This study reports and compares the adsorption and dissociation of water on oxidized and reduced CeO 2 (100) and CeO 2 (111) thin films. Water adsorbs dissociatively on both surfaces. On fully oxidized CeO 2 (100) the resulting surface hydroxyls are relatively stable and recombine and desorb as water over a range from 200 to 600 K. The hydroxyls are much less stable on oxidized CeO 2 (111), recombining and desorbing between 200 and 300 K. Water produces 30% more hydroxyls on reduced CeO 1.7 (100) than on oxidized CeO 2 (100). The hydroxyl concentration increases by 160% on reduced CeO 1.7 (111) compared to oxidized CeO 2 (111). On reduced CeO 1.7 (100) most of the hydroxyls still recombine and desorb as water between 200 and 750 K. Most of the hydroxyls on reduced CeO 1.7 (111) react to produce H 2 at 560 K, leaving O on the surface. A relatively small amount of H 2 is produced from reduced CeO 1.7 (100) between 450 and 730 K. The differences in the adsorption and reaction of water on CeO X (100) and CeO X (111) are attributed to different adsorption sites on the two surfaces. The adsorption site on CeO 2 (100) is a bridging site between two Ce cations. This adsorption site does not change when the ceria is reduced. The adsorption site on CeO 2 ( 111) is atop a single Ce cation, and the proton is transferred to a surface O in a site between three Ce cations. When the CeO X (111) is reduced, vacancy sites are produced which allows the water to adsorb and dissociate on the 3-fold Ce cation sites. Recently, Molinari et al. 9 calculated that dissociation is favored

The growth of large-area hexagonal boron nitride (h-BN) monolayers on catalytic metal substrates is a topic of scientific and technological interest. We have used real-time microscopy during the growth process to study h-BN chemical vapor deposition (CVD) from borazine on Ru(0001) single crystals and thin films. At low borazine pressures, individual h-BN domains nucleate sparsely, grow to macroscopic dimensions, and coalescence to form a closed monolayer film. A quantitative analysis shows borazine adsorption and dissociation predominantly on Ru, with the h-BN covered areas being at least 100 times less reactive. We establish strong effects of hydrogen added to the CVD precursor gas in controlling the in-plane expansion and morphology of the growing h-BN domains. High-temperature exposure of h-BN/Ru to pure hydrogen causes the controlled edge detachment of B and N and can be used as a clean etching process for h-BN on metals.

Results of an experimental and computational study of CO 2 adsorption onto a CeO X (100) thin-film surface are reported. For both oxidized CeO 2 (100) and reduced CeO 1.7 (100), a 5 L dose of CO 2 at 180 K resulted in mainly carbonate ([CO 3 ] 2− ) on the surface with a minute amount of physisorbed CO 2 that desorbed by 250 K based on C 1s and O 1s photoemission and C k-edge NEXAFS. No evidence for the formation of a carboxylate intermediate was indicated. Angle-dependent C k-edge NEXAFS revealed that the carbonate species was oriented parallel to the surface suggesting a tridentate configuration. Various adsorption geometries were tested using DFT PBE+U calculations. The most stable configuration was a carbonate with its molecular plane parallel to the surface and each O atom bonded to two Ce cations. Through temperature-programmed desorption (TPD), it was determined that CO 2 was the sole reaction product. CO was not detected in the TPD for the reduced surface, indicating that reoxidation of a reduced CeO 2-X (100) surface by CO 2 did not occur. TPD and photoemission indicated that the coverage and the thermal stability of the [CO 3 ] 2− intermediate were greater on partially reduced CeO 1.7 (100) compared to CeO 2 (100).

Methanol was adsorbed on oxidized and reduced CeOX(100) thin films to probe the active sites and reaction selectivity of these surfaces compared to those of CeOX(111). Roughly twice as much methoxy was formed on oxidized CeO2(100) compared to that formed on CeO2(111). In addition to more methoxy, hydroxyl is also more stable on CeO2(100). Unlike on CeO2(111), however, methanol on CeO2(100) produced CO, CO2, and H2 in addition to water and formaldehyde. The behavior of CeO2(100) is related to its surface structure, which provides greater access to Ce cations and therefore more active adsorption sites and more highly undercoordinated Ce and O. The undercoordinated O may explain the enhanced dehydrogenation activity leading to CO and H2 formation. The reduction of ceria leads to increased methanol uptake on both CeO2 - X(100) and CeO2 - X(111). However, although the uptake doubled on reduced CeO2 - X(111) compared to the oxidized surface, it increased by only 10% on reduced CeO2 - X(100) compared to that on fully oxidized CeO2(100). Reduction of both surfaces leads to a greater production of CO and H2. Reaction on all surfaces progresses rapidly from methoxy to products. There is no spectroscopic evidence of formyl or formate intermediates. On CeOX(100), carbonate is detected that decomposes into CO2 at high temperature.