and average pore diameter) experienced a considerable drop as seen in Table 1. In fact, BET surface area of 10%Ni/SBA-15 catalyst decreased by 20% compared with SBA-15 support whereas a reduction of 25% was observed for total pore volume. The inevitable decline in the physical attributes of 10%Ni/SBA-15 catalyst was possibly due to NiO particle agglomeration and pore blockage.

X-ray diffraction measurements The wide-angle X-ray diffraction patterns of SBA-15 support, fresh and spent 10%Ni/SBA-15 catalysts are displayed in Fig. 2. The Joint Committee on Powder Diffraction Standards (JCPDS) database was used as references for interpreting the X-ray diffractograms of specimens [44]. All diffractograms shown in Fig. 2 exhibited a broad peak between 15 and 35 with center at 22.6 typical for amorphous SiO2 framework of SBA-15 support (JCPDS card No. 29e0085) [45]. Additionally, the uniform bell-shaped curves for the amorphous SiO2 peak of SBA15 support, fresh and spent 10%Ni/SBA-15 catalysts would be indicative of unchanged structure or high stability of

Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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Fig. 1 e N2 adsorption-desorption isotherms and pore size distribution (inset) of (a) SBA-15 support and (b) 10%Ni/SBA-15 catalyst.

mesoporous siliceous material. The distinctive peaks detected at 2q ¼ 37.2 , 43.3 , 62.9 and 75.5 can be indexed as (101), (012), (220) and (311) reflections of NiO phase (JCPDS card No. 47e1049) for the fresh 10%Ni/SBA-15 catalyst (cf. Fig. 2(b)). As seen in Fig. 2(c), the active metallic Ni0 phase was observed for the spent 10%Ni/SBA-15 catalyst with characteristic peaks located at 2q values of 44.6 , 51.9 and 76.5 associated with the corresponding (111), (200) and (220) planes reasonably due to the reduction of Ni2þ to Ni0 particles during H2 pretreatment. However, small intensity peaks typical for NiO phase were also detected for spent catalyst. This observation would be due to the partial re-oxidation of Ni0 phase to NiO particles during BRM reaction with the co-existence of strong CO2 and H2O oxidizing agents in feedstock. Although the re-oxidation of active metal to metal oxide is reportedly one of the main factors inducing catalytic deactivation [46], this behavior could be inevitable because of the nature of BRM reaction using two oxidizing agents as reactants. Indeed, in the study of SRM over 20%Ni/SiO2 catalyst (having only H2O oxidizing agent), Matsumura and Nakamori also detected the formation of NiO phase on spent catalyst owing to H2O oxidation of

metallic Ni0 phase [47]. Thus, it could be more difficult to suppress Ni re-oxidation in the case of BRM reaction. However, as seen in Fig. 2(c), the significantly high intensity of Ni0 phase compared with NiO phase suggested that the reoxidation of metallic Ni0 phase could be relatively low. The mean crystallite size of NiO particles, dNiO was estimated by Scherrer equation as about 14.5 nm (see Table 1) whilst that of NiO and Ni (dNi) phases (estimated from the most intense peaks at corresponding 2q of 43.3 and 44.6 ) for spent catalyst was around 38.2 nm and 34.1 nm, respectively. The considerable increase in crystallite size of the spent catalyst in comparison with fresh catalyst could be due to unavoidable sintering effect at high reaction temperature of 1073 K in agreement with other studies [18e20]. Interestingly, the typical reflection peak of graphitic carbon at 26.2 [32] was not observed for the spent catalyst, suggesting the negligible formation of crystalline carbon due to the presence of strong oxidizing agents (CO2 and H2O reactants) simultaneously gasifying carbonaceous species formed during BRM reaction from catalyst surface and hence preventing catalyst from deactivation. Kumar et al. also observed the absence of

Table 1 e Textural attributes of SBA-15 support and 10%Ni/SBA-15 catalyst. Physical properties

Average BET surface area (m2 g
1)

Total pore volumea (cm3 g
1)

Average pore diameter (nm)

Average crystallite size of NiO phase, dNiO (nm)

SBA-15 support 10%Ni/SBA-15 catalyst

669.5 538.6

1.2 0.9

7.1 6.6

e 14.5b

a b

Total pore volume is calculated using Barret-Joyner-Halenda (BJH) method at P/P0 ¼ 0.99. Average crystallite size of NiO phase computed for the most intense NiO peak at 2q of 43.3 ((012) plane) using Scherrer equation [40].

Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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Fig. 2 e XRD patterns of (a) SBA-15 support, (b) fresh 10%Ni/SBA-15 catalyst and (c) spent 10%Ni/SBA-15 catalyst obtained from BRM at 1073 K, PCH4 ¼ 45 kPa, PCO2 ¼ 15 kPa and PH2 O ¼ 30 kPa.

Fig. 3 e FTIR spectra of (a) SBA-15 support, (b) fresh 10%Ni/SBA-15 catalyst and (c) spent 10%Ni/SBA-15 catalyst obtained from BRM at 1073 K, PCH4 ¼ 45 kPa, PCO2 ¼ 15 kPa and PH2 O ¼ 30 kPa. Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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graphitic carbon on the surface of Ni-based pyrochlore catalyst during BRM reaction at 1023 K due to the steam gasification of partially dehydrogenated CxH1-x or C(s) species to CO and H2 mixture [48].

FTIR spectroscopy analysis Fig. 3 illustrates the FTIR spectra of SBA-15 support, fresh and spent 10%Ni/SBA-15 catalysts. For all samples, the highest absorbance band centred at 1050 cm
1 was ascribed to the condensed siloxane network due to the presence of the asymmetric stretching of SieOeSi lattice vibration [45]. In addition, the other bands detected at around 440 and 810 cm
1 for all samples belonged to the corresponding asymmetric and symmetric stretching vibrations of SieO bond [49]. Additionally, the low-intensity peak observed at ca. 965 cm
1 (black circle in Fig. 3) was attributed to the vibrations of silanol group (SieOH) [50]. The presence of SieO, SieOeSi bonds and SieOH group would further confirm the formation of mesoporous SBA-15 support consistent with XRD results (see Fig. 2) and other studies [49]. The identical FTIR spectra for SBA-15 support, fresh and spent 10%Ni/SBA-15 catalysts would also suggest that the mesoporous structure of SBA-15 support was negligibly altered or preserved during NiO addition and BRM reaction at high reaction temperature of 1073 K. Moreover, the typical vibration of NieO bond in NiO particles at 425 cm
1 was not visibly observed probably due to the overlapping of strong and broad SieO vibration peak belonging to silica support [51].

H2 temperature-programmed reduction measurement Fig. 4 shows the reduction profile of 10%Ni/SBA-15 catalyst during H2-TPR measurement. The H2-TPR test was also

7

performed for SBA-15 support for comparison purpose and investigating support stability during H2 activation. As seen in Fig. 4(a), no peaks were observed for SBA-15 support indicating the resistance of SBA-15 support to H2 reducing agent at high reduction temperature. Thus, three detected peaks (deconvoluted peaks P1, P2 and P3) for 10%Ni/SBA-15 catalyst would belong to the reduction of NiO particles (cf. Fig. 4(b)). However, Ni2þ phase was reportedly reduced to metallic Ni0 phase via one-step process without the formation of any intermediate oxidation states [35]. Therefore, the H2 consumption peaks appeared at different reduction temperature (peaks P1, P2, and P3 at 660, 690 and 770 K, respectively) should be due to the reduction of NiO particles with different degrees of metalsupport interaction [32]. The low temperature peak P1 at 660 K was associated with the reduction of bulky NiO crystallites located on the external surface of SBA-15 support with weak metal-support interaction to Ni0 phase while the broad shoulder (peak P2) centred at 690 K could be due to the reduction of medium NiO particles moderately attached to SBA-15 support [32,35]. In addition, the tiny hump (peak P3) at around 770 K could be attributed to the reduction of NiO particles having smallest size and hence possessing the strongest interaction with support [52]. As seen in Fig. 4, the intensity of deconvoluted peak P1 was significantly higher than that of peaks P2 and P3 indicating that the number of bulky NiO particles was superior to the quantity of medium or small NiO particles on the surface or inside the mesoporous channels of SBA-15 support, respectively. Additionally, there was no detectable peak beyond 850 K suggesting the complete reduction of NiO to Ni0 phase. Hence, the reduction temperature of 973 K was employed in this study to assure that NiO particles were completely reduced to active metallic Ni0 phase prior to BRM reaction.

