(499e) The Effect Pd Size on TiO2 for Photocatalytic NOx Removal

Sotiris E. Pratsinis

Particle Technology Laboratory, Institute of Process Engineering,

Department of Mechanical & Process Engineering, ETH Zurich, Switzerland

Reducing the particle size of noble metals on ceramic supports can maximize noble metal performance and minimize its use (Flytzani-Stephanopoulos & Gates, Annu. Rev. Chem. Biomol. Eng. 2012, 3:545). 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 (Fujiwara et al., ACS Catalysis, 2017, 6:1887). Annealing such materials at appropriate temperatures leads to photocatalytic NO_x removal in a standard ISO reactor up to10 times faster than that of commercial TiO₂ (P25, Evonik). Such superior performance can be attained by only 0.1 wt% Pd loading on TiO₂ (Fujiwara & Pratsinis, AIChE J.2017, 63:139-146).

Annealing these flame-made powders in air up to 600 ^oC decreases also 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 (STEM) and XRD. The single Pd atoms and clusters on TiO₂ are stable up to, at least, 600 ^oC for 2 hours in air but at 800 ^oC 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 ^oC they diffuse and coalesce. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals NO adsorption on single, double, 3- and 4-fold 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 NO_x removal at much higher Pd contents. Here, the number of isolated Pd sites in different Pd/TiO₂ catalysts is quantitatively compared.