(371a) Invited - Solar Light NO Removal By Flame-Made Sub-Nano Pd Clusters on Nano-TiO2

Noble metal clusters deposited on metal oxide supports are broadly used. Size reduction of such noble metal particles is a classic way to enhance their activity with minimizing the use of expensive noble metals. Recently, loading of extremely small Pd clusters (~ 0.5 nm) on TiO₂ exhibits superior photocatalytic activity for NO removal¹ and its further size reduction from the clusters to the isolated atoms have a great potential to maximize the NO_x removal. Here, TiO₂ with different Pd loadings (0-3%) were prepared by scalable flame aerosol technology to attain atomically dispersed Pd atoms on TiO₂. The photocatalytic NO (1 ppm) removal efficiency of those Pd/TiO₂ under solar light (100 mW/cm²) was evaluated based on standard ISO procedures.

Under solar light irradiation, these materials remove NO_x 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 ^oC for 2 hours in air. However, a fraction of them is reduced into relatively large (> 1 nm) metallic Pd nanoparticles by annealing in N₂ at 400 ^oC for 2 hours, 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.

1. Fujiwara, K.; Müller, U.; Pratsinis, S. E., Pd Subnano-Clusters on TiO₂ for Solar-Light Removal of NO, ACS Catalysis, 6, 1887-1893 (2016).