Spectroscopic Studies of the Reaction of Water with Metal Oxide Surfaces

Abstract

The reaction of water with mineral surfaces is one of the most fundamental natural reactions in nearsurface environments, yet little is known in detail about its mechanism. We have undertaken a series of synchrotron-based photoemission and L-edge studies of the reaction of water with several clean metal oxide surfaces under ultra-high vacuum conditions. Surfaces studied include MgO(100), CaO(100), etA1203 (0001) and (1-102), ~-Fe203 (0001) and (1-102), Fe304 (001) and (111), and ~-FeOOH. Most of the samples were synthetic single crystals prepared by bulk growth (MgO, CaO, et-A1203) or multiple beam epitaxial (MBE) growth on appropriate lattice-matched substrates (~t-Fe203 on otA1203 and Fe304 on MgO). We also studied the reaction of water on natural single crystal samples of ~-Fe203, Fe304, and ~z-FeOOH. Each of these samples was cleaved or sputtered and annealed in vacuum, and surface contamination was checked by XPS. LEED patterns are consistent with unreconstructed surfaces in each case. Because these materials are insulators, surface charging during photoemission experiments was mitigated using a low energy electron flood gun operated at voltages of 4 6 eV. Water exposures at various water vapour pressures (ranging from 10 -9 Tort to >1 Ton') surface for fixed exposure times (typically 3 minutes) were accomplished using a water dosing needle placed near the sample. Additional exposures to bulk water were accomplished by removing samples from the UHV chamber into a N2-filled glove bag and immersing the sample in DDI water for 10 minutes then returning the sample to the UHV chamber for analysis. In all cases of water vapour exposure, we observed initial reaction of water vapour with surface defect sites, resulting in the formation of surface hydroxyl groups at coverages of less than 0.1 monolayer (ML). After this initial dissociative chemisorption reaction, no further changes in the Ols, O2s, and valence band spectra were observed until a threshold pressure was reached, which varied with the surface studied. In the case of MgO(100) and CaO(100), these threshold pressures were 10 -4 Ton. and 10 -9 Ton., respectively [1]. Comparison of the MgO Ols photoemission spectra with similar spectra of MgO(100) surfaces immersed in bulk water and of polycrystalline Mg(OH)2 indicates that water chemisorbs dissociatively in two distinct stages on 'low defect' MgO(100) surfaces, forming surface hydroxyl groups. The first stage occurs at water vapour exposures ~ 10 _4 Torr for 3 min.] and involves dissociative chemisorption of water on terrace sites, which is not predicted by recent first-principles calculations. The apparent sticking coefficient for the first reaction stage (~> 0.16) is about four orders of magnitude larger than that for the second reaction stage (~> 3 x 10-5), suggesting that the second reaction stage requires significantly more activation energy than the first stage. Our results also suggest that the hydroxylation reaction is not sensitive to exposure time below a threshold pressure o f 10 4 Ton.. Although both kinetic and thermodynamic interpretations are possible, a thermodynamic analysis of the hydroxylation reaction (using bulk solid free energies) predicts approximately the same threshold pressure