Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane

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Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane Aybu¨ke Leba 1, Ramazan Yıldırım* _
azic¸i University, Department of Chemical Engineering, Bebek, Istanbul, Bog 34342, Turkey

highlights

graphical abstract


Various structural forms of nickelcobalt catalysts were tested for DRM activity.
NieCo impregnated MgO wash coated over monolith gave the best performence.
It was stable for 48 h with reduced coke deposition with 3% O2 in feed.
Such

structured

catalysts

can

improve the commercialization of DRM process.

article info

abstract

Article history:

In this study, catalytic activity of carbon dioxide reforming of methane was investigated

Received 7 October 2019

over nickel-cobalt catalysts in various structural forms. Catalytic activity tests were per-

Received in revised form

formed at the temperatures of 600e800  C in a micro-flow quartz reactor. SEM-EDX, XRD

27 November 2019

and XPS studies were also performed to understand the surface morphology of the cata-

Accepted 5 December 2019

lysts. The results showed that 8 wt%Ni-2wt.%Co on wash-coated MgO over monolithic

Available online xxx

structure led to highest catalytic performances with CH4 and CO2 conversions of 83% and 89% respectively as well as H2/CO ratio of 0.95 at 750  C. SEM-EDX and XPS results of

Keywords:

catalyst spent at 750  C also showed considerable amount of coke formation; however, the

Carbon dioxide reforming

use of 3% oxygen in the feed suppressed the coke formation significantly. The catalyst was

Dry reforming

stable for 48 h in the presence of O2 (3%) added feed at the temperature of 750  C.

NieCo catalyst

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Structured catalyst Monolith Nanowire

* Corresponding author. E-mail address: [email protected] (R. Yıldırım). 1
ı, Osmaniye, 80000, Turkey Present Address: Osmaniye Korkut Ata University, Department of Chemical Engineering, Fakıus‚ag https://doi.org/10.1016/j.ijhydene.2019.12.020 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Introduction Dry reforming of methane (DRM) has received great attention for the last few decades as it uses two main greenhouses gases (CH4 and CO2) at the same time and provides synthesis gas (a mixture of hydrogen and carbon monoxide) with a suitable H2/CO ratio of approximately one for the production of valuable chemicals through processes such as Fisher-Tropsh synthesis [1e3]. The reaction (Eq. (1)) is highly endothermic and requires minimum 550  C operation temperature [4]. It is possible to use natural gas as well as the landfill gas and biogas from other biological sources in DRM [5e8].   CH4 þ CO2 42H2 þ 2CO DH298K ¼ 247 kJ mol

(1)

The main DRM reaction is highly endothermic, and accompanied by some side reactions such as reverse water gas shift reaction (RWGS; Eq. (2)) consuming H2 and CO, the methane cracking reaction (Eq. (3); above 553e557  C) or Boudourad reactions (Eq. (4); below 700  C) causing carbon deposition and catalyst deactivation [9e11]. CO2 þ H2 4CO þ H2 O

  DH298K ¼ 41kJ mol

(2)

CH4 4 C þ 2H2

  DH298K ¼ 75kJ mol

(3)

2CO 4 C þ CO2

  DH298K ¼ 172kJ mol

(4)

Therefore, from the industrial point of view, the development of an effective and economic catalyst to prevent these reactions with high activity and selectivity as well as longterm stability is crucial. For this purpose, several catalysts, mostly Ni based, have been developed so far; noble metal based catalysts such as Pt, Ru, Rh etc. have been also suggested as promising DRM catalysts with good stability but they have high cost [12,13]. Non-noble metal based catalysts such as Ni catalysts, on the other hand, suffer from coke formation and frequently experienced sintering problem even though they seem to be good alternatives for the process with their low cost and remarkable activity [14e16]. In order to improve the catalytic activity of Ni catalysts, together with long term stability and good resistance to coke formation and sintering, various precautions have been tested such as the utilization of several support types like Al2O3, MgO, SiO2 and ZrO2 [17e21], the addition of promoters such as CeO2, Mg, La, Mn, Zr, TiN and Co [22e27], the change of the catalyst preparation method and particle size [15,28e30], the usage of the bimetallic catalysts [31e33] changing reduction temperature [34] and calcination conditions [35e37]. It was argued, in various publications, that the basic supports such as MgO, CaO, BaO, K2O, and Ca2SiO4 should be used to increase CO2 activation ability and hence suppress coke formation [38e40]. Mesoporous supports such as SBA-15 has also widely studied since it has high surface area and pore size [4,25]. Several researchers also reported that the addition of second metal such as Co or Fe to Ni catalyst was beneficial to reduce coke formation with reasonable activity [41e46]. The synergic effect between bimetallic NieCo catalysts is the main reason for this superiority of the catalyst [4]. Co takes part in the oxidation of surface carbon [44]. Luisetto et al. [31] reported

that bimetallic CoeNi (Co 3.75 wt%, Ni 3.75 wt%) catalysts over CeO2 support showed better catalytic activity at 800  C compared to monometallic ones with Ni (7.5 wt%) and Co (7.5 wt%), respectively, due to the intrinsic property of NieCo alloy. Moreover, CoeNi and Co based catalysts were ended up with low carbon formation stemming from carbon deposition preventing property of Co and strong chemical interaction with Ni. Wu et al. [2] also indicated in their study that the good DRM catalytic stability of 4.5Ni0$5Co/SBA15 prepared by the b-cyclodextrin modified co-impregnation method as related to enrichment of the catalyst with of trace amount Co which prevented the sintering of metal particles and enabled the adsorption of CO2. Zhang et al. [33] stated that among NieMe (Me ¼ Co, Fe, Cu and Mn), NieCo/MgO catalyst showed excellent catalytic performance in their study as a cause of good metal dispersion, strong metal-support interaction, solid-solution as well as the synergetic effect. As it was mentioned above, the particle size of NieCo clusters play a crucial role on low amount of carbon formations. For instance, it has been suggested that particle size of below 10 nm on inert CeZrO2 support and below 20e45 nm on highly reducible CeZrO2 positively affect the carbon deposition while above these sizes this impact becomes less important [47,48]. Gao et al. [43] investigated the catalytic performance of bimetallic NieCo catalyst over silica support in CDRM. The addition of Co into Ni catalyst was believed to enable the controlling of Ni metal size and increase the interaction of Ni-silica support as well as inhibit the Ni sintering and oxidation through the electron transfer of Co. They also concluded that the catalytic activity of this catalyst remained the same over 30 h without any carbon deposition or metal sintering. There has been also some work combining the dry reforming with other reforming technologies such as autothermal reforming process [49] or partial oxidation [50]. Most of the studies in the literature were conducted in a fixed bed reactor with the particulate form of the catalyst while there were also a few studies performed over the monoliths [49,51]. Metal and ceramic monolithic structures having many channels inside separated by thin walls [52] offer some superiorities against the packed bed reactors such as low pressure drop (thus low energy loss) and higher mechanical strength, and they can be also easily scaled up by just adjusting the number of channels [53e57]. In case of DRM reactions, wash-coated monolithic Rh/Al2O3 catalyst [49], monolithic Ni/Al2O3 catalyst [58], micro-fibrous structured Ni/ Al2O3 catalyst [59], Ni-based monolithic catalysts over SBA-15/ Al2O3/FeCrAl [60], Ni supported on g-Al2O3 promoted by Ru both in packed and monolithic reactors [61] have been investigated. In literature, there are also works that integrated nanostructured materials to monolithic honeycomb structures to improve their properties such as enhanced surface area, thermal stability and high catalyst utilization efficiency for various reaction systems such as nitric oxide CH4 and CO oxidation [62e64]; monolithic honeycombs can act either as support or an active material depending on the reaction. They can also provide precise and optimum microstructure control as well as the applicability in correlating materials structure with properties [65]; additionally, they reduce the diffusion distance and amount of material use [64].

