Hybrid photocatalysts composed of titania modified with plasmonic nanoparticles and ruthenium complexes for decomposition of organic compounds

Abstract

Plasmonic photocatalysts were prepared by deposition of 2wt% of gold or silver nanoparticles (NPs) on commercial titania particles with different structural properties. Ruthenium(II) complexes with phosphonic and carboxylic acid binding groups were synthesized and adsorbed on bare titania and noble metal-modified titania. The structural properties of the samples were characterized by DRS, XPS, XRD, STEM, TLC, 1H NMR and 2D-COSY. Large surface area, small crystallite sizes, low pH value, nature of the deprotected phosphonate binding groups, and pre-deposited nanoparticles of noble metals enhanced the adsorption yield. Modification caused titania activation toward visible light (>450nm) for 2-propanol oxidation and for methanol dehydrogenation under UV/vis irradiation. The modified samples exhibited enhanced activity under UV/vis irradiation for acetic acid oxidation depending on the kind of modifiers and properties of titania, i.e., 2–6, 3–9 and 1–3-fold enhancement was observed after modification with NPs of gold, NPs of silver and Ru(II) complex. The time-resolved microwave conductivity (TRMC) method proved that higher photocatalytic activity of modified titania under UV irradiation was caused by scavenging of mobile electrons by NPs of noble metals, and therefore decreasing the recombination between charge carriers. The photocatalytic activity of hybrid photocatalysts under UV/vis was influenced by the nature of the plasmonic metal and structural properties of the metal and titania, e.g., crystallite size and polymorphic form. Different kinds of action were observed for two plasmonic metals in two reaction systems under UV irradiation, e.g., modification with an Ru(II) complex caused 12-times faster dehydrogenation of methanol for silver-modified large anatase titania (ST41) and hardly changed the activity of gold-modified samples, while during acetic acid oxidation, only hybrid photocatalysts composed of gold NPs and Ru(II) complex exhibited a slight increase of photocatalytic activity (1.1–1.3 times). The difference between gold and silver hybrid photocatalysts might have been caused by differences in surface charges of metallic deposits, i.e., the surface of silver NPs was mainly positively charged, while gold was zero-valent. Therefore, it is thought that the Ru(II) complex bound also to the surface of positively charged silver. Under visible light irradiation, fine titania modified with an Ru(II) complex exhibited the highest level of photocatalytic activity. The presence of an Ru(II) complex highly enhanced photocatalytic activity of titania modified with plasmonic NPs. However, NPs of plasmonic metals hindered the photoactivity of Ru(II)-titania.