Improving the selectivity of photocatalytic NOx abatement through improved O2 reduction pathways using Ti0.909W0.091O2Nx semiconductor nanoparticles: from characterisation to photocatalytic performance

In this paper we provide detailed insight into the electronic-crystallographic-structural relationship for Ti0.909W0.091O2Nx semiconductor nanoparticles, explaining the mutual electronic and magnetic influence of the photo-induced, stable N- and W-based paramagnetic centres, their involvement in the photo-induced charge carriers trapping and their role in improving the nitrate selectivity of the photocatalytic oxidation of NOx to nitrates. In particular, reduced tungsten species in various crystallographic environments within the anatase host lattice were observed to play a fundamental role in storing and stabilising photo-generated electrons. Here we show how these reduced centres can catalyse multi-electron transfer events without the need for rare and expensive platinum group metals (PGMs). This allows for a versatile and elegant design of redox potentials. As a result, electron transfer processes that are kinetically inaccessible with metal oxides such as TiO2 can now be accessed, enabling dramatic improvements in reaction selectivity. The photocatalytic abatement of NOx towards non-toxic products is exemplified here and is shown to pivot on multiple routes for molecular oxygen reduction. The same rationale can furthermore be applied to other photocatalytic processes. The observations described in this work could open new exciting avenues in semiconductor photocatalysis for environmental remediation technologies, where the optimisation of molecular oxygen reduction, together with the pollutant species to be oxidised, becomes a central element of the catalyst design, without relying on the use of rare and expensive PGMs.