Steam reforming of acetic acid over nickel/kapok-derived biochar: the effect of nickel exposure on the evolution of reaction intermediates and coke formation

Kapok and its biochar derived via pyrolysis have fibrous structures, which might facilitate metal dispersion in the use of kapok-biochar as a carrier for metal/biochar catalysts. Herein, a Ni/biochar catalyst with a nickel content of 5% or 20% was prepared via the pyrolysis of kapok and further evaluated in the steam reforming of acetic acid. The results suggested that the interaction between the biochar and nickel species was not strong enough to hinder the aggregation of metallic nickel species. The deposits formed from the secondary condensation of volatiles or volatile-nickel interactions during pyrolysis could partially cover nickel species, thus reducing the activity of the Ni/biochar catalyst for steam reforming, especially that of the 5% Ni/biochar with a lower Ni loading. The characterization of steam reforming using in situ IR techniques showed that reaction intermediates bearing -OH and -C-H species could be rapidly converted over Ni/biochar but carbonyl intermediates were more chemically inert, acting as important precursors of coke. The high availability of nickel in 20% Ni/biochar facilitated the conversion of olefinic C&z.dbd;C to aromatic C&z.dbd;C and the production of coke in a carbon nanotube form with a smooth outer surface and clearly identified inner cavities. Nevertheless, coke formed over 5% Ni/biochar was more significant (32 wt% versus 24 wt% over 20% Ni/biochar). Moreover, the accumulation and transformation of oxygen-containing species over 5% Ni/biochar, originating from the low availability of nickel for gasification, generated coke with a carbon nanotube form with a very rough surface. The fibrous structure of the biochar derived from kapok promotes the dispersion of nickel on Ni/biochar catalysts and improves their catalytic activity for the steam reforming of acetic acid.