450281 Invited - Solar Light NO Removal By Flame-Made Sub-Nano Pd Clusters on Nano-TiO2

Tuesday, November 15, 2016: 12:30 PM
Peninsula (Hotel Nikko San Francisco)
Sotiris E. Pratsinis, Department of Mechanical and Process Engineering, Particle Technology Laboratory, ETH Zurich, Zurich, Switzerland

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 NOx 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 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 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).

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