Single Pd atoms on TiO2 dominate photocatalytic NOx removal

Reducing the particle size of platinum-group metals (PGM) down to single atom can maximize catalyst performance while minimizing their use. Here Pd/TiO2 particles are made by flame spray pyrolysis with closely controlled Pd-content. The fraction of Pd on the TiO2 surface is determined by leaching. For 0.025-1 wt.% of nominal Pd-content, 40-70% of it is on the TiO2 surface dramatically enhancing its solar-light photocatalytic NOx removal. Most importantly, the fraction of isolated Pd atoms on TiO2 is quantified by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using NO as probing molecule and BaSO4 as internal standard. Isolated Pd atoms are identified by the linear NO adsorption peak on cationic single Pd atoms. Their fraction on flame-made Pd/TiO2 linearly increases up to 0.1 wt.% of nominal Pd-content and above that, formation of Pd clusters or particles takes place. In contrast, for photodeposited Pd (0.1 and 1 wt.%) on TiO2, isolated Pd atoms are not found on its surface. The NO removal efficiency linearly increases with the mass fraction of isolated Pd atoms on TiO2, regardless of the presence of Pd clusters or particles. The superior performance of isolated Pd sites is attributed to their high resistance to nitrate poisoning and high selectivity for the conversion of NO to nitrate compared to that of nitrate to NO2.