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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In the present work, catalytic performance of DRM was evaluated on nickel-cobalt catalysts in various structural forms to contribute to the search of an active catalyst in an easily scalable and low pressure drop structure; as far as we know, there is no such comprehensive work to improve the structure of catalyst for DRM process in literature. The 8 wt% Ni-2wt.%Co, which was found to be effective in previous works [34,66] was used in all experiments. The preliminary works over particulate 8 wt %Ni-2wt.%Co wash-coated on cordierite monolith indicated that the performance could be improved by modifying the structure. Hence, several structural forms of support such as particulate MgO (Im_P), nanorod MgO (Im_NR), particulate MgO washcoted over monolith (Im_P_M) and nanorod washcoated over monolith (Im_NR_M) were investigated. The structure of the active part (NieCo) was also changed to form Ni0·5Co2·5O4 nanoarrays, which were grown over monolith (NW_M). Finally, a small amount of oxygen (1e3%) was introduced to the feed to suppress the coke formation with minimum inverse effect on the product distribution.

Experimental Catalyst preparation Particulate NieCo/MgO (Im_P) catalysts were prepared by incipient wetness impregnation technique with an active metal loading of 8 wt%Ni and 2 wt%Co. MgO powder in the vacuum flask was first mixed with an ultrasonic mixer under vacuum for 30 min. Then, proper amount of Ni(NO3)2$6H2O and Co(NO3)2$6H2O dissolved in deionized water were coimpregnated. After this, the resulting slurry was dried at 120  C for 12 h, and then calcined at 500  C for 5 h. The metal loading, calcination conditions and the reduction conditions given below were taken from the work by Huo et al. [34] over particulate NieCo/MgO catalysts to focus to the effects of structural form of the catalyst. To prepare a NieCo catalyst impregnated on particulate MgO wash coted over monolith (Im_P_M), the commercial ceramic cordierite (2MgO$2Al2O3$5SiO2) with the channel with of 0.9 mm were cut into the piece with the dimensions of 8 mm diameter and 18 mm length, washed with acetone to remove the possible impurities and dried in the oven at 120  C for 2 h. The clean monoliths were put into solution of appropriate amount of MgO and deionized water and then mixed with an ultrasonic mixer for 40 min. Every 10 min, excess MgO was flushed out by air with the help of a syringe; then they were dried for 40 min in a microwave oven with a power of 160 W. Approximately, 20 wt% of MgO was coated on bare monolith and the excess MgO were removed by scrubbing with the help of a needle of syringe. After wash-coating process, proper amount of Ni(NO3)26H2O and Co(NO3)26H2O were dissolved in deionized water injected drop by drop to the channels of the monolith. All channels and surfaces of the monoliths were wetted by droplets. The coated monoliths, which contained catalytic material equivalent to 50 mg particulate catalyst were dried at 120  C for 4 h and calcined at 500  C for 5 h. MgO nanorods were synthesized using the similar procedure of Al-Hazmi et al. [67]. The proper amount of urea and

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magnesium acetate hydrate were dissolved in 25 ml and 75 ml distilled water respectively, and mixed separately for 30 min with magnetic stirrer; then the urea solution was added to magnesium acetate hydrate solution and mixed for another 5 min. The well mixed solutions then transferred into 100 ml Teflon autoclave and put into the oven at 180  C for 2 h. After the autoclave was cooled to room temperature, the solution in the autoclave was transferred into centrifuge tube for the centrifugation at 4100 rpm for 10 min. Then the products were collected from the tube and filtered with distilled water and then ethanol to reduce agglomeration. The final products were dried at 60  C for 24 and then calcined at 600  C for 1 h. In order to obtain 8 wt%Ni e 2 wt%Co over MgO nanorod (Im_NR), proper amount of NieCo active metal solutions were impregnated, dried and calcined as in the NieCo/MgO particulate catalyst synthesis. For the preparation of nanorod wash coated over monolith (Im_NR_M), the appropriate amount of synthesized nanorod MgO mixed in distilled water and the slurry directly washcoated over the monolith followed by calcination at 600  C. The active metal solution was added by injection, and the monolith was dried and calcined as described for NieCo/MgO monolith synthesis above. Finally, Ni0$5Co2$5O4 nanowire synthesized over the monoliths (NW_M) were prepared following a similar procedure used by Ren et al. [63]. The cordierite monoliths were cut and cleaned as in the preparation method of NieCo Monolith catalyst. 0.5 mol/lt Ni(NO3)26H2O and 0.5 mol/lt Co(NO3)26H2O were dissolved in deionized water. The monoliths were put into active metal solutions and mixed ultrasonically. 5e10 mol urea was then added into the active metal solution in ultrasonic mixing and the solution was kept at 90  C for 2 h. The solution was then put in oven to complete the synthesis at 90  C for 10 h. After the reaction processes, the monolith was removed from the solution and softly rinsed by distilled water, flushed out by air with the help of a syringe and dried at 80  C for 4 h, and annealed at 300  C for 4 h with ramping 20  C/min.

Catalytic activity test Activity tests were performed in a packed-bed, downward tubular quartz reactor with a 775 mm length and 10 mm inner diameter. 50 mg particulate (Im_P), nanorod (Im_NR) or a monoliths containing wash coated catalysts (Im_P_M and Im_NR_M) equivalent to 50 (±0.1) mg particulate catalyst were placed to the reactor with the help of quartz wool; the amount of active catalyst was 65 mg for Ni0$5Co2$5O4 for the nanowire (NW_M) because it could not possible to obtain exact amount each time (50 mg would not be exactly equivalent to others either due to the completely different molecular formula). Before each activity test, the catalysts were reduced with H2 gas of 25 ml min1 at 800  C for 1 h. NW_M catalyst was also tested without reduction and with reduction at 600  C for 1 h. Total flow rate was about 70 ml min1 (10 ml min1 of total flow was N2 as internal standard while 60 ml min1 was 1 gas hourly reactant gases): this corresponds 84000 mlg1 cath space velocity (GHSV) over a 50 mg catalyst. In O2 tests 42000 1 GHSV was used, and 0e3% O2 was added to the feed. mlg1 cath The product stream was analysed by Schimadzu GC 2014 Gas

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Chromatograph with a Carboxen1000 column. First data was taken after 1 h of all reactant gases introduced.

Catalyst characterization SEM-EDX analysis of fresh-reduced and used catalyst samples were conducted using Philips XL30 ESEM-FEG system at Bogazici University Advanced Technologies R&D Laboratory. The crystallographic structures of the monolithic catalyst samples were analysed by Rigaku D/MAX-Ultimaþ/PC X-Ray diffraction (XRD) equipment with Cu Ka radiation (l ¼ 0.154 nm) in a scanning angle with a range of 3e90 at a rate of 2 /min and with an accelerating voltage of 40 kV and a current of 40 mA. The nature of metal species on the catalyst samples were investigated by Thermo Scientific X-ray Photoelectron Spectrometer (XPS) with a source gun of Al K Alpha XRay 000 and with a spot size of 400 mm in the same laboratory. The binding energy scale of the XPS spectra was adjusted to the C1s line.

