Noble Metal Promoted TiO2 from Silver-Waste Valorisation: Synergism between Ag and Au

Wastewaters from precious metal industries contain high amounts of noble metals, but their efficient recycling is hindered by the wastewater complex composition. Here, we propose an innovative approach for the efficient recovery of noble metals contained in these metal-enriched wastewaters as precursors for the synthesis of noble metal nanoparticles (NPs) and supported metal catalysts. Silver NPs were synthesized from Ag-enriched wastewater and then deposited on TiO2 to prepare photocatalysts. Then, further promotion of the photocatalytic activity of Ag-modified TiO2 was achieved by the addition of as little as 0.5 wt.% of Au. STEM-EDS analyses proved that Au NPs were located on Ag or AgOx nanoparticles. The contact between the two metal-containing NPs results in charge transfer effects, appreciable both in terms of oxidation states determined by XPS and of optical properties. In particular, the plasmon band of Au NPs shows photochromic effects: under UV light irradiation, bimetallic samples exhibit a blue-shift of the plasmon band, which is reversible under dark storage. The activity of the materials was tested towards ethanol photodegradation under UV light. Adding 0.5 wt.% Au NPs resulted in a promoted activity compared to Ag-TiO2, thus showing synergistic effects between Au and Ag. Ethanol was completely converted already after 1 h of UV irradiation, acetaldehyde was formed as the main oxidation product and fully degraded in less than 180 min. Notably, bimetallic samples showed ethylene formation by a parallel dehydration mechanism.