Syngas Production via Methane Dry Reforming over Ceria-Magnesia

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Kinetics, Catalysis, and Reaction Engineering

Syngas Production via Methane Dry Reforming over Ceria-Magnesia Mixed Oxide Supported Nickel Catalysts Basem M. Al-Swai, Noridah Osman, Mohamad Sahban Alnarabiji, Adesoji Adediran Adesina, and Bawadi Abdullah Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03671 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Industrial & Engineering Chemistry Research

Syngas Production via Methane Dry Reforming over Ceria-Magnesia Mixed Oxide Supported Nickel Catalysts Basem M. Al–Swai a, Noridah Osman a, Mohamad Sahban Alnarabiji a, Adesoji A. Adesina b, and Bawadi Abdullah *a,c a

Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri

Iskandar, 32610 Perak, Malaysia b ATODATECH c

LLC, Pasadena, CA, USA 91101

CO2 Utilization Group, Institute Contaminant Management for Oil and Gas, Universiti

Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia *Corresponding author: [email protected] and [email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Dry reforming of methane (DRM) is becoming an appealing research topic due to the urgent need to minimize global warming and the demand for alternative energy resources. However, DRM commercialization and industrial scale are limited by the deactivation of the applied catalysts. In this work, Ni-based catalysts supported on CeO2–MgO mixed oxides (0–20% CeO2 molar content) were prepared and employed in DRM. The support was synthesized via coprecipitation method followed by impregnation of Ni metal. The catalysts prepared were characterized by XRD, BET, TPR, XPS and FESEM techniques. The catalytic performance of the catalysts was evaluated in a fixed–bed continuous reactor with an equimolar (CH4/CO2) ratio at 1073 K. The addition of CeO2, as a promoter to the support, altered the interaction between Ni and MgO and modulated the properties of the catalysts toward an excellent activity performance and multi-walled carbon nanotubes (MWCNTs) production. CeO2 significantly enhanced the BET surface area, promoted Ni dispersion and improved the reducibility of the catalyst. Among the obtained catalysts, Ni/15%CeO2-MgO achieved the maximum conversion of both CO2 (95.2% ) and CH4 (93.7%) without significant deactivation during the reaction. The superior catalytic performance of the aforementioned catalyst is due to the presence of a high quantity of active Ni sites and high Ce+3/Ce+4 ratio that promoted the formation of oxygen vacancies. With the aid of TPO, FESEM, TEM and Raman Spectroscopy analysis, it was found that the amorphous carbon encapsulated the active sites of the catalysts, in the absence of Ce, which suppress the syngas production significantly. Whereas, the introduction of Ce not only decreased the deposited carbon but also changed the type of the later to MWCNTs, which had positive effects on the activity of the catalyst. KEYWORDS: Ni/CeO2–MgO; amorphous carbon; solid solution; carbon deposition; MWCNTs


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1. Introduction Recently, there has been a growing concern about the high price and limited availability of fossil fuels. Since the industrial revolution, global emissions of greenhouse gases have been exorbitant, leading to the phenomenon of global warming.1 The current estimations of the world’s proved natural gas reserves indicate a vast resource of CH4, particularly since the emergence of shale gas.2 Nonetheless, much of this reserve found in regions far from the industrial complexes; pipelines for transportations may not be available, and liquefaction for shipping is expensive.3 Besides, there are various sources of CH4 other than natural gas, such as methane hydrates and fermented organic waste.4,5 Catalytic reactions are highly important for green energy production.6-8 Reforming of methane is a method to convert CH4 into syngas, a mixture of H2 and CO that is important feedstock used for the synthesis of many valuable chemicals and fuels. Various technologies have been used, notably, steam reforming of methane (SRM), partial oxidation of methane (POM) and dry reforming of methane (DRM).9,10 Recently, DRM has gained renewed interest among researchers for several intriguing advantages; firstly, the production of syngas via DRM provides an ideal equal molar ratio of H2/CO of unity, providing a suitable building block to oxygenated compounds and synthetic fuels through Fischer–Tropsch process.3 Secondly, the economic and the environmental concerns since DRM utilizes two main sources of harmful greenhouse gases as a feedstock.11 In addition, DRM provides a way of natural gas fields with high CO2 content; reaching up to 70% in some natural gas fields, avoiding the complication and high cost of separation.12 Generally, the reaction (Eq.(1)) of DRM is catalyzed by both noble metals such as Ru, Pt, Rh, Pd, and Ir metals and non-noble metals (Fe, Co, Ni) for their high activity.13-15 3 ACS Paragon Plus Environment


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∆H298 = 247.9 kJ mol -1

CH4 + CO2→2CO + 2H2

(1)

Despite the fact that noble metals have superior ability to cleave the bond C-H and better resistance to carbon formation, Ni-based catalysts have been preferred due to their availability, comparable activity, affordability as well as a variety of morphologies.8,9,15 However, the major drawback for industrial application is the sintering and carbon formation that induce the deactivation of the catalysts.5,9,16 Therefore, Ni catalysts are being extensively studied to decrease the carbon formation tendency and enhance their stability by employing appropriate catalyst composition, preparation methodology, pretreatment and adding modifiers, such as alkaline or noble metals.15,17,18 Another strategy that led to significant research activity is the addition of second metal to have bimetallic Ni-based catalysts. The synergetic effect between Ni and the second metals is reported to have high resistance against carbon formation by changing the Ni surface properties.12 Recently, catalysts with spinel, perovskite core/yolk shell and hollow

structures19 have been

demonstrated.10, 20, 21 Ni on hollow structural materials have been extensively studied and show the high resistance against sintering because of the high surface area that leads to high mass transfer efficiency.22 Various types of carbon formation, including amorphous and carbon nanotubes, originate mainly from the methane decompensation (MD) represented by (Eq. (2) and CO disproportionation (reverse Boudouard reaction (BR)), according to Eq. (3). (MD) CH4→C + 2H2

∆H298 = + 75 kJmol -1

(2)

(BR) 2CO→CO2 + C

∆H298 = –172 kJmol -1

(3)