Thermodynamic equilibrium analysis The thermodynamic equilibrium product analysis was performed by HSC Chemistry 6.0 software using Gibbs free energy minimization technique in order to determine the consistency of the experimental results with the thermodynamic limits. In these calculations, CO2, CH4, H2, CO, H2O and solid graphite were taken as the products.

Results and discussion In this section, the characterizations of fresh and spent catalyst were discussed first. Then activity tests of catalysts performed at 600e800  C, flow rate of 70 ml min1 and CH4/CO2 of 1.0 will be discussed; this will be followed by the discussion of time on stream tests as the initial indicators for the stability of various structures. After that, the effects of small amount of oxygen on the coke deposition will be presented for the most promising structural form; the stability test for the final catalyst will be also discussed briefly.

Characterization of the fresh and spent catalysts Fig. 1 shows the SEM images of 8 wt%Ni-2wt.%Co over MgO coated monolithic structures (Im_P_M) of reduced-fresh (Fig. 1aec), spent at 750  C for 8 h in the absence (Fig. 1def) and in the presence of 3% O2 in the feed (Fig. 1gei). The support seemed to be uniformly coated on the monolithic structure as a thin film (Fig. 1a, b); the dark points in Fig. 1b are natural pore structure of the monoliths. It is not easy to distinguish Ni and Co active metals in Fig. 1c; however, they were well dispersed over the support as to EDX mapping (not shown). The average value of EDX analysis of the images taken from three different point in 2000x (Fig. 1b) with 60 scans gave 7.7 wt% Ni, and 1.9 wt% Co, which are reasonably close to the targeted loadings indicating that the active metals are well dispersed on the MgO coated monolith. As it can be seen from Fig. 1d, the coke deposition over the monolithic piece was apparent from even the colour of used

monolithic piece compared with the coated but unused monolith presented in Fig. 1a. The coke deposition was also observable in the lateral SEM image of used monolithic channel Fig. 1e, f; EDX analysis also showed that the average carbon deposited over the monolith surface was 41.3 wt% (it was 2.6 wt% for unused monolith) while the Ni and Co concentrations dropped to 4.6 wt% and 1.3 wt%, respectively. As discussed in Introduction in details, various investigators found that the Co addition normally decreases coke formation, and attributed this to several effects like ability of Co oxidizing surface carbons [44], synergic interactions between N and Co [4] and formation of small NiCo clusters preventing coke formation [47,48]. However, it seems that the coke formation was not completely suppressed in our case. Then a small amount of oxygen (3%) was added to the feed to see whether the coke deposition was successfully depressed without affecting the conversion and product distribution (Fig. 1gei). Although there are works in the literature aiming to combine dry reforming with partial oxidation (oxyCO2 reforming) [68], auto-thermal reforming [49], steam reforming [69] or to regenerate the deactivated catalyst with cyclic O2 addition [70], only 1e3% O2 was used in this work to test the possibility of reducing coke formation without fundamentally changing the product distribution of dry reforming process. The comparison of Fig. 1a, d and g indicates that 3% O2 worked well and significantly prevented the coke deposition. The EDX mapping for C element in the absence and presence of 3% oxygen also verify this result; C content on the surface changed from 41 wt% to 13.3 wt% (Ni and CO were 5.3 wt% and 1.5 wt% respectively); as will be discussed letter the addition of 3% oxygen did not change the product distribution significantly. Fig. 2 shows the SEM images of MgO nanorods (or nanotubes), which seem to be synthesized quiet successfully; the walls made of much smaller spherical MgO nanoparticles (Fig. 2a, b). As seen from Fig. 2c, d, the impregnated active metals formed thinner nanowires and nano sheets over the MgO (Im_NR); the nonorod/nanowire structure of MgO was deformed significantly during impregnation. Han et al. [71] also obtained a flower-like nano-architecture composed of nano-sized flakes in MgOeAl2O3 aerogel samples for CO2 capture process. In addition, Zhang et al. [72] observed a similar morphology (thin sheets, which were intercrossed with each other) in the sample of Ni/MgO containing 15 wt % Al2O3 for dry reforming of methane. The SEM images of Im_NR catalyst, which was spent at 750  C for 8 h (Fig. 2e, f) showed that nanowires and nanosheets of active metals were mostly preserved after reaction even though the nanostructure of the MgO support was further deformed. The SEM images of NieCo impregnated-MgO nanorod wash coated over monolith (Im_NR_M) are given in Fig. 3a, b, which showed a good dispersion of MgO structures over the monolith. Additionally, the impregnation of Ni and Co did not make any significant deformation on the nanorod structure as it did in the absence of monolith (Fig. 2); apparently fixing the MgO nanorods over the surface of monolithic helped them to preserve their structure against the adverse effects of impregnation procedure. The tiny nanowire structures (probably Ni and Co) appeared after impregnation on sole MgO nanorods were not observed here. Interestingly, this catalyst

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 1 e Images of fresh-reduced NieCo/MgO coated monolith (Im_P_M) fresh-reduced (a), spent one in the absence of O2 (d), spent one in the presence of O2 (3%) and their SEM internal views in 200x (b), (e), (h) and in 50000x (c), (f), (i), respectively.

also preserved its structure much better than sole MgO 1 nanorods after the reaction at 750  C with an 84000 mlg1 cath GHSV for 8 h as well. Compared to the SEM tests results in wash-coated-MgO structured catalyts (Im_P_M) in Fig. 1e, f, the carbon formation was lower (avg. 4 wt% compared to 41.3 wt%) in the nonarod structured monolithic catalyst (Im_NR_M). The SEM results of Ni0$5Co2$5O4 over monolith (NW_M) in Fig. 4aec showed that the nanowires over ball-like Ni0$5Co2$5O4 structures, which were similar to those developed by Ren et al [63], were formed. Although the ball-like structures are concentrated in some region of monolith (even occasionally on top of each other), the distribution was quite homogeneous. The preliminary study of the reduced samples of the nanowires under H2 flow at 800  C showed thicker nanowire structured and it was concluded that these nanowires could be sensitive to temperature and reactant flow. Hence, the tests were also repeated without reduction and reduction at 600  C. The images shown in Fig. 4 are for the fresh (a-c) and spent (df) samples reduced at 600  C for 1 h before reaction. As it is apparent from Fig. 4def, the morphology of the catalyst could not be preserved well during the reaction; while the nanowire structures turned into smoother clusters, large amount of

carbon wires (white spaghetti like structures in Fig. 4f) also formed. This catalyst also lost its stability in a short time as it will be discussed below. The reduced-fresh and spent (both in the absence and presence of oxygen) NieCo on MgO wash coated over monolith (Im_P_M) catalyst was characterized further using XRD and XPS. In XRD images in Fig. 5, the main peaks at 2q ¼ 10.35, 18, 18.94 21.66, 26.3, 28.38, 29.36, 33.82, 54.22, 69.64 in all samples clearly correspond to the cordierite monolith while peaks at 2q ¼ 43, 62 are for cubic MgO. Generally, the spectra of used catalyst in the presence of 3% O2 is much closer to the fresh catalyst compared with the spectra of the used catalyst without oxygen; apparently, the presence of 3% oxygen helped to preserve the structure of catalyst while it prevented coke deposition as it shown in Fig. 1. This is also apparent from the crystal size of MgO. The crystallite size of the MgO peaks at around 2q ¼ 62 was calculated by the Scherrer equation with the measurement of the FWHM (full width at half maximum) of the diffraction peaks; the size was found to be 20 nm for the reduced samples. The size increased to 36 nm in the spend samples in the absence of oxygen while it was measured as 20 nm over the reacted samples with 3% oxygen in the feed. There may be also NiO and CoO in the catalyst,

