PAN-templated synthesis of layered Co3O4 nanosheets for rapid reduction of 4-NP to 4-AP: Improved formation of unique Co3+-rich (440) facet on the crystal lattice of Co3O4

The efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using Co3O4 in NaBH4(aq) requires the precise control of hydride (Hδ-) and protic hydrogen (Hδ+) generation. Currently, this is achieved using (311) facet-dominated Co3O4 with Co2+-rich edges that favor Hδ- activation leading to poor performance. Herein, we employ the ladder-like structure of oxidized polyacrylonitrile (PAN) to fine-tune the growth of the (311)-to-(440) facets in layered Co3O4 nanosheets. This yields a balanced distribution of Co3+/Co2+ edge sites, leading to enhanced catalytic activity. Our catalyst is prepared by air-annealing a mixture of cobalt acetate (Co(OAc)2) and PAN at 500 oC. The mass ratio of Co(OAc)2 to PAN dictates the (311)/(440) facet ratio (i.e., Y). Specifically, a ratio of 0.25/0.4 (P-0.25-500) gives Y= 1.12. Meanwhile, at ratios of 0.05/0.4 (P-0.05-500) and 0.5/0.8 (P-0.5-500), Y increases to 1.25 and 1.27, respectively. XPS analysis confirms that the surface Co3+/Co2+ distribution (X) on P-0.25-50, P-0.05-500, and P-0.5-500 is 0.43, 0.41, and 0.39, respectively, highlighting a direct relationship between the (440) exposure and Co3+ concentration. The rate of 4-NP reduction (i.e., kapp) on P-0.25-500, i.e., ~2.84×10-2 s-1 is 3.1x and 3.6x faster than P-0.05-500 (~9.16×10-3 s-1) and P-0.5-500 (~7.81×10-3 s-1), respectively. P-0.25-50's performance is comparable to that of state-of-the-art noble metal catalysts and higher than commercial Co3O4, and it maintains a decent catalytic retention of 88.84% after six recycles, demonstrating high stability. Our research presents a simple yet highly effective method for achieving balanced Hδ+ and Hδ- activation edges in Co3O4, resulting in superior 4-NP reduction activity.