Ultrafast time-resolved quantum cascade laser diagnostic for revealing the role of surface formate species in the photocatalytic oxidation of methanol

Abstract

In the present work, a new quantum cascade laser (QCL)-assisted operando FT-IR setup was used to obtain direct insight into the relationship between the overall catalytic performance of TiO₂ photocatalysts in the gas phase photooxidation of methanol at 25 °C, and the kinetics of intermediate surface formate species. Apparent rates of formation (R_app,f) and reaction (R_app,r) of surface formates over three different TiO₂ photocatalysts (P25, commercial anatase, and homemade anatase) have been determined through UV on–off cycles under flowing methanol and oxygen. We show that the R_app,r follows the same trend as the overall catalytic activity in terms of total yield of methylformate and carbon dioxide, namely the following: TiO₂ P25 ≈ commercial anatase > homemade anatase. So, the faster the reaction of the surface formates, the higher the catalytic methanol oxidation activity. Thereby, we demonstrate, to our knowledge for the first time, the direct relationship between the conversion of surface formate species and the catalytic performance of the photocatalysts. Through photocatalytic tests without oxygen in the reaction feed, we also show that the crystallinity of TiO₂ impacts the availability of its lattice oxygen to contribute for both the formation and conversion of formate species. Indeed, the absence of flowing oxygen reduces the R_app,f over TiO₂ CA by a factor 2 while it does not affect that over TiO₂ P25, and it reduces the R_app,r over TiO₂ CA about 4 times more than that over TiO₂ P25.