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It is always understood that carbon deposition encapsulates the active metal and produce adverse effects to the reaction. However, the literature shows that certain types of carbon species can be less toxic from deactivation point of view.23,24 Furthermore, He et al.25 proved that carbon formation in the form of Carbon Nanotubes (CNTs) could bring positive consequences and improve the conversion and stability of the catalyst during the reforming of toluene. The nature of the support plays a significant key role in developing catalysts with good activity and stability for DRM. Wang and Ruckenstein26 investigated the catalyst behavior of Rh catalysts supported on CeO2, Nb2O5, Ta2O5, TiO2, and ZrO2; (reducible oxides) and γ–Al2O3, La2O3 MgO, SiO2, and Y2O3; (irreducible oxides). The study reported MgO, CeO2, γ–Al2O3 and ZrO2 as auspicious supports for dry reforming of methane based on their activity and stability. A similar comparison was conducted and reported superior catalytic performance over MgO and Al2O3 and the quick deactivation over SiO2, TiO2, and ZrO2 due to the weak interaction with NiO.27 Researchers aim to decrease the vulnerability of catalyst deactivation by employing suitable support with high surface basicity since materials rich in strong acid sites are inclined to cause coke deposition.28 MgO is an alkaline earth metal oxides that has high thermal stability and pronounced surface basicity and may, therefore, be used as industrial catalyst support.28,29 Furthermore, MgO is a unique carrier for Ni catalysts as Ni can diffuse in magnesia lattice creating NiO– MgO solid solution.30 However, MgO was reported to have very low BET surface area, and the reduction pre-treatment of Ni/MgO catalyst has to be performed at a very high temperature (>1173 K).31-33 In addition, supports or modifiers with high availability of oxygen on the surface are recommended to be used, since oxygen vacancies enhance the 5 ACS Paragon Plus Environment


adsorption and dissociation of CO2.34 Ceria is one of the materials that release and store oxygen because of the simultaneous reduction of Ce4+ to Ce3+ associated with the creation of oxygen vacancies.35 It was reported that the addition of CeO2 to ZrO2 formed a solid solution with high oxygen storage and stabilized textural properties.36 Similarly, several authors reported the excellent thermal resistance, improved dispersion of metals and phase stability agent for Al2O3 when CeO2 was added.19,35,37,38 Akri et al.39 reported that the addition of cerium to Ni–Ce catalysts supported on illite clay enhanced the surface area and Ni dispersion as a result of mesoporosity development and smaller Ni particles. Therefore, better support would result from mixed oxides by combining more than one criteria of a suitable catalyst, such as basicity, an oxygen storage and redox properties. To the best of the authors’ knowledge, CeO2–MgO mixed oxides supported Ni catalysts for DRM have not been investigated in details. Recently, Al–Doghachi et al.40 reported the effect of CeO2 on the catalytic performance of Pt, Pd, Ni/MgO trimetallic catalyst. However, the influence of ceria loading on MgO supporting Ni monometallic in DRM is highly required to offer a fundamental understanding of the role of ceria as a support promotor in the reaction process. Thus, in the present work, Ni/CeO2–MgO catalyst with various CeO2 loading was investigated for the DRM reaction with the aim to assess the influence of the binary oxide support on catalytic performance and carbon formation. The prepared catalysts were characterized using different techniques including XRD, BET, FESEM, TPR and XPS. The catalytic performance for DRM was tested in a fixed bed reactor at 1073 K, (CH4/CO2) ratio of one and GHSV = 36000 cm3 gcat-1 h-1. Additionally, the TPO, FESEM, TEM and Raman spectroscopy was used to characterize the spent catalysts to gain insight into the nature of carbon deposited on the surface of the catalyst.


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2. Experimental Section 2.1 Catalyst preparation CeO2–MgO mixed oxide supports (5, 10, 15 and 20 wt.% CeO2) were prepared via coprecipitation method. In a typical experiment, stoichiometric quantities of Mg(NO3)2·6H2O and Ce(NO3)2·6H2O were dissolved in deionized water under mild stirring until the formation of a clear solution. The pH value of the solution was maintained at about 9–9.5 by dropwise addition of the precipitation agent, ammonia solution (28 wt.% in water), with constant stirring at 353 K for. When the pH value stabilized at 9.5, the stirring and heating were stopped, and the mixture was aged overnight at room temperature. After that, the precipitate was filtered and washed with warm deionized water. The resulting slurry was dried at 393 K overnight and then calcined at 1073 K for 3 h in the static air. The calcined mixed oxide supports are designated as xCeO2–MgO, in which x presents the weight percentage of ceria (5, 10, 15, and 20 wt.%). Pure MgO was also prepared for comparison purpose by adopting the same precipitation method. Ni metal with constant 10 wt.% loading was introduced by impregnation to the previously prepared xCeO2–MgO. The support was impregnated with nickel nitrate Ni(NO3)2.6H2O solution to form approximately 10 wt.% Ni in the final catalyst. Then the excess water was evaporated under constant stirring at 353 K and further dried at 292 K for 12 h. The catalysts were obtained after calcination at 1073 K for 3 h. The calcination temperature of 1073 K chosen as previous studies suggested better stability and influence on the diffusion of Ni2+ ions into the MgO lattice23.



2.2 Catalyst Characterization N2 physisorption isotherm was used to measure the textural properties of catalysts at 77 K using gas adsorption analyzer (Tristar 3020, Micromeritics). The surface area was calculated using Brunauer–Emmett–Teller (BET) method in a relative pressure range (p/po) 0.05–0.20. Additionally, the pore volume was calculated from the amount of N2 adsorbed at a relative (p/po) of 0.99. Field emission scanning electron microscope (FESEM) equipped with EDC (Zeiss Supra 55VP) was used to study the morphologies of the samples. The morphologies of the spent catalysts were further observed by transmission electron microscopy TEM using Hitachi H7100 transmission electron microscopy. Powder X-ray diffraction(XRD) patterns were recorded on the calcined catalysts with Bruker D8B Advance X-ray diffractometer in the 2θ =10–90o angular range. The present phases in the sample were identified using the ICDD database by comparing the patterns with the reference from the database. The average crystallite sizes were estimated with the help of Scherrer’s equation. H2-TPR analyses of the catalyst were conducted to study the behavior of the catalyst and active sites using Thermo Finnigan TPD/R/O 1100 equipped with a thermal conductivity detector and a mass spectrometer. In a typical analysis, about 0.05 g of catalyst was placed in a reactor and treated at 580 K for 60 min in N2 (20 mLmin-1). The catalyst samples were degassed under a flow of hydrogen (5% in N2) under heating from 40 to 1270 K. Temperature-programmed-oxidation (TPO) of the spent catalysts were performed with the same equipment and the same procedures of H2–TPR with the amount 0.2 g of the spent


