*2.2. Nanoparticle Synthesis*

Au-based nanoparticle synthesis was performed as previously reported [24]. As a general approach, a suitable amount of the metal precursors (HAuCl<sup>4</sup> and PdCl2) were dissolved in water. Once dissolved, glucose, NaOH and the stabilizing agent (PVP) were added to the solution and allowed to react for 2.5 min at 95 ◦C under solvent reflux. Then, the resulting nanoparticle suspension was concentrated using 50 kDa Amicon Ultra filters (Millipore, Burlington, MO, USA) to eliminate excess water. The concentrated suspension was impregnated onto TiO<sup>2</sup> in order to achieve a metal loading (pristine Au or Au + Pd with 6/1 metal ratio) of 1.5 wt.%. After the impregnation, solvent was evaporated by thermal treatment at 120 ◦C. Alternatively, another batch of the Au6Pd<sup>1</sup> NPs colloidal suspension was washed with formic acid in two subsequent filtrations to replace water.
