Multidentate Interaction of Methylamine with Rutile TiO2 (110)

Aiming for a comprehensive expertise of the adsorption and activation of small organic molecules on transition metal oxides, the chemistry of methylamine on rutile TiO2 (110) is examined combining surface science experiments with first-principles calculations. Besides the physisorption of a methylamine multilayer, three types of methylamine adsorbates were ascertained: adsorption via hydrogen bonding toward bridging oxygen atoms, adsorption of methylamine at one individual Ti-5c site, and an adsorbate occupying two Ti(5c )sites in an unprecedented multidentate bonding geometry. In all three adsorbate classes, multiple interactions with the TiO2 substrate as well as intermolecular dispersion forces are identified to be crucial to explain the molecule-surface interaction and concomitant adsorption geometries. While the Ti-N bond surely makes the largest contribution to the molecules' adsorption energy, smaller forces such as hydrogen bonds between NH-O and CH-O moieties are shown to be the central key for understanding the chemistry of methylamine on titania surfaces. Comparing different adsorption geometries, these multidentate adsorption motifs can boost the thermal stability in relation to ordinary adsorption, in the case of the adsorbate occupying two Ti-5c sites by more than 100 K or several tens of kJ per mol being thermally stable up to more than 500 K. Surprisingly, the multidentate adsorption is remarkably sensitive to the adsorption conditions and especially does not occur for elevated adsorption temperatures. Curiously enough, low adsorption temperatures thus yield high-temperature desorption species. Our findings are particularly important for the understanding of (photo-)chemical reactions, which usually requires an extensive knowledge of adsorbate-substrate interactions and the bonding situation toward the catalysts surface.