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catalyst and 5% O2/N2 gas mixture was applied. Prior to the TPO analysis, the spent catalysts were homogenized in an agate mortar. Raman spectra were recorded using Raman spectrometer (Renishaw inVia) equipped with a 532 nm Ar–ion laser beam under ambient conditions. XPS measurement was performed for the unreduced catalysts on an XSAM800 spectrometer with Al K Alpha (hv = 1486.6 eV). The region of interest for the narrow scan corresponded to Ni 2p, Ce 3d, and O 1s photoelectron signals were obtained. The spectra were collected with a pass energy of 200 eV and aperture of 400 µm × 700 µm radiation. Charging effects were corrected by adjusting the binding energies using C 1s peak at 284.8 eV. from carbon contamination. 2.3 Catalytic performance The DRM reaction was performed in a tubular furnace reactor, consisting of 3 sections: gases supply, tube reactor, and products analysis as shown in. Fig. S1. The tube reactor is made of quartz with 10 mm internal diameter operating at atmospheric pressure. In details, 100 mg of each catalyst is placed in the middle of the tube reactor and sandwiched by quartz wool layers to minimize heat and mass transfer limitation and keep the bed at the desired location. Before the real reaction, a 50%H2/N2 at 40 mL min-1 and a temperature of 1073 K were applied for 1 h to reduce the catalyst in–situ. After the reduction, the reactor temperature was brought 873 K in a flow of nitrogen. After that, a mixture, equimolar of CH4, CO2 and N2, was introduced and the activity test is performed at 1073 K under the flow rate of 60 mL min-1. The decrease in catalyst activity at 1073 K was observed up to 6 h on stream. The products and reactants were analyzed using an online gas chromatograph



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equipped with a thermal conductivity detector (TCD). The calculations of CH4 and CO2 conversions (XCH4 and XCO2), as well as syngas ratio (H2/CO), were estimated as follows:

XCH4(%) =

XCO2(%) =

FCH4, in - FCH4, out FCH4, in FCO2, in - FCO2, out

H2/CO =

FCO2, in FH2, out FCO, out

× 100

(4)

× 100

(5)

× 100

(6)

3. Results and Discussion 3.1 Catalysts Characterization The bulk phase features of the MgO support and the catalysts were identified using XRD. Fig. 1 displays the XRD patterns of the MgO, NiO, NiO.MgO and CeO2. The characteristic peaks of cubic MgO phase are located on the following sharp peaks 2θ= 36.9o, 42.8o, 62.2o,74.6o, and 78.5o. [ICDD file no: 98–005–6143] in correspondence with (111), (200), (220), (311), and (222) Miller indices. XRD pattern of NiO is similar to that of MgO, and it is difficult to distinguish between them. However, the addition of Ni to the MgO support results in the higher intensity of MgO patterns, which could be due to the change in the crystallinity of MgO.23 On the other hand, Long et al.41 ascribed the increase in the intensity of MgO patterns to the formation of a NiMgOx solid solution and the efficient substitution of Mg2+ by Ni2+ during the impregnation. The formation of NiO.MgO solid solution is usually ascribed to the approximate ionic


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radii of Ni+2 (0.072 nm) and Mg+2 (0.065), and the cubic crystal type of both NiO and MgO.30 Therefore, Ni/MgO, NiO, and NiO.MgO have a similar prominent reflection that can be assigned to NiO, MgO, a NiO.MgO solid solution, or a combination of the three phases in unknown concentration without significant changes to the crystalline structure that can be detected by XRD.42 Moreover, the diffraction peaks at 2θ= 28.5°, 33.1°, 47.5°, 56.3°, 59.2°, 69.4°,76.7°, 78.6° and 88.4° appear as a result of the cubic phase of ceria which could be indexed to (111), (002), (022), (113), (222), (004), (133), (024) and (224) planes [ICDD file no: 98–005–3995]. The patterns obtained show an increase in the intensity of the diffraction peaks of CeO2 and become stronger as CeO2 content increased from 5 to 20 wt.%. On the contrary, the diffraction peaks of MgO gets weaker and progressively broadens. This indicates that CeO2 inhibited the crystal formation in Ni/MgO and prevented the interaction between NiO and MgO42 as well as improved the active site dispersion.43 It was reported that Mg2+ has (0.072 nm) radii which is significantly lower than that of Ce4+ (0.101 nm). Therefore, it can be revealed that Mg2+ incorporated into Ce4+ during support calcination process. This result is in line with the XPS analysis as will be demonstrated later. The interaction between Ce–Mg–O can also be confirmed by the slight shift of CeO2 patterns when CeO2 was added to MgO as shown in the XRD figure (cf. Fig. S2). Similar behavior was reported by Surender and his colleagues44 when Mg2+ was partially inserted into La3+ which resulted in a shift of La2O3 XRD peaks. They also reported the enhancement of metal dispersion and the high reducibility as a result of the interaction between La and MgO. In the current study, the observation of Ce–Mg–O interaction was supported by TPR, where the reduction peak of Ni shifted to a lower temperature due to the resistance to the formation of NiO.MgO solid solution.



Determining NiO crystalline size is difficult because of the similar structure of MgO and NiO. Therefore, Mg (200) plane of XRD is used to estimate the average crystalline size of the catalyst by Debye–Scherrer equation (cf. Table 1). It is found that the crystalline size decreases when CeO2 is added to the support MgO. It was reported by Al–Doghachi et al.28 that the addition of CeO2 may hinder the agglomeration of magnesia and decrease the crystalline size. The decrease is more pronounced with the addition of 5% CeO2. Further increase in CeO2 did not influence the crystal size significantly. Furthermore, the formation of smaller crystallite size is in line with BET surface area results in which catalysts with CeO2 exhibit higher surface area (cf. Table 1).

