TPD characterization of Al-OD-Si sites at the interface of bilayer Al0.42Si0.58O2/Ru(0001) thin-films.

Abstract

Temperature programmed desorption (TPD) has been used to isolate and investigate water interactions unique to bilayer Al0.42Si0.58O2/Ru(0001) via comparison with identical water TPD experiments on a polymorphically equivalent bilayer SiO2/Ru(0001) sample. Relative to SiO2, the aluminosilicate shows evidence of additional desorption processes leading to TPD peaks near 400, 500, and 600 K, with the highest temperature feature exhibiting excellent agreement to previous results documenting infrared monitored loss of Al-OD-Si sites from nominally identical samples. The second order desorption energy estimated from this peak is ~1.6 eV, which is considerably lower than that reported for conventional zeolites, but potentially on par with desorption features reported from other minerals within the feldspar family. Quantification of the peak intensity suggests a ~5% likelihood for formation during water condensation and subsequent desorption. The lower temperature features have been assigned to desorption of molecular water bound at the excess hydroxyl sites. Each hydroxyl appears to bind 2 additional water molecules, which then desorb in a stepwise sequential process. The same low temperature TPD features are also apparent after creating silanol groups on (Al-free) SiO2/Ru(0001) via electron bombardment of supported ice layers, indicating similar water-OH-film interactions independent of specific OH site type.