Fig. 4 e H2 temperature-programmed reduction of (a) SBA-15 support and (b) 10%Ni/SBA-15 catalyst. Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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HR-TEM measurement xCH4 /Cx H1
x þ In order to examine the morphological attributes and the distribution of NiO particles on SBA-15 substrate, the fresh 10%Ni/SBA-15 catalyst was analyzed by HR-TEM measurement as shown in Fig. S1 (see supplementary data). HR-TEM images revealed that SBA-15 support possessed a hexagonal mesoporous structure with the long-range order and textural uniformity of porous channels (see Figs. S1(a) and (b)). Additionally, the parallel fringes with the spacing of about 10 nm (the distance between two bright strips as seen in the magnified inset of Fig. S1(b)) in agreement with results reported by Li et al. [45] can be assigned to the typical and intrinsic structure of mesoporous SBA-15 support [53]. It can be clearly seen from HRTEM micrographs that NiO nanoparticles with different particle sizes were randomly distributed throughout the SBA-15 support. Fig. S1 shows that most NiO particles were located outside the pore channels of SBA-15 support because the pore size of SBA-15 support (about 7.1 nm) was substantially smaller than the average NiO crystallite size (14.5 nm) as seen in Table 1. The unavoidable NiO agglomeration was due to the migration of NiO particles from the pore channels to the external support surface at elevated calcination temperature of 1073 K employed to ensure catalytic thermal stability during BRM reaction [32]. Based on their varying particle sizes, NiO particles were classified as: large (red circles), moderate (broken dark blue circles) and small (white square) NiO particles as shown in Fig. S1 of supplementary data. Additionally, it seemed that the population of large NiO particles was superior to that of moderate particles followed by small particles in agreement with H2-TPR results (cf. Fig. 4). In addition, these aggregated NiO particles on support surface possibly blocked the pore channels of SBA-15 support, resulting in a considerable and undesirable drop in BET surface area of 10%Ni/SBA-15 catalyst compared with SBA15 support (see Table 1).

Catalytic activity of BRM reaction Effect of CH4/(CO2 þ H2O) ratio As seen in Eq. (4), the stoichiometric feed composition for BRM reaction is CH4:CO2:H2O ¼ 3:1:2. Since CO2 and H2O oxidizing agents play an important role in the catalytic performance and stability of BRM reaction, the influence of CH4 to (CO2 þ H2O) ratio on BRM reaction metrics is investigated in this section by keeping CO2 and H2O partial pressure constant at 15 kPa and 30 kPa, respectively and varying CH4 partial pressure of 25, 35 and 45 kPa. The effect of CH4/ (CO2 þ H2O) ratio on reactant conversion (Xi), H2/CO ratio, product selectivity (Sj) and yield (Yj) at 1073 K is shown in Fig. 5. As seen in Fig. 5(a), CH4 conversion experienced a considerable enhancement from about 61.6% to 81.1% with decreasing CH4/(CO2 þ H2O) ratio from 1 (stoichiometric condition) to 0.56. This observation would be due to the increasing gasification rate of deposited carbon (CxH1-x, x  1) produced from methane decomposition (see Eq. (14)) in CH4deficient or oxidizing agents-rich feedstock as seen in Eqs. (15) and (16).

