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Water reactions with oxide surfaces are of key importance in many areas of basic and applied surface science. Over the years a great deal of information on water reactions has been obtained using photoelectron spectroscopy (PES). However, traditional PES requires strict UHV conditions far removed from conditions under which most environmentally, biologically, and catalytically important reactions take place. Although UHV studies have contributed to the understanding of water reactions on dehydrated surfaces at a fundamental level, the possibility of different behavior under high water pressure have not been thoroughly investigated. In this study we used a differentially pumped spectrometer that allows acquisition of spectra to the Torr-range of pressures to study reactions of water vapor with clean (111) mineral surfaces of environmentally common magnetite (Fe3O4 ) at near ambient temperatures. The high analyzer efficiency at the beam line 11.0.2 at Advanced Light Source (ALS) allowed acquisition of O 1s data within seconds of dosing which allows control of surface contamination. The results indicate surface hydroxylation at above 10-4 Torr of pH2O. The amount of hydroxyl species increases steadily with increasing water vapor pressure, but saturates at 0.01 to 1 Torr pH2O where molecular water at the surface and in the gas phase are detected in addition to surface OH. The presence of surface water causes some adsorption of oxidized carbon-based contaminants; their effect on surface reactions can be minimized by fast data acquisition, but they must be considered in quantitative data analysis of isotherms and high pressure isobars. As in prior PES studies evacuation to UHV after high pH2O exposures removes surface molecular water but leaves most of the surface hydroxyls. This suggests that results from dehydrated surface should be similar to earlier traditional UHV studies. However, the results of this work are very different from prior UHV reports, where much larger hydroxylation levels and hydroxyls chemical shifts were observed. The reasons for this discrepancy, most likely related to surface contamination in prior studies, are discussed. |