PNNL

Abstract

Anatase TiO2 is used extensively in a wide range of heterogeneous and photocatalysis processes. As such, understanding its interactions with prototypical organics such as methanol is of high importance. In this study, we employ a cross-disciplinary approach and study adsorption and dissociation of methanol on the (101) surfaces of anatase TiO2 single crystals (SCs) and faceted nanoparticles (F-NPs). While scanning tunneling microscopy was used to follow the spatial distribution of adsorbed methanol molecules and methoxy and hydroxy species on SC TiO2(101), comparative temperature programmed desorption and infrared reflection absorption spectroscopy measurements were performed on both SC and F-NP TiO2(101). We find that adsorption of molecular methanol at 80 K on SC TiO2(101) leads to the formation of chains along the titania rows. These chains are metastable and fall apart upon annealing due to the repulsion of neighboring molecules. Further, we find that on SC TiO2(101) methanol deprotonates to produce neighboring methoxy and hydroxyl groups following the annealing of high coverages to room temperature (RT). The coverage of methoxy and hydroxyl groups can be increased with repeated adsorption and annealing cycles, and the annealing above room temperature leads to the recombinative desorption of methanol. On F-NP TiO2(101) surfaces, we find that methanol adsorbs both molecularly and dissociatively at RT. With increasing temperature, molecularly adsorbed methanol gradually converts to methoxy. Therefore, parallel behavior in adsorption and surface reaction is found on both SC and F-NP TiO2(101). An added layer of complexity for the latter is that Ti-OH groups on the nanoparticle surfaces, likely associated with defects, also play an important role in methanol adsorption and surface reaction.