Pd Subnano-Clusters on TiO₂ for Solar-Light Removal of NO

Palladium subnano-clusters (<1 nm) on TiO₂ nanoparticles have been prepared in one step by flame aerosol technology. Under solar light irradiation, these materials remove NOx 3 or 7 times faster than commercial TiO₂ (P25, Evonik) with or without photodeposited Pd on it. X-ray photoelectron spectroscopy (XPS) reveals that such photodeposited Pd consists of metallic Pd along with several Pd oxidation states. In contrast, flame-made Pd subnano-clusters on TiO₂ dominantly consist of an intermediate Pd oxidation state between metallic Pd and PdO. In that intermediate state, the Pd subnano-clusters are stable up to, at least, 600 °C for 2 h in air. However, a fraction of them are reduced into relatively large (>1 nm) metallic Pd nano-particles by annealing in N₂ at 400 °C for 2 h, as elucidated by XPS and scanning transmission electron microscopy. The Pd subnano-clusters interact with oxygen defects on the TiO₂ surface, as shown by Raman spectroscopy. This interaction suppresses CO adsorption on Pd, as observed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), analogous to strong metal–support interactions (SMSI) of nano-sized noble metals on TiO₂.