| AVS 55th International Symposium & Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS1+NC-ThA |
| Session: | Water-Surface Interactions |
| Presenter: | J.T. Newberg, Lawrence Berkeley National Lab. |
| Authors: | J.T. Newberg, Lawrence Berkeley National Lab. D.E. Starr, Lawrence Berkeley National Lab. S. Yamamoto, Stanford Synchrotron Radiation Lab. S. Kaya, Stanford Synchrotron Radiation Lab. H. Ogasawara, Stanford Synchrotron Radiation Lab. T. Kendelewicz, Stanford University M. Salmeron, Lawrence Berkeley National Lab. G.E. Brown, Stanford University A.R. Nilsson, Stanford Synchrotron Radiation Lab. H. Bluhm, Lawrence Berkeley National Lab. |
| Correspondent: | Click to Email |
The MgO(100) substrate is one of the most widely studied surfaces for water adsorption.1 However, fundamental questions about whether water adsorbs molecularly or dissociatively under ambient conditions remains unanswered. This has been due in part to the lack of an in situ, chemically specific, surface sensitive technique. CO2 is an important greenhouse gas, and the carbonation of MgO in mineral deposits has been suggested as a potential medium for CO2 sequestration.2 Here we present results from the investigation of the interaction of water with MgO(100)/Ag(100) films using ambient pressure XPS (AP XPS). With AP XPS we can quantitatively probe the water film thickness along with the chemical speciation of the solid substrate, while in equilibrium with water vapor. We have characterized the uptake of water on MgO at water pressures from 10-9 to 1 Torr, up to a maximum of 25% relative humidity (RH). In addition, we monitored the interaction of CO2 with the metal-oxide surface. At room temperature, both MgO hydroxylation and molecular water adsorption were observed at < 10-6 Torr. At ~0.1% RH about 0.3 ML of molecular water was observed (1 ML = 0.31 nm). However, at this RH the surface of MgO was completely passivated with an overlayer of hydroxide that has a thickness similar to that of brucite (Mg(OH)2, 1 ML = 0.48nm). As the RH was increased to 25% RH, the Mg-hydroxide overlayer thickness remained at ~1 ML, while the molecular water film increased to ~1.5 ML. Preliminary results for CO2 showed some dependency of RH on the reactivity towards the metal-oxide surface. The formation of a brucite-like overlayer is consistent with a favorable Gibbs free energy for the bulk reaction of liquid and gas phase water with MgO (-27 and -36 kJ/mol, respectively). A similar phenomenon was observed with AP XPS for water on hematite (Fe2O3).3 These results indicate that even under the lowest ambient RH values in the environment, metal-oxides that have thermodynamically stable hydroxides are chemically transformed at the surface due to thin film water. Thus, the presence of thin film water can have implications for how mineral surfaces interact with organic, biological and inorganic species in the environment.
1 M.A. Henderson 2002 Surf. Sci Rep. 46 1.
2 T. Koljonen et al. 2004 Energy 29 1521.
3 S. Yamamoto et al. publication in preparation.