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 2 e SEM Images of fresh MgO nanorods (a-b), fresh NieCo impregnated MgO nanorods (c-d), spent NieCo impregnated MgO nanorods (eef) (Im_NR) in 10000£ and 20000£ magnifications.

however it is impossible to be sure because their peaks has close 2q with MgO, hence they may be overlapping. Huo et al. [34] has stated that diffraction peaks at 2q ¼ 75, 79 indicates the formation of solid solution in the form of either NieMgeO or CoeMgeO, or their composites. Such peak was observed in the spectra of spent catalysts (without oxygen) possibly supporting the results of article mentioned above although it is not fully conclusive. The carbon peak was not observed in XRD analysis probably due to the fact that it was overlapped with the peaks of cordierite monolith. XPS analysis was also conducted to measure the binding energies of the components in the reduced and spent NieCo over MgO wash coated monolithic samples (Im_P_M). The detailed Ni 2p and Co 2p scans of freshreduced and used samples in the presence of 3% oxygen was given in Fig. 6. Unfortunately, the detailed XPS scans of the reacted samples in the absence of oxygen could not be clearly obtained due to significant coke formation. As seen in Fig. 6, both Ni 2p and Co 2p scans have a complex shape having a mixture of core level and satellite features.

In the reduced samples in Fig. 6a, b, the binding energies (BEs) at 856.3 eV and 873.9 eV show the Ni 2p3/2 and Ni 2p1/2 spin orbit peaks while BEs at 780 eV and 796.7 eV are Co 2p3/2 and Co 2p1/2 spin orbit peaks. The other peaks as labelled in Fig. 6a, b are the satellites of these spin orbits. The peak at 856 eV can be related to oxidized Ni due to the interaction of mixed oxide support, MgO or hydroxyl groups. The one at 853 eV was only accepted as the sign of metallic Ni in the study of Walker et al. [73]. Similarly, Xu et al. [74] indicated the peaks at 852.0 eV as the reduction state of Ni and the ones at 855.1 eV as the oxidation states of Ni. The peaks at 855.5 and 852.2 eV in Ni 2p3/2 spectra were also accepted as the Niþ2 and Ni0 phases of the nickel respectively while the ones at 780.7 and 777.4 eV in the Co 2p3/2 spectra were stated as Coþ2 and Co0, respectively [75]. According to Fan et al. [76], the binding energies of Niþ2 and þ2 Co were shifted from 855.4 to 861.8 and from 784 eV to 779.9 eV in the XPS studies of NieCo bimetallic catalysts. As it seen in Fig. 6, both Ni 2p and Co 2p scans were significantly changed after the reaction in the presence of oxygen. The

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 3 e SEM Images of fresh (a,b) and spent (c, d) NieCo impregnated-MgO nanorod wash coated over monolith (Im_NR_M).

Fig. 4 e SEM images of fresh (a, b, c) and spent (d, e, f) Ni0·5Co2·5O4 (NW_M) after reduced at 600  C. changes in Ni 2p scan and Co 2p scans in Fig. 6c, d can be attributed to the formation NiO and CoO during the reaction.

Performances of %8 wt.Ni-%2 wt.Co over various MgO support structures The catalytic activities of %8 wt.Ni-%2 wt.Co catalysts impregnated over various MgO/monolith structures are given in Fig. 7. All structural forms were found to be highly

active; however, the wash coated monolithic catalysts (Im_P_M and Im_NR_M) performed better than their particulate (Im_P) and nanorod (Im_NR) MgO alone counterparts at low temperatures while the one with particulate MgO wash coated over monolith (Im_P_M) had the best performance in terms of both CH4 and CO2 as well as H2/CO product distribution (closer to unity). The CH4 conversion started from 25% (for Im_NR) and 40% (for Im_P_M) and increased with increasing temperature for all forms as

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 5 e The XRD spectra of the samples. (1) bare monolith, reduced (2) MgO coated monolith, (3) NieCo/MgO/monolith (Im_P_M), (4) used NieCo/MgO monolith without O2, (5) used NieCo/MgO monolith with O2 (3%).

expected; the conversion reached to almost %96 when the temperature increased further to 800  C. The similar trends were also observed for CO2 conversion while the H2/CO ratio also approached to unity with increasing temperature. The most important results from this part is that the same CH4 conversion was obtained at about 50  C lower temperatures in monolithic forms; the difference diminishes as temperature increases. It can be concluded from these results that the use of monolithic support enhances the process significantly; the necessity of lower temperatures for an economically feasible process was well as its low pressure drop and easy to scale up characteristics makes the monolithic structures as an effective

support. There are also works in literature supporting these results. For example, Soloviev et al. [58] over Nickelalumina on cordierite honeycomb monolith and Liu et al. [51] over the Ni catalysts on SiC monolithic also observed high performances. Miguel et al. [77] found that Ni/MgO catalyst coated metal monolith also showed higher stability and higher conversion than particulate catalyst for steam reforming of methane process. This was probably due to the fact that monolithic forms enhanced the heat transfer and surface area-volume relation. Luisetto et al. [61] have also found that CH4 and CO2 conversions of NieRu cordierite monolith were higher than those of powdered catalysts. The chemical composition of cordierite (2MgO$2Al2O3$5SiO2) may also contribute the results

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 6 e XPS scan results for (a)-Ni and (b)-Co over fresh-reduced catalyst (Im_P_M); (c)-Ni, (d)-Co over spent catalyst (Im_P_M) in the presence of 3% oxygen (Im_P_M).

considering that these oxides are also used in DRM as support; however, we cannot verify that at this stage. According to the thermodynamic analysis using Gibbs free energy minimization method, the methane conversion over all catalysts was significantly below the equilibrium conversion; however, the methane conversions almost reached to the thermodynamic equilibrium value when the temperature approached to 800 . In the case of CO2 conversion, the experimental values over all catalysts erroneously exceeded the thermodynamic limits as it is also observed by other investigators [78]. This was probably due to the unaccounted or wrongly accounted carbon source indeed we could account about 90e95% carbon at low temperatures while it was close to 100% at higher temperatures at which both thermodynamic and experimental CH4 and CO2 conversions reached to approximately 95%.

Performances of Ni0·5Co2·5O4 nanowire grown on monolith DRM performances of over Ni0$5Co2$5O4 nanowire over monolithic structures (NW_M) were also tested at 600  Ce800  C, and the results were plotted in Fig. 8; unreduced and reduced NW_M at 600  C and 800  C were used for this purpose. Although 8 wt%

Ni - 2 wt%Co/MgO and Ni0$5Co2$5O4 are not comparable due to the significant difference in metal content, we presented them in the same figure to see their performance together. The results over particulate MgO wash-coated over monoliths (Im_P_M), which was the best in previous test given above, was also added for comparison. As it seen from the figure, CH4 and CO2 conversions are slightly lower than those over Im_P_M at low temperatures. This probably due to the fact that the catalyst could not preserve its structure as discussed in characterization; this will be more obvious when the result of time on stream test are presented below. When the catalyst was unreduced, no conversion was observed at 600 even though 30e40% conversions were obtained in the presence of reduction (the tests were repeated several time and the same result was obtained). Probably, the catalyst was not active without the reduction process, and it may be reduced by the produced H2 with the increasing temperature.

