Surface sodium functionalization of ordered mesoporous Co3O4 controls the enhanced simultaneous catalytic removal of soot and NOx

Abstract

Nanocasting is a common strategy for creating nanostructured porous materials. The last step of the nanocasting process is the removal of the hard template to obtain the final replica. Here using Co₃O₄ as a model catalyst for the simultaneous soot–NO_x removal reaction, we illustrate that the silica mold removal step, using hot sodium hydroxide (NaOH) or dilute hydrofluoric acid (HF) solutions, will significantly influence the final catalytic activity. Ordered mesoporous Co₃O₄ using HF to remove the template (m-Co₃O₄–F) exhibited slightly higher activity over bulk Co₃O₄ synthesized by the sol–gel method (b-Co₃O₄–S). While mesoporous Co₃O₄ obtained with a similar process using NaOH (m-Co₃O₄–Na) exhibited excellent activity in soot combustion and NO_x reduction. The yield of N₂ for m-Co₃O₄–Na was almost twice the amount of b-Co₃O₄–S. Experimental results reveal that NO_x is stored as unstable nitrate species over m-Co₃O₄–Na due to the existence of homogeneous sodium species introduced by the removal process, hence favoring the oxidation of soot and reduction of NO_x. This result not only highlights the importance of the NO reaction pathway in the simultaneous soot–NO_x removal reaction but also affords a facile route to functionalize mesoporous oxide surface properties by varying the removal procedures, which unlocks strategies to optimize other reactions employing mesoporous materials synthesized by the nanocasting method.