Paper SS+EN-MoM2
Infrared Reflection-Absorption Spectroscopy Study of Adsorption and Photo-Decomposition of Formic Acid on Reduced and Defective Rutile TiO2 (110) Surfaces
Monday, November 10, 2014, 8:40 am, Room 315
Adsorption and photo-decomposition of formic acid on rutile TiO2(110) have been investigated with infrared reflection-absorption spectroscopy (IRRAS) employing p- and s-polarized light along the [001] and [1-10] crystal directions. The single crystal surfaces were prepared either by sputtering and annealing in ultra-high vacuum (UHV) to obtain a reduced surface (r-TiO2), or by sputtering alone to create a rough, highly defective surface (sp-TiO2). Results are compared with corresponding measurements in synthetic air on rutile nanocrystals performed. IRRAS spectra obtained on r-TiO2 and rutile nanocrystals are very similar (Fig. S1), and show that in both cases formic acid dissociates and is predominately adsorbed as a bridging bidentate formate species,1 demonstrating that the adsorption structure on the nanocrystals is determined by interactions with majority (110) surfaces. In contrast, the IRRAS spectra on sp-TiO2 are different (Fig. S1), with only minor spectral features associated with (110) surfaces, which can be explained by changed adsorption geometry due to bonding to low-coordinated Ti3+ atoms. IRRAS measurements in UHV on thin nanoporous rutile films, made by reactive DC sputtering, were performed to compare the adsorption geometry of formate with that for single crystal surfaces and nanoparticles. The UV-induced rate of formate photo-decomposition is about 30 times higher on rutile nanocrystals in synthetic air compared with sp-TiO2 under UHV conditions, and even larger than on r-TiO2.2 These differences are explained by the lack of oxygen and limited hydroxyl coverage under UHV conditions (thus quenching electron scavenging by adsorbed O2 and lowering OH radical formation), and by strong bonding of formate on (110) surfaces which lowers the reactivity on r-TiO2 further. Our results suggest that surface reaction studies of formic acid conducted at elevated pressures on rutile nanocrystals can be accurately modelled with single crystal studies conducted in UHV.1 A. Mattsson, S-L. Hu, K, Hermansson, L. Österlund, Journal of Chemical Physics 140 (2014) 034705
2 L. Österlund, Solid State Phenomena 162 (2010) 203-219