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Palladium subnano-clusters (2 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.
Fujiwara et al. (Mon,) studied this question.