In situ synthesis of metal (Bi0)–semiconductor [BiOX (X = Cl, Br, and I)] nanocomposites as a highly effective catalyst for the reduction of 4-nitrophenol to 4-aminophenol

Abstract

The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) has been efficiently catalyzed by in situ generated Bi⁰–BiOX (X = Cl, Br, I) nanocomposites in the presence of sodium borohydride (NaBH₄). The hierarchical 3D arrays of precursors (BiOCl, BiOBr, and BiOI) have been synthesized by a facile co-precipitation method at ambient temperature. The metallic Bi⁰ is generated in situ by partial reduction of the Bi³⁺ ions in BiOX (X = Cl, Br, and I) mediated by NaBH₄ during the conversion of 4-NP to 4-AP. In situ generated Bi⁰–BiOX shows better catalytic activity than Bi⁰–BiOX composite formed by mixing individually synthesized metallic Bi⁰ particles with semiconducting BiOX nanoparticles. The Bi⁰–BiOI nanocomposite demonstrated higher catalytic activity (k_app = 0.529 min⁻¹) compared to Bi⁰–BiOCl (k_app = 0.095 min⁻¹) and Bi⁰–BiOBr (k_app = 0.098 min⁻¹) due to its efficient conversion into metallic Bi⁰. The approximate percentage composition of in situ generated metallic Bi⁰ on the surface of BiOCl, BiOBr, and BiOI is nearly 10, 17, and 25%, respectively. Further, the reduction of 4-NP has been completed within 24, 22, and 6 minutes by in situ generated Bi⁰–BiOCl, Bi⁰–BiOBr, and Bi⁰–BiOI nanocomposites from BiOCl, BiOBr, and BiOI, respectively. Extensive studies have been conducted to generate a better understanding of the actual catalytic sites present on the surface of the catalysts.