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[Display omitted] • Much more coke forms in upper-layer Ni/KIT-6 in steam reforming of acetic acid. • Used Ni/KIT-6 is more effective to inhibit coking than fresh counterpart Ni/KIT-6. • More O-containing species form over used catalyst via influence from original coke. • O-containing species pass down to lower layer and promote polymeric coke formation. • Used catalyst in upper layer reduces the stability and crystallinity of coke. Deactivated or coked catalyst from steam reforming may not always be treated as a solid waste, which might be potentially used as a sacrificial or protective layer of catalyst for reducing coke formation of catalyst in the lower bed in the same steam reforming process. In this study, steam reforming of acetic acid over Ni/KIT-6 at 600 °C was conducted with used Ni/KIT-6 in upper layer and freshly reduced Ni/KIT-6 in lower layer (dual-bed system). The results suggested that significantly more coke (3 to 4 times) formed on upper-layer catalyst (used or fresh catalyst) than on lower-layer fresh catalyst. The upper-layer used catalyst was more effectively than counterpart of fresh catalyst for mitigating coking in lower-layer catalysts. The in-situ IR technique suggested that carbonaceous species on the used catalyst impacted or reacted with the intermediates newly formed, generating more oxygen-containing species with functionalities like C O C and C O of anhydride or ketones/aldehyde form. These oxygen-containing species could polymerize/condense and pass down to the lower bed, forming more polymeric coke of lower thermal stability. Differently, more unsaturated hydrocarbon species formed on the upper-layer fresh catalyst, making the coke of highly aromatic nature with a higher thermal stability and crystallinity. Additionally, modification of reaction intermediates with the used catalyst in upper layer also formed carbon nanotubes of irregular structures with coarse surface. [ABSTRACT FROM AUTHOR]