Nanostructured systems to combat NOx air pollution through Vis-light activated nanoarchitectonics: how, where and why…?

Nitrogen oxide (NO_x) gases generated from various anthropogenic sources have a highly detrimental impact on both human health and the environment. Among the possible routes for their efficient removal from the atmosphere (DeNO_x), a mandatory issue in compliance with the ever more stringent regulations, photocatalytic processes offer sustainable and eco-friendly toolkits for NO_x elimination at parts per billion (ppb) levels. To date, a great deal of work has been performed on UV-activated photocatalysts based on TiO₂, but real-world applications require the use of Vis-light activated materials to effectively harness solar energy, a renewable and largely available natural resource. In this general framework, the present review provides an original summary of recent advances in the preparation, characterization and functional validation of Vis-light activated DeNO_x photocatalysts free from TiO₂, an overview which, to our knowledge, is not available in the literature. In particular, the attention is concentrated on tailored nanostructure control in the target materials, with particular focus on nano- and heterocomposites enabling improved charge carrier separation and, hence, enhanced performance. The main issues in preparation and characterization, with particular regard to the catalyst stability and selectivity towards NO_x conversion into harmless nitrates, are discussed in relation to selected material categories, with an eye on multi-faceted design strategies and on the outlook for Vis-light activated TiO₂-free DeNO_x photocatalysts. This overview is expected to inspire frontier advancements in the mastering of Vis-light activated photocatalysts for air purification, an issue of key importance to promote effective sustainable development.