Time on stream tests Fig. 9 shows the time on stream tests (TOS) results for all catalysts mentioned above at 750  C for 8 h. Both CH4 and CO2

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 7 e The effect of temperature on the catalytic activity of NieCo over various MgO structured catalysts. a) CH4 conversion, ¡1 . b) CO2 conversion c) H2/CO ratio. CH4/CO2 ¼ 1, F/W ¼ 84000 mlg¡1 cath

conversions of NieCo impregnated over nanorod, monolithic, nanorod-monolithic structured catalysts were not changed, as the indicator of stability while the activity of Ni0$5Co2$5O4 nanowire grown over monolith (NW_M) decreased with increasing time on stream; the methane conversion decreased from 84% to 75% while CO2 conversion declined from 91% to 85%. Although, the decrease over the reduced nanowires were slower, it was still significant, and it seems to be due to the deformation of the entire structure as it was presented in Fig. 4; the H2/O ratio was also changed significantly with increasing time on stream for this catalyst.

Performance of %8 wt.Ni-%2 wt.Co over particulate MgO wash-coated MgO on monolith (with 1e3% oxygen) The possibility of reducing coke formation with oxygen, without fundamentally changing the product distribution, was also tested; a small amount of oxygen (1e3%) was added to see feed for this purpose. The results are given Fig. 10; the oxygen was completely consumed in all runs, and no temperature fluctuation was observed due to the oxygen addition. The test performed without oxygen in the feed was also added for comparison. CH4 conversion was

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 8 e The effect of temperature on the catalytic activity of various NieCo catalysts. a) CH4 conversion, b) CO2 conversion c) ¡1 . H2/CO ratio. T ¼ 750  C, CH4/CO2 ¼ 1, F/W ¼ 84000 mlg¡1 cath increased a few percent while CO2 conversion decreased as expected; H2/CO ratio was also increased slightly. The decrease of CO2 conversion in the presence of oxygen may be due to the fact that O2 is more reactive than CO2; the oxygen may be also oxidizing coke and CO producing additional CO2 [70]. Nikoo and Amin [78] has also declared that it is possible to avoid carbon formation at a temperature 800  C and to have a syngas yields of 90% and unity ratio of H2/CO with oxygen use; O'Connor et al. [79] also reported a similar result (however, O2 ratio was higher than ours in both studies).

These results suggest that the biogas from landfills and other biological sources, as the feedstock in dry reforming of methane, could be also used since the landfill gas has approximately 45e55% CH4, 30e40% CO2, 10e15% N2, 0e5% O2.

Stability of %8 wt.Ni-%2 wt.Co over particulate MgO washcoated on monolith The stability tests of the NieCo/MgO over cordierite catalyst (Im_P_M) was also performed for 48 h in the presence of 3% O2 1 at 750  C; the F/W was 42000 mlg1 cath ; CH4/CO2 feed ratio was

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 9 e Time on stream test results of various NieCo based structured catalysts. a) CH4 conversion, b) CO2 conversion, c) H2/ ¡1 . CO ratio. T ¼ 750  C, CH4/CO2 ¼ 1, F/W ¼ 84000 mlg¡1 cath

Fig. 10 e Activity results for NieCo/MgO cordierite monolithic (Im_P_M) catalyst in presence of O2 (0e3%) over cordierite ¡1 . monolithic catalyst. (1) CO2 conversion, (2) CH4 conversions, (3) H2/CO. T ¼ 750  C, CH4/CO2 ¼ 1, F/W ¼ 42000 mlg¡1 cath Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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Fig. 11 e Stability test results of NieCo/MgO monolith (Im_P_M) catalysts. T ¼ 750  C, CH4/CO2 ¼ 1, O2 ¼ 3%, F/W ¼ 42000 ¡1 . mlg¡1 cath

one. As in Fig. 11 shown, the CH4 conversion, CO2 conversion and H2/CO ratio were almost constant (with some small fluctuation) for 48 h. This indicates the potential of the catalyst, even though it may not be sufficient to show the stability for longer periods. The coke deposition at the end of the test was also lower than that observed in the absence of oxygen (even in much shorter times on stream).

Conclusions The effects of various structured nickel-cobalt catalysts such as impregnated (8 wt%Ni-2wt.%Co) on MgO, wash-coated MgO over monoliths, MgO nanorods and their monolithic form as well as Ni0$5Co2$5O4 nanowires on monolithic structures on the catalytic activity of carbon dioxide reforming of methane were investigated. Catalytic activity tests were performed at 600e800  C and CH4/CO2 ratios of 1.0. The structure of the MgO support highly influenced the catalytic performances; among all structures tested, 8 wt%Ni-2wt.%Co over MgO wash-coated monolith led to higher catalytic activity and stability as well as low coke formation in the presence of low amount of oxygen; this was also indicated by SEM-EDX and XPS analyses. CH4 and CO2 conversions were 83% and 89% respectively with the H2/CO ratio of 0.95 at 750  C while these values were 70%, 80% and 0.89 respectively for particulate catalyst. In the case of nanorod structure, both Ni-based MgO nanorods and their wash-coated form over monolith resulted lower catalytic activity than Ni-based MgO wash-coated monolith; however, it should be noted that they seemed to be more resistant to coke formation than the particulate MgO containing catalysts as the SEM analysis indicated. Ni0.5Co2.5O4 nanowire structures also showed high catalytic activity at the same conditions; however, they were not stable.

The catalytic activity of 8 wt%Ni-2wt.%Co on the MgO wash-coated over monolith was also performed in the presence of 1e3% O2 in the feed, and it was observed that 3% oxygen significantly decreased coke deposition without negatively affecting the performance; the same catalyst in the presence of 3% O2 was also found to be stable showing no significant changes in performance for 48 h at these operating conditions. As a result, it can be concluded that, in addition to low pressure drop and easily be scaled up properties, its high catalytic performance, low coke deposition characteristic and high stability make the monolithic NieCo/MgO catalyst a good candidate for the dry reforming of natural gas as well as land fill and other bio gases.

Acknowledgement This work was supported by Bogazici University Research Fund through Project 15A05D3.

references

[1] Bian Z, Suryawinata IY, Kawi S. Highly carbon resistant multicore-shell catalyst derived from Ni-Mg phyllosilicate nanotubes@silica for dry reforming of methane. Appl Catal, B 2016;195:1e8. https://doi.org/10.1016/j.apcatb.2016.05.001. [2] Wu H, Liu J, Liu H, He D. CO2 reforming of methane to syngas at high pressure over bi-component NiCo catalyst: the anticarbon deposition and stability of catalyst. Fuel 2019;235:868e77. https://doi.org/10.1016/j.fuel.2018.08.105. [3] Matei-Rutkovska F, Postole G, Rotaru CG, Florea M, Parvulescu VI, Gelin P. Synthesis of ceria nanopowders by microwaveassisted hydrothermal method for dry reforming

