Paper SS+AS+HC-MoA1
Scanning Tunneling Microscopy Studies of Hydrogen adsorption on the RuO₂(110) Surface

Monday, November 7, 2016, 1:40 pm, Room 104E

Understanding of hydrogen/oxide interactions is important for a variety of fundamental and applied processes. By using high resolution scanning tunneling microscopy (STM), we probed the adsorption of H₂ (or D₂) on model catalyst RuO₂(110) surface, which has wide range of applications in heterogeneous catalysis, hydrogen storage, and many other energy related areas. Well-defined RuO₂(110) surface exposes alternating rows of bridge-bonded oxygen atoms (O_b) and five-fold-coordinated Ru atoms (Ru_cus). STM data indicate that hydrogen molecule dissociates even at 5 K, whereas one hydrogen adatom adsorbs on top of the Ru_cus site (producing a hydrate, H-Ru_cus, species) and the second on top of the adjacent O_b site (forming a bridging hydroxyl, H-O_b, species), generating an H-Ru_cus/H-O_b pair. For the low hydrogen coverage, the dissociated H-Ru_cus/H-O_b pairs adsorb on every alternate Ru_cus/O_b sites adopting a (2×1) registration. When RuO₂(110) surface adopts a such registration of the H-Ru_cus/H-O_b pairs locally, hydrogen starts to adsorb molecularly on top of the Ru_cus sites in between the adjacent dissociated hydrogen-pairs. With further increase of hydrogen coverage, linear arrays of H₂ molecules are formed along Ru_cus rows. The saturation coverage of the hydrogen on the RuO₂(110) surface is observed to be ~0.75 ML, where 1 ML is designated as the Ru_cus site density on the stoichiometric RuO₂(110) surface (5.06x10¹⁴ cm^-2). Upon annealing the hydrogen-covered RuO₂(110) surface, H₂ molecules from the linear array desorb around 110 K. On the other hand, the H-R_cus species of H-R_cus/H-O_b pair transforms (via a proton transfer) into another H-O_b group, across-row from original H-O_b group, producing crosswise H-O_b/H-O_b pair at temperatures above ~250 K.