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

• Published in: Journal of environmental chemical engineering Vol. 11; no. 6; p. 111496
• Main Authors: Dabaro, Mintesinot Dessalegn, Appiah-Ntiamoah, Richard, Guye, Meseret Ethiopia, Kim, Hern
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-12-2023
• Subjects: 4-aminophenol; 4-nitrophenol; Catalyst; High-index facet; Layered-nanosheet; Structure-directing; 4-aminophenol; Layered-nanosheet; High-index facet; 4-nitrophenol; Catalyst; Structure-directing
• ISSN: 2213-3437
• DOI: 10.1016/j.jece.2023.111496

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 °C. 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.
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•Porous layered Co3O4 nanosheets exposes (440) facets and tuned Co3+/Co2+ ratio.•PAN has served as a soft-templating agent for Co3O4 nanosheets.•(440) facets generate more chemisorbed active oxygen species, unlike (311) facets.•Unique (440) facets exposed on Co3O4 nanosheets is best for 4-NP reduction.•(440) generates more of Co3+, catalytically active site to generate Hδ+.