  5x
1 H2 2 

Cx H1
x þ xCO2 /2xCO þ

 1
x H2 2

(14)

(15)

and Cx H1
x þ xH2 O/xCO þ

  1þx H2 2

(16)

Indeed, in the thermodynamic study of BRM reaction, Jang et al. also reported that coke formation was suppressed at low CH4/(CO2 þ H2O) ratio of about 0.5 [26]. Additionally, Qin et al. employed in situ isotope-labelled 13CO2 for studying the mechanistic pathway of BRM reaction and proposed that the adsorbed atomic oxygen, Oad formed by the dissociative H2O and CO2 adsorption could react with adsorbed carbon species produced from the dissociative adsorption of CH4 on catalyst surface to generate CO gas [24]. Thus, in CH4-deficient environment, the excess Oad species could result in increasing CH4 consumption. However, as seen in Fig. 5(a), a significant decline in CO2 conversion was observed from 58.9% to 47.2% with decreasing CH4/(CO2 þ H2O) ratio from 1 to 0.56 most likely due to the presence of inadequate amount of CH4 reactant considered as a limiting reactant for reacting with CO2-excess feed composition. Interestingly, H2/CO ratio improved considerably with rising CH4/(CO2 þ H2O) and reached to about 2.14 (at stoichiometric feed composition) which is close to the ideal H2/CO ratio of 2 (favored for downstream FTS [6,54,55]) in agreement with other studies [15]. The obtained H2/CO ratio of 2.14 slightly higher than the theoretical H2/CO ratio of 2 for BRM reaction at stoichiometric condition (cf. Eq. (4)) indicated the predominance of simultaneous SRM side reaction. As seen in Fig. 5(b), both H2 selectivity and yield decreased significantly with reducing CH4/(CO2 þ H2O) ratio from 1 to 0.56 but the inverse trend was experienced for selectivity and yield of CO. This could be due to the simultaneous occurrence of thermodynamically favored RWGS reaction (see Eq. (5)) [17], consuming H2 and generating CO product and hence decreasing H2/CO ratio (cf. Fig. 5(a)).

Effect of CO2/(CH4 þ H2O) ratio In order to investigate the role of CO2 oxidizing agent on BRM performance, CO2 partial pressure was varied from 10 to 20 kPa whilst PH2 O and PCH4 were maintained at 30 kPa and 45 kPa, respectively. As seen in Fig. 6(a), both CH4 and CO2 conversions substantially increased with rising CO2/ (CH4 þ H2O) ratio from 0.13 to 0.27 reasonably owing to the enhancement of MDR reaction in CO2-rich feedstock in agreement with other studies [26]. In addition, H2/CO ratio decreased considerably from 2.65 to 1.88, which is less than the stoichiometric H2/CO ratio of 2 for BRM, with growing CO2 composition further confirming the dominance of MDR reaction with CO2-excess reactants. In fact, SRM and MDR reactions reportedly yielded H2/CO ratio of 3 [6] and less than 1 [6,11], respectively. Thus, the predominant MDR side reaction during BRM would result in a lower H2/CO ratio than 2. Additionally, as shown in Fig. 6(b), increasing CO2 feed composition

Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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 x x x ( 2 0 1 8 ) 1 e1 4

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Fig. 5 e Effect of CH4/(CO2 þ H2O) feed ratio on (a) reactant conversions and H2/CO ratio; and (b) product selectivity and yield at PCO2 ¼ 15 kPa, PH2 O ¼ 30 kPa, PCH4 ¼ 25e45 kPa and 1073 K for 10%Ni/SBA-15 catalyst.

improved CO selectivity but reduced H2 selectivity corroborated with the considerable drop in H2/CO ratio (see Fig. 6(a)). However, both H2 and CO yields were enhanced significantly with growing CO2/(CH4 þ H2O) ratio due to the improvement of CO2 gasification (see Eq. (15)) of partially dehydrogenated CxH1-x species generated from methane cracking (cf. Eq. (14)) into H2 and CO products.

Effect of H2O/(CH4 þ CO2) ratio The influence of H2O oxidizing agent composition on BRM reaction was also examined by varying H2O partial pressure from 15 to 30 kPa at constant CH4 and CO2 partial pressure of 45 kPa and 15 kPa, respectively with reaction temperature of 1073 K. As seen in Fig. 7(a), CO2 conversion was greatly increased from 58.9% to 73.5% with decreasing H2O/

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Fig. 6 e Effect of CO2/(CH4 þ H2O) feed ratio on (a) reactant conversions and H2/CO ratio; and (b) product selectivity and yield at PCO2 ¼ 10e20 kPa, PH2 O ¼ 30 kPa, PCH4 ¼ 45 kPa and 1073 K for 10%Ni/SBA-15 catalyst.

