International Journal of Hydrogen Energy, Vol.45, No.35, 17299-17310, 2020
Characterization and catalytic behavior of hydrotalcite-derived Ni-Al catalysts for methane decomposition
A series of Ni catalysts were prepared from Ni-Al hydrotalcite-like compounds (HTlcs) by varying the Ni/Al molar ratio (1-4) and calcination temperature (773-1173 K) of HTlcs. The catalysts were reduced with H-2 at 1073 K and tested for CH4 decomposition at 773-923 K on a thermal gravimeter. Various techniques including N-2 physical adsorption, XRD, H-2-TPR, XPS, HAADF-STEM, TEM, and Raman were applied to characterize the catalysts and the as-produced carbon. The characterizations show that calcination of Ni-Al HTlcs leads to Ni(Al)O solid solution and minor NiO and/or NiAl2O4 spinel may be formed depending on the Ni/Al ratio and calcination temperature; upon reduction at 1073 K, most nickel species are reduced to metallic Ni. In CH4 decomposition, carbon yield shows a volcano-type dependence on the Ni content with the optimum Ni/Al ratio equal to 3. On the otherhand, carbon yield is affected by the calcination temperature of the Ni3Al HTlcs to a small extent. Carbon yield is also significantly affected by the reaction temperature, which decreases remarkably with a rise of temperature to 923 K. TEM and Raman indicate that fish-bone carbon nanofibers are formed at 773-823 K, whereas multi-walled carbon nanotubes are formed at 873-923 K. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.