Figure 1: XRD patterns of the MgO support and the catalysts with different CeO2 content. XRD analysis was also performed for the Ni/MgO and Ni/15% CeO2–MgO spent catalysts, after 6 h of DRM reaction, and shown in Fig. S3. The MgO, CeO2 cubic phases were present as indicated by their distinct diffraction peaks. Also, the result demonstrates 12 ACS Paragon Plus Environment

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the presence of carbonaceous species carbon at around 25° with higher crystallinity over Ni/15% CeO2–MgO. Similarly, the average crystalline size of the spent catalysts is calculated using the highest peak in the XRD patterns using the Scherrer equation. The crystalline sizes are found to be similar to the crystalline size of the fresh catalysts, 32.1 and 22.0 nm for Ni/MgO and Ni/15% CeO2–MgO, respectively. This indicates a minor influence of sintering in the catalysts. The structural properties of Ni catalysts supported on CeO2-MgO mixed oxide are shown in Table 1. and compared with Ni/MgO. Ni/MgO catalyst has BET surface area of 20.4 m2g-1, whereas catalysts with CeO2 have higher values (35.9 m2g-1, 40.6 m2g-1, 47.7 m2g-1 for catalysts with 5, 10, 15 wt.% CeO2, respectively). This indicates that the addition of CeO2 to MgO significantly influence the structural properties of the catalysts obtained because of the higher surface area of CeO2 compare to MgO.45 However, the surface area reduced to 39.0 m2g-1 when the amount of CeO2 was 20 wt.%. CeO2 was re thetheported to enhance the surface area due to the development of mesoporosity.39 The average pore volume and pore size follow the same trend as the BET; they show an increase with the increase in CeO2 (up to 15 wt.%) and slightly reduced beyond that as shown in Table 1. The presence of CeO2 helps the dispersion of nickel particles on the support and prevent the high agglomeration of particles (as indicated by TPR, XPS, FESEM as well as enhancing CH4 and CO2 conversion), leading to an improvement in BET surface area and pore volume. The N2 adsorption/desorption isotherms of the catalysts (cf. Fig. 2) is categorized as type IV isotherm which is a typical isotherm for mesoporous materials (2–50 nm), according to IUPAC classification. In addition, the isotherm has H3 hysteresis loops at the



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high relative pressure from 0.8 to 1.0, characteristic of a large mesoporous and large pore volumes in the samples.

Table 1 Textural properties of the prepared catalysts Catalysts

BET surface Pore volume Pore size Average crystal Carbon areaa (m2/g) (cm3/g) (nm) sizeb (nm) formation ratec

Ni/MgO

20.4

0.05

12.5

31.5

265.5

Ni/5%CeO2–MgO

35.9

0.21

22.5

22.9

60.8

Ni/10% CeO2–MgO

40.6

0.24

25.1

20.6

47.2

Ni/15% CeO2–MgO

47.7

0.26

26.7

20.3

44.1

Ni/20% CeO2–MgO

39.0

0.23

22.6

20.5

30.2

[a] Specific surface area calculated by the BET method. [b] Determined by the Debye–Scherrer equation of the Mg (200) plane of XRD. [c] defined as mg C/gcat*h measured by TPO

Mesoporous materials favor the dispersion of Ni metal particles and limited the growth of Ni particles during the preparation and subsequent reaction because of the confinement effect.46 Fig. 2 (inset) presents the pore size distribution as calculated by the Barrett–Joyner–Halenda (BJH) method of nitrogen isotherm. The BJH pore size confirms the formation of mesoporous materials with an average size of 4 nm. Another broad pore size culminated around 15 nm appeared with the addition of 20 wt.% CeO2. It is reported that small pores offer a large area of the active surface, contributing to the high diffusion and simultaneously increasing the dispersion of supported metal which in turn increases the activity of the catalysts.47


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Figure 2: N2 adsorption-desorption isotherm curves and (inset) pore size distribution of the prepared catalysts The TPR profiles of the catalysts with different CeO2 content are shown in Fig.3 while; Table 2 shows the reduction peak temperatures and the reducibility of the catalysts. The H2–TPR profile of the Ni/MgO shows a broad reduction peak at 800 K (region II) and 1270 K (region IV).The lower peak is ascribed to the reduction of Ni2+ located at the surface sites of the catalyst or to the reduction of uninfluenced NiO by MgO support.23 While the higher peak corresponds to the reduction of complex NiO species that have a strong interaction with MgO support.23,48 Previous studies showed that most active forms of NiO undergo progressive incorporation into the MgO lattice and forms a solid solution.23,

42

Higher calcination temperature (>873 K) has a strong influence on the diffusion of NiO into NiO.MgO solid solution which is difficult to be reduced to Ni metal.23



However, the addition of CeO2 to the support causes a considerable growth of the more easily reducible NiO forms leading to the appearance of noticeably sharp peaks in the region I as shown in Fig. 3. The first peak centered at 629 K, 613 K, 574 K and 598 K for catalysts Ni/5%CeO2–MgO, Ni/10%CeO2–MgO, Ni/15CeO2–MgO and Ni/20%CeO2– MgO, respectively might be due to the reduction of NiO to Ni0 which was not influenced by the MgO support. Clearly, the H2–TPR profile of the catalysts with CeO2 shows that the first peak is shifted to a lower reaction region indicating the strong influence of Ce. The possible reasons for the shift of NiO reduction peak might be because of the fact that CeO2 inhibits the formation of NiO.MgO solid solution.42 Besides, many authors have pointed out the role of electron transfer in oxide–reductive processes that resulted from the coexistence of Ce3+/Ce4+ redox pair which is confirmed by XPS (cf. Fig. 6).49,50 The addition of CeO2 to MgO to form the binary oxide support improved the oxygen storage capacity and the redox properties of the catalysts. The redox properties of the support enabled more efficient transfer of electrons. Hence an increase in electron density donated by CeO2 played a crucial role in enhancing the reducibility of the catalysts.51,52 A weak peak at region II at a temperature of 752 K, 763 K, 744 K and 744 K is recorded for Ni/5%CeO2–MgO, Ni/10%CeO2–MgO, Ni/15CeO2–MgO and Ni/20%CeO2– MgO, respectively. These peaks might be assigned to the reduction of the outer layers of Ce4+.35,47,53 Another reduction temperature centered around 1023 K is ascribed to the bulk reduction of CeO2 in the composite catalysts and is less pronounced for Ni/5%CeO2–MgO catalyst.42 According to literature, the TPR profile of Ce shows a peak at around 673 K and 1063 K due to the reduction of the surface layer and the bulk phase, respectively.47,54 On the other hand, Das et al.55 reported two reduction peaks of Ni/CeO2 – one at 623 K and


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another at 725 K and were ascribed to the reduction of surface oxygen species and surface reduction of Ce4+ to Ce3+ species. The difference in the reported ranges of temperature reveals the different degree of interaction and the evolvement of different species in the consumption of H2. Finally, all the catalysts with CeO2 composite show higher temperature reduction at about 1270 K which is ascribed to the intimate interaction of NiO with MgO. This strong interaction resulted in from the formation of solid solution where NiO and MgO are miscible because of their similar crystalline structure. In general, it is imperative to say that CeO2 brought an increase in oxygen mobility and enhanced the reducibility of the catalysts. Similar findings were reported in another study where CeO2 was added to Al2O3 support for methane decomposition.37

