Insight into the role of metal/oxide interaction and Ni availabilities on NiAl mixed metal oxide catalysts for methane decomposition

Abstract

Ni loaded mixed metal oxides (MMO) catalysts were systematically synthesized with different Ni/Al ratios through layered double hydroxides (LDH) decomposition, fully characterized using various techniques, and tested in catalytic methane decomposition reaction to study the properties of active nickel species on supports. H2 chemisorption indicated the Ni metallic surface areas on Ni0.5Al-MMO and Ni1Al-MMO were 17.1 and 21.1 m2/g, respectively, which were much higher than those on Ni0.5/Al2O3 and Ni1/Al2O3 impregnated catalysts (at 8.8 and 8.5 m2/g, respectively). However, the MMO catalysts gave the lower carbon nanotubes (CNTs) yield compared with the impregnated catalysts. Form the XPS, XRD and H2-TPR characterizations, it were found that there existed a strong metal and support interaction, leading to less NiO being reduced as the active sites. When Ni/Al ratios increased to 2 and 3, the CNT yields for the NixAl-MMO catalysts were much higher than those of impregnated Ni catalysts, up to 2.02 gc/gcat. and 4.55 gc/gcat., vs 1.36 gc/gcat. and 1.29 gc/gcat., respectively. From H2 chemisorption, we found that NixAl-MMO (x = 2 or 3) catalysts had much higher metallic surface areas (22.3 m2/g and 24.6 m2/g), while they were only 4.9 m2/g and 4.3 m2/g on the impregnated catalysts at the same Ni loadings. It suggests MMO catalysts have more reduced Ni active sites available for methane decomposition reaction, attributing to the proper metal-support interactions. SEM and TEM results display the uniform Ni particle sizes existing on MMO catalysts, giving a nice correlation to the inner diameter of CNTs produced.