Carbon Cocatalysts Self-Destruction in Photocatalytic Air Purification

The most promising strategy for photocatalyst design involves loading cocatalysts to enhance charge-carrier separation, with carbon nanomaterials perceived as being more environmentally and economically feasible than noble metals. However, this study reveals a critical limitation: in atmospheric environments where reactant density is relatively low, the generation of OH radicals triggers the breakdown of the C-C network in carbon cocatalysts rather than degrading air pollutants. This self-destruction of carbon cocatalysts not only reduced long-term efficiency but also led to their mineralization into CO2, causing unintended secondary air pollution. We verified the self-destruction mechanism of carbon nanotubes, graphene oxides, and carbon nanofibers loaded onto TiO2 through microscopic and colorimetric analyses in air purification applications. Although these composites initially exhibited higher removal efficiencies for various gaseous pollutants, their performances over an extended period reverted to that of pristine TiO2, while contributing to additional CO2 emission. An economic analysis comparing carbon cocatalysts with noble metals, factoring in material costs, accumulated depreciation, and CO2 emission taxes, revealed that carbon cocatalysts are less economically favorable than noble metal cocatalysts, especially when they are used in practice over the long term. Contrary to perception, using carbon cocatalysts in photocatalytic air purification may offset their benefits, indicating the need for a paradigm shift from focusing on initial cost and removal efficiency to considering the ultimate environmental relevance.