Figure 3: TPR profiles of the catalysts: (a) Ni/MgO, (b) Ni/5%CeO2-MgO, (c) Ni/10%CeO2-MgO, (d) Ni/15%CeO2-MgO, (e) Ni/20%CeO2-MgO



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Moreover, based on TPR–H2 results in Table 2, Ni/15%CeO2–MgO has the highest total H2 consumption compared to the other catalysts which could explain the higher activity of the catalyst. The highest H2 consumption was due to the optimum loading of CeO2 that resulted in lower crystalline size, lower binding energy of Ni and higher dispersion. Furthermore, the total reducibility can be also related to the BET surface area in Table 1 where the increase in BET enhances reducibility too. The addition of more than 15% CeO2 resulted in higher metal support interaction between the support and Ni due to the synergetic effect with Ce and lowered H2 consumption.56

Table 2 Reduction peaks temperatures and H2-gas adsorbed of the catalysts Catalysts

Region I K

Region II K

Region III K

Region IV. K

Amount H2-gas adsorbed µmole/g

Ni/MgO

-

692

1270

261.5

Ni/5%CeO2-MgO

629

752

1024

1270

430.4

Ni/10% CeO2-MgO

613

763

1036

1270

465.3

Ni/15% CeO2-MgO

575

744

1041

1270

542.0

Ni/20% CeO2-MgO

598

744

1050

1270

485.6

Fig. 4 presents the FESEM images of Ni/MgO, Ni/5%CeO2–MgO, Ni/10%CeO2– MgO, Ni/15CeO2–MgO and Ni/20%CeO2–MgO samples indicating a clear change in the surface of the samples as the amount of ceria added to magnesia increased. Ni/MgO structure is composed of agglomerated spheroid particles connected, with grains that are denser and the microstructure has low porosity corresponds to the residual pores.


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Figure 4: FESEM images of catalysts: (a) Ni/MgO, (b) Ni/5%CeO2-MgO, (c) Ni/10%CeO2MgO, (d) Ni/15%CeO2-MgO, (e)Ni/20%CeO2-MgO Catalysts with CeO2 have very small grained crystals with regular shapes and sizes between 20–50 nm. Das et al.

55

reported that that in CeO2 doped Ni/SiO2, particle sizes were

reduced due to the dispersive character of CeO2 and the creation of surface and bulk oxygen vacancies. The images show aggregates of smaller sizes and spherical-like in shape probably formed during the thermal decomposition. The particles are aggregated to each 19 ACS Paragon Plus Environment


other on the catalyst surface generating pores and developing porous texture. The morphology of all the samples is staghorn-coral- like morphology. Higher content of ceria in the catalyst resulted in some branches having deer-antlers-like (Figs. 4d and e). Toemen et al.57 reported the same morphology with the addition of 65 wt.% Ce to Ru/Mn–65/Al2O3. This morphology plays a crucial role in the catalysts activities by providing space for the access of active metal to adsorb CO2 and H2 molecules on the surface of the catalyst.57 Furthermore, the surface area obtained for the catalysts with ceria is much higher than a catalyst with pure MgO, (cf. Table 1) which is in line with the high porosity. XPS was used to investigate the electronic structures of all catalyst samples in terms of oxidation state and species of surface atoms. Figs. 5, 6 and 7 display the high–resolution XPS spectrum of Ni2p, Ce3d, and O1s elements and their corresponding binding energy values (BE), respectively. Table 3 presents Ni 2p3/2 BEs and the surface atomic compositions of Ni and Ce relative to Mg in all the catalysts. Ni2p XPS with a typical Ni 2p3/2 spectrum (Fig. 5) consists of two main peaks: primary peak positioned within the range 854.01855.86 eV for Ni 2p3/2 and its satellite peak to simulate the charge–transfer is displayed at 860.81859.92 eV. In general, the catalysts have higher Ni 2p3/2 BE than the BE of pure NiO (853.8-854.5 eV)48,58,59, which indicates the electron transfer from Ni to Mg and the interaction of Ni2+ with the support.60 With the increase in CeO2 content from 0 to 15 wt.% in the support, the Ni 2p3/2 BEs gradually shifts from 855.86 for Ni/MgO to a lower value reaching 854.01 eV for Ni/15%CeO2–MgO sample. This indicates that the addition of Ce to the support decreased the interaction of Ni species to the MgO support, which was observed in the TPR profile presented in Fig. 3. The pronounced reduction in the BE can reveal also the change in


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chemical environment or Ce structure surrounding the Ni ions.56 However, there is a slight increase in the BE when the CeO2 reaches 20 wt.% which is ascribed to the synergetic effects between Ni metals and CeO2 and the existence of strong metal–support interaction.56 This result could be matched well with TPR results in Fig. 3. and Table 2; the reduction degree of Ni2+ ions increases with increasing CeO2 content (up to 15 wt.% Ce) and decreases with the further addition of Ce.

Figure 5: XPS narrow scans for Ni 2p (a) Ni/MgO, (b) Ni/5%CeO2-MgO, (c) Ni/10% CeO2MgO, (d) Ni/15% CeO2-MgO, (e) Ni/20% CeO2-MgO In the case of Ce 3d, the spectrum is broad and complex because of multiple oxidation states and spin-orbital splitting.50 The deconvolution of the spectrum reveals sets of peaks that represent Ce 3d5/2 and Ce 3d3/2 spin-orbital components and confirm the existence of Ce13 and Ce(III) states (Fig. 6).61 The labels used in identifying Ce 3d XPS peaks in Fig. 6 are established by comparison with literature data.38 The bonds marked u are ascribed to Ce 3d3/2, and that labeled v represent the Ce3d5/2. The band labeled, v′ and



u′ are the characteristics of Ce3+, and the other six bands u′′′ and ν′′′, u′′ and ν′′, and u and ν are ascribed to Ce4+.61,62 Moreover, the Ce3+/Ce4+ ratio could be determined from the integral area under the corresponding peaks of (v′ and u′ ) to (u′′′, ν′′′, u′′, ν′′, u and ν) as illustrated in Table 3.62 Noticeably, the percentage concentration of Ce3+ increases from 25.68 to 30.57 % with increase in CeO2 from 5 to 15%, and then decreased to 28.55 % with further increase of CeO2 to 20%. The presence of Ce3+ can cause charge imbalance, resulting in the generation of oxygen vacancies via the shift from Ce3+ to Ce4+, which increases the amount of oxygen.36 For the DRM, surface adsorbed oxygen plays a significant role due to its mobility and redox performance.63 This result is consistent with the reduction analysis and the spectrum of O 1s.

