Catalytic Decomposition of Methane to Hydrogen and Carbon Nanofibers over Ni−Cu−SiO2 Catalysts

Highly active and stable Ni−Cu−SiO2 catalysts are prepared by a coprecipitation method and are employed for direct decomposition of methane to hydrogen and carbon nanofibers at 650 °C and atmospheric pressure. The influence of Cu content is investigated over Ni−Cu−SiO2 samples with different Cu/Si ratios. The activity results revealed that a certain amount of Cu could enhance methane decomposition activity of Ni. The influence of catalyst calcination temperature is also explored, and it is concluded that calcination at 450 °C is enough for good catalytic performance of Ni−Cu−SiO2 samples. The physicochemical characteristics of fresh catalysts are characterized by BET-SA, X-ray diffraction (XRD), scanning electron microscopy–energy dispersive X-ray (SEM-EDX), temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) analyses. The deactivated catalysts are analyzed by BET-SA, XRD, Raman, transmission electron microscopy (TEM), and carbon hydrogen nitrogen sulfur (CHNS) techniques. The TEM pictures displayed that the deposited carbon is nanofibers in nature. The Raman spectra distinguished the presence of ordered (G-band) and defective (D-band) carbon and disorders resulting from lattice distortion (D′-band) structures of carbon. TPR analysis revealed the low-temperature reduction of NiO (Ni2+ to Ni0) in the presence of Cu and suggests that Cu produces spillover hydrogen, which considerably accelerates the nucleation of the Ni metal in these reduction conditions and enhances the reducibility of Ni2+. The XPS analysis of Ni−Cu−SiO2 indicated that the main line of Ni 2p at BE of 855 eV implies a change in the chemical state of nickel from NiO to NiSiO3. However, XRD analysis did not show the diffraction lines due to NiSiO3 phase. It is observed that a catalyst composition of Ni−Cu−SiO2 (60:25:15) calcined at 450 °C showed better activity and longevity over the other compositions.