Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

John T. Newberg; David E. Starr; Susumu Yamamoto; Sarp Kaya; Tom Kendelewicz; Erin R. Mysak; Soeren Porsgaard; Miquel B. Salmeron; Gordon E. Brown; Anders Nilsson; Hendrik Bluhm

doi:10.1016/j.susc.2010.10.004

Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

John T. Newberg, David E. Starr, Susumu Yamamoto, Sarp Kaya, Tom Kendelewicz, Erin R. Mysak, Soeren Porsgaard, Miquel B. Salmeron, Gordon E. Brown, Anders Nilsson, Hendrik Bluhm

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Abstract

To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H₂O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)₂ upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H₂O, OH and Ox thickness changes as a function of relative humidity.

Original language	English
Pages (from-to)	89-94
Number of pages	6
Journal	Surface Science
Volume	605
Issue number	1-2
DOIs	https://doi.org/10.1016/j.susc.2010.10.004
Publication status	Published - 2011 Jan

Keywords

Catalysis
Geochemistry
Magnesium oxide
Surface chemistry

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10.1016/j.susc.2010.10.004

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title = "Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS",

abstract = "To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H2O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)2 upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H2O, OH and Ox thickness changes as a function of relative humidity.",

keywords = "Catalysis, Geochemistry, Magnesium oxide, Surface chemistry",

author = "Newberg, {John T.} and Starr, {David E.} and Susumu Yamamoto and Sarp Kaya and Tom Kendelewicz and Mysak, {Erin R.} and Soeren Porsgaard and Salmeron, {Miquel B.} and Brown, {Gordon E.} and Anders Nilsson and Hendrik Bluhm",

year = "2011",

month = jan,

doi = "10.1016/j.susc.2010.10.004",

language = "English",

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pages = "89--94",

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T1 - Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

AU - Newberg, John T.

AU - Starr, David E.

AU - Yamamoto, Susumu

AU - Kaya, Sarp

AU - Kendelewicz, Tom

AU - Mysak, Erin R.

AU - Porsgaard, Soeren

AU - Salmeron, Miquel B.

AU - Brown, Gordon E.

AU - Nilsson, Anders

AU - Bluhm, Hendrik

PY - 2011/1

Y1 - 2011/1

N2 - To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H2O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)2 upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H2O, OH and Ox thickness changes as a function of relative humidity.

AB - To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H2O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)2 upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H2O, OH and Ox thickness changes as a function of relative humidity.

KW - Catalysis

KW - Geochemistry

KW - Magnesium oxide

KW - Surface chemistry

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Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

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