Reaction of Water Vapor with alpha-Al203 and alpha-Fe203(0001) Surfaces: Synchrotron X-ray Photoemission Studies and Thermodynamic Calculations
- Stanford University
- VISITORS
- 7664
- US FISH& WILDLIFE SERV
- BATTELLE (PACIFIC NW LAB)
X-ray photoemission experiments were carried out to study the reaction of water vapor with clean (0001) surfaces of single-crystal corundum (alpha-Al2O3) and single-crystal, thin-film hematite (alpha-Fe2O3) at constant reaction time (3 min) as a function of water vapor pressure[p(H2O)] and, in separate experiments, at longer reactions times(=30 min, with total exposure ranging from 1.8 L to 1.8 x 10 (10) L at constant p(H2)). A two-stage reaction was observed for each oxide surface, with dissociative chemisorption of water occurring mainly at defect sites below threshold pressures of {approx}1 Torr and {approx}10 (-4) Torr for the corundum and hematite (0001) surfaces, respectively. Extensive hydroxylation of these surfaces was found to occur above the respective threshold pressures. Longer exposures to water vapor below these threshold pressures did not result in increased hydroxylation; however, longer exposures above these threshold pressures resulted in increased hydroxylation, as expected. Th e threshold pressure of alpha-Al2O3 was accurately predicted using a simple equilibrium thermodynamic model for the conversion of corundum to gibbsite (Al(OH)3). In contrast, the measured threshold pressure for hematite is more than five orders of magnitude lower than the predicted threshold pressure for the conversion of hematite to FeOOH or Fe(OH)3. This difference between observation and prediction of hematite (0001) is not completely understood, but may be due to the presence of a passivating layer of Fe-hydroxide that reduces the surface energy of the hydroxylated hematite (0001).
- Research Organization:
- Pacific Northwest National Lab., Richland, WA (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC06-76RL01830
- OSTI ID:
- 15003092
- Report Number(s):
- PNNL-SA-30290; KP1301020; TRN: US0402429
- Journal Information:
- Surface Science, Vol. 417; Other Information: PBD: 8 Nov 1998
- Country of Publication:
- United States
- Language:
- English
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