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

14

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

international journal of hydrogen energy xxx (xxxx) xxx

of methane. Int J Hydrogen Energy 2016;14:2512e25. https:// doi.org/10.1016/j.ijhydene.2015.12.097. Erdogan B, Arbag H, Yasyerli N. SBA-15 supported mesoporous Ni and Co catalysts with high coke resistance for dry reforming of methane. Int J Hydrogen Energy 2018;48:1396e405. https://doi.org/10.1016/ j.ijhydene.2017.11.127. Serrano-Lotina A, Daza L. Long-term stability test of Ni-based catalyst in carbon dioxide reforming of methane. Appl Catal A 2014;474:107e13. https://doi.org/10.1016/ j.apcata.2013.08.027. Lavoie JM. Review on dry reforming of methane, a potentially more environmentally-friendly approach to the increasing natural gas exploitation. Front Chem 2014;2:811e7. http:// doi:10.3389/fchem.2014.00081. Cruz PL, Navas-Anguita Z, Iribarren ZD, Dufour J. Exergy analysis of hydrogen production via biogas dry reforming. Int J Hydrogen Energy 2018;43:11688e95. https://doi.org/10.1016/ j.ijhydene.2018.02.025. Calgaro CO, Perez-Lopez OW. Biogas dry reforming for hydrogen production over Ni-M-Al catalysts (M ¼ Mg, Li, Ca, La, Cu, Co, Zn). Int J Hydrogen Energy 2019;44:17750e66. https://doi.org/10.1016/j.ijhydene.2019.05.113. Budiman AW, Song SH, Chang TS, Shin CH, Choi MJ. Dry reforming of methane over cobalt catalysts: a literature review of catalyst development. Catal Surv Asia 2012;16:183e97. http://doi:10.1007/s10563-012-9143-2. Pakhare D, Spivey J. A review of dry (CO2) reforming of methane over noble metal catalysts. Chem Soc Rev 2014;47:7813e37. http://doi:10.1039/c3cs60395d. Mofrad BD, Rezaei M, Hayati-Ashtiani M. Preparation and characterization of Ni catalysts supported on pillared nanoporous bentonite powders for dry reforming reaction. Int J Hydrogen Energy 2019;44:27429e44. https://doi.org/ 10.1016/j.ijhydene.2019.08.194. re P, Reddy GK, Loridant S, Takahashi A, Deliche Reddy BM. Reforming of methane with carbon dioxide over Pt/ZrO2/SiO2 catalystsdeffect of zirconia to silica ratio. Appl Catal A 2010;389:92e100. http://doi:10.1016/j. apcata.2010.09.007. Liu D, Cheo WNE, Lim YWY, Borgna A, Lau R, Yang Y. A comparative study on catalyst deactivation of nickel and cobalt incorporated MCM-41 catalysts modified by platinum in methane reforming with carbon dioxide. Catal Today 2010;154:229e36. http://doi:10.1016/j.cattod.2010.03.054. ¨ ner D. Dry reforming of methane over CeO2 Ay H, U supported Ni, Co and NieCo catalysts. Appl Catal, B 2015;179:128e38. https://doi.org/10.1016/ j.apcatb.2015.05.013. Abdollahifar M, Haghighi M, Babaluo AA, Talkhoncheh SK. Sono-synthesis and characterization of bimetallic NieCo/ Al2O3eMgO nanocatalyst: effects of metal content on catalytic properties and activity for hydrogen production via CO2 reforming of CH4. Ultrason Sonochem 2016;31:173e83. https://doi.org/10.1016/j.ultsonch.2015.12.010. Du X, Zhang D, Shi L, Gao R, Zhang J. Coke- and sinteringresistant monolithic catalysts derived from in situ supported hydrotalcite-like films on Al wires for dry reforming of methane. Nanoscale 2013;5:2659e63. http://doi:10.1039/ c3nr33921a. Sengupta S, Deo G. Modifying alumina with CaO or MgO in supported Ni and NieCo catalysts and its effect on dry reforming of CH4. J CO2 Util 2015;10:67e77. https://doi.org/ 10.1016/j.jcou.2015.04.003. Damyanova S, Pawelec B, Arishtirova K, Fierro JLG. Ni-based catalysts for reforming of methane with CO2. Int J Hydrogen Energy 2012;37:15966e75. https://doi.org/10.1016/ j.ijhydene.2012.08.056.

[19] Rong-jun Z, Guo-fu X, Ming-feng L, Yu W, Hong N, Da-dong L. Effect of support on the performance of Ni-based catalyst in methane dry reforming. J Fuel Chem Technol 2015;43:1359e65. https://doi.org/10.1016/S1872-5813(15)30040-2. [20] Bradford MCJ, Vannice MA. Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity. Appl Catal Gen 1996;142:73e96. https://doi.org/10.1016/0926-860X(96)00065-8. [21] Pizzolitto C, Pupulin E, Menegazzo F, Ghedini E, Michele AD, Mattarelli M, Cruciani G, Signoretto M. Nickel based catalysts for methane dry reforming: effect of supports on catalytic activity and stability. Int J Hydrogen Energy 2019;44:28065e76. https://doi.org/10.1016/ j.ijhydene.2019.09.050. [22] Amin R, Liu B, Huang ZB, Zhao YC. Nickel based catalysts for methane dry reforming: effect of supports on catalytic activity and stability. Int J Hydrogen Energy 2016;41:807e19. https://doi.org/10.1016/j.ijhydene.2015.10.063. [23] Khajenoori M, Rezaei M, Meshkani F. Dry reforming over CeO2-promoted Ni/MgO nano-catalyst: effect of Ni loading and CH4/CO2 molar ratio. J Ind Eng Chem 2015;21:717e22. https://doi.org/10.1016/j.jiec.2014.03.043. [24] Yao L, Zu J, Peng X, Tong D, Hu C. Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane. Int J Hydrogen Energy 2013;38:72688e7279. https://doi.org/10.1016/ j.ijhydene.2013.02.126. [25] Chotirach M, Tungasmita S, Tungasmita DN, Tantayanon S. Titanium nitride promoted Ni-based SBA-15 catalyst for dry reforming of methane. Int J Hydrogen Energy 2018;43:21322e32. https://doi.org/10.1016/ j.ijhydene.2018.09.205. [26] Chein RW, Fung W. Syangas production via dry reforming of methane over CeO2 modified Ni/Al2O3 catalysts. Int J Hydrogen Energy 2019;44:14303e15. https://doi.org/10.1016/ j.ijhydene.2019.01.113. [27] Ghani NAA, Azapour A, Muhammad AFS, Abdullah B. Dry reforming of methane for hydrogen production over NieCo catalysts: effect of Nb-Zr promoters. Int J Hydrogen Energy 2019;44:20881e8. https://doi.org/10.1016/ j.ijhydene.2018.05.153. [28] Huang F, Wang R, Yang C, Driss H, Chu W, Zhang H. Catalytic performances of Ni/mesoporous SiO2 catalysts for dry reforming of methane to hydrogen. J Energy Chem 2016;25:709e19. https://doi.org/10.1016/j.jechem.2016.03.004. [29] Sajjadi SM, Haghighi M, Rahmani F. Sol-gel synthesis of NiCo/Al2O3-MgO-ZrO2 nanocatalyst used in hydrogen production via reforming of CH4/CO2 greenhouse gases. J Nat Gas Sci Eng 2015;22:9e21. https://doi.org/10.1016/ j.jngse.2014.11.014. [30] Jing J, Yang Z, Huo J, Bai H, Li W. Metal precursor impregnation sequence effect on the structure and performance of Ni-Co/MgO catalyst. Int J Hydrogen Energy 2019;44:8089e98. https://doi.org/10.1016/ j.ijhydene.2019.02.079. [31] Luisetto I, Tuti S, Bartolomeo ED. Co and Ni supported on CeO2 as selective bimetallic catalyst for dry reforming of methane. Int J Hydrogen Energy 2012;37:15992e9. https:// doi.org/10.1016/j.ijhydene.2012.08.006. [32] Meshkani F, Rezaei M. Nanocrystalline MgO supported nickel-based bimetallic catalysts for carbon dioxide reforming of methane. Int J Hydrogen Energy 2010;35:10295e301. http://doi:10.1016/j.ijhydene.2010.07.138. [33] Zhang J, Wang H, Dalai AK. Development of stable bimetallic catalysts for carbon dioxide reforming of methane. J Catal 2007;249:300e10. http://doi:10.1016/j.jcat.2007.05.004. [34] Huo J, Jing J, Li W. Reduction time effect on structure and performance of Ni-Co/MgO catalyst for carbon dioxide

