SMSIs in nanosilver–TiO2 form visible-light active crystalline TiOx while SOSIs through Pd–Ox–Ti stabilize single Pd atoms on TiO2.

Palladium subnano-clusters (<1 nm) on TiO 2 nanoparticles have been prepared in one step by flame aerosol technology. Under solar light irradiation, these materials remove NO x 3 or 7 times faster than commercial TiO 2 (P25, Evonik) with or without photodeposited Pd on it. Xray photoelectron spectroscopy (XPS) reveals that such photodeposited Pd consists of metallic Pd along with several Pd oxidation states. In contrast, flame-made Pd subnanoclusters on TiO 2 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 N 2 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 TiO 2 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 TiO 2 .

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Reducing the particle size of noble metals on ceramic supports can maximize noble metal performance and minimize its use. Here Pd clusters onto nanostructured TiO2 particles are prepared in one step by scalable flame aerosol technology while controlling the Pd cluster size from a few nanometers to that of single atoms. Annealing such materials at appropriate temperatures leads to solar photocatalytic NOx removal in a standard ISO reactor up to 10 times faster than that of commercial TiO2 (P25, Evonik). Such superior performance can be attained by only 0.1 wt.% Pd loading on TiO2. Annealing these flame‐made powders in air up to 600 °C decreases the amorphous TiO2 fraction and increases its crystal and particle sizes as observed by x‐ray diffraction (XRD) and N2 adsorption. The growth of single Pd atoms to Pd clusters on TiO2 prepared at different Pd loading and annealing conditions was investigated by scanning transmission electron microscopy and XRD. The single Pd atoms and clusters on TiO2 are stable up to, at least, 600 °C for 2 h in air but at 800 °C they grow into PdO nanoparticles whose fraction is comparable with the nominal Pd loading. Hence, most of Pd atoms are on the TiO2 surface where at 800 °C they diffuse and coalesce. Diffuse reflectance infrared Fourier transform spectroscopy reveals NO adsorption on single, double, three and fourfold coordinated Pd atoms depending on their synthesis and annealing conditions. The peak intensity of NO adsorption sites involving multiple Pd atoms is substantially lower in TiO2 containing 0.1 wt.% than 1 wt.% Pd but that intensity from single Pd atoms is comparable. This indicates the dominance of isolated Pd atoms compared to clusters in Pd/TiO2 containing 0.1 wt.% Pd that match or exceed the photocatalytic NOx removal of Pd/TiO2 of higher Pd contents. © 2016 American Institute of Chemical Engineers AIChE J, 63: 139–146, 2017

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