Redox Dynamics of Pt and Cu Nanoparticles on TiO2 during the Photocatalytic Oxidation of Methanol under Aerobic and Anaerobic Conditions Studied by In Situ Modulated Excitation X-ray Absorption Spectroscopy

In situ modulated excitation X-ray absorption spectroscopy (ME-XAS) at the Pt L3- and Cu K-edges has been successfully used in this work to shed light on the controversial role that Pt and Cu nanoparticles (NPs) on TiO2 have in photocatalysis. TiO2 photocatalysts, either bare or modified by Pt and Cu NPs, synthesized in a single step by flame spray pyrolysis, were tested in the gas-phase photocatalytic oxidation of methanol to CO2 under both aerobic (CH3OH + 3/2O2 → CO2 + 2H2O) and anaerobic (CH3OH + H2O → CO2 + 3H2) conditions. Both Pt-and Cu-containing TiO2 showed lower activity than bare TiO2 under aerobic conditions, while higher H2 and CO2 production rates were attained under anaerobic conditions in the presence of the Pt co-catalyst, with copper-containing TiO2 having in this case an activity similar to that of bare titania. ME-XAS, coupled with phase-sensitive detection, proved in fact that Pt NPs efficiently capture the electrons photopromoted in the conduction band of TiO2 and are the active sites for H2 or H2O evolution under anaerobic or aerobic conditions, respectively. Much more complex is the role of Cu-NPs on titania, with ME-XAS revealing that copper is simultaneously present in different oxidations states, depending on the reaction conditions. Cu NPs are not the reaction active sites under anaerobic conditions, while a Cu(II)/Cu(I) redox switching occurs under chopped irradiation under aerobic conditions.