Active yet extremely durable Co3O4 spheroids of different texture without/with Au deposition for CO oxidation

Abstract

Spherically shaped Co₃O₄ particles were synthesized by one-pot solvothermal treatment of Co(NO₃)₂ in n-octanol that is free of structure-directing agents or pore formers. Au nanoparticles (2–4 nm) dispersed on the Co₃O₄ substrates were fabricated using deposition–precipitation method. The as-synthesized Co₃O₄ (without calcination) and the corresponding Au-containing catalyst achieved complete CO oxidation at 90 °C and 80 °C, respectively. Upon calcination, the condensed Co₃O₄ formed on which uniform dispersion of small-sized flat Au entities (3.0 ± 0.6 nm) with large Au–Co₃O₄ interfaces was established, showing complete CO oxidation at 110 °C. These two types of catalysts were found to be extremely durable even when operated in a period beyond 70 h under certain conditions. The calcined Co₃O₄-based Au catalyst can outperform Au/d-Co₃O₄ in both activity and stability when subjected to a pre-reaction at 350 °C for 5 h. The yolk–shell type Co₃O₄@SiO₂ catalysts synthesized by controllable acid-etching of Co₃O₄ cores demonstrated an optimal Co₃O₄ core–SiO₂ shell interaction and a suitable Co₃O₄ core particle size for CO oxidation. Both Co₃O₄ substrates and Au/Co₃O₄ systems were found to encounter substantial activity enhancement by in situ pretreatment. The pretreatment resulted in (i) transformation of AuO_x to Au⁰, (ii) higher fraction of surface Co³⁺, and (iii) suitably lower concentration of surface oxygen adspecies, accounting for the enhanced activities.