Dry reforming of methane for syngas production over Ni–Co-supported Al2O3–MgO catalysts
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This research project focuses on the development of catalysts for syngas production by synthesizing Ni–Co bimetallic catalyst using aluminum oxide (Al2O3) and magnesium oxide (MgO) as the catalyst support. Ni/Al2O3 (CAT-1), Ni–Co/Al2O3 (CAT-2) and Ni–Co/Al2O3–MgO (CAT-3) nanocatalysts were synthesized by sol–gel method with citric acid as the gelling agent, and used in the dry reforming of methane (DRM). The objective of this study is to investigate the effects of Al2O3 and MgO addition on the catalytic properties and the reaction performance of synthesized catalysts in the DRM reactions. The characteristics of the catalyst are studied using field emission scanning electron microscope (FESEM), Brunauer–Emmett–Teller (BET), X-ray powder diffraction (XRD), transmission electron microscopy, H2-temperature programmed reduction, CO2-temperature programmed desorption and temperature programmed oxidation analysis. The characteristics of the catalyst are dependent on the type of support, which influences the catalytic performances. FESEM analysis showed that CAT-3 has irregular shape morphology, and is well dispersed onto the catalyst support. BET results demonstrate high surface area of the synthesized catalyst due to high calcination temperature during catalysts preparation. Moreover, the formation of MgAl2O4 spinel-type solution in CAT-3 is proved by XRD analysis due to the interaction between alumina lattice and magnesium metal which has high resistance to coke formation, leading to stronger metal surface interaction within the catalyst. The CO2 methane dry reforming is executed in the tubular furnace reactor at 1073.15 K, 1 atm and CH4/CO2 ratio of unity to investigate the effect of the mentioned catalysts. Ni–Co/Al2O3–MgO gave the highest catalyst performance compared to the other synthesized catalysts owning to the strong metal–support interaction, high stability and significant resistance to carbon deposition during the DRM reaction.
KeywordsCatalyst development Dry reforming Bimetallic catalysts Catalysis Sol–gel Support
Recently, the global warming issue is getting crucial due to the substantial dependence on petroleum-based energy that leads to the increment of greenhouse-gas emissions within the atmosphere . The concentration of CO2 in the atmosphere has currently increased by about 1.5 ppm/year which indicated that if there is about 5.3 × 1021 g air, the rate of CO2 increase is about 8 billion tons per year . These realities encouraged the study on the development of CO2 reforming of methane that would effectively reduce the level of CO2 and CH4 within the atmosphere.
The development of cost-effective catalysts having higher catalytic activity and more considerable resistance to carbon formation is one of the most concerning issues to commercialize DRM reaction in the industries . One of the approaches applied for the DRM reaction to develop high carbon resistance of Ni-based catalyst is the addition of second metal . The addition of a non-noble metal such as cobalt is more preferable from the economic point of view . Xu et al.  proved that the ratio of Ni/Co is closely allied to the catalytic activity of the bimetallic Ni/Co catalysts supported with the γ-Al2O3 and doped with La2O3. The catalyst having the Ni/Co ratio of 7/3 shows the greatest CH4 and CO2 conversion. If the amount of cobalt increases, the catalytic activity decreases. Bimetallic catalyst having ratio of 5/5 and 3/7 exhibits lower catalytic activity than monometallic Ni catalyst .
Conventionally, Al2O3 is the most proper support for most of the catalytic materials owning to mechanical strength, stability at high temperature and also good textural properties . Although it has been commercially used, the coke deposition is one of the drawbacks of using Al2O3 due to its acidic properties. MgO and CeO2, which are known as the alkaline and alkaline earth oxides, are being used as modifiers of Ni-based catalysts  to enhance the metallic dispersion, improve the metal–support interaction, reduce sintering and improve the thermal stability [23, 27]. The basicity of the catalyst is predicted to be increased by the incorporation of MgO and CeO2. The catalyst basicity improves the adsorption of CO2 which prevents the formation of coke on the catalyst surface [14, 29, 33].
In present work, the influence of bimetallic Ni–Co catalysts supported with Al2O3 and Al2O3–MgO in DRM reaction, synthesized by the sol–gel method, is being studied. The synthesized catalysts were characterized and analyzed in tubular furnace reactor to explain the effect of supports on reaction performances to enhance the reactant conversions as well as to minimize the coke formation in DRM reaction.
The materials used for the catalysts preparation were Ni(NO3)2·6H2O (EMSURE ACS, 99%) and Co(NO3)2·6H2O (HmbG Chemicals, 97%) as the active metals; Al(NO3)3·9H2O (HmbG Chemicals, 98.5%) and Mg(NO3)2·6H2O (EMSURE ACS, 99%) as the catalyst supports; and citric acid as the gelling agent. High-purity CO2, CH4, N2 and H2 (Linde) were used as the laboratory gases for the reaction.
The new Ni–Co bimetallic catalysts were synthesized by direct sol–gel method  as it has high potential in producing high homogeneity composition, and improves the particle size distribution in nanoscale levels that could lead to the high catalytic performance [1, 16]. The Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Al(NO3)3·9H2O, Mg(NO3)2·6H2O, and citric acid were dissolved in deionized water. The continuously stirred mixture was heated using hot plate at 333.15 K until the gel was formed. Consequently, the resulting gel was dried in an oven at 383.15 K overnight, and then calcined in the furnace at 1173.15 K for 5 h. The other samples were also been prepared by the above procedure.