Figure 6: XPS narrow scans for Ce 3d (a) Ni/5%CeO2-MgO, (b) Ni/10% CeO2-MgO, (c) Ni/15% CeO2-MgO, (d) Ni/20% CeO2-MgO Fig. 7 shows three distinct oxygen species for O 1s on the surface of the catalyst, Ni/MgO and additional peak at low BE (527 eV) for catalysts contain ceria which might be due to the differential charging of the oxide peak.64 The three overlapping peaks observed 22 ACS Paragon Plus Environment

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in the O 1s BE region are located at 529.1±0.4 eV64, 530.5±0.2 eV59 and 532.±0.7 eV38, which are similar to what the one reported in the literature.65 The first peak, at BE of 529 eV is attributed to the lattice oxygen64, and the other two overlapping peaks at 530 and 531 eV are assigned to surface adsorbed oxygen59 and surface oxygen of hydroxyl species or adsorbed water species38, respectively. The intensity of the peak ascribed to hydroxyl species at 531 eV appears to be more pronounced on Ni/MgO than the catalysts with CeO2. This result can be justified by the high affinity between the surface chemisorbed OH and MgO, which remains with fraction even after thermal treatment at a temperature as high as 2273 K.66

Figure 7: XPS narrow scans for O 1s of catalysts: (a) Ni/MgO, (b) Ni/5%CeO2-MgO, (c) Ni/10%CeO2-MgO, (d) Ni/15%CeO2-MgO, (e)Ni/20%CeO2-MgO The ratio of Ob species for all the catalysts is shown in Table 3. It can be seen that the amount of chemisorbed oxygen59 increases with the addition of CeO2 up to 15 wt.% and decreases when the support contains 20 wt.% CeO2. Therefore, Ce4+ increases on the 23 ACS Paragon Plus Environment


expenses of Ce3+ which drive the reduction of Ob from 65.09 to 46.79 for Ni/15%CeO2MgO and Ni/20%Ce2-MgO, respectively. Furthermore, the ratio of Ce+3 is in accordance with Ob which resulted from the defect oxide during the reduction of Ce+4 to Ce+3 and, consequently, promotes the formation of oxygen vacancies. It is known that the substitution of lower valence metal ions creates oxygen vacancies to maintain the charge balance in Ce as Mg substitute Ce4+, consequently increasing OSC of CeO2.67 The increase of oxygen migration influences the redox properties which are proven by TPR. Lee et al.65 stated that the surface chemisorbed oxygen59 is the most highly active oxygen during the reaction (selective catalytic oxidation (SCO) of NH3 to N2) due to higher mobility. Therefore, the oxygen that is adsorbed to the surface upon the formation of Ce3+, positively affects the relative contribution of the CH4 or the CO2 activation routes towards carbon formation, as well as on the reactivity of the various carbons towards H2 and O2. Quantitative analysis of Ni/Mg, Ce/Mg and Ni/(Mg+Ce) reveals a large variation in composition based on the synthesis conditions (CeO2 content). It can be observed that the surface Ni/Mg ratio increases with the increase in CeO2. For example, the ratio increases from 1.55 for Ni/MgO to 2.58 for Ni/20%CeO2–MgO. Similarly, the surface Ce/Mg ratio increases from 1.07 to 1.58 when CeO2 content increases from 5 wt.% to 15 wt.% is indicating that the addition of CeO2 in the range of 515 wt.% form a non–equilibrium Ce– Mg–O solid solution. This result is in good agreement with XRD (cf. Fig. 1) where the intensity of MgO diffraction peaks become lower with the addition of CeO2 due to the reaction of MgO with CeO2 in the calcination process to form the solid Ce–Mg–O. However, when CeO2 content further increases, the Ce/Mg ratio for Ni/20%CeO2–MgO sample decreases to 1.06. Hence, by observing the increment of Ni/Mg ratio, it can be


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noticed that the ratio is enhanced by approximately 0.15% with every 5 wt.% addition of CeO2 to MgO till CeO2 content reaches 15 wt.%. However, there is a remarkable increase in the Ni/Mg ratio (0.43%) when CeO2 content increases from 15 to 20 wt.%, consequently, percentage Ce3+ reduced from 30.57 to 28.55%. To the best of our knowledge, there are no similar studies to this work for the comparative purpose. However, we found a work where CeO2 formed a solid solution with rare-earth oxides over wide composition range by incorporation into CeO2 or occupying defect sites or surface vacancies of the CeO2 crystals. However, there could be a saturation point for the solid solution, and excessive Ce would result in Ce4+ to preferentially deposit on the surface of the support which facilitates the Ni-Ce interaction67. Table 1: BE and surface atomic concentration (%) of the catalysts

Sample

BE of Ni 2p3/213

Composition by XPS (atomic ratio) *100 Ni/Mg

Ce/Mg

Ni/(Mg+Ce)

Ratio% [Ce3+]

1.55

Oβ in OT

Ni/MgO

855.86

1.55

39.62

Ni/5%CeO2-MgO

855.55

1.70

1.07

1.68

25.68

43.63

Ni/10%CeO2-MgO

854.76

1.96

1.32

1.94

27.49

56.39

Ni/15%CeO2-MgO

854.01

2.15

1.58

3.36

30.57

65.09

Ni/20%CeO2-MgO

854.22

2.58

1.06

3.73

28.55

46.79

In other words, the addition of 20% CeO2 to MgO causes the formation of an oversaturated solid solution. Hence, it is revealed that majority of catalysts tend to enrich its outer surfaces via specific metal or metal oxide as a consequence of segregation effect because of the affinity of the catalyst to reduce the outer surface energy.68 It was reported that Ce has a lower surface energy (0.7 j/m2 )69 than magnesia (1.16 j/m2).70 Based on the 25 ACS Paragon Plus Environment


above discussion and by observing the variation in the ratio between Ni/Mg and Ce/Mg as well as the Ce3+ content between Ni/15%CeO2-MgO and Ni/20%Ce2-MgO, it can be suggested that Ni species are highly dispersed on the surface of the segregated CeO2 which causes a significant reduction in Ce/Mg ratio. The literature reported the positive effect of the formation of Ce-Mg-O on Ni dispersion and the high reducibility by freeing more Ni from the NiO.MgO solid solution.67 Similar effects were reported when CeO2 was added to Al2O3 which significantly enhanced the activity of the catalyst.38