(CH4 þ CO2) ratio from 0.5 to 0.25 (or lowering H2O partial pressure from 30 to 15 kPa) but an opposite behavior was observed for the conversion of CH4. The enhancement of XCO2 and the decline in XCH4 in H2O-deficient environment could be due to the prevalence of MDR reaction (see Eq. (3)) and the reverse Boudouard (or CO2 gasification) side reaction

consuming CO2 to gasify deposited carbon from methane decomposition (cf. Eq. (6)). Additionally, the CO2 conversion was superior to that of CH4 conversion at low H2O/(CH4 þ CO2) ratio of 0.25e0.33 further confirming the dominant MDR over SMR in inadequate H2O feed composition. However, at the stoichiometric condition (H2O/(CH4 þ CO2) ratio of 0.5), CH4

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Fig. 7 e Effect of H2O/(CH4 þ CO2) feed ratio on (a) reactant conversions and H2/CO ratio; and (b) product selectivity and yield at PH2 O ¼ 15e30 kPa, PCH4 ¼ 45 kPa, PCO2 ¼ 15 kPa and 1073 K for 10%Ni/SBA-15 catalyst.

conversion was greater than CO2 version as SRM appeared to be more predominant than MDR. This could be attributed to the less endothermic character of SRM than MDR reaction due to the inferior stability of H2O reactant in SRM to that of CO2 reactant in MDR [25]. Additionally, as seen in Fig. 7(a), a

significant drop in H2/CO ratio from 2.14 to 1.83 with a decrease in H2O/(CH4 þ CO2) ratio from 0.5 to 0.25 was assigned to the enhancement of RWGS side reaction in H2Odeficient or CO2-rich feedstock [56]. In fact, the decline in selectivity and yield of H2 and the improvement of CO

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Table 2 e Summary of total carbon deposition on catalyst surface after BRM reaction at different feedstock ratios. Investigation factor

Reaction conditions

Ratio

Total carbon deposition (%)

1.00 0.78 0.56 0.27 0.20 0.13 0.50 0.33 0.25

0.95 0.79 0.13 0.70 0.95 2.28 0.95 2.50 4.80

Reactant partial pressure (kPa) CH4/(CO2 þ H2O) ratio

CO2/(CH4 þ H2O) ratio

H2O/(CH4 þ CO2) ratio

CH4

CO2

H2O

45 35 25 45 45 45 45 45 45

15 15 15 20 15 10 15 15 15

30 30 30 30 30 30 30 20 15

selectivity and CO yield in H2O-deficient feedstock could further confirm the increasing rate of RWGS during BRM reaction.

TPO measurements of spent catalysts The TPO analysis was carried out for all spent catalysts to quantitatively measure the amount of carbon deposition during each BRM run. The influence of feed ratio on TPO weight profiles is shown in Figs. S2-S4 (see supplementary data). The weight profiles for all spent catalysts show an initial weight loss before 375 K because of moisture removal. The gradual decline in sample weight from 375 to 625 K was assigned to deposited carbon oxidation (see Figs. S2-S4 in supplementary data). The calculated total carbon deposition (%) on used catalysts at different BRM reaction conditions is summarized in Table 2. The TPO results showed a strong correlation between the quantity of deposited carbon and catalytic activity of the 10%Ni/SBA-15 catalyst for BRM. As CH4/(CO2 þ H2O) ratio was increased from 0.56 to 1, the total carbon deposition rose from 0.13% to 0.95%. Interestingly, as previously discussed about Fig. 5(a), CH4 conversion experienced a noticeable drop with rising CH4/(CO2 þ H2O) ratio from 0.56 to 1. This behavior could evidently deduce that utilizing CH4-rich or oxidizing agentsdeficient feedstocks increased the rate of CH4 decomposition to deposited carbon which in turn resulted in the decline of CH4 conversion. However, as seen in Table 2, the total carbon deposition ranging from 0.13% to 0.95% was significantly lower than that of conventional steam [7,57] or dry [58,59] reforming processes at similar reaction conditions over SBA15 supported Ni catalysts. The negligible carbon deposition from BRM reaction was rationally due to the presence of two oxidizing agents (CO2 and H2O), hindering carbon formation. The beneficial effect of individual oxidizing agents on carbon resistance was also examined by TPO measurements for spent catalysts obtained from BRM reactions at varying CO2/ (CH4 þ H2O) or H2O/(CH4 þ CO2) ratios. Table 2 shows that increasing CO2/(CH4 þ H2O) ratio from 0.13 to 0.27 induced a considerable decrease in carbon formation from 2.28% to 0.7%, implying that the rising CO2 gasification of CxH1-x species in CO2rich feedstock would enhance carbon resistance. Thus, the decline in total carbonaceous deposition with growing CO2/ (CH4 þ H2O) ratio contributed to improve both CH4 and CO2

