Autocatalytic Surface Hydroxylation of MgO(100) Terrace Sites Observed under Ambient Conditions

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Abstract

We have investigated the reaction of water vapor with the MgO(100) surface using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), which permits the study of the chemical composition of the MgO/water vapor interface at p(H2O) in the Torr range. Water dissociation on thin MgO(100) films of 4–5.5 monolayers (ML) grown on Ag(100) was studied under isobaric conditions at p(H2O) ranging from 0.005 to 0.5 Torr and temperatures from 380 to −10 °C, up to a maximum relative humidity (RH) of 20%. At RH < 0.01% dissociative adsorption occurs only at defect sites (∼0.08 ML), while terrace sites remain unreactive toward water dissociation. In the range 0.01 < RH < 0.1% there is an abrupt onset of dissociative adsorption at terrace sites which saturates at 1 ML at 0.1% RH, and is accompanied by an increase in molecular water adsorption. At 20% RH there is ∼1 ML of molecularly adsorbed water interacting with a fully hydroxylated interface on MgO(100). The observed onset of hydroxylation near 0.01% RH is suggested to be due to water molecules aggregating at the surface, leading to an autocatalytic dissociation of water at MgO(100) terrace sites.

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