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The reaction of methane decomposition is an endothermic process that produces hydrogen without any impurities as a gas product and carbon nanotubes/carbon nanofibers as a solid product. In this study, NiO–MgO catalysts with different nickel loadings (20, 30, 40, and 50 wt. %) were synthesized using the mechanochemical method. The Physicochemical features of the synthesized catalysts were characterized by various methods such as BET, XRD, SEM, TPO, and H2-TPR. The prepared samples were tested in the methane decomposition process at various temperatures under the GHSV value of 48,000 ml/(gcat h). Among the prepared samples, the 50wt.%NiO–MgO exhibited the best activity (40% methane conversion at 575 °C) in this process due to its higher concentration of active metal and better catalyst reducibility. Also, the effect of different parameters such as feed ratio, GHSV, and calcination temperature was studied on the activity of this catalyst. The results showed that with the increase of GHSV and feed ratio, the methane conversion decreased due to the lesser contact time and diminishing the proportion of accessible active sites per methane entrance molecules. Moreover, the results showed that with the rise of calcination temperature from 500 to 700 °C, the methane conversion decreased due to the sintering of nickel particles. The results also showed that the addition of 15 wt.% Cr2O3 to the catalyst formulation improved the catalytic performance and lifetime in the thermocatalytic decomposition of methane due to the higher reducibility and dispersion of the active species on the catalyst surface. [ABSTRACT FROM AUTHOR]

Abstract Thermocatalytic decomposition of methane is proposed to be an economical and green method to produce CO x -free hydrogen and carbon nanomaterial. In present work, 60 wt% Ni/Al 2 O 3 catalysts with different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by co-impregnation method to investigate promotional effects of these metal additives on the activity and stability of 60 wt% Ni/Al 2 O 3 and find out a really effective promoter for decomposition of methane. The catalyst was characterized by N 2 adsorption/desorption, X-ray diffraction, scanning electron microscopy, inductively coupled plasma optical emission spectrometer and hydrogen temperature programmed reduction. While metal additives (5 wt%) were added into 60 wt% Ni/Al 2 O 3 , the activity stability of 60 wt% Ni/Al 2 O 3 was improved and the CH 4 conversion of 60 wt% Ni/Al 2 O 3 was also improved except Zn addition. Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al 2 O 3 significantly and the CH 4 conversion of 5 wt% Mn-60 wt% Ni/Al 2 O 3 was ∼80%. Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al 2 O 3 and the CH 4 conversion of 5 wt% Cu-60 wt% Ni/Al 2 O 3 decreased from 61% to 45% after 250 min of reaction time. Carbon nanomaterials formed in the thermocatalytic decomposition process were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer and Raman spectroscopy. Carbon deposits consist of amorphous carbon and carbon nanofibers. Graphical abstract Effect of different 5 wt% M-60 wt% Ni/Al 2 O 3 catalysts on methane conversion. Image 1 Highlights • Performance of Ni/Al 2 O 3 with different metal additions for TCD of CH 4 was studied. • The catalytic activity of Ni/Al 2 O 3 was improved by adding Cu, Mn, Pd, Co, Fe, Mg. • Carbon nanofibers were formed on all Ni-based catalysts. [ABSTRACT FROM AUTHOR]

Ni/Al 2 O 3 catalysts with nickel loadings from 10 to 40 wt.% prepared by wet impregnation have been tested for the thermocatalytic decomposition of propane in the temperature range from 700 to 1100 °C in a fixed bed reactor. Full conversion of propane was observed in all cases. The hydrogen yield was found to increase at higher reaction temperature, at lower temperatures significant quantities of CH 4 were observed in the product gas. The Ni loading determined stability of the catalyst. After performing the catalytic decomposition reaction for a certain time, the carbon formed on the catalyst surface was removed by combustion with air. During this regeneration process carbon monoxide was always the main product. Repeated cycles of decomposition and regeneration were carried out. By using an automated control device, it was possible to ensure a continuous operation of the reactor for 65 h. For this period, stable H 2 yield was observed with a catalyst containing 40 wt.% Ni supported by Al 2 O 3 . Fresh and spent catalysts were analyzed by BET, X-ray fluorescence (XRF), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Carbon formed by the decomposition reaction was analyzed by thermogravimetric analysis (TGA) and SEM. [ABSTRACT FROM AUTHOR]

• TCD TOF increases with Ni(0) particle size from 4.0–19.4 nm. • TCD is a structure sensitive reaction and is independent of support composition. • Large Ni(0) nanoparticles are selective towards CNT formation. • Small Ni(0) nanoparticles are selective towards graphitic carbon layers formation. • The deactivation is caused by a decrease in Ni nanoparticle size during reaction. Thermocatalytic decomposition of methane (TCD) is a promising approach for producing CO 2 -free hydrogen and solid carbon co-product. In this study, a series of Al 2 O 3 - and MgAl 2 O 4 -based Ni catalysts, prepared with varying synthesis and pretreatment methods, were evaluated for methane TCD performance at 650 °C and characterized before and after reaction to elucidate activity-structure relationships. We found that methane TCD turnover increases with Ni particle size. Further, large Ni particle sizes (i.e., >20 nm) are selective toward the formation of carbon nanotubes (CNTs), while small Ni particle sizes (i.e., <10 nm) are selective toward the formation of graphitic carbon layers. The formation of graphitic carbon layers block access to Ni active sites, thus rendering the catalyst inactive more quickly than when CNTs are produced. Additionally, the catalyst deactivation observed with time-on-stream is due to the fragmentation of Ni particles into smaller Ni particles followed by their encapsulation with graphitic carbon layers. [ABSTRACT FROM AUTHOR]