Synthesis of TiO2/copper-based oxide photocatalytic composites from copper smelting flotation slag for photocatalytic H2 evolution

Abstract

Pure TiO₂ and its composites can be used for hydrogen production by a two-step photocatalytic reduction: photocatalytic reduction of Pt (IV) and subsequent H₂ evolution. Here, photocatalysts were synthesized from copper smelting flotation slags from Uzbekistan for hydrogen production. The crystallinity of the two main phases of the slag, CuFe₂O₄ and Fe₃O₄, was increased by calcinating the slag (Cal900Air5h). Subsequently, the Cal900Air5h was composited with TiO₂ by in-situ hydrothermal reaction (TiO₂-5 %Cal900Air5h_HT). The TiO₂-5 %Cal900Air5h_HT catalyzed the production of a significant amount of H₂, 2.6 times more than pure TiO₂. Moreover, the most interesting result is that the TiO₂-5 %Cal900Air5h_HT showed H₂-generating activity not only higher than pure TiO₂ and physically mixed composite (TiO₂-5 %Cal900Air5h_PM), but also higher than the reagent-based hydrothermally fabricated TiO₂ composite (TiO₂-5 %CuFe₂O₄, Fe₃O₄, CuO_HT). This suggested other amorphous phases including Si, Ca, and Al in Cal900Air5h, which are insulators by themselves, might contribute in an indirect way to the photocatalytic reactions. A possible mechanism of the photocatalytic H₂ evolution is as follows: The band structure determined using various techniques indicated that a Type II heterojunction formed mainly between TiO₂ and CuFe₂O₄ and less between TiO₂ and CuO in the calcined material. Therefore, when the composite was exposed to light, electrons were excited in both phases of TiO₂ and CuFe₂O₄ (slightly CuO) in the calcined slag, and then the excited electrons were transferred from CuFe₂O₄ and CuO to TiO₂ through the heterojunction. The electrons were accumulated on the TiO₂ side, and the photocatalytic reduction of H₂O molecules into H₂ on the surfaces of Pt (0) nanoparticles serving as a cocatalyst. This study uniquely explored the recycling of copper slag as an econtomical and sustainable photocatalyst for hydrogen evolution to achieve carbon neutrality.