conversions (cf. Fig. 6(a)). The same behavior for the amount of carbon deposition was also observed with rising H2O/(CH4 þ CO2) ratio from 0.25 to 0.50. As seen in Table 2, the total carbon deposition declined from 4.80% to 0.95% with increasing H2O/ (CH4 þ CO2) ratio owing to rising steam gasification rate [60,61]. Therefore, the reduction in deposited carbon on catalyst surface resulted in an enhancement of CH4 conversion as seen in Fig. 7(a).

Conclusions SBA-15 supported Ni catalyst has been successfully synthesized using incipient wetness impregnation of Ni(NO3)2 on SBA-15 support previously prepared by hydrothermal technique. The inevitable drop in BET surface area from 669.5 m2 g
1 (SBA-15 support) to 538.6 m2 g
1 (10%Ni/SBA-15 catalyst) was indicative of successful NiO particle penetration into the mesoporous cylindrical channels of SBA-15 support and hence pore blockage. A considerable increase in crystallite size was observed for both Ni (34.1 nm) and NiO (38.2 nm) phases of spent catalyst obtained after BRM at stoichiometric condition and 1073 K was due to unavoidable metal sintering at elevated reaction temperature. The identical FTIR spectra and un-altered N2 adsorption-desorption isotherms of SBA-15 support, fresh and spent 10%Ni/SBA-15 catalysts would suggest that mesoporous structure of SBA-15 support was well preserved during NiO addition and BRM reaction. NiO nanoparticles were completely reduced during H2-TPR at beyond 850 K and the reduction temperature of NiO to metallic Ni0 phase was dependent of metal-support interaction associated with NiO crystallite size, location and confinement effect. The oxidizing agents (CO2 and H2O) played a key role in controlling carbon formation during BRM reaction due to the capability of gasifying the partially dehydrogenated CxH1-x or C(s) species to CO and H2 mixture. In fact, no evidence of graphite was observed on catalyst surface by XRD measurements. The decrease in CH4/(CO2 þ H2O) ratio increased significantly CH4 conversion from 61.6% to 81.1% while an opposite trend was observed for CO2 conversion. H2/CO ratio can be easily adjusted by altering CH4, CO2 or H2O partial pressure within 1.69e2.65. Interestingly, an ideal H2/CO ratio of about 2.14 close to the composition of metgas was achieved at stoichiometric feed composition and was preferred for downstream FTS to produce long-chain hydrocarbons. Additionally, increasing CO2/

Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136

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(CH4 þ H2O) ratio considerably enhanced H2 and CO yields because of rising CO2 gasification rate of partially dehydrogenated intermediate species. In addition, H2/CO ratio decreased from 2.14 to 1.83 with reducing H2O/(CH4 þ CO2) ratio from 0.5 to 0.25 reasonably due to the growing parallel RWGS reaction in H2O-deficient feed composition.

Acknowledgements The authors would like to acknowledge the financial support provided by the Universiti Malaysia Pahang (UMP Research Grant Scheme, RDU170326). Doctoral Scholarship Scheme (DSS) conferred to Sharanjit Singh by UMP is also greatly appreciated.

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.07.136.

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Please cite this article in press as: Singh S, et al., Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.136