Modified indophenol blue method enables reliable routine quantification of photocatalytically produced ammonia in aqueous sulfite electrolyte†

Abstract

Extensive research efforts in the chemical engineering community have been devoted to transforming the ammonia synthesis process from the energy-intensive and capital-heavy Haber–Bosch method to alternative green production methods. Photocatalytic and photoelectrochemical methods have emerged as pivotal alternative strategies attributed to environmental sustainability. Sulfides are vital photoactive materials with potential in photocatalytic and photoelectrochemical water splitting and CO₂ reduction, aided by economic sacrificial reagents like sulfite to prevent photocorrosion. However, most of the previously reported ammonia quantification methods in aqueous sulfite electrolyte systems malfunction due to high concentrations of Na⁺ or K⁺ ions or reactions between sulfite ions, sulfide ions, and chromogenic reagents. These obstacles exclude sulfide catalysts from photocatalytic and photoelectrochemical ammonia synthesis, significantly hindering field development. We developed a modified indophenol blue method to reliably and economically quantify ammonia in aqueous sulfite electrolyte using the advanced oxidation method. Furthermore, a standardized operating procedure is established to eliminate potential false-positive results. This work not only provides guidance to reliably and economically quantify ammonia in aqueous sulfite electrolyte but also paves the way for the development of advanced sulfide catalysts for photocatalytic and photoelectrochemical ammonia synthesis applications.