Investigation of bimetallic Pt-Co and Pt-Ni catalysts in the photoreduction of nitrate

Photocatalytic nitrate (NO₃¯) reduction represents a novel and transformative technology that has the potential to produce harmless gaseous byproducts. The photocatalytic denitrification process is frequently accompanied by the creation of undesirable nitrite (NO₂¯) or ammonium (NH₄⁺), resulting in poor selectivity for dinitrogen (N₂). The catalytic reduction of NO₃¯ ions utilizing bimetallic catalysts necessitates the presence of a noble metal, alongside a promoting transition metal. The transition metal facilitates the reduction of NO₃¯ to NO₂¯ ions through a redox mechanism, which subsequently results in its oxidation. Furthermore, the noble metal's function is to maintain the transition metal in its lower oxidation states via hydrogen spillover. Among oxide photocatalysts, titanium dioxide (TiO₂) has found significant application in photocatalysis and the cleanup of environmental contaminants.

In this report, we present the deposition of platinum (Pt), cobalt (Co), and nickel (Ni) onto the surface of TiO₂ powder through a successive impregnation method. The influence of non-noble metal incorporation over Pt-TiO₂ catalyst was studied by means of powder X-ray diffraction (XRD), diffuse reflectance UV-Vis, and temperature programmed reduction in hydrogen (H₂-TPR) analyses and photoluminescence spectroscopy (PL). The initial activity evaluation focused on the catalytic reduction of NO₃¯ with H₂. The following stage involved the evaluation of the photocatalytic activity of the catalysts by monitoring the reduction of NO₃¯ under the illumination of a UV lamp. A primary goal is to assess these materials for their optimal selectivity in generating benign end products (such as N₂). In the tests conducted under UV light irradiation, the selectivity toward N₂ was around 68%, which is approximately 1.5 times greater than the values obtained during denitrification without light exposure. A preliminary test for water splitting using the Pt-Ni/TiO₂ sample yielded hydrogen. It is hypothesized that hydrogen is generated, yet it is utilized in the process of nitrate photoreduction.