Reaction of water vapor with α-Al2O3(0001) and α-Fe2O3(0001) surfaces:: synchrotron X-ray photoemission studies and thermodynamic calculations

被引:242
作者
Liu, P [1 ]
Kendelewicz, T
Brown, GE
Nelson, EJ
Chambers, SA
机构
[1] Stanford Univ, Dept Geog & Environm Sci, Surface & Aqueous Geochem Grp, Stanford, CA 94305 USA
[2] Stanford Linear Accelerator Ctr, Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA
[3] Pacific NW Lab, Environm Mol Sci Lab, Richland, WA 99352 USA
基金
美国国家卫生研究院;
关键词
aluminum oxide; iron oxide; single-crystal surfaces; surface defects; synchrotron radiation photoelectron spectroscopy; water;
D O I
10.1016/S0039-6028(98)00661-X
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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 reaction times (=30 min, with total exposure ranging from 1.8 L to 1.8 x 10(10) L) at constant p(H2O). A two-stage reaction was observed for each oxide surface, with dissociative chemisorption of water occurring mainly at defect sites below threshold pressures of approximate to 1 Torr and approximate to 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. The threshold pressure of alpha-Al2O3 was accurately predicted using a simple equilibrium thermodynamic model for the conversion of corundum to gibbsite (AI(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 for 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). (C) 1998 Published by Elsevier Science B.V. All rights reserved.
引用
收藏
页码:53 / 65
页数:13
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