3.2 Catalyst Performance Evaluation The performance of the catalysts (Ni/xCeO2–MgO) for the DRM (expressed as % conversion of CH4 and CO2) was studied for 6 h on stream at 1073 K and GHSV = 36000 cm3 gcat-1 h-1 as shown in Fig. 8. The catalysts supported on CeO2–MgO mixed oxide show remarkably higher conversions in comparison to the one supported on pure magnesia (Ni/MgO). From Fig. 8 (a, b), CH4 and CO2 conversion increases with the increase in the amount of CeO2 from 5% to 15%. Then decreases for the catalyst with 20% CeO2. The CH4 and CO2 conversion for Ni/15%CeO2–MgO reached 95.2% and 93.7%, respectively and remained constant for the 6 h reaction time. However, Ni/MgO had 82.4% initial conversion for CO2 and 68.6% for CH4 and dropped to 62.1% and 59.1% after 6 h reaction time for CO2 and CH4, respectively revealing the positive role of CeO2 on the conversion and stability of the catalysts. The conceivable results of Ni/15%CeO2–MgO catalyst is due to the properties that are enhanced with the introduction of 15 wt.% CeO2 as an optimum loading. The catalyst has the highest BET surface area, lower binding energy with the support that led to higher reducibility. 26 ACS Paragon Plus Environment

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Despite the feeding of an equimolar amount of CH4 and CO2 in the reforming reaction with a stoichiometric ratio of 1 to 1 for CH4 and CO2, the CO2 conversion (Fig. 8 (b)) is higher than the corresponding CH4 conversion for all catalysts during the whole stream. This could be attributed to the reverse water gas shift reaction (RWGS) (Eq. (7)), taking place during the reaction. The RWGS consumes an additional amount of CO2 to yield CO and H2O which is also confirmed by the ratio of syngas being less than unity (Fig. 8 (c)). However, the reaction over Ni/20%CeO2–MgO results in higher and stable H2/CO (about 1.03) indicating a smaller contribution from RWGS.28

Figure 8:Screening of catalysts for DRM reaction (a) Conversion of CH4, %, (b) Conversion of CO2, % and (c) H2/CO Molar ratio at 1073 K. feed composition CH4: CO2.: N2 = 1:1:1 and GHSV = 36000 cm3 gcat-1 h-1 27 ACS Paragon Plus Environment


CO2 + H2→CO + H2O

∆H298 = 41.0 KJ mol -1

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(7)

To assess the effect of reaction temperature on the conversion of CH4 and CO2 DRM was performed over Ni/15CeO2–MgO in the range of 9731173 K. CH4 conversion is expected to increase at elevated temperature due to the strong endothermic nature of DRM (Eq. (1)). From Fig. 9, CH4 conversion increases from 68.4% to 93.1% then 96.2%, corresponding to the temperatures 973 K, 1073 K and 1173 K, respectively. The CO2 exhibits similar behavior, showing an increase, with an increase in the reaction temperature (reaching 95.2% at 1073 K). However, at 1073 K the conversion did not increase, rather it slightly decreased to 93%. The slight decrease of CO2 conversion might be due to the suppression of RWGS (Eq. (7)) It was reported that carbon formation is most severe at a lower temperature which is the reason for lower conversion where carbon accumulation is faster than carbon oxidation.12,55 It is noted that the H2/CO ratio increases from 0.85 to 1.2 at 1173 K, indicating dry reforming is favored and CH4 has higher conversion than CO2. It also indicates the suppression of RWGS at elevated temperature.

Figure 9: The influence of temperature on the catalytic activity of Ni/15CeO2-MgO for the 1:1 ratio of CH4: CO2. and GHSV = 36000 cm3 gcat-1 h-1 28 ACS Paragon Plus Environment

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Fig. 10 shows the stability test of Ni/15%CeO2-MgO catalyst at 1073 K and (CH4/CO2) ratio of one conducted for 30 h. The catalyst shows incredible stability over the studied time on stream. The initial conversion is 92.9 % for CH4 and 93.5 for CO2, and only 4% and 3% loss of CH4 and CO2 conversions was observed after 30 h time on stream. The plausible performance of the catalyst can be related to several factors perceived from the characterization of the catalyst. These factors include the high reducibility and high amount of active metal freed by the interaction of Ce and MgO and the participation of the Ce2O2 and the surface oxygen that tackle the formation of carbo. Besides, High concentration (30.57) of Ce3+ strongly influenced the activation and the stability of the catalysts. For comparison purpose, catalytic performance for different catalysts system was presented in Table S1 and compared with the current study. It can be noticed that the studied catalyst show comparable activity and stability with the reported catalyst. Table S1 also presented the types of carbon formed during the reaction with different catalytic systems and can be noted that most of the studies reported the amorphous and filaments carbon type. It is noteworthy to mention that the experiment had to be stopped not because of the catalyst deactivation but rather because of the pressure build-up resulted from carbon formation on the inner wall of the reactor tube situated under the catalyst bed. This phenomenon has been reported and explained in thermodynamic terms using a carbon limit diagram by Kawasaki et al.71. The risk of carbon accumulation happens when O/C and H/C ratios lie below a line at each temperature. In our study, the accumulated carbon was collected after 30 h of reaction and characterized using XRD as shown in the picture Figs. S4 and S4 (inset).



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Figure 10: Long term stability test of Ni/15CeO2-MgO at 1073 K, 1:1 ratio of CH4: CO2 and GHSV = 36000 cm3 gcat-1 h-1 Recently, a bi–functional mechanism for dry reforming became widely accepted and reported by several authors to explain their results.28,38 It has been postulated that DRM reaction is not solely occurring on the active metal surface, but also on the metal support interfacial region.72 CH4 adsorbs and dissociates over Ni0 sites leaving reactive hydrogen and CHx as shown in the schematic diagram (Fig. S5). Then the CHx species react with oxygen atoms in the CeO2 lattice to CO and H2.73 The redox properties (Ce4+/Ce3+) of the support form oxygen vacancies with a positive charge, thus possessing high affinities with the oxygen atom (which is negatively charged) in CO2 molecular. The presence of oxygen vacancies adsorbs/dissociates CO2 and facilitate the gasification of CHx species to produce CO and H2 according to Eqs. (8) and (9).74 Besides, part of the resulting chemisorbed oxygen species reacts with Ce-based support restoring the reduction of Ce4+ and producing oxygen vacancies (Eq. (10)). Ce2O3 + CO2→2CeO2 + CO