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

international journal of hydrogen energy xxx (xxxx) xxx

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

reforming of methane. Int J Hydrogen Energy 2014;39:21015e23. https://doi.org/10.1016/ j.ijhydene.2014.10.086. Usman M, Daud WMA, Hazzim W, Abbas F. Dry reforming of methane: influence of process parametersda review. Renew Sust Energ Rev 2015;45:710e44. https://doi.org/10.1016/ j.rser.2015.02.0261364-0321/&2015. Wang YH, Liu HM, Xu BQ. Durable Ni/MgO catalysts for CO2 reforming of methane: activity and metalesupport interaction. J Mol Catal A Chem 2009;299:44e52. http://doi:10. 1016/j.molcata.2008.09.025. Al-Fatesh AS, Abu-Dahrieh JK, Atia H, Armbruster U, Ibrahim AA, Khan WU, Abasaeed AE, Fakeeha AH. Effect of pre-treatment and calcination temperature on Al2O3-ZrO2 supported Ni-Co catalysts for dry reforming of methane. Int J Hydrogen Energy 2019;44:21546e58. https://doi.org/10.1016/ j.ijhydene.2019.06.085. Monroy TG, Abella LC, Gallardo SM, Hinode H. ZrO2Promoted Ni/MgO catalyst for methane dry reforming. J Mater Sci Eng 2012;2:544e9. re T, Wasserschaff G, Schunk S, Milanov A, Titus J, Roussie € ser R. Dry reforming of Schwab E, Wagner G, Oeckler O, Gla methane with carbon dioxide over NiOeMgOeZrO2. Catal Today 2016;270:68e75. https://doi.org/10.1016/ j.cattod.2015.09.027. Asencios YJO, Bellido JDA, Assaf EM. Synthesis of NiOeMgOeZrO2 catalysts and their performance in reforming of model biogas. Appl Catal Gen 2011;397:138e44. http://doi:10.1016/j.apcata.2011.02.023. Son IH, Lee SJ, Song IY, Jeon W, Jung I, Jeong D, Shim J, Jang W, Roh H. Study on coke formation over Ni/g-Al2O3, CoNi/g-Al2O3, and Mg-Co-Ni/g-Al2O3 catalysts for carbon dioxide reforming of methane. Fuel 2014;136:194e200. https://doi.org/10.1016/j.fuel.2014.07.041. Zhang J, Wang H, Dalai AK. Effects of metal content on activity and stability of Ni-Co bimetallic catalysts for CO2 reforming of CH4. Appl Catal A 2008;339:121e9. http://doi:10. 1016/j.apcata.2008.01.027. Gao X, Tan Z, Hidajat K, Kawi S. Highly reactive Ni-Co/SiO2 bimetallic catalyst via complexation with oleylamine/oleic acid organic pair for dry reforming of methane. Catal Today 2017;281:250e8. https://doi.org/10.1016/ j.cattod.2016.07.013. Estephane J, Aouad S, Hany S, Khoury BE, Gennequin C, Zakhem HE, Nakat JE, Aboukais A, Aad EA. CO2 reforming of methane over Ni-Co/ZSM5 catalysts. Aging and carbon deposition study. Int J Hydrogen Energy 2015;40:9201e8. https://doi.org/10.1016/j.ijhydene.2015.05.147. Medeiros RLBA, Macedo HP, Melo VRM, Oliveir AAS, Barros JMF, Melo MAF, Melo DMA. Ni supported on Fe-doped MgAl2O4 for dry reforming of methane: use of factorial design to optimize H2 yield. Int J Hydrogen Energy 2016;41:14047e57. https://doi.org/10.1016/ j.ijhydene.2016.06.246. Theofanidis SA, Galvita VV, Poelman H, Marin GB. Enhanced carbon-resistant dry reforming Fe-Ni catalyst: role of Fe. ACS Catal 2015;5:3028e39. https://doi.org/10.1021/ acscatal.5b00357.  P, Crnivec IGO, Pintar A. Biogas to syngas conversion Djinovic without carbonaceous deposits via the dry reforming reaction using transition metal catalysts. Catal Today 2015;253:155e62. https://doi.org/10.1016/j.cattod.2015.01.039.  P, Pintar A. Stable and selective syngas production Djinovic from dry CH4-CO2 streams over supported bimetallic transition metal catalysts. Appl Catal, B 2017;206:675e82. https://doi.org/10.1016/j.apcatb.2017.01.064. Kohn MP, Castal MJ, Farrauto RJ. Auto-thermal and dry reforming of landfill gas over a Rh/gAl2O3 monolith catalyst.

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64]

