Redox Dynamics of Pt and Cu Nanoparticles on TiO<sub>2</sub> during the Photocatalytic Oxidation of Methanol under Aerobic and Anaerobic Conditions Studied by In Situ Modulated Excitation X-ray Absorption Spectroscopy
Gian Luca Chiarello, Massimo Bernareggi, Elena Selli
Abstract
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 TiO 2 have in photocatalysis.TiO 2 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 CO 2 under both aerobic (CH 3 OH + 3/2O 2 → CO 2 + 2H 2 O) and anaerobic (CH 3 OH + H 2 O → CO 2 + 3H 2 ) conditions.Both Ptand Cu-containing TiO 2 showed lower activity than bare TiO 2 under aerobic conditions, while higher H 2 and CO 2 production rates were attained under anaerobic conditions in the presence of the Pt co-catalyst, with copper-containing TiO 2 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 TiO 2 and are the active sites for H 2 or H 2 O 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.