(8)

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4CeO2 + C→2Ce2O3 + CO2

(9)

2Ce3 + + 1/2O2→2Ce4 + + O2 -

(10)

The abundant oxygen vacancies supply the catalyst with active oxygen which reacts with deposited carbon. Hence, contribute to prolonging the life of the catalyst activity during DRM reaction.72 This mechanism can be clearly observed from the TPO result (cf. Fig. 11) where the amorphous carbon is significantly eliminated from the catalysts with mixed oxide support, indicating the role of CeO2. As mentioned earlier, CeO2 is used as a promoter for MgO support to acquire the combination of the high thermal stability and basicity of MgO and oxygen storage and release of CeO2.30, 73 The basicity of the catalysts plays an important role in DRM reaction by adsorbing and activating CO2. However, Ashok et al.75 reported that excessive basicity could facilitate the Boudouard reaction and promote the oxidation of active metal, resulting in catalyst deactivation. Therefore, there should be a balance between the basicity and the high availability of active sites. Charisiou et al.73 reported that promotion of CeO2 over Al2O3 increased Ni dispersion and reacted to Al2O3 to form CeAlO3–like species that play a role in the removal of carbon residues. Similarly, in this study, we can see the significant improvement effect of the solid solution of CeO2–MgO in improving CH4 and CO2 conversions. Zhang et al.67 suggested that MgO–CeO2 support interaction provides oxygen species on the catalyst surface that convert the intermediate species resulted from dehydrogenation of ethanol to CO2 and H2 during ethanol steam reforming reaction. 3.3 Post reaction characterization



Fig. 11 presents the O2–TPO profile of the spent Ni/MgO, Ni/5%CeO2–MgO, Ni/10%CeO2–MgO, Ni/15%CeO2–MgO and Ni/20%CeO2–MgO catalysts after 6 h of reaction at 1073 K. The signal intensity of the O2 consumption represents the amount of carbon deposited. Three kinds of carbon deposited are identified and designated as Cα, Cβ, and Cγ.25 The species Cα which is oxidized at around 673 K is amorphous carbon overlaying the metal surface. While the Cβ species at around 873 K and Cγ above 923 K are assigned to graphitic and carbon nanotube, respectively. Ni/MgO analysis shows peaks at 703 K and 856 K indicating the existence of Cα and Cβ species, respectively with the noticeably high intensity of Cα. On the other hand, the peaks for catalysts with CeO2 promoted support show the three species of carbon with a very low Cα peak. Cα species are mainly formed by decomposition of methane at the early stage of dry reforming while Cβ and Cγ are formed during subsequent reaction on stream.25 The Ce acts as an oxidant that reacts easily with amorphous carbon Cα, while the CNTs are difficult to reform in the reaction conditions due to their high degree of graphitization.76 The ceria provides more surface–active oxygen species through the redox Ce3+/Ce4+ cycling. Catalysts promoted with rare earth elements exhibit strong adsorption of CO2, which contributes to the process of carbon removal.34 In additionally, the amount of carbon deposited on the used catalysts followed the order Ni/MgO > Ni/5%CeO2–MgO > Ni/10%CeO2–MgO > Ni/15%CeO2– MgO > Ni/20%CeO2–MgO as shown in Table 1. This suggests that a significant amount of carbon is deposited over the spent Ni/MgO while CeO2 modified catalysts showed a relatively low amount of carbon. The low carbon deposition on Ni/20%CeO2–MgO in comparison to Ni/15%CeO2–MgO might be associated with the lower activity of the former. Therefore, by comparing the carbon depositions with catalyst activity, it can be


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deduced that Ni/15%CeO2–MgO has the lowest carbon deposition. Furthermore, Ni/20%CeO2–MgO has a more pronounced amorphous carbon type Cα peak compared to the catalysts with CeO2, which indicates the lower activity of the catalyst.

Figure 11: O2-TPO profiles of the spent catalysts after 6 h of DRM reaction at 1073 K To get further insight into the carbon deposition, spent Ni/MgO and Ni/15%CeO2–MgO catalysts were studied by FESEM(cf. Fig. 12) and TEM (cf. Fig. 13). The FESEM images are presented in Fig 12. (a) (for Ni/MgO) and (b) (for Ni/15%CeO2–MgO) after reforming at 1073 K for 6 h. Carbon formed on Ni/MgO exhibits interlaced nano–sheet-like characteristics. The catalyst particles are encapsulated by nano–sheet-like carbon which greatly inhibits the contact of methane with the catalyst. This type of carbon provides proof to explain the low activity of bare Ni/MgO; with zero ceria. However, the catalyst with 15 wt.% ceria forms nanotubes or nanofibers carbon confirming the result of TPO (cf. Fig. 11).



Figure 12: FESEM images with different magnifications of the spent (a and b) Ni/MgO, (c and d) Ni/15%CeO2-MgO after reaction at 1073 K and 1:1 ratio of CH4: CO2 for 6 h. The morphology of the carbon formed on the spent catalysts was further observed by TEM as shown in Figs. 13 (a) and (b). Ni/MgO shows a complex of amorphous carbon network as can be seen from the dark spots on the image. The nano-sheet-like morphology was also observed at higher magnification Fig. 13 (a) (inset). On the other hand, carbon deposited on Ni/15%CeO2–MgO was found to be nanotubes and Fig.13 (b) (inset) confirm the presence of Multi-Walled Carbon Nanotubes (MWCNTs) as they are large and comprise of tubes stacked one inside the other. Raman spectroscopy is used to further characterize the structure and graphitization of the deposited carbon formed onto the used catalysts Ni/MgO and Ni/15%CeO2–MgO. The results confirm the formation of different carbon structures on the catalysts during the


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reaction as shown in Fig. 14. Two distinct peaks are shown at 1342 cm−1 and 1583 cm−1 and attributed to D–band and G–band respectively. The G–band is related to the tangential stretching mode of all pairs of sp2 atoms in both rings and chains which associated to the graphitic carbon structure. However, the D–band is attributed to sp2 sites and represent the disordered carbon structure (such as amorphous carbon or defects within graphic lattice).25 The intensity ratio between D–band and G–band is used to evaluate the quality of CNTs, where the lower value of ID/IG ratio (

Syngas Production via Methane Dry Reforming over Ceria-Magnesia

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