Mass composition for synthesized catalysts
Description of catalyst
Composition of catalyst
15% Ni, 85% Al2O3
10.5% Ni, 4.5% Co, 85% Al2O3
Support: Al2O3 and MgO
10.5% Ni, 4.5% Co, 63.75% Al2O3, 21.25% MgO
FESEM was employed to determine the morphological change by scanning the catalyst samples with a high-energy beam of electrons using a Zeiss Supra_55 VP. Brunauer–Emmett–Teller (BET) analysis was used to determine the specific surface area of a sample—including the pore size distribution. 0.5 g catalyst was used for each analysis using a Micromeritics ASAP 2020. The degassing temperature was set at 383 K to remove the moisture and other adsorbed gases from the catalyst surface.
Furthermore, the phase compositions of the synthesized catalysts were defined by X-ray powder diffraction (XRD) analysis using a Bruker D8B advanced X-ray diffractometer which were recorded in the range 2θ = 20°–80°. The crystallite size, t, was estimated from X-ray line broadening using the Scherrer’s formula, t = 0.9·λ/(B cos(θ)), where λ is the X-ray wavelength (Cu Kα radiation 0.154 nm) and B the full-width half-maximum of the Bragg diffraction angle θ. To study the metal dispersion on the catalyst support, transmission electron microscopy (TEM) was conducted using a HITACHI instrument which operated at 120.0 kV.
Moreover, the reducibility performance of the synthesized catalysts was determined by H2-temperature programmed reduction (H2-TPR) analysis technique on a Thermo Finnigan (TPDRO 1100) instrument equipped with a thermal conductivity detector (TCD) in two-stage processes, which are pretreatment and analysis. CO2-temperature programmed desorption (CO2-TPD) was used to study the basic properties of the synthesized catalysts. The result is gained from the TP-5000 equipment coupled with a Hiden QIC-20 mass spectrometer. For post-reaction analysis, TPO was used with the same equipment and procedures of H2-TPR. 0.2 g of the spent catalysts and 5% of O2/He gas mixture were introduced for the TPO analysis.
Catalytic performance evaluation and testing
The performance of Ni-based supported catalysts in the DRM was studied in a tubular furnace reactor under atmospheric pressure. 0.2 g of the catalyst was held in the middle of the reactor tube between two layers of quartz wool, and the reactor was electrically heated in a furnace. The reactor was purged with N2 gas at 100 mL min−1 to provide an inert atmosphere in the reactor prior to starting the experiment. The reduction process was then carried out at a H2 flow rate of 20 mL min−1 and at a temperature of 1023.15 K for 1 h to activate the catalyst. Then, nitrogen gas was purged again in the reactor until the gas chromatography system showed a complete disappearance of hydrogen gas before the reaction initiation. Methane gas and carbon dioxide with a flow rate of 20 mL min−1 for each gas were introduced to the reactor for every run. The reaction took 8 h for every run, and the sample was taken every hour. The effluent from the reactor then was analyzed by an online gas chromatograph (Agilent 7890) with a thermal conductivity reactor (TCD) and a flame ionization detector (FID).
Results and discussion
Textural properties for synthesized catalysts
Pore volume (cm3/g)
Pore size (Å)
Catalytic performance in DRM
At the early reaction time, CAT-1 and CAT-2 have higher conversions for both CH4 and CO2 compared to CAT-3. However, the conversion of CAT-1 and CAT-2 keep on decreasing, implying that they are unstable throughout the reaction time. On the other hand, CAT-3 is the most stable but less active. This may be due to the low BET surface area of CAT-3 compared to the other two catalysts. However, the basic properties of MgO improve the support interaction and have high stability performance throughout the reaction. Also, the cooperation between Ni and Co results in high performance of catalysts in the DRM reaction. The employment of MgO also produces MgAl2O4 spinel type that enhances the carbon resistance of the catalyst. The average conversions of CH4 and CO2 for CAT-3 are higher, which are 79.17% and 84.82%, respectively, compared to the conversion of CH4 and CO2, which were only 55.7% and 60.9%, obtained from the results of Ni/Al2O3 catalyst prepared by Min et al.  with the reaction temperature of 1073.15 K and pressure of 1 atm.
A comparison between the three synthesized Ni–Co bimetallic catalysts is accomplished. The addition of MgO as the catalyst support into the bimetallic catalyst supported Al2O3 decreases the BET surface area and pore volume as MgO has low surface area which causes pore-filling during the catalyst preparation. XRD analysis proves that there is a formation of MgAl2O3 spinel-type solid solution which has high resistance to coke formation and high metal–support interaction. Hence, CAT-3 that is Ni–Co/Al2O3–MgO gives the highest catalyst performance compared to the other synthesized catalysts due to the addition of MgO which enhances the metal–support interaction, and suppresses the carbon formation in DRM reaction that can lead to the high stability and activity performance of the catalysts.
The authors thankfully acknowledge Universiti Teknologi PETRONAS, Malaysia in providing the necessary facilities to conduct the project.
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