Cobalt Oxides with Various 3D Nanostructured Morphologies for Catalytic Reduction of 4-Nitrophenol: A Comparative Study

Catalytic reduction processes represent a promising approach to treat the toxic nitro-aromatic pollutant, 4-nitrophenol (4-NP), via converting 4-NP to 4-aminophenol (4-AP). Many efforts have been invested into developing non-precious metal oxides for the conversion of 4-NP to 4-AP. As Co3O4 is validated as a useful non-precious metal catalyst for 4-NP reduction, it is crucial to probe into the relationship of morphology-property-reactivity of Co3O4 catalysts for optimizing design of Co3O4 for 4-NP reduction. Thus, the catalytic activity of Co3O4 for 4-NP is particularly investigated in this study through examining 3-dimensional (3D) Co3O4 catalysts (3DCoOs) with different nanostructured morphologies, including the conical-Co3O4 (CCO), stacked-Co3O4 (SCO), needled-Co3O4 (NCO) and floral-Co3O4 (FCO), for 4-NP reduction. A remarkably enhanced activity in the reduction of 4-NP is realized over all 3DCoOs than commercial Co3O4 catalysts. In particular, CCO shows the highest catalytic performance (k =0.6882 min-1 and Ea = 37.4 kJ mol-1), whereas FCO exhibits the least catalytic activity, with k =0.4709 min-1 and Ea = 47.1 kJ mol-1. These values are far more superior than those obtained by the commercial Co3O4 and the reported catalysts. All four 3DCoOs also show stable recyclability with complete 4-NP reduction over five successive cycles. These results and findings suggest that 3DCoOs are certainly advantageous Co-based catalysts for 4-NP reduction, and, through this study, the relationship between morphology, property and reactivity would be correlated to offer useful insights to design and apply 3D nanostructured Co3O4 catalysts for 4-NP reduction