Synergism of transition metal (Co, Ni, Fe, Mn) nanoparticles and "active support" Fe3O4@C for catalytic reduction of 4-nitrophenol.

Research reports, up to date, on supports for non-noble metal catalyst focus mainly on tuning their surface functionality and increasing surface area to maximize metal loading for high catalytic reduction of 4-nitrophenol. However, the "passive" role of these supports leads to inefficient hydride formation on the metal surface which limits catalytic activity. Herein, we present Fe3O4@porous-conductive carbon (Fe3O4@C-A) core-shell structure as an "active" support for non-noble metals (M = Co, Ni, Fe, and Mn) nanoparticles. Fe3O4@C-A was prepared by annealing Fe3O4@dense-carbon (Fe3O4@C) under N2. The resultant M-Fe3O4@C-A catalysts show high catalytic performance at very low metal loading, while non-noble metals supported on a "passive" support (Fe3O4@C) shows very low activity even at high metal loading. The significant difference in catalytic activity is ascribed to the synergistic effect amongst Fe3O4, conductive carbon and metal nanoparticles which leads to efficient hydride formation. Amongst the prepared catalysts, Ni-Fe3O4@C-A and Co-Fe3O4@C-A show the best catalytic activity, completing 4-nitrophenol reduction within 50 s and 80 s, respectively, in the presence of NaBH4. This result is comparable with previously reported noble-metal-based nanocomposites. In addition, Co-Fe3O4@C-A shows high recyclability in 5 consecutive catalytic reactions. In the broader context, our finding highlights how an "active support" together with non-noble metals can provide an efficient mechanism for hydride formation, subsequently accelerating the catalytic reduction of 4-nitrophenol.