15

Appl Catal, B 2010;94:125e33. http://doi:10.1016/j.apcatb. 2009.10.029. Nematollahi B, Rezaei M, Khajenoori M. Combined dry reforming and partial oxidation of methane to synthesis gas on noble metal catalysts. Int J Hydrogen Energy 2011;36:2969e78. http://doi:10.1016/j.ijhydene.2010.12.007. Liu H, Li S, Zhang S, Wang J, Zhou G, Chen L, Wang X. Catalytic performance of novel Ni catalysts supported on SiC monolithic foam in carbon dioxide reforming of methane to synthesis gas. Catal Commun 2008;9:51e4. https://doi:10. 1016/j.catcom.2007.05.002. Deutschmann O, Schwiedernoch R, Maier LI, Chatterjee D. Natural gas conversion in monolithic catalysts: interaction of chemical reactions and transport phenomena. Stud Surf Sci Catal 2001;136:251e8. https://doi.org/10.1016/S0167-2991(01) 80312-8. Heck RM, Gulati S, Farrauto RJ. The application of monoliths for gas phase catalytic reactions. Chem Eng J 2001;82:149e56. https://doi.org/10.1016/S1385-8947(00)00365-X. Giroux T, Hwang S, Liu Y, Ruettinger W, Shore L. Monolithic structures as alternatives to particulate catalysts for the reforming of hydrocarbons for hydrogen generation. Appl Catal, B 2005;56:95e110. http://doi:10.1016/j.apcatb.2004.07. 013. Ciambelli P, Palma V, Palo E. Comparison of ceramic honeycomb monolith and foam as Ni catalyst carrier for methane autothermal reforming. Catal Today 2010;155:92e100. http://doi:10.1016/j.cattod.2009.01.021. Ryu JH, Lee KY, La H, Kim HJ, Yang JI, Jung H. Ni catalyst wash-coated on metal monolith with enhanced heattransfer capability for steam reforming. J Power Sources 2007;171:499e505. http://doi:10.1016/j.jpowsour.2007.05.107. € o € nu¨m GN, Yildirim R. Water gas shift activity of Au-Re Ozy catalyst over microstructured cordierite monolith washcoated by ceria. Int J Hydrogen Energy 2016;41:5513e21. https://doi.org/10.1016/j.ijhydene.2016.02.025. Soloviev SO, Kapran AY, Orlyk SN, Gubareni EV. Carbon dioxide reforming of methane on monolithic Ni/Al2O3-based catalysts. J Nat Gas Chem 2011;20:184e90. http://doi:10.1016/ S1003-9953(10)60149-1. Chen W, Sheng W, Cao F, Lu Y. Microfibrous entrapment of Ni/Al2O3 for dry reforming of methane: heat/mass transfer enhancement towards carbon resistance and conversion promotion. Int J Hydrogen Energy 2012;37:18021e30. https:// doi.org/10.1016/j.ijhydene.2012.09.080. Wang K, Li X, Ji S, Huang B, Li C. Preparation of Ni-based metal monolithic catalysts and a study of their performance in methane reforming with CO2. Chem Sus Chem 2008;1:527e33. http://doi:10.1002/cssc.200700078. Luisetto I, Sarno C, Felicis DD, Basoli F, Battocchio C, Tuti S, Licoccia S, Bartolomeo ED. Ni supported on g-Al2O3 promoted by Ru for the dry reforming of methane in packed and monolithic reactors. Fuel Process Technol 2017;158:130e40. https://doi.org/10.1016/j.fuproc.2016.12.015. Ren Z, Guo Y, Zhang Z, Liu C, Gao P. Nonprecious catalytic honeycombs structured with three dimensional hierarchical Co3O4 nano-arrays for high performance nitric oxide oxidation. Mater Chem A 2013;1:9897e906. http://doi:10. 1039/c3ta11156c. Ren Z, Botu V, Wang S, Meng Y, Song W, Guo Y, Ramprasad R, Suib SL, Gao P. Monolithically integrated spinel MxCo3-xO4 (M ¼ Co, Ni, Zn) nanoarray catalysts: scalable synthesis and cation manipulation for tunable low-temperature CH4 and CO oxidation. Angew Chem Int Ed 2014;53:7223e7. http://doi: 10.1002/anie.201403461. Guo Y, Rena Z, Xiao W, Liua C, Sharmaa H, Gaoa H, Mhadeshwara A, Gao P. Robust 3-D configurated metal oxide nano-array based monolithic catalysts with ultrahigh

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020

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[65]

[66]

[67]

[68]

[69]

[70]

[71]

international journal of hydrogen energy xxx (xxxx) xxx

materials usage efficiency and catalytic performance tenability. Nano Energy 2013;2:873e81. https://doi.org/ 10.1016/j.nanoen.2013.03.004. Du S, Tang W, Guo Y, Binder A, Kyriakidou EA, Toops TJ, Wang S, Ren Z, Hoang S, Gao P. Understanding low temperature oxidation activity of nanoarray-based monolithic catalysts: from performance observation to structural and chemical insights. Emiss Control Sci Technol 2017;3:18e36. https://doi.org/10.1007/s40825-0160054-y. Takanabe K, Nagaoka K, Nariai K, Aika K. Titania-supported cobalt and nickel bimetallic catalysts for carbon dioxide reforming of methane. J Catal 2005;232:268e75. https:// doi.org/10.1016/j.jcat.2005.03.011. Al-Hazmi F, Alnowaiser F, Al-Ghamdi AA, Al-Ghamdi AA, Aly MM, Al-Tuwirqi RM, El-Tantawye F. A new large e scale synthesis of magnesium oxide nanowires: structural and antibacterial properties. Superlattices Microstruct 2012;52:200e9. http://doi:10.1016/j.spmi.2012.04.013. Chen L, Zhu Q, Wu R. Effect of Co-Ni ratio on the activity and stability of Co-Ni bimetallic aerogel catalyst for methane Oxy-CO2 reforming. Int J Hydrogen Energy 2011;36:2128e36. http://doi:10.1016/j.ijhydene.2010.11.042. Danilova MM, Fedorova ZA, Kuzmin VA, Zaikovskii VI, Porsin AV, Krieger TA. Combined steam and carbon dioxide reforming of methane over porous nickel based catalysts. Catal Sci Technol 2015;5:2761e8. http://doi:10.1039/ c4cy01614a. Assabumrungrat S, Charoenseri S, Laosiripojana N, Kiatkittipong W, Praserthdam P. Effect of oxygen addition on catalytic performance of Ni/SiO2.MgO toward carbon dioxide reforming of methane under periodic operation. Int J Hydrogen Energy 2009;34:6211e20. http://doi:10.1016/j. ijhydene.2009.05.128. Han SJ, Bang Y, Kwon HJ, Lee HC, Hiremath V, Song IK, Seo JG. Elevated temperature CO2 capture on nanostructured MgOeAl2O3 aerogel: effect of Mg/Al molar ratio.

[72]

[73]

[74]

[75]

[76]

[77]

[78]

[79]

Chem Eng J 2014;242:357e63. https://doi.org/10.1016/ j.cej.2013.12.092. Zhang L, Zhang Q, Liu Y, Zhang Y. Dry reforming of methane over Ni/MgO-Al2O3 catalysts prepared by two-step hydrothermal method. Appl Surf Sci 2016;389:25e33. https:// doi.org/10.1016/j.apsusc.2016.07.063. Walker DM, Pettit SL, Wolan JT, Kuhn JN. Synthesis gas production to desired hydrogen to carbon monoxide ratios by tri-reforming of methane using NieMgOe(Ce,Zr)O2 catalysts. Appl Catal A 2012;445:61e8. https://doi.org/ 10.1016/j.apcata.2012.08.015. Xu Y, Long H, Wei Q, Zhang X, Shang S, Dai X, Yin Y. Study of stability of Ni/MgO/g-Al2O3 catalyst prepared by plasma for CO2 reforming of CH4. Catal Today 2013;211:114e9. https:// doi.org/10.1016/j.cattod.2013.03.007. Zhang F, Wang N, Yang L, Li M, Huang L. NieCo bimetallic MgO-based catalysts for hydrogen production via steam reforming of acetic acid from bio-oil. Int J Hydrogen Energy 2014;39:18688e94. https://doi.org/10.1016/ j.ijhydene.2014.01.025. Fan M, Abdullah AZ, Bhatia S. Utilization of greenhouse gases through carbon dioxide reforming of methane over NieCo/MgOeZrO2: preparation, characterization and activity studies. Appl Catal, B 2010;100:365e77. http://doi:10.1016/j. apcatb.2010.08.013. Miguel N, Manzanedo J, Arias PL. Active and stable Ni-MgO catalyst coated on a metal monolith for methane steam reforming under low steam-to-carbon ratios. Chem Eng Technol 2012;35:2195e203. http://doi:10.1002/ceat.201200259. Nikoo MK, Amin NAS. Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation. Fuel Process Technol 2011;92:678e91. http://doi: 10.1016/j.fuproc.2010.11.027. O'Connor AM, Ross JRH. The effect of O2 addition on the carbon dioxide reforming of methane over Pt/ZrO2 catalysts. Catal Today 1998;46:203e10. https://doi.org/10.1016/S09205861(98)00342-3.

Please cite this article as: Leba A, Yıldırım R, Determining most effective structural form of nickel-cobalt catalysts for dry reforming of methane, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.020