Atomically dispersed Pd on nanostructured TiO2 for NO removal by solar light

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 degrees C decreases the amorphous TiO2 fraction and increases its crystal and particle sizes as observed by x-ray diffraction (XRD) and N-2 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 degrees C for 2 h in air but at 800 degrees 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 degrees 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. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 139-146, 2017